Advancements in Nanoenabled Membrane Distillation for a Sustainable Water‐Energy‐Environment Nexus

The emergence of nano innovations in membrane distillation (MD) has garnered increasing scientific interest. This enables the exploration of state‐of‐the‐art nano‐enabled MD membranes with desirable properties, which significantly improve the efficiency and reliability of the MD process and open up opportunities for achieving a sustainable water‐energy‐environment (WEE) nexus. This comprehensive review provides broad coverage and in‐depth analysis of recent innovations in nano‐enabled MD membranes, focusing on their role in achieving desirable properties, such as strong liquid‐repellence, high resistance to scaling, fouling, and wetting, as well as efficient self‐heating and self‐cleaning functionalities. The recent developments in nano‐enhanced photothermal‐catalytic applications for water‐energy co‐generation within a single MD system are also discussed. Furthermore, the bottlenecks are identified that impede the scale‐up of nanoenhanced MD membranes and a future roadmap is proposed for their sustainable commercialiation. This holistic overview is expected to inspire future research and development efforts to fully harness the potential of nano‐enabled MD membranes to achieve sustainable integration of water, energy, and the environment.

desalinated water or reclaimed water, is considered a vital and indispensable component of urban water supply systems. [4]onventional technologies, like reverse osmosis (RO)accounting for over 60% of total installed desalination capacityand multi-stage flash (MSF) and multi-effect distillation (MED), together making up 34%, [5,6] are prevalent but face significant challenges, such as high energy consumption and costs and complex operational and maintenance issues. [7]Recently, sustainable desalination technologies that prioritize maximal freshwater output with minimal energy usage, expenses, and environmental impacts are gaining significant attention for their potential to foster a sustainable WEE nexus. [8,9]ne such emerging desalination technology is membrane distillation, a hybrid membrane-thermal process that operates on the principles of thermally driven vapor transport across a hydrophobic membrane to extract water vapor from saline water and effectively reject non-volatile solutes.

Membrane Distillation (MD)
MD is a non-isothermal separation process that originated in the 1960s when Bodell patented an apparatus designed to derive potable water from an aqueous mixture, [10] a discovery followed by several other patents and scholarly articles. [11,12]After a period of initial interest, MD remained relatively unexplored until the 1980s, when the advent of enhanced membranes, like the commercially available polytetrafluoroethylene (PTFE) membranes by Gore & Associates, brought it back into focus. [13]Since then, MD has undergone substantial enhancements and has been thoroughly researched for applications such as desalination, wastewater treatment, resource recovery, and food processing (Figure 1).
Early MD research was centered on developing membranes that could withstand high temperatures and pressures.Subsequently, the focus shifted towards the development of membranes with improved hydrophobicity, aimed at enhancing separation efficiency by avoiding the mixing of hot and cold streams.Progress in material science and fabrication techniques have given rise to membranes with increased flux and superior separation efficiency. [14]Innovative module designs and process configurations have broadened the applications of MD, allowing it to compete with more established separation technologies. [15]Today, MD continues to be a vigorous area of research, with ongoing efforts to further refine its performance and expand its range of applications.
The operating principle of MD relies on thermally driven vapor transport (i.e., mass transfer) across a non-wetted hydrophobic membrane.The microporous membrane allows vapor passage, while its hydrophobicity prevents liquid infiltration.The vapor pressure gradient, created by the temperature difference between the hot feed and cold permeate sides, drives vapor diffusion through the membrane.The transported vapor condenses on the permeate side, releasing its latent heat and yielding purified water. [16,17]The heat of condensation can be recovered J. Guo School of Chemical Engineering and Technology Xi'an Jiaotong University Xi'an 710049, China or removed by a coolant, which maintains the temperature difference and perpetuates the vapor pressure gradient.MD involves concurrent mass and heat transfers, with mass transfer referring to the conveyance of vapors and gases through the membrane pores.Theoretical models for mass transfer have suggested different transport mechanisms, including Knudsen flow, Poiseuille flow, and ordinary molecular diffusion models or a combination thereof.Heat transfer refers to the transport of heat through both the membrane pores and the membrane matrix, including the latent heat carried by the transferred vapor as well as the heat conducted through either the membrane material or the membrane pores. [18]Latent heat drives the process and represents about 50-80% of the total heat transferred, offering an opportunity to enhance the efficiency of MD through latent heat recovery, as will be discussed later.Conversely, conduction, which results in energy loss, has a less significant impact on the process's efficiency.In MD, heat also transfers through convection at the feed and permeate boundary layers, causing temperature polarization-a phenomenon that limits MD's efficiency which will be discussed in greater detail in a later section.The mechanism and general principles of MD are depicted in Figure 2.
MD processes can be classified based on the configuration of the membrane module and the method used for heat and mass transfer.The main types of MD processes, which differ only in the permeate side configuration, are direct contact (DCMD), vacuum (VMD), air gap (AGMD), and sweeping gas (SGMD) membrane distillation.With feed and permeate directly contacting the membrane, DCMD is the simplest and serves as the foundation for other MD processes.VMD applies vacuum pressure on the permeate side to enhance the vapor pressure gradient, allowing for higher fluxes and better separation efficiency but increasing the risk of pore wetting and requiring additional energy input to sustain the vacuum.AGMD introduces an air layer to reduce conduction; however, this configuration amplifies mass transfer resistance, as the permeate has to bypass the air barrier, leading to lower fluxes compared to DCMD.SGMD employs an inert sweeping gas on the permeate side to carry vapors and condense them outside the membrane module, reducing mass transfer resistance and resulting in higher fluxes, but increasing energy demand due to the requirement for gas circulation.
[21] MD has tremendous potential for seawater and brackish water desalination, thanks to its high salt rejection capability and tolerance for high salinity levels. [22]Additionally, MD has been utilized to treat municipal and industrial wastewaters containing organic and inorganic contaminants, achieving high separation efficiency and water recovery. [15]Furthermore, MD facilitates the recovery of valuable compounds from aqueous solutions, such as nutrients from wastewater or minerals from industrial effluents.Moreover, MD's ability to be operated at low feed temperatures has broadened its application in food processing, such as milk concentration, [23] recovery of volatile aroma compounds, [24] and juice concentration. [25]In recent years, MD has undergone a paradigm shift in its application, extending beyond its conventional role as a liquid separation technology, and is now also Timeline of MD development showing major milestone events, initial MD projects/pilot plans, and major technology developers.Developments in MD started in the early 1960s with some patents and papers but then suffered a regression in the 1970s due to low fluxes before gaining momentum again in the 1980s with the development of membranes and membrane modules.Black square symbols (■) refer to major milestones; blue downward triangle symbols (▼) refer to initial MD projects/pilot plans; and purple upward triangle symbols (▲) refer to main technology developer companies.The timeline was structured based on the literature review as well as the references. [14,18].
being utilized for synergistic applications in water and energy production. [26]he selection of membrane material plays a crucial role in determining the effectiveness of MD processes, as it profoundly influences the membrane's performance and durability.Membrane materials must possess specific characteristics, such as hydrophobicity, high porosity, robust thermal stability, and chemical resistance, to ensure optimal performance. [18]Traditionally, two key categories of membrane materials have been employedpolymers and ceramic-each with its unique strengths and limitations.
Polymeric materials have gained widespread use in MD membranes due to their inherent hydrophobicity, cost-effectiveness, and ease of fabrication.Notable polymeric materials used in MD include polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), polypropylene (PP), and polyethylene (PE).These materials are valued for their hydrophobicity, which is essential for efficient vapor transport during the MD process.However, they face significant challenges related to limited thermal and chemical stability.[29] Additionally, these materials may be susceptible to fouling and scaling, which can compromise long-term performance.
In contrast, ceramic membranes, composed of materials like alumina, zirconia, and titania, offer superior stability at high temperatures and exhibit excellent resistance to harsh chemical environments, making them ideal candidates for MD applications in demanding conditions.However, their fabrication is more complex and typically involves advanced techniques that can be time-consuming and costly, which may limit their attractiveness for large-scale commercial applications. [30,31]Furthermore, ceramic membranes can be brittle, making them more susceptible to mechanical damage or breakage during handling and operation.These limitations, along with their relatively higher cost compared to polymeric membranes, have led to a careful consideration of their use in specific MD applications where their unique properties outweigh these challenges.
To date, MD has undergone significant advancements and garnered attention for its potential for treating seawater and wastewater, its energy-efficient operation, and its cost-effectiveness compared to conventional distillation systems.MD offers a viable solution to address challenges faced by various industries and society at large by providing a cost-effective approach for desalination, demineralization, and concentration.However, despite its promise, commercial adoption of MD is still in its early stages when compared to more established technologies like reverse osmosis (RO) and nanofiltration (NF).Yet, there is substantial scope for innovation, optimization, exploration of novel Figure 2. Schematic of the MD process with illustrations of (i) the vaporization/condensation, (ii) the mechanism of transport through the pores of the membrane, and (iii) heat transfer resistances in MD.The water evaporates at the liquid/air interface of the feed side, travels as a gas through the membrane pores, and condenses back to pure water at the air/liquid interface of the permeate side (inset i).Simultaneous mass and heat transfer take place during MD, where mass transfer refers to the transport/flux of vapors and gases through the membrane pores, while heat transfer refers to the transport of heat through both the membrane pores and membrane matrix (i.e., the latent heat accompanied within the transferred vapor as well as the heat transferred by conduction through either the membrane material or the membrane pores).In MD, the vapor is believed to transport through the pore via two main mechanisms: Knudsen flow and molecular diffusion flow (inset ii).In addition to vaporization and conduction heat losses, heat is also lost through convection at the feed and permeate sides (inset iii).Local spatial distribution of temperature and salt concentration in the direction normal to the membrane is shown in the main figure, featuring temperature and concentration polarizations.The heat losses at the feed and permeate boundary layers result in a temperature difference ΔT m (i.e., effective driving force) smaller than the applied bulk temperature difference ΔT b , resulting in temperature polarization and consequently, energy inefficiency.Similarly, the accumulation of salts at the feed-side membrane surface due to continuous separation results in a higher salt concentration than the bulk feed.T and C refer to temperature and concentration, respectively; subscripts b and m refer to the properties in the bulk and at the membrane surface, respectively; subscripts f and p denote the properties of the feed and permeate solutions, respectively.
applications, and better integration with other technologies to further enhance the capabilities of MD.

Where MD Stands Out among Other Membrane and Thermal Desalination Technologies
MD holds several advantages over other membrane-based (e.g., RO), and thermal-based (e.g., MSF and MED) technologies.One of the significant benefits of MD is its remarkable compatibility with renewable and unconventional heating sources, such as low-grade industrial waste heat and solar thermal and geothermal energy, for driving the process.This makes MD particularly suitable for energy-efficient desalination and treating highsalinity brines.Industrial low-grade waste heat holds great potential to realize energy-efficient MD, especially with the large volume of waste heat discharged from industrial processes and power plants.The utilization of waste heat in MD not only helps maintain its energy efficiency but also promotes a sustainable WEE nexus by reducing environmental hazards such as thermal air and aquatic pollution, which can have adverse effects on human and aquatic health and contribute to global warming. [32]eyond waste heat, MD can also harness low-grade solar thermal and geothermal energy. [33]Low-temperature solar thermal collectors are economical, emissions-free, and can be assembled from simple components. [34,35]Similarly, low-grade geothermal energy is abundant and stable. [36,37]'s simplicity compared to other membrane-based technologies is another appealing feature, especially for applications where space is limited.MD systems require minimal and less complex equipment and have a simpler design compared to RO and other thermal processes, which necessitate high-pressure pumps and intricate machinery.MD's lower pretreatment and membrane cleaning/changing requirements contribute to its streamlined design and smaller footprint.Particularly, MD proves advantageous for small-scale decentralized water purification systems in remote areas that face infrastructural challenges and lack centralized systems, especially when coupled with renewable energy sources. [38]Moreover, the compact and modular design of the MD system makes it suitable for applications with varying treatment requirements based on location and time, such as shale gas wastewater, without compromising efficiency. [39]omplete rejection of nonvolatiles, including dissolved solids, macromolecules, and colloids, is another key advantage for MD.This makes MD a promising technology for sustainable water treatment, especially where the rejection of non-volatile solutes, such as dissolved salts, is critical for system success.Moreover, MD's tolerance for high salinities further enables it to treat challenging hypersaline wastewater, such as those from oil/gas production, with salt levels exceeding RO's limit of 80 g L −1 (up to 360 g L −1 ). [41]In these applications, RO is constrained to about a 50% recovery rate due to osmotic pressure, [42] whereas, not being a pressure-driven process, MD can achieve a 98% recovery rate, capable of treating challenging hypersaline wastewater and purifying RO brines for potable reuse. [43]MD is robust for this application, whereas MSF/MED carries high capital costs. [44]Additionally, the hypersaline tolerance of MD allows it to be coupled with brine crystallizers to achieve zero liquid discharge (ZLD), in which MD is used to concentrate brines before crystallizing solids. [45,46]The environmental impacts from disposing of these solid wastes, typically at landfill sites, are significantly lower than the disposal of hypersaline brines in liquid form. [47]Moreover, the output of these crystallizers can be exploited as salts or used in the production of salt by-products.

Where Does MD Fall Behind?
While MD presents a viable solution to several challenges in desalination and wastewater treatment, it nonetheless exhibits some techno-economic limitations.A major drawback is that it requires high thermal energy consumption to vaporize bulk feedwater, resulting in increased costs and scalability constraints.For instance, in MD, a high vapor flux is achieved by maintaining a high and constant ΔT across the hydrophobic membrane; however, this is conventionally achieved by continuously heating the bulk feedwater.In other words, a large volume of water must be pre-heated before it flows through the MD module.Heating the bulk feed is particularly energy-intensive, constituting the majority of MD's total energy demand.This substantial energy requirement poses a major impediment to MD's widespread adoption as it escalates both the economic and environmental costs associated with the process.The high thermal energy consumption of MD stems from its inherent energy inefficiency due to the large enthalpy of vaporization (ΔH vap ) required for phase change distillation, [48] which is 2-3 orders of magnitude greater than the thermodynamic minimum (Gibbs free energy of separation, ΔG sep ) for ion separation processes such as RO. [7,49]With no latent heat recovery, MD necessitates ≈600× ΔH vap versus 2-3× ΔG sep for RO to treat an equivalent volume of water. [7,40]Approaches to enhance MD's energy efficiency encompass latent heat recovery, [35,50] innovative membranes and membrane configurations, minimizing conductive heat loss, [40] and the integration of renewable energy.These strategies aim to increase the gain output ratio (GOR), a metric used to evaluate the efficiency of distillation processes including MD, which measures the distillate produced per energy input (ΔH vap ).Recovering latent heat for preheating can increase GOR beyond the baseline of 1.0 (referring that the latent heat for evaporating 1 kg of water to produce 1 kg of distillate). [40]ulk water heating not only requires high thermal energy but also aggravates temperature polarization (TP), causing a reduction in the surface temperature of the membrane−water interface relative to the bulk value and leading to decreased mass-transport rates across the membrane, which thereby decreases MD's thermal efficiency and water production rates. [51,52] [53,7]A standard MD process can experience a 50−70% reduction in the practical driving force due to the TP effect alone. [7]Both the high energy requirements and TP contribute to MD's energy inefficiency and are considered significant hurdles impeding the large-scale and practical application of the MD process.MD's low water production rate due to lack of applied pressure, which generally results in lower permeate flux than pressurized RO systems, further restricts its large-scale application.Although less vulnerable than RO membranes, standard MD membranes are also susceptible to scaling, fouling, and wetting issues, which decreases performance efficiency and shortens membrane lifespan.With high-salinity feeds, where MD outperforms other membrane-based processes, inorganic fouling (i.e., scaling) becomes a major problem that can hinder the process if not properly addressed.The fouling/scaling layer can impose additional mass transfer resistance and restrict the hydrodynamic mixing at the membrane/feed interface, which results in flux reductions and exacerbate the TP effect, respectively. [54,55]Organics in the feedwater and low surface tension molecules can also wet the hydrophobic pores of MD membranes by attaching to the membrane surface or reducing the feed's surface tension, allowing liquids to penetrate the membrane pores and impede vapor transport.Figure 3 provides a summary of the areas where MD excels or falls short compared to other dominant membrane and thermal desalination technologies.
Recent advancements in nanotechnology have offered practical and viable solutions to address all of MD's intrinsic limitations, including high thermal energy consumption, temperature polarization, scaling/fouling/wetting control, and low water productivity by utilizing innovative nanoenabled self-heating, microstructured, and self-cleaning membranes.Subsequent sections will elaborate on these membranes and how nanomaterials are integrated in their fabrication to enhance the energy efficiency and system performance of MD.

Significance and Application of Nanotechnology in MD
Nanotechnology has emerged as a revolutionary field with immense potential to transform various industries, and MD is no exception.The integration of nanotechnology in MD involves the utilization of nanomaterials (NMs), i.e., materials with unique properties and structures at the nanoscale.NMs, including carbon-based NMs (i.e., carbon nanotubes (CNT), graphene, and carbon black (CB)), metallic-based NMs (i.e., pure metals, metal oxides, and metal derivatives), and metal-organic frameworks (MOFs), can be incorporated into the membrane matrix or coated onto the membrane surface to improve key membrane properties and overall MD performance.Figure 4 presents an overview of the applications of nanotechnology for the development of novel MD membranes and processes.
As discussed in Section 1.2.above, traditional MD membrane materials face several challenges.Recently, with the emergence of nanotechnology, a transformative shift has occurred in the realm of MD membranes.Nanoenabled membranes have harnessed the unique physio-chemical properties of nanomaterials (NMs), paving the way for novel membrane designs that bridge the gap between traditional polymers and ceramics.These innovative membrane types, including mixed-matrix and nanocomposite membranes, have brought forth solutions to the challenges posed by traditional membrane materials while unlocking new capabilities in MD technology.Mixed matrix membranes integrate inorganic fillers into polymers to enhance stability while preserving processability and cost benefits, [56][57][58] while nanocomposite  [40] According to the original table: "size data is based on plant data from DesalData (Global Water Intelligence, Oxford, UK), where effective small-scale operation refers to a produced water flow rate of <1000 m 3 day −1 ; energy efficiency is determined from several reviews; for use of low-grade energy, three blocks (excellent) refers to <70 °C, while one block (poor) is >110 °C or unamenable to heat input as in RO; minimal pretreatment performance is determined by comparing chemical additive costs relative to RO, where the other technologies shown are 50-66% of RO (two blocks) or less than 50% (three blocks); lifespan cost data was included from several sources including DesalData, where three blocks is <1 $ m −3 and one block is >10 $ m −3 ."61][62] Moreover, conventional MD membrane materials are limited in their ability to repel both high and low-surface-tension liquids effectively.They are also prone to fouling, scaling, and wetting, which can compromise their long-term performance.In contrast, the addition of carefully selected NMs offers a breakthrough solution.These nanomaterials endow MD membranes with enhanced liquid-repellence, increased resistance to fouling and scaling, enhanced transport properties, and improved selfcleaning functionalities.This transformative effect is driven by the unique properties of nanomaterials, such as their high surface area, surface energy modulation, and tunable surface chemistry, which make them highly suitable for MD applications.By incorporating NMs into MD membranes, we bridge the gap between the limitations of traditional materials and the demands of modern applications.The nanoenabled MD membranes combine the best of both worlds, offering enhanced performance, improved stability, and increased efficiency, ultimately propelling us toward more sustainable and effective water and energy solutions.
Thus far, researchers have been able to achieve remarkable enhancements in various key aspects of the MD membranes and process by utilizing NMs.Nanoparticles (NPs) and nanofibers have been extensively used to improve the surface area, hydrophobicity, pore size, and thermal conductivity of MD membranes.By incorporating NPs, such as titanium dioxide (TiO 2 ), CNTs, or zinc oxide (ZnO), into the membrane matrix, researchers have been able to create nanoporous structures within the membranes that increase the surface area and provide additional pathways for water vapor transport, resulting in higher vapor flux and improved separation efficiency.
Additionally, it became possible to endow enhanced hydrophobicity to MD membranes, which leads to improved vapor permeation and enhanced anti-wetting and anti-scaling properties, by incorporating NMs through surface modifications or the deposition of hydrophobic nanocoating that reduce the interfacial energy between the membrane and liquid water.The optimization and precise tuning of pore sizes through nanotechnology have enabled better control over separation characteristics, resulting in improved selectivity and reduced fouling potential.Furthermore, the incorporation of NMs with low thermal conductivity has reduced the heat transfer efficiency in MD, leading to more efficient energy utilization and enhanced performance in the MD process.
The role of nanotechnology in MD extends beyond the enhancement of physiochemical properties and overall performance.It has also paved the way for the development of membranes with unique characteristics, such as self-heating, selfcleaning, and smart sensing and monitoring capabilities.The use of photothermal, plasmonic, and conductive NPs to enable self-heating properties in membranes has recently emerged as a promising alternative approach to traditional bulk feedwater heating by offering localized feedwater heating at the membrane surface.Other nanocoatings comprising photo-or thermoresponsive NPs can be applied to develop self-cleaning membranes capable of reducing fouling and enhancing their long-term performance and durability without any chemical addition.Moreover, incorporating NMs with sensing capabilities into the membrane matrix can yield smart MD membranes capable of acquiring real-time information about the process conditions. [63]hese smart membranes can monitor variables such as temperature, pressure, or concentration gradients, offering opportunities for real-time process monitoring and providing valuable feedback for process optimization and control.
The advancements in photothermal and photocatalytic MD have paved the way for the development of novel simultaneous processes that combine the benefits of both.Photothermal MD harnesses the self-heating properties of NMs to enhance the temperature gradient across the membrane and improve energy efficiency, while photocatalytic MD utilizes the photocatalytic activity of NPs to degrade organic compounds and reduce fouling on the membrane surface.By integrating both photothermal and photocatalytic functionalities into MD, researchers have been able to create membranes that not only provide efficient heat transfer but also possess self-cleaning capabilities.
This integration has also provided a glimpse into developing multifunctional MD processes and has led researchers to rethink other applications, especially in dealing with climate change.For instance, green hydrogen production is challenging as it requires clean energy and water.With the fast development of photothermal and photocatalytic MD systems, H 2 O and H 2 co-generation became possible by applying photothermal catalytic (PTC) gel in MD. [64] Moreover, the utilization of PTC membrane technology to convert atmospheric or oceanic CO 2 into renewable fuels and chemicals along with water production exhibits great potential for reducing CO 2 emissions and realizing carbon neutrality. [65]s researchers continue to explore and develop new NMs and fabrication techniques, the potential for further enhancements in MD through nanotechnology is vast.Figure 5 illustrates the commonly used NMs and fabrication techniques for developing the development of nanoenabled MD membranes.The subsequent sections will provide a more comprehensive discussion of the different nanoenabled MD membranes developed for performance enhancement, thermal efficiency management, and simultaneous processes of the WEE nexus.

Advanced Nanoenabled MD Membranes for Performance Enhancement
As discussed in Section 2 earlier, the application of nanotechnology in MD has targeted the enhancement of key aspects of MD membranes and the overall process.One area of focus has been the development of advanced nanoenabled membranes with tailored physicochemical and surface properties to improve MD performance.To this end, the utilization of NPs in MD membranes aims to address three major technical challenges: wetting, fouling, and low membrane flux.The role of nanotechnology in MD membranes in dealing with these challenges lies in the incorporation of NMs mainly to i) manipulate surface hydrophobicity and (ii) target and degrade foulants/pollutants, thus improving anti-wetting and anti-fouling properties, ultimately leading to flux enhancements.These advancements have not only enhanced the overall efficiency and reliability of MD but have also opened up new opportunities for addressing the challenges in the WEE nexus.In this section, we will discuss in detail the latest advancements in various types of nanoenabled MD membranes and their contributions to achieving superior performance enhancements.

Nanoenabled MD Membranes for Liquid-Repellence and Low Adhesion Effects
The application of NMs to manipulate membrane hydrophobicity has been a key area in developing advanced MD membranes with special wetting properties.These include superhydrophobic, omniphobic, and Janus (i.e., composite) membranes developed to mitigate wetting, fouling, and scaling issues.Before discussing the role of NMs in these membranes, we will first briefly discuss the principles behind the wettability of membrane surfaces and the use of such terminologies.
The wetting behavior of a droplet on a solid surface, e.g., a membrane, is determined by the interfacial interactions between the three phases involved -liquid, solid surface, and vapor.This is quantified by the contact angle (CA), which is the angle between the liquid-vapor and solid-liquid interfaces.Surfaces with CA > 90°are hydrophobic, while CA < 90°indicates hydrophilic-ity.Young's equation relates the interfacial energies of the three phases to the CA on an ideal flat surface (Figure 6A). [66]However, it does not account for real-world rough surfaces.The Wenzel and Cassie-Baxter models were developed to describe wetting on rough surfaces.The Wenzel model introduces a roughness factor and assumes the full penetration of the liquid in surface cavities (i.e., homogenous wetting; Figure 6A). [67]The Cassie-Baxter model considers surfaces with small asperities where the liquid rests atop the roughness peaks, trapping air underneath (i.e., heterogeneous wetting; Figure 6A). [68]uperhydrophobic membranes, characterized by a water contact angle exceeding 150°, exhibit Cassie-Baxter state wetting owing to their low surface energy and surface roughness, which are attributes that can be accomplished through nanotechnology and bestows upon them a water-repellent characteristic.Moreover, the trapped air pockets in the surface roughness lead to increased water vapor-air interfacial area, enhancing water vapor flux through the membrane.The development of superhydrophobic membranes has significant potential for improving MD performance and efficiency.Superhydrophobic membranes are bioinspired from the self-cleaning and water-repellent properties of lotus leaves, also known as the "lotus effect" (see Figure 6C-F).Creating these biomimetic membranes involves modifying both Figure 6.Wetting states and membranes with special wetting properties.A) Schematic of a droplet placed onto (i) a flat surface in Young's state, (ii) a rough surface in Wenzel state, and (iii) a rough surface in Cassie-Baxter state.In the Wenzel state, the droplet wets the grooves of the membrane surface, whereas in the Cassie-Baxter state, the droplet is sustained on top of the surface asperities, trapping air underneath.Authors' own.B) Schematic of non-wettable rough surfaces in Cassie-Baxter state.The flat microstructure surfaces display the largest contact area between the droplet (blue) and the surface (gray), while the contact area is reduced and minimized in nanostructure and hierarchical surfaces, respectively.Reproduced with permission. [71]opyright 2016, Royal Society of Chemistry.C-F) SEM micrographs and images of a water droplet sitting on Example of a natural water-repelling superhydrophobic surface of Lotus leaves (Nelumbo nucifera) showing SEM micrographs and images of a water droplet sitting on the leaf surface.The surface consists of micro-and nanostructures.C,D,F) Reproduced with permission. [73]Copyright 2011, Elsevier.E) Reproduced with permission. [71]opyright 2016, Royal Society of Chemistry.G-I) Example of a natural water-and oil-repelling, omniphobic surface of springtails (Collembola), the omniphobicity of the surface demonstrated by which the plastron surroundings the whole body upon immersion into H) water and I) olive oil.Scale bars: 1 mm.Reproduced with permission. [71]Copyright 2016, Royal Society of Chemistry.J-L) The hierarchical structure of omniphobic surfaces as illustrated in J) SEM image showing the morphology of the superomniphobic membrane with the second re-entrant level of PVDF spherulites; K) schematic showing the third re-entrant level of SiNPs; L) schematic showing the minimal contact area between the water and the surface of the omniphobic membrane thus maintaining a stable non-wetting Cassie-Baxter state.Reproduced with permission. [74]Copyright 2020, Elsevier.M) The mechanism of high-and low-adhesion effects on hydrophobic and Janus membranes, respectively, as illustrated by the interaction between oil droplets and the membrane surface.Reproduced with permission. [75]Copyright 2017, Elsevier.N) The mitigation of wetting, scaling, and fouling by different membranes with special wetting properties.Typical hydrophobic membranes are susceptible to all three failure mechanisms.Superhydrophobic and omniphobic membranes are typically effective in mitigating scaling and wetting, respectively.Janus (h) (effective in mitigating oil fouling) and Janus (o) (effective for simultaneous wetting and fouling mitigation) are composite membranes with a hydrophilic (underwater oleophobic) surface coating layer and respectively hydrophobic and omniphobic substrates.An omniphobic-slippery membrane combines both superhydrophobicity and omniphobicity.The uncertainty or limited information for the effectiveness evaluation of fouling mitigation is indicated by a "*" superscript.Reproduced with permission. [76]opyright 2021, Elsevier.
the surface chemistry, through hydrophobic functionalization, and the surface morphology, by means of nano/micro-texturing and NPs deposition to create a composite solid-air-liquid interface that resists water penetration.Even surfaces with moderate interfacial free energy can exhibit non-wettability if they possess a sufficient number of nano/micro-protrusions to trap air. [69,70]ese surface asperities can entrap air and prevent direct contact between the membrane surface and the liquid droplet, as illustrated in Figure 6B.However, superhydrophobic membranes have limitations such as low durability and inadequate repellence of low-surface-tension (LST) water-miscible liquids (e.g., surfactants and alcohols) or oils.
Omniphobic membranes (also referred to as amphiphobic membranes) address these limitations by displaying oleophobicity and resisting wetting by both high and low-surface tension liquids.These membranes also draw inspiration from nature, from nature species such as springtails that respire through their cuticle despite the surrounding medium (Figure 6G-I). [71]hile both surface chemistry and morphology remain important for omniphobicity, the morphology, especially the vertical re-entrant curvature, is critical. [71]Specifically, hierarchical reentrant surface curvatures are a must in omniphobic membranes (see Figure 6J-L), while surface chemistry plays a less crucial role.This hierarchical structure forms a plastron-an entrapped air layer formed around the surface when immersed in liquids, enabling a stable Cassie-Baxter state. [71,72]Ms enable precise control over surface properties for creating superhydrophobicity and/or omniphobicity.Their nanoscale dimensions facilitate the creation of rough, hierarchical structures that mimic natural non-wetting surfaces, leading to enhanced water repellency and improved anti-wetting and lowadhesion properties.Additionally, the unique properties of NMs, such as their high aspect ratio, large surface area, and chemical reactivity, contribute to the overall performance enhancement of superhydrophobic and omniphobic membranes in MD applications.Carbon-based NMs, such as functionalized CNTs and graphene, exhibit excellent hydrophobic characteristics, making them ideal candidates for enhancing the surface hydrophobicity of MD membranes.Metal-based NMs, including NPs and nanowires, create nanoscale surface patterns and roughness, in addition to providing extra benefits such as antimicrobial properties or catalytic activities, which can be advantageous in specific MD applications as will be discussed later.Polymer-based nanofibers and NPs offer versatility and ease of functionalization to achieve tailored superhydrophobic and omniphobic properties.Moreover, hybrid NMs and composites offer synergistic effects for optimal non-wettability.
The successful incorporation of NMs into superhydrophobic and omniphobic membranes requires precise control over their distribution and attachment to the membrane surface.NM incorporation techniques can be broadly categorized into coating techniques that deposit them onto the membrane surface and embedding techniques that incorporate them into the membrane matrix.Coating approaches include layer-by-layer assembly of oppositely charged nanolayers, chemical vapor deposition of precursor gases, and electrochemical deposition by applying a potential.These result in conformal nanocoatings imparting desired properties.Embedding can occur during in-situ synthesis, by adding NMs during fabrication or post-fabrication, or by infiltrating NMs after fabrication through techniques such as dipcoating, spin-coating, spray-coating, or impregnation, wherein the membrane is immersed in or sprayed with a solution containing the NMs.
The deposition or incorporation of NPs increases roughness and creates re-entrant morphologies necessary for superhydrophobicity and omniphobicity.For example, coating with fluorinated SiNPs produced a superomniphobic membrane with macro, micro and nanoscale roughness capable of repelling both water and oils. [74]Due to the application of nanotechnology via SiNPs coating, calculations showed that the omniphobic membrane possessed a significant liquid-vapor inter-face with most of the water droplet contact area being with air pockets between adjacent SiNPs, while only a small portion of water coming into contact with the solid membrane surface (Figure 6J-L).Intercalating silica NPs into graphene oxide nanosheets increased the membrane's roughness and wetting resistance. [77]In another investigation, Wei et al. achieved micro-and nanoscale roughness by growing Ag NPs in situ. [78]ecently, Aziz et al. constructed flower-like and rod-like reentrant structures on a ceramic hollow fiber membrane by tethering TiO2 NPs followed by hydrothermal treatment to achieve omniphobicity, [79] reinforcing that re-entrant hierarchical surface texture is essential to synthesizing omniphobic membranes.Reentrant bead morphologies were made by electrospraying fluorinated ZnO NPs on a PVDF substrate.The omniphobicity of the membrane was confirmed by experimentally measured CAs of ≈160°, 130°, 129°, and 126°for water, sodium dodecyl sulfate (SDS), ethanol, and oil, respectively. [80]In summary, NM incorporation techniques enable precise control over the distribution and attachment of NPs, creating superhydrophobic and omniphobic membranes with enhanced anti-wetting for improved MD performance.Tuning NM properties and membrane surface morphology at the nanoscale is key to optimizing non-wettability.
Although superhydrophobic and omniphobic membranes successfully resist water penetration, their resistance to fouling, particularly oil-based, is not sufficiently robust.The amplification of their hydrophobicity increases hydrophobic interactions with foulants, inevitably causing fouling. [81,82]While omniphobic membranes demonstrate repellence towards oils in air, they acquire oleophilic properties when submerged, enabling underwater oil accumulation and pore blockage, albeit without pore wicking due to their hierarchical structure. [76]In this regard, nanotechnology enables the fabrication of Janus membranes, characterized by asymmetric wettability, to combat fouling.These membranes incorporate a hydrophobic substrate and a hydrophilic surface layer.The precisely controlled NM-based hydrophilic surface layer in Janus membranes forms a surface hydration shell that renders the membrane oleophobic underwater, thereby preventing oil fouling. [83]This hydrophilic layer also greatly minimizes foulant interactions with the membrane surface, resulting in the low adhesion properties of Janus membranes.The mechanism of high-adhesion and lowadhesion of foulants (oil droplets) over traditional hydrophobic membranes and Janus membranes, respectively is illustrated in Figure 6M.
Wang and Lin pioneered the application of Janus membranes in MD, crafting a Janus membrane by spray-coating a superhydrophilic skin layer on a hydrophobic substrate. [84]This layer, composed of a perfluorooctanoate/chitosan (PFO/CTS) nanoparticle-polymer composite with SiNPs, imbued the membrane surface with the nanoscale toughness essential for superhydrophilicity.Even in the air, the nanomodified surface exhibited underwater oleophobicity.When tested with oily feeds, Janus membranes show excellent antifouling due to the hydration layer's repulsive force that prevented oil droplet adhesion.When tested with the same feed solution but stabilized by surfactants, the Janus membrane was highly susceptible to pore wetting, despite preserving its fouling resistance property, as confirmed by the absence of flux decline.
Huang et al. also developed a Janus membrane by spray-coating a similar SiNPs-CTS/PFO nanoparticle-polymer composite on the surface of a fibrous hydrophobic electrospun PVDF-HFP substrate. [85]In MD tests using an oil-in-water emulsion feed, this Janus membrane demonstrated superior fouling resistance with minimal oil adhesion.The incorporation of SiNPs in the hydrophilic coating imparted the surface with underwater superoleophobicity and contributed to the membrane's low adhesion effects.
The hydrophilic surface layer of Janus membranes also improves flux and selectivity.Hydrophilic NPs deposited onto the Janus membranes enhance surface hydrophilicity, thereby promoting water vapor transport.Asymmetric wetting enabled by selective NPs deposition on each side facilitates vapor-liquid separation and reduces fouling.This selective surface modification allows the membranes to repel unwanted substances while promoting water vapor passage, leading to improved flux and antifouling properties.Moreover, functionalized NPs can serve as active sites for specific interactions, such as ion exchange, catalytic reactions, or molecular recognition.Incorporating NPs with tailored surface chemistry on the membrane's active side can enable selective transport and separation of specific solutes or contaminants.
NMs facilitate the engineering of Janus MD membranes with the hydrophilic active layer facing either the feed or permeate side, depending on the application and desired properties.Fouling resistance has been attained by applying the hydrophilic layer at the feed side while maximizing permeate flux was achieved by an active hydrophilic layer facing the permeate side.For instance, Fe 3 O 4 NPs sandwiched between hydrophilic PDA layers produced a Janus membrane with a stable flux and rejection ratio with the active side facing the feed.The deposition of Fe 3 O 4 NPs created a hierarchical rough morphology, enhancing surface roughness and preventing wetting. [86,87]In another study, covalently crosslinked MXene nanosheets provided high flux and antifouling performance when the active side was oriented towards the permeate. [88]lthough superhydrophobic membranes best resist scaling, omniphobic membranes best resist wetting, and Janus membranes best resist oil fouling (Figure 6N), [76] combining these surfaces using functionalized NPs leads to multifunctional membranes.Janus membranes with an omniphobic substrate and hydrophilic coating, also known as Janus (o) membranes, exhibit resistance to both wetting and fouling.For instance, Huang et al. applied a NPs-based superhydrophilic coating layer to an omniphobic substrate, resulting in a Janus membrane that demonstrated both anti-wetting and anti-fouling properties. [85]mniphobic-slippery membranes with superhydrophobicity and omniphobicity mitigate wetting and scaling (Figure 6N). [89]However, wetting, scaling, and fouling involve complex synergistic mechanisms with different compounds.Complex feed solutions containing several wetting and fouling compounds can trigger multiple failure mechanisms simultaneously, which are challenging to differentiate.One failure mechanism can trigger another, such as wetting induced by fouling and scaling.Nanotechnology plays a crucial role in elucidating these mechanisms, thereby advancing MD technology and its applications in the sustainable WEE nexus.

Nanoenabled MD Membranes for Self-Cleaning and Smart-Sensing Effects
The application of NMs in MD has advanced beyond developing specially wettable membranes for mitigation of wetting, scaling, and fouling (see Section 3.1.1above), paving the way for the development of novel MD membranes with exceptional self-cleaning and stimuli-responsive capabilities.By harnessing the unique properties of NMs, these membranes have mainly targeted pollutants degradation and enhanced fouling resistance, improved membrane performance, and real-time process monitoring and control.This section highlights the role of NMs in the design and fabrication of such novel MD membranes.Specifically, we explore the applications of photocatalytic, thermo-responsive, and electrically conductive self-cleaning membranes, shedding light on their mechanisms and potential for advancing MD into broader industrial domains.Here, self-cleaning refers to the photocatalytic and conductive effects of pollutants' degradation and cleaning, not to be mistaken with the superhydrophobic selfcleaning membranes discussed in the previous section.
Photocatalytic membranes are photo-responsive self-cleaning membranes that have emerged as an effective anti-fouling strategy in MD.They achieve fouling resistance and enable the cleaning of fouled membranes through the photocatalytic degradation of contaminants by generating reactive oxygen species (ROS) under light exposure, primarily hydroxyl radicals, which possess strong oxidation properties. [90]When foulants come into contact with the photocatalytic membrane surface exposed to visible or UV light, the ROS attack and break down the organic compounds, microorganisms, and other contaminants, resulting in their degradation.Unlike other treatment methods that generate secondary pollutants, photocatalysts allow complete pollutant degradation without generating secondary ones. [91]owever, a central concern in photocatalytic MD is how to adapt the inherently slower photocatalytic degradation process to meet the demands of high-flux water purification within MD systems.To address this challenge, researchers have explored various strategies, such as photocatalyst engineering, optimizing the photocatalyst loading on the membrane surface, enhancing the light absorption efficiency, and improving the MD module design.Nanostructured photocatalysts, such as graphene oxide, titanium dioxide NPs, and carbon nanotubes, have emerged as powerful tools to expedite photocatalytic reactions.Their high surface area and unique surface chemistry enable more efficient interaction with contaminants, leading to rapid degradation.Additionally, optimizing the loading of photocatalysts on the membrane surface is a crucial strategy to enhance photocatalytic degradation kinetics within an MD system.This involves achieving an optimal balance between the quantity and distribution of photocatalytic NPs or NMs on the membrane surface.Key considerations to achieve such optimization include uniform dispersion of photocatalytic NMs, controlling the density of photocatalysts loaded on the membrane, and developing anchoring techniques that securely bind NMs to the membrane surface to prevent their detachment during operation.Moreover, efficient light delivery to the membrane surface is another critical aspect of adapting photocatalytic degradation to high-flux MD.Innovative reactor designs, such as optical fibers, light-guiding structures, and specially designed reactor geometries ensure uniform and efficient light distribution across the membrane.These designs enable better penetration and utilization of incident light, particularly in high-flux MD applications.Furthermore, MD module design enhancements are crucial for integrating photocatalytic degradation into high-flux MD systems.These improvements can range from optimized flow patterns and multi-stage MD configurations to temperature control.Flow patterns and multi-stage configurations can promote prolonged contact between contaminated water and the photocatalytic membrane surface, and allow for multiple passes through the photocatalytic process, increasing pollutant removal efficiency.Maintaining the optimal temperature range for photocatalytic reactions is crucial for achieving efficient pollutant degradation.Temperature control systems are integrated into MD modules to ensure steady operation.By incorporating these strategies, researchers aim to overcome the challenges of slower photocatalytic degradation kinetics in high-flux MD applications.
By incorporating these strategies, researchers aim to overcome the challenges of slower photocatalytic degradation kinetics in high-flux MD applications.Furthermore, the synergy between nanoenhanced MD membranes and advanced photocatalytic materials is a promising avenue.The incorporation of tailored NMs can significantly enhance photocatalytic activity, making it more suitable for high-flux MD applications.This synergy opens up exciting possibilities for simultaneous water purification and energy generation within a single MD system, which we discuss further in the proceeding sections.These advancements not only expedite the degradation process but also maintain the overall performance and integrity of the MD system, ensuring efficient pollutant removal while maximizing water flux.
The fabrication of photocatalytic self-cleaning MD membranes involves integrating NMs into the membrane matrix or coating them on the surface to enable redox reactions.Techniques like sol-gel, electrospinning, hydrothermal growth, and physical deposition ensure uniform and stable nanomaterial incorporation, maximizing photocatalytic performance.Diverse NMs, including CdS, TiO 2 , CuO, ZnO, and graphitic carbon nitride (g-C 3 N 4 ), have been studied extensively for incorporation in membrane fabrication due to their strong oxidizing ability, high turnover number, high thermal and chemical stability, and potential for degrading micropollutants, among others. [92,93]2D materials are of particular interest in photocatalytic MD processes due to their abundant active sites, conductivity, and chemical stability. [94][97] Incorporating these NMs provides self-cleaning that ultimately leads to enhancing water quality by reducing fouling and degrading contaminants, as well as tunability of properties, such as photocatalytic activity, surface properties, and stability, for customized membrane designs to meet specific application requirements.
Several studies have demonstrated the effectiveness of photocatalytic self-cleaning membranes in MD applications, not only to impart anti-fouling properties but also to facilitate the regeneration of fouled membranes.For instance, TiO 2 NPs in a PVDF membrane achieved near complete dye removal and >90% flux recovery under UV-cleaning when treating Congo red wastewater. [98]Ning et al. synthesized an AgCl/MIL-100(Fe) nanomaterial (AM) and employed it to coat a photocatalytic layer on a PTFE substrate through an adhesive-assisted vacuum filtration method (Figure 7A). [99]When tested in DCMD under visible light with nitrobenzene (NB) as the model contaminant for evaluating the removal performance of semi-volatile organic compounds (s-VOCs) under visible light, the photocatalytic AM/PTFE membrane achieved an increased NB removal rate of 87.84%, compared to 63.44% achieved by the pristine PTFE membrane.After 5 NB removal cycles, the AM/PTFE membrane maintained a stable performance with a final s-VOCs rejection of >84.84%.Both adsorption and photocatalysis were proposed as the combined mechanism for NB removal, with volatile NB molecules first adsorbing on the surface of the photocatalytic membrane, and the NMs, under visible light irradiation, producing ROS that effectively degrade and remove NB (Figure 7B).Similarly, incorporating ZnO NPs into a PTFE membrane enabled dye removal and 94% flux recovery after UV irradiation cleaning of the fouled membrane. [100]In this way, photocatalytic NM incorporation can improve the efficiency, stability, water quality of MD processes and reduce the need for membrane cleaning.Precise nanomaterial fabrication and integration are key to maximizing the photocatalytic performance for superior antifouling and water treatment capabilities.
Thermo-responsive self-cleaning membranes utilize heatingsensitive NMs that change hydrophobicity based on temperature.Heat-sensitive NMs are hydrophilic and swell at feed temperatures lower than the critical temperature and become hydrophobic and shrink above the critical temperature. [101]Such temperature-based tuning of hydrophobicity enables the removal of the surface-deposited fouling layer to achieve fouling reversibility in MD, thereby contributing to maintaining a high permeate flux.While heating-sensitive NMs have been applied in other membrane processes like ultrafiltration, [102,103] thermoresponsive membranes have just begun to be studied for MD.However, the studies so far have demonstrated that thermoresponsive nanocomposite membranes can provide effective selfcleaning and antifouling in MD via fouling reversibility and enable post-MD membrane regeneration under thermal actuation.
In one study, poly(N-isopropylacrylamide) (PNIPAM) NPs were synthesized and coated on SiNPs to obtain uniform particle sizes.Then, a mixture of PDVF and SiNPs-PNIPAM was casted on a PTFE substrate via phase inversion to yield a thermoresponsive PNIPAM/PVDF/PTFE composite membrane. [87]Unlike the pristine PVDF/PTFE which maintained oleophilicity, the composite PNIPAM/PVDF/PTFE showed reversible oleophobicity based on temperature.In fouling tests with BSA-containing NaCl feed, both the pristine and engineered membranes experienced ≈40% flux decline, however, the latter showed much superior flux recovery (99%) through thermal cleaning due to the PNIPAM thermal actuation, compared to the pristine membrane (59%).Tests with microalgae-containing seawater gave similar results.Below the lower critical solution temperature (LCST) of 32 °C, PNIPAM swelled and showed hydrophilic properties, while above it, PNIPAM shrunk and showed hydrophobic properties.This tunable hydrophobicity based on temperature enabled self-cleaning effects by which the regeneration of fouled membranes became possible.
Recently, Li et al. fabricated a thermo-responsive MD composite membrane by electrospinning a layer of PNIPAM/PS on the surface of a PTFE support (Figure 7C). [104]When tested  B) the mechanism of its enhanced in-situ removal of semi-volatile organic compounds with (i) and (ii) showing the passage of nitrobenzene in the pristine PTFE membrane and the photocatalytic membrane in dark, respectively, while (iii) achieving the removal and degradation of NB with the photocatalytic membrane under visible light.Reproduced with permission. [99]Copyright 2022, Elsevier.C) Schematic of the preparation process of the thermo-responsive PNIPAM/PS-PTFE composite membrane.D) Schematic of the mechanism of the PNIPAM/PS-PTFE composite membrane's self-cleaning property.PNIPAM is a thermo-responsive material which swells and shows hydrophilic properties below its lower critical solution temperature (LCST) of 32 °C and shrinks and shows hydrophobic properties at temperatures higher than the LCST, thus endows the self-cleaning characteristic.Reproduced with permission. [101]Copyright 2023, Elsevier.E) Schematic of a DCMD module with electrical repulsion of the conductive SWCNT/PVDF membrane when connected to a DC power supply, and F) its wetting mitigation mechanism.During MD operation, surfactants adhere to the pristine PVDF membrane, reducing its hydrophobicity and causing performance degradation (left), while, with the application of a DC power supply, electrical repulsion prevents surfactants from adhering to the membrane surface (right).Reproduced with permission. [106]Copyright 2022, Elsevier.G) Anti-scaling performance of the conductive CNT/PVA/PP membrane as a result of electrophoretic mixing, and H) graphical representation of the electrophoretic mixing anti-scaling mechanism of the conductive membrane: (i) a concentration polarization (CP) layer forms near the uncharged membrane surface; (ii) application of 2 V DC potential, with the membrane as cathode, forms an electrical double layer (EDL) near membrane surface; (iii) electrophoretic mixing occurs within the CP layer due to EDL disruption and reformation caused by the polarity switching of the membrane.Reproduced with permission. [107]Copyright 2020, American Chemical Society.
in DCMD, the PNIPAM/PS-PTFE membrane showed a 1.6fold higher permeate flux than that of a commercial PTFE membrane.In fouling tests, thermal actuation cleaning enabled ≈100% flux recovery for the PNIPAM/PS-PTFE composite membrane versus 75-80% for the pristine PTFE.Similarly, the enhanced anti-fouling resistance and self-cleaning properties of the PNIPAM/PS-PTFE were attributed to its thermo-responsivity mechanism (Figure 7D).Despite these promising results, however, there are still complications to be resolved in the preparation of photocatalytic membranes, such as the agglomeration of NPs, which makes it difficult to obtain uniform dispersion, and NPs' weak loading stability, as will be discussed later in this review.
NMs have also been recently utilized for the development of electrically conductive membranes that possess controllable transport properties.By applying voltage, these membranes can repel organic foulants and inactivate microorganisms to pre-vent or reduce fouling at relatively low energy cost. [105][108] When integrated into MD membranes, the conductive pathways formed by CNTs or graphene enable efficient electron transfer across the membrane, improving performance.Upon application of voltage, the incorporated NMs create an electric field at the membrane surface, repelling oppositely charged foulants and preventing their adhesion.Additionally, the presence of these NMs disrupt fouling layer formation by inhibiting foulant attachment, thereby enhancing the anti-fouling capabilities of the MD membrane further.Moreover, the enhanced electrical conductivity improves heat and mass transfer, leading to increased flux rates and improved overall membrane efficiency.When coupled with additional equipment, such as resistance measuring instruments, conductive membranes can also enable early detection of membrane wetting, which can help prevent process failure or irreversible membrane damage.
Studies have reported promising antifouling results with NMmodified conductive membranes.For instance, CNTs incorporated into polymeric membranes improved antifouling.Kim et al. filter-coated multiple layers (1-4 layers) of single-wall carbon nanotubes (SWCNTs) on a commercial PVDF substrate to yield a conductive MD membrane (Figure 7E). [109]When tested in a DCMD module with a saline feed containing a high concentration of foulants (500 mg L −1 BSA and HA in 35 g L −1 NaCl), the conductive SWCNT/PVDF membrane applied with 1 V did not experience any flux decline during a 50 h experiment, even under high recovery rate conditions.In contrast, the pristine PVDF and the SWCNT/PVDF without voltage application showed severe reduction in flux, implying the formation of a fouling layer.The membrane also showed oil/surfactant repulsion that contributed to wetting prevention.The fouling and wetting mitigation of the SWCNT/PVDF membrane was attributed to the electric repulsion forces between the similarly charged membrane surface and the foulants/wetting agents (Figure 7F).
Another study reported that spray-coating CNTs on a commercial PP substrate, thereby increasing the membrane's hydrophilicity and lowering its surface roughness, with an application of low alternating potentials (2 V), reduced scaling decline due to CaSO 4 and silicate (by 30%) as well as flux decline (by 64%) compared to the uncoated PP membrane. [107]Additional experimentation with AC conditions concluded that the enhanced performance of the conductive CNT/PVA is due to the combination of membrane surface properties, applied electric potential, and solution pH (in the case of silicate). [107]Based on the findings, the authors proposed an electrophoretic mixing (Figure 7G) mechanism capable of nearly eliminating CaSO 4 and silicate scaling on the electrically conductive CNT/PVA membrane (Figure 7H).
Jiang et al. developed a conductive membrane by coating CNTs on a PTFE membrane and evaluated the anti-biofouling efficacy of capacitor mode and resistor mode DCMD for the treatment of surface water. [108]The conductive membrane was also employed for in-situ observation of membrane wetting, and the experimental results demonstrated that in both operation modes, the negatively charged CNT-PTFE membrane significantly hindered bacterial adhesion, leading to the elimination of more than 88% of the bacteria upon energization.The membrane also exhibited effective electrical repulsion against negatively charged silicate crystals, thereby effectively reducing scaling issues.The authors hypothesized that the anti-fouling behavior in the capacitor mode was due to the electrostatic repulsion of bacteria cells by the negative electrode and the destruction of cells by the generated electrocatalyzed ROS, whereas the enhanced anti-fouling properties in the resistor mode were a result of the increased temperature of membrane surface caused by Joule heating effects (which is further discussed in the following section).
Finally, it is worth mentioning that although nanoenabled photocatalytic and electrocatalytic MD membranes exhibit promise in degrading a wide range of contaminants, these membranes have limitations when it comes to broad-spectrum pollutant removal.Non-organic contaminants, such as heavy metals and metalloids, are not easily degraded by photocatalysis and electrocatalysis.Similarly, highly stable compounds, notably perflu-orinated compounds exhibit resistance to degradation due to their complex molecular structures.Large molecules and macromolecules, such as proteins, cellulose, and lignin, pose a challenge for degradation using these membranes due to their size and intricate compositions.Additionally, insoluble pollutants, including certain types of oils, greases, and hydrophobic compounds, do not readily interact with the catalyst surfaces, limiting their degradation efficiency.Furthermore, microplastics, which are small plastic particles, are not effectively degraded by photocatalysis and electrocatalysis due to their low reactivity and resistance to degradation processes.While these membranes have limitations in degrading such pollutants, they still offer valuable capabilities for targeted pollutant removal, and ongoing research aims to address and overcome these challenges for more comprehensive solutions.

Advanced Nanoenabled MD Membranes for Thermal Efficiency Management in MD
Thermal management in MD refers to the implementation of effective strategies for alleviating thermal energy consumption, mitigating temperature polarization, and diminishing conductive and convective heat losses during MD operations.This section comprehensively discusses the role of advanced photothermal, plasmonic, electrically conductive, and highly porous nanostructured materials and advanced membrane configurations in improving MD's energy efficiency.The schematics of the different nanoenabled thermal management strategies adopted in MD are illustrated in Figure 8.

Photothermal Membranes
The advent of light-to-heat converting NMs has sparked renewed interest in the old concept of solar distillation, revitalizing it as a green technology for improving MD's thermal efficiency and promoting sustainable desalination.As depicted in Figure 9A, photothermal NMs harvest solar energy from solar radiation and convert them into thermal energy to facilitate phase change from liquid to water vapors.Photothermal membrane distillation (PMD), which combines MD with specially designed surfaceheating membranes by incorporating photothermal NMs in the fabrication process, [110] not only provides the simplest system for solar distillation but also possesses the highest solar conversion efficiency among other photoactivation processes, including photovoltaic (i.e., conversion into electrical energy) and solar collectors. [111]PMD is based on the same principles as traditional MD processes, but unlike standard MD which requires high thermal energy to pre-heat the entire bulk feed up to 60 °C before it enters the module, PMD utilizes localized interfacial heat generated by the light-to-heat converting NMs on the surface or within the membrane to vaporize water directly at the feed/air interface, thereby significantly improving the energy efficiency of the process. [112,113]Moreover, in PMD, localized heating results in a higher feed temperature at the membrane/feed interface than the bulk feed temperature, thus maximizing ΔT across the membrane to reduce input heating energy and diminish temperature polarization simultaneously. [114]The choice of NMs for localized heating and their integration strategies with hydrophobic membranes play a vital role in harnessing and converting solar energy to thermal energy in PMD.A number of studies have explored different NMs and membrane configurations for efficient photothermal conversion and solar vaporization in MD.The selection of NMs for PMD is influenced by multiple factors, such as their efficiency, cost-effectiveness, durability, and versatility.The efficacy of NMs in PMD systems is primarily determined by their ability to absorb incident light and convert it into heat, which is referred to as photothermal conversion efficiency.This light-to-heat conversion efficiency expressed as solar absorptance (SA) is a crucial parameter in the design of efficient PMD systems that refers to the ratio of total absorbed solar radiation to incident light radiation and ranges from 0 to 1. Photothermal NMs can be tailored for specific applications by modifying their size, shape, composition, and surface properties.They can be broadly classified into two categories based on their chemical composition, structure, and photothermal conversion mechanism: carbon-based NMs, such as CNTs, graphene, and carbon NPs, [115][116][117] and plasmonic metallic NPs such as Cu, Au, Al, Ag NPs and others.Additionally, other materials including ceramics, MXenes, and MOFs also exhibit favorable properties and demonstrate promising photothermal effects.
Plasmonic NPs were among the first NMs to be utilized in PMD due to their excellent photothermal properties, biocompatibility, and ease of synthesis.These NPs can be incorpo-rated into the membrane or applied as a coating on the membrane surface to create a photothermal layer that absorbs solar energy and converts it into heat.The first study on PMD was conducted by Politano et al. in 2017, in which a plasmonic membrane was fabricated by incorporating unmodified Ag NPs into the PVDF matrix.The size of Ag NPs was highlighted as a factor that plays an essential role in maximizing the plasmonic properties of the prepared membranes.Upon UV irradiation, the plasmonic membrane showed an 11-fold higher distillate flux than pristine membranes operated under the same conditions. [118]lternative photothermal NMs, such as carbon-based ones, were then explored as a possible solution to prohibitively expensive plasmonic NPs with narrow wavelength absorption. [119]ased on lattice vibration, molecules of carbonaceous photothermal NMs undergo excitation and relaxation of electrons upon exposure to solar irradiation, resulting in thermal vibration converting solar energy into thermal energy. [120]CNTs have high mechanical strength, excellent thermal conductivity, and efficient photothermal properties, making them suitable for use as photothermal agents.Other carbon-based materials, such as carbon black, activated carbon, and carbon fibers, have also been investigated for their potential use in PMD.Wu et al. reported a SiO 2 /Au nanoshell and carbon black-coated photothermal MD membrane, which brought up to a 33% increase in permeate flux when irradiated under 1 sun. [121]igure 9. Nanoenabled thermal management in membrane distillation.A) Schematic diagram of photothermal membrane distillation (PMD).Reproduced with permission. [127]Copyright 2021, Elsevier.B) Fabrication and performance of a superhydrophobic photothermal MXene membrane: (i) electrospraying of MXene nanosheets on a PVDF support membrane, (ii) cross-sectional SEM image of the PM-PVDF membrane showing an MXene-engineered layer on the surface, (iii) the membrane cell for lab-scale PMD test, and (iv) the desalination performance of PM-PVDF and C-PVDF membranes under different solar intensities.Reproduced with permission. [115]Copyright 2022, Springer Nature.C) Schematic diagrams of different configurations of nanoenabled MD membranes: embedding configuration, bilayer configuration, and isolation configuration.Reproduced with permission. [128]Copyright 2021, John Wiley and Sons.D) Schematic diagram of direct surface heating membrane distillation (SHMD) using electricity as external energy with the hBN protective coating SS-mesh.Reproduced with permission. [129]Copyright 2020, Springer Nature.E) A DC-powered self-heating triple-layered membrane with the COOH-modified MWCNTs as the heating material.Reproduced with permission. [130]Copyright 2023, Elsevier.F) SEM images of (i) MAF-4 nanoparticles and (ii) the PVDF-MAF-4 nanofiber with reduced thermal conductivity, and (iii) their MD performance.Reproduced with permission. [131]opyright 2022, Elsevier.
Recently, emerging 2D semiconductors NMs have also received considerable interest in PMD.Unlike conductive materials, the photothermal mechanism in semiconductors is nonradiative relaxation.When the energy of incident light is similar to or higher than the bandgap, energy is released via protons via radiative relaxation or phonons via nonradiative relaxation as the photoexcited electron-hole pairs return to a low-level state. [120,122]Among others, titanium carbide (Ti 3 C 2 T x , T = ─F, ─O, and ─OH) is an emerging 2D MXene with interlayered 2D channels that exhibit beneficial traits as an efficient photothermal material for PDM systems. [95,96]Ti 3 C 2 MXene shows a remarkable light-to-heat solar conversion efficiency and broad solar absorption bandwidth due to a unique LSPR effect in high nearinfrared. [97,123]126] Zhang et al. fabricated a superhydrophobic photothermal MXene membrane by electrospraying the surface of a commercial PVDF membrane with polymer-based MXene solution to form a nanosphere-assembled MXene-engineering layer (Figure 9B).The prepared membrane demonstrated a water contact angle of ≈172°, denoting its suitability in PMD applications, in addition to the photothermal capabilities of MXene achieving a distillate flux of 2.88 NMs. [115]MOFs are another type of metal-based NMs that have been studied for their use in PMD.MOFs have a high surface area, tunable pore size, and excellent stability, making them suitable as the membrane matrix in PMD.They can also be engineered to have photothermal properties by incorporating metal ions or NPs into the MOF structure.Other metal-based materials such as titanium dioxide (TiO 2 ), aluminum oxide (Al 2 O 3 ), and zinc oxide (ZnO) have also been explored for their potential use in PMD.
Other parameters including, membrane configuration, transmembrane heat loss, wettability (i.e., having a hydrophobic surface to achieve liquid-vapor separation), permeability (affected by the photothermal layer/membrane properties, such as pore size, porosity, and thickness), and stability of the layer and membrane also greatly affect the light absorption and conversion efficiency of photothermal NMs. [132]The different configurations of nanoenabled membranes (i.e., embedding, bilayer, and isolation) are shown in Figure 9C.In general, coating the photothermal NMs as a separate layer on the membrane's surface offers better insulation and thus reduces heat loss compared to a design with photothermal NMs embedded into the membrane matrix.A number of studies have also attempted to address wetting, fouling, and scaling issues in PMD by modifying photothermal NMs or membranes.One study designed a carbon-based omniphobic-photothermal nanocomposite membrane by combining a PVDF membrane with 1H,1H,2H,2Hperfluorodecyltriethoxysilane (FAS17) modified CB nanoparticles with the aim to enhance wetting resistance.The fabricated membrane exhibited photothermal characteristics and excellent wetting resistance against surfactants of up to 0.4 mM SDS, aided by the omniphobic coating on the hydrophilic CB membrane, which had low surface energy that endowed high resistance to surfactants. [117]Another study coated a CNT-based photother-mal hydrophobic membrane with a hydrophilic PDA layer to obtain anti-scaling properties.The added PDA layer improved antiscaling by lowering mass transfer resistance and high salt solubility.Also, PDA's broad light absorption increased the photothermal efficiency and enhanced the heat transfer efficiency between the membrane surface and feed. [133]nvestigations have also been conducted to determine the scalability of PMD for practical applications.Functionalized CB NPs with polyvinyl alcohol (PVA) were deposited onto a PDAcoated PVDF membrane, in which the CB NPs with broad lightabsorbing properties allowed excellent and efficient localized photothermal heating.A distillate flux of 5.38 LMH and 0.55 LMH and solar energy efficiencies of over 20% and 53.8% were obtained for the small-scale and pilot-scale PMD with this modified membrane, respectively. [134]A pilot scale PMD with an area of 37.5 cm 2 was performed with a membrane loaded with Fe 3 O 4 NPs by vacuum filtration and obtained a lab-scale permeate flux of 0.97 LMH, which increased to 21.99 LMH under 3 kW m −2 solar radiation and a flow rate of 4L min −1 .Furthermore, the Fe 3 O 4 NPs exhibited high light absorption and subsequent conversion into heat. [135]

Electrothermal Nanoenabled Membranes (Joule Heating Membranes)
Electrothermal MD (ETMD), or joule heating MD, is another emerging self-heating MD technique that utilizes an electrically conductive NMs-based membrane to generate localized heat and facilitate water separation from other substances present in a solution via phase transformation.Joule heating, also known as resistive heating and ohmic heating, is a phenomenon that involves the conversion of electric energy into heat energy when an electric current passes through a conductive material.As the current flows, the electrons in the conductor collide with the atoms, thereby producing heat. [136]The amount of heat generated is proportional to the square of the current and the resistance of the conductor.Joule heating has recently been coupled with MD (i.e., JHMD) to provide localized feedwater heating through electrically conductive materials either embedded into the membrane's matrix, coated on the membrane surface, or positioned in the boundary layer near the MD membrane surface.In a typical ETMD configuration, heat is generated by applying an electric voltage on a conductive membrane interface to increase the temperature at the feed boundary layer.It counters the detrimental effects of TP, thereby providing a promising avenue for improving the energy efficiency of the MD process.
The conductive materials and resultant membranes are the core of ETMD and typically consist of two major components: conductive polymers or NMs that provide electrical conductivity and a hydrophobic porous membrane layer that provides the structural support and pathway for vapor transport.Since the conductive property is imparted to the membrane through material functionalization, the properties of the materials play a significant role in ETMD performance.An ideal electrothermal material should possess high electrical conductivity, corrosion resistance, thermal conductivity, porosity, and superb electric insulation from the surrounding saline water.
Recently, carbon-based NMs, including CNTs, [137,138] carbon nanostructures (CNS), [139] graphene, [140,141] and reduced graphene oxide (rGO), [142] have been widely studied in the fabrication of electrically conductive membranes due to their excellent conductivity, uniform heating, and hydrophobicity.Ahmed et al. reported the compounding effect of electrothermal surface heating and auxiliary feed heating for effective ETMD operation.Electrically conductive CNS films were pressed on commercial polypropylene (PP) membranes to form conductive membranes for use in joule heating MD.After optimization of the CNS mass loading, the membrane, with the conductive layer in direct contact with the feed solution, was tested in AGMD with intermittent electrical field application.The membranes showed an improved average permeate flux by 78% with a 25% enhancement in the GOR, achieving higher thermal energy efficiency. [139]ew developments in ETMD have been emerging recently, that show promise for reaching new milestones for MD processes.For instance, as a pioneer in employing CNTs in MD applications, Dudchenko et al. demonstrated that controlling the frequency of an applied alternating current to a porous thinfilm CNT/polymer composite Joule heating membrane at high potentials can prevent CNT degradation in ionizable environments like high-salinity brines.High-frequency operation enabled this porous membrane to be used as flow-through heating elements and self-heating membranes that directly heated high-salinity brines at the water/vapor interface of the MD element.This approach achieved high single-pass recoveries approaching 100%, surpassing standard MD recovery limits. [143]s shown in Figure 9D, Zuo et al. reported their efforts to address electrochemical corrosion by coating hexagonal boron nitride (hBN) on a conductive stainless-steel mesh. [129]Recently, Subrahmanya et al. reported a new three-layer flow-through in situ evaporation membrane (FTIEM) process for efficient desalination of high-salinity water.Figure 9E demonstrates the structure of the FTIEM, where COOH-MWCNTs-PVA was used as electrical heating materials and PEI-PDA-SBA-15 as its top layer for moisture retention, salt screening, and thermal insulation properties.The FTIEM has achieved the lowest specific heating energy of 0.024 KWh L −1 with maximum flux and salt rejection of 29.25 LMH and 99.99%, respectively. [130]Joule heating has also been applied to improve the photothermal membrane's efficiency and water production.Huang et al. fabricated a 3-layer composite membrane with PDMS-MWCNTs-PVDF that operate under sunlight and alternating current.The membrane showed simultaneous photothermal and Joule heating effects, resulting in higher thermal efficiency and freshwater production than the combined values of the two independent strategies. [144]

Nanoenabled Membranes with Reduced Thermal Conductivity (Thermally Insulating Membranes)
In addition to having high chemical and thermal resistance, the ideal MD membrane material should have low thermal conductivity to minimize conductive heat loss during MD operation.While several polymer-based materials (such as PVDF, PTFE, and PP) have been utilized for MD membrane fabrication, their thermal conductivities exceed 0.050 W m −1 K −1 .This high thermal conductivity can result in the transfer of heat energy through the membrane material, leading to a decrease in energy efficiency and water production rates.Advanced nanoenabled membranes can be fabricated by incorporating highly porous nanostructured materials with low thermal conductivities, such as MOFs, zeolites, mesoporous silica NPs, porous carbon NMs, aerogels, and others, into the polymeric matrix.Modifications with these highly porous nanostructured materials can create a network of interconnected pores with a high surface area, which in turn, reduces the thermal conductivity and minimizes conductive heat transfer from feed to permeate through the membrane.
Wu et al. reported the metal azolate framework-4 (MAF-4) PVDF nanofiber membrane (Figure 9F).The highly porous structure of MAF-4 provided additional vapor channels and diminished heat conductivity from 0.05496 (PVDF) to 0.04072 W m −1 K −1 (MAF-PVDF), while the flux increased from 17.5 to 27.9 LHM due to the improved thermal isolation. [131]Yang et al. successfully fabricated low thermal conductivity -Y 2 Si 2 O 7 membranes (0.497 W m −1 K −1 at 32 °C, and 0.528 W m −1 K −1 at 100 °C) via tape-casting and sintering. [145]The hydrophobic membrane achieved a high water flux of 10.07 LMH in water desalination experiments using a sweeping gas MD system and exhibited stable performance over 400 h.
Recent studies have revealed that the interface between the feed solution and the membrane is critical for heat conduction and mass transportation in MD.Consequently, surface engineering has garnered increasing attention as a promising approach to address these limitations.In contrast to nanocomposite membranes, surface engineering involves attaching NMs to the membrane surface in a restricted manner, forming a thin coating layer that acts as a thermal insulation layer.This layer effectively alters the properties at the liquid-solid interface, reducing thermal losses and improving MD performance.Surface engineering also encompasses nanostructure formation, which generates turbulence and microbubbles near the membrane surface, enhancing thermal efficiency.Li et al. utilized SiO 2 aerogel blending and coating to decrease the thermal conductivity of MD membranes. [146]The findings revealed that the coating process was more efficient than the blending process.Hou et al. adopted a different strategy by using nanostructured wood (nanocellulose) as the main material and fabricated a highly thermally efficient hydrophobic nano wood membrane for MD applications. [147]The nanomembrane had high porosity (89 ± 3%), a hierarchical pore structure that facilitated water vapor transportation, and low thermal conductivity (0.040 W m −1 K −1 ) in the transverse direction, thereby demonstrating excellent intrinsic vapor permeability (1.44 ± 0.09 LMH) and thermal efficiency (≈71% at 60 °C) that is highly desirable for MD applications.While surface engineering provides a promising direction for utilizing NMs with insulating properties, it is susceptible to instability, particularly under harsh operating conditions.

Advanced Nanoenabled MD Membranes for Synergistic Applications
Advances in nanotechnology have enabled innovative synergistic applications of MD processes beyond conventional seawater desalination and wastewater treatment.These applications mainly entail integrating photocatalytic functionalities with photothermal MD for synergistic water and energy applications.Examples include concurrent photothermal desalination and photocatalytic hydrogen (H 2 ) generation in a single system, simultaneous freshwater production and CO 2 conversion to renewable fuels, and synergistic water production and photodegradation using advanced nanoenabled photothermal-catalytic membranes (Figure 10).Resource recovery, such as ammonia from liquid waste streams, is another function that is considered for synergistic application with water production, photodegradation, and selfcleaning, as it is integral for achieving zero liquid discharge.[150][151][152][153][154][155][156][157][158][159] Table 1 summarizes the recent studies of nanoenabled MD membranes for synchronized MD waterenergy applications.

Photothermal-Catalytic Membranes for Simultaneous Water and Hydrogen Production
Green H 2 produced using renewable energy sources, like solar or wind power, is emerging as a promising alternative to fossil fuels and is gaining significant interest as a potential solution for reducing greenhouse gas emissions and addressing the impacts of climate change.H 2 possesses high energy density and can be produced without carbon emissions, making it a promising fuel for various applications, including transportation, electricity production, and industrial processes.What makes H 2 combustion even more attractive is the fact that it only produces water vapor as a by-product.Recently, solar-driven water splitting using a semiconductor material for photocatalytic H 2 generation has been receiving enormous attention as a complementary and renewable solution.Photocatalytic seawater splitting is an innovative approach for producing green H 2 that leverages the potential of photocatalysts to convert sunlight into chemical energy.These photocatalysts facilitate the water-splitting reaction, generating hydrogen and oxygen directly from seawater under solar irradiation.In general, the reaction mechanism of photocatalytic water splitting for H 2 production involves light absorption by semiconductor photocatalysts to generate electron (e−)-hole (h+) pairs, charge separation and migration to the surface of semiconductor photocatalysts, and surface reactions for water reduction and oxidation.This process not only eliminates the need for providing ultrapure water as a feedstock but also reduces the energy consumption associated with conventional water electrolysis.Moreover, photocatalytic seawater splitting can potentially utilize less expensive materials, lowering the overall cost of H 2 production.
The recent integration of photocatalysis in photothermal MD provided a stepping-stone for achieving a sustainable WEE nexus.By utilizing direct solar energy, both photothermal and photocatalytic systems eliminate the high energy requirements of standard MD and H 2 generation.Moreover, these systems do not require PV solar panels, thermal collectors, or batteries for the efficient and direct utilization of solar energy, making them more cost-effective and easier to implement than traditional solar energy systems.The core of such an integrated system lies in the type of photothermal and catalyst NMs that are responsible for  11G. [26]The gel structure provided efficient solar energy absorption, excellent thermal management, and abundant reaction sites.This hybrid system achieved a solar vaporization rate of ≈1.49LMH and a H 2 generation rate of ≈3260 μmol m −2 h −1 under one sun.Pornrungroj et al. demonstrated a UV light absorbing RhCrO x -Al:SrTiO 3 photocatalyst deposited visible and infrared light absorbing porous carbon SVG for the concurrent water and hydrogen generation from open water sources.One significant achievement of this study is the stable performance of the integrated SVG-PC setup on long-term, large-scale tests. [168]o our best knowledge, the research on H 2 production through PTC MD is still in the very early stages.Nonetheless, numerous studies have documented the combination of solar-driven evaporation with photocatalytic H 2 generation, emphasizing on the need to develop new high-efficiency photocatalytic membrane materials.Although previous research has shown the remarkable ability of photocatalytic membranes in H 2 generation, their overall solar-to-hydrogen (STH) conversion efficiency was only around 1%, which needs to be raised closer to the practical limitation of 10%. [170]Improvements in membrane surface structure and the design of specific membrane configurations for H 2 generation are necessary to reach this goal.Synthesizing novel photocatalytic materials, developing efficient membrane structures, and constructing customized membrane modules to enhance the mass transfer for gas-liquid bi-phases are all areas requiring future attention.This field is still ripe with opportunities and obstacles, waiting for researchers to explore.

Photothermal-Catalytic Membranes for Synergistic Water Purification and CO 2 Conversion to Renewable Fuels
The alarming growth of CO 2 has led to the investigation of strategies to reduce emissions or to capture and convert CO 2 into renewable fuels and chemicals to achieve carbon neutrality.The CO 2 reduction reaction (CO 2 RR) is an effective method to convert this greenhouse gas into value-added products.Photocatalytic CO 2 RR mimics the natural photosynthesis reaction, in which solar energy is harvested to drive the catalytic reaction.The reduction reaction involves light absorption,  [163] Copyright 2019, Elsevier.B) Schematic representation of the rGO-CNT membrane for water-tetrahydrofuran (THF) separation.Reproduced with permission. [176]opyright 2020, American Chemical Society.C) Depiction of NiSe and CoSe nanofillers-coated membranes for photothermal membrane crystallization.Reproduced with permission. [183]Copyright 2023, Elsevier.D) Schematic diagram and mechanism of the S-scheme AgCl/MIL-100(Fe) heterojunction nanocomposite-embedded membrane's photodegradation of sulfamethazine.Reproduced with permission. [184]Copyright 2022, Elsevier.E) Graphical illustration and mechanism of the BiOBr/Ag/AgBr nanocomposite coated carbon fiber cloth for pollutant photodegradation.Reproduced with permission. [185]Copyright 2021, Elsevier.F) Schematic diagram and dye photodegradation mechanism of the photothermal-catalytic MXene-PVA-TiO 2 membrane.Reproduced with permission. [162]Copyright 2023, Elsevier.G) Schematic representation of PTC gel for desalination and hydrogen cogeneration system.Reproduced with permission. [26]Copyright 2020, John Wiley and Sons.H) Graphical illustration of solar absorber with TiO 2 -Au NW/NP for clean water production and CO 2 reduction.Reproduced with permission. [169]Copyright 2023, Springer Nature.I) Illustration of a double-stage solar catalytic membrane distillation system for solar water and solar fuel production by a PAN/CNTs/ZnIn 2 S 4 /PPy membrane.Reproduced with permission. [65]opyright 2023, Elsevier.
the separation of charge carriers to form electron-hole pair, and the catalytic reaction.Products from the reduction reaction can become solar fuels such as H 2 , CO, CH 3 OH, CH 4 , and more are formed by a series of electron transfers. [171]ased on the photocatalyst's bandgap energy, different products can be generated to enhance the selectivity of a specific solar fuel.
Similar to the water-H 2 co-hydrogen generation MD process, the advent of nanotechnology had researchers rethink possible strategies to combine CO 2 RR with MD for synchronized clean water production and CO 2 reduction.An integrated photothermal-catalytic MD system can not only reduce the concentration of atmospheric CO 2 but also generate re-newable fuels while reducing energy consumption, forming a synergistic interdependence between the WEE nexus.Liu et al. became the first to explore a PTC hybrid system by introducing secondary in situ growth of Cu 2 O NPs onto a PPycoated photothermal spherical evaporator with a hydrophilic polyamide 6,6 (PA66) nanofiber support.The PPy NPs exhibited a photothermal effect, while Cu 2 O NPs exhibited a photocatalytic effect, allowing the PA66/PPy/Cu 2 O evaporator to harness solar energy for clean water production and solar fuel (62.55 μmol g cat −1 h −1 CO, 7.21 μmol g cat −1 h −1 CH 4 ) generation. [172]nother study designed a 3D TiO 2 -Au nanowire/nanoparticles (NW/NPs) solar absorber for PTC desalination and CO 2 reduction (Figure 11H).The solar absorber was fabricated by growing TiO 2 NWs onto a carbon cloth fiber via hydrothermal synthesis, followed by the deposition of Au NPs via sputtering.The TiO 2 -Au NW/NPs solar absorber exhibited efficient solar vapor conversion efficiency of approximately 90% (1.35 LMH) with a heat-insulated water channel, in addition to CO 2 reduction.In 5 h of simulated 1 sun irradiation, solar fuel yields were 0.015 μmol cm −2 CO and 0.066 μmol cm −2 CH 4. [169]   Han et al. also investigated PTC MD via a double-stage solar catalytic membrane distillation (DSCMD) system, as illustrated in Figure 11I.CNTs and ZnIn 2 S 4 were integrated into PAN fibers via electrospinning and further coated with PPy NPs.Their work demonstrated a distillate flux of 1.45 LMH with CO production rate of 21.29 μmol g cat −1 h −1 .The enhanced photocatalytic activity was achieved owing to the added heat source from photothermal NPs. [65]While the innovative hybrid PTC MD system's simultaneous solar water and solar fuel generation and minimal energy consumption show promise for long-term operation, in-depth investigations are needed to increase the yield of solar fuels without compromising the system's efficiency.

Nanoenabled Membranes for Concurrent Wastewater Treatment and Resource Recovery
In addition to water-energy co-generation, research efforts have also been made on recovering value-added resources from seawater and waste streams.The advancements in MD systems have opened new doors to unlock its potential for recovering valuable components from wastewater, instead of only their removal.Ammonia, a major pollutant found in wastewater, [173] is one of the components that can be effectively recovered using MD.Several studies have reported ammonia recovery [46] using commercial flat sheet or hollow fiber membranes, [173][174][175] however, the limitations of commercially available membranes, in terms of durability, efficiency, and energy demand, have led researchers to investigate the application of advanced nanoenabled membranes.Guo et al. studied the recovery of ammonia from ammonia-rich wastewater via MD using a PVDF-HFP nanofibrous membrane incorporated with Nafion ionomer and MWCNTs (Figure 11A).This honeycomb nanostructured Nafion membrane demonstrated 59.74% recovery of ammonia, which was three times greater than the commercial PVDF membrane (18.6%−22.0%).The authors ascribed this performance to the CNTs/Nafion membrane's superior gas permeation properties. [163]In another study, functionalized CNTs were immobilized into the polymer membrane (CNIM) for ammonia separation via MD with a lowtemperature feed (40 °C).Compared to the pristine PTFE membrane, the CNIM exhibited improved performance in terms of flux (63% higher) and ammonia removal efficiency, which was attributed to the preferred chemisorption of ammonia on f-CNTs. [164]The successful incorporation of f-CNTs into nanofibers via electrospinning was further confirmed to be an excellent strategy for the recovery of ammonia from anaerobic digestion effluent by another study, in which the integration of NMs into the nanofibrous membrane increased the ammonia separation factor by up to 60% compared to pristine membranes. [165]ike the previously mentioned study, carboxylic functional groups enhanced the removal performance due to its absorption characteristics. [165]mmonia recovery was further investigated by coating membranes with photothermal CB NMs for application in PMD.CB NPs were coated onto a PVDF membrane via spray-coating and tested for recovery of ammonia.Under solar irradiation, a 30.8% increase in the mass transfer coefficient was achieved in comparison with the pristine PVDF membrane, which was attributed to the generation of localized heat by CB.When comparing the CB coated membrane's photothermal efficiency with and without solar irradiation, the mass transfer coefficient under solar irradiation was more than 93% higher, implying a substantial difference between these conditions. [166]In a recent study, a nanoembedded membrane's ammonia recovery efficiency was reported to be over 95%, with the membrane demonstrating anti-fouling and anti-wetting properties.This superhydrophobic membrane was fabricated by grafting FeOOH NPs onto the membrane by hydrothermal synthesis, followed by fluorination.Modifications of the FeOOH nano-reentrant structures of the membrane created a super-repulsive surface, while the fluorosilane layer led to low surface energy, contributing to the wettability and fouling-resistant properties.The membrane's superior performance indicated its excellent potential for sustaining longterm operations for recovering ammonia from ammonia-rich wastewater. [167]anoenabled membranes have also been applied in MD for the recovery of organic solvents such as isopropanol (IPA) and tetrahydrofuran (THF) from aqueous-organic solvent mixtures. [176,177]These organic solvents form an azeotrope with water, which makes it difficult for the two components to be separated by traditional distillation methods. [177,178]One study studied a CNTs-immobilized membrane (CNIM) for the separation of IPA via sweep gas MD (SGMD).The presence of CNTs on the hydrophobic membrane caused alterations to the membrane morphology to promote the transport of IPA while inhibiting the penetration of water.The preferential sorption and rapid desorption of IPA on the CNTs led to a 132% increase in the mass transfer coefficient at 50 °C (42 mL min −1 , 10 vol% IPA) and a separation factor of 350% at 70°C when compared with a pristine PTFE membrane. [177]The recovery of THF from feed using MD has been investigated by Gupta et al., for which a hybrid membrane was developed by integrating GO, rGO, CNTs, and hybrid combinations of the nanocarbons into PTFE membranes (Figure 11B).Their findings show that the rGO-CNT-immobilized membrane achieved the highest THF flux and best separation factor (350%) due to the preferential sorption of THF on this hybrid nanocarbon.The graphene sheets contributed to the nanocapillary effect, while the frictionless surface of CNTs contributed to the activated diffusion of THF. [176][181][182] A study introduced nucleation sites onto the membrane by growing Si NPs in-situ onto PAN nanofibers with (3-Aminopropyl) triethoxysilane.The introduction of Si NPs allowed the hydrophilic PAN membrane to exhibit omniphobic characteristics with high wetting resistance, increased surface roughness, and a low contact angle.After a 24 h test, the Li-ion concentration of the feed exceeded 1100 ppm from the initial 360 ppm simulated geothermal brine, signifying successful Li enrichment. [179]Another study inserted plasmonic nanofillers into the membrane to investigate mineral recovery by MDC.The solar-powered MDC for NaCl recovery utilized commercial PVDF membranes coated with either NiSe or CoSe nanoparticles (Figure 11C).The NaCl growth rate for CoSe nanofillers was 1.52107 m s -1 , whereas that for NiSe nanofillers was 1.65107 m s -1 . [183]lthough MDC is promising for recovering valuable minerals, it reported applicability has so far been limited to the lab scale due to the high cost and energy involved in purifying the collected minerals.Concurrent PMD and crystallization present be an effective approach to reduce energy consumption, transforming existing photothermal membrane crystallizers into PMD-crystallization.

Synchronized Water Production, Photodegradation, and Self-Cleaning
Photocatalytic technology has been recognized as an environmentally friendly and cost-effective solution for recalcitrant environmental pollutant degradation.However, for its commercial viability, efficient degradation rates and effective regeneration of photocatalyst NMs are essential.With the recent advancements in designing and synthesizing nanostructured photocatalyst materials, integrating MD with photocatalysis offer great opportunities to achieve these goals.The elevated temperature in MD can accelerate the molecular motion increase collisions between the molecules and the catalyst, which, in turn, can significantly contribute to improving the efficiency of photocatalyst materials for photodegradation.The integration of MD with photocatalysis can be performed in two different manners.The first involves the integration of a photocatalytic reaction device with an MD apparatus to form a hybrid system.In this configuration, the photocatalyst NMs are dispersed in the feed solution, and the reaction occurs in an independent light reaction chamber.At the same time, the MD apparatus recovers the used catalyst.The second entails immobilizing the photocatalyst on the MD membrane surface, whereby the feed solution contacts the catalyst on the membrane surface and undergoes degradation on-site under solar irradiation.
Ning et al. reported an innovative AgCl/MIL-100(Fe) PTFE photocatalytic membrane, designed for degrading semi-volatile organic compounds (s-VOCs) in wastewater, as shown in Figure 11D. [99]As s-VOCs can pass through the membrane and enter the permeate side, leading to a decrease in the quality of the produced water, the degradation of VOCs in the feed is of great importance.Despite having photocatalytic effects that could effectively remove pollutants under visible light irradiation in a one-stop reaction, the degradation effectiveness is limited by the catalyst's performance and the surface area.The emergence of high-performance nano-photocatalytic materials has recently opened new possibilities for developing highperformance photocatalytic membranes.Sun et al. developed a photothermal catalytic membrane based on MXene for effective desalination and photodegradation (Figure 11F).The membrane surface was covered with the 2D/3D hierarchical structure of MXene-TiO 2 , which acted as a solar absorber to convert light into heat and facilitate the degradation reaction.The 2D/3D structure increased the surface area, resulting in a removal rate of over 95% for methylene blue in the feed water. [162]n addition to photolytic degradation coupled with water purification, the incorporation of photocatalysts to membranes can improve fouling resistance and fouling recovery in MD, thereby replacing chemical cleaning.Guo et al. fabricated a BiOBr/Ag photocatalytic membrane with regeneration potential under visible light, as depicted in Figure 11E.The evenly distributed photocatalyst coated on the PVDF-HFP polymer nanofiber through electrospraying increased the hydrophobicity of the membrane while also enhancing fouling resistance by photodegradation.The incorporation of Ag successfully narrowed the band energy from 2.9 to 2.47 eV, resulting in an expanded solar absorption range. [161]

Most Widely Used NMs for MD Membranes
As discussed in the review, rapid advancement in synthesis and characterization techniques in nanotechnology has introduced many novel NMs that have superior properties over conventional materials in MD operation.This section summarizes the most widely used NMs reported in MD operation, comprehensively covering the distinctive characteristics of each material along with the techniques employed to incorporate it into the membrane matrix or onto the surface, and all the specific enhancements it provides to MD operations.The category of materials primarily encompasses carbon-based NMs, such as carbon black NPs, CNTs, graphene, and its derivatives.Additionally, it includes metal-based NMs, such as pure metals, metal oxides, metal derivatives, and metal-organic frameworks (MOFs).Table 1 presents the most researched NMs in MD along with major enhancements endowed by their incorporation into the membranes.Table 2 presents the most researched NMs in MD along with major enhancements endowed by their incorporation into the membranes.

Carbon Nanotubes (CNTs)
CNTs exhibit remarkable electrical and thermal conductivity, surface area, and hydrophobicity, which make them highly attractive for enhancing the performance of polymeric membranes in MD applications. [186]The incorporation of CNTs into the polymeric or ceramic matrix offers several desirable features for MD, including significant enhancements in vapor flux and reductions in energy requirements via self-heating.A recent study by Sun et al. grew CNTs on a metal substrate via the chemical vapor deposition of gas precursors.Much-enhanced flux was attributed to the achievement of very low liquid-solid contact (accounting for only 3.23% of the superhydrophobic interface with a water contact angle of 170°) that enabled water vapor to rapidly move through the membrane structure and effectively resist saline feed by creating a Cassie-Baxter state. [187]The enhancement in flux was also elaborated by AN et al. hypothesizing that CNTs facilitated the mass , and molecular diffusion. [188]Xie et al. utilized the spray coating method to deposit distinct amounts of CNTs on three commercial microporous polymer membranes (PTFE, PVDF, and PP) and reported a threefold increase in water flux for the CNT-deposited PTFE membranes, which was attributed to the increase in its specific surface area. [116]CNT composite MD membranes prepared via the chemical vapor deposition method require expensive raw materials.A recent study by Li et al. employed waste polypropylene plastic as a carbon source to grow CNT on membrane support prepared from coal fly ash having a hierarchically oriented porous structure and utilized this Janus carbon composite membrane as a low-cost and environmentally favorable alternative in MD operation. [189]Alexander et al. spray-coated a 15 μm hydrophilic layer of carboxylated CNTs and PVA on a hydrophobic porous PTFE membrane.By applying alternating current at high frequencies, direct heating of the feed occurred at the water/vapor interface, which provided a much superior performance in MD than standard units (achieving high single-pass recoveries up to 100%), and no CNT degradation occurred in highly ionizable media, such as a concentrated brine of 100 g L −1 . [143]Another work fabricated a CNT fiber membrane by coaxial wet spinning using water as the core solution and oxidized CNT-PVB mixture as the shell solution.Subsequent calcination at a high temperature of 800 °C in an inert atmosphere of argon rendered the CNT microfibers to have a hol-low structure and porosity, the narrow channels providing localized space for efficiently vaporizing the feed.PDMS coating via the impregnation method further enhanced hydrophobicity, improving the flux of this electrothermal membrane from 4.05 to 24.27 LMH as input electric power increased from 0.5 W to 2.5 W. [137] CNTs, due to their broadband light adsorption, have also been utilized as photothermal additives in electrospun PVDF membranes to endow anti-scaling properties. [190]A recent work deposited a uniform layer of micropatterned CNTs on a commercial PTFE membrane by electrospraying, which achieved a significant reduction in salt deposition during the MD operation. [191]1.

Graphene and Its Derivates
Graphene, the first 2D NM discovered in 2004, possesses ultrahigh surface area and electrical conductivity as well as good tensile strength and bactericidal activity.Due to these qualities, graphene and its derivatives are also frequently utilized as fillers in MD membranes to enhance vapor flux, fouling resistance, and energy efficiency.One study fabricated PVDF membranes by electrospinning with graphene nanoplatelets (500 nm) of various loadings from 0 to 10 wt% of the polymer.The membrane modified with the optimized graphene loading of 5 wt% showed a higher LEP of 186.8 kPa and flux of 22.9 LMH compared to 139 kPa and 4.8 LMH exhibited by a pristine PVDF membrane. [192]Graphene films grown by top-down CVD technique have shown great promise in MD operation due to their excellent hydrophobicity and mechanical strength.However, pores as vapor transport channels required for MD operation are typically generated post-synthesis in graphene film in an energy-consuming complex process.Seo et al. carried out a fundamental study in which multilayer graphene on polycrystalline nickel support was grown by the CVD method and transferred to commercial PTFE support membrane where grain boundaries overlapping with each other was demonstrated as efficient water permeation channels in MD operation instead of pores. [193]Further, graphene synthesis was achieved using soybean oil as a low-cost and safe carbon source compared to costly and flammable precursor gases which enabled the fabrication of a macroscale 4 cm 2 membrane for real-world applicability processing 0.5 L water from Sydney Harbour in a day. [193]Reduced graphene oxide (rGO) is another promising graphene derivative for MD operation as it offers low-cost, facile, and scalable solution-based synthesis compared to graphene grown on solid substrates.However, the reduction process (removal of oxygen-containing functional groups) of GO can significantly vary based on the method utilized and affect the MD performance as it controls the hydrophobicity of the graphene surface.This effect was studied by Ahmed et al.where GO used as filler in film-casted PVDF membrane is chemically reduced by ascorbic acid to varying degrees by altering the reaction time, and the C/O ratio measured by XPS was used as a measure of the reduction of GO. [194] MD performance experiments revealed that flux increased initially as the C/O value raised from 2.3 to 5.5 but went down on further increase suggesting there is an optimum reduction degree of GO for MD membranes. [194]raphene derivatives are also used to reduce the problem of heat transport across membranes.A recent study demonstrated that thermal resistance can be significantly enhanced by depositing octadecylamine (a long-chain hydrocarbon) functionalized graphene oxide layer onto the PVDF substrate, leading to a reduction in temperature polarization across the membrane and increasing the salt rejection.Electrically conductive graphene is also used as a coating material for membranes to raise the temperature of feed via an electrothermal mechanism. [195]Tan et al. fabricated a graphene layer via laser irradiation on a PDMS-coated PES membrane.This Joule heating on the Janus membrane decreased the requirement of specific heating energy by up to 53.9%.Utilizing the remarkable antimicrobial activity of graphene, [196,197] Zeng et al. created a GOQD coating layer via covalent linkage on an amino-functionalized PVDF membrane.This is because GO-QDs, owing to their unique structure with a large number of active edges, efficiently stopped the formation of biofilm on the membrane surface by creating oxidative stress that deactivated bacterial cells better than other antibacterial nanocarbons such as GO. [198]A novel graphene derivative, 2D graphitic carbon nitride (g-C 3 N 4 ), is also widely applied in MD applications due to its excellent mechanical and photocatalytic properties.A recent study has incorporated g-C 3 N 4 as a filler in PVDF membrane for MD operation.Apart from improving flux, C 3 N 4 addition greatly improved membrane tensile strength and LEP favorable for robust operation in harsh conditions. [199]

Carbon Black (CB)
CB NPs, due to their excellent light adsorption and hydrophobic characteristics, have been utilized in MD to achieve photothermal and antifouling properties.A recent work fabricated a nanocomposite membrane through the non-solvent induced phase separation (NIPS) technique by casting a PVDF solution on a commercial support doped with Vulcan type CB NPs that are 30 to 60 nm in diameter.This not only improved membrane structure by increasing the pore size and porosity but also endowed photothermal properties to the membrane, where the irradiated surface reached high temperatures up to 60 °C. [200]nother study deposited CB NPs and hydrolyzed FTCS particles on a commercial PVDF membrane surface through a facial spraying method.Aside from superior light adsorption with 66.8% efficiency, this design resulted in a unique structure having multilevel roughness with interconnected micro/nano channels that imparted omniphobicity to the membrane surface.This was particularly beneficial for suppressing fouling when treating oil-contaminated and highly saline water, as evidenced by the stable flux maintained over 48 h, which is not possible with a commercial membrane. [201]Chen et al. fabricated a Janus membrane by grafting carbon quantum dots (surface functionalized by Na + ions) on a PEI/PDA polymer-coated PVDF membrane.The highly hydrophilic top hydration layer controlled oil fouling, whereas the intermediate hydrophobic nanoporous polymer layer repelled surfactants by size exclusion, thereby rendering a membrane with anti-wetting characteristics. [202]Activated carbon with microporosity for efficient vapor transport and high surface area is another promising carbon NM for MD membranes.However, agglomeration of these small-sized AC particles in the PVDF membrane leads to an undesired effect on performance.To overcome this issue, the esterification of AC particles with fluoroalkyl groups greatly improved its dispersibility in polymer membranes compared to conventional acid treatment used for surface functionalization. [203]2.Metal-Based NMs

Pure Metals
Ag NPs are widely used in MD to control fouling by organic pollutants or biofilms due to their photocatalytic and antibacterial properties.Guo et al. coated BiOBr/Ag NPs on an electrospun PVDF nanofiber membrane by electrospraying method to achieve self-cleaning by degrading of dye foulants. [161]A recent work immobilized Ag NPs in situ on a Janus polydopaminecoated PVDF hollow fiber membrane, which prevented the adhesion of various foulants.It was hypothesized that Ag NPs repaired most of the micro defects present within the PDA hydrophilic layer, thereby reducing the membrane's wetting propensity.Ag NPs also restricted the adhesion of bacterial cells (Bacillus sp.BF1 strain) and the consequent development of thick biofilms which were observed in both pristine and PDA-coated PVDF membranes. [204]Like carbon black, metallic NPs, such as gold (Au), also possess strong photothermal properties.When PVDF membranes surface were coated with SiO 2 /Au core-shell NPs by polydopamine-assisted covalent bonding, these uniform-sized 160 nm nanoshells having a 120 nm silica core, and a 20 nm Au shell facilitated evaporation via highly localized heat transfer at the water-air interface, which helped overcome the issue of temperature polarization and achieve better energy. [121]

Metal Oxides
Metal oxides have been utilized in MD with promising results for overcoming some of the limitations encountered with the use of noble metals, such as the high cost of Ag and toxicity caused by its leaching.The photocatalytic properties of TiO 2 are utilized extensively in MD operation. [26]When TiO 2 nanorods of 700−800 nm in length were grown on the electrospun PVDF nanofibers using a hydrothermal technique, the resulting membrane showed strong resistance to fouling and wetting-as the unique pineneedle-like hierarchical nanostructured membrane trapped large air inside its voids and minimized the water contact using only the very tips of TiO 2 nanorods. [205]TiO 2 NPs have also demonstrated antibacterial characteristics in MD, achieved by the generation of reactive oxygen species and the reaction of metal cations with thiol groups of cell membranes. [206]ow-cost transition metal magnetite Fe 3 O 4 NPs are also used in MD mainly for controlling membrane fouling and wetting.For example, confining Fe 3 O 4 in the top skin layer of the PVDF membrane via reverse filtration reduced internal fouling of the membrane by increased surface hydration, where amphiphilic foulants, such as humic acid (HA), accumulate more on the surface by adsorption over metallic NPs. [207]Similarly, adding 0.015 wt% FeOOH nanorods to a PVDF-co-HFP membrane during synthesis via phase-inversion technique significantly increased membrane wetting resistance. [208]Localized heating of Fe NPs via magnetic induction can also be used for creating selfheating MD membranes. [209]ilica (SiO 2 ) NPs are highly porous (84%) and hydrophobic, having the potential to significantly enhance the flux and microbial resistance in MD membranes. [210]Efome et al. studied the varying effect of adding spherical SiO 2 NPs (10-20 nm) from 1 to 10 wt% to a PVDF membrane prepared by phase inversion method where flux enhancement was attributed to the unique morphology of finger-like macro-voids, as opposed to a denser sponge-like structure, and the increase in pore size despite the decrease of porosity [211] A PVDF membrane with omniphobic surface was fabricated by amino functionalization and subsequent spraying of a solution mixture of SiO 2 NPs, silane, and fluorocarbon surfactant to coat the surface by electrostatic adsorption. [212]imilarly, an antiscaling PVDF membrane was fabricated using Si NPs coating and fluorination, where the corrugated surface increased feed turbulence and suppressed temperature polarization to enable superior scaling prevention against a feed containing high salt. [213]inc oxide (ZnO) NPs possess excellent photocatalytic functions which makes them promising candidates for fabricating self-cleaning membranes against various foulants encountered in MD during wastewater treatment.This was explored with MD feed containing dye foulants by Huang et al.where calcination of the zinc acetate precursor added to the PTFE nanofibers before electrospinning achieved a very uniform dispersion of ZnO NPs in the polymer matrix compared to the formation of agglomerates when ZnO NPs were added directly. [100]To control the wetting of PVDF membranes, an omniphobic surface was developed by seed-mediated growth of 1.3 μm long ZnO nanoneedles of 60 nm diameters that look like nano sea urchins. [214]

Metal Derivatives
Various metal derivatives have been employed in MD to enhance performance, such as metal carbides/nitrides, which exhibit promising plasmonic properties for engineering self-heating MD membranes.Titanium nitride (TiN) NPs were electro-sprayed on a commercial PVDF membrane functionalized with a PDA crosslinker to have a highly durable covalent coating of a plasmonic thin layer of 11 μm thickness which provided a high photothermal efficiency. [215]MXene, a recently discovered material with 2D layers of inorganic metal carbides, possesses excellent electrical conductivity and strong light adsorption characteristics owing to its black color.Mustakeem et al. coated 2D Ti 3 C 2 T x MXene on commercial PTFE membranes by vacuum filtration.where fast response in self-heating of feedwater, revealed its potential to be used in intermittent lighting conditions. [216]To decrease the material consumption and operational cost, Jiang et al. synthesized composite nanosheets of MXene and magnetic Fe 3 O 4 using solvothermal reduction.Besides achieving high flux in photothermal MD, their method allowed the complete recycling and reuse of the NPs. [217]n recent work, Sun et al. mixed MXenes with photocatalytic TiO 2 NPs to fabricate a nanocomposite PVDF membrane with photodegradation capability (95%) for methylene blue dye. [162]imilarly, Zhang et al. fabricated a MXene-porphyrin hydrogel that achieved solar evaporation in MD along with rhodamine dye photodegradation. [151]They further exploited the highly conductive nature of MXene for electrospraying polymer NPs to form uniform hierarchical nanostructures and achieved superhydrophobicity with a water contact angle of ≈172°that yielded strong wetting resistance to the membrane. [112]Wastewater for desalination often contains organic pollutants which can cause fouling during MD operation. [218]A recent work utilized a novel thermocatalyst strontium ferrate doped with cerium (Sr 0.85 Ce 0.15 FeO 3− ) synthesized by solution-combustion method which can degrade bisphenol-A pollutants at 65 °C by reactive oxygen species, without any additional external stimuli (such as light or current) typically required for photo or electrocatalysts. [158]Due to the promising plasmonic properties of low-cost transition metal selenides, CoSe and NiSe NPs of sizes ranging from 20 to 50 nm were fabricated by a solvothermal technique and spray-coated on a thin porous PDMS membrane supported by a commercial PVDF membrane, which reduced the cost by 11 and 24 times, respectively, compared to noble metal Ag NPs. [219]

Metal-Organic Frameworks (MOFs)
3D highly porous nanoarchitectures of various MOFs having uniform shape and size, in which metal ions are connected to various organic linkers, have been used extensively for enhancing flux in MD.Cheng et al. incorporated Aluminum-based MOF NPs in a PVDF membrane, where a 52.4% increase in membrane porosity was observed with a 1 wt% MOF addition.This decreased the heat loss across the membrane and increased the thermal efficiency as high as 46.2%.Moreover, the reduction in pore tortuosity and the presence of an interconnected pore network reduced the mass transport of water vapors, thereby bringing an improvement in vapor flux. [220]Another study spray-coated zincbased zeolitic imidazolate frameworks (ZIF-8), which increased flux by 37.6-53.8%by generating a superhydrophobic (WCA ≈ 165°) surface via the hierarchical roughness of dual-scale ZIF-8 NPs of 266.6 nm and 1310 nm sizes. [221]The addition of highly hydrophobic 0.75 wt% ZIF-71 NPs to an electrospun PVDF-HFP nanofibrous membrane resulted in a flux increase of 284% and 949% than those respectively achieved by pristine and commercial PVDF membranes.Apart from excellent salt rejection, the negatively charged composite membrane with a water contact angle of 135°effectively rejected up to 100% of the various negative dyes in colored wastewater via electrostatic repulsion. [222]It was also reported that adding 8 wt% zinc metal azolate framework-4 (MAF-4) to PVDF when fabricating an electrospun membrane improved the vapor flux by up to 60% due to the increased availability of vapor transfer channels and superior heat insulation provided by MAF-4.Furthermore, the surface modification by bonding fluorinated molecules on surface Zn atoms of MAF-4 realized better scaling resistance. [131]Attempts at adding Fe-BTC (iron 1,3,5-benzene dicarboxylate (F300)) MOF at various loadings (1, 3, and 5 wt%) to an electrospun PVDF membrane led to a 55% improvement in flux compared to the pristine PVDF due the creation of larger average pore size and greater porosity.No Fe 2+ was detected up to the ppt level in the permeate after the 180 min experiment that inductively coupled mass spectrometry (ICP-MS), showing no breakage or leakage of MOF NPs from the membrane surface. [223]MOFs have also been utilized as nanofillers to control membrane fouling and wetting issues as demonstrated by Wei et al. who synthesized zirconium-based MOF (Ui0-66) NPs which were surface modified by hydrophilic PVA incorporated in a hydrophobic PTFE membrane by vacuum filtration. [224]

Bottlenecks in Scaling Up and Commercializing Nanoenhanced MD Membranes
The application of nanoenabled MD technology raises a dilemma, where on one hand, it has the potential to effectively address the fundamental limitations of MD and achieve sustainable improvements in MD systems, while on the other hand, its implementation entails complex techno-economic challenges related to scaling up and commercializing the technology.In spite of extensive research efforts, remarkable breakthroughs, and outstanding outcomes at the laboratory level, the progress in the scaling up of nanoenabled MD membranes for practical applications has been relatively slow.There are multifaceted obstacles that must be addressed prior to the widespread commercialization and implementation of these membranes in MD.To gain a deeper understanding of the challenges facing this field, we classified the critical current and imminent issues into three different categories: 1) associated with NMs, 2) challenges related to nanoenabled membranes, and 3) the efforts toward commercialization and market acceptance.

Challenges and Uncertainties Related to NMs
The mass-scale production of NMs with consistent properties and quality remains the major obstacle. [225]Synthesizing precisely controlled nanoparticles, nanotubes, or nanosheets at a large scale is often challenging and involves specialized equipment and expertise.[228] Furthermore, the expense of precursor materials and the complexity of the synthesis process contribute to the higher overall cost of nanoenabled membranes, rendering them less competitive when compared to traditional polymeric or ceramic membranes used in MD.NMs often necessitate specialized precursor materials for their synthesis.These materials can be costly due to their distinctive properties, intricate manufacturing processes, or limited availability, particularly in the case of noble metals. [225]Additionally, the synthesis of NMs typically involves intricate and precise procedures, such as those employed for CNTs, [229] graphene, [230] MOFs, [231] and others.These procedures often require advanced equipment, controlled environments, specialized expertise, and rigorous quality control measures.Both the cost of precursor materials and the complexity of the synthesis process directly impact the production cost and complexity of nanoenabled membranes.The potential toxicity of NMs is another significant factor that limit their commercial in MD membranes.The NMs may release from the nanoenabled membranes through leaching, membrane degradation, or disposal.[234] Besides, the regulatory frameworks for NMs have yet to be established or are still evolving, which creates uncertainty for manufacturers and researchers/industries seeking to produce and use them.

Challenges Related to Nanoenabled Membranes
The fabrication of nanoenabled MD membranes has primarily been limited to lab-scale production, employing small-batch methods such as electrospinning, phase inversion, or coating techniques.Before these membranes can reach the stage of commercialization, several technical aspects must be overcome, including the complexity of membrane fabrication processes, ensuring the stability and durability of the membranes, managing performance trade-offs, establishing the compatibility between NMs and polymers, and the validating membrane performance.
The fabrication of nanoenabled MD membranes, from achieving superhydrophobicity to endowing photothermalcatalytic properties, involves complex and time-consuming membrane fabrication processes.][237] Other methods necessitate harsh reaction conditions and hazardous reagents, such as concentrated alkaline solutions or long-chain fluorinated compounds. [238,239]n addition, precise control over the NM dispersion, membrane/coating thickness, pore structure, pore density, and other crucial characteristics presents a significant challenge in achieving the desired and reproducible physicochemical and surface properties in membranes.While this may be feasible and straightforward for producing 1 × 1 cm nanoenabled membranes at a small scale, it poses a significant challenge regarding upscale production (1 × 1 m or larger) of membranes with consistent quality and optimal characteristics.
Ensuring the long-term stability and durability of nanoenabled membranes is critical to their consistent and reliable performance over an extended period in MD.These characteristics are highly dependent on the interactions between the NMs and the polymer matrix, exposure to harsh conditions such as high temperatures or aggressive chemicals or contaminants, NMs degradation or agglomeration over time, and potential fouling or scaling. [227,64,240]This is particularly relevant to membranes that are fabricated by coating NMs on the surface of the support membrane, placing the NMs in direct contact with the raw water and harsh operating conditions.Existing literature often claims stability and durability based on lab-scale testing, which typically involves synthetic feedwaters of limited complexity or shorter testing periods ranging from minutes to days.There lack rigorous testing of nanoenhanced MD membranes under real-world conditions, which is critical for validating the performance and reliability of these membranes.In contrast to lab-scale studies, realworld MD applications involve highly complex feedwaters that often contain diverse contaminants, surfactants, and corrosive substances.Furthermore, variations in feed properties, such as surface tension with temperature, can lead to intricate interactions between the feed solution and the membrane surface.Any potential degradation, detachment, or leaching of NMs from the nanoenabled MD membranes can compromise the vital functional attributes they are sought for.
Membrane selectivity (ability to separate desired solutes), permeability (vapor flux through the membrane), and efficiency (performance and energy) are interconnected measures that need to be carefully balanced, as emphasis on one leads to trade-offs with others.Striking the desired balance between these factors is often challenging in the design and testing of nanoenabled MD membranes.Studies improving the selectivity of membranes for specific contaminants or reducing membrane fouling/scaling negatively affect the vapor flux or efficiency of the membrane, or sometimes vice versa, eventually limiting the overall efficiency of the MD process. [83]For instance, creating bi-layer nanoenabled membrane structures, such as Janus membranes, photothermal membranes, or conductive membranes, with an additional active layer coated on the hydrophobic membranes may significantly reduce membrane fouling potential and improve energy and performance efficiency but at the cost of reducing distillate flux due to the additional mass flow resistance of the coating layer.Similarly, there exists a trade-off between wetting resistance and water vapor permeability in omniphobic membranes, as these membranes usually exhibit high wetting resistance but relatively lower vapor flux than conventional hydrophobic membranes. [235]

Limited Collaborative Efforts, Market Acceptance, and Competition
The adoption of nanoenabled membranes in the MD industry greatly relies on their ability to outperform existing membranes in terms of separation performance, membrane longevity, energy efficiency, and cost-effectiveness.One major constraint at the current stage is the lack of sufficient collaborations between researchers, manufacturers, and industrial users.As aforementioned, exceptionally performing nanoenabled MD membranes developed in the lab may not be scalable or cost-effective for commercial production and use.For instance, researchers may develop a nanoenhanced MD photothermal membrane using Au [241] or Ag [242] NPs with excellent separation and photothermal conversion properties, or produce superhydrophobic/omniphobic membranes with superior properties, but these membrane may be techno-economically unviable for scale-up and commercialization due to the use of expensive precursors materials or complex fabrication techniques.As most scientific studies on nanoenabled membranes do not include the costs of developing such membranes, it is challenging for the manufacturer and industrial users to assess their competitiveness relative to conventional MD membranes.
On the other hand, manufacturers may not fully understand the potential benefits of or market demand for nanoenhanced MD membranes without proper collaboration with researchers and industrial users, making them hesitant in investing in their large-scale production.The economic viability of mass-producing nanoenabled membranes, which is crucial for motivating manufacturers to invest, will hinge on future advancements.Notably, if there is a significant breakthrough in MD performance through the application of nanoenabled membranes, it can greatly enhance the prospects for their commercialization and expand their market potential.These developments will play a pivotal role in determining the economic feasibility and driving the investment in mass production of these membranes.Similarly, for the industrial end-users, cost (including capital and operating costs) and performance are critical considerations for selecting membranes for their processes.Currently, nanoenabled membranes face challenges in terms of cost and performance, which limit their competitiveness compared to existing technologies for practical industrial applications.More collaborative efforts among researchers, manufacturers, and industrial users to take on the risks and opportunities presented by nanoenhanced MD technology are required to bring the scaling-up, commercialization, and adoption of nanoenhanced MD membranes closer to our reach.

Doing Right in The Future
To start with, it is important to note that traditional MD membranes are not without sustainability concerns, spanning from the use of petroleum-based precursors, waste production, inefficient chemical use to low fouling/wetting resistance potential, low energy efficiency, and high emissions.The inherent challenges associated with traditional MD membranes create opportunities for the emergence of nanoenabled membranes as a more sustainable alternative.
Given the present scenario, selecting economically and environmentally sustainable NMs, innovative and facile synthesis techniques, and ensuring the safety of engineered membranes should be among the top priorities of researcher communities.Continued research efforts should be centered on bringing down the cost of complexity associated with nanoenabled MD membranes by devising facile and innovative techniques for producing these membranes.For instance, redirecting attention toward cost-efficient and earth-abundant NM precursors for developing nanoenabled membranes can significantly alleviate their economic and environmental impacts.An example of this is the utilization of carbon-based NMs instead of noble metals to confer light-to-heat conversion properties to membranes.This development has significantly advanced the practical application of photothermal MD membranes for real-world applications. [121]n addition, enhancing the evaluation and understanding of the environmental impacts stemming from the synthesis and postsynthesis processes of NMs for MD membranes at an early stage in the design process will help to produce materials that can achieve their functional water treatment goals with minimal deleterious outcomes.Assessing and addressing the potentially adverse effects of NMs should be of utmost priority for successfully scaling up and deploying nanoenabled MD membranes.Despite that, the utilization of potentially toxic NMs [243] should be avoided in membrane fabrication unless their immobilization in membrane is guaranteed or a sustainable disposal strategy is devised.
Advances in membrane designs, fabrication techniques, and rigorous testing beyond the lab scale in real-world conditions remain the critical factor between exceptional lab performances and scale-up reality.By optimizing membrane structures and properties, researchers can ensure robust performance in practical scenarios with fluctuating feed compositions, fouling, and varying operating conditions.For the successful scale-up and commercialization of these membranes, it is crucial for the research community to prioritize the development of simpler yet effective nanoenabled membrane fabrication techniques, which not only yield consistent and reproducible high-efficiency nanoenhanced membranes but also be practical and appealing to manufacturers and end users in terms of performance and cost.For instance, vacuum coating of NMs on a membrane surface might seem like a simple and facile technique to produce nanocoated MD membranes; however, whether it will yield a durable coating layer that has practical implications in commercial MD or attract manufacturers' attention remain questionable.By focusing on streamlining the efficient fabrication process while maintaining the desired membrane properties, researchers can enable easier adoption of these membranes on a larger scale, making them more accessible and commercially viable.Moreover, these nanoenabled MD membranes should be evaluated in long-term operation in natural conditions to ensure their stability and consistent performance across a wide range of real applications.For instance, the nanoenabled photothermal and photocatalytic membranes requiring sunlight as the main MD driving force, should be tested under natural sunlight conditions to validate their practical applications.
Putting the pieces together from where we stand today, transitioning nanoenabled MD membranes from the laboratory scale to large-scale production and commercialization still requires significant investments and collaborations between researchers, manufacturers, industry partners, and regulatory bodies.To achieve comprehensive growth in this field, it is important to promote a positive industry outlook toward adopting cutting-edge nanoenabled MD membranes and develop a forward-thinking plan that enables a unified approach to address the inherent challenges proactively.The lack of collaboration with manufacturers and industrial end users may misalign researchers' efforts from their needs.Effective communication among stakeholders is crucial to understand each party's requirements.For instance, scientists must learn more about potential applications based on industry requests, while the industry needs to be informed about new technologies to consider potential applications.With academic researchers focusing more on advancing the fundamental understanding of the materials science, transport phenomena, and process engineering aspects of nanoenabled membranes and MD process, the manufacturers and industry partners should lend a hand on developing scalable manufacturing processes, pilot-scale testing, and ultimately bringing nanoenhanced MD membranes to the market.Hence, all stakeholders need to work together to ensure that these advanced membranes are developed, produced, and implemented collaboratively in a coordinated manner to meet the needs of all parties involved.
While much work remains to be done to bridge the gap and reduce disparities between vision and reality, the sharing of knowledge from academia to industry, as well as ongoing collaborative efforts, suggest that in the next ten years, the cumulative knowledge of fundamental and applied nanotechnology will enable the development and commercialization of the next generation of sustainable nanoenhanced membranes for MD.Considering that doubts and negative perspectives do not contribute to the development of enduring products, it becomes crucial for membrane manufacturers and industries to unwaveringly uphold their belief as early adopters.The critical question for all stakeholders is whether the nano innovations in MD can penetrate the market profitably over an extended period.The emerging applications of nanoenabled photothermalcatalytic MD processes for green H 2 generation, CO 2 conversion into renewable fuels and chemicals, and resource recovery present a promising path toward a more sustainable and profitable MD industry.These processes enhance energy efficiency, utilize renewable resources, reduce CO 2 emissions, and generate value-added products, aligning with sustainability objectives while providing economic benefits to industry stakeholders.Further advancements in these innovative applications will garner attention from stakeholders and instil greater confidence in the technology, particularly when compared to existing MD membranes.

Figure 1 .
Figure1.Timeline of MD development showing major milestone events, initial MD projects/pilot plans, and major technology developers.Developments in MD started in the early 1960s with some patents and papers but then suffered a regression in the 1970s due to low fluxes before gaining momentum again in the 1980s with the development of membranes and membrane modules.Black square symbols (■) refer to major milestones; blue downward triangle symbols (▼) refer to initial MD projects/pilot plans; and purple upward triangle symbols (▲) refer to main technology developer companies.The timeline was structured based on the literature review as well as the references.[14,18].

Figure 3 .
Figure 3.Comparison of MD with three dominant membrane and thermal desalination technologies, including reverse osmosis (RO), multi-effect distillation (MED), and multi-stage flash (MSF), in terms of three levels of characteristics shown with different background color intensity.Each level includes several performance metrics, and the levels go from being more beneficial for MD (darker background color indicating that MD is largely compatible/suitable with the metric under consideration) to less beneficial for MD (brighter background color indicating that the metric under consideration is partially or not well addressed by MD technology).The figure was redrawn based on a reported table.[40]According to the original table: "size data is based on plant data from DesalData (Global Water Intelligence, Oxford, UK), where effective small-scale operation refers to a produced water flow rate of <1000 m 3 day −1 ; energy efficiency is determined from several reviews; for use of low-grade energy, three blocks (excellent) refers to <70 °C, while one block (poor) is >110 °C or unamenable to heat input as in RO; minimal pretreatment performance is determined by comparing chemical additive costs relative to RO, where the other technologies shown are 50-66% of RO (two blocks) or less than 50% (three blocks); lifespan cost data was included from several sources including DesalData, where three blocks is <1 $ m −3 and one block is >10 $ m −3 ."

Figure 4 .
Figure 4. Applications of nanotechnology for the development of novel MD membranes and processes.The coating and incorporation of NMs in MD membranes are divided into three main categories based on the acquired properties: performance enhancement, thermal efficiency management, and simultaneous processes for a sustainable WEE nexus.

Figure 5 .
Figure 5. (Top row) Most frequently used NMs employed in MD; (middle row) most frequently used techniques for fabricating nanoenabled MD membranes; (bottom row) less frequently used techniques for fabricating nanoenabled MD membranes.

Figure 7 .
Figure7.Application of nanomaterials in photoresponsive (i.e., photocatalytic), thermo-responsive, and conductive self-cleaning MD membranes.A) Schematic of the AgCl/MIL-100(Fe)/PTFE photocatalytic MD membrane under visible light and B) the mechanism of its enhanced in-situ removal of semi-volatile organic compounds with (i) and (ii) showing the passage of nitrobenzene in the pristine PTFE membrane and the photocatalytic membrane in dark, respectively, while (iii) achieving the removal and degradation of NB with the photocatalytic membrane under visible light.Reproduced with permission.[99]Copyright 2022, Elsevier.C) Schematic of the preparation process of the thermo-responsive PNIPAM/PS-PTFE composite membrane.D) Schematic of the mechanism of the PNIPAM/PS-PTFE composite membrane's self-cleaning property.PNIPAM is a thermo-responsive material which swells and shows hydrophilic properties below its lower critical solution temperature (LCST) of 32 °C and shrinks and shows hydrophobic properties at temperatures higher than the LCST, thus endows the self-cleaning characteristic.Reproduced with permission.[101]Copyright 2023, Elsevier.E) Schematic of a DCMD module with electrical repulsion of the conductive SWCNT/PVDF membrane when connected to a DC power supply, and F) its wetting mitigation mechanism.During MD operation, surfactants adhere to the pristine PVDF membrane, reducing its hydrophobicity and causing performance degradation (left), while, with the application of a DC power supply, electrical repulsion prevents surfactants from adhering to the membrane surface (right).Reproduced with permission.[106]Copyright 2022, Elsevier.G) Anti-scaling performance of the conductive CNT/PVA/PP membrane as a result of electrophoretic mixing, and H) graphical representation of the electrophoretic mixing anti-scaling mechanism of the conductive membrane: (i) a concentration polarization (CP) layer forms near the uncharged membrane surface; (ii) application of 2 V DC potential, with the membrane as cathode, forms an electrical double layer (EDL) near membrane surface; (iii) electrophoretic mixing occurs within the CP layer due to EDL disruption and reformation caused by the polarity switching of the membrane.Reproduced with permission.[107]Copyright 2020, American Chemical Society.

Figure 10 .
Figure 10.Advanced photothermal-catalytic membrane distillation processes for clean water production, photodegradation of organic pollutants, photocatalytic hydrogen generation (water splitting), and CO 2 reduction (solar fuel production).

Table 2 .
Most researched NMs in MD along with major performance enhancements endowed by their incorporation into polymer membranes ( not/seldom used; frequently used; heavily used).was collected from the Scopus Database based on an extensive search using relevant keywords.The authors caution that some studies may be missed despite extensive literature review.Readers may read this table as a general guide to understanding the utilization of specific NMs in various roles in MD and rationally designing novel NMs for improved MD operation based on their structure/property-function relationship; b) H 2 production; c) CO 2 reduction to fuels.transfer of vapors in an electrospun PVDF membrane via various mechanisms such as Knudsen diffusion (molecule−wall collision), viscous flow (molecule-molecule collision)

Table 1 .
Recent studies of nanoenabled MD membranes for the water-energy-environment nexus.