The coral‐inspired steam evaporator for efficient solar desalination via porous and thermal insulation bionic design

The solar‐driven interfacial evaporation (SIE) technology shows great prospects in seawater desalination and sewage treatment, but it is unable to obtain highly efficient and high‐quality clean nontoxic water at low cost. Here, a novel biodegradable hydrogel‐based solar evaporator (BBH‐L) with a bionic coral structure taking Chinese ink as the solar absorber was developed. This evaporator consists of chitosan/polyvinyl alcohol hydrogel and a loofah substrate. The average evaporation rate and efficiency of BBH‐L reach 4.37 kg/(m2·h) and 98.2%, respectively, under one sun illumination (1 kW/m2), which are attributed to its excellent thermal localization and water transporting abilities. Meanwhile, high salt resistance enables BBH‐L to achieve efficient desalination and purification of other unconventional water. Heavy metal ions in seawater can be effectively removed by chelation and forming hydrogen bonds in hydrogels. This study is anticipated to provide new possibilities to enhance evaporation performance and reduce the costs of water treatment systems.

][3] The world's water storage volume is up to about 1.4 billion km 3 , of which only 2% can be used as fresh water. 4Simultaneously, hundreds of millions of people worldwide are drinking unhealthy brackish water, and a host of household sewage is disposed into the water system every year, which will seriously harm human health, and damage the ecology. 5,6Thus, developing efficient water treatment technologies to address water scarcity is exceedingly desired.
The solar-driven interfacial evaporation (SIE) technology, an emerging promising sustainable strategy, uses abundant solar energy to purify seawater, brackish water, and household sewage to alleviate water scarcity.7][18][19][20][21][22][23][24] Photothermal conversion materials, as the protagonist of SIE, can enhance solar energy capture and evaporation efficiency, and reduce phase transition enthalpy. 25,26To date, plenty of photothermal conversion materials have been explored, such as carbon-based materials, 27 ultra-black absorbers, 28 plasmonic nanoparticles, 29,30 nonplasmonic metal nanoparticles, 31 and semiconductors. 32Nevertheless, these materials usually exhibit a limited range of evaporation rates with high costs, and their freshwater productivity is still insufficient to meet the current practical demands. 33Thus, overcoming this obstacle is indispensable for the further development of solar purification technologies to improve freshwater evaporation.
Fortunately, the hydrogel-based solar evaporator can achieve a markedly high evaporation rate (~4.1 kg/(m 2 •h)) under natural sunlight (1 kW/m 2 ). 34Hydrogel, a threedimensional network polymer material with physical or chemical cross-linking points, usually has a great number of hydrophilic groups in polymeric molecular meshes, which can be combined with water molecules, lowering energy consumption of evaporation and accelerating the evaporation rate. 35Furthermore, the porous structures and microchannels in hydrogel can ensure water supply from the evaporator to the surface, which constitutes a continuous process.The solar absorber is a critical part of the hydrogel solar evaporator, which can further enhance solar energy capture and conversion efficiency. 36The commonly used solar absorbers including polypyrrole, 37,38 polydopamine, 39 and graphene oxide 40 usually have complex preparation processes, poor thermal stability, and high costs.Chinese ink (CI), which has a history of more than 2000 years, is one of the traditional "four treasures of Chinese study" in China with mature production technology and no poisoning.It is also a necessary material for ancient Chinese painting and calligraphy. 41In recent years, the application of CI in photothermal conversion is gradually expanding.3][44] Inspired by the photothermal conversion of ink-based nanofluids, we believe CI can also be used as a light absorber of SIE.
6][47] Coral reefs, whose structures are formed by stone corals, are natural biomass materials with porous structures. 48Inspired by this, a novel biomass hydrogel-based solar evaporator with a bionic coral structure (BBH-L) was developed in this paper (Scheme 1).It consists of two parts.One part is biomass hydrogel (BBH) prepared from polyvinyl alcohol (PVA) and chitosan (CS), and possesses high hydrophilicity and strong water supply capacity.The CI was S C H E M E 1 Schematic illustration of the structure and the solar steam generation mechanism of BBH-L used as the solar absorber in the hydrogel preparation.The other important part is the substrate of the evaporator, which can make the evaporator float on the surface of saline water and reduce heat loss as much as possible.Loofah, 49 a naturally degradable biomass with a multi-layer filamentous structure, was used as the substrate of BBH-L.Moreover, the BBH-L can remove salt ions from seawater, owing to the existence of chelation and hydrogen bonds in BBH.This evaporator achieves a water evaporation rate of up to 4.37 kg/(m 2 •h) under one sun illumination (1 kW/m 2 ), and thus stands out from biomass solar evaporators.It also offers a new example of using solar energy to continuously and efficiently obtain purified water at a low cost from unconventional water, such as seawater, brackish water, and household sewage.

| Characterization
Inspired by coral, we prepared the hydrogels with different PVA/CS mass ratios (1:0, 1:1, 1:2, and 2:1), labeled as BBH 1, BBH 2, BBH 3, and BBH 4, respectively, to modulate the properties of the polymers.Figure 1A exhibits a circular hydrogel in diameter of about 3 cm, and the hydrogel is black owing to the uniform distribution of CI in the hydrogel network.The scanning electron microscopy (SEM) images of BBH 2 after freeze-drying are shown in Figure 1B.BBH 2 presents a coral-like skeleton structure compared with Figure 1A.The surface of the coral has pores of varying sizes, which can enhance water transport via capillary action.Compared with other hydrogels (Supporting Information: Figure S2), BBH 2 presents smoother interconnected pores in the size of 50 µm.Under higher magnification, the polymer skeleton shows the shape and wall thickness of the pores are uniform and regular, which will better achieve the management of water molecules.Furthermore, the average pore diameter of hydrogel is 7.75 µm, which is consistent with the SEM (Supporting Information: Figure S3).
Fourier transform infrared (FT-IR) spectroscopy was used to analyze the chemical compositions of polymers (Figure 1C).In the spectrum of PVA (green curve), the peaks at 3376, 2928, and 1723 cm −1 are attributed to the −OH stretching vibration, −CH stretching of methyl, and −C═O stretching of carbonyl, respectively.The peak of CS at 3392 cm −1 (orange curve) corresponds to the combinations of peaks of −NH 2 and −OH stretching vibrations, and the peak at 165 cm −1 is ascribed to the −CONH 2 of chitosan.In addition, the red-shift of BBH (blue curve) at 3411 cm −1 implies the formation of hydrogen bonds between the hydroxyls in PVA (at 3376 cm −1 ) and CS (at 3392 cm −1 ).The combination of characteristic peaks of PVA and CS is found in BBH 2. Thus, these results indicate the successful preparation of the hybrid polymer and are in agreement with the thermogravimetric analysis data (Supporting Information: Figure S4).
Hydrogel is a viscoelastic polymer.The additives infiltrated into the skeleton will change the storage and dissipation of energy, which are expressed by storage modulus (G′) and loss modulus (G″), respectively. 50The G′ and G″ of BBHs are shown in Figure 1D and Supporting Information: Figure S5.The dynamic frequency sweep experiments of all hydrogels show wide linear viscoelastic regions, which reveals the crosslinking state of hydrogels.G′ is larger than G″ in each case, confirming the cross-linked skeleton of the polymers.Furthermore, the G′ and G″ of BBH 1 are the lowest among all hydrogels, which suggests that the addition of CS increases the elasticity of polymer skeleton and the relative sliding ability between chains. 51On the contrary, the G′ and G″ of BBH 3 are significantly lower, indicating that blindly increasing the CS leads to a weakened skeleton.The highest G′ and G″ of BBH 2 reflect that its PVA/CS mass ratios are appropriate, suggesting BBH 2 has more crosslinking points and stronger mechanical properties.

| Evaporation performance of BBHs
The mass changes with hydrogel and pure water were recorded when the temperature stabilization (30 min of preheating), which represented the quality of water evaporation.Obviously, the amount of water evaporated based on the hydrogels under one sun's radiation was about six times higher than the natural evaporation of pure water (Figure 2A).
The evaporative properties of hydrogels can be improved by adjusting the weight ratio of PVA/CS.Especially, the evaporation rate of BBH 2 (3.55 kg/ (m 2 •h)) is much higher than BBH 1 (2.58 kg/(m 2 •h)) and even 7.9 times that of pure water (0.45 kg/(m 2 •h)).BBH 3 presents a large water content in the molecular network, but the evaporation rate is only 3.08 kg/ (m 2 •h), which is because more energy is used to heat the bulk water instead of evaporation, and will be further analyzed in the water content experiment.These results indicate BBH 2 has the most outstanding evaporation performance.Therefore, the introduction of loofah at BBH 2 was selected as the substrate of BBH-L (inset in Figure 2B and Supporting Information: Figure S6).The porous structure of the loofah provides the evaporator with a suitable water path, which can obtain sufficient water supply and ensure effective water evaporation.At the same time, BBH 2 is tightly surrounded by 0.5 cm-thick PVC foams.This layer can be used for thermal insulation to prevent vapor escape from the sidewall and reduce heat conduction loss during evaporation.BBH-L exhibits an excellent evaporation rate of 4.37 kg/(m 2 •h), which is 1.22 times that of BBH 2 (Figure 2B).This result indicates the introduction of loofah can optimize the evaporation performance of hydrogels.
The evaporation rate of hydrogel under dark conditions was recorded to calculate the energy efficiency of BBHs (Supporting Information: Figure S7A).The evaporation efficiency η pt can be calculated by the following formula: where ṁ is the water evaporation rate; H equ is the equivalent vaporization enthalpy of water, which can be estimated by evaporating water under dark conditions with identical energy input (U in ) (shown in Supporting Information: Figure S7); E in is the total solar radiation received.The η pt of the hydrogel and pure water systems are shown in Figure 2C, red dot.Under one solar illumination, the evaporation efficiency of the hydrogel was up to three times that of pure water (26%), which was mainly because the addition of CI enhanced the solar absorption capacity of the hydrogel and the water evaporation rate (ṁ).Furthermore, the evaporation efficiency of BBH-L reached 98.2%, which was because the loofah increased water transport capacity, reducing energy loss and allowing more heat to be used in the evaporation.The energy efficiency loss (2%) may be due to the change in the dark evaporation rate with the change of ambient humidity.Therefore, the influence of the environment on evaporation efficiency cannot be eliminated by subtracting the dark evaporation rate under the initial environmental conditions. 56Furthermore, convection loss and thermal radiation loss are also the main reasons for evaporation efficiency loss. 31herefore, high evaporation efficiency can be achieved by insulating foam and loofah substrate, but energy efficiency loss is still inevitable.
The relatively low efficiency of BBH 1 and BBH 3 further illustrates that a great water management system is essential for evaporation.The water evaporating rate under one sun illumination directly reflects the potential of vapor generation under ambient conditions.Thus, the evaporation rate and efficiency of BBH-L were compared with some recently reported data (Figure 2D), which showed new evaporator achieved record-high evaporation rates and evaporation efficiency.

| State of water restricted inside the BBHs
Sustainable water supply is a critical performance of an ideal evaporator.The hydrogel surface with certain hydrophilicity can quickly transfer from the bulk water to the evaporation surface of the hydrogel.The contact angle is an effective means to assess the hydrophilicity of a material, and the measured contact angles of freezedried BBH 2 are shown in Figure 3A.The water droplet was absorbed immediately after it touched the upper surface of the hydrogel (about 0.5 s, video in Supporting Information), which proves it has excellent hydrophilicity and can effectively supplement the evaporated water.Furthermore, the contact angle test of other hydrogels is shown in Supporting Information: Figure S8.
The swelling performance of the composite hydrogel can be adjusted by changing the mass ratio of PVA/CS.The saturated water content (Q s ) of hydrogels can be calculated by formula (2) where W and W d are the weights of the hydrogel sample after fully swelling in water and under a completely dry state, respectively.The water contents of BBH 1, BBH 2, BBH 3, and BBH 4 were 2.01, 4.04, 6.34, and 3.61 g/g, respectively.The results of water content are positively correlated with the proportion of CS molecular chains (Figure 3B), indicating the water content can be tuned by adjusting the weight ratio of PVA/CS.The increase in saturated water content also means the hydration performance of the hydrogel evaporator is enhanced, which is conducive to the formation of intermediate water and greatly reduces the energy required for evaporation.However, the too-high saturated water content will lead to the use of more energy to heat water, resulting in increased heat loss and a lower evaporation rate.Consequently, BBH 2 provides an appropriate saturated water content that ensures it achieves a stable and effective evaporation rate, which is consistent with the results of the evaporation experiments.
According to the difference in intermolecular hydrogen bonding, the water in the hydrated polymer network can be classified into three types (Figure 3C): free water (FW, normal water-water bonding), bound water (BW, water-polymer bonding) and intermediate water (IW, weakened water-water bonding).The frequency of stretching vibration peaks in Raman spectra decreases when the molecules form hydrogen bonds.Therefore, the state of water in a hydrogel can be analyzed by fitting the four stretching vibration peaks of the hydroxyl in the Raman spectrum.The peaks at 3080 and 3250 cm −1 in Figure 3D,E are attributed to low-density water containing four hydrogen bonds, which can be expressed as FW.The other two peaks near 3420 and 3620 cm −1 are due to the high-density water with no or weak hydrogen bond, which can be described as IW.The molar ratio of IW to FW in BBH 2 is higher than that in BBH 1 (1.084:1 vs. 0.98:1), suggesting that BBH 2 contains more IW and that IW is activated water and requires less energy to break the weak hydrogen bonds formed by water molecules. 52Therefore, BBH 2 exhibits excellent evaporation performance under the same energy input.Furthermore, the continuous evaporation of IW leads to the formation of negative pressure inside the hydrogel, which can achieve the selfregulation of water and help to replenish water molecules in time. 57These results suggest that a higher proportion of IW can effectively improve the water transmission effect to achieve better evaporation.

| Molecular dynamics (MD) simulations of PVA/CS chains
MD simulations were performed to investigate the evaporation process of water on the BBH surface.A sandwich-like hydrogel evaporation system with dimensions of 14.0 × 13.6 × 25.0 nm was created, within which the PVA surface was attached by CS chains with a mass ratio of 1:1 according to our preparation of BBH.The initial state of the BBH evaporation system is shown in Figure 4A.To prevent the periodic boundary movement of water molecules along the z-axis, two solid walls were placed at the top (z = 22.71 nm) and bottom (z = 0.57 nm) of the system model.Meanwhile, the surface was located at z ≈ 4.0 nm with 9272 water molecules beneath the surface in a rectangular block with dimensions of 13.8 nm × 13.4 nm × 1.5 nm.During the simulation, the water molecules with a z-coordinate larger than 19.5 nm were considered vaporized.The interactions between water molecules and PVA/ CS chain's surface were governed by the PCFF force field.NVT ensemble was applied during the simulation, and the system temperature was controlled by Nosé-Hoover thermostat 58,59 at 300 K. Figure 4B-D exhibit the evaporation process of water molecules in the hydrogel, and it can be seen that water molecules can be completely passed through BBH surface, indicating that it has suitable hydrophilicity.The simulation results were consistent with the experimental observations.At the end of the simulation, as shown in Figure 4E, 3.27% of water molecules cross from the bottom of the hydrogel and evaporate from the surface into the air layer within 10 ns, which indicates that water molecules escape from the surface of BBH quickly.In conclusion, MD simulations reveal the influence of BBH on the evaporation behavior of water molecules at the microscopic level.The PVA/CS chains could control the distribution of water at the evaporation interface, and the hydrophilic surface could improve the water evaporation rate of the hydrogels.

| Photothermal management behavior of BBHs
With CI loaded into the hydrogel as the solar absorber, BBH presents excellent solar absorption (≈97%) over a wide range from 400 to 2500 nm (Figure 5A).Exceptional solar absorption means that sunlight can be effectively "trapped" on the surface of the hydrogels.The photothermal performance and thermal localization ability of the hydrogels were evaluated by tracking the temperatures of the evaporation surface and bulk water under one sun.As illustrated in Figure 5B, the surface temperature of BBH-L increased rapidly within 5 min and reached equilibrium after about 30 min.Furthermore, the maximum temperature gap between the surface and the bulk water was nearly 20 °C.The above results indicate that the BBH-L evaporator has excellent thermal localization ability, which can limit heat mainly to the central surface and can greatly reduce heat loss.
Figure 5C represents the trend of average surface temperatures during the evaporation of BBHs (0, 5, 8, 10, and 15 min).The average surface temperatures of BBHs were recorded by infrared thermal imaging.Obviously, the temperatures of all hydrogel samples increased rapidly to more than 30 °C within 5 min.Especially, the average surface temperature of BBH 3 rose the slowest and reached only 37.6 °C in 15 min (Supporting Information: Figure S9), which was because its largest equilibrium water content called for more energy to heat the bulk water.In contrast, the average surface temperature of BBH 2 reached 42.2 °C in 15 min, which further indicates BBH 2 has an appropriate saturated water content that allows the maximum amount of heat to be used in the evaporation.
Moreover, the average surface temperature of BBH-L increased from 25.1 °C to 38.9 °C within 8 min and only rose by 1.3 °C after 15 min.This result indicates that the introduction of loofah at BBH 2 further enhances the water management ability of the hydrogel, which allows more heat to be used efficiently during evaporation.The above analysis is consistent with the experimental results of temperature rise.Accordingly, the advantages mentioned above provide BBH-L with a fast photothermal response and excellent thermal localization ability, which will significantly improve its evaporation performance in practical applications.The number of evaporated water molecules.(The top and bottom walls are dark gray; the carbon, oxygen, nitrogen, and hydrogen atoms in the surface are light gray, red, green, and white, respectively; the oxygen and hydrogen atoms in the water molecules are light blue and white).

| Seawater desalination performance
The solar-driven seawater desalination performance of BBH-L was evaluated with artificial seawater samples in three representative salinities (Baltic sea [8‰], World sea [35‰], and Dead sea [250‰]).As displayed in Figure 6A, the evaporation rates of the three samples were over 4.0 kg/ (m 2 •h) and were even up to 4.07 kg/(m 2 •h) in the Dead sea with the highest salt concentration.This result suggests that BBH-L can work stably in high-salinity water.
The desalination capability is one of the critical indicators to evaluate hydrogel-based evaporators.Accordingly, the total dissolved solids (TDS) of the three artificial seawater samples before and after desalination were measured.After desalination, the TDS of all samples was significantly reduced by about three-four orders of magnitude and from 6720, 37000, and 300000 to 24, 32, and  42 mg/L, respectively, which meet the drinking water quality standards specified by the WHO (1000 mg/L) and the US Environmental Protection Agency (EPA, 500 mg/L).Furthermore, a real seawater sample was used in seawater desalination experiments with the BBH-L.As shown in Figure 6B, inductively coupled plasma emission spectroscopy (ICP-OES) was used to trace the concentrations of five metal salt ions (Na + , K + , Ca 2+ , Mg 2+ , and Ba 2+ ).After purification, the concentrations of the five ions in the real seawater sample were significantly reduced by three-four orders of magnitude, and the total concentration was only 10 mg/L.These results prove that solar-driven seawater desalination based on the BBH-L is effective.
Generally, the salt concentration in BBH-L will increase as the water evaporates.The high salt content will cause hydrogel channel blockage, which will affect the evaporation rate.Therefore, the anti-salt scale function is an important premise for the long-term use of a solar evaporator.Thus, 1.5 g NaCl was added to the surface of the evaporator to test its salt scale resistance (Figure 6C).The salt crystals began to dissolve after 10 min and completely dissolved after 30 min.This was because the hydrogel was capable of continuous water delivery.After dissolution by the water transported to the surface, the salt will automatically diffuse into the water below due to the concentration difference, and then the salt scale on the surface will disappear.In other words, the formation of salt scale will decelerate interfacial light evaporation, but will not interrupt the continuous evaporation process.The salt resistance and self-cleaning function of BBH-L further ensure the continuity of desalination and pollution reduction.
In practical application, the solar evaporator needs to float in seawater or other unconventional water for a long time.Therefore, the number of cycles is another important parameter of the evaporator.As shown in Figure 6D, BBH-L can maintain a stable seawater evaporation rate of

| Brackish water desalination performance
Brackish water is usually collected from shallow strata and thus contains more Mg 2+ than seawater.Long-time drinking of brackish water will cause certain harm to the body.Therefore, the brackish water was treated with evaporation.After desalination, the TDS was reduced from 2750 to 21 mg/L, meeting the drinking water quality standards specified by the WHO and EPA.The concentrations of the five ions (Na + , K + , Ca 2+ , Mg 2+ , and Ba 2+ ) present in brackish water decreased significantly to 6.8, 3.2, 2.4, 4.3, and 2.8 mg/L, respectively, and the removal rate was up to 99% (Figure 7).These results indicate BBH-L has excellent effect in treating brackish water.

| Household sewage desalination performance
Similarly, household sewage samples at three pHs (2, 7, 13) were evaporated treated.This acid-base span usually covers all domestic sewage and detergent conditions.The experimental results are shown in Figure 8.Clearly, the evaporation rates of the three sewage samples were all up 4.0 kg/(m 2 •h) and were not significantly different compared to pure water.This comparison indicates that BBH-L can achieve efficient sewage treatment with high evaporation rates.The pHs of the water collected after evaporation were further tested (Figure 8, blue line).Compared to the standard color card, the pHs of the collected water are 6.5, 6.8, and 7.3, respectively, meeting the drinking water standard provided

| Outdoor solar desalination of BBH-L
Under natural sunlight, an outdoor solar desalination experiment with seawater was conducted from 7:00 to 19:00 at Chengdu University of Technology, and the experiment device was demonstrated in Supporting Information: Figure S11.The real-time changes of humidity, solar radiation angle, and wind speed were carefully tracked (Figure 9A).During the experiment, the humidity gradually decreased before 13:00 and then stabilized at about 45%.The wind speed reached 3.15-3.26m/s from 13:00 to 19:00.Meanwhile, the solar radiation intensity variation was consistent with the incident angle of solar radiation.As displayed in Figure 9B, the maximum radiation flux of 0.746 kW/m 2 and the highest evaporation rate of 3.24 kg/(m 2 •h) were reached at 13:00.Furthermore, the daily yield reached 24.5 kg/m 2 under the average solar illumination of 0.45 kW/m 2 .The results suggest that BBH-L can achieve high yield of freshwater under natural light conditions.

| CONCLUSION
In summary, we demonstrate a novel biodegradable hydrogel-based solar evaporator with a bionic coral structure (BBH-L) that achieves high efficiency and low-cost solar steam generation even in treatment of high-concentration seawater and wastewater.The evaporator is fabricated from the PVA/CS hydrogel and the loofah substrate.CI is used as a light absorber to enhance sunlight absorption.The BBH-L shows an ultrafast evaporation rate up to 4.37 kg/(m 2 •h) and an evaporation efficiency of 98.2% under one sun illumination.Meantime, the artificial seawater with different salt concentrations and the sewage with different pHs after treatment all meet the drinking water quality standards specified by the WHO (1000 mg/L) and the EPA (500 mg/ L).The potentials of the BBH-L for removing heavy metal ions or balancing pH values of wastewater are demonstrated experimentally.This work displays great promise for high-quality clean water production in a lowcost, low-energy and nontoxic way.

F
I G U R E 1 Chemical and structural characterization of the BBH.(A) Photograph of the as-prepared BBH; (B) SEM images at different magnifications showing; (C) Fourier transform infrared spectra of BBH, PVA, CS, and CI indicating the possibility of BBH synthesis; (D) Storage modulus (G′) and loss modulus (G″) of the BBHs.

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I G U R E 3 State of water restricted inside the BBHs.(A) Dynamic contact angle of BBH 2; (B) Saturated water content in BBHs per gram of the corresponding xerogel; (C) Schematic of the mechanism of moisture regulation in hydrogels; (D, E) Raman spectrum fitting curve of the energy region of BBHs: O-H stretching mode of water.

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I G U R E 4 (A) The initial state of molecular simulation of BBH.(B)-(D) The evaporation process of water molecules in the hydrogel.(E)

F I G U R E 5
Photothermal management behavior of BBHs.(A) UV-Vis-NIR spectra of BBH; (B) Bulk water temperature and surface temperature of BBH-L recorded by infrared thermal imaging; (C) Changes on average surface temperatures of hydrogel samples with the irradiation time.

F I G U R E 6
Seawater and brackish water desalination performance of the BBH-L.(A) Evaporation rates and the total dissolved solids (TDS) of the BBH-L in three artificial seawater samples; (B) Concentrations of main ions in seawater before and after desalination; (C) Water evaporation rate during the desalination of BBH-L immersed in seawater up to 30 days; (D) Antisalt crystallization performance of BBH-L.

4. 1
kg/(m 2 •h) after 30 days of continuous reuse under one sun, showing excellent durability.The results indicate that BBH-L has high reliability in long-term actual purification of unconventional water.

F I G U R E 7
Concentration of main ions in brackish water before and after desalination F I G U R E 8 Evaporation rates of the BBH-L in household sewage with three pHs and the pHs after desalination by the WHO (in the range of 6.5-8.5).The above results show that BBH-L has remarkable desalination performance and can be applied to treat different types of household sewage and other unconventional water.