Due to their hydrophilic character and charge, microbial polysaccharide based membranes are being developed for solvent dehydration by pervaporation, and for wastewater treatment, where they demonstrated a high adsorption capacity for aromatic compounds, dyes and heavy metal ions. They also have been applied for the development of components for electronic devices.
Solvent dehydration processes have a high economic and environmental relevance in the pharmaceutical, fine-chemistry, and chemical industry. Among the available dehydration techniques, pervaporation is attractive due to its relative simplicity of operation and high selectivity, making possible to circumvent the formation of azeotropes. Additionally, pervaporation may operate under mild conditions, which translates into a process economy.
Hydrophilic polymers, such as, polyvinyl alcohol, polysulfone, polyamides, among others have been selected as membrane material for the dehydration of various solvents. Membranes of polyvynilalcohol (PVA) have been commercialized by Sulzer Chemtech due to their excellent water perm-selective properties.
With growing environmental concern, it is very important to obtain polymers from renewable sources and many efforts have been devoted to the development of new membranes with high separation performance and reliability. However, these new membranes should present a good compromise between flux and selectivity and also chemical and mechanical stability, when compared with commercial membranes.
Among the biopolymeric materials used in hydrophilic pervaporation, polysaccharides have received much attention, due to their good selectivity and high flux. Chitosan and sodium alginate are examples of polysaccharides that have already been tested in pervaporation for dehydration of solvents, such as ethanol, isopropanol, tetrahydrofuran, and acetone, with high separation performance in terms of selectivity and water flux.[103-106]
Although they show an excellent affinity for water, as membrane materials they lack mechanical strength and stability in aqueous solutions. Membranes with enhanced water resistance and water selectivity have been developed using selected strategies, such as polymer cross-linking, incorporation of inorganic particles in the polymer matrix and blends or self-assembly of layer-by-layer polyelectrolyte polymers.
The degree of crosslinking affects the flux, selectivity and stability behavior of the membranes. A decrease of water permeability is expected with increasing crosslinking, but an improved selectivity and long-term stability is obtained. Composite membranes are often used, since they can offer a higher flux due to a much thinner thickness of the active membrane supported on a porous substrate, which should present negligible resistance to mass transfer.
Stable sodium alginate membranes using glutaraldehyde or ionic cross-linking with multivalent metal ions (e.g. Ca2+, Al3+) were obtained, by suppressing excessive swelling. Chitosan membranes are also extremely hydrophilic and can lose integrity in aqueous solutions, thus cross-linking and blend strategies were also employed.
Multilayer polyion membranes can be obtained using a layer-by-layer deposition method, in alternate mode, with chitosan as polycation and polyanion polymers, such as hydroxyethylcellulose, cellulose acetate and cellulose sulfate. These membranes demonstrated an excellent dehydration performance as shown in Table 3. The values of the selectivity and flux obtained for the ethanol dehydration by pervaporation with 10 wt % of water in the feed, range from 1000 to 10,000 and the fluxes are always higher than 100 g/m2 h.[104, 107, 108]
Table 3. Selectivity and Flux for Ethanol Dehydration by Pervaporation with Feed Water Content of 10 wt %
|Membrane||T (°C)||Selectivitya||Flux (g/m2 h)||References|
|Sodium alginate and cellulose blend||30||1175||170||103|
|Ca2+ crosslinked sodium alginate||50||300||230||107|
|GAb crosslinked sodium alginate||60||1000||300||108|
|Al3+ Cr3+ crosslinked sodium alginate||70||2750||942||109|
|GAb crosslinked chitosan||50||6000||1100||110|
|GAb crosslinked chitosan/sodium alginate||60||1000||210||111|
It is necessary to take into account that, the flux increases with higher feed concentration, higher temperature, and with lower membrane thickness. So, the performance of the process should be quantified in terms of permeability and selectivity which allow for describing the intrinsic properties of the separation membranes and compare results obtained at different experimental conditions.
Regarding microbial biopolymers there is, so far, not much work on their application as membranes for pervaporation. Bacterial cellulose membranes have been used for ethanol dehydration. For feed compositions containing less than 30% water, the selectivity toward water was in the range of 125 to 287 and the flux was higher than 100 g/m2 h. Recently, it has been reported a new extracellular polysaccharide (GalactoPol) produced with a low cost, abundant carbon source, the glycerol byproduct of the biodiesel industry, using Pseudomonas oleovorans. Two types of membranes, homogeneous (EPS) and composite of EPS with polyethersulfone (PES) as support (EPS-PES), were developed and used for ethanol dehydration by pervaporation. The homogeneous membrane, at a water feed concentration of 5.0% (w/w), showed a water/ethanol selectivity of 110. For the composite membrane a denser EPS polymer was used leading, under the same operating conditions, to much higher water/ethanol selectivity (3000). Moreover, the mechanical resistance was also improved in comparison with the homogeneous membrane, due to the physical characteristics of the commercial support used.
The fluxes obtained were lower than the reported previously, 11 to 22 g/m2 h, however the temperature used was 30°C and the water concentration in the feed was only 5 wt %. Increasing the water concentration in the feed to 10 wt % the fluxes increased to 40 to 60 g/m2 h and the selectivity decreased to 69 and 134. A higher water concentration leads to membrane swelling and higher mobility of the polymer chains. As a consequence, permeability increases and selectivity decreases.
These new membranes may become an interesting alternative to commercial hydrophilic pervaporation membranes for the dehydration of ethanol. Strategies, for further improvement should include optimization of polymer cross-linking conditions, in order to improve selectivity for higher concentration of water in the feed stream. Additionally, it will be important to evaluate the performance of these new membranes in other industrially relevant processes, such as the separation of polar and non-polar compounds in organic mixtures, and also for solvent-resistant nanofiltration processes.
Water and Wastewater Treatment
Polysaccharide-based materials have demonstrated good removal capabilities for certain pollutants, such as aromatic compounds, dyes, and heavy metal ions as compared with other commercial sorbents currently used in wastewater treatment processes. Sorbents containing polysaccharides possess a high capacity and high rate of adsorption, high detoxifying efficiency, and selectivity. They can be used in the form of insoluble beads, gels, sponges, capsules, films, membranes, or fibers. There has been a recent interest on the development of sorbents based on natural polysaccharides, mostly focused on the use of chitin, starch and their derivatives. Additionally, many microbial polysaccharides have been reported to have metal binding capacity[115-118] and have been proposed as possible alternatives to the traditional sorbents used.
Other Emerging Uses
There is still limited use of membranes based on microbial polysaccharides for other industrial applications. Nevertheless, there are promising reports of the investigation of curdlan and bacterial cellulose[50-56] for the development of components for electronic devices.
Bacterial cellulose (BC) exhibits a nanofibrous porous network structure with high strength and low density. The relatively stable and inert nature of BC allows the incorporation of metallic, ceramic and polymeric materials into its porous structure, which imparts BC biomaterials increased functionality. Promising research reports about the use of BC for the development of electronic components include: the synthesis of electrically conducting BC by the incorporation of multiwalled carbon nanotubes; an organic LED (light-emitting diode) fabricated with an electroluminescent BC–acrylic resin composite (Figure 4); an electro-active LiCl-impregnated BC composite; electronic paper made of BC embedded in an electrochromic dye; and magnetic composites synthesized by incorporation of ferrite, copper, and/or niquel nanoparticles.[54-56]
Phosphate-containing bacterial cellulose (PCBC) has also been proposed as a fire retardant compound. Compared with cellulose, PCBC produced by cultivation of Gluconacetobacter subsp. xylinus using corn steep liquor as a nitrogen source had lower decomposition temperature and higher char formation, which are fire-retardant characteristics.
Bacterial cellulose has already been used by Sony for several years for the fabrication of diaphragms for electroacoustic transducers in several products, such as earphones or loudspeakers.[59, 60] BC membranes have acoustic response in a wider frequency range, higher crystallinity and higher Young's modulus than the traditionally used plant cellulose membranes.