Desalination by pervaporation: A review

Desalination via pervaporation has the potential to be an efficient way of getting fresh water from non-potable saline sources with the advantages of a high rejection of salt and the capability of coping with high-salinity solutions. This overview of desalination via pervaporation mainly focuses on membrane materials, transport mechanisms and the effect of operating parameters on PV performance. Almost all kinds of membranes ever reported in desalination are mentioned, including those based on polymers, inorganic materials and their hybrids, all of which show reasonably performance with adequate flux and excellent salt rejection. The comparison of pervaporation with existing conventional reverse osmosis and membrane distillation processes and several strategies for further improvement of pervaporation performance are discussed. © 2016 Elsevier B.V. All rights reserved.

PV is a promising method for desalination. The main advantage of PV over RO is the ability to cope with a wide feed concentrate range without increasing cost, in contrast with RO. Almost all PV membranes mentioned, which are based on polymers, inorganic materials and their hybrids, show reasonable performance with adequate fluxes and excellent salt rejection. Unlike the PV separation of liquid mixtures where the volatile organic selectivity is always a concern for application, the nonvolatility nature of salts accounts for the high rejection irrespective of the variation in solute concentration, operation conditions and the degree of membrane hydrophilicity. In most cases, the rejection can be as high as 99.99%, which indicates a potential for producing ultrapure water. Transport mechanism of PV desalination is system-specific depending on hydrogen-bonding, ion-dipole interactions and/or ion-ion interactions between the feed salt solutions and the membranes with different chemistry and structure. In summary, it is proposed that water molecules are preferentially dissolved/adsorbed and diffuse through the membrane while salts are rejected by a dense membrane or retained by a porous charged or uncharged membrane through size exclusion and/or charge exclusion. The mass transport mechanism in non-porous uncharged polymeric membranes can be described by solution-diffusion theory. For the ion-exchange membranes, a higher capacity in increasing water flux has been observed beyond the value calculated from Fick's equation. The separation mechanism in porous inorganic membranes is based on molecular sieving (or size exclusion). As for the charged porous inorganic membranes, the mechanism of mass transfer and salt rejection can be explained jointly by size exclusion. Interestingly, in some cases, it is possible that a higher water flux of the membrane can be obtained in the presence of salt water than in pure water, assumably due to the ion exchange between the charged membranes and the adsorbed certain ions from the feed solution. However, the slow penetration/diffusion of trace amount of salt into and pass through both inorganic and polymeric membranes is observed sometimes. The fundamental studies are needed to understand and ascertain the mechanism of the transport of ions and water in charged or uncharged, and dense or molecularly porous PV membranes for desalination. Many improvements need to be made in membrane design. Thin film composite membranes or hybrid membranes may make a contribution towards a higher flux. The flux for the better membranes, such as cellulose membranes, PVA/PAN composite membrane, SiO2/PVA hybrid membrane, zeolite ZSM-5 membrane and graphene oxide/PAN composite membrane, ranges from 6.7 to 65 kg/m2·h. To achieve yet higher flux without decreasing the selectivity is a major research challenge. More effort is needed to make more open and thinner membranes with improved surface hydrophilicity and especially improved membrane nanostructure to lower the mass transfer resistance. Optimization of operating conditions can also be useful. Temperature is a crucial parameter because of the increase in diffusivity and reduction in flow viscosity that occurs on heating. High temperatures, mechanical energy in the form of a vacuum and a high flow rate of feed and sweeping flow can be called upon to enhance the flux. More research should be done to improve the efficiency of PV process by effectively reducing the concentration polarization in both sides of membrane. The relationship between the adsorption and fouling on membrane and the type and concentration of salts in the feed are needed to be established. Generally, continuous improvements in PV performance with respect to permeability and stability in the future will increase the competitiveness of the process. Greater efforts should be made to address this possible solution to overcoming global water scarcity for acquiring potable water. Acknowledgments This work was financially supported by the Fundamental Research Funds for the Central Universities of China (Grant No.xjj2013075).