(155b) Opportunities for Scalable Desalination

Belfort, G., Rensselaer Polytechnic Institute

Reverse osmosis (RO) has matured from a laboratory curiosity at the University of Florida and the University of California at Los Angeles in the early 1960s to an accepted industrial desalination process widely considered as the least energy intensive and most economically attractive technology for separating salt from water.  Plans are now in progress in Japan to build a megaton plant (1,000 m3/day).  With this wonderful success story, where are the opportunities for improvement?  They include development of smooth, low fouling, chorine stable and long lasting membranes.  Can we replace interfacial polarization as a mean of producing salt rejecting rough and chlorine susceptible membranes?  To address this question fully, we need to understand the limitations of RO, consider fundamental principles of the molecular behavior of water and salt at solid interfaces (i.e. synthetic membranes), and critically assess the limitations such as concentration polarization and fouling.  In this presentation, we will first summarize the major technological breakthroughs associated with the success of large-scale RO including the reduction in energy use.  While fouling and concentration polarization offer inherent limitations, they are dealt with through pretreatment (removal of foulants), chemistry and hydrodynamics.  Despite its successes, the molecular basis of salt separation by RO is still controversial: Gluekauff, Matsuura and Sourirajan, and Belfort have suggested molecular mechanisms that depend on dielectric, bound water and ion hydration arguments, respectively.  Given the importance of water-polymer interactions (hydrogen bonding), polar membrane materials such cellulose acetate and aromatic polyamides are widely used.  However, the lack of a molecular-level understanding means that a rational design strategy for improving RO efficiency is missing.  Recent experiments with oxidized graphene and single walled carbon nanotubes, together with simulations of water near polar and non-polar interfaces suggests exciting possibilities for high-flux desalination membranes.  These results indicate that water moves through non-polar nano-pores with little or no friction, with very high water fluxes using small amounts of energy.  Recent attempts to take advantage of these ideas are limited by concentration polarization.  Increasing the mass transfer coefficient through improved fluid mechanics will be needed to remove the polarized salt and reduce salt leakage. Nature-inspired transport, through the well-known aquaporin channels in biological membranes, uses a mixture of intermolecular forces to optimize selectivity.   In summary, we plan to discuss how integrating recent molecular insights with decades of industrial advances will provide exciting opportunities for scalable desalination.