(129a) Membrane Desalination: Limitations and Opportunities

Meeting: AIChE Annual Meeting

Session: Membrane Research Activities Around the World"

Host: Zhongyi Jiang
<zhyjiang@tju.edu.cn>

Date:  3-8 November, 2013

Location: SF, CA

"Membrane Desalination: Limitations and Opportunities"

Georges Belfort¹

¹Howard P. Isermann Department of  Chemical and Biological
Engineering, and

Center for Biotechnology
and Interdisciplinary Studies

Rensselaer Polytechnic
Institute

Troy NY 12180-3590, USA

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.