(287c) Charged and Hydrophilized Polybenzimidazole (PBI) Membranes for Forward Osmosis

Forward osmosis is the movement of water across a selectively permeable membrane.  The driving force for water permeation through the membrane is the difference in osmotic pressure between the feed and draw solutions.  Polybenzimidazole (PBI) is a material with excellent chemical resistance and high mechanical and thermal stability that is a promising material for forward osmosis separations.  Drawbacks associated with the use of PBI as a membrane material include low hydrophilicity and surface charge neutrality at neutral pH values. 

In this study, PBI membranes were cast using the phase-inversion technique in the form of asymmetric flat sheets, and membrane surfaces were functionalized using different modifying agents with the goal of increasing hydrophilicity, increasing surface charge, and reducing membrane pore sizes.  An increase on the negative charge of the membrane surface was expected to yield an increased rejection of ions and of charged species in the feed solution. An increase in hydrophilicity was expected to reduce fouling propensity and enhance wettability of the membrane surface. Lastly, a reduction in pore sizes was expected to allow for greater steric effects.  In order to modify the membranes, the surfaces of the membranes were first activated with 4-(chloromethyl) benzoic acid (CMBA).  The modifying agents selected for membrane functionalization included: taurine, para-phenylene diamine, ethylene diamine, and poly(acrylamide-co-acrylic acid).

Membranes were characterized using Fourier transform infrared spectroscopy in attenuated reflectance mode (FTIR-ATR), zeta potential, environmental scanning electron microscopy (ESEM), contact angle measurements, and total organic carbon (TOC).  Functionalization, surface charge, increased hydrophilicity, and reduced pore size were all verified.  Pure water permeability and monovalent salt rejection were tested in a pressure driven mode for comparison between both virgin and modified membranes. Salt rejection was investigated using various sodium chloride feed concentrations, and a range of pH values.

Flat sheet membranes were also tested in forward osmosis (FO) applications.  The FO process involved use of an ammonium bicarbonate draw solution and a sodium chloride feed solution.  All modified membranes showed enhanced water permeability and increased salt rejection over the unmodified membrane surface.