(90a) Cation Exchange in Forward Osmosis: Elucidating Impacts of Membrane and Solution Chemistry

Thin film composite (TFC) membranes designed for use in forward osmosis (FO) have recently reached the ubiquity previously occupied by the cellulose triacetate membranes. FO processes make use of an osmotic pressure gradient to drive water flux across a semi-permeable membrane. The solution having a higher osmotic pressure, or draw solution, can make use of a myriad of different solutes from nanoparticles, to organic solvents, to electrolytes. Electrolyte draw solutions are commonly used in many large scale applications of forward osmosis processes.

                A unique interaction between draw and feed solution containing differing electrolytes solutions and the thin film composite membrane chemistry has been noted in recent studies. This unique interaction is the exchange of cations across the membrane, which is present as high forward and reverse permeation of cations between the draw and feed solutions. It has been hypothesized that this behavior originates for carboxylic acid functional groups forming part of the chemical structure of the polyamide selective layer of TFC membranes. This functional group is shared by weak acid cation exchange resins which are characteristically sensitive to pH. This study seeks to further investigate the cation exchange behavior in TFC membranes with focus upon two primary goals:

1. To understand the importance of pH in affecting the bi-directional transport of cations across the selective layer of a commercial TFC FO membrane.
2. To form a TFC membrane possessing has superior resilience to cation exchange.

In this study potassium chloride has been selected as a model draw solute serving as an analog for the draw solutions envisaged for forward osmosis desalination processes. This selection eliminates the use of a draw solution which contains weak acids or bases, and the use of draw feed solutes having a metal cation allow for easy quantification of ion concentrations using atomic absorption spectroscopy. Water flux and ion fluxes were observed for a feed and draw solution kept at identical pHs, buffered at pHs of 2, 4, 6, 8, and 10. It was found that cation flux dropped significantly at pHs lower than 6.

New TFC membranes tailored for FO were prepared of differing selective layer chemistries to measure the rate of cation transport between the previously mentioned draw solution of potassium chloride and feed solution of sodium chloride.  The selective layers of these membranes were formed on a composite support layer prepared from polysulfone cast over electrospun polyvinylidiene fluoride fibers. Membranes having this support layer were found to have low structural parameters and give adequate performance in FO. For the membranes prepared in this portion of the study differences in both water and ion flux were observed.