About five RO membranes (e.g., LE-440, XLE-440) and about two NF membrane (e.g., NF-90), provided by several companies, were and will be characterized according to pure water permeability (PWP), molecular weight cutoff (MWCO), hydrophobicity (contact angle), and surface charge (zeta potential). In bench-scale experiments, solute rejections are determined with pure water (Milli-Q) as well as synthetic feed-waters with adjusted pH and ionic strength.
Stirred cell tests are being performed as dynamic adsorption tests, with results compared against static (isotherm) adsorption tests to describe solute partitioning into the membrane. The experimental approach of bench-scale cross-flow tests with flat-sheet specimens involves determining rejections from synthetic waters over a range of Jo/k ratios and/or recoveries. It is noteworthy that rejections of compounds of intermediate hydrophobicity by candidate membranes were observed to be less than salt rejections reported for these membranes, suggesting that transport of these solutes through these membranes is facilitated by solute-membrane interactions. Diffusion cell measurements are being performed using actual membrane specime diffusion coefficients that, when compared to solute diffusion coefficients in water, describe hindered or facilitated solute transport through a membrane.
Data derived from cross-flow and diffusion cell tests will be used as a basis in formulating a solute transport model, delineating transport by convection versus diffusion. In addition, the role of hydrogen bonding and the influence of membrane fouling are being further explored. To date, we have observed greater rejection of (negatively) charged compounds than neutral compounds, and greater rejection of non-polar than polar compounds.