Separation of ionic species by polymeric nanofiltration membranes in crossflow membrane filtration: Implications for arsenic removal

The separation of ionic species by polymeric nanofiltration membranes during crossflow membrane filtration is investigated. A local model for predicting membrane performance (permeate flux and ion separation) is developed which considers both the accumulation of retained ions at the membrane surface (concentration polarization) and transport of ions through porous nanofiltration membranes. Experiments are performed to delineate the membrane properties of two selected nanofiltration membranes and to examine the separation of ionic species by the membranes. Experimental results are compared with model predictions to test the validity of the coupled model. The removal of arsenic by reverse osmosis and nanofiltration membranes is studied as an example of the application of crossflow membrane filtration for separation of ionic species from water.; Model predictions are performed for a negatively charged (anionic) membrane and an uncharged (neutral) membrane. The effects of membrane properties, membrane channel properties, and test conditions on separation of single salt and multiple salt solutions are examined. Performance predictions indicate that both Donnan and steric exclusion effects play a role in the separation of ionic species by porous nanofiltration membranes. Experiments with the NF-45 and BQ01 nanofiltration membranes indicate that the membranes show separation behavior which is generally consistent with model predictions for an anionic membrane and a neutrally charged membrane, respectively.; Greater than 90 percent removal of both As(V) and As(III) species by reverse osmosis and tight nanofiltration membranes was observed for the operating conditions and test solution compositions investigated. The high separation of arsenic species was attributed to their relatively large molecular weights. On the other hand, charged As(V) species were rejected by porous nanofiltration membranes at a much higher rate compared to uncharged As(III) species. In addition, separation of charged As(V) species was strongly dependent on test solution pH while separation of uncharged As(III) species was unaffected by changes in test solution pH. Separation of arsenic species by porous nanofiltration membranes was governed by both Donnan and steric exclusion effects.