Nanofiltration for trace organic contaminant removal: Structure, solution, and membrane fouling effects on the rejection of perfluorochemicals

The use of nanofiltration (NF) membranes for water recycling requires an improved understanding of the factors that govern rejection of potentially harmful organic trace contaminants. Rejections of 15 perfluorochemicals (PFCs)-5 perfluorinated sulfonates, 9 perfluorinated carboxylates, and perfluorooctane sulfonamide (FOSA)-by four nanofiltration membranes (NF270, NF200, DK, and DL) were measured. Rejections for anionic species were > 95% for MW > 300 g/mol. FOSA (MW = 499 g/mol), which is uncharged at the pH of deionized water (5.6), was rejected as little as 42% (DL membrane). Decreasing the pH to less than 3 decreases rejection by up to 35%, effectively increasing the MWCO of NF270 by > 200 g/mol, while a 2500 mg/L NaCl equivalent increase in ionic strength reduces rejections < 1%. An alginate fouling layer increases transmission, where quantifiable, by factors of 4-8. Accumulation of PFCs on membranes was measured after the completion of rejection experiments. Based on rejection kinetics and the extent of sorption, we infer that two different sorption processes are significant: charged species adsorb quickly to the membrane surface, whereas the uncharged FOSA absorbs within the membrane matrix in a much slower process.