Recovery of phosphate using multilayer polyelectrolyte nanofiltration membranes

Nanofiltration (NF) is an attractive technique for selective recovery of phosphate from waste waters. This work examines the potential of NF membranes prepared by alternating layer-by-layer deposition of polycations and polyanions on porous substrates for selective recovery of phosphate from chloride-containing solutions. These membranes are attractive because they contain a high surface charge, and the minimal thickness of the polyelectrolyte skins allows a high flux. In the best case we examined, at pH 8.4 poly(styrene sulfonate) (PSS)/poly(diallyldimethylammonium chloride) (PDADMAC) films deposited on a porous alumina support showed a 98% rejection of phosphate, a chloride/phosphate selectivity of 48, and a solution flux of 2.4 m3/m2-day at 4.8 bar. By way of comparison, commercial NF 90 membranes exhibited low chloride/phosphate selectivity with more than 97% rejection for both chloride and phosphate at a solution flux of only 1.2 m3/m2-day. The rejection of phosphate depends on the pH of the feed solution, with rejection decreasing at lower pH where a large fraction of phosphate is H2PO4−. Even at pH 5.6, however, phosphate rejection was 86% and chloride/phosphate selectivity was 6 when using (PSS/PDADMAC)4PSS-coated membranes. The selectivity of PSS/PDADMAC membranes varies significantly with the number of deposited layers, confirming that NF properties are very sensitive to film structure.