Removal of toxic ions (chromate, arsenate, and perchlorate) using reverse osmosis, nanofiltration, and ultrafiltration membranes

摘要

Rejection characteristics of chromate, arsenate, and perchlorate were examined for one reverse osmosis (RO, LFC-1), two nanofiltration (NF, ESNA, and MX07), and one ultrafiltration (UF and GM) membranes that are commercially available. A bench-scale cross-flow flat-sheet filtration system was employed to determine the toxic ion rejection and the membrane flux. Both model and natural waters were used to prepare chromate, arsenate, and perchlorate solutions (approximately 100 μg L~(-1) for each anion) in mixtures in the presence of other salts (KC1, K_2SO_4, and CaCl_2); and at varying pH conditions (4,6, 8, and 10) and solution conductivities (30, 60, and 115 mS m~(-1)). The rejection of target ions by the membranes increases with increasing solution pH due to the increasingly negative membrane charge with synthetic model waters. Cr(VI), As(V), and ClO_4~- rejection follows the order LFC-1 (>90%) > MX07 (25-95%) ≌ ESNA (30-90%) > GM (3-47%) at all pH conditions. In contrast, the rejection of target ions by the membranes decreases with increasing solution conductivity due to the decreasingly negative membrane charge. Cr(Vl), As(V), and ClO_4~- rejection follows the order CaCl_2 < KCI ≌ K_2SO_4 at constant pH and conductivity conditions for the NF and UF membranes tested. For natural waters the LFC-1 RO membrane with a small pore size (0.34 nm) had a significantly greater rejection for those target anions (>90%) excluding NO_3~-(71-74%) than the ESNA NF membrane (11-56%) with a relatively large pore size (0.44 nm), indicating that size exclusion is at least partially responsible for the rejection. The ratio of solute radius (r_(i,s)) to effective membrane pore radius (r_p) was employed to compare ion rejection. For all of the ions, the rejection is higher than 70% when the r_(i,s)/r_p ratio is greater than 0.4 for the LFC-1 membrane, while for di-valent ions (CrO_4~(2-), SO_4~(2-), and HASSO_4~(2-)) the rejection (38-56%) is fairly proportional to the r_(i,s)/r_p ratio (0.32-0.62) for the ESNA membrane.