Arsenic removal using oxidative media and nanofiltration

Abstract

Nanofiltration (NF) is a promising drinking water treatment technology for arsenic removal; however, most of the research on NF treatment of arsenic has used synthetic water. In this investigation, a pilot membrane system treated groundwater naturally contaminated with arsenic to test the performance of two NF membranes and one reverse osmosis (RO) membrane, both with and without oxidizing pretreatment using manganese dioxide (MnO2). The arsenic concentration in the groundwater was ~ 40 μg/L, mostly present as arsenite, a neutral species. Without the oxidizing pretreatment, the two NF membranes provided almost no removal whereas the RO membrane provided ~ 25 to 50% arsenic removal, depending on operating conditions. Following pretreatment with MnO2, the treated arsenic concentration dropped to < 4 μg/L (90% removal) for all three membranes. The substantially improved performance for these negatively charged membranes was attributed to the oxidation of the neutrally charged arsenite to negatively charged arsenate. These results indicate that where arsenite is present, facilities with RO or NF processes can dramatically enhance their arsenic removal by adding a membrane‐compatible oxidation step such as MnO2 filtration.