Removal of perchlorate using reverse osmosis and nanofiltration membranes

Rejection characteristics of perchlorate (ClO_4-) were examined for commercially available reverse osmosis (RO) and nanofiltration (NF) membranes. A bench-scale dead-end stirred-cell filtration system was employed to determine the toxic ion rejection and the membrane flux. Model water solutions were used to prepare ClO_4- solutions (approximately, 1,000 μg/L) in the presence of background salts (NaCl, Na₂SO₄, and CaCl₂) at various pH values (3.5, 7, and 9.5) and solution ionic strengths (0.001, 0.01, and 0.01 M NaCl) in the presence of natural organic matter (NOM). Rejection by the membranes increased with increasing solution pH owing to increasingly negative membrane charge. In addition, the rejection of the target ion by the membranes increased with increasing solution ionic strength. The rejection of ClO_4- was consistently higher for the RO membrane than for the NF membrane and ClO_4- rejection followed the order CaCl₂ < NaCl < Na₂SO₄ at conditions of constant pH and ionic strength for both the RO and NF membranes. The possible influence of NOM on ClO_4- rejection by the membranes was also explored.