Implication of zeta potential at different salinities on boron removal by RO membranes

Abstract

Boron removal by reverse osmosis (RO) membranes has been improved through raised pH and its removal mechanism was suggested as either charge repulsion or size exclusion. In the present study, boron removal by different RO membranes at different salinities was investigated along with their respective zeta potentials. While the impact of salinity on zeta potential of RO membranes was similar, its impact on boron removal by brackish water reverse osmosis (BWRO) membranes was different from that by SWC4+ and ESPAB membranes. RO membranes used in this study showed negative zeta potential value at high pH. However, at pH 9 and with higher salinity, their zeta potentials shifted to positive values. Boron removal by BWRO membranes decreased with increasing salinity at pH 9. The shift of zeta potential towards positive values at higher salinity suggested that charge repulsion mechanism became less dominant. Boron removal by ESPAB and SWC4+ decreased initially when NaCl concentration was increased towards 2000 mg/L at pH 9. However, removal increased slowly when NaCl was beyond 2000 mg/L. This observation suggested that boron removal by these membranes at low salinity was partially contributed by charge repulsion mechanism. At higher salinity, size exclusion could be the dominant factor for boron removal by SWC4 and ESPAB.