Boron removal and zeta potential of RO membranes: impact of pH and salinity

Boron removal is a challenging task for any seawater desalination plant employing a single-pass reverse osmosis (RO) system. With a pK(a) value of 9.25, more than 99% boron removal is normally achieved at pH >10.5. As pK(a) value is theoretically lower at higher salinity, an increase of borate ion and a corresponding increase in boron removal could be expected as salinity increases. However, boron removals by CPA2 and SWC4+ membranes were found to decrease or, sometimes, unchanged at higher salinity where membrane zeta potentials shifted from negative values to positive values were observed at higher salinity at both pH 7 and 9. The decrease in boron removal by CPA2 membrane with increasing salinity at pH 9 could be attributed to the reduced charge repulsion mechanism. Similarly, boron removal by SWC4+ membrane decreased with increasing salinity. However, the removal efficiency reached its lowest value at 2000 mg/l of NaCl at pH 9. The subsequent increase in boron removal efficiency at salinity higher than 2000 mg/l could be attributed to size exclusion mechanism. At pH 7, the trend of boron removal by CPA2 membrane was similar to that observed at pH 9. However, the corresponding trend of SWC4+ membrane at pH 7 was different from that observed at pH 9. With a negligible amount of borate at pH 7, reduced boron removal by CPA2 membrane observed at higher salinity might be attributed to enhanced diffusion. In contrast, the relatively stable boron removal at pH 7 by SWC4+ membrane observed at all salinities suggested that size exclusion could be the dominant mechanism.