Pressure-driven membrane processes for boron and arsenic removal: pH and synergistic effects

This study analyzed novel synergistic effects on rejection of arsenic and boron during membrane-based treatment. The effect of pH on rejection was characterized for pure and mixed arsenate, arsenite and borate (As(V), As (III), and B respectively) solutions. The favorable effect of high pH was strongest for B and As(III) due to the similar charge evolution in the pH 7-11 range. While only significant for arsenate, both arsenic species exhibited higher rejections when boron was present. Analysis of watersheds in southern Peru revealed a correlation between arsenic and boron; synthetic river water containing 10 mg/L B (0.97 mmol/L) and 1 mg/L As (0.013 mmol/L) was subsequently tested. Both reverse osmosis (BW30) and nanofiltration membranes (NF90) produced a system permeate that complied with the WHO standards (1 mg/L B and 0.01 mg/L As) above pH 9 in these representative, contaminated waters. The combination of increasing the pH from 7 to 9.5 and the synergistic effect doubled the allowable concentration of As(V) in the feed for BW30 from 0.5 mg/L to 1.0 mg/L. In addition to informing treatment of surface and ground waters, findings could be applied to the reuse of high pH gold cyanidation wastewaters and enhancing boron rejection during seawater desalination.