Enhanced boron rejection by NF/RO membranes by complexation with polyols: measurement and mechanisms

Boron rejection by nanofiltration (NF) and reverse osmosis (RO) membranes in the presence of glycerol, mannitol and sorbitol was investigated as a function of feed solution pH and boron:polyol molar ratio. In the presence of polyols, significant boron rejection improvement was obtained and the extent of the impact was directly related to the stability constant of the boron-polyol complex. Polyols could complex with boron in either the boric acid or borate anion forms; however the complexation between polyol and boric acid appeared to be incomplete. With and without the presence of polyols, boron rejection was strongly pH dependent. The increase in boron rejection due to polyol addition was higher for the NF membrane compared to the RO membrane. A boron:polyol molar ratio of 1:1 appeared to be adequate. The presence of polyols did not cause any observable membrane fouling. Results reported here suggest that the addition of polyols could allow NF membranes to be effectively used for boron removal.