Modification of Polyamide Membranes by Selected Molecules for Improved Boron Rejection

Shiran Shultz, Chemical Engineering, Technion-Israel Institute of Technology, Haifa, Israel
Raphael Semiat, Chemical Engineering, Technion-israel Institute Of Technology, Haifa, Israel
Viatcheslav Freger, Chemical Engineering, Technion-israel Institute Of Technology, Haifa, Israel

Commercial polyamide RO membranes, though effective in terms of flux and salt removal, have a few drawbacks, in particular, poor rejection of boron (B) and thus are often unable to remove B to the required level in a one-pass sea desalination process. This complicates the process and available solutions increase the cost of desalinated water by 15-20%. Here we explore in-situ modification procedure that can significantly increase their boron rejection of polyamide RO membranes. Based on a hypothesis that increased hydrophobicity of the selective polyamide layer may help disrupt water-boric acid association and decouple water permeation from boron permeation, the proposed modification incorporates suitable modifying molecules, Such molecules have to combine a hydrophobic moiety and a reactive group that can chemically or physically bind to polyamide layer and thus tighten its structure and increase selectivity, a combination met by aliphatic amines. The results show that the proposed treatment with sufficiently long (up to 12 carbons) aliphatic amines may indeed reduce the boron passage by a factor of 2 to 4, without impairing the salt rejection of the membrane. The selectivity improvement comes at the expense of some flux reduction, but the flux-selectivity tradeoff improves compared to commercial polyamide membranes.