Enhancing boron rejection on electrically conducting reverse osmosis membranes through local electrochemical pH modification

Abstract

Abstract Reverse-osmosis (RO) has become a prevailing technology for seawater desalination. While RO removes the majority of ions in seawater, the removal of small, uncharged contaminants remains challenging. Boron existing as uncharged boric acid (B (OH)3) in seawater is partially removed during the RO process, requiring further treatment if the desalinated seawater will be used for certain crop irrigation. Boron removal efficiency is significantly improved under a high pH condition where non-charged B(OH)3 turns into negatively charged borate (B(OH)4−). Therefore, RO desalination plants often exploit a double-pass configuration, where the pH of the RO permeate from the first pass is chemically increased above the pKa of B(OH)3, and is treated again using RO membrane (the second pass). Although this process can achieve high boron removal efficiency, it incurs substantial operating and capital costs. In this study, commercial RO membranes are coated with carbon nanotubes forming an electrically conducting, porous layer. We explore the impact of applied cathodic potentials on boron and salt rejection, membrane flux, and fouling. We demonstrate that applying cathodic potentials can elevate the near-membrane pH, which dramatically increases boron rejection. However, the higher pH results in membrane scaling (Mg(OH)2), although the formed scaling does not dramatically reduce membrane flux.