Ion exchange resin – Bipolar membrane electrodialysis hybrid process for reverse osmosis permeate remineralization: Cation exchange resins equilibria and kinetics

Reverse osmosis (RO) membrane technology is widely used for producing high-quality drinking water. Yet RO permeate is by itself acidic (pH = 5.5 to 6.0), unbuffered and has low mineral content, therefore post treatment i.e., remineralization is mostly required. An ion exchange resin – bipolar membrane electrodialysis hybrid process was developed for sustainable RO permeate remineralization. Fundamental phenomena in the recovery of calcium and magnesium by ion exchange to remineralize reverse osmosis permeate were investigated. Sorption equilibrium and mass transfer kinetics were investigated for weakly acidic (Amberlite IRC747, Amberlite IRC748, Lewatit S8227) and strongly acidic (DOWEX Marathon MSC) cation exchange resins. Most suitable resin for the remineralization process should have high selectivity for calcium and magnesium and low selectivity for monovalent ions to avoid adding undesired ions to the remineralised water downstream as well as relatively fast mass transfer kinetics. The isotherms were correlated with the stoichiometric ion exchange isotherm and the Langmuir-Freundlich (Sips) isotherm. All resins showed high selectivity for ions with higher valence, but weakly acidic cation exchange (WAC) resins showed significantly lower selectivity towards monovalent ions than the strongly acidic cation exchange resin. The influence of each resin functional group, charge density and degree of protonation was shown to have a major effect on the resin selectivity. Amberlite IRC748 had the lowest selectivity (KNH4+/Na+ = 0.77
0.19) and removal (46%) for ammonium in a single-component system. The mass transfer rate was found to be controlled by intraparticle diffusion rather than film diffusion. Amberlite IRC748 is recommended for use in a remineralization process where divalent ions are present because of its favourable sorption and higher mass transfer kinetics (Ks = 8.65
0.58 × 10−12, 7.95
0.38 × 10−12 m2/s for calcium and magnesium, respectively).