The way to increase the monovalent ion selectivity of FujiFilm® anion-exchange membranes by cerium phosphate modification for electrodialysis desalination

Abstract

Recently, the selectivity of ion-exchange membranes and its control have been the subject of numerous fundamental and applied studies. This paper reports an enhanced monovalent anion selectivity of hybrid membrane based on the FujiFilm® Type I anion-exchange membrane and in situ synthesized cerium phosphate (CeP) with a gradient distribution over the membrane thickness. The Cl/SO4-selectivity coefficient (P(Cl/SO4)) of hybrid membranes reached 6.2. The effect of modification on the Cl/SO4 selectivity passed through a minimum as the total concentration of the external solution increased from 0.04 M to 1 M. For low concentration electrolytes, Cl/SO4-selectivity improving was attributed to partial neutralization of positive fixed charges by negative fixed charges of cerium phosphate, which causes an effective decrease of ion-exchange capacity and reduction of electrostatic selectivity factor favouring sulfate transport; for high concentration electrolytes, to the formation of narrow transport channel between the surface of the inorganic nanoparticle and the pore wall, which causes a decrease of sulfate transport due to the size sieving effect. The hybrid membranes and cerium phosphate were characterized using ATR FTIR and 31P MAS NMR spectroscopy, XRD analysis and scanning electron microscopy with EDX microanalysis.