Transport numbers of various anions relative to chloride ions in modified anion-exchange membranes during electrodialysis

Abstract

Modified anion-exchange membranes were prepared by the reaction of a copolymer membrane composed of chloromethylstyrene and divinylbenzene with polyethylenepolyamines such as ethylenediamine, tetraethylenepentamine and polyethyleneimine (molecular weight: 70000) and then trimethylamine. The change in transport numbers of various anions such as bromide, nitrate, sulfate and fluoride, relative to chloride ions, was evaluated by electrodialysis of the mixed solution of the respective sodium salt and sodium chloride. In general, the anion-exchange membranes, modified with polyethylenepolyamines, selectively permeated nitrate and bromide ions rather than chloride ions, compared with the anion-exchange membrane with benzyl trimethylammonium groups though the degree of change differed with the nature of the polyethylenepolyamines. On the other hand, fluoride ions and sulfate ions were difficult to permeate through the modified membranes. In particular, sulfate ions were difficult to permeate through the anion-exchange membrane which had been reacted with tetraethylenepentamine. The change in the relative transport number is mainly related to the increase or decrease in affinity of the respective anions to the modified membranes, which implies a change in the hydrophilicity of the membranes relative to the hydration of the anions.