Changing permselectivity between halogen ions through anion exchange membranes in electrodialysis by controlling hydrophilicity of the membranes

Abstract

To change the permselectivity between halogen ions through anion exchange membranes in electrodialysis, the anion exchange membranes were modified: impregnation of compounds which have ether bonds into the membrane and formation of a cationic polysoap layer on the membrane surface, led to control of the hydrophilicity of the membrane. Evaluation was performed by measurement of the transport numbers of fluoride, bromide and iodide ions relative to chloride ions. The permeation of the less hydrated bromide anions decreased and that of the strongly hydrated fluoride anions increased relative to chloride ions by impregnating compounds containing ether bonds into the membranes, which was a consequence of the increase in the hydrophilicity of the membranes. Notably, permeation of chloride ions through the membrane exceeded that of bromide ions in the presence of ethylene glycols, which is the opposite behaviour of those of conventional anion exchange membranes. On the contrary, to examine the effect of decreasing the hydrophilicity on the permselectivity between halogen ions, a hydrophobic layer [poly(N-dodecyl 4-vinylpyridinium bromide)] was formed on the desalting side of the membrane surface. Although fluoride ions permeated only with difficulty through the membrane owing to formation of the layer, permeation of bromide and iodide ions was enhanced by the layer. In particular, the transport number of iodide ions relative to chloride ions reached 35.5 (without the layer 4.8). Controlling the hydrophilicity of the anion exchange membranes is an effective method to change the permeation behavior of halogen ions through anion exchange membranes.