Membrane filters for the concentration of trace elements in water: distribution of ion pairs between membrane filter and aqueous phases

Abstract

The sorption of ion pairs on membrane filters (MFs) has been studied by taking the membrane filter as one of the homogeneous phases. The sorbability of some ionic species and the sorption abilities of different types of MF were evaluated in terms of the sorption constant defined by K_sor,CA=[(C⁺,A^–)_f]/[C⁺][A^–], where C⁺, A^– and (C⁺,A^–) refer to the cation, anion and the ion pair, respectively, and f refers to the filter phase. The values of K_sor,CA were determined for many combinations of ionic complexes of cobalt(III) with pyridylazophenols, either as cations or anions, and oppositely charged organic ions having different alkyl chain lengths, with MFs made of different materials: nitrocellulose (NC; Toyo Advantec), acetylcellulose (AC; Fuji Film), regenerated cellulose (RC; Toyo Advantec) and polyethersulphone (PS; Toyo Advantec). For a given cobalt complex ion, the value of log K_sor,CA increased linearly with the increasing number of carbons in the counter ion. Membrane filters made of different materials showed different sorption abilities, the order being NC > PS > RC > AC. It was shown that the surface area of the MF is of greater significance than the volume of the matrix of the MF in determining the sorption constant.