Permeation selectivity in the electro-dialysis of mono- and divalent cations using supported liquid membranes

Abstract

We investigated in detail the permeation selectivity in the electro-dialysis of Na⁺, K⁺, Mg²⁺ and Ca²⁺ in both binary and quaternary mixtures using a supported liquid membrane (SLM). The SLM consisted of the organic liquid 2-nitrophenyl octyl ether (NPOE) containing a lipophilic anion, i.e. tetrakis[3,5-bis(trifluoromethyl)phenyl]borate, as the cation-exchanging site, which was used to fill the pores of the supporting membrane Accurel^R. We first determined the electro-phoretic mobilities of the migrating cations in single salt solutions, yielding: Na⁺ > K⁺ > Mg²⁺ > Ca²⁺. This order reflects the different size of the migrating cations. The monovalent cations Na⁺ and K⁺ migrate in the dehydrated state and the divalent cations Ca²⁺ and Mg²⁺ migrate in a (partly) hydrated state, a conclusion was supported by Karl Fisher titrations. Both binary and quaternary salt experiments showed a permeation selectivity in the following order: K⁺ > Na⁺ > Ca²⁺ > Mg²⁺. Since this order does not correlate with the order of electro-phoretic mobilities, we have determined the ion-exchange selectivity constant (K_ex) and found: K⁺ > Ca²⁺ > Mg²⁺ ≈ Na⁺. We conclude that the overall permeation selectivity is determined by the combination of ion-exchange selectivity and electro-phoretic mobility of the cations present in the membrane.