Transition metal ion exchange in zeolites. Part 1.—Thermodynamics of exchange of hydrated Mn2+, Co2+, Ni2+, Cu2+ and Zn2+ ions in ammonium mordenite

Abstract

Ion exchange of Mn²⁺, Co²⁺, Ni²⁺, Cu²⁺ and Zn²⁺ in ammonium mordenite has been studied in aqueous solution over a pH range of 4–7 at 25°C. Values of the thermodynamic equilibrium constant K_a and the standard free energy of exchange ΔG^⊖ were calculated for reversible systems. In no case at 25°C did the degree of exchange of metal ions exceed 50%, and pH and anion were found to have no significant effect on the equilibria. A thermodynamic affinity sequence of Mn²⁺ > Cu²⁺ > Co²⁺∼ Zn²⁺ > Ni²⁺ was established and thermodynamic affinities of the zeolite for pairs of metal ions were calculated using the triangle rule. Selectivity quotients indicated that mordenite would preferentially remove manganese(II) or copper(II) from a binary solution mixture of one of these ions with cobalt(II), nickel(II) or zinc(II).