Ion exchange equilibria of the gold cyanide complex in aqueous and mixed solvent environments

Abstract

Ion exchange equilibria are presented for [ ]− − Au(CN)2 / Cl , [ ]− − Au(CN)2 / SCN and SCN− / Cl− in aqueous solution, and in various mixed solvents, at 303K using Purolite A500 as the ion-exchanger. The mixed solvents investigated include water-acetone, water-dimethylsulfoxide (DMSO) and water-N-methyl-2-pyrrolidone (NMP). In aqueous solution, the selectivity of Purolite A500 for a given anion increases in the order: [ ]− − < − < Cl SCN Au(CN)2 . This selectivity sequence confirms the high affinity of the ion exchange resin for the [ ]− Au(CN)2 species. In mixed solvents, however, the selectivity of Purolite A500 for [ ]− Au(CN)2 decreases with an increase in the composition of the organic solvent in the external solution. Mixed solvents containing greater than 60 mol% organic solvent are preferred for the displacement of [ ]− Au(CN)2 from the resin. The effectiveness of a given type of mixed solvent generally increases in the following order: DMSO &it acetone &it NMP. The ion exchange equilibria are correlated using the Law of Mass Action, modified with activity coefficients, to determine the equilibrium constant for each binary system. The fitted values of the equilibrium constants are consistent with the trends observed in the ion exchange isotherms. The accuracy of the correlation results in the mixed solvent systems range from 1 to 10% and this is similar to the level of accuracy obtained for the ion exchange equilibria in aqueous solution. From these results it can be concluded that the Law of Mass Action is equally valid in mixed solvent systems. The variation in the equilibrium constant with mixed solvent composition, for a given binary system, correlates well with the dielectric constant of the mixed solvent. For a given value of the dielectric constant, however, the equilibrium constant, however, the equilibrium constant is dependent on the type of mixed solvent. A fundamental relationship is derived between the equilibrium constants and the Gibbs energies of transfer associated with the solvation of the ions in the mixed solvents. Based on this relationship, the redistribution of ions between the pore solution and the bulk mixed solvent, appears to be the most significant factor that governs the selectivity of the resin in mixed solvent systems.