An XAS study of zinc speciation in aqueous acetate solutions at 25-200 °C

Solutions of zinc(II) acetate in acetic acid/sodium acetate buffer solutions of varying concentration were examined by X-ray absorption spectroscopy (XAS) at temperatures between 25 and 200 °C. The influence on speciation by increasing the ratio of available acetate to Zn concentration was marked, and followed a similar trend to that observed with increasing temperature. End-member X-ray absorption spectra were observed at both the low and high temperature/acetate concentration regimes. Fitting of the EXAFS spectra gave Zn-O bonding distances in the range 1.95-2.05 Å, which are comparable to available crystallographic data. Thermodynamic modelling of the XANES data shows ZnAc+ is important at lower acetate concentration, while ZnAc2(aq) predominates solutions with acetate concentrations above 1.5 m at 25 °C, and the evolution of the XANES spectra shows that ZnAc2(aq) becomes more important with increasing temperature. Therefore acetate has been shown to be a valid Zn transport ligand under hydrothermal conditions.