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.