Coupling stable isotope analyses and X-ray absorption spectroscopy to investigate the molecular mechanism of zinc sorption by calcite

Calcite plays a pivotal role in regulating the mobility and fate of zinc (Zn) in natural environments. Despite its significance, the mechanism of Zn sorption on calcite surfaces, particularly the transitional dynamics from surface adsorption to precipitation, remains unclear. This research studied the sorption behavior of Zn on calcite across a wide range of reaction times, pH values, and Zn concentrations. The underlying sorption mechanisms were examined through a combination of Zn stable isotope measurements and X-ray absorption fine structure (XAFS) spectroscopy. At pH 6.5 and a low Zn concentration of 5 μM, the surface coverage reached 0.9 μmol/m2, accompanied by a pronounced Δ66Znsorbed-aqueous of + 0.40 ‰, which is indicative of a tetrahedral inner-sphere surface complexation mechanism. Conversely, at pH ≥ 7.5 and a higher Zn concentration (100 μM), the surface coverage surpassed 57.6 μmol/m2, resulting in diminished Zn isotope fractionation (+0.20 ‰), suggesting the formation of hydrozincite precipitates. These results, integrated with the XAFS analysis, revealed a continuous transition from inner-sphere tetrahedral surface complexes to hydrozincite precipitates as the pH and/or Zn concentration increased. Notably, the sensitivity of Zn isotope fractionation to distinct Zn sorption mechanisms was supported by an inverse linear relationship between Zn isotope fractionation and the Zn-O bond distance. This study significantly advances our understanding of Zn sorption mechanisms on calcite by demonstrating that the surface of calcite may have catalyzed hydrozincite precipitation when the bulk solution was undersaturated with respect to hydrozincite. The synergistic application of Zn stable isotopes and XAFS spectroscopy provides a robust framework for probing metal-mineral interactions under environmentally relevant conditions.