The role of pH on Cr(VI) partitioning and isotopic fractionation during its incorporation in calcite

The Cr(VI) incorporation and chromium stable isotope composition in calcite has been studied in experiments performed in the pH range between 8.0 and 10.6 at constant temperature (T = 25 ± 1 °C), precipitation rate (rp = 10−7.7±0.2 mol m−2 s−1) and total aqueous Cr(VI) concentration (Cr(VI) = 49.6 ± 1.3 mM). The obtained results indicate that Cr(VI) incorporation in calcite is pH-dependent and it is likely significantly affected by the formation of the aqueous CaCrO40 species. The experimental findings suggest that during calcite growth at pH < 9.4 Cr(VI) uptake in the solid phase is likely controlled by the initial adsorption and subsequent incorporation of CaCrO40 complexes, whereas this process is balanced by the additional uptake of aqueous CrO42− species when calcite forms at pH ≥ 9.4. This distinct mechanism of Cr(VI) incorporation into calcite is further confirmed by the Cr(VI) isotope fractionation between calcite and the precipitating solution. Owing to the longer CrO bond lengths in aqueous CaCrO40 compared to CrO42− species the lighter 52Cr(VI) isotope is preferentially abundant in the aqueous CaCrO40. The preferential uptake of the isotopically lighter CaCrO40 in the growing calcite results in Cr(VI) isotope fractionation, Δ53Crcalcite-solution = δ53Cr calcite – δ53Crsolution, as low as −0.7‰ at pH 8. In contrast, at pH > 9.4 the smaller contributions of CaCrO40 in the total concentration of Cr(VI) in calcite yields in a Δ53Crcalcite-solution value close to 0‰.