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 (r_p = 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 CaCrO₄ ⁰ 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 CaCrO₄ ⁰ complexes, whereas this process is balanced by the additional uptake of aqueous CrO₄ ²⁻ 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 Cr[sbnd]O bond lengths in aqueous CaCrO₄ ⁰ compared to CrO₄ ²⁻ species the lighter ⁵²Cr(VI) isotope is preferentially abundant in the aqueous CaCrO₄ ⁰. The preferential uptake of the isotopically lighter CaCrO₄ ⁰ in the growing calcite results in Cr(VI) isotope fractionation, Δ⁵³Cr_{calcite-solution} = δ⁵³Cr _calcite – δ⁵³Cr_solution, as low as −0.7‰ at pH 8. In contrast, at pH > 9.4 the smaller contributions of CaCrO₄ ⁰ in the total concentration of Cr(VI) in calcite yields in a Δ⁵³Cr_{calcite-solution} value close to 0‰. Our results imply that the chromium isotope tracer system applied to calcite, as an environmental proxy for the reconstruction of ocean redox conditions, is not solely a mirror of redox effects in the aqueous fluid from which the carbonates precipitate, but additionally is controlled by the pH of the forming fluid and consequently by the relative stability and the distribution of aquo-complexes. Speciation calculations for calcite precipitated from seawater and include the presence of CaCrO⁰ complex, predict isotope fractionation values that lay within −0.67‰ < Δ⁵³Cr_{calcite-solution} < −0.43‰, and come in excellent agreement with the experimental results of this study at similar pH conditions. This theoretical model predicts that calcite formation under pH conditions below 8.5 results in depletion of ⁵³Cr(VI) in the growing calcite crystal. In contrast, the Cr(VI) isotopic composition of precipitating calcite in alkaline solutions can be reasonably ascribed to directly depict the Cr(VI) isotopic signature of the aqueous solution from which calcite forms, at least within the range of the experimental conditions.