Cadmium isotope fractionation during Cd-calcite coprecipitation: Insight from batch experiment

Cadmium (Cd) calcite coprecipitation experiments were conducted to constrain the possible Cd isotope fractionation mechanism under different conditions. The changes in Cd/Ca molar ratio, HCO₃^- concentration, addition of Mg²⁺, Na₂EDTA, or ionic strength (I_NaNO3) did not affect the composition of the precipitation but resulted in substantial change in mineral morphology. Cd exhibited an apparent isotope fractionation in the process of coprecipitation with calcite, and the fractional coefficient α_CaCO3-Cd(aq) was less than one, indicating that the solution is preferentially enriched in heavy isotopes. The Cd mainly existed in the form of Cd(H₂O)₆²⁺ in the solution before the reaction and was dominated by Cd(NO₃)₂, CdNO₃⁺, Cd(EDTA)^2- at the end of the experiments. The different isotopic ratios of Cd between the liquid phase and the solid phase can be explained by changes in Cd speciation which have different bond lengths of CdO. Expect for the experiments with Na₂EDTA and high ionic strength, the measured isotope ratios under different conditions plotted around the theoretical Rayleigh fractionation curve. This demonstrates that Cd isotopes almost reach equilibrium with Cd-calcite coprecipitation under the different experiment conditions and the Cd isotope fractionation mechanism is closely related to the thermodynamic isotope fractionation. In the experiment with Na₂EDTA and high ionic strength, the formation of Cd(EDTA)^2- and ion occlusion at surface sites inhibit Cd²⁺ entering the calcite and the isotopes of the solid and liquid phases do not reach equilibrium. As the result, obtained experiment data shift from the Rayleigh fractionation curve indicating the Cd isotope fractionation mechanism is closely related to the kinetic isotope effect. In general, the study results indicate that the Cd/Ca molar ratio, ionic strength, HCO₃^-, Na₂EDTA, and Mg²⁺ concentrations of the solution can cause Cd isotope fractionation in different degrees during Cd-calcite coprecipitation. Cd isotope ratios might potentially provide important information on Cd mobilization and transportation in groundwater environment.