Cadmium isotope fractionation during adsorption onto calcite

Adsorption on mineral surfaces is a crucial process controlling (bio)geochemical cycling of Cd, and Cd isotopic behaviour during adsorption is an important but little investigated aspect of Cd isotope systematics. This work looked at the fractionation of Cd isotopes during Cd adsorption onto calcite, which is an important sink of Cd but isotopic behaviour has only previously been investigated in co-precipitation experiments. The Cd isotope composition of calcite adsorbed components was determined to be lighter and consistent with equilibrium fractionation in all experiments. And degree of fractionation observed in our experiments is not influenced by the Cd/CaCO3 ratio, pH value or Cd/PO43- ratio in the studied range. In MQ water, the observed fractionation value alpha(sorbed-aqueous) is 0.99934 but this alpha value is affected by the presence of other compounds. Notably, alpha(sorbed-aqueous) = 0.99984, a reduced but resolvably lower value fractionation, was observed in the presence of PO43- which may be an important factor to be considered when interpreting Cd isotope signatures in nature. The enrichment of lighter Cd isotopes may result from highly distorted octahedral Cd-O-6 sites of inner-sphere Cd complexes on calcite, indicated by extended X-ray absorption fine structure (EXAFS). However, near identical coordination environments of the aqueous and sorbed Cd species in the presence of PO43- likely led to the reduced Cd isotope fractionation. This work suggests adsorption of Cd onto carbonates minerals may produce significant Cd isotope fractionation between fluids and solid phase. Our observations offer an alternative process apart from co-precipitation when interpreting Cd isotope ratios in the presence of calcite in paleo-geological studies and could be helpful to better understand Cd (bio)geochemical cycling.