Experimental Investigation of Cadmium Isotope Fractionation during Adsorption on Montmorillonite and Kaolinite

Cadmium (Cd) isotopes have recently emerged as novel tracers of Cd sources and geochemical processes. Widespread clay minerals play a key role in Cd migration due to their strong adsorption capacity, but the mechanism of Cd isotope fractionation during adsorption onto clay minerals is poorly understood. Here, we experimentally investigated the adsorption mechanisms of Cd on montmorillonite (2:1) and kaolinite (1:1) by using extended X-ray absorption fine structure (EXAFS) spectroscopy. Our results show that lighter Cd isotopes are preferentially adsorbed on clay minerals, with Δ^114/110Cd_{dissolved-adsorbed} values of 0.03 ± 0.02‰ for montmorillonite and 0.03 ± 0.03‰ for kaolinite. These values are independent of the pH, initial Cd concentration, and ionic strength. EXAFS indicate that the adsorbed Cd forms octahedral outer-sphere surface complexes with an average Cd-O bond length of 2.25-2.26 Å, identical to Cd-O bonding environment in the Cd(NO₃)₂ solution. Our results demonstrate that the extent of Cd isotope fractionation is constrained by structural changes (including the bond length and coordination number) in surface complexes. These findings suggest that Cd adsorption on clay minerals has a minimal impact on the variability of Cd isotopic compositions in soils and sediments, implying that variations in Cd isotope ratios in soils and sediments primarily reflect changes in Cd sources.