Cadmium isotope fractionation during adsorption–desorption on clay minerals and soil

Purpose Cadmium (Cd) isotope fractionation related to various components in soils provides useful information on Cd biogeochemical behaviour in the natural environment. Existing studies focus solely on Cd isotope fractionation during adsorption on environmentally relevant surfaces (e.g., clay minerals) but poorly document Cd isotopic variation during desorption, particularly in the case of partial desorption. Studying Cd isotopic behaviour during adsorption–desorption on clay minerals and soil benefits Cd fate tracing. Materials and methods Here, Cd isotope fractionation during adsorption–desorption was investigated on kaolinite and montmorillonite at different solution pH and initial Cd concentrations, and in an albic soil rich in clay minerals but low in organic matter and iron-manganese oxides. In addition, the clay particle fraction (< 2 μm) of the albic soil was also separated and used for adsorption–desorption experiment and isotopic analysis. Results and discussion Solution pH and Cd concentration significantly affected Cd adsorption at different sites and Cd desorption rate on the surfaces of the two clay minerals but led to negligible Cd isotope fractionation during adsorption and desorption. This strongly suggests similar Cd isotope fractionations induced by the two different adsorption sites on the clay minerals. Negligible isotope fractionation was also observed during adsorption or desorption in the albic soil and its clay particle fraction (< 2 μm; Δ 114/110 Cd adsorption  = − 0.07 to − 0.06‰ and Δ 114/110 Cd desorption  = − 0.08 to − 0.05‰). Cadmium adsorption on montmorillonite which dominated Cd adsorption on the soil studied and the clay particle fraction is suggested as the explanation for the negligible Cd isotope fractionation during soil adsorption and desorption. Conclusions This work provides preliminary data on Cd isotope fractionation during adsorption and desorption in actual soils with multiple reactive surfaces and indicates that the contribution of kaolinite and montmorillonite to Cd isotope fractionations during adsorption/desorption in soils may be neglected, and in some clay-rich soils ion exchange may not lead to significant Cd isotope fractionation. Graphical abstract