Theoretical isotope fractionation of cadmium during complexation with organic ligands

Abstract Cadmium (Cd) isotopes are an important tool to better understand both inorganic and organic geochemistry of Cd, and organic ligands play a key role to control the toxicity and mobility of Cd in living organisms and also in terrestrial and aquatic environments. Knowledge of the equilibrium isotope fractionation of Cd with organic ligands is crucial to further advance Cd isotope source and process tracing in the field of biogeochemistry. In this study, we calculated reduced partition function ratios (103lnβ) of Cd isotopes in various organic Cd complexes by density functional theory. The calculation results show that the 103lnβ of 114Cd/110Cd for these complexes are decreased in the order of Cd(Hcit)(H2cit)− > Cd(cit)(H2O)3− > CdH(cit)(H2O)4 > CdEDTA > Cd(his)2H2O > Cd(cit)24− ≈ Cd(H2O)62+ > Cd(H2O)42+ > Cd(cys)(H2O)32+ > Cd(GS)2(H2O)22− > Cd(DMPS)(H2O)2− > Cd(DMPS)24− at 0–100 °C, and heavy Cd isotopes preferably bind to oxygen and nitrogen donor atoms while light Cd isotopes bind to sulfur donor atoms of organic ligands. Thus, the previously observed immobilization of light Cd isotopes in living organisms could be related to Cd detoxification processes with sulfur. The predicted equilibrium isotope fractionation will strengthen Cd isotopes as a process tracing tool in these systems and will improve the understanding of Cd isotope cycling in aquatic and terrestrial systems.