Sulfur-Driven Cadmium Isotope Fractionation by Low-Molecular Weight Organic Ligands: Experimental Evidence from the Donnan Membrane Technique

Organic ligands have been suggested to play a key role in controlling root-to-shoot transport and isotope fractionation of cadmium (Cd) in crops. In this work, we used the Donnan membrane technique to determine the Cd equilibrium isotope fractionation that is induced by the chelation of Cd with low-molecular weight organic ligands. While a model ligand with oxygen and nitrogen functional groups (EDTA, R-OH, R-NH) induced no significant isotope fractionation between complexed and free Cd2+ (Δ114/110CdCdEDTA – freeCd = −0.08 ± 0.12‰; n = 4), the model ligand with sulfur donor groups (DMPS, R-SH) induced a strong isotope fractionation (Δ114/110CdCdDMPS – freeCd = −0.66 ± 0.20‰; n = 4). These experimental results indicate that binding of Cd to thiols plays a major role in retaining light Cd isotopes in roots and shoots, thereby minimizing transfer to cereal grains.