Binding and adsorption energy of Cd in soils and its environmental implication for Cd bioavailability

The interaction of Cd and other heavy metals with soil colloidal particles controls the sequestration, mobility and bioavailability of Cd in soils. In this study, the binding (oG(bi)) and adsorption (oG(ad)) energies of Cd on colloidal particles of 18 soils were determined by the Wien effect method. The binding energy of Cd on soil colloidal particles varied from 5.3 to 9.9 kJ mol(-1), depending on the soil characteristics including pH, Mn-oxide content and dissolved organic carbon in the soil. The Cd adsorption energy correlated positively with Mn-oxide content and pH. In parallel, the extended X-ray absorption fine structure (EXAFS) spectroscopy was used to determine the speciation of Cd in Cd-saturated soil samples, which revealed that the outer-sphere Cd was the predominant species, accounting for 32.2-73.7% of the total adsorbed Cd, and positively correlated to the binding and adsorption energies. Humic acid-Cd (10.4-42.2%) and montmorillonite-Cd (2.5-51.2%) were also major species that were identified by EXAFS spectroscopy. The toxicity (log EC50) of Cd in soils to three organisms (earthworm, Collembola and Chinese cabbage) was found to correlate positively with the binding energies, indicating the predictive capability of using binding energies of Cd in different soils as an indicator for evaluating Cd bioavailability and toxicity in soils.