Multi‐element stable isotopic dilution and multi‐surface modelling to assess the speciation and reactivity of cadmium and copper in soil

Chemical extraction, multi‐element stable isotopic dilution (ID) and multi‐surface modelling were used to investigate the lability of cadmium (Cd) and copper (Cu) in nine types of soil with different properties and contaminated or not with Cd and Cu. The chemical extraction and ID analyses both showed that Cd was more labile than Cu in all the soil types studied. From the ID results, 32.8–93.3% of total Cd and 14.7–71.8% of total Cu were isotopically exchangeable after 3 days of equilibration. A single extraction in 0.43 m HNO₃ gave similar results to the 3‐day ID assay for Cu in most of the soils and for Cd in the non‐calcareous soils. However, an eight‐step selective sequential extraction (SSE) procedure gave different results from the ID assay for both metals. Predictions of the multi‐surface model for the amounts of Cd and Cu adsorbed, based on measured metal ion activities in the soil solution and the concentrations of reactive surfaces in the soil, agreed with the ID results. The model predicted that soil organic matter was the predominant sorbent for Cd and Cu in the soils and that manganese oxide was the least important sorbent. The contributions of iron oxides to sorption were predicted to be small except in soil with a high pH and little organic matter. The predicted sorption on different soil components did not match SSE measurements.