Study on Isotopic Fractionation during Cadmium Migration in Contaminated Soil from Industrial Sites

The migration and transformation of cadmium (Cd) after entering the soil can induce varying degrees of isotopic fractionation. Elucidating the underlying fractionation mechanisms is crucial for the accurate identification of Cd pollution sources. However, existing studies are largely limited to static adsorption experiments simulating single soil components, and there is a paucity of research on the multi-process coupling of Cd isotopic fractionation mechanisms in complex soil systems. In this study, a laboratory soil column setup was used to simulate the dynamic leaching of site soil using an acidic blank solution (control treatment) and a Cd-contaminated solution (Cd treatment). By combining analyses of Cd concentration, speciation, and isotopic composition, the migration and transformation behavior of Cd in soil and its isotopic fractionation characteristics were systematically investigated. The results showed that Cd migration in the Cd-treated soil column exhibited typical dynamic three-stage characteristics: “Complete adsorption−Breakthrough−Saturation”. During this process, the Cd isotopic composition of the leachate exhibited significant variations (Δ¹¹⁴/¹¹⁰Cd_{source-leachate} = −0.32‰ to 0.21‰), and the soil preferentially adsorbed the light Cd isotopes from the pollution source (Δ¹¹⁴/¹¹⁰Cd_{soil-leachate} = −0.04‰ ± 0.00‰), which is consistent with Rayleigh fractionation. Under acidic leaching conditions, the residual fraction of Cd in the control soil column decreased, accompanied by a significant negative shift in the Cd isotopic composition (Δ¹¹⁴/¹¹⁰Cd_{original soil-current soil} = 0.16‰), with the surface soil preferentially releasing heavy Cd isotopes. In contrast, Cd in the Cd-treated soil column was mainly present in the acid-extractable fraction, exhibiting a clear positive shift (Δ¹¹⁴/¹¹⁰Cd_{original soil-current soil} = −0.24‰). When the fractionation effects during migration were ignored, the source apportionment of Cd in the soil column by applying the isotopic binary mixing model yielded results that differed significantly from those obtained via mass balance calculations. The relative error was minimal in the soil layer near the pollution source (0 to 5 cm, RE = 0.16%) and increased significantly with depth, reaching up to 38.02%. These findings indicate that the differential fractionation effects caused by the migration stages of pollutants should be taken into consideration in Cd isotopic source tracing at actual contaminated sites.