Cadmium mobility in three contaminated soils amended with different additives as evaluated by dynamic flow-through experiments.

As arable land has become an important sink for cadmium (Cd), soil is being recognized as a major source of metals to the food chain. It becomes, therefore, essential to investigate metal mobility in contaminated soils and to identify suitable remediation strategies. For this, immobilization of Cd was evaluated in contaminated stagnic anthrosol: S1, gleysol: S2 and fluvisol: S3 under flow through conditions. Ten treatments including control were tested alone or in composite form firstly at natural Cd contents (0.58-0.69 mg kg-1). Here, T2 (lime), T5 (biochar) and T10 (composite amendment) were found better in reducing the Cd concentration in the soils' leachates, so, their efficacy was further investigated in the same soils of higher Cd contents (1 and 2 mg kg-1 imposed by soil spiking). Amendments significantly reduced the leachate metal contents especially in 1 mg kg-1 spiked soils. Characterization of T2, T5 and T10 revealed their structural transformations in all the studied soil types, while active functional groups e.g. C-O, CO, O-H, Si-O-Si, ester and alcoholic groups were notably involved in Cd precipitation or adsorption on amendments surface. Variations in Cd speciation in these soils exhibited the exchange of Cd to more stable fractions with tested amendments. These continuous-flow experiments confirmed the strong efficiency of T2, T5 and T10 in reducing the Cd concentration in the leachate of three soils. This study has strong implications in understanding the role of different amendments in controlling the fate, leaching behavior and immobilization of Cd in diverse soil types.