Adsorption behavior of cadmium in response to soil surface charge

I investigated the adsorption behavior of cadmium, both as Cd²⁺and complexed as CdEDTA²⁻, on four soils. Christiana silt loam (Typic Paleudult), Lima loam (Typic Hapludalf), and Vergennes silt loam (Glossaquic Hapludalf) all exhibited low zero points of charge (ZPC ≃ 2) and thus were negatively charged over the entire pH range typical of soil systems. Dothan clay (Plinthic Paleudult), in contrast, had a ZPC around pH 6.0, indicating an electropositive surface at its field pH (4.7). The pH-dependent adsorption of Cd²⁺was nearly identical for the four soils. When complexed as Cd-EDTA²⁻, Cd was adsorbed by the positively charged Dothan soil, suggesting that strong chelation does not necessarily preclude substantial metal adsorption. In soils with high ZPCs, complexed heavy metals may be less mobile and available than their unbound counterparts.