Speciation and complexation of cadmium in extracted soil solutions

We report the soil solution speciation of Cd in 64 field-collected contaminated soils containing between 0.1 and 38 mg Cd kg(-1). The soils were analyzed for pH (3.5-8.1), soil organic matter (8.0-108 g C kg(-1)), total dissolved Cd (0.03-182 mu g Cd L-1), dissolved organic carbon (1.5-12 mg C L-1), and free Cd2+ (10(-1)0-2 x 10(-7) M). Free Cd2+ activity in solution was determined using differential pulse anodic stripping voltammetry (DPASV), assuming DPASV is sensitive to easily dissociated inorganic ion-pairs and free Cd2+ while excluding organic complexes. The solid/liquid partition coefficient(K-d) varied over a range from 10 to 100 000, and the fraction of the dissolved Cd present in solution as the estimated free Cd2+ species varied between 0 and 60% but averaged about 20%. The dissolved concentrations of Cd and the free Cd2+ activity in the soil solutions of contaminated soils of different origins can be predicted with reasonable accuracy using a simple competitive adsorption model dependent on pH and total metal loading.