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 μg Cd L-1), dissolved organic carbon (1.5−12 mg C L-1), and free Cd2+ (10-10−2 × 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 (Kd) 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.