Predicting soil-water partition coefficients for cadmium

Equilibrium batch measurements were made of the adsorption of Cd(II) on 15 New Jersey soils. The soils were characterized in terms of surface properties and chemical composition, particularly organic matter and metal oxides. The adsorption of Cd(II) followed the Langmuir relation and increased with increasing pH between pH 3 and pH 10. At constant pH and metal concentration, greater adsorption was observed for soils with higher organic matter content. To better understand the mechanism of adsorption, the experimental results for the adsorption of Cd by the 15 soils were tested using a partition coefficient model that related the adsorption of the Cd(II) to the soil components: organic matter, iron oxides, aluminum oxides, and manganese oxides. This model was not successful when applied to measurements at the natural soil pH because of the competition of protons with Cd(II) for available sites. However, at constant pH, partition coefficients obtained from experimental data were highly correlated with those calculated for a partition coefficient between Cd(II) and organic matter alone. Normalization of the partition coefficients, K-d, for the organic matter content of the soils, K-om, greatly improved the relationship between the partition coefficient and pH (R(2) increased from 0.799 to 0.927). This suggests that the surficial adsorption sites are principally composed of organic matter. For the 24-h equilibration period employed, diffusion of Cd(II) through this superficial organic matter coating to underlying sorptive materials, including metal oxides, is unimportant in the partitioning of Cd(II).