Estimation of Cd 2+ and Ni 2+ activities in soils by chelation

Abstract

Cadmium and Ni solubility in soils is of concern due to their biological toxicity, especially to humans. Free Cd 2+ and Ni 2+ activities in both contaminated and uncontaminated soils were determined using the chelation method. The measured Cd 2+ and Ni 2+ activities were highly pH dependent with correlation coefficients of 0.99 and 0.80 in the contaminated, and 0.98 and 0.94 in the uncontaminated soils, respectively. The measured Cd 2+ and Ni 2+ activities in the contaminated soils averaged 1.42 and 0.73 log units higher than those in the uncontaminated soils, and can be predicted for these soils using the equations: log(Cd 2+) = (7.64 ± 0.27) − 2pH and log(Ni 2+) = (7.82 ± 0.85) − 2pH. The Cd 2+ and Ni 2+ activities in the uncontaminated soils can be divided into two groups based on soil pH. The Cd and Ni solubilities in soils of pH > 6.90 were close to soil-Cd and soil-Ni, which are reference values for the solubility of Cd and Ni in most agricultural soils, and can be predicted using the equations: log(Cd 2+) = (6.22 ± 0.18) − 2pH, and log(Ni 2+) = (7.09 ± 0.38) − 2pH. Solubility in soils of pH < 6.9 were undersaturated with respect to soil-Cd and soil-Ni. Adsorption-desorption may be important in regulating Cd 2+ and Ni 2+ activities in the uncontaminated soils of low pH, whereas precipitation-dissolution process may operate in soils of high pH and/or high Cd or Ni levels. The results indicate that these contaminated soils contained elevated free Cd and Ni concentrations and remedial actions are needed to reduce their levels.