Adsorption and solid–solution compositional relationships of cadmium in tropical savannah soils from Northern Nigeria

Abstract

Increasing accumulation of cadmium (Cd) in urban and peri-urban savannah soils under long-term wastewater irrigation has been reported in some studies. Because of the insidiousness of Cd in soil ecosystems and the human health implications after the consumption of Cd in food and vegetables containing residues of the metal, we investigated its adsorption and solubility controls in selected urban soils. The latter were spiked with Cd at the rate of 200 mg Cd kg⁻¹ soil and allowed to react at pH 4–8 for 25 days. Cadmium adsorption was stronger at higher pH, with a maximum at about pH 8, just below its first hydrolysis constant (pK ₁ = 10.1). Total Cd in the soil solution (Cd_T) was speciated into free Cd²⁺ and Cd complexes. Free Cd²⁺ accounted for between 66% and 88% of Cd_T. The dominant Cd complexes were Cd–fulvate, CdCl⁺ and CdOH⁺ in two of the three experimental field soils, while Cd–fulvate and were the dominant ones in the third. Free Cd²⁺ was undersaturated with regard to the Cd₃(PO₄)₂–β-Ca₃(PO₄)₂ solubility isotherm in the soil solution at pH ≤ 7.0, while above pH 7.0 Cd²⁺ in the solution was supersaturated with respect to Cd₃(PO₄)₂–β-Ca₃(PO₄)₂ solubility, but undersaturated with respect to the octavite (CdCO₃) solubility isotherm. The slopes of the −log [Cd²⁺] versus pH relations indicated that mechanisms other than the solid phase formation controlled Cd²⁺ concentrations in the soils.

Keywords:

• adsorption
• cadmium activities
• cadmium complexes
• Cd-solid phases

Acknowledgments

The facilities offered by the Department of Soil science, Ahmadu Bello University, and the National Institute for Research in Chemical Technology (NARICT) are acknowledged by the senior author. We are grateful to the Volkswagen Stiftung, Hannover, Germany, for partially supporting this research financially under the Urban Food Project (No. I/82 189) within the collaborative program “Resources, their dynamics, and sustainability–capacity–development in comparative and integrated approaches.”