Evaluation of Heavy Metals in Soil Using Modified BCR Sequential Extraction

Recent studies have shown that strong acid digestions for the determination of heavy metals in soils and sediments can be misleading when assessing environmental effects. Therefore, this study adopted the use of modified community bureau of reference (BCR) sequential extraction scheme in order to overcome the limitation. The physicochemical properties of the soil such as pH, cation exchange capacity (CEC) and soil organic matter (SOM) were determined. The pH, CEC and SOM were found to be 5.90, 0.0023 cmol/kg and 99.8% respectively. The concentrations of the heavy metals in the solution were also measured using atomic absorption spectrophotometry. Pseudo total metal content in the soil (aqua-reqia digestions) for cadmium, lead, chromium, nickel, zinc and copper were 0.12, 0.74, 0.48, 0.26, 15.9 and 11.0 mg/kg respectively. These concentrations are within the World Health Organization (WHO) permissible limit in agricultural soils. The heavy metals were partitioned into four fractions (exchangeable, reducible, oxidizable and residual fractions) in the soil using modified BCR sequential extraction. Cd, Zn and Cu were predominantly in the first two fractions, thus, mobile and bioavailable for plant uptake, while Pb, Cr and Ni were found in the last two fractions (less mobile and immobile residual fractions). Thus, Cd, Zn and Cu may pose high environmental risk. The results indicate the reliability of the Modified BCR scheme in risk assessment of heavy metals in soils.