Operational speciation of lead, cadmium, and zinc in farmlands around a lead polluted goldmine in northern Nigeria

Abstract

This study was undertaken to evaluate the geochemical fractions and risk potential of lead (Pb), cadmium (Cd) and zinc (Zn) in farmlands around a polluted goldmine in Dareta, northern Nigeria. The total heavy metal concentrations were obtained  through a mixed acid digestion. A modified sequential extraction procedure was  used in separating the total metal concentrations into five operationally defined  fractions. The results showed extremely high concentrations of total Pb (26 – 2,247 mg kg^-1), and Cd (2 - 69 mg kg^-1) in the soils. The fractionation results also showed very high concentrations of Pb and Cd held on the easily mobilized fractions.  However, in terms of proportions, the bulk of the metal concentrations were   partitioned to the residual fraction while the exchangeable fraction held the least.   The concentrations of Zn recorded were below the thresholds employed. Lead and   Cd, showed strong enrichment, with both elements recording enrichment factors of   32.8 and 83.0, and I-geo factors of 4.4 and 5.8, respectively. Although the bulk of  the metals were partitioned to the residual fractions, the concentration of metals   held onto the easily mobilized fractions such as the exchange, oxide and organically bound fractions may make the metals bioavailable on the long run with possible   pollution risk on this soil, especially when used for agricultural purposes.

Keywords: Heavy metal contamination, Metal fractionation, Pollution index