CHEMICAL SPECIATION AND BIOAVAILABILITY OF SOME SELECTED HEAVY METALS IN FARMLAND SOILS AROUND WARRI REFINERY, DELTA STATE, NIGERIA

Author: Obaro Ajovi

Abstract

The aim of this study was to investigate the chemical speciation and bioavailability of selected heavy metals in farmland soils around Warri refinery, Delta State, Nigeria. Soil samples were collected from 12 farmlands around the refinery at different depths. The concentration of heavy metals were determined using atomic absorption spectrometry (AAS) after digestion with aqua-regia. The chemical forms of the heavy metals were determined using the Tessier sequential extraction method. The results showed that the concentrations of the metals ranged from 1.02 to 11.16 mg kg-1, 5.59 to 42.38 mg kg-1, 11.06 to 70.08 mg kg-1 and 39.18 to 182.35 for As, Pb, Cr and Zn respectively. Cadmium was below the detection limit of the instrument. The chemical speciation of the heavy metals indicated that the residual and organic fractions were the predominant fractions of the heavy metals. The speciation of As and Pb followed the order residual> organic> carbonate> Fe- Mn oxide> exchangeable while that of Cr and Zn followed the order residual> organic> Fe- Mn oxide > carbonate> exchangeable. The mobility factors for the heavy metals ranged from 11.90-89.91, for As, 12.61-57 for Pb, 6.64-22.30 for Cr and 4.05-11.77 for Zn following the order As> Pb> Cr> Zn. The relatively high mobility factors observed for As and Pb indicate high mobility, lability and bioavailability for As and Pb in the soils studied. The results suggest that As and Pb in these soils pose health risk to humans through the plant uptake dietary route.

Dr. S.A. Osakwe