Chemical Fractionation of Heavy Metals in the Soil of Auto-Mechanic Workshops in Akure, Ondo State, Nigeria

Abstract

Total metal concentrations and sequential extraction scheme were employed to investigate the potential environmental risk of seven important metals in the soil of selected auto-mechanic workshops in Akure metropolis, Ondo State, Nigeria. Soil physicochemical parameters were also determined with the following results: pH ranged from 5.50-6.32, total organic carbon from 0.96-1.56%, particle size indicates the soil to be mainly sandy, Cation Exchange Capacity (CEC) ranged from 7.00-9.00 Cmol/Kg. The concentrations (mg/kg) for the seven metals in Ilesha-Garage soil samples were 4.25±1.06 (Cd), 17.00±0.00 (Cr), 20.75±0.35 (Cu), 4750.00±0.00 (Fe), 4.00±0.70 (Ni), 40.50±0.70 (Pb), 40.75±0.35 (Zn) while the mean concentrations of metals (mg/kg) in Oyemekun soil samples were 7.50±0.00 (Cd), 22.00±0.70 (Cr), 33.25±0.35 (Cu), 6475.00±0.35 (Fe), 5.00±0.01 (Ni), 60.00±0.00 (Pb) and 48.50±0.70 (Zn) respectively. Cadmium content in the soil samples exceeded the target values of the Department for Petroleum Resources (DPR) regulatory Standards for soil. Sequential extraction by fractionation reveals that the average percentage concentration of  Cd, Cr, Ni, Pb and Fe were more prevalent in residual fractions, while Zn and Cu were found more in Iron-Manganese Oxides, carbonate in addition to residual fractions. The heavy metals speciation of the soil samples indicates that the heavy metals were predominately found in both the residual and non-residual fractions suggestive of control by geogenic and anthropogenic sources respectively. Eco-toxicological assessment of the soil samples using the mobility factor indices revealed the following sequence: Lead (31.11%) > zinc (21.11%) > Nickel (20.75%) > Copper (12.24%) > Chromium (10.73%) > Cadmium (0.80%) > Iron (0.09%). The pollution index assessment indicates that most of the metals fall within the “slight contamination” and “moderate contamination” range in the soil.