Effects of Blighia sapida derived Biochar Amendment on Mobility and Bioavailability of Heavy Metals in Lead-Acid Battery Contaminated Soil

Abstract

This study was carried out to investigate the effects of Blighia sapida derived Biochar on the mobility, bioavailability and toxicity of Heavy metals in a  lead-acid battery contaminated soil. Soil samples were collected from three different lead-acid battery charging workshops in Benin city, Edo  state, Nigeria using systematic grid sampling method to form a composite sample. The Blighia sapida (ukpe) hardwood was crushed to dust and  pyrolysed in a microwave furnace at 350°C for 1 hour 20 minutes using slow pyrolysis condition. The lead-acid battery contaminated soil, Blighia  sapida (ukpe) hardwood derived biochar were characterized using standard methods. The lead-acid battery contaminated soils were amended with  the Blighia sapida hardwood derived biochar (5%, 10%, 15%, 20%). The geochemical fractions of heavy metals were determined using modified  European Community Bureau of Reference (BCR) three-step sequential extraction procedures and total heavy metal content with the aid of Atomic  Absorption Spectrophotometer (AAS) VGP 210 model. The total heavy metals in the lead-acid battery contaminated soil was 127.50 mg/kg Pb, 0.32  mg/kg Cr, 0.03 mg/kg Cd, 0.18 mg/kg Ni and 135.00 mg/kg Fe. The total heavy metal content in the Blighia sapida (ukpe) hardwood derived biochar  was found to be 0.23 mg/kg Pb, 0.18 Cr, 0.01 mg/kg Cd, 0.01 mg/kg Ni and 0.43 mg/kg Fe. The results obtained from the pot experiment showed  that the heavy metal content in lead-acid battery contaminated soil amended with Blighia sapida hardwood derived biochar were found to decrease  as the percentage of amender increases (5%>10%>15%>20%). From the fractionation process, the bioavailable metals were 30 mg/kg Pb,  0.08 mg/kg Cr, 0.11 mg/kg mg/kg Ni and 0.19 mg/kg Fe from the fraction that is water soluble, mobile and adsorbed to carbonate while Cd was  below detection limit of the instrument used. The mobility factors of the metals were 23.97% Pb, 26.67% Cr, 68.75% Ni and 0.15% Fe, with Ni being  more mobile and bioavailable to plant. The mobility factors and bioavailability indices of the metals were found to reduce as the percentage of  amender reduces. The result obtained from this study indicates that Blighia sapida (ukpe) hardwood derived biochar was effective in reducing the  mobility, bioavailability and toxicity of heavy metals in lead-acid battery polluted soil.