Comparative studies on the effects of biochar and cow dung amendments on the mobility, bioavailability and toxicity of heavy metals in lead-acid contaminated soil

Abstract

This study compared the heavy metals remediation potency of biochar and cow dung in soils contaminated by lead-acid batteries. Soil samples were gathered from three different battery charging shops in Benin City at the depth of 0-15cm using systematic grid sampling method. Hardwood was pyrolyzed at 350°C for 80minutes and cow dung was air dried and pound into powder. The soil was characterized using standard methods, and was fractionated using modified European Community Bureau of Reference (BCR) three-step sequential extraction procedure while the concentration of heavy metals was determined using Atomic Absorption Spectrophotometer (AAS). The total metal concentration in the lead-acid contaminated soil was 127.50mg/kg Pb, 0.32mg/kg Cr, 0.03mg/kg Cd, 0.18mg/kg Ni and 135.00mg/kg Fe. The contaminated soil was remediated with biochar and as well with cow dung at 5%, 10%, 15% and 20%. The soil was also amended using biochar-cow dung mixture (1:1, 2:1 and 1:2). The outcome revealed that the metal content in lead-acid contaminated soil remediated with biochar and cow dung were found to generally decrease as the percentage of amenders increases, with biochar being further efficient in lowering the levels of heavy metals in the contaminated soil. However, biochar when mixed with cow dung at 2:1 ratio at 20% amendment gives a better result than when used separately. Pot experiment was designed and the amounts of heavy metals bioavailable to plant were determined. The mobility factors and bioavailability indices of the metals reduced as the percentage of amenders’ increases. The result obtained from this study revealed that although biochar and cow dung were effective amenders, biochar was more potent in lowering the mobility, bioavailability and toxicity of heavy metals in lead-acid contaminated soil. Biochar when mixed with cow dung at 2:1 ratio gave the best result in remediation of lead-acid contaminated soil.