Immobilisation and bioavailability of toxic metals in lettuce (Lactuca sativa L.) cultivated in contaminated soil amended with peanut and sheanut shells biochar

Low-cost and eco-friendly techniques are required to restore or reclaim metal contaminated land or industry areas. The study assessed the effects of peanut and sheanut shells biochars on the phytoavailability of toxic metal ions in lettuce planted in polluted soil and the human health risk of its consumption. The experiment was done for eight months using biochar produced from peanut shell and sheanut shell at 350 ± 5 °C and 700 ± 5 °C, respectively for the amendments. The pH of the industrial area soil was 6.85 and electrical conductivity was 158.80 µS/cm with a sandy loam texture. The highest reduction in total phytoavailability of the metals was obtained in soils amended with peanut shell biochar produced at 700 °C (PB700, 65–83%), followed by the combination of peanut and sheanut shells biochar produced at 700 °C (PS700, 61–79%), sheanut shells biochar produced at 350 °C (SB350, 66–68%), peanut shell biochar produced at 350 °C (PB350, 53–72%), the combination of biochar produced from peanut and sheanut shells at 350 °C (PS350, 46–68%) and biochar produced from sheanut shell at 350 °C (SB350, 31–48%) treatments at 10% (w/w). The peanut and sheanut shell biochar reduced the phytoavailability of Pb, Cd, and Hg in the lettuce in the soil compared to the spiked control which was statistically significant in terms of Pb. The effectiveness in reducing toxic metals may be attributed to the physicochemical properties of the peanut and sheanut shell biochar that can improve soil quality and agronomic value. Peanut and sheanut shell biochars were particularly effective in reducing the toxic metal concentrations in lettuce. Based on the results, however, the amount of Cd and Hg that lettuce contributes to the overall body burden of toxic metals can be regarded as insignificant, and there is no discernible risk to consumers'health from eating lettuce. Therefore, more focus should be placed on metal content, especially lead content, to guarantee food security and lower the health hazards related to food intake.