Heavy Metal Bioaccumulation by the Anecic Earthworm, Alma nilotica (Grube 1855) and the Pollution Level of Biochar-amended E-waste-contaminated Soil

Abstract

Background: Soil heavy metal pollution is a significant environmental challenge that adversely affects soil fertility, food security, and human health, thereby hindering the achievement of global sustainability goals. This pollution arises from overexploitation, uncontrolled waste dumping, informal recycling activities, and improper soil use, primarily through leaching and volatilization. Heavy metals resist organic detoxification and bioaccumulate, leading to prolonged half-lives in soils. Aims: To investigate the effect of amending e-waste contaminated soils with maize cob-based biochar on heavy metal bioavailability to Alma nilotica and soil pollution level. Methodology: Experimental design. Laboratory of Applied Biology and Ecology (LABAE) and Laboratory of Soil Science and Environmental Chemistry at the University of Dschang, August 2024 and July 2025. The e-waste-contaminated soil was obtained from an informal e-waste recycling site located in Bonaberi, Douala, Cameroon. Surface soil samples were collected from 10 random points using a soil auger to a depth of 20 cm The bioaccumulation bioassay followed the OECD (2010) test guidelines No.317 for Bioaccumulation on terrestrial oligochaetes. Soil samples were collected, dried and analyzed for the studied metals. Earthworms were exposed to both the culture soil, e –e-waste soil with and without biochar and biochar alone for a period of 35days. A soil sample and an earthworm were sampled after every 4days. Each of the samples was analysed for Pb, Cu, Cd, Ni, Zn. Results: Results indicated that the amended soil had improved pH, % organic carbon, and Cation Exchange Capacity. Biochar amendment reduced the heavy metal contamination and pollution levels by 59% and 64% respectively, especially when considering their bioavailable fractions in soil. Exposure of earthworms resulted in continuous Cd, Cu, Ni, Pb, and Zn uptake and simultaneous reduction in soil heavy metal concentrations. The heavy metal uptake by the earthworms was reduced by 51.6% to 68.8% in the amended soil, resulting in higher heavy metal concentrations in the amended soils and indicating reduced metal bioavailability in the biochar-amended e-waste soil. Consequently, the heavy metal bioaccumulation factors in the amended soil were reduced by between 72.7% and 82.8%. Earthworms proved to be a good indicator of the heavy metals' bioavailable fraction in soil. 59% and 64% reductions are for contamination and pollution indices based on bioavailable components. Conclusion: These results have important implications for soil remediation using the biochar approach. However, these should be directly linked to earthworm growth and reproduction to ensure long-term protection of soil biota. The findings advocate for the strategic use of biochar, enhancing soil quality and contributing to sustainable remediation practices in areas affected by e-waste pollution. Overall, this research provides valuable insights into the benefits of biochar as a soil amendment in contaminated environments, while also emphasising the need for further investigation into its long-term effects on soil biota and the environment.