Bacterial lead vermicomposting as a sustainable strategy for remediating lead contamination in soil: a synergistic approach integrating bioremediation and nano-bioremediation.

Soil contamination by heavy metals, particularly lead (Pb), is a critical environmental concern, affecting soil fertility, agricultural productivity, and human health. Conventional remediation techniques are often costly and environmentally invasive. This study investigates an integrated bioremediation strategy combining the phytoremediation potential of Sorghum bicolor, microbial consortia (Bacillus haynesii and Priestia megaterium) derived from vermicompost, and zinc oxide nanoparticles (ZnO NPs) for the remediation of Pb-contaminated soil. The objective is to evaluate the individual and synergistic effects of these approaches on Pb reduction and assess the contribution of bacterial strains in enhancing metal uptake and removal. Alluvial soil, solid waste, and vermicompost were collected for bioremediation assessment. Alluvial soil was analysed for nutrient content and plant growth suitability; solid waste was analysed for heavy metal contamination to establish a realistic Pb concentration for experimental soil contamination; and vermicompost was used as a source of Pb-tolerant bacteria. Physicochemical analysis confirmed elevated Pb levels in the solid waste, which served as the reference matrix for simulating Pb-contaminated soil conditions. In a controlled pot culture experiment, Sorghum bicolor was grown in all treatments. Bioremediation trials included treatments with monocultures and a consortium of B. haynesii and P. megaterium, applied at two inoculum volumes (5 mL and 25 mL). An additional treatment combined the 25 mL microbial consortium with ZnO nanoparticles (0.6 mg/kg). These variations were designed to evaluate Pb removal efficiency based on bacterial inoculum, nanoparticle supplementation, and their effect on plant uptake and soil remediation over time. Physicochemical analysis of three distinct sample types alluvial soil, solid waste, and vermicompost-revealed elevated Pb concentrations specifically in the solid waste, which was used as the reference matrix for subsequent bioremediation experiments. The combination of S. bicolor with the microbial consortium (Bacillus haynesii and Priestia megaterium) significantly improved Pb removal compared to monocultures. The addition of ZnO NPs further accelerated Pb reduction, achieving a 98.7% decrease in Pb concentration by Day 30, compared to Day 60 in bacterial-only treatments. S. bicolor served as a bioindicator, and Pb uptake in its tissues was specifically assessed in the highest contamination group (74.7 mg/kg) to evaluate translocation and accumulation. The combination of plant, microbes, and ZnO NPs showed the highest overall remediation efficiency. This study highlights the potential of integrating phytoremediation using Sorghum bicolor with microbial consortia and ZnO NPs as an eco-friendly and effective approach for Pb-contaminated soil remediation. The synergistic interaction reduced detoxification time while enhancing Pb removal. However, a limitation of the study is the exclusive use of alluvial soil; future studies should evaluate the effectiveness of this strategy in diverse soil types to improve its field applicability.