Immobilization of Heavy Metals in Soil: A Sustainable Bioremediation and Reuse with <i>Bacillus subtilis</i> and Alternate Calcium Sources

Abstract

Heavy-metal contamination in soil due to rapid industrialization poses a persistent environmental challenge, demanding effective and sustainable remediation solutions. Bacillus subtilis, a resilient bacterial strain, has proven to be a promising candidate for bioremediation due to its remarkable ability to immobilize heavy metals. While traditional cementation reagents employing calcium chloride (CaCl2) can increase heavy-metal leaching, this study explores alternative calcium sources to enhance the effectiveness of Bacillus subtilis-mediated bioremediation. Through a comprehensive experimental design guided by Response Surface Methodology, the impact of various calcium sources on the immobilization of cadmium, nickel, and zinc in contaminated red soil was investigated. The bacteria were found to be efficient in precipitating heavy metals into carbonates and less leachable forms of heavy metals. The findings reveal that utilizing calcium hydroxide (Ca(OH)2) at an optimal concentration of 0.625 M in the cementation reagent significantly minimized heavy-metal leaching. This approach achieved a remarkable 97% reduction in cadmium leachability and substantial reductions of 85% for nickel and 98% for zinc. This study provides evidence for a novel and sustainable bioremediation strategy, harnessing the power of Bacillus subtilis in conjunction with optimized calcium sources to effectively remediate heavy-metal-contaminated soils. The bacteria also impart strength to the soil, which can be considered a construction material.