Synergistic Experimental and Quantum Chemical Investigation of Heavy Metal Immobilization in Industrial Soils

Industrial discharge from the textile and dyeing industries in Tamil Nadu’s Tiruppur district has severely contaminated the soil with heavy metals, posing significant ecological and human health risks. This study assesses vermicomposting as a sustainable and eco-friendly bioremediation strategy for detoxifying these heavy metal-contaminated soils. Industrially polluted soil samples were subjected to vermicomposting using Eisenia fetida with organic amendments such as cattle dung and decomposed plant residues under controlled conditions for 45–60 days. Changes in soil properties and heavy metal concentrations were measured before and after treatment using Atomic Absorption Spectrophotometry (AAS). The results showed a substantial reduction in all analysed heavy metals with cadmium removal being the most effective (0.9516 to 0.1355 ppm) followed by chromium (0.2980 to 0.0773 ppm), nickel (0.6855 to 0.4218 ppm) and lead (0.5423 to 0.3128 ppm). To understand the molecular mechanisms behind metal immobilisation Density Functional Theory (DFT) calculations and Molecular Electrostatic Potential (MEP) analysis were used to examine interactions between heavy metals and vermicompost-derived humic substances. DFT results indicated favourable binding energies and reduced HOMO-LUMO energy gaps confirming the high affinity of carboxyl, hydroxyl and phenolic groups for metal ions. MEP mapping revealed electron-rich regions as dominant metal-binding sites supporting strong adsorption and complexation. The combined experimental and computational findings suggest metal reduction occurs through synergistic mechanisms involving earthworm bioaccumulation, microbial-mediated redox transformations, humification and stable metal-organic complex formation. Crucially, the microbial reduction of toxic hexavalent chromium to the less mobile trivalent form was key to chromium detoxification. Beyond metal stabilisation, vermicomposting significantly increased soil organic carbon, nutrient availability (nitrogen, phosphorus and potassium) and microbial activity. This indicates effective soil health restoration. The final metal concentrations were well below internationally accepted permissible limits for agricultural soils. Overall, this study demonstrates that Eisenia fetida -based vermicomposting, supported by DFT and MEP insights, is a cost-effective low-energy and environmentally safe strategy for remediating and sustainably managing heavy metal-contaminated industrial soils.