Bioremediation of toxic metals in coal overburden spoil using indigenous bacterial and fungal strains

Coal overburden spoils generated from opencast mining are often enriched with heavy metals such as nickel (Ni), copper (Cu), and chromium (Cr), posing long-term environmental risks. This study investigates the bioremediation potential of native, multi-metal-tolerant bacterial and fungal strains pre-isolated from coal overburden materials. Four bacterial and seven fungal isolates were selected based on metal tolerance and mutual compatibility. These were used to formulate four microbial consortia, which were immobilized in alginate beads and applied to coal overburden samples under controlled conditions. Consortium-4 (containing all selected isolates) achieved the highest removal efficiencies-Cr (60.52%), Cu (29.31%), and Ni (25.91%)-over 60 days, accompanied by a shift in pH from highly acidic to moderately acidic. Scanning electron microscopy (SEM) confirmed metal-induced morphological adaptations in the microbes. The findings highlight the effectiveness of indigenous microbial consortia as a sustainable and low-cost bioremediation strategy for heavy metal mitigation in coal mining environments.