Performance evaluation of gold (III) bio-reduction by bacterial strains and a consortium isolated from gold mine effluent

• Indigenous bacteria from mine waste achieve >98 Au (III) reduction within 24 h. • Gram-negative bacteria outperformed Gram-positive by 6% via electron transfer. • Mixed consortium reached 94.5% Au (III) reduction through synergistic interactions. • Effective bioreduction of low-grade gold solutions (3–6 ppm) typical of mining effluents. The bioreduction of Au (III) to Au (0) offers a sustainable and eco-friendly alternative to conventional gold recovery methods, reducing environmental impacts associated with chemical processing. This study investigated the bioreduction capabilities of indigenous bacterial strains and a microbial consortium isolated from gold tailings and mine wastewater. Aerobic batch experiments were conducted at pH 7 and 35 ± 2°C over 24 h, with initial Au (III) concentrations of 3 and 6 ppm, to compare the reduction efficiencies of individual strains and a mixed consortium. Gram-negative bacteria ( Stenotrophomonas maltophilia, Klebsiella pneumoniae, Acinetobacter bereziniae ) achieved superior reduction efficiencies of 98.33–99.8 %, outperforming the Gram-positive Bacillus cereus (92.9 %), likely due to differences in cell wall structure, with Gram-negative strains leveraging outer membrane proteins and efficient extracellular electron transfer mechanisms, while B. cereus relies on biosorption via its thicker peptidoglycan layer. The consortium, combining all four strains, reached 94.5 % efficiency, reflecting synergistic interactions through resource partitioning and metabolite recycling. Scanning electron microscope (SEM), Energy-dispersive X-ray spectroscopy (EDS), X- ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) confirmed the formation of Au (0) nanoparticles, with an 8.5 % crystalline gold content in the consortium despite low starting concentrations. These findings highlight the potential of indigenous bacteria for efficient gold bio- recovery from mining effluents and tailings, demonstrating a viable bio- hydrometallurgical process that integrates waste treatment with precious metal extraction. The technology offers significant advantages for mining operations through reduced chemical consumption, lower energy requirements, and simultaneous environmental remediation, supporting the industry’s transition toward sustainable extraction practices and circular resource utilization in mineral processing operations.