Precious metal recovery using biotechnological approaches: Biosynthesis of functional nanomaterials

Abstract

Gold mining industries generate significant residues e.g., aqueous waste, waste rock, and tailings, potentially containing unrecovered gold, heavy metals, and toxic substances. Therefore, this thesis explores the mineralogy and geochemistry of solid tailings from two Saudi gold mines, investigating the potential use of Geobacter sulfurreducens for bioprocessing PMs in both lab-synthesised solutions and mineral processing solutions from Sukhaybarat Mine, Saudi Arabia. Quartz, albite, micas, sulphide, and clay minerals were commonly found in tailings from Sukhaybarat and Mahd Adh Dhahab Mines. The presence of heavy metals suggested their association with sulphide minerals. Low concentrations of Au and Ag were detected, except for a notably high Au concentration (>25 ppm) in the Sukhaybarat sample. The preventive measures implemented help reduce the possibility of the surrounding areas being adversely affected. Bioreduction experiments were conducted in synthetic systems for Au(III) and Ag(I), both individually and as bimetallic compounds. The results showed the high efficiency of G. sulfurreducens in reducing Au(III) and Ag(I) to nano-scale zero-valent metal precipitates, with H₂ proving to be a significantly more efficient electron donor, achieving removal rates (>90%) for Au(III) and Ag(I) compared to sodium acetate. XRD confirmed the presence of Au(0) and Ag(0) NPs, particularly in monometallic samples with H2 and cells. TEM showed differences in NPs sizes and distributions on bacterial cells between H2 and sodium acetate systems. XPS was also used to identify Au species and assess the extent of Au(III) reduction by G. sulfurreducens using sodium acetate. The fitted XPS data suggested the presence of mainly Au(0) and a minor portion of Au(I). In mineral processing solutions, G. sulfurreducens and H2 showed efficient removal of dissolved Au and Ag, reaching >90%. TEM showed Au and Ag nanoparticles adhering to the surface of bacterial cells, ranging in size from 5 to 10 nm. The alkaline pH (9.1) of these solutions could be a contributing factor to the observed small size of NPs. This study presents a novel biorecovery approach using G. sulfurreducens, showing its efficacy in removing dissolved Au and Ag, especially from cyanide-containing solutions, highlighting its potential as a promising candidate for precious metal recovery in the mining sector, including mining effluents. These findings contribute to the development of sustainable mining practices, aligning with global efforts toward eco-friendly resource exploitation.