Enhanced Long-term Reduction of High-level Au(III) with the Presence of NO3− in a H2-based Membrane Biofilm Reactor

Abstract

Increased mining and ore processing of gold (Au) are leading to waters contaminated with Au(III) ions, and a common co-contaminant is nitrate (NO₃⁻). Here, we demonstrate that a hydrogen (H₂)-based membrane biofilm reactor (MBfR) enabled synergistic co-reductions of NO₃⁻ to N₂ and Au(III) to elemental Au° for over 250 days of continuous operation. Au(III) was reduced to Au⁰ nanoparticles (Au⁰NPs) that were retained within the biofilm's extracellular polymeric substances. NO₃⁻ and Au(III) were > 95% reduced at steady state for a wide range of influent conditions: NO₃⁻-N at 1 or 4 mM; Au(III) at 100, 200, or 500 mg/L. Metal-tolerant denitrifiers Azonexus, Pannoibacter, Thermomonas, and Cupriavidus were enriched, as were genes encoding metal reductases. The rate of Au(III) reduction was positively correlated with the abundance of NO₃⁻ and NO₂⁻ reductases, which supports the role of these reductases in Au(III) reduction. Remarkably, the Au(III)-reduction efficiency remained above 90% in the highly acidic condition, despite NO₂⁻ accumulation due to incomplete NO₃⁻ reduction; thus, the microbial community was resilient against environmental perturbation. By providing a mechanistic basis for Au recovery using the MBfR, this study establishes the MBfR as a promising and sustainable technology for treating wastewaters containing valuable metals, such as gold, in coordination with microbial denitrification.