Bio-Electrochemical process for Metal and Sulfur Recovery from Acid Mine Drainage

Abstract

Water recycling and resource recovery from acid mine drainage (AMD) are increasingly being regarded as desirable practices with direct benefits for the environment and the operational and economic viability of the resources sector.This thesis proves the concept of a novel bioelectrochemical system (BES) for the direct electrode-driven resource recovery and practically permanent AMD treatment. The technology consists of a two-cell bioelectrochemical setup to enable the removal of sulfate from the ongoing reduction-oxidation sulfur cycle, thereby also reducing salinity, without external addition of chemicals.In particular, the goals of this thesis are: (i) to enrich a sulfate reducing bacterial community capable of directly utilising a carbon based cathode as electron donor for autotrophic sulfate reduction, or via bioelectrochemically-produced H2; (ii) to elucidate the electron flux pathways of autotrophic sulfate reduction and microbial interactions in cathodic mixed cultures; (iii) to develop a high-rate sulfate reducing bioelectrochemical reactor based on high surface area electrode materials; (iv) to design and implement a combined chemical-free bioelectrochemical process that enables the sulfur, metal and water recovery from AMD.In order to elucidate whether cathodes can effectively release electrons and act as the only electron donor to support sulfate reduction process, the effect of cathode potential and inoculum source were evaluated using electrochemical tools, including the recording of chronoamperometry and cyclic/linear sweep voltammetry (Chapter 5). Electrochemical and off-gas analysis coupled to liquid phase sampling was carried out to determine the electron fluxes from the electrode to the final electron acceptor (sulfate) during autotrophic sulfate reduction (Chapter 6). Fluorescence in situ hybridization (FISH) and digital image analysis (DAIME) of the microbial communities in z-stack confirmed the microbial stratification. After obtaining a well-functioning biocathode for autotrophic sulfate reduction, its performance was experimentally optimised in terms of high-surface area electrode materials, like multi-wall carbon nanotubes on reticulated vitreous carbon (MWCNT-RVC) and carbon granules (CG) (Chapter 7). Finally, a novel BES was tested for AMD treatment (Chapter 8). This thesis reported the effect of inoculum and cathode potential on the successful enrichment of an autotrophic sulfate-reducing biocathode controlled at -0.9 V vs. standard hydrogen electrode (SHE). This study proved for the first time that high rates of autotrophic sulfate reduction (29 ± 3 g SO42--S m-2 d-1) are mainly driven via hydrogen produced at the same cathode, with 95±0.04% Coulombic efficiency towards sulfide production. Moreover, the relative abundance of the biofilm-forming sulfate-reducing bacteria (SRBs) enriched on the carbon cloth cathodes (46.1± 3.9%) showed the remarkable ability to consume hydrogen at a rate of 3.9 ± 0.5 mol H2 m-2 d-1, outcompeting methanogens and homoacetogens for the hydrogen without the need to add chemical inhibitors. The findings of this thesis show that inexpensive CG can achieve higher current-tosulfide efficiencies at lower power consumption than the nano-modified three-dimensional MWCNT-RVC. Sequencing analysis of the 16S rRNA gene on day 58 revealed that MWCNT-RVC retained the bacteria and archaea population from the inoculum, while CG electrode surface was able to select for bacteria over archaea.The feasibility to integrate a two-stage BES was proven with a lab-scale setup for AMD treatment. Using AMD, the BES operation enhanced the sulfate reduction rate (SRR) to 946 ± 18 g SO42--S m-3 d-1, which corresponds to 189 ± 4 g SO42- -S m-2d-1. The power consumption was 10 kWh kg-1 of S0 recovered with an effective removal of sulfate-S to less than 550 mg L-1 (85 ± 2% removal).In addition, the BES operation drove the removal and recovery of the main cations Al, Fe, Mg, Zn at rates of 151 ± 0 g Al m−3 d−1, 179 ± 1 g Fe m−3d−1, 172 ± 1 Mg m−3d−1 and 46 ± 0 g Zn m−3 d−1 into a concentrate stream (containing 263 ± 2 mg Al, 279 ± 2 mg Fe, 152 ± 0 mg Mg and 90 ± 0 mg Zn per grams of solid precipitated after BES treatment).The solid metal-sludge was 2 times less voluminous and 9 times more readily settleable than metal-sludge precipitated using NaOH. The continuous BES treatment also demonstrated the concomitant precipitation of rare earth elements + yttrium (REY) per grams of solid, with up to 498 ± 70 µg Y, 166 ± 27 µg Nd, 155 ± 14 µg Gd, among other high-value metals.The proven integrated process enhances the potential for mining water recycling worldwide by achieving sulfur recovery in elemental form and recovery of metals at low concentrations from mining and mineral processing wastewater.