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Abstract

Scaling up microbial fuel cells (MFCs) for remediation of acid rock drainage is challenging due to the intersection of interdependent biological, electrochemical and geochemical processes. Shewanella oneidensis MR-1-based microbial fuel cells (MFCs) were employed to recover metals and neutralize acid rock drainage (ARD) in a system containing mixed copper and iron metals. A methodical experimental approach was used to design and construct a continuous flow-based reactor that included the following steps- 1. electrode sizing in batch mode, 2. electrode configuration (anode-cathode connections) evaluation in batch mode, 3. electrode configuration testing in custom-made small-scale flow cells to assess impact of transition from batch to flow-based MFCs. Subsequently, the bench-scale flow based MFC was constructed, in which >96% copper was recovered selectively from the mixed copper-iron metal ARD solution. To improve current and power generation in MFCs, a chemical treatment method of graphite felts was investigated using potassium dichromate and sulphuric acid. Contrary to expected results, high capacitance and internal area of the electrodes (50x than untreated) only resulted in a small current enhancement (~10% higher than untreated felts) due to occurrence of power overshoot at higher current densities. The phenomenon persisted in graphite felts even on increasing nitrogen presence on the felts via hydrazine treatment. Presence of S. oneidensis MR-1 was limited on the surface as well as the interior of the electrode which was attributed to lower biocompatibility of the electrode and poor access to the electrode’s interior by the cells due to the intermeshed nature of the graphite felt. To improve macro-porosity, a conductive three-dimensional electrode was fabricated via 3D printing using PLA. Prepared PLA scaffolds were treated with acetone to increase hydrophilicity, functionalized with reduced graphene oxide (rGO) via hypophosphite reduction and then nickel electroless plated to fabricate electrodes with resistance <1 Ω. In an MFC, the electrodes modified with poly-pyrrole resulted in high current densities of 4.7 A/m² (~50x higher than graphite felts) compared to 0.12 A/m² from graphite felts of comparable projected surface area. In combination, the described reactor design and electrode materials are critical to scaling up MFCs for ARD treatment application.