Abstract
Anode potential has been shown to be a critical factor in the rate of acetate removal in microbial electrolysis cells (MECs), but studies with fermentable substrates and set potentials are lacking. Here, we examined the impact of three different set anode potentials (SAPs; −0.25, 0, and 0.25 V vs. standard hydrogen electrode) on the electrochemical performance, electron flux to various sinks, and anodic microbial community structure in two-chambered MECs fed with propionate. Electrical current (49–71%) and CH4 (22.9–41%) were the largest electron sinks regardless of the potentials tested. Among the three SAPs tested, 0 V showed the highest electron flux to electrical current (71 ± 5%) and the lowest flux to CH4 (22.9 ± 1.2%). In contrast, the SAP of −0.25 V had the lowest electron flux to current (49 ± 6%) and the highest flux to CH4 (41.1 ± 2%). The most dominant genera detected on the anode of all three SAPs based on 16S rRNA gene sequencing were Geobacter, Smithella and Syntrophobacter, but their relative abundance varied among the tested SAPs. Microbial community analysis implies that complete degradation of propionate in all the tested SAPs was facilitated by syntrophic interactions between fermenters and Geobacter at the anode and ferementers and hydrogenotrophic methanogens in suspension.
Highlights
In METs fed fermentable substrates, methane generation by hydrogenotrophic methanogens is an important sink of electrons, and a coulombic loss[16,17]
The ohmic drop compensation analysis (Supplementary Figure S2) using the peak current achieved at individual batch cycles (Supplementary Figure S1) and the carbon fiber brush anode resistance measured by electrochemical impedance spectroscopy, confirms that the ohmic drop in the MECs operated at different SAPs was minimal
In our earlier study[30], we demonstrated that propionate degradation in MECs occurred via a microbial partnership between fermenters and Geobacter, and low propionate removal (80% removal) was observed at 36 mM compared to complete removal at 4.5 mM30
Summary
In METs fed fermentable substrates, methane generation by hydrogenotrophic methanogens is an important sink of electrons, and a coulombic loss[16,17]. Hari et al demonstrated that propionate degradation at the anode of MECs operated at an applied voltage of 0.7 V using a power source occurs via a microbial partnership between fermenters and Geobacter[30] They showed that multiple paths of electron flow to current (via acetate/H2 or acetate/formate) could occur simultaneously during propionate oxidation regardless of the propionate concentration tested. In order to better understand the impact of anode potential on a fermentable substrate, the effect of three different SAPs (−0.25, 0 and 0.25 V vs standard hydrogen electrode, SHE) was examined on MEC performance using propionate in terms of its degradation rate, electron fluxes to various sinks (current, CH4 and undefined sinks), and microbial community structure using two-chamber reactors that minimized hydrogen gas crossover from the cathode
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