Significance of Biological Hydrogen Oxidation in a Continuous Single-Chamber Microbial Electrolysis Cell

摘要

A single-chamber microbial electrolysis cell (MEC) that used a high density of nonmetal-catalyst carbon fibers as the anode achieved high volumetric current densities from 1470 ± 60 to 1630 ± 50 A/m~3 for a hydraulic retention time of 1.6-6.5 h. The high current density was driven by a large anode surface area and corresponded to a volumetric chemical oxygen demand (COD)-removal rate of 27-49 kg C0D/m~3·d. Observed H_2 harvesting rates were from 2.6 ± 0.10 to 4.3 ± 0.46 m~3 H_2/ m~3·d, but the H_2 production rates computed from the current densities were 16.3-18.2 m~3 H_2/m~3·d. Tracking all significant electron sinks (residual acetate, H_2, CH_4, biomass, and soluble microbial products (SMP)) in the single-chamber MEC showed that H_2 reoxidation by anode-respiring bacteria recycled H_2 between the cathode and the anode, and this caused the large discrepancy in H_2 production and harvest rates. H_2 recycle accounted for 62-76% of observed current density, and this made the observed Coulombic efficiency 190-310% at steady state. Consequently, the cathodic conversion efficiency was only 16-24%. The current density added by H_2 recycle also increased the applied voltage from ～0.6 V to ～1.5 V for the highest H_2 harvest rate (4.3 m~3 H_2/m~3·d). CH_4 generation consistently occurred in the continuous single-chamber MEC, and its electron fraction of consumed acetate was 7-25%. Because of methane formation and biomass/SMP accumulation, the overall H_2 recovery was moderate at 1.8-2.0 mol of H_2/mol of acetate in the MEC. Thus, this study illustrates that a single-chamber MEC with a high anode surface area can generate high volumetric rates for COD removal and H_2 generation, but H_2 recycle and methanogenesis present significant challenges for practical application.