Hydrogen production by geobacter species and a mixed consortium in a microbial electrolysis cell

Abstract

A hydrogen utilizing exoelectrogenic bacterium (Geobacter sulfurreducens) was compared to both a nonhydrogen oxidizer (Geobacter metallireducens) and a mixed consortium in order to compare the hydrogen production rates and hydrogen recoveries of pure and mixed cultures in microbial electrolysis cells (MECs). At an applied voltage of 0.7 V, both G. sulfurreducens and the mixed culture generated similar current densities (ca. 160 A/m3), resulting in hydrogen production rates of ca. 1.9 m(3) H2/m3/day, whereas G. metallireducens exhibited lower current densities and production rates of 110 +/- 7 A/m3 and 1.3 +/- 0.1 m3 H2/m3/day, respectively. Before methane was detected in the mixed-culture MEC, the mixed consortium achieved the highest overall energy recovery (relative to both electricity and substrate energy inputs) of 82% +/- 8% compared to G. sulfurreducens (77% +/- 2%) and G. metallireducens (78% +/- 5%), due to the higher coulombic efficiency of the mixed consortium. At an applied voltage of 0.4 V, methane production increased in the mixed-culture MEC and, as a result, the hydrogen recovery decreased and the overall energy recovery dropped to 38% +/- 16% compared to 80% +/- 5% for G. sulfurreducens and 76% +/- 0% for G. metallireducens. Internal hydrogen recycling was confirmed since the mixed culture generated a stable current density of 31 +/- 0 A/m3 when fed hydrogen gas, whereas G. sulfurreducens exhibited a steady decrease in current production. Community analysis suggested that G. sulfurreducens was predominant in the mixed-culture MEC (72% of clones) despite its relative absence in the mixed-culture inoculum obtained from a microbial fuel cell reactor (2% of clones). These results demonstrate that Geobacter species are capable of obtaining similar hydrogen production rates and energy recoveries as mixed cultures in an MEC and that high coulombic efficiencies in mixed culture MECs can be attributed in part to the recycling of hydrogen into current.

FIG. 1.

The MEC single-chamber reactor design used in this study is shown. A reference electrode is shown extending from the front of the reactor.

FIG. 2.

Current densities (A), anode potentials (B), and gas production (C) recorded at E_AP = 0.7 V for G. sulfurreducens (GS), mixed culture (MC), G. metallireducens (GM), and the control (CNTRL; no inoculum).

FIG. 3.

Current densities (A), anode potentials (B), and gas production (C) recorded at E_AP = 0.4 V for G. sulfurreducens (GS), a mixed culture (MC), G. metallireducens (GM), and the control (CNTRL; no inoculum).

FIG. 4.

Current densities recorded during hydrogen utilization tests (no acetate provided) at an applied voltage of E_AP = 0.7 V for G. sulfurreducens (GS), a mixed culture (MC), G. metallireducens (GM), and the control (CNTRL; no inoculum).

FIG. 5.

CV recorded at 10 mV/s from −0.8 V to + 0.2 V for G. sulfurreducens (GS), a mixed culture (MC), and the control (CNTRL; no inoculum).

FIG. 6.

Current densities recorded as a function of applied voltage for G. sulfurreducens (GS) and a mixed culture (MC).

FIG. 7.

SEM images of the control (A and D), G. sulfurreducens (B and E), and a mixed culture (C and F) anodes at the end of all batches. Images were taken after day 63 for G. sulfurreducens and day 47 for the mixed culture. The bars in panels A to C represent 100 μm. The bars in panels D to F represent 10 μm.