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72% confidence

This study demonstrates that Acidimicrobiaceae sp. A6, a Feammox bacterium, can oxidize NH4+ to NO2− in microbial electrolysis cells (MECs) using a graphite anode as electron acceptor instead of ferric iron. Pure and enrichment cultures achieved NH4+ removal of 0.6–0.66 mM over 3 weeks, comparable to batch controls with Fe(III), while sustaining similar biomass. The electron shuttle AQDS was essential for pure cultures (monotonic increase in current and NH4+ removal with AQDS concentration) but less critical for enrichment cultures. Maximum current density reached 4.2 A m⁻³ and coulombic efficiency 16.4% under constant stirring and 0.3 V applied voltage. NO2− was not detected due to abiotic reaction with trace Fe(II) transferred with inoculum.

Key insights

  • Threshold AQDS concentration drives a monotonic increase in both current density and NH4+ removal in pure A6 cultures, with zero current and zero NH4+ removal in the absence of AQDS despite presence of NH4+.

    So what. Operators must supply or generate electron shuttles (AQDS or natural organic matter) to enable Feammox-MEC function; pure cultures cannot operate without them, but enrichment cultures may self-generate shuttles over time.

  • Mechanism Cyclic voltammetry shows oxidation peaks at −0.03 ± 0.025 V vs. Ag/AgCl with AQDS and −0.11 ± 0.035 V without AQDS, corresponding to ΔG° of −60.78 kJ mol⁻¹ and −14.47 kJ mol⁻¹ respectively.

    So what. AQDS shifts the anode potential requirement toward less positive values, improving thermodynamic favorability; this explains why AQDS enables current production at lower applied voltages.

  • Method NO2− was not detected in MECs despite NH4+ removal, because trace Fe(II) transferred with the bacterial inoculum (Table S1) abiotically oxidizes NO2− to N-gas forms, as confirmed by a control test showing NO2− disappearance in presence of Fe(II).

    So what. Researchers must either minimize Fe carryover in inoculum or use acetylene inhibition to track true NO2− production; apparent NH4+ removal without NO2− detection does not prove the reaction is incomplete.

  • Trade-off Mixing regime change from orbital shaker to magnetic stirring extended MEC operation from 2 weeks to >1 month and increased maximum current density from 3.2 to 4.2 A m⁻³, with coulombic efficiency rising from 4.33% to 16.4%.

    So what. Mechanical stability of electrode connections is a critical design constraint; gentle internal stirring outperforms whole-reactor shaking for long-term Feammox-MEC operation.

  • Performance AQDS effect on current production is much greater in pure A6 cultures than enrichment cultures (Fig. 2), and the authors predict AQDS will not be needed long-term in enrichment cultures due to organic matter accumulation from cell turnover.

    So what. Enrichment cultures may be more practical for scale-up because they reduce or eliminate the need for exogenous electron shuttle addition over time.

Opportunity areas6

Specific gaps in this paper, with an experiment that would close each.

  • High No systematic variation of applied voltage (Vapp) between 0.3 and 0.7 V to identify optimal operating window and trade-off between current density, coulombic efficiency, and NH4+ removal rate.

    Two different Vapp values (0.7 V and 0.3 V) were used in different experiments, but no controlled comparison within the same mixing regime. The paper states 'MECs were connected in parallel to a programmable power supply...with a constant external applied voltage (Vapp) set at 0.3 V' for later experiments but earlier used 0.7 V, yet does not explain why or test the intermediate range.

    Try. Vapp sweep from 0.2 to 0.8 V in 0.1 V increments, holding all other variables constant (magnetic stirring, 0.15 mM AQDS, pure A6), measuring current density, coulombic efficiency, and NO2− production (with acetylene inhibition) over 2-week periods.

  • High No variation of AQDS concentration in enrichment cultures to determine the threshold below which enrichment cultures also require exogenous shuttle, and to quantify the rate of organic shuttle generation.

    Fig. 3 shows monotonic increase in current and NH4+ removal with AQDS in pure cultures (0 to 0.3 mM tested), but enrichment cultures were only tested at 0.15 mM AQDS. The paper predicts AQDS becomes unnecessary over time but provides no data on enrichment cultures at zero or low AQDS concentrations.

    Try. Enrichment culture MECs at 0, 0.05, 0.1, 0.15, 0.3 mM AQDS, sampled weekly for 8 weeks, measuring current density, NH4+ removal, and dissolved organic carbon (DOC) to track shuttle generation.

  • High No measurement of N2O or other gaseous N products; only NO2− was tracked, and it was not detected due to Fe(II) reaction. True nitrogen balance and product distribution remain unknown.

    The paper states 'NO2− concentration measurements in MECs were below the detection limit' and explains this by abiotic Fe(II) oxidation, but does not use acetylene inhibition in the main MEC experiments (only mentioned as done in prior enrichment culture work). Without N2O or N2 quantification, the fate of 6 electrons per NH4+ is unclear.

    Try. Repeat pure and enrichment culture MECs with acetylene (10% v/v headspace) to inhibit N2O reduction, measure NO2−, N2O, and N2 via gas chromatography and ion chromatography, calculate electron balance and product selectivity.

  • Medium No variation of pH (currently fixed at 5–5.5) to test whether lower pH improves current density or coulombic efficiency, or whether the system can tolerate neutral pH for practical applications.

    The paper states 'The pH of the medium was initially set to 5–5.5 because the Feammox process works best under acidic conditions with pH below 6.3' but does not experimentally test pH as a variable. All experiments were conducted at the same pH range.

    Try. Pure A6 MECs at pH 4.5, 5.0, 5.5, 6.0, 6.5 with all other variables constant, measuring current density, coulombic efficiency, and A6 biomass over 3 weeks.

  • Medium No measurement of cathode potential or hydrogen production rate; only applied voltage between anode and cathode is reported. Cathode overpotential and H2 yield are not characterized.

    The paper describes the cathode as 'stainless steel mesh' and states 'H+ is reduced to form H2 at the cathode' but provides no data on cathode potential, H2 gas production, or cathode efficiency. The coulombic efficiency of 16.4% suggests significant electron loss, but it is unclear whether this is due to anode-side or cathode-side inefficiency.

    Try. Install reference electrode at cathode, measure cathode potential vs. SHE during operation, quantify H2 production via gas chromatography, calculate cathode coulombic efficiency and compare to anode-side electron balance.

  • Medium No test of continuous-flow or fed-batch operation; all experiments are batch with no substrate replenishment. Practical applicability to wastewater treatment requires flow-through performance data.

    The paper states 'Results presented here are a first step towards the development of a Feammox-bacteria-based system for anaerobic ammonium oxidation in bioelectrochemical reactors' and mentions 'long-term continuous-flow reactor applications' as motivation, but all experiments are closed-batch systems with initial NH4+ of 5 mM and no replenishment.

    Try. Continuous-flow MEC with pure A6 culture, inlet NH4+ 5 mM, hydraulic retention time (HRT) 1, 3, 7 days, measure steady-state current density, NH4+ removal efficiency, and NO2− production (with acetylene inhibition).

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What they did

System
MEC

What worked

Coulombic efficiency16.4%best of 3

Abstract

Schematic of a biotic MEC with Feammox bacterium Acidimicrobiaceae sp. A6 culture. NH 4 + is oxidized to NO 2 − at the anode and H + is reduced to form H 2 at the cathode.

Keywords

ElectrolysisCathodeAmmoniumAnodeInorganic chemistryNuclear chemistry

Identifiers

DOI
10.1039/c9ew00366e
Journal
Environmental Science Water Research & Technology
Year
2019
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Extraction

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