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This study demonstrates that a tartrate-EDTA-Fe(III/II) electron mediator (EM) in the catholyte of a bioelectrochemical ammonia recovery system dramatically improves performance by providing a highly soluble oxidizing equivalent (50 mM) instead of relying on dissolved oxygen (~7 mg/L). At pH 10 and RC-A = 12, the EM system achieved 6.9 ± 0.2 g NH4+-N m−2 d−1 recovery—3.8× higher than the non-EM control—while enabling operation at RC-A = 0.5 with comparable performance to prior work using RC-A = 288. The tartrate stabilizes the iron complex under alkaline conditions, and the midpoint potential (−0.075 V vs. SHE at pH 10) enables efficient cathode reduction with minimal overpotential.

Key insights

  • Threshold Tartrate-EDTA-Fe complex requires a tartrate:EDTA-Fe molar ratio ≥0.33 to prevent iron precipitation; at 1:1 ratio, total dissolved Fe remained stable at ~50 mM over 140 h at pH 10, whereas at 0.2:1 it dropped to 42.64 ± 0.45 mM.

    So what. Operators must maintain tartrate:EDTA-Fe ≥1:1 molar ratio in alkaline catholytes to prevent loss of redox capacity and maintain stable mediator concentration.

  • Mechanism The EM system (SEM-1A) achieved 1.20 ± 0.02 A m⁻² stable current density, 36% higher than the oxygen-limited non-EM system with dual aeration (SNonEM-2A: 0.883 ± 0.004 A m⁻²), despite both having sufficient electron acceptor supply.

    So what. Tartrate-EDTA-Fe(III) reduction kinetics are superior to oxygen reduction on unmodified carbon electrodes; use EM when catalyst-free electrodes are preferred or when kinetic advantage is needed.

  • Trade-off Oxidizing equivalent supply rate in SEM-1A (579 C h⁻¹) was 50× higher than SNonEM-1A (10.28 C h⁻¹), yet consumption was only 41.4 C h⁻¹, indicating RC-A can be reduced from 12 to 0.5 while maintaining comparable NH4+-N recovery (6.9 vs. prior work at 288).

    So what. Reduce catholyte recirculation rate dramatically (24-fold lower) when using EM, cutting pumping energy and stripping gas flow while maintaining recovery rate—critical for scale-up economics.

  • Mechanism NH4+-N recovery efficiency (removed vs. recovered) was 85–91% across all modes; the gap was attributed to microbial assimilation at the bioanode rather than stripping losses.

    So what. Design anode chambers to minimize heterotrophic assimilation (e.g., via selective enrichment or anode potential control) to close the 9–15% gap and improve overall N recovery.

  • Threshold At RC-A = 0.5, SEM-1A current density dropped to ~0.6 A m⁻² and NH4+-N recovery rate fell to ~3.5 g N m⁻² d⁻¹, indicating a lower operating window threshold below which mediator regeneration becomes rate-limiting.

    So what. Operators should not reduce RC-A below 0.5–1.0 without accepting proportional loss in recovery rate; balance recirculation cost against performance target.

Opportunity areas6

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

  • High No investigation of N2O or NO2− accumulation during operation; nitrogen speciation in anode and cathode chambers not reported.

    Paper focuses on ammonia recovery but does not measure whether denitrification or nitrification intermediates form, which is critical for understanding N-balance and potential emissions.

    Try. Measure NO3−, NO2−, N2O, and N2 in anode and cathode chambers across RC-A range 0.5–12 and current densities 0.6–1.2 A m⁻² using ion chromatography and gas chromatography.

  • Medium Cathode potential was not systematically varied; only one cathode potential (−0.2 V vs. SHE) used in chronoamperometry test.

    Section 3.1 shows CV redox peaks at −0.157 V (reduction) and −0.007 V (oxidation); operating window and trade-offs between potential and current/recovery rate not explored.

    Try. Vary cathode potential from −0.3 to +0.1 V vs. SHE at fixed RC-A and measure current density, NH4+-N recovery rate, and mediator oxidation state to map operating window.

  • Medium Dissolved oxygen concentration in catholyte not measured during steady-state operation in SEM-1A; only reported for SNonEM-1A (~7.1 mg/L).

    Section 3.2.1 states DO was 'saturated (~7.1 mg L⁻¹)' in SNonEM-1A but no DO data given for SEM-1A, making it unclear whether residual O2 contributes to mediator regeneration.

    Try. Measure DO continuously in SEM-1A catholyte outlet and in stripping column inlet/outlet across RC-A 0.5–12 to quantify O2 contribution to Fe(II) reoxidation.

  • Medium No variation of EM concentration; only 50 mM tartrate-EDTA-Fe tested in main experiments.

    Section 3.1 tested tartrate:EDTA-Fe ratios but not absolute EM concentration; optimal concentration for cost–performance trade-off not determined.

    Try. Test 10, 25, 50, 100 mM tartrate-EDTA-Fe at RC-A 1–12 and measure NH4+-N recovery rate, current density, and mediator stability to identify concentration threshold.

  • Medium Anode chamber microbial community and acetate oxidation kinetics not characterized; 9–15% gap between removed and recovered NH4+-N attributed to assimilation but not verified.

    Section 3.2.2 states TAN remained stable and gap was 'likely due to microbial assimilation' but no biomass, enzyme activity, or 13C-acetate tracing provided.

    Try. Measure anode biofilm biomass, acetate consumption rate, and heterotrophic growth rate; use 13C-labeled acetate to quantify assimilation vs. oxidation.

  • Low No long-term stability test (>7 days) of tartrate-EDTA-Fe in operating BES; only 140 h static stability and 67 h chronoamperometry shown.

    Section 3.1 and Fig. S2 show short-term stability but operational stability over weeks/months not demonstrated; iron leakage into anode chamber via CEM not measured.

    Try. Operate SEM-1A for 30 days at RC-A 1–6; measure dissolved Fe in anode chamber weekly and track current density and recovery rate to assess mediator loss and CEM permeability.

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What they did

No methodology extracted yet.

What worked

Current density1.20A/m²best of 3

Abstract

Traditional bioelectrochemical systems (BESs) coupled with stripping units for ammonia recovery suffer from an insufficient supply of electron acceptors due to the low solubility of oxygen. In this study, we proposed a novel strategy to efficiently transport the oxidizing equivalent provided at the stripping unit to the cathode by introducing a highly soluble electron mediator (EM) into the catholyte. To validate this strategy, we developed a new kind of iron complex system (tartrate-EDTA-Fe) as the EM. EDTA-Fe contributed to the redox property with a midpoint potential of -0.075 V (vs. standard hydrogen electrode, SHE) at pH 10, whereas tartrate acted as a stabilizer to avoid iron precipitation under alkaline conditions. At a ratio of the catholyte recirculation rate to the anolyte flow rate (R C-A ) of 12, the NH 4 + -N recovery rate in the system with 50 mM tartrate-EDTA-Fe complex reached 6.9 ± 0.2 g N m -2 d -1 , approximately 3.8 times higher than that in the non-EM control. With the help of the complex, our system showed an NH 4 + -N recovery performance comparable to that previously reported but with an extremely low R C-A (0.5 vs. 288). The strategy proposed here may guide the future of ammonia recovery BES scale-up because the introduction of an EM allows aeration to be performed only at the stripping unit instead of at every cathode, which is beneficial for the system design due to its simplicity and reliability.

Keywords

Stripping (fiber)AmmoniaOxidizing agentTartrateCathodeInorganic chemistry

Identifiers

DOI
10.1016/j.ese.2022.100186
PubMed
36158760
Journal
Environmental Science and Ecotechnology
Year
2022
Extraction

Reported parameters

26 extracted values

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No 3D model is mapped to this paper yet. Parameter ranges above still place reported values on the literature distribution.