MESSAI.iO

AI summary

72% confidence

This perspective reviews microbial electrochemical systems (BES) for bioremediation, emphasizing how electrode-driven electron transfer can overcome pollutant persistence caused by lack of suitable electron donors or acceptors. The paper surveys applications including denitrification, perchlorate reduction, reductive dechlorination, hydrocarbon oxidation, and PAH removal, highlighting that cathode potential controls intermediate formation (nitrite, N2O), energy consumption varies widely (0.34–19 kWh/kg NO₃⁻-N), and METland® wetland systems can operate under hybrid aerated/anoxic modes. Key advantages are precise control, no chemical residue, and low energy when designed properly; challenges include microbial knowledge gaps, electrode integration into subsurface settings, and dynamic control algorithms.

Key insights

  • Mechanism Cathode potential directly controls denitrification intermediates and biocathode stability; different potentials drive presence of undesirable nitrite and N2O.

    So what. Operators must carefully set and monitor cathode potential as a primary control knob; potential windows exist where intermediates accumulate—identify these for your target pollutant before deployment.

  • Trade-off Energy consumption for nitrate removal spans 100-fold range: in situ MFC 0.34 kWh/kg NO₃⁻-N vs. MEC 19 kWh/kg NO₃⁻-N, driven by configuration and external power input.

    So what. Choose MFC over MEC for nitrate if electron donor is available on-site; if MEC is necessary, expect 10–50× higher energy cost and justify by pollutant concentration or regulatory deadline.

  • Threshold Cathode potential window for TCE dechlorination is narrow: 0.26 V (vs. SHE) gave highest rate (1 mmol/L·d), but 0.06 V or 0.46–0.66 V reduced rates by 7–49%; at 0.45 V, >60% current diverted to methane generation.

    So what. For TCE, operate within ±0.2 V of 0.26 V (vs. SHE); outside this window, competing methanogenesis or other pathways consume current and reduce target removal—use potentiostat feedback control.

  • Mechanism METland® hybrid aerated/anoxic operation achieves both COD and nutrient removal with Geobacter outcompeting under mixed redox; nitrification occurs under aerated downflow conditions.

    So what. If your site has both organic and nitrogen pollution, test hybrid aerated/anoxic METland® design; nitrification in the aerated zone may enable coupled nitrification–denitrification without separate reactors.

  • Performance Specific power consumption for denitrifying MEC (6.8 × 10⁻³ kWh/g NO₃⁻-N) is ~30× lower than membrane bioreactor (2.04 × 10⁻² kWh/g NO₃⁻-N), but absolute energy cost remains high due to external power requirement.

    So what. MEC is energy-efficient per unit pollutant removed, but total cost depends on pollutant concentration; use for low-concentration groundwater where conventional methods require large volumes.

Opportunity areas6

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

  • High No systematic study of N2O accumulation vs. cathode potential during denitrification; paper mentions N2O as undesirable intermediate but provides no quantitative data on N2O yield, threshold potentials, or conditions favoring complete reduction to N2.

    Section 3.1 states 'cathode potential influences... the presence of undesirable intermediates (nitrite and nitrous oxide)' but provides no N2O data, only references to nitrite control in other studies.

    Try. Vary cathode potential from −0.2 to −0.8 V (vs. SHE) in denitrifying MEC; measure NO₃⁻, NO₂⁻, N2O, and N2 at each potential; identify N2O accumulation window and conditions for complete reduction.

  • High No investigation of C/N ratio or HRT effects on denitrification rate and intermediate formation in BES; paper mentions HRT as an operational parameter but provides no data on how HRT interacts with cathode potential or current density.

    Section 3.1 states 'Operational conditions including nitrate load, hydraulic retention time (HRT) and process configuration also impact the denitrification rate and the energy consumption' but no quantitative results are given.

    Try. Test HRT range 2–24 h at fixed cathode potential (e.g., −0.4 V vs. SHE); measure NO₃⁻ removal, NO₂⁻ and N2O accumulation, and current density; identify minimum HRT for complete denitrification without intermediate buildup.

  • Medium No data on catholyte pH effects on denitrification or nitrification in BES; METland® section mentions alkaline cathode effluent for water softening but does not quantify pH or its impact on nitrogen cycling.

    Section 3.7 mentions 'alkaline effluent generated in the cathode of MEC' but no pH values, pH range, or pH-dependent kinetics are provided for nitrogen removal.

    Try. Operate denitrifying MEC at fixed current density while monitoring and controlling catholyte pH (6.5–8.5); measure denitrification rate, NO₂⁻ and N2O accumulation, and microbial community composition as functions of pH.

  • Medium No comparison of ammonia recovery vs. denitrification in the same BES; paper focuses on denitrification and mentions ammonia recovery only in passing (autotrophic denitrifiers reduce carbon requirements) but provides no data on ammonia oxidation, recovery, or trade-offs.

    Section 4 states 'For groundwater remediation with low nitrate and COD concentrations, the autotrophic denitrifiers in the microbial electrochemical process reduce the carbon requirements' but no ammonia data or recovery yields are given.

    Try. Design a two-chamber MES with anode for ammonia oxidation and cathode for nitrate reduction; vary current density and measure ammonia oxidation rate, nitrate reduction rate, and N2O formation; assess feasibility of simultaneous ammonia recovery and denitrification.

  • Medium No systematic study of dissolved oxygen (DO) effects on denitrification or nitrification in BES; METland® operates under hybrid aerated/anoxic conditions but no DO profiles, oxygen penetration depth, or DO-dependent kinetics are reported.

    Section 3.7 states 'METland® has been recently proved to be effective even under down-flow aerated mode' and 'such aerobic conditions favoured nitrification' but no DO measurements or DO-dependent rate data are provided.

    Try. Operate METland® or similar system with controlled aeration; measure DO profile along flow path, nitrification rate, denitrification rate, and N2O formation as functions of aeration rate; identify optimal DO window for coupled nitrification–denitrification.

  • Medium No data on current density effects on denitrification rate, intermediate formation, or energy efficiency; paper mentions current as a control parameter but provides no quantitative relationship between current density and nitrogen removal kinetics.

    Section 3.1 states 'MECs showed higher denitrification rates due to the external energy input' but no current density values, rate vs. current relationships, or optimal current windows are given.

    Try. Operate denitrifying MEC at fixed cathode potential (e.g., −0.4 V vs. SHE) while varying current density (0.1–10 A/m²); measure NO₃⁻ removal rate, NO₂⁻ and N2O accumulation, and energy consumption; identify current density for maximum removal with minimal intermediates.

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Abstract

Lack of suitable electron donors or acceptors is in many cases the key reason for pollutants to persist in the environment. Externally supplementation of electron donors or acceptors is often difficult to control and/or involves chemical additions with limited lifespan, residue formation or other adverse side effects. Microbial electrochemistry has evolved very fast in the past years - this field relates to the study of electrochemical interactions between microorganisms and solid-state electron donors or acceptors. Current can be supplied in such so-called bioelectrochemical systems (BESs) at low voltage to provide or extract electrons in a very precise manner. A plethora of metabolisms can be linked to electrical current now, from metals reductions to denitrification and dechlorination. In this perspective, we provide an overview of the emerging applications of BES and derived technologies towards the bioremediation field and outline how this approach can be game changing.

Keywords

BioremediationElectrochemistryEnvironmental scienceEnvironmental chemistryBiochemical engineeringWaste managementChemistryEngineeringContaminationBiologyEcologyElectrode

Identifiers

DOI
10.1016/j.ese.2020.100013
Journal
Environmental Science and Ecotechnology
Year
2020
Extraction

Reported parameters

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