MESSAI.iO

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This paper evaluates surface-modified SS304L stainless steel electrodes for microbial electrochemical sensors (MES) applied to water quality monitoring. Four modification techniques—electrochemical reduction, carbon powder coating, flame oxidation, and graphene coating—were tested using synthetic pollutants (4-nitrophenol and potassium nitrate). Carbon-coated and graphene-coated anodes showed superior electron transfer efficiency and biocompatibility, with graphene achieving R² = 0.8785 for potassium nitrate detection. The work demonstrates that carbon-based surface modifications enhance MES performance by increasing surface carbon content (48.4–78.6%) and reducing iron exposure, though thick biofilms on carbon-coated electrodes increased internal resistance.

Key insights

  • Trade-off Flame oxidation increased oxygen content from 2.4% to 19.9%, forming iron oxide layers that protect against corrosion and support microbial attachment without promoting thick biofilm growth, resulting in lower internal resistance (32.7 Ω) than carbon coatings.

    So what. If internal resistance is a bottleneck, flame oxidation offers a middle ground between biocompatibility and conductivity; prioritize it over carbon coating when electrode fouling is a concern.

  • Trade-off Carbon-coated and graphene-coated electrodes showed the highest current density response (8.0 and 7.2 A/m² respectively at >0.2 V scan potential) but also the highest internal resistance (51.2 and 41.6 Ω), indicating thick, dense biofilm and extracellular polymer matrix formation that impedes electron transfer.

    So what. Carbon coatings maximize initial current but risk biofilm-induced resistance buildup; monitor impedance over time and consider periodic biofilm disruption or lower-resistance alternatives if long-term stability is critical.

  • Threshold Potassium nitrate (NO₃⁻) caused initial current density decline followed by gradual recovery during stepwise addition (100 mL/h for 6 h), but sudden addition of 1 L caused sharp decline across all electrodes, indicating nitrate acts as a competing electron acceptor and becomes toxic at high concentration.

    So what. Operate MES at low to moderate nitrate levels (<100 mg/L stepwise) to avoid metabolic inhibition; if high-concentration nitrate removal is needed, expect transient current loss and allow recovery time.

  • Trade-off Electrochemical reduction produced the lowest internal resistance (17.5 Ω) due to thin, less-dense biofilm formation, but also the lowest initial current density (3.0 A/m²), reflecting poor biocompatibility compared to carbon-based materials.

    So what. Electrochemical reduction is suitable only if low impedance and fast response are prioritized over high current output; not recommended for pollutant detection requiring high sensitivity.

  • Threshold 4-Nitrophenol (4-NP) at low stepwise concentrations (100 mL/h, 1 g/L) stimulated current density increase, but sudden addition of 1 L caused sharp decline, indicating microbial adaptation at low dose but toxicity inhibition at high dose (>150 mg/L equivalent).

    So what. For 4-NP detection, use stepwise dosing protocols to avoid toxicity-induced signal loss; if sudden high-concentration exposure occurs, expect 24–48 h recovery time before resuming normal operation.

Contradictions1

Tensions the paper itself points to. Each side quotes the passage it comes from.

  • Against prior work Potassium nitrate current response contradicts prior literature expectation.

    One side

    Stepwise addition of potassium nitrate (100 mL/h, 1 g/L for 6 h) caused initial current density decline followed by gradual recovery.

    “Ianya menunjukkan penurunan awal dalam ketumpatan arus diikuti dengan pemulihan secara beransur-ansur, menunjukkan penyesuaian mikrob kepada pencemar. Ini bercanggah dengan kajian lepas di mana ketumpatan arus menaik dengan penambahan 25 mg”
    Other side

    Prior work showed current density should increase with potassium nitrate addition at 25 mg/L.

    “Ini bercanggah dengan kajian lepas di mana ketumpatan arus menaik dengan penambahan 25 mg/L kalium nitrat (Gokhale et al. 2015).”

    Authors’ reading. Authors explain that nitrate acts as a competing electron acceptor in anaerobic conditions; microbes preferentially use nitrate over the anode because nitrate reduction is more energy-favorable. Initial decline reflects metabolic shift; recovery occurs as microbes re-establish anode-based electron transfer. Difference from prior work may reflect higher initial nitrate concentration (1 g/L vs 25 mg/L) or different microbial community composition.

Opportunity areas6

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

  • High No systematic variation of applied potential (voltage) across the MES; only 0.5 V was tested. Cathode and anode potentials relative to Ag/AgCl reference were not independently controlled or reported.

    Cyclic voltammetry (Figure 5) shows oxidation onset varies by electrode type (-0.4 V to -0.2 V), but the fixed 0.5 V operating point may not be optimal for all modifications. Potentiostatic control could reveal optimal operating windows and trade-offs between current density and internal resistance.

    Try. Apply potentials from -0.6 V to +0.2 V (anode) and -0.8 V to -0.2 V (cathode) vs Ag/AgCl; measure current density, internal resistance, and pollutant removal rate for each electrode type. Identify voltage windows that maximize current while minimizing biofilm resistance.

  • High No measurement of dissolved oxygen (DO) in the catholyte or anolyte; no control of anode/cathode oxygen exposure or pH.

    Nitrogen cycling (denitrification, nitrification) and nitrite/N2O accumulation depend critically on DO and pH. The paper tests potassium nitrate but does not report whether denitrification, nitrification, or N2O production occurred. Catholyte pH and anode DO are not mentioned.

    Try. Measure DO (0–100% saturation), pH (6–8), and nitrogen species (NO₃⁻, NO₂⁻, NH₄⁺, N₂O) via IC and gas chromatography during potassium nitrate tests. Vary anode DO (0–100%) and catholyte pH (6–8) to map operating windows for complete denitrification vs N2O accumulation.

  • High Hydraulic retention time (HRT) and carbon-to-nitrogen ratio (C/N) not explicitly controlled or varied. Nutrient medium composition fixed at 0.250 M sodium acetate and 0.125 M ammonium chloride.

    C/N ratio and HRT are critical for nitrogen removal pathways and microbial community selection. The paper does not report the molar C/N ratio or test how changes in acetate:ammonia ratio affect nitrate reduction or N2O production.

    Try. Vary C/N ratio from 2:1 to 20:1 (acetate:ammonia) and HRT from 6 h to 48 h. Measure current density, nitrogen species removal, and N2O/NO₂⁻ accumulation. Identify C/N and HRT windows that maximize denitrification efficiency and minimize N2O slip.

  • Medium No long-term stability or biofilm characterization; experiments appear to run for ~27 h total. No microbial community analysis (16S rRNA, metagenomics) or biofilm thickness measurement.

    Internal resistance varies widely by electrode type (17.5–51.2 Ω), suggesting different biofilm properties, but biofilm composition, thickness, and temporal evolution are not characterized. Long-term performance (weeks to months) is unknown.

    Try. Run each electrode modification for 30–90 days with weekly sampling. Measure biofilm thickness (SEM cross-section), microbial community composition (16S rRNA), and internal resistance over time. Correlate biofilm properties with current density and pollutant removal rate.

  • High Nitrite (NO₂⁻) accumulation not measured or reported. Only potassium nitrate (NO₃⁻) and 4-nitrophenol tested; no direct measurement of nitrogen species transformation.

    Potassium nitrate tests show current density changes but do not confirm whether NO₃⁻ was reduced to NO₂⁻, N₂O, or N₂. Nitrite accumulation is a common problem in bioelectrochemical denitrification and indicates incomplete reduction.

    Try. During potassium nitrate tests, measure NO₃⁻, NO₂⁻, NH₄⁺, and N₂O concentrations via IC and gas chromatography at 2–4 h intervals. Quantify nitrogen mass balance and identify which electrode modification minimizes NO₂⁻ accumulation and maximizes complete denitrification.

  • Medium No comparison of MES performance to conventional denitrification or nitrification systems; no energy efficiency or cost-benefit analysis.

    The paper claims MES advantages (low cost, portability, self-powered) but does not quantify energy recovery, compare to activated sludge or biofilm reactors, or estimate capital/operating costs.

    Try. Measure power output (W/m² electrode area) during potassium nitrate and 4-NP tests. Calculate energy recovery per kg pollutant removed. Compare to conventional treatment energy demand and cost per unit removal.

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Abstract

Microbial Electrochemical Sensor (MES) is classified as a self-powered electrochemical biosensor and utilising biofilm electrode used as a sensor. Conventional techniques such as ion chromatography (IC), highperformance liquid chromatography (HPLC), and chemical oxygen demand (COD) require large, expensive, and non-portable equipment. Therefore, MES has gained significant attention due to its advantages, including low cost, easy operation, and portability. Stainless steel (SS404L) is widely used in various applications but exhibits lower biocompatibility compared to carbon-based materials when used as an anode. This MES is constructed using modified electrodes through techniques such as carbon coating, electrochemical reduction, flame oxidation, and graphene coating. A nutrient medium stock is periodically supplied daily to enrich the electroactive microbes and maintain the biosensor’s performance at a stable level. Samples are collected and analyzed during pollutant testing to determine pH, conductivity, IC, and COD values. These parameter values are correlated with MES signal data to assess each biosensor’s performance in terms of reaction rate. FESEM-EDX and RAMAN analysis indicated an increase in O content from 2.4% to 19.9% for flame oxidation. For carbon and graphene coatings, carbon content increased from 4.1-9.1% to 48.4-78.6%. The carbon coating exhibited an R² value of 0.7023 for tests using 4-nitrophenol. Meanwhile, for potassium nitrate, the graphene coating demonstrated the highest R² value of 0.8785. A higher R² value indicates a stronger correlation and suggests better performance in electrode modification within MES. This highlights the significance of MES results in water quality monitoring, which can be understood and applied more effectively.

Keywords

Aerospace engineering

Identifiers

DOI
10.17576/jkukm-2024-36(6)-13
Journal
Jurnal Kejuruteraan
Year
2024
Extraction

Reported parameters

3 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.