Differences in Contaminants Removal Efficiency and Electricity Production in Disposing Leachate with Chemical-Cathode and Aerobic Bio-Cathode MFC

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

• High No measurement of N₂O or NO production in either cathode chamber; only NO₃⁻–N and NO₂⁻–N tracked. Given that ABMFC is aerobic, denitrification intermediates (N₂O, NO) may be produced and lost to gas phase.

  Fig. 6 shows NO₃⁻–N accumulation but NO₂⁻–N 'hardly changed' (text); nitrogen mass balance is incomplete. ABMFC cathode is aerobic, so denitrification is unlikely, but nitrification and subsequent incomplete reduction could produce N₂O.

  Try. Measure headspace gas composition (N₂O, NO, N₂) in both cathode chambers over 45 days; use isotope tracing (¹⁵N) to quantify N₂O yield from ammonia oxidation.

• High No variation of cathode potential or anode potential; both systems operated at fixed external resistance (1500 Ω) with no potentiostatic control. Operating window for nitrogen speciation (NH₃ vs NO₃⁻ vs N₂O) unknown.

  Text states 'MFCs were operated at atmospheric pressure and room temperature' with fixed 1500 Ω load; no mention of potential control. Cathode potential in ABMFC likely drifted with biofilm growth (internal resistance increased from 600 to 700 Ω).

  Try. Operate ABMFC at fixed cathode potentials (−0.2, 0, +0.2, +0.4 V vs SHE) and measure NO₃⁻–N, NO₂⁻–N, N₂O, and ammonia removal at each potential; identify threshold for denitrification vs nitrification.

• High No measurement of dissolved oxygen in ABMFC cathode chamber; oxygen availability and mass transfer rate not quantified. Cathode biofilm thickness and oxygen penetration depth unknown.

  Text states 'The dissolved oxygen in ABMFC cathode chamber was from air' but no DO measurements reported. Biofilm accumulation on cathode (mentioned as cause of performance decline) would limit oxygen diffusion and shift cathode chemistry.

  Try. Monitor dissolved oxygen (DO) in ABMFC cathode chamber continuously; measure biofilm thickness and oxygen penetration depth via microelectrode profiling; correlate DO depletion with NO₃⁻–N accumulation.

• Medium No variation of HRT (hydraulic retention time) or feed rate; both systems operated in batch mode with periodic substrate addition. Nitrogen removal kinetics and steady-state accumulation unknown.

  Text states 'regularly supplying the ferricyanide or carbon source in cathode' but no details on feed schedule or HRT. 45-day batch operation does not reveal whether nitrogen accumulation is kinetic (incomplete removal) or equilibrium (saturation).

  Try. Operate both systems in semi-continuous mode at HRTs of 5, 10, 20 days; measure NO₃⁻–N accumulation rate and steady-state concentration as function of HRT.

• Medium No measurement of catholyte pH in either system; pH is critical for ammonia speciation (NH₃ vs NH₄⁺) and nitrification kinetics. ABMFC cathode pH likely increased due to oxygen reduction (OH⁻ production).

  Text does not report pH in cathode or anode chambers during operation. Ammonia diffusion (Fig. 5) depends on pH gradient; nitrification in ABMFC cathode (if occurring) would consume H⁺ and raise pH.

  Try. Monitor pH in both chambers continuously; measure ammonia speciation (NH₃ vs NH₄⁺) at different pH values; quantify nitrification rate in ABMFC cathode as function of pH.

• Medium No analysis of microbial community composition in anode or cathode biofilms; mechanism of nitrogen removal (heterotrophic nitrification, denitrification, anammox) not identified.

  Text mentions 'microbial communities are phylogenetically diverse' and references Geobacter enrichment but provides no 16S rRNA or metagenomic data for this study. Nitrogen cycling pathway (abiotic vs biotic) unclear.

  Try. Perform 16S rRNA gene sequencing and metagenomics on anode and cathode biofilms at days 0, 15, 30, 45; identify nitrifiers, denitrifiers, and other nitrogen-cycling taxa; correlate community composition with NO₃⁻–N and NO₂⁻–N dynamics.