Optimizing total ammonia–nitrogen concentration for enhanced microbial fuel cell performance in landfill leachate treatment: a bibliometric analysis and future directions

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

• High No data on cathode potential, anode potential, or applied voltage ranges tested in the 18 ammonia-inhibition studies; critical for designing bioelectrochemical nitrogen recovery.

  The paper identifies ammonia inhibition as the main performance-limiting factor but does not extract or synthesize operating potentials from the reviewed literature. Cathode potential controls denitrification vs. nitrite accumulation; anode potential affects proton generation and microbial community composition.

  Try. Systematic variation of cathode potential (−0.4 to −0.8 V vs. Ag/AgCl) and anode potential (−0.2 to +0.3 V) at fixed TAN 400–800 mg L⁻¹, measuring power density, ammonia removal rate, and N₂O/NO₂⁻ accumulation.

• High No quantitative data on N₂O or nitrite (NO₂⁻) accumulation in any of the 18 reviewed studies; critical for assessing environmental impact and nitrogen recovery efficiency.

  The keyword co-occurrence map (Fig. 5A) identifies 'nitrite' as a keyword in the blue cluster (bioelectricity generation), but the paper provides no values, thresholds, or operating conditions that minimize N₂O or NO₂⁻ formation.

  Try. Measure N₂O and NO₂⁻ concentrations in cathode and anode effluent across TAN 400–800 mg L⁻¹, cathode potentials −0.4 to −0.8 V, and HRT 12–48 h; quantify nitrogen recovery as NH₃ or NH₄⁺ vs. loss as N₂O.

• High No data on hydraulic retention time (HRT) or C/N ratio optimization; both are mentioned as keywords but not synthesized with ammonia inhibition outcomes.

  The keyword map shows 'HRT' in the green cluster (mechanism research), but the paper does not extract or compare HRT values (e.g., 12 h vs. 48 h) or C/N ratios tested in the 18 ammonia studies.

  Try. Test HRT 12, 24, 36, 48 h and C/N ratio 5:1, 10:1, 20:1 at fixed TAN 400–800 mg L⁻¹ and cathode potential −0.6 V; measure ammonia removal, power density, and denitrification efficiency.

• High No data on catholyte pH or pH buffering strategies to mitigate ammonia inhibition via proton depletion; mechanism is identified but not operationalized.

  The paper states 'NH₃ can act as a proton acceptor, reducing the availability of protons in the anode compartment' but does not extract pH ranges, buffer types, or pH control strategies from the 18 reviewed studies.

  Try. Maintain catholyte pH at 6.5, 7.0, 7.5, 8.0 using phosphate or bicarbonate buffer at fixed TAN 600 mg L⁻¹ and cathode potential −0.6 V; measure ammonia removal rate, proton flux, and power density.

• Medium No comparison of ammonia removal mechanisms (biosorption, volatilization, electrochemical oxidation, nitrification–denitrification) across the 18 studies; unclear which pathway dominates under different potentials.

  The paper identifies ammonia inhibition and mentions 'denitrification' as a keyword but does not synthesize which removal pathways are active at different cathode/anode potentials or how to favor recovery (NH₃ stripping) over oxidation or denitrification.

  Try. Use ¹⁵N-labeled NH₄⁺ tracer to quantify fate (N₂O, N₂, NO₃⁻, NH₃ gas, biosorbed) at cathode potentials −0.4, −0.6, −0.8 V and anode potentials −0.2, 0, +0.2 V; identify potential windows for selective recovery.

• Medium No data on microbial community composition or functional gene expression (nir, nor, nos, amoA) in relation to ammonia inhibition or nitrogen cycling; mechanism remains phenomenological.

  The keyword map shows 'microbial community' in the green cluster, but the paper does not extract 16S rRNA, metagenomics, or transcriptomics data linking community structure to ammonia tolerance or nitrogen removal pathways.

  Try. Profile anode and cathode biofilm 16S rRNA and functional genes (nir, nor, nos, amoA, hao) at TAN 400, 600, 800 mg L⁻¹ and cathode potentials −0.4, −0.6, −0.8 V; correlate community shifts with ammonia removal and N₂O production.