Research Progress in Photocatalytic-Coupled Microbial Electrochemical Technology in Wastewater Treatment

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

• High No quantitative operating windows reported for cathode potential, current density, or HRT in nitrogen cycling (denitrification, nitrification, or ammonia recovery). The paper cites one study (Chen et al. [22]) with COD/NO3− = 0.75 but does not systematically vary C/N ratio, applied voltage, or residence time to map denitrification performance.

  Section 2.1 and 3.1 mention nitrogen removal but provide only single-point data; no dose–response curves, threshold potentials, or HRT optimization for NO3− or NO2− reduction are presented.

  Try. Vary cathode potential (−0.4 to −1.0 V vs. Ag/AgCl), current density (0.5–5 A/m²), and HRT (4–24 h) in photoanode–biocathode system; measure NO3−-N, NO2−-N, and N2O accumulation at each condition.

• High N2O accumulation and selectivity toward N2 vs. N2O not discussed. No data on whether photocatalytic coupling suppresses or exacerbates N2O slip during denitrification.

  Section 2.1 mentions denitrification to 'nitrogen gas' but does not quantify N2O production or measure selectivity; this is critical for environmental compliance and greenhouse gas mitigation.

  Try. Measure N2O, NO, NO2, and N2 gas products via gas chromatography during denitrification in photoanode–biocathode system across applied potentials (−0.6 to −1.0 V); correlate with dissolved oxygen and pH.

• High Catholyte pH effects on nitrogen cycling not characterized. No data on how pH (6–8) influences denitrification rate, N2O selectivity, or ammonia recovery in photocatalytic-coupled systems.

  Section 3.1 discusses organic pollutant degradation under varying conditions but does not isolate pH as a variable for nitrogen species reduction or recovery.

  Try. Operate photoanode–biocathode system at pH 6.0, 7.0, and 8.0; measure NO3−-N removal rate, N2O/N2 ratio, and any NH4+ production; track dissolved oxygen and redox potential.

• Medium Dissolved oxygen (DO) threshold for switching between aerobic and anaerobic nitrogen pathways not defined. Wang et al. [73] showed aerobic conditions improve 2-CP degradation, but no DO setpoint or range is specified for nitrogen cycling.

  Section 3.1 states 'appropriate amount of oxygen' improved degradation but does not quantify DO concentration (mg/L or % saturation) or its effect on denitrification vs. nitrification.

  Try. Operate photocatalytic-coupled system at controlled DO levels (0, 1, 2, 4, 8 mg/L) and measure NO3−-N, NO2−-N, NH4+-N, and N2O production; identify DO threshold for denitrification vs. nitrification dominance.

• Medium Nitrite (NO2−) accumulation not reported in any nitrogen removal study. No data on whether photocatalytic coupling causes NO2− buildup or enables complete reduction to N2.

  Section 2.1 and 2.4 discuss nitrate and nitrite reduction but do not measure intermediate NO2− concentrations or report whether it accumulates under any operating condition.

  Try. Measure NO3−-N, NO2−-N, and N2O simultaneously during denitrification in photoanode–biocathode system; vary applied potential and current density to identify conditions favoring NO2− accumulation.

• Medium Ammonia recovery or nitrification not addressed in any photocatalytic-coupled system. All nitrogen studies focus on denitrification; no data on whether these systems can recover NH4+ or perform nitrification.

  Abstract and Section 3 mention 'nitrogen oxides' removal but do not explore ammonia-based nitrogen recovery pathways or nitrification in photocatalytic-coupled MES.

  Try. Test photocathode–bioanode system for ammonia recovery from NO3− or direct ammonia oxidation; measure NH4+-N production and selectivity under varying cathode potentials (−0.4 to −0.8 V) and pH (6–8).