Effects of the Feeding Solution Composition on a Reductive/Oxidative Sequential Bioelectrochemical Process for Perchloroethylene Removal

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

• High No measurement of N₂O or NO₂⁻ accumulation during nitrate reduction at −450 mV SHE; the paper assumes complete reduction to N₂ but provides no gas chromatography or ion chromatography data for intermediate N species.

  Nitrate reduction is reported as 'complete' (Figure 6b shows inlet 20 mg/L → outlet ~0 mg/L) but the reduction product is never identified. At −450 mV SHE, incomplete reduction to N₂O or NO₂⁻ is possible and would affect coulombic efficiency calculation and environmental impact.

  Try. Measure N₂O, NO, and NO₂⁻ in reductive reactor headspace and effluent using GC-ECD and ion chromatography across −350 to −550 mV SHE cathode potential range; quantify N balance.

• High No systematic variation of cathode potential to identify the operating window where RD is favored over SO₄²⁻ reduction; only −450 mV SHE is tested with synthetic groundwater.

  The paper states prior work tested −550 mV SHE (reference 24) but does not compare RD vs SO₄²⁻ reduction selectivity across a potential range. At −450 mV SHE, SO₄²⁻ reduction dominates (45% CE); more negative potentials might suppress it or enhance RD.

  Try. Operate reductive reactor at −350, −400, −450, −500, −550 mV SHE with synthetic groundwater; measure RD rate, SO₄²⁻ removal rate, and D. mccartyi abundance as functions of potential.

• Medium No investigation of ionic strength or trace nutrient supplementation to restore D. mccartyi abundance without changing the groundwater composition; the paper identifies low ionic strength (0.83 mS/cm) and missing nutrients as the cause but does not test remediation.

  The authors conclude 'the decrease of the RD rate was probably related to the different composition of the feeding solution, both in terms of nutrients and ionic strength' but do not experimentally validate this by adding back NaCl or trace metals to synthetic groundwater.

  Try. Prepare three synthetic groundwater variants: (1) baseline (0.83 mS/cm), (2) + NaCl to 2.68 mS/cm, (3) + trace metals (Fe, Ni, Co, Mo); measure PCE removal rate, D. mccartyi 16S rRNA, and RD coulombic efficiency.

• Medium No characterization of the unrecovered current (25 mA or 39% of total during synthetic groundwater period); the paper acknowledges it but does not identify the electron sink.

  Text states 'the unrecovered current, i.e., the current that was not justified by the identified reduction reactions, reached a similar value' and attributes it to 'electron loops of species such as hydrogen' but provides no direct measurement of H₂ production or oxidation rates.

  Try. Measure H₂ concentration in reductive reactor headspace and effluent using GC-TCD; calculate H₂ production rate and compare to unrecovered current; test with and without internal counter-electrode.

• Medium No measurement of catholyte pH or buffering capacity during synthetic groundwater operation; pH can affect both RD kinetics and SO₄²⁻ reduction product speciation (H₂S vs HS⁻).

  The paper does not report pH in either feeding solution or reactor effluent. Synthetic groundwater may have lower buffering capacity than mineral medium, affecting microbial metabolism and product distribution.

  Try. Monitor catholyte pH continuously during synthetic groundwater operation; measure H₂S and HS⁻ speciation in effluent; test pH buffering with added bicarbonate.

• Low Oxidative reactor developed unintended RD capacity (TCE removal, cisDCE accumulation) but no systematic study of how to suppress it or exploit it; the paper treats it as an artifact.

  The authors note 'the presence of D. mccartyi in the oxidative reactor was likely due to the presence of the internal graphite counter electrode' and show 86±3% TCE removal, but do not test whether this is beneficial (removes incomplete RD products) or detrimental (wastes oxidative capacity).

  Try. Compare oxidative reactor performance with and without internal counter-electrode; measure VC and Eth removal efficiency as a function of counter-electrode potential (−0.5 to +0.2 V SHE).