Overview
Shi and colleagues tested whether cathode aeration improves Cr(VI) reduction in a lab-scale Leersia hexandra constructed wetland-microbial fuel cell (CW-MFC). The source is primary remediation evidence for synthetic chromium wastewater, not food or ingredient occurrence evidence. The best-performing condition was 4.5 mg/L dissolved oxygen (DO), where the system reached 520 mV, 93.73% COD removal, and 97.77% Cr(VI) removal while shifting chromium toward Cr(III) and residual/bound forms in the substrate and plant tissues.
Key numbers
Reactor setup and wastewater
The study built five up-flow CW-MFC devices with gravel, ceramsite, activated-carbon anode/cathode layers, stainless-steel mesh collectors, L. hexandra plants, and anaerobic-sludge inoculum. The synthetic wastewater contained 300 mg/L COD, 80 mg/L Cr(VI), 100 mg/L total nitrogen, 5 mg/L total phosphorus, and pH 6.5-8.0.
Aeration rates of 0, 2, 4, 6, and 8 L/min produced average cathode DO concentrations of 3.0, 3.5, 4.0, 4.5, and 5.0 mg/L. The system was run until water quality and voltage stabilized; the full study lasted about 7 months.
Removal and electricity generation
From 3.0 to 4.5 mg/L DO, COD removal increased from 81.56% to 93.73%, and Cr(VI) removal increased from 84.82% to 97.77%. The maximum voltage was 520 mV at 4.5 mg/L DO. The authors report that pollutant removal and electricity generation declined when DO increased further to 5.0 mg/L.
Substrate chromium
Table 1 reports total chromium (Cr), Cr(VI), and Cr(III) in the anode and cathode substrate in mg/kg.
| DO (mg/L) | Total Cr anode | Total Cr cathode | Cr(VI) anode | Cr(VI) cathode | Cr(III) anode | Cr(III) cathode |
|---|---|---|---|---|---|---|
| 3.0 | 7988.54 | 8386.46 | 572.50 | 448.33 | 7416.04 | 7938.13 |
| 3.5 | 8295.83 | 8517.71 | 487.50 | 416.25 | 7808.33 | 8101.46 |
| 4.0 | 9045.83 | 9525.00 | 453.75 | 246.25 | 8592.08 | 9278.75 |
| 4.5 | 14945.83 | 16350.00 | 357.92 | 204.17 | 14587.92 | 16145.83 |
| 5.0 | 10132.29 | 12778.13 | 422.08 | 241.67 | 9710.21 | 12536.46 |
The highest total substrate Cr was 31,295.83 mg/kg at 4.5 mg/L DO, split between 14,945.83 mg/kg in the anode and 16,350.00 mg/kg in the cathode. The authors report that the cathode had higher Cr(III) than the anode under each DO condition and interpret the 4.5 mg/L condition as promoting Cr(VI) reduction.
At 4.5 mg/L DO, BCR-extracted substrate Cr forms also peaked. The anode values were residual 6447.01 mg/kg, oxidizable 3795.45 mg/kg, reducible 2385.42 mg/kg, and weak-acid extractable 2304.21 mg/kg. The cathode values were residual 6668.86 mg/kg, oxidizable 4301.67 mg/kg, reducible 2786.99 mg/kg, and weak-acid extractable 2230.58 mg/kg. The authors summarize the effective Cr state at 31% and the stable state at 69%.
Plant chromium
Table 2 reports total Cr, Cr(VI), and Cr(III) in L. hexandra roots, stems, and leaves in mg/kg.
| DO (mg/L) | Total root | Total stem | Total leaf | Cr(VI) root | Cr(VI) stem | Cr(VI) leaf | Cr(III) root | Cr(III) stem | Cr(III) leaf |
|---|---|---|---|---|---|---|---|---|---|
| 3.0 | 8428.13 | 1428.13 | 1191.67 | 1927.08 | 635.42 | 483.33 | 6501.04 | 792.71 | 708.33 |
| 3.5 | 9913.54 | 1851.04 | 1365.63 | 1616.67 | 495.83 | 472.92 | 8296.88 | 1355.21 | 892.71 |
| 4.0 | 13070.83 | 2402.08 | 1913.54 | 1104.17 | 481.25 | 450.00 | 11966.67 | 1920.83 | 1463.54 |
| 4.5 | 18869.79 | 3642.71 | 3070.83 | 641.67 | 418.75 | 385.42 | 18228.13 | 3223.96 | 2685.42 |
| 5.0 | 14313.54 | 2588.54 | 2187.50 | 1081.25 | 441.67 | 414.58 | 13232.29 | 2146.88 | 1772.92 |
At 4.5 mg/L DO, total Cr peaked in roots (18,869.79 mg/kg), stems (3642.71 mg/kg), and leaves (3070.83 mg/kg). Root Cr was much higher than stem or leaf Cr across conditions. The same condition also produced the highest reported Cr(III) values: roots 18,228.13 mg/kg, stems 3223.96 mg/kg, and leaves 2685.42 mg/kg.
Plant chemical-form extraction showed maximum root values at 4.5 mg/L DO: residual 10,273.44 mg/kg, HCl-extractable 2626.25 mg/kg, H2O-extractable 1577.75 mg/kg, ethanol-extractable 1344.31 mg/kg, HAc-extractable 1771.19 mg/kg, and NaCl-extractable 1296.69 mg/kg. The residual form accounted for 55% of root Cr. Stem forms also peaked at 4.5 mg/L DO: residual 2024.06 mg/kg, HCl-extractable 480.00 mg/kg, H2O-extractable 350.69 mg/kg, ethanol-extractable 220.13 mg/kg, HAc-extractable 311.25 mg/kg, and NaCl-extractable 324.25 mg/kg. In leaves, residual Cr peaked at 1303.75 mg/kg and accounted for 43% of leaf Cr at 4.5 mg/L DO.
Microbial community
Aeration increased microbial diversity: the no-aeration sample (A1) had Shannon 3.73, Chao 514.44, Ace 499.83, Simpson 0.04, and Shannoneven 0.60; the aerated sample (A2) had Shannon 4.77, Chao 515.52, Ace 515.37, Simpson 0.02, and Shannoneven 0.77.
At phylum level, the system mainly included Proteobacteria, Chloroflexi, Bacteroidetes, Firmicutes, Synergistetes, Planctomycetes, and Acidobacteria. At genus level, the authors report Geobacter at 2.09% without aeration and 3.66% with aeration, and they identify Geobacter enrichment as a likely contributor to electricity generation and Cr(VI) reduction.
Methods (brief)
The authors operated five lab CW-MFC reactors under different aeration rates and used outlet water samples to calculate COD and Cr(VI) removal. DO was measured in situ. COD used the dichromate method, and aqueous Cr(VI) used diphenylcarbohydrazide spectrophotometry. Substrate samples were collected by five-point sampling. Plant roots, stems, and leaves were dried, ground, and sieved. Cr(VI) was extracted by alkali digestion; total Cr was extracted by acid digestion and measured by flame atomic absorption spectrometry. Plant Cr chemical forms used sequential extraction with ethanol, water, NaCl, acetic acid, HCl, and residual digestion. Substrate Cr forms used an improved BCR sequential extraction. Microbial communities were characterized with 16S rRNA high-throughput sequencing. Plant standard material and blanks were used for quality control, with recoveries between 90% and 110%.
Implications
Certification: This source does not support any food, ingredient, or product occurrence claim. It is relevant to chromium speciation discipline and remediation context: total Cr capture alone is not the endpoint; the paper tracks Cr(VI) reduction toward lower-toxicity Cr(III) and residual/bound fractions.
Courses: Useful case study for coupled chemical speciation, plant uptake, and microbial redox remediation. It also shows why high removal percentages must be read beside residual Cr burdens in substrate and plant tissue.
App: Route as Cr(VI) wastewater remediation context. Do not apply the 80 mg/L synthetic influent, substrate mg/kg, or plant-tissue mg/kg values to consumer water, foods, or crop benchmark pools.
Related evidence
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