Insight into the performance discrepancy of GAC and CAC as air-cathode materials in constructed wetland-microbial fuel cell system

Constructed wetland-microbial fuel cell (CW-MFC) has exhibited the performance discrepancy between using granular activated carbon (GAC) and columnar activated carbon (CAC) as air-cathode materials. No doubt, this is linked with electrochemical performance and decontaminants characteristics in the CW-MFC system. To provide insight into this performance discrepancy, three CW-MFCs were designed with different carbon-material to construct varied shapes of air-cathodes. The results showed that the ring-shaped cathode filled with GAC yielded a highest voltage of 458 mV with maximum power density of 13.71 mW m^-2 and >90% COD removal in the CW-MFC system. The electrochemical characteristics and the electron transport system activity (ETSA) are the driven force to bring the GAC a better electron transportation and oxygen reduction reaction (ORR). This will help elucidating underlying mechanisms of different activated carbon for air-cathode and thus promote its large application.

Biological modification in air-cathode microbial fuel cell: Effect on oxygen diffusion, current generation and wastewater degradation

Oxygen diffusion in the anodic chamber is the major limitation of air-cathode microbial fuel cell (MFC) design. To address this drawback, the application of microbial (Escherichia coli EC) patch on cathode was tested. Pseudomonas aeruginosa BR was used as exoelectrogen during the study. The MFC reactor with a patch had a better electron transfer rate, degraded 94.64% of synthetic wastewater (BR_SyW) and its current generation was increased by 95.66%. The maximum power density recorded for BR_SyW was 259.34 ± 7.28 mW/m². Application of patch in real wastewater (BR + Sludge) condition registered 63.18% of wastewater degradation, increment in current generation (59.71%) and decreased the charge transfer and ohmic resistances by 97.95% and 97.01% respectively. Apart from hindering oxygen diffusion and better current generation, this simple design also worked as a two-step degradation system. Thus, such MFC reactor is a potential candidate for wastewater management and green energy generation.

Efficient degradation of refractory pollutant in a microbial fuel cell with novel hybrid photocatalytic air-cathode: Intimate coupling of microbial and photocatalytic processes

A microbial fuel cell-photocatalysis system with a novel photocatalytic air-cathode (MFC-PhotoCat) was proposed for synergistic degradation of 2,4,6-trichlorophenol (TCP) with simultaneous electricity generation. Stable electricity generation of 350 mV was achieved during 130 days of operation. Besides, 50 mg L^-1 TCP was completely degraded within 72 h, and the rate constant of 0.050 h^-1 was 1.8-fold higher than MFC with air-cathode without N-TiO₂ photocatalyst. Degradation pathway was proposed based on the intermediates detected and density functional theory (DFT) calculation, with two open-chain intermediates (2-chloro-4-keto-2-hexenedioic acid and hexanoic acid) detected. Furthermore, hierarchical cluster and PCoA revealed significant shifts of microbial community structures, with enriched exoelectrogen (55.2% of Geobacter) and TCP-degrading microbe (7.1% of Thauera) on the cathode biofilm as well as 61.8% of Pseudomonas in the culture solution. This study provides a promising strategy for synergic degradation of recalcitrant contaminants by intimate-coupling of MFC and photocatalysis.

Innovative air-cathode bioelectrochemical sensor for monitoring of total volatile fatty acids during anaerobic digestion

Bioelectrochemical sensors have proven attractive as simple and low-cost methods with high potential for online monitoring of volatile fatty acids (VFA) in the anaerobic digestion (AD) process. Herein, an innovative dual-chamber air-cathode microbial fuel cell was developed as biosensor for VFA monitoring. The response of the biosensor was nonlinear and increased along with the concentration of VFA mixture increase (2.8-112 mM). Meanwhile, the relationship was linear with low VFA levels (<14 mM) within 2-5 h reaction. High concentrations of bicarbonate decreased the voltage. Stirring speeded up the response and amplified the signal but reduced the saturation concentration (approximately 30 mM) and therefore narrowed the detection range. The applicability of the biosensor was further validated with the effluents from an AD reactor during a start-up period. The VFA concentrations measured by the biosensor were well correlated with the gas chromatographic measurement. The results demonstrate that this biosensor with a novel design could be used for VFA monitoring during the AD process. Based on the 16S rRNA gene sequencing, the dominant microbiomes in the biofilm were identified as Geobacter, Hydrogenophaga, Pelobacter, Chryseobacterium, Oryzomicrobium, and Dysgonomonas.

A loop of catholyte effluent feeding to bioanodes for complete recovery of Sn, Fe, and Cu with simultaneous treatment of the co-present organics in microbial fuel cells

A loop of catholyte effluent feeding to the bioanodes of air-cathode microbial fuel cells (MFCs) achieved complete recovery of mixed Sn(II), Fe(II) and Cu(II), with simultaneous treatment of the co-present organics in synthetic wastewater of printed circuit boards (PrCBs). This in-situ utilization of caustic in the cathodes and the neutralization of acid in the anodes achieved superior metal recovery performance at an optimal hydraulic retention time (HRT) of 24 h. Cathode chambers primarily removed Sn of 91 ± 4% (bottom: 74 ± 3%; electrode: 17 ± 1%), Fe of 89 ± 8% (bottom: 64 ± 4%; electrode: 25 ± 2%), and Cu of 92 ± 7% (electrode: 63 ± 5%; bottom: 29 ± 1%), compared to Sn of 9 ± 3% (electrode: 7 ± 1%; bottom: 2 ± 1%), Fe of 9 ± 3% (electrode: 8 ± 3%; bottom: 1 ± 0%), and Cu of 7 ± 3% (electrode: 4 ± 1%; bottom: 3 ± 1%) in the bioanodes. Bacterial communities on the anodes were well evolutionarily developed after the feeding of catholyte effluent, with the increase in abundance of Rhodopseudomonas and Geobacter, and the shift from Thiobacillus and Acinetobacter to Pseudomonas, Comamonas, Aeromonas and Azospira. This loop of cathodic effluent feeding to the bioanodes of MFCs may represent a unique method for complete metal recovery with simultaneous extraction of renewable electrical energy from the co-present organics. This study also offers new insights into the development of compact microbial electro-metallurgical processes for simultaneous recovery of value-added products from PrCBs processing wastewaters and accomplishing the national wastewater discharge standard for both metals and organics.

Electricity generation from banana peels in an alkaline fuel cell with a Cu2O-Cu modified activated carbon cathode

Low-cost and highly active catalyst for oxygen reduction reaction is of great importance in the design of alkaline fuel cells. In this work, Cu₂O-Cu composite catalyst has been fabricated by a facile laser-irradiation method. The addition of Cu₂O-Cu composite in activated carbon air-cathode greatly improves the performance of the cathode. Our results indicate the enhanced performance is likely attributed to the synergistic effect of high conductivity of Cu and the catalytic activity of Cu₂O towards the oxygen reduction reaction. Furthermore, an alkaline fuel cell equipped with the composite air-cathode has been built to turn banana peels into electricity. Peak power density of 16.12Wm^-2 is obtained under the condition of 3M KOH and 22.04gL^-1 reducing sugar, which is higher than other reported low-temperature direct biomass alkaline fuel cells. HPLC results indicate the main oxidation products in the alkaline fuel cell were small organic acids.

Effective phosphate removal for advanced water treatment using low energy, migration electric-field assisted electrocoagulation

A migration electric-field assisted electrocoagulation (MEAEC) system was developed to increase phosphate removal from domestic wastewater, with reduced energy consumption, using a titanium charging (inert) electrode and a sacrificial iron anode. In the MEAEC, an electric field was applied between the inert electrode (titanium) and an air cathode to drive migration of phosphate anions towards the sacrificial anode. Current was then applied between the sacrificial anode (Fe or Al mesh) and the air cathode to drive electrocoagulation of phosphate. A MEAEC with the Fe electrode using primary clarifier effluent achieved 98% phosphate removal, producing water with a total phosphorus of 0.3 mg/L with <6 min total treatment time (five cycles; each 10 s inert electrode charging, and 1 min electrocoagulation), at a constant current density of 1 mA/cm². In the absence of the 10 s charging time, electrocoagulation required 15 min for the same removal. With an aluminum anode and the same phosphorus removal, the MEAEC required 7 cycles (7 min total treatment, 1 min 10 s total charging), while conventional electrocoagulation required 20 min. The energy demand of Fe-MEAEC was only 0.039 kWh/m³ for 98% phosphate removal, which was 35% less than with the Al-MEAEC of 0.06 kWh/m³, and 28% less than that previously obtained using an inert graphite electrode. Analysis of the precipitate showed that a less porous precipitate was obtained with the Al anode than with the Fe anode. The phosphorus in precipitate of Fe-MEAEC was identified as PO₄^3- and HPO₄^2-, while the Fe was present as both Fe²⁺ and Fe³⁺. Only HPO₄^2- and Al³⁺ were identified in the precipitate of the Al-MEAEC. These results indicated that the MEAEC with a titanium inert charging electrode and iron anode could achieve the most efficient phosphate removal with very low energy demands, compared to previous electrochemical approaches.

Electric field induced salt precipitation into activated carbon air-cathode causes power decay in microbial fuel cells

As a promising design for the real application of microbial fuel cells (MFCs) in wastewater treatment, activated carbon (AC) air-cathode is suffering from a serious power decay after long-term operation. However, the decay mechanism is still not clear because of the complex nature of contaminations. Different from previous reports, we found that local alkalinization and natural evaporation had an ignorable effect on cathode performance (∼2% decay on current densities), while electric field induced salt precipitation (∼53%) and biofouling (∼37%) were dominant according to the charge transfer resistance, which decreased power desities by 36% from 1286 ± 30 to 822 ± 23 mW m^-2 in 6 months. Biofouling can be removed by scrapping, however, electric field induced salt precipitation under biofilm still clogged 37% of specific area in catalyst layer, which was even seen to penetrate through the gas diffusion layer. Our findings provided a new insight of AC air-cathode performance decay, providing important information for the improvement of cathodic longevity in the future.

A simple method for preparing a binder-free paper-based air cathode for microbial fuel cells

In this paper, we proposed a simple method for preparing a binder-free air-cathode using carbonized kraft paper as the support and iron (II) phthalocyanine (FePc) as the catalyst. The results indicated that the oxygen reduction reaction (ORR) performance of the air-cathodes was dependent on the fabrication steps. A cathode (KP-HT-FePc-HT) fabricated by pyrolyzing kraft paper at 1000°C followed by FePc heat treatment at 700°C showed the highest P_max of 830±31mWm^-2 compared to FePc/KP-HT (363±48mWm^-2) prepared by direct pyrolysis. X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), Raman spectroscopy and electrochemical tests showed that the superior electrocatalytic activity of KP-HT-FePc-HT was attributable to its higher content of pyridinic-N. This study demonstrated that the FePc/KP-based binder-free air-cathode had the advantages of low cost, easy fabrication, environmental benefits, and good scalability and therefore could serve as a good alternative for the air-cathode of MFCs.

Continuous treatment of high strength wastewaters using air-cathode microbial fuel cells

Treatment of low strength wastewaters using microbial fuel cells (MFCs) has been effective at hydraulic retention times (HRTs) similar to aerobic processes, but treatment of high strength wastewaters can require longer HRTs. The use of two air-cathode MFCs hydraulically connected in series was examined to continuously treat high strength swine wastewater (7-8g/L of chemical oxygen demand) at an HRT of 16.7h. The maximum power density of 750±70mW/m² was produced after 12daysof operation. However, power decreased by 85% after 185d of operation due to serious cathode fouling. COD removal was improved by using a lower external resistance, and COD removal rates were substantially higher than those previously reported for a low strength wastewater. However, removal rates were inconsistent with first order kinetics as the calculated rate constant was an order of magnitude lower than rate constant for the low strength wastewater.

Microbial fuel cells using Cellulomonas spp. with cellulose as fuel

Cellulomonas fimi, Cellulomonas biazotea, and Cellulomonas flavigena are cellulose-degrading microorganisms chosen to compare the degradation of cellulose. C. fimi degraded 2.5 g/L of cellulose within 4 days, which was the highest quantity among the three microorganisms. The electric current generation by the microbial fuel cell (MFC) using the cellulose-containing medium with C. fimi was measured over 7 days. The medium in the MFC was sampled every 24 h to quantify the degradation of cellulose, and the results showed that the electric current increased with the degradation of cellulose. The maximum electric power generated by the MFC was 38.7 mW/m², and this numeric value was 63% of the electric power generated by an MFC with Shewanella oneidensis MR-1, a well-known current-generating microorganism. Our results showed that C. fimi was an excellent candidate to produce the electric current from cellulose via MFCs.

The enhancement of ammonium removal from ethanolamine wastewater using air-cathode microbial fuel cells coupled to ferric reduction

A microbial fuel cell (MFC) with biological Fe(III) reduction was implemented for simultaneous ethanolamine (ETA) degradation and electrical energy generation. In the feasibility experiment using acetate as a substrate in a single-chamber MFC with goethite and ammonium at a ratio of 3.0(mol/mol), up to 96.1% of the ammonium was removed through the novel process related to Fe(III). In addition, the highest voltage output (0.53V) and maximum power density (0.49Wm(-2)) were obtained. However, the ammonium removal and electrical performance decreased as acetate was replaced with ETA. In the long-term experiment, the electrical performance markedly decreased where the voltage loss increased due to Fe deposition on the membranes.