Xiangtong Zhou

Coupling interaction of cathodic reduction and microbial metabolism in aerobic biocathode of microbial fuel cell

Certain mixed consortia colonized on aerobic biocathodes can improve the 4-electron oxygen reduction of cathodes; however, the coupling interaction of the cathodic reaction and microbial metabolism remains unclear. To better understand the abovementioned interaction evolved in the cathodic process, biocathodes were enriched using nitrifying sludge and operated at various NH4Cl and NaHCO3 concentrations in both the open and closed external circuit conditions. Based on the variation of the nitrification and cathodic oxygen reduction activities, it was shown that the oxygen reduction process, to some extent, relied on the nitrification activity of the biocathode; the external electrons from the cathode, in turn, might benefit the nitrifying bacteria selected in the MFC habitat by entering the electron transfer chains as the energy source. Nitrifiers, including Nitrosomonas sp., Nitrospira sp. and Nitrobacter sp., were detected in all the biocathodes that were cultured in different conditions, even the ones cultured without NH4Cl in the medium. These findings provided valuable insights into the possible working mechanism of biocathodes.

A combined system of microbial fuel cell and intermittently aerated biological filter for energy self-sufficient wastewater treatment.

Energy self-sufficiency is a highly desirable goal of sustainable wastewater treatment. Herein, a combined system of a microbial fuel cell and an intermittently aerated biological filter (MFC-IABF) was designed and operated in an energy self-sufficient manner. The system was fed with synthetic wastewater (COD = 1000 mg L−1) in continuous mode for more than 3 months at room temperature (~25 °C). Voltage output was increased to 5 ± 0.4 V using a capacitor-based circuit. The MFC produced electricity to power the pumping and aeration systems in IABF, concomitantly removing COD. The IABF operating under an intermittent aeration mode (aeration rate 1000 ± 80 mL h−1) removed the residual nutrients and improved the water quality at HRT = 7.2 h. This two-stage combined system obtained 93.9% SCOD removal and 91.7% TCOD removal (effluent SCOD = 61 mg L−1, TCOD = 82.8 mg L−1). Energy analysis indicated that the MFC unit produced sufficient energy (0.27 kWh m−3) to support the pumping system (0.014 kWh m−3) and aeration system (0.22 kWh m−3). These results demonstrated that the combined MFC-IABF system could be operated in an energy self-sufficient manner, resulting to high-quality effluent.

Evaluation of an integrated continuous stirred microbial electrochemical reactor: Wastewater treatment, energy recovery and microbial community.

A continuous stirred microbial electrochemical reactor (CSMER) was developed by integrating anaerobic digestion (AD) and microbial electrochemical system (MES). The system was capable of treating high strength artificial wastewater and simultaneously recovering electric and methane energy. Maximum power density of 583±9, 562±7, 533±10 and 572±6 mW m(-2) were obtained by each cell in a four-independent circuit mode operation at an OLR of 12 kg COD m(-3) d(-1). COD removal and energy recovery efficiency were 87.1% and 32.1%, which were 1.6 and 2.5 times higher than that of a continuous stirred tank reactor (CSTR). Larger amount of Deltaproteobacteria (5.3%) and hydrogenotrophic methanogens (47%) can account for the better performance of CSMER, since syntrophic associations among them provided more degradation pathways compared to the CSTR. Results demonstrate the CSMER holds great promise for efficient wastewater treatment and energy recovery.

Cascade degradation of organic matters in brewery wastewater using a continuous stirred microbial electrochemical reactor and analysis of microbial communities.

A continuous stirred microbial electrochemical reactor (CSMER), comprising of a complete mixing zone (CMZ) and microbial electrochemical zone (MEZ), was used for brewery wastewater treatment. The system realized 75.4 ± 5.7% of TCOD and 64.9 ± 4.9% of TSS when fed with brewery wastewater concomitantly achieving an average maximum power density of 304 ± 31 m W m−2. Cascade utilization of organic matters made the CSMER remove a wider range of substrates compared with a continuous stirred tank reactor (CSTR), in which process 79.1 ± 5.6% of soluble protein and 86.6 ± 2.2% of soluble carbohydrates were degraded by anaerobic digestion in the CMZ and short-chain volatile fatty acids were further decomposed and generated current in the MEZ. Co-existence of fermentative bacteria (Clostridium and Bacteroides, 19.7% and 5.0%), acetogenic bacteria (Syntrophobacter, 20.8%), methanogenic archaea (Methanosaeta and Methanobacterium, 40.3% and 38.4%) and exoelectrogens (Geobacter, 12.4%) as well as a clear spatial distribution and syntrophic interaction among them contributed to the cascade degradation process in CSMER. The CSMER shows great promise for practical wastewater treatment application due to high pre-hydrolysis and acidification rate, high energy recovery and low capital cost.

Effects of azide on electron transport of exoelectrogens in air-cathode microbial fuel cells.

The effects of azide on electron transport of exoelectrogens were investigated using air-cathode MFCs. These MFCs enriched with azide at the concentration higher than 0.5mM generated lower current and coulomb efficiency (CE) than the control reactors, but at the concentration lower than 0.2mM MFCs generated higher current and CE. Power density curves showed overshoot at higher azide concentrations, with power and current density decreasing simultaneously. Electrochemical impedance spectroscopy (EIS) showed that azide at high concentration increased the charge transfer resistance. These analyses might reflect that a part of electrons were consumed by the anode microbial population rather than transferred to the anode. Bacterial population analyses showed azide-enriched anodes were dominated by Deltaproteobacteria compared with the controls. Based on these results it is hypothesized that azide can eliminate the growth of aerobic respiratory bacteria, and at the same time is used as an electron acceptor/sink.

Effects of azide on current generation and microbial community in air-cathode MFCs

Azide is known to be a respiratory inhibitor, which can disrupt electron transfer in the process of aerobic respiration. It has been proposed for preventing the reduction of oxygen in the anode compartment of MFC-based biosensors, but has also been found to function as an electron acceptor in recent research. However, there are few reports about the effects of azide on the structure and composition of the microbial community in air-cathode MFCs, as well as on their corresponding performance. Therefore, the current generation, electroactivity and community structure of anodic biofilms were investigated using air-cathode MFCs acclimated with (1.5 mM) and without azide. The enrichment process was much slower in the presence of azide compared to the control. Biofilms enriched with and without azide were found to produce similar voltammograms, but the difference lay in the current intensity of the predominant peaks. Pyrosequencing indicated that the distribution of microbes at the genus level was more uniform, with Geobacter and Ignavibacterium being the dominant genera on both biofilms, although the community of the azide-enriched film was less diverse than that of the control. These results demonstrate that the microbial community enriched with azide was not significantly altered compared to the control and the difference in the maximum current or peak current of cyclic voltammograms (CVs) was thought to be related to the amount of biomass.

Microwave-assisted synthesis of nitrogen-doped activated carbon as an oxygen reduction catalyst in microbial fuel cells

A nitrogen-doped activated carbon (NDAC) as a cathode catalyst in microbial fuel cells (MFCs) was synthesized by a microwave-assisted method using ammonium carbonate as a nitrogen source. The prepared NDAC showed a higher BET surface area of up to 1717.8 m2 g−1 and a total pore volume of 0.79 cm3 g−1. X-ray photoelectron spectroscopic analysis demonstrated that N was successfully doped on the surface of AC in three species, corresponding to pyrrolic N, pyridinic N and pyridine-N-oxide. Compared with untreated AC, the NDAC exhibited better electrocatalytic activity for the oxygen reduction reaction in rotating disk electrode tests, with a current density of 12.4 mA cm−2 at a set potential of −0.8 V (vs. SCE) (AC, 11.3 mA cm−2) and an electron transfer number of 3.14 (AC, n = 2.83). MFCs equipped with a NDAC cathode achieved a higher maximum power density of 471 ± 11 mW m−2 when fed with domestic wastewater, which was 1.3 times higher than that of the AC cathode. It also displayed long-term operation stability when dealing with real wastewater, indicating a promising cathode catalyst for MFCs towards practical applications.

Simultaneous current generation and ammonia recovery from real urine using nitrogen-purged bioelectrochemical systems

Bioelectrochemical systems (BESs) offer a strategy for treating source-separated urine with current generation, but the high content of ammonia is still a challenge for sustainable maintenance of BESs due to ammonia inhibition. Therefore, an integrated BES setup was developed to overcome this problem by ammonia recovery. This setup, working in closed circuit mode with nitrogen purging (CN), allowed for the produced ammonia to be continuously channeled to an absorption bottle. In addition, control reactors in closed circuit (CC) or in open circuit mode (OC) were also run for comparison. A maximum power density of 310.9 ± 1.0 mW m−2 was obtained for the CN reactor, and 127.1 ± 0.9 mW m−2 was obtained for the CC reactor. Total nitrogen (TN) removal efficiency (84.9% ± 2.2%) from urine was considerably higher in the CN reactor than it was in the CC (29.7% ± 6.7%) or OC (30.0% ± 8.2%) reactor. In the CN reactor, 52.8% ± 3.6% of the TN was recovered in the form of NH3-N, with a NH3 recovery rate of 435.7 ± 29.6 gN m−3 d−1. The improved performance of the CN reactor was attributed to the mitigation of ammonia inhibition to the anode electro-activity. 16S rDNA sequencing showed that no Anammox and nitrifiers were detected on the anodes and cathodes. Overall, nitrogen purging provides the urine-fed BESs with a useful approach for maintaining the system performance by ammonia recovery.

Electricity generation by biocathode coupled photoelectrochemical cells

Biocathode coupled photoelectrochemical cells (Bio-PEC) have the potential for electricity generation and pollutant removal, with the simultaneous utilization of both solar energy and bioenergy. However, their performance is influenced by many factors. In this work, crucial parameters, including the pollutant type, electrolyte concentration and gas atmosphere of the photoanode, were investigated to optimize the operation of the Bio-PEC in terms of electricity generation.

Azide as an oxidant in the cathodic reaction of bioelectrochemical systems (BESs)

Azide could be used as an oxidant in the cathodic reaction in BESs, and produced a peak current of 0.37 mA. An estimation based on electron balance showed that azide reduction at the abiotic cathode was an eight-electron reaction, with NH3 as the only reduction product.