| Azide has widely been used in industrial products such as automobile air bags and explosive detonators. Ammonia in municipal wastewater mainly originates from urine hydrolysis. Both substances are toxic to many species. Microbial fuel cells are green technologies, capable of removing chemicals in water by using them as electron donors or acceptors. Although azide has been proved to be inhibitory to aerobic respiration, it’s still unknown whether it can participate in anaerobic respiration at bioanode in MFCs. Urine has been confirmed to be available as “fuel” of MFCs, but the influence of high concentration of ammonia from urine hydrolysis on sustainable MFC operation needs to be further investigated. In this study, sodium azide and ammonia produced by urine hydrolysis were used as the research objects to investigate the characteristics of MFCs removing both substances, and to explore their removal mechanisms.The investigation of azide removal was carried out. Single-chamber MFCs with azide at higher concentrations(> 0.2 m M)produced lower current compared to the controls. Similar phenomenon was also exhibited in coulombic efficiencies(CEs). Despite original concentrations, azide was partially removed from MFCs, showing azide could be as electron acceptors for bioanodes. The characteristics of MFC removing azide were also investigated. The soluble COD(s COD) removal efficiencies of these reactors with azide at lower concentrations were 92.0 ± 5.6 %(0.1 m M), 93.5 ± 4.9 %(0.2 m M), comparable to that(91.5 ± 3.5 %) of the controls, but higher than that of reactors with azide at higher concentrations: 82.0 ± 2.8 %, 0.5 m M; 81.5 ± 6.3 %, 1.0 m M; 82.0 ± 4.2 %, 1.5 m M. The maximum power density of these reactors decreased with increasing azide concentration, from 810.9(0 m M) to 330.3 m W/m2(1.5 m M). Power density and electrode polarization curves of these reactors showed overshoot at higher azide concentrations, indicating that the electro-activities of bioanodes were inhibited by azide especially at higher concentrations. Electrochemical impedance spectroscopy(EIS) was used to investigate the electron transfer kinetics of bioanodes, and the results showed that charge transfer resistance of the bioanodes was increased by azide at higher concentrations, indicating that a part of electrons were used for azide reduction at anode.Pyrosequencing, microbial community and chemical analyses were used to explore the mechanisms of MFCs removing azide. Despite being enriched with azide or not, anodes were dominated by Geobacter(60.0 %, 0 m M; 69.3 %, 1.5 m M), indicating that azide didn’t change the composition of the dominant bacterial populations. To further investigate whether extracellular electron transfer pathways(EETP) of bioanodes were induced and changed, bioanodes were electrochemically analyzed using cyclic voltammetry(CV). Bioanodes enriched with and without azide produced similar voltammograms, with same mid-potentials at-0.37 ± 0.01 V, indicating that azide didn’t change EETP of azide-enriched bioanodes compared to controls. In addition, the maximum current of azide-containing reactors slowly increased over time and approached to that of controls, suggesting that azide influenced bacterial growth on anodes, also reflected by differences of peak height of CVs. The reduction products of azide at bioanode and abiotic cathode were analyzed using two-chamber MFCs. The shared reaction product ammonia was detected, and azide reduction at abiotic cathode was an 8-electron reaction, suggesting that azide removal could occur on both electrodes.The factors affecting performance of urine-fed MFC were investigated. The maximum power densities of urine-fed MFCs decreased over time, from 143.5 ± 4.9 to 51.1 ± 1.0 m W/m2. The decrease in performance was mainly attributed to the severe polarization of anodes. Given the potential inhibition of free ammonia on bioanode activities, systems(CN) constructed by coupling nitrogen purging and urine-fed MFCs were used to remove excess free ammonia, while the urine-fed MFC operating under open(OC) and closed circuits(CC) were used as the controls. The maximum power density of 310.9 ± 1.0 m W/m2 was obtained for the CN reactor, and 127.1 ± 0.9 m W/m2 for the CC reactor. Total nitrogen(TN) removal efficiency(84.9 ± 2.2 %) from urine was much higher in the CN reactors than in the CC(29.7 ± 6.7 %) or OC(30.0 ± 8.2 %) reactors. As part of TN in the CN reactor, 52.8 ± 3.6 % of TN was recovered in the form of NH3-N, with a recovery rate of 435.7 ± 29.6 g-N/(m3·d). For the treated urine, there existed 214 mg/L of NH3-N in the CN reactor, 1087 mg/L in the OC reactor and 822 mg/L in CC reactor. In addition, salt precipitation appeared on anodes in the OC or CC reactors, but not in the CN reactors. Thus, sustainable maintenance of urine-fed MFC requires continuous ammonia removal.The mechanism of the combined process removing ammonia by coupling MFC and nitrogen purging was investigated based on microbial community and chemical analyses for urine samples. In all reactors, NO2--N was not found in original and treated urine, and NO3--N concentration decreased by one order of magnitude. Microbial community analysis showed that no nitrifiers or anaerobic ammonia oxidation(Anammox) bacteria and denitrifiers were detected on all the electrodes. Thus, the combination of ammoniation, diffusion/electromigration and the subsequent volatilization was the only pathway for ammonia removal in urine-fed systems. |
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