Microbial fuel cell (MFC) technology has the function of decarburization and denitrification and is considered advantageous in treating wastewater with a low carbon-nitrogen ratio. In the current study, MFC treats different concentrations of nitrate nitrogen. And compared with abiotic electrode treatment and microbial treatment only, MFC showed obvious treatment advantages. The best treatment effect was recorded when the nitrate-nitrogen (NO3-N) concentration was 414 mg/L. The running of the MFC system showed that it produced a stable output voltage up to 0.17 V within 160 h; And showed a power density up to 40.18 mW/m2, which was 1.58 times of M1 (138 mg/L, 25.49 mW/m2), 4.48 times of non-biological electrode; The removal rate of NO3-N was 97.48%, but the lack of anode electron supply resulted in the incomplete reduction of nitrate nitrogen and accumulation of nitrite-nitrogen (188.83 mg/L). Nitrate-nitrogen concentration in the study had no significant impact on microbial population diversity. However, the relative abundance of Proteobacteria increased from 47.3% to 65.4% when NO3-N concentration increased from 138 to 414 mg/L. Results from this study are promising for a theoretical basis for microbial cathode denitrification.
Research Article
Effect of nitrate on power generation and nitrogen removal and bacterial community in Microbial fuel cell (MFC)
https://doi.org/10.21203/rs.3.rs-1820657/v1
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Microbial fuel cell
Biological cathode
Nitrogenous wastewater
Microorganism
Farming wastewater, kitchen wastewater, feedlot wastewater, pharmaceutical wastewater, etc., all contain high nitrogenous pollutants (Herzog et al., 2008; Meng et al., 2015; Roy et al., 2020). The nitrogenous wastewater will cause severe pollution of the water environment and threaten human health without not effectively treated and recycled (Chen et al., 2017; Lu et al., 2014; Roy et al., 2020; Zhang et al., 2021). The physical method has low efficiency in treating nitrogenous sewage. The chemical method has a high cost and secondary pollution. The electrochemical process has high energy consumption. Microbial fuel cell (MFC) is an eco-friendly bio-electrochemical device that converts the chemical energy contained in the organic matrix into electrical energy through the respiration of microorganisms. MFC has the dual advantages of microbial power generation and waste treatment (Budihardjo et al., 2021; Xu et al., 2017). Therefore, MFC is recognized as a promising and challenging technology to overcome environmental problems (Pandey et al., 2016).
The principle of MFC denitrification is developed based on the traditional nitrification-denitrification code. Park et al. (1999) first detected that microbial receiving electrons from the cathode electrode on the biofilm electrode were reduced to nitrogen. Nitrate (NO3-N) and nitrite (NO2-N) in the cathode of MFCs are removed as electron acceptors, which through electrochemical reduction or autotrophic denitrification (Desloover et al., 2011). Part of the progress of MFCs denitrification has been achieved through different designs and devices (He et al., 2009; Zhang & Angelidaki, 2013). Zhang Fei et al. (Zhang & He, 2012b) found that dual-cathode MFCs can simultaneously generate electricity and remove nitrogen. At present, dual-cathode MFCs can achieve better performance at a lower nitrogen loading rate and remove more than 90% of total nitrogen (TN). Then, they further studied tubular double cathode MFCs to achieve nitrification of the outer cathode and denitrification of the inner cathode. The removal rate of organic matter was 85% ~ 99%, and the removal rate of ammonia nitrogen was 96% (Zhang & He, 2012a). Virdis et al. (2010) studied the reduction reaction of NO3-N as the electron acceptor in MFCs cathode. The system realized simultaneous nitrification and denitrification by optimizing the oxygen supply in the aerated cathode chamber, and the highest nitrogen removal efficiency was 94%. Puig et al. (2011) and Desloover et al. (2011) also demonstrated similar results. The biocathodes using NO2-N as electron acceptor showed a TN removal rate of 48% and a removal rate of 7.6 g (NO2--N) m-3/d under the mode of continuous operation for 4 hours.
Although the removal rate of TN by cathodic (autotrophic) denitrification in MFC is generally lower than that by heterotrophic denitrification, autotrophic microorganisms only need a few carbon sources and produce less sludge due to their slow growth (Wang et al., 2009; Zhang et al., 2011). For wastewater with low carbon content, removing nitrogen economically and effectively is the current research direction. MFCs technology also has the function of carbon removal. Therefore, autotrophic denitrification MFCs is a promising technology to treat low organic carbon wastewater.
2.1 Treatment of carbon cloth and proton exchange membrane
Treatment of carbon cloth: The purchased carbon cloth was cut into a size of 4×2.5 cm and used as the cathode electrode and anode of the microbial fuel cell. The carbon cloth electrode removed surface impurities through acidification, the effective surface area was increased, and the conductivity was enhanced. Specific method: The carbon cloth electrode was acidified in 1 mol/L hydrochloric acid solution for 2 h. After completion, the electrode was cleaned with deionized water until the surface was neutral. Finally, it was dried in a constant temperature drying oven at 60°C for use.
Treatment of proton exchange membrane: 3% H2O2 solution was added to the beaker to immerse the proton exchange membrane and maintained for 1 h at 80℃ to remove impurities on the surface of the proton exchange membrane. Then soak in distilled water at 80℃ for 1 h to remove residual H2O2 solution. The residual H2O2 solution was acidified in a 0.15 mol/L sulfuric acid solution for 1 h. After the acidification was completed, the proton exchange membrane was washed twice with distilled water at 80°C. The proton exchange membrane treatment was completed and stored in distilled water at room temperature for later use.
2.2 MFC start-up and operation
MFC system was composed of one cathode and one anode chamber with the working volumes of 220 mL, separated with a proton exchange membrane and chosen same electrode material. The cathode, anode, and external load were connected in series, and the outer DC rheostat box adjusted the resistance value of MFC. MFC was operated at the room temperature of 32±1℃, pH 7, and the hydraulic retention time per cycle was 10 d. Anolyte consisting of the following components (per liter); 11.67 g citric acid, 0.1 g EDTA, 3 g Fe2(SO4) 3, 0.226 g (NH4)2SO4, 0.009 g KH2PO4, and deionized water. Catholyte consisting of KNO3, NaHCO3, KH2PO4. Before using anode liquid and cathode liquid, NaOH solution and hydrochloric acid solution were used to adjust to neutral, and nitrogen gas was introduced for 20 minutes to ensure an anaerobic environment. Inoculated the anode and cathode with anaerobic sludge and added the configured anolyte and catholyte to connect the external circuit. When the open circuit voltage was stable, MFC was considered running successfully.
The experiment was divided into two stages: in the first stage, three MFCs were set up. The first group with connected circuit but not inoculated with microorganism on the cathode was CMFC group, the second group with inoculated microorganism but not connected with external circuit was KMFC group, and the third group with inoculated microorganism and connected with external circuit was MFC group. In the first stage, NO3-N concentration in the cathode was 138 mg/L, and the MFC system operated for 10 days; In the second stage, the initial concentration of NO3-N in the cathode liquid of the MFC group was 138 mg/L, 276 mg/L, 414 mg/L, and 552 mg/L respectively. Each cycle operated for 10 days.
2.3 Analysis and calculation
The battery voltage acquisition system consists of a data acquisition card, computer, and wire. The voltage at both ends of MFC external resistance was collected once every 60 s using a data acquisition card (mps-010602) and stored in a computer for storage. The power density was calculated by the current per unit area. Cyclic Voltammetry (CV) experiments were conducted with the potential ranging from −800 to 800 mV with a scan rate of 25 mV/s. The Nyquist curve analyzed the internal resistance of the battery. Electrochemical impedance spectroscopy (EIS) was carried out in a frequency range of 0.01 Hz to 105 Hz using a 10 mV sinusoidal disturbance voltage. The microbial community was analyzed by high-throughput sequencing. Chemical oxygen demand (COD) was determined by the rapid digestion method, and coulomb efficiency (CE) was determined by the change of COD in one cycle.
The anodic CE was calculated using Eq. (1):
Where CE is the coulomb efficiency (%); 
is molar mass of oxygen; t is one cycle reaction time (h); I is recorded current; 4 is the number of electrons exchanged per mole of oxygen; F is Faradaic constant, (F=96485.33 C/mol); V is working volume of anode chamber (m3); ∆COD is changed in chemical oxygen demand.
The cathodic CE was calculated based on Eq. (2) as follow:
∆e- is the number of electrons consumed by cathode denitrification.
The voltage drop calculated by the Tafel equation was expressed by (3):
E is overvoltage (V); a is intercept between Tafel curve and potential axis; b is Tafel slope; ix is current density (A/cm2).
The MFC labels for CMFC, 138 mg/L, 276 mg/L, 414 mg/L, and 552 mg/L NO3-N, are M0, M1, M2, M3, and M4, respectively.
3.1 Electrochemical data
3.1.1 Output voltage
As shown in Fig. 1, the output voltage of CMFC was stable from 0 h to 70 h and increased slowly. Its output voltage was lower than that of the MFC group, which indicated that microbial inoculation could improve the utilization efficiency of electrons. In the MFC group, different initial NO3-N concentrations were added to simulate wastewater. The output voltage of MFC increased for a while and then expanded to a stable period, which could last for a long time. The duration of an output voltage of MFC with different initial NO3-N concentrations was different. The output voltage of M1 dropped to 0 V at 70 h, and then increased suddenly at 96 h, 144 h, 216 h, and 240 h, and then continued to decreased. The rising time of the voltage coincides with the sampling time. Oxygen enters the cathode chamber during sampling, and oxygen acts as an electron acceptor in the cathode chamber for reduction reaction, making the circuit form a complete closed-loop again. The voltage value of M2 dropped to 0 V at 130 h, and the fluctuation time of the output voltage was the same as M1. Therefore, the output voltage fluctuation in the later stage of MFC increased dissolved oxygen concentration in the catholyte. The highest voltage of M3 lasts the longest, about 160 h. The output voltage of the initial M4 began to decrease gradually after 50 h. The stable output voltages of M1, M2, M3, and M4 were 0.12 V, 0.13 V, 0.17 V, and 0.11 V. MFC output voltage was closely related to NO3-N concentration. With the continuous denitrification at the cathode, NO3-N was reduced. The final concentration of electron acceptor decreased, limiting the cathode reaction, reducing the output voltage at the later stage of MFC operation. The increase of initial NO3-N concentration provided more substrates for cathode microorganisms. The electrons transferred from the anode could be used by the organism as much as possible. MFC output voltage could be increased with the increase of NO3-N concentration (138 ~ 414 mg/L), but it was inhibited when the concentration reached 525 mg/L.
3.1.2 Power density curve
The output power density curves of the five groups of experiments showed the trend of increasing with the current density, and the output power density first increased to the peak and then decreased (Fig. 2). The coincidence degree of the curve before the peak value of the output power density was higher than that after the peak value of the output power density, which indicated that NO3-N concentration had a significant effect on the output power density. The maximum output power densities of M0, M1, M2, M3 and M4 were 8.96 mW/m2, 25.49 mW/m2, 32.81 mW/m2, 40.18 mW/m2 and 28.41 mW/m2, respectively. When the NO3-N concentration was 138 ~ 414 mg/L, the output power of MFC could be increased, but a higher NO3-N concentration inhibited the output power of MFC. The current density and output power density of MFC were significantly higher than those of CMFC so that the cathode microorganism can improve the utilization of electrons.
3.1.3 Internal resistance
The internal resistance of MFC is reflected by the slope of the voltage-current fitting line, also known as apparent internal resistance, which is composed of internal ohmic resistance and non-ohmic internal resistance. The voltage-current proper curves of CMFC and MFC with different NO3-N concentrations are shown in Fig. 3.
The internal resistances of M0, M1, M2, M3, and M4 were 5018.7 Ω, 1022.9 Ω, 835.4 Ω, 741.4 Ω, and 935.9 Ω, respectively. The change of MFC internal resistance showed that when NO3-N concentration was 138 ~ 414 mg/L, the internal resistance of MFC was decreasing. The increase of NO3-N concentration represents the increase of electron acceptor concentration in the cathode, promoting the electron transfer between anode and cathode and reducing the internal resistance of MFC. When the NO3-N concentration was 552 mg/L, the internal resistance of MFC increased to 935.9 Ω because the high NO3-N concentration inhibited the denitrifying microbial activity, resulting in the decrease of electron utilization rate. The comparison of internal resistance between CMFC and MFC showed that cathode microorganisms promote the reduction reaction of cathode and significantly reduce the internal resistance of MFC.
3.2 Electrochemical analysis
Bioelectrochemistry involves biocatalysis related to charging separation, which is usually associated with charge transfer. Charge transfer can occur uniformly in solution or unevenly on the electrode surface.
3.2.1 Capacitance analysis
Cyclic voltammetry is a robust method to detect the thermodynamic and electrochemical activity of electron transfer in MFC. The closed curve area represents the total amount of charge moving in and out of the redox reaction in MFC, which is positively correlated with the charge storage capacity of MFC.
Table 1. Capacitance of CMFC and MFC
|
Initial concentration |
CMFC |
MFC 138 mg/L |
MFC 276 mg/L |
MFC 414 mg/L |
MFC 552 mg/L |
|
Capacitance (mF) |
0.0920 |
0.7614 |
1.2239 |
1.2646 |
1.0249 |
The capacitance of CMFC was much lower than that of MFC, and the cathode inoculation improved the power generation performance of MFC. When the NO3-N initial concentration was 138 ~ 552 mg/L, the capacitance of MFC first increased and then decreased with the increase of initial concentration (Fig. 4). NO3-N had a toxic effect on denitrifying microorganisms, resulting in the weakening of microbial activity. The initial concentration of nitrate reached 552 mg/L, which inhibited the activity of denitrifying organisms (Table 1).
3.2.2 Electrochemical impedance spectroscopy
EIS was usually used to measure the internal resistance of MFC. EIS spectra explored the charge transfer impedance at the electrode interface, traditionally expressed as the semicircular part of the high-frequency region and the linear part of the diffusion limiting process or mass transfer process for the low-frequency region (Fig. 5).
Table 2. Internal resistance of CMFC and MFC
|
MFC |
Ohmic resistance（Ω） |
Charge transfer internal resistance（Ω） |
|
CMFC |
22.87 |
140.12 |
|
MFC-138 mg/L |
33.97 |
137.63 |
|
MFC-276 mg/L |
29.41 |
132.69 |
|
MFC-414 mg/L |
48.88 |
124.72 |
|
MFC-552 mg/L |
26.7 |
141.8 |
Table 2 lists the different internal resistances of CMFC and MFC groups. The internal resistance of ions in the electrolyte and the contact resistance between the electrode and active material will affect internal ohmic resistance. The internal ohmic resistance of CMFC was the smallest. That was to say, after inoculated microorganisms on the cathode, electrons were transferred between the cathode electrode and microorganisms, which made the internal ohmic resistance increase. Ohmic internal resistance did not show a consistent trend in the MFC group because the dominant microbial community may be different under different NO3-N concentrations, which made the internal resistance of MFC change non directionally. When NO3-N initial concentration was 138 ~ 414 mg/L, the internal resistance of charge transfer of the MFC group gradually decreased with the increase of the initial concentration of nitrate nitrogen. Manohar and Mansfeld (2009) Showed that the decrease of internal resistance in MFC could increase the attachment of microorganisms and the output of power density. When the initial concentration of NO3-N was 552 mg/L, the internal resistance of charge transfer of the MFC group increased to 141.8 Ω, which also indicated that a too high NO3-N concentration would reduce some microbial activities.
3.2.3 Tafel analysis
Tafel analysis was a technique to evaluate the dynamic activity of electrodes. It mainly correlated the exchange current density, charge transfer resistance, charge transfer coefficient, and Tafel slope with the overpotential of electrodes (Rajesh et al., 2015). Various obstacles need to be overcome in degrading electrons from anode to cathode, resulted in energy loss and reduced the conversion efficiency of a fuel cell. At a lower current density, its loss had a more significant impact. Tafel curve parameters of MFC are shown in Fig. 6 and Table 3.
Table 3. Tafel curve slope and exchange current density of CMFC and MFC
|
MFC |
CMFC |
MFC 138 mg/L |
MFC 276 mg/L |
MFC 414 mg/L |
MFC 552 mg/L |
|
ix (mA/m2) |
0.1872 |
1.714 |
2.321 |
2.530 |
1.856 |
|
b (mV/dec) |
0.8774 |
0.7478 |
0.6896 |
0.5891 |
0.7060 |
The current exchange density (ix) represents the "exchange rate" between the reactant and the product state in the equilibrium state. The higher the value, the lower the reaction energy barrier in MFC and the faster the reaction rate. The i1, i2, and i3 were 1.714 mA/m2, 2.321 mA/m2, and 2.530 mA/m2, indicating that the increase of NO3-N concentration was helpful to improve the power generation performance of MFC. The i4 dropped to 1.856 mA/m2, indicating that the excessive NO3-N concentration reduced the current exchange density of MFC, which was consistent with the change of NO3-N removal rate (3.3) also reflected the adverse effect of high NO3-N concentration on microbial growth. The slope of the Tafel curve was directly proportional to the reaction efficiency. M3 slope showed that when microorganisms catalyze organic matter, the overpotential was the lowest, the reaction was more accessible, which was consistent with the results of internal resistance analysis (3.1.3).
3.3 Remove of nitrate nitrogen and COD
According to the sampling of cathode solution during operation, the content changes of nitrate nitrogen and nitrite nitrogen were measured, and the change curves of nitrate nitrogen and nitrite nitrogen were drawn（Fig. 7a, b）.
The removal rate by CMFC was only 8.27%, which showed that the effect of NO3-N reduction could not be achieved by CMFC only. The NO3-N removal rate of KMFC can reach 43.58%, which was 5.3 times that of CMFC. However, the effluent NO3-N concentration of KMFC was 77.86 mg/L, which still could not meet the effluent quality standard.
The NO3-N removal rate of M1, M2, and M3 finally reached 93.31~99.53%, and M4 was only 59.52%. During the whole operation period, the NO3-N removal rate of M1 increased rapidly from 0 h to 100 h, and the removal rate reached 93.62%. It showed that the NO3-N reduction rate was faster during this period. After 100 h operation, the rising curve of NO3-N removal rate tended to be flat because most of the NO3-N was reduced, and the final electron acceptor content was too low, the nitrate reduction rate decreased. The NO3-N removal rate of M2 increased slowly after 168 h, while M2 and M3 increased with the increase of operation time. The NO3-N removal rate was directly related to the output voltage of the MFC. M3 had the highest output voltage and a higher removal rate of nitrate nitrogen. The NO3-N removal rate was directly related to the output voltage of the MFC. M3 had the highest output voltage and a higher removal rate of NO3-N. The increase of NO3-N concentration triggered biomass secretion and promoted biomass adsorption on the electrode surface and cathode liquid interface (Valenzuela et al., 2021). Within a specific range, the activity of denitrifying microorganisms was positively correlated with NO3-N concentration. Still, the activity of denitrifying microorganisms will be inhibited when NO3-N attention exceeded the degradation ability of the organism itself.
NO3-N as denitrification nitrogen source, NO2-N accumulation was inevitable (Xie et al., 2021). The accumulation of NO2-N in M0, M1, M2, M3, M4, and KMFC were 2.05 mg/L, 49.27 mg/L, 21.96 mg/L, 280.17 mg/L, 188.83 mg/L, and 27.15 mg/L. The increase of NO2-N concentration in the cathode indicated that NO3-N had not been completely reduced. The reduction of NO3-N to nitrogen involves sequence domination of several intermediate reduction steps (NO2-, NO, and N2O). The accumulation of NO2-N indicated that the expected reduction step of NO2-N does not occur. NO2-N had biological toxicity to microorganisms, and its high concentration can inhibit the activity of microorganisms and form carcinogenic N-nitroso compounds (Patel et al., 2021a). As shown in Fig. 7b, the accumulation of NO2-N increased with the increase of the initial NO3-N concentration. The accumulation of NO2-N in M1 and M2 had the same trend. During 0-144 h, NO2-N concentration was always in the accumulation state and reached the maximum value (86.33 mg/L and 94.58 mg/L) at 144 h, then the content of NO2-N began to decline, the removal rate of NO3-N increased slowly. The main material reduced in the cathode chamber changes from NO3-N to NO2-N. The accumulation of NO2-N in M3 and M4 increased continuously, while M4 was lower than that in M3 from 144 h. It showed that the initial concentration of NO3-N reached 552 mg/L, and the content of NO2-N in the solution was too high, inhibiting the microbial activity and weakens the denitrification of the cathode. No ammonia nitrogen was detected during the whole operation period because NO3-N was converted to ammonia nitrogen only when the electrons were excessive (Kato et al., 2012). NO2-N accumulation indicated that the cathode electron supply of MFC was insufficient, and ammonia nitrogen cannot be detected.
The COD removal rate of MFC had been rising all the time (Fig. 7c). Still, its voltage (3.1.1) began to decline at different operation times, which showed that COD in the anode was used for power generation and used for the everyday life activities of microorganisms. The COD removal rate was an indicator of microbial utilization of organic matter. The higher the COD removal rate, the more organic matter degraded by microorganisms, and the stronger the power generation capacity of MFC. The COD removal rate of CMFC (48.9%) was lower than that of the MFC group, indicated that cathodic inoculation of microorganisms would promote the removal of NO3-N and the catalysis of anodic microorganisms to organic matter. Mo et al. (2021) found that the coupling of biocathode and anode can improve denitrification and organic matter degradation simultaneously. The COD removal rates of M1, M2, and M3 increased with initial concentration (68.8 ~ 81.3%). Mo et al. (2021) reported that increasing the initial concentration of NO3-N in the cathode was conducive to improving the removal rate of COD in the anode.
The Coulomb efficiencies of M0, M1, M2, M 3 and M4 were 1.4%, 8.1%, 14.3%, 26.1% and 12.1%, respectively; The cathode Coulomb efficiencies were 0.49%, 3.7%, 8.0%, 6.8% and 6.5% (Fig. 8). The changing trend of coulomb efficiency was consistent with that of COD removal and power generation performance. The coulomb efficiency of the cathode was much lower than that of MFC, and the electron loss was large. Within a specific range, as the concentration of NO3-N increased, the utilization rate of cathode electrons increased. However, too high NO3-N concentration would cause too high a concentration of NO2-N and hinder electron transfer. However, the COD removal rate of M4 decreased to 78.1%. A high concentration of NO3-N inhibited the activity of denitrifying bacteria. The accumulation of NO2-N was also toxic to microorganisms, resulting in the reduction rate of the cathode, and electrons could not be consumed in time, which limited the efficiency of anode microorganisms in catalyzing organic matter.
3.4 Biofilm cathode microbial community
The MFC labels for denitrification sludge, 138 mg/L NO3-N, 276 mg/L NO3-N, 414 mg/L NO3-N, 552 mg/L NO3-N, and KMFC are A0, A1, A2, A3, A4, and A5, respectively. Cathode microorganisms limited the denitrification rate and electricity production of cathode microorganisms. The genome of the microbial population was used to analyze the effect of inoculated microorganisms on MFC.
The predominant phyla in MFC belonged to Proteobacteria, Chloroflexi, Planctomycetes, Actinobacteria, Bacteroidetes, and Firmicutes (Fig. 9a). Proteobacteria were the most dominant phylum in A0 (37.7%), A1 (47.3%), A2 (61.8%), A3 (62.6%), A4 (65.4%), A5(64.7%). As the concentration of NO3-N increases, the relative abundance of Proteobacteria increases significantly, which was suitable for the treatment of high-concentration NO3-N wastewater. The relative abundance of Chloroflexi, Planctomycetes, Firmicutes, and Actinobacteria in A1 was higher than that in A5, and these bacteria phyla may have the ability to accept electrons at the cathode.
The predominant orders in MFC belonged to Burkholderiales, Hydrogenophilales, Rhodocyclales, Planctomycetales, Rhizobiales, and Anaerolineales (Fig. 9b). In A0 and A5 groups, the relative abundance of Burkholderiales, Hydrogenophilales, and Rhodocyclales increased after acclimation, indicating that these three orders were the main denitrifying bacteria. In A1 and A5 groups, the relative abundance of Burkholderiales, Planctomycetales, and Anaerolineales increased, which meant that they were electroactive microorganisms with good electron reception to the cathode electrode. The relative abundance of Burkholderiales began to decrease when the NO3-N concentration reached 414 mg/L, indicating that the effect of NO3-N concentration was greater than that of current. The relative abundance of Hydrogenophilales increased with the increase of NO3-N concentration, which showed that Hydrogenophilales was one of the main denitrifying bacteria. The relative abundance of Rhodocyclales increased significantly with the growth of NO3-N concentration, and it was resistant to high NO3-N concentration. Griessmeier et al. (2017) found that Rhodocyclales is more suitable for the treatment of high NO3-N concentration wastewater. Mohan et al. (2004) detected a new cytochrome c in the periplasmic space of Rhodocyclales, which has a specific effect on Planctomycetales. The relative abundance of Planctomycetales decreased gradually with the increase of NO3-N concentration. Too high NO3-N concentration or NO2-N accumulation inhibited its growth activity. The relative abundance of Planctomycetales was the same in A2 and A3, indicating that the effect of current and NO3-N concentration reached a balance. As the concentration of NO3-N continued to increase, the inhibitory effect on microorganisms exceeded the promoting effect of current, and the relative abundance decreased.
The predominant genera in MFC belonged to Sulfuritalea，Anaerolineae，Ardenticatena，Thauera，and Thiobacillus (Fig. 9c). Sulfuritalea is a kind of facultative autotrophic bacteria, which is more common in freshwater lakes. It is suitable for treating low nitrogen wastewater and can reduce nitrate under anaerobic conditions (Kojima & Fukui, 2011). From A1 and A5, the relative abundance of Sulfuritalea and Anaerolineae in A1 increased. These two bacteria may be able to obtain electrons from the electrode, which needs further study. In A1, A2, A3, and A4, the relative abundance of Ardenticatena was 0.88%, 1.77%, 2.13%, and 2.67%, respectively. The relative abundance showed an increasing trend. With the increase of nitrate concentration, the growth and reproduction of Ardenticatena were better. Kawaichi et al. (2018) and Guo et al. (2021) reported that Ardenticatena was a denitrifying bacterium with the ability of anode and cathode extracellular electron transfer. Thauera is a typical denitrifying bacteria genus that can directly convert NO2-N into gaseous nitrogen (Patel et al., 2021b). Liu et al. (2015) reported that Thauera is the main electrochemically active denitrifying bacteria. Among A1-4, the relative abundance of Thauera in A3(35.67%) was the highest, which may be due to the higher nitrite nitrogen content and output voltage in this group.
MFC, abiotic electrode, and microbial treatment were evaluated to treat high nitrogen-containing wastewater in this study, removal percentages of 99.53%, 8.27%, and 43.58% of NO3-N, respectively. MFC is thus promising in solving the challenges associated with the treatment of high concentration nitrogen-containing wastewater. This study’s second objective was to explore reported divergent impacts on the MFC treatment effect when different NO3-N concentrations were added to the cathode. Results showed that the best treatment effect was recorded when the NO3-N attention was 414 mg/L in the MFC. It produced a stable output voltage up to 0.17 V within 160 h; Power density up to 40.18 mW/m2; Removal percentages of 97.48%, 81.3% of NO3-N and COD, respectively. In a specific range, the increase of NO3-N concentration can improve the treatment effect of MFC. NO3-N concentration in the study had a significant impact on the relative abundance of microorganisms.
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No competing interests reported.