graphitic carbon nitride coated granular activated carbon cathode successfully treated organic acids industrial wastewater with residual nitric acid

Journal Pre-proofs A Microbial fuel cell system with manganese dioxide/titanium dioxide/graphitic carbon nitride coated granular activated carbon cathode successfully treated organic acids industrial wastewater with residual nitric acid Qian Zhang, Lifen Liu PII: DOI: Reference:

S0960-8524(20)30261-3 https://doi.org/10.1016/j.biortech.2020.122992 BITE 122992

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Bioresource Technology

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Please cite this article as: Zhang, Q., Liu, L., A Microbial fuel cell system with manganese dioxide/titanium dioxide/ graphitic carbon nitride coated granular activated carbon cathode successfully treated organic acids industrial wastewater with residual nitric acid, Bioresource Technology (2020), doi: https://doi.org/10.1016/j.biortech. 2020.122992

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A Microbial fuel cell system with manganese dioxide/titanium dioxide/graphitic carbon nitride coated granular activated carbon cathode successfully treated organic acids industrial wastewater with residual nitric acid

Qian Zhang a， Lifen Liu a, b, ＊ a

MOE Key lab of Industrial Ecology and Environmental Engineering, School of Environmental

Science and Technology, Dalian University of Technology, Dalian, China, 116024 b School ＊

of Ocean Science & Technology, Dalian University of Technology, Panjin, China, 124221

Corresponding Author （email: [email protected])

Abstract To meet the urgent demands for sustainable and efficient, environmental-friendly wastewater treatment, a Microbial fuel cell reactor system with MnO2/TiO2/g-C3N4 (manganese dioxide/ titanium dioxide/graphitic carbon nitride) @GAC (granular activated carbon) electrode was developed. It was both efficient and energy-saving in treating organic acid wastewater generated in Nylon production, with high-concentration COD and residual nitric acid. The MnO2/TiO2/g-C3N4 catalyst was deposited on GAC via in-situ growth and sol-gel method. The COD, NH4+-N and NO3-N was efficiently removed (respectively 98%, 99% and 99%). The COD removal capacity (17.77 kg COD m-3d-1) and the maximum power density (1176.47 mW m-3) was respectively 36.83% and 65.29% higher than the GAC cathode system. The anodic and cathodic microbial consortiums in MFC were analyzed and compared. The MnO2/TiO2/g-C3N4@GAC MFC system is technically feasible and cost-effective in treating industrial wastewater. Keywords: Microbial fuel cell; Electro-catalyst; Wastewater treatment 1. Introduction The organic acid industrial wastewater, produced in Nylon production process, has high concentrations of inorganic nitrogen and COD, it is highly oxidative and very acidic (pH is about 1, high content nitric acid), also contains Vanadium heavy metal ions, therefore poses serious impact on the environment , threat the ecological balance and human health(Xia et al. 2019). Currently its

treatment involves methods such as neutralization, mixing and other physical/chemical/biological techniques. Conventional industrial wastewater treatment methods, such as chemical precipitation, oxidization, adsorption, activated sludge/SBR/MBR, have problems such as high treatment cost, poor effluent quality and secondary pollution etc. (Kim et al., 2018; Lee et al., 2014; Xia et al., 2019). Due to the high concentration of organic complex components, strong oxidizing acid and odor, it is difficult to be treated successfully using one single method (Kim et al., 2018). Therefore, how to efficiently treat the organic acid industrial wastewater, recover the energy environmentalfriendly, and lower the energy consumption and chemical reagents consumption, it is necessary to develop new sustainable process applying bio-electrochemical principle and integrating other efficient electrochemical technology by designing self-sustained/self -biased fuel cell reactor. Microbial Fuel Cell (MFC) technology is a new technology that combines biological and electrochemical technologies (Gao et al., 2018b). MFC is applied in wastewater treatment, generating electric energy while degrading organic matter, realizes direct conversion of pollutants to electric energy, and directly convert substrate into small molecular inorganic substances such as CO2 and H2O (Logan et al., 2006; Rabaey and Verstraete, 2005). It is considered a clean energy technique that can turn waste into treasure (Ge et al., 2013). And it can handle a large range of influent COD and has a short start-up time (Xia et al., 2019a). Although there are potential disadvantages such as poor effluent quality, but that can be totally avoided by integrating MFC with MBR or other intensified electrochemical catalytic electrodes. The selection of suitable cathode materials and catalysts can effectively improve not only the oxygen reduction reaction (ORR) rate, but also the effluent quality and power output (Li et al., 2019; Tiwari et al., 2017; Zou et al., 2019). Therefore, the integration of MFC and the scaling ups have been studied as better choices for wastewater treatment. Electrodes with activated carbon (AC) have been widely used, as the carbonized and activated carbonaceous materials/adsorbent, is non-crystalline but with fine graphite crystallite. It has not only large specific surface area, rich porous structure, but also high mechanical strength and stable properties, and excellent acid/alkali resistance. Research about its application in water pollution control and as supports of catalyst has been extensive (Liang et al., 2018; Pedersen et al., 2019;

Vatankhah et al., 2019). Li et al. (Li et al., 2017a) added FeOOH/TiO2/GAC in the cathode chamber of MBR/MFC system, that not only reduced membrane fouling, but also achieved a power density of 5.1 W/m3 and an antibiotic removal rate of over 99%. Fazli et al. (Fazli et al., 2019) used bioelectrochemical cell combined with granular activated carbon (GAC) to treat spent caustic water, improved pollutant removal efficiency by 25% and OCV by 5 times. Catalysts are necessary for MFC systems, often TiO2 and its composites are used for visible light photo-electrochemical and/or electrochemical applications. Compared with other metal oxides semiconductor materials, TiO2 has a relatively high polarity of Ti-O bonds, and the water molecule adsorbed on the surface is dissociated by polarization, hydroxyl groups are easily formed. This surface hydroxyl group can improve the performance of TiO2. In liquid (especially polar) medium, TiO2 particles will adsorb opposite charged molecules due to electrostatic interactions, forming a diffused electric double layer. Doped or modified TiO2 has been widely used in cathode and/or anode (Gu et al., 2015; Li et al., 2017a; Liu et al., 2019; Yang et al., 2018; Yu et al., 2016; Zhao et al., 2019). According to Nishikiori et al. (Nishikiori et al., 2019) silica on titanium dioxide demonstrated high activity for degradation of organic molecules. According to Zhang et al. (Zhang et al., 2015) TiO2/RGO/Cu (II) composite degraded phenol and the degradation rate increased by nearly 3 times compared with unmodified TiO2. Another catalyst, CN (g-C3N4), is cheap and has fairly high activity, is recently used for many chemical reactions (Zhang et al., 2017), contaminants degradation and wastewater treatment (Rabé et al., 2019; Shen et al., 2017; Zhang et al., 2015; Zhang et al., 2016). However, the electrochemical and photocatalytic activity of pure CN is not satisfactory due to quick recombination of excited charges, therefore is less used in fuel cells and other electrochemical processes. But, due to its deep negative conduction band, excellent electron collection and transmission properties, its modification or compounding with other substances is expected to improve its electron transfer efficiency in MFCs. Yan et al. (Yan et al., 2019) prepared g-C3N4/Fe2O3 through the facile one-pot calcination, the oxidation efficiency of bisphenol A is improved, and a cost-effective Visible MFC/CNFe/PDS system is constructed.

The third worth mentioning catalyst is MnO2, a costing-effective and environmentally benign metal oxide, has been also widely used in wastewater treatment, especially in bio-electrochemical water treatment systems (Gao et al., 2018a, 2018b). MnO2 combined with other materials could effectively improve the catalytic performances. For example, Liu et al. (Liu et al., 2018) used graphene oxide and potassium permanganate to form MnO2/GO composite. Aided by H2O2, the MnO2/GO composite exhibits good performance in degrading methylene blue. Zhao et al. (Zhao et al., 2018) prepared a heterostructure of Mn2O3/Mn3O4/MnO2, which is superior to single-valent manganese catalysts, mainly manifested in improved surface area, reduced isoelectric point, enhanced light absorption and effective charge separation. For the above-mentioned reasons, the objectives of this study are to prepare a new MnO2/TiO2/gC3N4@GAC electrode as the MFC cathode for industrial wastewater treatment. To do this, MnO2, TiO2 and g-C3N4 were deposited via in situ growth, sol-gel method, and calcination method. Then, scanning electronic microscopy and electrochemical methods were used to characterize the MnO2/TiO2/g-C3N4@GAC material. Finally, the performance of MFCs with the MnO2/TiO2/gC3N4@GAC electrode in treating high-concentration industrial organic acid wastewater were evaluated. Eventually, the function of MnO2/TiO2/g-C3N4@GAC electrode in MFC system, fed with high-concentration industrial organic acid wastewater was elucidated. 2. Materials and methods 2.1. The preparation of MnO2 doped granular activated carbon (MnO2@GAC) 100 g granular activated carbon (GAC, 0.8-1 mm) were soaked in 300 mL (0.05 mol/L) potassium permanganate solution for 3 h. After being taken out, the GAC was washed with deionized water for several times, and dried at 100 ℃. Then it was placed in a tubular furnace and calcined at 350 ℃ for 3 h with a heating rate of 5 ℃/min, during which N2 was continuously injected, and finally washed and dried to produce MnO2@GAC for use. 2.2. The preparation of MnO2/TiO2/g-C3N4 doped granular activated carbon (MnO2/TiO2/gC3N4@GAC) First, putting 10 g melamine into an alumina crucible with cover, and calcining it in the muffle oven at 550 ℃ for 4 h, the heating rate was 5 ℃/min to obtain a pale yellow solid (Yu et al., 2016).

Then grinding it, sieving it and mixing it evenly in 18.5% hydrochloric acid solution (20 mg/mL) for 6 h ultrasound. Next the solution was centrifuged and washed with deionized water until Ph=7, and dried at 105 ℃ to obtain g-C3N4 powder for later use (Rabé et al., 2019). In a 1000 mL beaker, 680 mL ethanol was added first, then 12 mL pure water and 0.4 mL concentrated hydrochloric acid (37 wt%) were added under magnetic stirring. After evenly mixing, 40 mL butyl titanate was slowly added, and continuous stirring was carried out (Li et al., 2017a). During the mixing, 0.74 g g-C3N4 was added (Yu et al., 2016), and the mixture continued to be stirred for 1 h and then stood for 48 h. Next MnO2@GAC was added to the above solution, stirred evenly and dried at 80 ℃. The dried solid was placed in a tubular furnace and calcined at 400 ℃ for 2 h (heating rate 5 ℃/min), during which N2 was continuously injected to obtain MnO2/TiO2/gC3N4@GAC electrode. 2.3. Construction of MnO2/TiO2/g-C3N4@GAC cathode coupled MFC system and operation MFC anode chamber design size was 5×5×15 cm, filled with GAC which was loaded with electrogenesis microorganism, filling rate was about 80%. The carbon rod 1 (C1, Ф5×100 mm) as the anode was inserted vertically into the center of the anode chamber, and the saturated calomel electrode was used as a reference electrode and inserted into the anode chamber in parallel with the C1. C1 and saturated calomel electrode were separately connected to the data acquisition system to continuously monitor the anode electricity production in real time. Anode chamber and the cathode chamber were connected by a proton exchange membrane, cathode chamber size was 5×4×15 cm, and filled with 80 g above-mentioned prepared MnO2/TiO2/g-C3N4@GAC. An aerator installed at the bottom of the cathode chamber to control the aeration rate. The electrons generated from the anode was transferred into the cathode chamber by the carbon rod 2 (C2) and wires, and C2 was connected to the data acquisition system. 1000 Ω outside resistance was connected between C1 and C2 in series. The wastewater continuously entered the bottom of the anode chamber through the inlet flow regulating device, then entered the bottom of the cathode chamber through the overflow device from the upper part of the anode chamber, finally, the treated effluent flowed out from the overflow device about 3 cm away from the top of the cathode chamber.

When the new MFC reactor with MnO2/TiO2/g-C3N4@GAC electrode was used to treat the organic acid industrial wastewater (pH≈1, COD≈8995.6 mg/L, NH4+-N≈204.5 mg/L, NO3--N≈914.6 mg/L), the influent in the early stage was diluted. Before entering the reactor, the pH should be adjusted to 5-6 with NaOH, and all experiments were performed at room temperature. The initial influent COD of the reactor was about 2000 mg/L. About 2 weeks, the water quality of the effluent, anode potential and the cell voltage were gradually stable. The ammonia nitrogen removal rate is an indicator of biofilm maturation in wastewater treatment, and the removal rate of mature biofilm is above 60%. In our system, the ammonia nitrogen removal rate was significantly higher than 60%, so the anode biofilm could be judged to be mature. The concentration of influent was sequentially increased, divided into four stages, and the corresponding COD was about 2700 mg/L; 4100 mg/L; 6500 mg/L; 9000 mg/L. The hydraulic retention time (HRT) in the early stages (1-3) of the reactor operation was 8 h. Then the HRT was extended to 12 h. When the influent water was undiluted organic acid industrial wastewater, the HRT was kept for 12 h. The system had been running continuously for more than 6 months. At each stage of operation, MnO2/TiO2/g-C3N4@GAC (cat./GAC) and pure GAC filled the MFC cathode chamber at a ratio of 1:0; 1:1; 0:1. During steady operation at each stage, influent and effluent quality, cell voltage, power generation density and polarization curve, etc. in MFC system were monitored. 2.4. Characterization of MnO2/TiO2/g-C3N4@GAC and electrochemical analysis The micro-morphologies of MnO2/TiO2/g-C3N4@GAC, MnO2@GAC and GAC were characterized and compared by scanning electronic microscopy (SEM) (NOVA NANOSEM 450, USA). The elementary composition and distribution of the cathodes was measured using energy dispersive X-ray detector (EDX) during SEM characterization. The cyclic voltammetry curve (CV) of cathode material was tested to characterize the activities using a potentiostat (Chenhua, China) and a three electrodes system. The prepared cathode was served as the working electrode whereas saturated calomel electrode (SCE, type of 232, 0.244 V vs SHE) and a Pt sheet electrode were used as the reference and the counter electrode, respectively. CV was tested between -0.5 and 0.6 V in 5 mM potassium ferricyanide solution and Na2SO4 solution with O2 or N2 over 30 min and the scan rate of 50 mV/s (Li et al., 2015; Li et al., 2017b). The pH of

the solution was not adjusted and was neutral. During the operation period, the anode potential and the cell voltage of the MFC were collected and recorded every 30 minutes by a data logger (Yikong, China). The method used to determine the polarization curve and power density curve of the system was the gradient resistance method. When the system was running stably, the anode and cathode of the system was disconnected from the external circuit to keep it open for more than 2 h. Then a variable resistance box was connected to form a new closed loop. With the resistance value of the variable resistance box changing from 99999.9 Ω to 10 Ω, the stable voltage under the corresponding resistance value was recorded in turn. Electrochemical impedance spectroscopy (EIS) analysis of system with different cathodes were tested. They were carried out in a frequency range of 100 kHz to 0.1 Hz with an alternating current signal of 5 mV amplitude by using a CHI760E electrochemical workstation (CH Instruments, Chenhua Instrument Co. China). EIS measurements were performed When the open potential was stable, and using the two-electrode mode to test measure internal resistance of the whole cell. The maximum energy consumption of degrading COD was calculated, using Eq. (1). 𝑃∆𝑡

𝐸𝐶 = 𝑉∆𝐶𝑂𝐷

(1)

Where P is the total maximum power consumption of equipment (influent peristaltic pump and aeration pump, W), Δt is the time (h), V is the volume of wastewater treated in Δt (L), ΔCOD is the concentration difference between the influent and effluent (mg/L). 2.5. Water quality analysis The water quality index such as COD, NH4+-N, NO3--N, NO2--N and TP were tested and analyzed using APHA standard methods every 24 h (Gao et al., 2017). We started sampling on the fourth day after each change of influent or cathode composition. The UV254 value of the effluent was measured at 254 nm after sampling. The UV-vis absorption spectra of the influent and effluent samples were determined using an ultraviolet spectrophotometer (model UV-5500, Yuanxi, China). Before the samples were tested, the baseline was calibrated with deionized water. The influent was diluted 10 times and then measured, and the effluent samples were directly measured after filtration. The samples were scanned in a 1 cm quartz cuvette in the range of 200-800 nm (Liu et al., 2018). 2.6. Microbial analysis

Biofilm samples taken from the anode chamber and the cathode chamber in the MFC during low and high concentration influent (COD: 4100 mg/L and 9000 mg/L) were extracted respectively. The samples needed to be pretreated, and they were rapidly cooled in liquid nitrogen after purified by centrifugation (Gao et al., 2017). The genomic DNA of the samples was extracted by CTAB or SDS method, and then the purity and concentration of the DNA was detected by agarose gel electrophoresis. The appropriate amount of sample DNA was taken in a centrifuge tube, and the sample was diluted to 1 ng/μL with sterile water. Using diluted genomic DNA as a template, based on the selection of sequencing regions, Polymerase Chain Reaction (PCR) was carried out by using specific primers with Barcode, Phusion® High-Fidelity PCR Master Mix with GC Buffer from New England Biolabs and high-efficiency high-fidelity enzymes to ensure amplification efficiency and accuracy. Primer corresponding area includes: 16S V4 region primers (515F and 806R) for identification of bacterial diversity; 18S V4 region primers (528F and 706R) for identification of eukaryotic microbial diversity; ITS1 region primers (ITS5-1737F and ITS2-2043R) for identification of fungal diversity. In addition, the amplified region also includes: 16S V3-V4/16S V4-V5; archaea 16S V4/V4-V8; 18S V9 and ITS2 regions. PCR products were detected by electrophoresis using 2% agarose gel. The samples were mixed in equal amounts according to the concentration of the PCR product. After thorough mixing, the PCR product was purified by agarose gel electrophoresis with 1×TAE concentration of 2%, and the target band was recovered by shearing. The product purification kit used was the Thermo Scientific GeneJET Glue Recovery Kit. The library was constructed using Thermofisher’s Ion Plus Fragment Library Kit 48 rxns library. After the constructed library had passed the Qubit quantification and library test, sequencing on machine was performed using Thermosper’s Ion S5TMXL (Langille et al., 2013; Liang et al., 2015; Lundberg et al., 2013; Santos et al., 2012; Xia et al., 2019; Zhang et al., 2019; Zhu et al., 2019). 3. Results and discussion 3.1. Material characterizations The micros-morphology changes during the preparation of MnO2/TiO2/g-C3N4@GAC were observed by scanning electron microscopy (SEM). Clearly, significant differences in surface structure were observed. The surface of pure GAC was rough and uneven; for MnO2@GAC, MnO2

was uniformly loaded on the surface of GAC; the surface porosity of MnO2/TiO2/g-C3N4@GAC was increased and evenly distributed. Part of the larger particles of MnO2 loaded in the previous step fell off and the surface structure became smoother. The EDX results indicated that MnO2 and TiO2 were successfully loaded onto the GAC, the weight percentage of Mn in MnO2@GAC and MnO2/TiO2/gC3N4@GAC was more than 8% (0% in GAC) and the weight percentage of Ti in MnO2/TiO2/gC3N4@GAC was about 2%-4% (0.04% in both GAC and MnO2@GAC). 3.2. Electrochemical performance The catalytic activities and ORR activities of MnO2/TiO2/g-C3N4@GAC and GAC were shown in Fig. 1. MnO2/TiO2/g-C3N4@GAC had obvious oxidation peak at ～0.328 V and reduction peak at ～0.129 V (Fig. 1a), and the catalyst was active in much wide range in potential. Moreover, MnO2/TiO2/g-C3N4@GAC also had a ORR peak at ～0.041 V (Fig. 1b). The results indicated that MnO2/TiO2/g-C3N4@GAC had more significant electrochemical catalytic properties than GAC. At each stage of the system operation (distinguished by influent), the voltage output of the system was shown in Fig. 2. In Fig. 2a to c, the MFC voltage output could also be divided into three stages, corresponding to: cathode composition was MnO2/TiO2/g-C3N4@GAC (cat. / GAC) and GAC at a ratio of 1:0; 1:1; 0:1. After the anode microorganisms in the system had been domesticated for about 2 weeks, the anode potential output remained relatively stable. When the cathode was filled with MnO2/TiO2/g-C3N4@GAC (1:0), the cell voltage was the highest, reaching a maximum of 0.439 V, which was 20% higher than the control group using GAC cathode (0:1). In Fig. 2a to d, as the concentration of influent increased, the anode potential output decreased, resulting in a decrease in the cell voltage output. The species of anode microorganisms changed with the increase of the influent concentration, and the electrogenic bacteria decreased slightly. And the increase of NO3- and NO2- in influent directly consumed part electrons generated by the anode, so the anode potential output decreased. However, under the same influent conditions, the anode potential remained relatively stable, and using MnO2/TiO2/g-C3N4@GAC cathode (1:0), the cell voltage was the highest in each influent stage. The trend of the cathode potential and the cell voltage was similar, indicating that the cathode potential was affected by the cathode catalytic activity and could further affect the

cell voltage. During the test, MnO2/TiO2/g-C3N4@GAC was reused without any physical or chemical cleaning. As the reaction time increased, the cathode catalyst may partly fall off, and microorganisms may adhere to the surface, resulting in a decrease in the catalytic activity of the cathode and the cathode potential. Like Fig. 2c, the cell voltage reduced by 0.04 V at the later stage. This phenomenon could be caused by the biofilm formed by the massive growth of microorganisms on the MnO2/TiO2/g-C3N4@GAC cathode. The large amount of microbial adhesion obscured the catalytic component of the surface of the GAC, so that the mass transfer of O2 was hindered, directly affecting the ORR rate, thus causing a decrease in voltage output (Gao et al., 2018a). In treating the high-concentration organic acid industrial wastewater, the energy production of the system was investigated. The results of MFC polarization curve and power density curve were shown in Fig. 3. As shown in Fig. 3a, the OCV measurement value of MFC was 0.209 V (cat. / GAC cathode) and 0.174 V (GAC cathode), respectively. Although this value was lower than those reported in previous studies, it was mainly due to the anodic microorganisms, and it could still prove that MnO2/TiO2/g-C3N4@GAC performance was better than pure GAC. The maximum power density was 711.76 mW/m3 (normalized anode chamber volume) with GAC cathode, and the corresponding current density was 3.82 A/m3 (Fig. 3b). Using MnO2/TiO2/g-C3N4@GAC cathode, the maximum power density of the MFC system reached 1176.47 mW/m3 at a current density of 6.95 A/m3 (Fig. 3b), the increase was 65.29%. The internal resistance of the system was about 91 Ω (cat. / GAC) and 147 Ω (GAC), respectively. Fig. 3c, d and Table 1 showed EIS test results of the system. The ohmic (R1), charge transfer (R2) and mass transfer (R3) resistance of the system using MnO2/TiO2/g-C3N4@GAC were all lower than the control group using GAC cathode. Although the measured values were lower than the internal resistances of the system obtained through the polarization curve, this could prove that using MnO2/TiO2/g-C3N4@GAC as the cathode of MFC was better than GAC. The catalyst supported on the surface and in the channel of GAC increased the ORR rate and reduced the activation energy required for the reaction. The reduction of internal resistance could effectively reduce the internal energy loss in the system and improve the efficiency of energy recovery. This result proved that using MnO2/TiO2/g-C3N4@GAC as the MFC cathode had a better

performance advantage. 3.3. Contaminants removal in MFC 3.3.1. COD removal in MFC The COD removal rate is an important index for assessing the wastewater treatment capacity of the system. In order to investigate the effect of MnO2/TiO2/g-C3N4@GAC on the removal of COD by MFC, this experiment was set up at different influent concentrations by using 20%-100% organic acid industrial wastewater. When other settings were unchanged, the COD concentration and removal rate of MFC effluent were compared (Fig. 4). When the influent COD concentration was about 2700 mg/L (Fig. 4a), there was no significant difference in the COD removal rate of effluent. The COD concentration of MFC anode effluent was about 800 mg/L. GAC had good conductivity and adsorption property, and MFC with GAC cathode could effectively remove part of pollutants. With the increase of COD concentration in influent, the difference in COD removal rate with different cathodes was more obvious (Fig. 4b, c, e), and the advantage of MnO2/TiO2/g-C3N4@GAC as a cathode were reflected. When the influent COD concentrations were about 4100 mg/L and 9000 mg/L respectively, the anode effluent COD concentrations were about 870 mg/L and 6460 mg/L, and the COD removed in MFC anode was about 3230 mg/L and 2540 mg/L, indicating that the treatment capacity of the anode for wastewater was slightly reduced with the increase of the influent concentration, which was consistent with the anodic energy production. When the influent COD concentration reached 6500 mg/L and HRT was 8 h, although the COD removal rate with MnO2/TiO2/g-C3N4@GAC cathode was over 90%, the effluent COD concentration was still high. Extending HRT to 12 h, the effluent COD could be reduced to below 100 mg/L (Fig. 4d). Therefore, when the system influent was undiluted high-concentration organic acid industrial wastewater, the HRT was kept for 12 h (Fig. 4e). When using GAC as the MFC cathode, the COD removal rate was about 70% (the average removal rate in MFC cathode was about 44.02%); the COD removal capacity was 12.99 kg COD m-3d-1; and the maximum energy consumption for degrading COD was 457.91 kWh/kg COD. Using MnO2/TiO2/g-C3N4@GAC cathode, the removal rate of COD was over 98% (the average

removal rate in MFC cathode was over 70.61%, the increase was approximately 60.4%); the COD removal capacity was 17.77 kg COD m-3d-1, the increase was 36.83%; and the maximum energy consumption of degrading COD was 334.66 kWh/kg COD, which had been reduced by 26.92%. Fig. 4f was the absorbance of the effluent measured at the wavelength of 254 nm, which was a great index of the concentration of organic matter in water. The result shown in Fig. 4f was consistent with the trend of the aforementioned COD test result. In this study, the removal rate of COD was more than 98%, the COD removal capacity reached 17.77 kg COD m-3d-1. The COD removal capacity was much higher than other similar studies, 4 times blackwater (Gao et al. 2019), twice slaughterhouse wastewater (Vidal et al. 2019), 8 times coking water (Zhu et al. 2019) and 24 times municipal wastewater (Liang et al. 2018). The above results indicated the MFC reactor with prepared novel MnO2/TiO2/g-C3N4@GAC electrode had the ability to treat high load industrial wastewater, with high organic acid and rich nitrate. 3.3.2. Nitrogen removal in MFC The concentrations and removal rates of the effluent NH4+-N and NO3--N of the MFC were shown in Fig. 5. As with the influent COD increase, the concentrations of the influent NH4+-N and NO3--N were also gradually increased. When cathode was cat. / GAC:GAC=1:0; 1:1, and influent NH4+-N concentrations were about 37 mg/L and 68 mg/L, the removal rate of NH4+-N reached 100%. When the influent concentration continued to increase (the third influent stage, influent NH4+-N=92 mg/L), the removal rate of NH4+-N decreased slightly. With the extension of HRT from 8 h to 12 h, the removal rate of NH4+-N increased again to over 99%. On the whole, when 100% MnO2/TiO2/g-C3N4@GAC was used as the MFC cathode, the effluent NH4+-N concentration was lower than 1mg/L, and the removal rate was maintained above 99% (excluding the third influent stage, HRT=8 h). In the degradation process of NO3--N, the advantage of MnO2/TiO2/g-C3N4@GAC as MFC cathode was more significant. Under different influent concentrations, the removal rate of NO3--N test result showed 100% MnO2/TiO2/gC3N4@GAC>50% MnO2/TiO2/g-C3N4@GAC+50% GAC>100% GAC (1:0>1:1>0:1). When the influent was undiluted organic acid industrial wastewater (Fig. 5a), with MnO2/TiO2/gC3N4@GAC cathode, the NH4+-N concentration of anode effluent was about 54 mg/L, the average

removal rates of the anode and the cathode were 73.39% and 26.36%, respectively. The NH4+-N removal capacity was 0.41 kg NH4+-N m-3d-1. The removal rate of NO3--N was over 99%, which was 19.48% higher than the control group using GAC cathode (Fig. 5b). The NO3--N concentration of anode effluent was about 180 mg/L, so the contribution rates of anode, GAC, MnO2/TiO2/g-C3N4@GAC in the process of NO3--N removal were 80.30%, 0.20%, and 19.48%, respectively. The NO3--N removal capacity was 1.83 kg NO3--N m-3d-1, the increase was 24.2%. The denitrification process could occur at both the anode and cathode of MFC. The anode contained a large amount of organic matter, so there was mainly a heterotrophic denitrification process, could simultaneously remove COD and nitrate and nitrite: (CH2O) n→CO2+e-; NO3-+5e+6H+→0.5N2+3H2O. In cathode, both NO3- and NO2- could act as cathode electron acceptors for cathodic electrochemical denitrification. The cathode nitrate reduction process was: NO3-→NO2→N2O→NO→N2. Ammonia nitrogen was converted into nitrate or nitrite by nitrification of nitrifying bacteria under aerobic conditions, or joint redox electrochemical reaction similar with biological anammox process. There were aerobic and anoxic regions in the cathode of MFC, which could realize simultaneous nitrification and denitrification. It can be seen from the results that part of the ammonia nitrogen had been converted at the anode, so there may be an anaerobic ammonia oxidation process. Under anaerobic conditions, ammonia nitrogen and nitrite nitrogen could be removed simultaneously. The principle was to use NH4+ as the electron donor and NO2- as the electron acceptor to generate N2: NH4++ NO2-→N2+2H2O. The NO2--N concentration in the effluent was shown in Fig. 5c. There was little NO2--N in influent. When using MnO2/TiO2/g-C3N4@GAC cathode, the NO2--N concentration in the effluent was significantly lower than the control group using GAC cathode. The NO2--N in the effluent was mainly formed by the transformation of NH4+-N and NO3--N. The above results proved that the system had a high denitrification capacity. In addition, the concentrations of TP in both influent and effluent were quite low. 3.3.3 UV-vis absorption spectra analysis The characteristic UV-vis absorption spectrum was related to the molecular structure of the

substance in the water sample, which could reflect the content of unsaturated bonds in the molecule, and the UV absorption value was positively correlated with the aromaticity and molecular complexity of the organic matter. The untreated organic acid industrial wastewater showed a high absorption peak in the range of 200-250 nm, indicating that the wastewater contained a large amount of polycyclic aromatic compounds and macromolecular organic matter with carbonyl and conjugated double bonds. The absorption peak of wastewater treated by MFC was significantly reduced. When MnO2/TiO2/g-C3N4@GAC was used as the MFC cathode, the UV-vis absorption spectrum of the effluent appeared a blue shift, indicating that the molecular coupling effect and condensation degree became weak and a large number of conjugated double bonds were destroyed. All of the above proved that the aromatics and macromolecular organic matter in effluent were significantly reduced. In addition, the removal of different organic species in wastewater samples could be easily reflected by means of the absorbance values at different characteristic wavelengths (Liu et al., 2018). As shown in Fig. 4f, the absorbance of the samples at a wavelength of 254 nm could indicate the aromaticity of the organic compounds in water. The larger the absorbance value, the more complicated the aromaticity. The absorbance of the organic acid industrial wastewater diluted 10 times at 254 nm was 0.330. Referring to Fig. 4f, when MnO2/TiO2/g-C3N4@GAC was used as the MFC cathode, the effluent absorbance at 254 nm was smaller, indicating that the aromaticity of the organic compound in water was smaller, which was consistent with the above analysis results. 3.4. The analysis of anode and cathode microorganisms 3.4.1 Diversity of microbial community We used 16S rDNA Amplicon Sequencing to analyze microorganisms before acclimation (L1), microorganisms in the anode chamber and cathode chamber (L2, L3, COD ≈ 4100 mg/L), microorganisms in the anode chamber and cathode chamber (L4, L5, COD ≈ 9000 mg/L). A total of 322197 effective sequences and 1455 OTUs were retrieved from five samples, and each subsample produced 47629-80178 effective sequences. In order to study the species composition diversity of the samples, the clean reads (or Effective Tags) of all samples were clustered, and the sequences were clustered into OTUs (Operational Taxonomic Units) with 97% identity. The OUTs numbers for the five samples were 172, 184, 151, 501, 447, respectively. The statistical analysis and rarefaction

curves summarizing the OTUs clustered from clean reads were showed in Table 2 and Fig. 6a, respectively. Good’s coverage revealed that the results represented the majority of bacterial 16S rRNA sequences present in each subsample, and the values ranged from 99.7% to 100.0% (Zhu et al., 2019). Alpha Diversity is used to analyze the microbial community diversity in the sample (Withincommunity). The single sample diversity analysis can reflect the richness and diversity of microbial communities in the sample. The Shannon and Simpson indices are used to assess the abundance and uniformity of microbial communities, and the greater the Shannon and Simpson indices value, the higher the uncertainty of the microbial community and the higher the species richness of the sample. The Chao1 and ACE indices are commonly used to estimate the number of species in the sample, with larger ACE and Chao1 representing higher bacterial abundance (Xia et al., 2019). As shown in Table 2, compared with L1 and L3, L2 had larger number of microorganisms and higher diversity of communities. This result indicated that the microbial community diversity of MFC anode microorganisms was increased after acclimation with organic acid wastewater. This was because the bacterial abundance of L2 increased due to the increase in the variety of nutrients after acclimation of organic acid wastewater. As the substrate degradation, the nutrient content decreased and the microbial bacterial abundance of the MFC cathode decreased. Compared with L2 and L3, the number of microorganisms and diversity of communities in L4 and L5 had a significant increase. It also proved that the increase of nutrients could increase the microbial community diversity. 3.4.2. Microbial community composition In order to study the changes of microbial community in MFC, the relative abundance of different groups at the phylum level was analyzed. As shown in Fig. 6b, c, the main phylum in sample L1 was Firmicutes, which accounted for 90.92%. This was because the microorganisms in L1 used glucose as a nutrient, and Firmicutes could convert glucose into simple molecules, providing energy to producers in glucose-fed MFC, so the proportion of Firmicutes in L1 was the largest (Jung and Regan, 2007). Proteobacteria was the dominant group in L2, followed by Bacteroidetes and Firmicutes, with relative abundances of 82.77%, 12.03%, and 3.20%, respectively. The βProteobacteria in Proteobacteria was responsible for the removal of organic matter and nutrients

(Cydzik-Kwiatkowska and Zielińska, 2016). The main role of Bacteroidetes was to break down macromolecules such as proteins, starch, cellulose and fiber (Yang et al., 2015). Compared with L2, Proteobacteria in L3 was significantly reduced to 45.82%, which may be related to the decrease of organic matter and nutrient content in the cathode substrate. The relative abundance of Bacteroidetes in L3 was 50.57%, which was superior to Proteobacteria. As the influent concentration increased, Proteobacteria became the dominant group in L4 and L5 again (respectively 91.13% and 82.64%). This result was consistent with the foregoing conclusion. Other less abundant phyla were also widely detected in subsamples, including Euryarchaeota, Actinobacteria, Acidobacteria, Tenericumutes, Verrucomicrobia and others. There were 0.04%0.24% unidentified Bacteria in the samples, indicating that there was an unknown new taxonomic group at the phylum level. According to previous studies, γ-Proteobacteria in Proteobacteria was ubiquitous in wastewater containing nitrate. The main genera of γ-Proteobacteria had good utilization of nitrate, which was also the reason why Proteobacteria was abundant in the treatment of industrial organic acid wastewater containing high concentration of nitrate nitrogen (Zhu et al., 2019). The species abundance cluster heat map of the five samples at the genus level were shown in Fig. 6d, e. The clustering results matched the species relative abundance at the phylum level of Fig. 6b, c. Bacteroides sp. in L1 was the dominant functional bacteria that converted organic matter in water into acids and alcohols, and the bacteria belonging to Firmicutes such as Clostridiales sp. could use carbon metabolites to generate electricity in MFC. Dechloromonas sp. was not only the main electrogenic microorganism in L2, but also denitrifying bacteria, which played an important role in the removal of nitrate nitrogen. Thauera genus was abundant in L2 and L4, it was a typical denitrifying bacterium and also a group of important functional groups with the ability to degrade a variety of aromatic pollutants. In the denitrification process, most of the Thauera bacteria could completely convert nitrate nitrogen into N2. Stappia sp. (in L4) was also widely used in the field of biodegradation. After the microorganisms were domesticated by organic acid industrial wastewater, the dominant group changed with the organic matter and nutrients to adapt to the change of water

quality change. In the case of controlled aeration, there were aerobic and anoxic zones at different locations of the MFC cathode. The presence of typical microorganisms such as Candidatus Nucleicultrix in L3 indicated that simultaneous nitrification and denitrification reactions could occur in the MFC cathode (Gao et al., 2017). Azoarcus (in L5) was an aerobic bacterium and a genus of nitrogenfixating bacteria, and could grow well in organic acids. Rhee et al. (Rhee et al., 1997) isolated a strain of Azoarcus from industrial wastewater and degraded pyridine in heterocyclic amines under the same route under aerobic and anaerobic conditions. Due to the difference in influent concentration, the microbial composition differed in MFC at different stages. However, the anode microorganisms were still dominated by denitrifying bacteria and electrogenic bacteria, and the cathode microorganisms were mainly aerobic and facultative bacteria in general. 4. Conclusions In this study, we prepared a novel MnO2/TiO2/g-C3N4@GAC electrode and developed a new MFC reactor with it for efficient treatment of high-concentration organic acid industrial wastewater, realizing efficient removal of contaminants, such as COD, NH4+-N, NO3--N, and electricity generation/energy recovery and in-situ use. During stable operation for more than 6 months, the electrode had maintained definite electrochemical activity and stability, without fouling. Adapting to high load acidic wastewater and long-term operation, it shows excellent performance and application prospects in treating industrial wastewater treatment, with very significant high treatment capacity 17.77 kg COD m-3d-1. Acknowledgements This work was supported by the National Natural Science Foundation of China (grant number 21677025). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at ****. References

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Figure captions Fig. 1. The catalytic properties of MnO2/TiO2/g-C3N4@GAC (cat. / GAC) and GAC: (a) the catalyst activities; (b) the ORR activities. Fig. 2. Electricity generation of the MFC. The influent COD was about: (a) 2700 mg/L; (b) 4100 mg/L; (c) 6500 mg/L; (d) 9000 mg/L. Staged operation: the ratio of cat. / GAC: GAC in 1st, 2nd and 3rd stage was 0:1; 1:1 and 1:0, respectively.

Fig. 3. Electrochemical performance: (a) Voltage-resistance curve; (b) Polarization curve and power density curve using GAC and MnO2/TiO2/g-C3N4@GAC. Nyquist plot of impedance spectroscopy: (c) Total cell of MFC using MnO2/TiO2/g-C3N4@GAC; (d) Total cell of MFC using GAC. Fig. 4. The concentration and removal rate of COD in effluent of MFC. The influent COD was about: (a) 2700 mg/L; (b) 4100 mg/L; (c) 6500 mg/L; (d) 6500 mg/L (HRT = 8 h, 12 h); (e) 9000 mg/L; (f) 9000 mg/L (UV254). The ratio of cat. / GAC: GAC was 0:1, 1:1 and 1:0. Fig. 5. The concentration and removal rate of NH4+-N, NO3--N and NO--N in effluent of MFC. The ratio of cat. / GAC: GAC was 0:1 and 1:0. Fig. 6. (a): Rarefaction curve of microorganisms in different groups. (b) (c): Relative abundance of 16S rRNA sequences of different groups at phylum levels. (d) (e): Heat map analysis of different groups at genus levels. L1: microorganisms before acclimation; influent COD≈4100 mg/L, L2: anode microorganisms, L3: cathode microorganisms; influent COD≈9000mg/L, L4: anode microorganisms, L5: cathode microorganisms.

Fig. 1. The catalytic properties of MnO2/TiO2/g-C3N4@GAC (cat. / GAC) and GAC: (a) the catalyst activities; (b) the ORR activities

Fig. 2. Electricity generation of the MFC. The influent COD was about: (a) 2700 mg/L; (b) 4100 mg/L; (c) 6500 mg/L; (d) 9000 mg/L. Staged operation: the ratio of cat. / GAC: GAC in 1st, 2nd and 3rd stage was 0:1; 1:1 and 1:0, respectively.

Fig. 3. Electrochemical performance: (a) Voltage-resistance curve; (b) Polarization curve and power density curve using GAC and MnO2/TiO2/g-C3N4@GAC. Nyquist plot of impedance spectroscopy: (c) Total cell of MFC using MnO2/TiO2/gC3N4@GAC; (d) Total cell of MFC using GAC.

Fig. 4. The concentration and removal rate of COD in effluent of MFC. The influent COD was about: (a) 2700 mg/L; (b) 4100 mg/L; (c) 6500 mg/L; (d) 6500 mg/L (HRT = 8 h, 12 h); (e) 9000 mg/L; (f) 9000 mg/L (UV254). The ratio of cat. / GAC: GAC was 0:1, 1:1 and 1:0.

Fig. 5. The concentration and removal rate of NH4+-N, NO3--N and NO--N in effluent of MFC. The ratio of cat. / GAC: GAC was 0:1 and 1:0.

Fig. 6. (a): Rarefaction curve of microorganisms in different groups. (b) (c): Relative abundance of 16S rRNA sequences of different groups at phylum levels. (d) (e): Heat map analysis of different groups at genus levels. L1: microorganisms before acclimation; influent COD≈4100 mg/L, L2: anode microorganisms, L3: cathode microorganisms; influent COD≈9000mg/L, L4: anode microorganisms, L5: cathode microorganisms.

48. Table 1 Comparison of the ohmic (R1), charge transfer (R2) and mass transfer (R3) resistance (Ω) of the system using cat. / GAC and GAC. R1

R2

R3

Cat. / GAC

8.6

3.2

39.8

GAC

10.7

7.4

63.3

Table 2 Alpha Diversity analysis index for different samples at 97% consistency threshold. Sample Observed

Shannon Simpson

Chao1

ACE

Good’s

PD

coverage

whole

(%)

tree

name

species

L1

172

1.99

0.47

190.60

195.19

99.9

19.87

L2

184

3.77

0.88

202.53

212.02

99.9

26.43

L3

151

3.85

0.84

168.27

163.62

100

22.38

L4

501

3.95

0.79

585.42

598.98

99.7

49.72

L5

447

3.77

0.85

514.80

541.61

99.8

53.16

Observed species were the total amount of sample OTUs.

49. Highlights 

The prepared MnO2/TiO2/g-C3N4@GAC had great electro-catalytic properties.



A MFC with the above cathode efficiently removed COD, NH4+-N and NO3--N.



The COD removal capacity was 17.77 kg COD m-3d-1.



Proteobacteria was dominant in MFC anode.

50. Graphical Abstract

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51. Credit Author Statement Qian Zhang: Conduct test and analysis, Data curation, Formal analysis, WritingOriginal draft preparation. Lifen Liu: System & concept design, Funding acquisition, Resources, Supervision, Writing- Reviewing and Editing. 52.

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