Huiyu Li ,Ming Gao ,Pan Wang ,Hongzhi Ma, *,Ting Liu ,Jin Ni ,Qunhui Wang,Tien-Chin Chang

1 Department of Environmental Engineering,University of Science and Technology Beijing,Beijing Key Laboratory of Resource-oriented Treatment of Industrial Pollutants,Beijing 100083,China

2 Key Laboratory of Cleaner Production and Integrated Resource Utilization of China National Light Industry,Beijing Technology and Business University,Beijing 100048,China

3 Institute of Environmental Engineering and Management,National Taipei University of Technology,Taipei 106,China

Keywords:Microbial fuel cell Oxygen reduction reaction Fermentation Catalyst Biomass Biotoxicity

ABSTRACT Bacterial cellulose doped with P and Cu was used as a catalyst for a microbial fuel cell (MFC) cathode,which was then used to treat ethanol fermentation stillage from food waste.Corresponding output power,coulombic efficiency(CE),and biological toxicity were detected.Through a series of characterization experiments,the addition of the cathode catalyst was found to improve catalytic activity and accelerate the consumption of the substrate.The resulting maximum output power was 572.16 mW·m-2.CE and the removal rate of chemical oxygen demand(COD)in the fermentation stillage by P-Cu-BC reached 26%and 64.5%,respectively.The rate of biotoxicity removal by MFC treatment reached 84.7%.The aim of this study was apply a novel catalyst for MFC and optimize the treatment efficiency of fermentation stillage.

1.Introduction

Owing to the depletion of fossil fuel reserves and various environmental issues,biomass as an alternative fuel source has attracted considerable interest [1,2].Along with economic development and improvement in living standards,the output of the food waste,a nutrient-rich and abundant biomass feedstock,has risen sharply;In China,the output has reached 60 million tons per year [3].Food waste is an invaluable resource because it is biodegradable and renewable and can produce high energy [4].Food waste disposal and energy recovery have gradually attracted the attention of environmental scientists.At present,many researchers use food waste to ferment ethanol,microbial lipids,and other biofuels,thus producing fermentation wastewater [5,6].

In recent years,the microbial fuel cell (MFC) technology have been used to remove chemical oxygen demand(COD),salinity,toxins,and other contaminants in wastewater and convert chemical energy present in organic/inorganic wastes in substrates into electricity,particularly through the use of electrogenerated bacteria[7,8].MFC is becoming a versatile renewable energy technology.Food waste fermentation stillage has high COD and biochemical oxygen demand (BOD) and is rich in sugar and other substances[9].Food waste fermentation stillage not only can be used as a substrate for MFC electricity production but also is effective in removing pollutants[10].Although some studies have indicated that food wastewater has some applications in MFC,few focused on treating ethanol fermentation stillage.Considering electricity production and stillage treatment in MFC operations remains a great challenge.Jiaet al.showed that the maximum output voltage of MFC with food waste as substrate was 320 mV after 600 h and the COD removal rate was 86.4% [11].Zhanget al.found that the output voltage of food waste water can be stabilized at 500 mV after 10 days and organic matter removal rate was 90% [12].MFCs become stable after wastewater treatment,and the oxygen reduction reaction (ORR) of its cathode is much slower than that of its anode [13,14].

It was found that adding a variety of non-noble metal doped carbon nanofiber catalysts to the MFC cathode can effectively improve the electrocatalytic activity [15,16].Among them,bacterial cellulose composed of ultrafine fibers has the advantages of good biocompatibility and one-dimensional nanostructure [17].Therefore,it is widely used in the field of functional materials,mainly including biological functional materials,optoelectronic materials,catalytic materials,etc.[18-20].The study demonstrated that bacterial cellulose doped with P and Cu can be pyrolyzed in an inert atmosphere after freeze-drying for the production of highquality ORR catalysts [21].And the electrocatalytic activities of these catalysts are superior to those of common carbon black,graphene oxide,and other common materials.The mass transfer rate of the counter electrode is faster than that of precious metal Pt.Therefore,a low-cost,high-activity,and high-stability cathode catalyst can be obtained by using bacterial cellulose as an excellent precursor of high-conductive carbon nano-fiber (CNF) aerogel.

In the present study,a novel cathode catalyst was applied to accelerate the cathode reaction in ineffectively disposed food waste fermentation stillate.This approached accelerated the degradation of organic matter in the substrate and improved the efficiency of the fermentation stillage.Performance in terms of MFC production and stillage degradation was determined by a series of electrochemical methods and COD,organic acid,and biotoxicity tests.The present work can provide a novel method for optimizing the performance of MFC treatment technologies applied to food waste fermentation stillage.

2.Experimental

2.1.Food waste fermentation stillage

Before ethanol fermentation,food waste was pretreated according to the methods described by Maet al.[22]and then mixed with water at a ratio of 2:1,and glucoamylase (Ao Bo Xing Company,China) was added as an enzyme of 100 U·g-1in a broth.Saccharified the solution at 60°C for 6 h,then the liquid was centrifuged at 4000 r·min-1for 10 min.The supernatant was inoculated with 10%dry yeast (Anqi Company,China) for fermentation.And the fermentation stillage obtained was used for the MFC substrate.

2.2.Catalyst and air-cathode fabrication

According to the methods of Liet al.[21],put the bacterial cellulose(Hainan Yide Food Co.,LTD.,China)into a beaker containing 50 ml of 0.02 mol·L-1H3PO4aqueous solution,and continuously rotated for 6 h at 100 r·min-1in a shaker at room temperature.Then continue to add 50 ml of 0.01 mol·L-1CuCl2aqueous solution to the beaker and rotate for 6 h.Three kinds of cathode catalysts were prepared by freeze-pyrolysis and the catalysts were named bacterial cellulose (BC),P-doped bacterial cellulose (P-BC),and P,Cu-doped bacterial cellulose (P-Cu-BC),and the control group was Pt.The morphology image and pore structure information has been provided in the Supplementary Material.The cathode film prepared in our previous study served as an air diffusion layer[23].The mass ratio of the abovementioned powdered catalyst to the Nafion PFSA polymer(Yier Sheng International Trade Co.,Ltd.)was 1:4.After dilution with isopropyl alcohol,it is applied to the cathode film multiple times to dry.

2.3.MFC configuration and operation

A MFC with a single-chamber air cathode and working volume of 120 ml was constructed as previously described[24].The bioanode was based on a carbon felt,and a cathode film coated with different catalysts were installed on the MFCs.The stillage was diluted 100 times with PBS buffer and injected into the reactor containing 0.62 g·L-1NH4Cl,5.54 g·L-1NaH2PO4·2H2O,23.08 g·L-1Na2HPO4·12H2O,0.26 g·L-1KCl,trace minerals (10 ml·L-1),and vitamins(1 ml·L-1)[25].In addition,the MFC was inoculated with activated sludge.When the microorganisms were attached to the carbon felt and the output voltage was maintained stable,the activation was regarded as completed.

2.4.Analysis

In the present study,electrochemical test was further explored on an electrochemical workstation (CHI660C,Shanghai Chenhua Instrument Co.,Ltd.)viaelectrochemical methods.Electrocatalytic activity was characterized by using cyclic voltammetry (CV).The reactor bioanode,the air cathode,and the Ag/AgCl electrode(saturated KCl) were used as the working electrode,the counter electrode,and the reference electrode,respectively.CV was performed from-700 mV to 500 mV with a scan rate of 5 mV·s-1.The polarization curves and power density curves of the MFCs were measured.The bioanode served as the working electrode,whereas the air cathode served as a counter and reference electrode under an open circuit voltage -100 mV with the sweeping rate of 0.5 mV·s-1[21].The COD was measured according to standard methods.The biotoxicity test was based on previous research[24].The coulombic efficiency (CE) of the MFC was calculated by the following formula [26]:

whereMis the molecular weight of oxygen(32),Fis Faraday’s constant,nis the number of electrons exchanged per mole of oxygen(4),Vanodicis the volume of liquid in the anodic chamber,and ΔCOD is the change in COD over timet.

3.Results and Discussion

3.1.ORR kinetics on different catalysts

To investigate the possible reasons for the ORR activity of the different catalysts,the reaction kinetics was studied using rotating-disk electrode (RDE) voltammetry in a 0.1 mol·L-1KOH electrolyte saturated with O2at 1600 rpm.The lsv curves of the various catalysts are presented in Fig.1.The onset potential of BC,P-BC,P-Cu-BC and Pt are -0.277 V,-0.124 V,-0.061 V and-0.026 V respectively.It can be found that the onset potential of P-Cu-BC is close to that of Pt,and both have similar ORR electrochemical activity.There is no well-defined diffusion limiting current plateau for all carbon samples,which is similar to other non-noble metal carbon-based ORR catalysts [27].The reason for this phenomenon may be due to the existence of a rather thick film on the rotating disk and the complex rough surface of the carbonbased catalyst,so the mass transfer layer is not smooth.Another possible explanation is that the electrocatalytic active sites of the sample are not uniform,which may cause the inclined current in response to polarization [28].

Fig.1. Rotating-disk electrode LSV at 1600 r·min-1 with different cathode catalysts in 0.1 mol·L-1 KOH.

3.2.Cyclic voltammetry curves of the MFCs

The redox activity of the electrode surface was characterized by CV.After the MFCs were stably operated for about 1 week,the CV results of the four MFCs were tested between the voltage range of-0.7 and 0.5 V(vs.Ag/AgCl).As shown in Fig.2,the three MFCs had obvious oxidation peaks.The maximum current generated by the P-Cu-BC reached 16.24 mA,and the maximum current of Pt was 13.16 mA.These results were consistent with those of previous studies[21];after sodium acetate was replaced with fermentation stillage,the anode still formed a stable biofilm.In addition,the microorganisms had certain activity,and the introduction of P and Cu promoted the operation of the whole system.When the voltage was retraced from -0.4 V to -0.6 V,the four catalysts had ORR at the cathode,as indicated by the reduction peaks of the four MFCs.Devalet al.[29] found that when using bioelectrochemical systems for waste disposal,electrogenic bacteria in MFC can fully use acetic acid to generate electrons.The ethanol fermentation stillage in the current study contained various organic acids and is suitable as a substrate for MFC.

Fig.2. CV curves for MFCs equipped with different cathode catalysts.

3.3.Power output performance of the MFCs

Polarization behavior was further explored as discussed in Section 2.4,and electricity production in the MFCs was observed.As shown in Fig.3,the maximum output power density of P-Cu-BC was 572.16 mW·m-2,which was slightly higher than that of Pt,511.88 mW·m-2.These values were 1.9 times and 1.4 times of those of BC (299.2 mW·m-2) and P-BC (404.86 mW·m-2),respectively.The maximum output power obtained in this study was lower than the use of MFC with pure substance as substrate,such as sodium acetate[30]because the complex organic matter in food waste was not easily degraded in MFC,thus limiting the power generation [31].However,in comparison with reported results of other recent MFC cathode catalysts,the excellent electrochemical performance of the P-Cu-BC cathode is exhibited in Table 1.Liet al.[35] analyzed the characteristics of organic matter in food waste before and after MFC treatment,studied the biodegradation and conversion of organic matter,and obtained the suitable method for recovering electricity.The result was a volume-based maximum output power of 5.6 W·m-3,which was lower than the P-Cu-BC results in the current study,5.99 W·m-3.At the same time,Jiaet al.[11]used MFC to treat food waste,and found that the maximum output power was 556 mW·m-2when the COD was(3200 ± 400) mg·L-1,which was lower than the maximum output power of 572.16 mW·m-2in the current study.

3.4.COD removed by MFCs

The removal rate of COD by MFC using different catalysts was obtained by ultraviolet spectrophotometry.The results were shown in Fig.4.The influent COD of P-Cu-BC was 1223 mg·L-1,and the effluent COD was 434 mg·L-1.The influent CODs of the other three MFCs was maintained at 1400 mg·L-1,and the effluent was 600 mg·L-1.Given that all the MFCs were mixed with the same PBS buffer,a large amount of organic matter in the stillage were observed,and organic matter was unevenly distributed.The COD removal rates of BC,P-BC,P-Cu-BC,and Pt were 54.8%,55.5%,64.5%,and 59.0%,respectively.Consistent with the electricity production effect,the MFC doped with P and Cu in the same time had the maximum COD removal rate,which was even slightly higher than that of Pt.The removal rate improved 1.5 times relative to the research results of Sakdaronnaronget al.[32] which obtained by using MFC to degrade food waste.These results indicated that the catalyst doping of P and Cu accelerated mass transfer rate and has a positive effect on the removal of COD in the substrate,and it could be used as a catalyst for simultaneous electricity generation and treatment of stillage.

Fig.3. (a) Polarization curves for MFCs equipped with different cathode catalysts,(b) Maximum current density and power density based on area and volume.

Table 1 Performance comparison of different MFC cathode catalysts

Fig.4. The COD concentration and COD removal rate of influent and effluent of four MFCs.

3.5.Coulombic efficiency of the MFCs

In the comprehensive assessment of combined power generation and stillage treatment,demonstrating that an MFC can recover electricity from food waste with efficient removal of the organic matters,CE was obtained.As shown in Fig.5,the CE results of BC and P-BC were almost equal.The result of P-Cu-BC was superior to that of Pt.The highest coulomb in this study was lower than those of treated ammonium and sulfide in wastewater by Chenet al.[36]possibly because of microbial processes,such as fermentation and methane production,which consumed available substrates to produce bioelectricity and reduced the CE results of the MFCs [33].But the highest coulomb was much higher than the results of Mohariret al.(11.8%) that of a MFC using food waste as a substrate[34].This was mainly attributed to the efficient cathode catalyst used in this study.It was conducive to the occurrence of the oxygen reduction reaction to and accelerated the mass transfer rate,thereby improving CE.Therefore,by using P-Cu-BC as a cathode catalyst,electricity generation and stillage treatment can be simultaneously achieved,and other biochemical reactions of substances in the substrate can be alleviated by microbial utilization.

Fig.5. Coulomb efficiency of four MFCs equipped with different cathode catalysts.

3.6.Biotoxicity test

Compared with the maximum output of MFC with pure substance as substrate[19],the current study with ethanol fermentation stillage as substrate was reduced from 1177.31 mW·m-2to 572.16 mW·m-2due to the biotoxicity of the stillage.The effect of influent and effluent of the four MFCs onV.fisheriwas presented in Fig.6.Luminescence inhibition rate varied among the four influent waters.On the one hand,this variation may be due to the uneven distribution of organic substances in the stillage stored for a period of time.On the other hand,biotoxicity was not linear with the presence of organic matter in the stillage.After a period of MFC treatment,the biotoxicity of the four effluents decreased.PCu-BC effluent showed the lowest toxicity toV.fisheri,with 7.85% luminescence inhibition and had the highest biotoxicity removal rate(84.7%).Accordingly,it greatly reduced biological toxicity.Jiet al.[37]found that the ethanol,acetic acid,propionic acid,and butyric acid accumulated with each other,this biological contribute to toxicity.Biological toxicity significantly inhibits the activity and stability of microorganisms in MFCs,thus affecting electricity production.At the same time,aromatic compounds are present in fermentation stillage [30],and microorganisms in MFCs easily degrade aromatic compounds in the hydrophilic and bioavailable fractions of polycyclic aromatic hydrocarbons(PAHs),resulting in a significant decrease in toxicity [38,39].Given the above reasons,the P-Cu-BC catalyst of this study reduced the overpotential and diffusion resistance of the MFC and further accelerated the indirect reduction,promoted microbial activity,and improved the toxicity removal rate.

Fig.6. Inhibition of V.fisheri luminescence and biotoxicity removal rate after exposure to influent and effluent of four MFCs.

3.7.Application prospect

As the key material of MFC system,cathode catalyst has lower cost and more advantages.In this study,a novel MFC cathode catalyst with Cu/C/P/O based on bacterial cellulose (0.5 g·m-2,38 USD·m-2)was developed.The catalyst came from cheap raw materials,was made byin-situreaction loading and there were no expensive metals in the catalyst component.Compared with common precious platinum (0.5 g·m-2,84 USD·m-2) [40],the catalyst based on bacterial cellulose has the advantage of optimizing capital cost when the power generation capacity is comparable.This study has a very good economic prospect for large-scale industrial application of MFC catalysts in wastewater treatment.

4.Conclusions

An MFC was used to treat ethanol fermentation stillage from food waste.A Cu and P-doped bacterial cellulose catalyst was used,which facilitated the transfer of electrons and the mass transfer rate of the electrode due to its three-dimensional pore structure.The use of these materials significantly improved catalytic activity and accelerated the consumption of the substrate.P-Cu-BC exhibited higher output power,CE,and biotoxicity removal rate than Pt.This research applied P-Cu-BC in MFC utilization and optimized MFC treatment for ethanol fermentation stillage.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This research was supported by the Open Research Fund Program of Key Laboratory of Cleaner Production and Integrated Resource Utilization of China National Light Industry (CP-2019-YB7).The authors also acknowledged the support by Fundamental Research Funds for the Central Universities(TW2019014).Also the support from Sino-US-Japan Joint Laboratory on Organic Solid Waste Resource and Energy Technology of USTB is appreciated.

Supplementary Material

Supplementary data to this article can be found online at https://doi.org/10.1016/j.cjche.2020.11.001.