Leilei Xiao

Fanghua Liu
email: fhliu@yic.ac.cn

Eric Lichtfouse

Peng Zhang

Dawei Feng

Fangbai Li

Methane production by acetate dismutation stimulated by Shewanella oneidensis and carbon materials: An alternative to classical CO2 reduction

niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

• Various technologies were used to verify methane production pathways.

• Direct acetate dismutation preference via increased exoelectrogenic activity.

• A new model for electron flow during syntrophic methanogenesis was revealed.

G R A P H I C A L A B S T R A C T A R T I C L E I N F O

Keywords: Acetoclastic methanogenesis Independent CO 2 reduction Shewanella oneidensis MR-1 Exogenous electron Electron transfer Methane production A B S T R A C T

Methane is a major greenhouse gas responsible of global warming and renewable energy, but the precise contribution of biomethane from microbial decomposition is vague because microbial mechanisms are not fully understood. CO 2 reduction and direct acetate dismutation are two main pathways for biomethane production, accounting for ~1/3 and 2/3 of produced methane in terrestrial ecosystems, respectively. A classical process explaining methane production involves CO 2 reduction by direct interspecies electron transfer (DIET). Herein, we hypothesized that methane could also be produced by direct acetate dismutation by syntrophy between electron-donating Shewanella oneidensis MR-1 and electron-accepting methanogens. We tested the effect of two conducting carbon materials, granular activated carbon and carbon nanotubes, on methane production. The electrical activity was monitored with a microbial fuel cell. We used CH 3 F, 13 C labelling, thermodynamics, DNA analysis and modelling to elucidate the mechanism. Results show that the rate of methane production increased by 29.0% using S. oneidensis MR-1, and by 36.2-40.7% using S. oneidensis MR-1 and conducting materials. 13 C labelling shows that about 94% of methane is produced by acetate dismutation. Findings further show that acetate dismutation is enhanced by exoelectrogenic activity, thus suggesting that electrons from S. oneidensis MR-1 are used to convert methyl groups into methane. Overall our results disclose DIET-acetate dismutation as an alternative mechanism of biomethane production, which is highly stimulated by carbon-based conductive

Introduction

As a potent greenhouse gas and feasible source of renewable energy, methane takes an active part in carbon and energy cycles. Biomethane is produced in anaerobic environments such as soils, sediments, and waste treatment plants [START_REF] Evans | An evolving view of methane metabolism in the Archaea[END_REF][START_REF] Thauer | Methanogenic archaea: ecologically relevant differences in energy conservation[END_REF][START_REF] Jeong | Evaluating the feasibility of pyrophyllite-based ceramic membranes for treating domestic wastewater in anaerobic ceramic membrane bioreactors[END_REF]. Since methanogenesis is the final step of organic matter degradation, limiting methanogenesis by geoengineering would induce accumulation of high amounts of organic C in soils and sediments, and thus would favor C sequestration. On the other hand, optimizing biomethane production is needed for the development of sustainable fuels. Nonetheless, microbial mechanisms ruling biomethane generation are not fully understood. The classical pathway for biomethane production is CO 2 reduction involving H 2 as reducing agent [START_REF] Stams | Electron transfer in syntrophic communities of anaerobic bacteria and archaea[END_REF]. Yet the H 2 diffusion rate is slow, which decreases the rate of methane generation. To optimize methane production, alternative strategies are thus needed. Here, recent studies have revealed an alternative mechanism, named direct interspecies electron transfer (DIET), which do not rely on the diffusion of gaseous compounds. DIET has been identified in wastewater digesters [START_REF] Morita | Potential for direct interspecies electron transfer in methanogenic wastewater digester aggregates[END_REF], and in pure cultures of Geobacter and methanogens [START_REF] Liu | Promoting direct interspecies electron transfer with activated carbon[END_REF][START_REF] Rotaru | A new model for electron flow during anaerobic digestion: direct interspecies electron transfer to Methanosaeta for the reduction of carbon dioxide to methane[END_REF].

Methanogenesis can be accelerated by fostering the syntrophy between electron-donating and electron-accepting partners. For example, methanogenesis is accelerated when either the electrical performance of Geobacter or the electrode potential are enhanced [START_REF] Yin | Enhanced methane production in an anaerobic digestion and microbial electrolysis cell coupled system with co-cultivation of Geobacter and Methanosarcina[END_REF][START_REF] Cheng | Direct biological conversion of electrical current into methane by electromethanogenesis[END_REF]. Methane production is also augmented by the addition of conductive materials, which increase the conductivity between electron-donating microbes and methanogens [START_REF] Park | Direct interspecies electron transfer via conductive materials: A perspective for anaerobic digestion applications[END_REF][START_REF] Mei | Novel Geobacter species and diverse methanogens contribute to enhanced methane production in media-added methanogenic reactors[END_REF][START_REF] Lei | Magnetite enhances anaerobic digestion and methanogenesis of fresh leachate from a municipal solid waste incineration plant[END_REF][START_REF] Lin | Improved efficiency of anaerobic digestion through direct interspecies electron transfer at mesophilic and thermophilic temperature ranges[END_REF]. Actually, DIET coupled with CO 2 reduction (DIET-CO 2 ) is considered as the sole mechanism explaining methane production by electric syntrophy [START_REF] Zhang | Electroconversion of carbon dioxide (CO 2 ) to low-carbon methane by bioelectromethanogenesis process in microbial electrolysis cells: The current status and future perspective[END_REF][START_REF] Nelabhotla | Electrochemically mediated CO 2 reduction for bio-methane production: a review[END_REF][START_REF] Zhao | Potentially shifting from interspecies hydrogen transfer to direct interspecies electron transfer for syntrophic metabolism to resist acidic impact with conductive carbon cloth[END_REF]. Yet, two methanogenic pathways are known to produce methane: CO 2 reduction and direct acetate dismutation (acetoclastic methanogenesis), which contribute to about 1/3 and 2/3 of biomethane production, respectively [START_REF] Conrad | Quantification of methanogenic pathways using stable carbon isotopic signatures: a review and a proposal[END_REF]. Here the possible involvement of exoelectrogenic bacteria and conductive materials in acetoclastic methanogenesis is unclear.

For instance, a review suggests that DIET-CO 2 reduction may not systematically explain the enhanced methane production by conductive materials [START_REF] Martins | Methane production and conductive materials: a critical review[END_REF]. Another insight suggesting the possible occurrence of other mechanisms is that DIET was evidenced only in cocultures of some electricigens, e.g. Geobacter spp., with methanogenic archaea, but major electroactive microorganisms such as Geobacter spp. are not always detected in improved methane production systems [START_REF] Van Steendam | Improving anaerobic digestion via direct interspecies electron transfer requires development of suitable characterization methods[END_REF]. Two recent studies suggested that conductive magnetite accelerates acetoclastic methanogenesis [START_REF] Fu | NanoFe3O4 as solid electron shuttles to accelerate acetotrophic methanogenesis by Methanosarcina barkeri[END_REF][START_REF] Inaba | Metatranscriptomic evidence for magnetite nanoparticle-stimulated acetoclastic methanogenesis under continuous agitation[END_REF]. First, Fu et al. showed that nanoFe 3 O 4 acts as solid electron shuttles to accelerate acetoclastic methanogenesis by Methanosarcina barkeri in pure cultures [START_REF] Fu | NanoFe3O4 as solid electron shuttles to accelerate acetotrophic methanogenesis by Methanosarcina barkeri[END_REF]. Second, Inaba et al. provided metatranscriptomic evidence for stimulated acetoclastic methanogenesis by magnetite nanoparticle under continuous agitation [START_REF] Inaba | Metatranscriptomic evidence for magnetite nanoparticle-stimulated acetoclastic methanogenesis under continuous agitation[END_REF]. These findings show that conductive minerals enhance the dismutation of acetate and, in turn, increase methanogenesis. Overall, these insights suggest that DIET-CO 2 reduction may not be the sole mechanisms explaining electromethanogenesis.

Until 2018, Geobacter spp. were the sole known bacterial species involved in DIET with methanogens [START_REF] Van Steendam | Improving anaerobic digestion via direct interspecies electron transfer requires development of suitable characterization methods[END_REF]. Bioaugmentation of Geobacter has been shown to promote methanogenesis [START_REF] Zhang | A novel bioaugmentation strategy to accelerate methanogenesis via adding Geobacter sulfurreducens PCA in anaerobic digestion system[END_REF]. Yet Geobacter is in theory not the optimal species for syntrophy by acetoclastic methanogenesis because Geobacter can use acetate as a substrate, and, as a consequence, Geobacter and methanogens are in competition for acetate. Alternatively, other electroactive microorganisms such as Shewanella spp. may be involved [START_REF] Logan | Electroactive microorganisms in bioelectrochemical systems[END_REF][START_REF] Ringeisen | High power density from a miniature microbial fuel cell using Shewanella oneidensis DSP10[END_REF]. For instance, S. oneidensis MR-1 consumes lactate to produce acetate and electrons [START_REF] Tang | A kinetic model describing Shewanella oneidensis MR-1 growth, substrate consumption, and product secretion[END_REF], which are substrates for methanogens. Therefore, there may be a potential syntrophy between Shewanella and methanogens by acetoclastic methanogenesis. However, strong experimental evidence is actually missing, despite observations of the co-occurrence of both Shewanella and methanogens in some environments [START_REF] Xiao | A potential contribution of a Fe(III)-rich red clay horizon to methane release: Biogenetic magnetitemediated methanogenesis[END_REF][START_REF] Guo | Mechanisms of metabolic performance enhancement during electrically assisted anaerobic treatment of chloramphenicol wastewater[END_REF].

Here we investigated the role of S. oneidensis on acetoclastic methanogenesis. We mainly focused on three points: 1, potential syntrophy: whether bioaugmentation of S. oneidensis MR-1 can enhance methane production; 2, electric syntrophy: whether the increased methane production performance is related to exoelectrogenic activity; 3, we hypothesized that methane production by acetoclastic methanogenesis stimulated by S. oneidensis MR-1 and carbon materials acts as an alternative to classical CO 2 reduction. The following sections present results on methane production, exoelectrogenic activity, microbial diversity, thermodynamics, and 13 C tracing.

Materials and methods

Bacteria growth and inoculation

Experiment 1: Luria-Bertani (LB) broth was inoculated with S. oneidensis MR-1 and incubated aerobically at 30 °C until the optical density (OD600) reached 2. Then, S. oneidensis MR-1 was included into new broth with a nitrogen blowing in advance to adapt anaerobic growth. The bacterial culture of about 100 mL was concentrated by refrigerated centrifugation at 6000 rpm and 4 °C in an anaerobic glove box (Coy Laboratory Products). The collected cells were dissolved in 1 mL oxygen-free phosphate buffer saline solution (50 mM, pH 7.2) as a seed solution.

Soil sample and batch experiments (Experiment 2)

We sampled a soil from the Yellow River Delta [START_REF] Li | A new insight into the strategy for methane production affected by conductive carbon cloth in wetland soil: Beneficial to acetoclastic methanogenesis instead of CO 2 reduction[END_REF][START_REF] Xiao | Stimulation of long-term ammonium nitrogen deposition on methanogenesis by Methanocellaceae in a coastal wetland[END_REF]. Straw was thus used as the carbon source during pre-incubation for enriching methanogenic archaea. In brief, 150 g soil, 1.5 g straw and 450 mL sterile water were poured into a 1000 mL serum bottle. The bottles were flushed 30 min with high-purity N 2 then incubated statically at 30 °C in the dark for five weeks. For S. oneidensis MR-1, its nearly inability for anaerobic acetate metabolism provided advantage that this kind of electricigens does not compete with methanogens for acetate [START_REF] Logan | Electroactive microorganisms in bioelectrochemical systems[END_REF]. During pre-incubation, the potential electron acceptors, such as sulfate and nitrate, are consumed accompanied by high amounts of methane production, of about 100,000 ppm.

Bioaugmentation was performed by adding 0.1 mL of S. oneidensis MR-1 seed solution, obtained from experiment 1, into each vial with the total volume about 11 mL. The volume occupied by the slurry and upper space is approximately 5.5 mL, respectively. The other treatment groups were vials with granular activated carbon (GAC) or carbon nanotubes (CNTs). Characterization of GAC is given by Liu et al. [START_REF] Liu | Promoting direct interspecies electron transfer with activated carbon[END_REF]. The final amount of GAC was 1 g/L. CNTs were characterized by scanning electron microscope (SEM), indication a diameter in the nanometer range (Fig. S1). For the experiments including CNTs, suspensions of 10 g/L CNTs in ultrapure water were N 2 flushed, then sterilized. CNTs suspension (100 μL) was added to vials with a total volume of 11 mL to reach a concentration of about 0.2 g/L. Other treatments including the control group, S. oneidensis MR-1 and GAC treatment received equal amount of oxygen-free and sterilized ultrapure water (0.1 mL). Acetate was used as the substrate for methanogens with the final acetate concentration about 20 mM according to a previous report [START_REF] Wagner | Methane fluxes in permafrost habitats of the Lena Delta: effects of microbial community structure and organic matter quality[END_REF]. Overall, all treatments contained the same amount of upper space and slurry plus with sterile water about 5.5 mL.

Vials were sacrificed in triplicate to measure the concentrations of methane, hydrogen and CO 2 after 1, 2, 3, 4, 5, 6, 7 and 8 days of incubation with a gas chromatography (GC; Agilent 7820A, USA) equipped with a flame ionization detector (FID) and a thermal conductivity detector (TCD). The column is Hayesep Q 80-100 mesh (6ft*1.8″* 2.0 mm). The low detection limits for methane and hydrogen are respectively 0.0008 mM and 0.0004 mM. High-pressure liquid chromatography (HPLC; Agilent 1260 Infinity) was used to measure the acetate concentration, with a minimum detectable limit of 0.02 mM.

Batch experiments for identifying methanogenic pathways by inhibitor

Experiment 3: CH 3 F, an inhibitor of acetoclastic methanogenesis [START_REF] Conrad | Quantification of methanogenic pathways using stable carbon isotopic signatures: a review and a proposal[END_REF], was applied at 1.5%v/v in N 2 in the vials to distinguish the respective contribution of acetoclastic methanogenesis and CO 2 reduction to total methane production. In addition to replacing the same amount of nitrogen with 1.5% CH 3 F gas, other operations can refer to experiment 2.

Carbon stable isotope fractionation for identifying methanogenic pathways

Experiment 4: Carbon stable isotope fractionation and the related calculations were conducted as well. CH 4 and CO 2 collected from the headspace were tested for obtaining the δ 13 C using a gas chromatograph combustion isotope ratio mass spectrometer (GC-C-IRMS) system (Thermo Fisher MAT253, Germany). The isotopes were quantified with a Finnigan MAT253 IRMS. Separation of CH 4 /CO 2 was performed in a Finnigan Precon. Gas (~1mL) was injected into a sample container (100 mL), which was filled with helium gas (99.999% purity) beforehand. A chemical trap was applied to scrub CO 2 from the sample. Methane was subsequently purified by cold trap with the help of Ni wires. Then, methane can be oxidized in a combustion reactor at 960 °C and converted to CO 2 and water. The combusted CO 2 was enriched by two liquid nitrogen cold traps and transferred into the IRMS for determination. The precision of repeated analyses was ± 0.2‰ when 1.3 nmol methane was injected. The abundance of 13 C in a sample is given relative to a standard using the δ notation: where PDB refers to the Pee Dee Belemnite carbonate that is used as standard which has a 13 C/ 12 C ratio of 0.0112372. A similar method was used to test δ 13 C-values of CO 2 . The chemical trap was replaced by a water trap. The α value can be calculated using the equation:

⎜ ⎟ ⎜ ⎟ = ⎡ ⎣ ⎢ ⎛ ⎝ ⎞ ⎠ ⎛ ⎝ ⎞ ⎠ - ⎤ ⎦ ⎥ × δ C C C C C 1
= + + α δ CO δ CH 1000 1000 13 2 13 4
. For the calculation of f ma , the equation was used as follow, = --

f ma δ δ C H δ δ mc mc ma 13 4 .
2.5. 13 C tracing with high abundance of 13 CH 3 COOH Experiment 5: some 13 C tracing experiments were conducted as well. Two kinds acetate with higher 13 C-labelled methyl, 1% and 3% 13 C/ 12 C, were spiked into vials, respectively, to follow the fate of the 13 C-labelled methyl. In addition to replacing 12 CH 3 COOH with 13 CH 3 COOH, other operations can refer to experiment 2 2.6. 16S rRNA gene sequencing for archaea Experiment 6: Archaeal (Arch519F and Arch915R) primers were used to perform amplification of the 16S rRNA gene. Three repetitions were carried out. An Illumina Miseq platform (Tiny Gene Bio-Tech (Shanghai) Co., Ltd.) was used for sequencing after the construction of the amplicon library. The OTU taxonomies (from phylum to species) were determined based on the NCBI database. The detailed analysis can refer to previous report [START_REF] Xiao | A potential contribution of a Fe(III)-rich red clay horizon to methane release: Biogenetic magnetitemediated methanogenesis[END_REF].

Electrochemical analysis

Experiment 7: A microbial fuel cell (MFC) was used to analyze the electron transfer efficiency to study whether S. oneidensis MR-1 and conductive materials can accelerate electron output and transfer. Htype double chamber MFCs were used and the chambers were separated by a cation exchange membrane (Ultrex CMI-7000). A 3.0 cm × 2.5 cm × 0.3 cm graphite plate was used as the electrode in each chamber and connected with titanium wire. A1 kΩ resistor was equipped in the external circuit. N 2 was flushed 20 min in the two chambers to remove air. 80 mL of slurry, which came from the same batch incubations as experiment 2, was added in the anode chamber and 80 mL oxygen-free K 3 [Fe(CN) 6 ] (0.05 M) was added to the cathode chamber. For the treatments with bioaugmentation, S. oneidensis MR-1 was added in the anode chamber. N 2 was flushed 20 min again in the two chambers to remove air and methane produced by pre-incubated soil. A data acquisition system (Model 2700, Keithley Instruments, USA) and ExcellLINX software were used to record the output voltage. The current density, C den (A/m 2 ) was calculated using the following expression: C den = U/RS, where U is the output voltage (V), R is the external resistance (Ω), and S is the surface area of the electrodes (m 2 ). For batch experiments without electrode, it is reasonable that these electrons may be utilized by methanogens. For electrochemical impedance spectroscopy (EIS) test can referring to Xu et al. [START_REF] Xu | Effect of anodes decoration with metal and metal oxides nanoparticles on pharmaceutically active compounds removal and power generation in microbial fuel cells[END_REF], the working electrode was anode, the cathode and Ag/AgCl electrode acted as the counter electrode and reference electrode, respectively. The EIS was conducted over a frequency range from 100 kHz to 0.01 Hz with a perturbation signal of 5 mV. The initial potential of EIS was the open circuit potential of MFCs. CVs were measured using an electrochemical workstation (CHI660e, Chenhua, China). Three electrodes were used same as EIS experiments. The working electrode had a scan voltage between -1.2 and 2.0 V (versus Ag/AgCl), and the scan rates were 20 -140 mV/s.

Thermodynamic analysis

Experiment 8: Concentrations of gases and acetate were used to calculate the ΔG of hydrogenotrophic methanogenesis and acetoclastic methanogenesis. ΔG of hydrogenotrophic methanogenesis can be calculated as:

= + * ⎧ ⎨ ⎩ ⎡ ⎣ ⎢ * ⎤ ⎦ ⎥ ⎫ ⎬ ⎭ C C C ΔG ΔG RT ln ( ) CH CO H 0 4 4 2 2
where ΔG 0 is ΔG at 273.15 K and 101.325 kPa; R, the ideal gas constant, 8.3145 J•mol -1 •K -1 ; T, the absolute thermodynamic temperature, 303.15 K; C CH4 , C CO2 , and C H2 are concentrations of methane, CO 2 , and H 2 , respectively, mol•L -1 . The calculation of ΔG 0 was detailed referring to a previous study [START_REF] Li | A new insight into the strategy for methane production affected by conductive carbon cloth in wetland soil: Beneficial to acetoclastic methanogenesis instead of CO 2 reduction[END_REF].

For acetoclastic methanogenesis, = + ΔG ΔG R 0 * ⎡ ⎣ ⎤ ⎦ + * * * { } RT N T ln 2.303 C C C pH CH CO acetate 4 2
holds, where the parameters are the same as above. C acetate is concentration of acetate, mol•L -1 ; N pH is pH value.

Statistical analysis

Data are presented as mean ± standard deviation of triplicate cultures expect for carbon isotope fractionation and calculation, in which two repetitions was conducted. All statistical analyses were performed with Origin 8.5 (Origin Lab Corporation, USA) software. Ttest was used to analyze the significance level, and a P value < 0.05 was considered statistically significant.

Results and discussion

Potential electric syntrophy between S. oneidensis MR-1 and methanogens

Increase of methane production performance with S. oneidensis MR-1 and conductive GAC/CNTs

We incubated soil suspensions with the S. oneidensis MR-1 and two conductive materials: GAC and CNTs (experiment 2), with CH 3 F (experiment 3). Results without CH 3 F show that addition of S. oneidensis MR-1 increased the rate of methane production, of 0.88 mM/day, versus that of the control, of 0.76 mM/day (p < 0.01) (Fig. 1a, experiment 2). Further addition of conductive materials increased the rate of methane production to 1.06 mM/day for GAC and to 1.02 mM/day for CNTs (p < 0.01. Results also show that the addition of CH 3 F highly inhibited methane production.

Our results reveal first that S. oneidensis MR-1 improved methane production. Indeed, although exoelectrogenic bacteria have been shown to favor DIET-mediated methane production [START_REF] Rotaru | A new model for electron flow during anaerobic digestion: direct interspecies electron transfer to Methanosaeta for the reduction of carbon dioxide to methane[END_REF][START_REF] Yin | Enhanced methane production in an anaerobic digestion and microbial electrolysis cell coupled system with co-cultivation of Geobacter and Methanosarcina[END_REF][START_REF] Park | Direct interspecies electron transfer via conductive materials: A perspective for anaerobic digestion applications[END_REF], only Geobacter bacterial species have been found so far to participate in DIET with methanogens [START_REF] Van Steendam | Improving anaerobic digestion via direct interspecies electron transfer requires development of suitable characterization methods[END_REF]. Shewanella species and methanogens have been previously detected simultaneously in the same niche [START_REF] Xiao | A potential contribution of a Fe(III)-rich red clay horizon to methane release: Biogenetic magnetitemediated methanogenesis[END_REF][START_REF] Guo | Mechanisms of metabolic performance enhancement during electrically assisted anaerobic treatment of chloramphenicol wastewater[END_REF], but no evidence was given for increasing methane production by exoelectrogenic activity. Second, the high increase of methane production by addition of conductive materials is explained by easier electron transfer between the electron donor S. oneidensis MR-1 and methanogens, thus suggesting an electric syntrophy. This hypothesis is strengthened by reports suggesting that carbon materials can act as electrode modifiers promoting direct electron transfer from S. oneidensis [START_REF] Liu | Intermittent contact of fluidized anode particles containing exoelectrogenic biofilms for continuous power generation in microbial fuel cells[END_REF][START_REF] Peng | Carbon nanotubes as electrode modifier promoting direct electron transfer from Shewanella oneidensis[END_REF].

Third, the high decrease of methane production by addition of CH 3 F demonstrates that acetate dismutation played a major role in methane production (Fig. 1a), beside the classical CO 2 reduction pathway. For instance, S. oneidensis MR-1 increased methane production only by 5% after 6 days of incubation with CH 3 F, whereas the increase reached 22.5% without CH 3 F. Calculations shows that acetoclastic methanogenesis accounted for 70-80% of produced methane, and, conversely, CO 2 reduction accounted for only 20-30% of produced methane (Table 1). Our findings are supported by pure culture and metatranscriptomic data showing that Fe 3 O 4 nanoparticles act as electron shuttles to stimulate acetoclastic methanogenesis [START_REF] Fu | NanoFe3O4 as solid electron shuttles to accelerate acetotrophic methanogenesis by Methanosarcina barkeri[END_REF][START_REF] Inaba | Metatranscriptomic evidence for magnetite nanoparticle-stimulated acetoclastic methanogenesis under continuous agitation[END_REF]. However, for this study and the two studies mentioned above [START_REF] Fu | NanoFe3O4 as solid electron shuttles to accelerate acetotrophic methanogenesis by Methanosarcina barkeri[END_REF][START_REF] Inaba | Metatranscriptomic evidence for magnetite nanoparticle-stimulated acetoclastic methanogenesis under continuous agitation[END_REF], the experiments were performed with acetate as the sole carbon source. The observations may be limited to the simplified experimental conditions, not to real environments containing different VFAs (e.g., propionate and butyrate) and other organic compounds whose metabolism can involve DIET (via GAC or CNTs). But based on the results of this study, it reveals that acetoclastic methanogenesis is a major mechanism of methane production by electric syntrophy, versus the classical CO 2 reduction pathway. These findings are further strengthened by the analysis of electrical properties and methanogenic pathways in the next section.

Consistency of increased methane production and exoelectrogenic activity

We studied the electrochemical properties of soil suspensions with S. oneidensis MR-1 and conductive materials without CH 3 F, using a microbial fuel cell (MFC) (experiment 7). Results show that S. oneidensis MR-1 highly increased the current density after 3 days of incubation (Fig. 2a). This finding is consistent with previous reports that current density showed significant improvement by S. oneidensis [START_REF] Ringeisen | High power density from a miniature microbial fuel cell using Shewanella oneidensis DSP10[END_REF][START_REF] Liu | Enhanced Shewanella biofilm promotes bioelectricity generation[END_REF]. Moreover S. oneidensis MR-1 increased the electrical quantity to 123.0 C, versus 79.5 C in the control (Fig. 2b), and reduced the resistance from 597.7 to 516.5 Ω/cm 2 (Fig. 2c). This phenomenon has been explained by better electron transfer mediated by S. oneidensis C-type cytochromes [START_REF] Carmona-Martinez | Cyclic voltammetric analysis of the electron transfer of Shewanella oneidensis MR-1 and nanofilament and cytochrome knock-out mutants[END_REF][START_REF] Nimje | Comparative bioelectricity production from various wastewaters in microbial fuel cells using mixed cultures and a pure strain of Shewanella oneidensis[END_REF]. Results also show that conductive materials further increased the current density, the electric quantity, and reduced the resistance by 46% for GAC and by 129% for CNTs. These findings are in agreement with the known increase of energy generation by electrogenic microbes when resistance is reduced [START_REF] Logan | Exoelectrogenic bacteria that power microbial fuel cells[END_REF][START_REF] Zhao | Ultrahigh Rate and Long-Life Sodium-Ion Batteries Enabled by Engineered Surface and Near-Surface Reactions[END_REF]. Capacitancevoltage (CV) curves further show that S. oneidensis MR-1 increased the peak current value, and that CNTs induced the largest peak current (Fig. 2d andS2). This finding is in agreement with previous studies showing that GAC and CNTs enlarge the peak current [START_REF] Peng | Carbon nanotubes as electrode modifier promoting direct electron transfer from Shewanella oneidensis[END_REF][START_REF] Zhang | Electrochemical characteristics of Shewanella loihica on carbon nanotubes-modified graphite surfaces[END_REF].

It has been concluded that all microorganism are capable of extracellular electron transfer with electrodes with the ability of DIET to date [START_REF] Van Steendam | Improving anaerobic digestion via direct interspecies electron transfer requires development of suitable characterization methods[END_REF]. Therefore, if the electron acceptor is a microorganism instead of electrode, it is very reasonable to occur an electric syntrophy process between S. oneidensis MR-1 and methanogens. Overall, our results imply that S. oneidensis MR-1 improved the exoelectrogenic activity and that conductive carbon materials further enhanced electron transfer. Specially, the perfect consistency between the increase of methane production (previous section) and exoelectrogenic activity by S. oneidensis MR-1 and GAC/CNTs strengthened the possibility that there is an electric syntrophy between S. oneidensis MR-1 and methanogens.

Population and abundance of archaea responding to the addition of S. oneidensis MR-1 and conductive materials

Methanosarcinaceae, which is the most metabolically versatile of the methanogenic archaea, had the highest abundance of archaea after the augmentation of S. oneidensis MR-1 (Fig. 3, experiment 6). Its abundance exceeded 62.2%. In sharp contrast, the abundance was only 8.63% in the control group. S. oneidensis MR-1 spurred a several-fold increase of Methanosarcinaceae abundance, reaching 7.21 times. Due to di-substrate consuming capacity of Methanosarcinaceae, we discussed in detail on the potential substrate, CO 2 or acetate, and syntrophic models in the next two parts.

The addition of GAC or CNTs resulted in a slight decrease in the abundance of Methanosarcinaceae, which is metabolically very versatile and capable of H 2 /CO 2 -utilizing methanogenesis. It is very interesting to note that both GAC and CNTs significantly improved the abundance of Methanosaetaceae, which is another methanogenic archaea via direct acetate dismutation to generate methane. The abundances in group of S. oneidensis MR-1, S. oneidensis MR-1 + GAC and S. oneidensis MR-1 + CNTs were 1.02, 14.33 and 5.15%, respectively. GAC and CNTs increased the multiple of Methanosaetaceae to 14.0 and 5.05 times. It was reasonable that Methanosarcinaceae and Methanosaetaceae played an important role in methane production in this study. Very recently, we incubated of soils from a Fe(III)-rich red clay horizon and found that the abundances of S. oneidensis and Methanosarcinaceae were improved simultaneously [START_REF] Xiao | A potential contribution of a Fe(III)-rich red clay horizon to methane release: Biogenetic magnetitemediated methanogenesis[END_REF]. The findings of this study explains well the experimental phenomena found in in situ 5 Calculated values of δ 13 CH 4 using ( 3) and ( 4) according to the equation shown in ( 3 ). 6 According to a previous review [START_REF] Whiticar | Carbon and hydrogen isotope systematics of bacterial formation and oxidation of methane[END_REF], the value of δ ma was calculated by assuming an ε of -21‰ and δ ma = δ 13 Cacetate + ε. Determined acetate value was -24.04 ± 0.23. 7 The percentage of acetoclastic methanogenesis to the total methane from the direct measurement of methane production with or without the inhibitor (CH 3 F). environmental samples.

Identify major methanogenic pathways: Acetoclastic methanogenesis

Negligible contribution of CO 2 reduction for methane production by hydrogenotrophic methanogenesis

Hydrogen and electrons are the two most important reducing power to reduce CO 2 . Hydrogen-producing microorganisms produced very little hydrogen (Fig. 4a, experiment 2). From the second day, hydrogen partial pressures were always below 1pa. The weak hydrogen-producing ability of related bacteria, such as negligible acetate oxidation progress, may cause this experimental phenomenon. It was found that threshold hydrogen partial pressure is generally greater than 10 Pa for hydrogenotrophic methanogenesis [START_REF] Thauer | Methanogenic archaea: ecologically relevant differences in energy conservation[END_REF]. Even though the produced hydrogen can be completely converted to methane, the theoretical value is only about 0.113 μM methane. The detected methane concentration was four orders of magnitude of this theoretical value.

We further analyzed the potential contribution of hydrogenotrophic methanogenesis according to ΔG values (Fig. 4b, experiment 8 with data from experiment 2). The values of ΔG fluctuated around zero, and S. oneidensis MR-1 and conductive materials did change the trend of ΔG. This provides additional evidence that S. oneidensis MR-1 and carbon materials were not conducive to the hydrogenotrophic methanogenesis.

As reported, electrons can also directly reduce CO 2 to produce methane, which was more advantageous than hydrogen as electron carrier. It has been pointed out that external electron transfer rates per cell pair (cp) are considerably higher for DIET (4.49*10 4 e -cp -1 s -1 ) compared with hydrogen-mediated IET (5.24*10 3 e -cp -1 s -1 ) using a reaction-diffusion-electrochemical approach in a three-dimensional domain [START_REF] Storck | Modelling extracellular limitations for mediated versus direct interspecies electron transfer[END_REF]. Therefore, we tried to analyse that whether the ignorable contribution of hydrogenotrophic methanogenesis was due to the more robust of direct electron transfer. Analysis of interspecies electron transfer via hydrogen diffusion can be reliable with reference to Cruz Viggi et al. [START_REF] Viggi | Magnetite particles triggering a faster and more robust syntrophic pathway of methanogenic propionate degradation[END_REF]. The calculated value of i H2 is about 1 × 10 -11 A (data from experiment 2). As showed in Fig. 2a, the maximum current intensity detected by electrochemical workstation in the range 4.8 × 10 -4 A, which was much higher than i H2 . Direct utilization of H 2 as reducing power to generate methane seemed to be negligible in this study.

S. oneidensis MR-1 triggered slight CO 2 reduction with electrons as reducing power compared to direct acetate dismutation

Acetoclastic methanogenesis is much more sensitive to CH 3 F compared to other methanogenic pathways. S. oneidensis MR-1 was not to be more beneficial to CO 2 reduction in this study. For the effects of S. oneidensis MR-1 on methanogenic pathway, the proportion of methane from CO 2 reduction was the highest with values about 23.07% (=100%-76.93%) and 21.04% (=100%-78.96%) in the control group (Table 1), and S. oneidensis MR-1 diminished the importance of CO 2 reduction to produce methane (Table 1). Therefore, the generated electrons may be more shunted to the acetate dismutation pathway with the presence of S. oneidensis MR-1.

GAC did not accelerate CO 2 reduction, and CNTs even showed a negative impact on CO 2 reduction. In contrast, an obvious promotion occurred when direct acetate dismutation pathway was involved (Fig. 1a). However, when the source of methane production is not analyzed in detail, overwhelming researches suggested that the enhancement of methane production is from an increase of CO 2 reduction [START_REF] Park | Direct interspecies electron transfer via conductive materials: A perspective for anaerobic digestion applications[END_REF][START_REF] Nelabhotla | Electrochemically mediated CO 2 reduction for bio-methane production: a review[END_REF][START_REF] Yin | Clarifying electron transfer and metagenomic analysis of microbial community in the methane production process with the addition of ferroferric oxide[END_REF][START_REF] Song | Enhanced electron transfer and methane production from low-strength wastewater using a new granular activated carbon modified with nano-Fe 3 O 4[END_REF]. According to the detailed analysis, this study showed that electromethanogenesis may come from the acetoclastic methanogenesis as an alternative to DIET-CO 2 reduction. experiment 8 with data from experiment 2). Although ΔG gradually became larger due to the decrease of acetate concentration and increase of methane concentration, the values of ΔG during the whole process were still negative, which indicates that this pathway can occur spontaneously. Moreover, if the physiological activities of methanogens and energy release are comprehensively considered, direct acetate dismutation actually generates more energy than CO 2 reduction in a nominal environment detailed in a previous study [START_REF] Bethke | The Thermodynamic Ladder in Geomicrobiology[END_REF]. In brief, for generating one mole of methane, 32 or 1 kJ of energy is available by acetoclastic methanogenesis or hydrogenotrophic methanogenesis, respectively, to Methanosarcina. This requirement may also be why the proportion of methane production from acetate dismutation, ~80%, was much higher than that from CO 2 reduction, ~20%, at the condition with sufficient acetate supply. In fact, with low concentration of acetate supply, an increase of electron transfer seemed to be still favorable to CO 2 reduction [START_REF] Xiao | Nano-Fe 3 O 4 particles accelerating electromethanogenesis on an hour-long timescale in wetland soil[END_REF].

Consistency of direct acetate dismutation and methane production by carbon balance analysis

No significant difference existed for acetate consumption in the first half of the experiment (Fig. 5a, experiment 2). Correspondingly, S. oneidensis MR-1 and GAC/CNTs did not cause more methane production (Fig. 1a). In the second half, the rate of acetate decrease was enhanced by S. oneidensis MR-1. GAC and CNTs stimulated acetate utilization to a greater extent compared with only S. oneidensis MR-1 addition with significant statistical differences (p < 0.01 at 4th, 5th and 6th day) (Fig. 5a). It is worth emphasizing, trend of methane production was in compliance with acetate consumption. S. oneidensis MR-1 significantly promoted methane production (p < 0.01 at 4th, 5th and 6th day), and in particular, GAC and CNTs further accelerated methane accumulation. Moreover, the time node at which the current density reached the maximum was basically consistent with the max difference of acetate consumption and the methane production for all four treatments, suggesting that electricity may play a very important role in the direct utilization of acetate to perform methanogenesis.

Carbon stable isotope fractionation and calculation confirmed a significant contribution of acetoclastic methanogenesis

To further confirm that produced methane mainly derived from direct acetate dismutation, the 13 C of methane and CO 2 were analyzed on the 5th and 6th days (experiment 4, data from experiment 2 and 3), when the obvious difference of methane concentration presented. Inhibition of acetoclastic methanogenesis resulted in lower δ 13 C-CH 4 (Table 1). The δ 13 C-CH 4 in the presence of CH 3 F represents the δ 13 C of methane from CO 2 reduction (δ 13 CH 4 (CO 2 ), i.e. δ mc ) [START_REF] Conrad | Quantification of methanogenic pathways using stable carbon isotopic signatures: a review and a proposal[END_REF]. δ mc is expected of being lower than the δ 13 C of total methane, which was actually observed. The fractionation factors of CO 2 reduction, α, were in a range of 1.041-1.046, which have been observed in anaerobic soils [START_REF] Penning | Quantification of carbon flow from stable isotope fractionation in rice field soils with different organic matter content[END_REF][START_REF] Kruger | Seasonal variation in pathways of CH 4 production and in CH 4 oxidation in rice fields determined by stable carbon isotopes and specific inhibitors[END_REF]. Specially, Krüger et al. showed that adding straw obviously affected the α value [START_REF] Kruger | Seasonal variation in pathways of CH 4 production and in CH 4 oxidation in rice fields determined by stable carbon isotopes and specific inhibitors[END_REF], which was about 1.04 compared to 1.07 without straw. It was very similar to the results showed in table 1 (α = 1.041-1.046). The same range of α value has been used for calculation of f ma during study on rice fields and laboratory study [START_REF] Penning | Quantification of carbon flow from stable isotope fractionation in rice field soils with different organic matter content[END_REF][START_REF] Susilawati | Carbon and hydrogen isotope fractionation during methanogenesis: A laboratory study using coal and formation water[END_REF].

On the fifth day, the proportion of produced methane from acetate dismutation was 67.08% in the control group (Table 1). S. oneidensis MR-1 and GAC and CNTs significantly increased the corresponding proportions to 72.67, 70.55 and 82.50%, respectively. Similarly, on the sixth day, the proportion in the control group was 73.57% by acetoclastic methanogenesis, and S. oneidensis MR-1 increased by nearly 10 percentage points to 82.70%. In particular, it increased again by more than 10% to 94.24 and 93.85% with the application of GAC and CNTs. Obviously, methane was mainly from direct acetate dismutation and further enhanced by S. oneidensis MR-1. More importantly, conductive materials further facilitated the completion of this process. It was also observed that conductive materials increased the proportion of acetoclastic methanogenesis in anaerobic granular sludge [START_REF] Xiao | Simultaneous intensification of direct acetate cleavage and CO 2 reduction to generate methane by bioaugmentation and increased electron transfer[END_REF]. Overall, the two methods, inhibitor and natural abundance stable isotope fractionation, reached perfectly consistent conclusions. For acetoclastic methanogenesis, methanogenic archaea convert acetate into methane and CO 2 by dismutation reaction (CH 3 COO -+ H + → CH 4 + CO 2 ). Here, the acetate carboxyl group is oxidized to CO 2 with the release of electrons (COOH -→ CO 2 + e -+ H + ). Then, electrons are used to reduce methyl groups to form CH 4 (CH 3

-+ e -+ H + → CH 4 ). It is reported that magnetite nanoparticles penetrate into the cell membrane of M. barkeri to function as a solid electron shuttle [START_REF] Fu | NanoFe3O4 as solid electron shuttles to accelerate acetotrophic methanogenesis by Methanosarcina barkeri[END_REF]. GAC is a large particle carbon-based material that cannot penetrate into the cell membrane. Our previous research found that GAC can promote the syntrophic methane production between Geobacter and Methanosarcina by co-culture experiment [START_REF] Liu | Promoting direct interspecies electron transfer with activated carbon[END_REF]. In detail, GAC greatly stimulated ethanol metabolism and methane production in co-cultures of Geobacter metallireducens and M. barkeri. Cells were attached to GAC, but not closely aggregated, suggesting little opportunity for biological electrical contacts between the species when GAC acted as a "conductor". For the mechanisms/functions of GAC in this study, the biggest possibility is that GAC provided the attachment sites for S. oneidensis MR-1 and methanogens. The electrons released by S. oneidensis MR-1 are more easily transferred to the methanogens via GAC. As more exogenous electrons are provided, the reduction of methyl groups may be accelerated in this study, which further stimulates the rapid completion of the entire methanogenesis process.

Tracing of artificial carbon isotope verified the contribution of acetoclastic methanogenesis

The oxidation of carboxyl groups occurs firstly during the direct acetate dismutation, and the electrons produced in this process can reduce methyl groups to produce methane [START_REF] Evans | An evolving view of methane metabolism in the Archaea[END_REF]. In theory, increasing the ratio of 13 CH 3 COOH/ 12 CH 3 COOH will enlarge the value of 13 CH 4 produced by direct dismutation. Under the premise of inhibiting acetoclastic methanogenesis, the 13 C-CO 2 was improved for both treatments with artificial abundance of acetate (Fig. 6a, experiment 5), suggesting that part of the 13 C-labeled methyl group was oxidized to CO 2 . This was consistent with the findings of natural abundance, which showed the occurrence of slight SAO (discussed in part 3.3.2). It should be noted that S. oneidensis MR-1 significantly enhanced the abundance of 13 C-CH 4 (p < 0.05) (Fig. 6b). It provided the robust evidence that exoelectrogenic bacteria can strengthen direct acetate dismutation. Carbon materials maintained a strong direct acetate dismutation capacity to make 13 C-CH 4 level stable. Collectively, direct acetate dismutation preference via increased electron donor and transfer presents an alternative strategy for syntrophic methanogenesis. For the potential for full-scale applications, this work implies that the process of direct acetate dismutation should be paid attention to when enhancing the methane production capacity of fermentation systems through bioaugmentation and/or conductive materials. For example, when a large amount of acetate accumulates due to the degradation of volatile fatty acids, in addition to strengthening syntrophic acetate oxidation coupled with CO 2 reduction to produce methane, the intervention of conductive materials to enhance the direct acetate dismutation pathway may also relieve the accumulation of acetate and even volatile fatty acids.

Conclusions

To date, long-standing concept that DIET-CO 2 reduction to generate methane introduced by conductive materials seems to be fully applicable to almost all scenario. Recently, some studies indicated the benefit of increased electron transfer on direct acetate dismutation [START_REF] Fu | NanoFe3O4 as solid electron shuttles to accelerate acetotrophic methanogenesis by Methanosarcina barkeri[END_REF][START_REF] Inaba | Metatranscriptomic evidence for magnetite nanoparticle-stimulated acetoclastic methanogenesis under continuous agitation[END_REF][START_REF] Li | A new insight into the strategy for methane production affected by conductive carbon cloth in wetland soil: Beneficial to acetoclastic methanogenesis instead of CO 2 reduction[END_REF]. However, these data are not enough to provide evidence that exoelectrogenic activity can affect direct acetate dismutation to generate methane. The findings in this study based on detailed experiments and confirmations expand electromethanogenic strategy that electric syntrophy, which can be strengthened by exoelectrogenic bacteria and electrical conductivity of carbon materials, can depend on acetoclastic methanogenic progress. Furthermore, for the first time, we tested the potential effect of S. oneidensis MR-1 on acetoclastic methanogenesis. This strategy can outcompete DIET-CO 2 reduction under some conditions with high acetate concentration. In addition to the well-known DIET-CO 2 reduction, this findings are very important complements to the strategy for biomethane production in natural environment and artificial anaerobic systems. Special for artificial fermentation with high-loading organic substance and acetate, more attention to the acetoclastic methanogenesis coupled with bioaugmentation by electroactive microorganisms is important for energy recovery.

Fig. 1 .

 1 Fig. 1. Effects of S. oneidensis MR-1 and GAC/CNTs on methanogenic activities of wetland soils. (a) Time-courses of methane concentrations in the enrichment cultures in the absence or presence of CH 3 F and (b) the methanogenic rates estimated from curves in (a). Where indicated, acetoclastic methanogenesis was inhibited by the addition of CH 3 F. Different letters above the bar represent a significant difference (p < 0.05) in the methanogenic rates among different treatments. Data are presented as the means of three independent experiments, and error bars represent standard deviations.

Fig. 2 .

 2 Fig. 2. Electrochemical characterizations of the performance of the MFC. Time-courses of current density (a) and the total electric quantity (b) in a 6-day experiment. The Nyquist plots (c) for the MFCs with the addition of S. oneidensis MR-1 and conductive GAC and CNTs. CVs at scan rate of 80 mV s -1 for four treatments (d).

Fig. 3 .

 3 Fig. 3. Communities of archaea at the family level in incubated soil. Relative abundances of less than 1% for archaea were classified into the group 'others'.

Fig. 4 .

 4 Fig. 4. Dynamics of hydrogen (a) and the ΔG values for hydrogenotrophic methanogenesis (b) within the batch experiment fed with acetate. Error bars represent standard deviations of triplicate tests.

Fig. 5 .

 5 Fig. 5. Dynamics of acetate within the batch experiment (a) and the ΔG values for acetoclastic methanogenesis (b). Error bars represent standard deviations of triplicate tests.

Fig. 6 .

 6 Fig. 6. Abundance of 13 CO 2 and 13 CH 4 with the application of 1% and 3% 13 CH 3 COOH. (a) 13 CO 2 of the four treatments in the presence or absence of CH 3 F; (b) abundance of 13 CH 4 affected by S. oneidensis MR-1 and conductive GAC.

Table 1

 1 Isotopic signatures (δ 13 C) of CH 4 and CO 2 produced in the vials for the four treatments and fractions of acetate-dependent CH 4 production (f ma ) as calculated from δ 13 C-values of CH 4 and CO 2 . δ 13 C-values of CH 4 and CO 2 produced in incubations of slurry in the presence of CH 3 F. δ 13 C-values of CO 2 produced in incubations of slurry without CH 3 F.

	Sampling time Treatments		δ 13 C-values of	δ 13 C-values of	α 3	δ 13 C-values of	δ 13 CH 4 (CO 2 ) i.e.	δ 13 C-values of	f ma (%) 6 (%) 7
				produced methane	produced CO 2	produced CO 2	δ mc	5	produced CH 4
				(with CH 3 F) 1	(with CH 3 F) 2	(without CH 3 F) 4	(without CH 3 F) i.e.
									δ 13 CH 4
	5th day	Control		-61.68 ± 2.13	-22.97 ± 0.81	1.044 ± 0.0 -20.65 ± 1.54	-64.15 ± 0.78 -51.27 ± 2.01	67.08	76.93
		S. oneidensis		-62.96 ± 3.65	-24.88 ± 0.84	1.045 ± 0.0 -20.47 ± 0.73	-66.87 ± 0.80 -50.94 ± 0.25	72.67	78.95
		S. oneidensis + GAC -64.14 ± 0.90	-22.29 ± 1.26	1.046 ± 0.0 -21.28 ± 0.47	-65.29 ± 1.20 -50.94 ± 0.85	70.55	81.04
		S. oneidensis + CNTs -63.79 ± 2.09	-23.12 ± 1.36	1.045 ± 0.0 -22.02 ± 0.15	-65.18 ± 1.31 -48.53 ± 2.84	82.30	80.43
	6th day	Control		-60.83 ± 0.11	-23.28 ± 0.31	1.041 ± 0.0 -22.45 ± 0.38	-61.75 ± 0.30 -49.39 ± 0.54	73.57	78.96
		S. oneidensis		-65.17 ± 0.40	-22.75 ± 0.23	1.045 ± 0.0 -23.36 ± 0.29	-64.83 ± 0.22 -48.39 ± 0.33	82.70	78.16
		S. oneidensis + GAC -64.84 ± 1.53	-22.86 ± 0.58	1.045 ± 0.0 -22.45 ± 0.76	-64.93 ± 0.55 -46.10 ± 1.03	94.24	80.67
		S. oneidensis + CNTs -62.09 ± 0.56	-22.70 ± 0.41	1.043 ± 0.0 -21.97 ± 1.57	-62.99 ± 0.39 -46.06 ± 0.21	93.85	83.65
	1,2 3 Calculated with the equation	(	α CO CH 2 4	=	+ δ CH Co δ Co 13 2 1000 + 13 4 ( 2) 1000	)	using δ 13 C-values of CH 4 and CO 2 in the presence of CH 3 F.

4

Acknowledgments

This research was financially supported by the National Natural Science Foundation of China (no. 41703075, 91751112 and 41573071), the Natural Science Foundation of Shandong Province, China (no. JQ201608 and ZR2016DQ12) and the Young Taishan Scholars Program (No. tsqn20161054).

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.

Appendix A. Supplementary data

Supplementary data to this article can be found online at https:// doi.org/10.1016/j.cej.2020.124469.

Supplementary Material

Methane production by acetate dismutation stimulated by Shewanella oneidensis and carbon materials: As an alternative to classical CO2 reduction

Leilei Xiao 1,3 , Fanghua Liu 1,2,3 *, Eric Lichtfouse 4 , Peng Zhang 5 , Dawei Feng 1,3 , Fangbai Li 6 Chemical Engineering Journal, Volume 389, 1 June 2020, 124469 https://doi.org/10.1016/j.cej.2020.124469