Conductive carbon nanoparticles inhibit methanogens and stabilize hydrogen production in microbial electrolysis cells

Comparative anti-methanogenic ability of green algae (C. reinhardtii) with/without nanoparticles: in vitro gas and methane production

Introduction

The purpose of this study was to investigate how in vitro gas production (GP) and ruminal fermentation characteristics were affected by increasing concentrations of green algae plant (C. reinhardtii) extracts in combination with nanoparticles MgO and MgS.

Methods

A solution containing 0.1 M MgCl2 was prepared in 300 mL for the green production of MgCl nanoparticles. The mixture was refluxed for two hours at 85°C using a reflux condenser after 10 mL of pomegranate plant extract was added. The green algal plant (C. reinhardtii), which has many non-toxic antioxidants, was used as a carbon source to produce carbon quantum dots (CQD). Chemical analysis was conducted in accordance with AOAC (2005) recommendations. Rumen fluid from recently slaughtered calves is used to produce in vitro gas immediately following slaughter. Analysis of variance (ANOVA) was performed on the obtained data from the in vitro study in a completely randomized design using the mixed model of SAS (version 9.4; Inc., Cary NC, USA).

Results and Discussion

The variance analysis results and the average values of the chemical compositions were significantly influenced by the extracts (all p < 0.0001). In this line, the values of net gas, pH, OMD, ME, NEl, and ME were found to be the highest for Algae + 50 MgO and the lowest for Algae + 50 MgS, respectively (all p < 0.0001). These promising results imply that extracts from C. Reinhardtii may be able to mitigate the adverse consequences of rumen fermentation. To precisely ascertain the impact particular Rhodophyta on greenhouse gas emissions, additional investigation is needed.

Dual-roles of carbon black to accelerate phosphorus recovery as vivianite

Carbon materials have been confirmed to promote phosphorus recovery as vivianite through enhancing dissimilatory iron reduction (DIR), which alleviates phosphorus crisis. Carbon black (CB) exhibits contradictory dual roles of cytotoxicity inducer and electron transfer bridge towards extracellular electron transfer (EET). Herein, the effect of CB on vivianite biosynthesis was investigated with dissimilatory iron reduction bacteria (DIRB) or sewage. With Geobacter sulfurreducens PCA as inoculum, the vivianite recovery efficiency increased accompanied with CB concentrations and enhanced by 39 % with 2000 mg·L^-1 CB. G. sulfurreducens PCA activated the adaptation mechanism of secreting extracellular polymeric substance (EPS) to resist cytotoxicity of CB. While in sewage, the highest iron reduction efficiency of 64 % was obtained with 500 mg·L^-1 CB, which was appropriate for functional bacterial selectivity like Proteobacteria and bio-transformation from Fe(III)-P to vivianite. The balance of CB's dual roles was regulated by inducing the adaptation of DIRB to gradient CB concentrations. This study provide an innovative perspective of carbon materials with dual roles for vivianite formation enhancement.

Long-term effects of soluble and insoluble ferric irons on anaerobic oxidation of methane in paddy soil

Iron-dependent anaerobic oxidation of methane (Fe-AOM) is an important process to reduce methane emissions into the atmosphere. It is well known that iron bioavailability largely influences microbial iron reduction, but the long-term effects of different ferric irons on soil Fe-AOM remain unknown. In this work, paddy soil in the ferruginous zone was collected and inoculated with insoluble ferrihydrite and soluble EDTA-Fe(III) for 420 days. Stable isotope experiments, activity inhibition tests, and molecular biological techniques were performed to reveal the activity, microbial community, and possible mechanism of paddy soil Fe-AOM. The results showed that ferrihydrite was a better electron acceptor for long-term Fe-AOM cultivation. Although EDTA-Fe(III) is highly bioavailable and could stimulate Fe-AOM activity for a short time, it restricted the activity increase in the long term. The abundances of archaea, iron-reducing bacteria (IRB), and gene mcrA largely increased after cultivation, indicating the important roles of mcrA-carrying archaea and IRB. Remarkably, archaeal communities were similar, but bacteria were totally different with different ferric irons. The results of the microbial community and activity inhibition suggested that Fe-AOM was performed likely by the cooperation between archaea (Methanomassiliicoccaceae or pGrfC26) and IRB in the cultures.

Management of Enteric Methane Emissions in Ruminants Using Feed Additives: A Review

In ruminants' metabolism, a surplus of hydrogen is removed from the reduction reaction of NAD⁺ (nicotinamide adenine dinucleotide) by the formation of methane by methanogenic bacteria and archaea methanogens. The balance of calculations between VFA (volatile fatty acids), CO₂, and CH₄ indicates that acetate and butyrate play a role in methane production, while the formation of propionate maintains hydrogen and therefore reduces methane production. CH₄ formation in ruminant livestock is not desired because it reduces feed efficiency and contributes to global warming. Therefore, numerous strategies have been investigated to mitigate methane production in ruminants. This review focuses on feed additives which have the capability of reducing methane emissions in ruminants. Due to the environmental importance of methane emissions, such studies are needed to make milk and meat production more sustainable. Additionally, the additives which have no adverse effects on rumen microbial population and where the reduction effects are a result of their hydrogen sink property, are the best reduction methods. Methane inhibitors have shown such a property in most cases. More work is needed to bring methane-reducing agents in ruminant diets to full market maturity, so that farmers can reap feed cost savings and simultaneously achieve environmental benefits.

Optimization of soil microbial fuel cell for sustainable bio-electricity production: combined effects of electrode material, electrode spacing, and substrate feeding frequency on power generation and microbial community diversity

BACKGROUND

Microbial fuel cells (MFCs) are among the leading research topics in the field of alternative energy sources due to their multifunctional potential. However, their low bio-energy production rate and unstable performance limit their application in the real world. Therefore, optimization is needed to deploy MFCs beyond laboratory-scale experiments. In this study, we investigated the combined influence of electrode material (EM), electrode spacing (ES), and substrate feeding interval (SFI) on microbial community diversity and the electrochemical behavior of a soil MFC (S-MFC) for sustainable bio-electricity generation.

RESULTS

Two EMs (carbon felt (CF) and stainless steel/epoxy/carbon black composite (SEC)) were tested in an S-MFC under three levels of ES (2, 4, and 8 cm) and SFI (4, 6, and 8 days). After 30 days of operation, all MFCs achieved open-circuit voltage in the range of 782 + 12.2 mV regardless of the treatment. However, the maximum power of the SEC-MFC was 3.6 times higher than that of the CF-MFC under the same experimental conditions. The best solution, based on the interactive influence of the two discrete variables, was obtained with SEC at an ES of 4.31 cm and an SFI of 7.4 days during an operating period of 66 days. Analysis of the experimental treatment effects of the variables revealed the order SFI < ES < EM, indicating that EM is the most influential factor affecting the performance of S-MFC. The performance of S-MFC at a given ES value was found to be dependent on the levels of SFI with the SEC electrode, but this interactive influence was found to be insignificant with the CF electrode. The microbial bioinformatic analysis of the samples from the S-MFCs revealed that both electrodes (SEC and CF) supported the robust metabolism of electroactive microbes with similar morphological and compositional characteristics, independent of ES and SFI. The complex microbial community showed significant compositional changes at the anode and cathode over time.

CONCLUSION

This study has demonstrated that the performance of S-MFC depends mainly on the electrode materials and not on the diversity of the constituent microbial communities. The performance of S-MFCs can be improved using electrode materials with pseudocapacitive properties and a larger surface area, instead of using unmodified CF electrodes commonly used in S-MFC systems.

Effects of ferroferric oxide on propionate methanogenesis in sequencing batch reactors: Microbial community structure and metagenomic analysis

This study investigated the effects of ferroferric oxide (Fe₃O₄) on propionate methanogenesis in anaerobic sequencing batch reactor (ASBR). Compared to ASBR_C (without Fe₃O₄ addition), the addition of 10 g/L Fe₃O₄ (ASBR_Fe) decreased the maximum methane production rate by 69.6 % when propionate was used as the sole substrate. The addition of Fe₃O₄ reduced the contents of humic substances, riboflavin and nicotinamide adenine dinucleotide in extracellular polymeric substances. Therefore, Fe₃O₄ inhibited interspecies electron transfer of microorganisms through electronic mediators. Microbial community analysis revealed that Fe₃O₄ addition increased the relative abundance of acetate oxidizing bacterium (Mesotoga), but decreased the abundance of hydrogenotrophic methanogen (Methanobacterium). Further metagenomics analysis indicated that Fe₃O₄ increased the abundance of acetate oxidation genes and decreased that of hydrogenotrophic methanogenesis, quorum sensing and V/A-type ATPase genes. Thus, Fe₃O₄ reduced propionate methanogenesis during anaerobic digestion. The overall results indicate that Fe₃O₄ addition inhibits methanogenesis for treatment of propionate-contaminated wastewater in ASBR.

Enhancing anaerobic methane production in integrated floating-film activated sludge system filled with novel MWCNTs-modified carriers

Conductive materials can enhance anaerobic methane production by accelerating interspecies electron transfer between electroactive bacteria and methanogens. However, the daily loss or less specific surface area of small/big size of conductive materials always limits their application in anaerobic digestion. In this study, the conductive multi-walled carbon nanotubes (MWCNTs) (15 wt% and 20 wt%) were mixed with high-density polyethylene (HDPE) and novel conductive suspended carriers were prepared. Results showed the conductivity of the novel conductive suspended carriers increased by 1-2 orders of magnitude comparing with HDPE carriers, as well as the attached biomass improved from 3.93 g/m² (HDPE carriers) to 5.82 g/m² (15 wt% MWCNTs-modified carriers) and 6.67 g/m² (20 wt% MWCNTs-modified carriers). Integrated floating-film activated sludge (IFFAS) filled with MWCNT-modified carriers showed significant advantages in chemical oxygen demand (COD) removal (removal efficiency increased by 3.6-37.2%) and methanogenic performance (cumulative methane increased by 12.28-62.91%) compared with the control reactor filled with conventional HDPE carriers when treating sodium propionate wastewater at the organic loading rates (OLR) of 11.3-26.3 kg COD/(m³∙d). SEM images and high-throughput sequencing results proved potential direct interspecies electron transfer (DIET) had been established successfully on the MWCNTs-modified carriers. The syntrophic electroactive bacteria (Geobacter, Thauera) and Methanotrix were enriched by 2.28-4.58% and 9.41-16.80% respectively owning to the addition of novel conductive carriers. This study proved IFFAS process filled with novel MWCNTs-modified suspended carriers showed great potential in establishing DIET to enhance anaerobic digestion in practical application.

Biomethane enhancement using reduced graphene oxide in anaerobic digestion of municipal solid waste

The present research investigated the impact of reduced graphene oxide (rGO) addition on the semi-continuous anaerobic digestion of the organic fraction of municipal solid waste (OFMSW) in the range of 0.5-10 gVolatileSolid(VS)/L_reactorday organic loading rates (OLR). Adding rGO enhanced the rate and yield of biomethane production, and the maximum biomethane increment rate was obtained as 110% at an OLR of 4.0 gVS/L_reactorday. However, after increasing the OLR to 6 gVS/L_reactorday, there was a dramatic decrease in biomethane production because of volatile fatty acid (VFA) accumulation. Methanotrix is the predominant archaeal genus at OLRs lower than 6 gVS/L_reactorday in reactors (89-97%). An increment in biomethane production was associated with the higher abundance of the Methanothrix genus in the rGO-supported reactor (rG) than in the control reactor (rC).

Enhanced anaerobic treatment of synthetic protein-rich wastewater promoted by organic xerogels

Carbon-based materials have been shown to enhance anaerobic digestion processes by promoting direct interspecies electron transfer in methanogenic consortia. However, little is known on their effects during the treatment of complex substrates, such as those derived from protein-rich wastewaters. Here, organic xerogels (OX) are tested, for the first time, as accelerators of the methanogenic activity of an anaerobic consortium treating a synthetic protein-rich wastewater. Three OX with distinct pore size distribution (10 and 1000 nm for OX-10 and OX-1000, respectively) and structural conformation (graphene oxide integration into OX-10-GO polymeric matrix) were synthesized. OX-1000 promoted the highest methane production rate (5.21 mL/g*h, 13.5% increase with respect to the control incubated without OX) among the synthesized OX. Additionally, batch bioreactors amended with OX achieved higher chemical oxygen demand (COD) removal (up to 88%) as compared to the control, which only showed 50% of COD removal. Interestingly, amendment of bioreactors with OX also triggered the production of medium-chain fatty acids, including caprylate and caproate. Moreover, OX decreased the accumulation of ammonium, derived from proteins hydrolysis, partly explained by their adsorption capacities, and probably involving their electron-accepting capacity promoting anaerobic ammonium oxidation. This is the first time that OX were successfully applied as methanogenic accelerators for the anaerobic treatment of synthetic protein-rich wastewater, increasing the methane production rate and COD removal as well as triggering the production of medium chain fatty acids and attenuating the accumulation of ammonium. Therefore, OX are proposed as suitable materials to boost the efficiency of anaerobic systems to treat complex industrial wastewaters.

Improvement of Direct Interspecies Electron Transfer via Adding Conductive Materials in Anaerobic Digestion: Mechanisms, Performances, and Challenges

Anaerobic digestion is an effective and sustainable technology for resource utilization of organic wastes. Recently, adding conductive materials in anaerobic digestion to promote direct interspecies electron transfer (DIET) has become a hot topic, which enhances the syntrophic conversion of various organics to methane. This review comprehensively summarizes the recent findings of DIET mechanisms with different mediating ways. Meanwhile, the influence of DIET on anaerobic digestion performance and the underlying mechanisms of how DIET mediated by conductive materials influences the lag phase, methane production, and system stability are systematically explored. Furthermore, current challenges such as the unclear biological mechanisms, influences of non-DIET mechanisms, limitations of organic matters syntrophically oxidized by way of DIET, and problems in practical application of DIET mediated by conductive materials are discussed in detail. Finally, the future research directions for practical application of DIET are outlined.

Impact factors and novel strategies for improving biohydrogen production in microbial electrolysis cells

Microbial electrolysis cell (MEC) system is an environmentally friendly method for clean biohydrogen production from a wide range of biowastes owing to low greenhouse gas emissions. This approach has relatively higher yields and lower energy costs for biohydrogen production compared to conventional biological technologies and direct water electrolysis, respectively. However, biohydrogen production efficiency and operating costs of MEC still need further optimization to realize its large-scale application.This paper provides a unique review of impact factors influencing biohydrogen production in MECs, such as microorganisms and electrodes. Novel strategies, including inhibition of methanogens, development of novel cathode catalyst, advanced reactor design and integrated systems, to enhance low-cost biohydrogen production, are discussed based on recent publications in terms of their opportunities, bottlenecks and future directions. In addition, the current challenges, and effective future perspectives towards the practical application of MECs are described in this review.

Proteomics reveal biomethane production process induced by carbon nanotube

Biomethane produced by methanogenic archaea is a main greenhouse resource of terrestrial and marine ecosystems, which strongly affects the global environment change. Conductive materials, especially nano-scale, show considerable intervention on biomethane production potential, but the mechanism is still unclear. Herein, we precisely quantified the absolute abundance of Methanosarcina spp. proteins affected by carbon nanotubes (CNTs) using tandem mass tag (TMT) proteomics technology. Among the 927 detectable proteins, more than three hundred, 304, showed differential expression. Gene Set Enrichment Analysis on KEGG pathways and GO biological processes revealed a trend of decreased protein synthesis induced by CNTs, suggesting these conductive nanomaterials may replace part of the cell structure and function. Interestingly, increased acetoclastic methanogenesis actually came at the expense of reduced protein synthesis in related pathways. CNTs stimulated biomethane production from acetate by stimulating intracellular redox activity and the -COOH oxidation process. These findings enhanced the understanding of the biomethane production process affected by conductive materials.

Insight into sludge anaerobic digestion with granular activated carbon addition: Methanogenic acceleration and methane reduction relief

In this study, the multiple effects of granular activated carbon (GAC) on sludge anaerobic digestion at ambient (16-24 °C), mesophilic (35 °C) and thermophilic (55 °C) temperature were investigated. After GAC addition, although the methane yields of raw sludge were reduced by 6.5%-36.9%, the lag phases of methanogenesis were shortened by 19.3%-30.6% and the reductions of methane yields were declined to only 5.9%-8.1% simultaneously for pretreated sludge. The inhibitory substances like phenols that generated by thermal pretreatment were reduced after GAC addition, which were demonstrated to be responsible for the methanogenic acceleration. Meanwhile, the methane reduction due to the non-selective adsorption by GAC could be mitigated by pretreatment and elevated temperature. Thus, a strategy coupling thermal pretreatment with detoxification by GAC was proposed to improve the methane production rate and avoid the negative effects during sludge anaerobic digestion with GAC addition.

Multi-Walled Carbon Nanotubes Enhance Methanogenesis from Diverse Organic Compounds in Anaerobic Sludge and River Sediments

Conductive nanomaterials affect anaerobic digestion (AD) processes usually by improving methane production. Nevertheless, their effect on anaerobic communities, and particularly on specific trophic groups such as syntrophic bacteria or methanogens, is not extensively reported. In this work, we evaluate the effect of multi-walled carbon nanotubes (MWCNT) on the activity of two different anaerobic microbial communities: an anaerobic sludge and a river sediment. Methane production by anaerobic sludge was assessed in the presence of different MWCNT concentrations, with direct methanogenic substrates (acetate, hydrogen) and with typical syntrophic substrates (ethanol, butyrate). MWCNT accelerated the initial specific methane production rate (SMPR) from all compounds, with a more pronounced effect on the assays with acetate and butyrate, i.e., 2.1 and 2.6 times, respectively. In the incubations with hydrogen and ethanol, SMPR increased 1.1 and 1.2 times. Experiments with the river sediment were performed in the presence of MWCNT and MWCNT impregnated with 2% iron (MWCNT-Fe). Cumulative methane production was 10.2 and 4.5 times higher in the assays with MWCNT-Fe and MWCNT, respectively, than in the assays without MWCNT. This shows the high potential of MWCNT toward bioenergy production, in waste/wastewater treatment or ex situ bioremediation in anaerobic digesters.

Enhanced anaerobic digestion of primary sludge with additives: Performance and mechanisms

Anaerobic digestion of primary sludge with different additives, namely nano magnetite, graphite powder, activated carbon powder and NiCl₂/CoCl_2, were evaluated by biomethane potential tests, kinetics modelling and microbial community analysis. Specific methane yields increased from 136 mL/g VS for primary sludge to 146 mL/g VS, 151 mL/g VS, and 152 mL/g VS for the addition of nano magnetite, graphite powder, and activated carbon powder at optimal dosages, respectively. The first order hydrolysis constant k_h increased from 0.488 d^-1 to 0.526 d^-1, 0.622 d^-1, and 0.724 d^-1, respectively. Microbial community analysis revealed that the abundance of key bacterial and archaeal populations was positively correlated with hydrolysis and methane production. The enhanced methane production with activated carbon powder was due to shifting methane formation pathway from acetoclastic to hydrogenotrophic methanogenesis. In contrast, nano magnetite and graphite powder additives enhanced the direct interspecies electron transfer evidenced by increased abundance of Methanosaeta and Methanolinea.

A slurry electrode integrated with membrane electrolysis for high-performance acetate production in microbial electrosynthesis

Microbial electrosynthesis (MES) technology employs electrotrophic microbes as biocatalysts to produce chemicals from CO₂. The application of a slurry electrode can enlarge the surface area to volume ratio, and membrane electrolysis (ME) for on-line extraction can solve the problem of product inhibition. This study constructed a novel dual-chamber ME-MES integrated system equipped with a slurry electrode, and the effect of concentration of powder-activated carbon (AC) in the catholyte on chemical production was also evaluated. The integrated system amended with 5 g L^-1 AC produced up to 13.4 g L^-1 acetate, showing a 179% increase compared with the control group without AC (4.8 g L^-1). However, further increasing the AC concentration to 10 and 20 g L^-1 resulted in decreased acetate production. A high concentration of AC showed higher antimicrobial activity to methanogens, as compared to acetogens. Amending AC exacerbated the process of electroosmosis. Also, amending AC with 0 to 10 g L^-1 decreased the electrochemical losses via both the membrane and electrolyte. The chemical production using H₂ or the electrode as electron donors showed a similar trend when amending AC. The present study provided important information for guiding future research to construct an efficient configuration of an MES bioreactor.

Adding carbon-based materials on anaerobic digestion performance: A mini-review

The anaerobic digestion is the adopted to remediate the pollutant and extract the bioenergy from the waste during the treatment. Effects of adding carbon-based materials on enhancement of digestion performance are studied in literature. This paper provided a mini review on the current research efforts on the traditional view on the cytotoxicity of carbon-based materials to the aquatic microorganisms and the novel "adding carbon-based material strategy" for improving the anaerobic digestion performances. The further research needs for comprehending the interactions between the added carbon materials, the substrates and the microorganisms and the impacts of adopting these additives on full-scale operations were highlighted.

Conductive materials in anaerobic digestion: From mechanism to application

Anaerobic digestion (AD) is an effective strategy combined advantages of maintaining the global carbon flux and efficient energy conversion. Various conductive materials (CMs) have been applied in anaerobic digesters to improve the performance of anaerobic fermentation and methanogenesis, including carbon-based CMs and metal-based CMs. Generally, CMs facilitated the AD thermodynamically and kinetically because they triggered more efficient syntrophic metabolism to increase electron capture capability and accelerate reaction rate as well as enhance the performance of AD stages (hydrolysis-acidification, methanogenesis). Besides, adding CMs into anaerobic digester is benefit to dealing with the deteriorating AD, which induced from temperature variation, acidified working condition, higher H₂ partial pressure, etc. However, few CMs exhibited inhibition on AD, including ferrihydrite, magnesium oxide, silver nanoparticles and carbon black. Inhibition comes from a series of complex factors, such as substrate competition, direct inhibition from Fe(III), Fe(III) reduction of methanogens, toxic effects to microorganisms and mass transfer limitation.