Conductive iron oxide minerals accelerate syntrophic cooperation in methanogenic benzoate degradation

Advances in microbial community, mechanisms and stimulation effects of direct interspecies electron transfer in anaerobic digestion

Anaerobic digestion (AD) has been proven to be an effective green technology for producing biomethane while reducing environmental pollution. The interspecies electron transfer (IET) processes in AD are critical for acetogenesis and methanogenesis, and these IET processes are carried out via mediated interspecies electron transfer (MIET) and direct interspecies electron transfer (DIET). The latter has recently become a topic of significant interest, considering its potential to allow diffusion-free electron transfer during the AD process steps. To date, different multi-heme c-type cytochromes, electrically conductive pili (e-pili), and other relevant accessories during DIET between microorganisms of different natures have been reported. Additionally, several studies have been carried out on metagenomics and metatranscriptomics for better detection of DIET, the role of DIET's stimulation in alleviating stressed conditions, such as high organic loading rates (OLR) and low pH, and the stimulation mechanisms of DIET in mixed cultures and co-cultures by various conductive materials. Keeping in view this significant research progress, this study provides in-depth insights into the DIET-active microbial community, DIET mechanisms of different species, utilization of various approaches for stimulating DIET, characterization approaches for effectively detecting DIET, and potential future research directions. This study can help accelerate the field's research progress, enable a better understanding of DIET in complex microbial communities, and allow its utilization to alleviate various inhibitions in complex AD processes.

Crystalline iron oxide mineral (magnetite) accelerates methane production from petroleum hydrocarbon biodegradation

Methane (CH4) emissions are a factor in climate change; in addition, CH4 production may affect reclamation of fluid fine tailings (FFT) in tailings ponds, and end-pit lakes (EPLs). In laboratory cultures, we investigated the effect of crystalline iron mineral (magnetite) on CH4 production from the biodegradation of hydrocarbons added to FFT collected from methanogenically more and less active sites in a demonstration EPL. Magnetite enhanced CH4 production from both sites, having a greater effect in more active FFT, where it increased the CH4 production rate as much as 48% (from 6.67 μmol d-1 to 9.87 μmol d-1) compared to FFT without magnetite. Correspondingly, magnetite hastened biodegradation of hydrocarbons (monoaromatics, n-alkanes and iso-alkanes), with a pronounced effect on o-xylene, ethylbenzene, m/p-xylenes, n-octane, n-nonane, and 2-methyloctane, where biodegradation rates increased by 46, 117, 11, 45, 28 and 37%, respectively, compared to FFT without magnetite. Little FeII was produced, suggesting that magnetite is not being used as an electron acceptor but rather functions as a conduit for electron transfer. Thus, magnetite may be a suitable amendment to enhance bioremediation of anaerobic environments contaminated with hydrocarbons. Importantly, our observations imply that magnetite may increase CH4 emissions from terrestrial ecosystems, thus affecting carbon budget estimations.

Nano Fe3O4 improved the electron donating capacity of dissolved organic matter during sludge composting

The effect of Fe3O4 nanoparticles (Fe3O4 NPs) on the electron transfer process in aerobic composting systems remains unexplored. In this study, we compared the electron transfer characteristics of DOM in sludge composting without additives (group CK) and with the addition of 50 mg/kg Fe3O4 NPs additive (group Fe). It was demonstrated that the electron transfer capacity (ETC) and electron donating capacity (EDC) of compost-derived DOM increased by 13%-29% and 40%-47%, respectively, with the addition of Fe3O4 NPs during sludge composting. Analyzing the composition and structure of DOM revealed that Fe3O4 NPs promoted the formation of humic acid-like substances and enhanced the aromatic condensation degree of DOM. Correlation analysis indicated that the increase in EDC of DOM was closely associated with the phenolic group in DOM and influenced by quinone groups and the degree of aromatization of DOM. The higher EDC and the structural evolution of DOM in group Fe reduced the bioaccessibility of Cu, Cr, Ni, Zn. This study contributes to a deeper understanding of the redox evolutionary mechanism of DOM in sludge composting and broadens the application of iron oxides additives.

Improvement of biotic nitrate reduction in constructed photoautotrophic biofilm-soil microbial fuel cells

The biotic nitrate reduction rate in freshwater ecosystems is typically constrained by the scarcity of carbon sources. In this study, 'two-chambers' - 'two-electrodes' photoautotrophic biofilm-soil microbial fuel cells (P-SMFC) was developed to accelerate nitrate reduction by activating in situ electron donors that originated from the soil organic carbon (SOC). The nitrate reduction rate of P-SMFC (0.1341 d-1) improved by ∼ 1.6 times on the 28th day compared to the control photoautotrophic biofilm. The relative abundance of electroactive bacterium increased in the P-SMFC and this bacterium contributed to obtain electrons from SOC. Biochar amendment decreased the resistivity of P-SMFC, increased the electron transferring efficiency, and mitigated anodic acidification, which continuously facilitated the thriving of putative electroactive bacterium and promoted current generation. The results from physiological and ecological tests revealed that the cathodic photoautotrophic biofilm produced more extracellular protein, increased the relative abundance of Lachnospiraceae, Magnetospirillaceae, Pseudomonadaceae, and Sphingomonadaceae, and improved the activity of nitrate reductase and ATPase. Correspondingly, P-SMFC in the presence of biochar achieved the highest reaction rate constant for nitrate reduction (kobs) (0.2092 d-1) which was 2.4 times higher than the control photoautotrophic biofilm. This study provided a new strategy to vitalize in situ carbon sources in paddy soil for nitrate reduction by the construction of P-SMFC.

Effects of Mixtures of Engineered Nanoparticles and Cocontaminants on Anaerobic Digestion

The widespread application of nanotechnology inevitably leads to an increased release of engineered nanoparticles (ENPs) into the environment. Due to their specific physicochemical properties, ENPs may interact with other contaminants and exert combined effects on the microbial community and metabolism of anaerobic digestion (AD), an important process for organic waste reduction, stabilization, and bioenergy recovery. However, the complicated interactions between ENPs and other contaminants as well as their combined effects on AD are often overlooked. This review therefore focuses on the co-occurrence of ENPs and cocontaminants in the AD process. The key interactions between ENPs and cocontaminants and their combined influences on AD are summarized from the available literature, including the critical mechanisms and influencing factors. Some sulfides, coagulants, and chelating agents have a dramatic "detoxification" effect on the inhibition effect of ENPs on AD. However, some antibiotics and surfactants increase the inhibition of ENPs on AD. The reasons for these differences may be related to the interactive effects between ENPs and cocontaminants, changes of key enzyme activities, adenosine triphosphate (ATP) levels, reactive oxygen species (ROS) production, and microbial communities. New scientific opportunities for a better understanding of the coexistence in real world situations are converging on the scale of nanoparticles.

Synergetic effect of nitrate on dissolved organic carbon attenuation through dissimilatory iron reduction during aquifer storage and recovery

Aquifer storage and recovery (ASR) is a promising water management technique in terms of quantity and quality. During ASR, iron (Fe) (hydr)oxides contained in the aquifer play a crucial role as electron acceptors in attenuating dissolved organic carbon (DOC) in recharging water through dissimilatory iron reduction (DIR). Considering the preference of electron acceptors, nitrate (NO3⁻), possibly coexisting with DOC as the prior electron acceptor to Fe (hydr)oxides, might influence DIR by interrupting electron transfer. However, this phenomenon is yet to be clarified. In this study, we systematically investigated the potential effect of NO3⁻ on DOC attenuation during ASR using a series of sediment columns representing typical aquifer conditions. The results suggest that DOC attenuation could be enhanced by the presence of NO3⁻. Specifically, total DOC attenuation was notably higher than that from the stoichiometric calculation simply employing NO3⁻ as the additional electron acceptor to Fe (hydr)oxides, implying a synergetic effect of NO3⁻ in the overall reactions. X-ray photoelectron spectroscopy analyzes revealed that the Fe(II) ions released from DIR transformed the Fe (hydr)oxides into a less bioavailable form, inhibiting further DIR. In the presence of NO3⁻, however, no aqueous Fe(II) was detected, and another form of Fe (hydr)oxide appeared on the sediment surface. This may be attributed to nitrate-dependent Fe(II) oxidation (NDFO), in which Fe(II) is (re)oxidized into Fe (hydr)oxide, which is available for the subsequent DOC attenuation. These mechanisms were supported by the dominance of DIR-relevant bacteria and the growth of NDFO-related bacteria in the presence of NO3⁻.

Reduced arsenic availability in paddy soil through Fe-organic ligand complexation mediated by bamboo biochar

The reuse of arsenic (As)-contaminated paddy fields is a global challenge because long-term flooding would result in As release due to the reductive dissolution of iron minerals. Biochar amendment is a common and effective remediation technique for As-contaminated paddy soil. However, the literature is still lacking in systematic research on the function of biochar in controlling the complexation of released dissolved organic matter (DOM) and iron oxides and its synergistic impact on the availability of As in flooded paddy soil. In the present study, bamboo biochar was prepared at different pyrolysis temperatures (300, 450 and 600 °C), as BB300, BB450 and BB600. Four paddy soil treatments including BB300, BB450, BB600 applications (1% ratio, m/m, respectively) and control (CK, no biochar application) were set and incubated for 60 d in flooding condition. The results showed that As availability represented by adsorbed As species (A-As) was mitigated by BB450 amendment compared with CK. The amendment of BB450 in paddy soil facilitated the complexation of HCl extractable Fe(III)/(II) and DOM and formation of amorphous iron oxides (e.g. complexed Fe species). Moreover, the abundance of Geobacteraceae and Xanthomonadaceae, as common electroactive bacteria, was promoted in the BB450 treated paddy soil in comparison to CK, which assisted to form amorphous iron oxides. The formed amorphous iron oxides then facilitated the formation of ternary complex (As-Fe-DOM) with highly stability, which could be considered as a mechanism for As immobilization after biochar was applied to the flooding paddy soil. Thus, the synergistic effect between amorphous iron oxides and electroactive stains could make main contribution to the passivation of released As in paddy soil under long-term flooding condition. This study provided a new insight for As immobilization via regulating iron-organic ligand complexation amendment with biochar in flooding paddy soil.

Structural and Functional Shifts in the Microbial Community of a Heavy Metal-Contaminated Soil Exposed to Short-Term Changes in Air Temperature, Soil Moisture and UV Radiation

The interplay between metal contamination and climate change may exacerbate the negative impact on the soil microbiome and, consequently, on soil health and ecosystem services. We assessed the response of the microbial community of a heavy metal-contaminated soil when exposed to short-term (48 h) variations in air temperature, soil humidity or ultraviolet (UV) radiation in the absence and presence of Enchytraeus crypticus (soil invertebrate). Each of the climate scenarios simulated significantly altered at least one of the microbial parameters measured. Irrespective of the presence or absence of invertebrates, the effects were particularly marked upon exposure to increased air temperature and alterations in soil moisture levels (drought and flood scenarios). The observed effects can be partly explained by significant alterations in soil properties such as pH, dissolved organic carbon, and water-extractable heavy metals, which were observed for all scenarios in comparison to standard conditions. The occurrence of invertebrates mitigated some of the impacts observed on the soil microbial community, particularly in bacterial abundance, richness, diversity, and metabolic activity. Our findings emphasize the importance of considering the interplay between climate change, anthropogenic pressures, and soil biotic components to assess the impact of climate change on terrestrial ecosystems and to develop and implement effective management strategies.

Dual role of birnessite on the modulation of acid production and reinforcement of interspecific electron transfer in anaerobic digestion

Achieving efficient anaerobic digestion of highly loaded substrates is one of the most challenging problems in the field of waste resourcing. Here, the addition of birnessite (2.0 g/L) to kitchen wastewater increased the acetate and final methane yields by 40.53 and 99.18 %, respectively, while reducing the yields of propionate and butyrate by 38.17 and 48.86 %, respectively. There were two main pathways for birnessite to enhance anaerobic digestion, one of which is to act as an electron acceptor, by inducing an alteration in the ratio of reduced-state coenzyme I in the microorganism, allowing the acid production process to proceed towards deeper oxidation. Another pathway enhances the interspecific electron transfer between bacteria and archaea and improves methane yield by optimizing the metabolic relationship. The Kyoto Encyclopedia of Genes and Genomes (KEGG) functional predictions suggest that the extracellular electron transport pathway of the microorganism is enhanced with the addition of birnessite and that its intracellular metabolic pathway is biased towards the nicotinamide adenine dinucleotide (NADH) generation pathway. This work demonstrated that anaerobic digestion facilitation by metallic minerals was not monolithic; that is, different properties of the minerals were employed to intensify the different stages of anaerobic digestion and obtain an amplification cascade.

Enhancing sludge methanogenesis with changed micro-environment of anaerobic microorganisms by Fenton iron mud

Conductive materials have been demonstrated to enhance sludge methanogenesis, but few researches have concentrated on the interaction among conductive materials, microorganisms and their immediate living environment. In this study, Fenton iron mud with a high abundance of Fe(III) was recycled and applied in anaerobic reactors to promote anaerobic digestion (AD) process. The results show that the primary content of extracellular polymeric substances (EPS) such as polysaccharides and proteins increased significantly, possibly promoting microbial aggregation. Furthermore, with the increment of redox mediators including humic substances in EPS and Fe(III) introduced by Fenton iron mud, the direct interspecies electron transfer (DIET) between methanogens and interacting bacteria could be accelerated, which enhanced the rate of methanogenesis in anaerobic digestion (35.21 ± 4.53% increase compared to the control). The further analysis of the anaerobic microbial community confirmed the fact that Fenton iron mud enriched functional microorganisms, such as the abundance of CO2-reducing (e.g. Chloroflexi) and Fe(III)-reducing bacteria (e.g., Tepidimicrobium), thereby expediting the electron transfer reaction in the AD process via microbial DIET and dissimilatory iron reduction (DIR). This work will make it possible for using the recycled hazardous material - Fenton iron mud to improve the performance of anaerobic granular sludge during methanogenesis.

Effect of Addition of Zero-Valent Iron (Fe) and Magnetite (Fe3O4) on Methane Yield and Microbial Consortium in Anaerobic Digestion of Food Wastewater

Direct interspecies electron transfer (DIET), which does not involve mediation by electron carriers, is realized by the addition of conductive materials to an anaerobic digester, which then activates syntrophism between acetogenic and methanogenic microorganisms. This study aimed to investigate the effect of the addition of two conductive materials, zero-valent iron (ZVI) and magnetite, on the methane production and microbial consortium via DIET in the anaerobic digestion of food wastewater. The operation of a batch reactor for food wastewater without the addition of the conductive materials yielded a biochemical methane potential (Bu), maximum methane production rate (Rm), and lag phase time (λ) of 0.380 Nm3 kg−1-VSadded, 15.73 mL day−1, and 0.541 days, respectively. Upon the addition of 1.5% ZVI, Bu and Rm increased significantly to 0.434 Nm3 kg−1-VSadded and 19.63 mL day−1, respectively, and λ was shortened to 0.065 days. Simultaneously, Methanomicrobiales increased from 26.60% to 46.90% and Methanosarcinales decreased from 14.20% to 1.50% as the ZVI input increased from 0% to 1.50%. Magnetite, at an input concentration of 1.00%, significantly increased the Bu and Rm to 0.431 Nm3 kg−1-VSadded and 18.44 mL day−1, respectively. However, although magnetite improves the efficiency of methanogenesis via DIET, the effect thereof on the methanogen community remains unclear.

Optimization of iron-enhanced anaerobic digestion of agro-wastes for biomethane production and phosphate release

In this study, the optimization of additives (polypyrrole magnetite nanocomposites (Ppy/Fe₃O₄) and antagonists (humic acid and arsenic oxide)) for simultaneous recovery of biomethane and phosphate release from enhanced anaerobic co-digestion of okra waste and pig manure was investigated. The pre-determined dosages of additives from our previous studies were used for the batch anaerobic digestion at different ratios under mesophilic conditions based on the two level-four factors central composite design (CCD) response surface methodology (RSM). After the anaerobic digestion processes, the biomethane yields were recorded and the digestates were characterized to determine the quantity of soluble phosphates. Both the independent variables and the responses were used to model and optimize the biogas yield and phosphate release conditions. The result showed that the maximum biomethane yield and P release were respectively 502.743 mLCH4/gVS and 168.674 mg/L at the optimum conditions of Ppy/Fe₃O₄ (20.0014 mg/L), HA (5.0018 mg/L), As (1.448 mg/L) and co-digestion (25.0001%). The response models predicted biomethane yield and P release to be 528.635 mLCH4/gVS and 164.405 mg/L respectively. All the response models were highly significant with appropriate goodness of fit and had prediction differences of 4.90% and 2.597% respectively for both biomethane yield and P release. Although both the accelerants and antagonists had influences on the anaerobic digestion processes by achieving enhanced biomethane production and P release, the influence of long exposure of anaerobic digestion processes to these additives on both responses is recommended for further investigation.

Effect of Benzyl Alcohol on Biomethanation from Lignite

Currently, biomethane obtained from coal resources, such as lignite and peat, serves as a sustainable biofuel urgently needed by the energy economy. To improve biomethane yield from lignite, the effects of different concentrations of benzyl alcohol, a degraded product of humic acid, on a biomethanation fermentation system were analyzed. The total biomethane yield, daily biomethane yield, coenzyme F₄₂₀, VFA (volatile fatty acids) concentration, alkalinity, and pH were used to determine the optimal benzyl alcohol concentration. The biomethanation fermentation system with 2000 mg/L benzyl alcohol produced up to 4.03 mL/g of biomethane, which was 1.15 times that produced from the control group. The coenzyme F₄₂₀, VFA, alkalinity, and pH of the system after adding 2000 mg/L benzyl alcohol were more preferable after adding other concentrations during the lignite biomethanation process. In summary, 2000 mg/L benzyl alcohol had a significantly positive effect on the lignite biomethanation fermentation system. When benzyl alcohol is added to the fermentation system, it accelerates the tricarboxylic acid cycle, which in turn produces more biomethane. Additionally, the self-supply of lignite microbial transformation nutrients from the perspective of chemical composition was explored as a novel approach in solving the common problem of low biomethane yield from a single lignite raw material. This also laid a foundation for subsequent steps through the adjustment of pretreatment conditions to ensure that the lignite pretreatment liquid contained increased benzyl alcohol, and a greater yield of biomethane can be produced after activated sludge addition.

Effect of magnetite particle size on propionate degradation in the propionate-based anaerobic system

The size effect of magnetite (Fe₃O₄) on the degradation of propionate (PA) in the PA-based anaerobic system was investigated. The sequential bench-scale experiments were conducted. Results showed that the effects of different sized magnetite particles on PA degradation varied, and reaction cycles also played a role in substrate removal/degradation. With the increase of reaction cycle, nano-magnetite promoted PA degradation and CH₄ production, which caused faster PA degradation rate (0.997 g/L·d) than the control group (CK) without magnetite (0.834 g/L·d), whereas the groups with micron- and millimeter-sized magnetite had slower PA degradation rates (0.746 and 0.636 g/L·d) than CK group. The particle size or surface characteristics of the magnetite may become the main factor determining the PA degradation rate. Furthermore, the analysis of PA conversion and volatile fatty acids (VFAs) distribution showed the C6-dismutation pathway, which converses PA to butyrate, enhanced by the introduction of magnetite. Microbial community analysis showed that PA was degraded mainly by methyl-malonyl-CoA (MMC) pathway. The relative abundance of Syntrophobacter that catalyze MMC pathway in the group with nano-magnetite were much higher after three reaction cycles at 39 %, as compared to micro-magnetite at 28 %, and millimeter-sized magnetite at 27 %, which contributed to faster degradation of PA. Functional enzyme-encoding genes for the four methanogenesis pathways were identified with reference to KEGG database entries. The methanogenesis pathway using acetate was the most abundant pathway in all groups. The observations have important implications for enhancing the PA removal in PA-inhibited anaerobic digester.

Effect of Fe3O4 nanoparticles on the performance of anaerobic digestion through electrochemical analysis

In view of the high cost and complex operation of the traditional chemical method for the determination of the anaerobic digestion (AD) process, this study investigated the feasibility of using electrochemistry analysis to determine the effect of Fe₃O₄ nanoparticles (NPs) on the AD process. The quantity of electric charge of Fe₃O₄ NPs modified graphite electrode in sodium acetate electrolyte was higher than that of graphite electrode. The addition of Fe₃O₄ NPs was beneficial to improve the electrochemical activity of the electrode. On this basis, enzymes and microorganisms were extracted from anaerobic reactors containing different concentrations of Fe₃O₄ NPs to make electrodes respectively to further investigate the effects of Fe₃O₄ NPs on enzyme and microbial activities in AD. The electrochemical analysis results of the enzyme electrodes and microbial electrodes showed that the quantity of electric charge produced by the group of 200 mg/L Fe₃O₄ NPs was the highest. The cumulative biogas production of 200 mg/L Fe₃O₄ NPs was 809 mL/g VS, and better than other groups. These results showed that 200 mg/L Fe₃O₄ NPs was the best dosage, and the electrochemical analysis has higher sensitivity in the AD detection.Highlight Electrochemistry was used to investigate the activities of enzymes and microorganisms in anaerobic digestion.The effects of Fe₃O₄ nanoparticles on different stages of anaerobic digestion were analyzed by electrochemical method.The feasibility of electrochemical analysis was verified from the perspective of microbiology.

Enhanced methane production in anaerobic digestion: A critical review on regulation based on electron transfer

Anaerobic digestion (AD) is a potential bioprocess for waste biomass utilization and energy conservation. Various iron/carbon-based CMs (e.g., magnetite, biochar, granular activated carbon (GAC), graphite and zero valent iron (ZVI)) have been supplemented in anaerobic digestors to improve AD performance. Generally, the supplementation of CMs has shown to improve methane production, shorten lag phase and alleviate environmental stress because they could serve as electron conduits and promote direct interspecies electron transfer (DIET). However, the CMs dosage varied greatly in previous studies and CMs wash out remains a challenge for its application in full-scale plants. Future work is recommended to standardize the CMs dosage and recover/reuse the CMs. Moreover, additional evidence is required to verify the electrotrophs involved in DIET.

Iron Compounds in Anaerobic Degradation of Petroleum Hydrocarbons: A Review

Waste and wastewater containing hydrocarbons are produced worldwide by various oil-based industries, whose activities also contribute to the occurrence of oil spills throughout the globe, causing severe environmental contamination. Anaerobic microorganisms with the ability to biodegrade petroleum hydrocarbons are important in the treatment of contaminated matrices, both in situ in deep subsurfaces, or ex situ in bioreactors. In the latter, part of the energetic value of these compounds can be recovered in the form of biogas. Anaerobic degradation of petroleum hydrocarbons can be improved by various iron compounds, but different iron species exert distinct effects. For example, Fe(III) can be used as an electron acceptor in microbial hydrocarbon degradation, zero-valent iron can donate electrons for enhanced methanogenesis, and conductive iron oxides may facilitate electron transfers in methanogenic processes. Iron compounds can also act as hydrocarbon adsorbents, or be involved in secondary abiotic reactions, overall promoting hydrocarbon biodegradation. These multiple roles of iron are comprehensively reviewed in this paper and linked to key functional microorganisms involved in these processes, to the underlying mechanisms, and to the main influential factors. Recent research progress, future perspectives, and remaining challenges on the application of iron-assisted anaerobic hydrocarbon degradation are highlighted.

Novel insight on ferric ions addition to mitigate recalcitrant formation during thermal-alkali hydrolysis to enhance biomethanation

Thermal-chemical pre-treatment has proven to facilitate the solubilization of organics and improvement in biogas generation from the organic fraction of municipal solid waste (OFMSW). However, the production of recalcitrant is inevitable when OFMSW is pretreated at high temperatures and alkali dosage. This study develops a strategy to use Fe³⁺ to reduce the formation of recalcitrant compounds, i.e., 5-HydroxyMethyl Furfural (5-HMF), furfurals, and humic acids (HA) during thermal-alkali pre-treatment. It was postulated that the formation of the recalcitrant compound during pre-treatment can be reduced by Fe³⁺ dosing to oxidize intermediates of Maillard reactions. A decrease in 5-HMF (45-49%) and furfurals (54-66%) was observed during Fe³⁺ (optimum dose: 10 mg/L) mediated thermal-alkali pre-treatment owing to the Lewis acid behavior of FeCl₃. The Fe³⁺ mediated assays show a substantial improvement in VS removal (28%) and biogas yield, i.e., 31% (292 mL/gVS_added) in 150 °C + 3 g/L NaOH, 34% (316 mL/gVS_added) in 175 °C + 3 g/L NaOH, and 36% (205 mL/gVS_added) in 200 °C + 3 g/L NaOH assays, over their respective controls (no Fe³⁺ dosing). The reducing property of Fe³⁺ rendered a low ORP (-345 mV) in the system than control, which is beneficial to the anaerobic microbiome. Electrical conductivity (EC) also shows a three-fold increase in Fe³⁺ mediated assays over control, promoting direct interspecies electron transfer (DIET) amongst microbes involved in the electrical syntrophy. The score plot and loading plots from principal component analysis (PCA) showed that the results obtained by supplementing 10 mg/L Fe³⁺ at 150, 175, and 200 °C were significantly different. The correlation of the operational parameters was also mutually correlated. This work provides a techno-economically and environmentally feasible option to mitigate the formation of recalcitrant compounds and enhance biogas production in downstream AD by improving the degradability of pretreated substrate.

Effects of different carbon sources on methane production and the methanogenic communities in iron rich flooded paddy soil

Various carbon sources as substrates and electron donors can produce methane via different metabolic pathways. In particular, the methane produced by rice cultivation has a severe impact on climate change. However, how Fe³⁺, the most abundant oxide in paddy soil, mediates the methanogenesis of different carbon sources is unknown. In this study, we investigated the effect of four carbon sources with different chain lengths (acetate, glucose, nonanoate, and starch) on CH₄ production and associated methanogens in iron-rich paddy soil over 90 days of anaerobic incubation. We found that glucose and starch were the more preferential substrates for liberating methane compared to acetate, and the rate was also faster. Nonanoate was unable to support methane production. Methanosarcinales and Methanobacteriales were the most predominant methanogenic archaea as shown by 16S rRNA gene sequencing, though their abundance changed over time. Additionally, a significantly higher content of iron-reducing bacteria was observed in the glucose and starch treatments, and it was significantly positively correlated with the copy number of the methanogenic mcrA gene. Together, we confirmed the methanogenic capacity of different carbon sources and their related microorganisms. We also showed that iron oxides play a central role in regulating methane emissions from paddy soils and need more attention to be paid to them.

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.

Characteristics of organic removal for supermarket wastewater treatment with an anaerobic baffled reactor and efficacy evaluation of changing HRT

An anaerobic baffled reactor (ABR) is one of the useful wastewater treatment technologies, but the knowledge about its treatment performance for actual wastewater with load fluctuation is limited. The organic removal performance of an ABR for treating supermarket wastewater was evaluated. The ABR, which consisted of eight columns, was examined under four hydraulic retention time (HRT) conditions of 19.4, 12.9, 8.0, and 4.4 h. As a result, the unfiltered chemical oxygen demand (COD) removal efficiency was 80 (±8) % at an HRT of 19.4 h. When the HRT was shortened to 12.9 h, the average unfiltered COD removal efficiency decreased to 58 (±15) %. However, it showed buffering effect against high load inflow in the first column, indicating that it is useful as a pretreatment system under this condition. At an HRT of 4.4 h, the unfiltered COD removal efficiency decreased to 9%, indicating the system failed. The results of the microbial community structure analysis showed that the detection frequency of acidogenic bacteria decreased in proportion to the extension of residence time in the reactor. These results indicate that the ABR is useful for the treatment of supermarket wastewater with load fluctuations as a main treatment system at a HRT of 19.4 h and as a pretreatment system at a HRT of 12.9 h.

Methanogen Productivity and Microbial Community Composition Varies With Iron Oxide Mineralogy

Quantifying the flux of methane from terrestrial environments remains challenging, owing to considerable spatial and temporal variability in emissions. Amongst a myriad of factors, variation in the composition of electron acceptors, including metal (oxyhydr)oxides, may impart controls on methane emission. The purpose of this research is to understand how iron (oxyhydr)oxide minerals with varied physicochemical properties influence microbial methane production and subsequent microbial community development. Incubation experiments, using lake sediment as an inoculum and acetate as a carbon source, were used to understand the influence of one poorly crystalline iron oxide mineral, ferrihydrite, and two well-crystalline minerals, hematite and goethite, on methane production. Iron speciation, headspace methane, and 16S-rRNA sequencing microbial community data were measured over time. Substantial iron reduction only occurred in the presence of ferrihydrite while hematite and goethite had little effect on methane production throughout the incubations. In ferrihydrite experiments the time taken to reach the maximum methane production rate was slower than under other conditions, but methane production, eventually occurred in the presence of ferrihydrite. We suggest that this is due to ferrihydrite transformation into more stable minerals like magnetite and goethite or surface passivation by Fe(II). While all experimental conditions enriched for Methanosarcina, only the presence of ferrihydrite enriched for iron reducing bacteria Geobacter. Additionally, the presence of ferrihydrite continued to influence microbial community development after the onset of methanogenesis, with the dissimilarity between communities growing in ferrihydrite compared to no-Fe-added controls increasing over time. This work improves our understanding of how the presence of different iron oxides influences microbial community composition and methane production in soils and sediments.

Independent of direct interspecies electron transfer: Magnetite-mediated sulphur cycle for anaerobic degradation of benzoate under low-concentration sulphate conditions

The aim of this study was to investigate the primary mechanism of magnetite promoting anaerobic degradation of aromatic compounds under the low-concentration sulphate conditions. Under influent conditions of benzoate at 50 mM-chemical oxygen demand (COD) and sulphate at 15 mM, magnetite promoted benzoate degradation (77.1% vs 56.3%), while the effluent sulphate concentration was slightly higher than that without magnetite (1.6 mM vs 0.7 mM), inconsistent with functional gene prediction that both sulphate respiration and sulphur compound respiration were relatively more active in the presence of magnetite. Remarkably, X-ray diffraction showed that, signal related to Fe₃O₄ faded away and finally was replaced by FeSO₄ and FeS, indicating that magnetite participated in benzoate degradation coupled to sulphate reduction via dissimilatory Fe(III) reduction. Further X-ray photoelectron spectroscopy showed that, signal related to S⁰ was only detected with magnetite, suggesting the possibility of re-oxidation of sulphide to elemental sulphur coupled to Fe(III) reduction. This was further supported by the increase in abundance of Desulfuromonas acetexigens capable of growing on Fe(III). In addition, magnetite specially enriched the chemolithotrophic sulphur-disproportionating microbes, Desulfovibrio aminophilus, which might proceed the disproportionation of elemental sulphur to sulphate and sulphide to achieve a sulphur cycle for benzoate degradation.

Effects of sludge thermal hydrolysis pretreatment on anaerobic digestion and downstream processes: mechanism, challenges and solutions

Thermal hydrolysis pretreatment (THP), as a step prior to sludge anaerobic digestion (AD), is widely applied due to its effectiveness in enhancing organic solids hydrolysis and subsequent biogas productivity. However, THP also induces a series of problems including formation of refractory compounds in THP cylinder, high residual ammonia and organic in the AD centrate, inhibition on downstream nitrogen removal process and reduction in UV-disinfection effectiveness during post-treatment. More attention should be paid on how to mitigate these negative effects. Despite intensive studies were carried out to reduce refractory compounds formation and enhance biological performance, there is limited effort to discuss the solutions to tackle the THP associated problems in a holistic manner. This paper summarizes the solutions developed to date and analyzes their technology readiness to assess application potential in full-scale settings. The content highlights the limitations of THP and proposes potential solutions to address the technological challenges.

Influencing mechanisms of siderite and magnetite, on naphthalene biodegradation: Insights from degradability and mineral surface structure

Biodegradation is the most economical and efficient process for remediating polycyclic aromatic hydrocarbons (PAHs) such as naphthalene (Nap). Soil composition is pivotal in controlling PAH migration and transformation. Iron minerals such as siderite and magnetite are the primary components of soil and sediment and play key roles in organic pollutant biodegradation. However, it is unclear whether siderite and magnetite promote or inhibit Nap biodegradation. The effects of siderite and magnetite on Nap biodegradation were investigated through batch experiments in this study. The results indicated that siderite increased Nap biodegradation efficiency by 7.87%, whereas magnetite inhibited Nap biodegradation efficiency by 3.54%. In the presence of siderite, Nap-degrading bacteria with acid-producing effects promoted siderite dissolution via metabolic activity, resulting in an increased Fe (II) concentration in solution which accelerated the iron reduction process and promoted Nap biodegradation. In addition, the presence of iron minerals altered the genus-level community structure. Anaerobic sulfate-reducing bacteria such as Desulfosporosinus occurred in the presence of siderite, indicating that sulfate reduction occurred in advance under the influence of siderite. In the presence of magnetite, Fe (III) in iron minerals were converted to Fe (II), and under the mediation of microorganisms, Fe (II) combined with carbonate to form secondary minerals (e.g., siderite). Secondary minerals were attached to the surface of magnetite, which inhibited magnetite dissolution and reduced the efficiency of Fe (III) utilization by microorganisms. Furthermore, as the reaction proceeds, acid-producing microorganisms promoted magnetite further dissolution, resulting in a longer duration of the Fe (III) reduction process. Bacteria utilizing sulfuric acid as the terminal electron acceptor consumed organic matter more rapidly than those using iron as the terminal electron acceptor. Therefore, magnetite inhibited Nap degradation. These observations enhance our understanding of the interaction mechanisms of iron minerals, organic pollutants, and degrading bacteria during the biodegradation process.

Application of electric potential improves ethanol production from xylose by active sludge

BACKGROUND

Low-cost raw materials such as lignocellulosic materials have been utilized in second-generation ethanol production process. However, the sequential and slow conversion of xylose into target products remains one of the main challenges for realizing efficient industrial lignocellulosic biorefinery.

RESULTS

By applying different constant potentials to different microbial electrolysis cells with xylose as the sole carbon source, we analyzed the output of metabolites, microbial community structures, electron flow, and carbon flow in the process of xylose electro-fermentation by domesticated activated sludge. The bioreactors produced currents when applying positive potentials. The peak currents of the + 0.242 V, + 0.542 V and + 0.842 V reactors were 0.96 × 10-6 A, 3.36 × 10-6 A and 6.43 × 10-6 A, respectively. The application of potentials promoted the xylose consumption, and the maximum consumption rate in the + 0.542 V reactor was 95.5%, which was 34.8 times that of the reactor without applied potential. The potential application also promoted the production of ethanol and acetate. The maximum ethanol yield (0.652 mol mol-1 xylose) was obtained in the + 0.842 V reactor. The maximum acetate concentration (1,874 µmol L-1) was observed in the + 0.842 V reactor. The optimal potential for ethanol production was + 0.842 V with the maximum ethanol yield and energy saving. The application of positive potential caused the microorganisms to carry out ethanol fermentation, and the application of negative potential forced the microorganisms to carry out acetic fermentation. The potential application changed the diversity and community structure of microorganisms in the reactors, and the two most significantly changed families were Paenibacillaceae and Bacillaceae.

CONCLUSION

The constructed microbial electrolysis cells with different potentials obtained better production yield and selectivity compared with the reactor without applied potential. Our work provides strategies for the subsequent fermentation processes with different needs.

Performance and mechanism of conductive magnetite particle-enhanced excess sludge anaerobic digestion for biogas recovery

The aim of this study was to evaluate the effect of magnetite particles on the anaerobic digestion of excess sludge. The results showed that methane production increased with the increase in magnetite dosage in the range of 0–5 g L⁻¹, and the cumulative methane production increased by 50.1% at a magnetite dosage of 5 g L⁻¹ compared with the blank reactor after 20 days. Simultaneously, numerous volatile fatty acids (VFAs) were produced at high magnetite dosages, providing the required substrates for methanogenesis. The concentration of magnetite addition was positively correlated with methane production, which proved that magnetite was beneficial for the promotion of the conversion of VFAs to methane. Moreover, the degradation efficiencies of proteins and carbohydrates reached 64% and 52.6% at the magnetite dosage of 5 g L⁻¹, respectively, and corresponding activities of protease and coenzyme F420 were 9.03 IU L⁻¹ and 1.652 μmol L⁻¹. In addition, the Methanosaeta and Methanoregula genus were enriched by magnetite, which often participate in direct interspecies electron transfer as electron acceptors.

Magnetite particles were applied to excess sludge anaerobic digestion. The methane production and sludge reduction were related to magnetite particle dosage, and the Methanosaeta and Methanoregula involved in the electron transfer were enriched.

Microbiological Surveillance of Biogas Plants: Targeting Acetogenic Community

Acetogens play a very important role in anaerobic digestion and are essential in ensuring process stability. Despite this, targeted studies of the acetogenic community in biogas processes remain limited. Some efforts have been made to identify and understand this community, but the lack of a reliable molecular analysis strategy makes the detection of acetogenic bacteria tedious. Recent studies suggest that screening of bacterial genetic material for formyltetrahydrofolate synthetase (FTHFS), a key marker enzyme in the Wood-Ljungdahl pathway, can give a strong indication of the presence of putative acetogens in biogas environments. In this study, we applied an acetogen-targeted analyses strategy developed previously by our research group for microbiological surveillance of commercial biogas plants. The surveillance comprised high-throughput sequencing of FTHFS gene amplicons and unsupervised data analysis with the AcetoScan pipeline. The results showed differences in the acetogenic community structure related to feed substrate and operating parameters. They also indicated that our surveillance method can be helpful in the detection of community changes before observed changes in physico-chemical profiles, and that frequent high-throughput surveillance can assist in management towards stable process operation, thus improving the economic viability of biogas plants. To our knowledge, this is the first study to apply a high-throughput microbiological surveillance approach to visualise the potential acetogenic population in commercial biogas digesters.

Co-digestion of primary sewage sludge with drinking water treatment sludge: A comprehensive evaluation of benefits

Anaerobic co-digestion of primary sludge with two types of drinking water treatment sludge (DWTS), namely iron- or aluminum-rich DWTS (Fe- or Al-DWTS) were systematically evaluated by biochemical methane potential tests, kinetic modelling, downstream process parameters and microbial community analysis. Specific methane yields decreased approximately 19% to 123 mL·g^-1 VS, while the hydrolysis constant k_h decreased from 0.21 d^-1 to 0.18 d^-1 for Fe-DWTS at 10% to 40% dosages. On the contrary, specific methane yields decreased 45-55% for Al-DWTS, and k_h decreased to 0.14 d^-1 at 40% dosage. Significant removals (>95%) of phosphate and hydrogen sulfide were observed for Fe- and Al-DWTS additions at 40% dosage. Microbial community analysis revealed that Al-DWTS increased the abundance of most hydrogenotrophic methanogens, while Fe-DWTS increased the abundance of acetoclastic methanogens. Kinetic modelling further revealed that Fe- and Al-DWTS additions affected the hydrolysis and methanogenesis process kinetics and the methane yield differently.

Crystalline iron oxides stimulate methanogenic benzoate degradation in marine sediment-derived enrichment cultures

Elevated dissolved iron concentrations in the methanic zone are typical geochemical signatures of rapidly accumulating marine sediments. These sediments are often characterized by co-burial of iron oxides with recalcitrant aromatic organic matter of terrigenous origin. Thus far, iron oxides are predicted to either impede organic matter degradation, aiding its preservation, or identified to enhance organic carbon oxidation via direct electron transfer. Here, we investigated the effect of various iron oxide phases with differing crystallinity (magnetite, hematite, and lepidocrocite) during microbial degradation of the aromatic model compound benzoate in methanic sediments. In slurry incubations with magnetite or hematite, concurrent iron reduction, and methanogenesis were stimulated during accelerated benzoate degradation with methanogenesis as the dominant electron sink. In contrast, with lepidocrocite, benzoate degradation, and methanogenesis were inhibited. These observations were reproducible in sediment-free enrichments, even after five successive transfers. Genes involved in the complete degradation of benzoate were identified in multiple metagenome assembled genomes. Four previously unknown benzoate degraders of the genera Thermincola (Peptococcaceae, Firmicutes), Dethiobacter (Syntrophomonadaceae, Firmicutes), Deltaproteobacteria bacteria SG8_13 (Desulfosarcinaceae, Deltaproteobacteria), and Melioribacter (Melioribacteraceae, Chlorobi) were identified from the marine sediment-derived enrichments. Scanning electron microscopy (SEM) and catalyzed reporter deposition fluorescence in situ hybridization (CARD-FISH) images showed the ability of microorganisms to colonize and concurrently reduce magnetite likely stimulated by the observed methanogenic benzoate degradation. These findings explain the possible contribution of organoclastic reduction of iron oxides to the elevated dissolved Fe²⁺ pool typically observed in methanic zones of rapidly accumulating coastal and continental margin sediments.

Link between characteristics of Fe(III) oxides and critical role in enhancing anaerobic methanogenic degradation of complex organic compounds

Fe(III) oxides have been investigated to accelerate anaerobic methanogenic degradation of complex organic compounds. However, the critical role linked to the characteristics of different types of Fe(III) oxides is still unclear. Study presented here performed a side-by-side comparison of four types of Fe(III) oxides including Fe(III)-citrate, ferrihydrite, hematite and magnetite to evaluate their effectiveness in methanogenic degradation of phenol. Results showed that, amorphous Fe(III)-citrate group showed the fastest phenol degradation and Fe²⁺ release among all the groups, followed by poorly crystalline ferrihydrite. Although Fe(III)-citrate group also showed the fastest methane production rate, the efficiency of electron recovery in methane production was only 58-78%, which was evidently lower than that in both crystalline hematite (86-89%) and magnetite (93-97%) groups. Methane production rate with non-conductive ferrihydrite was nearly same as that with conductive magnetite, both of which were significantly higher than that with semi-conductive hematite. X-ray Diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) analysis showed that sludge collected from hematite and magnetite group still respectively presented a relatively intact characteristic spectra involved in hematite and magnetite. Differently, the characteristic spectra involved in ferrihydrite was not evident in sludge collected from ferrihydrite group, whereas the characteristic spectra involved in magnetite was detected. Microbial community analysis showed that, both Fe(III)-citrate and ferrihydrite specially enriched Fe(III)-reducing bacteria capable of degrading phenol into fatty acids (Trichococcus and Caloramator) via dissimilatory Fe(III) reduction. Fe(III)-citrate also stimulated the growth of Syntrophus capable of degrading phenol/benzoate into acetate and proceeding direct interspecies electron transfer (DIET). In magnetite and hematite group, the abundance of Enterococcus species evidently increased, and they might proceed DIET with Methanothrix species in syntrophic conversion of fatty acids into methane.

Recent advances in methanogenesis through direct interspecies electron transfer via conductive materials: A molecular microbiological perspective

Conductive materials can serve as biocatalysts during direct interspecies electron transfer for methanogenesis in anaerobic reactors. However, the mechanism promoting direct interspecies electron transfer in anaerobic reactors, particularly under environments in which diverse substrates and microorganisms coexist, remains to be elucidated from a scientific or an engineering point of view. Currently, many molecular microbiological approaches are employed to understand the fundamentals of this phenomenon. Here, the direct interspecies electron transfer mechanisms and relevant microorganisms identified to date using molecular microbiological methods were critically reviewed. Moreover, molecular microbiological methods for direct interspecies electron transfer used in previous studies and important findings thus revealed were analyzed. This review will help us better understand the phenomena of direct interspecies electron transfer using conductive materials and offer a framework for future molecular microbiological studies.

Novel magnetic loofah sponge biochar enhancing microbial responses for the remediation of polycyclic aromatic hydrocarbons-contaminated sediment

Magnetic activated carbon and magnetic biochar have been widely used for contaminants removal due to the advantages of sequestration and recovery. However, the remediation function and microbial response of conductive magnetic carbonaceous materials for treating organic contaminated sediment are poorly understood. In this study we applied novel three-dimensional mesh magnetic loofah sponge biochar (MagLsBC), made from natural agricultural product, to remediate polycyclic aromatic hydrocarbons (PAHs)-contaminated sediment. Compared to other carbon-based materials, MagLsBC achieved the high reduction of PAHs content and bioavailability in sediment by respectively 31.9 % and 38.1 % after 350 days. Microbial analysis showed that MagLsBC amended sediment had different community diversity, structure and enriched dominant species associated with the aromatic hydrocarbon metabolism. And MagLsBC amendment significantly increased the aromatic compounds degradation function, which was not observed in other treatments, and methanogenesis function. Further analysis revealed that the enhanced microbial responses in MagLsBC amended sediment were related with the high conductivity of MagLsBC. These results give the new insights into the effect of magnetic carbon materials on microbial community and organic pollutants degradation function during the long period amendment, demonstrating MagLsBC as an effective material with the biostimulation potential for the risk control of PAHs contamination.

Sparking Anaerobic Digestion: Promoting Direct Interspecies Electron Transfer to Enhance Methane Production

Anaerobic digestion was one of the first bioenergy strategies developed, yet the interactions of the microbial community that is responsible for the production of methane are still poorly understood. For example, it has only recently been recognized that the bacteria that oxidize organic waste components can forge electrical connections with methane-producing microbes through biologically produced, protein-based, conductive circuits. This direct interspecies electron transfer (DIET) is faster than interspecies electron exchange via diffusive electron carriers, such as H2. DIET is also more resilient to perturbations such as increases in organic load inputs or toxic compounds. However, with current digester practices DIET rarely predominates. Improvements in anaerobic digestion associated with the addition of electrically conductive materials have been attributed to increased DIET, but experimental verification has been lacking. This deficiency may soon be overcome with improved understanding of the diversity of microbes capable of DIET, which is leading to molecular tools for determining the extent of DIET. Here we review the microbiology of DIET, suggest molecular strategies for monitoring DIET in anaerobic digesters, and propose approaches for re-engineering digester design and practices to encourage DIET.

Inhibition mitigation of methanogenesis processes by conductive materials: A critical review

Methanogenesis can be promoted by the addition of conductive materials. Although stimulating effects of conductive materials on methane (CH₄) production has been extensively reported, the crucial roles on recovering methanogenic activities under inhibitory conditions have not been systematically discussed. This critical review presents the current findings on the effects of conductive materials in methanogenic systems under volatile fatty acids (VFAs), ammonia, sulfate, and nano-cytotoxicity stressed conditions. Conductive materials induce fast VFAs degradation, avoiding VFAs accumulation during anaerobic digestion. Under high ammonia concentrations, conductive materials may ensure sufficient energy conservation for methanogens to maintain intracellular pH and proton balance. When encountering the competition of sulfate-reducing bacteria, conductive materials can benefit electron competitive capability of methanogens, recovering CH₄ production activity. Conductive nanomaterials stimulate the excretion of extracellular polymeric substances, which can prevent cells from nano-cytotoxicity. Future perspectives about unraveling mitigation mechanisms induced by conductive materials in methanogenesis processes are further discussed.

Dynamics of hydrocarbon mineralization characterized by isotopic analysis at a jet-fuel-contaminated site in subtropical climate

Release of benzene, toluene, ethylbenzene, and xylene (BTEX) as components of the light non-aqueous phase liquids (LNAPL) contaminates soil and groundwater. Assessing the mechanisms of degradation and mineralization of BTEX in groundwater helps understand the migration of the dissolved plume, enabling the reduction of risks to humans. Here, we studied the fate of ethylbezene, m,p-xylenes and o-xylenes and the accompanying formation of methane in a Cenozoic lateritic aquifer in Brazil by compound-specific carbon stable isotope analysis (CSIA), to gain insights into the complex dynamics of release and biodegradation of BTEX in the LNAPL source zone. The enrichment of ∂¹³C in aromatic compounds dissolved in groundwater compared to the corresponding compounds in LNAPL indicate that CSIA can provide valuable information regarding biodegradation. The isotopic analysis of methane provides direct indication of oxidation mediated by aquifer oxygenation. The ∂¹³C-CO₂ values indicate methanogenesis prevailing at the border and aerobic biodegradation in the center of the LNAPL source zone. Importantly, the isotopic results allowed major improvements in the previously developed conceptual model, supporting the existence of oxic and anoxic environments within the LNAPL source zone.

Potential of biogas production from manure of dairy cattle fed on natural soil supplement rich in iron under batch and semi-continuous anaerobic digestion

This study provides a novel method for improving the anaerobic digestion (AD) of Holstein dairy manure (HDM) by the direct addition of Mineraso (MnS), a natural soil-derived supplement, to the feed of Holstein dairy cattle (HDC). MnS is chiefly composed of approximately 69.08% Fe₃O₄ and was supplemented at rates of 0 (F1), 25 (F2), and 50 (F3) g/head of HDC/d for two months. The HDM was then examined for non-absorbed iron prior to the batch and semi-continuous bench AD experiments. The results revealed that MnS enhanced CH₄ generation in F2 and F3 by 25% and 42%, respectively, in the batch experiments compared to that of F1. Additionally, the gas yield improved in F2 and F3 by 45% and 66%, respectively, over the control after 7 d in the bench experiments. Therefore, supplementing animals with MnS represents a sustainable and economic approach to enhancing CH₄ yields.

Stimulation of methane production from benzoate with addition of carbon materials

Huge amounts of wastewater that contain aromatic compounds such as benzene and phenols are discharged worldwide. Benzoate is a typical intermediate in the anaerobic transformation of those aromatic compounds. In this study, electrically conductive carbon-based materials of granulated activated carbon (GAC), multiwalled carbon nanotubes (MwCNTs), and graphite were evaluated for the ability to promote the benzoate degradation. The results showed that 82-93% of the electrons were recovered in CH₄ production from benzoate. The carbon materials stimulated benzoate degradation in the sequence of GAC (5 g/L) > MwCNTs (1 g/L) ~ Graphite (0.1 g/L) > Control. Acetate was the only detected intermediate in the process of benzoate degradation. Taxonomic analyses revealed that benzoate was degraded by Syntrophus to acetate and H₂, which were subsequently converted to methane by Methanosarcina (both acetoclastic methanogens and hydrogenotrophic methanogens) and Methanoculleus (hydrogenotrophic methanogens), and direct interspecies electron transfer (DIET) of Desulfovibrio and Methanosarcina. Thus, these results suggest a method to effectively enhance the removal of aromatic compounds and methane recovery.

Clarifying the synergetic effect of magnetite nanoparticles in the methane production process

Magnetite nanoparticles (Fe₃O₄ NPs) were applied in an anaerobic semi-continuous tank reactor (ASTR) to investigate its effect on the anaerobic digestion (AD) of acetate synthetic wastewater. The Fe₃O₄ NPs corrosion could create a more favorable micro-environment to enhance the methanogens activity. The chemical oxygen demand (COD) removal efficiency and methane production in test (ASTR_T) were 31.1% and 101.5% higher than those in control (ASTR_C). With the addition of Fe₃O₄ NPs, the concentration of key coenzyme (F₄₂₀ and M) increased from averagely 0.523 and 5.352 μmol/g-VSS to 0.956 and 9.267 μmol/g-VSS, and the content of soluble microbial products (SMPs) significantly increased. Additionally, the high-throughput 16S rRNA gene sequencing further confirmed that the percentage of hydrogen-utilizing methanogens (Methanolinea) was up to 62.6% of total archaeal sequences. Fe₃O₄ NPs addition would accelerate electrons transfer from acids oxidizers to syntrophic methanogenesis, further stimulate acids oxidizers to decompose acetate to H₂/CO₂, and finally facilitate more methane production.

Iron oxide alleviates acids stress by facilitating syntrophic metabolism between Syntrophomonas and methanogens

Anaerobic digestion (AD) is a promising technology for food waste management, but frequently restricted with long lag phase as a consequent of acidification. Two laboratory experiments were conducted to investigate the effects of iron materials on food waste AD. Experiment 1 compared the effects of iron oxide (IO) and zero valent iron (ZVI) on AD performance. The results showed that both IO and ZVI could enhance methane (CH₄) generation, but IO showed better performance regarding the reduction of lag phase. The lag phase of the reactor supplemented with IO was 17.4% and 42.7% shorter than that of the reactor supplemented with ZVI and the control, respectively. Based on these results, experiment 2 was designed to examine the role of IO in alleviation of acid stress at high substrate to inoculum (SI) ratio. The results showed that supplemented IO into reactor could ensure a successful methanogenesis when operating at high SI ratio, while IO-free reactor was failed to generate CH₄ although operating for 77 days. Supplementing IO into the reactor after 48 h of digestion could restore the CH₄ generation, though its lag phase was 2.6 times of the reactor supplemented with IO at the beginning of the digestion. Microbial community structure analysis revealed that IO could simultaneously enrich Syntrophomonas and methanogens (i.e. Methanobacterium, Methanofollis and Methanosarcina), and might promote electron transfer between those two types of microbes, which were critical for achieving an effective methanogenesis.

Performance of a novel IAHD-DSR process with methane and sulfide as co-electron donors

A novel integrated autotrophic and heterotrophic denitrification- denitrifying sulfide removal (IAHD-DSR) process was established in this study for biogas desulfurization to simultaneously remove nitrogen in wastewater. The study demonstrated that the system could utilize methane and sulfide as co-electron donors to replace organic carbon source in IAHD process. Three batch tests (B1, B2 and B3) were set up with IAHD sludge to explore how the novel process works. According to mass balance in B2, methane oxidation and sulfide oxidation contributed 18.75 % and 71.25 % to nitrate removal, respectively; however, the contribution of methane oxidation to total nitrogen (TN) removal reached 84.36 %. Sulfide was mainly responsible for the reduction of nitrate to nitrite, while the methane was for nitrite to nitrogen gas in the presence of insufficient sulfide as electron donors. The TN removal in B2 was almost the same as in normal IAHD-DSR process B3-C. The functional genes mcrA and pmoA responsible for methane oxidation were detected in all three batches, with the abundance of 2.23 ×10⁶ copies/(g dry soil) for mcrA in B1 being the highest in three batches. The sulfide addition in B2 increased the abundance of gene pmoA, indicating the enhancement of nitrite reduction coupled with methane oxidation.

Enhanced Anaerobic Digestion of Long Chain Fatty Acid by Adding Magnetite and Carbon Nanotubes

This study investigated the impact of stimulating direct interspecies electron transfer (DIET), by supplementing nano-sized magnetite (nFe3O4, 0.5 g Fe/g VSS) and carbon nanotubes (CNT, 1 g/L), in anaerobic digestion of oleic acid (OA) at various concentrations (0.10 - 4.00 g chemical oxygen demand(COD)/L). Both supplementations could enhance CH4 production, and its beneficial impact increased with increased OA concentration. The biggest improvements of 114% and 165% compared to the control were achieved by nFe3O4 and CNT, respectively, at OA of 4 g COD/L. The enhancement can be attributed to the increased sludge conductivity: 7.1 ± 0.5 (control), 12.5 ± 0.8 (nFe3O4-added), and 15.7 ± 1.1 µS/cm (CNT-supplemented). Dissolved iron concentration, released from nFe3O4, seemed to have a negligible role in improving CH4 production. The excretion of electron shuttles, i.e., humic-like substances and protein-like substances, were found to be stimulated by supplementing nFe3O4 and CNT. Microbial diversity was found to be simplified under DIET-stimulating conditions, whereby five genera accounted for 88% of the total sequences in the control, while more than 82% were represented by only two genera (Methanotrix concilli and Methanosarcina flavescens) by supplementing nFe3O4 and CNT. In addition, the abudance of electro-active bacteria such as Syntrophomonas zehnderi was significantly increased from 17% to around 45%.

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.

Micro-level evaluation of organic compounds transformation in anaerobic digestion under feast and famine conditions assisted by iron-based materials - Revealing the true mechanism of AD enhancement

Conductive materials have been applied to assist syntrophic metabolism in anaerobic digestion. However, their role in the transformation of organic compounds, particularly recalcitrant compounds, has not been revealed. In this study, iron-based materials - magnetite nanoparticles and Fe²⁺- were employed to explore their effects on the transformation of different organic matters in anaerobic system. Prompted methane production rates and quantity in iron-based materials groups were found due to the improved solubilization of organic particles, enhanced degradation of recalcitrant compounds, and maintained microbial activity under substrate-limited conditions. Specifically, the proportion of the reducing functional groups (C-C/H or CC) and O/C ratio were always significantly lower in iron-based materials supplemented groups (Fe groups) compared to Control group, despite hydrolysis was greatly enhanced in Fe groups. The greater dehydrogenation oxidation was confirmed in the presence of iron-based materials. The remaining humic-like substances (HS), a typical type of recalcitrant compound, was about 2.5 times higher in Control group (221.2 ± 5.3 mg/L-C) compared to Fe groups after 30 days degradation. By tracking the aromaticity of HS and individual compounds at molecular level, this study reveals that iron-based materials were more effective in stimulating the degradation of aliphatic moieties than the aromatic moieties of recalcitrant compounds. When readily biodegradable substrates were limited, Fe groups continued methane generation by using recalcitrant compounds (e.g. thiethylperazine and fluvoxamino acid) as carbon source, and the microbial activity was maintained according to higher relative abundance of protonated nitrogen and continuous methanogenesis activity at starvation phase.

Enhancing direct interspecies electron transfer in syntrophic-methanogenic associations with (semi)conductive iron oxides: Effects and mechanisms

Anaerobic digestion is an effective biological treatment process that produces methane by degrading organic compounds in waste/wastewater. It is a complicated microbial process by metabolic interactions among different types of microorganisms. In this process, efficient interspecies electron transfer between secondary fermenting bacteria and methanogens is the critical process for fast and effective methanogenesis. In syntrophic metabolism, hydrogen or formate has been considered as the conventional electron carrier transferring electrons from secondary fermenting bacteria to hydrogenotrophic methanogens. Recently, direct interspecies electron transfer (DIET) without the involvement of dissolved redox mediators is arousing great concerns and has been regarded as a more efficient and thermodynamically favorable interspecies electron transfer pathway for methanogenesis. Interspecies electron exchange through DIET is accomplished via the membrane-bound cytochromes or conductive pili. Several kinds of exogenously-added conductive or semiconductive iron oxides have been discovered to greatly enhance anaerobic methanogenesis through promoting DIET. Different (semi)conductive iron oxides give a boost to DIET through different mechanisms based on the physicochemical properties of the iron oxides and the reciprocal interactions between iron oxides and functional microorganisms. In this review, the current understanding of interspecies electron transfer in syntrophic-methanogenic consortions is summarized, the effects and deep-rooted mechanisms of (semi)conductive iron oxides on methanogenesis and DIET are discussed, and possible future perspectives and development directions are suggested for DIET via (semi)conductive iron oxides in anaerobic digestion.

Evaluating the effect of biochar on mesophilic anaerobic digestion of waste activated sludge and microbial diversity

This study compared the effects of sewage sludge-derived pyrochar (PC300, PC500, and PC700) and hydrochar (HC180, HC240, and HC300) on mesophilic anaerobic digestion of waste activated sludge (WAS). It was demonstrated that hydrochar can better promote the methane production compared with pyrochar. The highest accumulative methane yield of 132.04 ± 4.41 mL/g VS_added was obtained with HC180 addition. In contrast, the PC500 and PC700 showed a slightly negative effect on methane production. Sludge-derived HC not only accelerated the solubilization and hydrolysis of sludge floc, but also improved the production of acetic acid and propionate, further resulting in improved methane production. Simultaneously, the syntrophic microbes facilitating direct interspecies electron transfer (DIET) such as Syntrophomonas, Peptococcaceae, Methanosaeta and Methanobacterium bred rapidly with the addition of HCs. These results indicated that the hydrochar is more ideal as the accelerant to promote the methane production from mesophilic anaerobic digestion of WAS than the pyrochar.