Biochar Modulates Methanogenesis through Electron Syntrophy of Microorganisms with Ethanol as a Substrate

Continuously feeding fenton sludge into anaerobic digesters: Iron species change and operating stability

Fenton sludge generated from the Fenton process contains a large number of ferric species and organic pollutants, which need to be properly treated before discharge. In this study, Fenton sludge as an Fe(III) source for dissimilatory iron reduction (DIR) was continuously added with increasing dosage into an anaerobic digester to enhance the treatment. Results showed continuously feeding Fenton sludge to the anaerobic digester did not deteriorate the performance and increased methane production and COD removal rate by 2.2 folds and 14.0%, respectively. The Fe content of sludge in the digester increased from 40.25 mg/g (dry weight) to 131.53 mg/g after continuously feeding for 77days, and then declined to 109.17 mg/g when the feeding was stopped. Mass balance analysis showed that 20.5 to 48.4% of Fe in the Fenton sludge was released to the effluent. After experiment, the ratio of reducible Fe species to the total Fe was 75.1%, which maintained the high activity in DIR. Microbial community analysis showed that iron-reducing bacteria were enriched with the addition of Fenton sludge and the sludge in the digester had a higher conductivity and capacitance to strengthen the electron transfer of DIR. All results suggested that feeding Fenton sludge into anaerobic digesters was a feasible method to dispose of Fenton sludge as well as to enhance the performance of anaerobic digestion.

Biochar facilitated bacterial reduction of Cr(VI) by Shewanella Putrefaciens CN32: Pathways and surface characteristics

Biochar can facilitate the microbial reduction of various pollutants in soil and groundwater environments, but its impact on Cr(VI) reduction by dissimilatory metal reducing bacteria (DMRB) remains to be systematically investigated. In this study, we prepared biochars at 500 °C and 700 °C from wheat straw and grass, and investigated the impact of these biochars on Cr(VI) reduction by a model DMRB, Shewanella Putrefaciens CN32 (CN32). Pristine biochars abiotically reduced Cr(VI), which decreased the concentration and toxicity of chromium to CN32 cells, and brought about higher overall Cr(VI) removal extent after CN32 were added sequentially; on the other hand, no enhancement effect were observed when biochars and CN32 were added simultaneously. Further tests between biologically reduced biochars and Cr(VI) revealed that the reaction rates between bioreduced biochars and Cr(VI) are relatively sluggish compared to that of direct Cr(VI) reduction by CN32, which prohibited biochars from directly accelerating the Cr(VI) reduction by CN32 in simultaneous-addition scenario. The relative importance of biochars' surface functional groups and surface areas on their reactivities towards Cr(VI) reduction were also investigated. This study deepened our understanding towards the role of biochar played during bacterial Cr(VI) reduction and could potentially contribute to optimizing the biochar-based Cr(VI) bioremediation strategies.

The effect of modified biochar on methane emission and succession of methanogenic archaeal community in paddy soil

Modified biochars have been widely applied in ameliorating environmental problems. However, the effect of modified biochar on suppressing CH₄ emission in rice paddy soil is not fully understood. In order to further study CH₄ regulation in paddy soil via the modification of biochar and explore its influence on key archaeal communities, two modified biochars were generated with the pre-treatment of nitric acid (NBC) and hydrogen peroxide (OBC), respectively, and a control group was setup with water-washed biochar (WBC). Results showed that NBC significantly suppressed CH₄ emission, followed by OBC and WBC, while NBC promoted the CO₂ emission. Besides, the addition of biochars inhibited the accumulation of acetate and H₂ in rice paddy soil, especially in the NBC treatment. 16S rRNA gene sequencing revealed that biochars amendment increased α-diversity of archaeal community and the modified biochars could mitigate the loss of α-diversity in the early stage of anaerobic incubation. Additionally, NBC amendment largely declined the relative abundance of methanogens (especially Methanosarcina) in archaeal community, while OBC and NBC promoted the relative abundance of Candidatus_Methanoperedens. Via Spearman's correlation coefficient analysis, NBC had positive correlations with Methanosaeta, and OBC showed a negative correlation with Methanocella. Overall, this study provided a practical way to regulate the CH₄ emission and associated methanogenic archaea via the amendment of different modified biochars in rice paddy soil.

The impact of powdered activated carbon types on membrane anti-fouling mechanism in membrane bioreactors

Dosing powdered activated carbon (PAC) has been proven to be an economical and effective method to mitigate membrane fouling. However, the effects of pretreated PAC with different redox properties on membrane fouling still need to be further investigated. Here, the impact of commercial PAC, oxidized-PAC, and reduced-PAC on membrane fouling was investigated in membrane bioreactors (MBRs). Surprisingly, the filtration cycles were extended from 12-36 h to 132-156 h only by dosing reduced-PAC and commercial PAC with a finial dosage of 3 g/L, which were provided with reductive properties. However, few improvements of filtration cycle (less than 50 h) were achieved by dosing oxidized-PAC in the same dosage, which had the same adsorption performance as reduced-PAC and commercial PAC. The biomass and foulant concentration suggested that the enhanced anti-fouling performances by PAC with reductive properties were mainly attributed to the reduction of extracellular polymer substances (EPS) and soluble microbial products (SMP) content in the bulk solutions after 14 days of continuous operation. The model foulant degradation tests and the confocal laser scanning microscope (CLSM) images of activated sludge further demonstrated that PAC with reductive properties directly affected the microbial activities by controlling the EPS and SMP concentrations in the bulk solution, thereby suppressing membrane fouling. Such a finding provides new insights into anti-fouling mechanisms that the redox properties of PAC played a decisive role in membrane fouling mitigation, and also provides a strategy to prolong the anti-fouling effects by restoring the reductive properties of PAC. KEY POINTS: • The anti-fouling mechanisms of PAC with reductive property were investigated. • Reductive property was the main reason for fouling control instead of adsorption. • PAC with reductive property hindered the sludge activity to produce fewer foulants.

Alleviating acid inhibition in anaerobic digestion of food waste: Coupling ethanol-type fermentation with biochar addition

In this study, ethanol-type fermentation pretreatment and adding two types of biochar prepared at 600 °C and 1000 °C (referred to as SS600 and SS1000) were combined to alleviate acid accumulation via strengthening direct interspecies electron transfer (DIET) during anaerobic digestion of food waste. Results demonstrated that ethanol production was about 11 g/L after the ethanol-type fermentation at pH of 4-5 for 4 days, accounting for 8.9% of the influent COD of the subsequent methanogenesis. After the ethanol-type fermentation pretreatment, average methane productions of digesters with SS600 and SS1000 addition increased by 86.3% and 64.9% to 618.1 ± 30.1 and 527.3 ± 25.4 mL/g VS under solid retention time (SRT) of 25 d respectively, and the conductivity of sludge increased by 95.3% and 65.3% compared to digester without biochar addition. Furthermore, adding biochar also could accelerate the recovery of acidification digester. The relative abundance of Methanothrix performing DIET were enriched with SS600. These results suggested that coupling ethanol-type fermentation with biochar addition could strengthen DIET to resist the shocks of high organic loading rate.

Understanding the mechanisms behind enhanced anaerobic digestion of corn straw by humic acids

Humic acids (HAs) are abundant on earth, yet their effects on anaerobic digestion (AD) of cellulosic substrate are not fully uncovered. The effects of HAs on AD of corn straw and the mechanisms behind were analyzed in this study. Results showed that the effects of HAs on methane yield were closely related to the total solids (TS) content. At relative high TS content of 5.0%, HAs benefited AD process by increasing 13.8% of methane yield, accelerating methane production rate by 43% and shortening lag phase time by 37.5%. Microbial community analysis indicated that HAs increased the relative abundance of syntrophic bacteria (Syntrophomonadaceae and Synergistaceae), facilitating the degradation of volatile fatty acids. HAs might act as electron shuttles to directly transfer electrons to hydrogenotrophic methanogens for CO₂ reduction to CH₄. This study provides a simple and efficient strategy to facilitate the AD of cellulosic substrate by HAs addition.

Pyrogenic carbon facilitated microbial extracellular electron transfer in electrogenic granular sludge via geobattery mechanism

Electroactive pyrogenic carbon (PC) is an intriguing candidate for realizing the ambitious goals of large-scale applications of microbial electrochemical technologies (METs). In this study, PC was employed to promote the extracellular electron transfer (EET) within the electrogenic granular sludge (EGS) by acting as an electron conduit. The pecan shell-derived PC prepared at three temperatures (600, 800, and 1000 ˚C) contained rich oxygenated-functional moieties (mainly quinone) on the surface, endowing a good electron transfer capacity (EEC). The maximum current density (J_max) of EGS with PC amendment outperformed the control EGS without PC amendment, i.e., 100-132 times higher than J_amx of EGS in the absence of PC. Among various pyrolysis temperatures, the PC derived from 600 ˚C produced the highest J_max (0.40 A/ m²), 0.67-times, and 0.33-times higher than that of PC derived from 800 and 1000 ˚C, respectively. Furthermore, more polysaccharides were secreted in extracellular polymeric substance with PC amendments. The microbial community analysis demonstrated that the PC favored the growth of electroactive bacteria over methanogens. The metabolic pathway revealed that PC induced more functional enzymes in the quinone biosynthesis and cytochrome c and heme synthesis, resulting in an enhanced EET. The EEC of PC was responsible for the EET enhancement effect via PC acting as a geobattery to wire up the EGS and electrodes. Overall, this study pinpoints the finding of PC role in a mixed electroactive biofilm and provides a wide scenario of the PC applications in MET at large scales.

Enhancing hydrogen peroxide activation of CuCo layered double hydroxide by compositing with biochar: Performance and mechanism

In this study, CuCo layered double hydroxide/biochar composite (CuCo LDH/BC) was prepared and employed for activating H₂O₂ to degrade ciprofloxacin (CIP), a common fluroquinolone antibiotic detected in water environment. The as-synthesized catalysts were also comprehensively characterized to study the physiochemical properties. For the catalytic activity, the degradation rate of CuCo LDH/BC to CIP was approximately 1.5 times higher than that of CuCo LDH. The improved catalytic activity can be ascribed to the synergistic effect between CuCo LDH and BC, such as more functional groups, accelerated electron transfer, and varied charge distribution. Meanwhile, CuCo LDH/BC/H₂O₂ could degrade CIP efficiently in a wider pH range comparing with CuCo LDH/H₂O₂, and the efficiency was approximately 84.7% at neutral pH within 90 min. The generation of OH, O₂^- and ¹O₂ in CuCo LDH/BC/H₂O₂ system were then verified by electron spin resonance (ESR) technology. The quenching experiments indicated that both non-radical pathway (¹O₂) and radical pathway (OH, O₂^-) led to CIP degradation, in which O₂^- and ¹O₂ made major contribution. Then, the intermediate products of CIP during catalytic reaction were monitored by high-performance liquid chromatography-mass spectrometry (HPLC-MS), and the environmental risk of these degradation intermediates was tested through seed germination experiments. This study tends to provide valuable information for LDH/BC application in heterogeneous Fenton-like reaction.

Control the greenhouse gas emission via mediating the dissimilatory iron reduction: Fulvic acid inhibit secondary mineralization of ferrihydrite

Reducing methane emission is of great importance to control the global greenhouse effect. Dissimilatory iron reduction (DIR) coupling of organic matter decomposition may suppress methane production via reducing primary electron donors available for methanogenesis. However, during DIR, the amorphous iron oxides (e.g., ferrihydrite) are easy to transform into more stable crystalline iron minerals, which slowdowns the rate of DIR. Humic substance (HS) with redox activity has been extensively reported to facilitate DIR via "electron shuttles" mechanism, yet little known about the effect of HS on mediating the mineralization of iron oxides and the subsequent influences on DIR and methanogenesis. To clarify this, ferrihydrite and fulvic acid (FA) (as the model substance of HS) were supplied to anaerobic methanogenesis systems. Results showed that FA could significantly decrease the formation of crystalline iron oxides, enhance DIR rate by 13.72% and suppress methanogenesis by 25.13% compared to ferrihydrite supplemented only. By X-ray absorption spectra analysis, it was found that FA could complex with ferrihydrite via forming a Fe-C/O structure on the second shell of Fe atom. Quantum chemical calculation further confirmed that FA reduced the adsorption energy between Fe(II) and ferrihydrite. Our study suggested that rational use of HS to mediate mineralization pathway of iron oxides could efficiently improve the availability of iron oxides to drive DIR and control the conversion of organics into CH₄ in natural or engineered systems.

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.

Microbial succession analysis reveals the significance of restoring functional microorganisms during rescue of failed anaerobic digesters by bioaugmentation of nano-biochar-amended digestate

Nano-biochar application was investigated for anaerobic digestion of orange peel waste. The application for methane production focused on the optimization of biochar feedstock, rescue of failed digesters, and microbial succession analysis. It showed that sewage sludge (SS) derived biochar had the highest performance enhancement among the different feedstocks, which could be ascribed to the improvement of electron transfer, interspecies hydrogen transfer, and supply of trace elements. Subsequently, nano SS biochar-amended digestate was evaluated for rescuing failed digesters, and the experimental results indicated its positive roles through gradual bioaugmentation operation. The dynamic analysis of microbial succession indicated the successful application was through the mechanism of restoring partially the functional microbial communities. The major reconstruction of functional microorganisms included bacteria phyla Hydrogenispora (24.5%) and Defluviitoga (18.8%) as well as methanogenic genera of Methanosarcina (41.5%) and Methanobacterium (27.3%). These findings would contribute to rescuing failed anaerobic digesters by bioaugmentation with biochar-amended digestate.

A Polyhydroxybutyrate (PHB)-Biochar Reactor for the Adsorption and Biodegradation of Trichloroethylene: Design and Startup Phase

In this work, polyhydroxy butyrate (PHB) and biochar from pine wood (PWB) are used in a mini-pilot scale biological reactor (11.3 L of geometric volume) for trichloroethylene (TCE) removal (80 mgTCE/day and 6 L/day of flow rate). The PHB-biochar reactor was realized with two sequential reactive areas to simulate a multi-reactive permeable barrier. The PHB acts as an electron donor source in the first “fermentative” area. First, the thermogravimetric (TGA) and differential scanning calorimetry (DSC) analyses were performed. The PHB-powder and pellets have different purity (96% and 93% w/w) and thermal properties. These characteristics may affect the biodegradability of the biopolymer. In the second reactive zone, the PWB works as a Dehalococcoides support and adsorption material since its affinity for chlorinated compounds and the positive effect of the “coupled adsorption and biodegradation” process has been already verified. A specific dechlorinating enriched culture has been inoculated in the PWB zone to realize a coupled adsorption and biodegradation process. Organic acids were revealed since the beginning of the test, and during the monitoring period the reductive dichlorination anaerobic pathway was observed in the first zone; no chlorinated compounds were detected in the effluent thanks to the PWB adsorption capacity.

Plastic-containing food waste conversion to biomethane, syngas, and biochar via anaerobic digestion and gasification: Focusing on reactor performance, microbial community analysis, and energy balance assessment

To manage the mixture of food waste and plastic waste, a hybrid biological and thermal system was investigated for converting plastic-containing food waste (PCFW) into renewable energy, focusing on performance evaluation, microbial community analysis, and energy balance assessment. The results showed that anaerobic digestion (AD) of food waste, polyethylene (PE)-containing food waste, polystyrene (PS)-containing food waste, and polypropylene (PP)-containing food waste generated a methane yield of 520.8, 395.6, 504.2, and 479.8 mL CH₄/gVS, respectively. CO₂ gasification of all the plastic-containing digestate produced more syngas than pure digestate gasification. Syngas from PS-digestate reached the maximum yield of 20.78 mol/kg. During the digestate-derived-biochar-amended AD of PCFW, the methane yields in the biochars-amended digesters were 6-30% higher than those of the control digesters. Bioinformatic analysis of microbial communities confirmed the significant difference between control and biochar-amended digesters in terms of bacterial and methanogenic compositions. The enhanced methane yields in biochars-amended digesters could be partially ascribed to the selective enrichment of genus Methanosarcina, leading to an improved equilibrium between hydrogenotrophic and acetoclastic methanogenesis pathways. Moreover, energy balance assessment demonstrated that the hybrid biological and thermal conversion system can be a promising technical option for the treatment of PCFW and recovery of renewable biofuels (i.e., biogas and syngas) and bioresource (i.e., biochar) on an industrial scale.

New insights into the mechanisms underlying biochar-assisted sustained high-efficient co-digestion: Reducing thermodynamic constraints and enhancing extracellular electron transfer flux

To clarify the roles of biochar in the anaerobic co-digestion of waste activated sludge (WAS) and food waste (FW), batch tests were conducted coupled with thermodynamics, extracellular electron transfer flux and microbial community analysis. Compared with the control group, biochar significantly facilitated the co-digestion at three periods, but its sustainable facilitation was mainly in the syntrophic methanogenesis of volatile fatty acids (VFAs). The thermodynamic analysis confirmed that biochar could alleviate limitations imposed by high hydrogen partial pressure during interspecies hydrogen transfer (IHT), the thermodynamic windows was expanded 137% and 92% in the syntrophic methanogenesis of acetate and propionate, respectively. Meanwhile, due to the redox capacity of biochar (4.85 and 0.35 μmol e^-/g biochar), the equivalent current of direct interspecies electron transfer (DIET) flux for syntrophic methanogenesis of acetate and propionate obtained were 1.0 × 10^-4 A and 0.9 × 10^-4 A, which were 10⁸ times than that of IHT. It should be noticed that the functional microorganisms like Methanosarcina which could participate DIET were only enriched on the surface of biochar, the dominant Methanothermobacter in suspended sludge probably indicate IHT was still the main pathway for syntrophic methanogenesis. Nevertheless, the DIET triggered by the redox-active moieties on the surface of biochar and the enhanced IHT by alleviating thermodynamic restrictions, promoted the syntrophic methanogenesis synergistically.

Stimulatory effects of biochar addition on dry anaerobic co-digestion of pig manure and food waste under mesophilic conditions

The stimulatory effect of biochar addition on dry anaerobic digestion (AD) has been rarely investigated. In this study, the effects of commonly used biochars (bamboo, rice husk, and pecan shell) on dry co-AD were investigated using mesophilic batch digesters fed with pig manure and food waste as substrates. The results show that the specific methane yield was mildly elevated with the addition of biochars by 7.9%, 9.4%, and 12.0% for bamboo, rice husk, and pecan shell-derived biochar additions, respectively. Biochar did facilitate the degradation of poorly biodegradable organics. In comparison, there was no significant effect on the peak methane production rate by the supplementation of the selected biochars. Among the three mechanisms of enhancing methanogenesis by biochar (buffering, providing supporting surface, and enhancing electron transfer), the first two mechanisms did not function significantly in dry co-AD, while the third mechanism (i.e., enhancing electron transfer) might play an important part in dry AD process. It is recommended that the utilization of biochar for the enhancement of biomethanation in dry AD should be more focused on mono digestion in future studies.

Carbon-based conductive materials enhance biomethane recovery from organic wastes: A review of the impacts on anaerobic treatment

Amongst the most important sustainable waste management strategies, anaerobic biotechnology has had a central role over the past century in the management of high-pollution load sources, such as food, agricultural and municipal wastes. During anaerobic digestion (AD), valuable by-products such as digestate and biogas are produced. Biogas (mainly composed of methane) is generated through a series of reactions between bacteria and archaea. Enhancement of AD process with higher methane yield, accelerated methane production rate, and shorter start-up time is possible via tapping into a novel methanogenic pathway discovered a decade ago. This fundamentally new concept that is a substitute to interspecies hydrogen transfer is called direct interspecies electron transfer (DIET). DIET, a thermodynamically more feasible way of electron transfer, has been proven to occur between bacteria and methanogens. It is well-documented that amendment of carbon-based conductive materials (CCMs) can stimulate DIET via serving as an electrical conduit between microorganisms. Therefore, different types of CCMs such as biochar and activated carbon have been amended to a variety of AD reactors and enhancement of process performance was reported. In this review, a comparative analysis is presented for enhancement of AD performance in relation to major CCM related factors; electrical conductivity, redox properties, particle size and dosage. Additionally, the impacts of AD operational conditions such as organic loading rate and temperature on CCM amended reactors were discussed. Further, the changes in microbial communities of CCM amended reactors were reviewed and future perspectives along with challenges for CCM application in AD have been provided.

Poorly conductive biochar boosting extracellular electron transfer for efficient volatile fatty acids oxidation via redox-mediated mechanism

This study explored the performances, and associated mechanisms of biochar promoting volatile fatty acids (VFA) oxidation via extracellular electron transfer (EET) pathway. It was found that in a bioelectrochemical system, adding biochar suspension remarkably enhanced electricity generation whatever acetate or propionate used as an electron donor. The maximum current density in biochar-assisted groups reached 1.6-2.2 A/m², which were 69.2-220.0% higher than that of control groups. The lower electrical resistance of anode in biochar-assisted groups was potentially attributed to the formed biofilm dominated by electro-active Geobacteraceae, and the electron donor type depending on dominant genus. In specific, with biochar assistance, Desulfuromonas enriched from 1.1% to 25.0% when acetate as an electron donor, and the relative abundance of Geobacter increased from 4.6% to 31.7% as dominant genus in propionate-added group. Electrochemical analysis uncovered that biochar hardly elevated sludge electrical conductivity, while the excellent redox-based electron exchange transfer capacity likely made biochar as a transient electron acceptor, which was more accessible than anode to support the metabolism of electroactive bacteria in the initial stage. Meanwhile, the porous surface area of biochar particle likely provided a "bridge" between suspended sludge and anode, to support a more directional evolution of electroactive bacteria on anode. This dual-function of biochar achieved a sustainable VFA oxidation via EET-based pathway.

Microbial investigations of new hydrogel-biochar composites as soil amendments for simultaneous nitrogen-use improvement and heavy metal immobilization

Agricultural sustainability is challenging because of increasingly serious and co-existing issues, e.g., poor nitrogen-fertilizer use and heavy metal pollution. Herein, we introduced a new poly(acrylic acid)-grafted chitosan and biochar composite (PAA/CTS/BC) for soil amendment, and provided a first microbial insight into how PAA/CTS/BC amendment simultaneously improved nitrogen cycling and immobilized heavy metals. Our results suggest that the PAA/CTS/BC amendment significantly promoted soil ammonium retention, and reduced nitrate accumulation, nitrous oxide emission and ammonia volatilization during the rice cultivation. The availability of various heavy metals (Fe, Mn, Cu, Zn, Ni, Pb, Cr, and As) markedly decreased in the PAA/CTS/BC amended soil, thereby reducing their accumulation in rice root. The PAA/CTS/BC amendment significantly altered the structure and function of soil microbial communities. Importantly, the co-occurrence networks of microbial communities became more complex and function-specific after PAA/CTS/BC addition. For example, the keystone species related to organic matter degradation, denitrification, and plant resistance to pathogen or stresses were enriched within the network. In addition to direct adsorption, the effects of PAA/CTS/BC on shaping microbial communities played dominant roles in the soil amendment. Our findings provide a promising strategy of simultaneous nitrogen-use improvement and heavy metal immobilization for achieving crop production improvement, pollution control, and climate change mitigation.

Long-term immobilization of soil metalloids under simulated aging: Experimental and modeling approach

Aging is an inevitable natural process, leading to faded performances of soil amendments. Understanding long-term aging features is crucial for the risk management of contaminated soil. In this study, a novel quantitative aging method, namely, the "soil coin" method, was developed, which can simulate the effects of natural aging on metal(loid) immobilization performances. To better depict the aging features, two models on the basis of conditional probability-induced failure were developed. To effectively immobilize soil arsenic (As) and antimony (Sb), magnesium (Mg) and iron (Fe) oxides were simultaneously introduced to either fresh or pre-oxidized biochar via a facile method. Although post-application aging is harmful, pre-aging (i.e., pre-oxidation using H₂O₂) in turn served as an effective means to introduce more metal oxides, thereby rendering better short-term and long-term effectiveness for metalloid immobilization. Experimental and modeling approaches suggested that precipitation accounted for long-term immobilization, while a constant aging rate is the key feature for a promising soil amendment. It is suggested that to further calibrate this method and better understand the immobilization performances in the long run, more evidence from the field is needed.

Bio-based carbon materials with multiple functional groups and graphene structure to boost methane production from ethanol anaerobic digestion

This study evaluated the effects of bio-based carbon materials on methane production by anaerobic digestion. The results showed that biochar and hydrochar can promote cumulative methane yield by 15% to 29%. However, there was no statistical significance (p > 0.05) between hydrochar and biochar produced at different temperature on methane production. 16S rRNA gene sequencing and bioinformatics analysis showed that biochar and hydrochar enriched microorganism that might participate in direct interspecies electron transfer (DIET) such as Pseudomonadaceae, Bacillaceae, and Clostridiaceae. The the surface properties of the modified biochar were characterized with BET, Raman, FTIR and XPS. Bio-based carbon materials with uniform dispersion provided a stable environment for the DIET of microorganisms and electrons are transferred through aromatic functional groups on the surface of materials. This study reveals bio-based carbon materials surface properties on methane production in anaerobic digestion and provides a new approach to recycling spent coffee grounds.

Anaerobic bacterial responses to carbonaceous materials and implications for contaminant transformation: Cellular, metabolic, and community level findings

Carbonaceous materials (CM) enhance the abundance and activity of bacteria capable of persistent organic (micro)pollutant (POP) degradation. This review synthesizes anaerobic bacterial responses to minimally modified CM in non-fuel cell bioremediation applications at three stages: attachment, metabolism, and biofilm genetic composition. Established relationships between biological behavior and CM surface properties are identified, but temporal relationships are not well understood, making it difficult to connect substratum properties and "pioneer" bacteria with mature microorganism-CM systems. Stark differences in laboratory methodology at each temporal stage results in observational, but not causative, linkages as system complexity increases. This review is the first to critically examine relationships between material and cellular properties with respect to time. The work highlights critical knowledge gaps that must be addressed to accurately predict microorganism-CM behavior and to tailor CM properties for optimized microbial activity, critical frontiers in establishing this approach as an effective bioremediation strategy.

Modification of hydrochar increased the capacity to promote anaerobic digestion

Hydrochar has been demonstrated to increase methane production rate during anaerobic digestion (AD) of organic wastes/wastewater by facilitating direct interspecies electron transfer (DIET). The present study compared the hydrochars prepared at different conditions (260 °C-1 h, 260 °C-8 h, 320 °C-1 h and 320 °C-8 h) on AD of glucose. Hydrochar prepared at lower temperature and residence time (260 °C-1 h) resulted in the highest methane production rate, which was 237% higher of control experiment without hydrochar. Modification of hydrochar (260 °C-1 h) by ball-milling further increased the capacity to increase methane production rate. Hydrothermal liquefaction (HTL) conditions affected the surface oxygen-containing functional groups that related with DIET, and hydrochar (260 °C-1 h) had higher peaks relating with C-O and O-H functional groups. Ball-milling enhanced the formation of such groups. Microbial analysis showed hydrochar (260 °C-1 h) by ball-milling resulted in the formation of different microbial communities as compared with control experiments, and Azospira and Methanosarcina were enriched, which might be involved in DIET.

The conductivity and redox properties of pyrolyzed carbon mediate methanogenesis in paddy soils with ethanol as substrate

Pyrolyzed carbon such as biochar and activated carbon could influence the methanogenesis in paddy soil, which is an important process controlling methane emission. Different electrochemical properties of pyrolyzed carbon may be involved in methanogenesis, such as conductivity and redox activity. However, their different roles have not been thoroughly illustrated before. In this study, we identified the roles of pyrolyzed carbon redox property and electron conductivity in methanogenesis with ethanol as a substrate, by comparing pyrolyzed carbon samples with sequential change of electrochemical properties. Right after the addition, pyrolyzed carbon with highest electron donating capacity (0.85 mmol/g) promoted the methane generation by 33.3%; while, other pyrolyzed carbon with higher electron accepting capacity and lower electron donating capacity than B4 inhibited the methane generation. The relative abundance of electroactive bacteria and certain methanogens increased with the pyrolysis temperature. The strict linear relationship between electroactive bacteria/certain methanogens and cyclic voltammetry peak currents of paddy soil implied that microbial structure was altered due to the improved the electron transfer situation by the electron shuttle ability of pyrolyzed carbon. This study could deepen our understanding about the effect of pyrolyzed carbon on methanogenesis process.

Effects and mechanisms of modified biochars on microbial iron reduction of Geobacter sulfurreducens

Biochar was proved as an electron shuttle to facilitate extracellular electron transfer (EET) of electrochemically active bacteria (EAB); however, its underlying mechanism was not fully understood. In this study, we aimed to further explore how the regulation of surface functional groups of biochar would affect the microbial iron reduction process of Geobacter sulfurreducens as a typical EAB. Two modified biochars were achieved after HNO₃ (NBC) and NaBH₄ (RBC) pretreatments, and a control biochar was produced after deionized water (WBC) washing. Results showed that WBC and RBC significantly accelerated microbial iron reduction of G. sulfurreducens PCA, while had no effect in the final Fe (II) minerals (e.g., vivianite and green rust (CO₃^2-)). Besides, Brunauer-Emmett-Teller (BET) surface area, electron spin resonance (ESR) and electrochemical measurements showed that larger surface area, lower redox potential, and more redox-active groups (e.g., aromatic structures and quinone/hydroquinone moieties) in RBC explained its better electron transfer performance comparing to WBC. Interestingly, NBC completely suppressed the Fe (III) reduction process, mainly due to the production of reactive oxygen species which inhibited the growth of G. sulfurreducens PCA. Overall, this work paves a feasible way to regulate the surface functional groups for biochar, and comprehensively revealed its effect on EET process of microorganisms.

Study on the performance of HNO3-modified biochar for enhanced medium temperature anaerobic digestion of food waste

Biochar can help promote direct interspecies electron transfer (DIET) and increase methane production; the surface redox groups play a constructive role in these processes. This study attempted to improve the anaerobic digestion (AD) performance by modifying biochar with HNO₃ to increase its redox activity. A comparative experimental study, raw biochar (BC0) and biochar treated with HNO₃ for 6 h (BC6), were conducted to investigate the effect of HNO₃ treatment on the medium temperature AD performance of food waste. Both BC0 and BC6 can enhance CH₄ yield and facilitate the degradation of volatile fatty acids. The enhanced yield of CH₄ was 36% for BC0 and 90% for BC6, respectively. Biochar can also enhance methanogenesis, presumably owing to direct interspecific electron transfer (DIET). Compared with BC0, BC6 had a higher redox activity and a smaller conductivity. It was supposed that BC0 mediated DIET through its conductivity, whereas BC6 accelerated DIET by surface redox groups.

Enhanced trichloroethylene biodegradation: Roles of biochar-microbial collaboration beyond adsorption

Trichloroethylene (TCE) is a pollutant widely found in groundwater, especially in the heavily contaminated industrial sites. Biological dechlorination method is environmentally friendly and low-cost. However, microorganisms grow slowly and their activity is susceptible to environmental fluctuations. This study used biochar as an additive to promote anaerobic biodegradation of TCE with mixed culture. Results showed that biochar with dose of 0.1-0.4% (w/v) brought a rapid initial decrease of TCE concentration by 39.4-88.8% in 24 h via adsorption mechanism. Biochar produced at 500 °C pyrolysis temperature (BC500) achieved the highest TCE adsorption in comparison to BC300 and BC700. Subsequently, a significantly shortened microbial stagnation phase (from 85 h to 37 h) was observed in the system with the presence of biochar. During the exponential growth phase, BC700 outperformed BC300 and BC500 in terms of TCE degradation efficiency. Electrochemical analysis demonstrated that BC700 possessed the greatest electron transfer capability. Finally, biochar shortened the time for achieving 100% removal of TCE by 54.5-69.7% (from approximate 330 h to 100-150 h). Even at high concentration of TCE (20-30 mg·L^-1) that could lead to serious microbial growth inhibition, the TCE degradation efficiency could be recovered in the presence of BC500. The high-throughput sequencing data revealed that biochar promoted the relative abundance of co-metabolizing dechlorinating microorganisms (Pseudomonas, Burkholderia) in the aqueous solution, and simultaneously led to the selective colonization of reductive dechlorinating microorganisms (Enterobacteriaceae, Clostridium) attached on biochar surface. On the other hand, biochar addition decreased the relative abundance of hydrogen-competing microorganisms, thereby forming an efficient co-metabolism-reductive dechlorination system. These findings allow a better understanding of the promotion mechanism of biochar for microbial dechlorination technology supporting the biochar-assisted bioremediation in practice.

Combined microbial transcript and metabolic analysis reveals the different roles of hydrochar and biochar in promoting anaerobic digestion of waste activated sludge

Hydrothermal pretreatment of waste activated sludge (WAS) could eliminate the rate limiting step of anaerobic digestion (AD) -hydrolysis. However, the high organic loading rate may cause acid accumulation, thus leading to an unstable system. This study compared the effect of different hydrochar (HC2-260°C and HC3-320°C) and biochar (BC5-500°C and BC7-700°C) on AD of hydrothermal pretreated WAS (HPS). Results demonstrated that hydrochar was superior to biochar in the methane yield and production rate, especially HC2. HC2 had the highest surface oxygen-containing functional groups that could facilitate direct interspecies electron transfer (DIET). The enhanced methane yield was related with the increased protein utilization, and hydrochar and biochar enriched different microbes related to protein degradation. Metabolomic analysis showed the significantly changed metabolites induced by hydrochar and biochar were involved in fatty acids and amino acids-related metabolism, indicating the rapid conversion of intermediated products, which was consistent with the microbial community structure results. Hydrochar and biochar also induced upregulation of metabolites related to microbial metabolic activity and extracellular electron transfer. Although biochar induced the same metabolic changes, the alterations of these metabolites were weaker than those of hydrochar. The results of this study offered new insights into the molecular mechanisms of enhanced AD of HPS by hydrochar and biochar.

Effects of feedstock and pyrolysis temperature of biochar on promoting hydrogen production of ethanol-type fermentation

Biochar has been shown to benefit fermentative hydrogen production. However, the influencing factors and key characteristics of its promoting function remained to be elucidated. This study investigated the effects of two crucial factors, feedstock and pyrolysis temperature, on the hydrogen production-promoting function of biochar in ethanol-type fermentation. The physicochemical characteristics and promoting effects of biochars prepared with five biomass wastes (coffee ground, corn stalk, Ginkgo biloba leaf, mealworm frass, and sugarcane bagasse) were determined. Sugarcane bagasse-derived biochar (SBBC) showed the best hydrogen production-promoting effect in ethanol-type fermentation. The physicochemical properties of biochar, such as pH, element composition and surface features, were significantly affected by pyrolysis temperature, but the promoting effects were not significantly changed. The hydrogen production-promoting effect of biochar in ethanol-type fermentation was mainly affected by feedstock instead of pyrolysis temperature. A potential promoting mechanism was proposed that biochar prepared at low temperature boosted the hydrogen production with redox activity, while that at high temperature achieved the promotion via cell growth enhancement. This study revealed the key promoting factor of biochar in ethanol-type fermentative hydrogen production, and provided novel insights for the promoting mechanism of biochar.

Algae biochar enhanced methanogenesis by enriching specific methanogens at low inoculation ratio during sludge anaerobic digestion

Carbon materials are promising in improving the performance of anaerobic digestion, however, interactive mechanisms between the carbon-based enhancement and operating parameters remained unclear. Using anaerobic digested sludge as inoculum, the effects of Taihu blue algae biochar (ABC) on methanogenesis at different inoculation ratios were investigated during sludge anaerobic digestion. Results showed that ABC enhanced methane productions at the lower inoculation ratios (4% and 1%, v/v), but not at the higher ratio (10%, v/v). Mechanism analysis demonstrated methanogenic improvements at the lower inoculation ratios were not owing to initial organic loading rate increments. Otherwise, ABC addition at the lower inoculation ratios were more favorable for the enrichment of Methanosarcina than the higher ratio, which might be benefit for methanogenesis through directed interspecies electron transfer. Thus, for the improvement of sludge anaerobic digestion, the microbial enrichments at different inoculation ratios would be more important than the merely biochar addition.

Comprehensive evaluation of manganese oxides and iron oxides as metal substrate materials for constructed wetlands from the perspective of water quality and greenhouse effect

Manganese oxides and iron oxides have been widely introduced in constructed wetlands (CWs) for sewage treatment due to their extensiveness in nature and their ability to participate in various reactions, but their effects on greenhouse gas (GHG) emissions remain unclear. Here, a set of vertical subsurface-flow CWs (Control, Fe-VSSCWs, and Mn-VSSCWs) was established to comprehensively evaluate which are the better metal substrate materials for CWs, iron oxides or manganese oxides, through water quality and the global warming potential (GWP) of nitrous oxide (N₂O), methane (CH₄), and carbon dioxide (CO₂). The results revealed that the removal efficiencies of chemical oxygen demand (COD), total nitrogen (TN), and total phosphorus (TP) in Mn-VSSCWs were all higher than that in Fe-VSSCWs, and manganese oxides could almost completely suppress the CH₄ production and reduce GWP (from 8.15 CO₂-eq/m²/h to 7.17 mg CO₂-eq/m²/h), however, iron oxides promoted GWP (from 8.15 CO₂-eq/m²/h to 10.84 mg CO₂-eq/m²/h), so manganese oxides are the better CW substrate materials to achieve effective sewage treatment while reducing the greenhouse gas effect.

Linking population dynamics to microbial kinetics for hybrid modeling of bioelectrochemical systems

Mechanistic and data-driven models have been developed to provide predictive insights into the design and optimization of engineered bioprocesses. These two modeling strategies can be combined to form hybrid models to address the issues of parameter identifiability and prediction interpretability. Herein, we developed a novel and robust hybrid modeling strategy by incorporating microbial population dynamics into model construction. The hybrid model was constructed using bioelectrochemical systems (BES) as a platform system. We collected 77 samples from 13 publications, in which the BES were operated under diverse conditions, and performed holistic processing of the 16S rRNA amplicon sequencing data. Community analysis revealed core populations composed of putative electroactive taxa Geobacter, Desulfovibrio, Pseudomonas, and Acinetobacter. Primary Bayesian networks were trained with the core populations and environmental parameters, and directed Bayesian networks were trained by defining the operating parameters to improve the prediction interpretability. Both networks were validated with Bray-Curtis similarly, relative root-mean-square error (RMSE), and a null model. A hybrid model was developed by first building a three-population mechanistic component and subsequently feeding the estimated microbial kinetic parameters into network training. The hybrid model generated a simulated community that shared a Bray-Curtis similarity of 72% with the actual microbial community at the genus level and an average relative RMSE of 7% for individual taxa. When examined with additional samples that were not included in network training, the hybrid model achieved accurate prediction of current production with a relative error-based RMSE of 0.8 and outperformed the data-driven models. The genomics-enabled hybrid modeling strategy represents a significant step toward robust simulation of a variety of engineered bioprocesses.

Enhancing anaerobic digestion process with addition of conductive materials

Anaerobic digestion is widely used for the treatment of wastewater for its low costs and bioenergy production, but the performances of anaerobic digestion often need improving in practical applications. The addition of conductive materials could lead to direct interspecies electron transfer (DIET) among the anaerobic microorganisms, and consequently enhance the efficiencies of anaerobic digestion. In this paper, the effects of DIET via conductive materials on chemical organic demand (COD) removal, volatile fatty acid (VFA) consumption and methane production were reviewed. The reports on the increase of conductive microorganisms due to the addition of conductive materials were discussed. Results regarding activities of microorganisms and morphology and properties of sludge were described and commented, and future research needs were also proposed which included better understanding of the roles of DIET in each step of anaerobic digestion, mechanisms of metabolism of pollutants in DIET-established systems and inhibition of excessive dosage of conductive materials.

Zeolitic imidazolate framework-derived porous carbon enhances methanogenesis by facilitating interspecies electron transfer: Understanding fluorimetric and electrochemical responses of multi-layered extracellular polymeric substances

Modulating microbial electron transfer during anaerobic digestion can significantly improve syntrophic interactions for enhanced biogas production. As a carbonaceous conductive material, zeolite imidazolate framework-67 (ZIF-67)-derived porous carbon (PC) was hypothesized to act as a microbial electron transfer highway and assessed with respect to understanding the fluorimetric and electrochemical responses of multilayered extracellular polymeric substances (EPS). The highest biomethane yield (614.0 mL/g) from ethanol was achieved in the presence of 100 mg/L PC prepared at a carbonization temperature of 800 °C (PC-800), which was 28.2% higher than that without PC addition. Electrochemical analysis revealed that both the redox peak currents and conductivity of the methanogenic sludge increased, while the free charge transfer resistance decreased with PC-800 addition. The conductive PC-800 potentially functioned as an abiotic electron conduit to promote direct interspecies electron transfer, thereby resulting in decreased expression of functional genes associated with electrically conductive pili (e-pili) and hemeproteins. Additionally, PC-800 stimulated the secretion of redox-active humic substances (HSs), and excitation emission matrix spectra analysis indicated that the largest increase in percent fluorescence response of HSs occurred in the tightly bound EPS (TB-EPS) with addition of PC-800. This was attributed to the strong complexation ability of PC-800 particles to hydroxyl/carboxylic/phenolic moieties of HSs contained in the TB-EPS. Microbial analysis revealed that syntrophic/exoelectrogenic bacteria such as Pelotomaculum and Syntrophomonas, as well as hydrogenotrophic/electrotrophic methanogens such as Methanoculleus and Methanobacterium, were enriched in methanogenic sludge with adding PC-800. This study provided comprehensive insights for understanding the interactions among ZIF-derived PC, methanogenic microorganisms and their multilayered EPS.

How can ethanol enhance direct interspecies electron transfer in anaerobic digestion?

Anaerobic digestion (AD) of organic waste to produce biogas is a mature biotechnology commercialised for decades. However, the relatively recent discovery of direct interspecies electron transfer (DIET) brings a new opportunity to improve the efficiency of biogas technology. DIET may replace mediated interspecies electron transfer (MIET) by efficient electron transfer between exoelectrogens and electrotrophic methanogens, thereby enhancing yields and rates of biogas production. Ethanol, as the initial electron donor in the discovery of the DIET pathway, is now a "hot topic" in the literature. Recent studies have indicated that ethanol in AD functions not only as the substrate, but also as the precursor to stimulate DIET by enriching exoelectrogens and electrotrophic methanogens for co-digesting complex organic wastes. This review aims to highlight the state of the art and recent advances in ethanol-based DIET in AD. The DIET associated reactions of ethanol oxidation and carbon dioxide reduction are assessed by thermodynamic analysis to reveal the extent of the potential for improvement of the AD processes that utilizes DIET pathways. Three ethanol-based DIET strategies are discussed: (1) ethanol as the sole substrate supplemented with conductive materials in AD, (2) ethanol co-digestion with complex substrates and (3) ethanol-type fermentation prior to AD. This review aims to chart the pathways for improved AD performance by utilizing ethanol-based DIET in specific treatments of biological wastes.

In Situ Spectroelectrochemical Characterization Reveals Cytochrome-Mediated Electric Syntrophy in Geobacter Coculture

Direct interspecies electron transfer (DIET) between microbial species prevails in some key microbial consortia. However, the electron transfer mechanism(s) in these consortia is controversial due to lack of efficient characterization methods. Here, we provide an in situ anaerobic spectroelectrochemical coculture cell (in situ ASCC) to induce the formation of DIET coculture biofilm on the interdigitated microelectrode arrays and characterize the electron transfer directly. Two typical Geobacter DIET cocultures, Geobacter metallireducens and wild-type Geobacter sulfurreducens (G.m&G.s) and G. metallireducens and a G. sulfurreducens strain deficient in citrate synthase (G.m&G.s-ΔgltA), were selected. In situ Raman and electrochemical Fourier transform infrared (FTIR) spectroscopy indicated that cytochromes are abundant in the electric syntrophic coculture. Cyclic voltammetry and potential step experiment revealed a diffusion-controlled electron transfer process and the electrochemical gating measurements further demonstrated a cytochrome-mediated electron transfer in the DIET coculture. Furthermore, the G.m&G.s-ΔgltA coculture displayed a higher redox conductivity than the G.m&G.s coculture, consistent with the existence of an intimate and efficient electrical connection between these two species. Our findings provide the first report of a redox-gradient-driven electron transport facilitated by c-type cytochromes in DIET coculture, supporting the model that DIET is mediated by cytochromes and suggest a platform to explore the other DIET consortia.

Biochar heavy metal removal in aqueous solution depends on feedstock type and pyrolysis purging gas

The effectiveness of biochar as a sorptive material to remove contaminants, particularly heavy metals, from water is dependent on biomass type and pyrolysis condition. Biochars were produced from pulp mill sludge (PMS) and rice straw (RS) with nitrogen (N₂) or carbon dioxide (CO₂) as the purging gas. The sorptive capacity of the biochars for cadmium(II), copper(II), nickel(II) and lead(II) was studied. The heavy metal adsorption capacity was mainly affected by biomass type, with biochars adsorption capacities higher for lead(II) (109.9-256.4 mg g^-1) than for nickel(II) (40.2-64.1 mg g^-1), cadmium(II) (29.5-42.7 mg g^-1) and copper(II) (18.5-39.4 mg g^-1) based on the Langmuir adsorption model. The highest lead(II) adsorption capacities for PMS and RS biochars were 256.4 and 133.3 mg g^-1, respectively, when generated using N₂ as the purging gas. The corresponding lead(II) adsorption capacities were 250.0 and 109.9 mg g^-1, respectively, when generated using CO₂ as the purging gas. According to the intraparticle diffusion model, 30-62% of heavy metal adsorption was achieved in 1 h; film diffusion was the rate-dominating step, whereas pore diffusion was a rate-limiting step. Ion exchange and complexation between heavy metals and biochar surface functional groups such as carbonyl and hydroxyl groups were effective mechanisms for heavy metal sorption from the aqueous solution. We conclude that proper selection of both the feedstock type and the purging gas is important in designing biochars for the effective removal of potentially toxic metals from wastewater.

Protein biomethanation: insight into the microbial nexus

Biomethanation of carbohydrates (e.g., lignocellulosic biomass) and lipids (e.g., waste oils) has been well studied. However, investigations on the biomethanation of protein-rich biowastes (PRBs) and associated microbial communities have not been reported. This review summarizes the challenges in the metabolic process of anaerobic digestion of PRBs and the microbial instability associated with it. We discuss the diversity of bacterial and archaeal communities via metagenomics under PRB mono- and codigestion. A stable community structure with enhanced metabolic activity is a core factor in PRB biomethanation. The application of strategies such as codigestion of PRBs with carbon-rich biomass and microbial stimulation/augmentation would make PRB biomethanation more feasible.

Genome-Centric Metatranscriptomics Analysis Reveals the Role of Hydrochar in Anaerobic Digestion of Waste Activated Sludge

Anaerobic digestion (AD) of waste activated sludge (WAS) has been widely used, while it poses problems including low methane yield and production rate. Hydrochar is produced by hydrothermal liquefaction of biomass; however, little is known about the role of hydrochar in promoting AD of WAS. The present study showed that hydrochar increased the methane production rate by 30.8% and yield by 31.4% of hydrothermal pretreated dewatered WAS. Hydrochar increased the methane production rate and yield by enhancing the acidification and methanogenesis processes. Genomic-centric metatranscriptomics were used to identify the metabolic activities and transcriptomic response of individual metagenome-assembled genomes that were enriched by hydrochar. Although Methanosarcina sp. FDU0106 had been shown unable to used H₂, it had the complete pathway for the reduction of CO₂ to methane. Syntrophomonas sp. FDU0164 expressed genes for extracellular electron transfer via electrically pili, suggesting that Syntrophomonas sp. FDU0164 and Methanosarcina sp. FDU0106 were exchanging electrons via direct interspecies electron transfer. The expression of pili was decreased, indicating that hydrochar could replace its roles. Additionally, Firmicutes sp. FDU0048, Proteiniphilum sp. FDU0082, and Aminobacterium mobile FDU0089 were related to the degradation of organics, which could be related to the enhanced methane yield.

Direct interspecies electron transfer mechanism in enhanced methanogenesis: A mini-review

The role of direct interspecies electron transfer (DIET) on enhancement of methanogenesis has been studied. This mini-review updated the current researches on the potential role of DIET on enhanced performance for anaerobic digestion of organic substrates with effective strategies implemented. Since most experimental observations correlated with the DIET mechanism are yet to be consolidated, this article categorized and discussed the current experimental observations supporting DIET mechanism for methanogenesis, mainly based on those with supplement of carbon materials, from which the prospects and challenges for further studies to confirm the role of DIET in anaerobic digestion processes were highlighted.

Promoting potential direct interspecies electron transfer (DIET) and methanogenesis with nitrogen and zinc doped carbon quantum dots

Although it has been demonstrated that one-dimensional, two-dimensional, and three-dimensional carbon nanomaterials can improve the CH₄ production of anaerobic digestion (AD), the effect of zero-dimensional carbon nanomaterials on AD have not been reported. To expand the application of carbon nanomaterials in AD, the effect of zero-dimensional carbon nanomaterials-carbon quantum dots (CDs) on various feedstocks (c.a. cellulose, glucose, ethanol, and vinegar residue) were investigated in this study. Results have shown that CH₄ yield from ethanol was increased by 24.59% (p = 0.396) after adding 5 g/L zinc doped carbon quantum dots (Zn-doped CDs) while that from vinegar residue was dramatically increased by 230% (p = 0.000) using 5 g/L nitrogen doped carbon quantum dots (N-doped CDs). In addition, photoluminescence demonstrated that CDs acted as a capacitor for transmitting and receiving electrons. Furthermore, co-occurrence network analysis revealed that Clostridiales might be used as a signal source to communicate with other species. This study firstly shifted the application of CDs from fluorescence to AD and manifested its positive impact on AD. In short, these findings provided a better understanding on the effects of CDs on different feedstocks of methanogenesis and revealed new evidence of stimulating methanogenesis via CDs.

Biochar and GAC intensify anaerobic phenol degradation via distinctive adsorption and conductive properties

The roles of biochar and granular activated carbon (GAC) in the enhancement of anaerobic phenol degradation were characterized through batch tests conducted at different phenol concentrations, coupled with adsorption kinetics, microbial community, and in-situ electrochemical analysis. Both biochar and GAC (15 g/L) led to markedly shorter lag times (t₀) by adsorbing dissolved phenol, and faster maximum CH₄ production rate (R_max) by triggering direct interspecies electron transfer (DIET) during a two-stage (adsorption then degradation) anaerobic phenol degradation. The high adsorption capacity of GAC helped achieve a shorter t₀, but less affected R_max of subsequent phenol degradation. Compared with GAC, which showed higher conductivity but no redox activity, biochar exhibited higher electron exchange capacity (6.57 μmol e^-/g). This higher electron exchange capacity stemmed from the diverse redox-active moieties, which resulted in a more efficient DIET. Meanwhile, the formation of wire-like appendages which linked the enriched DIET partners (such as Syntrophorhabdus and Methanosaeta) on biochar probably futher enhanced the electron transfer. However, hydrogenotrophic methanogenesis was still the main pathway for syntrophic phenol degradation in the suspended sludge. The in-situ analysis also confirmed that biochar and GAC acted as geobatteries and geoconductors, respectively, and that the stimulation of DIET was persistent.

Magnetite-contained biochar derived from fenton sludge modulated electron transfer of microorganisms in anaerobic digestion

Biochar, with redox moieties or conjugated π-bond, can act as electron shuttle or conductor to facilitate electron transfer of syntrophic metabolism to enhance anaerobic digestion. High pyrolysis temperature (>500 ℃) is usually required to prepare conductive biochar, which however may cause biochar to loss redox moieties such as quinone/hydroquinone that are capable of serving as electron shuttle. Considering that magnetite is an excellent conductor which has been applied in improving syntrophic metabolism of anaerobic digestion, a novel magnetite-contained biochar was prepared using iron-rich Fenton sludge as raw material in this study. Amorphous iron oxides of Fenton sludge were transformed into magnetite at 400 ℃ of pyrolysis, while redox quinone/hydroquinone moieties of biochar were preserved well. Correspondingly, this magnetic biochar owned both high capacitance and excellent conductivity. When supplementing the biochar into an anaerobic digestion system, methane production was significantly enhanced. This study also offered a new approach to recycle Fenton sludge that is regarded as hazardous material.

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.

Microbial community shift via black carbon: Insight into biological nitrogen removal from microbial assemblage and functional patterns

Understanding the ecological relationship of microbial community under external stimulation is crucial for environmental restoration. Black carbon (e.g., biochar) have been widely deemed as a strategy to enhance pollutants removal because of its structure and redox-active property. However, the underlying ecological mechanism of microbial community under black carbon addition is poorly understood. The major purposes of this study were to determine the microbial assemblage and functional patterns via null model test, network analysis, and function prediction. The results showed that the nitrate removal efficiency of modified black carbon system achieved 46.44%. Both deterministic and stochastic processes were significant for mediating the microbial assemblage and the deterministic process dominated (>95%) the assemblage of community. Keystone taxa in the black carbon systems, involving Sulfuricella, Allorhizobium, and Nitrospira, stimulated the shift of community composition regarding the nitrogen removal. The existence of black carbon and the biotic interactions increased biological nitrate utilization and promoted nitrogen removal. Overall, this study presents the mechanism of bacterial community assembly and provides insight into biological nitrogen removal from microbial ecological perspective.

The role of humic substances in mitigating greenhouse gases emissions: Current knowledge and research gaps

Humic substances (HS) constitute a highly transformed fraction of natural organic matter (NOM) with a heterogeneous structure, which is rich in electron-transferring functional moieties. Because of this feature, HS display a versatile reactivity with a diversity of environmentally relevant organic and inorganic compounds either by abiotic or microbial processes. Consequently, extensive research has been conducted related to the potential of HS to drive relevant processes in bio-engineered systems, as well as in the biogeochemical cycling of key elements in natural environments. Nevertheless, the increase in the number of reports examining the relationship between HS and the microorganisms related to the production and consumption of greenhouse gases (GHG), the main drivers of global warming, has just emerged in the last years. In this paper, we discuss the importance of HS, and their analogous redox-active organic molecules (RAOM), on controlling the emission of three of the most relevant GHG due to their tight relationship with microbial activity, their abundance on the Earth's atmosphere, and their important global warming potentials: carbon dioxide (CO₂), methane (CH₄) and nitrous oxide (N₂O). The current knowledge gaps concerning the microbial component, on-site occurrence, and environmental constraints affecting these HS-mediated processes are provided. Furthermore, strategies involving the metabolic traits that GHG-consuming/HS-reducing and -oxidizing microbes display for the development of environmental engineered processes are also discussed.

Coconut husk biochar amendment enhances nutrient retention by suppressing nitrification in agricultural soil following anaerobic digestate application

Anaerobic digestate and biochar are by-products of the biogasification and pyrolysis of agricultural wastes. This study tested the hypothesis that combined application of anaerobic pig/cattle manure digestate and coconut husk (CH) biochar can improve soil nutrient conditions, whilst minimizing atmospheric and groundwater pollution risks. Microcosms simulated digestate application to agricultural soil with and without CH biochar. Ammonia volatilization and nutrient leaching were quantified after simulated heavy rainfalls. Archaeal and bacterial community and abundance changes in soils were quantified via next generation sequencing and qPCR of 16S rRNA genes. Nitrifying bacteria were additionally quantified by qPCR of functional genes. It was found that CH biochar retarded nitrate leaching via slower nitrification in digestate-amended soil. CH biochar reduced both nitrifying archaea and bacteria abundance in soil by 71-83 percent in the top 4 cm soil layer and 66-80 percent in the deeper soil layer one month after the digestate application. Methanotroph abundances were similarly reduced in the CH biochar amended soils. These findings demonstrate combined benefits of anaerobic digestate and CH biochar application which are relevant for the development of a more circular rural economy with waste minimization, renewable energy production, nutrient recycling and reduced water pollution from agricultural land.

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.

Biochar based sorptive remediation of steroidal estrogen contaminated aqueous systems: A critical review

Remediation of steroidal estrogens from aqueous ecosystems is of prevailing concern due to their potential impact on organisms even at trace concentrations. Biochar (BC) is capable of estrogen removal due to its rich porosity and surface functionality. The presented review emphasizes on the adsorption mechanisms, isotherms, kinetics, ionic strength and the effect of matrix components associated with the removal of steroidal estrogens. The dominant sorption mechanisms reported for estrogen were π-π electron donor-acceptor interactions and hydrogen bonding. Natural organic matter and ionic species were seen to influence the hydrophobicity of the estrogen in multiple ways. Zinc activation and magnetization of the BC increased the surface area and surface functionalities leading to high adsorption capacities. The contribution by persistent free radicals and the arene network of BC have promoted the catalytic degradation of adsorbates via electron transfer mechanisms. The presence of surface functional groups and the redox activity of BC facilitates the bacterial degradation of estrogens. The sorptive removal of estrogens from aqueous systems has been minimally reviewed as a part of a collective evaluation of micropollutants. However, to the best of our knowledge, a critique focusing specifically and comprehensively on BC-based removal of steroidal estrogens does not exist. The presented review is a critical assessment of the existing literature on BC based steroidal estrogen adsorption and attempts to converge the scattered knowledge regarding its mechanistic interpretations. Sorption studies using natural water matrices containing residue level concentrations, and dynamic sorption experiments can be identified as future research directions.

Mitigating methane emission via annual biochar amendment pyrolyzed with rice straw from the same paddy field

To develop an economic and sustainable biochar application strategy for mitigating methane emission from paddy fields, a four-year field experiment was conducted to compare two biochar amendment methods. The annual low (AL) rate pyrolyzed biochar returning method used the same amount of biochar as was harvested from rice straw in the field, 2.8 t ha^-1 yr^-1. The high single (HS) biochar returning method consisted of a single application of 22.5 t ha^-1 biochar only in the first year, 2015. Our results showed that the AL biochar returning strategy prevailed over the HS strategy in mitigating methane emission from paddy fields. On average, AL and HS could reduce methane emissions by 41% and 38.25% in four years, respectively. Methane accumulation per unit rice production was 45.8% and 43.1% in AL and HS, respectively. AL showed a stable effect on mitigating methane emission over four successive years, which resulted from the continuously increasing methanotrophs due to annual fresh biochar application. Aged biochar weakened the promotion of methanotrophs, leading to lower methane reduction rates in HS than in AL in the 4 years. Our results indicate that AL is a highly sustainable strategy for methane mitigation in paddy fields due to its high efficiency, practical operation, and economical acceptance.