Biochar facilitates ferrihydrite reduction by Shewanella oneidensis MR-1 through stimulating the secretion of extracellular polymeric substances

Integrated evaluation for advanced removal of nitrate using novel solid carbon biochar/corncob/PHBV composite: Insight into electron transfer and metabolic pathways

This study developed a novel Biochar/Corncob/PHBV (BCP) composite material, integrating the electron transfer capability of biochar, the cost-effectiveness of corncob, and the sustained carbon release performance of PHBV. The BCP system achieved a maximum nitrate removal efficiency of 97.3 %, significantly outperforming the single PHBV system (91.05 %), while effectively reducing nitrous oxide and other greenhouse gas emissions. It also demonstrated stable carbon release and enhanced electron transfer capabilities, contributing to a more sustainable denitrification process. The physical and chemical characterization of BCP confirmed that its superior performance is attributed to the uniformly distributed functional groups (e.g., CO and -COOH) on the surface and its porous structure, which facilitated electron transfer and microbial adhesion. Metagenomic and microbial analyses further revealed that BCP enriched functional genera such as Cellulomonas and Chryseobacterium and significantly increased the abundance of key functional genes related to nitrate reduction (e.g., NaR and NiR), enhancing organic matter decomposition and microbial nitrogen transformation. Beyond improving nitrate removal efficiency compared to PHBV, the BCP material offers practical engineering value by addressing carbon source limitations in long-term wastewater treatment applications. Its enhanced electron transfer and microbial enrichment suggest strong potential for application in constructed wetlands, biofilters, and other decentralized wastewater treatment systems. The study demonstrates that the BCP composite is not only a viable alternative to traditional PHBV but also a cost-effective and environmentally friendly material with broad applicability in nitrogen pollution control.

Biochar-assisted control of antibiotic-resistant bacteria and methane yield optimization in two-stage anaerobic digestion under organic load and antibiotic stress

This study explores the interactions between microbial communities, antibiotic resistance, and biogas production in anaerobic digestion systems, focusing on the acidogenic (AP) and methanogenic (MP) phases under varying organic loads, cefazolin (CEZ) exposure, and biochar supplementation. High organic loading (10 g/L glucose) significantly suppressed CEZ-resistant bacteria (CEZ-r) during the AP phase. However, their abundance markedly rebounded in MP, rising from 0.30 % to 36.28 % in control, indicating phase-specific dynamics. CEZ residues increased CEZ-r by 2.49 % and 9.30 % at 0 and 5 g/L glucose during AP. Although AP suppressed CEZ-r to 0.23 % in the CEZ-added reactor at 10 g/L glucose, MP rebounded CEZ-r to 8.30 %. In addition, CEZ exposure reduced methane yields by up to 28.14 %, likely due to the suppression of Methanosaetaceae and impaired acetic acid conversion. In contrast, biochar addition effectively reduced CEZ-r abundance to below 1.00 % at moderate to high organic loads and alleviated CEZ-induced inhibition on methane production. Biochar also enhanced Methanosaetaceae abundance (up to +6.55 %) compared to the control and promoted more efficient substrate utilization, possibly by facilitating direct interspecies electron transfer. These findings emphasize the role of organic load and digestion phase in shaping antibiotic resistance and system performance. Furthermore, biochar addition effectively mitigates the negative impacts of antibiotic residues, stabilizes microbial communities, and enhances biogas production.

Physical Contact between Bacteria and Carbonaceous Materials: The Key Switch Triggering Activated Carbon and Biochar to Promote Microbial Iron Reduction

Carbonaceous materials, including activated carbon and pyrolytic carbon, have been recognized for about over a decade as effective electron shuttles or conductive materials in promoting microbial Fe(III) mineral reduction. However, recent studies reveal inhibitory effects, sparking debates about their overall impact. We hypothesized that the physical contact between bacteria and carbon is an overlooked yet critical factor in determining whether carbon promotes or inhibits microbial Fe(III) reduction. Using systems containing Shewanella oneidensis MR-1, activated carbon, and ferrihydrite, we investigated how carbon-iron oxide aggregate structure affects Fe(III) reduction kinetics. At low activated carbon-to-iron oxide ratios (C/Fe = 5:7 by mass), ferrihydrite aggregated with carbon, forming carbon-encapsulated particles that suppressed Fe(III) reduction rates. Conversely, at higher ratios (C/Fe = 100:7), the ferrihydrite dispersed on the carbon surface, enhancing both the rate and extent of Fe(III) reduction. Tests with 11 different carbonaceous materials (activated carbon and biochar) all confirmed that the microstructure of iron oxides─whether encapsulating or dispersed─on carbon surfaces is critical for determining Fe(III) reduction rates. This insight resolves the debate on whether carbonaceous materials promote or inhibit Fe(III) mineral reduction and enhances our understanding of their roles in biogeochemical processes and environmental remediation.

Insights into the enhanced uranium reduction efficiency through extracellular polymeric substances from Desulfovibrio vulgaris UR1 induced by mediating materials

Understanding the role of extracellular polymeric substances (EPS) in the microbial reduction of uranium accelerated by mediating materials is crucial for enhancing the bioremediation of uranium-contaminated wastewater. In this study, biochar- and magnetite-loaded Desulfovibrio vulgaris UR1 exhibited significantly higher uranium reduction efficiency, with increases of 1.52 and 1.44 times respectively within one day. After loading with mediating materials, the charge transfer resistance of EPS was reduced, facilitating the extracellular electron transfer process. The increase of redox components, such as aromatic compounds and flavins, in EPS explained the enhanced extracellular electron transfer capacity. Moreover, the higher α-helix content in extracellular proteins could promote electron hopping. Proteomics analysis showed that extracellular proteins involved in iron-sulfur cluster binding, oxidoreductase activity, and electron transfer were significantly up-regulated, which facilitated the rapid microbial reduction of uranium. These findings provide valuable insights into the in-depth development of bioremediation technology for uranium-contaminated wastewater.

Utilization of Algal Biochar for Biopassivation of Copper Sulfide Tailings to Reduce Acid Mine Drainage

Acid mine drainage (AMD) has serious impacts on the environment. To inhibit the generation of AMD from copper sulfide tailings at the source, in this paper, a strategy is developed for promoting the biopassivation of copper sulfide tailings using algal biochar, and the effects of the pyrolysis temperature and concentration of algal biochar on the passivation efficiency and stability are investigated. The results reveal that the introduction of algal biochar during the biopassivation of copper sulfide tailings significantly enhances the tailings passivation effect of the tested Acidithiobacillus ferrooxidans strain and greatly stabilizes the formed passivation layer. Algal biochar prepared with a pyrolysis temperature of 300 °C and applied at a concentration of 6 g/L not only optimizes biopassivation but also significantly improves the stability of the passivation layer. The complex mechanisms of algal biochar in this system include regulating the pH and oxidation‒reduction potential of the reaction system, effectively adsorbing microbial cells, efficiently aggregating metal cations in solution, stimulating the synthesis of extracellular polymeric substances, and accelerating electron transfer. This research offers a novel method for the benign treatment of copper sulfide tailings and resource utilization of algae.

Enhanced chloramphenicol biodegradation and sustainable electricity generation via co-cultured electroactive biofilms modified with in-situ self-assembled gold nanoparticles and reduced graphene oxide

Bioelectrochemical technology emerges as a promising approach for addressing the challenge of antibiotic residue contamination. This research innovated by incorporating in-situ self-assembled gold nanoparticles (Au-NPs) and reduced graphene oxide (rGO) into a co-cultured electroactive biofilm (EAB) of Raoultella sp. DB-1 and Shewanella oneidensis MR-1 (Au-rGO@R/S-C). Supported by the rGO scaffold, the embedding of Au-NPs at key intercellular sites, and the adhesion of extracellular polymeric substance (EPS), the Au-rGO@R/S-C EAB enhanced the bioelectrochemical performance of the inoculated microbial fuel cell (MFC), with a 34.4% increase in the maximum voltage output and a 1.95-fold rise in the maximum power density, enabling the complete degradation of 100 mg/L chloramphenicol within 24 h. Notably, the Au-rGO@R/S-C EAB adapted to increased chloramphenicol stress by amplifying EPS secretion, especially with an elevated protein/polysaccharide ratio. Further analysis indicated a positive correlation between excessive production of EPS, particularly the increase in tightly bound EPS, and the stability in balancing the self-protection and extracellular electron transfer efficiency of the Au-rGO@R/S-C EAB under environmental stress. Our findings present a crucial strategy for the rational engineering of EABs, leveraging their dual potential in both environmental remediation and clean energy production.

Vanadium (V) reduction and the performance of electroactive biofilms in microbial fuel cells with Shewanella putrefaciens

The electron transfer ability of biofilms significantly influences the electrochemical activity of microbial fuel cells (MFCs). However, there is limited understanding of pentavalent vanadium (V(V)) bioreduction and microbial response characteristics in MFCs. In this study, the effect of gradient concentrations of V(V) on the performance of EABs with Shewanella putrefaciens in MFCs was investigated. The results showed that as V(V) concentration increased (0-100 mg/L), the voltage output, power densities, polarization, and electrode potential decreased. V(V) was found to act as an electron acceptor and was reduced during MFCs operation, with a yield of 83.16% being observed at 25 mg/L V(V). Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) indicated declining electrochemical performance of the MFCs with escalating V(V) concentration. The content of protein and polysaccharide from extracellular polymeric substances (EPS) in anodic biofilms increased to 66.75 and 49.15 mg/L at 75 mg/L V(V), respectively. Three-dimensional fluorescence spectroscopy confirmed increased humic substances in EPS extraction with V(V) exposure. The functional genes narG, nirK, and gor involved in V(V) reduction were upregulated with rising V(V) concentration through quantitative polymerase chain reaction (qPCR) analysis. Additionally, riboflavin, cytochrome c, nicotinamide adenine dinucleotide (NADH), and electron transport system activity (ETSA), key indicators for assessing electron transfer behavior, exhibited a negative correlation with various V(V) concentrations, decreasing by 31.81%, 57.14%, 67.39%, and 51.41%, respectively, at a concentration of 100 mg/L V(V) compared to the blank control. These findings contribute valuable insights into the response of EABs to V(V) exposure, presenting potential strategies for enhancing their effectiveness in the treatment of vanadium-contaminated wastewater.

Cr(VI) bioreduction enhanced by the electron transfer between flavin reductase and persistent free radicals

Persistent free radicals (PFRs) in biochar are an important electron shuttle for mediating electron transfer, which has significant impact on the biogeochemical redox reactions. Although the influence of biochar on the extracellular electron transfer (EET) for redox cycle has been extensively studied, the molecular mechanism for promoting the EET with PFRs remains poorly understood. This study investigated the oxygen-centered PFRs-mediated Cr(VI) reduction by Shewanella oneidensis MR-1 (MR-1) and exhibited the molecular mechanism of electron transfer between flavin substances and PFRs. Results showed that the Cr(VI) bioreduction rate by MR-1 increased from 31% to 70% with the addition of biochar. Electrochemical results illustrated that biochar increased biocurrent generation in the Cr(VI) bioreduction process. 3D-EEM and LC/MS spectra indicated that MR-1 secreted the flavin mononucleotide (FMN) reductase that relied on the [H] to provide the electrons. Electron paramagnetic resonance spectra illustrated that PFRs in biochar accepted the electrons from FMN reductase and stored those bioelectrons. Because of the oxidation of FMN, the electron transfer from FMN reductase to PFRs would increase the intracellular reactive oxygen species, which further produced the extracellular ·O2-. The reduced PFRs released the bioelectrons, accelerating the Cr(VI) reduction by ·O2-. Together with the results of the mutant strains experiment, it was found that the EET by c-cytochrome and free radicals contributed to the Cr(VI) bioreduction by 7.1% and 92.9%, respectively. These findings revealed that the PFRs could participate in the EET process and promote the redox reactions, providing a new approach for enhancing the remediation of heavy metal pollution by microorganisms and suggesting the important role of PFRs in the electron transfer process.

Multi-dimensional perspectives into the pervasive role of microbial extracellular polymeric substances in electron transport processes

During the process of biological treatment, most microorganisms are encapsulated in extracellular polymeric substances (EPS), which protect the cell from adverse environments and aid in microbial attachment. Microorganisms utilize extracellular electron transfer (EET) for energy and information interchange with other cells and the outside environment. Understanding the role of steric EPS in EET is critical for studying microbiology and utilizing microorganisms in biogeochemical processes, pollutant transformation, and bioenergy generation. However, the current study shows that understanding the roles of EPS in the EET processes still needs a great deal of research. In view of recent research, this work aims to systematically summarize the production and functional group composition of microbial EPS. Additionally, EET pathways and the role of EPS in EET processes are detailed. Then factors impacting EET processes in EPS are then discussed, with a focus on the spatial structure and composition of EPS, conductive materials and environmental pollution, including antibiotics, pH and minerals. Finally, strategies to enhance EET, as well as current challenges and future prospects are outlined in detail. This review offers novel insights into the roles of EPS in biological electron transport and the application of microorganisms in pollutant transformation.

Performances of a novel BAF with ferromanganese oxide modified biochar (FMBC) as the carriers for treating antibiotics, nitrogen and phosphorus in aquaculture wastewater

In this paper, a biological aerated filter (BAF) based on ferromanganese oxide-biochar (FMBC) was constructed to investigated the removal performance and mechanism for conventional pollutants and four kinds of antibiotic, in contrast of conventional zeolite loaded BAF (BAF-A) and bamboo biochar filled BAF (BAF-B). Results showed that the average removal efficiency of total nitrogen (TN), total phosphorus (TP) and antibiotics in a FMBC-BAF (named by BAF-C) were 52.97 ± 2.27%, 51.58 ± 1.92% and 70.36 ± 1.00% ~ 81.65 ± 0.99% respectively in running period (39-100 d), which were significantly higher than those of BAF-A and BAF-B. In the BAF-C, the expression of denitrification enzyme activities and the secretion of extracellular polymeric substance (EPS) especially polyprotein (PN) were effectively stimulated, as well as accelerated electron transfer activity (ETSA) and lower electrochemical impedance spectroscopy (EIS) were acquired. After 100 days of operation, the abundance of nitrogen, phosphorus and antibiotic removal functional bacteria like Sphingorhabdus (4.52%), Bradyrhizobium (1.98%), Hyphomicrobium (2.49%), Ferruginibacter (7.80%), unclassified_f_Blastoca tellaceae (1.84%), norank_f_JG30-KF-CM45 (6.82%), norank_f_norank_o_SBR1031 (2.43%), Nitrospira (2.58%) norank_f_Caldilineaceae (1.53%) and Micropruina (1.11%) were enriched. Mechanism hypothesis of enhanced performances of nutrients and antibiotics removal pointed that: The phosphorus was removed by adsorption and precipitation, antibiotics removal was mainly achieved through the combined action of adsorption and biodegradation, while nitrogen removal was realized by biologic nitrification and denitrification in a FMBC-BAF for aquaculture wastewater treatment.

From Bulk to Nano: Formation, Features, and Functions of Nano-Black Carbon in Biogeochemical Processes

Globally increasing wildfires and widespread applications of biochar have led to a growing amount of black carbon (BC) entering terrestrial ecosystems. The significance of BC in carbon sequestration, environmental remediation, and the agricultural industry has long been recognized. However, the formation, features, and environmental functions of nanosized BC, which is one of the most active fractions in the BC continuum during global climate change, are poorly understood. This review highlights the formation, surface reactivity (sorption, redox, and heteroaggregation), biotic, and abiotic transformations of nano-BC, and its major differences compared to other fractions of BC and engineered carbon nanomaterials. Potential applications of nano-BC including suspending agent, soil amendment, and nanofertilizer are elucidated based on its unique properties and functions. Future studies are suggested to develop more reliable detection techniques to provide multidimensional information on nano-BC in environmental samples, explore the critical role of nano-BC in promoting soil and planetary health from a one health perspective, and extend the multifield applications of nano-BC with a lower environmental footprint but higher efficiency.

Removal of dibutyl phthalate (DBP) by bacterial extracellular polymeric substances (EPS) via enzyme catalysis and electron transmission

Phthalic acid esters (PAEs) showed high environmental risk due to the widely existence and toxicity. Microbial-excreted extracellular polymeric substances (EPS) showed potential of degrading organic compounds. In this study, the degradation ability and the mechanisms of EPS from two bacteria (PAEs degrader Gordonia sihwensis; electrochemically active strain Shewanella oneidensis MR-1) were investigated. Results showed that EPS of the two bacteria had different composition of C-type cytochromes, flavins, catalase, and α-glucosidase. The removal of dibutyl phthalate (DBP) by total EPS were 68% of G. sihwensis and 72% for S. oneidensis. For both bacteria, the degradation rates k of EPS were as TB-EPS > LB-EPS > S-EPS. The degradation mechanisms of EPS from the two bacteria showed difference with electrochemical active components mediated electron transmission for S. oneidensis MR-1 and enzymes catalysis for G. sihwensis. Results of this study illustrated the variation of the contribution of active components of EPS to degradation.

A review of microplastics on anammox: Influences and mechanisms

Microplastics (MPs) are prevalent in diverse environmental settings, posing a threat to plants and animals in the water and soil and even human health, and eventually converged in wastewater treatment plants (WWTPs), threatening the stable operation of anaerobic ammonium oxidation (anammox). Consequently, a comprehensive summary of their impacts on anammox and the underlying mechanisms must be provided. This article reviews the sources and removal efficiency of MPs in WWTPs, as well as the influencing factors and mechanisms on anammox systems. Numerous studies have demonstrated that MPs in the environment can enter WWTPs via domestic wastewater, rainwater, and industrial wastewater discharges. More than 90% of these MPs are found to accumulate in the sludge following their passage through the treatment units of the WWTPs, affecting the characteristics of the sludge and the efficiency of the microorganisms treating the wastewater. The key parameters of MPs, encompassing concentration, particle size, and type, exert a notable influence on the nitrogen removal efficiency, physicochemical characteristics of sludge, and microbial community structure in anammox systems. It is noteworthy that extracellular polymer secretion (EPS) and reactive oxygen stress (ROS) are important impact mechanisms by which MPs exposure affects anammox systems. In addition, the influence of MPs exposure on the microbial community structure of anammox cells represents a crucial mechanism that demands attention. Future research endeavors will delve into additional crucial parameters of MPs, such as shape and aging, to investigate their effects and mechanisms on anammox. Furthermore, the effective mitigation strategies will also be developed. The paper provides a fresh insight to reveal the influences of MPs exposure on the anammox process and its influence mechanisms, and lays the groundwork for further exploration into the influence of MPs on anammox and potential mitigation strategies.

Promotion of anaerobic biodegradation of azo dye RR2 by different biowaste-derived biochars: Characteristics and mechanism study by machine learning

The addition of biochar resulted in a 31.5 % to 44.6 % increase in decolorization efficiency and favorable decolorization stability. Biochar promoted extracellular polymeric substances (EPS) secretion, especially humic-like and fulvic-like substances. Additionally, biochar enhanced the electron transfer capacity of anaerobic sludge and facilitated surface attachment of microbial cells. 16S rRNA gene sequencing analysis indicated that biochar reduced microbial species diversity, enriching fermentative bacteria such as Trichococcus. Finally, a machine learning model was employed to establish a predictive model for biochar characteristics and decolorization efficiency. Biochar electrical conductivity, H/C ratio, and O/C ratio had the most significant impact on RR2 anaerobic decolorization efficiency. According to the results, the possible mechanism of RR2 anaerobic decolorization enhanced by different types of biochar was proposed.

Robust rehabilitation of anammox system by granular activated carbon under long-term starvation stress: Microbiota restoration and metabolic reinforcement

This article investigates the buffering capacity and recovery-enhancing ability of granular activated carbon (GAC) in a starved (influent total nitrogen: 20 mg/L) anaerobic ammonium oxidation (anammox) reactor. The findings revealed that anammox aggregated and sustained basal metabolism with shorter performance recovery lag (6 days) and better nitrogen removal efficiency (84.9 %) due to weak electron-repulsion and abundance redox-active groups on GAC's surface. GAC-supported enhanced extracellular polymeric substance secretion aided anammox in resisting starvation. GAC also facilitated anammox bacterial proliferation and expedited the restoration of anammox microbial community from a starved state to its initial-level. Metabolic function analyses unveiled that GAC improved the expression of genes involved in amino acid metabolism and sugar-nucleotide biosynthesis while promoted microbial cross-feeding, ultimately indicating the superior potential of GAC in stimulating more diverse metabolic networks in nutrient-depleted anammox consortia. This research sheds light on the microbial and metabolic mechanisms underlying GAC-mediated anammox system in low-substrate habitats.

A review of microbial responses to biochar addition in anaerobic digestion system: Community, cellular and genetic level findings

The biochar is a well-developed porous material with various excellent properties, that has been proven with excellent ability in anaerobic digestion (AD) efficiency promotion. Current research is usually focused on the macro effects of biochar on AD, while the systematic review about the mechanisms of biochar on microbial behavior are still lacking. This review summarizes the effects and potential mechanisms of biochar on microorganisms in AD systems, and found that biochar addition can provide habitats for microbial colonization, alleviate toxins stress, supply essential nutrients, and accelerate interspecies electron transferring. Moreover, it also improves microbial community diversity, facilitates EPS secretion, enhances functional enzyme activity, promotes functional genes expression, and inhibits the antibiotic resistance genes transformation. Future research directions including biochar targeted design, in-depth microbial mechanisms revelation, and modified model development were suggested, which could promote the widely practical application of of biochar-amended AD technology.

Enhancing start-up strategies for anammox granular sludge systems: A review

The anaerobic ammonium oxidation (anammox) process has been developed as one of the optimal alternatives to the conventional biological nitrogen removal process because of its high nitrogen removal capacity and low energy consumption. However, the slow growth rate of anammox bacteria and its high sensitivity to environmental changes have resulted in fewer anammox sludge sources for process start-up and a lengthy start-up period. Given that anammox microorganisms tend to aggregate, granular-anammox sludge is a frequent byproduct of the anammox process. In this study, we review state-of-the-art strategies for promoting the formation of anammox granules and the start-up of the anammox process based on the literature of the past decade. These strategies are categorized as the transformation of alternative sludge, the addition of accelerators, the introduction of functional carriers, and the implementation of other physical methods. In addition, the formation mechanism of anammox granules, the operational performance of various strategies, and their promotion mechanisms are introduced. Finally, prospects are presented to indicate the gaps in contemporary research and the potential future research directions. This review functions as a summary guideline and theoretical reference for the cultivation of granular-anammox sludge, the start-up of the anammox process, and its practical application.

Enhanced microbial degradation mediated by pyrogenic carbon toward p-nitrophenol: Role of carbon structures and iron minerals

Pyrogenic carbon (PC) including black carbons and engineered carbons can mediate the extracellular electron transfer to facilitate the biogeochemical reaction with organic pollutants. Yet, the role of carbon structures and iron minerals on PC-mediated microbial degradation is still lacking of understanding. Herein, we studied the electrochemical properties of PCs produced from varied feedstock with regards to the mediated degradation of p-nitrophenol (PNP) by Shewanella putrefaciens CN32 in anoxic system. Mediated degradation by PCs was enhanced by facilitating extracellular electron transfer through oxygenated group and graphitic structure. Graphitic crystallites improved the electron-accepting capacity (as suggested by ID/IG and EAC) and diminished the electrochemical impedance (as suggested by Rct), contributing to PNP degradation under the anoxic system. Furthermore, more interfacial adsorption was conducive to the mediated reduction by the graphitic structure on PCs of high-temperature. In the presence of iron minerals, both hematite and goethite significantly facilitated PC-mediated degradation, which could be ascribed to the enhancement of the electron-donating capacity of microorganism and the accumulation of the reductive-state PCs by the interaction with generated Fe(II). This work paves a feasible way to the technical design on the remediation of phenolic contaminants by PC-mediated microbial degradation in environment.

Enhanced microbial reduction of Cr(VI) in soil with biochar acting as an electron shuttle: Crucial role of redox-active moieties

Biochar has been proven to participate in the biotic reduction of hexavalent chromium (Cr(VI)) in environment since its involvement may accelerate the extracellular electron transfer (EET). However, roles of the redox-active moieties and the conjugated carbon structure of biochar in this EET process remain unclear. In this study, 350 °C and 700 °C were selected to produce biochar with more O-containing moieties (BC350) or more developed conjugated structures (BC700), and their performances in the microbial reduction of soil Cr(VI) were investigated. Our results showed that BC350 presented a 241% increase of Cr(VI) microbial reduction after 7-day incubation, much higher than that of BC700 (39%), suggesting that O-containing moieties might play more important roles in accelerating the EET process. Biochar, especially BC350 could serve as an electron donor for microbial anaerobic respiration, but its contribution (73.2%) as an electron shuttle for EET was dominant to the enhanced Cr(VI) reduction. The positive correlation between electron exchange capacities (EECs) of pristine and modified biochars and the corresponding maximum reduction rates of Cr(VI) evidenced the crucial role of redox-active moieties in electron shuttling. Moreover, EPR analysis suggested the nonnegligible contribution of semiquinone radicals in biochars to the accelerated EET process. This study demonstrates the crucial role of redox-active moieties, i.e., O-containing moieties in mediating the EET process during the microbial reduction of Cr(VI) in soil. Findings obtained will advance the current understanding of biochar as an electron shuttle participating in the biogeochemical processes of Cr(VI).

Mechanistic insights into aggregation process of graphene oxide and bacterial cells in microbial reduction of ferrihydrite

Microbial reduction of ferrihydrite is prevalent in natural environments and plays an important role in reductive dissolution of Fe(III) minerals. With consistent release of anthropogenic graphene oxide (GO) into water bodies, new changes in the Fe(III)-reducing microorganisms/ferrihydrite binary system demand attention. Herein, we focused on the interaction of GO and bacterial cells in view of colloidal stability and interfacial forces, and on the consequences for microbial ferrihydrite reduction. The results showed that the addition of GO decreased the bioreduction efficiency of ferrihydrite down to 1/15 of the control. Meanwhile, the GO nanosheets were found not depositing on ferrihydrite but spontaneously aggregating with Shewanella spp., the representative dissimilatory Fe(III) reduction bacterial species. Using the extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) theory and atomic force microscopy (AFM), the aggregation process can be interpreted in three steps according to the interaction energy calculation, namely, colloidal instability, reversible aggregation and irreversible aggregation. The motility of living cells seems the reason inducing the colloidal instability between GO and bacteria. While, the aggregation remains reversible even the secondary minimum achieved at the separation distance of 8.74-9.24 nm from XDLVO. When the separation distance <5.74-6.01 nm, the adhesion work predominates and causes irreversible aggregation, validated by AFM. Additionally, the probable ecological risks raised by this aggregation behavior for the imbalance of iron biogeochemical cycle were demonstrated.

Effects of biochar on anaerobic treatment systems: Some perspectives

Many anaerobic activities involve carbon, nitrogen, iron, and sulfur cycles. As a well-developed porous material with abundant functional groups, pyrolytic biochar has been widely researched in efforts to promote microbial activities. However, the lack of consensus on the biochar mechanism has limited its practical application. This review summarizes the effects of different pyrolysis temperatures, particle sizes, and dosages of biochar on microbial activities and community in Fe(III) reduction, anaerobic digestion, nitrogen removal, and sulfate reduction systems. It was found that biochar could promote anaerobic activities by stimulating electron transfer, alleviating toxicity, and providing suitable habitats for microbes. However, it inhibits microbial activities by releasing heavy metal ions or persistent free radicals and adsorbing signaling molecules. Finding a balance between the promotion and inhibition of biochar is therefore essential. This review provides valuable perspectives on how to achieve efficient and stable use of biochar in anaerobic systems.