Effects of nano-sized MnO2 on methanogenic propionate and butyrate degradation in anaerobic digestion

Fe-Mn binary oxides improve the methanogenic performance and reduce the environmental health risks associated with antibiotic resistance genes during anaerobic digestion

Increasing evidence indicates that metal oxides can improve the methanogenic performance during anaerobic digestion (AD) of piggery wastewater. However, the impacts of composite metal oxides on the methanogenic performance and risk of antibiotic resistance gene (ARG) transmission during AD are not fully understood. In this study, different concentrations of Fe-Mn binary oxides (FMBO at 0, 250, 500, and 1000 mg/L) were added to AD to explore the effects of FMBO on the process. The methane yield was 7825.1 mL under FMBO at 250 mg/L, 35.2% higher than that with FMBO at 0 mg/L. PICRUSt2 functional predictions showed that FMBO promoted the oxidation of acetate and propionate, and the production of methane from the substrate, as well as increasing the abundances of most methanogens and genes encoding related enzymes. Furthermore, under FMBO at 250 mg/L, the relative abundances of 14 ARGs (excluding tetC and sul2) and four mobile gene elements (MGEs) decreased by 24.7% and 55.8%, respectively. Most of the changes in the abundances of ARGs were explained by microorganisms, especially Bacteroidetes (51.20%), followed by MGEs (11.98%). Thus, the methanogenic performance of AD improved and the risk of horizontal ARG transfer decreased with FMBO, especially at 250 mg/L.

Allophane improves anaerobic digestion of chicken manure by alleviating ammonia inhibition and intensifying direct interspecies electron transfer

Synthesized allophane was employed in anaerobic digestion of chicken manure to improve the stability and methane production under ammonia inhibition. Adding 0.5 %, 1.0 % and 1.5 % (w/w) allophane increased the methane production by 261 ∼ 350 % compared with the group without allophane addition. Further investigation indicated that the maximum adsorption capacity of allophane for NH4+-N achieved at 261.9 mg/g; it suggested that allophane adsorption potentially alleviated the ammonia inhibition, which also was reflected by the increase in the activity of the related enzyme, such as coenzyme F420. Moreover, allophane addition also intensified the direct interspecies electron transfer (DIET) in anaerobic digestion; it can be well supported by the increased relative abundance of Methanosaeta and Methanosarcina involved in the DIET. Overall, the improved anaerobic digestion via alleviating ammonia inhibition and intensifying DIET by allophane was elucidated comprehensively, which can contribute to the development of a functional additive for efficient anaerobic digestion in practical application.

Boosting anaerobic digestion of long chain fatty acid with microbial electrolysis cell combining metal organic framework as cathode: Biofilm construction and metabolic pathways

The role of metal organic framework (MOF) modified cathode in promoting long chain fatty acid (LCFA) methanation was identified in microbial electrolysis cell coupled anaerobic digestion (MEC-AD) system. The maximum methane production rate of MEC-AD-MOF achieved 49.8 ± 3.4 mL/d, which increased by 41 % compared to MEC-AD-C. The analysis of bio-cathode biofilm revealed that microbial activity, distribution, population, and protein secretion prompted by MOF cathode, which in turn led to an acceleration of electron transfer between the cathode and microbes. Specifically, the relative abundance of acetate-oxidizing bacterium (Mesotoga) in MEC-AD-MOF was 1.5-3.6 times higher than that in MEC-AD-C, with a co-metabolized enrichment of Methanobacterium. Moreover, MOF cathode reinforced LCFA methanation by raising the relative abundance of genes coded key enzymes involved in CO2-reducing pathway, and elevating the tolerance of microbes to LCFA inhibition. These results indicate that MOF can enhance biofilm construction in MEC-AD, thereby improving the treatment performance of lipid wastewater.

Effects of Fe/Fe-Mn oxides loaded biochar on anaerobic degradation of typical phenolic compounds in coal gasification wastewater: Performance and mechanism

In this study, two kinds of magnetic biochar (BC) were synthesized by loading Fe (FeBC) and Fe-Mn oxides (FMBC) and their effects on anaerobic phenolics degradation were investigated. Compared with BC/FMBC, FeBC addition achieved the superior phenolics biodegradation even for 3,5-xylenol. Compared with control, FeBC addition enhanced CH4 production by 100.1 % with the lag time shortened from 9.5 days to 6.6 days while it increased to 11.2 days with FMBC addition. FeBC addition activated adsorption-biodegradation and Fe (III) reduction with the improved electron transfer activity, adenosine triphosphate and cytochrome C concentrations. Abundant phenol degrading bacteria, electroactive bacteria, syntrophic partners could be enriched by FeBC addition, contributing to the enhanced benzoyl-CoA and methanogenesis pathways. However, this enhancement was inhibited by FMBC addition owing to the accumulation of reactive oxygen species. This study provided novel insights into the application of magnetic BC to enhanced anaerobic treatment of phenolic wastewater.

Enhanced ammonia nitrogen and phenol removal by immobilized bacteria through composite mycelium pellet-driven quinone redox cycle

The composite mycelium pellet (CMP) was coupled with Pseudomonas sp. Y1 (CMP-Y1) to remove phenol and ammonia nitrogen (NH4+-N). The CMP was formed by the self-assembly of fungal mycelium with sponge iron (SIO), gallic acid (GA), and oxalic acid. The results showed that CMP with abundant pore size and successful internal loading of sponge iron containing iron nanoparticles. CMP could induce GA redox cycle to form Fenton-like reaction and thus achieve efficient phenol removal (93.32%, 24 h). Meanwhile, the removal efficiencies of phenol, NH4+-N, and chemical oxygen demand (COD) using CMP-Y1 at 12 h were 93.71, 92.40, and 89.00%, respectively. The increase in the electron transfer activity of strain Y1 by the addition of CMP could facilitate the nitrogen removal processes. In addition, high-throughput sequencing results indicated the abundance of antioxidant and repair genes was increased, which might be a strategy of strain Y1 to cope with oxidative stress. This strategy provided the possibility for the practical application of the combination of advanced oxidation and biological treatment, and offered new insights into the symbiotic system of fungi and bacteria.

Emerging Strategies for Enhancing Propionate Conversion in Anaerobic Digestion: A Review

Anaerobic digestion (AD) is a triple-benefit biotechnology for organic waste treatment, renewable production, and carbon emission reduction. In the process of anaerobic digestion, pH, temperature, organic load, ammonia nitrogen, VFAs, and other factors affect fermentation efficiency and stability. The balance between the generation and consumption of volatile fatty acids (VFAs) in the anaerobic digestion process is the key to stable AD operation. However, the accumulation of VFAs frequently occurs, especially propionate, because its oxidation has the highest Gibbs free energy when compared to other VFAs. In order to solve this problem, some strategies, including buffering addition, suspension of feeding, decreased organic loading rate, and so on, have been proposed. Emerging methods, such as bioaugmentation, supplementary trace elements, the addition of electronic receptors, conductive materials, and the degasification of dissolved hydrogen, have been recently researched, presenting promising results. But the efficacy of these methods still requires further studies and tests regarding full-scale application. The main objective of this paper is to provide a comprehensive review of the mechanisms of propionate generation, the metabolic pathways and the influencing factors during the AD process, and the recent literature regarding the experimental research related to the efficacy of various strategies for enhancing propionate biodegradation. In addition, the issues that must be addressed in the future and the focus of future research are identified, and the potential directions for future development are predicted.

Internal driving mechanism of microbial community and metabolic pathway for psychrophilic anaerobic digestion by microbial electrolysis cell

The system that microbial electrolysis cell coupled anaerobic digestion (termed MEC-AD) with metal organic framework-modified cathode was operated under different voltage levels (0-1.2 V) at 20 °C. The maximum methane yield increased to 0.23 ± 0.01 LCH₄ g^-1COD at 0.9 V, with 28% improvement compared to 0 V (0.18 ± 0.01 LCH₄ g^-1COD). Moreover, total volatile fatty acid and propionate accumulation decreased by 32% and 15% at 0.9 V, indicating the system has potential to alleviate acidity suppression. Acidogens and electroactive microorganisms was clearly enriched with increasing applied voltage. Specifically, the abundance of Smithella increased, which could degrade propionate to acetate. Methanosaeta was dominant, accounting for ca. 40.1%∼55.1% of the archaea community at 0.3-1.2 V. Furthermore, the system reinforced psychrophilic methanogenesis by activating important enzymes involved in related metabolism pathways. Overall, this study provides perspective on the future practical application for the regulation of psychrophilic AD in electrochemically integrated bioreactors.

Response mechanisms of anaerobic fermentative sludge to zinc oxide nanoparticles during medium-chain carboxylates production from waste activated sludge

The conversion of waste activated sludge (WAS) into medium chain carboxylates (MCCs) has attracted much attention, while investigations of the impacts of ZnO nanoparticles (NPs) on this process are sparse. The present study showed that 8 mg/g-TSS of ZnO NPs have little effects on all key steps and the activity of anaerobes, and finally leading to unchanged MCCs production. Although 30 mg/g-TSS of ZnO NPs weakly inhibited the hydrolysis, acidogenesis, and chain elongation process, WAS solubilization was enhanced, thus, the improvement was enough to offset inhibition, also resulting in an insignificant impact on overall MCCs production. However, the improvement with ZnO NPs dosages above 100 mg/g-TSS was not sufficient to offset the biological inhibition, thus inducing negative impact on overall MCCs production. The decline of EPS induced by Zn²⁺ and generation of excessive reactive oxygen species (ROS) were the main factors responsible for the inhibitory effects of ZnO NPs on lower activity of anaerobes. For chain elongation process, the discriminative Clostridium IV (as MCCs-forming bacteria) with a strong adaptation to ZnO NPs (300 mg/g-TSS) was observed. The present study provided a deep understanding related to the effects of ZnO NPs on the production of MCCs production from WAS and identified a zinc resistance anaerobe, which would be significant for the evaluation of influence and alleviation of inhibition induced by ZnO NPs on the carbon cycle of organic wastes.

Nonmonotonic effect of CuO nanoparticles on medium-chain carboxylates production from waste activated sludge

The growing applications of CuO nanoparticles (NPs) in industrial and agriculture has increased their concentrations in wastewater and subsequently accumulated in waste activated sludge (WAS), raising concerns about their impact on reutilization of WAS, especially on the medium-chain carboxylates (MCCs) production from anaerobic fermentation of WAS. Here we showed that CuO NPs at 10-50 mg/g-TS can significantly inhibit MCCs production, and reactive oxygen species generation was revealed to be the key factor linked to the phenomena. At lower CuO NPs concentrations (0.5-2.5 mg/g-TS), however, MCCs production was enhanced, with a maximum level of 37% compared to the control. The combination of molecular approaches and metaproteomic analysis revealed that although low dosage CuO NPs (2.5 mg/g-TS) weakly inhibited chain elongation process, they displayed contributive characteristics both in WAS solubilization and transport/metabolism of carbohydrate. These results demonstrated that the complex microbial processes for MCCs production in the anaerobic fermentation of WAS can be affected by CuO NPs in a dosage-dependent manner via regulating microbial protein expression level. Our findings can provide new insights into the influence of CuO NPs on anaerobic fermentation process and shed light on the treatment option for the resource utilization of CuO NPs polluted WAS.

Size-dependent effects of polystyrene microplastics on anaerobic digestion performance of food waste: Focusing on oxidative stress, microbial community, key metabolic functions

Polystyrene (PS) microplastics (MPs) are widely existed in food waste (FW) due to the usage of plastic food-packaging. However, the effects and mechanisms of PS MPs with different sizes on anaerobic digestion (AD) performance of FW have not been comprehensively studied yet. Herein, the impacts of different PS MPs sizes (1 mm, 100 µm and 1 µm) with 20, 200 particles/g-TS were investigated. Results showed that 20 particles/g-TS PS MPs decreased cumulative methane production by 1.46-18.11 %, while the higher levels (200 particles/g-TS) significantly inhibited by 9.14-33.08 % (p < 0.05) compared with control group. The inhibiting effects were enhanced as particle size smaller. Physicochemical analysis indicated that MPs prolonged organic matter hydrolysis, weakened the volatile fatty acids metabolism and inhibited methanogenesis-related microorganisms (Synergistetes, Proteiniphilum and Methanosarcina). Small-sized MPs could induce more reactive oxygen species causing cell toxicity and suppressed key enzymes (α-glucoside, protease, acetate kinases and F₄₂₀) activities, thereby restraining methane production. The analyses of acetyl-CoA synthase and methyl-coenzyme M reductase functional genes illustrated that small-sized MPs negatively affected acetoclastic methanogenesis pathways. Overall, these results provide new insights into the size-dependent effects on AD performance induced by PS MPs.

Interrelating EPS, soluble microbial products and metal solubility in a methanogenic consortium stressed by nickel and cobalt

The relationships between extracellular polymeric substances (EPS), soluble microbial product production, metal solubility, and methanogenic activity were investigated. The individual, and joint, toxic effects of nickel and cobalt on methanogenic consortia fed with glucose as model substrate were studied using biomethane potential assays. Cobalt was found to be less toxic to methanogens than nickel at each concentration tested, and the combined effects of Ni and Co on methane production in the bimetal experiment was higher than the sum of the effects of each metal alone. The protein content of EPS, and extracellular soluble protein fractions, decreased with increasing concentrations of total metals. Meanwhile, no significant change in response to metal stress was apparent for carbohydrate content of EPS or extracellular soluble carbohydrate. Decreasing protein content of EPS was accompanied by reduced methanogenic activity and an increase in the soluble metal fraction. The strong associations observed between these variables could be due to the critical role of EPS in protecting microbial cells against nickel and cobalt stress, possibly by capturing metal cations through their functional groups, thus reducing metal availability to the microbial cells in the methanogenic consortia underpinning the anaerobic digestion process.

Urchin-like hollow SiO2@γ-MnO2 microparticles for the rapid degradation of organic dyes

In this paper, using hollow silica microspheres as carriers, we developed a facile one-pot method for the preparation of hollow SiO₂@MnO₂ composite microparticles. Under a certain proportion of hollow silica microspheres and manganese salt, a novel kind of hollow urchin-like SiO₂@γ-MnO₂ microparticles was obtained. The structure and morphology of the composite microparticles were characterized by XRD, SEM and TEM. On this basis, using rhodamine B and methyl orange as model molecules, the oxidative degradation ability of the hollow SiO₂@γ-MnO₂ microparticles for organic dyes in water was investigated through UV-vis analysis technology. The urchin-like SiO₂@γ-MnO₂ microparticles showed excellent performance for the rapid oxidative degradation of organic dyes under acidic conditions. This study indicated that γ-MnO₂ loaded on hollow materials can be used as an efficient tool for treating organic dye wastewater, and shows broad application prospects for solving environmental problems in the related industry.

In this paper, using hollow silica microspheres as carrier, we developed a facile method for preparation of a novel kind of hollow urchin-like SiO₂@γ-MnO₂ microparticles with excellent performance for rapid oxidation degradation of organic dyes.

Impact of electro-conductive nanoparticles additives on anaerobic digestion performance - A review

Anaerobic digestion (AD) is a biochemical process that converts waste organic matter into energy-rich biogas with methane as the main component. Addition of electric electro-conductive, such as that nanoparticles (NP), has been shown to improve biogas generation. Interspecies electron transfer and direct interspecies electron transfer (DIET) using conductive materials is one of the mechanisms responsible for observed increases in CH₄. This article discusses the effect of the type and size of electro-conductive NPs on improving microbial degradation within AD systems, as well as the effect of electro-conductive NPs on microbial community shifts and syntrophic metabolism. Limitations and future perspectives of using NPs in an AD system is also discussed.

Influence of Tubificidae Limnodrilus and electron acceptors on the environmental fate of BDE-47 in sediments by (14)C-labelling

2,2',4,4'-tetrabromodiphenyl ether (BDE-47) was difficult to degrade in sediments. In this study, the environmental behavior of BDE-47 with/without the effect of benthos (Tubificidae Limnodrilus) and electron acceptors in sediments was investigated using C-14 tracer. Generally, extractable residues of BDE-47 were dominant in sediment and posed high environment risk. The amount of non-extractable residues (NERs) accounted for 39.0% of initial radioactivity in oxic sediments was significantly higher than those in anoxic sediments (17.6%). Most of NERs were localized in the humin fraction and presented as sequestrated forms. Under oxic conditions, the present of Limnodrilus significantly increased the proportion of NERs in sediment. Limnodrilus accumulated 34.2% of initial radioactivity. Under anoxic conditions, the addition of iron (Ⅲ) [Fe(III)], sulfate and nitrate reduced the environmental risk of BDE-47 with the increase of NERs formation, while manganese (IV) [Mn(IV)] addition had no effect on the formation of NERs. The present of Limnodrilus and electron acceptors promoted the production of metabolites. Meanwhile, BDE-47 changed the microbial community structure of sediments. These findings indicated that the environmental behavior and risk of BDE-47 was affected by benthos and electron acceptors, and the high proportion of sequestrated NERs posed high bioactivity and toxic threat to ecological environment.

The role of conductive nanoparticles in anaerobic digestion: Mechanism, current status and future perspectives

In the current scenario, alternative energy sources are the need of the hour. Organic wastes having a larger fraction of biodegradable constituents present a sustainable bioenergy source. It has been reported that the calorific value of biogas generated by anaerobic digestion (AD) is 21-25 MJ/m³ with the treatment which makes it an excellent replacement of natural gas and fossil fuels and can reduce more than 80% greenhouse gas emission to the surroundings. However, there are some limitations associated with the AD process for instance ammonia build-up at the first stage reduces the rate of hydrolysis of biomass, whereas, in the last stage it interferes with methane formation. Owing to special physicochemical properties such as high activity, high reactive surface area, and high specificity, tailor-made conductive nanoparticles can improve the performance of the AD process. In the AD process, H₂ is used as an electron carrier, referred as mediated interspecies electron transfer (MIET). Due to the diffusion limitation of these electron carriers, the MIET efficiency is relatively low that limits the methanogenesis. Direct interspecies electron transfer (DIET), which enables direct cell-to-cell electron transport between bacteria and methanogen, has been considered an alternative efficient approach to MIET that creates metabolically favorable conditions and results in faster conversion of organic acids and alcohols into methane. This paper discusses in detail the application of conductive nanoparticles to enhance the AD process efficiency. Interaction between microbes in anaerobic conditions for electron transfer with the help of CNPs is discussed. Application of a variety of conductive nanomaterials as an additive is discussed with their potential biogas production and treatment enhancement in the anaerobic digestion process.

Effects of nano metal oxide particles on activated sludge system: Stress and performance recovery mechanism

Nano metal oxide particles (NMOPs) are widely used in daily life because of their superior performance, and inevitably enter the sewage treatment system. Pollutants in sewage are adsorbed and degraded in wastewater treatment plants (WWTPs) depending on the microbial aggregates of activated sludge system to achieve sewage purification. NMOPs may cause ecotoxicity to the microbial community and metabolism due to their complex chemical behavior, resulting in a potential threat to the safe and steady operation of activated sludge system. It is of great significance to clarify the influencing mechanism of NMOPs on activated sludge system and reduce the risk of WWTPs. Herein, we first introduce the physicochemical behavior of six typical engineering NMOPs including ZnO, TiO₂, CuO, CeO₂, MgO, and MnO₂ in water environment, then highlight the principal mechanisms of NMOPs for activated sludge system. In particular, the performance recovery mechanisms of activated sludge systems in the presence of NMOPs and their future development trends are well documented and discussed extensively. This review can provide a theoretical guidance and technical support for predicting and evaluating the potential threat of NMOPs on activated sludge systems, and promoting the establishment of effective control strategies and performance recovery measures of biological wastewater treatment process under the stress of NMOPs.

The impacts of metal-based engineered nanomaterial mixtures on microbial systems: A review

The last decade has witnessed tremendous growth in the commercial use of metal-based engineered nanomaterials (ENMs) for a wide range of products and processes. Consequently, direct and indirect release into environmental systems may no longer be considered negligible or insignificant. Yet, there is an active debate as to whether there are real risks to human or ecological health with environmental exposure to ENMs. Previous research has focused primarily on the acute effects of individual ENMs using pure cultures under controlled laboratory environments, which may not accurately reveal the ecological impacts of ENMs under real environmental conditions. The goal of this review is to assess our current understanding of ENM effects as we move from exposure of single to multiple ENMs or microbial species. For instance, are ENMs' impacts on microbial communities predicted by their intrinsic physical or chemical characteristics or their effects on single microbial populations; how do chronic ENM interactions compare to acute toxicity; does behavior under simplified laboratory conditions reflect that in environmental media; finally, is biological stress modified by interactions in ENM mixtures relative to that of individual ENM? This review summarizes key findings and our evolving understanding of the ecological effects of ENMs under complex environmental conditions on microbial systems, identifies the gaps in our current knowledge, and indicates the direction of future research.

A review of mechanisms underlying the impacts of (nano)microplastics on anaerobic digestion

The presence of (nano)microplastics in domestic wastewater and their subsequent release to the aquatic environment via the discharge of treated sewage has raised significant concerns. Previous studies have also identified their excessive accumulation in sewage sludge. Anaerobic digestion is one of the most used sludge stabilization methods in wastewater treatment plants. Therefore, understanding the potential effects of (nano)microplastics on anaerobic digestion has been receiving increasing attention from researchers. This article provides a comprehensive review of mechanisms underlying the impacts of (nano)microplastics on anaerobic digestion. Notably, this review covers mechanisms of inhibition/enhancement of anaerobic digestion by (nano)microplastics and their potential impacts on biochemical pathways, key enzymes, functional genes, and microbial communities investigated to date. Moreover, potential environmental risks of biosolids contaminated with (nano)microplastics were highlighted. Finally, knowledge gaps and future research needs were outlined. This review will guide more standardized studies in the future, covering both fundamental and engineering aspects.

Static-magnetic-field coupled with fly-ash accelerant: A powerful strategy to significantly enhance the mesophilic anaerobic-co-digestion

The anaerobic-co-digestion (AcoD) of dairy-manure (DM) and aloe-peel-residue (ALR) waste is enhanced by combining static-magnetic-field (SMF) and fly-ash (FA). Varying SMF intensities (5-30 mT) were applied to the co-digestion digesters containing the optimum FA concentration (1.5 wt.%), which were selected from co-digestion systems with a varying FA (0-2 wt.%). All experimental groups exhibit the greater COD removal rates (51.56-64.19%) and cumulative biogas yields (604.14-671.64 mL/g VS) than reference group (37.77% and 433.19 mL/g VS). The digester with optimum FA concentration (1.5 wt.%) under 5 mT shows the highest biogas yield (671.64 mL/g VS), and exhibits superior digestate stability (45.4%) and fertility (7.01%) for fertilizer utilization. A powerful strategy for understanding the underlying mechanism of the SMF and FA accelerant in an enhanced AcoD system is proposed. This work documents an innovative technique for an enhanced AcoD system using the SMF coupled with FA accelerant.

Characterization and significance of extracellular polymeric substances, reactive oxygen species, and extracellular electron transfer in methanogenic biocathode

The microbial electrolysis cell assisted anaerobic digestion holds great promises over conventional anaerobic digestion. This article reports an experimental investigation of extracellular polymeric substances (EPS), reactive oxygen species (ROS), and the expression of genes associated with extracellular electron transfer (EET) in methanogenic biocathodes. The MEC-AD systems were examined using two cathode materials: carbon fibers and stainless-steel mesh. A higher abundance of hydrogenotrophic Methanobacterium sp. and homoacetogenic Acetobacterium sp. appeared to play a major role in superior methanogenesis from stainless steel biocathode than carbon fibers. Moreover, the higher secretion of EPS accompanied by the lower ROS level in stainless steel biocathode indicated that higher EPS perhaps protected cells from harsh metabolic conditions (possibly unfavorable local pH) induced by faster catalysis of hydrogen evolution reaction. In contrast, EET-associated gene expression patterns were comparable in both biocathodes. Thus, these results indicated hydrogenotrophic methanogenesis is the key mechanism, while cathodic EET has a trivial role in distinguishing performances between two cathode electrodes. These results provide new insights into the efficient methanogenic biocathode development.

Insight into the surface activity of defect structure in α-MnO2 nanorod: first-principles research

The contribution of defect structure to the catalytic property of α-MnO₂ nanorod still keeps mysterious right now. Using microfacet models representing defect structure and bulk models with high Miller index, several parameters, such as cohesive energy, surface energy, density of state, electrostatic potential, et al., have been used to investigate the internal mechanism of their chemical activities by first-principles calculation. The results show that the trend in surface energies of microfacet models follows as E_surface[(112 × 211)] > E_surface[(110 × 211)] > E_surface[(100 × 211)] > E_surface[(111 × 211)] > E_surface[(112 × 112)] > E_surface[(111 × 112)], wherein all of them are larger than that of bulk models. So the chemical activity of defect structure is much more powerful than that of bulk surface. Deep researches on electronic structure show that the excellent chemical activity of microfacet structure has larger value in dipole moments and electrostatic potential than that of bulk surface layer. And the microfacet models possess much more peaks of valent electrons in deformantion electronic density and molecular orbital. Density of state indicates that the excellent chemical activity of defect structure comes from their proper hybridization in p and d orbitals.

Variation of volatile fatty acid oxidation and methane production during the bioaugmentation of anaerobic digestion system: Microbial community analysis revealing the influence of microbial interactions on metabolic pathways

Anaerobic digestion (AD) is widely used on waste treatment for its great capability of organic degradation and energy recovery. Accumulation of volatile fatty acids (VFAs) caused by impact loadings often leads to the acidification and failure of AD systems. Bioaugmentation is a promising way to accelerate VFA degradation but the succession of microbial communities usually caused unpredictable consequences. In this study, we used the sludge previously acclimated with VFAs for the bioaugmentation of an acidified anaerobic digestion system and increased the methane yield by 8.03-9.59 times. To see how the succession of microbial communities affected bioaugmentation, dual-chamber devices separated by membrane filters were used to control the interactions between the acidified and acclimated sludges. The experimental group with separated sludges showed significant advantages of VFA consumption (5.5 times less final VFA residue than the control), while the group with mixed sludge produced more methane (4.0 times higher final methane yield than the control). Microbial community analysis further highlighted the great influences of microbial interaction on the differentiation of metabolic pathways. Acetoclastic methanogens from the acclimated sludge acted as the main contributors to pH neutralization and methane production during the early phase of bioaugmentation, and maintained active in the mixed sludge but degenerated in the separated sludges where interactions between sludge microbiotas were limited. Instead, syntrophic butyrate and acetate oxidation coupled with nitrate and sulfate reduction was enriched in the separated sludges, which lowered the methane conversion rate and would cause the failure of bioaugmentation. Our study revealed the importance of microbial interactions and the functionality of enriched microbes, as well as the potential strategies to optimize the durability and efficiency of bioaugmentation.

Enhanced direct interspecies electron transfer with transition metal oxide accelerants in anaerobic digestion

Transition metal compounds have been widely used to enhance the anaerobic digestion (AD) performance, while the role of transition metal compounds in enhancing AD performance remains unclarified. In this work, the function of transition metal oxide accelerants (tantalum oxide, niobium oxide, hafnium oxide, and tungsten oxide) in enhanced AD systems was investigated from experimental and theoretical standpoints. Higher biogas production (565.01-617.85 mL/g VS), chemical oxygen demand degradation rate (67.17%-70.45%), total solids and volatile solids reduction rates (29.76%-34.71%, 51.83%-60.88%) were achieved in AD systems with transition metal oxide accelerants than the control (327.08 mL/g VS, 56.65%, 22.65%, and 41.18%). The first-principle density functional theory calculations, electron exchange capacity analysis, and the 16S rRNA gene pyrosequencing demonstrated superior electron transfer and exchange capacities as well as microbial consortia development in transition metal oxides-induced DIET mechanism. This work provides a promising strategy for understanding the function of high-performance accelerants in AD systems.

Influence of polyether sulfone microplastics and bisphenol A on anaerobic granular sludge: Performance evaluation and microbial community characterization

The retention of polyether sulfone (PES) and bisphenol A (BPA) in wastewater has received extensive attention. The effects of PES and BPA on the removal of organic matter by anaerobic granular sludge were investigated. We also analyzed the changes in the electron transport system and the effects on the composition of extracellular polymeric substances (EPS), as well as alternations of the microbial community in the anaerobic granular sludge. In the experimental groups which received BPA, the removal of the chemical oxygen demand (COD) were significantly suppressed, which an average removal efficiency of less than 65%, 30% lower than that of the control group. In the loosely-bound EPS (LB-EPS) excitation-emission matrix (EEM) spectra, the absorption peak of tryptophan disappeared when the BPA pollutants was added, which it was present in the control group without added pollutants. The addition of PES and BPA also affected protease, acetate kinase, and coenzyme F₄₂₀ activities in the anaerobic granular sludge. Especially, the coenzyme F₄₂₀ reduced from 0.0045 to 0.0017 μmol/L in the presence of PES and BPA. The relative abundance of Spirochaetes decreased in the presence of PES and BPA, while the relative abundance of Bacteroidetes increased from 12.98% to 22.87%. At the genus level, in the presence of PES and BPA, the relative abundance of Acinetobacter increased from 2.20% to 9.64% and Hydrogenophaga decreased sharply from 15.58% to 0.12%.

Role of biomass-derived carbon-based composite accelerants in enhanced anaerobic digestion: Focusing on biogas yield, fertilizer utilization, and density functional theory calculations

The performance of anaerobic digestion (AD) can be improved by the addition of accelerants. Three types of biomass-derived carbon-based composites (Co/C, CoO/C, and Co₃O₄/C) were used as accelerants to investigate the effect on AD systems in this work. These accelerants significantly improved the cumulative biogas yield (576-585 mL/g VS), and the total chemical oxygen demand degradation rate (68.48-71.11%) compared to the reference group (435.8 mL/g VS, 50.74%). The digestates with accelerants exhibited exceptional stability (59.24-63.67%) and superior fertilizer utilization (3.50-4.55%). In addition, first-principle density functional theory (DFT) calculations were conducted to provide the theoretical basis for the direct interspecies electron transfer (DIET), and a general strategy was proposed to help understand the enhanced methanogenesis pathway induced by the biomass-derived carbon-based composites. These important findings provide a novel avenue for the development of composite accelerants for AD systems.

Comparative facilitation of activated carbon and goethite on methanogenesis from volatile fatty acids

To provide insight into direct interspecies electron transfer (DIET) via carbon-based materials and ferric oxides, the effects of three conductive materials (i.e. activated carbon (AC), iron modified activated carbon (FEAC) and goethite (FEOOH)), on methanogenesis from volatile fatty acids (VFAs) were evaluated. Under the acid stress (~4 g/L VFAs), the maximum methane yield of 266 mL/g-chemical oxygen demand (COD) was found in the FEOOH supplemented reactor, which was 48% higher than that of AC reactor. The reasons for the enhanced activity of the electron transport chain and extracellular electron transfer ability by FEOOH include: 1) the activation on iron-containing enzymes that involved in methanogenesis and acidogenesis; 2) selective enrichment on functional microorganism. The higher electron donating capacities (EDC) value of FEOOH may be a triggering factor on the growth of Syntrophomonadaceae, which perform DIET with methanogens (Methanosaeta and Methanosarcina) for the syntrophic degradation of VFAs.

Coupling the phenolic oxidation capacities of a bacterial consortium and in situ-generated manganese oxides in a moving bed biofilm reactor (MBBR)

Phenolic wastewater containing phenol and 4-chlorophenol pose a risk to the environment and to human health. Treating them using chemical-biological coupling method is challenging. In this study, manganese oxidizing bacteria (MnOB) were enriched in moving bed biofilm reactor (MBBR) using synthetic phenol wastewater (800 mg L^-1) to facilitate in situ production of biogenic manganese oxides (BioMnOx) after 90 days of operation. Then, 4-chlorophenol (4-CP) was added to the MBBR to simulate mixed phenolic wastewater. Comparing the MBBR (R1) without feeding Mn(II) and the MBBR with BioMnOx (R2) production, R2 exhibited robust phenol and 4-CP removal performance. 16S rRNA gene sequencing was employed to determine the microbial community. Subsequently, a batch experiment demonstrated that partly purified BioMnOx does not exhibits a capacity for phenol removal, but can efficiently remove 4-CP. Interestingly, 5-chloro-2-hydroxymuconic semialdehyde was found in the products of 4-CP degradation, which was the unique product of 4-CP degradation by catechol 2,3-dioxygenase (C23O). In both reactors, only catechol 1,2-dioxygenase (C12O) activity from microbes can be detected, indicating that the existence of BioMnOx provide an alternative pathway in addition to microbe driven 4-CP degradation. Overall, MBBR based MnOB enrichment under high phenol concentration was achieved, and 4-CP/phenol removal can be accelerated by in situ-formed BioMnOx. Considering the C23O-like activity of BioMnOx, our results suggest a new coupling strategy that involves nanomaterials and a microbial consortium.

The performance and microbial community in response to MnO2 nanoparticles in anammox granular sludge

The response of anaerobic ammonium oxidation (anammox) process and granular sludge in the upflow anaerobic sludge blanket reactor was analyzed under long-term exposure to MnO₂ nanoparticles (NPs) (1-200 mg L^-1). The addition of 200 mg L^-1 MnO₂ NPs had a significantly positive effect on nitrogen removal and this system exhibited excellent performance, with a total nitrogen removal efficiency of 93.1%. Moreover, the specific anammox activity enhanced with increasing concentrations of MnO₂ NPs up to the maximum value of 657.3 ± 9.3 mg TN g^-1 VSS d^-1 under MnO₂ NPs concentration of 200 mg L^-1. This value was approximately 1.6-fold higher than that of the reactor in the absence of MnO₂ NPs. The extracellular polymeric substances and settling velocity were both increased with MnO₂ NPs addition. Meanwhile, the high-throughput sequencing results revealed that MnO₂ NPs increased the relative abundance of dominant bacteria (Candidatus Kuenenia) from 17.3% at the absence of MnO₂ NPs to 23.9% at 200 mg L^-1 MnO₂ NPs, which resulted in a higher efficiency of biological nitrogen removal on the anammox system. These results indicated that MnO₂ NPs enhanced nitrogen removal performance of anammox process.

Deposition Kinetics of Colloidal Manganese Dioxide onto Representative Surfaces in Aquatic Environments: The Role of Humic Acid and Biomacromolecules

The initial deposition kinetics of colloidal MnO₂ on three representative surfaces in aquatic systems (i.e., silica, magnetite, and alumina) in NaNO₃ solution were investigated in the presence of model constituents, including humic acid (HA), a polysaccharide (alginate), and a protein (bovine serum albumin (BSA), using laboratory quartz crystal microbalance with dissipation monitoring equipment (QCM-D). The results indicated that the deposition behaviors of MnO₂ colloids on three surfaces were in good agreement with classical Derjaguin-Landau-Verwey-Overbeek (DLVO) theory. Critical deposition concentrations (CDC) were determined to be 15.5 mM NaNO₃ and 9.0 mM NaNO₃ when colloidal MnO₂ was deposited onto silica and magnetite, respectively. Both HA and alginate could largely retard the deposition of MnO₂ colloids onto three selected surfaces due to steric repulsion, and HA was more effective in decreasing the deposition rate relative to alginate. However, the presence of BSA can provide more attractive deposition site and thus lead to greater deposition behavior of MnO₂ colloids onto surfaces. The dissipative properties of the deposited layer were also influenced by surface type, electrolyte concentration, and organic matter characteristics. Overall, these results provide insights into the deposition behavior of MnO₂ colloids on environmental surfaces and have significant implications for predicting the transport potential of common MnO₂ colloids in natural environments and engineered systems.