Bioreduction of Antimonate by Anaerobic Methane Oxidation in a Membrane Biofilm Batch Reactor

Microbial synergy mechanism of hydrogen flux influence on hydrogen-based partial denitrification coupled with anammox in a membrane biofilm reactor

The hydrogen-based partial denitrification coupled with anammox (H2-PDA) biofilm system effectively achieves low-carbon and high-efficiency biological nitrogen removal. However, the effects and biological interaction mechanism of H2 flux with the H2-PDA system have not yet been understood. This study assessed the effects of H2 flux on interactions among anammox bacteria (AnAOB), denitrifying bacteria (DB), and sulfate-reducing bacteria (SRB) coexisting in a H2-PDA system. Results showed the simultaneous removal of 40 mg/L ammonium nitrogen (NH4+-N) and 50 mg/L nitrate nitrogen (NO3--N) in the H2-PDA system at a flux of 0.13-0.14 e- eq/(m2·d) without additional organic carbon. Candidatus_Brocadia was involved in H2 oxidation and was negatively associated with the heterotrophic Thauera genus (DB). The expression of nirS and dsrA was increased to 5.6 × 105 copies/gSS and 2.1 × 105 copies/gSS, respectively, with excessive H2 flux (0.17 e- eq/(m2·d). This study provides technical guidance for understanding and applying the H2-PDA technology for low-carbon wastewater treatment.

Anaerobic oxidation of methane driven by different electron acceptors: A review

Methane, the most significant reduced form of carbon on Earth, acts as a crucial fuel and greenhouse gas. Globally, microbial methane sinks encompass both aerobic oxidation of methane (AeOM), conducted by oxygen-utilizing methanotrophs, and anaerobic oxidation of methane (AOM), performed by anaerobic methanotrophs employing various alternative electron acceptors. These electron acceptors involved in AOM include sulfate, nitrate/nitrite, humic substances, and diverse metal oxides. The known anaerobic methanotrophic pathways comprise the internal aerobic oxidation pathway found in NC10 bacteria and the reverse methanogenesis pathway utilized by anaerobic methanotrophic archaea (ANME). Diverse anaerobic methanotrophs can perform AOM independently or in cooperation with symbiotic partners through several extracellular electron transfer (EET) pathways. AOM has been documented in various environments, including seafloor methane seepages, coastal wetlands, freshwater lakes, soils, and even extreme environments like hydrothermal vents. The environmental activities of AOM processes, driven by different electron acceptors, primarily depend on the energy yields, availability of electron acceptors, and environmental adaptability of methanotrophs. It has been suggested that different electron acceptors driving AOM may occur across a wider range of habitats than previously recognized. Additionally, it is proposed that methanotrophs have evolved flexible metabolic strategies to adapt to complex environmental conditions. This review primarily focuses on AOM, driven by different electron acceptors, discussing the associated reaction mechanisms and the habitats where these processes are active. Furthermore, it emphasizes the pivotal role of AOM in mitigating methane emissions.

Enhanced Extracellular Electron Transfer in Magnetite-Mediated Anaerobic Oxidation of Methane Coupled to Humic Substances Reduction: The Pivotal Role of Membrane-Bound Electron Transfer Proteins

Humic substances are organic substances prevalent in various natural environments, such as wetlands, which are globally important sources of methane (CH4) emissions. Extracellular electron transfer (EET)-mediated anaerobic oxidation of methane (AOM)-coupled with humic substances reduction plays an important role in the reduction of methane emissions from wetlands, where magnetite is prevalent. However, little is known about the magnetite-mediated EET mechanisms in AOM-coupled humic substances reduction. This study shows that magnetite promotes the reduction of the AOM-coupled humic substances model compound, anthraquinone-2,6-disulfonate (AQDS). 13CH4 labeling experiments further indicated that AOM-coupled AQDS reduction occurred, and acetate was an intermediate product of AOM. Moreover, 13CH313COONa labeling experiments showed that AOM-generated acetate can be continuously reduced to methane in a state of dynamic equilibrium. In the presence of magnetite, the EET capacity of the microbial community increased, and Methanosarcina played a key role in the AOM-coupled AQDS reduction. Pure culture experiments showed that Methanosarcina barkeri can independently perform AOM-coupled AQDS reduction and that magnetite increased its surface protein redox activity. The metatranscriptomic results indicated that magnetite increased the expression of membrane-bound proteins involved in energy metabolism and electron transfer in M. barkeri, thereby increasing the EET capacity. This phenomenon potentially elucidates the rationale as to why magnetite promoted AOM-coupled AQDS reduction.

Microbial mediated remediation of heavy metals toxicity: mechanisms and future prospects

Heavy metal pollution has become a serious concern across the globe due to their persistent nature, higher toxicity, and recalcitrance. These toxic metals threaten the stability of the environment and the health of all living beings. Heavy metals also enter the human food chain by eating contaminated foods and cause toxic effects on human health. Thus, remediation of HMs polluted soils is mandatory and it needs to be addressed at higher priority. The use of microbes is considered as a promising approach to combat the adverse impacts of HMs. Microbes aided in the restoration of deteriorated environments to their natural condition, with long-term environmental effects. Microbial remediation prevents the leaching and mobilization of HMs and they also make the extraction of HMs simple. Therefore, in this context recent technological advancement allowed to use of bioremediation as an imperative approach to remediate polluted soils. Microbes use different mechanisms including bio-sorption, bioaccumulation, bioleaching, bio-transformation, bio-volatilization and bio-mineralization to mitigate toxic the effects of HMs. Thus, keeping in the view toxic HMs here in this review explores the role of bacteria, fungi and algae in bioremediation of polluted soils. This review also discusses the various approaches that can be used to improve the efficiency of microbes to remediate HMs polluted soils. It also highlights different research gaps that must be solved in future study programs to improve bioremediation efficency.

Biological oxidation of As(III) and Sb(III) by a novel bacterium with Sb(III) oxidase rather than As(III) oxidase under anaerobic and aerobic conditions

Sb and As are chemically similar, but the sequences and structures of Sb(III) and As(III) oxidase are totally distinct. It is thus interesting to explore whether Sb(III) oxidase oxidizes As(III), and if so, how microbial oxidations of Sb(III) and As(III) influence one another. Previous investigations have yielded ambiguous or even erroneous conclusions. This study aimed to clarify this issue. Firstly, we prepared a consortium of Sb(III)-oxidizing prokaryotes (SOPs) by enrichment cultivation. Metagenomic analysis reveals that SOPs with the Sb(III) oxidase gene, but lacking the As(III) oxidase gene are predominant in the SOP community. Despite this, SOPs exhibit comparable Sb(III) and As(III)-oxidizing activities in both aerobic and anaerobic conditions, indicating that at the microbial community level, Sb(III) oxidase can oxidize As(III). Secondly, we isolated a representative cultivable SOP, Ralstonia sp. SbOX with Sb(III) oxidase gene but without As(III) oxidase gene. Genomic analysis of SbOX reveals that this SOP strain has a complete Sb(III) oxidase (AnoA) gene, but lacks As(III) oxidase (AioAB or ArxAB) gene. It is interesting to discover that, besides its Sb(III) oxidation activities, SbOX also exhibits significant capabilities in oxidizing As(III) under both aerobic and anaerobic conditions. Moreover, under aerobic conditions and in the presence of both Sb(III) and As(III), SbOX exhibited a preference for oxidizing Sb(III). Only after the near complete oxidation of Sb(III) did SbOX initiate rapid oxidation of As(III). In contrast, under anaerobic conditions and in the presence of both Sb(III) and As(III), Sb(III) oxidation notably inhibited the As(III) oxidation pathway in SbOX, while As(III) exhibited minimal effects on the Sb(III) oxidation. These findings suggest that SOPs can oxidize As(III) under both aerobic and anaerobic conditions, exhibiting a strong preference for Sb(III) over As(III) oxidation in the presence of both. This study unveils a novel mechanism of interaction within the Sb and As biogeochemical cycles.

Reaction pathways and Sb(III) minerals formation during the reduction of Sb(V) by Rhodoferax ferrireducens strain YZ-1

Antimony (Sb), a non-essential metalloid, can be released into the environment through various industrial activities. Sb(III) is considered more toxic than Sb(V), but Sb(III) can be immobilized through the precipitation of insoluble Sb2S3 or Sb2O3. In the subsurface, Sb redox chemistry is largely controlled by microorganisms; however, the exact mechanisms of Sb(V) reduction to Sb(III) are still unclear. In this study, a new strain of Sb(V)-reducing bacterium, designated as strain YZ-1, that can respire Sb(V) as a terminal electron acceptor was isolated from Sb-contaminated soils. 16S-rRNA gene sequencing of YZ-1 revealed high similarity to a known Fe(III)-reducer, Rhodoferax ferrireducens. XRD and XAFS analyses revealed that bioreduction of Sb(V) to Sb(III) proceed through a transition from amorphous valentinite to crystalline senarmontite (allotropes of Sb2O3). Genomic DNA sequencing found that YZ-1 possesses arsenic (As) metabolism genes, including As(V) reductase arsC. The qPCR analysis showed that arsC was highly expressed during Sb(V)-reduction by YZ-1, and thus is proposed as the potential Sb(V) reductase in YZ-1. This study provides new insight into the pathways and products of microbial Sb(V) reduction and demonstrates the potential of a newly isolated bacterium for Sb bioremediation.

Archaea are better adapted to antimony stress than their bacterial counterparts in Xikuangshan groundwater

Archaea are important ecological components of microbial communities in various environments, but are currently poorly investigated in antimony (Sb) contaminated groundwater particularly on their ecological differences in comparison with bacteria. To address this issue, groundwater samples were collected from Xikuangshan aquifer along an Sb gradient and subjected to 16S rRNA gene amplicon sequencing and bioinformatic analysis. The results demonstrated that bacterial communities were more susceptibly affected by elevated Sb concentration than their archaeal counterparts, and the positive stimulation of Sb concentration on bacterial diversity coincided with the intermediate disturbance hypothesis. Overall, the balance of environmental variables (Sb, pH, and EC), competitive interactions, and stochastic events jointly regulated bacterial and archaeal communities. Linear fitting analysis revealed that Sb significantly drove the deterministic process (heterogeneous selection) of bacterial communities, whereas stochastic process (dispersal limitation) contributed more to archaeal community assembly. In contract, the assembly of Sb-resistant bacteria and archaea was dominated by the stochastic process (undominated), which implied the important role of diversification and drift instead of selection. Compared with Sb-resistant microorganisms, bacterial and archaeal communities showed lower niche width, which may result from the constraints of Sb concentration and competitive interaction. Moreover, Sb-resistant archaea had a higher niche than that of Sb-resistant bacteria via investing on flexible metabolic pathways such as organic metabolism, ammonia oxidation; and carbon fixation to enhance their competitiveness. Our results offered new insights into the ecological adaptation mechanisms of bacteria and archaea in Sb-contaminated groundwater.

A review on sources of soil antimony pollution and recent progress on remediation of antimony polluted soils

Antimony (Sb) is a serious toxic and non-essential metalloid for animals, humans, and plants. The rapid increase in anthropogenic inputs from mining and industrial activities, vehicle emissions, and shoot activity increased the Sb concentration in the environment, which has become a serious concern across the globe. Hence, remediation of Sb-contaminated soils needs serious attention to provide safe and healthy foods to humans. Different techniques, including biochar (BC), compost, manures, plant additives, phyto-hormones, nano-particles (NPs), organic acids (OA), silicon (Si), microbial remediation techniques, and phytoremediation are being used globally to remediate the Sb polluted soils. In the present review, we described sources of soil Sb pollution, the environmental impact of antimony pollution, the multi-faceted nature of antimony pollution, recent progress in remediation techniques, and recommendations for the remediation of soil Sb-pollution. We also discussed the success stories and potential of different practices to remediate Sb-polluted soils. In particular, we discussed the various mechanisms, including bio-sorption, bio-accumulation, complexation, and electrostatic attraction, that can reduce the toxicity of Sb by converting Sb-V into Sb-III. Additionally, we also identified the research gaps that need to be filled in future studies. Therefore, the current review will help to develop appropriate and innovative strategies to limit Sb bioavailability and toxicity and sustainably manage Sb polluted soils hence reducing the toxic effects of Sb on the environment and human health.

Antimony toxicity upon microorganisms from aerobic and anaerobic environments

Antimony (Sb) is a toxic and carcinogenic metalloid that can be present in contaminated water generated by mining operations and other industrial activities. The toxicity of Sb (III) and Sb (V) to aerobic microorganisms remains limited and unexplored for anaerobic microorganisms involved in hydrogen (H₂) and methane (CH₄) production. This study aimed to evaluate the toxicity of Sb (III) and Sb (V) upon aerobic and anaerobic microorganisms important in biological wastewater treatment systems. Sb (III) was more toxic than Sb (V) independently of the test and environment evaluated. Under aerobic conditions maintained in the Microtox assay, Sb (V) was not toxic to Allivibrio fischeri at concentrations as high as 500 mg/L, whereas Sb (III) caused just over 50% inhibition at concentration of 250 mg/L after 5 min of exposure. In the respirometry test, for the specific oxygen uptake rate, the concentrations of Sb (III) and Sb (V) displaying 50% inhibition were 0.09 and 56.2 mg/L, respectively. Under anaerobic conditions, exposure to Sb (III) and Sb (V) led to a decrease in microorganisms activity of fermentative and methanogenic processes. The results confirm that the microbial toxicity of Sb depends on its speciation and Sb (III) displays a significantly higher inhibitory potential than Sb (V) in both aerobic and anaerobic environments.

Antimony (Sb) pollution control by coagulation and membrane filtration in water/wastewater treatment: A comprehensive review

Antimony (Sb) pollution in the water environment caused by the large-scale mining of Sb ore and the wide use of Sb-containing products seriously endangers human health and poses a great threat to the ecological environment. Coagulation is one of the most cost-effective technologies for Sb pollution control in water/wastewater treatment and has been widely used. However, a comprehensive understanding of Sb pollution control by coagulation, from fundamental research to practical applications, is lacking. In this work, based on the current status of Sb pollution in the water environment, a critical review of the Sb removal performance and mechanism by coagulation and related combined processes was carried out. The influencing factors of Sb removal performance by coagulation are introduced in detail. The internal mechanisms and improvement strategies of Sb removal by oxidation/reduction-coagulation and coagulation-membrane filtration technologies are emphasized. Moreover, given the development of Sb-removing coagulants and the resource utilization of Sb-containing sludge, future perspectives of coagulation for Sb removal are discussed. As the first review in this field, this work will illuminate avenues of basic research and practical applications for Sb and Sb-like pollution control in water/wastewater treatment.

Effect of nitrate and sulfate coexistence on hydrogen autotrophic reduction of antimonate (Sb(V)) and microbial community structures

Hydrogen autotrophic bioreduction of antimonate (Sb(V)) to antimonite (Sb(III)) is an alternative approach for removing antimony (Sb) from water. This study investigated Sb(V) reduction kinetics and the effects of various parameters on the Sb(V) removal performance in a hydrogen autotrophic reaction system (HARS). Sb(V) reduction in the HARS was well fitted to the Michaelis-Menten model, showing a positive correlation between the reaction rate and biomass. The maximum specific substrate removal rates were 0.29-4.86 and 6.82-15.87 mg Sb(V)/(g·VSS·h) at initial Sb(V) concentrations of 500 μg/L and 10 mg/L, respectively. Coexisting nitrate significantly inhibited Sb(V) reduction, and the inhibition intensified with increasing nitrate concentration. However, coexisting sulfate had a positive effect on Sb(V) reduction, and the sulfate effectively enhanced total antimony (TSb) removal performance by generating sulfide from sulfate reduction. Illumina high-throughput sequencing technology was used to determine the changes in microbial community structure during different periods in the HARS, revealing the effects of co-existing ions on the dominant Sb(V) reducing bacteria. In the HARS, Longilinea and Terrimonas were the dominant genera in the presence of nitrate, and Longilinea was the dominant genus in the presence of sulfate, at initial Sb(V) concentration of 500 μg/L. When the concentration of Sb(V) was 10 mg/L, Longilinea and Thauera were the dominant genus in the HARS for treating water co-polluted with nitrate and sulfate, respectively. These results provide a theoretical basis of the application of HARS for the bio-remediation of Sb(V) contaminated water.

Vanadate reducing bacteria and archaea may use different mechanisms to reduce vanadate in vanadium contaminated riverine ecosystems as revealed by the combination of DNA-SIP and metagenomic-binning

Vanadium (V) is a transitional metal that poses health risks to exposed humans. Microorganisms play an important role in remediating V contamination by reducing more toxic and mobile vanadate (V(V)) to less toxic and mobile V(IV). In this study, DNA-stable isotope probing (SIP) coupled with metagenomic-binning was used to identify microorganisms responsible for V(V) reduction and determine potential metabolic mechanisms in cultures inoculated with a V-contaminated river sediment. Anaeromyxobacter and Geobacter spp. were identified as putative V(V)-reducing bacteria, while Methanosarcina spp. were identified as putative V(V)-reducing archaea. The bacteria may use the two nitrate reductases NarG and NapA for respiratory V(V) reduction, as has been demonstrated previously for other species. It is proposed that Methanosarcina spp. may reduce V(V) via anaerobic methane oxidation pathways (AOM-V) rather than via respiratory V(V) reduction performed by their bacterial counterparts, as indicated by the presence of genes associated with anaerobic methane oxidation coupled with metal reduction in the metagenome assembled genome (MAG) of Methanosarcina. Briefly, methane may be oxidized through the "reverse methanogenesis" pathway to produce electrons, which may be further captured by V(V) to promote V(V) reduction. More specially, V(V) reduction by members of Methanosarcina may be driven by electron transport (CoMS-SCoB heterodisulfide reductase (HdrDE), F₄₂₀H₂ dehydrogenases (Fpo), and multi-heme c-type cytochrome (MHC)). The identification of putative V(V)-reducing bacteria and archaea and the prediction of their different pathways for V(V) reduction expand current knowledge regarding the potential fate of V(V) in contaminated sites.

Toxic effects of antimony in plants: Reasons and remediation possibilities-A review and future prospects

Antimony (Sb) is a dangerous heavy metal (HM) that poses a serious threat to the health of plants, animals, and humans. Leaching from mining wastes and weathering of sulfide ores are the major ways of introducing Sb into our soils and aquatic environments. Crops grown on Sb-contaminated soils are a major reason of Sb entry into humans by eating Sb-contaminated foods. Sb toxicity in plants reduces seed germination and root and shoot growth, and causes substantial reduction in plant growth and final productions. Moreover, Sb also induces chlorosis, causes damage to the photosynthetic apparatus, reduces membrane stability and nutrient uptake, and increases oxidative stress by increasing reactive oxygen species, thereby reducing plant growth and development. The threats induced by Sb toxicity and Sb concentration in soils are increasing day by day, which would be a major risk to crop production and human health. Additionally, the lack of appropriate measures regarding the remediation of Sb-contaminated soils will further intensify the current situation. Therefore, future research must be aimed at devising appropriate measures to mitigate the hazardous impacts of Sb toxicity on plants, humans, and the environment and to prevent the entry of Sb into our ecosystem. We have also described the various strategies to remediate Sb-contaminated soils to prevent its entry into the human food chain. Additionally, we also identified the various research gaps that must be addressed in future research programs. We believe that this review will help readers to develop the appropriate measures to minimize the toxic effects of Sb and its entry into our ecosystem. This will ensure the proper food production on Sb-contaminated soils.

Dissimilatory and Cytoplasmic Antimonate Reductions in a Hydrogen-Based Membrane Biofilm Reactor

A hydrogen-based membrane biofilm reactor (H₂-MBfR) was operated to investigate the bioreduction of antimonate [Sb(V)] in terms of Sb(V) removal, the fate of Sb, and the pathways of reduction metabolism. The MBfR achieved up to 80% Sb(V) removal and an Sb(V) removal flux of 0.55 g/m²·day. Sb(V) was reduced to Sb(III), which mainly formed Sb₂O₃ precipitates in the biofilm matrix, although some Sb(III) was retained intracellularly. High Sb(V) loading caused stress that deteriorated performance that was not recovered when the high Sb(V) loading was removed. The biofilm community consisted of DSbRB (dissimilatory Sb-reduction bacteria), SbRB (Sb-resistant bacteria), and DIRB (dissimilatory iron-reducing bacteria). Dissimilatory antimonate reduction, mediated by the respiratory arsenate reductase ArrAB, was the main reduction route, but respiratory reduction coexisted with cytoplasmic Sb(V)-reduction mediated by arsenate reductase ArsC. Increasing Sb(V) loading caused stress that led to increases in the expression of arsC gene and intracellular accumulation of Sb(III). By illuminating the roles of the dissimilatory and cytoplasmic Sb(V) reduction mechanism in the biofilms of the H₂-MBfR, this study reveals that the Sb(V) loading should be controlled to avoid stress that deteriorates Sb(V) reduction.

Coupled Aerobic Methane Oxidation and Arsenate Reduction Contributes to Soil-Arsenic Mobilization in Agricultural Fields

Microbial oxidation of organic compounds can promote arsenic release by reducing soil-associated arsenate to the more mobile form arsenite. While anaerobic oxidation of methane has been demonstrated to reduce arsenate, it remains elusive whether and to what extent aerobic methane oxidation (aeMO) can contribute to reductive arsenic mobilization. To fill this knowledge gap, we performed incubations of both microbial laboratory cultures and soil samples from arsenic-contaminated agricultural fields in China. Incubations with laboratory cultures showed that aeMO could couple to arsenate reduction, wherein the former bioprocess was carried out by aerobic methanotrophs and the latter by a non-methanotrophic bacterium belonging to a novel and uncultivated representative of Burkholderiaceae. Metagenomic analyses combined with metabolite measurements suggested that formate served as the interspecies electron carrier linking aeMO to arsenate reduction. Such coupled bioprocesses also take place in the real world, supported by a similar stoichiometry and gene activity in the incubations with natural paddy soils, and contribute up to 76.2% of soil-arsenic mobilization into pore waters in the top layer of the soils where oxygen was present. Overall, this study reveals a previously overlooked yet significant contribution of aeMO to reductive arsenic mobilization.

Enhanced biological antimony removal from water by combining elemental sulfur autotrophic reduction and disproportionation

Antimony (Sb), a toxic metalloid, has serious negative effects on human health and its pollution has become a global environmental problem. Bio-reduction of Sb(V) is an effective Sb-removal approach. This work, for the first time, demonstrates the feasibility of autotrophic Sb(V) bio-reduction and removal coupled to anaerobic oxidation of elemental sulfur (S⁰). In the S⁰-based biological system, Sb(V) was reduced to Sb(III) via autotrophic bacteria by using S⁰ as electron donor. Meanwhile, S⁰ disproportionation reaction occurred under anaerobic condition, generating sulfide and SO₄^2- in the bio-systems. Subsequently, Sb(III) reacted with sulfide and formed Sb(III)-S precipitate, achieving an effective total Sb removal. The precipitate was identified as Sb₂S₃ by SEM-EDS, XPS, XRD and Raman spectrum analyses. In addition, it was found that co-existing nitrate inhibited the Sb removal, as nitrate is the favored electron acceptor over Sb(V). In contrast, the bio-reduction of co-existing SO₄^2- enhanced sulfide generation, followed by promoting Sb(V) reduction and precipitation. Illumina high-throughput sequencing analysis revealed that Metallibacterium, Citrobacter and Thiobacillus might be responsible for Sb(V) reduction and S⁰ disproportionation. This study provides a promising approach for the remediation of Sb(V)-contaminated water.

Antimony redox processes in the environment: A critical review of associated oxidants and reductants

The environmental behavior of antimony (Sb) has recently received greater attention due to the increasing global use of Sb in a range of industrial applications. Although present at trace levels in most natural systems, elevated Sb concentrations in aquatic and terrestrial environments may result from anthropogenic activities. The mobility and toxicity of Sb largely depend on its speciation, which is dependent to a large extent on its oxidation state. To a certain extent, our understanding of the environmental behavior of Sb has been informed by studies of the environmental behavior of arsenic (As), as Sb and As have somewhat similar chemical properties. However, recently it has become evident that the speciation of Sb and As, especially in the context of redox reactions, may be fundamentally different. Therefore, it is crucial to study the biogeochemical processes impacting Sb redox transformations to understand the behavior of Sb in natural and engineered environments. Currently, there is a growing body of literature involving the speciation, mobility, toxicity, and remediation of Sb, and several reviews on these general topics are available; however, a comprehensive review focused on Sb environmental redox chemistry is lacking. This paper provides a review of research conducted within the past two decades examining the redox chemistry of Sb in aquatic and terrestrial environments and identifies knowledge gaps that need to be addressed to develop a better understanding of Sb biogeochemistry for improved management of Sb in natural and engineered systems.

Desulfurivibrio spp. mediate sulfur-oxidation coupled to Sb(V) reduction, a novel biogeochemical process

Antimony (Sb) contamination released from mine tailings represents a global threat to natural ecosystems and human health. The geochemical conditions of Sb tailings, which are oligotrophic and replete in sulfur (S) and Sb, may promote the coupled metabolism of Sb and S. In this study, multiple lines of evidence indicate that a novel biogeochemical process, S oxidation coupled to Sb(V) reduction, is enzymatically mediated by Desulfurivibrio spp. The distribution of Desulfurivibrio covaried with S and Sb concentrations, showing a high relative abundance in Sb mine tailings but not in samples from surrounding sites (i.e., soils, paddies, and river sediments). Further, the metabolic potential to couple S oxidation to Sb(V) reduction, encoded by a non-canonical, oxidative sulfite reductase (dsr) and arsenate reductase (arrA) or antimonate reductase (anrA), respectively, was found to be common in Desulfurivibrio genomes retrieved from metal-contaminated sites in southern China. Elucidation of enzymatically-catalyzed S oxidation coupled to Sb(V) reduction expands the fundamental understanding of Sb biogeochemical cycling, which may be harnessed to improve remediation strategies for Sb mine tailings.

Metabolic potential of anaerobic methane oxidizing archaea for a broad spectrum of electron acceptors

Methane (CH₄) is a potent greenhouse gas significantly contributing to the climate warming we are currently facing. Microorganisms play an important role in the global CH₄ cycle that is controlled by the balance between anaerobic production via methanogenesis and CH₄ removal via methanotrophic oxidation. Research in recent decades advanced our understanding of CH₄ oxidation, which until 1976 was believed to be a strictly aerobic process. Anaerobic oxidation of methane (AOM) coupled to sulfate reduction is now known to be an important sink of CH₄ in marine ecosystems. Furthermore, in 2006 it was discovered that anaerobic CH₄ oxidation can also be coupled to nitrate reduction (N-DAMO), demonstrating that AOM may be much more versatile than previously thought and linked to other electron acceptors. In consequence, an increasing number of studies in recent years showed or suggested that alternative electron acceptors can be used in the AOM process including Fe^III, Mn^IV, As^V, Cr^VI, Se^VI, Sb^V, V^V, and Br^V. In addition, humic substances as well as biochar and perchlorate (ClO₄^-) were suggested to mediate AOM. Anaerobic methanotrophic archaea, the so-called ANME archaea, are key players in the AOM process, yet we are still lacking deeper understanding of their metabolism, electron acceptor preferences and their interaction with other microbial community members. It is still not clear whether ANME archaea can oxidize CH₄ and reduce metallic electron acceptors independently or via electron transfer to syntrophic partners, interspecies electron transfer, nanowires or conductive pili. Therefore, the aim of this review is to summarize and discuss the current state of knowledge about ANME archaea, focusing on their physiology, metabolic flexibility and potential to use various electron acceptors.

Antimony contamination and its risk management in complex environmental settings: A review

Antimony (Sb) is introduced into soils, sediments, and aquatic environments from various sources such as weathering of sulfide ores, leaching of mining wastes, and anthropogenic activities. High Sb concentrations are toxic to ecosystems and potentially to public health via the accumulation in food chain. Although Sb is poisonous and carcinogenic to humans, the exact mechanisms causing toxicity still remain unclear. Most studies concerning the remediation of soils and aquatic environments contaminated with Sb have evaluated various amendments that reduce Sb bioavailability and toxicity. However, there is no comprehensive review on the biogeochemistry and transformation of Sb related to its remediation. Therefore, the present review summarizes: (1) the sources of Sb and its geochemical distribution and speciation in soils and aquatic environments, (2) the biogeochemical processes that govern Sb mobilization, bioavailability, toxicity in soils and aquatic environments, and possible threats to human and ecosystem health, and (3) the approaches used to remediate Sb-contaminated soils and water and mitigate potential environmental and health risks. Knowledge gaps and future research needs also are discussed. The review presents up-to-date knowledge about the fate of Sb in soils and aquatic environments and contributes to an important insight into the environmental hazards of Sb. The findings from the review should help to develop innovative and appropriate technologies for controlling Sb bioavailability and toxicity and sustainably managing Sb-polluted soils and water, subsequently minimizing its environmental and human health risks.

A novel granular sludge-based and highly corrosion-resistant bio-concrete in sewers

Bio-concrete is known for its self-healing capacity although the corrosion resistance was not investigated previously. This study presents an innovative bio-concrete by mixing anaerobic granular sludge into concrete to mitigate sewer corrosion. The control concrete and bio-concrete (with granular sludge at 1% and 2% of the cement weight) were partially submerged in a corrosion chamber for 6 months, simulating the tidal-region corrosion in sewers. The corrosion rates of 1% and 2% bio-concrete were about 17.2% and 42.8% less than that of the control concrete, together with 14.6% and 35.0% less sulfide uptake rates, 15.3% and 55.6% less sulfate concentrations, and higher surface pH (up to 1.8 units). Gypsum and ettringite were major corrosion products but in smaller sizes on bio-concrete than that of control concrete. The total relative abundance of corrosion-causing microorganisms, i.e. sulfide-oxidizing bacteria, was significantly reduced on bio-concrete, while more sulfate-reducing bacteria (SRB) was detected. The corrosion-resistance of bio-concrete was mainly attributed to activities of SRB derived from the granular sludge, which supported the sulfur cycle between the aerobic and anaerobic corrosion sub-layers. This significantly reduced the net production of biogenic sulfuric acid and thus corrosion. The results suggested that the novel granular sludge-based bio-concrete provides a highly potential solution to reduce sewer corrosion.

Microbial reduction of Cr(VI) in the presence of Ni, Cu and Zn by bacterial consortium enriched from an electroplating contaminated site

The bioremediation of Cr(VI) has been intensively reported in recent years, while little information about Cr(VI)-reducing consortium enriched from in-situ contaminated soil has been revealed, specifically the functional genes involved. In this study, we verified a Cr(VI) reduction process by a consortium enriched from in-situ contaminated soil through enzymatic analysis. The chromate reductase gene ChrR has been successfully amplified and further analyzed, provided solid evidence to prove the Cr(VI) bio-reduction was an enzyme-mediated process. Meanwhile, the analysis of metabolic pathways demonstrates that the consortium could detoxicate and resist Cr(VI) and co-existing metals (Ni²⁺, Zn²⁺ and Cu²⁺) through membrane transport and DNA repair process. The co-existing heavy metals Zn and Cu had a relatively significant negative and positive effects on Cr(VI) reduction respectively, which may play important roles in the Cr(VI) contaminated soil bioremediation.

Effects of antimony on anaerobic methane oxidization and microbial community in an antimony-contaminated paddy soil: A microcosm study

Anaerobic methane oxidation (AOM) plays an important role in sinking global methane and thereby in constraining climate change. Anthropogenic activities in antimony (Sb) mines have resulted in Sb contamination in rice fields, which are among the dominant methane sources. However, the knowledge of effects of Sb on AOM in paddy soils and the microbiota involved in this process remains limited. Herein, Sb was amended into the paddy soil to investigate the effects of Sb contamination on AOM and the microbial communities such as methanotrophs. Significant inhibition of AOM was observed in the treatment with Sb addition in comparison with the treatment without Sb addition. The significant increases in the abundance of the mcrA genes, responsible for methane production and oxidation, were observed in the treatment with/without Sb addition. In contrast, no significant increases in the copy number of the mcrA gene were detected in the treatment with the addition of the methanogenic and methanotrophic inhibitor 2-bromoethanesulfonate (BES). These results suggested that Sb contamination might inhibit only AOM but not methane production. In addition, amplicon high-throughput sequencing showed that the Sb addition impaired the diversity of microbial communities and impacted the biotic interactions in the soil. However, the abundance of bacterial methane-oxidizing phylum NC10 and its biotic connections with other microbes were enhanced by the addition of Sb. Pseudogulbenkiania and Methanosarcina dominated the bacterial and archaeal communities in the treatment without Sb addition, while the bacteria Ramlibacter and Geothrix and the archaea Methanomethylovorans were the most abundant genera in the treatment with Sb addition. These analyses of microbial communities indicated that Sb addition had significant effects on both the compositions of bacterial and archaeal communities. This study expands our knowledge of the effects of Sb contamination on AOM and the microbial (especially methanotrophs) diversity and composition in paddy soils.

Limiting factors of heavy metals removal during anaerobic biological pretreatment of municipal solid waste landfill leachate

An anaerobic biofilm reactor was used to pretreat a typical municipal solid waste landfill leachate. It was challenging to remove Fe, Pb, and Ni to meet the discharge-to-sewer standards at a hydraulic retention time (HRT) typically used in previous studies. This work further systematically studied the factors that limited the metal removal. The HRT limited metal removal because the required metal sulfides precipitation time was more than 3.5 times of the HRT. Sulfide availability only slightly limited the metal removal since adding sulfate above the stoichiometric requirement improved the metal removal by only 5-11%. Via experiments combined with modeling, it was found that metal bisulfide was the dominant complex that limited Fe removal, but humic acids-metal complex was the dominant complex that limited the removal of Pb and Ni. When the total dissolved sulfide concentration is <18 mg/L, humic substances are more limiting the removal of the three metals than bisulfide. On the other hand, when the total dissolved sulfide concentration is >250 mg/L, bisulfide is more limiting than humic substances.

Production of two morphologically different antimony trioxides by a novel antimonate-reducing bacterium, Geobacter sp. SVR

A novel dissimilatory antimonate [Sb(V)]-reducing bacterium, strain SVR, was isolated from soil of a former antimony (Sb) mine. Strain SVR coupled Sb(V) reduction to acetate oxidation with an apparent reduction rate of 2.4 mM d^-1. The reduction of Sb(V) was followed by the precipitation and accumulation of white microcrystals in the liquid medium. The precipitates were initially small and amorphous, but they eventually developed to the crystal phase with a length > 50 µm. Strain SVR removed 96% of dissolved Sb as the precipitates. An X-ray diffraction analysis indicated that the microcrystals were the orthorhombic Sb trioxide (Sb₂O₃), i.e., valentinite. Phylogenetic and physiological analyses revealed that strain SVR is a member of the genus Geobacter. The cell suspension of strain SVR incubated with acetate and Sb(V) at pH 7.0 was able to form valentinite. Interestingly, at pH 8.0, the cell suspension formed another crystalline Sb₂O₃ with a cubic structure, i.e., senarmontite. Our findings provide direct evidence that Geobacter spp. are involved in Sb(V) reduction in nature. Considering its superior capacity for Sb removal, strain SVR could be used for the recovery of Sb and the individual productions of valentinite and senarmontite from Sb-contaminated wastewater.

Making good use of methane to remove oxidized contaminants from wastewater

Being an energetic fuel, methane is able to support microbial growth and drive the reduction of various electron acceptors. These acceptors include a broad range of oxidized contaminants (e.g., nitrate, nitrite, perchlorate, bromate, selenate, chromate, antimonate and vanadate) that are ubiquitously detected in water environments and pose threats to human and ecological health. Using methane as electron donor to biologically reduce these contaminants into nontoxic forms is a promising solution to remediate polluted water, considering that methane is a widely available and inexpensive electron donor. The understanding of methane-based biological reduction processes and the responsible microorganisms has grown in the past decade. This review summarizes the fundamentals of metabolic pathways and microorganisms mediating microbial methane oxidation. Experimental demonstrations of methane as an electron donor to remove oxidized contaminants are summarized, compared, and evaluated. Finally, the review identifies opportunities and unsolved questions that deserve future explorations for broadening understanding of methane oxidation and promoting its practical applications.

Rapid formation of granules coupling n-DAMO and anammox microorganisms to remove nitrogen

Granular sludge exhibits unique features, including rapid settling velocity, high loading rate and relative insensitivity against inhibitors, thus being a favorable platform for the cultivation of slow-growing and vulnerable microorganisms, such as anaerobic ammonium oxidation (anammox) bacteria and nitrite/nitrate-dependent anaerobic methane oxidation (n-DAMO) microorganisms. While anammox granules have been widely applied, little is known about how to speed up the granulation process of n-DAMO microorganisms, which grow even slower than anammox bacteria. In this study, we used mature anammox granules as biotic carriers to embed n-DAMO microorganisms, which obtained combined anammox + n-DAMO granules within 6 months. The results of whole-granule 16S rRNA gene amplicon sequencing showed the coexistence of anammox bacteria, n-DAMO bacteria and n-DAMO archaea. The microbial stratification along granule radius was further elucidated by cryosection-16S rRNA gene amplicon sequencing, showing the dominance of n-DAMO archaea and anammox bacteria at inner and outer layers, respectively. Moreover, the images of cryosection-fluorescence in situ hybridization (FISH) verified this stratification and also indicated a shift in microbial stratification. Specifically, n-DAMO bacteria and n-DAMO archaea attached to the anammox granule surface initially, which moved to the inner layer after 4-months operation. On the basis of combined anammox + n-DAMO granules, a practically useful nitrogen removal rate (1.0 kg N/m³/d) was obtained from sidestream wastewater, which provides new avenue to remove nitrogen from wastewater using methane as carbon source.

Factors affecting antimonate bioreduction by Dechloromonas sp. AR-2 and Propionivibrio sp. AR-3

The microbial reduction of antimonate (Sb(v)) to antimonite (Sb(iii)), which forms insoluble Sb compounds, is a promising approach to remove antimony (Sb) from wastewater. Among the bacterial strains capable of reducing Sb(v) via anaerobic respiration that have been isolated to date, Dechloromonas sp. AR-2 and Propionivibrio sp. AR-3 are promising agents because they can grow aerobically and reduce Sb(v) under both anaerobic and microaerobic conditions. In this study, the effects of temperature, pH, electron donors, and coexisting electron acceptors on Sb(v) reduction and Sb removal by strains AR-2 and AR-3 were investigated to assess the usefulness of the strains in practical Sb treatment scenarios. Efficient Sb(v) reduction and removal by the two strains occurred over a relatively wide temperature range (15-35 °C) and neutral pH (6-7). In contrast, the carbon sources usable by these strains as electron donors for Sb respiration were limited to simple fatty acids such as acetate and lactate. Although strain AR-2 used nitrate and AR-3 used nitrate and arsenate as electron acceptors for anaerobic respiration in addition to Sb(v), the co-presence of other electron acceptors did not inhibit Sb(v) reduction. These results suggest that strains AR-2 and AR-3 can be potentially used in the practical treatment of Sb(v)-containing wastewater.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s13205-021-02703-0.

Reduction of Sb(V) by coupled biotic-abiotic processes under sulfidogenic conditions

Increasing use and mining of antimony (Sb) has resulted in greater concern involving its fate and transport in the environment. Antimony(V) and (III) are the two most environmentally relevant oxidation states, but little is known about the redox transitions between the two in natural systems. To better understand the behavior of antimony in anoxic environments, the redox transformations of Sb(V) were studied in biotic and abiotic reactors. The biotic reactors contained Sb(V) (2 mM as KSb(OH)₆), ferrihydrite (50 mM Fe(III)), sulfate (10 mM), and lactate (10 mM), that were inoculated with sediment from a wetland. In the abiotic reactors, The interaction of Sb(V) with green rust, magnetite, siderite, vivianite or mackinawite was examined under abiotic conditions. Changes in the concentrations of Sb, Fe(II), sulfate, and lactate, as well as the microbial community composition were monitored over time. Lactate was rapidly fermented to acetate and propionate in the bioreactors, with the latter serving as the primary electron donor for dissimilatory sulfate reduction (DSR). The reduction of ferrihydrite was primarily abiotic, being driven by biogenic sulfide. Sb and Fe K-edge X-ray absorption near edge structure (XANES) analysis showed reduction of Sb(V) to Sb(III) within 4 weeks, concurrent with DSR and the formation of FeS. Sb K-edge extended X-ray absorption fine structure (EXAFS) spectroscopy analysis indicated that the reduced phase was a mixture of S- and O-coordinated Sb(III). Reduction of Sb(V) was not observed in the presence of magnetite, siderite, or green rust, and limited reduction occurred with vivianite. However, reduction of Sb(V) to amorphous Sb(III) sulfide occurred with mackinawite. These results are consistent with abiotic reduction of Sb(V) by biogenic sulfide and reveal a substantial influence of Fe oxides on the speciation of Sb(III), which illustrates the tight coupling of Sb speciation with the biogeochemical cycling of S and Fe.

Heavy metal reduction coupled to methane oxidation：Mechanisms, recent advances and future perspectives

Methane emission has contributed greatly to the global warming and climate change, and the pollution of heavy metals is an important concern due to their toxicity and environmental persistence. Recently, multiple heavy metals have been demonstrated to be electron acceptors for methane oxidation, which offers a potential for simultaneous methane emission mitigation and heavy metal detoxification. This review provides a comprehensive discussion of heavy metals reduction coupled to methane oxidation, and identifies knowledge gaps and opportunities for future research. The functional microorganisms and possible mechanisms are detailed in groups under aerobic, hypoxic and anaerobic conditions. The potential application and major environmental significances for global methane mitigation, the elements cycle and heavy metals detoxification are also discussed. The future research opportunities are also discussed to provide insights for further research and efficient practical application.

Antimonate removal by diatomite modified with Fe-Mn oxides: application and mechanism study

In this study, diatomite coated with Fe-Mn oxides (DFMO) was synthesized through calcination. The adsorption of antimonate (Sb(V)) by DFMO was studied, and environmental factors affecting the adsorption were investigated. The components of DFMO were identified as γ-Fe₂O₃, γ-MnO₂, and SiO₂, in the presence of diatomite covered with nanoscale metal oxides. Batch experiments were carried out to evaluate the antimonate adsorption performance in aqueous solution. Results showed that maximum Sb(V) adsorption capacity of DFMO reached 10.7 mg/g at pH 4, corresponding to 22.2 mg/g per unit metal oxides. Antimonate adsorption occurred on heterogenous surface, following the Freundlich and Pseudo-second order model. Overall, antimonate adsorption was favored at acidic condition due to low point of zero charge. However, when treating electroplating wastewater, neutral pH condition exhibited a higher efficiency than acidic pH, because co-existing ions in electroplating wastewater significantly affects antimony adsorption. Further investigation showed that among different potential co-existing ions, fluoride can strongly inhibit the adsorption of antimonate at 5 mg/L under pH 4. Density functional theory (DFT) analysis confirmed that adsorption energy on DFMO follows: HF < F^- < Sb(OH)₆^-, indicating that fluoride is easier to bind with DFMO compared to antimonate, especially under pH 3.5 at which fluoride exists as HF. Moreover, the competitive adsorption of fluoride toward antimonate indicated the necessity of pre-treatment like neutralization and precipitation before adsorption process.

Microbial antimonate reduction and removal potentials in river sediments

Antimony (Sb), a toxic metalloid, exists mainly as Sb(V) and Sb(III) in the aquatic environment. Sb(V) displays greater solubility and can be reduced to insoluble Sb(III) compounds by microbial activities under anaerobic conditions, thus affecting the environmental fate of Sb. This study was conducted to evaluate the potential of Sb(V) reduction and removal from the aqueous phase by microbial communities existing in river sediments with and without the impact of Sb mining activities. Among the 14 tested sediment samples, which were collected from an urban river without Sb impact and a river flowing through mining area, microbial communities in two samples could reduce and remove Sb(V) in the presence of high concentrations of sulfate, whereas those in other six samples could reduce Sb(V) even under low sulfate concentrations, indicating the relatively wide distribution of microbial Sb(V) reduction potential in the environment, irrespective of the anthropogenic impact. The Sb(V) reduction and removal abilities under different sulfate levels also suggested the presence of multiple types of Sb(V) reduction and removal pathways, including the direct Sb(V) reduction by anaerobic respiration, indirect (chemical) Sb(V) reduction by sulfide produced by microbial sulfate reduction, and their combination. Furthermore, analysis of microbial communities in two enrichment cultures, which were constructed from sediment samples with Sb(V) reduction ability under the minimum sulfate condition and maintained Sb(V) removal ability during 28-d enrichment process, revealed possible contribution of several microbial taxa such as Azospira, Chlostridium, Dechloromonas, Dendrosporobacter, and Halodesulfovibrio to Sb(V) reduction in sediment microbial communities.

Recycling of Pd(0) catalysts by magnetic nanocomposites-microbial extracellular polymeric substances@Fe3O4

Palladium (Pd) is extremely expensive due to its scarcity and excellent catalytic performance. Thus, the recovery of Pd has become increasingly important. Herein, microbial extracellular polymeric substances (EPS) and magnetic nanocomposite EPS@Fe₃O₄ were applied to recover Pd catalysts from Pd(II) wastewater. Results indicated that Pd(II) was reduced to Pd (0), which was then adsorbed by EPS (101.21 mg/g) and EPS@Fe₃O₄ (126.30 mg/(g EPS)). After adsorbing Pd, EPS@Fe₃O₄ could be collected by magnetic separation. The recovered Pd showed excellent catalytic activity in the reduction of methylene blue (MB). The pseudo-second-order kinetic model and Redlich-Peterson model best fit the adsorption results. According to spectral analysis, Pd(II) was reduced to Pd (0) by chemical groups in EPS and EPS@Fe₃O₄, and the hydroxyl had a chelating effect on adsorbed Pd. Therefore, EPS@Fe₃O₄ is an efficient adsorbent for recovering Pd from Pd(II) wastewater.

How the Soil Microbial Communities and Activities Respond to Long-Term Heavy Metal Contamination in Electroplating Contaminated Site

The effects of long-term heavy metal contamination on the soil biological processes and soil microbial communities were investigated in a typical electroplating site in Zhangjiakou, China. It was found that the soil of the electroplating plant at Zhangjiakou were heavily polluted by Cr, Cr (VI), Ni, Cu, and Zn, with concentrations ranged from 112.8 to 9727.2, 0 to 1083.3, 15.6 to 58.4, 10.8 to 510.0 and 69.6 to 631.6 mg/kg, respectively. Soil urease and phosphatase activities were significantly inhibited by the heavy metal contamination, while the microbial biomass carbon content and the bacterial community richness were much lower compared to noncontaminated samples, suggesting that the long-term heavy metal contamination had a severe negative effect on soil microorganisms. Differently, soil dehydrogenase was promoted in the presence of Chromate compared to noncontaminated samples. This might be due to the enrichment of Sphingomonadaceae, which have been proven to be able to secrete dehydrogenase. The high-throughput sequencing of the 16S rRNA gene documented that Proteobacteria, Actinobacteria, and Chloroflexi were the dominant bacterial phyla in the contaminated soil. The Spearman correlation analysis showed the Methylobacillus, Muribaculaceae, and Sphingomonadaceae were able to tolerate high concentrations of Cr, Cr (VI), Cu, and Zn, indicating their potential in soil remediation.

Laboratory scale bioreactor designs in the processes of methane bioconversion: Mini-review

Global methane emissions have been steadily increasing over the past few decades, exerting a negative effect on the environment. Biogas from landfills and sewage treatment plants is the main anthropogenic source of methane. This makes methane bioconversion one of the priority areas of biotechnology. This process involves the production of biochemical compounds from non-food sources through microbiological synthesis. Methanotrophic bacteria are a promising tool for methane bioconversion due to their ability to use this greenhouse gas and to produce protein-rich biomass, as well as a broad range of useful organic compounds. Currently, methane is used not only to produce biomass and chemical compounds, but also to increase the efficiency of water and solid waste treatment. However, the use of gaseous substrates in biotechnological processes is associated with some difficulties. The low solubility of methane in water is one of the major problems. Different approaches have been involved to encounter these challenges, including different bioreactor and gas distribution designs, solid carriers and bulk sorbents, as well as varying air/oxygen supply, the ratio of volumetric flow rate of gas mixture to its consumption rate, etc. The aim of this review was to summarize the current data on different bioreactor designs and the aspects of their applications for methane bioconversion and wastewater treatment. The bioreactors used in these processes must meet a number of requirements such as low methane emission, improved gas exchange surface, and controlled substrate supply to the reaction zone.

Microbial Perchlorate Reduction Driven by Ethane and Propane

Previous studies demonstrated that methane can be used as an electron donor to microbially remove various oxidized contaminants in groundwater. Natural gas, which is more widely available and less expensive than purified methane, is potentially an alternative source of methane. However, natural gas commonly contains a considerable amount of ethane (C₂H₆) and propane (C₃H₈), in addition to methane. It is important that these gaseous alkanes are also utilized along with methane to avoid emissions. Here, we demonstrate that perchlorate (ClO₄^-), a frequently reported contaminant in groundwater, can be microbially reduced to chloride (Cl^-) driven by C₂H₆ or C₃H₈ under oxygen-limiting conditions. Two independent membrane biofilm reactors (MBfRs) supplied with C₂H₆ and C₃H₈, respectively, were operated in parallel to biologically reduce ClO₄^-. The continuous ClO₄^- removal during long-term MBfR operation combined with the concurrent C₂H₆/C₃H₈ consumption and ClO₄^- reduction in batch tests confirms that ClO₄^- reduction was associated with C₂H₆ or C₃H₈ oxidation. Polyhydroxyalkanoates (PHAs) were synthesized in the presence of C₂H₆ or C₃H₈ and were subsequently utilized for supporting ClO₄^- bio-reduction in the absence of gaseous alkanes. Analysis by reverse transcription-quantitative polymerase chain reaction (RT-qPCR) showed that transcript abundance of bmoX (encoding alpha hydroxylase subunit of C₂H₆/C₃H₈ monooxygenase) was positively correlated to the consumption rates of C₂H₆/C₃H₈, while pcrA (encoding a catalytic subunit of perchlorate reductase) was positively correlated to the consumption of ClO₄^-. High-throughput sequencing targeting 16S rRNA, bmoX, and pcrA indicated that Mycobacterium was the dominant microorganism oxidizing C₂H₆/C₃H₈, while Dechloromonas may be the major perchlorate-reducing bacterium in the biofilms. These findings shed light on microbial ClO₄^- reduction driven by C₂H₆ and C₃H₈, facilitating the development of cost-effective strategies for ex situ groundwater remediation.

Membrane processes

This literature review provides a review for publications in 2018 and 2019 and includes information membrane processes findings for municipal and industrial applications. This review is a subsection of the annual Water Environment Federation literature review for Treatment Systems section. The following topics are covered in this literature review: industrial wastewater and membrane. Bioreactor (MBR) configuration, membrane fouling, design, reuse, nutrient removal, operation, anaerobic membrane systems, microconstituents removal, membrane technology advances, and modeling. Other sub-sections of the Treatment Systems section that might relate to this literature review include the following: Biological Fixed-Film Systems, Activated Sludge, and Other Aerobic Suspended Culture Processes, Anaerobic Processes, and Water Reclamation and Reuse. This publication might also have related information on membrane processes: Industrial Wastes, Hazardous Wastes, and Fate and Effects of Pollutants.

Isolation and Characterization of Facultative-Anaerobic Antimonate-Reducing Bacteria

Microbial antimonate (Sb(V)) reduction is a promising approach to remove Sb(V) from wastewater. However, current knowledge regarding microbial Sb(V) reduction is limited to strictly anaerobic conditions. This study was the first to isolate three facultative-anaerobic Sb(V)-reducing bacterial strains from the sludge collected from a wastewater treatment facility in an antimony products plant. Two of the isolated strains, designated Dechloromonas sp. AR-2 and Propionivibrio sp. AR-3, were characterized based on their Sb(V)-reducing abilities. When cultivated under anaerobic conditions with Sb(V) and acetate as the electron acceptor and donor, respectively, both strains could efficiently reduce 5.0 mM Sb(V), removing most of it from the water phase within 7 d. Along with Sb(V) reduction by the strains, white precipitates, which were likely amorphous Sb(OH)₃ solids, were formed with a minor generation of soluble antimonite. Additionally, respiratory Sb(V) reduction by both strains occurred not only under anaerobic but also microaerobic conditions. It was suggested that Sb(V) reduction and the growth abilities of the strains under microaerobic conditions presented a substantial advantage of the use of strains AR-2 and AR-3 for practical applications to Sb(V)-containing wastewater treatment.

Microbial selenate reduction in membrane biofilm reactors using ethane and propane as electron donors

Selenate (Se(VI)) contamination in groundwater is one of major concerns for human health, in particular in shale gas extraction sites. Microbial selenate reduction coupled to methane (CH₄) oxidation has been demonstrated very recently. Little is known whether ethane (C₂H₆) and butane (C₃H₈) are able to drive selenate reduction, although they are also important components in shale gas. In this study, we demonstrated Se(VI) bio-reduction could be achieved using C₂H₆ and C₃H₈ as electron donors and carbon sources. Scanning electron microscopy coupled to energy dispersive X-ray spectroscopy (SEM-EDX) confirmed elemental Se (Se⁰) was the major final product formed from Se(VI) bio-reduction. Polyhydroxyalkanoates (PHAs) were generated in the biofilms as the internal electron-storage materials, which were consumed for sustaining Se(VI) bio-reduction in absence of C₂H₆ and C₃H₈. Microbial community analysis showed that two genera capable of oxidizing gaseous alkanes dominated in the biofilms, including Mycobacterium (in both C₂H₆ and C₃H₈-fed biofilms) and Rhodococcus (in C₃H₈-fed biofilm). In addition, several potential Se(VI) reducers (e.g., Variovorax) were detected in the biofilms. Investigation of Communities by Reconstruction of Unobserved States analysis supported that predictive genes associated with alkanes oxidation, denitrification and PHAs cycle were enriched in the biofilms. These findings offer insights into the process of selenate reduction driven by C₂H₆ and C₃H₈, which ultimately may help to develop a solution to use shale gas for groundwater remediation, especially near shale gas exploitation sites.

NC10 bacteria promoted methane oxidation coupled to chlorate reduction

The strictly anaerobic serum bottles were applied to investigate methane oxidation coupled to chlorate (ClO₃^-) reduction (MO-CR) without exogenous oxygen. 0.35 mM ClO₃^- was consumed within 20 days at the reduction rate of 17.50 μM/d, over three times than that of ClO₄^-. Chlorite (ClO₂^-) was not detected throughout the experiment and the mass recovery of Cl^- was over 89%. Isotope tracing results showed most of ¹³CH₄ was oxided to CO₂, and the electrons recovery reached to 77.6%. Small amounts of ¹³CH₄ was consumed for DOC production probably through aerobic methane oxidation process, with oxygen generated from disproportionation reaction. In pMMO (key enzyme in aerobic oxidation of methane) inhibition tests, ClO₃^- reduction rate was slowed to 7. 0 μmol/d by 2 mM C₂H₂, real-time quantitative PCR also showed the transcript abundance of pMMO and Cld were significantly dropped at the later period of experiment, indicating that the O₂ disproportionated from ClO₂^- was utilized to active CH₄. NC10 bacteria Candidatus Methylomirabilis, related closely to oxygenic denitrifiers M. oxyfera, was detected in the system, and got enriched along with chlorate reduction. Several pieces of evidence supported that NC10 bacteria promoted CH₄ oxidation coupled to ClO₃^- reduction, these oxygenic denitrifiers may perform ClO₂^- disproportionation to produce O₂, and then oxidized methane intracellularly.

A mixed consortium of methanotrophic archaea and bacteria boosts methane-dependent selenate reduction

Though methane-based selenate reduction has been reported, neither the selenate load nor the removal rate could satisfy practical applications, thus limiting this technique to bio-remediate selenate pollution. In the present study, using a membrane biofilm batch reactor (MBBR), we successfully enriched a consortium performing methane-dependent selenate reduction, with enhanced reduction rates from 16.1 to 28.9 μM-day^-1 under a comparable Se concentration to industrial wastewaters (i.e., ~500 μM). During active reduction, 16S rRNA gene copies of Archaea and Bacteria were both increased more than one order of magnitude. Clone library construction and high-throughput sequencing indicated that Methanosarcina and Methylocystis were the only methane-oxidizing microorganisms. The presence of 20 mM bromoethanesulphonate or 0.15 mM acetylene both significantly, but not completely, inhibited methane-dependent selenate reduction, indicating the concurrent contributions of methanotrophic archaea and bacteria. Fluorescence in situ hybridization (FISH) revealed that archaea directly adhered to the surface of the membrane while bacteria were in the outer layer, together forming the mature biofilm. This study highlights the crucial role of both methanotrophic archaea and bacteria in methane-dependent selenate reduction, and lays foundations in applying methane to bio-remediate practical selenate pollution.

High-rate anaerobic decolorization of methyl orange from synthetic azo dye wastewater in a methane-based hollow fiber membrane bioreactor

Anaerobic biological techniques are widely used in the reductive decolorization of textile wastewater. However, the decolorization efficiency of textile wastewater by conventional anaerobic biological techniques is generally limited due to the low biomass retention capacity and short hydraulic retention time (HRT). In this study, a methane-based hollow fiber membrane bioreactor (HfMBR) was initially inoculated with an enriched anaerobic methane oxidation (AOM) culture to rapidly form an anaerobic biofilm. Then, synthetic azo dye wastewater containing methyl orange (MO) was fed into the HfMBR. MO decolorization efficiency of ∼ 100 % (HRT = 2 to 1.5 days) and maximum decolorization rate of 883 mg/L/day (HRT = 0.5 day) were obtained by the stepwise increase of the MO loading rate into the methane-based HfMBR. Scanning electron microscopy (SEM) and fluorescence in situ hybridization (FISH) analysis visually revealed that archaea clusters formed synergistic consortia with adjacent bacteria. Quantitative PCR (qPCR), phylogenetic and high-throughput sequencing analysis results further confirmed the biological consortia formation of methane-related archaea and partner bacteria, which played a synergistic role in MO decolorization. The high removal efficiency and stable microbial structure in HfMBR suggest it is a potentially effective technique for high-toxic azo dyes removal from textile wastewater.

Leaching behavior of Sb and Br from E-waste flame retardant plastics

The improper disposal of E-waste flame retardant plastics laden with antimony (Sb) and bromine (Br) has brought enormous environmental hazards, however, rare information on the effective removal of Sb and Br is available. In this study, through building an alkaline sulfide system under hydrothermal conditions, Sb and Br were simultaneously extracted from flame retardant plastic with high efficiency of 85.60% and 90.13%, respectively. Sulfur ion through mass transfer reacted with encapsulated Sb₂O₃ to form safe and non-toxic SbS₃^3-. Alkaline solution trapped the Br through substitution or neutralization reaction to inhibit the formation of brominated organic compounds. The results showed that the optimum temperature, residence time, Na₂S and NaOH concentration for hydrothermal removal of Sb and Br were 220 °C, 2 h, 50 g/L and 20 g/L. The results also revealed that both Na₂S and NaOH played an interrelated role in the process of Sb removal. However, NaOH was the only factor controlling the process of debromination. Moreover, the FTIR structure of plastic after alkaline sulfide hydrothermal treatment remained unchanged, which implies that the treated plastic can be reused, and is an added advantage of this technology. The TG-DTG analysis proved the effectiveness of alkaline sulfide hydrothermal treatment in removing Sb and Br.

Microbial vanadate and nitrate reductions coupled with anaerobic methane oxidation in groundwater

Vanadate contaminant in groundwater receives increasing attentions, but little is known on its biogeochemical transformation with gaseous electron donors. This study investigated bio-reduction of vanadate coupled with anaerobic methane oxidation and its relationship with nitrate reduction. Results showed 95.8 ± 3.1% of 1 mM vanadate was removed within 7 days using methane as the sole electron donor. Tetravalent vanadium compounds were the main reduction products, which precipitated naturally in groundwater environment. The introduction of nitrate inhibited vanadate reduction, though both were reduced in parallel. Accumulations of volatile fatty acids (VFAs) were observed from methane oxidation. Preliminary microbial community structure and metabolite analyses indicated that vanadate was likely reduced via Methylomonas coupled with methane oxidation or through synergistic relationships between methane oxidizing bacteria and heterotrophic vanadate reducers with VFAs served as the intermediates.

Humic substances as electron acceptors for anaerobic oxidation of methane driven by ANME-2d

Humic substances (humics) are ubiquitous in terrestrial and aquatic environments where they can serve as electron acceptors for anaerobic oxidation of organic compounds. Methane is a powerful greenhouse gas, as well as the least reactive organic molecule. Anaerobic oxidation of methane (AOM) coupled to microbial reduction of various electron acceptors plays a crucial role in mitigating methane emissions. Here, we reported that humics could serve as terminal electron acceptors for AOM using enriched nitrate-reducing AOM microorganisms. AOM coupled to the reduction of humics was demonstrated based on the production of ¹³C-labelled carbon dioxide, and AOM activity was evaluated with different methane partial pressures and electron acceptor concentrations. After three-cycle reduction, both AOM activity and copy numbers of the archaea 16S rRNA and mcrA genes were the highest when anthraquinone-2,6-disulfonic acid and anthraquinone-2-sulfonic acid were electron acceptors. The high-throughput sequencing results suggested that ANME-2d were the dominant methane oxidation archaea after humics reduction, although the partner bacteria NC10 trended downward, other reported humics reduction bacteria (Geobactor and Anammox) appeared. The potential electron transfer models from ANME-2d to humics were proposed. These results enable a better understanding of available electron acceptors for AOM in natural environments and broaden our insight into the significant role of ANME-2d.

Microbial antimonate reduction with a solid-state electrode as the sole electron donor: A novel approach for antimony bioremediation

The anaerobic antimonate [Sb(V)] reduction with a solid-state electrode serving as the sole electron donor was demonstrated by employing a bioelectrochemical system. The highest Sb(V) reduction efficiency was observed at the biocathode potential of -0.7 V versus standard hydrogen electrode using a cathode potential range from -0.5 V to -1.1 V. The scanning electron microscopy and energy dispersive X-ray spectroscopy indicated that both amorphous and crystallized Sb₂O₃ were formed as products of Sb(V) reduction. The irreversible recovery of bioelectrochemical Sb(V), when the cathode potential deviated from the optimal potential, was explained through the alteration in microbial communities, which was further elucidated by the next-generation sequencing of 16S rRNA gene amplicons. Chryseobacterium koreense and Stenotrophomonas nitritireducens were the dominant species of microbial consortia at Sb(V)-reducing biocathodes. This study revealed a novel option for bioremediation of Sb at underground contaminated sites, where the delivery of organic electron donors is limited or ineffective.

Anaerobic methane oxidation coupled to chromate reduction in a methane-based membrane biofilm batch reactor

Chromate can be reduced by methanotrophs in a membrane biofilm reactor (MBfR). In this study, we cultivated a Cr(VI)-reducing biofilm in a methane (CH₄)-based membrane biofilm batch reactor (MBBR) under anaerobic conditions. The Cr(VI) reduction rate increased to 0.28 mg/L day when the chromate concentration was ≤ 2.2 mg/L but declined sharply to 0.01 mg/L day when the Cr(VI) concentration increased to 6 mg/L. Isotope tracing experiments showed that part of the ¹³C-labeled CH₄ was transformed to ¹³CO₂, suggesting that the biofilm may reduce Cr(VI) by anaerobic methane oxidation (AnMO). Microbial community analysis showed that a methanogen, i.e., Methanobacterium, dominated in the biofilm, suggesting that this genus is probably capable of carrying out AnMO. The abundance of Methylomonas, an aerobic methanotroph, decreased significantly, while Meiothermus, a potential chromate-reducing bacterium, was enriched in the biofilm. Overall, the results showed that the anaerobic environment inhibited the activity of aerobic methanotrophs while promoting AnMO bacterial enrichment, and high Cr(VI) loading reduced Cr(VI) flux by inhibiting the methane oxidation process.

Methane oxidation coupled to vanadate reduction in a membrane biofilm batch reactor under hypoxic condition

This study shows vanadate (V(V)) reduction in a methane (CH₄) based membrane biofilm batch reactor when the concentration of dissolved oxygen (O₂) was extremely low. V(IV) was the dominant products formed from V(V) bio-reduction, and majority of produced V(IV) transformed into precipitates with green color. Quantitative polymerase chain reaction and Illumina sequencing analysis showed that archaea methanosarcina were significantly enriched. Metagenomic predictive analysis further showed the enrichment of genes associated with reverse methanogenesis pathway, the key CH₄-activating mechanism for anaerobic methane oxidation (AnMO), as well as the enrichment of genes related to acetate synthesis, in archaea. The enrichment of aerobic methanotrophs Methylococcus and Methylomonas implied their role in CH₄ activation using trace level of O₂, or their participation in V(V) reduction.

Methane oxidation coupled to perchlorate reduction in a membrane biofilm batch reactor

A specially designed CH₄-based membrane biofilm batch reactor (MBBR) was applied to investigate anaerobic methane oxidation coupled to perchlorate reduction (AnMO-PR). The 0.21 mM ClO₄^- added in the first stage of operation was completely reduced in 28 days, 0.40 mM ClO₄^- was reduced within 23 days in stage 2, and 0.56 mM of ClO₄^- was reduced within 30 days in stage 3. Although some chlorate (ClO₃^-) accumulated, the recovery of Cl^- was over 92%. Illumina sequencing of the 16S rRNA gene documented that the bacterial community was mainly composed by perchlorate-reducing bacteria (PRB), methanotrophic bacteria, and archaea. Real-time quantitative PCR showed the archaeal 16S rRNA and mcrA genes increased as more ClO₄^- was reduced, and the predominant archaea belonged to Methanosarcina mazei, which is related to ANME-3, an archaeon able to perform reverse methanogenesis. Several pieces of evidence support that ClO₄^- reduction by the MBBR biofilm occurred via a synergism between Methanosarcina and PRB: Methanosarcina oxidized methane through reverse methanogesis and provided electron donor for PRB to reduce ClO₄^-. Because methanotrophs were present, we cannot rule out that they also were involved in AnMO-PR if they received O₂ generated by disproportionation of ClO₂^- from the PRB.