Zero valent iron simultaneously enhances methane production and sulfate reduction in anaerobic granular sludge reactors

Deciphering Fe@C amendment on long-term anaerobic digestion of sulfate and propionate rich wastewater: Driving microbial community succession and propionate metabolism

This study evaluated the reflection of long-term anaerobic system exposed to sulfate and propionate. Fe@C was found to efficiently mitigate anaerobic sulfate inhibition and enhance propionate degradation. With influent propionate of 12000mgCOD/L and COD/SO42- ratio of 3.0, methane productivity and sulfate removal were only 0.06 ± 0.02L/gCOD and 63 %, respectively. Fe@C helped recover methane productivity to 0.23 ± 0.03L/gCOD, and remove sulfate completely. After alleviating sulfate stress, less organic substrate was utilized to form extracellular polymeric substances for self-protection, which enhanced mass transfer in anaerobic sludge. Microbial community succession, especially for alteration of key sulfate-reducing bacteria and propionate-oxidizing bacteria, was driven by Fe@C, thus enhancing sulfate reduction and propionate degradation. Acetotrophic Methanothrix and hydrogenotrophic unclassified_f_Methanoregulaceae were enriched to promote methanogenesis. Regarding propionate metabolism, inhibited methylmalonyl-CoA degradation was a limiting step under sulfate stress, and was mitigated by Fe@C. Overall, this study provides perspective on Fe@C's future application on sulfate and propionate rich wastewater treatment.

L-tryptophan anaerobic fermentation for indole acetic acid production: Bacterial enrichment and effects of zero valent iron

Indole acetic acid (IAA) as a plant hormone, was one of the valuable products of anaerobic fermentation. However, the enriching method remained unknown. Moreover, whether zero valent iron (ZVI) could enhance IAA production was unexplored. In this work, IAA producing bacteria Klebsiella (63 %) was enriched successfully. IAA average production rate and concentration were up to 3 mg/L/h and 56 mg/L. With addition of 1 g/L ZVI, IAA average production rate and concentration was increased for 2 and 3 folds. Mechanisms indicated ZVI increased Na+K+-ATP activity and electron transport activity for 2 folds and 1 fold. Moreover, macro transcription determined indole pyruvate pathway activity like primary-amine oxidase, indole pyruvate decarboxylase and aldehyde dehydrogenase were increased for 146 %, 187 %, and 557 %, respectively. Therefore, ZVI was suitable for enhancement IAA production from mixed culture anaerobic fermentation.

Effects of zero-valent iron added in the flooding or drainage process on cadmium immobilization in an acid paddy soil

Zero-valent iron (ZVI) is a promising material for the remediation of Cd-contaminated paddy soils. However, the effects of ZVI added during flooding or drainage processes on cadmium (Cd) retention remain unclear. Herein, Cd-contaminated paddy soil was incubated for 40 days of flooding and then for 15 days of drainage, and the underlying mechanisms of Cd immobilization coupled with Fe/S/N redox processes were investigated. The addition of ZVI to the flooding process was more conducive to Cd immobilization. Less potential available Cd was detected by adding ZVI before flooding, which may be due to the increase in paddy soil pH and newly formed secondary Fe minerals. Moreover, the reductive dissolution of Fe minerals promoted the release of soil colloids, thereby increasing significantly the surface sites and causing Cd immobilization. Additionally, the addition of ZVI before flooding played a vital role in Cd retention after soil drainage. In contrast, the addition of ZVI in the drainage phase was not conducive to Cd retention, which might be due to the rapid decrease in soil pH that inhibited Cd adsorption and further immobilization on soil surfaces. The findings of this study demonstrated that Cd availability in paddy soil was largely reduced by adding ZVI during the flooding period and provide a novel insight into the mechanisms of ZVI remediation in Cd-contaminated paddy soils.

Mechanisms of anaerobic treatment of sulfate-containing organic wastewater mediated by Fe0 under different initial pH values

The anaerobic treatment of sulfide-containing organic wastewater (SCOW) is significantly affected by pH, causing dramatic decrease of treatment efficiency when pH deviates from its appropriate range. Fe0 has proved as an effective strategy on mitigating the impact of pH. However, systematic analysis of the influence mechanism is still lacking. To fill this gap, the impact of different initial pH values on anaerobic treatment efficiency of SCOW with Fe0 addition, the change of fermentation type and methanogens, and intra-extracellular electron transfer were explored in this study. The results showed that Fe0 addition enhanced the efficacy of anaerobic treatment of SCOW at adjusted initial pH values, especially at pH 6. Mechanism analysis showed that respiratory chain-related enzymes and electron shuttle secretion and resistance reduction were stimulated by soluble iron ions generated by Fe0 at pH 6, which accelerated intra-extracellular electron transfer of microorganisms, and ultimately alleviated the impact of acidic pH on the system. While at pH 8, Fe0 addition increased the acetogenic bacteria abundance, as well as optimized the fermentation type and improved the F420 coenzyme activity, resulting in the enhancement of treatment efficiency in the anaerobic system and remission of the effect of alkaline pH on the system. At the neutral pH, Fe0 addition had both advantages as stimulating the secretion of respiratory chain and electron transfer-related enzymes at pH 6 and optimizing the fermentation type pH 8, and thus enhanced the treatment efficacy. This study provides important insights and scientific basis for the application of new SCOW treatment technologies.

Long-term transformation of nanoscale zero-valent iron explains its biological effects in anaerobic digestion: From ferroptosis-like death to magnetite-enhanced direct electron transfer networks

Nanoscale zero-valent iron (nZVI) has been extensively used for environmental remediation and wastewater treatment. However, the biological effects of nZVI remain unclear, which is no doubt a result of the complexity of iron species and the dynamic succession of microbial community during nZVI aging. Here, the aging effects of nZVI on methanogenesis in anaerobic digestion (AD) were consecutively investigated, with an emphasis on deciphering the causal relationships between nZVI aging process and its biological effects. The addition of nZVI in AD led to ferroptosis-like death with hallmarks of iron-dependent lipid peroxidation and glutathione (GSH) depletion, which inhibited CH₄ production during the first 12 days of exposure. With prolonged exposure time, a gradual recovery (12-21 days) and even better performance (21-27 days) in AD were observed. The recovery performance of AD was mainly attributed to nZVI-enhanced membrane rigidity via forming siderite and vivianite on the outer surface of cells, protecting anaerobes against nZVI-induced toxicity. At the end of 27-days exposure, the significantly increased amount of conductive magnetite simulated direct interspecies electron transfer among syntrophic partners, improving CH₄ production. Metagenomic analysis further revealed that microbial cells gradually adapted to the aging of nZVI by upregulating functional genes related to chemotaxis, flagella, conductive pili and riboflavin biosynthesis, in which electron transfer networks likely thrived and the cooperative behaviors between consortium members were promoted. These results unveiled the significance of nZVI aging on its biological effects toward multiple microbial communities and provided fundamental insights into the long-term fates and risks of nZVI for in situ applications.

Hydrochar mediated anaerobic digestion of bio-wastes: Advances, mechanisms and perspectives

Bio-wastes treatment and disposal has become a challenge because of their increasing output. Given the abundant organic matter in bio-wastes, its related resource treatment methods have received more and more attention. As a promising strategy, anaerobic digestion (AD) has been widely used in the treatment of bio-wastes, during which not only methane as energy can be recovered but also their reduction can be achieved. However, AD process is generally disturbed by some internal factors (e.g., low hydrolysis efficiency and accumulated ammonia) and external factors (e.g., input pollutants), resulting in unstable AD operation performance. Recently, hydrochar was wildly found to improve AD performance when added to AD systems. This review comprehensively summarizes the research progress on the performance of hydrochar-mediated AD, such as increased methane yield, improved operation efficiency and digestate dewatering, and reduced heavy metals in digestate. Subsequently, the underlying mechanisms of hydrochar promoting AD were systematically elucidated and discussed, including regulation of electron transfer (ET) mode, microbial community structure, bio-processes involved in AD, and reaction conditions. Moreover, the effects of properties of hydrochar (e.g., feedstock, hydrothermal carbonization (HTC) temperature, HTC time, modification and dosage) on the improvement of AD performance are systematically concluded. Finally, the relevant knowledge gaps and opportunities to be studied are presented to improve the progress and application of the hydrochar-mediated AD technology. This review aims to offer some references and directions for the hydrochar-mediated AD technology in improving bio-wastes resource recovery.

A review on sulfur transformation during anaerobic digestion of organic solid waste: Mechanisms, influencing factors and resource recovery

Anaerobic digestion (AD) is an economical and environment-friendly technology for treating organic solid wastes (OSWs). OSWs with high sulfur can lead to the accumulation of toxic and harmful hydrogen sulfide (H₂S) during AD, so a considerable amount of studies have focused on removing H₂S emissions. However, current studies have found that sulfide induces phosphate release from the sludge containing iron‑phosphorus compounds (FePs) and the feasibility of recovering elemental sulfur (S⁰) during AD. To tap the full potential of sulfur in OSWs resource recovery, deciphering the sulfur transformation pathway and its influencing factors is required. Therefore, in this review, the sulfur species and distributions in OSWs and the pathway of sulfur transformation during AD were systematically summarized. Then, the relationship between iron (ferric compounds and zero-valent iron), phosphorus (FePs) and sulfur were analyzed. It was found that the reaction of iron with sulfide during AD drove the conversion of sulfide to S⁰ and iron sulfide compounds (FeS_x), and consequently iron was applied in sulfide abatement. In particular, ferric (hydr)oxide granules offer possibilities to improve the economic viability of hydrogen sulfide control by recovering S⁰. Sulfide is an interesting strategy to release phosphate from the sludge containing FePs for phosphorus recovery. Critical factors affecting sulfur transformation, including the carbon source, free ammonia and pretreatment methods, were summarized and discussed. Carbon source and free ammonia affected sulfur-related microbial diversity and enzyme activity and different sulfur transformation pathways in response to varying pretreatment methods. The study on S⁰ recovery, organic sulfur conversion, and phosphate release mechanism triggered by sulfur deserves further investigation. This review is expected to enrich our knowledge of the role of sulfur during AD and inspire new ideas for recovering phosphorus and sulfur resources from OSWs.

Decoupling Fe0 Application and Bioaugmentation in Space and Time Enables Microbial Reductive Dechlorination of Trichloroethene to Ethene: Evidence from Soil Columns

Fe⁰ is a powerful chemical reductant with applications for remediation of chlorinated solvents, including tetrachloroethene and trichloroethene. Its utilization efficiency at contaminated sites is limited because most of the electrons from Fe⁰ are channeled to the reduction of water to H₂ rather than to the reduction of the contaminants. Coupling Fe⁰ with H₂-utilizing organohalide-respiring bacteria (i.e., Dehalococcoides mccartyi) could enhance trichloroethene conversion to ethene while maximizing Fe⁰ utilization efficiency. Columns packed with aquifer materials have been used to assess the efficacy of a treatment combining in space and time Fe⁰ and aD. mccartyi-containing culture (bioaugmentation). To date, most column studies documented only partial conversion of the solvents to chlorinated byproducts, calling into question the feasibility of Fe⁰ to promote complete microbial reductive dechlorination. In this study, we decoupled the application of Fe⁰ in space and time from the addition of organic substrates andD. mccartyi-containing cultures. We used a column containing soil and Fe⁰ (at 15 g L^-1 in porewater) and fed it with groundwater as a proxy for an upstream Fe⁰ injection zone dominated by abiotic reactions and biostimulated/bioaugmented soil columns (Bio-columns) as proxies for downstream microbiological zones. Results showed that Bio-columns receiving reduced groundwater from the Fe⁰-column supported microbial reductive dechlorination, yielding up to 98% trichloroethene conversion to ethene. The microbial community in the Bio-columns established with Fe⁰-reduced groundwater also sustained trichloroethene reduction to ethene (up to 100%) when challenged with aerobic groundwater. This study supports a conceptual model where decoupling the application of Fe⁰ and biostimulation/bioaugmentation in space and/or time could augment microbial trichloroethene reductive dechlorination, particularly under oxic conditions.

Enhanced Cd(II) immobilization in sediment with zero-valent iron induced by hydrogenotrophic denitrification

In this study, an integrated system of Fe⁰ and hydrogenotrophic microbes mediated by nitrate (nitrate-mediated bio-Fe⁰, NMB-Fe⁰) was established to remediate Cd(II)-contaminated sediment. Solid phase characterization confirmed that aqueous Cd(II) (Cd(II)_aq) was successfully immobilized and enriched on iron surface due to promoted iron corrosion driven by hydrogenotrophic denitrification and subsequent greater biomineral production such as magnetite, lepidocrocite and green rust. Compared to a Cd(II)_aq removal of 21.1% in overlying water of the nitrate-mediated Fe⁰ (NM-Fe⁰) system, the NMB-Fe⁰ system obtained a much higher Cd(II)_aq removal of 83.1% after 7 d remediation. The leaching test and sequential extraction results also showed that the leachability of Cd(II) decreased by 75.9% while the residual fraction of Cd(II) increased by 185.7% in comparison with untreated sediment. Besides, the Cd(II)_aq removal raised with the increase of nitrate concentration and Fe⁰ dosage, further revealing the promotion effect of nitrate on Cd(II) removal by bio-Fe⁰. This study highlighted the involvement of bio-denitrification in the remediation of Cd(II)-contaminated sediment by Fe⁰ and provided a new insight to enhance its reactivity and applicability for Cd(II) immobilization.

Roles of zero-valent iron in anaerobic digestion: Mechanisms, advances and perspectives

With the rapid growth of population and urbanization, more and more bio-wastes have been produced. Considering organics contained in bio-wastes, to recover resource from bio-wastes is of great significance, which can not only achieve the resource recycle, but also protect the environment. Anaerobic digestion (AD) has been proved as one of the most promising strategies to recover bio-energy from bio-wastes, as well as to realize the reduction of bio-wastes. However, the conventional interspecies electron transfer is sensitive to environmental shocks, such as high ammonia, organic pollutants, metal ions, etc., which lead to instability or failure of AD. The recent findings have proved that the introduction of zero-valent iron (ZVI) in AD system can significantly enhance methane production from bio-wastes. This review systematically highlighted the recent advances on the roles of ZVI in AD, including underlying mechanisms of ZVI on AD, performance enhancement of AD contributed by ZVI, and impact factors of AD regulated by ZVI. Furthermore, current limitations and outlooks have been analyzed and concluded. The roles of ZVI on underlying mechanisms in AD include regulating reaction conditions, electron transfer mode and function of microbial communities. The addition of ZVI in AD can not only enhance bio-energy recovery and toxic contaminants removal from bio-wastes, but also have the potential to buffer adverse effect caused by inhibitors. Moreover, the electron transfer modes induced by ZVI include both interspecies hydrogen transfer and direct interspecies electron transfer pathways. How to comprehensively evaluate the effects of ZVI on AD and further improve the roles of ZVI in AD is urgently needed for practical application of ZVI in AD. This review aims to provide some references for the introduction of ZVI in AD for enhancing bio-energy recovery from bio-wastes.

Biogas upgrading to methane and removal of volatile organic compounds in a system of zero-valent iron and anaerobic granular sludge

The current study presented a novel process of biogas upgrading to biomethane (higher than 97%) based on anaerobic sludge and zero-valent iron (ZVI) system. When ZVI was added into an aquatic system with anaerobic granular sludge (AnGrSl) under anaerobic abiotic conditions, H₂ was generated. Then, the H₂ and CO₂ were converted by the hydrogenotrophic methanogens to CH₄. Biogas upgrading to biomethane was achieved in 4 days in the AnGrSl system (50 g L^-1 ZVI, initial pH 5 and 20 g L^-1 NaHCO₃). In this system, when zero-valent scrap iron (ZVSI) was added instead of ZVI, a more extended period (21 days) was required to achieve biogas upgrading. X-ray diffraction (XRD) analysis revealed that the materials in a reactor with CO₂ or biogas headspace, exhibited a mixture of ferrite and the iron carbonate phase of siderite (FeCO₃), with the latter being the dominant phase. VOCs analysis in raw biogas (in the system of anaerobic sludge and ZVI) highlighted the reduction of low mass straight- and branched-chain alkanes (C₆-C₁₀). Also, H₂S and NH₃ were found to be substantially reduced when the anaerobic sludge was exposed to ZVI compared to the cases where ZVI was not added. This study found that simultaneously with biogas upgrading, VOCs, H₂S and NH₃ can be removed in a system of ZVI or ZVSI and AnGrSl under aquatic anaerobic conditions.

Promotion of nitrogen removal in a zero-valent iron-mediated nitrogen removal system operated in co-substrate mode

In this study, the performance and mechanism of nitrogen removal were investigated in a zero-valent iron-mediated nitrogen removal system operated in co-substrate mode with sodium acetate as the organic carbon source. The results showed that the additional organic matter had the capacity to promote NH4+-N and total inorganic nitrogen (TIN) removal with efficiencies of 91.09% and 84.10%, and increases of 60.06% and 75.32% compared with the control group, respectively. The organic matter also stimulated the production of extracellular polymer substances that reduced the passivation and toxicity of iron to microorganisms. The ammonia oxidation activity was 2.5 times higher than that in the control group, and the anammonia oxidation activity and denitrification activity were substantially higher than in the control group with TIN removal efficiencies of 1.02 and 1.19 mgN/(gVSS·d), respectively. In addition, the organic matter increased the enrichment of the heterotrophic denitrification bacterium Diaphorobacter and facultative iron salt-based bacterium Dechloromonas.

Roles of Fe-C amendment on sulfate-containing pharmaceutical wastewater anaerobic treatment: Microbial community and sulfur metabolism

The effects of multiple two-phase anaerobic treatment involving acidification coupling Fe-C on sulfate-containing chemical synthesis-based pharmaceutical wastewater treatment were investigated. Fe-C was added as a filler with 25% vol. to acidogenic reactors for semi-continuous operation. The results suggested that Fe-C amendment promoted sulfate removal efficiency by 47.5% and shortened the reaction time by 50% in the acidogenic phase. With mitigation of sulfate inhibition, SCOD removal efficiency and methane production were further increased by 24.6% and 398% compared to direct raw wastewater anaerobic digestion, respectively, in methanogenic phase. The results of sulfate removal kinetics confirmed a 150% increase of removal rate in acidogenic phase. However, the apparent kinetic microbial sulfate removal constant without Fe-C amendment was maintained at approximately 0.06 h^-1. The Fe-C amendment not only increased the relative abundance of Methanothrix and Desulfovibrio for sulfate reduction but also enriched unclassified_p__Chloroflexi and unclassified_c__Deltaproteobacteria for acidification. Metagenomic results indicated that Fe-C enhanced dissimilatory sulfate reduction and PAPS synthesis of assimilatory step. The hydrogen sulfide production through the 3-mercaptopyruvate to pyruvate pathways was also enhanced. Butyrate-oxidizing genes were increased synchronously to convert butyrate to acetate.

Membrane disruption boosts iron overload and endogenous oxidative stress to inactivate Escherichia coli by nanoscale zero-valent iron

The inactivation of microorganisms by nanoscale zero-valent iron (nZVI) was extensively reported, but what happens inside the cells is rarely explored. Herein, we revealed that nZVI caused the drastic increase of intracellular iron concentrations, which subsequently catalyzed the Haber-Weiss reaction to produce high levels of endogenous reactive oxygen species (ROSs) and inactivated E. coli cells by oxidative damage of DNA, evidenced by the significantly higher inactivation efficiencies of E. coli mutant strains deficient in iron uptake regulation and DNA repair than the parental strain. The intracellular iron levels, endogenous ROSs levels and the inactivation efficiencies of E. coli were positively correlated. The permeabilized cytomembrane due to the close contact between nZVI and E. coli was responsible for the iron overload. This work demonstrates experimentally for the first time that nZVI causes iron overload and endogenous oxidative stress to inactivate E. coli, thus deepening our knowledge of the nZVI antimicrobial mechanism.

Multi-Agent Evolutionary Game in the Recycling Utilization of Sulfate-Rich Wastewater

Current industrial development has led to an increase in sulfate-rich industrial sewage, threatening industrial ecology and the environment. Incorrectly treating high-concentration sulfate wastewater can cause serious environmental problems and even harm human health. Water with high sulfate levels can be treated as a resource and treated harmlessly to meet the needs of the circular economy. Today, governments worldwide are working hard to encourage the safe disposal and reuse of industrial salt-rich wastewater by recycling sulfate-rich wastewater (SRW) resources. However, the conflict of interests between the SRW production department, the SRW recycling department, and the governments often make it challenging to effectively manage sulfate-rich wastewater resources. This study aims to use the mechanism of evolutionary game theory (EGT) to conduct theoretical modelling and simulation analysis on the interaction of the behaviour of the above three participants. This paper focuses on the impact of government intervention and the ecological behaviour of wastewater producers on the behavioural decisions of recyclers. The results suggest that the government should play a leading role in developing the SRW resource recovery industry. SRW producers protect the environment in the mature stage, and recyclers actively collect and recover compliant sulfate wastewater resources. Governments should gradually deregulate and eventually withdraw from the market. Qualified recyclers and environmentally friendly wastewater producers can benefit from a mature SRW resources recovery industry.

Recent Advances and Perspectives of Nanotechnology in Anaerobic Digestion: A New Paradigm towards Sludge Biodegradability

Anaerobic digestion (AD) is the strategy of producing environmentally sustainable bioenergy from waste-activated sludge (WAS), but its efficiency was hindered by low biodegradability. Hence, the usage of nanomaterials was found to be essential in enhancing the degradability of sludge due to its nanostructure with specific physiochemical properties. The application of nanomaterials in sludge digestion was thoroughly reviewed. This review focused on the impact of nanomaterials such as metallic nanoparticles, metal oxide nanoparticles, carbon-based nanomaterials, and nanocomposite materials in AD enhancement, along with the pros and cons. Most of the studies detailed that the addition of an adequate dosage of nanomaterial has a good effect on microbial activity. The environmental and economic impact of the AD enhancement process is also detailed, but there are still many existing challenges when it comes to designing an efficient, cost-effective AD digester. Hence, proper investigation is highly necessary to assess the potency of utilizing the nanomaterials in enhancing AD under various conditions.

Enhancing methanogenesis of anaerobic granular sludge by incorporating Fe/Fe oxides nanoparticles aided with biofilm disassembly agents and mediating redox activity of extracellular polymer substances

Anaerobic granular sludge (AGS) is a promising technology for organic wastewater treatment and energy recovery. In this study, three different kinds of Fe and Fe oxides nanoparticles (Fe₃O₄, Fe₂O₃ and ZVI) were tried to be incorporated into AGS through direct loading or aided with biofilm disassembly agents of norspermidine and D-tyrosine, which was aimed to enhance methane production capacity of AGS via increasing redox activity of extracellular polymer substance (EPS) and interspecies electron transfer. Despite the loading methods, incorporation of Fe and Fe oxides nanoparticles into AGS increased methane production capacity remarkably, with an enhancement of 36.49-85.17%, 20.37-204.95% and 189.71-243.32%, respectively, for the Fe₃O₄, Fe₂O₃ and ZVI loaded AGS. Pretreatment of AGS using biofilm disassembly agents helped to incorporate more Fe and Fe oxides into the inner structure of AGS, which further enhanced methane production capacity by 48.68% and 184.58%, respectively, for the Fe₃O₄ and Fe₂O₃ loaded AGS. Loading Fe and Fe oxides into AGS not only introduced exogenous conductive substances and Fe(III)/Fe(II) redox couples into EPS matrix of AGS, but also stimulated the production of redox active components of flavins and c-Cyts. All these factors may contribute to the reduced resistance of EPS, enhanced interspecies electron transfer and methane production capacity of AGS. This study provides a novel strategy and facile method to accelerate interspecies electron transfer and enhance methane production for matured AGS.

Zero-valent iron mediated alleviation of methanogenesis inhibition induced by organoarsenic roxarsone

Zero-valent iron (ZVI) can enhance anaerobic digestion, and has great potential to alleviate/eliminate methanogenesis inhibition. Little is known about the feasibility of utilizing ZVI to alleviate methanogenesis inhibition that is caused by typical animal feed additive roxarsone in livestock wastewater. In this study, the role of ZVI on alleviating roxarsone-induced methanogenic inhibition and its mechanisms were investigated. With the increase of roxarsone concentration from 5 to 50 mg/L, the inhibition of methanogenesis increased from 3.0% to 65.7%. This inhibition was alleviated by 80.7% and 57.2% when 1.0 and 10.0 g/L ZVI were added, respectively. Due to ZVI addition, an efficient arsenic immobilization onto ZVI (45.4-85.8%) was achieved mainly through the formation of FeAsO₄ precipitate and adsorption by ZVI. Under the function of ZVI, hydrogenotrophic methanogenic activity was obviously restored. The microbial community analysis indicates that the ZVI-regulated alleviation on the methanogenesis inhibition was attributed to the enrichment of Methanobacterium and Methanosarcina. The findings from this study demonstrate that ZVI addition is an effective way for treatment of organoarsenic-contaminated wastewater.

The role of iron-based nanoparticles (Fe-NPs) on methanogenesis in anaerobic digestion (AD) performance

Several strategies have been proposed to improve the performance of the anaerobic digestion (AD) process. Among them, the use of various nanoparticles (NPs) (e.g. Fe, Ag, Cu, Mn, and metal oxides) is considered one of the most effective approaches to enhance the methanogenesis stage and biogas yield. Iron-based NPs (zero-valent iron with paramagnetic properties (Fe⁰) and iron oxides with ferromagnetic properties (Fe₃O₄/Fe₂O₃) enhance microbial activity and minimise the inhibition effect in methanogenesis. However, comprehensive and up-to-date knowledge on the function and impact of Fe-NPs on methanogens and methanogenesis stages in AD is frequently required. This review focuses on the applicative role of iron-based NPs (Fe-NPs) in the AD methanogenesis step to provide a comprehensive understanding application of Fe-NPs. In addition, insight into the interactions between methanogens and Fe-NPs (e.g. role of methanogens, microbe interaction and gene transfer with Fe-NPs) beneficial for CH₄ production rate is provided. Microbial activity, inhibition effects and direct interspecies electron transfer through Fe-NPs have been extensively discussed. Finally, further studies towards detecting effective and optimised NPs based methods in the methanogenesis stage are reported.

Enhancing anaerobic digestion of pharmaceutical industries wastewater with the composite addition of zero valent iron (ZVI) and granular activated carbon (GAC)

Anaerobic digestion of pharmaceutical wastewater is challenged by its contained toxic compounds which limits the stability and efficiency of methane production and organic degradation. In this study, zero valent iron (ZVI) and granular activated carbon (GAC) were added with different strategies to improve anaerobic digestion of pharmaceutical wastewater. The results confirmed synergy effects of ZVI + GAC for both COD removal (increased by 13.4%) and methane production (increased by 11.0%). Furthermore, ZVI + GAC improved the removal of pharmaceutical intermediates, in particular, the residues (%) of dehydroepiandrosterone (DHEA) and 2,2'-methylenebis(6-tert-butyl-4-methylphenol) were only 30.48 ± 6.53 and 39.92 ± 4.50, and effectively reduced biotoxicity. The promoted results were attributed to the establishment of direct interspecies electron transfer (DIET). Microbial community analysis revealed that ZVI + GAC decreased species evenness and richness in bacterial whereas increased in archaeal. The relative abundance of acetotrophic methanogens decreased but hydrogenotrophic and methylotrophic methanogens increased, which broadening the pathway of methane production.

Iron-based biochar derived from waste-activated sludge enhances anaerobic digestion of synthetic salty organic wastewater for methane production

Herein, synthetic iron-based biochar was used in anaerobic digestion of synthetic salty organic wastewater for methane production. The iron-based biochar synthesized at different pyrolysis temperatures improved methane production. An optimal methane production of 551 ± 4.0 mL/L was achieved by adding iron-based biochar prepared at 700℃. The rate of hydrolysis-acidification and methanogenesis was promoted by iron-based biochar as the NaCl concentration was less than 20 g/L. However, the catalytic effect of iron-based biochar on methane production of saline wastewater failed during the NaCl concentration of 40 g/L due to the complete suppression of methanogenesis. Analyzing the methanogenic activity of iron-based biochar modified anerobic systems and characterizing the physical-chemical properties of iron-based biochar demonstrated that the iron oxides and/or zero-valent iron generated on the biochar surface increased methane production. This study highlights the potential benefits of iron-rich sludge-based biochar on enhanced anaerobic digestion and treatment of salty organic wastewater.

Application of pyritic sludge with an anaerobic granule consortium for nitrate removal in low carbon systems

Granules recovered from a highly reduced anaerobic digester were capable of active nitrogen removal in the absence of exogenous electron donors, averaging 0.25 mg mgNO3--N /gVSS/d over 546 days of operation. Electron mass balance indicated that about half the influent nitrate was converted to ammonia via DNRA and another half denitrified. This capacity was associated with an onion-like structure of multiple layers enriched in reduced iron and sulfur, and a complex microbial community shown by metagenomic sequencing to consist of multiple physiological groups and associated activities, including methanogenesis, denitrification, dissimilatory nitrate reduction to ammonia (DNRA), iron oxidation and reduction, and sulfur reduction and oxidation. Nitrate reduction was supported by both entrained organic material and reduced iron and sulfur species, corresponding to 2.13 mg COD/gVSS/d. Batch incubations showed that approximately 15% of denitrified nitrate was coupled to the oxidation of sulfur derived from both sulfate respiration and granular material enriched in iron-sulfide. Inhibition of sulfate reduction resulted in redirection of electron flow to methanogenesis and, in combination with other batch tests, showed that these granules supported a complex microbial community in which cryptic redox cycles linked carbon, sulfur, and iron oxidation with nitrate, sulfate, iron, and carbon dioxide reduction. This system shows promise for treatment of nitrate contaminated ground water without addition of an external organic carbon source as well as wastewater treatment in combination with (granular) sludge elimination leading in a net reduction of solid treatment costs.

L-Cysteine Synthase Enhanced Sulfide Biotransformation in Subtropical Marine Mangrove Sediments as Revealed by Metagenomics Analysis

High sulfides concentrations can be poisonous to environment because of anthropogenic waste production or natural occurrences. How to elucidate the biological transformation mechanisms of sulfide pollutants in the subtropical marine mangrove ecosystem has gained increased interest. Thus, in the present study, the sulfide biotransformation in subtropical mangroves ecosystem was accurately evaluated using metagenomic sequencing and quantitative polymerase chain reaction analysis. Most abundant genes were related to the organic sulfur transformation. Furthermore, an ecological model of sulfide conversion was constructed. Total phosphorus was the dominant environmental factor that drove the sulfur cycle and microbial communities. We compared mangrove and non-mangrove soils and found that the former enhanced metabolism that was related to sulfate reduction when compared to the latter. Total organic carbon, total organic nitrogen, iron, and available sulfur were the key environmental factors that effectively influenced the dissimilatory sulfate reduction. The taxonomic assignment of dissimilatory sulfate-reducing genes revealed that Desulfobacterales and Chromatiales were mainly responsible for sulfate reduction. Chromatiales were most sensitive to environmental factors. The high abundance of cysE and cysK could contribute to the coping of the microbial community with the toxic sulfide produced by Desulfobacterales. Collectively, these findings provided a theoretical basis for the mechanism of the sulfur cycle in subtropical mangrove ecosystems.

L-Cysteine Synthase Enhanced Sulfide Biotransformation in Subtropical Marine Mangrove Sediments as Revealed by Metagenomics Analysis

High sulfides concentrations can be poisonous to environment because of anthropogenic waste production or natural occurrences. How to elucidate the biological transformation mechanisms of sulfide pollutants in the subtropical marine mangrove ecosystem has gained increased interest. Thus, in the present study, the sulfide biotransformation in subtropical mangroves ecosystem was accurately evaluated using metagenomic sequencing and quantitative polymerase chain reaction analysis. Most abundant genes were related to the organic sulfur transformation. Furthermore, an ecological model of sulfide conversion was constructed. Total phosphorus was the dominant environmental factor that drove the sulfur cycle and microbial communities. We compared mangrove and non-mangrove soils and found that the former enhanced metabolism that was related to sulfate reduction when compared to the latter. Total organic carbon, total organic nitrogen, iron, and available sulfur were the key environmental factors that effectively influenced the dissimilatory sulfate reduction. The taxonomic assignment of dissimilatory sulfate-reducing genes revealed that Desulfobacterales and Chromatiales were mainly responsible for sulfate reduction. Chromatiales were most sensitive to environmental factors. The high abundance of cysE and cysK could contribute to the coping of the microbial community with the toxic sulfide produced by Desulfobacterales. Collectively, these findings provided a theoretical basis for the mechanism of the sulfur cycle in subtropical mangrove ecosystems.

Mini art review for zero valent iron application in anaerobic digestion and technical bottlenecks

Zero valent iron (ZVI) has been used extensively to control environmental pollution owing to its strong reducibility and low cost. Herein, we evaluate the impact of ZVI (iron scrap and ZVI powder with different scales) on anaerobic digestion (AD) reactor performance improvement and syntrophic relationship stimulation among various microbial groups in the methanogenesis process. In recent studies, ZVI addition significantly enhanced methane and volatile fatty acid (VFA) yields and alleviated excessive acidification, ammonia accumulation, and odorous gas production. Further, we reviewed the changes in enzyme activity and microbial metabolism after the addition of ZVI throughout the reaction process. Certain innovative technologies, such as bioelectrochemical system assistance and combined usage of conductive materials, may improve AD performance compared to the use of ZVI alone, the mechanism of which has been discussed from various viewpoints. Furthermore, the primary technical bottlenecks, such as poor mass transfer efficiency in dry AD and high ZVI dosage, have been illustrated, and syntrophic methanogenesis regulated by ZVI addition can be further studied by conducting theoretical research.

Enhanced refractory organics removal by sponge iron-coupled microbe technology: performance and underlying mechanism analysis

Sponge iron (SFe) is a zero-valent iron (Fe⁰) composite with a high-purity and porous structure. In this study, SFe was coupled with microorganisms that were gradually domesticated to form a Fe⁰/iron-oxidizing bacteria system (Fe⁰-FeOB system). The enhancement effect of the Fe⁰-FeOB system on refractory organics was verified, the mechanism of its strengthening action was investigated, and the relationship and influencing factors between the Fe⁰ and microorganisms were revealed. The average removal rates of the Fe⁰-FeOB system were 8.98%, 5.69%, and 40.67% higher than those of the SBR system for AF, AN, and NB wastewater treatment, respectively. With the addition of SFe, the microbial community structure was gradually enhanced with a large number of FeOB were detected. Moreover, the bacteria with strong iron corrosion and Fe(II) oxidation abilities plays a critical role in improving the Fenton-like effect. Interestingly, the variation trend of ⋅OH was fairly consistent with that of Fe(II). Thus, the main drivers of the Fenton-like effect are biological corrosion and metabolism. Consequently, microbial degradation and Fenton-like effect contributed to the degradation performance of the Fe⁰-FeOB system. Among them, the microbial degradation accounted for 96.09%, of which the biogenic Fenton effect accounted for 8.9%, and the microbial metabolic activity accounted for 87.19%. However, the augmentation of the Fe⁰-FeOB system was strongly dependent on SFe for the strengthening effect of microorganisms disappeared after leaving the SFe 35 days.

Core-shell ZVI@carbon composites reduce phosphate inhibition of ZVI dissolution and enhance methane production in an anaerobic sewage treatment

Inhibitory effects of phosphate on zero-valent iron (ZVI) dissolution have been studied mainly focusing on a single chemical system. Little is known about inhibitory effects and the mechanism of phosphate on ZVI dissolution within a bioreactor during long-term operation. This study demonstrates the feasibility of achieving energy recovery from phosphate-containing domestic sewage using an efficient anaerobic reactor with micro-sized or nano-sized ZVI addition. The results indicate that the chemical oxygen demand (COD) removal and methane production are enhanced by ZVI addition. A maximum COD removal efficiency of 89% and methane content of 60% was achieved. However, the strengthening effect of ZVI on methane production is weakened by the presence of phosphate in domestic sewage. Analyzing the variations of Fe²⁺ ions and phosphate concentrations and characterizing the micro-morphology of corroded ZVI proved that the generated Fe²⁺ ions reacts with phosphate and forms a passivation layer on the ZVI surface, inhibiting further dissolution of ZVI. As an improved alternative, we chose the double layered core-shell structured ZVI@carbon composite as an excellent candidate to reduce the inhibitory effects of phosphate on ZVI dissolution. In this way, the direct formation of precipitates on the ZVI surface can be avoided due to the protective carbon layer which adjusts the ion transfer. Adding ZVI@carbon composites accelerate the methane content by 16%. To our knowledge, this is the first report on adding ZVI@carbon composites to promote the anaerobic metabolism in studies, which are focusing on reducing the inhibition of ZVI by phosphate.

Oral Iron Supplementation—Gastrointestinal Side Effects and the Impact on the Gut Microbiota

Iron deficiency anaemia (IDA) is a worldwide healthcare problem affecting approximately 25% of the global population. The most common IDA treatment is oral iron supplementation, which has been associated with gastrointestinal (GI) side effects such as constipation and bloating. These can result in treatment non-adherence and the persistence of IDA. Intravenous iron does not cause GI side effects, which may be due to the lack of exposure to the intestinal lumen. Luminal iron can cause changes to the gut microbiota, aiding the promotion of pathogenic species and decreasing beneficial protective species. Iron is vital for methanogenic archaea, which rely on iron for growth and metabolism. Increased intestinal methane has been associated with slowing of intestinal transit, constipation, and bloating. Here we explore the literature to understand a potential link between iron and methanogenesis as a novel way to understand the mechanism of oral iron supplementation induced GI side effects.

Nano zero-valent iron: A pH buffer, electron donor and activator for chain elongation

Chain elongation produce medium chain carboxylates, which are important precursors to many pharmaceuticals, antimicrobials and biofuels. Results in the presented investigations show that the supply of nano zero-valent iron (NZVI) can enhance caproate production. The highest caproate concentration achieved amounted to 27.2 mmol/L when 5 g/L NZVI were added, which was about 100% higher than the control. The study also showed increase of ethanol oxidation and decrease of butyrate and butanol with NZVI addition. Mechanism study showed NZVI can stimulate caproate production by preventing pH to fall below 5.4 through displacement reaction. Electron balance analysis displayed that NZVI provides extra electron by promoting ethanol oxidation and its dissolution. H₂ was the potential electron shuttle between NZVI and chain elongators; High throughput sequencing showed function of NZVI on reshaping of microbial communities, especially enriching Oscillibacter Marseille-P3260, a kind of chain elongator and Corynebacterium which possesses fatty acid biosynthesis and iron utilization.

Integrated remediation of sulfate reducing bacteria and nano zero valent iron on cadmium contaminated sediments

Integrated-remediation technologies on heavy metal polluted sediments have received much attention. In this study, Cd contaminated sediments were treated with various conditions: sulfate reducing bacteria (SRB) only and SRB combined with different dosages of nano zero valent iron (nZVI (0.5-10 mg/g)). The immobilization of Cd was found in all remediation treatments according to the decreases of mobile Cd and the increases of more stable Cd compared with control. Five typical SRBs (Desulfobulbaceae, Desulfobacteraceae, Syntrophobacteraceae, Desulfovibrionaceae and Desulfomicrobiaceae) were identified having significant influences on Cd speciation transformation and they could stabilize Cd into sulfide precipitation through dissimilatory sulfate reduction (DSR). The ANOVA results of mobilization index and Cd concentration in overlying water both demonstrated that integrated-remediation systems with 5 mg/g and 10 mg/g of nZVI (Fe5 and Fe10 systems, respectively) presented better immobilization performance than conventional SRB only system (P < 0.05). It is confirmed that nZVI could stimulate the SRB bio-immobilization possibily through providing electrons and enhancing enzyme activities during DSR. The XPS analyses and Pourbaix diagrams revealed that mackinawite may be produced in the Fe10, resulting in the possible formation of Cd-S-Fe. This study indicates that integrated-remediation of SRB and nZVI have great potential in Cd immobilization of sediments, especially with higher addition of nZVI.

Performance prediction of ZVI-based anaerobic digestion reactor using machine learning algorithms

The use of zero-valent iron (ZVI) to enhance anaerobic digestion (AD) systems is widely advocated as it improves methane production and system stability. Accurate modeling of ZVI-based AD reactor is conducive to predicting methane production potential, optimizing operational strategy, and gathering reference information for industrial design in place of time-consuming and laborious tests. In this study, three machine learning (ML) algorithms, namely random forest (RF), extreme gradient boosting (XGBoost), and deep learning (DL), were evaluated for their feasibility of predicting the performance of ZVI-based AD reactors based on the operating parameters collected in 9 published articles. XGBoost demonstrated the highest accuracy in predicting total methane production, with a root mean squared error (RMSE) of 21.09, compared to 26.03 and 27.35 of RF and DL, respectively. The accuracy represented by mean absolute percentage error also showed the same trend, with 14.26%, 15.14% and 17.82% for XGBoost, RF and DL, respectively. Through the feature importance generated by XGBoost, the parameters of total solid of feedstock (TS_f), sCOD, ZVI dosage and particle size were identified as the dominant parameters that affect the methane production, with feature importance weights of 0.339, 0.238, 0.158, and 0.116, respectively. The digestion time was further introduced into the above-established model to predict the cumulative methane production. With the expansion of training dataset, DL outperformed XGBoost and RF to show the lowest RMSEs of 11.83 and 5.82 in the control and ZVI-added reactors, respectively. This study demonstrates the potential of using ML algorithms to model ZVI-based AD reactors.

Enhancement of microbial redox cycling of iron in zero-valent iron oxidation coupling with deca-brominated diphenyl ether removal

Iron-redox cycling microorganisms are important for understanding the biogeochemical iron and play key roles in zero-valent iron (ZVI) mediated environmental bioremediation. Their influence on ZVI oxidation coupling with organic contaminant removal is of particular interest but is still poorly understood. The objective of this research was to study microbial redox cycles of iron in ZVI oxidation and deca-brominated diphenyl ether (deca-BDE) removal. It was found that iron-oxidizing bacteria (IOB) enhanced ZVI oxidation by using iron as the sole electron donor. Iron-reducing bacteria (IRB) with high activity of Fe (III) reduction, also significantly accelerated rather than inhibited ZVI oxidation. ZVI oxidation activity was increased from 3.42% to 24.28% by IOB and 19.49% by IRB. When deca-BDE was present in the medium, ZVI oxidation activity by IOB and IRB was increased from 2.67% to 48.33% and 64.33%, respectively. However, no co-accelerating effect of IOB and IRB occurred but rather a neutralizing influence on ZVI oxidation was detected with iron-redox cycling bacteria (IORB). ZVI oxidation activity by IORB only increased to 13.14% and 37.0% in the absence and presence of deca-BDE, respectively. Meanwhile, IRB also exhibited the highest removal activity of deca-BDE. Approximately 71.67% of deca-BDE was removed by IRB, compared to 18.91% by IOB and 43.24% by IORB. Deca-BDE significantly influenced the effects of iron-metabolizing microorganisms on ZVI oxidation by altering the composition of microbial communities. Pseudomonas, Paenibacillus, and Sporolactobacillus were the key genera influencing ZVI oxidation and deca-BDE removal. Sporolactobacillus was firstly reported to be able to stimulate both ZVI oxidation and deca-BDE removal. Pseudomonas accelerated ZVI oxidation but had no significant contribution to deca-BDE removal. However, Paenibacillus inhibited both Fe(III) reduction and deca-BDE removal. It is expected that continuous integration of ZVI oxidation and organic contaminant removal can be achieved by regulating the key genera in iron-metabolizing microbial communities.

Synergistic Zerovalent Iron (Fe0) and Microbiological Trichloroethene and Perchlorate Reductions Are Determined by the Concentration and Speciation of Fe

Trichloroethene (TCE) and perchlorate (ClO₄^-) are cocontaminants at multiple Superfund sites. Fe⁰ is often used during TCE bioremediation with Dehalococcoides mccartyi to establish anoxic conditions in the aquifer. However, the synergy between Fe⁰ abiotic reactions and microbiological TCE and ClO₄^- reductions is poorly understood and seldom addressed in the literature. Here, we investigated the effects of Fe⁰ and its oxidation product, Fe²⁺, at field-relevant concentrations in promoting microbial TCE and ClO₄^- reductions. Using semibatch microcosms with a Superfund site soil and groundwater, we showed that the high Fe⁰ concentration (16.5 g L^-1) expected during Fe⁰in situ injection mostly yielded TCE abiotic reduction to ethene/ethane. However, such concentrations obscured dechlorination by D. mccartyi, impeded ClO₄^- reduction, and enhanced SO₄^2- reduction and methanogenesis. Fe²⁺ at 0.25 g L^-1 substantially delayed conversion of TCE to ethene when compared to no-Fe controls. A low concentration of aged-Fe⁰ synergistically promoted microbiological TCE dechlorination to ethene while achieving complete ClO₄^- reduction. Collectively, these results illustrate scenarios relevant at or downstream of Fe⁰ injection zones when Fe⁰ is used to facilitate microbial dechlorination. Results also underscore the potential detrimental effects of Fe⁰ and bioaugmentation cultures coinjection for in situ treatment of chlorinated ethenes and ClO₄^-.

Enhanced anaerobic co-digestion of waste activated sludge and food waste by sulfidated microscale zerovalent iron: Insights in direct interspecies electron transfer mechanism

The enhancement of zerovalent iron (ZVI) on anaerobic digestion (AD) has been proved, but there are still some problems that constrain the large-scale application of ZVI, such as the destruction of cell membrane and the inhibition of methanogenesis led by rapid H₂ accumulation. Aiming at these problems, sulfidated microscale zerovalent iron (S-mZVI) was employed to evaluate its effect on anaerobic co-digestion (AcoD) of waste activated sludge (WAS) and food waste (FW). Experimental results showed that S-mZVI promoted the direct interspecies electron transfer (DIET) between specific bacteria and methanogens, resulting in higher methane yield. At S-mZVI 10 g/L, the cumulative methane yield and ETS activity reached 264.78 mL/g-VS and 24.62 mg INTF/(g-TS h), which was 1.33 and 1.83 times that of blank. Microbiological analysis demonstrated that the abundance of DIET-related microorganisms such as Syntrophomonas, Methanosarcina and Methanobacterium increased with the increasing dosage of S-mZVI.

The bio-chemical cycle of iron and the function induced by ZVI addition in anaerobic digestion: A review

Zero-valent iron (ZVI) is known to be an additive in facilitating waste treatment and improving biogas production in anaerobic digestion (AD) systems. This review concentrates on the chemical cycle of iron as well as the function of the iron cycle in the removal of four kinds of pollutants: organic carbon, nitrogen, sulphur and phosphorus, which are commonly encountered in waste treatment. In recent studies, the addition of ZVI to an AD system promoted the in-situ production of CH₄ from CO₂, enabling carbon capture through biotechnology. Additionally, using iron-carbon microbial electrolytic cells in AD systems in order to accelerate electron transport, as well as specific pollutant degradation mechanisms, are illustrated in the present study. Particularly, the main factors affecting the removal efficiency of contaminants in a ZVI-AD system such as pH, VFA/ Alkalinity (ALK), oxidation-reduction potential and particle size are reviewed. According to the above characteristics, combined with technical model and economic analyses, an AD system based on ZVI was considered to be an economical, efficient and carbon-neutral pollutant treatment technology. Accordingly, Iron-based AD is suggested to be a promising and sustainable approach orientated to a circular economy, which may be applied to many waste treatments fields.

Improving CH4 production and energy conversion from CO2 and H2 feedstock gases with mixed methanogenic community over Fe nanoparticles

To achieve methanogenic community optimization and improve the conversion efficiency of CO₂ to CH₄, Fe nanoparticles were used to promote the Methanothermobacter abundance in methanogens, which significantly increased the conversion efficiency of CO₂ and H₂ feedstock gases to CH₄ product. High-throughput 16S rRNA gene sequencing analysis revealed that Methanothermobacter abundance markedly increased from 7 to 16% when the Fe nanoparticles concentration increased from 0 to 1.5 g/L_R (the working volume in the bioreactor). Therefore, the CH₄ yield significantly promoted from 0.105 to 0.186 L/L_R. However, when the Fe nanoparticles concentration was further increased to 2 g/L_R, methanogenesis was inhibited due to toxic effects. The electron transfer constant k_app of anaerobic sludge increased by 32.8-fold to 5.77 × 10^-2 s^-1 when the Fe nanoparticles concentration increased from 0 to 1.5 g/L_R, which significantly promoted carbon conversion efficiency from 52.9 to 92.9% and energy conversion efficiency from 46.3 to 76.9%.

Evolutions of antibiotic resistance genes (ARGs), class 1 integron-integrase (intI1) and potential hosts of ARGs during sludge anaerobic digestion with the iron nanoparticles addition

In this work, we investigated the impact of iron nanoparticle, including magnetite nanoparticles (Fe₃O₄ NPs) and nanoscale zero-valent iron (nZVI), on the anaerobic digestion (AD) performance. Moreover, the evolutions of antibiotic resistance genes (ARGs), class 1 integrons-integrase (intI1) and potential hosts of ARGs were also investigated. The optimal addition of Fe₃O₄ NPs and nZVI to promote methane production was 0.5 g/L and 1 g/L, which led to 22.07% and 23.02% increase in methane yield, respectively. The degradation rate of organic matter was also enhanced with the addition of Fe₃O₄ NPs or nZVI. The results of high-throughput sequencing showed that the reactors with iron NPs exhibited significant differences in microbial community structure, compared to the reactors with the non‑iron NPs. Iron NPs have caused the relative abundance of the dominant bacteria (Proteobacteria, Firmicutes and Actinobacteria) generally decreased, while the dominant archaea (Euryarchaeota) increased in AD sludge. Quantitative PCR results revealed that iron NPs accelerated the reductions in total absolute abundance of ARGs, especially a beta-lactamase resistance encoded gene (blaOXA). Network analysis displayed that the attenuation of ARGs was mainly attributed to the decline of potential hosts (Proteobacteria, Firmicutes and Actinobacteria). Meanwhile, environmental factors (such as pH, soluble chemical oxygen demand and heavy metals) were also strongly correlated with ARGs.

Effects of different conductive nanomaterials on anaerobic digestion process and microbial community of sludge

The effects of four conductive nanomaterials (nano-carbon powder, nano-Al₂O₃, nano-ZnO, nano-CuO) on sludge anaerobic digestion (AD) performance and microbial community were investigated through a 36-day fermentation experiment. Results showed that biogas production enhanced by 16.9% and 23.4% with nano-carbon powder and nano-Al₂O₃ added but decreased by 90.2% and 17.3% with nano-ZnO and nano-CuO. Total solids (TS) removal efficiency was increased by 38.73% and 27.11% with nano-carbon powder and nano-Al₂O₃ added but decreased by 70.67% and 43.70% with nano-ZnO and nano-CuO. Kinetic analysis indicated four conductive nanomaterials could shorten the lag phase of AD sludge with an average rate of 51.75%. 16S rRNA amplicon sequencing results demonstrated microbes such as Syntrophomonas and Methanosaeta were enriched in nano-carbon powder and nano-Al₂O₃ reactors. However, microbial community diversity and richness were both inhibited by adding nano-ZnO and nano-CuO. Redundancy analysis (RDA) revealed that genera belong to Firmicutes and Chloroflexi could conduce to methanogenesis process.

Theoretical framework for the estimation of H2S concentration in biogas produced from complex sulfur-rich substrates

A theoretical framework was developed and validated for the estimation of H₂S concentration in biogas produced from complex sulfur-rich effluents. The modeling approach was based on easy-to-obtain data such as biological biogas potential (BBP), chemical oxygen demand, and total sulfur content. Considering the few data required, the model fitted well the experimental H₂S concentrations obtained from BBP tests and continuous bioreactors reported in the literature. The model supported a correlation coefficient (R²) of 0.989 over the experimental data, obtaining average and maximum errors of ~ 25 and ~ 35%, respectively. The theoretical framework yielded good estimations for a wide range of experimental H₂S concentrations (0.2 to 4.5% in biogas). This modeling approach is, therefore, a useful tool towards anticipating the H₂S concentration in biogas produced from sulfur-rich substrates and deciding whether the installation of a desulfurization technology is required or not.

Advances in heavy metal removal by sulfate-reducing bacteria

Industrial development has led to generation of large volumes of wastewater containing heavy metals, which need to be removed before the wastewater is released into the environment. Chemical and electrochemical methods are traditionally applied to treat this type of wastewater. These conventional methods have several shortcomings, such as secondary pollution and cost. Bioprocesses are gradually gaining popularity because of their high selectivities, low costs, and reduced environmental pollution. Removal of heavy metals by sulfate-reducing bacteria (SRB) is an economical and effective alternative to conventional methods. The limitations of and advances in SRB activity have not been comprehensively reviewed. In this paper, recent advances from laboratory studies in heavy metal removal by SRB were reported. Firstly, the mechanism of heavy metal removal by SRB is introduced. Then, the factors affecting microbial activity and metal removal efficiency are elucidated and discussed in detail. In addition, recent advances in selection of an electron donor, enhancement of SRB activity, and improvement of SRB tolerance to heavy metals are reviewed. Furthermore, key points for future studies of the SRB process are proposed.

Optimization ofS/Fe ratio for enhanced nitrobenzene biological removal in anaerobicSystem amended withSulfide-modified nanoscale zerovalent iron

Anaerobic reduction of nitrobenzene (NB) can be efficiently enhanced bySupplementing withSulfide-modified nanoscale zerovalent iron (S-nZVI). In thisStudy,S/Fe ratio ofS-nZVI was further optimized for enhancing biological NB removal in anaerobicSystem amended withS-nZVI and inoculated by anaerobicSludge. The results indicated that the performance andStability of the coupled anaerobicSystem for NB reduction and aniline formation were remarkably improved byS-nZVI atS/Fe molar ratio of 0.3 (0.3S-nZVI). TheSecretion of extracellular polymericSubstances (EPS), transformation of volatile fatty acids (VFAs), yield of methane and activity ofSeveral key enzymes could be efficiently improved by 0.3S-nZVI. Furthermore,Species related to NB reduction, fermentation, electroactivity and methanogenesis could be enriched in 0.3S-nZVI coupled anaerobicSystem, with remarkable improvement in the biodiversity observed. ThisStudy demonstrated thatSulfidation would be a promising method to improve the performance of nZVI in coupled anaerobicSystems for the removal of recalcitrant nitroaromatic compounds from wastewater.

Improved Algal Sludge Methane Production and Dewaterability by Zerovalent Iron-Assisted Fermentation

This study investigated the methane production improvement of algal sludge by zerovalent iron (ZVI)-assisted anaerobic digestion. The zerovalent iron added were 0.5, 2, 5, 10, and 20 g·ZVI/g·TS (total solid). The results indicated that the addition of ZVI at 2, 5, 10, and 20 g·ZVI/g·TS has improved the methane production 1.07, 1.24, 1.41, and 1.46 times as compared with no ZVI added. The dewaterability of treated algal sludge has improved 1.06, 1.08, 1.08, and 1.11 times as compared with no ZVI addition. The biochemical methane production test results fitted to both one-substrate and two-substrate models. The one-substrate model indicated that the hydrolysis rate k has increased 8.21, 7.07, 9.39, 3.50, and 5.07 times as compared with R1 where no ZVI was added. The two-substrate model implied that the rapid hydrolysis rate k _rapid values were 5.23, 4.5, 5.98, 2.23, and 3.23 times as compared with R1. The one-substrate model predicted that the value of methane production was in high correlation with the actual value (R ² > 0.98). The addition of ZVI in algal sludge for methane production without an extra pretreatment process has improved the hydrolysis rate and methane production. This has the potential to be developed as an effective and economic technology in resource recovery from algal sludge.

Enhanced methane production by alleviating sulfide inhibition with a microbial electrolysis coupled anaerobic digestion reactor

Anaerobic digestion (AD) of organics is a challenging task under high-strength sulfate (SO₄^2-) conditions. The generation of toxic sulfides by SO₄^2--reducing bacteria (SRB) causes low methane (CH₄) production. This study investigated the feasibility of alleviating sulfide inhibition and enhancing CH₄ production by using an anaerobic reactor with built-in microbial electrolysis cell (MEC), namely ME-AD reactor. Compared to AD reactor, unionized H₂S in the ME-AD reactor was sufficiently converted into ionized HS^- due to the weak alkaline condition created via cathodic H₂ production, which relieved the toxicity of unionized H₂S to methanogenesis. Correspondingly, the CH₄ production in the ME-AD system was 1.56 times higher than that in the AD reactor with alkaline-pH control and 3.03 times higher than that in the AD reactors (no external voltage and no electrodes) without alkaline-pH control. MEC increased the amount of substrates available for CH₄-producing bacteria (MPB) to generate more CH₄. Microbial community analysis indicated that hydrogentrophic MPB (e.g. Methanosphaera) and acetotrophic MPB (e.g. Methanosaeta) participated in the two major pathways of CH₄ formation were successfully enriched in the cathode biofilm and suspended sludge of the ME-AD system. Economic revenue from increased CH₄ production totally covered the cost of input electricity. Integration of MEC with AD could be an attractive technology to alleviate sulfide inhibition and enhance CH₄ production from AD of organics under SO₄^2--rich condition.

Impact of coexistence of sludge flocs on nitrous oxide production in a granule-based nitrification system: A model-based evaluation

A common operational status of granule-based reactor is the inevitable coexistence of sludge flocs. Such hybrid system could have a profound impact on nitrous oxide (N₂O) production in nitrifying process. In this work, a mathematical model is employed to evaluate the key role of the coexistence of sludge flocs on N₂O production in a granule-based nitrifying system for the first time, by considering both nitrifier denitrification and hydroxylamine oxidation pathways. The modelling results show that the N₂O production gradually decreases with the increase of the percentage of sludge flocs in the total biomass (10-60%). More N₂O is tended to be generated in sludge flocs which has lower N₂O production capacity compared to granular biomass, thus lowering the total N₂O production. The relative contributions of two N₂O production pathways are only affected by bulk dissolved oxygen (DO) for the sludge flocs in the hybrid system, whereas those are affected by both bulk DO and the fractions of sludge flocs for the granular biomass. The results reveal a substantial effect of the coexistence of sludge flocs on N₂O production in granule-based nitrifying process, which should not be ignored in future design and operation.

Influence of nanoscale zero-valent iron and magnetite nanoparticles on anaerobic digestion performance and macrolide, aminoglycoside, β-lactam resistance genes reduction

The effect of nanoscale zero-valent iron (NZVI) and magnetite nanoparticles (Fe₃O₄ NPs) on anaerobic digestion (AD) performance was investigated through a series of 100-day semi-continuous mesophilic anaerobic digestions. The results indicated that biogas production had increased by 24.44% and 21.66% with the addition of 0.5 g/L Fe₃O₄ NPs and 1.0 g/L NZVI, respectively. Besides, the abundance of five widespread antibiotic resistance genes (ARGs) (ermF, ermA, ermT, aac(6')-IB, blaOXA-1) was also studied. The decrease in abundance of aac(6')-IB and blaOXA-1 was observed during the AD process with an average removal rate of 95.69% and 44.82%, respectively. Most of the ARGs, especially ermA and ermT, were less abundant in NZVI group compared with control group. The overall results suggested that the addition of NZVI and Fe₃O₄ NPs contributed to a better sludge anaerobic digestion performance, and NZVI was beneficial to the removal of some ARGs.

Continuous sulfur biotransformation in an anaerobic-anoxic sequential batch reactor involving sulfate reduction and denitrifying sulfide oxidization

The pathways and intermediates of continuous sulfur biotransformation in an anaerobic and anoxic sequential batch reactor (AA-SBR) involving sulfate reduction (SR) and denitrifying sulfide oxidization (DSO) were investigated. In the anoxic phase, DSO occurred in two sequential steps, the oxidation of sulfide (S^2-) to elemental sulfur (S⁰) and the oxidation of S⁰ to sulfate (SO₄^2-). The oxidation rate of S^2- to S⁰ was 3.31 times faster than that of S⁰ to SO₄^2-, resulting in the accumulation of S⁰ as a desired intermediate under S^2--S/NO₃^--N ratio (molar ratio) of 0.9:1. Although, approximately 60% of generated S⁰ suspended in the effluent, about 40% of S⁰ retained in the sludge, which could be further oxidized or reduced in anoxic or anaerobic phase. In anoxic, S⁰ was subsequently oxidized to SO₄^2- under S^2--S/NO₃^--N ratio of 0.5:1. In anaerobic, S⁰ coexist with SO₄^2- (in fresh wastewater) were simultaneously reduced to S^2-, and the reduction rate of SO₄^2- to S^2- was 3.17 times faster than that of S⁰ to S^2-, resulting in a higher production of S⁰ in subsequent anoxic phase. Microbial community analysis indicated that SO₄^2-/S⁰-reducing bacteria (e.g. Desulfomicrobium and Desulfuromonas) and S^2-/S⁰-oxidizing bacteria (e.g. Paracoccus and Thermothrix) co-participated in continuous sulfur biotransformation in the AA-SBR. A conceptual model was established to describe these main processes and key intermediates. The research offers a new insight into the reaction processes optimization for S⁰ recovery and simultaneous removal of SO₄^2- and NO₃^- in an AA-SBR.

Synergistic effect of magnetite and zero-valent iron on anaerobic degradation and methanogenesis of phenol

Anaerobic digestion is widely employed for treating phenol-containing wastewater, but there are still some drawbacks such as slow phenol degradation rate and vulnerable acetoclastic methanogens. Coupling of magnetite (Fe₃O₄) and zero valent iron (ZVI) was firstly used to enhance anaerobic digestion of phenol. The results indicated an obvious synergistic effect was generated with coupling of Fe₃O₄ and ZVI during the whole anaerobic digestion of phenol. The phenol degradation rate and methane production of Fe₃O₄/ZVI-added group were increased by 8.8-23.1% and 11.9-31.6%, respectively compared with Fe₃O₄-added group, and enhanced by 5.9-17.1% and 4.4-18.3%, respectively compared with ZVI-added group. ZVI improved the growth of hydrogenotrophic methanogens and Fe₃O₄ enhanced the growth of syntrophic acetate-oxidizing bacteria. Finally, the syntrophic interaction between acetate-oxidizing bacterium and hydrogenotrophic methanogens played a vital role on the synergistic effect of Fe₃O₄ and ZVI on the whole anaerobic phenol digestion.