Morphological, Microstructural, and In Situ Chemical Characteristics of Siderite Produced by Iron-Reducing Bacteria.

Dissimilatory iron-reducing bacteria (DIRB) oxidize organic matter or hydrogen and reduce ferric iron to form Fe(II)-bearing minerals, such as magnetite and siderite. However, compared with magnetite, which was extensively studied, the mineralization process and mechanisms of siderite remain unclear. Here, with the combination of advanced electron microscopy and synchrotron-based scanning transmission X-ray microscopy (STXM) approaches, we studied in detail the morphological, structural, and chemical features of biogenic siderite via a growth experiment with Shewanella oneidensis MR-4. Results showed that along with the growth of cells, Fe(II) ions were increasingly released into solution and reacted with CO32- to form micrometer-sized siderite minerals with spindle, rod, peanut, dumbbell, and sphere shapes. They are composed of many single-crystal siderite plates that are fanned out from the center of the particles. Additionally, STXM revealed Fh and organic molecules inside siderite. This suggests that the siderite crystals might assemble around a Fh-organic molecule core and then continue to grow radially. This study illustrates the biomineralization and assembly of siderite by a successive multistep growth process induced by DIRB, also provides evidences that the distinctive shapes and the presence of organic molecules inside might be morphological and chemical features for biogenic siderite.

Green synthesis of Fe3O4@ceramsite from sludge improving anaerobic digestion performance of waste activated sludge.

Anaerobic digestion (AD) is a promising technique for waste management, which can achieve sludge stabilization and energy recovery. This study successfully prepared Fe3O4@ceramsite from WAS and applied it as an additive in sludge digestion, aiming to improve the conversion of organics to biomethane efficiency. Results showed that after adding the Fe3O4@ceramsite, the methane production was enhanced by 34.7% compared with the control group (88.0 ± 0.1 mL/g VS). Further mechanisms investigation revealed that Fe3O4@ceramsite enhanced digesta stability by strong buffering capacity, improved sludge conductivity, and promoted Fe (III) reduction. Moreover, Fe3O4@ceramsite has a larger surface area and better porous structure, which also facilitated AD performance. Microbial community analysis showed that some functional anaerobes related to AD such as Spirochaeta and Smithella were enriched with Fe3O4@ceramsite treatment. Potential syntrophic metabolisms between syntrophic bacteria (Syntrophomonas, associated with DIET) and methanogens were also detected in the Fe3O4@ceramsite treatment AD system.

Comparative investigations on the incorporation of biogenic Fe products into anaerobic granular sludge of different sources: Fe loading capacity, physicochemical properties, microbial community and long-term methanogenesis performance.

Anaerobic granular sludge (AGS) has been regarded as the core of lots of advanced anaerobic reactors. Formation of biogenic Fe products and their incorporation into AGS could influence interspecies electron transfer and methanogenesis performance. In this study, with anaerobic granular sludge (AGS) from different sources (brewery, chemical plant, paper mill, citric acid factory, and food factory) as the research targets, the formation of biogenic iron products in AGS through the biologically induced mineralization process was studied. Furthermore, the influences of physicochemical properties and microbial community on methanogenesis were investigated. Results showed that all the AGS of different sources possessed the capacity to form biogenic Fe products through dissimilatory iron-reduction process, and diverse Fe minerals including magnetite (Fe3O4), hematite (Fe2O3), goethite (FeOOH), siderite (FeCO3) and wustite (FeO) were incorporated into AGS. The AGS loaded with Fe minerals (Fe-AGS) showed increased conductivity, magnetism and zeta-potential comparing to the control. Those Fe-AGS of different sources demonstrated different methanogenesis performance during the long-term operation (50 days). Methane production was increased for the Fe-AGS of citric acid (6.99-32.50%), food (8.33-37.46%), chemical (2.81-7.22%) and brewery plants (2.27-2.81%), but decreased for the Fe-AGS of paper mill (54.81-72.2%). The changes of microbial community and microbial correlations in AGS as a response to Fe minerals incorporation were investigated. For the Fe-AGS samples with enhanced methane production capability, it was widely to find the enriched populations of fermentative and dissimilatory iron reducing bacteria Clostridium_sensu_stricto_6, Bacteroidetes_vadinHA17 and acetoclastic methanogens Methanosaeta, and positive correlations between them. This study provides comprehensive understanding on the effects of incorporation biogenic Fe products on AGS from different sources.

Unveiling the mechanisms of Fe(III)-loaded chitosan composite (CTS-Fe) in enhancing anaerobic digestion of waste activated sludge.

Anaerobic digestion (AD) of waste activated sludge (WAS) is usually limited by the low generation efficiency of methane. Fe(III)-loaded chitosan composite (CTS-Fe) have been reported to effectively enhanced the digestion of WAS, but its role in promoting anaerobic sludge digestion remains unclear. In present study, the effects of CTS-Fe on the hydrolysis and methanogenesis stages of WAS anaerobic digestion were investigated. The addition of CTS-Fe increased methane production potential by 8%-23% under the tested conditions with the addition of 5-20 g/L CTS-Fe. Besides, the results demonstrate that the addition of CTS-Fe could effectively promote the hydrolysis of WAS, evidenced by lower protein or polysaccharides concentration, higher soluble organic carbon in rector adding CTS-Fe, as well as the increased activity of extracellular hydrolase with higher CTS-Fe concentration. Meanwhile, the enrichment of Clostridia abundance (iron-reducing bacteria (IRBs)) was observed in CTS-Fe adding reactor (8.9%-13.8%), which was higher than that in the control reactor (7.9%). The observation further suggesting the acceleration of hydrolysis through dissimilatory iron reduction (DIR) process, thus providing abundant substrates for methanogenesis. However, the presence of CTS-Fe was inhibited the acetoclastic and hydrogenotrophic methanogenesis process, which could be ascribed to the Fe(III) act as electron acceptor coupled to methane for anaerobic oxidation. Furthermore, coenzyme F420 activity in the CTS-Fe added reactor was 34.9% lower than in the blank, also abundance of microorganisms involved in hydrogenotrophic methanogenesis was decreased. Results from this study could provide theoretical support for the practical applications of CTS-Fe.

Polycaprolactone-Modified Biochar Supported Nanoscale Zero-Valent Iron Coupling with Shewanella putrefaciens CN32 for 1,1,1-Trichloroethane Removal from Simulated Groundwater: Synthesis, Optimization, and Mechanism.

1,1,1-Trichloroethane (1,1,1-TCA) is a typical organochloride solvent in groundwater that poses threats to human health and the environment due to its carcinogenesis and bioaccumulation. In this study, a novel composite with nanoscale zero-valent iron (nZVI) supported by polycaprolac-tone (PCL)-modified biochar (nZVI@PBC) was synthesized via solution intercalation and liquid-phase reduction to address the 1,1,1-TCA pollution problem in groundwater. The synergy effect and improvement mechanism of 1,1,1-TCA removal from simulated groundwater in the presence of nZVI@PBC coupling with Shewanella putrefaciens CN32 were investigated. The results were as follows: (1) The composite surface was rough and porous, and PCL and nZVI were loaded uniformly onto the biochar surface as micro-particles and nanoparticles, respectively; (2) the optimal mass ratio of PCL, biochar, and nZVI was 1:7:2, and the optimal composite dosage was 1.0% (w/v); (3) under the optimal conditions, nZVI@PBC + CN32 exhibited excellent removal performance for 1,1,1-TCA, with a removal rate of 82.98% within 360 h, while the maximum removal rate was only 41.44% in the nZVI + CN32 treatment; (4) the abundance of CN32 and the concentration of adsorbed Fe(II) in the nZVI@PBC + CN32 treatment were significantly higher than that in control treatments, while the total organic carbon (TOC) concentration first increased and then decreased during the culture process; (5) the major improvement mechanisms include the nZVI-mediated chemical reductive dechlorination and the CN32-mediated microbial dissimilatory iron reduction. In conclusion, the nZVI@PBC composite coupling with CN32 can be a potential technique to apply for 1,1,1-TCA removal in groundwater.

Electrolocation? The evidence for redox-mediated taxis in Shewanella oneidensis.

Shewanella oneidensis is a dissimilatory metal reducing bacterium and model for extracellular electron transfer (EET), a respiratory mechanism in which electrons are transferred out of the cell. In the last 10 years, migration to insoluble electron acceptors for EET has been shown to be nonrandom and tactic, seemingly in the absence of molecular or energy gradients that typically allow for taxis. As the ability to sense, locate, and respire electrodes has applications in bioelectrochemical technology, a better understanding of taxis in S. oneidensis is needed. While the EET conduits of S. oneidensis have been studied extensively, its taxis pathways and their interplay with EET are not yet understood, making investigation into taxis phenomena nontrivial. Since S. oneidensis is a member of an EET-encoding clade, the genetic circuitry of taxis to insoluble acceptors may be conserved. We performed a bioinformatic analysis of Shewanella genomes to identify S. oneidensis chemotaxis orthologs conserved in the genus. In addition to the previously reported core chemotaxis gene cluster, we identify several other conserved proteins in the taxis signaling pathway. We present the current evidence for the two proposed models of EET taxis, "electrokinesis" and flavin-mediated taxis, and highlight key areas in need of further investigation.

Molecular evidence for the adaptive evolution of Geobacter sulfurreducens to perform dissimilatory iron reduction in natural environments.

The electrically conductive pili (e-pili) of Geobacter species enable extracellular electron transfer to insoluble metallic minerals, electrodes and other microbial species, which confers biogeochemical significance and global prevalence on Geobacter in diverse anaerobic environments. E-pili are constructed by truncated PilA which is considered to have evolved from full-length pilin by gene fission under positive evolutionary selection. However, this hypothesis is based on phylogenetic analysis and has not yet been experimentally confirmed. Here, we reconstructed an ancestral strain of G. sulfurreducens (designated COMB) carrying full-length PilA by combining genes GSU1496 and GSU1497. The results demonstrated that strain COMB expressed and assembled the full-length fused PilA and exhibited an outer membrane c-type cytochrome profile similar to the wild-type strain. Surprisingly, the generated COMB-pili were also conductive, indicating the evolution of truncated PilA did not occur for conductivity. Moreover, strain COMB minimally reduced Fe(III) oxides but maintained its ability to respire electrodes, demonstrating the truncation of pilin enables iron respiration. This study provides the first experimental evidence that the truncation of pilin in Geobacter species confers adaption to Fe(III)-mineral-mediated selective pressures, and suggests an evolutionary event during which the separation of the GSU1497 gene helped Geobacter survive and thrive in natural environments.

Effects of Dissolved Organic Matter on the Bioavailability of Heavy Metals During Microbial Dissimilatory Iron Reduction: A Review.

Dissolved organic matter (DOM), a type of mixture containing complex structures and interactions, has important effects on environmental processes such as the complexation and interface reactions of soil heavy metals. Furthermore, microbial dissimilatory iron reduction (DIR), a key process of soil biogeochemical cycle, is closely related to the migration and transformation of heavy metals and causes the release of DOM by carbon-ferrihydrite associations. This chapter considers the structural properties and characterization techniques of DOM and its interaction with microbial dissimilated iron. The effect of DOM on microbial DIR is specifically manifested as driving force properties, coprecipitation, complexation, and electronic shuttle properties. The study, in addition, further explored the influence of pH, microorganisms, salinity, and light conditions, mechanism of DOM and microbial DIR on the toxicity and bioavailability of different heavy metals. The action mechanism of these factors on heavy metals can be summarized as adsorption coprecipitation, methylation, and redox. Based on the findings of the review, future research is expected to focus on: (1) The combination of DOM functional group structure analysis with high-resolution mass spectrometry technology and electrochemical methods to determine the electron supply in the mechanism of DOM action on DIR; (2) Impact of DOM on differences in structure and functions of plant rhizosphere in heavy metal contaminated soil; and (3) Bioavailability of DOM-dissociative iron-reducing bacteria-heavy metal ternary binding on rhizosphere heavy metals under dynamic changes of water level from the perspective of the differences in DOM properties, such as polarity, molecular weight, and functional group.

Impact of organic acids and sulfate on the biogeochemical properties of soil from urban subsurface environments.

Urban subsurface environments are often different from undisturbed subsurface environments due to the impacts of human activities. For example, deterioration of underground infrastructure can introduce elevated levels of Ca, Fe, and heavy metals into subsurface soils and groundwater. Likewise, leakage from sewer systems can lead to contamination by organic C, N, S, and P. However, the impact of these organic and inorganic compounds on biogeochemical processes including microbial redox reactions, mineral transformations, and microbial community transitions in urban subsurface environments is poorly understood. Here we conducted a microcosm experiment with soil samples from an urban construction site to investigate the possible biotic and abiotic processes impacted when sulfate and acetate or lactate were introduced into an urban subsurface environment. In the top-layer soil (0-0.3 m) microcosms, which were highly alkaline (pH > 10), the major impact was on abiotic processes such as secondary mineral precipitation. In the mid-layer (2-3 m) soil microcosms, the rate of Fe(III)-reduction and the amount of Fe(II) produced were greatly impacted by the specific organic acid added, and sulfate-reduction was not observed until after Fe(III)-reduction was complete. Near the end of the incubation, some genera related to syntrophic acetate oxidation and methanogenesis were observed in the lactate-amended microcosms. In the bottom-layer (7-8 m) soil microcosms, the rate of Fe(III)-reduction and the amount of Fe(II) produced were affected by the concentration of amended sulfate. Sulfate-reduction was concurrent with Fe(III)-reduction, suggesting that Fe(II) production was likely due to abiotic reduction of Fe(III) by sulfide produced by microbial sulfate reduction. The slightly acidic initial pH (~5.8) of the mid-soil system was a major factor controlling sequential microbial Fe(III) and sulfate reduction versus parallel Fe(III) and sulfate reduction in the bottom soil system, which had a neutral initial pH (~7.2). 16S rRNA gene-based community analysis revealed a variety of indigenous microbial groups including alkaliphiles, dissimilatory iron and sulfate reducers, syntrophes, and methanogens tightly coupled with, and impacted by, these complex abiotic and biogeochemical processes occurring in urban subsurface environments.

Effects of dissimilatory iron reduction on acetate production from the anaerobic fermentation of waste activated sludge under alkaline conditions.

Anaerobic digestion of waste activated sludge (WAS) to produce acetate has recently attracted growing interest, while the slow hydrolytic acidification of sludge and the consumption of acetate by methanogens both decrease the yield of acetate. In this study, Fe3O4 was added to a WAS anaerobic digester under alkaline conditions (pH = 10). The concentration of short-chain fatty acids (SCFA) during WAS anaerobic fermentation was found to be affected positively by Fe3O4. The maximal SCFA production of the Fe3O4-added digester was 3619.4 mg/L, while the maximal SCFA production in the control was 2899.7 mg/L. The increase of SCFA with Fe3O4 was mainly resulted from the increase in acetate accumulation (accounting for 90%), because Fe3O4 stimulated dissimilatory iron reduction (DIR) that participated in the decomposition of complex organics and the transformation of pronionate and butyrate into acetate. Further investigation showed that each step of hydrolytic-acidification process was promoted except the homoacetogenesis. The activity of enzymes and abundance of microbes relevant to hydrolysis and acidification were in agreement with the above results.

Influence of iron (hydr)oxide mineralogy and contents in aquifer sediments on dissolved organic carbon attenuations during aquifer storage and recovery.

Aquifer storage and recovery (ASR) is a promising approach for managing water resources that enhances water quality through biogeochemical reactions occurring within aquifers. Iron (hydr)oxides, which are the predominant metallic oxides in soil, play a crucial role in degrading dissolved organic carbon (DOC), primarily through a process known as dissimilatory iron reduction (DIR). However, the efficiency of this reaction varies depending on the mineralogy and composition of the aquifer, and this understanding is essential for adequate water quality in ASR. The objective of this study is to investigate the impact of iron (hydr)oxide on acetate, as an organic carbon source, attenuation during the ASR. To achieve this, three sets of laboratory sediment columns were prepared, each containing a different type of iron (hydr)oxide minerals: ferrihydrite, goethite, and hematite. Following an acclimation period of 28 days to simulate the microcosm within an aquifer, the columns were continuously supplied with the simulated river water spiked with acetate (DOC 40-60 mg L-1), and the acetate concentration in the effluent was monitored. The result revealed that the column containing ferrihydrite achieved 97% acetate attenuation through DIR with anoxic conditions (DO < 0.1 mg L-1), while the goethite and hematite columns exhibited limited attenuation rates of 40 and 50%, respectively. Furthermore, the efficiency of acetate attenuation in the ferrihydrite columns increased with the content of ferrihydrite but experienced a rapidly declined at higher contents (3-4%), possibly due to the partial conversion of ferrihydrite to goethite as a result of the interaction between ferrihydrite and the Fe(II) produced during DIR. Additionally, an analysis of the microbial community demonstrated that microorganisms known to possess the ability to reduce iron (hydr)oxides under anaerobic conditions were abundant in the ferrihydrite columns.

New insights into inhibition of high Fe(III) content on anaerobic digestion of waste-activated sludge.

The impacts of the increased iron in the waste-activated sludge (WAS) on its anaerobic digestion were investigated. It was found that low Fe(III) content (< 750 mg/L) promoted WAS anaerobic digestion, while the continual increase of Fe(III) inhibited CH4 production and total chemical oxygen demand (TCOD) removal. As the Fe(III) content increased to 1470 mg/L, methane production has been slightly inhibited about 5 % compared with the group containing 35 mg/L Fe(III). Particularly, as Fe(III) concentration was up to 2900 mg/L, CH4 production, and TCOD removal decreased by 43.6 % and 37.5 %, respectively, compared with the group with 35 mg/L Fe(III). Furthermore, the percentage of CO2 of the group with 2900 mg/L Fe(III) decreased by 52.8 % compared with the group containing 35 mg/L Fe(III). It indicated that Fe(II) generated by the dissimilatory iron reduction might cause CO2 consumption, which was confirmed by X-ray diffraction that siderite (FeCO3) was generated in the group with 2900 mg/L Fe(III). Further study revealed that Fe(III) promoted the WAS solubilization and hydrolysis, but inhibited acidification and methane production. The methanogenesis test with H2/CO2 as a substrate showed that CO2 consumption weakened hydrogenotrophic methanogenesis and then increased H2 partial pressure, further causing VFA accumulation. Microbial community analysis indicated that the abundance of hydrogen-utilizing methanogens decreased with the high Fe(III) content. Our study suggested that the increase of Fe(III) in sludge might inhibit methanogenesis by consuming or precipitating CO2. To achieve maximum bioenergy conversion, the iron content should be controlled to lower than 750 mg/L. The study may provide new insights into the mechanistic understanding of the inhibition of high Fe(III) content on the anaerobic digestion of WAS.

Enhancing the microbial advanced oxidation of P-nitrophenol in sediment through accelerating extracellular respiration with electrical stimulation.

Microbial advanced oxidation, a fundamental process for pollutant degradation in nature, is limited in efficiency by the weak respiration of indigenous microorganisms. In this study, an electric field was employed to enhance microbial respiration and facilitate the microbial advanced oxidation of p-nitrophenol (PNP) in simulated wetlands with alternation of anaerobic and aerobic conditions. With intermittent air aeration, an electric field of 0.8 V promoted extracellular electron transfer to increase Fe2+ generation through dissimilatory iron reduction and the production of hydroxyl radicals (•OH) through Fenton-like reactions. As a result, the PNP removal rate of the electrically-stimulated group was higher than that of the control (72.15 % vs 46.88 %). Multiple lines of evidence demonstrated that the electrically-induced polarization of respiratory enzymes expedited proton-coupled electron transfer within the respiratory chain to accelerate microbial advanced oxidation of PNP. The polarization of respiratory enzymes with the electric field hastened proton outflow to increase cell membrane potential for adenosine triphosphate (ATP) generation, which enhanced intracellular electron transportation to benefit reactive oxygen species generation. This study provided a new method to enhance microelectrochemical remediation of the contaminant in wetlands via the combination of intermittent air aeration.

Mechanistic insights into sulfadimethoxine degradation via microbially driven Fenton reactions.

Biodegradation, while cost-effective, is hindered by the requirement for specialized microorganisms and co-contaminants. Innovative biological technologies like the microbially driven Fenton reaction, hold promise for enhancing degradation efficiency. However, the intricate biochemical processes and essential steps for effective degradation in such systems have remained unclear. In this study, we harnessed the potential of the microbially driven Fenton reaction by employing Shewanella oneidensis MR-1 (MR-1). Our approach showcased remarkable efficacy in degrading a range of contaminants, including sulfadimethoxine (SDM), 4,4'-dibromodiphenyl ether (BDE-15) and atrazine (ATZ). Using SDM as a model contaminant of emergent contaminants (ECs), we unveiled that biodegradation relied on the generation of hydroxyl radicals (•OH) and involvement of oxidoreductases. Transcriptomic analysis shed light on the pivotal components of extracellular electron transfer (EET) during both anaerobic and aerobic periods. The presence of reactive oxidizing species induced cellular damage and impeded DNA repair, thereby affecting the Mtr pathway of EET. Moreover, the formation of vivianite hindered SDM degradation, underscoring the necessity of maintaining iron ions in the solution to ensure sustainable and efficient degradation. Overall, this study offers valuable insights into microbial technique for ECs degradation, providing a comprehensive understanding of degradation mechanisms during aerobic/anaerobic cycling.

Synergetic effect of nitrate on dissolved organic carbon attenuation through dissimilatory iron reduction during aquifer storage and recovery.

Aquifer storage and recovery (ASR) is a promising water management technique in terms of quantity and quality. During ASR, iron (Fe) (hydr)oxides contained in the aquifer play a crucial role as electron acceptors in attenuating dissolved organic carbon (DOC) in recharging water through dissimilatory iron reduction (DIR). Considering the preference of electron acceptors, nitrate (NO3⁻), possibly coexisting with DOC as the prior electron acceptor to Fe (hydr)oxides, might influence DIR by interrupting electron transfer. However, this phenomenon is yet to be clarified. In this study, we systematically investigated the potential effect of NO3⁻ on DOC attenuation during ASR using a series of sediment columns representing typical aquifer conditions. The results suggest that DOC attenuation could be enhanced by the presence of NO3⁻. Specifically, total DOC attenuation was notably higher than that from the stoichiometric calculation simply employing NO3⁻ as the additional electron acceptor to Fe (hydr)oxides, implying a synergetic effect of NO3⁻ in the overall reactions. X-ray photoelectron spectroscopy analyzes revealed that the Fe(II) ions released from DIR transformed the Fe (hydr)oxides into a less bioavailable form, inhibiting further DIR. In the presence of NO3⁻, however, no aqueous Fe(II) was detected, and another form of Fe (hydr)oxide appeared on the sediment surface. This may be attributed to nitrate-dependent Fe(II) oxidation (NDFO), in which Fe(II) is (re)oxidized into Fe (hydr)oxide, which is available for the subsequent DOC attenuation. These mechanisms were supported by the dominance of DIR-relevant bacteria and the growth of NDFO-related bacteria in the presence of NO3⁻.

Charging-discharging cycles of geobattery activated carbon enhance iron reduction and vivianite recovery from wastewater.

Vivianite as a significant secondary mineral of dissimilatory iron reduction (DIR) exhibits marvelous potential to solve eutrophication as well as phosphorus shortage. Geobattery represents by natural organic matters (NOM) with rich functional groups influences bioreduction of natural iron mineral. Activated carbon (AC) which contains abundant functional groups is expected to serve as geobattery, but there remains insufficient understanding on its geobattery mechanism and how it benefits the vivianite formation. In this study, the charging and discharging cycle of "geobattery" AC enhanced extracellular electron transfer (EET) and vivianite recovery was demonstrated. Feeding with ferric citrate, AC addition increased vivianite formation efficiency by 141 %. The enhancement was attributed to the electron shuttle capacity of storage battery AC, which was contributed by the redox cycle between CO and O-H. Feeding with iron oxides, huge gap of redox potential between AC and Fe(III) minerals broke through the reduction energy barrier. Therefore the iron reduction efficiency of four Fe(III) minerals was accelerated to the same high level around 80 %, and the vivianite formation efficiency were increased by 104 %-256 % in pure culture batches. Except acting as storage battery, AC as a dry cell contributed 80 % to the whole enhancement towards iron reduction, in which O-H groups were the dominant driver. Due to the rechargeable nature and considerable electron exchange capacity, AC served as geobattery playing the role of both storage battery and dry cell on electron storaging and transferring to influence biogeochemical Fe cycle and vivianite recovery.

Sustainable disposal of Fenton sludge and enhanced organics degradation based on dissimilatory iron reduction in the hydrolytic acidification process.

Fenton sludge generated in the flocculation stage of the Fenton oxidation process contains significant amounts of ferric iron and organic pollutants, which require proper treatment. Previous studies have demonstrated that adding Fenton sludge to an anaerobic digester can decompose some of the organic pollutants in the Fenton sludge to lower its environmental risk, but iron gradually accumulates in the reactor, which weakens the sustainability of the method. In this study, Fenton sludge was introduced into a hydrolytic acidification reactor with a weak acid environment to relieve the iron accumulation as well as improve the degradation of organic matter. The results showed that the added Fenton sludge acted as an extracellular electron acceptor to induce dissimilatory iron reduction, which increased chemical oxygen demand (COD) removal and acidification efficiency by 16.1% and 19.8%, respectively, compared to the group without Fenton sludge. Along with the operation, more than 90% of the Fe(III) in Fenton sludge was reduced to Fe(II), and part of them was released to the effluent. Moreover, the Fe(II) in the effluent could be used as flocculants and Fenton reagents to further decrease the effluent COD by 29.8% and 44.5%, respectively. It provided a sustainable strategy to reuse Fenton sludge to enhance organic degradation based on the iron cycle.

Dissimilatory iron reduction contributes to anaerobic mineralization of sediment in a shallow transboundary lake.

Dissimilatory iron reduction (DIR) coupled with carbon cycling is increasingly being recognized as an influential process in freshwater wetland soils and sediments. The role of DIR in organic matter (OM) mineralization, however, is still largely unknown in lake sediment environments. In this study, we clarified rates and pathways of OM mineralization in two shallow lakes with seasonal hydrological connectivity and different eutrophic situations. We found that in comparison with the domination of DIR (55%) for OM mineralization in Lake Xiaoxingkai, the contribution of methanogenesis was much higher (68%) in its connected lake (Lake Xingkai). The differences in rates and pathways of sediment OM mineralization between the two lakes were attributed to higher concentrations of carbonate associated iron oxides (Fecarb) in Lake Xiaoxingkai compared to Lake Xingkai (P = 0.002), due to better deposition mixing, more contributions of terrigenous detrital materials, and higher OM content in Lake Xiaoxingkai. Results of structural equation modeling showed that Fecarb and total iron content (TFe) regulated 25% of DIR in Lake Xiaoxingkai and 76% in Lake Xingkai, accompanied by a negative effect of TFe on methanogenesis in Lake Xingkai. The relative abundance and diversity of Fe-reducing bacteria were significantly different between the two lakes, and showed a weak effect on sediment OM mineralization. Our findings emphasize the role of iron minerals and geochemical characterizations in regulating rates and pathways of OM mineralization, and deepen the understanding of carbon cycling in lake sediments.

Carbon nanotubes accelerates the bio-induced vivianite formation.

Vivianite widely existed in digested sludge and activated sludge as a potential phosphate resource recovered from wastewater treatment plants (WWTPs). As an important product of extracellular electron transfer (EET) and biological iron reduction, the production of vivianite can be enhanced by conductive materials. Carbon nanotubes (CNTs) with excellent electrical conductivity have been reported to promote electron transfer, which was applied in wastewater treatment to accelerate the degradation of the contaminants. However, the impact of CNTs on vivianite formation was barely reported. In this study, the iron reduction, vivianite recovery, and the biotoxicity of CNTs were investigated in order to determine the influence of CNTs towards the vivianite production. The enhancement of vivianite production after CNTs adding reached up to 17 % by promoting the electron transfer between dissimilative iron-reducing bacteria (DIRB) and Fe(III). However, at the initial stage (0-24 h), Fe(III) reduction efficiency decreased by 81 % after inoculating with sewage sludge, which was attributed to CNTs destroying of the cell membrane (as indicated by SEM, CLSM and AFM analysis). The biotoxicity of CNTs stimulated DIRB to secret extracellular polymeric substances (EPS) and form bio-flocs to resist the physical puncture. After 48 h, the proportion of living DIRB in 1000 mg/L CNTs batch increased to 98 %, which was 79 % higher than 12 h. As a result, the vivianite recovery of raw sewage with 1000 mg/L CNTs increased to 44 ± 1 %, which was 33 % higher than that in the CNT-0.

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

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