Green rust formation during Fe(II) oxidation by the nitrate-reducing Acidovorax sp. strain BoFeN1

Remediation of arsenic- and nitrate-contaminated groundwater through iron-dependent autotrophic denitrifying culture

Groundwater contamination with arsenic and nitrate poses a pressing concern for the safety of local communities. Bioremediation, utilizing Fe(II)-oxidizing nitrate reducing bacteria, shows promise as a solution to this problem. However, the relatively weak environmental adaptability of a single bacterium hampers practical application. Therefore, this study explored the feasibility and characteristics of a mixed iron-dependent autotrophic denitrifying (IDAD) culture for effectively removing arsenic and nitrate from synthetic groundwater. The IDAD biosystem exhibited stable performace and arsenic resistance, even at a high As(III) concentration of 800 μg/L. Although the nitrogen removal efficiency of the IDAD biosystem decreased from 71.4% to 64.7% in this case, the arsenic concentration in the effluent remained below the standard (10 μg/L) set by WHO. The crystallinity of the lepidocrocite produced by the IDAD culture decreased with increasing arsenic concentration, but the relative abundance of the key iron-oxidizing bacteria norank_f_Gallionellaceae in the culture showed an opposite trend. Metagenomic analysis revealed that the IDAD culture possess arsenic detoxification pathways, including redox, methylation, and efflux of arsenic, which enable it to mitigate the adverse impact of arsenic stress. This study provides theoretical understanding and technical support for the remediation of arsenic and nitrate-contaminated groundwater using the IDAD culture.

In situ incubation of iron(II)-bearing minerals and Fe(0) reveals insights into metabolic flexibility of chemolithotrophic bacteria in a nitrate polluted karst aquifer

Groundwater nitrate pollution is a major reason for deteriorating water quality and threatens human and animal health. Yet, mitigating groundwater contamination naturally is often complicated since most aquifers are limited in bioavailable carbon. Since metabolically flexible microbes might have advantages for survival, this study presents a detailed description and first results on our modification of the BacTrap© method, aiming to determine the prevailing microbial community's potential to utilize chemolithotrophic pathways. Our microbial trapping devices (MTDs) were amended with four different iron sources and incubated in seven groundwater monitoring wells for ∼3 months to promote growth of nitrate-reducing Fe(II)-oxidizing bacteria (NRFeOxB) in a nitrate-contaminated karst aquifer. Phylogenetic analysis based on 16S rRNA gene sequences implies that the identity of the iron source influenced the microbial community's composition. In addition, high throughput amplicon sequencing revealed increased relative 16S rRNA gene abundances of OTUs affiliated to genera such as Thiobacillus, Rhodobacter, Pseudomonas, Albidiferax, and Sideroxydans. MTD-derived enrichments set up with Fe(II)/nitrate/acetate to isolate potential NRFeOxB, were dominated by e.g., Acidovorax spp., Paracoccus spp. and Propionivibrio spp. MTDs are a cost-effective approach for investigating microorganisms in groundwater and our data not only solidifies the MTD's capacity to provide insights into the metabolic flexibility of the aquifer's microbial community, but also substantiates its metabolic potential for anaerobic Fe(II) oxidation.

Microbial response and recovery strategy of the anammox process under ciprofloxacin stress from pure strain and consortia perspectives

Ciprofloxacin (CIP) poses a high risk of resistance development in water environments. Therefore, comprehensive effects and recovery strategies of CIP in anaerobic ammonia oxidation (anammox) process were systematically elucidated from consortia and pure strains perspectives. The anammox consortia was not significantly affected by the stress of 10 mg L-1 CIP, while the higher concentration (20 mg L-1) of CIP caused a dramatic reduction in the nitrogen removal performance of anammox system. Simultaneously, the abundances of dominant functional bacteria and corresponding genes also significantly decreased. Such inhibition could not be mitigated by the recovery strategy of adding hydrazine and hydroxylamine. Reducing nitrogen load rate from 5.1 to 1.4 kg N m-3 d-1 promoted the restoration of three reactors. In addition, the robustness and recovery of anammox systems was evaluated using starvation and shock strategies. Simultaneously, antibiotic resistance genes and key metabolic pathways of anammox consortia were upregulated, such as carbohydrate and energy metabolisms. In addition, 11 pure stains were isolated from the anammox system and identified through phylogenetic analysis, 40 % of which showed multidrug resistance, especially Pseudomonas. These findings provide deep insights into the responding mechanism of anammox consortia to CIP stress and promote the application of anammox process for treating wastewater containing antibiotics.

Boosting the denitrification efficiency of iron-based constructed wetlands in-situ via plant biomass-derived biochar: Intensified iron redox cycle and microbial responses

Considering the unsatisfied denitrification performance of carbon-limited wastewater in iron-based constructed wetlands (ICWs) caused by low electron transfer efficiency of iron substrates, utilization of plant-based conductive materials in-situ for improving the long-term reactivity of iron substrates was proposed to boost the Fe (III)/Fe (II) redox cycle thus enhance the nitrogen elimination. Here, we investigated the effects of withered Iris Pseudacorus biomass and its derived biochar on nitrogen removal for 165 days in ICWs. Results revealed that accumulate TN removal capacity in biochar-added ICW (BC-ICW) increased by 14.7 % compared to biomass-added ICW (BM-ICW), which was mainly attributed to the synergistic strengthening of iron scraps and biochar. The denitrification efficiency of BM-ICW improved by 11.6 % compared to ICWs, while its removal capacity declined with biomass consumption. Autotrophic and heterotrophic denitrifiers were enriched in BM-ICW and BC-ICW, especially biochar increased the abundance of electroactive species (Geobacter and Shewanella, etc.). An active iron cycle exhibited in BC-ICW, which can be confirmed by the presence of more liable iron minerals on iron scraps surface, the lowest Fe (III)/Fe (II) ratio (0.51), and the improved proportions of iron cycling genes (feoABC, korB, fhuF, TC.FEV.OM, etc.). The nitrate removal efficiency was positively correlated with the nitrogen, iron metabolism functional genes and the electron transfer capacity (ETC) of carbon materials (P < 0.05), indicating that redox-active carbon materials addition improved the iron scraps bioavailability by promoting electron transfer, thus enhancing the autotrophic nitrogen removal. Our findings provided a green perspective to better understand the redox properties of plant-based carbon materials in ICWs for deep bioremediation in-situ.

Acidovorax PSJ13, a novel, efficient polyacrylamide-degrading bacterium by cleaving the main carbon chain skeleton without the production of acrylamide

Given the environmental challenge caused by the wide use of polyacrylamide (PAM), an environmental-friendly treatment method is required. This study demonstrates the role of Acidovorax sp. strain PSJ13 isolated from dewatered sludge in efficiently degrading PAM. To be specific, the strain PSJ13 can degrade 51.67% of PAM in 96 h (2.39 mg/(L h)) at 35 °C, pH 7.5 and 5% inoculation amount. Besides, scanning electron microscope, X-ray photoelectron spectroscopy, liquid chromatography-mass spectrometry and high-performance liquid chromatography were employed to analyze samples, and the nitrogen present in the degradation products was investigated. The results showed that the degradation of PAM by PSJ13 started from the side chain and then mainly the -C-C- main chain, which produced no acrylamide monomers. As the first study to report the role of Acidovorax in efficiently degrading PAM, this work may provide a solution for industries that require PAM management.

Highly Efficient Coremoval of Nitrate and Phosphate Driven by a Sulfur-Siderite Composite Reactive Filler toward Secondary Effluent Polishing

Reactive fillers consisting of reduced sulfur and iron species (SFe-ReFs) have received increasing attention in tertiary wastewater treatment for nitrate and phosphate coremoval. However, the existing SFe-ReFs suffer from either low performance (e.g., pyrrhotite and pyrite) or unsatisfactory use in terms of combustible risk and residual nonreactive impurities (e.g., sulfur mixing with natural iron ores). Here, we developed a new type of sulfur-siderite composite ReF (SSCReF) with a structure of natural siderite powders eventually embedded into sulfur. SSCReFs exhibited many excellent properties, including higher mechanical strengths and hardness and especially much poorer ignitability compared to pure sulfur. By using SSCReF to construct packed-bed reactors, the highest denitrification and dephosphorization rates reached 829.70 gN/m3/d (25 wt % siderite) and 36.70 gP/m3/d (75 wt % siderite), respectively. Dephosphorization was demonstrated to be dependent on sulfur-driven denitrification, in which the acid produced from the later process promoted Fe(II) dissolution, which then directly combined with phosphate to form vivianite or further converted into phosphate adsorbents (ferrihydrite, a green rust-like compound). Water flush was an effective way to finally wash out these surface deposited Fe-P compounds, as well as those nonreactive impurities (Si and Al-bearing compounds) detached from SSCReF. Such a highly efficient and safe SSCReF holds considerable application potential in secondary effluent polishing.

Identifying the origin and fate of dissolved U in the Boeun aquifer based on microbial signatures and C, O, Fe, S, and U isotopes

The uranium inventory in the Boeun aquifer is situated near an artificial reservoir (40-70 m apart) intended to supply water to nearby cities. However, toxic radionuclides can enter the reservoir. To determine the U mobility in the system, we analyzed groundwater and fracture-filling materials (FFMs) for environmental tracers, including microbial signatures, redox-sensitive elements and isotopes. In the site, U mass flux ranged from only 9.59 × 10-7 µg/L/y to 1.70 × 10-4 µg/L/y. The δ18O-H2O and 14C signatures showed that groundwater originated mainly from upland recharges and was not influenced by oxic surface water. We observed U accumulations (∼157 mg/kg) in shallow FFMs and Fe enrichments (∼226798 mg/kg) and anomalies in the 230Th/238U activity ratio (AR), 230Th/234U AR, δ56Fe and δ57Fe isotopes, suggesting that low U mobility in shallow depths is associated with a Fe-rich environment. At shallow depths, anaerobic Fe-oxidizers, Gallionella was prevalent in the groundwater, while Acidovorax was abundant near the U ore deposit depth. The Fe-rich environment at shallow depths was formed by sulfide dissolution, as demonstrated by δ34S-SO4 and δ18O-SO4 distribution. Overall, the Fe-rich aquifer including abundant sulfide minerals immobilizes dissolved U through biotic and abiotic processes, without significant leaching into nearby reservoirs.

Biodegradation of nitrate and p-bromophenol using hydrogen-based membrane biofilm reactors in parallel

ABSTRACTP-bromophenol (4-BP) is a toxic halogenated phenolic organic compound. The conventional treatment processes for 4-BP elimination are costly and inefficient, with complete mineralization remaining a challenge for water treatment. To overcome these limitations, we investigated the treatment of 4-BP in a membrane biofilm reactor (MBfR) using hydrogen as an electron donor. The pathway of 4-BP degradation within the H2-MBfR was investigated through long-term operational experiments by considering the effect of nitrate and 4-BP concentrations, hydrogen partial pressure, static experiments, and microbial community diversity, which was studied using 16S rRNA. The results showed that H2-MBfR could quickly remove approximately 100% of 4-BP (up to 20 mg/L), with minimal intermediate product accumulation and 10 mg/L of nitrate continuously reduced. The microbial community structure showed that the presence of H2 created an anaerobic environment, and Thauera was the dominant functional genus involved in the degradation of 4-BP. The genes encoding related enzymes were further enhanced. This study provides an economically viable and environmentally friendly bioremediation technique for water bodies that contain 4-BP and nitrates.

Novel Insight into Microbially Mediated Nitrate-Reducing Fe(II) Oxidation by Acidovorax sp. Strain BoFeN1 Using Dual N-O Isotope Fractionation

Microbially mediated nitrate reduction coupled with Fe(II) oxidation (NRFO) plays an important role in the Fe/N interactions in pH-neutral anoxic environments. However, the relative contributions of the chemical and microbial processes to NRFO are still unclear. In this study, N-O isotope fractionation during NRFO was investigated. The ratios of O and N isotope enrichment factors (18ε:15ε)-NO3- indicated that the main nitrate reductase functioning in Acidovorax sp. strain BoFeN1 was membrane-bound dissimilatory nitrate reductase (Nar). N-O isotope fractionation during chemodenitrification [Fe(II) + NO2-], microbial nitrite reduction (cells + NO2-), and the coupled process [cells + NO2- + Fe(II)] was explored. The ratios of (18ε:15ε)-NO2- were 0.58 ± 0.05 during chemodenitrification and -0.41 ± 0.11 during microbial nitrite reduction, indicating that N-O isotopes can be used to distinguish chemical from biological reactions. The (18ε:15ε)-NO2- of 0.70 ± 0.05 during the coupled process was close to that obtained for chemodenitrification, indicating that chemodenitrification played a more important role than biological reactions during the coupled process. The results of kinetic modeling showed that the relative contribution of chemodenitrification was 99.3% during the coupled process, which was consistent with that of isotope fractionation. This study provides a better understanding of chemical and biological mechanisms of NRFO using N-O isotopes and kinetic modeling.

Enrichment and response of iron-metabolizing microorganisms and metabolic genes in the contaminated area of stratified stacking coal gangue dumps, Northern China

In the Xishan coalfield of northern China, the stratified stacking of soil and gangue was applied to limit the acid pollution from high-sulfur coal gangue. In this study, we found that stratified stacking can effectively neutralize the acidity, with the pH value of gangue-leaching water being 6.02-8.13. In contrast to the acidic contaminated area, most of the microorganisms in the study area sediment were neutrophilic, with the main genera being Arthrobacter, Pseudorhodobacter, Pseudomonas, and Rhodoferax. A variety of iron- and sulfur-metabolizing bacteria was discovered in the gangue-leaching sediment, with the total relative abundance ranging from 4.20 to 23.75%, of which the iron-reducing bacteria (FeRB) accounted for the highest percentage. The distributions of these functional microorganisms in the samples were significantly influenced by Fe and S. The co-occurrence network analysis revealed a significant positive correlation between the iron- and sulfur-metabolizing bacteria in the sediment (93.75%), indicating a strong reciprocal symbiotic relationship between these bacteria. The iron and sulfur metabolism genes in the sediment were predicted and compared based on the Tax4Fun functional prediction method. Results showed that functional genes related to iron metabolism were highly expressed in the gangue-leaching sediment. This study enhances the understanding of iron and sulfur metabolism in gangue-leaching contaminated areas.

Soil Microbial Community Involved in Nitrogen Cycling in Rice Fields Treated with Antiozonant under Ambient Ozone

Ethylenediurea (EDU) can effectively mitigate the crop yield loss caused by ozone (O3), a major, phytotoxic air pollutant. However, the relevant mechanisms are poorly understood, and the effect of EDU on soil ecosystems has not been comprehensively examined. In this study, a hybrid rice variety (Shenyou 63) was cultivated under ambient O3 and sprayed with 450 ppm EDU or water every 10 days. Real time quantitative polymerase chain reaction (RT-qPCR) showed that EDU had no significant effect on the microbial abundance in either rhizospheric or bulk soils. By applying both metagenomic sequencing and the direct assembly of nitrogen (N)-cycling genes, EDU was found to decrease the abundance of functional genes related to nitrification and denitrification processes. Moreover, EDU increased the abundance of genes involved in N-fixing. Although the abundance of some functional genes did not change significantly, nonmetric multidimensional scaling (NMDS) and a principal coordinates analysis (PCoA) suggested that the microbial community structure involved in N cycling was altered by EDU. The relative abundances of nifH-and norB-harboring microbial genera in the rhizosphere responded differently to EDU, suggesting the existence of functional redundancy, which may play a key role in sustaining microbially mediated N-cycling under ambient O3. IMPORTANCE Ethylenediurea (EDU) is hitherto the most efficient phytoprotectant agent against O3 stress. However, the underlying biological mechanisms of its mode of action are not clear, and the effects of EDU on the environment are still unknown, limiting its large-scale application in agriculture. Due to its sensitivity to environmental changes, the microbial community can be used as an indicator to assess the environmental impacts of agricultural practices on soil quality. This study aimed to unravel the effects of EDU spray on the abundance, community structure, and ecological functions of microbial communities in the rhizosphere of rice plants. Our study provides a deep insight into the impact of EDU spray on microbial-mediated N cycling and the structure of N-cycling microbial communities. Our findings help to elucidate the mode of action of EDU in alleviating O3 stress in crops from the perspective of regulating the structure and function of the rhizospheric soil microbial community.

High-rate iron sulfide and sulfur-coupled autotrophic denitrification system: Nutrients removal performance and microbial characterization

Iron sulfides-based autotrophic denitrification (IAD) is a promising technology for nitrate and phosphate removal from low C:N ratio wastewater due to its cost-effectiveness and low sludge production. However, the slow kinetics of IAD, compared to other sulfur-based autotrophic denitrification (SAD) processes, limits its engineering application. This study constructed a co-electron-donor (FeS and S⁰ with a volume ratio of 2:1) iron sulfur autotrophic denitrification (ISAD) biofilter and operated at as short as 1 hr hydraulic retention time (HRT). Long-term operation results showed that the superior total nitrogen and phosphate removals of the ISAD biofilter were 90-100% at 1-12 h HRT, with the highest denitrification rate up to 960 mg/L/d. Considering low sulfate production, HRT of 3 h could be the optimal condition. Such superior performance in the ISAD biofilter was achieved due to the interactions between FeS and S⁰, which accelerated the denitrification process and maintained the acidity-alkalinity balance. Metagenomic analysis found that the enriched nitrate-dependent iron-oxidizing (NDFO) bacteria (Acinetobacter and Acidovorax), sulfur-oxidizing bacteria (SOB), and dissimilatory nitrate reduction to ammonia (DNRA) bacteria likely supported stable nitrate reduction. The metabolic pathway analysis showed that completely denitrification and DNRA, coupled with sulfur oxidation, disproportionation, iron oxidation and phosphate precipitation with FeS and S⁰ as co-electron donors, were responsible for the high-rate nitrate and phosphate removal. This study provides the potential of ISAD as a highly efficient post-denitrification technology and sheds light on the balanced microbial S-N-Fe transformation.

Autotrophic denitrification using Fe(II) as an electron donor: A novel prospective denitrification process

As a newly identified nitrogen loss pathway, the nitrate-dependent ferrous oxidation (NDFO) process is emerging as a research hotspot in the field of low carbon to nitrogen ratio (C/N) wastewater treatment. This review article provides an overview of the NDFO process and summarizes the functional microorganisms associated with NDFO from different perspectives. The potential mechanisms by which external factors such as influent pH, influent Fe(II)/N (mol), organic carbon, and chelating agents affect NDFO performance are also thoroughly discussed. As the electron-transfer mechanism of the NDFO process is still largely unknown, the extensive chemical Fe(II)-oxidizing nitrite-reducing pathway (NDFO_chem) of the NDFO process is described here, and the potential enzymatic electron transfer mechanisms involved are summarized. On this basis, a three-stage electron transfer pathway applicable to low C/N wastewater is proposed. Furthermore, the impact of Fe(III) mineral products on the NDFO process is revisited, and existing crusting prevention strategies are summarized. Finally, future challenges facing the NDFO process and new research directions are discussed, with the aim of further promoting the development and application of the NDFO process in the field of nitrogen removal.

Coupled C, H, N, S and Fe biogeochemical cycles operating in the continental deep subsurface of the Iberian Pyrite Belt

Microbial activity is a major contributor to the biogeochemical cycles that make up the life support system of planet Earth. A 613 m deep geomicrobiological perforation and a systematic multi-analytical characterization revealed an unexpected diversity associated with the rock matrix microbiome that operates in the subsurface of the Iberian Pyrite Belt (IPB). Members of 1 class and 16 genera were deemed the most representative microorganisms of the IPB deep subsurface and selected for a deeper analysis. The use of fluorescence in situ hybridization allowed not only the identification of microorganisms but also the detection of novel activities in the subsurface such as anaerobic ammonium oxidation (ANAMMOX) and anaerobic methane oxidation, the co-occurrence of microorganisms able to maintain complementary metabolic activities and the existence of biofilms. The use of enrichment cultures sensed the presence of five different complementary metabolic activities along the length of the borehole and isolated 29 bacterial species. Genomic analysis of nine isolates identified the genes involved in the complete operation of the light-independent coupled C, H, N, S and Fe biogeochemical cycles. This study revealed the importance of nitrate reduction microorganisms in the oxidation of iron in the anoxic conditions existing in the subsurface of the IPB.

Water-energy-greenhouse gas nexus of a novel high-rate activated sludge-two-stage vertical up-flow constructed wetland system for low-carbon wastewater treatment

Municipal wastewater treatment which is associated with high energy consumption and excessive greenhouse gas (GHG) emissions, has been facing severe challenges toward carbon emissions. In this study, a high-rate activated sludge-two-stage vertical up-flow constructed wetland (HRAS-TVUCW) system was developed to reduce carbon emissions during municipal wastewater treatment. Through carbon management, optimized mass and energy flows were achieved, resulting in high treatment efficiency and low operational energy consumption. The carbon emission of the HRAS-TVUCW system (i.e., 0.21 kg carbon dioxide equivalent/m³ wastewater) was 4.1-folds lower than that of the conventional anaerobic/anoxic/aerobic (A²O) process. Meanwhile, the recovered energy from the HRAS-TVUCW system increased its contribution to carbon neutrality to 40.2%, 4.6-folds higher than that of the A²O process. Results of functional microbial community analysis at the genus level revealed that the controlled dissolved oxygen allocation led to distinctive microbial communities in each unit of HRAS-TVUCW system, which facilitated denitrification efficiency increase and carbon emissions reduction. Overall, the HRAS-TVUCW system could be considered as a cost-effective and sustainable low-carbon technology for municipal wastewater treatment.

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.

Unveiling microbial community and function involved in anammox in paddy vadose under groundwater irrigation

The extensive application of nitrogen fertilizer in intensive irrigation areas poses a potential threat to groundwater. Given that the vadose zone acts as a buffer zone for the underground entry of surface pollutants, an in-depth understanding of its microbial community structure and function was crucial for controlling groundwater nitrogen pollution. In this study, soil samples from paddy vadose under groundwater irrigation with different depths (G1: 6.8 m, G2: 13.7 m, G3: 15.6 m, and G4: 17.8 m) were collected to unravel the differences in microbial community structure and function at different vadose depths (0-250 cm), as well as their relationship with soil properties. Results showed some differences among soil physicochemical factors under groundwater irrigation with different depths and that some electron acceptors were more abundant than others under deep groundwater irrigation (G2-G4). Remarkable differences in microbial communities under shallow- and deep-groundwater irrigation were found. The high abundances of anammox bacteria Candidatus_Brocadia in G2 and G3 indicated that deep groundwater irrigation was beneficial to its enrichment. Iron-reducing bacteria Anaeromyxobacter and sulfate-reducing bacteria Desulfovibrio were widely distributed in vadose zone and possessed the potential for anammox coupled with Fe(III)/sulfate reduction. Norank_f_Gemmatimonadaceae had nitrate- and vanadium-reducing abilities and could participate in anammox in vadose zone. Dissimilatory nitrate reduction to ammonia (DNRA) bacteria Geobacter facilitated Fe(II)-driven DNRA and thus provided electron donors and acceptors to anammox bacteria. Soil nutrients and electron donors/acceptors played important roles in shaping microbial community structure at phylum and genus levels. Microorganisms in vadose zone under groundwater irrigation showed good material/energy metabolism levels. Deep groundwater irrigation was conducive to the occurrence of anammox coupled with multi-electron acceptors. Our findings highlight the importance of understanding the structure and function of microbial communities in paddy vadose under groundwater irrigation and reveal the potential role of indigenous microorganisms in in-situ nitrogen removal.

Characterization of arsenic-metabolizing bacteria in an alkaline soil

Arsenite (As(III)) is more toxic, mobilizable and bioavailable than arsenate (As(V)). Hence, the transformations between As(III) and As(V) are crucial for the toxicity and mobility of arsenic (As). However, As transformation and microbial communities involved in alkaline soils are largely unknown. Here we investigate two major pathways of As transformation, i.e., As(III) oxidation and As(V) reduction, and identify the bacteria involved in the alkaline soil by combining stable isotope probing with shotgun metagenomic sequencing. As(III) oxidation and significant increase of the aioA genes copies were observed in the treatments amended with As(III) and NO₃^-, suggesting that As(III) oxidation can couple with nitrate reduction and was mainly catalyzed by the microorganisms containing aioA genes. As(V) reduction was detected in the treatments amended with As(V) and acetate where the abundance of arrA gene significantly increased, indicating that microorganisms with arrA genes were the key As(V) reducers. Acidovorax, Hydrogenophaga, and Ramlibacter were the putative nitrate-dependent As(III) oxidizers, and Deinococcus and Serratia were the putative respiratory As(V) reducers. These findings will improve our understanding of As metabolism and are meaningful for mapping out bioremediation strategies of As contamination in alkaline environment.

Hematite-promoted nitrate-reducing Fe(II) oxidation by Acidovorax sp. strain BoFeN1: Roles of mineral catalysis and cell encrustation

Although nitrate-reducing Fe(II) oxidizing (NRFO) bacteria can grow sustainably in natural environments, numerous laboratory studies suggested that cell encrustation-induced metabolism limitations and cell death occurred more seriously in the absence of natural minerals. Hence, a study on how natural minerals could affect NRFO is warranted. This study examined the impact of hematite on NRFO by Acidovorax sp. BoFeN1 with different electron donors (acetate and Fe(II), acetate alone, and Fe(II) alone) and with nitrate as an electron acceptor. When acetate and Fe(II) were used as the electron donors, the amount of Fe(II) oxidation and nitrate reduction was enhanced in the presence of hematite, whereas no promotion was observed when only acetate was added as an electron donor. Under the conditions with only Fe(II) added as an electron donor, the level of Fe(II) oxidation was increased from 3.07 ± 0.06 to 3.92 ± 0.02 mM in the presence of hematite and nitrate reduction was enhanced. This suggests that hematite promotes microbial nitrate reduction by accelerating the biological oxidation of Fe(II). The main secondary minerals were goethite and lepidocrocite. After adding hematite, the assemblage of iron minerals on the cell surface decreased, and the cell crusts became thinner, indicating that hematite effectively mitigated cell encrustation. Furthermore, hematite accelerated the chemical oxidation of Fe(II) by nitrite. Hence, hematite can promote the NRFO of Acidovorax sp. BoFeN1 via two possible pathways: (i) hematite acts as nucleation sites to mitigate cell encrustation; (ii) hematite catalyzes the biological and chemical oxidation of Fe(II) through the mineral catalysis effects. This study highlights the importance of existing iron minerals on NRFO and sheds light on the survival strategy of NRFO bacteria in anoxic subsurface environments.

Review on corrosion and corrosion scale formation upon unlined cast iron pipes in drinking water distribution systems

The qualified finished water from water treatment plants (WTPs) may become discolored and deteriorated during transportation in drinking water distribution systems (DWDSs), which affected tap water quality seriously. This water stability problem often occurs due to pipe corrosion and the destabilization of corrosion scales. This paper provides a comprehensive review of pipe corrosion in DWDSs, including corrosion process, corrosion scale formation, influencing factors and monitoring technologies utilized in DWDSs. In terms of corrosion process, corrosion occurrence, development mechanisms, currently applied assays, and indices used to determine the corrosion possibility are summarized, as well as the chemical and bacterial influences. In terms of scale formation, explanations for the nature of corrosion and scale formation mechanisms are discussed and its typical multilayered structure is illustrated. Furthermore, the influences of water quality and microbial activity on scale transformation are comprehensively discussed. Corrosion-related bacteria at the genus level and their associated corrosion mechanism are also summarized. This review helps deepen the current understanding of pipe corrosion and scale formation in DWDSs, providing guidance for water supply utilities to ensure effective measures to maintain water quality stability and guarantee drinking water safety.

Arsenic and nitrate remediation by isolated FeOB strains coupled with additional ferrous iron in the iron-deficient arid soils

Remediation of As(III) by use of Fe(II) oxidation bacteria (FeOB) in iron-rich soils has been reported, but seldom used in the iron-deficient soil of arid areas. This study was aimed at selecting native bacterial strains to remediate As pollution in arid soils, coupled with the addition of Fe(II). The used methods included: The selection of two FeOB strains; XRD for solid phase identification based on peaks; SEM with EDS for morphology identification of newly formed minerals with chemical compositions; XPS for surface chemistry of the minerals; FTIR for functional groups of precipitates and 3DEEM for EPS determination, etc. The results were as follows: Sharp decrement curves of As(III) and NO₃^- with Fe(II) and total Fe contents and increment of NO₂^-; NH₄⁺ fluctuating during the experimental period of 11 days; and precipitation of Fe(III) hydroxides together with As(III) with broken FeOBs due to encrustation in the SEM scan. It was concluded that two selected Pseudomonas strains have NAFO functionality by addition of iron as iron reduction-oxidation pair in the arid soil, further potentially fixing NH₄⁺ while As(III) can be effectively remediated through the FeOB participation in forms of adsorption and co-precipitation of Fe(OH)₃ through an oxidation of Fe(II) process.

Effect of Environmental pH on Mineralization of Anaerobic Iron-Oxidizing Bacteria

Freshwater lakes are often polluted with various heavy metals in the Anthropocene. The iron-oxidizing microorganisms and their mineralized products can coprecipitate with many heavy metals, including Al, Zn, Cu, Cd, and Cr. As such, microbial iron oxidation can exert a profound impact on environmental remediation. The environmental pH is a key determinant regulating microbial growth and mineralization and then influences the structure of the final mineralized products of anaerobic iron-oxidizing bacteria. Freshwater lakes, in general, are neutral-pH environments. Understanding the effects of varying pH on the mineralization of iron-oxidizing bacteria under neutrophilic conditions could aid in finding out the optimal pH values that promote the coprecipitation of heavy metals. Here, two typical neutrophilic Fe(II)-oxidizing bacteria, the nitrate-reducing Acidovorax sp. strain BoFeN1 and the anoxygenic phototrophic Rhodobacter ferrooxidans strain SW2, were selected for studying how their growth and mineralization response to slight changes in circumneutral pH. By employing focused ion beam/scanning electron microscopy (FIB–SEM) and transmission electron microscopy (TEM), we examined the interplay between pH changes and anaerobic iron-oxidizing bacteria and observed that pH can significantly impact the microbial mineralization process and vice versa. Further, pH-dependent changes in the structure of mineralized products of bacterial iron oxidation were observed. Our study could provide mechanical insights into how to manipulate microbial iron oxidation for facilitating remediation of heavy metals in the environment.

Salinity Impact on Composition and Activity of Nitrate-Reducing Fe(II)-Oxidizing Microorganisms in Saline Lakes

Nitrate-reducing Fe(II)-oxidizing (NRFeOx) microorganisms contribute to nitrogen, carbon, and iron cycling in freshwater and marine ecosystems. However, NRFeOx microorganisms have not been investigated in hypersaline lakes, and their identity, as well as their activity in response to salinity, is unknown. In this study, we combined cultivation-based most probable number (MPN) counts with Illumina MiSeq sequencing to analyze the abundance and community compositions of NRFeOx microorganisms enriched from five lake sediments with different salinities (ranging from 0.67 g/L to 346 g/L). MPN results showed that the abundance of NRFeOx microorganisms significantly (P < 0.05) decreased with increasing lake salinity, from 7.55 × 10³ to 8.09 cells/g dry sediment. The community composition of the NRFeOx enrichment cultures obtained from the MPNs differed distinctly among the five lakes and clustered with lake salinity. Two stable enrichment cultures, named FeN-EHL and FeN-CKL, were obtained from microcosm incubations of sediment from freshwater Lake Erhai and hypersaline Lake Chaka. The culture FeN-EHL was dominated by genus Gallionella (68.4%), while the culture FeN-CKL was dominated by genus Marinobacter (71.2%), with the former growing autotrophically and the latter requiring an additional organic substrate (acetate) and Fe(II) oxidation, caused to a large extent by chemodenitrification [reaction of nitrite with Fe(II)]. Short-range ordered Fe(III) (oxyhydr)oxides were the product of Fe(II) oxidation, and the cells were partially attached to or encrusted by the formed iron minerals in both cultures. In summary, different types of interactions between Fe(II) and nitrate-reducing bacteria may exist in freshwater and hypersaline lakes, i.e., autotrophic NRFeOx and chemodenitrification in freshwater and hypersaline environments, respectively. IMPORTANCE NRFeOx microorganisms are globally distributed in various types of environments and play a vital role in iron transformation and nitrate and heavy metal removal. However, most known NRFeOx microorganisms were isolated from freshwater and marine environments, while their identity and activity under hypersaline conditions remain unknown. Here, we demonstrated that salinity may affect the abundance, identity, and nutrition modes of NRFeOx microorganisms. Autotrophy was only detectable in a freshwater lake but not in the saline lake investigated. We enriched a mixotrophic culture capable of nitrate-reducing Fe(II) oxidation from hypersaline lake sediments. However, Fe(II) oxidation was probably caused by abiotic nitrite reduction (chemodenitrification) rather than by a biologically mediated process. Consequently, our study suggests that in hypersaline environments, Fe(II) oxidation is largely caused by chemodentrification initiated by nitrite formation by chemoheterotrophic bacteria, and additional experiments are needed to demonstrate whether or to what extent Fe(II) is enzymatically oxidized.

Oligotrophic Growth of Nitrate-Dependent Fe2+-Oxidising Microorganisms Under Simulated Early Martian Conditions

Nitrate-dependent Fe2+ oxidation (NDFO) is a microbially mediated process observed in many anaerobic, low-nutrient (oligotrophic) neutral-alkaline environments on Earth, which describes oxidation of Fe2+ to Fe3+ in tandem with microbial nitrate reduction. Evidence suggests that similar environments existed on Mars during the Noachian epoch (4.1-3.7 Ga) and in periodic, localised environments more recently, indicating that NDFO metabolism could have played a role in a potential early martian biosphere. In this paper, three NDFO microorganisms, Acidovorax sp. strain BoFeN1, Pseudogulbenkiania sp. strain 2002 and Paracoccus sp. strain KS1, were assessed for their ability to grow oligotrophically in simulated martian brines and in a minimal medium with olivine as a solid Fe2+ source. These simulant-derived media were developed from modelled fluids based on the geochemistry of Mars sample locations at Rocknest (contemporary Mars soil), Paso Robles (sulphur-rich soil), Haematite Slope (haematite-rich soil) and a Shergottite meteorite (common basalt). The Shergottite medium was able to support growth of all three organisms, while the contemporary Mars medium supported growth of Acidovorax sp. strain BoFeN1 and Pseudogulbenkiania sp. strain 2002; however, growth was not accompanied by significant Fe2+ oxidation. Each of the strains was also able to grow in oligotrophic minimal media with olivine as the sole Fe2+ source. Biomineralised cells of Pseudogulbenkiania sp. strain 2002 were identified on the surface of the olivine, representing a potential biosignature for NDFO microorganisms in martian samples. The results suggest that NDFO microorganisms could have thrived in early martian groundwaters under oligotrophic conditions, depending on the local lithology. This can guide missions in identifying palaeoenvironments of interest for biosignature detection. Indeed, biomineralised cells identified on the olivine surface provide a previously unexplored mechanism for the preservation of morphological biosignatures in the martian geological record.

Elucidating heterogeneous iron biomineralization patterns in a denitrifying As(iii)-oxidizing bacterium: implications for arsenic immobilization

Anaerobic nitrate-dependent iron(ii) oxidation is a process common to many bacterial species, which promotes the formation of Fe(iii) minerals that can influence the fate of soil and groundwater pollutants, such as arsenic. Herein, we investigated simultaneous nitrate-dependent Fe(ii) and As(iii) oxidation by Acidovorax sp. strain ST3 with the aim of studying the Fe biominerals formed, their As immobilization capabilities and the metabolic effect on cells. X-ray powder diffraction (XRD) and scanning transmission electron microscopy (STEM) nanodiffraction were applied for biomineral characterization in bulk and at the nanoscale, respectively. NanoSIMS (nanoscale secondary ion mass spectrometry) was used to map the intra and extracellular As and Fe distribution at the single-cell level and to trace metabolically active cells, by incorporation of a ¹³C-labeled substrate (acetate). Metabolic heterogeneity among bacterial cells was detected, with periplasmic Fe mineral encrustation deleterious to cell metabolism. Interestingly, Fe and As were not co-localized in all cells, indicating delocalized sites of As(iii) and Fe(ii) oxidation. The Fe(iii) minerals lepidocrocite and goethite were identified in XRD, although only lepidocrocite was identified via STEM nanodiffraction. Extracellular amorphous nanoparticles were formed earlier and retained more As(iii/v) than crystalline "flakes" of lepidocrocite, indicating that longer incubation periods promote the formation of more crystalline minerals with lower As retention capabilities. Thus, the addition of nitrate promotes Fe(ii) oxidation and formation of Fe(iii) biominerals by ST3 cells which retain As(iii/v), and although this process was metabolically detrimental to some cells, it warrants further examination as a viable mechanism for As removal in anoxic environments by biostimulation with nitrate.

Reduction of Chlorinated Ethenes by Ag- and Cu-Amended Green Rust

Chlorinated ethenes have been used extensively as solvents, degreasers, and dry-cleaning agents in a range of commercial and industrial applications. This has created a legacy of contaminated soils and groundwater, particularly with respect to perchloroethylene (PCE; a.k.a. tetrachloroethene—C2Cl4), and trichloroethylene (TCE; a.k.a. trichloroethene—C2HCl3), prompting the development of a wide array of treatment technologies for remediation of chlorinated ethene-contaminated environments. Green rusts are highly redox-active layered Fe(II)-Fe(III) hydroxides that have been shown to be facile reductants for a wide range of organic and inorganic pollutants. The reduction of chlorinated ethenes [vinyl chloride (VC); 1,1-dichloroethene(11DCE), cis-1,2-dichloroethene (c12DCE), trans-1,2-dichloroethene (t12DCE), TCE, and PCE] was examined in aqueous suspensions of green rust, alone as well as with the addition of Ag(I) (AgGR) or Cu(II) (CuGR). Green rust alone was ineffective as a reductant for the reductive dechlorination for all of the chlorinated ethenes. Near-complete removal of PCE was observed in the presence of AgGR, but all other chlorinated ethenes were essentially non-reactive. Partial removal of chlorinated ethenes was observed in the presence of CuGR, particularly 11DCE (34%), t12DCE (51%), and VC (66%). Significant differences were observed in the product distributions of chlorinated ethene reduction by AgGR and CuGR. The effectiveness of Ag(I)- and Cu(II)-amended green rusts for removal of chlorinated ethenes may be improved under different conditions (e.g., pH and interlayer anion) and warrants further investigation.

Anaerobic microbial corrosion of carbon steel under conditions relevant for deep geological repository of nuclear waste

We examined microbial corrosion of carbon steel in synthetic bentonite pore water inoculated with natural underground water containing microorganisms over a period of 780-days under sterile and anaerobic conditions. Corrosion behaviour was determined using the mass loss method, SEM-EDS analysis and Raman spectroscopy, while qualitative and quantitative changes in the microbial community were analysed using molecular-biological tools (16S rDNA amplicon sequencing and qPCR analysis, respectively). Corrosion rates were significantly higher in the biotic environment (compared with an abiotic environment), with significant localisation of corrosion attacks of up to 1 mm arising within 12-months. Nitrate reducing bacteria, such as Pseudomonas, Brevundimonas and Methyloversatilis, dominated the microbial consortium, the high abundance of Methyloversatilis correlating with periods of highest localised corrosion penetrations, suggesting that this bacterium plays an important role in microbially influenced corrosion. Our results indicate that nitrate-reducing bacteria could represent a potential threat to waste canisters under nuclear repository conditions.

Temperature dependence of nitrate-reducing Fe(II) oxidation by Acidovorax strain BoFeN1 - evaluating the role of enzymatic vs. abiotic Fe(II) oxidation by nitrite

Fe(II) oxidation coupled to nitrate reduction is a widely observed metabolism. However, to what extent the observed Fe(II) oxidation is driven enzymatically or abiotically by metabolically produced nitrite remains puzzling. To distinguish between biotic and abiotic reactions, we cultivated the mixotrophic nitrate-reducing Fe(II)-oxidizing Acidovorax strain BoFeN1 over a wide range of temperatures and compared it to abiotic Fe(II) oxidation by nitrite at temperatures up to 60°C. The collected experimental data were subsequently analyzed through biogeochemical modeling. At 5°C, BoFeN1 cultures consumed acetate and reduced nitrate but did not significantly oxidize Fe(II). Abiotic Fe(II) oxidation by nitrite at different temperatures showed an Arrhenius-type behavior with an activation energy of 80±7 kJ/mol. Above 40°C, the kinetics of Fe(II) oxidation were abiotically driven, whereas at 30°C, where BoFeN1 can actively metabolize, the model-based interpretation strongly suggested that an enzymatic pathway was responsible for a large fraction (ca. 62%) of the oxidation. This result was reproduced even when no additional carbon source was present. Our results show that at below 30°C, i.e., at temperatures representing most natural environments, biological Fe(II) oxidation was largely responsible for overall Fe(II) oxidation, while abiotic Fe(II) oxidation by nitrite played a less important role.

Photoferrotrophy and phototrophic extracellular electron uptake is common in the marine anoxygenic phototroph Rhodovulum sulfidophilum

Photoferrotrophy allows anoxygenic phototrophs to use reduced iron as an electron donor for primary productivity. Recent work shows that freshwater photoferrotrophs can use electrons from solid-phase conductive substances via phototrophic extracellular electron uptake (pEEU), and the two processes share the underlying electron uptake mechanism. However, the ability of marine phototrophs to perform photoferrotrophy and pEEU, and the contribution of these processes to primary productivity is largely unknown. To fill this knowledge gap, we isolated 15 new strains of the marine anoxygenic phototroph Rhodovulum sulfidophilum on electron donors such as acetate and thiosulfate. We observed that all of the R. sulfidophilum strains isolated can perform photoferrotrophy. We chose strain AB26 as a representative strain to study further, and find that it can also perform pEEU from poised electrodes. We show that during pEEU, AB26 transfers electrons to the photosynthetic electron transport chain. Furthermore, systems biology-guided mutant analysis shows that R. sulfidophilum AB26 uses a previously unknown diheme cytochrome c protein, which we call EeuP, for pEEU but not photoferrotrophy. Homologs of EeuP occur in a range of widely distributed marine microbes. Overall, these results suggest that photoferrotrophy and pEEU contribute to the biogeochemical cycling of iron and carbon in marine ecosystems.

Reactions of nitrite with goethite and surface Fe(II)-goethite complexes

Chemodenitrification-the abiotic (chemical) reduction of nitrite (NO₂^-) by iron (II)-plays an important role in nitrogen cycling due in part to this process serving as a source of nitrous oxide (N₂O). Questions remain about the fate of NO₂^- in the presence of mineral surfaces formed during chemodenitrification, such as iron(III) (hydr) oxides, particularly relative to dissolved iron(II). In this study, stirred-batch kinetic experiments were conducted under anoxic conditions (to mimic iron(III)-reducing conditions) from pH 5.5-8 to investigate NO₂^- reactivity with goethite (FeOOH_(s)) and Fe(II)-treated goethite using wet chemical and attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy. Nitrite removal from solution by goethite was more rapid at pH 5.5 than at pH 7 and 8. Spectral changes upon nitrite adsorption imply an inner-sphere surface interaction (monodentate and bidentate) at pH 5.5 based on ATR-FTIR spectra of the nitrite-goethite interface over time. In iron(II)-amended experiments at pH 5.5 with high aqueous Fe(II) in equilibrium with goethite, nitrous oxide was generated, indicating that nitrite removal involved a combination of sorption and reduction processes. The presence of a surface complex resembling protonated nitrite (HONO) with an IR peak near ~1258 cm^-1 was observed in goethite-only and iron(II)-goethite experiments, with a greater abundance of this species observed in the latter treatment. These results might help explain gaseous losses of nitrogen where nitrite and iron(II)/goethite coexist, with implications for nutrient cycling and release of atmospheric air pollutants.

Mechanistic Study of Ni(II) Sorption by Green Rust Sulfate

The sorption of Ni(II) by green rust sulfate (GR-sulfate) was studied in anoxic pre-equilibrated suspensions at pH 7.0 and pH 7.8 with combined batch kinetic experiments, X-ray diffraction measurements, and Ni K-edge X-ray absorption spectroscopy (XAS) analyses. Continuous removal of aqueous Ni(II) was observed over the course of the reaction (1-2.5 weeks) at both pH values, with no concurrent changes in aqueous Fe(II) levels or detectable mineralogical modifications of the GR sorbent. XAS results indicate that Ni(II) is not retained as mononuclear adsorption complexes on the GR surface but rather incorporated in the octahedral layers of an Fe^II_0.67-xNi^II_xFe^III_0.33(OH)₂-layered double hydroxide (LDH) phase with 0 < x < 0.67. The combined macroscopic and spectroscopic data suggest that Ni(II) substitutes into the GR lattice during Fe(II)-catalyzed recrystallization of the sorbent and/or forms secondary Ni(II)/Fe(II)-Fe(III)-LDH phases with a higher stability than that of GR, complemented likely by Ni(II)-Fe(II) exchange at GR particle edges. The results of this study reveal GR to be a dynamic sorbent that engages in dissolution-reprecipitation and exchange reactions, causing extensive incorporation of trace metal Ni(II)_aq. Additional work is needed to further define the mechanisms involved and to assess the sorptive reactivity of GR with other trace metal species.

Fe(II) Redox Chemistry in the Environment

Iron (Fe) is the fourth most abundant element in the earth's crust and plays important roles in both biological and chemical processes. The redox reactivity of various Fe(II) forms has gained increasing attention over recent decades in the areas of (bio) geochemistry, environmental chemistry and engineering, and material sciences. The goal of this paper is to review these recent advances and the current state of knowledge of Fe(II) redox chemistry in the environment. Specifically, this comprehensive review focuses on the redox reactivity of four types of Fe(II) species including aqueous Fe(II), Fe(II) complexed with ligands, minerals bearing structural Fe(II), and sorbed Fe(II) on mineral oxide surfaces. The formation pathways, factors governing the reactivity, insights into potential mechanisms, reactivity comparison, and characterization techniques are discussed with reference to the most recent breakthroughs in this field where possible. We also cover the roles of these Fe(II) species in environmental applications of zerovalent iron, microbial processes, biogeochemical cycling of carbon and nutrients, and their abiotic oxidation related processes in natural and engineered systems.

Geochemical and microbial characteristics of seepage water and mineral precipitates in a radwaste disposal facility impacted by seawater intrusion and high alkalinity

The construction of an underground facility can dramatically change the quality, flow direction, and level of groundwater. It may also impact subsurface microbial composition and activity. Groundwater quality was monitored over eight years in two observational wells near an underground disposal facility on the east coast of South Korea. The results showed dramatic increases in dissolved ions such as O₂, Na, Ca, Mg, and SO₄ during facility construction. Seepage water samples downgradient from the silos and tunnels, and precipitates deposited along the seepage water flow path were collected to determine the impact inside the disposal facility. X-ray analysis (powder X-ray diffraction (pXRD) and X-ray absorption fine structure (XAFS)) were used to characterize the mineral precipitates. Microbial community composition was determined by 16S rRNA gene sequencing. The seepage water composition was of two types: Ca-Cl and Ca-Na-HCO₃. The ratio of Cl and δ¹⁸O showed that the Ca-Cl type seepage water was influenced by groundwater mixed with seawater ranging from 2.7% to 15.1%. Various sulfate-reducing bacteria were identified in the Ca-Cl type seepage water, exhibiting relatively high sulfate content from seawater intrusion. Samples from the Ca-Na-HCO₃ type seepage water had an extremely high pH (>10) and abundance of Hydrogenophaga. The precipitates observed along the flow path of the seepage water included calcite, ferrihydrite, green rust, and siderite, depending on seepage water chemistry and microbial activity. This study suggests that the construction of underground structures creates distinct, localized geochemical conditions (e.g., high alkalinity, high salinity, and oxic conditions), which may impact microbial communities. These biogeochemical changes may have undesirable large-scale impacts such as water pump clogging. An understanding of the process and long-term monitoring are essential to assess the safety of underground facilities.

An evolving view on biogeochemical cycling of iron

Biogeochemical cycling of iron is crucial to many environmental processes, such as ocean productivity, carbon storage, greenhouse gas emissions and the fate of nutrients, toxic metals and metalloids. Knowledge of the underlying processes involved in iron cycling has accelerated in recent years along with appreciation of the complex network of biotic and abiotic reactions dictating the speciation, mobility and reactivity of iron in the environment. Recent studies have provided insights into novel processes in the biogeochemical iron cycle such as microbial ammonium oxidation and methane oxidation coupled to Fe(III) reduction. They have also revealed that processes in the biogeochemical iron cycle spatially overlap and may compete with each other, and that oxidation and reduction of iron occur cyclically or simultaneously in many environments. This Review discusses these advances with particular focus on their environmental consequences, including the formation of greenhouse gases and the fate of nutrients and contaminants.

Positive effects of zeolite powder on aerobic granulation: Nitrogen and phosphorus removal and insights into the interaction mechanisms

Aerobic granular sludge is considered one of the most promising biological wastewater treatment technologies of the 21st century. However, the long granulation time and poor treatment effect on N and P have severely limited its popularity and large-scale application. In this study, we systematically examine the strengthening effects of zeolite powder on granulation, N and P removal, and their interaction mechanisms. The addition of zeolite powder decreased sludge granulation time to 18 d, and improved average N and P removal rates by 4.48% and 2.22%, respectively. The multi-pore and nutrient-rich environment of the zeolite powder is beneficial for maintaining microbial activity and granular stability. Moreover, its adsorption to N and P enriches their respective removal strains, improving their removal efficiency.

An efficient way to achieve stable and high-rate ferrous ion-dependent nitrate removal (FeNiR): Batch sludge replacement

Ferrous ion can be used as electron donor for denitrification in the ferrous ion-dependent nitrate removal (FeNiR). To prevent the FeNiR performance decrease caused by iron encrustation, a modified FeNiR process with batch sludge replacement was developed. Based on the decay kinetics of sludge mass and sludge activity, the sludge retention time (SRT) was determined as 40 days in the modified FeNiR process. To keep the FeNiR rate at 0.70 kg-N/(m³·d), the sludge replacement amount was 25% of total sludge every 10 days. The FeNiR efficiency stabilized around 70%. The batch sludge replacement could be an effective method to offset the active sludge decay caused by iron encrustation, and therefore led to the good FeNiR performance. The wasted FeNiR sludge was found to adsorb phosphate at a rate of 0.9 mg-P/(g VS min). The modified FeNiR process was proposed to be coupled with phosphate removal, achieving the co-removal of nitrate and phosphate. The coupled technology is promising due to the less consumption of resources and energy, as well as the less production of excessive sludge.

Phosphorus Recovery from Wastewater Prominently through a Fe(II)-P Oxidizing Pathway in the Autotrophic Iron-Dependent Denitrification Process

Phosphorus (P) recovery from wastewater can be completed by iron-involved autotrophic denitrification via forming Fe(III)-P precipitates and/or adsorbing P onto Fe(III) oxyhydroxides. However, so far, most studies focused on the final P-containing products, while the P-capturing pathways in such a process remain unclear. In this work, autotrophic iron-dependent denitrification (AIDD) was used as a typical anoxic iron-involved P-capturing biosystem to investigate the main P recovery pathways. The AIDD biosystem showed a relatively stable capability of capturing P coupled with nitrate reduction. Direct formation of amorphous Fe(II)-P precipitates after the phosphate was fed, followed by microbially driven oxidation into Fe(III)-P minerals, was found to be the primary pathway for the P capture. In addition, adsorption of phosphate onto the formed iron oxyhydroxides also contributed to the P recovery. This work provides better understanding about recovering P in AIDD and iron-involved denitrification and highlights the important roles of iron oxidizers in the iron-related biological wastewater treatment processes.

The role of natural Fe(II)-bearing minerals in chemoautotrophic chromium (VI) bio-reduction in groundwater

To date, comparatively little is known about the role of natural Fe(II)-bearing minerals in bioremediation of chromium (VI) contaminated aquifers subject to chemoautotrophic conditions. This work employed four kinds of Fe(II)-bearing minerals (pyrite, mackinawite, wustite, and magnetite) as inorganic electron donors to support Cr(VI) bio-reduction. In batch experiments, mackinawite (FeS) performed best, with Cr(VI) removal efficiency of 98.1 ± 1.21 % in 96 h. Continuous column experiments lasting 180 d implied that groundwater chemistry and hydrodynamics influenced the Cr(VI) removal process. A breakthrough study suggested that biotic and abiotic contributions to Cr(VI) reduction were 76.0 ± 1.12 % and 24.1 ± 1.43 %, respectively. Cr(VI) was reduced to insoluble Cr(III), whereas Fe(II) and S(-II) in mackinawite were finally oxidized to Fe(III) and sulfate. Mackinawite evolved progressively into pyrrhotite. High-throughput 16S rRNA gene sequencing indicated that mackinawite-driven Cr(VI) reduction was mediated through synergistic interactions of microbial consortia; i.e. autotrophs as Acidovorax synthesized volatile fatty acids as metabolic intermediates, which were consumed by Cr(VI) reducers as Geobacter. Genes encoding enzymes for S oxidation (soxB) and Cr(VI) reduction (chrA, yieF) were upregulated. Cytochrome c participating in Fe(II) oxidation increased significantly. This work advances the development of sustainable techniques for Cr(VI) polluted groundwater remediation.

Iron as electron donor for denitrification: The efficiency, toxicity and mechanism

For the treatment of low C/N wastewaters, methanol or acetate is usually dosed as electron donor for denitrification but such organics makes the process costly. To decrease the cost, iron which is the fourth most abundant element in lithosphere is suggested as the substitution of methanol and acetate. The peak volumetric removal rate (VRR) of nitrate nitrogen in the ferrous iron-dependent nitrate removal (FeNiR) reactor was 0.70 ± 0.04 kg-N/(m³·d), and the corresponding removal efficiency was 98%. Iron showed toxicity to cells by decreasing the live cell amount (dropped 56%) and the live cell activity (dropped 70%). The toxicity of iron was mainly expressed by the formation of iron encrustation. From microbial community data analysis, heterotrophs (Paracocccus, Thauera and Azoarcus) faded away while the facultative chemolithotrophs (Hyphomicrobium and Anaerolineaceae_uncultured) dominated in the reactor after replacing acetate with ferrous iron in the influent. Through scanning electron microscope (SEM) and transmission electron microscope (TEM), two iron oxidation sites in FeNiR cells were observed and accordingly two FeNiR mechanisms were proposed: 1) extracellular FeNiR in which ferrous iron was bio-oxidized extracellularly; and 2) intracellular FeNiR in which ferrous iron was chemically oxidized in periplasm. Bio-oxidation (extracellular FeNiR) and chemical oxidation (intracellular FeNiR) of ferrous iron coexisted in FeNiR reactor, but the former one predominated. Comparing with the control group without electron donor in the influent, FeNiR reactor showed 2 times higher and stable nitrate removal rate, suggesting iron could be used as electron donor for denitrification. However, further research works are still needed for the practical application of FeNiR in wastewater treatment.

Mineral characterization of the biogenic Fe(III)(hydr)oxides produced during Fe(II)-driven denitrification with Cu, Ni and Zn

The recovery of iron and other heavy metals by the formation of Fe(III) (hydr)oxides is an important application of microbially-driven processes. The mineral characterization of the precipitates formed during Fe(II)-mediated autotrophic denitrification with and without the addition of Cu, Ni, and Zn by four different microbial cultures was investigated by X-ray fluorescence (XRF), Raman spectroscopy, scanning electron microscopy equipped with energy dispersive X-Ray analyzer (SEM-EDX), Fourier transform infrared spectroscopy (FTIR) and X-ray Powder Diffraction (XRD) analyses. Fe(II)-mediated autotrophic denitrification resulted in the formation of a mixture of Fe(III) (hydr)oxides composed of amorphous phase, poorly crystalline (ferrihydrite) and crystalline phases (hematite, akaganeite and maghemite). The use of a Thiobacillus-dominated mixed culture enhanced the formation of akaganeite, while activated sludge enrichment and the two pure cultures of T. denitrificans and Pseudogulbenkiania strain 2002 mainly resulted in the formation of maghemite. The addition of Cu, Ni and Zn led to similar Fe(III) (hydr)oxides precipitates, probably due to the low metal concentrations. However, supplementing Ni and Zn slightly stimulated the formation of maghemite. A thermal post-treatment performed at 650 °C enhanced the crystallinity of the precipitates and favored the formation of hematite and some other crystalline forms of Fe associated with P, Na and Ca.

Bioaccumulation of potentially toxic elements by submerged plants and biofilms: A critical review

The accumulation of potentially toxic elements (PTEs) in aquatic ecosystems has become a global concern, as PTEs may exert a wide range of toxicological impacts on aquatic organisms. Submerged plants and the microorganisms attached to their surfaces, however, have displayed great potential as a means of coping with such pollution. Therefore, it is crucial to understand the transport pathways of PTEs across sediment and organisms as well as their accumulation mechanisms in the presence of submerged plants and their biofilms. The majority of previous studies have demonstrated that submerged plants and their biofilms are indicators of PTE pollution in the aquatic environment, yet relatively little is known about PTE accumulation in epiphytic biofilms. In this review, we describe the transport pathways of PTEs in the aquatic environment in order to offer remarkable insights into bioaccumulation mechanisms in submerged plants and their biofilms. Based on the literature cited in this review, the roles of epiphytic biofilms in bioaccumulation and as an indicator of ecosystem health are discussed.

Antimonite Binding to Natural Organic Matter: Spectroscopic Evidence from a Mine Water Impacted Peatland

Peatlands and other wetlands are sinks for antimony (Sb), and solid natural organic matter (NOM) may play an important role in controlling Sb binding. However, direct evidence of Sb sequestration in natural peat samples is lacking. Here, we analyzed solid phase Sb, iron (Fe), and sulfur (S) as well as aqueous Sb speciation in three profiles up to a depth of 80 cm in a mine water impacted peatland in northern Finland. Linear combination fittings of extended X-ray absorption fine structure spectra showed that Sb binding to Fe phases was of minor importance and observed only in the uppermost layers of the peatland. Instead, the dominant (to almost exclusive) sequestration mechanism was Sb(III) binding to oxygen-containing functional groups, and at greater depths, increasingly Sb(III) binding to thiol groups of NOM. Aqueous Sb speciation was dominated by antimonate, while antimonite concentrations were low, further supporting our findings of much higher reactivity of Sb(III) than Sb(V) toward peat surfaces. Insufficient residence time for efficient reduction of antimonate to antimonite currently hinders higher Sb removal in the studied peatland. Overall, our findings imply that Sb(III) binding to solid NOM acts as an important sequestration mechanism under reducing conditions in peatlands and other high-organic matter environments.

Metabolic Inactivity and Re-awakening of a Nitrate Reduction Dependent Iron(II)-Oxidizing Bacterium Bacillus ferrooxidans

Microorganisms capable of anaerobic nitrate-dependent Fe(II) (ferrous iron) oxidation (ANDFO) contribute significantly to iron and nitrogen cycling in various environments. However, lab efforts in continuous cultivation of ANDFO strains suffer from loss of activity when ferrous iron is used as sole electron donor. Here, we used a novel strain of nitrate-dependent Fe(II)-oxidizing bacterium Bacillus ferroxidians as a model and focused on the physiological activity of cells during ANDFO. It was shown that B. ferrooxidans entered a metabolically inactive state during ANDFO. B. ferrooxidans exhibited nitrate reduction coupled with Fe(II) oxidation, and the activity gradually declined and was hardly detected after 48-h incubation. Propidium monoazide (PMA) assisted 16S rRNA gene real-time PCR suggested that a large number of B. ferrooxidans cells were alive during incubation. However, ²H(D)-isotope based Raman analysis indicated that the cells were metabolically inactive after 120-h of ANDFO. These inactive cells re-awakened in R2A medium and were capable of growth and reproduction, which was consistent with results in Raman analysis. Scanning electron microscopy (SEM) observation and x-ray diffraction (XRD) revealed the formation of Fe minerals in close proximity of cells in the Fe(II)-oxidizing medium after Fe(II) oxidation. Overall, our results demonstrated that continued ANDFO can induce a metabolically inactive state in B. ferrooxidans, which was responsible for the loss of activity during ANDFO. This study provides an insight into the ANDFO process and its contribution to iron and nitrogen cycling in the environments.

Fabricating biogenic Fe(III) flocs from municipal sewage sludge using NAFO processes: Characterization and arsenic removal ability

This study involved fabricating biogenic Fe(III) flocs enriched from municipal sludge using microbial nitrate-dependent anaerobic Fe(II)-oxidizing (NAFO) processes. The research focused on bacterial community compositions and physicochemical properties of the biogenic Fe(III) flocs and their ability to adsorb arsenic (As). High-throughput sequencing analysis showed that significant microbial succession occurs in the raw sludge after the NAFO processes. The predominant bacterial communities in the biogenic Fe(III) flocs included Rhodanobacter, Parvibaculum, Gemmatimonas and Segetibacter genera. Microscopic and spectroscopic analyses included scanning electron microscopy - energy disperse spectroscopy (SEM-EDS), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and Raman spectroscopy. These tests indicated that biogenic Fe(III) flocs were a mixture of NAFO bacteria and nanosized, poorly crystalline Fe(III) oxide particles. Batch experiments showed that after 120 min of reaction time, more than 95% of As(III) and As(V) (at an initial concentrations of 0.25 mg/L) were effectively removed with 120 ppm biogenic Fe(III) flocs. In addition, biogenic Fe(III) flocs removed As more effectively than abiotic Fe(III) flocs. These findings indicated that biogenic Fe(III) flocs produced from municipal sludge using NAFO processes performed well in removing As.

Variations regularity of microorganisms and corrosion of cast iron in water distribution system

Corrosion, one of the most common problems of metal pipe for water supply, generally leads to poor water quality, bacteria proliferation, water capacity decrease and other problems. As microorganisms affect corrosion by changing the characteristics of metal surface, the mechanism of microbial corrosion still remains unclear. The corrosion behavior of ductile cast iron is implemented in the dynamic flow and static conditions, in which variations of water quality and microbial community are analyzed in details. The results show that if the corrosion rate of ductile cast iron decreases, the corrosion of cast iron will result in a lower DO and a higher total iron in bulk water. The number of microorganisms is not a decisive factor of corrosion, even though the counts of bacteria had a close relationship with DO. On the basis of the detection of the 10 kinds of nitrate-reducing bacteria by Miseq sequencing, NRB of the biofilm biomass accounts for 18.3% on the 30th day and 20.5% on the 55th day. Even though aerobic NRBs go into the biofilm later than the facultative anaerobic NRBs, the growth of the anaerobic NRBs is not affected.

Sub-micron level investigation reveals the inaccessibility of stabilized carbon in soil microaggregates

Direct evidence-based approaches are vital to evaluating newly proposed theories on the persistence of soil organic carbon and establishing the contributions of abiotic and biotic controls. Our primary goal was to directly identify the mechanisms of organic carbon stabilization in native-state, free soil microaggregates without disrupting the aggregate microstructure using scanning transmission x-ray microscopy coupled with near edge x-ray absorption fine structure spectroscopy (STXM-NEXAFS). The influence of soil management practices on microaggregate associated-carbon was also assessed. Free, stable soil microaggregates were collected from a tropical agro-ecosystem in Cruz Alta, Brazil. The long-term experimental plots (>25 years) comparing two tillage systems: no-till and till with a complex crop rotation. Based on simultaneously collected multi-elemental associations and speciation, STXM-NEXAFS successfully provided submicron level information on organo-mineral associations. Simple organic carbon sources were found preserved within microaggregates; some still possessing original morphology, suggesting that their stabilization was not entirely governed by the substrate chemistry. Bulk analysis showed higher and younger organic carbon in microaggregates from no-till systems than tilled systems. These results provide direct submicron level evidence that the surrounding environment is involved in stabilizing organic carbon, thus favoring newly proposed concepts on the persistence of soil organic carbon.

Azo dye degradation pathway and bacterial community structure in biofilm electrode reactors

In this study, the degradation pathway of the azo dye X-3B was explored in biofilm electrode reactors (BERs). The X-3B and chemical oxygen demand (COD) removal efficiencies were evaluated under different voltages, salinities, and temperatures. The removal efficiencies increased with increasing voltage. Additionally, the BER achieved maximum X-3B removal efficiencies of 66.26% and 75.27% at a NaCl concentration of 0.33 g L^-1 and temperature of 32 °C, respectively; it achieved a COD removal efficiency of 75.64% at a NaCl concentration of 0.330 g L^-1. Fourier transform infrared spectrometry and gas chromatography-mass spectrometry analysis indicated that the X-3B biodegradation process first involved the interruption of the conjugated double-bond, resulting in aniline, benzodiazepine substance, triazine, and naphthalene ring formation. These compounds were further degraded into lower-molecular-weight products. From this, the degradation pathway of the azo dye X-3B was proposed in BERs. The relative abundances of the microbial community at the phylum and genus levels were affected by temperature, the presence of electrons, and an anaerobic environment in the BERs. To achieve better removal efficiencies, further studies on the functions of the microorganisms are needed.