Redox Cycling Of Iron Research Articles

Decoupling of redox processes from soil saturation in Arctic tundra

Permafrost thaw in warming Arctic landscapes alters hydrology and saturation-driven biogeochemical processes. Models assume that aerobic respiration occurs in drained soils while saturated soils support methanogenesis; however, saturated soils maintain redox gradients that host a range of anaerobic metabolisms. We evaluated how redox potential and redox-active solutes vary with soil moisture in the active layer of permafrost-affected acidic and non-acidic tundra hillslopes. Oxidizing conditions persisted in highly permeable organic horizons of both unsaturated tussock tundra and saturated wet sedge meadows. Redox potential decreased with depth in all soils as increasing soil bulk density restricted groundwater flow and oxygen diffusion. High concentrations of dissolved iron, phosphate, and organic carbon coincided with redox boundaries below the soil surface in acidic tundra, indicating active iron redox cycling and potential release of adsorbed phosphate during iron (oxyhydr)oxide dissolution. In non-acidic tundra, weatherable minerals affected nutrient dynamics more than redox-driven iron cycling, especially in low-lying, saturated areas where thaw reached mineral soils. The role of thaw depth and the ability of saturated soils to maintain oxidizing conditions in organic surface layers highlight the importance of soil physical properties and hydrology in predicting biogeochemical processes and greenhouse gas emissions.

The successive reduction of iodate to iodide driven by iron redox cycling

Ferrous iron (Fe(II)) produced by microbial Fe(III) reduction and reactive oxygen species (ROS) generated from aerobic Fe(II) oxidation can mediate iodate (IO3−) reduction and iodide (I−) oxidation, respectively. Nevertheless, how Fe redox cycling under redox fluctuating conditions drives transformation of iodine species remain unclear. In this study, Shewanella oneidensis MR-1 wildtype (WT) and its mutant △dmsEFAB, which lost the ability to enzymatically reduce IO3−, were chosen to conduct ferrihydrite/goethite/nontronite culture experiments under consecutive cycles of anoxic reduction of Fe(III) and re-oxidation of Fe(II) by O2 to reveal the role of Fe redox cycling in the transformation of iodine species. The results showed that both surface-adsorbed and mineral structural Fe(II) chemically reduced IO3−. Chemical IO3− reduction by biogenic Fe(II) was slower than enzymatic IO3− reduction by WT. Compared to △dmsEFAB cultures, WT cultures all showed higher Fe(II) concentrations under anoxic conditions but lower cumulative •OH under oxic conditions, which imply the chemical reaction between I− and ROS. I− oxidation by ROS, however, did not lead to a significant production of IO3− compared with I− formed under anoxic conditions. Consequently, Fe redox cycling successively reduced IO3− to I−, which highlights vital roles of Fe(III)-reducing bacteria in I− formation and mobilization in environments.

Iron-modified biochars and their aging reduce soil cadmium mobility and inhibit rice cadmium uptake by promoting soil iron redox cycling

Iron (Fe) modified biochar has been widely used for cadmium (Cd) contaminated soil remediation. However, the accompanying anions introduced during the modification process potentially affect the behavior of Cd in soil. In this study, we investigated the distinct Cd immobilization mechanisms by Fe2(SO4)3 modified biochar (FSBC) and Fe(NO3)3 modified biochar (FNBC) in a two-year pot experiment. Results showed that both FSBC and FNBC significantly reduced Cd concentrations in rice grains by 23%–42% and 30%–37% compared to pristine biochar (BC). Specifically, NFBC promoted the formation of amorphous Fe oxides by enhancing the NO3−-reducing Fe(II) oxidation process, which significantly increased Fe/Mn oxide-bound Cd and decreased soil CaCl2-extractable Cd. For FSBC, the introduction of SO42− significantly promoted the formation of Fe plaques by enhancing the Fe(III) reduction process, which blocked the Cd transfer from the soil to the rice roots. More importantly, after two years of biochar application, an organo-mineral complex layer is formed on the biochar surface, which immobilized a large amount of Cd. The Cd immobilization on the surface of aged biochar could be due to the fixation by the secondary Fe oxides within the organo-mineral layer and the complexation by the surface functional groups. The result of laser ablation inductively coupled plasma mass spectrometry showed that the Cd content on aged FNBC and FSBC was 5.9 and 2.6 times higher than on aged BC. This might be attributed to the Fe-modified biochar's higher electron exchange capability (EEC), which promoted the development of organo-mineral complexes. Notably, the EEC of biochar was maintained during its aging process, which may keep the biochar surface active and facilitate continual Cd immobilization. This study revealed the complex mechanisms of soil Cd immobilization with Fe-modified biochar, providing new insights into sustainable biochar environmental remediation.

Hierarchically ordered porous carbon crystals for nanoconfined and sustainable fenton oxidation of water pollutants

In this work, we developed a hierarchically multi-faceted ordered porous carbon crystal (HOPC) for generating a favorable chemical microenvironment to intensify both Fe3+ binding and H2O2 utilization efficiency. The confined pore space and surface functionality enable •OH and micropollutants to be concentrated in the pores, thus significantly accelerating pollutant degradation kinetics (0.034 min−1) with reduced chemical consumption (0.081 mM H2O2). Theoretical calculations and in-situ characterizations reveal that both CO and pyrrolic N act as active sites bridging Fe3+, facilitating the transfer of outermost e− from the p-orbital of active sites (O and N) to the d-orbital of iron, effectively reducing Fe3+ to Fe2+ and enhancing the open-circuit potential of HOPC-Fe3+ complexes, as well as facilitating e− extraction from H2O2. For the first time, H2O2 was unveiled to be both an electron donor and acceptor to cooperate with the iron redox cycle for sustainable •OH generation toward fast pollutant decontamination.

Activation of iron oxides through organic matter-induced dissolved oxygen penetration depth dynamics enhances iron-cycling driven ammonium oxidation in microaerobic granular sludge

The iron redox cycle can enhance anammox in treating low-strength ammonia wastewater. However, maintaining an effective iron redox cycle and suppressing nitrite-oxidizing bacteria in a one-stage partial nitritation and anammox (PN/A) process poses challenges during long-term aeration. We proposed a novel and simple strategy to achieve an efficient iron redox cycle in an iron-mediated anoxic-microaerobic (A/O) process by controlling organic matter (OM) at medium-strength levels (30–110 mg COD/L) in microaerobic granular sludge (MGS)-dominated reactor. The developed A/O process consistently achieved >90 % OM removal and >75 % nitrogen removal. Medium-strength OM varied the penetration depths of dissolved oxygen (DO) in MGS, regulating redox conditions and promoting redox reactions across MGS layers, thus activating accumulated inert iron oxides. Ammonia-oxidizing bacteria (Nitrosomonas), iron-reducing bacteria (e.g., Ignavibacterium, Geobacter), and anammox bacteria (Ca. Kuenenia) coexisted harmoniously in MGS. This coexistence ensured high anammox and Feammox rates along with a robust iron redox cycle, thereby mitigating the adverse impacts of fluctuating DO and OM on one-stage PN/A process stability. The identification of iron reduction-associated genes within Ca. Kuenenia, Ignavibacterium, and Geobacter suggests their potential roles in supporting Feammox coupled in one-stage PN/A process. This study introduces an iron-cycle-driven A/O process as an energy-efficient alternative for simultaneous carbon and nitrogen removal from low-strength wastewater.

Promotion of aromatic amino acids of extracellular polymeric substance targeted transformation via sulfite mediated iron redox cycling in sludge solid-liquid separation

Highly hydrophilic extracellular polymeric substance (EPS) with gel-like structure seriously plagues the development of sludge deep dewatering. Oxysulfur radicals-based oxidation driven by iron-bearing mineral proposes a promising strategy for effective EPS decomposition. However, the transformation and involved interaction mechanisms of aromatic proteins are still controversial due to the complex EPS structure. Herein, sulfite mediated siderite (denoted as Fe(II)/S(IV)) was developed for targeted transformation aromatic amino acids in EPS oxidation to strengthen sludge solid-liquid separation. The enhanced sludge dewaterability were benefited from the Fe(II)/S(IV) bonded interaction assisted by Fe3+/Fe2+ as redox interface that facilitating the release of intracellular bound water via diminish the hydrophily and bind strength with solid protons. The amide region nitrogen of aromatic amino acids (especially tyrosine and tryptophan) originating from EPS presented looser structure and lower spatial site resistance, which were attributed to the exposure of hydrophobic sites in amino groups after Fe(II)/S(IV) treatment. Furthermore, the effective decline of aromatic amino acids in inner layer-EPS (loosely bound EPS and tightly bound EPS) was directed from Fe-N targeted interaction by triggering a series of sulfate-based radical chain reactions. The good correlation between electron transfer amount (R2 = 0.926) and Fe-N (R2 = 0.925) with bonding interaction demonstrated that the complexation of aromatic amino acids with Fe sites on siderite/sulfite via Fe-N bonds, accounting for efficient sludge solid-liquid separation. This study deepens the understanding of sludge organic matter targeted transformation and provides a tactic for iron-based conditioning of sludge.

Neglected role of iron redox cycle in direct interspecies electron transfer in anaerobic methanogenesis: Inspired from biogeochemical processes

Anaerobic digestion is an indispensable technical option towards green and low-carbon wastewater treatment, with interspecies electron transfer (IET) playing a key role in its efficiency and operational stability. The exogenous semiconductive iron oxides have been proven to effectively enhance IET, while the cognition of the physicochemical-biochemical coupling stimulatory mechanism was circumscribed and remains to be elucidated. In this study, semiconductive iron oxides, α-Fe2O3, γ-Fe2O3, α-FeOOH, and γ-FeOOH were found to significantly enhance syntrophic methanogenesis by 76.39, 72.40, 37.33, and 32.64% through redirecting the dominant IET pathway from classical interspecies hydrogen transfer to robust direct interspecies electron transfer (DIET). Their alternative roles as electron shuttles potentially substituting for c-type cytochromes were conjectured to establish an electron transport matrix associated with conductive pili. Distinguished from the conventional electron conductor mechanism of conductive Fe3O4, semiconductive iron oxides facilitated DIET intrinsically through the capacitive Fe(III/II) redox cycles coupled with secondary mineralization. The growth of Aminobacterium, Sedimentibacter, and Methanothrix was enriched and the gene copy numbers of Geobacteraceae 16S ribosomal ribonucleic acid were selectively flourished by 2.0-∼4.5- fold to establish a favorable microflora for DIET pathway. Metabolic pathways of syntrophic acetogenesis from propionate/butyrate and CO2 reduction methanogenesis were correspondingly promoted. The above findings provide new insights into the underlying mechanism of iron minerals enhancing the DIET-oriented pathway and offer paradigms for redox-mediated energy harvesting biological wastewater treatment.

Red mud/steel slag three-dimensional electrochemical system for the removal of organic pollutant with Na2S2O3 as electrolyte: Contribution of •OHads

The dynamic changes in the iron redox cycle (Fe2+/Fe3+) played a crucial role in the electro-Fenton degradation of organic pollutants. The synthesis of composite particle electrodes RMSSx:y from a mixture of steel slag (SS) and red mud (RM) was applied in a three-dimensional electrochemical system for the remediation of hydrochloric tetracycline (TCH) in wastewater, utilizing Na2S2O3 as an electrolyte. Further, some investigations were conducted to determine the effects of RM/SS, electrolyte, particle electrode dosage, cell voltage, and pH on TCH degradation efficiency. Under optimal conditions such as 8 g/L RMSS5:5 particle electrodes, a cell voltage of 4 V, an initial pH of 3, and 6 mM Na2S2O3 electrolyte, the degradation rate of TCH in wastewater was 98.3 %. Meanwhile, its total organic carbon (TOC) was reduced by 88 % within 60 min. Cyclic experiments with consistently high degradation efficiency demonstrated the stability of the particle electrode RMSS5:5. The practical applicability of RMSS5:5 was further confirmed through its successful treatment of various industrial wastewaters, including dyeing (DW), chemical (CW), and pharmaceutical (PW) effluents, achieving chemical oxygen demand (COD) removal rates of 67.8 %, 67.6 %, and 65.0 % respectively. The enhancement of TCH degradation through the utilization of Na2S2O3 as the electrolyte was primarily attributed to the complexation and reduction effects exerted by S2O32- ions. These effects prevented the leaching and precipitation of iron ions, facilitated the reduction of Fe (III), and consequently enabled •OHads to play a dominant role in the degradation process. These findings demonstrated the potential of RMSS5:5 in efficiently degrading emerging contaminants, thereby offering an innovative approach for utilizing SS and RM as functional materials in electro-Fenton processes.

Methanogenesis in the presence of oxygenic photosynthetic bacteria may contribute to global methane cycle

Accumulating evidences are challenging the paradigm that methane in surface water primarily stems from the anaerobic transformation of organic matters. Yet, the contribution of oxygenic photosynthetic bacteria, a dominant species in surface water, to methane production remains unclear. Here we show methanogenesis triggered by the interaction between oxygenic photosynthetic bacteria and anaerobic methanogenic archaea. By introducing cyanobacterium Synechocystis PCC6803 and methanogenic archaea Methanosarcina barkeri with the redox cycling of iron, CH4 production was induced in coculture biofilms through both syntrophic methanogenesis (under anoxic conditions in darkness) and abiotic methanogenesis (under oxic conditions in illumination) during the periodic dark-light cycles. We have further demonstrated CH4 production by other model oxygenic photosynthetic bacteria from various phyla, in conjunction with different anaerobic methanogenic archaea exhibiting diverse energy conservation modes, as well as various common Fe-species. These findings have revealed an unexpected link between oxygenic photosynthesis and methanogenesis and would advance our understanding of photosynthetic bacteria’s ecological role in the global CH4 cycle. Such light-driven methanogenesis may be widely present in nature.

Effect of pyrite on the treatment of chlorophenolic compounds with zero-valent iron-Fenton process under uncontrolled pH conditions: reaction mechanism and biodegradability

This current study explored the effect of pyrite on the treatment of chlorophenolic compounds (CP) by Fenton process with micron-sized zero-valent iron (ZVI) as the catalyst. The experiments were conducted in batch reactors with 100 mg L−1 CP, 0–0.02 M H2O2, and variable pyrite and ZVI doses (0–1 g L−1). Our findings show that while the reactor with 1 g L−1 ZVI as the only catalyst achieved only 10% CP removal efficiency due to rapid ZVI surface passivation and ZVI particle aggregation, the CP removal efficiency increased with increasing pyrite dose and reached 100% within couple of minutes in reactors with 0.8 g L−1 pyrite and 0.2 g L−1 ZVI. The CP removal was mainly driven by the oxidative treatment of CPs with some strong radicals such as hydroxyl radicals (•OH) while the adsorption onto the catalyst surface was only responsible for 10 to 25% of CP removals, depending on the type of CP studied. The positive impact of pyrite on CP removal by the ZVI/H2O2 system could be attributed to the ability of pyrite to (1) create an acidic environment for optimum Fenton process, (2) provide support material for ZVI to minimize ZVI particle agglomeration, and (3) stimulate iron redox cycling for improved surface site generation. Following oxidative Fenton treatment, the degradation intermediate products of CPs, including some aromatic compounds (benzoquinone, hydroquinone, etc.) and organic acids (e.g., acetic acid), became more biodegradable in comparison to their mother compounds. Overall, the treatment systems with a mixture of ZVI and pyrite as catalyst materials could offer a suitable cost-effective technology for the treatment of wastewater containing biologically non- or low-degradable toxic compounds such as chlorophenols.

New insights into dual metals boost PMS activation over iron tungstate: The role of surface hydroxyl

The activity of iron oxychloride (FeOCl) in advanced oxidation processes (AOPs) was limited by the redox cycle of Fe(III)/Fe(II). To resolve this problem, FeOCl loading on WS2 (FW-X) was successfully synthesized with the different ratios of Fe/W (X = 0.5, 1, 2, 3) to active peroxymonosulfate (PMS). Among them, FW-3 displayed excellent catalytic performance which activated 0.5 mM PMS to degrade 97 % thiamphenicol (TAP) in 45 min. FW-3/PMS performed well in a wide pH range (3–10). Besides, the anions (Cl-,NO3-,H2PO4-,HPO42-andHCO3-) and humic acid inhibited the oxidation of TAP in varying degrees. During the catalytic process, H2O and PMS were adsorbed on the surface of FW-3 with the formation of surface hydroxyl groups (Fe–OH) and Fe–HOOSO3. Surface hydroxyls as a bridge promoted the electron transfer between O–O bond and Fe sites. Then O–O bond broke to generate reactive oxygen species (ROS, HO·, SO4∙-, Fe(IV), O2∙- and 1O2), and SO4∙- was the dominant ROS for the degradation of TAP. Additionally, WS2 as a co-catalyst broke the limitation of the iron redox cycle. Under the attack of these ROS, TAP produced eighteen intermediates through four possible degradation pathways. According to ecotoxicity assessment, toxicity of some intermediates produced from path of S-C bond breakage was less harmful than TAP. After three consecutive cycles, FW-3 maintained outstanding catalytic performance. Additionally, FW-3/PMS possessed a good non-selectivity that could remove different contaminants. In a word, this paper proved the potential of FW-3/PMS in practical wastewater remediation.

Sulfur cycling likely obscures dynamic biologically-driven iron redox cycling in contemporary methane seep environments.

Deep-sea methane seeps are amongst the most biologically productive environments on Earth and are often characterised by stable, low oxygen concentrations and microbial communities that couple the anaerobic oxidation of methane to sulfate reduction or iron reduction in the underlying sediment. At these sites, ferrous iron (Fe2+) can be produced by organoclastic iron reduction, methanotrophic-coupled iron reduction, or through the abiotic reduction by sulfide produced by the abundant sulfate-reducing bacteria at these sites. The prevalence of Fe2+in the anoxic sediments, as well as the availability of oxygen in the overlying water, suggests that seeps could also harbour communities of iron-oxidising microbes. However, it is unclear to what extent Fe2+ remains bioavailable and in solution given that the abiotic reaction between sulfide and ferrous iron is often assumed to scavenge all ferrous iron as insoluble iron sulfides and pyrite. Accordingly, we searched the sea floor at methane seeps along the Cascadia Margin for microaerobic, neutrophilic iron-oxidising bacteria, operating under the reasoning that if iron-oxidising bacteria could be isolated from these environments, it could indicate that porewater Fe2+ can persist is long enough for biology to outcompete pyritisation. We found that the presence of sulfate in our enrichment media muted any obvious microbially-driven iron oxidation with most iron being precipitated as iron sulfides. Transfer of enrichment cultures to sulfate-depleted media led to dynamic iron redox cycling relative to abiotic controls and sulfate-containing cultures, and demonstrated the capacity for biogenic iron (oxyhydr)oxides from a methane seep-derived community. 16S rRNA analyses revealed that removing sulfate drastically reduced the diversity of enrichment cultures and caused a general shift from a Gammaproteobacteria-domainated ecosystem to one dominated by Rhodobacteraceae (Alphaproteobacteria). Our data suggest that, in most cases, sulfur cycling may restrict the biological "ferrous wheel" in contemporary environments through a combination of the sulfur-adapted sediment-dwelling ecosystems and the abiotic reactions they influence.

Siderite with chelator as high-efficiency Fenton reagent to degrade naphthalene via ferrous liberation and carbon dioxide radical anion-mediated iron redox cycle

Siderite (FeCO3) has been verified to be an effective iron material in Fenton-like process, but large dose of FeCO3 and oxidants limits its application. Here, chelators were successfully used to enhance the degradation of naphthalene (NAP). When 1.0 mM of hydrogen peroxide (H2O2) and 1.0 g L−1 of FeCO3 were applied, in the presence of 0.2 mM oxalic acid (OA), 0.1 mM nitrilotriacetic acid (NTA), or 0.1 mM DL-tartaric acid (DL-TA), 99.3 %, 100 %, or 96.7 % of NAP was degraded within 24 h, respectively. The different roles of chelators were confirmed. For OA and DL-TA, due to weak complexation with iron ions, acid effect and generation of carbon dioxide radical anion (CO2−) contributed to high concentration of Fe2+, while for NTA, chelation and CO2− were two major factors. In addition, the excellent potential application of H2O2/FeCO3/chelator was determined because of (1) lower environmental risk of chemicals; (2) strong tolerance to water matrixes; (3) the excellent stability of FeCO3 confirmed by surface characterizations and cyclic experiment, as NAP was completely degraded after the 6th cycle. At last, H2O2/FeCO3/chelator had superiority over other reduced iron minerals due to lower running cost in lab study, better reusability, and environmental friendliness. Therefore, it is expected that H2O2/FeCO3/chelator technique can provide an effective alternative to Fenton-like process in remediation of NAP contaminated sites.

Methylene blue treatment with zero-valent iron/pyrite/H2O2 system under static and continuous flow conditions: Reaction mechanism and toxicity evaluation

Laboratory batch and continuous flow studies, coupled with surface and toxicity analysis, were performed to evaluate methylene blue treatment by heterogeneous Fenton process using zero-valent iron (ZVI) and pyrite as co-catalyst. The use of pyrite in batch reactors significantly enhanced methylene blue treatment by the ZVI/H2O2 system because of improved iron redox cycling. The continuous-flow experiments revealed that the reactor performance increased in the order of: ZVI/H2O2 < pyrite/H2O2 < ZVI/pyrite/H2O2 under dynamic flow conditions. The methylene blue treatment by the ternary ZVI/pyrite/H2O2 system was described by an initial degradation of methylene blue with *OH radicals, followed by the adsorption and/or co-precipitation of degradation intermediates with some spherical particles. The surface analysis showed that these spherical particles formed, even at pH less than 4. The genotoxicity and cytotoxicity tests performed on mouse embryonic fibroblast (3 T3-L1) and human embryonic kidney (HEK293) cell lines showed that the Fenton treatment of methylene blue using the ZVI/pyrite/H2O2 system resulted in the formation of degradation species with much lower toxicity levels relative to methylene blue. Moreover, the Fenton degradation species of methylene blue significantly enhanced the metabolic activity of several bacterial strains, including E. coli ATCC 8739 and P. aeruginosa PAO1.

Distinct photochemistry of adsorbed and coprecipitated dicarboxylates with ferrihydrite: Implications for iron reductive dissolution and carbon stabilization

Although ligand-promoted photodissolution of ferrihydrite (FH) has long been known for low molecular weight organic acids (LMWOAs), such as oxalate (Oxa) and malonate (Mal), photochemistry of coprecipitated FH with Oxa and Mal remains unknown, despite the importance of these mineral-organic associations in carbon retention has been acknowledged recently. In this study, ferrihydrite-LMWOAs associations (FLAs) were synthesized under circumneutral conditions. Photo-dissolution kinetics of FLAs were compared with those of adsorbed LMWOAs on FH surface and dissolved Fe-LMWOAs complexes through monitoring Fe(II) formation and organic carbon decay. For aqueous Fe(III)-LMWOAs complexes, Fe(II) yield was controlled by the initial concentration of LMWOAs and nature of photochemically generated carbon-centered radicals. Inner-sphere mononuclear bidentate (MB) configuration dominated while LMWOAs were adsorbed on the FH surface. MB complex of FH-Oxa was more photoreactive, leading to the rapid depletion of Oxa. Oxa can be readsorbed but in the form of binuclear bidentate and outer-sphere complexation, with much lower photoreactivity. While LMWOAs was coprecipitated with FH, the combination mode of LMWOAs with FH includes surface adsorption with a mononuclear bidentate structure and internal physical inclusion. Higher content of LMWOAs in the FLAs promoted the photo-production of Fe(II) as compared to pure FH, while it was not the case for FLAs containing moderate amounts of LMWOAs. The distinct photochemistry of adsorbed and coprecipitated Fe-LMWOAs complexes is attributed to ligand availability and configuration patterns of LMWOAs on the surface or entrapped in the interior structure. The present findings have significant implications for understanding the photochemical redox cycling of iron across the interface of Fe-organic mineral associates.

Promoting denitrification via high electron conversion ratio in the iron valence Cycle: Prospects for Pseudomonas sp. JM-7 application in environmental engineering

The interdependence between iron and nitrogen in the environment is of utmost importance, and this study has explored the intricate relationship between denitrification and iron valence cycling using Pseudomonas sp. strain JM-7 (P. JM-7) as a model bacteria. Our research demonstrated that P. JM-7 had the ability to effectuate electron transfer from the periplasmic space to electron acceptors, such as nitrate, via Cytochrome c-proteins (c-Cyts). We found that iron valence state transformations with Fe(II)/Fe(III) promoted P. JM-7′s denitrification of nitrate coupled to yeast extract powder (YEP) oxidation. Denitrification rates depended on temperature (>21 °C) and pH (6–9). During logarithmic growth, 5 g/L YEP enabled 350 mg/L nitrate removal, increasing to 1650 mg/L in stable growth. Addition of 4 mM Fe(II)/Fe(III) increased denitrification to 500 mg/L during logarithmic growth. The dynamic test results displayed that compared to the SiO2 filler control group, iron-containing fillers could facilitate P. JM-7 to reduce 100 mg/L nitrate with 50 mg/L YEP electrons, corresponding to 6 times the maximum electron transfer efficiency in the static test. Overall, iron redox cycling significantly enhanced P. JM-7 denitrification, imparting insights into iron–nitrogen interactions. In summary, our research imparts valuable insights into the nexus between iron and nitrogen in the environment, highlighting the potential application of P. JM-7 in environmental engineering. Our findings indicate that P. JM-7 exhibits a significant denitrification capability, and further research is recommended to investigate its ability to remove phosphorus and promote sustainable environmental remediation practices.

New insights into nitrogen removal by divalent iron-enhanced moving bed biofilm reactor: Performance, interfacial interaction and co-occurrence network

A divalent iron-mediated moving bed biofilm reactor with intermittent aeration was developed to enhance the nitrogen removal at low carbon-to-nitrogen ratios. The study demonstrated thatammonia removal increased from 51 ± 4 % to 79 ± 4 % and nitrate removal increased from 72 ± 5 % to 98 ± 4 % in phases I-IV, and 2–5 mg·L-1 of divalent iron significantly increased the anoxic denitrification process. Divalent iron stimulated the secretion of extracellular polymeric substances, which facilitated the formation of cross-linked network between microbial cells. Furthermore, the cycle between divalent and trivalent iron decreased the energy barrier between the biofilm and the pollutant. The microbial community further revealed that Proteobacteria (relative abundance: 40–48 %) andBacteroidota(relative abundance: 31–37 %) were the dominant phyla, supporting the synchronous nitrification and denitrification processes as well as the lower accumulation of nitrite. In conclusion, iron redox cycling significantly enhanced the nitrogen removal. This study proposes a viable strategy for the efficient treatment of nutrient wastewater.

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.

Inhalable iron redox cycling powered nanoreactor for amplified ferroptosis-apoptosis synergetic therapy of lung cancer

Inhalable iron redox cycling powered nanoreactor for amplified ferroptosis-apoptosis synergetic therapy of lung cancer

Elucidating phosphorus removal dynamics in a denitrifying woodchip bioreactor

Denitrifying woodchip bioreactors (DBRs) are an established nitrate mitigation technology, but uncertainty remains on their viability for phosphorus (P) removal due to inconsistent source-sink behaviour in field trials. We investigated whether iron (Fe) redox cycling could be the missing link needed to explain P dynamics in these systems. A pilot-scale DBR (Aotearoa New Zealand) was monitored for the first two drainage seasons (2017–2018), with supplemental in−field measurements of reduced solutes (Fe2+, HS−/H2S) and their conjugate oxidised species (Fe3+/SO42−) made in 2021 to constrain within-reactor redox gradients. Consistent with thermodynamics, the dissolution of Fe3+(s) to Fe2+(aq) within the DBR sequentially followed O2, NO3− and MnO2(s) reduction, but occurred before SO42− reduction. Monitoring of inlet and outlet chemistry revealed tight coupling between Fe and P (inlet R2 0.94, outlet R2 0.85), but distinct dynamics between drainage seasons. In season one, outlet P exceeded inlet P (net P source), and coincided with elevated outlet Fe2+, but at ⁓50 % lower P concentrations relative to inlet Fe:P ratios. In season 2 the reactor became a net P sink, coinciding with declining outlet Fe2+ concentrations (indicating exhaustion of Fe3+(s) hydroxides and associated P). In order to characterize P removal under varying source dynamics (i.e. inflows vs in-situ P releases), we used the inlet Fe vs P relationship to estimate P binding to colloidal Fe (hydr)oxide surfaces under oxic conditions, and the outlet Fe2+ concentration to estimate in-situ P releases associated with Fe (hydr)oxide reduction. Inferred P-removal rates were highest early in season 1 (k = 0.60 g P m3 d−1; 75–100 % removal), declining significantly thereafter (k = 0.01 ± 0.02 g P m3 d−1; ca. 3–67 % removal). These calculations suggest that microbiological P removal in DBRs can occur at comparable magnitudes to nitrate removal by denitrification, depending mainly on P availability and hydraulic retention efficiency.