Influence of Physical Perturbation on Fe(II) Supply in Coastal Marine Sediments

Biogeochemical cycles of iron: Processes, mechanisms, and environmental implications

The iron (Fe) biogeochemical cycle is critical for abiotic and biological environmental processes that overlap spatially and may compete with each other. The development of modern molecular biology technologies promoted the understanding of the electron transport mechanisms of Fe-cycling-related microorganisms. Recent studies have revealed a novel pathway for microaerophilic ferrous iron (Fe(II))-oxidizers in extracellular Fe(II) oxidation. In addition, OmcS, OmcZ, and OmcE nanowires on the cell surface have been shown to promote electron transfer between microorganisms and their environment. These processes affect the fate of pollutants in directly or indirectly ways, such as greenhouse gas emissions. In this review, these advances and the environmental implications of the Fe cycle process were discussed, with a particular focus on the mechanisms of intracellular or extracellular electron transport in microorganisms.

Arsenic immobilization and greenhouse gas emission depend on quantity and frequency of nitrogen fertilization in paddy soil

Nitrogen (N) fertilization in paddy soils decreases arsenic mobility and methane emissions. However, it is unknown how quantity and frequency of N fertilization affects the interlinked redox reactions of iron(II)-driven denitrification, iron mineral (trans-)formation with subsequent arsenic (im-)mobilization, methane and nitrous oxide emissions, and how this links to microbiome composition. Thus, we incubated paddy soil from Vercelli, Italy, over 129 days and applied nitrate fertilizer at different concentrations (control: 0, low: ∼35, medium: ∼100, high: ∼200 mg N kg-1 soil-1) once at the beginning and after 49 days. In the high N treatment, nitrate reduction was coupled to oxidation of dissolved and solid-phase iron(II), while naturally occurring arsenic was retained on iron minerals due to suppression of reductive iron(III) mineral dissolution. In the low N treatment, 40 μg L-1 of arsenic was mobilized into solution after nitrate depletion, with 69 % being immobilized after a second nitrate application. In the non-fertilized control, concentrations of dissolved arsenic were as high as 76 μg L-1, driven by mobilization of 36 % of the initial mineral-bound arsenic. Generally, N fertilization led to 1.5-fold higher total GHG emissions (sum of CO2, CH4 and N2O as CO2 equivalents), 158-fold higher N2O, and 7.5-fold lower CH4 emissions compared to non-fertilization. On day 37, Gallionellaceae, Comamonadaceae and Rhodospirillales were more abundant in the high N treatment compared to the non-fertilized control, indicating their potential role as key players in nitrate reduction coupled to iron(II) oxidation. The findings underscore the dual effect of N fertilization, immobilizing arsenic in the short-term (low/medium N) or long-term (high N), while simultaneously increasing N2O and lowering CH4 emissions. This highlights the significance of both the quantity and frequency of N fertilizer application in paddy soils.

Remobilization of Cd caused by iron oxide phase transformation and Mn2+ competition after stabilization by nano zero valent iron

Stabilization techniques are vital in controlling Cd soil pollution. Nano zero valent iron (nZVI) has been extensively utilized for Cd remediation owing to its robust adsorption and reactivity. However, the environmental stress-induced stability of Cd after nZVI addition remains unclear. A pot experiment was conducted to evaluate the Cd bioavailability in continuously flooded (130 d) soil after stabilization with nZVI. The findings indicated that nZVI application did not result in a decline in Cd concentration in rice, as compared to the no-nZVI control. Additionally, nZVI simultaneously increased the available Cd concentration, iron-manganese oxide-bound (OX) Mn fraction, and relative abundance of Fe(III)-reducing bacteria, but it decreased OX-Cd and Mn availability in soil. Cadmium in rice tissues was positively correlated with the available Cd in soil. The results of subsequent adsorption tests demonstrated that CdO was the product of Cd adsorption by the nZVI aging products. Conversely, Mn2+ decreased the adsorption capacity of Cd-containing solutions. These results underscore the crucial role of both biotic and abiotic factors in undermining the stabilization of nZVI under continuous flooding conditions. This study offers novel insights into the regulation of nZVI-mediated Cd stabilization efficiency in conjunction with biological inhibitors and functional modification techniques.

Hydroxyl radical production by abiotic oxidation of pyrite under estuarine conditions: The effects of aging, seawater anions and illumination

Pyrite is widely distributed in estuarine sediments as an inexpensive natural Fenton-like reagent, however, the mechanism on the hydroxyl radical (HO·) production by pyrite under estuarine environmental conditions is still poorly understood. The batch experiments were performed to investigate the effects of estuarine conditions including aging (in air, in water), seawater anions (Cl-, Br- and HCO3-) and light on the HO· production by pyrite oxidation. The one-electron transfer dominated the process from O2 to HO· induced by oxidation of pyrite. The Fe (oxyhydr)oxide coatings on the surface of pyrite aged in air and water consumed hydrogen peroxide while mediating the electron transfer, and the combined effect of the two resulted in a suppression of HO· production in the early stage of aging and a promotion of HO· production in the later stage of aging. Corrosion of the surface oxide layers by aggressive anions was the main reason for the inhibition of HO· production by Cl- and Br-, and the generation of Cl· and Br· may also play a role in the scavenging of HO·. HCO3- increased the average rate of HO· production through surface-CO2 complexes formed by adsorption on the surface of pyrite. The significant enhancement of HO· production under light was attributed to the formation of photoelectrons induced by photochemical reactions on pyrite and its surface oxide layers. These findings provide new insights into the environmental chemical behavior of pyrite in the estuary and enrich the understanding of natural remediation of estuarine environments.

Effect of Stoichiometry on Nanomagnetite Sulfidation

Magnetite (Mt) has long been regarded as a stable phase with a low reactivity toward dissolved sulfide, but natural Mt with varying stoichiometries (the structural Fe(II)/Fe(III) ratio, x_stru) might exhibit distinct reactivities in sulfidation. How Mt stoichiometry affects its sulfidation processes and products remains unknown. Here, we demonstrate that x_stru is a master variable controlling the rates and extents of sulfide oxidation by magnetite nanoparticles (11 ± 2 nm). At pH = 7.0-8.0 and the initial Fe/S molar ratio of 10-50, the partially oxidized magnetite (x_stru = 0.19-0.43) can oxidize dissolved sulfide to elemental sulfur (S⁰), but only surface adsorption of sulfide, without interfacial electron transfer (IET), occurs on the nearly stoichiometric magnetite (x_stru = 0.47). The higher initial rate and extent of sulfide oxidation and S⁰ production are observed with the more oxidized magnetite that has the higher electron-accepting capability from surface-complexed sulfide (S(-II)_(s)). The FeS clusters formed from magnetite sulfidation can be oxidized by the most oxidized magnetite with x_stru = 0.19 but not by other magnetite particles. A linear relationship between the Gibbs free energy of reaction and the surface area-normalized initial rate of sulfide oxidation is observed in all experiments under the different conditions, suggesting the S(-II)_(s)-magnetite IET dominates magnetite sulfidation at high Fe/S molar ratios and near-neutral pH.

Nano-scale analysis of uranium release behavior from river sediment in the Ili basin

Due to the limitations of the conventional water sample pretreatment methods, some of the colloidal uranium (U) has long been misidentified as "dissolved" phase. In this work, the U species in river water in the Ili Basin was classified into submicron-colloidal (0.1-1 μm), nano-colloidal (0.1 μm-3 kDa) and dissolved phases (< 3 kDa) by using high-speed centrifugation and ultrafiltration. The U concentration in the river water was 5.39-8.75 μg/L, which was dominated by nano-colloidal phase (55-70%). The nano-colloidal particles were mainly composed of particulate organic matter (POM) and had a very high adsorption capacity for U (accounting for 70 ± 23% of colloidal U). Sediment disturbance, low temperature, and high inorganic carbon greatly improved the release of nano-colloidal U, but high levels of Ca²⁺ inhibited it. The simulated river experiments indicated that the flow regime determined the release of nano-colloidal U, and large amounts of nano-colloidal U might be released during spring floods in the Ili basin. Moreover, global warming increases river flow and inorganic carbon content, which may greatly promote the release and migration of nano-colloidal U.

Redox Behavior of Secondary Solid Iron Species and the Corresponding Effects on Hydroxyl Radical Generation during the Pyrite Oxidation Process

During the pyrite oxidation process, aqueous ferrous/ferric ions (Fe²⁺/Fe³⁺), as well as surface-adsorbed Fe²⁺/Fe³⁺, have been widely recognized to dominate hydroxyl radical (^•OH) generation, while this study reveals that the secondary solid iron species also play non-negligible roles. Based on the different forms and the presence of sites, the secondary solid iron species were classified as Fe_coat (iron-containing coating on the pyrite surface) and Fe_dep (ex situ-deposited iron (oxyhydr)oxide that is not in contact with pyrite). Instead of participating in building a stubborn passivation layer on the pyrite surface, Fe_coat is easy to fall off from the pyrite surface as the oxidation of pyrite deepens, while large fractions of Fe_dep and Fe_coat are found to be extractable with nitrilotriacetic acid (NTA). Achieved by cyclically oxidizing pyrite within different NTA levels (0/0.1/10 mM), Fe_coat and Fe_dep were proved to have distinct redox behavior during the pyrite oxidation process. Amorphous Fe_dep, originated from the hydrolyzation of dissolved Fe³⁺, accelerates the nonradical decay of hydrogen peroxide (H₂O₂); as a result, the accumulation of Fe_dep always decreases the ^•OH production during the pyrite oxidation process. However, part of Fe_dep adsorbs on the pyrite surface through electrostatic attraction and converts into Fe_coat. The electron conduction between Fe_coat and pyrite was verified, which accelerates the oxidative dissolution of pyrite, produces reactive Fe(II), and therefore favors ^•OH generation. This study improves our understanding of the redox behavior of pyrite in complex media such as natural processes and practical engineering systems.

Flooding and drainage induced abiotic reactions control metal solubility in soil of a contaminated industrial site

Intense industrialization has led to the increasing leaching risk of metals into groundwater at heavily polluted industrial sites. However, metal dissolution in polluted industrial soils has been neither fully investigated nor quantified before. In this study, the dissolution of Zn, Ni, and Cu in soil from a heavily contaminated industrial site during a flooding-drainage period was investigated by sequential extraction, geochemical modelling, and X-ray absorption near edge structure spectroscopy. The results showed a steady decrease in metal solubility during both reduction and oxidation stages. During reduction, with limited decrease in Eh (>100 mV), formation of carbonate precipitates rather than sulfide precipitates and adsorption on soil solids was responsible for Zn and Ni dissolution, whereas bound to soil organic matter (SOM) and iron oxides dominated Cu dissolution, due to its lower concentration and higher affinity to SOM and iron oxides compared to Zn and Ni. During oxidation, the acidity caused by ferrous oxidation was buffered by calcite dissolution, while metal precipitation ceased and adsorption on soil surface controlled metal solubility. The metal solubility and speciation during the flooding-drainage process were quantitatively predicted by geochemical model. The findings demonstrate that due to high metal concentrations and weak microbial effect in the industrial soil, metal release was largely regulated by abiotic reactions rather than biotic reactions, which is somehow different from that of the wetland or rice field soils.

Growth of microaerophilic Fe(II)-oxidizing bacteria using Fe(II) produced by Fe(III) photoreduction

Iron(II) (Fe(II)) can be formed by abiotic Fe(III) photoreduction, particularly when Fe(III) is organically complexed. Light-influenced environments often overlap or even coincide with oxic or microoxic geochemical conditions, for example, in sediments. So far, it is unknown whether microaerophilic Fe(II)-oxidizing bacteria are able to use the Fe(II) produced by Fe(III) photoreduction as electron donor. Here, we present an adaption of the established agar-stabilized gradient tube approach in comparison with liquid cultures for the cultivation of microaerophilic Fe(II)-oxidizing microorganisms by using a ferrihydrite-citrate mixture undergoing Fe(III) photoreduction as Fe(II) source. We quantified oxygen and Fe(II) gradients with amperometric and voltammetric microelectrodes and evaluated microbial growth by qPCR of 16S rRNA genes. We showed that gradients of dissolved Fe(II) (maximum Fe(II) concentration of 1.25 mM) formed in the gradient tubes when incubated in blue or UV light (400-530 nm or 350-400 nm). Various microaerophilic Fe(II)-oxidizing bacteria (Curvibacter sp. and Gallionella sp.) grew by oxidizing Fe(II) that was produced in situ by Fe(III) photoreduction. Best growth for these species, based on highest gene copy numbers, was observed in incubations using UV light in both liquid culture and gradient tubes containing 8 mM ferrihydrite-citrate mixtures (1:1), due to continuous light-induced Fe(II) formation. Microaerophilic Fe(II)-oxidizing bacteria contributed up to 40% to the overall Fe(II) oxidation within 24 h of incubation in UV light. Our results highlight the potential importance of Fe(III) photoreduction as a source of Fe(II) for Fe(II)-oxidizing bacteria by providing Fe(II) in illuminated environments, even under microoxic conditions.

Seasonal Fluctuations in Iron Cycling in Thawing Permafrost Peatlands

In permafrost peatlands, up to 20% of total organic carbon (OC) is bound to reactive iron (Fe) minerals in the active layer overlying intact permafrost, potentially protecting OC from microbial degradation and transformation into greenhouse gases (GHG) such as CO₂ and CH₄. During the summer, shifts in runoff and soil moisture influence redox conditions and therefore the balance of Fe oxidation and reduction. Whether reactive iron minerals could act as a stable sink for carbon or whether they are continuously dissolved and reprecipitated during redox shifts remains unknown. We deployed bags of synthetic ferrihydrite (FH)-coated sand in the active layer along a permafrost thaw gradient in Stordalen mire (Abisko, Sweden) over the summer (June to September) to capture changes in redox conditions and quantify the formation and dissolution of reactive Fe(III) (oxyhydr)oxides. We found that the bags accumulated Fe(III) under constant oxic conditions in areas overlying intact permafrost over the full summer season. In contrast, in fully thawed areas, conditions were continuously anoxic, and by late summer, 50.4 ± 12.8% of the original Fe(III) (oxyhydr)oxides were lost via dissolution. Periodic redox shifts (from 0 to +300 mV) were observed over the summer season in the partially thawed areas. This resulted in the dissolution and loss of 47.2 ± 20.3% of initial Fe(III) (oxyhydr)oxides when conditions are wetter and more reduced, and new formation of Fe(III) minerals (33.7 ± 8.6% gain in comparison to initial Fe) in the late summer under more dry and oxic conditions, which also led to the sequestration of Fe-bound organic carbon. Our data suggest that there is seasonal turnover of iron minerals in partially thawed permafrost peatlands, but that a fraction of the Fe pool remains stable even under continuously anoxic conditions.

Influence of Fe(III) source, light quality, photon flux and presence of oxygen on photoreduction of Fe(III)-organic complexes - Implications for light-influenced coastal freshwater and marine sediments

Iron(III) photoreduction is an important source of Fe(II) in illuminated aquatic and sedimentary environments. Under oxic conditions, the Fe(II) can be re-oxidized by oxygen (O₂) forming reactive O-species such as hydrogen peroxide (H₂O₂) which further react with Fe(II) thus enhancing Fe(II) oxidation rates. However, it is unknown by aquatic sediments how the parameters wavelength of radiation, photon flux, origin of Fe(III) source and presence or absence of O₂ influence the extent of Fe(II) and H₂O₂ turnover. We studied this using batch experiments with different Fe(III)-organic complexes mimicking sedimentary conditions. We found that wavelengths <500 nm are necessary to initiate Fe(III) photoreduction and that the photon flux, wavelength and identity of Fe(III)-complexing organic acids control the kinetics of Fe(III) photoreduction. The formation of photo-susceptible Fe(III)-organic complexes did not depend on whether the Fe(III) source was biogenically produced, poorly-crystalline Fe(III) oxyhydroxides or chemically synthesized ferrihydrite. Oxic conditions caused chemical re-oxidation of Fe(II) and accumulation of H₂O₂. The photon flux, wavelength and availability of Fe(III)-complexing organic molecules are critical for the balance between concurrent Fe(III) photoreduction and abiotic Fe(II) oxidation and may even lead to a steady-state concentration of Fe(II) in the micromolar range. These results help understand and predict Fe(III) photoreduction dynamics and in-situ formation of Fe(II) in oxic or anoxic, illuminated and organic-rich environments.

Poorly crystalline Fe(Ⅱ) mineral phases induced by nano zero-valent iron are responsible for Cd stabilization with different soil moisture conditions and soil types

Nano zero-valent iron (nZVI) is a promising remediation material for Cd-contaminated soil, but questions remain regarding the effects of nZVI-induced Fe oxides on Cd availability with different soil types and moisture conditions. To identify the changes in Cd availability and Fe mineral phases resulting from the application of nZVI, three types of Cd-spiked soils with 0.1% nZVI amendment were incubated under different moisture conditions with water-holding capacities (WHCs) of 30%, 60%, and 180%. The availability of Cd was significantly decreased in yellow and black soils amended with nZVI, with fewer changes being observed in cinnamon soil. The limited effect of nZVI on Cd stabilization was due to the extremely low content of poorly crystalline Fe phases in cinnamon soil. The Cd stabilization efficiency of nZVI was higher in the flooding soils (180% WHC) than in the non-flooding yellow and black soils (30% and 60% WHC, respectively). Moreover, the addition of nZVI promoted the formation of less-available forms of Cd (Fe-oxide-bound Cd in yellow soil and Fe-oxide-bound and organic-material-bound Cd in black soil) under the flooding condition. The decrease in extractable Cd was strongly related to the increase in poorly crystalline Fe(Ⅱ) mineral phases among the three soils and various soil moisture contents. Although 0.1% nZVI amendment induced the dissolution of Mn oxides, it did not hinder the Cd stabilization in the three soils. Overall, this study indicates that increased amounts of poorly crystalline Fe(Ⅱ) compounds due to nZVI amendment play a critical role in the stabilization of Cd in soils.

Accelerated bioremediation of a complexly contaminated river sediment through ZVI-electrode combined stimulation

Complexly contaminated river sediment caused by reducible and oxidizable organic pollutants is a growing global concern due to the adverse influence on ecosystem safety and planetary health. How to strengthen indigenous microbial metabolic activity to enhance biodegradation and mineralization efficiency of refractory composite pollutants is critical but poorly understood in environmental biotechnology. Here, a synergetic biostimulation coupling electrode with zero-valent iron (ZVI) was investigated for the bioremediation of river sediments contaminated by 2,4,6-tribromophenol (TBP, reducible pollutant) and hydrocarbons (oxidizable pollutants). The bioremediation efficiency of ZVI based biostimulation coupling electrode against TBP debromination and hydrocarbons degradation were 1.1-3 times higher than the electrode used solely, which was attributed to the shape of distinctive microbial communities and the enrichment of potential dehalogenators (like Desulfovibrio, Desulfomicrobium etc.). The sediment microbial communities were significantly positively correlated with the enhanced degradation efficiencies of TBP and hydrocarbons (P < 0.05). Moreover, the coupled system predominately increased positive microbial interactions in the ecological networks. The possible mutual relationship between microbes i.e., Thiobacillus (iron-oxidizing bacteria) and Desulfovibrio (dehalogenator) as well as Pseudomonas (electroactive bacteria) and Clostridium (hydrocarbons degraders) were revealed. This study proposed a promising approach for efficient bioremediation of complexly contaminated river sediments.

Habitat heterogeneity induces regional differences in sediment nitrogen fixation in eutrophic freshwater lake

Biological nitrogen fixation (BNF) in sediments is an important source of bioavailable nitrogen in aquatic systems. However, the effect of habitat change caused by eutrophication on nitrogen fixation within sediments is still unclear. In this study, nitrogen fixation rates and diazotroph diversities in sediments with heterogeneous ecological status in one eutrophic lake were investigated by using an isotope tracer method and sequencing of nitrogen-fixing (nif) genes. The results showed that both nitrogenase activity (NA) and nifH abundance in sediments of blooms area were higher than those in vegetation-dominated habitats. Correlation analysis showed that NA was correlated closely to nifH abundance, dissolved sulfide, and iron. The diazotrophic assemblage contained mainly Proteobacterial sequences belonging to Cluster I and III, and the variations of diazotrophic community could be explained by total nitrogen content, total phosphorus content, organic matters, sulfides, ammonium and iron content. Moreover, the co-occurrence network analysis showed the Alphaproteobacteria shaped the major interactions in diazotrophic community, and sediment properties had stronger effect on diazotrophic community in cyanobacteria-dominated habitat. This study revealed that habitat heterogeneity in eutrophic lakes shaped different succession of BNF in sediments and cyanobacterial blooms significantly improved the nitrogen-fixing activity in sediments, which broadened our understanding of nitrogen cycle and nutrient management in eutrophic freshwater lakes.

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.

Acid mine drainage formation and arsenic mobility under strongly acidic conditions: Importance of soluble phases, iron oxyhydroxides/oxides and nature of oxidation layer on pyrite

Acid mine drainage (AMD) formation and toxic arsenic (As) pollution are serious environmental problems encountered worldwide. In this study, we investigated the crucial roles played by common secondary mineral phases formed during the natural weathering of pyrite-bearing wastes-soluble salts (melanterite, FeSO₄·7H₂O) and metal oxides (hematite, Fe₂O₃)-on AMD formation and As mobility under acidic conditions (pH 1.5-4) prevalent in historic tailings storage facilities, pyrite-bearing rock dumps and AMD-contaminated soils and sediments. Our results using a pyrite-rich natural geological material containing arsenopyrite (FeAsS) showed that melanterite and hematite both directly-by supplying H⁺ and/or oxidants (Fe³⁺)-and indirectly-via changes in the nature of oxidation layer formed on pyrite-influenced pyrite oxidation dynamics. Based on SEM-EDS, DRIFT spectroscopy and XPS results, the oxidation layer on pyrite was mainly composed of ferric arsenate and K-Jarosite when melanterite was abundant with/without hematite but changed to Fe-oxyhydroxide/oxide and scorodite when melanterite was low and hematite was present. This study also observed the formation of a mechanically 'strong' coating on pyrite that suppressed the mineral's oxidation. Finally, As mobility under acidic conditions was limited by its precipitation as ferric arsenate, scorodite, or a Fe/Al arsenate phase, including its strong adsorption to Fe-oxyhydroxides/oxides.