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Iron(II)-bearing trioctahedral smectites (saponites) form during anoxic alteration of basaltic rock. They are predicted to have been widespread on the early Earth and are observed in the oceanic subsurface today. Smectite structures, including the occupancy of sites in the octahedral sheet, affect iron redox behavior but the rates and products of trioctahedral smectite oxidation have been largely unexplored to date. In this study we synthesized two Fe(II)-bearing trioctahedral smectites, one moderate (22 wt% Fe) and one high (27 wt% Fe) in iron content. We then examined the rate, extent, and products of their oxidation by dissolved oxygen, nitrite, and hydrogen peroxide. Dissolved oxygen caused partial oxidation of Fe(II) in the smectites with 14 to 43% of Fe(II) unoxidized after 20 to 30 days of exposure. The rate and extent of oxidation correlated with the dissolved oxygen concentration and the Fe(II) content of the clay. The incomplete oxidation in these experiments is consistent with the mixed-valent trioctahedral smectites observed in oxidized natural samples but contrasts with the complete reoxidation by oxygen shown by chemically- or microbially-reduced dioctahedral smectites. Oxidation of structural Fe(II) by 5 mmol L−1 nitrite was negligible for the moderate-iron smectite and yielded only ∼17% oxidation after 54 days of reaction for the high-iron smectite. Hydrogen peroxide caused rapid and near-complete oxidation of both clays. Powder X-ray diffraction, variable-temperature Mössbauer spectroscopy, and extended X-ray absorption fine structure spectroscopy together detected no crystalline or short-range-ordered secondary phases and show that oxidized iron remained in the trioctahedral smectite structure. The recalcitrant Fe(II) pool in oxidized trioctahedral smectites exists in less distorted sites than Fe(II) in the initial clays. Its unreactive nature at prolonged reaction times indicates an elevated redox potential generated by the local coordination environment. Slower oxidation rates create a larger recalcitrant Fe(II) pool, suggesting kinetic competition between oxidation and a process involved in redox stabilization, such as electron exchange between octahedral iron sites or deprotonation of hydroxyl groups in the structure. The resistance to complete oxidation of trioctahedral ferrous smectites and their full retention of iron demonstrates that transitions from anoxic to oxic conditions generate mixed-valence smectites rather than a mixture of new phases. Identifying the diagenetic products of mixed-valent trioctahedral smectites may provide an indicator in the rock record of past redox cycling. Substantial portions of structural Fe(II) in trioctahedral smectites display slow abiotic oxidation kinetics and represent potential electron donors for both microaerophilic iron oxidizing and nitrate-reducing, iron-oxidizing microorganisms in altered mafic rocks and related settings. [ABSTRACT FROM AUTHOR]

The long‐term stability of soil carbon (C) is strongly influenced by organo‐mineral interactions. Iron (Fe)‐oxides can both inhibit microbial decomposition by providing physicochemical protection for organic molecules and enhance rates of C mineralization by serving as a terminal electron acceptor, depending on redox conditions. Restoration of floodplain hydrology in montane meadows has been proposed as a method of sequestering C for climate change mitigation. However, dissimilatory microbial reduction of Fe(III) could lead to C losses under increased reducing conditions. In this study, we explored variations in Fe‐C interactions over a range of redox conditions and in soils derived from two distinct parent materials to elucidate biochemical and microbial controls on soil C cycling in Sierra Nevada montane meadows. Soils derived from basalt showed greater rates of Fe(III)‐reduction at increasing soil moisture levels than granitic soils. Increases in Fe(III) reduction, however, were only associated with elevated rates of C mineralization in one basalt soil. Known Fe(III)‐reducing taxa were present in all samples but neither the relative abundance nor richness of Fe(III)‐reducers corresponded with measured rates of Fe(III) reduction. Under reducing conditions, Fe(III)‐reduction was only coupled to C mineralization in the soil with the greatest amount of Fe‐oxide bound C. However, Fe‐oxide ‐bound C was below theoretical limits for C sorption onto Fe‐oxides and not detectable in all soils. Overall, our results suggest that "what's there" in terms of soil chemistry may be a more important driver of C mineralization coupled to Fe(III) reduction than "who's there" in the microbial community. Plain Language Summary: The ability of soils to sequester and store carbon may depend, in large part, on associations between soil minerals and carbon molecules. Iron is associated with some of the oldest and most persistent soil carbon. However, in saturated soils microbial utilization of iron can lead to the loss of soil carbon. In this study, we explored how microbial utilization of iron under different soil moisture levels impacts rates of soil carbon loss and whether geologic differences in parent material or microbial community composition explain patterns between sites. To test this, we incubated soils from basalt and granitic parent material at different levels of soil moisture and tested rates of iron utilization and carbon decomposition. Differences in parent material impacted microbial utilization of iron but increases in iron utilization were only coupled to soil carbon decomposition in one soil. Iron reducing bacteria were present in all soils, regardless of rates of iron utilization, and differences in microbial community composition did not impact rates of iron utilization or soil carbon decomposition. Overall, our results suggest that site‐specific soil chemistry may impact iron‐carbon interactions more than microbial community composition or parent material. Key Points: Differences in parent material were associated with differences in rates of Fe(III) reduction but not C mineralizationMixotrophic ability of many Fe(III)‐reducers may partially explain the lack of pattern observedSoil chemistry may exert stronger controls over dissimilatory Fe(III)‐reduction than microbial community composition [ABSTRACT FROM AUTHOR]

Water electrolysis is recognized as a green hydrogen production technology, but the high voltage required for anodic oxygen evolution reaction restricts the practical application. In this work, a Fe(II)/Fe(III) redox mediated SO2 conversion is proposed to couple the cathodic hydrogen evolution reaction to achieve sulfur dioxide conversion and hydrogen production at low voltage. The onset potential of Fe(II) electrooxidation to Fe(III) is as low as 0.75 VRHE (vs reversible hydrogen electrode). Ex situ ultraviolet spectroscopy (UV) spectrum and ion chromatography indicate that SO2 in electrolyte can reduce Fe(III) to Fe(II), completing the Fe(II)/Fe(III) redox cycle as well as the conversion of SO2 to sulfuric acid. The assembled flow cell electrolyzer requires a low operating voltage of 0.97 V at 10 mA cm−2 and shows good performance under both acidic and neutral conditions. This study proposes an innovative energy saving and environment friendly strategy for simultaneous hydrogen production and sulfur dioxide capture based on low‐cost catalyst materials. [ABSTRACT FROM AUTHOR]

• Humus can enhance the denitrification ability of strain ZP7 at low C/N ratio. • Multiple pollutants can be effectively removed in water remediation systems. • Bio-immobilized carriers effectively control the release of pollutants in sediment. • The complex of iron and humus enhanced microbial activity. • Gene expression related to the redox cycle of nitrogen and iron was enhanced. The combined pollution and oligotrophic characteristics of surface water led to poor self-purification capacity of water bodies. In this study, humic acid (HA) and fulvic acid (FA) were used to promote the denitrification process of strain Zoogloea sp. ZP7. Subsequently, iron and different humus (HA and FA) composites were encapsulated by polyvinyl alcohol (PVA) and sodium alginate (SA) to prepare two biological immobilization (BI) carriers Fe-HA@PVA/SA (FHB) and Fe-FA@PVA/SA (FFB), which immobilized strain ZP7. The BI materials were added to the water remediation system model and operated for three stages (synthetic wastewater, actual polluted surface water, sediment-contaminated surface water) for 48 days. The results showed that FHB (FFB) could remove up to 89.7 % (88.6 %), 90.5 % (89.5 %), 82.2 % (81.5 %), and 90.4 % (80.8 %) of total nitrogen, nitrate, COD Mn , and phosphate from the actual polluted surface water within 16 days of stage II. In addition, the incorporation of FHB and FFB was effective in controlling the release of organic matter and heavy metals from the sediments. Microbial community analysis showed that Zoogloea became the dominant species in actual water bodies. KEGG database analysis illustrated that the expression of genes related to denitrification and iron redox cycle was enhanced. This work provides a novel approach into the in-situ bioremediation of actual nutrient-poor water bodies. [Display omitted] [ABSTRACT FROM AUTHOR]

This research aims to understand colouring technologies in 5th–7th centuries glass imported to Atlantic Britain by correlating the iron (Fe) and manganese (Mn) ratios and oxidation states with colour. Despite having a similar matrix chemical composition and concentrations of Fe and Mn oxides, these vessels display different colours (from green to yellow/amber, sometimes with purple streaks). Colour changes can be induced by controlling the reduction-oxidation reactions that occur during glass production, which are influenced by the raw materials, furnace and melt atmosphere, and recycling. To evaluate these parameters, reference glasses were prepared, following the composition of Late Antique archaeological glass recovered from Tintagel (UK) and Whithorn (UK). A corpus of archaeological and experimental glass samples was analysed using bulk Fe and Mn K-edge x-ray absorption near edge structure (XANES) spectroscopy, micro-XANES and micro x-ray fluorescence ( μ -XRF) at beamline ID21, at the European Synchrotron Radiation Facility. Fe and Mn XANES spectra of the archaeological glass indicate that Fe and Mn are in a similar oxidation state in all the yellow samples, predominantly Fe ^3+ and Mn ^2+ . No detectable difference in Mn and Fe oxidation state occurs in the purple streaks compared to the yellow glass bulk but μ -XRF maps of the distribution of Fe and Mn show that Mn is more concentrated in the purple streaks. This indicates that the purple colour of the streaks is mainly due to a higher Mn/Fe ratio and persistence of more oxidised manganese in the purple areas, even though it is difficult to detect. Many archaeological fragments appear pale green in transmitted light but amber in reflected light. XANES studies detected the presence of surface layers where manganese is more oxidised. This layer is believed to scatter transmitted and reflected light differently and might be responsible for the optical features of the archaeological glass.

Iron (Fe) deficiency is common in agricultural crops and affects millions of people worldwide. Translocation of Fe in the xylem is a key step for Fe distribution in plants. The mechanism controlling this process remains largely unknown. Here, we report that two Arabidopsis ferroxidases, LPR1 and LPR2, play a crucial and redundant role in controlling Fe translocation in the xylem. LPR1 and LPR2 are mainly localized in the cell walls of xylem vessels and the surrounding cells in roots, leaves, and stems. Knockout of both LPR1 and LPR2 increased the proportion of Fe(II) in the xylem sap, and caused Fe deposition along the vascular bundles especially in the petioles and main veins of leaves, which was alleviated by blocking blue light. The lpr1 lpr2 double mutant displayed constitutive expression of Fe deficiency response genes and overaccumulation of Fe in the roots and mature leaves under Fe-sufficient supply, but Fe deficiency chlorosis in the new leaves and inflorescences under low Fe supply. Moreover, the lpr1 lpr2 double mutant showed lower Fe concentrations in the xylem and phloem saps, and impaired 57Fe translocation along the xylem. In vitro assays showed that Fe(III)-citrate, the main form of Fe in xylem sap, is easily photoreduced to Fe(II)-citrate, which is unstable and prone to adsorption by cell walls. Taken together, these results indicate that LPR1 and LPR2 are required to oxidize Fe(II) and maintain Fe(III)-citrate stability and mobility during xylem translocation against photoreduction. Our study not only uncovers an essential physiological role of LPR1 and LPR2 but also reveals a new mechanism by which plants maintain Fe mobility during long-distance translocation in the xylem. The ferroxidases LPR1 and LPR2 are localized in the call wall of xylem vessels and the surrounding cells and play a crucial role in iron translocation along the xylem of Arabidopsis plants. The two enzymes oxidize ferrous iron for the formation of ferric-citrate complexes and maintain the stability and mobility of ferric-citrate complexes during xylem translocation against light-induced reduction. [ABSTRACT FROM AUTHOR]

Layered oxides for Na‐ion batteries containing Fe have attracted strong interest mainly due to their low cost. However, full oxidation of Fe3+ to Fe4+ is rarely seen before O‐redox sets in and is typically accompanied by voltage and capacity fade on cycling. On charging P2‐Na0.67[Fe0.5Mn0.5]O2, Fe3+ is oxidized to only ≈Fe3.3+ before the onset of O‐redox. O‐redox occurs when the Na content is sufficiently low (Na ≈0.3) to permit the transition from P‐type to O‐type stacking, thus enabling Fe3+ migration to the Na layer. Fe3+ migration generates cation vacancies in the transition metal layer, forming □‐O‐□ configurations, which trigger the onset of O‐redox. In contrast, doping this material with Mg2+ to form P2‐Na0.67[Fe0.25Mn0.6Mg0.15]O2 allows full oxidation of Fe3+ to Fe4+ before the Na content is low enough to favor O‐type stacking. During O‐redox, Mg2+ is displaced into the Na layers instead of Fe. Mg substitution enables greater reversibility of the Fe3+/Fe4+ redox couple and significantly suppresses Fe migration, which is responsible for the voltage and capacity fade observed for P2‐Na0.67Fe0.5Mn0.5O2. [ABSTRACT FROM AUTHOR]

Serpentinization, the water-driven alteration of olivine-rich rocks, is a fundamental process contributing to planetary habitability and the maintenance of rock-hosted life on Earth. Serpentinization generates molecular hydrogen via the reduction of water and the coupled oxidation of Fe, which partitions into serpentine, brucite and magnetite. Magnetite formation controls the magnetic properties of serpentinites, and researchers have highlighted a link between increasing magnetic susceptibility and decreasing density of peridotites with increasing degree of serpentinization. Several analytical and theoretical studies have suggested that these increases in magnetization and decreases in density are accompanied by increasing porosity. Here, we investigate the potential correlation between porosity generation and serpentinization by analyzing the geochemical composition, Fe redox state, and porosity of 28 abyssal serpentinites recovered via drilling from the Atlantic Ocean subseafloor. Principal component analysis applied to the resulting dataset revealed no direct links between these properties, which we suggest results from the introduction of variability due to low-temperature seafloor weathering processes. Seafloor weathering can modify the porosity of serpentinites by dissolving brucite, and it affects the Fe redox state of serpentinites by inducing the replacement of magnetite by Fe(III) minerals like maghemite, hematite, and goethite. Moreover, we also suggest that ophiolitic serpentinites would not show direct correlations between porosity and Fe redox state, due to geochemical transformations induced by metamorphism and porosity reworking linked to deformation induced by tectonic stresses. Ultimately, our results demonstrate that weathering processes can significantly affect the chemical and physical properties of serpentinites and thus must be considered in geochemical and geophysical evaluations of these rocks. • Literature studies suggest link between porosity and iron redox in serpentinites. • Porosity and geochemistry of abyssal serpentinites were characterized. • Statistical analysis revealed no significant correlations. • Correlations are obscured by seafloor weathering processes. • Characterization of full range of serpentinization could reveal hidden correlations. [ABSTRACT FROM AUTHOR]

The occurrence of chromium picolinate (Cr(pic) 3) in environment has attracted raising concerns on its fate and the associated risks. Herein, the photoinduced oxidation of Cr(pic) 3 in the presence of ferric ions (Fe(III)) under simulated sunlight and natural solar light irradiation were investigated. Cr(pic) 3 was stable under dark or without Fe(III). 87.9 % of Cr(pic) 3 (C 0 = 1.0 μM) was degraded in the presence of 50 μM Fe(III) after 90 min simulated sunlight irradiation at initial pH of 4.0. •OH was the main cause for Cr(pic) 3 oxidation, it attacked the chromium center to generate hexavalent chromium (Cr(VI)) and picolinic acid (k = 5.9 ×108 M−1·s−1). Picolinic acid could be further oxidized to NH 4 + and small organics. Relative higher Fe(III) content (25 – 75 μM) and Cr(pic) 3 concentration (0.5 – 2.0 μM) promoted both of Cr(pic) 3 degradation and Cr(VI) accumulation. While, the degradation of Cr(pic) 3 decreased with pH at the range of 3.0 – 8.0, more Cr(VI) was accumulated at pH 5.0 and 6.0. The co-existence of inorganic ions and dissolved organic matter (DOM) in river water inhibited Cr(pic) 3 oxidation by scavenging the •OH formed and shielding the light. 8.0 – 16.7 μg/L of Cr(VI) was accumulated after 9.0 h simulated sunlight irradiation of Cr(pic) 3 in river water matrix ([Fe(III)] 0 = 50 – 100 μM). The generation of Cr(VI) under solar light was slower than that under simulated sunlight due to the weaker light intensity (43.2 – 85.0 mW/cm2 vs. 750 – 1300 mW/cm2). These results consistently suggest photoinduced oxidation of Cr(pic) 3 in environment generates the toxic Cr(VI), which deserves significant attention. [Display omitted] • Fe(III) significantly enhanced the photodegradation of Cr(pic) 3. • •OH dominated and directly involved in Cr(pic) 3 degradation. • Degradation of Cr(pic) 3 released Cr(VI). • Highest Cr(VI) accumulation occurred at near neutral pH. • Water matrix dramatically affected the photodegradation of Cr(pic) 3. [ABSTRACT FROM AUTHOR]

[Display omitted] • Anammox sludge drives iron reduction using extracellular polymeric substances. • Fe(II) promotes partial DNRA pathway of anammox with nitrite as end products. • Accumulated nitrite is further eliminated with ammonium via anammox pathway. • Nitrogen removal efficiency of the iron-assisted anammox process reaches 95%. Anammox is recognized as a prospective alternative for future biological nitrogen removal technologies. However, the nitrate by-products produced by anammox bacteria limit its overall nitrogen removal efficiency below 88 %. This study introduced Fe(III) into the anammox bioreactor to enhance the nitrogen removal efficiency to approximately 95 %, surpassing the biochemical limit of 88 % imposed by anammox stoichiometry. Anammox sludge was demonstrated to utilize extracellular polymeric substances to reduce Fe(III) into Fe(II), and this process promoted the dominance of Ca. Brocadia. The iron addition improved the abundance of narGHI genes and facilitated the partial dissimilatory nitrate reduction to ammonium, with nitrite as the end product. The accumulated nitrite was then eliminated through the anammox pathway, along with the excess ammonium (30 mg/L) in the influent. Overall, this study deepens our understanding of the enhanced nitrogen removal triggered by Fe(III) in anammox sludge and offers an effective approach to boost anammox process. [ABSTRACT FROM AUTHOR]

The mantles of both Earth and Mars are more oxidized than would be expected based on low pressure equilibration of molten silicate and alloy during their magma ocean stages. High pressure silicate-alloy equilibration in a magma ocean can produce appreciable ferric iron in the silicate, leading to comparatively oxidized near surface conditions and overlying atmospheres. Upon crystallization, this may feasibly be sufficient to account for oxygen fugacities prevailing in basalt source regions of Earth and Mars. Experiments and first principles studies affirm that Fe3+ is stabilized at high pressure, but to date there has been no model that accounts accurately for the combined effects of melt composition, temperature, pressure, and oxygen fugacity on magma ocean Fe3+/FeT. We calibrate a new model for Fe3+/FeT as a function of temperature, pressure, melt composition, and f O2 which reproduces Fe3+/FeT for experimental peridotite liquids and which incorporates differences in FeO and Fe 2 O 3 liquid heat capacities into a potentially realistic temperature function. For the effects of pressure, two versions of the model are implemented based on recent equations of state (EOS), though only the EOS of Deng et al. (2020) is applicable to pressures relevant to metal-silicate equilibration in a deep terrestrial magma ocean. For Earth, metal-silicate equilibration at 28–53 GPa, 2300–4100 K, and f O2 set by plausible mantle and core compositions produces Fe3+/FeT between 0.034 and 0.10, with variation mostly owing to differences in assumed temperatures. For Mars, different proposed mantle compositions produce Fe3+/FeT ratios that range from 0.026 for FeO* of 13.5 wt.% up to 0.038 for FeO* of 18.1 wt.%. Although significant Fe3+ may be present in magma oceans owing to high pressure equilibration with alloy, the budget of Fe 2 O 3 in crystallized mantles is expected to be modified from that in the molten state. An important additional factor is the influence of Cr, which is Cr2+ in molten silicate equilibrated with alloy and Cr3+ in terrestrial upper mantles. Production of Cr3+ and Fe2+ by reaction with Cr2+ and Fe3+ during crystallization can destroy much of the Fe 2 O 3 present during the magma ocean stage. Considering the stability of Cr2+ in olivine and the temperature-dependent partitioning of Cr3+ between mantle silicates, we construct an empirical model for the fraction of Cr that is Cr 2 O 3 in solid spinel peridotite as a function of temperature and f O2. For Earth, at least 0.35 wt.% Fe 2 O 3 is destroyed by oxidation of magma ocean CrO and for Mars, more than 0.55 wt.% Fe 2 O 3 should be destroyed. Consequently, either the terrestrial and martian magma oceans were significantly more enriched in Fe 2 O 3 than their present-day upper mantles or other processes contributed to oxidation of the latter. Over-enrichment of Fe 2 O 3 in the magma oceans is plausible only if terrestrial metal-silicate equilibration occurred above 3300 K and if the martian mantle contains >17 wt.% FeO*. Subsolidus disproportionation of ferrous iron may have contributed to the present-day redox state of the Earth's mantle, and late accretion of chondrite-like material and hydrogen degassing also likely affected the solidified mantles of both Earth and Mars. [ABSTRACT FROM AUTHOR]

The deep ocean is generally considered to have changed from anoxic in the Precambrian to oxygenated by the Late Paleozoic (∼420–400 Ma) due to changes in atmospheric oxygen concentrations. When the transition occurred, that is, in the Early Paleozoic or not until the Late Paleozoic, is less well constrained. To address this, we measured Fe3+/ΣFe of volcanic rocks, sheeted dykes, gabbros, and ultramafic rocks from the Early Paleozoic (∼485 Ma) Bay of Islands (BOI) ophiolite as a proxy for hydrothermal alteration in the presence or absence of O2 derived from deep marine fluids. Combining this data with previously published data from the BOI indicates that volcanic rocks are oxidized relative to intrusive crustal rocks (0.35 ± 0.02 vs. 0.19 ± 0.01, 1 standard error), which we interpret to indicate that the volcanic section was altered by marine‐derived fluids that contained some dissolved O2. We compare our results directly to the Macquarie Island and Troodos ophiolites, drilled oceanic crust, previously compiled data for ophiolitic volcanic rocks, and newly compiled data for ophiolitic intrusive rocks. These comparisons show that the BOI volcanic (but not intrusive) rocks are oxidized relative to Precambrian equivalents, but are less oxidized relative to Late Paleozoic to modern equivalents. We interpret these results to indicate that the Early Paleozoic ocean contained dissolved O2, but at concentrations ∼2.4× lower than for the Late Paleozoic to today. Key Points: We report Fe3+/ΣFe in volcanic and intrusive crustal rocks and ultramafic rocks from the Early Paleozoic Bay of Islands (BOI) ophioliteFe3+/ΣFe of the BOI volcanic rocks are elevated compared to Precambrian systems but lower than Late Paleozoic to modern systemsThis difference indicates deep‐ocean O2 levels in the Early Paleozoic were elevated compared to the Precambrian but lower than today [ABSTRACT FROM AUTHOR]

Abstract The deep ocean is generally considered to have changed from anoxic in the Precambrian to oxygenated by the Late Paleozoic (∼420–400 Ma) due to changes in atmospheric oxygen concentrations. When the transition occurred, that is, in the Early Paleozoic or not until the Late Paleozoic, is less well constrained. To address this, we measured Fe3+/ΣFe of volcanic rocks, sheeted dykes, gabbros, and ultramafic rocks from the Early Paleozoic (∼485 Ma) Bay of Islands (BOI) ophiolite as a proxy for hydrothermal alteration in the presence or absence of O2 derived from deep marine fluids. Combining this data with previously published data from the BOI indicates that volcanic rocks are oxidized relative to intrusive crustal rocks (0.35 ± 0.02 vs. 0.19 ± 0.01, 1 standard error), which we interpret to indicate that the volcanic section was altered by marine‐derived fluids that contained some dissolved O2. We compare our results directly to the Macquarie Island and Troodos ophiolites, drilled oceanic crust, previously compiled data for ophiolitic volcanic rocks, and newly compiled data for ophiolitic intrusive rocks. These comparisons show that the BOI volcanic (but not intrusive) rocks are oxidized relative to Precambrian equivalents, but are less oxidized relative to Late Paleozoic to modern equivalents. We interpret these results to indicate that the Early Paleozoic ocean contained dissolved O2, but at concentrations ∼2.4× lower than for the Late Paleozoic to today.

Oxygen (O2) limitation contributes to persistence of large carbon (C) stocks in saturated soils. However, many soils experience spatiotemporal O2 fluctuations impacted by climate and land‐use change, and O2‐mediated climate feedbacks from soil greenhouse gas emissions remain poorly constrained. Current theory and models posit that anoxia uniformly suppresses carbon (C) decomposition. Here we show that periodic anoxia may sustain or even stimulate decomposition over weeks to months in two disparate soils by increasing turnover and/or size of fast‐cycling C pools relative to static oxic conditions, and by sustaining decomposition of reduced organic molecules. Cumulative C losses did not decrease consistently as cumulative O2 exposure decreased. After >1 year, soils anoxic for 75% of the time had similar C losses as the oxic control but nearly threefold greater climate impact on a CO2‐equivalent basis (20‐year timescale) due to high methane (CH4) emission. A mechanistic model incorporating current theory closely reproduced oxic control results but systematically underestimated C losses under O2 fluctuations. Using a model‐experiment integration (ModEx) approach, we found that models were improved by varying microbial maintenance respiration and the fraction of CH4 production in total C mineralization as a function of O2 availability. Consistent with thermodynamic expectations, the calibrated models predicted lower microbial C‐use efficiency with increasing anoxic duration in one soil; in the other soil, dynamic organo‐mineral interactions implied by our empirical data but not represented in the model may have obscured this relationship. In both soils, the updated model was better able to capture transient spikes in C mineralization that occurred following anoxic–oxic transitions, where decomposition from the fluctuating‐O2 treatments greatly exceeded the control. Overall, our data‐model comparison indicates that incorporating emergent biogeochemical properties of soil O2 variability will be critical for effectively modeling C‐climate feedbacks in humid ecosystems. [ABSTRACT FROM AUTHOR]

Serpentinized peridotite is reacting with groundwater in the subsurface of the Samail ophiolite in Oman. Although these rocks are partially to completely serpentinized, they interact with a groundwater aquifer containing hyperalkaline fluids rich in H2 and CH4. Since the mechanisms by which H2 production may continue at low temperatures (<50°C) are not fully understood, core recovered during the Oman Drilling Project provides an excellent opportunity to study the mineralogy and Fe speciation in highly serpentinized harzburgite recording multiple stages of water/rock interaction. In Hole BA3A, early hydration of olivine and pyroxene, which likely occurred at temperatures of ∼100°C–200°C, produced serpentine, Fe‐rich brucite, awaruite, and little magnetite. Notably, Fe‐rich brucite is only preserved at >∼100 m depth in the core. Fe‐rich brucite is nearly absent within two near‐surface reaction zones where later stages of reaction are recorded, which include replacement of Fe‐rich brucite by Fe(III)‐bearing serpentine, and increases in the proportion of other Fe(III)‐bearing phases such as magnetite and hydroandradite. Thus, Fe‐rich brucite at depth represents substantial stored capacity for H2 production that can continue at low temperature, even after primary olivine and pyroxene are exhausted, thereby sustaining habitable conditions for microbial life. Plain Language Summary: When ultramafic rocks from Earth's mantle come into contact with water, they undergo a set of hydration reactions leading to the formation of secondary minerals such as serpentine and brucite. Such "serpentinization reactions" also often involve the oxidation of iron, which can result in the production of hydrogen gas. Many models predict abundant hydrogen production at high temperatures >250°C, but the potential reactions that may produce hydrogen are more enigmatic at lower temperatures. Using rock cores drilled from the Samail ophiolite in Oman, we show that in highly serpentinized rocks in contact with hyperalkaline, low‐temperature, hydrogen‐rich fluids, multiple stages of reactions can be observed. By analyzing changes in mineralogy with depth, it is possible to identify reaction fronts where rocks that initially formed abundant Fe(II)‐bearing minerals such as brucite are now interacting with modern groundwaters at low‐temperature to form Fe(III)‐bearing minerals including magnetite, hydroandradite, and Fe(III)‐bearing serpentine. These reactions are likely responsible for the production of hydrogen gas that is able to support a rock‐hosted microbial community, and show that habitable conditions can be produced even during late, non‐hydrothermal stages of ultramafic rock alteration. Key Points: Subsurface serpentinites from the Samail ophiolite contain distinct reaction fronts with implications for H2 production and habitabilityWhere Fe(II)‐rich brucite is present, it has the potential to fuel future H2 productionFe(II,III)‐rich serpentine, hydroandradite, and magnetite all record past H2 production at low temperatures <200°C [ABSTRACT FROM AUTHOR]

Roman glass is studied here by means of optical absorption spectroscopy, in order to provide an objective method to quantitatively evaluate colour. The dataset is composed of 165 soda-lime silicate glass samples from various western European sites, mainly dated from the first to fourth century AD, and containing variable amounts of iron, manganese and/or antimony. Iron redox ratios and colour coordinates (based on the CIELab colour system) are determined and put in relation with the thickness of samples and their manganese/antimony contents. Results reveal thickness as a crucial parameter when discussing glass hues, thus leading to a differentiation between the ‘intrinsic’ and ‘perceived’ colour of glass objects (i.e. the colour of the object with the thickness normalised to 1 mm, and that with its original thickness, respectively). Apart from HIMT and purple glass, the concentration of ferrous iron appears to be correlated with a* — a colourimetric parameter determining how green the glass is. Significant relations of antimony/manganese contents versus iron redox and glass colour are also considered, resulting in quantitative arguments to entitle antimony-decoloured glass as the most oxidised and colourless glass. [ABSTRACT FROM AUTHOR]

• Gillespite (BaFeSi4O10) can be crystallized in a reduced Ba Fe silicate glass. • Pt is an effective nucleating agent for gillespite. • Glass-ceramics comprising gillespite, sanbornite, and cristobalite have been made. • The amount of gillespite in the glass-ceramic increases with Fe+2 concentration. The discovery of a new glass-ceramic based on the mineral phase gillespite (BaFeSi 4 O 10) is reported. Glasses having the stoichiometric barium iron silicate composition were melted to obtain a range of iron redox states. Gillespite was found to precipitate only if the iron reduction ratio, [Fe+2:total Fe], in the glass was greater than 0.5. Under this condition, surface crystallization was always observed, with some large, internal gillespite crystals forming when the reduction ratio was ≥0.7. A uniform, fine-grained glass-ceramic was achieved when Pt was added as a nucleating agent. The phase assemblage typically comprised gillespite as the major phase with sanbornite (low-BaSi 2 O 5) as a secondary phase and minor amounts of cristobalite: the gillespite crystals forming rosettes around the Pt nuclei. The relative amount of gillespite in the glass-ceramic could be increased by increasing the iron reduction ratio. Gillespite glass-ceramics have a red color which could make them useful as Cd-free red pigments. [ABSTRACT FROM AUTHOR]

Abstract Oxisols are the dominant soil type in humid tropical and subtropical regions and are subjected to both drying–rewetting (DRW) cycles and fluctuating oxygen (O 2) availability driven by warm temperatures and abundant rainfall in surface layers. Drying-rewetting cycles and O 2 fluctuations may critically affect the microbial transformation of plant litter and subsequent stabilization as mineral-associated organic carbon (MAOC), but experimental data are still limited. We examined the impacts of DRW cycles, and variable O 2 regimes with constant moisture, on carbon (C) and iron (Fe) dynamics in a degraded Oxisol (under long-term fallow) with added plant residues. In laboratory incubations (>3 months), both DRW cycling and fluctuating O 2 availability induced a flush of respiration and a temporary increase in microbial biomass C (MBC) following soil rewetting or O 2 exposure, although MBC was consistently suppressed in these treatments relative to the control (60% water holding capacity under constantly aerobic condition). Consequently, DRW cycles significantly increased but O 2 fluctuations significantly decreased cumulative C mineralization relative to the control. Concentrations of short-range-ordered Fe oxides peaked immediately after litter addition and decreased five-fold during the remainder of the experiment. Mineral-associated C (defined as the chemically dispersed <53 μm soil fraction) increased 42–64% relative to initial values but was significantly lower under DRW cycling and fluctuating O 2 relative to the control. Correspondingly, these treatments had greater fine particulate organic C (53–250 μm), despite increased CO 2 production under DRW cycling. Our data indicate the potential for rapid and significant accrual of MAOC in a degraded Oxisol, but environmental factors such as DRW cycling and fluctuating O 2 can inhibit the conversion of plant litter to MAOC—possibly by suppressing microbial biomass formation and/or microbial transformations of organic matter. Graphical abstract Unlabelled Image Highlights • Degraded Oxisols have the potential for rapid accrual of mineral-associated C. • Both dry-rewetting (DRW) cycles and O 2 fluctuations suppress microbial growth. • Neither DRW cycles nor O 2 fluctuations drive substantial iron redox cycling. • Both DRW cycles and O 2 fluctuations reduce mineral-associated C formation. [ABSTRACT FROM AUTHOR]

Abstract This review discusses the chemical mechanisms of ascorbate-dependent reduction and solubilization of ferritin's ferric iron core and subsequent release of ferrous iron. The process is accelerated by low concentrations of Fe(II) that increase ferritin's intrinsic ascorbate oxidase activity, hence increasing the rate of ascorbate radical formation. These increased rates of ascorbate oxidation provide reducing equivalents (electrons) to ferritin's core and speed the core reduction rates with subsequent solubilization and release of Fe(II). Ascorbate-dependent solubilization of ferritin's iron core has consequences relating to the interpretation of 59Fe uptake sourced from 59Fe-lebelled holotransferrin into ferritin. Ascorbate-dependent reduction of the ferritin core iron solubility increases the size of ferritin's iron exchangeable pool and hence the rate and amount of exchange uptake of 59Fe into ferritin, whilst simultaneously increasing net iron release rate from ferritin. This may rationalize the inconsistency that ascorbate apparently stabilizes 59Fe ferritin and retards lysosomal ferritinolysis and whole cell 59Fe release, whilst paradoxically increasing the rate of net iron release from ferritin. This capacity of ascorbate and iron to synergise ferritin iron release has pathological significance, as it lowers the concentration at which ascorbate activates ferritin's iron release to within the physiological range (50–250 μM). These effects have relevance to inflammatory pathology and to the pro-oxidant effects of ascorbate in cancer therapy and cell death by ferroptosis. Graphical abstract fx1 Highlights • Low ascorbate and labile iron concentrations synergise ferritin iron mobilization. • Ascorbate and iron synergise ferritin's intrinsic ascorbate oxidase activity. • Ascorbate-induced reduction of ferritin's core raises its iron exchange capacity. • Raised exchange capacity increases ferritin's ascorbate-dependent 59Fe uptake. • Ferritin's raised 59Fe exchange is often confused with increased net iron uptake. • Synergetic ascorbate- iron-ferritin iron release has pathological implications. [ABSTRACT FROM AUTHOR]

Iron redox cycle plays a primary role in controlling the mobility of P in sediments. It is crucial to better understand how lanthanum (La) modified bentonite (LMB, Phoslock ® ), an increasingly employed capping agent, immobilizes P from sediments by altering Fe redox-coupled P cycling. Batch adsorption experiments found that LMB effectively adsorbed Fe(II) with a capacity of 8.51 mg g −1 . Fe(II)-preloaded LMB effectively retained P during a 518-hour equilibration, while up to 16.7% of adsorbed P was release-sensitive in LMB without Fe(II) preloading. A 60-day incubation experiment was performed using sediment cores, with an LMB amendment dosage of up to 200 LMB/P mob (w/w, P mob denotes the amount of mobile P in the surface 40 mm sediment layer). The concentrations of pore water soluble reactive P (SRP) and labile P were measured by high resolution dialysis (HR-Peeper) and by diffusive gradient in thin films (DGT), respectively, at a vertical millimeter scale. They stratified into static layers with extremely low concentration distribution in the top 16–22 mm sediments (mean SRP ≤ 0.28 mg L −1 and mean DGT-labile P ≤ 0.051 mg L −1 ) and active layers with decreased upward diffusion potential (≤5.85 for SRP and ≤12.7 for DGT-labile P) below the static layer, when the applied dosage reached 60 LMB/P mob . The LMB amendment reduced the pore water Fe and DGT-labile Fe in sediments, while considerable amounts of Fe and Fe-bound P existed in the LMB binding layer (25% of the total P in 200 LMB/P mob treatment). These findings show that the adsorption of Fe by LMB plays a significant role in the stabilization of LMB-bound P, possibly by adsorbing release-sensitive P initially bound to the rhabdophane surface. LMB adsorbed Fe and P were not released until the redox potential decreased to extremely reductive conditions (−150 mV to −300 mV), possibly due to the re-adsorption of Fe and P by LMB. This study reveals synergistic effects of Fe adsorption and provides new insight into the immobilization mechanisms of P by LMB application. [ABSTRACT FROM AUTHOR]

The effectiveness of sediment dredging for the control of internal phosphorus (P) loading, was investigated seasonally in the eutrophic Lake Taihu. The high-resolution dialysis (HR-Peeper) and diffusive gradients in thin films (DGT) techniques were used to measure the concentrations of soluble Fe(II) and soluble reactive P (SRP) as well as DGT-labile Fe/P in the non-dredging and post-dredging sediments. The P resupply kinetics from sediment solids were interpreted using DGT Induced Fluxes in Sediments (DIFS) modeling. The results showed no obvious improvement in water and sediment quality after dredging for 6 years, due to their geographical proximity (a line distance of approximately 9 km). However, dredging significantly decreased the concentrations of soluble Fe(II)/SRP and DGT-labile Fe/P in sediments, with effects varying at different depths below the sediment-water interface; More pronounced effects appeared in January and April. The diffusive flux of pore water SRP from sediments decreased from 0.746, 4.08 and 0.353 mg/m 2 /d to 0.174, 1.58 and 0.048 mg/m 2 /d in April, July and January, respectively. DIFS modeling indicated that the P retention capability of sediment solids was improved in April in post-dredging site. Positive correlations between pore water soluble Fe(II) and SRP as well as between DGT-labile Fe and P, reflect the key role of Fe redox cycling in regulating dredging effectiveness. This effect is especially important in winter and spring, while in summer and autumn, the decomposition of algae promoted the release of P from sediments and suppressed dredging effectiveness. Overall, the high-resolution HR-Peeper and DGT measurements indicated a successful control of internal P loading by dredging, and the post-dredging effectiveness was suppressed by algal bloom. [ABSTRACT FROM AUTHOR]

The surplus of exogenous and endogenous phosphate in submerged paddy fields could increase the risk of algal blooms, the photosynthesis of which might further influence the redox processes of iron. This work investigated the effects of biochar on photosynthetic microorganism growth and iron redox under different phosphate (P) levels to understand the dynamics of P and thereby control non-point source pollution by biochar addition. Paddy soils were incubated anaerobically with phosphate and biochar addition under controlled illumination conditions to determine the variation in chlorophyll a ( Chl a ), ferrous iron [Fe(II)], soil pH and water-soluble phosphate (W-P) with incubation time. Biochar addition significantly inhibited the photosynthetic microorganism growth, with Chl a decreased by 4.74–15.78 mg·g − 1 when compared with the control. Fe(III) reduction was significantly stimulated in response to biochar addition, while Fe(II) oxidation was inhibited because of the suppression of photosynthetic microorganism growth. The enhanced Fe(III) reduction and suppressed Fe(II) oxidation decreased the P solubility in the tested soils. These findings provide a cost-effective approach for inhibiting photosynthetic microorganism growth in paddy field and valuable insight into the effect of iron cycling on P retention for further management of eutrophication from exogenous and endogenous P loading. [ABSTRACT FROM AUTHOR]

The structure of molten Fe 2 SiO 4+ x has been studied using both high-energy X-ray diffraction and Fe K-edge X-ray absorption near-edge structure (XANES) spectroscopy, combined with aerodynamic levitation and laser beam heating. A wide range of Fe 3+ contents were accessed by varying the levitation and atmospheric gas composition. Diffraction measurements were made in the temperature ( T ) and oxygen partial pressure ranges 1624(21) < T < 2183(94) K (uncertainties in parentheses) and −5.6(3) < ΔFMQ < +2.8(5) log units (relative to the Fayalite-Magnetite-Quartz buffer). Iron K-edge XANES measurements covered the ranges 1557(33) < T < 1994(36) K and −2.1(3) < ΔFMQ < +4.4(3) log units. Fe 3+ contents, x = Fe 3+ /ΣFe, estimated directly from the pre-edge peaks of the XANES spectra varied between 0.15(1) and 0.40(2). While these agree in some cases with semi-empirical models, notable discrepancies are discussed in the context of the redox kinetics and the limitations in both the models and in the calibrations used to derive oxidation state from XANES spectra. XANES pre-edge peak areas imply average Fe–O coordination numbers, n FeO , close to 5 for all Fe 3+ /ΣFe. Diffraction measurements yielded values of 4.4(2) < n FeO < 4.7(1). There is limited evidence for a linear trend n FeO ( x ) = 4.46(3) + 0.4(1) x . Asymmetric Fe–O bond length distributions peak at around 1.96 Å and have a shoulder arising from longer interatomic distances. Mean r FeO lie close to 2.06 Å, consistent with n FeO close to 5. These observations suggest that Fe 2+ is less efficient at stabilizing tetrahedral Fe 3+ compared to large monovalent alkali cations. Comparison of in-situ XANES estimates of Fe 3+ /ΣFe in the melts to those of the quenched solids obtained from XANES as well as Mössbauer spectroscopy indicate rapid oxidation during cooling, enabled by stirring of the melt by the levitation gas flow. As such, the oxidation state of hot komatiitic and other highly fluid melts may not be retained, even during rapid cooling, as it is for cooler basaltic and more silicic magmas. [ABSTRACT FROM AUTHOR]

[Display omitted] • Treatment of Cr(pic) 3 using Fenton process was first proposed. • Decomplexing of Cr(pic) 3 by oxidation facilitated the overall Cr removal. • Simultaneous Cr(pic) 3 oxidation and Cr removal were achieved. • Cr(VI) barely accumulated in aqueous phase. Chromium picolinate (Cr(pic) 3) is widely used for feeding animals. Most Cr(pic) 3 administered to animals is excreted without change. The land use of animal waste introduces Cr(pic) 3 to the environment. Transformation of Cr(pic) 3 in the environment releases hexavalent chromium (Cr(VI)), posing a chromium contamination risk. Because chromium (in trivalent valent) is complexed by picolinic acid (pic), removal of chromium in Cr(pic) 3 form via direct adsorption and precipitation methods is rather difficult. Release of chromium from Cr(pic) 3 first and then removal of it through the combination of adsorption and precipitation is advisable. To this end, the treatment of Cr(pic) 3 using the Fenton process was first investigated in this work. Treatment parameters were optimized for Cr(pic) 3 degradation and chromium removal (i.e., initial and final pH of 3.0 and 7.0, and the molar ratio of Fe(II) and H 2 O 2 was 1:2). Adverse effects of the water constituents could be mitigated by flexible adjustment of the Fenton reagent dosages and the external addition of iron ions. The residual chromium in the aqueous phase was simultaneously lowered to the required standard, and Cr(VI) was barely detected in the aqueous phase. Mechanisms of Cr(pic) 3 degradation and chromium removal were proposed by the intermediates determined. The results obtained indicate that the Fenton process is robust for degrading Cr(pic) 3 and removing chromium simultaneously and is promising for treating Cr(pic) 3 -containing wastewater. [ABSTRACT FROM AUTHOR]

[Display omitted] • The reactive co-transport of Fe2+ and antibiotic in quartz sand column was studied. • Fe2+ oxidizes on quartz sand to form weak crystallinity lepidocrocite. • Fe2+ inhibits antibiotics transport in column, inhibitory intensity OTC > OFL > SMX. • OTC and OFL promote the Fe2+ transport and inhibit the lepidocrocite formation. • Fe2+ and Fe2+ reach equilibrium by circulation in OTC and Fe2+ co-transport system. Iron redox is often coupled with the degradation and conversion of co-existing organic pollutants, however, their co-transport behavior in hyporheic zone is not clear. In this study, the characteristics and mechanism of reactive co-transport of Fe2+ and antibiotics during groundwater discharge were revealed by quartz sand column simulation. It was found that Fe2+ inhibited the transport of three antibiotics sulfamethoxazole (SMX), ofloxacin (OFL), and oxytetracycline (OTC) in the quartz column, and the average inhibitory intensity was OTC (∼80%) > OFL (∼20%) > SMX (∼2%). Meanwhile, the content of dissolved oxygen in the column decreased from more than 7 mg/L to 3 mg/L. Fe2+ transport in the column was promoted with the increase of OTC and OFL concentrations (2–50 mg/L), as Fe2+-OTC and Fe2+-OFL complexes inhibited the formation of lepidocrocite and induced Fe2+/ Fe3+ cycling during co-transport of Fe2+ and OTC. In contrast, the interaction between SMX and Fe2+ was not obvious due to the electrostatic repulsion. Due to the prevalence of Fe2+ and antibiotics in groundwater system (especially with high antibiotic contamination), this study provides a new perspective on the potential environmental impacts of Fe and pollutants co-transport during groundwater discharge. [ABSTRACT FROM AUTHOR]

The viscosity of two series of synthetic alkali silicates, corresponding to Al-bearing pantelleritic and Al-free tri-silicate compositions, has been investigated as a function of temperature, iron redox and Na/(Na + K) ratio. Low temperature (708–1000 K) viscosities were determined by the micropenetration technique in the 10 9.6 to 10 13.6 Pa s range. The effect of Fe 2 + /Fe tot , from 0.15 to 0.86, was explored for [Na/(Na + K)] ratios from 0 to 1. The results demonstrate a strong decrease of viscosity with the replacement of Na for K in both the Al-bearing and Al-free compositions. In the Al-bearing pantelleritic glasses (Ebu) the viscosity as well as the activation energy for viscous flow decrease with an increase of Fe 2 + /Fe tot from 0.15 to 0.76. In contrast, no measurable changes in viscosity occur in the Al-free tri-silicate glasses (NFS and KFS) as Fe 2 + /Fe tot ratio varies from 0.15 to 0.86. The comparison of the pantelleritic glasses with peralkaline compositions from the literature stresses the strong influence of iron redox on the overall viscosity of such melts. This suggests that the contribution of iron species should be accounted for in the calculation of agpaitic index (AI) if magma Fe 3 + contents are known. One peculiarity of peralkaline magmas is the excess of alkali to alumina, which generally results in lower viscosities of pantelleritic liquids compared with more common metaluminous rhyolites and exerts a primary control of the rheological behaviour of these melts. Our new viscosity data, combined with existing literature, allow more accurate constraints on the nature and eruption of the pantelleritic magmas. Moreover, the present experimental study can be used to improve the viscosity models that mostly do not take in consideration parameters as iron redox in the viscosity prediction. [ABSTRACT FROM AUTHOR]

Sodium-alkaline earth-silicate glasses, of nominal molar composition 16Na 2 O–10RO–74SiO 2 (R = Ca, (Ca,Mg) and Mg) doped with 0.5 wt.% of Fe 2 O 3 , were studied by UV–Visible–NIR absorption spectroscopy and electron paramagnetic resonance (EPR) at X- and Q-band to understand the structural control of Fe 3+ optical absorption properties as a function of iron redox and glass composition. By comparing with a set of [4] Fe 3+ , [5] Fe 3+ and [6] Fe 3+ crystalline references, optical absorption spectra indicate the presence of 5-fold Fe 3+ in addition to a majority of tetrahedral Fe 3+ . The combination of Q- and X-band EPR data shows Fe 3+ partition among isolated, distributed sites and Fe-rich clusters, providing unique insight into the distortion of isolated Fe 3+ sites. It demonstrates also the peculiar character of the residual Fe 3+ sites that exist in reduced glasses. Changing Ca to Mg increases the amount of tetrahedral Fe 3+ sites and decreases their distortion. The presence of Mg also reduces the amount of isolated rhombic Fe 3+ sites and promotes the formation of clusters. These clusters confirm the non-homogeneous structure of silicate glasses, as well as the preference of Fe 3+ for a more calcic than magnesian environment in sodic (Ca,Mg) glasses. [ABSTRACT FROM AUTHOR]

The CR chondrites carry one of the most pristine records of the solar nebula materials that accreted to form planetesimals. They have experienced very variable degrees of aqueous alteration, ranging from incipient alteration in their matrices to the complete hydration of all of their components. In order to constrain their chemical alteration pathways and the conditions of alteration, we have investigated the mineralogy and Fe oxidation state of silicates in the matrices of 8 CR chondrites, from type 3 to type 1. Fe-L edge X-ray Absorption Near Edge Structure (XANES) was performed on matrix FIB sections using synchrotron-based scanning transmission X-ray microscopy (STXM). The Fe 3 + / ∑ Fe ratio of submicron silicate particles was obtained and coordinated with TEM observations. In all the least altered CR chondrites (QUE 99177, EET 87770, EET 92042, LAP 02342, GRA 95229 and Renazzo), we find that the matrices consist of abundant submicron Fe-rich hydrated amorphous silicate grains, mixed with nanometer-sized phyllosilicates. The Fe 3 + / ∑ Fe ratios of both amorphous and nanocrystalline regions are very high with values ranging from 68 to 78%. In the most altered samples (Al Rais and GRO 95577), fine-grained phyllosilicates also have a high Fe 3 + / ∑ Fe ratio (around 70%), whereas the coarse, micrometer-sized phyllosilicates are less oxidized (down to 55%) and have a lower iron content. These observations suggest the following sequence: submicron Fe 2+ -amorphous silicate particles were the building blocks of CR matrices; after accretion they were quickly hydrated and oxidized, leading to a metastable, amorphous gel-like phase. Nucleation and growth of crystalline phyllosilicates was kinetically-limited in most type 3 and 2 CRs, but increased as alteration became more extensive in Al Rais and GRO 95577. The decreasing Fe 3 + / ∑ Fe ratio is interpreted as a result of the transfer of Fe 3+ from silicates to oxides during growth, while aqueous alteration progressed (higher temperature, longer duration, change of fluid composition). In a fully closed system, equilibrium thermodynamics suggest that the water to rock ratios, typically assumed to be low (<1) for chondrites, should primarily control the iron valency of the silicates and predict a lower Fe 3 + / ∑ Fe ratio. Such a high Fe 3 + / ∑ Fe value could be accounted for, however, if the system was partially open, at least with respect to H 2 (and other gases as well). Rapid degassing of the fluid would have favored more oxidizing fluid conditions. Recently proposed scenarios involving some degree of water D/H increase through Rayleigh isotopic fractionation are supported by these results. [ABSTRACT FROM AUTHOR]

The crystallization tendencies of five series of Na 2 O–FeO–Fe 2 O 3 –P 2 O 5 (NFP) glasses with different O/P (3.0–3.5) and Fe/P (0.13–0.67) ratios were studied. Characteristic temperatures, including the glass transition temperature (T g ) and crystallization temperature (T x h ), were obtained using differential thermal analysis (DTA), and liquidus temperatures (T L ) were determined by microscopic evaluation of heat-treated samples. The compositional dependence of glass structure and the characteristic temperatures are discussed. The glass stability (GS) against crystallization is described using parameters based on the characteristic temperatures. For the glass series with O/P ratios near the pyrophosphate composition (O/P ~ 3.5), the glass stability goes through a minimum value around Fe/P ~ 0.3 to 0.4, which corresponds to the crystalline phase NaFeP 2 O 7 and Na 7 Fe 3 (P 2 O 7 ) 4 . [ABSTRACT FROM AUTHOR]

Lignin decomposition is critically linked to terrestrial carbon (C) cycle due to the enormous C mass of lignin and its importance in controlling overall rates of litter decomposition. Interactions between lignin and iron (Fe) minerals have been increasingly recognized as key mediators of lignin decomposition in experimental studies. However, we still lack a quantitative understanding of how Fe minerals interact with microbes to control lignin decomposition. Here, we leveraged experimental results from an incubation of Fe-rich soil, in which lignin decomposition rates were measured at aerobic conditions after four levels of pre-treated O 2 availability, to examine microbe-Fe (MiFe) interactions in lignin decomposition with a MiFe model. We quantified how Fe redox cycling interacted with microbial activities to control lignin decomposition via data-model integration. Our results showed that the MiFe model with time-dependent growth and mortality functions better represented CO 2 release from lignin decomposition (R2 ranging from 0.96 to 0.97) than models assuming either first-order or Michaelis-Menten kinetics. Reduction of Fe(III) to Fe(II) after pre-treatments with lower O 2 availability stimulated the Fenton reaction to break down macro-molecular lignin into small molecules available to microbes. The small molecules of lignin and necromass bounded with oxidized Fe and were protected from decomposition. After 1-year incubation, the model implied that most of C stabilized with Fe minerals was derived from small molecular lignin C. Our quantitative analysis of microbe-Fe interactions sheds new light on lignin decomposition and preservation and helps improve model prediction of soil C persistence under global change. • Observed nonlinear lignin decomposition cannot be explained by traditional C models. • A microbe-iron interactive model was developed to describe nonlinear decomposition. • Iron plays dual roles on lignin decomposition. • Plant-derived lignin C contributes most to mineral associated organic matter. [ABSTRACT FROM AUTHOR]

In straw return fields, nitrogen-fertilizers are added to mitigate microbial competition for nitrogen with plants. However, in arsenic (As)-contaminated paddy fields, the specific effects of different nitrogen fertilizers on As mobility after straw incorporation and the interactions among iron(Fe)/carbon(C)/nitrogen(N)/As are not well understood. In the reported microcosm experiment we monitored As-mobility as a function of different dosages of KNO 3 , NH 4 Cl and rice straw incorporation. Addition of both KNO 3 and NH 4 Cl significantly inhibited the As mobilization induced by straw incorporation. Following the KNO 3 addition, the As concentration in porewater dropped by 51–66% after 2 days of the incubation by restraining Fe reduction and enhancing Fe oxidation. High-dose NH 4 Cl addition reduced As in porewater by 22–43% throughout the incubation by decreasing porewater pH. High-throughput sequencing results demonstrated that KNO 3 addition enriches both the denitrifying and Fe-oxidizing bacteria, while diminishing Fe-reducing bacteria; NH 4 Cl addition has the opposite effect on Fe-reducing bacteria. Network analysis revealed that As and Fe concentrations in porewater were positively correlated with the abundance of denitrifying and Fe-reducing bacteria. This study broadens our insight into the As biogeochemistry associated with the N/C/Fe balance in soil, which are of great significance for agronomic management and mitigation the risk of As-contaminated paddy fields. [Display omitted] • KNO 3 immobilized As by enhancing Fe oxidation and restraining Fe dissolution. • NH 4 Cl immobilized As by decreasing soil pH and inhibiting anammox. • KNO 3 enriched iron-oxidizing bacteria but reduced iron-reducing bacteria. • NH 4 Cl addition increased the relative abundance of iron-reducing bacteria. • KNO 3 showed short-term but NH 4 Cl exhibited long-term As immobilization function. [ABSTRACT FROM AUTHOR]

As antibiotic-resistant bacterial strains emerge and pose increased global health risks, new antibacterial agents are needed as alternatives to conventional antimicrobials. Naturally occurring antibacterial clays have been identified which are effective in killing antibiotic-resistant bacteria. This study examines a hydrothermally formed antibacterial clay deposit near Crater Lake, OR (USA). Our hypothesis is that antibacterial clays buffer pH and Eh conditions to dissolve unstable mineral phases containing transition metals (primarily Fe), while smectite interlayers serve as reservoirs for time release of bactericidal components. Model pathogens ( Escherichia coli ATCC 25922 and Staphylococcus epidermidis ATCC 14990) were incubated with clays from different alteration zones of the hydrothermal deposit. In vitro antibacterial susceptibility testing showed that reduced mineral zones were bactericidal, while more oxidized zones had variable antibacterial effect. TEM images showed no indication of cell lysis. Cytoplasmic condensation and cell wall accumulations of <100 nm particles were seen within both bacterial populations. Electron energy loss analysis indicates precipitation of intracellular Fe-oxide nanoparticles (<10 nm) in E. coli after 24 h. Clay minerals and pyrite buffer aqueous solutions to pH 2.5-3.1, Eh > 630 mV and contain elevated level (mM) of soluble Fe (Fe and Fe) and Al. Our interpretation is that rapid uptake of Fe impairs bacterial metabolism by flooding the cell with excess Fe and overwhelming iron storage proteins. As the intracellular Fe oxidizes, it produces reactive oxygen species that damage biomolecules and precipitates Fe-oxides. The ability of antibacterial clays to buffer pH and Eh in chronic non-healing wounds to conditions of healthy skin appears key to their healing potential and viability as an alternative to conventional antibiotics. [ABSTRACT FROM AUTHOR]