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Continental margin sediments represent a potentially large, but poorly constrained, source of iron to surface waters to support primary productivity. To investigate this problem, we examined iron redox cycling in the sediments of ...
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Continental margin sediments represent a potentially large, but poorly constrained, source of iron to surface waters to support primary productivity. To investigate this problem, we examined iron redox cycling in the sediments of Santa Monica Basin (SMB), CA as it relates to benthic sources of iron to the water column. Our results show that iron redox cycling in SMB sediments results in the formation of a thin layer at the sediment surface (< 2 cm thick) that is highly enriched in amorphous, reactive iron oxides. Such oxides, when re-suspended into the water column, may be a source of bioaccessible iron to primary producers. Calculations show that the resuspension flux of reactive amorphous iron oxides from SMB sediments may be up to two orders of magnitude greater than benthic fluxes of soluble dissolved iron from most continental margin sediments. The factors which favor the formation of this surface sediment layer enriched in reactive amorphous iron oxides appear to be related to the dissolved O-2 level in SMB bottom waters, and its impact on iron redox cycling and the occurrence of sediment bioturbation. These factors are not necessarily unique to SMB, and other sediment sites on the open continental margin that lie within the O-2 minimum zone may also have similar properties. Furthermore, expansion of O-2 minimum zones due to global warming could possibly increase the areal extent of such environments, and potentially act as a negative feedback on rising atmospheric CO2 by providing additional iron to stimulate primary production. Further work examining iron biogeochemistry in low O-2 continental margin settings will be needed to more critically examine these possibilities, along with studies that better define the role sediment resuspension fluxes may play as an iron source to surface waters. Such efforts will ultimately lead to an improved understanding of the role that ocean deoxygenation may play in enhancing this sediment iron source to support surface ocean primary production.
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The iron (Fe) cycle is one of the most important marine and lacustrine geochemical cycles and is closely related to environmental redox conditions, which directly affects carbon, sulfur, phosphorus cycling. Analysis of Fe speciati...
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The iron (Fe) cycle is one of the most important marine and lacustrine geochemical cycles and is closely related to environmental redox conditions, which directly affects carbon, sulfur, phosphorus cycling. Analysis of Fe speciation (including Fe-ox, Fe-mag, Fe-carb, and Fe-py) of organic-rich shale samples provides new insights into the controversial redox conditions during the Chang 7 sedimentary period. Our results show that Fe-carb and Fe-py of the Chang 7 organic-rich shale are the main forms of reactive Fe pools, indicating that reactive Fe mainly existed in the form of Fe2+ in sediments. With a few exceptions, most samples fall into the ferruginous region in crossplots of Fe-py/Fe-HR versus Fe-HR/Fe-T, indicating that pore water environments during early diagenesis were ferruginous accompanied by intermittent euxinic. Previous works examining the size of framboidal pyrite, the molar ratio of organic carbon and total phosphorus (P), and nitrogen isotope values suggest the bottom water environments conditions were oxic-suboxic accompanied by intermittent anoxic. We argue these patterns may be explained by the proximity of the redox boundary of depositional environment to the sediment-water interface during most of the Chang 7 period, with short-term fluctuations. Euxinic pore water conditions correspond to anoxic bottom water conditions and elevated total organic carbon (TOC) content, indicating that these environmental conditions would increase the flux of P diffused from sediments. As a result, lacustrine primary productivity would be elevated, creating conditions conducive to the accumulation of organic matter that is characteristic of the Chang 7 sedimentary period. This work will further our understanding of the accumulation mechanism of organic matter in the Chang 7 shale, the principal source rock for the Mesozoic oil-bearing system in the Ordos Basin.
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Elements that can occur in more than one valence state, such as Fe, C and S, play an important role in Earth's systems at all levels, and can drive planetary evolution as they cycle through the various geochemical reservoirs. Subd...
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Elements that can occur in more than one valence state, such as Fe, C and S, play an important role in Earth's systems at all levels, and can drive planetary evolution as they cycle through the various geochemical reservoirs. Subduction introduces oxidised Fe, C and S in sediments, altered ocean crust, and partially serpentinised lithospheric mantle to the relatively reduced mantle, with short- and long-term consequences for the redox state of the mantle. The distribution of redox-sensitive elements in the mantle controls the redox state of mantle-derived material added to the lithosphere and atmosphere, such as arc volcanic gases and the magmas that form arc-related ore deposits.The extent of mantle oxidation induced by subduction zone cycling can be assessed, albeit with large uncertainties, with redox budget calculations that quantify the inputs and outputs to subduction zones. Literature data are augmented by new measurements of the chemical composition of partially serpentinised lithospheric mantle from New Caledonia and ODP 209. Results indicate that there is a net addition of Fe (55±13×10 ~(12)molyear ~(-1)), C (4.6±4.0×10 ~(12)molyear ~(-1)), S (2.4±0.9×10 ~(12)molyear ~(-1)), and redox budget (5-89×10 ~(12)molyear ~(-1)) at subduction zones. Monte Carlo calculations of redox budget fluxes indicate that fluxes are 46±12×10 ~(12)molyear ~(-1) entering subduction zones, if input and output parameters are assumed to be normally distributed, and 46-58×10 ~(12)molyear ~(-1) if input and output parameters are assumed to be log-normally distributed.Thus, inputs into subduction zones for Fe, C, S and redox budget are in excess of subduction zone outputs. If MORB and plume-related fluxes are taken into account then Fe, C and S fluxes balance, within error. However, the redox budget does not balance, unless the very lowest estimates for the extent of slab oxidation are taken. Thus it is likely that subduction continuously increases the redox budget of the mantle, that is, there is addition of Fe, C and S that are oxidised relative to the Fe, C and S in the mantle.The fate of this redox budget can be constrained by consideration of element mobility under mantle conditions. If slab fluids are assumed to be dominantly aqueous and relatively low salinity then fluxes of Fe ~(3+), C ~(4+), and S ~(6+) are limited to less than 10 ~9, 2.3×10 ~(12)molyear ~(-1) and 2×10 ~(12)molyear ~(-1) respectively by the low solubility of these elements in slab-derived fluids. Nevertheless, such fluxes can produce the increased f _(O2) inferred for sub-arc mantle from arc lavas after around 10Ma subduction.The rest of the redox budget added by the subduction process is likely to be carried to the deep mantle by the slab, and mix slowly with the whole mantle reservoir, depending on the timescale of reincorporation of subducted lithosphere into the mantle. Simple mixing calculations indicate that these fluxes will only cause a measurable difference to mantle redox on a 1. Ga timescale, which is longer than the 550. Ma during which redox budget fluxes are likely to have been at present day levels. However, measurable effects, with potential consequences for the Earth's evolution may be expected in the future.
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Iron minerals influence the environmental redox behaviour and mobility of metals including the long-lived radionuclide technetium. Technetium is highly mobile in its oxidized form pertechnetate (Tc(VII)O_4), however, when it is re...
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Iron minerals influence the environmental redox behaviour and mobility of metals including the long-lived radionuclide technetium. Technetium is highly mobile in its oxidized form pertechnetate (Tc(VII)O_4), however, when it is reduced to Tc(IV) it immobilizes readily via precipitation or sorption. In low concentration tracer experiments, and in higher concentration XAS experiments, pertechnetate was added to samples of biogenic and abiotically synthesized Fe(II)-bearing minerals (bio-magnetite, bio-vivianite, bio-siderite and an abiotically precipitated Fe(II) gel). Each mineral scavenged different quantities of Tc(VII) from solution with essentially complete removal in Fe(II)-gel and bio-magnetite systems and with 84±4% removal onto bio-siderite and 68±5% removal onto bio-vivianite over 45 days. In select, higher concentration, Tc XAS experiments, XANES spectra showed reductive precipitation to Tc(IV) in all samples. Furthermore, EXAFS spectra for bio-siderite, bio-vivianite and Fe(II)-gel showed that Tc(IV) was present as short range ordered hydrous Tc(IV)O_2-like phases in the minerals and for some systems suggested possible incorporation in an octahedral coordination environment. Low concentration reoxidation experiments with air-, and in the case of the Fe(II) gel, nitrate-oxidation of the Tc(IV)-labelled samples resulted in only partial remobilization of Tc. Upon exposure to air, the Tc bound to the Fe-minerals was resistant to oxidative remobilization with a maximum of ~15% Tc remobilized in the bio-vivianite system after 45 days of air exposure. Nitrate mediated oxidation of Fe(II)-gel inoculated with a stable consortium of nitrate-reducing, Fe(II)-oxidizing bacteria showed only 3.8±0.4% remobilization of reduced Tc(IV), again highlighting the recalcitrance of Tc(IV) to oxidative remobilization in Fe-bearing systems. The resultant XANES spectra of the reoxidized minerals showed Tc(IV)-like spectra in the reoxidized Fe-phases. Overall, this study highlights the role that Fe-bearing biogenic mineral phases have in controlling reductive scavenging of Tc(VII) to hydrous TcO_2-like phases onto a range of Fe(II)-bearing minerals. In addition, it suggests that on reoxidation of these phases, Fe-bound Tc(IV) may be octahedrally coordinated and is largely recalcitrant to reoxidation over medium-term timescales. This has implications when considering remediation approaches and in predictions of the long-term fate of Tc in the nuclear legacy.
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As an essential nutrient and energy source for the growth of microbial organisms, iron is metabolically cycled between reduced and oxidized chemical forms. The resulting flow of electrons is invariably tied to reactions with other...
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As an essential nutrient and energy source for the growth of microbial organisms, iron is metabolically cycled between reduced and oxidized chemical forms. The resulting flow of electrons is invariably tied to reactions with other redox-sensitive elements, including oxygen, carbon, nitrogen, and sulfur. Therefore, iron is intimately involved in the geochemistry, mineralogy, and petrology of modern aquatic systems and their associated sediments, particulates, and porewaters. In the geological past, iron played an even greater role in marine geochemistry, as evidenced by the vast deposits of Precambrian iron-rich sediments, the "banded iron formations." These deposits are now being used as proxies for understanding the chemical composition of the ancient oceans and atmosphere.
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A green, high-efficiency, and wide pH tolerance water remediation process has been urgently acquired for the increasingly exacerbating contaminated water. In this study, a Fe~(3+)/persulfate (Fe~(3+)/PS) system was employed and en...
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A green, high-efficiency, and wide pH tolerance water remediation process has been urgently acquired for the increasingly exacerbating contaminated water. In this study, a Fe~(3+)/persulfate (Fe~(3+)/PS) system was employed and enhanced with a green natural ligand cysteine (Cys) for the degradation of quinclorac (QNC). The introduction of Cys into the Fe~(3+) /PS system widened the effective pH range to 9 with a superior removal rate for QNC. The mechanism revealed that the Fe~(3+)/Cys/PS system can enhance the ability of degrading QNC by accelerating the Fe~(3+)/Fe~(2+) redox cycle, maintaining Fe~(2+) concentration and thereby generating more HO~· and SO_4~(·-). The impact factors (i.e., pH, concentrations of PS, Fe~(3+) and Cys) were optimized as well. This work provides a promising strategy with high catalytic activity and wide pH tolerance for organic contaminated water remediation.
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The electronic configuration of oxygen (O_2) does not allow it to react directly with wine reductants such as polyphenols. It relies on the catalytic intervention of iron (Fe), which redox cycles between its ferrous (Fe(Ⅱ)) and f...
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The electronic configuration of oxygen (O_2) does not allow it to react directly with wine reductants such as polyphenols. It relies on the catalytic intervention of iron (Fe), which redox cycles between its ferrous (Fe(Ⅱ)) and ferric (Fe(Ⅲ)) states. O_2 oxidizes Fe(Ⅱ) to Fe(Ⅲ), and Fe(Ⅲ) then oxidizes polyphenols. Low concentrations of copper accelerate oxidation, and nucleophiles, especially sulfite, promote polyphenol oxidation. In wine that is protected from air, Fe exists mainly as Fe(Ⅱ), but the Fe(Ⅲ):Fe(Ⅱ) concentration ratio increases immediately on air exposure, stabilizing at varying speeds and values. The oxidation of Fe(Ⅱ) in air-saturated model wine and the reduction of Fe(Ⅲ) by a catechol under nitrogen in model wine were examined separately to better understand the oxidative process. The Fe(Ⅲ) produced when Fe(Ⅱ) reacted with O_2 slows the reaction. As in wine, it was important to include sulfite to remove the intermediate hydrogen peroxide, which also oxidizes Fe(Ⅱ). The reaction was pseudosecond-order in Fe(Ⅱ), indicating that the transfer of both electrons to O_2 is rate determining. Similarly, when Fe(Ⅲ) was reduced by the catechol, the Fe(Ⅱ) produced inhibited the reaction, which overall followed a pseudosecond-order rate law in Fe(Ⅲ). The rate of Fe(Ⅱ) oxidation was slower than the rate of Fe(Ⅲ) reduction, but when the reactions occurred together, as in wine oxidation, Fe(Ⅲ) and Fe(Ⅱ) concentrations equilibrated such that their rates equalized. Under the conditions studied, this occurred at 32% Fe(Ⅲ). This equilibrium was attained quickly, as is the case in red wine. These findings on the oxidative process should help explain the relationships between wine composition, redox state, and Fe(Ⅲ):Fe(Ⅱ) concentration ratios.
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? 2023 Elsevier LtdThe aim of this paper was to investigate the influence of Fe (III) on humification and free radicals evolution. The experimental data showed that the experimental group (CT) with Fe2(SO4)3 had a better degree of...
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? 2023 Elsevier LtdThe aim of this paper was to investigate the influence of Fe (III) on humification and free radicals evolution. The experimental data showed that the experimental group (CT) with Fe2(SO4)3 had a better degree of humification than the control group (CK). The humic substances (HS) content was 10% higher in CT (23.94 mg·g?1) than in CK (21.54 mg·g?1) in the final. Fe (III) contributed significantly to the formation of free radicals in HS. The amount of H2O2 in CT increased to 74.8 mmol·kg?1, while CK was only 46.5 mmol·kg?1. The content of semiquinone free radical was 10.32 × 1011 spins/mm3 in CT, 5.11 × 1011 spins/mm3 in CK in the end. Several iron-reducing bacteria were detected in composting, among which Paenibacillus was dominant. The above findings suggested that the application of Fe2(SO4)3 enhanced the iron reduction synergistic quinone redox cycling and promoted the generation of free radicals during the humification of composting.
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The reformer sponge iron cycle produces hydrogen from hydrocarbons with high efficiencies and a high degree of purity. In this process, the gas purification step is performed by a cyclic redox reaction of the contact mass (iron ox...
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The reformer sponge iron cycle produces hydrogen from hydrocarbons with high efficiencies and a high degree of purity. In this process, the gas purification step is performed by a cyclic redox reaction of the contact mass (iron oxide) with synthesis gas and steam at high temperatures. In order to get information about the lifetime of the contact mass, investigations of the cycle behaviour of the contact mass have been carried out. First experiments showed that iron oxide pellets without additives were deactivated quickly for the redox reaction due to sintering effects. SiO_2, CaO and Al_2O_3 were added to the iron oxides, and the effects of different compositions were investigated. The contact mass was characterized before cycling, after 10, and 20 cycles by XRD, SEM, mercury porosimetry and Raman spectroscopy. It was shown that a higher content of SiO_2 prevents sintering of iron species during reduction and the subsequent oxidation with water vapour over 20 cycles at 800℃. A quartz content below 6.5 wt% prevented the contact mass from reacting already after 5 cycles.
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It is generally believed that clay minerals can protect organic matter from degradation in redox active environments, but both biotic and abiotic factors can influence the redox process and thus potentially change the clay-organic...
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It is generally believed that clay minerals can protect organic matter from degradation in redox active environments, but both biotic and abiotic factors can influence the redox process and thus potentially change the clay-organic association. However, the specific mechanisms involved in this process remain poorly understood. In this study, model organic compound 12-Aminolauric acid (ALA) was selected to intercalate into the structural interlayer of nontronite (an iron-rich smectite, NAu-2) to form an ALA intercalated NAu-2 composite (ALA-NAu-2). Shawanellaputrefaciens CN32 and sodium dithionite were used to reduce structural Fe(III) to Fe(II) in NAu-2 and ALA-NAu-2. The bioreduced ALA-NAu-2 was subsequently re-oxidized by air. The rates and extents of bioreduction and air re-oxidation were determined with wet chemistry methods. ALA release from ALA-NAu-2 via the redox process was monitored. Mineralogical changes after iron redox cycle were investigated with X-ray diffraction, infrared spectroscopy, and scanning and transmission electron microscopy. At the beginning stage of bioreduction, S. putrefaciens CN32 reductively dissolved small and poorly crystalline particles and released intercalated ALA, resulting a positive correlation between ALA release and iron reduction extent (<12%). The subsequent bioreduction (reduction extent from 12-30%) and complete air re-oxidation showed no effect on ALA release. These results suggest that released ALA was largely from small and poorly crystalline NAu-2 particles. In contrast to bioreduction, chemical reduction did not exhibit any selectivity in reducing ALA-NAu-2 particles, and a considerable amount of reductive dissolution was responsible for a large amount of ALA release (>80%). Because bacteria are the principal agent for mediating redox process in natural environments, our results demonstrated that the structural interlayer of smectite can serve as a potential shelter to protect organic matter from oxidation.
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