Kinetics and mechanisms of iron redox reactions in silicate melts: The effects of temperature and alkali cations

The kinetics and the mechanisms of iron redox reactions in molten Fe-bearing pyroxene compositions have been investigated by Raman spectroscopy and X-ray absorption Near Edge Structure (XANES) experiments at the iron K-edge. The former experiments have been made only near the glass transition whereas the latter have also been performed from about 1300 to 2100 K. The same kinetics are observed with both techniques. They are described by characteristic times that depend primarily on temperature and not on the initial redox state. At high temperatures, where both kinds of reactions could be investigated, these times are similar for oxidation and reduction. From these characteristic times we have calculated as a function of temperature and composition a parameter termed effective redox diffusivity. For a given melt, the diffusivities follow two distinct Arrhenius laws, which indicate that the mechanisms of the redox reaction are not the same near the glass transition and at high temperatures. As is now well established, diffusion of divalent cations is the dominant mechanism at low temperatures but the enhanced kinetics observed for alkali-bearing melts indicate that Li⁺ and Na⁺ also participate in ionic transport. At superliquidus temperatures, in contrast, diffusion of oxygen represents the dominant mechanism.     

Probing the oxidation-reduction properties of terrestrially and microbially derived dissolved organic matter

Dissolved organic matter (DOM) has been shown to be an integral component in biogeochemical electron transfer reactions due to its demonstrated ability to facilitate redox reactions. While the role of DOM as a facilitator of electron transfer processes has been demonstrated, greater knowledge would lead to better understanding of the structural components responsible for redox behavior, such as quinones and nitrogen and sulfur (N/S) functional groups. This investigation uses direct scan voltammetry (DSV) coupled with fluorescence and NMR spectroscopy as well as thermochemolysis gas chromatography mass spectrometry (GC-MS) and X-ray photoelectron spectroscopy (XPS) to elucidate the organic moieties responsible for facilitating electron transfer reactions. We contrast electrochemical properties and structural details of three organic matter isolates from diverse sources; Great Dismal Swamp DOM (terrestrially derived, highly aromatic), Pony Lake DOM (microbially derived, highly aliphatic) and Toolik Lake (terrestrially derived, photochemically and microbially altered) with juglone (a redox-active model quinone). Aromatic and phenolic constituents were detected (by ¹³C NMR) and recovered (by thermochemolysis GC-MS) from all three fulvic acid samples, highlighting the ubiquity of these compounds and suggesting that the quinone-phenol redox couple is not limited to DOM derived from lignin precursors. The range of hydroxy-benzene and benzoic acid derivatives may explain the lack of a single pair of well-defined oxidation and reduction peaks in the DSV scans. The presence of a wide-range of hydroxylated benzoic acid isomers and other redox-active aromatic residues implies that native DOM possesses overlapping redox potentials analogous to their characteristic range of pK_a values.     

Evaluation of the processes affecting vertical water chemistry in an alluvial aquifer of Mankyeong Watershed, Korea, using multivariate statistical analyses

Vertical variations of redox chemistry and groundwater quality were investigated in an alluvial aquifer beneath an agricultural area, in which deep groundwaters are free of NO₃, Fe, and Mn problems that are frequently encountered during the development of alluvial groundwaters. This study was performed to identify and evaluate vertical chemical processes attenuating these chemical species in the study area. For this study, the processes affecting groundwater chemistry were identified by factor analysis (FA) and the groundwater samples collected from six multilevel samplers were hierarchically classified into three different redox zones by cluster analysis (CA) based on the similarity of geochemical features. FA results indicated three major factors affecting the overall water chemistry: agricultural activities (factor 1), redox reactions (factor 2), and remnant seawater (factor 3). The groundwater quality in the study area was revealed to be controlled by a series of different redox reactions, resulting in different redox zones as a function of depth. It was also revealed that the low Fe and Mn levels in the groundwater of the deeper part are associated with sulfate reduction, which led to precipitation of Fe as iron sulfide and adsorption of Mn on it.     

Iron-Molybdenum Isotopes and the Chemical Evolution of Ancient-Oceans

Iron(Fe) is abundant in nature while molybdenum(Mo) is the most abundant transition metal in seawater. Due to their high sensitivity to the redox state of the environment, the isotopic compositions of Fe and Mo as well as variations have been widely used to probe the redox conditions and the evolution of ancient ocean chemistry in favor of improved analytical techniques. Here, we summarized isotopic fractionation mechanisms and natural distribution of both iron and molybdenum isotopes, and further we summarized and partially reinterpreted the redox evolution of ancient oceans through time based on available Fe-Mo data compiled in this study. The process that causes the largest iron isotope fractionation is redox reaction and the iron in oxidation state is generally enriched in ⁵⁶Fe. Biotic and abiotic pyrite formations also produce a large Fe isotope fractionations. Isotopic fractionation of molybdenum in seawater is mainly caused by the adsorption process of dissolved Mo onto ferromanganese oxides or hydroxides in sediments. Fe-Mn (hydro)oxides tend to adsorb isotopically light molybdenum resulting in the isotopic composition of Mo in seawater heavier. However, the Mo sinks in euxinic settings cause almost no molybdenum isotope fractionation. The Fe Mo isotope isotopic records through geological timegenerally suggest similar ocean redox evolution: Oceans older than 2.3 Ga was mainly dominated by ferruginous condition, and there was a slight increase in oxygen content between 2.6 and 2.5 Ga. Earth’s surface was initially oxidized during 2.3 to 1.8 Ga, during which euxinic deposition of sulfide was elevated. Euxinic waters may have expanded greatly between 1.8 and 0.8 Ga, and after that, Earth’s surface had being gradually oxidized and the euxinic waters shrank substantially.Finally, suggestions are proposed for further work on the Fe-Mo isotope research in the context of ancient ocean chemistry.     

Speciation and natural attenuation of arsenic and iron in a tidally influenced shallow aquifer

The mobility of subsurface arsenic is controlled by sorption, precipitation, and dissolution processes that are tied directly to coupled redox reactions with more abundant, but spatially and temporally variable, iron and sulfur species. Adjacent to the site of a former pesticide manufacturing facility near San Francisco Bay (California, USA), soil and groundwater arsenic concentrations are elevated in sediments near the prior source, but decrease to background levels downgradient where shallow groundwater mixes with infiltrating tidal waters at the plume periphery, which has not migrated appreciably in over two decades of monitoring. We used synchrotron X-ray absorption spectroscopy, together with supporting characterizations and sequential chemical extractions, to directly determine the oxidation state of arsenic and iron as a function of depth in sediments from cores recovered from the unsaturated and saturated zones of a shallow aquifer (to 3.5 m below the surface). Arsenic oxidation state and local bonding in sediments, as As-sulfide, As(III)-oxide, or As(V)-oxide, were related to lithologic redox horizons and depth to groundwater. Based on arsenic and iron speciation, three subsurface zones were identified: (i) a shallow reduced zone in which sulfide phases were found in either the arsenic spectra (realgar-like or orpiment-like local structure), the iron spectra (presence of pyrite), or both, with and without As(III) or As(V) coordinated by oxygen; (ii) a middle transitional zone with mixed arsenic oxidation states (As(III)–O and As(V)–O) but no evidence for sulfide phases in either the arsenic or iron spectra; and (iii) a lower oxidized zone in the saturated freshwater aquifer in which sediments contained only oxidized As(V) and Fe(III) in labile (non-detrital) phases. The zone of transition between the presence and absence of sulfide phases corresponded to the approximate seasonal fluctuation in water level associated with shallow groundwater in the sand-dominated, lower oxic zone. Total sediment arsenic concentrations showed a minimum in the transition zone and an increase in the oxic zone, particularly in core samples nearest the former source. Equilibrium and reaction progress modeling of aqueous-sediment reactions in response to decreasing oxidation potential were used to illustrate the dynamics of arsenic uptake and release in the shallow subsurface. Arsenic attenuation was controlled by two mechanisms, precipitation as sulfide phases under sulfate-reducing conditions in the unsaturated zone, and adsorption of oxidized arsenic to iron hydroxide phases under oxidizing conditions in saturated groundwaters. This study demonstrates that both realgar-type and orpiment-type phases can form in sulfate-reducing sediments at ambient temperatures, with realgar predicted as the thermodynamically stable phase in the presence of pyrite and As(III) under more reduced conditions than orpiment. Field and modeling results indicate that the potential for release of arsenite to solution is maximized in the transition between sulfate-reduced and iron-oxidized conditions when concentrations of labile iron are low relative to arsenic, pH-controlled arsenic sorption is the primary attenuation mechanism, and mixed Fe(II,III)-oxide phases do not form and generate new sorption sites.     

古海洋活性铁循环研究进展及对白垩纪缺氧 富氧 沉积转变的启示

铁作为地壳中丰度最高的元素之一,广泛参与到一系列地球化学循环中。现代海洋中的铁主要来源于河流、冰川和风的铁氧化物颗粒和溶解铁的输入。陆源输入的铁氧化物在有机质埋藏、降解的早期成岩作用过程中,发生一系列转化过程而埋藏下来,该过程被称作活性铁循环。氧化 强氧化条件利于沉积物中氧化铁的持续产生或者至少保持不被溶解的状态,从而形成棕色-红色沉积物;还原条件利于沉积物中铁氧化物的溶解,形成菱铁矿、黄铁矿(铁硫化物) 等形式的埋藏,并可能造成溶解铁在海洋内的迁移。Raiswell、Canfield、Poulton等通过对现代典型海洋环境活性铁循环研究,提出了一系列用于判别古海洋氧化 还原条件的活性铁指标体系,并成功地将太古宙以来的古海洋划分成为含铁的大洋、硫化的大洋和氧化的大洋等3个演化阶段。由于活性铁的不同形态对磷具有不同的生物地球化学效应,将造成“氧化条件下磷的优先埋藏、缺氧条件下优先释放的现象”。磷是海洋生产力的限制性元素,铁和磷循环的上述耦合关系将造成“缺氧的大洋生产力越高,富氧的大洋生产力越低”现象的出现。目前已在白垩纪古海洋缺氧 富氧沉积中初步证实了上述反馈关系的存在,但是对活性铁埋藏形式对该特殊沉积的贡献还需要进一步的工作。     

A comparative study of iron and manganese diagenesis in continental slope and deep sea basin sediments off Uruguay (SW Atlantic)

Pore water and solid phase from surface sediments of the continental slope off Uruguay and from the Argentine Basin (southwestern Atlantic) were investigated geochemically to ascribe characteristic early diagenetic reactions of iron and manganese. Solid-phase iron speciation was determined by extractions as well as by Mössbauer spectroscopy. Both methods showed good agreement ( <6% deviation) for total-Fe speciation. The proportion of easy reducible iron oxyhydroxide relative to total-Fe oxides decreased from the continental slope to the deep sea which is attributed to an increase in crystallinity during transport as well as to a general decrease of iron mobilization. The product of iron reoxidation is Fe oxyhydroxide which made up less than 5% of total Fe. In addition to this fraction, a proportion of smectite bound iron was found to be redox reactive. This fraction made up to 10% of total Fe in sediments of the Argentine Basin and was quantitatively extracted by 1?N HCl. The redox reactive Fe(+II) fraction of smectite was almost completely reoxidized within 24?h under air atmosphere and may therefore considerably contribute to iron redox cycling if bioturbation occurs. In the case of the slope sediments we found concurrent iron and manganese release to pore water. It is not clear whether this is caused by dissimilatory iron and manganese reduction at the same depth or dissimilatory iron reduction alone inducing Mn(+IV) reduction by (abiotic) reaction with released Fe²⁺. The Argentine Basin sediment showed a significant manganese solid-phase enrichment above the denitrification depth despite the absence of a distinct pore-water gradient of Mn. This implies a recent termination of manganese mobilization and thus a non-steady-state situation with respect to sedimentation or to organic carbon burial rate.     

利用铁屑腐蚀电池原位修复被氯代烃污染地下水的研究进展

简述了氯代烃的主要物理性质和用途，认为铁屑腐蚀电池处理氯代烃污水是基于氧化还原反应、铁屑中活性炭微粒对氯代烃的吸附与对反应的催化、氯离子对氧化膜的破坏和微电池的电场效应等原理；简要总结了迄今利用铁屑去除氯代烃的室内实验和利用原位铁屑反应墙处理地下水中的氯代烃污染研究所取得的成果，认为这种反应墙是一种效果好，成本低，维护方便，有望投入商业运行的最佳方法，并指出了其存在的问题。     

Electrochemical mass-transport in overburden: a new model to account for the formation of selective leach geochemical anomalies in glacial terrain

Recent studies have reported the presence of geochemical anomalies spatially related to mineralisation but which overlie thick glacial overburden. Existing models developed to account for similar anomalies over thin overburden do not adequately explain their presence in thick, young overburden, and to date no new model has been advanced that does. Problems with the existing models are discussed and a new theory is advanced that proposes electrochemically induced mass transport to account for the presence of geochemical anomalies over thick glacial drift. An upward increasing redox gradient exists in most surficial geological materials. Sub-vertical electronic conductors such as graphite or metallic sulphides in bedrock can provide a ‘short-circuit' route across this redox field between reducing agents abundant at depth and oxidising agents abundant in shallower areas. As electrons move up the conductor, oxidising agents in overlying overburden are consumed and a negative redox anomaly develops above the conductor relative to surrounding overburden. High redox gradients in this area induce the rapid migration of reduced anions away from the top of the conductor resulting in the development of a redox-front between reduced and oxidised areas. This front continues to migrate outward and upward at a quantifiable rate that far exceeds that of chemical diffusion until it encounters a continuous source of oxidising agents. The final result may be the propagation of the redox anisotropy to ground surface and the development of a permanently reduced ‘column' between bedrock mineralisation and surface. Higher redox gradients between mineralisation and the ground surface at the edges of the ‘column' relative to its centre result in the focusing of ionic current at the edges. ‘Rabbit-ear' anomalies in surface materials could result from both upward movement of reduced anions to the flanks and inward movement of oxidised cations. Depletions in soil media for iron and manganese and co-precipitating elements are expected in the centre of the column due to their higher mobility in reduced environments.     

Decompression mechanism of ferric iron reduction in tektite melts during their formation in the impact process

The analysis of available data on the Fe³⁺/Fe²⁺ ratio of impact-produced glasses showed that tektites and some other types of impact glasses are reduced compared with the precursor target material. Possible reasons for the change in the degree of iron oxidation in the impact process are still debatable. Based on the analysis of redox reactions in relatively simple systems with iron in different oxidation states (Fe-O and SiO₂-FeO-Fe₂O₃) and the available data on the influence of temperature, oxygen partial pressure (pO₂), and total pressure (P _tot) on the Fe³⁺/Fe²⁺ ratio of silicate melts, a model was proposed suggesting that the lower Fe³⁺/Fe²⁺ values of tektites formed in the impact process compared with the initial target material could be related to the characteristics of oxygen regime during the decompression stage following shock compression. One of the main prerequisites for the occurrence of reduction reactions involving iron and other elements is the attainment of high temperatures (>1800–2000°C) at a certain stage of decompression, providing the complete melting and partial evaporation of the material. When the vapor pressure in the system becomes equal to the total pressure during adiabatic decompression, a further decrease in P _tot will be inevitably accompanied by a decrease in pO₂ and, correspondingly, partial reduction of Fe³⁺ to Fe²⁺ in the melt. The reactions of decompression reduction occur under closed-system conditions and do not require oxygen removal from the system. The higher the temperature and Fe³⁺/Fe²⁺ ratio of the melt, the more extensive iron reduction can be observed during the final stages of decompression. If the temperatures attained during decompression after an impact event are sufficient (>2500–3000°C) for the complete evaporation of the material, the melt produced during subsequent condensation must be significantly more reduced than the initial material. The final stage of the impact process is characterized by a catastrophic expansion of the explosion cloud, condensation, and rapid cooling. During this stage, the system is already not closed. The quenched glasses of this stage record the redox state of earlier melts. In addition, they can contain microinclusions of the products of nonequilibrium vapor condensation with iron compounds of different oxidation states, including metallic iron and iron oxides (wüstite, magnetite, and hematite).     

Etude des eaux thermominérales carbogazeuses du Massif Central Français. I. Potentiel d'oxydo-réduction et comportement du fer

The thermomineral waters from Massif Central are iron rich waters of sodium bicarbonate type. The aim of this work is to provide more information about the geochemistry of iron, which is clearly related to the redox potential in this type of water. Thermodynamic calculations were used, and the distribution of iron was found to be controlled by the solubility of siderite. The oxidation potential that can be measured with calomel and platinum electrodes corresponds to an electrochemical equilibrium which involves a ferric hydroxide or oxyhydroxide. Finally E_Pt values measured in the field are in good agreement with values predicted from the equilibrium between siderite and amorphous Fe(OH)₃, the association of which acts as a redox buffer. Dissolved oxygen is not in equilibrium with the ferrous-ferric couple and does not affect the measured potential. Even when of local theoretical significance as a thermodynamic variable, this redox potential represents only partial equilibrium conditions.     

Early diagenetic influences on iron transformations in a freshwater lake sediment

Iron transformations in a calcium carbonate rich fresh-water sediment were studied by analyzing the relevant constituents of both interstitial water and solid matter. Analysis of interstitial water shows that the observed redox sequence NO⁻₃/NH⁺₄, MnO₂/Mn(II), FeOOH/Fe(II), SO²⁻₄/S(−II) is roughly in agreement with that predicted by the Gibbs Free Energy for the corresponding reactions. In contrast to marine sediments, these redox transitions occur in the uppermost sediments, i.e., at depths of 0–4 cm.

Deeper in the sedimentary sequence, the depth profile for dissolved iron exhibits a steady non-linear increase up to 400 μmol dm⁻³. In this anoxic zone, according to thermodynamic predictions, iron (II)-minerals such as iron sulfide, siderite, and vivianite should precipitate while Fe(III) oxides should be completely dissolved. However, microscopic analysis showed that Fe(III) oxides were present throughout the studied sediment. Furthermore, scanning electron microscope/energy dispersive spectroscopy analysis suggests the presence of iron sulfide could be verified but not that of siderite or vivianite. These observations indicate kinetic control of iron transformations.

We have investigated the importance of kinetic control of iron distribution in anoxic sediments using a diagenetic model for dissolved iron(II). A rough estimate of time scales for dissolution and precipitation rates was made by imposing limiting boundary conditions. Using the calculated rate constant, we established that more than 1000 years would be required for the complete dissolution of Fe(III) oxides, which is agreement with our observations and experimental data from the literature. Calculated precipitation rates of Fe(II) for a given mineral phase such as siderite yield a maximum value of 3 μg_(FeCO3) g⁻¹_{(dry sediment)} yr⁻¹. Such low rates would explain the absence of siderite and vivianite.

Finally, it can be inferred from the Mn_T/Fe_T ratio in the sediments that this ratio depends on the redox conditions of the sediment-water interface at the time of deposition. Thus, this ratio can be used as “paleo-redox indicator” in lacustrine sediments.     

Oxidation state of iron and extensive redistribution of sulfur in thermally modified Stardust particles

Two representative thermally modified Stardust samples were investigated by analytical transmission electron microscopy in order to decipher their iron oxidation state after the strong thermal episode due to the capture in aerogel. Their dominant microstructure consists of evenly distributed rounded Fe-Ni-S nano-droplets within a silica-rich glassy matrix. The mineralogy and associated redox state of iron is assessed using a Fe-Mg-S ternary diagram on which ferromagnesian silicates, sulfides and metal can be represented and potentially compared with any other extraterrestrial material. In this diagram, all the data (bulk and local analysis of silicates, sulfide + metal) scatter along a mixing line between the Mg corner and the average composition of the iron-sulfide. There is an obvious genetic relationship between the different phases observed in such samples, further supported by the very low concentration of iron in the glassy matrix. Silicate glasses contain a significant concentration of dissolved sulfur probably present as MgS complexes. This chemical signature is typical of highly reduced environments. These secondary microstructures were established during the high temperature stage of the capture. A significant part of the Fe-droplets formed in situ by reduction at high temperature of ferromagnesian silicates (olivine and pyroxenes) during the impact. At this stage, the indigenous sulfides destabilized and sulfur readily volatilized as S₂, diffused into molten materials and condensed later onto the Fe-precipitates that formed in the silicate melt. This scenario is supported by the structure of Fe-Ni-S beads with a metal core and a sulfide rim. It will be difficult to derive reliable information on the redox state of 81P/Wild 2 particles based on bulk analyses of whole tracks because particles found along the walls of tracks suffered strong reduction reactions, contrary to terminal particles that may have preserved their pristine redox state. The capture effect must be taken into account for comparison of Wild 2 particles with other chondritic material.     

The effect of dissolved water on the oxidation state of iron in natural silicate liquids

Ferric-ferrous ratios have been measured in 22 experiments on three natural compositions equilibrated at known temperature (950°–1100° C) and oxygen fugacity, and at water-saturated conditions over a pressure range from 0.05 to 0.2 GPa. There does not appear to be any reaction between the melt and the capsule material that affects the redox state of the iron in the melt. An empirical expression for the anhydrous behavior of the redox state of iron in each of these compositions has also been determined at 1 bar as a function of temperature and oxygen fugacity. A direct comparison of the hydrous ferric-ferrous values with the calculated anhydrous values shows that the dissolution of water in a per-alkaline rhyolite, andesite, and an augite minette has no effect on the redox state of the iron in these melts. This result parallels the effect of water on sulfide speciation in basaltic melts, and confirms published results on experimental hydrous basalts.     

Early diagenetic reactions in interbedded pelagic and turbiditic sediments in the Nares Abyssal Plain (western North Atlantic): Consequences for the composition of sediment and interstitial water

Measured pore-water concentrations of iron in interbedded pelagic and turbiditic sediments from the Nares Abyssal Plain are in excellent agreement with sediment colour and measured redox potential. The organic carbon content of these sediments appears to define the redox conditions and consequently the porewater and solid-phase concentration of constituents that are involved in early diagenetic reactions. In the turbiditic sediments the concentration of NO₃⁻ generally goes to zero within a sediment depth of 1 m, whereas at 8 m in a pelagic core from the same area the concentration of NO₃⁻ is still higher than it is in the bottom water. The pore-water concentration of Mn²⁺ in the turbiditic sediments increases sharply down to a depth of approximately 3 m and from thereon remains nearly constant due to saturation with respect to Mn, Ca-CO₃. The pore water of the turbiditic sediments is also saturated with respect to calcite. The few “diagenetic spikes” in the pore-water concentration of NO₃⁻ and Mn²⁺ and the concentration/depth profile of dissolved iron, H₄SiO₄ and phosphate all clearly demonstrate the inhomogeneous nature of interbedded pelagic and turbiditic sediments. The simultaneous occurrence of peaks of dissolved iron/silica and of sediment intervals with a relatively high organic carbon content is attributed to enhanced early diagenetic reactions associated with the decomposition of organic matter in these specific intervals. Linked with these reactions is the irregular pore-water concentration of phosphate, which is shown to originate partly from the oxidation of organic matter, but mainly from the desorption of phosphate from iron oxide. Potential concentrations of phosphate are calculated from the stoichiometric early diagenetic reactions and compared with measured concentrations. Due to the unique combination of low porosity and relatively high sedimentation rates, the sediments from the Nares Abyssal Plain are an ideal basis for the study of such interbedded sequences of pelagic and turbiditic deposits.     

Arsenic redox transformation by humic substances and Fe minerals

The toxicity and mobility of the redox-active metalloid As strongly depends on its oxidation state, with As(III) (arsenite) being more toxic and mobile than As(V) (arsenate). It is, therefore, necessary to know the biogeochemical processes potentially influencing As redox state to understand and predict its environmental behavior. The first part of this presentation will discuss the quantification of As redox changes by pH-neutral mineral suspensions of goethite [α-Fe^IIIOOH] amended with Fe(II) using wet-chemical and synchrotron X-ray absorption (XANES) analysis (Amstaetter et al., 2010). First, it was found that goethite itself did not oxidize As(III). Second, in contrast to thermodynamic predictions, Fe(II)–goethite systems did not reduce As(V). However, surprisingly, rapid oxidation of As(III) to As(V) was observed in Fe(II)–goethite systems. Iron speciation and mineral analysis by Mössbauer spectroscopy showed rapid formation of ⁵⁷Fe–goethite after ⁵⁷Fe(II) addition and the formation of a so far unidentified additional Fe(II) phase. No other Fe(III) phase could be detected by Mössbauer spectroscopy, EXAFS, scanning electron microscopy, X-ray diffraction or high-resolution transmission electron microscopy. This suggests that reactive Fe(III) species form as an intermediate Fe(III) phase upon Fe(II) addition and electron transfer into bulk goethite but before crystallization of the newly formed Fe(III) as goethite.The second part of the presentation will show that semiquinone radicals produced during microbial or chemical reduction of a humic substance model quinone (AQDS, 9,10-anthraquinone-2,6-disulfonic acid) can react with As and change its redox state (Jiang et al., 2009). The results of these experiments showed that these semiquinone radicals are strong oxidants and oxidize arsenite to arsenate, thus decreasing As toxicity and mobility. The oxidation of As(III) depended strongly on pH. More arsenite (up to 67.3%) was oxidized at pH 11 compared to pH 7 (12.6% oxidation) and pH 3 (0.5% oxidation). In addition to As(III) oxidation by semiquinone radicals, hydroquinones that were also produced during quinone reduction, reduced As(V) to As(III) at neutral and acidic pH values (less than 12%) but not at alkaline pH. In an attempt to understand the observed redox reactions between As and reduced/oxidized quinones present in humic substances, the radical content in reduced AQDS solutions was quantified and Eh-pH diagrams were constructed. Both the radical quantification and the Eh-pH diagram allowed explaining the observed redox reactions between the reduced AQDS solutions and the As.In summary these studies indicate that in the simultaneous presence of Fe(III) oxyhydroxides, Fe(II), and humic substances as commonly observed in environments inhabited by Fe-reducing microorganisms, As(III) oxidation can occur. This potentially explains the presence of As(V) in reduced groundwater aquifers.     

Trace element cycling in a subterranean estuary: Part 1. Geochemistry of the permeable sediments

Subterranean estuaries are characterized by the mixing of terrestrially derived groundwater and seawater in a coastal aquifer. Subterranean estuaries, like their river water-seawater counterparts on the surface of the earth, represent a major, but less visible, hydrological and geochemical interface between the continents and the ocean. This article is the first in a two-part series on the biogeochemistry of the subterranean estuary at the head of Waquoit Bay (Cape Cod, MA, USA). The pore-water distributions of salinity, Fe and Mn establish the salt and redox framework of this subterranean estuary. The biogeochemistry of Fe, Mn, P, Ba, U and Th will be addressed from the perspective of the sediment composition. A second article will focus on the groundwater and pore-water chemistries of Fe, Mn, U and Ba.Three sediment cores were collected from the head of Waquoit Bay where the coastal aquifer consists of permeable sandy sediment. A selective dissolution method was used to measure the concentrations of P, Ba, U and Th that are associated with “amorphous (hydr)oxides of iron and manganese” and “crystalline Fe and Mn (hydr)oxides.” The deeper sections of the cores are characterized by large amounts of iron (hydr)oxides that are precipitated onto organic C-poor quartz sand from high-salinity pore waters rich in dissolved ferrous iron. Unlike Fe (hydr)oxides, which increase with depth, the Mn (hydr)oxides display midcore maxima. This type of vertical stratification is consistent with redox-controlled diagenesis in which Mn (hydr)oxides are formed at shallower depths than iron (hydr)oxides. P and Th are enriched in the deep sections of the cores, consistent with their well-documented affinity for Fe (hydr)oxides. In contrast, the downcore distribution of Ba, especially in core 3, more closely tracks the concentration of Mn (hydr)oxides. Even though Mn (hydr)oxides are 200-300 times less abundant than Fe (hydr)oxides in the cores, Mn (hydr)oxides are known to have an affinity for Ba which is many orders of magnitude greater than iron (hydr)oxides. Hence, the downcore distribution of Ba in Fe (hydr)oxide rich sediments is most probably controlled by the presence of Mn (hydr)oxides. U is enriched in the upper zones of the cores, consistent with the formation of highly reducing near-surface sediments in the intertidal zone at the head of the Bay. Hence, the recirculation of seawater through this type of subterranean estuary, coupled with the abiotic and/or biotic reduction of soluble U(VI) to insoluble U(IV), leads to the sediments acting as a oceanic net sink of U. These results highlight the importance of permeable sediments as hosts to a wide range of biogeochemical reactions, which may be impacting geochemical budgets on scales ranging from coastal aquifers to the continental shelf.     

成矿作用过程中赤铁矿—磁铁矿之间非氧化还原转变

自然界中铁氧化物的主要存在形式为赤铁矿和磁铁矿,两者之间的相互转变一直是人们关注和研究的热点。磁铁矿和赤铁矿之间的相互转变一直被认为是一个氧化还原反应的结果,反应的发生与一定的氧化剂或还原剂密切相关。然而,近年来一个铁氧化物之间的非氧化还原反应机制被提出,这种非氧化还原反应机制对于认识和了解复杂的成矿作用具有重要的意义。本文阐述了自然界中铁氧化物之间的相互交代结构,对BIF研究和实验学两方面的证据进行了综述,认为这种非氧化还原反应可能存在于很多不同类型的成矿作用过程之中。这种赤铁矿和磁铁矿之间的非氧化还原反应机制具有重要的理论和实际意义:一方面,仅靠地质作用过程中出现磁铁矿或赤铁矿现象不一定就能判别其形成流体的氧化还原状态;另一方面,它可以为勘探含后生赤铁矿的铁矿床提供新的找矿思路,进一步指导深埋在古风化面以下铁矿体的寻找。     

Mineral-organic-microbe interactions: Environmental impacts from molecular to macroscopic scales

The role of microbes in geological processes is discussed with particular reference to the geochemical cycle involving iron. Microbial oxidation of Fe(II) minerals can occur via at least three mechanisms, the most important involving acidophilic prokaryotes which promote oxidation of iron sulphides. Accelerated breakdown of arsenopyrite is a good example, where multi-step electrochemical reactions are facilitated by the presence of organisms such as Leptospirillum ferrooxidans. Other organisms actively promote the reduction of Fe(III) to more soluble Fe(II). Reduction rates are highly variable, depending on mineral substrate, with oxyhydroxides being most reactive. Proper understanding of such redox processes requires knowledge of interactions at the molecular scale. Advances are being made through genetic studies of relevant organisms, and of mineral surfaces as exemplified by our experimental and computational studies of iron oxides such as magnetite, the reaction of which with simple organic molecules shows diverse behaviour. Mineral-organic interactions precede formation of bacterial biofilms, which can create local geochemical environments causing mineral precipitation. Biofilms and precipitate phases can have a major influence on fluid flow through fractures or porous media as we demonstrate using experiments from micro- to macro-scales.     

Microbial respiratory quinones as indicator of ecophysiological redox conditions

The bacterial respiratory quinones and membrane phospholipid fatty acids (PLFA) were measured to test the biochemical responses to the redox conditions after the respiration of diverse electron acceptors by microorganisms. Shewanella putrefaciens strain CN32 was examined for its growth with O₂, nitrate, ferrihydrite, ferric citrate, and sulfite as electron acceptors. The same parameters were also measured for Desulfovibrio desulfuricans strain G-20, Geobacter metallireducens strain GS-15, Thioploca spp., two strains of magnetotactic bacteria (Magneteospirilum magnetotactium marine vibrioid strain MV-1 and M. sp. strain AMB-1), and environmental sediments. Microorganisms with aerobic respiratory of oxygen (MV-1 and AMB-1) have high ratios of monounsaturated to saturated straight chain PLFA and ubiquinone to menaquinone ratios; while those that conduct strict anaerobic respirations (G-20 with sulfate and GS-15 with ferric iron) have low ratios of monounsaturated to saturated straight chain PLFA and uniquinone to menaquinone ratios. The facultative respiratory of nitrate (Thioploca) has these parameters in the middle. The ratios of menaquinones to ubiquinones in CN32 cells systematically increase according to the increase of redox potential and bioavalibility of electron acceptors. The correlation between σUQ-n/σMK-n ratios and redox conditions indicates the structure of respiratory quinone responses sensitively to the microbial ecophysiological conditions.