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1.
The surface structure of α-Fe2O3(0 0 0 1) was studied using crystal truncation rod (CTR) X-ray diffraction before and after reaction with aqueous Fe(II) at pH 5. The CTR results show the unreacted α-Fe2O3(0 0 0 1) surface consists of two chemically distinct structural domains: an O-layer terminated domain and a hydroxylated Fe-layer terminated domain. After exposing the α-Fe2O3(0 0 0 1) surface to aqueous Fe(II), the surface structure of both co-existing structural domains was modified due to adsorption of Fe at crystallographic lattice sites of the substrate, resulting in six-coordinated adsorbed Fe at the surface. The average Fe-O bond lengths of the adsorbed Fe are consistent with typical Fe(III)-O bond lengths (in octahedral coordination), providing evidence for the oxidation of Fe(II) to Fe(III) upon adsorption. These results highlight the important role of substrate surface structure in controlling Fe(II) adsorption. Furthermore, the molecular scale structural characterization of adsorbed Fe provides insight into the process of Fe(II) induced structural modification of hematite surfaces, which in turn aids in assessing the effective reactivity of hematite surfaces in Fe(II) rich environments.  相似文献   

2.
Jarosite is an important mineral on Earth, and possibly on Mars, where it controls the mobility of iron, sulfate and potentially toxic metals. Atomistic simulations have been used to study the incorporation of Al3+, and the M2+ impurities Cd, Cu and Zn, in the (0 1 2) and (0 0 1) surfaces of jarosite. The calculations show that the incorporation of Al on an Fe site is favorable on all surfaces in which terminal Fe ions are exposed, and especially on the (0 0 1) [Fe3(OH)3]6+ surface. Incorporation of Cd, Cu or Zn on a K site balanced by a K vacancy is predicted to stabilize the surfaces, but calculated endothermic solution energies and the high degree of distortion of the surfaces following incorporation suggest that these substitutions will be limited. The calculations also suggest that incorporation of Cd, Cu and Zn on an Fe site balanced by an OH vacancy, or by coupled substitution on both K and Fe sites, is unfavorable, although this might be compensated for by growth of a new layer of jarosite or goethite, as predicted for bulk jarosite. The results of the simulations show that surface structure will exert an influence on uptake of impurities in the order Cu > Cd > Zn, with the most favorable surfaces for incorporation being (0 1 2) [KFe(OH)4]0 and (0 0 1) [Fe3(OH)3]6+.  相似文献   

3.
Molecular dynamics simulations of water in contact with the (0 0 1) and (0 1 0) surfaces of orthoclase (KAlSi3O8) were carried out to investigate the structure and dynamics of the feldspar-water interface, contrast the intrinsic structural properties of the two surfaces, and provide a basis for future work on the diffusion of ions and molecules in microscopic mineral fractures. Electron density profiles were computed from the molecular dynamics trajectories and compared with those derived experimentally from high-resolution X-ray reflectivity measurements by Fenter and co-workers [Fenter P., Cheng L., Park C., Zhang H. and Sturchio N. C. (2003a) Structure of the orthoclase (0 0 1)- and (0 1 0)-water interfaces by high-resolution X-ray reflectivity. Geochim. Cosmochim. Acta67, 4267-4275]. For each surface, three scenarios were considered whereby the interfacial species is potassium, water, or a hydronium ion. Excellent agreement was obtained for the (0 0 1) surface when potassium is the predominant interfacial species; however, some discrepancies in the position of the interfacial peaks were obtained for the (0 1 0) surface. The two surfaces showed similarities in the extent of water ordering at the interface, the activation energies for water and potassium desorption, and the adsorption localization of interfacial species. However, there are also important differences between the two surfaces in the coordination of a given adsorbed species, adsorption site densities, and the propensity for water molecules in surface cavities and those in the first hydration layer to coordinate to surface bridging oxygen atoms. These differences may have implications for the extent of dissolution in the low-pH regime since hydrolysis of Si(Al)OSi(Al) bonds is a major dissolution mechanism.  相似文献   

4.
The structure and mechanism of cation sorption at the (0 0 1) muscovite-water interface were investigated in 0.01 and 0.5 m KCl, CsCl, and CaCl2 and 0.01 m BaCl2 solutions at slightly acidic pH by high-resolution X-ray reflectivity. Structural relaxations of atom positions in the 2M1 muscovite were small (?0.07 Å) and occurred over a distance of 30 to 40 Å perpendicular to the interface. Cations in all solutions were sorbed dominantly in the first and second solution layers adjacent to the mineral surface. The derived heights of the first solution layer in KCl and CsCl solutions, 1.67(6)-1.77(7) and 2.15(9)-2.16(2) Å, respectively, differ in magnitude by the approximate difference in crystallographic radii between K and Cs, and correspond closely to the interlayer cation positions in bulk K- and Cs-mica structures. The first solution layer heights in CaCl2 and BaCl2 solutions, 2.46(5)-2.56(11) and 2.02(5) Å, respectively, differ in a sense opposite to that expected based on crystallographic or hydrated radii of the divalent cations. The derived ion heights in all solutions imply that there is no intercalated water layer between the first solution layer and the muscovite surface. Molecular compositions were assigned to the first two solution layers in the electron density profiles using models that constrain the number density of sorbed cations, water molecules, and anions by considering the permanent negative charge of the muscovite and average solution density. The models result in partial charge balance (at least 50%) by cations sorbed in the first two layers in the 0.01 m solutions and approximately full charge balance in the 0.5 m solutions. Damped oscillations of model water density away from the first two solution layers agree with previous X-ray reflectivity results on the muscovite (0 0 1) surface in pure water.  相似文献   

5.
Structural characterization of iron oxide-water interfaces provides insight into the mechanisms through which these minerals control contaminant fate and element cycling in soil, sedimentary, and groundwater systems. Ordering of interfacial water and structural relaxations at the hematite (1 1 0) surface have been investigated in situ using high-resolution specular X-ray reflectivity. These measurements demonstrate that relaxations are constrained to primarily the top ∼5 Å of the surface. Near-surface iron atoms do not relax substantially, although the uppermost layer displays an increased distribution width, while the undercoordinated oxygens on the surface uniformly relaxed outward. Two sites of adsorbed water and additional layering of water farther from the surface were observed. Water fully covers the (1 1 0) surface and appears to form a continuous network extending into bulk solution, with positional order decreasing to that of a disordered bulk fluid within 1 nm. The arrangement of water is similar to that on the hematite (0 1 2) surface, which has a similar surface topography, although these surfaces display different vibrational amplitudes or positional disorder of adsorbed water molecules and average spacings of near-surface layered water. Comparison between these surfaces suggests that interfacial water ordering on hematite is controlled primarily by surface structure and steric constraints and that highly ordered water is likely common to most hematite-water interfaces.  相似文献   

6.
Computer modelling techniques were used to elucidate the hydration behaviour of three iron (hydr)oxide minerals at the atomic level: white rust, goethite and hematite. A potential model was first adapted and tested against the bulk structures and properties of eight different iron oxides, oxyhydroxides and hydroxides, followed by surface simulations of Fe(OH)2, α-FeO(OH) and α-Fe2O3. The major interaction between the adsorbing water molecules and the surface is through interaction of their oxygen ions with surface iron ions, followed by hydrogen-bonding to surface oxygen ions. The energies released upon the associative adsorption of water range from 1 to 17 kJ mol−1 for Fe(OH)2, 26 to 80 kJ mol−1 for goethite and 40 to 85 kJ mol−1 for hematite, reflecting the increasing oxidation of the iron mineral. Dissociative adsorption at goethite and hematite surfaces releases larger hydration energies, ranging from 120 to 208 kJ mol−1 for goethite and 76 to 190 kJ mol−1 for hematite.The thermodynamic morphologies of the minerals, based on the calculated surface energies, agree well with experimental morphologies, where these are available. When the partial pressures required for adsorption of water from the gas phase are plotted against temperature for the goethite and hematite surfaces, taking into account experimental entropies for water, it appears that these minerals may well be instrumental in the retention of water during the cyclic variations in the atmosphere of Mars.  相似文献   

7.
The fate of the oxoanion arsenate in diverse systems is strongly affected by its adsorption on the surfaces of iron (oxyhydr)oxide minerals. Predicting this behavior in the environment requires an understanding of the mechanisms of arsenate adsorption. In this study, the binding site and adsorption geometry of arsenate on the hematite (0 1 2) surface is investigated. The structure and termination of the hematite (0 1 2)-water interface were determined by high resolution X-ray reflectivity, revealing that two distinct terminations exist in a roughly 3:1 proportion. The occurrence of multiple terminations appears to be a result of sample preparation, and is not intrinsic to the hematite (0 1 2) surface. X-ray standing wave (XSW) measurements were used to determine the registry of adsorbed arsenate to the hematite structure, and thus the binding site and geometry of the resulting surface complex. Arsenate forms a bridging bidentate complex on two adjacent singly coordinated oxygen groups on each of the two distinct terminations present at the hematite surface. Although this geometry is consistent with that seen in past studies, the derived As-Fe distances are longer, the result of the topology of the FeO6 octahedra on the (0 1 2) surface. As EXAFS-derived As-Fe distances are often used to determine the adsorption mechanism in environmental samples (e.g., mine tailings, contaminated sediments), this demonstrates the importance of considering the possible sorbent surface structures and arrangements of adsorbates when interpreting such data.As multiple functional groups are present and multiple binding geometries are possible on the hematite (0 1 2) surface, the XSW data suggest that formation of bridging bidentate surface complexes on singly coordinated oxygen sites is the preferred adsorption mechanism on this and most other hematite surfaces (provided those surfaces contain adjacent singly coordinated oxygen groups). These measurements also constrain the likely reaction stoichiometry, with only the protonation state of the surface complex undetermined. Although bridging bidentate inner-sphere surface complexes comprised the majority of the adsorbed arsenate present on the hematite (0 1 2) surface, there is an additional population of sorbed arsenate species that could not be characterized by the XSW measurements. These species are likely more disordered, and thus more weakly bound, than the bridging bidentate complexes, and may play a role in determining the fate, transport, and bioavailability of arsenate in the environment. Finally, the possibility of obtaining species-specific XSW measurements by tuning the incident beam energy to specific features in a XANES spectrum is described.  相似文献   

8.
The kinetics of Fe(III) precipitation in synthetic buffered waters have been investigated over the pH range 6.0-9.5 using a combination of visible spectrophotometry, 55Fe radiometry combined with ion-pair solvent extraction of chelated iron and numerical modeling. The rate of precipitation, which is first order with respect to both dissolved and total inorganic ferric species, varies by nearly two orders of magnitude with a maximum rate constant of 16 ± 1.5 × 106 M−1 s−1 at a pH of around 8.0. Our results support the existence of the dissolved neutral species, Fe(OH)30, and suggest that it is the dominant precursor in Fe(III) polymerization and subsequent precipitation at circumneutral pH. The intrinsic rate constant of precipitation of Fe(OH)30 was calculated to be allowing us to predict rates of Fe(III) precipitation in the pH range 6.0-9.5. The value of this rate constant, and the variation in the precipitation rate constant over the pH range considered, are consistent with a mechanism in which the kinetics of iron precipitation are controlled by rates of water exchange in dissolved iron hydrolysis species.  相似文献   

9.
Many geochemical reactions that control the composition of natural waters, contaminant fate and transport, and biogeochemical element cycling take place at the interface between minerals and aqueous solutions. A fundamental understanding of these important processes requires knowledge of the structure of mineral-water interfaces. High-resolution specular X-ray reflectivity was used to determine the structure of the hematite (0 1 2)-water interface. Relaxation of the surface was observed to be minor, and water was found to order near the hematite surface. Two sites of adsorbed water are inferred to be ordered laterally, one bridging between triply coordinated functional groups and the other bridging between the singly coordinated functional groups on the surface, as steric constraints limit the possible arrangements of water molecules occurring at the observed heights above the hematite surface. Relaxations of the hematite and corundum (0 1 2) surfaces, which are isostructural, are similar and limited primarily to the top most layer of the structures. No significant changes to the interfacial stoichiometry (i.e., partial occupancy of surface species) are observed in either case. The structure of interfacial water is similar on the hematite and corundum (0 1 2) surfaces as well, although water appeared to be less well ordered on the hematite surface. This may be due to expected differences in the oxygen exchange rates from surface functional groups or the apparent better matching of the corundum oxygen lattice to the natural structural ordering in water, and suggests that the dielectric constant gradients of interfacial water may differ on the two surfaces. Similar charging behavior is expected for these surfaces as similar types of surface functional groups are exposed. Although generally similar, subtle differences in the reactivity of hematite and corundum (0 1 2) surfaces to arsenate adsorption, and possibly the adsorption of other species, may be related to the difference in ordering of interfacial water observed in this study.  相似文献   

10.
Adsorption of Rb+ and Sr2+ at the orthoclase (0 0 1)-solution interface is probed with high-resolution X-ray reflectivity and resonant anomalous X-ray reflectivity. Specular X-ray reflectivity data for orthoclase in contact with 0.01 m RbCl solution at pH 5.5 reveal a systematic increase in electron density adjacent to the mineral surface with respect to that observed in contact with de-ionized water (DIW). Quantitative analysis indicates that Rb+ adsorbs at a height of 0.83 ± 0.03 Å with respect to the bulk K+ site with a nominal coverage of 0.72 ± 0.10 ions per surface unit mesh (55.7 Å2). These results are consistent with an ion-exchange reaction in which Rb+ occupies an inner-sphere adsorption (IS) site. In contrast, X-ray reflectivity data for orthoclase in contact with 0.01 m Sr(NO3)2 solution at pH 5.3 reveal few significant changes with respect to DIW. Resonant anomalous X-ray reflectivity was used to probe Sr2+ adsorption and to image its vertical distribution. This element-specific measurement reveals that Sr2+ adsorbs with a total coverage of 0.37 ± 0.02 ions per surface unit mesh, at a substantially larger height (3.28 ± 0.05 Å) than found for Rb+, and with a relatively broad density distribution (having a root-mean-square width of 1.88 ± 0.08 Å for a single-peak model), implying that Sr2+ adsorbs primarily as a fully-hydrated outer-sphere (OS), species. Comparison to a two-height model suggests that 13 ± 5% of the adsorbed Sr2+ may be present as an IS species. This partitioning implies a ∼5 kJ/mol difference in free energy between the IS and OS Sr2+ on orthoclase. Differences in the partitioning of Sr2+ between IS and OS species for orthoclase (0 0 1) and muscovite (0 0 1) suggest control by the geometry of the IS adsorption site. Results for the OS distribution are compared to predictions of the Poisson-Boltzmann equation in the strong coupling regime, which predicts an intrinsically narrow vertical diffuse ion distribution; the OS distribution might thus be thought of as the diffuse ion profile in the limit of high surface charge.  相似文献   

11.
Various iron-bearing primary phases and rocks have been weathered experimentally to simulate possible present and past weathering processes occurring on Mars. We used magnetite, monoclinic and hexagonal pyrrhotites, and metallic iron as it is suggested that meteoritic input to the martian surface may account for an important source of reduced iron. The phases were weathered in two different atmospheres: one composed of CO2 + H2O, to model the present and primary martian atmosphere, and a CO2 + H2O + H2O2 atmosphere to simulate the effect of strong oxidizing agents. Experiments were conducted at room temperature and a pressure of 0.75 atm. Magnetite is the only stable phase in the experiments and is thus likely to be released on the surface of Mars from primary rocks during weathering processes. Siderite, elemental sulfur, ferrous sulfates and ferric (oxy)hydroxides (goethite and lepidocrocite) are the main products in a water-bearing atmosphere, depending on the substrate. In the peroxide atmosphere, weathering products are dominated by ferric sulfates and goethite. A kinetic model was then developed for iron weathering in a water atmosphere, using the shrinking core model (SCM). This model includes competition between chemical reaction and diffusion of reactants through porous layers of secondary products. The results indicate that for short time scales, the mechanism is dominated by a chemical reaction with second order kinetics (k = 7.75 × 10−5 g−1/h), whereas for longer time scales, the mechanism is diffusion-controlled (DeA = 2.71 × 10−10 m2/h). The results indicate that a primary CO2- and H2O-rich atmosphere should favour sulfur, ferrous phases such as siderite or Fe2+-sulfates, associated with ferric (oxy)hydroxides (goethite and lepidocrocite). Further evolution to more oxidizing conditions may have forced these precursors to evolve into ferric sulfates and goethite/hematite.  相似文献   

12.
For the purpose of improving fundamental understanding of the redox reactivity of magnetite, quantum-mechanical calculations were applied to predict Fe2+ availability and electron hopping rates at magnetite (1 0 0) surfaces, with and without the presence of adsorbed water. Using a low free energy surface reconstruction (½-Fetet layer relaxed into the Feoct (1 0 0) plane), the relaxed outermost layer of both the hydrated and vacuum-terminated surfaces were found to be predominantly enriched in Fe2+ within the octahedral sublattice, irrespective of the presence of adsorbed water. At room temperature, mobile electrons move through the octahedral sublattice by Fe2+-Fe3+ valence interchange small polaron hopping, calculated at 1010-1012 hops/s for bulk and bulk-like (i.e., near-surface) environments. This process is envisioned to control sustainable overall rates of interfacial redox reactions. These rates decrease by up to three orders of magnitude (109 hops/s) at the (1 0 0) surface, and no significant difference is observed for vacuum-terminated versus hydrated cases. Slower hopping rates at the surface appear to arise primarily from larger reorganization energies associated with octahedral Fe2+-Fe3+ valence interchange in relaxed surface configurations, and secondarily on local charge distribution patterns surrounding Fe2+-Fe3+ valence interchange pairs. These results suggest that, with respect to the possibility that the rate and extent of surface redox reactions depend on Fe2+ availability and its replenishment rate, bulk electron hopping mobility is an upper-limit for magnetite and slower surface rates may need to be considered as potentially rate-limiting. They also suggest that slower hopping mobilities calculated for surface environments may be amenable to Fe2+-Fe3+ site discrimination by conventional spectroscopic probes.  相似文献   

13.
The uptake of Eu(III) by crystalline calcium silicate hydrate (C-S-H) phases 11 Å tobermorite and xonotlite has been investigated by the combined use of time-resolved laser fluorescence spectroscopy (TRLFS) and extended X-ray absorption fine structure (EXAFS) spectroscopy. Eu(III) doped tobermorite and xonotlite samples with varying metal loading (0.4, 7 and 35 μmol Eu/g solid phase) and reaction time (1-570 days) were investigated. The structural environment of Eu(III) taken up by tobermorite and xonotlite was found to depend on both parameters.At high Eu(III) loading (7 μmol Eu/g solid phase), TRLFS data indicated presence of three Eu(III) species with different fluorescence lifetimes after 1 day reaction time. The emission lifetimes deduced for the different species correspond to ∼4.7, ∼1 and 0 water molecules in the first coordination sphere, thus suggesting the presence of one surface species forming an inner-sphere surface complex and two species incorporated in the crystal structure. After longer contact times (90 days, 570 days), the surface species was not observed. At the lower Eu(III) loading (0.4 μmol Eu/g solid phase) and reaction times between 1 and 310 days only two Eu(III) species with ∼1-2 and 0 water molecules were detected, corresponding to Eu(III) being incorporated in the crystal structure. The results from EXAFS showed that the distances between Eu(III) and neighboring Ca and Si atoms in Eu(III) doped tobermorite increase after prolonged reaction time. Furthermore, the number of neighboring Ca and Si atoms was found to increase with time. This study demonstrates that binding into the structure of 11 Å tobermorite and xonotlite is the dominant mode of Eu(III) immobilization after long reaction time. This finding is essential for an overall assessment of the safe disposal of actinides in deep geological repositories for radioactive waste, as incorporation into the crystal structure suggests long-term immobilization in the repository environment.  相似文献   

14.
Ordering of interfacial water at the hematite and corundum (0 0 1)-water interfaces has been characterized using in situ high resolution specular X-ray reflectivity measurements. The hematite (0 0 1) surface was prepared through an annealing process to produce a surface isostructural with corundum (0 0 1), facilitating direct comparison. Interfacial water was found to display a similar structure on this pair of isostructural surfaces. A single layer of adsorbed water having a large vibrational amplitude was present on each surface and additional ordering of water extended at least 1 nm into the bulk fluid, with the degree of ordering decreasing with increasing distance from the surfaces. Consistent with prior studies of the (0 1 2) and (1 1 0) surfaces of hematite and corundum, the configuration of water above the (0 0 1) surfaces is primarily controlled by the surface structure, specifically the arrangement of surface functional groups. However, interfacial water at the (0 0 1) surfaces displayed significantly larger vibrational amplitudes throughout the interfacial region than at other isostructural sets of hematite and corundum surfaces, indicating weaker ordering. Comparison of the vibrational amplitudes of adsorbed water on a series of oxide, silicate, and phosphate mineral surfaces suggests that the presence or absence of a substantial interfacial electrostatic field is the primary control on water ordering and not the surface structure itself. On surfaces for which charge originates dominantly through protonation-deprotonation reactions the controlling factor appears to be whether conditions exist where most functional groups are uncharged as opposed to the net surface charge. The doubly coordinated functional groups on hematite and corundum (0 0 1) surfaces are largely uncharged under slightly acidic to circumneutral pH conditions, leading to weak ordering, whereas singly coordinated groups on (0 1 2) and (1 1 0) surfaces of these phases are always charged, even when the net surface charge is zero, and induce strong water ordering. Surfaces lacking structural charge can thus be divided into two distinct classes that induce either strong or weak ordering of interfacial water. Surface functional group coordination is the ultimate control on this division as it determines the charge state of such groups under different protonation configurations. Ion adsorption and electron transfer processes may differ between these classes of surfaces because of the effect of water ordering strength on interfacial capacitances and hydrogen bonding.  相似文献   

15.
The structure and dynamics of water on muscovite mica (0 0 1) surfaces have been investigated by molecular dynamics simulations. X-ray reflectivity profiles highly reflecting the interfacial structure are directly calculated and compared with those of experiments. The direct comparison has validated the usefulness of MD simulations to understand the real interfacial structure of the mica−water system. We observed five distinguished peaks in the density profile of oxygen present in water, and these peaks are attributable to the water molecules directly adsorbed on mica, hydrated to the K+ ions on the mica surface, and ordered due to hydrogen bonds between hydrated K+ ions. The hydrated K+ ions make an inner-sphere complex and have an explicit first hydration shell with a radius of 3.6 Å and a hydration number of 2.9. The change of the viscosity of water located above 1 nm apart from the mica surface was not observed. This feature is in good agreement with a recent experimental study in which the shear measurement was conducted using a surface forces apparatus. The increase of the viscosity by a factor of ca. 2-3 relative to that of the bulk water was observed at water located within 1 nm from the isolated mica surface.  相似文献   

16.
The dissolution behavior of the barite (0 0 1) surface in pure water at 30 °C was investigated using in situ Atomic Force Microscopy (AFM), to better understand the dissolution mechanism and the microtopographical changes that occur during the dissolution, such as steps and etch pits. The dissolution of the barite (0 0 1) surface started with the slow retreat of steps, after which, about 60 min later, the <hk0> steps of one unit cell layer or multi-layers became two-step fronts (fast “f” and slow “s” steps) with a half-unit cell layer showing different retreat rates. The “f” step had a fast retreat rate (≈(14 ± 1) × 10−2 nm/s) and tended to have a jagged step edge, whereas the “s” step (≈(1.8 ± 0.1) × 10−2 nm/s) had a relatively straight front. The formation of the “f” steps led to the formation of a new one-layer step, where the front of the “s” step was overtaken by that of the immediate underlying “f” step. The “f” steps also led to the decrease of the <hk0> steps and the increase in the percentage of stable steps parallel to the [0 1 0] direction during the dissolution.Etch pits, which could be observed after about 90 min, were of three types: triangular etch pits with a depth of a half-unit cell, shallow etch pits, and deep etch pits. The triangular etch pits were bounded by the step edges parallel to [0 1 0], [1 2 0], and [] and had opposite orientations in the upper half and lower half layers. Shallow etch pits that had a depth of two or more half-unit cell layers had any two consecutive pits pointing in the opposite direction of each other. The triangular etch pit appeared to grow by simultaneously removal of a row of ions parallel to each direction from the three step edges. At first, deep etch pits were elongated in the [0 1 0] direction with a curved outline and then gradually developed to an angular form bounded by the {1 0 0}, {3 1 0}, and (0 0 1) faces. The retreat rate of the (0 0 1) face was much slower than those of the {1 0 0} and {3 1 0} and tended to separate into two rates ((0.13 ± 0.01) × 10−2 nm/s for the deep etch pits derived from a screw dislocation and (0.07 ± 0.01) × 10−2 nm/s for those from other line defects).The changes in the dissolution rate of a barite (0 0 1) surface during the dissolution were estimated using the retreat rates and densities of the various steps as well as the growth rates, density, and areas of the lateral faces of the deep etch pits that were obtained from this AFM analysis. Our results showed that the dissolution rate of the barite (0 0 1) surface gradually increased and approached the bulk dissolution rate because of the change in the main factor determining the dissolution rate from the density of the steps to the growth and the density of the deep etch pits on the surface.  相似文献   

17.
Oxidation of pyrite by hydrogen peroxide (H2O2) at millimolar levels has been studied from 4 to 150 °C in order to evaluate isotopic effects potentially associated with radiolytic oxidation of pyrite. Gaseous, aqueous, and solid phases were collected and measured following sealed-tube experiments that lasted from 1 to 14 days. The dominant gaseous product was molecular oxygen. No volatile sulfur species were recovered from any experiment. Sulfate was the only aqueous sulfur species detected in solution, with sulfite and thiosulfate below the detection limits. X-ray diffraction patterns and images from scanning electron microscopy reveal solid residues composed primarily of hydrated ferric iron sulfates and sporadic ferric-ferrous iron sulfates. Hematite was detected only in solid residue produced during high temperature experiments. Elemental sulfur and/or polysulfides are inferred to be form on reacting pyrite surface based on extraction with organic solvents. Pyrite oxidation by H2O2 increases in rate with increasing H2O2concentration, pyrite surface area, and temperature. Rates measured in sealed-tube experiments at 25°C, for H2O2 concentration of 2 × 10−3 M are 8.8 × 10−9 M/m2/sec, which are higher than previous estimates. A combination of reactive oxygen species from H2O2 decomposition products and reactive iron species from pyrite dissolution is inferred to aggressively oxidize the receding pyrite surface. Competing oxidants with temperature-dependent oxidation efficiencies results in multiple reaction mechanisms for different temperatures and surface conditions. Sulfur isotope values of remaining pyrite were unchanged during the experiments, but showed distinct enrichment of 34S in produced sulfate and depletion in elemental sulfur. The Δsulfate-pyrite and Δelemental sulfur-pyrite was +0.5 to +1.5‰ and was −0.2 to −1‰, respectively. Isotope data from high-temperature experiments indicate an additional 34S-depleted sulfur fraction, with up to 4‰ depletion of 34S, in the hematite. Sulfur isotope trends were not influenced by H2O2 concentration, temperature, or reaction time. Results of this study indicate that radiolytically produced oxidants, such as hydrogen peroxide and hydroxyl radicals, could efficiently oxidize pyrite in an otherwise oxygen-limited environment. Although H2O2 is generally regarded as being of minor geochemical significance on Earth, the H2O2 molecule plays a pivotal role in Martian atmospheric and soil chemistry. Additional experimental and field studies are needed to characterize sulfur and oxygen isotope systematics during radiolytical oxidation of metallic sulfides and elemental sulfur.  相似文献   

18.
The oxygen isotope fractionation factor of dissolved oxygen gas has been measured during inorganic reduction by aqueous FeSO4 at 10−54 °C under neutral (pH 7) and acidic (pH 2) conditions, with Fe(II) concentrations ranging up to 0.67 mol L−1, in order to better understand the geochemical behavior of oxygen in ferrous iron-rich groundwater and acidic mine pit lakes. The rate of oxygen reduction increased with increasing temperature and increasing Fe(II) concentration, with the pseudo-first-order rate constant k ranging from 2.3 to 82.9 × 10−6 s−1 under neutral conditions and 2.1 to 37.4 × 10−7 s−1 under acidic conditions. The activation energy of oxygen reduction was 30.9 ± 6.6 kJ mol−1 and 49.7 ± 13.0 kJ mol−1 under neutral and acidic conditions, respectively. Oxygen isotope enrichment factors (ε) become smaller with increasing temperature, increasing ferrous iron concentration, and increasing reaction rate under acidic conditions, with ε values ranging from −4.5‰ to −11.6‰. Under neutral conditions, ε does not show any systematic trends vs. temperature or ferrous iron concentration, with ε values ranging from −7.3 to −10.3‰. Characterization of the oxygen isotope fractionation factor associated with O2 reduction by Fe(II) will have application to elucidating the process or processes responsible for oxygen consumption in environments such as groundwater and acidic mine pit lakes, where a number of possible processes (e.g. biological respiration, reduction by reduced species) may have taken place.  相似文献   

19.
Schwertmannite (Fe8O8(OH)6SO4) is a common Fe(III)-oxyhydroxysulfate mineral in acid-sulfate systems, where its formation and fate strongly influence water quality. The present study examines transformation of schwertmannite to goethite (FeOOH), as catalyzed by interactions with Fe(II) in anoxic aquatic environments. This study also evaluates the role of the Fe(II) pathway in influencing the formation of iron-sulfide minerals in such environments. At pH > 5, the rates of Fe(II)-catalyzed schwertmannite transformation were several orders of magnitude faster than transformation in the absence of Fe(II). Complete transformation of schwertmannite occurred within only 3-5 h at pH > 6 and Fe(II)(aq) ? 5 mmol L−1. Model calculations indicate that the Fe(II)-catalyzed transformation of schwertmannite to goethite greatly decreases the reactivity of the Fe(III) pool, thereby favoring SO4-reduction and facilitating the formation of iron-sulfide minerals (particularly mackinawite, tetragonal FeS). Examination of in situ sediment geochemistry in an acid-sulfate system revealed that the rapid Fe(II)-catalyzed transformation was consistent with an abrupt shift from an acidic Fe(III)-reducing regime with abundant schwertmannite near the sediment surface, to a near-neutral mackinawite-forming regime where goethite was dominant. This study demonstrates that the Fe(II) pathway exerts a major influence on schwertmannite transformation and iron-sulfide formation in anoxic acid-sulfate systems. These findings have important implications for understanding acidity dynamics and trace element mobility in such systems.  相似文献   

20.
This study examined the effects of humic acids (HA) and Fe(III)–HA complex on the photodegradation of atrazine, one of the most widely used herbicides. It was shown that the photolysis of atrazine proceeded via first-order reaction kinetics and that atrazine photodegradation was inhibited by the presence of HA, whereas the rate for atrazine photolysis was promoted in solutions containing both HA and Fe(III). Interactions of Fe(III) with HA were characterized by SEM, EDX, UV–Vis and FTIR, revealing that Fe(III)–HA complex was formed by ligand exchange between oxygen groups of HA and Fe(III). Using fluorescence spectrometry the stability constant (Kc) and the fraction of fluorophores available for complexation (f) were obtained as log Kc = 4.28 and f = 74%. Photoformed Fe(II) by ligand-to-metal charge-transfer (LMCT) within the Fe(III)–HA complex was the most important factor involved in photolysis of atrazine, since Fe(II) was the reactant to generate hydroxyl radical. Thus, the rate of atrazine photodegradation in natural sunlit waters is dependent on both the amount of iron present and the interaction between HA and iron.  相似文献   

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