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1.
Fixation of Mo in sulfidic environments is believed to be preceded by conversion of geochemically passive MoO42− to particle-reactive thiomolybdates (MoOxS4−x2−). In aqueous solution, these transformations are general-acid catalyzed, implying that proton donors can accelerate both the forward and reverse reactions. Here, we explore whether mineral surfaces also catalyze thiomolybdate interconversions. The rate of MoS42− hydrolysis is investigated in the presence and absence of natural kaolinite (KGa-1b) and synthetic Al2O3 and SiO2 phases. Comparison of rates achieved with these phases suggests that the Al oxyhydroxide component in kaolinite furnishes the catalytic activity. An anhydrous Al2O3 phase is catalytically inactive until hydrated (and therefore protonated). Surface kinetics with kaolinite at mildly alkaline pH are consistent with rate limitation by formation or decomposition of monomeric surface complexes; oligomeric surface intermediates may become important at MoS42− > 20 μmol/L, higher than is likely to be found in nature. The pH dependence of the kaolinite-catalyzed reaction suggests that weak-acid surface sites promote hydrolysis. Intermediate thiomolybdates or molybdate appears to compete for active sites, inhibiting MoS42− hydrolysis. Catalysis of MoOS32− hydrolysis is also observed but has not been studied systematically. Thiomolybdate hydrolysis is inhibited slightly by sulfate and more strongly by phosphate. Low NaCl concentrations (<10−2 mol/L) promote hydrolysis, but higher NaCl concentrations retard the reaction to a small extent. A mechanism is postulated involving expansion of the coordination number around Mo from 4 to 6 under the influence of the surface. The effective concentration of surface sites available to Mo in sediment pore waters is likely to be large enough to greatly accelerate thiomolybdate hydrolysis and sulfidation. Possibly this explains why Mo capture in seasonally or intermittently anoxic environments often occurs through processes operating within sediments but not in overlying waters.  相似文献   

2.
Molybdenum (Mo) isotope studies in black shales can provide information about the redox evolution of the Earth’s oceans, provided the isotopic consequences of Mo burial into its major sinks are well understood. Previous applications of the Mo isotope paleo-ocean redox proxy assumed quantitative scavenging of Mo when buried into sulfidic sediments. This paper contains the first complete suite of Mo isotope fractionation observations in a sulfidic water column and sediment system, the meromictic Lake Cadagno, Switzerland, a small alpine lake with a pronounced oxygen-sulfide transition reaching up to H2S ∼ 200 μM in the bottom waters (or about 300 μM total sulfide: ΣS2− = H2S + HS− + S2−). We find that Mo behaves conservatively in the oxic zone and non-conservatively in the sulfidic zone, where dissolved Mo concentrations decrease from 14 nM to 2-8 nM across this transition. Dissolved Mo in the upper oxic waters has a δ98Mooxic = 0.9 ± 0.1‰, which matches that of the riverine input, δ98Moriver = 0.9 ± 0.1‰. In the deeper sulfidic waters, a subaquatic source delivers Mo at 1.55 ± 0.1‰, but the dissolved Mo is even heavier at δ98Mosulfidic = 1.8‰. Sediment traps in the sulfidic zone of the lake collect particles increasingly enriched in Mo with depth, with δ98Mo values significantly fractionated at −0.8‰ to −1.2‰ both near the chemocline and in the deepest trap. Suspended particulates in the sulfidic waters carry lighter Mo than the ambient dissolved Mo pool by ∼0.3-1.5‰. Sedimentary Mo concentrations correlate with total organic carbon and yield Mo levels which are two orders of magnitude higher than typical crustal values found in rocks from the catchment area. Solid-phase Mo in the sediment shows a slightly positive δ98Mo trend with depth, from δ98Mo = 1.2‰ to 1.4‰ while the pore waters show dramatic enrichments of Mo (>2000 nM) with a relatively light isotope signature of δ98Mo = 0.9-1.0‰.These data are explained if Mo is converted to particle-reactive oxythiomolybdates in the sulfidic waters and is fractionated during removal from solution onto particles. Isotope fractionation is expressed in the water column, despite the high sulfide concentrations, because the rate of Mo removal is fast compared to the slow reaction kinetics of thiomolybdate formation. However, elemental and isotopic mass balances show that Mo is indeed quantitatively removed to the lake sediments and thus the isotopic composition of the sediments reflects sources to the sulfidic water. This efficient Mo drawdown is expected to occur in settings where H2S is very much in excess over Mo or in a restricted setting where the water renewal rate is slow compared to the Mo burial rate. We present a model for the Mo isotope fractionation in sulfidic systems associated with the slow reaction kinetics and conclude that quantitative removal will occur in highly sulfidic and restricted marine systems.  相似文献   

3.
This report describes a new form of arsenian pyrite, called As3+-pyrite, in which As substitutes for Fe [(Fe,As)S2], in contrast to the more common form of arsenian pyrite, As1−-pyrite, in which As1− substitutes for S [Fe(As,S)2]. As3+-pyrite has been observed as colloformic overgrowths on As-free pyrite in a hydrothermal gold deposit at Yanacocha, Peru. XPS analyses of the As3+-pyrite confirm that As is present largely as As3+. EMPA analyses show that As3+-pyrite incorporates up to 3.05 at % of As and 0.53 at. %, 0.1 at. %, 0.27 at. %, 0.22 at. %, 0.08 at. % and 0.04 at. % of Pb, Au, Cu, Zn, Ni, and Co, respectively. Incorporation of As3+ in the pyrite could be written like: As3++yAu++1-y(□)⇔2Fe2+; where Au+ and vacancy (□) help to maintain the excess charge. HRTEM observations reveal a sharp boundary between As-free pyrite and the first overgrowth of As3+-pyrite (20-40 nm thick) and co-linear lattice fringes indicating epitaxial growth of As3+-pyrite on As-free pyrite. Overgrowths of As3+-pyrite onto As-free pyrite can be divided into three groups on the basis of crystal size, 8-20 nm, 100-300 nm and 400-900 nm, and the smaller the crystal size the higher the concentration of toxic arsenic and trace metals. The Yanacocha deposit, in which As3+-pyrite was found, formed under relatively oxidizing conditions in which the dominant form of dissolved As in the stability field of pyrite is As3+; in contrast, reducing conditions are typical of most environments that host As1−-pyrite. As3+-pyrite will likely be found in other oxidizing hydrothermal and diagenetic environments, including high-sulfidation epithermal deposits and shallow groundwater systems, where probably kinetically controlled formation of nanoscale crystals such as observed here would be a major control on incorporation and release of As3+ and toxic heavy metals in oxidizing natural systems.  相似文献   

4.
Small hexagonal and triangular platelets of molybdenite (MoS2), 5 to 25 m in diameter, were identified in phenocrysts and matrix glass of unaltered felsic volcanic rocks from Pantelleria, Italy. The MoS2 occurs commonly in pantellerites (peralkaline rhyolites), rarely in pantelleritic trachytes, and never in trachytes. The occurrence of euhedral MoS2 platelets in all phenocryst phases, in matrix glass, and even in some melt inclusions indicates that MoS2 precipitated directly from the peralkaline melt. Despite MoS2 saturation, the melt (glass) contains greater than 95% of the Mo in Pantellerian rocks: X-ray fluorescence analyses of 20 whole rocks and separated glasses show that whole rocks consistently contain less Mo than corresponding matrix glasses, the differences being in proportion to phenocryst abundances. The Mo contents increase with differentiation from trachytes (2–12 ppm) to pantellerites (15–25 ppm) and correlate positively with incompatible elements such as Th, Y, and Nb. The Mo concentrations, as determined by secondary ion mass spectrometry, are essentially the same in matrix glasses and melt inclusions, showing that Mo did not partition strongly into a volatile fluid phase during outgassing. The high Mo contents of the pantellerites (relative to metaluminous magmas with 1–5 ppm) may be due to several factors: (1) the enhanced stability of highly charged cations (such as Mo6+, U4+, and Zr4+) in peralkaline melts; (2) the rarity of Fe-Ti oxides and litanite into which Mo might normally partition; (3) reduced volatility of Mo in low fO2, H2O-poor (1–2 wt%) peralkaline magmas. Geochemical modeling indicates that the precipitation of MoS2 can be explained simply by the drop in temperature during magmatic differentiation. The occurrence of MoS2 in pantellerites may result from their high Mo concentrations and low redox state (Ni/NiO=-2.5) relative to metaluminous magmas, causing them to reach MoS2 saturation at magmatic temperatures. The apparent absence of MoS2 microphenocrysts in more oxidized, metaluminous rhyolites may indicate that Mo is dissolved primarily as a hexavalent ion in those magmas.  相似文献   

5.
Pyrite is an environmentally significant mineral being the major contributor to acid rock drainage. Synchrotron based SPEM (scanning photoelectron microscopy) and micro-XPS (X-ray photoelectron spectroscopy) have been used to characterise fresh and oxidised pyrite (FeS2) with a view to understanding the initial oxidation steps that take place during natural weathering processes. Localised regions of the pyrite surface containing Fe species of reduced coordination have been found to play a critical role. Such sites not only initiate the oxidation process but also facilitate the formation of highly reactive hydroxyl radical species, which then lead the S oxidation process.Four different S species are found to be present on fresh fractured pyrite surfaces: S22−(bulk) (4-fold coordination), S22−(surface) (3-fold coordination), S2− and S0/Sn2− (metal deficient sulfide and polysulfide respectively). These species were found to be heterogeneously distributed on the fractured pyrite surface. Both O2 and H2O gases are needed for effective oxidation of the pyrite surface. The process is initiated when O2 dissociatively and H2O molecularly adsorb onto the surface Fe sites where high dangling bond densities exist. H2O may then dissociate to produce OH radicals. The adsorption of these species leads to the formation of Fe-oxy species prior to the formation of sulfoxy species. Evidence suggests that Fe-O bonds form prior to Fe-OH bonds. S oxidation occurs through interactions of OH radicals formed at the Fe sites, with formation of SO42− occurring via S2O32−/SO32− intermediates. The pyrite oxidation process is electrochemical in nature and was found to occur in patches, where site specific adsorption of O2 and H2O has occurred. Fe and S oxidation was found to occur within the same area of oxidation probably in atomic scale proximity. Furthermore, the O in SO42− arises largely from H2O; however, depending on the surface history, SO42− formed early in the oxidation process may also contain O from O2.  相似文献   

6.
A thin film of marcasite, FeS2, was synthesized under vacuum and its structure and reactivity under oxidizing conditions was investigated by means of diffraction and surface analytical techniques, respectively. Synthesis of the film was carried out by codepositing Fe and S2 onto a Ta support. The thickness of the film could be varied from approximately 10 Å to 1 μm. High-resolution S 2p synchrotron-based photoemission showed S22−, with undetectable amounts of S2− impurity that is typically present on natural sample surfaces. X-ray diffraction of the micron-thick films showed that the film crystallized in the marcasite phase of FeS2. Atomic force microscopy indicated that the thin film had a nanometer-scale roughness suggesting the film contained defects such as steps and kinks. X-ray photoelectron spectroscopy studies found the thin marcasite film to be more reactive than natural pyrite (the most ubiquitous FeS2 dimorph) after exposure to a gaseous O2/H2O environment on the basis of the amount of sulfate formation. Likely the oxidation of marcasite was dominated by its short-range order (e.g., presence of steps), because the density of nonstoichiometric defect sites (e.g., S2−) was low as assessed by photoelectron spectroscopy.  相似文献   

7.
We re-evaluate the cycling of molybdenum (Mo) and rhenium (Re) in the near-surface environment. World river average Mo and Re concentrations, initially based on a handful of rivers, are calculated using 38 rivers representing five continents, and 11 of 19 large-scale drainage regions. Our new river concentration estimates are 8.0 nmol kg−1 (Mo), and 16.5 pmol kg−1 (Re, natural + anthropogenic). The linear relationship of dissolved Re and in global rivers (R2 = 0.76) indicates labile continental Re is predominantly hosted within sulfide minerals and reduced sediments; it also provides a means of correcting for the anthropogenic contribution of Re to world rivers using independent estimates of anthropogenic sulfate. Approximately 30% of Re in global rivers is anthropogenic, yielding a pre-anthropogenic world river average of 11.2 pmol Re kg−1. The potential for anthropogenic contribution is also seen in the non-negligible Re concentrations in precipitation (0.03-5.9 pmol kg−1), and the nmol kg−1 level Re concentrations of mine waters. The linear Mo- relationship (R2 = 0.69) indicates that the predominant source of Mo to rivers is the weathering of pyrite. An anthropogenic Mo correction was not done as anthropogenically-influenced samples do not display the unambiguous metal enrichment observed for Re. Metal concentrations in high temperature hydrothermal fluids from the Manus Basin indicate that calculated end-member fluids (i.e. Mg-free) yield negative Mo and Re concentrations, showing that Mo and Re can be removed more quickly than Mg during recharge. High temperature hydrothermal fluids are unimportant sinks relative to their river sources 0.4% (Mo), and 0.1% (pre-anthropogenic Re). We calculate new seawater response times of 4.4 × 105 yr (τMo) and 1.3 × 105 yr (τRe, pre-anthropogenic).  相似文献   

8.
Microbial SO42− reduction limits accumulation of aqueous As in reducing aquifers where the sulfide that is produced forms minerals that sequester As. We examined the potential for As partitioning into As- and Fe-sulfide minerals in anaerobic, semi-continuous flow bioreactors inoculated with 0.5% (g mL−1) fine-grained alluvial aquifer sediment. A fluid residence time of three weeks was maintained over a ca. 300-d incubation period by replacing one-third of the aqueous phase volume of the reactors with fresh medium every seven days. The medium had a composition comparable to natural As-contaminated groundwater with slightly basic pH (7.3) and 7.5 μM aqueous As(V) and also contained 0.8 mM acetate to stimulate microbial activity. Medium was delivered to a reactor system with and without 10 mmol L−1 synthetic goethite (α-FeOOH). In both reactors, influent As(V) was almost completely reduced to As(III). Pure As-sulfide minerals did not form in the Fe-limited reactor. Realgar (As4S4) and As2S3(am) were undersaturated throughout the experiment. Orpiment (As2S3) was saturated while sulfide content was low (∼50 to 150 μM), but precipitation was likely limited by slow kinetics. Reaction-path modeling suggests that, even if these minerals had formed, the dissolved As content of the reactor would have remained at hazardous levels. Mackinawite (Fe1 + xS; x ? 0.07) formed readily in the Fe-bearing reactor and held dissolved sulfide at levels below saturation for orpiment and realgar. The mackinawite sequestered little As (<0.1 wt.%), however, and aqueous As accumulated to levels above the influent concentration as microbial Fe(III) reduction consumed goethite and mobilized adsorbed As. A relatively small amount of pyrite (FeS2) and greigite (Fe3S4) formed in the Fe-bearing reactor when we injected a polysulfide solution (Na2S4) to a final concentration of 0.5 mM after 216, 230, 279, and 286 days. The pyrite, and to a lesser extent the greigite, that formed did sequester As from solution, containing 0.84 and 0.23 wt.% As on average, respectively. Our results suggest that As precipitation during Fe-sulfide formation in nature occurs mainly in conjunction with pyrite formation. Our findings imply that the effectiveness of stimulating microbial SO42− reduction to remediate As contamination may be limited by the rate and extent of pyrite formation and the solubility of As-sulfides.  相似文献   

9.
The fractionation of the isotopes of Mo between different geological environments has recently been determined to high accuracy using mass spectrometry (Barling et al., 2001). Fractionation is observed between Mo in seawater, where it exists primarily in the form of the Mo(VI) anion molybdate, MoO4−2, and in oxic sediments, where the Mo is isotopically lighter than in sea water by ∼1.8‰ (in terms of the 97Mo, 95Mo isotope pair). EXAFS evidence exists for a five- or six-coordinate Mo environment in the Fe,Mn oxyhydroxides of ferromanganese nodules (Kuhn et al., 2003). In sediment regimes which are anoxic and sulfidic (sometimes referred to as euxinic), where the Mo(VI) is expected to exist as a sulfide, no fractionation is observed compared to seawater. This is presumably because of the stoichiometric conversion of the Mo from MoO4−2 to MoS4−2 (Erickson and Helz, 2000) and then to other sulfides. If the conversion is stoichiometrically complete, mass balance requires the same isotopic distribution in reactant and product. This is a result of the very high equilibrium constant for this reaction. Thus, to understand isotopic fractionation processes both the equilibrium constants for the isotopic fractionation reactions and the equilbrium constants for transformation of one chemical compound to another must be considered.We here present quantum mechanical calculations of the isotopic fractionation equilibrium constants for the isotopes 92Mo and 100Mo between MoO4−2, MoO3(OH)−, MoO2(OH)2, MoO3, MoO3(OH2)3, MoS4−2 and a number of other oxidic and sulfidic complexes of Mo. The fractionation equilibrium constants are calculated directly from the computed vibrational, rotational and translational contributions to the free energy in the gas-phase using quantum methods. Calculated vibrational frequencies and ratios of frequencies for different isotopomers are first obtained using a number of different quantum methods and compared with available experimental data to establish the most reliable methodology.We have also calculated free energy changes in aqueous solution for a range of reactions of MoO4−2 and MoO2(OH)2 with H2O and H2S. We present evidence for the instability of the monomeric octahedral species Mo(OH)6 commonly assumed to exist in acid solution and suggest highly distorted six-coordinate MoO3(OH2)3 or three-coordinate MoO3 as better representations of the species present. We have also calculated visible-UV absorption spectra to support our interpretation of the speciation. MoO3 is calculated to be isotopically lighter than MoO4−2 by 1.6‰, consistent with the experimentally observed difference between sea water and oxic sediments. We explain the isotopic lightness of oxic sediments as arising from an intermediate step in which a three coordinate MoO3 species is formed in aqueous solution, and subsequently attaches to the surface of a Fe,Mn oxyhydroxide mineral.  相似文献   

10.
《Chemical Geology》2007,236(3-4):228-246
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11.
The speciation of aqueous dissolved sulfur was determined in hydrothermal waters in Iceland. The waters sampled included hot springs, acid-sulfate pools and mud pots, sub-boiling well discharges and two-phase wells. The water temperatures ranged from 4 to 210 °C, the pHT was between 2.20 and 9.30 at the discharge temperature and the SO4 and Cl concentrations were 0.020-52.7 and <0.01-10.0 mmol kg−1, respectively. The analyses were carried out on-site within ∼10 min of sampling using ion chromatography (IC) for sulfate (SO42−), thiosulfate (S2O32−) and polythionates (SxO62−) and titration and/or colorimetry for total dissolved sulfide (S2−). Sulfite (SO32−) could also be determined in a few cases using IC. Alternatively, for few samples in remote locations the sulfur oxyanions were stabilized on a resin on site following elution and analysis by IC in the laboratory. Dissolved sulfate and with few exceptions also S2− were detected in all samples with concentrations of 0.02-52.7 mmol kg−1 and <1-4100 μmol kg−1, respectively. Thiosulfate was detected in 49 samples of the 73 analyzed with concentrations in the range of <1-394 μmol kg−1 (S-equivalents). Sulfite was detected in few samples with concentrations in the range of <1-3 μmol kg−1. Thiosulfate and SO32− were not detected in <100 °C well waters and S2O32− was observed only at low concentrations (<1-8 μmol kg−1) in ∼200 °C well waters. In alkaline and neutral pH hot springs, S2O32− was present in significant concentrations sometimes corresponding to up to 23% of total dissolved sulfur (STOT). In steam-heated acid-sulfate waters, S2O32− was not a significant sulfur species. The results demonstrate that S2O32− and SO32− do not occur in the deeper parts of <150 °C hydrothermal systems and only in trace concentrations in ∼200-300 °C systems. Upon ascent to the surface and mixing with oxygenated ground and surface waters and/or dissolution of atmospheric O2, S2− is degassed and oxidized to SO32− and S2O32− and eventually to SO42− at pH >8. In near-neutral hydrothermal waters the oxidation of S2− and the interaction of S2− and S0 resulting in the formation of Sx2− are considered important. At lower pH values the reactions seemed to proceed relatively rapidly to SO42− and the sulfur chemistry of acid-sulfate pools was dominated by SO42−, which corresponded to >99% of STOT. The results suggest that the aqueous speciation of sulfur in natural hydrothermal waters is dynamic and both kinetically and source-controlled and cannot be estimated from thermodynamic speciation calculations.  相似文献   

12.
To better understand reaction pathways of pyrite oxidation and biogeochemical controls on δ18O and δ34S values of the generated sulfate in acid mine drainage (AMD) and other natural environments, we conducted a series of pyrite oxidation experiments in the laboratory. Our biological and abiotic experiments were conducted under aerobic conditions by using O2 as an oxidizing agent and under anaerobic conditions by using dissolved Fe(III)aq as an oxidant with varying δ18OH2O values in the presence and absence of Acidithiobacillus ferrooxidans. In addition, aerobic biological experiments were designed as short- and long-term experiments where the final pH was controlled at ∼2.7 and 2.2, respectively. Due to the slower kinetics of abiotic sulfide oxidation, the aerobic abiotic experiments were only conducted as long term with a final pH of ∼2.7. The δ34SSO4 values from both the biological and abiotic anaerobic experiments indicated a small but significant sulfur isotope fractionation (∼−0.7‰) in contrast to no significant fractionation observed from any of the aerobic experiments. Relative percentages of the incorporation of water-derived oxygen and dissolved oxygen (O2) to sulfate were estimated, in addition to the oxygen isotope fractionation between sulfate and water, and dissolved oxygen. As expected, during the biological and abiotic anaerobic experiments all of the sulfate oxygen was derived from water. The percentage incorporation of water-derived oxygen into sulfate during the oxidation experiments by O2 varied with longer incubation and lower pH, but not due to the presence or absence of bacteria. These percentages were estimated as 85%, 92% and 87% from the short-term biological, long-term biological and abiotic control experiments, respectively. An oxygen isotope fractionation effect between sulfate and water (ε18OSO4-H2O) of ∼3.5‰ was determined for the anaerobic (biological and abiotic) experiments. This measured value was then used to estimate the oxygen isotope fractionation effects between sulfate and dissolved oxygen in the aerobic experiments which were −10.0‰, −10.8‰, and −9.8‰ for the short-term biological, long-term biological and abiotic control experiments, respectively. Based on the similarity between δ18OSO4 values in the biological and abiotic experiments, it is suggested that δ18OSO4 values cannot be used to distinguish biological and abiotic mechanisms of pyrite oxidation. The results presented here suggest that Fe(III)aq is the primary oxidant for pyrite at pH < 3, even in the presence of dissolved oxygen, and that the main oxygen source of sulfate is water-oxygen under both aerobic and anaerobic conditions.  相似文献   

13.
Pyrite dissolution in acidic media   总被引：2，自引：0，他引：2  
Oxidation of pyrite in aqueous solutions in contact with air (oxygen 20%) was studied at 25°C using short-term batch experiments. Fe2+ and SO42− were the only dissolved Fe and S species detected in these solutions. After a short period, R = [S]tot/[Fe]tot stabilized from 1.25 at pH = 1.5 to 1.6 at pH = 3. These R values were found to be consistent with previously published measurements (as calculated from the raw published data). This corresponds to a nonstoichiometric dissolution (R < 2) resulting from a deficit in aqueous sulfur. Thermodynamics indicate that S(−I) oxidation can only produce S(s)0 and SO42− under these equilibrium conditions. However, Pourbaix diagrams assuming the absence of SO42− indicate that S2O32− and S4O62− can appear in these conditions. Using these species the simplest expected oxidation mechanism is
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14.
We examined the solubility, mineralogy and geochemical transformations of sedimentary Fe in waterways associated with coastal lowland acid sulfate soils (CLASS). The waterways contained acidic (pH 3.26-3.54), FeIII-rich (27-138 μM) surface water with low molar Cl:SO4 ratios (0.086-5.73). The surficial benthic sediments had high concentrations of oxalate-extractable Fe(III) due to schwertmannite precipitation (kinetically favoured by 28-30% of aqueous surface water Fe being present as the FeIII species). Subsurface sediments contained abundant pore-water HCO3 (6-20 mM) and were reducing (Eh < −100 mV) with pH 6.0-6.5. The development of reducing conditions caused reductive dissolution of buried schwertmannite and goethite (formed via in situ transformation of schwertmannite). As a consequence, pore-water FeII concentrations were high (>2 mM) and were constrained by precipitation-dissolution of siderite. The near-neutral, reducing conditions also promoted SO4-reduction and the formation of acid-volatile sulfide (AVS). The results show, for the first time for CLASS-associated waterways, that sedimentary AVS consisted mainly of disordered mackinawite. In the presence of abundant pore-water FeII, precipitation-dissolution of disordered mackinawite maintained very low (i.e. <0.1 μM) S−II concentrations. Such low concentrations of S−II caused slow rates for conversion of disordered mackinawite to pyrite, thereby resulting in relatively low concentrations of pyrite (<300 μmol g−1 as Fe) compared to disordered mackinawite (up to 590 μmol g−1 as Fe). This study shows that interactions between schwertmannite, goethite, siderite, disordered mackinawite and pyrite control the geochemical behaviour of sedimentary Fe in CLASS-associated waterways.  相似文献   

15.
Pyrite dissolution and interaction with Fe(II), Co(II), Eu(III) and U(VI) have been studied under anoxic conditions by solution chemistry and spectroscopic techniques. Aqueous data show a maximal cation uptake above pH 5.5. Iron (II) uptake can explain the non-stoichiometric [S]aq/[Fe]aq ratios often observed during dissolution experiments. Protonation data corrected for pyrite dissolution resulted in a proton site density of 9 ± 3 sites nm−2. Concentration isotherms for Eu(III) and U(VI) sorption on pyrite indicate two different behaviours which can be related to the contrasted redox properties of these elements. For Eu(III), sorption can be explained by the existence of a unique site with a saturation concentration of 1.25 × 10−6 mol g−1. In the U(VI) case, sorption seems to occur on two different sites with a total saturation concentration of 4.5 × 10−8 mol g−1. At lower concentration, uranium reduction occurs, limiting the concentration of dissolved uranium to the solubility of UO2(s).Scanning electron microscopy and micro-Raman spectrometry of U(VI)-sorbed pyrite indicate a heterogeneous distribution of U at the pyrite surface and a close association with oxidized S. X-ray photoelectron spectroscopy confirms the partial reduction of U and the formation of a hyperstoichiometric UO2+x(s). Our results are consistent with a chemistry of the pyrite surface governed not by Fe(II)-bound hydroxyl groups, but by S groups which can either sorb cations and protons, or sorb and reduce redox-sensitive elements such as U(VI).  相似文献   

16.
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17.
An anomalous enrichment in marine sulfate δ34SSO4 is preserved in globally-distributed latest Ediacaran-early Cambrian strata. The proximity of this anomaly to the Ediacaran-Cambrian boundary and the associated evolutionary radiation has invited speculation that the two are causally related. Here we present a high-resolution record of paired sulfate (δ34SSO4) and pyrite (δ34Spyr) from sediments spanning ca. 547-540 million years ago (Ma) from the Ara Group of the Huqf Supergroup, Sultanate of Oman. We observe an increase in δ34SSO4 from ∼20‰ to ∼42‰, beginning at ca. 550 Ma and continuing at least through ca. 540 Ma. There is a concomitant increase in δ34Spyr over this interval from ∼ −15‰ to 10‰. This globally correlative enrichment, here termed the Ara anomaly, constitutes a major perturbation to the sulfur cycle. The absolute values of δ34Spyr reported here and in equivalent sections around the world, require the isotopic composition of material entering the ocean (δ34Sin) to be significantly more enriched than modern (∼3‰) values, likely in excess of 12‰ during the late Ediacaran-early Cambrian. Against this background of elevated δ34Sin, the Ara anomaly is explained not by increased fractionation between sulfate and pyrite (Δδ34S), but by an increase in pyrite burial (fpyr), most likely driven by enhanced primary production and sequestration of organic carbon, consistent with earlier reports of elevated organic carbon burial and widespread phosphorite deposition.  相似文献   

18.
The mobility and transport of gold in low-temperature waters and brines is affected by the aqueous speciation of gold, which is sensitive in particular to pH, oxidation and halide concentrations. In this study, we use UV-Vis spectrophotometry to identify and measure the thermodynamic properties of Au(III) aqueous complexes with chloride, bromide and hydroxide. Au(III) forms stable square planar complexes with hydroxide and halide ligands. Based on systematic changes in the absorption spectra of solutions in three binary systems NaCl-NaBr, NaCl-NaOH and NaBr-NaOH at 25 °C, we derived log dissociation constants for the following mixed and end-member halide and hydroxide complexes: [AuCl3Br]−, [AuCl2Br2]−, [AuBr3Cl]− and [AuBr4]−; [AuCl3(OH)]−, [AuCl2(OH)2]−, [AuCl(OH)3]− and [Au(OH)4]−; and [AuBr3(OH)]−, [AuBr2(OH)2]− and [AuBr(OH)3]−. These are the first reported results for the mixed chloride-bromide complexes. Increasing temperature to 80 °C resulted in an increase in the stability of the mixed chloride-bromide complexes, relative to the end-member chloride and bromide complexes. For the [AuCl(4−n)(OH)n]− series of complexes (n = 0-4), there is an excellent agreement between our spectrophotometric results and previous electrochemical results of Chateau et al. [Chateau et al. (1966)]. In other experiments, the iodide ion (I−) was found to be unstable in the presence of Au(III), oxidizing rapidly to I2(g) and causing Au to precipitate. Predicted Au(III) speciation indicates that Au(III) chloride-bromide complexes can be important in transporting gold in brines with high bromide-chloride ratios (e.g., >0.05), under oxidizing (atmospheric), acidic (pH < 5) conditions. Native gold solubility under atmospheric oxygen conditions is predicted to increase with decreasing pH in acidic conditions, increasing pH in alkaline conditions, increasing chloride, especially at acid pH, and increasing bromide for bromide/chloride ratios greater than 0.05. The results of our study increase the understanding of gold aqueous geochemistry, with the potential to lead to new methods for mineral exploration, hydrometallurgy and medicine.  相似文献   

19.
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   

20.
The acidophilic iron-oxidizing bacterium, Acidithiobacillus ferrooxidans, plays a part in the pyrite oxidation process and has been widely studied in order to determine the kinetics of the reactions and the isotopic composition of dissolved product sulphates, but the details of the oxidation processes at the surface of pyrite are still poorly known. In this study, oxygen and sulphur isotopic compositions (δ18O and δ34S) were analyzed for dissolved sulphates and water from experimental aerobic acidic (pH < 2) pyrite oxidation by A. ferrooxidans. The oxidation products attached to the pyrite surfaces were studied for their morphology (SEM), their chemistry (Raman spectroscopy) and for their δ18O (ion microprobe). They were compared to abiotically (Fe3+, H2O2, O2) oxidized pyrite surface compounds in order to constrain the oxidation pathways and to look for the existence of potential biosignatures for this system.The pyrite dissolution evolved from non-stoichiometric (during the first days) to stoichiometric (with increasing time) resulting in dissolved sulphates having distinct δ18O (e.g. +11.0‰ and −2.0‰, respectively) and δ34S (+4.5‰ and +2.8‰, respectively) values. The “oxidation layer” at the surface of pyrite is complex and made of iron oxides, sulphate, polysulphide, elemental sulphur and polythionates. Bio- and Fe3+-oxidation favour the development of monophased micrometric bumps made of hematite or sulphate while other abiotic oxidation processes result in more variable oxidation products. The δ18O of these oxidation products at the surface of oxidized pyrites are strongly variable (from ≈−40‰ to ≈+30‰) for all experiments.Isotopic fractionation between sulphates and pyrite, Δ34SSO4-pyrite, is equal to −1.3‰ and +0.4‰ for sulphates formed by stoichiometric and non-stoichiometric processes, respectively. These two values likely reflect either a S-S or a Fe-S bond breaking process. The Δ18OSO4-H2O and Δ18OSO4-O2 are estimated to be ≈+16‰ and ≈−25‰, respectively. These values are higher than previously published data and may reflect biological effects. The large δ18O heterogeneity measured at the surfaces of oxidized pyrites, whatever the oxidant, may be related (i) to the existence of local surface environments isolated from the solution in which the oxidation processes are different and (ii) to the stabilization at the pyrite surface of reaction intermediates that are not in isotopic equilibrium with the solution. Though the oxygen isotopic composition of surface oxidation products cannot be taken as a direct biosignature, the combined morphological, chemical and isotopic characterization of the surfaces of oxidized pyrites may furnish clues about a biological activity on a mineral surface.  相似文献   

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