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1.
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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2.
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   

3.
Experiments were conducted to investigate (i) the rate of O-isotope exchange between SO4 and water molecules at low pH and surface temperatures typical for conditions of acid mine drainage (AMD) and (ii) the O- and S-isotope composition of sulfates produced by pyrite oxidation under closed and open conditions (limited and free access of atmospheric O2) to identify the O source/s in sulfide oxidation (water or atmospheric molecular O2) and to better understand the pyrite oxidation pathway. An O-isotope exchange between SO4 and water was observed over a pH range of 0–2 only at 50 °C, whereas no exchange occurred at lower temperatures over a period of 8 a. The calculated half-time of the exchange rate for 50 °C (pH = 0 and 1) is in good agreement with former experimental data for higher and lower temperatures and excludes the possibility of isotope exchange for typical AMD conditions (T  25 °C, pH  3) for decades.Pyrite oxidation experiments revealed two dependencies of the O-isotope composition of dissolved sulfates: O-isotope values decreased with longer duration of experiments and increasing grain size of pyrite. Both changes are interpreted as evidence for chemisorption of molecular O2 to pyrite surface sites. The sorption of molecular O2 is important at initial oxidation stages and more abundant in finer grained pyrite fractions and leads to its incorporation in the produced SO4. The calculated bulk contribution of atmospheric O2 in the dissolved SO4 reached up to 50% during initial oxidation stages (first 5 days, pH 2, fine-grained pyrite fraction) and decreased to less than 20% after about 100 days. Based on the direct incorporation of molecular O2 in the early-formed sulfates, chemisorption and electron transfer of molecular O2 on S sites of the pyrite surface are proposed, in addition to chemisorption on Fe sites. After about 10 days, the O of all newly-formed sulfates originates only from water, indicating direct interaction of hydroxyls from water with S at the anodic S pyrite surface site. Then, the role of molecular O2 is as proposed in previous studies: acting as electron acceptor only at the cathodic Fe pyrite surface site for oxidation of Fe(II) to Fe(III).  相似文献   

4.
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   

5.
Pyrite plays the central role in the environmental issue of acid rock drainage. Natural weathering of pyrite results in the release of sulphuric acid which can lead to further leaching of heavy and toxic metals from other associated minerals. Understanding how pyrite reacts in aqueous solution is critical to understanding the natural weathering processes undergone by this mineral. To this end an investigation of the effect of solution redox potential (Eh) and various anions on the rate of pyrite leaching under carefully controlled conditions has been undertaken.Leaching of pyrite has been shown to proceed significantly faster at solution Eh of 900 mV (SHE) than at 700 mV, at pH 1, for the leach media of HCl, H2SO4 and HClO4. The predominant effect of Eh suggests electrochemical control of pyrite leaching with similar mechanism(s) at Eh of 700 and 900 mV albeit with different kinetics. Leach rates at 700 mV were found to decrease according to HClO4 > HCl > H2SO4 while at 900 mV the leach rate order was HCl > HClO4 > H2SO4. Solution Fe3+ activity is found to continually increase during all leaches; however, this is not accompanied by an increase in leach rate.Synchrotron based photoemission electron microscopy (PEEM) measurements showed a localised distribution of adsorbed and oxidised surface species highlighting that pyrite oxidation and leaching is a highly site specific process mediated by adsorption of oxidants onto specific surface sites. It appears that rates may be controlled, in part, by the propensity of acidic anions to bind to the surface, which varies according to , thus reducing the reactive or effective surface area. However, anions may also be involved in specific reactions with surface leach products. Stoichiometric dissolution data (Fe/S ratio), XPS and XRD data indicate that the highest leach rates (in HCl media at 900 mV Eh) correlate with relatively lower surface S abundance. Furthermore, there are indications that solution Cl− assists oxidation especially at higher Eh through the prevention of surface S0 buildup at reactive surface sites.  相似文献   

6.
A detailed experimental study was conducted to investigate mechanisms of pyrite oxidation by determining product yields and oxygen isotopic fractionation during reactions between powdered pyrite (FeS2) with aqueous hydrogen peroxide (H2O2). Sealed silica-tube experiments utilized aliquots of pyrite that were reacted with 0.2 M H2O2 for 7 to 14 days at 4 to 150 °C. No volatile sulfur species were detected in any experiment. The only gaseous product recovered was elemental oxygen inferred to result from decomposition of H2O2. Aqueous sulfate (Saq) was the only sulfur product recovered from solution. Solid hydrated ferric iron sulfates (i.e., water-soluble sulfate fraction, Sws) were recovered from all experiments. Ferric oxide (hematite) was detected only in high temperature experiments.Reactants were selected with large differences in initial δ18O values. The oxygen isotopic compositions of oxygen-bearing reactants and products were analyzed for each experiment. Subsequent isotopic mass-balances were used to identify sources of oxygen for reaction products and to implicate specific chemical reaction mechanisms. δ18O of water did not show detectable change during any experiment. δ18O of sulfate was similar for Saq and Sws and indicated that both H2O and H2O2 were sources of oxygen in sulfate. Low-temperature experiments suggest that H2O-derived oxygen was incorporated into sulfate via Fe3+ oxidation, whereas H2O2-derived oxygen was incorporated into sulfate via oxidation by hydroxyl radicals (HO). These two competing mechanisms for oxygen incorporation into sulfate express comparable influences at 25 °C. With increasing reaction temperatures from 4 to 100 °C, it appears that accelerated thermal decomposition and diminished residence time of H2O2 limit the oxygen transfer from H2O2 into sulfate and enhance the relative importance of H2O-derived oxygen for incorporation into sulfate. Notably, at temperatures between 100 and 150 °C there is a reversal in the lower temperature trend resulting in dominance of H2O2-derived oxygen over H2O-derived oxygen. At such high temperatures, complete thermal decomposition of H2O2 to water and molecular oxygen (O2) occurs within minutes in mineral-blank experiments and suggests little possibility for direct oxidation of pyrite by H2O2 above 100 °C. We hypothesize that a Fe-O2 mechanism is responsible for oxygenating pyrite to sulfate using O2 from the preceding thermal decomposition of H2O2.  相似文献   

7.
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   

8.
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   

9.
A previous contribution from our laboratory reported the formation of hydrogen peroxide (H2O2) upon addition of pyrite (FeS2) to O2-free water. It was hypothesized that a reaction between adsorbed H2O and Fe(III), at a sulfur-deficient defect site, on the pyrite surface generates an adsorbed hydroxyl radical (OH•).
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10.
The mechanism of pyrite oxidation in carbonate-containing alkaline solutions at 80 °C was investigated with the help of rate experiments, thermodynamic modeling and diffuse reflectance infrared spectroscopy (DRIFTS). Pyrite oxidation rate increased with pH and was enhanced by addition of bicarbonate/carbonate ions. The carbonate effect was found to be limited to moderately alkaline conditions (pH 8-11). Metastable Eh-pH diagrams, at 25 °C, indicate that soluble iron-carbonate complexes (FeHCO3−, FeCO30, Fe(CO3)(OH)− and FeCO32−) may coexist with pyrite in the pH range of 6-12.5. Above pH 11 and 13, the Fe(II) and Fe(III) hydroxocomplexes, respectively, become stable, even in the presence of carbonate/bicarbonate ions. Surface-bound carbonate complexes on iron were also identified with DRIFTS as products of pyrite oxidation in addition to iron oxyhydroxides and soluble sulfate species. The conditions under which thermodynamic and DRIFTS analyses indicate the presence of carbonate compounds also correspond to those in which the fastest rate of pyrite oxidation in carbonate solutions was observed. Following the Singer-Stumm model for pyrite oxidation in acidic solutions, it is assumed that Fe(III) is the preferred pyrite oxidant under alkaline conditions. We propose that carbonate ions facilitate the electron transfer from soluble iron(II)-carbonate to O2, increase the iron solubility, and provide buffered, favorable alkaline conditions at the reaction front, which in turn favors the overall kinetics of pyrite oxidation. Therefore, the electron transfer from sulfur atoms to O2 is facilitated by the formation of the cycle of Fe(II)-pyrite/Fe(III)-carbonate redox couple at the pyrite surface.  相似文献   

11.
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12.
The 2 site protolysis non electrostatic surface complexation and cation exchange (2SPNE SC/CE) sorption model has been used over the past decade or so to quantitatively describe the uptake of metals with oxidation states from II to VI on 2:1 clay minerals; montmorillonite and illite. One of the main features in this model is that there are two broad categories of amphoteric edge sorption sites; the so called strong (SSOH) and weak (SW1OH) sites. Because of their different sorption characteristics, it was expected that the coordination environments of the surface complexes on the two site types would be different. Zn isotherm data on two montmorillonites, Milos and STx-1, were measured and modelled using the 2SPNE SC/CE sorption model. The results were used to define the most favourable experimental conditions under which Zn sorption was either dominated by the strong (SSOH, ∼2 mmol kg−1) or by the weak sites (SW1OH, ∼40 mmol kg−1). Highly oriented self-supporting films were prepared for polarised extended X-ray absorption fine structure (P-EXAFS) investigations.Montmorillonites often contain Zn incorporated in the clay matrix. The Zn bound in this form was quantified and the results from the analysis of the P-EXAFS spectra were taken into account in the interpretation of the spectra measured at low Zn loadings (∼2 mmol kg−1) and medium Zn loadings (∼30 mmol kg−1). The Zn spectra on the “strong sites” exhibited a pronounced angular dependency and formed surface complexes in the continuity of the Al-octahedral sheets at the montmorillonite edges. In contrast, the Zn “weak site” spectra showed only a weak angular dependency. The spectroscopic evidence indicates the existence of two distinct groups of edge surface binding sites which is consistent with a multi-site sorption model and in particular with the strong/weak site concept intrinsic to the 2SPNE S/CE sorption model.  相似文献   

13.
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   

14.
The rates of Sb(III) oxidation by O2 and H2O2 were determined in homogeneous aqueous solutions. Above pH 10, the oxidation reaction of Sb(III) with O2 was first order with respect to the Sb(III) concentration and inversely proportional to the H+ concentrations at a constant O2 content of 0.22 × 10−3 M. Pseudo-first-order rate coefficients, kobs, ranged from 3.5 × 10−8 s−1 to 2.5 × 10−6 s−1 at pH values between 10.9 and 12.9. The relationship between kobs and pH was:
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15.
Enthalpies of formation of ferrihydrite and schwertmannite were measured by acid solution calorimetry in 5 N HCl at 298 K. The published thermodynamic data for these two phases and ε-Fe2O3 were evaluated, and the best thermodynamic data for the studied compounds were selected.Ferrihydrite is metastable in enthalpy with respect to α-Fe2O3 and liquid water by 11.5 to 14.7 kJ•mol−1 at 298.15 K. The less positive enthalpy corresponds to 6-line ferrihydrite, and the higher one, indicating lesser stability, to 2-line ferrihydrite. In other words, ferrihydrite samples become more stable with increasing crystallinity. The best thermodynamic data set for ferrihydrite of composition Fe(OH)3 was selected by using the measured enthalpies and (1) requiring ferrihydrite to be metastable with respect to fine-grained lepidocrocite; (2) requiring ferrihydrite to have entropy higher than the entropy of hypothetical, well-crystalline Fe(OH)3; and (3) considering published estimates of solubility products of ferrihydrite. The ΔG°f for 2-line ferrihydrite is best described by a range of −708.5±2.0 to −705.2±2.0 kJ•mol−1, and ΔG°f for 6-line ferrihydrite by −711.0±2.0 to −708.5±2.0 kJ•mol−1.A published enthalpy measurement by acid calorimetry of ε-Fe2O3 was re-evaluated, arriving at ΔH°f (ε-Fe2O3) = −798.0±6.6 kJ•mol−1. The standard entropy (S°) of ε-Fe2O3 was considered to be equal to S° (γ-Fe2O3) (93.0±0.2 J•K−1•mol−1), giving ΔG°f (ε-Fe2O3) = −717.8±6.6 kJ•mol−1. ε-Fe2O3 thus appears to have no stability field, and it is metastable with respect to most phases in the Fe2O3-H2O system which is probably the reason why this phase is rare in nature.Enthalpies of formation of two schwertmannite samples are: ΔH°f (FeO(OH)0.686(SO4)0.157•0.972H2O) = −884.0±1.3 kJ•mol−1, ΔH°f (FeO(OH)0.664(SO4)0.168•1.226H2O) = −960.7±1.2 kJ•mol−1. When combined with an entropy estimate, these data give Gibbs free energies of formation of −761.3 ± 1.3 and −823.3 ± 1.2 kJ•mol−1 for the two samples, respectively. These ΔGf° values imply that schwertmannite is thermodynamically favored over ferrihydrite over a wide range of pH (2-8) when the system contains even small concentration of sulfate. The stability relations of the two investigated samples can be replicated by schwertmannite of the “ideal” composition FeO(OH)3/4(SO4)1/8 with ΔG°f = −518.0±2.0 kJ•mol−1.  相似文献   

16.
金在黄铁矿表面沉淀机理的实验研究   总被引：6，自引：0，他引：6       下载免费PDF全文
为了研究金在黄铁矿表面沉淀的机理,于室温、常压,在氯化物溶液中进行了黄铁矿粉末吸附金的实验。在不同pH的溶液中,黄铁矿均可吸附金,而且pH值明显地影响吸附速率。扫描电镜观察表明,反应后黄铁矿粒表面有金晶体形成。XPS研究得知,黄铁矿光片与含金氯化物溶液反应后表面有A0存在;硫在反应初期为S0、S2O32-,随后转变为SO42-,而铁成为Fe3+.黄铁矿中的Fe2+和S22-是溶液中金的还原剂。金在黄铁矿表面沉淀可能涉及吸附、还原和晶体生长等过程。  相似文献   

17.
The oxidation of Fe(II) with H2O2 at nanomolar levels in seawater have been studied using an UV-Vis spectrophotometric system equipped with a long liquid waveguide capillary flow cell. The effect of pH (6.5 to 8.2), H2O2 (7.2 × 10−8 M to 5.2 × 10−7 M), HCO3− (2.05 mM to 4.05 mM) and Fe(II) (5 nM to 500 nM) as a function of temperature (3 to 35 °C) on the oxidation of Fe(II) are presented. The oxidation rate is linearly related to the pH with a slope of 0.89 ± 0.01 independent of the concentration of HCO3−. A kinetic model for the reaction has been developed to consider the interactions of Fe(II) with the major ions in seawater. The model has been used to examine the effect of pH, concentrations of Fe(II), H2O2 and HCO3− as a function of temperature. FeOH+ is the most important contributing species to the overall rate of oxidation from pH 6 to pH 8. At a pH higher than 8, the Fe(OH)2 and Fe(CO3)22− species contribute over 20% to the rates. Model results show that when the concentration of O2 is two orders of magnitude higher than the concentration of H2O2, the oxidation with O2 also needs to be considered. The rate constants for the five most kinetically active species (Fe2+, FeOH+, Fe(OH)2, FeCO3, Fe(CO3)22−) in seawater as a function of temperature have been determined. The kinetic model is also valid in pure water with different concentrations of HCO3− and the conditions found in fresh waters.  相似文献   

18.
Quantum-mechanical calculations allow resolving and quantifying in detail important aspects of reaction mechanisms such as spin transitions and oxygen dissociation that can be the major rate-limiting steps in redox processes on sulfide and oxide surfaces. In addition, this knowledge can help experimentalists in setting up the framework of rate equations that can be used to describe the kinetics of, e.g., oxidation processes. The unique molecular crystal structure of realgar, As4S4 clusters held together by van der Waals bonds, allows for a convenient quantum-mechanical (q.m.) cluster approach to investigate the thermodynamics and kinetic pathways of oxidation. The interaction of As4S4 clusters with oxygen and co-adsorbed ions provides a model system for understanding the molecular-scale processes that underpin empirically-derived rate expressions, and provides clues to the oxidation mechanisms of other sulfides and oxides. Two activated processes are shown to dominate the kinetics of oxidation by molecular oxygen: (i) a paramagnetic 3O2 to diamagnetic 1O2 spin transition and (ii) oxygen dissociation on the surface, in that order. The activation energies for the spin transition and O2 dissociation step were determined to be 1.1 eV (106 kJ/mol) and 0.9 eV (87 kJ/mol), respectively, if molecular oxygen is the only reactant on the surface. In the case of As4S4, q.m. calculations reveal that 3O2 transfers its spin to the cluster and forms a low-spin, peroxo intermediate on the surface before dissociating. The adsorption of a hydroxide ion on the surface proximate to the 3O2 adsorption site changes the adsorption mechanism by lowering the activation energy barriers for both the spin transition (0.30 eV/29 kJ/mol) and the O2 dissociation step (0.72 eV/69 kJ/mol). Thus, while spin transition is rate limiting for oxidation with O2 alone, dissociation becomes the rate-limiting step for oxidation with co-adsorption of OH−. First-principles, periodic calculations of the realgar surface show that the energetics and structural changes that accompany oxidation of As4S4 clusters on the surface are similar to those involving individual As4S4 clusters. Thus, assuming that an As4S4 cluster with an adsorbed hydroxyl group is a reasonable approximation of the surface of As4S4 at high pH, the theoretically calculated oxidation rate (∼1 × 10−10 mol m−2 s−1) is of the same order as empirically-derived rates from experiments at T = 298 K, pH = 8, and similar dissolved oxygen concentrations. In addition, the co-adsorption of other anions found in alkaline waters (i.e. carbonate, bicarbonate, sulfate, and sulfite) were shown to energetically promote the oxidation of As4S4 (on the order of 5-40 kJ/mol depending on the co-adsorbed anion, OH−, , , , or , and accounting for changes in the hydration of products and reactants). The effect of the co-adsorbate on the kinetics and thermodynamics of oxidation is due to each adsorbate modifying the electronic and structural environment of the other adsorption site.Activation-energy barriers due to spin transitions are rarely discussed in the literature as key factors for controlling oxidation rates of mineral surfaces, even though the magnitude of these barriers is enough to alter the kinetics significantly. The attenuation of the activation energy by co-adsorbed anions suggests the possibility of pH− or p(co-adsorbate)-dependent activation energies that can be used to refine oxidation rate laws for sulfide minerals and other, especially semiconducting minerals, such as oxides.  相似文献   

19.
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   

20.
Reports of the high ion content of steam and low-density supercritical fluids date back to the work of Carlon [Carlon H. R. (1980) Ion content of air humidified by boiling water.J. Appl.Phys.51, 171-173], who invoked ion and neutral-water clustering as mechanism to explain why ions partition into the low-density aqueous phase. Mass spectrometric, vibrational spectroscopic measurements and quantum chemical calculations have refined this concept by proposing strongly bound ion-solvent aggregates and water clusters such as Eigen- and Zundel-type proton clusters H3O+·(H2O)m and the more weakly bound water oligomers (H2O)m. The extent to which these clusters affect fluid chemistry is determined by their abundance, however, little is known regarding the stability of such moieties in natural low-density high-temperature fluids. Here we report results from quantum chemical calculations using chemical-accuracy multi-level G3 (Curtiss-Pople) and CBS-Q theory (Peterson) to address this question. In particular, we have investigated the cluster structures and clustering equilibria for the ions and H3S+·(H2O)m(H2S)n, where m ? 6 and n ? 4, at 300-1000 K and 1 bar as well as under vapor-liquid equilibrium conditions between 300 and 646 K. We find that incremental hydration enthalpies and entropies derived from van’t Hoff analyses for the attachment of H2O and H2S onto H3O+, and H3S+ are in excellent agreement with experimental values and that the addition of water to all three ions is energetically more favorable than solvation by H2S. As clusters grow in size, the energetic trends of cluster hydration begin to reflect those for bulk H2O liquids, i.e. calculated hydration enthalpies and entropies approach values characteristic of the condensation of bulk water (ΔHo = −44.0 kJ mol−1, ΔSo = −118.8 J K mol−1). Water and hydrogen sulfide cluster calculations at higher temperatures indicate that a significant fraction of H3O+, and H3S+ ions exists as solvated moieties.  相似文献   

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