Arsenite sorption on troilite (FeS) and pyrite (FeS2)

Arsenic is a toxic metalloid whose mobility and availability are largely controlled by sorption on sulfide minerals in anoxic environments. Accordingly, we investigated reactions of As(III) with iron sulfide (FeS) and pyrite (FeS₂) as a function of total arsenic concentration, suspension density, sulfide concentration, pH, and ionic strength. Arsenite partitioned strongly on both FeS and FeS₂ under a range of conditions and conformed to a Langmuir isotherm at low surface coverages; a calculated site density of near 2.6 and 3.7 sites/nm² for FeS and FeS₂, respectively, was obtained. Arsenite sorbed most strongly at elevated pH (>5 to 6). Although solution data suggested the formation of surface precipitates only at elevated solution concentrations, surface precipitates were identified using X-ray absorption spectroscopy (XAS) at all coverages. Sorbed As was coordinated to both sulfur [d(As-S) = 2.35 Å] and iron [d(As-Fe) = 2.40 Å], characteristic of As coordination in arsenopyrite (FeAsS). The absorption edge of sorbed As was also shifted relative to arsenite and orpiment (As₂S₃), revealing As(III) reduction and a complete change in As local structure. Arsenic reduction was accompanied by oxidation of both surface S and Fe(II); the FeAsS-like surface precipitate was also susceptible to oxidation, possibly influencing the stability of As sorbed to sulfide minerals in the environment. Sulfide additions inhibit sorption despite the formation of a sulfide phase, suggesting that precipitation of arsenic sulfide is not occurring. Surface precipitation of As on FeS and FeS₂ supports the observed correlation of arsenic and pyrite and other iron sulfides in anoxic sediments.     

Redox potential (Eh) and anion effects of pyrite (FeS2) leaching at pH 1

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.     

Iron isotope fractionation between pyrite (FeS2), hematite (Fe2O3) and siderite (FeCO3): A first-principles density functional theory study

In addition to equilibrium isotopic fractionation factors experimentally derived, theoretical predictions are needed for interpreting isotopic compositions measured on natural samples because they allow exploring more easily a broader range of temperature and composition. For iron isotopes, only aqueous species were studied by first-principles methods and the combination of these data with those obtained by different methods for minerals leads to discrepancies between theoretical and experimental isotopic fractionation factors. In this paper, equilibrium iron isotope fractionation factors for the common minerals pyrite, hematite, and siderite were determined as a function of temperature, using first-principles methods based on the density functional theory (DFT). In these minerals belonging to the sulfide, oxide and carbonate class, iron is present under two different oxidation states and is involved in contrasted types of interatomic bonds. Equilibrium fractionation factors calculated between hematite and siderite compare well with the one estimated from experimental data (ln α⁵⁷Fe/⁵⁴Fe = 4.59 ± 0.30‰ and 5.46 ± 0.63‰ at 20 °C for theoretical and experimental data, respectively) while those for Fe(III)_aq-hematite and Fe(II)_aq-siderite are significantly higher that experimental values. This suggests that the absolute values of the reduced partition functions (β-factors) of aqueous species are not accurate enough to be combined with those calculated for minerals. When compared to previous predictions derived from Mössbauer or INRXS data [Polyakov V. B., Clayton R. N., Horita J. and Mineev S. D. (2007) Equilibrium iron isotope fractionation factors of minerals: reevaluation from the data of nuclear inelastic resonant X-ray scattering and Mössbauer spectroscopy. Geochim. Cosmochim. Acta71, 3833-3846], our iron β-factors are in good agreement for siderite and hematite while a discrepancy is observed for pyrite. However, the detailed investigation of the structural, electronic and vibrational properties of pyrite as well as the study of sulfur isotope fractionation between pyrite and two other sulfides (sphalerite and galena) indicate that DFT-derived β-factors of pyrite are as accurate as for hematite and siderite. We thus suggest that experimental vibrational density of states of pyrite should be re-examined.     

Arsenic incorporation into FeS2 pyrite and its influence on dissolution: A DFT study

FeS₂ pyrite can incorporate large amounts of arsenic (up to ca. 10 wt%) and hence has a strong impact on the mobility of this toxic metalloid. Focussing on the lowest arsenic concentrations for which the incorporation occurs in solid solution, the substitution mechanisms involved have been investigated by assuming simple incorporation reactions in both oxidising and reducing conditions. The solution energies were calculated by Density Functional Theory (DFT) calculations and we predict that the formation of AsS dianion groups is the most energetically favourable mechanism. The results also suggest that the presence of arsenic will accelerate the dissolution and thus the generation of acid drainage, when the crystal dissolves in oxidising conditions.     

Pyrite (FeS2) oxidation: A sub-micron synchrotron investigation of the initial steps

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/S_n²⁻ (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.     

Rates of oxygen exchange between the Al2O8Al28(OH)56(H2O)26(aq) (Al30) molecule and aqueous solution

Rates of steady exchange of oxygens between bulk solution and the largest known aluminum polyoxocation: Al₂O₈Al₂₈(OH)₅₆(H₂O)₂₆¹⁸⁺(aq) (Al₃₀) are reported at pH≈4.7 and 32-40°C. The Al₃₀ molecule is a useful model for geochemists because it is ≈2 nm in length, comparable to the smallest colloidal solids, and it has structural complexity greater than the surfaces of most aluminum (hydr)oxide minerals. The Al₃₀ molecule has 15 distinct hydroxyl sites and eight symmetrically distinct bound waters. Among the hydroxyl bridges are two sets of μ₃-OH, which are not present in any of the other aluminum polyoxocations that have yet been studied by NMR methods. Rates of isotopic equilibration of the μ₂-OH and μ₃-OH hydroxyls and bound water molecules fall within the same range as we have determined for other aluminum solutes, although it is impossible to determine rate laws for exchange at the large number of individual oxygen sites. After injection of ¹⁷O-enriched water, growth of the ¹⁷O-NMR peak near 37 ppm, which is assigned to μ₂-OH and μ₃-OH hydroxyl bridges, indicates that these bridges equilibrate within two weeks at temperatures near 35°C. The peak at +22 ppm in the ¹⁷O-NMR spectra, assigned to bound water molecules (η-OH₂), varies in width with temperature in a similar fashion as for other aluminum solutes, suggesting that most of the η-OH₂ sites exchange with bulk solution at rates that fall within the range observed for other aluminum complexes. Signal from one anomalous group of four η-OH₂ sites is not observed, indicating that these sites exchange at least a factor of ten more rapidly than the other η-OH₂ sites on the Al₃₀.     

K-H3OFe3(SO4)2(OH)6铁矾的XRD研究

根据X射线衍射(XRD)分析发现: A Fe₃(SO₄)₂(OH)₆(A=K⁺、H₃O⁺)系列铁钒的XRD数据十分相近,难以用XRD区别,需通过能谱(EDS)辅助分析,才能区分此类铁矾。另外,此类铁矾的003和107面网间距d随K⁺含量增大而增大,且呈一元三次方程的关系;而033和220面网间距d随K⁺含量增大而减小,呈一元二次方程的关系。对该现象从铁矾晶体结构方面进行解释:K⁺、H₃O⁺离子位于较大空隙中,且沿着Z轴方向排列,当K⁺、H₃O⁺离子之间相互替换时,会导致该铁矾晶体结构在Z轴方向有较明显的变化。     

Oxidation of Sb(III) to Sb(V) by O2 and H2O2 in aqueous solutions

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, k_obs, 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 k_obs and pH was:

     

Free energy of formation for green rust sodium sulphate (NaFe6Fe3(OH)18(SO4)2(s))

In a recent study, sulphate-bearing green rust (GR_SO4) was shown to incorporate Na⁺ in its structure (NaFe^II₆Fe^III₃(OH)₁₈(SO₄)_2(s); GR_Na,SO4). The compound was synthesised by aerial oxidation of Fe(OH)_2(s) in the presence of NaOH. This paper reports on its free energy of formation .Freshly synthesised GR_Na,SO4 was titrated with 0.5 M H₂SO₄ in an inert atmosphere at 25 °C, producing dissolved Fe²⁺ and magnetite or goethite. Solution concentrations, PHREEQC and the MINTEQ database were used to calculate reaction constants for the reactions:

     

The kinetics of oxygen exchange between the GeO4Al12(OH)24(OH2)12(aq) molecule and aqueous solutions

We report rates of oxygen exchange with bulk solution for an aqueous complex, ^IVGeO₄Al₁₂(OH)₂₄(OH₂)₁₂⁸⁺(aq) (GeAl₁₂), that is similar in structure to both the ^IVAlO₄Al₁₂(OH)₂₄(OH₂)₁₂⁷⁺(aq) (Al₁₃) and ^IVGaO₄Al₁₂(OH)₂₄(OH₂)₁₂⁷⁺(aq) (GaAl₁₂) molecules studied previously. All of these molecules have ε-Keggin-like structures, but in the GeAl₁₂ molecule, occupancy of the central tetrahedral metal site by Ge(IV) results in a molecular charge of +8, rather than +7, as in the Al₁₃ and GaAl₁₂. Rates of exchange between oxygen sites in this molecule and bulk solution were measured over a temperature range of 274.5 to 289.5 K and 2.95 < pH < 4.58 using ¹⁷O-NMR.Apparent rate parameters for exchange of the bound water molecules (η-OH₂) are k_ex²⁹⁸ = 200 (±100) s⁻¹, ΔH^‡ = 46 (±8) kJ · mol⁻¹, and ΔS^‡ = −46 (±24) J · mol⁻¹ K⁻¹ and are similar to those we measured previously for the GaAl₁₂ and Al₁₃ complexes. In contrast to the Al₁₃ and GaAl₁₂ molecules, we observe a small but significant pH dependence on rates of solvolysis that is not yet fully constrained and that indicates a contribution from the partly deprotonated GeAl₁₂ species.The two topologically distinct μ₂-OH sites in the GeAl₁₂ molecule exchange at greatly differing rates. The more labile set of μ₂-OH sites in the GeAl₁₂ molecule exchange at a rate that is faster than can be measured by the ¹⁷O-NMR isotopic-equilibration technique. The second set of μ₂-OH sites have rate parameters of k_ex²⁹⁸ = 6.6 (±0.2) · 10⁻⁴ s⁻¹, ΔH^‡ = 82 (±2) kJ · mol⁻¹, and ΔS^‡ = −29 (±7) J · mol⁻¹ · K⁻¹, corresponding to exchanges ≈40 and ≈1550 times, respectively, more rapid than the less labile μ₂-OH sites in the Al₁₃ and GaAl₁₂ molecules. We find evidence of nearly first-order pH dependence on the rate of exchange of this μ₂-OH site with bulk solution for the GeAl₁₂ molecule, which contrasts with Al₁₃ and GaAl₁₂ molecules.     

Actinide crystal-liquid partitioning for clinopyroxene and Ca3(PO4)2

Using fission and alpha track radiography techniques, we have measured partition coefficients (D) for the actinide elements Th, U and Pu between diopsidic clinopyroxene, whitlockite [β-Ca₃ (PO₄)₂] and silicate liquid at 20kbar. Equilibrium partitioning at the crystal-liquid interface is assumed, and corrections for actinide zoning have been applied to the measured D values. Reproducibility for both actinide and minor element D values is carefully examined as a criterion for crystal-liquid interface equilibrium. The data are mostly compatible with interface equilibrium except for experiments at high cooling rates ( ? 30 deg/hr). Partition coefficients for Th/U/Pu of about 0.002/0.002/0.06 are measured for clinopyroxene and 1.2/0.5/3.4 for whitlockite. At an oxygen fugacity of 10^?8.5, Pu is much more readily incorporated into the crystalline phases than is U or Th because of the importance of trivalent Pu. The D_Pu(cpx) is similar to D(cpx) of the light rare earths supporting the concept of Pu/(rare earth) dating.     

Oxygen isotope partitioning during oxidation of pyrite by H2O2 and its dependence on temperature

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 (S_aq) was the only sulfur product recovered from solution. Solid hydrated ferric iron sulfates (i.e., water-soluble sulfate fraction, S_ws) 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 S_aq and S_ws 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.     

First-principles study of (MgH2SiO4)? n (Mg2SiO4) hydrous olivine structures. I. Crystal structure modelling of hydrous olivine Hy-2 a (MgH2SiO4) ? 3(Mg2SiO4)

 Recently, the Hy-2a hydrous olivine (MgH₂ SiO₄)·3(Mg₂SiO₄) occurring as nanometre-sized inclusions in mantle olivines has been found by TEM, and has been suggested to be a new DHMS phase (Khisina et al. 2001). A model of the crystal structure of Hy-2a has been proposed as a 2a-superstructure of olivine with one Me²⁺ -vacant octahedral layer in the (1 0 0) plane per Hy-2a unit cell (Khisina and Wirth 2002). In the present study the crystal structure of Hy-2a hydrous olivine is optimized by ab initio calculations. The aims of this study are: (1) verification of the suggested models of Hy-2a hydrous olivine structure; (2) calculation of the most stable configurations for Hy-2a structure with minimum static lattice energy, by assuming a possible formation of Me²⁺ vacancies in either M1 or M2 octahedral sites; (3) determination of the position of protons and hydrogen bonds in the Hy-2a structure. Several different possible configurations of the Hy-2a structure are optimized. The results support the idea of a stable olivine structure with ordered planar-segregated OH-bearing defects oriented parallel to (1 0 0). The data obtained indicate a preferred stability of the Hy-2a structure with the protons associated with M1 vacancies and bonded with O1 and O2 oxygen sites. The relative energy values of the optimized Hy-2a structure configurations correlate as a rule with the average shifts of atoms from their positions in pure forsterite structure. Received: 7 February 2002 / Accepted: 23 October 2002     

Density functional theory study and kinetic analysis of the formation mechanism of Al30O8(OH)56(H2O)26 (Al30) in aqueous solution

The formation mechanism of Al₃₀O₈(OH)₅₆(H₂O)₂₆¹⁸⁺ (Al₃₀) has been investigated by the density functional theory based on the supermolecule model and kinetic analysis on the ²⁷Al nuclear magnetic resonance (NMR) experimental results in monitoring Al₃₀ synthesis process. The theoretical chemistry calculations on the four possible schemes show that δ-Na-Al₁₃ is the reasonable intermediate followed by the substitution of Na with Al to form δ-Al₁₄, and Na⁺ plays an important role in stabilizing the intermediate (δ-Na-Al₁₃) in the transformation. The kinetic analysis on the ²⁷Al NMR experimental data indicates that ε-Al₁₃ decomposes and isomerizes in the formation of Al₃₀, while Al monomers facilitate the decomposition of ε-Al₁₃ and so the isomerization of ε-isomers to δ-isomers effectively. The favorable formation mechanism of Al₃₀ includes three steps: (1) ε-Al₁₃ decomposes and rearranges into the isomer δ-Al₁₃; (2) Na⁺ reacts with δ-Al₁₃ to stabilize the intermediate δ-Na-Al₁₃, followed by Al monomers replacing Na to form δ-Al₁₄; (3) δ-Al₁₄ reacts with the Al monomers in the solution to finally form Al₃₀. Both Al monomers and Na⁺ are important in the transformation. Al monomers are the basic building units and helpful to the isomerization while Na⁺ can well stabilize the isomer δ-Al₁₃ to yield intermediate δ-Na-Al₁₃. The results also show that other isomers of ε-Al₁₃ (β-Al₁₃ and α-Al₁₃) form in the formation of Al₃₀, and their calculated ²⁷Al NMR tetrahedral resonance shifts are consistent with the experimental ²⁷Al NMR tetrahedral signals in the preparation process of Al₃₀.     

Solubility of Fe-ettringite (Ca6[Fe(OH)6]2(SO4)3 · 26H2O)

The solubility of Fe-ettringite (Ca₆[Fe(OH)₆]₂(SO₄)₃ · 26H₂O) was measured in a series of precipitation and dissolution experiments at 20 °C and at pH-values between 11.0 and 14.0 using synthesised material. A time-series study showed that equilibrium was reached within 180 days of ageing. After equilibrating, the solid phases were analysed by XRD and TGA while the aqueous solutions were analysed by ICP-OES (calcium, sulphur) and ICP-MS (iron). Fe-ettringite was found to be stable up to pH 13.0. At higher pH-values Fe-monosulphate (Ca₄[Fe(OH)₆]₂(SO₄) · 6H₂O) and Fe-monocarbonate (Ca₄[Fe(OH)₆]₂(CO₃) · 6H₂O) are formed. The solubilities of these hydrates at 25 °C are: