Calculation of the visible-UV absorption spectra of hydrogen sulfide,bisulfide, polysulfides,and As and Sb sulfides,in aqueous solution

Recently we showed that visible-UV spectra in aqueous solution can be accurately calculated for arsenic (III) bisulfides, such as As(SH)₃, As(SH)₂S^- and their oligomers. The calculated lowest energy transitions for these species were diagnostic of their protonation and oligomerization state. We here extend these studies to As and Sb oxidation state III and v sulfides and to polysulfides S _n ^2- , n = 2–6, the bisulfide anion, SH^-, hydrogen sulfide, H₂S and the sulfanes, S _n H₂, n = 2–5. Many of these calculations are more difficult than those performed for the As(iii) bisulfides, since the As and Sb(v) species are more acidic and therefore exist as highly charged anions in neutral and basic solutions. In general, small and/or highly charged anions are more difficult to describe computationally than larger, monovalent anions or neutral molecules. We have used both Hartree-Fock based (CI Singles and Time-Dependent HF) and density functional based (TD B3LYP) techniques for the calculations of absorption energy and intensity and have used both explicit water molecules and a polarizable continuum to describe the effects of hydration. We correctly reproduce the general trends observed experimentally, with absorption energies increasing from polysulfides to As, Sb sulfides to SH^- to H₂S. As and Sb(v) species, both monomers and dimers, also absorb at characteristically higher energies than do the analogous As and Sb(III)species. There is also a small reduction in absorption energy from monomeric to dimeric species, for both As and Sb III and v. The polysufides, on the other hand, show no simple systematic changes in UV spectra with chain length, n, or with protonation state. Our results indicate that for the As and Sb sulfides, the oxidation state, degree of protonation and degree of oligomerization can all be determined from the visible-UV absorption spectrum. We have also calculated the aqueous phase energetics for the reaction of S₈ with SH^- to produce the polysulfides, S _n H^-, n = 2–6. Our results are in excellent agreement with available experimental data, and support the existence of a S₆ species.     

A study of speciation of Sb in bisulfide solutions by X-ray absorption spectroscopy.

Direct evidence of the structure of thioantimonide species in alkaline aqueous solutions is provided by X-ray absorption spectroscopy. Twenty solutions containing thioantimonide species were prepared by dissolving stibnite (Sb₂S₃) in deoxygenated aqueous NaHS solutions; the solution pH range was 8–14, the [Sb_tot] 1–100 mM and the [HS⁻] 0.009–2.5 M. The structural environment of the dissolved Sb was determined by EXAFS analysis of the Sb K-edge over the temperature range 80–473 K.Many of the solutions contain a species with Sb bonded to four S atoms at 2.34 Å, consistent with the presence of a [Sb(V)S₄³⁻] species, demonstrating that oxidation of Sb(III) to Sb(V) has occurred on dissolution. There is evidence that the complementary reduced phase is H₂. In three solutions, the Sb has three nearest neighbor S atoms and two of these solutions have an additional S shell of two atoms at 2.9Å, with one showing evidence of an Sb shell at 4.15 Å. This provides evidence of the presence of multimeric Sb(V) thioantimonide species. Analysis of several solutions reveals the presence of a species with three Sb–S interactions of 2.41–2.42 Å, supporting the presence of a Sb(III) species such as Sb₂S₂(SH)₂. Six solutions have S coordination numbers from 2.7–4 Å and Sb–S distances of 2.37–2.39 Å, and are likely to contain mixtures of at least two species in concentrations such that each make a significant contribution to the EXAFS. There was no clear relationship between either [Sb_tot] or [HS⁻] and the type of species present, but Sb(III) species were only present in the solutions with high pH. The effect of temperature was most significant in one solution, where at 423 K partial hydrolysis occurred and the presence of a species such as Sb₂S₂(OH)₂, with an Sb–O distance of 1.91 Å, is indicated.The study provides new information on the coordination environment of thioantimonide species, complementary to previous studies and provides a basis for a better understanding of Sb speciation in aqueous solutions found in hydrothermal systems, anoxic basins and man-made, high pH environments. In particular it demonstrates the need for Sb(V) to be considered in theoretical and experimental studies of such systems. However, more definitive interpretation of some of the data is inhibited by the presence of mixtures of species and the lack of information on the outer coordination shells that would confirm the presence of multimeric species.     

Geochemical evolution of Sb migration species in carbonate and thermal waters in relation to Sb hydrogenic mineralization

Based on the synthesis of hydrogeochemical materials on Sb occurrence in carbonate and thermal waters and thermodynamic simulations, genetic analysis was conducted of the transformations of probable Sb migration species (particularly oxygen-bearing and sulfide ones), and their transformations were calculated depending on the main parameters of hydrogeochemical systems (\(P_{CO_2 } \), T, R/W, Eh, and pH). The oxygen 2HSbO ₂ ⁰ + 3H₂S = Sb₂S₃ + 4H₂O (2SbO ₂ ^? + 3HS^? + 5H⁺ = Sb₂S₃ + 4H₂O) and sulfide HSb₂S ₄ ^? + H⁺ = Sb₂S₃ cr + H₂S (Sb₂S ₄ ^2? + 2H⁺ = Sb₂S₃cr + H₂S) models for the genesis of hydrogenic Sb₂S₃(cr) were simulated. Information on occurrences of carbonate and thermal waters in various regions worldwide was generalized, and the reasons were identified for the geochemical separation of As and Sb in carbonate and thermal waters. The causes and conditions of an increase in Sb concentrations in thermal waters were revealed, and Sb migration species in carbonate and thermal waters were identified for various parameters of hydrogeochemical systems. Variations in Sb speciation were demonstrated for hydrogeochemical systems depending on their boundary conditions (\(P_{CO_2 } \), T, and R/W). Models were outlined responsible for the precipitation of Sb₂S₃(cr) from carbonate and thermal waters.     

Arsenic and Antimony in Groundwater Flow Systems: A Comparative Study

Arsenic (As) and antimony (Sb) concentrations and speciation were determined along flow paths in three groundwater flow systems, the Carrizo Sand aquifer in southeastern Texas, the Upper Floridan aquifer in south-central Florida, and the Aquia aquifer of coastal Maryland, and subsequently compared and contrasted. Previously reported hydrogeochemical parameters for all three aquifer were used to demonstrate how changes in oxidation–reduction conditions and solution chemistry along the flow paths in each of the aquifers affected the concentrations of As and Sb. Total Sb concentrations (Sb_T) of groundwaters from the Carrizo Sand aquifer range from 16 to 198 pmol kg⁻¹; in the Upper Floridan aquifer, Sb_T concentrations range from 8.1 to 1,462 pmol kg⁻¹; and for the Aquia aquifer, Sb_T concentrations range between 23 and 512 pmol kg⁻¹. In each aquifer, As and Sb (except for the Carrizo Sand aquifer) concentrations are highest in the regions where Fe(III) reduction predominates and lower where SO₄ reduction buffers redox conditions. Groundwater data and sequential analysis of the aquifer sediments indicate that reductive dissolution of Fe(III) oxides/oxyhydroxides and subsequent release of sorbed As and Sb are the principal mechanism by which these metalloids are mobilized. Increases in pH along the flow path in the Carrizo Sand and Aquia aquifer also likely promote desorption of As and Sb from mineral surfaces, whereas pyrite oxidation mobilizes As and Sb within oxic groundwaters from the recharge zone of the Upper Floridan aquifer. Both metalloids are subsequently removed from solution by readsorption and/or coprecipitation onto Fe(III) oxides/oxyhydroxides and mixed Fe(II)/Fe(III) oxides, clay minerals, and pyrite. Speciation modeling using measured and computed Eh values predicts that Sb(III) predominate in Carrizo Sand and Upper Floridan aquifer groundwaters, occurring as the Sb(OH)₃⁰ species in solution. In oxic groundwaters from the recharge zones of these aquifers, the speciation model suggests that Sb(V) occurs as the negatively charged Sb(OH)₆⁻ species, whereas in sufidic groundwaters from both aquifers, the thioantimonite species, HSb₂S₄ ⁻ and Sb₂S₄ ²⁻, are predicted to be important dissolved forms of Sb. The measured As and Sb speciation in the Aquia aquifer indicates that As(III) and Sb(III) predominate. Comparison of the speciation model results based on measured Eh values, and those computed with the Fe(II)/Fe(III), S(-II)/SO₄, As(III)/As(V), and Sb(III)/Sb(V) couples, to the analytically determined As and Sb speciation suggests that the Fe(II)/Fe(III), S(-II)/SO₄ couples exert more control on the in situ redox condition of these groundwaters than either metalloid redox couple.     

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:

     

The behavior of molybdenum and its isotopes across the chemocline and in the sediments of sulfidic Lake Cadagno, Switzerland

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 δ⁹⁸Mo_oxic = 0.9 ± 0.1‰, which matches that of the riverine input, δ⁹⁸Mo_river = 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 δ⁹⁸Mo_sulfidic = 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.     

Calculation of the UV-visible spectra and the stability of Mo and Re oxysulfides in aqueous solution

Erickson and Helz (2000) established that molybdate, MoO₄⁻², is quickly and completely transformed to thiomolybdate, MoS₄⁻², by reaction with sulfide. They monitored the equilibria and kinetics of this process by measuring the UV-visible spectra of solutions containing the different oxythiomolybdate species. There is interest in the analogous reactions for the ReO₄⁻ species but little experimental data. We have therefore calculated quantum-chemically the equilibrium constants in solution for the sulfidation reactions of both MoO₄⁻² and ReO₄⁻, as well as recalculating their UV-visible charge-transfer spectra. Calculations using configuration interaction singles and time-dependent hybrid Hartree-Fock density functional methods give good absolute values and trends in UV-vis energies for both series. For MoO₄⁻² the calculated equilibrium constants for the various sulfidation steps match reasonably well against the experimental values (within two log K units). For the ReO₄⁻ sulfidation reaction the first two steps are considerably less favorable than for MoO₄⁻², suggesting that the “geochemical switch” of Erickson and Helz, a rapid transformation of oxyanion to thioanion highly dependent on sulfide concentration, will be less effective in the Re case. However, both our calculations and experiment indicate that ReO₄⁻ and ReS₄⁻ are both easier to reduce than their Mo analogs, so that reduction of these Re(VII) species will be the preferred mechanism for their removal from seawater. A previous suggestion that the neutral species MoO₄H₂ is actually the hydrated octahedral Mo(OH)₆ species is found to be incorrect, but the MoO₃(OH₂)₃ species, a highly distorted six-coordinate complex, is almost competitive in energy with MoO₄H₂ plus two H₂O.     

Studies of equilibrium, structure, and dynamics in the aqueous Al(iii)-oxalate-fluoride system by potentiometry, C and F NMR spectroscopy

The AlOx_1-3 (Ox = oxalate) species were identified in 0.6 M aqueous NaCl by ¹³C nuclear magnetic resonance (NMR). Rate constants and activation parameters for intramolecular cis/trans isomerization of the Werner-type AlOx₂⁻ complex (k(298 K) = 5 s⁻¹, ΔH^# = 67 ± 5 kJ mol⁻¹, ΔS^# = −6 ± 6 J mol⁻¹ K⁻¹, the rate determining step could be the breaking of the Al-O(C=O) bond) and a very slow intermolecular ligand exchange reaction of AlOx₃³⁻ complex and the free ligand (k₃₀(298 K) = 6.6 · 10⁻⁵ s⁻¹, ΔH^# = 164 ± 17 kJ mol⁻¹, ΔS^# = 225 ± 51 J mol⁻¹ K⁻¹, D/I_d mechanism) were determined by dynamic 1D and 2D ¹³C NMR measurements. Mixed complexes, AlFOx, AlFOx₂^2-, AlF₂Ox⁻, and AlF₂Ox₂^3-, with overall stability (logβ) of 11.53 ± 0.03, 15.67 ± 0.03, 15.74 ± 0.02, and 19.10 ± 0.04 were measured by potentiometry using pH- and fluoride-selective electrodes and confirmed by ¹³C and¹⁹F NMR. The role of these complexes in gibbsite dissolution was modeled. The mixed Al(III)-Ox^2--F⁻ complexes have to be considered as the chemical speciation of Al(III) in natural waters is discussed.     

Thermodynamic model for arsenic speciation in sulfidic waters: A novel use of ab initio computations

Recent discoveries demonstrate that the chemistry of arsenic in sulfidic waters is much more complex that previously believed. One implication is that all earlier thermodynamic data on stabilities of As thioanions require revision. Previously used experimental approaches for determining As thioanion stabilities may be inadequate to deal with the full range of complexity. Here we use computational as well as empirical information to construct a provisional model for equilibrium As thioanion distributions in sulfidic waters. Whereas previous authors have argued for either As(III) or As(V) thioanions, the new model predicts that both are important and can occur simultaneously under commonly encountered pH and ΣS^−II conditions. At the order of magnitude level, the model reasonably predicts the solubility of As₂S₃ in sulfidic solutions, provides tentative peak assignments for published Raman spectroscopic data and plausibly accounts for how sulfide modifies the bacterial toxicity of As. The model yields a thermodynamic justification for how sulfide, which is usually regarded as a reducing agent, can counter-intuitively drive oxidation of As(III) to As(V), as has been observed both in the laboratory and in the field. Despite its uncertain accuracy, the model serves as a useful source of new, testable hypotheses about As geochemistry and highlights crucial experimental data needs.     

Antimony speciation in saline hydrothermal fluids: A combined X-ray absorption fine structure spectroscopy and solubility study

Solubility of senarmontite (Sb₂O₃, cubic) in pure water and NaCl-HCl aqueous solutions, and local atomic structure around antimony in these fluids were characterized using in situ X-ray absorption fine structure (XAFS) spectroscopy at temperatures to 450 °C and pressures to 600 bars. These experiments were performed using a new X-ray cell which allows simultaneous measurement of the absolute concentration of the absorbing element in the fluid, and atomic environment around the absorber. Results show that aqueous Sb(III) speciation is dominated by the complex in pure water, mixed Sb-hydroxide-chloride complexes in acidic NaCl-HCl solutions (2 m NaCl-0.1 m HCl), and by Sb-chloride species in concentrated HCl solutions (3.5 m HCl). Interatomic Sb-O and Sb-Cl distances in these complexes range from 1.96 to 1.97 Å and from 2.37 to 2.47 Å, respectively. These structural data, together with senarmontite solubility determined from XAFS spectra, were complemented by batch-reactor measurements of senarmontite and stibnite (Sb₂S₃, rhombic) solubilities over a wide range of HCl and NaCl concentrations from 300 to 400 °C. Analysis of the whole dataset shows that Sb(III) speciation in high-temperature moderately acid (pH > 2-3) Cl-rich fluids is dominated by mixed hydroxy-chloride species like Sb(OH)₂Cl° and Sb(OH)₃Cl⁻, but other species containing two or three Cl atoms appear at higher acidities and moderate temperatures (?300 °C). Calculations using stability constants retrieved in this study indicate that mixed hydroxy-chloride complexes control antimony transport in saline high-temperature ore fluids at acidic conditions. Such species allow for a more effective Sb partitioning into the vapor phase during boiling and vapor-brine separation processes occurring in magmatic-hydrothermal systems. Antimony hydroxy-chloride complexes are however minor in the neutral low- to moderate-temperature solutions (?250-300 °C) typical of Sb deposits formation; the antimony speciation in these systems is dominated by Sb(OH)₃ and potentially Sb-sulfide species.     

The photoelectron spectra of some Tl-Sb sulphosalts

The results of X-ray induced photoelectron spectroscopy (XPS) experiments on several phases of the ternary system Tl-Sb-S are reported. The binding energies of the inner S, Sb and Tl electrons increase with increasing quantities of Sb and decreasing amounts of Tl in these compounds. This is explained by the influences of the proportions of the bonded metals on the effective electron affinity of S. The higher proportions of the more electronegative element bonded to S cause the increase of its effective electron affinity. The results for Tl₂S (carlinite), Tl₃SbS₃, TlSbS₂ (weissbergite), TlAsS₂ (lorandite) and Sb₂S₃ (antimonite) can be interpreted in this way. The results for Tl₄S₃ suggest a predominantly covalent character of bonding for both Tl(III) and Tl(I), which are present in this sulfide. From comparison with Tl₃SbS₄ it could be supposed that Tl(III)-S bond has a more covalent character than Sb(V)-S bond. The results for Tl₃SbS₄ are in agreement with crystal structure data and the results of Moessbauer spectroscopy. For AsS (realgar) the binding energies of the inner electrons of As and S significantly increase, showing that the electrons in molecular orbitals are less strongly bonded to individual atoms, as compared to pure elements. The results for the amorphous TlSb₅S₈ (corresponding in composition to parapierrotite) suggest that in amorphous compound the Tl-S bonding is stronger and the coordination of Tl more regular than in a crystalline one.     

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.     

Estimates of the second dissociation constant of H2S from the surface sulfidation of crystalline sulfur

The adsorption of hydrogen sulfide (Γ_H2S) and protons (Γ_H⁺) on the surface of crystalline sulfur was investigated experimentally in H₂S-bearing solutions at temperatures of 25, 50, and 70°C, NaCl concentrations of 0.1 and 0.5 mol/dm⁻³ and log C_H⁺ values in the range −2.3 to −5. At all temperatures, the dominant process on the surface of the sulfur was deprotonation, and the average values of Γ_H2S were very close to the highest values determined for Γ_H⁺. This finding, combined with the lack of detectable proton adsorption in H₂S-free solutions, suggests that proton adsorption/desorption on the surface of sulfur occurs through formation of ≡ S − H₂S complexes in the presence of H₂S.We propose that this complexation represents sulfidation of the sulfur surface, a process analogous to hydroxylation of oxide surfaces, and that the sulfidation can be described by the reaction: ≡ S + H₂S = ≡SSH₂⁰ β° The deprotonation of the ≡ SH° complex occurs via the reaction: ≡ SSH₂⁰ = ≡SSH⁻ + H⁺ β⁻ Values of 2.9, 2.8, and 2.9 (± 0.23) were obtained for −log β⁻ at 25, 50, and 70°C, respectively. These data were employed to estimate the second dissociation constant for hydrogen sulfide in aqueous solutions using the extrapolation method proposed by Schoonen and Barnes (1988) and yielded corresponding values for the constant of 17.4 ± 0.3, 15.7, and 14.5, respectively. The value for 25°C is in very good agreement with the experimentally determined values of Giggenbach (1971) at 17 ± 0.1; Meyer et al. (1983) at 17 ± 1; Licht and Manassen (1987) at 17.6 ± 0.3; and Licht et al. (1990) at 17.1 ± 0.3.     

Arsenic mobility in the ambient sulfidic environment: Sorption of arsenic(V) and arsenic(III) onto disordered mackinawite

Arsenate, As(V), sorption onto synthetic iron(II) monosulfide, disordered mackinawite (FeS), is fast. As(V) sorption decreases above the point of zero surface charge of FeS and follows the pH-dependent concentration of positively charged surface species. No redox reaction is observed between the As(V) ions and the mineral surface over the time span of the experiments. This observation shows that As(V) dominantly forms an outer-sphere complex at the surface of mackinawite. Arsenite, As(III), sorption is not strongly pH-dependent and can be expressed by a Freundlich isotherm. Sorption is fast, although slower than that of As(V). As(III) also forms an outer-sphere complex at the surface of mackinawite. In agreement with previous spectroscopic studies, complexation at low As(V) and As(III) concentration occurs preferentially at the mono-coordinated sulfide edge sites. The K_d (L g⁻¹) values obtained from linear fits to the isotherm data are ∼9 for As(V) and ∼2 for As(III). Stronger sorption of As(V) than As(III), and thus a higher As(III) mobility, may be reflected in natural anoxic sulfidic waters when disordered mackinawite controls arsenic mobility.     

Reduced sulfur species in a stratified seawater lake (Rogoznica Lake, Croatia); seasonal variations and argument for organic carriers of reactive sulfur

Over a period of a year, Hg⁰-reactive, total reduced sulfur species (RSS_T), as well as a non-volatile fraction that cannot be gas-stripped at pH ∼2 (RSS_NV), have been measured by voltammetry in a stratified, saline lake. In the hypolimnion, RSS_T is dominated by unusually high (up to 5 mM) dissolved divalent sulfur (S^−II), present as H₂S + HS⁻ and as inorganic polysulfides (H_xS_n^x−2). Less abundant RSS_NV is attributed to dissolved zero-valent sulfur (S⁰) in inorganic polysulfides. Assuming negligible contribution of organic S⁰ species in the hypolimnion, the equilibrium distribution of polysulfide ions is calculated; S₅²⁻ is found to predominate. In the epilimnion, all RSS_T consists of RSS_NV within analytical uncertainty. Through spring and summer, RSS_T and RSS_NV display little vertical or seasonal variation, but they increase dramatically when stratification breaks down in autumn. Based on decay rate, RSS during mixing events is attributed to dissolved S₈ from oxidation of sulfide and decomposition of inorganic polysulfides. This hypothesis quantitatively predicts precipitation of elemental sulfur in a year when colloidal sulfur was observed and predicts no precipitation in a year when it was not observed. Except during mixing events, the entire water column is undersaturated with respect to both rhombic sulfur and biologic sulfur, and the limited variations of RSS exclude hydrophobic and volatile aqueous S₈ as a major species. During such periods, RSS (typically 8 nM) may be associated with organic carbon, perhaps as adsorbed S₈ or as covalently bound polysulfanes or polysulfides. The hypolimnion is viewed as a zero-valent sulfur reactor that creates S⁰-containing, dissolved organic macromolecules during stable stratification periods. Some are sufficiently degradation-resistant and hydrophilic to be dispersed throughout the lake during mixing events, subsequently giving rise to ∼10⁻⁸ M RSS in the oxic water column. Voltammetrically determined RSS in oxic natural waters has often been described as “sulfide” or “metal complexed sulfide”, implying an oxidation state of S^−II; we argue that RSS in oxic Rogoznica Lake waters is mainly S⁰.     

Kinetics of the leaching of mechanically activated berthierite,boulangerite and franckeite

In this study, changes in surface area, morphology and leachability of antimony from mechanically activated berthierite—FeSb₂S₄, boulangerite—Pb₅Sb₄S₁₁ and franckeite—FePb₅Sn₃Sb₂S₁₄ by a high-energy planetary mill were investigated. It appears that a selective extraction of antimony from these complex sulphosalts in alkaline solution of sodium sulphide is positively affected by mechanical activation. The influence of milling on mineral particle size and shape was studied by scanning electron microscopy. The temperature dependencies of berthierite alkaline leaching were investigated in an interval of 323–363 K. Resulting experimental activation energies E _a were 0.11 and 6.78 kJ mol⁻¹ for mechanically activated berthierite due to a break of Arrhenius plot. The values E _a are characteristic for a process controlled by diffusion as the rate-controlling step of leaching reaction.     

Antimony adsorption on kaolinite in the presence of competitive anions

Antimony (Sb) emissions to the environment are increasing, and there is a dearth of knowledge regarding Sb fate and behavior in natural systems. In natural systems, the presence of competitive anions may compete with Sb for adsorption sites on mineral surfaces, hence increasing its potential bioavailability. Accordingly, the adsorption of Sb(III) on kaolinite was investigated in the presence of competitive anions. Kinetic studies suggest that adsorption reaction of Sb(III) on kaolinite is rapid initially and becoming slow after 12 h both in binary Sb(III)–kaolinite system and in ternary Sb(III)-competitive anion–kaolinite system. The presence of PO₄ ^3? has a much stronger and more obvious promotive effect on the adsorption of Sb(III) on kaolinite compared with the other two anions. The adsorption data of Sb(III) on kaolinite in the absence and presence of competitive anions at three temperatures were successfully modeled using Langmuir (r ² > 0.95) and Freundlich (r ² > 0.95) isotherms. Accompanied the adsorption of Sb(III) on kaolinite, significant oxidation of Sb(III) to Sb(V) had occurred under the experimental conditions used in this study. The presence of kaolinite which has a larger specific surface area could increase the contact area between the adsorbed Sb(III) and oxygen in the bulk solution, which promoted the oxidation rate of Sb(III) to Sb(V).     

Interaction between sulphide and H2O in silicate melts

Reaction between dissolved water and sulphide was experimentally investigated in soda-lime-silicate (NCS) and sodium trisilicate (NS3) melts at temperatures from 1000 to 1200 °C and pressures of 100 or 200 MPa in internally heated gas pressure vessels. Diffusion couple experiments were conducted at water-undersaturated conditions with one half of the couple being doped with sulphide (added as FeS or Na₂S; 1500-2000 ppm S by weight) and the other with H₂O (∼3.0 wt.%). Additionally, two experiments were performed using a dry NCS glass cylinder and a free H₂O fluid. Here, the melt was water-saturated at least at the melt/fluid interface. Profiling by electron microprobe (sulphur) and infrared microscopy (H₂O) demonstrate that H₂O diffusion in the melts is faster by 1.5-2.3 orders of magnitude than sulphur diffusion and, hence, H₂O can be considered as a rapidly diffusing oxidant while sulphur is quasi immobile in these experiments.In Raman spectra a band at 2576 cm⁻¹ appears in the sulphide - H₂O transition zone which is attributed to fundamental S-H stretching vibrations. Formation of new IR absorption bands at 5025 cm⁻¹ (on expense of the combination band of molecular H₂O at 5225 cm⁻¹) and at 3400 cm⁻¹ was observed at the front of the in-diffusing water in the sulphide bearing melt. The appearance and intensity of these two IR bands is correlated with systematic changes in S K-edge XANES spectra. A pre-edge excitation at 2466.5 eV grows with increasing H₂O concentration while the sulphide peak at 2474.0 eV decreases in intensity relative to the peak at 2477.0 eV and the feature at 2472.3 eV becomes more pronounced (all energies are relative to the sulphate excitation, calibrated to 2482.5 eV). The observations by Raman, IR and XANES spectroscopy indicate a well coordinated S²⁻ - H₂O complex which was probably formed in the glasses during cooling at the glass transition. No oxidation of sulphide was observed in any of the diffusion couple experiments. On the contrary, XANES spectra from experiments conducted with a free H₂O fluid show complete transformation of sulphide to sulphate near the melt surface and coexistence of sulphate and sulphide in the center of the melt. This can be explained by a lower H₂O activity in the diffusion couple experiments or by the need of a sink for hydrogen (e.g., a fluid which can dissolve high concentration of hydrogen) to promote oxidation of sulphide by H₂O via the reaction S²⁻ + 4H₂O = SO₄²⁻ + 4H₂. Sulphite could not be detected in any of the XANES spectra implying that this species, if it exists in the melt, it is a subordinate or transient species only.