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.     

Chalcopyrite leaching: The rate controlling factors

The processes that determine the rate of chalcopyrite leaching are central to understanding how chalcopyrite (CuFeS₂) behaves under the environmentally adverse conditions of acid rock drainage. To this end the effect of the acid anion on chalcopyrite leach rates using a variety of acidic media (H₂SO₄, HClO₄, HCl and H₂SO₄ with 0.25 M NaCl) under carefully controlled solution conditions (pH 1 and 2, Eh 750 mV (SHE) and 75 °C) has been examined. These conditions have been chosen to enable sufficient leach rates for accurate experimental determination and to compare to the previous mechanistic analysis carried out by Harmer et al. (2006).Extensive surface analysis of leach residues demonstrated that variations in the surface speciation could not be responsible for the observed variations in leach rate. The rate of Cu release, however, was found to be first order with respect to Fe³⁺ activity and inversely proportional with respect to H⁺ activity to the power of 0.7:

     

The effect of sulfur on the partitioning of Ni and other first-row transition elements between olivine and silicate melt

The effect of sulfur dissolved as sulfide (S²⁻) in silicate melts on the activity coefficients of NiO and some other oxides of divalent cations (Ca, Cr, Mn, Fe and Co) has been determined from olivine/melt partitioning experiments at 1400 °C in six melt compositions in the system CaO-MgO-Al₂O₃-SiO₂ (CMAS), and in derivatives of these compositions at 1370 °C, obtained from the six CMAS compositions by substituting Fe for Mg (FeCMAS). Amounts of S²⁻ were varied from zero to sulfide saturation, reaching 4100 μg g⁻¹ S in the most sulfur-rich silicate melt. The sulfide solubilities compare reasonably well with those predicted from the parameterization of the sulfide capacity of silicate melts at 1400 °C of O’Neill and Mavrogenes (2002), although in detail systematic deviations indicate that a more sophisticated model may improve the prediction of sulfide capacities.The results show a barely discernible effect of S²⁻ in the silicate melt on Fe, Co and Ni partition coefficients, and also surprisingly, a tiny but resolvable effect on Ca partitioning, but no detectable effect on Cr, Mn or some other lithophile incompatible elements (Sc, Ti, V, Y, Zr and Hf). Decreasing Mg# of olivine (reflecting increasing FeO in the system) has a significant influence on the partitioning of several of the divalent cations, particularly Ca and Ni. We find a remarkably systematic correlation between and the ionic radius of M²⁺, where M = Ca, Cr, Mn, Fe, Co or Ni, which is attributable to a simple relationship between size mismatch and excess free energies of mixing in Mg-rich olivine solid solutions.Neither the effect of S²⁻ nor of Mg#^ol is large enough by an order of magnitude to account for the reported variations of obtained from electron microprobe analyses of olivine/glass pairs from mid-ocean ridge basalts (MORBs). Comparing these MORB glass analyses with the Ni-MgO systematics of MORB from other studies in the literature, which were obtained using a variety of analytical techniques, shows that these electron microprobe analyses are anomalous. We suggest that the reported variation of with S content in MORB is an analytical artifact.Mass balance of melt and olivine compositions with the starting compositions shows that dissolved S²⁻ depresses the olivine liquidus of haplobasaltic silicate melts by 5.8 × 10⁻³ (±1.3 × 10⁻³) K per μg g⁻¹ of S²⁻, which is negligible in most contexts. We also present data for the partitioning of some incompatible trace elements (Sc, Ti, Y, Zr and Hf) between olivine and melt. The data for Sc and Y confirm previous results showing that and decrease with increasing SiO₂ content of the melt. Values of average 0.01 with most falling in the range 0.005-0.015. Zr and Hf are considerably more incompatible than Ti in olivine, with and about 10⁻³. The ratio / is well constrained at 0.611 ± 0.016.     

Arsenic, iron and sulfur co-diagenesis in lake sediments

Profiles of porewater pH and dissolved As, Fe, Mn, sulfate, total sulfide (ΣS^−II), total zero-valent sulfur (ΣS⁰), organic carbon and major ion concentrations, as well as those of solid As, acid-volatile sulfide (AVS), total S, Fe, Mn, Al, organic C, ²¹⁰Pb and ¹³⁷Cs were determined in the sediment of four lakes spanning a range of redox and geochemical conditions. An inverse modeling approach, based on a one-dimensional transport-reaction equation assuming steady-state, was applied to the porewater As profiles and used to constrain the net rates of reactions involving As (). The model defines depth intervals where As is either released to (positive ) or removed from (negative ) the porewaters.At two of the sites, whose bottom water were oxygenated at sampling time, a production zone ( = 12 × 10⁻¹⁸ mol cm⁻³ s⁻¹-71 × 10⁻¹⁸ mol cm⁻³ s⁻¹) is inferred a few cm below the sediment-water interface, coincident with sharp porewater As and Fe peaks that indicate an intense coupled recycling of As and Fe. This process is confirmed by solid As and Fe maxima just below the sediment surface. In these two lakes a zone of As consumption ( = −5 × 10⁻¹⁸ mol cm⁻³ s⁻¹ to −53 × 10⁻¹⁸ mol cm⁻³ s⁻¹), attributed to the slow adsorption of As to authigenic Fe oxyhydroxides, occurs just above the production zone. A second-order rate constant of 0.12 ± 0.03 cm³ mol⁻¹ s⁻¹ is estimated for this adsorption reaction.Such features in the porewater and solid profiles were absent from the two other lakes that develop a seasonally anoxic hypolimnion. Thermodynamic calculations indicate that the porewaters of the four lakes, when sulfidic (i.e., ΣS^−II ? 0.1 μM), were undersaturated with respect to all known solid As sulfides; the calculation also predicts the presence of As^V oxythioanions in the sulfidic waters, as suggested by a recent study. In the sulfidic waters, the removal of As ( = −1 × 10⁻¹⁸ mol cm⁻³ s⁻¹ to −23 × 10⁻¹⁸ mol cm⁻³ s⁻¹) consistently occurred when saturation, with respect to FeS_(s), was reached and when As^V oxythioanions were predicted to be significant components of total dissolved As. This finding has potential implications for As transport in other anoxic waters and should be tested in a wider variety of natural environments.     

Cu and Fe chalcopyrite leach activation energies and the effect of added Fe

The leaching kinetics of chalcopyrite (CuFeS₂) concentrate in sulfuric acid leach media with and without the initial addition of Fe³⁺ under carefully controlled solution conditions (E_h 750 mV SHE, pH 1) at various temperatures from 55 to 85 °C were measured. Kinetic analyses by (i) apparent rate (not surface area normalised), and rate dependence using (ii) a shrinking core model and (iii) a shrinking core model in conjunction with Fe³⁺ activity, were performed to estimate the activation energies (E_a) for Cu and Fe dissolution.The E_a values determined for Cu and Fe leaching in the absence of added Fe³⁺ are within experimental error, 80 ± 10 kJ mol⁻¹ and 84 ± 10 kJ mol⁻¹, respectively (type iii analyses E_a are quoted unless stated otherwise), and are indicative of a chemical reaction controlled process. On addition of Fe³⁺ the initial Cu leach rate (up to 10 h) was increased and Cu was released to solution preferentially over Fe, with the E_a value of 21 ± 5 kJ mol⁻¹ (type ii analysis) suggestive of a transport controlled rate determining process. However, the rate of leaching rapidly decreased until it was consistently slower than for the equivalent leaches where Fe³⁺ was not added. The resulting E_a value for this leach regime of 83 ± 10 kJ mol⁻¹ is within experimental error of that determined in the absence of added Fe³⁺. In contrast to Cu release, Fe release to solution was consistent with a chemical reaction controlled leach rate throughout. The Fe release E_a of 76 ± 10 kJ mol⁻¹ is also within experimental error of that determined in the absence of added Fe³⁺. Where type (ii) and (iii) analyses were both successfully carried out (in all cases except for Cu leaching with added Fe³⁺, <10 h) the E_a derived are within experimental error. However, the type (iii) analyses of the leaches in the presence of added Fe³⁺ (>10 h), as compared to in the absence of added Fe³⁺, returned a considerably smaller pre-exponential factors for both Cu and Fe leach analyses commensurate with the considerably slower leach rate, suggestive of a more applicable kinetic analysis.XPS examination of leached chalcopyrite showed that the surface concentration of polysulfide and sulfate was significantly increased when Fe³⁺ was added to the leach liquor. Complementary SEM analysis revealed the surface features of chalcopyrite, most likely due to the nature of the polysulfide formed, are subtly different with greater surface roughness upon leaching in the absence of added Fe³⁺ as compared to a continuous smooth surface layer formed in the presence of added Fe³⁺. These observations suggest that the effect of Fe³⁺ addition on the rate of leaching is not due to the change in the chemical reaction controlled mechanism but due to a change in the available surface area for reaction.     

Copper partitioning in a melt-vapor-brine-magnetite-pyrrhotite assemblage

The effect of sulfur on the partitioning of Cu in a melt-vapor-brine ± magnetite ± pyrrhotite assemblage has been quantified at 800 °C, 140 MPa, f_O2 = nickel-nickel oxide (NNO), logf_S2=-3.0 (i.e., on the magnetite-pyrrhotite curve at NNO), logf_H2S=-1.3 and logf_SO2=-1. All experiments were vapor + brine saturated. Vapor and brine fluid inclusions were trapped in silicate glass and self-healed quartz fractures. Vapor and brine are dominated by NaCl, KCl and HCl in the S-free runs and NaCl, KCl and FeCl₂ in S-bearing runs. Pyrrhotite served as the source of sulfur in S-bearing experiments. The composition of fluid inclusions, glass and crystals were quantified by laser-ablation inductively coupled plasma mass spectrometry. Major element, chlorine and sulfur concentrations in glass were quantified by using electron probe microanalysis. Calculated Nernst-type partition coefficients (±2σ) for Cu between melt-vapor, melt-brine and vapor-brine are , , and , respectively, in the S-free system. The partition coefficients (±2σ) for Cu between melt-vapor, melt-brine and vapor-brine are , , and , respectively, in the S-bearing system. Apparent equilibrium constants (±1σ) describing Cu and Na exchange between vapor and melt and brine and melt were also calculated. The values of are 34 ± 21 and 128 ± 29 in the S-free and S-bearing runs, respectively. The values of are 33 ± 22 and60 ± 5 in the S-free and S-bearing runs, respectively. The data presented here indicate that the presence of sulfur increases the mass transfer of Cu into vapor from silicate melt. Further, the nearly threefold increase in suggests that Cu may be transported as both a chloride and sulfide complex in magmatic vapor, in agreement with hypotheses based on data from natural systems. Most significantly, the data demonstrate that the presence of sulfur enhances the partitioning of Cu from melt into magmatic volatile phases.     

The kinetics of iodide oxidation by the manganese oxide mineral birnessite

The kinetics of iodide (I⁻) and molecular iodine (I₂) oxidation by the manganese oxide mineral birnessite (δ-MnO₂) was investigated over the pH range 4.5-6.25. I⁻ oxidation to iodate proceeded as a two-step reaction through an I₂ intermediate. The rate of the reaction varied with both pH and birnessite concentration, with faster oxidation occurring at lower pH and higher birnessite concentration. The disappearance of I⁻ from solution was first order with respect to I⁻ concentration, pH, and birnessite concentration, such that −d[I⁻]/dt = k[I⁻][H⁺][MnO₂], where k, the third order rate constant, is equal to 1.08 ± 0.06 × 10⁷ M⁻² h⁻¹. The data are consistent with the formation of an inner sphere I⁻ surface complex as the first step of the reaction, and the adsorption of I⁻ exhibited significant pH dependence. Both I₂, and to a lesser extent, sorbed to birnessite. The results indicate that iodine transport in mildly acidic groundwater systems may not be conservative. Because of the higher adsorption of the oxidized I species I₂ and , as well as the biophilic nature of I₂, redox transformations of iodine must be taken into account when predicting I transport in aquifers and watersheds.     

Dissolution of arsenopyrite (FeAsS) and galena (PbS) in the presence of desferrioxamine-B at pH 5

Microorganisms and higher plants produce biogenic ligands, such as siderophores, to mobilize Fe that otherwise would be unavailable. In this paper, we study the stability of arsenopyrite (FeAsS), one of the most important natural sources of arsenic on Earth, in the presence of desferrioxamine (DFO-B), a common siderophore ligand, at pH 5. Arsenopyrite specimens from mines in Panasqueira, Portugal (100-149 μm) that contained incrustations of Pb, corresponding to elemental Pb as determined by scanning electron microscopy-electron diffraction spectroscopy (SEM-EDX), were used for this study. Batch dissolution experiments of arsenopyrite (1 g L⁻¹) in the presence of 200 μM DFO-B at initial pH (pH₀) 5 were conducted for 110 h. In the presence of DFO-B, release of Fe, As, and Pb showed positive trends with time; less dependency was observed for the release of Fe, As, and Pb in the presence of only water under similar experimental conditions. Detected concentrations of soluble Fe, As, and Pb in suspensions containing only water were found to be ca. 0.09 ± 0.004, 0.15 ± 0.003, and 0.01 ± 0.01 ppm, respectively. In contrast, concentrations of soluble Fe, As, and Pb in suspensions containing DFO-B were found to be 0.4 ± 0.006, 0.27 ± 0.009, and 0.14 ± 0.005 ppm, respectively. Notably, the effectiveness of DFO-B for releasing Pb was ca. 10 times higher than that for releasing Fe. These results cannot be accounted for by thermodynamic considerations, namely, by size-to-charge ratio considerations of metal complexation by DFO-B. As determined by SEM-EDX, elemental sample enrichment analysis supports the idea that the Fe-S subunit bond energy is limiting for Fe release. Likely, the mechanism(s) of dissolution for Pb incrustations is independent and occurs concurrently to that for Fe and As. Our results show that dissolution of arsenopyrite leads to precipitation of elemental sulfur, and is consistent with a non-enzymatic mineral dissolution pathway. Finally, speciation analyses for As indicate variability in the As(III)/As(V) ratio with time, regardless of the presence of DFO-B or water. At reaction times <30 h, As(V) concentrations were found to be 50-70%, regardless of the presence of DFO-B. These results are interpreted to indicate that transformations of As are not imposed by ligand-mediated mechanisms. Experiments were also conducted to study the dissolution behavior of galena (PbS) in the presence of 200 μM at pH₀ 5. Results show that, unlike arsenopyrite, the dissolution behavior of galena shows coupled increases in pH with decreases in metal solubility at t > 80 h. Oxidative dissolution mechanisms conveying sulfur oxidation bring about the production of {H⁺}. However, dissolution data trends for arsenopyrite and galena indicate {H⁺} consumption. It is plausible that the formation of Pb species is dependent on {H⁺} and {OH⁻}, namely, stable surface hydroxyl complexes of the form (pH₅₀ 5.8) and for pH values 5.8 or above.     

An integrated sulfur isotope model for Namibian shelf sediments

In this study the sulfur cycle in the organic-rich mud belt underlying the highly productive upwelling waters of the Namibian shelf is quantified using a 1D reaction-transport model. The model calculates vertical concentration and reaction rate profiles in the top 500 cm of sediment which are compared to a comprehensive dataset which includes carbon, sulfur, nitrogen and iron compounds as well as sulfate reduction (SR) rates and stable sulfur isotopes (³²S, ³⁴S). The sulfur dynamics in the well-mixed surface sediments are strongly influenced by the activity of the large sulfur bacteria Thiomargaritanamibiensis which oxidize sulfide (H₂S) to sulfate () using sea water nitrate () as the terminal electron acceptor. Microbial sulfide oxidation (SOx) is highly efficient, and the model predicts intense cycling between and H₂S driven by coupled SR and SOx at rates exceeding 6.0 mol S m⁻² y⁻¹. More than 96% of the SR is supported by SOx, and only 2-3% of the pool diffuses directly into the sediment from the sea water. A fraction of the produced by Thiomargarita is drawn down deeper into the sediment where it is used to oxidize methane anaerobically, thus preventing high methane concentrations close to the sediment surface. Only a small fraction of total H₂S production is trapped as sedimentary sulfide, mainly pyrite (FeS₂) and organic sulfur (S_org) (∼0.3 wt.%), with a sulfur burial efficiency which is amongst the lowest values reported for marine sediments (<1%). Yet, despite intense SR, FeS₂ and S_org show an isotope composition of ∼5 ‰ at 500 cm depth. These heavy values were simulated by assuming that a fraction of the solid phase sulfur exchanges isotopes with the dissolved sulfide pool. An enrichment in H₂S of ³⁴S towards the sediment-water interface suggests that Thiomargarita preferentially remove H₂³²S from the pore water. A fractionation of 20-30‰ was estimated for SOx (ε_SOx) with the model, along with a maximum fractionation for SR (ε_SR-max) of 100‰. These values are far higher than previous laboratory-based estimates for these processes. Mass balance calculations indicate negligible disproportionation of autochthonous elemental sulfur; an explanation routinely cited in the literature to account for the large fractionations in SR. Instead, the model indicates that repeated multi-stepped sulfide oxidation and intracellular disproportionation by Thiomargarita could, in principle, allow the measured isotope data to be simulated using much lower fractionations for ε_SOx (5‰) and ε_SR (78‰).     

A first principles study of the oxidation energetics and kinetics of realgar

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.     

Zinc complexation in aqueous sulfide solutions: Determination of the stoichiometry and stability of complexes via ZnS(cr) solubility measurements at 100 °C and 150 bars

The solubility of ZnS_(cr) was measured at 100 °C, 150 bars in sulfide solutions as a function of sulfur concentration (m(S_total) = 0.02-0.15) and acidity (pH_t = 2-11). The experiments were conducted using a Ti flow-through hydrothermal reactor enabling the sampling of large volumes of solutions at experimental conditions, with the subsequent concentration and determination of trace quantities of Zn. Prior to the experiments, a long-term in situ conditioning of the solid phase was performed in order to attain the reproducible Zn concentrations (i.e. solubilities). The ZnS_(cr) solubility product was monitored in the course of the experiment. The following species were found to account for Zn speciation in solution: Zn²⁺ (pH_t < 3), (pH_t 3-4.5), (pH_t 5-8), and ZnS(HS)⁻ (pH_t > 8) (pH_t predominance regions are given for m(S_total) = 0.1). Solubility data collected in this study at pH_t > 3 were combined with the ZnS_(cr) solubility product determined at lower pH to yield the following equilibrium constants (t = 100 °C, P = 150 bars):

     

Thermodynamic properties of anhydrous smectite MX-80, illite IMt-2 and mixed-layer illite-smectite ISCz-1 as determined by calorimetric methods. Part I: Heat capacities, heat contents and entropies

The heat capacities of the anhydrous international reference clay minerals, smectite MX-80, illite IMt-2 and mixed-layer illite-smectite ISCz-1, were measured by low temperature adiabatic calorimetry and differential scanning calorimetry, from 6 to 520 K (at 1 bar). The samples were chemically purified and Na-saturated. Dehydrated clay fractions <2 μm were studied. The structural formulae of the corresponding clay minerals, obtained after subtracting the remaining impurities, are K_0.026Na_0.435Ca_0.010(Si_3.612Al_0.388) (Al_1.593Mg_0.228Ti_0.011)O₁₀(OH)₂ for smectite MX-80, K_0.762Na_0.044(Si_3.387Al_0.613) (Al_1.427Mg_0.241O₁₀(OH)₂ for illite IMt-2 and K_0.530Na_0.135(Si_3.565Al_0.435)(Al_1.709Mg_0.218Ti_0.005)O₁₀(OH)₂for mixed-layer ISCz-1. From the heat capacity values, we determined the molar entropies, standard entropies of formation and heat contents of these minerals. The following values were obtained at 298.15 K and 1 bar:

	(J mol−1 K−1)	S0 (J mol−1 K−1)
Smectite MX-80	326.13 ± 0.10	280.56 ± 0.16
Illite IMt-2	328.21 ± 0.10	295.05 ± 0.17
Mixed-layer ISCz-1	320.79 ± 0.10	281.62 ± 0.15

Full-size table

     

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 activity coefficients of Fe(III) hydroxide complexes in NaCl and NaClO4 solutions

The osmotic coefficients of FeCl₃ at 25 °C from 0.15 to 1.7 m [Rumyantsev et al., Z. Phys. Chem., 218, 1089-1127, 2004] have been used to determine the Pitzer parameters (β⁽⁰⁾, β⁽¹⁾ and C^?) for FeCl₃. Since the differences in the Pitzer coefficients of rare earths in NaCl and NaClO₄ are small, the values of Fe(ClO₄)₃ have been estimated using the differences between La(ClO₄)₃ and LaCl₃. The Pitzer coefficients for FeCl₃ combined with enthalpy and heat capacity data for the rare earths can be used to estimate the activity coefficients of Fe³⁺ in NaCl over a wide range of temperatures (0 to 50 °C) and ionic strength (0 to 6 m).The activity coefficients of Fe³⁺ in NaCl and NaClO₄ solutions have been used to determine the activity coefficients of Fe(OH)²⁺ in these solutions from the measured first hydrolysis constants of Fe³⁺ [Byrne et al., Mar. Chem., 97, 34-48, 2005]. The activity coefficients of , Fe(OH)₃ and from 0 to 50 °C have also been determined from the solubility measurements of Fe(III) in NaCl solutions [Liu and Millero, Geochim. Cosmochim Acta, 63, 3487-3497, 1999]. These activity coefficients have been fitted to the Pitzer equations. These results can be used to estimate the speciation of Fe(III) with OH⁻ in natural waters with high concentrations of NaCl from 0 to 50 °C.     

Sphalerite oxidation pathways detected by oxygen and sulfur isotope studies

Sphalerite oxidation is a common process under acid-mine drainage (AMD) conditions and results in the release of , Zn and potentially toxic trace metals, which can pollute rivers and oceans. However, there are only a few studies on the mechanisms of aerobic sphalerite oxidation. Oxygen and S isotope investigations of the produced may contribute to the understanding of sphalerite oxidation mechanisms so helping to interpret field data from AMD sites. Therefore, batch oxidation experiments with an Fe-rich sphalerite were performed under aerobic abiotic conditions at different initial pH values (2 and 6) for different lengths of time (2–100 days). The O and S isotope composition of the produced indicated changing oxidation pathways during the experiments. During the first 20 days of the experiments at both initial pH values, molecular O₂ was the exclusive O source of . Furthermore, the lack of S isotope enrichment processes between and sphalerite indicated that O₂ was the electron acceptor from sphalerite S. As the oxidation proceeded, a sufficient amount of released Fe(II) was oxidized to Fe(III) by O₂. Therefore, electrons could be transferred from sphalerite S sites to adsorbed hydrous Fe(III) and O from the hydration sphere of Fe was incorporated into the produced as indicated by decreasing δ¹⁸O_SO4 values which became more similar to the δ¹⁸O_H2O values. The enrichment of ³²S in relative to the sphalerite may also result from sphalerite oxidation by Fe(III).The incorporation of O₂ into during the oxidation of sphalerite was associated with an O isotope enrichment factor ε_SO4–O2 of ca. −22‰. The O isotope enrichment factor ε_SO4–H2O was determined to be ?4.1‰. A comparison with O and S studies of other sulfides suggests that there is no general oxidation mechanism for acid-soluble sulfides.