Polysulfide Pathway Research Articles

Micrometric pyrite catalyzes abiotic sulfidogenesis from elemental sulfur and hydrogen

Hydrogen sulfide (H2S) in environments with temperatures below 100 °C is generally assumed to be of microbial origin, while abiotic H2S production is typically restricted to higher temperatures (T). In this study, we report an abiotic process for sulfidogenesis through the reduction of elemental sulfur (S0) by hydrogen (H2), mediated by pyrite (FeS2). The process was investigated in detail at pH 4 and 80 °C, but experimental conditions ranged between 40 and 80 °C and pH 4–6. The experiments were conducted with H2 as reducing molecule, and µm-sized spherical (but not framboidal) pyrite particles that formed in situ from the H2S, S0 and Fe2+ present in the experiments. Fe monosulfides, likely mackinawite, were identified as potential pyrite precursors. The absence of H2 production in controls, combined with geochemical modelling, suggests that pyrite formation occurred through the polysulfide pathway, which is unexpected under acidic conditions. Most spherical aggregates of authigenic pyrite were composed of nanometric, acicular crystals oriented in diverse directions, displaying varying degrees of organization. Although it was initially hypothesized that the catalytic properties were related to the surface structure, commercially sourced, milled pyrite particles (< 50 μm) mediated H2S production at comparable rates. This suggests that the catalytic properties of pyrite depend on particle size rather than surface structure, requiring pyrite surfaces to act as electron shuttles between S0 and H2.

A methodological framework to study the behavior and kinetic influence of V, Mn, Co, Ni, Cu, Zn, As, Se and Mo during pyrite formation via the polysulfide pathway at ambient temperature

Pyritization links the biogeochemical cycles of Fe, S and C to those of trace elements (TE) such as As, Ni, Cu, Co, Mn, Zn, Se and Mo in reducing environments. The scavenging modes of such impurities in pyrite are parameters of importance for evaluating to which extent pyrite can immobilize TE under diverse (sub)surface conditions. Furthermore, determining the fate of TE during pyritization is a prerequisite to evaluate the ability of pyrite to record the chemical signature of surrounding waters at the time of precipitation. Here, we provide a methodological framework to study the incorporation of TE during a reaction sequence leading to pyrite formation via the polysulfide pathway. Laboratory syntheses were conducted by reacting a FeS precursor with elemental sulfur – both readily obtained by reducing FeCl3 with Na2S in solution under strict anoxia – at ambient temperature and in the presence of aqueous V, Mn, Co, Ni, Cu, Zn, As, Se and Mo in a TE/Fe molar ratio of 0.5 mol%. Within this simplified framework, our experiments aimed at reproducing the early formation of pyrite in water column aggregates/bottom sediments before the late diagenetic aging phases. Solids and liquids were sampled over time during the reaction sequence leading to pyrite formation, up to 3100 h, and further analyzed by ICP-OES, XRD, XRF and STEM-EDXS to obtain information on (1) the kinetics of pyrite formation and (2) the extent of TE incorporation in the solid phase, during and after the complete conversion of FeS to FeS2. Pyrite formation kinetic is observed to be strongly influenced by the identity of the added TE, with the delay before nucleation decreasing in the order Ni >> Mn, Co > Cu, Zn, Se > Control, V >> As, Mo. During the full reaction sequence, i.e. FeS plus S(0) conversion to FeS2, Mn remains in the liquid phase, Ni, Co, V, Cu, and Se quickly precipitate and remain sequestered overtime, Zn and Mo precipitate and are further released into solution, and As is partially incorporated in the solid. Elemental mapping at the microscale using STEM-EDXS show that, except for Mn, the TE are associated with pyrite. Moreover, while TE are homogeneously distributed at the particle scale for Co, Cu, Zn, As, and Se, Ni-pyrite exhibited an enriched core. The solid-liquid partition coefficients of each TE during our pyritization experiments in our simplified model system are calculated and ranks as follows: Cu ≥ Ni ≥ Co > Se > V > Mo >> As > Zn >> Mn. These partitions are discussed in the light of values reported in the literature from natural settings. The important differences observed exemplify the complexity to predict TE pyrite-water partition in sedimentary environments, in particular when considering additional multifactorial processes such as the local biogeochemistry of micro-environments, as well as the partial sequestration of TE by non-sulfide minerals such as authigenic clays and carbonates.

Mineralogy-dependent sulfide oxidation via polysulfide and thiosulfate pathways during weathering of mixed-sulfide bearing mine waste rock

Tracking the S oxidation pathway in sulfide-bearing mine waste-rock piles is complicated by variations in water content, O2 and Fe3+ concentrations, microbial diversity, mineralogy, the occurrence of a variety of S species associated with incomplete oxidation, and nonlinear coupling between physicochemical processes. Synchrotron-based S K-edge X-ray absorption near edge structure (XANES) spectroscopy facilitates the identification and quantification of S species (i.e., S2−, S22−, S8, S2O32−, and SO42−) in a weathered mine waste-rock pile containing pyrite, sphalerite, galena, pyrrhotite, and chalcopyrite. Mineralogy-dependent polysulfide and thiosulfate pathways both affect the S oxidation within the waste-rock pile. Currently, the polysulfide pathway, with S8 as the intermediate S species, generates high concentrations of dissolved metals (from the rapid oxidation of monosulfides including sphalerite, galena, pyrrhotite, and chalcopyrite). As these monosulfides are depleted, the thiosulfate pathway of pyrite oxidation, including intermediate oxidation products (i.e., S8 and S2O32−), will become the dominant oxidation pathway, resulting in the potential for generation of additional acidic drainage. This information highlights the importance of identifying the sulfide-mineral oxidation reaction pathways when attempting to estimate acid generation potential, and when developing remediation strategies for the storage and management of sulfide-bearing waste rock.

Influence of trace level As or Ni on pyrite formation kinetics at low temperature

Pyrite formation at low temperature during early diagenesis in (sub-)surface sediments is an essential step of Fe and S biogeochemical cycles and the presence of this ubiquitous mineral of surface environments is often used as an indicator of paleo-redox conditions. Pathways of pyrite formation are usually discussed in environmental settings by involving a variety of nanosized Fe-S mineralogical precursors as a function of the local geochemical conditions. However, the influence of trace element impurities such as Ni and As in the solution at the time of pyrite formation has been poorly studied, whereas specific chemical signatures of trace elements are commonly observed in sedimentary pyrites. A better understanding of the impact of Ni and As incorporation at trace levels on pyrite formation is essential to help refining the use of these elements as paleo-redox indicators and to evaluate the role of pyrite as a sink regulating the biogeochemical cycle of potentially toxic trace elements. In this study, we have performed syntheses of pyrite at low temperature via the polysulfide pathway using aqueous Fe(III) and H2S in the presence of trace amounts of Ni(II) (0.001 mol%Fe) and As(III) (0.001 mol%Fe). Analysis of the solids collected at different time steps over the course of the experiments using X-Ray absorption spectroscopy at both the Fe and S K-edges shows that pyrite starts to precipitate within 5 days in presence of Ni(II) and within 32 days in presence of As(III), while the control experiment showed an intermediate precipitation rate of 14 days. Shell-by-shell analysis of Fe K-edge EXAFS data shows that the initial mineralogical precursors are the same in all the experiments and correspond to poorly-crystalline FeS (3.0 ± 0.1 Fe-S@2.25 Å; 1.7 ± 0.2 Fe-Fe@2.67 Å). In addition, XANES qualitative analysis suggests the incorporation of small amounts of Fe(III) within these FeS precursors. Synchrotron-based XRD and WAXS-PDF analysis of the starting solids show that in addition to S(0), the FeS precursors correspond to a continuum of FeS particles that ranges from tetragonal nanocrystalline FeS (a = 3.70(2) Å, c = 5.24(7) Å, MCDab = 41 ± 4 Å MCDc = 21 ± 2 Å) to cluster-type FeS (MCDabc < 8.4 ± 4.3 Å). We propose that Ni(II) and As(III) have a different type of interaction with these FeS precursors, resulting respectively in an increase and a decrease in the rate of pyrite nucleation. While Ni(II) would incorporate within the structure of the FeS precursors, As would interact with (poly)sulfides in solution to form thio-As, possibly binding or precipitating onto FeS surfaces and thus slowing FeS transformation to FeS2. Given that both Ni and As were introduced at trace levels in our experiments, these results suggest that the occurrence of trace amounts of impurities could have a strong influence on pyrite precipitation kinetics in natural settings such as pore-scale microenvironments. In addition to emphasizing the importance of trace elements such as Ni or As on the persistence of mobile colloidal FeS species in anoxic conditions, the results of the present study also point to the importance of considering the actual nature of the impurities when using pyrite composition for ancient environments and past climates reconstruction.

A novel framework for interpreting pyrite-based Fe isotope records of the past

Abstract The variation in the iron isotopic composition (δ56Fe) of sedimentary pyrite is often interpreted to reflect the degree of Fe redox cycling in modern and ancient environments. However, the degree to which precipitation pathways, isotopic exchange, and precipitation rates can affect the isotopic fractionation associated with pyrite precipitation from aqueous Fe(II) (Fe(II)aq) is poorly understood. In this study, pyrite is precipitated at 80 °C in batch reactors through the H2S and polysulfide pathways, in which the precipitation rates and the concurrent formation of a greigite (Fe3S4) phase is modulated by the amount of initially added elemental sulfur and aqueous molybdenum. Our results indicate an average apparent isotopic fractionation (δ56Fepyrite - δ56FeFeSx, where FeSx includes FeS, Fe(II)aq, and greigite) of −0.51 ± 0.22‰ throughout the experiments irrespective of precipitation pathways and greigite formation. Early-stage precipitation is associated with ∼0.3‰ larger isotopic fractionation than late-stage precipitation, possibly indicating either a rate-dependent kinetic isotope effect (KIE) or a different isotopic fractionation factor for early-stage pyrite nucleation compared to later-stage growth. Overall, the magnitude of the apparent isotopic fractionation is significantly smaller than the

Sulfidation of Magnetite Nanoparticles – Following the Polysulfide Pathway

The in‐situ sulfidation of sub‐10 nm sized Fe3O4 and partial‐substituted CoxFe3–xO4 (x = 0.25–0.75) nanoparticles by hot‐injection of sulfur dissolved in oleylamine (OA) is strongly time‐ and temperature dependent. Fe3O4 nanoparticles are completely converted within 5 minutes into Fe3S4 nanoparticles with preservation of the spinel structure at 250 °C, whereas higher reaction temperatures and prolonged reaction times result in additional sulfur uptake and formation of pyrite FeS2 nanoparticles. The formation of the cubic (Co,Fe)S2 phase was observed in sulfidation reactions of CoxFe3–xO4, even with low Co substitution levels (x = 0.25). The chemical composition, morphology, and phase purity of the resulting nanoparticles were investigated by EDX, XRD, TEM, and XPS.

Function of microorganism and reaction pathway for carrollite dissolution during bioleaching

The function of microorganism and dissolution reaction pathway of carrollite in the bioleaching process were investigated. The results showed that both indirect and contact mechanisms influenced the leaching process. The dissolution of carrollite was significantly accelerated when bacteria were adsorbed on the mineral surface, indicating that the contact mechanism significantly affected the dissolution of carrollite. During bioleaching, the sequence of oxidation state of the sulfur moiety of carrollite was as follows: S−2→S0→S+4→S+6. Elemental sulfur precipitated on the mineral surface, indicating that the dissolution of carrollite occurred via the polysulfide pathway. The surface of carrollite was selectively corroded by bacteria, and oxidation pits with different sizes were observed at various sites. Elemental sulfur, sulfate and sulfite were present on the surface of carrollite during the leaching process, and may have formed a passivation layer on mineral surface.

Iron monosulfide accumulation and pyrite formation in eutrophic estuarine sediments

This study investigates iron (Fe) and sulfur (S) cycling in sediments from the eutrophic Peel–Harvey Estuary in Western Australia, which is subject to localized accumulation of strongly reducing, organic- and sulfide-rich sediments. Sedimentary iron was mostly present in highly reactive form (on average 73% of total Fe) and showed extensive sulfidization even in surface sediments, despite being overlain by a well-mixed oxygenated water column. This indicates that, under eutrophic marine conditions, Fe sulfidization may be driven by reductive processes in the sediment without requiring oxygen depletion in the overlying waters. Strong enrichments in iron monosulfide (FeS>300μmolg−1) were observed in fine-grained sediment intervals up to 45cm depth. This metastable Fe sulfide is commonly restricted to thin subsurface sediment intervals, below which pyrite (FeS2) dominates. Our findings suggest inhibition of the dissolution–precipitation processes that replace FeS with FeS2 in sediments. Rates of pyrite formation based on the FeS2 profiles were much lower than those predicted by applying commonly used kinetic equations for pyrite formation. Dissolved H2S was present at millimolar levels throughout the investigated sediment profiles. This may indicate that (i) pyrite formation via reaction between dissolved Fe (including Fe clusters) and H2S was limited by low availability of dissolved Fe or (ii) reaction kinetics of pyrite formation via the H2S pathway may be relatively slow in natural reducing sediments. We propose that rapid burial of the FeS under anoxic conditions in these organic-rich reducing sediments minimizes the potential for pyrite formation, possibly by preventing dissolution of FeS or by limiting the availability of oxidized sulfur species that are required for pyrite formation via the polysulfide pathway.

Comparison of pyrite (FeS2) synthesis mechanisms to reproduce natural FeS2 nanoparticles found at hydrothermal vents

Here we report the synthesis of pyrite in micrometer and nanometer sizes and with varied morphology. The purpose of these syntheses was to mimic natural pyrite nanoparticles recently found at hydrothermal vents, and to better understand the formation processes of these nano and micrometer sized particles. The duration of synthesis, surfactant used, and chemical mechanism of pyrite formation were all found to have an effect on resultant size and morphology. Yields of pyrite were sometimes low as our goal was to stop growth quickly after nucleation, in order to mimic hot vent fluids (350°C) emanating to cold ocean bottom waters (2°C). Natural nanoparticles forming under these conditions have currently been identified at diverse hydrothermal vents including Lau Basin, the Mid Atlantic Ridge (MAR), and the East Pacific Rise (EPR). These natural nanoparticles were variable in size and morphology, with aggregates of 50–350nm diameter being the most prevalent. Syntheses occurring via the H2S pathway mimicked natural nanoparticles more closely than syntheses via the polysulfide pathway. One synthesis reported is suggestive of a pyrite/pyrrhotite equilibrium, indicating that we have succeeded in mimicking redox conditions similar to those that occur at certain hydrothermal vents.

Progress in bioleaching: fundamentals and mechanisms of bacterial metal sulfide oxidation—part A

Bioleaching of metal sulfides is performed by a diverse group of microorganisms. The dissolution chemistry of metal sulfides follows two pathways, which are determined by the mineralogy and the acid solubility of the metal sulfides: the thiosulfate and the polysulfide pathways. Bacterial cells can effect this metal sulfide dissolution via iron(II) ion and sulfur compound oxidation. Thereby, iron(III) ions and protons, the metal sulfide-attacking agents, are available. Cells can be active either in planktonic state or in forming biofilms on the mineral surface; however, the latter is much more efficient in terms of bioleaching kinetics. In the case of Acidithiobacillus ferrooxidans, bacterial exopolymers contain iron(III) ions, each complexed by two uronic acid residues. The resulting positive charge allows an electrostatic attachment to the negatively charged pyrite. Thus, the first function of complexed iron(III) ions is the mediation of cell attachment, while their second function is oxidative dissolution of the metal sulfide, similar to the role of free iron(III) ions in non-contact leaching. In both cases, the electrons extracted from the metal sulfide reduce molecular oxygen via a redox chain forming a supercomplex spanning the periplasmic space and connecting both outer and inner membranes. In this review, we summarize some recent discoveries relevant to leaching bacteria which contribute to a better understanding of these fascinating microorganisms. These include surface science, biochemistry of iron and sulfur metabolism, anaerobic metabolism, and biofilm formation. The study of microbial interactions among multispecies leaching consortia, including cell-to-cell communication mechanisms, must be considered in order to reveal more insights into the biology of bioleaching microorganisms and their potential biotechnological use.

Formation and burial of pyrite and organic sulfur in mud sediments of the East China Sea inner shelf: Constraints from solid-phase sulfur speciation and stable sulfur isotope

Solid-phase sulfur speciation and stable sulfur isotopic compositions are used to elucidate the formation and burial of pyrite-sulfur (Spy) and organic sulfur (OS) at three selected sites in mud sediments of the East China Sea (ECS) inner shelf, and to infer potential factors influencing the preservation of Spy and OS in the sediments. Our results in combination with previous studies show that the overall reactivity of sedimentary organic matter (OM) is low, while OM at the site impacted by frequent algal-bloom events displays somewhat enhanced reactivity. We observed characteristically low contents of acid volatile sulfide (AVS) and Spy in the sediments, which can be attributed to low sulfate reduction rate due to high redox potential together with limited availability of labile OM. Several geochemical features, for example, persistent occurrence of S0, good coupling among the profiles of AVS, S0 and Spy, and large 34Spy depletion, all suggest that the polysulfide pathway and disproportionation are likely involved in the pyrite formation. Organic sulfur amounts in the sediments are at the lower end of OS contents reported in many other marine sediments around the world. The sources of OS are both biosynthetic and diagenetic, with the biosynthetic OS being the major share (59–73%). In one site studied (C702), enhanced accumulation of OS within the upper layers (14cm) is believed to be associated with frequent algal-bloom events. Net burial fluxes of Spy and OS in the three sites studied range from 0.27 to 0.82mmol/m2/d and from 0.22 to 0.74mmol/m2/d, respectively. Sedimentation rate and algal-bloom events are two important factors influencing the spatial variability of Spy and OS burial fluxes in the whole shelf.

FISH Analysis of Bacterial Attachment to Copper Sulfides in Bioleaching Processes

Bioleaching is the biological conversion of an insoluble metal compound into a water soluble form. In this process metal sulfides are oxidized to metal ions and sulfate by acidophilic microorganisms capable of oxidizing Fe2+ and/or sulfur-compounds. The metal solubilization from sulfide minerals is a chemical process which requires Fe3+ reduction. It is an environmentally friendly technique and an economical method for recovering metals that requires low investment and operation costs. In this work we studied the bioleaching of two kinds of acid-soluble copper sulfides, one easily leached by mesophilic bacteria (covellite), and the other one refractory to their activity (chalcopyrite), in acidic media with or without Fe2+ ions. We studied attached and planktonic populations of autotrophic bacteria, such as Acidithiobacillus ferrooxidans, Acidithiobacillus thiooxidans and Leptospirillum ferrooxidans in pure or mixed cultures. The influence of a heterotrophic microorganism, Acidiphilium cryptum, was also studied. Attachment was evaluated with fluorescence staining and FISH using four specific probes. L. ferrooxidans showed highest initial attachment in all cases. The presence of Ap. cryptum increased the cell attachment compared with the autotrophic pure cultures. It was possible to correlate experimental data with a mechanism of bacterial-metal sulfide oxidation, the polysulfide pathway for acid- soluble metal sulfides.

Transformation of Reactive Iron Minerals in a Permeable Reactive Barrier (Biowall) Used to Treat TCE in Groundwater

Iron and sulfur reducing conditions generally develop in permeable reactive barrier systems (PRB) constructed to treat contaminated groundwater. These conditions allow formation of FeS mineral phases. FeS readily degrades TCE, but a transformation of FeS to FeS2 could dramatically slow the rate of TCE degradation in the PRB. This study uses acid volatile sulfide (AVS) and chromium reducible sulfur (CRS) as probes for FeS and FeS2 to investigate iron sulfide formation and transformation in a column study and PRB field study dealing with TCE degradation. Solid phase iron speciation shows that most of the iron is reduced and sulfur partitioning measurements show that AVS and CRS coexist in all samples, with the conversion of AVS to CRS being most significant in locations with potential oxidants available. In the column study, 54% of FeS was transformed to FeS2 after 2.4 years. In the field scale PRB, 43% was transformed after 5.2 years. Microscopy reveals FeS, Fe3S4 and FeS2 formation in the column system; however, only pyrite formation was confirmed byX-ray diffraction. The polysulfide pathway is most likely the primary mechanism of FeS transformation in the system, with S0 as an intermediate species formed through H2S oxidation.

Trace element diagenesis in pyrite-rich sediments of the Achterwasser lagoon, SW Baltic Sea

The early diagenesis of trace elements (V, Cr, Co, Cu, Zn, As, Cd, Ba, U) in anoxic sediments of the Achterwasser, a shallow lagoon in the non-tidal Oder estuary in the Baltic Sea, was investigated in the context of pyrite formation. The dissolved major redox parameters show a two-tier distribution with transient signals in the occasionally re-suspended fluid mud layer (FM) and a permanently established diagenetic sequence in the sediment below. Intense microbial respiration leads to rapid depletion of O 2 within the uppermost mm of the FM. The reduction zones of Mn, Fe and sulfate overlap in the FM and in the permanently anoxic sediment section which appears to be a typical feature of estuarine sediments, under low-sulfate conditions. Degrees of pyritization (DOP) range from 50% in the FM to remarkably high values > 90% at 50 cm depth. Pyrite formation at the sediment surface is attributed to the reaction of Fe-monosulfides with intermediate sulfur species via the polysulfide pathway. By contrast, intense pyritization in the permanently anoxic sediment below is attributed to mineral growth via adsorption of aqueous Fe-sulfide complexes onto pyrite crystals which had originally formed in the surface layer. The studied trace elements show differential behavior patterns which are closely coupled to the diagenetic processes described above: (i) Zn, Cu and Cd are liberated from organic matter in the thin oxic layer of the sediment and diffuse both upwards across the sediment/water boundary and downwards to be trapped as monosulfides, (ii) V, Cr, Co and As are released during reductive dissolution of Mn- and Fe-oxyhydroxides, (iii) U removal from pore water occurs concomitantly to Fe reduction in the FM and is attributed to reduction of U(VI) to U(IV), (iv) the Ba distribution is controlled by reductive dissolution of authigenic barite in the sulfate reduction zone coupled with upward diffusion and re-precipitation. The incorporation of trace elements into pyrite is most intense for Co, Mn and As, intermediate for Cu and Cr and little to negligible for U, Zn, Cd, V and Ba. The observed trend is largely in agreement with previous studies and may be explained with differing rates for ligand exchange. Slow and fast ligand exchange and thus precipitation kinetics are also displayed by downcore increasing (Mn, Cr, Co and As) or constantly low (Zn, Cu, Cd) pore water concentrations. The downward increasing degrees of trace metal pyritization (DTMP) for Co, Cu, Zn and As are, in analogy to pyrite growth, assigned to adsorption of sulfide complexes or As oxyanions onto preexisting pyrite minerals.

Combined Test for Bioleaching Activities

Both, the employment of leaching bacteria for metal winning as well as the mitigation of bioleaching processes at AMD/ARD sites, require reliable monitoring methods for assessing bacterial activities. Therefore, we have developed a robust and rapid test system combining two sensitive analytical techniques: quantification of heat evolution by microcalorimetry and determination of all relevant inorganic sulfur species by chromatographic methods (IC and HPLC). Generally, only about 1 g of sample is sufficient for a complete analysis. The combined test has been applied to various leaching biotopes such as bioreactors, columns, heaps and natural sites. The bacterial activity of diverse sulfidic materials such as lignite and coal wastes, pure metal sulfides and complex ores has been investigated. In our labs, microcalorimetry can be performed in the range of 5 to 80 °C, covering most of the temperature spectrum of leaching bacteria. Hence, the heat evolution values of samples as a direct measure for calculating leaching rates can be obtained at nearly all relevant in situ temperatures. The combination with sulfur species determination results in additional information on leaching mechanisms (thiosulfate or polysulfide pathway) and general leaching performance (e. g. accumulation of sulfur intermediates).

Greigite: a true intermediate on the polysulfide pathway to pyrite.

The formation of pyrite (FeS2) from iron monosulfide precursors in anoxic sediments has been suggested to proceed via mackinawite (FeS) and greigite (Fe3S4). Despite decades of research, the mechanisms of pyrite formation are not sufficiently understood because solid and dissolved intermediates are oxygen-sensitive and poorly crystalline and therefore notoriously difficult to characterize and quantify.In this study, hydrothermal synchrotron-based energy dispersive X-ray diffraction (ED-XRD) methods were used to investigate in situ and in real-time the transformation of mackinawite to greigite and pyrite via the polysulfide pathway. The rate of formation and disappearance of specific Bragg peaks during the reaction and the changes in morphology of the solid phases as observed with high resolution microscopy were used to derive kinetic parameters and to determine the mechanisms of the reaction from mackinawite to greigite and pyrite.The results clearly show that greigite is formed as an intermediate on the pathway from mackinawite to pyrite. The kinetics of the transformation of mackinawite to greigite and pyrite follow a zero-order rate law indicating a solid-state mechanism. The morphology of greigite and pyrite crystals formed under hydrothermal conditions supports this conclusion and furthermore implies growth of greigite and pyrite by oriented aggregation of nanoparticulate mackinawite and greigite, respectively. The activation enthalpies and entropies of the transformation of mackinawite to greigite, and of greigite to pyrite were determined from the temperature dependence of the rate constants according to the Eyring equation. Although the activation enthalpies are uncharacteristic of a solid-state mechanism, the activation entropies indicate a large increase of order in the transition state, commensurate with a solid-state mechanism.

Sulfur isotope partitioning during experimental formation of pyrite via the polysulfide and hydrogen sulfide pathways: implications for the interpretation of sedimentary and hydrothermal pyrite isotope records

We show that the sulfur isotopic composition of sedimentary and hydrothermal pyrite is a good approximation of the average sulfur isotopic composition of the dissolved sulfide sources from which the pyrite formed. Consequently, pyrite sulfur isotope systematics normally provide little evidence of the pyrite-forming mechanism in most natural systems. Stable sulfur isotope partitioning during pyrite (FeS 2) synthesis via the polysulfide and H 2S pathways was investigated between 80 and 120 °C. Iron monosulfide (FeS) was reacted with hydrogen sulfide (H 2S) or tetrasulfide (S 4 2−) in aqueous solution under strictly anoxic conditions. The results provide independent confirmation of the hydrogen sulfide and polysulfide mechanisms. The measured isotopic composition of the synthesized pyrite is compared with (1) isotopic mixing models of the reactant reservoirs and (2) predictions based on the suggested mechanisms for the hydrogen sulfide and polysulfide pathways for pyrite formation. The isotopic composition of the pyrite product is consistent with the result predicted from the reaction mechanisms. Pyrite produced via the H 2S pathway has a composition reflecting isotopic contributions from both FeS and H 2S reservoirs. Pyrite formed via the polysulfide pathway inherits an isotopic composition dominated by the polysulfide reservoir. In both cases, solubility driven isotope exchange between FeS and aqueous S species contribute to the final pyrite composition. We show that published experimental sulfur isotope data for pyrite formation which apparently support conflicting pyrite-forming pathways, are consistent with pyritization via the polysulfide and H 2S pathways. Formation rates of natural pyrite, however, may be too slow compared to solubility exchange for the influence of the reaction pathway on the isotopic composition to be significant.

Bioleaching review part A:

Bioleaching of metal sulfides is caused by astonishingly diverse groups of bacteria. Today, at least 11 putative prokaryotic divisions can be related to this phenomenon. In contrast, the dissolution (bio)chemistry of metal sulfides follows only two pathways, which are determined by the acid-solubility of the sulfides: the thiosulfate and the polysulfide pathway. The bacterial cell can effect this sulfide dissolution by "contact" and "non-contact" mechanisms. The non-contact mechanism assumes that the bacteria oxidize only dissolved iron(II) ions to iron(III) ions. The latter can then attack metal sulfides and be reduced to iron(II) ions. The contact mechanism requires attachment of bacteria to the sulfide surface. The primary mechanism for attachment to pyrite is electrostatic in nature. In the case of Acidithiobacillus ferrooxidans, bacterial exopolymers contain iron(III) ions, each complexed by two uronic acid residues. The resulting positive charge allows attachment to the negatively charged pyrite. Thus, the first function of complexed iron(III) ions in the contact mechanism is mediation of cell attachment, while their second function is oxidative dissolution of the metal sulfide, similar to the role of free iron(III) ions in the non-contact mechanism. In both cases, the electrons extracted from the metal sulfide reduce molecular oxygen via a complex redox chain located below the outer membrane, the periplasmic space, and the cytoplasmic membrane of leaching bacteria. The dominance of either At. ferrooxidans or Leptospirillum ferrooxidans in mesophilic leaching habitats is highly likely to result from differences in their biochemical iron(II) oxidation pathways, especially the involvement of rusticyanin.

Sulfides in Sandy Sediments: New Insights on the Reactions Responsible for Sedimentary Pyrite Formation

The formation of sedimentary iron sulfides was studied in sandy sediments of the Laguna Madre, TX, in order to better understand how this process operates in sediments where reactive iron is likely to be limiting for sulfide mineral formation. These sediments usually had reactive iron and total reduced sulfide concentrations one to two orders of magnitude less than in typical shallow water terrigenous muds, but organic-C concentrations typical of fine-grained sediments due to the extensive presence of seagrass beds. This resulted in moderate (0–150 μm) dissolved H2S concentrations with maximum concentrations in the upper (3–:5 cm) root zone. Based on citrate dithionite extractable-Fe the degree of sulfidization was usually 100% or greater. Acid volatile sulfides (AVS) typically comprised roughly 60% of total reduced sulfur and the proportion of AVS generally increased instead of decreasing with depth. The unusual proportion of TRS as AVS and persistence of AVS are attributed to exceptionally slow pyrite formation kinetics. The probable reasons for these slow reaction kinetics are the high (>7.8) pH of the sediments, which favors the slow polysulfide pathway for pyrite formation, high (typically about 2–4 mm) dissolved organic carbon concentrations that inhibit growth of pyrite and the low concentration of reactants which greatly increases the average transport distances necessary for diffusion controlled reactions.

Kinetics of pyrite formation by the H 2S oxidation of iron (II) monosulfide in aqueous solutions between 25 and 125°C: The rate equation

A kinetic study of the reaction Fe S ( s ) + H 2 S ( aq ) = FeS 2 + H 2 ( g ) where FeS is precipitated FeS, H 2S (aq) is aqueous H 2S, FeS 2 is pyrite, and H 2(g) is hydrogen gas, shows that the rate between 25 and 125°C can be described by the equation d FeS 2 / dt = k ( FeS ) ( c H 2 S ( aq ) ) where k the second order rate constant varies between 1.03 × 10 −4L mol −1 s −1 at 25°C and 3.20 × 10 −3 L mol −1 s −1 at 125°C. The rate constant shows a sigmoidal temperature dependence with an average Arrhenius activation energy of 35 kJ mol −1. The reaction is surprisingly fast at ambient temperatures with up to 50% reaction being completed within one day. The direct dependence of the rate on cH 2S (aq) means that the rate is pH dependent for any fixed total sulfide concentration. In typical sulfidic aquatic systems and sediments 9 × 10 −13 to 9 × 10 −8 mol FeS 2 per L sediment will be formed each day by this process. This is equivalent to approximately 3 × 10 −10 to 3 × 10 −5 mol FeS 2 per g sediment per year. At pH = 7, for the same total sulfide and FeS constraints, the rate of pyrite formation 1.5 × 10 −9 to 1.5 × 10 −4 mol FeS 2 per g sediment per year. In hydrothermal systems, such as deep ocean vents, the rate of pyrite formation by oxidation of FeS by H 2S at 125°C assuming a typical H 2S concentration of 1 mM is 3.2 × 10 −6 mol L −1 s −1 per mol FeS. A 1 million tonne pyrite deposit could form from a solution containing 1 mmol FeS and 1 mmol H 2S by this process in 1000 years at a flow rate of 0.3 Ls −1. The fluid would have a H 2 concentration of 3 × 10 −9 M. The process is by far the most rapid of the pyrite-forming reactions hitherto identified. Alternative pyrite-forming processes involving HS −, rather than H 2S, as the reaction requires an additional oxidising agent to maintain electron balance. These pathways may involve reactants such as polysulfides or intermediaries such as greigite, Fe 3S 4. In natural systems, therefore, the H 2S process will tend to be favored in strictly anoxic environments. In transitional environments, with limited molecular oxygen contents, pyrite formation through the polysulfide pathway or a solid state process may become more important. However, the generally low concentrations of polysulfides and greigite observed in natural pyrite-forming environments might suggest that these processes are always subordinate. The fast nature of the H 2S oxidation process suggest that it makes a significant contribution to the global sulfur cycle. The process provides a straightforward explanation of a number of observations regarding pyrite formation in natural systems, including pyrite formation in strictly anoxic environments and rapid pyrite formation. Hydrogen generation through this process in aquatic and sedimentary systems provides an important alternative metabolite for microorganisms.