Pristine Pyrite Research Articles

Regulating sulfur vacancies formation on mechanochemically modified pyrite to steer non-radical molecular oxygen activation for water purification

Ubiquitous pyrite (FeS2) in the earth’s crust features strong reducing capacity, presenting a promising alternative for molecular oxygen (O2) activation to facilitate water purification, but the interfacial electron transfer efficiency was remarkably retarded due to the existed passivation layer and limited reactive sites on the surface of pristine pyrite. Herein, we proposed an effective mechanochemical approach to unlock the potential of pyrite for O2 activation and refractory pollutant degradation. Compared to the inert pristine one, the ball milling-treated pyrite (BMPs) could achieve efficient sulfamethoxazole (SMX) degradation with a rate constant of 0.23 h−1. By a combination of characterizations, batch experiments, and density function theory (DFT) calculations, a clear panorama delineated the consecutive O2 adsorption and activation processes was established. Ball milling treatment could import rich sulfur vacancies (SVs) on pyrite, which unshifted the p-band center of adjacent S for improved O2 adsorption, also narrowed the band gap of BMPs to promote the interfacial electron transfer rate for O2 activation. Singlet oxygen (1O2) was identified as the dominant reactive species for SMX degradation, such non-radical oxidation feature rendered the system with remarkable activity and environmental robustness. Our work lay a foundation for constructing robust pyrite towards stable and efficient environmental remediation applications.

Hydrophobic modification of pyrite with a composite of sodium oleate and SiO2 nanoparticles to inhibit its oxidation for controlling acid mine drainage

The formation of acidic mine drainage (AMD) is mainly due to the oxidation of pyrite in the presence of water. In this study, the environmentally friendly Sodium oleate (SO) was used as hydrophobic agent and SiO2 nanoparticles were added as a filler material to modify pyrite. Na-oleate and nano-SiO2 nanoparticles form a hydrophobic passivation film directly on the pyrite surface in a very simple process.Excellent antioxidant capacity was achieved with the addition of 6% SO and 1 wt% SiO2 at a reaction temperature of 70 °C. Chemical leaching experiments and electrochemical experiments were used to characterise the antioxidant properties of this hydrophobic passivated film. The results showed that the pH of the leachate could be maintained above 5.7, a 2.6 increase over raw pyrite. In addition, the reduction rate of SO42- and total Fe concentration could reach over 83.69% and 97.91%, respectively. SO-SiO2-treated pyrite exhibits lower oxidation rates in electrochemistry and an increased contact angle of 70.1° compared to pristine pyrite. Finally, this paper details the mechanism of the action of SO-SiO2 on the surface of pyrite by scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) analysis. The results show that sodium oleate can combine with iron ions to form iron oleate wrapped on the surface of pyrite, while silica nanoparticles attach to pyrite by physical adsorption, filling the pores on the surface of hydrophobic film layer and effectively protecting pyrite from oxidation.

Highly efficient activation of sulfite by surface oxidized pyrite for atrazine degradation: Experimental and DFT study

Surface oxidized pyrite (SOPy-90) was synthesized via calcination and showed much higher catalytic ability for sulfite than the pristine pyrite to degrade atrazine (ATZ). The impacts of SOPy-90 dosage, sulfite doses, and dissolved O2 on the ATZ elimination by SOPy-90/sulfite system were studied. Furthermore, β-FeOOH was generated on the surface of oxidized pyrite, which played a significant role in the catalysis of sulfite by SOPy-90. The reaction pH (4.0–7.0) was acidified to 4 by adding SOPy-90, which was beneficial for the sulfite activation by both material and dissolved Fe2+ to produce SO3•−. The generated SO3•− was further oxidized by dissolved O2 to form SO4•− as the predominant species, confirmed by the quenching experiments and EPR analysis. Besides, the Fe(II)/Fe(III) cycle was accelerated by the low-valent S(−II) on the SOPy-90 surface, thus enhancing the SOPy-90 catalysis capacity. The density functional theory (DFT) calculations revealed that Fe(II) was more likely to activate HSO3− to generate SO3•− on the SOPy-90 surface. The SOPy-90/sulfite system has good reusability and showed excellent performance to decompose ATZ in real water treatment.

Insight into the Influence of Mineralogical Properties of Pristine Pyrite on Its Bioleaching with Thermophiles

ABSTRACT To expound the relationship between bioleaching behaviors and mineralogical properties of pristine pyrite samples, chemical compositions, semiconductive properties, surface morphologies, and chemical species of synsedimentary pyrite (SSP) and mesothermal pyrite (MTP) were compared, accompanying with bioleaching behaviors analysis by three thermophiles. Mineralogical characterizations showed that both pyrites belonged to p-type semiconductors with an S/Fe ratio higher than 2, and the arsenic content in MTP was as high as 0.6%. The fractured surface of SSP had more kinks and steps, and higher concentrations of S2- and oxides, as compared with that of MTP. Bioleaching results indicated that SSP was leached faster than MTP. The more kinked and stepped structures and the larger contents of S2- and oxides on pyrite surfaces were the dominant reasons for the faster bioleaching kinetics of SSP. The high arsenic content in MTP limiting the cell growth was another cause for its slower leaching.

Textural, compositional and isotopic characteristics of pyrite from the Zaozigou gold deposit in West Qinling, China: Implications for gold metallogeny

The Zaozigou gold deposit (ZZGD) located within the West Qinling Orogen in China, with 142 t of Au at an average grade of 2.69 g/t, is hosted in a sequence of metasediments and altered dacitic to granodioritic rocks. The majority of gold is present as invisible gold in disseminated arsenian pyrite. Here we investigate the morphological and compositional evolution of the pyrite grains and constrain the ore-forming fluid source(s) and genetic models related to the ore-forming processes through the application of Scanning electron microscopy, electron probe microanalysis, in-situ laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) and fsLA-MC-ICP-MS analyses. Based on texture and trace element concentrations, we identify five generations of pyrite (namely, Py1, Py2, Py3, Py4, and Py5). Among these, the pre-ore framboidal pyrite (Py1), which is characterized by relatively low concentrations of As, Au, Co, Ni, and Sb, has negative δ34S values of −31.07 to −20.79‰, which we correlate to bacterial reduction of marine sulfate during sedimentation and diagenesis. The auriferous Py2 and Py5, which occur as either porous aggregates or epitaxial overgrowth on earlier pristine pyrite, contain relatively high concentrations of As, Sb, Tl, and Au compared with pyrite in the other generations. Both (Py2 and Py5) display a relatively narrow range of negative δ34S values, ranging from −5.66 to −1.16‰, which are interpreted to result from a magmatic source. Gold-free Py3 and Py4 exhibit relatively variable δ34S values of −17.1 to −6.02‰, indicating that a small amount of sulfur in the sedimentary strata was added to the magmatic hydrothermal systems during fluid-rock interaction. Sulfur isotopes and LA-ICP-MS mapping of pyrite grains revealed that the gold endowment of the ZZGD is associated with multiple mineralizing events and multiple magmatic and hydrothermal activities. We propose that gold in the arsenian pyrite from the ZZGD was scavenged in ore-forming fluids by chemical adsorption onto the As-rich, Fe-deficient, and actively growing surfaces of pyrite. Our results indicate that the primary source of the sulfur, gold, and mineralizing fluids was likely fluids from magmatic hydrothermal systems in the West Qinling orogen.

Two-stage fluid pathways generated by volume expansion reactions: insights from the replacement of pyrite by chalcopyrite

Volume expansion reactions involved in mineral–fluid interactions are linked to a number of geological processes, including silicate weathering, retrograde metamorphism, and mineralization. However, the effect of volume expansion on replacement reactions remains unclear. Here, we demonstrate that reactions associated with volume expansion during the replacement of pyrite by chalcopyrite involve two competing processes. The reaction is initially augmented because of the development of reaction-induced fractures in the pyrite. However, these fractures are subsequently filled by compacted products, which ultimately disrupts the contact and interaction between bulk fluids and the pristine pyrite surface. These competing processes indicate that replacement reactions are both augmented and inhibited by volume expansion reactions during pyrite replacement.

In situ construction of Z-scheme FeS2/Fe2O3 photocatalyst via structural transformation of pyrite for photocatalytic degradation of carbamazepine and the synergistic reduction of Cr(VI)

Pyrite is the most abundant sulfide semiconductor mineral with excellent optical properties. However, few reports have investigated its photocatalytic activity because of the low photogenerated carrier separation efficiency. In this work, a Z-scheme FeS2/Fe2O3 composite photocatalyst was fabricated in situ via structural transformation of pyrite through heat treatment. A remarkably enhanced photocatalytic performance was observed over the FeS2/Fe2O3 composite photocatalyst. Compared with the pristine pyrite, the degradation efficiency of carbamazepine (CBZ) reached 65% at the added hexavalent chromium (Cr(Ⅵ)) concentration of 20 mg/L and the Cr(Ⅵ) was nearly completely reduced in the mixed system using FeS2/Fe2O3 within 30 min under simulated solar light irradiation. The enhanced photocatalytic activity can be attributed to the efficient separation and transfer of photogenerated carriers in the FeS2/Fe2O3 composite photocatalyst. This facilitated the generation of •OH, hole (h+) and •O2− species, which participated in the photocatalytic reaction with CBZ. Based on the measurement of the active species and electric properties, a Z-scheme electron transfer pathway was proposed for the FeS2/Fe2O3 composite photocatalyst. This work broadens the application potential of pyrite in environmental remediation.

Comparison of the reaction kinetics and mechanisms of Sb(III) oxidation by reactive oxygen species from pristine and surface-oxidized pyrite

With the rapid increase in the antimony (Sb) concentration in the environment, the environmental risk and pollution control of Sb has gradually attracted wide attention. The oxidation of Sb by pyrite influences its toxicity and mobility. Pyrite is oxidized by dissolved oxygen (DO) in an aerobic environment to form a surface oxide coating. However, the effect of the surface oxide coating on the reaction activity of pyrite is poorly understood. In this study, the influence of oxidation products on the surface morphology of pyrite and its interfacial reaction with Sb were investigated comprehensively. The amount of surface oxide coating increased with increasing pH. The main oxidation products under acidic conditions were mainly iron (II) hydroxy complex, and those under neutral conditions were iron (III) hydroxy complex. In a suspension of surface-oxidized pyrite (SOP), the oxidation efficiency of Sb(III) decreased significantly, and inhibition became more significant with increasing pH. As a result of the formation of the surface oxide coating, the contents of hydrogen peroxide (H2O2) and hydroxyl radicals (·OH), which are the main oxidants of Sb(III), were found to be lower in the SOP suspension than in pristine pyrite. The reasons are as follows. On the one hand, surface oxidation products decompose H2O2. On the other hand, they retard the production of H2O2 and ·OH by inhibiting Fe(II) release into the solution and O2 diffusion into the surface of pyrite. In addition, the adsorption capacity of Sb was enhanced after aqueous oxidation because Fe hydroxy complex have a strong affinity to Sb and large specific surface area. Aqueous oxidation of pyrite greatly influences the geochemical cycling and existence of Sb in the environment. The study lays a solid foundation for assessing the environmental risk of and developing pollution controls for Sb.

Non-Native Pyrite Performs Better Than Native Marcasite CoSe2 for Electrochemical Oxygen and Hydrogen Evolution Reaction: A Theoretical Investigation

Efficient bifunctional electrocatalyst for water splitting is essential for replacing fossil-fuel energy sources with clean energy-dense hydrogen fuel (142 MJ/kg). Efficient electrocatalyst can be obtained by either increasing active site density or specific activity on individual active sites. The active site densities can be increased through roughening the potential energy surface or exposing the facets which has higher active site densities. The specific activity can be increased through modulation of strain or charge densities on active sites which can be achieved through introduction of dopants, defects or stabilization of “non-native phases” that are all the other crystalline and amorphous states that differ in terms of discrete translational symmetry in the sub-surface region from the “native” phase (or bulk ground-state). While for a given composition, there is a unique native state for a given set of thermodynamic condition while, there can be many non-native structures having different bond-angles, bond-distances and surface atom densities from the native phase, leading to different electrocatalytic properties. In this context, polymorphic engineering via stabilizing ‘non-native phase’ offers a potential approach for improving both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) electrocatalysts and its activity. The beneficial effect of polymorphic engineering with regards to bifunctional electrochemical OER and HER is demonstrated by first principle calculation by taking CoSe2 as a model electrocatalyst which has marcasite (Space Group-58) and pyrite (Space Group-205) as the native (N) and non-native (NN) structures, respectively. The first principle computations predict pyrite (NN) structure of CoSe2 would have better electrochemical activity towards OER and HER than its marcasite (N) counterpart which is confirmed through experimental results in literature too. Though the co-ordination number of Co remains same in both the structures, the co-ordination symmetry surrounding Co atom varies. This results in differential charge distribution in constituting Co- and Se-atoms consequently resulting in variable density of state (DOS) near Fermi level (Figure 1) thereby affecting the binding energies (BE) of reaction intermediates of OER and HER. Pyrite (NN) phase of CoSe2 has a greater electron density near Fermi Level in comparison to its marcasite (N) counterpart due to differential co-ordination symmetry. A greater electron density near Fermi-level is indicative of lower work function and consequently lower polarization resistance during water splitting. A greater electron density near Fermi level is contributed by Co-3d orbitals which is the common active site for both OER and HER. The greater electron density and lower work function in Pyrite (NN) results in stronger metal-hydrogen BE (0.03 eV) resulting in lower overpotential of HER. Hydrogen adsorption on Se sites occurs only at higher HER overpotential due to weak Se-hydrogen BE (0.59 eV). This results in observation of twin Tafel slopes during HER on CoSe2 electrocatalyst as the potential determination step (PDS) switches from Volmer to Heyrovsky step with participation of Se during HER. The lower work function and higher electron density near Fermi level in Pyrite (NN) structure results in weaker metal-oxygen bond thereby promoting multi-electron OER activity. The OER intermediates (-OH, -O, -OOH) has a higher BE over Co- than Se-sites. The transformation of Oads à HOOads on Co-sites of CoSe2 (001) structure is the potential determination step with an onset potential of 1.66 V (vs RHE).The desorption of O2 from Se site is found to be the potential-determination-step (PDS) for OER (η=0.79 V). Furthermore, pristine CoSe2 acts as a precursor for OER which undergoes dissolution to form a surface Co-O structure which has a greater activity than pristine pyrite CoSe2 surfaces (η=0.31 V). This energetics is more favourable for pyrite (NN) structure than marcasite (N) structure for dissolution process to form surface Co-O structure due to stronger Co-Se bonds present in the latter case. Furthermore, point-defects which can aid both OER and HER, can be more easily formed in pyrite (NN) structure than marcasite (N) structure due to the aforementioned reason. The present study underlines the importance of stabilization of non-native structures which has a great potential to produce higher electrocatalytic activity thus providing greater options in search of better water splitting electrocatalyst. Figure 1

Geochemical and micro-textural fingerprints of boiling in pyrite

Abstract The chemical composition, textures and mineral associations of pyrite provide key information that help elucidate the evolution of hydrothermal systems. However, linking the compositional and micro-textural features of pyrite with a specific physico-chemical process, e.g., boiling versus non-boiling, remains elusive and challenging. In this study we examine pyrite geochemical and micro-textural features and relate these results to pyrite-forming processes at the active Cerro Pabellon Geothermal System (CPGS) in the Altiplano of the northern Chile. We integrate electron microprobe analysis (EMPA) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) data with micro-textural observations of pyrite and associated gangue minerals recovered from a ∼500 m long drill core that crosscuts the argillic, sub-propylitic and propylitic alteration zones of the CPGS. Additionally, we carried out a Principal Component Analysis (PCA) in order to inspect and understand the main data structure of the pyrite geochemical dataset. The concentrations of precious metals (Au and Ag), metalloids (As, Sb, Se, Bi and Tl), and base and heavy metals (Cu, Co, Ni and Pb) in pyrite from the CPGS are significant. Among the elements analyzed, As, Cu and Pb are the most abundant with concentrations that vary from a few parts per million (ppm) to wt% levels (up to 4.4 wt% of As, 0.5 wt% of Cu and 0.2 wt% of Pb). Based on contemporaneous gangue mineral associations and textures, the mechanisms of pyrite precipitation in the CPGS were inferred. Pyrite formed during vigorous boiling is characterized by relatively high concentrations of As, Cu, Pb, Ag and Au and lower concentrations of Co and Ni compared to pyrite formed under different conditions. These anhedral to euhedral pyrite grains display zones with a porous texture and abundant mineral micro- to nano-inclusions (mainly galena and chalcopyrite) indicating a formation by rapid crystallization. In contrast, pyrite formed under gentle boiling (more gradual cooling and less abrupt physico-chemical variations than in vigorous boiling) to non-boiling conditions is characterized by a higher concentration of Co and Ni, and relatively low concentrations of As, Cu, Pb, Ag and Au. Texturally, these pyrites form aggregates of euhedral and pristine pyrite crystals with scarce pores and mineral inclusions suggesting formation under steadier physico-chemical conditions. Our results show that pyrite can not only record the chemical evolution of hydrothermal fluids, but can also provide critical information related to physico-chemical process such as boiling and phase separation. Since boiling of aqueous fluids is a common phenomenon occurring in a variety of pyrite-forming environments, e.g., active continental and seafloor hydrothermal systems, and porphyry Cu-epithermal Au-Ag deposits, pyrite compositional and textural features are a valuable complement for discriminating and tracking boiling events in modern and fossil hydrothermal systems.

Pyrite-based mixtures as composite electrodes for lithium-sulfur batteries

Pyrite-based composite materials (Li/S) were prepared by high-energy ball milling of FeS2 with one or more metal sulfides, Li2S, Cu2S, and CuS, in various molar ratios and under different conditions. Prolonged grinding resulted in the formation of mixed compounds rather than single-phase lithiated chalcopyrite. The composite materials were primarily investigated via XPS, XRD, and TOF-SIMS characterization methods, and performance of the materials as composite cathodes was evaluated via analysis of dQ/dV curves. Evidence of main active phases in the materials was found, including, Cu-doped FeS2 (pyrite) and/or nonstoichiometric chalcopyrite (CuxFeyS2 − z). TOF-SIMS measurements exhibited further evidence of formation of partially lithiated pyrite mixtures. Lithium cells comprising mixed pyrite cathodes demonstrate superior electrochemical performance, namely, 98–100% coulombic efficiency and 300–320 mAh g−1 (per gram of active material) after 150 cycles over a voltage range of 1.2–2.8 V at 0.1 C-rate, whereas pristine FeS2/Li cells exhibit only 220 mAh g−1 at similar operating conditions. Lowering of cut-off discharge voltage from 1.2 to 1.0 V of the FeS2/Li cells with FeS2-rich mixed cathodes is accompanied by about 15% capacity enhancement (350–450 mAh g−1 after 200 cycles). The cells with mixed pyrite composite cathodes exhibit an increase in maximum pulse power capability from 5 to 15 W/g and C-rate from 0.3 to 2.5, which points towards higher electrical conductivity of the active material as compared to the pristine pyrite.

Sorption of Differently Charged Gold Nanoparticles on Synthetic Pyrite

Sorption of nanoparticles on mineral surfaces has been well recognized for its environmental implication. For certain gold ore deposits, sorption of gold nanoparticles (AuNPs) on pyrite may constitute a critical step in the metallogenesis process, yet it has not been adequately investigated. In this work, we synthesized negatively charged, positively charged AuNPs and pyrite respectively, and experimentally studied sorption of AuNPs on pyrite in an anaerobic environment. The effects of AuNPs charge characteristics, pH, hexadecyl trimethyl ammonium bromide (CTAB) concentration, and ionic strength were evaluated. For the negatively charged AuNPs, almost no sorption was observed owing to the electrostatic repulsion between the AuNPs and pyrite surface (with a measured isoelectric point of 2.0). In contrast, positively charged AuNPs could be significantly adsorbed on pyrite through electrostatic attraction, with the sorption extent decreasing with the increase of pH (2.2–9.1). However, the adsorption of CTAB from the solution was found to increase with the increase of pH. Furthermore, deliberately added CTAB seemed to inhibit the sorption of AuNPs. Our study not only demonstrates that the electrostatic interaction is the dominant mechanism in the sorption of AuNPs on pristine pyrite, but also reveals the important competitive relation between AuNPs and CTAB in the sorption process.

Textural and trace element evolution of pyrite during greenschist facies metamorphic recrystallization in the southern Apuan Alps (Tuscany, Italy): Influence on the formation of Tl-rich sulfosalt melt

Several small pyrite ± baryte ± iron-oxide orebodies (Buca della Vena, Canale della Radice, Fornovolasco, Monte Arsiccio and Pollone) are hosted in the metamorphic rocks of the southern Apuan Alps, northern Tuscany, Italy. These deposits are exceptionally thallium-rich, expressed in part by assemblages comprising rare Tl-sulfosalts. Using a variety of techniques including laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) on fifty-four samples, the textural and trace element evolution of pyrite during greenschist facies metamorphism has been described. Five generations of pyrite have been recognised. Pyrite-1 (py1) and pyrite-2 (py2) (framboidal and colloform pyrite, respectively) likely represent pristine generations and are rich in trace elements. Pyrite-3 (py3) forms fine-grained disseminations often aligned with the metamorphic fabric and likely formed during the earliest stages of deformation associated with metamorphism. Pyrite-4 (py4) forms coarse, euhedral, inclusion-rich crystal clusters and likely formed as pristine pyrite generations began to recrystallize. Pyrite-5 (py5) is typically coarse, relatively ‘clean’ and often euhedral. It is interpreted to be the end-product of pyrite recrystallization, with grain size increasing and micro-inclusions being expelled late on the prograde path, then granoblastic annealed textures forming during retrograde cooling. Pyrite (especially primary pyrites; py1 and py2) is particularly rich in As (up to 17,400 ppm), Sb (up to 5100 ppm) and Tl (up to 4200 ppm), while Ni, Pb and Mn may be present above 1000 ppm, and Hg, Co, Cu, Zn, Mo, Bi and Ag may all be present above 100 ppm. Some trace elements, particularly As, Ni and Co, are commonly zoned in late metamorphic pyrite overgrowths. The incorporation of some trace elements into pyrite seems to have been facilitated by the presence of Sb in a similar way that As generally facilitates Au incorporation. Concentrations of most trace elements decrease from py3 to py4 at Fornovolasco and Monte Arsiccio, while in the other deposits the opposite trend is observed. Concentrations of most trace elements drop significantly from py4 to py5, where recrystallization of pyrite liberates a large proportion of As, Sb, Tl, Pb, Hg, Cu, Zn, Ag and Mn, likely during retrograde cooling and annealing. The release of these elements from pyrite during metamorphic recrystallization has directly facilitated the formation of late-stage sulfosalts, especially at Monte Arsiccio, where textural evidence suggests sulfosalt assemblages intimately associated with py5 were mobilised as melts during greenschist facies metamorphism. At Monte Arsiccio, around 75% of Tl hosted in early recrystallized pyrite is released upon complete recrystallization. As such, mass balance calculations show that all Tl contained in Tl-sulfosalts could be supplied from locally recrystallizing pyrite, and that in total, more than 250 tons of Tl could potentially have been liberated from the pyrite orebody at Monte Arsiccio during metamorphism. This study highlights the significant quantities of Tl and other metals that may be hosted in pyrite, and also the potential role that metamorphic recrystallization may play in mobilising and (re)-concentrating (or indeed dispersing) metals in a pyrite-dominant ore system.

As(V) and As(III) reactions on pristine pyrite and on surface-oxidized pyrite

Reactions of As(III) and As(V) with pyrite were investigated using pristine pyrite (produced and reacted in a rigorously anoxic environment with PO2<10−8atm) and using surface-oxidized pyrite (produced under anoxic conditions, exposed to air, then stored and reacted under rigorously anoxic conditions). Results with surface-oxidized pyrite were similar to previously reported arsenic-pyrite results. However As(III) adsorbed over a broader pH range on pristine pyrite than on surface-oxidized pyrite, As(V) adsorbed over a narrower pH range on pristine pyrite than on surface-oxidized pyrite, and adsorbed As(V) on pristine pyrite was reduced to As(III) but adsorbed As(V) was not reduced with surface-oxidized pyrite. Reduction of As(V) with pristine pyrite was first-order in total As(V), Fe(II) was released, and sulfur was oxidized. The proposed mechanism for pyrite oxidation by As(V) was similar to the published mechanism for oxidation by O2 and rates were compared. The results can be used to predict the removals of As(V) and As(III) on pyrite in continuously anoxic environments or on pyrite in intermittently oxic/anoxic environments. Rigorous cleanup and continuous maintenance of strictly anoxic conditions are required if commercial or produced pyrites are to be used as surrogates for pristine pyrite.

Ab initioand x-ray photoemission spectroscopy study of the bulk and surface electronic structure of pyrite (100) with implications for reactivity

Ab initio quantum chemical modeling is used to predict differences in electronic structure between the (100) surface and bulk of pyrite. The density functional generalized gradient approximation (CASTEP) is used to obtain the density of states for the valence band of pyrite bulk and surface states. For this purpose the pyrite (100) surface is cleaved along all possible layers of the bulk, thus yielding a bulk terminated surface, a surface with the S-S dimer bond broken, and an irregularly cleaved fractured surface with exposed surface monomers, respectively. Furthermore, the hybrid functional B3LYP (CRYSTAL03) is used to predict S $2p$ and Fe $2p$ core line shifts for pyrite surface states. Experimental x-ray photoelectron spectroscopy (XPS) data for these core lines show the presence of two kinds of S surface states: surface ${\mathrm{S}}^{2\ensuremath{-}}$ monomers at a S $2{p}_{3∕2}$ binding energy (BE) of $161.2\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$, and ${(\mathrm{S}\text{\ensuremath{-}}\mathrm{S})}^{2\ensuremath{-}}$ surface dimer states at a S $2{p}_{3∕2}$ BE of $162.0\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$, compared to the S $2{p}_{3∕2}$ BE of bulk pyrite at $162.7\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. Investigation of the ab initio valence band density of states for these different surfaces supports our conventional and synchrotron radiation XPS data, collected for the pristine pyrite surface, which suggests the presence of bivalent S monomers and monovalent S dimer surface states. The Fe $2p$ surface XPS displays several multiplets (implying high spin configuration) to higher binding energy than the bulk Fe $2p$ signal, which can be ascribed to surface state contributions. The quantum chemical simulation predicts an S $2p$ core level shift of $0.69\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ between the S bulk and S surface dimers, in good agreement with the $0.6\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ found in XPS measurements. Evidence for surface ${\mathrm{Fe}}^{2+}$ and ${\mathrm{Fe}}^{3+}$ surface states can be seen in the Fe projected valence band density of states, also confirming the interpretation of the photoemission spectra. From the B3LYP calculation, the surface contributions to the Fe $2p$ BE peak lead to a broadening of the signal, as observed by experiment.

Gelatin/DMSO: a new approach to enhancing the performance of a pyrite electrode in a lithium battery

We have studied the electrochemical behavior of natural pyrite (FeS 1.9, n-type semiconductor) treated nonaqueously with dimethylsulfoxide (DMSO) solvent and also with a gelatin/DMSO solution. Composite electrodes (comprised of pyrite, polyvinilidene fluoride, polyethylene oxide and carbon) were characterized in a lithium cell at room temperature by cyclic voltammetry and galvanostatic measurements; the electrolyte used was LiPF 6 in a solution of ethylene carbonate and dimethyl carbonate (1 mol l −1). The gelatin/DMSO treatment greatly improved the reversible specific capacity of a pyrite electrode. For galvanostatic discharge/charge at a current density of 0.4 mA cm −2 and between voltage limits of 3.2 and 1.1 V, its reversible specific capacity at the 15th cycle equaled 275 mA h g −1, an impressive value compared to less than 25 mA h g −1 for a pristine pyrite electrode.

Synchrotron XPS evidence for Fe2+-S and Fe3+-S surface species on pyrite fracture-surfaces, and their 3D electronic states

A X-ray photoelectron Fe 2p 3/2 spectrum of a pristine pyrite fracture surface was collected using synchrotron radiation with the source tuned to 800 eV. Comparison of this highly surface sensitive Fe 2p spectrum with Fe 2p spectra collected by conventional means (1487 eV AlKa source) reveals that the high binding energy tail of the pyrite Fe 2p 3/2 line results primarily from Fe surface state contributions. The three major contributions to the spectrum are interpreted to be: (1) Fe 2+ resident on bulk sites; (2) Fe 2+ resident on surfaces, edges and corners; (3) Fe 3+ surface states produced during fracture by an auto-redox reaction involving Fe and S. The intense main peak is ascribed to the bulk state, whereas the high binding energy tail of the spectrum is composed primarily of Fe 2+ and Fe 3+ surface state contributions. Fe 2+ on bulk sites is octahedrally coordinated (O h symmetry). All valence electrons of Fe on bulk sites are paired (diamagnetic) and a singlet photopeak at 707 eV is consequently produced. Fracture produces Fe 2+ surface states with lower coordination than bulk sites. Fe 2+ located at surfaces, edges and corners experiences modified Ligand Field Stabilization Energies (LFSE) which results in stabilization of the d z 2 orbital and destabilization of the dxy orbital. Promotion of a dxy electron to the d z 2 orbital makes surface Fe 2+ surface states paramagnetic resulting in multiplet splitting of their associated photopeaks. The Fe 3+ surface state is necessarily paramagnetic and its photoemissions are consequently multiply split. Analysis of photopeak structures and binding energy splittings of Fe 2+ and Fe 3+ surface states demonstrates that they are located at the appropriate binding energies, and span the appropriate energy range, to satisfactorily explain the high binding energy tail on of the Fe 2p 3/2 spectrum.

X-ray photoelectron spectroscopic study of a pristine pyrite surface reacted with water vapour and air

Pristine pyrite fracture surfaces, exposed for 7 h to water vapour at low pressure (10 −5 Pa), display no change to their Fe(2 p) or S(2 p) X-ray photoelectron (XPS) spectrum, but oxygen deposition occurs as H 2O, OH − and O 2− (74, 19, and 7%, respectively). An additional 24 h exposure to air (80% humidity) causes the proportions of oxygen species to change dramatically, with OH − and O 2− increasing to 55 and 25% and H 2O decreasing to 20%. These changes are accompanied by development of a broad Fe(III) peak of the Fe(2 p) spectrum, produced by oxidation of Fe(II) to Fe(III) and formation of Fe(III)-oxyhydroxide surface species. There is, however, no sulphate peak developed in the S(2 p) spectrum during the 24 h exposure to air. The XPS data demonstrate that formation of Fe(III)-oxyhydroxides precedes sulphate formation, hence rates of redox reactions producing sulphate or other oxygen-bearing S species are initially slower than redox reactions leading to the formation of Fe(III)-oxyhydroxide surface species. Exposure to air for an additional 9 days produces no appreciable change to the O(1 s) or Fe(2 p) spectrum, but small amounts of sulphate are observed in the S(2 p) spectrum. After production of sulphate species, Fe(III)-sulphate salts probably form at the surface of pyrite by reaction of sulphate with previously formed Fe(III)-oxyhydroxides. The Fe(2p 3 2 ) spectrum of the vacuum-fractured pyrite surface reveals a high energy tail on the major Fe(II) peak. The tail may result from the metallic character of Fe in pyrite. The binding energy and shape of the tail, however, are accurately predicted from S-bonded Fe(III) spectral peaks observed in the pyrrhotite Fe(2 p) spectrum; consequently, the presence of Fe(III) bonded to sulphur in the near-surface of pyrite is another reasonable explanation for the tail. Disulphide (S 2− 2), monosulphide (S 2−), and polysulphide ( S 2− n , n>2) are present on the vacuumfractured pyrite surface at 85, 10 and 5%, respectively. Monosulphide decreases, and disulphide increases proportionately, during the first 24 h exposure to air. After a total of 10 days exposure, monosulphide decreases to half its original value, polysulphide increases appreciably, and sulphate and thiosulphate are present at 1.8 and 2.3 at. %, respectively. There is little change to disulphide content during the entire experiment. Two explanations are offered for the presence of the three S species on the vacuum-fractured pyrite surfaces. Approximately 5% of S is present as polysulphide ( S 2− n ) and 10% as monosulphide (S 2−). This 1:2 ratio is obtained if some disulphide is “disproportionated” to polysulphide and monosulphide where, on average, four atoms of S are included in the polysulphide species (S 2− 4). An alternative explanation includes ferric iron; if present, it may give rise to complex charge compensation involving “disproportionation” of disulphide to monosulphide (for charge balance) and polysulphide.

A XANES study of S L-edges in sulfide minerals

Pyrite, FeS2, commonly referred to as “Fool’s gold” is the most common sulfide in the Earth’s surface region. Not only is the mineral ubiquitous, but the reactivity of pyrite is of central importance in a devastating environmental issue known as acid mine drainage (AMD) and in beneficial commercial processes such as mineral benefaction, which can range from the desulfurization of coal to the isolation of copper or gold ores. Pyrite has even been postulated to be a key constituent of a prebiotic iron–sulfur world existing at the high pressure and temperature conditions common to hydrothermal vents at the oceanic floor. The development of an atomic level picture of the structure and reactivity of pyrite is paramount to understanding the chemistry of this mineral in these wide-ranging environments.This contribution focuses on research carried out over the past three decades that has used modern surface science tools to understand the reactivity of pyrite surfaces. An understanding of the reactivity of the pyrite surfaces has benefited from studies using a wide range of experimental techniques that range from vacuum-based experiments utilizing electron and photon spectroscopies, and probe microscopy to in situ studies using infra-red spectroscopy. Synchrotron-based techniques that include photoelectron spectroscopy and X-ray absorption spectroscopy have played a large role in both these environments. These techniques have perhaps been the most useful in establishing the structure of the pristine pyrite surface. Related iron sulfides are also briefly introduced in this review including pyrrhotite (FexS1−x) and the dimorph of pyrite, marcasite. The surface reactivity of these sulfides exhibit both similarities and differences to pyrite, and help to bring forward the unique activity of pyrite in both environmentally and technologically important conditions.