Ab initiosimulation of water interaction with the (100) surface of pyrite

Inhibition of acid rock drainage with iron-silicate or phosphate film: in rainy and submerged environments

Iron phosphate-based coating and iron silicate-based coating were used to inhibit the oxidation of sulfide minerals in rainy and submerged environments. The inhibiting effectiveness of coating agents on the oxidation of iron sulfide minerals was investigated using pyrite and rock samples resulting from acid drainage. The film formed with both surface-coating agents was identified by pyrite surface analysis. It was also confirmed that the formation of coatings varies depending on the crystallographic orientation. The inhibitory effects under rainy and submerged conditions were investigated using column experiments. Submerged conditions accelerated deterioration compared to that under rainy conditions. Iron phosphate coating had a significantly better oxidation-inhibitory effect (84.86-98.70%) than iron silicate coating (56.80-92.36%), and at a concentration of 300 mM, H+ elution was inhibited by more than 90% throughout the experiment. Furthermore, methods for effective film formation were investigated in terms of producing Fe3+; (1) application of coating agents mixed with oxidant (H2O2), (2) application of coating agent after the use of the oxidant. In a rainy environment, applying iron phosphate-based coating using the sequential method showed oxidation inhibition effects for cycles 1-9, whereas applying the mixed material showed effects for cycles 9-13. The use of a surface-coating agent after applying an oxidant did not inhibit oxidation. The surface coating agent and the oxidizing agent should be applied as a mixture to form a film.

Emerging Trends of Computational Chemistry and Molecular Modeling in Froth Flotation: A Review

Froth flotation is the most versatile process in mineral beneficiation, extensively used to concentrate a wide range of minerals. This process comprises mixtures of more or less liberated minerals, water, air, and various chemical reagents, involving a series of intermingled multiphase physical and chemical phenomena in the aqueous environment. Today's main challenge facing the froth flotation process is to gain atomic-level insights into the properties of its inherent phenomena governing the process performance. While it is often challenging to determine these phenomena via trial-and-error experimentations, molecular modeling approaches not only elicit a deeper understanding of froth flotation but can also assist experimental studies in saving time and budget. Thanks to the rapid development of computer science and advances in high-performance computing (HPC) infrastructures, theoretical/computational chemistry has now matured enough to successfully and gainfully apply to tackle the challenges of complex systems. In mineral processing, however, advanced applications of computational chemistry are increasingly gaining ground and demonstrating merit in addressing these challenges. Accordingly, this contribution aims to encourage mineral scientists, especially those interested in rational reagent design, to become familiarized with the necessary concepts of molecular modeling and to apply similar strategies when studying and tailoring properties at the molecular level. This review also strives to deliver the state-of-the-art integration and application of molecular modeling in froth flotation studies to assist either active researchers in this field to disclose new directions for future research or newcomers to the field to initiate innovative works.

Unraveling the Nature of Hydrogen Bonds of "Proton Sponges" Based on Car-Parrinello and Metadynamics Approaches

The nature of intra- and intermolecular non-covalent interactions was studied in four naphthalene derivatives commonly referred to as "proton sponges". Special attention was paid to an intramolecular hydrogen bond present in the protonated form of the compounds. The unsubstituted "proton sponge" served as a reference structure to study the substituent influence on the hydrogen bond (HB) properties. We selected three compounds substituted by methoxy, amino, and nitro groups. The presence of the substituents either retained the parent symmetry or rendered the compounds asymmetric. In order to reveal the non-covalent interaction properties, the Hirshfeld surface (HS) was computed for the crystal structures of the studied compounds. Next, quantum-chemical simulations were performed in vacuo and in the crystalline phase. Car-Parrinello molecular dynamics (CPMD), Path Integral Molecular Dynamics (PIMD), and metadynamics were employed to investigate the time-evolution changes of metric parameters and free energy profile in both phases. Additionally, for selected snapshots obtained from the CPMD trajectories, non-covalent interactions and electronic structure were studied. Quantum theory of atoms in molecules (QTAIM) and the Density Overlap Regions Indicator (DORI) were applied for this purpose. It was found based on Hirshfeld surfaces that, besides intramolecular hydrogen bonds, other non-covalent interactions are present and have a strong impact on the crystal structure organization. The CPMD results obtained in both phases showed frequent proton transfer phenomena. The proton was strongly delocalized in the applied time-scale and temperature, especially in the PIMD framework. The use of metadynamics allowed for tracing the free energy profiles and confirming that the hydrogen bonds present in "proton sponges" are Low-Barrier Hydrogen Bonds (LBHBs). The electronic and topological analysis quantitatively described the temperature dependence and time-evolution changes of the electronic structure. The covalency of the hydrogen bonds was estimated based on QTAIM analysis. It was found that strong hydrogen bonds show greater covalency, which is additionally determined by the proton position in the hydrogen bridge.

A theoretical study of adsorption on iron sulfides towards nanoparticle modeling

Surface modification of zero-valent iron (nZVI) nanoparticles, which are frequently used in the removal of chlorinated hydrocarbons from contaminated groundwater, can increase their surface stability without significant loss of reactivity. Sulfidation is a process during which thin iron sulfide phases are formed on nZVI particles. In this work, the adsorption capability of two iron sulfide minerals (mackinawite and pyrite) and ZVI with respect to two small polar molecules (H2O and H2S) and trichloroethylene (TCE) was modeled by using the quantum mechanics (QM) approach. High-level QM methods used on cluster models helped in benchmarking and validation of density functional theory methods used on periodic slab models of the (001) surfaces of iron sulfides and the (111) surface of ZVI. This careful computational treatment was necessary for achieving reliable results because modeled iron containing compounds represent computationally demanding systems. The results showed that adsorption was strongly affected by surface topology, accessibility of surface sites, and the shape of adsorbed molecular species. The mackinawite surface is practically hydrophobic having weak interactions with polar molecules (about -5/-6 kcal mol-1), in contrast to the surfaces of pyrite and ZVI (adsorption energies are about three times larger). On the other hand, the adsorption of weakly polar planar TCE molecule is relatively strong and similar for all three surfaces (in the range of -11 to -17 kcal mol-1). Moreover, it was shown that the dominant component of the adsorption energy of TCE had originated from dispersion interactions, which were less important for small polar molecules.

Effect of anion reorientation on proton mobility in the solid acids family CsHyXO4(X = S, P, Se,y= 1, 2) fromab initiomolecular dynamics simulations

The high temperature phases of the solid acids CsHSeO₄, CsHSO₄and CsH₂PO₄show extraordinary high proton conductivities, which are enabled by the interplay of high proton transfer rates and frequent anion reorientation.

CO2 Adsorption and Activation on the (110) Chalcopyrite Surfaces: A Dispersion-Corrected DFT + U Study

We have used the density functional theory within the plane-wave framework to understand the reconstruction of most stable (110) chalcopyrite surfaces. Reconstructions of the polar surfaces are proposed, and three different possible nonpolar terminations for the (110) surface, namely, I, II, and III, are investigated. A detailed discussion on stabilities of all three surface terminations is carried out. It is generally observed that the (110) chalcopyrite surfaces encounter significant reconstruction in which the metal Fe and Cu cations in the first atomic layer considerably move downward to the surface, while the surface S anions migrate slightly outward toward the surface. We also investigated the adsorption of the CO₂ molecule on the three terminations for the (110) surface by exploring various adsorption sites and configurations using density functional theory calculations, in which long-range dispersion interactions are taken into consideration. We show that the CO₂ molecule is adsorbed and activated, while spontaneous dissociation of the CO₂ molecule is also observed on the (110) surfaces. Structural change from a neutral linear molecule to a negatively charged (CO₂ ^-δ) slightly or considerably bent species with stretched C-O bond distances are highlighted for description of the activation of the CO₂ molecule. The results address the potential catalytic activity of the (110) chalcopyrite toward the reduction and conversion of CO₂ to the organic molecule, which is appropriate to the production of liquid fuels.

Energy dispersive spectrometry and first principles studies on the oxidation of pentlandite

Experimental and computational studies were carried out to investigate the oxidation of pentlandite (Fe4.5Ni4.5S8). The oxidation product was first analyzed by energy dispersive spectrometry to reveal the elemental distribution at the cross section. Our experimental study shows that the Fe atoms in pentlandite migrated to the surface and were preferentially oxidized to form a thin layer of Fe2O3, whereas the Ni atoms remained at the center of the grain. Furthermore, density functional theory calculations were performed to investigate the adsorption and diffusion of atomic oxygen as well as the adsorption and dissociation of molecular oxygen on the (001) and (010) surfaces of pentlandite. From the calculated adsorption energies of atomic oxygen at the different sites of the (001) and (010) surfaces, we found that oxidation of the Fe sites was preferable to that of the Ni sites when exposed to an oxidizing atmosphere. For molecular oxygen adsorption on the surfaces of pentlandite, the bridge sites (Fe-Ni and Fe-Fe) were found to be the most favorable adsorption sites. The dissociative adsorption of O2 is thermodynamically more favorable than the molecular adsorption. Calculated dissociation barriers show that the oxidation is feasible during high temperature roasting.

Interactions of Oxygen and Water Molecules with Pyrite Surface: A New Insight

Pyrite is the most common sulfide in nature, and it is well-known for its roles in acid mine drainage, flotation separation of useful metal (Cu, Pb, Zn, and Mo) sulfide minerals, optoelectronic and photovoltaic application, pneumoconiosis, and even in the origin of life. However, the detailed oxidation behaviors of pyrite are still unclear and not well-understood. New oxidation pathways by O₂ on the pyrite (100) surface have been found in this work for the first time using density functional theory simulation; that is, besides Fe sites, S sites are also possible oxidation sites in the initial oxidation state of pyrite, where easier and stronger oxidation may occur. This is the first time to confirm the other researchers' conjecture on the direct oxidation of S sites, which explains the isotopic composition experiments that a minor amount of O₂ is permanently incorporated into SO₄^2- during pyrite oxidation (O in SO₄^2- is mainly derived from water). We constructed various H₂O-O₂ coadsorption models on the pyrite surface by considering the adsorption sequence of H₂O and O₂. It is found that the H₂O molecule undergoes step-wise dissociation in the presence of the O₂ molecule. Hydroxyl radical •OH is the reactive oxygen species during H₂O dissociation. Cyclic voltammetric measurements confirm the presence of •OH. In addition, H₂O₂ may also be formed on the surface in terms of H₂O-then-O₂ sequence adsorption.

A kinetic model of water adsorption, clustering and dissociation on the Fe3S4{001} surface

The interaction of water with catalyst surfaces is a common process which requires investigation. Here, we have employed density functional theory calculations to investigate the adsorption of up to ten water molecules on the {001} surface of greigite (Fe₃S₄), which owing to its redox properties, is of increasing interest as a catalyst, e.g. in electro-catalysis. We have systematically analyzed and characterized the modes of water adsorption on the surface, where we have considered both molecular and dissociative adsorption processes. The calculations show that molecular adsorption is the predominant state on these surfaces, from both a thermodynamic and kinetic point of view. We have explored the molecular dispersion on the surface under different coverages and found that the orientation of the molecule, and therefore the surface dipole, depends on the number of adsorbed molecules. The interactions between the water molecules become stronger with an increasing number of water molecules, following an exponential decay which tends to the interaction energy found in bulk water. We have also shown the evolution of the infra-red signals as a function of water coverage relating to the H-bond networks formed on the surface. Next we have included these results in a classical micro-kinetic model, which introduced the effects of temperature in the simulations, thus helping us to derive the water cluster size on the greigite surface as a function of the initial conditions of pressure, temperature and external potential. The kinetic model concluded that water molecules agglomerate in clusters instead of wetting the surface, which agrees with the low hydrophilicity of Fe₃S₄. Clusters consisting of four water molecules was shown to be the most stable cluster under a wide range of temperatures and external potential.

DFT-D2 simulations of water adsorption and dissociation on the low-index surfaces of mackinawite (FeS)

The adsorption and dissociation of water on mackinawite (layered FeS) surfaces were studied using dispersion-corrected density functional theory (DFT-D2) calculations. The catalytically active sites for H2O and its dissociated products on the FeS {001}, {011}, {100}, and {111} surfaces were determined, and the reaction energetics and kinetics of water dissociation were calculated using the climbing image nudged elastic band technique. Water and its dissociation products are shown to adsorb more strongly onto the least stable FeS{111} surface, which presents low-coordinated cations in the surface, and weakest onto the most stable FeS{001} surface. The adsorption energies decrease in the order FeS{111} > FeS{100} > FeS{011} > FeS{001}. Consistent with the superior reactivity of the FeS{111} surface towards water and its dissociation products, our calculated thermochemical energies and activation barriers suggest that the water dissociation reaction will take place preferentially on the FeS nanoparticle surface with the {111} orientation. These findings improve our understanding of how the different FeS surface structures and the relative stabilities dictate their reactivity towards water adsorption and dissociation.

Periodic DFT+U investigation of the bulk and surface properties of marcasite (FeS2)

Marcasite FeS₂ and its surfaces properties have been investigated by Hubbard-corrected density functional theory (DFT+U) calculations.

Catalytic water dissociation by greigite Fe3S4 surfaces: density functional theory study

The iron sulfide mineral greigite, Fe3S4, has shown promising capability as a hydrogenating catalyst, in particular in the reduction of carbon dioxide to produce small organic molecules under mild conditions. We employed density functional theory calculations to investigate the {001},{011} and {111} surfaces of this iron thiospinel material, as well as the production of hydrogen ad-atoms from the dissociation of water molecules on the surfaces. We systematically analysed the adsorption geometries and the electronic structure of both bare and hydroxylated surfaces. The sulfide surfaces presented a higher flexibility than the isomorphic oxide magnetite, Fe3O4, allowing perpendicular movement of the cations above or below the top atomic sulfur layer. We considered both molecular and dissociative water adsorption processes, and have shown that molecular adsorption is the predominant state on these surfaces from both a thermodynamic and kinetic point of view. We considered a second molecule of water which stabilizes the system mainly by H-bonds, although the dissociation process remains thermodynamically unfavourable. We noted, however, synergistic adsorption effects on the Fe3S4{001} owing to the presence of hydroxyl groups. We concluded that, in contrast to Fe3O4, molecular adsorption of water is clearly preferred on greigite surfaces.

Prebiotic NH3 Formation: Insights from Simulations

Simulations of prebiotic NH₃ synthesis from NO₃⁻ and NO₂⁻ on pyrite surfaces under hydrothermal conditions are reported. Ab initio metadynamics calculations have successfully explored the full reaction path which explains earlier experimental observations. We have found that the reaction mechanism can be constructed from stepwise single atom transfers which are compatible with the expected reaction time scales. The roles of the hot-pressurized water and of the pyrite surfaces have been addressed. The mechanistic picture that emerged from the simulations strengthens the theory of chemoautotrophic origin of life by providing plausible reaction pathways for the formation of ammonia within the iron-sulfur-world scenario.

Theoretical Design by First Principles Molecular Dynamics of a Bioinspired Electrode-Catalyst System for Electrocatalytic Hydrogen Production from Acidified Water

Bacterial di-iron hydrogenases produce hydrogen efficiently from water. Accordingly, we have studied by first-principles molecular-dynamics simulations (FPMD) electrocatalytic hydrogen production from acidified water by their common active site, the [FeFe]H cluster, extracted from the enzyme and linked directly to the (100) surface of a pyrite electrode. We found that the cluster could not be attached stably to the surface via a thiol link analogous to that which attaches it to the rest of the enzyme, despite the similarity of the (100) pyrite surface to the Fe4S4 cubane to which it is linked in the enzyme. We report here a systematic sequence of modifications of the structure and composition of the cluster devised to maintain the structural stability of the pyrite/cluster complex in water throughout its hydrogen production cycle, an example of the molecular design of a complex system by FPMD.

Quantifying the origin of inter-adsorbate interactions on reactive surfaces for catalyst screening and design

The adsorption energy of reactant molecules and reaction intermediates is one of the key descriptors of catalytic activity of surfaces and is commonly used as a metric in screening materials for design of heterogeneous catalysts. The efficacy of such screening schemes depends on the accuracy of calculated adsorption energies under reaction conditions. These adsorption energies can depend strongly on interactions between adsorbed molecules in the adlayer. However, these interactions are typically not accounted for in screening procedures that use DFT-based zero-coverage adsorption energies. Identifying the physical mechanisms behind these interactions is essential to model realistic catalyst surfaces under reaction conditions and to understand the dependence of adsorption energies on reaction parameters like surface strain and composition. This article describes a method to quantitatively resolve the observed inter-adsorbate interactions into various direct adsorbate-adsorbate interactions (i.e. Coulombic and steric) and surface-mediated interactions (i.e. adsorbate-induced surface relaxation and change in electronic structure) by combining density functional theory and cluster-expansion calculations of coverage-dependent adsorption energies. The approach is implemented on a model catalyst surface of FeS2(100) reacting with H2S molecules. We find that the adsorption energy of H2S molecules can be affected by over 0.55 eV by the repulsive inter-adsorbate interactions caused primarily by the adsorbate-induced changes to the electronic structure of the FeS2 surface. These interactions also show a strong monotonic dependence on surface strain, being three times stronger on compressively strained surfaces than on surfaces under tensile strain. The large magnitude of inter-adsorbate interactions as well as their strong dependence on lattice strain demonstrate the need for using coverage-dependent adsorption energies for more accurate screening, for example for strained catalytic systems like core-shell and overlayer structures.

Pyrite in contact with supercritical water: the desolation of steam

The supercritical water and pyrite interface has been studied by DFT calculations. A surprisingly dry surface has been found which points to a new reactivity under extreme conditions which has relevance in the iron–sulfur world prebiotic chemistry of the early Earth.

Electronic and structural properties of bulk arsenopyrite and its cleavage surfaces – a DFT study

We have investigated the structural and electronic properties of arsenopyrite and its cleavage surface formation using a density functional/plane waves method. QTAIM and ELF were applied for investigating the nature of the bonding in arsenopyrite.

Is trabecular bone permeability governed by molecular ordering-induced fluid viscosity gain? Arguments from re-evaluation of experimental data in the framework of homogenization theory

It is generally agreed on that trabecular bone permeability, a physiologically important quantity, is governed by the material׳s (vascular or intertrabecular) porosity as well as by the viscosity of the pore-filling fluids. Still, there is less agreement on how these two key factors govern bone permeability. In order to shed more light onto this somewhat open issue, we here develop a random homogenization scheme for upscaling Poiseuille flow in the vascular porosity, up to Darcy-type permeability of the overall porous medium "trabecular bone". The underlying representative volume element of the macroscopic bone material contains two types of phases: a spherical, impermeable extracellular bone matrix phase interacts with interpenetrating cylindrical pore channel phases that are oriented in all different space directions. This type of interaction is modeled by means of a self-consistent homogenization scheme. While the permeability of the bone matrix equals to zero, the permeability of the pore phase is found through expressing the classical Hagen-Poiseuille law for laminar flow in the format of a "micro-Darcy law". The upscaling scheme contains pore size and porosity as geometrical input variables; however, they can be related to each other, based on well-known relations between porosity and specific bone surface. As two key results, validated through comprehensive experimental data, it appears (i) that the famous Kozeny-Carman constant (which relates bone permeability to the cube of the porosity, the square of the specific surface, as well as to the bone fluid viscosity) needs to be replaced by an again porosity-dependent rational function, and (ii) that the overall bone permeability is strongly affected by the pore fluid viscosity, which, in case of polarized fluids, is strongly increased due to the presence of electrically charged pore walls.

Effects of sulfur-deficient defect and water on rearrangements of formamide on pyrite (100) surface

The efficient formation of HCN/HNC from formamide (FM) combining the advantages of water-assistance, self-catalyzed reactions, and the mineral surfaces was investigated. Periodic density functional theory calculations with plane-wave pseudopotential basis sets were performed to study the interaction of FM with pyrite (100) ideal and defect surfaces. Effects of sulfur vacancy defect and water on tautomerization and rearrangement barriers of FM on the (100) surface were evaluated. Calculated results show that FM adsorbs more strongly on the defect surface than on the ideal surface, with the lowest adsorption energy on the defect surface being -22 kcal/mol. The energy barriers for rearrangements of FM on these two surfaces being close to each other suggests that the adsorptions on the surfaces have small effects on the energy barriers. The energy barriers for formimic acid isomer formations are 44.5 and 46.0 kcal/mol, and those of aminohydroxymethylene formations are 72.6 and 71.9 kcal/mol on the ideal and defect surfaces, respectively. A reduction of ∼30 kcal/mol in tautomerization energy barriers is observed in water-assisted process on the defect surface. Because this reduction is close to that of the gas-phase reactions, the catalytic effect is clearly due to the presence of water molecule instead of the interaction with the surface. In this case, the pyrite surfaces with the ability to accumulate reactive species only play the role of connecting bridges between the two steps of the proposed reaction mechanism: the water-assisted rearrangement and the self-catalyzed dehydration.

Interaction of Oxygen and Water with the (100) Surface of Pyrite: Mechanism of Sulfur Oxidation

We present a density-functional study of the adsorption and reactions of oxygen and water with the (100) surface of pyrite. We find that dissociative adsorption is energetically favorable for oxygen, forming ferryl-oxo, Fe(4+)═O(2-), species. These transform easily to ferric-hydroxy, Fe(3+)-OH(-), in the presence of coadsorbed water, and the latter fully covers the surface under room conditions. A mechanism for surface oxidation is identified, which involves successive reactions with molecular oxygen and water, and leads to the complete oxidation of a surface sulfur to SO4(2-). The crucial recurring process is the surface O(2-) and OH(-) species acting as proton acceptors for incoming water molecules. Using a recently proposed method, we examine the oxidation state changes of the surface ions and the electron flow during the adsorption and oxidation processes. The oxidation mechanism is consistent with isotopic labeling experiments, suggesting that the oxygens in SO4(2-) from gas-phase oxidation are derived from water.

Highly spin-polarized conducting state at the interface between nonmagnetic band insulators: LaAlO3/FeS2 (001)

First-principles density functional calculations demonstrate that a spin-polarized two-dimensional conducting state can be realized at the interface between two nonmagnetic band insulators. The (001) surface of the diamagnetic insulator FeS(2) (pyrite) supports a localized surface state deriving from Fe d orbitals near the conduction band minimum. The deposition of a few unit cells of the polar perovskite oxide LaAlO(3) leads to electron transfer into these surface bands, thereby creating a conducting interface. The occupation of these narrow bands leads to an exchange splitting between the spin subbands, yielding a highly spin-polarized conducting state distinct from the rest of the nonmagnetic, insulating bulk. Such an interface presents intriguing possibilities for spintronics applications.

Glycine at the Pyrite−Water Interface: The Role of Surface Defects

Ab initio molecular dynamics simulations were performed in order to study chemisorption, electronic properties, and desorption of glycine at wet pyrite surfaces focusing on the role of surface point defects. The change in the electronic structure and its influence on the chemical reactivity of the free FeS(2)(100) surface due to sulfur vacancies was studied in detail yielding several adsorption modes of glycine and water molecules. Energetically preferred adsorption modes were furthermore investigated in the presence of hot pressurized water mimicking "Iron Sulfur World" prebiotic conditions. The metadynamics Car-Parrinello technique was employed to map the free energy landscape including paths and barriers for desorption of glycine from such wet defective surfaces. The ubiquitous sulfur vacancies are found to increase the retention time of the adsorbed amino acid by many orders of magnitudes in comparison to the ideal pyrite-water interface. The importance of these findings in terms of a possible two-dimensional primordial chemistry on mineral surfaces is discussed.

Ab initio simulations of desorption and reactivity of glycine at a water-pyrite interface at "iron-sulfur world" prebiotic conditions

Glycine at the interface of a pyrite surface (001) FeS2, and bulk water at high pressure and temperature conditions relevant to the "iron-sulfur world" scenario of the origin of life is investigated by theoretical means. Car-Parrinello molecular dynamics is used in order to study the desorption process of the zwitterionic form of this amino acid using two different adsorption modes, where either only one or both oxygens of the carboxylate group are anchored to surface iron atoms. It is found that the formation of stabilizing hydrogen bonds plays a key role in the detachment process, leading to longer retention times for the bidentate adsorption mode. In addition, the chemical reactivity of this heterogeneous system is probed by calculating the Fukui functions as site-specific reactivity indices. The most prominent targets for both nucleophilic and electrophilic reactions to occur are surface atoms, whereas the reactivity of glycine is only slightly affected upon anchoring.

Mechanistic aspects of pyrite oxidation in an oxidizing gaseous environment: an in situ HATR-IR isotope study

The reaction of FeS2 (pyrite) with gaseous H2O, O2, and H2O/O2 was investigated using horizontal attenuated total reflection Fourier transform infrared spectroscopy (HATR-FTIR). Spectra were interpreted with the aid of hybrid molecular orbital/density functional theory calculations of sulfate-iron hydroxide clusters. Reaction of pyrite in gaseous H2O led primarily to the formation of iron hydroxide on pyrite. Exposure of the pyrite to gaseous O2 after exposure to H2O vapor led to the formation of sulfur oxyanions that included SO42-. Isotopic labeling experiments showed that after this exposure sequence the oxygen in the sulfate product was primarily derived from the H2O reactant. If, however, pyrite was exposed to gaseous O2 prior to pure H2O vapor, both SO42- and iron oxyhydroxide became significant products. Isotopic rabeling experiments using the O2-then-H2O sequence showed that the oxygen in the SO42- product was derived from both H2O and O2. The results indicate that H2O and O2 exhibit a competitive adsorption on pyrite, with H2O blocking surface sites for O2 adsorption. The extent of oxygen incorporation from either the H2O or the O2 component into the surface-bound sulfur oxyanion product appears to be a strong function of the relative concentration ratio of the reactant H2O and O2.