Pyrite, one of the most important minerals to catalyze redox reactions in nature and a bulk low-spin Fe mineral, needs to provide high-spin Fe on surfaces to moderate spin-forbidden transitions. Here, the spin state of pyrite is investigated using density functional theory (DFT) calculations on cluster and periodic models. The energies of clusters FexS2x (where x = 4, 8, 16, and 32) were calculated as a function of total spin and different up/down spin configurations. The undercoordinated Fe on surfaces, edges, and corners were found to provide intermediate and high-spin Fe necessary for catalysis. Generally, the lower the crystal field splitting energy (CFSE), Δ, for a particular Fe atom, the higher is the spin density. Pyrite bulk (3D) and surfaces (2D) (+ water to mimic aqueous systems) were examined. The calculated bulk band gap (0.95 eV) is in excellent agreement with previous reports. For the surface, a conducting state is predicted. The calculated CFSE for bulk Fe(ii) in pyrite (∼2.2 eV) agrees with previous CFT results; due to surface states, this CFSE decreases to ∼1 eV on terraces. This study highlights the importance of accurately describing the spin state of pyrite.

Barite dissolution rates and features were investigated under conditions that combine a variety of organic ligands with different background electrolyte solutions. The organic ligands included low molecular weight organic compounds (LMWO) that can be produced by halophilic bacteria and are common in natural environments, as well as synthetic chelators that were previously studied in barite dissolution. Background electrolyte solutions included deionized water, 0.72M NaCl and 2M NaCl solutions designed to simulate freshwater, seawater, and brine fluids, respectively. Barite dissolution rates and corresponding rate constants normalized for saturation states were calculated for most experiments. Equal or lower rates and rate constants for pure LMWO relative to controls at all background electrolyte concentrations and pH values indicated that LMWO do not promote barite dissolution. Synthetic chelators showed higher rates and rate constants at pH 10 at static conditions than controls, indicating enhanced barite dissolution. Distinctive etch pit morphologies on the barite (001) face were observed in the presence of synthetic chelators, suggesting a direct contact dissolution mechanism by synthetic chelators. The effect of synthetic chelators on barite dissolution was significantly reduced in the presence of 2 M NaCl and/or at pH 6.5 due to salting out effects. Under hypersaline conditions, barite dissolution rates and rate constants by bacterial activities were up to four times higher than those of synthetic chelators. The wide range of conditions studied in this work provides insights into prediction of barite dissolution behavior in various natural environments as well as in industrial treatment procedures. (C) 2018 Elsevier Ltd. All rights reserved.

Radium in hydraulic fracturing wastewaters derives from two isotopically distinct end-members in the shale, labile 228Ra hosted by mineral surfaces (226Ra/228Ra atom ratio ~250) and exchangeable 226Ra hosted by organic surfaces (226Ra/228Ra ~10,000). Here we use mass balance and isotope mixing models to reconcile extraction of Ra from these phases with mechanisms of Marcellus wastewater production. Radium isotopic mass balance requires that the characteristic water-rock ratio between wastewater and shale is exceedingly low, on the order of 0.04, and that this ratio decreases with time during wastewater production. An evolving water-rock interaction drives increasing Ra concentrations (=[Ra]) and 226Ra/228Ra ratios during wastewater production, all mediated by increasing [Ca2+] that favors desorption of 226Ra from organics. Our observations and models of Ra isotope geochemistry are best reconciled with observations of water and salinity mass balance, δ18O, Na-Br-Cl, and 87Sr/86Sr if wastewater is produced by mixing of injected fluids with a limited volume of pore brine (on the order of 13% by volume), accompanied by contemporaneous extraction of excess alkaline earth elements by water-rock exchange. Validated using Ra isotope data, this model attributes the extreme salinity and [Ra] in wastewaters to the progressive, hydrologic enrichment of injected fluids during hydraulic fracturing.

High concentrations of barium (Ba), strontium (Sr) and radium (Ra) are present in both the liquid and suspended solid portions of wastewater produced from hydraulic fracturing. These high concentrations often require special treatment in which the solid and liquid portions are separated and then independently treated prior to disposal. The solids are typically disposed in landfills while the liquids are further treated, recycled for future hydraulic fracturing, or disposed via injection wells. Finding optimal treatment methods of both the solid and the liquid fractions requires a thorough understanding of potential Ra mobility from both the raw suspended solids and mineral precipitates formed during treatment. Using a sequential extraction procedure, we found that, without treatment, more than 50% of Ra-226 in the suspended solids was associated with soluble salts and readily exchangeable fractions. When the liquid portion of the wastewater was treated by mixing with acid mine drainage (AMD), which contained high sulfate concentrations, approximately 80-97% of the total Ra-226 in the mixture solution is found in the insoluble sulfate fraction of the precipitate. The activity of Ra-226 sequestered in the precipitated solid sulfate fractions is positively correlated with the Sr/Ba ratio of the wastewater-AMD solution. We discuss implications of these findings for effective long-term management of elevated radium in both solid and liquid wastes.

Hydraulic fracturing of the Marcellus Shale produces wastewaters that are hypersaline and highly enriched in isotopes of radium. Radium is understood to derive from the Marcellus Shale itself, but its source phases and their contributions to wastewater production have not been described. Using sequential extractions and experimental leachates, we characterize two distinct end-members that could contribute Ra to wastewaters, (1) a mineral phase, which hosts labile Ra-228 and has Ra-226/Ra-228 atom ratios similar to 250, and (2) an organic phase, which hosts exchangeable 226 Ra and has Ra-226/Ra-228 similar to 10,000. In leaching experiments we observed rapid extraction of Ra from these phases, with high ionic strength solutions leaching up to 14% of Ra from the shale in just hours. Radium concentrations and 226 Ra/ 228 Ra ratios increase with [Ca2+] of the leaching solution, and solutions approaching 1 M Ca2+ produce Ra-226/Ra-228 ratios compatible with Marcellus wastewaters. In contrast, pure water removes < 0.5% of Ra from the shale with low Ra-226/Ra-228 ratios incompatible with wastewaters. Experimental results and wastewater data together provide a coherent picture, that the distinctive Ra isotopic signature of Marcellus wastewaters results from contemporaneous water-rock interactions that promote desorption of Ra-226 from organics during hydraulic fracturing.

Fluids injected into shale formations during hydraulic fracturing of black shale return with extraordinarily high total-dissolved-solids (TDS) and high concentrations of barium (Ba) and radium (Ra). Barite, BaSO4, has been implicated as a possible source of Ba as well as a problematic mineral scale that forms on internal well surfaces, often in close association with radiobarite, (Ba, Ra) SO4. The dissolution of barite by abiotic processes is well quantified. However, the identification of microbial communities in flowback and produced water necessitates the need to understand barite dissolution in the presence of bacteria. Therefore, we evaluated the rates and mechanisms of abiotic and microbially-mediated barite dissolution under anoxic and hypersaline conditions in the laboratory. Barite dissolution experiments were conducted with bacterial enrichment cultures established from produced water from Marcellus Shale wells located in northcentral Pennsylvania. These cultures were dominated by anaerobic halophilic bacteria from the genus Halanaerobium. Dissolved Ba was determined by ICP-OES and barite surfaces were investigated by SEM and AFM. Our results reveal that: 1) higher amounts of barium (up to similar to 5 x) are released from barite in the presence of Halanaerobium cultures compared to brine controls after 30 days of reaction, 2) etch pits that develop on the barite (001) surface in the presence of Halanaerobium exhibit a morphology that is distinct from those that form during control experiments without bacteria, 3) etch pits that develop in the presence of Halanaerobium exhibit a morphology that is similar to the morphology of etch pits formed in the presence of strong organic chelators, EDTA and DTPA, and 4) experiments using dialysis membranes to separate barite from bacteria suggest that direct contact between the two is not required in order to promote dissolution. These results suggest that Halanaerobium increase the rate of barite dissolution in anoxic and high ionic strength solutions. Additionally, the increase in rate occurs without direct microbe-mineral contact suggesting that metabolites secreted by the bacteria may be responsible for promotion of dissolution. The findings of this study have implications for understanding barium cycling in marine/hypersaline environments, release of barium (and associated radium) from waste solids generated from energy and mining industries, as well as potential for developing new anti-scaling chemicals. (C) 2016 Elsevier Ltd. All rights reserved.

The specific Eh-pH conditions and mechanism(s) for the reduction of arsenite, As(III), by pyrite are incompletely understood. A fundamental question is what role the pyrite surface plays in the reduction process. We used electrochemical methods to evaluate the reduction of As(III) under controlled redox conditions. As(III) reduction to elemental As(0) occurs on the pyrite surface under suboxic-reducing conditions and is promoted at low pH. Remarkably, As(III) reduction on pyrite occurs at similar potentials to those for reduction on platinum metal, suggesting a similar mechanism and kinetics for these surfaces. The onset for As(III) reduction at pH ≤ 3.5 coincides with the potential for hydrogen electroadsorption on pyrite, E ≈ +0.1 V (versus RHE). Batch reactions show that As(III) is reduced on pyrite at the Eh-pH predicted by the electrochemical study. X-ray photoelectron spectroscopy reveals that, at pH ≤ 3.5, a significant fraction of the surface arsenic (30-60%) has an oxidation state consistent with As(0). Here, we propose a mechanism whereby atomic hydrogen that forms on ferric (hydr)oxide surface layers promotes As(III) reduction at low Eh and pH. Insights provided by this study will have implications for understanding the controls on dissolved As(III) concentrations in suboxic-anoxic environments.

Hydraulic fracturing is an important technological advance in the extraction of natural gas and petroleum from black shales, but water injected into shale formations in the fracturing process returns with extraordinarily high total-dissolved-solids (TDS) and high concentrations of barium, Ba. It is generally assumed that high TDS comes from the mixing of surface water (injected fluid) with Na-Ca-Cl formation brines containing elevated Ba, but the mechanisms by which such mixing might occur are disputed. Here we show that Ba in water co-produced with gas could originate from water-rock reactions, with Ba levels observed in produced waters reached on a time scale relevant to hydraulic fracturing operations. We examined samples from three drill cores from the Marcellus Shale in Pennsylvania and New York to determine the possible water-rock reactions that release barium during hydraulic fracturing. Two samples, one containing microcrystalline barite (BaSO4) and one without barite, contain elevated concentrations of Ba relative to the crustal average for shale rocks. A third sample is slightly depleted in Ba relative to the crustal average. Micro-XRF measurements and SEM/EDS analysis combined with chemical sequential extraction methods reveal that a majority of the Ba in all samples (55-77 wt.%) is present in clays and can only be leached from the rock by dissolution in hydrofluoric acid. Thus, a majority of barium in our samples is relatively inaccessible to leaching under hydraulic fracturing conditions. However, the balance of Ba in the rocks is contained in phases that are potentially leachable during hydraulic fracturing (e.g., soluble salts, exchangeable sites on clays, carbonates, barite, organics).We next studied how shale reacts with water at elevated temperatures (80 degrees C), low Eh (-100 to -200 mV), and a range of ionic strengths (IS = 0.85-6.4) that emulate conditions prevalent at depth during hydraulic fracturing. Our experimental results indicate that the amount of Ba released from the bulk rock has a positive correlation with the ionic strength of the reacting fluid. Between 5 and 25% of the total Ba in the rock can be leached from shale under ionic strength conditions and leachate compositions typical of produced waters over a contact time of just 7 days. We suggest that reductive weathering of black shale occurs during hydraulic fracturing due to: 1) Ba2+ in clays exchanging with Na+ and Ca2+ ions that are present in high concentrations in produced water, and 2) increased solubility and dissolution kinetics of barite under high ionic strength conditions. At the low Eh conditions prevalent during hydraulic fracturing the sulfate deficient water allows Ba to be dissolved into the produced water. Based on Ba yields determined from laboratory leaching experiments of Marcellus Shale and a reasonable estimate of the water/rock mass ratio during hydraulic fracturing, we suggest that all of the Ba in produced water can be reconciled with leaching directly from the fractured rock. (C) 2015 Elsevier Ltd. All rights reserved.

A barium-enriched interval of Marcellus Shale (Middle Devonian Oatka Creek Formation) from a core in Chenango County, NY contains similar to 100 mu m diameter ellipsoidal grains with variable mineralogical compositions between pure barite and pure pyrite endmembers. Petrographic characterization and in-situ sulfur isotope analysis by Secondary Ion Mass Spectrometry (SIMS) was performed to better understand the diagenetic conditions under which these grains form and are preserved in the shale. Textural relationships suggest partial to complete pseudomorphic replacement of ellipsoidal barite by pyrite. Spatially, the ellipsoidal grains are concentrated in discrete layers parallel to original bedding and intervals within these layers often contain grains with similar degrees of replacement. The fraction of barite replaced by pyrite between these intervals can vary significantly, which is remarkable considering these intervals are separated by stratigraphic distances on the order of mm to cm in the shale (depths equivalent to deposition over 10' s-1000' s of years).The mean delta S-34 of barite and pyrite in ellipsoidal grains is 63.3 +/- 3.6% and 2.2 +/- 3.0%, respectively, indicating that the grains are authigenic. Mass balance calculations based on density and stoichiometric differences between barite and pyrite indicate that reduction of sulfate from barite alone cannot be the sole source of sulfur in the replaced grains: only similar to 23% of sulfur in pyrite comes from the dissolution of barite while the remainder derives from an additional source with delta S-34 = -17.6 +/- 1.3%. We suggest that pseudomorphic replacement of barite led first to the formation of greigite (Fe3S4), where one mole of sulfur was provided by barite and the other three moles of sulfur were contributed by FeS(aq); the latter formed by reaction of Fe2+ with sulfide from microbial sulfate reduction. Transformation of greigite to pyrite occurred via the sulfur addition and/or iron loss pathways. These observations suggest the following mechanism for the replacement of barite by pyrite in the ellipsoidal barite grains: (1) burial of authigenic barite below the sulfate-methane transition zone (SMTZ), and (2) partial to complete dissolution of the grain and concomitant precipitation of greigite (and its subsequent transformation to pyrite) in the presence of pore water depleted in sulfate and enriched in FeS(aq) and polysulfides. We suggest that closely-spaced intervals containing different barite to pyrite ratios may reflect fine-scale temporal shifts or fluctuations in the position of the SMTZ due to variable rates of methanogenesis and/or sedimentation during diagenesis. (C) 2016 Elsevier Ltd. All rights reserved.

Pentavalent uranium is generally treated as an unstable intermediate when uranyl, U(VI)O22+, is reduced to U4+. However, mineral surfaces have been shown to stabilize pentavalent uranium, thus hindering further reduction (Ilton et al., 2005, 2010). The subject of this study is to identify the kinetic pathways that lead to U(V)O2+ being a metastable species. Electrochemical methods provide an in situ approach for the investigation of the intermediate reaction of U(V)O2+ on the surfaces of magnetite. Redox reactions of uranyl ions on particulate (∼3μm) and bulk magnetite surfaces were investigated using cyclic voltammetry and potential step chronoamperometry using cavity microelectrodes and bulk (planar) mineral electrodes. The estimated redox potentials are consistent with the standard redox potential of UO22+/UO2+, indicating UO22+ is first reduced to UO2+ on the surfaces of both powder and bulk magnetite. The one-electron reduction of UO22+ to UO2+ was further confirmed by directly measuring the number of electrons transferred during the reduction process on the bulk magnetite electrode. Based on the charge conservation analysis and the positive correlation between the pH and the peak current for the UO2+ transformation to UO22+, the peak corresponding to the oxidation of U4+ to UO22+ was assigned in the voltammograms of particulate magnetite. The presence of U4+ indicates that the disproportionation of UO2+ (2U(V)↔U(IV)+U(VI)) is occurring on the surface of particulate magnetite within the timeframe of the experiment. The lack of a peak for U4+ in voltammograms for bulk magnetite suggests that the rate of the UO2+ disproportionation reaction is slower on bulk magnetite than that on particulate magnetite. The catalytic property of particulate magnetite surfaces on the disproportionation reaction is explained by its ability to adsorb and desorb protons, which could facilitate the proton-coupled disproportionation reaction of UO2+. This increased catalytic activity and related adsorption and desorption kinetics of protons may be related to the increased number of under-coordinated surface sites near step edges on the magnetite powder.

Ever since humans discovered how to separate metal from its ore mineral, preserving its metallic luster has been a driving force in the advancement of materials science. In modern times, developing materials that will contain and isolate nuclear waste has pushed corrosion science to new limits. We must now predict corrosion rates over geologic time scales, upwards of a million years. This article reviews the electrochemical basics that underpin metal and mineral corrosion and uses that to understand the case study of copper corrosion in nuclear-waste containers. Electrochemistry can also explain electron-transfer processes on mineral surfaces and so offer insight into weathering and environmentally relevant natural redox processes, such as those forming supergene metal deposits.

Hematite occurs in various geologic settings including igneous, metamorphic, and sedimentary rocks as well as in soils. However, it frequently occurs at low concentrations, especially in soils, where it may be <1% by weight. Because hematite has the potential to be an indicator of oxidizing and climatic conditions in soils and paleosols, it is important to understand its limit of detection. In this paper we examine the limits of detection of hematite visually and with diffuse reflectance spectrophotometry (DRS) and X-ray diffraction (XRD). To accomplish this we used a sample set consisting of "knowns" or calibration samples. These known samples consisted of 15 different matrices of varying mineral composition into which hematite in 7 different concentrations ranging from 0.01 to 4% by weight were mixed. Including the 0% hematite, our calibration data set consisted of 120 samples. Visually, hematite can be detected at a concentration of 0.01% by weight in a light matrix and 0.5% in the darkest of our matrices. However, because of metamerism, visual techniques cannot specifically identify hematite. We find that for both DRS and XRD the limit of detection is also dependent on the matrix. For XRD the limit of detection for hematite in bulk samples is about 1%. For DRS the limit of detection depends on the data reduction technique used. The commonly used Kubelka-Munk remission function and its first and second derivatives can easily identify hematite at the 0.5% level. However, the first derivative of the percent reflectance curve can detect hematite at 0.01% by weight in a light matrix and 0.05% in a dark matrix. We suggest that the first derivative of DRS curves is the best currently available method for qualitatively detecting the mineral hematite at low concentrations found in soils, sediments, and rocks.Work described in this paper may be applied in several situations. Our study of visual limits of hematite detection should aid field geologists in assessing hematite content. Analysis of color wavelength bands may also have application in remote sensing by indicating which bands are most sensitive to hematite, reported to be an important constituent of the martian surface. Furthermore, this study could help clarify remotely sensed terrestrial albedo changes, especially the Sahara/Sahel transition where the sediments change from light, quartz-dominated to dark, hematite-dominated. Our study also points out that with laboratory-based spectra the first derivative of the reflectance curve is the most sensitive transform for processing spectral data for hematite, thereby allowing concentrations as low as 0.01% to be detected.

The formation of chondrules, matrix and other components of chondrites is not yet fully understood. Knowing their relative formation ages and genetic relationships can provide crucial information on their formation conditions. Short lived radionuclides can provide new insights into the chronology of the formation of early solar system materials. In the past, the 26Al-26Mg system has been mainly used for this purpose, but recently the 182Hf-182W system emerged as a promising tool to date chondrule formation. Thus, components of chondrites that never underwent high temperature metamorphism, like CV3 chondrites, are well suited candidates for Hf-W investigations. Earlier studies of Allende chondrules with the Al-Mg und PbPb systems indicate that they formed up to 3.2 Ma after CAI formation1-4. We prepared 23 Allende and 13 Vigarano separates for Hf and W isotope measurements, covering both reduced and oxidized CV chondrites. These include pure handpicked chondrule and matrix fractions as well as magnetic separates and bulk aliquots. The separates all define isochrons which indicates a contemporaneous formation of all components. The ages defined by the isochrons suggest formation of Allende and Vigarano components within 3 Ma after CAI formation. These ages are consistent with chondrule formation ages obtained from other isotopic systems. Notably, W and Hf concentrations in chondrule and matrix fractions from Allende and Vigarano vary considerably. Chondrules from Allende exhibit high Hf/W ratios (2.5 to 3.0), whereas matrix and strong magnetic fractions exhibit low Hf/W ratios (0.5 to 0.9). Unlike in Allende, Vigarano contains chondrules with extremly low Hf/W ratios. As suggested from petrological observations, these low ratios can be attributed to the presence of metal inclusions within the reduced Vigarano chondrules.

There are many studies describing the influence of parameters such as pH, pCO(2), and complexing ligands on the sorption of the aqueous uranyl species onto mineral surfaces. However, few of these studies describe the reduction reaction mechanisms and the factors that influence the rate of reduction, despite the fact that the oxidation state of uranium is the most important factor controlling the mobility of uranium. In this study, the energetics and kinetics of the U(VI) reduction half-reaction on pyrite, hematite, and magnetite were investigated by electrochemical methods using a powder microelectrode (PME) as the working electrode. Anodic and cathodic peaks corresponding to the 1 e(-) redox couple, U(VI)/U(V), were identified in cyclic voltammograms of pyrite, hematite, and magnetite at pH 4.5. A second oxidation peak, corresponding to the oxidation of U(IV), was identified and provides evidence for the formation of reduced uranium phase(s) on the mineral surfaces. In addition, uranium-containing precipitates were identified on pyrite surfaces after polarization in a PME. This study identifies the disproportionation of U(V) species on the surface as a possible rate-limiting step in the two-step U(VI) reduction mechanism: (1) charge transfer to form U(V) followed by, (2) a disproportionation reaction that forms U(IV) and U(VI). The Tafel slope (i.e., the derivative of the electrode potential with respect to log [current]) was used to evaluate electrochemical mechanisms. High Tafel slopes (>220 mV/(logunit of current) on all minerals evaluated) suggest that uranyl reduction is mediated by insulating (hydr)oxide layers that are present on the semiconducting mineral surfaces. The onset potential for uranyl reduction was determined for pyrite (>+0.1 V vs. Ag/AgCl), and hematite and magnetite (between-0.02 and-0.1 V vs. Ag/AgCl). The onset potential values establish a baseline kinetic parameter that can be used to evaluate how solution conditions (e. g., dissolved reductants, complexing ligands, and polarizing ions) affect the kinetics of uranyl reduction.The results of this study demonstrate the potential for using PMEs to evaluate redox potentials and mechanisms for U(VI) reduction by Fe-oxides and sulfides under more complex solution conditions as well as other environmentally-relevant mineral-analyte systems. However, it should be noted that the determination of redox kinetics using Butler-Volmer theory has limitations when applied to semiconductor mineral electrodes. Charge depletion in semiconductor surface states can affect the kinetic values obtained for redox reactions on the surface. These limitations and a discussion of the flat band potential are considered in the interpretation of U redox kinetics in this study. (C) 2013 Elsevier Ltd. All rights reserved.