Dissolution Of Pyrite Research Articles

The genesis of an extremely acidic perched aquifer within roasted pyrite waste in a fully urbanized area (Zaragoza, Spain)

Contaminated groundwater is a serious problem in developed countries. The abandonment of industrial waste may lead to acid drainage affecting groundwater and severely impacting the environment and urban infrastructure. We examined the hydrogeology and hydrochemistry of an urban area in Almozara (Zaragoza, Spain); built over an old industrial zone, with pyrite roasting waste deposits, there were acid drainage problems in underground car parks. Drilling and piezometer construction, and groundwater samples revealed the existence of a perched aquifer within old sulfide mill tailings, where the building basements interrupted groundwater flow, leading to a water stagnation zone that reached extreme acidity values (pH < 2). A groundwater flow reactive transport model was developed using PHAST to reproduce flow and groundwater chemistry, in order to be used as a predictive tool for guiding remediation actions. The model reproduced the measured groundwater chemistry by simulating the kinetically controlled pyrite and portlandite dissolution. The model predicts that an extreme acidity front (pH < 2), coincident with the Fe (III) pyrite oxidation mechanism taking dominance, is propagating by 30 m/year if constant flow is assumed. The incomplete dissolution of residual pyrite (up to 18 % dissolved) predicted by the model indicates that the acid drainage is limited by the flow regime rather than sulfide availability. The installation of additional water collectors between the recharge source and the stagnation zone has been proposed, together with periodic pumping of the stagnation zone. The study findings are expected to serve as a useful background for the assessment of acid drainage in urban areas, since urbanization of old industrial land is rapidly increasing worldwide.

Geochemistry of Upper Cretaceous bauxite deposits, Zagros Fold Thrust Belt, SW Iran: Paleoenvironment and provenance constraints

Iranian's karst bauxites, as part of the Irano-Himalayan bauxite belt, are related to four structural zones, including the Zagros Fold-Thrust Belt (ZFTB). The ZFTB consists of 7–12 km of the Paleozoic to Pliocene carbonate sediments deposited on the Precambrian basement and Arabian continental margin. In the ZFTB occur several karst bauxite deposits between the Cenomanian-Turonian shallow-marine argillaceous of the Sarvak Formation and the Santonian-Campanian carbonates of the Ilam Formation. Compositional data of karst bauxite deposits may address paleogeographic restoration and record paleoclimate, and here we discuss a dataset including the Mombi, Bidgol, Tang-e Pirzal, and Bagoushi bauxite deposits in the ZFTB. Although the ZFTB karst bauxites show similar texture, they are different in composition, reflecting different paleoenvironmental formation conditions. In most of the studied bauxites, the composition of ooidal concretions indicates the occurrence of long periods of humid tropical climate followed by short and intermittent dry-to-wet conditions. In the Bidgol deposit, a different pattern is recognized, reflecting the predominance of dry climates and characterized by low aqueous silica activity and low water activity. The formation of iron oxyhydroxide minerals is likely due to a multistage process involving the circulation of Fe-rich solutions in micro-cracks or veins due to pyrite and chamosite dissolution and Fe2+ oxidization. In the ZFTB bauxites, two main trends of Ce/Ce* distribution with depth, a proxy of paleo-redox evolution during bauxitization, are recognized. In the Bidgol and Mombi deposits, Ce/Ce* is negative in the basal portion and increases upward. The positive Ce-anomalies in the upper parts of these deposits are due to ceranite formation under oxidizing environment near the surface. In the Bagoushi and Tang-e Pirzal deposits, Ce/Ce* is close to the unit with the notable exceptions of a few samples characterized by significant and positive Ce/Ce* anomalies. This feature suggests that cerianite, originally ubiquitous in these deposits, was affected by congruent dissolution through cerium reduction with Ce3+ transportation by acidic downward percolating solutions as fluoride complexes (REEF2+ and REEF2+), carbonate complexes (REECO3+) and carbonate–fluoride complexes (REECO3F) followed by fluorocarbonate minerals' precipitation. This process has been repeated several times with maximum distribution throughout the profile due to environmental changes, including periodic fluctuations in the groundwater level. Finally, the Sm/Nd, Nb/Ta, and Eu/Eu* provenance proxies indicate that the protolith(s) from which the ZFTB bauxite deposits are derived is UCC-like, thus excluding supply of mafic material and pointing toward a source material primarily derived from the bedrock argillaceous sediments. In addition, several samples record provenance from siliciclastic craton-derived sediment, suggesting that the ZFTB was connected with a continental margin during the Jurassic-early Cretaceous.

Genesis and mineralization implications of dissolution–regrowth pyrite in the large Qukulekedong Au–Sb deposit, East Kunlun, NW China

The Qukulekedong deposit is a recently discovered large Au–Sb deposit in the western part of the East Kunlun Orogenic Belt, NW China. Pyrite with a characteristic dissolution–regrowth texture has developed in the high-grade Au ore of the deposit. In this study, information on the mineralogical properties, elements distribution, and in situ S isotopic composition of the dissolution–regrowth pyrite was used to interpret its genesis and relationship to Au concentration. The pyrite grains are composed of a core, mantle, and rim. The core consists of pyrite with δ34S values ranging from 1.02 ‰ to 1.99 ‰, indicating a neutral pH, medium-oxygen fugacity environment. The mantle consists of sphalerite, sericite, quartz, and ankerite with dissolution texture, indicating pyrite dissolution and replacement in a neutral pH, high-oxygen fugacity environment. The rim consists of pyrite with fine arsenopyrite inclusions and recovers the crystal shape of the pyrite, which have δ34S values ranging from 1.60 ‰ to 2.67 ‰. This indicates that the rim was formed via regrowth in a neutral pH, low-oxygen fugacity environment. The dissolution of the mantle and regrowth of the rim represent an oxygen fugacity increasing–decreasing process in the formation of pyrite. The increase in oxygen fugacity could remobilize Au from dissolved pyrite, forming a highly Au-concentrated hydrothermal fluid. The decrease in oxygen fugacity triggered the supersaturation and precipitation of Au, forming an Au-rich rim. Combining geological characteristics, the pyrite likely had a magmatic sulfur origin and formed via a dissolution-regrowth process caused by the depressurization–oxidation associated with rock fractures. The dissolution-regrowth pyrite can be used as a mineralogical indicator for regional Au exploration.

Estonian Phosphate Rock Dissolution in Hydrochloric Acid: Optimization of Acid Dosage and Concentration

The increasing need for phosphorus and rare earth elements (REEs) has initiated the studies of new mineral deposits and new complex processing technologies. Estonian phosphate rock (EPR) resources, which are not in use, are estimated to be more than 3 billion metric tons or 800 million tons of P2O5. The experiments of dissolution of three different EPR samples in hydrochloric acid were carried out with the aim of studying the impact of the chemical and mineralogical composition of EPR on the leaching process. The leaching of P, Ca, Mg, and consumption of H+ ions depend on HCl concentration and dosage. The solubility of fluorine and REEs are also influenced by CaF2 and REEs-phosphates precipitation. Fe solubility depends on the mineralogical composition of EPR but also on particle size, acid dosage, pH, and phosphorus content in the solution. The dissolution of pyrite is much lower than the solubility of carbonate apatite. Dolomite dissolution depends on the acid dosage and the fractional composition of EPR. Dolomite dissolution also rests lower than that of apatite. For all the samples studied, the best regression models that describe P, Mg, and Ca solubility and the optimum concentration of HCl for phosphorus dissolution were found using mathematical modeling.

Comparative Experiments on the Role of CO2 in the Gold Distribution between Pyrite and a High-Salinity Fluid

Experimental studies were conducted to identify the physical and chemical features of gold’s behaviour in hydrothermal processes linked to ore formation and involving CO2 in oxidized deposits. With the aid of the autoclave method, in a temperature range of between 200 and 400 °C, the isochoric dependences of the PVT parameters of concentrated sulphate chloride fluids were plotted, both in the presence and absence of CO2. Our experiments established that concentrated sulphate–chloride fluids (22 wt % Na2SO4 + 2.2 wt % NaCl) that lack CO2 are characterized by a wide supercritical temperature range, with homogenization temperatures of between 250 and 325 °C. In the presence of CO2, the same type of fluids showed heterogenization at a molar fraction of XCO2 = 0.18 (t = 192 °C, P = 176 bar). The process of homogenization for these low-density and high-salinity fluids was impossible at temperatures between 375 and 400 °C and at pressures between 600 and 700 bar. The behaviour of gold was studied during its interaction with a basic composition fluid of sulphate–chloride. We applied the autoclave method under the conditions of a simultaneous synthesis of pyrite and gold dissolution (metallic Au), at a temperature of 340 °C and at a pressure of 440 bar. High Au concentrations (up to 4410 ppm of Au in CO2-bearing fluids) were attained at high gold solubilities (up to 13.5 ppm in the presence of CO2), owing to the process of Au reprecipitation within the pyrite phase. We did not detect Au in the pyrite when we used the XRD or SEM methods, which suggested that it might be present as invisible gold. High values of the distribution coefficient (KD = CAu(solid)/CAu(solution)) in the fluids lacking (KD = 62) and bearing CO2 (KD = 327) empirically confirmed the possibility that gold concentrates in pyrite in structurally non-binding forms.

Fe/S oxidation-coupled arsenic speciation transformation mediated by AMD enrichment culture under different pH conditions

Arsenic (As) speciation transformation in acid mine drainage (AMD) is comprehensively affected by biological and abiotic factors, such as microbially mediated Fe/S redox reactions and changes in environmental conditions (pH and oxidation-reduction potential). However, their combined impacts on arsenic speciation transformation remain poorly studied. Therefore, we explored arsenic transformation and immobilization during pyrite dissolution mediated by AMD enrichment culture under different acidic pH conditions. The results for incubation and mineralogical transformation of solid residues show that in the presence of AMD enrichment culture, pH 2.0, 2.5, and 3.0 are more conducive to the formation of jarosites and ferric arsenate, which could immobilize high quantities of dissolved arsenic by adsorption and coprecipitation. The pH conditions significantly affect the initial adsorption of microbial cells to the minerals and the evolution of microbial community structure, further influencing the biodissolution of pyrite and the release and oxidation process of Fe/S. The results of Fe/S/As speciation transformation of the solid residues show that the transformation of Fe, S, and As in solution is mainly regulated by pH and potential values, which imposed significantly different effects on the formation of secondary minerals and thus arsenic oxidation and immobilization. The above results indicated that arsenic transformation is closely related to the Fe/S oxidation associated with pyrite bio-oxidation, and this correlation is critically regulated by the pH conditions of the system.

Petrographic, mineralogical, morphological and organic constraints of the Permian shaly-coal in the Tuli Basin of Limpopo-Area Karoo-Aged basin, South Africa: Implication for potential gas generation

The transition to a low-carbon energy source from coal has become imperative to mitigating the escalating climate change crisis. This study aims to investigate the organic-rich shale core samples of the Limpopo-Area Karoo Basin, retrieved from the borehole at depth 480–580 m. These samples were subjected to petrographic, mineralogical, morphological, and kerogen-type analysis to investigate potentials for shale gas generation. The petrographic analysis reveals maceral group mainly comprised of vitrinite group with some liptinite and inertinite and mineral compositions. The preponderance of pyrite framboids (1.6–12%) indicate redox of FeS2 as a precursor to methane gas generation in anoxic condition. The x-ray diffractometer (XRD) reveals the presence of quartz, albite, microcline, dolomite, pyrite and clay minerals which alluded to the mineral composition indicated by the petrographic analysis. The clay mineral component consists of montmorillonite, illite, chlorite, and mixed layered illite/smectite (I/S) in the Madzaringwe samples. The representative scanning electron microscopy-Energy Dispersive X-ray (SEM-EDX) images of the studied shales depict a combination of organic-matter, groundmass mineral, micro-fracture pore structures reflecting polyframboidal pyrite and carbonate dissolution morphology. The total organic carbon (TOC) contents averaging at 47 wt%, indicating an excellent source rock. The Rock-Eval 6 programmed pyrolysis samples showed S2 value (15.25–16.47 mg HC/g rock) with an average of 15.69 mg HC/g rock and Hydrogen index (HI) (34.0–37.0 mg HC/g TOC) indicating a Type-III Kerogen dominance prone to gas generation. The shale showed Tmax values (464–470 °C) averaging at 467.2 °C, yielding a thermally mature condensate wet-gas. This study reveals that the Permian carbonaceous rock tends to generate gas which can be hosted mainly by the organic matter pore structures, inorganic and micro-fracture pore structures.

Mineralogy and geochemistry of pattern formation in zebra rock from the East Kimberley, Australia

Rhythmic patterns are widespread in geological materials. A particularly striking, macroscale example is zebra rock from the East Kimberley region of northwestern Australia. The rock is famous for its distinctive, rhythmically ordered, iron-oxide pattern, which transforms an Ediacaran-aged siltstone into an attractive semi-precious gemstone. Several different formation mechanisms of this pattern have been proposed in previous studies, with the two most prominent being redoximorphic banding in acid-sulfate soils and Liesegang banding in an acidic hydrothermal system. Using a combination of mineralogy, geochemistry and geological context, this study attempts to confirm both the occurrence and relative timing of acid-sulfate fluid rock interactions in zebra rock and seeks to determine whether pedogenic processes or hydrothermal alteration can better explain the origin of these patterns. We present the first evidence that the iron-oxide banding developed simultaneously with a period of aluminosilicate dissolution, clay precipitation, and fluid flow, consistent with the infiltration of an acidic fluid. This conclusion was evidenced through the hexagonal-platelet morphology of the hematite pigment, kaolinite-hematite textural relationships, and a paleoflow direction preserved in the asymmetric intensity of the hematite concentration. However, while the mineralogy of zebra rock strongly suggests interactions with acid-sulfates, the origin and temperature of the fluid could not be conclusively determined. Supporting a hydrothermal origin, a thorough analysis of zebra rock mineralogy revealed a mineral assemblage consistent with advanced argillic hydrothermal alteration, wherein pyrophyillite, kaolinite, and dickite indicate minimum palaeotemperatures upwards of 120 °C and the presence of alunite and a svanbergite-woodhouseite solid solution suggests oxidising, acidic (pH <5) conditions. The low Rb/Sr ratio and relative immobility of rare earth elements in most zebra rock deposits are also consistent with an acidic hydrothermal origin. Further support was also observed in the mineralogical trends between examined outcrops, grading from alunite-type to kaolinite/dickite-type facies in a south-west direction. But despite this evidence, the acid-sulfate soil hypothesis could not be refuted and was itself supported by the large number of pyrite dissolution voids both underlying and within the patterned layer. Furthermore, the consistent compaction of reduction spheroids, dissolution voids, and pseudomorphic inclusions within the light banding of zebra rock are in agreement with near-surface supergene weathering, ruling out hypogene hydrothermal alteration. A mechanism of pattern formation is proposed whereby zebra rock banding is formed by the Liesegang phenomenon, driven by the oxidation of Fe2+ ions during the infiltration of an Fe2+-rich, acid-sulfate fluid into oxidising host sediments.

CO2 Reaction-Diffusion Experiments in Shales and Carbonates

The evaluation of caprock integrity and reservoir efficiency is critical for safe CO2 geological storage management. It is therefore important to investigate geochemical reactions between CO2-rich fluids and host rocks and their contribution in retaining CO2 at depth. This study deals with diffusive reaction experiments on shales and carbonate samples cored from an offshore structure in the Malaysian basin, a potential target for CO2-enhanced gas recovery. The aim is to evaluate the CO2 reaction front velocity in a typical shaly caprock and the mineral response of the reservoir. Rock samples were characterized in terms of texture, chemistry, and mineralogy by X-ray diffraction, electron microscopy (SEM), microanalysis (EDS), infrared spectroscopy (FT-IR), rock geochemistry (XRF), and mercury injection capillary permeability (MICP). Performed analyses show mineralogical alteration induced by CO2 as it penetrated into the samples. Carbonate dissolution and weathering of pyrite to form secondary carbonates belonging to siderite-ankerite series were observed along two reaction fronts. Estimated diffusion coefficients of CO2 are two orders of magnitude lower than CO2(aq) molecular diffusion in pure water and from half to an order of magnitude lower than diffusivity computed on unaltered sample, highlighting the important effect of gas–water–rock reactions on the CO2(aq) diffusivities in shales and carbonates. Results obtained in this study provide an insight regarding the effect of geochemical reactions on CO2 transport and represent a further discussion point on the diffusion coefficients.

Experimental insight into the dissolution behavior and interaction of key components in shale-persulfate system

Shale-persulfate interaction is ubiquitous in hydraulic fracturing, but the underlying geochemical reaction and its impact on the shale matrix are still not well understood. In this study, key components, including pyrite, calcite, chlorite, and kerogen, were chosen to react with sodium persulfate (Na2S2O8) at reservoir temperature. Results showed the oxidation-dominated reaction in the pyrite-persulfate system turned to acidification in calcite/chlorite-persulfate. In the pyrite-rich kerogen (PRK), however, kerogen competed with pyrite for persulfate consumption. An increased concentration of Na2S2O8 enhanced the dissolution of pyrite, PRK, and chlorite but resulted in the mass increase of solid in the calcite-Na2S2O8 system because of gypsum precipitation. The concentration of dissolved cations significantly correlated with the mass change of the corresponding components during persulfate stimulation, demonstrating the feasibility of water chemistry for evaluating the potential matrix alteration. Reaction in the treatment of persulfate on a multi-component mixture further suggested a complex interaction. The higher amounts of PRK, calcite, and chlorite led to the enhanced dissolution of Fe, Ca, and Mg, respectively. Various chemical reactions also influenced the dissolution behavior of cations. At low pyrite dosage, chlorite was acidly etched and slightly contributed to the dissolution of Fe. The dissolution of calcite and precipitation of gypsum affected the behavior of Ca. Element Mg mainly came from chlorite, but persulfate hydrolysis and PRK oxidation also changed its dissolution. Based on the total mass change of the multi-component mixture after persulfate treatment, a binary model was established to predict the mass change and quantify the potential matrix alteration during shale-persulfate interaction. This model could be used for selecting a potential reservoir during the in situ application of persulfate stimulation for shale permeability enhancement.

Geochemistry and palaeo-hydrogeology of the weathered zone in the Opalinus Clay

The effects of weathering and decompaction on the Opalinus Clay in northern Switzerland were studied on the basis of a 146 m long drillcore profile. Below 6 m of Quaternary sediments, the rock down to 16 m depth is affected by partial or complete dissolution of siderite, calcite, pyrite and organic matter, with goethite as the only identified product. The reduction level of Fe (Fe2+/Fetot) in the rock increases with depth from <20% to >80% in unweathered rock below 24 m. Porosity reaches 0.39 in the shallowest samples before dropping to normal values of around 0.13 below 30 m. Pore-water composition obtained by rock squeezing evolves with depth from fresh water of the Ca-SO4-HCO3 type to a 10–12 g/L Na-SO4-Cl type in unweathered rock below 40 m. Ground-water samples taken in the weathered zone indicate reducing conditions at present and therefore reflect a remaining redox buffering capacity even in the strongly oxidised rock. The interpretation of the rock properties, in conjunction with the chemical and isotopic signatures of ground and pore waters, suggests that substantial weathering occurred primarily during stages with a lower ground-water table in the Pleistocene, providing access to oxygen via gas diffusion across the partially unsaturated zone. Three evolutionary stages can be distinguished: 1) Long-term pyrite oxidation during Pleistocene surface exposure under dry climate and diffusive transport of SO42− into the formation. This explains the higher SO42− pore-water concentrations even in the unweathered rock at depth when compared to regional data from deep boreholes. 2) Build-up of the curved Cl− and water-isotope diffusion profiles since ∼50–60 ka with a depth penetration of 40–80 m. 3) Holocene rise of the water table, leading to self sealing of fractures and the establishment of a diffusive regime in the weathered zone. Under saturated conditions, the depth penetration of rock oxidation is quite limited, which illustrates the substantial buffering capacity of the Opalinus Clay against external geochemical disturbances.

Extracellular polymeric substances overproduction strategy in Ferroplasma acidiphilum growth for biooxidation of low-grade gold bearing ore: Role of monosaccharides

The adhesion of microorganisms to surfaces and the affecting factors is important in biomining pretreatment. In this research, the novelty is focused on studying the monosaccharide's impact on the adaptability and adhesivity of Ferroplasma acidiphilum for oxidization of sulfide-bearing ore containing pyrite harboring 98 % of gold in its crystal lattice. d-sucrose increased EPS production with the highest amount of pyrite dissolution (69 %) as compared to the other types of monosaccharides (d-galactose and d-fructose). Addition of 0.8 wt% d-sucrose enhanced the production of ferric ions 65 % for the ore load of 20 wt% while for the addition of 0.4 wt%, the ferric ions concentration was maximum up to 95 %. The results indicated that the addition of both yeast extract and d-sucrose with the concentration of 0.4 wt% enhanced the EPS (Extracellular Polymeric Substances) biovolume fraction from 7.5 to 32.5 v/v %.

Pyrite Dissolution in the Cretaceous Yogou Formation of the Niger (Chad) Basin: Implications for Basin Evolution under a Rift Tectonic Setting.

This is the first study of pyrite minerals in the entire West and Central African Rift System (WCARS). Several polished organic-rich core samples from the Cretaceous Yogou Formation of the Niger (Chad) Basin located in the WCARS were investigated for their pyrite content using FE-SEM and SEM-EDS imaging techniques. An attempt was made to classify the types and provenance of the pyrites and to highlight the control of rift fractures on the oxidation and dissolution of pyrites in the region. Three major types of pyrites are present in the studied formation, including euhedral pyrite (EPy) crystals, pyrite framboids (FPy), and sunflower pyrites (SPy). A statistical analysis of 307 FPy shows that the framboids are diagenetically formed with an average diameter of 6.61 μm. SPy is present in a relatively low amount compared to framboids. The pyrites underwent a variety of diagenetic modifications, from mechanical compaction to oxidation, dissolution, and recrystallization. Unoxidized pyrites primarily contain Fe, S, and C, but oxidized pyrites also contain O, Al, and Si. There is a strong correlation between the fractures and the spatial distribution of the physicochemical alteration of the pyrite in the study. Dissolution in relatively deep-buried samples occurs mainly along fracture planes. The fractures provide a pathway for oxidants and other metal elements to reach the pyrites. The pattern of pyrite dissolution reflects the timing of fracture formation and fracture activities as a purveyor or drainage for fluids in the organic-rich samples investigated. The pyrites are associated intimately with organic matter (OM); thus, the relationship between the fracture and the pyrites' transformation is significant in the assessment of organic matter preservation at deep-burial depth.

Mineralogical, magnetic and geochemical data constrain the pathways and extent of weathering of mineralized sedimentary rocks

The oxidative weathering of sulfidic rock can profoundly impact watersheds through the resulting export of acidity and metals. Weathering leaves a record of mineral transformation, particularly involving minor redox-sensitive phases, that can inform the development of conceptual and quantitative models. In sulfidic sedimentary rocks, however, variations in depositional history, diagenesis and mineralization can change or overprint the distributions of these trace minerals, complicating the interpretation of weathering signatures. Here we show that a combination of bulk mineralogical and geochemical techniques, micrometer-resolution X-ray fluorescence microprobe analysis and rock magnetic measurements, applied to drill core samples and single weathered fractures, can provide data that enable the development of a geochemically consistent weathering model.This work focused on one watershed in the Upper Colorado River Basin sitting within the Mesaverde Formation, a sedimentary sandstone bedrock with disseminated sulfide minerals, including pyrite and sphalerite, that were introduced during diagenesis and subsequent magmatic-hydrothermal mineralization. Combined analytical methods revealed the pathways of iron (Fe), carbonate and silicate mineral weathering and showed how pH controls element retention or release from the actively weathering fractured sandstone. Drill core logging, whole rock X-ray diffraction, and geochemical measurements document the progression from unweathered rock at depth to weathered rock at the surface. X-ray microprobe analyses of a 1-cm size weathering profile along a fracture surface are consistent with the mobilization of Fe(II) and Fe(III) into acidic pore water from the dissolution of primary pyrite, Fe-sphalerite, chlorite, and minor siderite and pyrrhotite. These reactions are followed by the precipitation of secondary minerals such as of goethite and jarosite, a Fe-(oxyhydr)oxide and hydrous Fe(III) sulfate, respectively. Microscale analyses also helped explain the weathering reactions responsible for the mineralogical transformations observed in the top and most weathered section of the drill core. For example, dissolution of feldspar and chlorite neutralizes the acidity generated by Fe and sulfide mineral oxidation, oversaturating the solution in both Fe-oxides. The combination of X-ray spectromicroscopy and magnetic measurements show that the Fe(III) product is goethite, mainly present either as a coatings on fracture surfaces in the actively weathering region of the core or more homogeneously contained within the unconsolidated regolith at the top of the core. Low-temperature magnetic data reveal the presence of ferromagnetic Fe-sulfide pyrrhotite that, although it occurs at trace concentrations, could provide a qualitative proxy for unweathered sulfide minerals because the loss of pyrrhotite is associated with the onset of oxidative weathering. Pyrrhotite loss and goethite formation are detectable through room-temperature magnetic coercivity changes, suggesting that rock magnetic measurements can determine weathering intensity in rock samples at many scales. This work contributes evidence that the weathering of sulfidic sedimentary rocks follows a geochemical pattern in which the abundance of sulfide minerals controls the generation of acidity and dissolved elements, and the pH-dependent mobility of these elements controls their export to the ground- and surface-water.

Aerobic respiration controls on shale weathering

The weathering of shale exerts an important control on the hydrochemical fluxes to river systems, thus influencing the global carbon, nutrient, and geochemical cycles. However, the quantitative understanding of shale weathering and its impact on global biogeochemical cycles remains inadequate due to the complex interplay between hydrological, biogeochemical, and physical processes. In this study, we develop a novel modeling approach to quantitatively interpret the long-term chemical weathering of shale occurring since the last glaciation period (15,000 years) leading to the present geochemical conditions. The model explicitly considers processes occurring across multiple phases and involved in the weathering, including: (i) the infiltration of meteoric water, (ii) the interactions between the water and the mineral assemblage via dissolution/precipitation reactions, (iii) the microbially-mediated oxidation of organic matter, (iv) the evolution of porosity induced by mineral reactions, and (v) the exchange of gases between the subsurface and the atmosphere. To implement and test our model, we conduct this study at a well-instrumented hillslope underlain by Mancos Shale and located at the East River study site, Western Colorado. Consistent with field observations, the model successfully reproduces the stratified weathering front, the complex spatial distribution of organic carbon, and the gaseous emissions of carbon dioxide from the subsurface. Model simulations show that aerobic respiration exerts a fundamental control on the weathering of shale. While previous studies have highlighted the diffusion of oxygen from the atmosphere as the primary mechanism for shale weathering, our model simulations demonstrate that aerobic respiration limits the propagation of oxygen in the shallow subsurface, and thereby inhibits the dissolution of pyrite at depth. Aerobic respiration is particularly favored in the top soil horizon due to the constant flux of oxygen from the atmosphere, the replenishment of fresh litter/plant-derived organic matter, and to a lesser extent the presence of fossil shale-associated organic matter. The acidic pore water generated through aerobic respiration within the shallow subsurface is transported to greater depths, where it sustains the dissolution of carbonate (dolomite in this example). Overall, our results demonstrate that the evolution of pyrite and carbonate depletion fronts are significantly different, and primarily depend on the ability of microorganisms to carry out microbial respiration and the various transport pathways of reactants controlling the mineral reactions under partially saturated conditions.

Vanadium selective separation enhancement from iron in black shale using oxalic acid due to FeS2 (1 0 0) surface passivation: A theoretical and experimental study

Iron impurity tends to precipitate and reduces vanadium product purity during vanadium extraction from black shale. Oxalic acid is an eco-friendly leachant which can enhance vanadium selective separation from iron impurity. In this paper, the separation mechanism was systematically investigated. The mineral phases transformations and thermodynamics analysis indicate that the FeS2 in black shale can react with H2C2O4, HC2O4-, C2O42- and O2, as a result, the pyrite surface was coated by ferrous oxalate passivation layer, which can depress pyrite dissolution reaction. Meanwhile, the density functional theory (DFT) calculations show that the H2C2O4, HC2O4- and C2O42- can reconstruct FeS2 (100) surface by adsorption, and the adsorption energy can be as low as −3.73 eV for C2O42- adsorption configuration. The electrons in Fe atoms were transferred to O atoms and the stable Fe-O bands together with closed seven/eight-membered rings structures were formed as passivation layer, which can weaken and reduce Fe atoms reactive activation for the FeS2 during oxalic acid leaching. This study reveals vanadium separation from iron due to pyrite passivation in presence of oxalic acid at an atomic level.

Insight into the Process and Mechanism of Water-Rock Interaction in Underground Coal Mine Reservoirs Based on Indoor Static Simulation Experiments.

During the storage of underground water reservoirs in coal mines, water–rock interaction occurred between mine water and collapsed rocks, resulting in improved mine water quality, but the water–rock process and mechanism have not been clarified. In this study, six sets of simulated experiments were designed to investigate the water–rock interaction between two types of roof collapse rocks and different water samples in the Daliuta coal mine. The ion ratio method and multivariate statistics are used to reveal the process and mechanism of water–rock interaction during the experimental process from the perspective of input and output water chemical characteristics and rock properties, respectively. The results show that the ion concentration of the effluent water is controlled by water–rock interaction, and the water–rock process mainly involves the dissolution of halite, silicate, pyrite, calcite, dolomite, magnesia chlorite, and gypsum precipitation and is accompanied by ion exchange. Mineral dissolution and precipitation are the most important factors affecting the ion abundance in the effluent. There are differences in the reaction rate, degree of reaction, and dissolution process during the experimental process of fine sandstone and mudstone. Fine sandstone is more reactive than mudstone in terms of reaction rate and degree of reaction, and fine sandstone is dissolved from surface pores to inside and around, while mudstone is generated in new dissolved pores. We found that rock type, ion concentration in the input water, and rock reaction period influence the water–rock interaction during the experiment. The results provide a reference for clarifying the water–rock interaction during the storage of underground water reservoirs in coal mines and predicting the water quality of the effluent.

Multimodal study of the impact of stimulation pH on shale pore structure, with an emphasis on organics behavior in alkaline environments

The tight nature of shale formations calls for hydraulic fracturing techniques able of altering the pore architecture to facilitate the production of hydrocarbon resources. The composition, pH, salinity, density and chemistry of Hydraulic Fracturing Fluids (HFF) vary substantially depending on reservoir characteristics. pH is among the most important stimulation fluid properties, ranging from very acidic, for carbonate reservoirs, to basic, in the case of clay-rich formations. pH regulates what geochemical reactions take place between the stimulation fluid and shale, as well as pore architecture alteration. The dissolution of carbonates and pyrite, and the precipitation of common minerals such as barite, gypsum and iron oxides in acidic environments have been extensively documented. In contrast, alkali stimulation environments and the role of organic components have received less attention. This research provides insight into the role of both minerals and organic components during alkaline stimulation, and the resulting pore architecture alterations. A set of reactive experiments are performed using three different shales, with varying organic matter (OM) content, at different pH (3, 6, 8, 10, 12). The analysis of pore architecture alteration induced by the reaction was performed via nitrogen (N2) gas adsorption and Time-Domain Nuclear Magnetic Resonance (TD-NMR). Inductively Coupled Plasma-Optical Emission Spectrometry (ICP-OES) was used to measure the composition of the stimulation fluid post-reaction. The system with a larger impact on the pore architecture (pH 12) underwent a more extensive analysis through the application of Focused Ion Beam Scanning Electron Microscopy (FIB-SEM) and x-ray Diffraction (XRD). Results show that mineral dissolution contributed to the creation of secondary porosity, mainly through dissolution of clays and silicates. We show that the abundance and distribution of organic matter both play a significant role in changes in pore architecture. Most samples showed an increase in inter-organic porosity, likely due to the organic acids within kerogen reacting with the stimulation fluid. Samples with a larger amount and more widespread distribution of organic matter show the most significant alterations. Finally, we also highlight the dissolution process of iron-rich minerals, including framboidal pyrite, as another important source of secondary porosity.

Redox Behavior of Secondary Solid Iron Species and the Corresponding Effects on Hydroxyl Radical Generation during the Pyrite Oxidation Process.

During the pyrite oxidation process, aqueous ferrous/ferric ions (Fe2+/Fe3+), as well as surface-adsorbed Fe2+/Fe3+, have been widely recognized to dominate hydroxyl radical (•OH) generation, while this study reveals that the secondary solid iron species also play non-negligible roles. Based on the different forms and the presence of sites, the secondary solid iron species were classified as Fecoat (iron-containing coating on the pyrite surface) and Fedep (ex situ-deposited iron (oxyhydr)oxide that is not in contact with pyrite). Instead of participating in building a stubborn passivation layer on the pyrite surface, Fecoat is easy to fall off from the pyrite surface as the oxidation of pyrite deepens, while large fractions of Fedep and Fecoat are found to be extractable with nitrilotriacetic acid (NTA). Achieved by cyclically oxidizing pyrite within different NTA levels (0/0.1/10 mM), Fecoat and Fedep were proved to have distinct redox behavior during the pyrite oxidation process. Amorphous Fedep, originated from the hydrolyzation of dissolved Fe3+, accelerates the nonradical decay of hydrogen peroxide (H2O2); as a result, the accumulation of Fedep always decreases the •OH production during the pyrite oxidation process. However, part of Fedep adsorbs on the pyrite surface through electrostatic attraction and converts into Fecoat. The electron conduction between Fecoat and pyrite was verified, which accelerates the oxidative dissolution of pyrite, produces reactive Fe(II), and therefore favors •OH generation. This study improves our understanding of the redox behavior of pyrite in complex media such as natural processes and practical engineering systems.

Effect of electric current intensity on performance of polycaprolactone/FeS2-based mixotrophic biofilm–electrode reactor

In this study, a bioelectrochemical system consisting of pyrite-based autotrophic denitrification (PAD) and heterotrophic denitrification (HD) was established to polish nitrate wastewater. The loading of electric current (EC) could stimulate the dissolution of pyrite. Appropriate EC (I ≤ 30 mA) was conducive to nitrate removal, too high EC (I = 40 mA) would inhibit nitrate removal and lead to an obvious accumulation of NO2–-N and NH4+-N. Microbial analysis revealed that the increase of EC could inhibit the diversity of heterotrophic microbes, but appropriate EC (I = 10 mA) could increase the diversity of autotrophic microbes. The EC loading was conducive to the enrichment of iron autotrophic denitrifiers (Ferritrophicum), pyrite-oxidizing bacteria (Thiobacillus, Sulfurimonas), and sulfur autotrophic denitrifiers (Dechloromonas, Thiobacillus, and Arenimonas). The EC loading enlarged the contribution of PAD, making PAD a dominant pathway in denitrification.