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At least 127 records · Page 7
All sourcesNASA NTRSDOE OSTI
DOE OSTI2024

Microbiological and geochemical characterization of As-bearing tailings and underlying sediments

Over the past 100 years, extensive oxidation of As-bearing sulfide-rich tailings from the abandoned Long Lake Gold Mine (Canada) has resulted in the formation of acid mine drainage (pH 2.0-3.9) containing high concentrations of dissolved As (similar to 400 mg L -1 ), SO 4 2- , Fe and other metals. Dissolved As is predominantly present as As(III), with increased As(V) near the tailings surface. Pore-gas O 2 is depleted to < 1 vol% in the upper 30-80 cm of the tailings profile. The primary sulfides, pyrite and arsenopyrite, are highly oxidized in the upper portions of the tailings. Elevated proportions of sulfide-oxidizing prokaryotes are present in this zone (mean 32.3% of total reads). The tailings are underlain by sediments rich in organic C. Enrichment in δ 34 S-SO 4 in pore-water samples in the organic C-rich zone is consistent with dissimilatory sulfate reduction. Synchrotron-based spectroscopy indicates an abundance of ferric arsenate phases near the impoundment surface and the presence of secondary arsenic sulfides in the organic-C beneath the tailings. The persistence of elevated As concentrations beneath the tailings indicates precipitation of secondary As sulfides is not sufficient to completely remove dissolved As. The oxidation of sulfides and release of As is expected to continue for decades. The findings will inform future remediation efforts and provide a foundation for the long-term monitoring of the effectiveness of the remediation program.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2022

Aerosol iron speciation and seasonal variation of iron oxidation state over the western Antarctic Peninsula

The iron (Fe) speciation and oxidation state have been considered critical factors affecting Fe solubility in the atmosphere and bioavailability in the surface ocean. In this study, elemental composition and Fe speciation in aerosol samples collected at the Palmer Station in the West Antarctic Peninsula were determined using synchrotron-based X-ray fluorescence (XRF) and X-ray Absorption Near-Edge Structure (XANES) spectroscopy. The elemental composition of coarse-mode (>1 mu m) Fe-containing particles suggests that the region's crustal emission is the primary source of aerosol Fe. The Fe minerals in these aerosol particles were predominantly hematite and biotite, but minor fractions of pyrite and ilmenite were observed as well. The Fe oxidation state showed an evident seasonal variation. Further, the Fe(II) content accounted for 71% of the total Fe in the austral summer, while this fraction dropped to 60% in the austral winter. Multivariate linear models involving meteorological parameters suggested that the wind speed, relative humidity, and solar irradiance were the factors that significantly controlled the percentage of Fe(II) in the austral summer. On the contrary, no relationship was found between these factors and the Fe(II) percentage in the austral winter, suggesting that atmospheric photoreduction and regional dust emission were limited. Moreover, the snow depth was significantly (p < 0.05) correlated with the aerosol Fe concentration, confirming the limiting effect of snow/ice cover on the regional dust emission. Given that the Antarctic Peninsula has experienced rapid warming during recent decades, the ice-free areas in the Antarctic Peninsula may act as potential dust sources.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2025

Biogeochemical controls on iron speciation and cycling across upland to shoreline gradients in freshwater and estuarine coastal soils (Lake Erie and Chesapeake Bay, United States)

Coastal environments are dynamic interfaces that mediate carbon and nutrient exchanges between terrestrial landscapes and open waters, and understanding the biogeochemical factors controlling these exchanges, particularly iron (Fe) redox transformations, is crucial for predicting coastal ecosystem functions. Here, we investigated the mechanisms controlling Fe speciation changes across upland-to-shoreline gradients in freshwater and estuarine soils using Fe K-edge X-ray absorption spectroscopy, solid and porewater composition analysis, and 16S rRNA sequencing analysis. We show that Fe transformations depend primarily on inundation patterns. In unsaturated uplands, Fe occurs as Fe(III) oxyhydroxides, mainly goethite (9–35 %), Fe(II,III)-phyllosilicates (39–89 %), and Fe(III)-organic species (0–61 %). Soils influenced by estuarine waters exhibit porewater sulfide concentrations reaching up to 221 μM, Fe- and S-cycling bacteria, and up to 81 % pyrite (FeS 2 ), indicating that sulfur-driven redox dynamics control Fe transformations. In lacustrine wetlands, Fe(III) reduction is indicated by porewater Fe(II) concentrations increasing to 1.0–2.1 mM, and ~10–15 % of Fe as Fe(II,III)-(hydr)oxides (green rust), vivianite (Fe 3 (PO 4 ) 2 ·8H 2 O), and/or adsorbed Fe(II) species. EXAFS data also indicate reduction of structural Fe(III) to Fe(II) in phyllosilicates. The presence of Fe- and S-cycling bacteria, as well as sulfide (0–10 μM), suggests that Fe-cycling is microbially driven and potentially coupled with cryptic S-cycling. Fe(II) oxidation was indicated above/near the water table by the presence of Fe(III) oxyhydroxides (ferrihydrite, lepidocrocite). Furthermore, negligible Fe(III) or sulfate reduction was observed at some water-saturated sites located at the upland-wetland transition, likely due to oxic (sub-)surface water inputs. Overall, our results highlight the importance of considering both Fe-speciation and hydro-biogeochemical dynamics when predicting Fe-cycling at coastal interfaces.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2022

Impact of Acid–Base Stimulation Sequence on Mineral Stability for Tight/Impermeable Unconventional Carbonate-Rich Rocks: A Delaware Basin Case Study

We report mineral precipitation due to reactions with injected fluids during unconventional fracture stimulation is a well-recognized problem. The goal of this study is to evaluate secondary mineral precipitation and permeability attenuation under chemical injection scenarios specific to the Delaware basin. Whole cylindrical cores (2.54 cm diameter and 2.54 cm height) and ground shale (150–250 μm) from the carbonate-rich Bone Spring Formation, Delaware Basin TX (Leonardian), were reacted at 80 °C and 85 bar using a hydraulic fracturing fluid (HFF) recipe and an injection sequence typical of the Delaware Basin. The reacted shales and solutions were analyzed using a variety of laboratory- and synchrotron-based techniques to characterize both the chemical and spatial distributions of secondary mineral precipitation and identify changes in permeability and mineralogy. This carbonate-rich shale (>84% calcite) rapidly neutralized the acidic HFF. Synchrotron-based X-ray fluorescence mapping coupled with X-ray absorption spectroscopy (both bulk and micro) showed that most of the iron was in an oxidized form prior to exposure to HFF and that almost all iron(II) became fully oxidized after the reaction. Scanning electron microscopy images of the ground shale samples primarily identified iron(oxyhydr)oxide microcrystals on grain surfaces. A few small isolated iron-rich areas also contained sulfur, suggesting that some pyrite was preserved when isolated within a calcite crystal but that most was oxidized. The rapid neutralization of the acid spearhead in these carbonate-rich samples demonstrates that the acid spearhead is useful for initiating fractures in extremely calcite-rich rocks but does little to enhance rock permeability. This suggests that the impact of the acid spearhead is significantly smaller for carbonate-rich shales compared to clay-rich shales, which has broad implications for acidizing in carbonate-rich shale formations and iron transformations within these shales.

58 GEOSCIENCES↗
DOE OSTI2022

Experimental Investigation of Barium Sources and Fluid–Rock Interaction in Unconventional Marcellus Shale Wells Using Ba Isotopes

Produced waters from unconventional Marcellus Shale gas wells have anomalously high barium (Ba) concentrations and yield some of the isotopically heaviest Ba measured to date. Experiments were conducted to constrain the source of Ba in these fluids and the controls on barite (BaSO 4 ) precipitation and dissolution in oil and gas wells. Experiments simulating the acidizing stage evaluated the solubility of pure barite and drilling mud in 2 M HCl at 80 °C for periods of 2, 6, and 48 h and resulted in <0.01% barite dissolution with no appreciable change in δ 138 Ba ( 138 Ba/ 134 Ba normalized to NIST standard 3104a). Static autoclave experiments conducted at 66 °C and 20.7 MPa with combinations of ground Marcellus Shale solids and/or barite-bearing drilling mud with synthetic low-Ba fracturing fluid resulted in decreased Ba concentrations in the fluid, with the largest decrease in the shale-only run. Fluid δ 138 Ba values increased by about 0.5‰ as Ba concentrations decreased, consistent with closed-system Rayleigh fractionation. Flow-through experiments in Marcellus Shale core conducted for 28 days resulted in effluent Ba concentrations an order of magnitude lower than the influent, while sulfate concentrations increased over time. Effluent δ 138 Ba values increased over the first 12 days and plateaued at about 1‰ higher than the influent. Modeling suggests a combination of the release of labile shale Ba and barite precipitation. This work indicates that the processes of Ba release from fluid–shale interactions and barite precipitation in fractures and the well bore, while capable of producing high δ 138 Ba fluids, are unlikely to generate fluids with high-Ba concentrations and δ 138 Ba values like those in Marcellus-produced waters. As a result, we find that the release of sulfate from shale pyrite oxidation rapidly catalyzes barite precipitation and that dissolution of drilling mud barite or natural barite in the shale is unlikely to be the major source of Ba in Marcellus-produced waters.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2022

Impact of Concurrent Solubilization and Fines Migration on Fracture Aperture Growth in Shales during Acidized Brine Injection

Acidic hydraulic fracturing fluid chemically and physically alters shale rock fabric during injection and shut-in, creating a “reaction-altered zone” along the fracture faces. To better characterize the variable thickness and composition of this reaction-altered zone under advective flow, we take a coupled experimental and modeling approach. A fluidic cell, with six fiducial markers, is first fabricated to keep the rock sample in place during the core floods and to allow image alignment of acquired images. Then, we conduct a series of reactive core floods in a clay-rich siliceous Wolfcamp shale sample with 10 wt % carbonate, using a synthetic fracturing fluid under no confining stress and at room temperature. High-resolution computed tomography (CT) scans are periodically conducted to observe the spatial alteration of the fracture network. We then perform scanning electron microscopy (SEM-EDS) on the two orthogonal surfaces (fracture surface and freshly cut profile face) to generate high-resolution elemental maps that show the change in mineralogy, both with distance along a given flow path along a fracture surface and with depth from the fracture surface into the shale matrix. These results are contrasted against a two-dimensional (2D) advection-diffusion reaction model developed previously for batch reactions between shale and synthetic fracturing fluids. The model simulates the geochemical interaction occurring at the fracture/matrix interface and penetrating into the shale matrix during the reactive core flood. Both model and experimental results show that the acidic brine is neutralized during the core flood, corresponding to an increase in fracture aperture as a function of fluid volume injected with the greatest change near the inlet. SEM-EDS scans reveal significant dissolution of carbonates on the fracture surface without pyrite oxidation. The reactive transport model indicates that carbonate depletion into the shale interior should be observable, yet SEM-EDS shows no discernible loss of carbonate in the orthogonal profile face. The combination of these observations suggests an additional fracture evolution mechanism in the reactive system, i.e., fines migration. We show that fines migration enhances the access of fracturing fluid to the matrix resulting in a more pronounced fracture widening. Finally, we conclude that coupled mineral dissolution and fines migration govern fracture aperture growth during acidized brine injection. In this work, we effectively show the underlying risk of relying solely on models that do not include an important (transport) process that can alter the system significantly and propose a combined chemomechanical mechanism for fracture evolution appropriate for this shale mineralogy.

58 GEOSCIENCES↗
DOE OSTI2022

Integrated Experimental and Modeling Study of Geochemical Reactions of Simple Fracturing Fluids with Caney Shale

Interactions between rock minerals and hydraulic fluids directly impact the geochemical and geomechanical properties of shale formations. However, the mechanisms of geochemical reactions in shale unconventional reservoirs remain poorly understood. Here, to investigate the geochemical reactions between shale and hydraulic fracturing fluids, a series of batch reactor experiments were undertaken. Three rock samples with different mineralogical compositions and three fluid samples of different compositions (deionized water, deionized water + 2% potassium chloride (KCl), and deionized water + 0.5% choline chloride (C 5 H 14 ClNO) were used. Experiments were undertaken at reservoir temperature and atmospheric pressure. Elemental compositions of effluents after 1, 3, 7, 14, 28 days were analyzed using Inductively Couple Plasma Mass Spectrometry (ICP-MS). Medical Computed Tomography (CT) scan and X-ray Fluorescence (XRF) were conducted on the entire core run to help upscale results obtained from rock-fluid interaction experiments. Geochemical modeling using a reactive simulator, TOUGHREACT, was undertaken to corroborate experimental results. Results show that lower pH triggered high dissolution rates in the rock samples, especially the carbonate components. As pH increased, the rate of dissolution declined significantly, though for most cases dissolution still continued. Observed dissolved silica concentrations were much higher than the quartz solubility, suggesting that much of the silica originates from more soluble silica polymorphs and possibly desorption from clay mineral exchange sites. Concentration of most elemental species in solution increased but aluminium and magnesium concentrations declined rapidly following initial entry into solution. Geochemical modeling corroborated the conclusions regarding mineral dissolution and precipitation observed from experiments, notably; the dissolution of calcite and oxidation of pyrite in reacted shale samples, the likely presence of silica polymorphs such as opal, chalcedony or amorphous silica in these samples, and the reduction of Al and Mg concentrations in solution by precipitation of secondary aluminosilicate phases. The de-flocculation of clay minerals during reaction implies fines migration after hydraulic fracturing. This is detrimental to reservoir productivity as clay fines are displaces and lodged within the micro and nano-fractures created during fracturing. The immediate consumption of aluminium and magnesium also has implications on blockage of hydrocarbon pathways due to precipitation of new minerals in these locations.

04 OIL SHALES AND TAR SANDS↗
DOE OSTI2025

Inorganic Alterations in Unconventional Shale Reservoirs: Importance of Additive and Base Fluid Chemistry

The effective development of unconventional petroleum systems requires the use of significant water resources. In an effort to reduce the consumption of freshwater resources for hydraulic fracturing, highly saline produced waters are increasingly recycled for use as a base fluid. However, there are significant knowledge gaps regarding potential water–rock interactions resulting from the introduction of produced waters and associated additives into shale reservoirs such as formation and deposition of mineral scale, which can negatively affect hydrocarbon production through wellbore restriction and damage to hydraulically generated fractures. To assess the impacts of field stimulation practices in the subsurface, a series of laboratory experiments were completed using (a) three distinct sedimentary rock formations of the Midland Basin (Texas, USA) and (b) additives with two different base fluids: municipal fresh water and clean brine. The experimental approach used relevant injection sequences and mixing ratios in specialized reactors for 3 weeks. Static pressurized experiments and nonpressurized time-resolved experiments were undertaken. The resulting solids and liquids were analyzed by using a variety of laboratory- and synchrotron-based techniques. The use of an acid spearhead (15% HCl) resulted in texturing of both clay-rich and calcareous shales, which can temporarily enhance porosity but subsequently result in mineral scale deposition. The primary matrix scale was Fe(III)-bearing phases, which occurred in all experiments regardless of base fluid chemistry. Additionally, strontium sulfate (SrSO 4 ) precipitated on shale surfaces when clean brines were used. It was concluded that clean brine was the main source of Sr 2+ species, while persulfate breaker degradation and oxidation of pyrite were the sources of SO 4 2– . Sulfate scaling was more pronounced in clay-rich shales, suggesting that Sr sorption is important for promoting celestite formation. This work demonstrates that mineral scale deposition is a complex phenomenon, whereby the type and proportions of various mineral phases are determined from reservoir alteration processes and coprecipitation of constituents from injection fluids. In conclusion, the experimental results shown here should be considered when evaluating different base fluids and additives in order to mitigate mineral precipitation in unconventional shale reservoirs, which could result in reservoir degradation.

Jew, Adam D. [SLAC National Accelerator Laboratory↗
DOE OSTI2020

Reactive Transport Modeling of Shale–Fluid Interactions after Imbibition of Fracturing Fluids

Injection of hydraulic fracturing fluid (HFF) into shale formations for unconventional oil/gas production results in chemical reactions in the shale matrix. Our recent experimental study determined the depths to which different types of reactions between the shale matrix and the HFF extended. In the present study, we built continuum-scale reactive transport models to understand the coupling of chemical reactions and the transport of aqueous species in these shale–HFF systems. Calibration of the model with our previous experimental results reveals that it takes hours to months for the shale matrix to completely neutralize the imbibed acids, depending primarily upon the carbonate content of the shale. Both the HFF pH and pore pH affect the location of barite precipitation, resulting in unique barite precipitation profiles extending millimeters into calcite-rich Eagle Ford shale but only tens of micrometers into low-carbonate Marcellus shale. In addition, dissolved oxygen and extracted bitumen are key to reproducing the experimental observation of Fe(III) (oxyhydr)oxide formation in the shale matrix as a result of pyrite dissolution in the shales. A comparison between the modeling results of porosity in the present study to experimentally measured permeabilities in our previous study suggests that chemical reactions occurring at a greater depth than the observable reaction zone might have impacted permeability. Our model serves as a benchmark for efficiently modeling water–rock interactions in similar systems where bulk rock samples react with a solution in batch reactors. Important reactive transport processes were ascertained via modeling, which allows for quantitative prediction of shale–HFF interactions in shale matrices given the shale and HFF compositions.

58 GEOSCIENCES↗
DOE OSTI2022

Giant Thermoelectric Power Factor Anisotropy in PtSb 1.4 Sn 0.6

In this work, we report on the giant anisotropy found in the thermoelectric power factor (S 2 σ) of marcasite structure-type PtSb 1.4 Sn 0.6 single crystal. PtSb 1.4 Sn 0.6 , synthesized using an ambient pressure flux growth method upon mixing Sb and Sn on the same atomic site, is a new phase different from both PtSb 2 and PtSn 2 , which crystallize in the cubic Pa$\bar{3}$ pyrite and Fm$\bar{3}$m fluorite unit cell symmetry, respectively. The large difference in S 2 σ for heat flow applied along different principal directions of the orthorhombic unit cell stems mostly from anisotropic Seebeck coefficients.

36 MATERIALS SCIENCE↗
DOE OSTI2021

The Relationship Between Topography, Bedrock Weathering, and Water Storage Across a Sequence of Ridges and Valleys

Abstract Bedrock weathering regulates nutrient mobilization, water storage, and soil production. Relative to the mobile soil layer, little is known about the relationship between topography and bedrock weathering. Here, we identify a common pattern of weathering and water storage across a sequence of three ridges and valleys in the sedimentary Great Valley Sequence in Northern California that share a tectonic and climate history. Deep drilling, downhole logging, and characterization of chemistry and porosity reveal two weathering fronts. The shallower front is ∼7 m deep at the ridge of all three hillslopes, and marks the extent of pervasive fracturing and oxidation of pyrite and organic carbon. A deeper weathering front marks the extent of open fractures and discoloration. This front is 11 m deep under two ridges of similar ridge‐valley spacing, but 17.5 m deep under a ridge with nearly twice the ridge‐valley spacing. Hence, at ridge tops, the fraction of the hillslope relief that is weathered scales with hillslope length. In all three hillslopes, below this second weathering front, closed fractures and unweathered bedrock extend about one‐half the hilltop elevation above the adjacent channels. Neutron probe surveys reveal that seasonally dynamic moisture is stored to approximately the same depth as the shallow weathering front. Under the channels that bound our study hillslopes, the two weathering fronts coincide and occur within centimeters of the ground surface. Our findings provide evidence for feedbacks between erosion and weathering in mountainous landscapes that result in systematic subsurface structuring and water routing.

58 GEOSCIENCES↗
DOE OSTI2024

Oxidized Sulfur Species in Slab Fluids as a Source of Enriched Sulfur Isotope Signatures in Arcs

Recycling of oxidized sulfur from subducting slabs to the mantle wedge provides simultaneous explanations for the elevated oxygen fugacity (fO 2 ) in subduction zones, their high hydrothermal and magmatic sulfur outputs, and the enriched sulfur isotopic signatures (i.e., δ 34 S > 0‰) of these outputs. However, a quantitative understanding of the abundance and speciation of sulfur in slab fluids consistent with high pressure experiments is lacking. Here we analyze published experimental data for anhydrite solubility in H 2 O-NaCl solutions to calibrate a high-pressure aqueous speciation model of sulfur within the framework of the deep earth water model. We characterize aqueous complexes, required to account for the high experimental anhydrite solubilities. We then use this framework to predict the speciation and solubility of sulfur in chemically complex fluids in equilibrium with model subducting mafic and ultramafic lithologies, from 2 to 3 GPa and 400 to 800°C at log fO 2 from FMQ-2 to FMQ+4. We show that sulfate complexes of calcium and sodium markedly enhance the stability of sulfate in moderately oxidized fluids in equilibrium with pyrite at fO 2 conditions of FMQ+1 to +2, causing large sulfur isotope fractionations up to 10‰ in the fluid relative to the slab. Such fluids could impart oxidized, sulfur-rich and high δ 34 S signatures to the mantle wedge that are ultimately transferred to arc magmas, without the need to invoke 34 S-rich subducted lithologies.

58 GEOSCIENCES↗
DOE OSTI2021

Superionic iron oxide–hydroxide in Earth’s deep mantle

Water ice becomes a superionic phase under the high pressure and temperature conditions of deep planetary interiors of ice planets such as Neptune and Uranus, which affects interior structures and generates magnetic fields. The solid Earth, however, contains only hydrous minerals with a negligible amount of ice. Here we combine high pressure and temperature electrical conductivity experiments, Raman spectroscopy and first-principles simulations to investigate the state of hydrogen in the pyrite-type FeO 2 H x (x ≤ 1), which is a potential H-bearing phase near the core–mantle boundary. We find that when the pressure increases beyond 73 GPa at room temperature, symmetric hydroxyl bonds are softened and the H + (or proton) becomes diffusive within the vicinity of its crystallographic site. Increasing temperature under pressure, the diffusivity of hydrogen is extended beyond the individual unit cell to cover the entire solid, and the electrical conductivity soars, indicating a transition to the superionic state, which is characterized by freely moving protons and a solid FeO 2 lattice. Furthermore, the highly diffusive hydrogen provides fresh transport mechanisms for charge and mass, which dictate the geophysical behaviours of electrical conductivity and magnetism, as well as geochemical processes of redox, hydrogen circulation and hydrogen isotopic mixing in Earth’s deep mantle.

58 GEOSCIENCES↗
DOE OSTI2020

Evidence of shock-compressed stishovite above 300 GPa

SiO 2 is one of the most fundamental constituents in planetary bodies, being an essential building block of major mineral phases in the crust and mantle of terrestrial planets (1–10 M E ). Silica at depths greater than 300 km may be present in the form of the rutile-type, high pressure polymorph stishovite (P4 2 /mnm) and its thermodynamic stability is of great interest for understanding the seismic and dynamic structure of planetary interiors. Previous studies on stishovite via static and dynamic (shock) compression techniques are contradictory and the observed differences in the lattice-level response is still not clearly understood. Here, laser-induced shock compression experiments at the LCLS- and SACLA XFEL light-sources elucidate the high-pressure behavior of stishovite on the lattice-level under in situ conditions on the Hugoniot to pressures above 300 GPa. We find stishovite is still (meta-)stable at these conditions, and does not undergo any phase transitions. This contradicts static experiments showing structural transformations to the CaCl 2 , α-PbO 2 and pyrite-type structures. However, rate-limited kinetic hindrance may explain our observations. These results are important to our understanding into the validity of EOS data from nanosecond experiments for geophysical applications.

58 GEOSCIENCES↗
DOE OSTI2024

The application of synthetic antibacterial minerals to combat topical infections: exploring a mouse model of MRSA infection

Abstract The development of new antibiotics has stalled, and novel strategies are needed as we enter the age of antibiotic resistance. Certain naturally occurring clays have been shown to be effective in killing antibiotic resistant bacteria. However, these natural clays are too variable to be used in clinical settings. Our study shows that synthetic antibacterial minerals exhibit potent antibacterial activity against topical MRSA infections and increase the rate of wound closure relative to controls. The antibacterial minerals maintain a redox cycle between Fe 2+ /Fe 3+ and the surfaces of pyrite minerals, which act as a semiconductor and produce reactive oxygen species (ROS), while smectite minerals act as a cation exchange reservoir. Acidic conditions are maintained throughout the application of the hydrated minerals and can mitigate the alkaline pH conditions observed in chronic non-healing wounds. These results provide evidence for the strategy of ‘iron overload’ to combat antibiotic resistant infections through the maintained release of Fe 2+ and generation of ROS via distinct geochemical reactions that can break the chronic wound damage cycle.

58 GEOSCIENCES↗
DOE OSTI2021

Persistent arsenate–iron( iii ) oxyhydroxide–organic matter nanoaggregates observed in coal

Understanding how natural nanoaggregates of iron (Fe) and organic matter (OM), currently identified in organic rich soil or peat, interact with metals and metalloids is environmentally significant. Here, coal is also organic-rich and exemplifies anoxic sedimentary environments with Fe usually as pyrite and not oxides. Here, we analyze the local structure of Fe (6880–21 700 mg kg –1 ) and As (45–5680 mg kg –1 ) in representative Guizhou coal samples using X-ray absorption near-edge structure and extended X-ray absorption fine structure (XANES and EXAFS) to illustrate how Fe(III) and As(V) are preserved in coal formed from reduced, organic-rich precursors. Arsenic XANES indicates that >80% of As exists as As(V) with <14% of As associated with sulfides in 5 Guizhou coal samples, confirming published but unexplained results. An As–Fe shell at 3.25–3.29 Å in the As EXAFS suggests that this As(V) is adsorbed on Fe(III) oxyhydroxides as evidenced by Fe EXAFS in these coal samples. Significantly, lower Fe–Fe coordination numbers (CN) of 0.6–1.1 relative to those in 2-line ferrihydrite (CN = 1.6) and goethite (CN = 2.1) suggest that these Fe(III) oxyhydroxides are likely Fe–OM nanoaggregates protected by OM encapsulation and adsorption of arsenate. Such structurally stabilized composites of As(V)–Fe(III)–OM may be more widely distributed and allow oxidized As and Fe to persist in other organic-rich, reducing environments.

54 ENVIRONMENTAL SCIENCES↗