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Since their introduction in 1929, Pauling’s five rules have been used by scientists from many disciplines to rationalize and predict stable arrangements of atoms and coordination polyhedra in crystalline solids; amorphous materials such as silicate glasses and melts; nanomaterials, poorly crystalline solids; aqueous cation and anion complexes; and sorption complexes at mineral-aqueous solution interfaces. The predictive power of these simple yet powerful rules was challenged recently by George et al. (2020), who performed a statistical analysis of the performance of Pauling’s five rules for about 5000 oxide crystal structures. They concluded that only 13% of the oxides satisfy the last four rules simultaneously and that the second rule has the most exceptions. They also found that Pauling’s first rule is satisfied for only 66% of the coordination environments tested and concluded that no simple rule linking ionic radius to coordination environment will be predictive due to the variable quality of univalent radii. We address these concerns and discuss quantum mechanical calculations that complement Pauling’s rules, particularly his first (radius sum and radius ratio rule) and second (electrostatic valence rule) rules. We also present a more realistic view of the bonded radii of atoms, derived by determining the local minimum in the electron density distribution measured along trajectories between bonded atoms known as bond paths, i.e., the bond critical point (rc). Electron density at the bond critical point is a quantum mechanical observable that correlates well with Pauling bond strength. Moreover, a metal atom in a polyhedron has as many bonded radii as it has bonded interactions, resulting in metal and O atoms that may not be spherical. O atoms, for example, are not spherical in many oxide-based crystal structures. Instead, the electron density of a bonded oxygen is often highly distorted or polarized, with its bonded radius decreasing systematically from ~1.38 Å when bonded to highly electropositive atoms like sodium to 0.64 Å when bonded to highly electronegative atoms like nitrogen. Bonded radii determined for metal atoms match the Shannon (1976) radii for more electropositive atoms, but the match decreases systematically as the electronegativities of the M atoms increase. As a result, significant departures from the radius ratio rule in the analysis by George et al. (2020) is not surprising. We offer a modified, more fundamental version of Pauling’s first rule and demonstrate that the second rule has a one-to-one connection between the electron density accumulated between the bonded atoms at the bond critical point and the Pauling bond strength of the bonded interaction. Pauling’s second rule implicitly assumes that bond strength is invariant with bond length for a given pair of bonded atoms. Many studies have since shown that this is not the case, and Brown and Shannon (1973) developed an equation and a set of parameters to describe the relation between bond length and bond strength, now redefined as bond valence to avoid confusion with Pauling bond-strength. Brown (1980) used the valence-sum rule, together with the path rule and the valence-matching principle, as the three axioms of bond-valence theory (BVT), a powerful method for understanding many otherwise elusive aspects of crystals and also their participation in dynamic processes. We show how a priori bond-valence calculations can predict unstrained bond-lengths and how bond-valence mapping can locate low-Z atoms in a crystal structure (e.g., Li) or examine possible diffusion pathways for atoms through crystal structures. In addition, we briefly discuss Pauling’s third, fourth, and fifth rules, the first two of which concern the sharing of polyhedron elements (edges and faces) and the common instability associated with structures in which a polyhedron shares an edge or face with another polyhedron and contains high-valence cations. The olivine [α-(MgxFe1–x)2SiO4] crystal structure is used to illustrate the distortions from hexagonal close-packing of O atoms caused by metal-metal repulsion across shared polyhedron edges. We conclude by discussing several applications of BVT to Earth materials, including the use of BVT to: (1) locate H+ ions in crystal structures, including the location of protons in the crystal structures of nominally anhydrous minerals in Earth’s mantle; (2) determine how strongly bonded (usually anionic) structural units interact with weakly bonded (usually cationic) interstitial complexes in complex uranyl-oxide and uranyl-oxysalt minerals using the valence-matching principle; (3) calculate Lewis acid strengths of cations and Lewis base strengths of anions; (4) determine how (H2O) groups can function as bond-valence transformers by dividing one bond into two bonds of half the bond valence; (5) help characterize products of sorption reactions of aqueous cations (e.g., Co2+ and Pb2+) and oxyanions [e.g., selenate (Se6+O4)2− and selenite (Se4+O3)2−] at mineral-aqueous solution interfaces and the important role of protons in these reactions; and (6) help characterize the local coordination environments of highly charged cations (e.g., Zr4+, Ti4+, U4+, U5+, and U6+) in silicate glasses and melts.

Injection of water-based hydraulic fracturing fluid (HFF) into tight shale gas/oil formations can increase formation permeability and enhance production rates, but this process frequently causes mi...

Knowledge of how arsenic (As) partitions among various phases in Fe-rich sulfidic environments is critical for understanding the fate and mobility of As in such environments. We studied the reaction of arsenite and arsenate sorbed on ferrihydrite nanoparticle surfaces with dissolved sulfide at varying S/Fe ratios (0.1–2.0) to understand the fate and transformation mechanism of As during sulfidation of ferrihydrite. By using aqueous As speciation analysis by IC-ICP-MS and solid-phase As speciation analysis by synchrotron-based X-ray absorption spectroscopy (XAS), we were able to discern the mechanism and pathways of As partitioning and thio-arsenic species formation. Our results provide a mechanistic understanding of the fate and transformation of arsenic during the codiagenesis of As, Fe, and S in reducing environments. Our aqueous-phase As speciation data, combined with solid-phase speciation data, indicate that sulfidation of As-sorbed ferrihydrite nanoparticles results in their transformation to trithioarsenate and arsenite, independent of the initial arsenic species used. The nature and extent of transformation and the thioarsenate species formed were controlled by S/Fe ratios in our experiments. However, arsenate was reduced to arsenite before transformation to trithioarsenate.

In this response to the comment by S. Peiffer, Environ. Sci.: Nano, 2021, we wish to first underline the main objective of the paper by V. Noel, et al., Environ. Sci.: Nano, 2020, 7, 2102–2116, which was to better characterize the chemical parameters controlling the generation of Fe–S-colloids under anaerobic conditions. Export of highly reactive FeS-compounds from reducing to more oxidizing environments has down-stream consequences for electron transfer and biogeochemical reactivity. Thus, detailed knowledge of formation, nature, and stability of these colloids is critical for developing conceptual models to predict Fe remobilization under sulfidic conditions in natural environments. However, our understanding of the biogeochemical behavior of Fe–S-colloids is not sufficient to develop such models. This should not be interpreted as indicating that S transformations and speciation in these systems are not important, as we have previously emphasized in several publications. On the contrary, we show that detailed examination of the Fe chemistry provides clear data in terms of the Fe–S-colloid composition (i.e., S-bearing colloids), as we demonstrate in this response.

Uranium and other radionuclides are prominent in many unconventional oil/gas shales and is a potential contaminant in flowback/produced waters due to the large volumes/types of chemicals injected into the subsurface during stimulation. To understand the stability of U before and after stimulation, a geochemical study of U speciation was carried out on three shales (Marcellus, Green River, and Barnett). Two types of samples for each shale were subjected to sequential chemical extractions: unreacted and shale-reacted with a synthetic hydraulic fracture fluid. A significant proportion of the total U (20-57%) was released from these three shales after reaction with fracture fluid, indicating that U is readily leachable. The total U released exceeds labile water-soluble and exchangeable fractions in unreacted samples, indicating that fluids leach more recalcitrant phases in the shale. Radiographic analysis of unreacted Marcellus shale thin sections shows U associated with detrital quartz and the clay matrix in the shale. Detrital zircon and TiO2 identified by an electron microprobe could account for the hot spots. This study shows that significant proportions of U in three shales are mobile upon stimulation. In addition, the extent of mobilization of U depends on the U species in these rocks.

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...

Hydraulic fracturing of unconventional oil/gas shales has changed the energy landscape of the U.S. Recovery of hydrocarbons from tight, hydraulically fractured shales is a highly inefficient process, with estimated recoveries of25% for natural gas and5% for oil. This review focuses on the complex chemical interactions of additives in hydraulic fracturing fluid (HFF) with minerals and organic matter in oil/gas shales. These interactions are intended to increase hydrocarbon recovery by increasing porosities and permeabilities of tight shales. However, fluid-shale interactions result in the dissolution of shale minerals and the release and transport of chemical components. They also result in mineral precipitation in the shale matrix, which can reduce permeability, porosity, and hydrocarbon recovery. Competition between mineral dissolution and mineral precipitation processes influences the amounts of oil and gas recovered. We review the temporal/spatial origins and distribution of unconventional oil/gas shales from mudstones and shales, followed by discussion of their global and U.S. distributions and compositional differences from different U.S. sedimentary basins. We discuss the major types of chemical additives in HFF with their intended purposes, including drilling muds. Fracture distribution, porosity, permeability, and the identity and molecular-level speciation of minerals and organic matter in oil/gas shales throughout the hydraulic fracturing process are discussed. Also discussed are analysis methods used in characterizing oil/gas shales before and after hydraulic fracturing, including permeametry and porosimetry measurements, X-ray diffraction/Rietveld refinement, X-ray computed tomography, scanning/transmission electron microscopy, and laboratory- and synchrotron-based imaging/spectroscopic methods. Reactive transport and spatial scaling are discussed in some detail in order to relate fundamental molecular-scale processes to fluid transport. Our review concludes with a discussion of potential environmental impacts of hydraulic fracturing and important knowledge gaps that must be bridged to achieve improved mechanistic understanding of fluid transport in oil/gas shales.

We have used synchrotron-based X-ray absorption spectroscopy (structure of Fe–S clusters), transmission electron microscopy (solid-phase crystallinity), Fourier-transform ion-cyclotron-resonance mass spectrometry (identity and composition of natural organic carbon compounds), inductively coupled plasma optical emission spectrometry (total aqueous Fe), and the revised ferrozine method (aqueous Fe(II) and Fe(III) concentrations) to determine the stability and nature of colloids generated by sulfidation of ferrihydrite nanoparticles in the absence and presence of organic compounds. We observed that reductive dissolution of ferrihydrite by aqueous sulfide generates nm-scale FeS clusters. Their subsequent aggregation, which promotes settling of FeS aggregates into the solid fraction, was directly correlated with sulfide/Fe ratio. At sulfide/Fe ratios ≤0.5, FeS clusters and larger colloids remained in suspension for at least 14 days (and up to several months). At sulfide/Fe ratios >0.5, sulfidation reaction rates were rapid and FeS cluster aggregation was accelerated. Moreover, the presence of organic compounds increased the time of suspension of FeS colloids, whereas increased ionic strength inhibited the generation of FeS colloids. We present a general conceptual model to predict when and where FeS colloids can form and enhance or inhibit the mobility of contaminants and nutrients associated with them. Our study indicates that in low-salinity fresh groundwater systems poor in sulfate (i.e. low sulfidation; lakes, floodplains, peatlands etc.), the ferrihydrite sulfidation reaction generates aqueous FeS clusters and larger colloids that remain suspended over long time periods, thus mobilizing a substantial fraction of the total aqueous Fe and S.

Hydraulic fracturing of unconventional shale reservoirs increases the fracture network surface area to access hydrocarbons from the low permeability rock matrix. Porosity and permeability of the matrix, through which hydrocarbons migrate to fractures, are important for determining production efficiency and can be altered by chemical interactions between shale and hydraulic fracturing fluids (HFFs). Here, we present results from an experimental study that characterizes the thickness of the alteration zone in the shale matrix after shale–HFF interactions. Experiments were conducted with whole cores submerged in HFF both with and without added barium and sulfate to promote barite scale formation. After 3 weeks of reaction at 77 bar and 80 °C, the cores were characterized using X-ray microtomography, synchrotron X-ray fluorescence microprobe imaging, and synchrotron X-ray absorption spectroscopy. Our results show that the thickness of the altered zone depends on shale mineralogical composition and varies for ...

Groundwater contamination by As from natural and anthropogenic sources is a worldwide concern. Redox heterogeneities over space and time are common and can influence the molecular-level speciation of As, and thus, As release/retention but are largely unexplored. Here, we present results from a dual-domain column experiment, with natural organic-rich, fine-grained, and sulfidic sediments embedded as lenses (referred to as "reducing lenses") within natural aquifer sand. We show that redox interfaces in sulfur-rich, alkaline aquifers may release concerning levels of As, even when sediment As concentration is low (

Nanopores (at least one dimension ≤100 nm and no dimension To address these knowledge gaps, we carried out batch experiments designed to reveal the molecular-level details of adsorption reaction products and attendant isotope fractionation of aqueous Zn(II) for synthetic, variably porous, amorphous silica (SiO2(am)) particles with average pore diameters ranging from 10 nm to 330 nm. Shell-by-shell fitting of Zn K-edge extended X-ray absorption fine structure (EXAFS) spectra reveals that Zn(II) (1) adsorbs as dominantly monodentate, corner-sharing complexes to silicate tetrahedra of all SiO2(am) particles examined, regardless of pore size, (2) is tetrahedrally coordinated by oxygen atoms in nanoporous silica with small pore sizes (≤10 nm), and (3) is present as a mixture of tetrahedrally and octahedrally coordinated surface complexes in silica with larger pore sizes (>10 nm). In addition, at low Zn(II) surface coverages ( 0.9 μmol m−2). Although molecular-level differences in Zn(II) adsorption complexes are apparent with respect to SiO2(am) pore size, we observed no quantifiable differences in attendant isotope fractionation or adsorption edge structure for Zn(II) uptake on nanoporous SiO2(am) surfaces. These results suggest that the differences in binding energy of tetrahedrally versus octahedrally coordinated Zn(II) complexes at the same site on SiO2(am) are small. Comparison of EXAFS spectra to the results of surface complexation modeling (SCM) of experimental Zn(II) uptake data indicates that monodentate Zn(II) adsorption on the SiO2(am) surface releases two protons through binding one site and passivating an adjacent site, which accounts for the modification of surface charge and surface complex structure during adsorption of Zn(II) cations. This study shows that adsorption of Zn(II) in variably porous SiO2(am) results in differences in surface complex geometry as pore size decreases and as surface coverage of Zn(II) increases. Although surface complexation modeling of Zn(II) adsorption edges on macroporous and nanoporous SiO2(am) does not independently identify these molecular-level differences, combination of SCM and molecular-level observations reveals unique insight into the overall adsorption reaction and neighboring surface site reactivity.

The recent increase in unconventional oil and gas exploration and production has prompted a large amount of research on hydraulic fracturing, but the majority of chemical reactions between shale minerals and organic matter with fracturing fluids are not well understood. Organic matter, primarily in the form of kerogen, dominates the transport pathways for oil and gas; thus any alteration of kerogen (both physical and chemical properties) upon exposure to fracturing fluid may impact hydrocarbon extraction. In addition, kerogen is enriched in metals, making it a potential source of heavy metal contaminants to produced waters. In this study, we reacted two different kerogen isolates of contrasting type and maturity (derived from Green River and Marcellus shales) with a synthetic hydraulic fracturing fluid for 2 weeks in order to determine the effect of fracturing fluids on both shale organic matter and closely associated minerals. ATR-FTIR results show that the functional group compositions of the kerogen is...

We used synchrotron-based X-ray absorption spectroscopy, transmission electron microscopy, and wet chemical analyses to study the sulfidation mechanism(s) and sulfur oxidation products from the reaction of ferrihydrite, goethite, and hematite nanoparticles with dissolved sulfide at different S/Fe molar ratios under anaerobic condition. Our results suggest that surface area alone does not explain the differences in reactivity of Fe(III)-(oxyhydr)oxide nanoparticles with dissolved sulfides; differences in atomic-level surface structure are also likely to play an important role. The higher reactivity of ferrihydrite leads to a faster sulfidation rate compared to that of goethite and hematite. We found that polysulfides as well as elemental sulfur are the major reaction products in the sulfidation of all three Fe(III)-(oxyhydr)oxide nanoparticles studied. We also found that thiosulfate and sulfate formed during the sulfidation of goethite and hematite but did not form in the case of ferrihydrite, suggesting that the slower reaction kinetics of goethite and hematite favors the formation of solid-phase thiosulfates and elemental sulfur in our experiments. In addition, our results revealed that the S/Fe ratio is a critical variable in the sulfidation reaction. Iron dissolution rates for ferrihydrite, goethite, and hematite nanoparticles were found to increase up to a S/Fe ratio of ≤0.5 and decline above this ratio, suggesting formation of FeS species. Similarly, Fe dissolution rates increased with increasing S/Fe ratios and remained an order of magnitude higher for ferrihydrite than for goethite and three times higher for ferrihydrite than for hematite. Sulfur-K-edge X-ray absorption near edge structure (XANES) spectroscopy revealed for the first time the mass distribution of these solid-phase sulfur oxidation products. In addition, we used Fe-K-edge XANES and extended X-ray absorption fine structure (EXAFS) spectroscopic analysis to follow the kinetics of FeS formation for the three types of Fe(III)-(oxyhydr)oxide nanoparticles, with varying S/Fe ratios. Ferrihydrite transformed completely to FeS in our experiments, but only 58% of the goethite and only 18% of the hematite transformed to FeS. These results have important environmental implications for Fe- and S-redox cycling and contaminant mobility and provide experimental evidence for the impact of S/Fe ratio on contaminant mobility in the systems studied, either by releasing surface-sorbed contaminants due to Fe(III)-reductive dissolution at lower S/Fe ratios or by trapping or co-precipitation of contaminants with FeS precipitation at higher S/Fe ratios.

One of the biggest environmental concerns caused by coal-fired power plants is the emission of mercury (Hg), which is toxic metal. To control the emission of Hg from coal-derived flue gas, it is important to understand the behavior and speciation of Hg as well as the interaction between Hg and solid materials in the flue gas stream. In this study, atomic-scale theoretical investigations using density functional theory (DFT) were carried out in conjunction with laboratory-scale experimental studies to investigate the adsorption behavior of Hg on hematite (α-Fe2O3). According to the DFT simulation, the adsorption energy calculation proposes that Hg physisorbs to the α-Fe2O3(0001) surface with an adsorption energy of -0.278 eV, and the subsequent Bader charge analysis confirms that Hg is slightly oxidized. In addition, Cl introduced to the Hg-adsorbed surface strengthens the Hg stability on the α-Fe2O3(0001) surface, as evidenced by a shortened Hg-surface equilibrium distance. The projected density of states (PDOS) analysis also suggests that Cl enhances the chemical bonding between the surface and the adsorbate, thereby increasing the adsorption strength. In summary, α-Fe2O3 has the ability to adsorb and oxidize Hg, and this reactivity is enhanced in the presence of Cl. For the laboratory-scale experiments, three types of α-Fe2O3 nanoparticles were prepared using the precursors Fe(NO3)3, Fe(ClO4)3, and FeCl3, respectively. The particle shapes varied from diamond to irregular stepped and subrounded, and particle size ranged from 20 to 500 nm depending on the precursor used. The nanoparticles had the highest surface area (84.5 m2/g) due to their highly stepped surface morphology. Packed-bed reactor Hg exposure experiments resulted in this nanoparticles adsorbing more than 300 μg Hg/g. The Hg LIII-edge extended X-ray absorption fine structure spectroscopy also indicated that HgCl2 physisorbed onto the α-Fe2O3 nanoparticles. IMPLICATIONS Atomic-scale theoretical simulations proposes that Hg physisorbs to the α-Fe2O3(0001) surface with an adsorption energy of -0.278 eV, and the subsequent Bader charge analysis confirms that Hg is slightly oxidized. In addition, Cl introduced to the Hg-adsorbed surface strengthens the Hg stability on the α-Fe2O3(0001) surface, as evidenced by a shortened Hg-surface equilibrium distance. The PDOS analysis also suggests that Cl enhances the chemical bonding between the surface and the adsorbate, thereby increasing the adsorption strength. Following laboratory-scale experiment of Hg sorption also shows that HgCl2 physisorbs onto α-Fe2O3 nanoparticles which have highly stepped structure.

Metal ion-mineral surface interactions and the attendant isotopic fractionation depend on the properties of the mineral surface and the local atomic-level chemical environment. However, these factors have not been systematically examined for phases of the same composition with different levels of surface disorder. We present pH-dependent adsorption edges, X-ray absorption spectra, and isotopic measurements to illustrate the effects of surface structural disorder and surface coverage on zinc(II) (Zn(II)) surface complexation and isotope fractionation. Our results demonstrate that Zn(II) surface complexes on quartz and amorphous silica (SiO 2(am) ) transition from octahedral to tetrahedral coordination by oxygen as surface coverage increases. In low ionic strength solutions (I = 0.004 M) and at low surface loadings (Γ −2 ), Zn(II) adsorbs to the quartz surface predominantly as outer-sphere octahedral complexes (R Zn-O = 2.05 A) with no significant isotopic fractionation (Δ 66/64 Zn aqueous-sorbed = −0.01 ± 0.06‰) from aqueous Zn(II). In contrast, under similar chemical conditions and surface loading, outer-sphere Zn(II) adsorption complexes are not observed on SiO 2(am) surfaces. At high ionic strength (I = 0.1 M) and low surface loading (Γ −2 ), inner-sphere, monodentate octahedral Zn(II) complexes (R Zn-O = 2.05–2.07 A) are observed on both quartz and SiO 2(am) surfaces. At the same ionic strength (I = 0.1 M) and higher surface loading (Γ = 0.6–1.4 μmol m −2 ), Zn(II) forms inner-sphere, monodentate tetrahedral complexes (R Zn-O = 1.98 A) at the quartz surface. On the SiO 2(am) surface under similar chemical conditions and surface loading, Zn(II) forms dominantly inner-sphere, monodentate tetrahedral complexes with shorter Zn O bond distances (R Zn-O = 1.94 A). Despite different coordination numbers, the measured equilibrium isotope fractionation factors for inner-sphere octahedral and tetrahedral complexes versus dissolved Zn, under the same conditions and on the same silica substrate, are not distinguishable beyond uncertainties. However, there is a larger measured equilibrium isotope fractionation with preferential sorption of heavy Zn as inner-sphere complexes on SiO 2(am) (Δ 66/64 Zn aqueous-sorbed = −0.94 ± 0.11‰) than on quartz (Δ 66/64 Zn aqueous-sorbed = −0.60 ± 0.11‰).The propensity for Zn(II) to occur in tetrahedral and octahedral coordination with oxygen may help explain these observations. We posit that the low energetic difference between octahedral and tetrahedral Zn(II) may be why changes in inner-sphere Zn(II) coordination numbers with increasing coverage do not manifest as distinguishable isotope fractionations or as an observable alteration to the macroscopic sorption edges. Thus, we attribute the larger magnitude of Zn isotope fractionation on SiO 2(am) compared to quartz to differences in the bonding environments on the two types of silica surfaces rather than to a change in the coordination number of Zn(II).

Mining activities over several millennia have resulted in a legacy of environmental contamination that must be mitigated to minimize ecosystem damage and human health impacts. Designing effective remediation strategies for mining and processing wastes requires knowledge of nano- and molecular-scale speciation of contaminants. Here, we discuss how modern nano- and molecular-level concepts and methods can be used to improve risk assessment and future management of contaminants that result from mining activities, and we illustrate this approach using relevant case studies.

As part of a larger study of the reactivity and mobility of uranyl (U(VI)O22+) cations in subsurface environments containing natural organic matter (NOM) and hydrous ferric oxides, we have examined the effect of reference humic and fulvic substances on the sorption of uranyl on 2-line ferrihydrite (Fh), a common, naturally occurring nano-Fe(III)-hydroxide. Uranyl was reacted with Fh at pH 4.6 and 7.0 in the presence and absence of Elliott Soil Humic Acid (ESHA) (0–835 ppm) or Suwanee River Fulvic Acid (SRFA) (0–955 ppm). No evidence was found for reduction of uranyl by either form of NOM after 24 h of exposure. The following three size fractions were considered in this study: (1) ≥0.2 μm (Fh–NOM aggregates), (2) 0.02–0.2 μm (dispersed Fh nanoparticles and NOM macro-molecules), and (3)

Hexavalent chromium is a water-soluble pollutant, the mobility of which can be controlled by reduction of Cr(VI) to less soluble, environmentally benign Cr(III). Iron(II/III)-bearing clay minerals are widespread potential reductants of Cr(VI), but the kinetics and pathways of Cr(VI) reduction by such clay minerals are poorly understood. We reacted aqueous Cr(VI) with two abiotically reduced clay minerals: an Fe-poor montmorillonite and an Fe-rich nontronite. The effects of ionic strength, pH, total Fe content, and the fraction of reduced structural Fe(II) [Fe(II)/Fe(total)] were examined. The last variable had the largest effect on Cr(VI) reduction kinetics: for both clay minerals, the rate constant of Cr(VI) reduction varies by more than 3 orders of magnitude with Fe(II)/Fe(total) and is described by a linear free energy relationship. Under all conditions examined, Cr and Fe K-edge X-ray absorption near-edge structure spectra show that the main Cr-bearing product is a Cr(III)-hydroxide and that Fe remains in the clay structure after reacting with Cr(VI). This study helps to quantify our understanding of the kinetics of Cr(VI) reduction by Fe(II/III)-bearing clay minerals and may improve predictions of Cr(VI) behavior in subsurface environments.

The use of hydraulic fracturing techniques to extract oil and gas from low permeability shale reservoirs has increased significantly in recent years. During hydraulic fracturing, large volumes of water, often acidic and oxic, are injected into shale formations. This drives fluid-rock interaction that can release metal contaminants (e.g., U, Pb) and alter the permeability of the rock, impacting the transport and recovery of water, hydrocarbons, and contaminants. To identify the key geochemical processes that occur upon exposure of shales to hydraulic fracturing fluid, we investigated the chemical interaction of hydraulic fracturing fluids with a variety of shales of different mineralogical texture and composition. Batch reactor experiments revealed that the dissolution of both pyrite and carbonate minerals occurred rapidly, releasing metal contaminants and generating porosity. Oxidation of pyrite and aqueous Fe drove precipitation of Fe(III)-(oxy)hydroxides that attenuated the release of these contaminants via co-precipitation and/or adsorption. The precipitation of these (oxy)hydroxides appeared to limit the extent of pyrite reaction. Enhanced removal of metals and contaminants in reactors with higher fluid pH was inferred to reflect increased Fe-(oxy)hydroxide precipitation associated with more rapid aqueous Fe(II) oxidation. The precipitation of both Al- and Fe-bearing phases revealed the potential for the occlusion of pores and fracture apertures, whereas the selective dissolution of calcite generated porosity. These pore-scale alterations of shale texture and the cycling of contaminants indicate that chemical interactions between shales and hydraulic fracturing fluids may exert an important control on the efficiency of hydraulic fracturing operations and the quality of water recovered at the surface.

Hydraulic fracturing of unconventional hydrocarbon reservoirs is critical to the United States energy portfolio; however, hydrocarbon production from newly fractured wells generally declines rapidly over the initial months of production. One possible reason for this decrease, especially over time scales of several months, is the mineralization and clogging of microfracture networks and pores proximal to propped fractures. One important but relatively unexplored class of reactions that could contribute to these problems is oxidation of Fe(II) derived from Fe(II)-bearing phases (primarily pyrite, siderite, and Fe(II) bound directly to organic matter) by the oxic fracture fluid and subsequent precipitation of Fe(III)-(oxy)hydroxides. The extent to which such reactions occur and their rates, mineral products, and physical locations within shale pore spaces are unknown. To develop a foundational understanding of potential impacts of shale iron chemistry on hydraulic stimulation, we reacted sand-sized (150–250 ...

The surface charging behavior as a function of pH and isoelectric points (IEPs) of single-crystal α-Al2O3 (0001) and (1102) and α-Fe2O3 (0001) was determined by streaming potential measurements using an electrokinetic analyzer. The IEPs of α-Al2O3 (0001) and (11-02)$(1\overline1 02)$ and α-Fe2O3 (0001) were found to be 4.5, 5.1, and 6.5, respectively. These IEP values for oriented single crystals of α-Al2O3 are in good agreement with literature values, whereas the new IEP value for α-Fe2O3 (0001) is significantly lower than four reported values (IEP = 8-8.5) for single-crystal α-Fe2O3 (0001) (Eggleston and Jordan 1998; Zarzycki et al. 2011; Chatman et al. 2013; Lutzenkirchen et al. 2013) and significantly higher than one (IEP = 4) recently measured by Lutzenkirchen et al. (2015) on a fresh α-Fe2O3 (0001) surface. Most of the single-crystal IEP values measured recently are lower than IEP values reported for polycrystalline α-Al2O3 and α-Fe2O3, which are generally in the pH range of 8 to 10. Calculations of the IEP values based on estimated Ka values of α-Fe2O3 and α-Al2O3 surfaces in contact with water as a function of defect type and concentration suggest that highly reactive surface defect sites (primarily singly coordinated aquo groups) on the Fe-and Al-oxide powders are possibly a major source of the surface charge differences between polycrystalline samples and their oriented single-crystal counterparts studied here. The results of this study provide a better understanding of the surface charging behavior of Fe and Al-oxides, which is essential for predicting complex processes such as metal-ion sorption occurring at mineral/water interfaces. (Less)

Competitive sorption of Pb(II) and Zn(II) on Shewanella oneidensis MR-1 biofilm-coated single-crystal α-Al 2 O 3 (1 −1 0 2) and α-Fe 2 O 3 (0 0 0 1) surfaces was investigated using long-period X-ray standing wave-florescence yield (LP-XSW-FY) spectroscopy. In situ partitioning of aqueous Pb(II) and Zn(II) between the biofilms and underlying metal-oxide substrates was probed following exposure of these complex interfaces to equi-molar Pb and Zn solutions (0.01 M NaNO 3 as background electrolyte, pH = 6.0, and 3-h equilibration time). At higher Pb and Zn concentrations (⩾10 −5 M), more than 99% of these ions partitioned into the biofilms at S. oneidensis /α-Al 2 O 3 (1 −1 0 2)/water interfaces, which is consistent with the partitioning behavior of both Pb(II) or Zn(II) in single-metal-ion experiments. Thus, no apparent competitive effects were found in this system at these relatively high metal-ion concentrations. However, at lower equi-molar concentrations (⩽10 −6 M), Pb(II) and Zn(II) partitioning in the same system changed significantly compared to the single-metal-ion systems. The presence of Zn(II) decreased Pb(II) partitioning onto α-Al 2 O 3 (1 −1 0 2) substantially (∼52% to ∼13% at 10 −7 M, and ∼23% to ∼5% at 10 −6 M), whereas the presence of Pb(II) caused more Zn(II) to partition onto α-Al 2 O 3 (1 −1 0 2) surfaces (∼15% to ∼28% at 10 −7 M, and ∼1% to ∼7% at 10 −6 M). The higher observed partitioning of Zn(II) (∼28%) at the α-Al 2 O 3 (1 −1 0 2) surfaces compared to Pb(II) (∼13%) in the mixed-metal-ion systems at the lowest concentration (10 −7 M) suggests that Zn(II) is slightly favored over Pb(II) for sorption sites on α-Al 2 O 3 (1 −1 0 2) surfaces under our experimental conditions. Competitive sorption of Pb(II) and Zn(II) at S. oneidensis /α-Fe 2 O 3 (0 0 0 1)/water interfaces at equi-molar metal-ion concentrations of ⩽10 −6 M showed that the presence of Pb(II) ions decreased Zn(II) partitioning onto α-Fe 2 O 3 (0 0 0 1) significantly (∼45% to −7 M, and ∼41% to 3% at 10 −6 M), whereas adding Zn(II) caused only small changes in Pb(II) partitioning (∼59% to ∼47% at 10 −7 M, and ∼26% to ∼23% at 10 −6 M), suggesting that Pb(II) strongly outcompetes Zn(II) for sorption sites on S. oneidensis -coated α-Fe 2 O 3 (0 0 0 1) surfaces. Our study implies that caution should be taken when applying results obtained from partitioning studies of single-metal-ion systems to mixed-metal-ion systems at complex biofilm/mineral interfaces.

Microbial biofilms are often present as coatings on metal-oxide surfaces in natural and industrial environments and may induce significant changes in the partitioning behavior and speciation of aqueous metal ions, which in turn can impact their transport and fate. In this study, long-period X-ray standing wave-fluorescence yield (LP-XSW-FY) spectroscopy was used to measure under in situ conditions the partitioning of aqueous Pb(II) and Zn(II) between multilayer Shewanella oneidensis MR-1 biofilms and highly polished, oriented single-crystal surfaces of α-Al2O3 and α-Fe2O3 as a function of metal-ion concentration and time at pH 6.0. We show that after 3-h exposure time, Pb(II) binds preferentially to the α-Al2O3 (1–102) and α-Fe2O3 (0 0 0 1) surfaces at low Pb concentration ([Pb] = 10−7 M) and then increasingly partitions into the biofilm coatings at higher concentrations (10−6 to 10−4 M). In contrast, Zn(II) partitions preferentially into the biofilm coating for both surfaces at all Zn concentrations studied (10−7 to 10−4 M). In comparison, the α-Al2O3 (0 0 0 1) surface has a low affinity for both Pb(II) and Zn(II), and the biofilm coatings are the dominant sink for both ions. These findings suggest that in the presence of S. oneidensis biofilm coatings, α-Al2O3 (0 0 0 1) is the least reactive surface for Pb(II) and Zn(II) compared to α-Al2O3 (1–102) and α-Fe2O3 (0 0 0 1). They also show that Zn(II) has a lower affinity than Pb(II) for reactive sites on α-Al2O3 (1–102) and α-Fe2O3 (0 0 0 1) at [Me(II)] of 10−7 M; at 10−5 M, the bulk of the metal ions partition into the biofilm coatings. At longer exposure times (20–24 h), both Pb(II) and Zn(II) increasingly partition to the metal-oxide surfaces at [Me(II)] = 10−5 M and pH 6.0, indicating possible reaction/diffusion-controlled sorption processes. Pb LIII-edge and Zn K-edge grazing-incidence extended X-ray absorption fine structure (GI-EXAFS) measurements suggest that both Pb(II) and Zn(II) ions may be complexed by carboxyl groups in S. oneidensis biofilms after 3-h exposure at pH 6.0 and [Me(II)] = 10−5 M. In contrast with Burkholderia cepacia, which was used in our previous studies of monolayer biofilm-coated metal-oxide surfaces (Templeton et al., 2001), S. oneidensis MR-1 forms relatively thick biofilm coatings (6–20 μm) that are rich in reactive functional groups and are expected to dominate metal-ion adsorption. Our results show that even thick and highly reactive biofilms like S. oneidensis do not cause much change in the intrinsic chemical reactivities of the underlying metal-oxide surfaces with respect to aqueous Pb(II) and Zn(II) and don’t block reactive sites on the metal-oxide surfaces; instead they reduce the rate of Pb(II) and Zn(II) sorption onto these surfaces.

The development of complex alteration layers on silicate mineral surfaces undergoing dissolution is a widely observed phenomenon. Given the complexity of these layers, most kinetic models used to predict rates of mineral–fluid interactions do not explicitly consider their formation. As a result, the relationship between the development of the altered layers and the final dissolution rate is poorly understood. To improve our understanding of the relationship between the alteration layer and the dissolution rate, we developed a spatially resolved surface kinetic model for olivine dissolution and applied it to a series of closed-system experiments consisting of three-phases (water (±NaCl), olivine, and supercritical CO 2 ) at conditions relevant to in situ mineral carbonation ( i.e. 60 °C, 100 bar CO 2 ). We also measured the corresponding δ 26/24 Mg of the dissolved Mg during early stages of dissolution. Analysis of the solid reaction products indicates the formation of Mg-depleted layers on the olivine surface as quickly as 2 days after the experiment was started and before the bulk solution reached saturation with respect to amorphous silica. The δ 26/24 Mg of the dissolved Mg decreased by approximately 0.4‰ in the first stages of the experiment and then approached the value of the initial olivine (−0.35‰) as the steady-state dissolution rate was approached. We attribute the preferential release of 24 Mg to a kinetic effect associated with the formation of a Mg-depleted layer that develops as protons exchange for Mg 2+ . We used experimental data to calibrate a surface kinetic model for olivine dissolution that includes crystalline olivine, a distinct “active layer” from which Mg can be preferentially removed, and secondary amorphous silica precipitation. By coupling the spatial arrangement of ions with the kinetics, this model is able to reproduce both the early and steady-state long-term dissolution rates, and the kinetic isotope fractionation. In the early stages of olivine dissolution the overall dissolution rate is controlled by exchange of protons for Mg, while the steady-state dissolution rate is controlled by the net removal of both Mg and Si from the active layer. Modeling results further indicate the importance of the spatial coupling of individual reactions that occur during olivine dissolution. The inclusion of Mg isotopes in this study demonstrates the utility of using isotopic variations to constrain interfacial mass transfer processes. Alternative kinetic frameworks, such as the one presented here, may provide new approaches for modeling fluid–rock interactions.

International audience; Physical and chemical transformations and biological responses of silver nanoparticles (AgNPs) in wastewater treatment systems are of particular interest because of the extensive existing and continually growing uses of AgNPs in consumer products. In this study, we investigated the transformation of AgNPs and AgNO3 during thermophilic anaerobic digestion and effects on selection or transfer of antibiotic resistance genes (ARGs). Ag2S-NPs, sulfidation products of both AgNPs and AgNO3, were recovered from raw and digested sludges and were analyzed by analytical transmission electron microscopy (TEM) and X-ray absorption spectroscopy (XAS). TEM and XAS revealed rapid (

Covering more than 70% of tropical and subtropical coastlines, mangrove intertidal forests are well known to accumulate potentially toxic trace metals in their sediments, and thus are generally considered to play a protective role in marine and lagoon ecosystems. However, the chemical forms of these trace metals in mangrove sediments are still not well known, even though their molecular-level speciation controls their long-term behavior. Here we report the vertical and lateral changes in the chemical forms of nickel, which accumulates massively in mangrove sediments downstream from lateritized ultramafic deposits from New Caledonia, where one of nature’s largest accumulations of nickel occurs. To accomplish this we used Ni K-edge Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy data in combination with microscale chemical analyses using Scanning Electron Microscopy coupled with Energy-Dispersive X-ray Spectroscopy (SEM-EDXS). After Principal Component and Target Transform analyses (PCA-TT), the EXAFS data of the mangrove sediments were reliably least-squares fitted by linear combination of 3-components chosen from a large model compound spectral database including synthetic and natural Ni-bearing sulfides, clay minerals, oxyhydroxides, and organic complexes. Our results show that in the inland salt flat Ni is hosted in minerals inherited from the eroded lateritic materials, i.e. Ni-poor serpentine (44–58%), Ni-rich talc (20–31%), and Ni-goethite (18–24%). In contrast, in the hydromorphic sediments beneath the vegetated Avicennia and Rhizophora stands, a large fraction of Ni is partly redistributed into a neoformed smectite pool (20–69% of Ni-montmorillonite), and Ni speciation significantly changes with depth in the sediment. Indeed, Ni-rich talc (25–56%) and Ni-goethite (15–23%) disappear below ∼15 cm depth in the sediment and are replaced by Ni-sorbed pyrite (23–52%) in redox-active intermediate depth layers and by pyrite (34–55%) in the deepest sediment layers. Ni-incorporation in pyrite is especially observed beneath an inland Avicennia stand where anoxic conditions are dominant. In contrast, beneath a Rhizophora stand closer to the ocean, where the redox cycle is intensified due to the tide cycle, partial re-oxidation of Ni-bearing pyrites favors nickel mobility, as confirmed by Ni-mass balance estimates and by higher Ni concentration in the pore waters. These findings have important environmental implications for better evaluating the protective role of mangroves against trace metal dispersion into marine ecosystems. They may also help in predicting the response of mangrove ecosystems to increasing anthropogenic pressure on coastal areas.

Mercury associated with fly ash is a significant contaminant released in flue gas emissions from coal-fired power plants. This work focuses on the association of Hg with other elements and phases as well as the molecular-level speciation of Hg in bulk and

An upward loss of Ni is commonly reported in the oxide-rich unit of Ni-laterite deposits developed over ultramafic rocks in tropical regions, especially in freely drained and deeply weathered regoliths. Because goethite is the major mineral constituent of such oxide-rich units, this Ni loss has been linked to compositional changes in goethite. In the present study, we have investigated possible correlations between Ni contents in the bulk laterite, and the evolution of goethite in terms of composition and crystallinity, in two Ni-rich and one Ni-poor lateritic profiles from New Caledonia. Ni K-edge Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy indicates that goethite hosts the main fraction of Ni in the three profiles investigated. Asbolane/lithiophorite identified as accessory minerals by X-ray diffraction (XRD) have little effect on the vertical variations in bulk Ni content in spite of the fact that the Ni contents of these Mn-oxides can be significant at certain depths. The gradual decrease in Ni content from the bottom to the top of the three Ni lateritic profiles correlates with a decrease in the Ni content of goethite as determined by electron probe micro-analysis. In addition, XRD data show that these compositional trends are linked to an increase of the mean coherent domain size of goethite. These observations support the hypothesis that Ni is expelled from goethite as it ages through successive dissolution and recrystallization cycles during the lateritization process. Comparison of laterites having different degrees of weathering suggests that this aging process could also play a significant role in the regional variability of Ni content in Ni-laterite deposits.

Injection of carbon dioxide into subsurface geologic reservoirs during geologic carbon sequestration (GCS) introduces an oxidizing supercritical CO2 phase into a subsurface geologic environment that is typically reducing. The resulting redox disequilibrium provides the chemical potential for the reduction of CO2 to lower free energy organic species. However, redox reactions involving carbon typically require the presence of a catalyst. Iron oxide minerals, including magnetite, are known to catalyze oxidation and reduction reactions of C-bearing species. If the redox conditions in the reservoir are modified by redox transformations involving CO2, such changes could also affect mineral stability, leading to dissolution and precipitation reactions and alteration of the long-term fate of CO2 in GCS reservoirs. We present experimental evidence that reservoirs with reducing redox conditions are favorable environments for the relatively rapid abiotic reduction of CO2 to organic molecules. In these experiments, an aqueous suspension of magnetite nanoparticles was reacted with supercritical CO2 under pressure and temperature conditions relevant to GCS in sedimentary reservoirs (95–210 °C and ∼100 bars of CO2). Hydrogen production was observed in several experiments, likely caused by Fe(II) oxidation either at the surface of magnetite or in the aqueous phase. Heating of the Fe(II)-rich system resulted in elevated P H 2 and conditions favorable for the reduction of CO2 to acetic acid. Implications of these results for the long-term fate of CO2 in field-scale systems were explored using reaction path modeling of CO2 injection into reservoirs containing Fe(II)-bearing primary silicate minerals, with kinetic parameters for CO2 reduction obtained experimentally. The results of these calculations suggest that the reaction of CO2 with reservoir constituents will occur in two primary stages (1) equilibration of CO2 with organic acids resulting in mineral–fluid disequilibrium, and (2) gradual dissolution of primary minerals promoting significant CO2 reduction through the release of Fe(II). The reduction of CO2 is identified as a new trapping mechanism that could significantly enhance the long-term stability of GCS reservoirs. Identification of reservoir characteristics that promote CO2 redox transformations could be used as an additional factor in screening geologic reservoirs for GCS.

International audience; Formation of Fe-sulfides in anoxic horizons of mangrove sediments makes this ecosystem a potential long-term sink for metal contaminants in the intertropical region. Increasing anthropogenic pressure on coastal areas can alter the physicochemical of mangrove sediments by modifying their redox state, affecting directly the rate of Fe-sulfides that mediate accumulation of metal contaminant. Here, we show that isolation from the sea, due to land use planning, directly modify the redox state of mangrove sediments from reducing condition to oxidizing condition, affecting the stability of Ni-accumulating Fe-sulfides. Unusual suboxic/oxic conditions are indeed observed at intermediate depths in these mangrove sediments and favor the oxidative dissolution of Ni-pyrite (Fe1-xNixS2) that initially formed under anoxic/suboxic conditions. This reaction leads to a significant release of aqueous HS-, Fe2+ and Ni2+ at the redox boundary. HS- and Fe2+ oxidize into SO42- and Fe3+ and precipitate as schwertmannite (Fe8O8(OH)(6)SO4), leading to acidification of the pore-waters. Meanwhile, aqueous Ni2+ is mostly leached downward in the underlying anoxic layers of the sediment where it sorbs at the surface of pyrite and/or incorporates in the structure of newly formed pyrites. These results emphasize the potential of Fe-sulfides for mitigating the impact of the oxidation of former Ni-rich mangrove sediments, as long as the anoxic conditions are preserved at depth. This assumption can be expanded to other divalent metals and should be applicable to a larger set of mangrove ecosystems worldwide.

Sulfur dimer (S22–) terminated pyrite FeS2{100} surfaces with a low energy electron diffraction (LEED) pattern of 2 × 1 symmetry are reported. The 2 × 1 symmetry correlates with the orientation of the anisotropic surface structure and external symmetry of macroscopic striations on the pyrite cube face. The basic condition to form these surfaces is a mild 200 V Ne+ sputter-cleaning procedure followed by a 570 K anneal of the sample in a 10–7 Torr S2(g) atmosphere. Controlled amounts of surface sulfur monomers (S2–) can be introduced by mild sputtering of the sulfur dimer terminated surfaces. At low monomer concentrations the surface displays the same characteristic 1 × 1 LEED pattern as that for fracture-generated surfaces. With increasing sulfur depletion, a (1/√2 × 1/√2)R45° LEED pattern emerges, and soft X-ray photoelectron spectroscopy (XPS) results show a sulfur dimer deficient near-surface region and a new high binding energy sulfur spectral component suggesting the presence of local coordination env...

Active and closed Hg mines are significant sources of Hg contamination to the environment, mainly due to large volumes of mine waste material disposed of on-site. The application of Hg isotopes as source tracer from such contaminated sites requires knowledge of the Hg isotope signatures of different materials potentially released to the environment. Previous work has shown that calcine, the waste residue of the on-site ore roasting process, can exhibit distinct Hg isotope signatures compared with the primary ore. Here, we report results from a detailed small-scale study of Hg isotope variations in calcine collected from the closed New Idria Hg mine, San Benito County, CA, USA. The calcine samples exhibited different internal layering features which were investigated using optical microscopy, micro X-ray fluorescence, micro X-ray absorption spectroscopy (mu-XAS), and stable Hg isotope analysis. Significant Fe, S, and Hg concentration gradients were found across the different internal layers. Isotopic analyses revealed an extreme variation with pronounced isotopic gradients across the internal layered features. Overall, delta Hg-202(+/- 0.10 parts per thousand, 2 SD) describing mass-dependent fractionation (MDF) ranged from -5.96 to 14.49 parts per thousand, which is by far the largest range of delta Hg-202 values reported for any environmental sample. In addition, Delta Hg-199 (+/- 0.06 parts per thousand, 2 SD) describing mass-independent fractionation (MIF) ranged from -0.17 to 0.21 parts per thousand. The mu-XAS analyses suggested that cinnabar and metacinnabar are the dominant Hg-bearing phases in the calcine. Our results demonstrate that the incomplete roasting of HgS ores in Hg mines can cause extreme mass-dependent Hg isotope fractionations at the scale of individual calcine pieces with enrichments in both light and heavy Hg isotopes relative to the primary ore signatures. This finding has important implications for the application of Hg isotopes as potential source tracers for Hg released to the environment from closed Hg mines and highlights the need for detailed source signature identification. (C) 2014 Elsevier Ltd. All rights reserved.

In order to evaluate the chemistry and kinetics of mineral carbonation reactions under conditions relevant to subsurface injection and storage of CO2, olivine alteration was studied at 60 °C and 100 bar CO2 pressure, including olivine dissolution and the formation of carbonate minerals. Batch experiments were performed with olivine (Fo92), water, CO2, and NaCl inside gold cells contained within rocking autoclaves. Two reproducible experiments yielded an initial (1 h) dissolution rate of 9.50 ± 0.10 × 10− 11 and a long-term (10–70 days) rate of 1.69 ± 0.23 × 10− 12 mol cm− 2 s− 1. The long-term rate is consistent with previously published rate laws at 4.5

Siliceous ferrihydrites are abundant nanoparticles in natural environments. Although it is well known that the physical properties of ferrihydrite are affected when formed in the presence of silicate oxoanions (SiO44−), the structure of siliceous ferrihydrites (SiFh), and the speciation of Si within these nanosolids are not well understood. In this study we evaluate the effects of Si (at concentrations ranging from 5 to 40 mol% Si) on synthetic ferrihydrite precipitates using structural data derived from synchrotron-based high energy X-ray scattering and pair distribution function (PDF) analysis, in combination with X-ray absorption near edge structure (XANES) spectroscopy, and transmission electron microscopy (TEM). Silicate oxoanions have a major effect on Fe(O,OH)x polyhedral polymerization and ferrihydrite particle growth, illustrated by the formation of smaller, poorly crystalline, structurally disordered/strained ferrihydrite nanocrystallites. Variation in Fh unit-cell parameters is suggested to arise from substantial particle size-induced structural disorder. As a result of this significant size-dependent structural disorder, it was not possible to identify evidence for Si4+ for Fe3+ substitution in these samples based on unit cell parameter variations or refinement of different structural models. Principal component analyses (PCA) and linear combination fits carried out on the PDFs suggest that iron partitions between several phases (e.g., ferrihydrite and an Fe-bearing amorphous silica phase (Amorph. SiO2 + Fe)) in these co-precipitates. A mechanism of co-precipitation is proposed, in which silicate binds to Fe polymers and Fh particles, thus inhibiting particle growth at low Si content. At higher Si content, SiO44− polymerization traps significant Fe, and we suggest that the occurrence of this second Fe pool limits further the availability of Fe required for ferrihydrite particle development. Such Si-ferrihydrite co-precipitates are expected to be more stable in natural environments with respect to reductive dissolution or transformation, and to impact the bioavailability of Fe(III).

Mineralogical and spectroscopic analyses were conducted on a lateritic Ni-deposit from Southern New Caledonia. Results show that Ni is incorporated in siderite (FeCO3) found between 37 and 40 m depth in the laterite and saprolite units of the regolith. SEM-EDXS analyses of siderite-rich samples indicate that a significant amount of nickel can be hosted by this crystalline phase (~0.8 wt% NiO). Ni and Fe K -edge extended X-ray absorption fine structure (EXAFS) spectroscopic analyses of the siderite-rich samples from the regolith as well as comparison with synthetic Ni-bearing and Ni-free siderites demonstrate isomorphous substitution of Ni2+ for Fe2+ in the siderite structure. Linear combination fitting (LCF) of the Ni K -edge EXAFS data reveals that this Ni-bearing siderite species accounts for more than 90% of the total Ni pool (1 wt% NiO) in the siderite-rich horizons of the regolith. In addition, LCF analysis of the EXAFS spectra indicates that goethite and serpentine are the major Ni hosts in the upper horizons (laterite) and lower horizons (saprolite) of the regolith, respectively. Formation of siderite, an uncommon mineral species in such oxidized environments, is attributed to the development of swampy conditions in organic-rich lateritic materials that accumulated at the bottom of dolines. These results thus show the importance of siderite as a host for nickel in lateritic Ni deposits that have been affected by late hydromorphic and reducing conditions.

Their Toxicity” L et al. reported that sulfidation of silver nanoparticles (Ag-NPs) works as a natural antidote to their toxicity. They found that reduction in the toxicity exerted in four diverse types of aquatic and terrestrial eukaryotic organisms is primarily associated with a postsulfidation decrease in Ag concentration, which is due to the lower solubility of Ag2S relative to elemental Ag (Ag). They also showed that the presence of chloride in the exposure medium for a given organism affects the toxicity outcomes by affecting Ag speciation. Transformation of metallic NPs to their sulfide forms (that is, sulfidation) commonly occurs as a part of the wastewater treatment processes. As most NPs enter the environment as a part of wastewater treatment plant effluent, the work done by Levard et al. is very interesting and important. We agree that the decreased Ag level after sulfidation may play a role in toxicity reduction. However, the corresponding role presented in the work by Levard et al. is likely to be overestimated. Our point lies in the fact that although the magnitude of the dissolution rate of Ag-NPs is one order higher than that of Ag sulfide NPs, the dissolution ratio of Ag-NPs is still low. It was reported that the dissolution equilibrium of the initial Ag-NPs was reached after one month and only about 2% was dissociated. This result is in agreement with that obtained by Levard et al. (see Figure 2). Thus, the possible maximum concentration of Ag dissociated from pristine Ag-NPs at the concentration of 2 mg/L (as indicated in Figure 4 with almost 100% lethality in Caenorhabditis elegans (C. elegans)) was no more than 0.04 mg/L. Meanwhile, the 50% growth inhibition dose (EC50) and the minimum observed dose causing 100% lethality in C. elegans within 24 h of exposure to Ag were 0.1 and 0.15 mg/L, respectively, in which C. elegans were exposed to EPA medium, same as the condition used in Figure 4 of Levard et al. As the possible maximum concentration of Ag dissociated from Ag-NPs is less than half of EC50 and only onefourth of the minimum concentration causing 100% lethality, the potential maximum concentration of dissociated Ag alone is unlikely to cause almost 100% lethality in C. elegans (see Figure 4). Therefore, there must be another cause for the marked difference in toxicity between pristine Ag-NPs and AgNPs that have undergone partial sulfidation. Given the same logic, toxicity reduction due to the formation of AgCl(s) in the presence of chloride might also not be as important as described by Levard et al. The authors made an important contribution in understanding sulfidation-induced reduction of toxicity of Ag-NPs. However, based on our above rationale, the real mechanism underlying toxicity reduction of Ag-NPs through sulfidation remains unknown, and more work should be done to elucidate this. We sincerely hope that this comment can help toward a better understanding of this interesting topic. Ze-hua Liu*,†,‡ Hua Yin†,‡ Zhi Dang†,‡ †College of Environment and Energy, South China University of Technology, Guangzhou 510006, Guangzhou, China ‡Key Lab Pollution Control & Ecosystem Restoration in Industry Cluster, Ministry of Education, Guangzhou 510006, Guangdong, China

What is mesoscale science? The modifier “meso” can mean different things to different communities. In many areas of science, “mesoscale” generally refers to a middle-ground domain of length, energy, or time where theories accurate at both lower and higher scales fail. In materials science, for example, mesoscale behavior often rises when quantum behavior begins to fade, collective effects become important, or statistical variation and defects appear, often at length scales larger than a few nm. However, for atmospheric scientists and ecologists, mesoscale means miles. For meterologists, mesoscale means hundreds to thousands of miles. The mesoscale arena for cosmologists is many light-years across.

International audience; Estimation of Fe3+/ΣFe ratios in materials at the submicrometre scale has been a long-standing challenge in the Earth and environmental sciences because of the usefulness of this ratio in estimating redox conditions as well as for geothermometry. To date, few quantitative methods with submicrometric resolution have been developed for this purpose, and most of them have used electron energy-loss spectroscopy carried out in the ultra-high vacuum environment of a transmission electron microscope (TEM). Scanning transmission X-ray microscopy (STXM) is a relatively new technique complementary to TEM and is increasingly being used in the Earth sciences. Here, we detail an analytical procedure to quantify the Fe3+/ΣFe ratio in silicates using Fe L2,3-edge X-ray absorption near edge structure (XANES) spectra obtained by STXM, and we discuss its advantages and limitations. Two different methods for retrieving Fe3+/ΣFe ratios from XANES spectra are calibrated using reference samples with known Fe3+ content by independent approaches. The first method uses the intensity ratio of the two major peaks at the L3-edge. This method allows mapping of Fe3+/ΣFe ratios at a spatial scale better than 50 nm by the acquisition of 5 images only. The second method employs a 2-eV-wide integration window centred on the L2 maximum for Fe3+, which is compared to the total integral intensity of the Fe L2-edge. These two approaches are applied to metapelites from the Glarus massif (Switzerland), containing micrometre-sized chlorite and illite grains and prepared as ultrathin foils by focused ion beam milling. Nanometre-scale mapping of iron redox in these samples is presented and shows evidence of compositional zonation. The existence of such zonation has crucial implications for geothermometry and illustrates the importance of being able to measure Fe3+/ΣFe ratios at the submicrometre scale in geological samples.

Mass-dependent fractionation (MDF) and mass-independent fractionation (MIF) of Hg isotopes provides a new tool for tracing Hg in contaminated environments such as mining sites, which represent major point sources of Hg pollution into surrounding ecosystems. Here, we present Hg isotope ratios of unroasted ore waste, calcine (roasted ore), and poplar leaves collected at a closed Hg mine (New Idria, CA, U.S.A.). Unroasted ore waste was isotopically uniform with δ(202)Hg values from -0.09 to 0.16‰ (± 0.10‰, 2 SD), close to the estimated initial composition of the HgS ore (-0.26‰). In contrast, calcine samples exhibited variable δ(202)Hg values ranging from -1.91‰ to +2.10‰. Small MIF signatures in the calcine were consistent with nuclear volume fractionation of Hg isotopes during or after the roasting process. The poplar leaves exhibited negative MDF (-3.18 to -1.22‰) and small positive MIF values (Δ(199)Hg of 0.02 to 0.21‰). Sequential extractions combined with Hg isotope analysis revealed higher δ(202)Hg values for the more soluble Hg pools in calcines compared with residual HgS phases. Our data provide novel insights into possible in situ transformations of Hg phases and suggest that isotopically heavy secondary Hg phases were formed in the calcine, which will influence the isotope composition of Hg leached from the site.