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Understanding lead (Pb) and antimony (Sb) speciation associated with the weathering of bullets at shooting ranges is essential for identifying species migration potential to local watersheds and for assessing the overall toxicity of shooting range soils. In the present study, we fired 2000 5.56 mm bullets into newly constructed and instrumented target berms composed of well-characterized test soils (sand, sandy loam, loamy sand, silt loam) and collected berm pore water runoff and soil samples over five summers (2011 to 2015). We tracked the chemical transformations of Pb and Sb released during bullet weathering as a function of time and soil properties. During 2014 summer, an amendment of ferrous chloride (FeCl2) with a calcium carbonate (CaCO3) buffer was added to a subset of the berms of each soil type to test this remediation strategy. Bulk speciation analysis coupled with micro-scale spectroscopic methods show that both Sb(III) and Sb(V) species are present in soil solution depending on the soil matrix type, but Sb(III) was not observed after 9 months of weathering. In general, Sb was found to be more mobile than Pb, attributable to the relatively low solubility of the dominant Pb phases present in the crust forming around bullet fragments and within soil. The oxidation of Pb(0) resulted in a mixture of lead oxide, lead carbonate, and lead sorbed onto iron(III) oxides. We found a higher degree of metal(loid) mobilization (higher dissolved metal concentrations) in the berms made from the sandy soils. In contrast, silt loam soil was found to be more effective at immobilizing metal(loid)s. Furthermore, we observed that an iron-oxide type amendment may be effective at further reducing Pb and Sb runoff. Results from this study provide insight into the fate and transport of metal(loid)s within small arms target ranges and address a potential method for metal(loid) immobilization.

A structural study of the surface complexation of Pb(II) on the ( 1 1 ¯ 02 ) surface of hematite was undertaken using crystal truncation rod (CTR) X-ray diffraction measurements under in situ conditions. The sorbed Pb was found to form inner sphere (IS) complexes at two types of edge-sharing sites on the half layer termination of the hematite ( 1 1 ¯ 02 ) surface. The best fit model contains Pb in distorted trigonal pyramids with an average Pb O bond length of 2.27(4) A and two characteristic Pb-Fe distances of 3.19(1) A and 3.59(1) A. In addition, a site coverage model was developed to simulate coverage as a function of sorbate-sorbate distance. The simulation results suggest a plausible Pb-Pb distance of 5.42 A, which is slightly larger than the diameter of Pb’s first hydration shell. This relates the best fit surface coverage of 0.59(4) Pb per unit cell at monolayer saturation to steric constraints as well as electrostatic repulsion imposed by the hydrated Pb complex. Based on the structural results we propose a stoichiometry of the surface complexation reaction of Pb(II) on the hematite ( 1 1 ¯ 02 ) surface and use bond valence analysis to assign the protonation schemes of surface oxygens. Surface reaction stoichiometry suggests that the proton release in the course of surface complexation occurs from the Pb-bound surface O atoms at pH 5.5.

Lead (Pb) and antimony (Sb) associated with bullets pose a long term contamination risk in shooting ranges because bullet fragments weather over years to decades. In this study we analyzed bullet and soil samples from a historic military shooting range in interior Alaska. Bulk speciation analysis coupled with micro-scale analytical methods demonstrate the presence of Sb(V) in octahedral coordination with 5.3(1) O and a second shell of 3(1) Fe atoms is the primary species present in the weathered bullet crust. However, trivalent Sb bound to 3.1(2) O atoms is likely the initial oxidation species as detected at the soil/alloy interface of a weathered bullet from this study. Similar analysis shows that cerussite (PbCO3), hydrocerussite (Pb3(CO3)2(OH)2), and litharge (PbO) comprise the majority of the Pb species in the weathering crust but Pb(II) adsorbed to Fe(III) oxides are also present in the soil distal to the source material. These results show differences in speciation between the weathering crust and soil fraction in shooting range samples and highlight the natural association of Pb and Sb with Fe. Understanding metal speciation is a critical first step in developing and implementing remediation strategies in small arms shooting ranges.

Most frozen soils contain unfrozen water – water that remains liquid at subfreezing temperatures – which affects a frozen soil's strength and contributes to frost heave. Among the variables that influence the flow of unfrozen water through a freezing soil is the presence of clay minerals and their adsorbed cations. Zeta potential (ζp) is one measure of clay mineral surface properties that may provide insight into the interaction of mineral surfaces with unfrozen water films; however, one gap in the literature is the lack of ζp measurements at subfreezing temperatures. The purpose of this research was to measure the ζp of six cation-treated soils as a function of temperature, including measurements at a subfreezing temperature. The results indicate that the magnitude of the ζp decreases as a function of temperature. The ζp of the divalent cation-treated clays generally had a smaller magnitude and was less sensitive to temperature than the monovalent cation treatments, suggesting that unfrozen water mobility (and thus frost heave) may be correlated with smaller ζp. Based on these results, it is hypothesized that the amounts of immobile unfrozen water may depend on the ζp at the shear plane. To complement the results of this research, we recommend additional tests to determine the viscosity of unfrozen water as a function of temperature.

Increasing air temperatures in the Arctic have the potential to degrade permafrost and promote the downward migration of the seasonally thawed active layer into previously frozen material. This may expose frozen soils to mineral weathering that could affect the geochemical composition of surface waters. Determining watershed system responses to drivers such as a changing climate relies heavily on understanding seasonal controls on freshwater processes. The majority of studies on elemental concentrations in Arctic river systems have focused on sampling only from spring snowmelt to the summer season. Consequently, there remains a limited understanding of surface water geochemistry, particularly with respect to trace metals, during late fall and early winter. To examine the variability of metal concentrations as a function of seasonality, we measured trace metal concentrations from spring melt to fall freeze-up in 2010 in two high Arctic watersheds: Imnavait Creek, North Slope, Alaska and Roche Mountanee Creek, Brooks Range, Alaska. We focused on aluminum (Al), barium (Ba), iron (Fe), manganese (Mn), nickel (Ni) and zinc (Zn). Concentrations of ‘dissolved’ (

Antimony (Sb) is a contaminant of concern that can be present in elevated concentrations in shooting range soils due to mobilization from spent lead/antimony bullets. Antimony in shooting range soils has been observed as either metallic Sb(0) or as Sb(V) immobilized by iron (hydr)oxides. The absence of Sb(III) in soils is indicative of rapid Sb(III) oxidation to Sb(V) under surface soil conditions. However, the major controls on antimony oxidation and mobility are poorly understood. To better understand these controls we performed multiple batch experiments under oxic conditions to quantify the oxidation and dissolution of antimony in systems where Sb(0) is oxidized to Sb(III) and further to Sb(V). We also tested how variations in the aqueous matrix composition and the presence of metallic lead (Pb) affect the dissolution, solid phase speciation, and oxidation of antimony. We monitored changes in the aqueous antimony speciation using liquid chromatography inductively coupled plasma mass spectrometry (LC-ICP-MS). To test which solid phases form as a result of Sb(0) oxidation, and therefore potentially limit the mobility of antimony in our studied systems, we characterized the partially oxidized Sb(0) powders by means of extended X-ray absorption fine structure (EXAFS) spectroscopy and powder X-ray diffraction (XRD). The observed oxidation of Sb(0) to Sb(III) and mobilization to solution is rapid: after 5–15 min of reaction the aqueous antimony concentration reached 50–600 lM. The amount of dissolved antimony and the rate of Sb(III) oxidation to Sb(V) in

article The KantishnaHills miningdistrictofinteriorAlaska,USA,locatedwithin DenaliNational ParkandPreserve,con- tains a number of antimony lode deposits, including Alaska's historically largest antimony producer, the Stam- pede mine. Oxidative weathering of sulfidic tailings and waste rock associated with historic mining operations has impacted water quality in the region. In the Stampede and Slate Creek watersheds, antimony and arsenic concentrations instream waters were as high as 720 μg/L and 239 μg/L, respectively. Antimony inall watersam- plesispredominantlypresentasSb(V),whereasarsenicwasdetectedinvaryingratiosofAs(III)andAs(V).Based onX-rayabsorptionspectroscopy(XAS)measurementsreducedAs(III)andSb(III)wereidentifiedinminewaste materials, whereas predominantly oxidized forms, As(V) and Sb(V), were found in downstream sediments. Elevated antimony concentrations extend for more than 8 km downstream from the antimony lodes, whereas arsenic quickly attenuates within 1.5 km. The difference between antimony and arsenic aqueous phase specia- tion suggests that antimony oxidation is more rapid than arsenic within this system. A high correlation is ob- served between antimony, arsenic, and iron concentrations in fine-fraction streambed sediments downstream of the source lodes. This suggests that sorption and co-precipitation with iron (hydr)oxides are important path- ways for the attenuation of antimony and arsenic in these interior Alaska watersheds. Further XAS characteriza- tion of the downstream sediments corroborates these observations and indicates that antimony is adsorbed to Fe-oxide phases as inner-sphere bi-dentate edge and corner sharing complexes. The trace element redox states, as well as downstream partitioning, are mainly controlled by iron speciation based on the strong correlation be- tween redox potentials calculated from iron (Fe(II)/Fe(III)) and arsenic (As(III)/As(V)).

This study reports on density functional theory (DFT) modeling of antimony adsorption at model environmental interfaces. Both Sb(III) and Sb(V) surface complexes were studied on hydrated alumina and hematite, at varying adsorption sites and with varying distal Sb coordination. We calculate the DFT reaction energies of a number of hypothetical heterogeneous interconversion reactions of Sb surface complexes with gas-phase water and oxygen, which predict that an octahedral Sb(V) surface complex is overall the most favorable. Additionally, the results suggest that several different heterogeneous pathways starting from distinct metastable Sb surface complexes are possible, including an antimonyl (Sb O) surface complex. A total of 28 Sb surface complexes are found through DFT geometry optimizations, the structural and energetic details of which are reported to guide future experimental studies and to form a basis for ongoing theoretical work including dynamic simulations.

We describe a simple method for making in-house , low concentration (0–100 ppm) pressed pellet standards for analyzing Ni, Cu, Mo, V and U in carbonate lithologies by wavelength dispersive X-ray fluorescence (WD-XRF) spectrometry. This method can easily be modified to include other elements of interest and to encompass a different range of concentrations. The purpose here was to circumvent the general lack of adequate geological certified reference materials (CRMs) having the necessary concentrations of the above five elements that are commonly used in paleoceanographic studies as proxies for changes in bioproductivity and paleoredox conditions in the water column and near the sediment–water interface. Standards were prepared by spiking 99.999% pure calcium carbonate powder with single element, inductively coupled plasma mass spectrometry (ICP-MS) standard stock solutions to yield final multi-element mixtures containing 0, 1, 2, 5, 10, 20, 50 and 100 ppm of Ni, Cu, Mo, V and U, independently verified using acid digestion ICP-MS and PROTrace XRF trace element analyses. The level of in-house standard homogeneity was determined statistically using ANOVA tests ( analyses of variance ) on count rate data generated for each analyte from three surface transects (50 × 500 μm) ablated on each standard using laser-ablation ICP-MS, demonstrating that all are statistically distinguishable and sufficiently homogenous for the intended purposes of larger spatial scale XRF measurements. The in-house standards were then used to refine preexisting XRF calibrations based on a broad suite of non-carbonate geological CRMs. The refined calibrations yield close agreement with ICP-MS and PROTrace XRF-derived values for the suite of in-house standards when analyzed repeatedly as ‘unknown samples’ by the XRF analytical protocol. The precision of sample preparation and analysis was assessed statistically by ANOVA tests on the results of repeated analyses of analyte concentrations for replicate XRF samples prepared from a representative carbonate sample, yielding reproducibility on the order of 1)

The use of geothermal fluids for the production of electricity poses a risk of contaminating surface waters when spent fluids are discharged into (near) surface environments. Arsenic (As) in particular is a common component in geothermal fluids and leads to a degradation of water quality when present in mobile and bioavailable forms. We have examined changes in arsenic speciation caused by quick transition from high temperature reducing conditions to surface conditions, retention mechanisms, and the extent of transport associated with the release of spent geothermal fluids at the Dachny geothermal fields (Mutnovsky geothermal region), Kamchatka, Russia — a high temperature field used for electricity production. In the spent fluids, the arsenic concentration reaches 9 ppm, while in natural hot springs expressed in the vicinity of the field, the As concentration is typically below 10 ppb. The aqueous phase arsenic speciation was determined using Liquid Chromatography (LC) coupled to an Inductively Coupled Plasma Mass Spectrometer (ICP-MS). The arsenic speciation in the bottom sediments ( − from the deep well fluids as a tracer. Analysis of the Extended X-ray Absorption Fine Structure (EXAFS) spectra shows that sediment phase arsenic is associated with both Al- and Fe-rich phases with a bi-dentate corner sharing local geometry. The geothermal waste fluids released in the surface water create a localized area of arsenic contamination. The extent of transport of dissolved As is limited to ~ 7 km downstream from the source, while As associated with bottom sediment travels ~ 3 km farther.

Composition B (Comp B) is a commonly used military formulation composed of the toxic explosive compounds 2,4,6-trinitrotoluene (TNT), and hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX). Numerous studies of the temporal fate of explosive compounds in soils, surface water and laboratory batch reactors have been conducted. However, most of these investigations relied on the application of explosive compounds to the media via aqueous addition and thus these studies do not provide information on the real world loading of explosive residues during detonation events. To address this we investigated the dissolution and sorption of TNT and RDX from Comp B residues loaded to pure mineral phases through controlled detonation. Mineral phases included nontronite, vermiculite, biotite and Ottawa sand (quartz with minor calcite). High Performance Liquid Chromatography and Attenuated Total Reflectance Fourier Transform Infrared spectroscopy were used to investigate the dissolution and sorption of TNT and RDX residues loaded onto the mineral surfaces. Detonation resulted in heterogeneous loading of TNT and RDX onto the mineral surfaces. Explosive compound residues dissolved rapidly (within 9 h) in all samples but maximum concentrations for TNT and RDX were not consistent over time due to precipitation from solution, sorption onto mineral surfaces, and/or chemical reactions between explosive compounds and mineral surfaces. We provide a conceptual model of the physical and chemical processes governing the fate of explosive compound residues in soil minerals controlled by sorption-desorption processes.

X-ray crystal truncation rod (CTR) diffraction under hydrated conditions at circum-neutral pH was used to determine the surface structure of Fe3O4(111) following a wet chemical mechanical polishing (CMP) preparation method. The best-fit model to the CTR data shows the presence of two oxygen terminated domains that are chemically inequivalent and symmetrically distinct in the surface contribution ratio of 75% oxygen octahedral-iron (OOI) termination (aO2.61–aO1.00–oh1Fe2.55–bO1.00–bO3.00–td1Fe1.00–oh2Fe1.00–td2Fe1.00–R) to 25% oxygen mixed-iron (OMI) termination (bO1.00–bO3.00–td1Fe0–oh2Fe1.00–td2Fe1.00–aO3.00–aO1.00–oh1Fe3.00–R). An adsorbed water layer could not be constrained in the best-fit model. However, bond-valence analyses suggest that both of the surfaces are hydro-oxo terminated. Furthermore, the topmost iron layers of both domains are inferred to be occupied with the redox active Fe2+ and Fe3+ cations indicating that these irons are the principle irons involved in controlling the surface reactivity of magnetite in industrial and environmentally relevant conditions.

Goethite({alpha}-FeOOH), an abundant and highly reactive iron oxyhydroxide mineral, has been the subject of numerous studies of environmental interface reactivity. However, such studies have been hampered by the lack of experimental constraints on aqueous interface structure, and especially of the surface water molecular arrangements. Structural information of this type is crucial because reactivity is dictated by the nature of the surface functional groups and the structure or distribution of water and electrolyte at the solid-solution interface. In this study we have investigated the goethite (1 0 0) surface using surface diffraction techniques, and have determined the relaxed surface structure, the surface functional groups, and the three dimensional nature of two distinct sorbed water layers. The crystal truncation rod (CTR) results show that the interface structure consists of a double hydroxyl, double water terminated interface with significant atom relaxations. Further, the double hydroxyl terminated surface dominates with an 89% contribution having a chiral subdomain structure on the (1 0 0) cleavage faces. The proposed interface stoichiometry is ((H{sub 2}O)-(H{sub 2}O)-OH{sub 2}-OH-Fe-O-O-Fe-R) with two types of terminal hydroxyls; a bidentate (B-type) hydroxo group and a monodentate (A-type) aquo group. Using the bond-valence approach the protonation states of the terminal hydroxyls are predictedmore » to be OH type (bidentate hydroxyl with oxygen coupled to two Fe{sup 3+} ions) and OH{sub 2} type (monodentate hydroxyl with oxygen tied to only one Fe{sup 3+}). A double layer three dimensional ordered water structure at the interface was determined from refinement of fits to the experimental data. Application of bond-valence constraints to the terminal hydroxyls with appropriate rotation of the water dipole moments allowed a plausible dipole orientation model as predicted. The structural results are discussed in terms of protonation and H-bonding at the interface, and the results provide an ideal basis for testing theoretical predictions of characteristic surface properties such as pK{sub a}, sorption equilibria, and surface water permittivity.« less

The ability to apply existing density functional theory-based modeling techniques to timely research problems in environmental chemistry is demonstrated by an ab initio thermodynamics investigation of stable hydrated oxide surface models and a comparative reactivity study of Pb(II) adsorption on two water-mineral interfaces with a common geometry, but distinct electronic structure. We emphasize the unique considerations required to produce chemically reasonable structural models for hydrated surfaces and surface complex structures, as well as how to use experimental insights to limit the extensive configuration space encountered in complex hydrated models relative to theoretical surface science done under idealized, ultra-high vacuum conditions.

The surface structure of {alpha}-Fe{sub 2}O{sub 3}(0 0 0 1) was studied using crystal truncation rod (CTR) X-ray diffraction before and after reaction with aqueous Fe(II) at pH 5. The CTR results show the unreacted {alpha}-Fe{sub 2}O{sub 3}(0 0 0 1) surface consists of two chemically distinct structural domains: an O-layer terminated domain and a hydroxylated Fe-layer terminated domain. After exposing the {alpha}-Fe{sub 2}O{sub 3}(0 0 0 1) surface to aqueous Fe(II), the surface structure of both co-existing structural domains was modified due to adsorption of Fe at crystallographic lattice sites of the substrate, resulting in six-coordinated adsorbed Fe at the surface. The average Fe-O bond lengths of the adsorbed Fe are consistent with typical Fe(III)-O bond lengths (in octahedral coordination), providing evidence for the oxidation of Fe(II) to Fe(III) upon adsorption. These results highlight the important role of substrate surface structure in controlling Fe(II) adsorption. Furthermore, the molecular scale structural characterization of adsorbed Fe provides insight into the process of Fe(II) induced structural modification of hematite surfaces, which in turn aids in assessing the effective reactivity of hematite surfaces in Fe(II) rich environments.

The structure of α - Fe 2 O 3 ( 1 1 ¯ 02 ) reacted with Fe(II) under anoxic conditions was studied using crystal truncation rod (CTR) diffraction. The CTR results show the crystalline termination of α - Fe 2 O 3 ( 1 1 ¯ 02 ) is modified due to adsorption of Fe(II) at crystallographic lattice sites. In addition, the binding sites for adsorbed Fe are similar for all studied conditions: reaction for 2 h at pH 5.0, for 34 d at pH 5.0, and for 5.5 h at pH 7.0. The occupancy of adsorbed Fe increases with both reaction time and pH, which is consistent with typical cation adsorption behavior on iron (hydr)oxide surfaces. The metal–oxygen bond lengths of the (ordered) surface Fe atoms are characteristic of Fe(III), which provides indirect evidence for oxidation of adsorbed Fe(II) and is consistent with recent studies indicating that Fe(III)-hydroxides are effective oxidants for dissolved ferrous iron. Grazing-incidence X-ray diffraction measurements indicate that no crystalline surface reaction products formed during the course of Fe(II) reaction. Overall, the structural characterization of the Fe(II) adsorption reaction results in an enhanced understanding of how reduced iron affects the structure, stability and reactivity of hematite.

The surface structure of α- Fe 2 O 3 ( 1 1 ¯ 0 2 ) was studied under two different surface preparation conditions using crystal truncation rod (CTR) diffraction. Wet chemical and mechanical polishing (CMP) at 298 K results in a crystalline surface termination in which the top layer of iron atoms is absent compared to the stoichiometric bulk termination. Annealing in air at 773 K resulted in a transformation of the surface to a structure consistent with hydroxylation of the stoichiometric termination. These results agree with theoretical predictions of Lo et al. [C.S. Lo, K.S. Tanwar, A.M. Chaka, T.P. Trainor, Phys. Rev. B 75 (2007) 075425] and clearly show an ambient pressure surface preparation path leading to a stoichiometric hydroxylated surface, which is apparently a meta-stable configuration at room temperature.

The structure of the hydroxylated a-Fe2O3ð1 102 Þ surface prepared via a wet chemical and mechanical polishing (CMP) procedure was determined using X-ray crystal truncation rod diffraction. The experimentally determined surface model was compared with theoretical structures developed from density functional theory (DFT) calculations to identify the most likely protonation states of the surface (hydr)oxo moieties. The results show that the hydroxylated CMP-prepared surface differs from an ideal stoichiometric termination due to vacancies of the near surface bulk Fe sites. This result differs from previous ultra high vacuum studies where two stable terminations were observed: a stoichiometric (1 · 1) termination and a partially reduced (2 · 1) reconstructed surface. The complementary DFT studies suggest that hydroxylated surfaces are thermodynamically more stable than dehydroxylated surfaces in the presence of water. The results illustrate that the best fit surface model has predominantly three types of (hydr)oxo functional groups exposed at the surface at circumneutral pH: Fe–OH2 ,F e 2–OH, and Fe3–O and provide a structural basis for interpreting the reactivity of model iron-(hydr)oxide surfaces under aqueous conditions. � 2006 Elsevier B.V. All rights reserved.

Chemical processes occurring at environmental interfaces (e.g. mineral–fluid, mineral–organic matter and mineral–biofilm interfaces) have a profound impact on the environmental fate and bioavailability of aqueous metals and other contaminant species. However, the direct analysis of molecular scale structure and properties of environmental interfaces, particularly under “high-pressure” or “wet” conditions is highly challenging. Synchrotron based X-ray scattering and spectroscopic approaches offer numerous advantages, such as the high penetrating power and molecular scale information inherent to X-ray techniques. Yet, the ability to localize information content to environmental interfaces requires challenging experimental configurations. Here, the application of grazing angle X-ray fluorescence techniques is reviewed, including the presentation of a model formalism that allows for quantitative analysis of fluorescent yield profiles and discussion of the experimental setup. Illustrative examples are discussed, particularly in the context of combining results of GI measurements with the results of other complementary interface probes such as crystal truncation rod diffraction and X-ray microprobe spectroscopic studies.

Evaluation of the long-term health risks of uranium contamination in soils, sediments, and groundwater requires a fundamental understanding of the various processes affecting subsurface transport of uranium, including adsorption processes at mineral/water interfaces. In this study, the sites of binding and surface complexation of U(VI) adsorbed on the (1102) surfaces of α-Al2O3 and α-Fe2O3 have been determined using crystal truncation rod (CTR) diffraction and grazing incidence extended X-ray absorption fine structure (GI-EXAFS) spectroscopy. The available binding sites on the (1102) surfaces were constrained through bond valence and steric analyses. On both surfaces, U(VI) forms uranyl-carbonato ternary complexes to surface oxygens that are singly coordinated to aluminum or iron. On the α-Al2O3 (1102) surface, a monodentate complex results, whereas on the α-Fe2O3 (1102) surface, the binding is bidentate to adjacent singly coordinated oxygen sites (i.e., binuclear). Differences in protonation of the singly coordinated oxygen atoms, surface charging, U(VI) aqueous speciation, substrate structure, or the electronic structure of surface functional groups may be the cause of these differences in adsorption geometry. Both XPS and CTR diffraction reveal higher U(VI) surface coverages on the α-Fe2O3 (1102) surface than on the α-Al2O3 (1102) surface. This difference cannot be the result of differences in defect concentration alone as CTR diffraction is not sensitive to U(VI) sorbed to defect sites, implying that the α-Fe2O3 (1102) surface has an intrinsically higher affinity for U(VI). The surface complexes observed in this study are different from the bidentate, mononuclear complexes typically derived for U(VI) on powdered aluminum- and iron-(oxyhydr)oxides and clay minerals using U LIII-edge EXAFS spectroscopy. However, the presence of monodentate, mononuclear and bidentate, binuclear complexes may have been overlooked in past EXAFS studies on such substrates, as these complexes have U-Al or U-Fe interatomic distances that are too large to be easily detected by EXAFS spectroscopy.

The structure of the hydroxylated hematite (0 0 0 1) surface was investigated using crystal truncation rod diffraction and density functional theory. The combined experimental and theoretical results suggest that the surface is dominated by two hydroxyl moieties—hydroxyls that are singly coordinated and doubly coordinated with Fe. The results are consistent with the formation of distinct domains of these surface species; one corresponding to the hydroxylation of the surface Fe-cation predicted to be most stable under UHV conditions, and the second a complete removal of this surface Fe species leaving the hydroxylated oxygen layer. Furthermore, our results indicate that the hydroxylated hematite surface structures are significantly more stable than their dehydroxylated counterparts at high water partial pressures, and this transition in stability occurs at water pressures orders of magnitude below the same transition for α-alumina. These results explain the observed differences in reactivity of hematite and alumina (0 0 0 1) surfaces with respect to water and binding of aqueous metal cations.

We have grown single-crystal Co ferrite (CoFe2O4) on MgO(0 0 1) by oxygen-plasma-assisted molecular beam epitaxy (OPA-MBE), and have characterized the composition, structure, surface morphology, and magnetic properties by a number of methods. The as-grown OPA-MBE material forms a perfect inverse spinel, in which all Co is in the 2+ formal oxidation state, and occupies octahedral sites within the cation sublattice. The OPA-MBE film surfaces are very flat, with mean roughnesses of only a few A, and exhibit large, stable magnetic domains. The measured moment per unit volume is 250 emu/cm3, and the saturation magnetization for films in the 1000 A film thickness range is ∼60% of that of bulk Co ferrite. The material also exhibits strain-dependent magnetic anisotropy that can be understood by considering the various contributions to the total magnetic energy. The overall quality of epitaxial Co ferrite grown on MgO by OPA-MBE is superior in every respect to that obtained using pulsed laser deposition as the growth method.

The structure of the α-Al2O3 (1 −1 0 2) surface was examined using crystal truncation rod (CTR) diffraction. The initial surface was prepared by Ar-ion sputter and anneal cycles in O2, which resulted in a clean (1×1) surface. CTR data were then collected for the clean and water-dosed surfaces (1×10−8–1.6 Torr) in a UHV diffraction chamber (base pressure≈1–3×10−9 Torr). Water dosing had little effect on the diffraction data, suggesting that the initially prepared surface was fully oxygenated. Least-squares analysis of the CTR data resulted in two best fit models, a relaxed stoichiometric bulk termination and a relaxed bulk termination with a zero occupancy for the first layer of aluminum atoms. Crystal chemistry considerations suggest that the second model is the most plausible if the surface oxygens are protonated.

Grazing-incidence XAFS spectroscopy was applied to study the sorption of Zn(II) on two crystallographically distinct surfaces of highly polished sapphire single crystals as a simplified analog for metal ion sorption on natural aluminum-(hydr)oxides. Experiments were performed both in situ (in contact with bulk solution) and ex situ in a humidified N2 atmosphere. The identification of an Al shell at roughly 3 A in all samples indicates that Zn(II) binds as an inner sphere complex on both the (0001) and (1–102) surfaces. The first shell Zn–O distances of 1.97–1.99 A suggest that Zn is in fourfold coordination with oxygen in the in situ samples. However, sample drying appears to have induced the formation of polynuclear surface complexes with first shell Zn–O distances closer to values expected for sixfold coordination (2.06–2.07 A). The results presented here show that in situ characterization of sorption products on single crystal surfaces using Grazing-incidence XAFS is feasible if solution conditions are chosen carefully.

Macroscopic measurements show that Pb(II) uptake on iron-(hydr)oxides can be altered significantly by dissolved carbonate (enhanced up to 18% at pH 5 and decreased above pH approximately 6.5 in analyses at 1 atm CO(2)). This study elucidates the molecular-scale processes giving rise to these macroscopic effects by characterizing the structures of Pb(II) sorption complexes formed on goethite (alpha-FeOOH) in the presence of carbonate using in situ Pb L(III)-EXAFS and ATR-FTIR spectroscopies. Bond valence and structural constraints are applied to develop mineral surface site-specific models for Pb sorption. Under all conditions studied (pH 5-7, Gamma(Pb)=0.4-4µmol/m(2), and P(CO(2))=0-1 atm), Pb(II) forms predominantly inner-sphere edge-sharing (bidentate and/or tridentate) complexes with Fe(O,OH)(6) octahedra (R(Pb-Fe) approximately 3.3 Å). Corner-sharing complexes (R(Pb-Fe) approximately 3.9 Å) are observed only in low pH (5) samples (P(CO(2)) 0-1 atm). Consistent with this pH sensitivity, site-specific analyses suggest that the relative abundance of corner-sharing sites reflects changes in the proton affinity of triply coordinated sites on the goethite (110) surface as suggested previously. FTIR results suggest the existence of ternary surface complexes in which carbonate groups bond to Pb as monodentate ligands. EXAFS data indicate that these ternary complexes are bound to the surface through Pb, forming metal-bridged (Type A) complexes. Findings are summarized as structural models and corresponding mineral surface site-specific chemical reactions. Copyright 2000 Academic Press.