David M. Sherman

Surface complexation of arsenic(V) to iron(III) (hydr)oxides: structural mechanism from ab initio molecular geometries and EXAFS spectroscopy

Abstract Arsenic(V), as the arsenate (AsO4)3− ion and its conjugate acids, is strongly sorbed to iron(III) oxides (α-Fe2O3), oxide hydroxides (α–,γ–FeOOH) and poorly crystalline ferrihydrite (hydrous ferric oxide). The mechanism by which arsenate complexes with iron oxide hydroxide surfaces is not fully understood. There is clear evidence for inner sphere complexation but the nature of the surface complexes is controversial. Possible surface complexes between AsO4 tetrahedra and surface FeO6 polyhedra include bidentate corner-sharing (2C), bidentate edge-sharing (2E) and monodentate corner-sharing (1V). We predicted the relative energies and geometries of AsO4-FeOOH surface complexes using density functional theory calculations on analogue Fe2(OH)2(H2O)nAsO2(OH)23+ and Fe2(OH)2(H2O)nAsO4+ clusters. The bidentate corner-sharing complex is predicted to be substantially (55 kJ/mole) more favored energetically over the hypothetical edge-sharing bidentate complex. The monodentate corner-sharing (1V) complex is very unstable. We measured EXAFS spectra of 0.3 wt. % (AsO4)3− sorbed to hematite (α-Fe2O3), goethite(α–FeOOH), lepidocrocite(γ–FeOOH) and ferrihydrite and fit the EXAFS directly with multiple scattering. The phase-shift-corrected Fourier transforms of the EXAFS spectra show peaks near 2.85 and 3.26 A that have been attributed by previous investigators to result from 2E and 2C complexes. However, we show that the peak near 2.85 A appears to result from As-O-O-As multiple scattering and not from As-Fe backscatter. The observed 3.26 A As-Fe distance agrees with that predicted for the bidentate corner-sharing surface (2C) complex. We find no evidence for monodentate (1V) complexes; this agrees with the predicted high energies of such complexes.

EFFECT OF INORGANIC AND ORGANIC LIGANDS ON THE MECHANISM OF CADMIUM SORPTION TO GOETHITE

Abstract Extended X-ray Absorption Fine Structure (EXAFS) spectroscopic data for Cd2+ sorbed on goethite in the presence of phosphate, sulphate and humate indicate that cadmium is surrounded by a first shell of ∼6 ± 1 O atoms at 2.3 ± 0.1 A. A second shell of approximately 2 Fe atoms at 3.8 ± 0.1 A is also observed. These interatomic distances suggest that Cd2+ is inner-spherically bound by bidentate double-corner-sharing adsorption on the (110) surface of goethite. In contrast, Cd2+ appears to form a precipitate in the presence of citrate and oxalate. The enhancement of Cd2+ adsorption in the presence of sulphate and phosphate is solely by electrostatic interaction, implying that the ligands sorb to sites other than those occupied by Cd2+. No ternary complexes are observed in the presence of these 2 inorganic ligands.

Sorption of As(V) on green rust (Fe4(II)Fe2(III)(OH)12SO4 · 3H2O) and lepidocrocite (γ-FeOOH): Surface complexes from EXAFS spectroscopy

Green rust (Fe4(II)Fe2(III)(OH)12SO4 · 3H2O) is an intermediate phase in the formation of iron (oxyhydr)oxides such as goethite, lepidocrocite and magnetite; current thinking is that it occurs in many soil and sediment systems. Green rust has been shown to reduce sorbed selenate and nitrate and, therefore, might presumeably reduce sorbed arsenate to the more toxic and mobile As(III) species. We have investigated the mechanism of As(V) sorption onto green rust and its fate during oxidation of green rust to lepidocrocite. EXAFS spectroscopy was used to determine the As speciation and coordination environment. We find that As(V) is not reduced to the more mobile and toxic As(III) form following equilibrium with green rust for 24 h. It remains adsorbed as (AsO4)3− by forming inner-sphere surface complexes. The same result is obtained whether As(V) is added prior to or after green rust nucleation. Two different inner sphere surface complexes are resolved: one results from edge-sharing between AsO4 and FeO6 polyhedra while the second results from and double-corner sharing between AsO4 tetrahedra and adjacent FeO6 polyhedra. During the oxidation of green rust to lepidocrocite, the (AsO4)3− remains preferentially bound to green rust and only sorbs onto lepidocrocite when all of the green rust has been oxidized. Sorption onto lepidocrocite occurs via an inner-sphere complex resulting from bidentate corner sharing between AsO4 tetrahedra and adjacent FeO6 octahedra.

The mechanism of cadmium surface complexation on iron oxyhydroxide minerals

Abstract Many sediment and soil systems have become significantly contaminated with cadmium, and earth scientists are now required to make increasingly accurate predictions of the risks that this contamination poses. This necessitates an improved understanding of the processes that control the mobility and bioavailability of cadmium in the environment. With this in mind, we have studied the composition and structure of aqueous cadmium sorption complexes on the iron oxyhydroxide minerals goethite (α-FeOOH), lepidocrocite (γ-FeOOH), akaganeite (β-FeOOH), and schwertmannite (Fe8O8(OH)6SO4) using extended X-ray adsorption fine structure spectroscopy. The results show that adsorption to all of the studied minerals occurs via inner sphere adsorption over a wide range of pH and cadmium concentrations. The bonding mechanism varies between minerals and appears to be governed by the availability of different types of adsorption site at the mineral surface. The geometry and relative stability of cadmium adsorption complexes on the goethite surface was predicted with ab initio quantum mechanical modelling. The modelling results, used in combination with the extended X-ray adsorption fine structure data, allow an unambiguous determination of the mechanism by which cadmium bonds to goethite. Cadmium adsorbs to goethite by the formation of bidentate surface complexes at corner sharing sites on the predominant (110) crystallographic surface. There is no evidence for significant cadmium adsorption to goethite at the supposedly more reactive edge sharing sites. This is probably because the edge sharing sites are only available on the (021) crystallographic surface, which comprises just ∼2% of the total mineral surface area. Conversely, cadmium adsorption on lepidocrocite occurs predominately by the formation of surface complexes at bi- and/or tridentate edge sharing sites. We explain the difference in extended X-ray adsorption fine structure results for cadmium adsorption on goethite and lepidocrocite by the greater availability of reactive edge sharing sites on lepidocrocite than on goethite. The structures of cadmium adsorption complexes on goethite and lepidocrocite appear to be unaffected by changes in pH and surface loading. There is no support for cadmium sorption to any of the studied minerals via the formation of an ordered precipitate, even at high pH and high cadmium concentration. Cadmium adsorption on akaganeite and schwertmannite also occurs via inner sphere bonding, but the mechanism(s) by which this occurs remains ambiguous.

Surface oxidation of pyrite under ambient atmospheric and aqueous (pH = 2 to 10) conditions: electronic structure and mineralogy from X-ray absorption spectroscopy

The nature of the surface oxidation phase on pyrite, FeS2, reacted in aqueous electrolytes at pH = 2 to 10 and with air under ambient atmospheric conditions was studied using synchrotron-based oxygen K edge, sulfur LIII edge, and iron LII,III edge X-ray absorption spectroscopy. We demonstrate that O K edge X-ray absorption spectra provide a sensitive probe of sulfide surface oxidation that is complementary to X-ray photoelectron spectroscopy. Using total electron yield detection, the top 20 to 50 A of the pyrite surface is characterized. In air, pyrite oxidizes to form predominantly ferric sulfate. In aqueous air-saturated solutions, the surface oxidation products of pyrite vary with pH, with a marked transition occurring around pH 4. Below pH = 4, a ferric (hydroxy)sulfate is the main oxidation product on the pyrite surface. At higher pH, we find iron(III) oxyhydroxide in addition to ferric (hydroxy)sulfate on the surface. Under the most alkaline conditions, the O K edge spectrum closely resembles that of goethite, FeOOH, and the surface is oxidized to the extent that no FeS2 can be detected in the X-ray absorption spectra. In a 1.667 × 10−3 mol/L Fe3+ solution with ferric iron present as FeCl3 in NaCl, the oxidation of pyrite is autocatalyzed, and formation of the surface iron(III) oxyhydroxide phase is promoted at low pH.

Surface oxidation of chalcopyrite (CuFeS2) under ambient atmospheric and aqueous (pH 2-10) conditions: Cu, Fe L- and O K-edge X-ray spectroscopy

Abstract X-ray absorption and emission spectra were used to characterize the surface of chalcopyrite after oxidation both in air and in air-saturated aqueous solution (pH = 2–10). For chalcopyrite oxidized in aqueous solution, the Cu and Fe L-edge spectra show that the surface oxidation layer is copper deficient. As the pH increases, O K-edge spectra reveal a change in the nature of the oxidation layer. An iron (hydroxy)sulfate is dominant at low pH, whereas FeOOH is the major surface phase under alkaline conditions. Fe2O3 may be present at intermediate pH. The surfaces of chalcopyrite samples oxidized in air consist of a mixture of copper oxides, FeOOH, and sulfate phases. Sulfate is much more abundant on the surface of air-oxidized chalcopyrite because of its high solubility in aqueous solution. Likewise, copper oxidation products can be observed in the O K-edge spectra of air-oxidized chalcopyrite in contrast to the aqueous samples.

Stability of possible Fe-FeS and Fe-FeO alloy phases at high pressure and the composition of the Earth's core

Abstract First-principles density functional calculations (beyond the local density approximation) are used to predict the equations of state (EOS) and formation energies of Fe-FeO and Fe-FeS alloys under the pressures of the Earths core. The accuracy of the static calculations is demonstrated from predicted equations of state and phase transitions of Fe, FeO and FeS. As indicated by the formation energies of Fe3O and Fe4O, solid solution between Fe and FeO remains energetically unfavorable up to core pressures. The instabilities are so large that no reasonable entropy term could stabilize an Fe-FeO solid solution at core temperatures. In contrast, solid solution between Fe and FeS becomes favored at core pressures as indicated by the formation energy of Fe3S. To the extent that the Earths inner core is not dense enough to be pure iron, it follows that the inner core is most likely an Fe-FeS alloy rather than an Fe-FeO alloy. This, however, requires that the melting point of FeS fall below that of Fe at core pressures. The much lower density of the outer core may reflect either the width of the “phase loop” in the Fe-FeS binary or presence of an additional light element which cannot be incorporated into solid iron. Even if an additional light element is present in the outer core, the Earth must be enriched in sulfur relative to potassium. TheK/S ratio of the Earth must reflect the segregation of the core as an Fe-FeS eutectic during the early differentiation of the Earth.

Crystal-chemistry of Ni in marine ferromanganese crusts and nodules

Abstract Marine ferromanganese crusts and nodules are highly enriched in transition metals such as Ni and Co, yet the crystal chemistry and mode of incorporation of these metals is poorly known. We characterized the crystal chemistry of Ni in two hydrogenetic Pacific ocean ferromanganese crust samples and a hydrogenetic nodule from the Madeira abyssal plain. Energy dispersive spectrometry shows that Ni is associated with the manganese oxide phases, in agreement with previous work. X-ray diffraction patterns show that the dominant Mn3+/4+ oxide is a phyllomanganate similar to hexagonal birnessite or δ-MnO2. Extended X-ray absorption fine-structure spectroscopy shows that the coordination environment of Ni results from structural incorporation into the phyllomanganate phase by replacement of Mn3+/4+. In contrast, Ni initially sorbs to freshly prepared synthetic birnessite by surface complexation over vacancy sites in the MnO2 layer. We propose that the transformation of Ni sorption from surface complexation to structural incorporation provides a potentially irreversible sink for Ni in seawater.

An extended x-ray absorption fine structure spectroscopy investigation of cadmium sorption on cryptomelane (KMn8O16)

The mobility of cadmium in the environment is strongly inhibited by sorption onto Fe and Mn (hydr)oxide minerals such as cryptomelane (KMn8O16). Adsorption experiments showed that cryptomelane was able to sorb two thirds of available cadmium from solution at pH as low as 2.0. Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy was used to determine whether cadmium had sorbed to the external surfaces of the mineral, or migrated into the ∼4.6 A diameter tunnels which exist in the cryptomelane structure. The Mn coordination environment around sorbed cadmium (4.9±1.0 Mn at 3.65±0.02 A) at pH 2.0 strongly indicates that the majority of sorbed cadmium is located inside the tunnels, and that it is displaced from the ideal tunnel cation position (special position 2a (0, 0, 0)). The oxygen coordination environment around cadmium (6.5±1.3 O at 2.24±0.02 A) is consistent with this conclusion, but also suggests that sorbed cadmium is partially hydrated. The dominant tunnel cation (K+) was not significantly released to solution during cadmium sorption. Thus, it seems likely that cadmium exchanged with H+ in the tunnels rather than K+. This is supported by the lowering of pH during cadmium adsorption and corresponding charge balance calculations. Cadmium sorption at the tunnel sites is likely to be energetically favourable because it allows occupation of those tunnel sites which K+ cannot fill, thus resulting in a more effective balancing of the negative structural charge in cryptomelane. This is the first EXAFS study of cation sorption on a mineral with the hollandite structure.

Aqueous speciation of yttrium at temperatures from 25 to 340°C at Psat: an in situ EXAFS study

Abstract The solvation environment of aqueous yttrium in chloride bearing solutions was characterised by EXAFS spectroscopy at temperatures from 25 to 340°C. Four solution compositions were considered containing 0.1 M YCl 3 , 0.1 M YCl 3 +0.23 M HCl+1.0 M NaCl, 0.1 M YCl 3 +0.23 M HCl+2.0 M NaCl, and 0.05 M Y(NO 3 ) 3 , respectively. Yttrium was found to be surrounded by an inner coordination shell of H 2 O, the coordination number decreasing from 9–10 to ∼8 with increasing temperature from 25 to 340°C. The Y–O interatomic distance is constant 2.37±0.02 A at temperatures up to 340°C. Yttrium chloride inner sphere complexing is negligible up to 340°C for all solutions, including that containing 2.5 M chloride. This observation implies that chloride inner sphere complexes play an insignificant role in the solubility and transport of yttrium, and by analogy the heavy rare-earth elements in sub critical crustal fluids. Data analysis of EXAFS spectra of the 0.1 M YCl 3 solution at temperatures in excess of 250°C indicate the presence of yttrium atoms located at ∼3.6 A and ∼4.9 A from the central yttrium atom. This latter observation is consistent with yttrium polyatomic species formation, or polymerisation.