Thomas P. Trainor

Structure and reactivity of the calcite-water interface

The zetapotential of calcite in contact with aqueous solutions of varying composition is determined for pre-equilibrated suspensions by means of electrophoretic measurements and for non-equilibrium solutions by means of streaming potential measurements. Carbonate and calcium are identified as charge determining ions. Studies of the equilibrium solutions show a shift of isoelectric point with changing CO(2) partial pressure. Changes in pH have only a weak effect in non-equilibrium solutions. The surface structure of (104)-faces of single crystal calcite in contact to solutions corresponding to those of the zetapotential investigations is determined from surface diffraction measurements. The results reveal no direct indication of calcium or carbonate inner-sphere surface species. The surface ions are found to relax only slightly from their bulk positions; the most significant relaxation is a ∼4° tilt of the surface carbonate ions towards the surface. Two ordered layers of water molecules are identified, the first at 2.35±0.05Å above surface calcium ions and the second layer at 3.24±0.06Å above the surface associated with surface carbonate ions. A Basic-Stern surface complexation model is developed to model observed zetapotentials, while only considering outer-sphere complexes of ions other than protons and hydroxide. The Basic-Stern SCM successfully reproduces the zetapotential data and gives reasonable values for the inner Helmholtz capacitance, which are in line with the Stern layer thickness estimated from surface diffraction results.

Pb(II) distributions at biofilm–metal oxide interfaces

The distribution of aqueous Pb(II) sorbed at the interface between Burkholderia cepacia biofilms and hematite (α-Fe2O3) or corundum (α-Al2O3) surfaces has been probed by using an application of the long-period x-ray standing wave technique. Attached bacteria and adsorbed organic matter may interfere with sorption processes on metal oxide surfaces by changing the characteristics of the electrical double layer at the solid–solution interface, blocking surface sites, or providing a variety of new sites for metal binding. In this work, Pb Lα fluorescence yield profiles for samples equilibrated with 10−7 to 10−3.8 M Pb(II) were measured and modeled to determine quantitatively the partitioning of Pb(II) at the biofilm–metal oxide interface. Our data show that the reactive sites on the metal oxide surfaces were not passivated by the formation of a monolayer biofilm. Instead, high-energy surface sites on the metal oxides form the dominant sink for Pb(II) at submicromolar concentrations, following the trend α-Fe2O3 (0001) > α-Al2O3 (11̄02) > α-Al2O3 (0001), despite the greater site density within the overlying biofilms. At [Pb] > 10−6 M, significant Pb uptake by the biofilms was observed.

Occurrence of Zn/Al hydrotalcite in smelter-impacted soils from northern France: Evidence from EXAFS spectroscopy and chemical extractions

Abstract Zinc speciation was studied by EXAFS spectroscopy, μ-SXRF elemental mapping, XRD, and chemical extraction methods in two smelter-impacted soils sampled near one of the largest Pb and Zn processing plants in Europe, which is located in northern France about 50 km south of Lille. The tilled and wooded soils chosen for study differ in Zn concentration (≈600 and 1400 mg/kg, respectively), soil pH (7.5 and 5.5, respectively), and organic matter content (1.5 and 6.4 wt% TOC, respectively). In both soils, the occurrence of Fe- and Zn-rich (up to 10 wt% Zn) slag particles ranging in size from a few micrometers to a few millimeters, was shown by m-SXRF elemental mapping of soil thin sections as well as by SEM and chemical analysis of different soil size fractions. For both soils, XRD analysis of the dense coarse fraction, which contains up to 10 wt% Zn, revealed the presence of a minor amount (1-1.5 wt%) of crystalline ZnS (sphalerite and wurtzite). In this fraction, EXAFS data show that Zn is mainly incorporated in the tetrahedral sites of a magnetite- franklinite solid solution. The clay fraction (<2 μm) represents the largest pool of Zn in both soils, with 77 and 62% of the total Zn in the tilled and wooded soils, respectively. However, XRD was not able to detect any Znbearing phases in this fraction. Comparison of Zn K-EXAFS data of untreated and chemically treated samples from the bulk (<2 μm) and the clay (<2 μm) soil fractions with Zn K-EXAFS data from more than 30 model compounds suggests that Zn is present in the following chemical forms: (1) Zn outer-sphere complexes, (2) Zn-organic matter inner-sphere complexes, (3) Zn/Al-hydrotalcite (Zn/ Al-HTLC), (4) phyllosilicates in which Zn is present in the dioctahedral layer at dilute levels, and (5) magnetite-franklinite solid solutions inherited from the smelting process. The presence of exchangeable Zn outer-sphere complexes and of Zn inner-sphere complexes on organic matter is indicated by the relative increase of second-neighbor contributions in the EXAFS RDFs after chemical treatments with 0.01 M CaCl2 and 0.1 M Na4P4O7. The occurrence of Zn/Al-HTLC is demonstrated by the persistence of a Zn-Zn pair correlation at 3.10 ± 0.04 Å (i.e., edge sharing ZnO6 octahedra in the trioctahedral layer structure) in EXAFS data of Na4P2O7 treated soil samples and its disappearance after treatment with 0.45 M HNO3. This latter treatment also revealed the occurrence of Znbearing phyllosilicate minerals, as shown by two Zn-Mg/Al/Si pair correlations at 3.05 ± 0.04 Å and 3.26 ± 0.04 Å, and of magnetite-franklinite solid solutions, as indicated by a Zn-Mn/Fe/Zn pair correlation at 3.50 ± 0.04 Å. Significant changes in the relative proportions of the different forms of Zn between the two soils explain their different responses to chemical treatments and emphasizes the relationships between solid state speciation and mobility of Zn in soils.

Sb(III) and Sb(V) Sorption onto Al-Rich Phases: Hydrous Al Oxide and the Clay Minerals Kaolinite KGa-1b and Oxidized and Reduced Nontronite NAu-1

We have studied the immobilization of Sb(III) and Sb(V) by Al-rich phases - hydrous Al oxide (HAO), kaolinite (KGa-1b), and oxidized and reduced nontronite (NAu-1) - using batch experiments to determine the uptake capacity and the kinetics of adsorption and Extended X-ray Absorption Fine Structure (EXAFS) Spectroscopy to characterize the molecular environment of adsorbed Sb. Both Sb(III) and Sb(V) are adsorbed in an inner-sphere mode on the surfaces of the studied substrates. The observed adsorption geometry is mostly bidentate corner-sharing, with some monodentate complexes. The kinetics of adsorption is relatively slow (on the order of days), and equilibrium adsorption isotherms are best fit using the Freundlich model. The oxidation state of the structural Fe within nontronite affects the adsorption capacity: if the clay is reduced, the adsorption capacity of Sb(III) is slightly decreased, while Sb(V) uptake is increased significantly. This may be a result of the presence of dissolved Fe(II) in the reduced nontronite suspensions or associated with the structural rearrangements in nontronite due to reduction. These research findings indicate that Sb can be effectively immobilized by Al-rich phases. The increase in Sb(V) uptake in response to reducing structural Fe in clay can be important in natural settings since Fe-rich clays commonly go through oxidation-reduction cycles in response to changing redox conditions.

Interaction of Aqueous Zn(II) with Hematite Nanoparticles and Microparticles. Part 1. EXAFS Study of Zn(II) Adsorption and Precipitation

Sorption of Zn(II)(aq) on hematite (alpha-Fe2O3) nanoparticles (average diameter 10.5 nm) and microparticles (average diameter 550 nm) has been examined over a range of total Zn(II)(aq) concentrations (0.4-7.6 mM) using Zn K-edge EXAFS spectroscopy and selective chemical extractions. When ZnCl2 aqueous solutions were reacted with hematite nanoparticles (HN) at pH 5.5, Zn(II) formed a mixture of four- and six-coordinated surface complexes [Zn(O,OH)4 and Zn(O,OH)6] with an average Zn-O distance of 2.04+/-0.02 A at low sorption densities (Gamma<or=1.1 micromol/m2). On the basis of EXAFS-derived Zn-Fe3+ distances of (3.10-3.12)+/-0.02 A, we conclude that both Zn(O,OH)6 and Zn(O,OH)4 adsorb on octahedral Fe3+(O,OH)6 or pentahedral Fe3+(O,OH)5 surface sites on HN as inner-sphere, mononuclear, bidentate, edge-sharing adsorption complexes at these low sorption densities. It is possible that polynuclear Zn complexes are also present because of the similarity of Zn and Fe backscattering. At higher Zn(II) sorption densities on hematite nanoparticles (Gamma>or=3.38 micromol/m2), we observed the formation of Zn(O,OH)6 surface complexes, with an average Zn-O distance of 2.09+/-0.02 A, a Zn-Zn distance of 3.16+/-0.02 A, and a linear multiple-scattering feature at 6.12+/-0.06 A. Formation of a Zn(OH)2(am) precipitate for the higher sorption density samples (Gamma>or=3.38 micromol/m2) is suggested on the basis of comparison of the EXAFS spectra of the sorption samples with that of synthetic Zn(OH)2am. In contrast, EXAFS spectra of Zn(II) sorbed on hematite microparticles (HM) under similar experimental conditions showed no evidence of surface precipitates even at the same total [Zn(II)(aq)] that resulted in precipitate formation in the nanoparticle system. Instead, Zn(O,OH)6 octahedra (d(Zn-O)=2.10+/-0.02 A) were found to sorb dominantly in an inner-sphere, bidentate, edge-sharing fashion on Fe3+(O,OH)6 octahedra at hematite microparticle surfaces, based on an EXAFS-derived Zn-Fe3+ distance of 3.44+/-0.02 A. CaCl2 selective extraction experiments showed that 10-15% of the sorbed Zn(II) was released from Zn/HN sorption samples, and about 40% was released from a Zn/HM sorption sample. These fractions of Zn(II) are interpreted as weakly bound, outer-sphere adsorption complexes. The combined EXAFS and selective chemical extraction results indicate that (1) both Zn(O,OH)4 and Zn(O,OH)6 adsorption complexes are present in the Zn/HN system, whereas dominantly Zn(O,OH)6 adsorption complexes are present in the Zn/HM system; (2) a higher proportion of outer-sphere Zn(II) surface complexes is present in the Zn/HM system; and (3) Zn-containing precipitates similar to Zn(OH)2(am) form in the nanoparticle system but not in the microparticle system, suggesting a difference in reactivity of the hematite nanoparticles vs microparticles with respect to Zn(II)(aq).

Selenium speciation and partitioning within Burkholderia cepacia biofilms formed on α-Al2O3 surfaces

Abstract The distribution and speciation of Se within aerobic Burkholderia cepacia biofilms formed on α-Al2O3 (1-102) surfaces have been examined using grazing-angle X-ray spectroscopic techniques. We present quantitative information on the partitioning of 10−6 M to 10−3 M selenate and selenite between the biofilms and underlying alumina surfaces derived from long-period X-ray standing wave (XSW) data. Changes in the Se partitioning behavior over time are correlated with microbially induced reduction of Se(VI) and Se(IV) to Se(0), as observed from X-ray absorption near edge structure (XANES) spectroscopy. Selenite preferentially binds to the alumina surfaces, particularly at low [Se], and is increasingly partitioned into the biofilms at higher [Se]. When B. cepacia is metabolically active, B. cepacia rapidly reduces a fraction of the SeO32− to red elemental Se(0). In contrast, selenate is preferentially partitioned into the B. cepacia biofilms at all [Se] tested due to a lower affinity for binding to the alumina surface. Rapid reduction of SeO42− by B. cepacia to Se(IV) and Se(0) subsequently results in a vertical segregation of Se species at the B. cepacia/α-Al2O3 interface. Elemental Se(0) accumulates within the biofilm with Se(VI), whereas Se(IV) intermediates preferentially sorb to the alumina surface. B. cepacia/α-Al2O3 samples incubated with SeO42− and SeO32− when the bacteria were metabolically active result in a significant reduction in the mobility of Se vs. X-ray treated biofilms. Remobilization experiments show that a large fraction of the insoluble Se(0) produced within the biofilm is retained during exchange with Se-free solutions. In addition, Se(IV) intermediates generated during Se(VI) reduction are preferentially bound to the alumina surface and do not fully desorb. In contrast, Se(VI) is rapidly and extensively remobilized.

Calculation of crystal truncation rod structure factors for arbitrary rational surface terminations

The technique of crystal truncation rod (CTR) diffraction is widely used for studying the structure of crystalline surfaces and interfaces. The theory and experimental details of the technique are well established; however, published methods for structure-factor calculations are typically based on a simple surface cell geometry. A method is presented for determining a surface coordinate system which results in a reciprocal lattice that is simply defined in terms of the surface termination. Based on this surface coordinate system, a general formalism for the calculation of CTR structure factors is re-derived, which may be easily applied to any surface that can be represented as a rational plane of a bulk crystal system.

Interaction of Zn(II) with hematite nanoparticles and microparticles: Part 2. ATR-FTIR and EXAFS study of the aqueous Zn(II)/oxalate/hematite ternary system.

Sorption of Zn(II) to hematite nanoparticles (HN) (av diam=10.5 nm) and microparticles (HM) (av diam=550 nm) was studied in the presence of oxalate anions (Ox2-(aq)) in aqueous solutions as a function of total Zn(II)(aq) to total Ox2-(aq) concentration ratio (R=[Zn(II)(aq)]tot/[Ox2-(aq)]tot) at pH 5.5. Zn(II) uptake is similar in extent for both the Zn(II)/Ox/HN and Zn(II)/Ox/HM ternary systems and the Zn(II)/HN binary system at [Zn(II)(aq)](tot)<4 mM, whereas it is 50-100% higher for the Zn(II)/Ox/HN system than for the Zn(II)/Ox/HM ternary and the Zn(II)/HN and Zn(II)/HM binary systems at [Zn(II)(aq)]tot>4 mM. In contrast, Zn(II) uptake for the Zn(II)/HM binary system is a factor of 2 greater than that for the Zn(II)/Ox/HM and Zn(II)/Ox/HN ternary systems and the Zn(II)/HN binary system at [Zn(II)(aq)]tot<4 mM. In the Zn(II)/Ox/HM ternary system at both R values examined (0.16 and 0.68), attenuated total reflectance Fourier transform infrared (ATR-FTIR) results are consistent with the presence of inner-sphere oxalate complexes and outer-sphere ZnOx(aq) complexes, and/or type A ternary complexes. In addition, extended X-ray absorption fine structure (EXAFS) spectroscopic results suggest that type A ternary surface complexes (i.e., >O2-Zn-Ox) are present. In the Zn(II)/Ox/HN ternary system at R=0.15, ATR-FTIR results indicate the presence of inner-sphere oxalate and outer-sphere ZnOx(aq) complexes; the EXAFS results provide no evidence for inner-sphere Zn(II) complexes or type A ternary complexes. In contrast, ATR-FTIR results for the Zn/Ox/HN sample with R = 0.68 are consistent with a ZnOx(s)-like surface precipitate and possibly type B ternary surface complexes (i.e., >O2-Ox-Zn). EXAFS results are also consistent with the presence of ZnOx(s)-like precipitates. We ascribe the observed increase of Zn(II)(aq) uptake in the Zn(II)/Ox/HN ternary system at [Zn(II)(aq)]tot>or=4 mM relative to the Zn(II)/Ox/HM ternary system to formation of a ZnOx(s)-like precipitate at the hematite nanoparticle/water interface.

Identification of Cr species at the aqueous solution-hematite interface after Cr(VI)-Cr(III) reduction using GI-XAFS and Cr L-edge NEXAFS

Surface-mediated redox processes and sorption reactions occurring at the solid-liquid interface are of importance to a broad variety of environmental and industrial pollution or corrosion/passivation problems. A more fundamental understanding of the mechanisms of these reactions is required in order to increase our ability to predict interfacial phenomena. In this study, gazing-incidence XAFS (GI-XAFS) and near-edge XAFS (both Land K-edge NEXAFS) techniques were used to investigate the chemical and structural characteristics of interfacial Cr(III) and Cr(VI) species on a molecular level. In particular, products of postulated electron transfer reactions between Cr(VI) and Fe(II) at the oxide-solution interface were investigated. Both techniques confirmed the reduction of Cr(VI) to Cr(III) mediated by a partially reduced hematite (O001) surface. The observed local structure of the interracial chromium species emphasizes the need for an improved, molecular-level conceptualization of reactions occurring at the solid-solution interface.

Geochemistry of Mineral Surfaces and Factors Affecting Their Chemical Reactivity

Publisher Summary This chapter focuses on mineral surfaces and some of the factors affecting their chemical reactivity with water and aqueous species. It presents a brief overview of the geochemistry of mineral surfaces, focusing on metal-oxide and metal-(oxy)hydroxide surfaces, including their dissolution mechanisms, development of electrical charge when in contact with aqueous solutions, and uptake of aqueous cations and anions. It also discusses some of the factors that control their chemical reactivity, including defect density, cooperative effects among adsorbates, intrinsic differences in surface properties such as isoelectric points, and differences in surface structure under hydrated conditions. It begins with a discussion of the most common minerals present in Earths crust, soils, and troposphere, as well as some less common minerals that contain common environmental contaminants. Following this is it provides a discussion of the nature of environmentally important solid surfaces before and after reaction with aqueous solutions, including their charging behavior as a function of solution pH; and the nature of the electrical double layer. It also describes how it is altered by changes in the type of solid present and the ionic strength and pH of the solution in contact with the solid. Furthermore, it deals with dissolution, precipitation, and sorption processes relevant to environmental interfacial chemistry. Finally, it explores some of the factors affecting chemical reactivity at mineral/aqueous solution interfaces.