George A. Parks

In Situ X-ray Absorption Study of Surface Complexes: Selenium Oxyanions on α-FeOOH

A novel application of x-ray absorption spectroscopy has provided structural information for ions sorbed at oxide-water interfaces. As an example, in situ extended x-ray absorption fine structure (EXAFS) measurements of adsorbed selenate and selenite ions at ah α-FeOOH(goethite)—water interface have been performed; these measurements show that selenate forms a weakly bonded, outer-sphere complex and that selenite forms a strongly bonded, inner-sphere complex. The selenite ion is bonded directly to the goethite surface in a bidentate fashion with two iron atoms 3.38 angstroms from the selenium atom. Adsorbed selenate has no iron atom in the second coordination shell of selenium, which indicates retention of its hydration sphere upon sorption. This method provides direct structural information for adsorbed species at solid-liquid interfaces.

Characterization of Aqueous Colloids by Their Electrical Double-Layer and Intrinsic Surface Chemical Properties

Electrical charge acquired by colloids or solid surfaces in aqueous systems plays a significant role in determining their properties. Homo- and heterocoagulation and flocculation, nonspecific ion adsorption and ion exchange, electrode process rates, and the configuration of organic macromolecules are all affected to some extent by surface charge or potential. Over the past decade, several lines of research and model development involving solids or surfaces as different as metallic mercury, virtually insoluble oxides, silver halides, proteins, polystyrene latexes, ionizable monolayers, and clays have converged. For colloids and hydrosols that contain acidic, basic, or amphoteric functional groups accessible at their surfaces, it is now possible to present a single, conceptually simple and easily visualized model capable of predictively modeling the development of surface charge and potential, the zeta potential of the solid, the conductivity of the sol, electrolyte adsorption densities, and related properties. It is our purpose to review the origins of this model, the methods of characterizing solids and colloids for its use, and the results obtained when it is applied to inorganic oxides, polystyrene latexes, and clays. We are convinced that the fundamental concepts embodied in the model provide a more realistic understanding and representation of ionic processes at surfaces than any single previous model, though it is a direct outgrowth of several earlier approaches. We hope that this review will stimulate a more unified view of organic and inorganic colloids and that it will trigger systematic study of the ionization and complexation reactions at surfaces and of the appropriate intrinsic equilibrium constants for surface functional groups.

Dissolution kinetics of magnesium silicates

Abstract Kinetic dissolution experiments on serpentine, forsterite and enstatite over a wide pH range at 25°C show that there is an initial rapid exchange of surface magnesium ions with hydrogen ions followed by a longer period of hydrogen exchange and extraction of internal magnesium and silicon, with the amount extracted proportional to t 1 2 . The parabolic exchange kinetics are consistent with either of two rate controlling mechanisms: nonsteady state diffusion of ions within the mineral and quasi-steady state diffusion of ions through a leached shell surrounding the mineral. Diffusion coefficients for magnesium are greater than for silicon for each of the minerals, leading to incongruent dissolution over moderate time periods. The diffusion coefficients decrease in the order forsterite > serpentine > enstatite. Eventually the parabolic exchange rates decrease to the rate of dissolution of all material at the aqueous interface. Hence, over very long periods the amount of silicon and magnesium dissolved is proportional to t and dissolution is congruent. In highly acid solutions dissolution rates are fast and this terminal condition is reached much sooner.

Quantitative arsenic speciation in mine tailings using X-ray absorption spectroscopy

Abstract Polarized infrared absorption spectra of thin, oriented single-crystal slabs of pectolite and serandite were recorded between 4000 and 350 cm-1 at 298 and 83 K. The spectra of both minerals show a broad absorption region parallel to the silicate chains (b direction) that is centered around 1000 cm-1 which is interrupted by a transmission window, and which is superimposed by sharp silicate, lattice, and overtone modes. This band is assigned to the OH stretching mode consistent with the alignment of the O-H O hydrogen bond parallel to b and the short O···O distance of 2.45-2.48 Å that was found in previous X- ray structure refinements. At 1396 cm-1 (pectolite) and 1386 cm-1 (serandite) an OH bending mode is observed in the IR spectra parallel to c. At low temperatures, this mode shifts up to higher frequencies (1403 cm-1 at 83 K in pectolite). whereas the down-shift of the OH stretching mode cannot be observed due to the extremely broad band width. The slightly higher energy of the bending mode in pectolite indicates a slightly stronger hydrogen bond with respect to serandite. However, the bond length in serandite is slightly shorter than that in pectolite. An asymmetric O-H ··· O bond is confirmed in pectolite and serandite through comparison with different materials with similar, very strong hydrogen bonds and low-energy OH stretching modes.

Silica diagenesis; I, Solubility controls

ABSTRACT The diagenetic sequence of silica is examined in terms of several thermodynamic and kinetic concepts. The relationship between solubility and surface area or particle size is sufficient to explain simple opal-A opal-CT quartz transformations, suggesting that the role of temperature and other physical parameters may not be as critical as previously thought. Complexation and adsorption in an impure system, when added to the model, allow for possibilities of opal-A transformation directly to quartz, of clay and zeolite formation from an opaline silica source, and of variations in silica transformation rates due to presence of impurities in the system.

Cd2+ uptake by calcite, solid-state diffusion, and the formation of solid-solution: Interface processes observed with near-surface sensitive techniques (XPS, LEED, and AES)

Abstract Cadmium uptake by calcite from aqueous solution was studied using techniques sensitive to the near-surface: X-ray photoelectron spectroscopy (XPS), low-energy electron diffraction (LEED), and Auger electron spectroscopy (AES). These techniques allowed direct observations of structure and bonding environments at the calcite surface. The results indicate that the main processes involved in cadmium uptake by calcite are adsorption and solid-state diffusion into the crystal, which leads eventually to the formation of solid-solution. Pure calcite crystals were cleaved from precleaned Iceland spar and were briefly exposed to aqueous solutions containing various concentrations of Cd2+, CO32−, ClO4−, and/or Cl−. Some Cd2+ was radiolabelled. LEED results demonstrate that the calcite surface is atomically ordered, even after hydration and cadmium uptake. γ-scintillation data from crystals exposed briefly to solutions of 109Cd2+ indicate that surface uptake ranged from the equivalent of about 1 to 4 monolayers. XPS analyses in the first 2 hours after exposure detected Cd within the top 30 A, but crystals stored in air or in ultra-high vacuum showed a decrease in Cd surface concentration with time such that after two days, Cd was barely detectable in the near-surface region. In other experiments, LEED verified the crystallinity of otavite (CdCO3) grown epitaxially over the {101} cleavage faces of calcite, and XPS showed almost no Ca in the near-surface on scans taken immediately after precipitation; but after storage for a month in ultrahigh vacuum, binding energy shifts and the presence of a Ca peak strongly suggested the development of solid-solution by diffusion through the solid. No Cd enrichment was observed at sites of surface defects using AES, indicating that solid-state diffusion into the mineral surface was not accomplished simply by migration along microfractures alone. This work suggests that solid-state diffusion may play a role in the rate and extent of uptake of certain trace metals from solution and probably leads to the formation of solid-solution in calcite and other carbonate minerals. It also suggests that the process of diffusion into the solid mineral host should be considered in hydrogeochemical models that intend to simulate and predict trace-metal mobility in carbonate terrains.

Differential redox and sorption of Cr (III/VI) on natural silicate and oxide minerals: EXAFS and XANES results

Synchrotron-based X-ray absorption fine structure (XAFS) spectroscopy was used to investigate the reduction of aqueous Cr(VI) to Cr(III) in magnetite-bearing soils from Cr-contaminated sites. Soils from two field sites were examined, showing that mixed-valence Cr(III/VI) effluent is reduced to Cr(III) when associated with the magnetite fraction of the soil, whereas the Cr effluent associated with non-Fe(II)-bearing minerals results in mixed Cr(III/VI) adsorbates or precipitated phases. The Fe{sup 2+} in magnetite, Fe{sup 2+}Fe{sub 2}{sup 3+}O{sub 4}, may act as an electron source for heterogeneous Cr(VI)-to-Cr(III) reduction, converting magnetite topotactically to maghemite, {gamma}-Fe{sub 2}{sup 3+}. The ratio of Cr(VI)/total Cr was determined by the height of the Cr(VI) XAFS pre-edge feature, which is due to a Is to 3d electronic transition. This pre-edge feature was calibrated as a function of Cr(VI)/Cr(III) using mixtures of Cr(III) and Cr(VI) model compounds. Environmental remediation of Cr-contaminated sites requires knowledge of chromium oxidation and speciation, and XAFS spectroscopy may be used to supply both types of information with minimal sample processing or data analysis. 36 refs., 9 figs., 2 tabs.

Surface complexation of Pb(II) at oxide-water interfaces: I. XAFS and bond-valence determination of mononuclear and polynuclear Pb(II) sorption products on aluminum oxides

Abstract Pb(II) sorption on Al2O3 powders was studied as functions of sorption density (from 0.5 to 5.2 μmoles/m2) and [Pb]eq (0.03–1.4 mM) in 0.1 M NaNO3 electrolyte solution using XAFS spectroscopy. At pH 6 and 7, Pb(II) ions were found to be fully hydrolyzed and adsorbed preferentially as mononuclear bidentate complexes to edges of AlO6 octahedra. At higher sorption densities (Γ ≥ 3.4 μmoles · m−2), XAFS results suggest the presence of dimeric Pb(II) surface complexes. A bond-valence model was used in conjunction with these results to constrain the compositions and reaction stoichiometries of adsorption complexes. We conclude that Pb(II) adsorption on alumina is aattributable to complexation by [ Al Al Al  O −1 2 ] and [ AlOH −1 2 ] surface functional groups. Several plausible Pb(II) adsorption reactions are proposed, based on these results, which provide a basis for chemically realistic descriptions of surface complexation of Pb(II) on aluminum oxides.

Sorption of Trace Elements on Mineral Surfaces: Modern Perspectives from Spectroscopic Studies, and Comments on Sorption in the Marine Environment

The partitioning (or sorption) of trace elements from aqueous solutions onto mineral surfaces and natural organic matter (NOM) has played a major role in determining the trace element content of natural waters. This review examines sorption processes on mineral surfaces for nine trace elements (Cr, Co, Ni, Cu, Zn, Sr, Cd, Hg, Pb), focusing on the results of modern x-ray spectroscopic studies. Such studies provide unique information on the structure and composition of sorption products, including their mode of attachment to mineral surfaces or functional groups in NOM under in situ conditions (i.e., with aqueous solution present at 25°C). The types of chemical reactions (acid-base, ligand exchange, redox, dissolution/reprecipitation) that can occur at mineral-aqueous solution interfaces are also reviewed, and some of the factors that affect the reactivity of mineral surfaces are discussed, including changes in the geometric and electronic structures of mineral surfaces when they first react with aqueous solutions and constraints on the bonding of adions to surface functional groups imposed by Pauling bond valence sums. A summary of electrical double layer (EDL) theory is presented, including the results of several recent x-ray spectroscopic and parameter regression studies of the EDL for metal-(oxyhydr) oxide-aqueous solution interfaces. The effects of common inorganic and organic complexants on the sorption of trace metal cations at mineral-solution interfaces are considered, in the context of spectroscopic studies where possible. The results of sorption studies of trace metal cations on NOM, common bacteria, and marine biomass are reviewed, and the effects of coatings of NOM and microbial biofilms on cation uptake on mineral surfaces are discussed, based on macroscopic and spectroscopic data. The objective here is to assess the relative importance of inorganic versus organic sorption processes in aquatic systems. The paper concludes with a discussion of the effects of water composition on trace element removal mechanisms, with the aim of providing an understanding of the effects of the high salinity of seawater on trace element sorption processes. The information presented in this review indicates that sorption processes on mineral, NOM, and microbial and algal surfaces, including true adsorption and precipitation, are highly effective at removing trace elements from natural waters and generally supports Krauskopfs (1956) conclusion that such processes are likely responsible for the present trace element concentrations in seawater.

Surface complexation of Pb(II) at oxide-water interfaces: II. XAFS and bond-valence determination of mononuclear Pb(II) sorption products and surface functional groups on iron oxides

Abstract Pb(II) sorption on goethite and hematite powders was studied at room temperature as a function of pH (6–8), sorption density (2–10 μmoles/m2), and [Pb]eq (0.2 μM – 1.2 mM) in 0.1 M NaNO3 electrolyte using XAFS spectroscopy. Pb(II) ions were found to be hydrolyzed and adsorbed as mononuclear bidentate complexes to edges of FeO6 octahedra on both goethite and hematite under all conditions. Hydrolysis of Pb(II) appears to be a primary source of proton release associated with surface complexation of Pb(II). A bond-valence model was used to relate the relative stabilities of iron-oxide surface functional groups and Pb(II) adsorption complexes to their structures and compositions. This combined approach suggests that Pb(II) adsorption occurs primarily at unprotonated [ Fe Fe Fe  O 1 2 ] sites and at [ Fe  OH 2 +1 2 ] sites. Several adsorption reactions are proposed. Comparison to EXAFS results from Pb(II) adsorption on aluminum oxides suggests that the edge lengths of surface AlO6 or FeO6 octahedra partially determine the reactivities and densities of available surface sites. The results of this study provide a basis for constructing chemically realistic descriptions of Pb(II) surface complexation reactions on Fe (hydr)oxides.