The interaction of the l-lactate ion (l-CH3CH(OH)COO−, lact−1) with hematite (α-Fe2O3) nanoparticles (average diameter 11 nm) in the presence of bulk water at pH 5 and 25 °C was examined using a ...combination of (1) macroscopic uptake measurements, (2) in situ attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy, and (3) density functional theory modeling at the B3LYP/6-31+G* level. Uptake measurements indicate that increasing lact−1(aq) results in an increase in lact−1 uptake and a concomitant increase in Fe(III) release as a result of the dissolution of the hematite nanoparticles. The ATR-FTIR spectra of aqueous lact−1 and lact−1 adsorbed onto hematite nanoparticles at coverages ranging from 0.52 to 5.21 µmol/m2 showed significant differences in peak positions and shapes of carboxyl group stretches. On the basis of Gaussian fits of the spectra, we conclude that lact−1 is present as both outer-sphere and inner-sphere complexes on the hematite nanoparticles. No significant dependence of the extent of lact−1 adsorption on background electrolyte concentration was found, suggesting that the dominant adsorption mode for lact−1 is inner sphere under these conditions. On the basis of quantum chemical modeling, we suggest that inner-sphere complexes of lact−1 adsorbed on hematite nanoparticles occur dominantly as monodentate, mononuclear complexes with the hydroxyl functional group pointing away from the Fe(III) center.
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 structure of thin-film water on a BaF(2)(111) surface under ambient conditions was studied using x-ray absorption spectroscopy from ambient to supercooled temperatures at relative humidity up to ...95%. No hexagonal ice-like structure was observed in spite of the expected templating effect of the lattice-matched (111) surface. The oxygen K-edge x-ray absorption spectrum of liquid thin-film water on BaF(2) exhibits, at all temperatures, a strong resemblance to that of high-density phases for which the observed spectroscopic features correlate linearly with the density. Surprisingly, the highly compressed, high-density thin-film liquid water is found to be stable from ambient (300 K) to supercooled (259 K) temperatures, although a lower-density liquid would be expected at supercooled conditions. Molecular dynamics simulations indicate that the first layer water on BaF(2)(111) is indeed in a unique local structure that resembles high-density water, with a strongly collapsed second coordination shell.
Polarization-dependent grazing incidence X-ray absorption spectroscopy (XAS) measurements were completed on oriented single crystals of magnesite MgCO3 and brucite Mg(OH)2 reacted with aqueous ...uranyl chloride above and below the solubility boundaries of schoepite (500, 50, and 5 ppm) at pH 8.3 and at ambient (P CO2 = 10–3.5) or reduced partial pressures of carbon dioxide (P CO2 = 10–4.5). X-ray absorption near edge structure (XANES) spectra show a striking polarization dependence (χ = 0° and 90° relative to the polarization plane of the incident beam) and consistently demonstrated that the uranyl molecule was preferentially oriented with its OaxialU(VI)Oaxial linkage at high angles (60–80°) to both magnesite (101̅4) and brucite (0001). Extended X-ray absorption fine structure (EXAFS) analysis shows that the “effective” number of U(VI) axial oxygens is the most strongly affected fitting parameter as a function of polarization. Furthermore, axial tilt in the surface thin films (thickness ∼ 21 Å) is correlated with surface roughness σ. Our results show that hydrated uranyl(-carbonate) complexes polymerize on all of our experimental surfaces and that this process is controlled by surface hydroxylation. These results provide new insights into the bonding configuration expected for uranyl complexes on the environmentally significant carbonate and hydroxide mineral surfaces.
Pristine silver nanoparticles (AgNPs) are not chemically stable in the environment and react strongly with inorganic ligands such as sulfide and chloride once the silver is oxidized. Understanding ...the environmental transformations of AgNPs in the presence of specific inorganic ligands is crucial to determining their fate and toxicity in the environment. Chloride (Cl–) is a ubiquitous ligand with a strong affinity for oxidized silver and is often present in natural waters and in bacterial growth media. Though chloride can strongly affect toxicity results for AgNPs, their interaction is rarely considered and is challenging to study because of the numerous soluble and solid Ag–Cl species that can form depending on the Cl/Ag ratio. Consequently, little is known about the stability and dissolution kinetics of AgNPs in the presence of chloride ions. Our study focuses on the dissolution behavior of AgNPs in chloride-containing systems and also investigates the effect of chloride on the growth inhibition of E.coli (ATCC strain 33876) caused by Ag toxicity. Our results suggest that the kinetics of dissolution are strongly dependent on the Cl/Ag ratio and can be interpreted using the thermodynamically expected speciation of Ag in the presence of chloride. We also show that the toxicity of AgNPs to E.coli at various Cl– concentrations is governed by the amount of dissolved AgCl x (x–1)– species suggesting an ion effect rather than a nanoparticle effect.
Synchrotron-based soft X-ray spectromicroscopy was used to probe nanometer-scale chemical heterogeneities of black carbon (BC) materials, including anthracite coal, coke, and activated carbon (AC), ...and to study their impact on the partitioning of one type of polychlorinated biphenyls (PCB-166: 2,3,4,4‘,5,6 hexachloro biphenyl) onto AC particles. Various carbon species (e.g., aromatic, ketonic/phenolic, and carboxylic functional groups) were found in all of the BC materials examined, and impurities (e.g., carbonate and potassium ions in anthracite coal) were identified in nanometer-scale regions of these samples. We show that these chemical heterogeneities in AC particles influence their sorption of hydrophobic organic compounds (HOCs). PCB-166 was found to accumulate preferentially on AC particles with the highest content of aromatic functionalities. These new findings from X-ray spectromicroscopy have the following implications for the role of BC materials in the environment: (1) the functional groups of BC materials vary on a 25-nanometer scale, and so does the abundance of the HOCs; (2) molecular-level characterization of HOC sorption preferences on AC will lead to an improved understanding of AC sorption properties for the remediation of HOCs in soils and sediments.
Sorption of contaminants such as arsenic (As) to natural Fe(III) (oxyhydr)oxides is very common and has been demonstrated to occur during abiotic and biotic Fe(II) oxidation. The molecular mechanism ...of adsorption- and co-precipitation of As has been studied extensively for synthetic Fe(III) (oxyhydr)oxide minerals but is less documented for biogenic ones. In the present study, we used Fe and As K-edge X-ray Absorption Near Edge Structure (XANES), extended X-ray Absorption Fine Structure (EXAFS) spectroscopy, Mössbauer spectroscopy, XRD, and TEM in order to investigate the interactions of As(V) and As(III) with biogenic Fe(III) (oxyhydr)oxide minerals formed by the nitrate-reducing Fe(II)-oxidizing bacterium
Acidovorax sp. strain BoFeN1. The present results show the As immobilization potential of strain BoFeN1 as well as the influence of As(III) and As(V) on biogenic Fe(III) (oxyhydr)oxide formation. In the absence of As, and at low As loading (As:Fe
≤
0.008
mol/mol), goethite (Gt) formed exclusively. In contrast, at higher As/Fe ratios (As:Fe
=
0.020–0.067), a ferrihydrite (Fh) phase also formed, and its relative amount systematically increased with increasing As:Fe ratio, this effect being stronger for As(V) than for As(III). Therefore, we conclude that the presence of As influences the type of biogenic Fe(III) (oxyhydr)oxide minerals formed during microbial Fe(II) oxidation. Arsenic-K-edge EXAFS analysis of biogenic As–Fe–mineral co-precipitates indicates that both As(V) and As(III) form inner-sphere surface complexes at the surface of the biogenic Fe(III) (oxyhydr)oxides. Differences observed between As-surface complexes in BoFeN1-produced Fe(III) (oxyhydr)oxide samples and in abiotic model compounds suggest that associated organic exopolymers in our biogenic samples may compete with As oxoanions for sorption on Fe(III) (oxyhydr)oxides surfaces. In addition HRTEM–EDXS analysis suggests that As(V) preferentially binds to poorly crystalline phases, such as ferrihydrite, while As(III) did not show any preferential association regarding Fh or Gt.
The disposal of basic sodium aluminate and acidic U(VI)−Cu(II) wastes in the now-dry North and South 300 A Process Ponds at the Hanford site resulted in a groundwater plume of U(VI). To gain insight ...into the geochemical processes that occurred during waste disposal and those affecting the current and future fate and transport of this uranium plume, the solid-phase speciation of uranium in a depth sequence of sediments from the base of the North Process Pond through the vadose zone to groundwater was investigated using standard chemical and mineralogical analyses, electron and X-ray microprobe measurements, and X-ray absorption fine structure spectroscopy. Near-surface sediments contained uranium coprecipitated with calcite, which formed due to overneutralization of the waste ponds with base (NaOH). At intermediate depths in the vadose zone, metatorbernite Cu(UO2PO4)2·8H2O precipitated, likely during pond operations. Uranium occurred predominantly sorbed onto phyllosilicates in the deeper vadose zone and groundwater; sorbed uranium was also an important component at intermediate depths. Since the calcite-bearing pond sediments have been removed in remediation efforts, uranium fate and transport will be controlled primarily by desorption of the sorbed uranium and dissolution of metatorbernite.
The solubility of Ag NPs can affect their toxicity and persistence in the environment. We measured the solubility of organic-coated silver nanoparticles (Ag NPs) having particle diameters ranging ...from 5 to 80 nm that were synthesized using various methods, and with different organic polymer coatings including poly(vinylpyrrolidone) and gum arabic. The size and morphology of Ag NPs were characterized by transmission electron microscopy (TEM). X-ray absorption fine structure (XAFS) spectroscopy and synchrotron-based total X-ray scattering and pair distribution function (PDF) analysis were used to determine the local structure around Ag and evaluate changes in crystal lattice parameters and structure as a function of NP size. Ag NP solubility dispersed in 1 mM NaHCO3 at pH 8 was found to be well correlated with particle size based on the distribution of measured TEM sizes as predicted by the modified Kelvin equation. Solubility of Ag NPs was not affected by the synthesis method and coating as much as by their size. Based on the modified Kelvin equation, the surface tension of Ag NPs was found to be ∼1 J/m2, which is expected for bulk fcc (face centered cubic) silver. Analysis of XAFS, X-ray scattering, and PDFs confirm that the lattice parameter, a, of the fcc crystal structure of Ag NPs did not change with particle size for Ag NPs as small as 6 nm, indicating the absence of lattice strain. These results are consistent with the finding that Ag NP solubility can be estimated based on TEM-derived particle size using the modified Kelvin equation for particles in the size range of 5–40 nm in diameter.
Sorption of Zn(II)(aq) on hematite (α-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 Å at low sorption densities (Γ ≤ 1.1 μmol/m2). On the basis of EXAFS-derived Zn−Fe3+ distances of (3.10−3.12) ± 0.02 Å, 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 (Γ ≥ 3.38 μmol/m2), we observed the formation of Zn(O,OH)6 surface complexes, with an average Zn−O distance of 2.09 ± 0.02 Å, a Zn−Zn distance of 3.16 ± 0.02 Å, and a linear multiple-scattering feature at 6.12 ± 0.06 Å. Formation of a Zn(OH)2(am) precipitate for the higher sorption density samples (Γ ≥ 3.38 μmol/m2) is suggested on the basis of comparison of the EXAFS spectra of the sorption samples with that of synthetic Zn(OH)2(am). 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 Å) 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 Å. 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).
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