V14B-01
Spectroscopic and Structural Study of the Protonation of Aquatic Natural Organic Matter from the Amazon river tributaries
The Amazon River basin contributes up to 20 percent of the total global flux of terrestrial dissolved organic carbon to the oceans. Because of this, elucidation of the properties on NOM generated in it is essential for understanding global and local cycles of organic carbon. Previous studies have examined the role Amazonian NOM in the speciation and transport of major cationic constituents and trace metals and other issues but, given the global role of the Amazon basin, more information is needed to further ascertain the chemistry and variability of Amazonian NOM. This study demonstrates that the deprotonation of NOM originating from small creeks feeding the Jau River a tributary the Rio Negro in the Amazon basin can be quantified based on measurements of pH effects on its absorbance and fluorescence spectra. NEXAFS, SEC and pyrolysis GC/MS. Differential absorbance (DA) spectra of these fractions contain bands with maxima located at ca. 240, 280, 315 and below 350 nm. The emergence of these features is associated with the engagement of phenolic, carboxylic and other functionalities. The results indicate the existence of structural and compositional differences between the colloidal and hydrophobic fractions that constitute an overwhelmingly important part of the NOM of the small and large black rivers of the Amazon basin. These differences will affect the fate of the mineral colloids and trace elements traveling in the basin together with the organic matter.
V14B-02
From crystal chemistry to colloid stability
Aqueous suspensions of ferrihydrite nanoparticles form a colloid with properties that can be understood using classical theories but which additionally exhibit the distinctive phenomenon of nanocluster formation. While use of in situ light and x-ray scattering methods permit the quantitative determination of colloid stability, interparticle interactions, and cluster or aggregate geometry, there are currently few approaches to predict the colloidal behavior of mineral nanoparticles. A longstanding goal of aqueous geochemistry is the rationalization and prediction of the chemical properties of hydrated mineral interfaces from knowledge of interface structure at the molecular scale. Because interfacial acid-base reactions typically lead to the formation of a net electrostatic charge at the surfaces of oxide, hydroxide, and oxyhydroxide mineral surfaces, quantitative descriptions of this behavior have the potential to permit the prediction of long-range interactions between mineral particles. We will evaluate the feasibility of this effort by constructing a model for surface charge formation for ferrihydrite that combines recent insights into the crystal structure of this phase and proposed methods for estimating the pKa of acidic surface groups. We will test the ability of this model to predict the colloidal stability of ferrihydrite suspensions as a function of solution chemistry.
V14B-03
Structural Aspects of Synthetic Ferrihydrite
The role of natural ferrihydrite in geochemical and biological systems, and use of synthetic ferrihydrite in technological and industrial applications, is attracting broad scientific attention. The importance of ferrihydrite in these systems is primarily related to its large amount of reactive surface area (>350 m2 g-1) which has been shown to effectively scavenge a variety of potential contaminants (e.g., arsenic, chromium). In general, the association of ferrihydrite with metals and metalloids through sorption and co-precipitation is expected to alter its reactivity and thus affect its overall role in aqueous geochemical systems. Such changes in the reactivity of ferrihydrite nanoparticles, typically with all dimensions less than ~7 nm, are inextricably related to their atomic structure, i.e., the 3-dimensional arrangement of atoms. Evaluating the structures of particles with extreme small particle sizes (<10 nm) and substantial disorder has proven difficult by conventional methods for structure determination which are most sensitive to either short-range order (X-ray absorption spectroscopy) or long-range periodicity (X-ray or electron diffraction). However, the recent application of high-energy X-ray total scattering coupled with pair distribution function (PDF) analysis is providing new insight into the structural aspects of ferrihydrite, a material with no known crystalline counterpart. The information obtainable both directly and indirectly from the PDF will be discussed primarily using examples of synthetic inorganically-derived ferrihydrites. This work on synthetic samples complements our investigations of natural ferrihydrites forming in acid mine drainage-impacted waters. Such natural samples are inherently more complex because they typically form in the presence of dissolved inorganic species such as silica, aluminum, chromium, sulphate, etc., as well as organic matter. The complexity of natural ferrihydrites necessitates the use of synthetically-derived samples in order to evaluate specific changes in certain fundamental aspects of these phases, i.e., size, shape, composition, and structure.
V14B-04
Nanoparticles in the Atmosphere
Nanoparticles are abundant and occur throughout the troposphere (you just inhaled several million!). Their high surface areas make them extremely reactive. They grow rapidly through coagulation and adsorption, and through those processes they affect cloud formation and thus climate as well as visibility and health. Their reactivity also makes them highly transient and challenging to study. Although transmission electron microscopy (TEM) is an excellent means for the study of both their chemical and physical features, significant problems exist with collecting and retaining such small particles on TEM grids. However, it is common to find nanoparticles trapped within relatively viscous organic materials that are widespread in the atmosphere and that then shield or impede these nanoparticles from further growth but also facilitate their measurement. They are generated continuously and in large numbers from vehicles and industries in urban areas and from vegetation and sea spray in rural areas. Nanoparticles represent a far richer and more complex world than is commonly recognized, and they contain more information than is commonly being recovered.
V14B-05
Formation of Uranyl-Silicate Nanoparticles at Ambient Conditions
Uranium(VI)-silicates are the dominant crystalline form of U(VI) at and near Earth's surface, but are difficult to form as pure phases under ambient conditions because of slow reaction kinetics aided by similar thermodynamic stabilities of the many possible minerals. We have investigated the effects of pH (2 to 11) and time (1 to 10 days) on the formation of U(VI)-silicates from initial solutions with U = 0.05 M and a fixed molar ratio of U:Si = 2:1, 1:1, 1:2, and 1:5 using high-energy X-ray scattering (HEXS), attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR), powder X-ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM) and solution thermodynamic modeling. Previously, we used HEXS to identify from solutions with U:Si = 1:2 at pH 5 to 9, aged for one day, a trimeric U-silicate structural unit, or synthon, approximately one nanometer in dimension with U-U correlation lengths of about 0.4 nm. This synthon is a structural building block in uranyl silicate minerals such as soddyite, boltwoodite, and weeksite. ATR-FTIR results on the full set of samples show systematic changes in peak positions along with appearance and disappearance of vibrational modes that occurred with reaction time, pH and/or U:Si ratio; whereas, XRD indicated only a crystalline Na-boltwoodite-like phase at pH 11 and without the correlation length-scale resolution of HEXS. HRTEM results show few particles in a matrix of material containing areas having the lower correlation length visible in HEXS data. The data show clearly different mixtures of solids, including silica, and precipitate sizes under all conditions that transform over the 1 to 10 day aging period. The experimental reactions simulate conditions in the subsurface at sites contaminated with uranium, and the results are relevant to processes of uranium adsorption and colloid formation. [This work is supported by DOE's Environmental Remediation Science Program].
V14B-06
Molecular- and Nano-Scale Structure and Reactivity of Biogenic Uranium(IV) Oxide
Bioremediation has been proposed and extensively researched as an in-situ immobilization strategy for uranium contamination in the subsurface with nanoparticulate uraninite (UO2) being the commonly reported product. Little detail is known about the structure and reactivity of this material, but based on comparison to its closest abiotic analog, UO2+x (0 < x < 0.25), we expect that it is complex and disordered and capable of structurally incorporating common groundwater cations. In addition, it has been predicted that the nanoparticulate form would induce strain and increase the solubility, and therefore reduce the effectiveness of this method as a remediation technology. In this study, the local-, intermediate- and long-range atomic and nano-scale structure of biogenic UO2 (formed at varying pH and divalent cation concentration, using Shewanella oneidensis strain MR-1) was characterized using EXAFS, SR-based powder diffraction and TEM. The lattice parameter of the nanoparticulate phase is seen to be consistent with bulk UO2. There is no evidence for hyperstoichiometry or strain of the UO2 particles, the latter indicating that surface energy is relatively modest. Similar results were obtained for biogenic UO2 particles produced by other metal reducing bacteria indicating that biological variability may play a minimal role in structure. In agreement with the structural analysis, the surface area-normalized dissolution rate of the biogenic UO2 was found to be comparable to that of coarser, synthetic UO2.00. Mn2+ was found to attenuate the particle size of biogenic UO2+xand to be structurally incorporated. This finding suggests that groundwater composition can have a pronounced impact on the structure and properties of biogenic uraninite.
V14B-07
Study of Hematite Nanoparticle Interaction With Zn(II), Oxalate, and Shewanella oneidensis Using ATR-FTIR, EXAFS, and STXM
The process occurring at the mineral nanoparticle-water interface is important due to the abundance of the mineral nanoparticles in natural environments. Hematite (α-Fe2O3) is one of the naturally occuring iron oxide mineral phases and it is also a commonly present in a nanoparticle form. In this study, sorption of Zn(II)(aq) on hematite nanoparticles (HN, avg. diam. 10.5 nm) and microparticles (HM, avg. diam. 550 nm) has been examined over a wide range of Zn(II)(aq) concentrations using Zn K-edge EXAFS spectroscopy. When reacted with HN at pH 5.5, a mixture of four- and six-coordinated surface complexes formed at low sorption densities (Γ < 196 μmol/g). Based on EXAFS-derived Zn and Fe distances of 3.12±0.02 Å, we conclude that tetrahedral Zn(II) sorbs on tetrahedral Fe(III) surface sites on HN as mononuclear bidentate edge sharing surface complex. At higher Zn(II) sorption densities on HN (Γ > 586 μmol/g), formation of surface precipitates is suggested. In contrast, EXAFS spectra of Zn(II) sorbed on HM showed no evidence of surface precipitates. Instead, octahedral Zn(II) sorbs in a bidentate edge sharing fashion on octahedral Fe(III)surface sites on HM. The combined EXAFS and selective chemical extraction results indicate that the reactivities of HN and HM differ. Sorption of Zn(II)(aq) on HN and HM was studied as a function of Zn(II)(aq) to Ox2-(aq) concentration ratio (R) at pH 5.5. Increased Zn(II)(aq) uptake was observed in the Zn/Ox/HN system relative to the Zn/Ox/HM system. Based on ATR-FTIR and EXAFS results, we conclude inner-sphere oxalate and outer-sphere ZnOx(aq) complexes, and/or type A ternary complexes form at or near HM-water interface regardless of R values. On HN surfaces, we found (1) at R = 0.15, inner-sphere oxalate and outer- sphere ZnOx(aq) complexes, (2) at R = 0.68, ZnOx(s)-like surface precipitate and possibly type B ternary surface complexes. We ascribe the observed increase of Zn(II)(aq) uptake in the HN ternary system relative to the HM ternary system to a greater number of available reactive surface sites for Zn(II)(aq) in the former which can be attributed to the formation of ZnOx(s)-like surface precipitate at the HN/water interface. Lastly, dissimilatory reduction of HN and HM was studied in batch cultures of Shewanella oneidensis MR-1. Notably higher iron reduction rates were observed for HN compared to HM when particle aggregation was not extensive. Scanning transmission x-ray microscopy (STXM) images and C K-edge and Fe L2,3-edge NEXAFS spectra showed the presence of Fe(II)-containing solids and HN in close proximity to S. oneidensis cells when reacted with HN, whereas no such spatial correlations were observed for HM systems.
V14B-08
Arsenite sequestration by nanosized ferrous minerals after bioreduction of arsenic-sorbed lepidocrocite by Shewanella putrefaciens
X-ray absorption spectroscopy was used in combination with high resolution transmission electron microscopy, X-ray powder diffraction, and Mössbauer spectroscopy to obtain detailed information on arsenic and iron speciation in the products of anaerobic reduction of pure and As(V)- or As(III)-adsorbed lepidocrocite (γ-FeOOH) by Shewanella putrefaciens ATCC 12099. We found that this strain is capable of using both Fe(III) in lepidocrocite and As(V) in solution or adsorbed on lepidocrocite surfaces as electron acceptors. Bioreduction of lepidocrocite in the absence of arsenic resulted in the formation of hydroxycarbonate green rust 1 [FeII4FeIII2(OH)12CO3: GR1(CO3)], which completely converted into ferrous- carbonate hydroxide (FeII2(OH)2CO3: FCH). Bioreduction of As(III)-adsorbed lepidocrocite also led to the formation of GR1(CO3) prior to formation of FCH, but the presence of As(III) slows down this transformation, leading to the co-occurrence of both phases. At the end of this experiment, As(III) was found to be adsorbed on the surfaces of GR1(CO3) and FCH. Bioreduction of As(V)-bearing lepidocrocite led directly to the formation of FCH in association with nanometer-sized particles of a minor As-rich Fe(OH)2 phase, with no evidence for green rust formation. At the end of this experiment, As(V) was fully converted to As(III) and dominantly sorbed at the surface of the Fe(OH)2 nanoparticles as oligomers binding to the edges of Fe(OH)6 octahedra in the octahedral layers of Fe(OH)2. These multinuclear As(III) surface complexes are characterized by As–As pairs at a distance of 3.32 ± 0.02 Å and by As–Fe pairs at a distance of 3.50 ± 0.02 Å and represent a new form of As(III) surface complexes. Chemical analyses show that the majority of As(III) produced in the experiments with As present is associated with iron-bearing hydroxycarbonate or hydroxide solids, reinforcing the idea that, at least under some circumstances, bacterial reduction can promote As(III) sequestration instead of mobilizing it into solution.