2009. "Morphology and Electronic Structure of the Oxide Shell on the Surface of Iron Nanoparticles." Journal of the American Chemical Society 131(25):8824–8832. doi:10.1021/ja900353f Abstract A iron nanoparticle exposed to air at room temperature will be instantly covered by an oxide shell of typical thickness of ~ 3 nm. This native oxide shell in combination with an underlying iron core determines the physical and chemical behavior of this type of core-shell nanoparticles. One of the great challenges for characterizing this type of nanoparticles is determination of the structure of the oxide shell, as it is FeO, Fe3O4, -Fe2O3, -Fe2O3, or anything else. Significant research effort, mostly based on x-ray diffraction and spectroscopy and electron diffraction and transmission electron microscopy imaging, has been made to determine the structure of this thin layer of iron oxide. Most of the experimental results have been framed with one of the known iron oxide structures, although it is not necessarily true that this thin layer of iron oxide consists of a standard iron oxide. In this paper, the structure of the oxide shell on iron nanoparticle is probed using electron energy loss spectroscopy (EELS) at O K-edge with a spatial resolution of several nanometers (individual particle). Two types of representative particles were studied: particles that are fully oxidized and core-shell particle which possesses a Fe core. We found that the O K-edge spectra collected on the oxide shell in the nanoparticles shows distinctive differences as compared with that of the known iron oxide. Based on finger printing and quantum mechanical calculations results, we conclude that the distances between the absorbing oxygen and the next-nearest neighbor oxygens are more widely distributed than that in bulk Fe3O4 for both of these two types of particles. For smaller and fully oxidized particles, there is also a broadened distribution between the absorbing oxygen and the nearest neighbor oxygens. These results clearly demonstrate that the coordination configuration in the oxide shell on Fe nanoparticle is defective as compared with that of their bulk counterpart. Of the two types particles examined in this work, the degree of disorder is larger for the smaller fully oxidized particles.
2009. "Multicomponent reactive transport modeling of uranium bioremediation field experiments." Geochimica et Cosmochimica Acta 73(20):6029-6051. Abstract Biostimulation field experiments with acetate amendment are being performed at a former uranium mill tailings site in Rifle, Colorado, to investigate subsurface processes controlling in situ bioremediation of uranium-contaminated groundwater. An important part of the research is identifying and quantifying field-scale models of the principal terminal electron-accepting processes (TEAPs) during biostimulation and the consequent biogeochemical impacts to the subsurface receiving environment. Integrating abiotic chemistry with the microbially mediated TEAPs in the reaction network brings into play geochemical observations (e.g., pH, alkalinity, redox potential, major ions, and secondary minerals) that the reactive transport model must recognize. These additional constraints provide for a more systematic and mechanistic interpretation of the field behaviors during biostimulation. The reaction network specification developed for the 2002 biostimulation field experiment was successfully applied without additional calibration to the 2003 and 2007 field experiments. The robustness of the model specification is significant in that 1) the 2003 biostimulation field experiment was performed with 3 times higher acetate concentrations than the previous biostimulation in the same field plot (i.e., the 2002 experiment), and 2) the 2007 field experiment was performed in a new unperturbed plot on the same site. The biogeochemical reactive transport simulations accounted for four TEAPs, two distinct functional microbial populations, two pools of bioavailable Fe(III) minerals (iron oxides and phyllosilicate iron), uranium aqueous and surface complexation, mineral precipitation, and dissolution. The conceptual model for bioavailable iron reflects recent laboratory studies with sediments from the Old Rifle Uranium Mill Tailings Remedial Action (UMTRA) site that demonstrated that the bulk (~90%) of Fe(III) bioreduction is associated with the phyllosilicates rather than the iron oxides. The uranium reaction network includes a U(VI) surface complexation model based on laboratory studies with Old Rifle UMTRA sediments and aqueous complexation reactions that include ternary complexes (e.g., calcium-uranyl-carbonate). The bioreduced U(IV), Fe(II), and sulfide components produced during the experiments are strongly associated with the solid phases and may play an important role in long-term uranium immobilization.
2008. "Aging of Iron Nanoparticles in Aqueous Solution: Effects on Structure and Reactivity." Journal of Physical Chemistry C 112(7):2286-2293. doi:10.1021/jp0777418 Abstract Aging (or longevity) is one of the most important and potentially limiting factors in the use of nano-Fe0 to reduce groundwater contaminants. We investigated the aging of FeH2 (Toda RNIP-10DS) in water with a focus on changes in (i) the composition and structure of the particles (by XPS, XRD, TEM, and bulk Fe0 content), and (ii) the reactivity of the particles (by carbon tetrachloride reaction kinetics and electrochemical corrosion potentials). Our results show that the FeH2 becomes more reactive between 0 and ~2 days aging, and then gradually loses reactivity over the next few hundred days. These changes in reactivity correlate with evidence for rapid destruction of the original Fe(III) oxide film on FeH2 during immersion and the subsequent formation of a new passivating mixed-valence Fe(II)-Fe(III) oxide shell. The behavior of “unaged” nano-Fe0 in the laboratory may be similar to that in field-scale applications for source-zone treatment due to the short reaction times involved. Long-term aged FeH2 acquires properties that are relatively stable over weeks or even months.
2008. "Ultraviolet stimulation of hydrogen peroxide production using aminoindazole, diaminopyridine, and phenylenediamine solid polymer complexes of Zn(II)." Journal of Photochemistry and Photobiology. A, Chemistry 197(2-3):245-252. doi:10.1016/j.jphotochem.2007.12.031 Abstract Hydrogen peroxide is a valuable chemical commodity whose production relies on expensive methods. If an efficient, sustainable, and inexpensive solar-mediated production method could be developed from the reaction between dioxygen and water then its use as a fuel may be possible and gain acceptance. Hydrogen peroxide at greater than 10 M possesses a high specific energy, is environmentally clean, and is easily stored. However, the current method of manufacturing H2O2 via the anthraquinone process is environmentally unfriendly making the unexplored nature of its photochemical production from solar irradiation of interest. Here the concentration and quantum yield of hydrogen peroxide produced in an ultraviolet (UV-B) irradiated environment using aromatic and nitrogen-heterocyclic ring complexes of zinc(II) as solid substrates was studied. The amino-substituted isomers of the substrates indazole, pyridine, and phenylenediamine solid polymer complexes are examined. Samples exposed to the ambient atmosphere (e.g., aerated) were irradiated with a low power lamp with emission from 280-360 nm. Irradiation of various zinc complexes revealed Zn-5-aminoindazole to have the greatest first-day production of 63 mM/day with a 37% quantum yield. Para-phenylenediamine (PPAM) showed the greatest long-term stability and thus suggests H2O2 is produced photocatalytically. Isomeric forms of the catalyst’s organic components (e.g., amino groups) did have an effect on the production. Irradiation of diaminopyridine isomers indicated 2,3-diamino and 3,4-diamino structures were the most productive, each generating 32 mM/day hydrogen peroxide. However, the 2,5-diamino isomer showed no peroxide production. A significant decrease in hydrogen peroxide production in all but PPAM was noticed in the samples, suggesting the possibility of a catalyst poisoning mechanism. The samples ability to produce H2O2 is rationalized by proposing a reaction mechanism and examining the stability of the resonance structures of the different isomers.
2008. "Evaluation of Characterization Techniques for Iron Pipe Corrosion Products and Iron Oxide Thin Films." Journal of Environmental Engineering (ASCE) 134(10):835-844. Abstract A common problem faced by drinking water studies is that of properly characterizing the corrosion products (CP) in iron pipescor synthetic Fe (hydr)oxides used to simulate the iron pipe used in municipal drinking-water systems. The present work compares the relative applicability of a suite of imaging and analytical techniques for the characterization of CPs and synthetic Fe oxide thin films and provide an overview of the type of data that each instrument can provide as well as their limitations to help researchers and consultants choose the best technique for a given task. Crushed CP from a water distribution system and synthetic Fe oxide thin films formed on glass surfaces were chosen as test samples for this evaluation. The CP and synthetic Fe oxide thin films were analyzed by atomic force microscopy (AFM), scanning electron microscopy (SEM), energy-dispersive spectroscopy, time-of-flight secondary ion mass spectrometry (ToF-SIMS), X-ray powder diffractometry (XRD), grazing incident diffractometry (GID), transmission electron microscopy (TEM), selected area electron diffraction, X-ray photoelectron spectroscopy (XPS), Fourier transform infrared, Mössbauer spectroscopy, Brunauer-Emmett-Teller N2 adsorption and Fe concentration was determined by the ferrozine method. XRD and GID were found to be the most suitable techniques for identification of the mineralogical composition of CP and synthetic Fe oxide thin films, respectively. AFM and a combined ToF-SIMS-AFM approach proved excellent for roughness and depth profiling analysis of synthetic Fe oxide thin films, respectively. Corrosion products were difficult to study by AFM due to their surface roughness, while synthetic Fe oxide thin films resisted most spectroscopic methods due to their limited thickness (118 nm). XPS analysis is not recommended for mixtures of Fe (hydr)oxides due to their spectral similarities. SEM and TEM provided great detail on mineralogical morphology.
2008. "Characterization Challenges for Nanomaterials." Surface and Interface Analysis 40(3-4):529-537. doi:10.1002/sia.2726 Abstract Nanostructured materials are increasingly subject to nearly every type of chemical and physical analysis possible. Because of their small feature size there is a significant focus on tools with high spatial resolution. Because of their high surface area, it is also natural to characterize nanomaterials using tools designed to analyze surfaces. Regardless of the approach, nanostructured materials present a variety of obstacles to adequate, useful and needed analysis. This paper provides short overviews to some of the issues and complications including: particle stability, environmental effects, specimen handling, surface coating, contamination and time. Some specific examples are provided from a our work focused on ceria nanoparticles and iron metal-core/oxide-shell nanoparticles in which we use a combination of tools for routine analysis including XPS, TEM, and XRD and apply other methods as needed to obtain essential information.
2008. "Potential method for measurement of CO2 leakage from underground sequestration fields using radioactive tracers." Journal of Radioanalytical and Nuclear Chemistry 277(1):85-89. doi:10.1007/s10967-008-0713-8 Abstract Reduction of anthropogenic carbon dioxide (CO2) release to the environment is a pressing challenge that should be addressed to avert the potential devastating effects of global warming. Within the United States, the most abundant sources of CO2 emissions are those generate from coal- or gas-fired power plants; one method to control CO2 emissions is to sequester it in deep underground geological formations. From integrated assessment models the overall leakage rates from these storage locations must be less than 0.1% of stored volume per year for long-term control. The ability to detect and characterize nascent leaks, in conjunction with subsequent remediation efforts, will significantly decrease the amount of CO2 released back into the environment. Because potential leakage pathways are not necessarily known a priori, onsite monitoring must be performed; the monitoring region in the vicinity of a CO2 injection well may be as large as 100 km2, which represents the estimated size of a supercritical CO2 bubble that would form under typical injection scenarios. By spiking the injected CO2 with a radiological or stable isotope tracer, it will be possible to detect ground leaks from the sequestered CO2 using fewer sampling stations, with greater accuracy than would be possible using simple CO2 sensors. The relative merits of various sorbent materials, radiological and stable isotope tracers, detection methods and potential interferences will be discussed.
2007. "Morphology and Oxide Shell Structure of Iron Nanoparticles Grown by Sputter-Gas-Aggregation." Nanotechnology 18(25):Art. No. 255603. doi:10.1088/0957-4484/18/25/255603 Abstract Much recent research effort has been made on the synthesis, characterization, and property evaluation of core-shell structured Fe nanoparticles. Fundamental properties of these particles depend on both their external crystal faceting planes and the nature of a protective oxide layer. In this paper, the crystal faceting planes and oxide coating structures of core-shell structured iron/iron oxide nanoparticles synthesized by a sputter-gas-aggregation process were studied using transmission electron microscopy (TEM), electron diffraction and Wulff shape construction. The particles grown by this process and deposited on a support at room temperature process have been compared with particles grown and deposited at high temperature as reported in literature. Most synthesis processes produce round particles for particles less than 20 nm in diameter. For larger particles crystallographic facets are observed. It has been found that the Fe nanoparticles formed at RT are invariantly faceted on the {100} lattice planes and truncated by the {110} planes at different degrees. Substantial fraction of particles are confined only by the 6 {100} planes (not truncated by the {110} planes), this contrasts with the Fe particles formed at high temperature (HT) for which a predominance of {110} planes has been reported. Furthermore, at RT no particle was identified to be only confined by the 12 {110} planes which is relatively common for the particles formed at HT. The Fe cubes defined by the 6 {100} planes show a characteristic inward relaxation along the <100> and <110> directions and the reason for this behavior is not fully understood. The oxide shell on the Fe {100} plane maintains an orientation relationship: Fe(001)//Fe3O4(001) and Fe[100]//Fe3O4[110], which is same as the oxide formed on a bulk Fe(001) through thermal oxidation. Orientation of the oxide that forms on the Fe{110} facets differs from that on Fe{001}, therefore, properties of core-shell structured Fe nanoparticle faceted primarily with one type of lattice plane may be fully different from that faceted with another type of lattice planes.
2007. "Electron Beam Induced Thickening of the Protective Oxide Layer around Fe Nanoparticles ." Ultramicroscopy 108(1):43-51. doi:10.1016/j.ultramic.2007.03.002 Abstract There are many circumstances in science where the process of measuring the properties of a system alters the system. An imaging process can exert an inadvertent effect on the object being observed. Consequently, what we observe does not necessarily represent what had been present before the observation. Normally this effect can be ignored if the consequence of such a change is believed not to be significant. The expansion of nanostructured materials has made high resolution transmission electron microscopy one of the indispensable tools for probing the characteristics of nano-materials. Modification of nanoparticles by the electron beam during their imaging has been widely noticed and this is generally believed to be due to electron beam induced heating effect, defect formation in the particles, charging of the particle, or excitation of surrounding gases. However, an explicit experimental identification of which process dominates is often very hard to establish. We report the thickening of native oxide layer on iron nanoparticle under electron beam irradiation. Based on atomic level imaging, electron diffraction, and computer simulation, we have direct evidence that the protecting oxide layer formed on Fe nanoparticle at room temperature in air continues to grow during an electron beam bombardment in the vacuum system typical of most TEM systems. Partial illumination of a nanoparticle and observation of the shell thickening conclusively demonstrates that many of the mechanisms postulated to explain such processes are not occurring to a significant extent. The observed growth is not related to the electron beam induced heating of the nanoparticle, or residual oxygen ionization, or establishment of an electrical field, rather it is related to electron beam facilitated mass transport across the oxide layer (a defect related process).
2007. "Comparing metal leaching and toxicity from high pH, low pH, and high ammonia fly ash." Fuel 86(10-11):1623-1630. doi:10.1016/j.fuel.2006.11.018 Abstract Previous work with both class F and class C fly ash indicated minimal leaching from most fly ashes tested. However, the addition of NOx removal equipment might result in higher levels of ammonia in the fly ash. We have recently been testing fly ash with a wide range of pH (3.7–12.4) originating from systems with NOx removal equipment. Leaching experiments were done using dilute CaCl2 solutions in batch and columns and a batch nitric acid method. All methods indicated that the leaching of heavy metals was different in the highest ammonia sample tested and the high pH sample. However, toxicity testing with the Microtox* system has indicated little potential toxicity in leachates except for the fly ash at the highest pH (12.4). When the leachate from the high pH fly ash was neutralized, toxicity was eliminated.
2007. "Uranium immobilization by sulfate-reducing biofilms grown on hematite, dolomite, and calcite." Environmental Science & Technology 41(24):8349-8354. doi:10.1021/es071335k Abstract Biofilms of sulfate-reducing bacteria Desulfovibrio desulfuricans G20 wereused to reduce dissolved U(VI)and subsequently immobilize U(IV) in the presence of uranium-complexing carbonates. The biofilms were grown in three identically operated fixed bed reactors, filled with three types of minerals: one noncarbonate-bearing mineral(hematite) and two carbonate-bearing minerals (calcite and dolomite). The source of carbonates in the reactors filled with calcite and dolomite were the minerals, while in the reactor filled with hematite it was a 10 mM carbonate buffer, pH 7.2, which we added to the growth medium. Our five-month study demonstrated that the sulfate-reducing biofilms grown in all reactors were able to immobilize/reduce uranium efficiently, despite the presence of uranium-complexing carbonates.
2007. "Mechanisms controlling soil carbon turnover and their potential application for enhancing carbon sequestration ." Climatic Change 80(1-2):5-23. Abstract Two major mechanisms, (bio)chemical alteration and physicochemical protection, stabilize soil organic carbon (SOC) and thereby control soil carbon turnover. With (bio)chemical alteration, SOC is transformed by biotic and abiotic processes to chemical forms that are more resistant to decomposition and, in some cases, more easily retained by sorption to soil solids. With physicochemical protection, biochemical attack of SOC is inhibited by organomineral interactions at molecular to millimeter scales. Stabilization of otherwise decomposable SOM can occur via sorption to soil surfaces, complexation with soil minerals, occlusion within aggregates, and deposition in pores inaccessible to decomposers and extracellular enzymes. Soil structure (i.e., the arrangement of solids and pores in the soil) is a master integrating variable that both controls and indicates the SOC stabilization status of a soil. To enhance SOC sequestration, the best option is to modify the soil physicochemical environment to favor the activities of fungi. Specific practices that accomplish this include minimizing tillage, maintaining a near-neutral soil pH and an adequate base cation exchange capacity (particularly Ca), ensuring adequate drainage, and minimizing erosion by water and wind. In some soils, amendments with various high-specific-surface micro- and mesoporous sorbents such as fly ash or charcoal can be beneficial.
2006. "Quantification of Volatile Organics in Soil Aging Experiments Using Fourier Transform Infrared Spectroscopy." Applied Spectroscopy 60(8):914-919. doi:10.1366/000370206778062156 Abstract Soil aging may have a significant influence on the success of environmental remediation strategies and the accuracy of numerical models that estimate contaminant transport. The focus of this study was to define the ability of online Fourier transform infrared (FT-IR) spectroscopy to monitor the concentration of halogenated volatile organic compounds in a process that simulates long-term slow accumulation of contaminants in soils.
2006. "Design of Sensor Networks for the Long term Monitoring of Geological Sequestration." Energy Conversion and Management 47(13-14):1968-1974. doi:10.1016/j.enconman.2005.09.010 Abstract We report here on the design of sensor networks, as a part of development of a prototypical field-deployable sensor/tracer technology for monitoring the rate of leakage of geologically sequestered CO2. The primary focus is on conservative-tracer technology for immediate leak detection. The design simulations are developed to answer several key questions about the sensor network layout. Simulation results indicate that the key parameters governing the monitoring network design are the sensor node spacing (S), sampling density D (no. of sensors/km2), minimum detection limit of sensor (L), injected tracer concentration (Co) and dispersed tracer concentration (Ct) in the environment.
2006. "Incorporation of Chromate into Calcium Carbonate Structure during Coprecipitation." Water, Air, and Soil Pollution. doi:10.1007/s11270-006-9242-7 Abstract To assess treatment technologies and establish regulatory framework for chromate-contaminated site remediation, it is imperative to know the exact chromium speciation in soil matrices. In an earlier study, Thornton and Amonette (1999) reported that some chromate in the bulk particles was not accessible to gaseous reductants or solution-phase extractants, based on XANES studies. We hypothesized that part of this non-extractable chromate may reside in the structure of minerals such as calcium carbonate. To test this hypothesis, a number of calcium carbonate precipitates were prepared in the presence of various concentrations of chromate during the precipitation, which could coprecipitate chromate, or by adding chromate after the precipitation was completed. Hydrochloric acid was used to dissolve calcium carbonate and therefore extract the coprecipitated and surface attached chromate. The results showed that the coprecipitated chromate was non-extractable by hot alkaline solution or phosphate buffer, but could be solubilized by HCl in proportional to the amount of calcium carbonate dissolved. The X-ray diffraction experiments revealed that the coprecipitation of chromate with calcium carbonate had an influence on its crystal structure: the higher the chromate concentration, the greater the ratio of vaterite to calcite.
2006. "Metal-based Nanoparticle Synthesis from the Rapid Expansion of Carbon Dioxide Solutions." Journal of Nanoscience and Nanotechnology 6(2):562-567. Abstract Synthesis of 10 to 500 nm diameter Fe-based nanoparticles is described. Nanometer-sized droplets of iron carbonyl are generated by the rapid expansion of CO2-based supercritical fluid solutions and are photolyzed in-flight using a UV lamp to remove the carbonyl groups. Solid metal particles are collected electrostatically. Upon air oxidation, the iron particles react rapidly to produce an iron oxide phase. Mo-based nanoparticles were similarly produced using a Mo(Co)6 precursor.
2006. "Magnesium inhibition of calcite dissolution kinetics." Geochimica et Cosmochimica Acta 70(3):583-594. Abstract We present evidence of inhibition of calcite dissolution by dissolved magnesium through direct observations of the (104) surface using atomic force microscopy (AFM) and vertical scanning interferometry (VSI). Far from equilibrium, the pattern of magnesium inhibition is dependent on solution composition and specific to surface step geometry. In CO2-free solutions (pH 8.8), dissolved magnesium brings about little inhibition even at concentrations of 0.8 x 10-3 molal. At the same pH, magnesium concentrations of less than 0.05 x 10-3 molal in carbonate-buffered solutions generate significant inhibition, although no changes in surface and etch pit morphology are observed. As concentrations exceed magnesite saturation, the dissolution rate shows little additional decrease; however, selective pinning of step edges results in unique etch-pit profiles, seen in both AFM and VSI datasets. Despite the decreases in step velocity, magnesium addition in carbonated solutions also appears to activate the surface by increasing the nucleation rate of new defects. These relationships suggest that the modest depression of the bulk rate measured by VSI reflects a balance between competing reaction mechanisms that simultaneously depress the rate through selective inhibition of step movement, but also enhance reactivity on terraces by lowering the energy barrier to new etch-pit formation.
2005. "Void Formation during Early Stages of Passivation: Initial Oxidation of Iron Nanoparticles at Room Temperature." Journal of Applied Physics 98(9):094308(7). doi:10.1063/1.2130890 Abstract The examination of nanoparticles allows study of some processes and mechanisms that are not as easily observed for films or other types of studies where sample preparation artifacts have been cause of some uncertainties. Exposure of most clean metals to air or oxygen results in the nearly instant formation of an oxide layer. Because this initial layer normally forms in a relatively uncontrollable manner, the atomic level understanding of the initial oxidation is limited in comparison to the abundant experimental observation and theoretical derivation on thickening of oxidation layer on metal surface at high temperature. We report in this letter mMicrostructurale characterization of iron nanoparticles oxide passivated with iron oxide shell nanoparticleswere studied using high resolution transmission electron microscopy (HRTEM) and high angle annular dark-filed (HAADF) imaging in aberration corrected scanning transmission electron microcopy (STEM). Voids were readily observed on both small single crystal -Fe nanoparticles formed in a sputtering process and the more complex particles created by reduction of an oxide by hydrogen. Although the formation of hollow spheres of nanoparticle has been engineered for Co at higher temperatures1, they occur for iron at room temperature and provide insight into the initial oxidation processes of iron. The examination of nanoparticles allows study of some processes and mechanisms that are not as easily observed for films or other types of studies where sample preparation artifacts have been cause of some controversy. For example, void formation has been noticed in the single crystal -Fe nanoparticles as a consequence of iron outward diffusion during the initial oxidation at room temperature. There exists a critical size of ~ 8 nm for which the iron has been fully oxidized, leading to a hollow iron oxide nanoparticle. For particles larger than the critical size, an iron/iron oxide core-shell structure was formed and voids reside at the interface between the oxide shell and the iron core. The present observation provides new insight for tailoring of metal/metal-oxide core-shell structured nanoparticles for applications related to optics, magnetism, and nanoelectronics.
2005. "Reoxidation of Reduced Uranium with Iron(III) (Hydr)Oxides under Sulfate-Reducing Conditions." Environmental Science and Technology 39(7):2059-2066. Abstract In cultures of Desulfovibrio desulfuricansG20 the effects of iron(III) (hydr)oxides (hematite, goethite, and ferrihydrite) on microbial reduction and reoxidation of uranium (U) were evaluated under lactate-limited sulfate-reducing conditions. With lactate present, G20 reduced U(VI) in both 1,4-piperazinediethanesulfonate (PIPES) and bicarbonate buffer. Once lactate was depleted, however, microbially reduced U served as an electron donor to reduce Fe(III) present in iron(III) (hydr)oxides. With the same initial amount of Fe(III) (10 mmol/L) for each iron(III) (hydr)oxide,reoxidation of U(IV) was greater with hematite than with goethite or ferrihydrite. As the initial mass loading of hematite increased from 0 to 20 mmol of Fe(III)/L, the rate and extent of U(IV) reoxidation increased. Subsequent addition of hematite [15 mmol of Fe(III)/L] to stationary-phase cultures containing microbially reduced U(IV) also resulted in rapid reoxidation to U(VI). Analysis by U L3-edge X-ray absorption near-edge spectroscopy (XANES) of microbially reduced U particles yielded spectra similar to that of natural uraninite. Observations by high-resolution transmission electron microscopy, selected area electron diffraction, and energy-dispersive X-ray spectroscopic analysis confirmed that precipitated U associated with cells was uraninite with particle diameters of 3-5 nm. By the same techniques, iron sulfide precipitates were found to have a variable Fe and S stoichiometry and were not associated with cells.
2005. "Characterization and Properties of Metallic Iron Nanoparticles: Spectroscopy, Electrochemistry, and Kinetics." Environmental Science and Technology 39(5):1221-1230. Abstract There are reports that nano-sized zero-valent iron (Fe⁰) exhibits greater reactivity than micro-sized particles of Fe⁰, which may impart advantages for groundwater remediation or other environmental applications. However, most of these reports are preliminary in that they leave a host of potentially significant (and often challenging) material or process variables either uncontrolled or unresolved. To better understand the reactivity of iron nanoparticles, we have used a variety of complementary techniques to characterize two widely studied nano Fe⁰ preparations: one synthesized by heat-reduction of goethite under H₂ (FeH₂) and the other by reductive precipitation with borohydride (FeBH). X-ray diffraction (XRD), transmission electron microscopy (STXM) showed particles of similar size (40-80 nm), but surface area measurements varied widely with method of measurement (4-60 m² g-¹). FeH₂ is a two-phase material consisting of ⍺-Fe⁰ and Fe₃O₄, doped with reduced sulfur, whereas FeBH is mostly metallic Fe with an oxide shell that is high in boron. Both materials exhibit corrosion potentials that are more negative than nano-sized Fe₂O₃, Fe₃O₄, micro-sized Fe⁰, or a solid Fe⁰ disk, consistent with their rapid reduction of oxygen, benzoquinone, and carbon tetrachloride. Benzoquinone-which presumably probes inner-sphere surface reactions-reacts more rapidly with FeBH than with FeH₂, whereas with carbon tetrachloride, FeBH and FeH₂ react at similar rates, presumably by outer-sphere electron transfer. Whether either material reacts more rapidly with the probes than micro-sized Fe⁰ is unclear due to uncertainties in the appropriate specific surface areas. The distribution of products from reduction of carbon tetrachloride is more favorable with FeH₂, which produces less chloroform than reaction with FeBH.
2005. "Uranium removal by sulfate reducing biofilms in the presence of carbonates ." Water Science and Technology 52(7):49-55. Abstract Hexavalent uranium [U(VI)] was immobilized in biofilms composed of the sulfate reducing bacteria (SRB), Desulfovibrio desulfuricans G20. The biofilms were grown in two flat-plate, continuous-flow reactors using lactate as the electron donor and sulfate as the electron acceptor. The growth medium contained uranium U(VI) and the pH was maintained constant using bicarbonate buffer. The reactors were operated for 5 months, and during that time biofilm activity and uranium removal were evaluated. The efficiency of uranium removal strongly depended on the concentration of uranium in the influent, and was estimated to be 30.4% in the reactor supplied with 3 mg/L of U(VI) and 73.9% in the reactor supplied with 30 mg/L of U(VI). TEM and SAED analysis showed that uranium in both reactors accumulated mostly on microbial cell membranes and in the periplasmic space. The deposits had amorphous or poor nanocrystalline structures.
2005. "Bioreduction of Natural Specular Hematite under Flow Conditions." Geochimica et Cosmochimica Acta 69(5):1145-1155. Abstract ABSTRACT-Dissimilatory reduction of Fe(III) by Shewanella oneidensis MR1 was evaluated using natural specular hematite as sole electron acceptor in an open system under dynamic flow conditions to obtain a better understanding of biological Fe(III) reduction in the natural environment...
2004. "Dissolution and Growth of (10(1) over-bar4) Calcite in Flowing Water: Estimation of Back Reaction Rates via Kinetic Monte Carlo Simulations." Journal of Crystal Growth 262(1-4):503-518. Abstract Although calcite is an important mineral for many processes, there ae been relatively few simulations of it's growth and dissolution behavior. Such simulations are complicated by the multitude of defect types and by the asymmetry of the crystal. The present work combined a kinetic Monte Carlo (KMC) technique with the Kossel crystal (100) simple cubic concept and the Blasius boundary layer model to simulate the simultaneous growth and dissoution of the (1014)calcite cleavage surface in flowing water. The objective was to determine the activation energies of the back reaction (growth) from those of the forward reaction (dissolution) by obtaining agreement with cleavage-step morphologies and step dissolution velocities previously measured using an atomic force microscope (AFM). Blasius boundary layer conditions for the flowing fluid defined a model that treated the solid, the dissolution/growth interface, and the fluid kinetics. Microscopic reversibility and the laws of large numbers gave an expression for the back reaction activation energies in terms of the forward reaction energies and the entropy of mixing, a quantity estimated from the concentration of desorbates in a very small fluid layer adjacent to the interface. The KMC simulations produced cleavage-step morphologies that were in qualitive agreement with observations from AFM. The kinetics were dominated by diffusion events on the solid/fluid interface and in the fluid, as expected. The relative magniyudes of the desorption and adsorption activation energies were consistent with experimental data, entropic arguments, and crystal roughening theories. Quantitive agrrement with measured step velocities was best when the boundary layer parameters were given physically reasonable values, indicating that the model is self consistent.
2004. "Copper Sorption Mechanisms on Smectites." Clays and Clay Minerals 52(3):321-333. Abstract Abstract– Due to the importance of clay minerals in metal sorption many studies have attempted to derive mechanistic models that describe adsorption processes. These models often include several different types of adsorption sites, including permanent charge sites and silanol and aluminol functional groups on the edges of clay minerals. The edge sites have similar pH-dependent adsorption properties as many oxide minerals. To provide a basis for development of adsorption models it is critical that molecular level studies be done to characterize sorption processes. In this study we conducted XAFS and ESR spectroscopic experiments on copper (II) sorbed on smectite clays using suspension pH and ionic strength as variables. At low ionic strength, results suggest that Cu is sorbing in the interlayers and maintains its hydration sphere. At high ionic strength Cu atoms are excluded from the interlayer and sorb primarily on the silanol and aluminol functional groups of the montmorillonite or beidellite structures. Interpretation of the XAFS and ESR spectroscopy results provides evidence that multinuclear complexes are forming on the edge sites. Fitting of EXAFS spectra revealed that the Cu-Cu atoms in the multinuclear complexes are 2.65 apart, and have coordination numbers near one. This structural information suggests that small Cu dimers are sorbing on the surface. These complexes are consistent with observed sorption on mica and amorphous silicon dioxide, as well as the Cu-bearing silicate minerals plancheite and shattuckite, yet are inconsistent with previous spectroscopic results for Cu sorption on montmorillonite. We hypothesize that the differences in sorption mechanisms on the edges of the montmorillonite are due to loading level. The results reported in this paper provide mechanistic data that will be valuable for modeling surface interactions of Cu with clay minerals, and predicting the geochemical cycling of Cu in the environment.
2004. "Reduction of Uranium(VI) under Sulfate-reducing Conditions in the Presence of Fe(III)-(hydr)oxides." Geochimica et Cosmochimica Acta 68(12):2639-2648. Abstract U(VI) dissolved in a modified lactate-C medium (either sulfate- or lactate-limited) was reacted with a mixture of an Fe(III)-(hydr)oxide mineral (hematite, goethite, or ferrihydrite) and quartz under anoxic conditions and equivalent mineral surface areas. After sorption equilibration, the suspensions were inoculated with a sulfate-reducing bacterium (SRB, Desulfovibrio desulfuricans G20). Inoculation of the suspensions containing sulfate-limited medium yielded significant SRB growth, along with concomitant reduction of sulfate and removal of U(VI) from solution. Inoculation of the suspensions containing lactate-limited medium yielded similar results while lactate was still present. Once the lactate was depleted, however, some of the U that had been removed from solution was re-solubilized in the hematite treatment and, to a lesser extent, in the goethite treatment. No re-solubilization was observed in the lactate-limited ferrihydrite treatment even after a prolonged incubation of four months. Analysis by U L3-edge XANES spectroscopy of mineral specimens sampled without inoculation yielded a typical U(VI) spectrum. Mineral specimens sampled at the end of the experiment yielded spectra similar to that of uraninite, thus providing strong evidence for SRB-promoted removal of U(VI) from solution by reductive precipitation of uraninite. Consequently, U re-solubilization was attributed to re-oxidation of the uraninite by Fe(III) present in the (hydr)oxide phases. Our results thus suggest that inoculation with SRB mediates reduction of soluble U(VI) to an insoluble U(IV) oxide so long as a suitable electron donor is available. Depletion of the electron donor may result in partial re-oxidation of the U(IV) to soluble U(VI) species when the surfaces of crystalline Fe(III) (hydr)oxides are incompletely reduced by reaction with SRB-generated sulfide.
2004. "Enhancement of Carbon Sequestration in US Soils." BioScience 54(10):895-908. Abstract Improved agricultural, forestry, and land management practices could be used to reduce atmospheric CO2 concentration; but only if they are technically feasible over large areas, economically attractive, and environmentally beneficial...
2004. "Prospects for Enhancing Carbon Sequestration and Reclamation of Degraded Lands with Fossil-fuel Combustion By-products." Advances in Environmental Research 8(3-4):425-438. Abstract Concern for the potential global change consequences of increasing atmospheric CO2 has prompted interest in the development of mechanisms to reduce or stabilize atmospheric CO2 .During the next several decades, a program focused on terrestrial sequestration processes could make a significant contribution to abating CO2 increases.The reclamation of degraded lands, such as mine-spoil sites, highway rights-of-way, and poorly managed lands, represents an opportunity to couple C sequestration with the use of fossil-fuel and energy by-products and other waste material, such as biosolids and organic wastes from human and animal sewage treatment facilities, to improve soil quality. Degraded lands are often characterized by acidic pH, low levels of key nutrients, poor soil structure, and limited moisture-retention capacity.Much is known about the methods to improve these soils, but the cost of implementation is often a limiting factor.However, the additional financial and environmental benefits of C sequestration may change the economics of land reclamation activities.The addition of energy-related by-products can address the adverse conditions of these degraded lands through a variety of mechanisms, such as enhancing plant growth and capturing of organic C in long-lived soil C pools.This review examines the use of fossil-fuel combustion by-products and organic amendments to enhance C sequestration and identifies the key gaps in information that still must be addressed before these methods can be implemented on an environmentally meaningful scale.
2004. "Synthesis of Colloidal Mn2+:ZnO Quantum Dots and High-TC Ferromagnetic Nanocrystalline Thin Films." Journal of the American Chemical Society 126(30):9387-9398. Abstract Abstract:We report the synthesis of colloidal Mn2+-doped ZnO (Mn2+:ZnO) quantum dots and the preparation of room-temperature ferromagnetic nanocrystalline thin films. Mn2+:ZnO nanocrystals were prepared by a hydrolysis and condensation reaction in DMSO under atmospheric conditions. Synthesis was monitored by electronic absorption and electron paramagnetic resonance (EPR) spectroscopies. Zn(OAc)2 was found to strongly inhibit oxidation of Mn2+ by O2, allowing the synthesis of Mn2+:ZnO to be performed aerobically. Mn2+ ions were removed from the surfaces of as-prepared nanocrystals using dodecylamine to yield high-quality internally doped Mn2+:ZnO colloids of nearly spherical shape and uniform diameter (6.1 ( 0.7 nm). Simulations of the highly resolved X-and Q-band nanocrystal EPR spectra, combined with quantitative analysis of magnetic susceptibilities, confirmed that the manganese is substitutionally incorporated into the ZnO nanocrystals as Mn2+ with very homogeneous speciation, differing from bulk Mn2+:ZnO only in the magnitude of D-strain. Robust ferromagnetism was observed in spin coated thin films of the nanocrystals, with 300 K saturation moments as large as 1.35 íB/Mn2+ and TC > 350 K. A distinct ferromagnetic resonance signal was observed in the EPR spectra of the ferromagnetic films. The occurrence of ferromagnetism in Mn2+:ZnO and its dependence on synthetic variables are discussed in the context of these and previous theoretical and experimental results.
2004. "Identification of Sporulated and Vegetative Bacteria using Statistical Analysis of Fourier Transform Mid-Infrared Transmission Data ." Applied Spectroscopy 58(2):203-211. Abstract A combined mid-infrared spectroscopic/statistical modeling approach for the discrimination and identification, at the strain level, of both sporulated and vegetative bacterial samples is presented. Transmission mode spectra of bacteria dried on ZnSe windows were collected using a Fourier-transform mid-infrared (FTIR) spectrometer. Five Bacillus bacterial strains (B. atrophaeus 49337, B. globigii, B. thuringiensis ssp. kurstaki, B. subtilis 49780, and B. subtilis 6051) were used to construct a reference spectral library and to parameterize a four-step statistical model for the systematic identification of bacteria. The statistical methods used included principal-component analysis (PCA), classification and regression trees (CART), and Mahalanobis-distance calculations. Internal cross-validation studies successfully classified 100% of the samples into their correct physiological state (sporulated or vegetative) and identified 67% of the samples correctly as to their bacterial strain. Analysis of thirteen blind samples, which included reference and other bacteria, nonbiological materials, and mixtures of both nonbiological and bacterial samples, yielded comparable accuracy. The chief advantage of this approach is the accurate identification of unknown bacteria, including spores, in a matter of minutes.
2004. "Uranium Immobilization by Sulfate-reducing Biofilms." Environmental Science and Technology 38(7):2067-2074 . Abstract Hexavalent uranium [U(VI)] was immobilized using biofilms of the sulfate-reducing bacterium (SRB) Desulfovibrio desulfuricans G20. The biofilms were grown in flat-plate continuous-flow reactors using lactate as the electron donor and sulfate as the electron acceptor. U(VI) was continuously fed into the reactor for 32 weeks at a concentration of 126 íM. During this time, the soluble U(VI) was removed (between 88 and 96% of feed) from solution and immobilized in the biofilms. The dynamics of U immobilization in the sulfate-reducing biofilms were quantified by estimating: (1) microbial activity in the SRB biofilm, defined as the hydrogen sulfide (H2S) production rate and estimated from the H2S concentration profiles measured using microelectrodes across the biofilms; (2) concentration of dissolved U in the solution; and (3) the mass of U precipitated in the biofilm. Results suggest that U was immobilized in the biofilms as a result of two processes: (1) enzymatically and (2) chemically, by reacting with microbially generated H2S. Visual inspection showed that the dissolved sulfide species reacted with U(VI) to produce a black precipitate. Synchrotron-based U L3-edge X-ray absorption near edge structure (XANES) spectroscopy analysis of U precipitated abiotically by sodium sulfide indicated that U(VI) had been reduced to U(IV). Selected-area electron diffraction pattern and crystallographic analysis of transmission electron microscope lattice-fringe images confirmed the structure of precipitated U as being that of uraninite.
2003. "Interlayer Structure and Dynamics of Cl-Bearing Hydrotalcite: Far Infrared Spectroscopy and Molecular Dynamics Modeling." American Mineralogist 88(Feb-Mar 2003):398-409. Abstract Comparison of the observed far-infrared (FIR) spectrum of Cl--containing hydrotalcite, [Mg3Al(OH)8]Cl?3H2O, with its power spectrum calculated using molecular dynamics (MD) computer simulation provides greatly increased understanding of the structure and vibrational dynamics in the interlayers of layered double hydroxides. The simulation model assumes an ordered Mg3Al arrangement in the octahedral layer and no constraints on the movement of any atoms or on the geometry and symmetry of the simulation supercell. Calculated anisotropic components of the individual atomic power spectra in combination with computed animations of the vibrational modes from normal mode analysis allow for reliable interpretations of the observed spectral bands. For the vibrations related to octahedral cation motions, bands near 145, 180 and 250 cm-1 are due dominantly to Mg vibration in the z direction (perpendicular to the hydroxide layers), Al vibration in the z direction and Mg and Al vibrations in the x-y plane (parallel to the hydroxide layers), respectively. The low frequency vibrational motions of the interlayer are controlled by a network of hydrogen bonds formed among interlayer water molecules, Cl- ions, and the OH groups of the main hydroxide layers. The bands near 40-70 cm-1 are related to the translational motions of interlayer Cl- and H2O in the x-y plane, and the bands near 120 cm-1 and 210 cm-1 are due largely to translational motions of the interlayer species in the z direction. The three librational modes of interlayer water molecules near 390, 450 and 540 cm-1 correspond to twisting, rocking and wagging hindered rotations, respectively. The spectral components of the interlayer Cl- motions are remarkably similar to those of bulk aqueous chloride solutions, reflecting the structural and dynamic similarity of the nearest-neighbor Cl- environments in the interlayer and in solution.
2003. "Identification of Bacterial Spores using Statistical Analysis of Fourier Transform Infrared Photoacoustic Spectroscopy Data ." Applied Spectroscopy 57(8):893-899. Abstract Fourier Transform Infrared Photoacoustic Spectroscopy (FTIR-PAS) has been applied for the first time to the identification and speciation of bacterial spores. With minimal preparation the spores were deposited into the photoacoustic sample cup and their spectra recorded. A total of 40 different samples of 5 different strains of Bacillus spores were analyzed: Bacillus subtilis ATCC 49760, Bacillus atrophaeus ATCC 49337, Bacillus subtilis 6051, Bacillus thuringiensis ssp. kurstaki, and Bacillus globigii Dugway. The statistical methods used included principal-component analysis (PCA), classification and regression trees (CART), and Mahalanobis-distance calculations. Internal cross-validation studies successfully classify the spores according to their bacterial strain in 38 of 40 cases (95%) and 36 of 40 (90%) in cross-validation. Analysis of fifteen blind samples, which included library and other spores, and nonbacterial materials, resulted in correct strain classification the blind samples that were members of the library and correct rejection of the nonbacterial samples.
2003. "Direct determination of the absolute electron density of nanostructured and disordered materials at sub-10 nm resolution ." Physical Review. B, Condensed Matter and Materials Physics 68(012201):1-3. Abstract Porous materials have found great use as catalysts and sorption media due to their large internal surface areas. The recent developments in synthesis methods for producing ordered porous materials have greatly broadened their applications, ranging from molecular sieves to microelectronics. To date, the internal structures of these materials have been inferred from either adsorption-isotherm measurements, X-ray diffraction, or electron microscopy where a model was first assumed. For thin materials (< 50 nm), a direct 3D imaging method using electron crystallography has recently been developed, but provides only relative electrostatic potential maps in most applications. Here we report a new approach for the determination of the absolute electron density of porous materials at sub-10 nm resolution using coherent X-rays. While the ultimate resolution is limited by the X-ray wavelength that is employed, in practice the resolution is limited by the CCD size and the exposure time. We anticipate this general approach can be used to directly determine the absolute electron density of nano-crystals and non-crystalline materials, a measurement which is currently beyond the capability of other methods.
2003. "Heteroepitaxial Growth of a Manganese Carbonate secondary Nano-Phase on the (101_4) Surface of Calcite in Solution." Surface Science 524(1-3):63-77. Abstract Heteroepitaxy of a manganese carbonate phase with nanometer dimensions on the (101_4) surface of calcite (CaCO₃) using an AFM has been observed in solution during dissolution of calcite when the ion activity product of Mn₂⁺ and CO₃₂- nears the solubility limit of MnCO₃. Growth-rate observations at different Mn concentrations, coupled with XPS and EPR measurements, suggest that the resulting phase is Mn₀․₅Ca₀․₅CO₃. These islands, while growing many microns in length along the [22_1] direction, have a uniform width in the range of 120-240 nm and a uniform height of approximately 2.7 nm, corresponding to nine atomic layers. The islands cease growing when they encounter step edges and have been observed to dissolve when undercut by a growing etch pit.
2003. "Variation in Calcite Dissolution Rates: A Fundamental Problem? ." Geochimica et Cosmochimica Acta 67(9):1623-1634. Abstract A comparison of published calcite dissolution rates measured far from equilibrium at a pH of ~ 6 and above shows well over an order of magnitude in variation. Recently published AFM step velocities extend this range further still. In an effort to understand the source of this variation, and to provide additional constraint from a new analytical approach, we have measured dissolution rates by vertical scanning interferometry. In areas of the calcite cleavage surface dominated by etch pits, our measured dissolution rate is 10?10.95 mol/cm2/s (PCO2 10?3.41 atm, pH 8.82), 5 to ~100 times slower than rates published in bulk powder experiments, although similar to rates derived from AFM step velocities. On cleavage surfaces free of local etch pit development, dissolution is limited by a slow, ?global? rate (10?11.68 mol/cm2/s). Although these differences confirm the importance of etch pit (defect) distribution as a controlling mechanism in calcite dissolution, they also suggest that ?bulk? calcite dissolution rates observed in powder experiments may derive substantial enhancement from grain boundaries having high step and kink density. We also observed significant rate inhibition by introduction of dissolved manganese. At 2.0 mM Mn, the rate diminished to 10?12.4 mol/cm2/s, and the well formed rhombic etch pits that characterized dissolution in pure solution were absent. These results are in good agreement with the pattern of manganese inhibition in published AFM step velocities, assuming a step density on smooth terraces of ~ 9 mm?1.
2003. "Toxicity of Al to Desulfovibrio desulfuricans." Applied and Environmental Microbiology 69(7):4057-4066. Abstract The toxicity of Al to Desulfovibrio desulfuricans G20 was assessed over a period of eight weeks in a modified lactate-C medium buffered at four initial pHs (5.0, 6.5, 7.2, and 8.3) and treated with five levels of added Al (none, 0.01, 0.1, 1.0, and 10 mM). At pH 5, cell populations decreased significantly and any effect of Al was negligible compared to that of pH. At higher pH, no direct impact of Al was seen at concentrations of soluble Al below 5x10-5 M. Soluble Al concentrations exceeded this level in the pH-6.5 and pH-7.2 treatments having total-Al concentrations = 1 mM and caused substantial and proportional decreases in cell populations. In contrast to the small size and vibrio morphology seen in most treatments, substantial numbers of large spirilloidal bacteria were seen in the pH-8.3 treatments, and their incidence was correlated with greater total-Al concentrations. This effect was believed to be a result of Mg deprivation stemming from adsorption/coprecipitation of Mg with Al hydroxy-sulfate gels rather than a direct response to high total-Al levels. Calculations of soluble-Al speciation suggested the presence of the Al13O4(OH)24(H2O)127+ "tridecamer" cation in the pH-6.5 and pH-7.2 treatments showing direct Al toxicity. Analysis of inoculated and control samples from the pH-6.5 and pH-8.3 10-mM Al treatments by 27Al nuclear-magnetic-resonance spectroscopy yielded broad 4800-Hz resonances at 58 ppm, near the 63-ppm resonance expected for the tridecamer cation. These resonances were independent of sample filtration through 25,000 MWCO membranes and attributed to self-assembled colloidal clusters of hydrophobic tridecamer species that had been neutralized by sorption of lactate and sulfate anions. Although indirect effects of Al addition were believed to have caused the lower cell populations observed in the pH-8.3 treatments, the NMR data suggest that direct effects of Al toxicity stemming from tridecamer colloids cannot be excluded.
2003. "Imaging the heterogeneity of mineral surface reactivity using Ag(I) and synchrotron X-ray microscopy." Physics and Chemistry of Minerals 30(9):559-569. Abstract Microscopic-scale imaging of reduced zones on the surfaces of minerals can be achieved by reaction with dilute Ag(I) solutions and subsequent analysis using synchrotron X-ray microscopy (XRM) above the Ag K-edge (25.5 keV). The principal reductant is Fe(II), but other reductants such as sulfide may contribute. Reduced zones may exist instrinsically, as in the structure of biotite and augite, or may be generated by reaction with chemical agents such as dithionite or treatment with sulfate-reducing bacteria (SRB). We demonstrate the method on flakes of specular hematite and biotite, as well as on thin sections of different rocks (arfvedsonitic granite, oolitic hematite, diabase, and quartz conglomerate) treated with SRB, and discuss possible artifacts that can occur. To our knowledge, this is the only microscopic technique that can image Fe(II) zones on the surface of an Fe-bearing mineral with monolayer sensitivity.
2003. "Heterogeneous Electron-Transfer Kinetics with Synchrotron 57Fe Mossbauer Spectroscopy." Geochimica et Cosmochimica Acta 67(12):2109-2116. Abstract In the first known kinetic application of the technique, synchrotron 57Fe-Mossbauer spectroscopy was used to follow the rate of heterogeneous electron transfer between aqueous reagents and a solid phase containing Fe. The solid, a synthetic 57Fe-enriched Fe(III)-bearing pyroaurite-like phase having terephthalate (TA) in the interlayer [Mg3Fe(OH)8(TA)0.5 2H2O], was reduced by Na2S2O4 and then re-oxidized by K2Cr2O7 using a novel flow-through cell. Synchrotron Mossbauer spectra were collected in the time domain at 30-s intervals. Integration of the intensity obtained during a selected time interval in the spectra allowed sensitive determination of Fe(II) content as a function of reaction time. Analysis of reaction end member specimens by both the synchrotron technique and conventional Mossbauer spectroscopy yielded comparable values for Mossbauer parameters such as center shift and Fe(II)/Fe(III) area ratios. Slight differences in quadrupole splitting values were observed, however. A reactive diffusion model was developed that fit the experimental Fe(II) kinetic data well and allowed the extraction of second-order rate constants for each reaction. Thus, in addition to rapidly collecting high quality Mossbauer data, the synchrotron technique seems well-suited for aqueous rate experiments due to the penetrating power of 14.4 keV X-rays and high sensitivity to Fe valence state.
2002. "Creation of a Subsurface Permeable Reactive Barrier Using In Situ Redox Manipulation." Chapter 6 in Handbook of Groundwater Remediation using Permeable Reactive Barriers: Applications to Radionuclides, Trace Metals, and Nutrients, ed. David L. Naftz, et al, pp. 163-192. Academic Press, San Diego, CA. Abstract An In Situ Redox Manipulation (ISRM) method for creating a permeable reactive barrier in the subsurface has been developed and a field-scale demonstration has been conducted at a chromate-contaminated site on the U. S. Department of Energy's Hanford site in southeastern Washington State. The ISRM treatment zone is created by reducing the ferric iron [Fe(III)] phases naturally present in the aquifer sediments to ferrous iron phases [mainly adsorbed Fe(II)] with a chemical reducing agent using an injection/withdrawal (i.e., push/pull) emplacement strategy. Sodium dithionite (Na2S2O4) is injected into the aquifer, provided a residence time sufficient to react the sediment, and any remaining unreacted reagent and reaction products are withdrawn from the aquifer. Standard groundwater wells are used, allowing treatment of contaminants too deep below the ground surface for conventional trench-and-fill technologies. Once in place, redox-sensitive contaminants migrating through this manipulated zone are destroyed (organic solvents) or immobilized (metals). In the spring of 1997, an ISRM treatability test was initiated at a chromate-contaminated site located within Hanford's 100-D Area. Analysis of groundwater samples collected from the reduced zone following emplacement of the barrier indicate that concentrations of hexavalent chromium in groundwater have decreased from a pre-emplacement concentration of approximately 1,000 ?g/L to below analytical detection limits (<8 ?g/L). Hexavalent chromium concentrations have also significantly decreased below baseline values in downgradient monitoring wells. Laboratory analysis of iron in the soil indicates the barrier should remain in place for approximately 23 years. If additional reductive capacity is needed, the barrier can be regenerated at a reduced cost using the original injection well network.
2002. "Methods for Determination of Mineralogy and Environmental Availability ." Chapter 5 in Soil Mineralogy with Environmental Applications, SSSA Book Series, vol. 7, ed. J.B. Dixon and D.G. Schulze, pp. 153-197. Soil Science Society of America, Madison , WI. Abstract Determining Mineralogy and Environmental Availability
2002. "X-ray absorption and magnetic studies of trivalent lanthanide ions sorbed on pristine and phosphate-modified boehmite surfaces." Langmuir 18 (26)(Dec 24, 2002):10128-10136. Abstract The feasibility of immobilizing radionuclides on mineral surfaces was examined in the absence and the presence of phosphate anions, using trivalent lanthanide ions (Eu3+, Gd3+, and Dy3+) as chemical analogues of trivalent actinide radionuclides. The amount of the lanthanide ions (Ln3+) sorbed on boehmite (gamma-AlOOH) surfaces dramatically increased on the presence of phosphate below pH 5. The structure of the sorbed lanthanide was determined by X-ray absorption spectroscopy, magnetic susceptibility measurements, and electron paramagnetic resonance spectroscopy. We proved Dy3+ forms precipitates on boehmite surfaces in the presence of phosphate, and Gd3+ both in the presence and absence of phosphate. In the presence of phosphate, however, these rare-earth cations react to from ultrafine particles of LnPO4 surface precipitates on boehmite surfaces.
2001. "Kinetics and Mechanism of Birnessite Reduction by Catechol." Soil Science Society of America Journal 65:58-66. Abstract Kinetics and Mechanism of Birnessite Reduction by Catechol
2001. "Microscopic Effects of Carbonate, Manganese, and Strontium ions on Calcite Dissolution." Geochimica et Cosmochimica Acta 65 (3):369-379. Abstract Aqueous dissolution of the (1014) surface of calcite was observed at pH near 9 using an atomic force microscope equipped with a fluid cell. The influence of carbonate, Sr, and Mn ion concentrations were observed. Carbonste ions were shown to have a step-specific effect on calcite dissolution. At ow levels (5 mu-M) of carbonate, the retreat rate of the more structually open [441]+steps was than the retreat rate of the structurally confined [441]-steps, leading to anisotropic dissolution. Increasing the carbonate level to 200 mu-M decreased the rate of retreat of both steps, but the [411]+step was slowed to a much greater extent making the dissolution nearly isotropic. At high levels (800 mu-M) of carbonate, the rate of retreat of the [441]+step was slower than that of the [441]-step making dissolution anisotropic in the opposite sense to that observed at low levels of carbonate. This decrease in step velocity at high carbonate levels was attributed to a corresponding increase in the reaction (i.e., precipitation) as the solution approached saturation with respect to calcite, and thus is related to the rate of incorporation of calcium cations into the structure. In addition to changing the rate, this back reaction also altered the shape of etch pits formed by dissolution. Strontium cations were also shown to have a step-specific effect on calcite dissolution similar to that of carbonate, suggesting that strontium is preferentially incorporated into the [441]-step to a greater extent than strontium. When the solution exceeded saturation with respect to rhodochrosite, calcite dissolution was nearly isotropic. These results suggest that the small manganese ion (r = 83 pm), is readily incorporated into both [441]+ and [441]-steps, in contrast to the larger Ca (r = 100 pm) and Sr (r = 131 pm) cations, which are preferentially incorporated into the [441]+step.
2001. "Detection of Trace Levels of Water in Oil by Photoacoustic Spectroscopy." Sensors and Actuators. B, Chemical 77(3):620-624. Abstract Detection of Trace Levels of Water in Oil by Photoacoustic Spectroscopy
2000. "Interaction of Aqueous Chromium Ions with Iron Oxide Surfaces." Chapter 14 in Nuclear Site Remediation: First Accomplishments of the Environmental Management Science Program. ACS Symposium Series, vol. 778, ed. Eller, P. Gary; Heineman, William R., pp. 212-246. American Chemical Society, Washington DC. Abstract To gain a more fundamental understanding of abiotic processes controlling reduction reactions of aqueous chromate and dichromate ions (Cr(VI)aq) in subsurface environments, we carried out molecular-level experimental and modeling studies of the interaction of water and Cr(VI)aq with well-characterized single crystal samples of synthetic and natural hematite and magnetite. A reductionist approach was adopted in which simplified model systems of increasing complexity were studied. Photoemission spectroscopy (PES), photo-electron diffraction, and vacuum STM were used to characterize the composition, atomic structure, and morpho-logy of clean surfaces of ⍺-Fe₂O₃(0001) and Fe₃O₄(100) grown by molecular beam epitaxy on single crystal substrates of ⍺-Al₂O₃(0001) and MgO(100), respectively.
2000. "Listening to Colloidal Silica Samples: Simultaneous Measurement of Absorbed and Scattered Light Using Pulsed-Laser Photoacoustics." Applied Spectroscopy 54(8):1142-1150.
2000. "Creation of a Subsurface Permeable Treatment Zone for Aqueous Chromate Contaminatioin Using In Situ Redox Manipulation." Ground Water Monitoring and Remediation 20(2):66-77. Abstract Creation of a Subsurface Permeable Treatment Zone for Aqueous Chromate Contaminatioin Using In Situ Redox Manipulation
2000. "Dechlorination of Carbon Tetrachloride by Fe(II) Associated with Goethite." Environmental Science and Technology 34:4606-4613. Abstract N/A
1999. "Hydrogen Sulfide Gas Treatment of Cr(VI)-Contaminated Sediment Samples from a Plating-Waste Disposal Site-Implications for In-Situ Remediation." Environmental Science and Technology 33:4096-4101. Abstract Hydrogen Sulfide Gas Treatment of Cr(VI)-Contaminated Sediment Samples from a Plating-Waste Disposal Site-Implications for In-Situ Remediation.
1999. "2H Solid-State NMR Investigation of Terephthalate Dynamics and Orientation in Mixed-Anion Hydrotalcite-like Compounds." Journal of Physical Chemistry B 103:5197-5203.
1999. "Synthesis of Smectite Clay Minerals: A Critical Review." Clays and Clay Minerals 47(5):529-554. Abstract Smectites are one of the most important groups of phyllosilicates found in soils and sediments, and certainly one of the most difficult to study. New information about the formation mechanisms, impact of structural features on surface properties, and long-term stability of smectites can best be gained from the systematic study of single-phase specimens. In most instances, these specimens can only be obtained through synthesis under controlled conditions. Syntheses of smectites have been attempted (1) at ambient pressure and low-temperature (< 100 C), (2) under moderate hydrothermal conditions (100-1000 C, pressures to several kbars), (3) under extreme hydrothermal conditions (>1000 C or pressures > 10 kbars), and (4) in the presence of fluoride. Of these approaches, syntheses performed under moderate hydrothermal conditions are the most numerous and the most successful in terms of smectite yield and phase- purity. Using hydrothermal techniques, high phase-purity can be obtained for beidellites and several transition-metal smectites. However, synthesis of montmorillonite in high purity remains difficult. Starting materials for hydrothermal syntheses include gels, glasses, and other aluminosilicate minerals. The presence of Mg2+ seems to be essential for the formation of smectites, even for phases such as montmorillonite which contain low amounts of Mg. Highly crystalline smectites can be obtained when extreme temperatures or pressures are used, but other crystalline impurities are always present. Although the correlation between synthesis stability fields and thermodynamic stability fields is good in many instances, metastable phases are often formed. Few studies, however, include the additional experiments (approach from under- and over-saturation, reversal experiments) needed to ascertain the conditions for formation of thermodynamically stable phases. Thorough characterization of synthetic products by modern instrumental and molecular-scale techniques is also needed to better understand the processes leading to smectite formation.
1999. "In Situ Redox Manipulation by Dithionite Injection: Intermediate-Scale Laboratory Experiments." Ground Water 37(6):884-889. Abstract In Situ Redox Manipulation by Dithionite Injection: Intermediate Scale Laboratory Experiments.
1999. "Laser Photoacoustic Spectroscopy: A Versatile Absorption-Spectroscopic Technique." American Laboratory 31(4):96S-108S. Abstract There is no abstract currently available for this item
1999. "In-situ Detection of Chromate Using Photoacoustic Spectroscopy." Applied Spectroscopy 53(6):735-740.