Anthropogenic activities and natural processes over time have led to the release of toxic heavy metal contaminants into the environment. As a consequence, there is an increasing number of illnesses caused by the exposure of humans to heavy metals and metalloids. The dissertation work presented here focused on the synthesis, characterization, and understanding of the surface chemistry, as well as the photo-reactivity, of a variety of iron (oxyhydr)oxide nano-materials that have relevance for the remediation of heavy metal contaminants, such as arsenic and chromium in aqueous environments. The research focused on the photo-induced reductive dissolution of a nano-dimensioned iron oxyhydroxide, ferrihydrite, in the presence of oxalate, the photo-induced arsenite oxidation, and the simultaneous redox transformation of arsenite and chromate in the presence of ferrihydrite and another environmentally relevant iron oxyhydroxide, goethite. The photo-reductive dissolution of ferrihydrite (using simulated solar radiation) in the presence of oxalic acid was investigated with surface sensitive in situ and ex situ techniques that included attenuated total reflectance Fourier transform infrared spectroscopy. Ferrihydrite at a solution pH of 4.5 exhibited an induction period where the rate of Fe(II) release was limited by a low concentration of adsorbed oxalate due to the site-blocking of carbonate that was intrinsic to the surface of the ferrihydrite starting material. The photo-induced decarboxylation of adsorbed oxalate also ultimately led to the appearance of carbonate reaction product (distinct from carbonate intrinsic to the starting material) on the surface. Ferrihydrite that was prepared under carbonate free condition showed a rapid release of Fe(II) upon irradiation and no induction period was observed. Arsenite [As(III)] oxidation in the presence of ferrihydrite and goethite was also investigated. Ferrihydrite or goethite when exposed to As(III) in the dark led to no change in the oxidation state of As(III) reactant. However, exposure of As(III) in the presence of ferrihydrite or goethite to simulated solar light resulted in the oxidation of As(III) and a reduction of surface Fe(III) leading to an overall increase in the total As removal. At a solution pH of 5, this conversion of As(III) to As(V) on ferrihydrite resulted in the partitioning of a stoichiometric amount of Fe(II) into the aqueous phase and the majority of the As(V) product remained bound to the ferrihydrite surface. In contrast, the As(III)/goethite system showed a different photochemical behavior in the absence or presence of dissolved oxygen. Under oxic conditions, in contrast to ferrihydrite, the majority of the As(V) product was in the aqueous phase and the relative amount of aqueous Fe(II) was significantly less than in the ferrihydrite circumstance. Experimental observations suggested that in the oxic environment, Fe(II) on the goethite surface was heterogeneously oxidized to Fe(III) by dissolved oxygen resulting in the formation of reactive oxygen species that led to the further oxidation of As(III) in solution. Similarly, various experimental investigations were conducted to test the simultaneous removal of As(III) and Cr(VI) from solution. Our results suggested that a surface mediated spontaneous electron transfer between As(III) and Cr(VI) occurred in the presence of Fe- and Al-(oxy)hydroxides. Both infrared and x-ray absorption spectroscopies were conducted to get more insight into the charge transfer reaction and mechanism of electron transfer reaction. In summary, the research discussed here should help to understand the details of oxidation/reduction reactions occurring at mineral-water interfaces. Perhaps more importantly, the methodologies discussed in this dissertation could potentially be novel and eco-friendly approaches for arsenite as well as hexavalent chromium remediation. / Chemistry

The adsorption and desorption behavior of trace element contaminants was evaluated solids—goethite and ruthenium oxide. The importance of anion displacement as a mechanism responsible for arsenic release from iron oxides was investigated on goethite. The adsorption and polymerization of silicate on goethite was examined as a function of surface concentration determine the influence of adsorbed silicate monomers and polymers on arsenite adsorption desorption. A kinetic model was employed to describe arsenite adsorption and desorption absence and presence of silicate. The potential environmental impacts of the research discussed. Hydrous and crystalline ruthenium oxides were extensively characterized traditional colloidal surface characterization techniques, dissolution experiments, and macro- spectroscopic experiments. The two ruthenium oxide phases exhibited large specific areas, a high density of reactive surface functional groups and the presence of multiple oxidation states in both solids. Enhanced dissolution of hydrous ruthenium oxide occurred presence of oxalate and ascorbate. While enhanced dissolution of the crystalline phase only in the presence of oxalate at pH 3. Results from the dissolution experiments were develop possible mechanisms for the oxalate and ascorbate promoted dissolution of ruthenium oxides. Macroscopic adsorption studies of arsenate adsorption on both ruthenium oxides examined over a broad pH (3-10) and initial solution concentration range (0.01 to Results from the adsorption studies indicate arsenate forms a stable surface complex with ruthenium oxide phases. Extended x-ray absorption fine structure spectroscopy and Pressure-jump relaxation studies indicates arsenate is specifically adsorbed the ruthenium oxide Chromate adsorption on ruthenium oxides was investigated as a function of pH and chromate solution concentration. Macroscopic adsorption studies and zeta measurements suggest chromate forms an inner-sphere surface complex with both oxide X-ray absorption near edge spectroscopy data indicates chromate (Cr(VI)) is reduced chromium (Cr(III)) on the ruthenium oxide surface. Modeling of the first Cr shell indicated two oxygen backscattering distances similar to the Cr-O atomic distances reported for coordinated to Cr(VI) and Cr(III) providing additional evidence for Cr(VI) reduction. / Ph. D.

Chapter 1 Dynamic Processes Occurring at the Cr^III_aq – Manganite (γ-MnOOH) Interface: Simultaneous Adsorption, Microprecipitation, Oxidation/Reduction and Dissolution The complex interaction between Cr^III_aq and manganite (γ-MnOOH) was systematically studied at room temperature over a pH range of 3 to 6, and within a concentration range of 10⁻⁴ to 10⁻² M CrOH²⁺_aq. Solution compositional changes during batch reactions were characterized by ICP and UVvis. The manganites were characterized before and after reaction with XPS, SEM, high-resolution FESEM, and EDS analysis. Fluid-cell AFM was used to follow these metal-mineral interactions in situ. The reactions are characterized by 1) sorption of Cr^III and the surface-catalyzed microprecipitation of Cr^III-hydroxy hydrate on manganite surfaces, 2) the acidic dissolution of the manganite, and 3) the simultaneous reductive dissolution of manganite coupled with the oxidation of Cr^III_aq to highly toxic Cr^VI_aq. Cr^III-hydroxy hydrate was shown to precipitate on the manganite surface while still undersaturated in bulk solution. The rate of manganite dissolution increased with decreasing pH due both to faster acid-promoted and Mn-reduction- promoted dissolution. Due to direct redox coupling with Mn reduction, Cr oxidation was most rapid in the lower pH range. Neither Mn^II nor Cr^VI were ever detected on manganite surfaces, even at the maximum rate of their generation. At the highest pHs of this study, Cr^III_aq was effectively removed from solution to form Cr^III-hydroxy hydrate on manganite surfaces and in the bulk solution, and manganite dissolution and Cr^VI_aq generation were minimized. All interface reactions described above were heterogeneous across the manganite surfaces. This heterogeneity is a direct result of the heterogeneous semiconducting nature of natural manganite crystals, and is also an expression of the proximity effect, whereby redox processes on semiconducting surfaces are not limited to next nearest neighbor sites. Chapter 2 Comparison of the Reactivity of Various Mn-Oxides with Cr^III_aq: Microscopic and Spectroscopic Observations of Dissolution, Cr-sorption and Cr and Mn Redox Interactions The interaction between Cr^III_aq and seven different Mn-oxides (6 monomineralic, 1 synthetic) have been observed in pH ~4.4 HNO₃ and pH ~4.4 ~10⁴ M Cr^III_aq solutions. For each mineral-solution interaction, the aqueous chemical concentrations (e.g. [Mn]_aq, [Cr]_aq, [Cr^VI_aq]) were measured with time. Reacted samples were examined by XPS to determine if, and to what extent, the surface chemical states of Cr, Mn and O had changed. Microscopic observations of the reacted surfaces were obtained using AFM and high-resolution, low-voltage FESEM. The solubility of the Mn-oxides in the acidic, non-Cr bearing solutions varied inversely with the average Mn valence, but did not show systematic behavior with respect to the mineral structure type (e.g. tunnel, layer, framework). This trend was interpreted as resulting from the relative ability of an adsorbed proton to polarize surface Mn-O bonds, with the polarizability being in the order Mn²⁺-O > Mn³⁺-O > Mn⁴⁺-O. For samples reacted with Cr^III_aq, the rate and extent of reductive dissolution was always greater than for acidic dissolution during the initial time period. The measured ratios of the [Mn]_aq : [Cr^VI]_aq were approximately in agreement with the values expected from the proposed stoichiometric reactions. Cr-uptake was observed to occur in undersaturated solutions as a result of adsorption, absorption and surface catalyzed precipitation. The chromium as detected by XPS was predominately Cr^III, however pyrolusite contained both Cr^III and Cr^VI. Previous studies have implicated a chromium surface precipitate to be responsible for the cessation of the Cr^III_aq oxidation reaction. Our surface sensitive FESEM and AFM observations tend to suggest that Cr-uptake is by isolated site binding, very small (<30 nm) surface clusters or monolayer scale films. Cr-uptake was followed by slow Cr-release on several of the solids (particularly the layered solids) after a substantial portion of the total aqueous Cr had been converted to Cr^VI_aq. The oxidizing ability of the different Mn-oxides for Cr^III_aq is evaluated with regards to the energy level of the redox couple (i.e. the redox potential) as compared with the Fermi energy level of the Mn-oxide. Although these energies were calculated rather than directly measured, the results indicate that electrons originating from adsorbed Cr^III ions may be transferred into the conduction band or more likely, into available surface states. The presence of an initial limited quantity of electron accepting surface states likely explains the observation of a rapid initial Cr^III-oxidation followed by much slower oxidation. The Mn-oxides that exhibited the greatest and longest lasting Cr^III-oxidizing power were the Mn-oxides containing Mn⁺, and in particular those containing Mn³⁺ and Mn⁺. It is believed that the combined presence of a reducible Mn ion (e.g. Mn³⁺) and a highly soluble Mn⁺ ion facilitates a sustained Cr^III-oxidation reaction because fresh surface is exposed during the reaction. / Ph. D.

Structural and electronic effects in hydrocarbon reactions over metal oxides have been examined by comparing the reactions of methyl (-CH₃) and methylene (=CH₂) fragments on three different oxide single crystal surfaces: α-Cr₂O₃(101̅2), α-Cr₂O₃(0001), and α-Fe₂O₃(101̅2). The intermediates have been generated through the decomposition of halogenated hydrocarbons. The primary reactions of methyl and methylene over α-Cr₂O₃ are methyl dehydrogenation to methylene, and methylene coupling (C-C bond formation) to ethylene (CH₂=CH₂). The different surface geometric structures of α-Cr₂O₃(101̅2) and (0001) lead to an increase in the activation barrier for methylene surface migration, a critical step in the coupling reaction, of 5.9 kcal/mol over the (0001) surface. For methyl dehydrogenation, differences in the local site pair (cation/anion) geometry and the proximity of surface lattice oxygen to the methyl group do not result in a significant difference in the barrier for dehydrogenation, suggesting that the surface anions play a minor role in the dehydrogenation of methyl on these surfaces. Electronic differences in the Fe³⁺ (𝑑⁵) and Cr³⁺ (𝑑³) cations on structurally-similar α-Cr₂O₃(101̅2) and α-Fe₂O₃(101̅2) surfaces lead to major differences in reaction selectivity. α-Cr₂O₃(101̅2) is nonreducible under the reaction conditions of this study, but α-Fe₂O₃(101̅2) is highly reducible due to the difference in the d electron configuration. Hydrocarbons are formed over α-Cr₂O₃(101̅2), but nonselective oxidation products (CO₂, CO, H₂O) are formed over the stoichiometric α-Fe₂O₃(101̅2) surface along with surface reduction. Reduction of the α-Fe₂O₃(101̅2) leads to a shift in the product selectivity towards formaldehyde (CH₂O) and ethylene. For the limited number of systems examined in this study, examples of geometric structure sensitive (methylene coupling) and structure insensitive (methyl dehydrogenation) reactions have been found on α-Cr₂O₃, and electronic effects are observed for the reactions on α-Cr₂O₃(101̅2) and α-Fe₂O₃. For the structure sensitive reaction, the differences in surface geometry impact the reactions kinetics over Cr₂O₃ but not the types of products formed, while the electronic differences give rise to dramatic changes in the selectivity associated with the very different products formed over α-Cr₂O₃(101̅2) and α-Fe₂O₃(101̅2). / Ph. D.

The oxidation of arsenic(III) by chromium(VI) requires the postulation of uncommon oxidation states as reactive intermediates. The possible intermediates of chromium are the divalent, tetravalent, and pentavalent states. Recently arsenic(IV) has been postulated as a reactive intermediate in certain oxidations of As(III). A detailed study of the reaction rate law in various solutions has been carried out in order to determine the nature of the intermediates found in the Cr(VI)-As(III) reaction. The effects of hydrogen ion and buffer concentrations have been determined. The mechanism postulated depends upon the solution in which the reaction occurs. The kinetics of the chromium(VI)-arsenic(III) reaction have been measured in perchloric acid solutions, acetic acid solutions, acetic acid-acetate buffers, ammonium nitrate solutions, and dihydrogen phosphatemonohydrogen phosphate buffers. The rate laws are: Perchloric Acid - -d[Cr(VI)]/ dt = [k₀ + k_H[H⁺] + k_H² [H⁺]²] (K’ [As (III)][Cr(VI)])/(l+K’[As(III)]) Acetic Acid-Acetate Buffers - -d[Cr(VI)]/ dt = [k₀ + k_HOAc[HOAc]} (K’ [As (III)][Cr(VI)])/(l+K’[As(III)]) Acetic Acid - -d[Cr(VI)]/ dt = [k₀ + k’_HOAc[HOAc]] (K’ [As (III)][Cr(VI)])/(l+K’[As(III)]) H₂PO₄⁻- HPO₄⁻² Buffers - -d[Cr(VI)]/ dt = (kK”[As(III)][Cr(VI)][H₂PO₄⁻])/(1+k”[H₂PO₄⁻]) Ammonium Nitrate Solutions - -d[Cr(VI)]/ dt = k₂ [As(III)][Cr(VI)] The reaction studied was the oxidation of As(III) by Cr(VI} according to the stoichiometry 3 As(III) + 2 Cr(VI) = 3 As(V) + 2 Cr (III) Rate data were obtained spectrophotometrically and by iodometric titration depending upon the region being investigated. At high As(III)-low Cr(VI), the Cr(VI) concentration was followed as a function of time at 350 mu using a Beckman DU spectrophotometer. The validity of Beer's law with respect to Cr(VI) was checked. At low As(III)-high Cr(VI), the unreacted As(III) concentration was determined by iodometric titration. In phosphate buffers, the HCrPO₇⁻² complex is indicated as an oxidizing agent. In the other systems, the following mechanism is consistent with the experimental facts. As(III) + HCrO₄⁻ [stacked right and left arrows with K above] As(III) · HCrO₄ As (III), As(III) · HCrO₄⁻ [right arrow with kₒ above] products, rate determining As(III) · HCrO₄⁻ + HOAc [right arrow with K_HOAc above] products, rate determining As(III) · HCrO₄⁻ + H⁺ [right arrow with k_H] products, rate determining In perchloric acid, H₂CrO₄is indicated as participating in the oxidation by the following steps: HCrO₄⁻ + H⁺ [stacked right and left arrows with Kₐ above] H₂CrO₄, ₂CrO₄ + As(III) [stacked right and left arrows with K above] As(III) · H₂CrO₄ The decomposition of As(III}•H₂CrO₄ is also acid catalyzed. In the mechanism described above, the products of the rate determining steps are arsenic(V) and chromium(IV). The existence of chromium(IV) is supported by the induced oxidation of ferrocyanide by the As(III)-Cr(VI) reaction. The chromium(IV) reacts immediately with chromium(VI) to form chromium(V) which can oxidize As(III) to As(V) directly. This is in agreement with the experimentally observed stoichiometry and accounts for the use of K' in the rate laws where K’ is actually 2K. The value of K is obtained from plots of 1/k_m versus 1/As(III). The value of K which describes all data reported is 17.2 1.mole⁻¹. In all three systems, perchloric acid, acetic acid, and acetic acid-acetate, the same kₒ is obtained. The specific rate constant for the acetic acid catalysis is 1.99 x 10⁻³ 1.⁻² mole² sec.⁻¹. Comparison of k"_HOAc terms for acetic acid and acetic acid-acetate solutions shows that k”_HOAc for the acid solution is larger than that for the buffers. This difference is explained on the basis of a medium effect. / Doctor of Philosophy

Yes / To interpret nitrogen sintering of chromium and manganese steels without the formation of deleterious oxides, but with manganese and carbon modifying the local microclimate, the role of the volatile Mn and carbothermic reactions were considered. Reduction of Cr2O3 by Mn vapour is always favourable. CO is an effective reducing agent, however, whereas at atmospheric pressure it will reduce FeO at ~730°C, temperatures some 500 and 700°C higher, i.e. above those for conventional sintering, are necessary for reducing Cr2O3 and MnO, respectively. Accordingly partial pressures must be considered and the sintering process is modelled at a conglomerate of several surface oxidised alloy particles surrounding a pore with graphite present and a tortuous access to the nitrogen-rich atmosphere containing some water vapour and oxygen. The relevant partial pressures were calculated and reduction reactions become thermodynamically favourable from ~200°C. Kinetics, however, dictates availability of CO and the relevant reactions are the water-gas, C + H2O = CO + H2 from ~500°C and the Boudouard, C + CO2 = 2CO, from ~700°C. Discussion of sintering mechanisms is extended to processing in semi-closed containers, also possessing specific microclimates.

Empirically estimated potentials were used to study the defects in Ni₃Al. A computer program which uses the conjugate gradient technique to obtain the relaxed structures was used. Atoms in the vicinity of the planar defects on the (111) plane were found to relax in two different oscillation modes namely the acoustic and optical modes. While the former is similar to the relaxation observed in pure Aluminum or Nickel, the latter was found to be a result of micromoments within the unit cell. The presence of atoms that differ in size and charge are believed to create micro-moments within each unit cell. The energy of a surface seem to depend directly on the atomic density of the terminating plane. A vacancy prefers to be formed near the boundary rather than in the bulk. The formation of an Nickel vacancy is preferred, also a Nickel vacancy has more activation energy to migrate to the boundary. The effect of the Σ = 5 boundary seems to be felt only until a distance of approximately 5 Å away from the boundary. / M.S.

The percentage of reduction in fatigue limit of AISI 4340 steel due to one chromium plating was 21 percent, based on the fatigue limit of unplated AISI 4340 steel. / Master of Science

Dairy cattle characteristically exhibit decreased milk yield and reproductive performance, as well as increased uterine infection rates during periods of high stress. Chromium supplementation has demonstrated efficacy as a feed additive capable of reducing the detrimental effects of stress. As such, its application in dairy production may help to alleviate economic losses associated with seasonal heat stress and the stress experienced during the early postpartum period. Therefore, the objective of the work described in this thesis was to evaluate the potential benefits of short-duration, high-dose chromium (Cr) supplementation in early postpartum dairy cows during the summer months. Multiparous, early postpartum cows (20.95 ± 0.21 DIM, 658.29 ± 13.61 kg) were assigned to one of two treatment groups: 1) normal TMR (Con; n=10) and 2) normal TMR + Cr propionate supplementation (CrPro; 12 mg/h/d Cr; n=12). Body weight (BW), milk yield, and feed intake were measured each day of the experiment. Ambient temperature and humidity were monitored, and the temperature-humidity index (THI) was calculated for the duration of the study as an indicator of the severity of the heat stress experienced by the cows. Transrectal ultrasonography was performed every three days to assess ovarian follicular and luteal dynamics. Respiration rates (RR), rectal temperatures (RT), and blood glucose were recorded concurrently with ovarian ultrasonography. Plasma was collected and used for analysis of progesterone concentrations. Every six days in conjunction with ultrasonography, endometrial cytology samples were collected via cytobrush from each cow to determine the incidences of subclinical endometritis (SCE), as determined by polymorphonuclear leukocyte (PMNL) %. There were no treatment-based differences in RR, RT, blood glucose, feed intake, milk yield, or BW. However, the supplementation strategy did improve reproductive parameters. Within the Con group, there was an increase in PMNL % between samples five and six. Furthermore, at cytology sample six, the Con group had a greater percentage of PMNL than the CrPro group (P=0.01). Chromium consumption did not affect the counts or sizes of most follicles, with the exception being the 6-9 mm category where the CrPro group had a greater average diameter and tended to have more follicles in this category. While CL numbers or size did not differ between treatments, the ratio of progesterone (P4) to corpus luteum (CL) volume was greater in the CrPro group compared to the Con group (P=0.03). The results from this study indicate that the proposed supplementation strategy does not influence DMI or milk yield in cows experiencing stress. Nonetheless, short-duration, high-dose Cr supplementation strategy could benefit reproductive performance and thereby limit economic losses experienced by dairy producers during periods of stress. / Master of Science / Dairy producers continue to experience monetary losses due to the decrease in production performance by dairy cows resulting from physiological changes in response to stress. Elevated ambient temperature and humidity conditions can lead to heat stress, which has been found to decrease both milk and reproductive performance on dairy operations. The period after calving, known as the postpartum period, can also lead to metabolic changes in a cow due to the stress of giving birth and beginning to produce milk. Chromium is an essential trace mineral that can be supplemented with cattle feed to improve the impacts of such stressful periods on production parameters. This study evaluated the effect of a short-duration, high-dose chromium supplementation strategy on reproduction and lactation of early postpartum dairy cattle during summer months. Twenty-two cows were assigned to two treatment groups: control (standard feed only) and chromium supplement (standard feed plus chromium propionate supplementation). Cows receiving the chromium supplement did not experience any changes in respiration rate, rectal temperature, blood glucose levels, feed intake, milk yield, or body weight. Reproductive analyses determined that cows receiving the chromium supplementation had a lower number of immune cells present in the uterus by the end of the experiment, which may be indicative of a healthier uterine environment. Chromium supplemented cows also had a greater level of progesterone concentration to corpus luteum volume compared to cows in the control group. These results indicate that the short-duration, high-dose chromium supplementation strategy could benefit reproductive performance and limit losses experienced by dairy cows under stress conditions.

No / (51)Cr-labeled, superparamagnetic, iron oxide nanoparticles ((51)Cr-SPIOs) and (65)Zn-labeled CdSe/CdS/ZnS-quantum dots ((65)Zn-Qdots) were prepared using an easy, on demand, exchange-labeling technique and their particokinetic parameters were studied in mice after intravenous injection. The results indicate that the application of these heterologous isotopes can be used to successfully mark the nanoparticles during initial distribution and organ uptake, although the (65)Zn-label appeared not to be fully stable. As the degradation of the nanoparticles takes place, the individual transport mechanisms for the different isotopes must be carefully taken into account. Although this variation in transport paths can bring new insights with regard to the respective trace element homeostasis, it can also limit the relevance of such trace material-based approaches in nanobioscience. By monitoring (51)Cr-SPIOs after oral gavage, the gastrointestinal non-absorption of intact SPIOs in a hydrophilic or lipophilic surrounding was measured in mice with such high sensitivity for the first time. After intravenous injection, polymer-coated, (65)Zn-Qdots were mainly taken up by the liver and spleen, which was different from that of ionic (65)ZnCl2. Following the label for 4 weeks, an indication of substantial degradation of the nanoparticles and the release of the label into the Zn pool was observed. Confocal microscopy of rat liver cryosections (prepared 2 h after intravenous injection of polymer-coated Qdots) revealed a colocalization with markers for Kupffer cells and liver sinusoidal endothelial cells (LSEC), but not with hepatocytes. In J774 macrophages, fluorescent Qdots were found colocalized with lysosomal markers. After 24 h, no signs of degradation could be detected. However, after 12 weeks, no fluorescent nanoparticles could be detected in the liver cryosections, which would confirm our (65)Zn data showing a substantial degradation of the polymer-coated CdSe/CdS/ZnS-Qdots in the liver.