Physical and Chemical Interactions Affecting U and V Transport from Mine Wastes

This research investigates the physical and chemical interactions that affect the transport of uranium (U) and vanadium (V) from uranium mine waste sites by integrating laboratory experiments and reactive transport modelling with various spectroscopy, microscopy, and diffraction techniques. The document consists of results from three related studies. In the first study (Chapter 3), the reactive transport of U and V was investigated by sequentially reacting mine wastes collected from Blue Gap/Tachee Claim-28 mine site, AZ with 10mM NaHCO3 (7.9) and 10mM CH3COOH (3.4) during continuous flow through column experiments. Under both of these conditions (pH 3.4 and 7.9), dissolution of U-V bearing minerals predominant at the site were identified as a key mechanism affecting the reactive transport of U and V. The equilibrium (Keq) and reaction rate constants (km) for U-V bearing mineral dissolution were estimated to be Keq = 10-44.81 and km = 4.8x10-13 mol cm-2 sec-1 at circumneutral conditions, and Keq = 10-38.65 and km = 3.2x10-13 mol cm-2 sec-1 under acidic conditions. These results, coupled with electron microscopy data, suggest that the release of U and V is affected by difference in solution pH and crystalline structure of U-V bearing minerals. Identifying the crystal chemistry of these U-V bearing minerals was the task in the second study (Chapter 4) of the dissertation. Using various diffraction and microscopy tools, the U-V bearing minerals were identified as hydrated carnotite. Finally, the mobility of U from co-occurring submicron U(IV) and U(VI) mineral phases in mine wastes from the Jackpile mine in Laguna Pueblo, NM was investigated under oxidizing conditions. Co-occurrence of U(VI) and U(IV) at a 19:1 ratio mostly as coffinite (USiO4) and U-phosphate was observed in these mine waste solids. The highest U release from these mine wastes was observed during batch reactions with 10 mM NaHCO3 solution containing ambient dissolved oxygen concentrations. Results from these investigations provide an improved understanding on the role of thermodynamics, crystallinity, stoichiometry, and solution chemistry in the reactive transport of U and V from mine wastes that affect the water quality of surface and ground water resources

Effects of residual disinfectants on the redox speciation of lead( ii )/( iv ) minerals in drinking water distribution systems

This study investigated the reaction kinetics on the oxidative transformation of lead(ii) minerals by free chlorine (HOCl) and free bromine (HOBr) in drinking water distribution systems. According to chemical equilibrium predictions, lead(ii) carbonate minerals, cerussite PbCO3(s) and hydrocerussite Pb3(CO3)2(OH)2(s), and lead(ii) phosphate mineral, chloropyromorphite Pb5(PO4)3Cl(s) are formed in drinking water distribution systems in the absence and presence of phosphate, respectively. X-ray absorption near edge spectroscopy (XANES) data showed that at pH 7 and a 10 mM alkalinity, the majority of cerussite and hydrocerussite was oxidized to lead(iv) mineral PbO2(s) within 120 minutes of reaction with chlorine (3 : 1 Cl2 : Pb(ii) molar ratio). In contrast, very little oxidation of chloropyromorphite occurred. Under similar conditions, oxidation of lead(ii) carbonate and phosphate minerals by HOBr exhibited a reaction kinetics that was orders of magnitude faster than by HOCl. Their end oxidation products were identified as mainly plattnerite β-PbO2(s) and trace amounts of scrutinyite α-PbO2(s) based on X-ray diffraction (XRD) and extended X-ray absorption fine structure (EXAFS) spectroscopic analysis. A kinetic model was established based on the solid-phase experimental data. The model predicted that in real drinking water distribution systems, it takes 0.6-1.2 years to completely oxidize Pb(ii) minerals in the surface layer of corrosion scales to PbO2(s) by HOCl without phosphate, but only 0.1-0.2 years in the presence of bromide (Br-) due the catalytic effects of HOBr generation. The model also predicts that the addition of phosphate will significantly inhibit Pb(ii) mineral oxidation by HOCl, but only be modestly effective in the presence of Br-. This study provides insightful understanding on the effect of residual disinfectant on the oxidation of lead corrosion scales and strategies to prevent lead release from drinking water distribution systems

Materials and Energy Recovery from E‑Waste Plastics

The objective of the current study was to investigate environmentally sustainable and energy efficient processes to recover value added material and energy from e-waste as a means to divert these nondegradable materials from landfills. We studied two different types of plastics (1) simple mixtures like polycarbonate/polyamide (PC/PA)) found in cell phone plastics for solvent extraction and (2) complex mixtures like PC/PA/acrylonitrile butadiene styrene (ABS)/poly methyl methacrylate (PMMA) found in many other e-waste streams for pyrolysis. Solvent extraction using <i>N</i>-methyl-2-pyrrolidone (NMP) was performed as an alternate to using dichloromethane (DCM) for selective dissolution and recovery of PC from simple mixtures of cell phone plastic (CPP) to avoid the use of chlorinated compounds. Using distillation a recovery of 89% and 87% pure PC was observed for NMP and DCM, respectively. Relatively close to the first run, the recycled NMP also recovered 87% of pure PC. However, in order to reduce energy consumption in the NMP solvent recovery step a nonsolvent approach using methanol precipitation was demonstrated as an alternate route. Energy consumption through methanol distillation (343.3 kJ kg^–1) was significantly lower than that of NMP (413.2 kJ kg^–1). For other e-waste plastic mixtures like PC/PA/ABS/PMMA, a pyrolysis approach was demonstrated for waste reduction to 57% potentially decomposable solid residue, while generating pyrolysis oil. The results obtained in this work contribute to the development of a commercial and sustainable process to recycle e-waste plastics effectively. The development of these effective recycling practices helps to reduce prevailing e-waste plastic related environmental pollution

Poly(amidoamine) Dendrimer Nanocarriers and Their Aerosol Formulations for siRNA Delivery to the Lung Epithelium

Small interfering RNA (siRNA)-based therapies have great promise in the treatment of a number of prevalent pulmonary disorders including lung cancer, asthma and cystic fibrosis. However, progress in this area has been hindered due to the lack of carriers that can efficiently deliver siRNA to lung epithelial cells, and also due to challenges in developing oral inhalation (OI) formulations for the regional administration of siRNA and their carriers to the lungs. In this work we report the ability of generation four, amine-terminated poly­(amidoamine) (PAMAM) dendrimer (G4NH2)–siRNA complexes (dendriplexes) to silence the enhanced green fluorescent protein (eGFP) gene on A549 lung alveolar epithelial cells stably expressing eGFP. We also report the formulation of the dendriplexes and their aerosol characteristics in propellant-based portable OI devices. The size and gene silencing ability of the dendriplexes was seen not to be a strong function of the N/P ratio. Silencing efficiencies of up to 40% are reported. Stable dispersions of the dendriplexes encapsulated in mannitol and also in a biodegradable and water-soluble co-oligomer were prepared in hydrofluoroalkane (HFA)-based pressurized metered-dose inhalers (pMDIs). Their aerosol characteristics were very favorable, and conducive to deep lung deposition, with respirable fractions of up to 77%. Importantly, siRNA formulated as dendriplexes in pMDIs was shown to keep its integrity after the particle preparation processes, and also after long-term exposures to HFA. The relevance of this study stems from the fact that this is the first work to report the formulation of inhalable siRNA with aerosol properties suitable to deep lung deposition using pMDIs devices that are the least expensive and most widely used portable inhalers. This study is relevant because, also for the first time, it shows that siRNA–G4NH2 dendriplexes can efficiently target lung alveolar epithelial A549 cells and silence genes even after siRNA has been exposed to the propellant environment

Electrocatalytic Reduction of Nitrate Using Magnéli Phase TiO₂ Reactive Electrochemical Membranes Doped with Pd-Based Catalysts

This research focused on synthesis, characterization, and application of point-of-use catalytic reactive electrochemical membranes (REMs) for electrocatalytic NO₃^– reduction. Deposition of Pd–Cu and Pd–In catalysts to the REMs produced catalytic REMs (i.e., Pd–Cu/REM and Pd–In/REM) that were active for NO₃^– reduction. Optimal performance was achieved with a Pd–Cu/REM and upstream counter electrode, which reduced NO₃^– from 1.0 mM to below the EPAs regulatory MCL (700 μM) in a single pass through the REM (residence time ∼2 s), obtaining product selectivity of <2% toward NO₂^–/NH₃. Nitrate reduction was not affected by dissolved oxygen and carbonate species and only slightly decreased in a surface water sample due to Ca²⁺ and Mg²⁺ scaling. Energy consumption to treat surface water was 1.1 to 1.3 kWh mol^–1 for 1 mM NO₃^– concentrations, and decreased to 0.19 and 0.12 kWh mol^–1 for 10 and 100 mM NaNO₃ solutions, respectively. Electrocatalytic reduction kinetics were shown to be an order of magnitude higher than catalytic NO₃^– reduction kinetics. Conversion of up to 67% of NO₃^–, with low NO₂^– (0.711 μM) and NH₃ formation (<10 μM), and low energy consumption obtained in this study suggest that Pd–Cu/REMs are a promising technology for distributed water treatment

Reactive Transport of U and V from Abandoned Uranium Mine Wastes

The reactive transport of uranium (U) and vanadium­(V) from abandoned mine wastes collected from the Blue Gap/Tachee Claim-28 mine site in Arizona was investigated by integrating flow-through column experiments with reactive transport modeling, and electron microscopy. The mine wastes were sequentially reacted in flow-through columns at pH 7.9 (10 mM HCO₃^–) and pH 3.4 (10 mM CH₃COOH) to evaluate the effect of environmentally relevant conditions encountered at Blue Gap/Tachee on the release of U and V. The reaction rate constants (<i>k</i>_<i>m</i>) for the dissolution of uranyl–vanadate (U–V) minerals predominant at Blue Gap/Tachee were obtained from simulations with the reactive transport software, PFLOTRAN. The estimated reaction rate constants were within 1 order of magnitude for pH 7.9 (<i>k</i>_<i>m</i> = 4.8 × 10^–13 mol cm^–2 s^–1) and pH 3.4 (<i>k</i>_<i>m</i> = 3.2 × 10^–13 mol cm^–2 s^–1). However, the estimated equilibrium constants (<i>K</i>_eq) for U–V bearing minerals were more than 6 orders of magnitude different for reaction at circumneutral pH (<i>K</i>_eq = 10^–38.65) compared to acidic pH (<i>K</i>_eq = 10^–44.81). These results coupled with electron microscopy data suggest that the release of U and V is affected by water pH and the crystalline structure of U–V bearing minerals. The findings from this investigation have important implications for risk exposure assessment, remediation, and resource recovery of U and V in locations where U–V-bearing minerals are abundant

Elevated Concentrations of U and Co-occurring Metals in Abandoned Mine Wastes in a Northeastern Arizona Native American Community

The chemical interactions of U and co-occurring metals in abandoned mine wastes in a Native American community in northeastern Arizona were investigated using spectroscopy, microscopy and aqueous chemistry. The concentrations of U (67–169 μg L^–1) in spring water samples exceed the EPA maximum contaminant limit of 30 μg L^–1. Elevated U (6,614 mg kg^–1), V (15,814 mg kg^–1), and As (40 mg kg^–1) concentrations were detected in mine waste solids. Spectroscopy (XPS and XANES) solid analyses identified U (VI), As (-I and III) and Fe (II, III). Linear correlations for the release of U vs V and As vs Fe were observed for batch experiments when reacting mine waste solids with 10 mM ascorbic acid (∼pH 3.8) after 264 h. The release of U, V, As, and Fe was at least 4-fold lower after reaction with 10 mM bicarbonate (∼pH 8.3). These results suggest that U–V mineral phases similar to carnotite [K₂(UO₂)₂V₂O₈] and As–Fe-bearing phases control the availability of U and As in these abandoned mine wastes. Elevated concentrations of metals are of concern due to human exposure pathways and exposure of livestock currently ingesting water in the area. This study contributes to understanding the occurrence and mobility of metals in communities located close to abandoned mine waste sites