Uranium-selective adsorbent materials for environmental radiation sensing

Nuclear nonproliferation efforts and treaty verification require portable, robust radiation detectors capable of detecting trace-levels of radionuclides in environmental matrices. Despite widespread interest, currently, there are no fieldable, radiation detection techniques capable of directly analyzing gross activity or isotopic distributions of uranium in environmental waters. Extractive scintillating resins in flow-cell detectors and alpha spectroscopy are two promising fieldable detection methods; however, the implementation of these detection methods in the field requires novel separation materials capable of isolating uranium from environmental waters. Organophosphorus-derivitized materials are a promising class of materials to fit this need and are reviewed in Chapter 1. Chapter 2 presents my work understanding the stability of extractive scintillating resins prepared by multiple methods. This chapter characterizes the stability of scintillating resins for ionizing radiation detection that were synthesized with 2-(1-naphthyl)-5-phenyloxazole (α-NPO) or 2-(1-naphthyl)-4-vinyl-5-phenyloxazole (v-NPO) fluorophore in polystyrene (PS) or poly(4-methyl styrene) (PVT) matrices. Leaching studies of the PS and PVT beads with methyl acetate show a 60% reduction in luminosity and 80% reduction in detection efficiency for α-NPO samples; while v-NPO resins retained detection properties. Degradation studies indicate the nitration of PS resins and the fluorophores after nitric acid exposure, resulting in a 100% reduction in optical properties; whereas PVT resins with v-NPO fluorophore maintained 20% detection efficiency. Heuristics are reported for designing stable scintillating resins. Chapter 3 describes the synthesis and characterization of phosphonic acid derivitized resins for the simultaneous concentration and detection of uranium. Resins in this study were prepared by a two-step procedure: (1) suspension polymerization followed by (2) phosphorylation and hydrolysis. Phosphonate hydrolysis was performed via strong acid or trimethylsilyl bromide (TMSBr)-mediated methanolysis. Fluorophore degradation was observed in the resin hydrolyzed by strong acid, while the resin hydrolyzed by TMSBr-mediated methanolysis maintained luminosity and showed hydrogen bonding-induced Stokes\u27 shift of ~100 nm. Flow cell detection efficiency of the TMSBr-mediatied methanolysis resin was found to decrease with increasing pH. Trends are discussed in terms of uranium speciation in solution. Experiments performed in pH 4 synthetic groundwater show that the resins can concentrate the uranyl cation from waters with high concentrations of competitor ions at near-neutral pH. Chapter 4 describes the optimization of design parameters for the implementation of extractive scintillating resin in flow cell detectors. This chapter details the application of extractive scintillating resin with covalently bound fluorophore, v-NPO, and ligand, methyl phosphonic acid, in flow-cell experiments utilizing varying column diameters, resin diameters and diffusion times. The detection efficiency was evaluated by standard offline liquid scintillation counting methods. Diffusion experiments were conducted to determine the effect of the location of the alpha decay on detection efficiency, and the data were compared to a diffusion model to assess the feasibility of relying on diffusion-enhanced detection efficiency in an online measurement. Chapter 5 describes the synthesis and characterization of phosphonic acid- and alkyl phosphate-derivitized extractive scintillating resins. Organophosphorus-derivitized resins were synthesized by two approaches: (1) synthesis of a 4-vinylbenzyl chloride containing polymer followed by solid-phase synthesis techniques to add the phosphonic acid moiety and (2) one-pot polymerization that directly incorporates an alkyl phosphate moiety. Resins were compared on the basis of capacity for uranium, detection efficiency, and volume to detection. The volume to detection was evaluated in neutral pH simulated ground waters (pH 4-8) using a Shewart-3σ alarm statistic applied to data collected in real-time in a flow-cell detector. Resins exhibited similar binding capacities (0.18 mmol g-1) and detection efficiencies (~40%); however, the alkyl phosphate resins more rapidly achieved the Shewart-3σ alarm criteria than the phosphonic acid resins. Thermodynamic models in Visual MINTEQ software and the pKa\u27s of the ligands are used to understand how the binding mechanisms for both functional groups may change as a function of pH. The data imply that as pH increases from 4 to 8, the binding mechanism shifts from ion-exchange to ligand-exchange. Chapter 6 describes our efforts to develop a high throughput analytical technique for waterborne isotopic analysis by using reactive, functional membranes as alpha spectroscopy substrates. In this work, alpha spectroscopy substrates were prepared by two methods: (1) physical deposition of a uranium-selective, water-soluble polymer film on ultrafiltration membranes and (2) grafting uranium-selective ligands from the surface of ultrafiltration membranes. Uranium was loaded onto the substrates by filtering uranium-contaminated water through the ultrafiltration membranes. The uranium-selective, water-soluble polymer was prepared by the copolymerization of 2-hydroxylethyl methacrylate and 2-ethylene glycol methacrylate phosphate. The ligand-grafted membrane was prepared by the UV-polymerization of 2-ethylene glycol methacrylate phosphate with N,N-methylene bisacrylimide. Membranes were characterized by Fourier-transform infrared spectroscopy before and after modification to support the deposition or grafting of the polymer on the membrane surface. The capacity for uranium, 1.9 mmol U g-1, was determined from equilibrium binding experiments. The effect of membrane preparation method and membrane pore size on peak resolution in the alpha spectrum was investigated for pure uranium containing solutions at pH = 6. To mimic more realistic conditions, the selectivity of the membrane was tested using uranium-233 in simulated groundwater. Both uranium-coated membranes prepared from distilled water and groundwater showed resolutions of 80-100 keV in the alpha spectrum and detection efficiencies of 12% for uranium-233. The membranes showed both high resolution and fast preparation time. The permeability of the polymer-coated, ultrafiltration membranes (MWCO 100 kDa) was determined to be 3.74 LMH kPa-1. Overall, this research demonstrates the ability to synthesize robust, uranium-selective materials for the direct concentration of uranium from ground water. Results from this research are laying the groundwork for the development of robust, portable radiation detection techniques capable of analyzing waterborne alpha-emitting radionuclides