Extractive Membranes for the Detection and Screening of Waterborne Plutonium

The development of rapid screening tools for special nuclear materials remains a crucial focus for nonproliferation efforts. Traditional approaches for the analysis of trace-level Pu isotopes in water requires tedious and time-consuming sample preparation steps that do not lend well to expeditious screening. Therefore, a novel analytical method that combines both Pu concentration and source preparation into a single detection system would make for an invaluable tool for nuclear security applications. Extractive membranes absorbers can help to fulfill this role as they are capable of concentrating Pu to detectable limits while subsequently serving as alpha spectrometry sample sources. In Chapter 1, I discuss the standard separation and sample preparation procedures for alpha spectrometry to screen waterborne Pu at concentrations below 10-12 M. Pu-extractive thin films and membranes are reviewed, and I detail methods used to develop the extractive copolymers and functional membranes presented throughout the dissertation. Chapter 2 describes my initial studies into the development of thin polymer-ligand films for Pu concentration and direct alpha spectrometry analysis. Submicron thin films were prepared through spin coating combinations of polystyrene (PS) with dibenzoylmethane, thenoyltrifluoroacetone and di(2-ethylhexyl) phosphoric acid (HDEHP) onto Si wafers. Of the three ligands tested, only films containing HDEHP showed significant Pu recovery at pH 2.3 and 6.3. Alpha spectrometry peak resolutions that rivaled conventional electrodeposited sources were obtained for the PS-HDEHP films over film thicknesses ranging from 30 to 250 nm. In an uptake study with a solution composed of greater than 90% Pu(V), it was observed that Pu recovery would increase over ten-fold when the HDEHP loading in the film was doubled. X-ray photoelectron spectroscopy (XPS) revealed that the ligand bloomed to the film surface in the samples with higher loadings of HDEHP. The XPS analysis also revealed that HDEHP was stripped from the films after contact with water at a circumneutral pH, but the films with higher HDEHP loadings retained some ligand even after a 12 h soak in water. Chapter 3 explores the fabrication of extractive thin-film composite (e-TFC) membranes for the rapid concentration and isotopic screening of Pu from a spiked water source. The e-TFC membranes were composed of a Pu-extractive copolymer spin coated onto an ultrafiltration membrane support. Phosphonate and phosphoric acid monomers were tested for their ability to create copolymers suitable for casting and extracting waterborne Pu at trace levels. These monomers were polymerized with either methyl methacrylate or 4-methylstyrene through bulk radical polymerization. In batch screening studies, it was observed that copolymers containing ethylene glycol methacrylate phosphate (EGMP) achieved the highest 242Pu recoveries from solutions at pH 4.2 and 6.8. A copolymer containing 5 % (w/w) EGMP to 4-methylstyrene exhibited a 242Pu distribution coefficient of 92.7 ± 18.5 L/kg at pH 6.8, and the copolymer was suitable for casting when dissolved in toluene. e-TFC membranes were prepared with the EGMP-based copolymer and tested in a direct filtration study. The e-TFC membranes were able to extract up to 10.2 ± 4.2 % Pu after filtering 10 mL of a solution bearing 4.53 Bq/mL 242Pu at pH 6.8. A 242Pu peak energy resolution of 71.7 ± 8.7 keV full-width at half-maximum was obtained for the e-TFC membranes and allowed for the distinction between 242Pu from 238Pu/241Am also present in the solution. Chapter 4 details the development of Pu-extractive membranes through non-solvent induced phase separation (NIPS). An innovative grafting procedure was performed to create a copolymer consisting of EGMP anchored to polyvinylidene (PVDF), which then can be used as a dope additive to cast PVDF membranes with Pu binding sites. Permeability studies were performed with membranes containing 10 wt% PVDF-g-EGMP, and the NIPS casting conditions were varied to study their effect on membrane permeability. Direct filtration studies showed that the membranes recovered up to 59.9 ± 3.0 % and 19.3 ± 3.5 % 238Pu from deionized and synthetic seawater solutions, respectively, after filtering 10 mL of solution at a concentration of ~0.5 Bq/mL 238Pu. Peak energy tailing was observed in the alpha spectra for the copolymer membranes, and SEM-EDS analysis indicated that PVDF-g-EGMP was distributed throughout the membrane and attributed to the loss in spectral resolution. The studies in my dissertation demonstrate that functional membranes for the detection and screening of waterborne Pu can be developed through relatively simple and effective measures. I hope that these studies will further aid the development of rapid and fieldable Pu detection systems and aid the advancement of nuclear security endeavors