(397n) Uranium Adsorption on Organophosphorus-Derivitized Extractive Scintillating Resins

Nuclear nonproliferation efforts and treaty verification require portable, robust radiation detectors capable of detecting trace-levels of radionuclides in environmental matrices. A recent development in environmental sensing is a portable, flow cell detector that utilizes extractive scintillating resin. Extractive scintillating resin serves the dual purpose of (1) concentrating the radionuclide of interest through adsorption and/or ion-exchange and (2) serving as a radiation transducer through scintillation. At the heart of this analytical method lies a separations challenge, as the incorporation of analyte-selectivity can enhance the sensitivity of the radiation measurement.

Uranium solution chemistry has a strong dependence on pH, ionic strength, solution composition (i.e. the presence of other ions) and the redox potential of the system. Due to its complex chemistry, there is no one material that is universally selective for uranium. Designing a uranium-selective separation material requires a careful consideration of the behavior of both ligand and uranium at a given set of conditions.

This contribution describes our efforts to synthesize extractive scintillating resins that selectively concentrate and detect uranium from near-neutral pH ground water. 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 achieve the Shewart-3σ alarm criteria than the phosphonic acid resins. Simulations utilizing Density Functional Theory (DFT) are used in conjunction with thermodynamic models in Visual MINTEQ software to understand how the binding mechanisms for both functional groups 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. Understanding the binding mechanisms of uranium with organophosphorus moieties will contribute to the rational design of future uranium-adsorbent materials for environmental systems.