Cations impact radical reaction dynamics in concentrated multicomponent aqueous solutions

Emily T Nienhuis; Trent R Graham; Nicolas L D'Annunzio; Malgorzata I Kowalska; Jay A LaVerne; Thomas M Orlando; Jacob G Reynolds; Donald M Camaioni; Kevin M Rosso; Carolyn I Pearce; Eric D Walter

doi:10.1063/5.0153132

Cations impact radical reaction dynamics in concentrated multicomponent aqueous solutions

J Chem Phys. 2023 Jun 14;158(22):224503. doi: 10.1063/5.0153132.

Authors

Affiliations

¹ Pacific Northwest National Laboratory, 902 Battelle Blvd., Richland, Washington 99354, USA.
² Washington River Protection Solutions, LLC, 2435 Stevens Center Pl., Richland, Washington 99354, USA.
³ Radiation Laboratory and Department of Physics and Astronomy, University of Notre Dame, 102 Radiation Research Building, Notre Dame, Indiana 46556, USA.
⁴ School of Chemistry and Biochemistry, Georgia Institute of Technology, 901 Atlantic Drive, Atlanta, Georgia 30332, USA.
⁵ Department of Crop and Soil Sciences, Washington State University, P.O. Box 646420, Pullman, Washington 99164, USA.

PMID: 37306956
DOI: 10.1063/5.0153132

Abstract

Ultraviolet (UV) photolysis of nitrite ions (NO2-) in aqueous solutions produces a suite of radicals, viz., NO·, O-, ·OH, and ·NO2. The O- and NO· radicals are initially formed from the dissociation of photoexcited NO2-. The O- radical undergoes reversible proton transfer with water to generate ·OH. Both ·OH and O- oxidize the NO2- to ·NO2 radicals. The reactions of ·OH occur at solution diffusion limits, which are influenced by the nature of the dissolved cations and anions. Here, we systematically varied the alkali metal cation, spanning the range from strongly to weakly hydrating ions, and measured the production of NO·, ·OH, and ·NO2 radicals during UV photolysis of alkaline nitrite solutions using electron paramagnetic resonance spectroscopy with nitromethane spin trapping. Comparing the data for the different alkali cations revealed that the nature of the cation had a significant effect on production of all three radical species. Radical production was inhibited in solutions with high charge density cations, e.g., lithium, and promoted in solutions containing low charge density cations, e.g., cesium. Through complementary investigations with multinuclear single pulse direct excitation nuclear magnetic resonance (NMR) spectroscopy and pulsed field gradient NMR diffusometry, cation-controlled solution structures and extent of NO2- solvation were determined to alter the initial yields of ·NO and ·OH radicals as well as alter the reactivity of NO2- toward ·OH, impacting the production of ·NO2. The implications of these results for the retrieval and processing of low-water, highly alkaline solutions that comprise legacy radioactive waste are discussed.