AbstractUltra-violet (UV) photolysis of nitrite ions (NO2-) in aqueous solutions produces a suite of radicals, namely 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, and the resulting products, 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 highly alkaline nitrite solutions using electron paramagnetic resonance spectroscopy with nitromethane as a spin trap. A comparison of data for the different alkali cations as a function of ionic strength 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- towards •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.
Published: July 8, 2023