Cations impact radical reaction dynamics in concentrated multicomponent aqueous solutions

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.

Radiation‐Induced Defects in Uranyl Trinitrate Solids

“…A recent paper by Nienhuis et al noted the formation of radicals in UV photolysis of nitrite was promoted in the presence of low-charge density cations, such as Cs + . [67] This suggests that the stability of the free radical may likely be influenced by the electrostatics of the surroundings that includes both the uranyl and alkali cations, but additional efforts are needed to confirm this hypothesis.…”

Effect of impurities on radical formation in gibbsite radiolysis

“…5,6 Interestingly, stable H atoms were also observed in the radiolysis of talc. 7 No significant H atom formation was observed in previous radiolytic studies of gibbsite, 8 or its polymorph bayerite (β-Al(OH) 3 ). 9 This difference in H atom observations between boehmite and gibbsite has been discussed in terms of H atom diffusion, 10 but there is a lack of experimental evidence.…”