Decoding the Three-Pronged Mechanism of NO₃^• Radical Formation in HNO₃ Solutions at 22 and 80 °C Using Picosecond Pulse Radiolysis

Abstract

With nitric acid (HNO) being at the core of nuclear technology through actinides separation and extraction processes, achieving a complete characterization of the complex processes involving concentrated HNO solutions under ionizing radiation equates bringing efficiency and safety into their operation. In this work, the three mechanisms contributing to the formation of nitrate radicals (NO) in concentrated nitric acid were investigated by measuring the radiolytic yield of NO in HNO solutions (0.5-23.5 M) at room (22.5 °C) and elevated (80 °C) temperatures on time scales spanning from picosecond to microsecond by pulse radiolysis measurements. We conclude that the formation yield of NO, just after the 7 ps electron pulse, is due to the direct effect and to the ultrafast electron transfer reaction between NO and the water cation radical, HO. The absolute formation yield of NO radicals due to the direct effect, G, is found to be (3.4 ± 0.1) × 10 mol·J, irrespective of the concentration and temperature. On longer time scales, >1 ns, an additional contribution to NO formation from the reaction between OH radicals and undissociated HNO is observed. The rate constant of this reaction, which is activation-controlled, was determined to be (5.3 ± 0.2) × 10 M·s for 22.5 °C, reaching a value of (1.1 ± 0.2) × 10 M·s at 80 °C.

Ultrafast Processes Occurring in Radiolysis of Highly Concentrated Solutions of Nucleosides/Tides

“…Recent wastewater treatment via electron-beam techniques requires extensive studies in concentrated aqueous solutions. To elucidate these processes, it is necessary to know the mechanisms involved in the radiation-mediated formation of radicals, and their yields in highly concentrated nitric acid solutions, (e.g., nitrate radical (NO 3 • ), nitrite radical (NO 2 • ) [33,34,35]) as well as some elementary reactions between organics and water radicals in complex systems.…”

“…Applying this above-mentioned technique, we have shown that three distinct processes are involved in the formation of secondary radicals such as phosphate and nitrate radicals [35,41]: (i) reaction of • OH with phosphate anions and undissociated H 3 PO 4 and HNO 3 (indirect effect), (ii) direct ionization of phosphoric acid (direct effect of ionizing radiation on solutes), and (iii) reaction of H 2 PO 4 − and NO 3 − with H 2 O •+ . On the other hand, by observing a decreasing initial yield of e sol − as a function of H 3 O + concentration at a similar timescale, we showed that the radiation-produced excess electrons reacted rapidly with H 3 O + [41].…”

Reactivity and energy level of a localized hole in liquid water

“…Electron transfer may occur when the water cation is formed close to a hole acceptor (i.e. in biological systems 75 or in concentrated solutions 76 ). water molecule at semiconductor-water interfaces would imply overcoming high energy barriers, thus suppressing this mechanism in photocatalytic water splitting (cf.…”

New insight on the simultaneous H2and HNO2production in concentrated HNO3aqueous solutions under alpha radiation

“…Due to the multi-component aspect of spent nuclear fuel, a multidirectional approach is needed, taking into account the alpha, beta and gamma radiation induced degradation. Many studies were dedicated to the radiolytic degradation of HNO 3 soon aer the development of the PUREX process in the '50s, providing a wealth of information on the radiolytic mechanism and yields of H 2 , NO 3 c and NO 2 À /HNO 2 in the g 3,7-25 or b radiolysis 8,16,17,22,[26][27][28][29][30][31][32][33][34][35][36][37] of aqueous nitrate and nitric acid solutions. However, fewer studies have been dedicated to the a radiolysis of these solutions, and the majority of these investigations were performed in the radiolysis of HNO 3 in the presence of radionuclides ( 243 Am, 241 Am, 244 Cm, 210 Po, 240 Pu, 238 Pu).…”