Hydroxyl radical, sulfate radical and nitrate radical reactivity towards crown ethers in aqueous solutions

Abstract

Reaction rate constants of crown ethers (12-crown-4, 15-crown-5, 18-crown-6) and their analogs 1,4-dioxane (6C2) with some important oxidative radicals, hydroxyl radical (OH), sulfate radical (SO4−) and nitrate radical (NO3), were determined in various aqueous solutions by pulse radiolysis and laser photolysis techniques. The reaction rate constants for 6C2 and crown ethers with OH and SO4− increase with the number of hydrogen atoms in the ethers, indicating that the hydrogen-atom abstraction is a dominant reaction between crown ethers and these two radicals. The presence of cations in solution has negligible effect on the rate constants of crown ether towards OH and SO4−. However, for the NO3, the rate constants are not proportional to the number of hydrogen atoms in ethers, and 12-crown-4 (12C4) is the most reactive compared with other crown ethers. Except 12C4 and 6C2, the cations in the aqueous solution affect the reactivities of 15-crown-5 (15C5) and 18-crown-6 (18C6). The cations with high binding stability for crown ether would improve the reactivity of 15C5. For the studied crown ethers, the reaction rate constants of these oxidative radicals have the order OH>SO4−>NO3. Furthermore, the formation of radicals after the reaction of crown ethers with sulfate radical could be observed in the range of 260–280nm using laser photolysis and pulse radiolysis. This is the first report on the kinetic behavior of crown ethers with NO3, and it would be helpful for the understanding of stability of crown ethers in the processing of spent nuclear fuel.