Radiation and Radical Chemistry of NO3-, HNO3, and Dialkylphosphoric Acids in Room-Temperature Ionic Liquids

Hydrophobic room-temperature ionic liquids (ILs) are considered as possible replacements for molecular diluents for nuclear separations, as well as the basis of new separations processes. Such applications may put the solvents both in high radiation fields and in contact with aqueous raffinate containing 1-6 M HNO3. In this study, we address the effect of the extracted nitrate and nitric acid on the radiation chemistry of hydrophobic ILs composed of 1-alkyl-3-methylimidazolium cations (and closely related systems). We demonstrate that the nitrate anion competes with the solvent cation as an electron scavenger, with most of the primary radical species converted to NO3 center dot 2- and NO2 center dot that initiate a complex sequence of radical reactions. In hydrophobic ILs equilibrated with 3 M HNO3, nearly all electrons released by the ionizing radiation are converted to NO2 center dot. While the reductive pathway is strongly affected by the nitrate and there is also some N-O bond scission via direct excitation, the extent of interference with the oxidative pathway is relatively small; the cation damage is not dramatically affected by the presence of nitrate as most of the detrimental radiolytic products are generated via the oxidative pathway. These results are contrasted with the behavior of dialkylphosphoric acids (a large class of extraction agents for trivalent metal ions). We demonstrate that IL solvents protect these dialkylphosphoric acids against radiation-induced dealkylation.