Uranium reactions with hydrogen peroxide studied by EPR–spin trapping with DMPO†

Abstract

Uranyl nitrate [UO₂(NO₃)₂] (1.0 × 10^–3M) is reacted with hydrogen peroxide (H₂O₂, 5 × 10^–3M) in the presence of the spin trap 5,5-dimethyl-4,5-dihydro-3H-pyrrole N-oxide (DMPO, 5 × 10^–2M) in acidic solutions. The reaction generates a 1∶2∶2∶1 quartet with hyperfine coupling constants, a_N = a_H^β = 1.50 mT. These values are consistent with reported values for the DMPO–OH spin adduct. It is possible that the uranous ion (UO²⁺), which corresponds to U^IV, generates hydroxyl radicals (˙OH) to give the observed DMPO–OH. This is suggested by two observations: (1) the intensity of the EPR spectrum is dependent on pH, reaching a maximum at pH = 0.6, which is consistent with the formation of UO²⁺ from UO₂²⁺ at lower pH values; and (2) the uranyl ion (UO₂²⁺) corresponds to U^VI and cannot be further oxidized. To determine whether ˙OH radicals generate DMPO–OH, the reaction is carried out in the presence of varying concentrations of ethanol (EtOH). The results indicate that at a low ratio of [EtOH]/[DMPO] the EPR signal corresponds to DMPO–OH, while at a [EtOH]/[DMPO] ratio ca. 1 the signal is mixed and equally intense for DMPO–OH and the hydroxyethyl adduct to DMPO (DMPO–EtOH). When the [EtOH]/[DMPO] ratio is ca. 10 the EPR signal is mainly that of the DMPO–EtOH adduct. This confirms that uranium reacts with H₂O₂ generating hydroxyl radicals.