Sonochemical redox reactions of Pu(iii) and Pu(iv) in aqueous nitric solutions

Abstract

The behavior of Pu(iv) and Pu(iii) was investigated in aqueous nitric solutions under ultrasound irradiation (Ar, 20 kHz). In the absence of anti-nitrous reagents, ultrasound has no effect on Pu(iv), while Pu(iii) can be rapidly oxidized to Pu(iv) due to the autocatalytic formation of HNO(2) induced by HNO(3) sonolysis. In the presence of anti-nitrous reagents (sulfamic acid or hydrazinium nitrate), Pu(iv) can be sonochemically reduced to Pu(iii). The reduction follows a first order reaction law and leads to a steady state where Pu(iv) and Pu(iii) coexist in solution. The reduction process is attributed to the sonochemical generation of H(2)O(2) in solution. The kinetics attributed to the reduction of Pu(iv) are however higher than those related to the formation of H(2)O(2) which, after several hypotheses, is explained by the sonochemical erosion of the titanium-based sonotrode. Titanium particles thereby generated can be solubilized under ultrasound and generate Ti(iii) as an intermediate species, a strong reducing agent able to react with Pu(iv).

Effect of ultrasonic frequency on H2O2 sonochemical formation rate in aqueous nitric acid solutions in the presence of oxygen

“…This observation is in agreement with the literature and results from the reaction of OH Å radicals with nitrate ions and the subsequent hydrolysis of the generated nitrate radical as discussed above (reactions (13) and (14)) [16,17,21] …”

“…Several authors have reported higher H 2 O 2 yield for Ar/(20-30 vol.%)O 2 gas mixtures. This was attributed to much more significant generation of both OH Å and HO 2 Å radicals [12,21,22]. According to these observations, one can assume that the sonication of a correctly tuned system combining nitric acid (in the presence of anti-nitrous reagent) and Ar/O 2 atmosphere may enhance H 2 O 2 yields.…”

“…However, in the presence of anti-nitrous reagents such as hydrazinium nitrate or sulfamic acid which scavenge HNO 2 by the reaction (11) and (12), H 2 O 2 can accumulate at a higher rate than that observed in pure water [20,21]. This phenomenon is attributed to the reaction of nitrate ions with OH Å radicals forming NO 3 Å radical species, which, after hydrolysis, yield hydrogen peroxide in solution (reactions (13) and (14)).…”

“…This reagent, having a low reactivity towards OH Å , has been already successfully used in nitric medium as HNO 2 scavenger [21]. Fig.…”

Ultrasound-assisted reductive dissolution of CeO2and PuO2in the presence of Ti particles

“…33,34 Recently, it was shown that 20 kHz ultrasound is a promising alternative for the in situ control of the Pu redox state in nitric solutions. 34,35 In heterogeneous systems, cavitation proceeds with the generation of shock waves and microjets which may significantly affect extended surfaces by the fragmentation of particles, erosion of surfaces, increase of mass transfer, and thickening of diffusion layers. 33,36 Significant improvements under ultrasonic treatment were reported for the oxidative dissolution of uranium carbide and metallic plutonium in nitric solutions.…”