Kinetic study of photoaccelerated U(IV)–U(VI) electron exchange reaction in hydrochloric acid

Abstract

A kinetic study of the photoaccelerated U(IV)–U(VI) electron exchange reaction has been carried out in hydrochloric acid. The variation of the quantum yield as a function of wavelength of the incident light indicates that the exchange reaction is accelerated only by the light absorbed by uranium(VI). The rates and quantum yields were determined as a function of uranium(IV) and uranium(VI) concentrations, light intensity and temperature under light irradiation of 365 and 436 nm, which correspond to the main absorption bands of uranium(VI). For light absorption by the weak spin-forbidden visible band of uranium(VI), the quantum yield of the exchange reaction is larger than that for light absorption by the spin-allowed intense UV band. The reaction appears to be complex and the quantum yield for both visible and UV bands follows virtually the same kinetic rate law. The proposed mechanism involves formation of uranium(V) from uranium(IV) and excited uranium(VI), a one electron transfer reaction between uranium(V) and uranium(IV), and uranium(V) and uranium(VI) and the disappearance of uranium(V) through the disproportionation to uranium(IV) and uranium(VI). The respective quantum yields at 365 and 436 nm had essentially the same activation energy for the overall exchange reaction; 14.8±0.4 and 15.6±0.8 kcal/mole, respectively. The photoacceleration mechanism seems to involve the reaction from the lowest triplet excited state of uranium(VI).