Reactivity of Zn in high-temperature water radiolysis.

Reactivity of transients involving Zn in high-temperature water radiolysis has been studied in the temperature range of 25-300 °C. The reduced monovalent zinc species were generated from an electron transfer process between the hydrated electron and Zn ions using pulse radiolysis. The Zn species can subsequently be oxidized by the radiolytically-produced oxidizing species: ˙OH, HO and ˙H. We find that the absorption of monovalent zinc is very sensitive to the pH of the medium. An absorption maximum at 306-311 nm is most pronounced at pH 7 and the signal then decreases in acidic media where the reducing electrons are competitively captured by protons. At pH values higher than 7, hydroxo-forms of Zn are created and the maximum of the absorption signal begins to shift to the red spectral region. We find that the optical spectrum of Zn cannot be fully explained in terms of a charge-transfer to solvent (CTTS) process, which was previously proposed. Reaction rates of most of the recombination reactions investigated follow the empirical Arrhenius relationship at temperatures up to 200 °C and have been determined at higher temperatures for the first time. A bimolecular disproportionation reaction of Zn is not observed under the conditions investigated.