Reactivity of Zn+aq in high-temperature water radiolysis

Abstract

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 Zn²⁺ ions using pulse radiolysis. The Zn⁺ species can subsequently be oxidized by the radiolytically-produced oxidizing species: ˙OH, H₂O₂ 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 Zn²⁺ 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⁺_aq 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⁺_aq is not observed under the conditions investigated.