A combined experimental and model analysis on the effect of pH and O2(aq) on γ-radiolytically produced H2 and H2O2

The effects of pH and dissolved O2 on the γ-radiolysis of water were studied at an absorbed dose rate of 2.5 Gy s−1. Argon- or air-saturated water with no headspace was irradiated and the aqueous samples were analyzed for molecular radiolysis products (H2 and H2O2) as a function of irradiation time. The experimental results were compared with computer simulation results using a comprehensive water-radiolysis kinetic model, consisting of the primary radiolysis production, subsequent reactions and related acid–base equilibria. Both the experimental and computer model results were discussed based on the steady-state kinetic analysis of smaller reaction sets consisting of key production and removal reactions. While the main production path for a water decomposition product is the primary radiolysis, the main removal path varies. For H2O2 the main removal path is the reactions with eaq− and OH, whereas for H2 it is the reaction with OH. As a result, the presence of a dissolved species, or a change in chemical environment, affects the concentrations of H2O2 and H2 through interaction with radicals eaq− and OH. Over a wide range of conditions, there exist quantitative but simple relationships between the radical and the molecular product concentrations. The experimental and model analyses show that dissolved oxygen increases the steady-state concentrations of H2O2 and H2 by reacting with OH and eaq–, and the impact of oxygen is more noticeable at pH below 8. The steady-state concentrations of water decomposition products are nearly independent of pH in the range 5–8. However, raising pH above the pKa value of the acid–base equilibrium of H (⇆eaq−+H+) significantly increases [H2O2] and [H2] at the expenses of [OH] and [eaq–]. At pH >10, the radiolytical production of O2 becomes significant, but at a finite rate. This considerably increases the time for the irradiated system to reach a steady state, and is responsible for different impacts on [H2O2] and [H2] due to radically produced O2, compared to impacts due to initially dissolved O2. Model sensitivity analysis has shown that at higher pHs (pH >10) transient species such as O2− and O3− play a major role in determining the steady-state concentration of molecular products H2 and H2O2. Further validation of the water radiolysis model, particularly at higher pHs, is also discussed.