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

摘要

The effects of pH and dissolved O2 on the gamma-radiolysis of water were studied at an absorbed dose rate of 2.5 Gys~(-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 steadgamma-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 ~·e_(aq)~-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 ~·e_(aq)~-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 steadgamma-state concentrations of H2O2 and H2 by reacting with"OH and ~·e_(aq)~-,and the impact of oxygen is more noticeable at pH below 8.The steadgamma-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(<->~·e_(aq)~-+H+~)significantly increases [H2O2] and [H2] at the expenses of [~·OH] and [~·e_(aq)~-].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 ~·O_2~-and ~·O_3~-play a major role in determining the steadgamma-state concentration of molecular products H2 and H2O2.Further validation of the water radiolysis model,particularly at higher pHs,is also discussed.