The effect of interfacial mass transfer on steady-state water radiolysis

The effect of aqueous–gas interfacial transfer of volatile species on the γ-radiolysis of water was studied as a function of gas-to-liquid volume ratio at various solution pHs and cover gas compositions. Water samples with cover-gas headspace were irradiated at an absorbed dose rate of 2.5Gys−1 and the radiolytic productions of H2 in the cover gas and H2O2 in the water phase were monitored as a function of irradiation time. The experimental results were compared with computer simulations using a water radiolysis kinetics model that included primary radiolysis, subsequent reactions of the primary radiolysis products in the aqueous phase, and aqueous–gas interfacial transfer of the volatile species H2 and O2. This study shows that the impact of the interfacial mass transfer strongly depends on pH. At pH≤8 (lower than the pKa of •H of 9.6) the effect of aqueous-to-gas phase transfer of the volatile species on the steady-state concentrations of the other radiolysis products is negligible. At higher pHs (≥8), radiolytic production of O2 is slow but considerable, which results in significant increase in the steady-state concentrations of H2 and H2O2 compared to those at lower pHs. Thus, in the presence of headspace, the interfacial transfer of both H2 and O2 becomes significant, and the aqueous concentrations of H2 and O2 are no longer independent of the interfacial surface area and water volume. Nevertheless, the accumulated mass of H2(g) in the headspace is proportional to the aqueous concentration of H2 at all pHs, and the gaseous concentration of H2 in the headspace can be used to infer the aqueous concentration of H2.