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

The effect of aqueous-gas interfacial transfer of volatile species on the gamma-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 2.5 Gy s(-1) and the radiolytic productions of H-2 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 H-2 and O-2. This study shows that the impact of the interfacial mass transfer strongly depends on pH. At pH <= 8 (lower than the pKa of center dot 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 O-2 is slow but considerable, which results in significant increase in the steady-state concentrations of H-2 and H2O2 compared to those at lower pHs. Thus, in the presence of headspace, the interfacial transfer of both H-2 and O-2 becomes significant, and the aqueous concentrations of H-2 and O-2 are no longer independent of the interfacial surface area and water volume. Nevertheless, the accumulated mass of H-2(g) in the headspace is proportional to the aqueous concentration of H-2 at all pHs, and the gaseous concentration of H-2 in the headspace can be used to infer the aqueous concentration of H-2. (C) 2010 Elsevier Ltd. All rights reserved.