Adsorption effects of electron scavengers and inorganic ions on catalysts for catalytic oxidation of sulfamethoxazole in radiation treatment

Abstract

This study aimed to investigate adsorption effects of electron scavengers (H2O2 and S2O82−) on oxidation performance for mineralization of sulfamethoxazole (SMX) in radiation treatment using catalysts (Al2O3, TiO2). Hydrogen peroxide (H2O2, 1 mM) as an electron scavenger showed weak adsorption onto catalysts (0.012 mmol g−1-Al2O3 and 0.004 mmol g−1-TiO2, respectively), leading to an increase in TOC removal efficiency of SMX within the absorbed dose of 30 kGy by 12.3% with Al2O3 and by 8.0% with TiO2. The weak adsorption of H2O2 onto the catalyst allowed it to act as an electron scavenger, promoting indirect decomposition reactions. However, high adsorption of S2O82− (1 mM) onto Al2O3 (0.266 mmol g−1-Al2O3) showed a decrease in TOC removal efficiency of SMX from 76.2% to 30.2% within the absorbed dose of 30 kGy. The high adsorption of S2O82− onto the catalyst inhibited direct decomposition reaction by reducing adsorption of SMX on catalysts. TOC removal efficiency for Al2O3 without electron scavengers in an acidic condition was higher than that in a neutral or alkaline condition. However, TOC removal efficiency for Al2O3 with S2O82− was higher in a neutral condition than in other pH conditions. This indicates that the pH of a solution plays a critical role in the catalytic oxidation performance by determining surface charges of catalysts and yield of reactive radicals produced from water radiolysis. In the radiocatalytic system, H2O2 enhances the oxidation performance of catalysts (Al2O3 and TiO2) over a wide pH range (3–11). Meanwhile, S2O82− is not suitable with Al2O3 in acidic conditions because of its strong adsorption onto Al2O3 in this study.