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

This study aimed to investigate adsorption effects of electron scavengers (H ₂ O ₂ and S ₂ O ₈ ^2- ) on oxidation performance for mineralization of sulfamethoxazole (SMX) in radiation treatment using catalysts (Al ₂ O ₃ , TiO ₂ ). Hydrogen peroxide (H ₂ O ₂ , 1 mM) as an electron scavenger showed weak adsorption onto catalysts (0.012 mmol g ^-1 -Al ₂ O ₃ and 0.004 mmol g ^-1 -TiO ₂ , respectively), leading to an increase in TOC removal efficiency of SMX within the absorbed dose of 30 kGy by 12.3% with Al ₂ O ₃ and by 8.0% with TiO ₂ . The weak adsorption of H ₂ O ₂ onto the catalyst allowed it to act as an electron scavenger, promoting indirect decomposition reactions. However, high adsorption of S ₂ O ₈ ^2- (1 mM) onto Al ₂ O ₃ (0.266 mmol g ^-1 -Al ₂ O ₃ ) 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 S ₂ O ₈ ^2- onto the catalyst inhibited direct decomposition reaction by reducing adsorption of SMX on catalysts. TOC removal efficiency for Al ₂ O ₃ without electron scavengers in an acidic condition was higher than that in a neutral or alkaline condition. However, TOC removal efficiency for Al ₂ O ₃ with S ₂ O ₈ ^2- 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, H ₂ O ₂ enhances the oxidation performance of catalysts (Al ₂ O ₃ and TiO ₂ ) over a wide pH range (3-11). Meanwhile, S ₂ O ₈ ^2- is not suitable with Al ₂ O ₃ in acidic conditions because of its strong adsorption onto Al ₂ O ₃ in this study.
Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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