Enhancement of TOC removal efficiency of sulfamethoxazole using catalysts in the radiation treatment: Effects of band structure and electrical properties of radiocatalysts

Photocatalysts in AOP have been used for the decomposition of recalcitrant organic pollutants. However, several technical issues associated with energy band gap of catalysts still limit the practical application of photocatalysts with sunlight and UV. Some catalysts with large energy band gap have been successfully used in the radiocatalytic systems using gamma radiation and electron beam. However, there has been little systematic studies for band structure and electrical properties of an appropriate radiocatalyst in the radiation treatment. Kinetic studies were conducted to evaluate the properties (conduction band, valence band, energy band gap, electron mobility, and stability) of catalysts suitable for the radiocatalytic system using high energy sources and the radiocatalytic oxidation performance for sulfamethoxazole (SMX). The lower the CB potential and/or the higher the VB potential was, the higher the oxidation performance of a catalyst was. Unlike the photocatalytic process (activating the catalysts with energy band gaps lower than 6.2 eV), the radiation system using a gamma ray could activate the catalyst with a comparatively large energy band gap (<1,240 keV). TOC removal efficiency for SMX (30 mg/L) with Al₂O₃ (2 g/L) as a radiocatalyst was enhanced up to 83.9 % at an absorbed dose of 5 kGy, compared to the control (radiation treatment without catalysts) showing 6.6 % TOC removal efficiency at the same condition. The stabilities of metal oxide catalysts were much higher (low self-degradation of 0.0006–0.01 % at 50 kGy) than those (0.07–0.44 % at 50 kGy) of metal sulfide catalysts. Since S²⁻ is less electronegative than O²⁻, the VB holes of metal sulfide catalyst are more prone to self-degradation. The radiation technology has a wide range of selection for catalysts, and can be an effective alternative-treatment system for the mineralization of recalcitrant organic chemicals.