Photocatalytic degradation of sulfonamides by Bi2O3-TiO2/PAC ternary composite: Mechanism, degradation pathway

The photocatalytic degradation of sulfamethoxazole (SMX), sulfadiazine (SDZ) and sulfamethazine (SMT) by Bi 2 O 3 -TiO 2 /PAC under solar light irradiation was studied. The results exhibited that Bi 2 O 3 -TiO 2 /PAC composite prepared by two-stage calcination method contained of rutile and anatase mixed crystal phases, and it had visible light response capability. In addition, Bi 2 O 3 -TiO 2 /PAC had a certain adsorption capacity for SMX, SDZ and SMT, and its saturated adsorption mass was 30.88, 11.28 and 26.24 mg/g, respectively. Compared to photolysis and adsorption process, the photocatalytic degradation efficiencies of the three sulfonamides were significantly improved, and more than 99 % of photocatalytic degradation efficiency was achieved. The EPR measurements showed that both hydroxyl radicals (·OH) and superoxide radicals (·O 2 − ) were produced in the photocatalytic process. However, trapping experiments confirmed that photo-generated hole (h + ) played a lead role in the photocatalytic degradation of SMX. Contrarily, the ·O 2 - was the main oxidative species for SDZ and SMT degradation. Furthermore, the effects of initial pH of solution, sodium bicarbonate, humic acid and natural water (lake and river water) on the photocatalytic degradation were investigated. The photocatalytic degradation pathways of Bi 2 O 3 -TiO 2 /PAC for sulfonamides were discussed. There were differences degradation pathways of the SDZ, SMT and SMX by Bi 2 O 3 -TiO 2 /PAC mainly reflected in: Hydroxylation of benzene ring, extrusion of SO 2 . While, similar pathways existed in the process of sulfanilamide degradation, such as the breaking of S N bond and the cracking of N-containing benzene ring.