Coupled heterogeneous photocatalysis using a P-TiO2-αFe2O3 catalyst and K2S2O8 for the efficient degradation of a sulfonamide mixture

Phosphorous-doped Ti-Fe mixed oxide (P-TiO2-αFe2O3) catalysts were prepared by the microwave-assisted sol-gel route and characterized using XRD, SEM, N2 physisorption, UV–vis diffuse reflectance, FTIR, and XPS. P-TiO2-αFe2O3 was evaluated during the degradation of a sulfonamide mixture (5 mg/L, each) under visible light. The photocatalytic process was optimized with a face-centered central composite design. Under optimal conditions (0.5 wt% of αFe2O3, pH 10, and 0.75 g/L of catalyst loading), the sulfate radical advanced oxidation process was carried out using 5 mM K2S2O8 (PS). P doping shifted the light absorption of P-TiO2-αFe2O3 in the visible light range owing to substitutional doping, while the coupling of P-TiO2 with α-Fe2O3 enhanced the absorption in the visible range, which resulted in an increase in the lifetime of the charge carriers and in a superior photoactivity of the P-TiO2-αFe2O3 catalyst in comparison to that of TiO2. The mineralization yield of the sulfonamides (SNs) mixture was enhanced in the presence of an electron acceptor (SO4− ), allowing nearly 69 % within 300 min with the P-TiO2-αFe2O3/PS system, while P-TiO2-αFe2O3 and K2S2O8 oxidation achieved only 27 % and 21 %, respectively. The biodegradability index was 0.48 using the P-TiO2-αFe2O3/PS system, indicating a less toxic effluent than the original compounds. Recycling tests demonstrated that P-TiO2-αFe2O3 exhibits good stability in activating PS for SNs degradation during three cycles. Two main intermediates (pyrimidine and hydroquinone) and their hydroxylated re-arrangements were detected during the degradation of the SNs by the coupled process. Oxalic, oxamic, sulfonic, and acetic acids were also identified as by-products from the degradation of the SNs.