Two-stage calcination composite of Bi2O3-TiO2 supported on powdered activated carbon for enhanced degradation of sulfamethazine under solar irradiation

In a previous study, we used a sol-hydrothermal method with a one-stage calcination in an air atmosphere at 400 degrees C to prepare composites of Bi2O3-TiO2 supported on powdered activated carbon (Bi-Ti/PAC-1). However, the photocatalytic efficiency of those composites was relatively low due to their high band gap and the single crystal phase of their TiO2. To improve the photocatalytic efficiency, we developed a two-stage calcination method with a first calcination in air at 300 degrees C followed by a second calcination in a N-2 atmosphere (at 500, 600, or 700 degrees C) to form the Bi2O3-TiO2 composites (Bi-Ti/PAC-2). Compared with Bi-Ti/PAC-1, the photocatalytic degradation efficiency of sulfamethazine (SMT) by Bi-Ti/PAC-2-700 increased by 32 % under solar light irradiation. Bi-Ti/ PAC-2-700 contained two TiO2 crystal phases, ruffle and anatase, and its band gap was 2.58 eV, far less than that of Bi-Ti/PAC-1. Electron paramagnetic resonance measurements and trapping experiments showed that the superoxide radical, hole, and hydroxyl radical each played an important role in the degradation of SMT, among which the superoxide radical predominated. The degradation pathway of SMT included four processes: oxidation of hydroxyl radical, SO2 extrusion, N-C and S-N bond cleavage. We also determined that solution pH, humic acid, and inorganic ions had varying effects on the photocatalytic degradation capability of Bi-Ti/PAC-2-700. We measured the efficiency of Bi-Ti/PAC-2-700 for SMT degradation in natural river and lake water and found it to be > 85 %, sufficient for practical applications. This study provides a green and efficient photocatalyst that can be applied to water treatment processes.