Study of the photocatalytic activity of CdS–ZnS nano-composite in the photodegradation of rifampin in aqueous solution

Abstract

The elimination of rifampin (RN) as a pollutant from an aqueous solution was performed via the photodegradation process by the coupled CdS–ZnS system under visible light irradiation. The catalyst was briefly characterized by Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), diffuse reflectance spectroscopy (DRS), and transmission electron microscope (TEM) techniques. The cubic ZnS structure and the hexagonal CdS structure with the crystallite size of 10 nm were detected from the XRD patterns. From the DRS results, the bandgap energies of 2.3, 3.9, and 2.4 eV were obtained for CdS, ZnS, and CdS–ZnS, respectively. The pH-values of 5.2 (5.25), 5.8 (5.8), and 5.9 (5.88) were obtained for the pHpzc of CdS, ZnS, and CdS–ZnS by two methods, respectively. Increased photocatalytic activity of the coupled ZnS–CdS system was achieved compared to the single systems and when the moles of CdS is 6 times higher than the ZnS component, the best activity was obtained. The optimal degradation conditions were the catalyst dose: 0.5 g/L, CRN: 8 mg/L, pH: 2, and irradiation time: 120 min. The rate constants of RN photodegradation and mineralization (from the chemical oxygen demand (COD) results) were 0.028 and 0.019 min−1, respectively. The mechanism pathway was similar to the type (II)-heterojunction system in which the accumulated holes in VB-ZnS, the accumulated electrons in CB-CdS, and hydroxyl radicals are more important reactive species in the photodegradation of RN molecules. Further, this mechanism is useful for hydrogen production by both the photogenerated electrons in CB-CdS and CB-ZnS positions. Another mechanism that can illustrate the photodegradation process is similar to the direct Z-scheme mechanism pathway in which the accumulated electrons in the CB-ZnS, the accumulated holes in the VB-CdS, and the superoxide radicals are more important active centers for the degradation of RN molecules. Also, the accumulated holes in the CB-CdS position are another active center to oxidize RN molecules. Besides, the accumulated holes in the CB-ZnS position are more suitable for hydrogen production. The photogenerated holes in the CB-CdS position are also capable to oxidize water molecules for oxygen evolution.