Photocatalytic degradation of 2,4-Dichlorophenoxyacetic acid using ZnO in thin film: Effect of sintering temperature and irradiation source
摘要
2,4-Dichlorophenoxyacetic acid degradation is crucial because it is commonly used as an herbicide in the agro-industry and gardens, leading to adverse health and environmental consequences. Even when ZnO is a potential photocatalyst in water pollution, it is generally used in powder form, requiring a subsequent washing process to get it back, which is a limiting factor for its large-scale application. In this work, ZnO films obtained by a simple and economical technique, such as sol–gel and sintered at various temperatures (Ts), were tested as photocatalysts for the degradation of this herbicide. Films were sintered at Ts of 200 °C, 300 °C, 400 °C, and 450 °C; the effect of this parameter on the 2,4-D photocatalytic degradation in the aqueous phase was evaluated for the first time and correlated with their morphological, structural, and optical properties. Two irradiation sources with similar powers were used to evaluate the films’ photocatalytic activity: germicide lamp and solar irradiation. The results show that ZnO crystallite size and photocatalytic reaction constant (k) increase as Ts rise. On the other hand, ZnO band gap energy decreases from 3.35 to 3.26 eV as Ts rises; the highest energy value is associated with the quantum confinement effect. The highest photocatalytic response is attributed to better crystalline quality and lower lattice strain in the films. The herbicide is degraded following a zero-order reaction kinetics with maximum reaction constants, k = 6.0 × 10–7 mol L−1 min−1 with germicide lamp and k = 3.3 × 10–7 mol L−1 min−1 with solar irradiation, both obtained using ZnO films sintered at the highest Ts. Parameters involved in the photodegradation were carefully measured and reported. The lower k value obtained using solar light is attributed to the lower ZnO optical absorption coefficients (at higher energies to its bandgap) than it obtained with UV light, leading to the generation of electron–hole carriers farther from the photocatalyst’s surface, decreasing the probability that they participate in the pollutant’s photodegradation.