Visible light driven heterojunction photocatalysts for removal of organic contaminants and bacterial inactivation

• Main Author: Zhu, Wenyu
• Other Authors: Zhou Yan
• Format: Theses and Dissertations
• Language: English
• Published: 2017
• Subjects: DRNTU::Engineering::Environmental engineering::Environmental pollution
• Online Access: http://hdl.handle.net/10356/73033
• Institution: Nanyang Technological University

The presence of various antibiotics and their residues in aquatic systems has become one of the emerging environmental issues. Heterogeneous photocatalysis, one of advanced oxidation process driven by solar energy, has been regarded as a promising technology to eliminate antibiotics as well as other refractory organic contaminants. Under proper light irradiation, different reactive oxygen species such as hydroxyl, superoxide and hydroperoxyl radicals can be generated which will subsequently lead to the degradation of organic matters. Currently, research focused on applying heterogeneous photocatalysis to treat antibiotics is still rare, and most studies used TiO2 with a wide band gap as candidate. Therefore, the major objective of this study is to explore other kinds of semiconductor-based heterojunctions to photodegrade recalcitrant organic compounds. In the first part of this study, 2D BiOCl nanodiscs were prepared via one-step solvothermal method. To enhance its photocatalytic activity, Ag nanoparticles (Ag NPs) were deposited on the surface of BiOCl to form BiOCl-Ag heterostructured composites as photocatalysts to eliminate sulfanilamide (SAM) which is the precursor of sulfonamide group under visible light irradiation. The photo-degradation efficiency of SAM was improved by 71.7% using BiOCl-Ag compared to pristine BiOCl, indicating that Ag deposition is effective to augment the photocatalytic activity of BiOCl. Furthermore, the existence of Ag NPs also endowed BiOCl-Ag composites significant and enhanced antimicrobial ability as verified by the disinfection effect on Escherichia Coli (E. coli) and Bacillus subtilis (B. subtilis). In the second part of this study, by optimizing the synthesis conditions, WO3 nanoplates with uniform size were obtained by a facile one step hydrothermal method while different amounts of Ag NPs were loaded onto WO3 nanoplates using a photo-reduction method to generate WO3-Ag composites. The performance of pure WO3 and WO3-Ag composites as visible light driven photocatalysts was estimated by degradation of SAM and inactivation of E. coli and B. subtilis. The operation conditions were carefully optimized and the results illustrated that WO3-Ag composites showed remarkably better photocatalytic efficiency than that of pure WO3 while their performance was found to be closely related to the amount of Ag deposited. In the third part of this study, two kinds of semiconductors, WO3 and g-C3N4, were combined together to construct different WO3-g-C3N4 heterojuntions via a facile hydrothermal method. Sulfamethoxazole (SMX), one of the most commonly utilized antibiotics from sulfonamide class, was adopted as a model organic contaminant to evaluate the degradation performance of as-prepared WO3-g-C3N4 composites under visible light illumination. In particular, SMX removal efficiency can be raised up largely to 91.7% when optimized WO3-g-C3N4 composites with initially 8 mg g-C3N4 being applied at a dosage of 1.0 g L-1. Such enhanced photocatalytic activity can be ascribed to the formation of direct solid-state Z-scheme heterojunctions between g-C3N4 and WO3, which could improve the photogenerated electron-hole pairs separation efficiency as well as the redox capability. At the same time, the larger surface area and extended visible light absorption capability can also contribute to the enhanced efficiency. In the last phase of this study, efforts were made to fabricate reduced graphene oxide-WO3 (RGO-WO3) composites to explore their potential degradation feasibility towards SMX as visible light driven photocatalsyts. Significantly improved SMX degradation (> 98%) was obtained by two as prepared RGO-WO3 composites. Moreover, these composites presented good stability and reusability making them promising candidates towards elimination of SMX. Furthermore, the photocatalytic intermediates of SMX were analyzed and the photodegradation pathway was proposed accordingly. Overall, the major contribution of this study is the successful construction of various highly efficient visible light driven heterogeneous composites; in particular, WO3 based composites were the most efficient. Moreover, the outstanding performance for sulfonamides degradation and excellent stability make them promising candidates in practical application to remove refractory organic pollutants from water systems.