Photocatalytic degradation of norfloxacin on different TiO2−X polymorphs under visible light in water

Abstract

Reduced TiO₂ (TiO_2−X) materials with different crystallographic structures were prepared and characterized. Cat.I-A, Cat.II-R, Cat.III-B are TiO_2−X with anatase, rutile and brookite structures, respectively, while the Cat.IV-A&R series are materials with anatase and rutile phases mixed in different ratios. All samples exhibit efficient photocatalytic activity for the degradation of norfloxacin (Nor) under visible light, and Cat.IV-A&R-4 is the best among the samples studied. Our results show that the photocatalytic activity is governed by different factors such as the specific surface area of the catalysts as well as the concentrations of Ti³⁺ and the density of oxygen vacancies in the photocatalytic materials. Mechanistic study of the materials demonstrates that photohole (h⁺) transfer contributes more to Nor degradation than reaction with ˙OH radicals and the other reactive oxygen species (ROSs). Intermediate species were characterized by HPLC-TOF-HRMS and HPLC-MS/MS to construct a general transformation mechanism of Nor on the family of TiO_2−X under visible light. The study shows that Nor adsorption onto TiO_2−X occurs by its heteroatoms followed by cleavage of its piperazine ring and hydroxylation of its quinolone ring under the attack of h⁺ and ˙OH radicals. The study could assist the further search for efficient photocatalytic materials for the degradation of organic pollutants.