Degradation of methylene blue by ozone oxidation activated by N and S co-doped biochar supported Co–Al layered double hydroxides

Abstract

Dye pollution has caused great harm to the ecological environment. In this paper, N and S co-doped biochar (K-NSBC) was obtained by a calcination–alkali activation method, then Co/Al layered double hydroxide (Co/Al-LDH) was anchored on it by a co-precipitation method to activate ozone (O₃) to degrade methylene blue (MB). When the O₃ concentration was 3.12 mg L⁻¹, catalyst dosage was 0.6 g L⁻¹, the MB concentration was 25 mg L⁻¹, and pH was 6.8, the degradation efficiency of MB reached 99.65%. The degradation efficiency still remained at 92.75% even after Co/Al-LDH@K-NSBC was recycled five times. The results of material characterization and quenching experiments show that the presence of oxygen vacancies (O_v) on the metal surface not only accelerates the exchange of oxygen and the generation of active free radicals, but also promotes the conversion of Co³⁺/Co²⁺ and the activation of O₃. Pyridine N, graphite N and thiophene S provide electron-rich groups, which promote the activation of O₃ to produce reactive oxygen species (ROS). Among them, ¹O₂ is the main ROS leading to the degradation of MB, and its intermediates ˙OH and ˙O₂⁻ also participate in the oxidation of MB. This study is expected to provide new ideas and directions for the design of highly efficient and stable ozonation activators and industrialization of non-phase catalytic ozonation degradation of dye wastewater.