ZnO Precursor’s ability to catalyze formation of reactive oxygen species to degrade aqueous organic pollutants

Abstract

Advanced oxidative processes (AOP) could mitigate emerging and persistent pollutants in drinking water. Yet there is a dearth of economical materials to generate the reactive oxygen species (ROS) that are responsible for pollutant degradation in AOPs. In this work, we design a zinc hydroxychloride monohydrate (ZHCM) silicon dioxide photocatalytic nanocomposite to generate ROS for non-selective degradation of drinking water contaminants. ZHCM is an intermediate phase in the sol–gel synthesis of ZnO-SiO2-Ag nanocomposites containing 63–100 % ZnO. Typically, ZHCM is calcinated around 500 °C to crystallize into ZnO, yet we find that this calcination step may decrease photocatalytic activity. ZHCM exhibited higher methylene blue degradation efficiency through both catalysis (31 % after 4 h) and photocatalysis (98 % after 4 h) compared to the ZnO composites (∼0% and 78 % after 4 h). ZHCM had the highest photodegradation efficiency for ciprofloxacin and caffeine. The increased catalytic and photocatalytic activity of these model pollutants is attributed to the generation of two ROS: superoxide anions in UV light and peroxide anions in the absence of light. These ROS were identified using a new modified biological assay measuring the chemiluminescence of luminol in the presence of horseradish peroxidase. Results are further verified via scavenger quenching tests. The ZHCM and ZnO-SiO2-Ag are characterized for their antimicrobial activity measured by E. coli growth inhibition zones, band gaps, crystalline domains, and surface areas. This work demonstrates the potential to synthesize, at lower temperatures, a photocatalyst with enhanced ROS generation and reduced UV dependency.