Efficient degradation of organic and inorganic contaminants in water using a ZnO-based heterojunction photocatalyst

Access to clean water is essential for sustaining life, yet organic and inorganic contaminants pose significant risks to public health and ecosystems. In this study, a ZnO-BiOI heterojunction photocatalyst was synthesized and characterized for the degradation of methyl orange (MO), 2-chlorobiphenyl (2CBP), cyanide, and thiocyanate. Structural analyses (SEM-EDS, XRD, optical, and thermal techniques) revealed a highly porous, crystalline morphology. Under optimal conditions, the photocatalyst achieved complete removal of thiocyanate, cyanide, and MO within 30, 35, and 120 minutes, respectively, and 60% removal of 2CBP in 150 minutes. Photocatalytic performance was influenced by catalyst loading and initial contaminant concentration. For example, MO degradation increased from 59% to 100% as catalyst mass increased from 2.5 mg to 40 mg within 120 minutes. However, excess catalyst led to agglomeration and turbidity, reducing efficiency. Higher contaminant concentrations also diminished degradation due to active site saturation and light-screening effects. Kinetic studies showed pseudo-first-order behaviour, with half-lives of 12, 141, and 4 minutes for MO, 2CBP, and thiocyanate, respectively. This study highlights the importance of parameter optimization and demonstrates the potential of ZnO-BiOI heterojunctions as sustainable photocatalysts for industrial and environmental water treatment.