Synthesis and Characterisation of Zinc Oxide Nanoparticles from Citrus sinensis Peel Extract and Their Photocatalytic Degradation of Textile Dyes in Wastewater

The photocatalytic degradation of persistent synthetic dyes in textile wastewater using green-synthesised metal oxide nanoparticles has emerged as a high-priority research domain in environmental chemistry, offering superior efficiency and reduced secondary pollution compared to conventional chemical treatment. This study synthesises zinc oxide nanoparticles using an aqueous extract of Citrus sinensis (sweet orange) peel as a capping and reducing agent, a material abundantly available from juice processing industries in South West Nigeria. The synthesis is optimised by varying extract concentration, precursor concentration, pH, and calcination temperature. Characterisation of the resulting nanoparticles employs X-ray diffraction for crystal phase identification and crystallite size calculation by the Scherrer equation, Fourier transform infrared spectroscopy for surface functional group analysis, scanning electron microscopy for morphological assessment, transmission electron microscopy for particle size distribution, and UV-visible diffuse reflectance spectroscopy for bandgap determination. Photocatalytic performance is evaluated in batch reactors using methylene blue and Congo red dye solutions under visible light irradiation, measuring dye concentration reduction by UV-visible spectrophotometry at timed intervals. The effect of catalyst dose, initial dye concentration, pH, and irradiation time on degradation efficiency is systematically investigated using response surface methodology. Degradation kinetics are modelled using the Langmuir-Hinshelwood pseudo-first-order rate equation. Reusability of the catalyst over five degradation cycles is assessed. Findings contribute mechanistic understanding of green nanoparticle-assisted dye degradation and provide scalability data relevant to textile wastewater treatment in Nigerian industrial zones. Keywords: zinc oxide nanoparticles, green synthesis, photocatalysis, textile wastewater, dye degradation