Green synthesized ZnO nanocatalysts for rapid and effective visible-light degradation of industrial dyes

Abstract

Synthetic dyes from industrial sources, particularly textiles, are major contributors to water pollution due to their non-biodegradable and toxic nature, posing serious environmental and health hazards. Semiconductor metal oxide nanomaterials have emerged as promising photocatalysts for dye degradation, owing to their activity, stability, and tunable structural–electronic properties. This study explores the visible-light-driven photocatalytic degradation of two hazardous dyes—Rose Bengal (RB) and Methylene Blue (MB)—using green-synthesized ZnO nanoparticles (NPs). The NPs were prepared via an eco-friendly route employing Tabernaemontana divaricata flower extract, producing three distinct samples: ZnO0 (17.5 nm), ZnO10 (20.8 nm), and ZnO15 (23.3 nm). Photocatalytic performance was evaluated under varying crystallite sizes, pH, catalyst dosages, and temperatures. Results revealed that activity increased with larger crystallite size, higher catalyst dosage, and elevated temperature, attributable to enhanced surface reactivity and reduced charge carrier recombination. ZnO15 achieved the highest efficiencies—99.14% (RB) and 99.42% (MB) degradation within 90 min—under optimized conditions, with rate constants of 0.07195 min⁻¹ and 0.06922 min⁻¹, respectively. The degradation followed pseudo-first-order kinetics according to the Langmuir–Hinshelwood model. Optimal RB degradation occurred at pH 6, whereas MB degradation peaked at pH 10, reflecting the influence of electrostatic interactions between dye molecules and the ZnO surface. Scavenger experiments indicated that hydroxyl radicals were the dominant reactive species for RB degradation, while photogenerated holes played the key role in MB degradation. ZnO15 maintained significant activity over five successive cycles, suggesting good reusability. This work demonstrates that green-synthesized ZnO is an efficient photocatalyst for dye degradation under visible light and systematically assesses the influence of crystallite size, pH, catalyst loading, and temperature. While promising for wastewater treatment applications, further studies are needed to validate performance in complex effluents and under long-term operational conditions.