Sustainable valorization of onion bulbs for the green synthesis of gold nanoparticles and catalytic degradation of methylene blue

The catalytic degradation of toxic synthetic dyes in wastewater remains a pressing environmental concern due to their adverse effects on ecosystems and public health. In this study, we present a green and sustainable method for synthesizing gold nanoparticles (AuNPs) using Allium cepa (onion) bulb extract — an agricultural byproduct with significant post-harvest and processing losses. The onion bulb extract served as a natural reducing and stabilizing agent, enabling the eco-friendly synthesis of AuNPs. The formation of AuNPs was evidenced by a ruby-red coloration and an absorption peak at 530 nm in UV–visible spectroscopy. Further characterization confirmed the production of ultrafine spherical nanoparticles (∼8 nm) with good colloidal stability (zetapotential: –24.40 mV). The biosynthesized AuNPs exhibited excellence catalytic activity toward the degradation of methylene blue, achieving 95% removal within 8 min. The reaction followed pseudo-first-order kinetics with a rate constant (k) of 0.3550 min−1. This study demonstrates the effective valorization of onion biomass into functional nanomaterials and highlights a sustainable pathway for wastewater treatment using green nanotechnology.