Comparative catalytic insights into green and commercial gold nanoparticles: synergistic catalytic reduction of organic pollutants

Abstract

The presence of toxic organic pollutants such as methyl orange (MO) and 4-nitrophenol (4-NP) in industrial effluents poses significant environmental and public health risks. This study reports a rapid, scalable, and eco-friendly microwave-assisted synthesis of gold nanoparticles (AuNPs) using Azadirachta indica leaf extract as a dual reducing-stabilizing agent. The synthesis parameters, including microwave power, irradiation time, and precursor concentration, were optimized to maximize catalytic activity. Comprehensive characterization via XRD, FTIR, TEM, FESEM, DLS, and UV-Vis spectroscopy confirmed the formation of crystalline, uniformly dispersed AuNPs (average size 11.90 ± 2.84 nm) exhibiting a distinct plasmonic resonance at 535 nm. A key novelty of this work is the direct comparison of biogenic AuNPs with commercial AuNPs for the NaBH₄-mediated reduction of MO and 4-NP, monitored by UV-Vis spectroscopy under identical conditions. Green-synthesized AuNPs achieved about 75% MO and 70% 4-NP catalytic reduction within 10 minutes, with apparent rate constants of 0.059 ± 0.002 min⁻¹ and 0.133 ± 0.066 min⁻¹, respectively. The superior performance is attributed to a synergistic effect, where phytochemical capping agents from Azadirachta indica enhance pollutant adsorption and electron transfer between NaBH₄ and the pollutant molecules via the AuNP core. Kinetic studies confirmed pseudo-first-order behavior. The proposed catalytic reduction mechanism involves efficient electron shuttling from BH₄⁻ to pollutant molecules mediated by AuNPs, with bio-organic ligands acting as active surface sites. This comparative and mechanistic approach establishes a sustainable nanocatalyst platform for reducing hazardous organic contaminants and offers practical applicability for wastewater treatment.