Greenly Synthesized Gold–Alginate Nanocomposites Catalyst for Reducing Decoloration of Azo-Dyes

Abstract

A green synthesis of gold nanoparticles (AuNPs) using aqueous solution of sodium alginate (SA) has been demonstrated in this work. The SA plays the roles of both reducing and stabilizing agent. The surface plasmon resonance (SPR) band of UV-Vis spectrum around 532[Formula: see text]nm confirmed the formation of AuNPs. The characterization through high resolution-transmission electron microscopic (HRTEM), energy dispersive X-ray (EDX) spectroscopy and X-ray diffraction (XRD) infer the as-synthesized AuNPs which were spherical in shape with a face cubic crystal (FCC) structure. The results obtained from zeta potential and dynamic light scattering (DLS) suggest the good stability and narrow size distribution of the AuNPs. The size-controlled AuNPs were obtained through adjusting the reaction parameters such as the concentration of SA, pH of the reaction mixture, temperature and the time of incubation. The as-synthesized AuNPs–SA conjugates were employed to prepare AuNPs–SA beads easily based on SA high affinity toward divalent metal ions. The resulting AuNPs–SA beads function as an efficient heterogeneous catalyst in reducing decoloration of azo-dye model compounds, acidic orange 7 (AO7) and reactive orange 5 (RO5), in the presence of sodium borohydride. The reaction rate constants, estimated based on the reduction reactions followed pseudo-first-order kinetics, suggest the high catalytic activity and stability of the AuNPs–SA beads.