Highly Efficient Recyclable Bismuth Nanocatalysts Fabricated Using a Facile One-Step Aqueous Method for Faster Reduction of Azo Dye Contaminants

Easily accessible robust synthesis of metallic nanoparticles (NPs) and their colloidal stabilization via successive surface functionalization with desired molecules are crucial for catalytic applications. In this research, tannic acid (TA)-functionalized bismuth (Bi)-based novel NPs were prepared via a simple in situ aqueous reduction of Bi3+ ions for the catalytic reduction of azo groups. The synthesis, morphology, and structure of Bi/TANPs were confirmed through spectroscopic, electron microscopic and X-ray diffraction analyses. The Bi/TANPs comprise Bi, carbon, oxygen and sodium as building components and possess a high negative surface charge of -58 mV, colloidal dispersity, thermal stability and crystalline structure. The Bi/TANPs are almost spherical shaped with an average diameter of 33 nm. The surface of the catalyst is mesoporous with a high specific surface area of 267 m2 g-1. The designed Bi/TANPs exhibit pH-specific affinity for azo dye molecules and reduced azo moieties in the presence of aqueous NaBH4 without requiring any hydrogen gas supply. The catalytic reduction efficiencies of Bi/TANPs against methylene blue and Congo red are almost 100%. These reduction reactions are very fast owing to the presence of TA moieties on the catalyst surface, which facilitate direct electron transfer to azo groups, and follow a pseudo-first-order kinetic model. The catalyst is mechanically recyclable, and shows a minimal loss (<3%) of its initial efficiency until the fifth cycle. This study not only developed an efficient catalyst for the remediation of azo dye-contaminated water, but also offers novel insights into the synergistic effects of TA and glycerin on the reduction mechanism of aqueous Bi3+ ions and the concomitant colloidal stabilization of Bi NPs.