Insights into catalytic reduction of dyes catalyzed by nanocomposite beads Alginate@Fe3O4: Experimental and DFT study on the mechanism of reduction

Abstract

In the present work, a facile pathway was developed to prepare magnetic alginate nanocomposite beads (M@AN). The prepared nanocomposite beads were evaluated as a catalyst for the degradation of methylene blue (MB) and orange G (OG) dyes in a simple and binary system under the presence of a reducing agent NaBH4. The in-situ formed magnetic nanoparticles (Fe3O4-NPs) were identified by visual observance, X-ray diffraction (XRD), UV–Vis DR Spectroscopy (UV–vis DR), Fourier transform infrared (FTIR) spectroscopy and energy dispersive x-ray (EDS). The obtained results revealed the formation of dispersed Fe3O4-NPs inside the beads linked with −COO groups of alginate biopolymer by a bidentate bridging coordination. Owing the presence and well-dispersed Fe3O4 nanoparticles in M@AN surface. The prepared catalyst was more efficient with the cationic MB dye in the simple and binary system. In the simple system, the rate constant was 0.0034 s−1 and 7.416 10−4 s−1 for MB and OG dye, respectively, whereas in the binary system the rate constant was 0.0012 s−1 and 5.096 10−4 s−1 for MB and OG dye, respectively. To have deep insight into the reduction mechanism of MB dye onto M@AN, the density functional theory (DFT) method was used. The results showed that the NaBH4 and MB reagents are firstly diffused onto the surface of the nanocatalyst via hydrogen banding, then, the reduction reaction was carried out by a nucleophilic attack of borohydride ion by hydrogenation on the double bond between carbon and nitrogen in –NC– group of MB which thereafter transformed into –NH–CH– bond. The theoretical results show good agreement with the experimental data, in which the calculated UV–visible spectrum of Leuco–MB product using DFT method shows an electronic transition at 300 nm which is a close value to the experimental.