Green Synthesis of Nanoscale Zero-Valent Iron Aggregates for Catalytic Degradation of Textile Dyes

The application of zero-valent iron nanoparticles (nZVI) is an area of growing interest, especially in the remediation processes of waters of different origin. The standard synthetic procedure for nZVI involves the chemical reduction of Fe2+ or Fe3+ with the solution of a strong reducing agent. This type of synthetic route leads to the formation of new toxic species. The green synthesis of nZVI using plant extracts is, therefore, an excellent alternative to the above-mentioned conventional methods. Within the scope of this work, the amorphous nZVI was synthesized by green tea mediated single-step method from Fe3+ as precursors, denoting an eco-friendly approach. Additionally, the physico-chemical characterization (TEM, SEM/EDS, size of particles/aggregates, XRD, BET) of nZVI was performed. The catalytic activity of the obtained nZVI was tested in a Fenton-like reaction of the removal/degradation of three representative industrial textile dyes (methylene blue, methyl orange and bromothymol blue) from water. In order to optimize the process parameters, the effect of homogenization treatment, nZVI and H2O2 concentration on catalytic performance were also done followed by degradation kinetic study using four different models. The results of this work have shown the exceptionally high efficiency of the green nZVI which is reflected in almost complete degradation of all tested dyes within very short period of time, despite their amorphous nature, aggregates arrangement and very low specific surface area. Furthermore, the examined degradation kinetics has excellently followed the BMG model, indicating that the degradation process of the investigated dyes proceeds via two stages (two-stage pattern)-a fast one and a much slower one, regardless of the used reaction conditions. Based on all gained results, particular degradation mechanisms were proposed. [GRAPHICS]