Green synthesis of akaganéite (β-FeOOH) nanocomposites as peroxidase-mimics and application for discoloration of methylene blue

Abstract

This work reports an environmentally benign and readily scalable process for production of akaganéite (β-FeOOH) nanocomposites by using abundant gallic acid or grape seed tannins and urea. Influences from those phytochemicals on the properties of β-FeOOH nanocomposites were investigated by X-ray powder diffraction, Fourier transform infrared spectroscopy, Thermogravimetric analysis, Scanning electron microscopy, Transmission electron microscopy, UV-Vis spectroscopy and Photoluminescence. The addition of 0.1% (w/v) grape seed tannins or gallic acid (640 mg L^-1) solution yielded single-crystalline β-FeOOH nanocomposites with reduced dimensions, increased porosities and BET surface area, and no oxidized impurities such as hematite (Fe₂O₃) were formed. The added grape seed tannins (S_0.8) or gallic acid together with less urea (0.8 M) produced β-FeOOH nanocomposites with higher activities as peroxidase mimics compared to those prepared with only urea (C_0.8). Moreover, S_0.8 was more efficient in methylene blue (MB) discoloration compared to C_0.8 at all three pH values of 4, 7 and 11, and the S_0.8-mediated MB degradation pathways at pH 4 and 7 were different from those at pH 11 due to the generation of different predominant oxidants. The overall MB discoloration efficacies by S_0.8 at pH 4, 7 and 11 were combinative effects of both physical adsorption and chemical reactions. These β-FeOOH nanocomposites possess great potential as peroxidase mimics for facile monitoring of excess hydrogen peroxide and applications in environmental remediation.

Keywords: (Photo)-fenton reactions; Controlled hydrothermal synthesis; Iron oxide/oxyhydride nanoparticles; Peroxidase-mimics; Phytochemicals.