Photodecolorisation of melanoidins in vinasse with illuminated TiO2-ZnO/activated carbon composite

Abstract

A hybrid photo-catalyst, TiO₂-ZnO, was synthesized by immobilizing ZnO on commercial TiO₂ (aeroxide P25). Activated carbon (AC) was subsequently used to support the hybrid, thus forming a TiO₂-ZnO/AC composite catalyst. Fourier transform infrared (FTIR) analysis and scanning electron microscopy integrated with energy-dispersive X-ray spectroscopy (SEM-EDX) investigations revealed successful catalyst synthesis. Optical properties of the hybrid determined from photoluminescence (PL) and Ultraviolet-visible (UV-vis) spectroscopy confirmed a restrained recombination of electron-hole pairs and reduced energy band gap due to a successful heterojunction formation. The prepared catalysts were used to photodecolorise vinasse in a 12-W UVC batch photoreactor. TiO₂-ZnO had improved photocatalytic activity compared with TiO₂ and ZnO separately. On supporting the hybrid onto AC, both adsorption and photocatalytic activities were further enhanced with improved overall color removal of 86% from 68%. Photodecolorisation followed the pseudo-first-order reaction model with the rate constant ([Formula: see text]) observed decreasing from 0.0701 to 0.0436 min^-1 on increasing the initial concentration from 5,000 to 14,000 ppm. The UV process was found to be 33-fold less energy intensive for color reduction as compared to total organic carbon (TOC) reduction. Formation of nitrates during the photodecolorisation process was attributed to the mineralization of nitrogen heteroatoms in the color-causing melanoidin compounds.

Keywords: Activated carbon; UV-photodegradation; energy demand; melanoidins; titanium dioxide; vinasse; zinc oxide.