Catalytic ozonation of caffeine with sepiolite: Effects of impregnation with zero-valent iron and ultrasound

Abstract

The study is about the oxidative destruction and mineralization of caffeine by catalytic ozonation using sepiolite, which is a low-cost natural clay mineral found abundantly in Anatolia. The results showed that while ozonation alone at pH 6.0 (the natural pH of the test solution) provided 72% caffeine decay and 18% carbon mineralization during 10-min and 60-min reaction, respectively; catalytic ozonation with sepiolite provided 96% conversion and 30% mineralization during equivalent pH and reaction times. The activity of the catalyst was further improved by immobilization of zero-valent iron (ZVI) on its surface to produce a nanocomposite (ZVI/SEP) with a massive surface area and a much more reactive surface. Integration of the process with high-frequency ultrasound (US) was found to further enhance the rate of caffeine degradation and the degree of carbon mineralization via the unique properties of ultrasound for enhancing the rate of mass transfer to the catalyst surface and the generation of hydroxyl radicals (HO center dot) via thermal fragmentation of water molecules in the collapsing cavity bubbles. Maximum process efficiency was obtained under the following optimized conditions: pH = 6.0, O-3 (aq.) = 4 mg L-1, SEP/ZVI = 1.0 g L-1, US frequency = 577 kHz, t = 5-min and 60-min for 100% caffeine and 57% TOC decay, respectively. The nanocomposite was also highly stable, for it was used consecutively in four cycles without significant loss in activity.