Photocatalytic Destruction of Caffeine on Sepiolite-Supported TiO2 Nanocomposite

The study is about the degradation of a common pharmaceutical and personal care product (PPCP) caffeine by photocatalysis under UV–visible light using pristine TiO2 (P-25) and a lab-made nanocomposite of sepiolite–TiO2. It was found that the dark adsorption of caffeine on P-25 was insignificant, but considerably high on the nanocomposite, owing to the high porosity and unique structure of sepiolite. The degradation of the compound in the presence of P-25 and Sep–TiO2 followed the pseudo-first order and exponential decay kinetics, with a rate constant of 0.12 min−1 and 0.50 min−1, respectively. The efficiency of both catalysts for carbon mineralization was better at acidic pH, but that of the nanocomposite was significantly higher at all pH than that of the unmodified P-25 (36.1 against 9.6% at pH 6.0). The loading of TiO2 on sepiolite was an important factor in the activity of the catalyst, as the maximum activity was observed at a loading ratio of 12.5 mmol per g sepiolite, which decreased at higher ratios of the semiconductor to sepiolite. Exposure of the treated samples to high-frequency ultrasound at pH 6 was found to enhance the degree of mineralization to 65.1 and 52.1% in the presence of the nanocomposite and P-25, respectively. The outcome was attributed to the unique properties of ultrasound for the cleaning of solid surfaces, for enhancing the mass transfer of solutes to heterogeneous interfaces, and for generation of excess hydroxyl radicals. Finally, the nanocomposite was found to be considerably stable, as it was easily recovered and used four times without a significant loss in activity.