Solar photocatalytic degradation of caffeine with titanium dioxide and zinc oxide nanoparticles

Abstract

This study investigates the photocatalytic degradation of caffeine (CAF) under solar light (290–1200 nm) irradiation using two different photocatalysts (i.e. TiO2 and ZnO). The adsorption of CAF on both photocatalysts follows Freundlich adsorption isotherm in the pH range of 6.0–8.0 with Kf values 0.0012 L mg−1 and 0.00097 L mg−1 for ZnO and TiO2 respectively at pH 7.0. A comprehensive parametric study shows that for both photocatalysts the optimum reaction conditions are pH 7.0, light intensity of 100 mW cm-2 and photocatalyst amount of 1.0 g L−1. The photocatalytic degradation of CAF follows pseudo first-order kinetics irrespective of the photocatalyst used. The apparent rate constants (Kapp) of solar photocatalytic degradation of CAF were determined as 0.032 min−1 and 0.024 min−1 for TiO2 and ZnO respectively. The degradation rate constant (Kr in mg L−1 min−1) of CAF with ZnO is directly proportional to the incident light intensity (I0.79). In the case of TiO2, it does not depend on I when I ≤ 60 mW cm-2. However, it increases when I > 60 mW cm-2. The present paper shows that solar light coupled with unmodified TiO2 or ZnO can be successfully used to degrade caffeine in water.