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

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 K-f 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 (K-app) 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 (K-r in mg L-1 min(-1)) of CAF with ZnO is directly proportional to the incident light intensity (I-0.79). 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.