Photocatalytic degradation of diclofenac sodium salt: adsorption and reaction kinetic studies

The photocatalytic oxidation of diclofenac, which is a non-steroidal anti-inflammatory medication, with TiO 2 illuminated with UV-A light under constant flow of pure oxygen has been investigated in our laboratories to evaluate the effect of initial reactant concentration on the reaction rate. Diclofenac adsorption experiments under dark conditions were also carried out to get information about the interaction between the organic molecule and the solid catalyst. Adsorption experiments at natural pH under dark conditions confirmed that the negatively charged diclofenac molecule is easily adsorbed on the positively charged surface of titania reaching equilibrium in less than 50 min. According to the Langmuir isotherm, the adsorption constant ( K eq ) and the maximum uptake of diclofenac ( q m ) on the surface of commercial TiO 2 are 21.1401 L mMol −1 DFC and 0.04013 mMoles DFC g −1 TiO 2 , respectively. Experimental results also indicate that the initial reaction rate of the photocatalytic degradation of diclofenac obeys the Langmuir–Hinshelwood–Hougen–Watson model with a kinetic constant K 1 = 0.08489 min −1 and an adsorption parameter K 2 = 14.12821 mM −1 L. Combined chemical analysis of the reaction samples by HPLC, TOC, and UV–vis spectroscopy indicated that the diclofenac is efficiently mineralized by photocatalysis via hydroxylation of the aromatic rings to form 2-aminophenol, catechol, benzene-triol, 2,6-dichloroaniline, and fumaric acid, which in turn are completely mineralized to CO 2 and water.