Photocatalytic degradation of organic compounds in aqueous solution by a TiO₂-coated rotating-drum reactor using solar light

Abstract

This paper deals with the degradation of aqueous phenol by a newly proposed rotating-drum reactor coated with a TiO₂ photocatalyst, in which TiO₂ powders loaded with Pt are immobilized on the outer surface of a glass-drum. The reactor can receive solar light and oxygen from the atmosphere effectively. It was shown experimentally that phenol can be decomposed rapidly by this reactor under solar light: with our experimental conditions the phenol with an initial concentration of 22.0 mg/dm³ was decomposed within 60 min and was completely mineralized through intermediate products within 100 min. The photonic efficiency under solar light was shown to take the value 0.00742 mol-C/Einstein. The photocatalytic decomposition processes of phenol by this reactor were also discussed on the basis of the Langmuir–Hinshelwood kinetic model.

Introduction

Photocatalytic oxidation reactions have the potential to completely mineralize organic compounds to carbon dioxide by solar light and to lead us to a ‘clean and green purification technology’ for treatment of polluted air and water (Ollis and Al-Ekabi, 1993, Parent et al., 1996). Several semiconductors such as TiO₂, Fe₂O₃, ZnO, ZnS, CdS, WO₃ are known to have photocatalytic properties (Domènech, 1993, Hasegawa and Mayo, 1986, Lepore et al., 1993). Among them, TiO₂ is the most widely used for its high photocatalytic activity, stability and non-toxicity (Nogueira and Jardim, 1996, Okamoto et al., 1985).

For air purification, some practical purification technologies using the TiO₂ photocatalyst have been achieved. For water purification, however, viable applications have not been realized yet, in spite of extensive basic research related to the decomposition of aqueous organic pollutants by a TiO₂ photocatalyst. In existing publications, the reactors with suspension of TiO₂ powder have been widely researched; in these reactors the water has to be continuously stirred during the reaction and the TiO₂ powder has to be separated from the treated water. In contrast to the powder, the film type photocatalyst with immobilized TiO₂ powder is attractive as it avoids such disadvantages.

The main purpose of the present study is to develop a new photocatalyst reactor for water purification which can be operated under solar light. We here propose a rotating-drum reactor coated with a TiO₂ photocatalyst, in which Pt-loaded TiO₂ powder is immobilized on the outer surface of a glass-drum. The reactor can receive solar light and oxygen from the atmosphere effectively: since the water film on the drum is very thin, the light can easily penetrate to the TiO₂ surface and O₂ in the atmosphere can diffuse rapidly to the TiO₂ surface, which leads to a rapid production of ^·OH and ^·OOH radicals.

The reactor was tested by decomposing aqueous phenol under solar light and its performance was evaluated. It was shown that phenol can be decomposed effectively by this reactor under solar light. The experiment using artificial ultraviolet light was also carried out and the results were compared with those obtained using solar light. The degradation and mineralization processes were also shown to be explained by the Langmuir–Hinshelwood kinetic model.