PHOTOCATALYTIC DEGRADATION OF 4-CHLOROPHENOL IN AERATED AQUEOUS TITANIUM-DIOXIDE SUSPENSIONS - A KINETIC AND MECHANISTIC STUDY

The photocatalytic degradation 4-chlorophenol (4-CP) in aqueous titani um dioxide suspensions has been studied and compared for two different catalysts: Sachtleben Hombikat UV 100 and Degussa P25. The influence of pH and initial 4-CP concentration on reaction rate and product dist ribution has been investigated in detail. Regarding the initial 4-CP c oncentration, the degradation kinetics formally can be discribed bg a Langmuir-Hinshelwood expression. While the degradation of 4-CP itself is hardly influenced by the pH, the overall mineralization rate (measu red in terms of total organic carbon reduction changes considerably, d ecreasing strongly at pH > 7. With P25 used as the photocatalyst the m ineralization of 4-CP is slightly faster than that with Hombikat UV 10 0, but smaller concentrations and. numbers oi intermediates are detect ed in the latter case. Most of the intermediates observed during this photocatalytic degradation process are identical with those identified during direct photolysis. In acidic solution (pH 3) hydroquinone and benzoquinone are the only intermediates when Hombikat UV 100 is the ph otocatalyst. Using P25 as a photocatalysts, seven additional intermedi ates, namely hydroxyhydroquinone, hydroxybenzoquinone, phenol, 4-chlor ocatechol, 4-hydroxyphenylbenzoquinone, 2,5,4'-trihydroxybiphenyl, and 5-chloro-2,4'-dihydroxybiphenyl, were found. Hydroquinone (HQ) and be nzoquinone (BQ) have bet li found to be the main intermediates of the photocatalytic 4-CP degradation. It could be shown that the HQ/BQ phot osystem acts as an electron relay which effectively short circuits the photocatalyst resulting in a reduced efficiency of the degradation of 4-CP and most probably other aromatic compounds. An electron shuttle mechanism is proposed to explain Une highly efficient short circuiting of the photocatalyst in the presence of these compounds. A degradatio n mechanism for a-chlorophenol is proposed and discussed to account in particular for the pH dependence of the 4-CP degradation.