Photolytic degradation of all chlorophenols with polyoxometallates and H2O2

The photolytic degradation of all chlorophenols(CPs) under UV and near VIS light, in the presence of H2O2 (0.1 M) or polyoxometallate (POM) photocatalyst PW12O403- (7 x 10(-4) M) in aqueous solutions has been studied. In both cases, the initial rates of photo degradation of all substrates are comparable, following first-order kinetics. In both cases the photodecomposition of substrates correlates with the position and the number of chlorine atoms in the ring. The slowest is the fully chlorinated pentachlorophenol (PCP). It has been found that chlorine substituents in meta positions accelerate the photodecomposition process. Thus. in sonic cases the position dominates the number of Cl atoms, for instance 3,4.5-trichlorophenol > 3.4-dichlorophenol > 3-chlorophenol. Similar results for a few Cps have been reported with TiO2 and H2O2. A target Compound, 2,4,6-trichlorophenol. was used in order to compare the degradation mechanisms, by both H2O2 and PW12O403-. The intermediates detected were 3,5-dichlorocatechol, 2,6-dichlorobenzoquinone, dihydroxytrichlorobenzene, trihydroxydichlorobenzene and dichlorohydroquinone. The same intermediates have been reported with TiO2. The similarity in the photodecomposition process of all CPs by three different methods, i.e. PW12O403-, TiO2 or H2O2 further supports the notion that all three act, mainly, through the generation of OH radicals. (C) 2002 Elsevier Science B.V. All rights reserved.