Photocatalytic degradation of 2-phenylphenol on TiO2 and ZnO in aqueous suspensions

The degradation of 2-phenylphenol (OPP), commonly used as fungicide, can be photocatalysed by TiO2 and ZnO. The latter is a little more efficient but cannot be used in acidic solution. In both cases, the main identified photoproducts are hydroquinone (HQ), p-benzoquinone (BQ), phenylhydroquinone (PHQ), phenylbenzoquinone (PBQ), 2,2 '- and 2,3-dihydroxybiphenyls (2,2 '- and 2,3-BPh(OH)(2)). The observed minor formation of 2-hydroxydibenzofuran is due to the photocyclisation of PBQ. The influence of various parameters is considered. The phototransformation of OPP is slightly favoured by increasing pH. It is enhanced by the addition of small amounts of H2O2, but it is inhibited by high concentrations of chloride, sulphate or nitrate. From the inhibiting effect of alcohols, it is deduced that the main degradation pathway involves hydroxyl radicals, but a direct oxidation by positive holes hi is probably not negligible. The observed slow photodegradation of ZnO in unbuffered solution is attributed to an electrochemical process. A mechanism is proposed for the photocatalytic transformation of OPP. (C) 2001 Elsevier Science B.V. All rights reserved.