THE PHOTODEGRADATION OF 2,3-BENZOFURAN AND ITS INTERMEDIATES, 2-COUMARANONE AND SALICYLALDEHYDE, IN TIO2 AQUEOUS SUSPENSIONS

2,3-Benzofuran (BZF), a mutagenic and carcinogenic compound, was chosen as a model water pollutant to study the TiO2 photocatalytic degradation of molecules which have two aromatic rings and a built-in oxygen heteroatom. For BZF and its two main degradation intermediates, salicylaldehyde (SA) and 2-coumaranone (2-CO), a relatively good correlation was found between the apparent first-order rate constant of disappearance (approximately 0.059 min(-1), 0.0365 min(-1) and 0.031 min(-1) respectively) and the measured extent of dark adsorption (about 0.43, 0.134 and 0.108 molecules per square nanometre of TiO2 respectively) or the logarithm of the l-octanol-water partition coefficient (2.67(exp), 1.75(exp) and 1.35(calc) respectively). This 1.75 illustrates the importance of surface phenomena for these classes of compounds. From the degradation of BZF, 2-CO and SA (studied separately), it was inferred that the larger number of BZF degradation intermediates resulted from the oxidation of the furan ring to produce 2-CO (and perhaps 3-CO), 2,3-dioxobenzofuran and SA, which was further oxidized/hydroxylated to salicylic acid, catechol and dihydroxybenzaldehydes (and perhaps to dioxobenzaldehydes and hydroxyparabenzoquinone or isomers). However, the identification of other compounds with one hydroxyl substituent on the benzene ring and an unopened furan ring demonstrated that the degradation proceeded by a variety of pathways. Although most of these intermediates could be formed by hydroxyl radicals, the involvement of other species cannot be ruled out; in particular, the direct formation of SA from BZF may result from the interaction of superoxide with BZF(.+). The efficiency of the photocatalytic pollutant degradation was illustrated by the observation that the maximum quantity of each intermediate was less than a few per cent of eliminated BZF.