Laurence Amalric

Assessment of the importance of the role of H2O2 and O2o−in the photocatalytic degradation of 1,2-dimethoxybenzene

Catalase, which catalyses the dismutation of H2O2 into H2O and O2, and Cu, Zn-super-oxide dismutase (SOD), which catalyses the dismutation of O2o− and H2O2, have been used to assess the importance of the role of these species formed in situ in TiO2 or ZnO aqueous suspensions. Variations in the apparent first-order rate constant, kapp, of the disappearance of the electron-rich aromatic 1,2-dimethoxybenzene (1,2-DMB; 0.145 mmol l−1, i.e. 20 mg l−1) were produced by these enzymes. Catalase activity under photocatalytic conditions was verified by adding H2O2. This enzyme (22 mg l−1, i.e., ca. 28,600 U) caused only a 20% decrease in kapp in the case of TiO2 and no effect in the case of ZnO, showing that H2O2 formed in situ is not essential for the elimination of 1,2-DMB. This observation does not mean that addition of H2O2 does not affect kapp. Indeed, it had a negative effect on ZnO and either a favourable or an unfavourable effect on TiO2 depending on the initial ratio [H2O2]/[1,2-DMB]. The influence of SOD (100 mg l−1 i.e., ca. 10,000 U) on kapp was much more pronounced than that of catalase. In the case of TiO2, the inhibition of Cu, Zn-SOD effect by CN− ions, the weak influence of Cu(No3)2 on kapp, and the change in the distribution of the degradation intermediates at equivalent transformation rates of 1,2-DMB, suggest that the effect of SOD is really due to the catalytic activity of this enzyme with respect to superoxide. This points to the important chemical role of this species in the photocatalytic degradation of pollutants, at least those structurally related to 1,2-DMB.

Correlation between the photocatalytic degradability over TiO2 in water of meta and para substituted methoxybenzenes and their electron density, hydrophobicity and polarizability properties

Abstract In an earlier study [Pichat et al. (1993) Photocatalytic Purification and Treatment of Water and Air, pp. 207–223. Elsevier, Amsterdam] the photocatalytic degradability over TiO2 of some chlorophenols in water was correlated to the 1-octanol-water partition coefficient (Kow) and to the Hammett constant (σ) of these compounds. To determine whether a similar correlation can be found for less closely structurally related aromatic pollutants, the case of methoxybenzenes (anisoles) substituted at the meta or para positions by F, Cl, NO2, OH, NH2 groups was investigated. The Hammett constant, σ, the Brown constant σ+ and the ionization potential were successively used as descriptors for the electron availability of these compounds. The best correlation was log kapp=0.20 log Kow−0.20σ+−0.033 MR−−0.43 (r=0.903) where kapp (in min−1) is the pseudo-first order rate constant of the pollutant disappearance in TiO2 aqueous suspensions and MR is the molar refractivity of the pollutant. With respect to σ, σ+ has the interest of introducing a greater difference between the meta and para derivatives. MR, which is proportional to the polarizability, is thought to reflect the pollutant-pollutant interaction and to amend the effect of Kow in accounting for the distribution of the pollutant molecules in the TiO2-water system. This descriptor induces much smaller differences between the various anisoles than Kow and, above all, σ+. The validity of methods to evaluate Kow when experimental values are not available or MR when the refractive index is unknown, is also discussed.

The photodegradation of 2,3-benzofuran and its intermediates, 2-coumaranone and salicylaldehyde, in TiO2 aqueous suspensions

Abstract 2,3-Benzofuran (BZF), a mutagenic and carcinogenic compound, was chosen as a model water pollutant to study the TiO 2 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 TiO 2 respectively) or the logarithm of the 1-octanol-water partition coefficient (2.67 exp , 1.75 exp and 1.35 calc respectively). This 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 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.