LABORATORY STUDY OF THE RATES AND PRODUCTS OF THE PHOTOTRANSFORMATIONS OF NAPHTHALENE ADSORBED ON SAMPLES OF TITANIUM-DIOXIDE, FERRIC-OXIDE, MUSCOVITE, AND FLY-ASH

The photodegradation of naphthalene (NPH), chosen as a model of polynuclear aromatic pollutants, has been studied in the presence of a layer of four water-insoluble inorganic solids which can be found in the troposphere (TiO2, Fe2O3, muscovite, and a fly ash sample). Direct photolysis of NPH is negligible at lambda > 340 nm. Dark adsorption of NPH on TiO2 (mainly anatase, nonporous, 50 m2 g-1) at 293 K corresponds to a surface coverage of ca. 50% at equilibrium. Under these conditions (saturated surface), the stationary-state photocatalytic degradation reaches 0.4 molecule nm-2 h-1 (lambda > 340 rim, radiant flux ca. 22 mW cm-1). Dioxygen is required and its partial pressure in air is such that the degradation is zero order in O2. Water vapor markedly increases the rate. The other particulates have also an effect, less important than that of TiO2, however quite noticeable with respect to surface area unit for the fly ash sample which contains 3.2% Fe2O3. Apart from 1,4-naphthoquinone, which is the main intermediate product in all cases, 2-naphthol, phthalide, phthaldialdehyde, phthalic acid, acetophenone, benzaldehyde, benzoic acid are also formed on dry TiO2. Depending on their volatility, these compounds are transferred to the gas phase or remain principally adsorbed on the solid particles where they are further transformed. For instance, phthalic acid (or anhydride) and benzoic acid are generated from 1,4-naphthoquinone. Degradation mechanisms are briefly discussed.