LABORATORY-SCALE AND PILOT-PLANT-SCALE PHOTODEGRADATION OF CHLOROALIPHATICS IN AQUEOUS-SOLUTION BY PHOTOCATALYTIC MEMBRANES IMMOBILIZING TITANIUM-DIOXIDE

The TiO2-mediated photodegradation of chloroaliphatics (dichloromethane, trichloroethene and mono-, di- and trichloroethanoic acids) was studied at 308 +/- 2 K (with the ratio between the hydrogen peroxide added and the stoichiometric amount (N) in the range 0-30), using PHOTOPERM(TM) CPP/313 membranes containing immobilized 30% +/- 3% TiO2, at laboratory scale (radiant power in the absorption range, 145 W) and in a pilot plant (radiant power in the absorption range, 31 W). In addition to this semiconductor, some proprietary photocatalytic systems, including stabilized preparations containing Co(III), V(v) and Fe(III) organometallic compounds, were immobilized in the photocatalytic membranes. The initial rate of photodegradation was studied as a function of the initial concentration of the substrates (5.0 x 10(-2)-5.0 x 10(-7) M) using the linearized form of the Langmuir-Hinshelwood equation, which was well fitted by the membranes over the whole range of concentration, and from which the rate constants k and equilibrium adsorption constants K were evaluated. The contributions of processes (a)-(d) ((a) photolysis of micropollutant, independent of the presence of membranes and oxidizing agent; (b) photodegradation due to UV and hydrogen peroxide; (c) photodegradation due to the semiconductor immobilized in the membrane; (d) the same as (c), but in the presence of a promoting photocatalytic system), which occur simultaneously in our experimental conditions, were measured. The contribution of process (d), particularly for a synergistic mixture of tri(tert-butyl) and tri-(isopropyl) vanadate(V) or iron(III) potassium oxalate as photocatalysts, is greater than that of all other processes, while UV degradation in the presence of hydrogen peroxide (b) represents one-tenth or less of the whole. The rationalization of k values for this integrated membrane process is discussed on the basis of their dependence on the membrane surface and on the square root of the radiation intensity.