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

The TiO2-mediated photodegradation of chloroaliphatics (dichloromethan e, trichloroethene and mono-, di- and trichloroethanoic acids) was stu died at 308 +/- 2 K (with the ratio between the hydrogen peroxide adde d and the stoichiometric amount (N) in the range 0-30), using PHOTOPER M(TM) CPP/313 membranes containing immobilized 30% +/- 3% TiO2, at lab oratory 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, inclu ding stabilized preparations containing Co(III), V(v) and Fe(III) orga nometallic compounds, were immobilized in the photocatalytic membranes . The initial rate of photodegradation was studied as a function of th e 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, whic h was well fitted by the membranes over the whole range of concentrati on, and from which the rate constants k and equilibrium adsorption con stants K were evaluated. The contributions of processes (a)-(d) ((a) p hotolysis of micropollutant, independent of the presence of membranes and oxidizing agent; (b) photodegradation due to UV and hydrogen perox ide; (c) photodegradation due to the semiconductor immobilized in the membrane; (d) the same as (c), but in the presence of a promoting phot ocatalytic system), which occur simultaneously in our experimental con ditions, were measured. The contribution of process (d), particularly for a synergistic mixture of tri(tert-butyl) and tri-(isopropyl) vanad ate(V) or iron(III) potassium oxalate as photocatalysts, is greater th an that of all other processes, while UV degradation in the presence o f hydrogen peroxide (b) represents one-tenth or less of the whole. The rationalization of k values for this integrated membrane process is d iscussed on the basis of their dependence on the membrane surface and on the square root of the radiation intensity.