PILOT-PLANT PHOTOMINERALIZATION OF DICHLOROMETHANE AND TETRACHLOROETHENE IN AQUEOUS-SOLUTION, BY PHOTOCATALYTIC MEMBRANES IMMOBILIZING TITANIUM-DIOXIDE AND PHOTOPROMOTERS

The TiO2-mediated photomineralization of 5.0x10(-6)-5.0x10(-7) M aqueo us solutions of dichloromethane (DCM), tetrachloroethene (PCE), and mo no-, di-, and trichloroethanoic acids (MCEA, DCEA, and TCEA respective ly) was studied at 296+/-2 K, using PHOTOPERM(R) CPP/313 membranes con taining immobilized 30+/-3 wt.% TiO2, and, in some of the runs with PC E, 7 wt.% of photocatalytic promoters based on Co-III, V-V, and Fe-III organometallic compounds. Disappearance of both substrate and total o rganic carbon (TOC) was examined, as well as chloride ion formation. T he apparent reaction order being unit in the range of concentration co nsidered, half times for these three kinetic processes have been measu red in a PHOTOPERM(R) WW membrane module, fitted with 1 m(2) of membra ne, with an absorbed radiating power of 31 W, and saturated with air. With PCE, the marked decrease of rate (to 1/25) with decreasing O-2 co ncentration (to 2-3 ppm), and the positive effects of promoters or of applying an anodic bias (enhancement of rate up to about 3/1) were asc ertained. The role of oxygen as scavenger for photogenerated electrons is much preferable to that of adsorbed substrate to scavenge these co nduction band electrons, both to explain the efficient action of photo promoters as oxidant scavengers or oxygen transporters, and redox proc esses involving formation of chlorinated intermediates, such as DCEA, during radical reactions, as TOC disappearance from DCEA, MCEA, and TC EA proceeds al a much slower rate than that experimentally observed fo r PCE. Copyright (C) 1996 Published by Elsevier Science Ltd.