Pilot-plant photomineralization of dichloromethane and tetrachloroethene in aqueous solution, by photocatalytic membranes immobilizing titanium dioxide and photopromoters

The TiO2-mediated photomineralization of 5.0×10−6−5.0×10−7 M aqueous solutions of dichloromethane (DCM), tetrachloroethene (PCE), and mono-, di-, and trichloroethanoic acids (MCEA, DCEA, and TCEA respectively) was studied at 296 ± 2 K, using PHOTOPERM® CPP/313 membranes containing immobilized 30 ± 3 wt.% TiO2, and, in some of the runs with PCE, 7 wt.% of photocatalytic promoters based on CoIII, VV, and FeIII organometallic compounds. Disappearance of both substrate and total organic carbon (TOC) was examined, as well as chloride ion formation. The apparent reaction order being unit in the range of concentration considered, half times for these three kinetic processes have been measured in a PHOTOPERM® WW membrane module, fitted with 1 m2 of membrane, with an absorbed radiating power of 31 W, and saturated with air. With PCE, the marked decrease of rate (to 1/25) with decreasing O2 concentration (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 ascertained. The role of oxygen as scavenger for photogenemted electrons is much preferable to that of adsorbed substrate to scavenge these conduction band electrons, both to explain the efficient action of photopromoters as oxidant scavengers or oxygen transporters, and redox processes involving formation of chlorinated intermediates, such as DCEA, during radical reactions, as TOC disappearance from DCEA, MCEA, and TCEA proceeds at a much slower rate than that experimentally observed for PCE.