PHOTOLYTIC AND PHOTOCATALYTIC DESTRUCTION OF FORMALDEHYDE IN AQUEOUS-MEDIA

Formaldehyde (HCHO) was decomposed in UV-irradiated aqueous suspension s of titanium dioxide. Initial HCHO levels in the range 60-1000 ppm, a nominal TiO2 dose of 1 g/liter, and a medium-pressure Hg lamp were em ployed in these experiments. Chemical oxygen demand measurements of th e solution before and after photocatalytic treatment revealed the mine ralization of HCHO to be complete only for the lower end (<100 ppm) of the concentration range. Direct ultraviolet photolysis of HCHO also o ccurred, but this pathway had an initial lag unlike the photocatalytic route. In the presence of TiO2 and ultraviolet irradiation, the direc t photolysis route was bypassed and the reaction proceeded dominantly by the photocatalytic route. The kinetics of the photocatalytic oxidat ion of HCHO were analyzed using several models. The effect of H2O2 add ition to the UV/TiO2 system was also probed as well as the homogeneous ultraviolet H2O2 approach for the treatment of HCHO. The latter exhib ited the fastest kinetics for the destruction of HCHO. Finally, the lo ng-term stability and photocatalytic activity of TiO2 were monitored w ith two substrates, namely, HCHO and trichloroethylene in two differen t media: viz. pristine water and a ''real-life'' water sample with bic arbonate alkalinity. No degradation in the photocatalyst performance w as noted over ten repeat use cycles in either case.