Oxidative degradation and fluorescence of a non-biodegradable brightener via titania suspensions: implications for the natural cycle

The light-induced degrdn. of Tinopal DMS-X, a stilbene type fluorescence whitening agent (FWA) which is quite resistant to biodegrdn., is described. This substance has been chosen for the study of the titania-induced degrdn. of polycyclic stilbenes since it is one of the most important compd. of its class found in a variety of washing effluents. TiO2-irradiated suspensions affected the complete mineralization within 2 h in spite of the low coverage of the TiO2 surface used during this study. Adding H2O2 as an irreversible electron acceptor shortened the degrdn. time. The influence of several reaction variables, i.e., brightener concn., type of TiO2 used, amt. of catalyst, initial pH, and gas atm., on the reaction under study is described. The influence of these parameters on the kinetics of brightener disappearance has been followed by fluorescence techniques. The mineralization taking place is reflected by the dissolved org. carbon (DOC) and the CO2 evolved during the reaction. This study shows that at relatively high concns. of brighteners (as used in this study), the 1O2 involvement during the photodegrdn. process is not meaningful. Titania is shown capable of catalyzing the photodegrdn. over many cycles without loosing its efficiency. The emission lifetime of Tinopal DMS-X was measured by monophoton counting techniques and was 3.4 ns. Lab. expts. showed that Tinopal DMS-X was not biodegradable up to 8 days and the photochem. pretreatment did not improve significantly the obsd. lack of biodegradability. Also the partially oxidized intermediates were found to be nonbiodegradable in 8 days when exposed to a non-acclimated culture. Tinopal DMS-X was effective in humic acid sensitization and the obsd. sensitization was related to the brightener concn. Exptl. observations are reported allowing to assign the energy transfer from the Tinopal DMS-X to the humic acid as proceeding via singlet mechanism.