Oxidative Degradation and Fluorescence of a Non-biodegradable Brightener via Titania Suspensions: Implications for the Natural Cycle

Abstract

Degradation induced by light is presented for Tinopal DMS-X a stilbene type fluorescence whitening agent (FWA) which is quite resistant to biodegradation. This substance has been chosen for the study of the titania induced degradation of polycyclic stilbenes since it is one of the most important compound 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 degradation time. The influence of several reaction variables like on the reaction under study like: brightener concentration, type of TiO₂ used, amount of catalyst, initial pH, gas atmosphere is reported. 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 organic carbon (DOC) and the CO₂ evolved during the reaction. This study shows that at relatively high concentrations of brighteners (as used in this study) the ¹O₂ involvement during the photodegradation process is not meaningful. Titania is shown capable of catalyzing the photodegradation 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. Laboratory experiments showed that Tinopal DMS-X was not biodegradable up to 8 days and the photochemical pretreatment used did not improve significantly the observed lack of biodegradability. Also the partial oxidized intermediates were found to be nonbiodegradable in 8 days when exposed to a non-acclimatcd culture. Tinopal DMS-X was effective in humic acid sensitization and the observed sensitization was related to the brightener concentration. Experimental observations are reported allowing to assign the energy transfer from the Tinopal DMS-X to the humic acid as proceeding via singlet mechanism.