Comparative performance of TiO2-rGO photocatalyst in the degradation of dichloroacetic and perfluorooctanoic acids

Abstract

Halogenated organic compounds are frequently characterized by their bioaccumulative nature, and long­term effects on human health. Their low biodegradability and their difficult removal by conventional technologies lead to their undesirable persistence in the environment. Titanium dioxide and reduced graphene oxide TiO2-rGO composites have shown promising results with enhanced photocatalytic degradation rates compared to bare TiO2 in the degradation of a good number of persistent pollutants. In this context, this work deepens on the influence of the type and number of halogen substitution on the photocatalytic degradation of halogenated organic compounds (HOCs). For the experimental analysis two HOCs, dichloroacetic acid (DCA) and perfluorooctanoic acid (PFOA), have been selected as both molecules differ in the type of halogen atoms, chlorine and fluorine, the number of halogen atoms, and in the length of the alkyl chain. The results showed that TiO2-rGO catalysts achieved a similar kinetic performance for the removal of the primary contaminant in terms of its degradation rate. Besides, TiO2-rGO composite successfully induced the release of all chlorine atoms from the DCA molecule, achieving its total mineralization. However, PFOA defluorination and mineralization rates were remarkably lower than its degradation rate. Although both contaminants released two halogen atoms step-by-step for the degradation of each molecule, PFOA dehalogenation was slowed down due to the generation of secondary products that retain fluorine. Therefore, we conclude that the composite photocatalysts showed a similar performance in terms of degradation rate of the primary contaminant independent on the length of the alkyl chain and on the number of halogen atoms, however, the mineralization and dehalogenation rates were strongly dependent on the number of halogen substituents and the length of the alkyl chain.