Photoassisted oxidative degradation of surfactants and simultaneous reduction of metals in titanium dioxide dispersions

Photoinduced oxidative degradation of both anionic (dodecyl sulfate) and cationic (cetyltrimethylammonium) surfactants can be efficiently realized by simultaneous reduction of chromium(VI) to chromium(III) and mercury(II) to its metallic form in aqueous systems containing colloidal titanium dioxide photocatalyst. The charge of the reactants (surfactant and metal ions) has considerable effect on the efficiency of the photoassisted processes. Similarly charged reactants in general show mutual inhibition, mostly due to the competition for the active sites on the surface of the catalyst, while oppositely charged detergent and metal ions promote the redox change of both components. The cetyltrimethylammonium–chromate system displays significant photoactivity even in the absence of titanium dioxide, due to the strong ion-pair formation resulting in direct electron transfer from the surfactant to the metallate ions. The initial pH in the studied systems may considerably affect both the Coulombic interactions between the reactants and the surface of the catalyst, and the driving forces of some of the redox key reactions.