Abstract
A popular surfactant used can be easily decomposed in aqueous titanium dioxide dispersion under aerated UV-illumination or solar exposure. The photocatalytic degradation mechanism for the chemical structures was revealed. The degradation for both the hydrophilic and hydrophobic groups proceeded competitively to mineralize easily to carbon dioxide via several oxidation steps. The optimal degradation conditions for the chemical structures of surfactants bearing functional moieties were investigated, of which the rate was as the following order : anionic surfactant > nonionic one > cationic one. With respect to the hydrophobic group in the surfactant structure, the degradation rate was aromatic ring > ethoxyl moiety > alkyl chain. The photooxidation processes were classified into two factors : the static factor of adsorption and electrostatics as well as the dynamic factor of charge separation and OH radical formation. Adsorption of surfactant on the TiO2 particle surface was inferred from the simulation of point charges. The atom (s) bearing the higher frontier electron density among the surfactant molecular skeleton were certainly attacked by OH radicals. For example, it is deduced. that ·OH radicals attack preferentially the carbon atoms bonded to the sulfonate groups on the aromatic rings, followed by further attack on the remaining carbon atoms of the aromatic rings first and then more slowly on the carbon atoms of the long hydrocarbon chain.
In addition, various chemical contaminants such as polymers, agrochemicals, dyes and endocrine disruptors can be also photocatalytically decomposed to non-toxic compounds. The activity for a pigment including TiO2 or ZnO in cosmetics of a sunscreen agent was assessed by relaxation or linearity of double helical DNA configuration. The photocurrent during the photodegradation of water-soluble organic substances could be generated by the “double photocurrent effect” through TiO2-fixed thin layer conductive electrodes. The possibility of application of “wet-solar cell” was discussed as for to a sustainable resource of organic contaminants.
