Dynamics and mechanistic features in the photocatalyzed oxidation of disulfonated anionic surfactants on the surface of UV-irradiated titania nanoparticles

Abstract

The photocatalyzed transformation of the anionic 4-dodecyl-1,1′-oxybisbenzenedisulfonate (DOBS) surfactant and its derivatives was carried out in aqueous TiO₂ dispersions under UV and aerated illumination conditions to examine the dynamics and the mechanistic features of the degradative processes. Spectroscopic and other methodologies were used to probe the break-up of the aromatic ring, the mineralization of the surfactant(s) to carbon dioxide, decay of total organic carbon (TOC) remaining in solution, formation of sulfate ions, and identification of oxidation intermediates (e.g., acetic acid, formic acid and acetaldehyde) by NMR spectroscopy. Adsorption of DOBS, in particular, and the other anionic surfactant derivatives in general, on the TiO₂ particle surface was inferred from the calculated point charges. Frontier electron densities of all atoms ascertained the positions of ˙OH radical attack on the DOBS molecular skeleton. A model for the initial adsorption behavior of DOBS through electrostatic forces between the anionic surfactant(s) and the cationic TiO₂ particle surface is proposed, aided by evidence from ζ-potential measurements. It is deduced that ˙OH radicals attack preferentially the carbon atoms bonded to the sulfonate groups on the DOBS 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 hydrophobic alkyl chain.