Acid Orange 7 (AO7) was decolorized and degraded by titaniabased
photocatalysis [51]. Sulfonated and polar intermediates
were identified using the LC-(+ESI)-MS technique (Scheme 3A). A
detailed reaction mechanism was proposed from the initial step
generating OH free radicals to the final products. It was shown that
hydroxylated AO7 underwent partial and complete oxidation of
the naphthalene ring to form stable intermediates {namely sodium
4-[(2,3-dihydroxy-1-naphthyl) diazenyl] benzene sulfonate,
4-[2-(4-sulfophenyl)hydrazino] pent-2-enedioic acid, and phenyldiazene-
carboxylic acid}. The opening of the naphthalene ring of
AO7 was explained by the attack of OH radical on the ortho position
with respect to the hydroxyl group of naphthalene ring to
form naphthaquinone. Further concerted cleavage of the naphthalene
ring resulted in the formation of carboxylic acids. LC-MS was
successfully employed to identify intermediates that could not be
identified by other techniques.