Degradation of naphthalenesulfonic acids by oxidation with ozone in aqueous phase

Sulfonic aromatic compounds are widely used in the textile industry for the synthesis of azo dyes. The presence of the sulfonic group endows these compounds with high water-solubility and resistance to biological treatment. The present work analysed the efficacy of oxidation with ozone in the treatment of waters containing naphthalene-1-sulfonic acid, naphthalene-1,5-disulfonic acid and naphthalene-1,3,6-trisulfonic acid. The reactivity to ozone significantly decreased with an increase in the number of sulfonic groups in the aromatic ring, and naphthalene-1,3,6-trisulfonic acid showed the lowest reaction rate. The stoichiometry of the direct ozonization reaction was around one mole of sulfonic acid per mole of ozone in all cases. The activation energy value of the direct ozonization reaction was very similar in all studied cases (37–42 kJ mol−1). The contribution of the direct reaction to the global oxidation at pH 2 was observed to decrease with more sulfonic groups in the aromatic ring, from a contribution of 69% for naphthalene-1-sulfonic acid to around 20% for naphthalene-1,3,6-trisulfonic acid. The radical reaction rate constant was found to be of the order of 109 × M−1 s−1 in all cases, although it showed a slight reduction as the number of sulfonic groups in the aromatic ring increased. A follow-up study of the oxidation products of each sulfonic aromatic acid revealed the presence of highly oxidized organic acids and sulfate ions. A theoretical study of the electronic density at the bond critical point showed that double bonds with highest electron density in the three acids are in positions 1–2, 3–4, 5–6 and 7–8. These bonds are initially attacked by ozone via 1,3-dipolar cycloaddition.