Effect of the carboxylic substituent on the reactivity of the aromatic ring during the wet oxidation of phenolic acids

The wet oxidation of three phenolic acids that appear frequently in several industrial wastewaters (salicylic acid, p-hydroxybenzoic acid and 5-hydroxyisophthalic acid) was studied and compared with the wet air oxidation of phenol at 423 K, 1.0 MPa and pH 2. Results showed that the oxidation of salicylic and 5-hydroxyisophthalic acids was faster than phenol oxidation, with conversions of 100% and 90%, respectively after 200 min. The high reactivity of these compounds was attributed to the low electronic density of the aromatic ring, due to the ortho position of the substituents in the case of the salicylic acid and due to the presence of two carboxylic groups in the case of the 5-hydroxyisophthalic acid. On the other hand, p-hydroxybenzoic acid had a lower conversion rate and degree of mineralization than phenol, explained by the opposing effects on the electronic density of the aromatic ring of the substituents in the para position. In all cases, a maximum point in the colour of the reaction media (an indication of the accumulation of quinone-like compounds) was observed when the conversion of the phenolic compound was approximately 90%. Iron(II) changed the reactivity order of the phenolic compounds studied and, for any given conversion value, increased the degree of mineralization.