Autoxidation Reactions of Different Aromatic o -Aminohydroxynaphthalenes That Are Formed during the Anaerobic Reduction of Sulfonated Azo Dyes

Mono- and disulfonated naphthalene derivatives with a hydroxy group in ortho -position to an amino group are commonly occurring products of the anaerobic reduction of azo dyes by microorganisms. These substituted o -aminohydroxybenzenes and o -aminohydoxynaphthalenes are oxygen-sensitive and decompose under aerobic conditions. To evaluate their behavior under aerobic conditions, 1-amino-2-hydroxynaphthalene-6-sulfonate [AHNS; a reduction product of Sunset Yellow FCF (FD&C Yellow No.6, C.I. No. 15985)], 1-amino-2-hydroxynaphthalene-3,6-disulfonate [AHNDS; a reduction product of Amaranth (Acid Red 27, C.I. No. 16185)], and 1,2,7-triamino-8-hydroxynaphthalene-3,6-disulfonate [TAHNDS; a reduction product of Naphthol Blue Black B (Acid Black 1, C.I. No. 20470)] were chemically prepared and exposed to air at neutral pH. The reactions were analyzed, and the autoxidation products were characterized mainly by LC−MS and UV/VIS spectroscopy. Autoxidation of AHNS led to the formation of a dimer. With AHNDS, 1,2-naphthoquinone-3,6-disulfonate was found as intermediate, which subsequently reacted to a disulfonated cinnamic acid derivative. From TAHNDS a stable autoxidation product with a deep blueish color was formed that was identified as a naphthoquinone imine derivative of the parent compound. The spontaneous autoxidation reactions were compared to the behavior of the o -aminohydroxynaphthalenes and their products in the presence of activated sludge under aerobic conditions over the period of 1 month. In the presence of activated sludge, a biological conversion of the stable autoxidation product of TAHNDS was observed.