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 orth o-position to an amino group are commonly occurring products of the anaerob ic reduction of azo dyes by microorganisms. These substituted o-aminohydrox ybenzenes and o-aminohydoxynaphthalenes are oxygen-sensitive and decompose under aerobic conditions. To evaluate their behavior under aerobic conditio ns, 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-hydroxynap hthalene-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-disulfona te [TAHNDS; a reduction product of Naphthol Blue Black B (Acid Black 1, C.I . No. 20470)] were chemically prepared and exposed to sir at neutral pH. Th e reactions were analyzed, and the autoxidation products were characterized mainly by LC-MS and UV/VIS spectroscopy. Autoxidation of AHNS led to the f ormation of a dimer. With AHNDS, 1,2-naphrhoquinone-3,6-disulfonate was fou nd as intermediate, which subsequently reacted to a disulfonated cinnamic a cid derivative. From TAHNDS a stable autoxidation product with a deep bluei sh color was formed that was identified as a naphthoquinone imine derivativ e of the parent compound. The spontaneous autoxidation reactions were compa red 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.