Effects of solution chemistry on the oxidative transformation of 1-naphthol and its complexation with humic acid

The extent of 1-naphthol (1-hydroxynaphthalene) complexation with humic acid (HA) was studied as a function of solution chemistry [pH (4-11), ionic strength, I (0.001 and 0.1 M LiCl), and dissolved O₂ (DO) concentration (0 and 8 mg L^-1)] using spectroscopic (fluorescence, UV absorbance) and macroscopic (equilibrium dialysis) techniques. 1-Naphthol is transformed by DO in aqueous solutions; oxidation increases with pH and I, producing (hydroxy)naphthoquinones and coupled reaction products. Quenching of 1-naphthol fluorescence by HA increased with equilibration time from 1 to 7 d. This time-dependent relationship was found to result from (a) weak complexation of 1-naphthol by HA and (b) oxidative transformation of 1-naphthol (slow reaction) resulting in the formation of secondary products that are more reactive with HA. Sorption of ¹⁴C-labeled compound (as measured by equilibrium dialysis) followed a pH-dependent trend with minimal removal below pH 7.0, a sharp increase over a narrow pH range, and maximum retention around pH 9.0. Effects of I were observed only between pH 8 and pH 10 where a 100-fold increase in Li⁺ concentration doubled the extent of sorption. Good agreement between fluorescence quenching and dialysis methods was obtained. Exclusion of DO from the reaction vessel resulted in only a moderate decrease in the amount of ¹⁴C sorption, which suggests that functional groups in HA may participate directly in electron-transfer reactions with 1-naphthol.