Complexation of 1-Naphthol by Humic and Fulvic Acids

Complexation of organic solutes with dissolved humic substances may have a significant effect on the chemistry and fate of contaminants in natural systems. The complexation of 1-naphthol with dissolved humic and fulvic acids (HA and FA) in 0.01 M KCl was studied using fluorescence quenching. No increase in quenching at elevated temperature and an increase in quenching ratio at higher viscosity both indicated that the quenching was due to the formation of naphthol-HA water-soluble complexes. Conditional equilibrium constants for the association of 1-naphthol with humic and fulvic acids were estimated from the Stern-Volmer equation, and ranged from 9.7 to 33.4 L/g C. In general, HA showed a stronger affinity for 1-naphthol than FA. The ionic strength of the solution, adjusted using KCl and CaCl₂, did not significantly affect the complexation of 1-naphthol with HA. The quenching ratio increased as the pH of the solution increased, suggesting that changes in the dispersion of HA polymers or additional binding mechanisms result in increased HA complexation of naphthol at higher pH. The complexation of 1-naphthol with HA was enhanced in the presence of Cu²⁺ and Zn²⁺ at a pH higher than the dissociation constant (pKₐ) of 1-naphthol (9.34). This suggests that cation bridging between functional groups of anionic organic solutes and humic or fulvic acids is an additional binding mechanism for the association of organic solutes with soluble humic substances.