Fluorescence characterization of the interaction of Al 3+ and Pd 2+ with Suwannee River fulvic acid in the absence and presence of the herbicide 2,4-dichlorophenoxyacetic acid

Abstract

In an effort to understand the role of environmental metal ions in the interaction of charged pesticides with humic substances, a fluorescence study of the interaction of the widely-used herbicide 2,4-dichlorophenoxyacetic acid (DCPAA) with Al 3+ and Pd 2+ and Suwannee River fulvic acid (SRFA) was undertaken. Initial fluorescence experiments on binary solutions clearly indicated that both Al 3+ and Pd 2+ strongly interact with both SRFA and DCPAA when alone in solution with the metal ion. Titrations of SRFA with Al 3+ at pH values of 4.0, 3.0 and 2.0 revealed decreased degrees of fluorescence emission enhancement (at λ emission, max=424 nm) with decreasing pH, consistent with the expected loss of rigidity in the SRFA–Al 3+ complexes formed as pH is lowered. In contrast, titrations of SRFA with Pd 2+ at all of these pH values resulted in significant fluorescence quenching. Al 3+ additions to solutions of DCPAA at pH values above the p K a (2.64) of DCPAA resulted primarily in significant changes in the wavelength of maximum emission (without significant quenching or enhancement of emission intensity), while Pd 2+ additions to DCPAA solutions resulted primarily in very significant fluorescence quenching. The DCPAA fluorescence results strongly support the formation of an Al 3+–DCPAA complex at pH values above the p K a of DCPAA. The fluorescence results obtained for solutions of Pd 2+ and DCPAA are best explained by a collisional quenching mechanism, that is, energy transfer from excited DCPAA molecules to Pd 2+ following the collision of these two species in solution. Excitation–emission matrix plots obtained on ternary solutions (at environmentally-relevant pH 4.0) containing SRFA, DCPAA and metal ions (i.e., either Al 3+ or Pd 2+) provides evidence (especially for systems containing Al 3+) for the existence of ternary complexes between fulvic acid species, the herbicide DCPAA and metal ion, suggesting (at least at pH 4.0, where the predominant DCPAA species is negatively-charged) that metal ions may function to “bridge” negatively-charged fulvic acids to negatively-charged pesticides.