Fluorescence characterization of the interaction of Al3+ and Pd2+ with Suwannee River fulvic acid in the absence and presence of the herbicide 2,4-dichlorophenoxyacetic acid

摘要

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 Al3+ and Pd2+ and Suwannee River fulvic acid (SRFA) was undertaken. Initial fluorescence experiments on binary solutions clearly indicated that both Al3+ and Pd2+ strongly interact with both SRFA and DCPAA when alone in solution with the metal ion. Titrations of SRFA with Al3+ at pH values of 4.0, 3.0 and 2.0 revealed decreased degrees of fluorescence emission enhancement (at lambda(emission, max) = 424 nm) with decreasing pH, consistent with the expected loss of rigidity in the SRFA-Al3+ complexes formed as pH is lowered. In contrast, titrations of SRFA with Pd2+ at all of these pH values resulted in significant fluorescence quenching. Al3+ additions to solutions of DCPAA at pH values above the pK(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 Pd2+ additions to DCPAA solutions resulted primarily in very significant fluorescence quenching. The DCPAA fluorescence results strongly support the formation of an Al3+-DCPAA complex at pH values above the pK(a) of DCPAA. The fluorescence results obtained for solutions of Pd2+ and DCPAA are best explained by a collisional quenching mechanism, that is, energy transfer from excited DCPAA molecules to Pd2+ 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 Al3+ or Pd2+) provides evidence (especially for systems containing Al3+) 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. (C) 2003 Elsevier Inc. All rights reserved. [References: 44]