Fluorescence characterization of the interaction Suwannee river fulvic acid with the herbicide dichlorprop (2-(2,4-dichlorophenoxy)propionic acid) in the absence and presence of aluminum or erbium

This study uses fluorescence spectroscopy to better understand the role of environmental metal ions in the interaction of charged herbicides with biochemical degradation product Suwannee River fulvic acid (SRFA). The interactions between the widely-used herbicide dichlorprop (2-(2,4-dichlorophenoxy)propionic acid) (DCPPA) with Al3+ and the comparative metal Er3+ were probed at pH 4.0. Fluorescence experiments on binary solutions at pH 4.0 clearly indicated that Al3+ and Er3+ strongly interact with both SRFA and DCPPA alone in solution as demonstrated by fluorescence quenching with DCPPA and enhancement with SRFA by Al3+ and fluorescence quenching of both SRFA and DCPPA fluorescence by Er3+. Titrating Al3+ or Er3+ to SRFA-DCPPA quenched SRFA fluorescence as compared to the SRFA-metal ion binary complexes. Formation constants were determined using the Ryan-Weber model for the titration data. The DCPPA fluorescence results strongly support the formation of DCPPA-Al3+ and DCPPA-Er3+ complexes at pH values above the pKa (3.0) of DCPPA. Excitation and emission data obtained on ternary solutions of SRFA-Al3+-DCPPA and SRFA-Er3+-DCPPA complexes at pH 4.0 suggest that at this pH where the predominant DCPPA species is negatively-charged, Al3+ and Er3+ metal ions may function to “bridge” negatively-charged fulvic acids to negatively-charged pesticides. Fluorescence data collected on UV-irradiated ternary complexes indicate that both metals can also bridge DCPPA interactions with SRFA under those conditions. The results of our studies suggest that creation of a herbicide-free boundary corridor is recommended near mines and runoff areas with metal ions in surface waters to control possible complexation among fulvic acids, DCPPA and metal ions that maintains these molecules in a bioavailable state to plants and animals.

Recorded fluorescence excitation and emission data indicate the formation of a SRFA-Al3+-DCPPA ternary complex as indicated by this isosbestic point.Figure optionsDownload as PowerPoint slide