Fluorescence study of the interaction of Suwannee River fulvic acid with metal ions and Al 3+-metal ion competition

Abstract

In this study emission and synchronous-scan fluorescence spectroscopy have been used to investigate the interaction of the class A (oxygen seeking ‘hard acid’) metal Al 3+, with Suwannee River fulvic acid (SRFA), as well as competition between Al 3+ and several other metal ions (Ca 2+, Mg 2+, Cu 2+, Pd 2+, La 3+, Tb 3+ and Fe 3+) for binding sites on SRFA. Of the four metal ions possessing very similar (and relatively low) ionic indices (Ca 2+, Mg 2+, Cu 2+ and Pd 2+) only the latter two paramagnetic ions significantly quenched SRFA fluorescence emission intensity. Of the four metal ions possessing very similar (and relatively low) covalent indices (Ca 2+, Mg 2+, La 3+ and Tb 3+) only the last paramagnetic ion (Tb 3+) significantly quenched SRFA fluorescence. None of these metals was able to significantly compete with SRFA-bound Al 3+ Fe 3+, which differs substantially from all of the other metals examined in this study in that it possesses a relatively high ionic index (but not as high as Al 3+) and a relatively low covalent index (but not as low as Al 3+), was able not only to quench SRFA fluorescence but also to compete (at least to some extent) with SRFA-bound Al 3+. Synchronous-scan fluorescence SRFA spectra taken in the absence and presence of Fe 3+ and/or Al 3+ support the view that these two metal ions can compete for sites on SRFA. The results of these fluorescence experiments further confirm the Al 3+, and metal ions that have electronic properties somewhat similar to Al 3+ (such as Fe 3+) are somewhat unique in their ability to interact strongly with binding sites on fulvic acids.