Fluorescence study of the interaction of Suwannee River fulvic acid with metal ions and Al3+-metal ion competition

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 Al3+, with Suwannee River fulvic acid (SRFA), as well as competition between Al3+ and several other metal ions (Ca2+, Mg2+, Cu2+, Pd2+, La3+, Tb3+ and Fe3+) for binding sites on SRFA. Of the four metal ions possessing very similar (and relatively low) ionic indices (Ca2+, Mg2+, Cu2+ and Pd2+) 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 (Ca2+, Mg2+, La3+ and Tb3+) only the last paramagnetic ion (Tb3+) significantly quenched SRFA fluorescence. None of these metals was able to significantly compete with SRFA-bound Al3+Fe3+, 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 Al3+) and a relatively low covalent index (but not as low as Al3+), was able not only to quench SRFA fluorescence but also to compete (at least to some extent) with SRFA-bound Al3+. Synchronous-scan fluorescence SRFA spectra taken in the absence and presence of Fe3+ and/or Al3+ support the view that these two metal ions can compete for sites on SRFA. The results of these fluorescence experiments further confirm the Al3+, and metal ions that have electronic properties somewhat similar to Al3+ (such as Fe3+) are somewhat unique in their ability to interact strongly with binding sites on fulvic acids.