Effects of aluminum-induced aggregation on the fluorescence of humic substances

Aluminum-induced aggregates of terrestrial and aquatic humic acid standards from the International Humic Substances Society are shown to be fluorescen t by means of a multiwavelength fluorescence anisotropy experiment in which the data was treated with a model for nonspherical particles. While aggreg ates of aquatic humic acids appear in the fluorescence signal at both short and long excitation wavelengths, aggregates of terrestrial humic acids are detected only at the long wavelength. Furthermore, the results indicate th at emission obtained at longer excitation wavelengths is representative of smaller particles. At pH 4, the aquatic humic acids appear to exist in an e xtended conformation, whereas the terrestrial humic acids show less extensi on. The size and shape of the fluorescent particles display a complex depen dence on Al concentration. Both enhancement and quenching of fluorescence a re observed in the total luminescence spectra upon Al addition. However, qu enching is shown to be the result of decreased humic acid concentration due to precipitation by Al rather than photophysical processes.