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Flourescence Spectroscopy of Humic Substances

Dep. of Soil and Environmental Sciences, Univ. of California, Riverside, CA 92521

Garrison Sposito^*

Dep. of Plant and Soil Biology, Univ. of California, Berkeley, CA 94720

* Corresponding author.

ABSTRACT

Fluorescence spectroscopy has been underutilized as a method to investigate the nature of humic substances. This study was conducted to obtain fluorescence excitation and emission spectra systematically for a variety of humic and fulvic acids of diverse origin. Solutions containing 50 g m^–3 humic or fulvic acid at pH values between 4 and 10 were investigated with excitation, emission, and synchronous-scan excitation modes of fluorescence spectroscopy. The excitation spectra were markedly different for humic vs. fulvic acid and could be used to distinguish unambiguously between the two kinds of humic substance in aqueous solution. The fulvic acid excitation spectra also differed as a function of provenance. The fluorescence emission spectra, although broad, showed maximum-intensity wavelengths that were lower for humic than for fulvic acids, indicating a greater degree of condensed, aromatic character in humic acids. Synchronous-scan excitation spectra of the humic substances showed complex lineshapes that depended sensitively on the type of humic substance as well as on the pH value. These spectra, which are unique for fluorescence processes, may have important potential in resolving condensed aromatic structures in humic and fulvic acids.

Contribution from the Dep. of Soil and Environmental Sciences, Univ. of California at Riverside.

Received for publication October 5, 1987.

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