Characterization, differentiation, and classification of humic substances by fluorescence spectroscopy

Fifty samples of humic acids and fulvic acids isolated from various soils and soil-related materials (including paleosols, peat, leonardite, composted and earth-worm-composted organic materials, sewage sludges, and materials synthesized by soil fungi) have been investigated by fluorescence spectroscopy in the emission, excitation, and synchronous-scan excitation modes. Emission spectra are generally characterized by a unique broad band showing a maximum wavelength and relative fluorescence intensity that depend mainly on the nature and origin of the humic material. Excitation and synchronous-scan excitation spectra generally exhibit a number of peaks and shoulders of relative intensity at wavelengths that also vary according to the type and source of the humic material. The observed, distinct fluorescence properties of humic substances provide useful diagnostic criteria for distinguishing between humic and fulvic acids from the same source and between humic or fulvic acids from various sources. As a consequence, a classification of humic substances is proposed on the basis of their fluorescence behavior. Hypotheses on the chemical nature of relevant fluorescing structures in the various humic materials are suggested by comparing experimental data with corresponding fluorescence data available for defined simple molecules. The long wavelengths and low intensities measured for the major fluorescence peaks of paleosol, soil, peat, and leonardite humic acid are mainly ascribed to the presence of linearly-condensed aromatic ring and other unsaturated bond systems, capable of a high degree of conjugation and bearing electron-withdrawing substituents such as carbonyl and carboxyl groups, and to their high molecular weight units. The short wavelengths and high intensities generally measured for main fluorescence peaks of compost and earth-worm-composted humic acid and soil and peak fulvic acid are associated with the presence of simple structural components of low molecular weight, low degree of aromatic polycondensation, low levels of conjugated chromophores, and bearing of electron-donating substituents such as hydroxyls, methoxyls, and amino-groups. In the Conclusions, emphasis is given to the utility of further investigations on fluorescence behavior of humic substances.