Estimating the Percent Aromatic Carbon in Soil and Aquatic Humic Substances Using Ultraviolet Absorbance Spectrometry

The aromatic C content of humic substances is an important chemical property that can be used to explain formation, source, and potential interactions of the humic substances with pesticides and other contaminant organics. The relationship between the UV absorptivity at 272, 254, and 205 nm of six aquatic fulvic acids, eight soil fulvic acids, and four water soluble organic C (WSOC) fractions, and their aromatic C content, as determined by 13C cross‐polarization magic angle spinning nuclear magnetic resonance (CPMAS NMR) spectroscopy was evaluated. The percent aromatic C (determined by NMR) for the pooled soil and aquatic fulvic acids and WSOC fractions data was poorly correlated (r < 0.7) with the UV absorptivity measurements at all three wavelengths. When the soil fulvic acid was considered individually, however, a relatively high correlation (r = 0.80 to 0.85, P < 0.05) resulted for the prediction of aromatic C content using UV absorptivity. Poor correlations were found for the other individual fractions. Our data suggest that the prediction of aromatic C content using UV absorptivity was applicable only for base‐extracted soil fulvic acids and not for the aquatic fulvic acids and soil WSOC fractions used in this study.