Spectroscopic characterization of humic acid fractions isolated from soil using different extraction procedures

To extract humic substances from soils, the sequential resin-alkali extraction technique has recently been introduced, providing for milder conditions as compared to the classical isolation procedure. In this study the results of spectroscopic investigations of resin- and alkali-extracted humic acids (HAs) are reported. Five HA fractions were isolated from bog soil (USA) with carboxylate resin (RCOONa) and 0.1 M NaOH by three protocols. They comprised the classical acid-alkali isolation procedure with benzene/methanol pre-treatment of soil (one alkali-extracted fraction), and the sequential resin-alkali extraction both with benzene/methanol pre-treatment of soil and without it (two resin- and two alkali-extracted fractions); then the HA fractions were de-ashed by HCl/HF and freeze-dried. The HA samples were analyzed for elemental composition (C, H, N and O) by Fourier-transformed infrared (FTIR), ultraviolet-visible, and fluorescence spectroscopy in emission and synchronous-scan excitation modes. All the HAs obtained had low ash contents and similar elemental composition. The FTIR evidence suggests that alkali-extracted HAs contain more aliphatics, carbohydrates, aromatics and/or amides as compared to resin-extracted material, which is relatively rich in carboxylic and phenolic groups. Resin-extracted HAs have higher A(254)/A(436) and E-4/E-6 ratios than the alkali-extracted samples. They also exhibit higher fluorescence emission maximum. The sequential resin-alkali extraction procedure appears to provide isolation of soil HA fractions which vary in nonhumic compounds inclusion and in some photophysical characteristics. (c) 2005 Elsevier B.V All rights reserved.