Sorption of Water-Soluble Organic Substances by Mineral Horizons of Podzol

Transformation of water-soluble organic substances entering mineral horizons of podzol from the forest litter was studied in a model column experiment. The concentration of organic matter in the solution after its interaction with the E horizon increased by 10-44% due to the release of native soluble organic matter. This was accompanied by changes in the quality of water-soluble organic matter: the portion of hydrophobic components increased, the coefficients of extinction at 254-280 nm decreased, and the portion of low-molecular-weight compounds slightly decreased with a corresponding increase in the portion of high-molecular-weight compounds. Upon the interaction of water-soluble organic substances with the E horizon, the pH values first decreased and then slightly increased, which could be due to sequential desorption of organic acids of different strengths. Dissolved organic substances from the E horizon were partly sorbed in the BF horizon at the rate of about 65 mg C/100 g, or 22% of C-org in the BF horizon. The sorption resulted in the removal of mainly hydrophobic medium- and high-molecular-weight (>7000 Da) organic substances from the solution. As judged from the coefficients of extinction in the UV area, the sorbed substances were characterized by a higher aromaticity in comparison with the substances remaining in the solution. The sorption of organic substances in the BF horizon was accompanied by the rise in the pH of the solution, which corresponded to binding of organic anions on the surface of iron hydroxides according to the mechanism of ligand exchange with OH ions.