Yulia Gennadyevna Izosimova

The content of benzoic acid in the water extract from the organic horizons of the podzolic soil was found to be 3,27 and 1,83 ppm in the horizons L and F, respectively, and it decreases to hundredths of ppm in the mineral horizons. It was established in an experiment, that with an increase in the benzoic acid equilibrium concentration of from 0,02 to 1,45 mmol/L, its sorption onto Ca-montmorillonite increases from 5,61 to 15,34 mmol/kg. Taking into account the content and mineralogical composition of the clay fraction, the expandable clays in the upper mineral horizons of the podzolic soil are capable to retain from 0,06 to 0,15 mmol of benzoic acid per 1 kg of soil within the same range of equilibrium concentrations. The linear equation and the Freundlich model provide the best fit to the measured data. It is assumed that, under experimental conditions, the main mechanisms of benzoic acid sorption were the cation bridging or water bridging and the electrostatic interaction of benzoate with positively charged silanol and aluminol groups on edges of montmorillonite particles. The absorption of benzoic acid in the interlayer space of the mineral wasn’t observed.

The fractionation of the clay fraction was carried out and the mineral composition of clay and its subfractions in the soil of the rhizosphere of the Norway maple (Acer platanoides L.) and in the bulk soil from the AELoa horizon of podzolic soil was studied. It was found that the clay fraction is dominated by a 1‒0.2 microns subfraction (56% in the rhizosphere of maple and 60% in the bulk soil) and contains 20% of 0.2‒0.06 microns subfraction in the rhizosphere and 27% of this fraction in the bulk soil. The content of 0.06‒0.02 and <0.02 microns subfractions did not exceed 10%. A significant increase was revealed in the proportion of 0.06‒0.02 microns subfraction and a decrease in the proportion of <0.02 microns subfraction in the rhizosphere compared with the bulk soil. The size fractions of clay both in the rhizosphere and in the bulk soil are differentiated by their mineral composition. The 1‒0.2 microns fraction is dominated by kaolinite; the proportion of micas increased and the proportion of kaolinite decreased in the 0.2‒0.02 microns fraction, while in the <0.02 microns fraction the proportion of labile minerals significantly increases. There was no significant difference in the mineral composition of the clay fraction and the 1‒0.2 microns subfraction. It was found that the 0.2‒0.02 microns subfraction in the rhizosphere contains significantly more labile minerals compared to the bulk one, which are a result of intensively occurring mica transformation processes into labile structures in the rhizosphere.

The interaction of water-soluble organic matter (DOM) in aqueous extracts obtained from H horizon samples of peaty-podzolic-gleyic soil on kaolinite and muscovite were studied. The extracts contained different amounts of water-soluble Corg. mainly with molecular masses of about 20 kDa, represented mainly by hydrophilic compounds, but with different ratios of hydrophobic and hydrophilic components and with different concentrations of compounds containing aromatic structures. It has been established that the sorption of DOM components under the experimental conditions depends on the Сorg. concentration in the extracts, on the ratio of hydrophilic and hydrophobic components, on the mineral : solution ratio and practically does not depend on pH. Hydrophobic components and aromatic compounds are more sorbed on kaolinite surface compared to muscovite. The observed decrease of C org. sorption on both minerals when sorption per unit area of external surface in the experiments with the mineral : solution ratio of 1:50 in comparison with the ratio of 1:10 may indicate both the heterogeneity of sorption centers on the surface of minerals and the competition for these centers between the components of DOM. No significant changes in the curves of molecular weight components of DOM after interaction with kaolinite and with muscovite were revealed.