In this regard we examined native soil samples with the purpose to find moisture conditions that alter the ratio of their activities, and to establish the relation between the chemical nature of a sorbate and the presence of SOM blocking effect on sorption as a consequence of clay-SOM interactions. Vapor-phase sorption isotherms of n-nonane, p-xylene (hydrocarbons - HC) and methanol (an aliphatic alcohol) on soils in the moisture range of 0-9% were obtained with the help of static headspace gas-chromatographic analysis method. Soil samples from seven separate horizons of dark-grey forest soil (Tatarstan republic, Russia) were analyzed, which had similar granulometric composition (34-38% of clay) but were significantly different in Corg (0-7.5%). The sorption values Vs of both HC (p-xylene and n-nonane) on oven-dried soil samples strongly decreased with the increase of Corg. This showed pronouncedly that HC interact with SOM more weekly than with clay compounds, and that furthermore, SOM blocks active sorption centers of soil clay surface. The decrease of Vs values of both HC (p-xylene and n-nonane) with the increase of Corg (from lower to upper soil layers) was also observed for soil samples moistened up to 2-4%. However, this tendency became weaker when moisture content increased. When the latter reached 5%, sorption values were practically independent from SOM contents. Moreover, at 6%, we even observed a small inversion of this dependence, i.e. the growth of Vs values with the increase of Corg contents. In comparison with HC sorption, our results for methanol were different. Here, Vs values for methanol sorbed by oven-dried soil layers did not change significantly with the increase of Corg. This indicates the similarity between sorption activities of non-hydrated mineral and organic soil constituents in relation to methanol. This result is surprising since it is known that alcohols are capable to interact specifically with soil clay minerals as opposed to HC, which realize only non-specific interactions. In this connection we have proposed that the main reason of such fact is different mechanisms of interaction of alcohols and HC with SOM. To verify this possibility, we studied the sorption of HC (benzene) and aliphatic alcohol (ethanol) by dried samples of humic acids extracted from dark-grey forest soil. As a result we have seen that Vs values for benzene on HA were insignificant (close to zero) in all isotherm range, whereas Vs values for ethanol were also insignificant up to activities of P/P0 = 0.25 but strongly increased (single-order) at activities above P/P0 = 0.25. Such strong positive "cooperative effect" was due to the change of spatial phase structure of HA caused by the initially sorbed alcohol molecules and resulting in the formation of additional binding centers.
Thus, we have demonstrated that relative sorption activities of soil clay and organic constituents depend not only on moisture but also on the chemical sorbate nature. The effect of sorption "blocking" caused by the coating of oven-dried mineral surface with SOM, has no universal character and is dependent on the nature of organic sorbates. * The work was supported by ISTC, Project #2419; RFBR, Project 06-04-49097a; and Grant of President of Russia MK-2439.2005.5.
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