Influence of Soil Organic Matter Composition on the Partition of Organic Compounds

The sorption at room temperature of benzene and carbon tetrachloride from water on three high-organiccontent soils (muck, peat, and extracted peat) and on cellulose was determined in order to evaluate the effect of sorbent polarity on the solute partition coefficients. The isotherms are highly linear for both solutes on all the organic matter samples, which is consistent with a partition model. For both solutes, the extracted peat shows the greatest sorption capacity while the cellulose shows the lowest capacity; the difference correlates with the polarto-nonpolar group ratio [(O + N)/C] of the sorbent samples. The relative increase of solute partition coefficient (K_om) with a decrease of sample polar content is similar for both solutes, and the limiting sorption capacity on a given organic matter sample is comparable between the solutes. This observation suggests that one can estimate the polarity effect of a sample of soil organic matter (SOM) on K_om of various nonpolar solutes by determining the partition coefficient of single nonpolar solute when compositional analysis of the SOM is not available. The observed dependence of K_om on sample polarity is used to account for the variation of K_om values of individual compounds on different soils that results from change in the polar group content of SOM. On the assumption that the carbon content of SOM in “ordinary soils” is 53-63%, the calculated variation of K_om is a factor of ~3. This value is in agreement with the limit of variation of most K_om data with soils of relatively high SOM contents.