SOIL AGGREGATION AS A SOURCE OF VARIATION IN SORPTION ISOTHERMS OF HYDROPHOBIC ORGANIC COMPOUNDS.

Nonlinear sorption isotherms have significant environmental implications because concentration-dependent mobility of hydrophobic organic compounds complicates model predictions in soil-water systems. To better understand the factors that affect the nonlinearity of sorption isotherms, we investigated the impact of aggregation of soil materials on isotherm shape for a model hydrophobic organic compound-phenanthrene. The isotherms were constructed from batch experiments using both 0.5- to 1.0-mm macroaggregates and <2-[mu]m microaggregates of two Iowa Mollisols. Sorption experiments were conducted in 0.01 M CaCl2 background electrolyte and also with disaggregated sorbents that were dispersed by using sonification or by shaking in 0.03 M NaCl. For the poorly aggregated, low-clay samples, both Freundlich sorption parameters (Kʹf and n) determined in the CaCl2 background were diminished by sonification, and shaking in NaCl increased the sorption capacity. In contrast, sorption isotherms for clay-rich macroaggregates and microaggregates were nonlinear (Freundlich exponent, n = 0.53-0.67) when sorption experiments were conducted in 0.01 M CaCl2. Isotherm nonlinearity decreased (n increased) upon dispersion of aggregates by NaCl treatment. Our results suggest that, in clay-rich soil material, aggregation may regulate accessibility to sorption locales, increase sorption isotherm nonlinearity, and decrease sorption capacity. These findings encourage caution when comparing, combining, or interpreting hydrophobic organic compound sorption parameters that have been determined on different soil materials by varying sample pretreatments or in varying matrices.