Role of pH in partitioning and cation exchange of aromatic amines on water-saturated soils

Predicting the reversible interactions between aromatic amines and soil is essential for assessing the mobility, bioavailability and exposure from con taminated sites. Reversible sorption mechanisms of aniline and alpha -napht hylamine were investigated by using single and binary solute sorption to fi ve soils at several pH values, and by applying a distributed parameter (DP) model. The DP model assumes linear partitioning of the neutral species int o soil organic matter domains and organic cation binding on negative-charge d sites with the exchange coefficients represented by a Gaussian probabilit y distribution. Sorption nonlinearity was attributed to cation exchange wit h varying site affinities, which was adequately simulated using the DP mode l. Greater uptake by hydrophobic partitioning and selectivity for cation ex change sites was observed for alpha -naphthylamine compared to aniline. Sor ption of alpha -naphthylamine was not impacted quantitatively by aniline un der those conditions examined; however, aniline sorption was reduced by alp ha -naphthylamine with the largest reduction occurring in the soil with the lowest pH. DP model simulations showed that although hydrophobic partition ing increases with soil-solution pH, cation exchange still contributes sign ificantly to the total sorption even at soil-solution pH values greater tha n pK(a) + 2. (C) 2001 Elsevier Science Ltd. All rights reserved.