Role of pH in partitioning and cation exchange of aromatic amines on water-saturated soils Hui Li a , Linda S. Lee a, * , Jose R. Fabrega b , Chad T. Jafvert b a Department of Agronomy, Purdue University, West Lafayette, IN 47907-1150, USA b School of Civil Engineering, Purdue University, West Lafayette, IN 47907-1284, USA Received 5 May 2000; accepted 26 September 2000 Abstract Predicting the reversible interactions between aromatic amines and soil is essential for assessing the mobility, bio- availability and exposure from contaminated sites. Reversible sorption mechanisms of aniline and a-naphthylamine were investigated by using single and binary solute sorption to ®ve soils at several pH values, and by applying a dis- tributed parameter DP) model. The DP model assumes linear partitioning of the neutral species into soil organic matter domains and organic cation binding on negative-charged sites with the exchange coecients represented by a Gaussian probability distribution. Sorption nonlinearity was attributed to cation exchange with varying site anities, which was adequately simulated using the DP model. Greater uptake by hydrophobic partitioning and selectivity for cation exchange sites was observed for a-naphthylamine compared to aniline. Sorption of a-naphthylamine was not impacted quantitatively by aniline under those conditions examined; however, aniline sorption was reduced by a- naphthylamine with the largest reduction occurring in the soil with the lowest pH. DP model simulations showed that although hydrophobic partitioning increases with soil±solution pH, cation exchange still contributes signi®cantly to the total sorption even at soil±solution pH values greater than pK a 2. Ó 2001 Elsevier Science Ltd. All rights reserved. Keywords: Competitive sorption; Speciation; Aromatic amine; Distributed parameter model; Cation exchange; Partitioning 1. Introduction Organic bases such as aniline and aniline derivatives are of environmental concern because of their potential toxicity, carcinogenicity, and the large mass of these produced annually as raw materials in the manufactur- ing of dyes, pesticides, varnishes and perfumes Schnell et al., 1989; Essington, 1994). Anilines also are found in wastes from coal gasi®cation and shale oil extraction processesandastransformationproductsinsoilstreated with carbamate herbicides Still and Herrett, 1975; Zachara et al., 1984). Our recent eorts to develop comprehensive quantitative relationships for describing their sorption to natural sorbents have included incor- poration of both reversible and irreversible processes Lee et al., 1997; Fabrega et al., 1998; Li and Lee, 1999; Fabrega et al., 2000; Li et al., 2000). Aromatic amines undergo primarily reversible sorption within the ®rst 24 h Lee et al., 1997), during which time the irreversible sorption and transformation processes are much less Li and Lee, 1999). Two primary mechanisms that are responsible for aromatic amine reversible sorption on soils are: i) ca- tion exchange of the protonated organic species with inorganic cations on minerals and soil organic matter SOM) Zachara et al., 1986; Fabrega et al., 1998); and www.elsevier.com/locate/chemosphere Chemosphere 44 2001) 627±635 * Correspondingauthor.Tel.:+1-765-494-8612;fax:+1-765- 496-2926. E-mail address: lslee@purdue.edu L.S. Lee). 0045-6535/01/$ - see front matter Ó 2001 Elsevier Science Ltd. All rights reserved. PII:S0045-653500)00511-7