Modeling competitive cation exchange of aromatic amines in water-saturatedsoils

Competitive association to several components of sail through ion exchange processes influences the fate of organic cations in the environment. To exa mine these processes, the distributions of aniline and 1-aminonaphthalene b etween aqueous 5 mM CaCl2 solutions and three different Indiana soils were evaluated. Solute ratios (Sr) of aniline to 1-aminonaphthalene of 0.4-4.7 w ere employed, and the soil solutions ranged in pH from 2.7 to 7.5, with all measurements made 24 h after the introduction of the chemicals to the soil s. Two previously proposed equilibrium models - the two-site (TS) and distr ibuted parameter (DP) models - were modified to predict competition. These models assume instantaneous equilibrium of the following reversible process es: (i) acid dissociation of the protonated organic base (BHaq+) in the aqu eous phase; (ii) ion exchange on the soil between the protonated organic ba se and inorganic divalent cations (DXaq2+ = Ca-aq(2+) + Mg-aq(2+)); and (ii i) partitioning of the nonionic species of aniline (B-aq) to soil organic c arbon. The TS model is a general mass action model that does not take into consideration cation exchange site heterogeneity, whereas the DP model cons iders association constants to these sites to be distributed in a log-norma l fashion. To describe competition for cation exchange sites within the DP model, it was necessary to add a correlation coefficient (rho) that relates the ion-exchange association constant (K-BH) probability density distribut ion functions of the two compounds. The value of rho is characteristic of e ach soil. Results indicate that competition has a greater effect at low pH values, where ion exchange is the predominant process. For all cases, these models capture the general trends in the soil-water distribution data of b oth amines. The DP model also captures the nonlinearity of the 1-aminonapht halene isotherms at low pH while at the same time capturing the nearly line ar isotherms of aniline as a competing organic base.