Modeling Competitive Cation Exchange of Aromatic Amines in Water-Saturated Soils
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Competitive association to several components of soil through ion exchange processes influences the fate of organic cations in the environment. To examine these processes, the distributions of aniline and 1-aminonaphthalene between 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 were 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 soils. Two previously proposed equilibrium modelsthe two-site (TS) and distributed parameter (DP) modelswere modified to predict competition. These models assume instantaneous equilibrium of the following reversible processes: (i) acid dissociation of the protonated organic base (B ) in the aqueous phase; (ii) ion exchange on the soil between the protonated organic base and inorganic divalent cations ( = C + M); and (iii) partitioning of the nonionic species of aniline (Baq) to soil organic carbon. The TS model is a general mass action model that does not take into consideration cation exchange site heterogeneity, whereas the DP model considers association constants to these sites to be distributed in a log-normal fashion. To describe competition for cation exchange sites within the DP model, it was necessary to add a correlation coefficient (ρ) that relates the ion-exchange association constant (KBH) probability density distribution functions of the two compounds. The value of ρ is characteristic of each 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 both amines. The DP model also captures the nonlinearity of the 1-aminonaphthalene isotherms at low pH while at the same time capturing the nearly linear isotherms of aniline as a competing organic base.