Modeling Abiotic Processes of Aniline in Water-Saturated Soils

Abstract

The long-term interactions of aromatic amines with soils are important in defining the fate and transport of these compounds in the environment. Abiotic loss of aniline from the aqueous phase to the soil phase occurs with an initial rapid loss due to reversible mass transfer processes, followed by a slow loss due to irreversible reactions. A kinetic model describing these processes in water-saturated soils was developed and evaluated. The model assumes that instantaneous equilibrium occurs for the following reversible processes: (i) acid dissociation of the protonated organic base (BH+ ) in the aqueous phase; (ii) ion exchange between inorganic divalent cations (D2+ = Ca2+ + Mg2+) on the soil and the protonated organic base; and (iii) partitioning of the nonionic species of aniline (Baq ) to soil organic carbon. The model assumes that irreversible loss of aniline occurs through reaction of Baq with irreversible sites (Cir) on the soil. A kinetic rate constant, kir, and the total concentration of irreversible sites, CT, were employed as adjustable model parameters. The model was evaluated with measured mass distributions of aniline between water (with 5 mM added CaCl2) and five soils ranging in pH (4.4−7.3), at contact times ranging from 2 to 1600 h. Some experiments were performed at different soil mass to water volume ratios. A good fit was obtained with a single value of kir for all soils, pH values, and soil−water ratios. To accurately predict soil−water distributions at contact times < 24 h, mass transfer of the neutral species to the soil was modeled as a kinetic process, again, assuming that ion exchange processes are instantaneous.