Modeling short-term soil-water distribution of aromatic amines

The applicability of a distributed parameter model for describing the distribution of aniline and alpha-naphthylamine between soil and aqueous phases at short contact times was assessed. Mass action equations considered in the distributed parameter model were (a) acid dissociation of the protonated organic amine (BH+); (b) sorption of the neutral species (B) to soil organic carbon through the partition coefficient K-oc; and (c) ion exchange of the protonated organic amine and inorganic divalent cations (Ca2+ + Mg2+ = D2+). The last reaction was expressed mathematically as separated association reactions for each cation to unoccupied cation exchange sites, with constants K-BH and K-D. A Gaussian distribution on log KBH values with mode mu and standard deviation a was employed. The overall model was expressed as a system of two nonlinear equations with two unknown values (BH+ and D2+). The model was evaluated with aniline and a-naphthylamine isotherms measured on three Indiana soils ranging in pH (4.5-7.0) and added calcium concentration (0.5-50 mM) after a 24 h contact period. The parameters K-oc, mu, and sigma for each amine were obtained by minimizing the sum of squared residuals between predicted and measured aqueous-phase organic amine concentrations (i.e., [B](aq) + [BH+](aq)) for all soil, pH, and CaCl2 conditions, simultaneously. Calculated isotherms were compared against those calculated with the general form of the speciation model in which a singular value of KBH was employed. The distributed parameter form of the model resulted in nonlinear calculated isotherms in general agreement with the curvature of alpha-naphthylamine measured isotherms. Skewed distributions on log K-BH were evaluated also.