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 aqueo us phases at short contact times was assessed. Mass action equations c onsidered in the distributed parameter model were (a) acid dissociatio n of the protonated organic amine (BH+); (b) sorption of the neutral s pecies (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 Gauss ian distribution on log KBH values with mode mu and standard deviation a was employed. The overall model was expressed as a system of two no nlinear equations with two unknown values (BH+ and D2+). The model was evaluated with aniline and a-naphthylamine isotherms measured on thre e Indiana soils ranging in pH (4.5-7.0) and added calcium concentratio n (0.5-50 mM) after a 24 h contact period. The parameters K-oc, mu, an d sigma for each amine were obtained by minimizing the sum of squared residuals between predicted and measured aqueous-phase organic amine c oncentrations (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 whi ch a singular value of KBH was employed. The distributed parameter for m of the model resulted in nonlinear calculated isotherms in general a greement with the curvature of alpha-naphthylamine measured isotherms. Skewed distributions on log K-BH were evaluated also.