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The applicability of a distributed parameter model for describing the distribution of aniline and α-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 K BH values with mode μ and standard deviation σ 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 α-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, μ, and σ 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 K BH was employed. The distributed parameter form of the model resulted in nonlinear calculated isotherms in general agreement with the curvature of α-naphthylamine measured isotherms. Skewed distributions on log K BH were evaluated also.

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 models--the two-site (TS) and distributed parameter (DP) models--were modified to predict competition. These models assume instantaneous equilibrium of the following reversible processes: (i) acid dissociation of the protonated organic base (BHaq+) in the aqueous phase; (ii) ion exchange on the soil between the protonated organic base and inorganic divalent cations (Daq2+ = Caaq2+ + Mgaq2+); 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 (rho) that relates the ion-exchange association constant (KBH) probability density distribution functions of the two compounds. The value of rho 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.

Abstract:This work analyzes hydroclimate projections in Panama toward the end of the 21 st century by employing the MRI-AGCM3.1 model. Understanding the impact of climate change on water resources is fundamental for a number of economic activities in Panama (i.e. Panama Canal operation, hydropower generation, and agriculture). Therefore, it is important to assess hydroclimatic impacts in specific basins using reliable Atmospheric Global Circulation Models (AGCMs) validated against actual field data. A 20-km mesh experiment was developed by using time-sliced analysis for current and future (2075-2099) periods. Uncertainty in climate projections were addressed by completing 60-km mesh AGCM ensemble experiments at three additional lower boundary conditions. Four regions in Panama were selected for detailed analysis: from east to west, Bocas del Toro, Veraguas, Panama Canal and Darien. Projections show significant precipitation increases from May and July to December for all regions except Bocas del Toro. In this region, a decrease in precipitation is expected between April and August. Total runoff for all regions followed the changes in precipitation as expected. Due to net radiation increases, projected evaporation did not appear to be affected by precipitation changes.

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