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Kinetic Modeling of Bioavailability for Sorbed-Phase 2,4-Dichlorophenoxyacetic Acid

^a Dep. of Civil and Environ. Eng., Michigan State Univ., East Lansing, MI 48824
^b Dep. of Crop and Soil Science, Michigan State Univ., East Lansing, MI 48824

View larger version (69K): [in a new window]   Fig. 1. Conceptive model for disappearance of 2,4-D in slurry systems, where S and C are sorbed and liquid phase concentration, respectively, fs is attached cell fraction, Kd is the sorbent-water distribution coefficient, and kl and ks are the liquid and sorbed phase degradation rate coefficients, respectively.

View larger version (13K): [in a new window]   Fig. 2. 2,4-D desorption from silica following a single dilution of the solution phase. Time points represent single measurements.

View larger version (19K): [in a new window]   Fig. 3. 2,4-D liquid-phase concentration versus incubation time. The two depicted lines were plotted using following values: Rsl (0.19), kl (0.0060), ks (0.032), and fs (0.28). Inoculum density was 1.2 x 107 CFU mL-1.

View larger version (16K): [in a new window]   Fig. 4. Enhanced transformation factor versus the ratio of silica to liquid. The estimated line was plotted using average values of kl (0.0060), ks (0.027), and fs (0.28). Inoculum density was 1.2 x 107 CFU mL-1.

View larger version (18K): [in a new window]   Fig. 5. Initial depletion rate vs. initial equilibrium concentration at a constant silica/liquid ratio. Inoculum density was 5.5 x 106 CFU mL-1. Values used for model were: kl (0.0045), ks (0.014), and fs (0.28).

View larger version (15K): [in a new window]   Fig. 6. The ratio effect of desorption rate coefficient to degradation rate coefficient on liquid-phase concentration over incubation time. Ceq is calculated using EB model with Ef value of one. Cd is calculated using Eq. [2] (overall mass balance equation with ks value of zero) and Eq. [9] (first-order desorption-rate equation). The Kd value is assumed 3.3. Rsl is 0.1. and kl is 0.006 min-1.