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CHAPTER 3 - Sorption Dynamics of Organic Compounds in Soils and Sediments
(1) (p. 46) Adsorption - refers to uptake stage of sorption, regardless of mechanism;
Desorption - refers to release stage.
(2) Sec. 3-1 Slow sorption kinetics
Freundlich eqn used to measure sorption at equilibrium:
= Ka Ca 0]
@) 1. "Equilibrium constant" obtained as (mg X/g soil) / (mgX/ml) over range of S Conc.
2. Hysteresis where desorptive pathway does not return to zero S at zero C.(Fig.3-1)
(4) (p. 49) (1) Resistant - not desorbed during the time of the experiment, but will eventually.
(2) Irreversible - fraction that cannot be recovered in its original form due to
transformation (chemical or biological).
5. (p. 49) - Resistant category: = subject of Chapter 3!
@ Table 3-1 (p. 50), pyrene sorption with incubation time ----~ >
propose "2-site" model for: (1) fast reaction (min) and (2) slow reaction (hrs,days)
(2) desorption may also follow as pattern of: (1) fast and (2) slow
G3) (p. 53) “Unextractable, bound residues” of pesticides in soil. Research question!
6. (p. 54) Sec. 3-2: Modeling of Sorption Kinetics : Mechanism considerations
qd) Diffusion through bulk water to soil surface
(2) Diffusion through pores of soil to surface of sorbing materials
@3) Diffusion through soil solids (organic matter gels) to sites
(4) Overcoming energy barriers at molecular level to phase transfer or sorption.
7. (p. 54) 3-2.1.1 Diffusion through aqueous solution:
(1) Fick's first law: F = D 8&C/éx [2]
(2) Laboratory experiments for batch sorption isotherms: mechanical mixing eliminates
diffusion as an important process. Conversely, saturated and unsaturated flow in vadose
zone soils, saturated flow in soils and sediments, and laboratory column experiments
are cases where advection occurs, but to a lesser degree than a mixed system in a flask!
(3) Advection may be absent altogether: lake bed sediment, small! pores of soil particle
aggregate, microvoids of soil organic matter or amorphous mineral matter.
(4) Diffusion coefficients in water (Dy29) at 25C for weak or nonelectrolytes of small
molecular size: ca. 0.5 to 1.5 x 10-5 cm? s-1_ [cm3/cm-s].
(5) Apparent diffusion coefficient (D,) for a solute in a porous medium containing solid
particles that are inert to the solute:
Ds = @Di20 [4]
where © is an empirical coefficient to account for solid tortuosity, (0.5 to 0.01).
(6) Effective diffusion coefficient (Def) < D, owing to surface sorption effects:
Derr will be inversely related to Kg: At high Kg values:
Det = Deo Det = Deo
~~) —_—______ [6]
9+ (1- 0)ps Ka (1— 6)Ps Ka
Therefore, as Ka increases, Deg decreases.
