Transport in Nonuniform Media - Transport and Fate of Chemicals in Soils: Principles and Applications - page 308

0.8

LANGMUIR

0.6

R1 R2

0.4

R2 R1

0.2

0.0

0 5 10 15 20 25 30

Pore Volume (V/V o )

FIGURE 9.4
Simulated breakthrough results for a two-layered soil column under different layering orders
(R1 → R2 and R2 → R1). Here R1 is a nonreactive layer and R2 is a reactive layer with Langmuir
adsorption.

boundary condition was used at the interface between the layers. Consistent
with the above finding, we found that for all parameters used in this study,
the layering sequence has no effect on the BTCs when Langmuir-type
adsorption was the dominant mechanism.

9.4.4 Kinetic Retention

In this section, first-, and nth-order reversible kinetics were considered as the
dominant retention mechanism; namely,

∂
∂

S
t

k θ

ρ

1

= CkS −

(9.10)

2

and

∂
∂

S
t

k θ

ρ

1

n

= CkS −

(9.11)

2

Values of the reaction order n used were 0.3, 0.7, and 1.0. The BTCs under
reverse layering orders showed a very good match regardless of the value of
the nonlinear parameter n . For all cases, a good match was also realized (see
Figure 9.5). We also carried out simulations where both layers were assumed
reactive. Other retention mechanisms considered included irreversible reac-
tion as well as second-order mechanism. Regardless of the retention mech-
anism, simulation results indicated that layered soils with reverse layering