Agriculture Reference
In-Depth Information
These reactions include Freundlich, Langmuir, and reversible and irreversible
first- and
n
th-order kinetics. Furthermore, simplified approaches can be used
to quantify the transport of reactive chemicals in soil profile, if equilibrium
reactions of the linear type are dominant for individual soil layers. Inclined
soil layers are common and observed in hilly regions and sloping soils.
9.6 Mixed Media
In this section, the transport of reactive chemicals in mixed soil systems
consisting of two or more porous media coupled with the second order
two-site model (SOTS) and multireaction and transport model (MRTM) are
presented. Applications and sensitivity analysis and examples for reactive
chemical transport in mixed media are discussed.
9.6.1 Multireaction and Transport Model
The MRTM was developed to describe the time-dependent behavior of
adsorption-desorption as well as transport of reactive chemicals in soils
(Selim and Amacher, 1997). This multipurpose model accounts for equilib-
rium and kinetic reactions of both the reversible and irreversible types. The
model used in this analysis can be presented in the following formulations:
θ
ρ
n
Sk
=
C
(9.13)
e
e
−+
∂
∂
S
t
θ
ρ
k
n
=
k
CkkS
(
)
(9.14)
1
2
3
k
∂
∂
S
t
i
=
kS
(9.15)
3
k
Here
C
is the concentration in solution (mg L
-1
),
S
e
is the amount retained on
equilibrium sites (mg kg
-1
),
S
k
is the amount retained on kinetic type sites
(mg kg
-1
), and
S
i
is the amount retained irreversibly (mg kg
-1
). Moreover,
K
e
is a dimensionless equilibrium constant,
k
1
and
k
2
(h
-1
) are the forward and
backward reaction rates associated with kinetic sites, respectively,
k
3
(h
-1
) is
the irreversible rate coefficient associate with the kinetic sites,
n
is the dimen-
sionless reaction order, θ is the soil water content (cm
3
cm
-3
), ρ
is the soil bulk
density (g cm
-3
), and
t
is the reaction time (h). At any time
t
, the total amount
of atrazine retained on all sites is:
SSSS
T
=++
(9.16)
e
k
i
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