Chemical and Physical Properties That Affect the Interaction Between Plants and Contaminated Groundwater - Introduction to Phytoremediation of Contaminated Groundwater - page 285

12.1.6 Uptake, Partitioning, and Transport
Conceptual Models

by Trapp (1995), diffusion into roots can be approximated
by diffusion into a cylinder (an 'ideal' root) such that

G r A ¼

2 L

p D e =

ln R 2

ð R 1 = R 1 Þ

(12.37) This section brings together the physiological structures of

plants in relation to water and vapor uptake outlined in
Chap. 3, the chemical partitioning described earlier in this
chapter, and the affect on the ultimate fate of

Where G r is the soil-root conductance (m/s), A is the
root surface area, L is the root length, D e is the effective
diffusion coefficient, to account to a reduction in D c
caused by the tortuous pathways that must be taken before
entry into a plant root on account of irregular-sized grains,
R 1 is the root radius, and R 2

the

contaminants commonly found in groundwater.
Chemicals in the roots zone can enter the plant through
the water or vapor phase. Simple uptake can be described as
entry by diffusion. Plant roots exposed to water and soil air
that contains a contaminant will take up the contaminant
until equilibrium is established between phases. These
interactions can be described by the various partition
coefficients discussed above. The first process to consider
from the plant perspective is the RCF . This in itself is related
to the log K ow . Keep in mind that roots are mostly water
(85%) with few lipids (less than 1%). By now, it should be
apparent that log K ow is a master variable when it comes to
contaminant fate in plants.
Chemicals can enter the plant through the apoplastic or
the symplastic pathways. To enter the transpiration stream,
however, the chemical has to be transferred into the
symplast, i.e., through the Casparian strip of the endodermal
layer of cells. As we saw earlier, the extent that this passage
can occur is related to the chemical lipophilicity. The TSCF ,
therefore, is related linearly to the log K ow . In a simple sense,
the mass flow of solute, N t (kg/s) is related directly to the
flow of water in the plant, Q w (m 3 /s) and the concentration of
the subject chemical C w :

R 1 is the length of diffusion
between the root and soil or water matrix. D e can be
estimated from air- and water-filled pore volumes,
according to

2 D w

D we ¼ y ð 10 = 3 Þ=e (12.38)

2 D g

D ae ¼ e y ½ ð Þ 10 = 3 =e (12.39)

where D we and D ae are the effective diffusion coefficients if
the pores are filled with water or air, respectively,

y is the

fraction of water-filled pores, e y is the fraction of air-

filled pores, is the total porosity, and D w and D a are the

molecular diffusion coefficients in water and air. The
amount of chemical available for diffusion also is
constrained by the physico-chemical partitioning of the
compounds in the medium.
The flux, D , across a membrane can be estimated as

e

D ¼ Pa i ð a o Þ (12.40)

N t ¼ QðÞ CðÞ (12.41)

where D is the diffusional flux (kg/m 2 /s), P is the membrane
permeability (m/s), and a is the activity inside, i , and outside,
o , the membrane (Trapp 2003). As might be expected,
compounds that have a higher lipophilicity tend to cross
cell membranes with greater ease, so P is directly related
to the K ow of the contaminant.
Cho et al. (2005) investigated the ability for plants to
take up and rapidly remove volatile organics such as TCE
and PCE from contaminated soils above the water table
by gas transport through the roots to the atmosphere.
They compared under laboratory conditions the volatili-
zation of such VOC in planted versus unplanted
treatments. They reported that volatilization was faster
and more complete in the unplanted treatments, and that
the presence of plants (grasses) actually decreased con-
taminant volatilization. Other studies (Ma and Burken
2003) reported that the component of chlorinated solvents
that were taken up by trees could be metabolized within
the plant, rather than released to the atmosphere. In either
case, the contaminant is removed from the contaminated
subsurface media.

Because of the TSCF discrimination, the concentration of
a chemical in the xylem C x (kg/m 3 )is

C x ¼ ð TSCF Þ CðÞ

(12.42)

12.1.6.1 Mechanistic Models
Trapp et al. (1994) integrated most of the primary partition
processes that occur between plants, air, water, soil, and
contaminants into one model, called PlantX. It was reviewed
as one of many models in a model comparison study by
Collins and Fryer (2003). These discussions are for neutral
contaminants only, not electrolytes or weak electrolytes.
PlantX considers the diffusion of contaminants in water or
air to roots using the partition coefficients of H and K ow and
the RCF , the movement of contaminants into the transpira-
tion stream using the TSCF , translocation in the xylem,
concentration changes caused by plant cell metabolism and
growth, and the diffusion from the leaves into the air.