Chemical and Physical Properties That Affect the Interaction Between Plants and Contaminated Groundwater - Introduction to Phytoremediation of Contaminated Groundwater - page 282

different TSCF for the same contaminant and trees. They
approached the TSCF issue by determining the partition coef-
ficient for the compound TCE between the phases present: air,
water, and wood. The dimensionless Henry's Law partition
coefficient for TCE is 0.82, air-wood is 74 L/kg, and water-
wood is 51 L/kg.
As was described in Chap. 3, water and solutes enter the
transpiration stream after following plant entry through the
symplastic or apoplastic pathways. Regardless of pathway,
at the endodermis all water and solutes must pass over the
cell membrane of the endodermal Casparian strip. It is here
where the physical and chemical properties of the solute
determine its potential for further entry into the plant's
water system. This potential for entry has been predicted
based on the log K ow . In fact, the strength of the relation
between log K ow and plant-contaminant uptake explains the
usefulness of TSCF as a master variable to describe contam-
inant entry into plants because it incorporates the differential
entry of contaminants across the living cell membranes in
the Casparian strip. Hence, the TSCF is a more defensible
surrogate for membrane permeability than perhaps even
log K ow .
We saw in a previous section that solutes gain entry into
plants by two main processes, passive uptake or active
uptake. In Chap. 4, it was described how the scientist De
Saussure in the early 1800s showed that even though solute
uptake by roots is dependent upon the soil solution concen-
tration, there was a selective membrane through which it
must purchase entry into the plant's vascular system. Many
chemicals are first taken up according to passive gradients
controlled by diffusion. Additional uptake, especially in
slowly transpiring plants, may be limited by diffusion, but
assisted by plant metabolism. Passive uptake is suggested if
the sorption (uptake) rate is a function of the solution con-
centration, as described by first-order kinetics. Active
metabolism is indicated if oxygen becomes limited, or if
the chemical concentration inside the plant exceeds that
outside the plant. Active transport, on the other hand,
involves the expenditure of plant resources to facilitate a
reaction, usually against thermodynamic gradients. In sum,
passive uptake processes occur along decreasing chemical
potential gradients, whereas active uptake occurs against
these chemical potential gradients.
Passive uptake, especially in the cortex cells outside of
the endodermal cells of the Casparian strip, is related line-
arly to contaminant concentrations. If present in a system
with no soil, just a water-solute solution, passive uptake can
be written as

external water, and k is the partition coefficient. Most
groundwater contaminants are partitioned into the plant by
passive processes, controlled by diffusion, water solubility,
and the permeability of the endodermal cell membranes.
However, the rate of uptake does not increase indefinitely,
on account of saturation of available sorption sites within the
plant. Ma and Burken (2002) showed that for TCE, there was
a liner relation between the TCE concentration in the root
solution and that measured in the transpiration stream of
plant cuttings. They reported that when TCE concentrations
ranged from 1 to 50 mg/L, the calculated TSCF was 0.26.
Just as there are limitations with the RCF , there are
limitations with the TSCF that need to be kept in mind. The
TSCF assumes equilibrium conditions and does not consider
the impact of whether or not the contaminant is present in the
subsurface as a dissolved or vapor phase. This is an artifact
of initial development of TSCF for nonvolatile herbicides.
Moreover, changes in concentration due to metabolism
for rhizosphere degradation also are not considered.
Plant uptake from groundwater is directly related to the
TSCF and the amount of water transpired:

Plant uptake ¼ TSCF C (12.22)

where TSCF is the transpiration stream concentration factor,
plant uptake is the volume of transpired water (L), and C is
the bulk pore-water concentration (mg/L). To account for
the removal of contaminant mass in the root zone prior to
uptake, this equation can be rewritten as

Plant uptake ¼ C ð 1 f Þ TSCF (12.23)

where f is the fraction of contaminant degraded in the rhizo-
sphere; f is 0 if no degradation is present and the resulting
plant uptake is not decreased (Golpalakrishnan et al. 2007).
Uptake of a contaminant that is dissolved in water in plants
that are actively transpiring can be taken up by the advection
of water.
The TSCF also assumes that the concentration of the
compound remains unchanged as it moves through the
xylem. This scenario most likely is the exception rather
than the rule, given the ability for contaminant detoxification
processes, as described later in this chapter.

12.1.3.5 K waterwood
Xylem is a potential site for chemical and physical reactions
to occur. The effect of these reactions on transpiration
stream solute concentrations can be approximated by a
water-wood partition coefficient, K lw , where the l stands
for lignin, as

C pt ¼ kC w (12.21)

where C pt is the concentration of the contaminant in the
plant, C w is the concentration of the contaminant in the

K lw ¼ C dw mg ð = g Þ= C l mg ð = L Þ (12.24)