Chemical and Physical Properties That Affect the Interaction Between Plants and Contaminated Groundwater - Introduction to Phytoremediation of Contaminated Groundwater - page 284

12.1.5 Root Uptake of Contaminants
in the Volatile Phase

Where K aw is the air-wood partition coefficient, C l the
concentration of organics in plants (in

m g/g), and C a the

concentration in air ( g/L); this is similar to the Henry's

law approach mentioned earlier in this chapter.
In order to examine the magnitude of diffusion of various
groundwater contaminants from trees likely to be planted
at phytoremediation sites, Baduru et al. (2008) provided
the first measurements of the diffusional loss from excised
tree tissue in the laboratory of common groundwater
contaminants following advective transport upward. For
each contaminant, the measured decrease in contaminant
concentration from the xylem to atmosphere pathway was
simulated using a 1-dimensional diffusion equation. The
higher the molecular weight of the compound examined,
the lower was its diffusivity (Baduru et al. 2008).
This relationship between contaminant fate and diffusiv-
ity can be used to calculate the potential for mass loss
through tree tissues such as the trunk, stems, or branches.
Baduru et al. (2008) suggest that the loss rate of a contami-
nant by volatilization can be estimated if the contaminant
diffusivity, D (cm 2 /s), diffusion pathway length, x , and the
surface area, A , mass, M , and tree-tissue density,

m

Up until now, we have considered the entry of dissolved-
phase contaminants into plants. Many priority pollutants
encountered at sites characterized by groundwater contami-
nation also will be present in the gaseous phase; this form of
contamination also can enter plants and needs to be consid-
ered as part of the overall phytoremediation strategy.
Volatilization is the transfer of a contaminant from the
liquid phase to a gas phase. We already saw how this process
drives the hydrologic cycle as water changes phases from
liquid to vapor. As was discussed in Chap. 2, the extent of a
contaminant's ability to change state is a function of its
vapor pressure, which essentially is a way to describe a
compound's gas solubility, rather than the compound's
water solubility.
An example perhaps better illustrates the potential for
volatile groundwater contaminants to enter plants.
Struckhoff et al. (2005) investigated the source of PCE
detected in cores of tree tissues at a site near New Haven,
MO. Both the subsurface soil and groundwater were
contaminated with PCE. There was a stronger relation
between PCE tree-core concentration and the soil PCE con-
centration than that of the groundwater PCE concentration.
As such, it could be assumed that PCE probably entered the
tree roots for subsequent partitioning into the water and then
translocated by diffusion. At the contaminated site, the loca-
tion of the trees cored was about 120 m (396 ft) from
the Missouri River. Depth to groundwater was about 7 m
(23.1 ft) but was higher if flooding occurred. The
partitioning between water and wood for PCE was measured
in the laboratory and found to be 49 L/kg. The partitioning
between air and wood was 8.1 L/kg.
One of the confounding problems encountered in
investigating the potential for volatile entry into plants is
that the gas-phase exchange between the groundwater and
the air from the unsaturated zone air is most likely not at
equilibrium. This situation will be more prevalent at sites
where the water table fluctuates in response to precipitation,
ET , or river stage.
The uptake of gas solutes is primarily a passive process
that depends on the movement of solute particles into plants
imposed by concentrations gradients. This movement was
related to the following equation by Fick in 1855:

r , are

related in the following expression

K v ¼ ð A = M Þ ð D =D x Þ ðÞ (12.33)

The use of this relationship is hindered by the range of
values reported for the diffusivities of various compounds
encountered at contaminated groundwater sites. For exam-
ple, the reported diffusivities, in D

10 7 cm 2 /s, for TCE
range from 0.01 to 25, for MTBE range from 1.78 to 8.00,
and for benzene range from 0.80 to 2.98, and these were
generated using hybrid poplar trees (Zhang et al. 1999; Ma
and Burken 2002; Baduru et al. 2008).
The partitioning of an organic solute in water in the leaf
also can partition onto the organic matter present in the
leaf itself, such as lipids, membranes, among other polar
and nonionic components of plant cells and tissues. The
membranes of vacuoles are composed of lipids, as is most
cell membranes. In sum, the resulting partition coefficient is
an estimate of the tendency for the solute, or compound, C ,
to dissolve into the organic lipid, or solvent, O , or water, W ,
such that

K ow ¼ C o = C w (12.34)

where K ow is the octanol/water partition coefficient, C o is the
solute concentration in the octanol phase (kg/m 3 ), and C w is
the solute concentration in the water phase (kg/m 3 ). This can
be related to the partitioning of solutes to plant lipids, K cl ,
such that

D ¼ D c d C = d x (12.36)

where D is the net movement of particles across a unit area,
D c is the diffusion coefficient, and d C /d x is the concentration
gradient. The D c is proportional to temperature and inversely
proportional to molecular weight. This diffusion can occur at
the cellular levels as well as the tissue level. As pointed out

K cl ¼ K ow (12.35)