Chemical and Physical Properties That Affect the Interaction Between Plants and Contaminated Groundwater - Introduction to Phytoremediation of Contaminated Groundwater - page 283

Where C dw is the contaminant concentration in dry wood,
and C l is the contaminant concentration in the transpiration
stream (Ma and Burken 2002). Similarly,

(external and internal-humidity, the xylem, cytoplasmic
water, and water stored in vacuoles), or atmospheric
gases-external and internal. For a volatile organic com-
pound such as a groundwater contaminant, the leaf interac-
tion with the atmosphere can drive the volatilization of a
compound into the atmosphere. These partitioning processes
will be discussed briefly here. The reader is referred to
Riederer (1995) for a comprehensive treatment of this topic.
As has been discussed already, the partitioning of a
chemical in solution to an atmosphere in contact with that
solution can be described using the Henry's Law constant
(Eq. 12.7 ). This transfer is based solely on the physical and
chemical properties of the solute, independent of the fact
that it is occurring in the living cells of the plant leaf.
Another useful model for the concentration ratio of a con-
taminant between air and water at the leaf-air interface is

K wood ¼ C wood = C water (12.25)

The partitioning of lipophilic compounds from water to
wood is a function of the lignin content of the wood because
lignin binds cellulose to provide structural integrity to wood,
and is hydrophobic and, therefore, can attract lipophilic
compounds as they move through the plant in the transpira-
tion stream (Golpalakrishnan et al. 2009). The strength of
this water-wood partitioning is directly related to the log K ow
of a compound and the lignin content of the plant in ques-
tion. For example,

Log K wood ¼ 27 þ 0 : 632 log K ow for oaks ð Þ (12.26)

K aw ¼ C air = C water (12.30)

and

Riederer (1995) presents an instructive model for relating
the various physico-chemical properties of potential solutes
to the potential for leaf concentration versus leaf loss, as

Log K wood ¼ 28 þ 0 : 668 log K ow for willows ð Þ (12.27)

An additional partitioning occurs as the water and solutes
are moving through the transpiration stream: the presence of
the large absorption potential of the wood itself. This
includes the compounds of cellulose and lignin, as previ-
ously discussed, which attract those contaminants that are
lipophilic. Trapp et al. (2003) calculated a K wood partition
coefficient

K la ¼ v a þ v w = KAW þ V l K la (12.31)

where K la is the gross leaf to air partition coefficient and V is
the volume fractions of the compartments relative to the total
plant leaf volume, where v i ¼ V i /V t as a way of normalizing

the magnitude of each process; leaf-cuticle partitioning is
not included here. Representative leaf water-air partition
coefficients, or log K aw , range from

K wood ¼ C wood = C w (12.28)

0.57 for toluene to

0.71,
respectively (Riederer 1995). If there is a high lipid content
of the leaf or low air volume in the stomata, the result will be
an increased leaf concentration of the compound. K la is
inversely proportional to K aw but proportional to K ow . For a
given K aw , the K la remains constant for compounds with log
K ow 's of less than 2.5. This fact is attributed to the dissolu-
tion of the chemical into the cellular water in the stomatal
opening of mesophyl cells.
Plants' release of organic contaminants to the atmosphere
as a vapor is an important mass-loss process and is based on
diffusion. The extent of this diffusion of a particular con-
taminant after uptake will be controlled by the position of
the stomata, the number and depth of the stomata, the con-
taminant concentration, and the diffusion coefficient of the
compound to air through various tissues from the xylem-
to-atmosphere pathway. These parameters can be used to
calculate a stomatal conductance for that compound
(Riederer 1995). Another approach is the following

3.85 for methanol, where the log K ow is 2.62 and

where C wood is the chemical concentration in the wood, and
C w is the chemical concentration in the plant transpiration
stream. As might be inferred, K wood is similar to the K ow in
such a way that Trapp et al. (2001) reported a linear regres-
sion relation of

Log K wood ¼ 0 : 27 þ 0 : 632 log K ow (12.29)

12.1.4 Leaf and Tree Tissue Processes That
Control Contaminant Loss

Leaves provide an interface between the water in the tran-
spiration stream and moistures levels in the atmosphere.
Leaves contain stomata, and these contain the invaginated
wet layers of mesophyll cells where by diffusion CO 2 enters
and O 2 and H 2 O exit. Contaminants in the transpiration
stream and water that supplies the cells in the stomatal
opening may partition within the following compartments:
the leaf lipids (internal-oils; external-waxy cuticle), water

K aw ¼ C l m ð g = g Þ= C a m ð g = L Þ (12.32)