Chemical and Physical Properties That Affect the Interaction Between Plants and Contaminated Groundwater - Introduction to Phytoremediation of Contaminated Groundwater - page 286

Trapp (2002) developed a simple dynamic model based
on the advective rather than diffusive uptake of nonionic
organics. The model is used to calculate the concentration
of a particular compound if a transpiration rate and
plant growth rate can be calculated or assumed. These
determinations are based on a mass balance approach, simi-
lar to that presented for water budgets in Chap. 2. Essentially

Another model is that of Paterson and Mackay (1995)
based on the fugacity of a contaminant. They describe a
system of models called SNAPS (Simulation Model Net-
work Atmosphere-Plant-Soil). Both types of models are
limited by their assumptions. Other factors that could affect
the contaminant concentration are ignored in these models.

12.1.7 Plants and Contaminant Interactions

Mass change ð M Þ¼ Input ð I Þ Loss ð m Þ (12.43)

If the input, I , of a chemical is constant and the loss of
mass, m , is proportional to the total mass, M , through a
constant, k , then

The uptake of contaminants released to groundwater such as
organic solvents or petroleum hydrocarbons will tend to
follow the passive uptake pathway of diffusion and osmosis
and will be retarded due to the physical-chemical properties
of the particular compound as it comes into contact with the
organic parts of the plant. This retardation is important, as
the bulk flow of water from soil to root hair to root xylem to
xylem to leaf to atmosphere is continual as long as stomata
are open. Passive uptake can be reduced to at least two
phases: (1) the root cells come into equilibrium with the
aqueous concentration in the pore water and (2) organic
transfer to the cell walls of the root epidermis by sorption.
The uptake of water and contaminants was studied by
Wild et al. (2005b). They point out that even though much
has been revealed about the entry of extracellular water into
the plant (see Chap. 3), little is known about how organic
solutes are transported. Wild et al. (2005b) used two-photon
excitation microscopy (TPEM) to observe the movement of
solutes that exhibit auto-fluorescence inside plant cells. They
observed the movement of anthracene through the epidermal
cells wall through the cell membrane into the epidermal
cytoplasm. The movement was diffusional, and rapid, with
penetration into the cytoplasm of the epidermis observed
within 72 h. Wild et al. (2005b) point out that their results
challenge the dogma that organic compounds are limited
to storage in lipophilic cell components and do not enter
the transpiration stream and,

d m

=

d t

¼ I k m

(12.44)

The solution to this equation at any time t from initial
conditions at time t

¼ 0, m o ,is

m o e k t e k t M t ¼ þ I = k 1 (12.45)

This model is dynamic up until steady-state conditions
where d m /d t

¼

0.
Mass can be converted to chemical concentration by
dividing the mass, m , by the total sample mass, M ,or
volume, V , such that

C ¼ m = M or C ¼ m = V (12.46)

Therefore, the mass-loss equation becomes a concentra-
tion-loss equation

d C = d t ¼ I km (12.47)

12.1.6.2 Empirical Models
Alternative empirical-based models exist to evaluate plant
uptake of neutral contaminants, including a simple regres-
sion-based model by Travis and Arms (1988) where

therefore, cannot undergo

transformation.
Plant leaves are exposed to the sun's radiation. The outer
layer of leaf cells, such as the cuticle, acts to permit entry of
particular wavelengths of light, much as the cell membrane
excludes certain solutes but permits water to enter. Although
harmful ultraviolet A (UVA) wavelengths are mostly
attenuated by these cells through carotenoid compounds,
some of this energy does penetrate deeper into leaf tissues.
Wild et al. (2005a) showed that some of this UVA can be
used by plants to photodegrade organic chemicals that enter
the plant.
For the fate of contaminants in the subsurface, Wild et al.
(2005a) looked at the entry of the PAHs anthracene and
phenanthrene, into root cells, an extension of their work
into the fate of foliar anthracene described above. Using
TPEM techniques, they observed no PAHs in the root cap

Log B v ¼

: ð :

log K ow

Þ

1 588 0 578 (12.48)

Where B v is the bioconcentration factor for plants as the
ratio of the chemical concentration in the shoots as dry
weight divided by the concentration in the soil. The log
K ow of the organic compounds (mostly pesticides) used for
the regression ranged from 1.15 to 9.35. The B v was then
converted to a BCF according to

1

BCF wet ¼ B vdry ð W Þ r wet =r dry (12.49)

where W is the plant water content and r the soil bulk

density.