Chemical and Physical Properties That Affect the Interaction Between Plants and Contaminated Groundwater - Introduction to Phytoremediation of Contaminated Groundwater - page 280

water, n -hexane, dichloromethane, and acetone of soils
contaminated with PAHs and can be used to relate the degree
of extraction with potential for plant bioavailability and
therefore uptake. These various preferential uptake or
removal mechanisms will ultimately decrease the contami-
nant concentration at the root interface.

Specifically, in most cases initial uptake of a chemical
can be described as following first-order, or concentration-
dependent, kinetics. For example

d Q at = d t ¼ K a Q m ð Q at Þ (12.15)

where Q at is the chemical amount retained on root surface at
time t , Q m is the chemical retained on root at maximum t ,
and K a

12.1.3.3 Root Concentration Factor
The efficiency of chemical sequestration into root tissues is
called the root concentration factor, or RCF . This relates that
entry of the contaminant into the roots through the vapor or
aqueous phases:

is the absorption rate constant. Integration of

Eq. 12.15 gives

Log Q m = Q m Q at ¼ K a t (12.16)

This equation can be used to estimate the potential for a
chemical to be accumulated at concentrations in the plant
above that present in solution. This also explains the deriva-
tion of the RCF . When the RCF is unity (1), the root con-
centration is equal

RCF ¼ C root = C w (12.12)

where C root is the solute concentration in the roots, and C w is
the solute concentration in water. RCF is essentially an
experimentally determined bioaccumulation factor in
which the concentration in the solution is related to the
plant concentration. Because plant cell membranes are com-
posed of a lipid bilayer, it acts to control the flow of
substances on the basis of lipophilicity or hydrophilicity.
Most compounds that are lipophilic can pass through the
membranes more easily than the more highly water soluble
compounds that are less lipophilic, as was experimentally
demonstrated by Shone and Wood (1974). As might be
expected, the RCF can be related to a compound's log K ow ,
with higher RCF for compounds with higher log K ow .
An equilibrium condition will be reached between the
solute concentrations in the root with that in the solution.
This condition will occur faster for root hairs and finer roots
than for large tap roots because of higher surface-to-volume
ratios for root hairs. Root interaction with contaminants can
occur along the entire length of the root, but passive-diffu-
sion-based uptake is limited to the unsuberized cell walls of
the root hairs. For most nonionic organic solutes, this equi-
librium will be controlled by diffusion. As this is occurring,
the solute also will have a tendency to sorb onto the organic
matter (lipids) present in the root itself. The extent to which
this will occur is controlled by the log K ow of the solute, with
more absorption occurring with solutes of higher log K ow .
The relation between the magnitude of RCF and log K ow was
experimentally indicated by Briggs et al. (1982), as

to the external concentration. An

RCF > 1 indicates that the plant has the ability for that

compound to accumulate in it at a higher concentration
than that external to the root.
The RCF can be considered as independent of concentra-
tion once equilibrium has been established between the
contaminant in the subsurface and its concentration in the
root. The uptake is not always dependent upon first-order
kinetics, because it is possible for little uptake to occur if all
sorption sites on the roots are filled (Collins et al. 2006), or if
they are actively growing. Moreover, RCFs are given as
constants for chemicals, but they may vary for different
plants. Variation is expected because the lipid content of
the epidermal cells differs between plants.
A potential shortcoming with comparing experimentally
determined RCFs with field observations is the assumption
that the log K ow is constant, even if metabolites are formed
in-situ. This assumption will lead to an underestimate of the
RCF (Thompson et al. 1998). Also, when the RCF is deter-
mined, what part of the root is actually involved in the
partitioning expression? The answer is not straightforward,
because roots consist of organic material, such as lipids, but
also of water and possibly gas in the cortex.

12.1.3.4 Transpiration Stream Concentration
Factor
The efficiency of a solute to flow to shoots from the roots can
be expressed in a manner similar to the RCF , called the
transpiration stream concentration factor ( TSCF ) such that

¼ 0 : 82 þ 0 : 03 K ow 0 : 77 RCF (12.13)

or TSCF ¼ C shoot = C w (12.17)

where C shoot is the solute concentration in the plant, and C w
is the solute concentration in water. The TSCF is essentially
an experimentally determined bioaccumulation factor in
which the concentration in the solution is related to the

Log RCF ð 0 : 82 Þ ¼ 0 : 77 log K ow 1 : 52 (12.14)

InthecaseofMTBE(log K ow ¼ 1.14), the RCF is

1.04.