United States
Environmental Protection
Agency
Risk Reduction
Engineering Laboratory
Cincinnati, OH 45268
Research and Development
EPA/600/SR-92/138 Oct. 1992
EPA Project Summary
Higher Plant Accumulation of
Organic Pollutants from Soils
Robert M. Bell
This work determines the effect of higher
plants on sites polluted by organic chemi-
cals and discusses the potential use of
plants as an In situ cleanup treatment
In situ cleanup systems have many ad-
vantages when compared with other
cleanup techniques. These systems treat
polluted soils, without excavating the bulk
of the polluted material, by detoxifying,
neutralizing, degrading, Immobilizing, or
otherwise rendering harmless the con-
taminants where they are found.
The first steps in developing an in situ
plant cleanup system for organically pol-
luted soils are to (1) determine the techni-
cal feasibility and cost effectiveness of
the method, (2) determine the availability
of suitable plant species or varieties, (3)
determine whether the site possesses op-
timal soil conditions, (4) conduct green-
house scale confirmatory uptake tests, and
(5) confirm that the plant materials that
have extracted the contaminants can be
disposed of in an environmentally safe
manner and that the plant mass and har-
vesting mechanics are realistically man-
ageabte.
This work is based primarily on litera-
ture review but also Includes greenhouse
experiments and field testwork. It is con-
cerned with the behavior of organic pol-
lutants in the plant-soil environment, plant
uptake and accumulation of organic pol-
lutants, and variation in uptake by different
plant species in different conditions.
The literature review involved keyword
searches Into suitable databases (includ-
ing Water Resources Abstracts, Biosis
Previews, Chemical and Biological Ab-
stracts, Agricola, and Phytotox) and re-
view of over 750 scientific publications for
information. Within this report greater
emphasis has been placed on the few
reports where sufficient details concerning
experimental methods to make compari-
sons is provided.
The greenhouse experiments Investigate
the actual extent of plant uptake of pollut-
ants from soils under known environmen-
tal conditions. The field testwork quan-
tifies natural effects.
The full report is not concerned with
foodchain effects where the plant may
accumulate pollutants, and animals feed-
ing on the plant may receive high doses
of the pollutant for subsequent effect Nor
does this report address effects of the
pollutant on the plant itself. As will be
seen, these effects result from interactions
between pollutant concentrations and a
variety of environmental effects.
This Project Summary was developed
by EPA's Risk Reduction Engineering
Laboratory, Cincinnati, OH, to announce
key findings of the research project that is
fully documented in a separate report of
the same title (see Project Report ordering
information at back).
Introduction
The full report is concerned with the be-
havior of organic compounds in soils and the
potential use of higher plants as an in situ
cleanup technique to clean up these polluted
soils. For polluted soils, in situ cleanup sys-
tems have many advantages over other
cleanup techniques. These systems encom-
pass methods of treating polluted soils, with-
out excavating the bulk of the polluted mate-
rial, by detoxifying, neutralizing, degrading,
immobilizing, or otherwise rendering harm-
less, contaminants where they are found. As
the polluted materials are not excavated, the
workforce is not exposed and pollutants do
not migrate from the site during excavation.
The first steps in developing an in situ plant
cleanup system for organic polluted soils are
to:
1. determine whether vegetative extrac-
tion from the contaminated soil has a
high probability of being the most tech-
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nfcaBy and cost-effective approach at
the specific site, realizing that this ap-
proach will require a substantial time
period and intensive agronomic man-
agement over that time;
2. determine whether suitable plant spe-
cfes (or varieties whhin a species) are
available to accomplish the desired
contaminant extraction;
3. determine whether the site possesses,
or can be readily modified to possess,
soil conditions that will support optimal
growth of the selected plant materials;
4. conduct greenhouse-scale confirma-
tory uptake tests; and
5, confirm that the plant materials that
have extracted soil contaminants can
be adequately disposed of in an envi-
- ronmentaliy safe manner and thaMhe
plant mass and harvesting mechanics
are realistically manageable.
No limits have been placed on the meth-
ods of soH cleanup by plants, ft could be that
the plant accumulates the pollutant for sub-
sequent disposal, or that the plant degrades
the pollutant, or that microbes attached to the
plant root degrade the pollutant
Using plants to ctean up organic polluted
soils, which may prove suitable in a number
of different pollutant situations, could be most
useful for soils contaminated by organic
chemicals to shallow depths, i.e., less than 2
meters. The maximum depth that plant roots
normally penetrate the soil is 2 meters. Such
furfaca situations are commonly encountered
from spills or leaks when the source of the
contamination is at or near the soil surface, ft
also occurs on many former dumping sites.
Using the ability of plants to accumulate
pollutants and then metabolizing them to
simple units would essentially be inexpensive
to establish and maintain, ft could, therefore,
prove extremely useful when vast volumes of
soil and sediment materials are polluted but
not to an immediately hazardous extent These
materials may be polluted by dust bfow of by
surface erosion resulting from adjacent con-
taminated sites.
The ability of plants to remove and accu-
mulate compounds from the soil is an essential
function of the plant Plants remove great
amounts of applied nitrogen and phosphorus
while they grow and thereby protect the
groundwaterfrom these nutrients. Plants may
also be able to extract excessive levels of
some micro-elements from polluted sites and
thereby rehabilitate the site for more normal
crops.
The full report, based on literature review,
greenhouse experiments, and field testwork,
is concerned with:
1. tha behavior of organic pollutants in
the plant-soil environment;
2. plant uptake of organic pollutants from
the soil; and
3. variation in the extent of plant uptake
by different plant species in different
conditions.
Literature Review
Behavior of Pollutants in the Plant-
Soil
The study of organic chemicals in the soil
environment has been limited to agricultural
chemicals (e.g., insecticides, pesticides, and
herbicides) and specific compounds that cause
a problem or persist in the soil for tang periods
(e.g., PCBs, PBBs). This has possibly oc-
curred because of the complexity of reactions,
the large number of compounds, and the cost
associated with their analysis.
, Many_ transformations .and .processes afc_
feet an organic chemical when it is in the soil
environment. The sum of these determine
both the compound's environmental impact
and its life. Soil and environmental factors
such as pH, cation exchange capacity, organic
matter, day content, and water content all
affect the rate and degree of these transfor-
mations. In a given situation (soil and envi-
ronmental conditions), however, these trans-
formations are dependent upon the physical
and chemical properties of the compound
and therefore vary between compounds.
The greatest influences on the potential for
all nontanfc organic chemicals to affect a
plant are the relationships and interactions
between their vapor, liquid, and adsorbed
phases in the soil, and their soil degradation
rates. These processes determine not only
the form of the compound that is available to
affect the plant but also the speed at which
the compound moves or spreads through the
soil to achieve its effect. The importance of
each of these processes will be discussed
separately.
The concentration of all organic compounds
in the soil decreases with time as tang as no
further additions occur. This decrease results -
from interactions between the compound and
a number of physical, biological, and chemical
parameters acting in the soil that either remove
the compound from the soil or alter its original
state.
Extensive research investigated the equi-
librium between the pollutant sorbed to the
soil and that in solution in the soil water. This
is often expressed as an adsorption isotherm,
that, at low concentrations, approximates a
straight line, giving rise to the equation
Cs = Kd{C,)
where Cs is the adsorbed concentration (g/kg
soil), C, is the solution concentration (g/m3 soil
solution), and Kd (nvVkg) is the slope of the
adsorption isotherm of the distribution coeffi-
cient. This immediately assumes complete
reversibility, or an equilibrium between these
phases, that may not strictly occur for some
chemicals.
Where the clay content of soils and sedi-
ments is relatively bw, pollutant adsorption
occurs primarily on the organic fraction of the
soil. The degree of adsorption of the non-
ionic pollutant is then dependent on the organic
carbon content of the soil or the sediment
Variation between materials, which exhfoft a
wide range of physicochemical properties, can
then be reduced by defining an organic carbon
distribution coefficient
'
where Kd is again the slope of the adsorption
isotherm in m3/kg, and f-,. is the organic car-
bon fraction in-the soiTor sediment.- This
equation ultimately assumes that all organic
matter behaves in the same manner.
Where the adsorption value of a particular
pollutant in a particular soil is not available or
has not been measured, a good correlation
has been found between the organic carbon
distribution coefficient, K,,,., above and the n-
octanolAvater partition coefficient, K^,, of the
chemical. The K^ is defined as the ratio of
the chemical concentration in octanol to that
in water, when an aqueous solution of the
chemical is mixed with octanol and then al-
lowed to separate.
The relationship between K^ and K^ with
different groups of environmentally active
chemicals and varying soil types has been
investigated with many authors reporting
equations similar to
tog KOO- 0.524 tagK^ + 0.62
This equation illustrates that as adsorption of
the pollutant to the soil increases, and thus
soil solution concentration of the pollutant de-
creases, log K^, increases.
To have the greatest effect upon a plant,
the organic compourKl'rnust stay within the"
vicinity of the plant root and not be quickly
leached away by mass flow.
The vapor partitioning of a compound in
the soil is important because the speed at
which a vapor spreads through the soil in the
vapor phase is considerably greater than that
in the solution phase. Even for chemicals with
relatively bw vapor densities, this transport
route has been shown to be significant
The relationship between the compart-
mentalizatbn of the compound between the
soil solution and the air spaces in the soil is
often described by Henry's Law and the extent
of partitioning is described by Henry's Con-
stant.
Those chemicals having a high vapor pres-
sure, and thus a relatively high Henry's Con-
stant, will easily move from the soil solution
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into the soil air, will be quickly lost from the
soil and the immediate vicinity of the plant
root, and will have a low overall effect within
the soil. They may subsequently be taken up
into the plant leaf from this vapor phase; this
is discussed later.
There have been no all-embracing studies
to determine the Henry's Constant above
which volatilization plays an important role in
the transport of the compound in the environ-
ment. It is not possible, therefore, to select a
Constant above which transport in the soil will
occur primarily by the vapor phase.
The effect of the vapor phase is strongly
influenced by the amount of water in the soil;
limitations in soil water mean limited partition
from the soil water to the soil air and on the
density or pore space of the soil itself.
the Transpiration Stream Concentration Fac-
tor (TSCF) has been proposed as
-[££•*- |jg herbicide in shoots per ml water transpired
Hg herbicide per ml uptake solution
In turn, it has been proposed that the uptake
of a chemical into a plant root could be de-
scribed by the Root Concentration Factor
(RCF), defined as
RCF =
concentration in root
concentration in external solution
and the analogous Stem Concentration Fac-
tor (SCF) as
SCF =
concentration in stem
concentration in external solution
Plant Uptake of Organic Pollutants
Uptake of chemicals from the soil into plants
can be both a complex process involving
compound-specific or active processes and/
or a passive process in which the chemical
accompanies the transpiration water through
the plant. When the former occurs, then a
rigorous relationship between the degree of
uptake and the physicochemical parameters
of the chemical cannot be expected although
some general trends may be evident. When
uptake into the plant is a passive process,
then relationships should exist.
ft is generally accepted that a chemical in
the soil enters a plant through four pathways:
1. root uptake into the conduction chan-
nels and subsequent transbcation by
the transpiration stream;
2. uptake from vapor in the surrounding
air;
3. uptake by external contamination of
shoots by soil and dust, folbwed by
retention in the cuticle or penetration
through it; and/or
4. uptake and transport in oil cells found
in oil-containing plants like carrots and
--- """cress." ' .....•-•
The amount of a pollutant found in a plant
growing in organic polluted soil will therefore
be the sum total of each of these transport
routes. Their respective importance depends
on the nature of the pollutant and the soil and
the environmental conditions during which
plant exposure occurs. Although the latter
two uptake routes may be significant in a
local context, they are not generally wide-
spread and can be discounted as major routes
of plant contamination. Because most reported
instances of plant uptake of soil-borne or-
ganic compounds make no attempt to distin-
guish the extent of the first two uptake routes,
the relative importance of each, under differ-
ent environmental conditions and for different
pollutants, cannot be assessed.
To describe the relationship between her-
bicide transport and water uptake into plants,
Although the concentration factor concept is
useful in describing the relative concentration
in a particular plant part, ft has many limitations.
These arise because the concentration of
organic chemicals, both within the soil and
within the plant, do not remain constant but
change with time. The concentration of any
organic chemical in the soil, or in nutrient
solution, may be depleted by plant uptake or
degradation; the concentration of an organic
chemical in a plant will also be reduced with
time both by degradation within the plant and
by the plant increasing its mass and thus
effectively diluting the chemical.
Research in the early 1980s with herbi-
cides related both the RCF and the TSCF to
the already defined K^ of the different groups
of herbicides under test. TSCF shows a bell-
shaped dependence on K^ with a broad
maximum around a K^ of 1.8. The RCF in-
creases with increasing K w and decreases
to a limiting value of less than unity for polar
compounds. The explanation for this is that at
K^ values bebw 1.8, translocatfon is limited
by the root concentration of the herbicide. At
-values above 1.8, transfocation is limited by
the rate of release of the sorbed lipophilic
chemical from the plant root into the transpi-
ration stream. All the TSCF in this investiga-
tion were, like earlier reports, bebw unity,
indicating that the chemicals under test moved
passively into the shoot with the transpiration
stream and were not taken up against a
concentratbn gradient.
In another experiment with more lipophilic
chemicals, the sorpttan of chemicals by mac-
erated roots was shown to be very dose to
the RCF above, but (in contrast to the RCF)
the sorptbn continued to fall sharply as the
lipophilicity decreased. There was a linear
relatbnship between the concentratbn factor
of the macerated roots and K^, shown as
tog RCF (macerated root) = 0.77 log K^ -1.52
This relationship suggests that sorptbn of
chemical by roots is the same whether the
root is living or dead; the sorptbn process of
chemicals to the root is therefore likely to be a
partitioning event.
In condusbn, if degradation of the chemi-
cal does not occur within the plant and plant
root uptake and transbcatbn of pollutants
from the soil is a passive process, then plant
uptake can be described as a series of con-
secutive partitions between the soil solids
and the soil water, the soil water and the
plant roots, and then the plant roots and the
transpiratbn stream, and the plant roots and
the plant leaves.
Pollutants with a highest fog K^ value, for
example dfoxin (6.14), PCBs (4.12-6.11),
some of the phthalate esters (those with val-
ues above 5.2), and the polycydic aromatic
hydrocarbons (4.07-7.66), are those most
likely to be accumulated by or in the root and
not be translocated out of ft. Those chemicals
with a bwer K^ (those that are lipophobb
and water soluble) are likely to be transfo-
cated within the plant and may reach signifi-
cant concentratbns within the plant leaves.
It also seems likely that for some of the
more volatile herbicides at least diffusion in
the vapor phase and subsequent uptake by
the shoot may be an important route of
chemical entry into the plant. Two processes
precede the penetratbn of chemicals in the
soil into leaf tissue by the air. The first pro-
cess concerns volatilizatbn of the chemical
from the soil. The volatilization depends on
the vapor pressure of the compound in the
soil pores and it varies according to ambient
temperatures, the water solubility of the com-
pound, and the adsorption capacity and
physical properties of the soil. The second
process involves deposition from the air onto
the leaf surface. As this deposition proceeds,
the vapor pressure of the chemical is de-
creased and more volatilizatbn occurs.
Variations In Pollutant Uptake by
Different Plant Species
A further variable affecting plant uptake of
soil-borne organfo pollutants is the type of
plant being exposed to the pollutant. No sys-
tematic examination has been made of plant
responses to organb chemicals in soil, al-
though it does appear that, as with plant
uptake of soil-borne heavy metals, there is
variation in uptake both between spedes and
within the same spedes on an individual level.
Some of these experimental variables were
investigated in a series of greenhouse stud-
ies and in a field assessment.
Greenhouse Studies
The greenhouse investigators consisted
of adding pure chemical to a soil to produce a
contaminated soil. Plants were then grown in
•U.S. Government Printing Office: 1992 — 648-080/60080
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this soil and their accumulation of the pollut-
ant was assessed. Although environmental
conditions in a greenhouse are normally very
different from those outside the greenhouse,
the greenhouse trials produced some very
interesting results.
In triate with hexachbrobenzene (HCB),
the plant root concentration of the pollutant
was, in many cases, greater than that occur-
ring in the soil; this showed that pollutant
accumulation had occurred. This accumula-
tion occurred to a greater extent with (east
oiganlc matter fraction within the soil me-
dium, and it occurred increasingly with in-
creasing time. In one experiment, 33% of the
sol-tome pollutant was accumulated from
the soB to the roots of radish within 67 days of
seed sowing. Between plant species, the abifity
to accumulate soil-borne pollutants varied.
This suggests that further work could identify
species with great affinity for cleanup.
Reid Assessment
The field trial consisted of collecting plants
actually growing on polluted soil at a polluted
site and then comparing the root and plant
leaf concentrations with those found in the
soil surrounding the root. The results from a
dfoxin (TCDD) polluted site near St. Louis,
MO, again showed that plant roots accumu-
late soil-borne pollutants with high log K^ to
concentrations many times those found in the
soil. Plant-leaf concentrations of most col-
lected plants were not detectable; however,
all three replicates of one species showed
high plant-leaf concentrations of TCDD and
this requires further investigation.
Recommendations
This investigation of the behavior of or-
ganic pollutants in soils and the accumulation
of these pollutants by plants has highlighted
the potential for using plants to degrade these
pollutants, either directly or through microbes
attached to the plant root. Further investiga-
tion into these areas is merited and the prob-
ability of achieving a plant cleanup system for
polluted soils remains high.
The full report was submitted in fulfillment
of Cooperative Agreement CR812845 by the
University of Liverpool under the sponsorship
of the U.S. Environmental Protection Agency.
Robert M. Ball is with the Environmental Advisory Unit, Ltd., Yorkshire House,
Liverpool L3 9AG, UK
P. fL Sferra is the EPA Project Officer (see below).
The complete report, entitled "Higher Plant Accumulation of Organic Pollutants
from Soils," (Order No. PB92-209 378/AS; Cost: $26.00, subject to
change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Risk Reduction Engineering Laboratory
U.S. Environmental Protection Agency
Cincinnati, OH 45268
United States
Environmental Protection Agency
Center for Environmental Research Information
Cincinnati, OH 45268
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