17
In Situ Gentle Remediation
Measures for Heavy
Metal-Polluted Soils
S.K. Gupta, T. Herren, K. Wenger, R. Krebs, and
T. Hari
CONTENTS
Introduction
Basic Concept for In Situ Gentle Decontamination and
Stabilization Approaches
Degree of Contamination and Severity of Risk
Types of Gentle Remediation Techniques
Stabilization
Increase of Soil pH by Liming
Increase Binding Capacity
Stabilization Through Plants
Stabilization Through Microorganisms
Decontamination
Controlled and Targeted Mobilization
Mobilization by Microorganisms
Organic and Inorganic Acids and Chelators
Capture of Mobilized Heavy Metals
Capture by Plants (Phytoremediation)
Capture by Microorganisms
Other Captors
Harvesting the Metal-Loaded Captors
Gentle Remediation: Chance or Utopia
References
INTRODUCTION
Over the course of recent decades, industrial and agricultural activities have led to
a considerable increase in heavy metal levels in different environmental compart-

ments, especially in soil. A large number of sites throughout the world are classified
as polluted. Although in most of these sites the risk for man, plants, and animals is
at present not very acute, soil quality and groundwater are severely affected. On
Copyright © 2000 by Taylor & Francis
certain polluted sites, there is the hazard of entry of pollutants into the food chain.
Besides reducing emissions, the development of a concept of risk management for
these polluted sites is an important task for soil protection. The hazard-alleviating
measures can be classified into three categories (Figure 17.1): (1) gentle in situ
remediation measures, (2) harsh soil use restrictive measures, and (3) harsh soil
destructive measures. The main goal of the last two harsh alleviating measures is to
avert hazards either to man, plant, or animals. The main goal of gentle in situ
remediation is to restore the multifunctionality of soil (soil fertility), which allows
a safe use of the soil (Krebs et al., 1998). The category of gentle remediation
measures consists of two main groups, stabilization (immobilization) and decontam-
ination.
Gentle remediation techniques are applied in situ and, therefore, no excavation
or transport of soil is necessary. The physical soil structure is maintained and may
even be improved during the remediation process. Besides their ecological advan-
tages, gentle remediation techniques may be economically advantageous. Costs for
gentle remediation may be orders of magnitude less than costs associated with harsh
physicochemical technologies. One problem associated with gentle remediation
techniques is that a longer curing time is generally required as compared to conven-
tional harsh techniques. Another restriction may be the use of living organisms (e.g.,
plants or microorganisms) for gentle processes; the soil texture, pH, salinity, pollutant
concentrations, and the presence of other toxins must be within the limits of tolerance
of such organisms (Cunningham et al., 1995a; Bollag and Bollag, 1995).
The main objectives of this chapter are to discuss and evaluate in situ gentle
remediation measures, to discuss the complementary relationship between stabili-
zation and decontamination, and to evaluate the possibilities of decontamination
with techniques other than the use of plants (phytoremediation).

FIGURE 17.1 Possible measures to reduce the hazard of a soil polluted with heavy metals.
Copyright © 2000 by Taylor & Francis
BASIC CONCEPT FOR IN SITU GENTLE
DECONTAMINATION AND STABILIZATION
APPROACHES
D
EGREE OF CONTAMINATION AND SEVERITY OF RISK
The decision to remediate a site or not is made on the basis of the degree of the risk
posed by a further spread of heavy metals. For polluted sites which pose a severe
risk of further spread of the contaminants, only harsh methods are suitable, but for
larger areas and soils with diffuse sources of pollution below a certain degree of
contamination, gentle remediation techniques are ecologically and economically
reasonable alternatives.
Three levels are considered important to assess the effects of any potentially
toxic metal species in soil (Tadesse et al., 1994): (1) background levels, (2) tolerable
levels, and (3) harmful pollutant levels. Soils with background levels contain no or
small amounts of anthropogenic trace elements. Background levels of heavy metals
in soils are highly variable and therefore detailed knowledge is fundamental for
regulations based on these levels (Frink, 1996). In the range of tolerable levels, soils
contain increased amounts of anthropogenic heavy metals. In this case, the multi-
functionality or the fertility of soil might be affected and toxicity symptoms on
vegetation or crop plants will become visible. Such soils are a potential risk to plants,
animals, or men and pathway-specific measures have to be taken. For soils with
harmful pollutant levels, an immediate remediation is required, because such soils
are a hazard for any use.
A concept was proposed by Gupta et al. (1996) for risk assessment and risk
management of heavy metal-polluted soils based on threshold values representing
the limits between the ranges defined by Tadesse et al. (1994; Figure 17.2). In soils
exceeding the “guide values” (tolerable levels), the long-term functionality of the
soil is no longer assured. In this case, the location of the heavy metal source and a

reduction of the emissions are the appropriate measures. Soils with heavy metal
concentrations above the “cleanup values” (limit between tolerable levels and harm-
ful polluting levels) are a hazard, and fast and stringent measures have to be taken.
A third value was inserted within the range of tolerable levels, the “trigger values.”
In soils exceeding the trigger values, either the land use must be changed or reme-
diation measures have to be taken according to the results of subsequent site-specific
investigations. This concept has been implemented in the Swiss Ordinance regarding
pollutant impacts on the soil (VBBO, 1998).
Gentle remediation methods may be used in soils with heavy metal concentra-
tions between the trigger value and the cleanup value. To choose the appropriate
method, determination of the type and extent of soil contamination is necessary.
Measurements of total heavy metal levels of a soil include both metal species
available to the biota and metal fixed in minerals that is normally not available to
plants or animals (Phillips and Chapple, 1995; Sims et al., 1997). Only the mobile
fraction of cations is available for plant uptake and poses a risk of being leached to
the groundwater. The mobile fraction may be defined as the fraction that may enter
a living receptor when in contact with it. In context of risk assessment, this fraction
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may induce a toxic effect or an impairment of quality on plants. There are different
approaches to estimate the extent of this fraction. In our laboratory, we use the
NaNO
3
-extractable fraction as approximation of mobile heavy metals. If only the
immobile and, therefore, not phytoavailable fraction of heavy metals in soil is high,
this may not have toxic effects on plants. In such soils, the contamination is stable,
but there might be a need to reduce the total content, because soil properties may
change due to natural processes or due to environmental effects such as acid rain,
increased decomposition of soil organic matter, or global climate change. Therefore,
it is not reasonable to focus decisions concerning remediation measures only on the
total content of heavy metals, but also on the different metal fractions in soil (Figure

17.3).
TYPES OF GENTLE REMEDIATION TECHNIQUES
Usually, only the mobile fractions of heavy metals in soil can be directly influenced
by gentle remediation methods. The equilibrium between soluble and insoluble
fractions may either be shifted toward more insoluble or toward more soluble heavy
metals. A decrease in the soluble fraction will stabilize the pollutants in the soil,
whereas an increase in the soluble fraction will not only increase the danger of a
further spread of the pollutants but will also make them more available for decon-
tamination. Therefore, the soluble fraction plays the central role in the decision on
the appropriate decontamination technique. In order to obtain an ecologically safe
decontamination, the maintenance of an optimal ratio between soluble and insoluble
heavy metals is necessary.
FIGURE 17.2 Concept of soil protection and values for remediation measures.
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There are principally two categories of remediation techniques of a contaminated
soil: stabilization and decontamination (Figure 17.4). The choice of the principal
category is mainly made on different site factors such as soil type, the nature and
distribution of pollution as well as the severity of the hazard, current land use, soil
pH, and cleanup goals (Gabriel, 1991). With knowledge of these major points, the
decision can be made as to whether a stabilization or a decontamination procedure
is preferable. Knowledge of the current land use will reveal whether or not changes
are needed. If the pollutants should be stabilized, the pH of the soil makes it clear
whether liming or another stabilization technique should be applied. When the final
FIGURE 17.3 Illustration of the central role of the soluble fraction of heavy metals in soil.
FIGURE 17.4 Concept of immobilization and decontamination of heavy metals in soil.
Copyright © 2000 by Taylor & Francis
goal is a complete decontamination of the soil, further investigations are necessary
to determine the appropriate decontamination technique. Today, most in situ reme-
diation techniques are still at an experimental stage and are not adapted to a large
spectrum of soil types or various pollutants.

In the following sections, known and new possible techniques are critically
evaluated and presented in detail. Our concept of gentle remediation is not restricted
to either stabilization or decontamination. In a remediation process, stabilization
may only be the first step which reduces the hazard and gives time to make detailed
investigations to optimize the following decontamination.
STABILIZATION
This strategy aims to reduce the immediate risk of uncontrolled heavy metal transfer
to the groundwater or to the biosphere (Conner, 1994; Vangronsveld et al., 1995).
To attain this aim, the heavy metal fraction available to plants in the soil has to be
reduced, which means that heavy metals are immobilized and the equilibrium
between soluble and insoluble fraction is intentionally shifted toward more insoluble
forms either by increasing soil pH or by increasing the binding capacity of the soil.
Nevertheless, the heavy metals remain in the soil. Therefore, the result of stabiliza-
tion is not a decontaminated but a stabilized soil where metals are transferred into
an inactive form. In the next section, recent stabilization (immobilization) techniques
that have been tested either under field conditions or under greenhouse conditions
are reviewed.
Increase of Soil pH by Liming
Immobilization can be achieved by increasing the soil pH, as described for zinc and
cadmium (Alloway and Jackson, 1991). Liming is used in agriculture to increase
the pH of acidic soils. Most experiments investigating the effect of liming on the
availability of heavy metals were made in soils that had received high doses of
sewage sludge. Little is known about the formation of complexes with soluble
organic substances and the effects of liming on the complexation.
Krebs et al. (1998) investigated heavy metal uptake of peas in limed and unlimed
plots treated with mineral fertilizer (control), sewage sludge, or pig manure. The
above-ground parts of field peas grown on limed soils contained lower heavy metal
concentrations than plants grown on fertilized, unlimed soils (Figure 17.5). The
highest reductions in zinc uptake, due to the addition of lime, was found in plants
grown on control plots. The zinc concentration decreased from 73 to 50 mg/kg dry

matter (DM) in seeds and from 59 to 19 mg/kg DM in crop residues. Cadmium
uptake was reduced even further by liming than zinc uptake. The maximal reduction
was again found on control plots, where cadmium concentrations of 213 μg/kg DM
were measured in crop residues from unlimed plots and only 67 μg/kg DM from
limed plots. Liming also led to a considerable reduction of copper uptake by seeds
and crop residues in all treatments, which was unexpected in view of the unchanged
NaNO
3
-extractable (mobile) copper concentrations (data not shown). An explanation
may be that the enhanced mobility was due to an increased formation of organic
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complexes of large molecular size that are less available to plant uptake than free
copper ions.
Increase Binding Capacity
Another way to immobilize heavy metals is to increase the metal-binding capacity
of the soil by the addition of clay minerals, iron oxides, or waste products such as
gravel sludge. Such additives reduce the mobility of heavy metals due to their large
specific surface and high cation exchange capacities. However, heavy metals can
readily be exchanged by other cations such as calcium and magnesium (van Bladel
et al., 1993). It is therefore important that the addition of binding agents does not
(or only slightly) affect the availability of nutritional cations for plants.
Several binding agents such as zeolite, beringite, hydrous manganese, or ferrous
oxides have been studied for the use as immobilizing agents under pot and field
conditions (Czupyrna et al., 1989; Didier et al., 1993; Greinert, 1995; Vangronsveld
et al., 1995). Lothenbach et al. (1998) compared the effectiveness of different binding
agents in zinc immobilization in batch experiments (Figure 17.6). Al-montmorillo-
nite significantly reduced dissolved concentrations of zinc in the pH 5 to 8 range.
In most studies, expensive and purified clay minerals were used. For the application
on agricultural soils, binding agents must be available in large quantities at a suffi-
ciently low cost.

Gravel sludge is a waste product of the gravel industry and, at least in Switzer-
land, is available in large quantities at a low price. Normally, this product contains
about 45% clay minerals, and the concentrations of heavy metals are much lower
than limit values of sewage sludge according to the Swiss Ordinance on Substances
(StoV, 1986). Relative to the heavy metal concentration already present in soils,
metal input due to the experimental application of the gravel sludge is insignificant.
A comparison between gravel sludge and Na-montmorillonite as binding addi-
tives in pot experiments was made by Lothenbach et al. (1998). Both additives
FIGURE 17.5 Reduction of the zinc, copper, and cadmium contents of seeds and crop residue
after lime application.
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reduced the soluble zinc fraction in soils by about a factor of eight (Figure 17.7).
In contrast to Na-montmorillonite, gravel sludge only slightly affected soil pH
(Figure 17.8). The addition of Na-montmorillonite had a negative effect on the yield
of red clover, whereas gravel sludge did not reduce the yield. The application of
gravel sludge was also studied in the field (Krebs et al., 1999). At all three experi-
mental sites investigated, the application of gravel sludge led to an increase in soil
pH, which can be attributed to the high CaCO
3
content (30%) of the gravel sludge.
In all treatments, the effects on NaNO
3
-extractable Cu concentrations were less than
on zinc. The concentrations in soil and the total plant uptake of zinc, copper, and
cadmium by ryegrass due to gravel sludge application were most strongly reduced
at site 1 (Figure 17.9). At the other sites, the effect on the concentrations and the
total metal uptake by ryegrass was less evident. Thus, gravel sludge treatments led
to a decrease in heavy metal uptake by plants, a varying level depending on the
plant and site characteristics.
FIGURE 17.6 Dissolved zinc concentrations in the presence of montmorillonite or Al-mont-

morillonite before (left) and after the addition of Ba(ClO
4
)
2
(right) as a function of pH.
FIGURE 17.7 Decrease of NaNO
3
-extractable zinc after the addition of binding agents.
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FIGURE 17.8 Effect of binding agents on biomass yield of red clover (Trifolium pratense;
top) and on soil pH (bottom).
FIGURE 17.9 Relative changes in copper, zinc, and cadmium contents of aerial parts of
ryegrass (Lolium perenne) due to the addition of gravel sludge.
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Stabilization Through Plants
A special form of stabilization is mediated by plants (phytostabilization). This
immobilization requires heavy metal-tolerant living plants that reduce the mobility
of heavy metals in soil by uptake and storage in the roots (Salt et al., 1995). The
stabilization effect of plants is also suitable for soils in which the heavy metals were
previously immobilized by methods mentioned above. The plants growing on these
soils will increase the stability of the heavy metals and prevent wind or water erosion
(Vangronsveld et al., 1995).
Stabilization Through Microorganisms
Different microorganisms have the ability to immobilize heavy metals in soils (Sum-
mers, 1992; Frankenberger and Losi, 1995), and it was suggested to use this ability
to immobilize metals through the management of specific microbial populations
(Morel et al., 1997). Immobilization mediated by microorganisms has the advantage
of being more selective in the binding of a unique heavy metal than that associated
with synthetic chemical sorbents. Microorganisms are able to stabilize soils by
concentrating heavy metals either in an active process, called bioaccumulation, or

by uptake processes that do not require energy, called biosorption (Bolton and Gorby,
1995). Furthermore, microorganisms can influence heavy metal solubility by direct
or indirect reduction. As an example, Cr(VI) (chromate, CrO
4
2-
) is mobile and toxic,
whereas the reduced form, Cr(III), is relatively nontoxic and nonmobile in the
environment (Bolton and Gorby, 1995). Similar reactions which precipitate metals
were reported for arsenic (As(III) to As(0)), uranium (U(VI) to U(IV)), or selenium
(Se(VI) to Se(0)). Sulfate-reducing bacteria are capable of precipitating metals as
metal sulfides (Farmer et al., 1995).
DECONTAMINATION
Decontamination involves several steps, finally resulting in a soil with reduced heavy
metal concentrations and restored soil quality (soil fertility). Today, soil decontam-
ination techniques use the ability of plants to extract heavy metals from soil. To
remove sufficient amounts of heavy metals by this technique, both high tissue
concentrations in the plant and high biomass yields are important. Plants known as
hyperaccumulators have high concentrations, but their biomass is usually very small.
Plants with a high biomass production normally take up small amounts of heavy
metals if only moderate concentrations are available. These plant characteristics, as
well as the availability of the heavy metal in soil, strongly influence the length of
time required for decontamination. In all known cases, the length of decontamination
time is in the range of one to several hundreds of years. An in situ decontamination
of heavy metal-contaminated soils is feasible if plant uptake of heavy metals is
strongly enhanced.
Controlled and Targeted Mobilization
The basic need of the decontamination process is the mobilization of heavy metals
to render them more accessible to the captor, which may be plants or natural or
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artificial exchangers. The mobilization process should be controlled in order to avoid

loss of heavy metals by leaching to the groundwater but at the same time provide
a maximum of soluble heavy metals available for removal. When used with living
organisms, the optimal concentration of soluble heavy metals allows maximal uptake
by the captor, but does not induce toxicity symptoms. Depending on the decontam-
ination method, it might be necessary to repeat the mobilization treatment several
times at a low dosage throughout the decontamination process (e.g., during the
vegetative period in phytoremediation). The interval between each treatment is
determined by the rate of the degradation process of the substance used for mobi-
lization (Figure 17.10), but certainly, the highest possible soluble heavy metal con-
centration in combination with an accumulating plant will drastically shorten the
decontamination time.
An experiment was performed by Wenger et al. (1997) to investigate the uptake
capacity of several plant species in a soil with different levels of soluble zinc. In
this pot experiment, zinc was not mobilized, but added as ZnNO
3
. The total extraction
of zinc after 25 days was highest by the tobacco plant (Table 17.1).
Mobilization by microorganisms
Microorganisms produce a wide range of different chelating agents which make
metals more accessible for plant uptake and therefore may also be useful for reme-
diation applications (Bolton and Gorby, 1995). Other microbial mobilization pro-
cesses for the mobilization of metals may involve the oxidizing reactions as men-
tioned previously for stabilization. Some microorganisms gain their energy from the
oxidation of reduced inorganic compounds such as sulfur or iron. The oxidation of
sulfides produces sulfuric acid that solubilizes the ions of different metals (Bolton
and Gorby, 1995) a process known as bioleaching (Tuovinen, 1990).
Organic and inorganic acids or chelators
Heavy metals can be mobilized by decreasing soil pH (Gupta, 1992; Herms and
Brümmer, 1980). By this method, the uptake of heavy metals by plants can be
FIGURE 17.10 Maintenance of an optimal mobilization range for heavy metals in soil

with no growth restriction for remediation organisms.
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increased by a factor of 2 to 3 (Hasselbach and Boguslawski, 1991). The addition
of inorganic acids, however, may lead to accumulation of the corresponding anions
such as nitrate, chloride, and sulfate in soil. Therefore, it is preferable to use organic
agents that are degradable by microorganisms.
Wallace et al. (1974) described the mobilization by synthetic chelates, e.g.,
ethylenedinitrilotetraacetic acid (EDTA) or nitrilotriacetic acid (NTA). Heavy metal
uptake by plants could be increased by the addition of EDTA or NTA to soil (Balmer
and Kulli, 1994; Jorgensen, 1993; Wallace et al., 1974). These synthetic ligands,
especially EDTA, are barely degradable by microorganisms. For this reason, it is
preferable to use soil-borne agents. Depending on their degradation rate, it might
be necessary to add such agents several times to maintain an optimal concentration
of soluble heavy metals during the decontamination process.
In laboratory studies, the effectiveness of different agents on zinc mobilization
was tested (Wenger et al., 1998). The synthetic chelator NTA mobilized about 15
times more zinc than other agents tested (Figure 17.11). Among the natural organic
acids, citric acid had the greatest solubilization effect on zinc, about three times
more than the other organic acids or nitric acid. The effects of soil treatment with
NTA as well as with citric acid on soil pH were very similar, but the metal complex
equilibrium constants of NTA are higher than those of the tested natural organic
acids (Martell and Smith, 1989). Therefore, it is assumed that the greater mobilizing
effect of NTA is based mainly on its greater complexing capacity. The degradation
of NTA is much slower than the degradation of the natural organic acids used to
mobilize zinc (Figure 17.12). By treatment with natural organic acids at 25 mmol/kg
soil, the mobilizing effect of the agents decreased after 17 days and the zinc con-
centration dropped below the levels measured at the lower acid concentration. Pos-
sibly, this effect occurred because the sudden large supply of soil-borne organic
agents led to an increased growth of microorganisms that live by degrading these
organic substances.

TABLE 17.1
Extraction Capacity of Tobacco, Birch, Knotgrass, and Mustard at Different
Levels of Soluble Zinc in a Pot Experiment
Zinc Added to Soil
as Zn(NO
3
)
2
.6H
2
O
NaNO
3
-
Extractable Zinc
Concentration
Zinc Extraction after 25 Days (mg/kg soil)
(µg/g soil) (µg/g soil) Tobacco Birch Knotgrass Mustard
0 6.2 1.9 1.3 1.0 0.7
100 14.2 2.7 1.5 1.3 0.7
170 17.5 3.5 1.8 1.6 0.6
330 31.0 4.7 1.6 2.1 0.4
500 45.3 4.0 1.2 1.7 0.3
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FIGURE 17.11 Mobilizing effect on NaNO
3
-extractable zinc by several concentrations of
synthetic and natural ligands and nitric acid.
FIGURE 17.12 Degradation kinetics of different organic acids and their effect on NaNO
3

-
extractable zinc concentrations by various doses of mobilizing agents (0, 5, and 25 mmol
agent/kg soil).
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Capture of Mobilized Heavy Metals
Capture of heavy metals involves specific binding to surfaces or the uptake into
living organisms that may be later removed from soil after a certain exposure time.
Similar to immobilization, the specificity of this process is very important; large
quantities of captured nutritional ions will induce deficiency symptoms for plants
or microorganisms, and their growth will be drastically reduced.
Capture by plants (phytoremediation)
Special metal-accumulating plants are used to take up, transport, and concentrate
metals from the soil into the harvestable parts of roots and above-ground shoots
(Salt et al., 1995). The restrictions of this method basically depend on the ability of
plants to take up and concentrate heavy metals. To reach the highest possible
extraction within the shortest possible time, the plants should achieve both a high
biomass yield and high heavy metal concentrations in their tissue. The biomass yield
depends on several different factors such as types of plant roots, yield potential, ion
absorption ability of plants, and tolerance of high concentrations. The concentration
in plants depends on mobile metal concentrations in the soil, root types, ion inter-
actions, and the hydrogen ion concentration.
Hyperaccumulator plants have an unusually high uptake of heavy metals from
polluted sites. However, the biomass production of these hyperaccumulator plants
is usually very low and the calculated decontamination times vary from one hundred
to several hundreds of years. Therefore, efforts were made to increase the biomass
of such plants by crossing them with related plants that have a larger biomass
(Cunningham and Ow, 1996). In recent years, clones of high biomass crop plants
that are able to accumulate increased levels of heavy metals were selected (Ow,
1993). Transgenic plants containing animal or plant genes encoding for different
types of metallothioneins were tested for their ability as phytoextraction plants

(Elmayan and Tepfar, 1994; Hattori et al., 1994; Pan et al., 1994). To understand
more about the mechanisms involved in heavy metal tolerance, attempts were also
made to research molecular mechanisms and genes leading to hyperaccumulation
in tolerant species (Brown et al., 1995a,b).
Capture by microorganisms
Similar to the immobilization of heavy metals, the uptake or biosorption by micro-
organisms may be used to capture heavy metals in soil (Bolton and Gorby, 1995).
Until now, such systems have mainly been used in the remediation of water, but
attempts are being made to use this system also in soil. For use as captors, micro-
organisms should not be dispersed in soil, but immobilized in a matrix with a certain
mechanical strength (Brierley, 1990; Volesky, 1990). The advantage of using micro-
organisms to capture metals are the low costs of production and the possibility to
adapt the microorganisms either by cultural conditions or genetic manipulations to
the environmental conditions at the remediation site (Bolton and Gorby, 1995).
A special type of removal is the volatilization of metals mediated by micro-
organisms. By this method metals are not concentrated and cannot be captured. The
contamination is only transferred from soil to the air. As an example, the soluble
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Hg(II) may be reduced directly by microorganisms to the volatile Hg(0) (Robinson
and Touvinen, 1984). Another reaction catalyzed by microorganisms is the methy-
lation that also usually leads to a volatilization of metals (Gadd, 1993). The vola-
tilization by methylation was shown for selenium, which was transformed to dim-
ethyl selenide and dimethyl diselenide (McCarty et al., 1995), and arsenic (Bachofen
et al., 1995). However, the methylation of metals can greatly enhance their toxicity
to humans (Bolton and Gorby, 1995).
Other captors
A first chemical approach is to introduce artificial resins into the soil. These resins
may function as ion exchangers or as specific chelators. There is little experience
with such applications, and the ideal form of such resins to allow easy removal has
yet to be found.

Besides artificial resins, natural products or byproducts may also be able to bind
specific heavy metals in the soil. Such products might have the form of a bulb, which
could be incorporated into soil and harvested with standard methods used in agri-
culture. Attempts are being made at our institute to prepare such bulbs made of
artificial or natural exchangers. Exchangers needed to fulfill criteria, such as stability
during application and harvesting, specificity for heavy metals, have no side effects
on soil functionality and soil organisms. Experiments are under way with several
artificial and natural exchangers to investigate their heavy metal-binding capacity
in the presence of metals and known salt concentrations.
Harvesting of Metal-Loaded Captors
Harvesting strongly depends on the type of captor. Principally, the disturbance of
soil structure during harvesting should be as low as possible. The use of standard
agricultural harvesting methods would render the whole decontamination process
less expensive, especially for larger areas.
Harvesting of phytoremediation plants will fulfill the criterion of standard agri-
cultural methods if most of the captured heavy metal is located in above-ground
plant parts. The removal of capturing bulbs or other structures consisting of artificial
or natural chelating agents from soil is not yet sufficiently developed. In the devel-
opment of such new methods, harvesting should be taken into account when these
structures are incorporated into the soil. Harvesting methods are available for plants
with edible parts below the ground (e.g., potatoes).
GENTLE REMEDIATION: CHANCE OR UTOPIA
As defined previously, gentle techniques involve the in situ remediation of heavy
metal-polluted soils by a combination of various biophysical and chemical treat-
ments. The concept of gentle remediation does not mean a complete decontamination
of a soil, but its aim is to reduce the risk of contamination by balancing between
ecological and economical needs. Gentle remediation does not consist only of a
single treatment, but of a series of different individual steps that may be optimally
adapted to the actual situation at a polluted site.
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To date, few worldwide studies have been conducted on gentle remediation
techniques under large-scale field conditions. The stabilization of heavy metal-
polluted soils by liming or by the addition of different binding substances has been
established in several field trials (Vangronsveld et al., 1995; Krebs et al., 1999). The
capture and the resulting removal of heavy metals from soil is still at an experimental
phase. Several experiments were made with plants for phytoextraction, but only a
few on a large scale under field conditions. Furthermore, plants that are able to
accumulate high concentrations of heavy metals while possessing a high biomass
yield are still lacking. Besides phytoextraction, other captors mentioned in this article
are in the conceptual state.
In the future, several steps of the decontamination strategy (see Figure 17.4)
will be improved and new procedures will be proposed. A scenario for possible
improvement of existing techniques is shown in Table 17.2. First, the time required
for a fivefold decrease of the heavy metal concentrations in soil with help of existing
crop plants is calculated. The time for zinc is between 200 and 250 years, and for
cadmium between 36 and 40 years. The total uptake of heavy metals by crop plants
may be increased by enhancing the yield either by breeding or by improved crop
production techniques. With an increase in yield of 25%, the remediation time may
be reduced to 160 to 200 years for zinc and 24 to 32 years for cadmium. To extend
the total uptake further, it is also important to increase the heavy metal concentration
in plant tissue. This might either be achieved by approaches to enhance the uptake
capacity of the plants or by increasing the availability of heavy metals in the soil
by mobilization. By the latter method, a reduction to 32 to 46 years is possible in
the case of zinc. For cadmium, the remediation time may even be reduced to 3 to
8 years.
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REFERENCES
Alloway, J.B. and A.P. Jackson. The behavior of heavy metals in sewage sludge-amended
soils. Sci. Total Environ. 100, 151-176, 1991.
Bachofen, R., L. Birch, U. Buchs, P. Ferloni, I. Flynn, G. Jud, H. Tahedi, and T.G. Chasten,

Volatilization of arsenic compounds by microorganisms, in Bioremediation of Inor-
ganics, Hinchee, R.E., J.L. Means, and D.R. Burris, Eds., Battelle Press, OH, 1995.
Balmer, M. and B. Kulli, Der Einfluss von NTA auf die Zink- und Kupferaufnahme durch
Lattich und Raigras. Diploma work, Institute of terrestrial ecology, ETH Zürich, 1994.
Bollag, J M. and W.B. Bollag, Soil contamination and the feasibility of biological remedia-
tion, in Bioremediation: Science and Applications, Skipper, H.D. and R.F. Turco,
Eds., Soil Science Society of America, Madison, WI, 1995.
TABLE 17.2
Extraction Capabilities of Maize and Tobacco in Soils Contaminated by
Zinc or Cadmium
High Yield
Varieties
a
Optimized Yield
b

(+25%)
Optimized Yield
Heavy Metals
Mobilized
c
Zinc Cadmium Zinc Cadmium Zinc Cadmium
Tobacco Yield [t/ha] 16 16 20 20 20 20
Content [kg/t
DM]
0.3 0.011 0.3 0.011 1.3 0.117
Total Uptake
[kg/ha y]
4.8 0.18 6.0 0.22 26.0 2.34
Reduction

Time [y]
250 36 200 29 46 3
Maize Yield [t/ha] 20 20 25 25 25 25
Content [kg/t
DM]
0.3 0.008 0.3 0.008 1.5 0.033
Total Uptake
[kg/ha y]
6.0 0.16 7.5 0.20 38 0.83
Reduction
Time [y]
200 40 160 32 32 8
Note: The heavy metal concentrations in the soil are assumed fivefold the guide value, that should
be reduced to the guide value (750 to 150 ppm for Zn and 4 to 0.8 ppm for Cd). For the calculations,
soil bulk density was assumed to be 1 g cm
-3
. Soil weight per hectare with a depth of 20 cm will
therefore be
a
Varieties that have a high yield capacity under good growth conditions.
b
Assuming that the yield can be increased 25% by breeding and optimal growth conditions.
c
The soluble heavy metal fraction is maintained at the highest possible level.
10
20 10 000
1000
2
2
.

,
.g cm
cm 100 cm m m ha
g kg
million kg ha
-3
-1 2 -1
-1
-1
∗
∗∗
()
=
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Bolton, H. and Y.A. Gorby, An overview of the bioremediation of inorganic contaminants,
in Bioremediation of Inorganics, Hinchee, R.E., J.L. Means, and D.R. Burris, Eds.,
Battelle Press, OH, 1995.
Brierley, C.L., Bioremediation of metal-contaminated surface and groundwaters. Geomicro-
biol. J. 8, 201-223, 1990.
Brown, S.L., R.L. Chaney, J.S. Angle, and A.J.M. Baker. Zinc and cadmium uptake by
hyperaccumulator Thlaspi caerulescens grown in nutrient solution. Soil Sci. Soc. Am.
J. 59, 125-133, 1995a.
Brown, S.L., R.L. Chaney, J.S. Angle, and A.J.M. Baker. Zinc and cadmium uptake by
hyperaccumulator Thlaspi caerulescens and metal tolerant Silene vulgaris grown on
sludge-amended soils. Environ. Sci. Technol. 29, 1581-1585, 1995b.
Conner, J.R., Chemical stabilization of contaminated soils, in Hazardous Waste Site Soil
Remediation, Wilson, D.J. and A.N. Clarke, Eds., Marcel Dekker, New York, 1994.
Cunningham, S.D., W.R. Berti, and J.W. Huang. Phytoremediation of contaminated soils.
Trends Biotechnol. 13, 393-397, 1995a.
Cunningham, S.D. and C.R. Lee, Phytoremediation: plant-based remediation of contaminated

soils and sediments, in Bioremediation: Science and Applications, Skipper, H.D. and
R.F. Turco, Eds., Soil Science Society of America, Madison, WI, 1995b.
Cunningham, S.D. and D.W. Ow. Promises and prospects of phytoremediation. Plant Physiol.
110, 715-719, 1996.
Czupyrna, G., R.D. Levy, A.J. MacLean, and H. Gold, In situ Immobilization of Heavy-Metal
Contaminated Soil. Noyes Data Corporation, NJ, 1989.
Didier, V., M. Mench, A. Gomez, A. Manceau, D. Tinet, and Ch. Juste, Réhabilitation de sols
pollués par le cadmium, évaluation de l’efficacité d’amendements minéraux pour
diminuer la biodisponibilité du cadmium. Contes Rendues Acad. Sci. 316, Série III,
83-88, 1993.
Elmayan, T. and M. Tepfar. Synthesis of a bifunctional metallothionein/beta-glucuronidase
fusion protein in transgenic tobacco plants as a means of reducing leaf cadmium
levels. Plant J. 6, 433-440, 1994.
Farmer, G.H., D.M. Updegraff, P.M. Radehaus, and E.R. Bates, Metal removal and sulfate
reduction in low-sulfate mine drainage, in Bioremediation of Inorganics, Hinchee,
R.E., J.L. Means, and D.R. Burris, Eds., Battelle Press, OH, 1995.
Frankenberger, W.T. and M.E. Losi, Applications of bioremediation in the cleanup of heavy
metals and metalloids, in Bioremediation: Science and Applications, Skipper, H.D.
and R.F. Turco, Eds., Soil Science Society of America, Madison, WI, 1995.
Frink, C.R., A perspective on metals in soils. J. Soil Contam. 5, 329-359, 1996.
Gadd, G.M., Microbial formation and transformation of organometallic and organometalloid
compounds. FEMS Microbiol. Rev. 11, 297-316, 1993.
Gabriel, P.F., Innovative technologies for contaminated site remediation: focus on bioreme-
diation. J. Air Waste Manage. Assoc. 41, 1657-1660, 1991.
Greinert, A., Clay as substances limiting phytotoxic influence of Pb, Zn, and Cd in sandy
soils, in Contaminated Soil ’95, van den Brink, W.J., R. Bosman, and F. Arendt, Eds.,
Kluwer Academic Publishers, The Netherlands, 1995.
Gupta, S.K., Mobilizable metal in anthropogenic contaminated soils and its ecological sig-
nificance, in Impact of Heavy Metals on the Environment, Vernet, J.P., Ed., Elsevier,
Amsterdam, 1992.

Gupta, S.K., M.K. Vollmer, and R. Krebs. The importance of mobile, mobilisable and pseudo
total heavy metal fractions in soil for three-level risk assessment and risk management.
Sci. Total Environ. 178, 11-20, 1996.
Copyright © 2000 by Taylor & Francis
Hamby, D.M Site remediation techniques supporting environmental restoration activities —
a review. Sci. Total Environ. 191(3), 203-224, 1996.
Hasselbach, G. and E. von Boguslawski, Bodenspezifische Einflüsse auf die Schwermetal-
laufnahme der Pflanzen, in Auswirkungen von Siedlungsabfällen auf Böden, Bode-
norganismen und Pflanzen, Sauerbeck, D. and S. Lübben, Eds., Berichte aus der
ökologischen Forschung, Band 6, 1991.
Hattori, J., H. Labbe, and B.L. Miki. Construction and expression of a metallothionein-beta-
glucuronidase gene fusion. Genome 37, 508-512, 1994.
Herms, U. and G. Brümmer, Einfluss der Bodenreaktion auf Löslichkeit un tolerierbare
Gesamtgehalte an Nickel, Kupfer, Zink, Cadmium und Blei in Böden und kompost-
ierten Siedlungsabfällen. Landwirtsch. Forsch. 33, 408-423, 1980.
Jorgensen, S.E., Removal of heavy metals from compost and soil by ecotechnological meth-
ods. Ecol. Eng. 2, 89-100, 1993.
Krebs, R., S.K. Gupta, G. Furrer, and R. Schulin, Solubility and plant uptake of metals with
and without liming sludge applied soils. J. Environ. Qual. 27, 18-23, 1998.
Krebs, R., S.K. Gupta, G. Furrer, and R. Schulin, Gravel sludge as binding additive in soils
polluted with zinc, copper, and cadmium — a field study. Water Air Soil Pollut. 1999.
In press.
Lothenbach, B., R. Krebs, G. Furrer, S.K. Gupta, and R. Schulin, Heavy metal immobilization
in soil by addition of montmorillonite, Al-montmorillonite, and gravel sludge: batch
and pot experiments. Eur. J. Soil Sci., 49, 141-148, 1998.
Martell, A.E. and R.M.Smith, Critical Stability Constants, 2nd ed., Plenum Press, New York,
1989.
McCarty, S.L., T.G. Chasteen, V. Stalder, and R. Bachofen, Bacterial bioremediation of
selenium oxyanions using a dynamic flow bioreactor and headspace analysis, in
Bioremediation of Inorganics, Hinchee, R.E., J.L. Means, and D.R. Burris, Eds.,

Battelle Press, OH, 1995.
Morel, J.L., G. Bitton, C. Schwartz, and M. Schiavon, Bioremediation of soils and waters
contaminated with micropollutants: which role for plants?, in Ecotoxicology:
Responses, Biomarkers and Risk Assessment, an OECD Workshop, Zelikoff, J.T., Ed.,
SOS Publications, Fair Haven, NJ, 1997.
Ow, D.W., Phytochelatin-mediated cadmium tolerance in Schizosaccharomyces pombe. In
Vitro Cell Dev. Biol. 29P. 213-219, 1993.
Pan, A., F. Tie, Z. Duau, M. Yang, Z. Wang, L. Li, Z. Chen, and B. Ru, Alpha-domain of
human metallothionein I-A can bind to metals in transgenic tobacco plants. Mol. Gen.
Genet. 242, 666-674, 1994.
Phillips, I. and L. Chapple, Assessment of a heavy metals-contaminated site using sequential
extraction, TCLP, and risk assessment techniques. J. Soil Contam. 4, 311-325, 1995.
Robinson, J.B. and O.H. Touvinen. Mechanisms of microbial resistance and detoxification of
mercury and organomercury compounds: Physiological, biochemical, and genetic
analyses. Microbiol. Rev. 48, 95-124, 1984.
Salt, D.E., M. Blaylock, N.P.B.A. Kumar, V. Dushenkov, B.D. Ensley, I. Chet, and I. Raskin.
Phytoremediation: a novel strategy for the removal of toxic metals from the environ-
ment using plants. Biotechnology 13, 468-474, 1995.
Sims, J.T., S.D. Cunningham, and M.E. Sumner. Assessing soil quality for environmental
purposes: roles and challenges for soil scientists. J. Environ. Qual. 26, 20-25, 1997.
StoV, Verordnung über umweltgefährdende Stoffe. Verordnung des Schweiz. Bundesrates,
Eidg. Drucksachen und Materialzentrale, Bern, Switzerland, SR 814.015, 1986.
Summers, A.O., The hard stuff: metals in bioremediation. Curr. Opin. Biotechnol. 3, 271-
276, 1992.
Copyright © 2000 by Taylor & Francis
Tadesse, B., J.D. Donaldson, and S.M Grimes. Contaminated and polluted land: a general
review of decontamination management and control. J. Chem. Technol. Biotechnol.
60, 227-240, 1994.
Tuovinen, O.H., Biological fundamentals of mineral leaching processes, in Microbial Mineral
Recovery, Ehrlich, H.L. and C.L. Brierly, Eds., McGraw-Hill, New York, 1990.

van Bladel, R., H. Halen, and P. Cloos, Calcium-zinc and calcium-cadmium exchange in
suspensions of various types of clays. Clay Minerals 28, 33-38, 1993.
Vangronsveld, J., F. Van Assche, and H. Clijsters. Reclamation of a bare industrial area
contaminated by non-ferrous metals: in situ metal immobilization and revegetation.
Environ. Pollut. 87, 51-59, 1995.
VBBO, Swiss Ordinance Relating to Impacts on the Soil, Verordnung des Schweiz. Bundes-
rates, Eidg. Drucksachen und Materialzentrale, Bern, Switzerland, SR 814.12, 1998.
Volesky, B., Removal and recovery of heavy metals by biosorbtion, in Biosorption of Heavy
Metals, B. Volesky, Ed., CRC Press, Boca Raton, FL, 1990.
Wallace, A., R.T. Mueller, and G. Alexander, Effects of high levels of NTA on metal uptake
of plants grown in soils. Agron. J. 66, 707-708, 1974.
Wenger, K. and S.K. Gupta, Kann eine kontrollierte Mobilisierung die Phytoextraktion von
Schwermetallen wesentlich erhöhen? Bull. BGS 21, 91-96, 1997a.
Wenger, K., T. Hari, S.K. Gupta, R. Krebs, R. Rammelt, and C.D. Leumann, Possible
approaches for in situ restoration of soils contaminated by zinc. ISCO – Proc., 1997b,
in press.
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