Phytoremediation of Contaminated Soil and Water - Chapter 17 pdf

Hari CONTENTS Introduction Basic Concept for In Situ Gentle Decontamination and Stabilization Approaches Degree of Contamination and Severity of Risk Types of Gentle Remediation Techniq

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

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.

BASIC CONCEPT FOR IN SITU GENTLE

DECONTAMINATION AND STABILIZATION

APPROACHES

DEGREE 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

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 NaNO3-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.

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.

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 NaNO3-extractable (mobile) copper concentrations (data not shown) An explanation may be that the enhanced mobility was due to an increased formation of organic

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 addiaddi-tives

FIGURE 17.5 Reduction of the zinc, copper, and cadmium contents of seeds and crop residue

after lime application

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 CaCO3 content (30%) of the gravel sludge

In all treatments, the effects on NaNO3-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(ClO4)2 (right) as a function of pH

FIGURE 17.7 Decrease of NaNO3-extractable zinc after the addition of binding agents

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.

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, CrO42-) 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