Soil Pollution Remediation

Soil Pollution see Pollution, Soil

Soil Pollution Remediation MD Ferna´ndez Rodrı´guez, MC Garcı´a Go´mez, N Alonso Blazquez and JV Tarazona, National Institute for Agriculture and Food Research and Technology (INIA), Madrid, Spain Ó 2014 Elsevier Inc. All rights reserved.

Introduction

3. Treatment technologies capable of decreasing the concentration of soil contaminants.

In developed countries, contaminated land is mostly the outcome of historical industrial use, prior to the implementation of environmental protection policies. Currently, ground contamination arises from leaks, spills, accidents, poor waste disposal practice, and uncontrolled discharges. The management of contaminated lands starts with the identification of soils requiring remediation. The second stage consists of setting the remedial objectives and developing the remedial strategy. Risk assessment techniques offer support for identifying and prioritizing sites requiring remediation, developing remedial objectives and cleanup standards, and selecting the most appropriate remedy for a particular location. Currently, cleanup objectives are usually based on the target concentration of contaminants derived from risk-based analysis. However, during remediation processes, changes in bioavailability and the formation of more toxic metabolites may challenge the efficiency of remediation in lowering soil toxicity. Bioassays performed with soil samples taken from the site can be used to complement chemical analyses to evaluate the quality of remediated soil. They provide a direct estimation of the toxicity produced by the combined effects of identified substances, unknown compounds, and transformation products. In this article, several different bioassays are presented for the direct ecotoxicological assessment of remediated soils and biomonitoring of remediation processes.

Some remedial measures, denominated in situ, manage directly the contaminated material in the ground. Other measures may require the contaminated material to be excavated before it can be treated, and they are called ex situ remedial treatment. The subsequent treatment of excavated soils can be performed on the site (‘on-site’ treatment), while other measures may require the contaminated material is transported and treated in special facilities (‘off-site’ treatment). The main advantage of in situ treatments is that soil can be treated without being excavated and transported resulting in significant cost savings and additional treatment-related environmental impacts. However, in situ treatments generally require longer time periods, are less effective, and the monitoring of the processes is more difficult because of the variability in soil characteristics. The ex situ treatments are more expensive but also quicker in getting more complete remediation of the contaminated land and providing more certainty about the uniformity of the treatment. The main remediation techniques are briefly described below. Although the affection of groundwater is a main issue in the remediation of contaminated lands, this article excludes technologies for its remediation.

Containment Technologies General Considerations in Remediation Processes The choice of remedial technology largely depends on the nature and degree of contamination, the intended function or usage of the remediated site, and the availability of innovative and cost-effective techniques. The traditional approach to the remediation of contaminated land has been to excavate and redeposit, usually as landfill. This approach is unsustainable and increasingly expensive. New techniques are available that help reduce the amount of waste going to landfill and eliminate the contamination of the site with the aim of reusing the land. Generally, several treatment technologies are combined at each site. The different types of remedial measures that are available can be categorized into three main groups: 1. Containment technologies that seek the isolation of the site, but without acting directly over the contaminants. 2. Immobilization technologies that reduce the mobility of the contaminants in the environment through both physical and chemical means.

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Containment technologies use conventional civil engineering techniques to isolate contaminated media from the surrounding environment, that is, blocking the pathways by which contaminants can reach the receptors without destroying them. These technologies donot require soil excavation and area low-moderate cost treatment group despite the required long-term monitoring and maintenance. In general, these technologies are used when potential hazards could be produced during excavation and removal of contaminated soil or when other treatment technologies are not available or have an unrealistic cost. Table 1 summarizes the main characteristics of these remediation technologies. Containment technologies include covers and barriers. Covers are designed to avoid the direct exposure of receptors to contaminated soil, minimize the infiltration of rainwater into the landfill in order to avoid leachate production and hence groundwater contamination, and prevent the emission of volatile contaminants in the atmosphere. The soil surface is capping with low-permeability materials that can be natural (soils and bentonite), civil engineering (concrete or bituminous asphalt),

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Soil Pollution Remediation

Table 1

Summary of remediation technologies based on containment and immobilization strategies

Technology

In situ/ ex situ

Containment technologies Covers/barriers In situ

Immobilization Solidification/ stabilization

Ex situ/ in situ

Contaminants/ application

Limitations

Advantages

Organic and inorganic contaminants

It requires careful design and implementation (specially excavation and backfilling of the trench) It is susceptible to failure or damage It does not treat for the contamination It is not a permanent solution It requires long-term monitoring (groundwaters and gases) Wall materials depend on the type of contaminants

Applicable to a wide range of contaminants, specially complex mixtures Applicable to large or small sites

Inorganics, including radionuclides and heavy metals

Weathering and water infiltration can affect the integrity of the stabilized mass causing contaminant mobility The selection of immobilizing agent requires treatability studies The process may increase the waste volume In situ, it is difficult to get a complete and uniform mix of the immobilizing agent with the soil In situ, the depth of contaminants may limit the process Volatile compound (VOC) may be released during the process High energy is required It needs special equipment and trained personal Soil moisture increases the time and cost of the process The soil must contain enough silica and alkali oxides to allow the vitreous mass formation The resulting material may limit future land use It does not support a vegetal cover

Reagents are widely available and inexpensive The maintenance of immobilized material can be reduced if proper conditions are maintained It greatly reduces the leaching of contaminants

Organics in case of asphalt batching

Vitrification

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In situ/ ex situ

Inorganics, including radionuclides and heavy metals Organics, although other methods are preferred

or geosynthetic. The design of covers is site specific and is based on one single layer or a complex multilayer system combining different materials. To apply this technique, it is necessary that the site does not undergo continuous periods of wet–dryness, which can damage the cover. Groundwater should be controlled through wells. Moreover, gases can be generated due to contaminant degradation and must be collected and treated. It is recommended to use this technique in conjunction with vertical barriers to avoid lateral dispersion of the contaminants. Barriers are used for restricting the movement of contaminant plumes in soil and groundwater. They consist of vertically excavated trenches filled with highly impermeable materials to form a subsurface wall. The most commonly used materials are cement/bentonite or concrete. Another technique for constructing vertical barriers is metal sheet piling. They are long structural sections with a vertical interlocking system that creates a continuous wall. To seal the wall, the interspace must be filled with cement/bentonite grout or polyurethane. Subsurface horizontal barriers are also used to decrease soil permeability and to control the percolation of contaminants. The technique may be applied in the saturated and unsaturated zones. Physical barriers have been used for decades. Therefore,

Applicable to a wide range of contaminants Applicable to a broad range of media: solids, liquids, and sludges The resulting glass structure is durable and resistant to leaching

the equipment and methodology are readily available; however, the wall materials must be selected for containing the specific contaminants. A particular case of physical barriers are the permeable reactive barriers (PRBs), which are receiving great attention for in situ cleanup of groundwater contamination. In reality, these are not containment but treatment techniques. PRBs intercept the contaminant plume, allowing the movement of groundwater while contaminants are immobilized or chemically transformed to less harmful substances. A wide variety of materials can be used in PRBs, such as zerovalent metals (e.g., iron metal), humic materials, oxides, surfactantmodified zeolites (SMZs), and oxygen- and nitrate-releasing compounds.

Immobilization Technologies These technologies confine contaminants in soil, reducing their mobility to prevent migration to other media or the contact with potential receptors. Immobilization is achieved by acting directly on the conditions in which contaminants are

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present in the soil. Processes can be physicochemical or thermal (Table 1).

the separation processes generating residues, which require treatment or disposal. Residue management should be added to the total project costs and may require additional permits.

Physicochemical Methods Solidification and Stabilization Solidification and stabilization, also referred to as waste fixation, act through both physical and chemical methods. Solidification refers to techniques that physically bind or encapsulate the contaminants within a stabilized mass of high structural integrity and does not necessarily involve a chemical interaction between the contaminants and the solidifying additives. Stabilization refers to techniques that chemically reduce the hazard potential of a waste by converting the contaminants into less soluble, less mobile, or less toxic forms. The main technologies include cement, asphalt or phosphate, or alkalis that raise the pH facilitating the precipitation and immobilization of some heavy metal contaminants. Solidification and stabilization are performed both ex situ and in situ. These technologies are used for the unsaturated soil zone. These technologies have limited effectiveness against organic substances including pesticides, except asphalt batching that destroys most organic contaminants. In the long term, the effects of weathering and water infiltration can affect the integrity of the stabilized mass resulting in contaminant mobility.

Thermal Methods Vitrification Vitrification uses a powerful source of energy to melt soil at extremely high temperatures, immobilizing most inorganic contaminants and destroying organic contaminants by pyrolysis and/or oxidation. Inorganic substances, such as metals and radionuclides, are incorporated into a glass structure which is generally strong, durable, and resistant to leaching. Some volatile metals, radioactive contaminants, and organic compounds may volatilize by gas treatment. The water vapor and the products of pyrolysis are collected and led to the gas treatment system to remove particles and other contaminants. This technology can be applied both in situ and ex situ. In ex situ, soil heating can be achieved through various systems (plasma, direct power, combustion, induction, or microwave), but the application of electric energy is the most widespread technique. The soil is placed in a furnace, where it undergoes an electric power to reach the temperature of 1100–1400
C. The temperature for in situ treatment is higher (1600–2000
C). Electrical energy is usually applied through graphite electrodes inserted into the soil to be treated. Soil vitrification is an extremely effective technology, destroying or immobilizing almost all contaminants.

Physicochemical Treatment Soil Flushing Soil flushing is an in situ treatment technology in which an aqueous solution is injected or infiltrated into the contaminated soil. This may occur within the unsaturated zone, the saturated zone, or both. The flushing solution increases the mobility or solubility of contaminants sorbed to the soil matrix. This solution may consist of surfactants, cosolvents, acids, bases, oxidants, chelants, solvents, or water. Contaminated groundwater and extraction fluids are captured and pumped to the surface using standard groundwater extraction wells. Finally, extraction fluids with the desorbed contaminants must be treated. Air emissions of volatile contaminants from recovered flushing fluids should be collected and treated too. Soil flushing is generally used in conjunction with other remediation technologies such as activated carbon, biodegradation, and pump and treat. Physical barriers such as slurry walls or sheet piles can be installed to prevent uncontrolled migration of the solvent and the contaminants. The main disadvantage is the potential risk of spreading contaminants into uncontaminated areas and the effects of flushing solution into the soil environment.

Soil Washing Soil washing is an ex situ technology to remove contaminants from the soil using two processes: physical separation and chemical leaching by aqueous solutions. This technique includes an initial process of homogenization in which the coarse particles are separated by differences in density. The physical separation is based on the fact that most organic and inorganic contaminants tend to bind to clay, silt, and inorganic particles. Thus, washing processes separate the fine (small) clay and silt particles from the coarser sand and gravel soil particles and concentrate the contaminants into a smaller volume of soil (sludge) that can be further treated by other methods such as incineration or bioremediation. The coarse soil fragments can be used as backfill. In the second process, the contaminants are selectively dissolved and then chemically transformed or recovered. The additives and reagents that are added to water depend on the nature of the contamination to be treated. In soils contaminated by multiple substances with different characteristics, the application of the technique usually requires a sequential process using different washing solutions. The contaminated water is treated with the technology suitable for the contaminants. The main advantage of soil washing is that it is a costeffective technique because it reduces the amount of the material that would require further treatment by another technology.

Treatment Technologies Chemical Extraction Treatment technologies may be classified into three main categories according to the processes used to remove, destroy, or modify the contaminants: (1) physicochemical technologies, (2) biological technologies, and (3) thermal technologies (Table 2). An important group of these techniques is based on

Chemical extraction is an ex situ process that separates metals and organic contaminants from soils using chemical extractants, while soil washing uses water or water with wash-improving additives. Physical separation steps are often used before chemical extraction to divide the soil into coarse and fine fractions.

Table 2

Summary of remediation technologies based on the treatment of contaminants

Technology

In situ/ex situ

Contaminants/application

Limitations

Advantages

Physicochemical Soil flushing

In situ

Mainly inorganic contaminants: metals, cyanides, radioactive Organic contaminants: NAPLs, VOCs, SVOCs Fuels, pesticides

The equipment is easy to build and operate in permeable soils It can mobilize a wide range of organic and inorganic contaminants Costs are moderated depending on the flushing solution

Soil washing

Ex situ

SVOCs Petroleum and fuel, metals Cyanide

Chemical extraction

Ex situ

Vapor extraction

In situ/ex situ

PCBs, VOCs Halogenated solvents Petroleum wastes Organically bound metals (solvent extraction) Heavy metals (acid extraction) VOCs Certain SVOCs Light fuels

More effective in coarse-grained soils Increased costs as a result of the use and recuperation of the surfactant or the cosolvent to be reused A long time may be necessary depending on the contaminant sorption to soil Flushing fluid must be controlled to avoid the migration of contaminants into uncontaminated areas Waste generation: flushing fluids and air emissions of volatile contaminants More effective in coarse-grained soils Production of a large amount of washing water to be treated Usually, silt and clay after washing processes require to be treated by other methods Soils with high clay content and high moisture levels hinder the success of the process Residual acid and solvent traces in treated soil

Electrokinetic

In situ

Mainly metals Anions Polar organics

Chemical Chemical oxidation

In situ (ISCO)/ ex situ

Inorganics Halogenated VOC, cyanide Fuels Phenols and sulfur compounds

Soil washing can remove many types of contaminants It may not be cost effective for small amounts of contaminants It allows to clean chemicals that are difficult to be removed using other technologies The extraction of contaminants is generally quicker than in situ methods

Oxidizing agents are nonselective Risk of incomplete oxidation and production of toxic intermediates Highly adsorbed compounds may limit the degradation process Treatments may alter soil properties It is not cost effective for high contaminant concentrations Potential hazards of chemicals to workers Uncontrolled exothermic reactions in the subsurface

Contaminants are destroyed It can be less costly and quicker than other removal technologies ISCO is particularly useful to remediate sites difficult to treat by other techniques Capability to oxidize DNAPLs (Continued)

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It treats a large volume of soil at reasonable costs Minimal disturbance in situ operations It can be applied at sites with free contaminants The wells and equipment are simple to install and maintain It helps the biodegradation of non-VOCs Addition of reagents is not required Ex situ treatment is uniform and easily monitored It is effective for fine-grained soils of low permeability, which are difficult to treat by other methods The contaminated soil solution is easily extracted from the point of collection

Soil Pollution Remediation

Concentration reductions greater than 90% are difficult to achieve Low effectiveness in low air permeability or stratified soils High moisture and organic content limit its effectiveness Costly treatment of extracted vapors No volatile contaminants present in the site may require additional technologies for remediation Ex situ, air emissions may occur during excavation Ex situ, a large amount of space is required Effectiveness is reduced if the soil moisture content is less than 10% Electrolysis of metallic electrodes may introduce corrosive products into the soil. Electrodes of carbon, graphite, or platinum (inert materials) avoid this concern Light soluble and strongly adsorbed contaminants limit the success of the technology Oxidation/reduction reactions can form undesirable products

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Table 2

Summary of remediation technologies based on the treatment of contaminantsdcont'd In situ/ex situ

Contaminants/application

Limitations

Advantages

Dehalogenation

Ex situ

Halogenated SVOCs Halogenated pesticides

Formation of reaction products that may be more toxic than the contaminants being treated Soils with high clay and moisture content increase treatment costs It is not cost effective for high contaminant concentrations

The technology is amenable to small-scale applications APEG dehalogenation is in addition to incineration, one of the few processes available to treat PCBs

Thermal Thermal desorption

Ex situ

Nonhalogenated VOCs PAHs PCBs Pesticides Fuels Some SVOCs

Can be a fast and effective method of cleaning heavily polluted soil Equipment cost is lower than other thermal methods Soil structure is not altered at treatment temperature < 400–500
C

Incineration

Ex situ

Explosives Chlorinated hydrocarbons PCBs Dioxins

Previous homogenization of particle size is usually required Clay and silty soils and high humic content soils increase reaction time Dewatering may be necessary (soil moisture > 30%) Residuals are generated (condensed contaminants, off-gases, wastewater) Corrosion problems in the equipment when the waste contains halogenated compounds High metal concentration in the soil may require further treatment before backfilling Previous homogenization of particle size is usually required Residuals are generated (ash, off-gases, wastewater) It destroys the soil structure Heavy metals can produce a bottom ash that requires stabilization Sodium and potassium form low-melting-point ashes that can attack the equipment Local public opposition

Biological Phytoremediation

In situ/ex situ

Metals Chlorinated solvents Petroleum hydrocarbons, PAHs Pesticides and explosives

Bioremediation

In situ/ex situ

Biodegradable organic chemicals: Petroleum hydrocarbons, solvents, PAHs Pesticides, etc.

Time-consuming process Treatment is limited to the upper soil profile Plants grow well only in moderately contaminated soil Contaminants may enter the food chain through animals that feed on the plants used in these processes It requires the disposal of the plants used in the treatment of metal contaminated soils Climatic and hydrologic conditions may limit the plant’s growth High concentrations of contaminants may be toxic to the microorganisms Previous feasibility studies are necessary In situ, successful biological treatment depends on the climatic conditions Low bioavailability of contaminants limits the success of the process In situ, microorganisms cannot reach deep contaminants Recalcitrant behavior of some contaminants (PCBs, polychlorinated phenols, and PAHs) Bioremediation slows down at low temperatures

Incineration can destroy some types of chemicals that other methods cannot It is quicker than other technologies It reduces the amount of material that must be moved to a landfill

Low costs Aesthetically pleasing technique It protects soil against erosion (spreading of contaminants) Promising use to remove organic contaminants by metabolism

It is simple to maintain In situ, it is applicable over large areas It is cost effective It leads to the destruction of the contaminants

Soil Pollution Remediation

Technology

Soil Pollution Remediation

The two major chemical extraction processes are acid extraction and solvent extraction. Acid extraction uses hydrochloric acid to extract metal contaminants from soils. The heavy metals are potentially suitable for recovery. Solvent extraction uses organic solvents (acetone, hexane, methanol, dimethyl ether, or triethylamine) as extractants. The extractants are treated for their regeneration and can be reutilized in site. This technique is commonly used in combination with other technologies, such as solidification/stabilization, incineration, or soil washing, depending on the site-specific conditions. Traces of solvent may remain within the treated soil matrix, so the toxicity of the solvent is an important consideration. Chemical extraction is used to clean up many chemicals that are difficult to remove from soil using other technologies.

Soil Vapor Extraction Soil vapor extraction (SVE) is used to remediate unsaturated zone soil. A vacuum is applied to the soil to induce the controlled flow of air and remove volatile and some semivolatile organic contaminants. It is usually an in situ technology; however, in some cases, it can be used as an ex situ technology. In in situ SVE, also known as soil venting or vacuum extraction, vacuum is applied to the soil through the wells near the source of contamination, creating a negative pressure gradient to induce the controlled flow of air and remove the contaminants from the soil through an extraction well. Extracted vapor is treated before being released into the atmosphere. The increased airflow through the subsurface can also stimulate the biodegradation of some of the contaminants, especially those that are less volatile. In areas of high groundwater levels, water depression pumps may be required to offset the effect of upwelling induced by the vacuum. In situ SVE can reach greater depth than methods that require removing the soil, the wells and the equipment are simple to install and maintain. Ex situ SVE is a full-scale technology in which soil is extracted and placed over a network of aboveground piping where a vacuum is applied to volatilize organic contaminants. The process includes a system for handling off-gases.

Electrokinetic Electrokinetic is an in situ innovative technique for the decontamination of soils contaminated with metals, anions, and polar organics. The principle of electrokinetic remediation relies upon the application of a low-intensity direct current through a porous solid medium between appropriately distributed electrode arrays, causing ions and water to move toward the electrodes. Contaminants are transported by two contributive processes: electromigration (migration of ions) and electroosmosis (movement of liquid containing ions). Electromigration is the main mechanism for the electroremediation process. Moreover, other electrolysis effects such as diffusion, adsorption, complexation, and precipitation reactions also contribute to the process. Contaminants are removed at the electrode by different methods such as electroplating; precipitation or co-precipitation; water pumping near the electrode; or complexation with ion exchange resins. Apolar organic compounds are transported by the electroosmosis-induced water flow. Therefore, the addition of

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surfactants is necessary to increase their solubility and facilitate the formation of micelles. The main advantage of electrokinetic is that it is effective for fine-grained soils of low permeability that are difficult to treat by other methods. The effectiveness of this technique has been demonstrated in laboratory and pilot studies. However, more field trials are necessary.

Chemical Oxidation/Reduction Chemical oxidation is applied to treat organic substances that are almost completely oxidized into H2O and CO2 or transformed into less toxic compounds. This method may be applied in situ or ex situ. In situ chemical oxidation (ISCO) is an innovative technology applicable to a wide variety of organic compounds present in subsurface environments. Several oxidants have been tried, but most commercial applications use hydrogen peroxide (typically used together with Fe(II) to form Fenton’s reagent) or ozone for the vadose zone and hydrogen peroxide (H2O2) or potassium permanganate (KMnO4) in the saturated zone. Recently, persulfate salts (Na2S2O8) are being used for ISCO applications, but they are relatively expensive and require thermal activation. The method is based on the direct injection of an aqueous solution of the oxidant agents into the subsurface using conventional injection wells or advanced injection techniques, such as deep soil mixing or hydraulic fracturing, in case of lowpermeability soils. ISCO is the chosen technique to remediate those sites considered difficult to treat using other technologies. A serious potential limitation to the use of oxidizing agents for soil treatment is that the oxidants are nonselective. A significant part of these reagents is consumed by oxidizable material present in the soil and groundwater. This is a major concern because the concentration of natural organic material in the soils may be lowered, which would result in a decrease in the sorption capacity of some organics limiting the efficiency of the ISCO treatment. Reductive technologies may also be applied to soil remediation. The addition of reductive agents to soil can be used as an in situ treatment technology. They have been successfully applied in small-scale field experiments to remediate soils contaminated with organic compounds, Cr(VI) or Se(VI). Organic chemical constituents in soil may be reduced using catalyzed powder metals (mainly iron) or sodium borohydride (NaBH4). Metals are reduced by the addition of acidification agents such as sulfur or other agricultural acidification agents (leaf litter or acid compost) and a reducing agent (ferrous sulfate).

Chemical Dehalogenation Chemical dehalogenation processes use chemical reagents to degrade hazardous halogenated molecules or to transform them into other less harmful compounds. Two processes are employed: alkaline polyethylene glycol (APEG) reagents and base-catalyzed decomposition (BCD). Both are ex situ processes requiring excavation. APEG is used to treat halogenated aromatic compounds in a batch reactor in which the contaminated soil and the reagent are mixed and heated. The reaction between the chlorinated compounds and APEG replaces the chlorine atoms reducing the toxicity. A variation of this reagent is the use of potassium

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hydroxide or sodium hydroxide/tetraethylene glycol, referred to as alkaline tetraethylene glycol (ATEG) that is more effective on halogenated aliphatic compounds. The technology is amenable to small-scale applications and may be used in combination with other technologies. APEG dehalogenation is one of the few processes, other than incineration, that has been successfully field tested for treating polychlorinated biphenyls (PCBs). BCD is a two-phase process applied to remediate soils and sediments contaminated with chlorinated organic compounds, especially PCBs, dioxins, and furans. The first phase consists of thermal desorption in a rotary reactor which may include mixing contaminated material with sodium bicarbonate. In the second phase, the volatilized contaminants are transferred into a reactor for dehalogenation by a catalytic hydrogenation. The process uses sodium hydroxide, hydrogen donor oil, and temperatures between 250 and 350
C.

Biological Treatments Biological treatments include two main technologies: bioremediation and phytoremediation. Bioremediation exploits the ability of microorganisms to degrade and detoxify organic contaminants. Two general approaches are commonly used: biostimulation and bioaugmentation. The most widely used bioremediation procedure is biostimulation, which consists of stimulating the site conditions for the development of indigenous microorganisms by optimizing conditions such as aeration, addition of nutrients, pH, and temperature control. In addition to the stimulation of autochthonous microorganisms, the technique can also involve the addition of adapted microbial degraders specific for the contaminants (bioaugmentation). Bioremediation methods may be used in situ (e.g., bioventing) or ex situ (e.g., land farming, composting, slurry bioreactors). Phytoremediation is the use of green plants and their associated microbial communities to reduce contaminant levels in soils, surface and groundwaters, and sediments. It is an in situ technology that can also be applied ex situ (e.g., hydroponics systems). In soils, some plants can stabilize certain environmental contaminants (phytostabilization) and uptake contaminants that can be degraded or transformed into harmless metabolites (phytodegradation), stored within the cell structures (phytoaccumulation), and volatilized into the atmosphere (phytovolatilization). Moreover, plant roots release a range of organic compounds that stimulate the activity of microorganisms in the rhizosphere increasing the rates of biodegradation (rhizosphere degradation). Plants may assist in the remediation of sites contaminated by a wide range of chemicals: metals, radionuclides, and organic compounds such as pesticides, solvents, and petroleum products. Biological treatments (bioremediation and phytoremediation) are now widely used as an alternative to the equivalent physicochemical methods because of their low cost, effectiveness, and environmental benefits. Moreover, phytoremediation improves the overall quality and texture of the soil at remediated sites and contribute to the restoration of habitat quality. These techniques will be more widely dealt within other articles.

Thermal Methods Thermal methods are ex situ treatment technologies that destroy or remove contaminants through exposure to high temperature in treatment cells, combustion chambers, or other means used to contain the contaminated media during the remediation process.

Thermal Desorption Thermal desorption is a technology of physical separation based on heating the contaminated soil to volatilize water and organic contaminants. Soils are heated in a thermal desorption system, the rotary dryer being the most commonly used equipment. The systems require the treatment of the off-gas to remove particles and contaminants. Its effectiveness depends on the contaminant. Decontaminated soil usually returns to the original site. Based on the operating temperature, these processes can be categorized into two groups: high-temperature thermal desorption ranging from 320 to 560
C and lowtemperature thermal desorption ranging from 90 to 320
C. Thermal desorption can be used in a place where some other cleanup methods cannot be used, such as at sites that have a high soil contamination, and can be a soil remediation method that is faster than others. Thermal methods may also be applied as an in situ technique. In this case, heat is applied to soil to volatilize semivolatile organic compounds (SVOCs), which can be extracted via collection wells and treated. It is a particular case of SVE. Heat can be introduced into the subsurface by electrical resistance heating, radio frequency heating, or injection of hot air or steam. Thermal methods can be particularly useful for dense nonaqueous phase liquids (DNAPLs) or light nonaqueous phase liquids (LNAPLs).

Incineration Incineration is a technology for ex situ thermal treatment based on the application of high temperature (870–1200
C) to the soil to burn harmful organic chemicals. Metals cannot be destroyed by this technique. The efficiency of a properly operated incinerator is very high, especially for PCBs and dioxins. Excavated contaminated soils can be incinerated on site or transported to an incinerator off site, although to burn PCBs and dioxins only off-site incinerators are permitted. There are various types of incinerator plant designs: circulating bed combustor, infrared combustion, rotary kiln, and fluidized bed that may be applied to soil incineration. The gases produced in the process must be treated to remove any remaining metal, acids (HCl, NOx, and SOx), and particles of ash before they are dispersed into the atmosphere. The soil or ash remaining in the incinerator after the burning and from gas treatment may be disposed into a landfill or buried on site. However, incineration significantly reduces the material for disposal. Incineration can destroy some types of contaminants that are not possible by other methods and is quicker than other technologies. This is important when a site must be cleaned up quickly to prevent harm to the people or the environment.

Soil Pollution Remediation

Other Treatment Technologies Natural Attenuation Natural attenuation (NA) is an in situ treatment to reduce the contamination in soil and groundwater using naturally occurring processes in soil. This treatment acts without human intervention and the activity focuses on the verification and monitoring of processes to assure their sustainability over time and their effectiveness. Moreover, the process must be carefully controlled and monitored to ensure that unaccepted risk for human health and ecosystems may appear mainly as a result of the migration of contaminants. NA may act by three different ways: (1) to destroy the contaminant by biodegradation or abiotic processes such as hydrolysis, (2) to reduce the concentrations through dilution or spreading by diffusion, dispersion, and volatilization, and (3) to immobilize contaminants (adsorption), and hence to reduce the bioavailability and toxicity. The effectiveness of NA highly depends on the site-specific conditions and the types of contaminants. This technique requires contaminants that are readily degradable; therefore, it is being applied mainly to petroleum hydrocarbons and nonhalogenated solvents. NA is a relatively simple technology compared to other remediation technologies. The costs are low and they are associated with initial evaluation and monitoring. However, it presents two main limitations. First, it requires more time than conventional remediation methods to get the same remaining concentration. Second, contaminants may migrate into the soil and groundwater during the remediation process and larger volumes of groundwater can become contaminated through dilution. Therefore, before applying this technique, it is necessary to estimate the movement of the contaminant plume and its extension during the cleanness process. NA can be used as an alternative to or in combination with active technologies. Usually, it is used as a complement to an active system of remediation and it is accepted as the only approach in a small number of cases.

Nanoparticles Nanoparticles (NPs) are of interest for environmental applications because they have larger surface areas per volume of material, which provides more reactive sites allowing for more rapid degradation of contaminants compared to the larger particles of the same bulk material. An increasing variety of nanoscale materials is being researched and applied as in situ contamination remediation technologies. The most widely studied NPs in remediation trials is the nanoscale zerovalent iron (nZVI), although other substances are being investigated: self-assembled monolayers on mesoporous supports (SAMMSÔ), dendrimers, carbon nanotubes, metalloporphyrinogens, and swellable organically modified silica. The nZVI are used mainly in the remediation of groundwater, although they are also applied to soil remediation. Iron particles are effective for the treatment of contaminants such as chlorinated organic solvents, organochlorine pesticides, PCBs, organic dyes, and various inorganic compounds. NPs are directly injected via gravity or under pressure and need less infrastructure than traditional techniques. However, there are some fundamental issues limiting their applications. A main

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question is the insufficient knowledge about their environmental behavior and their toxicity to the potentially affected receptors. Another main disadvantage of using NPs is the agglomeration of particles among them or to the soil surface. It increases the particle size, reducing the effectiveness of these materials and their subsurface mobility. Modifications of NPs have been made to decrease this agglomeration; for example, adding coatings or encasing in emulsified vegetable oil droplets. More research is needed to understand the fate, transport, and the potential toxicity of nanoscale materials in the environment.

Sustainable Issues Contaminated land management has been traditionally considered intrinsically sustainable despite the remediation processes consuming a large amount of energy, generating large amounts of atmospheric emissions including greenhouse gases, and creating a risk of injury for its workers. Management decisions have not taken into account many of these impacts when selecting the remediation method. However, a more comprehensive approach to soil remediation should incorporate sustainability assessments. Sustainable (green) remediation involves balancing the benefits of remedial action with the impacts of these actions, without forgetting the long-term protection of human health and the environment. It is based on the three elements of sustainable development: environment (conservation of natural resources and biodiversity), economy (balancing economic viability), and society (enhancement of the quality of life in surrounding communities). Sustainability can be considered either in decisions that lead to the overall remediation project including site development and future land use or targeted to the selection of the most proper remediation option. Sustainability evaluation is a key point to be applied in a comparative sense where the ‘best option’ is selected, generally based on the assessment indicators. These indicators or metrics can be qualitative (e.g., wildlife and flora conservation, worker safety, local residents’ safety and quality of life, and potential for litigation) or quantitative (e.g., carbon dioxide emissions, energy consumption, direct costs, water usage, duration of work, and local job creation). Indicators are combined using different qualitative or quantitative tools. Some qualitative methods yield a relative ranking and are considered hybrids. These approaches include decision tables and multicriteria analysis that use scores (the magnitude of an effect) and weighting (the importance), which are mathematically processed. Some quantitative tools suited to evaluate sustainability of remediation projects are life-cycle assessment, net environmental benefit analysis, and cost–benefit analysis.

Biological Tools for the Assessment of Contaminated and Remediated Soils Traditionally, remediation objectives are focused on the concentration of the contaminant that is judged acceptable based on the established standards or guidelines or following

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a site-specific risk analysis. The chemical analysis of contaminated and remediated soils is carried out with those contaminants expected according to the historical use of the site. However, not all contaminants present in the soil, including metabolites or transformation products, may be considered during the analysis. Furthermore, the chemical analysis by itself does not allow an integration of the combined effects of the contaminants present at a contaminated site as well as the bioavailability changes during the remediation process. Biological tools contribute to identify contaminated soils, to follow the success of the remediation process in terms of biota, and to assess the final state of the soil as well as to monitor the evolution of the ecosystems after its clearness. Monitoring can be done by simple and cheap standardized bioassays or suitable biomarkers. These can verify the decline in toxicity due to transformation, dissipation, or reduction in the bioavailability, quantifying the impact of the remediation method by measuring directly the effects on biota. The potential toxicity of the chemical compounds added or produced during the remediation process, remaining in the remediated soil, is also integrated in the measurements. Higher-tier methods would be restricted to monitor the evolution of the ecosystem after its clearness, when the intended use of soil is natural. These methods are briefly described below.

different physicochemical properties of the test and control soils. Differences in soil properties between the control/reference and the test soils may affect the test results. Ideally, assays should be performed with a reference soil, which is a soil with the same characteristics as that of the test soil but noncontaminated. To find a proper reference soil on the site not affected by the contamination or far away but with similar properties can be problematic, making difficult the data interpretation except when toxicity is still observed after a large dilution. To correctly assess soils, a battery of tests has to be conducted with different test species to cover differences in species sensitivity. Typically, soil toxicity is tested on soil invertebrates (e.g., earthworms), terrestrial plants, and soil microorganisms; and leachates from the contaminated soil in algae and aquatic invertebrates. Fish are often not used to diagnose the toxicity of a soil due to the large volume of leachates required in the assay and because of the ethical issues intended to reduce testing on vertebrates. An alternative is the use of in vitro assays with fish cell lines to overcome these obstacles. Single-species assays are very good tools for the diagnosis of the soil at the starting stage in terms of toxicity to identify the most sensitive organisms and to verify the effectiveness of soil remediation.

Single-Species Toxicity Tests

Microcosms

Ecotoxicity tests performed at different levels of biological organization, from molecular level (biochemical changes) to ecosystem function, can be modified for the direct measurements of soil toxicity. Most assays are performed at the organism level, constituting the single-species toxicity tests. The measured end points may be lethal or sublethal effects (e.g., inhibition of growth or reproduction). Many single-species assays have been standardized by different organizations (ISO, OCDE, USEPA, ASTM, etc.) to harmonize their use, with the aim of reducing the variability and comparing the results. The OECD list of standardized methods on soil and water organisms is presented in Table 3. These methods may be applied with some modifications to the assessment of contaminated soils for their initial ‘ecodiagnostic’ and to remediated soils. The toxicity of contaminated or remediated soils may be tested using two approaches: at a single concentration of the test soil and at different dilutions of the test soil. In the first case, assays are performed with a control or reference soil and with nondiluted samples of the test soils. The results of the toxicity assessment are compared using statistical tools such as ANOVA analysis and are expressed as a percentage of inhibition with respect to the control. In the second approach, different dilutions of the test soil are obtained using a control soil or reference soil obtaining a contamination gradient. Quantitative dilution–response relationship is determined by regression analyses (e.g., logit, probit, and Spearman–Karber). The first approach is fast and cheap and allows obtaining directly the toxicity results at a percentage of inhibition between 0 and 100% for each sample. Regression models provide estimations of the parameters of interest with higher sensitivity and precision. Moreover, the use of different concentrations allows detecting cross effects such as hormesis or effects due to

An alternative to the battery of single-species assays is the use of multispecies systems that account for species interactions. These systems include simultaneously several organisms in the same unit allowing not only the interaction among the species but also evolving them under the same exposure conditions along with the assay. These systems are more realistic and complex than single-species test. Usually, the soil microcosm consists of a column of soil with a device to collect the leachate from below. Water is applied at regular times to simulate rainwater percolating through the soil, and is collected using a funnel. Two types of soil microcosms have been often used: intact soil cores with autochthonous soil organisms and artificial assemblages adding test organisms on sieved soil columns. For ecotoxicological testing of soil quality, the latter approach seems to be more appropriate. It allows determining the toxicity in a set of selected organisms cultivated in the laboratory and therefore not previously exposed to contaminants.

Biomarkers Biomarkers measure biological response (biochemical, cellular, physiological, or behavioral changes), at the organism level or below, that could be associated with the exposure to one or more contaminants. This biological response can be specific or nonspecific. At the present time, specific biomarkers are only available for a very limited number of chemicals and metals. Nonspecific biomarkers may be useful as an integrative measure of a set of stressors (not only chemicals but also nonchemical factors). Biomarkers are very sensitive indicators whose main goal is to serve as early warning signals for predicting adverse effects at higher biological organization levels (population or

Table 3

Standardized methods that can be applied to the toxicity assessment of contaminated and remediated soils (OECD Guidelines)

Assay Terrestrial compartment Earthworm, acute toxicity tests Terrestrial plant test: seedling emergence and seedling growth test Soil microorganisms: nitrogen transformation test

End point (measured variables)

OECD 207

Eisenia fetida Eisenia andrei Terrestrial plants

2 Weeks

LC50

Mortality (adult survival)

2 or 3 Weeks

LC50 NOEC

Seedling emergence and inhibition of growth (biomass, shoot height, etc.)

Soil microbial autochthonous populations Soil microbial autochthonous populations E. fetida E. andrei Enchytraeus albidus Enchytraeus sp. Folsomia candida Folsomia fimetaria

4 Weeks

EC50

Nitrogen transformation activity of soil microorganisms

4 Weeks

EC50

Carbon transformation activity of soil microorganisms

8 Weeks

NOEC

6 Weeks 4 Weeks 3 Weeks

ECx NOEC ECx NOEC

Reproductive output (cocoon production and viability, juvenile hatching) Reproductive output (number of juveniles/ adult) Reproductive output (number of juveniles/ adult)

72 h

ErCx

Growth inhibition of cultures (number of cells, fluorescence, optical density, etc.)

OECD 202

Pueraria subspicata Desmodesmus subspicatus Anabaena flos-aquae Daphnia magna

48 h

EC50

Mortality (immobilization)

OECD 211

Daphnia magna

21 Days

NOEC

OECD 203

Onchorhynchus mykiss Conchita carpio Pimephales promelas

96 h

LC50

Reproductive output (number of living offspring/parent alive) Mortality (adult survival)

OECD 208

OECD 216

Earthworm reproduction test

OECD 222

Enchytraeid reproduction test

OECD 220

Collembolan reproduction test in soil

OECD 232

OECD 201

Soil Pollution Remediation

Organism

OECD 217

Daphnia sp., acute immobilization test Daphnia magna Reproduction Test Fish, acute toxicity test

Effect/ concentration data

Reference

Soil microorganisms: carbon transformation test

Aquatic medium Freshwater alga and cyanobacteria, growth inhibition test

Exposure period

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community), indicating the presence of unacceptable levels of contamination. The use of biomarkers in soil characterization has some main limitations. In some cases, it is difficult to relate the physiological response to the effects at a higher level (e.g., impacts on survival, growth, or reproduction, or other end points related to population decline). The application of biomarkers in actual field assessment has not been validated yet. Effects are usually measured at the subcellular or suborganism level. Several biomarkers are associated with stress responses. Examples in case of earthworms are the measurement of the lysosomal membrane stability of coelomocytes, the lysosomal accumulation of lipofuscin in chloragogenous tissue and of neutral lipids in coelomic cells, and the determination of plasma membrane Ca2þ-ATPase activity or metallothionein content. In plants, biomarkers are related to the photosynthetic activity (decrease in chlorophyll fluorescence), the activation and synthesis of stress proteins, phytohormones, or phenolic compounds, and the production of high quantities of reactive oxygen species. For microbial function, the measurement of enzymatic activities (e.g., phosphatase and dehydrogenase) is very common. Biomarkers at suborganism or organism level are related with damage on tissues, biological fluids, or alterations in the behavior. In earthworms, histopathological makers such as bruising, swelling, tissue necrosis, and extrusion of coelomic fluid have been reported, especially in clitellum region. Avoidance of media can also be shown as an example of behavioral changes. In plants, different reactions to stress situations can be considered biomarkers of contamination such as inhibition of stomatal opening and respiration rate and chlorosis.

Biological Field Observations Biological field observations are another biological alternative for the direct assessment of contamination effects in situ. They can be used in the last stage of remediation process to control the recuperation of the natural abundance of communities in situ, in those cases where the future land use of remediated soil is natural use. The selection of the group of organisms for the monitoring program is based on the ecological relevance and practicability (e.g., sampling and identification). Microorganisms and soil invertebrates (enchytraeids, nematodes, earthworms, and mites) are frequently used. These organisms are responsible for key ecological processes. Information on the community may be desirable. Unusual species assemblages are recognized by comparing with reference sites. If a proper reference location is not available, only major ecosystem changes can be detected. Bioaccumulation of contaminants in the lowest food chains (producers and primary consumers) can also be studied. It provides information on the transfer of contaminants through the food web. Biomarkers may also be used in monitoring programs. In summary, the integration of chemical analysis and biological tools is required to manage the remediation of contaminated lands. There are a variety of biological tools for the assessment of both contaminated and remediated soils. Currently, there is an increased interest in the application of the bioassays for the risk assessment of contaminated soils. However, the use for remediation purposes is more limited due

to the low sensitivity of these assays. The use of long-term assays using more sensitive parameters such as sublethal effects is recommended for the observation of toxicity differences between initial soils and samples obtained after remediation treatments. The use of biomarkers can provide warnings of biological impact despite that for most of them there is not a relationship between physiological response and populationlevel parameters. Finally, the biological field observation of the site should be desirable to study the natural evolution of the site, when the intended use of the soil is natural use.

See also: Environmental Risk Assessment, Terrestrial; Site-Specific Environmental Risk Assessment; Pollution, Soil; Environmental Biomarkers; Multispecies Environmental Testing Designs; Environmental Toxicology; Ecotoxicology; Toxicity Testing, Aquatic.

Further Reading Calow, P. (Ed.), 1997. Handbook of Ecotoxicology, vol. I. Blackwell Science, Oxford. Fernández, M.D., Babín, M., Tarazona, J.V., 2010. Application of bioassays for the ecotoxicity assessment of contaminated soils. Methods Mol. Biol. 599, 235–262. Hugget, R.J., Kimerle, R.A., Mehrle, P.M., Bergman, H.L., 1992. Biomarkers, Biochemical, Physiological and Histological Markers of Anthropogenic Stress. Lewis, Boca Raton, FL. Khan, F.I., Husain, T., Hejazi, R., 2004. An overview and analysis of site remediation technologies. J. Environ. Manage. 71 (2), 95–122. Keddy, C.J., Greene, J.C., Bonnell, M.A., 1995. Review of whole-organism bioassays: soil, freshwater sediment, and freshwater assessment in Canada. Ecotoxicol. Environ. Saf. 30, 221–251. Knacker, T. (Ed.), 2004. Ring-Testing and Field-Validation of a Terrestrial Model Ecosystem (TME). Ecotoxicology, vol. 13, pp. 5–176 (special issue). Loibner, A.P., Szolar, O.H.J., Braun, R., Hirmann, D., 2003. Ecological assessment and toxicity screening in contaminated land analysis. In: Thompson, K.C., Nathanail, C.P. (Eds.), Chemical Analysis of Contaminated Land. Blackwell Publishing Ltd, Oxford, UK, pp. 229–267. McCarthy, J.F., Shugart, L.R., 1990. Biomarkers of Environmental Contamination. Lewis, Boca Raton, FL, USA. Schaeffer, A., Van den Brink, P.J., Heimbach, F., Hoy, S.P., de Jong, F.M.W., Römbke, J., et al., 2010. Semi-Field Methods for the Environmental Risk Assessment of Pesticides in Soil. CRC Press, Boca Raton, FL. Sheppard, S.C., 1997. Toxicity testing using microcosms. In: Tarradellas, J., Bitton, G., Rosell, D. (Eds.), Soil Ecotoxicology. Lewis, Boca Raton, FL, pp. 345–373. Smith, L.A., Means, J.L., Chen, A., Alleman, B., Chapman, C.C., Tixier Junior, J.S., et al., 1995. Remedial Options for Metals-Contaminated Sites. Lewis Publishers by CRC Press Inc. ISBN 1-56670-180-5. Stephenson, G.L., Kuperman, R.G., Linder, G.L., Visser, S., 2002. Toxicity Tests for Assessing Contaminated Soils and Ground Water. In: Sunahara, G.I., Renoux, A.Y., Thellen, C., Gaudet, C.L., Pilon, A. (Eds.), Environmental Analysis of Contaminated Sites. John Wiley and Sons Ltd, New York, NY, pp. 25–43. Swartjes, F.A., 2011. Dealing with Contaminated Sites: From Theory Towards Practical Application. Springer, NY. Van Gestel, C.A., Van Der Waarde, J.J., Derksen, J.G., van der Hoek, E.E., Veul, M.F.X.W., Bouwens, S., et al., 2001. The use of acute and chronic bioassays to determine the ecological risk and bioremediation efficiency of oil-polluted soils. Environ. Toxicol. Chem. 20, 1438–1449. Walker, C.H., Hopkin, S.P., Sibly, R.M., Peakall, D.B., 2001. Principles of Ecotoxicology. Taylor and Francis, New York.

Relevant Websites http://www.nicole.org/WorkingGroups/WGSustainableRemediation/ NICOLE (Network for Contaminated Land in Europe) http://www.sustainableremediation.org/ The Sustainable Remediation Forum (SURF.USA)

Soil Pollution Remediation

http://www.cluin.org/ Contaminated Site Clean-Up Information (CLU-IN) CLU-IN - The Hazardous Waste Clean-up Information (CLU-IN) Sponsored by the U.S. EPA Office of Superfund Remediation and Technology Innovation http://www.frtr.gov/matrix2/top_page.html. Remediation technologies screening matrix.The Federal Remediation Technology Roundtable (FRTR) http://oecd.org/ OECD (Organization for Economic Cooperation and Development). The OECD's Online Library of Statistical Databases, Books and Periodicals.

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http://www.oecd-ilibrary.org/environment/oecd-guidelines-for-the-testing-ofchemicals_chem_guide_pkg-en Organization for Economic Cooperation and Development (OECD). Guidelines for testing of chemicals. Section 2: Effects on biotic systems. Last updated July 2011. http://nepis.epa.gov/Exe/ZyPURL.cgi?Dockey¼200083PQ.txt USEPA (1990). Handbook on in situ treatment of hazardous-wastes contaminated soils. Rapport. EPA/ 540/2-90/002. pp. 169.