Techniques for polluted soils remediation. A review.

The soil remediation is considered an essential part of environmental engineering. Then, the main techniques of this area will be briefly described.

1. Definition of soil

From the environmental point of view, soil is defined as the thin top layer of the earth's crust (lithosphere) lying between the bedrock and the surface. It is composed of mineral particles, organic matter, water, air and living organisms.

Figure 1. Definition of soil.

2. What is considered a polluted soil?

According to the Law 10/1998 of Residues, polluted soil is that soil whose physical, chemical or biological characteristics have been negatively affected by the presence of hazardous components of human origin, in a concentration and at a risk to human health or the environment, according to the criteria and standards to be determined by the Government.

3. What is the remediation of polluted soils?

The remediation of polluted oils comprises a set of procedures by contention, removal or destruction of pollutants, which allows full or partial recovery of soil functions. The large number of existing techniques can be grouped according to their operating characteristics or purpose. Thus, according to the purpose of treatment, a group is formed by immobilization technologies or contention of contaminants, while another comprises the various treatments to remove, by withdrawing or transformation.

Furthermore, according to the location of the soil during the treatment, there are two types of techniques: at its original position, in situ, and those used after the soil excavation: ex situ. In addition, ex situ treatments can be performed in the field itself (on site) or elsewhere (off site).

4. Isolation techniques

These are a group of techniques for immobilization or contention of soil pollutants. They are divided into four types:

• Sealing. These techniques comprise coating and screen technologies (screens with grout, concrete, cement injection and  through chemical or synthetic membranes).

• Vitrification in situ. This technique is based on the heating and subsequent melting of the soil at a very high temperature (1600-2000 °C) using an electrical current. The current is sent to two electrodes that are introduced into the polluted area, causing the melting of the soil between them.

• Volatility reduction, by reducing the volume of air filled pores, decreasing temperature or sealing the surface layer.

• Stabilization and solidification. The aim of this technique is to reduce the solubility, reactivity or mobility of the pollutants through the modification of their chemical status or physical immobilization with a stabilization agent (stabilization); or to transform the residue into a solid residue, with an easier and more safe manipulation, reducing the risks of volatilization, lixiviation or leaks (solidification).

5. Decontamination techniques

In turn, these techniques seek soil decontamination by its withdrawal or transformation. They are divided into physicochemical, biological and thermal techniques.

5.1 Physicochemical decontamination techniques

The main physicochemical techniques are described below.

• Soil vapor extraction. This is a in situ technique which is applied in the saturated area of the soil for the extraction on the volatile organic compounds (VOCs). It consists on drilling well above the water table, in which a vacuum in generated, so that the VOCs contained in the soil are pumped.

• Air sparging. This is a in situ technique which is applied in the saturated area of the soil and in the area of capillary suction, and it is complementary to the vapor extraction. It involves the introduction of air below the water table through vertical or horizontal ducts. The injected air makes the saturated zone to sparge and the vapor-liquid equilibrium is shifted, volatilizing the insoluble VOCs, that are usually extracted by a vapor extraction system.

• Aeration. It is considered a method for removing volatile compounds. The soil is excavated and a thin layer of around 20 cm is poured. Then, after some time, the VOCs are volatilized.

• Water pumping. This is a in situ technique that involves the drilling of wells to reach the polluted aquifer for the extraction of harmful soluble compounds. After the water is removed, it follows a purification method.

• Flushing in situ. This technique consist of the injection (through wells, ditches, sprinkler systems or infiltration systems) of a rinse solution to help the mobilization contaminants for subsequent pumping to the surface through extraction wells. The extracted solution is treated in the surface and the effluent from this treatment can be recycled as new rinse solution.

• Soil washing. This is a ex situ technique in which the soil is treated in specific facilities where the pollutants are removed through chemical and physical processes. The procedure consist of employing a washing solution formed by water and chemical additives and a mechanical dust separation process.

• Electrokinetic treatment. This involves the application of a electric current of low intensity between two ceramic electrodes introduced in the polluted area to cause the migration of charged species. Metal ions, ammonium ions and organic compounds positively charged will migrate towards the cathode; while chloride anions, cyanides, fluorides, nitrates and negatively charged organic compounds will migrate towards the anode.

• Chemical treatments in situ. Consist on the partial or total removal of pollutants through chemical agents. In order to access to the polluted area, mixing processes or injection processes of the chemical products are needed.

• Permeable reactive barriers. This in situ technique involves the intercept of ground water to remove the pollutants through physical, chemical or biological processes. This is a reactive system completed by an additional system which leads the water flow into the barrier.

Figure 2. Excavation of polluted soils.

5.2 Biological decontamination techniques

 These techniques include the different possibilities of bioremediation (in situ, ex situ on site and ex situ off site) and the phytoremediation in situ.

• Biodegradation in situ. In turn, it is subdivided in:

- In situ bioestimulation or accelerated bioremediation. This technique aims to accelerate the natural biodegradation process by providing nutrients and /or new microorganisms to an area polluted with organic compounds, to carry out its transformation into harmless compounds.

- Bioventing. This in situ biological treatment technology combines the VOCs mechanic venting with the usage of indigenous microorganisms for the degradation of organic compounds adsorbed by the soil in the unsaturated area.

- Bioslurping. This technology uses vacuum and the activity of microorganisms for the remediation of soils polluted by HCs. It consists of extracting the air in the soil through vacuum, which stimulate the flow of new air in this area, which allows the increase of activity of microorganisms that are able to degrade organic compounds.

• Biodegradation ex situ on site. In turn, it is subdivided in:

- Landfarming. This ex situ technique consists of excavating the polluted soils and disposing them on a impermeable surface (usually a geomembrane). This geomembrane is situated on a surface near the polluted area or in a small excavated pool. On it the soil is disposed for its treatment. This system also has a drainage system for the collection of leaks, which should receive a later treatment.

- Biostacks. In this ex situ technique the soil polluted by HCs is extracted and disposed in a treatment area or pool previously excavated, for the decontamination with microorganisms. The piles of soil do not usually exceed 2-3 meters and can be covered by impermeable plastic to control the volatilization of VOCs, which should be treated before being released to the atmosphere.

- Compost. The compost of polluted soils is a controlled biological process in which the organic compounds are converted into harmless substances by aerobic microorganisms, resulting a final stabilized product called compost, which is useful in agriculture.

Figura 3. Compost facility.

• Biodegradation ex situ on site. This technique involves soil excavation and transport to specific facilities where it will be treated in bioreactors. The soil is introduced into the bioreactor, which normally consists of a cylindrical horizontal fermenter which rotates on its axis, promoting the mixing of the contaminated ground. Nutrients, oxygen and microorganisms are introduced if necessary.

• Phytoremediation in situ. It consist of the usage of plants for removal, transfer, stabilization or degradation of pollutants. The plants act as biological filters which decompose or stabilizes metals, or degrade organic compounds.  

5.3 Thermal  decontamination techniques

Incineration and thermal desorption are included in this group.

• Incineration. The soil is burned at 870-1200 ºC in the presence of oxygen and then, destroying by thermal oxidation, halogen compounds and other organic compounds mainly refractory, while metals are not destroyed.

• Thermal desorption. This ex situ treatment consists of warming the soil at 250-600 ºC to evaporate the VOCs or volatile metal such as Hg. The polluted gases generated are separated from the clean air using a gas collection equipment.

To sum up, in the next table it is possible to find the way of application, speed, costs and treatable pollutants.

Table 1. Main features of soil treatment technologies.

FEATURES OF SOIL TREATMENT TECHNOLOGIES
Technique	Place	Speed	Cost	Treatable pollutants
Screen technologies	In situ	slow	low	high toxicity pollutants
Vitrification in situ	In situ	medium	high	high toxicity pollutants
Volatility reduction	In situ	temporary  solution	low	VOCs
Estabilization/solidification	In situ ex situ	fast	low	heavy metals, radiactive materials
Vapor extraction	In situ	medium	low	VOCs, oil derivatives
Air inyection	In situ	medium	low	VOCs
Aeration	Ex situ	slow	low	VOCs
Water pumping	In situ	fast	low	Soluble compounds
Flushing	In situ	medium	medium	Phenols metals oils, soluble compounds, organic compounds
soil washing	Ex situ	fast	medium	Metals, oil derivatives, VOCs,pesticides
Electrokinetic treatment	In situ	medium	high	Metals,  organic compounds
Chemical treatments	In situ	fast	medium	PCBs, other organic compounds
Reactive barriers	In situ	slow	medium	Metals, halocarbons, oil derived HCs , other organic compounds
Bioestimulation in situ	In situ	slow	low	oil derived HCs,pesticides, solvents, wood preservatives, other organic compounds.
Bioventing	In situ	medium	low	Oil derived HCs, non chlorinated solvents,pesticides, wood preservatives, other organic compounds.
Bioslurping	In situ	medium	low	Oil derived HCs
Landfarmig	Ex situ	medium	low	refinery sludges
Biostacks	Ex situ	medium	low	VOCs, HCs, pesticides
Compost	Ex situ	medium	low	Explosives, biodegradable organic compounds
Biodegradation off site	Ex situ	medium	high	Artillery resiues, VOCs, PCBs, pesticides
Phytoremediation in situ	In situ	slow	low	Metals, pesticides, solvents, explosives,oil derived HCs .
Incineration	Ex situ	fast	high	All kind of organic compounds
Thermal desorption	Ex situ	fast	medium	Organic compounds from refinery wastes, coal tar wastes, wood industry residues, soils contaminated with creosote, hydrocarbons, pesticides, paint residues.

Author Silvia Fernández Castejón. Chemical Engineer.  Complutense University of Madrid.

Bibliography: 

http://www.conectapyme.com/files/medio/guia_suelos_contaminados.pdf

http://www.miliarium.com/Prontuario/TratamientoSuelos/Welcome.asp

http://www.revistaecosistemas.net/index.php/ecosistemas