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