Methods of Soil Environmental Remediation - Environmental Geomechanics - page 425

1 5.5.1.3 Neutralization
This process consists of chemically neutralizing the physicochemical
characteristics of the pollutant. Thus, pollution with a basic product can be
neutralized by using the appropriate amount of acid. Taking this principle as a
starting point, various processes have developed.
One of these processes, called “ion exchange”, is based on neutralization by ion
substitution. It consists of eliminating the ionic pollutants contained in solution in
the ground or water from the nappe by adsorption on a solid material (ion
exchanger). The ionic pollutants are thus replaced by an equivalent quantity of non-
polluting ions. This technique is mainly used to remove metal pollutants.
There is also a process of neutralization by active carbon. This process uses
active carbon to adsorb the pollutants dissolved in the ground. This process is also
used for gas cleaning, and therefore supplements the techniques of vacuum
extraction or stripping. This technique makes it possible to treat organic pollutants
with a high molecular weight and ebullition point and a low solubility and polarity.
15.5.2. Extraction technologies
15.5.2.1. Vacuum extraction
The extraction of volatile compounds from the ground, also known as vacuum
extraction or soil aeration, is an effective pollution control technique for treating
unsaturated soil contaminated with volatile or semi-volatile organic compounds. The
basic principle of this technique rests on putting the contaminated soil under
pressure, followed by suction and treatment of the polluted vapors using a vacuum
pump. The extraction systems of the volatile compounds typically use a series of
extraction shafts installed in the unsaturated zone to collect the organic vapors from
the soil containing the volatile pollutants (Figure 15.6).
The air flow is introduced into the soil by a pressure gradient applied across
these vertical shafts. Pressure inside the extractions shafts moves air from the soil to
the extraction shafts. The extracted vapors are generally treated at the surface before
being released into the atmosphere or reinjected into the soil. It should be noted that
the working criteria are governed by the chemical composition of the contaminants,
the vapor flows through the unsaturated zone, the geological nature of the soil, the
thickness of the contaminated soil and the depth of the movement of the nappe.