Environmental Engineering Reference
In-Depth Information
physically contain subterranean water or the emanating lixiviat of a polluted site.
The concept of a reactive barrier is based on the fact that the aureole of pollution is
stopped by a permeable or active wall containing materials (oxidoreductive
reactants) chosen in order to make the contaminants harmless before they pass to the
other side of the barrier or wall.
It is certain that the design of these types of barriers requires a comprehension of
the mechanisms of attenuation. [THO 93] identified the ionic exchanges (sorption),
precipitation, dilution and neutralization as being the principal mechanisms of
attenuation for several inorganic chemical solutions. However, with regard to heavy
metals, the two principal mechanisms of attenuation are the ionic exchange or
adsorption, and precipitation.
The ionic exchange or adsorption can be increased in a passive isolation device
by employing the additive materials that will increase the cation capacity of
exchange (CCE) or the total capacity of adsorption of the barrier. For example, the
use of organically modified clay having a CCE of 65 meq/100g or zeolites with a
CCE of 250 meq/100 g as additive material was described by [ALL 96, EVA 90,
RIC 99] among others. The precipitation potential can be improved by adding
materials that will increase the pH of the permeant liquid. For example, the addition
of quicklime, calcic hydrated lime (Ca(OH) 2 ) or of dolomitic quicklime can help to
increase the pH of an acid solution during its migration through the barrier in order
to:
− neutralize the pH of the solution;
− precipitate the metals contained in the solution;
− decrease the permeability and improve the total performance of the barrier.
In all the cases, special attention must be given to the initial permeability of the
treatment zone and to the chemical or other processes that can lead to its change in
the short- or long-term. (For example, an increase in permeability can be prejudicial
if the water level in the reactive zone is sensitive to permeability).
5.3.2.1 . Case study: decontamination of manure on an old factory site in Italy
[GRA 99] presented the case of a factory site with manure located in Italy which
was acquired, after its outage, by the local municipality. Evaluation of the quality of
the grounds and subterranean water showed contamination with heavy metals, such
as arsenic, copper and lead. A program of decontamination was developed
combining the excavation of the grounds and landfill as well as the construction of
reactive barriers to reduce the concentration of heavy metals in the legal
subterranean water levels.
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