Jaglal 1991). Desorption consists of an initial rap-
product and thus the waste must also be consid-
id phase followed by a continual slower phase.
ered potentially hazardous. Chemical neutraliza-
As an example, Di Cesare and Smith (1994) re-
tion methods can be more effective in reducing
ported that Pavlostathis and Mathavan (1992)
contamination and are discussed briefly.
found approximately 80 to 90% of the TCE sorbed
by a silty clay soil was desorbed in the first day.
Aqueous cleaning methods
They also found that the longer the soil was ex-
posed to TCE (2.5, 5.5, or 15.5 months), the more
Detergent cleaning
TCE appeared to remain bound (i.e., the less des-
There are numerous approaches to aqueous
orption occurred).
cleaning. Typically, small items can be cleaned by
As with the permeable surfaces, there has been
soaking in the cleaning solution (in a bath); clean-
very little study of desorption of organic contam-
ing by hand using a cloth, sponge, brush, or simi-
inants from impermeable surfaces. Parker et al.
lar device; ultrasonic cleaning in a bath; or ma-
(1990) found no desorption of any organic ana-
chine washing. Larger items are typically cleaned
lytes from stainless steel that had been exposed to
in place by circulating the cleaning solution and
ppm levels of a suite of organic solutes (VOCs,
rinse solutions.
nitroaromatics, and a nitramine) for six weeks.
Water acts as a solvent medium for contami-
These results were expected since there had been
nants, depending upon their solubility. Water can
no sorption of these analytes by the stainless steel
also act as a dispersal medium for substances that
surfaces during the study. More hydrophobic
do not dissolve but can be carried in suspension.
contaminants would most likely be adsorbed by
An aqueous cleaning solution must first wet the
these surfaces and could then desorb.
contaminated surface. Wetting means that the li-
quid solution readily spreads over the entire sur-
face. If wetting does not take place, the liquid
beads up and will run off any vertical or inclined
DECONTAMINATION METHODS
surface. When a cleaning solution wets a contam-
A surface can be decontaminated either by
inated surface, the contaminant is separated from
physically removing the contaminants or by chem-
the solid surface by the cleaning film. The wet-
ically neutralizing them. Methods that physically
ting ability of liquids varies: water has less wet-
remove contaminants include several aqueous
ting power than ammonia, benzene, acetone, and
cleaning methods, such as detergent washes and
most oils (Sandia Laboratories 1969). However,
high-pressure cleaning methods. High-pressure
the wetting ability of water can be increased by
cleaning methods involve either using a high-pres-
reducing its surface and interfacial tension; this is
sure washer or steam cleaning. For organics with
done by adding a surface active agent, or surfac-
poor solubility in water, cleaning or rinsing with
tant, such as soap or detergent.
less polar organic solvents is a viable alternative.
For aqueous systems, surfactants are organic
There are a number of other methods that physi-
compounds that have a two-component molecu-
cally remove contaminants (e.g., using abrasives,
lar structure; one component is hydrophilic and
water-soluble and the other is hydrophobic and
heating, etc.). However, many of these methods
relatively water insoluble (Di Cesare and Smith
are destructive and do not appear practical or nec-
1994). Surfactants are commonly categorized by
essary for most routine field cleaning and thus
the ionic character of the hydrophilic component
are not included in this discussion. Most currently
or long chain. The long chain may be negatively
used methods for decontaminating groundwater
charged (anionic), positively charged (cationic),
sampling devices utilize some combination of
or neutral (nonionic). Amphoteric or ampholytic
aqueous and nonaqueous cleaning methods. Al-
surfactants are also available; they also ionize in
though methods that physically remove contami-
solution but the long chain can carry either a pos-
nants from surfaces can be effective for decon-
itive or negative charge, depending upon the pH
taminating impermeable surfaces, permeable
of the solution (Osipow 1962).
materials can still contain considerable quantities
A detergent solution removes contaminants
of contaminants within the internal structure, and
from a surface in the following manner:
this can reduce the efficacy of these methods. An-
1) It wets the contaminated surfaces,
other problem with these methods is that the con-
2) It removes the contamination by either roll-
taminants are often still contained in the waste
ing it up and displacing it from the surface,
3
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