Air Stripping
| Air
stripping is a process where volatile organic compounds (VOCs) transfer
from water to air by bubbling a stream of air through the water. The
volatile compounds vaporize into the bubbles. |
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Volatilization
into the bubbles operates on the same principle that vaporizes water
into air. The greater the surface of the water exposed to air, the
greater the rate of evaporation in total mass per time. Introducing air
bubbles to the water increases the surface area of the contaminated
water exposed to air many-fold.
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In the case of contaminated groundwater, the stripping takes place inside the aquifer. Aquifer is a word coined from Latin aqua (water) and ferre
(to bear). An aquifer is a formation of earth materials (rock, sand,
clay, silt, and gravel) that bears water. The water resides in and
moves through the pores. The aquifer part of the formation is also
called the ”saturated zone,” because the pores are saturated
(completely filled) with water.
Stripping is a process that transfers contaminants from one medium to another. In the case of air stripping
as it applies to cleanup of groundwater, the air moves through the
water as bubbles and strips the contaminants. The contaminants enter
the air bubbles and get carried upward, collected and transferred to an
aboveground unit for separation or destruction.
Air
stripping is used to separate VOCs from water. Some compounds that have
been successfully separated from water using air stripping include
gasoline, benzene, toluene, ethylbenzene, xylenes, chloroethane, TCE,
DCE, and PCE.
To understand how air stripping works, click on Henry’s Law
for a brief illustration. Henry’s Law measures the extent to which a
chemical will separate from water and enter into the air. It
establishes characteristic numbers (constants) for substances. The
higher the constant, the more likely is for the respective substance to
separate from water. Air stripping is effective only for substances
having Henry’s Law constants greater than 0.01. Here are some
comparative values:
carbon monoxide
oxygen
carbon dioxide
tetrachloroethene (PCE)
trichloroethene (TCE)
hydrogen sulfide
benzene
methyl tert-butyl ether (MTBE)
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0.001
0.001
0.034
0.06
0.10
0.10
0.18
1.70
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naphthalene
PCBs
hydrogen chloride
ammonia
methanol
ethanol
hydrogen peroxide
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2.1
3.0
20
61
220
220
71000
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R. Sander: Henry’s law constants ( http://www.mpch-mainz.mpg.de/~sander/res/henry.html)
For example, the table shows that MTBE will strip nearly 10 times faster than benzene and 17 times faster than TCE.
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