46    UPTAKE AND RELEASE OF HAZARDOUS CHEMICALS BY
SOIL MATERIALS
Ju-Chang Huang, Professor
Brian A. Dempsey, Assistant Professor
Shoou-Yuh Chang, Assistant Professor
Hossein Ganjidoost, Graduate Research Assistant
Department of Civil Engineering
University of Missouri-Rolla
Rolla, Missouri 65401
INTRODUCTION
In many places of the nation, ground water has been contaminated with various concentrations of
chloroorganic solvents, such as 1,1,1-trichloroethane (TCA), trichloroethylene (TCE), and tetrachlo-
roethylene or perchloroethylene (PCE). Most of the contaminations have originated from the previous dump of these chemicals into unsealed ponds or landfill sites [1]. Since these organic solvents are
highly volatile, a good portion of these chemicals has escaped into the air upon their disposal.
However, once covered by soil and refuse, or having seeped into the subsurface environment, little
volatilization can occur. As the chloroorganic solvents are seeping into and through the ground, they
can be adsorbed by minerals and particulate organic materials. This adsorption process tends to
increase the retention time of volatile organic compounds (VOCs) in the soil. They travel more slowly
than inert and unreactive salts. Nevertheless, plumes of these solvents have been detected quite far
away from the initial site of contamination, indicating only a moderate ability to be adsorbed by soils
and minerals [2,3].
In many areas heavily polluted with VOCs, plans have been made or are being considered to pump
ground water to the surface and then subject it to air stripping to remove the highly volatile chloroorganic solvents [4], Although this kind of remedial action appears to be appropriate in removing
volatile chloroorganics, it is difficult to estimate the length of pumpage required to achieve adequate
cleanup of the subsurface water. This is because one of the major factors governing the future
transport and cleanup of these chloroorganic solvents in the subsurface environment is the sorption
and desorption reactions of these organics in the saturated and unsaturated soils. For a typical
polluted area, a fairly accurate account of the total amount of organic solvents disposed on a given
site may be known. However, since a large portion of the disposed VOCs may have been lost into the
atmosphere through volatilization, the exact amount of these chemicals entering the ground, their
distribution along the soil depth (in the unsaturated zone) as well as in the polluted plume (beneath the
ground water table) are normally difficult to define, particularly when the acquired field data show a
lack of consistency. This has commonly happened because the volatility of these organic solvents has
often introduced a significant degree of experimental errors in the chemical analysis unless proper
extraction and analytical protocols are followed. In general, the chlorinated solvents may be present
in the pure form (perhaps in droplets held in the soil matrix by capillary forces), adsorbed on the
sediment phase, or in the aqueous phase. The chemical distributions between the latter two phases are
described by adsorption isotherms. By establishing accurate adsorption and desorption isotherms for
a specific adsorbate-adsorbeni combination, the following advantages may be derived:
1. The adsorption isotherm will allow an engineer to make an estimate of the total quantity
of each organic solvent that has been retained on the saturated sediment phase based on
its aqueous concentration found in ground water.
2. The adsorption isotherm will also allow him to check the validity of the field data which
have been collected from the polluted plume in any contaminated site. This may be done
by comparing the chloroorganic concentrations in both the sediment and the aqueous
phases and then determining whether these data coincide with the established isotherm
relationships.
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