42    SOIL REMEDIATION USING SUPERCRITICAL FLUID
EXTRACTION
Lori E. Seeker, Graduate Student
Richard G. Zytner, Assistant Professor
Warren H. Stiver, Assistant Professor
School of Engineering, University of Guelph
Guelph, Ontario, Canada
INTRODUCTION
Contaminated land is "land that contains substances that, when present in sufficient quantities or
concentrations, are likely to cause harm, directly or indirectly, to man, to the environment, or on
occasions to other targets."1 This contamination can result from waste disposal at designated landfills
or uncontrolled dumps, from accidental spills and leaks, and from atmospheric deposition. The
potential hazards associated with soil contamination include migration through the soil to streams,
rivers, and groundwater, volatilization to air, affecting plant life and ultimately entering the food
chain. Consequently, soils contaminated with persistent organic chemicals must be remediated to
prevent potentially adverse health effects and make land suitable for future use.
Many techniques are available to remediate or reclaim contaminated soils. The method selection
process will be dependent on many factors, including, the type and extent of contamination, the
potential threat to identified targets, and the intended use of the land. Detailed reviews of available
treatment options have been provided by Smith,' Cairney,2 Kostecki and Calabrese,3 Arendt,4 and
Rees.5 However, excavation followed by landfilling still remains the most commonly applied solution6
due to its quickness in implementation and its relatively low cost.
A new remediation technique with potential is supercritical fluid extraction (SFE). SFE is viewed as
an on-site, ex-situ process in which the soil is returned to the site following treatment. The process
involves the excavation of the contaminated soil, placing it in a reactor in which supercritical carbon
dioxide (SC-C02) can be introduced. The SC-C02, which is a powerful solvent, extracts the contaminants, leaving behind a clean biologically active soil7 that requires no further treatment. SFE appears
to be well suited for all soil types, especially for sites involving high molecular weight, low volatility,
low reactivity organic chemicals. These chemicals have relatively high solubility and excellent mass
transfer characteristics in SCFs.
Application of SFE is hindered by the lack of thermodynamic partition coefficients and mass
transfer coefficients that ultimately govern the timing and cost of application for a given site. It is
probable that these coefficients are dependent on the system operating conditions, soil type, and
contaminants present. Therefore, these parameters must be determined for a wide range of circumstances in order to design optimal soil remediation equipment and assess the feasibility of SFE for a
given situation.
Carbon dioxide is the most frequently used supercritical fluid because of its low critical pressure
and temperature, 7.4 MPa and 31°C, respectively.8 Additionally, carbon dioxide is nonflammable,
nontoxic at low use levels, noncorrosive to stainless steel and plastic,9 inexpensive, well-known, safe
to use, and readily available in high purity. The required high pressure is an operational problem, but
the full-scale commercial application of SFE in coffee decaffeination has shown that this can be
overcome.
In this study, the thermodynamic partition coefficient between different soils and supercritical
carbon dioxide for various immiscible chemicals were experimentally determined. The procedure that
was initially developed by Gray et al.10 was modified such that equilibrium was attained between soils
having an "oil phase" and supercritical carbon dioxide. The quantity of solute extracted by the
supercritical carbon dioxide, as well as the quantity remaining on the soil, was measured and used to
calculate a partition coefficient for each chemical. This paper presents and discusses these laboratory
results for both dry and wet soils.
49th Purdue Industrial Waste Conference Proceedings, 1994 Lewis Publishers, Chelsea, Michigan 48118. Printed in
U.S.A.
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