18    SOIL REMEDIATION USING
SUPERCRITICAL CARBON DIOXIDE
Douglas J. Gray, Graduate Student
Richard G. Zytner, Assistant Professor
Warren H. Stiver, Assistant Professor
School of Engineering, University of Guelph
Guelph, Ontario, Canada
INTRODUCTION
Soil that has become contaminated with hazardous organic compounds as a result of human
activity may require remediation. This remediation requirement is driven by a demonstrated health
risk to either the ecosystem or community. Often, a change in land usage is the actual trigger
mechanism for the identification of sites requiring cleanup.
In the search for cost effective and efficient methods of soil remediation, one new technology that is
gaining some attention is the use of supercritical fluids (SCF). Supercritical fluids offer an exciting
range of possibilities for chemical separations and "should lead to some innovative applications,
particularly in the food, polymer and pharmaceutical industries, and in environmental protection."'
New commercial application processes include: coffee decaffeination, hops extractions, catalyst
regeneration, extraction of organic wastes from water and supercritical fluid chromatography.1 For
environmental applications, there are currently a number of commercial instruments available for
analytical extractions of a wide range of sample matrices using supercritical fluids. CF Systems
Corporation has developed a system for large scale site remediation that uses supercritical propane
and butane to extract organic contaminants from soils and sediments.2
A limitation of many innovative technologies for soil remediation is the inability to reasonably
estimate the cost and timing involved in their application at a particular site. For Supercritical Fluid
Extraction (SFE), the mass transfer coefficients and the thermodynamic partition coefficients are
required to complete this estimation and these two parameters are currently largely unavailable. It is
likely that these parameters are a function of operating conditions, soil type and the contaminant
involved. Research must determine these values for a wide range of situations to assist in the design
and optimization of large-scale soil remediation systems.
In this research, the thermodynamic partition coefficient between soil and supercritical carbon
dioxide for various chemicals was experimentally determined in the laboratory. Carbon dioxide was
chosen as the fluid because it is nontoxic, has favourable kinetic and thermodynamic properties, is
readily available, leaves no harmful soil residue and is relatively safe to handle.
An apparatus was constructed such that equilibrium was attained between chemically spiked soil
and supercritical carbon dioxide. The quantity of chemical solute extracted by the supercritical carbon
dioxide was then measured and used to calculate a partition coefficient for each chemical. These
results along with the experimental protocol and safety considerations are discussed in this paper.
BACKGROUND
For a given site, the most appropriate remediation technology is dependent on contaminant and soil
properties. Some of the common technologies currently in use, include:
• excavation and landfill disposal
• excavation and thermal treatment
• in situ soil washing
• soil vapour extraction
• bio-remediation
48th Purdue Industrial Waste Conference Proceedings, 1993 Lewis Publishers, Chelsea, Michigan 48118. Printed in
U.S.A.
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