PNNL-13501 - Pacific Northwest National Laboratory

strategy option at DOE sites (Brady et al. 1999). The first
comprehensive assessment of natural attenuation for
groundwater cleanup was recently reported (Macdonald
2000). A key assumption made in measuring the potential
success of natural attenuation is the likelihood that release
of hydrophobic organic contaminants from resistant
fractions in the subsurface environment occur on time
scales commensurate with natural attenuation process
rates (such as microbial degradation). Growing
acceptance of the natural attenuation option by regulators
and the public as a viable cleanup option will depend on
being able to demonstrate such assumptions are correct
suggesting that there would be acceptable risk in its
application.
One method for providing support for acceptable risk
would be to quantify the magnitude of the resistant
fractions for different hydrophobic organic contaminant
classes for a range of soil and sediment types found across
the DOE complex. Unfortunately, laboratory study of
hydrophobic organic contaminant resistant fractions has
been confounded by the inability to accurately evaluate
the performance of traditional analytical methodologies to
quantify resistant fractions in naturally aged soils and
sediments. Traditional methods for artificially aging soils
and sediments have proven inadequate because contact
times require years for adsorption and diffusion processes
to achieve the extent of contaminant penetration found in
naturally aged soils and sediments. These problems
might be overcome by use of supercritical carbon dioxide
for loading soils and sediments with hydrophobic organic
contaminants. The solubility of hydrophobic organic
contaminants in supercritical carbon dioxide is
controllable through the adjustment of temperature and
pressure. The favorable properties of supercritical carbon
dioxide (i.e., high diffusivity and low viscosity) have the
potential for allowing rapid incorporation of hydrophobic
organic contaminants into the natural organic component
of soils and sediments.
Results and Accomplishments
Phenanthrene was loaded onto nonporous glass beads and
a Hanford formation soil using the closed loop
supercritical carbon dioxide system. Both materials had
similar surface areas (2 versus 9 m 2 /g), and the Hanford
soil had an organic carbon content of 0.03%.
Phenanthrene rapidly established equilibrium with the
nonporous beads with no organic carbon content. In
contrast, the Hanford formation soil showed a continuous
slow uptake of phenanthrene likely attributable to
adsorption/diffusion into the soil organic matter
(Figure 1b versus 1a). Phenanthrene continued to adsorb
(diffuse) into the Hanford soil after 24 hours of exposure
(Figure 1b versus 1c). Adsorption and diffusion into the
organic matter was expected to be more extensive in the
24-hour loaded Hanford soil than the 5-hour loaded
Hanford soil. Differences in resistance of phenanthrene
to aqueous soil column leaching was expected to be
observed between 5-hour and 24-hour aqueous desorption
experiments. After approximately 7 hours, desorption
occurred slower for the 24-hour loaded soil than for the
5-hour loaded soil (Figure 2). Differences in the extent of
adsorption/diffusion into Hanford soil organic matter
(5-hour versus 24-hour loading) are likely responsible for
differences in the observed leachate behavior. Evidence
of a greater resistant fraction was demonstrated for the
24-hour loaded soil.
Figure 1. Supercritical fluid loading profiles for nonporous
glass beads and Hanford soil
Figure 2. Aqueous desorption of phenanthrene from 5-hour
and 24-hour loaded Hanford soil
Summary and Conclusions
Phenanthrene artificially loaded onto Hanford Site soil
samples showed release behavior consistent with what
would be expected for hydrophobic organic contaminants
releasing from a naturally aged soil or sediment. The
artificially aged soils demonstrated the presence of a
Earth System Science 231