PNNL-13501 - Pacific Northwest National Laboratory

Most of the permeable reactive barriers have been
constructed using trench-and-fill techniques, such as
digging a trench in the flow path of a contaminant plume
and backfilling the trench with reactant material (Gillham
et al. 1994; Yamane et al. 1995) or auger techniques (Puls
et al. 1995). Although the trench-and-fill approach is
effective, it is limited to sites where the aquifer material is
porous, as opposed to fractured, depths that are shallower
than about 20 m, and aquifers which are relatively thin
(because excavations must be dewatered during
construction). Although many sites have these
characteristics, many important sites do not. Because of
these factors and the fact that trench construction tends to
be the single greatest expense for this remediation
technique, innovative methods for installation of reactive
barriers in the subsurface are needed, especially in
geologic settings where current barrier installation
technologies are of limited practical value (fractured
media and/or at depths of greater than 20 m).
Previous work at our Laboratory demonstrated an
innovative approach for emplacement of an in situ
treatment zone composed of zero-valent iron (Kaplan et
al. 1994; Kaplan et al. 1996; Cantrell and Kaplan 1997).
In this approach, colloidal size (1 to 3 micron diameter)
zero-valent iron particles are injected as a suspension into
the subsurface. As the suspension of particles moves
through the aquifer material, the particles are filtered out
on the surfaces of the aquifer matrix. As a result of the
high density of the zero-valent iron particles (7.6 g/cm 3 ),
it appears that the primary removal mechanism of zerovalent
iron colloids in aqueous solution passing through
sand columns is gravitational settling (Kaplan et al. 1996;
Cantrell and Kaplan 1997).
Later work at the Laboratory using shear-thinning or
pseudoplastic viscosity amendments media was shown to
greatly improve the emplacement of the colloidal zerovalent
iron suspensions in porous media (Cantrell et al.
1997a,b). In contrast to a Newtonian fluid, whose
viscosity is independent of shear rate, certain non-
Newtonian fluids are shear-thinning, a phenomena in
which the viscosity of the fluid decreases with increasing
shear rate (Chhabra 1993). By increasing the viscosity of
the colloidal zero-valent iron suspension through the
addition of a shear-thinning polymer, the rate of
gravitational settling of the zero-valent iron particles will
decrease, however, an increase in viscosity of the
injection suspension will also have the adverse effect of
decreasing the effective hydraulic conductivity of the
porous media. The use of shear-thinning fluids in the
formulation of a zero-valent iron colloid suspension will
result in a high viscosity near the suspended zero-valent
iron particles (due to low shear stress of the fluid near the
particles) and a lower viscosity near the porous media
surface, where the fluid is experiencing a high shear
stress. These properties allow the development of a zerovalent
iron colloid suspension solution which is viscous
enough to keep the zero-valent iron particles in
suspension for extended time periods, but will not cause
an adverse decrease in hydraulic conductivity.
The results of this work have demonstrated that shearthinning
fluids greatly improves the emplacement profile
of suspensions of micron-size zero-valent iron particles in
porous media relative to suspensions without shearthinning
fluids. These fluids will also permit the use of
much lower flow rates than would be possible without
them. Lower flow rates are desirable because they will
increase the distance from the well that the slurry can be
effectively injected, decreasing the number of injection
wells required to emplace the barrier, thereby decreasing
the installation cost of the barrier. This will greatly
increase the range of subsurface environments where this
emplacement technology can be used.
Following the experiments conducted with pure quartz
sand, further column injection experiments were
conducted with natural Hanford sediments. Problems
were encountered during the experiments using natural
material. It was found that the shear-thinning polymer
was adsorbing to the sediments and/or precipitating
within the sediment creating large back pressures during
the injection. The subject of this work is to identify and
test a suitable polymeric shear-thinning material that does
not interact significantly with aquifer materials and major
ions in solution.
Results and Accomplishments
A literature search of commercially available shearthinning
polymers, resulted in the identification of several
potential candidate materials that could be used for further
testing. One particlularly promising compound (referred
to here as AMX) was subjected to batch and column
adsorption tests. The results indicated that this compound
does not adsorb to the natural minerals in Hanford
sediments to any significant extent. Essentially no
adsorption in the batch experiments was observed. A
slight loss of the compound may have occurred in the
column adsorption experiment, but this was attributed to
loss in dead-end pores as opposed to true chemical
adsorption.
Viscosity measurements were conducted with various
concentrations of the compound to determine the best
concentration to make up slurries for injection of the
micron-sized iron particles. We determined that an AMX
Earth System Science 255