Handbook of Solvents - George Wypych - ChemTech - Ventech!

14.21.2 Predicting cosolvency 1001
or saturated zone upgradient of the contaminated area. The solvent with the removed contaminants
is then extracted downgradient and treated above ground. Precise formulations
for the water/cosolvent mixture need to be determined by laboratory and pilot studies in order
to achieve the desired removal. 50-53
A few field-scale evaluations of this technique were carried out at Hill Air Force Base,
Utah, where the aquifer had been severely contaminated by jet fuel, chlorinated solvents,
and pesticides during 1940s and 1950s. These contaminants had formed a complex
non-aqueous phase liquid (NAPL) containing more than 200 constituents, which covered
the surfaces of soil particles and was trapped in pores and capillaries over the years. One of
the evaluations consisted of pumping ternary cosolvent mixture (70% ethanol, 12% n-pentanol,
and 18% water) through a hydraulically isolated test cell over a period of 10 days, followed
by flushing with water for another 20 days. 54,55 The removal efficiency varied from
90-99% at the top zone to 70-80% at the bottom near a confining clay layer. Similar removal
efficiencies were obtained from another test cell using a combination of cosolvent n-pentanol
and a surfactant at a total of 5.5 wt % of the flushing solution. 56 In order to remove gasoline
residuals at a US Coast Guard base in Traverse City, Michigan, it was demonstrated
that the contaminants were mobilized when cosolvent 2-propanol was used at 50% concentration,
while methanol at either 20% or 50% showed little effect. 57 Cosolvent flushing was
also proven to be effective in treating NAPLs which were denser than water. Methanol,
isopropanol, and t-butanol were used in treating soils contaminated with tri- and tetra-chlorinated
ethylenes. 58
The applicability of solvent flushing, however, is often limited by the characteristics
of the soil, especially the particle size distribution. While sandy soils may result in uncontrolled
fluid migration, clayey soils with particles size less than 60 μm are often considered
unsuitable for in situ solvent flushing due to low soil permeability. In an attempt to remove
PAHs from poorly permeable soils, Li, et al. 59 investigated the possibility of combining
cosolvent flushing with the electrokinetic technique. Electrokinetic remediation involves
application of a low direct electrical current to electrodes that are inserted into the ground.
As water is continuously replenished at anodes, dissolved contaminants are flushed toward
the cathode due to electroosmosis, where they can be extracted and further treated by various
conventional wastewater treatment methods. Their column experiment of removing
phenanthrene from soil was moderately successful with the assistance of cosolvent
n-butylamine at 20%(v). Retardation factor (ratio of the water linear velocity to that of the
chemical) of phenanthrene was reduced from 753 in pure water to 11 by the presence of
n-butylamine, and 43% of the phenanthrene was removed after 127 days or 9 pore volumes.
However, significant removal of phenanthrene was not attained in their experiments with
acetone and hydrofuran as cosolvents.
14.21.2.4 Experimental observations
Numerous experimental data exist in the literature on the solubility of organic solutes, including
both drugs and environmental pollutants, in various mixtures of water and
cosolvents. Experimental observations are often illustrated by plotting the logarithm of solubility
of the solute versus the volume fraction of cosolvent in the solvent mixture. A few
examples of solubilization curves are shown in Figure 14.21.2.1, which shows three typical
situations for solutes of different hydrophobicity in the mixture of water and ethanol.
The classification of solute/cosolvent/water systems based on their relative polarity
was suggested by Yalkowsky and Roseman. 61 Solutes which are less polar than both water