Remediation Technologies - Environmental Investigation and Remediation - page 329

organic contaminants, including benzene, toluene, 1,4-dioxane, acetonitrile, acetone, and methanol.
Several types of commercially available GAC were also tested for comparison. Of the chemicals
evaluated, 1,4-dioxane had the lowest overall adsorption rates, as well as the most variability in sorp-
tion rates during the tests. The only GAC capable of effective removal of 1,4-dioxane was made from
hard nutshells, which achieved a little more than 50% removal. Johns et al. (1998) concluded that the
low removal rates could have been due to competition for sorption sites with other adsorbates, steric
hindrance, reduced intraparticle diffusion, or lower GAC binding afi nity for 1,4-dioxane.
GAC has been reported to effectively remove 1,4-dioxane from extracted groundwater at the
Stanford Linear Accelerator Center (SLAC) in California (USEPA, 2006; Sabba and Witebsky,
2003). On the basis of the evaluation of several different kinds of GAC by Johns et al. (1998), it is
unlikely that the reductions noted at SLAC are due solely to the sorption to the GAC. The GAC unit
at the SLAC site has moderate inl uent concentrations of 1,4-dioxane (average 725 g/L) and very

low l ow rates (~0.5 gpm). The observed reduction in 1,4-dioxane concentration (

μ

g/L in
efl uent) may result from cometabolic biodegradation, in which a cocontaminant, in this case tetra-
hydrofuran (THF), supports biological activity while a by-product of the biological activity (i.e., an
enzyme) effectively destroys the target contaminant. Cometabolic degradation of 1,4-dioxane in
the presence of THF is well documented in the literature (Zenker et al., 2000; Parales et al., 1994)
and is discussed further in Section 7.6 . Further discussion of the GAC unit at SLAC is provided in
Section 8.5.4.

< 10 μ

7.2.3 O RGANOCLAYS
Organoclays have been used as adsorbents at wood-treating sites for polynuclear aromatic hydro-
carbon and creosote removal. They are typically bentonite clays (primarily montmorillonite),
chemically modii ed to make them hydrophobic (water repelling) and organophilic (oil attracting).
One application uses quaternary amines to produce an ion exchange capacity of ~70-90
milliequivalents/g (Alther, 2006). Amine chains bind to the clay particles and attach to organic
compounds in the inl uent water, binding contaminants to the organoclay material. Organoclays
are sometimes mixed with GAC, or a secondary carbon unit may be used on the efl uent to achieve
higher removal efi ciencies. Alther (2006) performed a number of laboratory tests to evaluate the
removal efi ciency of organoclays on a variety of contaminants, including 1,4-dioxane. Laboratory
batch tests were run by combining water containing 1,4-dioxane with nonionic organoclay in glass
vials and by shaking for one day. Aqueous concentration of 1,4-dioxane was reduced by only 2.8%
from levels as high as 958,000

μ g/L, which are higher than those that are typically found on con-

taminated sites.
A proprietary modii cation technology used by Aqua Technologies of Wyoming, Inc. (A.B.
Brown, personal communication, July 2 and 5, 2007), resulted in an organoclay product (ET-1)
capable of removing up to 83% of 1,4-dioxane, from 726,500 to 126,000

g/L. Test waters were
provided by a plastics manufacturer in South Carolina from a contaminated site. The results were
verii ed by an independent laboratory using Environmental Protection Agency Method 8260 with
selected-ion monitoring (SIM). Although this removal rate is impressive, the very high concentra-
tions in the extracted groundwater are not typical of most hazardous waste sites contaminated with
1,4-dioxane, and the treatment end points are well above most 1,4-dioxane standards.

μ

7.2.4 P ALLADIUM -111
Azad et al. (2000) evaluated the sorption characteristics of 1,4-dioxane on palladium-111 ( 111 Pd).
Their research indicated that sorption occurred at very low temperatures (-193°C) in a multilayer
that desorbs to leave an overlayer at a somewhat higher temperature (-113°C). The 1,4-dioxane
overlayer desorbs and thermally decomposes at ~27°C. The temperatures involved in this process
are not likely to be achieved cost-effectively in environmental remediation projects.