Case Studies of 1,4-Dioxane Releases, Treatment, and Drinking Water Contamination - Environmental Investigation and Remediation - page 398

1600

Influent
Effluent

1400

1200

1000

800

600

400

200

0

FIGURE 8.8 1,4-Dioxane concentrations in SLAC GAC inl uent and efl uent.

bacteria or actinomycetes capable of 1,4-dioxane metabolism, and the lag vessel acquires the micro-
bial community of the lead vessel over time.
Speculating on the serendipitous cometabolism of 1,4-dioxane under passive conditions by the
fortuitous and accidental culturing of just the right microbes may be viewed as wishful thinking;
however, the discovery of PM-1, a mixed-culture microbial consortium capable of biodegrading
MTBE, was made in a very similar setting. The PM-1 consortium was recovered by the University
of California, Davis graduate students from a compost-based bioi lter used for gas-phase treatment
of vapors at a Water Pollution Control Plant operated by the Los Angeles Sanitation District (Hanson
et al., 1998). The GAC at SLAC has not yet been cultured to identify the possible presence of 1,4-
dioxane-degrading microbes.
The second speculation for a possible mechanism by which the GAC at SLAC may remove
1,4-dioxane is absorption into the GAC pores, as opposed to adsorption onto GAC surfaces. The
very slow rate of l ow, 0.2 gpm, coupled with the 106-cubic-foot capacity in each GAC vessel (634
gallons in both vessels, accounting for a 40% void volume) leads to a slow bed-volume exchange
(0.45 bed volumes per day) and a long contact duration (53 h of transit time through the two GAC
vessels, i.e. 2.2 days). Under these conditions, diffusion of 1,4-dioxane into the pore structures of
activated carbon may play a much greater role than would be the case at higher l ow rates. Although
1,4-dioxane may be easily displaced by methyl ethyl ketone or other compounds from the surface of
the GAC by competitive sorption (see Section 3.3.2.7), it may remain diffused in pore spaces unavail-
able to other compounds, depending on the polar state of the predominant 1,4-dioxane conformation
and the electrostatic charge properties of the GAC. 1,4-Dioxane's polarity in its dominant conforma-
tion may favor its diffusion into pores spaces over nonpolar compounds that may be more likely to
adsorb to the surfaces. The interaction of 1,4-dioxane and GAC pores by diffusion was not studied
to support this speculation, but the opportunity for diffusion to play a more dominant role under very
low l ow conditions is nevertheless a possible explanation for 1,4-dioxane removal by GAC.
The grade of carbon selected for removal of VOCs at SLAC is Westates ® coconut shell-based
GAC—AquaCarb ® 1230C, which is designed to remove poorly adsorbable organics from water such
as MTBE and other trace-level organics. AquaCarb 1230C is an acid-washed pH-neutral GAC with