Environmental Engineering Reference
In-Depth Information
signii cant role in removal of volatile organic compounds from the atmosphere in coastal regions
where marine boundary layers occur (Li and Pirasteh, 2006). The reaction of atomic chlorine and
1,4-dioxane follows the general form
Cl
+
C
4
H
8
O
2
→
HCl
+
C
4
H
7
O
2
.
(3.17)
The product of the above reaction, C
4
H
7
O
2
, is a dioxane alkyl peroxy radical, which degrades to
EDF, a toxic by-product with an atmospheric residency of 24 days (Maurer et al., 1999).
3.1.4.3 Photo-Oxidation in Water
Hydroxyl radicals are present at concentrations on the order of 10
−17
mol/L in illuminated surface
waters (Hemond and Fechner, 1994). Hydroxyl radicals form when an organic chromophore
absorbs light and reacts with water to form hydrogen peroxide (H
2
O
2
). The formation and accumu-
lation of H
2
O
2
in natural waters are correlated with the concentration of naturally occurring humic
substances (Cooper et al., 1988). Hydrogen peroxide results from a reaction with superoxide, O
−
,
which forms when oxygen is reduced by free electrons generated by light-absorbing substances
through photoionization or energy transfer. Superoxide can also form by electron transfer from
reduced metals such as titanium dioxide (Cooper et al., 1988). As a source of hydroxyl radicals,
hydrogen peroxide plays a signii cant role in photo-oxidation of contaminants in natural surface
waters, and it may react directly with pollutants. The measured hourly photochemical accumula-
tion rate of H
2
O
2
in surface water exposed to midday sunlight ranges from 2.7 × 10
−7
to
48 × 10
−7
mol/L in waters with dissolved organic carbon ranging from 0.53 to 18 mg/L, respec-
tively. The measurements were made at 24.3° northern latitude, where the midday sunlight inten-
sity is 0.4 W/m
2
in the wavelength range 295-385 nm (Cooper et al., 1988). H
2
O
2
will break into
two hydroxyl radicals (OH
•
) if it absorbs a sufi ciently energetic photon.
The aqueous photo-oxidation half-life of 1,4-dioxane in water has been estimated to be as low as
67 days and as high as 9.1 years (Dorfman and Adams, 1973; Howard et al., 1991). Estimated photo-
oxidation half-lives for additional stabilizer compounds and chlorinated solvents are tabulated in
Table 3.6
.
3.2 SURFACE-WATER FATE AND TRANSPORT PROCESSES
Solvent wastes discharged to surface water enter a dynamic environment dominated by moving
water, biological processes, sedimentation, sunlight, and a variety of chemical reactions. Although
discharge of solvent wastes to surface water has been effectively eliminated with the adoption and
successful implementation and enforcement of regulations to protect streams, rivers, and lakes, the
discharge of 1,4-dioxane to surface water continues today. Untreated 1,4-dioxane may enter sur-
face-water bodies as the efl uent of groundwater treatment systems designed to remove chlori-
nated solvents such as air strippers and granular activated carbon treatment vessels, which are not
effective at removing 1,4-dioxane. Other stabilizer compounds that are not effectively removed
by conventional pump-and-treat technologies employed at solvent release sites may also be dis-
charged to surface-water bodies. Therefore, the surface-water fate and transport processes for
solvent-stabilizer compounds may warrant evaluation for managing the remediation of chlori-
nated solvent sites.
The transport processes governing contaminants in surface water are advection, diffusion, and sorp-
tion to suspended sediment. Diffusion, wind-induced currents, and thermal stratii cation are important
in lakes, whereas advective l ow governed by gravity and water velocity is important in streams and
rivers. Advection of low to moderate concentrations of dissolved organic compounds generally pro-
ceeds independently of chemical properties; however, diffusion is limited by compound-specii c prop-
erties. Surface-water transport processes are well studied and will not be described further; a concise
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