Environmental Fate and Transport of Solvent-Stabilizer Compounds - Environmental Investigation and Remediation - page 123

not result in degradation in the majority of compounds; instead, absorbed energy is released through
l uorescence, and the compounds return to their beginning energy state (Hemond and Fechner,
1994). Photolysis, also called direct photodegradation, is more likely to occur in compounds that
have double carbon bonds, such as alkenes (e.g., perchloroethylene and trichloroethylene) and aro-
matic rings (e.g., benzene and toluene). Shorter-wavelength light has higher frequency and energy
and is therefore the primary agent in photolysis. Visible light in the wavelength range from 280 to
730 nm (nanometers) is primarily responsible for photolysis of chemicals in the upper atmosphere
(Seinfeld, 1986). Photolysis of some compounds may occur in both the atmosphere and in surface-
water bodies; however, because light in the lower atmosphere has longer wavelengths, many com-
pounds are not directly photolyzed.
The intensity of sunlight passing through the atmosphere is decreased through absorption by
ozone and other atmospheric gases and by molecular and aerosol scattering. Essentially no light is
transmitted to the Earth's surface atmosphere (the troposphere) at wavelengths less than 295 nm,
and there is a sharp decrease in intensity in the 280-320 nm wavelength range due mainly to ozone
absorption. Sunlight in this wavelength interval, often called UV-B radiation, causes sunburn as
well as direct photolysis of many pollutants residing in the upper layers of surface-water bodies
(Zepp and Cline, 1977).

3.1.4.1.1 Photolysis of 1,4-Dioxane
1,4-Dioxane is photolyzed at wavelengths shorter than the wavelengths of light penetrating the tro-
posphere, that is, less than 290 nm; therefore, direct photolysis is inconsequential to the atmospheric
fate of 1,4-dioxane. Nevertheless, photolysis experiments hold interest for developing treatment
technologies involving UV light (see Chapter 7) and for understanding the various means by which
the 1,4-dioxane molecule can be disassembled.
Laboratory experiments on photolysis of 1,4-dioxane produced volatile solid by-products, includ-
ing p -formaldehyde and trioxane. Proposed pathways (Hentz and Parrish, 1971) for direct photolysis
of 1,4-dioxane vapor at 1470 Å (147 nm) include

n
ææÆ+
482

h

CHO CH 2CHO,

(3.10)

24 2

n
ææÆ+

h

CHO CH CHO + H + CO,

(3.11)

482 24 2 2

n
ææÆ+
482

h

C H O H [Unidentified product].

(3.12)

2

1,4-Dioxane absorbs UV light in the wavenumber range * 52200 - 60510 cm −1 (165-191 nm),
producing an absorption curve with a single peak, which may be the superposition of absorption
associated with the two symmetrical oxygen atoms on the ether ring (Pickett et al., 1951). A more
detailed pathway for direct photolysis of 1,4-dioxane exposed to light at 190 nm for 200 h was elic-
ited in later studies, as shown in Figure 3.1 .
The hydrogen abstraction from 1,4-dioxane ([B] in Figure 3.1) is disputed; other researchers i nd
the primary mechanism for 1,4-dioxane photolysis to be scission of the CO bond rather than cleavage
of the CH bond (Houser and Sibbio, 1977). Photolysis experiments on liquid 1,4-dioxane conducted
at 185 nm produced “an exceedingly complex mixture” including six major components and
50 minor components. The products included ethylene glycol, glycolaldehyde, dioxanone, hydroxym-
ethyldioxane, and a variety of ethers, alcohols, aldehydes, and some carbonyl compounds (Houser
and Sibbio, 1977). Vapor-phase photolysis of 1,4-dioxane at 147 nm produced formaldehyde and
ethylene (Hentz and Parrish, 1971).

* Wavenumber (in cm -1 ) is the inverse of wavelength.