35    DESTRUCTION OF VAPOR PHASE HALOGENATED
METHANES BY MEANS OF ULTRAVIOLET
PHOTOLYSIS
Gregory A. Loraine, Research Assistant
William H. Glaze, Chair
Dept. of Environmental Science & Engineering
University of North Carolina-Chapel Hill, North Carolina 27599
INTRODUCTION
At 68% of the hazardous waste sites with contaminated groundwater in the US, the pump and treat
method of groundwater remediation is used.' The most common method of removing volatile organics from water is air stripping. While air stripping transfers the volatiles from the water to the air, it
does not decompose them. Granular activated carbon (GAC) will remove the organics from the air
but this also is a phase transfer not a treatment.
Vacuum Ultraviolet (VUV, 200 nm) photolysis may provide a treatment option for certain organic
compounds. Carbon tetrachloride, chloroform, trichlorotrifluoroethane and other halogenated
organics are major groundwater contaminants.2 These compounds also absorb strongly in the VUV
range. Air streams from stripping of water contaminated with these compounds can be treated with
UV to degrade these compounds.3"6 From the study of halocarbon photodegradation in the stratosphere, absorption coefficients and quantum yields for photolytic processes for these compounds are
known. Because of their ubiquity in the environment and the strong data base available for these
compounds, the photolysis of carbon tetrachloride (CCL4), chloroform (CHCI,), and 1,1,2-
trichlorotrifluoroethane (C12FC2F2C1) was investigated.
The most commonly used sources for UV light are low-pressure mercury arc lamps and xenon flash
lamps.7 The mercury arc lamp emits a UV spectrum consisting of discrete lines. Most of the UV (>
90%) is at 254 nm and 185 nm. Five to 30% of the UV output is at the 185 nm line.7 Ozone producing
lamps are made of fused silica which is transparent to this line. However, the power output of these
lamps is limited by the self absorption of the emitted UV by Hg atoms.
The spectra of xenon flash lamps are different then those of mercury arc lamps. When a high
current is sent through a tube filled with xenon, a plasma is formed which emits from the 1R to the
UV-C region (300-200 nm) in a continuum approximating black-body radiation at a particular temperature. Increasing the current density increases the plasma temperature and shifts the continuum
toward the UV.7 Optimum plasma temperature is between 6000 and 8000°C, much higher then the
melting point of silica. This limitation is overcome by using micropulses of current which allow the
formation of the plasma without destroying the lamp. High intensities of up lo several hundred watts
per square inch of lamp surface can be generated from a small flash lamp.
A dielectric barrier discharge xenon-xenon excimer lamp was constructed for this study. In an
excimer lamp an unstable excited dimer, an excimer, is formed by passing a high voltage across a small
gap filled with a gas, in this case xenon. Excimer emissions are low wavelength UV with narrow
emissions peaks. The Xe-Xe excimer has a maximum peak at 172 nm with a peak hall-width of 14
nm.8
In Figure 1 the emission spectra of a low pressure Hg arc lamp, a xenon flash lamp, and a Xc-Xc
excimer lamp are plotted with the molar absorptivities of 02, O,, H20, and CCI4. Water and oxygen
strongly absorb UV light below 185 nm. Carbon tetrachloride has much larger absorptivity below 185
nm than at longer wavelengths. Thus by utilizing lower wavelength UV sources the photodissociation
of these molecules can be accomplished with lower lamp intensities and consumption of electricity
then is possible using conventional sources.
47th Purdue Industrial Waste Conference Proceedings,  1992 Lewis Publishers, Inc., Chelsea. Michigan 481 IK.
Printed in U.S.A.
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