<>EPA
United States
Environmental Protection
Agency
EPA/540/SR-93/516
July 1993
SUPERFUND INNOVATIVE
TECHNOLOGY EVALUATION
Emerging Technology
Summary
Destruction of Organic
Contaminants in Air Using
Advanced Ultraviolet Flashlamps
This summary describes a new pro-
cess for photo-oxidation of volatile or-
ganic compounds (VOCs) in air using
an advanced ultraviolet (UV) source,
and a pulsed xenon flashlamp. The
flashlamps have greater output at 200
to 250 nm than medium-pressure mer-
cury lamps at the same power and,
therefore, cause much more rapid di-
rect photolysis of VOCs, including me-
thylene chloride (CH2CI2), chloroform
(CHCI3), carbon tetrachloride (CCg, 1,2-
dichloroethane (1,2-DCA), 1,1,1-
trichloroethane (TCA), Freon 113, and
benzene. The observation of quantum
yields greater than unity indicate the
involvement of chain reactions for
trichloroethene (TCE), perchloroethene
(PCE), 1,1-dichloroethene (DCE), CHCI3,
and CH2CI2.
TCE was examined more closely be-
cause of its widespread occurrence and
very high destruction rate. Two full-
scale air emission control systems for
TCE were constructed at Purus and
tested at a Lawrence Livermore National
Laboratory (LLNL) Superfund site. The
systems were operated at flash frequen-
cies of 1 to 30 Hz, temperatures be-
tween 33 and 60 °C, flows up to 300
standard cubic feet per minute (scfm),
and 100 scfm, at concentrations up to
260 part per million per volume (ppmv)
and 10,600 ppmv of TCE, respectively.
Residence times ranged from 5 to 75
sees. In all cases, except at the lowest
flash frequency, greater than 99% re-
moval of TCE was observed. Careful
attention was paid to product forma-
tion and mass balances. The main ini-
tial photo-oxidation product of TCE was
dichloroacetyl chloride (DCAC), which
upon further photolysis was converted
in part to dichlorocarbonyl (phosgene
or DCC) and possibly formyl chloride,
and ultimately to HCI and CO2. Further
treatment of photo-oxidation products
is recommended for full-scale opera-
tion.
This Project Summary was developed
by EPA's Risk Reduction Engineering
Laboratory, Cincinnati, OH, to announce
key findings of the research project
that is fully documented in a separate
report of the same title (see Project
Report ordering information at back).
Introduction and Background
Many environmental remediation sites
are polluted with volatile organic com-
Printed on Recycled Paper
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pounds (VOCs). Some of these sites are
amenable to remediation by vacuum-in-
duced soil venting and groundwater air
stripping methods. VOC air emission con-
trols for restoration activities, however, are
becoming required by regulatory agencies.
We report the application of a pulsed xe-
non lamp (flashlamp) as a UV light source
for the photo-oxidation of some VOCs in
air.
Previously, the only light source that
was routinely used for UV photolyses on
a large scale was the mercury discharge
lamp and doped variations thereof.
Flashlamps discharge electrical energy
through a fill gas in short micro second
(/js) pulses and remain off for relatively
long periods of milliseconds (ms). Because
flashlamps have higher temperatures
C>13,000 K) and pressures than continu-
ous lamps, the emission is shifted to
shorter wavelengths, Figure 1. The xenon
flashlamp has a maximum output at 230
nm and a significant output at wavelengths
as low as 200 nm, whereas the mercury
lamp has most of its output at wavelengths
above 250 nm. A shift in peak output from
254 to 230 nm is significant because it
corresponds to a 1 to 2 order of magni-
tude increase in absorptivity of many
VOCs, thereby greatly enhancing the rates
of direct photolysis.
Initially, laboratory experiments were
performed on saturated and unsaturated
chlorinated hydrocarbons in air to screen
compounds for treatability. The kinetics of
photo-oxidation were studied, and appar-
ent quantum yields were determined for
the disappearance. Efforts were made to
characterize the photo-oxidation products
of TCE.
A full-scale photoreactor was built for
the photo-oxidation of TCE and was tested
at LLNL Site 300 at Building Complex
834. This summary contains information
on the laboratory screening studies and
performance data collected at the LLNL
site on the photochemical treatment pro-
cess for TCE. The TCE destruction effec-
tiveness and the yields of the main oxida-
tion products were characterized under
various operating conditions, including
flowrates of 100 to 290 cfm and TCE
concentrations of 30 to 10,000 ppmv.
These results, combined with toxicologi-
cal data, were used to estimate the oper-
ating conditions suitable for reducing the
total toxicity from TCE and its residual
products by 99% with the use of UV pho-
tolysis alone.
Experimental Methods
Laboratory Experiments: Pilot-
Scale Photolyses
Air mixtures were irradiated in a 208-L
steel, cylindrical reactor containing two
small fans for mixing. A high intensity, 6-
15
10
Xe Flashlamp
18.6% <300nm
Med Pressure Hg Lamp
11.4% <300 nm
180
200
220 240
Wavelength (nm)
260
280
300
Figure 1. Emission spectrum fora mercury lamp versus a 6-in. Xenon flashlamp. (Both lamp
outputs normalized to 3675 W input).
in. xenon flashlamp was inserted in the
middle of the reactor through its side. All
photolyses were performed at atmospheric
pressure, and the gas temperature ranged
from 300 K to approximately 340 K.
Known volumes of reagents were in-
jected into the reactor by syringe, allowed
to mix, photolyzed, and analyzed by gas
chromatography with photoionization or
electron capture detection. No reaction was
observed in laboratory light or in the reac-
tor with the lamp off. CO2 measurements
were made in one run using a Horiba PIR-
2000 CO2 monitor.
Field Measurements:
Photoreactors
In the field studies at LLNL, the process
stream was pumped from the extraction
wells, through a heat exchanger to cool,
and sent into two types of photoreactors.
The Air-2 reactor (not shown) is a large
steel cylinder, 4 ft in diameter by 8 ft in
length, with a volume of 101 ft3. Four
xenon lamps are distributed about the cen-
ter of the cylinder and point radially in-
ward. The process stream flows from one
end of the cylinder to the other. Air-3
(Figure 2) is a Purus-patented reactor con-
sisting of four disc-shaped stainless steel
chambers. Each chamber is 42 in. in di-
ameter by 6.1 in. high with a volume of
4.1 ft3 exposed to the light source. The
lamps in Air-3 are positioned in the center
axis of each chamber. The process flow
enters the bottom of each chamber, around
a deflection plate, in towards the lamp,
and then out the top. The chambers in
Air-3 are configured so that the flow is
split through two sets of chambers in se-
ries. Some experiments were conducted
with an enriched TCE air stream by gas
injection from a pressurized liquid, TCE-
filled, stainless-steel bubbler before the
reaction chambers.
Analyses
Atypical sampling session involved set-
ting the process flowrate, adjusting the
TCE concentration, and alternately taking
at least three input and output TCE
samples while photolyzing. The impinger
samples were collected during the moni-
toring of the TCE concentration and were
connected to the reactor port by 0.25-in.
i.d. Teflon tubing.
TCE was analyzed by gas chromatog-
raphy with a 30-m J&W 624 capillary col-
umn and photoionization detection.
Samples were drawn by a gas-tight sy-
ringe at septum-sealed sample ports where
the process flow entered or exited the
reactor. TCE standards were prepared in
volume-calibrated, glass sampling bulbs
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Xe flashlamp
assembly
Cylindrical 1
photo-reactor J^
1 .V
Suprasil Y | f~~l
lamp hniJtintj T ^'r 1
Yonn/i 11M^ 1
1 8 1
1 — ' — 1 1 Valve 1 ' 1
iSo
A^\
ports
XXX Va/ve
Process flo w CX! '
output
D_
-J 1
1 ' ' J
s
1 /6fc /
Va/ve J
A
TCE oufpuf
septum port
nput
m port
<
l?O Process flow -^
/npuf
Figure 2. Schematic ofAir-3 Photochemical Reactor.
by injecting liquid TCE into the bulbs; the
detection limit was approximately 0.01
ppmv.
DCC was analyzed by EPA Method TO-
6: 3 L of gas were collected in a series of
impingers filled with 30 ml of a 2% v/v
aniline/toluene solution, which reacts with
DCC to give carbanilide. After solvent
evaporation and take-up in acetonitrile,
carbanilide was determined by high pres-
sure liquid chromatography (HPLC) with
an octadecylsilyl column and UV detec-
tion at 254 nm. The detection limit was
0.02 ppmv DCC, and the average recov-
ery of carbanilide, based on standard
samples run through the blowdown proce-
dure, was 106 ± 19% (95% confidence
interval).
DCAC was determined as methyl
dichloroacetate after collection and
derivatization in methanol impingers.
Analysis involved gas chromatography with
electrolytic conductivity detection. The de-
tection limit was approximately 0.05 ppmv
DCAC.
Total HCI and hydrolyzable organic chlo-
rine were determined with the use of wa-
ter impingers. The samples were analyzed
by using EPA Method 325.3, a titrimetric
method employing mercury nitrate to de-
termine the chloride yield.
In one set of experiments, volatile or-
ganic compounds were determined by EPA
Method TO-14 with the use of evacuated
stainless steel (SUMMA) canisters and
were analyzed by Coast to Coast Analyti-
cal Services, San Luis Obispo, CA. Agree-
ment was reasonable; the Purus analyses
agreed with the standards within 10% and
the Coast to Coast analyses within 25%.
Results and Discussion
Laboratory Experiments:
Photolysis Kinetics and
Quantum Yields
Table 1 summarizes the results of the
laboratory experiments conducted in the
pilot-scale reactor. CCI4 was used as an
actinometer assuming it has a disappear-
ance quantum yield of 1.0, based on the
literature data at 214 nm. The apparent
quantum yields are averaged over the
wavelength range of overlap of the com-
pound absorbances and the emission
spectrum of the lamp. Benzene had a low
quantum yield, consistent with the ability
of aromatic compounds to intersystem
cross, fluoresce, and thermally decay by
modes that do not result in bond cleav-
age. The first four halogenated compounds
in Table 1 have quantum yields near unity,
indicating that simple C-CI bond cleavage
is highly efficient, as expected by analogy
to CCI4. Nevertheless, these compounds
photolyzed relatively slowly because they
absorb light weakly. Even shorter wave-
lengths than those available from the cur-
Table 1. First Order Decay Coefficients and Wavelength-Averaged Disappearance Quantum
Yields with a 2.756 kW Xenon Lamp
Compound
CCI4
CCIfCCIF2
Benzene
CH2CICH2CI
CFCI3
CCI3CH3
CH2CI2
CHCI3
1,1 -DCE
PCE
TCE
TCE + ethene
k(sec-1)"
0.00432
0.00093
0.0019
0.0024
0.0036
0.0041
0.0070'
0.0366'
1.24'
1.7'
5.5'
0.075
£ 'A,e\ 4
I \^voc
1.0
5.09
0.067
A/.D.#
1.18
0.79
4.60
1.79
0.0389
0.0134
0.0236
0.0236
kvoc
kcci4
1.0
0.22
0.44
N.D.
0.84
0.94
1.62
8.47
287
394
1300
17
Apparent
1.0
1.1
0.029
N.D.
0.99
0.74
7.5
15
11
5.3
31
0.4
' Initial rate constants are taken for non-log-linear curves.
# N.D.= not determined.
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rent Purus flashlamps are needed for a
commercially viable, direct, photolysis pro-
cess for these compounds.
The rate constants highlighted with an
asterisk in Table 1 exhibited non-first or-
der behavior, i.e., after about an order of
magnitude loss, the rate constant started
to decline. This same set of compounds
photolyzed more rapidly (especially the
chloroolefins) and had quantum yields
greater than one. These results all point
to the occurrence of a chlorine atom chain
reaction in these cases, and this conclu-
sion is verified by literature studies using
chlorine gas to initiate chlorine atom reac-
tions. Furthermore, when ethene was
added in large excess to TCE, the TCE
loss became first-order and much slower
because ethene is an effective scavenger
of chlorine atoms.
Chain Photo-oxidation
Mechanism for TCE
Below we show that DCAC is the main
initial photo-oxidation product of TCE in
air. The following chain mechanism is con-
sistent with all the product and kinetic
information:
HCIC=CCI2+ hv -» HCIC=CO + Cl»
Cl« + HCIC=CCI2 -» HCI2C-CCI2'
HCI2C-CCI2' + 02 -> HCI2C-CCI2OO'
2HCLC-CCLOO -> 2HCLC-CCLO + O.
HCI2C-CCI20
HCI2C-CCIO
(DCAC)
Similar pathways can be written for the
other compounds that undergo chain de-
composition. A common feature of the
mechanisms for chain reacting compounds
is that a Cl» atom reacts with them to
generate a carbon-centered radical that
has chlorine substitution, which can ulti-
mately cleave a Cl« atom and propagate
the chain. CCI4 and the freons photolyze
to the same type of radical but cannot
form a chain because Cl» atoms do not
react with C-CI or C-F bonds.
Experiments with olefin mixtures dem-
onstrated that co-contaminants can cause
both sensitization and inhibition of pho-
tolysis. Thus, TCE and PCE can be ex-
pected to sensitize the photo-oxidation of
the DCE isomers and vinyl chloride be-
cause the former are better light absorb-
ers and the latter react with Cl» atom more
readily. However, addition of chloroolefins
will not sensitize the photoreactions of the
chain promoters CHCI3 and CH CI2 be-
cause the chloroolefins enhance the rate
of Cl« atom scavenging as well as Cl»
production.
Field Experiments
Figure 3 and Table 2 give TCE disap-
pearance data and product data from ex-
periments in Air-3 at LLNL Site 300. A
larger data set over a broader range of
conditions in Air-2 gave similar results and
conclusions.
TCE Removal Efficiency
Over the range of experimental vari-
ables covered, TCE was photo-oxidized
to >99% conversion except at the lowest
flash frequencies and number of lamps.
This conclusion held true even at TCE
concentrations up to 10,000 ppmv and
flowrates up to 300 scfm (data not shown).
At the highest concentrations and optimal
conditions, conversions of 99.9996% were
achieved. Estimation of the TCE destruc-
tion efficiency was often limited by the
0.01 ppmv detection limit of the gas chro-
matographic method.
TCE Oxidation Products
Figure 3 shows the evolution of prod-
ucts during the photolysis of TCE in Air-3
at the field site. The data are taken from
Table 2 and converted to an exposure
time normalized to the standard condi-
tions of four lamps operating at 30 Hz.
DCAC is the principal initial organic prod-
uct, formed in > 85% yield from the lost
TCE. With further exposure, DCAC was
consumed and formed about 20% DCC
(Figure 3) and about 2% trichloroacetic
acid (TCAC; data not shown). The DCC
and TCAC yields did not equal the DCAC
loss, presumably because these com-
pounds also photolyze or because other
DCAC photolysis pathways exist that do
not form DCC or TCAC. The unidentified
1.5 2 4
Irradiation Time (seconds)
Figure 3. Product yields from the photolysis of TCE in Air-3.
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Table 2. Summary of Field Results with the Air-3 Photoreactor '
Freq.
(Hz)
30
30
30
30
15
15
5
5
1
1
No. of
3.7-kW
Lamps
4
4
4
2
4
2
4
2
4
2
• Res.
Time
(sec)
9.6
10.1
10.4
4.6
10.1
4.8
10.4
4.8
9.3
4.8
TCE
Destruction
(%)
>99.99
>99.99
>99.99
99.92
>99.99
>99.99
>99.99
>99.99
99.16
86.57
Mole%
DCC*
N.D.s
19.6
26.0
17.3
21.3
12.4
8.3
9.3
12.3
6.9
Mole %
DCAC*
25.8
24.4
34.6
53.7
N.D.
64.5
72.2
75.2
81.8
86.2
Mole %
Cl-
61.6
89.9
91.4
55.3
68.2
43.2
41.9
38.6
35.8
35.8
Chlorine
Balance
(Mole%)
78.8
106.2
114.5
91.1
N.D.
86.2
90.0
88.8
90.3
93.3
* Flowrate = 100 dm, initial [TCE] = 100 ppmv.
t Dichlorocarbonyl (phosgene).
* Dichloroacetyl chloride.
* Not detectable.
carbon compounds must contain either no
chlorine or only hydrolyzable forms of chlo-
rine, because a good mass balance on
chlorine is obtained (93 ±23 %). The total
chlorine recovery was defined as:
Fraction chlorine recovered =
(moles chloride + 2 x moles
DCAC) / (3 x moles TCE lost)
because the measured chloride was the
sum of gaseous HCI and hydrolyzable or-
ganic chlorine, such as DCC, formyl chlo-
ride, and the carbonyl chloride of DCAC.
Thus formyl chloride is a likely unidenti-
fied form of both chlorine and carbon. TO-
14 whole air sampling tests verified that
the concentrations of nonhydrolyzable
chlorine compounds are low: the chloro-
form yield was 0.65% of the TCE input;
the carbon tetrachloride yield, 0.15%; and
the methylene chloride yield, 0.05%. A
mass balance for carbon cannot be deter-
mined based on the measurements per-
formed in this study; however, a prelimi-
nary CO2 measurement indicated that most
of the carbon is converted to CO2 with
enough light exposure.
Estimation of Parameters to
Achieve Recommended
Treatment Levels at LLNL Site
300
Because of the formation of toxic prod-
ucts, the efficacy of treatment at LLNL
Site 300 cannot be measured simply in
terms of TCE removal. The major product
DCAC has approximately 40 times greater
long-term toxicity than TCE, and DCC
(phosgene) exhibits acute toxicity. Thus,
these products would need to be removed
by further photolysis or other post-treat-
ment before emission to the atmosphere.
A risk assessment indicated that the most
one can emit is 0.025% DCAC or 0.45%
DCC of the initial TCE concentration to
reach the goal of 99% reduction in initial
toxicity. Our data indicate that a flowrate
of 13 to 20 cfm in Air-2 or Air-3 with four
3.7-kW lamps can achieve the desired
DCAC reduction. It is uncertain if the DCC
concentration would be low enough when
this DCAC treatment goal is reached. DCC
could easily be removed with a water
scrubber, where it would would rapidly
hydrolyze to CO2 and HCI. Traces of
DCAC, however, would also hydrolyze to
dichloroacetic acid (DCAA) and HCI, and
with reasonably low water flows, the re-
sidual DCAA would still be at least an
order of magnitude above the proposed
drinking water limit of 0.2 ppb. Use of a
relatively dry scrubber, such as slaked
lime is a possibility because it would trap
both DCC and DCAC. However, DCAA is
likely to leach out when the lime is
landfilled. Promising approaches include
using very small flows of water and treat-
ing them by incineration or other thermal
processes.
Conclusions
Kinetics of VOC Photo-
oxidations
The low-wavelength emission of the
pulsed xenon lamps allows direct photoly-
sis of many VOCs, particularly chlorinated
compounds and freons, that is not pos-
sible with commercial mercury lamps. Nev-
ertheless, light absorption by some VOCs
is still weak enough at 230 nm that either
photosensitization or an even lower-wave-
length source is needed for the photoly-
ses to be rapid enough for commercializa-
tion at present. On the other hand, very
rapid and efficient destruction is observed
for compounds that undergo chain reac-
tions initiated by light, notably TCE, PCE,
and DCE, in order.
TCE Photo-oxidation Products
The main product (> 85%) from the
chain photo-oxidation of TCE is DCAC.
Further oxidation of DCAC is approximately
100 times slower than the photolysis of
TCE and forms DCC in about 20% yield,
TCAC U 2%), CHCI3 (-0.65%), CCI4
(-0.15%), CH2CI2 (-0.05%), and possibly
formyl chloride and DCAA. Evidence was
found that the carbon-containing products
are eventually converted to CO,, with
enough exposure.
Estimation Of Process
Parameters for Remediation
Although both full-scale reactors dem-
onstrated very efficient removal of TCE,
the formation of undesirable intermediates
required that their toxicity be taken into
consideration. A reduction in toxicity for
TCE of 99% requires that the residual
DCAC concentration be 0.025% of the
TCE input concentration, and the DCC
concentration must be 0.45% of the TCE
input concentration. The maximum flowrate
that meets the DCAC reduction goal at
LLNL using four 3.7-kW lamps was esti-
mated to be between 13 and 20 cfm. At
this level of treatment, the DCC concen-
tration may still be excessive and addi-
tional treatment may be needed. Scrub-
bing with water under these conditions
would rapidly hydrolyze the DCC to CO
and HCI and the DCAC to DCAA and HCI.
The accumulation of even a trace of DCAA
may, however, result in a disposal prob-
lem for the water because the expected
EPA drinking water limit for DCAA is so
low (-0.2 ppb).
Recommendations
Further studies on the use of low-wave-
length lamps for the destruction of VOCs
should be directed at (1) verifying the ef-
fectiveness of dry or wet scrubbers to
remove acidic photo-oxidation products,
(2) developing thermal or other methods
for post-treatment of products such as
DCAA present in the water after scrub-
bing, and (3) examining the use of shorter-
wavelength UV lamps or catalysts for pho-
tolysis of a broader range of VOCs. Purus
will examine some of these issues to-
gether with Argonne National Laboratory
in continued demonstrations at the De-
partment of Energy Savannah River site.
The full report was submitted in fulfill-
ment of Cooperative Agreement No. CR-
818209-01-0 by Purus, Inc., under the
sponsorship of the U.S. Environmental Pro-
tection Agency.
•&U.S. GOVERNMENT PRINTING OFFICE: 1993 - 750-071/80034
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Mark D. Johnson, Werner Haag, and Paul G. BIystone are with Purus, Inc., San
Hose, CA 95134. Paul F. Daley is with Lawrence Livermore National
Laboratory, Livermore, CA 94550.
Norma Lewis is the EPA Project Officer (see below).
The complete report, entitled "Destruction of Organic Contaminants in Air Using
Advanced Ultraviolet Flashlamps," (Order No. PB93-205383; Cost: $17.50,
subject to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Risk Reduction Engineering Laboratory
U. S. Environmental Protection Agency
Cincinnati, OH 45268
United States
Environmental Protection Agency
Center for Environmental Research Information
Cincinnati, OH 45268
Official Business
Penalty for Private Use
$300
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