Heterogeneous photoreaction of tetrachloroethene-air mixture on halloysite particles

Article abstract of DOI:10.1021/es9900095

The heterogeneous photodegradation of tetrachloroethylene in air proceeded on clay particles (halloysite) under photoillumination (> 300 nm) with an induction period. The reaction with the released chlorine atoms accelerated the photodegradation of tetrachloroethylene. The relative humidity and tetrachloroethylene partial pressure did not affect the induction period and the removal rate. Based on the diffuse reflectance UV-visible spectra and the physical properties of the halloysite particles, various tentative photoreaction mechanisms were proposed, e.g., the tetrachloroethylene-oxygen molecule charge-transfer complex. The photoreaction rate was estimated based on a simulated calculation, which can reproduce the tetrachloroethylene initial pressure dependence vs. time for tetrachloroethylene and its degradation products. Since the atmospheric lifetime of tetrachloroethylene via the gas-phase reaction with OH radicals is 140 days, potential photoreactions on the atmosphere-soils interface may affect the movement of tetrachloroethylene in the environment in the case that soils contain several weight percent of reactive clays, such as the halloysite particles.

Full text of DOI:10.1021/es9900095

Environ. Sci. Technol. 2000, 34, 2484-2489
the reaction conditions such as temperature on the
CCl dCCl decay and the degradation product formation was
Heterogeneous Photoreaction of
Tetrachloroethene-Air Mixture on
Halloysite Particles
2
2
examined. A possible reaction scheme was discussed on the
basis of the characteristics of halloysite and diffuse reflectance
UV-vis absorption spectra of halloysite particles exposed to
a CCl
released by the reaction to the photodegradation of
CCl dCCl on halloysite particles was also discussed.
2 2
dCCl -air mixture. The contribution of Cl atoms
S H U Z O K U T S U N A , *
T A K A S H I I B U S U K I , A N D K O J I T A K E U C H I
2
2
National Institute for Resources and Environment, 16-3
Onogawa, Tsukuba, 305-8569, Japan
Experimental Section
The experiments were performed in a closed-circulation
reactor made of Pyrex glass. It contains a magnetically driven
glass pump circulating the gas mixture and a multireflection
long path cell with a 3 m path length interfaced to an FTIR
spectrometer (JEOL, WINSPEC50). It was described elsewhere
in more detail (8) except for the new removal cell (9). The
The heterogeneous photodegradation of tetrachloroethene
in air proceeded on clay particles (halloysite) under
photoillumination (>300 nm) with an induction period. The
induction period and the tetrachloroethene removal rate
were not sensitive to the tetrachloroethene partial pressure
or the relative humidity. The photodegradation of
tetrachloroethene was accelerated by the reaction with
the released Cl atoms. Several tentative photoreaction
mechanisms such as that via the tetrachloroethene-oxygen
molecule charge-transfer complex were proposed based
on the diffuse reflectance UV-vis spectra and the physical
properties of the halloysite particles. The photoreaction
rate was estimated on the basis of a simulated calculation,
which can reproduce the tetrachloroethene initial pressure
dependence versus time for tetrachloroethene and its
degradation products.
3
total volume of the reactor was 0.85 dm . The new removal
cell was made from Pyrex glass with a quartz window. It has
a loop injector with 12 ports. Clay particles were piled up
loose in a small plate. The plate was set in the center of the
loop injector on which a quartz plate was put to more
efficiently expose the clay particles to the gas mixture. After
the removal cell was connected to the closed-circulation
reactor, its temperature was kept constant using a heater or
temperature-controlled water bath.
A commercial grade reagent of CCl
cal, 99%) was used without further purification. The gas
mixture of CCl dCCl in air was prepared by evaporation of
a liquid sample of CCl dCCl , followed by dilution with
2 2
dCCl (Wako Chemi-
2
2
2
2
synthetic air (Takachiho Kagaku Co., 99.9999%) by measuring
each partial pressure with an absolute pressure gauge (MKS
Baratron 122AA). So was the gas mixture of trichloroethene
(
CHCldCCl
Chemical, 99.5%). The gas mixture of 1,1-dichloroethene
CH dCCl ) was prepared by mixing a standard gas mixture
2
) by evaporation of a liquid sample (Wako
Introduction
(
2
2
2 2
Tetrachloroethene (CCl dCCl ) has been widely used as a
solvent for metal parts, semiconductor washing, and dry
cleaning. It causes groundwater pollution which poses a
in nitrogen (Takachiho-Kogyo Co.) with oxygen. The hu-
midified gas mixture was prepared by passing part of the gas
mixture through water in a bottle (Ichinose-type humidifier).
The examined clay particles of kaolinite and halloysite
were obtained from the Iwamoto Mineral Co. Their structure,
BET surface area, and components are shown in Table 1.
Table 1 does not include ignition loss, which mainly consisted
of crystalline OH groups. The pretreatment of clay particles
was carried out under a dry or humidified air flow. Fifty
milligrams of clay particles placed in the removal cell was
kept at a fixed temperature (typically 393 K) for an hour under
serious environmental problem (1). Since most of the CCl
2
d
CCl used is emitted into the atmosphere, much attention
2
has been given to its concentrations in the atmosphere from
the standpoint of human health and welfare. The attack by
the OH radicals is accepted as the dominant path for the
tropospheric degradation of these volatile compounds. The
lifetime of CCl dCCl has been estimated at 140 days based
2 2
on the reaction rate with OH (2).
Potential photoreactions on the atmosphere-soils in-
2 2
terface may effect the movement of CCl dCCl in the
environment, though it is restricted to the upper millimeter
of soils (3). In laboratory experiments, it has been pointed
3
-1
0
.2 dm min of flowing synthetic air at atmospheric pressure
and then cooled to the reaction temperature (typically 298
K). In the experiment carried out under a humidified
atmosphere, the clay particles were followed by exposure to
2 2
out that volatile compounds including CCl dCCl adsorbed
on particulate matter such as sand and silica are mineralized
under photoillumination, even at wavelengths longer than
-1
0
.2 dm3 min of the synthetic air humidified at a definite
relative humidity for an additional 1 h at the reaction
temperature.
2
90 nm (4), but the intermediates in the heterogeneous
photochemical reactions were not discussed. We investigated
the heterogeneous photoreactions of CCl dCCl and trichlo-
roethene (CHCldCCl ) with TiO particles and found these
The experimental procedure was divided into the fol-
lowing three steps. During the initial 30 min, the reactant
gas mixture was circulated without contact to the clay
particles. Subsequently, the gas-circulating route was changed
so that the gas mixture flowed over the clay particles in the
dark for 1 h. Finally, photoillumination with light of a
wavelength longer than 300 or 360 nm was carried out using
a 500 W xenon short arc lamp (Ushio Co.) and a UV30 or L38
optical filter (Hoya Co.), respectively. The measured intensity
2
2
2
2
volatile compounds were decomposed fast enough to serve
as a possible sink (5).
In this study, the heterogeneous reactions of CCl dCCl
2 2
were examined on two kinds of clay particles (kaolinite and
halloysite) which are among the main components of soils
(
6, 7). CCl
2 2
dCCl was only decomposed by a heterogeneous
-
2
was about 6 mW cm at 365 nm using a UV sensor (USHIO,
UVD-365PD). The reaction temperature was set between 283
and 313 K by putting the removal cell in a temperature-
controlled water bath.
photoreaction on the halloysite particles. The influence of
*
Corresponding author phone: 81-298-61-8163; fax: 81-298-61-
8
158; e-mail: kutsuna@nire.go.jp.
2
4 8 4
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 34, NO. 12, 2000
10.1021/es9900095 CCC: $19.00

2000 Am erican Chem ical Society
Published on Web 05/04/2000

TABLE 1. Properties of the Examined Clays
chemical component/wt %
2
-1
watera
structure
BET area/m g
SiO2
Al2O3
Fe2O3
TiO2
K2O
CaO
Na2O
MgO
kaolinite
halloysite
thin leaf
tube
16.0
29.4
40.5
50.4
36.1
35.7
0.1
0.2
0.5
0.1
2.1
trace
0.1
0.0
0.5
2.6
0.2
0.0
a
Water contents were calculated from the decrem ent of the weight by heating clay particles at 473 K for 2 h in am bient air.
The IR spectra of CCl
were measured at a 0.5 cm resolution with 50 accumula-
tions. The absolute concentration measurements of CCl
CCl , CO , and CO were based on the calibration curves which
2 2
dCCl and its degradation products
-
1
2
d
2
2
were prepared based on the IR peak areas (PA) of the
concentration-prepared standard gases. The peak areas for
-
1
-1
the region of 890.0-939.9 cm (PA1), 2290.1-2389.9 cm
-
1
(
PA2), and 2050.0-2230.1 cm (PA3) were used for the
quantification of CCl dCCl , CO , and CO, respectively. The
absorption coefficients (base 10) were 3.60 × 10 mTorr
cm for PA1 of CCl dCCl
and 1.31 × 10 mTorr cm for
PA3 of CO. The partial pressure of CO (PCO2/ mTorr) was
calculated based on the calibration curve that PCO2 ) 2.15 ×
2
2
2
-
4
-1
-
2
-4
-1
-2
2
2
2
2
3
PA2 + 0.0661 × PA2 + 0.00034 × PA2 for the 3 m path
length. The absolute concentration measurements of COCl
and HCl were based on the published absorption coefficient
2
-
5
-1
-1
-1
(
base 10) of 2.00 × 10 mTorr cm for the 1832.3 cm
-6 -1
2
peak of COCl
and 2.40 ×10 mTorr cm-1 for the maximum
-
1
peak in the region of 2740.1 to 2715.1 cm of HCl (10). For
3
CCl C(O)Cl, we used the measurement of the pressure-
prepared gas mixture. Since the IR peaks of HCl were also
measured, the prepared partial pressure was corrected by
subtracting the partial pressure of HCl in order to deduce
FIGURE 1. Time-course of CCl
2
dCCl
2
partial pressure in air without
clays, with kaolinite or halloysite particles at 298 K. The partial
pressure of the degradation products in the experiment with
halloysite particles is also plotted. The gas-circulating route was
changed at 30 min to expose clay particles to the gas mixture.
Illumination (>300 nm) was started at 90 min.
3
the partial pressure of CCl C(O)Cl. The absorption coefficient
-
6
-1
-1
-1
of 7.03 × 10 mTorr cm for the 1026.9 cm peak of
CCl C(O)Cl was obtained. Since COCl also shows an IR
absorption at 1026.9 cm-1, the absorbance of CCl
C(O)Cl
was corrected by subtracting that of COCl , that is, the value
3
2
3
2
-
1
of 0.013 × (OD at 1832.3 cm ).
the halloysite particles was much greater than that on the
kaolinite particles. In the dark, the CCl
did not change. Under photoillumination, CCl
pressure remained almost constant in both cases without
clay particles and with the kaolinite particles. However, for
the halloysite particles, the CCl dCCl partial pressure slightly
2 2
decreased during the initial 1 h of photoillumination and
then rapidly decreased.
2 2
Figure 2 shows the IR spectra of the CCl dCCl gas mixture
on halloysite particles before photoillumination and at 78
min and 158 min of photoillumination. Trichloroacetyl
The CCl dCCl or its degradation products adsorbed on
2
2
2
dCCl
2
partial pressure
dCCl partial
clay particles were measured with a diffuse reflectance UV-
vis spectrometer. The diffuse reflectance UV-vis absorption
spectra were measured using a flat quartz cell and a
spectrometer (Hitachi Co., U-3500) equipped with an internal
2
2
4
diffuse reflectance attachment. A pellet of MgSO was used
as the reference. The pretreatment of clay particles and/ or
the introduction of the gas mixture in a flat quartz cell were
carried out using a glass vacuum line. The clay particles were
evacuated at liquid nitrogen temperature in order to prevent
the water content being releasing from them, and then at
room temperature they were exposed to 620 Torr of synthetic
air. Under 620 Torr of air atmosphere they were heated at
chloride (CCl
3 2
C(O)Cl), phosgene (COCl ), carbon dioxide
(
CO ), carbon monoxide (CO), and hydrogen chloride (HCl)
2
were detected as the degradation products. Figure 1 also
shows the change in the partial pressure of these degradation
3
93 K for an hour and cooled to ambient temperature. They
were again evacuated at liquid nitrogen temperature and
then placed in the spectrometer and measured at ambient
temperature. After a sample of clay particles in a vacuum
was measured, it was exposed to the CCl dCCl -air mixture
2 2
at a specific pressure using a glass vacuum line for the
subsequent measurement.
products for halloysite particles. CCl
increased as the CCl dCCl rapidly decreased after the
induction period. CCl C(O)Cl decreased after almost all of
CCl dCCl was consumed.
Diffuse Reflectance UV-vis Spectra of Clay Particles
Exposed to CCl dCCl -Air Mixture. Figure 3 shows the
diffuse reflectance UV-vis spectra of halloysite and kaolinite
particles in a vacuum and under the CCl dCCl -air mixture.
3 2
C(O)Cl and COCl
2
2
3
2
2
2
2
Results and Discussion
2
2
Photodegradation of CCl
Figure 1 shows the change in the CCl
in the experiment when the CCl dCCl
2
dCCl
2
on Halloysite Particles.
dCCl partial pressure
-air mixture flowed
Both the halloysite and kaolinite particles have absorptions
in the wavelength range longer than 300 nm. Although the
CCl dCCl absorption spectrum in the gas phase is shorter
2 2
2
2
2
2
over no clay particles and the halloysite or kaolinite particles.
The decrease in partial pressure at 30 min was the sum of
the decrease due to the adsorption onto the clay particles
and the decrease due to the dilution attending the change
of the gas-circulating route. The latter corresponds to a 23%
decrease in partial pressure. The amount of adsorption on
than 240 nm (11), a broad absorption between 250 and 400
nm was observed only for the case of the halloysite particles
(spectrum c in Figure 3). An explanation of this absorption
is that it was due to the absorption of the CCl
charge-transfer complex in the electrostatic field of the
halloysite layer analogous to the alkene-O charge-transfer
2
dCCl
2
-O
2
2
VOL. 34, NO. 12, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
2 4 8 5

FIGURE 4. Dependence on the relative humidity or the pretreatment
temperature for CCl dCCl -air-halloysite at 298 K. Plots A and C
were examined under dry conditions, and plot B was examined at
0%RH. Plots A and B were examined on halloysite particles
2
2
4
pretreated at 393 K, and plot C was examined on those pretreated
at 333 K. Illumination (>300 nm) was started at 90 min.
2 2
FIGURE 2. IR spectra of CCl dCCl -air mixture on halloysite
particles before photoillumination and at 78 min and 158 min
photoillumination (>300 nm) at 298 K.
FIGURE 5. The dependence on the partial pressure of CCl
in the reaction of CCl dCCl -air mixture on halloysite particles at
98 K. Illumination (>300 nm) was started at 90 min. The symbols
of 0, O, 4 and 3 indicate the result of the CCl dCCl partial pressure
at 0 min ([C Cl ) of 46, 117, 166 and 268 mTorr, respectively. The
lines of 0, O, 4, and 3 indicate the calculated results based on
the scheme shown in Table 2 for [C Cl of 46, 117, 166, and 268
mTorr, respectively.
2 2
dCCl
2
2
2
FIGURE 3. Diffuse reflectance UV-vis absorption spectra of
2
2
halloysite (upper) and kaolinite (lower) particles. Spectra a, d and
2
4 0
]
b, e were measured in a vacuum and in the presence of CCl
2 2
dCCl
2
4 0
]
(9.3 Torr) in air (620 Torr), respectively. Spectra c and f are the
subtracted spectra of (b - a) and (e - d), respectively. The ordinate
indicates the Kubelka-Munk function.
for the relation between 166 and 268 mTorr of CCl
CO formation rate was independent of the CCl dCCl
pressure. The formation ratio of CCl
the increasing CCl dCCl partial pressure, while that of COCl
was almost constant.
2
dCCl
partial
C(O)Cl increased with
2
.
complex in zeolite (12). The electric static field could be
stronger in halloysite particles than in kaolinite particles due
to the amphoteric nature of the layer in halloysite (13).
2
2
2
3
2
2
2
Dependence of CCl
Product Form ation on the Reaction Conditions. Figure 4
shows the time-course of the CCl dCCl partial pressure in
2 2
dCCl Decay and the Degradation
The temperature dependence was examined in the range
of 283-313 K. The photodegradation rate was not sensitive
2
to the reaction temperature. The CO formation rate is
2
2
humidified air (B) and when the pretreatment temperature
of the halloysite particles was 333 K (C). Both time-profiles
were similar to that with halloysite particles pretreated at
dependent upon the reaction temperature and increased
with increasing temperature. The formation ratio of CCl C-
(O)Cl and COCl was not sensitive to the reaction temperature.
Contribution of Cl Atom s Released to the Photodeg-
radation of CCl dCCl . The time-profile of the CCl dCCl
3
3
93 K and in dry air (A), indicating that the photodegradation
2
rate was not sensitive to either the pretreatment temperature
or the relative humidity appropriate to the atmosphere.
2
2
2
2
Figure 5 shows the time-course of the CCl
ratio, CO partial pressure, the formation ratio of CCl
and COCl to the CCl dCCl decay at different partial
pressures of CCl dCCl . The CCl dCCl residence ratio decay
was not sensitive to the CCl dCCl partial pressure. Its rate
was rather high at the lower CCl dCCl partial pressure except
2
dCCl
2
residence
decay, that is, the rapid decrease after the induction time,
suggests that the photoreaction may include an autocatalytic
2
3
C(O)Cl
2
2
2
process in which the released Cl atoms attack CCl
confirm this, the photoreaction was examined in the presence
of ethene (CH dCH ) as a scavenger of Cl atoms. Although
CH dCH did not decay by itself on the halloysite particles
2 2
dCCl . To
2
2
2
2
2
2
2
2
2
2
2
2
2
4 8 6
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 34, NO. 12, 2000

FIGURE 6. Time-course of the partial pressure of CCl
2
dCCl
2
, CH
2
d
CH , and their degradation products for CCl dCCl -CH
2
2
2
2
dCH
2
-air-
halloysite at 298 K. Illumination (>300 nm) was started at 90 min.
in air during photoillumination (> 300 nm), it decreased in
the presence of CCl
rate of CCl dCCl was smaller, and the induction time was
longer than in the case shown in Figure 1. The formation of
CCl C(O)Cl was not observed in the gas phase. This result
indicates that part of the Cl atoms released from CCl dCCl
attacked CH dCH so that the reaction of the Cl atoms with
CCl dCCl was restricted. The expected products such as
CH ClC(O)H due to the reaction of Cl atoms with CH dCH
2 2
dCCl as shown in Figure 6. The decay
2
2
FIGURE 7. Time-course of the partial pressure of CH
2 2
dCCl (A),
3
CHCldCCl (B), and CCl dCCl (C) in air through the reaction on the
2
2
2
2
2
halloysite particles at 298 K under photoillumination at wavelengths
longer than 300 nm (open circle) or 360 nm (closed circle).
Illumination was started at 90 min.
2
2
2
2
2
2
2
were probably not observed because of their further de-
composition.
seems unclear. The diffuse reflectance UV-vis spectra shown
in Figure 3 suggests that the absorption of light by the CCl
2
d
The point concerning atmospheric chemistry is whether
Cl atoms will go into the gas phase or not. If all the Cl atoms
were released from the clay particles, they will be diluted in
the environment to a concentration at which the reaction
with them may be insignificant. This can be estimated from
CCl -O charge-transfer complex in the electrostatic field of
2
2
the halloysite layer may initiate the reaction. On the other
hand, since clay particles can absorb light with wavelengths
shorter than 400 nm, a photosensitized reaction is also a
possible mechanism. If the acid sites such as defects on the
clay surfaces served active sites for the photoreaction, the
presence of the water content would interfere with the
reaction (9). However, the photoreaction rate was insensitive
to the relative humidity as shown in Figure 4. An additional
possible scheme was that photocatalytic components such
as titanium dioxide might initiate the photodecomposition
of CCl dCCl . The anatase type of titanium dioxide was
the relative ratio of the CCl
2
dCCl
2
decay rate to the CH
dCCl
is 0.82 ( 0.01 mTorr min or 0.37 ( 0.01 mTorr
2
d
2
or
CH
CH
2
decay rate (R). Since the decay rate of CCl
2
-
1
2
dCH
2
-
1
min , respectively, from the slope of each line which was
linearly fitted with regard to the data after 180 min, the
2 2 2 2
observed ratio of the CCl dCCl decay rate to the CH dCH
decay rate (Robs) is 2.2. On the other hand, the ratio (Rga s) is
estimated to be less than 1.1 when the reaction of Cl atoms
2
2
with CCl
2
dCCl
2
or CH
2
dCH
2
occurred only in the gas phase.
reported as a component in kaolinite particles (15). For the
latter two possible mechanisms, it is not clear why the
reaction did not proceed on kaolinite particles. We also
examined the heterogeneous photodegradation of CHCld
Cl
Rga s ) (k
[Cl][CCl dCCl ])/
CCl2)CCl2
2
2
Cl
(
k
[Cl][CH dCH ]) < 1.1
CH2)CH2 2 2
CCl
CHCldCCl
2
or CH
2
dCCl
or CH
2
. The heterogeneous photodegradation of
dCCl proceeded not only on the halloysite
2
2
2
The gas-phase reaction rate constant with the Cl atom of
Cl
Cl
particles but also on the kaolinite particles where the
decreasing rate was less and the induction period was longer
than for the halloysite particles. These results suggested that
the kaolinite particles also had a photochemical activity,
which was lower than the halloysite particles. Figure 7 plots
the partial pressures of three kinds of chloroethenes against
the period of the photoillumination at wavelengths longer
than 300 or 360 nm for the halloysite particles. Under
CCl
2
dCCl
2
(k CCl2)CCl2) or CH
2
-10
dCH
2
(k CH2)CH2) is (2.6-4.5)
10- and (0.4-1.7) × 10
11
3
-1 -1
×
cm molecule
s
at 298 K
(
14), respectively. The result of Robs > Rga s indicates that the
2 2
reaction of CCl dCCl proceeds not only in the gas-phase
but also in the small pores and/ or on the surface of the
halloysite particles before they go into the gas phase. Since
the amount of CCl
particles was much greater than that of CH
be larger than Rga s. Hence, the contribution of Cl atoms to
the photodegradation of CCl dCCl can be worth evaluating
2
dCCl
2
adsorption onto the halloysite
2
dCH
2
, Robs can
photoillumination (>360 nm), CCl
for 3 h, and the decreasing rates of CHCldCCl
CCl were less, and their induction periods were longer than
2
dCCl
2
did not decrease
2
and CH d
2
2
2
although coexisting substances could interfere with it in the
environment.
2
under photoillumination (>300 nm). It seems that the rate-
determining step may be correlated to the extent to which
chloroethenes served as an electron donor, but it is still
unclear as to which of the above three possible mechanisms
initiated the reaction.
2 2
Reaction Schem e of Photodegradation of CCl dCCl on
Halloysite Particles. Though the rapid decrease after the
induction time was mainly caused by the attack of Cl atoms
as already mentioned, the initial step of the photoreaction
VOL. 34, NO. 12, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
2 4 8 7

2 2
TABLE 2. Schemes of Photoreaction of CCl dCCl -Air-Halloysite Particles
*
1
s: surface area of clay particles per unit volum e; n
s
: adsorption site density; γia: accom m odation coefficient; Q
1
: ratio of unoccupied sites
. s ) 17.3 cm 2 cm . n
-3
is assum ed to be 10 cm -2. γia is assum ed to be 1.
15
*2
P is the ratio of unoccupied sites to nps. nps is the adsorption
to n
site density where CCl
of 5/6 or 30/31 was fitted to reproduce the form ation ratio of CO
s
s
1
dCCl can be photochem ically activated. nps is assum ed to be 2 orders of m agnitude sm aller than n
2 s
. * The branching ratio
3
2
/CO. *4 The branching ratio of 12/13 was fitted to reproduce the form ation ratio
2
*
5
*6
of CCl
3
C(O)Cl/COCl
2
.
The rate constant was referred to the literature (14). The reaction rate of 26 was referred to the literature (18). That of
*7
2
2 was assum ed to be equal to that of 26. The branching ratio of 23/24 was referred to the literature (2). *8
to nss. nss is the adsorption site density where CCl C(O)Cl or COCl can be oxidized. nss was assum ed to be 2 orders of m agnitude sm aller than
1
S is the ratio of unoccupied sites
3
2
s
n .
In the experiment where the photoillumination was
restarted after the CCl dCCl -air mixture flowed over
halloysite particles for 40 min under photoillumination and
for 1 h in the dark, the CCl dCCl decreased just after
photoillumination. This indicates that the degradation
intermediates capable of emitting Cl atoms accumulated on
the halloysite particles at 298 K.
2 2
bution to the CCl dCCl decay of the photodecomposition
or the reaction with Cl as described below.
2
2
2
On the basis of this scheme, the time-profile of CCl d
CCl and its degradation products was simulated using the
2
commercial software program FACSIMILE (AEA Technology).
In Table 2, the subscript “ad” indicates the species adsorbed
2
2
on the surface. The (P) value indicates CCl
on the sites where CCl dCCl can be photochemically
activated. That of (S) indicates CCl C(O)Cl or COCl adsorbed
on the sites where they can be oxidized. Until these concepts
2 2
dCCl adsorbed
A tentative reaction scheme suitable to the CCl
partial pressure dependence shown in Figure 5 is shown in
Table 2. The reaction of CCl dCCl with O in the electrostatic
field of the halloysite particles was assumed as the initial
step of the photoreaction. The intermediate of CCl dCClO
2
dCCl
2
2
2
3
2
2
2
2
were introduced into the reaction scheme, the CCl
pressure dependence of the induction period in the CCl
CCl decay and the formation rates of CO and COCl
2
dCCl
2
2
d
2
2
2
2
2
could
was adopted from analogy with the gas-phase or liquid-phase
reactions (16, 17). Most of the reactions referred to the gas-
phase reactions, and the scheme may include inadequate
ones and/ or overlook some reactions. Nevertheless, the
scheme could reproduce the observed time-profile of the
gas-phase species and play a part in evaluating the contri-
not be reproduced. The calculated results are plotted by lines
in Figure 5.
On the basis of the simulated calculation, each contribu-
tion to the CCl
2 2
dCCl decay of the photodecomposition
(reaction 3 in the scheme), the surface reaction with Cl ((9))
2
4 8 8
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 34, NO. 12, 2000

and the gas-phase reaction with Cl ((20)), was calculated to
be 11, 78, and 3%; 5, 88, and 4%; 3, 90, and 4%; 2, 92, and
(2) Tuazon, E. C.; Atkinson, R.; Aschmann, S. M.; Goodman, M. A.;
Winer, A. M. Int. J. Chem. Kinet. 1988, 20, 241.
(
3) Miller, G. C.; Hebert, V. R.; Miller, W. W. In Reactions and
Movement of Organic Chemicals in Soils; SSSA Special Publica-
tion No. 22; Sawhney, B. L., Brown, K., Eds.; Madison, WI, 1989;
pp 99-110.
5
%, for the CCl
mTorr, respectively. It proved that CCl
decomposed by the surface reaction with Cl atoms which
were released through the decomposition of CCl dCCl and/
2
dCCl
2
initial pressure of 46, 117, 166, and 268
2
dCCl was mostly
2
2
2
(
4) Gab, S.; Schmitzer, J.; Thamm, H. W.; Parlar, H.; Korte, F. Nature
1977, 270, 331.
or its degradation products. For the environmental soils, part
of the Cl atoms released could react with coexisting species
(5) Kutsuna, S.; Ebihara, Y.; Nakamura, K.; Ibusuki, T. Atmos.
Environ. 1993, 27A, 599.
(6) Saigusa, M. In Soil Chemistry; Japan Chemical Society, Ed.;
Gakkai Shuppan Center: Tokyo, 1989; Chapter 5.
and hence the reactions of CCl
restricted. The examination of the influence of coexisting
materials to the reaction of CCl dCCl with Cl still remains
as a subject to evaluate the reaction rate in the environment.
If the reactions with Cl are completely restricted, the CCl
CCl decay will be determined only by the photodecompo-
sition. In that case, the removal rate constant of gaseous
CCl dCCl is represented as k3f [O ) s. Its
maximum value, which corresponds to Q ) 1, is calculated
2 2
dCCl with Cl could be
2
2
(
7) Zielke, R. C.; Pinnavaia, T. J.; Mortland, M. M. In Reactions and
Movement of Organic Chemicals in Soils; SSSA Special Publica-
tion No. 22; Sawhney, B. L., Brown, K., Eds.; Madison, WI, 1989;
pp 81-97.
2
d
2
(
8) Kutsuna, S.; Toma, M.; Takeuchi, K.; Ibusuki, T. Environ. Sci.
Technol. 1999, 33, 1071.
2
2
2 s
] (k1f/ k1r)(nps/ n
1
(9) Kutsuna, S.; Takeuchi, K.; Ibusuki, T., J. Geophys. Res. 2000, 105,
6611.
10) Hanst, P. L.; Hanst, S. T. Quantitative Reference Spectra for Gas
Analysis; Infrared Analysis Inc.; CA, 1990.
-
7
-1
to be 6 × 10 s min using the values shown in Table 2
(
where s is the surface area of clay particles per unit volume
2
-3
(
cm cm ) in the environment. It corresponds to (2400/ s)
(
(
(
11) Berry, M. J. J. Chem. Phys. 1974, 61, 3114.
days of lifetime by assuming that the sunlight duration is
half a day. If the air mass under 1 m height on the ground
is considered, w wt % (percent by weight) of halloysite
12) Blatter, F.; Frei, H. J. Am. Chem. Soc. 1993, 115, 7501.
13) Theng, B. K. G. The Chemistry of Clay-Organic Reactions; Adam
Hilger: London, 1974; Chapter 1.
14) DeMore, W. B.; Sander, S. P.; Golden, D. M.; Hampson, R. F.;
Kurylo, M. J.; Howard, C. J.; Ravishankara, A. R.; Kolb, C. E.;
Molina, M. J. Chemical Kinetics and Photochemical Data for
Use in Stratospheric Modeling; Evaluation No. 12, JPL Publication
2
particles in the soil at ground level corresponds to 7.5 w cm
(
-
3
cm of s based on the assumption that the density of the
-
3
soils is 2.5 g cm ; sunlight reached the upper 1 mm of the
soils. Accordingly, the lifetime of CCl dCCl in the air mass
can be estimated as (320/ w) days. Since the atmospheric
lifetime of CCl dCCl through the gas-phase reaction with
2
2
9
7-4; Pasadena, CA, 1997.
(
(
15) Murad, E. Am. Mineralogist 1997, 82, 203.
16) Calderwood, T. S.; Neuman, R. C., Jr.; Sawyer, D. T. J. Am. Chem.
Soc. 1983, 105, 3337.
17) Russell, J. J.; Seetula, J. A.; Gutman, D.; Senkan, S. M. J. Phys.
Chem. 1989, 93, 1934.
18) Lightfoot, P. D.; Cox, R. A.; Crowley, J. N.; Destriau, M.; Hayman,
G. D.; Jenkin, M. E.; Moortgat, G. K.; Zabel, F. Atmos. Environ.
1992, 26A, 1805.
2
2
OH radicals is 140 days, potential photoreactions on the
atmosphere-soils interface may effect the movement of
2 2
CCl dCCl in the environment in the case that soils contain
several to tens wt % of reactive clays such as the halloysite
particles examined. The dependence on the sunlight intensity
and the interference of coexisting species such as organic
acids in the soils are remaining subjects in order to estimate
(
(
2 2
the role of the photoreactions of CCl dCCl on clay particles
in the environment.
Received for review January 5, 1999. Revised manuscript
received September 8, 1999. Accepted March 20, 2000.
Literature Cited
(
1) Smith, J. A.; Cary, T. C.; Kammer, J. A.; Kile, D. E. Environ. Sci.
Technol. 1990, 24, 676.
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