Photolysis of 2-chlorophenol dissolved in surfactant solutions

Article abstract of DOI:10.1021/es9703279

The photochemical degradation and dechlorination of 2-chlorophenol (2CP) dissolved in surfactant solutions was studied. The initial degradation and dechlorination of 2CP followed apparent first-order kinetics. The quantum yields of 2CP degradation and dechlorination were enhanced in the presence of nonionic surfactant Brij 35 and anionic surfactant sodium dodecyl sulfate. A larger enhancement was observed at surfactant concentrations greater than the critical micelle concentration and was attributed to partitioning of 2CP into the micelles. Furthermore, the study of 2CP photolysis in hexane, methanol and dimethoxyethane suggested that the surfactant can serve as a hydrogen atom source in promoting 2CP degradation.The photochemical degradation and dechlorination of 2-chlorophenol (2CP)(0.1 g/L) dissolved in surfactant solutions has been studied. The initial degradation and dechlorination of 2CP followed apparent first-order kinetics in nearly all the test solutions. The quantum yields of 2CP degradation and dechlorination were enhanced in the presence of nonionic surfactant Brij 35 and anionic surfactant sodium dodecyl sulfate (SDS). A larger enhancement was observed at surfactant concentrations greater than the critical micelle concentration (cmc) and was attributed to partitioning of 2CP into the micelles. Furthermore, the study of 2CP photolysis in hexane, methanol, and dimethoxyethane suggested that the surfactant can serve as a hydrogen atom source in promoting 2CP degradation. Two 2CP phototransformation pathways in surfactant solutions were identified. The electronically excited anionic form of 2CP led to cyclopentadienecarboxylic acid formation through a ring contraction via the Wolff rearrangement Electronic excitation of the undissociated form of 2CP led to the formation of catechol via a nucleophilic displacement of chloride in the presence of water. Phenol also observed as a photoproduct in non-nucleophilic solvents that can act as hydrogen atom donors. Phenol was also demonstrated to sensitize the 2CP transformation to phenol in micellar media.

Full text of DOI:10.1021/es9703279

Environ. Sci. Technol. 1997, 31, 3581-3587
bene to a ketene has been suggested to account for cyclo-
Photolysis of 2-Chlorophenol
petandienic acid formation. A mechanism involving 2CP
excited-state nucleophilic displacement of chlorine by water
is thought to account for catechol formation (7-10). The
quantum yield for photodepletion (λex ) 296 nm) of the anion
of 2CP in basic solution (pH > 9) is reported to be (0.30 (
Dissolved in Surfactant Solutions
Z H O U S H I , † M I C H A E L E . S I G M A N , * , † , ‡
†
M R I G A N K A M . G H O S H , A N D
0
.03), which is 10-fold higher than that in the acidic solution
‡
R E Z A D A B E S T A N I
(
pH < 5) (8). Neither the product distribution nor the
Department of Civil and Environmental Engineering and
Center for Environmental Biotechnology,
University of Tennessee, Knoxville, Tennessee 37996-2010,
and Chemical and Analytical Sciences Division,
Oak Ridge National Laboratory, P.O. Box 2008, MS 6100,
Oak Ridge, Tennessee 37831-6100
quantum yields are reported to be influenced by presence of
oxygen (7, 8).
Photolysis of 2CP in aqueous solution under laboratory
conditions appears promising as a remediation technology.
Surfactant soil washing provides a method of removing 2CP
from the soil matrix. When a surfactant concentration is
above its critical micellar concentration (cmc), micelles form
that can effectively solvate hydrophobic contaminants from
soil (11-14). Micelles have also been reported to enhance
the photodegradation of contaminants (15-24). Because of
the amphiphilic structures of surfactants and the hydrophobic
interior of micelles, the micelles can impose micellar effects
The photochemical degradation and dechlorination of
2-chlorophenol (2CP) (0.1 g/L) dissolved in surfactant solutions
has been studied. The initial degradation and dechlorination
of 2CP followed apparent first-order kinetics in nearly all
the test solutions. The quantum yields of 2CP degradation
and dechlorination were enhanced in the presence of nonionic
surfactant Brij 35 and anionic surfactant sodium dodecyl
sulfate (SDS). A larger enhancement was observed at
surfactant concentrations greater than the critical micelle
concentration (cmc) and was attributed to partitioning
of 2CP into the micelles. Furthermore, the study of 2CP
photolysis in hexane, methanol, and dimethoxyethane sug-
gested that the surfactant can serve as a hydrogen atom
source in promoting 2CP degradation. Two 2CP phototrans-
formation pathways in surfactant solutions were identified.
The electronically excited anionic form of 2CP led to
cyclopentadienecarboxylic acid formation through a ring
contraction via the Wolff rearrangement. Electronic excitation
of the undissociated form of 2CP led to the formation of
catechol via a nucleophilic displacement of chloride in the
presence of water. Phenol also observed as a photoproduct
in non-nucleophilic solvents that can act as hydrogen atom
donors. Phenol was also demonstrated to sensitize the
(
i.e., cage, preorientation, localization, microviscosity, polar-
ity, and counterion effects) to modulate photodegradation
reactions (15). Furthermore, surfactants may also act as
hydrogen sources to increase quantum yields, even at
surfactant concentrations below the cmc. Chu and Jafvert
(
22) reported that the decay quantum yield of hexachlo-
robenzene (HCB) was about 1 order of magnitude greater in
nonionic surfactant micellar solutions (Brij 35, Brij 58, Tween
2
0, and Tween 80) than in water alone. Photochemical
dechlorination was reported to be the major mechanism of
HCB degradation. It has been observed that dechlorination
of seven polychlorinated biphenyl (PCB) congeners was
enhanced by the presence of Brij 58 or sodium dodecyl sulfate
(SDS) micelles in acetonitrile/ water mixtures also containing
dissolved sodium borohydride (23). The quantum yields,
which increased with the degree of PCB chlorination, were
2
.5-108-fold greater with the surfactant present than those
without surfactant. Additionally, fewer side reactions were
observed with surfactant present, and oxygen had little effect
on the photolysis rate. Shi et al. (24) showed that irradiation
(
254 nm) of 2,2′,4,4′PCB in Triton X-100 and TiO
2
suspension
resulted in 99% PCB degradation and 49% recovery of the
total chlorine as chloride in 20 min.
2CP transformation to phenol in micellar media.
Although photolysis in surfactant solutions is becoming
an increasingly active research area, information relating to
the role of surfactant micelles in photochemical degradation
of environmental contaminants is very limited. Photolysis
of chlorophenols in surfactant solutions has not, to our
knowledge, been studied previously. The purpose of this
work is to study the photochemical behavior of 2CP in
surfactant solutions. By comparison of results with and
without surfactant present, this work is helpful in further
understanding the role of the surfactant.
Introduction
2
-Chlorophenol (2CP) is an EPA priority pollutant (1). It is
widely used in pulp, paper, herbicide, and pesticide industries
and has been identified in industrial effluents, soils, and
finished drinking waters (1-3). Due to its recalcitrance to
general biodegradation (4-6) and potential to act as a
precursor to the formation of more toxic compounds, such
as polychlorinated phenols formed during the disinfection of
water and wastewater (1), this xenobiotic compound poses
a serious threat to environmental ecosystems and a chal-
lenging problem for environmental remediation.
Experimental Section
Chem icals. 2CP was purchased from Fisher Scientific
Company and further purified by chromatography on a
chromatotron (Model 7924T, Harrison Research) eluting with
dichloromethane. After purification, the 2CP was found to
be >99.5% pure by gas chromatograph (GC) (HP 5890 Series
II) with a flame ionized detector (FID). Nonionic surfactant,
polyoxyethylene lauryl ether (Brij 35), anionic surfactant
sodium dodecyl sulfate (SDS, 98% pure), phenol (>99% pure),
catechol (>99% pure), and cyclopentanecarboxylic acid
Photolytic transformation is proposed as one of the major
pathways of 2CP degradation in aquatic environments (7-
1
0). Direct irradiation of undissociated 2CP in aqueous
solution is reported to produce cyclopentadienecarboxylic
acid and catechol, while photolysis of the anionic form of
2
CP results in the formation of cyclopentadienecarboxylic
acid as the only photoproduct (7-10). A ring contraction
mechanism involving a Wolff rearrangement of an acylcar-
(
>99%) were obtained from Aldrich Chemical Co. and used
*
To whom correspondence should be addressed: at Oak Ridge
without further purification. The characteristics of the
surfactants and 2CP are listed in Table 1. All solvents used
National Laboratory. Phone: (423)-574-2173; fax: (423)-574-4902;
e-mail: sigmanme@ornl.gov.
S0013-936X(97)00327-1 CCC: $14.00

1997 Am erican Chem ical Society
VOL. 31, NO. 12, 1997 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
3 5 8 1

pH of the mobile phase H
2 2
O was preadjusted to 2 using H -
TABLE 1. Characteristics of Surfactants and 2CP
SO The concentration of chloride ion, generated from
4
.
dechlorination of 2CP, was determined by a chloride selective
electrode (Orion Model 96-17) and confirmed with a colo-
rimetric method as given by Iwasaki et al. (26). All samples
were analyzed in duplicate.
Photolysis of 2CP for product identification was carried
out using 80 mL of 2CP solutions placed in quartz tubes (25
cm diameter) with Teflon-lined caps. After photolysis,
samples were acidified to pH 2 using 18.6% HCl, followed by
trade
name
cmc (M)a
MW HLBb Naggrc
chemical formula
-
4
3
Brij 35 C12H25(OCH2CH2)23OH 1.59 × 10
1198 22.2 40
288 51.4 62
-
SDS
CH3(CH2)11OSO3Na
8.06 × 10
chemical
formula
solubility
(M)
d
pKad pKa* e log Kowd
2.6 2.17
name
MW
2
CP
ClC6H4OH
0.182
128.6 8.5
three batch extractions with equal volumes of CH
CH Cl fractions were combined in a flask and dried over
anhydrous granular Na SO (>99%, 10-60 mesh, E. M.
Scientific). The CH Cl extracts from samples containing
2 2
Cl . The
a
Determ ined by surface tension m easurem ent at 22 ( 1 °C.
2
2
b
Hydrophile-lipophile balance, calculated using Davies and Rideal
equation (16). c
aggr, aggrigation num ber, i.e., average num ber of
surfactant m olecules per m icelle (17). Average of the data in ref 25.
2
4
N
d
2
2
e
surfactants were further transferred into another flask con-
taining 5-20 g of predried silica gel (20-600 mesh) to remove
surfactants. Silica gel (20 g) was also used to avoid emulsion
formation in the SDS (4 g/ L) solution during extraction for
product identification. This large amount of silica gel possibly
caused the failure of identification of OH-bearing products
such as catechol and cyclopentadienecarboxylic acid, which
can strongly be absorbed to the silica gel surface. Finally, the
extracts were passed through a glass wool filter (pore size
a
Excited-state pK (7).
0
.47 mm, Whatman), concentrated under vacuum, and
redissolved in 1 mL of CH
2
2
Cl containing 1-naphthol (99%,
Fisher Scientific Co.) as the internal standard for product
analysis.
Photolysis products were identified by GC-FID and GC-
MS (Hewlett Packard 5890 and 5989B), and identities were
verified by matching the retention times as well as the mass
spectra with authentic compounds. Cyclopentanecarboxylic
acid was used to quantitate the yield of cyclopentadienecar-
boxylic acid. Both the GC and GC-MS were equipped with
a HP-1 capillary column (0.2 mm × 12 m, 250 mm film
thickness). Phenol was quantitatively determined in the
presence of co-eluting 2CP by GC-MS using selected ion
monitoring (SIM). The selected ions for 2CP and phenol were
chosen as 128.0 and 94.0, respectively.
FIGURE 1. UV-VIS spectra of 2CP in water (pH 7.2), 2CP in surfactant
solutions, and UV lamp photon flux spectral profile. Lamp photon
flux data points measured with 1-nm bandpass (see text).
in this study were HPLC grade. All other chemicals were
reagent grade and used without further purification.
UV-VIS spectra were measured by a Cary 4E UV-VIS
spectrophotometer in 1 mm path length cells. Fluorescence
was measured by a Fluorolog-2 spectrofluorometer (Spex
Industries, Inc) using 10 mm cells and right angle fluorescence
collection.
Methods. 2CP (0.1 g/ L) solutions for photolysis were
prepared by syringe addition of 19.8 mL of pure 2CP liquid
(
specific gravity: 1.2634 g/ mL at 20 °C relative to the density
of water at 4 °C) into a 250-mL flask containing 200 mL of
prepared surfactant standard solutions. More surfactant
standard solution was added to the flask to give 250 mL total
volume. The solution was stirred overnight by a magnetic
stirrer before use. Surfactant standard solutions were pre-
pared in deionized water at two concentrations, 0.1 and 4.0
g/ L, for each of the two surfactants. Solutions of 2CP (0.1
g/ L) in deionized water and in organic solvents were also
prepared in a similar fashion. All the solutions were prepared
without pH manipulation or degassing unless otherwise
stated.
Results and Discussions
Distribution of 2CP between Aqueous an d Micellar
Pseudophase. Before evaluating the photolytic behavior of
2CP in surfactant solutions, the distribution of 2CP between
the micellar and aqueous pseudophase was determined. The
linear solvation energy relationship (LSER), given in eq 1,
allows calculation of the micellar pseudophase incorporation
-
1
coefficient, K
s
(M ) (27):
log K )
s
To determine the rate of 2CP photolysis, 350 mL of the
prepared solution was pipetted into a quartz cell with 1 mm
path length and closed with a Teflon-lined cap. The solutions
were irradiated in a Rayonet photoreactor (Model RPR-100,
Southern New England Ultraviolet) fitted with a merry-go-
round unit. Lamp maximum output (at 310 nm, supplied by
c + a∑
R + b â + pπ + rR + v(V / 100) (1)
2 2 2 2 x
∑
In eq 1, ∑R , ∑â , π , R , and V (cm mol-1) are parameters
3
2
2
2
2
x
that account for solute hydrogen bond acidity, hydrogen bond
basicity, dipolarity, excess molar refraction, and molar volume,
and c is a multi-linear regression constant. System-dependent
coefficients corresponding to ∑R , ∑â , π , R , and V are a,
b, p, r, and v, respectively. Literature values for the parameters
(28, 29) and coefficients (27) are listed in Table 2.
The micellar pseudophase incorporation coefficient, Ks,
is also defined as (27)
1
4
Southern New England Ultraviolet) was 2.32 × 10 photons
cm , and the total output (in the range of 250-370 nm)
was 1.0 × 10 photons cm
2
2
2
2
x
-
2
-1
s
1
6
-2 -1
s
(Figure 1). The photon flux
was determined at the face of the cell used for rate measure-
ments using an IL 700 spectral radiometer (International Inc.)
with 1 nm bandpass. The 2CP remaining in solution was
determined at various photolysis times using a HP1090 liquid
chromatograph (LC) (Hewlett Packard, Inc.) equipped with
a Hypersil ODS column (i.d. × length ) 2.1 × 200 mm, C18
K ) S / (S C )
(2)
s
mic
aq
D
where Smic and Saq are the concentration of solute in micellar
and aqueous pseudophase, respectively. The micellized
with particle size 5 mm). The solvent ratio of CH
3
CN:H
2
O
was linearly changed from 20:80 to 70:30 over 16 min. The
D
surfactant concentration (C ) is defined as the difference
3
5 8 2
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 31, NO. 12, 1997

Changes in the UV-VIS spectra of 2CP during photolysis
TABLE 2. Parameters and Coefficient of LSER Applied to
Surfactants and 2CP
are illustrated in Figure 2. The spectra in water and Brij 35
(
0.1 g/ L) solutions show very similar behavior, and both exhibit
isosbestic points. When the surfactant concentration was
increased above the cmc (4 g/ L), the spectra were very
different from what was observed for aqueous solutions. No
isosbestic points were observed, and the maximum of the
absorption band shifted blue as the reaction proceeded. These
results suggest that different photochemistry occurs in the
Brij 35 micelles. Similar spectra were observed for 2CP in
SDS, indicating that the above conclusions also apply for
SDS solutions, even though partitioning into the SDS micelles
is less than partitioning into Brij 35 micelles.
surfactant
c
a
b
p
r
v
Brij 35
SDS
-1.39
-0.62
1.62
-0.08
-3.83
-1.84
-0.37
-0.57
1.63
0.32
3.65
3.25
solute
∑r2
0.32
∑â2
0.31
π2
R2
Vx
2
CP
0.88
0.853
89.75
between the total surfactant concentration (C
) C - cmc). At the surfactant level of 4 g/ L utilized in
this study, C
t
) and the cmc
(
C
D
t
-
3
-3
Photolysis Rates and Quantum Yields. Photodegradative
losses of 2CP in water and surfactant solutions are plotted in
D
values were 3.18 × 10 and 5.83 × 10 M for
Brij 35 and SDS, respectively.
The molar solubility ratio (MSR), molar partitioning
coefficient (K
the form of ln(C
graph demonstrates that 2CP degradation, like that of
-chlorophenol (31), followed apparent first-order kinetics
0 t
/ C ) as a function of time in Figure 3. The
m
) (30), and micellar occupancy number (Nocc
)
4
can be related to K
s
by the following equations (Nocc is defined
in all solutions except Brij 35 and SDS at surfactant con-
centrations above the cmc. Degradation of 2CP in micellar
solutions of both Brij 35 and SDS above their cmc obviously
deviates from a simple first-order rate law at later photolysis
times; however, the initial rates (photolysis time e10 min) in
these solutions can be estimated using the first-order rate
as the average number of 2CP molecules partitioning in each
micelle):
Smic
MSR ) CD ) KsSaq
(3)
-1
law. The initial rates characterized by rate constant k
at photolysis times e10 min, are listed in Table 4.
p
(min ),
Smic
The commonly used formula for calculating quantum
yields in weakly absorbing solutions incorporates integrated
photon flux values over specified spectral regions (usually
Xm (Smic + CD)
Ks
K )
)
)
(4)
(5)
m
Xa
S V
V (K S + 1)
aq
w
w
s aq
∼
10 nm) and average molar extinction coefficients over the
same region (31-33). The lamp spectral profile in Figure 1
is not a plot of integrated light flux over 10-nm regions, but
rather the figure shows the photon flux measured at 10-nm
increments using a spectral radiometer having a 1 nm
bandpass. Equation 6 was used to calculate 2CP degradation
quantum yields. This equation is functionally equivalent to
that previously given elsewhere (31-33):
Smic
Nocc
)
) K S N
s aq aggr
CD
Naggr
In eq 4, X
pseudophase, X
solute in aqueous pseudophase (X
The molar volume of water (V
m
is the mole fraction of the solute in micellar
m
) Smic/ (Smic + C
D
). The mole fraction of the
) is defined as X
a
a
) Saq
V
w
.
-
1
jkp
w
) has a value of 0.0181 M at
φ )
(6)
2
5 °C, and Naggr is the micellar aggregation number (Table 1).
Values of K , and Nocc obtained
2.303∑Iiꢀi
s
, Smic, Saq, Smic/ Saq, MSR, K
m
from eqs 1-5 are listed in Table 3. Calculating these values
utilizes the knowledge that the total solute, STot, is known and
that STot ) Smic + Saq. The calculated values given in Table
In eq 6, I
i i
(photons cm- s-1) and ꢀ (M-1 cm-1) are the
2
extrapolated flux of incident light (described below) and the
measured molar absorptivity of 2CP at wavelength λ . The
and ꢀ in
must be in units of s . The photon
3
reveal that 2CP molecules are more highly associated with
i
2
0
constant j in eq 6 has a value of 6.023 × 10 for I
l
l
Brij 35 micelles than with SDS micelles. To the extent that
micellar association effects the photochemical degradation
of 2CP, differences in quantum efficiencies and product
distributions should be observed for 2CP photodegradation
in Brij 35 and SDS micellar solutions.
-
1
the given units, and k
p
flux values were linearly extrapolated (to every l nm) between
the measured points on the smooth profile (Figure 1) and
multiplied by the measured 2CP ꢀ values, and the summation
was carried out as shown in eq 6. Extrapolation of photon
flux values between successively measured points can be
made with good accuracy due to the broad and structureless
nature of the spectral profile of the lamp. All of the light
absorbed by the aqueous system is absorbed by the 2CP, and
the path length of the light is assumed to be equal to the cell
thickness of 1 mm. The calculated quantum yields of
degradation are given in Table 4.
Characteristics of UV-VIS Absorbance and Fluorescence
Em ission. UV-VIS absorption spectra of 2CP and the
photolysis lamp spectral profile are shown in Figure 1. Both
the shape and intensity of the 2CP (0.1 g/ L) absorption spectra
in surfactant solutions (0.1 g/ L, below the cmc) were very
similar to that in water. When the surfactant concentration
was increased to the 4 g/ L (>cmc), the absorption intensity
decreased in SDS, although the spectral shape did not change.
In Brij 35 solution (4 g/ L), however, the absorption intensity
increased and the vibrational fine structure was lost, although
the wavelength corresponding to the maxim absorbance (273
nm) did not shift. The change in the 2CP spectra reflect the
new molecular environment within the micelle and the
partitioning of 2CP into the micelle. The partitioning of 2CP
into Brij 35 micelles is greater than into SDS micelles, as
revealed by the data in Table 3.
Photodechlorination of 2CP in water and surfactant
-
solutions was confirmed by the production of free Cl , which
was measured by a chloride selective electrode and by a
colorimetric method (26). The chloride concentrations
obtained by the two methods were within experimental error.
The photochemical production of the free chloride, measured
electrochemically, is plotted in Figure 4 using eq 7 (34). In eq
7
, C
∞
is the chloride concentration at infinite time and is set
equal to the initial concentration of 2CP.
The fluorescence quantum yield from the undissociated
form of 2CP in water has been reported to be as low as Φ )
.002 (9). We were unable to detect fluorescence from 2CP
in any water or surfactant solution, even when the aqueous
solutions were adjusted to pH 2 using HCl.
0
(C - C )
∞
0
ln
) kt
(7)
(
C - C )
∞ t
VOL. 31, NO. 12, 1997 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
3 5 8 3

s m
TABLE 3. LSER Estimated K , Smic, Saq, MSR, K , and Nocc in Solutions of 2CP (0.1 g/L) and Surfactant (4.0 g/L)
surfactant
Ks (M-1)
Smic (M-1
)
Saq (M-1
)
Smic/Saq
MSR
Km
Nocc
-
-
4
4
-4
-4
Brij 35
SDS
191
29.6
2.94 × 10
1.15 × 10
4.84 × 10
6.63 × 10
0.607
0.173
0.0924
0.0196
9660
1604
3.70
1.22
The quantum yields in aqueous solution without surfac-
tant, compiled in Table 4, are close to the previously reported
values (7-9). With the presence of surfactants, the initial
quantum yield of the degradation was slightly enhanced in
SDS solution at sub-cmc level. Further increasing the SDS
concentration to 4 g/ L (above the cmc) resulted in no
significant increase in the rates of dechlorination or degrada-
tion; however, an increase in the quantum yield is observed.
This apparent discrepancy is accounted for by the change in
absorption spectra for 2CP in 4 g/ L surfactant concentrations
(
vide supra). In Brij 35, small changes in the degradation
and dechlorination quantum yields are observed below the
cmc; however, a sizable increase in quantum yields is observed
above the cmc (4.0 g/ L surfactant). The initial modest
degradation quantum yield enhancement appears to coincide
with the presence of the surfactant molecules in solution,
while the larger effect is observed for both surfactants upon
the formation of micelles. All the rates in 4.0 g/ L surfactant
solutions were accelerated and showed deviation from first-
order kinetics as the reaction proceeded.
To further study whether the observed acceleration could
be attributed to product-induced decomposition, 2CP pho-
tolysis was studied in the presence of added phenol. Solutions
of 2CP (0.1 g/ L) in Brij 35 (4 g/ L) solutions with varied
concentrations of added phenol (0-0.1 g/ L) were irradiated,
and chloride formation was monitored (Figure 5). It was
found that the 2CP initial dechlorination rate increased as
the phenol concentration increased (Figure 6), confirming
that a photoproduct (phenol) can sensitize 2CP decomposi-
tion.
FIGURE 2. Comparison of UV-VIS spectra of 2CP in water and Brij
5 solutions at various photolysis times.
3
The measured quantum yields of dechlorination are
comparable to the quantum yields of degradation in all water
or surfactant solutions examined, with the possible exception
of 2CP in SDS below the cmc. This result strongly suggests
that degradation of 2CP occurs mainly through an initial
dechlorination pathway. Similar results have been found in
studies of the photolysis of HCB and PCBs in surfactant
solutions (22-24). In SDS below the cmc, the degradation
efficiency is greater than the dechlorination efficiency. This
could be due to a non-dechlorination degradation path;
however, no products were identified to support this hy-
pothesis.
Photolysis Products and Mechanism . The photolysis
products and their yields in water and surfactant solution are
listed in Table 5. In water solution (pH 7.1) without surfactant,
catechol and cyclopentadienecarboxylic acid were the only
identified degradation products. Our results agree with those
reported by Boule et al. (8-10), who propose that cyclo-
pentadienecarboxylic acid results primarily from the 2CP
anion undergoing a Wolff rearrangement. Catechol is thought
to arise from a water nucleophilic displacement of chloride
from the singlet excited state of 2CP. The mass balance
FIGURE 3. Photodegradation of 2CP (0.1 g/L) in water and surfactant
solutions. Degradation rates follow first order in water and surfactant
<
cmc solutions and deviate from the first order in surfactant >cmc
solutions.
(
products plus remaining 2CP) was only 44% in water (pH )
The initial chloride concentration (C
0
) is zero, and the
7.1), and the low yield may be due to partial photooxidation
of these primary products (8). In surfactant solutions, when
Brij 35 and SDS concentrations were at 0.1 g/ L (below cmc),
phenol was identified as a photoproduct in addition to
catechol and cyclopentadienecarboxylic acid. The surfactant
is thought to serve as the hydrogen atom source for the 2CP
transformation to phenol, as shown in Figure 7. The
mechanism in Figure 7 shows two pathways to phenol, one
by C-Cl homolysis followed by free radical hydrogen atom
abstraction and the second involving a triplet carbene
mechanism requiring abstraction of two H atoms. The
concentration at time t is denoted as C
t
. Chloride production
followed a first-order rate law in water and surfactant solutions
at concentration below the cmc; however, chloride production
in micellar solutions of both Brij 35 and SDS above their cmc
values deviated from a simple first-order rate law at photolysis
times exceeding 10 min. The initial rates (photolysis time e
1
0 min) in these solutions can be approximated using the
first-order rate law. The initial rate constants and quantum
yields of dechlorination were calculated in the same fashion
as for 2CP degradation and are also given in Table 4.
3
5 8 4
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 31, NO. 12, 1997

TABLE 4. 2CP (0.1 g/L) Initial Photolysis Rate and Quantum Yield
rate, kp (min-1)
a
half-life, t1/2 (min)
quantum yield, Φ
a
solutions
H2O
Brij 35 (0.1 g/L)
Brij 35 (4.0 g/L)
SDS (0.1 g/L)
SDS (4.0 g/L)
degradation
dechlorination
degradation
dechlorination
degradation
dechlorination
0.0285 ( 0.0049
0.0272 ( 0.0009
0.0438 ( 0.0012
0.0348 ( 0.0001
0.0339 ( 0.0015
0.0222 ( 0.0023
0.0294 ( 0.0026
0.0394 ( 0.0030
0.0230 ( 0.0018
0.0289 ( 0.0028
24.3
25.5
15.8
19.9
20.4
31.2
23.6
17.6
30.1
24.0
0.0561
0.0639
0.0921
0.0665
0.0917
0.0437
0.0691
0.0829
0.0439
0.0806
a
Calculated using the data within 10-m in photolysis.
FIGURE 4. Chloride production from photodechlorination of 2CP (0.1
g/L) in water and surfactant solutions, as described by eq 7 (see
text). Dechlorination rates follow first order in water and surfactant
FIGURE 6. Initial photodechlorination rates for 2CP in Brij 35 (4 g/L)
as the function of sensitizer (phenol) concentrations.
<
cmc solution and deviate from the first order in surfactant >cmc
Photolysis of 2CP in dimethoxyethane (DME; 99.9%,
Aldrich) led to phenol formation (80% mass balance). This
result demonstrates that the polyoxyethylene (POE) func-
tionality in Brij 35 surfactants can serve as a good hydrogen
source for 2CP transformation to phenol. Photolysis of 2CP
in 25% (v/ v) of DME in water at pH 10 (above the 2CP ground-
solutions.
state pK
a
of 8.5) (2, 25) gave a high yield (38.7%) of
cyclopentadienecarboxylic acid and low yield (1.0%) of
phenol. When the pH of the solution was reduced to 2 (below
the 2CP singlet excited-state pK of 2.6) (7), catechol (11.4%)
a
was formed along with phenol (31.2%) and cyclopentadien-
ecarboxylic acid (23.9%). Formation of catechol at a pH below
the excited-state pK of 2CP likely results from an excited-
a
state nucleophilic displacement of chloride by water, as
discussed above. The observed high yield of phenol at pH
)
2 and low yield of phenol at pH ) 10 suggest that the major
mechanism to phenol formation is through a free radical
reaction (homolytic C-Cl scission or electron transfer
sensitized decomposition). Formation of cyclopentadien-
ecarboxylic acid at pH ) 2 (below the 2CP singlet excited-
FIGURE 5. Chloride production from photodechlorination of 2CP (0.1
g/L) in Brij 35 (4 g/L) sensitized by varying phenol concentrations.
state pK
a
of 2.6) is attributed to rapid dissociation of the 2CP
excited state.
Phenol has previously been suggested to sensitize the
photodegradation of 2CP by a charge transfer dehalogenation
reaction (7):
carbene mechanism would require that intersystem crossing
in the initially formed singlet carbene be kinetically competi-
tive with the Wolf rearrangement. Product studies from 2CP
photolysis in dimethoxyethane (vita infra) lend support to
this proposed carbene-mediated mechanism of phenol
formation. The mass balance was higher in the Brij 35
surfactant solutions than in water (pH ) 7.1). In solutions
of Brij 35 and SDS surfactants at 4 g/ L (above cmc), phenol
was the only observed product. Furthermore, it was found
that the yields of phenol were greater at surfactant concen-
trations above the cmc than those below the cmc. Phenol
yields and material balances were also found to be greater
in Brij 35 solutions than in SDS solutions. Increased yields
of phenol in Brij 35 and SDS micelles is consistent with 2CP
solubilization within the micelles and phenol formation by
a hydrogen atom abstraction reaction.
OH
OH
O•
OH
OH
Cl
*
+
•
RH
+
+ HCl
2
+ HCl
In our study, the fact that the dechlorination rate of 2CP in
the Brij 35 (4 g/ L) was promoted by the presence of added
phenol in the solutions confirmed the sensitizing action of
phenol. At a 2CP concentration of 0.1 g/ L, Nocc (the average
number of 2CP molecules per micelle) is calculated to be 3.7
in Brij 35 (Table 3). At the highest phenol concentration
investigated (0.1 g/ L), Nocc for phenol in Brij 35 is estimated
VOL. 31, NO. 12, 1997 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
3 5 8 5

TABLE 5. 2CP Photoproducts and Transformation Percentage
initial
pH
photolysis
time (min)
transformation as
degraded 2CP (%)
mass blance as
initial 2CP (%)
a
b
solutions
H2O
products
7.1
30
catechol
acidc
11.6
1.4
44
Brij 35 (4.0 g/L)
Brij 35 (0.1 g/L)
3.9
7.5
20
30
phenol
catechol
acidc
56.0
16.3
1.7
90
86
phenol
phenol
catechol
acidc
5.6
3.0
21.7
7.1
SDS (4.0 g/L)
SDS (0.1 g/L)
7.5
7.6
20
30
24
45
phenol
phenol
acidc
phenol
catechol
acidc
0.2
DME
3
1
45.0
38.7
1.0
11.4
23.9
31.2
80
58
2
5% DME
10
2
2
5% DME
1
84
phenol
a
Transform ation ) Cprod/(C
0
- C
t
t
) × 100%, where Cprod, C
0
c
t
, and C are the concentrations of product 2CP at tim e zero and 2CP at tim e t (M),
× 100. Cyclopentadienecarboxylic acid.
b
respectively. Mass balance ) (C
+ sum of Cprod)/C
0
form cyclopentandienic acid in aqueous solution. In the
absence of a good nucleophile, the ketene can photolyze to
give cyclopentadienyl carbene. When CH bonds are present
in surfactants (Brij 35 and SDS) or organic solvents (DME,
methanol, and hexane), the cyclopentadienyl carbene can
undergo a concerted insertion into the bond or abstract a
hydrogen atom followed by radical coupling to form a new
covalent bond. In cyclohexane, we observed the formation
of cyclohexanone, dicyclohexyl ether, and cyclohexene as
solvent-derived products, which suggest that the cyclopen-
tadienyl carbene is reacting from a triplet state.
The excited state of the undissociated 2CP can undergo
a nucleophilic displacement of chloride by water to yield
catechol. Phenol can be derived from the same excited state
by homolytic scission of the C-Cl bond followed by hydrogen
abstraction or by chloride elimination and deprotonation
following electron transfer sensitization. The resulting radical
leads to phenol production by hydrogen atom abstraction.
Role of Surfactant in Photolysis. The surfactant plays
two important roles in the promotion of 2CP photolysis: (1)
the formation of micelles supplies a nonaqueous “cage” that
retains phenol and catechol allowing for electron transfer
sensitized degradation of 2CP in a higher local concentration
and (2) the polyoxyethylene chain and/ or hydrocarbon chain
of surfactant can act as the H source to allow for phenol
production by reductive dechlorination of 2CP. When the
surfactant concentration is below the cmc, surfactants only
serve as the H source.
FIGURE 7. Mechanism of 2CP phototransformation.
to be 4.76 (assuming that the presence of 2CP does not effect
the partitioning of phenol into the micelles). These occupa-
tion numbers support the proposed phenol acceleration of
2
CP photolysis by electron transfer. Further, at 0.1 g/ L 2CP
in Brij 35 (4 g/ L), on average in the rapidly forming and
dissociating micelles, phenol produced by photolysis can
sensitize the decomposition of associated 2CP molecules
occupying the same micelle.
Sum m ary. A detailed mechanism has been given for the
photochemical decomposition of 2CP in dilute aqueous
solutions and in surfactant solutions. Secondary photolysis
of a ketene formed through a Wolff rearrangement of the
initial carbene has been demonstrated in dilute solutions. It
has also been shown that surfactants can likely serve as a
hydrogen source to assist in phenol production upon 2CP
photolysis. Additionally, phenol has been demonstrated to
catalyze photochemical decomposition of 2CP in micellar
media.
Photolysis of 2CP (0.1 g/ L) was also examined in cyclo-
hexane, hexane, and methanol. In each case, phenol was the
minor product, and the substituted cyclopentadiene deriva-
tives were the major products. The cyclopentadiene deriva-
tives of cyclohexane and hexane result from trapping
cyclopentadienyl carbene. The carbene is formed by sec-
ondary photolysis of cyclopentadienyl ketene, the product of
the Wolff rearrangement. In methanol, the ketene is trapped
by the nucleophilic solvent to give the cyclopentadienecar-
boxylic acid methyl ester.
Acknowledgments
An overall mechanism of the photochemical degradation
of 2CP is given in Figure 7. Dissociation of chloride from 2CP
excited anion results in the formation of a proposed carbene
intermediate that can further undergo a Wolff rearrangement
The authors gratefully acknowledge the insight and helpful
suggestions of Dr. Tom Autrey of Pacific Northwest Labora-
tory. This research was sponsored by the Division of Chemical
Sciences, Office of Basic Energy Sciences, U.S. Department
of Energy under Contractor DE-AC05-96OR22464 with Oak
Ridge National Laboratory, managed by Lockheed Martin
Energy Research Corp.
(
8) to form cyclopentadienyl ketene or intersystem cross to
the triplet manifold, eventually leading to phenol production
as discussed above. The ketene can undergo hydrolysis to
3
5 8 6
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 31, NO. 12, 1997

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X
ES9703279
X
Abstract published in Advance ACS Abstracts, October 1, 1997.
VOL. 31, NO. 12, 1997 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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