Novel photocatalytic mechanisms for CHCl3, CHBr3, and CCl3CO2 degradation and the fate of photogenerated trihalomethyl radicals on TiO2

Article abstract of DOI:10.1021/es960157k

The photocatalytic degradation of CHCl₃, CHBr₃, CCl₄, and CCl₃CO₂^- is investigated in aqueous TiO₂ suspensions. A common intermediate, the trihalomethyl radical, is involved in the degradation of each substrate except for CCl₃CO₂^-, CHCl₃ and CHBr₃ are degraded into carbon monoxide and halide ions in the absence of dissolved oxygen. The anoxic degradation proceeds through a dihalocarbene intermediate, which is produced by sequential reactions of the haloform molecule with a valence band hole and a conduction band electron. Carbon dioxide and halide ion are formed as the primary products during CHCl₃ degradation in the presence of oxygen. Under these conditions, the trihalomethyl radicals react rapidly with dioxygen. At pH > 11, degradation of the haloforms is enhanced dramatically. This enhancement is ascribed to photoenhanced hydrolysis. The secondary reactions of the trichloromethyl radical generated during CCl₄ photolysis is strongly influenced by the nature of the electron donors. Both ·CCl₃ and Cl^- production increase substantially when 2-propanol is present as an electron donor. A new photocatalytic mechanism for CCl₃CO₂^- degradation, which involves the formation of a dichlorocarbene intermediate, is proposed.

Full text of DOI:10.1021/es960157k

Environ. Sci. Technol. 1997, 31, 89-95
for CHCl3 degradation, which was based on the reaction of
Novel Photocatalytic Mechanisms
for CHCl3, CHBr3, and CCl3CO -
Degradation and the Fate of 2
Photogenerated Trihalomethyl
trichloromethyl peroxyl radical (Cl3COO•) whose chemistry
was well understood from homogeneous radiolysis (7-10).
Trichloromethyl radicals in oxygenated aqueous solutions
react rapidly with dioxygen (vide infra, eq 3) at a rate near
the diffusion limit (k ) 3.3 × 109 M-1 s-1) (10).
Our recent studies (11, 12) on the photocatalytic reduction
of CCl4, however, demonstrated that •CCl3 on the TiO2 surface
could react not only with an oxygen molecule but also directly
Radicals on TiO
2
•
with a conduction-band electron (eq 4). The CCl3 radical
W O N YO N G C H O I † A N D
M I C H A E L R . H O F F M A N N *
can be formed on TiO2 surface via CHCl3 oxidation through
valence band (VB) hole transfer (eq 1) or from CCl4 reduction
through conduction band (CB) electron transfer (eq 2). The
subsequent reactions of •CCl3 should follow similar pathways
in both systems.
W. M. Keck Laboratories, California Institute of Technology,
Pasadena, California 91125
CHCl3 + hvb+ (or OH) f •CCl3 + H+ (or H2O) (1)
•
The photocatalytic degradation of CHCl3, CHBr3, CCl4, and
CCl3CO - is investigated in aqueous TiO suspensions. A
CCl4 + ecb- f •CCl3 + Cl-
•CCl3 + O2 f •OOCCl3
(2)
(3)
(4)
2
2
common intermediate, the trihalomethyl radical, is involved
in the degradation of each substrate except for CCl3CO -.
2
CHCl3 and CHBr3 are degraded into carbon monoxide and
halide ions in the absence of dissolved oxygen. The anoxic
degradation proceeds through a dihalocarbene intermedi-
ate, which is produced by sequential reactions of the
haloform molecule with a valence band hole and a conduction
band electron. Carbon dioxide and halide ion are formed
as the primary products during CHCl3 degradation in the
presence of oxygen. Under these conditions, the trihalomethyl
radicals react rapidly with dioxygen. At pH > 11, degrada-
tion of the haloforms is enhanced dramatically. This
enhancement is ascribed to photoenhanced hydrolysis. The
secondary reactions of the trichloromethyl radical generated
during CCl4 photolysis is strongly influenced by the nature
•CCl3 + ecb- f :CCl2 + Cl-
•OOCCl3 f f CO2 + 3Cl-
:CCl2 + H2O f CO + 2HCl
(5)
(6)
The above argument opens up the possibility for recognition
of a new CHCl3 degradation pathway in which there is a redox-
mediated short circuiting of the VB holes and CB electrons.
The new pathway could make the photocatalytic degradation
of CHCl3 possible in the absence of dissolved oxygen by
producing CO instead of CO2 as a final product (eq 6).
In order to test the possibility of the alternative mechanism
(eqs 1, 4, and 6), the photocatalytic degradation of CHCl3 is
re-examined. In this work, we explore evidence for this novel
photocatalytic mechanism for CHCl3 and CHBr3 photooxi-
dation. Reactions of trichloromethyl radical, which are
generated from the CCl4/ electron donor system, are compared
to those of the CHCl3 system in an attempt to ascertain the
general fate of the trihalomethyl radical on illuminated TiO2.
In addition, trichloromethyl radical generation from trichlo-
roacetate photooxidation via a photo-Kolbe process (eq 7)
(1, 13, 14) is investigated.
•
of the electron donors. Both CCl3 and Cl- production
increase substantially when 2-propanol is present as an
electron donor. A new photocatalytic mechanism for CCl3CO -
2
degradation, which involves the formation of a dichloro-
carbene intermediate, is proposed.
Introduction
Halogenated organic compounds are the most frequent and
important chemical contamitants in a wide range of environ-
ments. In addition, their general toxicity raises serious
concerns about their impacts on human health. The deg-
radation of halogenated compounds is known to proceed
through a series of free radical reactions. TiO2 photocatalysis,
which has been demonstrated to be effective for mineraliza-
tion of a variety of organic compounds (1-3), involves the
production of free radicals on the illuminated TiO2 surface.
However, our knowledge about the fate of carbon-centered
radicals generated during photocatalysis is far from complete.
The reaction mechanisms in heterogeneous radical chem-
istry are often different from their homogeneous counterparts
due to the influence of the solid surface (4). However, many
suggested mechanisms for photocatalytic reactions are
inferred from well-documented homogeneous reactions.
Previous studies (5, 6) proposed a photocatalytic mechanism
Cl3CCO2- + hvb+ f •CCl3 + CO2
(7)
Experimental Section
Materials, Apparatus, and Operation. Degussa (P25) TiO2,
which is known to be a mixture of 80% anatase and 20% rutile
with an average particle size of 30 nm and BET surface area
of ∼50 m2/ g (15), was used as a photocatalyst without further
treatment. All TiO2 suspensions were prepared at a con-
centration of 0.5 g/ L in water purified with a Milli-Q UV Plus
system (resistivity, 18.2 MΩ‚cm). The TiO2 suspension was
dispersed by simultaneous sonication and shaking in an
ultrasonic cleaning bath (Branson 5200). The suspension
was then bubbled with O2, N2, or a mixture (O2/ N2) at a fixed
ratio for 1 h before addition of the organic substrate. The
suspension was stirred magnetically throughout each experi-
ment. Saturated solutions of CCl4 (5 mM), CHCl3 (63 mM),
and CHBr3 (12 mM) were prepared by stirring in an excess
of each organic liquid in water, and solutions of a desired
concentration were then prepared by dilution. For anoxic
(i.e., oxygen free) photodegradation experiments, separate
* To whom correspondence should be addressed; e-mail:
mrh@cco.caltech.edu; phone: 818-395-4391; fax: 818-395-3170.
† Present address: Atmospheric Chemistry Element, Earth and
Space Sciences Division, Jet Propulsion Laboratory, Pasadena, CA
91109.
9
S0013-936X(96)00157-5 CCC: $14.00
1996 Am erican Chem ical Society
VOL. 31, NO. 1, 1997 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 8 9

saturated stock solutions were prepared under nitrogen. Fresh
stock solutions of CHBr3 were made daily because of its slow
hydrolytic decomposition. The pH of the suspension was
adjusted with 1 N HClO4 or 1 N NaOH and was measured
before and after the irradiation. CCl4 (Baker), CHCl3 (Baker),
CCl3COONa (Aldrich), and CHBr3 (Aldrich) were used as
received.
phase were sampled, filtered, and injected into the same glass
vial descibed above. In order to drive aqueous carbonate
species into the gas phase (for CO2 analysis), 100 µL of
concentrated sulfuric acid was added. The vial was then
shaken and left for at least 30 min for equilibration before GC
analysis. Standard solutions of sodium carbonate for CO2
analysis were prepared by the same method.
Irradiations were performed with a 1000-W Xe arc lamp
(Spindler and Hoyer) operated at 910 W. Light was filtered
through a 10-cm IR water filter and, when light intensities
needed to be measured, a UV band pass filter (310-400 nm,
Corning). The filtered light was focused through a convex
lens onto a reactor cell loaded with the TiO2 suspension. Light
intensity measurements were performed by chemical acti-
nometry using (E)-R-(2,5-dimethyl-3-furylethylidene) (iso-
propylidene) succinic anhydride (Aberchrome 540) (16). A
typical light intensity through a UV band pass filter was ∼1.4
The Cl- production after the photolysis was measured
with an Orion chloride ion-selective electrode (Model 96-
17B). Gaseous samples from the gas collecting tube and the
head space of the glass vials were analyzed by a GC (Carle
AGC series 400) equipped with a thermal conductivity detector
and columns of Porapak QS and 5-Å molecular sieve particles.
Helium was used as the carrier gas. Duplicate measurements
were made for each sample with an injection volume of 20
µL. Standards of gaseous CO and CO2 were made by mixing
each gas with He at known ratios in the gas collecting tube.
The total numbers of CO and CO2 molecules generated during
the photolysis were calculated by using Henry’s law constants
based on the assumption of liquid-gas equilibrium.
× 10-3 Einstein L-1 min-1
.
Two distinct types of photolysis experiments were carried
out. One set of experiments was focused on the formation
of halogenated intermediates and halide ions and the
disappearance of substrate compounds as a function of
irradiation time; another set was focused on the determination
of gaseous CO and CO2 generation after 2 h of irradiation.
The two sets of photolysis experiments were performed in
different reactors as described below.
Photolysis and Analysis of Halogenated Com pounds and
Halide Ions. For the experiments in which halogenated
intermediates and halide ions were determined, a 35-mL
quartz reactor cell was used. After gas saturation, reagents
(an aliquot of the saturated stock solution and alcohols as an
electron donor for CCl4 degradation) were added into the
reactor with minimal head space through a rubber septum.
Light was irradiated through a UV band pass filter.
Sample aliquots were obtained with a 1-mL syringe, filtered
through a 0.45-µm nylon filter, and injected into a 2.5-mL
glass vial having a screwtop cap and a Teflon-faced septum.
Halogenated compounds and their degradation intermediates
were extracted with 0.5 mL of pentane immediately after
sampling. Sample vials were stored at 4 °C in the dark up to
48 h before analysis. The degradation and formation of
halogenated compounds and intermediates were followed
chromatographically with a Hewlett-Packard (HP) 5880A gas
chromatograph (GC) equipped with a 63Ni electron capture
detector and a HP-5 column (crosslinked 50% PhMe silicone,
25m × 0.32 mm × 1.05 µm). Nitrogen was used as the carrier
gas. The GCs were calibrated daily with external standards
(CCl4, CHCl3, CHBr3, C2Cl4, and C2Cl6) and duplicate mea-
surements were made for each sample. The formation of
perbromoethylene (C2Br4), whose authentic standard was not
available, was identified with a GC (HP 5890 II) connected to
a mass selective detector (HP 5972A). The aqueous phase in
the sampling vial was analyzed by ion-exchange chroma-
tography (IC) for halide ions. The IC system was a Dionex
Bio-LC system equipped with a conductivity detector and a
Dionex OmniPac PAX-500 column (8 µm × 5 mm × 250 mm).
Photolysis and Analysis of Gaseous Products. The
experiments determining CO and CO2 used a Pyrex reactor
with a total volume of 100 mL. The Pyrex reactor was
connected to a gas collection tube (total volume 145 mL, Ace
Glass) through a glass joint, which was evacuated with a closed
stopcock. After N2 or O2 purging, 0.5 mL of CHCl3 or CCl4 was
added directly into the suspension. By the end of photolysis,
most of CHCl3 dissolved into the aqueous suspension while
excess CCl4 droplets remained at the bottom of the reactor.
A He-filled balloon was then attached to a top opening of the
reactor in order to collect gaeous product evolved during the
photolysis. The full band irradiation (with no filter) lasted
for 2 h. At the end of each photolysis, the evacuated gas
collection tube was filled with the gas mixture in the balloon
and taken for GC analysis. Two aliquots of 1-mL aqueous
Results
Results of the photocatalytic degradations of CHCl3 and CHBr3
under N2 saturation are shown in Figure 1. Even though
dioxygen was considered to be essential for the degradation
of these compounds (5 ,6, 17, 18), they were decomposed
slowly in the N2-saturated photocatalytic systems. The
linearity of halide production over the whole irradiation period
indicates that the contribution from the residual oxygen,
which remains even after vigorous N2 purging, to total halide
production is insignificant. In particular, the degradation
rates were greatly enhanced at pH 12 for both CHCl3 and
CHBr3. Although CHCl3 and CHBr3 can be degraded through
base-catalyzed hydrolysis (eqs 8-10) in the absence of light
(19, 20), the hydrolysis rate at pH 12 is small compared to the
net photolysis rate as shown in Figure 1.
CHX3 + OH- f CX3- + H2O (X ) Cl- or Br-) (8)
CX3- T :CX2 + X-
(9)
:CX2 + H2O f CO + 2H+ + 2X-
(10)
The measured quantum yields for the halide production were
ΦCHCl ) 0.021 (CHCl3 at pH 12), ΦCHCl ) 0.0034 (CHCl3 at pH
3
3
5), ΦCHBr ) 0.018 (CHBr3 at pH 12), and ΦCHBr ) 0.0019 (CHBr3
3
3
at pH 5).
The effects of the dissolved oxygen concentration on the
photolysis rate of CHCl3 are shown in Figure 2. These results
show that O2 increases the net dechlorination rate. The
stoichiometry for the complete mineralization of CHCl3 to
CO2 in the presence of oxygen is (5, 17)
CHCl3 + H2O + 1/ 2O2 f CO2(g) + 3HCl(aq)
∆H298 ) -113.1 kcal/ mol (11)
For CHCl3 photodegradation in an air-saturated solution
(Figure 2a), the Cl- production, however, went beyond the
level of concentration that corresponded to the stoichiometric
consumption of dissolved O2. This result is not consistent
with the observations of Kormann et al. (5). They observed
that [Cl-] increased linearly up to ∼1.4 mM, at which point
the chloride production rate abruptly decreased to a much
smaller value as the O2 was depleted.
The normal effect of [O2] on the dechlorination rate (Figure
2b) exhibited Langmurian dependence (21, 22) except that
9
9 0 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 31, NO. 1, 1997

FIGURE 2. (a) Chloride generation from the photolysis of 6 mM
CHCl3 solutions (pH 4.5) that were saturated with O , air, and N2,
i
2
respectively, prior to irradiation. The dotted line represents the
chloride level corresponding to the stoichiometric consumption of
dissolved oxygen (0.25 mM) in an air-saturated solution. (b)
Dependence of the photocatalytic chloride generation rates in 6
mM CHCl3 solution on the dissolved oxygen concentration. The
dechlorination rates were measured over an initial 1-h photolysis
period. The concentration of dissolved oxygen was varied by
changing the ratio of the flow rates of O and N2 bubbling gases and
2
calculated by assuming that the dissolved oxygen concentration in
an O -saturated solution was 1.27 mM. The solid line is a fit to eq
2
12.
lent of O2 adsorption constant, and kCl,0 is a photodechlo-
rination rate in the absence of dissolved oxygen. The fitted
values were kCl ) 11.5 µM min-1, KO2 ) 3.4 × 103 M-1 and kCl,0
) 3.4 µM min-1. Mills et al. (23) reported a value of KO2 ) 3.6
× 103 M-1 in a TiO2 (Degussa P25)/ 4-chlorophenol system
while Okamoto et al. (24) obtained KO2 ) 8.9 × 103 M-1 in a
FIGURE 1. Production of halide ions from the photocatalytic
decomposition of (a) 6 mM CHCl3 and (b) 6 mM CHBr3 and (c) the
production of halogenated byproducts at pH 12 (C Cl4 and C Cl6 from
2
2
6 mM CHCl3 solution and C Br4 from 6 mM CHBr3 solution) as a
2
function of irradiation time. Suspensions were saturated with N2
prior to the photolysis in a sealed reactor at the initial pH 5 or pH
12. The halide generations from dark hydrolysis of the same substrates
at pH 12 are shown as well. There was no hydrolysis at pH 5. The
TiO2 (anatase)/ phenol system. However, a value of KO
)
2
(1.3 ( 0.7) × 105 M-1, which Kormann et al. (5) reported for
a TiO2 (Degussa P25)/ chloroform system, appears to be an
overestimate.
concentration of C Br4 in panel c could not be quantified since its
2
authentic sample was not available.
The pH-dependent dechlorination rates and formation of
intermediates during CHCl3 degradation are compared
between O2- and N2-saturated suspensions in Figure 3.
Production of Cl- in O2-saturated solutions increased expo-
nentially at pH > 10, while the N2-saturated systems showed
a moderate increase in [Cl-]. The calculated hydrolysis (25)
of CHCl3 as a function of pH (dotted line in Figure 3a) shows
that the Cl- production from hydrolysis was about 100-1000
times smaller than that due to photolysis. The formation of
C2Cl6 and C2Cl4 (Figure 3b) was only significant at pH > 11.5,
the dechlorination rate was non-zero at [O2] ) 0.0 mM. The
data of Figure 3 are fitted to
d[Cl-]
dt
KO [O2]
2
) k
+ kCl,0
(12)
Cl 1 + KO [O2]
2
where kCl is a photodechlorination rate constant in the
presence of dissolved oxygen, KO2 is a photochemical equiva-
9
VOL. 31, NO. 1, 1997 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 1

TABLE 1. Chloride Production (µM) after 1-h Photolysisa of 6
mM Trichloroacetate Solution on TiO as a Function of pH
2
pH
2.1
3.0
5.8
12.1
O2-saturated
N2-saturated
870
840
700
680
520
740
400
420
a
Light illum ination through an IR water filter and a UV band pass
filter onto a 35-m L quartz reactor with an intensity of 1.4 × 10-3 Einstein
L-1 m in-1
.
observation is consistent to those of Chemseddine and Boehm
(14). The dechlorination rates increased gradually with
lowering pH.
In our previous study (11), we reported that the photore-
ductive decomposition of CCl4 on TiO2 was strongly depend-
ent on the nature of the electron donors. In order to
investigate how the electron donor affects the fate of
trichloromethyl radical (generated through eq 2) on TiO2, the
production of chloride and chlorinated byproducts (CHCl3,
C2Cl6, and C2Cl4), which are derived from the trichloromethyl
radical, were measured in the CCl4/ electron donor system
and summarized in Table 2. The chlorinated byproduct
formation varied greatly as a function of the specific electron
donor. The addition of 2-propanol resulted in enhanced
concentration of byproducts, while methanol showed a
moderate effect. tert-Butanol had little effect.
The production of CO2, CO, and Cl- from the photolysis
of CHCl3 is compared in Table 3. CO was found as the main
product of chloroform photolysis in the absence of O2 and
at high pH. At pH < 11 and in the absence of dissolved oxygen,
CHCl3 degradation was slow. In O2-saturated suspensions,
on the other hand, CO2 was determined to be the principle
product and CO was the minor product even at high pH. At
low pH under N2-saturation, the analytical method was not
accurate and sensitive enough to distinguish between back-
ground CO2 and photogenerated CO2 and CO. The stoichio-
metric balance between Cl- and gaseous products was
reasonable. However, the Cl- level was consistently a little
higher than the sum of CO and CO2; this result suggests the
presence of other byproducts.
FIGURE 3. Effect of the initial pH of TiO suspension on the
2
photocatalytic production of (a) chloride and (b) C Cl4 and C Cl6 in
2
2
6 mM CHCl3 solutions, which were saturated with O or N2 prior to
2
irradiation. Concentrations were measured after 1-h irradiation. The
dotted line in panel a represents calculated pH-dependent chloride
production after 1-h dark hydrolysis.
Similar analyses for CCl4 are presented in Table 4. Both
CO and CO2 were detected as photolysis products. In
addition, photolysis of the added electron donors in the
absence of CCl4 did not produce detectable levels of CO.
Therefore, CO appears to have originated directly from CCl4
degradation. Even though the chloride production under O2
saturation was about doubled from that under N2 saturation,
CO2 generation was enhanced more substantially in the O2-
saturated system. Based on this observation, it appears that
the majority of evolved CO2 originated from the electron donor
oxidation and not from CCl4 reduction. The sum of total CO
and an undetermined fraction of total CO2 (which came from
CCl4) accounts for only a small fraction of total Cl-. In the
present photolysis system, where excess CCl4 droplets re-
mained at the bottom of reactor, most of chlorinated radical
intermediates seemed to react with the electron donor or to
be scavenged into the CCl4 liquid phase, thus preventing their
subsequent degradation.
and their concentrations were higher in N2-saturated solutions
than in O2-saturated solutions.
Due to the high substrate concentration (6 mM) used in
this study, dimerization products of the intermediate radicals
were detected (eqs 13-15).
2•CCl3 f Cl3CCCl3
2:CCl2 f Cl2CdCCl2
2:CBr2 f Br2CdCBr2
(13)
(14)
(15)
The time-dependent production of the dimerized products
from CHCl3 and CHBr3 degradation at pH 12 are shown in
Figure 1c. Both C2Cl6 and C2Cl4 from CHCl3 degradation were
detected as byproducts while only C2Br4 (not C2Br6) was
detected from CHBr3 degradation. The concentration of
C2Cl6 reached its maximum in 1 h. The concentrations of
C2Cl4 and C2Br4 rose continuously. In control reactions, which
were performed in the dark, a trace amount of C2Br4 and 0.2
µM C2Cl4 were formed after 2.5 h from a dimerization of
hydrolytically produced dihalocarbenes (eqs 8 and 9).
In order to examine the possibility of trichloromethyl
radical production during trichloroacetate photooxidation
(eq 7), C2Cl6 and C2Cl4 were sought as byproducts from the
photolysis of trichloroacetate on TiO2. No C2Cl6 was detected
throughout the photolysis runs listed in Table 1. However,
a trace of C2Cl4 was found in some cases. Few differences in
the dechlorination rates were found under O2 and N2; this
Discussion
Experimental results presented above clearly show that CHCl3
and CHBr3 are photocatalytically degraded into CO in the
absence of dissolved oxygen. These results are consistent
with a “short-circuit” mechanism that involves reactions of
•
CHCl3 with a VB hole (or OH) (eq 1) and a CB electron (eq
4). The overall stoichiometry for this degradation pathway
can be written as
CHCl3 + H2O f CO(g) + 3HCl(aq)
∆H298 ) -45.6 kcal/ mol (16)
9
9 2 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 31, NO. 1, 1997

TABLE 2. Concentrations of Cl-, CCl6, and C Cl4 Produced after 1-h Photolysisa of 1 mMCCl4 in Air-Equilibrated TiO Suspensions
2
2
2
in the Presence of tert-Butanol (t-BuOH), 2-Propanol (2-PrOH), and Methanol as Electron Donors
b
pH 2.7
pH 5
pH 12
Cl-
CHCl3
C Cl6
2
C Cl4
2
Cl-
CHCl3
C Cl6
2
C Cl4
2
Cl-
CHCl3
C Cl6
2
C Cl4
2
(mM)
(µM)
(µM)
(µM)
(mM)
(µM)
(µM)
(µM)
(mM)
(µM)
(µM)
(µM)
no added alcohol
0.1 M t-BuOH
0.1 M 2-PrOH
0.1 M CH3OH
0.02
0.22
1.9
ndc
nd
51.3
4.7
traced
trace
0.51
nd
nd
9.1
0.56
0.03
0.17
2.2
nd
trace
trace
0.56
0.08
nd
nd
20.1
0.34
0.09
0.81
2.5
nd
nd
20.6
4.4
trace
trace
trace
trace
nd
trace
7.5
2.2
75.1
1.2
0.04
0.08
0.14
2.1
9.9
a
Light illum ination through an IR water filter and a UV-band pass filter onto a 35 m L quartz reactor with an intensity of 1.4 × 10-3 Einstein L-1
m in-1
.
Initial pH before irradiation. nc, not detected. Trace am ount <30 nM.
b
c
d
case, the enhanced rates were attributed to an increase in the
rate of CB electron transfer and the base-catalyzed hydrolysis
of dichlorocarbene. The same general arguments can be
applied to the reactions of eqs 4 and 6. However, there is no
reason why these effects should be enhanced in O2-saturated
suspensions. In addition, CO2 production (Table 3) in the
presence of O2 was enhanced significantly at high pH.
Furthermore, the rates of formation of C2Cl6 and C2Cl4 (Figure
3b) also show drastic increases above pH 11. However, during
CCl4 photoreduction the same byproducts were shown to
decrease at high pH (11, 12). This trend is supported by the
data presented in Table 2.
In order to account for these observations, a third
mechanistic pathway for CHCl3 and CHBr3 degradation is
proposed. Even though the base-catalyzed hydrolyses of the
haloforms (eqs 8-10) are much slower than the corresponding
photolysis rates (Figures 1 and 3a), they do increase expo-
nentially with pH. The rate-determining steps in the hy-
drolyses involve the release of a halide ion from a trihalo-
carbanion (eq 9) (19). On the photoilluminated TiO2 surface,
however, an alternative pathway for a trihalocarbanion is
possible as follows.
TABLE 3. CO , CO, and Cl- Generation after 2-h Photolysisa
2
of CHCl3 on TiO
2
-
NCO
NCO
NCl
NCO + NCO
2
2
pH/ saturated gas
(µmol) (µmol) (µmol)/3c
(µmol)
(12.2, 11.9)b/N2
(12.2, 11.6)/O2
(12.2, 12.2)/no light
(11.9, 11.6)/N2
(11.9, 10.3)/O2
(11.4, 10.9)/N2
(11.4, 3.2)/O2
(10.2, 8.6)/N2
(10.2, 2.7)/O2
(5.0, 4.1)/N2
3
160
3
3
162
1
49
1
52
1
149
12
9
56
3
11
ndd
nd
nd
nd
nd
187
187
17
97
163
25
90
4
152
172
12
59
165
12
49
1
60
2
47
52
1
41
(5.0, 2.7)/O2
41
a
Light illum ination through an IR water filter only onto a 100 m L
Pyrex reactor; [TiO2] ) 0.5 g/L in the presence of near saturated CHCl3
(0.5 m L/100 m L) in the reactor. bThe first and second num bers in the
parentheses are the pH of the suspension before and after the photolysis,
c
respectively. The num bers of chloride ions are divided by 3 to m ake
them equivalent to those of CO2 and CO on per CHCl3 m olecule basis.
d
nd, not detected within the sensitivity lim it of this experim ent.
CX3- + hvb+ f •CX3
(17)
TABLE 4. CO , CO, and Cl- Generation after 2-h of
Photolysisa o2f CCl4 on TiO with tert-Butanol (t-BuOH) or
2
The observed polarographic cathodic wave of trichloromethyl
radical (•CCl3 + e- f CCl3-) has a half-wave potential of E1/ 2
≈ 0.0 V (vs NHE) (26). Since the VB hole potential of TiO2
2-Propanol (2-PrOH) Present as Electron Donors
-
NCO
(µmol)
NCO
(µmol)
NCl
2
b
pH /gas/electron donorc
(µmol)/4d
(at pH 7) is +2.7 V, there is a large thermodynamic driving
•
force for the reaction of eq 17. When O2 is present, the CX
3
12.2/N2/t-BuOH
12.2/O2/t-BuOH
12.2/N2/2-PrOH
12.2/O2/2-PrOH
5.0/N2/2-PrOH
5.0/O2/2-PrOH
4
33
1
86
1
4
1
11
3
1
nd
25
38
58
115
9
radical is rapidly scavenged via eqs 3 and 5. As a consequence,
the equilibrium deprotonation step (eq 8) is shifted to the
right. This, in turn, results in an increase in the rates of CHCl3
degradation and byproduct formation with increasing pH. In
36
48
•
the absence of O2, the CX radical reacts with a CB electron
3
a
to form :CX (eq 4); this pathway is consistent with the slower
Light illum ination through an IR water filter only onto a 100 m L
2
increase of Cl- production in N2-saturated systems as shown
in Figure 3a. Since the above phenomenon is closely related
to the net effects of base-catalyzed hydrolysis, we name the
process “photoenhanced hydrolysis”.
Pyrex reactor; [TiO2] ) 0.5 g/L in the presence of excess liquid CCl4 (0.5
b
m L) in the reactor. Initial pH of the suspension before the light
c
illum ination. The electron donor concentration for each experim ent
d
was 0.1 M. The num bers of chloride ions are divided by 4 to m ake
them equivalent to those of CO2 and CO on per CCl4 m olecule basis.
The proposed photocatalytic mechanisms for CHCl3 and
CHBr3 degradation on TiO2 are summarized in Figure 4. The
mechanism of Figure 4a represents photooxidation in the
presence of dissolved oxygen, as proposed by Kormann et al.
(5). This mechanism is the major pathway for a haloform
degradation under normal conditions in which oxygen is
present and hydrolysis is negligible (pH < 10). The mech-
anism of Figure 4b becomes a major contributor under anoxic
conditions. The rate of this reaction in the absence of oxygen
is much slower than that achieved in pathway a because there
is no efficient CB electron scavengers available except for the
transient, trihalomethyl radicals. When a sufficient amount
of oxygen is present at the TiO2 surface, the mechanisms of
Figure 4, panels a and b, proceed concurrently. The
experimental observation (Figure 2a) that Cl- generation in
an air-saturated TiO2 suspension was linear beyond the
stoichiometric consumption of O2 supports this argument.
However, this reaction (eq 16) is preferred thermodynamically
only under anoxic conditions. In O2-saturated suspensions,
most of the •CCl3 radicals react with O2 (eq 3) to produce CO2
as suggested previously (5, 8). Therefore, the heterogeneously
photogenerated trichloromethyl radical exhibits two branch-
ing pathways; the branching ratio is dependent on the
availability of dioxygen on the surface.
Even though both pathways involving the trichloromethyl
radical lead to the full degradation of CHCl3, several experi-
mental observations can not be readily explained by these
alternative mechanisms. For example, the CHCl3 dechlori-
nation rates (Figure 3a) increase rapidly at pH > 10 in O2-
saturated suspensions while d[Cl-]/ dt rises slowly in N2-
saturated suspensions. Similar behavior was reported for
the kinetics of CCl4 dechlorination on TiO2 (11, 12). In that
9
VOL. 31, NO. 1, 1997 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 3

FIGURE 4. Three proposed photocatalytic mechanisms of CHX (X ) Cl or Br) degradation on TiO and a dark hydrolysis reaction are
3
2
illustratively compared.
At higher pH, the rate of dehalogenation can be increased
due to enhancement in both the CB electron-transfer pathway
and dihalocarbene hydrolysis. The mechanism of Figure 4c
becomes important only when deprotonation of the haloform
(eq 8) occurs at pH > 10. When oxygen is present, the
acceleration of the degradation rate is dramatic. Without
oxygen, the degradation pathway is similar to the mechanism
of Figure 4b. However, at this time, it is not clear how much
of the net anoxic degradation can be attributed to mechanisms
b and c, respectively. The intrinsic hydrolysis pathway of
Figure 4d is insignificant under the conditions employed in
this study.
experimentally. The chain propagation of eqs 18 and 19 is
inoperative in the CCl4/ tert-butanol system because tert-
butanol does not form R-hydroxyalkyl radicals. The extent
of chain propagation in our study seems to be limited due
to the competition from O2 addition to the carbon-centered
radicals.
The net production of C2Cl6 in the CCl4/ ROH systems is
less than that of C2Cl4 while the CHCl3 system showed the
opposite trend. At pH 12, only traces of C2Cl6 were found
(Table 2) in contrast to its dramatic increase in the CHCl3
system at the same pH (Figure 3b). This behavior is consistent
with the following secondary reaction induced by the electron
donor (eq 20) (27).
In contrast to the CHCl3 system, the trichloromethyl radical
in CCl4 photoreduction appears only in the presence of
appropriate electron donors. Since the electron donors (ROH)
scavenge VB holes efficiently and induce secondary reactions
through the formation of R-hydroxyalkyl radicals (11, 27, 28),
the chemistry of •CCl3 with efficient electron donors is
RR′C˙ OH + •CCl3 f RR′CO + :CCl2 + HCl
(20)
•
The oxidation potentials of CH2OH (-0.74 V vs NHE) and
•C(OH)(CH3)2 (-1.06 V) (27) are strong enough to directly
•
substantially different from that of CCl3 radical generated
•
reduce CCl3 (0.0 V). The reduced surface concentration of
without electron donors. As shown in Table 2, the specific
electron donors have a strong influence on the production
of Cl- and chlorinated byproducts. The relative effect of a
wide range of electron donors on CCl4 dechlorination has
been discussed previously (11).
The rates of formation of chlorinated hydrocarbon byprod-
ucts and Cl- are enhanced substantially when 2-propanol is
employed as an electron donor (Table 2). This effect can be
explained by the following chain propagation steps:
•CCl3 results in fewer net dimerizations to produce C2Cl6 while
at the same time :CCl2 dimerization to produce C2Cl4 is
enhanced.
In general, photocatalytic degradation of organic com-
pounds on illuminated TiO2 is thought to be initiated through
hydrogen atom abstraction by surface-bound •OH radical (13,
14). Therefore, organic compounds having no abstractable
-
hydrogen atoms such as CCl4 and CCl3CO2 exhibit much
slower overall degradation rates (13, 14, 18, 30). In the case
of CCl3CO2-, the slow degradation rates were attributed to
either the photo-Kolbe reaction (eq 7) (1, 3, 14) and/ or the
photoreduction by CB electrons (eq 21) (14, 31), although it
was not clear which was the predominant pathway mech-
anism.
•CCl3 + (CH3)2CHOH f CHCl3 + (CH3)2C˙ OH (18)
CCl4 + (CH3)2C˙ OH f •CCl3 + (CH3)2CO + HCl (19)
Considering the bond strength of H-C(CH3)2OH (91 ( 1 kcal/
mol) and H-CCl3 (94 ( 1 kcal/ mol) (29), this chain propaga-
tion sequence reaction is energetically feasible. With metha-
nol C-H bond strength of 94 ( 2 kcal/ mol, the chain
propagation of eqs 18 and 19 should be less efficient as seen
CCl3CO2- + ecb- f •CCl2CO2- + Cl-
(21)
Supporting evidence for the photo-Kolbe mechanism is
that CO2 is formed from trichloroacetate degradation in N2-
9
9 4 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 31, NO. 1, 1997

(3) Ollis, D. F., Al-Ekabi, H., Eds. Photocatalytic Purification and
Treatment of Water and Air; Elsevier: Amsterdam, 1993.
(4) Mao, Y.; Scho¨ neich, C.; Asmus, K.-D. J. Phys. Chem. 1992, 96,
8522.
(5) Kormann, C.; Bahnemann, D. W.; Hoffmann, M. R. Environ. Sci.
Technol. 1991, 25, 494.
(6) Bahnemann, D.; Bockelmann, D.; Goslich, R. Solar Energy Mater.
1991, 24, 564.
(7) Ko¨ ster, R.; Asmus, K.-D. Z. Naturforsch. 1971, 26B, 1108.
(8) Mo¨ nig, J.; Bahnemann, D.; Asmus, K.-D. Chem.-Biol. Interact.
1983, 45, 15.
(9) Mo¨ nig, J.; Asmus, K.-D. J. Chem. Soc. Perkin Trans. 2 1984, 2057.
(10) Lal, M.; Scho¨ neich, C.; Mo¨ nig, J.; Asmus, K.-D. Int. J. Radiat.
Biol. 1988, 54, 773.
(11) Choi, W.; Hoffmann, M. R. Environ. Sci. Technol. 1995, 29, 1646.
(12) Choi, W.; Hoffmann, M. R. J. Phys. Chem. 1996, 100, 2161.
(13) Mao, Y.; Scho¨ neich, C.; Asmus, K.-D. J. Phys. Chem. 1991, 95,
10080.
saturated TiO2 suspensions (14). On the other hand, Chem-
seddine and Boehm (14) reported that CO2 production under
-
-
N2 decreased in the order of CCl3CO2 > CHCl2CO2
>
CH2ClCO2- > CH3CO2-, which is consistent with a reaction
initiated by CB electrons. We should expect the opposite
order if the photo-Kolbe mechanism is working because
electron density at the carboxyl group is decreasing with an
increasing number of chlorine atoms. In this study, we did
not detect C2Cl6 from the fairly concentrated solution (6 mM)
of trichloroacetate, even though it was formed during the
photodegradation of CHCl3 and CCl4.
In order to resolve this mechanistic dilemma, we propose
the following photocatalytic mechanism of trichloroacetate
degradation, which is initiated by eq 21 and then followed
by eqs 22 and 23.
(14) Chemseddine, A.; Boehm, H. P. J. Mol. Catal. 1990, 60, 295.
(15) Degussa Technical Bulletin Pigments No. 56; Degussa: Hanau-
Wolfgang, 1990.
O
Cl
(16) Heller, H. G.; Langan, J. R. J. Chem. Soc., Perkin Trans. 2 1981,
341.
–
•
C
C
O-
:CCl2 + • CO2
(22)
(23)
(17) Pruden, A. L.; Ollis, D. F. Environ. Sci. Technol. 1983, 17, 628.
(18) Hsiao, C.-Y.; Lee, C.-L.; Ollis, D. F. J. Catal. 1983, 82, 418.
(19) Robinson, E. A. J. Chem. Soc. 1961, 1663.
(20) March, J. Advanced Organic Chemistry, 3rd ed.; John Wiley &
Sons: New York, 1985.
(21) Pelizzetti, E.; Pramuro, E.; Minero, C.; Serpone, N.; Borgadello,
E. In Photocatalysis and Environment; Schiavello, M., Ed.; NATO
ASI Series; Kluwer Academic Publication: Dordrecht, The
Netherlands, 1988; p 527.
Cl
Cl
–
+
•CO2 + hvb
CO2
O
Cl
:CCl2 + Cl– + CO2
(24)
C
C
O-
heat
Cl
(22) Al-Ekabi, H.; Serpone, N.; Pelizzetti, E.; Minero, C.; Fox, M. A.;
Draper, R.B. Langmuir 1989, 5, 250.
(23) Mills, A.; Morris, S. J. Photochem. Photobiol. A: Chem. 1993, 71,
75.
(24) Okamoto, K.-I.; Yamamoto, Y.; Tanaka, H.; Itaya, A. Bull. Chem.
Soc. Jpn. 1985, 58, 2023.
(25) The hydrolysis rate constant was taken from Schwarzenbach, R.
P.; Gschwend, P. M.; Imboden, D. M. Environmental Organic
Chemistry; John Wiley & Sons: New York, 1993; p 368.
(26) Bansal, K. M.; Henglein, A.; Sellers, R. M. Ber. Bunsen-Ges. Phys.
Chem. 1974, 78, 569.
A similar thermolytic decarboxylation mechanism (eq 24) of
trichloroacetate is known (20). In agreement with our
mechanism proposed above, Mao et al. (13) detected oxalate
as a minor product from the photocatalytic degradation of
trichloroacetate. The oxalate molecule can be formed from
the dimerization of •CO2- radicals. Since the carboxyl group
of the trichloroacetate molecule has a very low electron density
due to the presence of three electron-withdrawing substitutes,
decarboxylation appears to result from self-rearrangement
(eq 22) rather than from the direct photo-Kolbe reaction.
Additional study of the trichloroacetate photodegradation
on TiO2 is needed in order to verify the proposed mechanism.
(27) Lilie, V. J.; Beck, G.; Henglein, A. Ber. Bunsen-Ges. Phys. Chem.
1971, 75, 458.
(28) Ross, A. B.; Neta, P. Rate Constants for Reactions of Aliphatic
Carbon-Centered Radicals in Aqueous Solution; NSRDS-NBS 70;
National Bureau of Standards: Washington, DC, 1982.
(29) CRC Handbook of Chemistry and Physics, 74th ed.; CRC Press:
Boca Raton, FL, 1993.
(30) Ollis D. F.; Hsiao, C.-Y.; Budiman, L.; Lee, C.-L. J. Catal. 1984,
88, 89.
(31) The half-wave potential of trichloroacetate reduction is -0.49
V (vs NHE), which is comparable to TiO2 CB electron potential
(-0.5 V at pH 7); The half-wave potential was taken from Meites,
T.; Meites, L. Anal. Chem. 1955, 27, 1531.
Acknowledgments
We thank the Advanced Research Projects Agency (ARPA)
and the Office of Naval Research (ONR) {N0014-92-J-1901}
for financial support. We also appreciate the help from Robert
Rossi and Janet Kesselman in GC analysis.
Literature Cited
(1) Hoffmann, M. R.; Martin, S. T.; Choi, W.; Bahnemann, D. W.
Received for review February 20, 1996. Revised manuscript
received August 16, 1996. Accepted August 26, 1996.X
Chem. Rev. 1995, 95, 69.
ES960157K
(2) Serpone, N., Pelizzetti, E., Eds. PhotocatalysissFundamentals
and Applications; Wiley-Interscience: New York, 1989.
X Abstract published in Advance ACS Abstracts, November 1, 1996.
9
VOL. 31, NO. 1, 1997 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 5