Radiolytic reduction of hexachlorobenzene in surfactant solutions: A steady-state and pulse radiolysis study

Article abstract of DOI:10.1021/es991098o

Steady-state and pulse radiolysis experiments have been performed to gain insight into the mechanism of hexachlorobenzene (HCB) degradation in nonionic surfactant (Plurafac RA-40) solutions. This understanding is important for the environmental application of radiolysis to remediate soils contaminated with chlorinated aromatic compounds or to treat surfactant solution wastes from soil washing processes. Steady-state experiments showed that, after an applied dose of 50 kGy, reductive dechlorination of HCB to trichlorobenzene occurs under reducing conditions. Under oxidizing conditions at the same dose, reductive dechlorination proceeds more slowly to yield tetrachlorobenzene. Radiolytic experiments on the surfactant alone showed that the reaction rate constant between hydroxyl radicals and RA-40 (1.09 x 10⁹ M^-1 s^-1) was nearly 2 orders of magnitude higher than that between hydrated electrons and RA-40 (2.0 x 10⁷ M^-1 s^-1). Reaction kinetics analysis indicates efficient hydroxyl radical scavenging by surfactant molecules and the production of secondary surfactant radicals, which are reductive in nature. Thus, we observe HCB dechlorination in surfactant solutions even under strongly oxidizing conditions. Steady-state and pulse radiolysis experiments have been performed to gain insight into the mechanism of hexachlorobenzene (HCB) degradation in nonionic surfactant (Plurafac RA-40) solutions. This understanding is important for the environmental application of radiolysis to remediate soils contaminated with chlorinated aromatic compounds or to treat surfactant solution wastes from soil washing processes. Steady-state experiments showed that, after an applied dose of 50 kGy, reductive dechlorination of HCB to trichlorobenzene occurs under reducing conditions. Under oxidizing conditions at the same dose, reductive dechlorination proceeds more slowly to yield tetrachlorobenzene. Radiolytic experiments on the surfactant alone showed that the reaction rate constant between hydroxyl radicals and RA-40 (1.09 × 10⁹ M^-1 s^-1) was nearly 2 orders of magnitude higher than that between hydrated electrons and RA-40 (2.0 × 10⁷ M^-1 s^-1). Reaction kinetics analysis indicates efficient hydroxyl radical scavenging by surfactant molecules and the production of secondary surfactant radicals, which are reductive in nature. Thus, we observe HCB dechlorination in surfactant solutions even under strongly oxidizing conditions.

Full text of DOI:10.1021/es991098o

Environ. Sci. Technol. 2000, 34, 3401-3407
(9-11). An alternative to incineration, radiolysis, has been
shown to be an economically competitive alternative for
contaminant destruction (12, 13).
Radiolytic Reduction of
Hexachlorobenzene in Surfactant
Solutions: A Steady-State and
Pulse Radiolysis Study
Recent work in our laboratory has focused on the radiolytic
treatment of hexachlorobenzene (HCB) -contaminated soils
and metal oxides using 60Co and high-energy electrons
produced by linear accelerators (LINACs) (12, 14). In soil
studies, complete HCB degradation only occurred with the
addition of surfactants to soil samples. Surfactant addition
was found to reverse the retarding effect of soil organic matter.
Furthermore, regardless of the conditions of reaction (i.e.,
oxidative or reductive environments created by the addition
of hydroxyl radical and hydrated electron scavengers), the
irradiation of contaminated soils amended with surfactants
resulted in reductive dechlorination reactions (12, 15).
In all our work, the addition of a nonionic surfactant has
determined the extent of radiolytic reaction in soil systems.
Although essential, the role played by the surfactant in the
complicated chemistry of radiolysis is unknown. The purpose
of this work is to investigate the behavior of a nonionic
surfactant, BASF Plurafac RA-40, under controlled radiolytic
conditions and to probe its influence on HCB transformation.
To date, few studies concerning the radiolytic degradation
of chlorinated organics solubilized within surfactants have
been undertaken (16-18). To our knowledge, this is the first
study of the radiolytic behavior of HCB in surfactant systems.
In addition to aiding in the understanding of our soil system,
this work is also directly applicable to the treatment of soil
washing effluents by radiolysis (19). Furthermore, insight
into the fundamental chemistry of radical reactions in
surfactant solutions enhances the development of strategies
to control and target selective chemical transformations as
well as the ability to design efficient radiolytic processes for
environmental applications.
G E O R G E A D A M Z A C H E I S , †
K I M B E R L Y A . G R A Y, * , † A N D
P R A S H A N T V . K A M A T ‡
Department of Civil Engineering, 2145 Sheridan Road,
Northwestern University, Evanston, Illinois 60208-3109, and
Notre Dame Radiation Laboratory, University of Notre Dame,
Notre Dame, Indiana 46556-0579
Steady-state and pulse radiolysis experiments have been
performed to gain insight into the mechanism of
hexachlorobenzene (HCB) degradation in nonionic surfactant
(Plurafac RA-40) solutions. This understanding is important
for the environmental application of radiolysis to remediate
soils contaminated with chlorinated aromatic compounds or
to treat surfactant solution wastes from soil washing
processes. Steady-state experiments showed that, after
an applied dose of 50 kGy, reductive dechlorination of HCB
to trichlorobenzene occurs under reducing conditions.
Under oxidizing conditions at the same dose, reductive
dechlorination proceeds more slowly to yield tetrachloroben-
zene. Radiolytic experiments on the surfactant alone
showed that the reaction rate constant between hydroxyl
radicals and RA-40 (1.09 × 109 M-1 s-1) was nearly 2
orders of magnitude higher than that between hydrated
electrons and RA-40 (2.0 × 107 M-1 s-1). Reaction kinetics
analysis indicates efficient hydroxyl radical scavenging
by surfactant molecules and the production of secondary
surfactant radicals, which are reductive in nature.
Thus, we observe HCB dechlorination in surfactant solutions
even under strongly oxidizing conditions.
This work involved steady-state radiolytic experiments,
which allowed for the identification of byproducts and the
calculation of yields in HCB surfactant solutions under
reductive and oxidative conditions. In addition, pulse ra-
diolysis studies were undertaken to calculate the reaction
rates between primary radiation products and HCB as well
as the surfactant, to monitor the transient intermediates
produced by these interactions, and to elucidate the pathway
of HCB dechlorination.
Radiation Chemistry
Introduction
The radiolytic transformations of solutes may be quantified
in terms of G, defined as the number of molecules formed
or lost per 100 eV of energy absorbed (20). In aqueous
solutions at pH 7, the G values of solvated electrons and
hydroxyl radicals (primary products) are about 2.7 and 2.8,
respectively. These radicals, or primary products, are the
major species formed from the radiolysis of water.
Reductive conditions may be established through the
addition of tert-butyl alcohol, a scavenger of hydroxyl radicals,
and purged with N2 to eliminate O2, an electron scavenger
(21):
In the United States, the contamination of soils with
hydrophobic and toxic chemicals is a vast problem, and
treatment is a high priority (1). An estimated 217 000 sites,
contaminated with chlorinated compounds, remain to be
cleaned up at an estimated cost of $187 billion (1996 $) (2).
In addition, an estimated 500 000 ton of dioxin contaminated
soils alone are in need of treatment (3). Recalcitrant chemicals
including dioxins and polychlorinated biphenyls (PCBs) are
resistant to remediation via less expensive technologies such
as bioremediation due to low bioavailability (4-6). In many
cases, incineration is the only feasible alternative for con-
taminant destruction (3, 7, 8). However, destruction of dioxins
and PCBs requires very high combustion temperatures, and
inadequate operating temperatures often leads to incomplete
contaminant combustion and the formation of byproducts
having similar or greater toxicity, such as chlorobenzofurans
OH• + (CH3)3COH f H2O + (CH3)2C˙ H2COH
(1)
(2)
•-
O2 + e- f O2
Conversely, oxidative conditions are created using N2O, an
electron scavenger (21):
* Corresponding author phone: (847)467-4252; fax: (847)491-4011;
e-mail: k-gray@nwu.edu.
† Northwestern University.
eaq- + N2O + H2O f N2 + 2(•OH)
(3)
‡ University of Notre Dame..
9
10.1021/es991098o CCC: $19.00
Published on Web 07/13/2000
2000 Am erican Chem ical Society
VOL. 34, NO. 16, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 3 4 0 1

Oxidation of chlorinated benzenes tends to proceed
through hydroxyl substitution at the para and ortho positions
(22). In many studies of halogenated benzenes, the product
of hydroxyl adducts is the hydroxycyclohexadienyl radical
(21, 23-26). Hydroxyl radical attachment may also occur at
a chlorine atom, resulting in the formation of a phenoxyl
radical (22). These products are readily identified by UV
spectroscopy (27).
Chlorinated aromatics are more reactive toward solvated
electrons due to electron-deficient aromatic rings resulting
from chlorine atom substitution (28). Carbon-chlorine bonds
may by rapidly cleaved through direct electron capture (17):
eaq- + ArX f (ArX)- f Ar• + X-
(4)
Electron capture leads to the formation of intermediate
electron adducts, which decay to form aryl radicals and halide
ions (21). For example, hexafluorobenzene (HFB) has been
reported to react quantitatively with electrons to yield a
pentafluorobenzyl radical and fluoride (29).
FIGURE 1. Steady-state, γ-radiolysis of HCB (175 µM) in RA-40 (31
mM) under reducing (9)(N2 and 1% tert-butyl alcohol) and oxidizing
(b) conditions (N2O purged) vs time.
Experimental Section
and a Restek RTX-5 nonpolar column (12). Helium was used
as a carrier gas, and an argon/ methane (P-5) gas mixture
was used for ECD makeup.
Tentative identification of reaction products by GC-ECD
was verified through the use of a Hewlett-Packard 6890 GC-
MS. Mass spectra were compared to NIST databases for
tentative identification. Mass spectra of samples were
compared to standards for final confirmation. For the GC-
MS work, a J&W Scientific DB-5 column (nonpolar) was used.
Sam ple Preparation. RA-40 surfactant was obtained from
the BASF Corporation (Mt. Olive, NJ). Chlorinated benzenes
were acquired from Aldrich (Milwaukee, WI). All chemicals
were of the highest purity and were used as received without
further purification. RA-40 is a nonionic, ethoxyl alcohol
surfactant (CH3(CH2)12-15[-OCH2CH2-]12-15OH) with an aver-
age molecular weight of 820 amu. All surfactant solutions
were prepared on a weight to weight basis in triple distilled
water. Because of the very low aqueous solubility of HCB
(1.75 × 10-8 M) (30), surfactant solutions were heated to 75
°C and stirred for 3 h to enhance HCB solubilization.
Pulse radiolysis experiments were performed in unbuf-
fered surfactant solutions without pH adjustment. A 31 mM
solution of RA-40 had a measured pH of 5.7.
Results
Steady-State, γ-Radiolysis. Steady-state radiolysis of 175 µM
HCB in a 31 mM RA-40 surfactant solution was carried out
under different scavenger conditions over a period of 12 h
(50 kGy). Figure 1 shows the decay of HCB following
γ-radiolysis in oxidative and reductive environments. The
overall rate of HCB disappearance was 2 times greater under
reducing conditions. These data were fit with a pseudo-first-
order HCB decay rate constant of 0.105 h-1 under reductive
conditions and 0.052 h-1 under oxidative conditions.
GC-ECD analysis of steady-state irradiated samples
indicated that the byproducts of HCB degradation were in
the form of reduced chlorinated benzenes. Products formed
under reducing conditions include pentachlorobenzene
(PeCB), tetrachlorobenzenes (TeCBs), and trichlorobenzenes
(TCBs) as illustrated in Table 1. Two isomers of TeCB (1,3,4,5-
TeCB and 1,2,3,5-TeCB) could not be individually quantified
due to coelution under the conditions of the GC method
employed. Some of these reduced forms of HCB were also
observed under oxidizing conditions, although in different
proportions.
Because of the high degree of chlorine substitution in
HCB, one would expect its reaction rate with hydroxyl radicals
to be slower than with hydrated electrons. Minero et al. found
that HFB was less reactive than pentafluorophenol toward
hydroxyl radicals (32). Likewise, Atkinson found that hydroxyl
radical reactivity decreased with increasing chlorine sub-
stitution in PCBs (33). Under the conditions of these
experiments, however, only reduced byproducts were ob-
served, regardless of the level of chlorine substitution in the
parent compound. As will be discussed later, this phenom-
enon is the direct result of hydroxyl radical scavenging by
surfactant molecules, which prevents the oxidation of HCB
and its byproducts.
Electron-rich solutions were created through the addition
of 1% (by volume) tert-butyl alcohol and by purging the
solution with nitrogen gas. Oxidizing conditions were created
by sparging solutions with nitrous oxide.
γ-Radiolysis. All steady-state radiolysis experiments
were conducted using a Shepard-109, 60Co source. 60Co is an
emitter of high-energy photons (γ-rays), having an average
energy of 1.25 MeV. The Shepard-109 is a concentric well-
type source with a dose rate of approximately 69 Gy/ min.
Dose rates were determined using Fricke dosimetry (31). All
aqueous surfactant solutions were irradiated in 7-mL closed
vials to ensure that solutions remained saturated with
appropriate scavengers, such as the utilization of N2O as a
scavenger of electrons.
Pulse Radiolysis. Pulse radiolysis experiments were
performed using a model TB-8/ 16-1S linear accelerator
(LINAC). This LINAC produces electron pulses, 50 ns in
duration, at 8 MeV energy. Solutions were continuously
pumped through a quartz cell, and radical kinetics were
monitored by UV absorption.
Extraction and Analysis. Surfactant solutions were ex-
tracted sequentially using hexane. An internal standard, 1,3,5-
tribromobenzene (4 mg/ L), was added prior to extraction to
correct responses for extraction efficiency and changes in
volume. This internal standard was used due to its structural
similarity to HCB and high response in electron capture
detectors (ECD). After internal standard and solvent addition,
solutions were centrifuged for 3 min at 890g to separate
surfactant/ water and hexane phases. The resulting hexane
phase was decanted, passed through a column of sodium
sulfate, and then stored for GC analysis.
G values for the 5-h (20.8 kGy) irradiated samples were
calculated for experiments performed under both reductive
and oxidative conditions. These values as well as measured
concentrations of reduced products are presented in Table
1.
A method for the detection and quantification of HCB
and reduced forms of HCB was developed in our laboratory
using a Hewlett-Packard series 4890 GC with ECD detection
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TABLE 1. Results of Steady-State Radiolysis: Byproduct Levels
and Yields at 20.8 kGy
residual
concn
(µM)
G
σG
σ
(molecules/
(molecules/
100 eV)
chemical
HCB
(µM)
100 eV)a
Reductive Environmentb
76.75
33.17
0.53
5.23
6.12
0.09
0.04
0.08
3.20 × 10-2 2.18 × 10-3
1.55 × 10-2 2.85 × 10-3
2.48 × 10-4 4.31 × 10-5
1.29 × 10-5 1.82 × 10-5
1.54 × 10-4 3.63 × 10-5
PeCB
1,2,3,4-TeCB
1,3,5-TCB
1,2,4-TCB
1,2,3-TCB
0.03
0.33
0.055 0.001 2.57 × 10-5 1.09 × 10-6
Oxidative Environmentc
HCB
PeCB
1,2,3,4-TeCB
119.85
16.06
1.14
0.34
2.00 × 10-2 1.91 × 10-4
7.49 × 10-3 1.58 × 10-4
0.046 0.002 2.16 × 10-5 1.08 × 10-6
a
b
Apparent values of G. Initial [HCB] ) 162.75 ( 4.35 µM. Reductive
conditions refer to the radiolysis of HCB solution containing 31 m M
RA-40 and 1% tert-butyl alcohol (N2-saturated). Initial [HCB] ) 145.38
( 2.88 µM. Oxidative conditions refer to the radiolysis of HCB solution
containing 31 m M RA-40 (N2O-saturated).
FIGURE 2. Pentachlorobenzene formation in RA-40 (31 mM) vs time
under reducing (9) and oxidizing (b) conditions.
c
As shown in Table 1, the G value for HCB transformation
under reducing conditions was approximately 0.03 in com-
parison to 0.02 in an oxidative environment. In general, under
oxidizing conditions, the concentrations of transformation
products and the corresponding G values were less than G
values under reducing conditions with observed differences
as great as an order of magnitude. However, given the very
large reaction rate constant (discussed later) between hy-
droxyl radicals and RA-40, hydroxyl radicals were equally
scavenged by tert-butyl alcohol and RA-40. (Note that kC H
-
O
10
4
[C4H10O] = kRA-40[RA-40] at the concentration of tert-butyl
alcohol used (135 mM) in the present experiments.) For this
reason as well as the fact that the applied rather than absorbed
dose, which is unknown for this type of heterogeneous
system, was used in the calculations, the G values in Table
1 are apparent G values.
FIGURE 3. Difference absorption spectra recorded (a) 12.5 (9), (b)
25.0 (b), and (c) 50.0 µs (2) after an electron pulse in a N2-purged,
1% tert-butyl alcohol solution of HCB (175 µM) and RA-40 (31 mM).
Insert: Byproduct growth at 280 nm.
Slower reaction rates and lower G values for the dechlo-
rination of polychlorinated benzenes can be attributed to
the differences in reduction potentials with increasing
chlorine substitution. A similar observation has also been
made in the study of dechlorination of PCBs under γ-radi-
olysis in transformer oils (34). Past research by Hilarides et
al. found G values in the range of 1.4-9.6 × 10-6 for the
degradation of 2,3,7,8-tetrachlorobenzo-p-dioxin (TCDD)
(100 ng/ g) when adsorbed to RA-40-amended soil (15). Similar
G values for the irradiation of TCDD (100 ng/ g) in ethanol,
acetone, and dioxane were calculated from the data taken
from Fanelli et al. (35). In general, the differences in G values
observed in the dioxin and HCB research are attributed to
differences in contaminant concentration. The concentration
of HCB used in these experiments was 1000 times greater
than the dioxin concentrations used by Hilarides et al. Table
1 does not show the measurement of all byproducts, as a
mass balance of carbon was not achieved. Closure of the
carbon mass balance is complicated in this system by our
inability to quantify all the reaction products, especially those
resulting from ring cleavage.
of HCB (175 µM) in aqueous RA-40 (31 mM). Under reducing
conditions, the difference absorption spectra at various times
showed the growth of a large peak around 270-280 nm as
illustrated in Figure 3 and its insert. The transient absorption
at early times (spectrum a in Figure 3) arises from the initial
attack of HCB by hydrated electrons. A small amount of
bleaching is observed at 320 nm representing the disap-
pearance of parent HCB and the formation of an electron
adduct. At this wavelength, the transient has a lower
absorption than the parent HCB. The electron adduct so
formed is unstable and undergoes dechlorination. The peak
in the region of 270-280 nm is attributed to the appearance
of a dechlorination product. The identification of these
intermediates is explained more fully in the discussion
section. The bimolecular rate constant for the formation of
this transient at 280 nm was found to be 8.3 × 107 M-1 s-1
(Figure 3, insert).
Pulse radiolysis experiments carried out with HCB/ RA-
40 solutions under oxidizing conditions (Figure 4) (i.e.,
solutions sparged with N2O) exhibited a difference spectrum
somewhat similar to that obtained under reducing conditions
(Figure 3). For example, the transient absorption spectrum
in the 265-310 nm region (spectrum a in Figure 4) is similar
to the transient absorption spectrum recorded at 25.0 µs in
Figure 3 (spectrum b). As shown in Table 1, similar stable
products are formed in the reducing and oxidizing environ-
ments but in different proportions. Yet, there are also some
spectral differences evident under oxidizing conditions. In
Figure 2 shows the formation of PeCB with time under
both reductive and oxidative environments. Initially, the
formation of PeCB is pseudo-first-order with the rate of PeCB
formation equaling the rate of HCB decay. With increasing
time, the growth of PeCB becomes asymptotic and eventually
decays over larger dose periods. These data further illustrate
the much slower rates of byproduct appearance under
oxidizing relative to reducing conditions.
Pulse Radiolysis of HCB in RA-40 Surfactant Solutions.
Pulse radiolysis experiments were performed using solutions
9
VOL. 34, NO. 16, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 3 4 0 3

FIGURE 4. Difference absorption spectra recorded (a) 12.5 (9), (b)
25.0 (b), and (c) 50.0 µs ([) after an electron pulse in a N2O-purged
solution of HCB (175 µM) and RA-40 (31 mM).
FIGURE 6. Reaction rate between HCB and electrons in a N2-purged,
1% tert-butyl alcohol solution of RA-40 (31 mM). Insert: Electron
decay at 600 nm in the presence of (a) 44 (9) and (b) 175 µM (+)
HCB.
reported two-step reduction processes in surfactant solutions
(37). The initial component consists of a fast diffusion-
controlled electron transfer between aqueous-phase sur-
factant radical monomers and a given solute, while the second
component arises from the slower electron transfer between
micelle-attached surfactant radicals to the solute.
Although the identification of the surfactant radicals is
not possible, these results establish the reducing properties
of the radicals. These reactions proceed as follows:
•OH + R-O-CH2CH2OH f
•
H2O + R-O-CH2 CHOH (5)
•
R-O-CH2 CHOH + MV2+
f
+
FIGURE 5. Difference absorption spectra recorded (a) 5 (9), (b) 10
(b), and (c) 20.0 µs (2) after an electron pulse in a N2O-purged
solution of methyl viologen (100 µM) and RA-40 (31 mM). Insert:
Growth of reduced methyl viologen at 390 nm.
•+
R-O-CH2CHOH + MV (6)
The presence of reducing surfactant radicals has also been
verified in earlier studies through the bleaching of ferricy-
anate, illustrating electron transfer between surfactant
radicals and an oxidizing agent in solution (38). In addition,
simple alcohol radicals such as ethanol have been shown to
be capable of electron transfer (39).
contrast to the phenomena displayed under reducing condi-
tions, the major peak in the 270-280 nm region decreases
and the bleaching of the 320 nm intermediate is not observed
under oxidizing conditions. These results suggest that the
reaction proceeds on different time scales by a differing
pathway but involves similar (270-280 nm) as well as different
transients, as illustrated in Figures 3 and 4.
Reaction Rates with Hydrated Electrons. Pulse radiolysis
experiments enabled us to obtain the bimolecular rate
constant for the reaction between HCB and hydrated
electrons in RA-40 (31 mM) surfactant solutions. As shown
in the insert of Figure 6, the decay rate of electrons (monitored
at 600 nm) increased at higher concentrations of HCB. A
bimolecular reaction rate constant of 1.1 (( 0.2) × 109 M-1
s-1 was determined for the reaction between hydrated
electrons and HCB (Figure 6). Several research groups have
observed similar reactivities of halogenated benzenes toward
hydrated electrons. For example, Schmelling et al. found the
rate constant for the reaction between electrons and de-
cachlorobiphenyl in Triton X-100 solution to be 2.6 × 109
M-1 s-1 (17). Shoute and Mittal reported a reaction rate
constant of 8.5 × 109 M-1 s-1 for hydrated electrons and HFB
in aqueous alcohol solutions (40).
The dependence of electron decay on the concentration
of RA-40 was also investigated. The insert of Figure 7A shows
that increasing concentrations of RA-40 leads to an increase
in the rate of electron decay. A bimolecular rate constant of
2.0 (( 0.5) × 107 M-1 s-1 was determined for the reaction
between RA-40 and hydrated electrons. Again, we found that
the reaction rate constant of this reaction was similar to the
reaction rate constant between electrons and Triton X-100
Surfactant Radicals as Secondary Reducing Radicals.
To establish the reactivity of the surfactant radicals produced
by •OH radical reaction, we added a known amount (10-4 M)
of methyl viologen into N2O-purged RA-40 (31.0 mM)
surfactant solutions. Since MV2+ is a good electron acceptor
•+
and the reduced form, MV , has a characteristic absorption
maximum at 390 and 605 nm (not shown) (36), we employed
it as a probe to track the formation of reductive, secondary
surfactant radicals. Under N2O-saturated conditions, hy-
drated electrons were not detected (monitored at 600 nm)
and thus did not directly reduce methyl viologen.
Figure 5 shows the transient spectrum obtained following
the radiolysis of RA-40 (N2O-purged) solution containing
methyl viologen. The spectral characteristics of the transient
absorption having a 390 nm maximum confirm the formation
•+
•+
of MV . Two distinct steps occur in the growth of MV , a
fast growth component in the 390 nm absorption (completed
within 30 µs) and a slow growth component over a period
of 200 µs (insert of Figure 5). The bimolecular rate constants
for these two processes are 5.9 × 108 and 2.0 × 108 M-1 s-1
,
respectively. This observation is similar to the previously
9
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Because of the low solubility of HCB and the high reactivity
•
of RA-40 toward OH radicals, we were unable to determine
the reaction rate constant between hydroxyl radicals and
HCB in surfactant solutions. Competition kinetic measure-
ments showed nearly no change in the (SCN)2•- absorbance
when the concentration of HCB was increased from 0 to 1.75
× 10-4 M. It is evident that, at low concentrations, HCB is
unable to compete with SCN- and RA-40 in the reaction with
hydroxyl radicals. In an analogous study of HFB, the reaction
rate constant with hydroxyl radicals was reported to be 1.4
× 109 M-1 s-1 (40). Even with an upper limit of the rate
constant on the order of 109 M-1 s-1 for the reaction between
•
•OH radical and HCB, the probability of OH reaction with
HCB becomes significantly smaller at the concentration ratio
of surfactant:HCB (170:1) used in the present experiment.
The HCB transformations observed during radiolysis in N2O-
purged solutions are therefore attributed to the secondary
radical reactions, i.e., reaction with reducing surfactant
radicals (eqs 9 and 10):
•OH + R-O-CH2CH2OH f
•
H2O + R-O-CH2 CHOH (9)
•
R-O-CH2 CHOH + C6Cl6 f
+
•-
C
R-O-CH2 HOH + [C6Cl6] (10)
Discussion
Under reductive conditions, the dechlorination of HCB in
RA-40 surfactant solutions proceeded at a nearly diffusion-
controlled rate constant of 1.1 × 109 M-1 s-1. As illustrated
by reaction rate constants, we observed that the reaction of
surfactant molecules and hydroxyl radicals dominates in an
oxidative environment. In these experiments the concentra-
tion of RA-40 was 170 times greater than HCB, resulting in
rapid hydroxyl radical scavenging and the production of
surfactant radicals. The reaction of the surfactant, an ethoxyl
alcohol, with hydroxyl radicals is analogous to that with
simple alcohols. For example, alcohols such as 2-propanol
readily react to form R-hydroxy radicals (k ) 1.9 × 109 M-1
s-1;pH 6) (42). Once produced, surfactant radicals can initiate
secondary reduction of HCB in a way similar to the reduction
of methyl viologen in N2O-saturated surfactant solutions.
This explains the slower rate of HCB depletion in the
γ-radiolysis of N2O-saturated surfactant solutions (Figure 1).
Secondary reduction of HCB by surfactant radicals occurred
at a rate half that of the reaction of HCB with hydrated
electrons.
FIGURE 7. (A) Reaction rate between RA-40 and electrons (N2-
purged and 1% tert-butyl alcohol). Insert: Electron decay at 600 nm
in the presence of (a) 1.15 (×), (b) 1.85 (+), and (c) 2.28 mM (9)
RA-40. (B) Reaction rate between RA-40 and hydroxyl radicals using
competition kinetics.
reported by Schmelling et al. (1.2 × 107 M-1 s-1) (17).
Furthermore, the reaction rate constant between surfactants
and electrons (2.0 × 107 M-1 s-1) is significantly lower than
that between HCB and electrons (1.1 × 109 M-1 s-1).
Role of Hydroxyl Radicals. Due to the low extinction
coefficient of hydroxyl radicals, competition kinetics was
employed to determine the reaction rate constant between
hydroxyl radicals and RA-40 in aqueous solutions (28).
Thiocyanate (SCN-) was added to a RA-40 surfactant solution
(N2O-purged) at a concentration of 10-4 M, resulting in the
two competing reactions, eqs 7 and 8, to determine the fate
Radiolysis of HCB solubilized in RA-40 solution results in
the reduction of HCB irrespective of the experimental
conditions (reductive or oxidative) created in these experi-
ments. In general, the reaction of electrons with substituted
halobenzenes leads to dechlorination and the formation of
radical species (22). This process occurs via the reactions 11
and 12 and is also illustrated in Scheme 1:
•
of OH radicals:
SCN
SCN- + •OH
-8 (SCN)2•- + OH- + H2O
(7)
•OH + R-O-CH2CH2OH f
•
H2O + R-O-CH2 CHOH (8)
C6Cl6 + eaq- f [C6Cl6]•- f •C6Cl5 + Cl-
(11)
•-
Absorbance of the (SCN)2 radical was monitored at 472
nm. As shown in Figure 7B, the reaction rate constant is
obtained by plotting the inverse of the (SCN)2•- absorbance
at 472 nm versus the molar ratio of RA-40 to SCN-. From
these data, the reaction rate between hydroxyl radicals and
RA-40 was determined to be 3.3 (( 0.4) ×109 M-1 s-1. In
comparison, the reported reaction rate constant between
hydroxyl radicals and Triton X-100 is 8.8 × 109 M-1 s-1 (41).
It is important to note that the reaction rate constant for the
reaction between RA-40 and hydroxyl radicals (Figure 7B;
3.3 × 109 M-1 s-1) is nearly 2 orders of magnitude greater
than the rate constant of reaction between RA-40 and
hydrated electrons (Figure 7A; 2.0 × 107 M-1 s-1).
+
[C6Cl6]•- Haq / H2O8 [C6Cl6H]• f •C6Cl5 + H+ + Cl- (12)
Electrons are likely to be transferred to a chlorine atom,
resulting in chloride expulsion and the formation of an aryl
radical (eq 11) or transferred to the aromatic ring, resulting
in protonation to form a hexadienyl radical (eq 12). Proto-
nation reactions have been found to occur in halogenated
benzenes (21, 29) and even with benzene (43). Ultimately,
hexadienyl radicals may further degrade to form an aryl
radical and hydrogen chloride. Past research has shown aryl
radicals to be efficient hydrogen abstractors (44).
9
VOL. 34, NO. 16, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 3 4 0 5

electron adducts and reduced forms of HCB or through
hydroxyl radical reactions with surfactant molecules (Route
2) to produce reducing radicals that undergo electron transfer
with HCB.
SCHEME 1. Reaction Mechanism for HCB Degradation in
RA-40a
Acknowledgments
G.A.Z. would like to thank Dr. Suresh Das for his teachings
on all subjects and good company. K.A.G. gratefully ac-
knowledges the support of The Center for Catalysis and
Surface Science at Northwestern University. P.V.K. acknowl-
edges the support of the Office of Basic Energy Sciences of
the Department of Energy. This is Contribution No. NDRL-
4152 from the Notre Dame Radiation Laboratory.
Literature Cited
(1) Davila, B.; Whitford, K. W.; Saylor, E. S. EPA Issues: Technology
Alternatives for the Remediation of PCB-Contaminated Soil and
Sediment; U.S. Environmental Protection Agency: Washington,
DC, 1993; EPA/ 540/ S-93/ 506.
(2) Cleaning up the Nation’s Waste Sites: Markets and Technology
Trends; U.S. Environmental Protection Agency: Washington,
DC, 1997; EPA 542-R-96-005A.
(3) Dioxin Treatment Technologies; U.S. Office of Technology
Assessment (OTA): Washington, DC, 1991; OTA-BP-O-93.
(4) Boldrin, B.; Tiehm, A.; Fritzche, C. Appl. Environ. Microbiol.
1993, 59, 1927-1930.
a
These results show that the surfactant plays an im portant role in
the radiolytic chem istry of HCB. In addition to hydrogen donation, the
presence of surfactant directs the reductive transform ation of HCB and
its lesser chlorinated byproducts. The reaction of surfactant m olecules
with hydroxyl radicals produces radicals that, in turn, reduce chlorinated
benzenes. These results provide additional insight into why only
reductive transform ations have been observed in our work with
contam inated soils and m ay suggest strategies to control radical
chem istry in environm ental m aterials.
(5) Thomas, J. M.; Yordy, J. R.; Amador, J. A.; Alexander, M. Appl.
Environ. Microbiol. 1986, 52, 290-296.
(6) Volkering, F.; Breure, A. M.; Sterkenberg, A.; van Andel, J. G.
Appl. Microbiol. Biotechnol. 1992, 36, 548-552.
(7) Arienti, M.; Wilk, L.; Jasinski, M.; Prominski, N. Dioxin Containing
Wastes: Treatment Technologies; Noyes Data Corp: Park Ridge,
NJ, 1988.
(8) Edwards, N. Water Environ. Technol. 1992, 4, 58-60.
(9) Buser, H. R.; Rappe, C. Chemosphere 1979, 3, 157-174.
(10) Jansson, B.; Sundsrum, G. In Chlorinated Dioxins and Related
Compounds-Impact on the Environment; Pergamon Press:
Oxford, 1982.
The reaction of protons with hydrated electrons to produce
hydrogen radicals occurs at near-diffusion-limited rates over
a pH range of 4-5 (45). While the reaction of aryl radicals
with hydrogen may occur, the yield of hydrogen radicals at
neutral pH is only G)0.6 (46). Since the pH of RA-40 solutions
in our experiments is 5.6, the concentration of hydrogen
radicals is not significant.
Since surfactants are a rich source of hydrogen, we propose
that surfactant molecules and aryl radicals can react to form
PeCB. Koster and Asmus found that HFB radical anions react
quantitatively to yield pentafluorobenzyl radical and fluoride
(29). Fluoride elimination was verified by conductivity
measurements in pulse radiolysis experiments in neutral
solutions. Conductivity was shown to decay with time,
evidence of electron adduct protonation, to yield cyclohexa-
dienyl radicals as in eq 10. Transients in the pulse spectra
were attributed to the presence of cyclohexadienyl radicals
over long irradiation pulses (∼5 µs).
(11) Ahling, B.; Lindskog, A. In Chlorinated Dioxins and Related
Compounds-Impact on the Environment; Pergamon Press:
Oxford, 1982.
(12) Zacheis, G. A.; Gray, K. A.; Kamat, P. V. Submitted for publication
in Environ. Sci. Technol.
(13) Gray, K. A.; Clelland, M. R. J. Adv. Oxid. Technol. 1998, 3, 22-36.
(14) Zacheis, G. A.; Gray, K. A.; Kamat, P. V. J. Phys. Chem. B 1999,
103, 2142-2150.
(15) Hilarides, R. J.; Gray, K. A. Environ. Sci. Technol. 1994, 28, 2249-
2258.
(16) Hilarides, R. J. Ph.D. Dissertation, University of Notre Dame,
1994.
(17) Schmelling, D. C.; Poster, D. L.; Chaychian, M.; Neta, P.;
Silverman, J.; Al-Sheikhly, M. Environ. Sci. Technol. 1998, 32,
270-275.
(18) Hilarides, R. J.; Gray, K. A.; Guzzetta, J.; Cortellucci, N.; Sommer,
C. Water Environ. Res. 1996, 68, 178-187.
(19) Griffiths, R. A. J. Haz. Mater. 1995, 40, 175-189.
(20) Klassen, N. V. In Primary Products in Radiation Chemistry; VCH
Publishers: New York, 1987; Chapter 2.
(21) Terzian, R.; Serpone, N.; Draper, R. B.; Fox, M. A.; Pelizzetti, E.
Langmuir 1991, 7, 3081-3089.
(22) Mohan, H.; Mudaliar, M.; Aravindakumar, C. T.; Rao, B. S. M.;
Mittal, J. P. J. Chem. Soc., Perkin Trans. 2 1991, 1387-1392.
(23) Pan, X.-M.; Schuchmann, M. N.; von Sonntag, C. J. Chem. Soc.,
Perkin Trans. 2 1993, 289-297.
(24) Merga, G.; Rao, B. M. S.; Mohan, H.; Mittal, J. P. J. Phys. Chem.
1994, 98, 9158-9164.
(25) Mohan, H.; Mittal, J. P. Chem. Phys. Lett. 1995, 235, 444-449.
(26) Stafford, U.; Gray, K. A.; Kamat, P. V. J. Catal. 1997, 167, 25-32.
(27) Stafford, U.; Gray, K. A.; Kamat, P. V. J. Phys. Chem. 1994, 98,
6343-6351.
(28) Buxton, G. V. In Radiation chemistry of the liquid state: (1)
water and homogeneous aqueous solutions; VCH Publishers:
New York, 1987.
(29) Koster, R.; Asmus, K.-D. J. Phys. Chem. 1973, 77, 749-755.
(30) Schwarzenbach, R. P.; Gschwend, P. M.; Imboden, D. M.
Environmental Organic Chemistry; John Wiley & Sons: New
York, 1993.
(31) ASTM. Standard method for using the Fricke dosimeter to measure
absorbed dose in water; ASTM: Philadelphia, 1984; E1026-84.
(32) Minero, C.; Pelizzetti, E. Langmuir 1994, 10, 692-698.
Later experiments with HFB in solutions at pH 14 by
Shoute and Mittel contradicted the findings of Koster and
Asmus (40). They stated that HFB, following electron capture,
does not undergo fluoride elimination and instead forms
dimers and trimers of HFB. However, Shoute and Mittel failed
to explain the conductivity findings of Koster and Asmus. In
addition, the presence of an electron adduct was not detected
in experiments at lower values of pH (<13). These spectra
were attributed to the presence of a hexafluorocyclodienyl
radical, a result of rapid protonation of the hexafluorobenzyl
radical. In addition, research with pentahalophenols has
shown that the reaction of electrons with pentachlorophe-
noxide ions results in chlorine expulsion and the formation
of hydroxyphenyl radicals (21). In this case, researchers were
unable to detect the presence of an electron adduct due to
rapid protonation of hydroxyphenyl radicals to yield phenoxyl
radicals.
Primarily on the basis of the results of Koster and Asmus,
we conclude that the transient spectra of the intermediate
at 320 nm to be that of the HCB electron adduct which
undergoes subsequent protonation to form cyclohexadienyl
radicals and, ultimately, pentachlorobenzene.
In summary, as shown in Scheme 1, reactions in our
surfactant system may occur either through direct electron
reaction with HCB (Route 1) involving the generation of
9
3 4 0 6 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 34, NO. 16, 2000

(33) Atkinson, R. Environ. Sci. Technol. 1987, 21, 305-307.
(34) Chaychian, M.; Silverman, J.; Al-Sheikhly, M.; Poster, D. L.; Neta,
P. Environ. Sci. Technol. 1999, 33, 2461-2464.
(41) Perkowski, J.; Mayer, J. J. Radioanal. Nucl. Chem. 1990, 141,
271-277.
(42) Wolfenden, B. S.; Wilson, R. L. J. Chem. Soc., Perkin Trans. 2
1982, 805-812.
(43) Studier, M. H.; Hart, E. J. J. Am. Chem. Soc. 1969, 91, 4068-4072.
(44) Fang, X.; Mertens, R.; von Sonntag, C. J. Chem. Soc., Perkin
Trans. 2 1995, 1033-1036.
(45) Gordon, S.; Hart, E. J.; Matheson, M. S.; Rabani, J.; Thomas, J.
K. J. Am. Chem. Soc. 1963, 85, 1375-1377.
(46) Fendler, E. J.; Fendler, J. H. Prog. Phys. Org. Chem. 1970, 7,
229-335.
(35) Fanelli, R.; Chiabrando, C.; Salmona, M.; Garattini, S.; Caldera,
P. G. Experientia 1978, 34, 1126-1127.
(36) Mao, Y.; Breen, N. E.; Thomas, J. K. J. Phys. Chem. 1995, 99,
9909-9917.
(37) Thomas, J. K. Radiation Chemistry: Principles and Applications.
In Radiation Chemistry of Colloidal Aggregates; VCH Publish-
ers: New York, 1987; Chapter 12.
(38) Almgren, M.; Grieser, F.; Thomas, J. K. J. Chem. Soc., Faraday
Trans. 1 1979, 75, 1674-1687.
Received for review September 23, 1999. Revised manuscript
received April 27, 2000. Accepted May 2, 2000.
(39) Peyton, G. R.; Bell, O. J.; Girin, E.; Lefaivre, M. H. Environ. Sci.
Technol. 1995, 29, 1710-1712.
(40) Shoute, L. C. T.; Mittal, J. P. J. Phys. Chem. 1993, 97, 379-384.
ES991098O
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