Kinetics and mechanism of the enhanced reductive degradation of CCl4 by elemental iron in the presence of ultrasound

Article abstract of DOI:10.1021/es980273i

Enhanced rates of sonolytic degradation of CCl₄ in the presence of Fe⁰ are demonstrated. In Ar-saturated solutions, the first-order rate constant for CCl₄ degradation is k(US) = 0.107 min^-1, whereas in the presence of Ar and Fe⁰, the apparent first-order rate constant is found to depend on the total surface area of elemental iron in the following fashion: k(obs) = (k(US) + k(Fe)⁰A(Fe)⁰) min^-1, where k(US) = 0.107 min^-1, k(Fe)⁰ = 0.105 L m^-2 min^-1 and A(Fe)⁰) = reactive surface area of Fe⁰ in units of m² L^-1. In the coupled ultrasound and iron system, the contribution to the overall degradation rate by direct reaction with Fe⁰ results in an overall rate enhancement by a factor of 40. These enhancements are attributed (1) to the continuous cleaning and chemical activation of the Fe⁰ surface by the combined chemical and physical effects of acoustic cavitation and (2) to accelerated mass transport rates of reactants to the Fe⁰ surfaces. Additional kinetic enhancements are due to the production of H⁺ during the course of the reaction. Furthermore, the concentrations of the principal reaction intermediates, C₂Cl₆ and C₂Cl₄, are influenced substantially by the total available surface area of Fe⁰. Enhanced rates of sonolytic degradation of CCl₄ in the presence of Fe⁰ are demonstrated. In Ar-saturated solutions, the first-order rate constant for CCl₄ degradation is k_US = 0.107 min^-1, whereas in the presence of Ar and Fe⁰, the apparent first-order rate constant is found to depend on the total surface area of elemental iron in the following fashion: k_obs = (k_US+k_Fe⁰A_Fe⁰) min^-1, where k_US = 0.107 min^-1, k_Fe⁰ = 0.105 L m^-2 min^-1, and A_Fe⁰) = reactive surface area of Fe⁰ in units of m² L^-1. In the coupled ultrasound and iron system, the contribution to the overall degradation rate by direct reaction with Fe⁰ results in an overall rate enhancement by a factor of 40. These enhancements are attributed (1) to the continuous cleaning and chemical activation of the Fe⁰ surface by the combined chemical and physical effects of acoustic cavitation and (2) to accelerated mass transport rates of reactants to the Fe⁰ surfaces. Additional kinetic enhancements are due to the production of H⁺ during the course of the reaction. Furthermore, the concentrations of the principal reaction intermediates, C₂Cl₆ and C₂Cl₄, are influenced substantially by the total available surface area of Fe⁰.

Full text of DOI:10.1021/es980273i

Environ. Sci. Technol. 1998, 32, 3011-3016
The chemical effects of ultrasound are due to the
Kinetics and Mechanism of the
phenomenon of acoustic cavitation (1). Sound travels
through a liquid as a wave consisting of alternating com-
pression and rarefaction cycles. If the sound wave has a
sufficiently high pressure amplitude, it can overcome the
intermolecular forces bonding the fluid. As a result, the liquid
will break down and voids will be created; that is, cavitation
bubbles will be formed. In most liquids, cavitation is initiated
with pre-existing microbubbles or weak spots, which are any
type of inhomogeneity in the fluid. The inhomogeneity can
be anything from particles to gas nuclei. These microbubbles
grow sequentially during the compression and rarefaction
cycles due to the phenomenon of rectified diffusion until
they reach a critical size; in subsequent compression cycles,
these cavities can collapse violently, releasing a large amount
of energy. This rapid implosion is accompanied by an
adiabatic heating of the vapor phase of the bubbles, which
yields localized but transient high temperatures and pres-
sures. Temperatures on the order of 5000 K have been
obtained experimentally (8), and pressures of the order of
Enhanced Reductive Degradation of
CCl by Elemental Iron in the
4
Presence of Ultrasound
H U I - M I N G H U N G A N D
M I C H A E L R . H O F F M A N N *
W. M. Keck Laboratories, California Institute of Technology,
Pasadena, California 91125
Enhanced rates of sonolytic degradation of CCl4 in the
0
presence of Fe are demonstrated. In Ar-saturated solutions,
the first-order rate constant for CCl4 degradation is kUS
-
1
0
)
0.107 min , whereas in the presence of Ar and Fe , the
apparent first-order rate constant is found to depend on
the total surface area of elemental iron in the following
1
000 bar have been calculated (9).
0
0
0
-1
Elemental iron (Fe ) is a mild reductant with an E
H
° )
fashion: kobs ) (kUS + kFe AFe ) min , where kUS ) 0.107
-
1
0
-2
-1
0
-0.44 V. It has been proposed as a suitable source of electrons
for the in situ remediation of contaminated groundwater
because of its low cost and nontoxicity (10). Fe0 barrier
systems have the potential to replace the pump-and-treat
systems for the remediation of contaminated groundwater
to reduce the chlorinated compounds to nonchlorinated
products (11, 12). The kinetics of carbon tetrachloride
min , kFe ) 0.105 L m min , and AFe ) ) reactive surface
area of Fe in units of m L . In the coupled ultrasound
and iron system, the contribution to the overall degradation
0
2
-1
0
rate by direct reaction with Fe results in an overall rate
enhancement by a factor of 40. These enhancements are
attributed (1) to the continuous cleaning and chemical
0
activation of the Fe surface by the combined chemical and
0
dechlorination in the presence of elemental Fe have been
physical effects of acoustic cavitation and (2) to accelerated
mass transport rates of reactants to the Fe surfaces.
reported by Tratnyek and co-workers (10, 13-15). In addition
to degrading chlorinated solvents, zero-valent metals may
prove to be useful for the reduction of nitro aromatic
compounds to anilines, which can be further degraded
microbiologically (16-19).
0
Additional kinetic enhancements are due to the production
+
of H during the course of the reaction. Furthermore,
the concentrations of the principal reaction intermediates,
C2Cl6 and C2Cl4, are influenced substantially by the total
Because acoustic cavitation can increase the surface area
of the reactive solids by causing particles to rupture,
0
available surface area of Fe .
0
sonication in the presence of Fe has been explored for the
dechlorination of trichloroethylene (TCE) (20). In this study,
0
we investigate the combination of ultrasound and Fe for
Introduction
CCl
4
degradation. Carbon tetrachloride is degraded by
Ultrasound is widely used for medical imaging, emulsifica-
tion, plastic welding, and a wide variety of synthetic ap-
plications (1). In addition, ultrasonic irradiation has been
applied as an advanced oxidation technology for water
treatment (2, 3). Upon ultrasonic irradiation, organic
compounds in water are degraded via several mechanisms.
Three main pathways, which involve hydroxyl radical oxida-
tion, pyrolytic degradation, and supercritical water reactions,
have been proposed (3). In the case of an aqueous solution,
water vapor present in the bubble is homolytically split to
pyrolysis reaction inside the bubble, by reduction on the
0
0
surface of Fe , and synergistically on Fe surface sites created
by ultrasonic effects. The combination of ultrasound and
0
Fe is a heterogeneous reaction system involving solid/ liquid,
gas/ solid, and gas/ liquid interfaces.
There are two modes of cavitational bubble collapse that
can affect the surface of solids (21). First, cavitational bubble
collapse directly on a surface may cause direct damage by
shock waves produced upon implosion. These bubbles are
formed on nuclei such as surface defects, entrapped gases,
or impurities on the surface of the material. Second,
cavitational collapse near the solid surface in the liquid phase
will cause microjets to hit the surface and produce a
nonsymmetrical shock wave. This phenomenon results in
the well-known cleaning action of ultrasound. As a conse-
quence of these events, more reactant surface area is readily
formed for further surface reactions.
•
•
yield H and OH radicals. Chemical substrates present within
the vapor phase or in the nearby liquid of the collapsing
•
bubbles are subject to direct attack by OH radical (4, 5).
2 4
Volatile compounds such as H S and CCl partition into the
gas phase (i.e., into the gaseous bubbles within the aqueous
solution) and undergo direct pyrolysis (1, 5, 6). Furthermore,
it has been found that hydrolysis reactions are accelerated
by several orders of magnitude in the presence of ultrasound.
These accelerated reaction rates have been attributed to the
existence of transient supercritical water during ultrasonic
irradiation (7).
4
In this paper, kinetics and mechanism of CCl sonolysis
0
in aqueous solution in the presence of two different Fe
0
samples are presented. In addition, the effect of Fe on the
fate and behavior of the primary reaction intermediates,
tetrachloroethene (C
examined.
2 4 2 6
Cl ) and hexachloroethane (C Cl ), is
*
Author to whom correspondence should be addressed. Phone:
(
626)395-4931; fax: (626)395-3170; e-mail: mrh@cco.caltech.edu.
S0013-936X(98)00273-9 CCC: $15.00
Published on Web 08/14/1998

1998 Am erican Chem ical Society
VOL. 32, NO. 19, 1998 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
3 0 1 1

Technical Background
TABLE 1. Physical Dimensions of the Different Frequency
Transducers
Mechanism of CCl
of CCl has been studied by several research groups (1, 3,
2-24). The overall reaction mechanism (3) can be written
as
4
Sonolysis. The sonolytic degradation
4
emitting
diam, cm
emitting
sonication
vol, mL
2
areas, cm2
frequency
2
0 kHz + Fe0 powder
1.27
1.27
1.27
1.27
245
285
0
)
9
))
20 kHz + Fe turnings
•
•
CCl4
8 Cl + CCl
(1)
3
)
9
))
8 Cl + :CCl2
4 2
CCl can also be degraded by H produced from the reaction
of H with Fe .
CCl4
2
(2)
(3)
(4)
(5)
+
0
•
•
CCl f Cl + :CCl
3
2
Fe + 2H f Fe2+ + H2
0
+
(15)
(16)
•
•
CCl + CCl f CCl + :CCl
-
+
3
3
4
2
CCl + H f HCCl + Cl + H
4
2
3
•
•
CCl + CCl f C Cl
6
3
3
2
The primary product observed after several hours of reaction
is chloroform. Neither C Cl nor C Cl is observed as an
2
4
2
6
where ))) indicates sonolysis. The trichloromethyl radical,
produced via the pyrolysis of CCl in eq 1, is an effective
scavenger of the hydroxyl radical as follows:
intermediate. Matheson and Tratynek reported a pH increase
during the course of the reaction that is consistent with the
stoichiometry of eq 14. In addition, they observed that the
4
0
•
•
rate of dehalogenation by Fe was faster at low pH (10).
CCl + OH f HOCCl
(6)
as
3
3
Experimental Procedures
HOCCl
follows:
3
reacts rapidly to yield phosgene, HCl, and CO
2
Commercially available organic high-purity reagents were
used without further purification. They included carbon
HOCCl f HCl + COCl2
(7)
(8)
3
tetrachloride, CCl
hexachloroethane, C
hene, C Cl , 99.9% (Sigma-Aldrich). Pentane (Omnisolv
4
, 99.9% (J. T. Baker); chloroform, LC grade;
2
6
Cl , 98% (Aldrich); and tetrachloroet-
COCl + H O f CO + 2HCl
2
2
2
2
4
grade, EM Science) was used as the carrier solvent for the
GC. Aqueous solutions were made with water obtained from
a MilliQ UV Plus water purification system (18.2 m Ω‚cm
resistivity).
Dichlorocarbene, which is produced via eq 2, can couple
with itself to form tetrachloroethylene
:
CCl + :CCl f C Cl
(9)
2
2
2
4
Two different sources of elemental iron were used in this
study. Iron powder was obtained from J. T. Baker, and
or it can react with water to form carbon monoxide and
hydrochloric acid as follows:
0
electrolytic iron turnings were obtained from Fluka. The Fe
turnings were of a coarser grain size (mostly >40 mesh) and
0
were cleaner and smoother than the Fe powder and had a
:
CCl + H O f 2HCl + CO
(10)
2
2
0
high metallic luster. The nominal purity of the Fe turnings
was 99.9%, with the remainder consisting primarily of iron
Chlorine atoms self-react to form molecular chlorine, which
hydrolyzes to yield hypochlorous acid and chloride ion:
oxides. Trace impurities were <0.02% by weight of C,
<0.008% by weight of S, <0.003% by weight of Si, <0.02% by
+
H O
weight of P, <0.02% by weight of Mn, and <5 ppm of Mg
(19). The desired concentrations of CCl in Ar-saturated water
were prepared by dilution of a saturated CCl stock solution
2
•
2
Cl f Cl2
8 HOCl + HCl
(11)
4
4
0
Mechanism of CCl
4
Reduction by Fe . The following
(5.1 mM), which was made by stirring excess CCl with Ar-
4
mechanism (eqs 12-16) for the reductive dehalogenation of
CCl by elemental iron has been proposed by Matheson and
Tratnyek (10).
saturated water.
Sonolyses were performed with a Branson 200 sonifier
operating at 20 kHz. The temperature was maintained at 15
4
°
C with a Haake A80 refrigerated bath and circulator to
compare the results with this study with those of Matheson
and Tratnyek (10). Replaceable titanium tips on the 20 kHz
transducer were polished and the transducer was tuned
before each use to give a minimum power output when
vibrating in air. The tuning process is a standard procedure
to bring the transducer into resonance as part of the complete
probe assembly. Sonolytic reactions at 20 kHz were per-
formed in a 300 mL airtight reactor cell. The physical
dimensions and fundamental characteristics of the trans-
ducer are given in Table 1; the reactor configuration is similar
to the design shown in Hua and Hoffmann’s paper (3).
4
In the kinetic experiments, 285 mL of 0.1 mM CCl solution
was transferred into a water-jacketed glass cell, which was
closed from the atmosphere. The bottom of the glass reactor
was designed with a 1 cm indentation in the center for
reflection of the sound waves and for an even distribution
of the cavitation bubbles in the solution. The reactor was
made gastight with two O-ring seals in the threaded Teflon
3
0 1 2
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 32, NO. 19, 1998

4
FIGURE 1. First-order plot for CCl degradation: (0) by ultrasound
0
alone; ([) by ultrasound + 2 g of Fe powder; the solid lines are
the fitting results.
FIGURE 3. Ratio of maximum concentration of C
2
Cl
4 2 6
and C Cl as
0
functions of [Fe ] with respect to CCl
4
initial concentration: (a) for
0
0
Fe powder; (b) for Fe turnings.
0
FIGURE 2. Effect of reactive surface area of Fe on the pseudo-
0
first-order rate constant for CCl
(
4
degradation by (a) Fe powder and
0
0
b) Fe turnings [∆k ) k (ultrasound + Fe ) - k (ultrasound)].
collar that connects the glass cell to the stainless steel probe.
In addition, sampling ports were sealed with Teflon valves
and covered with rubber septa. Sample aliquots were
withdrawn with a 1 mL Hamilton syringe, filtered through
a 0.45 µm nylon filter into a 2.5 mL glass vial with a PTFE/
silicone septum-lined threaded cap, and mixed with 0.5 mL
-
FIGURE 4. Fitting results for [Cl ] concentration by eq 37: (a)
0
ultrasound only; (b) ultrasound + 2 g of Fe powder.
of pentane. For quantification of the concentrations of CCl
4
,
C
2
Cl , and C Cl , 0.5 µL samples of the pentane extract were
4
2
6
analyzed with an HP 5880A gas chromatograph with an
electron capture detector (GC-ECD) operated in the splitless
mode and equipped with an HP-5 column. The instrument
was calibrated with commercial standards. Duplicate mea-
surements were made for each sample. Time-sequenced
samples were analyzed immediately by GC-ECD. The
aqueous phase in the sample vial was analyzed by ion-
Results and Discussion
4
Experiments were performed at an initial CCl concentration
-4
of 1 × 10 M and an initial pH of 7. Degradation of CCl
4
was found to be >99% of its initial concentration after 90
min of exposure to ultrasound. Loss of CCl was found to
4
be <2% in the control experiments without ultrasound and
iron. The pH at the completion of the reaction under
ultrasound was 3.5; and with the combination of ultrasound
-
exchange chromatography (IC) for Cl . The IC system was
0
a Dionex Bio-LC system equipped with a conductivity
detector and a Dionex OmniPac AS-II column. The ultra-
sonic power output was measured by conventional calo-
rimetry (21). The measured output power employed in this
study was 62 W.
and Fe , the final pH was 6 (presumably due to the formation
-
0
of OH from Fe reduction according to eq 14).
First-order plots of [CCl ] versus time during sonolysis
4
0
and Fe reduction of an Ar-saturated solution at 20 kHz are
shown in Figure 1. The slopes from linear regression of the
VOL. 32, NO. 19, 1998 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
3 0 1 3

2
+
-
-
data yield the observed rate constants for CCl
The reaction rate for CCl degradation by ultrasound in the
presence of iron was also found to be first-order. In Figure
4
degradation.
Fe + CCl + H O f Fe + CHCl + OH + Cl
(20)
4
2
3
4
)))
9
CHCl3
8 :CCl + HCl
2
(21)
(22)
2
+
, the results for enhanced rate constants [∆k ) k (ultrasound
0
0
Fe ) - k (ultrasound)] for different sizes of the Fe particles
(
i.e., surface area) are shown. The iron powder had a higher
:CCl2 + :CCl2 f C2Cl4
reactive surface area than iron turnings; as a consequence,
0
4
Another possible pathway involves CCl sorption on the Fe
surface, which subsequently reacts with
during sonolysis as follows:
0
∆
k was enhanced by a factor of 8 for Fe powder at the same
0
•
CCl3 produced
Fe mass concentration. Figure 2 also shows that the rate
constants for CCl
4
reduction increased linearly with increas-
0
-1
ing mass of Fe per unit reaction volume (g L ) until it reached
a saturation value for mass concentrations larger than 30 g
)
9
))
•
•
CCl4
8 CCl + Cl
(23)
(24)
(25)
3
-
1
L
(for iron turnings).
Assuming a constant specific surface area of 0.005 m / g
2
1
•
1
2+
-
CCl + / Fe f CCl + / Fe + Cl
0
4
2
3
2
for Fe turnings (19), the following relationship for the reaction
0
rates with the Fe surface area was obtained by linear
2•
CCl f C Cl
6
regression:
3
2
k (min-1) ) kFe
+ 0.001 ) 0.105 (L m min-1) ×
-2
The reactions of eqs 20-25 help to explain the observed
higher concentrations of C Cl and C Cl in the combination
Fe / CCl system.
4
∆
0
AFe
0
2
4
2
6
2
-1
-1
AFe0 (m L ) + 0.001 min
(17)
0
Kinetic Analysis. The final reaction product observed
-
1
-
The differential rate constant (∆k) is in units of min and
during CCl
4
sonolysis is Cl . The overall degradation
the surface area concentration, AFe0, is in units of m2
-1
L
.
mechanism can be simplified as follows:
These results (eq 17) can be compared directly to the results
reported by Matheson and Tratnyek (10), who measured the
)
)) k1
0
rate of CCl
4
degradation by Fe without ultrasound as follows:
•
•
CCl4
8 CCl + Cl
(26)
(27)
(28)
(29)
3
k ) 0.0025AFe
0
+ 0.017
(18)
k2
2•
CCl3 98 C2Cl6
This comparison confirms that ultrasonic irradiation en-
k3
-
0
C Cl6
9
8 aCl + other products
hances the rate of CCl
4
degradation due to Fe by a factor of
0 compared to Fe in the absence ultrasound.
The higher degradation rate contributed by Fe0 in our
2
0
4
k4
•
•
Cl + H O f f HCl + OH
2
study can be attributed to the indirect chemical effects
associated with the continuous ultrasonic cleaning and
with the corresponding kinetic equations
0
activation of the Fe surface, the enhanced rate of mass
transport resulting from the turbulent effects of cavitation,
and the lower pH caused by CCl sonolysis. The shock wave
4
d[CCl ]/ dt ) -k [CCl ]
(30)
(31)
(32)
(33)
(34)
4
1
4
and microjets formed during cavitational bubble collapse
are primarily responsible for the cleaning actions of ultra-
sound. Transient cavitation results in turbulent flow condi-
tions within the reactor that enhance overall mass transport.
The iron corrosion accelerated by ultrasound may also
•
•
2
d[ CCl ]/ dt ) k [CCl ] - 2k [ CCl ]
3
1
4
2
3
•
2
d[C Cl ]dt ) k [ CCl ] - k [C Cl ]
2
6
2
3
3
2
6
•
•
contribute to the reductive dehalogenation of CCl
4
via eq 16.
d[ Cl]/ dt ) k1[CCl4] - k4[ Cl]
The effect of a drop in pH during sonication is to produce
0
-
•
a faster rate of reduction of Fe . Since the sonolytic
d[Cl ]/ dt ) k [ Cl] + ak [C Cl ]
4
3
2
6
+
degradation of CCl
4
produces H according to eqs 10 and 11,
we expect to see a faster rate in the combined system. The
Fe reduction reaction involving H is as follows:
The solution to the coupled differential eqs 30-34 can be
written as
0
+
Fe + CCl + H f Fe2+ + CHCl + Cl
+
-
(19)
k1
4
3
-k3t
- e-k1t)
[
C Cl ] )
[CCl ] (e
(35)
2
6
4 0
2
(k - k )
1
3
C
2
Cl
sonolysis discussed previously (3, 25). However, these
intermediates were not measured by Matheson and Tratnyek
10). The principal products observed during CCl reduction
sonolysis, the
increased with time and
4 2 6
and C Cl were observed as intermediates during
CCl
4
ak1
-
-k1t
-k3t
[
Cl ] ) [CCl ] 1 - e
+
(1 - e ) -
4 0
[
2(k - k )
1 3
(
4
ak3
0
by Fe are CHCl
3
and CH
2
Cl
2
. During CCl
Cl
4
1 - e-k1t) (36)
(
2(k - k )
1 3
]
concentrations of C
2
Cl and C
4
2
6
then decreased during continued sonication after reaching
maximum concentrations. The maximum concentrations
-
for C
2
Cl
6
and Cl , respectively (25). On the basis of our
of C
concentration of Fe in Figure 3. The measured C
Cl
concentrations increase with increasing Fe0 mass
concentration up to 10 g/ L. This effect can be explained by
the production of higher levels of chloroform (CHCl ) from
by Fe . We have
sonolysis produces more C Cl
(25). The overall reactions consistent with these
observations are as follows:
2
Cl
4
and C
2
Cl
6
are shown as functions of the mass
previous measurements, we know that k
degradation rate constant for sonolysis alone, ≈ k
corresponding C Cl degradation rate constant. Thus, eq 36
can be further simplified to
1
, which is the CCl
4
0
2
Cl
4
and
3
, the
C
2
6
2
6
3
the reductive dehalogenation of CCl
previously shown that CHCl
than C Cl
4
[Cl ] ≈ [Cl ] (1 - e-kCl-t)
-
-
(37)
0
∞
3
2
4
0
2
6
The effect of Fe can be treated as a simple step addition to
the mechanism of eqs 26-29 as follows:
3
0 1 4
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 32, NO. 19, 1998

k′
1
•
-
1
2+
CCl + / Fe
9
8 CCl + Cl + / Fe
(38)
(39)
4
2
3
2
4
TABLE 2. Comparison of k Values from CCl Degradation and
- 0 0
Cl Production: (a) Fe Powder and (b) Fe Turnings
As a consequence, eq 30 can be rewritten as
-
1
-
-1
kCCl , min
kCl , min
4
-
(k1+k′)t
a) Fe0 powder m ass concn, g/L
0
(
[
CCl ] ) [CCl ] e
4 4 0
0.13
0.15
0.20
0.20
0.20
0.13
0.14
0.16
0.18
0.18
2
8
1
.04
.16
6.33
where k′ (k′ ) ∆k) is the enhanced CCl
4
degradation rate in
the presence of Fe . Because Fe has a negligible influence
on the rate of C Cl degradation (18), C Cl
degraded by Fe0
can be ignored relative to the C Cl sonolysis. This allows
0
0
2
6
2
6
24.49
(b) Fe0 turnings m ass concn, g/L
0
2
6
the modification of eqs 35 and 36 to yield eqs 40 and 41 as
follows:
0.107
0.113
0.115
0.118
0.122
0.125
0.086
0.094
0.100
0.099
0.098
0.096
7
1
2
.02
4.0
1.0
(
k1 + k′)
-k3t
-(k +k′)t
1
28.1
42.1
[
C Cl ] )
[CCl ] (e
- e
)
(40)
2
6
4 0
2
(k + k′ - k )
1 3
-
-(k1+k′)t
[
Cl ] ) [CCl ] 1 - e
+
4
0
{
reaction intermediates, C
4 2 6
Cl and C Cl , which are observed
2
during the course of the reaction, appear to be influenced
a(k + k′)
1
-k3t
0
(
1 - e ) -
significantly by total available surface area of Fe . In
2
(k + k′ - k )
0
1
3
conclusion, the combination of ultrasound and Fe has a
ak3
positive synergistic effect on the dehalogenation of chlori-
nated hydrocarbons.
-(k +k′)t
1
(
1 - e
)
(41)
}
2
(k + k′ - k )
1
3
-
Acknowledgments
The [Cl ] concentration versus time predicted by eq 37 is
used to fit all of the kinetic data and obtain kCl- values as a
We appreciate the contributions by Dr. Weavers about the
details of the kinetics and mechanisms. Financial support
from the Office of Naval Research (NAV5-N0001492J1901;
NAV1-N47408-97-M-0771) and the Department of Energy
(DOE 1 963472402) is gratefully acknowledged.
0
function of [Fe ]. In Figure 4, the fitted results for ultrasound
0
alone and ultrasound + 2 g ofFe powder systems are shown.
Furthermore, the comparison of the rate constant, kCl-, for
-
[
Cl ] production and the rate constant, kCCl4, for CCl
4
0
degradation as a function of [Fe ] is shown in Table 2. If we
suppose that eq 41 can be simplified to eq 37, the kCl- obtained
Literature Cited
from Figure 4b embodies the contribution of k′, the enhanced
0
CCl
4
degradation rate by Fe . As mentioned above, k′ is larger
for Fe powder than Fe turnings. On the basis of the data
summary in Table 2, we see that k′ contributes more to kCl-
which shows greater increases for Fe powder than for Fe
turnings. From these comparisons, it is clear that kCl-
increases with [Fe ] but the relative magnitude of the increase
is less than that for kCCl4. This discrepancy could be due to
the contribution of k′ in eq 39; the difference between kCl-
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0
0
(
2) Cheung, H. M.; Bhatnagar, A.; Jansen, G. Environ. Sci. Technol.
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3) Hua, I.; Hoffmann, M. R. Environ. Sci. Technol. 1996, 30, 864-
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,
1
0
0
(
8
0
(4) Kotronarou, A.; Mills, G.; Hoffmann, M. R. J. Phys. Chem. 1991,
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(
(
(
5) Kotronarou, A.; Mills, G.; Hoffmann, M. R. Environ. Sci. Technol.
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99, 2335-2342.
0
and kCCl4 is consistent with increasing Fe mass concentration.
The degradation rate constant for ultrasound alone in
-
1
-3 -1
this study is 0.107 min ()1.8 × 10
this is equivalent to a first-order rate constant of 4.3 × 10
adjusted to 50 W in 95 mL through power density
normalization. This value is faster than that reported by
s ) at 62 W in 285 mL;
-
3
-
1
s
(8) Suslick, K. S.; Hammerton, D. A.; Cline, R. E. J. Am. Chem. Soc.
986, 108, 5641-5642.
1
-
3
-1
(9) Shutilov, V. A. Fundamental Physics of Ultrasound; Gordon &
Breach Science: New York, 1988.
Hua and Hoffmann (3) (i.e., 3.3 × 10
s
in 50 W and 95
mL). The relative enhancement may have been due to a
power density effect in which increased power results in
enhanced chemical effects instead of simply heating the
solution due to relative inefficiencies in energy transfer (26).
(
10) Matheson, L. J.; Tratnyek, P. G. Environ. Sci. Technol. 1994, 28,
2045-2053.
(11) Fairweather, V. Civil Eng. 1996, 66, 44-48.
(
(13) Tratnyek, P. G.; Johnson, T. L.; Scherer, M. M.; Eykholt, G. R.
12) Tratnyek, P. G. Chem. Ind. 1996, 499-503.
0
For the combination of ultrasound and Fe , Reinhart et al.
also observed similar rate enhancements for the reduction
of trichloroethylene (TCE) by Fe (20). In addition, they
reported an increase in the final pH of the mixture in the
Groundwater Monit. Rem. 1997, 17, 108-114.
14) Scherer, M. M.; Westall, J. C.; Ziomek-Moroz, M.; Tratnyek, P.
(
0
G. Environ. Sci. Technol. 1997, 31, 2385-2391.
(15) Johnson, T. L.; Fish, W.; Gorby, Y. A.; Tratnyek, P. G. J. Contam.
Hydrol. 1998, 29, 377-396.
0
TCE-Fe system compared with the application of ultrasound
(
(
(
16) Gillham, R. W.; Ohannesin, S. F. Groundwater 1994, 32, 958-
alone.
9
67.
In this study, we demonstrated that the rate of sonolytic
can be enhanced by the presence of Fe .
The higher apparent rates of reaction in the coupled US/ Fe
system compared to Fe reaction alone can be attributed to
the continuous cleaning and activation of the Fe surface by
the chemical and physical effects of ultrasound. Further-
more, faster mass transport rates resulting from the hydro-
dynamic cavitation and production of H during the course
17) Lipczynskakochany, E.; Harms, S.; Milburn, R.; Sprah, G.;
Nadarajah, N. Chemosphere 1994, 29, 1477-1489.
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Technol. 1996, 30, 2634-2640.
0
degradation of CCl
4
0
0
0
(19) Agrawal, A.; Tratnyek, P. G. Environ. Sci. Technol. 1996, 30, 153-
1
60.
(
20) Reinhart, D. R.; Clausen, C.; Geiger, C.; Ruiz, N.; Afiouni, G. F.
Enhancement of In-Situ Zero-Valent Metal Treatment of
Contaminated Groundwater. In Non-aqueous Phase Liquids in
Surface Environment: Assessment and Remediation; American
Society of Civil Engineers: Washington, DC, 1996; pp 323-332.
+
of the reaction also contribute to the observed rate en-
hancements. The relative concentrations of the principal
VOL. 32, NO. 19, 1998 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
3 0 1 5

(
(
21) Mason, T. J. Practical Sonochemistry: User’s Guide to Application
in Chemistry and Chemical Engineering; Ellis Horwood: Chi-
chester, U.K., 1991.
(26) Hung, H.-M.; Hoffmann, M. R. The Effect of Frequency,
Hydrostatic pressure, Temperature and Sonication Power on
Sonochemical Reactions. Unpublished results.
22) Wu, J. M.; Huang, H. S.; Livengood, C. D. Environ. Prog. 1992,
1
1, 195-201.
(
(
23) Francony, A.; Petrier, C. Ultrason. Sonochem. 1996, 3, S77-S82.
24) Bhatnagar, A.; Cheung, H. M. Environ. Sci. Technol. 1994, 28,
Received for review March 17, 1998. Revised manuscript
received June 1, 1998. Accepted June 15, 1998.
1
481-1486.
(
25) Hung, H.-M.; Hoffmann, M. R. Environ. Sci. Technol. 1998,
submitted for publication.
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