Environ. Sci. Technol. 1998, 32, 3194-3199
with each successive ultrasonic frequency cycle until they
Kinetics and Mechanism of the
Sonolytic Destruction of Methyl
tert-Butyl Ether by Ultrasonic
reach a critical resonance frequency size that results in the
violent collapse of gas bubbles. The rapid implosion of
cavitation bubbles is accompanied by adiabatic heating of
the vapor phase of the bubble that yields localized transient
high temperature and pressure. Temperatures and pressures
obtained upon bubble collapse have been estimated to be
on the order of 4200 K and 975 bar, respectively (11).
Temperatures exceeding 5000 K have been reported (12) in
the ultrasonic cavitation of organic and polymeric liquids.
Water vapor under these conditions undergoes a thermal
Irradiation in the Presence of Ozone
J O O N - W U N K A N G
Department of Industrial Environment & Health, Yonsei
University, Wonju Campus, 234 Maeji, Wonju, Korea 220-710
•
•
dissociation to yield extremely reactive radicals, H and OH ,
M I C H A E L R . H O F F M A N N *
•
and in the presence of O
2
yields HO
2
. As a result, organic
W. M. Keck Laboratories, California Institute of Technology,
Pasadena, California 91125
compounds present near bubble/ water interface can undergo
thermal decomposition, and/ or secondary reactions take
place between solute molecules and the reactive radicals.
Even though the basic physics and chemistry of reactions of
cavitation are fairly well understood, many questions con-
cerning detailed reaction mechanisms remain unanswered.
The kinetics and mechanism of the sonolytic degradation
of methyl tert-butyl ether (MTBE) have been investigated
at an ultrasonic frequency of 205 kHz and power of 200 W
In particular, the combined process of ozonation and
sonolysis needs to be explored further. Thus, the primary
objective of this study is to determine the kinetics and
mechanism of the sonolytic reaction of ozonation and
sonolysis for the rapid degradation of MTBE in the aqueous
solution.
-
1
L . The observed first-order degradation rate constant
-
4
-1
for the loss of MTBE increased from 4.1 × 10
s
to 8.5
-
4
-1
×
10
s
as the concentration of MTBE decreased
from 1.0 to 0.01 mM. In the presence of O3, the sonolytic
rate of destruction of MTBE was accelerated substantially.
The rate of MTBE sonolysis with ozone was enhanced by
a factor of 1.5-3.9 depending on the initial concentration
of MTBE. tert-Butyl formate, tert-butyl alcohol, methyl acetate,
and acetone were found to be the primary intermediates
and byproducts of the degradation reaction with yields of 8,
Experimental Methods
MTBE (99.9%; EM Science), tert-butyl formate (99%; Aldrich),
tert-butyl alcohol (99%; EM Science), acetone (99.5%; EM
Science), methyl acetate (99.5%, Aldrich), sodium bicarbonate
(Reagent Grade; EM Science), and other chemicals were used
without further purification.
5
, 3, and 12%, respectively. A reaction mechanism
The ultrasonic irradiations were performed with an
Ultrasonic Transducer USW 51 (AlliedSignal ELAC Nautik,
Inc.) in a glass and titanium reactor operating at 205 kHz
involving three parallel pathways that include the direct
pyrolytic decomposition of MTBE, the direct reaction of MTBE
with ozone, and the reaction of MTBE with hydroxyl
radical is proposed.
2
with an active acoustic vibration area of 25 cm . All reactions
were performed in a 500-mL double-walled (cooling jacket)
reaction chamber as described previously (7). The reactor
had four sampling ports to withdraw aqueous samples and
to allow gases to be introduced and vented. The interior
diameter of the reaction vessel was 6 cm. Temperature was
maintained constant at 20 °C throughout all kinetic runs.
Introduction
Since 1990, MTBE (methyl tert-butyl ether) has been used as
a gasoline additive to reduce volatile organic carbon emis-
sions from motor vehicles. However, the new gasoline
additives such as MTBE have had negative environmental
impacts on water quality due to leakage of petroleum
products from the underground storage tanks. For example,
MTBE, which is highly soluble (e.g., 0.35-0.71 M (1)), has
been detected at high concentrations in groundwater. MTBE
is reported to be resistant to both aerobic and anaerobic
microbial degradation (2), and it is poorly adsorbed on
granular activated carbon (GAC). In addition, due to its low
volatility, air stripping is not a viable process for the removal
of MTBE from water (1). Velleitner et al. (3) investigated the
Sonolyses were performed in water purified by a MilliQ
UV Plus system (R > 18 MΩ). Solutions containing O
prepared by bubbling ozone that was generated via the corona
discharge process on O with an Orec ozonator (model V10-
3
were
2
-1
0) into deionized water at a flow rate of 100 mL min through
a glass fritted diffuser until the desired aqueous-phase ozone
concentration was obtained. The target aqueous ozone
solution was monitored by UV spectrophotometry, using the
-
1
-1
3
molar extinction coefficient of 3300 M cm for O in water
at 254 nm (13).
reaction of MTBE and O
3
in water, and they reported that 5.5
All kinetic runs were made in the batch mode without
adding a buffer. The initial pH values of the solutions were
in the range of 6.6-6.8. MTBE stock solutions (100 mM)
were prepared and stored at 4 °C until use. After being
saturated, the ozone solutions were prepared, an appropriate
volume of the MTBE stock solution was spiked into 500 mL
of water saturated with ozone and was mixed with bubbling
ozone gas for a few more seconds at a low flow rate to
minimize any sparging effects. After the ozone supply valve
was closed, the t ) 0 samples were taken, and the sonolytic
irradiation was initiated. Typical irradiation intensity was
mol of ozone/ mol of MTBE was required for 80% net
destruction of MTBE. Barreto et al. (4) degraded MTBE
photocatalytically and found that the degradation rate was
significantly faster than the direct reaction between MTBE
3
and O .
Over the last several years, ultrasonic irradiation has been
explored for the treatment of chemical contaminants in
aqueous solution (5-10). The chemical effects of sonolysis
are the direct result of the formation of cavitation mi-
crobubbles. The sonolytically induced microbubbles grow
-
1
at a power density at 200 W L . During each 60-min kinetic
run, 1.0 mL of sample was taken at appropriate time intervals
and stored in 20-mL Teflon-faced aluminum-sealed vials. In
*To whom correspondence should be addressed. Phone:
(
818)395-4931; fax: (818)395-3170; e-mail: mrh@cco.caltech.edu.
3
1 9 4
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 32, NO. 20, 1998
S0013-936X(97)00874-2 CCC: $15.00
1998 Am erican Chem ical Society
Published on Web 09/04/1998