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Journal of Hazardous Materials B89 (2002) 197–212
Kinetics and products of reactions of MTBE with
ozone and ozone/hydrogen peroxide in water
Marie M. Mitani
a
, Arturo A. Keller
a
,
∗
, Clifford A. Bunton
b
,
Robert G. Rinker
c
, Orville C. Sandall
c
a
Bren School of Environmental Science and Management, University of California, 4666 Physical Sciences
North, Santa Barbara, CA 93106-5131, USA
b
Department of Chemistry and Biochemistry, University of California, Santa Barbara, CA, USA
c
Department of Chemical Engineering, University of California, Santa Barbara, CA, USA
Received 30 August 2000; received in revised form 30 June 2001; accepted 9 July 2001
Abstract
Methyl-
t
-butyl-ether (MTBE) has become a prevalent groundwater pollutant due to its high
volume use as a nationwide gasoline additive. Given its physicochemical properties, it requires new
treatment approaches. Both aqueous O
3
and a combination of O
3
/H
2
O
2
, which gives
•
OH, can
remove MTBE from water, making use of O
3
a viable technology for remediation of groundwater
from fuel contaminated sites. Rate constants and temperature dependencies for reactions of MTBE
with O
3
or with
•
OH at pH 7.2, in a range of 21–45
◦
C (294–318 K) were measured. The second-order
rate constant for reaction of MTBE with O
3
is 1
.
4
×
10
18
exp(
−
95.4/
RT
)(M
−
1
s
−
1
), and for reaction
of MTBE with
•
OH produced by the combination of O
3
/H
2
O
2
is 8
.
0
×
10
9
exp(
−
4.6/
RT
)(M
−
1
s
−
1
),
with the activation energy (kJ mol
−
1
) in both cases. At 25
◦
C, this corresponds to a rate constant of
27 M
−
1
s
−
1
for ozone alone, and 1
.
2
×
10
9
M
−
1
s
−
1
for O
3
/H
2
O
2
. The concentration of
•
OH was
determined using benzene trapping. Products of reactions of O
3
and O
3
/H
2
O
2
with MTBE, including
t
-butyl-formate (TBF),
t
-butyl alcohol (TBA), methyl acetate, and acetone, were determined after
oxidant depletion. A reaction pathway for mineralization of MTBE was also explored. Under
continuously stirred flow reactor (CSTR) conditions, addition of H
2
O
2
markedly increases the rate
and degree of degradation of MTBE by O
3
. © 2002 Elsevier Science B.V. All rights reserved.
Keywords:
MTBE; Ozone; Reaction pathway; Hydroxyl radical; Kinetics; Byproducts; TBA; TBF
1. Introduction
Methyl-
t
-butyl-ether (MTBE) has been used as a gasoline oxygenate in the US for over
two decades. It eliminates the need for leaded gasoline and is the most common fuel oxy-
∗
Corresponding author. Tel.:
+
1-805-893-7548; fax:
+
1-805-893-7612.
E-mail address:
keller@bren.ucsb.edu (A.A. Keller).
0304-3894/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved.
PII: S0304-3894(01)00309-0
198
M.M. Mitani et al. / Journal of Hazardous Materials B89 (2002) 197–212
Nomenclature
concentration of benzene in outlet (mol l
−
1
)
C
Bf
concentration of benzene in inlet (mol l
−
1
)
C
Bo
concentration of MTBE in outlet (mol l
−
1
)
C
mf
concentration of MTBE in inlet (mol l
−
1
)
C
mo
concentration of
•
OH (mol l
−
1
)
C
OH
outlet concentration of O
3
(mol l
−
1
)
C
O
3
rate constant for reaction of MTBE with O
3
(M
−
1
s
−
1
)
k
1
rate constant for reaction of MTBE with
•
OH (M
−
1
s
−
1
)
k
2
rate constant for reaction of benzene and
•
OH (M
−
1
s
−
1
)
k
3
global rate of disappearance of benzene (M s
−
1
)
R
B
global rate of disappearance of MTBE (M s
−
1
)
R
m
volumetric flow rate of outlet stream (ml s
−
1
)
v
f
volumetric flow rate of inlet stream (ml s
−
1
)
v
o
V
r
volume of reactor (l)
genate used to reduce air pollution and increase octane ratings [1]. MTBE may comprise
up to 15% by volume of gasoline, and it became the second highest volume chemical pro-
duced in the US in 1997 [2]. The high volume use as well as the chemical characteristics of
this gasoline additive have resulted in contaminated water supplies around the world where
MTBE is used as a gasoline additive.
MTBE is very water soluble, making its movement in the environment almost as fast
as groundwater, with practically no retardation due to sorption on soil particles. Once re-
leased, MTBE is quite persistent due to its molecular structure, i.e. the presence of the
t
-butyl
group, which inhibits environmental degradation under normal conditions and strongly in-
hibits natural biodegradation [3,4]. This results in widespread contamination when MTBE
escapes into the environment. A major concern arises from leaking underground fuel tanks
that contaminate groundwater at much higher concentrations than surface sources. Con-
tamination of lakes and rivers by two-stroke gasoline engines is also a problem [5]. MTBE
uncontained in the environment inevitably results in groundwater pollution, and it was the
second most frequently detected chemical in samples of shallow ambient groundwater from
the US Geological Survey’s National Water Quality Assessment Program [6].
Although, recent progress in in situ treatment has been reported, there are many cir-
cumstances where aboveground treatment is required [7]. Some methods simply separate
MTBE from water, such as air stripping or GAC adsorption, while others involve oxida-
tion to harmless products [8]. Although, separation techniques may be less expensive than
oxidation, they require an additional step for the treatment or disposal of the pollutant.
Ozonation has been shown to be a viable option in the treatment of waste and drinking
water. With the development of large scale ozone (O
3
) generators and lower operating costs,
there has been increasing interest in using O
3
to remove compounds that are difficult or
too expensive to remove by other methods. In some cases, O
3
treatment alone adequately
degrades contaminants to meet water quality standards [9].
M.M. Mitani et al. / Journal of Hazardous Materials B89 (2002) 197–212
199
O
3
may either react directly with organic compounds, or decompose generating more
reactive species, such as the hydroxyl radical (
•
OH), which control subsequent oxidation
reactions [10].
•
OH is one of the most important oxidants due to its high reactivity and
unselectivity towards organic compounds. The addition of hydrogen peroxide (H
2
O
2
)to
O
3
in water generates
•
OH, thereby increasing the oxidative capabilities of the system.
Appendix A presents O
3
and O
3
/H
2
O
2
chemistry that is applicable to this study. The rate
of decomposition of O
3
in water,
R
O
3
(mol l
−
1
min
−
1
) can be calculated using the rate
equation derived by Sotelo et al. [20]:
10
5
exp
−
[O
3
]
4964
T
R
O
3
=
3
.
26
×
+
5
.
69
10
18
exp
−
[OH
−
]
0
.
5
[O
3
]
1
.
5
10130
T
×
(1)
There are several recent studies of MTBE oxidation using ultrasonic irradiation in the
presence of ozone, UV/H
2
O
2
, or simply ozone [11–13]. Optimally, O
3
oxidation should
completely mineralize MTBE to CO
2
and H
2
O. Byproducts from incomplete oxidation
of MTBE during ozonation are of great concern because they may be as toxic, or more,
than MTBE.
t
-Butyl-formate (TBF) and
t
-butyl alcohol (TBA) are major initial products
in many oxidative reactions of MTBE [4,8,13]. From recent toxicologic studies, TBF and
TBA may pose greater health hazards than MTBE [14]. Therefore, it is important to identify
the various products of reactions between MTBE and O
3
or O
3
/H
2
O
2
, under conditions of
incomplete oxidation.
This study focuses on the reaction of MTBE with O
3
and O
3
/H
2
O
2
after oxidant de-
pletion, as well as the kinetics of MTBE oxidation and the intermediate products formed
from reaction, in order to better understand the oxidation process for treatment of MTBE
contaminated water.
2. Experimental methods
2.1. Oxidation kinetics
To determine the order of the reaction between MTBE and ozone, a stirred, 2000 ml
batch reactor was used at a constant temperature. MTBE or ozone was monitored at various
times with the other reactant in large excess. The reaction rates, at various initial concen-
trations of either MTBE or ozone, were extrapolated to zero time to obtain the reaction
order with respect to each reactant. Initial concentrations for MTBE ranged from 0.34 to
1.2 ppm (0.0035–0.014 mM), and initial ozone concentrations ranged from 6.0 to 12.1 ppm
(0.13–0.25 mM). For both sets of experiments, ozone was bubbled into a buffered solution
in the batch reactor until equilibrium conditions were attained, and then MTBE was quickly
injected and stirred continuously. Five to eight sets of samples were taken in timed intervals
of 10 s for MTBE and 30 s for ozone and analyzed immediately.
To determine the rate of reaction, kinetic studies were carried out by using a 1000 ml
continuous flow stirred reactor (CFSR) system. Inlet streams of aqueous MTBE and O
3
200
M.M. Mitani et al. / Journal of Hazardous Materials B89 (2002) 197–212
saturated water were pressure fed into the reactor. The O
3
stream used the pressure from
the O
3
generator and the MTBE solution reservoir was pressurized with nitrogen, and flow
rates of the inlet streams were controlled with calibrated rotometers. The flow rate was
0.3 ml s
−
1
for each reactant solution. Experiments with added H
2
O
2
involved a reference
reactant, benzene, in order to quantify
•
OH. When H
2
O
2
was added, flow rates were in-
creased to 1.5 ml s
−
1
.H
2
O
2
was added to the flask containing MTBE and benzene, where
previous experiments showed that H
2
O
2
by itself does not react with MTBE or benzene
at the concentrations and temperatures used for this study. Other studies indicate similar
results [4,13]. The reactor was placed in a constant temperature bath for temperature con-
trol, with experiments performed in a range of 18–50
◦
C. At steady state, inlet and outlet
samples were withdrawn by using 20 ml syringes, and the samples were analyzed for the
organic reactants and products and for O
3
. Samples were analyzed immediately after being
withdrawn from the reactor, or within a 30 min time period where no change in concentra-
tion was observed. Inlet O
3
concentrations in aqueous solution ranged from 6.1 to 6.7 ppm
(0.13–0.14 mM). MTBE concentrations ranged from 8.7 to 11.8 ppm (0.10–0.13 mM) and
benzene concentrations were in the range from 7.5 to 11.1 ppm (0.10–0.14 mM). Reaction
conditions were designed so that there was residual MTBE and benzene in the outlet.
2.2. Product formation
Product studies were conducted batchwise in 40 ml amber vials with known volumes and
concentrations of organic substrates (MTBE, TBF, or TBA in water). A known amount of
aqueous O
3
, and O
3
/H
2
O
2
when applicable, was added to the vial and allowed to react until
there were no further reaction. This was indicated by no change in the substrate or products
concentrations over time. Concentrations of unreacted substrate and identifiable products
were then measured in a gas chromatograph/mass spectrometer (GC/MS), as explained in
more detail below. Initial O
3
concentrations in aqueous solution ranged from 4.8 to 6.0 ppm
(0.10–0.11 mM). MTBE, TBF, TBA, and acetone concentrations initially ranged from 7.4 to
14.5 ppm (0.08–0.20 mM). In all batch reactions, O
3
was the limiting reactant with residual
unreacted organic compounds. All reactions were at room temperature, 22–24
◦
C.
MTBE (Sigma–Aldrich), TBF (Aldrich), TBA (Aldrich), methyl acetate (Aldrich), ace-
tone (Fisher), and benzene (Fisher) at purities >99% were used. H
2
O
2
(30%) (Fisher) was
diluted as necessary. A mole ratio of approximately 0.5–0.6 of H
2
O
2
to O
3
was chosen
on the basis of the stoichiometry of their reaction [15], but some kinetic experiments were
repeated with a higher mole ratio of approximately 1. All aqueous solutions were prepared
with Milli-Q water (Barnstead), buffered to pH 7.2, which is in the generally accepted
range for wastewater treatment [16]. Potassium phosphate buffer (Fisher) at 30 mM was
used for all solutions. The O
3
solutions were obtained by sparging the oxygen/O
3
gas mix-
ture from a Welsbach O
3
Generator (Model T-408) into water. The indigo dye method [17]
was used to measure the O
3
concentration, with absorbance of unreacted dye measured in
a spectrophotometer (Spectronic Instruments).
A Hewlett-Packard 5890 gas chromatograph equipped with a Hewlett-Packard 5970 mass
selective detector was used to qualitatively and quantitatively analyze reactants and products.
A solid-phase microextraction (SPME) fiber was used to extract organic components from
the aqueous reaction mixture and they were then thermally desorbed from the fiber in the
M.M. Mitani et al. / Journal of Hazardous Materials B89 (2002) 197–212
201
injection block of the GC, kept at 250
◦
C [18]. A 65
m polydimethylsiloxane/divinylbenzene
SPME fiber was used for most of the analyses and reproducibly and quantifiably extracted
MTBE, benzene, TBF, TBA, acetone (Me
2
CO), and methyl acetate (MeOAc). Concentra-
tions for all organic compounds were based on calibration standards which were carried
out at the same pH and temperature as the reaction conditions. Formic acid and acetic
acid were detectable by using this fiber, but the extraction was not reproducible. The
100
m polydimethylsiloxane SPME fiber gave a higher extraction of MTBE, but did
not extract the more soluble organic compounds and therefore was not generally used.
For both fibers, an exposure time of 2 min with stirring and a desorption time of 1 min
were used. The fiber was injected into a VOCOL (Supelco) capillary column (30 m
×
m) with a temperature ramp programmed to 100
◦
C for 3 min and in-
creased 20
◦
C min
−
1
0
.
25 mm
×
1
.
5
to 150
◦
C. All analyses were made in duplicate with a reproducibility
±
of
10%.
3. Results
3.1. Kinetic studies
The initial rates of reaction of aqueous O
3
and MTBE show that reactions are first
order with respect to O
3
and MTBE individually; i.e. second-order overall. Mechanis-
tically, this is most likely due to the activating effect of O
3
attack on methoxy hydro-
gen [19]. However, decomposition products of aqueous O
3
, namely
•
OH, may react with
MTBE or intermediates, and in some cases may be the predominant oxidant during ox-
idation by O
3
. The formation of
•
OH involves reaction of O
3
and the hydroxide ion
(initiation).
The calculated rate of O
3
decomposition by reaction with hydroxide ion (OH
−
), produc-
ing
•
OH using Eq. (1), accounts for approximately 10% of the rate of disappearance of O
3
in our system. This indicates that reaction of MTBE with
•
OH is not negligible and must
be included in the rate expression. Also, the disappearance of O
3
is approximately three to
four times faster than the disappearance of MTBE, indicating that O
3
is reacting with other
species such as OH
−
, various oxygen radicals produced by O
3
decomposition, and other
products of MTBE oxidation. MTBE may also react with species other than O
3
or
•
OH,
but we assume that these reactions are negligible.
Based on these considerations, the following kinetic rate expression in the stirred flow
reactor is applicable:
R
m
=
k
1
C
O
3
C
mf
+
k
2
C
OH
C
mf
(2)
where
R
m
is global rate of disappearance of MTBE (M s
−
1
),
k
1
the rate constant for reac-
tion of MTBE with O
3
(M
−
1
s
−
1
),
k
2
the rate constant for reaction of MTBE with
•
OH
(M
−
1
s
−
1
),
C
m
the concentration of MTBE (mol l
−
1
),
C
mf
the outlet concentration of MTBE
(mol l
−
1
),
C
O
3
the outlet concentration of O
3
(mol l
−
1
),
C
OH
is the concentration of
•
OH
(mol l
−
1
).
R
m
can also be related to the operating conditions, using a mass balance:
R
m
=
v
o
C
mo
−
v
f
C
mf
V
r
(3)
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