Hydrolysis of tert-butyl methyl ether (MTBE) in dilute aqueous acid

Article abstract of DOI:10.1021/es001431k

The aqueous acid hydrolysis of MTBE and other ether oxygenates was studied at pH ≥ 1. MTBE and other oxygenates were susceptible to acid hydrolysis at moderate pH values. MTBE was sensitive to acid-catalyzed hydrolysis reaction that generates tert-butyl alcohol and methanol as products. The reaction was first-order with respect to the concentration of MTBE and hydronium ion with a second-order rate constant of about 0.9 × 10^-2/M-hr at 26°C. MTBE could be rapidly hydrolyzed by acidic ion-exchange resins at near neutral pH. Pseudo first-order rate constants were observed to be as high as 0.03 and 0.12/hr at 25° and 35°C, respectively. These results were discussed in terms of their possible implications for the treatment and environmental fate of MTBE and other gasoline oxygenates.

Full text of DOI:10.1021/es001431k

Environ. Sci. Technol. 2001, 35, 3954-3961
spanning nearly 150 years of the chemical literature. The
Hydrolysis of tert-Butyl Methyl Ether
MTBE) in Dilute Aqueous Acid
first observation of ether cleavage was made in 1861, the
reaction taking place in concentrated hydroiodic acid (7).
The first report of the cleavage of alkyl ethers was published
by Silva in 1875, work also performed in concentrated
hydroiodic acid (8). In 1893, the first systematic study of the
effect of ether structure on reaction rate was performed (9).
At the same time, the first observation of the much faster
cleavage of tertiary ethers was made (10). In this experiment,
the tertiary ether was cleaved by aqueous hydroiodic acid in
15 min at room temperature. Hughes and Ingold (11) later
determined that an ether containing a secondary or tertiary
group proceeds via an A1 carbonium-ion type mechanism,
whereas primary ethers proceed via an A2 bimolecular
nucleophilic substitution mechanism. Since the hydrolysis
of ethers in dilute acid is slow, it has been less studied than
solvolysis of esters and alkyl halides. However, the hydrolytic
cleavage of several relatively labile ethers has been ac-
complished in 0.5 M p-toluenesulfonic acid (12). The first
observation of MTBE cleavage was made in 1932, where the
reaction was observed to proceed in a matter of hours at 0
(
K I R K T . O ’ R E I L L Y, * , † M I C H A E L E . M O I R , †
‡
C H R I S T I N E D . T A YL O R ,
‡
C H R I S T Y A . S M I T H , A N D
‡
M I C H A E L R . H YM A N
Chevron Research and Technology Co., P.O. Box 1627,
Richmond, California 94802 and Department of
Microbiology, North Carolina State University,
Raleigh, North Carolina 27695
tert-Butyl methyl ether (MTBE) is generally considered to
be resistant to chemical transformation in aqueous
solution. This lack of reactivity has led to concerns of the long-
term impacts of MTBE in groundwater. Although hydrolysis
in the presence of strong acids has been recognized
as a mechanism for MTBE transformation, it has been
discounted as a significant reaction under environmental
conditions. In this study, we have examined the fate of MTBE
and other ether oxygenates under moderately acidic
conditions (g pH 1). The results demonstrate that MTBE
is sensitive to acid-catalyzed hydrolysis reaction that
generates tert-butyl alcohol (TBA) and methanol as products.
The reaction is first-order with respect to the concentration
of MTBE and hydronium ion with a second-order rate
°
C in 27% HCl (13). In the present study, the aqueous acid
hydrolysis of MTBE and other ether oxygenates was studied
at pH 1 and above. This study was initiated after observing
unexpected high concentrations of tert-butyl alcohol in
experiments where the bacterial transformation of MTBE
had been halted by acidification. The results of the present
study demonstrate that MTBE and other ether oxygenates
are susceptible to acid hydrolysis at moderate pH values.
The results also show that MTBE can be rapidly hydrolyzed
by acidic ion-exchange resins at near neutral pH. The results
of these experiments are discussed in terms of their possible
impacts on the understanding of the environmental fate of
MTBE and possible treatment methodologies for MTBE-
contaminated groundwater.
-
2
-1 -1
constant of about 0.9 × 10
M
h
at 26 °C. Commercially
available acidic ion-exchange resins were also shown to
catalyze the hydrolysis of MTBE at near neutral pH. Pseudo-
first-order rate constants were observed to be as high
as 0.03 h at 25 °C and 0.12 h at 35 °C. These findings
are discussed in terms of their possible implications for
the treatment and environmental fate of MTBE and other
gasoline oxygenates.
-
1
-1
Experimental Section
MTBE (99.9%) was obtained from either Burdick and Jackson
(
Muskegon, MI) or Aldrich Chemical Co., (Milwaukee, WI).
tert-Amyl methyl ether (TAME) (97%), tert-butyl ethyl ether
ETBE) (99%), tert-amyl alcohol (TAA) (99%), and tert-butyl
(
alcohol (TBA) (99.5%) were obtained from Aldrich Chemical
Co.
Introduction
Analytical Methods: The rates of hydrolysis of MTBE,
TAME, and ETBE were determined in glass serum vials (25
mL). The vials were prepared by first filling the vials
tert-Butyl methyl ether, commonly abbreviated as MTBE and
used as a gasoline oxygenate, is generally considered to be
resistant to chemical transformation in water at ambient
conditions (1, 2). This lack of chemical reactivity has led to
concerns over of the long-term impacts of MTBE contami-
nation in groundwater. Although the chemical literature (3)
clearly indicates that alkyl ethers are sensitive to hydrolysis
under strongly acidic conditions, acid-catalyzed hydrolysis
of MTBE has been discounted as a mechanism for MTBE
transformation under environmental conditions (4).
The hydrolysis of a range of organic chemicals in the
environment has been discussed by Mill (5). In this review,
the hydrolysis of epoxides is discussed and is well-known to
be catalyzed by both acid and base. However among the
ethers, epoxides are an exception and alkyl ethers are stable
to base and are only reactive in acid (6). The study of ether
cleavage and hydrolysis has a long and important history
completely with sodium pyrophosphate (Na
4
P
2
O
7
2
‚10H O)
buffer (0.2 M) that had previously been adjusted to the desired
pH (1-7) using concentrated HCl. Pyrophosphate was
selected as the buffer for these experiments since it offers
superior buffering capacity as compared with phosphate at
pH 2 and below. However, pyrophosphate is known to convert
to orthophosphate in aqueous acid (14). The second-order
rate constant for pyrophosphate degradation at 26 °C is 1 ×
-
4
-1 -1
1
0
M
h , while the second-order rate constant for MTBE
-2
-1 -1
hydrolysis is 1 × 10
M
h . Consequently, pyrophosphate
buffer can be considered to be stable on the time scale of
MTBE hydrolysis. The filled vials were then sealed with
Teflon-faced Mininert valves (Alltech, Deerfield, IL). The
excess buffer displaced from the vial by the valve was vented
through a syringe needle (22 gauge) inserted through the
valve seal. To obtain the desired ether concentrations in the
incubation vials, varying volumes of buffer (up to 344 µL)
were removed and were then replaced by an equal volume
of ether added as a neat compound. Unless otherwise stated,
*
Corresponding author phone 510-242-5365; fax: 510-242-1954;
kito@chevron.com.
†
‡
Chevron Research and Technology Co.
North Carolina State University.
3
9 5 4
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 35, NO. 19, 2001
10.1021/es001431k CCC: $20.00

2001 Am erican Chem ical Society
Published on Web 08/23/2001

all reactions were conducted at 26 °C in a temperature-
controlled orbital incubator operated at 150 rpm. The pH of
all reaction mixtures was determined both before and after
the incubation period (typically 96 h). The solution pH was
determined using an Accumet combination electrode and
an Accumet AR15 pH meter (Fisher Scientific, Fairlawn, NJ)
equipped with an automatic temperature compensation
probe. The electrode apparatus was calibrated using a four-
point calibration (pH 1.0, 3.0, 5.0, and 7.0) using commercially
prepared buffer solutions (Fisher Scientific, Fairlawn, NJ).
The standards (0.05 M) were as follows: pH 1.0 (potassium
chloride-HCl), pH 3.0 (potassium biphthalate-HCl), pH 5.0
SCHEME 1. Possible Mechanisms for the Acid Hydrolysis of
MTBE
(
potassium biphthalate-NaOH), and pH 7.0 (potassium
phosphate (monobasic)-NaOH).
The production of TBA and methanol from MTBE, TBA,
and ethanol from ETBE and the production of TAA and
methanol from TAME were determined by gas chromatog-
raphy. Samples (2 µL) of the reaction mixture were removed
from the sealed incubation vials with microsyringes and were
directly injected into a Shimadzu (Kyoto, Japan) GC8A gas
chromatograph equipped with a flame ionization detector.
The reactants and products were separated on a stainless
steel column (6′ × 1/ 8′′) filled with Porapak Q (60-80 mesh)
(
Alltech, Deerfield, IL). The gas chromatograph was operated
with a column temperature of 160 °C, an injector temperature
of 200 °C, and a detector temperature of 220 °C. Nitrogen
was used as carrier gas at a flow rate of 15 mL/ min. The
hydrolysis of TAME and ETBE (both at 100 mM) was also
evaluated at pH values of 2 and 7 in experiments conducted
using buffered solutions with sodium pyrophosphate. Quan-
tification of MTBE, TAME, ETBE, TAA, and TBA was
performed using external standard calibration.
In a second laboratory, the hydrolysis of MTBE was
measured initially in 30 mL VOA vials containing 0.01 M
sulfuric acid with an initial MTBE concentration of 1.13 mM.
The vials were sampled by removing the lid, and pipeting 2
mL of the reaction mixture into gas chromatography (GC)
autosampler vials. One microliter samples of the solution
were analyzed for the concentration of MTBE, TBA, and
methanol by splitless injection into a Hewlett-Packard 6890
GC (Wilmington, DE) equipped with a 15 m × 0.32 J&W DB-1
column (Folsom, CA) and FID detector. The GC was operated
with an oven temperature program starting at 60 °C for 4
min followed by a 30 °C/ min temperature ramp with a final
temperature of 220 °C. The injector and detector temper-
atures were held at 250 and 275 °C, respectively. Helium
flowing at a rate of 2.8 mL/ min was used as the carrier gas.
Quantification of MTBE and TBA was performed using
external standard calibration.
shown in Scheme 1. A rapid preequilibrium with a proton
donating species (hydronium ion or the conjugate form of
a weak acid HA for specific or general acid catalysis
respectively) forming the conjugate acid of MTBE is followed
by water-mediated carbon-oxygen cleavage yielding TBA
and methanol while regenerating the acid catalyst. The
overwhelming evidence in the literature cited previously is
consistent with a reaction where the tertiary butyl C-O bond
cleaves forming the t-butyl carbocation (Scheme 1, step 2a).
This mechanism would be categorized as the limiting
unimolecular A1 reaction in the Ingold terminology (16).
Alternatively, the intermediate conjugate acid of MTBE may
be attacked by a molecule of water in a biomolecular A2
reaction (Scheme 1, step 2b). However, step 2b is not
consistent with the observation that tertiary ethers react much
faster than secondary or primary ethers (6). If hydronium
ion were the only catalytic species operating and assuming
steady state in any intermediates produced the rate of change
in the concentration of TBA for this mechanism is given by
eq 1:
+
d/ dt [TBA] ) k [MTBE][H ]
(1)
3
The hydrolysis of MTBE on strongly acidic ion-exchange
resins was investigated. Amberlite IR-120+ (Rohm and Haas)
and IR-36 WET (Rohm and Haas) were activated by placing
them in a 500-mL glass column and slowly passing 3 bed-
volumes of 1 N HCl. The resins were then rinsed with distilled
water until the column effluent pH was the same as the
distilled water (ca. pH 5.5). Forty milliliter VOA vials were
half-filled with resin and then filled with a solution containing
where [H+] is the concentration of hydronium ion H O
+
and
3
k3 ) k k / (k [H O] + k ). The bimolecular mechanism
1
2
-1
2
2
(Scheme 1, step 2b), while not likely, is kinetically indistin-
guishable from the first mechanism when water is used as
the solvent. For the competing mechanism k ) k k / (k +
3
1
2
2
k-1), however, k-1 . k , so the two integrated rate expressions
are identical. At low conversion of MTBE such as that
observed in this work the initial rate can be used to extract
2
1
1.3 mM MTBE. The pH of the solution in contact with the
the rate constant, kobs:
resin was 5. The vials were kept at 25 or 35 °C in a shaking
water bath at 100 rpm. Following sampling of the water for
analysis, excess water was decanted and replaced with a
known mass of methanol to recover sorbed MTBE and TBA.
Both methanol and water was analyzed for MTBE and TBA
by the GC method in the second laboratory above. The weight
of each vial was determined at each step of the experiment
to permit calculation of the concentration of MTBE and TBA
and the mass of material recovered.
d/ dt[TBA] ) kobs
(2)
(3)
The integrated rate expression becomes:
+
[
TBA] - [TBA] ) k [MTBE] [H ]t
0 3 0
where kobs is the slope of a plot of [TBA] versus time and is
+
Hydrolysis Mechanism and Kinetics. By analogy to the
hydrolysis of carboxylic esters, the hydrolysis of ethers in
dilute aqueous acid may proceed via acid catalysis (15) as
equal to k
3
[MTBE]
0
[H ], where [TBA]
0
and [MTBE]
0
are the
initial concentrations of TBA and MTBE, respectively. In the
case of general acid catalysis, the situation becomes sub-
VOL. 35, NO. 19, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
3 9 5 5

stantially more complex. The expression for kobs in aqueous
solution becomes:
+
kobs/ [MTBE] ) kwaterk3H[H ] + k
[HA ] + ... +
1
0
3HA1
k3HAn[HA ] (4)
n
for the n possible proton donating species including water.
The usual way to test for general acid catalysis is to hold the
pH and ionic strength constant and vary the amount of
general acid. For a polyprotic weak acid such as pyrophos-
phoric acid, eq 4 becomes:
+
kobs/ [MTBE] ) k
+ k3H[H ] + k
[H A] + ... +
3H4A 4
0
water
3
-
k3HA[HA ] (5)
FIGURE 1. Production of tert-butyl alcohol from MTBE in 0.2 M
sodium pyrophosphate at pH 2 and 26 °C. The initial concentration
of MTBE ranged from 10 to 100 mM. The solid lines are the lines
obtained by linear regression of the data points.
While it is reasonable that general acid catalysis occurs
in the hydrolysis of ethers, it will be virtually impossible to
detect kinetically in the case of MTBE. Observation of general
acid catalysis requires that proton transfer be a rate-limiting
step. The pK
a
values for the conjugate acids of both MTBE
The mechanism of the hydrolysis of MTBE by strongly acidic
and ETBE are -2.89 and -2.84, respectively (17). Since the
ion-exchange resins is hypothesized to proceed by a scheme
+
first step in Scheme 1 involves a proton transfer from a very
similar to that found in Scheme 1 where H
3
O is substituted
strong acid to water, k-1 must have a value of about 101
s
0
M
-1
by a proton residing on the acid-washed cationic exchange
resin. In the degenerate case where there is no limitation in
the number of reactive sites on the ion-exchange resin and
assuming that MTBE sorbs onto the resin at an active site,
the rate law becomes:
-
1
+
, close to the diffusion limit (18). If [M] and [MH ] are
concentrations of MTBE and its conjugate acid, respectively,
+
+
2.89
+
[
{
5
H
3
3
O ][M]/ [MH ] ) 10 . From Scheme 1, [H O ][M]/
+
10
-1 -1
[MH ][H2O]} ) k-1/ k
1
. Since k-1 ) 10
M
s
1
and k )
1
0
2.89
M-1 s-1 ) 7 × 10 M s-1. Consequently,
8
-1
5.5 × 10 / 10
the protonation of MTBE cannot be the rate-limiting step.
d/ dt[TBA] ) k k k / (k-1k-s)[MTBE][Hr]
(9)
1
2 s
This effect is simply amplified for acids that are weaker than
+
H
3
O . Even if protonation can occur by the weaker acids in
where k and k are the rate constants for the sorption and
s
-s
+
solution protonation by H
3
O
will dominate the kinetics,
desorption of MTBE on the resin and where [H ] is the
r
concentration of acid sites on the resin. The measurement
of TBA and MTBE is complicated by the fact that two species
are partitioned between the aqueous phase and the ion-
exchange resin. The total mass of TBA and MTBE is measured
at each point in the time series and the integrated rate law
becomes:
and the former process will not be detectable. It is likely for
this reason that in the analogous hydrolysis of esters, ketals,
and acetals, general acid catalysis is rarely observed
(
18-21).
The concentration of hydronium ion is calculated from
the definition of pH:
+
H ] ) 10-pH/ y
[
H+
(6)
ln(1 - xtba) ) -kobs
t
(10)
where yH+ is the molar activity coefficient of the hydronium
ion. The molar activity coefficient yH+ is calculated from the
Debye-H u¨ ckel equation modified by Robinson (22, 23) that
for hydronium ion becomes eq 7:
where xtba is the fraction of MTBE that has hydrolyzed to TBA
and where kobs ) k ].
1 2 s r
k k / (k-1k-s)[H
Results and Discussion
In an initial experiment, TBA produced during the acid
A I
x
hydrolysis of 1.13 mM MTBE was measured in 0.01 M H SO
2
4
-
log yH
+
)
-0.2I
(7)
I + 9B
x
I
at 25 °C. As expected, the rate of TBA formation was found
to be exceedingly slow. The hydrolysis was followed for 48
days at which time the concentration of TBA had reached
just 0.06 mM. Using eq 3, the second-order rate constant
where A and B are adjustable parameters that depend on
temperature and dielectric constant of the solvent and where
I is the ionic strength of the solution. The ionic strength
dependence of the ionization constants for pyrophosphoric
acid is known (14, 24). With known pK values at any ionic
a
strength, the measured pH of the solution, and a method of
calculating the distribution diagram for a polyprotic acid
25), the actual concentration of hydronium ions can be
calculated.
When a significant amount of MTBE is hydrolyzed, as
found in the experiments using strongly acidic ion-exchange
resins, eqs 2 through 5 cannot be used. In this case, at constant
was found to be 4.4 × 10-3
-1
-1. To obtain results in a
M
h
more timely manner, the concentration of MTBE was
increased in subsequent experiments (Table 1, experiments
2 to 5). The rate of production of TBA from the hydrolysis
of MTBE was measured at pH 2 in 0.2 M sodium pyrophos-
phate (Na P O ) at 26 °C. Solutions of MTBE ranging from
(
4
2
7
10 to 100 mM were prepared in 0.2 M sodium pyrophosphate
titrated to pH 2 with concentrated hydrochloric acid. The
+
[H ] was calculated from the measured pH using eqs 6 and
7. The concentration of TBA in the reaction mixture plotted
as a function of time is shown in Figure 1. Methanol was also
detected as a product of MTBE hydrolysis. The agreement
of the second-order rate constants extracted using eq 3 shown
in Table 1 demonstrates that the production of TBA is first-
order with respect to MTBE concentration and is consistent
with the mechanism shown in Scheme 1. Another series of
experiments was conducted (Table 1, experiments 6 to 12)
to measure the effect of pH on the reaction rate. The data
+
[
H ], eq 1 integrates to produce eq 8.
+
ln[MTBE] - ln[MTBE] ) -k [H ]t
(8)
0
3
The second-order rate constant k
from a plot of the natural logarithm of the concentration of
TBA versus time by dividing the slope by [H ], while the
pseudo-first-order rate constant kobs is the slope of this plot.
3
in eq 8 is extracted
+
3
9 5 6
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 35, NO. 19, 2001

TABLE 1. Observed and Extracted Second Order Rate Constants for the Hydrolysis of MTBE in Sodium Pyrophosphate Buffer at 26
°
C
-log[H+]
from eq 6 and 7
k3 (× 10 M h-1
2
-1
)
initial [MTBE]
(mM)
[Na4P2O7]/ionic
strength (M)
pH
(measured)
experiment
kobs (mM h-1)
from eq 5
-
-
-
-
-
-
-
-
-
-
3
3
3
3
2
2
3
3
3
5
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
10
25
50
0.2/0.95
0.2/0.95
0.2/0.95
0.2/0.95
0.2/0.91
0.2/0.90
0.2/0.95
0.2/0.96
0.2/0.97
0.2/0.99
0.2
2.00
2.00
2.00
2.00
1.10
1.53
2.01
2.25
2.50
3.01
4,5,6,7
2.06
2.06
2.06
2.06
1.09
1.63
2.07
2.32
2.57
3.08
not calculated
not calculated
1.96
1.96
2.00
2.04
2.11
1.15 ( 0.20 × 10
2.27 ( 0.09 × 10
4.09 ( 0.29 × 10
8.18 ( 0.20 × 10
7.68 ( 0.40 × 10
2.06 ( 0.20 × 10
7.52 ( 1.10 × 10
3.83 ( 0.40 × 10
2.29 ( 0.50 × 10
5.42 ( 0.44 × 10
negligible
1.32 ( 0.18
1.04 ( 0.09
0.94 ( 0.13
0.94 ( 0.05
0.95 ( 0.05
0.88 ( 0.09
0.88 ( 0.13
0.80 ( 0.08
0.85 ( 0.18
0.65 ( 5.4
not calculated
not calculated
0.63 ( 0.07
0.64 ( 0.11
0.63 ( 0.05
0.90 ( 0.12
1.01( 0.12
3.33 ( 0.38
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
1
1
1
1
1
1
1
1
1
1
a
0.2
12
negligible
-
-
-
3
3
3
0.01/0.06
0.02/0.11
0.05/0.23
0.1/0.47
0.2/0.95
0.5/2.45
2.05
2.02
2.05
2.05
2.05
2.05
6.90 ( 0.76 × 10
7.00 ( 1.20 × 10
6.38 ( 0.55 × 10
-
3
8.27( 1.08 × 10
7.86 ( 0.94 × 10
1.33 ( 0.15 × 10
-
-
3
2
2.40
a
2 4
The experim ent conducted at pH 12 was perform ed in 0.2 M NaH PO titrated to pH 12 with 2 M NaOH.
are likewise consistent with a reaction involving a uni-
molecular rate-limiting step and is most similar to the acid
hydrolysis of acetals.
To test for the existence of general acid catalysis, a series
of experiments was undertaken in which the rate of produc-
tion of TBA from MTBE was measured at buffer concentra-
tions ranging from 0.01 to 0.2 M (Table 1, experiments 14 to
1
9). The second-order rate constants were extracted from
the data using eq 3. The second-order rate constant increases
with the total concentration of acid and suggests the operation
of more than one catalytic species; in other words, the
hydrolysis of MTBE may be proceeding by general acid
catalysis. However, in light of the argument above regarding
the rate of protonation of MTBE and deprotonation of its
conjugate acid, it is very unlikely that general acid catalysis
can be observed in the hydrolysis of MTBE. Since the ionic
strength of the reaction mixture was not controlled (due to
experimental limitations), it is far more likely that the increase
in rate due to the increased concentration of pyrophosphate
FIGURE 2. Catalysis of MTBE hydrolysis by pyrophosphate at an
ionic strength of 1.0 M. The closed circles are the observed rate
constant for formation of TBA. Superimposed is the distribution
diagram for pyrophosphate at the specified ionic strength (14, 44).
These data show that the reaction can be catalyzed significantly
N
arises from a kinetic salt effect. The rate of A1 and S 1
+ -
only by H O , H P O , and H P O . The contributions from water
3 3 2 7 4 2 7
and the other pyrophosphate species are insignificant. An argument
reactions is accelerated by the presence of noncommon ions,
and an explanation of this phenomenon was developed by
Ingold et al based solely on ionic strength considerations
in the text indicates that the hydrolysis is dominated by the pathway
+
involving H
3
O .
(
35). In acid-catalyzed hydrolysis, salts may be expected to
accelerate a reaction proceeding by the A2 limiting mech-
anism, but to a lesser degree. Any attempt to uncover
mechanistic details from salt effects is fraught with peril (20),
but the results from the current study can be compared to
earlier work where the mechanism is known by independent
means. The Br o˜ nsted-Bjerrum equation (36), a linear free
energy relationship that is derivable from activated complex
theory, governs the effect of ionic strength on the rate of
reaction.
in Table 1 shows that, within experimental error, the reaction
is first-order with respect to hydronium ion concentration,
-
2
-1 -1
the average value being approximately 0.9 × 10
M
h .
Consistent with the literature, the rate of hydrolysis of MTBE
at pH 12 is insignificant, indicating that the reaction is not
catalyzed by hydroxide ions. The observed rate constants
plotted in Figure 2 are superimposed over the distribution
diagram for pyrophosphoric acid. The possible involvement
of pyrophosphate species in the rate determining step is
discussed below.
k / k ) y yH+/ y‡
(11)
e
0
B
Compelling evidence for the A1 reaction in the hydrolysis
of MTBE comes from the measurement of the Arrhenius
‡
activation parameters and the entropy of activation, ∆S .
where k
e
is the rate constant in any medium, k
, y +, and y are
H ‡
0
is the rate
‡
Hydrolyses with a positive entropy of activation, ∆S , have
constant for the reference medium, and y
B
previously been determined to react via an A1 mechanism,
while those having a negative ∆S proceed via an A2
the molar activity coefficients for a base, B (in this case MTBE),
the hydrated proton, and the transition state, respectively.
‡
mechanism (Table 2). Using the data found in Table 3 and
By inspection of eq 11, an increase in y
increase in the rate constant. It is generally believed (20) that
the addition of salts predominantly affects the value of y
while the ratio yH+/ y varies to a smaller degree in the opposite
direction with y varying similar to y . To investigate this
B
will result in an
extracting the activation parameters in the usual way (34)
‡
-1
-1
yields a value for ∆S of 6.5 ( 2.6 cal mol K , consistent
with the A1 mechanism. The relative large values for the
Arrhenius activation energy and the preexponential factor
B
‡
‡
B
VOL. 35, NO. 19, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
3 9 5 7

TABLE 2. Activation Parameters for the Hydrolysis of MTBE and Related Compounds
Ea
∆H‡
∆S
‡
-
1
-1
substrate
log A
(kcal mol ) (kcal mol-1) (cal mol K-1) mechanism
ref
MTBE (aqueous acid)
t-butyl form ate
t-butyl acetate
14.5 ( 5.8 27.5 ( 1.0 27.5 ( 1.0
6.5 ( 2.6
15.4
13.1
10.4
6.8
6.9
13.1
6.1
2.4
A1
A1
A1
A1
A1
A1
A1
A1
A1
A2
A2
A2
A2
A2
this work
Fredlein & Lauder (26)
16.6
16.1
28.8
27.5
29.2
26.4
25.4
22.7
15.7
28.7
14.2
13.6
17.3
19.0
31
28.8
27.5
29.2
26.4
25.4
22.7
15.7
28.7
14.2
13.6
17.3
19.0
31.0
Adam , Lauder & Stim son (27)
Stim son & Watson (28)
Koskikallio & Whalley (29)
Koskikallio & Whalley (29)
Koskikallio & Whalley (29)
Withey & Whalley (30)
Withey & Whalley (30)
Church, Pankow & Tratnyek (31)
Fredlein & Lauder (26)
t-butyl 2,4,6-trim ethylbenzoate 15.5
dim ethoxym ethane
diethoxym ethane
dim ethoxyethane
triethoxyethane
trioxane
t-butyl form ate
t-butyl form ate
t-butyl acetate
14.7
14.7
16.1
14.6
13.8
8.2
7.8
7.9
11.9
11.3
-23.0
-24.9
-24.4
-6.0
-9
Adam , Lauder & Stim son (27)
Long, Pritchard & Stafford (32)
Koskikallio & Whalley (33)
ethylene oxide
diethyl ether
MTBE (ion-exchange resin)
10.6 ( 5.9 23.3 ( 2.0 23.3 ( 2.0
-7.8 ( 6.8
see text this work
ether (ETBE) in experiments conducted at pH 2.0 (Table 6).
The hydrolysis of TAME resulted in the accumulation of both
tert-amyl alcohol (TAA) and methanol as hydrolysis products.
The hydrolysis of ETBE generated both TBA and ethanol as
products. At pH 2, the rate of hydrolysis for both ETBE and
TABLE 3. Observed and Extracted Second Order Rate
Constants for the Hydrolysis of MTBE in Sodium
Pyrophosphate Buffer
-
log[H+]
k
3
(× 10 M h-1
2
-1
)
temp [Na
4
P
2
O
7
]
pH from eqs
(meas) 6 and 7
k
obs
-3
-1
TAME was found to be 1.62 × 10
either TAME or ETBE was observed over a 4-day reaction in
experiments conducted at pH 7.0. The pK for the conjugate
h . No hydrolysis of
(°C)
(M)
(mM h-1)
from eq 5
2
6
7
6
7
0.2
0.2
0.2
0.2
1.10
1.14
2.01
2.04
1.09
1.15
2.07
2.11
7.68 ( 0.40 × 10-
2
1
3
2
0.95 ( 0.19
4.61 ( 0.52
0.88 ( 0.13
4.94 ( 0.12
a
-
3
2
3
3.27 ( 0.19 × 10
acid of ETBE is slightly higher than that of MTBE (-2.84
versus -2.89, respectively), but this fact can only explain
about 25% of the difference in the extracted rate constant.
Insufficient experimental evidence is available to fully explain
the faster hydrolysis of ETBE and TAME relative to MTBE
but considering the salt effect discussed above, the solvation
of the transition state clearly plays an important role.
-
7.52 ( 0.11 × 10
3.83 ( 0.45 × 10-
TABLE 4. Values of Activity Coefficients for MTBE, ETBE, and
TAME, log y
B
ionic strength (M)
MTBE
ETBE
TAME
Having demonstrated the hydrolysis of MTBE and other
ether oxygenates in aqueous acid, experiments were con-
ducted to determine whether hydrolysis of ethers could be
achieved using acidic ion-exchange resins. The hydrolysis of
carboxylic acid esters on acidic ion-exchange resins has been
reported previously (20, 45). In addition, MTBE is synthesized
commercially in the gas phase reaction of isobutylene and
methanol over acidic ion-exchange resins, so it can expected
that the hydrolysis of MTBE could be carried out by this
catalyst as well. Figure 3 shows the appearance of TBA and
the disappearance of MTBE plotted as a function of time for
the strongly acidic ion-exchange resin IR-120+. As compared
to the hydrolysis in aqueous acid, kobs for hydrolysis on acidic
ion-exchange resins is much larger (Table 7). During the
analysis of MTBE adsorbed onto the ion-exchange resin, it
0
1
2
.967
.955
.922
0.222
0.460
0.669
0.271
0.567
0.785
0.248
0.500
0.700
phenomenon further, the activity coefficients for MTBE,
ETBE, and TAME were measured according to the solubility
method of Long and McDevitt (37) in aqueous NaCl solution.
The water solubilities of MTBE, ETBE, and TAME were found
to be 4.86, 1.36, and 1.14 g/ 100 mL, respectively, in good
agreement with the literature values (38). Quantification of
MTBE, ETBE, and TAME was performed by gas chromatog-
raphy using the conditions found in the Experimental Section.
The logarithm of the activity coefficients for MTBE, ETBE,
and TAME are reported in Table 4. In the work of Paul, Long,
and McIntyre (39-41) on the hydrolysis of acetals, the salt
effect was found to be determined partly by the size of the
base. The current work is consistent with this view (Table 5)
where the increase in rate constant due to added salt is mainly
s
was found that the value of k / k-s is approximately 1.3 for
both resins. This and the number of acid sites on the resin
allows an estimate of the second-order rate constant to be
-
1
made. With kobs 0.03 h , [H
1
r
] ) 2 equiv/ L and with k
s
/ k-s
h
)
-2
-1, in
.3 yields a second-order rate constant of 1.2 × 10
B
due to the increase in y . It is noteworthy that the magnitude
good agreement with the second-order rate constant obtained
of the salt effect is consistent with those seen for the hydrolysis
of methylal that is known to proceed via an A1 mechanism.
However, it would be imprudent to claim that mechanistic
information can be deduced from the observed salt effect.
The theories proposed separately by McTigue (42, 43) and
Bunnett (44) could be used to measure the effect of hydration
on the activities of the species present in hydrolytic reactions.
However, McTigue’s observations could have been explained
‡
in solution. Interestingly, the negative value of ∆S obtained
for the ion-exchange resin appears to be inconsistent with
the A1 limiting mechanism (Table 2). However, a positive
‡
∆
S value should not necessarily be expected in heteroge-
neous catalysis since the loss of entropy associated with
moving from solution to the sorbed state is included in the
‡
experimental value of ∆S . Whalley warns of this possibility
‡
(
46), and so the negative ∆S value observed for catalysis on
simply by the change in y
changes in the term y are due to changes in the activity
of water so that log k ) w log a , is based on observations
B
. Bunnett’s proposition, that
ion-exchange should not be taken as being diagnostic of
mechanism in this case.
B ‡
y
H+/ y
e
/ k
0
w
made in more strongly acidic media than used in the current
work.
Rates of hydrolysis similar to that of MTBE were observed
for tert-amyl methyl ether (TAME) and ethyl tert-butyl ethyl
The rate of MTBE hydrolysis by the ion-exchange resins
is too slow to have an immediate application in pump and
-
1
-1
treat systems. With the observed rate of 0.03 h (0.72 d
)
at 25 °C, the hydraulic retention time of MTBE in a reactor
3
9 5 8
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 35, NO. 19, 2001

TABLE 5. Salt Effects in Acid Hydrolysis of Ethers and Esters
slope of log ke/k0
vs. ionic strength
slope of log yB
vs. ionic strength
substrate
mechanism
A1
presum ed A1
presum ed A1
A1
A2
A1
ref
MTBE
ETBE
TAME
0.31
0.23
0.28
0.25
0.21
0.12
0.23
this work
this work
this work
not m easured
not m easured
0.22
0.04
0.36
t-butyl acetate
m ethyl chloroacetate
m ethylal
McTigue & Watkins (43)
McTigue & Watkins (43)
Long & McIntyre (41)
TABLE 6. Observed and Extracted Second Order Rate Constants for the Hydrolysis of ETBE and TAME in Sodium Pyrophosphate
Buffer
kobs (mM h-1)
k3 (× 10 M h-1) from eq 5
2
-1
ether initial conc (mM) conc of Na4P2O7 pH (measured) -log[H+] from eq 6 and 7
-
-
3
3
ETBE
TAME
ETBE
TAME
100
100
100
100
0.2
0.2
0.2
0.2
2.00
2.00
7.00
7.00
2.07
2.07
7.15
7.15
1.26 ( 0.22 × 10
1.26 ( 0.14 × 10
negligible
1.66 ( 0.30
1.66 ( 0.15
not calculated
not calculated
negligible
FIGURE 4. Calculated dissolved MTBE concentration at steady state
in a sorptive resin column with the conditions discussed in the text.
FIGURE 3. Hydrolysis of MTBE to TBA over the strongly acidic
ion-exchange resin IR120+. Duplicate measurements are shown.
concentration at point x in the column (mg/ L), x ) column
length (m), Vx ) linear velocity of the MTBE through the
TABLE 7. Observed Rate Constants for the Hydrolysis of MTBE
on Strongly Acidic Ion Exchange Resins
-1
column (m/ d), k ) transformation rate or kobs (d ), and Vx
is a function of the sorptive characteristics of the solid matrix
strong
acid
Vx ) Vw/ K
(13)
temp
°C)
sites
k
obs (h-1)
k
3
(M-1 h-1
from eq 8
)
(
resin
(equiv/L)
from eq 10
k
obs (d-1)
where Vw ) linear velocity of water in the column (m/ d), K
) sorptive capacity of the matrix [(mg of MTBE/ kg of resin)/
2
5
6
5
IR-120+
2.0
2.0
1.9
0.030 ( 0.002 0.72 ( 0.05 0.012( 0.02
0.124 ( 0.008 2.97 ( 0.19 0.048( 0.09
0.026 ( 0.002 0.62 ( 0.05 0.011( 0.02
3
2
IR-120+
(
mg of MTBE/ L)].
IR-36 WET
The California MTBE Research Partnership funded an
evaluation of sorbent resins for MTBE treatment (47).
Carbonaceous resins, such as Ambersorb 563 and Ambersorb
575, had the highest sorptive capacity for MTBE. Depending
on the test system, K ranges from 1000 to over 10 000. Davis
and Powers (48) conducted Freudlich isotherms on a number
of sorbents and calculated a K of 1210 for Ambersorb 563
and 1140 for Ambersorb 575. These resins have a density of
about 1 g/ mL.
would have to be over 6 days to result in a 99% decrease in
MTBE concentration. To have a reactor of a practical size,
one needs to separate the retention time of the MTBE from
the hydraulic retention time. Material such as activated
carbon or sorbent resins increase the retention time of
organics in water treatment systems. Although the ion-
exchange resins evaluated in this study do not sorb MTBE
to a significant degree, sorbent resins are available (47, 48).
The question was posed whether a resin that has both the
hydrolysis catalyzing activity determined in this study and
realistic sorptive characteristics would permit the design of
a reasonably sized treatment system. A 1-D plug flow model
that includes the influence of both sorption and transforma-
tion (49) was used to evaluate this question. The model
predicts the concentration distribution across the reactor
column at steady state:
The model allows one to evaluate the influence of changes
in flow rate, influent concentration, sorptive capacity, and
transformation rate. Key assumptions are that sorption rates
are fast relative to flow and that sorbed constituents are
subject to transformation. When the transformation rate is
0, the steady-state solution is Cx ) Co, or a saturated sorbent
column. For the modeling exercise, a column with a cross-
2
sectional area of 1 m (diameter 1.13 m) was assumed. Given
the porosity of a packed resin bed of about 50%, this results
-
1
3
-1
in a Vx of 2 m d for each m d of aqueous flow. As k and
K increase, the concentration Cx decreases along the length
of the column. Figure 4 shows the steady-state solution
assuming a resin has a kobs of 0.35 and a K of 600. These are
-
k(x/ Vx)
Cx ) Co e
(12)
where Co ) influent concentration (mg/ L), Cx ) dissolved
VOL. 35, NO. 19, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
3 9 5 9

about one-half the value of these characteristics for available
resins. The steady state effluent concentration of a 2-m
column with a flow rate of 5 gpm and an influent of 115 µM
MTBE (10 mg/ L) would be 0.06 µM (0.005 mg/ L). This
evaluation suggests that adding a strong acid functionality
to carbonaceous resins may result in a material that would
be effective at transforming MTBE in a reasonably sized
treatment system. According to the resin vendors questioned,
such a resin does not currently exist. The assumption that
the sorbed MTBE is available for transformation needs to be
tested.
The effluent would contain the two alcohol products. This
technology would not be suitable for drinking water treatment
but could have applicability for pump and treat systems where
the effluent is discharged to a POTW. Understanding the fate
of the alcohol products will be important in assessing the
impact of these findings on groundwater protection and
possible treatment processes. Methanol should be readily
degraded under either aerobic or anaerobic conditions (50,
potentially impacted by the current findings is the role of
anaerobic microbial degradation of MTBE. To date, all of the
isolated MTBE-degrading microbial cultures are aerobic
(59, 60), and this is consistent with a role for oxygenase
enzymes in the initial cleavage of the ether bond (61, 62).
However, there have been some limited indications of
anaerobic MTBE degradation (51, 52, 63, 64). In these
instances, the mechanism of MTBE degradation is unknown.
While anaerobic conditions obviously exclude the possibility
of an oxygenation reaction directed at MTBE, an enzyme-
catalyzed hydrolysis of MTBE could potentially occur under
either aerobic or anaerobic conditions. The enzymes that
may be involved in the anaerobic degradation of MTBE are
not known at this time. However, it is possible that hydrolytic
enzymes catalytically similar to serine-containing peptidases
and carboxylic ester hydrolases could catalyze the hydrolysis
of MTBE. For example, the imidazole group associated with
the active site histidine residue in chymotrypsin functions
as an acid catalyst and also promotes the nucleophilic attack
of activated substrates by water (65). It is also notable that
the cleavage of the C-O-C bond in glycosidic bonds involves
an enzyme-catalyzed hydrolysis (6). For example, lysozyme
hydrolytically cleaves the â-(1-4) glycosidic bonds between
N-acetylglucosamine and N-acetylmuramic acid, the two
carbohydrate derivatives found in bacterial cell walls (65).
White et al. (66) also discussed the possibility of a hydrolytic
enzymatic mechanism in their review of the biological
scission of ether bonds. Although they presented limited
examples of ether hydrolyses, they felt that they may not
have much importance since, as they state, “ethers are ...
highly stable to hydrolysis even in the presence of mild acids”.
The results presented in this present study suggest that ether
oxygenates may be sufficiently susceptible to hydrolysis to
be subject to enzymatic cleavage.
5
1). In contrast, TBA degrading activity is less well character-
ized, although it has been detected at some (50, 52), but not
all (51), sites that have been investigated for biodegradation
processes.
The rate of MTBE hydrolysis in aqueous solution at near
neutral pH is too slow to have a significant impact on the
fate of this compound. At pH 7, the half-life of MTBE is
thousands of years. The observation of surface-catalyzed
hydrolysis may have implications for MTBE fate in the some
subsurface environments. The role of soils in catalyzing the
hydrolysis of organic compounds has been recently discussed
(
53, 54). Wei et al (53) studied the influence of clay minerals
on the hydrolysis of carbamate pesticides. A dilute suspension
10 mg/ L) of montmorillonite clay, a common soil constitu-
(
ent, resulted in a 100-fold increase in the rate of hydrolysis
at pH 6. These and other published results (57) suggest a
surface acidity of clay that can be 2 to 4 pH units lower than
the bulk solution. The high acidity of clays is due to both
Br o˜ nsted and Lewis acid sites (58). Although the acidity of
clays decreases as water content increases, clays can be used
as solid acids in aqueous solutions (58). The observed rate
of clay-catalyzed hydrolysis is higher in drier soils (57). The
similarity of clay to the ion-exchange resins evaluated in this
study is highlighted by the observation that clays can catalyze
the same MTBE synthesis reaction as IR-36WET (58).
Although the stability of MTBE at many sites indicates that
abiotic transformation may not be common, the high surface
area of clays, coupled with the high solids-to-water ratio of
the subsurface, suggests a possibility of surface-catalyzed
reactions.
Monitoring TBA concentrations in groundwater is be-
coming increasingly important due to both requests from
regulators and its role in evaluating the fate of MTBE.
California has proposed an action level for TBA in drinking
water of 0.16 µM or 12 µg/ L (55). Groundwater samples
collected for the analysis of volatile organic compounds are
typically preserved by lowering the pH to 2 or less by the
addition of a strong acid (56). Equation 1 can be used to
estimate the generation of TBA in preserved samples. At pH
Acknowledgments
A portion of this work was funded by the American Petroleum
Institute. The opinions expressed are those of the authors
and not necessarily those of the funding agency.
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Received for review June 28, 2000. Revised manuscript re-
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