United States
Environmental Protection
Agency
Environmental Sciences Research
Laboratory
Research Triangle Park NC 27711
Research and Development
EPA-600/S3-84-072 Aug. 1984
&EPA Project Summary
Reactions of Dissolved
Pollutants with Ozone in
Aqueous Solutions
C. H. Kuo
The kinetics of aqueous-phase oxida-
tion of selected hydrocarbons by ozone
at 5 to 35°C were investigated. The
experiments were conducted in aque-
ous solutions with pH values ranging
from 2 to 7 using a stopped-flow
spectrophotometer interfaced to a data-
acquisition system.
Ozonation reactions of olefinic com-
pounds, including cyclohexene, cyclo-
pentene, and 1-pentene were much
faster in the aqueous solutions than in
the vapor phase. The average rate
constants of the biomolecular reactions
were 4 x 106 L gmol-1 s-1 or larger, and
were nearly independent of the pH
value and temperature. Experimental
results indicated that saturated hydro-
carbons such as cyclohexane, cyclopen-
tane, hexane, and pentane were not
reactive with dissolved ozone. Benzene
and toluene reacted with ozone at
moderate rates in acidic solutions, but
the ozonation of benzene was very rapid
in neutral solutions. Orders of the
ozonation reactions of aromatic com-
pounds changed with acidity, indicating
a possible shift in the mechanisms of
the reactions.
Preliminary tests by chromatography
showed that acids and aldehydes were
formed in ozonation of the olefins and
that oxidation of benzene produced
benzoquinone and hydroquinone. The
detection of high-molecular-weight com-
pounds in the solutions indicated the
polymerization of some species in the
reactions.
This Project Summary was developed
by EPA's Environmental Sciences Re-
search Laboratory, Research Triangle
Park, NC, to announce key findings of
the research project that is fully docu-
mented in a separate report of the same
title (see Project Report ordering infor-
mation at back.)
Introduction
Hydrocarbons are among the major
pollutants emitted into the atmosphere
from mobile and stationary sources such
as automobiles, petroleum refineries,
and chemical manufacturing facilities.
Although atmospheric hydrocarbons vary
considerably in structure, many species
are reactive and may be oxidized to form
secondary and tertiary pollutants. One of
the most important oxidizing agents in
the atmosphere is ozone (Os) produced by
photochemical reactions in the strato-
sphere. A fraction of the O3 diffuses into
the troposphere and contributes to the
background Oa concentration in all at-
mospheres. Oxidation of the atmospheric
hydrocarbons by Oj in the vapor and
liquid phases can produce chemical spe-
cies that are hazardous to public health
and the environment. Under overcast,
high-humidity conditions, dissolved pol-
lutants may be oxidized to secondary and
tertiary aerosols in the liquid phase.
Rates and mechanisms of the conversion,
however, are not well known.
The present research was undertaken
to investigate the kinetics of the oxidation
of some saturated hydrocarbons, olefins,
and aromatic compounds by Oa in the
aqueous phase. The compounds chosen
for this study include benzene, cyclohex-
ane, cyclohexene, cyclopentane, eyclo-
pentene, hexane, pentane, 1-peritene,
and toluene. Although the oxidation rates
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of some of these pollutants in organic
solvents are reported in the literature,
little information is available concerning
the kinetics of reactions m the aqueous
phase.
A stopped-f low spectrophotometer (Dur-
rum Model D-110) was applied to conduct
the kinetic experiments in solutions of
various pH values and temperatures.
Absorbance data were collected through
an automatic data acquisition system
interfaced to the spectrophotometer. The
effects of acidity and temperature on the
rates of the reactions were investigated.
Identifications of products of the oxidation
reactions were also attempted using a
gas chromatograph (Hewlett-Packard Mod-
el 5840A).
Procedure
The kinetic experiments were conduct-
ed at temperatures varying from 5 to
35°C in aqueous solutions with pH values
ranging from 2 to 7. A buffer solution was
prepared by adding appropriate amounts
of chemicals such as HCI, Na2HP04,
NaH2P04, Na2SC>4, or NaOH in distilled
water for control of the pH value and ionic
strength. Ozone gas was produced from a
Welsbach Model T-408 ozonator using
extra dry, pure oxygen. Any possible
impurities in the buffer solution were
oxidized by bubbling 03gas in the solution
for several hours. Unreacted Oa was
displaced by bubbling off nitrogen gas.
The aqueous solution of a pollutant was
prepared by adding a known quantity of
the pollutant reactant (less than the
solubility limit) to the buffer solution and
agitating with a magnetic stirrer for
several hours to a few days until homo-
geneity was achieved. Also, a portion of
the buffer solution was used to prepare
the Oa solution.
The spectrophotometer system was
calibrated in accordance with procedures
specified by the manufacturer. By circu-
lating coolant water, the temperature in
the spectrophotometer system was main-
tained at the desired value during an
experiment. The aqueous solutions of the
pollutant reactant and 03 were kept at the
same temperature in an isothermal bath.
Before an experimental run, a portion of
the Oa solution was removed and stored
in a reservoir syringe in the spectropho-
tometer. Another reservoir syringe was
used to store the aqueous solution of the
pollutant reactant. The two drive syringes
were then filled from the two storage
syringes containing the two reactants in
the separate solutions with an identical
pH value and at the same temperature
After data from the reaction system and
sampling specifications were entered
onto a computer terminal, a flow actuator
was triggered. This resulted in simulta-
neous activations of both the stopped-
flow spectrophotometer and data-acqui-
sition systems. Absorbances of the mixed
reacting solution were recorded as a
function of reaction time and stored in the
computer memory. After the termination
of sampling of a run or a series of
experiments, the absorbance data were
recalled from data files in the data-acqui-
sition system and analyzed by appropriate
methods.
Analyses of the reaction products were
performed using the gas chromatograph.
Injection samples were prepared by mix-
ing two reactant solutions or by bubbling
of Oa in a solution containing a pollutant.
After complete depletion of Q3, aqueous
samples were injected directly into col-
umns with packings such as 5% FFAP to
obtain chromatograms of the reaction
products. To use other columns in which
aqueous samples were not acceptable,
the reaction products had to be extracted
from the aqueous solution by ether and
concentrated before injection. The range
of column temperature and rate of tem-
perature increase were programmed to
achieve best results in separation of the
reaction products. Retention times of the
reaction products were compared with
those of standard reagents to identify the
products of reaction. Also, blank tests
were performed to detect any contami-
nants in the buffer solution
Results
Oxidation of several saturated hydro-
carbons including cyclohexane, cyclopen-
tane, hexane, and pentane by Oa in
aqueous solutions was investigated. Ab-
sorbance data for these systems were
best fitted by three-halves-order kinetics
with respect to 03 concentration. These
kinetics of reaction were identical to
those of self decomposition of Oa in the
absence of any contaminant. Also, the
apparent rate constants calculated for
these systems are nearly identical to the
rate constants for decomposition reac-
tions at the same conditions as the
experiments. These results suggest that
theozonation reactions were insignificant
compared with the decomposition reac-
tion of ozone in the solutions.
Olefmic compounds such as cyclo-
hexene, cyclopentene, and 1-pentene
were very reactive with Oa at rates of
ozonation much faster than the corre-
sponding rates of Oa decomposition. The
kinetics of ozonation of cyclohexene were
second order with first order each i
concentration of both reactants. Th
ozonation rate was not significantly a
fected by temperature variation, and th
half-life of reaction of cyclohexene wa
less than one millisecond. At 25°C, th
second-order rate constant increase
slightly with acidity in the pH range of 21
7, and an average rate constant of 3.9
106 L gmol-1 s-1 was calculated. Th
second-order rate constants for cycloper
tene-Oaand 1-pentene-Oa reactions wer
estimated to be on the order of 1 x 107
gmol-1 s-1 though accurate determina
tions were not possible because of Nmita
tions of the stopped-flow apparatus
Since the rate constants for the ozonatio
reactions of these compounds in th
vapor phase varied from 3 x 103 to 4 x 10
L gmol-' s-1, the above results sugges
that Oa can react with the olefinic com
pounds much faster in the aqueous phas
than in the vapor phase. By using a 5°
FFAPandaChromosorb 101 glass columi
in the gas chromatograph, acetic acic
acetaldehyde, and butyraldehyde wen
detected as products of ozonation o
cyclopentene and pentene in the solu
tions In addition, several compound
with molecular weights ranging from 8!
to 249 were traced, but the identities o
the individual compounds have not beei
established. Although the formation o
high-molecular-weight compounds ha:
not been reported in the ozonation reac
tions in the vapor phase, the acids am
aldehydes have been detected. Experi
mental results also indicated that thi
ttoichiometric ratio was unity for th<
reaction between cyclohexene and Os.
The stoichiometric ratio of benzene t<
Oa in the ozonation reaction varied fron
0.97 to 1.26. Thus, it may be reasonabli
to consider the ratio as unity for th<
reaction in aqueous solutions. The kinet
ics of oxidation of benzene were one-hal
order each in concentrations of benzene
and Oa, and the rate constant increaset
with pH value and temperature. Fo
example, for the reactions in aqueou:
solutions with a pH value of 3, the rat*
constants increased from 5.5 x 10~4 tc
0.012 L/s as the temperature changec
from 5 to 35°C. In neutral solutions, the
fast reaction was first order with respec
to 03 concentration and independent o
benzene concentration. Again, the ratt
constants increased from 7.8 to 12.2 L/<
in the temperature range of 5 to 25°C. Ar
OV-17 glass column was used in the ga;
chromatograph to detect products o
benzene ozonation. Following the separa
tion, a mass spectrometer was used tc
obtain mass spectra of the individua
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species. Benzoquinone (p-qumone) was
identified in all samples. In addition,
hydroquinone (1, 4-dihydroxy benzene)
was also detected in the neutral solutions.
Several spectra with molecular weights
above 108 were detected, but definite
identification of these products has not
been possible. The results of the kinetic
experiments and product analyses seem
to suggest that there might be a shift in
the mechanism of ozonation depending
upon the acidity
The reaction between toluene and O3
was faster in acidic and slower in neutral
solutions than the benzene-Oa reaction.
In the acidic solutions, the ozonation
reaction of toluene was first order with
respect to both concentrations of toluene
and O3 In the neutral solutions, the
reaction was first order in Os only with a
rate constant of 0.19 L/sat25°C. Similar
to the ozonation of benzene, mechanisms
of the ozonation of toluene might be
different at various acidities as suggested
by the results of the kinetic studies.
Conclusions
Saturated hydrocarbons including cyclo-
hexane, cyclopentane, hexane, and pen-
tane studied in this research were not
very reactive with Os in the aqueous
phase. The three-halves-order rate con-
stants for these systems are nearly
identical to the corresponding rate con-
stants for self decomposition of 63 in the
solutions at similar conditions.
Cyclohexene can react rapidly with Os
in an aqueous solution at a half-life of
less than one millisecond. The kinetics of
ozonation were first order with respect to
both concentrations of 03 and cyclohex-
ene. The reaction was influenced very
little by the pH value and temperature of
the solutions. Although reliable informa-
tion on very rapid kinetics of ozonation of
cyclopentene and 1 -pentene could not be
obtained because of equipment limita-
tions, the reaction rate constants are
estimated to be of the order of 1 x 1 O7 L
gmol-1 s-1. These reactions were many
times faster than the rates of ozonation of
the olefmic compounds in the vapor
phase. Preliminary tests by the method of
gas chromatography indicated that in
addition to acids and aldehydes, some
high-molecular-weight compounds were
formed in the ozonation reactions in the
solutions
The rate of ozonation of benzene was
much faster than that of toluene in
neutral solutions, though the two reac-
tions proceeded only at moderate rates in
acidic solutions The kinetics of ozonation
were first order with respect to Os but
independent of benzene or toluene con-
centration in a neutral solution. In acidic
solutions, however, the ozonation'kinetics
were influenced by concentrations of all
reactants. In the reaction between ben-
zene and ozone, benzoquinone was
formed in the acidic solutions, and further
reaction to hydroquinone was detected in
the neutral solutions. Contrary to common
belief, benzene was found to be reactive
with Oi in the aqueous phase. The
mechanisms of ozonation might be dif-
ferent depending upon the pH values of
the solutions.
The stoichiometric ratio of near unity
was determined in this research for the
reactions of benzene and cyclohexene
with 03, suggesting that the ozonation
reactions are bimolecular in nature. Pos-
sible shift in mechanisms of the ozonation
reactions with acidity was indicated by
changes in orders of reactions of the
aromatic compounds at various pH values.
C. H. Kuo is with Mississippi State University. Mississippi State, MS 39762.
J. L. Durham is the EPA Project Officer (see below).
The complete report, entitled "Reactions of Dissolved Pollutants with Ozone in
Aqueous Solutions," (Order No. PB 84-211 218; Cost: $10.00, subject to
change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, I/'A 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Environmental Sciences Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park. NC 27711
* U S GOVERNMENT PRINTING OFFICE. 1984 — 759-015/7772
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Agency
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