United States
Environmental Protection
Agency
Atmospheric Sciences Research ^
Laboratory - • ;~^"
Research Triangle Park NC 27711 / f , \s
Research and Development
EPA/600/S3-85/031 June 1985
Project Summary
Reactions of Ozone with
Organics in Aqueous Solutions
C. H. Kuo and H. M. Barnes
Rates of ozonation of some aromatic
pollutants in the aqueous phase were
studied by the stopped-flow technique.
The kinetic experiments were conduct-
ed in distilled water and in aqueous
solutions of pH range from 2 to 7 at 5 to
35°C.
Aromatic amines including aniline and
a-naphthylamine-ozone reaction was
enhanced by temperature, but the faster
reaction between aniline and ozone re-
mained at a nearly constant rate for all
temperatures.
The order of reaction between toluene
and ozone varied with acidity though
the rate of reaction was moderate.
Polycyclic aromatic hydrocarbons in-
cluding naphthalene, phenanthrene,
and anthracene were reactive with
ozone according to second order ki-
netics. Rate constants of the reactions
increased as the temperature increased
and acidity of the solutions decreased.
This Project Summary was developed
by EPA's Atmospheric 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
Aromatic compounds are among the
major pollutants emitted into the atmos-
phere from mobile and stationary sources
such as automobiles, petroleum refiner-
ies, and chemical manufacturing and fuel
combustion facilities. Although atmos-
pheric organics 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 atmos-
phere is ozone produced in photochemi-
cal reaction in the stratosphere. A frac-
tion of the ozone diffuses into the tropo-
sphere and contributes to the background
ozone concentration in all atmospheres.
Oxidation of the organic compounds by
ozone in vapor and liquid phases can
result in formation of hazardous and/or
toxic products. Under overcast, high-
humidity conditions, dissolved pollutants
may be oxidized in the liquid phase to
produce secondary and tertiary aerosols.
Rates and mechanisms of the conversion,
however, are not well known. The pres-
ent research, therefore, was undertaken
to investigate kinetics of oxidation of
toluene, naphthalene, phenanthrene,
anthracene, aniline, and a-naphthyla-
mine by ozone in the aqueous phase.
A stopped-flow spectrophotometer(Dur-
rum Model D-110) was applied to conduct
the kinetic experiments in distilled water
and in aqueous solutions of various pH
values and temperatures. Absorbance
data were collected through an automatic
data acquisition system interfaced to the
spectrophotometer. Effects of the temper-
ature and acidity on the oxidation rate
were investigated.
Procedure
The kinetic experiments were con-
ducted at temperatures varying from 5 to
35°C in distilled water and 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, or
Na2S04 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 oxy-
gen. Any possible impurities in the buffer
solution were oxidized by bubbling ozone
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gas in the solution for several hours, and
unreacted ozone was displaced by bub-
bling of nitrogen gas. An aqueous solu-
tion of a pollutant was prepared by adding
a known quantity of the pollutant reactant
(less than the solubility limit) to the buffer
solution and agitated by a magnetic stirrer
for several hours to a few days to achieve
homogeneity. Also, a portion of the buffer
solution was utilized to prepare the ozone
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 two aqueous solutions
of the pollutant reactant and ozone were
kept at the same temperature in an isoth-
ermal bath before an experimental run. A
portion of the ozone solution was then
removed and stored in a reservoir syringe
in the spectrophotometer. Another reser-
voir syringe was filled with the aqueous
solution of the pollutant reactant. Two
separate drive syringes in the stopped-
flow apparatus then were filled from the
two storage syringes containing the two
reactants in the separate solutions of an
identical pH value at the same tempera-
ture. Following the entry of data of the
reaction system and sampling specifica-
tions through a computer terminal, a flow
actuator was triggered. This resulted in
simultaneous activations of both the
stopped-f low spectrophotometer and data
acquisition systems. Absorbances of the
mixed reacting solution were recorded as
function of reaction time and stored in the
computer memory. After the termination
of sampling of a run or a series of experi-
ments, the absorbance data were recalled
from data files in the data acquisition sys-
tem and correlated and analyzed using
appropriate rate equations.
Results
Analine and cr-naphthylamine were
very reactive with ozone and constituents
of buffer solutions. To avoid errors in
measurements, therefore, the kinetic
experiments were carried out only in
distilled water. The overall kinetics of
ozonation of both aniline and a-naphthyl-
amine were second order with first order
each in ozone and the organic reactants.
For the aniline-ozone reaction, the rate
was nearly independent of temperature
with an average second order rate con-
stant of 2.4 x 105 l/M-s. The ozonation
rate of cr-naphthylamine was slightly
lower with the second order rate constant
changing from 5.3 x 104 to 1.3 x 106 l/M-s
as the temperature increased from 5 to
35°C. Tests also were made to determine
the reactivity of these compounds with
hydrogen perioxide in the aqueous phase.
No significant change in the absorbance
of a mixed solution of the hydrogen
peroxide and the organic species was
detected. The result seemed to suggest
that neither aniline nor a-naphthylamine
was reactive with hydrogen peroxide in
the absence of a catalyst in the solutions.
As expected, toluene reacted with
ozone at a moderate rate in the aqueous
phase. The ozonation reaction was second
order in acidic solutions, and the rate
constant varied from 10.5 to 53.3 l/M-s
in the temperature range of 10 to 35°C. In
neutral buffer solutions, the reaction was
first order with respect to the ozone
concentration but was nearly independ-
ent of the concentration of toluene. As
the temperature increased from 10 to
35°C, the first order rate constant in-
creased from 0.02 to 1.6 l/s. Although
the rate of the toluene-ozone reaction
was slightly higher than that of the
benzene-ozone reaction investigated in
an earlier work, the phenomenon of
dependence of the reaction order on the
acidity was observed in both systems.
This might be indicative of a shift in
reaction mechanisms with pH value of a
buffer solution for both the toluene-ozone
and benzene-ozone reactions.
The simplest molecule of polynuclear
aromatic hydrocarbons, naphthalene was
much more reactive than the simple aro-
matic hydrocarbons such as benzene and
toluene with ozone in aqueous solutions.
The kinetics of ozonation of naphthalene
was first order with respect to concentra-
tions of both ozone and naphthalene. The
reaction rate was enhanced by tempera-
ture, and at 25°C, the second order rate
constant increased from 850 to 3,750
l/M-s as the pH value of aqueous solu-
tions increased from 3 to 7.
The ozonation of phenanthrene seemed
to be controlled by initial attachment of
ozone molecules at the 9,10 bond of phe-
nanthrene because of its double-bond
character. The overall kinetics can be
treated as second order, the reaction rate
was promoted by both temperature and
pH value of an aqueous solution. At 25°C,
the reaction rate constant increased from
1.94 x 104 l/M-s in strongly acid solu-
tions to 4.75 x 104 l/M-s in neutral solu-
tions. Activation energies of the reaction
were estimated to be 7 kcal/mole at the
pH value of 3 and 23 kcal/mole in other
acidic and neutral solutions.
Experiments also were carried out
study the rate of reaction between anthr
cene and ozone in the aqueous phas
The reaction was extremely fast with
half reaction life of less than a few m
liseconds. Analyses of the absorban
data indicated that the second order ra
constant is about 2 x 107 l/M-s in i
acidic solutions at temperatures varyii
from 10 to35°C.
The results of this research proje
indicate quantitatively the order of d
creasing reactivity of aromatic hydrocs
bons with ozone as follows: anthracene
phenanthrene > naphthalene > toluen
Although this reactivity trend has bee
suggested by several earlier investig
tors, very little information is available
the literature regarding the kinetics ai
rates of the ozonation reactions. Th
research also shows that aniline and
naphthylamine are very reactive wi
ozone according to the second ord
kinetics. The rate data were obtained
assist EPA's Risk Assessment unit inve
tigation of the organic pollutants.
Conclusions
Aromatic amines can be very reactiv
with ozone, as well as reagents of buffe
solutions. In distilled water, aniline re
acted with ozone according to secon
order kinetics, and the reaction rat
constant is nearly independent of tern
perature between 10 to 35°C at 2.4 x 1C
l/M-s. The reaction between cr-naphthyl
amine and ozone was slightly slower, th
second order rate constant is enhance
by temperature, increasing from 0.53
106 l/M-s in the temperature range of!
to35°C.
The present research confirmed the
polycyclic aromatic hydrocarbons in gen
era! are more reactive than simple arc
matic hydrocarbons with ozone in th
aqueous phase oxidation. Of the aromati
compounds investigated, the reactio
between anthracene and ozone was th
fastest with the second order rate con
stant of 2 x 107 l/M-s in acidic solutions
The ozonation of phenanthrene also wa
fast and the rate constant increased wit
pH value and temperature. At 25°C, th
second order rate constant increased fror
1.94 x 10" to 4.75 x 10" l/M-s as the pi
value varied from 2.2 to 7.0. The secon
order reaction between naphthalene an
ozone was moderate with the rate con
stant varying from 850 to 3750 l/M-s a
the pH value increased from 3 to 7 a
25°C. As expected, the rate of ozonatio
of toluene was lowest among the system
of oxidation reactions of aromatic hydrl
-------
carbons investigated. Similar to the ozon-
ation of benzene, the order of reaction of
toluene changed with acidity, indicating a
possible shift in the mechanism of reac-
tion.
C. H. Kuo is with Mississippi State University, Mississippi State, MS 59762.
H. M. Barnes is the EPA Project Officer (see below).
The complete report, entitled "Reactions of Ozone with Organics in Aqueous
Solutions, "f Order No. PB 85-191 171 /AS; Cost: $ 10.00, subject to change/ will
be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Atmospheric Sciences Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
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