Kinetics of Bromacil Ozonolysis

Article abstract of DOI:10.1021/jf970651n

Chemical oxidation processes have been used successfully in the degradation of organic pollutants, yet information is limited concerning the kinetic descriptions of the reaction mechanisms. In this study, the kinetics of bromacil (5-bromo-3-sec-butyl-6-methyluracil, a herbicide) ozonolysis was examined. From laboratory observations, a mechanism was proposed by which direct ozone attack occurred and the degradation pathway proceeded via two parallel reactions. The program MLAB was used to provide a numerical solution for the system of differential equations that described the mechanism. Rate parameters were determined using the slowest reaction system (H₂O₂/O₃). The kinetic model was then tested on a system with only bromacil and on a system containing a radical scavenger. This mathematical model is reasonably consistent with the experimental observations that the addition of hydrogen peroxide significantly reduces the formation of the byproduct responsible for the residual phytotoxicity of the waste stream.

Full text of DOI:10.1021/jf970651n

1630
J. Agric. Food Chem. 1998, 46, 1630−1636
Kin etics of Br om a cil Ozon olysis
Alba Torrents,*,† Brent G. Anderson,†,‡ and Cathleen J . Hapeman‡
Environmental Engineering Program, Department of Civil Engineering, University of Maryland,
College Park, Maryland 20742, and Environmental Chemistry Laboratory, Natural Resources Institute,
Agricultural Research Service, U.S. Department of Agriculture, Beltsville, Maryland 20705
Chemical oxidation processes have been used successfully in the degradation of organic pollutants,
yet information is limited concerning the kinetic descriptions of the reaction mechanisms. In this
study, the kinetics of bromacil (5-bromo-3-sec-butyl-6-methyluracil, a herbicide) ozonolysis was
examined. From laboratory observations, a mechanism was proposed by which direct ozone attack
occurred and the degradation pathway proceeded via two parallel reactions. The program MLAB
was used to provide a numerical solution for the system of differential equations that described the
mechanism. Rate parameters were determined using the slowest reaction system (H2O2/O3). The
kinetic model was then tested on a system with only bromacil and on a system containing a radical
scavenger. This mathematical model is reasonably consistent with the experimental observations
that the addition of hydrogen peroxide significantly reduces the formation of the byproduct
responsible for the residual phytotoxicity of the waste stream.
Keyw or d s: Bromacil; ozonolysis; ozonation; kinetics; wastewater treatment; modeling
INTRODUCTION
the phytotoxicity of the herbicides and, thus, to increase
the biodegradability of the pesticides.
In recent years, elimination of pesticide waste has
received considerable attention with the onset of stron-
ger governmental regulations (Kruger and Seiber, 1984;
Bourke et al., 1992). Furthermore, the agricultural
community has become increasingly aware of the envi-
ronmental impact of and problems associated with the
improper disposal of pesticide waste. It is estimated
that a single pesticide application spray rig generates
20 000 L of wash water per year (Nye, 1984). The long-
term effects of depositing these excess pesticide solu-
tions and equipment rinsates on the soil to degrade have
become a issue to the pesticide applicator because the
pesticides can be persistent in the environment and can
contaminate surface water and groundwater (Aharon-
son, 1987; Parsons and Witt, 1988.)
Bromacil (I, Figure 1), a nonselective and somewhat
persistent herbicide that inhibits photosynthesis, is used
for general weed control on noncrop lands and citrus
orchards (Worthing and Hance, 1991). Recent work has
shown that bromacil-containing wastewater retained
some of its phytotoxic effects after ozonolysis, thus
diminishing the overall usefulness of the oxidation
process (Acher et al., 1994). Bromacil ozonolysis was
shown to give rise to three products: 3-sec-butyl-5-
acetyl-5-hydroxyhydantion (II), 3-sec-butylparabanic
acid (III), and 3-sec-butyl-5,5-dibromo-6-methyl-6-hy-
droxyuracil (IV) (Figure 1). The structures of these
compounds were verified by NMR and mass spectral
data (Acher et al., 1994; Hapeman et al., 1997). The
mechanism for the formation of these three products
was shown to consist of three parallel pathways (Hape-
man et al., 1997). In a prior study it was observed that
the formation of IV was the result of a rapid reaction
between bromacil and Br2 (Acher et al., 1994). Finally,
ozonation processes, in general, proceed via a hydroxy
radical (•OH) process or by direct ozone attack on the
substrate (Masten and Davies, 1994). In other experi-
ments, the oxidation of bromacil was found to involve
Oxidation processes have been used for the treatment
of industrial wastewater, contaminated groundwater,
and landfill leachates. Many of these oxidation pro-
cesses do not completely mineralize the parent com-
-
pounds (that is, breakdown to CO2, H2O, NH3, or NO3
and inorganic salts). Efforts have been underway to
develop a binary remediation strategy whereby the
waste stream is oxidized followed by microbial miner-
alization in a controlled bioreactor (Somich et al., 1990;
Heinzle et al., 1992, 1995; Lee et al., 1992; Hu and Yu,
1994; Hapeman et al., 1995; Massey, 1995; Stockinger
et al., 1995). This binary process has been successfully
demonstrated with pesticide waste using ozone as the
oxidant and a waste-stream-tolerant microorganism
(Hapeman et al., 1995). In this system, the primary
purpose of the chemical oxidation stage was to decrease
•
direct ozone attack as opposed to a OH process (Hape-
man et al., 1997).
During the ozonolysis of bromacil, bromide ions would
be generated as a result of II and III production.
Bromide could then be oxidized by ozone to hypobromate
(OBr-), which in turn protonates and could attack
bromacil to form IV via an electrophilic addition across
the C5-C6 double bond (March, 1977). However, over
an extended period (>24 h), IV has been shown to
decompose to re-form bromacil, and it is this process
that is responsible for the residual phytotoxicity in the
treated waste (Acher et al., 1994). These results clearly
demonstrated that quantification of byproduct formation
* Author to whom correspondence should be addressed (e-
mail alba@eng.umd.edu).
† University of Maryland.
‡ U.S. Department of Agriculture.
S0021-8561(97)00651-1 CCC: $15.00 © 1998 American Chemical Society
Published on Web 03/20/1998

Kinetics of Bromacil Ozonolysis
J. Agric. Food Chem., Vol. 46, No. 4, 1998 1631
5-11 and has been thoroughly described elsewhere
(Staehelin and Hoigne´, 1982; Haag and Hoigne´, 1983;
Hoigne´ et al., 1985; Chelkowska et al., 1992; von Gunten
and Hoigne´, 1994). In the presence of hydrogen perox-
ide, the reduction of HOBr by H2O2 to form Br- must
also be considered (eq 11) (von Gunten et al., 1996). The
reactions of HOBr, BrO3-, H2O2, H+, and H2O with
ozone (eqs 9, 10, and 12-14) are very slow relative to
the other reactions in this system and will have a
negligible effect on the overall kinetics of the process.
Thus, the concentrations of bromide and related species
can then be described with regard to the bromacil
system by the differential eqs 15-18. An additional
equation, d[H2O2]/ dt, was required to describe the
second-order decay of the H2O2 concentration (pKa of
H2O2 ) 11.6) (eq 19).
F igu r e 1. Ozonolysis of bromacil (Hapeman et al., 1997).
OBr- + H+ f HOBr
is equally as important as quantifying the disappear-
ance of the parent compound.
k5 ) 6 × 105 µM-1 min-1
(5)
(6)
To optimize the oxidation of bromacil and concomi-
tantly minimize the formation of the undesirable phy-
totoxic products, an understanding of the reactions,
mechanisms, and the kinetics involved is required.
Substrate transformation can be described by a series
of chemical reactions that are governed by rate laws.
The kinetic parameters can then be incorporated into a
system of differential equations, providing a mathemati-
cal description of the mechanism that is necessary for
the full-scale implementation of the chemical process.
The purpose of this paper is to describe the previously
presented mechanism (Figure 1, Hapeman et al., 1997)
mathematically and to determine the rate parameters
from the resultant system of differential equations.
These rate parameters determined under specific ex-
perimental conditions will subsequently be used to
ascertain the ability of the model to predict the fate of
bromacil in different matrixes.
O3 + Br- f O2 + Br-
k6 ) 0.0108 µM-1 min-1
O3 + OBr- f 2O2 + Br-
k7 ) 0.0318 µM-1 min-1
(7)
(8)
k8
O3 + OBr- 98 BrO2- + O2
k8 ) 0.006 µM-1 min-1
BrO2- + O3 fast8 BrO3
-
O3 + HOBr f
O3 + BrO3- f
k9 < 1 × 10-7 µM-1 min-1
(9)
THEORY
The mechanism for the bromacil ozonolysis, which has
been shown to afford three products (II, III, and IV)
via direct ozone attack along three parallel pathways,
can be described by a series of differential equations (eqs
1-4). These equations, however, can be solved only if
the change in the concentrations of HOBr and O3 are
known with respect to time.
k10 < 1 × 10-8 µM-1 min-1 (10)
H2O2 + HOBr f Br-
k11 ) 4.2 µM-1 min-1
(11)
d[I]/dt ) -k2[I][O3] - k3[I][O3] - k4[I][HOBr] +
k-4[IV] (1)
O3 + H2O2 f
k12 ) 6 × 10-7 µM-1 min-1 (12)
d[II]/dt ) k2[I][O3]
d[III]/dt ) k3[I][O3]
(2)
(3)
(4)
O3 + H+
f
k13 ) 2.4 × 10-8 µM-1 min-1
(13)
d[IV]/dt ) k4[I][HOBr] - k-4[IV]
O3 + H2O f
k14 ) 3 × 10-8 µM-1 min-1 (14)
HOBr is formed in situ via the following sequence of
reactions. Aqueous bromide is generated as a result of
bromacil degradation and concomitant formation of
products II and III. Bromide then reacts with ozone to
form hypobromate (OBr- ). Equation 5 is diffusion
controlled, k5 ) 6 × 105 µM-1 min-1 (Stumm and
Morgan, 1981). Under the experimental conditions of
this work (pH is 5.5), the equilibrium is shifted toward
HOBr because the pKa of HOBr is 8.8. The fate of
bromine in a solution containing ozone is shown in eqs
d[Br-]/dt ) k2[I][O3] + k3[I][O3] - k6[O3][Br-] +
k7[O3][OBr-] + k11[H2O2][HOBr] (15)
d[OBr-]/dt ) -k5[H+][OBr-] + k6[O3][Br-] -
k7[O3][OBr-] - k8[O3][OBr-] (16)

1632 J. Agric. Food Chem., Vol. 46, No. 4, 1998
Torrents et al.
d[HOBr-]/dt ) -k4[I][HOBr] + k5[H+][OBr-] -
k11[H2O2][HOBr] (17)
down between runs. Several sets of experiments were con-
ducted, giving rise to similar results.
Ozon olysis in th e P r esen ce of ter t-Bu tyl Alcoh ol.
Solutions of 382 µM bromacil and three different concentra-
tions of tert-butyl alcohol (0, 13, or 53 mM tert-butyl alcohol),
d[BrO3-]/dt ) k8[O3][OBr-]
(18)
(19)
•
a OH scavenger, were subjected to ozonolysis on the same day
under exactly the same conditions in a manner similar to that
above. Several sets of experiments were conducted, giving rise
to similar results.
d[H2O2]/dt ) -k11[H2O2][HOBr]
HP LC An a lyses. Samples were analyzed directly by HPLC
employing two Gilson (Middleton, WI) model 303 HPLC pumps
equipped with Gilson systems controller software and a Gilson
model 116 UV detector (210 and 222 nm monitored). Separa-
tions were achieved using 40% acetonitrile/phosphoric acid
buffer (pH 2) at a flow rate of 1.0 mL/min on an Ultrasphere
C18 (ODS), 5 µm, end-capped, 4.6 mm × 25 cm, steel-jacketed
column (Beckman Instruments, Inc., Fullerton, CA). A stan-
dard curve for bromacil was obtained from a prepared stock
solution and diluted accordingly to known concentrations over
the required range of experimental analysis.
Quantification of products II-IV was carried out by subject-
ing a 400 mL solution of 1.53 mM bromacil spiked with 30
µCi of [2-14C]bromacil to ozonolysis. Samples were removed
periodically and assayed directly by HPLC to analyze for
bromacil and breakdown products. Effluent fractions were
collected at 15 s intervals and analyzed on a Beckman model
LS6000IC liquid scintillation counter (Columbia, MD) using
ScintiVerse II cocktail (Fisher Chemical). Chromatograms
were reconstructed at each reaction time interval from the
amount of radioactivity detected in each fraction. In addition
to bromacil, three major products were observed in the 14C
chromatogram (24% II, 56% III, and 20% IV); several very
minor products (<1%) that were at the level of detection were
not quantified (Hapeman et al., 1997). The area of each peak
(in dpm‚min) corresponded to a known mass as determined
from the total counts present in each run. The calculated mass
was plotted versus the area of the peak obtained using the
UV detector for each run. These standard curves were used
in subsequent runs to determine the concentrations of products
II-IV.
The rate of change in the ozone concentration in the
aqueous phase, d[O3]/dt, is a function of the mass
transfer coefficient, kLa, from the gas phase (Bin´, 1995;
Haas and Vamos, 1995), the rate of decomposition in
water, k0 ) 4200 µM-1 min-1 (Staehelin and Hoigne´,
1982; Hoigne´ et al., 1985), and the rate of decomposition,
ki, due to the presence of organic and inorganic sub-
strates. This is described in eq 20, where P is the
partial pressure of ozone in the gas phase, H ) 0.082
atm‚m3‚(g‚mol)-1 (Haas and Vamos, 1995) is the Henry’s
law constant for ozone, and Si is the concentration of
substrates available to react with ozone. The rate of
change in the ozone concentration in the aqueous phase
with respect to this system is described by eq 21.
d[O3]/dt ) kLa[(P/H) - [O3]] - ko[O3][OH-] -
n
k S [O ] (20)
∑
i
i
3
i
d[O3]/dt ) kLa[(P/H) - [O3]] - ko[O3][OH-] -
k2[O3][I] - k3[O3][I] - k5[OBr-][H+] - k6[O3][Br-] -
k7[O3][OBr-] - k8[O3][OBr-]
MATERIALS AND METHODS
Rea gen ts. Bromacil was obtained gratis from Agan Ltd.,
Israel. Recrystallization from 2-propanol yielded chromato-
graphically pure compound (needle crystals, mp 158-159 °C).
Stock solutions of bromacil were prepared with ultrapure
water (18 MΩ, Modulab, Type I HPLC, Continental Water
System Corp., San Antonio, TX). Ozone was generated using
a PCI model GL-1B ozone generator (PCI Ozone Corp., West
Caldwell, NJ ) from pure oxygen. tert-Butyl alcohol was
obtained from Fisher Chemical (Fair Lawn, NJ ) and was used
as a hydroxyl radical scavenger. Hydrogen peroxide, used to
promote the generation of hydroxyl radicals, was obtained from
Fisher as a 30-35% (by weight) technical grade product.
Ozon olysis of Br om a cil. Solutions of 382 µM bromacil
were subjected to ozonolysis at room temperature (20 ( 2 °C)
in a 550 mL batch reactor with a fine sintered glass disk in
the bottom for the introduction of ozone, a liquid sampling
valve, and a gas outlet line (Figure 2). Ozone (in oxygen) was
fed into the reactor at a constant rate of 1 L/min, and the
stream was maintained by use of mass flow controllers. Ozone
concentrations in the feed line (∼1.3% w/w ozone/oxygen or
500 µmol/min) and the off gas line were determined using an
ozone monitor (model HC-12, PCI Ozone Corp.). Samples were
removed at fixed time intervals via the sampling valve at the
bottom of the reactor, and the aqueous ozone concentration
(∼11 µM) was measured colorimetrically using the indigo
method (Bader and Hoigne´, 1981). Other samples were
obtained, purged with nitrogen, and analyzed immediately by
HPLC as described below. Although the solution was not
buffered, the pH remained ∼5.5 ( 0.2.
Kin etic Mod elin g. A kinetic model consisting of a series
of differential equations was used to describe the experimen-
tally observed concentrations of bromacil and oxidation prod-
ucts II-IV. MLAB (Civilized Software, Inc., Bethesda, MD),
a program that employs the Marquardt-Levenberg curve-
fitting method and minimizes the summation of the squares
between the experimental data and a theoretical model, was
used to determine a numerical solution to the system of
nonlinear differential equations. The kinetic parameters, k2,
k3, and k4, were determined from the best fit of the observed
concentrations of I-IV. The model was then used to predict
the concentrations of I-IV from the calculated values of k2,
k3, and k4 in a distilled water system and a radical scavenger
system.
RESULTS
Ozon olysis of Br om a cil. Ozonolysis of an aqueous
solution containing only bromacil led to efficient sub-
strate disappearance and rapid formation of the prod-
ucts II and III; a slight lag was observed in the
formation of IV (Figure 3). This pattern is clearly
indicative of a two parallel reaction process for the
formation of II and III and the dependence of IV on
the concomitant release and oxidation of bromide. In
all experiments, II-IV accounted for >99% of the initial
bromacil, indicating that the formation of other byprod-
ucts was not significant. Further oxidation of II-IV
was minimal, and during the initial phase of the
reaction, re-formation of bromacil via product IV deg-
radation was a very minor process. Therefore, k-4 was
assumed to be 0.
Ozon olysis in th e P r esen ce of Hyd r ogen P er oxid e.
Solutions of 382 µM bromacil and three different concentra-
tions of hydrogen peroxide (0, 0.07, 3.4, or 6.9 mM H2O2) were
subjected to ozonolysis in a manner similar to that above on
the same day under exactly the same conditions; that is, the
generator was stabilized prior to the first run and was not shut
Deter m in a tion of k2, k3, a n d k4. Three bromacil
systems were examined: (1) bromacil, (2) bromacil with

Kinetics of Bromacil Ozonolysis
J. Agric. Food Chem., Vol. 46, No. 4, 1998 1633
F igu r e 2. Experimental ozonolysis flow diagram.
hydrogen peroxide, and (3) bromacil with tert-butyl
alcohol, a hydroxy radical scavenger. Previous studies
showed that the degradation of bromacil was very fast
in all three systems, with the slowest degradation rate
of bromacil observed in the peroxide system (Anderson,
1996; Hapeman et al., 1997). The system with hydrogen
peroxide was significantly slower than either the system
with only ozone or the system with tert-butyl alcohol
and was used to determine k2, k3, and k4.
An ASCII script was encoded with the necessary
information for MLAB and included eqs 1-4, 15-19,
and 21, the literature values for k5, k6, k7, k8, and k11,
and the experimental concentrations of I-IV, H2O2, and
O3. The calculated rate parameters from the O3/H2O2
system over the period of 0-15 min (no bromacil
remained after 15 min) were as follows: k2 ) 0.0078 (
0.0007 µM-1 min-1, k3 ) 0.030 ( 0.002 µM-1 min-1, and
k4 ) 47 ( 4 µM-1 min-1 (R2 ) 0.992). Figure 4 shows
a good fit of the experimental data and the predicted
concentrations of bromacil and its degradation products.
To determine the effect of hydrogen peroxide concen-
tration on the product formation, ozonolysis was con-
ducted at three different H2O2 concentrations. The
results are shown in Figure 5 and clearly illustrate that
while the overall rate of bromacil degradation decreased
by <25% at the highest H2O2 concentrations, the
amount of IV formed per mole of bromacil degraded
decreased by as much as 83%. The different H2O2
concentrations were used as inputs for the initial
conditions in model script, and the same trends as above
were observed.
Testin g th e Kin etic Mod el on a Distilled Wa ter
System . The ozonolysis of bromacil was conducted in
distilled water without hydrogen peroxide. The kinetic
model was tested on this system. The ASCII script was
modified to include the determined values for k2, k3, and
k4 from the previously described hydrogen peroxide/
ozone system, and the initial concentration of hydrogen
peroxide was set to 0. The model was used to predict
the concentrations of I-IV as a function of time. The
resultant theoretical curves are shown in Figure 6 and
are reasonably consistent with the experimental data:
R2 ) 0.980, where R2 is a determination coefficient that
provides an indication of the goodness of fit for all four
concentration curves of I-IV with the actual data.
Testin g th e Kin etic Mod el on a System Con ta in -
in g a Ra d ica l Sca ven ger . A third reaction system
•
that consisted of bromacil and tert-butyl alcohol, a OH

1634 J. Agric. Food Chem., Vol. 46, No. 4, 1998
Torrents et al.
F igu r e 4. Modeling the ozonolysis of bromacil in the presence
of 6.9 mM H2O2. Symbols represent the experimental data,
and the lines correspond to the model. Error bars represent
the standard deviation for the three experiments performed
on different days under the same reaction conditions. Note the
differences in the concentrations of IV and III between this
experiment and those shown in Figures 6 and 7.
F igu r e 3. Ozonolysis of an aqueous solution of bromacil. Error
bars represent the standard deviation for the five experiments
performed on different days under the same reaction condi-
tions.
scavenger, was examined. The same script was used
to predict concentrations of bromacil and II-IV as a
function of time. The resultant predicted curves are
shown in Figure 7 and are reasonably consistent with
the experimental data: R2 ) 0.980.
DISCUSSION AND CONCLUSIONS
The mechanism for the bromacil ozonolysis has been
shown to proceed via a direct ozone reaction, as opposed
to oxidation by hydroxy radicals, by which II and III
are formed concomitantly and IV is formed after bro-
mide is released from these reactions. Bromacil deg-
radation was significantly enhanced under conditions
in which the hydroxy radicals are scavenged and
prevented from reacting with ozone. However, the
amount of the phytotoxic precursor (IV) to bromacil
degraded remained constant (Anderson, 1996). Alter-
natively, the addition of hydrogen peroxide decreased
the rate of bromacil degradation and significantly
altered the ratio of byproducts. The presence of hydro-
gen peroxide increases the concentration of hydroxy
radicals that can react with ozone, thereby decreasing
the ozone available to react with bromacil and more
importantly bromide. At the same time, hydrogen
peroxide reacts with the HOBr that is formed, decreas-
ing the steady state concentration of HOBr at any given
time. Thus, the formation of IV is decreased consider-
ably (up to 83% under the conditions of this study). This
further confirms the findings that the addition of
hydrogen peroxide in ozonation treatment processes is
particularly efficient at controlling the formation of
brominated byproducts (Griffini and Iozzelli, 1996).
A kinetic model was used to determine the rate
parameters and accurately predict the concentrations
F igu r e 5. Effect of hydrogen peroxide concentration on the
overall observed bromacil degradation rate [([I]0 - [I])/[I]0] and
on the formation of the phytotoxic precursor byproduct, IV
(Anderson, 1996; Hapeman et al., 1997). [I]0 is the initial
concentration of bromacil, and [I] is the concentration at t )
6 min.
of the products formed in different matrixes. The
predicted data were reasonably consistent with that
observed. The presence of other matrix components,
such as organic matter, other radical scavengers, anions,

Kinetics of Bromacil Ozonolysis
J. Agric. Food Chem., Vol. 46, No. 4, 1998 1635
F igu r e 6. Modeling the ozonolysis of bromacil in distilled
water. Symbols represent the experimental data, and the lines
correspond to the model. Error bars represent the standard
deviation for the five experiments performed on different days
under the same reaction conditions.
F igu r e 7. Modeling the ozonolysis of bromacil in the presence
of 53 mM tert-butyl alcohol. Symbols represent the experi-
mental data and the lines correspond to the model. Error bars
represent the standard deviation for the three experiments
performed on different days under the same reaction condi-
tions.
k6 ) rate constant for the reaction of Br- with O3 (eq 6)
and cations, may effect the distribution of byproducts
and the degradation rate of substrate. While additional
studies may be needed to address the influence of matrix
components on the chemistry of ozone and HOBr, the
above model is a necessary first step for optimization
of the ozonolysis process, which requires the formation
of the phytotoxic precursor to be minimized. Therefore,
for the degradation of bromacil, conditions that favor
hydroxy radical processes (high pH and H2O2 addition)
are preferred. Furthermore, this study clearly demon-
strates that a mechanistic understanding of the degra-
dation process and its mathematical description can aid
in visualization of the reaction conditions that will best
meet treatment needs.
(µM-1 min-1
)
k7 ) rate constant for the reaction of OBr- with O3 (eq 7)
(µM-1 min-1
k8 ) rate constant for the reaction of OBr- with O3 (eq 8)
(µM-1 min-1
k9 ) rate constant for the reaction of HOBr with O3 (eq 9)
(µM-1 min-1
k10 ) rate constant for the reaction of BrO3 with O3 (eq
10) (µM-1 min-1
k11 ) rate constant for the reaction of HOBr with H2O2 (eq
11) (µM-1 min-1
k12 ) rate constant for the reaction of O3 with H2O2 (eq
12) (µM-1 min-1
k13 ) rate constant for the reaction of H+ with O3 (eq 13)
(µM-1 min-1
k14 ) rate constant for the reaction of H2O with O3 (eq 14)
)
)
)
-
)
)
)
)
NOMENCLATURE
(µM-1 min-1
)
[Br-] ) concentration of bromide (µM)
[BrO3-] ) concentration of bromate (µM)
ki ) rate constant for the decomposition of ozone due to
the presence of organic and inorganic substrates (eq 20)
H ) Henry’s law constant for ozone [atm‚m3‚(g‚mol)-1
[HOBr] ) concentration of hypobromous acid (µM)
[H2O2] ) concentration of hydrogen peroxide (µM)
[I] ) concentration of bromacil (I) (µM)
]
(µM-1 min-1
)
kLa ) mass transfer coefficient from the gas phase to liquid
phase (eq 20) (min-1
ko ) rate constant of the decomposition ozone in water (eq
20) (µM-1 min-1
)
[I]0 ) concentration of bromacil at t ) 0 (µM)
[II] ) concentration of product II (µM)
)
[O3] ) concentration of ozone in solution (µM)
[OBr-] ) concentration of hypobromite ion (µM)
P ) partial pressure of ozone in the gas phase (atm)
Si ) concentration of substrates available to react with
ozone (eq 20) (µM)
[III] ) concentration of product III (µM)
[IV] ) concentration of product IV (µM)
k2 ) rate parameter for the formation of II from I (eq 1)
(µM-1 min-1
k3 ) rate parameter for the formation of III from I (eq 1)
(µM-1 min-1
k4 ) rate parameter for the formation of IV from I (eq 1)
(µM-1 min-1
k-4 ) rate parameter for the formation of I from IV (eq 1)
(min-1
k5 ) rate constant for the protonation of OBr- (eq 5) (µM-1
min-1
)
t ) reaction time (min)
)
LITERATURE CITED
)
Acher, A. J .; Hapeman, C. J .; Shelton, D. R.; Muldoon, M. T.;
Lusby, W. R.; Avni, A.; Water, R. Comparison of formation
of biodegradation of bromacil oxidation products in aqueous
solutions. J . Agric Food Chem. 1994, 42, 2040-2047.
)
)

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Aharonson, N. Potential contamination of groundwater by
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Received for review J uly 28, 1997. Revised manuscript received
J anuary 27, 1998. Accepted J anuary 27, 1998. Mention of
specific products or suppliers is for identification and does not
imply endorsement by U.S. Department of Agriculture to the
exclusion of other suitable products or suppliers.
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J F970651N