Radiation-induced reactions of 2,4,6-trinitrotoluene in aqueous solution

Article abstract of DOI:10.1021/es970688o

Radiolysis of aqueous solutions of TNT was examined to provide fundamental information concerning the reactions of TNT with radical species in water. χ-Radiation was used in conjunction with radical scavengers to compare yields for radiation-induced TNT transformation under oxidizing end reducing conditions and in the presence and absence of oxygen. Pulse radiolytic techniques were employed to determine rate constants and absorption spectra for the reactions of TNT with the hydroxyl radical and the aqueous electron. TNT was rapidly transformed under both reducing (1% tert- butyl alcohol, N₂ sparged) and oxidizing (N₂0 sparged) conditions although rates under reducing conditions were greater. The initial yield for transformation of a 350 μmol L^-1 TNT solution under reducing conditions was 0.14 μmol/J as opposed to 0,10 μmol/J measured in oxidizing conditions. The reactions of TNT with reduced oxygen species were found to be highly inefficient in aqueous solution. Although TNT is transformed by both oxidizing and reducing radicals, TNT degradation yields in the absence of a radical scavenger were low, indicating that under these conditions there were significant secondary reactions in which the species resulting from reactions between TNT and the primary radicals further reacted to reform the parent compound. The bimolecular rate constant for the reaction between TNT and ·OH was determined to be 4.3 x 10⁸ mol^-1 s^-1. Byproduct analyses from χ- radiolysis suggest that hydroxyl radical abstraction of a methyl hydrogen to form the trinitrotoluyl radical is an initial oxidative reaction. The bimolecular rate constant for the reaction between TNT and e(aq)^- was measured as 3.5 x 10¹⁰ mol^-1 s^-1. These results provide quantitative and qualitative insight into the reactions between TNT and the various aqueous radicals produced in many remediation processes. A study of aqueous trinitrotoluene (TNT) solutions radiolysis with gamma radiation in conjunction with radical scavengers provides insights into the reactions between TNT and the various aqueous radicals produced in many remediation processes. TNT was rapidly transformed under both reducing and oxidizing conditions, although rates under reducing conditions were higher. Low TNT degradation yields in the absence of a radical scavenger indicate secondary reactions in which the species resulting from reactions between TNT and the primary radicals further reacted to reform the parent compound. Byproduct analyses suggest that OH radical abstraction of a methyl hydrogen to form the trinitrotoluyl radical is an initial oxidative reaction.

Full text of DOI:10.1021/es970688o

Environ. Sci. Technol. 1998, 32, 971-974
sites where manufacture or demilitarization of munitions
Radiation-Induced Reactions of
2,4,6-Trinitrotoluene in Aqueous
Solution
D A N I E L C . S C H M E L L I N G , †
K I M B E R L Y A . G R A Y, * , ‡ A N D
P R A S H A N T V . K A M A T * , §
was performed (1, 2). Certain efforts to degrade TNT in water
for purposes of environmental remediation have focused on
the use of processes which rely on the generation of aqueous
radicals. Such processes include UV/ ozone/ peroxide com-
binations (3-6) and photocatalysis using TiO2 (7-11). This
situation has created interest in the reactions of TNT with
radical species formed in water. However, although TNT
has been shown to readily undergo both oxidative and
reductive transformations (12), few studies have examined
relative reaction rates of TNT with different types of radicals.
Moreover, rate constants for the reactions of TNT with the
primary radicals formed from water are, to the best of the
authors’ knowledge, currently unavailable in the literature.
Radiolysis has been shown to be an effective technique
for elucidating radical reaction mechanisms and rates. The
interaction of water with ionizing radiation, such as γ-rays
or a high energy electron beam, results in the formation of
a suite of radical species in which hydroxyl radicals (•OH)
and hydrated electrons (eaq-) are predominant. Through
the use of selective scavengers, the reactions of a solute in
dilute aqueous solution with these radicals can be investi-
gated. In the current study, radiolysis of aqueous solutions
of TNT has been examined. γ-Radiation has been utilized
in conjunction with radical scavengers to compare yields for
radiation-induced TNT transformation under oxidizing and
reducing conditions and in the presence and absence of
oxygen. Pulse radiolytic techniques have been employed to
determine rate constants and absorption spectra for the
reactions of TNT with the hydroxyl radical and the aqueous
electron. These data are intended to fill a current gap in the
literature by providing fundamental information concerning
the reactions of TNT with radical species in water and,
thereby, facilitate the application of remediation technologies
to TNT contaminated sites.
Department of Civil Engineering and Geological Science,
University of Notre Dame, Notre Dame, Indiana 46556
Radiolysis of aqueous solutions of TNT was examined
to provide fundamental information concerning the reactions
of TNT with radical species in water. γ-Radiation was
used in conjunction with radical scavengers to compare
yields for radiation-induced TNT transformation under
oxidizing and reducing conditions and in the presence and
absence of oxygen. Pulse radiolytic techniques were
employed to determine rate constants and absorption spectra
for the reactions of TNT with the hydroxyl radical and
the aqueous electron. TNT was rapidly transformed under
both reducing (1% tert-butyl alcohol, N2 sparged) and
oxidizing (N2O sparged) conditions although rates under
reducing conditions were greater. The initial yield for
transformation of a 350 µmol L-1 TNT solution under
reducing conditions was 0.14 µmol/J as opposed to 0.10
µmol/J measured in oxidizing conditions. The reactions
of TNT with reduced oxygen species were found to be highly
inefficient in aqueous solution. Although TNT is transformed
by both oxidizing and reducing radicals, TNT degradation
yields in the absence of a radical scavenger were low,
indicating that under these conditions there were significant
secondary reactions in which the species resulting from
reactions between TNT and the primary radicals further
reacted to reform the parent compound. The bimolecular
Experimental Section
Chem icals. The 2,4,6-trinitrotoluene used for experimental
work was purchased from Chemservice (West Chester, PA).
3,5-Dinitroaniline was purchased from Aldrich (Milwaukee,
WI). Certified standards of 2,4,6-trinitrotoluene, 1,3,5-
trinitrobenzene, 2-amino-4,6- dinitrotoluene, 4-amino-2,6-
dinitrotoluene, 2,4-dinitrotoluene, and 2,6-dinitrotoluene
were purchased from AccuStandard Inc. (New Haven, CT)
and were used for identification and quantification. All
compounds were of the highest purity available from these
vendors and were used without further purification. tert-
Butyl alcohol (Fisher) was used as a radical scavenger.
Nitrogen and oxygen gases were ultrahigh purity, and nitrous
oxide was U. S. P. grade. The water used in all experiments
and analyses was ultrapure (18 MΩ‚cm) prepared using a
Milli-Q purification system.
•
rate constant for the reaction between TNT and OH was
determined to be 4.3 × 108 mol-1 s-1. Byproduct analyses
from γ-radiolysis suggest that hydroxyl radical abstraction
of a methyl hydrogen to form the trinitrotoluyl radical is
an initial oxidative reaction. The bimolecular rate constant
-
for the reaction between TNT and eaq was measured
as 3.5 × 1010 mol-1 s-1. These results provide quantitative
and qualitative insight into the reactions between TNT
and the various aqueous radicals produced in many reme-
diation processes.
γ-Radiolysis. γ-Radiolysis was carried out with a Shep-
herd 109 60Co source. The Shepherd 109 is a concentric well-
type source which had an average dose rate of 67.7 Gy/ min
during the period in which the experiments were conducted.
Solutions for γ-irradiation were prepared by mixing reagents
with a 220 µmol L-1 TNT stock solution which was then
saturated with either N2, O2, or N2O by sparging for 90 min.
HPLC sample vials of 5 mL were filled with the solutions and
sealed with gastight PTFE septa. Sealed vials were then
wrapped in paraffin film to further inhibit gas transfer
between the solution and the atmosphere.
Introduction
The secondary explosive TNT (2,4,6-trinitrotoluene) is a
prevalent contaminant in water, sediment, and soil at many
* To whom correspondence should be sent.
† University of Notre Dame. Current address: U.S. Environmental
Protection Agency, NRMRL-WSWRD, Mailstop 690, 26 W. Martin
Luther King Dr., Cincinnati, OH 45268. E-mail: schmelling.daniel@
epamail.epa.gov.
‡ Department of Civil Engineering, 2145 Sheridan Road, North-
western University, Evanston, Illinois 60208-3109. Fax: 847-491-4011;
e-mail: k-gray@nwu.edu.
Pulse Radiolysis. Pulse radiolysis experiments were
performed using the linear accelerator at the Notre Dame
Radiation Laboratory. The accelerator had an output of 1-5
§
Notre Dame Radiation Laboratory, Notre Dame, Indiana 46556.
Fax: 219-631-8068; e-mail: Kamat@marconi.rad.nd.edu.
9
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VOL. 32, NO. 7, 1998 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 7 1

Gy/ pulse. Absorbance-time profiles at various wavelengths
were recorded immediately after pulsing aqueous solutions
of TNT. Solutions used to examine hydroxyl radical attack
were saturated with N2O while solutions utilized to analyze
reductive attack contained 1% tert-butyl alcohol and were
deaerated using N2 gas. The measured absorbance values
were calibrated against the extinction coefficient of the
thiocyanate radical at 472 nm (7580 L mol-1 cm-1). All pulse
radiolysis experiments were conducted at neutral pH.
Analysis. Analyses for TNT concentration and interme-
diate species were made using a Waters HPLC (712 WISP,
600E pump, 996 PDA). The column was a Supelcosil RP-C18
and the eluant was methanol:25 mmol of phosphate buffer
(pH ) 3.5) in an isocratic ratio of 40:60. Absorbance was
measured from 215 to 600 nm using the PDA detector.
Intermediate species were identified in all cases by matching
both HPLC retention time and absorbance spectra with that
of standards.
Results and Discussion
FIGURE 1. Transformation of TNT in aqueous solution induced by
γ-radiation under oxidizing and reducing conditions.
γ-Radiolysis. γ-Radiolysis was used in this work to generate
radical species in water. Primary radical and molecular yields
resulting from irradiation of water with pH 3-13 by low linear
energy transfer (LET) radiation, such as fast electrons and
γ-rays, are given in eq 1 (13):
superoxide anion, O2-. The yield for TNT transformation
during the initial period of the irradiation was 0.013 µmol/ J,
which was approximately 90% less than was measured in the
N2 sparged solution containing 1% tert-butyl alcohol. This
result demonstrates that O2 initially scavenged a majority of
the primary reducing radicals. Moreover, this yield suggests
that reactions of TNT with the products of oxygen reduction
are inefficient in aqueous solution. As a result of reactions
with radicals, the dissolved oxygen concentration dropped
as the irradiation continued. Consequently, the fraction of
reducing radicals reacting with TNT increased, and the TNT
decay rate accelerated until, after a dose of approximately
2 kGy, the rate was similar to that measured in the N2 sparged
solution containing 1% tert-butyl alcohol. The yield for TNT
transformation between 2 and 3 kGy was 0.11 µmol/ J.
No byproducts were identified from the γ-radiolytic
transformation of TNT under the reducing conditions
described for Figure 1. Compounds which were investigated
but not observed include 2-am ino-4,6-dinitrotoluene,
4-amino-2,6-dinitrotoluene, and 3,5-dinitroaniline. Under
oxidizing conditions 2,4,6-trinitrobenzoic acid (TNBA) was
identified as a byproduct. The concentration of TNBA was
approximately 16 µmol L-1 at a radiation dose of 4 kGy,
indicating that oxidation of the methyl group of TNT was an
initial transformation step. Oxidation of TNT to trinitroben-
zoic acid has been demonstrated by several methods
employing strong oxidants (14, 15) including aqueous TiO2
photocatalysis (7) where the primary oxidant is the hydroxyl
radical (16).
In Figure 2 the radiation induced degradation of TNT in
oxygen and nitrogen sparged solutions containing no ad-
ditional radical scavengers is shown. The initial yields for
TNT transformation under these conditions were 0.048 and
0.029 µmol/ J for O2 and N2 saturated solutions, respectively.
From data in Figure 1, discussed earlier, it may be assumed
that, upon irradiation of the O2 saturated solution, the primary
reducing radicals initially were almost entirely scavenged by
O2 to form reduced oxygen species. This situation is similar
to irradiation of the nitrous oxide saturated solution, shown
in Figure 1, where the primary reducing radicals were
scavenged by N2O, except that N2O forms hydroxyl radicals
after reacting with eaq-. Thus, the yield of hydroxyl radicals
in the N2O sparged solution shown in Figure 1 is believed to
have been approximately twice that of the O2 sparged solution
in Figure 2. The initial yield for TNT transformation in the
O2 saturated solution (0.048 µmol/ J), displayed in Figure 2,
was approximately half that measured in the N2O sparged
solution (0.10 µmol/ J) in Figure 1. This is further evidence
g(eaq-) ) g(•OH) ) g(H3O+) ) 0.28; g(H) )
0.062; g(H2) ) 0.047; g(H2O2) ) 0.072 (µmol/ J) (1)
When molecular oxygen is present, it reacts rapidly with the
aqueous electron and the hydrogen atom to form the
superoxide radical anion, O2-, and hydroperoxy radical, HO2.
Nitrous oxide also readily scavenges aqueous electrons,
forming nitrogen gas and the hydroxyl radical (see
eq 2). Consequently, saturation of a solution with N2O
eaq- + N2O f N2 + O-
O
- + H2O f •OH + OH-
(2)
(CH3)3COH + OH f (CH3)2 •CH2COH + H2O (3)
produces highly oxidizing conditions upon irradiation and
effectively doubles the hydroxyl radical yield. In contrast,
adding sufficient concentrations of a hydroxyl radical scav-
enger, such as tert-butyl alcohol (see eq 3), in conjunction
with deaerating the solution to remove molecular oxygen,
ensures that the predominant reacting radicals are reducing.
Relative rates of TNT transformation in dilute aqueous
solution were examined under oxidizing and reducing
conditions using γ-radiation. Oxidizing conditions were
created by saturating TNT solutions with N2O prior to
irradiation and reducing conditions were induced by adding
1% (v/ v) tert-butyl alcohol to the stock TNT solution and
then sparging with N2 gas. The results of this comparison
are shown in Figure 1. TNT was rapidly transformed under
both reducing (tert-butyl alcohol, N2 sparged) and oxidizing
(N2O sparged) conditions. The initial yield for TNT trans-
formation under reducing conditions was 0.14 µmol/ J, which
was approximately 40% higher than the initial yield of 0.10
µmol/ J measured in oxidizing conditions. Thus, these data
indicate that, while TNT is labile to both oxidative and
reductive transformations, degradation is more efficient
under reducing conditions.
Also shown in Figure 1 are data from γ-irradiation of a
220 µmol L-1 TNT solution containing 1% tert-butyl alcohol
that was sparged with O2 so that the dissolved O2 concentra-
tion was 1.3 mmol L-1 prior to irradiation. In these solution
conditions tert-butyl alcohol served to scavenge the oxidizing
radicals while the reducing radicals were expected to react
with dissolved O2 to form reduced oxygen species such as
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FIGURE 3. Absorbance spectrum of the species formed 20-25 µs
after the pulse radiolysis of a N2O saturated 350 µmol L-1 aqueous
solution of TNT.
FIGURE 2. γ-Radiation induced transformation of TNT in oxygen
and nitrogen sparged solutions containing no additional scavengers.
that TNT is readily degraded by the hydroxyl radical but is
relatively unreactive with reduced oxygen species in aqueous
solution.
•
Although TNT is transformed by both eaq- and OH, the
yield for TNT transformation in the nitrogen sparged solution,
as shown in Figure 2, was only 0.029 µmol/ J. This low yield
suggests that in the absence of a radical scavenger there
were significant secondary reactions in which the species
resulting from reactions between TNT and the primary
radicals further reacted to reform the parent compound.
Reactions of this type have been discussed in previous work
(17). The yield for TNT transformation in the deaerated
solution was initially much lower than was observed in the
oxygen sparged solution in Figure 2, where O2 served as an
electron and hydrogen atom scavenger. However, as the O2
concentration decreased at higher radiation doses, the decay
rates for the two solutions became essentially the same.
Pulse Radiolysis. Pulse radiolysis was used to determine
the rate constants for the reactions of TNT with •OH and eaq
-
and to acquire spectral information on the radical species
that result from these reactions. As before, solution condi-
tions in which •OH was the predominant radical were created
by sparging with N2O. The absorbance spectrum of the
species formed 20-25 µs after the pulse radiolysis of a N2O
saturated 350 µmol L-1 solution of TNT is shown in Figure
3. The spectrum displays a broad absorbance in the UV
region with a primary maximum at 310 nm and a secondary
maximum at 390 nm. The structure of this radical species
has not been identified but, based on byproduct analyses
from γ-radiolysis discussed above, it is proposed to result
predominantly from hydroxyl radical abstraction of a methyl
hydrogen to form the trinitrotoluyl radical. The pseudo-
first-order rate constant for the formation of this species was
measured for five different TNT concentrations in the range
75-350 µmol L-1 by monitoring absorbance at 310 nm (see
Figure 4). A linear least-squares curve fit of these rate
constants vs TNT concentration indicates that the bi-
molecular rate constant for the reaction between TNT and
FIGURE 4. Pseudo first-order rate constant as a function of TNT
concentration for the reaction between •OH and TNT. Absorbance
measured at 310 nm.
•OH is 4.3 × 108 mol-1 s-1
.
-
The reaction between TNT and eaq was studied by
creating reducing conditions as discussed earlier. The
absorbance spectrum of the species formed 100 ns after the
pulse radiolysis of a nitrogen sparged 350 µmol L-1 TNT
solution containing 1% tert-butyl alcohol is shown in Figure
5. The spectrum displays a very strong and sharply defined
absorbance maximum at 300 nm. On the basis of previous
radiolytic studies of other nitroaromatic compounds, this
species is proposed to be an electron adduct with the electron
FIGURE 5. Absorbance spectrum of the species formed 100 ns after
the pulse radiolysis of a nitrogen sparged 350 µmol L-1 aqueous
solution of TNT containing 1% tert-butyl alcohol.
localized on one of the electronegative nitro groups (18).
The pseudo-first-order rate constants for the scavenging of
aqueous electrons by TNT were determined at four TNT
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VOL. 32, NO. 7, 1998 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 7 3

Acknowledgments
D.C.S. and K.A.G. would like to acknowledge the support of
the National Science Foundation (Grant BCS91-57948). The
authors would also like to thank the Center for Bioengineering
and Pollution Control at UND for the use of their facilities.
P.V.K. acknowledges the support of the Office of Basic Energy
Sciences of the Department of Energy. This is contribution
no. 4021 from the Notre Dame Radiation Laboratory.
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(4) Kearney, P. C.; Zeng, Q.; Ruth, J. M. Chemosphere 1983, 12,
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FIGURE 6. Pseudo-first-order rate constant as a function of TNT
concentration for the reaction between eaq- and TNT. Absorbance
measured at 300 nm.
(5) Wentsel, R. S.; Sommerer, S.; Kitchens, J. F. Treatment of
Explosives Contaminated Lagoon Sediment-Phase 1. Literature
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Agency; Aberdeen Proving Ground, MD, 1981; NTIS # 91-18792.
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P. K. Photocatalytic Degradation of Aqueous TNT and RDX;
Presented at I&EC Special Symposium; American Chemical
Society; Atlanta, GA, September 1994.
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P. K. Photocatalysis for the Destruction of Aqueous TNT, RDX,
and HMX; Sandia National Laboratory; Albuquerque, NM, 1994;
SAND-94-3048C.; CONF-941255-1.
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1996, 3, 77.
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(12) Schmelling, D. C.; Gray, K. A.; Kamat, P. V. Environ. Sci. Technol.
1996, 30, 2547.
(13) Spinks, J. W.; Woods, R. J. An Introduction to Radiation Chemistry,
3rd ed.; John Wiley & Sons, Inc.: New York, 1990.
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and Structure; McGraw-Hill Book Co.: New York, 1977; pp 1096-
1098.
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(18) Greenstock, C. L. Radiation Chemistry of Amines, Nitro and
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Compounds and Their Derivatives, Part 1; John Wiley-
Interscience: New York, 1982; pp 291-318.
(19) Anbar, M.; Bambenek, M.; Ross, A. Selected Specific Rates of
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concentrations in the range 102-350 µmol L-1 by monitoring
absorbance at 300 nm (see Figure 6). A linear least-squares
curve fit of these rate constants vs TNT concentration
indicates that the bimolecular rate constant for the reaction
between TNT and eaq is 3.5 × 1010 M-1 s-1. This value is
-
similar to those reported for the reaction of eaq- with other
nitroaromatic compounds (19, 20) and is diffusion controlled.
These rate data support the conclusion, noted in the previous
•
section, that TNT will react with both OH and eaq- but the
rate of reaction with the aqueous electron is significantly
higher.
Im plications of Radiolysis Studies. The treatment of
organic wastes in aqueous systems by currently available
advanced oxidation processes relies primarily on the gen-
eration of hydroxyl radicals. However, in a previous study
of the photocatalytic degradation of TNT in water, using TiO2
and UV light, we demonstrated that reductive transformations
caused by conduction band electrons played a significant
role in facilitating the overall degradation of TNT (12). This
behavior indicated a potential advantage of photocatalysis
over other advanced oxidation processes in the treatment of
TNT contaminated water. In the present study, we have
-
separately resolved reduction and oxidation of TNT by eaq
•
and OH radicals. TNT was shown to have a significantly
higher rate of reaction with eaq- in comparison to OH, and
•
to degrade more rapidly under reductive conditions. While
-
the reduction potential of eaq is greater than that of TiO2
conduction band electrons, these data clearly suggest that
reductive transformation of TNT is a more favorable initial
degradation step. In addition, our data demonstrate a
relatively low rate of reaction between TNT and reduced
oxygen species. Given the low concentrations of TNT
typically present in explosives contaminated water and the
high rates of reaction between O2 and reducing radicals, these
data show that deaerated conditions are necessary in order
to effect reductive degradation of TNT efficiently. Incor-
poration of these observed kinetics into reactor design,
through the utilization of both reductive and oxidative
degradation mechanisms, will aid in the development of more
efficient methods for treating materials contaminated by
nitroaromatic pollutants such as TNT.
Received for review August 5, 1997. Revised manuscript
received December 18, 1997. Accepted January 2, 1998.
ES970688O
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