Electrochemical treatment of 2,4,6-trinitrotoluene and related compounds

Article abstract of DOI:10.1021/es001465s

This work involves electrolysis of nitrotoluene congeners, which are persistent pollutants that enter the environment as a consequence of their manufacture and use as explosives. Reduction to aminotoluenes occurred with high current efficiency at a variety of cathodes, at potentials -0.5 to -1 V vs SCE. The products were formed in high chemical yield and with excellent mass balance. Preliminary experiments were also carried out to find methods of removing the electrolysis products from solution by oxidative oligomerization. The most satisfactory method was partial reoxidation at a Ti/IrO₂ anode, suggesting an overall remediation technology in which reduction is followed by reoxidation of the spent catholyte in the anode compartment of the same electrolytic cell. This work involves electrolysis of nitrotoluene congeners, which are persistent pollutants that enter the environment as a consequence of their manufacture and use as explosives. Reduction to aminotoluenes occurred with high current efficiency at a variety of cathodes, at potentials -0.5 to -1 V vs SCE. The products were formed in high chemical yield and with excellent mass balance. Preliminary experiments were also carried out to find methods of removing the electrolysis products from solution by oxidative oligomerization. The most satisfactory method was partial reoxidation at a Ti/IrO₂ anode, suggesting an overall remediation technology in which reduction is followed by reoxidation of the spent catholyte in the anode compartment of the same electrolytic cell.

Full text of DOI:10.1021/es001465s

Environ. Sci. Technol. 2001, 35, 406-410
chemicals and the “green” issue of using only electrons as
Electrochemical Treatment of
2,4,6-Trinitrotoluene and Related
Compounds
reagents (14). Electrochemical technologies also offer the
prospect of relatively low capital cost, modular design,
operation at atmospheric pressure and temperature, and the
possibility of higher energy efficiency than thermal treatment
or photolysis. However, a significant drawback is that parasitic
reactions, such as electrolysis of water, often compete with
electrolysis of the contaminant(s) and lower energy efficiency.
The electrochemical reduction of nitroaromatic com-
pounds has long been known (15). Nitrobenzene undergoes
a four-electron reduction to phenylhydroxylamine in aqueous
solution at pH > 4 and a six-electron reduction to aniline at
lower pH. Reduction takes place in several steps involving
sequential electrochemical and chemical reactions (16-18).
An important consideration in the present context is that
reduction occurs at low negative potentials, at which parasitic
electrolysis of water is minimized.
J A M E S D . R O D G E R S A N D
N I G E L J . B U N C E *
School of Engineering and Department of Chemistry and
Biochemistry, University of Guelph, Guelph,
Ontario, Canada N1G 2W1
This work involves electrolysis of nitrotoluene congeners,
which are persistent pollutants that enter the environment
as a consequence of their manufacture and use as explosives.
Reduction to aminotoluenes occurred with high current
efficiency at a variety of cathodes, at potentials -0.5 to -1
V vs SCE. The products were formed in high chemical
yield and with excellent mass balance. Preliminary
experiments were also carried out to find methods of
removing the electrolysis products from solution by oxidative
oligomerization. The most satisfactory method was
Experimental Section
2,4-Dinitrotoluene (2,4-DNT), 2,6-dinitrotoluene (2,6-DNT),
and their reduction products 2-amino-4-nitrotoluene (2A4NT),
4-amino-2-nitrotoluene (4A2NT), 2,4-diaminotoluene (2,4-
DAT), and 2,6-diaminotoluene (2,6-DAT) were obtained from
Aldrich; 2,4,6-trinitrotoluene, 2-amino-4,6-dinitrotoluene (2-
ADNT), 4-amino-2,6-dinitrotoluene (4-ADNT), 2,4-diamino-
6-nitrotoluene (2,4-DANT), and 2,6-diamino-4-nitrotoluene
(2,6-DANT) were obtained from Chemservice. 2,4,6-Triami-
notoluene (TAT) was a gift from Dr. J. Hawari of the National
Research Council of Canada. Purities of all compounds were
verified by HPLC to be g 98%. Horseradish peroxidases
(HRP: Sigma) consisted of a Type I (EC 1.11.1.7, RZ 1.3) and
a Type II (EC 1.11.1.7, RZ 1.9) with activities of 220 and 240
units mg-1, respectively. One unit of activity is defined as the
number of µmol of hydrogen peroxide converted per minute
at pH 7.4 at 25 °C.
Voltam m etry. Reduction potentials were determined in
N2-purged solutions at a dropping mercury electrode using
a Radiometer polarographic analyzer with a platinum bulb
as a counter electrode and a saturated calomel reference
electrode. Conditions for differential pulse polarography
(DPP) were as follows: purge time 1000 s, initial potential
) 0 mV, final potential ) -1200 mV, step duration ) 0.4 s,
step amplitude ) 1 mV, pulse duration ) 40 ms, and pulse
amplitude ) 25 mV.
Electrolysis. Electrolyses were carried out amperostati-
cally or potentiostatically, controlled by a Pine RDE4 po-
tentiostat with flow-through, sandwich type cells constructed
from Plexiglas pieces with dimensions of 90 × 15 × 45 mm.
The center of each Plexiglas piece was bored out to provide
2.5 mL anode and cathode compartments, which were
separated by a Nafion 417 cation exchange membrane.
Anolyte and catholyte solutions were delivered from 60 mL
glass reservoirs and were pumped at 7 mL min-1 upward
through the cell (to facilitate removal of gas bubbles) using
a peristaltic pump. Connection to the pump head was by
means of LFL-Tygon; Teflon tubing was used for connections
between the pump and the reservoirs.
A three-electrode system was used, with electrode po-
tentials measured vs external Ag/ AgCl. Anodes (counter
electrodes) were either Pt foil or IrO2/ Ti (10 cm2). Cathodes
(10 cm2) were constructed from solid Pb, Ni-plated Ni wire
(0.5 mm diameter), Raney nickel, Pt, or reticulated carbon
(RTC). Pb and Ni were obtained from Aldrich, RTC from ERG
Materials. The Ni-plated cathode was prepared by elec-
trodepositing a 1.4 M NiSO4/ NiCl2 solution for 2 h at 15 mA
cm-2 onto Ni wire. The Raney Ni cathode was prepared by
partial reoxidation at a Ti/IrO anode, suggesting an overall
2
remediation technology in which reduction is followed
by reoxidation of the spent catholyte in the anode compartment
of the same electrolytic cell.
Introduction
The large scale manufacture and use of nitroaromatic
compounds such as 2,4,6-trinitrotoluene (TNT), 1,3,5-trini-
trobenzenene, dinitrotoluenes (2,4-DNT, 2,6-DNT), tetryl,
and picric acid as explosives has led to significant contami-
nation of soils (1) and groundwater. The U.S. ceased
production of TNT in the mid-1980s, but environmental
contamination has continued because of the open detonation
and burning of explosives at army depots and artillery ranges
and demilitarization at ordnance disposal sites (2). Nitroaro-
matic explosives are toxic: for example, the 14-day LC50 values
toward the guppy (Poecilia reticulzata) are 12.5 mg L-1 for
2,4-DNT and 18 mg L-1 for 2,6-DNT (3). TNT urinary
metabolites are mutagenic, but the major documented effects
on exposed humans are hemolysis, hepatotoxicity, and
changes in hepatic enzyme levels (4).
Nitrotoluenes are recalcitrant toward chemical or bio-
logical oxidation and hydrolysis (5). Biological treatment of
nitrotoluenes is limited by their toxicity at high concentrations
to microorganisms and sometimes produces recalcitrant or
toxic byproducts (6). Chemical treatments may be energy
intensive (e.g., rotary kiln incineration), be ineffective at low
concentrations, or may cause other environmental problems
such as NOx emissions.
Electrolysis, the use of electrical energy to drive an
otherwise unfavorable chemical reaction, is a developing
technology for environmental remediation. Recent examples
of its use include treatment of chromium (7), cyanides (8),
ethanol (9), EDTA (10), dye wastes (11), methanol (12), and
landfill leachates (13). Attractions of electrochemical tech-
nology include the low costs of electricity compared with
* Corresponding author phone: (519)824-4120 ext. 3962; fax: (519)-
766-1499; e-mail: bunce@chembio.uoguelph.ca.
9
4 0 6 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 35, NO. 2, 2001
10.1021/es001465s CCC: $20.00
2001 Am erican Chem ical Society
Published on Web 12/09/2000

TABLE 1. Comparison of Nitroaromatic Potential Peak Values at a DME under Different Conditions (vs SCE)
substrate
Hetmana (23)
Pearsonb (22)
this workc
TNT
2,4-DNT
2,6-DNT
-0.35, -0.55, -0.9 V
-0.68, -0.88 V
N/A
-0.10, -0.20, -0.31 V
-0.18, -0.30 V
-0.24, -0.40 V
-0.11, -0.23, -0.34 V
-0.10, - 0.19 V
-0.10, -0.18 V
a
b
30 m L of pyridine, 7 m L of 1 M KNO3, 35 m L of 2 M NH3NO3, 28 m L of H2O, pH 7.6. 5% ethanol, 95% 0.01 M potassium hydrogen phthalate
c
- HCl, pH 2.5. 0.05 M phosphate at pH 2.
moistening a 4:1 mixture of Raney Ni with nickel powder
and compressing the mixture at 10 000 psi followed by
leaching with 3 M NaOH for 1 h at 70-80 °C, cf. Velin-
Prikidanovics and Lessard (19). The latter authors used a
similar procedure except that they leached for 8 h.
The supporting electrolyte was 0.05-0.1 M disodium
hydrogen phosphate adjusted to pH 2. Oxidation experiments
employed an IrO2/ Ti anode and Pb cathode.
Chrom atography. Organic compounds were analyzed by
HPLC, consisting of a Waters model 501 pump (flow rate
0.5-1.0 mL/ min), Rheodyne injector containing a 20 µL
sample loop, and Waters model 440 UV absorbance detector
operated at 254 nm. The detector output was processed with
Peaksimple software. Separation was achieved on a reverse
phase C18 (25 cm × 4.6 mm) stainless steel column, using
40:60 to 70:30 (v/ v) MeOH:sodium phosphate buffer (pH 2)
as the mobile phase. All solvents used were Fisher Scientific
HPLC grade.
FIGURE 1. Reduction of 2 mM 2,6-DNT as a function of electrode
material at current density 10 mA cm-2.
TABLE 2. Current Efficiency (%) for the Reduction of a 2.5 mM
2,4-DNT Solution over 1 h of Electrolysis at a Ni Plated Ni
Cathode
current, mA cm2 efficiency, % current, mA cm2 efficiency, %
A Hewlett-Packard Model 5890 Series II gas chromato-
graph with a Hewlett-Packard Model 5971 mass selective
detector was used to identify volatile products. Separation
was achieved with a 30 m DB5MS column (i.d. 0.25 mm)
using He as carrier gas. Injector and detector temperatures
were 225 °C and 250 °C, respectively. The oven temperature
program was 60 °C, ramped at 7 °C/ min for 15 min, at 15
°C/ min for 9 min, and then held constant at 300 °C for 6 min.
Sample introduction was by solid-phase microextraction
(SPME) using a 100 µm poly(dimethylsiloxane)-coated fiber,
which was conditioned at 200 °C prior to use (20). Extraction
consisted of immersing the fiber for 30 min in a stirred 2 mL
aliquot of the aqueous sample, which was adjusted to pH
6-8 with KOH to protect the fiber and to deprotonate
products containing NH2 groups.
Nonvolatile electrolysis products were identified using
either probe mass spectrometry with electrospray ionization
or by LC/ MS, with separation effected on a 10 cm C18 column
(mobile phase 40% H2O: 60% CH3CN, flow rate 0.7 mL/
min). In these cases, electrolysis employed ammonium
acetate as the supporting electrolyte. Samples were run in
APCI [(M + 1) and (M - 1)] mode and then subjected to
MS/ MS for positive identification.
Peroxidase Oxidation. Enzymatic oxidation employed a
25 mL batch reactor containing 10 mL of HRP solution (0.2
mg/ mL: ∼50 units/ mL) prepared in 0.05 M sodium phos-
phate buffer (pH 7) and 1 mL of the aminotoluene derivative
(36 mM for 2,6-DAT and 2,4-DAT, 5 mM for TNT reduction
products). To this solution was added H2O2 (1 mL of a 36 mM
solution for DAT, 5 mM for TNT reduction products) prepared
in the same buffer. Aliquots were taken at specific time
intervals, filtered through a 0.45 µm filter, quenched by the
addition of a few drops of concentrated sulfuric acid, and
analyzed by HPLC for reactant loss.
1.3
6.3
12.5
69.7
45.1
23.1
18.8
25.0
16.2
13.0
molecule, and since each peak was shown to be associated
with reduction by ∼6 electrons, we concluded that each nitro
group was reduced completely before the reduction of
another nitro group commenced. Peak potentials for the
reduction of nitroaromatic compounds are strongly influ-
enced by the experimental conditions, especially pH, because
reduction is a proton-consuming reaction. This is shown by
the comparative data in Table 1.
ArNO2 + 7H+ + 6e- f ArNH3+ + 2H2O
2,6-Dinitrotoluene was the first target for electrolysis,
because the symmetry of the molecule limits the number of
reduction products. Figure 1 shows the results of electrolysis
of 2,6-DNT at different cathodes using a current density of
10 mA cm-2. The reactivity order was Ni-plated Ni wire > Pb
> Raney Ni > reticulated carbon (RTC) > Pt. The data for
Pt and RTC are plotted on a linear scale (left-hand ordinate);
all others are fitted logarithmically, consistent with pseudo-
first-order kinetics (right-hand ordinate). The poor perfor-
mance of Pt is consistent with the lack of response in
voltammetric experiments and is consistent with previous
studies (21). In the case of Raney Ni, which functions as a
reductant by means of electrocatalytic hydrogenation, its
inability to reduce DNT may be because the cell potential
was insufficiently negative, even though aromatic compounds
reduce more easily than water (22).
Current efficiency is the ratio of electrons used in the
desired transformation of the analyte to the total number of
electrons passed. Allowing for the number of electrons (six)
to reduce NO2 to NH2, current efficiency for 2,6-DNT ranged
from <5% for Pt to 81% at a Ni plated Ni wire cathode at 0.1
mA cm-2. Table 2 shows how current efficiency changed
with current density for 2,4-DNT as the substrate. Practical
remediation involves a compromise between low current
density (highest efficiency) and high current density (rapid
Results and Discussion
Differential pulse polarography was used to determine the
reduction potentials of the target nitro compounds under
our experimental conditions (pH 2, chosen because explo-
sives wastes are generally acidic due to the method of
preparation). Each nitrotoluene exhibited the same number
of polarographic peaks as the number of nitro groups in the
9
VOL. 35, NO. 2, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 4 0 7

FIGURE 2. The reduction of different nitroaromatics (2.5 mM, pH
2) at a Ni-plated cathode at a fixed potential of -1.00 V vs SCE:
2,4-DNT 2, s -; 2,6-DNT ×, s - - - ; TNT 9, - - -.
FIGURE 4. Products detected by LC/MS in the electrolytic reduction
of 2,6-DNT.
FIGURE 3. Reduction of 2,4,6-TNT (2.5 mM, pH 2) fitted to a first-
order relationship as a function of potential (V vs SCE) at a Ni-
plated Ni cathode.
FIGURE 5. Mass balance for the reduction of 2.2 mM 2,6-DNT solution
at pH 2, potential -0.9 V vs SCE, Ni cathode.
diaminomononitro stage only with difficulty. Mass balances
were determined using HPLC after preparing calibration
curves with authentic samples, allowing both compound
identification from the retention time and quantitation from
the calibration curve. Figure 5 shows the mass balance from
2,6-DNT in a potentiostatic experiment (-0.9 V vs SCE, Ni
cathode) as a function of time. 2,6-DAT was the major
product; 2A6NT was present early in the electrolysis,
consistent with it being an intermediate, and the mass
balance was close to 100% early in the electrolysis. The mass
balance declined after 4 h, at which time the solution changed
to a reddish color, consistent with autoxidation of 2,6-DAT
by atmospheric oxygen, while the recirculated solution was
in the reservoir. Related observations were made by Musolino
et al. (25) who found that the catalytic hydrogenation products
from 2,4-DNT oxidized to 4,4′-dinitro-2,2′-azoxytoluene, 2,4′-
dinitro-4,2′-azoxytoluene, and 2,2′-dinitro-4,4′-azoxytoluene
(26) upon exposure to the atmosphere.
Analogous results were obtained upon electrolysis of 2,4-
DNT. In this case both 2A4NT and 4A2NT were observed as
intermediates, with 2,4-DAT being the major product (data
not shown). The following differences were noted with TNT:
first, TAT was never more than a minor product, even at long
electrolysis times; second, the mass balance was only ∼76%
even at low conversion. Among the reasons may be uncer-
tainties in the nominal concentrations of the standards, which
were provided by the supplier to only one significant figure,
and the extreme air-sensitivity of TAT causing losses during
the time when it was recirculated into the reservoir.
Oxidation of Electrolysis Products. Although the forego-
ing results demonstrate that electrolysis is capable of reducing
nitrotoluene explosives, the resulting anilines are also toxic
substances. Aniline, for example, is a hematopoietic poison
that can cause methemoglobinemia, and chronic exposure
can cause sarcoma (27). However, aromatic amines are easily
oxidized, most frequently not to nitro compounds but rather
to insoluble oligomers through a series of one-electron
oxidations. In this work we considered several routes for
oxidation of the electrolysis products, including air sparging,
reaction). The high current efficiency at the Ni wire is well
above the 10% efficiency required to compete, neglecting
capital and electrode costs, with the raw material costs for
other treatment methods (23).
Potentiostatic experiments were carried out at the Ni
plated Ni cathode at -1.00 V vs SCE, at which potential the
voltammetric studies indicated that all the nitrotoluene
congeners would be reduced at the diffusion-controlled rate.
Accordingly, all the nitroaromatic substrates were reduced
at very similar rates (Figure 2), showing that reduction was
mass transfer limited under these conditions. At less negative
potentials, the rate of reduction changed with the cathode
potential, as exemplified for TNT in Figure 3. First-order
kinetics were observed only at potentials more negative than
-0.5 V, indicating the range over which reduction becomes
mass transfer limited.
Development of a remediation technology demands that
the identities and yields of the products be determined and
that a mass balance be established. Products were identified
experimentally by a combination of GC-MS and LC-MS. From
2,6-DNT, using GC-MS analysis with SPME extraction, we
observed the starting material (m/ z 182) and the monoamino
product 2A6NT (m/ z 152). We did not detect hydroxylamine
intermediates, presumably because of thermal instability (24),
nor the fully reduced 2,6-DAT (m/ z 122), due to difficulty of
extraction from aqueous solution, even at pH 8, at which the
amino groups should be deprotonated. Analysis by LC-MS
using atmospheric pressure chemical ionization (APCI) in
both positive (M + H)+ and negative (M - H)- ion modes
allowed the observation of the bolded ions in Figure 4,
including both hydroxylamine intermediates and 2,6-DAT,
thus confirming the reduction pathway. Further evidence
that m/ z 123 represented 2,6-DAT was obtained by MS/ MS
experiments: m/ z 123 from both authentic DAT and the
electrolysis product gave rise to daughter ions at m/ z 79,
106, and 108.
Analogous results were obtained from 2,4-DNT and TNT,
except that the latter compound reduced beyond the
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4 0 8 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 35, NO. 2, 2001

the following ranges: granular activated carbon adsorption,
$0.03-$3.00 per m3; UV oxidation, $0.32-$1.70 per m3;
composting/ bioslurry, $21-$32 per m3; and incineration,
$82 m-3 (34). Ignoring infrastructure requirements and
considering only electricity costs, at 6¢ per kWh and a
potential difference of 8 V across the cell, we can estimate
$3.60 per mol of nitro groups treated, equivalent to $0.65 for
treating 1 m3 of a 1 mM solution. This outperforms HRP, for
which Taylor et al. (35) calculated the cost for 1 mM solutions
of chlorophenols between $2 and 60 per m3, depending on
the congener.
TABLE 3. Removal Efficiencies of Aminotoluene Congeners
with Type 1 HRP Peroxidase
substrate
removal, %
substrate
removal, %
2,4-DAT
2,6-DAT
2,6-DANT
65
99.6
74.1
2,4-DANT
4-ADNT
2-ADNT
55.8
32.5
17.1
hydrogen peroxide with catalysis by horseradish peroxidase
(HRP), and electrochemical oxidation. The first two of these
methods were ex situ, to form any precipitates outside the
electrochemical cell and thus to minimize fouling of the
electrodes.
Acknowledgments
We thank the Natural Sciences and Engineering Research
Council of Canada for financial support and Dr. J. Hawari of
the National Research Council of Canada for the gift of a
sample of triaminotoluene.
Air sparging was of limited success; it led to a similar
coloration of the solution as was observed during long-tem
electrolysis, but only ∼30% of 2,6-DAT was removed.
Enzymatic polymerization involved treatment of diamino-
toluene congeners with two different horseradish peroxi-
dases. These enzymes are stable from 5-55 °C and at pH
6-9 (28). Table 3 shows the results from a series of 10 min
incubations with HRP/ H2O2: as soon as the hydrogen
peroxide was added, the solutions developed a similar color
to that observed in the air oxidation, and a noticeable
precipitate formed. Although the removal of 2,6-DAT oxida-
tion products from solution was efficient, neither 2,4-DAT
nor 2,4-DANT nor 2,6-DANT (partial reduction products from
TNT) were removed efficiently. Parallel results have been
obtained in the chlorophenol series, where the 2,4-dichlo-
rophenol is rather unreactive (29) compared with other
chlorophenol congeners. In that case, coprecipitation of 2,4-
dichlorophenol with other chlorophenols improved its
removal efficiency, but we did not have a similar success
when 2,6-DAT was used to coprecipitate 2,4-DAT. Likewise,
the mixed diaminomononitrotoluenes obtained as major
products from TNT were not oxidized efficiently by HRP/
H2O2, perhaps because of deactivation by the remaining nitro
group.
In approaching electrochemical oxidation of the polyami-
notoluenes, the prospective advantage was to couple the
anodic oxidation of the aminotoluenes to the cathodic
reduction of the nitrotoluenes, thereby taking advantage of
the inherent oxidation/ reduction properties of an electro-
chemical cell. The concern was that precipitation within the
cell might cause electrode fouling, a problem that is well-
known in the electrochemical oxidation of chlorophenols
(30-32). In previous work (33), we found that anode fouling
was most prominent under conditions that water oxidation
occurred concurrently with chlorophenol oxidation; by
keeping the anode potential to low positive values, electrode
fouling and inactivation could be minimized.
Linear sweep voltammagrams of 2,6-DAT using a platinum
bulb anode showed a peak at +0.62 V vs SCE (not shown)
and loss of anode activity unless the anode was regenerated
by heating in a flame. Although the latter observation
indicates fouling at the Pt anode, we were able to oxidize
2,6-DAT with 70% efficiency at a Ti/ IrO2 anode at pH 2, by
keeping the anodic potential low (0.8 V vs SCE; flow rate
through the cell was 7 mL/ min). At this potential repeated
trials showed no loss of anode activity, consistent with
electrode fouling not being a severe problem. This result
suggests electrolytic reduction, followed by partial reoxidation
of the spent catholyte in the anode compartment of the same
electrochemical cell, as a complete treatment for the nitro-
toluenes. The advantage of anodic reoxidation is that no
additional electrical costs are incurred, because oxidation of
anilines occurs at less positive potentials than the oxidation
of water, which would otherwise occur at the anode.
We briefly considered the cost of electrolytic remediation
of nitrotoluene explosives. Literature data suggest costs in
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