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TAML Activator /Peroxide Catalyzed Facile Oxidative Degradation of the Persistent Explosives Trinitrotoluene and Trinitrobenzene in Micellar Solutions Soumen Kundu, Arani Chanda, Sushil K. Khetan, Alexander D Ryabov, and Terrence J. Collins Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/es4000627 • Publication Date (Web): 15 Apr 2013 Downloaded from http://pubs.acs.org on April 26, 2013

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Environmental Science & Technology

1

TAML Activator /Peroxide Catalyzed Facile

2

Oxidative Degradation of the Persistent Explosives

3

Trinitrotoluene and Trinitrobenzene in Micellar

4

Solutions

5

Soumen Kundu, Arani Chanda,

6

Collins

7

ǂ

†

Sushil K. Khetan, Alexander D. Ryabov, and Terrence J.

,*

Department of Chemistry, Carnegie Mellon University, Pittsburgh, PA 15213, United States

8 9 10

KEYWORDS Homogeneous Oxidation, Peroxides, Iron, Pollutant Remediation, Nitroaromatics, Micelles

11 12 13 14

ACS Paragon Plus Environment

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1

ABSTRACT

2

TAML activators are well known for their ability to activate hydrogen peroxide to oxidize

3

persistent pollutants in water. The trinitroaromatic explosives, 2,4,6-trinitrotoluene (TNT) and

4

1,3,5-trinitrobenzene (TNB), are often encountered together as persistent, toxic pollutants. Here

5

we show that an aggressive TAML activator with peroxides boosts the effectiveness of the

6

known surfactant/base promoted breakdown of TNT and transforms the surfactant induced

7

nondestructive binding of base to TNB into an extensive multistep degradation process.

8

Treatment of basic cationic surfactant solutions of either TNT or TNB with TAML/peroxide

9

(hydrogen peroxide and tert-butylhydroperoxide, TBHP) gave complete pollutant removal for

10

both in 75% of the nitrogen and ≥20% of the carbon converted to nitrite/nitrate and

11

formate, respectively. For TNT, the TAML advantage is to advance the process toward

12

mineralization. Basic surfactant solutions of TNB gave the colored solutions typical of known

13

Meisenheimer complexes which did not progress to degradation products over many hours.

14

However with added TAML activator, the color was bleached quickly and the TNB starting

15

compound was degraded extensively toward minerals within an hour. A slower surfactant-free

16

TAML activator/peroxide process also degrades TNT/TNB effectively. Thus, TAML/peroxide

17

amplification effectively advances TNT and TNB water treatment giving reason to explore the

18

environmental applicability of the approach.

19

20

21


2

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1


INTRODUCTION

2

2,4,6-Trinitrotoluene (TNT) was the most widely used military explosive for missiles, bombs,

3

and torpedoes in both World War I and II.1-9 1,3,5-Trinitrobenzene (TNB) is a side product from

4

the industrial production of TNT and is also an explosive10, 11 that has been used in mining and

5

military applications.12 As a result of both this widespread historic use and more recent

6

demilitarization activities, TNT and TNB are major persistent contaminants in the soil and

7

ground water of production sites, ammunition plants, firing ranges, and open burning and

8

detonation areas.6-8 The US Environmental Protection Agency (EPA) health advisory level for

9

TNT in water is less than 0.002 mg/L (2 ppb).13 However, soil concentrations in some of these

10

contaminated sites ranges from 10–12000 mg/kg (10,000–12,000,000 ppb).14 The US

11

Department of Defense (DOD) has determined that of the more than 1000 sites in the US which

12

are heavily contaminated by explosive residuals,15 more than 95% are contaminated with TNT

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and 87% of these have TNT ground water concentrations above the permissible level.15 Since

14

photochemical oxidative degradation of TNT leads to TNB, it is a co-contaminant with TNT in

15

most of these sites.11 TNT is a US EPA priority pollutant. It is remarkably environmentally

16

persistent and is toxic, carcinogenic5,

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adverse effects of TNT include hemolysis, hepatotoxicity, and alterations of the hepatic enzyme

18

system.5 Once adsorbed on soil, TNT has an extremely slow mobility. Over time, it mostly

19

undergoes reductive transformations to form arylamines, arylhydroxyamines, azoxy- and azo-

20

compounds, which are equally or more toxic than TNT.8 Compared to TNT, toxicological data

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on TNB is sparse; TNB is a reproductive toxicant in rats.17, 18

6

or mutagenic16 to various organisms. Recorded human

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Because TNT and TNB are major soil and groundwater pollutants, scientists have worked for

23

decades to improve the environmental, technical and cost performances of degradation


3


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1

technologies.

2

peroxymonosulfate are unable to oxidize TNT.9 Advanced oxidation processes (AOP) that do

3

decompose TNT include Fenton’s reagent,19 photo-Fenton oxidation,6, 8 TiO2 photocatalysis,1, 20

4

heterogeneous iron-oxide/H2O2 treatment,9 UV radiation both without, and with H2O2.1

5

Alternative degradation approaches for TNT involve alkaline hydrolysis,3, 7, 21 zero-valent iron

6

treatments,22 and composting to induce microbial reduction.23,

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contaminated with high levels of TNT is expensive and presents an explosion risk.3 Among

8

existing approaches, surfactant/base treatment has shown high efficacy for TNT treatment25 and

9

we pick up on this in detail below by showing how TAML/peroxide can advance the approach

10

Traditional

oxidizing

agents

such

as

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permanganate,

24

persulfate,

and

Direct incineration of soil

and make it also applicable to TNB.

11

While TNB is much less studied than TNT, it is the bigger challenge because it is significantly

12

more resistant to oxidation.26 Some approaches developed for TNT have been applied also to

13

degrading TNB, e.g., photocatalytic remediation1 and microbial reduction10. However, all of the

14

existing nitroaromatic explosives remediation technologies have practical limitations. For

15

example, Fenton chemistry occurs at acidic pH (2-3) and requires relatively high loadings of iron

16

salts.19 Photocatalysis by TiO2 requires UV light to be directed at contaminated samples either as

17

sunlight or from an artificial source focused in specialized reaction chambers.1, 20 Heterogeneous

18

iron-oxide/H2O2 treatment is a prohibitively slow.9 Alkaline hydrolysis delivers little TNT

19

mineralization,3, 7 and produces polymeric materials of unknown toxicity.7 In most cases, TNT

20

biotransformations require the co-metabolism of additional compounds resulting in

21

complications of dosing and delivery.24 Surfactant/base treatments that are effective for TNT are

22

ineffective for TNB treatment.27 Thus, remediation technologies for the trinitroaromatics still

23

remain far from ideal.


4

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In this contribution, we show 1) how the known surfactant/base degradation of TNT25 can be

2

sped up for cationic surfactant CTAB (cetyl trimethylammonium bromide)/hydroxide by

3

addition of hydrogen peroxide, 2) how the addition of TAML® activator (1, Figure 1) produces a

4

deeper-reaching oxidation toward mineralization for TNT and, 3) how TNB, which is known to

5

form a Meisenheimer complex with surfactant/base, does not undergo further oxidation in

6

CTAB/hydroxide until both TAML activator and tert-butylhydroperoxide (TBHP) are added to

7

unleash a rapid, deep oxidation.

8

TAML activators are small molecule mimics of peroxidase and short-circuited P450

9

enzymes28, 29 that function effectively at concentrations in the range of parts-per-billion ( 99%) removed by

22

1/H2O2 in 28 min (5, Figure 2). However, on closer examination, it was found that TNT was

23

removed with comparable efficacy by CTAB/hydroxide/peroxide in the presence or absence of 1

42, 45

In this case, CTAB (5×10−4 M) addition produced a marked


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(see text later). This testifies to the remarkable effects of surfactant micellar catalysis on

2

inducing TNT degradation which are well established25 and points to the high electrophilicity of

3

TNT and to the vulnerability to degradation of the resulting nucleophile-TNT adducts.

4

Nevertheless, TAML catalyst addition has value in this case as it deepens the degradation toward

5

mineralization as will now be explained.

6 7

CTAB micellar solutions in the presence of base (3, Figure 2) and base/H2O2 (4, Figure 2)

8

show fast disappearance of TNT (> 99%) as described by S. M. Ahmed et al (surfactant/base).25

9

The electron withdrawing NO2 groups render trinitroaromatics susceptible to addition reactions

10

with nucleophiles and bases.47 Base (OH−) is known to induce a variety of processes with TNT

11

including (i) the nucleophilic substitution of nitro and methyl groups by hydroxide forming

12

phenols, (ii) the nucleophilic addition of hydroxide to the C3 and C5 carbons, and (iii) the

13

deprotonation of the methyl group (pKa = 10.548 in DMSO) giving the trinitrobenzylic carbanion

14

and the system has been deployed for remediating soil and ground water.7 At pH 11 and 12, the

15

combined effect of these processes requires 120 h and 24 h, respectively, to achieve >95% TNT

16

(starting concentration 4.6×10−4 M) transformation.3

17

Under the pH 10 conditions employed for this study, TNT is stable over 80 min (1, Figure 2).

18

Addition of CTAB (5×10−4 M) to a colorless solution of TNT (1×10−4 M) at pH 10 produced the

19

known immediate color change to reddish brown associated with TNT deprotonation and adduct

20

formation, {TNT,OH−} and fast degradation ensued (3, Figure 2).49 A similar color was observed

21

when H2O2 (5×10−2 M) was also added and the degradation was slightly accelerated (4, Figure

22

2)—deprotonation of H2O2 (pKa ≈ 11.2-11.650) gives the potent HO2- nucleophile36, 51 which is

23

likely responsible for the acceleration (see text later).


8

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1

In each of these degradation systems, the TNT removal process is expected to be complicated,

2

passing through myriad species en route to mineralization assisted by oxygen from the air and

3

peroxide when added. It is assumed that the more extensive the degradation, the better any

4

particular system is as a candidate for real world applications—minerals are the ideal endpoint.

5

The quantification of nitrite and nitrate from TNT nitrogen provides a relatively straightforward

6

method of estimating whether extensive degradation is occurring. The ultimate insight would

7

come from also following in detail the evolving fate of the TNT carbon. While detailed

8

investigations of step-by-step TAML/peroxide degradations have been undertaken by us in the

9

past,34, 37, 41 this type of intricate study would be exceptionally difficult in the presence of the

10

surfactant. Nevertheless, we have been able to detect various intermediates (see below) and

11

quantify the relatively easy to observe formate ion for each system. Formate quantification adds

12

weight to NO2-/NO3- analyses as indicators of a degree to which system proceeded toward

13

mineralization.

14

Emmrich has reported that basic hydrolysis at pH 12 and 13 produced 50% and 67% TNT

15

nitrogen mineralization, respectively.3 In this study at pH 10, OH−/CTAB treatment of TNT

16

produced NO2− (15.3±0.2%) and NO3− (3.7±0.3%) with no observable formate after 80 min

17

(Figure 3). With OH−/H2O2/CTAB, NO2− (45±1%) and NO3− (15±1%) formation increased, but

18

formate was again not observed (Figure 3). With OH−/1/H2O2/CTAB, NO2− (60±1%%) and

19

NO3− (22±2%%) formation increased further and formate (20% on total carbon) was also

20

observed (Figure 3). Overall, the TAML catalyzed process produced significantly more

21

NO2−/NO3− (82%) than the OH−/CTAB (19%) and OH−/H2O2/CTAB (60%) processes. More

22

importantly, formate was not detected for the TAML-free processes at >99% TNT removal

23

(Figure 3). Therefore, these results illustrate the superior performance of 1/H2O2/CTAB


9


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compared to the uncatalyzed processes. TAML activator catalyzed oxidations are usually

2

associated with significant mineralization of carbon to CO and CO2 that is typically estimated by

3

total organic carbon (TOC) analysis.34,

4

here by the high background organic content.

52

However, a similar TOC determination is precluded

5

6 7

Figure 2. Effect of CTAB on the removal of TNT. Conditions: [TNT] = 1×10−4 M, [1] = 2×10−6

8

M, [H2O2] = 5×10−2 M, [CTAB] = 5×10−4 M, 0.01 M carbonate, pH 10, room temperature. 1:

9

TNT+OH−, 2: TNT+ OH−+1+H2O2, 3: TNT+ OH−+CTAB, 4: TNT+ OH−+CTAB+H2O2, 5:

10

TNT+OH−+1+H2O2+CTAB. OH− represents the base in the pH 10 buffered reaction medium

11

TNT concentrations were measured by HPLC (see SI for details).


10

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1 2

Figure 3. Mineralization of nitrogen to nitrite and nitrate and carbon to formate during the

3

degradation of TNT in a micellar (CTAB) pseudophase in the presence of OH− (A), OH−/H2O2

4

(B), and 1/H2O2 (C). Conditions: [TNT] = 1×10−4 M, [1] = 2×10−6 M, [H2O2] = 5×10−2 M,

5

[CTAB] = 5×10−4 M, 0.01 M carbonate, pH 10, room temperature. Minerals were analyzed for

6

each reaction when TNT could not be detected (>99% TNT degradation, Figure 2, details of the

7

quenching method can be found in the SI)

8 9

Rate Evaluations for the TNT Hydrolysis and Oxidation Reactions

10

The rate constants (kobs) for the three TNT removal processes, OH−/CTAB (kOH),

11

OH−/H2O2/CTAB (kH2O2+OH) and 1/H2O2/CTAB (kcat), were systemically evaluated. In each case,

12

over variable [CTAB] (0.1–0.5 mM) the disappearance of TNT was exponential (e.g., Figure

13

4A). Linearization of the TNT exponential decay profile resulted in straight lines passing through

14

the origin (e.g., Figure 4B). The plots were satisfactorily linear (e.g., Figure 4B) for at least 3

15

half-lives indicating a pseudo-first order process in TNT. The pseudo-first order rate constants

16

(kOH, kH2O2+OH, kcat) were calculated from the slopes of the straight lines (Table 1). For the basic


11


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1

hydrolysis of TNT (OH−/CTAB), the rate constant kOH increased with increasing [CTAB] (Table

2

1). As mentioned before, TNT hydrolysis was not observed in the absence of CTAB (1, Figure

3

4A). Interestingly, CTAB at concentrations lower than the cmc (0.42 mM) also showed an

4

enhanced rate of hydrolysis of TNT compared to buffered aqueous medium (Figure 4A). This

5

can be explained by the formation of pre-micelles53 where substrates and surfactants interact at

6

concentrations below the cmc with the pre-micelles accelerating the reaction.54-58 Removal of

7

TNT by excess H2O2 in basic buffered reaction media, in the presence of variable [CTAB] (0.1–

8

0.5 mM), was also studied. The calculated rate constant (kH2O2+OH) pertains to the combined

9

effects of two processes, i.e., the reactions of TNT with OH− and H2O2 in the presence of CTAB.

10

Assuming that these are parallel reactions (i.e., kH2O2+OH = kOH + kH2O2), the observed pseudo-first

11

order rate constant (kH2O2) for the reaction between TNT and H2O2 was calculated (kH2O2+OH −

12

kOH) (Table 1). The observed rate constant kH2O2 increases at higher [CTAB], but the increase is

13

more prominent at [CTAB] ≥ 0.4 mM (Table 1), a value close to the measured cmc of CTAB

14

(0.43 mM). Micelles significantly changes acid-base equilibria and cationic micelles binding

15

hydroxide in the charged Stern layer are known to increase the degree of deprotonation of weak

16

acids.59 In this case, formation of cationic CTAB micelles might result in the increased

17

deprotonation of H2O2 and generation of the powerful HO2− nucleophile, consistent with the

18

higher values of kH2O2 at [CTAB] ≥ 0.4 mM. The rate behavior of the oxidative degradation of

19

TNT by 1 (2×10−6 M)/H2O2 (5×10−2 M) was also studied in the presence of variable [CTAB]. As

20

with the uncatalyzed processes, the rate of 1/H2O2 induced TNT degradation (kcat) increased with

21

increasing [CTAB]. The times for complete oxidation of TNT (>99%) at 0.2, 0.3, 0.4, and 0.5

22

mM CTAB were 300, 130, 45, and 28 min, respectively. A slight increase in kcat was observed

23

compared to the individual 1/H2O2-free TNT hydrolyses (kOH and kH2O2) and a ~3-fold rate


12

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1

increase (kcat vs kOH and kH2O2) emerged for [CTAB] ≥ 0.4 mM. The rate of the catalytic reaction

2

(kcat) is comparable to the combined background reactions (kH2O2+OH) for all [CTAB] (Table 1),

3

emphasizing that the value of 1 for TNT degradation derives from the catalysis of subsequent

4

steps after the initial TNT transformation reaction(s).

5

We propose a tentative mechanism for TNT oxidation by 1/H2O2 in CTAB micellar solutions

6

consistent with the evaluated rate constants and the higher TNT carbon and nitrogen

7

mineralization for the TAML catalyzed process (Figure 3). The mechanism involves the

8

reactions of hydroxide or HO2− with TNT in micellar solutions (slow step) producing electron

9

rich species (Meisenheimer complexes or deprotonated TNT) that are more easily oxidized than

10

the starting TNT. These electron-rich charged intermediates are then oxidized by 1/H2O2 (fast

11

steps) in a system that moves significantly toward mineralization of TNT carbon (20% formate)

12

and nitrogen (82%). Without 1/H2O2, these intermediates are likely transformed into phenolic

13

compounds (as mentioned before) with accompanying release of nitrite and nitrate (Figure 3).

14

1/H2O2 is comparatively ineffective for the oxidation of the starting TNT in the absence of

15

CTAB (2, Figure 2) and likely remains so even in the presence of CTAB. The rate of TNT

16

removal does not give information on the mechanism(s) of TAML-catalyzed steps, as the

17

reaction intermediates are oxidized by 1/H2O2 after the rate determining process. TAML

18

catalysis advances an already established hydrolytic (OH−) and a likely perhydrolytic TNT

19

removal process into a deep oxidation process overall.


13


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1 2

Figure 4. Dependence of the disappearance of TNT on the CTAB concentration. Conditions:

3

[TNT] = 1×10−4 M, pH 10 (0.01 M carbonate), room temperature. 1: [CTAB] = 0.0 mM, 2:

4

[CTAB] = 0.2 mM, 3: [CTAB] = 0.3 mM, 4: [CTAB] = 0.4 mM, 5: [CTAB] = 0.5 mM,

5 6

Table 1. Pseudo-first order rate constants for the disappearance of TNT at varying [CTAB]. [CTAB] / mM kOH×102

7 8

a,b

kH2O2+OH×102

a,b

kH2O2×102

a,c

kCat×102

a,b

0.2

1.6±0.1

2.303±0.006

0.7±0.1

1.80±0.07

0.3

3.2±0.1

4.1±0.3

0.9±0.3

3.6±0.2

0.4

5.4±0.2

9.9±0.6

4.5±0.6

12.7±0.4

0.5

7.7±0.2

16.4±0.6

8.7±0.6

20.9±0.9

a) min-1, b) Calculated from the slopes of the linearized exponential decay profile (e.g. Figure 4B), c) Calculated from kH2O2+OH − kOH.

9 10

Intermediates formed during TNT degradation

11

In search for intermediates, the reaction mixture for the degradation of TNT (1×10-4 M) by 1

12

(2×10-6 M)/H2O2 (5×10-2 M)/CTAB (5× 10−4 M) was analyzed after ~ 5 min by HPLC and by


14

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1

atmospheric pressure chemical ionization-mass spectrometry (APCI-MS) (see the Experimental

2

Section, SI for details). The reaction mixture was found to contain 2,4,6-trinitrophenol (TNP),

3

1,3,5-trinitrobenzene (TNB) as by far the dominant products (HPLC). The identities were

4

confirmed by comparing the MS fragmentation patterns of the molecular ion peaks (obtained

5

from the reaction medium) with standard compounds (Table S1) and by matching HPLC

6

retention times with standard compounds. Additional distinctly minor products in the reaction

7

medium were assigned the likely identities of dinitrotoluene, dinitrocresol, and 3-hydroxy-

8

trinitrotoluene (Scheme S1) by matching observed and theoretical m/z values in the APCI mass

9

spectrum (Table S1). Analysis of the final reaction mixture did not show the presence of the

10

above mentioned intermediates, indicating their oxidation by TAML/H2O2.

11 12 13

Oxidation of TNB by 1/H2O2 and 1/TBHP in CTAB Within the famously persistent nitroaromatic explosives family,60,

61

TNB is the ultimate

14

hurdle and is significantly more difficult to degrade than TNT.26 Ultimately, any totally

15

satisfactory degradation process for nitroaromatic explosives must work well for TNB. The TNT

16

approach described above was employed for TNB. Addition of TNB to pH 10 buffer solution in

17

the presence of CTAB resulted in the formation of a red solution (Figure 5A). The UV-vis

18

spectrum exhibited maxima at 435 and 513 nm associated with the formation of a σ-

19

Meisenheimer complex between TNB and OH− (Figure 6B).62 An APCI mass spectrum of this

20

solution showed an ion at m/z = 230 (see Figure S1) consistent with {TNB,OH−}. The MS/MS

21

fragmentation pattern (see Figure S2) of the m/z 230 ion matched that of {TNB,OH−}.63 In

22

contrast with TNT, TNB in basic CTAB micellar solution (0.01 M carbonate, pH 10) did not

23

proceed to measurable disappearance of TNB, even after 5 h as demonstrated by HPLC


15


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1

experiments. This is a significant difference between TNT and TNB. The formation of a stable

2

{TNB,OH−} complex has been reported for other cationic surfactants.27 1/H2O2/CTAB greatly

3

accelerated TNB oxidation compared to the modest (8%) degradation of TNB found with

4

1/H2O2.

5

The lower resistance of TNT than TNB to oxidative decay is well established for nitroaromatic

6

decontamination technologies1,

26

7

methyl group.26 Under identical conditions ([1] = 2×10-6 M, [H2O2] = 5×10-2 M, [CTAB] = 5×10-

8

4

9

TNT are 400 and 28 min, respectively. For the TNB degradation process, a higher concentration

10

of CTAB (1×10−3 M) reduced the reaction time to 320 min. Still, such sluggishness is a barrier to

11

real world applications. Thus, alternative primary oxidants were explored with the aim of finding

12

a faster TNB degradation protocol.

and has been attributed to the oxidative vulnerability of the

M, 0.01 M carbonate buffer, pH 10, r.t), the times required for complete removal of TNB and

13

For TAML-catalyzed oxidation of anthropogenic pollutants, H2O2 is always going to be the

14

most desirable peroxide as a green oxidant that is widely deployed in biochemistry and that

15

produces water as the only by-product.64 Other peroxide oxidants are capable of supporting faster

16

processes. For example, TBHP (tert-butylhydroperoxide) was found to be advantageous over

17

H2O2 for the deactivation of anthrax surrogate spores of Bacillus atrophaeus by 1/CTAB.42 This

18

advantage was also found here for the oxidation of TNB by 1/TBHP/CTAB with complete TNB

19

oxidation at 56 and 100 min at [CTAB] 1.00 and 0.5 mM, respectively (5 and 4, Figure 6A).

20

Moreover, in 4 h without CTAB, 1/TBHP was capable of oxidizing 95% of the TNB sample (3,

21

Figure 6A), compared to only 8% for 1/H2O2 under otherwise similar conditions suggesting the

22

suitability of this CTAB-free process for exploration in environmental applications. Higher

23

[CTAB] again resulted in higher mineralization of the TNB nitrogen to nitrite and nitrate (Figure


16

Page 17 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

6B). TBHP alone produced no loss of TNB after 4 h (1, Figure 6A). CTAB (1 mM)/TBHP

2

resulted in 17% removal of TNB in 4 h (2, Figure 6A). Thus, unlike TNT, the background

3

reactions (OH− or TBHP) for TNB removal in CTAB micellar solutions are much slower than

4

the TAML catalyzed process. Treatment of TNB (1×10−4 M) with 1 (2×10−6 M)/TBHP (5×10−2

5

M) in CTAB (1×10−3 M) micellar solution released 48±1% nitrogen as NO2−, 31±3% nitrogen as

6

NO3−, and 34% of the carbon as formate. Thus, the release of minerals (nitrate, nitrite) and a

7

small acid anion (formate) in high yields highlights the efficacy of the 1/TBHP/CTAB system for

8

remediation of TNB.

9

The reaction mixture following the treatment of TNB (1×10-4 M) with 1 (2×10-6 M)/TBHP

10

(5×10-2 M) was analyzed by Micritox® aquatic toxicity (Vibrio fisheri) testing. The EC50 (15

11

min) values of the treated reaction mixture and untreated TNB were 0.68% and 0.31%,

12

respectively. Thus, 1/TBHP treatment of TNB produced a 50% reduction in toxicity.

13

14 15

Figure 5. (A) UV-vis spectral changes of TNB upon addition of CTAB. (B) Proposed reversible

16

formation of a σ-bonded Meisenheimer complex {TNB,OH−}. Conditions: [TNB] = 1×10-4 M,

17

[CTAB] = 1×10-3 M, 0.01 M carbonate, pH 10, room temperature.


17


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 29

1 2

Figure 6. (A) Degradation of TNB by 1/TBHP in the presence of CTAB. (B) Percentage of

3

nitrogen in TNB mineralized at variable CTAB concentrations. Conditions: [TNB] = 1×10-4 M,

4

[1] = 2×10-6 M, [TBHP] = 5×10-2 M; [CTAB] = 1 mM or 0.5 mM, 0.01 M carbonate, pH 10,

5

room temperature. 1: TBHP, 2: TBHP + CTAB (1 mM), 3: 1 + TBHP, 4: 1 + TBHP + CTAB

6

(0.5 mM), 5: 1 + TBHP + CTAB (1 mM).

7

Intermediates formed during TNB degradation

8

The degradation of TNB by 1/TBHP/CTAB resulted in the formation of TNT (remarkably)

9

and 2,4,6-trinitrophenol (TNP) as intermediates (Scheme S2). These intermediates were detected

10

qualitatively. 1/TBHP oxidation of the TNB Meisenheimer complex (Figure 5B) to TNP is the

11

likely pathway to this product. It is known that the interaction of 1 and TBHP produces acetone

12

and a likely explanation involves elimination of CH3• from tBuO•.42 Acetone was indeed

13

identified in the reaction mixture (HPLC, see the Experimental Section). Based on the data

14

presented here, we cannot speculate on a possible mechanism for the formation of TNT as an

15

intermediate during TNB oxidation. TNT and TNP were identified by HPLC and APCI-MS.


18

Page 19 of 29

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1

Thus both TNT and TNB are oxidatively degraded with ease by 1-activated H2O2 or TBHP in

2

CTAB micellar solutions. TNT and (even more so) TNB are exceptionally persistent pollutants.

3

The 1-catalyzed oxidation processes resulted in high mineralization of nitrogen and extensive

4

oxidation of carbon in both cases. For TNT degradation, the high mineralization was likely

5

achieved through attack of hydroxide and hydroperoxide anion on the parent TNT to form

6

charged intermediates, followed by their oxidation by 1/H2O2. Reaction of TNB with 1/TBHP

7

produces significant TNB oxidation, which is enhanced in the presence of CTAB. The facile

8

degradation of TNB through TAML catalysis is a major step forward in treating nitroaromatic

9

explosives contaminated water.

10 11 12 13

ASSOCIATED CONTENT

14

Supporting Information. Experimental Details including Methods, Figures, and Schemes. This

15

material is available free of charge via the Internet at http://pubs.acs.org.

16

AUTHOR INFORMATION

17

Corresponding Author

18

*E-mail: [email protected]; phone: +1 412 268 6335; fax: +1 412 268 1061

19

Present Addresses

20

† Currently Senior Scientist at Eisai Inc., 4 Corporate Drive, Andover, MA 01810, United States

21

ACKNOWLEDGMENT


19


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Page 20 of 29

1

T.J.C. thanks the Heinz Endowments for support. Soumen Kundu thanks the R. K. Mellon

2

Foundation for a Presidential Fellowship in the Life Sciences (Carnegie Mellon University). We

3

thank Professor Mark Bier at the Center for Molecular Analysis (CMU) and Longzhu Q. Shen

4

for their help with APCI-MS instrumentation. We thank Hauck Environmental Engineering

5

Laboratories, Carnegie Mellon University for providing access to the Ion Chromatography

6

instrument.

7 8

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To be found under


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TOC Art

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