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Transformation of Hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) by Permanganate

Chanat Chokejaroenrat University of Nebraska-Lincoln

Steven Comfort University of Nebraska - Lincoln, [email protected]

Clifford E. Harris Albion College

Daniel D. Snow University of Nebraska-Lincoln, [email protected]

David A. Cassada University of Nebraska-Lincoln, [email protected]

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Chokejaroenrat, Chanat; Comfort, Steven; Harris, Clifford E.; Snow, Daniel D.; Cassada, David A.; Sakulthaew, Chainarong; and Satapanajaru, Tunlawit, "Transformation of Hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) by Permanganate" (2011). Faculty Publications from The Water Center. 22. https://digitalcommons.unl.edu/watercenterpubs/22

This Article is brought to you for free and open access by the Water Center, The at DigitalCommons@University of Nebraska - Lincoln. It has been accepted for inclusion in Faculty Publications from The Water Center by an authorized administrator of DigitalCommons@University of Nebraska - Lincoln. Authors Chanat Chokejaroenrat, Steven Comfort, Clifford E. Harris, Daniel D. Snow, David A. Cassada, Chainarong Sakulthaew, and Tunlawit Satapanajaru

This article is available at DigitalCommons@University of Nebraska - Lincoln: https://digitalcommons.unl.edu/ watercenterpubs/22 Published in Environmental Science & Technology 45:8 (2011), pp. 3643–3649; doi: 10.1021/es104057v Copyright © 2011 American Chemical Society. Used by permission.

Submitted June 16, 2010; revised March 11, 2011; accepted March 15, 2011; published online March 31, 2011.

Transformation of Hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) by Permanganate

Chanat Chokejaroenrat,1 Steve D. Comfort,2 Clifford E. Harris,3 Daniel D. Snow,4 David Cassada,4 Chainarong Sakulthaew,2,5 and Tunlawit Satapanajaru 6

1. Department of Civil Engineering, University of Nebraska–Lincoln, Lincoln, Nebraska 68588-0531, USA 2. School of Natural Resources, University of Nebraska–Lincoln, Lincoln, Nebraska 68583-0915, USA 3. Department of Chemistry, Albion College, Albion, Michigan 49224, USA 4. Water Sciences Laboratory, University of Nebraska–Lincoln, Lincoln, Nebraska 68583-0844, USA 5. Department of Veterinary Technology, Kasetsart University, Bangkok, Thailand 10900 6. Department of Environmental Science, Kasetsart University, Bangkok, Thailand 10900

Corresponding author — S. D. Comfort, tel 402 472-1502, fax 402 472-7904, email [email protected]

Abstract − The chemical oxidant permanganate (MnO4 ) has been shown to effectively transform hexa- hydro-1,3,5-trinitro-1,3,5-triazine (RDX) at both the laboratory and field scales. We treated RDX − with MnO4 with the objective of quantifying the effects of pH and temperature on destruction kinetics and determining reaction rates. A nitrogen mass balance and the distribution of reaction products were used to provide insight into reaction mechanisms. Kinetic experiments (at pH ~7, 25 − − °C) verified that RDX−MnO4 reaction was first-order with respect to MnO4 and initial RDX con- centration (second-order rate: 4.2 × 10−5 M−1 s−1). Batch experiments showed that choice of quench- ing agents (MnSO4, MnCO3, and H2O2) influenced sample pH and product distribution. When − MnCO3 was used as a quenching agent, the pH of the RDX−MnO4 solution was relatively un- − changed and N2O and NO3 constituted 94% of the N-containing products after 80% of the RDX was transformed. On the basis of the preponderance of N2O produced under neutral pH (molar ra- − tio N2O/NO3 ~5:1), no strong pH effect on RDX−MnO4 reaction rates, a lower activation energy − than the hydrolysis pathway, and previous literature on MnO4 oxidation of amines, we propose − that RDX−MnO4 reaction involves direct oxidation of the methylene group (hydride abstraction), followed by hydrolysis of the resulting imides, and decarboxylation of the resulting carboxylic ac- ids to form N2O, CO2, and H2O.

Introduction in removing RDX from tainted waters, the reaction rates and − mechanisms by which MnO4 transforms RDX (and other ex- Hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) is a common plosives) have not been thoroughly studied.(4) While a car- − groundwater contaminant at numerous military sites where bon mass balance of the RDX−MnO4 reaction has been ob- munitions were either formulated, manufactured, or used in served,(2) a similar nitrogen mass balance for this reaction has − not been reported. military exercises. Permanganate (MnO4 ) is an oxidant com- monly used with in situ chemical oxidation (ISCO) and has One analytical challenge to identifying degradation prod- − been widely accepted for treating chlorinated ethenes. Past ucts in a MnO4 matrix is that the solution is highly colored − (i.e., purple), so colorimetric and UV detection techniques are research has shown that MnO4 preferentially attacks com- − pounds with carbon−carbon double bonds, aldehyde groups, not possible unless samples are quenched to remove MnO4 or hydroxyl groups and is attracted to the electron-rich region before analysis. However, the choice of quenching agent may of chlorinated alkenes.(1) Although RDX possesses none of influence pH or product distribution and further complicates − these characteristics, laboratory studies performed by Adam et understanding the RDX−MnO4 reaction mechanism. − The transformation of RDX by various treatments has re- al.(2) showed that MnO4 could effectively transform and min- 14 vealed several possible reaction pathways. These include di- eralize RDX (i.e., ~87% recovered as CO2). Moreover, a pilot- scale demonstration at the Nebraska Ordnance Plant further rect ring cleavage, nitro-group reduction, concerted decompo- − sition, and N-denitration.(5-12) While intermediates produced supported MnO4 as a possible in situ treatment for RDX-con- taminated groundwater.(3) Despite demonstrating efficacy by some of these pathways are fleeting and difficult to mea-

3643 3644 Chokejaroenrat et al. in Environmental Science & Technology 45 (2011)

− − sure, the end products produced are often similar (N2O, NO2 , The aqueous RDX solution was treated with 4.20 mM MnO4 . − + − NO3 , NH4 ) but produced in different ratios depending Aqueous RDX solutions without MnO4 (n = 3) were used as on the reaction mechanisms. In this paper, we report results controls and monitored at each temperature. Samples were pe- from laboratory investigations designed to describe the kinet- riodically collected and quenched with MnCO3 as described − ics of the RDX−MnO4 interaction, quantify the effect of tem- and analyzed for RDX by HPLC. perature on RDX destruction kinetics, and provide a nitrogen 4-NDAB Experiments. To determine the stability of − − mass balance of the RDX−MnO4 reaction. On the basis of ex- 4-NDAB in the presence of MnO4 , we conducted batch exper- perimental results, possible mechanisms by which RDX is de- iments with 4-NDAB as the starting substrate. Batch experi- − graded by MnO4 are proposed. ments were performed in a 250-mL Erlenmeyer flask contain- ing 100 mL of 4-NDAB (0.04 mM) covered with parafilm and Experimental Section agitated with an orbital shaker at ambient temperature (24 °C). 4-NDAB was treated with 4.20, 8.40, 16.81, and 33.61 mM of − Details of the chemical standards, analytical instruments MnO4 . Samples were collected every 30 min and quenched (e.g., HPLC, IC, GC/ECD, and UV spectrophotometer), anal- with MnCO3 as previously described. 4-NDAB was immedi- ysis of N-containing gases, RDX purification procedures used ately analyzed by HPLC. for mass balance experiments, and experimental controls are RDX Nitrogen Mass Balance Experiment. Aque- provided in Supporting Information (SI-1, SI-2, SI-3). ous RDX (0.10 mM) prepared from purified RDX was placed Aliquot Sample Preparation. To accurately quan- into a 10-mL serum vial (Wheaton, Millville, NJ). The vial tify RDX and degradation product concentrations during ox- was closed with a silicone septum with zero headspace and − crimped with an aluminum cap. So that we could precisely cal- idation by MnO4 , samples were quenched to prevent further RDX transformation. To avoid interferences during RDX and culate the nitrogen mass balance, each vial was weighed be- degradate analyses, three different quenching agents were fore and after introducing any chemicals. Once the vial was sealed, a 21G (i.e., needle guage no. 21) needle and a 3-mL sy- tested (MnSO4, MnCO3, and H2O2). The choice of quenching agent was found to influence pH and product distribution ringe were inserted through the septum. Helium gas was then (see Supporting Information, SI-4). The two quenching agents added to the 21G needle to push out 2.5 mL of solution into the 3-mL syringe. The 21G needle was removed and another sy- we used most frequently included MnCO3 (0.10 g per mL of ringe was inserted into the He headspace where MnO − stock sample unless otherwise stated) and MnSO4·H2O [0.10 mL of 4 solution was introduced to produce an initial concentration a MnSO4 solution (0.10 g/mL) per mL of sample]. The typi- − cal quenching procedure involved removing 1-mL aliquots of 33.61 mM. Because the added MnO4 stock replaced a por- − tion of the He headspace gas back into the 3-mL syringe, exper- from the RDX−MnO4 batch reactors, placing the aliquots in a 1.5-mL centrifuge tube, adding the quenching agent, and cen- imental treatments began at ambient pressure (t = 0 d). Vials were again weighed to determine the precise volume of so- trifuging at 14 000 rpm for 10 min. When MnCO3 was used, an addition 5 min of shaking on a vortex was performed be- lution and headspace in each replicate. To avoid possible gas fore centrifuging. The supernatant was then transferred to an losses through the needled-pierced septa, the septa were sealed HPLC or an IC vial and stored at 4 °C until analysis. with thermoplastic adhesive. Each vial was then covered with − RDX Kinetic Experiments. Kinetic experiments aluminum foil to prevent MnO4 photodegradation(14, 15) be- were performed under batch conditions by placing 150 mL of fore shaking on a reciprocal shaker until analysis. RDX solution in 250-mL Erlenmeyer flasks and agitating on Sacrificial sampling occurred at 0, 1, 2, 3, 6, 9, 15, 19, 23, and an orbital shaker. Solution samples were taken every 2−3 d, 28 d. Each replicate (n = 4) was used to analyze three differ- ent types of analytes. For N O gas production, 0.5 mL of head- quenched with MnSO4, and analyzed for RDX by HPLC. We 2 space gas was removed and injected directly into GC/ECD. initially prepared RDX solutions by spiking 150 mL of H2O − with 1.04 mL of RDX stock solutions prepared in acetone, but For changes in solution concentrations of RDX and NO3 / − NO −, 2.0 mL of sample were quenched with MnCO . One al- acetone was found to facilitate the decomposition of MnO4 2 3 at alkaline pH and prevent further degradation of RDX >10 d iquot (1.0 mL) was analyzed by HPLC while the other was − − (see Supporting Information, SI-5). Consequently, all aqueous used to quantify NO3 /NO2 by IC. Each replicate solution − RDX solutions were prepared by dissolving purified crystal- was also analyzed for MnO4 with a UV spectrophotometer to − line RDX in water over several days. ensure uniformity in MnO4 concentrations among replicates. To determine reaction rates, kinetic experiments fixed the initial RDX concentration at 0.09 mM and samples were Results and Discussion − treated with MnO4 in excess by varying concentrations be- − tween 4.20 and 84.03 mM. Likewise, using initial MnO4 con- RDX Kinetics Experiments. Treating aqueous RDX − centrations at 33.61 mM, we treated varying concentrations with varied MnO4 concentrations resulted in a wide range of RDX (0.01−0.09 mM). Both experiments were conducted in of RDX destruction rates (i.e., pseudo-first-order rate, kobs = −1 neutral pH (~7) at room temperature (25 °C). Reaction rates 0.02−0.37 d , Figure S11A). By plotting log[kobs] versus log − were then determined by the initial rate method.(13) The rate [MnO4 ]O (Figure 1A), the calculated slope (β) of this regres- law describing a second-order reaction between RDX and sion was 0.98 ± 0.06 (r2 = 0.99) and indicates that reaction was − − MnO4 is presented in Supporting Information (SI-6). first-order with respect to MnO4 . Likewise, kinetic experi- − RDX−MnO4 Temperature Experiment. To quan- ments estimated the reaction order with respect to RDX (α). − tify the effect of temperature on the RDX−MnO4 reaction Upon treatment of varying RDX concentrations with 33.61 − (i.e., second-order rate constant, k′′), we performed experi- mM MnO4 , the initial reaction rates (r0; based on Equation ments in 150-mL glass bottles containing 100 mL of RDX (0.02 S8) were approximated from the tangent of the concentration− mM). Treatment temperatures were 20, 35, 50, and 65 °C and time curves (Figure S12). The log of the initial reaction rate held constant for 2−3 h prior to the start of the experiment. (log[r0]) was plotted against initial RDX concentration to es- Transformation of Hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) by Permanganate 3645

Figure 1. (A) Plot of pseudo-first-order rate constants for RDX − degradation vs [MnO4 ]O. Aqueous RDX (0.09 mM) was treated − with MnO4 ranging from 4.20 to 84.03 mM. (B) Plot of initial rates of RDX degradation vs [RDX]0 ranging from 0.01 to 0.09 mM when − treated with 33.61 mM MnO4 . Figure 2. Temporal changes in RDX concentration in aqueous so- − timate reaction order for RDX (α). Results indicated α values lution treated with 4.20 mM of MnO4 at 20, 35, 50, or 65 °C. Bars very close to 1, also verifying the reaction is first-order with re- indicate sample standard deviations (n = 3). spect to RDX (Figure 1B). Both sets of kinetic experiments (Figure 1) demonstrate that − the initial reaction between RDX and MnO4 is second-order (i.e., α = β = 1) with a rate constant (k′′) of 4.16 × 10−5 M−1 s−1 (±0.22 × 10−5) (Equation S6). A compilation of destruction rates − of various contaminants by MnO4 (4) revealed that a number − of contaminants may react with MnO4 as fast as or faster than the chlorinated ethenes, the groundwater contaminants most − commonly treated by MnO4 . One of those contaminants re- ported to have a second-order rate constant similar to that of the chlorinated ethenes was TNT.(4) Given that TNT and RDX are often cocontaminants in the field, a parallel set of kinetic experiments were performed with TNT (see Supporting Infor- mation, Figures S11B and S13). These experiments concluded − that the TNT−MnO4 reaction is first-order with respect to − TNT and MnO4 or second-order overall (Figure S14) with a k′′ of 1.18 × 10−3 M−1 s−1 (±0.02 × 10−3). While the second-or- der rate constant observed for RDX in this study is similar to Figure 3. Arrhenius plot of second-order rate constants. those reported,(2, 4) our k′′ for TNT is lower than that reported by Waldemer and Tratnyek (0.03 M−1 s−1 (4)) but still 28-fold − higher than what we observed for RDX, indicating a large dif- RDX with MnO4 at four temperatures to further elucidate the ference in reactivity between these two explosives. This obser- temperature dependency of the reaction at elevated tempera- vation is perhaps not surprising and undoubtedly related to tures. Results showed the RDX destruction rates were signif- differences in chemical classes (nitramine vs nitroaromatic). icantly increased with increasing temperature with pseudo- − −1 Effect of Temperature on RDX−MnO4 Reac- first-order rates ranging from 0.01 to 0.89 d (Figure 2A). At tion. Albano et al.(3) previously reported that RDX transfor- 20 °C, RDX concentration was only reduced by 30% after 30 d mation rates were slowed 3-fold under temperatures indica- (k = 0.01 d−1) while complete RDX transformation (100%) was tive of aquifer conditions (11.5 vs 23 °C). We treated aqueous achieved within 6 d at 65 °C (k = 0.89 d−1). A comparison of 3646 Chokejaroenrat et al. in Environmental Science & Technology 45 (2011)

Figure 4. Loss of 4-NDAB concentration following treatment of Figure 5. Nitrogen mass balance results from tracking loss of RDX − − 4-NDAB aqueous solution with various concentrations of MnO4 and production of N2O and NO3 . Bars indicate sample standard and quenching with 0.10 g MnCO3 (per mL). Bars indicate sample deviations (n = 4). standard deviations (n = 3). controls (temperature only) showed that RDX was relatively We also used 4-NDAB as the starting substrate by treat- − stable at temperatures ≤50 °C but decreases in RDX concentra- ing it with varying concentrations of MnO4 . Results showed − tions were observed at 65 °C, albeit at a slower rate than when that 4-NDAB is much more quickly transformed by MnO4 − MnO4 was also present (Figure 2B). A comparison of previ- than RDX, with transformation occurring within hours (Fig- ously reported destruction rates shows that, in order to get the ure 4) versus days for RDX (Figure S6A). 4-NDAB is likely ox- − destruction rate we observed at 65 °C with 4.20 mM MnO4 , idized faster than RDX because oxidation of the formamide − Adam et al.(2) needed 168 mM MnO4 at room temperature (a group to a carbamic acid and the subsequent decarboxylation 40-fold higher concentration). are known to be very fast reactions.(22-24) While 4-NDAB

Computed pseudo-first-order constants (kobs, Figure 2A) was found to be a dead-end product of RDX via photodeni- were converted to second-order rate constants at β = 1 based tration,(8) aerobic biodegradation,(5) and alkaline hydroly- − on Equation S6 (see Supporting Information, SI-6, SI-7, and sis,(6) we showed that it was not stable in MnO4 (Figure 4). − Table S2). The temperature dependency was further calcu- Treatment of 4-NDAB with MnO4 also produced N2O. A sep- lated by using an Arrhenius plot (Figure 3). The activation en- arate N-mass balance attempt showed that >50% of the N in − ergy for the reaction between RDX and MnO4 in the temper- 4-NDAB was converted to N2O. In contrast to what was ob- − ature range 20−65 °C was 77.48 ± 5.13 kJ/mol (Figure 3) with served with RDX (see below), NO3 was not formed in the · − an Arrhenius parameter (i.e., ln A) of 25.77 ± 1.96 L/mol min. 4-NDAB−MnO4 reaction. For comparison, the temperature dependency of RDX hydro- Nitrogen Mass Balance Experiment. Using pu- lysis from previously published work is also plotted (Figure rified RDX, we prepared an aqueous solution and treated it − − 3). Results show that the RDX−MnO4 reaction is less temper- with MnO4 . Initial experiments screened for a variety of N- − − + ature sensitive than alkaline hydrolysis and second-order rate containing products (NO2 , NO3 , N2O, NH4 , 4-NDAB, and − − constants for RDX−MnO4 are considerably lower than the MEDINA), but only NO3 and N2O were found to be formed rate constants for alkaline hydrolysis observed under similar in significant quantities. Temporal tracking of RDX, 2N O, and − temperatures. NO3 revealed that, after approximately 25% of the RDX had 4-NDAB Experiments. Past research has shown that been transformed, a 99% N balance was obtained (day 2, Fig- 4-NDAB is an RDX degradation product after ring cleavage ure 5). By day 9, roughly 80% of the RDX had been trans- − for both abiotic and biological treatments,(8, 9, 16) such as aer- formed and RDX, N2O, and NO3 still constituted 95.6% of the obic biodegradation(16-19) and alkaline hydrolysis.(6) In aero- N balance. During the time when most of the RDX was trans- bic degradation, 1 mol of RDX yields 1 mol of 4-NDAB and 2 formed (i.e., 0−9 d), roughly 5 times as much N2O was pro- − − mols of NO2 .(20) Adam et al.(2, 21) also separately reported duced than NO3 (molar basis). The production of N2O de- − observing 4-NDAB during treatment with MnO4 or ozone. clined after 9 d, and concentrations reached a plateau by day − − While Adam et al.(2) also used peroxide to quench MnO4 , 15; NO3 production continued with a slow steady increase which would explain the detection of NDAB (Figure S6B), until 28 d, the time when RDX was no longer detectable (Fig- their experiments with ozone required no quenching agent. ure 5). The nitrous oxide production was calculated by sum- (21) A question surrounding 4-NDAB detection during RDX ming the direct headspace measurement plus the calculated − degradation is whether it is formed during the RDX−MnO4 dissolved liquid phase concentration in equilibrium with the reaction or if it is just a product of the quenching process (see measured gas phase concentration.(25) Possible reasons why

Supporting Information, SI-4). N2O production did not continue to mirror RDX loss after 15 d Because only a trace of 4-NDAB (Figure S6) was observed include the inability of our microcosms to retain the headspace − in the RDX−MnO4 reaction when MnCO3 was used as a gases, the relationship between dissolved (i.e., calculated) and quenching agent, additional explanations for its lack of detec- headspace N2O concentrations changing as headspace pres- tion were pursued. In testing the stability of 4-NDAB under sure (i.e., N2O production) increased, or other nitrogen gases the different pH values, we found that 4-NDAB was relatively or dissolved species being produced. stable for the first 10 d (>90% remaining) (Figure S8). There- The possibility of other N-containing gases (NO, NO2, N2) − fore, the stability of NDAB was not influenced by the pH of forming from the RDX−MnO4 reaction was investigated us- our treatments. ing 15N-RDX but could not be confirmed [see Supporting In- Transformation of Hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) by Permanganate 3647

− Figure 6. Proposed RDX degradation pathway of RDX-MnO4 reaction under neutral pH. formation, SI-1(analysis of N-containing gas)]. Previous at- temperatures, and activation energies), and the lack of read- tempts to obtain a nitrogen balance during treatment of RDX ily identifiable carbon-containing intermediates (other than − have had mixed results. When 4-NDAB is a significant prod- a trace of 4-NDAB), the initial step in the RDX−MnO4 reac- uct, good N-balances have been obtained. For instance, Bal- tion is likely rate-limiting. On the basis of the various exper- akrishnan et al.(6) studied alkaline hydrolysis of RDX (pH imental treatments imposed, we believe this initial step can 10−12.3) and found 90.7% of nitrogen mass balance [i.e., be described in one of two ways. When solution pH was alka- − + NO2 (16.2%), N2O (25.6%), NH3/NH4 (1.3%), and 4-NDAB line, either as an artifact of the quenching agent (Figure S6B) (47.6%)]. When reductive treatments to RDX have been im- or purposely increased (Figure S6C), then it is probable that − posed, N-balances have generally been low (e.g., References 26 the first step in the RDX−MnO4 reaction is similar to the pre- and 27). viously reported hydrolysis pathway (N-denitration;(6) see RDX Degradation Mechanism. RDX is known to be de- Supporting Information, SI-9, Figure S18). This mechanism graded within days (~15 d) by base hydrolysis (pH 10) at am- would produce 4-NDAB, which was observed. And although bient temperature(6) and within hours to minutes at elevated 4-NDAB is a relatively stable intermediate during alkaline hy- − temperatures.(28, 29) We show that RDX was degraded by drolysis,(28) we show that 4-NDAB reacts with MnO4 at a − MnO4 at neutral pH over several days at room temperature much faster rate than RDX (Figures 2, 4, and S6A). Under alka- (Figure S6A) and that increasing temperature increased de- line pH then, 4-NDAB was apparently produced by hydroly- − struction rates (Figures 2 and 3). On the basis of results ob- sis faster than it is oxidized by MnO4 (Figures S6B and S6C), tained from the various experiments (quenching agents, pH, which allowed it to be detected. 3648 Chokejaroenrat et al. in Environmental Science & Technology 45 (2011)

Under neutral pH, which is more indicative of treating Supporting Information, providing details of experi- − aqueous RDX with MnO4 (pH 7.2 observed), we believe the mental procedures and further explanation of results, is pre- initial step is an oxidation mechanism that begins with ab- sented following the References. − straction of a hydride from the methylene carbon by MnO4 causing a carbocation to form (Figure 6). This proposed step is Acknowledgment — Funding was provided in part by the − EPA Region 7 and the Environmental Security Technology Cer- supported by past research on MnO4 −amine reactions.(30-32) Permanganate oxidation of amines has been shown to proceed tification Program (ESTCP), Project ER-0635. Partial support was in one of two ways, namely single electron transfer (SET) from also provided by the University of Nebraska School of Natural Re- the amine nitrogen and hydride or hydrogen atom abstraction sources and Water Center. This paper is a contribution of Agri- from the carbon. A detailed consideration of both possible first cultural Research Division Projects NEB-38-071. The authors also steps (SET vs hydride loss) for RDX oxidation is presented in gratefully acknowledge Ms. Melissa S. Love and Dr. Daniel M. Supporting Information (SI-8, Figure S17). In brief, theoretical Steffenson (Chemistry Department, Albion College, Albion, MI), for their suggestions and contribution to this study. explanations and experimental observations indicate that SET − will dominate the reactivities of tertiary amines with MnO4 but hydrogen abstraction becomes more prominent in sec- References ondary and primary amines.(31, 32) Second-order rate con- stants have also been shown to significantly decrease when the 1. Oberle, D. W.; Schroder, D. L.Design considerations for in-situ mechanism shifts from SET to hydrogen atom loss.(31) More- chemical oxidation. In Chemical Oxidation and Reactive Barri- over, when the initial intermediate can be stabilized with res- ers: Remediation of Chlorinated and Recalcitrant Compounds; onance as previously shown for benzylamine, the rate deter- Wickramanayake, G. B., Gavaskar, A. R.,Chen,, A. S. C., Eds.; mining step proceeds by loss of hydride (or hydrogen atom) Battelle Press: Columbus, OH, 2000; pp 91− 99. rather than SET.(30) Given that the initial carbocation interme- 2. Adam, M. L.; Comfort, S. D.; Snow, D. D.Remediating RDX-con- diate proposed for RDX (Figure 6) would be more stable than taminated ground water with permanganate: Laboratory in- the intermediate formed by SET (Figure S17), we believe the vestigations for the Pantex perched aquifer J. Environ. Qual. 2004, 33 ( 6) 2165– 2173 hydride loss mechanism would be operative. 3. Albano, J. A.; Comfort, S. D.; Zlotnik, V.; Halihan, T.; Burbach, Following through with the proposed mechanism (Figure M.; Chokejaroenrat, C.; Onanong, S.; Clayton, W.In situ chem- 6), the carbocation intermediate would react with water by hy- ical oxidation of RDX-contaminated ground water with per- drolysis to form a C−O bond (alcohol) and the resulting N,N- manganate at the Nebraska Ordnance Plant Ground Water − dinitroaminol with MnO4 itself to form an imide. It is well Monit. Rem. 2010, 30 ( 3) 96– 106 − established that MnO4 oxidizes alcohols to carbonyl com- 4. Waldemer, R. H.; Tratnyek, P. G.Kinetics of contaminant deg- pounds,(33) but the mechanisms have been shown to change radation by permanganate Environ. Sci. Technol. 2006, 40 ( 3) completely as pH, reagent, and structure are varied.(34-37) 1055– 1061 To our knowledge an oxidation mechanism for the proposed 5. Fournier, D.; Halasz, A.; Spain, J.; Fiurasek, P.; Hawari, N,N-dinitroaminol has not been studied, so no specific mech- J.Determination of key metabolites during biodegradation anism for this conversion (aminol to imide) is presented. Once of hexahydro-1,3,5-trinitro-1,3,5-triazine with Rhodococcus sp formed, however, water would attack the imide carbon (hy- strain DN22 Appl. Environ. Microbiol. 2002, 68 ( 1) 166– 172 drolysis) to open the ring and lead to a carbamic acid/amide 6. Balakrishnan, V. K.; Halasz, A.; Hawari, J.Alkaline hydrolysis anion. The carbamic acid/amide ion would then undergo de- of the cyclic nitramine explosives RDX, HMX, and CL-20: New insights into the degradation pathways obtained by the obser- carboxylation and liberation of CO2. The accelerated rate of N,N-dinitroimide hydrolysis and decarboxylation has been vation of novel intermediates Environ. Sci. Technol. 2003, 37 ( previously observed in N-nitrourea chemistry.(38-40) This 9) 1838– 1843 Halasz, A.; Groom, C.; Zhou, E.; Paquet, L.; Beaulieu, C.; De- same three-step cycle of oxidation, hydrolysis, and decarbox- 7. schamps, S.; Corriveau, A.; Thiboutot, S.; Ampleman, G.; Du- ylation would continue leading to the production of CO and 2 bois, C.; Hawari, J.Detection of explosives and their degrada- H2NNO2 (nitramide), which would rapidly be converted to tion products in soil environments J. Chromatogr., A 2002, 963 N2O(6) and water (Figure 6). ( 1−2) 411– 418 While the proposed mechanisms are presented separately 8. Hawari, J.; Halasz, A.; Groom, C.; Deschamps, S.; Paquet, L.; (Figures 6 and S18) and in a stepwise fashion, it is possible Beaulieu, C.; Corriveau, A.Photodegradation of RDX in aque- that several of these steps occur simultaneously. The domi- ous solution: A mechanistic probe for biodegradation with nant mechanism, however, can be inferred by the distribution Rhodococcus sp Environ. Sci. Technol. 2002, 36 ( 23) 5117– 5123 of nitrogen-containing degradation products observed. RDX 9. Hawari, J.Biodegradation of RDX and HMX: From basic re- − hydrolysis is reported to produce N2O, NO2 , NH3, and N2 in search to field application. In Biodegradation of Nitroaromatic the proportions 3.2:4.7:2.6:1, respectively.(28) The hydride re- Compounds and Explosives; Spain, J. C.; Hughes, J. B.; Knack- moval mechanism predicts that if RDX is exclusively degraded muss, H. J., Eds.; CRC Press: Boca Raton, FL, 2000; pp 277− 310. 10. McCormick, N. G.; Cornell, J. H.; Kaplan, A. M. The Anaerobic by oxidation, only N2O would be produced; any nitrate ob- served would have to result from hydrolysis of postoxidation Biotransformation of RDX, HMX, and Their Acetylated Deriv- intermediates (Figure 6). Restated, if the solution chemistry is atives; NATICK/TR-85/007; U.S. Army Natick Research and Development Center: Natick, MA, 1984. dominated by oxidation, we should observe ratios of N2O/ NO − which strongly favor N O. If the process is mainly hy- 11. McCormick, N. G.; Cornell, J. H.; Kaplan, A. M.Biodegradation 3 2 of hexahydro-1,3,5-trinitro-1,3,5-triazine Appl. Environ. Micro- drolysis followed by oxidation, the same ratio would strongly − − biol. 1981, 42 ( 5) 817– 823 favor NO3 . Our experiments indicate that RDX−MnO4 re- − 12. Hoffsommer, J. C.; Kubose, D. A.; Glover, D. J.Kinetic isotope action produces N2O and NO3 in a molar ratio of about 5:1, effects and intermediate formation for the aqueous alkaline ho- leading us to conclude that oxidation processes are dominant mogenous hydrolysis of 1,3,5-triaza-1,3,5-trinitrocyclohexane under the conditions we report. (RDX) J. Phys. Chem. 1977, 81 ( 5) 380– 385 Transformation of Hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) by Permanganate 3649

13. Casado, J.; Lopez-Quintela, M. A.; Lorenzo-Barral, F. M.The denitration in Clostridium bifermentans HAW-1 Appl. Microbiol. initial rate method in chemical kinetics: Evaluation and experi- Biotechnol. 2003, 63 ( 2) 187– 193 mental illustration J. Chem. Educ. 1986, 63 ( 5) 450– 452 27. Gregory, K. B.; Larese-Casanova, P.; Parkin, G. F.; Scherer, M. 14. Mathews, J. H.; Dewey, L. H.A quantitative study of some M.Abiotic transformation of hexahydro-1,3,5-trinitro-1,3,5-tri- photochemical effects produced by ultra-violet light J. Phys. azine by FeII bound to magnetite Environ. Sci. Technol. 2004, Chem. 1913, 17 ( 3) 211– 218 38 ( 5) 1408– 1414 15. Zimmerman, G.Photochemical decomposition of aqueous per- 28. Heilmann, H. M.; Wiesmann, U.; Stenstrom, M. K.Kinetics of manganate ion J. Chem. Phys. 1955, 23 ( 5) 825– 832 the alkaline hydrolysis of high explosives RDX and HMX in 16. Fournier, D.; Trott, S.; Hawari, J.; Spain, J.Metabolism of the aqueous solution and adsorbed to activated carbon Environ. aliphatic nitramine 4-nitro-2,4-diazabutanal by Methylobac- Sci. Technol. 1996, 30 ( 5) 1485– 1492 terium sp strain JS178 Appl. Environ. Microbiol. 2005, 71 ( 8) 29. Monteil-Rivera, F.; Paquet, L.; Giroux, R.; Hawari, 4199– 4202 J.Contribution of hydrolysis in the abiotic attenuation of RDX 17. Bernstein, A.; Ronen, Z.; Adar, E.; Nativ, R.; Lowag, H.; and HMX in coastal waters J. Environ. Qual. 2008, 37 ( 3) Stichler, W.; Meckenstock, R. U.Compound-specific isotope 858– 864 analysis of RDX and stable isotope fractionation during aero- 30. Wei, M. M.; Stewart, R.The mechanisms of permanganate oxi- bic and anaerobic biodegradation Environ. Sci. Technol. 2008, dation. VIII. Substituted benzylamines J. Am. Chem. Soc. 1966, 42 ( 21) 7772– 7777 88 ( 9) 1974– 1979 18. Fournier, D.; Halasz, A.; Spain, J.; Spanggord, R. J.; Bottaro, 31. Rosenblatt, D. H.; Davis, G. T.; Hull, L. A.; Forberg, G. J. C.; Hawari, J.Biodegradation of the hexahydro-1,3,5-trini- D.Oxidations of amines. V. Duality of mechanism in the re- tro-1,3,5-triazine ring cleavage product 4-nitro-2,4-diazabu- actions of aliphatic amines with permanganate J. Org. Chem. tanal by Phanerochaete chrysospotium Appl. Environ. Microbiol. 1968, 33 ( 4) 1649– 1650 2004, 70 ( 2) 1123– 1128 32. Mata-Perez, F.; Perez-Benito, J. F.Kinetics and mechanisms of 19. Bhushan, B.; Trott, S.; Spain, J. C.; Halasz, A.; Paquet, L.; Ha- oxidation of methylamine by permanganate ion Can. J. Chem. wari, J.Biotransformation of hexahydro-1,3,5-trinitro-1,3,5-tri- 1987, 65 ( 10) 2373– 2379 azine RDX by a rabbit liver cytochrome p450: Insight into the 33. Rawalay, S. S.; Shechter., S.Oxidation of primary, second- mechanism of RDX biodegradation by Rhodococcus sp strain ary, and tertiary amines with neutral permanganate. Simple DN22 Appl. Environ. Microbiol. 2003, 69 ( 3) 1347– 1351 method for degrading amines to aldehydes and ketones J. Org. 20. Jackson, R. G.; Rylott, E. L.; Fournier, D.; Hawari, J.; Bruce, Chem. 1967, 32 ( 10) 3129– 3131 N. C.Exploring the biochemical properties and remedia- 34. Kurz, J. L.Transition state characterization for the permanga- tion applications of the unusual explosive-degrading P450 nate oxidation of fluoral hydrate J. Am. Chem. Soc. 1964, 86 ( system XplA/B Proc. Natl. Acad. Sci. U.S.A. 2007, 104 ( 43) 11) 2229– 2232 16822– 16827 35. Lee, D. G.; Noureldin, N. A.Heterogeneous permanganate 21. Adam, M. L.; Comfort, S. D.; Snow, D. D.; Cassada, D.; Mor- oxidations. 3. Mechanism of the oxidation of alcohols by hy- ley, M. C.; Clayton, W.Evaluating ozone as a remedial treat- drated copper permanganate J. Am. Chem. Soc. 1983, 105 ( 10) ment for removing RDX from unsaturated soils J. Environ. 3188– 3191 Eng. 2006, 132 ( 12) 1580– 1588 36. Lee, D. G.; Chen, T.The oxidation of alcohols by permanga- 22. Joh, T. H.; Ross, R. A.; Reis, D. J.A simple and sensitive as- nate. A comparison with other high-valent transition-metal ox- say for dopamine-β-hydroxylase Anal. Biochem. 1974, 62 ( 1) idants J. Org. Chem. 1991, 56 ( 18) 5341– 5345 248– 254 37. Banerji, K. K.Kinetics and mechanism of the oxidation of al- 23. Pal, B. C.; Doherty, D. G.; David, G.Explosive mixtures Chem. iphatic alcohols by acid permanganate Bull. Chem. Soc. Jpn. Eng. News 1981, 59 ( 17) 47 1973, 46 ( 11) 3623– 3624 24. Fadda, F.; Argiolas, A.; Melis, M. R.; Montis, G. D.; Gessa, G. 38. Lobanova, A. A.; Sataev, R. R.; Popov, N. I.; II’yasov, S. G. L.Suppression of voluntary ethanol consumption in rats by Chemistry of urea nitro derivatives: I. Synthesis of nitra- gamma-butyrolactone Life Sci. 1983, 32 ( 13) 1471– 1477 mide from N,N′-dinitrourea Russ. J. Org. Chem. 2000, 36 ( 2) 25. Hudson, F.Standard operating procedure: Sample prepara- 164– 167 tion and calculations for dissolved gas analysis in water sam- 39. Lobanova, A. A.; II’yasov, S. G.; Popov, N. I.; Sataev, R. ples using a GC headspace equilibration technique; U.S. EPA: R.Chemistry of urea nitro derivatives: II. Synthesis of nitra- Washington, DC, 2004; http://www.epa.gov./ne/info/test- mide from N,N′-dinitrourea. New reactions of nitramide Russ. methods/pdfs/RSKsop175v2.pdf (accessed February 25, 2009). J. Org. Chem. 2002, 38 ( 1) 1– 6 26. Zhao, J. S.; Paquet, L.; Halasz, A.; Hawari, J.Metabolism of 40. II’yasov, S. G.; Lobanova, A. A.Nitration of N-alkyl-N′- hexahydro-1,3,5-trinitro-1,3,5-triazine through initial reduction nitroureas Russ. J. Org. Chem. 2002, 38 ( 12) 1806–1807 to hexahydro-1-nitroso-3,5-dinitro-1,3,5-triazine followed by

Supporting information follows.

1 Supporting Information for:

2

3 Transformation of Hexahydro-1,3,5-trinitro-1,3,5-triazine

4 (RDX) by Permanganate

5 6 CHANAT CHOKEJAROENRAT †, STEVE D. COMFORT ‡,*, CLIFFORD HARRIS §, 7 DANIEL D. SNOW II, DAVID CASSADA II, CHAINARONG SAKULTHAEW ‡, AND 8 TUNLAWIT SATAPANAJARU ┴ 9 10

11 * Corresponding author phone: 402-472-1502; fax: 402-472-7904; e-mail: [email protected]. 12 † Department of Civil Engineering, University of Nebraska-Lincoln 13 ‡ School of Natural Resources, University of Nebraska-Lincoln 14 § Department of Chemistry, Albion College, MI 15 II Water Science Laboratory, University of Nebraska-Lincoln

16 ⊥ Department of Environmental Science, Kasetsart University, Bangkok, Thailand

17 18 19 20 21 22

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23 24 25

26 Contents

27 SI-1. Additional Experimental Section 28 Figs. S1; S2 29 SI-2. RDX Purification 30 SI-3 Experimental Controls 31 Fig. S3; Table S1; Figs. S4; S5 32 SI-4. Effect of quenching agents on RDX degradation products 33 Figs. S6; S7; S8 34 SI-5. RDX Batch Experiments (Autocatalysis of permanganate) 35 Figs. S9; S10 36 SI-6. Kinetic Models 37 Figs. S11; S12; S13; S14 38 SI-7. Temperature dependency 39 Table S2 40 SI-8 Single electron transfer versus hydride (or hydrogen) atom removal 41 Fig. S15; S16; S17 42 SI-9. Proposed RDX degradation via proton abstraction 43 Fig. S18 44 SI-10. References 45 46

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47 SI-1. Additional Experimental Section 48 Chemical Standards. Commercial-grade RDX (~90% purity) was obtained from 49 the Fort Detrick U.S. Biomedical Research and Development Laboratory (Frederick, 50 MD). 4-nitro-2,4-diazabutanal, (4-NDAB, >99% purity) was custom synthesized by SRI

51 International (Menlo Park, CA). Sodium permanganate (NaMnO4, 40% by weight) and

52 potassium permanganate (KMnO4) were obtained from Fisher Scientific (Pittsburgh,

53 PA). Reagent grade hydrogen peroxide (H2O2, 30% v/v), methanol, manganous sulfate • 54 (MnSO4 H2O) (J.T.Baker, Phillipsburgh, NJ), and manganous carbonate (MnCO3, 55 99.9%, metals basis) (Alfa Aesar, Ward Hill, MA) were used as purchased. All solvents 56 used in this research were HPLC grade (Fisher Scientific, Springfield, NJ). An analytical

57 RDX standard (100 µg/mL) in a 50:50 acetonitrile-methanol matrix was purchased from

- + 58 AccuStandard (New Haven, CT). Nitrate (NO3 ), Ammonium (NH4 ) (1000 mg/L, GFS

- 59 Chemicals, Columbus, OH) and nitrite (NO2 ) (1000 mg/L, Absolute Standards Inc.,

60 Hamden, CT) standards were used as purchased. Nitrous oxide (N2O) standards were 61 prepared from the 2% stock gases (mole basis) obtained from Scott Specialty Gases 62 (Plumsteadville, PA). 63 High-Performance Liquid Chromatography (HPLC). Temporal changes in 64 RDX and degradate concentrations were quantified at a 220 nm by HPLC equipped with 65 a photodiode array detector (Shimadzu Scientific Instruments, Columbia, MD). Peak

66 separations were performed by injecting 20 µL of sample into a Supelcosil LC-8, 250 x

67 4.6 mm, (Supelco, Sigma-Aldrich Corporation, PA) or a Fluophase PFP perfluorinated 68 column, 250 x 4.6 mm, coupled with a guard column (Thermo Scientific, MA). A variety 69 of mobile phases and flow rates (0.50-1.50 mL/min) were tested to separate peaks but

70 the typical mobile phase was an isocratic mixture of methanol and H2O (30:70), or

71 acetonitrile and H2O (50:50) at a flow rate of 0.75 mL/min.

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- - + 72 Ion Chromatography (IC). Analysis of NO2 /NO3 and NH4 were performed with 73 a Dionex DX-120 Ion Chromatograph (Sunnyvale, CA) with suppressed conductivity 74 detection (conductivity detector, CDM-3). For anion analysis, separation was performed 75 with an AS-15 IonPac column, 250 x 4.0 mm, using an eluent of 38 mM NaOH at a flow 76 rate of 1 mL/min. For cation analysis, separation was performed with a CS12A IonPac

77 column, 250 x 4.0 mm, using an isocratic eluent of 5.5 mM H2SO4 at a flow rate of 1.2 78 mL/min. The injection volume for both analyses was 25 µL. To effectively analyze

- 79 samples by IC, RDX samples treated with MnO4 were quenched with MnCO3. 80 Gas Chromatography/Electron Capture Detector (GC/ECD). Nitrous oxide

- 81 (N2O) emitted from the RDX-MnO4 reaction was measured by direct injection into a 82 Hewlett-Packard (Palo Alto, CA) 6890 GC operated with a HP-Plot column (Molecular

83 sieve 5A) 30 m/0.53 mm (50 µm film thickness) and electron capture detector (ECD). A

84 P-5 gas (a mixture gas of 95% Argon and 5% CH4) was used as a carrier gas for the 85 GC system. The GC oven was equilibrated at least two hours at 225°C before analysis.

- 86 UV-Spectrophotometer. Changes in MnO4 concentrations were monitored by 87 diluting solution with Ultra Pure water in 20-mL vials and quantifying concentrations with 88 a HACH Spectrophotometer DR2800 (HACH Company, Loveland, CO) at a wavelength

- 89 of 525 nm. A test of whether colloidal MnO2 interfered with quantification of MnO4 is 90 presented in SI-3. 91 Analysis of N-containing gases.

- 92 Nitrogen Gas (N2) To determine if N2 gas was a product of the RDX-MnO4 93 reaction, experiments were conducted under vacuum in a Rittenburg tube, a two-legged 94 Y-shaped tube (Fig. S1), containing crystalline RDX (both 14N-RDX and 15N-RDX) in

- 15 95 one side and concentrated MnO4 solution in the other. Uniformly labeled, [U- N]RDX, 96 (15N abundance of 97 atom%) was purchased from PerkinElmer (Waltham, MA). Prior to 97 starting the reaction (i.e., mixing), all gases were evacuated through a vacuum line

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- 98 while the MnO4 solution was simultaneously frozen. Once the frozen solution melted, 99 we mixed it with the crystalline RDX in the other side. The tube was then immersed in

100 water (~20°C) to confirm no leakage and avoid atmospheric gas contamination. We

101 also mirrored this experiment without vacuuming so as to monitor the RDX 102 concentration by HPLC. When RDX was completely degraded, gas emission was drawn

103 by a vacuum system passing through a cold trap to freeze all gases but N2 gas (Fig. S2; 104 (1)). Gas samples were then collected in sample bulb and cryogenically transferred to 105 an Optima Dual Inlet mass spectrometer (VG Isotech, Colchester, VT). 106 Results indicated that no increase in gas pressure was observed during the

107 sample transfer and full scan measurement showed that, very little, if any N2 gas (m/z =

108 28, 29, 30) formed during treatment. The primary reaction gas formed, N2O, was 109 trapped in the preparation line but was not analyzed on the instrument. 110 111 112 113 114 115 116 117 118 119 120 121 122 123

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124 125 126 127 128 129 130 131 132 133 134 135 136

137

- 138 Figure S1: (Left) The Rittenburg tube containing MnO4 solution was frozen in liquid

- 139 nitrogen while all gases were being vacuumed. (Right) thawed RDX- MnO4 140 solution after mixing. 141 142 143 144 145 146 147 148 149

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150 151 152 153 154 155 156 157 158 159 160 161

162

163 Figure S2: Experimental system for trapping of N2 gas. When RDX was completely 164 degraded, all gases were evacuated from the Rittenburg tube (Lower Circle) and

165 trapped in the vacuum system except N2 gas which was forced to the gas-tight 166 sampling tube (Upper Circle). 167

168 NOx gases (i.e., NO2 and NO) Besides the N2 and N2O gases, we also

169 investigated the production of NOx gases (i.e., NO2 (nitrogen dioxide) and NO (nitric

- 170 oxide)) to determine if they were released during the RDX-MnO4 reaction. NO2 is 171 known to produce from the reaction of concentrated nitric acid and copper and is a toxic 172 brownish gas with a pungent acid odor. However, in the diluted solution of nitric acid 173 and copper, water molecules cause the reaction to produce NO instead. Although we

- 174 did not observe a distinct brownish color of NO2 during the RDX- MnO4 reaction, we

175 attempted to identify NO2 and other possible transformation products by treating 5 mL of

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176 saturated RDX (12.1 mg 15N-labeled and 0.4 mg non-labeled RDX) with 168.067 mM of

- 177 MnO4 in a 12-mL vial with a gastight septum. Each vial was degassed for 5 min and 178 purged with Helium for 5 min by the acid injector (3.2 psi, Gilson, Middleton, WI) at a

-1 179 flow rate of 20.5 mL min . NaMnO4 (0.2 mL of 40% by weight) was injected into a vial 180 by a gastight syringe. The temperature was controlled in a Precision 180 Series water 181 bath at 60°C (Precision Scientific Co., Baltimore, MD) to increase RDX destruction rate. 182 At 11 d, a 10 µL gas sample was removed from the vial and injected directly into a 183 Hewlett-Packard 5890 GC (Palo Alto, CA) an Agilent 5972 quadrupole mass 184 spectrometer. The N gases were separated on a 30 m/0.32 mm PLOT Moleseive 185 column (J&W Scientific, Folsom, CA). The instrument was calibrated using Helium 186 reference gas.

- 187 Results indicated that NO2 and NO were not detectable during the RDX-MnO4

188 reaction. A complicating factor, however, is that if NOx gasses (i.e., NO or NO2) are

- 189 liberated during the treatment of RDX with MnO4 , it will be difficult to quantify because

- - - 190 MnO4 provides an excellent means of removing NO by oxidizing it to NO2 and NO3 ,

- 191 depending on pH (2-4). Alkaline or acidic MnO4 has also been shown to be capable of

192 trapping NOx gas emission from soils (5-8). 193 194 195 196 197 198 199 200 201

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202 SI-2. RDX Purification 203 The commercial grade RDX contains ~90% RDX and ~10% HMX (octahydro- 204 1,3,5,7-tetranitro-1,3,5,7-tetrazocine). To remove interferences and degradation artifacts 205 associated with HMX, we removed the HMX by preparing a concentrated RDX solution 206 (in acetonitrile) and purified to ≥99% RDX by using a Waters 2695 HPLC (Waters Corp., 207 Milford, MA) with a temperature-controlled (30°C) Kromasil C18 column, 250 x 4.6 mm, 208 (Thermo Scientific, MA) and Photodiode Array Detector (Waters 2996, Waters Corp., 209 Milford, MA). The flow rate for this purification procedure was 1.5 mL/min with a

210 repeated injection volume of 25 µL. A mobile phase of methanol (in H2O) was used with 211 the following gradient: 60:40 for 9 min followed by 90:10 for 3.5 min and 60:40 for the 212 remainder of the run (~7.5 min). A Spectrum CF-2 fraction collector was used to isolate 213 the RDX peak eluting from the column. The RDX fractions were combined and 214 concentrated by the RapidVap evaporation system (Labconco, Kansas city, MO) in

215 which a cylindrical receptacle was swirled and blown by N2 gas at 50°C until dry. 216 217 218 219 220 221 222 223 224 225 226 227

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228 SI-3. Experimental Controls 229 A series of experiments were performed under batch conditions to verify that

- 230 RDX destruction rates by MnO4 were similar when the initial pH was controlled or

231 allowed to drift as the reaction proceeded (Fig. S3), the use of MnCO3 as a quenching 232 agent did not significantly influence sample pH or temperature (Table S1), RDX

233 concentrations after quenching with MnCO3 were stable and not subject to hydrolysis

- 234 (Fig. S4), and that quantification of MnO4 concentrations were not influenced by

235 colloidal MnO2 (Fig. S5). 236 1.2 237 1.0 238 0.8 239 0.6 240 0.4 241 0.2 242 RDX Concentration (C/Co) 0.0 243 8 244 6 - 245 MnO4 concentration 4.20 mM 246 4.20 mM (control pH) 4 8.40 mM pH 8.40 mM (control pH) 247 16.81 mM 16.81 mM (control pH) 2 33.61 mM 248 33.61 mM (control pH) 84.03 mM 84.03 mM (control pH) 249 0 250 0 2 4 6 8 10 1214 Time (day) 251

- 252 Figure S3. Changes in RDX concentration and pH by various concentrations of MnO4 253 under controlled and unbuffered pH.

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254 255

- 256 Table S1. Changes in pH and temperature of RDX-MnO4 solution following

257 quenching with various mass of MnCO3.

pH before pH after Temp before Temp after MnCO3 quenching quenching quenching quenching (g per mL of (°C) (°C) sample) 0.00 g 5.88 25 0.03 g 5.82 6.15 25 24 0.04 g 5.81 6.02 25 23 0.05 g 5.85 6.02 25 23 0.06 g 5.80 5.93 25 23 0.07 g 5.74 5.86 25 22.5 258 259 260 261 262 263 264 265 266 267 268

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269 270 271 272 1.2 273 - MnO4 concentration 274 1.0 4.20 mM 8.40 mM 275 16.81 mM 0.8 33.61 mM 276 4.20 mM (rerun) 8.40 mM (rerun) 277 0.6 16.81 mM (rerun) 33.61 mM (rerun) 278 0.4 279

280 0.2

281 RDX Concentration (C/Co) 282 0.0 0 4 8 12 16 20 24 28 283 284 Time (day) 285 286 287 Figure S4. Temporal changes in RDX concentrations following treatment with varying

- 288 MnO4 concentrations. Solid symbols signify concentrations of samples analyzed 289 immediately, open symbols are the same samples analyzed 9 d later. 290 291 292 293 294

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1.2 295 296 1.0

297 0.8 298 0.6 299 300 0.4 301 0.2 302 RDX Concentration (C/Co) 303 0.0 304 90 305

M) 75 MnO - concentration* 306 m 4 4.20 mM 307 60 8.40 mM 16.81 mM 308 45 33.61 mM 84.03 mM 309 30

310 Concentration ( - 4 311 15 312 MnO 0 313 0 2 4 6 8 1012 14 314 Time (day) 315 316

- 317 Figure S5. Changes in RDX and MnO4 concentrations following treatment with varying

- - 318 MnO4 concentrations. Solid symbols indicate MnO4 concentrations determined without 319 filtration, open symbols with filtration (0.45µm glasswool filter).

320

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321 322 SI-4. Effect of quenching agents on RDX degradation products 323 To determine the effect of quenching agents on RDX degradation products,

- 324 aqueous solutions of RDX (0.07 mM) were treated with 33.61 mM of MnO4 . We initially

325 prepared RDX solutions by spiking 150 mL H2O with 1.04 mL of RDX stock solutions

- - 326 prepared in acetone but the acetone-MnO4 reaction resulted in autocatalysis of MnO4 327 at alkaline pH and prevented further degradation of RDX >10 d (see Supporting 328 Information; SI-5). Consequently, all aqueous RDX solutions were prepared by

- 329 dissolving purified crystalline RDX in water over several days. Once MnO4 was added 330 to RDX solutions to initiate the reaction, samples were periodically collected and

331 quenched with MnCO3 or H2O2. Quenching with MnCO3 (pH = 6.7) was performed as

332 described in the main manuscript. When quenched with 30% H2O2 (0.04 mL per mL of

333 sample), samples were required to mix continuously to control H2O2 consumption. The

334 pH of samples quenched with H2O2 were found to increase significantly (pH = 11.5). To

335 elucidate this pH effect, one set of batch samples were quenched with MnCO3, and we

336 increased the pH to that observed with the H2O2 by adding NaOH. Temporal changes in

- - 337 RDX, 4-nitro-2,4-diazabutanal (4-NDAB), NO3 , and NO2 concentration were monitored 338 by using HPLC and IC.

- 339 Results indicated that an RDX solution treated with MnO4 led to complete RDX 340 transformation within 15 d (Fig. S6A). To quantify temporal changes in RDX 341 concentrations, samples were removed from the batch reactors every 2 to 3 d and

- 342 chemically quenched to remove MnO4 and prevent further RDX oxidation. While

343 MnSO4 is commonly used as a quenching agent (9-13) and does not interfere in HPLC

- - 344 analysis of RDX (12-13), the sulfate liberated interferes with NO2 and NO3 analyses by

345 ion chromatography (IC). By using MnCO3, we avoided this interference during IC

346 analysis. However, the disadvantage of using MnCO3 is that, at the concentrations of

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- 347 quenching agents used, MnCO3 takes longer than MnSO4 to quench the MnO4 . Given 348 the typical time course of the batch experiments (15 d), we compared RDX destruction 349 rates from the same batch experiment and observed similar RDX destruction rates (Fig. 350 S7). 351 Another consideration is that the quenching agent can alter the pH of the sample 352 and possibly influence product formation or stability. When samples were quenched

353 with MnSO4, solution pH decreased from ~7.2 (before quenching) to pH 2.6 after 354 quenching as predicted by the following reaction (Eq. S1). 355

+ 2- + 356 2NaMnO4 + 3MnSO4 + 2H2O
2Na + 3SO4 + 4H + 5MnO2 [Eq. S1] 357

- 358 When RDX-MnO4 solutions were quenched with MnCO3 (Eq. S2), sample pH 359 (after quenching) remained near 6.7, which is closer to the pH of the unquenched RDX-

- 360 MnO4 solution. 361

+ - + 362 2NaMnO4 + 3MnCO3 + 2H2O
2Na + 3HCO3 + 5MnO2 + H [Eq. S2] 363

- 364 Product analysis during the RDX-MnO4 reaction showed that when MnCO3 was

- - 365 used as a quenching agent, we observed NO3 production in the reaction but no NO2 366 and only a trace of 4-NDAB (~0.004 mM) (Fig. S6A).

- 367 Peroxide was also used as a quenching agent. Here, peroxide reacts with MnO4 368 by the following reaction (Eq. S3): 369

+ - 370 2NaMnO4 + 4H2O2
2Na + 2OH + Mn2O3 + 3H2O + 4O2 [Eq. S3] 371

- 372 Because OH is liberated, the pH of samples quenched with H2O2 increased (pH

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- - 373 ~11.5) and we observed NO2 , NO3 , and 4-NDAB (Fig. S6B). Although the magnitude of

- 374 NO3 generated was similar to what we observed when MnCO3 was used as a 375 quenching agent (Fig. S6A, S6B), RDX destruction kinetics were much faster (k = 1.83 376 d-1). Because we suspected excess peroxide may have contributed to RDX destruction,

377 we conducted an experiment where RDX solution was treated with H2O2 in the same

378 ratio as used in quenching process (0.04 mL of 30% H2O2 to 1 mL RDX or 1.2% (v/v)

- 379 H2O2; no MnO4 ). Results showed that RDX concentration was not significantly affected,

- - 380 pH remained constant, and RDX degradation products (NO2 NO3 , and 4-NDAB) were 381 not observed.

382 The peroxide concentration used in this control experiment (RDX + H2O2 only)

- 383 was higher than what the quenched RDX-MnO4 samples would have experienced

384 because most, if not all, of the H2O2 would have reacted with the MnO4. Therefore, the 385 increased RDX destruction kinetics observed (Fig. S6B) does not appear to be directly

386 related to the presence of excess peroxide. Rather, catalytic decomposition of H2O2 into

- 387 various radicals (i.e., superoxide anion (O2 ), hydroperoxide radical (HO2•), and hydroxyl 388 radical (•OH)) may have played a role in the enhanced degradation of RDX (Fig. S6B).

389 Although MnO2 surfaces can enhance oxidation reactions (14), this precipitate, which

- 390 forms during RDX-MnO4 reaction (12), is also a catalyst for decomposition of H2O2 and

- - 391 both O2 and HO2• are favored at high pH (15-16). O2 itself is known to be capable of

392 degrading RDX (17). Furthermore, during the quenching process, Mn2O3 is liberated 393 (Eq. S3) and can simultaneously act as a catalyst for degradation of organic compounds

394 in the presence of H2O2 (15, 18). Another possibility is that the alkaline pH created

395 during the quenching process (Eq. S3) facilitated H2O2 decomposition into •OH which 396 contributed to RDX degradation. Moreover, Gates-Anderson et al. (19) observed that in

- 397 strongly basic solutions (pH > 9) •OH can also be generated from MnO4 and directly 398 oxidize organic contaminants. These explanations support a seven-fold increase of

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399 RDX destruction kinetics (Figs. S6A, S6B). 400 Finally, an elevated temperature may also have been responsible for greater

401 RDX destruction in the H2O2 quenched samples. Heilmann et al. (20) showed that 402 alkaline hydrolysis rates of RDX in aqueous solution dramatically increased at high

o 403 temperature (50 C). We observed that using H2O2 as a quenching agent caused a rapid

o - 404 9 C increase in sample temperatures. Because H2O2-MnO4 reaction is exothermic, it is 405 reasonable that the combination of alkaline pH and heat may have contributed to RDX

- 406 degradation (See also Effect of Temperature on RDX-MnO4 Reaction in the main 407 manuscript). 408 Given that the treatment of RDX with peroxide alone did not cause an increase in

- - 409 pH or the production of NO2 and 4-NDAB, the alkaline pH created by the H2O2-MnO4 410 reaction was likely responsible for the degradation products observed. To test this

411 further, we again used MnCO3 as a quenching agent and artificially raised the pH of the

412 samples before and after quenching to pH 11.5 (similar to what was observed with H2O2 413 as a quenching agent). Results showed RDX degradation was slower than when

- 414 peroxide was used to quench the MnO4 and closer to the results obtained when MnCO3 415 was used without pH adjustment (Fig. S6A, k = 0.26 d-1; Fig. S6C, k = 0.33 d-1). This 416 observation lends credence to the possibility that peroxide radicals may have been

- 417 involved during the quenching of MnO4 with H2O2 (Fig. S6B). Using MnCO3 + alkaline

- 418 pH also produced NO2 and 4-NDAB as reaction products (Fig. S6C). Two known RDX 419 degradation schemes involve the removal of one nitro group (denitration) with the 420 intermediate methylenedintramine (MEDINA) or two nitro groups and the formation of 4-

- 421 NDAB (e.g. (21)). Thus, the detection of nitrite during the RDX-MnO4 reaction (with

422 H2O2 quenching agent or MnCO3 + alkaline pH) is likely a result of the alkalinity

- 423 stabilizing NO2 and preventing further transformation. Numerous reports have shown

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424 that nitrite is more persistent at alkaline pH (22-23). Balakrishnan et al. (24) also found

- 425 NO2 as an endproduct of RDX hydrolysis.

426 80 60 70 427 60 RDX 50 - 428 50 NO2 40 4-NDAB 40 -1 - 30 k = 0.26 d NO3 429 30 20

) 20 (A) MnCO3

430 -3 (pH=6.7) NO 10 10 2

431 - 0 0 NO ,

M x 10 80 m

432 60 3 -

70 ( Concentration -1 433 60 k = 1.83 d 50 50 40 434 40 30 435 30 20 20 (B) H2O2 (pH=11.5) 436 10 10 m M x 10 x M 437 0 0 80 60 438 70 -3 ) -1 RDX, 4-NDAB Concentration ( 60 k = 0.33 d 50 439 50 40 40 440 30 30 (C) MnCO 441 3 20 20 (pH=11.5) 10 442 10 0 0 443 0 2 4 6 8 10 12 14 16 Time (day) 444 445 Figure S6: Changes in RDX concentration and production of degradation products (4-

- - 446 NDAB, NO2 , and NO3 ) when quenched with; (A) 0.10 g MnCO3 (per mL); (B)

447 0.04 ml 30% H2O2 (per mL, pH ~ 11.5); and (C) 0.10 g MnCO3 (per mL) in which 448 sample solutions pH was raised to 11.5 before and after quenching. Bars indicate 449 sample standard deviations (n = 3).

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450 1.0

451 ) o 0.8 Control (RDX + MnSO ) 452 4 MnO - + RDX (Quenched with MnSO ) 0.6 4 4 Control (RDX + MnCO 453 3) MnO - + RDX (Quenched with MnCO ) 4 3 454 0.4 -1 -1 k = 0.174 d k = 0.169 d 455 RDX Conc (C/C 0.2 456 0.0 457 0 5 10 15 20 458 Time (d) 459

460 Figure S7: Comparison of RDX degradation kinetic rates when quenched with MnSO4

461 or MnCO3. Bars indicate sample standard deviations (n = 3). 462 463 1.2 )

464 o 1.0 465 0.8 466 0.6 467 Control (~pH =7) Buffer at pH = 5 468 0.4 Buffer at pH = 7 Buffer at pH = 9 Buffer at pH = 11 469 0.2 4-NDAB Concentration (C/C Concentration 4-NDAB 470 0.0 471 0 5 10 15 20 25 Time (d) 472 473 Figure S8: The effect of pH on 4-NDAB stability. Bars indicate sample standard 474 deviations (n = 3). 475

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476 SI-5. RDX Batch Experiments (Facilitated Decomposition of permanganate)

- 477 To evaluate the effects of initial MnO4 concentration on RDX destruction rates, 478 we conducted the batch experiment by treating 150 mL of aqueous 14C-RDX (0.02 mM,

-1 - 479 30000 dpm mL , uniformly ring-labeled) and varying MnO4 concentration from 8.40 mM

- 480 to 168.07 mM. Each MnO4 concentration was replicated three times. Temporal samples

481 were periodically collected and quenched with MnSO4 as described in analytical section 482 and monitored for the loss of RDX by HPLC and 14C-activity by Liquid Scintillation 483 Counter (LSC). 484 Results indicated that treating aqueous (i.e., distilled water) RDX with 168.067

- -1 485 mM of MnO4 reduced RDX concentrations to zero within 10 d (k = 0.49 d ) (Fig. S9A).

- 486 Lower MnO4 concentrations (8.40-42.02 mM) reduced RDX destruction rates and

- 487 overall removal. For instance, when RDX was treated with 8.40 mM of MnO4 488 destruction rates decreased ~70% (k = 0.14 d-1) and only 29% of the initial RDX was 489 removed within 10 d (25). These results are similar to those reported by Adam et al. 490 (12) but differ in that temporal decrease in both RDX and 14C concentrations (Fig. S9B)

- 491 reached a plateau after ~10 d. The reasons MnO4 failed to continually transform and 492 mineralize RDX beyond 10 d was investigated by monitoring temporal changes in pH

- 493 and MnO4 concentrations.

- - 494 By repeating the experiment with 84.03 mM of MnO4 and monitoring MnO4 and 495 pH (Fig. S10A, S10B, S10C), we observed an increase in pH from 6.5 to > 8. Using

- 496 higher MnO4 concentrations (126.05, 168.07 mM) also produced similar changes in pH.

- 497 This increase in pH coincided with a significant decrease in MnO4 concentration (Fig. 498 S10B). By contrast, when a pH-stat (Metrohm Titrino 718S; Brinkman Instruments, 499 Westbury) maintained the pH at 7, RDX concentrations did not plateau but continued to

- 500 decrease and very little consumption of MnO4 was observed (Fig. S10A, S10B). It is

- 501 clear that in the unbuffered treatment, the rapid decrease in MnO4 concentration

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502 coincided with the lack of further RDX destruction beyond 7 d (i.e., plateau). We believe

- 503 the loss of MnO4 was likely caused by a facilitated decomposition of permanganate at

- 504 alkaline pH. But alkaline pH alone was not solely responsible for the loss of MnO4 . 505 Adam et al. (12) evaluated the effect of pH on RDX destruction kinetics and reported no 506 pH effect in the range 4.1 to 11.3. A comparison of procedures used by Adam et al. (12) 507 and our protocol revealed that a higher percentage of acetone was used in our batch 508 reactors. This occurred by using RDX stock solutions prepared in acetone (both 509 unlabeled and 14C-labeled) to spike the aqueous solutions with RDX. Although the 510 volume of acetone spiked into the aqueous batch reactors was relatively low (1.04 mL

511 acetone/150 mL H2O), when this same concentration of acetone was added to 84.03

- - 512 mM of MnO4 without RDX, a similar decrease in MnO4 was observed (Fig. S10D, 513 S10E, S10F); similarly, when aqueous RDX solutions were prepared without acetone,

- 514 the pH remained constant (Fig. S10F) and MnO4 consumption was negligible (Fig. 515 S10E). The plateau in RDX loss observed (Figs. S9, S10) resulted from the reaction of

- 516 acetone with MnO4 and likely included the oxidation of acetone to oxalic acid and the

- 2+ 517 reaction of oxalic acid with MnO4 to form Mn , which is known to facilitate the

- 518 decomposition of MnO4 .

- 519 While the accelerated removal of MnO4 was traced back to the use of acetone 520 and subsequent formation of carboxylic acids in our batch reactors (Fig. S10), the 521 implications of this observation may be more than just an experimental anomaly. Oxalic

- 522 acid is a product of the TCE-MnO4 reaction (26). Li et al. (27) also showed that oxalate 523 was a primary oxidation product of the explosive TNT (2,4,6-trinitrotoluene) during

2+ 524 treatment with Fe and H2O2 (i.e., Fenton oxidation). Thus, situations may arise where

- 525 oxalate (or other carboxylic acids) are present and cause excessive MnO4 526 decomposition if the pH is not monitored and prevented from becoming alkaline. 527

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1.2

528 ) (A) o 529 1.0

530 0.8

-1 k = 0.14 d 531 0.6 -1 k = 0.18 d 532 -1 0.4 k = 0.31 d 533 - 0.2 -1 MnO Conc. 534 k = 0.37 d 4 RDX Concentration (C/C -1 Control -1 k = 0.41 d k = 0.489 d 535 0.0 8.40 mM 21.01 mM 536 42.02 mM 1.2 84.03 mM 537 (B) 126.05 mM 168.07 mM 1.0 )

538 o

539 0.8 k = 0.07 d-1

-1 540 0.6 k = 0.14 d -1 k = 0.32 d 541 0.4

C Activity (C/C -1 k = 0.40 d 542 14 -1 0.2 k = 0.41 d -1 543 k = 0.48 d 0.0 544 0 5 10 15 20 25 30 545 Time (d) 546 547 Figure S9: Loss of RDX and 14C-activity in aqueous solution treated with various

- 548 concentrations of MnO4 . Solution samples were quenched with MnSO4. Bars indicate

549 sample standard deviations (n = 3). 550

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551 1.2 552

) 1.0 o

553 Unbuffered Control 0.8 RDX + Acetone Control pH at 7.0 MnO - 554 0.6 (pH Stat = 7.0) 4 - MnO4 + RDX + Acetone 0.4 - 555 MnO4 + Acetone MnO - + RDX RDX Conc (C/C Conc RDX 0.2 4 556 (A) (D) 0.0 557 1.2 )

558 o 1.0 559 0.8 0.6

560 (C/C Conc 4 0.4 561 0.2 KMnO (B) (E) 562 0.0 563 9 8 564 7 565 6

pH 5 566 4 567 3 (C) (F) 568 2 0 2 4 6 8 10 12 14 16 0 2 4 6 8 10 12 14 16 18 569 Time (d) 570

- 571 Figure S10: (A-C) Changes in RDX, MnO4 concentration, and pH in the presence of

- 572 acetone following treatment with 84.03 mM of MnO4 (i.e., Unbuffered control,

- 573 and control pH at 7). (D-F) Changes in RDX, MnO4 concentration, and pH

574 with/without acetone. Solution samples were quenched with MnSO4. Bars

575 indicate sample standard deviations (n = 3).

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576 SI-6. Kinetic Models 577 While second-order expressions are commonly used to describe contaminant

- - 578 destruction rates by MnO4 (28-32), if MnO4 is in excess, the reaction can also be 579 described by a pseudo first-order expression (12, 33). Like many other second-order

- 580 reactions between contaminant and MnO4 , the general rate equation can be written as:

581 [Eq. S4]

582 [Eq. S5]

583 [Eq. S6] 584 Where is a reaction order with respect to RDX, is a reaction order with

- 585 respect to MnO4 , is a reaction rate, is a second-order rate constant, and is a

- 586 pseudo-order rate constant. By varying the initial concentration of MnO4 and measuring 587 by fitting the results into a pseudo first-order equation by regression analysis using

- 588 computer software SigmaPlot Version 10.0 (34), the value of with respect to MnO4 589 can be obtained by a log-log form of Eq. S6: 590 [Eq. S7] 591 Likewise, by varying the initial concentration of RDX and measuring the reaction 592 rate, the value of with respect to RDX can be determined by a log-log form of Eq. S5.

593 To evaluate for the reaction rates, we used the initial reaction rate (ro) by approximating 594 the tangent to the concentration time-curve (35); therefore, Eq. S5 can then be 595 expressed as: 596 [Eq. S8] 597 Second-order rates ( ) were then derived from pseudo first-order rates ( ) by 598 the relationship in Eq. S6.

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599

600 1.2 - -1 - 4.20 mM MnO4 ; kobs = 0.02 d A. RDX - MnO4 - -1 601 8.40 mM MnO4 ; kobs = 0.03 d 1.0 - -1 16.81 mM MnO4 ; kobs = 0.06 d 602 - -1 0.8 33.61 mM MnO4 ; kobs = 0.12 d - -1 603 84.03 mM MnO4 ; kobs = 0.37 d 604 0.6

605 0.4 606 0.2 607 RDX Concentration (C/Co) 608 0.0 0 5 10 15 20 25 30 609 Time (day) 610 611 1.2 - 612 B. TNT-MnO4 - -1 1.0 4.20 mM MnO4 ; kobs = 0.36 d - -1 613 8.40 mM MnO4 ; kobs = 0.74 d - -1 0.8 16.81 mM MnO4 ; kobs = 1.54 d 614 - -1 33.61 mM MnO4 ; kobs = 3.45 d - -1 615 0.6 84.03 mM MnO4 ; kobs = 12.19 d 616 0.4 617 0.2 618 TNT Concentration (C/Co) 619 0.0 0 20 40 60 80 100 620 Time (hour) 621 622 Figure S11: Loss of RDX (A) and TNT (B) when treated with various concentrations of

- 623 MnO4 . Note differences in time scales. Bars indicate sample standard deviations 624 (n = 3).

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625 626 627 628 1e-4 - 629 33.61 mM MnO4 -1 8e-5 RDX = 1.12e-05 M; |ro| = 2.84e-06 M d 630 -1 RDX = 2.25e-05 M; |ro| = 5.87e-06 M d 631 -1 6e-5 RDX = 4.50e-05 M; |ro| = 1.09e-05 M d -1 632 RDX = 6.75e-05 M; |ro| = 1.65e-05 M d RDX = 9.00e-05 M; |r | = 1.90e-05 M d-1 633 4e-5 o 634 635 2e-5 RDX Concentration (M) 636 0 637 0 5 10 15 20 25 30 35 638 Time (day) 639 640 641 642 643 Figure S12: Loss of RDX (initial concentrations ranging from 0.01 to 0.09 mM) when

- 644 treated with MnO4 at 33.61 mM. Bars indicate sample standard deviations (n = 645 3). 646 647

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1e-4 648 - A. 4.20 mM MnO4 8e-5 649 -1 TNT = 1.12e-05 M; |ro| = 6.03e-06 M d -1 650 TNT = 2.25e-05 M; |ro| = 1.08e-05 M d 6e-5 -1 TNT = 4.50e-05 M; |ro| = 2.15e-05 M d -1 651 TNT = 6.75e-05 M; |ro| = 3.72e-05 M d 4e-5 652 2e-5

653 TNT Concentration (M) 0 654 1e-4 - 655 B. 8.40 mM MnO4 8e-5 -1 TNT = 1.12e-05 M; |ro| = 1.23e-05 M d 656 -1 TNT = 2.25e-05 M; |ro| = 1.69e-05 M d 6e-5 -1 657 TNT = 4.50e-05 M; |ro| = 4.49e-05 M d -1 TNT = 6.75e-05 M; |ro| = 6.98e-05 M d 658 4e-5

659 2e-5 TNT Concentration (M)

660 0 661 1e-4 - C. 33.61 mM MnO4 662 8e-5 -1 TNT = 1.12e-05 M; |ro| = 3.15e-05 M d TNT = 2.25e-05 M; |r | = 8.35e-05 M d-1 663 6e-5 o -1 TNT = 4.50e-05 M; |ro| = 1.19e-04 M d -1 664 TNT = 6.75e-05 M; |ro| = 1.76e-04 M d 4e-5 TNT = 9.00e-05 M; |r | = 3.33e-04 M d-1 665 o 2e-5 666 TNT Concentration (M) 667 0 0 20 40 60 80 100 668 Time (hour) 669 670 Figure S13: Loss of TNT (initial concentrations ranging from 0.01 to 0.09 mM) when

- 671 treated with MnO4 at 4.20 (A), 8.40 (B), or 33.61 mM (C). Bars indicate sample 672 standard deviations (n = 3). 673

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-1.2 674 A. Pseudo first-order rate constants vs [MnO -] -0.8 4 o 675 ) -0.4 676 -1 0.0 ] (d 677 0.4 obs 678 0.8

679 Log [K 1.2 2 680 1.6 Slope = 1.171 + 0.043 (R = 0.996) 2.0 681 -2.4-2.2-2.0-1.8-1.6-1.4-1.2-1.0 - 682 Log [MnO4 ]o (M) 683 -5.6 B. Initial rates vs [TNT]o 684 -5.2

685 ) -4.8 -1 -4.4 686

] (M d -4.0 687 o -3.6 688 Log [r -3.2 - 2 4.202 mM MnO4 ; Log[ro] = -0.2881 + 1.001 Log[TNT]0 (R = 0.9956) - 2 689 8.403 mM MnO4 ; Log[ro] = 0.0224 + 1.009 Log[TNT]0 (R = 0.9784) -2.8 - 2 33.613 mM MnO4 ; Log[ro] = 0.5373 + 1.012 Log[TNT]0 (R = 0.9804) 690 -2.4 691 -4.0 -4.2 -4.4 -4.6 -4.8 -5.0

692 Log [TNT ]o (M) 693 694 695 Figure S14: (A) Plot of pseudo first-order rate constants for TNT degradation vs

- - 696 [MnO4 ]. Aqueous TNT (0.09 mM) was treated with MnO4 ranging from 4.20 to

697 84.03 mM. (B) Plot of initial rates of TNT degradation vs. [TNT]o ranging from

- 698 0.01 to 0.09 mM when treated with 4.20, 8.40, or 33.61 mM MnO4 . 699

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700 SI-7. Temperature dependency

- 701 In RDX-MnO4 temperature experiment, the pseudo first-order rates were 702 evaluated at four different temperatures. The activation energy, , can be determined 703 using a plot of the Arrhenius equation, as follows: 704

705 [Eq. S9]

706 707 Where is the empirical Arrhenius factor or pre-exponential factor; is gas constant 708 (8.314 J/K·mol); and is the absolute temperature (K). The logarithm of the second- 709 order rate constants ) are plotted against the reciprocal temperature (1/T) to 710 determine the Arrhenius factor A and the E/R value from its linear least-squares fit (20, 711 36-37).

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712 713 714 Table S2. Temperature Dependency of Kinetic Rates for Treatment of 0.02 mM RDX

- 715 with 4.20 mM MnO4 716

a a, b a a T kRDX1 kRDX2 kRDX2 Ln kRDX2 1/T (°C) (d-1) (L mol-1 d-1) (L mol-1 min-1) (1/K) 20 0.02 3.52 0.00 -6.01 0.0034 (0.00) (0.13) (0.00) (0.04) 35 0.06 14.22 0.01 -4.62 0.0032 (0.00) (0.21) (0.00) (0.01) 50 0.35 84.21 0.06 -2.84 0.0031 (0.01) (2.22) (0.00) (0.03) 65 0.89 212.65 0.15 -1.91 0.0030 717 (0.03) (7.16) (0.01) (0.03)

718 a Parenthetic values represent standard error of estimates. b kRDX2 = kRDX1/CMnO4- 719 720 721

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722 723 SI-8. Single electron transfer versus hydride (or hydrogen atom) removal 724 Based on supporting literature (38-40), two key ideas emerge: 725 1) Two different mechanisms are observed in amino oxidations 726 a) single-electron transfer (SET) at the amine nitrogen and 727 b) hydride (or hydrogen) abstraction from the carbon; 728 2) The electron density on the amine nitrogen determines the operative mechanism. 729 730 Specifically, electron-poor amines or those with resonance stabilized 731 intermediates tend to be oxidized by hydride abstraction. When these specific principles 732 and the principles of organic oxidation chemistry are applied to RDX, the problem 733 simplifies somewhat. For instance, there are only four distinct sites for oxidation of 734 RDX: an oxygen atom, a nitro nitrogen atom, an amine nitrogen atom, or a carbon. This 735 is illustrated below. 736 737 O A) oxygen B) nitro nitrogen 738 O2N N N N O 739

740 N C) amine nitrogen (Single Electron Transfer) 741 NO 2 D) carbon (loss of hydrogen atom or hydride ion) 742 743 744 Figure S15: Possible sites for oxidation of RDX. 745 746 Oxidation at the oxygen atom or a nitro nitrogen atom would give extremely 747 unstable intermediates since they place positive charge on electronegative oxygen or

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748 an already electron deficient nitro group nitrogen, respectively. The only two 749 reasonable sites for oxidation of RDX remaining are the exact two options carefully 750 studied by the cited authors (38-40). That is, oxidation at the amine nitrogen (by SET) 751 or oxidation at the carbon (by hydride abstraction, see S16, S17 below).

752 753 Figure S16: Overall comparison of SET versus hydride abstraction of RDX. 754 755 Because the amino nitrogens in RDX are extremely electron-poor, the hydride 756 loss will tend to dominate the reaction. This is probably because the intermediate 757 resulting from an initial single-electron transfer would produce an intermediate having 758 two positive charges on the adjacent nitrogen atoms, as shown above. Such an 759 intermediate would be much less stable than the proposed carbocation which 760 maximizes the distance of the two positive charges from each other, and places one of 761 them on the more electropositive carbon atom. 762 763 764

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765 766 Figure S17: A comparison of initial first steps via single electron transfer (A) versus 767 hydride removal (B). 768

- 769 When we considered both possible first-steps in the RDX-MnO4 reaction (SET 770 vs. Hydride loss, Fig. S15), we believe the strongly electron-withdrawing nitro groups 771 would tend to destabilize any cation intermediate. This destabilizing effect, however, 772 would be minimized for the carbocation intermediate formed via hydride abstraction (Fig 773 S17B) compared to the aminium ion formed by SET (Fig. S17A) because: 1) the 774 carbocation is further from the nitro group than the aminium ion and is therefore less 775 destabilized by inductive effects, 2) carbon is more electropositive than nitrogen, and 776 thus less destabilized by the cation, 3) resonance stabilization for the carbocation can 777 occur but is completely absent for the aminium intermediate. These theoretical 778 explanations are supported by the experimental observations that 1o, 2o and 3o 779 alkylamines having all their electron density isolated on the nitrogen tend to be oxidized 780 by SET (38-39), while amines with resonance distributed electron density like 781 benzylamine clearly undergo loss of hydride (or hydrogen atom) (40). Thus, theory and 782 experiment indicate that the carbocation intermediate will be more stable and therefore 783 formed more quickly than the aminium cation intermediate in RDX.

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784 785 SI-9. Proposed RDX degradation via proton abstraction

786 787

- 788 Figure S18: Proposed RDX degradation via proton abstraction and oxidation via MnO4 789 under alkaline pH.

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790 791 SI-10. References 792 (1) Ross, P.J.; Martin, A.E. A rapid procedure for preparing gas samples for 793 nitrogen-15 determination. Analyst 1970, 95(1134), 817-822. 794 (2) Sada, E.; Kumazawa, H.; Hayakawa, N. Absorption of NO in aqueous solutions

795 of KMnO4. Chem. Eng. Sci. 1977, 32(10), 1171-1175. 796 (3) Brogren, C.; Karlsson, H.T.; Bjerle, I. Absorption of NO in an alkaline solution of

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