Decomposition of Adsorbed VX on Activated Carbons Studied by 31P

For adsorption experiments, carbon granules of diameter 0.1−2.8 mm, which fitted ... Chemical shifts were referenced to external trimethyl phosphate...
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Environ. Sci. Technol. 2006, 40, 3952-3958

Decomposition of Adsorbed VX on Activated Carbons Studied by 31P MAS NMR ISHAY COLUMBUS,† DANIEL WAYSBORT,† LIORA SHMUELI,‡ IDO NIR,‡ AND D O R O N K A P L A N * ,‡ Departments of Organic Chemistry and Physical Chemistry, Israel Institute for Biological Research, P.O. Box 19, Ness Ziona 74100, Israel

The fate of the persistent OP nerve agent O-ethyl S-[2(diisopropylamino)ethyl] methylphosphonothioate (VX) on granular activated carbons that are used for gas filtration was studied by means of 31P magic angle spinning (MAS) NMR spectroscopy. VX as vapor or liquid was adsorbed on carbon granules, and MAS NMR spectra were recorded periodically. The results show that at least 90% of the adsorbed VX decomposes within 20 days or less to the nontoxic ethyl methylphosphonic acid (EMPA) and bis(S-2-diisopropylaminoethane) {(DES)2}. Decomposition occurred irrespective of the phase from which VX was loaded, the presence of metal impregnation on the carbon surface, and the water content of the carbon. Theoretical and practical aspects of the degradation are discussed.

1. Introduction The fate of adsorbed organophosphorus (OP) chemical warfare agents (CWA) on activated carbon is extremely important to personnel in charge of the disposal of used chemical protective equipment. OP-CWA (nerve agents) are strongly physisorbed on activated carbon, due to their relatively low vapor pressure and their high affinity toward surfaces. Hence, carbon filters effectively remove nerve agents from air. On the other hand, toxic substances that are retained by physical interactions alone are liable to pose environmental risks such as desorption of the toxic vapors into the atmosphere, release of contaminated carbon dust formed by attrition and wear, and pollution of groundwater by contact of the contaminated carbon granules with ambient water and precipitation. To evaluate the potential risks and choose an appropriate disposal procedure, the fate of the adsorbed toxic agent must be known. So far, only scarce information about adsorbed nerve agents on activated carbon has been published (1-3). The present study is focused on the fate of the persistent nerve agent O-ethyl S-[2-(diisopropylamino)ethyl] methylphosphonothioate (VX) on activated carbon. In acidic, neutral, or weakly basic aqueous solutions, the hydrolysis of VX is very slow (4). In strongly basic solutions the hydrolysis is rapid; however, one of the products is the highly toxic S-[2-(diisopropylamino)ethyl] methylphosphonothioate (EA2192) (4). Recent studies have shown that dispersed VX on various solid matrixes, including naturally occuring sands and concrete, decomposes to ethyl methylphosphonic acid * Corresponding author e-mail: [email protected]. † Department of Organic Chemistry ‡ Department of Physical Chemistry. 3952

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(EMPA) within days (5, 6). It is therefore conceivable that VX would also decompose when adsorbed on activated carbon. Apart from some very recent data (3), no information on the stability of VX on carbon is available. We present here a 31P magic angle spinning (MAS) NMR study of the fate of VX on activated carbons, including, for the first time, an impregnated carbon (ASC-whetlerite) that is extensively used for removal of CWA from the atmosphere. High concentrations of VX on carbon, of the order of the saturation concentration that exists in the front-end carbon layer in a filter, were created. MAS NMR was chosen to monitor the adsorbate directly and rapidly. The effects of the phase (vapor or liquid) from which VX was loaded and the relative humidity on the stability of the adsorbed VX were investigated.

2. Experimental Section Activated Carbons. The following activated carbons were used: BPL (coal-based, nonimpregnated, 6 × 16 mesh size, a product of Calgon, United States), ASC (whetlerite; manufactured from BPL by impregnation with salts of chromium, copper, and silver; 6 × 16 or 12 × 30 mesh size; Calgon, United States), and PICACTIF-TA EXTRA (nutshell-based, nonimpregnated; 12 × 20 mesh size; a product of PICA, France). These carbons are designed primarily for gas filtration and are highly microporous. Literature micropore volumes are 0.43 and 0.33 cm3/g for BPL and ASC carbons, respectively (7). The micropore volume of the PICACTIF carbon was found to be ≈0.47 cm3/g. Carbons were ovendried at 105-110 °C for at least 3 h and were stored in screwcap Pyrex-glass jars until used. For adsorption experiments, carbon granules of diameter 0.1-2.8 mm, which fitted inside the NMR rotor, were chosen. Some experiments were performed with a modified carbon (designated M-ASC), prepared by removal of the impregnation compounds from ASC carbon. The protocol was adapted from the standard analysis (8) of a similar type of impregnated carbon (ASZM-TEDA; Calgon, United States). The carbon was refluxed (3×) with aqueous 10% HCl and then washed (6×) with boiling deionized water until the liquid phase was colorless. The carbon residue was oven-dried for ≈1 h at 105 °C and for 17 h at 100-150 °C under vacuum (p e 1 Torr). Humidification. Activated carbon samples were exposed to constant relative humidities (RH) of 22%, 53%, or 75% in a closed jar containing a saturated aqueous solution of CH3COOK, Mg(NO3)2, or NaCl, respectively. Water uptake was determined from weight increment of the carbon sample ((0.0001 g). VX Adsorption from the Vapor Phase. Dilute VX vapors in air were generated and dynamically loaded onto carbon in a compact flow apparatus. The generator comprised a glass-wool plug in a short tube. A volume of 50-100 µL of VX was dispersed into the wool plug. A quantity of ca. 300 mg of carbon was packed into a short adsorption column (diameter 2 cm, height 0.2 cm) connected directly to the generator. A circle of HEPA filter paper was installed between the generator and the carbon layer, to retain VX droplets. A flow (2 L/min) of dry, warm air (dew point -77.8 °C, temperature 45 °C) was passed through the apparatus. The exterior of the apparatus was heated to 40 °C. Within 24-48 h, the carbon layer adsorbed enough VX (of the order of 10 mg) for acceptable 31P MAS NMR spectra (see below). Postevaporation 31P NMR analysis of the residue in the generator revealed that no significant decomposition of VX had occurred. 10.1021/es052226d CCC: $33.50

 2006 American Chemical Society Published on Web 05/12/2006

FIGURE 1. Selected 31P MAS NMR spectra of adsorbed VX vapor on dry BPL carbon. Spectra were recorded at (a) 1 h, (b) 4 d, and (c) 13 d after sample preparation. Adsorption from the Liquid Phase. An ca. 25 mg sample of carbon granules was packed into a 4 mm diameter ZrO2 MAS NMR rotor. Neat VX (ca. 3 mg) was applied via syringe onto the center of the sample. The rotor was closed with a tightly fitting Kel-F cap. When maximum dryness of the sample was sought, the above operations were performed in a glovebox under dry nitrogen. NMR. 31P MAS NMR spectra were obtained at 202 MHz on a 500 MHz Bruker Avance spectrometer, equipped with a 4 mm standard CP MAS probe, using direct excitation (no CP). The spinning rate was 8 kHz. Chemical shifts were referenced to external trimethyl phosphate (TMP) as 0 ppm. Pulse delay was 2 s, which is considered sufficient for relaxation since (a) literature values of the 31P relaxation times (T1) of similar-sized molecules on a solid matrix (concrete) are about 0.3 s or less (9) and (b) pulse delay times of 1-2 s were found to be sufficient for 31P relaxation in OP esters on carbon (10). The number of transients per spectrum varied between 200 and 1000, depending on the carbon type and the concentration of adsorbed VX. For comparison purposes, spectra were recorded in identical conditions.

3. Results and Discussion VX on BPL Carbon. Spectra of VX loaded from the vapor phase onto BPL carbon are shown in Figure 1. In the first spectrum (Figure 1a) a broad (δυ1/2 ≈ 1300 Hz) VX signal is located at 47 ppm. By comparison, literature chemical shifts of VX in organic solutions and adsorbed VX on alumina, silica, or CaO are 50-53 ppm (5, 11, 12). Since the volume of loaded VX was less than the micropore volume, we attribute the above resonance line to adsorbed VX in the micropores. Our assignment is in line with previous assignments of MAS NMR spectra of adsorbates on microporous carbons (10, 13-15). The upfield shift of the adsorbate resonance relative to the pure substance resonance is caused by shielding of the 31P nucleus by aromatic ring systems on the pore wall. The width of the adsorbate resonance line is a result of the superposition of exchange-broadened resonance lines, which originate from a distribution of pores of various sizes and having various arrangements of aromatic rings on the surface (15). Degradation of Adsorbed VX. The signal of adsorbed VX on BPL carbon disappears with time, and a broad signal (δυ1/2 ≈ 1700 Hz) due to a reaction product appears at 16-19 ppm (Figure 1a-c). The reaction products were extracted from the carbon with solvents and were identified in solution by means of NMR and GC-MS analysis. The 31P NMR spectrum of an ethanolic extract of the carbon consisted of a single peak that was assigned to ethyl methylphosphonic acid (EMPA). The assignment was verified by spiking the ethanolic solution with authentic EMPA. Notably, there was

SCHEME 1. Decomposition of VX on Activated Carbon

no evidence for the toxic byproduct EA-2192. The organosulfur degradation product of VX was identified as bis(S-2diisopropylaminoethane) {(DES)2} on the basis of 1H and 13C NMR spectra and GC-MS analysis of the extract. It thus appears that adsorbed VX in the micropores on BPL carbon decomposes predominantly to EMPA and diisopropylaminoethanethiol (DESH), which is quickly oxidized and transformed to (DES)2 (Scheme 1). The rapid transformation of DESH to (DES)2 on carbon was separately confirmed by loading authentic DESH onto a BPL carbon sample. The sample was extracted with chloroform after 3 days. The only discernible signals in the 1H and 13C NMR spectra of the extract were due to (DES)2. The finding that the product of VX is (DES)2, rather than a sulfonate, rules out the possibility that VX is oxidized on carbon. The intensity of the EMPA signal in the 31P MAS NMR spectra (Figure 1c) was found to be slightly lower than that of the parent VX signal. This effect appears to be the result of EMPA-surface interactions, as will be explained later. In a mixed solution of EMPA and DESH (or (DES)2), the respective 31P and 13C NMR spectra were found to manifest the existence of intermolecular interactions between the two compounds. For example, the 31P NMR chemical shifts of authentic EMPA (in CDCl3) in the absence and presence of 1:1 molar DESH were found to be 29.9 and 19.4 ppm, respectively. Presumably, EMPA forms hydrogen bonds with the amine group of DESH in solution. The same hydrogen bonds are perhaps formed in the VX decomposition mixture on carbon. Adsorption of Liquid vs Vapor. In contradistinction to the spectra of adsorbed VX vapor, spectra of freshly prepared liquid VX on BPL carbon (Figure 2a,b) displayed two partially overlapping signals of adsorbed VX at 48 ppm (δI; δυ1/2 ≈ 900 Hz) and 55 ppm (δII; δυ1/2 ≈ 300 Hz). These signals represent VX populations in two magnetically distinct groups of adsorption sites. Following a previous interpretation of the VOL. 40, NO. 12, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 2. Selected 31P MAS NMR spectra of adsorbed VX liquid on dry BPL carbon. Spectra were recorded at (a) 25 min, (b) 35 min, (c) 85 min, and (d) 225 min after sample preparation.

FIGURE 3. Selected 31P MAS NMR spectra of adsorbed VX liquid on dry PICACTIF carbon. Spectra were recorded at (a) 20 min, (b) 140 min, (c) 18 h, (d) 25 h, and (e) 4 d after sample preparation. spectra of liquid OP esters on microporous carbons (15), we assign δI to VX in the micropores (mainly) and the mesopores and δII to VX in the macropores and on external granule surfaces. The chemical shift of δII reflects a relatively weak shielding since in these sites the adsorbate molecules are weakly adsorbed and are remote from the aromatic rings (15). In nearly all of our carbon samples, the intensity of δI was found to be higher (usually by a factor of g3) than the intensity of δII, indicating that most of the adsorbate resides in micropores. The δI/δII ratio varied from sample to sample. This result was expected since the adsorbate had been applied in the form of a small drop onto a relatively large layer of carbon granules. With time, both δI and δII disappeared gradually, and a broad EMPA signal emerged (Figure 2b-d). The overall time of conversion of VX to EMPA was variable, as will be described 3954

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later. In most cases, such as the one shown in Figure 2, δII vanished at a faster rate than δI. This could mean that the two adsorption sites differ in decomposition kinetics. In addition, the δII signal might have decayed by migration of the adsorbate to the micropores, as previously observed for other OP molecules on activated carbons (14). VX on Other Nonimpregnated Carbons. The behavior of adsorbed VX on other types of carbon was studied in order to ascertain that the degradation to relatively nontoxic products is not unique to BPL. VX was loaded from the liquid phase onto PICACTIF-TA and M-ASC carbons, and spectra were recorded periodically. A spectrum of VX on PICACTIF carbon is shown in Figure 3. For both carbons, an initial distribution of VX between two types of adsorption sites and gradual decomposition of VX to EMPA were observed.

FIGURE 4. Selected 31P MAS NMR spectra of adsorbed VX vapor on dry ASC carbon. Spectra were recorded at (a) 1 h, (b) 3 d, and (c) 7 d after sample preparation.

FIGURE 5. Selected 31P MAS NMR spectra of adsorbed VX liquid on dry ASC carbon. Spectra were recorded at (a) 65 min, (b) 140 min, (c) 21 h, (d) 46 h, (e) 3 d, (f) 6 d, (g) 9 d, (h) 13 d, and (i) 20 d after sample preparation. VX on ASC Carbon. The fate of VX on whetlerites is of special interest, in view of their wide use in filters for the removal of CWA from air (16). The impregnation neutralizes weakly adsorbed vapors, such as hydrogen cyanide and cyanogen chloride, by chemical reaction. Spectra of VX loaded

from the vapor and liquid phases on ASC carbon are shown in Figures 4 and 5, respectively. The spectra feature a low S/N and broad lines, relative to the spectra of VX on nonimpregnated carbons. In the initial spectrum of adsorbed VX liquid on ASC carbon (Figure 5a), VOL. 40, NO. 12, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 6. 31P MAS NMR spectra of adsorbed EMPA liquid (3 µL/50 mg) on dry BPL (a) and ASC (b) carbons. The left-side shoulder at 29 ppm in (a) is assigned to adsorbate in the macropores and on external granule surfaces. Spectra were obtained under identical conditions. line broadenings caused an extensive overlap of δI with δII. We attribute these broadenings to the presence of the paramagnetic Cr3+ and Cu2+ ions. VX on ASC carbon decomposed gradually to EMPA, which appeared as a weak, broad signal at 13-16 ppm (Figures 4 and 5). Thus, the presence of the ASC impregnation compounds does not affect the decomposition path. Figures 4 and 5 show that the resonance of EMPA on ASC carbon is dramatically broadened and attenuated. This phenomenon is not related to VX or its other decomposition products (DESH, etc.), since it was observed with pure EMPA, as shown by the spectra of neat EMPA on BPL and ASC carbons, respectively (Figure 6). Wagner et al. claimed that the MAS NMR signals of EMPA on concrete are attenuated by strong interactions between EMPA and the solid matrix (5, 9). Likewise, we attribute the attenuation of the EMPA signal to strong interactions between EMPA and the impregnation compounds on the surface of ASC carbon. Presumably, such interactions limit the motional freedom of the adsorbate and thereby broaden the NMR signal. In addition, we hypothesize that the EMPA-surface interactions on ASC carbon shorten the distance between the 31P nucleus and the Cr3+ and/or Cu2+ ions, thereby enhancing paramagnetic broadening. The extreme broadening of the EMPA signal ruled out the evaluation of mass balance from the spectra of decomposing VX on ASC carbon. Degradation Times of Adsorbed VX on Dry Activated Carbons. The time periods t0.5 and t0.9 from the first recording of the spectrum up to 50% and 90% decompositions, respectively, of the adsorbed VX were taken as indicators of the VX decomposition rate. For most of the t0.5 estimations, the percentage of decomposition was taken as the ratio of the area under the VX peaks and the sum of areas under the peaks of VX and EMPA (method I). In cases where the EMPA peak partially overlapped with the right-hand spinning sideband of the VX peak, the sideband area was subtracted from the area of the EMPA peak. In spectra where the S/N declined progressively, such as spectra of VX on ASC carbon, the percentage of decomposition was taken as the ratio of the area under the residual VX peaks and the area under the VX peaks in the first recorded spectrum (method II). Values of t0.9 were evaluated exclusively by method II. The uncertainty in the t0.5 and t0.9 values is estimated as (20%. Not all samples provided both t0.5 and t0.9. The ratio t0.9/t0.5, taken as a rough measure of the reaction order, was calculated from t0.5 and t0.9 extracted from the same set of spectra. For a first-order reaction, t0.9/t0.5 ≈ 3.3. Decay times of adsorbed VX vapor and VX liquid on dried activated carbons are summarized in Tables 1 and 2, respectively. The degradation time data of adsorbed VX vapor (Table 1) indicate that about 90% of the VX population in micropores 3956

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TABLE 1. Degradation Times of Adsorbed VX Vapor on Dried Activated Carbonsa BPL db

t0.5 t0.9 t0.9/t0.5

3 d, 6 13 d, 16 db 3-4

ASC not observed 7 d, 7 d

a Carbon granule diameters were 0.7-1.3 mm. dried for 3 h in a vacuum at 100-150 °C.

b

The sample was

TABLE 2. Degradation Times of Adsorbed VX Liquid on Dried Activated Carbonsa BPL

PICACTIF-TA

M-ASC

ASC

t0.5

0.3-1 h (5) 2-5 d (4)

1 h, 2 h (2)

1 h, 1 d (2)

1-6 h (3) 2-7 d (7)

t0.9

1-4 h (4) 2-12 d (4b)

1 d, 4 d (2)

4 d, 6 d (2)

2.5-7 h (3) 1-20 d (9)

t0.9/t0.5

1.8-2 (4) 2.5-50 (4)

12, 96 (2)

6, 96 (2)

1.1-1.5 (2) 3-8 (8)

a Carbon granule diameters were 0.1-2.8 mm. Data are divided into subgroups for readability (see text). The number of determinations per subgroup is given in parentheses. b Two samples were dried for 3 h at 100-150 °C in a vacuum (e1 Torr) prior to VX loading. For these samples, t0.9 was 10-12 d.

decomposes within about 2 weeks, even when the carbon had been extensively dried. The data for adsorbed VX liquid (Table 2) span a wide range and include mostly long degradation times (days) as well as short degradation times (hours). Except for two samples of VX liquid on ASC carbon, which gave a t0.9/t0.5 of 1.1-1.5, the t0.9/t0.5 data of Table 1 and Table 2 point against autocatalytic degradation. However, the most important conclusion from Tables 1 and 2 is that, for all carbons, 50% and 90% decompositions are reached within e7 d and e20 d, respectively, irrespective of the phase (vapor/liquid) from which VX had been adsorbed and the extent of predrying. The variability of the measured degradation times for adsorbed VX liquid may have been caused by the initial nonuniform spreading of VX across the carbon granules since a later redistribution would be unlikely. Further variability of the degradation rates could be introduced by the variable distribution of VX among the two groups of adsorption sites, as observed from the variable δI/δII ratio in the MAS NMR spectra. A detailed analysis of the degradation of specific adsorbate populations was considered impractical since, at least for slowly decomposing samples, the VX population in the macropores probably declines by migration into the micropores, as mentioned above. Previously, migration times

of about 18 h for OP esters on a coconut-based activated carbon have been reported (14). A comparison of the degradation times of vapor-loaded VX (Table 1) with the degradation times of liquid-loaded VX (Table 2) provides some insight into the decomposition kinetics. In nearly all of our samples, the majority of adsorbed VX initially occupied the micropores. Yet the micropore-VX degradation times (Table 1) fall among the subgroup of long degradation times of adsorbed VX liquid (Table 2). Thus, decomposition appears to be slowest in the micropores, although in these pores a relatively large fraction of the adsorbate molecules is in direct contact with the surface. We explain this apparent discrepancy in terms of pore blocking by EMPA and (DES)2. Since these compounds are less volatile than VX, they are probably better adsorbed than VX. Once formed by VX decomposition, they will stay attached to the pore wall near the decomposition site and effectively block the passage of other adsorbate molecules. Since the degradation products of VX on activated carbon are EMPA and (DES)2, it would seem logical to assume that the degradation reaction is hydrolysis. Stoichiometric hydrolysis of the amount of VX in our carbon samples would require a water content of 0.2-0.6%. The presence of such quantities of water in our samples could not be excluded, either because of incomplete predrying of the carbon or due to adsorption of atmospheric humidity during sample preparation. In addition, water is the byproduct of the oxidation of DESH to (DES)2. In an equimolar mixture of neat VX and water, byproduct EMPA reacts with another VX molecule, which then decomposes to EMPA, etc., in an autocatalytic process (17). On carbon, strong binding of EMPA to the micropore walls, as described above, would likely interfere with the above reaction. Interference by accumulated surface-bound EMPA can explain the cases of decreased degradation rates (high t0.9/t0.5) in our experiments (Table 2). Since degradation of VX was observed even on vacuumdried carbon samples, we cannot exclude the possibility of direct reaction between adsorbed VX and oxygenated active groups that are part of the carbon surface. Conceivably, such reaction could be triggered by traces of water and proceed catalytically afterward. We note, however, that the same degradation occurred with activated carbons differing from each other in the composition of surface groups and in basicity. More information about the surface chemistry of these carbons is required for further elucidation of the reaction mechanism. Influence of Relative Humidity. In realistic scenarios of respiratory protection against toxic vapors, the carbon layer of the filter may contain pre-adsorbed water, due to previous exposures of the equipment to ambient humid air, as well as co-adsorbed water. Hence, the question, whether the presence of water accelerates the degradation of adsorbed VX on carbon, is highly relevant to the disposal of chemical protective equipment. In the present study, data on the degradation of adsorbed VX liquid on ASC carbon, that was exposed to humid air (RH ) 22-75%) either before or after VX loading were acquired. The spectra of VX on humidified carbon resembled the spectra of VX on dry carbon (Figure 5), except for the occasional appearance of an additional weak, narrow VX signal, at about 63 ppm, in the first spectrum after sample preparation. This signal is attributed to solubilized VX in water in macropores and on the external granule surfaces (5). The same gradual degradation of VX to EMPA as with dry carbon was observed. No EA-2192 was detected. Decay times of adsorbed VX on ASC carbon under humid conditions are given in Table 3. The results show that, in the presence of adsorbed water, the t0.5 values are in the range of 1-6 days and the t0.9 values

TABLE 3. Decay Times of Adsorbed VX Liquid on Humidified ASC Carbona sample no.

water content (%) prior to VX loading

RH (%)

1b 2 3c 4 5 6c 7c 8c

0-2 1-2 0-2 18 32 0-2 0-2 0-2

0 22 53 53 75 75 75 75

t0.5 (d) 2 4 4 1 3-6