Photoassisted Reaction of Sulfur Mustard under UV Light Irradiation

The photoassisted reaction of sulfur mustard (HD) in both the vapor and droplet states under UV light irradiation was investigated. It was found that ...
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Environ. Sci. Technol. 2005, 39, 8742-8746

Photoassisted Reaction of Sulfur Mustard under UV Light Irradiation GUO-MIN ZUO, ZHEN-XING CHENG,* GUO-WEN LI, LIAN-YUAN WANG, AND TING MIAO Contribution from the No.3 Department, Institute of Chemical Defense, P.O. Box 1048, Beijing, 102205, China

The photoassisted reaction of sulfur mustard (HD) in both the vapor and droplet states under UV light irradiation was investigated. It was found that HD molecules in either the gas or the condensed phase could be easily converted into other chemicals under the irradiation of a germicidal lamp. The products detected upon reaction suggested that the photoassisted reaction of HD molecules in the gas phase produced a kind of nontoxic heavy polymer, and this method seemed to be applicable for decontamination of air. Nevertheless, the photoassisted reaction of HD droplets would produce a series of products containing -SCH2CH2Cl or -OCH2CH2Cl groups, some of which were proven to be even more toxic than HD. Therefore, it was not an effective method for the decontamination of HD droplets. The obtained experimental results would indicate that two possible pathways might be involved in the destruction of HD molecules: (1) HD molecules may undergo a photochemical reaction upon absorbing photons of sufficient energy, which leads to cleavage the C-S bond in HD molecules at the primary step, or (2) HD molecules could be oxidized by the photogenerated ozone.

Introduction The chemical warfare agent sulfur mustard (bis(2-chloroethyl) sulfide or HD) is a highly toxic vesicant. It can destroy proteins and other components of living things via alkylation with the -SCH2CH2Cl group. Accordingly, detoxification methods of HD may include nucleophilic substitution, HCl elimination, and deep oxidation (1). The detoxification process mostly is fulfilled through destruction of the C-Cl bond, S-C bond, and even the whole molecule. Metal oxides such as MgO and CaO, especially nanosized, were known to effectively degrade HD adsorbed onto their surfaces through hydrolysis and elimination of HCl (2-4). In aqueous solution, another decontamination method based on oxidation was believed to be much more efficient, and HD could be oxidized rapidly into the nonvesicant sulfoxide and a small amount of vesicant sulfone (5-6). Over the past few decades, it emerged that photocatalysis had a favorable technical potential to mineralize a range of toxic substances and was regarded as the most promising technology to chemically purify polluted air (7). Attempts to use photocatalysis techniques to detoxify chemical warfare agents (CWAs) and their simulants were demonstrated to be principally feasible (8-15). Particularly, photocatalytic oxidation of an HD simulant such as 2-chloroethyl ethyl sulfide (2-CEES) (11, 13-16), 2-phenethyl-2-chloroethyl sulfide (10), * Corresponding author phone: 86-10-69760164; fax: 86-1069760161; e-mail: [email protected]. 8742

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as well as some other thioethers (17-19) were frequently reported. The experimental results indicated that photocatalytic degradation of 2-CEES mainly occurs through oxidation of sulfur, cleavage of the S-C bond, and oxidation of carbon (12). It should be noted that photodegradation of these substances was scarcely mentioned in the literature. It was reported that photodegradation was not a significant process to detoxify HD because HD did not absorb UV light above 290 nm (20). No data on actual photodegradation or reaction rates in the atmosphere were located. In this work, the photoassisted reaction of HD in vapor and droplets under UV light irradiation was studied. Gas chromatography/mass spectrometry (GC/MS), liquid chromatography/mass spectrometry (LC/MS), and NMR approaches were employed to identify the reaction products.

Experimental Section Materials. HD has a purity >95%. All solvents were of analytical grade. CAUTION: In view of its high toxicity, HD should be handled only by trained personnel using applicable safety procedures. Photoassisted Reaction of HD Vapor. A self-designed photoreactor was used to study the photoreaction of HD vapor, and a detailed description of the photoreactor has been reported previously (21). It was composed of a quartz light window of approximately 100 cm2 and a columnar stainless steel chamber 10 cm in diameter and 25 cm in length, both of which were adhered together with thermo-melting adhesive to form a space of approximately 3.3 L. HD vapor was generated by a method of static vacuum, i.e., to degas the reactor, to inject HD liquid into the chamber with a microsyringe, and then to introduce air upon complete vaporization. An 8 W germicidal lamp (type ZSZ8) with 253.7 nm peak density or a black lamp (type HSZ8) with 365 nm peak density was used as the illuminator. UV light was introduced into the reactor through the quartz light window. The irradiation intensity was determined with an ultraviolet irradiator (Photoelectric Instrument Factory of Beijing Normal University) close to the inner surface of the light window. The photoassisted reactions were carried out under the conditions of room temperature at 20-25 °C and relative humidity at 40-50%. Photoassisted Reaction of HD Droplets. The photoreaction of HD droplets was carried out in a glass tank as the photoreactor using a quartz glass plate as a light window to cover the tank. HD liquid (10 drops) was uniformly distributed onto a glass plate of 10 × 5 cm2 in the tank with a microsyringe. Each droplet had a volume around 1 µL and formed a round spot around 1-2 mm on the glass plate; separation distances for the droplets were kept around 1 cm to avoid overlapping each other. A ZSZ8 type germicidal lamp was used as a UV light illuminator to provide an irradiation intensity of 0.4 mW/cm2 at the bottom of the tank. Analysis of HD and Reaction Products. UV-vis absorption spectra of HD were obtained with a UV-vis spectrophotometer (Aglient-8453) with a scan range of 190-900 nm. Ethanol was used as a solvent and a blank. Quantitative analysis of HD in vapor or droplets was performed by a colorimetric method. For determining the concentration of HD in vapor, one could use a fresh dried airflow to purge the reactor, and the sweeping gas was absorbed by cooled ethanol. To determine the amount of HD in the droplets upon the photoreaction, the residue on the glass plate was also extracted by ethanol. The concentration of HD in the solution was measured by a colorimetric method. It should be noted that this method is based on a 10.1021/es050708j CCC: $30.25

 2005 American Chemical Society Published on Web 10/12/2005

FIGURE 1. UV-vis absorption spectra of HD in ethanol. Concentrations of HD in the solution are 0.1, 0.05, 0.025, and 0.01 M from top to bottom, respectively. The length of the quartz absorption cell is 10 mm. color reaction between -CH2CH2Cl in HD and thymolphthalein and some species containing -CH2CH2Cl might possibly lead to a positive reaction to enlarge the analysis results. For identification of the reaction products, both dichloromethane and methanol were used to extract the residue of HD droplets irradiated for 5 h. Dichloromethane extractant was analyzed using an Agilent 6890/5973 gas chromatograph/ mass selective detector, equipped with a HP-5 capillary column employing the temperature ramp 40-280 °C at 15 °C/min. And qualitative analysis of the methanol extractant was performed with an Agilent 1100 liquid chromatograph/ mass spectrometer, equipped with a nonpolar column (Zorbax SB-C18 column, 30 mm × 2.1 mm × 3 µm), with a mobile phase consisting of 0.1% acetic acid at a rate of 0.2 mL/min, column temperature 40 °C, and scan range over 50-2000 m/z. The residue of HD droplets irradiated for 5 h was washed into CDCl3. 13C{1H} and 1H NMR spectra were obtained using a Varian Mercury Vx300 NMR spectrometer. The observation frequencies were 75 MHz for 13C and 300 MHz for 1H. Chemical shifts were referenced to external CS2 (0 ppm) and TMS (0 ppm), respectively.

Results and Discussion UV-Vis Absorption Spectra of HD. As presented in Figure 1, 0.1 M HD in ethanol had two strong absorption bands in the UV region centered at 210 and 235 nm, respectively. When the concentration of HD in solution decreased, the absorption peak at 235 nm would sharply decrease until it vanished, while the peak at 210 nm only showed a small decrease. This suggested that the absorption at 235 nm was much weaker than that at 210 nm. Photoassisted Reaction of HD Vapor. As presented in Figure 2, when HD vapor was directly exposed to the UV light from germicidal lamp, the concentration of HD vapor quickly decreased, and 99% of HD vapor was eliminated after 3 h of irradiation. Furthermore, the elimination rate of HD vapor was observed to slow after 30 min; this was possibly due to the slow desorption of HD from the photoreactor wall and the slow diffusion of HD in the gas phase. However, black lamp irradiation showed nearly no effect on the elimination of HD vapor; this was probably because the photon energy of the black lamp was too weak to excite the HD molecules. Thus, the following photoassisted reaction experiments were carried out with a germicidal lamp as the illuminator.

FIGURE 2. Elimination of HD vapor in air under UV light irradiation. The initial concentration of HD was 0.42 mg/L; the irradiation intensity was 0.6 mW/cm2 for a germicidal lamp and 0.96 mW/cm2 for a black lamp.

FIGURE 3. Elimination of HD droplets in the presence/absence of UV light irradiation. The irradiation intensity upon HD droplets was 0.4 mW/cm2 from a 20 W germicidal lamp. Photoassisted Reaction of HD Droplets. It is known that HD is slightly volatile at room temperature (0.11 Torr at 20 °C, 0.95 mg/L) and HD droplets may slowly be vaporized and diffused into the surrounding air. The experimental results revealed that about 30% of HD droplets would be lost after stewing for 3 h in the tank, as shown in Figure 3. When UV light from the germicidal lamp irradiated the reactor through a quartz window, the elimination rate of HD droplets seemed to be accelerated. More than 70% of HD droplets were eliminated after being irradiated for 3 h. This suggested that UV irradiation might also lead to the destruction of HD molecules in the condensed phase. Photoassisted Reaction Products. It was experimentally demonstrated that HD vapor in air could be destructively eliminated under UV light irradiation, and some sticky substance (called “light window fog”) was observed to form on the inner surface of the quartz window. This substance, which was quite similar to a kind of polymer formed during the photoreaction, was nonvolatile and difficult to dissolve in solvents. Thereby, LC/MS and NMR methods were employed to identify the reaction product. However, it was not yet possible to obtain any information on the light window fog by LC/MS. This was possibly because its molecular weight was too great to pass through the liquid chromatographic column. The 1H NMR spectrum of the light window fog gave complex structural information, as presented in Figure 4. One could clearly observe a single resonance at 7.2 ppm, which likely indicated the existence of -OH groups. And two VOL. 39, NO. 22, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 4. 1H NMR spectrum of the light window fog.

FIGURE 5. GC/MS total ion chromatogram for the residue of HD droplets after 5 h of irradiation.

TABLE 1. Reaction Products Identified by GC/MS of HD Droplets Irradiated for 5 h no. 1 2 3 4 5 6

name

structural formula

bis(2-chloroethyl) disulfide 2-chloroethyl dichloroethyl disulfide 2-chloroethyl 2-chloroethoxyethyl disulfide bis(2-chloroethyl) trisulfide sesquimustard 2-chloroethyl 2-chloroethylthioethyl disulfide

ClCH2CH2SSCH2CH2Cl Cl2C2H3SSCH2CH2Cl ClCH2CH2SSCH2CH2OCH2CH2Cl ClCH2CH2SSSCH2CH2Cl ClCH2CH2SCH2CH2SCH2CH2Cl ClCH2CH2SCH2CH2SSCH2CH2Cl

peak groups appeared at 2.8 and 3.6 ppm, which were quite similar to the resonances of the HD molecule, and this would suggest that some species containing -CH2CH2- groups might still remain in the products. The light window fog also yielded several broad peaks less than 2.0 ppm, which were quite similar to those of a macromolecular substance or even a polymer. Therefore, the light window fog appeared to be a mixture of photoreaction products upon UV light irradiation of the HD vapor, and a kind of polymer might possibly be produced as a final product. The photodegradation residue of the HD droplets was also extracted with CDCl3 and analyzed in the same way by NMR spectrometer. Both 13C NMR and 1H NMR spectra of the residue of HD droplets being irradiated for 5 h were closely similar to that of the neat HD. It seemed that the -CH2CH2Cl group was not significantly destroyed during the photoassisted reaction of HD in the condensed phase. 8744

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The GC/MS spectrum with total ion chromatography of HD droplets irradiated for 5 h is presented in Figure 5. Several compounds as the photoreaction products were distinctively detected and identified, and the possible structural formulas determined on the basis of retention time and mass spectra are presented in Table 1. It was notable that all of the identified species contained -SCH2CH2Cl or -OCH2CH2Cl groups, and this was quite consistent with the obtained NMR analysis results. Thus it can be seen that photoassisted reaction of liquid HD molecules under UV light irradiation should possibly occur via cleavage of S-C bonds, instead of significant destruction of the -CH2CH2Cl group. In addition, these products were also known to have a positive reaction against thymolphthalein as in the colorimetric method used in Figure 3, and it would enlarge the quantitative analysis results of HD in the droplets. Possibly, the detection of HD in Figure 3

FIGURE 6. Liquid chromatogram with extracted ion chromatograms for HD droplets after 5 h of irradiation.

TABLE 2. Reaction Products Identified by LC/MS of HD Droplets Irradiated for 5 h no.

name

structural formula

mass

1 2

2-chloroethyl sulfoacid 1,4-dithiocyclohexane 1-oxide

ClCH2CH2S(O2)OH

M - H+)143 M + H+)137

3

mustard sulfoxide

ClCH2CH2S(O)CH2CH2Cl

M + H+)175

after 3 h of irradiation was mainly attributable to the reaction products containing -CH2CH2Cl groups. Polar compounds involved in the products were analyzed by LC/MS equipped with a nonpolar column. The obtained liquid chromatogram with the extracted ion chromatograms (M + H+ and M - H+) is presented in Figure 6. One could observe that three species were detected in this chromatogram. THe LC/MS spectrum mostly provides only the molecular ion (M + H+ or M - H+). One could only analyze these compounds based on the retention time in the liquid chromatographic column and the M ( H+ peaks. The possible structural formulas of the detected species were therefore deduced, as presented in Table 2. This provided an evidence for the oxidation of sulfur in HD and its derivatives by the photogenerated ozone. Thus it can be seen that the photoassisted reaction of HD in the condensed phase only resulted in a series of products with light molecular weights, while it produced significant polymerization to produce a polymer-like light window fog in the gas phase. For a photoreaction in the condensed phase, the exited molecule might easily be quenched by the surrounding molecules, which might possibly decrease the extent of the reaction. However, the light window fog has no positive reaction against thymolphthalein, which indicated that it has no vesicant toxicity. Nevertheless, most of the identified products for the photoassisted reaction of HD droplets have -SCH2CH2Cl or -OCH2CH2Cl groups, which indicated that they would still have vesicant toxicity. Some species such as bis(2-chloroethyl) disulfide and sesquimustard were known to be even more toxic than HD. It followed that UV light irradiation could sufficiently eliminate HD vapor and form nontoxic polymers in the gas phase, which seemed to be applicable for air decontamination, but it was still not an effective method for decontamination of HD droplets. Aspects of the Mechanism. As previously demonstrated by experimental results, molecules of HD in either the gas or the condensed phase would be easily converted into other chemicals under UV light irradiation. The detected reaction products strongly suggested that the photoassisted reaction of HD molecules in the gas phase produced a kind of heavy polymer, whereas that in the condensed phase only went on until it formed a series of species upon cleavage of the C-S

bond. Besides, the identified oxidized products show that photogenerated ozone was also involved in the photodegradation pathway. Therefore, two main pathways to destroy HD molecules in both the gas phase and the condensed phase under UV light irradiation might be involved. First, HD molecules could absorb a photon with sufficient energy and transit to an excited state and may subsequently undergo a photoreaction via cleavage of C-S bonds in the primary step. hv

ClCH2CH2SCH2CH2Cl 98 ClCH2CH2S• + •CH2CH2Cl (1) As a free-radical-initiated reaction, the secondary photochemical process would be extremely complicated, and it was difficult to go into detail upon the whole reaction process. Second, UV light is able to activate O2 in air to produce O3, which could also lead to the destruction of HD molecules and produce the corresponding sulfoxide. O3

ClCH2CH2SCH2CH2Cl 98 ClCH2CH2(SdO)CH2CH2Cl + O2 (2)

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Received for review April 13, 2005. Revised manuscript received September 12, 2005. Accepted September 12, 2005. ES050708J