Decontamination of mustard sulfur and VX by sodium percarbonate complexed with 1-acetylguanidine as a novel activator

The decontamination of chemical warfare agents (CWAs) is of great importance for nuclear, biological and chemical defense as well as counter-terrorism (Yang et al. 1992). In recent years, the chemical weapon was still sporadically used, such as the chemical attack that occurred on August 21, 2013 in the city of Damascus, Syria (Hakeem & Jabri 2015). In 2017, Novichok, as a series of nerve agents, entered into people′s vision, which might be similar in toxicity to VX (O-ethyl-S-(2-diisopropylamino-ethyl)-methyl phosphonothioate) (Nepovimova & Kuca 2018; Kloske & Witkiewicz 2019). Therefore, many martial scientists still paid great attention to decontaminating chemical warfare agents (Love et al. 2011; Zhao et al. 2018; Bigley & Raushel 2019).

It's a trend that the peroxide-based decontaminants were preferred by the researchers because of their high performance, non-corrosive and environmental-friendly merits. It has been clarified that the decontamination performance of inorganic peroxides, like H₂O₂, sodium percarbonate, sodium perborate, etc., was remarkably poorer than that of the organic peroxides, like magnesium monoperoxyphthalate (MMPP) (Lion et al. 1997). To achieve high decontamination efficiency and stability, a series of decontamination formula based on inorganic peroxides were developed where the catalysts, activators and surfactants were added, such as Decon Green (Wagner et al. 2001, 2002), DF-200 decontamination foam (Tucker & Comstock 2004) and the sodium percarbonate/tetraacetylethylenediamine (SPC/TAED) (Davies & Deary 1991; Qi et al. 2013) combined decontaminant. As an example, Decon Green as an environmentally benign decontaminant mainly composed of H₂O₂, potassium carbonate, potassium molybdate, propylene carbonate, and Triton X-100 additives (Wagner et al. 2001). The DF-200 decontamination foam developed by Sadia National Laboratory applied H₂O₂ and glycerol diacetate as activator, this formula can generate peracetic acid ion in aqueous solution (Magnaud et al. 1998; Tucker & Comstock 2004), which was quite effective toward decontaminating soman (GD), sulfur mustard (HD) and VX. SPC as a solid form of hydrogen peroxide (Zhang et al. 2021) can react with TAED in aqueous solution to yield peracetic acid as a stronger oxidant which was used to decontaminate CWAs (Davies & Deary 1991; Qi et al. 2013). It's well known peroxide hydrogen and TAED as a bleaching agent had been widely used in treating fabrics. What's noted that, it has been found that the addition of an asymmetrically substituted alkyl or 1-acetylguanidine compound to the peroxide bleaching bath significantly enhances the peroxide bleaching performance, leading to a whiter and brighter end-product (Cai 2007). Compared with TAED, the guanidine-based bleaching activators would be apparent as one of the most popular activators on the cellulosic materials market. Therefore, guanidine derivatives as a potential activator with SPC might be applied for the decontamination of CWAs.

In this work, an aqueous decontamination formula based on the activation reaction of SPC and 1-acylguanidine (ACG) was researched for degradation of chemical warfare agents HD and VX. The active species was detected by Raman and ¹³C NMR spectroscopy. Some factors were optimized, such as the molar ratio of active to ACG ((O)/(ACG)), pH value, molar ratio of active oxygen to CWA ((O)/(CWA)), reaction temperature for decontamination efficiency and so on. The degradation pathways of HD and VX were discussed based on the detected degradation products.

The toxic agents of HD and VX had a purity >95%, which were all attained from Institute of NBC Defense. Sodium percarbonate with 13% active oxygen content was purchased from Jinke Chemical Industry Co., Ltd in Zhejiang; ACG was obtained from Fisher Chemicals (Leicester, UK); Dichloromethane (GC grade) as solvent was obtained from Bafang Agents Inc. (Beijing, China). Because of their high toxicity, chemical warfare agents HD and VX were handled only by trained personnel using appropriate safety procedures.

In a glass vial of 20 mL, 10 mL 0.3 mol/L–0.5 mol/L of SPC, ACG, SPC/ACG and peracetic acid solution were detected by Raman spectroscopy separately, in order to identify the generated active species at the room temperature (20 ± 5 °C). Raman measurements were performed on a portable Raman spectrometer (RFS1000, Shanghai Oceanhood Optoelectronics Tech Co., LTD) equipped with 785 nm laser source.

All the samples were submitted to ¹³C NMR (AVANCE III HD 500 MHz, Bruker German) experiment, such as ACG and SPC/ACG solution. Samples of all substances were prepared in 5 mm NMR tubes, at concentration ranges between 40 and 150 mg/mL.

The solid decontaminant was mainly composed of SPC, ACG and sodium phosphate buffer. The decontaminant solutions with difference concentration and pH value were prepared according to the different ratio of all ingredients. For the decontamination reaction, each decontamination ingredient was placed into a 50 mL flask and stirred for 5 min at 1,320 r/min under a certain temperature to achieve thermal equilibrium. Then, HD or VX liquid was injected into the flask with a micro-syringe under continuous stirring. Finally, sampling and sample preparation were accomplished at the different reaction time (e.g. 2, 5 and 20 min) in order to determine the decontamination efficiency and degradation products. Three drops of 10% sodium thiosulfate (Na₂S₂O₃) solution as terminator was added into each sample to neutralized the redundant peroxides before extraction.

The residual CWAs and their non-polar degradation products could be quickly separated from the mixture of decontamination solution by organic solvent extraction (Davies & Deary 1991; Qi et al. 2012). To determine the residual HD and VX, the extraction agents ligarine and dichloromethane were used, respectively. The volume ratio of decontamination solution and extraction agent was 1:1, and the extraction time was 10 min. After the organic and aqueous phases were well separated, the organic solvent was sucked out for analysis. The residual HD in ligarine was quantitatively analyzed with a colorimetric method (Franke 1977). The residual VX in dichloromethane was quantitatively analyzed by an Agilent 6890N gas chromatography (Agilent Technologies, Palo Alto, CA, USA), equipped with a flame ionization detector (FID) and a HP-5 capillary column (30 m × 0.32 mm × 0.25 μm), employing the temperature ramp 70–280 °C at 15 °C/min (Qi et al. 2012).

For identification of the degradation products of HD and VX, dichloromethane was also used to extract the non-polar species in the decontamination solution. The samples were analyzed by an Agilent 7890A/5975C GC/MS, equipped with a HP-5-MS (30 m × 0.25 mm × 0.25 μm) capillary column employing the temperature ramp 40–280 °C at 15 °C/min. The polar degradation products in the decontamination solution were detected and identified by LC/MS. Firstly, the final solution was dried in a gentle stream of nitrogen, then the solid residue was extracted with methanol. The extractant was analyzed with an Agilent 1100 LC/MS equipped with a non-polar column (Zorbax SBC18 column, 30 mm × 2.1 mm × 3 μm). The mobile phase was 0.1% acetic acid aqueous solution at a rate of 0.2 mL/min, the column temperature was 40 °C, and the scan range was 50–1,000 m/z. In addition, the polar organic degradation products of VX were also analyzed by a pre-column derivatization method, where the aqueous sample was evaporated to dryness under a N₂ stream, the residue was alkylated via diazomethane (CH₂N₂) as an alkylation agent (Moore & Reed 1973), then dichloromethane solvent was used to extract the derivatized products. The dichloromethane sample was also analyzed by GC/MS with the same condition as above.

Ion chromatograph spectrometry (IC) was used to analyze the possible ionic products in decontamination solution upon long time of reaction. A Metrohm IC-761 ion chromatograph spectrometry (IC, Metrohm China Ltd, Hong Kong, China) equipped with a conductivity detector and an anion-exchange column Metrosep-A-Supp 5 (50 × 3 mm, Metrohm China Ltd, Hong Kong, China) was applied for analysis. The flow-rate of the eluent (3.2 mmol/L Na₂CO₃ and 1.0 mmol/L NaHCO₃) was kept at a rate of 0.7 mL/min.

It was known that hydrogen peroxide and sodium carbonate would be generated when SPC dissolved in water, so one can clearly observe the peak at 874 cm⁻¹ which was assigned to the stretching vibration of ‘-O-O-’ structure in H₂O₂ molecule, and the peak at 1,065 cm⁻¹ which was assigned to the stretching vibration of CO₃²⁻ ion. In the Raman spectrum of ACG solution, the peak at 927 cm⁻¹ could be assigned to the stretching vibration of N = C-N structure. It should be noted that in the spectrum of complexed solution SPC/ACG, the peak at 874 cm⁻¹ and 927 cm⁻¹ disappeared, as well as two new peaks at 824 cm⁻¹ and 1,009 cm⁻¹ appeared. This suggested that some new activated species generated. As compared to the Raman spectrum of PAA, the characteristic peak at 891 cm⁻¹ was not found in SPC/ACG complexed solution. It indicated that the activation of SPC/ACG did not conduct by the pathway (1) in Figure 1. The peaks at 824 cm⁻¹ and 1,009 cm⁻¹ may be assigned to the stretching vibration of ‘N = C-O-’ and ‘-O-O-’ structure in the functional group of peroxy imide acid (HN = C-OOH), respectively. Meanwhile, the peak of the shear vibration of ‘-NH₂’ should be (1,590–1,650) cm⁻¹, which was not detected in SPC/ACG complexed solution.

According to the above experimental results, a solid-state decontaminant formula based on SPC and ACG could be attained, where the mass ratio of SPC, ACG, sodium dihydrogen phosphate and disodium hydrogen phosphate was approximate to 1:1:1:1. Further, the decontamination performance of SPC/ACG formula was tested for HD and VX. When the concentration of the decontamination solution was 4% and the volume ratio of decontamination solution to CWAs droplets was 200:1, HD and VX were nearly completely degraded within 30 min at the temperature range from 0 °C to 45 °C, as listed in Table 2. Furthermore, it was verified in further experiments that the volume ratio of decontamination solution to CWAs could be decreased to 50:1 while the mass concentration of SPC/ACG formula was increased to 10%.

The polar compounds in the decontamination solution after one day were analyzed by LC/MS approach. Only three species were detected as negative ion (M ± H⁺) in the decontamination solution. The obtained liquid chromatogram with extracted ion chromatograms (M ± H⁺) were presented in Fig. S1. One could analyze these compounds based on the retention time and the M-H⁺ peaks. The possible structural formulas of the detected species were therefore deduced, as presented in Table 4. These detected sulfonic acid compounds were quite consistent with that observed in the references (Greenhalgh & Bannard 1959; Qi et al. 2013). Furthermore, S atom of partial HD would be finally oxidized to SO₄²⁻, which was detected by IC. The mineralization ratio of S atom in the total of HD was calculated to be about 10% after one day.

Table 4

Possible degradation products of HD identified by LC/MS

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Similarly, the non-polar degradation products of VX extracted by dichloromethane, but only N,N-diisopropylformamide was detected by GC/MS, as shown in Fig. S2. However, several polar degradation products of VX were detected by LC/MS after one day of degradation, as seen in the extracted ion chromatograms (M ± H⁺) in Fig. S3. The possible structural formulas of the detected species were therefore deduced, such as such as N,N-diisopropylhydroxylamine, diisopropylcarbamic acid, ethyl methylphosphonic acid (EMPA) and 2-(diisopropylamino)ethanesulfonic acid, 2-(hydroxy(isopropyl)amino)ethane-1-sulfonic acid, as presented in Table 5. These products were also detected by a precolumn derivation approach, as shown in Fig. S4. Almost 16% of the S atoms were oxidized into SO₄^2– as detected by IC after one day. The mineralization ratio of S atom was less than that of VX in the SPC/TAED aqueous solution (Lion et al. 1997). The product ethyl methylphosphonic acid (EMPA) detected was consistent with the literatures (Greenhalgh & Bannard 1959; Winemiller et al. 2003; Qi et al. 2013). The PO₃^3– or PO₄³⁻ ion was not detected by IC, which suggested that the P-C bond appeared quite stable according to the products detected by GC/MS. It can be seen from the molecular structure of some of detected products that oxidation had occurred on the carbon atom next to the nitrogen (α-C) in VX molecule to produce aldehyde, ketone and carboxylic acid. The nitrogen atom in VX molecule can be oxidized into oxime and NO₃^– ions which were detected by IC. The mineralization ratio of N atom in VX molecule was calculated to be about 31%. Nevertheless, the NO₃^– ion was not detected by IC during the degradation process of VX in SPC/TAED aqueous solution (Qi et al. 2013).

Table 5

Possible degradation products of VX identified by LC/MS

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In the aqueous solution, ACG as a peroxide activator could react with SPC to produce acetyl peroxyl imide acid as proved by Raman and ¹³C NMR spectrum. The SPC/ACG complexed solution behaved quite efficiently towards decontaminating HD and VX. On the basis of the results on influence factors such as molar ratio of (O)/(ACG), (O)/(CWA), and the pH value, an optimized decontaminant formula was obtained, where the molar ratio of (O)/(ACG) was 1:1 while the pH value was about 9. To achieve the decontamination percentage over 99%, the molar ratio of active oxygen to CWA ((O)/(CWA)) needed to be at least 3 for HD and 7 for VX. What's important is that the SPC/ACG formula had good performance for decontamination of HD and VX, which would be used to decontaminate CWAs when the mass concentration was from 4% to 10% at the temperature range 0 °C to 45 °C. Meanwhile, based on the identified degradation products, the degradation pathways of HD and VX were deduced. It indicated that HD molecule was destroyed by oxidation and elimination reaction, while the degradation pathway of VX molecule mainly involved oxidation and nucleophilic substitution reaction.

The present research was funded by National Key Research and Development Program of China (2016YFC0801303).

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.