Study on detoxification property of alkaline-modified MoO42--H2O2 decontaminants against PhSMe under subzero environment

The decontaminant activated by MoO42- (MoO42--H2O2) suitable for subzero environment shows strong oxidizing ability and weak nucleophilicity due to its acid. In this paper, in order to improve nucleophilicity and retain oxidation of MoO42--H2O2 as far as possible, NH3 and NaOH were used as alkaline modifiers, and PhSMe was used as a simulant of HD to study the oxidation rate and products of sulfides by alkaline-modified MoO42--H2O2 below zero. Results showed that the reaction rate constants decreased with the increase of pH in both NH3 and NaOH modified MoO42--H2O2 at -20°C, and the relative ratio of sulfone to sulfoxide increased especially at pH>9. The reaction activation energy Ea of PhSMe oxidation in the alkaline-modified MoO42--H2O2 decontaminants was lower than that in the MoO42--H2O2 decontaminant, which indicated that the sensitivity of the oxidation reaction rate to temperature in MoO42--H2O2 was reduced after modification.


Introduction
Although most chemical warfare agents (CWAs) stocks have been destroyed, they still appeared in some local wars or terrorist attacks, such as the suspected nerve agent attack events in Syria in 2015 and the death of Kim Jongnam in 2017 [1] . Decontamination, as a technical process to remove/neutralize contaminants from contaminated objects, plays an extremely important role [2,3]. CWAs still have a great hazard to personnel under subzero environment, hence it is necessary to destroy the toxic chemicals for the contaminated area where personnel and equipment pass, contaminated personnel and equipment. For this, it is obviously very important to study detoxification property of decontaminants applied under subzero environment.
The main decontaminants against CWAs include hypochlorite, base and peroxide. Hypochlorite cannot be used normally below zero due to its high freezing point. Alkali-alcohol-amine decontaminants which can be used under subzero environment show strong corrosiveness to the environment due to its strong basicity. While peroxide decontaminants have developed rapidly in recent years due to their high reactivity with CWAs and low corrosiveness to metals, rubbers, plastics, etc. The wellknown ones are DF-200 [4] and Decon Green [5] developed in the United States. Moreover, the hydrogen peroxide decontaminant with a low freezing point can retain high reactivity under subzero environment through adding additives, activators, etc.
The mechanisms of detoxification against CWAs include oxidation and nucleophilicity. H2O2 activated by MoO42-(MoO42--H2O2) shows strong oxidizing ability and weak nucleophilicity due to its acid, so it has high speed of oxidation on HD (mustard, bis (2-chloroethyl) sulfide) and limited degradation on GD (Soman, pinacoly methylphosphonofluoridate), which mainly been decontaminated through nucleophilic reaction. In order to improve nucleophilicity and retain oxidation of MoO42--H2O2 as far as possible, NH3 and NaOH were used as alkaline modifiers in the paper.
Thioanisole (PhSMe), with a similar structure to HD and weaker toxicity, has been usually used as a reaction simulator in the decontamination of HD [6]. In this work, PhSMe was used as the probe molecule to compare the oxidation performance between MoO42--H2O2 and MoO42--H2O2 modified by NH3 and NaOH. The reaction rate and detoxification products with pH were detected below zero. Through comparing the differences in rate constants and detoxification products in three decontamination solutions, their detoxification property with PhSMe was discussed.

Droplet detoxification experiments
The experiments were done in 25 mL test tubes. Firstly, 20 μL PhSMe in 25 mL test tubes and pre-prepared decontaminants were put in a low temperature constant temperature stirring reaction bath (DHJF-8002) at a preset temperature for 30 minutes. Secondly, 1 mL decontaminant was added to the test tube. After reacting for a certain time under stirring at 800 r/min (i.e. revolutions/minute), 6 mL 15% Na2SO3 and 10 mL extractant were added, and immediately shake for 20 s. After standing for 15 min, the extractant layer solution was taken for residual PhSMe determination.

The detection for PhSMe
With a specific absorption at 254 nm [7], PhSMe can be quantitatively detected by a UV-Visible Spectrophotometer (Evolution 220). There were no components that interfered with the ultraviolet absorption of PhSMe in the extractant layer solution.

Optimization for extraction
The extraction using n-hexane or DCM was investigated by comparing the PhSMe extraction ratios after reacting for the zero time. The results were showed in Table 1. Comparing the extraction ratio with n-hexane and DCM, it was found that they all have good extraction capacity for PhSMe in different decontaminants and it showed a slightly better extraction ratio with n-hexane (above 83%) than that with DCM n-hexane. So n-hexane was selected as extractant in the following experiments.
The actual residual concentration of PhSMe (c t , mg·mL -1 ) in reaction time of t (min) could be determinated by correction with the extraction ratio in zero reacting time (Y, %) as formula (1), where c t ' was the detected residual concentration of PhSMe, mg·mL -1 .

Kinetic study of detoxification in decontaminant solution
The oxidative detoxification against HD and PhSMe in H 2 O 2 decontaminant was regarded as a quasi-first order reaction [8,9] , and the first order kinetic rate constant (k, min -1 ) could be calculated through formula (2), where c 0 was initial PhSMe added into decontaminant solutions.    At the same pH, the difference in the reaction rate constant k of PhSMe in NH 3 -MoO 4 2--H 2 O 2 and NaOH-MoO 4 2--H 2 O 2 decontaminants may come from two aspects. On the one hand, under weakly alkaline conditions, the primary oxidation of PhSMe by peroxide was generally considered to be carried out through a solvent-assisted oxygen transfer mechanism [11,12] , and the oxidation reaction rate was linearly related to the pK a of the solvent [12] . The pK a in NH 3 -MoO 4 2--H 2 O 2 was higher than that in NaOH-MoO 4 2--H 2 O 2 , so the reaction rate is correspondingly fast in NH 3 -MoO 4 2--H 2 O 2 . On the other hand, the higher the cation concentration was, the faster the apparent oxidation reaction rate was under the same temperature and pH conditions. The cation (NH 4 + ) concentration in NH 3 -MoO 4 2--H 2 O 2 was higher than that (Na + ) in NaOH-MoO 4 2--H 2 O 2 , so the reaction rate k was high in NH 3 -MoO 4 2--H 2 O 2 , too.

Effects of pH on the oxidation products of PhSMe in alkaline-modified MoO4 2--H2O2 solution
The effect of pH on the oxidation products of PhSMe was investigated in alkali-modified MoO 4 2--H 2 O 2 solution after the reaction time 30 min at -20°C. The peak area ratio of sulfone to sulfoxide (A sulfone /A sulfoxide ) in different decontaminants with pH was shown in Figure 3.  Figure 3 showed that the relative content of sulfone had an upward trend with the increase of pH. The trend was not significant when the pH was less than 9, and the peak area ratio of sulfone to sulfoxide was less than 0.15 when the pH was about 9. The increase trend was more obvious when pH was more than 9.
Due to the weak alkalinity of NH 3 , the content of highly toxic sulfone as a deep oxidation product could not too high even if it was added excessively in actual use.

Effect of temperature on the detoxification property against PhSMe in three decontaminants solution
Two alkaline-modified decontaminants with pH close to 9 were chosen to investigate the PhSMe kinetic rate constants of the detoxification within the temperature range (-25C~-10°C), and compared with MoO 4 2--H 2 O 2 decontaminant. Figure 4 showed the good linear relationship between 1/T and lnk. The detoxification kinetic constants for oxidation of PhSMe in three decontaminant solutions at different temperatures were shown in Table 3.
The activation energy of the detoxification was calculated according to the Arrhenius equation (3).
Where，E a is the reaction activation energy, J·mol -1 ; T is the reaction temperature, K; A is the pre-exponential factor, min -1 ; R is the ideal gas constant.  This was also reflected in the change relations of the reaction rate constants k in the three decontaminants at the same temperature. At -10℃, rate constants k of PhSMe was obviously higher in the MoO 4 2--H 2 O 2 decontaminant (1.28 min -1 ) than that in NH 3 -MoO 4 2--H 2 O 2 (0.82 min -1 ) and NaOH-MoO 4 2--H 2 O 2 (0.49 min -1 ). While at -25℃, rate constants k of PhSMe was less in the MoO 4 2--H 2 O 2 decontaminant (0.23 min -1 ) than that in NH 3 -MoO 4 2--H 2 O 2 (0.27 min -1 ).
The difference in the apparent reaction rate constant k at the same temperature may be mainly due to the effect of the modified decontaminants on the pH. The pH values of the MoO 4 2--H 2 O 2 ， NH 3 -MoO 4 2--H 2 O 2 and NaOH-MoO 4 2--H 2 O 2 were 6.4, 8.9 and 9.0, respectively. With the addition of inorganic alkaline, the alkalinity of the decontaminants was improved, but the oxidation electrode potential of the system was reduced, which reduced the rate of the primary oxidation reaction. At the same time, the addition of inorganic alkaline also changed the kinetic mechanism of the reaction, resulting in a slight decrease in E a , which reduced the sensitivity of the reaction rate to temperature.

Conclusion
Results showed that the reaction rate constants k decreased with the increase of pH in both NH 3 and NaOH modified MoO 4 2--H 2 O 2 at -20℃, and the relative ratio of sulfone to sulfoxide increased especially when pH was more than 9. The E a of PhSMe oxidation in the alkaline-modified MoO 4 2--H 2 O 2 decontaminants was lower than that in the MoO 4 2--H 2 O 2 decontaminant, which indicated that the sensitivity of the oxidation reaction rate to temperature in MoO 4 2--H 2 O 2 was reduced after modification.
Hence, compared with MoO 4 2--H 2 O 2 decontaminant, the oxidization of the alkali-modified decontaminants was slightly lower due to the increase of alkalinity, but the sensitivity of the oxidation reaction rate to temperature was obviously reduced, especially for NH 3 -modified decontaminant. It provided a formula design idea for further development and design of decontaminations with both oxidation and nuclephilic properties, which was suitable for subzero environment.