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LOW-TEMPERATURE, LIQUID-PHASE PROCESSES 118 H (Mustard) For H and other mustard agents, insolubility, inclusion of thickeners, and the formation of gels and solid deposits during long storage present problems in carrying out chemical detoxification. Possibilities for overcoming these problems will be discussed after the chemistry is considered. Hydrolysis of mustard. Mustard agent in water solution hydrolyzes rapidly, but for demilitarization purposes the reaction is impaired by both physical and chemical factors. The physical problem is the extraordinarily low solubility of mustard in water. Investigators who have determined rate constants for mustard hydrolysis carried out their studies in mixed solvents, such as aqueous acetone (Bartlett and Swain, 1949; Yang et al., 1990c) or aqueous ethanol (Yang et al., 1987). The chemical problem is that the intermediate products are cyclic or oligomeric sulfonium salts, which are relatively unreactive and which moreover have the potential for slowly reforming mustard (Yang et al., 1990c). An estimate for the mustard hydrolysis rate constant at 90°C can be made, based on the enthalpy of activation ( ) of 18.5 kcal/mol reported by Yang et al. (1987) and on the rate constant 0.261 rain-1 at 25°C tabulated by Ward and Seiders (1987). (They pointed out reasons to consider this value as being of uncertain quality.) The estimate is 18.1 rain-1 at 90°C, or half-life 2.3 seconds. The actual hydrolysis reaction is thus very fast; if the solvent is just water, the rate-limiting step is the dissolution of mustard in water rather than the hydrolysis. Formation of the troublesome oligomeric sulfonium salt can be avoided if a strong nucleophile is present in the water; hydroxide ion fulfills this role. In principle, it can be supplied either as NaOH or calcium hydroxide Ca (OH)2.
LOW-TEMPERATURE, LIQUID-PHASE PROCESSES 119 In NaOH solutions, mustard reacts with an initial (rate-limiting) internal displacement to form a cyclic sulfonium salt (Bartlett and Swain, 1949; Ward and Seiders, 1984). This reaction is then followed by further reaction with NaOH. However, the surface reaction of water on liquid mustard agent is reported to form a complex set of ionic products, which then diffuse into the bulk phase. Overall performance of this system has been rated as unsatisfactory for demilitarization purposes (Durst et al., 1988). In contrast, reaction of mustard with Ca(OH)2 in water at 90° to 100°C was reported (Reichert, 1975), although this reaction seems not to have been used for large-scale demilitarization. It was used in England to convert 125-gallon batches of HD (mustard) to thiodiethanol in an apparatus not much more complicated than a tub (Eq. 7). The committee considers this method to be very promising but advises further study before any use is made of it. Reaction with oxidizing agents. HD can be oxidized in the liquid phase by several strong oxidizing agents. All of these reactions are limited by HD's low solubility in water. Approaches to increasing the contacting between reactants include improving the physical dispersion and forming microemulsions of HD. As discussed above, the rate of reaction at the HD surface appears to be fast enough to prevent significant diffusion of HD into the aqueous phase. Even though HD solubility may increase with temperature, corresponding increases in reaction rates may still prevent penetration into the aqueous phase of dissolved HD. Regardless of the specific mechanism, overall reaction rate will be controlled by the HD-liquid surface area. Mechanical emulsification (as in milk homogenization) can reduce droplet size to a few micrometers, forming a very large surface area (about 5 m2/gram, for a droplet of 1 µm in diameter). Work on physical dispersion should be an important componem of a program on liquid-phase detoxification of HD. The need for dispersion with gelled or solidified agent should also be included in such a program. A surfactant and a cosurfactant (such as butyl alcohol) in proper proportions, along with a hydrocarbon, readily form an emulsion with HD so freely dispersed that it appears dear. A compound less toxic but very similar to mustard (CH3CH2SCH2CH2Cl instead of ClCH2CH2SCH 2CH2Cl) was found
LOW-TEMPERATURE, LIQUID-PHASE PROCESSES 120 to react rapidly with sodium hypochlorite in microemulsion (Menger and Elrington, 1991). The half-life of reactant was about 3 seconds at room temperature. This method for bringing mustard into effective contact with sodium hypochlorite suffers a disadvantage in that the amount of the waste streams generated is increased. A typical microemulsion is composed of about 50 percent water and 50 percent organic compounds. Reaction with sodium hypochlorite. Mustard reacts readily with hypochlorite ion to form oxidation products of reduced toxicity. This chemistry has been used for decontamination since World War I (Eq. 8). As in NaOH hydrolysis, an impediment is the low solubility of mustard in water. Although hypochlorite salts are water soluble, a water solution of sodium hypochlorite, although good for decontamination of thin layered spills, can make such poor contact with bulk HD that very little reaction occurs. Reaction with peracids. Mustard is very rapidly oxidized by m-chloroperbenzoic add, at a rate described as ''too fast to measure'' (Yang, 1992c). The products are both mustard sulfoxide and mustard sulfone (Eq. 9; in the equation, R is generalized notation for part of a molecule). However, m-chloroperbenzoic acid is a moderately expensive laboratory chemical; moreover, if it were used, the by-product RCOOH would be m-chlorobenzoic acid, a product requiring further conversion. The use of ammonium peroxydisulfate in strong acid at 100°C for complete oxidation of the mustard surrogate thiodiethanol was reported (Cooper, 1992). This treatment would probably detoxify HD as would OXONE. Operation at 100°C, as for calcium hydroxide hydrolysis, may greatly increase the effectiveness of these and other reactions involving oxidizing agents. Other oxidizing agents of interest include a variety of peroxy acid salts and H2O2. H2O2 is of special interest because it is inexpensive and has water as its sole by-product. OXONE, persulfuric acid, and other peracids are commercially available and could probably also be used. Safety and performance need to be established.
