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LOW-TEMPERATURE, LIQUID-PHASE PROCESSES 112 The following sections address the applicability of specific chemical processes to the agents of greatest interest. GB (Sarin) Reaction with sodium hydroxide in water. The hydrolysis process in which GB (isopropyl methylphosphonofluoridate, or Satin) is reacted with sodium hydroxide (NaOH) in water, a process often called neutralization, was studied intensively by the Army a decade ago (Eq. 1). Trial runs, including some at full scale, were conducted. On the basis of that experience, the process was abandoned in 1982 in favor of the current incineration technology. Major considerations were the large residue of hydrolysis products and problems encountered in scaling up the mixing of GB with aqueous NaOH solution (Flamm et al., 1987; Coale and DePew, 1992). Large quantities of residual salts were formed because of the use of excess NaOH, which was used in an attempt to destroy GB completely. Although it is now evident that apparently incomplete reactions were analytical artifacts related to a diester impurity in GB, acceleration might be achieved by use of small mounts of catalysts, especially ortho-iodosobenzoate salts. The latter accelerate reactions of structural analogs of GB with sodium hydroxide (Moss et al., 1984). As to the problem of residual salts, use of just slightly more than enough NaOH to satisfy the requirements of the balanced equation (Eq. 1 above) should be sufficient.
LOW-TEMPERATURE, LIQUID-PHASE PROCESSES 113 In view of the high reactivity of GB with hydroxide ion in water (rate constant, k=23.7 liter/mole/second), it is probable that solutions of ammonia or low-molecular-weight amines in water would effectively detoxify it (Gustafson and Martell, 1962). This would need to be verified by research. Note that use of NH4OH instead of NaOH would change the nature of the waste stream from the process. The products from reaction of GB with aqueous NaOH are only slightly hazardous and are suitable for shipment, storage, or further processing. They will contain some of the mentioned diester, a substance not very hazardous itself and a contaminant in stockpiled GB, but convertible to GB on reaction with hydrofluoric acid (HF). Maintaining an excess of NaOH is therefore indicated. Although discontinued by the Army in favor of incineration (Flamm et al., 1987), this process has been used in the United Kingdom and other countries to destroy relatively small stocks of GB (see Chapter 3; Manley, 1992a, b). The hydrolysis of VX and GB by NaOH and Ca(OH)2 is further discussed below. Reaction with alkali in an alcohol The process of reacting agent with alkali in an alcohol may also be called neutralization. In this process, agent is combined with a solution of NaOH or potassium hydroxide (KOH) in an alcohol solvent. The alcohol may be methanol (McAndless, 1992a), 2-methoxyethanol (EPA, 1991), polyethylene glycol (Picardi et al., 1991), or (in principle) ethylene glycol. In Canadian experience, the solutions remaining after destruction of agents with KOH in methanol were then incinerated (McAndless, 1992a). Equation 2 shows the reactions that occur in this process. For H, or mustard, its reactions with KOH in methanol are shown; in other alcohols, the alkyl group in the ether (instead of CH3) would be that contributed by the structure of the alcohol.1 The ratio of the ether product to divinyl sulfide will depend on conditions. In the case of GB, probably some replacement of fluorine by the alkoxy group of the alcohol (e.g., minus-OCH3 with methanol) occurs, forming an intermediate that reacts further to form products of the type shown. This type of process is suitable for destruction of GB and H. There are doubts about its suitability for destruction of VX. Although reactions with VX are fast (Durst, 1992; Yang, 1992a-d), it appears that a by- product is a highly toxic compound also obtained from reaction of VX with NaOH in water 1 Most alcohols are composed of a hydroxyl group (OH) attached to a group composed of carbon and hydrogen atoms, such as C2H5 in the case of ethyl or grain alcohol. Thus, ethyl alcohol is C2H5OH.
LOW-TEMPERATURE, LIQUID-PHASE PROCESSES 114 (Epstein, 1992a,b). This compound is described below in the discussion of VX hydrolysis. Energetics such as TNT are expected to react with methanolic NaOH or KOH, to yield azoxy compounds (Hickinbottom, 1957). Similar reactions are expected with solutions of these alkalis in the other alcohols mentioned. Other energetics may also react, depending on their chemical structures. The U.S. Army decontaminating agent DS2 is a solution of NaOH (2 percent) in 2-methoxyethanol (28 percent) and diethylenetriamine (70 percent). Its action is based on hydrolysis chemistry like that described here. KOH in 2-methoxyethanol is the active reagent in the proprietary DeChlor/KGME process (EPA, 1991), which, although intended for destruction of polychlorinated biphenyls (PCBs), should also be effective with GB and H. Solutions of KOH in polyethylene glycols are used in other proprietary processes to destroy PCBs and dioxins, such as the Galson and General Electric KPEG processes (Picardi et al., 1991). The reagents were developed to manage toxic materials (aryl chlorides) far less reactive than the agents H, GB, and VX. The general class of processes described here appears to be broadly applicable to agent destruction. A disadvantage common to these processes is the relatively large quantity of organic waste produced. Acid-catalyzed hydrolysis. The rate of acid-catalyzed hydrolysis depends on the concentration of hydrochloric acid (HCl) or sulfuric acid and on the temperature. At a hydrogen-ion concentration of 1 mol/L (about 4 percent HCl in water), the half-life of agent is 138 minutes, which implies 99.9999
LOW-TEMPERATURE, LIQUID-PHASE PROCESSES 115 percent destruction in 27.6 minutes at 25°C (77°F) (Epstein, 1992b). This rate pertains to the first stage of reaction shown in Equation 3; the second stage is thought to be slower. (Its rate constant needs to be determined.) The products of the first step lack the enormous toxicity of GB. In principle, hydrolysis of diisopropyl methylphosphonate (the diester of structure shown above) should also be catalyzed by acids, but the actual reaction rate appears not to have been measured. GB undergoes some spontaneous hydrolysis in water. The acid product, HF, would be expected to accelerate the hydrolysis. It is therefore conceivable that dissolving GB in water and letting it stand for considerable time would allow the spontaneous reaction and ensuing autocatalysis to completely destroy the GB. A preferred alternative would be to recycle the HF-containing hydrolysis products to eliminate the need for adding other acids. A second reactor would be needed to serve as a source of acid and to attain the desired level of agent conversion. A possible disadvantage of acid-catalyzed hydrolysis is that corrosion of process equipment is likely to be more severe than for reaction of GB with aqueous NaOH. The chemical reaction described in Equation 3 is well known from laboratory studies (Epstein, 1992a,b). Additional laboratory work should be conducted to determine rate constants for diester hydrolysis and the second stage of GB hydrolysis, to allow comparison with the rate constant for reaction with NaOH and to form a basis for pilot plant work if this alternative is to be pursued. Reaction with ethanolamine. When dissolved and heated with ethanolamine (a commercial product), GB reacts to form products of lower toxicity that are suitable to store or ship for further processing (Eq. 4). Because the products contain a diester, further processing to destroy the
