Appendix G
Use of Tracking Compounds to Assess the Performance of a Treatment Technology

The effectiveness of a waste destruction process can be tracked by following the disappearance of a chemical compound that is especially resistant to the process conditions. The working assumption is that, if the tracking compound is completely destroyed, other less stable chemicals will also be absent in the process effluents. The fate of certain chemical species of particular regulatory or public concern, such as persistent organic pollutants (POPs) or toxic heavy metals, may also be used to track the performance of treatment technologies.

NERVE AGENTS

Sarin, Soman, and VX

The common nerve agents sarin (GB), soman (GD), and VX are derivatives of methylphosphonic acid (MPA)— chemical formula: CH3PO(OH)2. In fact, the presence of a methyl-phosphorus chemical bond is a key feature setting these agents apart from the organophosphorus compounds widely used as agricultural insecticides. Hydrolysis of the nerve agents yields esters of MPA or MPA itself. MPA is very resistant to further hydrolysis and to oxidative processes. This characteristic makes it useful as a tracking compound, allowing us to assure the absence of more toxic organophosphorus compounds in a waste stream.

MPA can be metabolized by selected species of bacteria but requires carefully controlled conditions, such as restriction of phosphate nutrient (DeFrank and Fry, 1996). It can also be oxidized by SCWO under harsh conditions to give phosphoric acid salts and oxidation products of the CH3 group (CO, CO2, and H2O). At 550°C and 4,000 psi, MPA is >99.9 percent oxidized at 14.4 seconds of contact time. In the presence of excess NaOH, the conversion is reduced to 95.2 percent under the same conditions.1

MPA is a white solid that is very soluble in water (>20 g/ L). MPA itself does not exhibit the extremely high order of toxicity of the nerve agents derived from it, but is sufficiently acidic to cause irritation or burns to the eyes, skin, respiratory tract, and mucous membranes (MDL, 1997). It is a CWC Schedule 2 intermediate subject to control under the treaty protocol.

Sulfur Mustard

To track the destruction of mustard agent, one may need to monitor for the presence of by-products rather than compounds derived from HD itself. Hydrolysis of HD produces thiodiglycol (TDG), a common industrial chemical, which— like MPA—is a CWC Schedule 2 intermediate. However, TDG is too easily oxidized by chemical or biochemical processes to be a good tracking compound. Some chemical oxidizing reagents convert TDG sequentially to a sulfoxide and a sulfone, which might track the disappearance of HD and TDG, but there are other oxidation pathways that bypass the TDG sulfoxide.

A better tracking compound may be the heterocycle 1,4-dithiane. Most HD is contaminated with 1,4-dithiane, which is present as a by-product of the HD manufacturing process. For example, the ton containers of HD stored at the Aberdeen Proving Grounds contain, on average, 1.5 percent dithiane (U.S. Army, 1996). This compound is sufficiently resistant to hydrolysis that its disappearance may signal the absence of HD. It is likely to be oxidized by chemical means, but no data on its oxidation by SCWO are known to the committee.

Other potential tracking compounds for the destruction of the carbon-chlorine chemical bonds associated with the toxicity of HD are the chlorinated hydrocarbons that usually are present as impurities in sulfur mustard (U.S. Army, 1996). The C-Cl bonds in 1,2-dichloroethane, 1,1,2,2-tetrachloroethane, tetrachloroethylene, and hexachloroethane are more resistant to hydrolysis than those bonds in the agent itself.

1  

E.F.Gloyna and L.Li, “Supercritical Water Oxidation of Methylphosphoric Acid,” presentation to the Committee on Alternative Chemical Demilitarization Technologies, August 28, 1997.



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Evaluation of Alternative Technologies for Disposal of Liquid Wastes from the Explosive Destruction System Appendix G Use of Tracking Compounds to Assess the Performance of a Treatment Technology The effectiveness of a waste destruction process can be tracked by following the disappearance of a chemical compound that is especially resistant to the process conditions. The working assumption is that, if the tracking compound is completely destroyed, other less stable chemicals will also be absent in the process effluents. The fate of certain chemical species of particular regulatory or public concern, such as persistent organic pollutants (POPs) or toxic heavy metals, may also be used to track the performance of treatment technologies. NERVE AGENTS Sarin, Soman, and VX The common nerve agents sarin (GB), soman (GD), and VX are derivatives of methylphosphonic acid (MPA)— chemical formula: CH3PO(OH)2. In fact, the presence of a methyl-phosphorus chemical bond is a key feature setting these agents apart from the organophosphorus compounds widely used as agricultural insecticides. Hydrolysis of the nerve agents yields esters of MPA or MPA itself. MPA is very resistant to further hydrolysis and to oxidative processes. This characteristic makes it useful as a tracking compound, allowing us to assure the absence of more toxic organophosphorus compounds in a waste stream. MPA can be metabolized by selected species of bacteria but requires carefully controlled conditions, such as restriction of phosphate nutrient (DeFrank and Fry, 1996). It can also be oxidized by SCWO under harsh conditions to give phosphoric acid salts and oxidation products of the CH3 group (CO, CO2, and H2O). At 550°C and 4,000 psi, MPA is >99.9 percent oxidized at 14.4 seconds of contact time. In the presence of excess NaOH, the conversion is reduced to 95.2 percent under the same conditions.1 MPA is a white solid that is very soluble in water (>20 g/ L). MPA itself does not exhibit the extremely high order of toxicity of the nerve agents derived from it, but is sufficiently acidic to cause irritation or burns to the eyes, skin, respiratory tract, and mucous membranes (MDL, 1997). It is a CWC Schedule 2 intermediate subject to control under the treaty protocol. Sulfur Mustard To track the destruction of mustard agent, one may need to monitor for the presence of by-products rather than compounds derived from HD itself. Hydrolysis of HD produces thiodiglycol (TDG), a common industrial chemical, which— like MPA—is a CWC Schedule 2 intermediate. However, TDG is too easily oxidized by chemical or biochemical processes to be a good tracking compound. Some chemical oxidizing reagents convert TDG sequentially to a sulfoxide and a sulfone, which might track the disappearance of HD and TDG, but there are other oxidation pathways that bypass the TDG sulfoxide. A better tracking compound may be the heterocycle 1,4-dithiane. Most HD is contaminated with 1,4-dithiane, which is present as a by-product of the HD manufacturing process. For example, the ton containers of HD stored at the Aberdeen Proving Grounds contain, on average, 1.5 percent dithiane (U.S. Army, 1996). This compound is sufficiently resistant to hydrolysis that its disappearance may signal the absence of HD. It is likely to be oxidized by chemical means, but no data on its oxidation by SCWO are known to the committee. Other potential tracking compounds for the destruction of the carbon-chlorine chemical bonds associated with the toxicity of HD are the chlorinated hydrocarbons that usually are present as impurities in sulfur mustard (U.S. Army, 1996). The C-Cl bonds in 1,2-dichloroethane, 1,1,2,2-tetrachloroethane, tetrachloroethylene, and hexachloroethane are more resistant to hydrolysis than those bonds in the agent itself. 1   E.F.Gloyna and L.Li, “Supercritical Water Oxidation of Methylphosphoric Acid,” presentation to the Committee on Alternative Chemical Demilitarization Technologies, August 28, 1997.

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Evaluation of Alternative Technologies for Disposal of Liquid Wastes from the Explosive Destruction System COMPOUNDS OF CONCERN High-temperature processes for posttreatment of EDS neutralents can yield chemical species that were not present in the neutralent. Some of these compounds—dioxins, for example—are persistent organic pollutants (POPs), which may pose significant risks to human health and the environment. Such compounds are the subject of much public concern, because they tend to accumulate in the environment and in fatty tissues in the human body. An international treaty calling for phasing out the production of 12 of the most prevalent POPs has been signed but not submitted to the U.S. Senate for ratification. Thus, tracking the potential formation of these compounds is an important metric of the technology’s performance. Chloride-containing wastes in particular are apt to generate POPs under temperature regimes such as those involved in quenching the gases resulting from incineration or plasma treatment of liquids. Certain agents (HD, CG, and L) contain high percentages of chloride, and their neutralents are chloride-rich. Dioxins and Furans Some high-temperature, vapor-phase alternatives to incineration, such as plasma arcs and secondary oxidizers, generate low levels of polychlorinated dibenzodioxins and dibenzofurans when used for processing chlorinated organic compounds such as sulfur mustard and lewisite. As explained below, these low-level by-products have aroused concern about the potential negative health effects of using alternative technologies of this type. For example, the stack gas from a PLASMOX plasma arc system contained detectable levels (0.25 ng TEQ/cu. m) of dioxins/furans in HD neutralent processing demonstrations early this year (Stone & Webster, 2001). As noted in the reference, the dioxins/ furans may have been an artifact of the specific test conditions, but this issue must be resolved by further tests. The organic chemicals commonly termed “dioxins” generally refer to about 30 polychlorinated and polybrominated dibenzodioxins and dibenzofurans out of the hundreds of possible chloro- and bromo-substituted isomers of these compounds. Polychlorinated dioxins are produced by many thermal processes, including incineration, wood-burning stoves, and natural phenomena. As a consequence, they are widely distributed in the environment. A major concern is that, when absorbed in the body, they persist in fatty tissues for long periods of time. They are associated with human health effects such as chloracne, even at extremely low concentrations. The mixture of dioxins to which humans may be exposed has been characterized as a “likely human carcinogen” by the EPA (EPA, 2000c). Of the dioxins, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) is the most toxic and is recognized as one of the most potent of all carcinogens, with EPA long having classified it as B2, or probable human carcinogen (EPA, 2000d). The National Toxicology Program has proposed removing TCDD from the list of carcinogens for which there is “sufficient evidence in experimental animals” and adding it to the list of chemicals “known to be human carcinogens” (U.S. Department of Health and Human Services, 2001). The recent EPA draft dioxin reassessment (EPA, 2000d) summarizes the weight of evidence on dioxin’s reproductive effects, immunological impacts, and developmental toxicity. These compounds are extremely stable in the environment and are found throughout the world at very low levels. According to the EPA (EPA, 200 1e), municipal incinerators are the largest single source of dioxin emissions (38.4 percent) and backyard refuse burning contributes 18.6 percent. The best of the hazardous waste incinerators, such as the U.S. Army’s stockpile chemical disposal facilities, have extremely low emissions of dioxins. Polychlorinated Biphenyls Polychlorinated biphenyls were used for decades as cooling fluids in electrical transformers because of their chemical inertness and thermal stability. They were also used widely as components of lubricants, paints, and copy paper. Their stability, which was an asset for industrial uses, has proved to be a liability in other contexts, because PCBs that have leaked into soil or streams can linger for decades. Their presence is of particular concern because several mixtures of PCBs have been shown to be carcinogenic in laboratory animals. “Based on sufficient evidence of carcinogenicity in experimental animals,” the National Toxicology Program considers that PCBs are “reasonably anticipated to be human carcinogens” (U.S. Department of Health and Human Services, 2001), and EPA has classified PCBs as Group B2, or probable human carcinogens (EPA, 2000c). However, recent studies found no link between risk of breast cancer and the presence of PCBs (Laden, 2001). In some situations, low levels of PCBs have been detected in emissions from incinerators, but the major source appears to be industrial spills and discharges. The discharges have been dramatically reduced since production of PCBs was restricted. Metals Several heavy metals that may occur in EDS neutralents are toxic. Special consideration should be given to their analysis and disposal. Generally speaking, their toxicity is inherent and is not destroyed by posttreatment processing. The general approach to dealing with them is to reduce their mobility and, hence, their bioavailability. Several posttreatments convert these metals to salts that have low solubility in water and are therefore resistant to leaching into groundwater. Leachability may be further reduced by stabilization with Portland cement or other reagent.

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Evaluation of Alternative Technologies for Disposal of Liquid Wastes from the Explosive Destruction System The committee has given particular consideration to the problems associated with arsenic, because it may occur in relatively high concentrations in neutralents derived from agents such as lewisite. Other heavy metals listed below can be expected to occur in lower concentrations because they derive from munition components such as fuzes, solders, and alloys. However, their toxicity is such that even low concentrations may pose significant risks. Arsenic Arsenic is not generally present in chemical munitions, but some chemical agents such as lewisite, adamsite, and clark-2 are organoarsenic compounds. These agents yield neutralents containing substantial amounts of arsenic in the form of soluble compounds such as sodium arsenite. The general approach to rendering the arsenic less dangerous to human health and the environment is to convert the soluble compounds to less soluble species such as an arsenate salt. Ferric arsenate has very low solubility in water. The toxicity of arsenic compounds is well known, but the lethality varies widely with the form of arsenic ingested or inhaled (Kaise et al., 1989). Two common organoarsenic compounds, methylarsonic acid and dimethylarsinic acid, have LD50 values of 1.8 and 1.2 g/kg, respectively. On the other hand, sodium arsenite (Na3AsO3), the usual hydrolysis product of lewisite agent, has an LD50 of 0.0045. Because of this general toxicity as well as the carcinogenicity of arsenic compounds, the allowable concentrations in drinking water and in the workplace are low. In 1976, the EPA acted under the Safe Drinking Water Act to propose an interim maximum concentration limit of 50 μg/L. This standard is in the process of being lowered, based in part on an NRC review of the scientific evidence concerning the carcinogenicity of arsenic compounds (NRC, In press). The report shows increased risk of cancer in humans from arsenic concentrations as low as 3 ppb, based on studies in both Taiwanese and U.S. populations. It is also considered to be a human carcinogen through exposure by inhalation. Lead Lead is often found at low concentrations in neutralents. It may derive from munition components such as solder or from detonating compounds such as lead azide. Lead compounds are strong neurotoxins and have been associated with developmental mental retardation in children. Like other divalent metal cations (e.g., zinc and cadmium), lead(II) salts can be removed from aqueous streams by precipitation with sulfide or by ion exchange. Mercury Mercury is sometimes present in low concentrations in neutralents. A potential source is the mercury fulminate present in some detonator compositions. Metallic mercury is sufficiently volatile that it can pose an inhalation danger in closed workspaces. A similar danger exists with organo-mercury compounds such as dimethylmercury. Mercury salts in an aqueous environment can be methylated by microorganisms to produce highly toxic monomethylmercury compounds. Elemental mercury vapor can be removed from gas streams (e.g., flue gas from coal-burning power plants) by adsorption, although with limited efficiency. Cadmium Cadmium is sometimes encountered in neutralents derived from EDS treatment of metal parts containing cadmium in an alloy or as a corrosion-resistant plating. The metal is toxic and is volatile when heated, but the major risk appears to lie in aqueous streams that may enter the environment.

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