Applications of Biodegradation in Chemical Demilitarization

Abstract

Nerve agents are decomposed by some microorganisms and by a variety of enzymes, but rate and engineering considerations have prevented their use in practical disposal systems. On the other hand, the hydrolysis products of both nerve and blister agents are good candidates for biodegradation. On this basis, the National Research Council recommended that the U. S. Army investigate a combination of neutralization and biodegradation as an alternative to incineration for destroying chemical agents. The Army responded with an integrated R&D program at its Edgewood Research, Development and Engineering Center, which has provided the basis for this review.

Mustard agent is recalcitrant in attempts at direct biodegradation, but responds well to a combination of hydrolysis and biotreatment. Hydrolysis by aqueous alkali yields thiodiglycol (TDG), S(CH₂CH₂OH)₂, plus a complex mixture of ether and thioether byproducts. On the other hand, hydrolysis with a large volume of hot water proceeds fairly cleanly to give an acidic solution of TDG. This dilute solution, after neutralization with NaOH, appears to be an attractive substrate for biodegradation by mixed cultures of microorganisms. Biological treatment in a sequencing batch reactor produces a relatively innocuous solution containing inorganic salts and traces of residual organics. A flow sheet for using these treatments for disposal of mustard agent is proposed along with identification of future R&D needs.

Hydrolysis of VX with hot aqueous NaOH solution yields primarily the ethyl ester (EMPA) of methylphosphonic acid and products derived from HSCH₂CH₂N(iPr)₂. These products are plausible candidates for biodegradation, but research on their biotreatment is still at an early stage. An interesting new development is the observation that VX reacts slowly with a stoichiometric quantity of water to form the 2-thioloethyl ammonium salt of EMPA. This finding raises the possibility that V agents might be detoxified in their storage containers and be biodegraded later after dilution and pH adjustment. Engineering aspects of such a scheme are postulated.

All neutralization-biodegradation technologies share the need for extensive development in the areas of:

• Disposal of biomass and liquid effluents from biotreatment;

• Techniques to monitor for agent and its hydrolysis products in aqueous solution;

• Cleaning and decontamination of storage containers and processing facilities.

Successful development will depend on completion of these generic tasks as well as solution of problems specific to the biodegradation of agent hydrolysates.

Biological degradation of chemical warfare agents or their hydrolysis products has been advocated as an alternative to disposal by incineration [1]. Although the U. S. National Research Council commended incineration as a relatively safe and effective procedure to destroy chemical agents, it also recommended accelerated research on neutralization as a means to destroy agents stored in bulk [2]. Because neutralization (defined here as chemical hydrolysis) does not destroy agents irreversibly, the NRC advocated that it be viewed as a preliminary treatment. It would be followed by another process that would convert the hydrolysis products to relatively innocuous materials for final disposal. The U. S. Army acted on the NRC recommendations and began an intensive research and development program on neutralization, both as a “stand-alone” process and as a pretreatment to be followed by biodegradation [3]. The work has focused on destruction of mustard blister agent and VX nerve agent, which are stored in bulk at Aberdeen, Maryland, and Newport, Indiana, respectively. This paper reviews recent research progress in the Army program and its potential for practical application.