This Strategic Environmental Research and Development Program project, WP-2614, was a collaborative effort among scientists at Aptim Federal Services (formerly CB&I Federal Services) and CDM Smith. The key technical objective of this project was to demonstrate electrochemical treatment of several representative explosive and propellant compounds at the bench-scale. Specific attention was given to determining the overall rates of treatment, delineating the major breakdown products of the process, performing carbon and nitrogen mass balances, and assessing the ability to effectively treat solid phase propellants.
This project used 1,3,5-hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) as a representative explosive compound and nitrocellulose as a representative solid phase propellant. Several types of mixed metal oxide (MMO) anodes, and well as a boron doped diamond (BDD) anode were screened. Both undivided and divided electrochemical cell configurations were tested, and constant current and constant voltage conditions were evaluated. Chemical analyses were performed to monitor both the degradation of the parent compound, and the formation/destruction of possible breakdown products. A longer term experiment was also conducted to assess electrode longevity and changes in efficiency.
This project produced solid proof-of-concept data that supports further investigation of electrochemical demilitarization of explosives and energetics. This project used a small-scale electrochemical system and single representative compounds (RDX and NC) to minimize the analytical complexity and allow multiple variables to be clearly compared.
For dissolved RDX (as a representative explosive), the key results of this project were as follows:
In an undivided cell configuration with constant current:
- Electrochemical degradation of dissolved RDX was demonstrated with both BDD and MMO anodes coupled to stainless steel (SS) cathodes.
- Degradation rates were slightly higher with the BDD anode than with the MMO anodes, but more net RDX breakdown products (nitrite and nitrate) were produced with the BDD anode.
- Modest changes in electrolyte concentration (10-fold) and flow rate (2-fold) only resulted in 10 to 20% changes in the RDX degradation rate coefficient.
In a divided cell configuration with constant current:
- Both anodic and cathodic degradation of dissolved RDX was demonstrated with BDD anode coupled to a SS cathode. This indicates the potential for sequential cathodic/anodic treatment with increased degradation efficiency (e.g., higher mass degraded per unit of energy expended).
- Robust RDX degradation was only observed cathodically with an IrO2 anode coupled to a SS cathode.
- Cathodic degradation rates were higher in the divided configuration compared to the undivided configuration, which was attributed to the more alkaline conditions in the catholyte in the divided cell configuration.
In a divided cell configuration with constant voltage:
- Cathodic RDX degradation rates with an IrO2 anode coupled to a SS cathode were relatively stable over repeated tests, indicating good longevity of the electrodes.
For NC, this project performed the first known demonstration of alkaline hydrolysis of solid nitrocellulose via electrochemically generated hydroxide using an IrO2 anode coupled to a SS cathode operated in divided cell configuration with constant voltage. The nitrite, nitrate, and carbonaceous breakdown products generated were partially degraded as the NC was hydrolyzed. The preliminary data indicated that the electrochemical process was 50-60% more efficient than bulk alkaline hydrolysis based on the rate of NC denitration, which would be expected to result in lower cost per kg of NC treated.
This research has added to the scientific knowledge base regarding potential technologies for demilitarization activities. While further testing is needed to optimize the conditions and generate information for cost-benefit analysis, these preliminary findings provide proof-of-concept that electrochemical processes can be employed to degrade explosives and propellants to non-toxic products. This approach would also provide an added level of worker safety, as bulk chemicals (e.g., caustic) do not need to be stored or handled.