In-Situ Electrochemical Remediation of Underwater Unexploded Ordinance
Dr. Kyoo Jo | U.S. Army Corps of Engineers, ERDC-CERL
This project focused on the development of technology for safe and cost effective remediation of unexploded ordnance (UXO) in the underwater environment with depths up to 35 meters using electrochemical reduction. The technology is aimed at the in-situ remediation of the secondary explosives contained in UXO and discarded munitions underwater. The methods provide a safe means for UXO remediation without accidental detonation or leakage of toxic chemicals. The objective of this project was to evaluate the processes by obtaining the critical data needed to evaluate their potential for success in this demanding real world application and to remediate munitions including small projectiles and mortars to small bombs.
Two key processes were investigated through this Strategic Environmental Research and Development Program (SERDP) Exploratory Development (SEED) project to provide proof of concept for an electrochemical remediation system for underwater UXO. These processes were 1) the electrochemical transformation of nitro containing explosives into stable compounds, and 2) the formation of an access hole through the munition casing by a gentle process of chemical machining.
Low-cost, high-surface area dual working electrode system for trinitrotoluene (TNT) and cyclotrimethylenetrinitramine (RDX) reduction was developed which is highly compatible with in-situ remediation. The dual working electrode system contains both solid electrodes for implanting into explosive material along with a flow through electrode for high efficiency reduction of solution phase material. The entire electrode was encased in a stainless tube to provide mechanical strength to the electrode. The tube also served as the counter electrode for the electrochemical system. The optimal conditions for the dual working electrode system were determined by studying the major variables, such as particle size of graphite particles used for fabricating the flow through working electrode, solution flow rate through the flow-through working electrode, supporting electrolytes, and the impact of binder on electrode fouling. The electrochemical reductions were performed by cyclic voltammetry and optimal scan rates for reducing/degrading the explosives up to 99% were determined. To measure reduction efficiency, the explosives and their by-products were measured before and after the electrochemical remediation process using Liquid Chromatography-Mass Spectrometer (LC-MS).
A chemical milling method was developed that optimized speed while providing sufficient hole quality to allow formation of a reliable seal around the access hole. The process that the project aimed to develop was speed over quality of finish. The main focus of this objective was to determine the etch rates and hole morphology for Si-steel and low carbon steel as a function of the etchant composition, convection, and temperature. The etch rates were significantly increased by supplying high rates of convection during the chemical milling process, which was in great contrast to conventional static chemical milling. Coaxial tubing system was developed for the high rates of convection where chemical milling solution was introduced through the center of the coaxial interface, and the spent solution extracted along the periphery. A negative pressure and suction was created by keeping the flow rate through the periphery greater than the coaxial. A soft suction cup formed a seal and suction held in place. The flow rates, etchant composition, effects of ions and temperature were optimized for fast chemical milling process.
Different explosives including TNT, dinitaminomelamine (DNAM), 2,4-dinitrotoluene (DNT), RDX were successfully reduced. With this dual electrode flow system, the time for electrochemical reduction has been significantly shortened. Any dissolved TNT, RDX solution can be electrochemically reduced within 100 cycles, which could be completed within 25 min. The project was also able to determine that aqua regia was ideal as the milling solution due to its effectiveness and versatility amongst various metal types. Furthermore the development of the coaxial tubing system was successful in securely attaching to metal surfaces and pump through the milling solution. The most crucial result was being able to chemical mill and punctures the wall of a 75mm artillery shell with this system in less than 2 hours. The results so far can be developed further into a complete functioning system to effectively neutralize UXOs.
Safe methods for in-situ remediation of underwater UXO are needed. This technology has the potential to form the basis of a remediation system that is safe, nearly leak-free, energy efficient, and contains the waste in a relatively small volume. A safe approach will protect humans and the environment from unintended detonation of old and unstable munitions. Removal of these objects will protect people from accidentally coming into contact with UXO. A virtually leak-free process will prevent the release of toxic chemicals into sensitive underwater ecosystems.