The presence of new insensitive high explosives (IHE) co-mingled with legacy munitions constituents (MC) has posed new challenges to treating explosives manufacturing waste streams and has prompted a demand for innovative treatment strategies. To meet the need for improved, cost-effective water treatment technologies for both legacy and new insensitive high explosives in wastewater, the investigators have developed a two-reactor system containing a flow-through electrochemical reactor that generates reductants based on reactive electrode surfaces and electrolysis of water, followed by a flow-through column containing a nanosized manganese dioxide catalyst supported on granular activated carbon sorbent for conversion of injected hydrogen peroxide to hydroxyl radicals. The reactor units use low power, feature inexpensive stainless steel electrodes, and is capable of sustained, long-term operation without electrode degradation. The objective of this work was to optimize the electrochemical and sorption-oxidation flow-through cells for the indiscriminate degradation of explosives with diverse physicochemical properties within wastewater.  

The central hypothesis of this project is that coupled reaction mechanisms produce favorable conditions for MC degradation, ring cleavage, and mineralization. A single chamber flow-through electrochemical reactor is envisioned wherein three modes of contaminant degradation are possible:

1. the direct reduction of MCs by electron transfer or adsorbed atomic hydrogen at cathode surfaces,
2. alkaline hydrolysis within a downstream zone of high pH, and
3. oxidation by in situ generated reactive oxygen species (ROS) such as hydroxyl radicals via electro-Fenton-like reaction.

Laboratory experiments with bench-scale column reactors were conducted to optimize the electrode materials, cathode dimensions, current operating conditions, hydrogen peroxide delivery, and catalyst loading. The reaction rates, extents, and pathways were evaluated for eight representative explosives. 

The optimized electrochemical reactor contained as a cathode stainless steel grade 316 wire mesh (#80) disks stacked in a length diameter ratio of 1:1, followed by a titanium/mixed metal oxide anode. Ideal cathode operating conditions were applied current near 450 mA and a contact time near 15 minutes. The optimized sorption-oxidation reactor contained activated carbon coated with manganese dioxide at a 5% Mn/C loading, while receiving direct injection of hydrogen peroxide at 2 mM concentration. The two-reactor system successfully removed all explosives studied, either individually or in mixture, to below reporting detection limits. System performance was slightly inhibited at high acidity or high organic co-solute presence. 

The results serve as a basis for material design, optimized operating conditions, and practical considerations for larger scale reactors. A kinetic model for cathodic processes provides a means for electrochemical reactor design. Reaction conditions are created using inert and cost-effective materials under low electric power. The technology is applicable to a variety of explosives in diverse water chemistries. Prior to manufacturing and implementing larger reactors, scale-up issues need further consideration and study, including further optimization for geometric similarities for both reactors, improved characterization for mass transfer limitations on reaction rates, better understanding of catalyst longevity, and assessment at higher flow rates. A fully realized treatment system should include multiple parallel electrochemical reactors followed by a centralized sorption-reaction column working in concert for explosives removal and mineralization. (Project Completion - 2022)

Compton, P., N.R. Dehkordi, P. Larese-Casanova, and A.N. Alshawabkeh. 2022. Activated Carbon Modifications for Heterogeneous Fenton-Like Catalysis, Journal of Chemical Engineering and Catalysts, 2022, Vol 1:203. doi.org/10.17303/jcec.2022.1.203.

Compton, P., N.R. Dehkordi, M. Knapp, L.A. Fernandez, A.N. Alshawabkeh, and P. Larese-Casanova. 2022. Heterogeneous Fenton-like Catalysis of Electrogenerated H₂O₂ for Dissolved RDX Removal, Frontiers in Chemical Engineering-Environmental Chemical Engineering, Vol. 4, 864816. doi.org/10.3389/fceng.2022.864816.

Dehkordi, N.R., M. Knapp, P. Compton, L.A. Fernandez, A.N. Alshawabkeh, and P. Larese-Casanova. Degradation of Dissolved RDX, NQ, and DNAN by Cathodic Processes in an Electrochemical Flow-Through Reactor, Journal of Environmental Chemical Engineering. Vol. 10, No. 3, 107865. doi.org/10.1016/j.jece.2022.107865.