Degradation of dissolved RDX, NQ, and DNAN by cathodic processes in an electrochemical flow-through reactor
Graphical Abstract
Introduction
The manufacturing and application of explosives can result in contaminated water and soil, posing risks to human health and to aquatic and terrestrial life [1], [2], [3], [4], [5]. In particular, manufacturing wastewaters may contain high concentrations of explosive compounds near solubility limits. Wastes from assembling formulations may also contain mixtures of explosives compounds. Formulations can be composed of both legacy munitions constituents such as RDX (hexahydro-1,3,5-trinitro-1,3,5-triazine) and new insensitive high explosives (IHEs) such as DNAN (2,4-dinitroanisole) or NQ (nitroguanidine). RDX, DNAN, and NQ are all C-N compounds containing nitro groups, but their physicochemical properties like solubility can vary widely when present in wastewaters [6].
Therefore, treatment methods are needed that can be applicable to explosive compounds with diverse properties and potentially diverse reactivities. Conventional treatment methods have been proven for legacy munitions, but they may still need adaptation to IHEs or could have some limitations. Physical removal is possible by activated carbon [7], but requires further treatment of spent media. Chemical treatment such as reduction by zero-valent iron [8], [9], [10] or alkaline hydrolysis [11], [12], [13] requires chemical addition and possible removal of Fe corrosion products. Advanced oxidation processes, although fast [14], [15], [16] are energy-intensive. Biological processes are not proven yet for many new IHEs.
Electrochemical treatment systems represent an alternative process that could offer some advantages for explosives wastewater treatment. Hosting reduction or oxidation reactions at electrodes could avoid chemical additions and reagent by-products, transformation could potentially be achieved entirely in situ, and current requirements may be low-energy. Electrochemical transformation of the explosives RDX [15], [17], [18], [19], [20], [21], [22] TNT [18], [19], [23], [24], 2,4-DNT [18], [19], [24], [25], 2,6-DNT [25] and NTO [26] have been reported, but studies of DNAN and NQ are lacking. Cathodic reduction of RDX tends to occur by reducing nitro groups to nitroso or amine groups and is a common initiation point of degradation [17], [19], [27]. Anodic oxidation of the explosive or its intermediates is also possible [19], [27], [28]. It has been recognized that first reducing explosives makes intermediate compounds that are more susceptible to oxidation reactions, and the combined strategy of reduction followed by oxidation in electrochemical systems [10], [18], [26], [29], [30]. Other systems using this strategy [10], [27], [29], [31], [32] have been found to more completely mineralize explosives.
This study examines the reductive treatment of these explosives occurring by cathodic processes within a flow-through column electrochemical reactor. Flowing water through a separated cathode and anode allows for localized regions of reduction at the cathode and oxidation at the anode. When operating potentials exceed that for water electrolysis, H+ reduction to H2 gas creates alkaline conditions which could potentially promote alkaline hydrolysis of explosives as a second reaction pathway. Electrolytic generation of alkaline conditions has previously been noted to contribute to RDX degradation within sand columns with redox conditions manipulated electrochemically [20]. How alkaline the electrochemical conditions become, and much the alkaline hydrolysis process contributes to explosives degradation within the cathode region, are therefore important questions. One drawback of flow-through column reactors, though, is that although the current can be applied instantly, the time to steady-state cathodic processes can be significantly long, up to several hours [33], [34]. In addition, longer cathodes allow for more contact time in the cathode, and so less expensive cathode materials are desired. So far for explosives, the more expensive Ti/MMO electrodes [15], [19], [20], [21] zero-valent metals [25], and carbon-based materials [17], [18], [24], [26], [28] have been used.
More recently, stainless steel (SS) has been evaluated as a cost-effective alternative electrode material for cathodic reduction of trichloroethylene (TCE) [33] and RDX [17], [27]. SS is a widely available material in various compositions, mesh sizes, and weaves. SS is an Fe-C metal allow with varying metal impurities for strength or corrosion resistance purposes. When Ni is present, it can act as a noble metal that can adsorb H2 to form atomic hydrogen (Hads) and promote catalytic hydrogenation as a secondary reductive process. Hads on nickel has previously been shown to reduce RDX [35], and some TCE reduction on SS cathodes has been attributed to Hads on Ni within SS [33]. Although cathodic reduction of explosives can occur at lower potentials to avoid H2 gas bubbles covering surfaces [36], kinetic rates may be slower. Higher potentials should increase reduction rates of explosives, which is needed in plug flow reactors when shorter hydraulic retention times in cathode regions are present when faster flow rates of treatment are desired. Higher operating potentials consequently produce H2 and alkaline conditions, which could be harnessed as side reactions for further explosives degradation in addition to direct electron transfer at cathode surfaces.
Therefore the objective of this work is to evaluate the use of SS to promote RDX, DNAN, and NQ degradation by three cathodic processes: direct electron transfer, reduction by Hads, and alkaline hydrolysis. The SS composition and configurations are evaluated with the goals to maximize explosives removal and to minimize time to steady-state removal conditions. RDX is chosen because it is a commonly manufactured explosive, and DNAN and NQ are chosen as representative new IHEs. The three are expected to have different susceptibilities to the three cathodic processes owing to their different structures—RDX a cyclic nitroaliphatic, DNAN a nitraromatic, and NQ a nitroaliphatic with a CN structure.
Section snippets
Explosives
All chemicals used in this study are analytical grade. RDX used for calibration was purchased from Sigma Aldrich or Lawrence Livermore National Laboratory. RDX used in experiments was synthesized through direct nitrolysis of hexamethylenetetramine (HA), [37] HA was incrementally added to stirred fuming nitric acid within an ice bath to keep the reaction temperature below 11 °C. The solids were filtered, rinsed with cold water, and then dried. To purify the solids and remove trace HMX, the
Electrochemical dynamics of RDX, NQ, and DNAN removal
RDX, NQ, and DNAN all decrease in concentration after passing through the cathode when 150 mA current is applied (Fig. 1). No explosives removal occurs when current is off, indicating explosives removal is likely due to a redox reaction, and losses of the compounds to sorption on surfaces or volatilization are not likely. The removal appears to occur in the cathode only because there is no further removal of each explosive after the anode. To further test this idea, a split cell batch reactor
Conclusions
A flow-through electrochemical reactor with a separated cathode and anode was evaluated for degradation of three explosive compounds, RDX, NQ, and DNAN. Separating the cathode from the anode provides an advantage of localizing conditions for reductive degradation and alkaline hydrolysis processes supported by the cathode, compared to conventional stirred batch reactors with more homogeneous conditions. Reduction of RDX, NQ, and DNAN in the cathode region was confirmed, and nearly entire removal
CRediT authorship contribution statement
Nazli Rafei Dehkordi: Investigation, Writing – original draft, Visualization. Michael Knapp: Investigation, Methodology. Patrick Compton: Investigation. Loretta A. Fernandez: Resources, Supervision. Akram N. Alshawabkeh: Resources, Conceptualization, Supervision. Philip Larese-Casanova: Conceptualization, Supervision, Writing – review & editing, Project administration, Funding acquisition.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was financially supported by the US Strategic Environmental Research and Development Program (project ER19-1130). Additional support was provided by the Superfund Research Program of the National Institute of Environmental Health Sciences (NIEHS), National Institutes of Health (NIH; grant number P42ES017198). Special thanks to Michael MacNeil and Kurt Braun for reactor vessel fabrication. This work made use of the Cornell Center for Materials Research Shared Facilities which are
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