(186f) Impact of Electrode Surface Morphology on the Electrochemical Reduction Kinetics of Nitroaromatics and Cyclic Nitramines Mixtures

Managing groundwater plumes of dissolved nitroaromatic and cyclic nitramine compounds is a high priority at military facilities where site management costs can be very high. A novel approach to control plume migration utilizes electrolytic permeable reactive barriers to degrade these chemical in situ. This research focuses on characterizing the relationship between electrode surface morphology and composition with the rate of electrochemical reduction for mixtures of cyclic nitramines and nitroaromatics. The electrode material studied consists of a mixed metal oxide (IrO₂/Ta2O₅) coated titanium mesh (Ti/MMO). The surface of previously unused electrodes is relatively smooth and free of cracks. Observed first-order heterogeneous rate constants, at cathode potentials of -1.2 V vs. the standard hydrogen electrode, for 1,3,5-trinitrotoluene (TNT) and hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) on fresh electrodes are approximately 0.15 and 0.1 cm min^-1 in experiments with a single reactant. These are roughly two to three orders of magnitude larger than reduction kinetics on Fe⁰ surfaces for TNT and RDX, respectively.

Two important factors that will affect the application of these electrolytic permeable reactive barriers have also been investigated. The first of these is the presence of contaminant mixtures; cyclic nitramines and nitroaromatics are often co-contaminants in the environment and mixture effects can be expected to alter the electrochemical reaction rates of TNT and RDX. To address this issue, electrode surface reaction kinetics were measured in multi-reactant batch degradation experiments using Ti/MMO cathodes. The target compounds were two nitroaromatics, TNT and 2,4-dinitrotoluene (2,4-DNT), and two cyclic nitramines, RDX and octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazine (HMX). When mixtures of species were present the inter-species competition for reaction sites impacted the observed kinetics according to a Langmuir-Hinshelwood-Hougen-Watson model of the reaction rate law. The second factor affecting implementation of this technology is the aging of the electrode material. After an extended period of use, pitting was observed at localized regions of the electrode. Electrochemical reduction kinetics of cyclic nitramines and nitroaromatic mixtures were re-evaluated using an artificially aged electrode that exhibited these surficial transformations. Surface morphology and composition evolution after artificial aging were assessed using scanning electron microscopy and energy dispersive x-ray spectroscopy. The correlation between surface structure/composition changes and mixture reaction kinetics will be discussed in this presentation.