Going out with a Bang: Absolute Kinetics of Removing Organic Energetic Compounds from Waters Using Hydroxyl Radicals

Organic energetic compounds (OECs) such as nitroaromatics, nitrate esters, and nitramines are widely used explosives that persist as toxic contaminants in water systems. Among them, 2,4,6-trinitrotoluene (TNT) is of particular concern due to its carcinogenicity and genotoxicity as well as its resistance to current remediation methods. Conventional treatment approaches, such as microbial degradation, phytoremediation, and phase-transfer methods are either inefficient or fail entirely to eliminate this contaminant. Advanced Oxidation Processes (AOPs), which generate highly reactive hydroxyl radicals (^•OH), represent a promising large-scale alternative for degrading persistent organic contaminants like OECs. However, the practical implementation of AOPs depends on the measurement of accurate kinetic data for ^•OH-OEC reactions. This study quantifies the absolute second-order rate constants for ^•OH radical reactions with six common OECs (TNT, picric acid (PA), tetryl, nitroglycerin (NG), ethylene glycol dinitrate (EGDN), and pentaerythritol tetranitrate (PETN)) in aqueous solution using pulse radiolysis and competition kinetics. Reaction kinetics were evaluated at environmentally relevant concentrations (3-6 μM), and TNT and PETN were further studied over 10-40 °C to determine Arrhenius activation parameters. Measured ^•OH reaction rate constants ranged from 0.73 × 10⁹ M^-1 s^-1 (EGDN) to 8.41 × 10⁹ M^-1 s^-1 (PETN), with TNT having a value of (1.14 ± 0.05) × 10⁹ M^-1 s^-1 at room temperature, in excellent agreement with corrected literature values. Activation energies were determined as 42.4 ± 4.3 kJ mol^-1 for TNT and 21.6 ± 1.5 kJ mol^-1 for PETN. Mechanistic kinetic comparison to model compounds suggests the hydroxyl radical reaction proceeds via hydrogen abstraction for both aliphatic and nitroaromatic species. These findings support the feasibility of hydroxyl radical-driven AOPs as an effective, scalable strategy for treating explosive-contaminated waters.