Linear Free Energy Relationship for Predicting the Rate Constants of Munition Compound Reduction by the Fe(II)-Hematite and Fe(II)-Goethite Redox Couples

Abstract

Abiotic reduction by iron minerals is arguably the most important fate process for munition compounds (MCs) in subsurface environments. No model currently exists that can predict the abiotic reduction rates of structurally diverse MCs by iron (oxyhydr)oxides. We performed batch experiments to measure the rate constants for the reduction of three classes of MCs (poly-nitroaromatics, nitramines, and azoles) by hematite or goethite in the presence of aqueous Fe²⁺. The surface area-normalized reduction rate constant (k_SA) depended on the aqueous-phase one-electron reduction potential (E_H¹) of the MC and the thermodynamic state (i.e., pe and pH) of the iron oxide-Fe_aq²⁺ system. A linear free energy relationship (LFER), similar to that reported previously for nitrobenzene, successfully captures all MC reduction rate constants that span 6 orders of magnitude: ${\log (k}_{SA})=(1.12\pm 0.04)[0.53\frac{E_H^1}{59mV}-(pH+pe)]+(5.52\pm 0.23)$. The finding that the rate constants of all the different classes of MCs can be described by a single LFER suggests that these structurally diverse nitro compounds are reduced by iron oxide-Fe_aq²⁺ couples through a common mechanism up to the rate-limiting step. Multiple mechanistic implications of the results are discussed. This study expands the applicability of the LFER model for predicting the reduction rates of legacy and emerging MCs and potentially other nitro compounds.

Keywords: iron oxides; linear free energy relationship; munition compounds; nitro group; pH+pe; reduction.