A key transformation pathway of munitions compounds (MCs) in the environment is reduction by redox-facile soil constituents, e.g., iron (III) minerals coupled to Fe(II)). Thus, under anoxic conditions, abiotic natural attenuation (NA) could represent a passive and cost-effective remediation strategy for MCs-contaminated soils and groundwater. To harness the reactivity of naturally occurring reductants, it is necessary to understand and predict the rate and extent of MC transformation. To date, there are no models capable of predicting the reactivity of MCs in soils and aquifer materials. This is in part due to the inability to 1) infer which soil constituent is responsible for the observed reactivity and 2) identify measurable parameters that can be linked to the geo-material’s reactivity. Our work with iron oxides-Fe(II) demonstrates that despite the structural differences among MCs (i.e., nitroaromatics, heterocyclic, and nitramines), a single linear free energy relationship (LFER) exists that can predict the surface area normalized reduction rate constants (kSA) based on the reducibility of the MC (first electron reduction potential, EH1), and the thermodynamic state of the reducing system (pH+pe). In this study, it is proposed that, just as with iron oxides-Fe(II) redox couples, the thermodynamic state of ferruginous soils and aquifer material could be used as a proxy for their observed reactivity towards MCs. The thermodynamic state of ferruginous soils at different extents of reduction (i.e., pH + pe, obtained from chemical redox titrations) was correlated to the measured pseudo first order rate constants (kObs) of the legacy (2,4,6-trinitrotoluene, TNT and 1,3,5-trinitro-1,3,5-triazinane, RDX) and the newly developed (3-nitro-1,2,4-triazol-5-one, NTO and 2,4-dinitroanisole, DNAN) MCs. The results show that despite the complexity of the reductant, a single LFER successfully predicts the reactivity of these structurally distinct MCs at a given reductant strength. Limiting the predictive capability of this LFER to easily attainable (measurable) parameters in the field (e.g., pH, pe), would allow site practitioners to apply the predictive model proposed in this work.

Linear free energy relationship for the prediction of munition compound reduction rate constants by ferruginous soils

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