Nitroaromatic compounds (NACs) are common munition constituents (MC) that contaminate military sites and require extensive remediation efforts. For this reason, it would be desirable to develop models that can predict the kinetics of NAC reduction in the subsurface. Iron oxides comprise a significant portion of the redox active fraction of soils and are thus key reductants in these systems. Previous studies of NAC reduction kinetics with iron oxides have focused on the concentration of sorbed ferrous iron as a key parameter due to the fact that it is believed to be a surface based reaction. To build a model that can predict reduction kinetics of these systems it is thus necessary to be able to predict the concentration of sorbed ferrous iron as a function of Fe(II) added, pH and surface area loading of the oxide. To this end, a model has been developed that utilizes surface acidity and surface complexation reactions to calculate sorbed iron concentrations. The equilibrium constants for those reactions, as well as the density of surface sites, are unique to each iron oxide and are fit through optimizations of both literature and independent data. The calculated amount of surface-complexed Fe(II) is then used as the reductant concentration for fitting oxide/NAC specific second order rate constants. Preliminary results with five oxides (goethite, hematite, lepidocrocite, ferrihydrite and magnetite) and four NAC/MCs (NB, 4-CN-NB, 4-Cl-NB, and NTO) have shown good correlation when comparing observed and predicted surface area normalized rate constants. This study allows us to move toward the goal of building a comprehensive model that is applicable toward predicting the timescales for NAC reduction under environmental conditions.

Two site model for the prediction of NAC/MC reduction kinetics with iron oxides

ACS Fall 2021

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