Thermodynamic Two-Site Surface Reaction Model for Predicting Munition Constituent Reduction Kinetics with Iron (Oxyhydr)oxides

Abstract

Iron (oxyhydr)oxides comprise a significant portion of the redox-active fraction of soils and are key reductants for remediation of sites contaminated with munition constituents (MCs). Previous studies of MC reduction kinetics with iron oxides have focused on the concentration of sorbed Fe(II) as a key parameter. To build a reaction kinetic model, it is necessary to predict the concentration of sorbed Fe(II) as a function of system conditions and the redox state. A thermodynamic framework is formulated that includes a generalized double-layer model that utilizes surface acidity and surface complexation reactions to predict sorbed Fe(II) concentrations that are used for fitting MC reduction kinetics. Monodentate-and bidentate Fe(II)-binding sites are used with individual oxide sorption characteristics determined through data fitting. Results with four oxides (goethite, hematite, lepidocrocite, and ferrihydrite) and four nitro compounds (NB, CN-NB, Cl-NB, and NTO) from six separate studies have shown good agreement when comparing observed and predicted surface area-normalized rate constants. While both site types are required to reproduce the experimental redox titration, only the monodentate site concentration controls the MC reaction kinetics. This model represents a significant step toward predicting the timescales of MC degradation in the subsurface.

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

“…We compiled kinetic data in the literature of MC and NAC reduction by iron oxide–Fe aq 2+ redox couples to test the LFERs proposed by Stewart et al and in this work. Because some authors did not report the equilibrium Fe aq 2+ concentration, which is required for computing the system redox potential, ,,, an Fe 2+ sorption model based on the generalized double-layer model of Dzombak and Morel was used to compute the amount of sorbed Fe 2+ in each study. The sorption model assumes two types of surface binding sites (monodentate and bidentate) and utilizes surface acidity and metal binding constants to account for Fe aq 2+ equilibrium sorption to a given iron oxide as a function of pH, oxide loading, and initial Fe aq 2+ concentration (prior to equilibration with the iron oxide).…”