Modeling the Reduction Kinetics of Munition Compounds by Humic Acids

Abstract

Dissolved organic matter (DOM) comprises a sizeable portion of the redox-active constituents in the environment and is an important reductant for the abiotic transformation of nitroaromatic compounds and munition constituents (NACs/MCs). Building a predictive kinetic model for these reactions would require the energies associated with both the reduction of the NACs/MCs and the oxidation of the DOM. The heterogeneous and unknown structure of DOM, however, has prohibited reliable determination of its oxidation energies. To overcome this limitation, humic acids (HAs) were used as model DOM, and their redox moieties were modeled as a collection of quinones of different redox potentials. The reduction and oxidation energies of the NACs/MCs and hydroquinones, respectively, via hydrogen atom transfer (HAT) reactions were then calculated quantum chemically. HAT energies have been used successfully in a linear free energy relationship (LFER) to predict second-order rate constants for NAC reduction by hydroquinones. Furthermore, a linear relationship between the HAT energies and the reduction potentials of quinones was established, which allows estimation of hydroquinone reactivity (i.e., rate constants) from HA redox titration data. A training set of three HAs and two NACs/MCs was used to generate a mean HA redox profile that successfully predicted reduction kinetics in multiple HA/MC systems.

Quinone Moieties Link the Microbial Respiration of Natural Organic Matter to the Chemical Reduction of Diverse Nitroaromatic Compounds

“…Different from 1.8 mg L –1 of humic acids, 1 μM AQDS, which can shuttle 2 μe – equivalents, supported the complete reduction of DNAN. The superior ability of the lowest concentrations of AQDS in shuttling electrons compared to the lowest concentrations of the humic acids can be explained by the quinone moieties of NOM not being as readily accessible as the quinone groups of AQDS due to NOM’s physical conformation. , Only a fraction of the quinone and hydroquinone moieties of humic acids are accessible to solutes due to the humic acids’ complex macromolecular structure, which is constantly rearranged as a function of environmental conditions. , …”

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 30 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, 14,16,38,39 an Fe 2+ sorption model 48 based on the generalized double-layer model of Dzombak and Morel 49 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).…”

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

“…Previous models have focused on a component-based approach to the electron content of the subsurface, with models developed to predict timescales of MC reduction by carbonaceous fractions of redox active media, such as hydroquinones , and humic acids . This work was undertaken to predict the timescales of MC reduction by iron (oxyhydr)­oxides, which represent another major redox-active fraction of the subsurface.…”