Nitroaromatic compounds (NACs) are common munition constituents 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. Humic acid (HA) comprises a sizeable portion of the redox-active media in the subsurface and is therefore a critical reductant for building a predictive model for NAC reduction kinetics. Such a model would require the inclusion of the energies associated with both the reduction of the NACs and the oxidation of the HA. The heterogeneous and unknown structure of HA, however, has previously prohibited reliable determination of such oxidation energies. To overcome this limitation, the redox moieties of soil humic acid were modeled as a collection of quinones of varying redox potentials. The reduction and oxidation energies of the NACs and hydroquinones, respectively, were then calculated quantum-chemically via a hydrogen atom transfer (HAT) mechanism. These are used in a recently developed linear free energy relationship (LFER) to predict second order rate constants. Furthermore, a relationship between the calculated oxidation energies and their respective standard reduction potentials of the quinones allows for the inclusion of hypothetical quinone functional groups that better represent the different redox moieties of the HA. To evaluate the validity of the model, the energetics and the concentrations of the quinone-like groups were fit to experimental HA reduction profiles, which characterize the potential as a function of the electron content of the humic acid. This was done concurrently with fits to the NAC reduction profile at various extents of HA reduction. Global fits were then performed across multiple NACs as a final determination of the energetics and distribution of the quinone-like groups in HA. The results indicate a potential solution for predicting the timescales of NAC reduction under realistic environmental conditions.

Modeling the reduction of nitroaromatics by humic acid using quantum chemically computed energies of reduction and oxidation half-reactions and quinone functional groups

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

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