Modeling the Reduction Kinetics of Munition Compounds by Humic Acids.

Area of Science:

• Environmental Chemistry
• Organic Geochemistry
• Computational Chemistry

Background:

• Dissolved organic matter (DOM) is a key environmental reductant for abiotic transformations.
• Predicting reaction kinetics requires understanding DOM oxidation and contaminant reduction energies.
• The complex nature of DOM hinders accurate determination of its oxidation energies.

Purpose of the Study:

• To develop a predictive kinetic model for abiotic transformations involving DOM and nitroaromatic compounds (NACs/MCs).
• To overcome limitations in determining DOM oxidation energies due to its heterogeneous structure.
• To model humic acids (HAs) as representative DOM and quantify their redox properties.

Main Methods:

• Utilized humic acids (HAs) as model DOM, representing their redox moieties as quinones.
• Calculated reduction/oxidation energies of NACs/MCs and hydroquinones via quantum chemistry (hydrogen atom transfer - HAT).
• Established a linear free energy relationship (LFER) between HAT energies and reaction rate constants.
• Correlated HAT energies with quinone reduction potentials to link reactivity to HA redox titration data.

Main Results:

• Developed a method to estimate hydroquinone reactivity from humic acid redox titration data.
• Successfully predicted second-order rate constants for NAC reduction by hydroquinones using LFER.
• Generated a mean HA redox profile from a training set of HAs and NACs/MCs.
• Demonstrated successful prediction of reduction kinetics in multiple HA/MC systems using the developed model.

Conclusions:

• Humic acids can effectively model DOM for studying redox reactions.
• The established relationships allow for estimation of DOM reactivity and prediction of contaminant transformation rates.
• This approach provides a pathway for building reliable kinetic models for environmental redox processes involving DOM.