Modeling the Reduction Kinetics of Munition Compounds by Humic Acids

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.溶解性有机质（Dissolved Organic Matter, DOM）占环境中氧化还原活性组分的相当大比例，是硝基芳香族化合物与弹药组分（Nitroaromatic Compounds and Munition Constituents, NACs/MCs）发生非生物转化的重要还原剂。构建此类反应的预测动力学模型，需要获取与NACs/MCs的还原反应以及DOM的氧化反应相关的能量参数。然而，DOM结构多相且未知，使其氧化能难以实现可靠测定。为突破这一局限，研究人员以腐殖酸（Humic Acids, HAs）作为DOM的模型物质，并将其氧化还原官能团建模为一系列不同氧化还原电位的醌类集合。随后通过量子化学计算，分别得到了经由氢原子转移（Hydrogen Atom Transfer, HAT）反应的NACs/MCs还原能与对苯二酚氧化能。氢原子转移能量已成功应用于线性自由能关系（Linear Free Energy Relationship, LFER），用于预测对苯二酚还原NACs的二级速率常数。此外，研究还建立了氢原子转移能量与醌类还原电位之间的线性关系，借此可通过腐殖酸氧化还原滴定数据估算对苯二酚的反应活性（即速率常数）。研究采用包含3种腐殖酸与2种NACs/MCs的训练集，构建了平均腐殖酸氧化还原分布模型，该模型成功预测了多组腐殖酸/弹药组分体系中的还原动力学过程。