Yin Shuning, Shen Qunfang, Liu Yong Dong, Zhong Rugang
Beijing Key Laboratory of Environmental and Viral Oncology, College of Life Science & Bioengineering, Beijing University of Technology, Beijing 100124, China.
Environ Sci Process Impacts. 2023 Mar 22;25(3):680-693. doi: 10.1039/d2em00501h.
Nitrate as a potential surrogate parameter for abatement of micropollutants, oxidant exposure, and characterizing oxidant-reactive DON during ozonation has attracted extensive attention, however, understanding of its formation mechanisms is still limited. In this study, nitrate formation mechanisms from amino acids (AAs) and amines during ozonation were investigated by the DFT method. The results indicate that -ozonation initially occurs to produce competitive nitroso- and ,-dihydroxy intermediates, and the former is preferred for both AAs and primary amines. Then, oxime and nitroalkane are generated during further ozonation, which are the important last intermediate products for nitrate formation from the respective AAs and amines. Moreover, the ozonation of the above important intermediates is the nitrate yield-controlling step, where the relatively higher reactivity of the CN moiety in the oxime compared to the general C atom in the nitroalkane explains why the nitrate yields of most AAs are higher than those from general amines, and it is the larger number of released C anions, which are the real reaction sites attacked by ozone, that leads to the higher nitrate yield for nitroalkane with an electron-withdrawing group bound to the C atom. The good relationship between nitrate yields and activation free energies of the rate-limiting step (Δ≠rls) and nitrate yield-controlling step (Δ≠nycs) for the respective AAs and amines verifies the reliability of the proposed mechanisms. Additionally, the bond dissociation energy of C-H in the nitroalkanes formed from amines was found to be a good parameter to evaluate the reactivity of the amines. The findings here are helpful for further understanding nitrate formation mechanisms and predicting nitrate precursors during ozonation.
硝酸盐作为臭氧氧化过程中微污染物去除、氧化剂暴露以及表征与氧化剂反应性溶解性有机氮的潜在替代参数已引起广泛关注，然而，对其形成机制的理解仍然有限。在本研究中，采用密度泛函理论（DFT）方法研究了臭氧氧化过程中氨基酸（AAs）和胺类物质形成硝酸盐的机制。结果表明，臭氧氧化最初会产生竞争性的亚硝基和α,β-二羟基中间体，对于氨基酸和伯胺而言，前者更为优先。然后，在进一步的臭氧氧化过程中会生成肟和硝基烷，它们分别是氨基酸和胺类物质形成硝酸盐的重要最终中间产物。此外，上述重要中间体的臭氧氧化是硝酸盐产率的控制步骤，肟中C=N部分相对于硝基烷中一般C原子具有较高的反应活性，这解释了为什么大多数氨基酸的硝酸盐产率高于一般胺类物质，并且是由于释放出更多的C阴离子，它们是被臭氧攻击的实际反应位点，导致与C原子相连有吸电子基团的硝基烷具有更高的硝酸盐产率。各氨基酸和胺类物质的硝酸盐产率与限速步骤（Δ≠rls）和硝酸盐产率控制步骤（Δ≠nycs）的活化自由能之间的良好关系验证了所提出机制的可靠性。此外，发现胺类物质形成的硝基烷中C-H的键解离能是评估胺类物质反应活性的一个良好参数。这里的研究结果有助于进一步理解臭氧氧化过程中硝酸盐的形成机制并预测硝酸盐前体。
