Given their environmental abundances, it has been long hypothesized that geochemical interactions between reactive forms of manganese and nitrogen may play important roles in the cycling of these elements. Indeed, recent studies have begun shedding light on the possible role of soluble, ligand-bound Mn(III) in promoting abiotic transformations under environmentally relevant conditions. Here, using the kinetic data of Karolewski et al. (Geochim Cosmochim Acta 293:365–378, 2021), we provide the chemical mechanism for the abiotic oxidation of nitrite (NO₂⁻) by Mn(III)-pyrophosphate, Mn^IIIPP, to form nitrate (NO₃⁻). Nitrous acid (HNO₂), not NO₂⁻, is the reductant in the reaction, based on thermodynamic and kinetic considerations. As soluble Mn(III) complexes react in a one-electron transfer reaction, two one-electron transfer steps must occur. In step one, HNO₂ is first oxidized to nitrogen dioxide, ·NO₂, a free radical via a hydrogen atom transfer (HAT) reaction. We show that this inner sphere reaction process is the rate-limiting step in the reaction sequence. In step two, ·NO₂ reacts with a second Mn^IIIPP complex to form the nitronium ion (NO₂⁺), which is isoelectronic with CO₂. Unlike the poor electron-accepting capability of CO₂, NO₂⁺ is an excellent electron acceptor for both OH⁻ and H₂O, so NO₂⁺ reacts quickly with water to form the end-product NO₃⁻ (step 3 in the reaction sequence). Thus, water provides the O atom in this nitrification reaction in accordance with the O-isotope data. This work provides mechanistic perspective on a potentially important interaction between Mn and nitrogen species, thereby offering a framework in which to interpret kinetic and isotopic data and to further investigate the relevance of this reaction under environmental conditions.

鉴于它们的环境丰度，长期以来人们一直假设锰和氮的活性形式之间的地球化学相互作用可能在这些元素的循环中发挥重要作用。事实上，最近的研究已经开始阐明可溶性配体结合的 Mn(III) 在环境相关条件下促进非生物转化的可能作用。在这里，使用 Karolewski 等人的动力学数据。 (Geochim Cosmochim Acta 293:365–378, 2021)，我们提供了亚硝酸盐 (NO ₂ ^- ) 通过 Mn(III)-焦磷酸盐、Mn ^III PP 非生物氧化形成硝酸盐 (NO ₃ ^- )的化学机制。根据热力学和动力学考虑，反应中的还原剂是亚硝酸 (HNO ₂ )，而不是 NO ₂ ^- 。由于可溶性 Mn(III) 络合物在单电子转移反应中发生反应，因此必须发生两个单电子转移步骤。在第一步中，HNO ₂首先通过氢原子转移（HAT）反应氧化成二氧化氮·NO ₂，​​一种自由基。我们表明，这种内球反应过程是反应序列中的限速步骤。在第二步中，·NO ₂与第二个Mn ^III PP络合物反应形成硝鎓离子(NO ₂ ⁺ )，其与CO ₂等电子。与 CO ₂较差的电子接受能力不同，NO ₂ ^{+对于 OH -}和 H ₂ O来说都是极好的电子受体，因此 NO ₂ ⁺与水快速反应形成最终产物 NO ₃ ^-（步骤 3）反应顺序）。因此，根据O-同位素数据，水在此硝化反应中提供了O原子。这项工作为锰和氮物质之间潜在的重要相互作用提供了机制视角，从而提供了一个解释动力学和同位素数据并进一步研究该反应在环境条件下的相关性的框架。