The Abiotic Nitrite Oxidation by Ligand-Bound Manganese (III): The Chemical Mechanism

Abstract

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.