NO_X (X=1 or 2) emitted from stationery/mobile sources are conventionally deemed as notorious, anthropogenic precursors of ultrafine particulate matters (PM2.5) because NO_X can undergo a series of SO₂-assisted photochemical transformative stages to finally evolve PM2.5 functioning as an air pollutant. Recently, a research group in South Korea rectifies the general notion of NO_X (vide supra) by proposing an interesting means to exploit NO_X in creative fashion.

Aqueous recalcitrant compounds including phenolics and bisphenol A are typically eliminated from water matrices via sedimentation with the use of coagulants or via degradation into H₂O and CO_Y (Y=1 or 2) with the injection of OH shuttles such as H₂O₂, O₃, etc. However, these methods require additional stages to recover coagulants or suffer from short lifespans and/or chemical instabilities innate to ^•OH, H₂O₂, and O₃, thus severely limiting the sustainability of H₂O purification processes currently being commercialized. As a substitute of ^•OH, NO₃^• can be particularly appealing due to its longer lifetime and/or greater oxidizing potential in comparison with ^•OH, ^•OOH, or O₂^•-, thereby being predicted to enhance the efficiency in degrading aqueous pollutants over the other radicals stated above. Nevertheless, NO₃^• production is not trivial and has a bunch of constraints such as the need of highly energized electrons in the presence of a radioactive element or highly acidic environments.

The resulting NO₃^• species were demonstrated to escalate degradation efficiency of textile wastewater by five- or seven-fold compared to those provided by conventional radicals (^•OH/^•OOH/O₂^•-). Of significance, the catalyst (NO₃^–-functionalized manganese oxide) discovered herein is ~30 % cheaper than a traditional commercial catalyst (iron salt) and is mass-producible. Of additional significance, the catalyst is reusable ten times or more. This is in contrast to a traditional catalyst that only guarantees one-time utilization in decomposing aqueous pollutants via homogeneous H₂O₂ scission (^•OH generation).The ^•OH → NO₃ technology  has been patented and sold to a domestic company (SAMSUNG BLUETECH). Given a plenty of merits imparted by the catalyst modified with NO₃^– functionalities, we basically expect to install the catalyst in a wastewater treatment unit so soon.

Jongsik Kim et al, Deciphering Evolution Pathway of Supported NO3• Enabled via Radical Transfer from •OH to Surface NO3– Functionality for Oxidative Degradation of Aqueous Contaminants, JACS Au (2021). DOI: 10.1021/jacsau.1c00124