Advance in water purification: Using radicalized NOX derivatives supported on metal oxides

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.

The Korea Institute of Science and Technology (KIST) has announced that a KIST research group with principal investigators of Dr. Jongsik Kim and Dr. Heon Phil Ha has collaborated with a research team led by Prof. Keunhong Jeong in the the Korea Military Academy (KMA) to graft NO₃^- species on a metal oxide via chemical fusion between NO_X and O₂ under a low thermal energy (≤ 150 °C). The resulting supported NO₃^- species can then be radicalized to generate NO₃^• analogues that serve as degraders of refractory organic substances present in a wastewater.

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.

Dr. Kim and co-workers make it viable under a wastewater including H₂O₂ and NO₃^--functionalized manganese oxide that surface manganese species (Mn²⁺/Mn³⁺) initially activate H₂O₂ for the formation of ^•OH, whereas ^•OH subsequently activates NO₃^- functionality for its transition into NO3^• (denoted as ^•OH → NO₃^•), all of which are evidenced by density functional calculation (DFT) techniques alongside with a bunch of control experiments.

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).

Dr. Kim remarks that “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.”

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The Korea Institute of Science and Technology(KIST). Founded as the first multidisciplinary government-funded research institute in Korea, KIST established a national development strategy based on science and technology and disseminated various essential industrial technologies. Now, half a century later, KIST is elevating Korea's status in the field of science and technology through world-leading fundamental technology R&D. Looking to the future, KIST will continue to strive to be a premier research institute, pursuing a brighter future for Korea and all of humanity.

This work is supported by the National Research Foundation of Korea (NRF) grant funded by government (Ministry of Science and ICT, Minister Hye-suk Lim) as well as the Future R & D and Young Fellow grants funded by KIST. This work is recently published in JACS Au, whose detail is JACS Au, 1 (2021) 1158.