Deciphering Evolution Pathway of Supported NO3• Enabled via Radical Transfer from •OH to Surface NO3- Functionality for Oxidative Degradation of Aqueous Contaminants

NO3 center dot can compete with omnipotent (OH)-O-center dot/SO4 center dot- in decomposing aqueous pollutants because of its lengthy lifespan and significant tolerance to background scavengers present in H2O matrices, albeit with moderate oxidizing power. The generation of NO3 center dot, however, is of grand demand due to the need of NO2 center dot/O-3, radioactive element, or NaNO3/HNO3 in the presence of highly energized electron/light. This study has pioneered a singular pathway used to radicalize surface NO3- functionalities anchored on polymorphic alpha-/gamma-MnO2 surfaces (alpha-/gamma-MnO2-N), in which Lewis acidic Mn2+/3+ and NO3- served to form (OH)-O-center dot via H2O2 dissection and NO3 center dot via radical transfer from (OH)-O-center dot to NO3- ((OH)-O-center dot -> NO3 center dot), respectively. The elementary steps proposed for the (OH)-O-center dot -> NO3 center dot route could be energetically favorable and marginal except for two stages such as endothermic (OH)-O-center dot desorption and exothermic (OH)-O-center dot-mediated NO3- radicalization, as verified by EPR spectroscopy experiments and DFT calculations. The Lewis acidic strength of the Mn2+/(3+) species innate to alpha-MnO2-N was the smallest among those inherent to alpha-/beta-/gamma-MnO2 and alpha-/gamma-MnO2-N. Hence, alpha-MnO2-N prompted the rate-determining stage of the (OH)-O-center dot -> NO3 center dot route ((OH)-O-center dot desorption) in the most efficient manner, as also evidenced by the analysis on the energy barrier required to proceed with the (OH)-O-center dot -> NO3 center dot route. Meanwhile, XANES and in situ DRIFT spectroscopy experiments corroborated that alpha-MnO2-N provided a larger concentration of surface NO3- species with bi-dentate binding arrays than gamma-MnO2-N. Hence, alpha-MnO2-N could outperform gamma-MnO2-N in improving the collision frequency between (OH)-O-center dot and NO3- species and in facilitating the exothermic transition of NO3- functionalities to surface NO3 center dot analogues per unit time. These were corroborated by a greater efficiency of alpha-MnO2-N in decomposing phenol, in addition to scavenging/filtration control runs and DFT calculations. Importantly, supported NO3 center dot species provided 5-7-fold greater efficiency in degrading textile wastewater than conventional (OH)-O-center dot and supported SO4 center dot- analogues we discovered previously.

Automated summary

This study introduces a novel method to radicalize NO3- functionalities on polymorphic α-/γ-MnO2 surfaces, enabling the transformation of NO3• to (OH)-O• and demonstrating the superior efficiency of α-MnO2-N in decomposing organic pollutants in water. The surface NO3- concentration and bi-dentate binding arrays provided by α-MnO2-N enhance the collision frequency between (OH)-O• and NO3- species, facilitating the exothermic transition of NO3- functionalities to surface NO3• analogues. These findings suggest that supported NO3• species are 5-7 times more efficient in degrading textile wastewater compared to conventional (OH)-O• radicals and supported SO4•- analogues.