ABNO-Functionalized Silica as an Efficient Catalyst for Enhancing Permanganate Oxidation of Emerging Contaminants.

Recent reports indicate that some soluble electron-shuttling compounds (or organic mediators) can accelerate reactions between permanganate (Mn(VII)) and contaminants of emerging concern. However, practical application is limited to homogeneous electron-shuttling compounds. This study reports on the development and application of a heterogeneous electron-shuttling catalyst for Mn(VII) reactions with bisphenol A (BPA). First, we screened a series of poly/monocyclic nitroxides, finding that 9-azabicyclo[3.3.1]nonane -oxyl (ABNO) provides the most significant enhancement of Mn(VII)/BPA reaction kinetics, where Mn(VII) oxidizes ABNO to BPA-reactive ABNO. Next, we immobilized ABNO onto silica (SiO) by covalent bonding of 9-azabicyclo[3,3,1]nonan-3-one-9-oxyl (keto-ABNO) via a 3-aminopropyltriethoxysilane bridge to yield an ABNO@SiO heterogeneous catalyst. The performance of ABNO@SiO in catalyzing Mn(VII)/BPA reactions is demonstrated, with BPA reaction kinetics being highly dependent on catalyst dosage and pH conditions. The stability of ABNO@SiO was retained at pH 5.0 and decreased slightly at pH 7.0 over five successive Mn(VII)/BPA reaction cycles. Kinetics modeling shows that BPA reacts with immobilized ABNO, Mn(VII), and in situ formed MnO. Moreover, ABNO can form via ABNO reactions with both Mn(VII) and the in situ formed MnO. These results indicate a promising strategy for developing practical heterogeneous catalysts for enhancing Mn(VII) reactivity and treatment applications.