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Enhanced removal of phenolic contaminants with permanganate oxidation mediated by SiO2@TEMPO: Performance, transformation pathway, and selectivity

Xiaoyan Chen,Jiangdong Cui,Xinyu Yang,Taoyue Yuan,Jiacheng Ge,Mengtao Ma,Shunke Ding,Wenhai Chu

The core bottleneck in water decontamination lies in trigging the selective removal of trace contaminants in the presence of complex water matrices, overcoming the inevitable consumption of oxidants and energy input by the water matrix. Here, the selective removal of phenolic contaminants with permanganate (PM) oxidation mediated by 2,2,6,6-tetramethylpiperidinooxy (TEMPO)-functionalized silica (SiO 2 @TEMPO) was firstly reported. SiO 2 @TEMPO was synthesized by covalently bonding 4-oxo-2,2,6,6-tetramethylpiperidinooxy onto SiO 2 through a 3-aminopropyltriethoxysilane bridge. The loading capacity of Wt% for nitrogen element in SiO 2 @TEMPO are 1.01–1.27 %. The degradation rate of tetracycline (around 1.50 min −1 ) with SiO 2 @TEMPO/PM are around 15 folds higher than that with PM, attributed to the formation of the reactive N-oxoammonium cation via single-electron transfer during PM oxidation. SiO 2 @TEMPO exhibited excellent stability and recyclability after five consecutive cycles of tetracycline degradation. Three transformation pathways (hydroxylation, demethylation, and dehydration) for tetracycline degradation by SiO 2 @TEMPO/PM were proposed based on the identification of 36 transformation products using HPLC/Q-Orbitrap HRMS. Thereinto, the attack of the N-oxoammonium cation in SiO 2 @TEMPO on the phenolic hydroxyl group to form corresponding hydroxylation and quinylation products accounted for the dominant degradation pathway. SiO 2 @TEMPO/PM also demonstrated remarkable adaptability across a wide pH range of 5.0–9.0, indicating its potential for practical applications. The significant resistance to the inhibitory effects of co-existing water matrix components (humic acid, cations and anions), along with the marked differences in degradation efficiency between phenolic and non-phenolic contaminants, demonstrate the selectivity of SiO 2 @TEMPO/PM towards phenolic contaminants. These results offer new insights into the selective removal of phenolic contaminants and pave the way for the development of efficient and sustainable strategies for water remediation.