Insights into the non-radical elimination mechanism of DOPO in trace TEMPO+ enhanced UVB/Periodate process

Periodate-based advanced oxidation processes (AOPs) have gained significant interest in water treatment. However, conventional periodate-based AOPs generate inorganic radicals such as hydroxyl radicals and periodate radicals, which are easily scavenged by water matrices, leading to an efficiency decrease of contaminant degradation. This study focuses on a novel non-radical dominated pollutant degradation process that employs 4-Acetamido-2,2,6,6-tetramethyl-1-oxopiperidinium (TEMPO+) as an electron shuttle to enhance the ultraviolet light B rays-emitting diodes (UVB-LEDs) activated periodate (PI) process, to rapid eliminate 9,10-Dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO, an organic phosphorus flame retardant parent compound). Results from electron paramagnetic resonance and probe compounds tests demonstrated that the introduction of a trace amount of TEMPO+ (0.10 μM) strongly enhanced DOPO degradation, primarily depending on a consistent cycle of TEMPO+/TEMPOH. The observed degradation rate constant (kobs-DOPO) in the UVB/PI/TEMPO+ process was measured at 2.42 × 10-3 s−1, which represents a 2-fold increase compared to the degradation rate of 1.21 × 10-3 s−1 in UVB/PI process. The degradation rate of DOPO within 15 min also increased from 60 % in UVB/PI process to 87 % in UVB/PI/TEMPO+ process. Besides, this enhancement of DOPO degradation still existed in complicated water matrix conditions including high concentrations of Cl-, HCO3– as well as in natural surface water. Furthermore, pathway analysis and toxicity assessment of the degradation products indicated that the UVB/PI/TEMPO+ process demonstrated superior detoxification capabilities for DOPO in comparison to the UVB/PI process. This study expounds a new mechanism of using LEDs to activate periodate and TEMPO+ as an electron shuttle to rapidly degrade pollutants and detoxification without deeply relying on radical pathways.