Degradation of N-Oxyl Radical Compounds by Fenton Reaction

2,2,6,6-Tetramethylpiperidine-1-oxyl radical (TEMPO) and their derivatives, 4-acetamide-TEMPO and 4-hydroxy-TEMPO, were dissolved in water, and their degradation behavior by Fenton reaction was studied under various conditions, considering the treatment of the N-oxyl radical-containing washing effluents formed during TEMPO-mediated oxidation of various celluloses. The N-oxyl radicals were treated with FeSO4 and H2O2 in water at pH 2.7 and 35 ̊C for various times, and time-dependent concentrations of the N-oxyl radicals were determined by electron spin resonance. When 0.29 mmol FeSO4 and 0.34 mmol H2O2 were used to the 100 mL solution containing 0.01 g N-oxyl radical (i.e., ~100 ppm), its concentration became below the detection limit (< 0.01 ppm) after Fenton reaction for 90 min. When 5% (v/v) ethanol, dimethylsulfoxide or acetone was present in the TEMPO-containing solution, similar degradation behavior by Fenton reaction was observed. The degradation products of TEMPO present in the supernatant and solid fractions formed during Fenton reaction were analyzed by gas-chromatography-mass spectroscopy, solid- and liquid-state 13C-NMR, ion chromatography and others. The results indicate that the N-oxyl radical compounds were mostly degraded to CO2, water and nitrate/nitrite ions by Fenton reaction. Thus, Fenton reaction is available for the treatment of washing effluents formed during TEMPO-mediated oxidation of celluloses.