Stability of 5,5-dimethyl-1-pyrroline-N-oxide as a spin-trap for quantification of hydroxyl radicals in processes based on Fenton reaction

Fenton reaction was used to produce hydroxyl radicals under conditions similar to AOPs with 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) as a spin trap agent in electron paramagnetic resonance (EPR) analysis. A theoretical kinetics model was developed to determine conditions under which the spin-adduct DMPO-OH is not further oxidized by Fe³⁺ and excessive radicals, so that hydroxyl radicals concentration could be accurately inferred. Experiments were designed based upon the model and H₂O₂ and Fe²⁺ concentrations were varied from 1 to 100 mM and from 0.1 to 10 mM, respectively, with a constant H₂O₂: Fe²⁺ ratio of 10:1. Results confirmed that DMPO concentration should be at least 20 times higher than the concentration of H₂O₂ and 200 times higher than iron concentration to produce stable DMPO-OH EPR signal. When DMPO: H₂O₂ ratio varied from 1 to 10, DMPO-OH could generate intermediates and be further oxidized leading to the apparition of an additional triplet. This signal was attributed to a paramagnetic dimer: its structure and a formation mechanism were proposed. Finally, the utilization of sodium sulfite and catalase to terminate Fenton reaction was discussed. Catalase appeared to be compatible with DMPO. However, sodium sulfite should be avoided since it reacted with DMPO-OH to form DMPO-SO₃.