Radical generation via sulfite activation on NiFe2O4 surface for estriol removal: Performance and mechanistic studies

We report a process using NiFe 2 O 4 nanocatalyst for the heterogeneous activation of sulfite ions for estriol (E3) oxidation. This NiFe 2 O 4 -sulfite system consisted of NiFe 2 O 4 as an catalyst, sulfite as a reductant, O 2 as an oxidant, and oxysulfur radicals (SO 3 − , SO 4 − and SO 5 − ) and hydroxyl radicals (HO ) as generated radicals. The experimental conditions for E3 removal were investigated, including the initial pH values, dissolved oxygen concentration, and NiFe 2 O 4 and sulfite ion dosages. The initial pH has a significant influence on the catalytic performance, and approximately 87.6% of E3 could be removed at pH 9.0. NiFe 2 O 4 nanocatalyst was characterized by high resolution and field emission transmission electron microscopy, X-ray diffractometry, and X-ray photoelectron spectroscopy. The catalytic reactivity and stability of the NiFe 2 O 4 nanocatalyst were investigated by consecutive repeated runs. Radicals generated in this NiFe 2 O 4 -sulfite system were identified via radical-scavenging and electron spin resonance experiments. SO 5 − was identified as major oxidative radicals for E3 removal, which is quite different from the previous studies on sulfite activation . The generation of >SO 3 2− -Ni(II)-OH on NiFe 2 O 4 surface played a critical role in sulfite auto-oxidation. Our results supplemented the mechanism of the sulfate radical generation with sulfite ion as a source.