Degradation of organic pollutants by NiFe2O4/peroxymonosulfate: efficiency, influential factors and catalytic mechanism

Abstract

Nickel ferrites (NiFe₂O₄) were prepared through thermal decomposition of homogeneous nickel oxalate and ferrous oxalate, and the product displayed typical spinel structure, small nanoparticle size (ca. 12 nm), high BET surface (53.5 m² g⁻¹), and good magnetic response (19.3 emu g⁻¹). The as-prepared NiFe₂O₄ was applied in heterogeneous catalysis to generate powerful radicals from peroxymonosulfate (PMS) for the removal of recalcitrant pollutant. Herein, benzoic acid (BA) was employed as a stable model organic pollutant, and it was found that NiFe₂O₄/PMS system could realize 82.5% degradation in 60 min and maintain its catalytic efficiency during four recycling experiments. Such a catalytic performance of NiFe₂O₄ was indeed superior to those from Fe₂O₃ (23.5%), Fe₃O₄ (48.0%), NiO (57.6%), and MnFe₂O₄ (63.8%). Although NiFe₂O₄ performed a slightly inferior BA degradation to CoFe₂O₄ (86.2%), its nickel leaching (0.265 mg L⁻¹) was much less than cobalt leaching (0.384 mg L⁻¹) from CoFe₂O₄. In addition, some potential influential factors, including the dosages of PMS and NiFe₂O₄, the initial pH value, ion strength, and the concentrations of bicarbonate, natural organic matter, halide, were systemically investigated. More importantly, NiFe₂O₄/PMS were also effective for BA removal under some actual water background conditions, especially in surface water and the finished water from drinking water treatment plant: the degradation efficiencies of BA were still close up to 60%. Sulfate and hydroxyl radicals were confirmed to be the main reactive species in the NiFe₂O₄/PMS system. XPS spectra revealed that Ni sites on the surface of NiFe₂O₄ were the primary active sites, and Raman spectra suggested that inner-sphere complexation between PMS and Ni sites derived peroxo species on the surface, which were further responsible for the efficient generation of radicals.