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

摘要

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 m(2) 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.