Activation of peroxymonosulfate by iron-based catalysts for orange G degradation: Role of hydroxylamine

Magnetite nanoparticles (Fe₃O₄) have received considerable and increasing attention and been applied as activators for peroxymonosulfate (PMS) to generate reactive radicals for degrading organic contaminants. However, the slow transformation of Fe(iii)/Fe(ii) limits its repetitive and widespread application. Hence, hydroxylamine (NH₂OH), a common reducing agent, was introduced to a Fe₃O₄/PMS system to accelerate the conversion of Fe(iii)/Fe(ii). With the addition of NH₂OH, orange G (OG) degradation was largely increased at pH 2.0-7.0, and the generation of reactive radicals (sulfate- and hydroxyl-radicals) was significantly promoted. NH₂OH was gradually degraded to N₂, N₂O, NO₂^-, and NO₃^-, while the eco-friendly gas, N₂, was considered as its major end product. It was surprising to find that the catalytic ability of Fe₃O₄ soared gradually in the ten consecutive runs. The effectiveness of NH₂OH for Fe₃O₄, Fe₂O₃ and iron sludge on activating PMS indicated its wide suitability for all the iron-based oxides/hydroxides. Continuous treatment of OG using 1.5 wt% Fe/Al₂O₃ catalyst (iron sludge immobilized on Al₂O₃) packed into a flow column reactor was performed, and no loss in activity over 30 d of operation was observed. Thus the present study provides a promising way to activate PMS for the rapid degradation of refractory organics and dispose the solid iron-based wastes.