Hydroxylamine promoted Fe(III)/Fe(II) cycle on ilmenite surface to enhance persulfate catalytic activation and aqueous pharmaceutical ibuprofen degradation

This study demonstrates a new system for the degradation of emerging pharmaceutical contaminants (e.g., ibuprofen) in water by coupling the naturally occurring ilmenite with hydroxylamine (HA) and persulfate (PS). Ilmenite was able to activate persulfate to generate sulfate radicals (SO4−·) and hydroxyl radicals (HO·). The radical generation was greatly improved by adding small amount of hydroxylamine into the solution, due to the efficient Fe(III)/Fe(II) cycle on the ilmenite surface promoted by HA, which was confirmed by X-ray photoelectron spectroscopy and electron paramagnetic resonance (EPR) spectroscopy analysis. SO4−· and HO· contributed comparably to ibuprofen degradation, which was verified by the radical scavenging tests. The degradation was enhanced with increasing ilmenite, PS and HA dosages, but the HA exhibited strong scavenging effect at its high concentrations. The ilmenite/PS/HA process worked well in the real treated wastewater, because the surface-controlled radical generation was less affected by the water matrix. However, the formation of bromate in the bromide-containing water by this process should be concerned. Ibuprofen was partially mineralized, and the degradation products were identified by ESI-tqMS. A radical-induced degradation pathway was proposed based on the product identification. This work provides the mechanistic insights on persulfate activation based on the surface-controlled catalytic processes. It also offers a new strategy to degrade emerging contaminants in water and sheds light on the environmental functions of natural minerals.