Metal-free black-red phosphorus as an efficient heterogeneous reductant to boost Fe3+/Fe2+ cycle for peroxymonosulfate activation

In this work, a novel metal-free black-red phosphorus (BRP) was prepared from red phosphorus (RP) and applied in Fe2+/peroxymonosulfate (PMS) process. Compared with that of RP, the contaminant degradation performance of BRP was significantly elevated due to the enhanced electron transfer from BRP to Fe3+. This enhancement was mainly induced by size decrease effect, the removal of oxidation layer and the partial phase conversion. Moreoevr, BRP avoided the radical quenching reaction caused by reductant itself, whereas it was inevitable using homogeneous reductant like hydroxylamine. More importantly, the system had a superior recyclability and strong resistance to natural water. Though concurrent side-reaction between PMS and BRP occured, multiple PMS dosage could remarkedly alleviated the side-reaction, thus elevating PMS utilization efficiency. The dominant BRP oxidation products included phosphite and phosphate. Interestingly, moderate increase of Fe3+ concentration could efficiently reduce the by-product formation via the prompt PMS activation by regenerated Fe2+. Our work clarified the acceleration mechanism of Fe3+/Fe2+ cycle by BRP and proposed the control strategy of by-prodoct formation.