Effects Of Heterogeneous Metals On The Generation Of Persistent Free Radicals (PFRs) As Critical Redox Sites In Iron-based Biochar For Persulfate Activation

Currently,persulfate-based advanced oxidation processes（PS-AOPs）are a research hotspot in the field of water pollution control.A great deal of research has been done on the use of biochar as a means of heterogeneous activation.In pursuit of higher activation performance,many complex materials have been designed using functionalization methods by metal or even mixed metals,involving mechanisms such as redox pairs cycling of Mⁿ⁺/M^（n-1）+,improved electrical conductivity and optimized structure of biochar.However,the effect of metal components on the generation of persistent free radicals（PFRs）within biochar was neglected.Moreover,little is known about the effect of interactions between metal components on the formation of PFRs within biochar when pyrolyzing biomass containing multiple metal components（e.g.,sewage sludges and hyperaccumulators）.Herein,iron-modified biochar（Fe O/BC）as well as biochar-supported bimetallic composites（Zn Fe O/BC,Co Fe O/BC,Ni Fe O/BC）were prepared using an impregnation-coprecipitation-pyrolysis method.Addition of heterogeneous metals（Zn,Co,Ni）resulted in different surface characteristics and structures of iron-based biochar（IBCs）.The iron component contributes to the PFRs generation.However,the situation varies when heterogeneous metals other than iron were further introduced into biomass.Due to the synergistic effect,the coupling of Fe with Co and Ni further enhances the PFRs strength in IBCs.While Zn inhibits the generation of PFRs by blocking the electron transfer from phenolic compounds to metal atoms centre during biomass pyrolysis.Furthermore,the performance of IBCs for catalytic PS activation towards TC degradation was evaluated.Positive correlation coefficients of 0.980 and 0.968 were found between k_obs values and PFRs intensities or the consumed PFRs intensities.Moreover,positive correlation coefficients of 0.870 and 0.786 between the peak intensities of DMPO-OH&SO₄ and PFRs intensities and the consumed PFRs intensities within IBCs were also observed.The results indicated that PFRs are the critical redox sites within IBCs for persulfate activation.Direct electron transfer mechanism was identified during persulfate activation by XPS analysis,electron quenching experiment of K₂Cr₂O₇ and linear sweep voltammetry（LSV）.This study provides new methods for PFRs regulation and updates the understanding of the PFRs formation within IBCs influenced by heterogeneous metals.