Effects of Heterogeneous Metals on the Generation of Persistent Free Radicals as Critical Redox Sites in Iron-Containing Biochar for Persulfate Activation

Metal components have critical influences on the generation of persistent free radicals (PFRs) in biochar. However, little is known about the effect on the formation of PFRs brought about by interactions among metal components when pyrolyzing biomass containing multiple metal components (e.g., sewage sludges and hyperaccumulators) for biochar. Herein, iron-modified biochar (FeO/BC) and iron-containing bimetallic oxide-loaded biochar (ZnFeO/BC, CoFeO/BC, and NiFeO/BC) were prepared. Due to the synergistic effect, the coupling of Fe with Co and Ni further enhances the abundance of PFRs in iron-containing biochars (IBCs) compared with FeO/BC catalysts, while Zn inhibits the generation of PFRs via blocking the electron transfer process during biomass pyrolysis. Furthermore, persulfate activation experiments on IBCs were conducted, and a positive correlation coefficient of 0.980 (or 0.974) was found between kobs values and PFR intensities (or the consumption rate of PFRs), indicating that PFRs are doubtless the critical redox sites. Finally, NiFeO/BC as a more efficient catalyst for PS activation was confirmed due to the higher adsorption energy and more transferred electrons based on experimental results and theoretical calculations. This study provides new methods for regulating PFRs in biochar and updates the understanding about heterogeneous metal-containing biomass pyrolysis processes.