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

Abstract

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 k obs 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.

Biochar-Derived Persistent Free Radicals: A Plethora of Environmental Applications in a Light and Shadows Scenario

“…Usually, free radicals are highly unstable and rapidly react with each other, thus being destroyed as soon as they form, with a consequent very short half-life. However, it has been found that in BC, some free radicals, named persistent free radicals (PFRs), like the radicals that naturally occur in the environment, known as environmental persistent free radicals (EPFRs), can remain stable for months and play a crucial role in the subsequent reactions of oxidative degradation carried out by BC containing them [25,107,125,126] (Figure 6). Unlike other free radicals, PFRs are resonance-stabilized since they are bound to the external or internal surface of solid particles of BC.…”

Reactive Oxygen Species (ROS)-Mediated Antibacterial Oxidative Therapies: Available Methods to Generate ROS and a Novel Option Proposal

“…If BC is conserved under vacuum, lifetime of PFRs could be infinite (no decay radicals), while when air-exposed they react with molecular oxygen in the air, decay over time, thus producing ROS. Similarly, in aqueous systems, PFRs act as transition metals like Fe 2+ , forming ROS, as well [301][302][303][304]. PFRs are categorized into three classes, i.e., oxygen-centered PFRs (OCPFRs), carbon-centered PFRs (CCPFRs), and oxygenated carbon-centered radicals (CCPFRs-O).…”