Persistent free radical-mediated Z-scheme charge transfer in ZnO-ZnS@Biochar: Bypassing redox potential compromise for efficient photocatalytic degradation

Metronidazole (MNZ) poses a significant environmental challenge due to its high bio-resistance and environmental persistence, resulting in refractory residues that are difficult to eliminate via traditional biochemical methods. Although ZnO-ZnS heterojunctions can facilitate charge separation, the traditional Type-II migration pathway inevitably directs photogenerated carriers toward energy bands with weaker redox potentials. This results in a redox potential compromise that severely limits the intrinsic reaction kinetics of the system, particularly in neutral environments. To address this critical issue, we developed a novel ZnO-ZnS@Biochar (HZSBC) heterojunction using straw biochar as an economical carbon host. The contribution of this work lies in discovering that interface-anchored persistent free radicals (PFRs) function as non-metallic solid-state electron relays. By inducing directional charge shuttling, these PFRs circumvent the aforementioned potential compromise, thereby establishing a Z-scheme charge transfer pathway that preserves exceptional redox capabilities. Via a dual-site anchoring-in situ transformation strategy, high-density oxygen-centered PFRs (spin concentration reaching 20.05 ×10 17 spins/g) were precisely engineered at the HZSBC interface, providing stable electron relay channels for the Z-scheme mechanism. Consequently, HZSBC achieved 90.2% MNZ removal within 110 min under neutral conditions (pH 7.0) with only 8 mM H 2 O 2 , exhibiting an apparent rate constant of 1.01 × 10 −2 min −1 and maintaining over 80% removal efficiency after five consecutive cycles. By overcoming the thermodynamic limitations of photocatalysis via PFRs-mediated interfacial band engineering, this work not only offers a new paradigm for the design of high-performance Z-scheme catalysts but also provides an ideal technical pathway for the deep coupling of advanced oxidation processes with downstream biological treatment units.