Collaborative optimization for the performance of ZnO/biochar composites on persulfate activation through plant enrichment-pyrolysis method

Abstract

A novel method of plant enrichment for ZnO nanoparticles (ZnO NPs) and pyrolysis was employed to gain the ZnO/biochar (ZnO/BC) composites. Comparing to pristine biochar and two zinc oxides/biochar composites obtained by conventional metal salts impregnation method, here the ZnO/BC exhibited distinct advantages in persulfate (PS) activation for tetracycline hydrochloride (TC) degradation. The activation mechanism analysis revealed that PS accepted electrons from persistent free radicals (PFRs) in ZnO/BC to directly or indirectly generate sulfate radicals (SO4∙-) and hydroxyl radicals (HO∙). Coincidentally, ZnO NPs promoted the secretion of total phenols and flavonoids, as precursors of PFRs, which accelerated the formation of more oxygen-centered PFRs with stronger catalytic ability and longer half-life. Moreover, owing to the tight combination of ZnO NPs with internal structures of the plant, higher porosity and large specific surface area in the biochar composites were favorable for reducing the electrochemical impedance of ZnO/BC, which could effectively facilitate the electron transfer process between PFRs and PS. Meanwhile, due to the injection of electrons during PFRs formation, slight narrow in the bandgap brought about the mild enhancement of photocatalytic properties for ZnO NPs. Overall, the method in this research not only provided a new strategy to acquire metal oxide/biochar composites with catalytic ability, but also opened up a novel way to collaboratively optimize the performance of biochar matrix and metal nanoparticles.