ZnO-doped lignin-based carbon as a catalyst for ciprofloxacin photocatalytic degradation

Herein, a lignin-based carbon loaded with zinc oxide (LC/ZnO) was synthesized from alkali lignin through a facile activated carbonization, and used for ciprofloxacin (CIP) photocatalytic degradation. The results showed that the prepared LC/ZnO composite possessed large surface area and high porosity, and the ZnO nanoparticles had a good distribution on the surface of LC. The absorption intensity of visible light of ZnO also remarkably increased after combination with LC, which all conducive to follow-up photo-induced electrons (e^-) and holes (h⁺) generation and separation. LC/ZnO displayed excellent performance and stability for CIP photodegradation in a large pH range in various simulative or actual water bodies, and the maximum degradation rate of CIP within 120 min reached up to 99.8 %. The effects of various anions and organic acid on CIP degradation accorded with the following order: humic acid (HA) > HCO₃^- > SO₄^2- > Cl^-. Scavenging experiments and electron spin resonance (ESR) analysis confirmed that ·OH and ·O₂^- free radicals as well as the photo-generated h⁺ were the main active species for CIP degradation over LC/ZnO. Finally, three possible degradation pathways together with potential mechanism of CIP photodegradation by LC/ZnO were also proposed based on ten detected intermediates.