Valorization of heavy metals enriched ALB adsorbents as effective ZnS-CdS/C photocatalysts for antibiotics removal

Reutilizing heavy metal enriched adsorbents into efficient photocatalysts for antibiotic removal is of great economic value. Herein, spent alkali-lignin biochar (ALB) with enriched Zn ²⁺/Cd ²⁺ ions is valorized into ZnS-CdS/C photocatalyst via a simple vulcanization method. ZnS-CdS/C (3:1) exhibits a superior photocatalytic removal efficiency of 98.6 % for levofloxacin (LEV), with about 60 % of the initial value remained after four cycles. It also shows good photocatalytic removal efficiencies of 92.1 %, 95.4 % and 84.0 % for enrofloxacin, ciprofloxacin and tetracycline, respectively. Electron paramagnetic resonance (ESR) shows that the introduction of ZnS and CdS in ALB promotes the generation of oxygen vacancies and persistent free radicals (PFRs). The photocatalytic LEV removal mechanism of ZnS-CdS/C indicates that both the abundant PFRs and photogenerated electrons serve as electron donors, efficiently reducing dioxygen to superoxide free radicals (·O ₂ ^-). The ·O ₂ ^- formed in the above two ways continuously degrade the LEV molecules. Finally, the biotoxicity and developmental toxicity of LEV degradation intermediates confirms that the ZnS-CdS/C degradation system can remove LEV effectively with reduced harm to the ecological environment.