Enhanced visible light-driven photodegradation of tetracycline by salicylic acid-modified graphitic carbon nitride and toxicity assessment.

The tetracycline (TC) in water has led to serious concern. Graphitic carbon nitride (g-CN) photocatalysts were produced via copolymerization of mono-benzene ring-mediated precursors (urea, melamine, and dicyandiamide) involving salicylic acid (SA) for TC degradation. The SA-modified g-CN samples showed improved visible light absorbance, transfer and separation of photogenerated electrons, and prospective photocatalytic application in TC degradation. As a result, the optimal SA-modified g-CN (2 wt% of SA) using urea (CNU-SA-2) showed 2 times higher TC degradation than that of pristine g-CN. The process of TC degradation was evaluated by the reduction of antibacterial activity and extensively studied by varying the types of TC, initial pH values, co-existing anions, and natural organic materials. In addition, the catalyst could be reused for at least four cycles, indicating good reusability. The main active species were revealed to be h and ·O by scavenging experiments and electron spin resonance. The CNU-SA-2 photocatalyst and TC intermediates during degradation had no adverse impact on zebrafish embryos. This work could provide a design strategy and a perspective on the practical application of g-CN-based photocatalysts for the treatment of wastewater containing antibiotics.