Effective degradation of norfloxacin on Ag3PO4/CNTs photoanode: Z-scheme mechanism, reaction pathway, and toxicity assessment

The removal of antibiotics was a concerned issue for human, while photoelectrocatalytic (PEC) technology displayed excellent degradation efficiency. Ag3PO4/carbon nanotubes (CNTs) was synthesized for PEC degradation of norfloxacin (NOR) in this work. The effects of applied bias voltage, Ag3PO4 content, pH, initial concentration of NOR, and supporting electrolyte concentration on the degradation efficiency of NOR were investigated. Under the optimal experimental conditions, the degradation efficiency of NOR could reach up to 93% within 30 min. Compared with photocatalytic (30%) and electrocatalytic (76%) degradation within 120 min, PEC degradation (100%) exhibited the highest degradation efficiency and a significant synergetic effect (74.9%). The prepared Ag3PO4/CNTs photoanode showed appropriate stability as the degradation efficiency of NOR remained 82% after 5 cycles. The excellent degradation efficiency of Ag3PO4/CNTs photoanode was explained based on the Z-scheme mechanism, which significantly promoted the separation of electron-hole pairs. During the degradation process, the contribution of h+ and ·O2– was the most major to oxidize NOR. Moreover, three degradation pathways were proposed by analysis of mass spectrometry. The toxicity of NOR and intermediates was approximately reduced 100% for Escherichia coli ATCC25922 (E. coli ATCC25922) in the PEC degradation process within 120 min. Based on the ecological structure–activity relationship (ECOSAR) program, some intermediates with higher toxicity generated compared with the parent compound in the PEC degradation process. Nevertheless, the PEC system showed low ecotoxicological risk according to the calculated result of ECOSAR. This work provided a Z-scheme mechanism for Ag3PO4/CNTs, which could degrade NOR and reduce its toxicity efficiently.