Alkali metal salt modulated visible photocatalytic degradation of tetracycline hydrochloride by g-C3N4: Degradation pathway, mechanism and toxicity assessment

• Novel graphitic carbon nitride synthesised from CaCl 2 -modified melamine precursors can significantly improve catalytic performance. • The free radicals that play a major role in this body system are ·O 2 − and ·OH, and the degradation mechanism is also proposed. • Two possible degradation pathways of tetracycline hydrochloride are proposed. • Toxicity changes before and after the degradation of tetracycline hydrochloride were evaluated by Chlorella vulgaris and luminescent bacteria ( Vibrio fischeri ). The frequent occurrence of tetracycline antibiotics in natural water systems poses a substantial risk to aquatic ecosystems and human health. Achieving efficient degradation of tetracyclines in aqueous environments using visible light is therefore of critical importance. In this study, CaCl 2 -modified melamine precursors were employed to synthesize CaCNx with varied microstructures through molten salt-assisted calcination, enabling high-efficiency visible-light-driven degradation of tetracycline in water. The results indicated that CaCNx demonstrated remarkable catalytic performance in degrading TC-HCl, with CaCN1 synthesized using 1 mol/L CaCl 2 in the precursor exhibiting the highest degradation efficiency, achieving a rate 3.58 times greater than that of CN. In the degradation of tetracycline hydrochloride (TC-HCl) by CaCN1, ·O 2 − was identified as playing a more significant role than ·OH. Furthermore, LC-MS analysis detected seven degradation by-products, which enabled the proposal of a degradation mechanism and two pathways for TC-HCl degradation by CaCN1. Toxicity validation tests revealed that the TC-HCl prodrug caused acute toxicity (p < 0.0001) to Chlorella vulgaris (p < 0.001) and Vibrio fischeri (p < 0.001), whereas its acute toxicity to C. vulgaris (p < 0.01) and V. fischeri (p < 0.01) was significantly reduced following degradation. This study presents a straightforward and efficient approach for visible light-driven degradation of tetracycline in situ within aqueous environments.