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

[Display omitted] •Novel graphitic carbon nitride synthesised from CaCl2-modified melamine precursors can significantly improve catalytic performance.•The free radicals that play a major role in this body system are ·O2− 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, CaCl2-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 CaCl2 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, ·O2− 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