Molten-Salt-Assisted Preparation of g-C3N4 for Photocatalytic Degradation of Tetracycline Hydrochloride: Degradation Mechanism, Pathway, and Toxicity Assessment

The sustainability of aquaculture tailwater plays a key role in the aquaculture industry. Photocatalytic degradation of recalcitrant antibiotics in aquaculture tailwater has emerged as a significant research focus, with gCN-based photocatalysis offering a promising approach. To address the issue of inefficient degradation associated with gCN, melamine was modified using NaCl solution, resulting in the synthesis of NaMe-x with distinctive microstructure through molten salt assistance. The ability of NaMe-x to degrade tetracycline hydrochloride (TC-HCl) was examined, including an analysis of its degradation pathway, intermediate products, mechanism, and toxicity of the by-products. The results demonstrated that NaCl-based precursor modification markedly enhanced the degradation capacity of gCN for TC-HCl, achieving a maximum degradation rate of 0.02214 min−1, which is 2.1 times higher than that of unmodified gCN. LC-MS analysis revealed intermediates at various degradation stages, and two potential pathways for TC-HCl degradation in the presence of NaMe-1 were identified. In this process, ·O2− and ·OH are the reactive radicals that play a dominant role, and their degradation mechanism is thus proposed. It was confirmed by toxicity experiments that the products after the degradation of TC-HCl by NaMe-1 were not significantly toxic to Chlorella vulgaris (p ˃ 0.05). However, it had a significant effect on Vibrio fischeri (p < 0.01). These findings suggest that the synthesis of NaMe-x via melamine precursor modification substantially improves the degradation performance of gCN and enhances the sustainability of aquaculture tailwater.