Conductive polymer/sulfur-doped graphitic carbon nitride with π-π conjugated structure for photocatalytic degradation of tetracycline in high and low salinity aquatic systems: Unveiling switching between free radical and non-radical pathways

Despite its eco-friendly nature, the use of photocatalysis in the treatment of high-salinity wastewater continues to pose significant challenges due to the unclear mechanism of how salinity impacts free radical generation. This study utilizes an electrostatic self-assembly method to integrate polythiophene (PTh) - a conductive polymer - with sulfur-doped graphitic carbon nitride (S-CN), thereby creating a composite of π-π conjugated photocatalyst (PTh/S-CN). Sulfur doping introduces an intermediate band gap in the catalyst, enhancing photon absorption. The hybridization also significantly broadens the light absorption spectrum of the composite, facilitating electron transfer to the triazine ring. By comparing the changes of free radicals during PTh/S-CN degradation of tetracycline (TC) in high-salinity and non-high-salinity waters, the formation processes of superoxide radical (O₂^-), hydroxyl radical (OH) and singlet oxygen (¹O₂) generation were elucidated. Experimental results showed that the degradation rate of TC by 5% PTh/S-CN reached 80.9% under visible light irradiation. High-salinity water bodies are more conducive to the production of ¹O₂. The degradation pathway of TC and the toxicity of its intermediates were evaluated. It was confirmed that high-salinity water favored ¹O₂ production when other conductive polymers (such as polyaniline (PANI) and polypyrrole (PPy)) were hybridized with S-CN. This study highlights the free radical transformation pathways in high-salinity and non-high-salinity waters, underscoring the key role of non-radical pathways in high salinity water. It also offers new insights for designing photocatalytic materials suitable for high salinity wastewater treatment.