Can environmental pharmaceuticals be photocatalytically degraded and completely mineralized in water using g-C3N4/TiO2 under visible light irradiation?—Implications of persistent toxic intermediates

• Acyclovir can be fully degraded within 90 min using g-C3N4/TiO2 under visible light.
• Acyclovir is difficult to be mineralized under same condition.
• Three intermediates produced during the photocatalytic process remain persistent.
• The aquatic toxicity of intermediate, guanine, was more toxic than acyclovir.

This study investigated the feasibility of photocatalytic degradation and detoxification of antiviral pharmaceuticals using a novel graphitic carbon nitride (g-C3N4)/TiO2 hybrid photocatalyst under visible light irradiation. The results indicated that acyclovir is difficult to be mineralized, although it could be completely degraded within 90 min using this high stable hybrid photocatalyst. Further investigation found that three main intermediates (P1, P2, and P3) produced during the photocatalytic process remain persistent, due to the low oxidation potential of the highest occupied molecular orbital (HOMO) of g-C3N4. The acute and chronic toxicities of acyclovir and these three intermediates were assessed at three trophic levels with theoretical calculated data obtained by the “ecological structure-activity relationships” program. The results found that toxicities of two of the intermediates P1 and P2 were lower than the toxicity of acyclovir to three levels tested organisms. However, the aquatic toxicity of the third intermediate P3, guanine, was more than double that of acyclovir, although most toxicity values still fell in the same toxic class except for the chronic impact on daphnia (acyclovir is harmful, and the guanine intermediary is toxic). This study's findings support the selection of new photocatalysts for purifying and detoxifying environmental pharmaceuticals in water.

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