Metal-free g-C3N4 photocatalysis of organic micropollutants in urban wastewater under visible light

Abstract

Heterogeneous photocatalysis under visible light was employed for the degradation of organic micropollutants (MPs) found in the biologically treated effluents of an urban wastewater treatment plant (UWWTP). The irradiation source consisted in 4 light emitting diodes (LEDs) with a nominal power of 400–500 W m–2 at 417 nm. Metal-free exfoliated graphitic carbon nitride (gCNT), which was synthetized through a simple thermal treatment using dicyandiamide as precursor, was employed as photocatalyst. The gCNT material was characterized using different techniques: N2 adsorption isotherms at 77 K, electron microscopy (scanning and transmission) and diffuse reflectance UV–Vis. The photocatalytic performance of the material was compared to the commercial TiO2-P25, confirming that gCNT led to a remarkable higher removal efficiency of the target MPs. Most of these MPs were removed in less than 10 min to levels below the limit of quantification (carbamazepine > isoproturon > clopidogrel > diclofenac > atenolol > bezafibrate > tramadol > venlafaxine > fluoxetine). Indirect photolysis, resulting from the secondary reactive oxidants generated during the irradiation of the aqueous matrix components, was found to partially contribute for the elimination of the occurring MPs. In addition, the gCNT photocatalyst was immobilized on glass rings for use under continuous mode operation, a minimum residence time of 25 min being required to attain significant removal efficiencies. Phytotoxicity experiments showed that heterogeneous photocatalysis did not enhance the toxicity of the wastewaters.