Photocatalytic degradation of organic micropollutants under UV-A and visible light irradiation by exfoliated g-C₃N₄ catalysts

In the present study, the photocatalytic performance of exfoliated graphitic carbon nitride (g-C₃N₄) catalysts, with enhanced properties and response in UV and visible light irradiation, was evaluated for the removal of selected contaminants i.e., diuron, bisphenol A and ethyl paraben. Commercial TiO₂ Degussa P25 was also used as a reference photocatalyst. The g-C₃N₄ catalysts demonstrated good photocatalytic activity which in some cases is comparable to TiO₂ Degussa P25 leading to high removal percentages of the studied micropollutants under UV-A light irradiation. In contrast to TiO₂ Degussa P25, g-C₃N₄ catalysts were also able to degrade the studied micropollutants under visible light irradiation. For all the studied g-C₃N₄ catalysts under both UV-A and visible light irradiation, the overall degradation rate decreases in the order of bisphenol A > diuron > ethyl paraben. Among the studied g-C₃N₄, the chemically exfoliated catalyst (g-C₃N₄-CHEM) showed superior photocatalytic activity under UV-A light irradiation due to its enhanced characteristics, such as pore volume and specific surface area and ~ 82.0 % in 6 min, ~75.7 % in 15 min and ~ 96.3 % in 40 min removals were achieved for BPA, DIU and EP, respectively. Under visible light irradiation, the thermally exfoliated catalyst (g-C₃N₄-THERM) demonstrated the best photocatalytic performance and the degradation ranged from ~29.5 to 59.4 % after 120 min. EPR data revealed that the three g-C₃N₄ semiconductors generate mainly O₂^•-, whereas TiO₂ Degussa P25 generates both HO^• and O₂^•-, the latter only under UV-A light irradiation. Nevertheless, the indirect formation of HO^• in the case of g-C₃N₄ should also be considered. Hydroxylation, oxidation, dealkylation, dechlorination and ring opening were the main degradation pathways. The process proceeded without significant alterations in toxicity levels. Based on the results, heterogeneous photocatalysis using g-C₃N₄ catalysts is a promising method for the removal of organic micropollutants without the formation of harmful transformation products.