Photocatalytic degradation of phenolic pollutants using N-methylquinolinium and 9-mesityl-10-methylacridinium salts

Abstract

[EN] The photodegradation of a mixture of phenolic pollutants including: phenol (P), orto-phenylphenol (OPP), 2,4,6-trichlorophenol (TCP) and pentachlorophenol (PCP) was accomplished using two organic cationic photocatalysts, namely N-methylquinolinium (NMQ(+)) and 9-mesityl-10-methylacridinium (Mes-Acr-Me+) salts, due to their singular photophysical and redox properties. On one hand, NMQ+ exhibits more energetic excited states and accordingly more favorable redox potentials than Mes-Acr-Me+; on the other hand, NMQ(+) absorption reaches only up to 380 nm, while Mes-Acr-Me+ extends in the visible up to 480 nm. Evaluation of the efficiency of both photocatalysts, revealed that the highest level of photodegradation was achieved when they were employed at 20% mol. Specifically, with NMQ(+), removal of the pollutants was completed within 24 h of irradiation. Even more, irradiation time could be shortened from 24 to 8 h, since high levels of removal were already achieved (93%, 100%, 100% and 82% for P, OPP, TCP and PCP, respectively). Albeit, Mes-Acr-Me+ was not as effective, and best results were obtained using 20% mol upon 24 h of irradiation. Under these conditions, removal of PCP was 80%, while TCP was 40%, OPP 30% and P resulted in the most recalcitrant contaminant with only 10% of removal. Next, NMQ(+) and Mes-Acr-Me+ were separately supported onto Zeolite Y, an inert inorganic support (Y-NMQ(+) and Y-Mes-Acr-Me+), and elemental analyses revealed a loading of ca. 13% and 15% weight for NMQ(+) and Mes-Acr-Me+, respectively. Upon heterogenization, in the case of Y-NMQ(+), the extent of removal was lower than the one achieved in the homogeneous photodegradations. On the contrary, performance of Y-Mes-Acr-Me+ improved, because of its enhanced photostability; thus, upon 46 h irradiation, 98%, 80%, 40% and 26% for PCP, TCP, OPP and P, respectively, was achieved. Moreover, their efficiency was maintained upon second use. Steady-state and time-resolved fluorescence quenching revealed that every pollutant was able to quench the singlet excited state of both 1(NMQ(+))* and 1(Mes-Acr-Me+)*, with kinetic rate constants in the order of the diffusion limit. Thus, Type I photooxidation happening through the singlet excited state of either photocatalyst was the main operating process in the photodegradation of the studied pollutants.