Synergistic effect of microwave irradiation and conjugated polymeric catalyst in the facile degradation of dyes

Abstract

Microwave-enhanced photodegradation of dyes is one of the emerging and promising technologies for waste water remediation. Microwaves effectively accelerate photocatalytic degradation, but only in the presence of a suitable photocatalyst such as TiO₂, ZnO and also when a microwave electrodeless lamp (MEL) substitutes a traditional lamp as a light source. As the existing inorganic photocatalysts have been proven to be potentially toxic to the aquatic environment, this remediation technique can be extremely simplified if the photocatalyst can be replaced by a benign catalyst which can work under microwave irradiation in the absence of any external light source. In the present study, an attempt is made for the first time to degrade and mineralize Orange G (OG) dye in a laboratory microwave oven at 30 °C, using an organic catalyst, poly(1-naphthylamine) (PNA), a conjugated polymer synthesized by an enzymatic method. PNA was characterized by relevant experimental techniques. The degradation was carried out by exposing the OG dye solutions to microwave irradiation for different time intervals in the absence of UV-vis radiation and TiO₂. PNA as a catalyst was found to enhance the dye degradation under microwave irradiation by almost two times as compared to its degradation under microwave irradiation alone. 100 ppm of OG dye solution was found to degrade up to 90% in 20 min at 30 °C in the presence of PNA. The same solution revealed mineralization up to 85% in 40 min as confirmed by the total organic content (TOC) analysis. With the help of LC-MS, seven intermediates were identified ranging between m/z 227 and m/z 97, on the basis of which a tentative degradation pathway for the dye has been proposed. Dye degradation in the presence of PNA as a microwave catalyst under the present experimental setup was found to yield results better than other photocatalytic or microwave-assisted photocatalytic degradation methods.