Multi-channel flow reactor design for the photocatalytic degradation of harmful dye molecules

This study investigates the design of a serpentine continuous photocatalytic reactor for the degradation of methylene blue (MB) dye as a model contaminant of the synthetic textile wastewater under sunlight irradiation using supported zinc oxide nanorods (ZnO NRs) catalyst. ZnO NRs were fabricated on glass substrates using the microwave-assisted hydrothermal method and characterized by different techniques. The characterized results show that wurtzite crystal structure ZnO NRs has native defects in the visible region. Water contact angle measurement shows the highest wettability of ZnO NRs surface at pH 8. Photocatalytic degradation was conducted with MB dye under sunlight in a batch system followed by a continuous flow reactor. In the batch reactor, three different concentrations of 10, 50, and 100 ppm MB were examined under sunlight. MB degradation reached 100%, 83%, and 62% under sunlight for 10, 50, and 100 ppm, respectively, after 3 h of irradiation. In the continuous flow reactor, 100 ppm MB was degraded 80%, 67%, 58%, and 46% at flow rates of 1.5, 2.5, 4.5, and 6.0 ml/min, respectively. The photocatalytic degradation of MB dye follows pseudo-first-order with the highest absorption rate constant of about 0.0066 h^-1 obtained for 1.5 ml/min flow rate. Furthermore, a multi-step process using a continuous flow reactor achieved ~ 90% MB degradation after five repeated steps. Moreover, the reusability of ZnO NRs catalysts was conducted for five repeated cycles using two different cleaning methods of used ZnO NRs, whereby the photo cyclic cleaning process achieved 71%, and the regular cleaning process attained only 45% degradation.