Photocatalytic degradation of reactive black 5 from synthetic and real wastewater under visible light with TiO2 coated PET photocatalysts

The photocatalytic removal of Reactive Black 5 (RB5) was investigated using a titanium dioxide-polyethylene terephthalate (TiO₂-PET) catalyst under visible (VIS) light. The sol-gel method was employed for the fabrication of the TiO₂-coated PET catalyst, which was then characterized using X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), and elemental mapping (MAP) analysis. This study examined the reaction kinetics using the one-factor-at-a-time (OFAT) approach and evaluates the effects of various parameters, including pH (3-11), catalyst dosage (0.1-1 g L^{- 1}), contact time (15-120 min), RB5 concentration (10-50 mg L^{- 1}), hydrogen peroxide (H₂O₂) content (2-100 mM), purging gases, organic compound types, and ionic strength, on the photocatalytic removal of RB5. Under optimal conditions (pH= 3, [RB5]_°= 20 mg L^{- 1}, nanocatalyst dosage= 0.5 g L^{- 1}), 99.99% of the dye was removed after 120 min. Increasing the RB5 concentration (10-50 mg L^{- 1}) resulted in a decrease in the observed reaction rate constant (k_obs) from 0.052 to 0.0017 min^{- 1}, while the calculated electrical energy per order (EEO) increased from 11.08 to 338.82 kWh m^{- 3}. Furthermore, the total operating cost of the light emitting diode (LED)/TiO₂-PET process (3 USD kg^{- 1}) was lower than that of other photocatalytic processes, including LED/TiO₂ (4.73 USD kg^{- 1}), LED/PET (40 USD kg^{- 1}), and LED (63.16 USD kg^{- 1}). The removal of RB5 was negatively affected by the presence of H₂O₂, O₂ and N₂ gases, organic compounds, and ionic species. Radical quenching experiments confirmed that hydroxyl radicals (^·OH) were the dominant reactive species responsible for RB5 degradation. The RB5 removal efficiency using the LED/TiO₂-PET method (99.99%) was significantly higher than that of the LED/TiO₂ method (63.42%). Desorption experiments demonstrated excellent catalyst stability, maintaining catalytic activity for up to five sequential cycles. GC-MS analysis identified several intermediate degradation products, including 1,2-benzenedicarboxylic acid, benzoic acid (2-amino-, methyl ester), benzene [(methylsulfonyl) methyl], phenol, 4-naphthalenedione, acetic acid, and propionic acid. Moreover, the removal efficiency in drinking water samples was approximately 63.31%, whereas for real textile wastewater samples, it reached 96.66%. Toxicity tests conducted on the final treated solutions confirmed no toxicity toward Daphnia magna, demonstrating the effectiveness of the LED/TiO₂-PET method in degrading both RB5 dye and its toxic by-products.