Sunlight-Driven Photocatalytic Degradation of Methylene Blue Using Oxygen-Doped g-C ₃ N ₄ : Process Efficiency and Life Cycle Assessment

Photocatalysis among various advanced oxidation processes is one of the efficient methods to degrade and mineralize the chemically complex dye molecule and produce intermediate and end products. Graphitic carbon nitride (g-C3N4) is an environmentally friendly potent metal-free photocatalyst because it works under visible light, is inexpensive, and is easy to prepare. However, the bulk g-C3N4 has certain limitations such as poor light absorption ability and high carrier recombination rate. In this study, oxygen-doped g-C3N4 (OCN) was synthesized by the calcination method. The prepared material was characterized by X-ray diffraction, field emission scanning electron microscopy, Fourier transform infrared spectroscopy, ultraviolet–visible diffuse reflection spectra, and photoluminescence spectrum analysis. The decolorization of methylene blue (MB) was studied under various operational conditions under sunlight irradiation. The effects of initial MB concentration, OCN dose, and initial pH of the solution on photocatalytic MB degradation were examined. The experimental results revealed that the decolorization efficiency of 10 mg/L of MB was found to be 99.25% for 1.4 g/L of OCN dose and approximately 66,000 lx of sunlight irradiation for 210 min duration. The degradation followed the first-order kinetics under different operational conditions. The degradation was more favorable for a higher pH range. The reproducibility was found to be within ±3.95% for the degradation of MB. A cradle-to-gate life cycle assessment (LCA) study was conducted to determine the environmental impact of the photocatalytic process. The end-point results of the LCA study confirmed a negative impact on human health (0.00027 disability-adjusted life years), ecosystem quality (26.74 PDF·m2·year), and climate change (207.88 kg CO2), and resource depletion (3,060.82 MJ) was mainly driven by the use of urea and electricity in the process.