Methylene blue beyond the dye: A critical review on its role as a benchmark pollutant in photocatalyst design

Methylene blue (MB), a cationic phenothiazine dye, has long served as a model pollutant in photocatalytic research due to its distinct spectroscopic features, stability, and ease of monitoring. This critical review revisits the MB role in photocatalyst evaluation. It explores its molecular structure, physicochemical behavior, degradation pathways, and limitations in experimental design. While MB provides valuable mechanistic insights into reactive oxygen species generation and surface interactions, its susceptibility to dye-sensitized degradation, tendency for strong adsorption, and risk of incomplete mineralization raise concerns about its environmental representativeness. The review emphasizes the need for methodological standardization and advocates integrating MB testing with complementary analyses, such as total organic carbon (TOC), high-performance liquid chromatography (HPLC), and toxicity assays, to ensure accurate interpretation of photocatalytic efficiency. Furthermore, it highlights the MB evolving role as a probe molecule in multi-pollutant testing frameworks designed for more realistic assessments. The continued use of MB in photocatalytic studies remains justified, provided its limitations are acknowledged and addressed through rigorous protocols. Ultimately, redefining the MB role from a simple dye to a calibrated benchmark supports the development of environmentally viable photocatalysts tailored for real-world remediation challenges.