Reductive Stability of Organic Dyes in Nanocatalyst-Assisted Sodium Borohydride Systems

Catalytic reduction of organic dyes using nanocatalysts and sodium borohydride (NaBH4) is widely regarded as an effective strategy for wastewater remediation. However, key mechanistic details underlying the long-term stability of dye removal have remained inadequately addressed. This work provides a systematic comparative investigation of the catalytic reduction behaviors of cationic dyes (methylene blue and rhodamine B) and anionic dyes (methyl orange and congo red) under nanoparticle catalysis with controlled atmospheric conditions. Our results demonstrate that cationic dyes undergo a reversible transition to colorless forms via NaBH4 reduction but are reoxidized back to their original-colored state upon exposure to atmospheric oxygen or hydrogen peroxide, confirming the nonpermanent nature of dye removal. In contrast, anionic dyes exhibit irreversible reduction, with decolorization persisting even after prolonged oxidant exposure. These findings, supported by UV–visible and Fourier-transform infrared spectroscopy, reveal critical roles of molecular structure and functional groups in dictating dye redox stability. By explicitly differentiating mechanistic pathways for cationic and anionic dyes, this study provides a new framework for evaluating nanocatalyst-assisted dye remediation. These insights underscore the need for rigorous mechanistic assessment of catalytic dye treatment strategies in environmental applications.