Oxidative decolorization of indigo carmine dye using spent cathode of Li-ion batteries

This paper proposes recycling spent cathodes (SC) from Li-ion batteries to decolorize indigo carmine (IC) solutions, thus eliminating the need for hydrogen peroxide (H₂O₂) or ultraviolet (UV) light exposure. The discoloration of indigo carmine is significantly influenced by the pH level, especially from 5 to 2. At lower pH values (pH ≤ 2), the decolorization kinetics are not affected by H⁺ ions and follow first-order kinetics. A 10-ppm solution at pH = 3 was decolorized in approximately 20 min, suggesting that SC from Li-ion batteries are promising for advanced oxidative processes. The study also emphasizes the risks associated with improper disposal of Li-ion batteries, as SC can chemically react with organic molecules similar to indigo carmine in landfills, without the presence of peroxide or sunlight. Mass spectrometry has helped identify the discoloration mechanism, which involves the oxidative reaction of indigo carmine leading to the formation of isatin acid.