Recycled cathode from Li-ion batteries applied to adsorption and demethylation of methylene blue for thionine formation

This study explores the demethylation of methylene blue in an acidic pH, utilizing spent cathode from Li-ion batteries. The spent cathode has a composition of LiMn ₂ O ₄ and a pcz close to pH = 2.1. In contact with methylene blue in an acidic medium (pH = 2), the spent cathode adsorbed and demethylated methylene blue to form thionine. This was confirmed by UV-vis and ESI measurements of the resulting solutions. The intermediate m/z = 300 was crucial for proposing the demethylation mechanism that is similar to a demethylation mechanism catalyzed by hemoproteins. FTIR measurements confirmed the adsorption of methylene blue onto the spent cathode. The rise in pH observed during demethylation can explain to the incomplete degradation of methylene blue stopping at the thionine stage. Consequently, the reaction involving spent cathode of Li-ion battery based on LiMn ₂ O ₄ may be applicable to the demethylation of other molecules in future research.