Upcycling spent LiCoO₂ cathodes into mixed-phase Co(OH)₂ for light-free bicarbonate–peroxide degradation of methylene blue

High-purity α/β-Co(OH)₂ precipitated from spent Li-ion-battery cathodes (LiCoO₂) was confirmed by XRD, FTIR, TG–DSC and SEM-EDS. In dark suspensions containing 50 mg Co(OH)₂, 10 mM NaHCO₃ and 4 mM H₂O₂ at pH ≈ 7, 10 ppm methylene blue was completely decolourised within 50 min, following pseudo-first-order kinetics. In-situ measurements showed dissolved Co²⁺ remained in the low-micromolar range, indicating that catalysis proceeds via transiently solubilised species rather than extensive leaching. ESI-MS and UV–Vis spectra revealed stepwise demethylation of MB to thionine (m/z 227) and smaller fragments (m/z 157), approaching mineralisation. The process thus unites sustainable cobalt recovery with an efficient, light-free advanced-oxidation route for dye remediation.