Degradation of Methylene Blue via H₂O₂/HCO₃⁻/Co²⁺ System Using Cobalt Recovered from Spent Li-Ion Batteries

The increasing environmental burden posed by synthetic dyes and electronic waste demands innovative, sustainable solutions. In this work, we present a green and efficient advanced oxidation process (AOP) employing cobalt ions recovered from spent Li-ion battery (LIB) cathodes to catalyze the degradation of methylene blue (MB), a model organic pollutant. The Co²⁺/HCO₃⁻/H₂O₂ system enabled complete decolorization of a 10 ppm MB solution within 10 minutes under mild conditions (pH ~ 8.3), with kinetic analysis revealing pseudo-zero-order behavior in MB and half-order dependence on Co²⁺, HCO₃⁻, and H₂O₂. UV–Vis spectroscopy confirmed the formation of the [Co(CO₃)₃]³⁻ complex, while electrospray ionization mass spectrometry (ESI-MS) revealed demethylated intermediates and smaller fragments, suggesting progressive mineralization. Mechanistic insights indicate the predominant formation of carbonate radicals (•CO₃⁻), as supported by isopropanol scavenging experiments. This study highlights the dual environmental benefit of cobalt recovery and wastewater treatment, offering a sustainable pathway for the valorization of electronic waste and the mitigation of textile dye pollution.