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 min under mild conditions (pH similar to 8.35), 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 (center dot 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.