Carbon recovered from Li-ion batteries for methylene blue adsorption: waste valorization toward environmental remediation

Graphitic carbon recovered from spent lithium-ion battery anodes was repurposed as a low-cost adsorbent for methylene blue removal from aqueous solutions, demonstrating a practical waste-valorization strategy. Adsorption was strongly pH-dependent, with the highest performance achieved at pH 11, where the Langmuir model showed excellent agreement (R² = 0.998) and a maximum adsorption capacity of 7.25 mg·g⁻¹. Isotherm analysis revealed an energetically heterogeneous surface, with adsorption becoming more distributed at neutral and alkaline pH. Thermodynamic evaluation confirmed a spontaneous, entropy-driven process governed predominantly by physisorption, as indicated by a small negative enthalpy change (ΔH° = −6.64 ± 0.47 kJ·mol⁻¹), negative Gibbs free energy values, and a positive entropy change. The adsorbent retained over 90% of its initial efficiency after seven regeneration cycles using dilute acid, highlighting its reusability. Overall, recycled graphitic carbon emerges as a simple, reusable, and environmentally friendly alternative for dye-contaminated wastewater treatment.