Non-Redox Removal of Indigo Carmine Using Recycled LiMn₂O₄ Cathode Materials

Recycled LiMn₂O₄ cathode material from spent lithium-ion batteries was investigated as an adsorbent for indigo carmine under non-redox conditions. At pH 7, a Langmuir monolayer capacity of 4.95 mg·g⁻¹ was obtained, which decreased sharply to 0.13 mg·g⁻¹ at pH 11 due to electrostatic exclusion rather than loss of adsorption sites. Thermodynamic analysis revealed a low positive enthalpy (ΔH° = 1.3 ± 0.1 kJ·mol⁻¹) and a high positive entropy (ΔS° = 100 ± 2 J·mol⁻¹·K⁻¹), indicating a weakly endothermic, entropy-driven adsorption process dominated by physisorption, while the negative Gibbs free energy (ΔG°₍₂₉₈₎ = −28.5 ± 0.6 kJ·mol⁻¹) confirms spontaneity at near-neutral pH. Poisson–Boltzmann analysis showed that the increase in zeta potential magnitude from − 20 to − 60 mV at alkaline pH restricts dye accessibility, yielding an effective charge of approximately − 2.2, consistent with IC²⁻ predominance. Despite the modest capacity, the material retained > 95% efficiency over multiple adsorption–regeneration cycles, highlighting recycled LiMn₂O₄ as a stable and reusable adsorbent for non-redox dye removal.