Adsorptive removal of indigo carmine using recycled LiMn2O4 cathode material under non redox conditions

Recycled LiMn 2 O 4 cathode material from spent lithium-ion batteries was investigated as a low-cost adsorbent for indigo carmine (IC) removal under non-redox conditions. The main novelty of this study lies in demonstrating that, in the absence of acid-driven redox processes, LiMn 2 O 4 can act as a stable adsorption surface, allowing the isolation of purely surface-controlled interactions. At pH 7, a Langmuir monolayer capacity of 4.95 mg·g −1 was obtained, which sharply decreased to 0.13 mg·g −1 at pH 11. This reduction is not associated with loss of active sites but rather with electrostatic exclusion effects, as confirmed by Poisson–Boltzmann analysis. Thermodynamic parameters (ΔH° = 1.3 ± 0.1 kJ·mol −1 , ΔS° = 100 ± 2 J·mol −1 ·K −1 , ΔG°₍₂₉₈₎ = −28.5 ± 0.6 kJ·mol −1 ) indicate a spontaneous, weakly endothermic, and entropy-driven process dominated by physisorption. The electrostatic model, supported by zeta potential measurements (−20 to − 60 mV), yields an effective dye charge of approximately −2.2, consistent with the predominance of IC 2− species. Despite its relatively low adsorption capacity, the material exhibits excellent operational stability, maintaining > 95% removal efficiency over multiple adsorption–regeneration cycles. These results demonstrate that recycled LiMn 2 O 4 is a robust and reusable adsorbent and highlight the critical role of electrostatic accessibility in controlling dye adsorption under non-redox conditions. Clinical trial number Not applicable.