<p>Recycled LiMn₂O₄ 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, LiMn₂O₄ 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 IC²⁻ species. Despite its relatively low adsorption capacity, the material exhibits excellent operational stability, maintaining &gt; 95% removal efficiency over multiple adsorption–regeneration cycles. These results demonstrate that recycled LiMn₂O₄ is a robust and reusable adsorbent and highlight the critical role of electrostatic accessibility in controlling dye adsorption under non-redox conditions.</p><p><?noindent??><b>Clinical trial number</b></p><p><?noindent??>Not applicable.</p>