Recycled Battery Cathode Material Pulls Toxic Blue Dye from Water Without Any Redox Chemistry

The blue dye that colors denim, candies, and medicines may soon face an unexpected adversary: the discarded remains of old cellphone batteries. In a study published in Discover Industrial Chemistry and Materials, a team of Brazilian researchers led by Eric M. Garcia at the Federal University of São João Del-Rei has shown that lithium manganese oxide (LiMn2O4) recovered from spent lithium-ion batteries can act as a stable, reusable adsorbent for removing indigo carmine, a widely used synthetic dye that resists conventional wastewater treatment. The work is notable less for raw capacity than for its mechanistic clarity, demonstrating for the first time how the dye clings to a recycled battery cathode purely through physical surface interactions, with no chemical degradation involved.

Indigo carmine, chemically known as disodium (2E)-3-oxo-2-(3-oxo-5-sulfonato-1,3-dihydro-2H-indol-2-ylidene)-5-indolinesulfonate, carries a molar mass of 466.36 grams per mole and appears throughout the textile, food, and pharmaceutical industries. Its high water solubility, aromatic structure, and sulfonate functional groups make it both persistent and difficult to strip from industrial effluents. Beyond aesthetic concerns, exposure carries documented risks including biotoxic effects and potential mutagenic and carcinogenic consequences, which makes effective treatment of dye-laden wastewater an environmental priority. Adsorption has long been considered one of the most attractive remediation routes because it is efficient, simple to operate, and inexpensive when low-cost or waste-derived materials are used.

The twist in this study lies in how the researchers deliberately avoided the chemistry that usually dominates when lithium manganese oxide meets this dye. Under acidic conditions, the spinel lattice of LiMn2O4 is redox-active: protons attack the structure, driving delithiation toward lambda-MnO2 and partial disproportionation to MnO, while manganese ions are reduced and the dye is oxidatively destroyed. Previous work, including earlier studies by the same group, exploited this acid-driven redox coupling for oxidative decolorization. But those experiments could not separate adsorption from chemical degradation. By working at pH 7 and pH 11, where manganese redox activity and acid-assisted lattice dissolution are suppressed, the team isolated purely surface-controlled interactions for the first time.

To build the adsorbent, the researchers manually disassembled commercial Nokia BL-4C cellphone batteries, chosen because their cathodes are predominantly LiMn2O4, and separated the cathode tape from the other components. A mild thermal pretreatment at 200 degrees Celsius for five hours in air was applied to volatilize residual organic solvents and electrolyte-derived species while preserving the spinel structure. The authors are careful to note that this step cannot remove polymeric binders such as PVDF, which degrade at higher temperatures, but scanning electron microscopy with energy-dispersive X-ray analysis showed a marked reduction in fluorine and phosphorus signals, consistent with partial cleaning of electrolyte residues such as LiPF6 and its decomposition products.

Characterization confirmed that the recovered material retained its essential identity. X-ray diffraction patterns indexed to the cubic spinel structure of LiMn2O4 in the Fd3-bar space group, matching the JCPDS reference 35-0782, and the pattern was essentially unchanged after dye uptake, showing no new crystalline phases or significant peak shifts. Fourier-transform infrared spectroscopy revealed Mn-O vibrational bands between 500 and 700 reciprocal centimeters characteristic of MnO6 octahedra in mixed Mn3+/Mn4+ valence states. After adsorption, new features appeared in the 900 to 1100 and 1480 to 1650 reciprocal centimeter regions, assigned to C-N and aromatic C=C stretching modes of the dye, alongside a broad 3100 to 3500 band reflecting hydrogen bonding between dye N-H and O-H groups and surface hydroxyl sites.

The quantitative heart of the study is the adsorption isotherm analysis. Using a fixed adsorbent dose of one gram per liter and dye concentrations from 10 to 100 milligrams per liter at 25 degrees Celsius, the Langmuir model fit the pH 7 data exceptionally well, yielding a monolayer capacity of 4.95 milligrams per gram with a correlation coefficient of 0.99. At pH 11, that capacity collapsed to just 0.13 milligrams per gram. Crucially, the Freundlich constant fell by nearly two orders of magnitude and the heterogeneity exponent dropped from 5.40 to 2.68, indicating weaker and less favorable adsorption under alkaline conditions, yet the high correlation coefficients at both pH values showed that the underlying adsorption mechanism remained intact.

The explanation for this dramatic pH dependence came from electrostatics rather than site loss. Zeta potential measurements showed that the recycled powder carries a point of zero charge near pH 3; above it, the surface becomes increasingly negative, ranging from about minus 20 millivolts at pH 7 to minus 60 millivolts at pH 11. Because indigo carmine exists predominantly as a dianionic IC2- species under these conditions, the increasingly negative surface electrostatically repels the dye, excluding it from the adsorption region. Applying a Poisson-Boltzmann-based electrostatic accessibility model in which the apparent Langmuir capacity scales with an exponential Boltzmann factor, the researchers found that matching the experimental capacity ratio required an effective dye charge of approximately minus 2.2, consistent with speciation diagrams showing the dianion accounts for roughly 95 percent of dissolved species even at pH 11.

Thermodynamics painted the picture of a physisorption-dominated process. The enthalpy change of 1.3 plus or minus 0.1 kilojoules per mole is small and positive, indicating a weakly endothermic interaction with no strong chemical bond formation. The entropy change of 100 plus or minus 2 joules per mole per kelvin is large and positive, reflecting the release of structured water molecules and counterions as the dye attaches to the surface. The resulting Gibbs free energy change of minus 28.5 plus or minus 0.6 kilojoules per mole at 298 kelvin confirms spontaneity, and the combination of low enthalpy with high entropy marks adsorption as entropy-driven rather than enthalpy-driven, the classic signature of physical adsorption on heterogeneous oxide surfaces.

Where the material truly distinguishes itself is durability. Although its maximum capacity trails many engineered adsorbents, including functionalized carbon nanotubes, modified graphene oxide composites, and mesoporous metal oxide nanoparticles, the recycled cathode maintained greater than 95 percent removal efficiency across repeated adsorption-regeneration cycles, retaining about 94 percent after 20 adsorption-desorption cycles with only 6 percent total efficiency loss. Regeneration involved simply filtering the powder and soaking it in dilute 0.01 molar hydrochloric acid for 24 hours. The weak physical binding that limits capacity also enables nearly complete dye desorption, minimizing irreversible fouling of active sites. Ultraviolet-visible spectroscopy reinforced the point: after adsorption, the characteristic dye bands at 610, 290, and 250 nanometers remained, merely diminished in intensity, showing the chromophore survived intact and no redox degradation or new degradation products formed.

The broader context makes the approach timely. Global production of spent lithium-ion batteries is projected to exceed 11 million tons by 2030, and cathode materials account for roughly 30 to 40 percent of battery mass while containing high-value metals and structurally robust oxides. Repurposing that waste stream as a functional water-treatment material embodies circular economy principles, converting an environmental liability into an asset. The authors position recycled LiMn2O4 not as a high-capacity engineered sorbent but as a low-cost, waste-derived, structurally stable, and reusable material whose performance hinges on electrostatic accessibility. By coupling isotherm modeling, thermodynamics, zeta potential measurements, and Poisson-Boltzmann theory, the study delivers something rarer than a new adsorbent: a quantitative framework predicting exactly how pH and surface charge govern dye uptake, a framework that could guide the rational deployment of recycled battery oxides in real wastewater treatment wherever near-neutral conditions prevail.

Subject of Research: Adsorptive removal of indigo carmine dye from water using recycled LiMn2O4 cathode material from spent lithium-ion batteries under non-redox conditions.

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

Article References: Garcia, E. M., Dias, T. M., Taroco, H. A., Melo, J. O. F., Mariz, C. S., & de Almeida Rocha, F. (2026). Adsorptive removal of indigo carmine using recycled LiMn2O4 cathode material under non redox conditions. Discover Industrial Chemistry and Materials, 1(1), Article 11. https://doi.org/10.1007/s44508-026-00012-z

Image Credits: AI Generated

DOI: 10.1007/s44508-026-00012-z

Keywords: lithium-ion battery recycling, LiMn2O4, indigo carmine, adsorption, wastewater treatment, physisorption, electrostatic accessibility, zeta potential, Langmuir isotherm, circular economy, dye removal, recycled cathode material