From Waste to Value: Recycled Li-ion Battery Cathode Catalyzes the Transformation of Sunset Yellow into Functional Aromatics

This study demonstrates, for the first time, the oxidative transformation of Sunset Yellow (SY) by a recycled Li-ion battery cathode material (SLBCP) rich in LiMn₂O₄ under mild acidic conditions (pH 1.5). The SLBCP catalysed the cleavage of the azo bond (–N = N–) in SY (50 mg·L⁻¹), yielding sulfanilic acid (SA) and 1-amino-2-naphthol-6-sulfonic acid (ANSA) as stable aromatic products, as confirmed by UV–Vis absorption and synchronous fluorescence spectroscopy. Kinetic analyses revealed that the decolourisation follows apparent first-order kinetics, with a calculated activation energy of 50 kJ·mol⁻¹, consistent with a surface-controlled Langmuir–Hinshelwood mechanism. XRD confirmed the formation of a Li₀.₂Mn₂O₄ phase after reaction, reflecting delithiation and surface restructuring. Cyclic voltammetry highlighted the role of Mn⁴⁺ sites in SY oxidation, with a progressive reduction peak current decay stabilizing at ~ 0.12 mA·cm⁻² after 50 min. COD analyses indicated selective bond cleavage without significant mineralisation, preserving the organic load. The findings showcase a low-energy, waste-to-value pathway, coupling battery cathode recycling with the generation of functional dye degradation products, and demonstrate the potential of SLBCP for selective oxidation processes.