Citrate-EDTA-H₂O₂ buffering leaching solution for Ni/Co/Mn recovery from spent lithium-ion battery black mass

The increasing global demand for lithium-ion batteries necessitates the development of environmentally sustainable recycling technologies for critical metal recovery. This study presents a novel citrate-EDTA buffered leaching system for recovery of Li, Co, Ni, and Mn from spent LIB cathode materials with mixed NMC/LiCoO₂ composition. The developed approach addresses limitations of conventional citric acid leaching through synergistic combination of citrate buffer (pH 4-6), Na₂EDTA as complexing agent, and H₂O₂ as reducing agent under mild conditions (50 °C). Thermodynamic analysis using Pourbaix diagrams demonstrated that the citrate-EDTA system significantly enhances metal solubility by forming stable chelate complexes and shifting redox boundaries to prevent passivation layer formation. Key parameters were optimized using response surface methodology and central composite rotatable plan to maximize metal recovery: 1.211 v/v% H₂O₂, 0.778 mol L^-1 citrate buffer, and 0.05 mol L^-1 Na₂EDTA. Kinetic studies revealed maximum metal leaching efficiencies at pH 5.0, solid-to-liquid ratio 1 : 20, and temperature 50 °C of Li-100.0%, Co-98.65%, Ni-90.69%, and Mn-82.87% under these mild conditions. Kinetic modeling using Avrami-Erofeev and Peleg equations revealed distinct leaching mechanisms: rapid delithiation followed by interfacial reaction control for Li and Co, while Ni and Mn exhibited diffusion-limited behavior with passivation effects. Comparative analysis demonstrated that the developed system is nearly as effective as traditional acid methods when operating at lower temperatures with less impact on the environment. Thermodynamic barrier analysis revealed the activation energy sequence: Co (92.1) > Ni (87.4) ≈ Mn (87.2) > Li (83.25) kJ mol^-1, confirming the mechanistic insights. This green chemistry approach offers significant advantages, including biocompatibility, mild operating conditions, and potential for industrial scale-up in sustainable battery recycling applications.