Recycling of iron phosphate residue from Lithium-extracted spent power battery: low-molecular-weight organic acids...

The rapid progress of electric vehicles has intensified global lithium recovery from spent LiFePO₄ batteries, generating substantial FePO₄ residue (FeP) with low economic value and limited utilization potential. Considering the insolubility and chemical inertness of this unique solid waste, we repurposed it through low-molecular-weight organic acids (oxalic acid/citric acid) activation, transforming it into free PO₄^3--rich materials (O-FeP and C-FeP) for heavy metal immobilization. Batch adsorption experiments combined with Visual MINTEQ modeling, spectroscopic characterization (XRD, FT-IR), and transmission electron microscopy (TEM) demonstrated distinct Pb/Cd stabilization mechanisms. For Pb, O-FeP demonstrated superior Pb adsorption capacity (823.9 mg/g) compared to FeP (153.0 mg/g), while C-FeP failed to show the enhancing effect brought by the activation (143.1 mg/g). In both systems, Pb removal primarily occurred via PO₄^3--mediated precipitation with formation of Pb₃(PO₄)₂, where oxalate further enhanced the adsorption capacity via PbC₂O₄ co-precipitation, whereas citrate exhibited a negative effect due to soluble Pb-citrate complexation. In the FeP system, Pb adsorption predominantly occurred through pseudomorphic transformation of Pb₅(PO₄)₃OH, driven by insolubility differences. For Cd removal, oxalic acid substantially enhanced the adsorption capacity, where citric acid produced only a limited improvement (O-FeP: 26.4 mg/g > C-FeP: 7.1 mg/g > FeP: 3.8 mg/g). This enhancement was mainly attributed to the formation of CdC₂O₄ in the O-FeP system, whereas Cd-citrate complexation only slightly improved Cd uptake in C-FeP, with phosphate-mediated precipitation playing a negligible role. This study provides a novel idea for addressing the final-stage residue in battery recycling and may be extended to other heavy metals.