Recycling of iron phosphate residue from Lithium-extracted spent power battery: low-molecular-weight organic acids activation for Pb/Cd adsorption.

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 PO43--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 PO43--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.