Engineering spin state of Fe(III) on resin for highly efficient removal of nickel(II)–citrate: Simultaneous decomplexation and recovery

A single-step displacement–capture strategy enables efficient separation of Ni(II)–citrate complexes. • High-spin Fe(III)-D001 composites efficiently decomplex Ni(II)–citrate and simultaneously recover Ni(II). • The strong complexation ability of high-spin Fe 3+ with citrate ligands is crucial for the decomplexation of Ni(II)–citrate. • Fe(III)-D001 exhibits effective decomplexation and Ni recovery in real wastewater. Heavy metal–ligand complex wastewaters pose persistent challenges for industrial water treatment and separation processes, as strong chelation by organic ligands severely limits the effectiveness of conventional adsorption and ion-exchange methods. Herein, we develop a single-step, regenerable separation strategy that integrates Fe(III)-driven metal displacement with ion-exchange capture for the efficient decomplexation of nickel–citrate complexes (Ni(II)–citrate) and simultaneous recovery of Ni(II). Fe(III)-functionalized cation-exchange resins (Fe(III)-D001) enriched with high-spin Fe(III) species were synthesized via a continuous ion-exchange process in aqueous solution. The resulting material exhibited effective removal of Ni(II)–citrate, achieving a maximum capacity of 87.74 mg g −1 . In contrast, Fe(III)-modified resins prepared in dimethylformamide, which predominantly contain low-spin Fe(III), showed markedly reduced removal performance. Mechanistic investigations reveal that high-spin Fe(III) acts as the displacement-active center, preferentially coordinating with citrate ligands and promoting the decomplexation of Ni(II)–citrate, while residual sulfonic acid groups on the resin enable the immediate capture of liberated Ni(II) via ion exchange. Fixed-bed operation further demonstrates stable separation performance under ion-rich conditions, highlighting the practical applicability of the proposed approach. This work provides a process-intensified route for treating chelated metal wastewaters and offers a promising separation strategy for complex industrial water matrices relevant to desalination and advanced wastewater treatment applications. Download: Download high-res image (271KB) Download: Download full-size image