Synergistic calcium enhancement of strontium adsorption in iron(II) oxalatophosphates elucidates the underlying rapid mechanisms

Efficient removal of Sr 2 + from radioactive wastewater is essential for the safe and sustainable development of nuclear power. Herein, three ferrous oxalatophosphates—(NH 4 ) 2 Fe 3 (HPO 4 ) 2 (C 2 O 4 ) 2 (NFPO), Na 2 [Fe(HPO 4 )C 2 O 4 ] (SFPO), and K 3 [Fe 4 (HPO 4 ) 2 (PO 4 )(C 2 O 4 ) 2 ]∙H 2 O (KFPO)—were synthesized via a one-step hydrothermal route and systematically evaluated for Sr 2+ capture. NFPO exhibited the best overall performance, featuring broad pH tolerance, thermal stability, and reusability. Notably, in the presence of Ca 2+ (optimal background concentration: 50 mg·L –1 ), Sr 2+ uptake was markedly enhanced, reaching equilibrium within 50 min and achieving an experimentally saturated capacity of 308.95 mg·g –1 (313 K). To improve handling and applicability, NFPO was immobilized into polyacrylonitrile microspheres (NFPO/PAN), which maintained effective dynamic adsorption in fixed-bed tests. Characterization results demonstrate that the crystalline framework remains largely preserved during adsorption, with Fe remaining in the divalent state and an ion-exchange trend involving NH 4 + . Density functional theory (DFT) calculations are further used to provide a plausible atomic-scale interpretation: Sr 2+ binding is energetically favored at oxygen-rich interfacial sites, while Ca 2+ can promote Sr 2+ capture by modulating local structure and electrostatic environment. This work highlights ferrous oxalatophosphates as promising adsorbents and provides an experimentally grounded and theoretically supported understanding for Sr 2+ remediation.