Iron powder (Fe, analytical grade, particle size < 50 µm), phosphoric acid (H₃PO₄, ≥ 85.0%), nitric acid (HNO₃, 65.0–68.0%), and sodium hydroxide (NaOH, ≥ 96.0%) were purchased from Sinopharm Chemical Reagent Co., Ltd. Anhydrous ethanol was obtained from Macklin Biochemical Co., Ltd. Europium nitrate hexahydrate (Eu(NO₃)₃·6H₂O, ≥ 99.9%) was supplied by Aladdin Reagent Co., Ltd. All solutions were prepared with deionized water.

Phosphorylated ball-milled zero-valent iron (P-ZVI_bm) was prepared via a mechanochemical approach. Iron powder (3.0 g) and KH₂PO₄ (0.7309 g) were mixed and placed into an agate ball-milling jar together with a defined amount of agate balls. To avoid oxidation of Fe during milling, nitrogen gas was purged into the jar for 30.0 min prior to sealing. The mixture was then subjected to planetary ball milling at 300.0 rpm for 3.0 h. The resulting material was collected and stored in vacuum-sealed bags and denoted as P-ZVI_bm. For comparison, ball-milled Fe powder without KH₂PO₄ was prepared under identical conditions and denoted as ZVI_bm.

The crystalline structure of the materials was analyzed using X-ray diffraction (XRD, Rigaku SmartLab SE, Cu-Kα radiation). Surface morphology was observed by scanning electron microscopy (SEM, TESCAN MIRA LMS) and transmission electron microscopy (TEM, JEOL JEM-F200). Surface functional groups were identified by Fourier transform infrared spectroscopy (FTIR, Thermo Nicolet iS5), while microstructural features and defects were investigated by Raman spectroscopy (Renishaw inVia). Elemental composition and chemical states were determined by X-ray photoelectron spectroscopy (XPS, LAUDA Scientific GmbH LSA-100X). P-ZVI_bm was prepared using a ball mill (PML10, Netzsch Instruments). The magnetic properties of ZVI_bm and P-ZVI_bm were measured using a vibrating sample magnetometer (VSM, LakeShore 7404, USA). The surface charges of ZVI_bm and P-ZVI_bm were determined using a solid surface zeta potential analyzer (SurPASS 3, Anton Paar, Austria).

Eu(III) stock solution (1 g·L⁻¹) was prepared as follows: 2.9353 g of Eu(NO₃)₃·6H₂O was placed in a beaker, dissolved in 10.0 mL aqua regia under heating and stirring, and subsequently evaporated to dryness in a fume hood. After cooling the residue to room temperature, it was redissolved in 50.0 mL of deionized water and diluted to a final volume of 1.0 L in a volumetric flask. Batch adsorption experiments were conducted using the equilibrium method. Eu(III) solutions of desired concentrations were prepared by dilution of the stock solution and adjusted to target pH using 0.1 mol·L⁻¹ HNO₃ or 0.1 mol·L⁻¹ NaHCO₃. In a typical run, 5.0 mg of adsorbent was added to 25.0 mL of Eu(III) solution and shaken at 180.0 rpm at a constant temperature for a specified time. The residual Eu concentration was measured by inductively coupled plasma mass spectrometry (ICP-MS).

The morphological and structural evolution of the zero-valent iron after ball-milling and phosphorylation modification (P-ZVI_bm) was systematically characterized (Figs. 1A–D). As shown in the schematic illustration (Fig. 1A), the phosphorylated surface of P-ZVI_bm provides abundant adsorption sites and facilitates Eu³⁺ immobilization through inner-sphere complexation and interface nucleation processes. SEM analysis (Fig. 1B) revealed that P-ZVI_bm displayed rougher, flake-like structures with visible surface defects and micro-pores. These morphological changes indicate that phosphate incorporation during mechanical activation promotes surface reconstruction and fragmentation, thereby enlarging the specific surface area and exposing more reactive sites[44–45]. Energy-dispersive X-ray spectroscopy (EDS) further confirmed the presence of Fe (92.08%), O (5.57%), and P (2.35%) in P-ZVI_bm, verifying the successful introduction of phosphate groups while maintaining Fe as the predominant component. X-ray diffraction (XRD) patterns (Fig. 1C) of P-ZVI_bm exhibited characteristic reflections at 2θ = 44.5° and 65.0°, corresponding to the (110) and (200) planes of metallic Fe⁰ (PDF#65-4899). The absence of new crystalline peaks (2θ = 44.5°) in P-ZVI_bm indicates that the Fe⁰ core structure was preserved after surface phosphorylation. However, the reduced intensity of Fe⁰ peaks suggests partial amorphization induced by high-energy milling and phosphate bonding. Fourier-transform infrared (FTIR) spectra (Fig. 1D) provide further evidence of chemical modification: absorption bands at 898.0 cm⁻¹, 1090.0 cm⁻¹, and 1280.0 cm⁻¹ correspond to P–O, Fe–O–P, and P = O vibrations[45], respectively, confirming stable coordination between phosphate groups and iron atoms. The Fe–O–P linkage is expected to enhance the chemical durability of ZVI in acidic media by forming a protective passivation layer that limits proton attack[46].

The adsorption performance of Eu³⁺ on ZVI_bm and P-ZVI_bm was systematically evaluated under varying conditions, including pH, solid-to-liquid ratio, contact time, concentration, and temperature (Fig. 2A). The speciation of Eu varies significantly with pH (Fig. 2B). At low pH (< 4), Eu³⁺ predominantly exists, whereas with increasing pH, hydrolyzed species such as Eu(OH)²⁺ progressively form in solution[47]. Figure 2C presents the magnetization curves of ZVI_bm and P-ZVI_bm. The Ms of ZVI_bm reaches 225.15 emu·g⁻¹, while that of P-ZVI_bm decreases to 73.82 emu·g⁻¹ due to phosphate surface modification. Nevertheless, both materials retain sufficient magnetic responsiveness, allowing efficient separation and recovery from Eu(III)-containing radioactive wastewater under an external magnetic field. To evaluate the electron transfer rate and corrosion resistance of the materials, Tafel tests were conducted using a three-electrode electrochemical system. As shown in Fig. 2D, the corrosion potentials of ZVI_bm and P-ZVI_bm are − 0.386 V and − 0.315 V, respectively. The positive shift in corrosion potential indicates that phosphate modification slightly reduces the electron transfer rate but significantly enhances the corrosion resistance of P-ZVI_bm. Considering that Eu exists predominantly as cations in aqueous solution, the surface charge of the adsorbent is expected to strongly influence the removal process. Therefore, the zeta potentials of the samples were measured over a pH range of 3–8, as shown in Fig. 2E. As the pH increased, the zeta potential of ZVI_bm decreased from + 28.34 mV to − 3.83 mV, while that of P-ZVI_bm exhibited a more pronounced shift from − 2.65 mV to − 50.03 mV. This pronounced negative shift for P-ZVI_bm is attributed to the introduction of PO₄³⁻ groups on the surface, which increases overall surface electronegativity[48–49]. The influence of the S/L ratio is presented in Fig. 2F. Increasing the S/L ratio from 0.1 to 0.6 g·L⁻¹ enhanced the overall removal efficiency; however, the adsorption capacity per unit mass peaked at S/L = 0.2 g·L⁻¹. The subsequent decrease in capacity at higher loadings is attributed to excessive unused sites and a diminished concentration gradient driving force. As shown in Fig. 2G, both adsorbents exhibited pronounced pH-dependent adsorption behavior within the acidic range (pH 2–6). The removal efficiency increased steadily with pH, reaching a maximum at pH = 3, where adsorption capacities were 7.92 mg·g⁻¹ for ZVI_bm and 121.76 mg·g⁻¹ for P-ZVI_bm. The low uptake under strong acidity was mainly due to proton competition and partial Fe⁰ dissolution, while the enhanced performance at higher pH reflected the deprotonation of surface hydroxyls and the increasing contribution of negatively charged Fe–O–P sites[50], which facilitated Eu³⁺ electrostatic attraction and inner-sphere complexation. These observations confirm that phosphate functionalization significantly improves acid tolerance and enhances active-site accessibility. As the initial Eu³⁺ concentration increased (Fig. 2H), the adsorption capacity rose sharply at low-to-moderate concentrations and then gradually reached saturation, yielding maximum capacities of 48.33 mg·g⁻¹ for ZVI_bm and 152.49 mg·g⁻¹ for P-ZVI_bm. Kinetic experiments (Fig. 2I) revealed rapid Eu³⁺ uptake during the initial stage, followed by equilibrium at 43.23 mg·g⁻¹ for ZVI_bm and 150.17 mg·g⁻¹ for P-ZVI_bm. Temperature-dependent adsorption results (Fig. 2J) demonstrated a clear enhancement in Eu³⁺ uptake with rising temperature, reflecting the endothermic nature of the process. The equilibrium capacity of P-ZVI_bm increased from 118.64 mg·g⁻¹ at 293 K to 155.21 mg·g⁻¹ at 313.0 K, whereas that of ZVI_bm increased modestly from 34.11 to 47.65 mg·g⁻¹ over the same range.

Post-adsorption analyses were conducted to elucidate the structural and chemical evolution of ZVI_bm and P-ZVI_bm after Eu³⁺ capture (Fig. 3A–H). SEM-EDS characterization revealed distinct morphological changes in ZVI_bm and P-ZVI_bm after Eu adsorption, showing surface alteration and discontinuous deposits. Elemental mapping indicated homogeneous distributions of Eu, Fe, O, and P on the ZVI_bm surface, whereas Eu on P-ZVI_bm appeared sparsely and locally concentrated. EDS quantification further showed that the Eu content on P-ZVI_bm (17.92 wt%) was nearly three times that of ZVI_bm, confirming the enhanced Eu affinity induced by phosphate functionalization. XRD results (Fig. 3C) provided structural evidence for distinct immobilization mechanisms. Both materials retained the characteristic Fe⁰ reflections at 2θ = 44.5° and 65.0°, but their intensities weakened after adsorption, indicating partial oxidation of the metallic core. For P-ZVI_bm, new diffraction peaks at 2θ = 25.8°, 28.2°, and 31.3° corresponded to crystalline Eu₃PO₇ phases (PDF#49-1024), confirming that phosphate groups on the surface acted not only as adsorption sites but also as active participants in interfacial precipitation with Eu³⁺. XPS spectra (Figs. 3D–G) further clarified the chemical states of key elements. The Eu 3d spectrum of P-ZVI_bm exhibited two well-defined peaks at 1134.5 eV and 1164.2 eV, characteristic of Eu³⁺. The P 2p signal decreased in intensity and shifted from 133.4 eV to 134.1 eV after adsorption, indicating coordination between phosphate oxygens and Eu³⁺ through Fe–O–P–Eu linkages. Concurrently, Fe 2p spectra showed the disappearance of metallic Fe⁰ (706.9 eV) and the emergence of Fe²⁺/Fe³⁺ peaks at 710.8 eV and 724.6 eV, evidencing oxidation of Fe⁰ into Fe₂O₃, Fe₃O₄, and Fe(OH)₃. The O 1s spectra displayed increased contributions from hydroxyl (Fe–OH) and phosphate (P–O–Fe, P–O–Eu) bonds, confirming the coexistence of redox transformation and chemical complexation during adsorption. Compared with ZVIbm, P-ZVIbm underwent more extensive surface oxidation and formed abundant reactive interfaces enriched with phosphate and iron oxyhydroxides, offering diverse coordination environments for Eu³⁺[44].

Density functional theory (DFT) calculations were employed to gain atomic-level insight into the adsorption mechanism of Eu³⁺ on phosphate-modified Fe–O surfaces (Fig. 4A–F). The optimized configurations reveal that Eu³⁺ strongly coordinates with surface oxygen atoms from phosphate and Fe–O–P groups, forming inner-sphere complexes with Eu–O bond lengths of approximately 2.1–2.3 Å and O–Eu–O angles between 77.929–79.545°. The corresponding adsorption energies progressively increase from − 8.015 eV for simple bidentate coordination to − 14.709 eV for multidentate bridging complexes, indicating that phosphate functionalization creates high-affinity sites capable of capturing Eu³⁺ through strong chemisorption. The enhanced stability of these configurations suggests that phosphate groups not only serve as anchoring ligands but also promote interfacial nucleation of Eu–phosphate species. Charge density difference analysis (Fig. 4E) displays pronounced electron accumulation around oxygen and depletion around europium, confirming substantial electron transfer from Eu³⁺ to the phosphate–oxygen framework and the mixed ionic–covalent nature of Eu–O bonding. This charge redistribution stabilizes Eu at the Fe–O–P interface and facilitates partial oxidation of surface Fe atoms. The projected density of states (Fig. 4F) further supports this conclusion, showing hybridization between Eu 4f/5d and O 2p orbitals near the Fermi level, accompanied by a narrowed band gap and shifted Fe 3d states. Together, these results establish that Eu³⁺ adsorption on P-ZVI_bm is dominated by inner-sphere complexation and chemisorptive interaction, wherein phosphate groups create electron-rich, multidentate coordination environments[51] that strongly bind Eu³⁺ and drive the formation of stable Eu–phosphate precipitates. This DFT-supported mechanism coherently explains the experimentally observed high adsorption energy, temperature-enhanced uptake, and structural evolution of the adsorbent, confirming that phosphorylation transforms the Fe⁰ surface into a robust, electronically active interface for selective Eu³⁺ immobilization and recovery.

To evaluate the practical applicability of the developed adsorbents, both ZVI_bm and P-ZVI_bm were tested for Eu³⁺ removal from actual mine wastewater containing multiple coexisting metal ions (Table 1 and Fig. 5A–D). The wastewater exhibited a complex matrix with high concentrations of competing cations such as Ce (1832.03 mg·L⁻¹), Gd (634.69 mg·L⁻¹), La (541.37 mg·L⁻¹), Sm (361.79 mg·L⁻¹), and other trace elements including Si, Ba, Ni, and Zn. These multicomponent conditions pose a significant challenge for selective adsorption of Eu³⁺, especially under slightly acidic conditions typical of mine drainage systems. As shown in Fig. 5A, both materials displayed effective Eu³⁺ uptake with increasing contact time, but the removal efficiency of P-ZVI_bm was substantially higher across all conditions. At equilibrium, P-ZVI_bm achieved a Eu³⁺ removal efficiency of 92.4%, compared with only 48.7% for ZVI_bm, confirming that phosphate functionalization significantly enhances adsorption performance in complex aqueous systems. The improvement is attributed to the strong inner-sphere complexation of Eu³⁺ with surface phosphate ligands and the formation of Eu–phosphate precipitates[52–53], which are less susceptible to ionic competition. The selectivity of P-ZVI_bm toward Eu³⁺ among the coexisting metal ions is presented in Fig. 5B. While both adsorbents showed some affinity for light rare-earth elements such as La³⁺, Ce³⁺, and Gd³⁺, the Eu³⁺ adsorption capacity of P-ZVI_bm was nearly fourfold higher than that of ZVI_bm, with minimal interference from high concentrations of neighboring REEs. This selectivity arises from the specific coordination geometry and bond energy matching between Eu³⁺ and Fe–O–P sites, as confirmed by DFT and XPS results. Furthermore, the negligible adsorption of common transition metals (Ni²⁺, Zn²⁺, Co²⁺) suggests that P-ZVI_bm possesses both chemical selectivity and environmental compatibility for rare-earth recovery from metallurgical effluents. The kinetics of Eu³⁺ removal in real wastewater (Fig. 5C) followed a rapid initial uptake phase, reaching equilibrium within 30.0 mins for P-ZVI_bm, compared to nearly 120.0 minutes for ZVI_bm. The faster equilibrium rate reflects improved surface accessibility and higher electrostatic attraction due to the negatively charged phosphate groups. In the presence of multiple ions, P-ZVI_bm maintained an adsorption capacity of 132.5 mg·g⁻¹, representing 86.0% of its performance in simulated single-ion systems, whereas ZVI_bm retained only 57.0%. This demonstrates the structural stability and anti-interference ability of the phosphorylated surface in realistic environments. The recyclability of P-ZVI_bm was further verified by five consecutive adsorption–desorption cycles using H₃PO₄ regeneration (Fig. 5D). The Eu³⁺ removal efficiency remained above 85.0% after five cycles, with only minor declines due to partial surface passivation and loss of active sites. In contrast, ZVI_bm exhibited a pronounced capacity drop to below 50% after the third cycle, resulting from severe corrosion and aggregation under acidic regeneration conditions. The retained magnetic response of P-ZVI_bm enabled facile separation from treated water by an external magnetic field, further supporting its suitability for practical wastewater applications. In summary, the results demonstrate that phosphorylation effectively transforms conventional ZVI into a highly stable, selective, and recyclable adsorbent capable of capturing and recovering Eu³⁺ from complex mine effluents. The enhanced affinity toward Eu³⁺, high tolerance to competing ions, and strong regeneration capacity confirm that P-ZVI_bm bridges the gap between laboratory-scale synthesis and field-scale application. This work provides a feasible pathway for resource recovery from rare-earth-associated radioactive wastewater, aligning with the principles of green metallurgy and sustainable waste valorization.