Fe3O4@C Core–Shell Nanoparticles: Hydrothermal Synthesis and Magnetic Removal of Heavy Metals from Water

Abstract

Core–shell Fe₃O₄@C nanoparticles were synthesized via a simple hydrothermal route followed by calcination at 450 °C. The resulting material displays a highly porous structure with a Brunauer–Emmett–Teller (BET) surface area of 238.18 m² g⁻¹, far exceeding typical Fe₃O₄ powders. When applied to aqueous solutions containing Pb²⁺, Cd²⁺, Cu²⁺ and Cr⁶⁺ at pH 3, the adsorbent achieved removal efficiencies of 100 %, 99.2 %, 96.6 % and 94.8 %, respectively. The magnetic core enables rapid and effortless separation of the particles from water, making this material a practical candidate for large‑scale water treatment. Moreover, the scalable synthesis approach offers a cost‑effective pathway to produce multifunctional core–shell nanostructures for environmental remediation, catalysis and energy applications.

Background

Heavy‑metal contamination poses serious risks to human health and ecosystems, prompting stringent regulations on industrial effluents. Conventional removal methods—ion exchange, coagulation, precipitation—often suffer from high operational costs, sludge generation, and incomplete metal removal. Nanostructured adsorbents with controlled morphology, such as hollow spheres, nanowires and nanotubes, have shown superior performance due to their high surface area and tunable chemistry. Core–shell architectures, in particular, combine the functional advantages of a magnetic core with a reactive shell, offering both high adsorption capacity and magnetic recoverability. Despite promising reports of Fe₃O₄–based core–shell composites, scalable, environmentally friendly synthesis routes remain limited. This study addresses that gap by presenting a straightforward hydrothermal procedure that yields Fe₃O₄@C hybrids with exceptional adsorption properties.

Experimental

Materials and Synthesis

Synthesis of Core–Shell Fe₃O₄ Hybrid Nanoparticle Aggregates

In a typical procedure, 0.72 g of Fe(NO₃)₃·9H₂O, 0.0086 g of NH₄H₂PO₄, 0.008 g of Na₂SO₄·10H₂O and 3 g of glucose were dissolved in 90 mL of distilled water. After stirring for 10 min, the mixture was transferred to a 100 mL Teflon‑lined stainless‑steel autoclave and heated at 180 °C for 48 h. The resulting black precipitate was washed with deionized water and ethanol, dried at 65 °C overnight, and calcined at 450 °C (3 °C min⁻¹) under a 4‑h CO/Ar flow. The calcination converts the Fe³⁺ precursor into Fe₃O₄ cores while carbonizing the organic matrix to form the shell, yielding Fe₃O₄@C core–shell aggregates (Scheme 1).

Synthesis route of the core–shell Fe₃O₄@C hybrid nanoparticle

Characterization

Phase composition was examined by X‑ray diffraction (Rigaku D/max‑A, Co Kα). Functional groups were identified via Fourier‑transform infrared spectroscopy (FTIR, Thermo Nicolet AVATARFTIR 360). Morphology and microstructure were visualized using SEM (AMRAY 1000B) and HR‑TEM (JEOL JEM‑2100, 200 kV). Nitrogen adsorption–desorption measurements (Micromeritics Tristar) provided BET surface areas and pore size distributions. Magnetic properties were measured with a vibrating‑sample magnetometer (VSM). Heavy‑metal adsorption experiments were conducted at room temperature: solutions containing 10 mg L⁻¹ Pb²⁺, Cd²⁺, Cu²⁺ or Cr⁶⁺ were adjusted to pH 3, stirred with 20 mg of Fe₃O₄@C, and aliquots were collected at specified intervals for atomic absorption spectroscopy (AAS, Hitachi Z2000).

Heavy Metal Ion Removal Experiments

Adsorption tests were performed in 50 mL volumes with 20 mg of Fe₃O₄@C. Samples were sampled at 0, 0.5, 1, 1.5, 2, 4, 6, 10 and 24 h, filtered, and analyzed by AAS to determine residual metal concentrations.

Result and Discussion

Physicochemical Characteristics of Core–Shell Fe₃O₄@C Nanospheres

X‑ray diffraction patterns (Fig. 1) confirm the formation of crystalline Fe₃O₄ (fcc) and graphitic carbon (002). SEM images (Fig. 2) reveal uniform microspheres (~700 nm) with a distinct core–shell morphology. HRTEM (Fig. 2e) shows lattice spacing of 0.297 nm, matching the (220) plane of Fe₃O₄, and SAED confirms single‑crystal quality. The high BET surface area (238.18 m² g⁻¹) and mesoporous pore distribution (7.5–9.1 nm) (Fig. 4) are attributed to the carbon shell and calcination process.

XRD patterns of core‑shell Fe₃O₄@C hybrid nanoparticle aggregates and its precursor (a—Fe₃O₄@C; b—precursor)

SEM image of prepared precursor (a). SEM images (b, c), TEM image (d), HRTEM micrograph (e), and SAED (f) of as‑synthesized core‑shell Fe₃O₄@C aggregates (calcined at 450 °C)

VSM measurements (Fig. 3) show a saturation magnetization of 53 emu g⁻¹, sufficient for magnetic separation yet lower than bare Fe₃O₄ microspheres due to the carbon shell.

Magnetization loop measurements

Nitrogen adsorption–desorption isotherm and BJH pore size distribution plot (inset) of the prepared sample before (a) and after (b) calcination

Uptake of Heavy Metal Ions by Fe₃O₄@C

Removal efficiencies (Fig. 5) reach 100 % for Pb²⁺, 99.2 % for Cd²⁺, 96.6 % for Cu²⁺, and 94.8 % for Cr⁶⁺ within 24 h at pH 3. The superior performance is attributed to the high surface area, abundant active sites and the magnetic core enabling rapid recovery.

Relationship between the removal efficiency and time for the adsorption of Pb(II), Cd(II), Cu(II) and Cr(VI) by Fe₃O₄@C (400 mg L⁻¹) at initial concentrations of 10 mg L⁻¹

FTIR Spectra of the Heavy Metal Loaded Fe₃O₄@C

FTIR analysis (Fig. 6) shows significant shifts in the O–H stretching (~3475 cm⁻¹), C=O (~1605 cm⁻¹) and metal–oxygen bands after Pb²⁺ adsorption, confirming strong metal–ligand interactions.

FTIR spectra of the prepared Fe₃O₄@C sample before (a) and after (b) adsorption of Pb(II)

Adsorption Kinetics

Adsorption follows a pseudo‑second‑order kinetic model (Eq. 1). The linear fit (R² = 0.999) confirms chemisorption dominates the process. Kinetic parameters are listed in Table 1.

\[\frac{t}{q_t}=\frac{1}{k_2 q_e^2}+\frac{1}{q_e}t\]

Adsorption Isotherm

Equilibrium data were best described by the Freundlich isotherm (R² = 0.9712), indicating heterogeneous surface adsorption. The Langmuir model did not fit the data adequately.

\[\lg q_e=\lg k_F+\frac{1}{n}\lg C_e\]

Thermodynamics Analysis

Arrhenius analysis yields an activation energy of 34.92 kJ mol⁻¹, characteristic of physisorption. This suggests the process is energetically favorable and reversible.

\[\ln k_2=\frac{A}{n}-\frac{E_a}{RT}\]

Conclusions

Fe₃O₄@C core–shell nanoparticles were produced via a green hydrothermal method and calcination. The resulting material combines a high surface area, mesoporosity and magnetic recoverability, achieving near‑complete removal of Pb, Cd, Cu and Cr species from water. The scalable, low‑cost synthesis and excellent adsorption performance position Fe₃O₄@C as a promising candidate for industrial wastewater treatment and other environmental applications.