Magnetite Nano‑Adsorbent from Mill Scale Waste Efficiently Removes Cu(II) from Water: Synthesis, Characterization, Adsorption Kinetics and Regeneration

Introduction

Water quality is critical for life, yet industrial growth has intensified heavy‑metal contamination, particularly copper(II) (Cu²⁺). While essential in trace amounts, elevated Cu²⁺ concentrations cause severe health risks and ecological damage, prompting stringent limits: WHO sets 2 mg L⁻¹ and EPA 1.3 mg L⁻¹. Conventional remediation methods (precipitation, ion exchange, membrane separation, coagulation, adsorption) each have drawbacks; adsorption stands out for its simplicity, cost‑effectiveness and high removal efficiency. However, many conventional adsorbents suffer from low surface area, weak binding, and poor regeneration.

Nanoparticles, especially magnetic Fe₃O₄, offer high surface‑to‑volume ratios, superparamagnetism, and easy magnetic separation, making them attractive for heavy‑metal removal. Yet, the use of magnetite derived directly from industrial waste remains underexplored. Here, we synthesize a magnetite nano‑adsorbent from steel mill‑scale waste via high‑energy ball milling, and systematically investigate its structural, magnetic, and adsorption characteristics for Cu²⁺ removal.

Materials and Methods

Materials and Chemicals

Mill‑scale chips were obtained from a Malaysian steel factory. Copper nitrate (Cu(NO₃)₂·3H₂O) was used to prepare a 1000 ppm stock solution. Deionized water (Milli‑Q) and 0.1 M HCl/NaOH were employed for solution preparation. A UV–Vis spectrophotometer (λ = 600 nm) quantified Cu²⁺ concentrations.

Synthesis of Magnetite Nano‑Adsorbent (MNA)

The raw chips were washed, dried at 104 °C, and milled to <100 µm. Non‑magnetic impurities were removed via magnetic separation (MST) and weakly magnetic particles were discarded using a Curie‑temperature separation (CTST). The resulting micron‑sized Fe₃O₄ was air‑dried and subjected to HEBM for 3, 5 or 7 h to produce nano‑sized magnetite (MNA‑3, MNA‑5, MNA‑7).

Characterization of Prepared Nano‑Magnetite Adsorbent (MNA)

Phase analysis employed XRD (Cu Kα, λ = 0.154 nm) and the Debye–Scherrer equation. Morphology was examined by HRTEM, FESEM–EDS. BET analysis (N₂ adsorption) provided specific surface area and pore size. Zeta potential was measured with a Malvern ZS Zeta sizer, and magnetic properties were assessed by VSM (0–13 kOe). FTIR spectra (400–4000 cm⁻¹) identified surface functional groups.

Adsorption Studies

Batch adsorption tests used 100 mL Cu²⁺ solutions (10–50 mg L⁻¹) in 250 mL flasks. Adsorbent dosages ranged from 10 to 50 mg. Experiments were conducted at 25 °C, pH 5.4, with stirring. The removal percentage and adsorption capacity were calculated via Eqs. (1) and (2). Contact times up to 240 min were evaluated.

Kinetic Study

Time‑resolved Cu²⁺ concentrations (0–50 min) were fitted to Lagergren’s pseudo‑first‑order and pseudo‑second‑order models (Eqs. (3) and (4)). R² values assessed model fit.

Regeneration Study

After adsorption, MNA was magnetically separated, treated with 0.1 M HCl for 180 min at 26 °C, rinsed to neutral pH, and dried at 60 °C for 1 h. The regenerated adsorbent was reused in three consecutive cycles; regeneration efficiency (RE) was calculated via Eq. (5).

Statistical Analysis

Data were analysed by one‑way ANOVA (SAS 9.4) with Duncan’s post‑hoc test (p < 0.05).

Results and Discussion

Structural and Phase Analysis

XRD confirmed the transformation of the raw waste into pure Fe₃O₄ after CTST. The 7‑h HEBM sample (MNA‑7) displayed characteristic Fe₃O₄ peaks with peak broadening indicative of nanocrystalline size (average crystallite size ≈ 9.8 nm). HRTEM images showed irregular particles averaging 11.2 nm. The increased FWHM with milling time correlated with reduced crystallite size.

Morphological and Microstructural Composition

HRTEM revealed a particle size distribution of 10–22 nm, with smaller particles produced at shorter milling times due to reduced agglomeration. FESEM–EDS confirmed the Fe:O ratio of ≈ 3:1, consistent with magnetite.

Magnetic Properties Analysis

VSM data showed saturation magnetization values of 21–27 emu g⁻¹, remanence 1.5–6.6 emu g⁻¹, and coercivity 200–270 G. MNA‑7 possessed the highest Ms (27 emu g⁻¹) and exhibited superparamagnetic behavior, facilitating magnetic separation.

Effect of Adsorption Parameters

Among the milling times, MNA‑7 delivered the highest adsorption capacity (4.41 mg g⁻¹) and removal efficiency (62.6 %). This superiority is attributed to its optimal particle size, surface area, and magnetic properties.

Surface Area Analysis

BET measurements of MNA‑7 yielded a specific surface area of 5.98 m² g⁻¹ and a pore volume of 0.011 cm³ g⁻¹. The N₂ isotherm displayed a Type III hysteresis, indicating weak adsorbate–adsorbent interactions typical for mesoporous Fe₃O₄.

EDS Analysis

Pre‑adsorption EDS showed Fe (78.3 %) and O (21.7 %). Post‑adsorption spectra revealed additional Cu (≈ 1.5 %), confirming Cu²⁺ uptake.

FTIR Analysis

Fe–O stretching bands at 525 and 576 cm⁻¹ persisted after adsorption, while new peaks around 500–600 cm⁻¹ and a broadened OH band at 3478 cm⁻¹ indicated Cu²⁺ coordination to surface hydroxyls.

Zeta Potential Analysis

The isoelectric point of MNA‑7 was pH 5.4. At pH > 5.4, the surface becomes negatively charged, enhancing electrostatic attraction to Cu²⁺ and explaining the optimal removal at this pH.

Batch Adsorption Analyses

Effect of Contact Time

Equilibrium was reached within 120 min; adsorption capacity increased rapidly in the first 10 min and plateaued thereafter. The maximum capacity of 4.41 mg g⁻¹ was achieved at 120 min.

Effect of Initial Concentration

Higher initial Cu²⁺ concentrations (10–50 mg L⁻¹) led to increased adsorption capacities, reflecting a higher driving force and more active site occupancy.

Effect of MNA Dosage

Adsorption capacity decreased with increasing dosage due to aggregation and reduced specific surface area per gram, while removal efficiency peaked at 0.05 g per 200 mL solution.

Effect of pH

Removal efficiency increased from 10.7 % at pH 2 to 62.6 % at the optimal pH 5.4, then declined slightly at higher pH values due to competing OH⁻ ions.

Copper Adsorption Kinetics

The pseudo‑second‑order model yielded R² = 0.999, indicating chemisorption involving electron exchange between Cu²⁺ and Fe₃O₄ surface sites. Pseudo‑first‑order fit was poorer (R² ≈ 0.96).

Copper Adsorption Isotherms

Both Freundlich and Temkin plots were linear; however, the Temkin isotherm provided a better fit (R² = 0.91), suggesting a decreasing heat of adsorption with coverage and indicating interaction among adsorbed species.

Regeneration and Desorption Study

Three regeneration cycles with 0.1 M HCl retained 70.9 % of the original capacity, with only a 10 % loss after the third cycle, demonstrating good reusability.

Conclusions

High‑energy ball milling of mill‑scale waste produced a magnetite nano‑adsorbent (MNA‑7) with 11.2 nm particles, 5.98 m² g⁻¹ surface area, and strong magnetic response. Under optimal conditions (pH 5.4, 0.05 g L⁻¹, 120 min), it removed 62.6 % of Cu²⁺ from aqueous solutions, achieving an adsorption capacity of 4.41 mg g⁻¹. Regeneration with 0.1 M HCl preserved 70.9 % of the capacity after three cycles. These results confirm that MNA‑7 is a low‑cost, efficient, and recyclable adsorbent for copper removal, and it represents a valuable route for waste valorisation and water treatment.

Availability of data and materials

The datasets generated during this study are available from the corresponding author upon reasonable request.