Mineral-Based Synthesis of CuFe₂O₄ Nanoparticles via Co-Precipitation and Microwave Techniques Using Leached Copper Solutions from Mined Minerals

Abstract

Environmental sustainability and responsible resource utilization are critical global challenges. In this work, we present a sustainable and circular-economy-based approach for synthesizing CuFe₂O₄ nanoparticles by directly utilizing copper oxide minerals sourced from Chilean mining operations. Copper sulfate (CuSO₄) was extracted from these minerals through acid leaching and used as a precursor for nanoparticle synthesis via both chemical co-precipitation and microwave-assisted methods. The influence of different precipitating agents—NaOH, Na₂CO₃, and NaF—was systematically evaluated. XRD and FESEM analyses revealed that NaOH produced the most phase-pure and well-dispersed nanoparticles, while NaF resulted in secondary phase formation. The microwave-assisted method further improved particle uniformity and reduced agglomeration due to rapid and homogeneous heating. Electrochemical characterization was conducted to assess the suitability of the synthesized CuFe₂O₄ for supercapacitor applications. Cyclic voltammetry (CV) and galvanostatic charge–discharge (GCD) measurements confirmed pseudocapacitive behavior, with a specific capacitance of up to 1000 F/g at 2 A/g. These findings highlight the potential of CuFe₂O₄ as a low-cost, high-performance electrode material for energy storage. This study underscores the feasibility of converting primary mined minerals into functional nanomaterials while promoting sustainable mineral valorization. The approach can be extended to other critical metals and mineral residues, including tailings, supporting the broader goals of a circular economy and environmental remediation.

1. Introduction

The global demand for vital minerals and metals, particularly copper, has recently escalated due to its significance in renewable energy technologies, electronics, and the construction of green infrastructure [1,2,3]. Applications in energy storage systems, solar photovoltaics, electric vehicles, and modern electronics depend on copper’s extraordinary electrical and thermal conductivity, corrosion resistance, and catalytic properties. Thus, meeting the objectives of the circular economy and reducing the environmental impact of the usage of mineral resources depend on the sustainable extraction and use of copper. With more than 25% of all copper produced worldwide, Chile continues to be the top copper producer in the world [4,5]. Although the supply of primary copper depends on conventional mining activities, these also produce large amounts of ore wastes, acid mine drainage, and other waste sources that present financial and environmental problems [6,7,8,9]. Energy-intensive and producing harmful emissions, traditional extraction methods including roasting and smelting call for the creation of alternative, low-impact processing technologies. In this regard, hydrometallurgical leaching, especially sulfuric acid leaching, has become a quick and environmentally friendly method for copper extraction from oxide minerals and ores [10,11,12,13].

Simulating industrial acid heap leaching chemistry at a laboratory scale, leaching systems enable the fast generation of high-purity copper sulfate (CuSO₄) solutions fit for downstream uses including electrowinning or nanomaterial synthesis [14,15]. Allowing the conversion of mining-derived materials into high-value functional materials, in turn reducing waste and resource loss, fits very nicely with the ideas of sustainable resource management and the circular economy [16,17,18,19]. Copper sulfate has one such exciting use in the synthesis of copper ferrite (CuFe₂O₄) nanoparticles, a class of spinel ferrites with special optical, magnetic, and catalytic qualities [20]. Applications for CuFe₂O₄ nanomaterials span photocatalysis, magnetic separation, sensors, energy storage, environmental remediation, and so forth [21,22,23,24,25,26,27,28]. Their great chemical stability, saturation magnetism, and capacity to absorb visible light make them perfect candidates for many purposes in energy and environmental systems. Among the several synthesis techniques for ferrite nanoparticles, the co-precipitation method is particularly simple, cheap, and able to generate nanoparticles with controlled stoichiometry and morphology [29,30,31]. The final phase purity, crystallite size, and particle shape of the ferrite product depend much on the choice of precipitating agents [32,33]. Commonly used agents including NaOH, Na₂CO₃, and NaF affect the nucleation and growth kinetics and can cause appreciable variations in particle structure and performance [34,35,36,37]. Apart from traditional co-precipitation, microwave-assisted synthesis has attracted interest for its benefits in terms of fast processing, energy economy, and the possibility to create homogeneous and crystalline nanoparticles with lowered agglomeration [38,39,40]. This approach is especially useful for manufacturing high-quality nanomaterials from aqueous precursors since microwaves offer consistent volumetric heating, which stimulates fast nucleation and particle growth.

In this work, CuSO₄ solutions were generated by acid leaching copper oxide minerals donated by Chilean state-owned mining company ENAMI Vallenar, Vallenar, Chile. Via co-precipitation and microwave-assisted techniques employing various precipitating agents, these leachates provided the copper source for the synthesis of CuFe₂O₄ nanoparticles. The ideal conditions for generating high-quality CuFe₂O₄ were found by means of an extensive analysis of the structural, morphological, and phase properties of the nanoparticles. This integrated approach valorizes mined minerals through low-impact processing and conversion into functional nanomaterials, thus addressing important sustainability goals. The results show that a circular approach to the use of critical minerals is supported by the efficient synthesis of CuFe₂O₄ from leached copper from mined minerals. Moreover, the study offers a platform for extending similar procedures to other mining-derived products or tailings, thus possibly lowering waste and promoting the growth of environmentally friendly technologies.

2. Experimental

2.1. Materials

The base material used for the synthesis and subsequent leaching processes consisted of mineral oxides donated by ENAMI Vallenar, a Chilean state-owned mining company. The facility is located on the outskirts of Vallenar, along the road to Alto del Carmen, at the coordinates −28.564918906044497, −70.74133301538157. These mineral samples originate from copper oxide ores acquired by ENAMI from small-scale miners in the Province of Huasco. All analytical-grade reagents, including iron(III) chloride (FeCl₃), iron(II) sulfate (FeSO₄), sodium hydroxide (NaOH), sodium fluoride (NaF), and sodium carbonate (Na₂CO₃), were purchased from Sigma-Aldrich and used without further purification.

2.2. Preparation of CuSO₄ Solution from Minerals

To prepare the Pregnant Leaching Solution (PLS), a leaching process was conducted using copper oxide minerals collected from the Taltal and Cobre Norte mining regions. For each test, 10.000 ± 0.001 g of mineral sample was used. The minerals were first ground to a fine particle size of -#325 mesh, increasing the reactive surface area and enhancing the leaching efficiency. The leaching process involved the addition of 100 mL of 0.1 M sulfuric acid (H₂SO₄) to the ground mineral in a 250 mL Erlenmeyer flask, as illustrated in Figure S1, which shows minerals in contact with sulfuric acid during the leaching process. This mixture simulates acid heap leaching chemistry but at a laboratory scale and with a finer particle size.

After acid addition, the samples were subjected to orbital shaking at 150 rpm for 30 min to improve the interaction between the acid and the mineral surface. This dynamic leaching setup accelerates metal dissolution compared to traditional static methods. The orbital shaking process is shown in Figure S2, where the samples are visibly agitated to enhance acid penetration and copper ion release.

Following the agitation step, the mixtures were filtered to separate the solid residues from the clear leachate containing dissolved copper. The filtration stage is shown in Figure S3, which captures the post-leaching separation process that yields the Pregnant Leaching Solution (PLS). This approach mimics industrial heap leaching, which typically uses coarser particles (¼ to 1 inch) and requires 3 to 4 weeks for copper extraction. However, the use of fine particles and mechanical agitation in this laboratory protocol significantly shortened the process to just 30 min, while achieving comparable dissolution efficiency. The resulting PLS, rich in Cu²⁺ ions, was then used to prepare a CuSO₄ solution, confirming the effectiveness of this rapid leaching technique for producing high-purity copper solutions suitable for downstream applications such as electrowinning, material synthesis, or analytical calibration standards.

2.3. Synthesis of CuFe₂O₄ Nanoparticles

CuFe₂O₄ nanoparticles were synthesized via a co-precipitation method using CuSO₄ solutions extracted from mining resources and FeSO₄ as precursor salts. The precipitation process was facilitated by the addition of NaOH, Na₂CO₃, or NaF, which acted as the precipitating agents. In the first step, CuSO₄ and FeSO₄ were dissolved in deionized water at room temperature to ensure the homogenous mixing of metal ions. Further, the ferric ions were added to form a divalent and trivalent metal ion mixture. The precipitating agent was then added dropwise under continuous stirring, leading to the formation of a precipitate. The pH was maintained between 9 and 10. The obtained precipitate was then subjected to multiple washing steps with deionized water to remove unreacted salts and byproducts such as Na₂SO₄ or NaCl. Magnetic separation was employed to collect the nanoparticles efficiently. Finally, the filtered product was dried at 200 °C for 2 h to enhance the crystallinity and phase purity of CuFe₂O₄ nanoparticles.

The microwave-assisted method was employed for the rapid and efficient synthesis of CuFe₂O₄ nanoparticles, utilizing a CuSO₄ solution from mining resources, FeSO₄ as precursor salts, and NaOH as the precipitating agent. In this approach, CuSO₄ and FeSO₄ were dissolved in deionized water at room temperature to ensure the uniform mixing of Cu²⁺ and Fe²⁺ ions. Further, the ferric ions were added to form a divalent and trivalent metal ion mixture. The NaOH solution was then added dropwise under continuous stirring to maintain a pH of 9 to 10, facilitating the controlled precipitation of Cu(OH)₂ and Fe(OH)₂. After the complete addition of NaOH, the reaction mixture was transferred to a commercial microwave oven and subjected to microwave irradiation for 2 min, with a 30 s step and a 1 min cooling period. The microwave energy promoted the rapid nucleation and crystallization of CuFe₂O₄, leading to the formation of nanosized particles with improved homogeneity. Following microwave treatment, the obtained precipitate was washed multiple times with distilled water to remove unreacted residues. Magnetic separation was employed to isolate the CuFe₂O₄ nanoparticles efficiently. The final product was then filtered and dried at 200 °C for 2 h, ensuring the removal of residual moisture and improving the structural integrity of the nanoparticles.

3. Characterizations

The structural properties of the synthesized CuFe₂O₄ nanoparticles were investigated using X-ray diffraction (XRD) analysis. XRD patterns were recorded using a Bruker D8 diffractometer equipped with a Cu Kα radiation source (λ = 1.5406 Å) operating in the 2θ range of 5–80°. The surface morphology and microstructural features of the as-prepared CuFe₂O₄ nanoparticles were examined using Scanning Electron Microscopy (SEM) with a ZEISS GeminiSEM 360, Oberkochen, Germany operating in NanoVP mode. The electrochemical properties were evaluated using cyclic voltammetry (CV) and galvanostatic charge–discharge (GCD) with a single-electrode setup on a CHI660E electrochemical workstation in 6 M KOH electrolyte. A three-electrode system was employed, where the working electrode was prepared by dispersing the synthesized material in N-Methyl-2-pyrrolidone (NMP) via ultrasonication to obtain a homogeneous viscous paste. A known amount of this paste (2 mg) was then cast onto a piece of nickel foam (NF) measuring 1 cm × 1 cm and dried overnight at 80 °C. In the three-electrode configuration, a Hg/HgO electrode served as the reference electrode, and a platinum wire was used as the counter electrode. Cyclic voltammetry measurements were carried out at various scan rates of 2, 5, 10, 20, 50, and 100 mV s⁻¹ within a potential window of 0 to 0.5 V.

4. Results and Discussions

The synthesized CuFe₂O₄ nanoparticles were characterized using XRD to confirm their crystallographic phase composition. Figure 1 shows the XRD patterns of CuFe₂O₄ nanoparticles prepared via the co-precipitation method using three different precipitating/reducing agents: (a) NaF, (b) Na₂CO₃, and (c) NaOH. The samples prepared with NaOH and Na₂CO₃ exhibited sharp and well-defined diffraction peaks corresponding to a single-phase formation of CuFe₂O₄. These peaks matched well with the standard diffraction data of copper ferrite, as per the JCPDS card No. 22–1086, confirming the formation of a pure spinel phase of CuFe₂O₄. This indicates that NaOH and Na₂CO₃ are effective in promoting the formation of a pure, single-phase CuFe₂O₄ structure during the co-precipitation process. In contrast, the sample synthesized using NaF displayed additional peaks in the XRD pattern, indicating the presence of secondary impurity phases along with the main CuFe₂O₄ phase. These secondary phases may arise due to an incomplete reaction or the formation of fluorinated byproducts, which suggests that NaF is less suitable as a precipitating agent for achieving phase-pure CuFe₂O₄ under the current synthesis conditions. Overall, the choice of precipitating/reducing agent plays a critical role in determining the phase purity and crystallinity of the final CuFe₂O₄ nanoparticles, with NaOH and Na₂CO₃ yielding the most favorable results for the formation of phase-pure copper ferrite.

To investigate the surface morphology and particle shape of the synthesized CuFe₂O₄ nanoparticles, FESEM analysis was performed. Figure 2 shows the FESEM micrographs of CuFe₂O₄ nanoparticles prepared via the co-precipitation method using three different precipitating agents (a) NaF, (b) Na₂CO₃, and (c) NaOH. The CuFe₂O₄ sample synthesized using NaF exhibited a disordered and highly agglomerated network-like morphology. The structure appeared gel-like, composed of irregularly shaped particles including plate-like, petal-shaped, and rod-like features. This heterogeneous and poorly defined morphology suggests incomplete phase formation and significant structural disorder. These morphological observations are in good agreement with the XRD results, which showed the presence of secondary impurity phases, indicating that NaF is not ideal for achieving phase-pure CuFe₂O₄ under the current synthesis conditions. In contrast, the CuFe₂O₄ nanoparticles synthesized with Na₂CO₃ showed a more uniform morphology, consisting predominantly of spherical particle clusters. These spherical aggregates were composed of smaller primary nanoparticles, and a few irregularly shaped particles were also observed within the cluster. The average size of these clusters was estimated to be around 100 nm. This microstructure is consistent with the XRD findings, which confirmed the formation of a single-phase CuFe₂O₄ spinel structure. The relatively uniform morphology suggests that Na₂CO₃ facilitates controlled nucleation and growth during co-precipitation, leading to improved particle uniformity. The sample prepared using NaOH exhibited the most well-defined and distinct nanoparticle morphology among the three. The FESEM images reveal dispersed individual nanoparticles with a range of shapes, including spherical, partially cubic, plate-like, and some irregular forms. Most of these particles were within the size range of a few tens of nanometers. Despite some shape diversity, the particles were relatively uniform and well separated, indicating a good degree of crystallinity and size control. These morphological characteristics align with the XRD results, which confirmed the formation of a pure CuFe₂O₄ phase with high crystallinity when NaOH was used as the precipitating agent.

To further evaluate the effect of the synthesis route on the structural characteristics of CuFe₂O₄ nanoparticles, microwave-assisted synthesis was employed using NaOH as the precipitating agent, and the results were compared with those obtained from the conventional co-precipitation method. Figure 3 shows the comparative XRD patterns of CuFe₂O₄ nanoparticles prepared via (a) the co-precipitation method and (b) the microwave-assisted chemical synthesis method. The XRD pattern of the CuFe₂O₄ synthesized via the microwave-assisted route exhibited sharp and well-defined peaks, indicating good crystallinity and the successful formation of a single-phase spinel structure. The absence of additional or unidentified peaks confirmed the phase purity, similar to that observed in the sample prepared through the co-precipitation method.

To estimate the crystallite size of the CuFe₂O₄ nanoparticles, the Scherrer equation was applied to the (311) peaks after Lorentzian peak fitting, carried out using Origin 8.5 software. Figure 4 shows the Lorentzian peak fitting of the XRD patterns for CuFe₂O₄ nanoparticles synthesized via (a) the co-precipitation method and (b) microwave-assisted chemical synthesis, which was used to estimate the crystallite size using the Scherrer equation. The average grain size was calculated to be approximately 28 nm for the co-precipitated CuFe₂O₄ and 30 nm for the microwave-assisted synthesized sample. The slightly larger crystallite size in the microwave-assisted sample suggests accelerated crystal growth under microwave irradiation, likely due to rapid and uniform volumetric heating, which promotes enhanced nucleation kinetics and crystal development. Despite this minor difference, both methods yielded highly crystalline and phase-pure CuFe₂O₄ nanoparticles. The comparison indicates that microwave-assisted synthesis is an effective alternative to the traditional co-precipitation approach, offering advantages such as shorter reaction times, better energy efficiency, and comparable or slightly improved crystallite size and structural purity. These results suggest that microwave-assisted synthesis can be considered a viable and scalable method for producing high-quality CuFe₂O₄ nanoparticles, especially in applications where fast synthesis and uniform nanostructure are essential.

Figure 5 presents the (a) FESEM micrograph and (b) particle size distribution curve of CuFe₂O₄ nanoparticles synthesized via microwave-assisted chemical synthesis, along with (c) the particle size distribution curve for CuFe₂O₄ nanoparticles prepared through the co-precipitation method. The FESEM image of the microwave-synthesized CuFe₂O₄ clearly reveals that the majority of the particles exhibit a spherical morphology with relatively uniform size and limited agglomeration. This suggests that the microwave-assisted method promotes a rapid and homogeneous nucleation environment, resulting in the formation of more isotropic particles. The particle size distribution analysis further supports this observation. The average particle size of the CuFe₂O₄ nanoparticles synthesized via the microwave-assisted method was estimated to be around 28 nm, with a relatively narrow size distribution ranging from 10 to 50 nm. In contrast, the CuFe₂O₄ nanoparticles obtained via the co-precipitation method exhibited a broader size distribution, ranging from 12 to 88 nm, with a higher average particle size of approximately 38 nm. This shift in particle size distribution indicates that the microwave-assisted synthesis method offers better control over particle size and uniformity compared to the conventional co-precipitation process. The narrower distribution and smaller average size observed in the microwave route are likely due to the fast, volumetric heating provided by microwave radiation, which reduces the time available for particle growth and aggregation.

These morphological findings correlate well with the XRD and Lorentzian peak fitting results, which showed a slightly smaller crystallite size for the microwave-assisted sample compared to the co-precipitated one. Although the crystallite sizes (from XRD) and particle sizes (from FESEM) differ due to their respective measurement principles, the trends are consistent: microwave synthesis results in smaller and more uniform particles, both at the grain and agglomerate levels. Overall, the combination of FESEM imaging and particle size distribution analysis highlights the advantages of microwave-assisted synthesis in achieving well-defined, uniform CuFe₂O₄ nanoparticles with reduced agglomeration and narrower size distribution—critical parameters for applications in catalysis, magnetic devices, and energy storage.

Further, the developed CuFe₂O₄ magnetic nanoparticles were evaluated for their suitability in electrochemical supercapacitor application. The CuFe₂O₄ electrode’s CV curves, measured at different scan rates of 2, 5, 10, 20, 50, and 100 mV/s in a typical three-electrode configuration, are shown in Figure 6. The presence of faradaic reactions linked to the Cu²⁺/Cu³⁺ and Fe²⁺/Fe³⁺ redox couples is suggested by the presence of small but broad humps that correspond to redox transitions, even though distinct and well-defined redox peaks are not sharply identified. These redox characteristics confirm the CuFeO₄ electrode material’s pseudocapacitive behavior.

The enclosed area under the CV curves expands larger as the scan rate increases, suggesting effective ion transport at the electrode–electrolyte interface and improved charge storage capacity. Materials displaying pseudocapacitive properties typically behave in this way. Furthermore, there is no distortion in the CV curves’ shape at any scan rate. Under dynamic cycling conditions, this stability suggests that the electrode material has good electrochemical reversibility and structural integrity. These findings demonstrate the encouraging electrochemical behavior of CuFeO₄ with regard to stability, redox activity, and rate handling.

To evaluate the electrochemical performance of the synthesized CuFe₂O₄ electrode material, GCD measurements were conducted within a potential window of 0.0 to 0.45 V at various applied current densities (2 A/g, 3 A/g, 4 A/g, 5 A/g, and 10 A/g). The corresponding GCD profiles are shown in Figure 7a. The GCD curves demonstrate quasi-triangular shapes, which are characteristic of pseudocapacitive behavior. While the ideal linear symmetry typically seen in EDLCs is not perfectly preserved, the discharge segments exhibit a notable initial IR drop between 0.45 V and ~0.38 V, especially at lower current densities. This drop is attributed to the internal resistance of the electrode and interface contact resistance. Following this initial voltage drop, the discharge curves proceed in a more linear manner, which is indicative of the faradaic charge storage associated with the redox-active nature of CuFe₂O₄. The specific capacitance (Cₛ) of the CuFe₂O₄ electrode was calculated from the discharge portion of the GCD curves using the following formula:

C_s = (I × Δt)/(m × ΔV),

where I is the discharge current (A), Δt is the discharge time (s), m is the mass of the active material (g), and ΔV is the potential window (V). Based on this calculation, the specific capacitance values at different current densities were found to be 1000.1 F/g at 2 A/g, 969.3 F/g at 3 A/g, 905.7 F/g at 4 A/g, 863.3 F/g at 5 A/g, and 535.5 F/g at 10 A/g. These results indicate that the electrode exhibits excellent capacitive behavior, particularly at lower current densities. Although a gradual decline in capacitance is observed with increasing current density, the retention of relatively high capacitance even at 10 A/g suggests good rate capability. This trend is typical for pseudocapacitive materials and is attributed to limited ion diffusion and less efficient redox reactions at higher charge/discharge rates. Figure 7b illustrates the relationship between specific capacitance and current density, showing a consistent trend that supports the electrode’s robustness under various operating conditions.

5. Conclusions

In this study, a sustainable and resource-efficient strategy was developed for synthesizing CuFe₂O₄ nanoparticles by directly utilizing copper-rich mineral materials sourced from Chilean mining operations. Copper sulfate (CuSO₄) was successfully obtained through the acid leaching of mined copper oxide minerals, and this leachate was subsequently used—after pH adjustment and concentration calibration—for the synthesis of CuFe₂O₄ nanoparticles via co-precipitation and microwave-assisted methods. The combination of FeSO₄ and ferric ions was employed to ensure a proper Cu²⁺:Fe³⁺/Fe²⁺ stoichiometry conducive to forming a stable spinel structure. Among the various precipitating agents tested, NaOH and Na₂CO₃ enabled the formation of highly crystalline, phase-pure CuFe₂O₄ nanoparticles, as evidenced by XRD analysis. In contrast, NaF resulted in secondary phases and disordered morphologies, indicating its unsuitability under the current synthesis conditions. FESEM observations confirmed that NaOH yielded better-defined morphologies and smaller particle sizes with reduced agglomeration, particularly when used in the microwave-assisted method. The microwave-assisted synthesis approach demonstrated several advantages over conventional co-precipitation, including rapid reaction kinetics, narrower particle size distribution (10–50 nm), and enhanced homogeneity. The crystallite sizes estimated using the Scherrer equation (28–30 nm) were consistent with SEM-based particle size trends. Electrochemical characterization, including CV, and GCD confirmed the pseudocapacitive behavior of the CuFe₂O₄ electrodes. The electrode exhibited a high specific capacitance of 1000.1 F/g at 2 A/g, with good rate capability, reinforcing its suitability for supercapacitor applications. Overall, this work highlights the feasibility of converting mined mineral resources into functional magnetic nanomaterials, contributing to sustainable resource utilization and circular economy principles. The developed methodology provides a scalable route not only for synthesizing CuFe₂O₄ but also for extending this approach to the valorization of other mineral-based sources including tailings toward high-value applications such as energy storage and environmental remediation. Future work will focus on optimizing synthesis parameters such as pH, temperature, and stirring rate to enhance phase purity and particle uniformity. Improved purification steps will be explored to obtain high-purity metal ions from leachates. Additionally, the method will be extended to other metals like Co and Ni, and the catalytic and magnetic properties of the synthesized CuFe₂O₄ will be evaluated. Scale-up and economic feasibility studies will also be undertaken.