Simulation study on selective recovery of Cu from Cu–Co–Ni composite system by Fe-struvite composite

Abstract

The selective recycling of heavy metals is of great economic and environmental importance to human production and life. In this paper, the Fe-struvite composite (FSC) was synthesized by chemical principles and crystal growth theory, and further experimental methods such as model fitting, batch passivation experiment, mineral characterization, and chemical determination were used to explore the fixation properties and mechanisms of FSC on Cu–Co–Ni. The results demonstrate that FSC is an irregular composite of nano zero-valent iron loaded struvite. The passivation of Cu by FSC is multilayer (Q_max = 166.67 mg g⁻¹), whereas the adsorption of Co/Ni is monolayer. The passivations of Cu, Co and Ni by FSC are all physicochemical adsorption processes dominated by chemisorption. Nano-zero-valent iron undergoes the redox reaction with Cu and is accompanied by the formation of Fe²⁺. Furthermore, NO₃⁻ weakly reduces the selective immobilization of Cu by FSC through reductive competition with Cu. Studies have shown that the passivation of Cu/Co/Ni by FSC is related to phase change, chelation/complexation, redox action and ion exchange, whereas the selective immobilization of Cu is related to differences in the physical properties of Cu/Co/Ni-PO₄ and Cu concentration. This study provides theoretical basis for the selective recovery of Cu using FSC.

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1. Introduction

Copper has superior electrical and thermal conductivity, ductility, corrosion resistance, and unique antimicrobial properties, which have made it an indispensable basic material in modern industrial and agricultural fields such as energy, healthcare, construction, and transport.^1–5 As the trend towards renewable energy, electrification and sustainability accelerates, the demand for copper increases significantly.⁵ Due to the scarcity of copper resources, our country needs to import a large amount of copper mineral resources from abroad every year.² However, Cu mineral resources are concentrated in countries such as Congo, Canada and Russia, making the supply chain vulnerable to international politics and environmental protection.^6,7 Therefore, the recycling of copper resources is one of the important ways to cope with the shortage of copper resources in China at present. Studies have shown that Co/Ni often forms composite deposits with Cu and other elements, leading to serious Co–Ni–Cu composite environmental heavy metal pollution during mineral extraction, which in turn poses a great threat to human health and ecological security.^8,9 Moreover, the production and processing of Cu generates a large amount of industrial wastewater (including Co, Ni, Cu, etc), and wastewater discharge will cause huge economic losses.^10–15 Therefore, the separation, extraction and recycling of copper resources is an important research topic to address the needs of social development and environmental pollution control.

Selective enrichment separation is one of the key technologies for the recovery and reuse of copper resources, and limited research has been conducted in this area. Yang et al. (2024) prepared functionalized Ui-66 composites using thiophene schiff bases, which achieved selective adsorption of copper ions by forming a special ligand structure with them.¹⁶ The ammonium pyrrolidine dithiocarbamate-modified biochar can achieve highly selective adsorption of copper ions through a unique chelating mechanism.¹⁷ Ji et al. (2021) have prepared unique gel particles using cross-linking and freeze-drying methods, which can achieve selective adsorption of copper ions through a slit-like structure.¹⁸ Jia et al. (2025) prepared surface ion-imprinted sodium alginate silica composites that not only selectively adsorb copper but also catalyze the formation of methanol from CO₂.¹⁹ Although some academic achievements have been made in the separation and extraction of copper ions, the high cost of material preparation, complexity of the process and low selective separation capacity mean the existing technology still needs further development.^20–22

Magnetic separation of heavy metals is one of the effective ways to reduce the cost of heavy metal wastewater treatment.²³ Huang et al. (2020) prepared sulfhydryl-functionalized magnetic covalent organic frameworks using Fe₃O₄, which can efficiently magnetically separate Hg²⁺ from wastewater.²⁴ Shewanella oneidenis enhances Cr(VI) passivation and magnetic separation of secondary products by mediating the synthesis of zero-valent iron nanoparticles.²⁵ Although both Fe₃O₄ and nano zero-valent iron are common magnetic source materials, nano zero-valent iron has been widely used in environmental pollution treatment of water, soil and groundwater due to its large surface area, powerful oxidation properties and high surface activity.^26–31 Therefore, the multifunctional composites formed by using zero-valent iron nanoparticles as a source of magnetism have a promising use in the efficient and selective recycling of heavy metal.^32,33

Struvite is a sub-stable phosphate mineral, which has an important impact on the transport and transformation of environmental heavy metals.^34–38 Our previous studies found that struvite exhibits highly selective adsorption properties for copper under certain specific circumstances, but the immobilization characteristics and mechanism are not clear.

Therefore, this article adopts chemical methods to synthesize Fe-struvite composite (FSC), and combines various experimental methods such as chemical analysis, batch adsorption, and mineral characterization to explore its competitive and selective passivation characteristics and mechanisms for Cu–Co–Ni. This study provides basic information on the separation and extraction of Cu by FSC.

2. Materials and methods

2.1. Material synthesis

The 200 mL of 80% aqueous ethanol solution was prepared using anhydrous ethanol and ddH₂O in a glass beaker, and 40 mL of the prepared ethanol solution was pipetted into a 50 mL EP tube using a pipette gun. The 1.0788 g KBH₄ was weighed on an analytical electronic balance and placed in a 50 mL EP tube containing 40 mL of ethanol solution and allowed to dissolve as much as possible. An analytical electronic balance was used to weigh 1.6218 g FeCl₃·6H₂O, which was poured into a 200 mL glass beaker containing the remaining ethanol solution. The beaker was placed on a magnetic rotator for stirring (400 rpm, 25 °C) and the KBH₄ solution was aspirated with a pipette gun and slowly added to the ferric chloride solution, which was spun for 10 min after the operation. The precipitates were then washed 3 times with ddH₂O and the supernatant was poured out after fixing the precipitates with a magnet.

The 100 mL ddH₂O was added to the above beaker containing the precipitate with magnetic stirring (400 rpm, 25 °C). An analytical electronic balance was used to weigh 2.3128 g Na₂HPO₄·12H₂O and 1.5274 g NH₄Cl and add them to the above beaker. Then, 0.7836 g MgCl₂·6H₂O was added to the aforementioned beaker, and the mixture was continuously stirred for 10 min. The precipitated compounds were rinsed several times with ddH₂O at the end of mixing and transferred to new 50 mL EP tubes. Finally, this EP tube was lyophilized and the material was used as an adsorbent for subsequent experiments.

2.2. Adsorption isotherm

CuCl₂·2H₂O, CoCl₂·6H₂O, NiCl₂·6H₂O and ddH₂O were used to prepare heavy metal solutions of different concentrations (Cu: 4.88–397.96 mg L⁻¹, Co: 4.95–406.94 mg L⁻¹, Ni: 4.45–378.36 mg L⁻¹). Subsequently, ddH₂O and the above chemicals were used to formulate aqueous solutions of different binary systems of heavy metals (Cu: 4.69–381.86 mg L⁻¹ + Ni: 4.78–394.12 mg L⁻¹, Cu: 4.71–387.74 mg L⁻¹ + Co: 4.83–409.49 mg L⁻¹, Co: 4.72–414.63 mg L⁻¹ + Ni: 4.73–387.42 mg L⁻¹), and binary heavy metal concentrations were combined in descending order of similar concentrations. Finally, different concentrations of ternary heavy metal solutions (Cu: 4.63–382.20 mg L⁻¹ + Co: 4.80–395.3 mg L⁻¹ + Ni: 4.86–415.66 mg L⁻¹) were prepared using ddH₂O and chemical reagents, and the ternary heavy metal concentrations were compounded according to the similar concentrations in order from the lowest to the highest.

The 20 mL solutions of the various heavy metals compounds at different concentrations described above were pipetted into several 50 mL EP tubes using a pipette gun, with three separate replicates at each concentration of each heavy metal compound. Several parts (0.01 g/serving) of the adsorbent were weighed using an analytical electronic balance. The experimental system was incubated for 24 h (25 °C, 100 rpm) after adsorbents were added to each of tubes. At the end of the adsorption, the samples were centrifuged (9000 rpm, 25 °C, 5 min) and the supernatant was collected. The concentrations of Cu, Co, Ni, Mg and Fe in the supernatant were determined using a flame atomic spectrophotometer (AAS, AA-6300C, Shimadzu) and the NH₄⁺ concentration was determined using a spectrophotometer (BD15/T 1458-2018). The eqn (1) was used to calculate the passivation capacity of heavy metals by adsorbent.³⁹

Furthermore, Langmuir and Freundlich equations were further used to fit the above experimental data.⁴⁰

Q_e = (C_o − C_e)/M × V (1)

Q_e: immobilization capacity at passivation equilibrium, mg g⁻¹; C_o: heavy metal initial concentration, mg L⁻¹; C_e: pollutant concentration at passivation equilibrium, mg L⁻¹; M: material utilization amount, g; V: experimental volume, mL.

2.3. Adsorption kinetic

CuCl₂·2H₂O, CoCl₂·6H₂O, NiCl₂·6H₂O and ddH₂O were used to prepare heavy metal solutions at specific concentrations for different composite systems (374.21 mg L⁻¹ Cu, 423.13 mg L⁻¹ Co, 347.95 mg L⁻¹ Ni, 385.78 mg L⁻¹ Cu + 405.53 mg L⁻¹ Co, 384.36 mg L⁻¹ Cu + 372.58 mg L⁻¹ Ni, 405.20 mg L⁻¹ Co + 379.83 mg L⁻¹ Ni, 379.06 mg L⁻¹ Cu + 381.06 mg L⁻¹ Co + 377.78 mg L⁻¹ Ni). Appropriate portions of 20 mL adsorption liquid of different composite systems were pipetted into 50 mL EP tubes using a pipette gun (three replicates per treatment). Multiple adsorbents (0.01 g/serving) were weighed using an analytical electronic balance and the adsorbent was added to each of the above 50 mL tubes. The experimental system was then incubated in a shaker with shaking (25 °C, 100 rpm). Three centrifuge tubes were centrifuged (25 °C, 9000 rpm, 5 min) at specific point in time (time points: 5–720 min). The concentration of Cu/Co/Ni in the supernatant was detected by AAS. The eqn (1) was used to calculate the passivation ability of Cu/Co/Ni. Based on the experimental results, the pseudo-first-order dynamic equation and pseudo-second-order dynamic equation were used to fit the experimental data.⁴¹

2.4. Material characterization

The morphology and elemental composition of the materials before and after adsorption were characterized using scanning electron microscopy microscopy and energy spectroscopy (SEM-EDS, Apreo 2S + – Oxford ultim max 65, Thermo Fisher Scientific). The mineral crystal structure and organic functional group before and after adsorption were studied using the X-ray diffractometer (XRD, BTX-526, Olympus, USA) and the infrared spectrometer (FTIR, Nexus 670, Thermo Nicolet). The magnetic strength and associated elemental valence states of the samples before and after adsorption were characterized using the vibrating sample magnetometer (VSM, LakeShore 7404, USA) and the X-ray photoelectron spectrometer (XPS, Thermo Fisher ESCALAB 250Xi, USA).

3. Results and discussion

3.1. Material characterization

The study of physicochemical properties is an important part of adsorbent, and the study results of material properties are shown in Fig. 1 and 2. During the course of the experimental manipulation, it is observed that the mineral changes from black to light grey, presumably forming a core–shell structure (Fe is inside and struvite is outside). The adsorption materials synthesized in this work exhibit an irregular morphology (Fig. 1A). The enlarged image shows that the surface of the mineral is rough and contains nanoscale bead-like structures in certain areas (Fig. 1B), which are thought to be nano zero-valent iron.^42,43 The EDS results show that the mineral consists of Mg, O, P, N, Fe, and C (Fig. 1C and D). The Mg, O, P, and N are the constituents of struvite. The element Fe is derived from zero-valent iron nanoparticles or synthetic raw materials, and C is supposed to be derived from carbon-based compounds such as ethanol during the synthesis of zero-valent iron.

Fig. 1 The characterization of the morphology and elemental composition of FSC (A and B) SEM; (C) Elemental electron image; (D) EDS.

Fig. 2 The physicochemical properties of FSC (A) XRD; (B) FTIR; (C) Hysteresis curve.

The XRD results show that the synthesized material mainly consists of crystalline struvite, but no peaks of nano zero-valent iron were compared (Fig. 2A), suggesting that the nano zero-valent iron may be an amorphous mineral.⁴⁴ The FTIR results reveal that organic functional groups (–OH/C=O/COO⁻) is observed on the surface of material (Fig. 2B), indicating that the material is an organic–inorganic composite.^45–47 Furthermore, the magnetic data show an overall elongated hysteresis curve (Ms = 22.12 emu g⁻¹, Mr = 7.13 emu g⁻¹, Hc = 609.79 O_e), indicating that the adsorption material is hard magnetic materials (Fig. 2C).⁴⁸ Based on the above results, it is clear that the material synthesized in this paper can be termed as Fe-struvite composite (FSC).

3.2. Adsorption isotherm

The adsorption isothermal parameters can well reveal the arrangement of heavy metals on the FSC, and the results of adsorption isothermal experiments of Cu/Co/Ni by FSC are shown in Fig. 3. The overall trend of Cu by FSC in different adsorption systems shows an increase with the initial Cu concentration (Fig. 3A). The presence of Co/Ni reduces the adsorption capacity of FSC for Cu, but to a lesser extent, which is attributed to the competitive adsorption between heavy metals (Fig. 3A). The adsorption of Co/Ni by FSC shows an overall trend of increasing and then levelling off, but the presence of other ions significantly reduces the adsorption of Co/Ni by FSC (Fig. 3B and C). The results of Fig. 3B and C show that the adsorption of Co/Ni is susceptible to competition from Cu, further revealing that the lower affinity of FSC for Co/Ni.⁴⁹ Additionally, the experimental data is further fitted using adsorption isothermal models (Table 1).

Fig. 3 The passivation of Cu/Co/Ni by FSC in different passivation systems (A) Cu; (B) Co; (C) Ni.

Table 1 The adsorption isothermal parameters

Treatment	Heavy metal	Langmuir			Freundlich
		Qmax (mg g^−1)	KL (L mg^−1)	R^2	ln KF (L mg^−1)	1/n	R^2
Unitary system	Cu	166.67	0.03	0.9283	2.82	0.39	0.9938
	Co	121.95	0.22	0.9995	3.62	0.30	0.6922
	Ni	113.64	0.55	1.0000	3.38	0.30	0.8886
Ternary system	Cu	135.14	0.02	0.6846	2.82	0.34	0.9363
	Co	28.33	0.01	0.4990	2.24	0.09	0.1729
	Ni	3.67	0.08	0.9292	1.84	0.16	0.6258

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The passivation of Cu by FSC is better fitted by the Freundlich equation (R² = 0.9938/0.9363) (Table 1), indicating that the passivation of Cu by FSC is a multilayer adsorption.⁵⁰ The immobilization of Co/Ni by FSC is better fitted by Langmuir equation (R² = 0.9995/0.4990 and 1/0.9292), revealing that the adsorption of Co/Ni by FSC is monolayer adsorption.⁵¹ Differences in the number of adsorbed layers may be related to the differences in hydration capacity of Cu/Ni/Co and their ability to bind to NH₄⁺. Furthermore, K_L and 1/n are between 0 and 1, demonstrating that the adsorption of Cu/Co/Ni by FSC is prone to occur.⁵² Further analysis shows that a strong competitive adsorption relationship is observed between Cu and Co/Ni (Table 2), which will provide useful information for the selective separation of Cu using FSC.

Table 2 The competition constants and interaction effects of Cu/Co/Ni in the ternary adsorption systems

Pollutants	Competitive constants		Interactive effect
	Qmixture/Qsingle	ΔY%
Cu	0.81	18.92	Poor antagonistic
Co	0.23	76.77	Strong antagonistic
Ni	0.03	96.77	Strong antagonistic

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3.3. Adsorption kinetic

The type of interaction force between the adsorbent and the adsorbate is one of the important research contents in adsorption experiments, the adsorption kinetics of Cu/Co/Ni by FSC are shown in Fig. 4 and Table 3. The passivation of Cu by FSC shows a first gradually increase and then tends to parallel (Fig. 4A), which may be related to the time and manner of formation of copper-containing phosphate compounds. The adsorption of Co/Ni by FSC is similar to the adsorption of Cu by FSC, but the presence of other ions inhibits the adsorption of Co/Ni by FSC (Fig. 4B and C), indicating that FSC has a high affinity for Cu. Furthermore, the kinetic model fitting parameters are presented in Table 3.

Fig. 4 The effect of immobilization time on the adsorption of Cu/Co/Ni by FSC in different passivation systems (A) Cu; (B) Co; (C) Ni.

Table 3 The adsorption kinetic parameters

Treatment	Heavy metal	Pseudo-first-order kinetic			Pseudo-second-order kinetic
		Qe (mg g^−1)	K1 (min^−1)	R^2	Qe (mg g^−1)	K2 (g mg^−1 min^−1)	R^2
Unitary system	Cu	91.74	39.75	0.7922	111.11	0.00	0.9944
	Co	105.26	2.17	0.8616	111.11	0.00	0.9995
	Ni	92.59	41.09	0.9962	85.47	0.00	0.9973
Ternary system	Cu	91.74	39.75	0.7922	111.11	0.00	0.9944
	Co	22.68	3.44	0.2278	18.66	0.00	0.8829
	Ni	19.65	2.36	0.3041	22.94	0.00	0.9896

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Table 3 shows that the passivation of Cu, Co and Ni by FSC are all well fitted by pseudo-second-order kinetic (R² = 0.9944/0.9944, 0.9995/0.8829 and 0.9973/0.9896), displaying that the adsorption of Cu/Co/Ni by FSC is a physicochemical adsorption dominated by chemisorption.⁵³ Moreover, the results of the adsorption kinetic show that FSC has a selective sequestration capacity for Cu in the ternary composite system (Fig. 4, S1 and Table 3), which is related to the difference in the K_sp of Cu/Co/Ni-PO₄ and Cu concentration (Tables S1 and 2). To further quantify the selective fixation ability of FSC on Cu, the results of the selectivity coefficient and distribution coefficient are shown in Table 4. Table 4 shows that the k value has an increasing trend with the increasing of passivation time, indicating that FSC can efficiently and selectively enrich and separate Cu from the composite heavy metal solution. Furthermore, the FSC also has magnetic separation properties (Fig. 2C), which may provide a viable option for the separation and extraction of Cu and the remediation of Cu contamination in field environments.

Table 4 The distribution and selectivity coefficient of Cu/Co/Ni in ternary system^a

Time (min)	Kd			k
	Cu	Co	Ni	Cu/Co	Cu/Ni
a Distribution coefficient Kd = Qt/Ct, selectivity coefficient k = Kd(Cu)/Kd(x), x = Co/Ni.
5	0.07	0.15	0.04	0.47	1.75
30	0.15	0.12	0.04	1.25	3.75
60	0.23	0.06	0.08	3.83	2.88
120	0.3	0.12	0.06	2.50	5.00
240	0.41	0.08	0.05	5.13	8.20
360	0.45	0.03	0.07	15.00	6.43
480	0.49	0.1	0.07	4.90	7.00
720	0.52	0.07	0.09	7.43	5.78

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4. Mechanism analysis

Revealing the immobilization mechanism of FSC on heavy metals helps to deepen the understanding of the interaction mechanism between FSC and Cu/Co/Ni, and the relevant results are shown in Fig. 5 and 6. The morphology of FSC adsorbed heavy metals changes significantly, with more secondary mineral particles appearing on the mineral surface (Fig. 5 A, A1, B, B1, C, C1, D and D1), and a large amount of Cu/Co/Ni is detected on the mineral surface (Fig. S2 and A2–D2). Additionally, the presence of Cu strongly reduces the content of Co/Ni (Fig. 5 and S2), which also indicates a strong competitive effect of Cu on Co/Ni.

Fig. 5 The morphological and electronic images of precipitates after adsorption of Cu/Co/Ni by FSC (A, A1, B, B1, C, C1, D, and D1) SEM; (A2–D2) Elemental electron images.

Fig. 6 The mechanistic analysis of Cu/Co/Ni passivation by FSC (A) XRD; (B) FTIR; (C) Hysteresis curve; (D) ion release capacity.

After passivation of Cu/Co/Ni by FSC, the mineral crystal structure collapses and no new secondary crystalline is detected (Fig. 6A), which may be related to the fickleness of the struvite crystal and its high affinity for Cu/Co/Ni. The vibrational peaks of the NH₄⁺ group are weakened or disappeared and the vibrational peaks of the PO₃⁻ group are weakened or red-shifted after the adsorption of Cu/Co/Ni by FSC (Fig. 6B), which is related to the combination of PO₃⁻ with Cu/Co/Ni and the release of NH₄⁺.⁵⁴ In addition, the –OH/C=O is red-shifted (Fig. 6B), which is related to the combination of –OH/C=O and Cu/Co/Ni.²² The magnetization strength of the secondary mineral reduces significantly (Ms = 3.15 emu g⁻¹, Mr = 0.84 emu g⁻¹, and Hc = 657.44 Oe) (Fig. 6C), which may be related to the decrease in zero-valent iron content. However, it is still possible to separate secondary mineral using magnetism in the presence of the externally enhanced magnetic field. Further studies show that the passivation of Cu/Co/Ni by FSC results in the release of NH₄⁺, Mg²⁺ and Fe²⁺ (Fig. 6D), suggesting that the adsorption of Cu/Co/Ni by FSC involves ion exchange. Zero-valent iron nanoparticles are highly oxidation, which may lead to changes in the valence state of Cu/Co/Ni, the results are shown in Fig. 7. The XPS results show the presence of Fe²⁺ and zero-valent iron in FSC (Fig. 7A), indicating that the FSC contains nano zero-valent iron. After adsorption of Cu/Co/Ni by FSC, the zero-valent iron disappear and only Fe²⁺ is detected (Fig. 7A–D), which indicates that the nano-zero-valent iron might undergo a redox reaction with Cu/Co/Ni. Further study shows that only a change in the valence state of Cu was detected (Fig. 7E–G), which may be related to factors such as differences in standard reduction potentials, the size of the kinetic barrier to the reaction and the chemical condition of the surface.

Fig. 7 The XPS of FSC after adsorption of heavy metals (A) Fe2p of FSC; (B) Fe2p of FSC after adsorption of Cu(II); (C) Fe2p of FSC after adsorption of Co(II); (D) Fe2p of FSC after adsorption of Ni(II); (E) Cu2p of FSC after adsorption of Cu(II); (F) Co2p of FSC after adsorption of Co(II); (G) Ni2p after FSC adsorption of Ni(II).

Additionally, reducing substances such as NO₃⁻ are commonly found in the environment and may affect the selective passivation of Cu by FSC. Therefore, the effect of reducing substances represented by NO₃⁻ on the selective passivation of Cu by FSC is investigated. The results shows that the presence of NO₃⁻ slightly reduces the selective adsorption capacity of FSC for Cu (Fig. S3). Fig. S4A shows that the presence of Co, Ni and NO₃⁻ leads to an increase in the slope of the curve at 933 eV together with an increase in the number of burrs, suggesting a weakening of the reduction of Cu. Fig. S4B shows that the presence of Co/Ni and NO₃⁻ significantly reduces the photoelectron intensity of Cu, which suggests that the presence of Co/Ni and NO₃⁻ inhibits the reduction action of Cu.

5. Conclusion

FSC is an irregularly shaped organic–inorganic mineral composite of nano zero-valent iron loaded struvite. The adsorption of Cu by FSC is multilayer, whereas the adsorption of Co/Ni is monolayer. The adsorption of Cu, Co and Ni by FSC are all physicochemical adsorption processes dominated by chemisorption. Phase change occurs in all of the Cu/Co/Ni adsorption by FSC, and both Mg²⁺ and NH₄⁺ are ion-exchanged with Cu/Co/Ni. Zero-valent iron nanoparticles undergo redox reactions with Cu accompanied by the formation of Fe²⁺, and NO₃⁻ weakly reduces the selective sequestration of Cu by FSC through competition with Cu for reduction action. Furthermore, FSC selectively passivates Cu in a variety of complex heavy metal ion systems. Studies have shown that the passivation of Cu/Co/Ni by FSC is related to phase change, ion exchange, redox and complexation/chelation, whereas the selective enrichment and separation properties of Cu is related to physical properties of Cu/Co/Ni-PO₄ and Cu concentration. This study provides theoretical basis for the selective recycling of Cu using FSC.

Author contributions

Xingxing Wang: conceptualization, investigation, experiments, data curation, data analysis & interpretation, writing – original draft, writing – review & editing; Xun Li: writing – original draft, writing – review & editing.

Conflicts of interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Data availability

Data will be made available on request.

Supplementary information is available. See DOI: https://doi.org/10.1039/d5ra03673a.

Acknowledgements

This study was supported by the Doctoral Scientific Startup Fund from Anqing Normal University (Grant No. 251052).

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