Selective Removal of Copper Ions from Fully Leached Solution of Lithium Iron Phosphate Using Copper Chelating Resin

Abstract

The wet recovery of spent lithium iron phosphate (LFP) batteries is severely hindered by the low efficiency of copper removal. Here, a new process has been developed using a copper-removing chelating resin with pyridine nitrogen, carboxyl, and hydroxyl groups for the selective separation of copper ions. This copper chelating resin achieved a copper removal efficiency of 96.99% and reduced the residual copper content to below 10 milligrams per liter, significantly outperforming the traditional iron powder method. The adsorption process is highly sensitive to pH, with the highest efficiency at pH 1.75. A concentration of 2.0 moles per liter of H₂SO₄ can achieve a desorption rate of approximately 95%. The adsorption process follows the Langmuir isothermal equation and the pseudo-second-order kinetic model, corresponding to single-layer chelated chemical adsorption. Mechanism studies have confirmed that the synergistic coordination effect of the multifunctional groups helps in the efficient capture of copper ions. This copper chelating resin exhibits excellent stability, reversibility, and reusability, providing a promising method for efficient copper removal and recovery in the wet metallurgical recycling of LFP.

1. Introduction

Lithium iron phosphate batteries (LiFePO₄) have demonstrated extensive application potential in fields such as new energy vehicles and communication base stations due to their abundant raw material resources, excellent safety performance, environmental friendliness, and outstanding economic efficiency [1,2,3]. With the arrival of the large-scale retirement stage of power batteries and the in-depth advancement of the “dual carbon” strategic goals, the efficient recycling and resource utilization of retired LiFePO₄ batteries have become an important issue that needs to be urgently addressed at present [4,5,6].

The recycling and utilization of spent lithium-ion batteries are of great significance for alleviating environmental pollution, conserving limited resources and promoting sustainable social development. The current main recycling technology routes include pyrometallurgy, hydrometallurgy and direct regeneration [7,8]. However, in the actual recycling process, since the positive electrode material and the negative electrode material are, respectively, firmly attached to the surface of the aluminum foil and copper foil current collectors through binders, during the physical disassembly steps such as crushing and peeling, impurity elements such as aluminum and copper are inevitably introduced into the leachate [9,10,11]. The presence of copper ions in the leachate is detrimental. It not only interferes with the subsequent metal separation and purification processes but may also cause lattice contamination or generate impurities during the regeneration of lithium iron phosphate materials, thereby adversely affecting the structural stability and electrochemical performance of the regenerated materials [12].

Therefore, achieving selective removal of copper in the lithium iron phosphate full immersion liquid system is one of the key pretreatment steps to ensure the quality of the recovered products [13,14]. For lithium iron phosphate acid leaching solutions, traditional chemical precipitation methods (such as sulfide precipitation and hydroxide precipitation) are often limited in application due to poor selectivity, easy co-precipitation loss of valuable metals (especially lithium), and the generation of a large amount of hazardous waste sludge. Although solvent extraction has relatively good selectivity, it has problems such as organic phase entrainment loss, easy emulsification, complex process and potential secondary pollution [15]. Although electrochemical methods have a relatively good effect on copper removal, they are usually accompanied by high energy consumption and complex equipment requirements [16,17]. In contrast, the adsorption separation technology of resins has shown great potential in the field of hydrometallurgical separation and purification due to its advantages such as simple operation, high design flexibility, environmental friendliness, excellent potential selectivity, and easy realization of continuous and coupled processes [18]. Especially chelating resins containing specific functional groups exhibit a strong selective adsorption capacity for transition metal ions such as Cu²⁺, and can still maintain good separation performance even under conditions of high concentrations of coexisting ions. Compared with traditional separation methods, resin copper removal has significant advantages such as less secondary waste generation, mild operating conditions, and the ability to regenerate and reuse the resin. However, at present, research on the application of ion-exchange resins in copper removal in lithium iron phosphate fully impregnated liquid systems is still relatively limited [19,20,21].

In this paper, the resin adsorption method was adopted to selectively remove copper from the full leaching solution of lithium iron phosphate. Based on chelation coordination reactions, the effects of different initial pH values, flow rates and other process parameters of solutions on the copper adsorption rate were investigated, as well as the influences of different desorbents, desorbent concentrations, and other processes on the copper desorption rate. The optimal process parameters for copper adsorption and desorption were determined. Combined with thermodynamic, kinetic studies and related detection methods, the adsorption and desorption mechanisms of copper by copper chelating resin, as well as the influence of impurity ions on the adsorption and desorption of copper, were determined. The research results aim to provide a theoretical basis and technical support for the efficient purification of leachate during the wet recovery process of lithium iron phosphate batteries and promote the development of green recycling processes for used lithium-ion batteries.

2. Materials and Methods

2.1. Raw Materials

A weakly alkaline chelating resin was selected for the highly selective adsorption of metals such as copper and nickel. This chelating resin has a polymer skeleton with uniform internal and external cross-linking, excellent adsorption kinetics, high adsorption capacity, and good mechanical strength. The water content ranges from 45% to 65%, the density is between 1.1 and 1.2 g/mL, and the particle size ranges from 0.4 to 1.2 mm.

The raw materials used in the experiment were the acid extract of retired LiFePO₄ obtained from the disassembly of retired batteries, provided by GEM.

Table 1. Main elemental composition of retired LiFePO₄ acid leaching solution.

Material	Cu (mg/L)	Fe (g/L)	Li (g/L)	P (g/L)	pH
LiFePO4 extract	65.50	29.25	7.90	13.40	1.5

shows the main elemental composition of the acid extract of retired LiFePO₄.

2.2. Test Method

2.2.1. Fourier Transform Infrared Spectroscopy (FT-IR)

FTIR spectra were recorded in attenuated total reflection (ATR) mode over the range of 4000–400 cm⁻¹ with a resolution of 4 cm⁻¹ and 32 scans. Carry out structural analysis of solid powders.

2.2.2. X-Ray Photoelectron Spectroscopy (XPS)

XPS analysis was performed using a monochromatic Al Kα X-ray source (hν = 1486.6 eV, 150 W). The C 1s peak at 284.8 eV was used for charge correction. Characterize the chemical environment of different elements on the particle surface.

2.2.3. Scanning Electron Microscopy-Energy Dispersive X-Ray Spectroscopy (SEM-EDS)

Morphology and elemental composition were characterized by field-emission SEM equipped with an EDS detector at an accelerating voltage of 10–20 kV. Non-conductive samples were sputter-coated with gold prior to imaging. Characterize the appearance of the product and the chemical composition of its microregions.

The specific models of the above three analytical instruments were listed in

Table 2. The analytical instruments.

Device Name	Unit Type
Fourier-transform infrared spectroscopy (FT-IR)	Nicolet iS50 (FT-IR) Thermo Fisher Scientific (Nicolet), Madison, WI, USA
X-ray photoelectron spectroscopy (XPS)	Escalab 250Xi (XPS) Thermo Fisher Scientific (VG Scienta), East Grinstead, West Sussex, UK
Scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDS)	Regulus 8220 (SEM-EDS) Hitachi High-Tech Corporation, Tokyo, Japan

.

2.3. Test Experiment

2.3.1. Static Adsorption Experiment

The pre-treated copper chelating resin was added to a conical flask, and a certain volume of lithium iron phosphate acid leaching solution containing copper was added. The static adsorption experiment was carried out in a constant-temperature water bath shaker at 25 °C and 200 r/min. After the adsorption was completed, the resin and solution were separated by filtration. The concentration of metal ions in the residual adsorption liquid was measured, and the ion adsorption rate was calculated. The calculation formula for the ion adsorption rate (β) is shown in Equation (1)

$$\beta ={\displaystyle \frac{C_0-C}{C_0}}\times 100\%$$

(1)

In the formula, C₀ represents the initial concentration of the solution (g/L), and C represents the concentration of the solution after adsorption (g/L).

2.3.2. Static Desorption Experiment

After washing the saturated adsorption copper chelating resin with ultrapure water 3 to 4 times, it was placed in a conical flask with the desorption liquid at the set liquid–solid ratio, and the desorption experiment was carried out under the conditions of 25 °C and 200 r/min. After desorption was completed, the copper chelating resin and solution were filtered and separated. The concentration of metal ions in the desorption liquid was measured, and the ion desorption rate was calculated. The calculation formula for the ion desorption rate (γ) is shown in Equation (2)

$$\gamma ={\displaystyle \frac{c\cdot V}{m}}\times 100\%$$

(2)

In the formula, m represents the mass of adsorbed ions in the copper chelating resin (g), c is the ion concentration in the solution (g/L), and V is the volume of the solution (L).

2.3.3. Dynamic Adsorption and Desorption Experiments

The dynamic experiment was conducted in a fixed-bed ion-exchange column with a diameter of Φ2.0 × 40 cm, and the flow rate was controlled by a peristaltic pump (Pump, Baoding, BT100-2J, Baoding Longer Precision Pump Co., Ltd., Baoding, Hebei, China). During dynamic adsorption, the lithium iron phosphate acid leaching solution containing copper was introduced into an exchange column filled with wet copper chelating resin. The effluent was collected at regular intervals according to the bed volume (BV), and the concentration of metal ions was determined. Before dynamic desorption, the saturated adsorption copper chelating resin was washed 3 to 4 times with ultrapure water, and then the desorption liquid was introduced for desorption. The copper-rich solution was collected regularly, and its ion concentration was analyzed.

2.3.4. Isothermal Adsorption Experiment

A total of 400 mL of lithium iron phosphate solution and 5 g of chelating copper resin were placed in a conical flask. Then, the sealed conical flask was placed in an oven shaker. The reaction was carried out at a speed of 300 revolutions per minute and a temperature of 25 °C for 6 h until the copper chelating resin reached the adsorption equilibrium. The concentration of Cu in the residual adsorption liquid was analyzed.

3. Results and Discussions

3.1. Comparison of Copper Removal Between Copper Chelating Resin and Iron Powder

In the acid leaching solution of lithium iron phosphate, with a feed solution pH of 1.5 and the addition amount of iron powder being 1.2 times the theoretical value, the reaction was carried out for 15 min. The effects of temperatures of 25, 40, 50, 60, and 70 °C on the copper removal effect by iron powder replacement were compared.

Different batches of full leaching solutions of lithium iron phosphate were taken. The reaction temperature was selected to be 60 °C, the leaching solution was 500 mL, the reaction time was 15 min, the amount of iron powder added was 1.2 times the theoretical usage amount of iron powder, the stirring speed was 300 r/min, and the copper ion content in the leaching solution was reduced to below 5 mg/L. Adsorption occurs at room temperature with a copper chelating resin volume of 20 g and a flow rate of 2 BV/h.

As shown in Figure 1a, heating the system from 25 to 50 °C can significantly accelerate the displacement reaction and alleviate the passivation of iron powder, with the most obvious improvement in copper removal efficiency. After the temperature reaches 50 °C, most of the free copper is removed, and further increasing the temperature has a limited impact on copper removal. When the temperature is higher than 60 °C, the side reaction of hydrogen evolution intensifies, leading to an increase in iron powder consumption. Considering the impurity removal effect and production cost, 60 °C is the optimal reaction temperature for copper removal from the lithium iron phosphate acid leaching solution using iron powder.

As shown in Figure 1b, the removal effect of copper in the full leaching solution of lithium iron phosphate was evaluated in multiple batches and compared with the copper removal process of iron powder. For the copper chelating resin copper removal process, the overall copper removal rate remained above 96%, with the highest reaching 99%. After treatment, the copper ion concentration in the leachate generally decreased to below 10 mg/L. In contrast, the copper removal efficiency of the iron powder copper removal process (Batch 1 and Batch 7) was significantly lower, with removal rates of only 57.65% and 58.99%, respectively. After treatment, a relatively high concentration of copper ions still remained in the system. The iron powder removal of copper has the following drawbacks: the reaction efficiency is prone to fluctuation; the copper removal is insufficient at low temperatures; the iron consumption increases at high temperatures; it is prone to introducing iron impurities and producing a copper-iron mixed slag.

The excellent copper-removal performance of the copper chelating resin method mainly stems from its selective coordination adsorption mechanism for Cu²⁺. The chelating functional groups on the copper chelating resin skeleton can form stable internal coordination complexes with Cu²⁺, enabling efficient adsorption even in complex systems where high concentrations of Fe³⁺/Fe²⁺ and phosphate ions coexist. Under the condition of similar initial copper concentrations, the copper chelating resin method demonstrates higher stability and repeatability in continuous batch operation, while the copper removal capacity of the iron powder method is limited, and it is difficult to achieve deep removal. The results show that the copper chelating resin achieves more efficient and controllable copper removal through the selective coordination adsorption of Cu²⁺, avoiding the problems of incomplete reaction and insufficient selectivity that are common in the iron powder displacement reaction.

Under the same leaching liquid conditions, the copper chelating resin method is significantly superior to the iron powder copper removal process in terms of copper removal efficiency, process stability, and terminal copper concentration control. It is more suitable for the deep purification of the lithium iron phosphate full leaching liquid system.

3.2. Influencing Factors of Chelating Resin for Separating Cu

3.2.1. The Influence of pH Values

Under the static adsorption condition of 10 g of copper chelating resin, 200 mL of filtrate volume, and 2 h of adsorption time, the influence of the initial pH value of the solution on the adsorption behavior of iron, copper, lithium and phosphorus was studied (Figure 2).

As the initial pH increased from 0.5 to 2.0, the adsorption rate of Cu rose significantly, exceeding 95% when pH ≥ 1.7, and then tended to stabilize under higher pH conditions. The pH of the solution has a significant impact on the adsorption of Cu. In contrast, the adsorption rates of Fe, Li and P remained at low levels throughout the pH range. The adsorption rate was around 5% and hardly changed with pH. The above results indicate that this copper chelating resin has a significant selective adsorption capacity for Cu under acidic conditions.

In order to more accurately analyze the adsorption effect of copper chelating resin on copper ions, it is necessary to investigate the influence of initial pH value, flow rate and other factors. The dynamic adsorption process of the copper chelating resin was investigated. Based on the results of the previous static test, dynamic adsorption tests were conducted under the conditions of a flow rate of 2 BV/h, a bed volume of 20 mL, and initial pH values of 1.0, 1.5, and 1.75 (Figure 3).

Under the pH 1.75 condition, the copper chelating resin exhibits high and stable adsorption performance throughout the process. The Cu adsorption efficiency remains basically close to 100% within 40 BV, indicating that the copper chelating resin has excellent anti-penetration ability and operational stability under this condition. When the pH is 1.5, the adsorption efficiency gradually decreases with the increase in BV, dropping to approximately 96% in the high-BV range. There is a certain degree of attenuation, but under this condition, the copper chelating resin still maintains a relatively high copper-removal capacity. In contrast, under the condition of pH 1.0, the adsorption efficiency of Cu decreased significantly and continued to decline with the increase in BV, from approximately 67% initially to approximately 55% at 40 BV, showing a distinct premature penetration phenomenon, indicating that an excessively low pH is not conducive to the effective adsorption of Cu.

In conclusion, the pH of the solution has a significant impact on the dynamic adsorption performance of Cu. Within the studied acidic range, a higher pH value is conducive to enhancing the adsorption efficiency and operational stability of Cu, while an excessively low pH will inhibit the adsorption process and reduce the effective adsorption capacity of the copper chelating resin. Therefore, the initial pH value of the optimal solution was selected as 1.75, which was consistent with the static experiment results.

3.2.2. The Influence of Flow Rate

Under the conditions of a bed volume of 20 mL and an initial pH value of 1.75, dynamic adsorption experiments were conducted to investigate the effect of liquid flow rate on the adsorption of copper (Figure 4). The faster the flow rate, the faster the copper concentration in the adsorbed residual liquid increases. The faster the flow rate, the shorter the contact time between the copper chelating resin and the solution, and the shorter the reaction time. This inhibits the effective coordination between Cu²⁺ and the functional groups of the copper chelating resin, resulting in less adsorption capacity of the copper chelating resin. Reducing the flow rate can prolong the contact time between the copper chelating resin and the solution and increase the ion diffusion time. Taking all factors into consideration, the flow rate is selected as 2 BV/h.

3.2.3. The Selection of Desorption Agents

In the process of copper chelating resin adsorbing metal ions, the selection of desorbent has a significant impact on the copper chelating resin regeneration efficiency, metal recovery rate and process economy. At present, the desorption of metal ions in chelating resins mainly relies on acidic solutions, complexing agents or their composite systems, which break the coordination bonds between metals and resins through proton competition, complexation or ion-exchange mechanisms. Therefore, during the experiment, only the desorption effects of different acids were compared. In the experiment, 2 mol/L hydrochloric acid solution, 2 mol/L sulfuric acid solution and 2 mol/L phosphoric acid solution were prepared to desorb the adsorbed substances from the saturated copper-removing chelating resin after the adsorption process was completed. The test conditions were as follows: temperature 25 °C, time 2 h, rotational speed 200 r/min, 10 g saturated copper chelating resin, and 100 mL desorption liquid (Figure 5).

As shown in Figure 5a, under the same experimental conditions, the desorption effect of H₂SO₄ is the best, with a desorption rate of approximately 95%. The desorption rates of both HCl and H₃PO₄ are less than 10%. The results show that under the action of high proton activity, the sulfuric acid system can more effectively break the coordination bonds between Cu²⁺ and the copper chelating resin functional groups, while the desorption ability of the chloride and phosphoric acid systems is relatively poor. The comparison of the desorption effects of Cu with different concentrations of sulfuric acid is shown in Figure 5b. The desorption rate of Cu increases with increasing H₂SO₄ concentration. When the concentration of sulfuric acid increased from 1.0 mol/L to 2.0 mol/L, the desorption rate rose from approximately 55% to approximately 95%. When the concentration of sulfuric acid was 3.0–4.0 mol/L, the desorption rate only increased slightly and tended to stabilize, eventually approaching 98–99%. This result indicates that within the lower concentration range, the concentration of sulfuric acid is the key factor affecting the desorption efficiency of Cu, while excessively high concentrations have a limited effect on improving the desorption effect. In conclusion, choosing 2.0 mol/L H₂SO₄ can achieve efficient desorption, which not only takes into account the desorption effect but also saves costs.

3.3. Mechanism of Adsorption–Desorption of Copper Chelating Resin

To better study and explore the adsorption and desorption processes of Cu by copper chelating resin, thermodynamic and kinetic analyses were conducted on the adsorption process of Cu. Meanwhile, FTIR, SEM-EDS and XPS analyses were carried out on the copper chelating resins obtained under different conditions during the adsorption and desorption experiments.

3.3.1. Isothermal Adsorption Model for Cu Adsorption by Copper Chelating Resin

A total of 400 mL of lithium iron phosphate full immersion solution and 5 g of copper chelating resin were placed in a conical flask, and then the sealed conical flask was put in a constant temperature shaker. The reaction was carried out at a rotational speed of 300 r/min and a temperature of 25 °C for 6 h until the copper chelating resin reached adsorption equilibrium. The concentration of Cu in the residual adsorption liquid was analyzed, and the obtained data were fitted using temperature models including Freundlich, Langmuir, and Temkin [22,23].

The data obtained from the experiment were calculated and fitted, and the results are shown in Figure 6 and

Table 3. Parameters in the Freundlich, Langmuir and Temkin equilibrium isotherms.

Parameters	Freundlich Isotherm			Langmuir Isotherm			Temkin Isotherm
	Kf	n	R2	Q0	KL	R2	bT	aT	R2
Values	1.613	3.167	0.946	10.718	0.021	0.999	1149.908	0.271	0.980

K_f, n, Freundlich capacity constant; Q₀ represents the adsorbent capacity (mg⋅g⁻¹); K_L is the Langmuir isothermal adsorption equilibrium constant (L/mg); a_T represents the equilibrium binding constant (L/mg); b_T is the adsorption heat related to the Temkin constant (J/mol); R² is the coefficient of determination.

. As can be seen from Figure 6 and Table 3, the fitting coefficients R² of the Freundlich, Temkin and Langmuir isothermal adsorption models are 0.946, 0.980 and 0.999, respectively. The fitting coefficients R² of the Freundlich and Temkin isothermal adsorption models are both smaller than that of the Langmuir isothermal adsorption model. The Langmuir isothermal adsorption model has the best fitting performance, with a fitting coefficient R² of 0.999. Therefore, except for the copper chelating resin, which exhibits copper adsorption that conforms to the Langmuir isothermal model, the copper adsorption is of a single-layer nature.

3.3.2. The Thermodynamic of Copper Adsorption by Copper Chelating Resin

Using the relevant thermodynamic parameters [24], the adsorption capacity of copper by the copper chelating resin can be calculated. The relevant data are presented in

Table 4. The thermodynamic parameters of copper adsorption by copper chelating resin.

T (K)	ΔG kJ/mol	ΔH kJ/mol	ΔS J/(mol/K)
298	−1.823	14.922	53.011
308	−2.019
318	−2.238
328	−2.380

ΔG represents the adsorption free energy (kJ/mol); ΔH represents the standard enthalpy change (kJ/mol); ΔS represents the entropy change (J/(mol/K)); T represents the reaction temperature (K).

and Figure 7. lnK was plotted against 1/T, and the ΔG, ΔH and ΔS values of the reaction were calculated according to formula [15]. The enthalpy change ΔH is 14.922 kJ/mol, and the entropy change ΔS is 53.01 J/(mol/K). The Gibbs free energy ΔG of the reactions at 25 °C, 35 °C, 45 °C and 55 °C was −1.823 kJ/mol, −2.019 kJ/mol, −2.238 kJ/mol and −2.380 kJ/mol, respectively. From the above-mentioned enthalpy change and Gibbs free energy variation, it can be known that increasing the temperature is conducive to the progress of the reaction. However, after calculation, the saturated adsorption capacities of the copper chelating resin at temperatures of 25 °C, 35 °C, 45 °C and 55 °C do not differ much, being 8.108 mg⋅g⁻¹, 8.2 mg⋅g⁻¹, 8.3 mg⋅g⁻¹ and 8.344 mg⋅g⁻¹, respectively. Therefore, room temperature can be selected during the copper chelating resin adsorption process.

3.3.3. The Adsorption Kinetics of Copper by Copper Chelating Resin

Based on the obtained data, the quasi-first-order dynamics, quasi-second-order dynamics, and Weber–Morris intraparticle diffusion dynamics models were selected for fitting [22,24]. For reversible reactions or liquid–solid phase equilibrium reactions, pseudo-first-order kinetics can be used to describe them, and the reaction process mainly involves physical adsorption. The pseudo-second-order kinetics reaction assumes that the adsorption rate is related to the adsorption capacity, that is, the adsorption rate is determined by the square of the number of adsorption sites on the resin surface, and the reaction rate is controlled by the chemical adsorption step. When the rate-controlling step is internal particle diffusion, the Weber–Morris internal particle diffusion model can be adopted.

Figure 8 shows the fitting of the pseudo-first-order kinetics (a), pseudo-second-order kinetics (b), and Weber–Morris (c) models for copper adsorption by copper chelating resin. It can be seen from Figure 8 and

Table 5. Kinetic constants of copper adsorption.

Parameters	Pseudo-First Order			Pseudo-Second Order			Weber–Morris
	Qe	K1	R2	Qe	K2	R2	C	K3	R2
Values	4.0626	0.2948	0.9916	8.0906	0.2018	0.9958	3.4667	1.722	0.9794

Q_e represents the amount of metal ions adsorbed per unit mass of resin at equilibrium (mg·g⁻¹); K₁ is the rate constant for the pseudo-first-order adsorption process (h⁻¹); K₂ is the pseudo-second-order adsorption rate constant (g/mg·h); K₃ is the internal diffusion rate constant (mg·g⁻¹·(t^1/2)⁻¹); C is related to the thickness of the boundary layer.

that the fitting coefficients R² of the pseudo-first-order dynamics model, the pseudo-second-order dynamics model and the Weber–Morris particle internal diffusion model are 0.9916, 0.9958 and 0.9794, respectively; that is, the fitting effect of the pseudo-second-order dynamics model is the best. The adsorption process of copper conforms to pseudo-second-order kinetics. The adsorption rate is related to the adsorption capacity. The theoretical adsorption capacity is 8.09 mg⋅g⁻¹, which is close to the actual adsorption capacity of the copper chelating resin, 8.20 mg⋅g⁻¹.

During the ion-exchange reaction between the resin and the solution, metal ions or exchangeable functional groups on the resin can migrate between the bulk solution and the resin interior via diffusion. Consequently, the ion-exchange reaction occurs not only on the external surface of the resin particles but also at the active sites within the particles. The ion-exchange process can be divided into liquid-film diffusion control, chemical reaction control, and particle diffusion control. As shown in Figure 9, through experimental simulation calculations, the fitting coefficients R² for liquid film diffusion control, particle diffusion control, and chemical reaction control are 0.9148, 0.9858, and 0.9769, respectively. This indicates that the adsorption reaction of copper on the copper chelating resin is controlled by particle diffusion. The adsorption kinetics followed the pseudo-second-order model, indicating chemisorption as the rate-controlling step, while the Weber–Morris plots showed multi-linearity without passing through the origin, suggesting that intraparticle diffusion also contributed to the rate-limiting process (mixed control).

3.3.4. Fourier Transform Infrared (FTIR) Spectrum Analysis

Figure 10 illustrates that the broad peak near 3426 cm⁻¹ was assigned to the O–H stretching vibration, mainly derived from adsorbed or coordinated water molecules. The broad peak at approximately 3058 cm⁻¹ corresponded to aromatic C–H stretching. The peaks at 1650, 1539, and 1444 cm⁻¹ were attributed to aromatic C=C/C=N stretching. The bands at 1118 cm⁻¹ and 1041 cm⁻¹ were characteristic vibrations of the pyridine ring, whose intensities increased in the adsorbed and desorbed copper chelating resins, indicating the influence of Cu adsorption.

The peak at 859 cm⁻¹ in the desorbed copper chelating resin was ascribed to the out-of-plane C–H vibration of the pyridine ring, which may be related to the removal of coordinated Cu from the pyridine ring during acid desorption. The peaks at 767 cm⁻¹ (out-of-plane C–H bending) and 617 cm⁻¹ (pyridine ring deformation) decreased sharply in intensity after desorption. The band at 511 cm⁻¹ was assigned to the antisymmetric stretching vibration of Cu–O (νas (Cu–O)), which appeared in the adsorbed copper chelating resin and weakened or disappeared after desorption, confirming the occurrence of Cu adsorption and desorption.

3.3.5. XPS Analysis

According to the C1s spectra, the peak at 284.8 eV was assigned to the C–C/C–H framework, whose binding energy remained unchanged after adsorption and desorption (Figure 11). The peaks at 286.06 eV and 287.99 eV were attributed to C–N and C=O/C–O bonds, respectively, and their binding energies increased after adsorption. These results confirm that the resin skeleton is highly stable, and only functional groups (C–N, C–O) participate in coordination. From the N1s spectra, the peak at 399.28 eV corresponded to free pyridine nitrogen (Py–N) in the pristine resin. After adsorption, a new peak at 401.67 eV assigned to Cu–N bonds appeared, with a binding energy increase of 2.38 eV, indicating that pyridine N is the main adsorption site for Cu ions, showing typical chelation characteristics. In the O1s spectra, peaks at 533.95 eV and 535.46 eV were ascribed to Cu–O and Cu–OCO coordination, respectively, demonstrating that both –OH and –COOH groups are involved in Cu²⁺ chelation. All O peaks shifted to higher binding energy after adsorption and fully recovered after desorption. The Cu2p spectra exhibited a characteristic peak of Cu 2p3/2 at 933.98 eV, confirming the adsorption of divalent Cu²⁺ via coordination chelation.

The pyridine N atoms (Py–N) donate lone-pair electrons to form stable Cu–N coordination bonds with Cu²⁺. Meanwhile, O atoms in –COOH and –OH groups participate in the formation of Cu–O bonds, further enhancing adsorption stability. The copper chelating resin framework remains intact throughout the process. After desorption, the characteristic peaks of N1s and O1s fully return to the original state, proving that the chelation adsorption is reversible and the copper chelating resin can be recycled.

3.3.6. SEM-EDS

Figure 12 illustrates that pristine resin exhibited a dense and intact surface structure. After Cu adsorption, the copper chelating resin underwent obvious swelling and formed a loose and porous structure, which was attributed to the coordination interaction between Cu²⁺ and the copper chelating resin as well as the expansion of the copper chelating resin framework.

EDS elemental mapping revealed that the pristine copper chelating resin showed almost no Cu signal. After saturation adsorption, the Cu signal was significantly enhanced and uniformly distributed, accompanied by a small amount of Fe co-adsorption. After desorption, the Cu signal was greatly weakened, and Fe was almost completely removed.

These results indicate that the copper chelating resin possesses excellent regeneration performance, efficient coordination adsorption of copper, and good adsorption–desorption recyclability, demonstrating promising potential for practical applications.

4. Conclusions

• The copper chelating resin demonstrated markedly higher efficiency for copper removal from lithium iron phosphate full leachate than iron powder cementation at room temperature, achieving up to 96.99% Cu removal with residual Cu concentrations below 10 mg/L.
• When the initial pH value of the lithium iron phosphate leaching solution is 1.75, the adsorption rate of copper by the copper chelating resin can reach 98%. Compared with HCl solution and H₃PO₄ solution, the desorption effect is the best when H₂SO₄ solution is used as the desorption medium. The desorption efficiency of copper can reach 95% when the loaded copper chelating resin is desorbed with 2 mol/L H₂SO₄ solution. Selective separation and enrichment of Cu have been achieved.
• The copper chelating resin’s adsorption of copper conforms to the Langmuir isothermal adsorption model and the pseudo-second-order kinetic law. The adsorption process was governed by a mixed control mechanism, with chemisorption as the dominant rate-limiting step and intraparticle diffusion also playing a role.
• This study demonstrates that the copper chelating resin enables efficient and selective Cu²⁺ adsorption via synergistic coordination. The copper chelating resin framework is stable, and its structure and performance can be fully recovered after desorption. This stable, reusable adsorbent shows high practical value for copper recovery.
• This research focuses on the practical demand for LFP battery recycling. Targeting the full leachate of LFP batteries, it fills the research gap in the selective removal of copper ions from such systems. Compared with traditional chemical precipitation, iron powder cementation and solvent extraction methods, the chelating resin adopted in this study can realize efficient and selective copper removal with the advantages of simple operation, low cost, environmental friendliness and higher safety. By deeply exploring the adsorption mechanism, this work provides a theoretical basis for the selective removal of copper from complex leachate systems and proposes a novel economical and recyclable copper removal method with a clarified reaction mechanism.