Selective Complexation and Leaching of Cobalt Using Histidine in an Alkaline Medium

Abstract

Considering the issues of significant ammonia volatilization loss and toxic gas emission associated with the conventional ammonia leaching method used in the resource utilization of cobalt-rich alloy slag, a novel approach involving selective complexation leaching of cobalt in an alkaline histidine solution has been proposed. Under conditions of 35 °C temperature, a molar ratio of histidine to cobalt of 1.5, pH of 8, a leaching period of 12 h, and a stirring speed of 300 rpm, the cobalt leaching rate from cobalt-rich alloy slag exceeds 95%. In contrast, the leaching rates for impurity metals such as iron, lead, and copper remain below 3%, demonstrating outstanding leaching selectivity. Leaching kinetics calculations indicate that the rate-controlling step is chemical reaction control, with an apparent activation energy of 64.32 kJ/mol. Through the use of FTIR and XPS characterization techniques, it has been confirmed that histidine molecules form a stable complex with cobalt ions via the dual coordination of the carboxyl (COO⁻) and amino (-NH₂) groups. This distinctive bifunctional synergistic coordination mechanism markedly enhances leaching selectivity and reaction efficiency.

1. Introduction

Co is a critical raw material, and it has been used extensively in the production of batteries, alloys, catalysts, ceramics, and other materials due to its special properties, which include ferromagnetism, corrosion resistance, and temperature-dependent crystal structure [1]. The lower grade of cobalt ores and the increased demand for cobalt have stimulated the development of methods for recovering Co from low-grade metallurgical waste resources. Cu-Co alloy slag is obtained from converter slag, which is reduced by electric furnace and water-quenched. The grade of cobalt in the alloy slag is much higher than that in the ore. Currently, the main methods for processing Co ore include hydrometallurgical and pyrometallurgical approaches [2,3,4,5]. Compared to pyrometallurgical processes, hydrometallurgical processes have demonstrated higher metal recovery efficiency, achieving both high product purity and substantial energy savings [6]. At present, the hydrometallurgical methods for extracting Co from cobalt ores involve acid leaching [7], alkaline leaching [8], and bioleaching [9]. In acid leaching processes, HCl, H₂SO₄, HNO₃, and other strong acid solutions are used as leaching agents, and harmful gases such as Cl₂, SO₃, or NO_x are released. The residual acids after leaching have a significant impact on the environment, which cannot be ignored [10].The ammonia leaching process has always been an effective method for extracting metals from minerals [11]. However, from an operational perspective, the volatile characteristics of ammonia lead to environmental and health risks. High concentrations of ammonia have been maintained throughout the leaching process, resulting in significant ammonia losses. Additionally, ammonia is explosive under high temperatures and pressure [12]. Amino acids (such as glycine) have been proven to be environmentally friendly and effective reagents, capable of selectively leaching various metals from a range of minerals under alkaline conditions [13]. Therefore, replacing ammonia with amino acids as leaching agents for metal recovery may be a better choice.

Eksteen and Oraby [14] were the first to propose the leaching of Cu and Au with glycine. Chen et al. [15] used glycine to leach Co and Li from spent lithium-ion batteries. The highest extractions of 97.07% for Co and 90.95% for Li were achieved at glycine concentration of 300 g/L, 10% H₂O₂, solid-to-liquid molar ratio of 1:100, temperature of 80 °C, and leaching time of 7 h. Manivanna et al. [16] achieved the cobalt leaching recovery of 89.7% from Li⁺ batteries (LiBs) using glycine at 100 °C, a slurry concentration of 13.8 g/L, and a glycine concentration of 1.24 mol/L. Eksteen et al. [17] employed an alkaline glycine leaching system to selectively extract Ni and Co successively from serpentine-rich low-grade sulfide ores. It was shown that Ni and Co could be selectively leached using glycine, but the leaching rate was very slow, with 83.5% of nickel and 76.3% of cobalt being leached in 672 h. The effectiveness of glycine-based metal recovery solutions was further validated using multiple technologies, including solvent extraction, sulfide precipitation, carbon adsorption, ion exchange, and chemical reduction [18]. The recyclability of glycine was confirmed by Eksteen and Oraby, particularly in multi-stage extraction systems and reused lean solutions. Although glycine has achieved significant breakthroughs in the leaching of precious metals, its large-scale application remains constrained by critical challenges, such as slow leaching kinetics and insufficient cost-effectiveness of amino acid reagents. The metal complexation efficacy of amino acids is directly governed by the structural configuration and electronic properties of their coordination groups. For instance, histidine, with its polydentate coordination structure—featuring an imidazole ring (π-electron donor), an amino group, and a carboxyl group, exhibits synergistic effects that enable the formation of highly stable cobalt complexes. This demonstrates superior coordination selectivity and leaching kinetics compared to conventional monodentate amino acids.

Histidine (His) is also a naturally basic amino acid, which contains an amino group (NH₂), a carboxyl group (-COOH), and an imidazole ring (1H-imidazole). Various molecular forms were exhibited by it at different pH values [19]. The isoelectric point (pI) of histidine is 7.59. It is polar, easily dissociable and hydrophilic, and is the only amino acid that is buffered at near neutral pH [20]. Metal-amino acid complexes with cyclic structures were formed by the amino and carboxyl groups of amino acids and metal ions in a certain ratio [21]. In contrast, glycine lacks such a buffering group, requiring stricter pH control (pH 10–12) for effective leaching. Currently, histidine has been successfully applied as a ligand in the synthesis of many metal nanoparticles, such as gold [22], silver [23], copper [24], etc. The leaching of gold was promoted by Eksteen and Oraby [25] with an amino acid mixture (glycine and histidine) under conditions of 0.1 mol/L amino acid, 1% H₂O₂, pH 11, and 60 °C. In this study, pure metal cobalt powder was first used as the research object to investigate the reaction mechanism of cobalt leaching with histidine. After systematically studying the influence of key leaching parameters, such as leaching temperature, reagent concentration, stirring speed, histidine/Co molar ratio, and pH, a series of characterization techniques were adopted. The leaching kinetics model of histidine on pure metal cobalt powder was established to further analyze the leaching mechanism of cobalt with histidine. Finally, Cu-Co alloy slag was used to verify the selective leaching effect of histidine on cobalt.

2. Materials and Methods

2.1. Materials

The chemical reagents used in the experiment, including lysine (C₆H₁₄N₂O₂), histidine (C₆H₉N₃O₂), glycine (C₂H₅NO₂), arginine (C₆H₁₄N₄O₂), citrulline (C₆H₁₃N₃O₃), theanine (C₇H₁₄N₂O₃), sulfuric acid (H₂SO₄), and sodium hydroxide (NaOH), were analytically pure reagents, purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). The cobalt used in the experiment is 99.99% cobalt powder with a particle size of 90% –200 mesh. The Cu-Co alloy slag sample was obtained from the converter slag during the copper smelting process through reduction smelting and water quenching. Detailed chemical composition of the alloy is listed in

Table 1. Chemical composition of the Cu-Co alloy slag.

Elements	Co	Cu	Fe	Pb	Ni
Wt.%	6.2	30.7	61.2	1.4	0.42

. All analytical samples were prepared with deionized water.

2.2. Leaching Experiment

A total of 1 g of cobalt powder was added to a certain volume of amino acid solution at different amino acid concentrations, leaching temperature, stirring speed, and leaching time. The pH of the solution was adjusted from 7 to 12 using 1% NaOH solution. After leaching, solid–liquid separation was conducted by filtration. The filter cake was dried at 105 °C for 3 h and weighed. The concentration of cobalt ions in the filtrate was analyzed using a flame atomic absorption spectrometer. The initial conditions were set as 0.3 mol/L amino acid concentration, temperature 25 °C, and 12 h leaching time.

A total of 10 g of Cu-Co alloy slag sample was used for leaching at an initial pH of 8, a temperature of 35 °C, a histidine concentration of 0.1 mol/L, and a histidine/Co molar ratio of 1.5:1. The mixtures were then stirred continuously for 6 to 24 h. Samples were extracted periodically to determine the ion concentrations of Co, Cu, Pb, and Fe. After the leaching process was completed, the residue was filtered, then dried at 105 °C for 3 h. The residual metals content in the filter slag were analyzed. The leaching rates of Co, Cu, Pb, and Fe were calculated.

2.3. Analytical Method

The chemical environment and valence state alterations of elements throughout the leaching process were analyzed using X-ray photoelectron spectroscopy (XPS Thermo Fisher Scientific, Waltham, MA, USA). The surface morphology of the leaching residue was scrutinized with a scanning electron microscope (SEM-EDS Regulus 8100, Chiyoda, Japan), while the functional group bonding changes of the amino acids before and after the leaching reaction were analyzed using Fourier transform infrared spectroscopy (FT-IR NEXUS 670, Nicolet, Stafford, TX, USA). The concentration of metal ions was quantified employing a flame atomic absorption spectrometer (AAS, 55B AA, Agilent Technologies Inc., Santa Clara, CA, USA), and the leaching rate of cobalt was computed as per Formula (1).

$${\mathrm{X}}_{\mathrm{M}\mathrm{e}}={\displaystyle \frac{{\mathrm{C}}_{\mathrm{M}\mathrm{e}}\times \mathrm{V}}{\mathrm{m}\times {\mathsf{\omega }}_{\mathrm{M}\mathrm{e}}}}\times 100\%$$

(1)

where X_Me is the leaching rate of cobalt, %; C_Me is the concentration of cobalt determined by flame atomic absorption spectrometry, mg/L; V is the volume of the filtered leaching solution, L; m is the mass of cobalt powder, mg; and ω_Me is the mass fraction of cobalt (purity), %.

3. Results and Discussion

3.1. Comparison of the Leaching Effects of Different Amino Acids on Cobalt

The effects of four different amino acids with varying numbers and positions of amino groups on cobalt leaching were studied, including glycine (Gly), histidine (His), lysine (Lys), and citrulline (Cit).

The structure of amino acids is influenced by pH, because their charge distribution and intermolecular interactions vary at different pH conditions. The results in Figure 1a demonstrate that histidine, lysine, and arginine all show good leaching effects on cobalt in the pH range of 7 to 10, while the leaching efficiency of cobalt from glycine increased with increasing pH. At pH 11, the leaching rates of histidine and lysine exceeded 95%, while the leaching efficiencies of arginine and glycine decreased to below 90%. At pH 12, the leaching efficiencies of the four amino acids on cobalt significantly declined. As a result, in subsequent experiments, the pH for glycine was set at 10, whereas the pH for the remaining three amino acids was set at 8.

The effect of leaching time on cobalt extraction efficiency of the above amino acids under suitable pH conditions is shown in Figure 1b. The experimental results indicated that the cobalt leaching rates of the four amino acids were different, among which histidine leached cobalt the fastest; all the cobalt was leached within 6 h. The order of the leaching rate was histidine, glycine, lysine, and citrulline. Therefore, the leaching times of histidine, glycine, lysine, and citrulline were set at 6, 7, 8, and 12 h, respectively, in the subsequent experiments.

Figure 1c shows that the rate of cobalt leaching increases with the increase in amino acid concentration. When the concentration of histidine was 0.1 mol/L, cobalt was completely leached. However, for other amino acids, the leaching rate of cobalt exceeds 90% only when their concentration is above 0.25 mol/L. Therefore, in subsequent experiments, the concentration of histidine was set at 0.1 mol/L, while the concentration of other amino acids was set at 0.25 mol/L.

The results in Figure 1d indicate that temperature has a minimal effect on the leaching of cobalt by amino acids relative to other leaching parameters. In summary, under the conditions of 0.1 mol/L histidine concentration, a histidine/Co molar ratio of 1.8:1, room temperature, pH 8, and leaching for 6 h, the cobalt was completely leached. Thus, histidine is the best choice of amino acid for subsequent experiments.

3.2. Parameter Optimization

3.2.1. Effect of Temperature

The effect of leaching temperature on the extraction of cobalt using histidine is shown in Figure 2a. Leaching temperature has a significant impact on the leaching efficiency of cobalt. The higher the temperature, the faster the leaching rate. Under the conditions of 0.1 mol/L histidine concentration, a histidine/Co molar ratio of 1.8:1, and pH 8, the complete leaching of cobalt was achieved in 3 h at 55 °C. At 45 °C and 35 °C, complete leaching was achieved in 5 h. At room temperature, complete leaching was achieved in 6 h. Based on the Arrhenius equation (Equation (2)), the rate constant K relates to temperature. This phenomenon is attributed to the increase in the rate constant K of leaching reaction kinetics at higher temperatures, which accelerates the Brownian motion in cobalt powder, improves the diffusion and mass transfer efficiency within the leaching system, and thus promotes the leaching efficiency of cobalt. The leaching kinetics shown below prove this view. The temperature of 35 °C was chosen for the subsequent experiments, where the leaching rate of cobalt reached 99% at 6 h.

$$\mathrm{K}=\mathrm{A} \mathrm{e}\mathrm{x}\mathrm{p}\left(-{\displaystyle \frac{\mathrm{E}\mathrm{a}}{\mathrm{R}\mathrm{T}}}\right)$$

(2)

where K is the rate constant, in min⁻¹; A is the pre-exponential factor, dimensionless; Ea is the apparent activation energy of the reaction, in kJ mol⁻¹; R is the molar gas constant, 8.314 J/K mol; and T is the thermodynamic temperature in K.

3.2.2. Effect of pH

To investigate the leaching efficiency of cobalt using histidine at different pH values, the leaching kinetics of cobalt were examined under controlled conditions: a histidine concentration of 0.1 mol/L, a histidine/Co molar ratio of 1.8:1, a temperature of 35 °C, and a stirring speed of 300 rpm. As shown in Figure 2b, when the pH value is 8, the leaching rate of cobalt using histidine reaches up to 95% after 4 h. In general, as long as the leaching time is sufficient (extended to 6 h), cobalt can be completely leached under various pH (pH 6–11). This phenomenon can be attributed to the buffering capacity of the histidine solution leaching system, which consists of weak bases and acids, maintaining pH stability within a certain range, and thus ensuring a sustained high cobalt extraction rate.

3.2.3. Effect of Stirring Speed

The increase in stirring rate promotes the diffusion of oxygen and histidine in the leaching system, enhancing mass transfer between the liquid phases and cobalt powder, which in turn accelerates the leaching rate of cobalt. As shown in Figure 2c, the leaching rate of cobalt is positively correlated with stirring rates from 150 to 350 rpm. This phenomenon indicates that the stirring rate not only enhances the relative motion between the solid phase and the liquid phase (helping the leaving out of leach products, i.e., complexed cobalt ions) during the leaching process but also increases the concentration gradient of reactants in the liquid film, thereby accelerating diffusion and improving leaching efficiency. After considering both leaching efficiency and energy consumption, the stirring speed of 300 rpm was determined, at which the cobalt leaching rate reached 99%.

3.2.4. Effect of Histidine Concentration

Figure 2d indicates that the leaching efficiency of cobalt increased with histidine concentration (Figure 2d), rising from 53% at 0.05 mol/L to 99% at 0.1 mol/L after 4 h. This trend is attributed to the enhanced availability of histidine ligands, which drive the complexation equilibrium toward Co²⁺ dissolution. At concentrations exceeding 0.1 mol/L, leaching efficiency plateaued (>95% after 4 h), indicating a transition from ligand-limited to surface reaction-limited kinetics. As shown in Figure 1c, the superior efficiency of histidine over glycine (requiring ≥0.25 mol/L) underscores the critical role of its imidazole group in stabilizing Co²⁺ complexes. The leaching reaction can be represented as (Equation (3)):

Co²⁺ + nHis → [Co(His)_n]²⁺

(3)

3.2.5. Effect of His/Co Molar Ratio

The molar ratio of histidine to cobalt is crucial for improving the leaching efficiency of cobalt. The results shown in Figure 2e indicate that the leaching rate of cobalt increases with the amount of histidine. When the molar ratio was 1.5:1, the leaching rate of cobalt reached over 95%. However, further increasing the amount of histidine does not significantly improve the leaching rate of cobalt. Therefore, an excessively high molar ratio will instead increase the consumption of histidine. Thus, the molar ratio of histidine to cobalt was set at 1.5:1.

3.2.6. Selective Leaching of Cu-Co Alloy Slag

To determine the selective leaching effect of histidine on Cu-Co alloy slag, the leaching rates of Cu, Fe, Co, and Pb in a copper-cobalt alloy slag are shown in Figure 3a. The results indicate that the leaching rate of Co (88%) by histidine is significantly higher than that of Fe, Pb, and Cu. The effect of leaching time on histidine leaching of alloy slag is shown in Figure 3b. When the leaching time was 12 h, the leaching rate of Co reached 95%, while the leaching rates of Fe, Pb, and Cu were 0.7%, 1%, and 1%, respectively. When the leaching time was extended to 24 h, the leaching rate of Co was further increased to 98%, while the leaching rates of other metals remain almost unchanged. The results demonstrate that the histidine leaching process has an excellent selective extraction effect on cobalt from alloy slag.

3.3. Characterization Analysis

The FT-IR spectra of the solid product from the leaching solution treated by freeze-drying (His-Co) and L-histidine are shown in Figure 4. The infrared spectrum of the histidine-cobalt sample showed significant differences compared to that of histidine. The positions of the main absorption peaks were obviously shifted and the intensity of the peaks changed, indicating a chemical reaction between histidine and cobalt. Compared with histidine, the N-H stretching vibration peak in His-Co was red-shifted, i.e., the frequency decreased from 1475 cm⁻¹ to 1464 cm⁻¹, indicating the weakening of the N-H bond strength and the complexation reaction between the amino group and Co²⁺ ions. The symmetric stretching vibration peak of COO- shifted from 1437 cm⁻¹ to 1414 cm⁻¹, which is due to the complexing of COO- with Co²⁺ ions, resulting in a decrease in the electron cloud density of the COO- group. The COOH absorption peak at 1633 cm⁻¹ narrows significantly, which is attributed to the weakening of intermolecular or intramolecular interactions, resulting in an increase in the degree of freedom of the molecule, and the vibration frequency is concentrated as a narrowing of the peak [26]. However, the peak at 1586 cm⁻¹ corresponds to the asymmetric CC bond stretching vibration of the imidazole ring [27], which is not present in the infrared spectrum of L-histidine. Furthermore, new characteristic peaks appeared in the spectrum of His-Co at 1343 cm⁻¹, 1250 cm⁻¹, and 923 cm⁻¹, which corresponded to the asymmetric stretching absorption peaks of NH^3⁺, C-N, C-N/N-H, and C-H, respectively [28,29,30]. The asymmetric stretching vibration peak of C-N shifted from 1114 cm⁻¹ to 1084 cm⁻¹, further confirming the involvement of the amino group in the reaction [31]. It is noteworthy that the strong absorption peak at 625 cm⁻¹ originates from the vibration of the imidazole ring. The Co-N vibration peak at 539 cm⁻¹ directly reveals the coordination relationship between the amino group and the cobalt ion. Based on FT-IR analysis, it is speculated that the amino group and COO- group in histidine played a complexing role in the cobalt leaching reaction.

Figure 5a presents the comprehensive XPS spectrum of the main elements in the leaching solution. To compare the peak binding energies of the elements in leaching solution to those of standard spectra, the analysis results are shown in Figure 5b–e. Figure 5b illustrates the C1s spectrum with three peaks at 284.8 eV, 285.79 eV, and 288.07 eV, corresponding to the orbital binding energies of C1s in sp_2- or sp_3-hybridized C-H or C-C bonds [32,33], the C=N in the imidazole group of histidine, and the C=O bond [34]. In Figure 5c, N1s peaks at 398.4 eV and 399.5 eV are attributed to C=N and CO-NH₂, respectively. Figure 5d shows that the O1s peaks of the main oxygen-containing groups are located at 530.8 eV and 532.5 eV, which belong to the bands of O-C-O and CoO [35], respectively. The content of O-C-O is dominant, indicating that there was still unreacted histidine in the leaching solution. The peaks of 802 eV and 796.1 eV shown in Figure 5e correspond to the Co 2p_1/2 orbital characteristic peaks of CoO and Co²⁺, respectively, while the peaks of 785.6 eV and 780.6 eV represent the Co 2p_3/2 orbital characteristic peaks of CoO and Co²⁺ [36]. The coexistence of O1s and Co2p peaks for CoO indicates the bonding of O atoms with Co ions. Based on these analyses, it is concluded that metallic Co was leached as Co²⁺ during the process. Considering the orbital binding energies of Co and C/O/N atoms in functional groups, it is inferred that electrons were donated to Co²⁺ by the amino and carboxyl groups of histidine during coordination. This finding is consistent with the XPS peak of Co²⁺ (796.1 eV), which match the reported values for Co-histidine complexes, confirming the formation of [Co(His)₂]²⁺. Additionally, the FT-IR results (N-H shift from 1475 to 1464 cm⁻¹) further support the proposed coordination mechanism via amino and carboxyl groups.

As shown in Figure 6a,b, the SEM analyses revealed the surface morphology characteristics of the original cobalt powder and the solid product from the leaching solution treated by freeze-drying. Compared with the irregular strip morphology of the original cobalt powder, the cobalt surface after reaction with histidine exhibits the phenomenon of leaching corrosion. Figure 6c shows the EDS analysis of the solid product from the leaching solution treated by freeze-drying; the content ratios of elements such as C, H, O, and Co were very close to those of Co(C₆H₉N₃O₂)₂. Combined with the previous FT-IR and XPS analyses, it was verified that O-C-O and -NH- in histidine formed metal ligands with cobalt ions. The mechanism of cobalt leaching by using histidine could be described as shown in Figure 7.

3.4. Leaching Kinetics

To describe the reaction kinetics mechanism of cobalt leaching using histidine, the film diffusion model (Equation (4)), the chemical reaction control model (Equation (5)), the product layer diffusion control model (Equation (6)), and mixing control model (Equation (7)) are used and fitted to identify the leaching process rate-controlling step [37,38].

$$\mathrm{X}=\mathrm{k}\mathrm{t}$$

(4)

$$1-{\left(1-\mathrm{X}\right)}^{{\displaystyle \frac{1}{3}}}=\mathrm{k}\mathrm{t}$$

(5)

$$1-{\displaystyle \frac{2}{3}}\mathrm{X}-{\left(1-\mathrm{X}\right)}^{{\displaystyle \frac{2}{3}}}=\mathrm{k}\mathrm{t}$$

(6)

$${\displaystyle \frac{1}{3}}\ln \left(1-\mathrm{X}\right)+{\left(1-\mathrm{X}\right)}^{{\displaystyle \frac{-1}{3}}}-1=\mathrm{k}\mathrm{t}$$

(7)

where X is the leaching rate of metal, t is the leaching time (h), and k is the constant of apparent rate.

As illustrated in Figure 8 and

Table 2. Fitting parameters of the equations for the three kinetic models for cobalt leaching using histidine.

Temperature (°C)	Internal Diffusion Control 1 − (2/3)x − (1 − x)2/3		Chemical Reaction Control 1 − (1 − x)1/3		Mixed Control 1/3ln(1 − x) + (1 − x)−1/3 − 1
	k1 (min−1)	R2	k2 (min−1)	R2	k2 (min−1)	R2
30	0.02493	0.846	0.05406	0.989	0.02732	0.716
35	0.03519	0.902	0.08991	0.998	0.04637	0.765
40	0.04110	0.911	0.13334	0.999	0.06148	0.747
45	0.04709	0.929	0.17980	0.999	0.07924	0.752

, the fitting coefficient (R₂) of the equation of the surface chemical reaction control model exceeds 0.98 at different temperatures, indicating that the surface chemical reaction control model has a good linear relationship with the cobalt powder leaching time and is significantly higher than the correlation coefficient obtained by the fitting of the internal diffusion control and mixing control of the solid product layer. Therefore, it is preliminarily believed that the histidine leaching process was controlled by surface chemical reactions, which is further confirmed by the calculation of the activation energy of the reaction. The apparent activation energy for the leaching reaction was calculated using the Arrhenius equation, the results for which are shown in Figure 9. It is widely acknowledged that the apparent activation energy (Ea) is less than 12.5 kJ/mol, indicating that the leaching process is diffusion-controlled in the product layer. If 12 < Ea < 41.8 kJ/mol, the leaching process is a mixed-controlled process. When Ea > 41.8 kJ/mol, the leaching process is a chemical reaction-controlled process [39].

The results shown in Figure 9 and

Table 3. The activation energy of the two control models.

Control Model	Slope k′	Correlation Coefficient (R2)	Ea (kJ/mol)
Chemical reaction control	7.74	0.99	64.32
Internal diffusion control	3.99	0.95	33.19

demonstrate that the surface chemical control kinetic model at different temperatures has a good fitting effect; the correlation coefficient (R₂) was 0.99. The apparent activation energy (Ea) for the cobalt leaching process was 64.32 kJ/mol, which was consistent with the activation energy Ea > 41.8 kJ/mol, indicating that the cobalt leaching process mainly conformed to the surface chemical control model and was consistent with the results obtained by fitting the kinetic model (Table 2).

A nucleation model based on the ideal sphere hypothesis is proposed to simulate the leaching process of solid particles. According to the model, the main chemical reaction occurs between the unreacted particle core and the surface of the phase boundary of the reaction product [40]. The leaching process occurs isotropically from the outer surface towards the interior of the spherical particles, preserving the core’s spherical configuration, as illustrated in Figure 10. The leaching steps mainly include the following: I. The leaching agent diffuses towards the surface of the cobalt powder; II. The leaching agent diffuses inward through the solid film; III. The leaching agent reacts with cobalt powder; IV. The product (Co-histidine complex) diffuses out of the reaction site; V. The product finally enters the solution through the diffusion layer [41].

3.5. Future Research Prospects

This study demonstrates that histidine-mediated selective leaching of cobalt is technically feasible. However, additional rigorous laboratory experiments are required before scaling up this technology for industrial or commercial applications. In practical leaching operations, recovery and recycling of the leaching solution should be implemented to reduce operational costs. Furthermore, potential synergies with existing hydrometallurgical technologies could be explored, such as combining histidine leaching with electrochemical processes or solvent extraction techniques. The results reveal relatively low nickel leaching efficiency by histidine (only 1% after 24 h). Future research could focus on optimizing leaching parameters (e.g., pH adjustment, redox agents) or incorporating histidine with auxiliary ligands like glycine to enhance nickel selectivity. Concurrently, conducting a life cycle assessment (LCA) to compare the environmental footprint of histidine leaching with conventional ammonia-based or acid-based methods would help evaluate the viability of histidine leaching as a versatile and sustainable alternative in hydrometallurgical processes.

4. Conclusions

The recovery of cobalt was efficiently achieved through a selective complexation leaching method utilizing histidine as a green leaching agent. Compared to glycine-based systems, histidine was demonstrated to rapidly form stable soluble complexes with Co²⁺ at lower concentrations across a broader alkaline pH range (6–11). In this study, histidine was innovatively introduced as a sustainable alternative to glycine for cobalt recovery, exhibiting significantly accelerated leaching kinetics and enhanced pH adaptability. The leaching process exhibited high selectivity toward cobalt. Under optimized conditions (initial pH 8, 35 °C, 0.1 mol/L histidine concentration, 300 rpm stirring speed, histidine/Co molar ratio of 1.5:1, and 12 h duration), extraction efficiencies of 95% Co, 0.7% Fe, 1% Pb, and 1% Cu were attained. Kinetic analysis revealed that cobalt extraction was governed by surface chemical reactions, with an activation energy of 64.32 kJ/mol. The coordination mechanism was confirmed through FT-IR and XPS analyses, which indicated that Co²⁺ in solution predominantly interacted with the -O=C-O- and -N-H functional groups of histidine, leading to the formation of stable chelate ring complexes. This study successfully validated the feasibility of employing histidine as a highly efficient and selective leaching agent for cobalt separation from cobalt-rich alloy slags.