Research on the adsorption of Co 2 + ions using halloysite clay and the ability to recover them by electrodeposition method

: In environmental research, along with discovering methods for adsorbing heavy metals, it is essential to comprehend the processes of desorption and recovery of these heavy metals from adsorbent materials and their reuse. In this study, halloysite (HAL) clay, obtained from the Thach Khoan, Vietnam, was utilized for the removal of Co 2 + ions from an aqueous solution, and the in ﬂ uence of di ﬀ erent factors on the adsorption properties of Co 2 + was investigated. Optimal conditions determined were 0.8 g HAL mass per 50 mL of solution, initial Co 2 + concentration of 40 mg·L − 1 , contact time of 80 min, pH 0 of 6.09, and room temperature of 30°C. Under these conditions, the adsorption e ﬃ - ciency and capacity obtained were 76.358 ± 0.981% and 1.909 ± 0.025 mg·g − 1 , respectively. The adsorption process followed the Langmuir adsorption isotherms, with a maximum monolayer adsorption capacity of 3.10206 ± 0.13551 mg·g − 1 , and exhibited a pseudo-second-order kinetic model. Desorption experiments were conducted using the electrochemical method with a deep eutectic solvent based on choline chloride and urea (reline). The results demonstrated that 94.11% of the Co metal could be recovered through electrodeposition after 5h, using an applied current of 7.5mA at 60°C. The HAL material was successfully regenerated following the desorption process.


Introduction
Halloysite (HAL) belongs to the kaolin mineral group, which includes minerals such as kaolinite, dickite, nacrite, and HAL itself.HAL occurs in two main polymorphs: a fully hydrated form with the chemical formula Al 2 Si 2 O 5 (OH) 4 •2H 2 O and a dehydrated form with the chemical formula Al 2 Si 2 O 5 (OH) 4 [1].HAL can exhibit various morphological forms, including tubular, spherical, and layered structures.In recent years, HAL has gained significant attention from scientists due to its unique properties, such as its tubular structure, non-toxicity, large surface area, high mechanical strength, and cost-effectiveness compared to nanotubular carbon.As a result, HAL has found applications in various fields, including pharmaceuticals, medicine, food, high-grade materials, agriculture, and environmental applications [2,3].In particular, HAL has proven to be an effective adsorbent for heavy metals [4][5][6][7][8][9][10][11][12].
Among the various methods available for treating water pollution caused by heavy metals, adsorption has emerged as a promising approach due to its simplicity, effectiveness, and broad applicability.In adsorption, the choice of adsorbent material plays a crucial role.Many studies have focused on utilizing natural and environmentally friendly materials as adsorbents for heavy metal removal.Cellulose has been investigated as an adsorbent for heavy metal removal [13,14].Similarly, chitosan has shown promise in adsorbing heavy metals [15,16].Hydroxyapatite [17], montmorillonite [18], natural clay [19], and plant straw [20] have also been explored as effective adsorbents for heavy metal adsorption.
There has been extensive research on heavy metal adsorption from the environment and industrial wastewater.However, studies focusing on the desorption and recovery of heavy metals from adsorbed materials and the regeneration of adsorbents for subsequent adsorption processes have received limited attention.Yet, the desorption of heavy metals from adsorbed materials is crucial to prevent secondary pollution and to enable the reuse of materials.Although some studies have been published on the desorption and recovery of heavy metals from hydroxyapatite [21][22][23][24][25][26][27] and clays [28][29][30], there are few published studies specifically addressing the desorption, recovery, and reuse of heavy metals from HAL [9,10,28].For instance, Itami and Yanai investigated the sorption/desorption properties of Cd and Cu on five clay types, highlighting the influence of the clays' charge characteristics and pH [28].Another study by Helios Rybicka et al. examined the adsorption/desorption behavior of Cd, Cu, Pb, Zn, and Ni on illite, beidellite, and montmorillonite.The results showed that the adsorption capacity of the clays was higher for Pb and Cu ions than for Ni, Zn, and Cd ions.Furthermore, the desorption properties varied, with Pb > Cd > > Cu > Ni > Zn for beidellite and Pb > Cd ∼ Cu > Ni > Zn for illite and montmorillonite using NaNO 3 as the eluent agent [29].
Cobalt is an essential trace element that is naturally present in all organs and cells of the body.It plays a vital role in synthesizing vitamin B12 and certain enzymes.However, excessive cobalt absorption at high concentrations can pose severe risks to humans, animals, and plants.Exposure to elevated levels of cobalt can lead to respiratory symptoms, lung damage, hypothyroidism, dermatitis, heart disease, and hearing and vision loss, among other health issues.Studies have demonstrated that plant mortality can occur when the cobalt concentration in the soil solution exceeds 10 mg•L −1 .
Similarly, fish can die when the cobalt concentration in water surpasses 5 mg•L −1 .To regulate and maintain safe levels, the Food and Agriculture Organization of the United Nations has set guidelines specifying that the cobalt content in irrigation water should not exceed 100 μg•L −1 , and the concentration of cobalt ions in wastewater must be kept below 1.0 mg•L −1 [31].These guidelines safeguard organisms' health and mitigate the potential adverse effects of elevated cobalt concentrations.Indeed, cobalt adsorption has been a subject of interest and research using various adsorbent materials.Different types of adsorbents have shown positive results in treating cobalt in water.Activated carbons produced from waste potato peels were also utilized for Co(II) adsorption from synthetic water.The resulting carbon materials had different surface areas depending on the processing temperature, and the maximum adsorption capacity reached up to 405 mg•g −1 at an optimum pH of 6 [32].The combination of HAL and cellulose into polyurethane foam also improved the adsorption capacity of the foam to metal ions in the solution.Various functional groups in HAL and cellulose contributed to electrostatic bonds between the adsorbent and metal ions, leading to increased adsorption capacity [33].However, despite these promising results with other metals, fewer reports have been published specifically on Co(II) adsorption using HAL.This indicates the need for more research to explore and optimize the potential of HAL as an effective adsorbent for cobalt ions.
In this study, we aim to adsorb Co 2+ ions using HAL clay as the adsorbent material.Subsequently, we focus on the desorption and recovery of cobalt from the adsorbed HAL material using an electrolysis process in a reline electrolyte, a non-toxic deep eutectic solvent (DES).DESs have gained attention in recent years.These solvents can be easily formed by mixing two safe components that are cost-effective, renewable, and biodegradable, capable of creating a eutectic mixture.Choline chloride (ChCl) is a commonly used component for generating DESs.Several publications have reported successful cobalt deposition from DESs formed with ChCl [34][35][36][37].Our study's main advantage lies in performing cobalt's desorption and recovery process simultaneously within an electrochemical cell.This approach allows for the reuse of the HAL adsorbent, minimizing waste and maximizing the efficiency of cobalt recovery.

Experimental method 2.1 Preparation of HAL powder
The HAL material was collected from the Lang Dong mine in Phu Tho province, Vietnam.After collection, the sample was mixed thoroughly and separated using the wet sieving method to achieve a particle size of less than 32 µm.The separated sample with a particle size of <32 µm was then dried at 60°C and finely ground using an agate mortar.The resulting HAL powder was used for subsequent analysis and experiments.

Preparation of DES
The DESs used in the study were prepared by mixing ChCl (Alfa Aesar, purity ≥98%) and urea (U; VWR Chemicals, NORMAPUR).ChCl was recrystallized from absolute ethanol (VWR Chemicals, NORMAPUR), followed by filtration and drying under vacuum.ChCl and U were then combined in a 1:2 molar ratio within a closed container.The mixture was stirred continuously for 3 h at 60°C until a homogeneous colorless liquid, known as ChCl-U (Reline) DES, was formed.

Determination of pH PZC of HAL powder
A mixture of 0.5 g HAL powder was combined with 50.0 mL of 0.01 M KNO 3 solution.The mixture was stirred for 60 min at room temperature.To adjust the initial pH values (pH 0 ), either a 0.1 M KOH solution or a 0.1 M HNO 3 solution was used within a pH range of 2.5-9.5.Once the equilibrium was reached, the pH values were measured again (pH f ).The point of zero charge (pH PZC ) was determined from the ΔpH = f(pH 0 ) plot, where ΔpH represents the difference between pH 0 and pH f .The pH PZC corresponds to the pH 0 value when ΔpH equals zero.

Adsorption experiments
The experiments were conducted at room temperature with continuous stirring at 400 rpm, a contact time studied from 10 to 120 min, pH from 2.37 to 6.99, mass of HAL changed from 0.3 to 1.2 g, initial concentration of Co 2+ verified between 10 and 80 mg•L −1 .After the adsorption process, the solution was filtered to remove the solid HAL material, and the remaining concentration of Co 2+ was determined using the Inductively coupled plasma mass spectrometry (ICP-MS) method on ICAP Q ICP-MS (Thermo Scientific, Germany) instrument.
The adsorption capacity and efficiency were calculated using Eqs. 1 and 2, respectively [38] as follows: The amount of metal ion adsorbed on the adsorbent at equilibrium (Q) is calculated using the equation Q = (C 0 − C) × V/m, where C 0 and C represent the initial and equilibrium concentrations (mg•L −1 ) of Co 2+ ions in the solution, respectively.V denotes the volume of the solution (L), and m is the mass of the adsorbent (g).
The experimental data obtained are analyzed using the Langmuir and Freundlich isotherm models [39].
Langmuir non-linear equation: Freundlich non-linear equation: where is the maximum adsorption capacity, K L is the Langmuir coefficient related to the adsorption energy, K F and n are the constants of the Freundlich model.
The adsorption kinetics is described by the pseudofirst-order and pseudo-second-order kinetic models using Eqs. 5 and 6, respectively [40].
where Q e is the adsorption capacity at equilibrium (mg•g −1 ), Q t is the adsorption capacity at time t (mg•g −1 ), and k 1 and k 2 are the pseudo-first-order (min•L) and pseudo-second-order (g•mg −1 •min −1 ) rate constants, respectively.Physicochemical characteristics of HAL before and after Co 2+ adsorption were analyzed by the following methods: The phase component was analyzed using X-ray diffraction (XRD) (Siemens D5000 diffractometer, CuKαradiation, λ = 1.54056Å, with a step angle of 0.030°, scanning rate of 0.04285°•s −1 , and 2θ in the range of 20-70°).The structure was analyzed by infrared spectroscopy (FT-IR) on a Nicolet iS10 instrument (Thermo Scientific, USA).Morphology and element components were analyzed by SEM-EDX on a FE-SEM-JSM-IT800 instrument (JEOL, Japan).

Electrochemistry experiments for desorption and recovery cobalt 2.5.1 Cyclic voltammetry scan in reline solvent
In the cyclic voltammetry scan experiments, a three-electrode system was used, and it was connected to an Autolab PGSTAT20 potentiostat from Metrohm.The working electrode (WE) used was a gold (Au) electrode with a geometric area of 0.0201 cm 2 .The reference electrode (RE) was an Ag/ AgCl electrode with a chloride-ion-saturated potassium chloride solution (Ag, AgCl|Cl − ), and the counter electrode (CE) was a platinum grid with a large area.A sample volume of 5 mL of reline DES containing either 0.5 g of Co(NO 3 ) 2 or 0.5 g of Co-HAL was used for the experiments.The scan potential range was set from −0.2 to −1.5 V, and the scan rate was 50 mV•s −1 .The experiments were carried out at a temperature of 60°C.Before the experiments, the WE (Au electrode) was polished using an alumina-water slurry on a smooth polishing cloth.It was then sonicated twice for 3 min and rinsed with Milli-Q water to ensure no remaining alumina.Finally, it was dried under a nitrogen atmosphere.The platinum CE was cleaned by flaming it until it reached a red glow.Before each experiment, the DES medium was purged with nitrogen for at least 20 min, and during all measurements, the system was kept under a nitrogen atmosphere.This careful preparation and control of the experimental conditions ensured accurate and reliable results during the cyclic voltammetry scans in the reline DES.The desorption of Co 2+ from Co-HAL and the subsequent electrodeposition of Co metal were performed in a threeelectrode electrochemical cell.The WE used was a gold plate with a geometric area of 1 cm 2 .The RE used was Ag/AgCl with a chloride-ion-saturated potassium chloride solution (Ag, AgCl|Cl − ), and the CE was a platinum grid with a large area.Cobalt recovery was carried out using the electrodeposition method in the reline solvent.The deposition potential was set to be less than or equal to −1.3 V, and the temperature was maintained at 60°C.Before starting the electrolysis process, the DES medium was purged with nitrogen for at least 20 min, and the nitrogen atmosphere was maintained throughout the electrolysis.Different applied current values of 1, 2, 3, 5, and 7.5 mA were used, and the electrolytic time was varied from 1 to 5 h.After the electrolysis process, the Co-HAL powder was filtered out from the mixture.Then, it underwent cleaning, drying, and the remaining Co in the powder was determined by the ICP-MS method.The surface of the Au electrode before and after electrolysis was analyzed by SEM-EDX.The phase component of initial HAL and reused HAL after the desorption process of Co 2+ were analyzed using XRD.

Effect of the experimental factors on
Co 2+ ions adsorption by HAL powder

pH PZC of HAL
The change in ΔpH against pH 0 is presented in Figure 1.
Based on the graph, it can be observed that ΔpH equals zero when the pH 0 value is 5.99.This indicates that the point of zero charge (pH PZC ) for the HAL material used in the study is determined to be 5.99.The pH PZC represents the pH value at which the surface charge of the material is neutral, indicating that the material has no net positive or negative charge.

Effect of contact time
The contact time between the solid adsorbent (HAL) and the metal ion solution (Co 2+ ) plays a crucial role in adsorption.To investigate the influence of contact time on HAL's adsorption ability for Co 2+ ions, experiments were conducted for 10-120 min using a 50 mg•L −1 Co 2+ solution.As shown in Table 1, the results indicate that as the contact time increased, the adsorption capacity and efficiency of the HAL for Co 2+ ions gradually increased.However, once the contact time reached 80 min, the adsorption capacity and efficiency became nearly stable.This suggests that after 80 min, the adsorption process of Co 2+ ions by HAL reached equilibrium, with no significant further increase in adsorption.Based on these findings, a contact time of 80 min was chosen for subsequent studies, representing the time required to achieve equilibrium in the Co 2+ adsorption process by HAL.

Effect of solution pH
The effect of pH on the Co 2+ adsorption capacity of HAL was investigated because changes in solution pH can influence the surface properties of the adsorbent and the form of metal ions in the solution.The study was conducted with the Co 2+ solution having a concentration of 50 mg•L −1 , and the pH values were varied around the pH PZC value of HAL, which was determined to be 5.99.However, it is essential to note that precipitation of Co 2+ hydroxide starts to occur at a pH of 7.8 [41].To accurately assess the amount of ion depletion in the solution due to adsorption on HAL, the pH was controlled to be less than 7.8.As shown in Table 2, the results demonstrated that within the investigated pH range, both the adsorption efficiency and capacity of Co 2+ ions by HAL increased as the pH increased.This can be explained by the protonation of HAL in acidic environments, resulting in a positively charged surface.Consequently, the number of adsorption sites for positive ions on HAL decreased, and competitive adsorption occurred between H + ions and Co 2+ ions on the surface of HAL, leading to a decrease in the adsorption ability of Co 2+ ions [40].As the pH increased, the positive charge density on the surface of HAL decreased, thereby enhancing the adsorption capacity of Co 2+ ions.Favorable adsorption of Co 2+ ions occurred when the pH was higher than the pH PZC value of HAL.To facilitate the treatment of larger quantities without the need for pH adjustment, an initial pH of 6.09 was selected for Co 2+ adsorption in subsequent studies.The adsorption efficiency and capacity reached 51.334 ± 1.047% and 2.567 ± 0.052 mg•g −1 at this pH, respectively.

Effect of material mass
When the mass of the solid adsorbent in the solution increases, the contact area between the adsorbent and the solution also increases.This increases the number of active sites available for adsorption, resulting in higher adsorption efficiency [41].This study observed that adsorption efficiency increased rapidly from 45.405 ± 0.824% to 71.308 ± 0.925% as the mass of HAL increased from 0.3 to 0.8 g (Figure 2).After reaching 0.8 g, further increases in the amount of adsorbent resulted in only slight improvements in adsorption efficiency.This is because the adsorption process reached equilibrium, and additional adsorbent did not significantly impact the overall efficiency.To achieve an appropriate magnitude of adsorption capacity and efficiency (2.228 ± 0.029 mg•g −1 and 71.308 ± 0.925%, respectively), a mass of 0.8 g of HAL was selected to study the adsorption of Co 2+ ions.This mass provided optimal adsorption performance, balancing the amount of adsorbent with the achieved efficiency.

Effect of Co 2+ concentration
The investigation of the adsorption process with varying initial concentrations of Co 2+ ions (20-80 mg•L −1 ) revealed significant findings, as shown in Table 3 and Figure 3.As the concentration of Co 2+ ions increased, the adsorption capacity of HAL gradually increased while the adsorption efficiency decreased.For low initial concentrations of Co 2+ ions, the adsorption of Co 2+ onto HAL was favorable due to the large contact area between the Co 2+ ions and the solid phase of HAL.However, as the initial concentration of Co 2+ ions increased, the amount of Co 2+ ions also increased.Nevertheless, the adsorption capacity of HAL reached saturation and did not increase further, resulting in a decrease in the adsorption efficiency [42].To achieve a high simultaneous  adsorption capacity and efficiency, an appropriate concentration of Co 2+ ions was selected within the range of 30-50 mg•L −1 .At a concentration of 40 mg•L −1 of Co 2+ ions, HAL's adsorption efficiency and capacity reached 76.358 ± 0.981% and 1.909 ± 0.025 mg•g −1 , respectively.This result is considered acceptable when comparing the adsorption efficiency of HAL for Co 2+ ions with other investigated metal ions, such as Cd 2+ with an efficiency of 51.45% and Pb 2+ with an efficiency of 79.3% [43], as well as As(III) with an efficiency of 82.4% [42].

Characterization of HAL before and after adsorption process
The HAL powder before and after Co 2+ adsorption was characterized through FT-IR, XRD, and SEM-EDX analyses (Figures 4 and 5    6  Le Thi Duyen et al.

Adsorption isotherm
In each experiment, 0.8 g of HAL was used to adsorb Co 2+ ions in a 50 mL solution with varying initial concentrations.The adsorption process was conducted at the natural pH of 6.09 and room temperature (25°C) for 80 min.The remaining Co 2+ concentration at equilibrium (C e ) was determined, and the adsorption capacity (Q) was calculated (Table 3).The Langmuir and Freundlich isotherm equations were constructed using Eqs. 3 and 4, respectively.The experimental data were plotted on the adsorption isotherm graphs (Figure 6).The Langmuir isotherm equation represents the maximum adsorption capacity (Q m ) and the Langmuir constant (K L ), while the Freundlich equation involves the experimental constants K F and n.The results presented in Table 4 indicate that the adsorption of Co 2+ on HAL follows the Langmuir isotherm adsorption model (R 2 = 0.98113, Sum of the Square of the Errors (SSE) = 0.07672).This finding is consistent with numerous published results that have utilized HAL as an adsorbent for heavy metals and organic pigments [4].The Langmuir isotherm model provides insights into the maximum adsorption capacity (3.10206) and the interaction between the adsorbate (Co 2+ ions) and the adsorbent (HAL) during adsorption.

Adsorption kinetic
The adsorption kinetic was studied based on the effect of adsorption time on the Co 2+ adsorption capacity by constructing graphs of the pseudo-first-order (Eq.5) and pseudosecond-order kinetics (Eq.6).The resulting graphs in Figure 7 exhibited non-linear lines for the pseudo-first-order and pseudo-second-order modes.The regression coefficients (R 2 ), adsorption rate constants (k) and the adsorption capacity at equilibrium (Q e ) were determined in Table 5.The results indicated that the regression coefficient of the pseudo-secondorder kinetic equation (R 2 = 0.94315) was much higher than that of the pseudo-first-order kinetic equation (R 2 = 0.67314) as well as the lower value of the SSE of the pseudo-second-order kinetic equation (SSE = 0.01223) than the pseudo-first-order kinetic equation (SSE = 0.07033).This result confirms that the adsorption of Co 2+ by HAL follows the pseudo-secondorder adsorption kinetic.The adsorption rate constant, k, was calculated to be 0.13905 g•mg −1 •min −1 .This indicates that the pseudo-second-order kinetic model better describes the adsorption process, which suggests a chemisorption mechanism involving strong bonding between the adsorbate (Co 2+ ions) and the adsorbent (HAL).The cyclic voltammetry result of Co-HAL in reline solvent suggests that Co 2+ ions can be effectively desorbed from the Co-HAL material and subsequently reduced to metallic Co on the surface of the Au electrode in the reline solvent.This electrochemical process allows for the recovery of the HAL adsorbent.The deposition of Co metal on the Au electrode surface can be achieved at a potential equal or less than −1.3 V.This possible range ensures the successful electrochemical recovery of Co from the Co-HAL material.

Effect of applied current and electrolytic time on recovery efficiency of cobalt
The recovery of cobalt was achieved through the electrodeposition method using the applied current technique.Figure 9 illustrates the process of cobalt deposition into the reline solvent through electrodeposition at the applied current of 7.5 mA.The potential reached −1.5 V and was quite stable during the electrolysis process.
The initial smooth surface of the Au electrode can be observed in Figure 10a.EDX spectroscopy analysis of the Au electrode confirmed the presence of characteristic

Table 5: Adsorption kinetic parameters
Pseudo-first-order kinetic equation Pseudo-second-order kinetic equation    6, providing valuable   insights into the effectiveness of the electrodeposition method for Co recovery.
The results presented in Table 6 demonstrate the variation in the recovery efficiency of Co based on the applied current and electrolytic time.It was observed that increasing the applied current resulted in a higher amount of Co being deposited on the surface of the Au electrode, leading to an increase in the recovery efficiency of Co.Similarly, increasing the electrolytic time also led to a higher amount of Co deposition and an increase in the recovery efficiency of Co. Notably, the recovery efficiency of Co reached 94.11% after 5 h of electrolysis at an applied current of 7.5 mA.This indicates that a significant amount of Co was successfully recovered from the Co-HAL material and deposited onto the surface of the Au electrode.These findings highlight the effectiveness of the electrodeposition method in the recovery of Co, with higher applied current and longer electrolytic time contributing to higher recovery efficiency and more significant deposition of Co metal.

Regeneration of HAL material
During the electrolysis process, Co 2+ ions were successfully desorbed from the Co-HAL material, allowing for the recovery of HAL material for further adsorption applications.Figure 11 presents the XRD pattern of the HAL material obtained after electrolysis, which closely resembles the XRD pattern of the initial HAL material.This indicates that the electrolysis process did not significantly alter the crystal structure of the HAL material.To assess the regeneration ability of the prepared sorbent, adsorption experiments were conducted under suitable conditions.After electrolysis, the obtained HAL material exhibited a Co 2+ adsorption capacity (Q) of 1.55 mg•g −1 and an adsorption efficiency (H) of 61.56%.This indicates that the regenerated HAL material retains a significant adsorption capacity and efficiency for Co 2+ ions.These results suggest that the electrolysis process effectively desorbs Co 2+ ions from the HAL material, allowing the sorbent's successful recovery and reuse for subsequent adsorption processes.

Conclusion
HAL clay has shown a high efficiency of 76.36% in removing Co 2+ ions from aqueous solutions.The equilibrium time for the adsorption process was determined to be 80 min.The adsorption isotherm results indicated that the adsorption of Co 2+ ions using HAL powder followed the Langmuir isotherm model, with a maximum monolayer adsorption capacity of 2.992 mg•g −1 .The adsorption kinetics data confirmed that the adsorption process of Co 2+ ions on HAL powder followed the pseudo-second-order kinetic model, with a high correlation coefficient (R 2 ) of 0.9998.This indicates that the adsorption process is well-described by this kinetic model.The recovery of cobalt from the adsorbed HAL material was achieved through electrodeposition on the surface of an Au electrode.A recovery efficiency of 94.11% was achieved after 5 h of electrolysis at an applied current density of 7.5 mA in the reline electrolytic solution.Significantly, the HAL adsorbent was not dissolved during the electrolysis process, demonstrating its stability and the possibility of regeneration.The obtained HAL material after desorption exhibited an adsorption capacity of 1.55 mg•g −1 and an adsorption efficiency of 61.56%.This indicates that the HAL adsorbent can be successfully reused after the desorption process, making it a sustainable and cost-effective solution for heavy metal removal from aqueous solutions.

2. 5 . 2
Desorption of Co 2+ and deposition of Co metal on the surface of an electrode

Figure 1 :
Figure 1: Determination of pH PZC of HAL powder.

Figure 2 :
Figure 2: The effect of material mass on the HAL's adsorption of Co 2+ at an initial concentration of 50 mg•L −1 , pH = 6.09, and contact time of 80 min.
).It can be observed that the FT-IR and XRD analysis graphs for both types of samples are similar and show no significant changes.The FT-IR spectra exhibit vibrations of O-H groups on the inner surface (at 3,695 and 3,622 cm −1 ), interlayer water (at 1,635 cm −1 ), Si-O (at 1,038 and 694 cm −1 ), Al-OH (at 914 cm −1 ), Al-O-OH (at 796, 752 cm −1 ), and Al-O-Si (540 cm −1 )[43].The XRD peaks at 12.4°, 25.0°, and 45.7°for kaolinite and at 20.0°, 26.7°, 35.1°, 38.4°, 55.0°and 62.5°for HAL are present in both sample types.SEM images of HAL after Co 2+ adsorption show the tubular structure characteristics.However, EDS analysis results indicate the presence of the Co 2+ elemental spectrum in the HAL powder after adsorption.These results suggest that HAL material has successfully adsorbed Co 2+ ions.However, due to the limited amount of adsorbed ions, it has not significantly affected the morphology of the HAL mineral, as well as the FT-IR and XRD analysis results.

Figure 3 :
Figure 3: The effect of Co 2+ concentration on the HAL's adsorption of Co 2+ at a dosage 0.8 g, pH = 6.09, and contact time of 80 min.

Figure 4 :
Figure 4: FT-IR spectra (a) and XRD pattern (b) of HAL before and after Co 2+ adsorption process.

Figure 5 :
Figure 5: SEM images and EDX results of HAL before (a) and after (b) Co 2+ adsorption process.

1 :Step 2 :
2+ out of Co-HAL and recovery of Co metal by electrodeposition method 3.2.1 Cyclic voltammograms (CVs) of Co(NO 3 ) 2 and Co-HAL in the electrolyte of reline solvent The results shown in Figure 8a demonstrate the presence of a reduction peak of Co 2+ at −1.25 V and an oxidation peak of Co 0 at −0.35 V in the cyclic voltammetry curve of reline containing Co(NO 3 ) 2 .This observation aligns with the reported CV of reline having Co 2+ in the literature [34,37].Figure 8b presents the results of the cyclic voltammetry curve of reline containing Co-HAL, showing a reduction peak of Co 2+ at −1.3 V and an oxidation peak of Co 0 at −0.55 V.These results indicate a slight shift in the oxidation-reduction peaks of cobalt between Co(NO 3 ) 2 /reline and Co-HAL/reline.The mechanism of the deposition and dissolution of cobalt on the Au electrode can be described as follows: Step The formation of a complex between Co 2+ from Co-HAL and Cl − from reline occurs, forming CoCl n 2−n .Subsequently, the reduction of Co 2+ (of CoCl n 2−n ) takes place on the surface of the electrode, leading to the formation of metallic Co: CoCl n 2−n + 2e → Co + n•Cl − Stripping of the Co metal occurs, converting it back to Co 2+ : Co − 2e → Co 2+

Figure 6 :
Figure 6: Langmuir (a) and Freundlich (b) isotherm plots for the adsorption of Co 2+ onto HAL.

Figure 7 :
Figure 7: Isotherm plots for the adsorption of Co 2+ onto HAL according to pseudo-first-order kinetic equation (a) and pseudo-second-order kinetic equation (b).
peaks corresponding to Au, aluminum (Al), and silicon (Si), which are components of the Au electrode.After the electrolysis process, a uniform deposition of Co metal was observed on the surface of the Au electrode, as shown in Figure 10b.The EDX spectroscopy analysis of the electrode after electrolysis still indicated the presence of Co metal, confirming the successful electrodeposition of Co onto the Au electrode.To assess the recovery efficiency of Co, cathodic polarization of the Au electrode was conducted at different applied current values and electrolytic times at a temperature of 60°C.The results of the recovery efficiency of Co are presented in Table

Figure 10 :
Figure 10: SEM-EDX of the surface of the Au electrode before (a) and after (b) electrolysis.

Funding information :
The authors express their gratitude for the financial support provided by the Ministry of Education and Training (project code: B2022-MDA-03), which has played a crucial role in completing this work.Author contributions: All authors contributed equally to this article.Conflict of interest: Authors state no conflict of interest.

Figure 11 :
Figure 11: XRD patterns of HAL before Co 2+ adsorption and after the desorption process.

Table 1 :
Effect of contact time on the HAL's adsorption of Co 2+ at an initial concentration of 50 mg•L −1 , HAL dosage 0.5 g, and pH = 6.09

Table 2 :
Effect of pH on the HAL's adsorption of Co 2+ at an initial concentration of 50 mg•L −1 , HAL dosage of 0.5 g, and contact time of 80 min

Table 6 :
Recovery efficiency of Co (H %) from 0.5 g Co-HAL at different applied currents and electrolytic times