<figure><img src="image_1.png" /></figure>

# Synthesis and characterization of a novel bispyridylmethylamine-based copolymer for Co(II) chelation

A. Giove<sup>a,b,c,*</sup>, K. Laatikainen<sup>a,d</sup>, F. Ibrahim<sup>b</sup>, Y. El Ouardi<sup>a</sup>, A. Sala<sup>b</sup>, S. Hietala<sup>e</sup>, E. Repo<sup>a</sup>, C. Branger<sup>b,**</sup>

## ARTICLE INFO

### Keywords:

Metal adsorption

Complex formation study

Bis(2-pyridylmethyl)amine

Hydrometallurgy

Selectivity

## ABSTRACT

Cobalt is a critical element used in several industries with limited natural sources. This study addresses the challenge of its recovery from solutions containing other transition metals, by synthesizing a novel chelating resin based on an original monomer, bis-AMP-MMA. bis-AMP-MMA was prepared through the introduction of a polymerizable methacrylic group into bis(2-pyridylmethyl)amine. The bis-AMP-MMA monomer was proven to form stable 1:1 and 1:2 stoichiometry complexes with Co(II) ions, indicating strong potential for cobalt recovery. To further assess its applicability, bis-AMP-MMA was copolymerized with ethylene glycol dimethacrylate (EGDMA) to prepare an adsorbent highly crosslinked resin, poly(bis-AMP-MMA-co-EGDMA). The new resin was characterized by FTIR, ¹³C CPMAS NMR, SEM and BET method. It showed a surface area of 153.9 m²/g, and demonstrated a maximum Co(II) adsorption capacity, qe, of 54.0 mg/g at pH 6.5. It also exhibited selectivity for Co(II) over Ni(II), Cd(II), and Mn(II) ions, with selectivity coefficients, k, between 1.13 and 2.09. Additionally, the resin showed good reusability, maintaining 93 % of its efficiency after the first cycle and 65 % after five cycles.

## 1. Introduction

The first application of cobalt is estimated to date back to the late Bronze Age, when this metal was used as a blue dye [1]. This application continues nowadays, since cobalt is involved in the production of blue and violet pigments for coloring porcelain and ceramics [2–4]. Due to its chemical properties, cobalt finds several other applications in modern society. Some examples are the production of superalloys used in jet engines and industrial turbines [5], the preparation of permanent surgical implants [6], and the catalysis of chemical reactions like CO₂ hydrogenation [7] and Fischer-Tropsch synthesis [8]. Other applications include the production of rare earth (RE) magnets such as Sm-Co-RE permanent magnetic materials [9] and cathodes for rechargeable lithium batteries [10,11]. Its wide range of applications makes cobalt a valuable element for several industrial sectors. However, sources of

cobalt are limited and 98 % of industrially available cobalt is obtained as by-product of copper or nickel mining [12].

The separation and recovery of cobalt from nickel-containing materials and solutions is a long-time challenge for the metallurgical industry due to the similarities in the chemical properties of these two elements. In recent decades, hydrometallurgical separation processes have partly substituted the classical pyrometallurgical methods for the recovery of metals. Hydrometallurgy, based on the treatment of water solutions, has established as a greener alternative to pyrometallurgy due to a lower energy consumption and a minimized production of toxic by-products [13]. Hydrometallurgical separation processes involve the dissolution of the raw material to release the target metal ions, which is solubilized together with other metal species and impurities [14]. The separation and recovery of the target metal is then performed with techniques such as precipitation [7], cementation [15], solvent

<footnote><sup>a</sup> Lappeenranta-Lahti University of Technology LUT, School of Engineering Science, Department of Separation Science, Yliopistonkatu 34, FIN-53850 Lappeenranta, Finland</footnote>

<footnote><sup>b</sup> Université de Toulon, MAPIEM, Toulon, France</footnote>

<footnote><sup>c</sup> University of Jyväskylä, Department of Chemistry, P.O. Box 35, FI-40014 Jyväskylä, Finland</footnote>

<footnote><sup>d</sup> Finnish Defence Research Agency, Paroistentie 20, FIN-34100 Lakiala, Finland</footnote>

<footnote><sup>e</sup> University of Helsinki, Department of Chemistry, PB 55, FIN-00014 Helsinki, Finland</footnote>

<footnote>* Corresponding author at: University of Jyväskylä, Department of Chemistry, P.O. Box 35, FI-40014 Jyväskylä, Finland.</footnote>

<footnote>** Corresponding author at: Université de Toulon, MAPIEM, Toulon, France.</footnote>

<footnote>E-mail addresses: alessio.a.giove@jyu.fi (A. Giove), branger@univ-tln.fr (C. Branger).</footnote>
extraction [16], ion exchange [17] and adsorption [18]. The efficiency of these techniques is strongly dependent on their selectivity for the target ion in presence of the other competitive species. To enhance the selectivity, several research groups have developed new materials characterized by specific interactions with the target metal. Some examples are the development of selective extractant [19], the synthesis of new precipitating agents [20] and the design of new chelators to be included within ion exchangers and adsorbents [21]. In particular, modifying commercial ligands with polymerizable groups offers a convenient and effective approach to preparing selective chelating resins [22–24].

In this context, the ligand bis(2-pyridylmethyl)amine (bis-AMP) was selected as precursor for the synthesis of a novel chelating monomer to prepare a new Co(II) adsorbent. This ligand is known to form 1:1 and 1:2 stoichiometry complexes with several metal ions including Cu(II), Ni(II), Co(II), Zn(II), Cd(II), Mn(II), and Ag(I) [25]. Additionally, the low pKₐ values for its three nitrogen atom-based basic functions, equal to 0.5–1.6, 2.1–2.7, and 3.4–4.1, make this ligand an efficient chelator in acidic conditions [26]. Both these properties of the bis-AMP ligand favor its application in the industrial production of commercial chelating resins. An example is represented by the bis-AMP-containing resin Dowex™ M4195 which was applied for the treatment of sulfide concentrates to produce copper and cobalt cathode products [27]. The new chelator proposed in this paper was synthetized by introducing a methacrylic group within the structure of the bis-AMP ligand. The introduction of the methacrylic group serves a dual purpose. First, it provides both a polymerizable function, thereby transforming bis-AMP from a ligand into a functional monomer. Second, it adds a carbonyl group which can act as an additional chelating group (Fig. 1).

The transformation of the bis-AMP ligand into a monomer enables its incorporation into a new resin through the formation of stable covalent bonds. This approach aims to prevent undesired release of ligand during the application of the resin in metal adsorption experiments. Notably, the addition of the carbonyl group introduces a new coordination site into bis-AMP structure. This is expected to enhance the stability of the complexes formed by the new monomer with the target ion of this study. To verify this hypothesis, the stability of the complexes formed between the newly synthesized bis-AMP-MMA monomer and Co(II) ions was investigated using an *in situ* spectroscopic method. The obtained results were compared to the stability constant values of the complexes formed with the bis-AMP ligand. A new resin incorporating the bis-AMP-MMA was finally prepared and applied in metal adsorption experiments to assess its adsorption capacity and selectivity for the target species.

<figure><img src="image_2.png" /><figcaption>Fig. 1. Structures of bis-AMP and bis-AMP-MMA.</figcaption></figure>

## 2. Material and methods

### 2.1. Reagents

The chemicals bis(2-pyridylmethyl)amine (97 %, bis-AMP), triethylamine (99 %, N(CH₂CH₃)₃), nickel nitrate hexahydrate (99.9 %, Ni(NO₃)₂·6H₂O), cadmium nitrate tetrahydrate (98 %, Cd(NO₃)₂·4H₂O), 2,2-azobis(2-methylpropionitrile) (98 %, AIBN), mineral oil (heavy), sodium hydroxide (≥ 98 %, NaOH), sulfuric acid (≥ 98 %, H₂SO₄) and magnesium sulfate (99.5 %, anhydrous, MgSO₄) were purchased from Sigma-Aldrich (Steinheim, Germany). Manganese nitrate tetrahydrate (≥ 98.5 %, Mn(NO₃)₂·4H₂O) and cobalt nitrate hexahydrate (≥ 98 %, Co(NO₃)₂·6H₂O) were purchased from Merck (Darmstadt, Germany) and VWR (Fontenay-Sous-Bois, France), respectively. N-(2-hydroxyethyl) piperazine-N-2-ethanesulfonic acid (≥ 99 %, HEPES) and hydrochloric acid (37 %, HCl) were purchased from Fisher Scientific (Loughborough, U.K.). Nitric acid (67–69 %, HNO₃) and hydrochloric acid (34–37 %, HCl) used for ICP-MS analysis were purchased from Romil (Espoo, Finland). All these chemicals listed above were used without further purifications.

Each monomer used in this work was purified to remove the polymerization inhibitors. The monomer methacryloyl chloride (97 %, C₄H₅ClO), purchased from Alfa Aesar, was distilled under reduced pressure at 40 °C. The monomers ethylene glycol dimethacrylate (98 %, EGDMA) and methyl methacrylate (99 %, MMA), purchased from Acros Organics (Geel, Belgium) and Sigma-Aldrich, respectively, were washed three times with a 10 % NaOH, twice with ultrapure water and dried with MgSO₄.

### 2.2. Equipment

#### 2.2.1. Nuclear magnetic resonance spectroscopy.

Liquid-state ¹H and ¹³C nuclear magnetic resonance (NMR) spectra were collected with a Bruker Advance 400-MHz spectrometer operating at 400 MHz. Solid-state ¹³C PMAS NMR spectra were collected with a Bruker Advance III spectrometer operating at 500 MHz with a double resonance CPMAS probehead. Chemical shifts (δ) are given in ppm. The multiplicity of signals in the ¹H NMR spectra are assigned as follows: singlet (s), doublet (d), triplet (t), and multiplet (m).

#### 2.2.2. UV–visible spectroscopy

UV–visible (UV–Vis) absorption spectra were collected with an Agilent 8453 UV–Vis spectrophotometer equipped with a thermostatic cuvette holder. UV–Vis spectra were collected at 80 °C in the wavelength range 300–900 nm, with a resolution of 1 nm. The samples were placed in a glass cuvette with a length path of 1 cm.

#### 2.2.3. Fourier transform infrared spectroscopy

Fourier transform infrared (FTIR) spectra were collected with a PerkinElmer FTIR Frontier spectrometer equipped with the attenuated total reflectance (ATR) sampling accessory. FTIR spectra were collected in the range of 4000–400 cm⁻¹ as result of an average of 8 scans with a resolution of 4 cm⁻¹.

#### 2.2.4. Inductively coupled plasma-mass spectroscopy

Metal ion concentration in the adsorption experiments were measured with inductively coupled plasma-mass spectroscopy (ICP-MS) using an Agilent 7900 spectrometer. The samples were diluted with ultrapure water containing 1 % of HCl and 1 % of HNO₃ in 10 mL metal-free tubes (Sarstedt). Each metal concentration was obtained as the average of three measurements of the same sample. The metal concentration measurements with a relative standard deviation (RSD) ≤ 5 % were considered as reliable data for the adsorption experiments.

#### 2.2.5. Scanning electron microscopy

The surface morphology figures of polymers were captured with
scanning electron microscopy (SEM) using a Hitachi SU3500 scanning electron microscope. The polymer particles were metallized with gold to increase their conductivity.

### 2.2.6. Nitrogen adsorption-desorption measurements

The nitrogen adsorption-desorption measurements were carried out at -196.15 °C on a Micromeritics Gemini V analyser. The sample was pre-conditioned at 80 °C under vacuum to eliminate the adsorbed gases and humidity. The specific surface area and pore size distribution of polymer particles were determined with Brunauer-Emmett-Teller (BET) [28] and Barrett-Joyner-Halenda (BJH) [29] methods.

### 2.3. Synthesis of a novel chelating monomer

The synthesis of bis-AMP-MMA monomer was carried out as follows. Two different solutions, named solution A and solution B, were prepared under argon atmosphere. Solution A was prepared by dissolving 0.89 mL (9.24 mmol) of methacryloyl chloride in 30 mL of anhydrous dichloromethane contained in a three-neck round-bottom flask cooled in an ice bath with NaCl. Solution B was prepared by dissolving 1.84 g (9.24 mmol) of bis-AMP and 1.30 mL (9.24 mmol) of trimethylamine in 10 mL of anhydrous dichloromethane. Solution B was then poured dropwise into the three-neck round-bottom flask containing solution A and stirred for 24 h under argon atmosphere. The reaction product was washed three times with water and dried with MgSO₄. 0.99 g of bis-AMP-MMA were finally obtained by removing the dichloromethane with a rotary evaporator (yield = 51 %) and characterized by liquid-state ¹H NMR and ¹³C NMR spectroscopies.

¹H NMR peaks (Fig. S.1.a) (CDCl₃, 400 MHz) δ (ppm): 8.50 (d, H₁, 2-pyridine), 8.09 (d, H₄, 2-pyridine), 7.76 (m, H₃, 2-pyridine), 7.35 (t, H₅, imidazole), 7.23 (t, H₂, 2-pyridine), 7.21 (m, H₅, imidazole), 5.43 (s, H₇, ethylene), 5.12 (s, H₆, ethylene), 2.13 (s, H₈, methyl).

¹³C NMR peaks (Fig. S.1.b) (CDCl₃, 400 MHz) δ (ppm): 173.3 (C₇, carbonyl), 156.7 (C₅',5, 2-pyridine), 149.6 (C₁',1, 2-pyridine), 140.4 (C₉, 1-ethylene), 136.6 (C₃',3, 2-pyridine), 122.4 (C₄',4, 2-pyridine), 121.1 (C₂',2, 2-pyridine), 115.5 (C₁₀, 2-ethylene), 54.3 (C₆', methylene), 49.7 (C₆, methylene), 20.7 (C₈, methyl).

### 2.4. Polymerization test

As a first step, 30 mL of mineral oil, previously flushed with argon, was introduced in a three-neck round bottom flask and heated at 80 °C under argon atmosphere. In a separate flask, 186 mg (0.695 mmol) of bis-AMP-MMA, 626 mg (6.26 mmol) of MMA and 4 mg (0.024 mmol) of AIBN were dissolved in 0.7 mL of DMSO-d₆ to form the pre-polymerization mixture. The monomers solution was stirred for 1 h under argon atmosphere and then quickly added to the pre-heated reactor to start the polymerization. The whole reaction lasted 48 h and two intermediate samplings were performed after 6 and 24 h. Both the intermediate sampling and the final reaction product were finally analyzed with ¹H NMR spectroscopy.

### 2.5. Complex formation study

A total of 15 samples were prepared as DMSO solutions in 3 mL glass cuvettes and were analyzed with a UV-Vis spectrometer in the wavelength range of 300-900 nm at 80 °C. The set of 15 UV-Vis spectra contained 13 spectra recorded at different bis-AMP-MMA/Co(II) ratios, while maintaining their combined concentrations at 0.1 M. Additionally, each dataset was supplemented with the spectra of bis-AMP-MMA and the Co(II) ions, both at a concentration of 0.1 M. The UV-Vis spectra related to bis-AMP-MMA complex formation study are shown in Fig. S.2. The UV-Vis spectra were finally processed with HypSpec program to extrapolate the complexes distribution curves (Fig. S.3) and to determine their stability constants at equilibrium, as previously described in the literature [30-32].

### 2.6. Synthesis of the chelating resin and poly(EGDMA)

The chelating resin based on bis-AMP-MMA (poly(bis-AMP-MMA-co-EGDMA)) was prepared via inverse suspension polymerization. 127 mL of mineral oil (continuous phase) were introduced into the reactor and heated to 80 °C under an argon atmosphere. In a separate flask, 743 mg (2.78 mmol) of bis-AMP-MMA and 4.5 mL of EGDMA (25.05 mmol) were dissolved in 3 mL of DMSO and stirred for 1 h under argon. After this time, 15 mg (0.09 mmol) of AIBN were added to form the pre-polymerization mixture (dispersed phase). The dispersed phase was quickly introduced into the reactor, and the polymerization was carried out for 6 h at 80 °C with a stirring rate of 400 rpm. The reaction product, consisting of polymer particles, was vacuum-filtered, washed with chloroform, and purified with a 1:1 acetone-chloroform mixture in a Soxhlet extractor for 24 h. The polymer particles were washed 5 times with 40 mL of 3 M NH₄OH and MilliQ water over a 24-h period. Finally, the poly(bis-AMP-MMA-co-EGDMA) particles were dried under vacuum at 50 °C, resulting in 2.6 g of dry product.

Poly(EGDMA) was prepared following the same procedure described for poly(bis-AMP-MMA-co-EGDMA), apart from the absence of bis-AMP-MMA in the pre-polymerization mixture, obtaining 2.8 g of dry product.

### 2.7. Co(II) adsorption experiments

The metal ion adsorption experiments were performed with poly(bis-AMP-MMA-co-EGDMA) particles. A fixed amount of adsorbent and 10 mL of Co(II) ion solution were introduced in a 15 mL Corning® tube. The amount of adsorbent was equal to 10 mg and the metal ions were introduced in solution as nitrate salts. The 15 mL Corning® tube was shaken at 200 rpm for 20 h at room temperature. After this, the obtained dispersion was centrifuged at 5000 rpm for 5 min and filtered with a 0.45 µm surfactant-free cellulose acetate filter. The filtered solution was finally diluted with ultrapure water containing 1 % of HCl and 1 % of HNO₃ in 10 mL metal-free tubes and analyzed with ICP-MS. The adsorption capacity values qₐ were calculated using Eq. 1.

$$q_a = \frac{(C_0 - C_e)}{m} \times V \quad (1)$$

where C₀ and Cₐ are the initial metal ion concentration and the metal ion concentration at equilibrium, respectively, m is the mass of the adsorbent, and V is the volume of the solution.

#### 2.7.1. Effect of pH

The effect of pH on the adsorption capacity was determined performing adsorption experiments on the target ion solutions prepared with different pH values. The target ion solution pH was adjusted with concentrated NaOH or HCl solutions.

#### 2.7.2. Effect of contact time on adsorption

The effect of contact time on adsorption was investigated by performing adsorption experiments at different contact time, ranging from 5 min to 24 h. The metal solutions pH value was equal to 6.5 and was adjusted with concentrated NaOH.

#### 2.7.3. Adsorption isotherms

The adsorption isotherms were obtained by performing adsorption experiments at different target ion concentrations, ranging from 0.02 to 1.0 g/L. The pH value was adjusted to 6.5 with concentrated NaOH. Data related to the adsorption isotherms were fitted using both the Langmuir (Eq. 2) [33] and Freundlich (Eq. 3) [34] models.

$$q_a = \frac{q_m K_L C_e}{1 + K_L C_e} \quad (2)$$

$$q_a = K_f C_e^{1/n} \quad (3)$$
where q_m is the maximum adsorption capacity, K_L is the Langmuir constant, K_F is the Freundlich constant related to the adsorption capacity and n is related to the adsorption strength.

### 2.7.4. Selectivity

The selectivity of the adsorbent was studied by performing adsorption experiments on binary solutions containing Co(II) coupled, in separate experiments, with Ni(II), Cd(II), and Mn(II) as competitive ions. Co(II) ion concentration was equal to 0.01 g/L in each selectivity experiment. Two different Co(II)/competitive ion ratios were investigated, including the values 1:1, 1:10. The pH values were adjusted to 6.5 with concentrated NaOH. Selectivity coefficient values k were calculated with Eq. 4.

$$k = \frac{K_{d,Co(II)}}{K_{d,Ni(II)}} \qquad (4)$$

where $K_{d,Co(II)}$ and $K_{d,Ni(II)}$ are the distribution coefficients of Co(II) ion and Ni(II) ions, respectively. The $K_d$ values were calculated with Eq. (5).

$$K_d = \frac{q_e}{c_e} \qquad (5)$$

### 2.7.5. Adsorbent reusability

The reusability of the adsorbent was studied for up to 5 adsorption-desorption cycles using NH₄OH 3 M as leaching agent. Each regeneration cycle consisted of an adsorption experiment followed by a 30 min wash with 10 mL of leaching agent in 15 mL Corning® tubes at 200 rpm. The polymer particles were then rinsed with MilliQ water and dried in a vacuum oven at 60 °C for 24 h before being applied in the following adsorption-desorption cycle.

The adsorption efficiency (adsorption efficiency_n%) was calculated after each adsorption-desorption cycle with Eq. (6):

$$\text{Adsorption efficiency}_n\% = \frac{q_{e,n}}{q_{e,1}} \qquad (6)$$

where $q_{e,n}$ is the adsorption capacity of the cycle n (with n equal to 1, 2, 3, 4 or 5) and $q_{e,1}$ is the adsorption capacity of the first cycle.

## 3. Results and discussion

### 3.1. Synthesis of a novel chelating monomer and polymerization test

Bis-AMP-MMA monomer was synthesized by coupling the bis-AMP ligand with methacryloyl chloride. Through this process, a methacrylic group was introduced into the bis-AMP structure, as illustrated in Fig. 2. This was confirmed by NMR by the presence of the vinyl protons peak at 5.18 ppm in the ¹H NMR spectrum (Fig. S.1.a), and by the vinyl carbon peaks at 140.4 ppm and 115.5 ppm in the ¹³C NMR spectrum (Fig. S.1.b).

<figure><img src="image_3.png" /><figcaption>Fig. 2. Synthesis of bis-AMP-MMA monomer.</figcaption></figure>

The synthesis of bis-AMP-MMA was followed by its copolymerization with MMA to evaluate its applicability in polymer preparation. The choice of MMA was driven by the similarity in its structure to EGDMA [35], the crosslinker used for the preparation of poly(bis-AMP-MMA-co-EGDMA). Additionally, the choice of MMA as comonomer allowed for the preparation of a linear and soluble copolymer whose composition could be easily characterized by liquid ¹H NMR. bis-AMP-MMA was introduced in a 1:9 ratio compared to MMA. The copolymer was prepared by inverse suspension polymerization, using DMSO-d₆ for the dispersed phase containing the monomers. These polymerization conditions were chosen to closely resemble those applied for the preparation of poly(bis-AMP-MMA-co-EGDMA). The whole test lasted 48 h, with intermediate sampling after 6 h and 24 h. The intermediate sampling and the final reaction product were analyzed with ¹H NMR spectroscopy (Fig. S.4) to determine the conversion percentage of each monomer into polymer and the fraction of bis-AMP-MMA within the copolymer ($F_{bis-AMP-MMA}$) at different reaction times (Table 1). A detailed description of the calculations to determine the conversion percentage of both monomers and $F_{bis-AMP-MMA}$ is provided in the supplementary data.

From the initial 1:9 bis-AMP-MMA/MMA ratio, a $F_{bis-AMP-MMA}$ into the final copolymer close to 10 % could be expected. However, after 6 h of reaction, only 14 % of the initial bis-AMP-MMA was converted into monomer and the $F_{bis-AMP-MMA}$ was limited to 1.7 % (Table 1). The limited reactivity of bis-AMP-MMA might arise from its high steric hindrance. When the reaction time was increased to 24 h and 48 h, only minor variations in $F_{bis-AMP-MMA}$ values were observed (Table 1). For this reason, a reaction time of 6 h was chosen for the preparation of poly (bis-AMP-MMA-co-EGDMA).

### 3.2. Complex formation study

The adsorption performance and selectivity of a chelating adsorbent are influenced by the strength and stability of the complexes formed by its chelating functions with the target ion. Therefore, investigating the formation of the complex between a new chelating monomer and the target ion is a useful tool to predict the efficiency of the corresponding new adsorbent.

The formation of monomer/metal complexes can be easily studied with an *in-situ* spectroscopic method, as previously described in literature [30–32]. The structure and species distribution of bis-AMP-MMA complexes with Co(II) was determined by analyzing the UV–Vis

<table><caption>Table 1
Time dependence of monomers conversion percentage and $F_{bis-AMP-MMA}$.</caption><thead><tr><td>Reaction time</td><td>bis-AMP-MMA conversion</td><td>MMA conversion</td><td>$F_{bis-AMP-MMA}$</td></tr></thead><tbody><tr><td>6 h</td><td>14.0 %</td><td>90.4 %</td><td>1.7 %</td></tr><tr><td>24 h</td><td>16.0 %</td><td>92.7 %</td><td>1.9 %</td></tr><tr><td>48 h</td><td>16.0 %</td><td>92.9 %</td><td>1.9 %</td></tr></tbody></table>
absorption spectra at different Co(II)/bis-AMP-MMA ratios (Fig. S.2), varying from 0.1 to 5.3 in the polymerization solvent (DMSO) at 80 °C. The complex distribution curves calculated with the HypSpec program at different Co(II)/bis-AMP-MMA ratios, together with the curves related to uncomplexed Co(II) ion and uncomplexed bis-AMP-MMA, are depicted in Fig. S.3. The complex formation study revealed that bis-AMP-MMA can form both 1:1 and 1:2 stoichiometry complexes with Co(II) ions, while 1:3 complexes were not detected. A similar result was also reported by Romary et al. for bis-AMP, who studied the formation of complexes of bis-AMP with several transition metals, including Co(II) ions [25]. When comparing bis-AMP-MMA and bis-AMP complexes with Co(II) ions, the 1:1 complexes had similar stability constants (7.72 ± 0.08 for bis-AMP-MMA and 7.74 for bis-AMP [25]). On the other hand, the stability constant for the 1:2 complex of bis-AMP-MMA with Co(II) ions was significantly higher (13.93 ± 0.17) than the one obtained with bis-AMP (5.31 [25]). As shown in Fig. 1, the main structural difference between bis-AMP and bis-AMP-MMA lies in the presence of the methacrylic group, which introduces a carbonyl functionality in the latter. This structural variation suggests that, in addition to the three nitrogen atoms already present in bis-AMP, the carbonyl oxygen in bis-AMP-MMA can also participate in the coordination of Co(II) ions, potentially leading to increased stability of the 1:2 complexes. From this result an efficient chelation of Co(II) ions by poly(bis-AMP-MMA-co-EGDMA) could be expected.

### 3.3. Synthesis and chemical characterization of poly(bis-AMP-MMA-co-EGDMA)

The adsorbent poly(bis-AMP-MMA-co-EGDMA) was prepared via inverse suspension polymerization introducing the functional monomer and the crosslinker, bis-AMP-MMA and EGDMA respectively, in a ratio equal to 1:9. The synthesis route for the preparation of poly(bis-AMP-MMA-co-EGDMA) is illustrated in Fig. 3.

The chemical structure of poly(bis-AMP-MMA-co-EGDMA) was studied by FTIR (Fig. 4.a) and ¹³C CP-MAS NMR (Fig. 4.b) spectroscopies. The presence of bis-AMP-MMA within the adsorbent particles was verified by comparing its spectra with those of poly(EGDMA).

The presence of poly(EGDMA) in the FTIR spectra was identified by its characteristic absorption bands (Fig. 4.a). These included the C=O stretching band at 1725 cm⁻¹, the C—O stretching band at 1144 cm⁻¹, and the -OCH₂ deformation vibration band at 1460 cm⁻¹ [36]. In poly (bis-AMP-MMA-co-EGDMA), distinct C=C and C≡N pyridine stretching bands at 1571 cm⁻¹ and 1591 cm⁻¹, as well as C=N aromatic amine stretching bands at 1295 cm⁻¹ and 1319 cm⁻¹, were present. These bands were not visible in the spectrum of poly(EGDMA), used as a reference material.

The presence of bis-AMP-MMA within the adsorbent particles was

also confirmed by the ¹³C CPMAS NMR spectrum of this material (Fig. 4.b). Poly(EGDMA) was the major component, and its characteristic signals were present in the spectrum. These signals included the carbonyl carbon (C=O) peak at 177.1 ppm, the pending vinyl carbon (C=CH₂) peak at 137.1 ppm, the ester carbon (—COO—) peak at 62.9 ppm, the quaternary carbon (>C<) peak at 45.8 ppm, the methylene carbon (—CH₂—) peak at 24.5 ppm, and the methyl carbon (—CH₃) peak at 18.4 ppm [37,38]. The presence of bis-AMP-MMA within the adsorbent was confirmed by the pyridine peaks in the aromatic carbon zone, including a weak peak at 157.9 ppm, along with a more intense peak at 124.5 ppm, which were not observed in the poly(EGDMA) spectrum. This result was consistent with the observation conducted with FTIR spectroscopy and provided an additional validation about the correct integration of bis-AMP-MMA within poly(bis-AMP-MMA-co-EGDMA) particles.

### 3.4. Morphological characterization of polymer particles

The chemical characterization of bis-AMP-MMA polymer particles was followed by the study of their surface morphology with SEM (Fig. 5.a). Additionally, the specific surface area, pore size, and pore volume were determined through nitrogen adsorption/desorption experiments (Fig. 5.b).

SEM images of poly(bis-AMP-MMA-co-EGDMA) (Fig. 5a) displayed an irregular, porous surface of the particles. The nitrogen adsorption-desorption isotherm (Fig. 5.b) exhibited, according to IUPAC classification [39], a Type II isotherm shape with a hysteresis loop H4. This result suggested the presence of narrow slit-like pores. The BJH [29] and BET [28] methods were used to determine the average pore diameters, total pore volumes, and specific surface areas of the adsorbent particles. The particles of poly(bis-AMP-MMA-co-EGDMA) presented a porous structure with a specific surface area of 153.9 m²/g and a total pore volume equal to 0.190 cm³/g. The average pore diameter was 4.9 nm, which classifies poly(bis-AMP-MMA-co-EGDMA) as a mesoporous material.

### 3.5. Co(II) adsorption study

The Co(II) adsorption experiments carried out with poly(bis-AMP-MMA-co-EGDMA) encompassed the effect of pH (Fig. 6.a), the time dependence of Co(II) adsorption (Fig. 6.b), the determination of Co(II) adsorption isotherms (Fig. 6.c), and the assessments of polymer selectivity and reusability (Fig. 6.d).

#### 3.5.1. Effect of pH and contact time dependence on Co(II) adsorption.

The first adsorption experiments were carried out to determine the optimum pH range and contact time for Co(II) adsorption with poly(bis-

<figure><img src="image_4.png" /><figcaption>Fig. 3. Synthesis route for the preparation of poly(bis-AMP-MMA-co-EGDMA).</figcaption></figure>
<figure><img src="image_5.png" /><figcaption>Fig. 4. a) FTIR spectra of poly(bis-AMP-MMA-co-EGDMA) and poly(EGDMA); b) $^{13}$C CPMAS NMR spectra of poly(bis-AMP-MMA-co-EGDMA) and poly(EGDMA).</figcaption></figure>

AMP-MMA-co-EGDMA) particles. The effect of pH on Co(II) adsorption capacity of poly(bis-AMP-MMA-co-EGDMA) was investigated within the pH range of 3.5 to 8 (Fig. 6.a). The most favorable adsorption performance was observed within pH 5 and 8, with a decrease in performance at pH 3.5. Given the stable performance in this pH range, pH 6.5 was selected for conducting the following adsorption experiments.

The impact of contact time on Co(II) adsorption capacity was investigated through adsorption experiments of varying durations up to 1500 min (Fig. 6.b). The adsorption capacity of Co(II) ions increased during the initial two hours and then reached a plateau. For this reason, the following adsorption experiments were carried out with a contact time exceeding two hours to ensure an efficient Co(II) adsorption.

### 3.5.2. Co(II) adsorption isotherms

The maximum adsorption capacity of poly(bis-AMP-MMA-co-EGDMA) was determined at room temperature through Co(II) adsorption experiments conducted at increasing initial concentration of Co(II) ions (Fig. 6.c). The adsorption capacity of poly(bis-AMP-MMA-co-EGDMA) exhibited a sharp increase in the Co(II) concentration range of 20–450 mg/L, after which a minor variation was observed. The maximum adsorption capacity was determined by fitting the adsorption data with the Langmuir model (Eq. 2, Table 2).

Langmuir's model offered a better correlation of the adsorption data in comparison to Freundlich's model (Table 2). This suggested the

presence of homogeneous binding sites with uniform adsorption energy and no interaction between adsorbate molecules on adjacent sites. The maximum adsorption capacity of poly(bis-AMP-MMA-co-EGDMA) was equal to 54.0 mg/g (916 µmol/g). When compared with the adsorption capacity of other Co(II) adsorbents described in the literature (Table 3), this value falls within the middle range of values.

### 3.5.3. Selectivity

The selectivity of poly(bis-AMP-MMA-co-EGDMA) was investigated at pH 6.5 using bi-component solutions containing Co(II) paired with one divalent metal ion M(II) among Ni(II), Cd(II), and Mn(II). The selectivity coefficient k (Eq. 4) values for each M(II)/Co(II) pair at different M(II)/Co(II) ratios are reported in Table 4.

When the M(II)/Co(II) ratio was set to 1, the adsorption of Co(II) resulted in selectivity coefficient values above 1 for all the examined competitive ions (Table 4). This observation indicated that this adsorbent exhibited a preference for Co(II) adsorption over other metal ions when introduced at equal concentrations into the solution. The selectivity of poly(bis-AMP-MMA-co-EGDMA) for Co(II) ions was also observed in presence of an excess of competitive ions (M(II)/Co(II) molar ratio set to 9). At this condition, there was only a slight variation in the selectivity coefficient values of poly(bis-AMP-MMA-co-EGDMA) with Ni(II), Cd(II), and Mn(II) as a competitive ions (Table 4). The results of this study made poly(bis-AMP-MMA-co-EGDMA) a promising
<figure><img src="image_6.png" /><figcaption>Fig. 5. a) SEM pictures of the surface polymer particles of poly(bis-AMP-MMA-co-EGDMA). b) Nitrogen adsorption-desorption isotherms of poly(bis-AMP-MMA-co-EGDMA) polymer particles.</figcaption></figure>

adsorbent for Co(II) ions in presence of other competitive species, an experimental condition which normally occurs in hydrometallurgical separation processes.

### 3.5.4. Reusability of the adsorbent

The regeneration of poly(bis-AMP-MMA-co-EGDMA) was studied through 5 Co(II) adsorption-desorption cycles (Fig. 6.d). The adsorption experiments were followed by the desorption of Co(II) ions with 3 M $NH_4OH$ as the Co(II) leaching agent. The material reusability was assessed by calculating the residual adsorption capacity after each adsorption cycle in relation to the first cycle with Eq. 6. The adsorption capacity decreased by 7 % after the first adsorption-desorption cycle, with a gradual decline in efficiency that led to a 33 % drop in performance by the fourth cycle. The performance reduction was stabilized in the fifth cycle, with a reduction of 35 %. This study demonstrated that this material can be efficiently reused for three adsorption-desorption cycles before facing a more significant performance drop in the fourth cycle.

## 4. Conclusions

In this work, a new chelating monomer based on bis(2-pyridylmethyl)amine (bis-AMP) ligand was synthesized and applied for the preparation of a Co(II) adsorbent. The new monomer, bis-AMP-MMA, was characterized through NMR spectroscopy to confirm the introduction of the polymerizable methacrylic group into its structure. A complex formation study between bis-AMP-MMA monomer and Co(II) ions revealed the formation of stable 1:1 and 1:2 metal/chelator stoichiometry complexes. The comparison of bis-AMP-MMA and bis-AMP complexes with Co(II) ions revealed similar stability constants for the 1:1 complexes. However, the bis-AMP-MMA 1:2 complex exhibited a much higher stability constant compared to bis-AMP, likely due to the enhanced chelating effect of the carbonyl group. The copolymerization of bis-AMP-MMA with EGDMA, used as crosslinker, led to the preparation of a new adsorbent resin named poly(bis-AMP-MMA-co-EGDMA).

The morphology of the new resin was studied with SEM and nitrogen adsorption-desorption experiments. The material was characterized by high roughness and its specific surface area, determined with BET method, was equal to 153.9 m²/g. The adsorbent resin was finally applied in Co(II) adsorption experiments. The maximum Co(II) adsorption capacity, determined at pH 6.5, was equal to 54.0 mg/g, standing in the middle range for Co(II) adsorbents. The new resin showed selectivity for Co(II) ions in presence of Ni(II), Cd(II) and Mn(II) as competitive ions, with selectivity coefficient values in the 1.13–2.09 range. Finally, the reusability of the material was tested up to five Co(II) adsorption-desorption cycles. The efficiency was reduced by only 7 % after the first cycle and up to 35 % in the fifth cycle, showing a good reusability of the new resin. These results suggest that the bis-AMP-MMA monomer and its corresponding resin are promising candidates for efficient cobalt recovery, contributing to advancements in hydrometallurgical separation processes.

### CRediT authorship contribution statement

**A. Giove:** Writing – review & editing, Writing – original draft, Methodology, Investigation, Formal analysis, Data curation, Conceptualization. **K. Laatikainen:** Writing – review & editing, Writing – original draft, Supervision, Project administration, Methodology, Investigation, Funding acquisition, Data curation, Conceptualization. **F. Ibrahim:** Writing – review & editing, Visualization, Investigation. **Y. El Ouardi:** Writing – review & editing, Visualization, Investigation. **A. Sala:** Writing – review & editing, Visualization, Investigation. **S. Hietala:** Writing – review & editing, Visualization, Investigation. **E. Repo:** Writing – review & editing, Visualization, Project administration. **C. Branger:** Writing – review & editing, Writing – original draft, Supervision, Project administration, Methodology, Investigation, Data curation, Conceptualization.
<figure><img src="image_7.png" /><figcaption>Fig. 6. a) Effect of pH on Co(II) adsorption capacity of poly(bis-AMP-MMA-co-EGDMA). Initial concentration of Co(II) ions = 1 g/L. b) Impact of contact time on the Co(II) adsorption capacity of poly(bis-AMP-MMA-co-EGDMA) at pH 6.5. Initial concentration of Co(II) ions = 500 mg/L. c) Co(II) adsorption with poly(bis-AMP-MMA-co-EGDMA) at pH 6.5. Adsorption data interpolated with Langmuir (dashed lines) and Freundlich (continuous lines) models. Range of initial Co(II) ions concentration: 20–1050 mg/L. d) Adsorption efficiency of poly(bis-AMP-MMA-co-EGDMA) in 5 adsorption-desorption cycles at pH 6.5 with 3 M NH₄OH as leaching agent. Initial concentration of Co(II) ions = 1 g/L.</figcaption></figure>

**Table 2**

Langmuir and Freundlich models parameters for Co(II) adsorption with poly(bis-AMP-MMA-co-EGDMA) at pH 6.5.

<table><thead><tr><th>Model</th><th>Parameter</th><th></th></tr></thead><tbody><tr><td rowspan="3">Langmuir</td><td>q<sub>e,max</sub> (mg/g)</td><td>54.0</td></tr><tr><td>K<sub>L</sub></td><td>0.008</td></tr><tr><td>R<sup>2</sup></td><td>0.926</td></tr><tr><td rowspan="3">Freundlich</td><td>1/n</td><td>0.783</td></tr><tr><td>K<sub>F</sub></td><td>0.114</td></tr><tr><td>R<sup>2</sup></td><td>0.893</td></tr></tbody></table>

**Table 3**

Maximum Co(II) adsorption capacity values of adsorbents described in literature and the one prepared in this work.

<table><thead><tr><th>Adsorbent</th><th>q<sub>max</sub><br>(mg/g)</th><th>Ref.</th></tr></thead><tbody><tr><td>Lignosulfonate-chitosan</td><td>386.0</td><td>[40]</td></tr><tr><td>(3-(((5-ethoxybenzenethiol)imino)methyl)-salicylic acid) anchored on mesoporous silica</td><td>185.2</td><td>[41]</td></tr><tr><td>Ethylenediaminetetraacetic acid-chitosan</td><td>63.0</td><td>[42]</td></tr><tr><td>Poly(bis-AMP-MMA-co-EGDMA)</td><td>54.0</td><td>This paper</td></tr><tr><td>Isatin-chisotan</td><td>53.5</td><td>[43]</td></tr><tr><td>Diethylenetriaminepentaacetic acid-chitosan</td><td>49.1</td><td>[42]</td></tr><tr><td>Kaolinite clay</td><td>30.5</td><td>[44]</td></tr><tr><td>Graphene oxide</td><td>21.3</td><td>[45]</td></tr><tr><td>Natural clay</td><td>12.9</td><td>[46]</td></tr></tbody></table>

**Table 4**

Selectivity coefficient values for Co(II) adsorption with poly(bis-AMP-MMA-co-EGDMA) at pH 6.5 with Ni(II), Cd(II), Mn(II), as competitive ions.

<table><thead><tr><th>M(II)</th><th>M(II)<br>Co(II)</th><th>k</th></tr></thead><tbody><tr><td>Ni(II)</td><td>1<br>9</td><td>1.50<br>2.09</td></tr><tr><td>Cd(II)</td><td>1<br>9</td><td>1.31<br>1.25</td></tr><tr><td>Mn(II)</td><td>1<br>9</td><td>1.13<br>1.37</td></tr></tbody></table>

**Declaration of competing interest**

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

**Acknowledgements**

The authors are thankful to the Academy of Finland for financial support through the project "New generation separation materials and sensors for environmental applications – Interactions between ligands and ions/molecules" (core project 23B350E9YT10).
# Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.reactfunctpolym.2025.106203.

## Data availability

Data will be made available on request.

## References

[1] R. Giannini, I.C. Freestone, A.J. Shortland, European cobalt sources identified in the production of Chinese famille rose porcelain, J. Archaeol. Sci. 80 (2017) 27–36, https://doi.org/10.1016/j.jas.2017.01.011.

[2] M. Llusar, A. Forés, J.A. Badenes, J. Calbo, M.A. Tena, G. Monrós, Colour analysis of some cobalt-based blue pigments, J. Eur. Ceram. Soc. 21 (2001) 1121–1130, https://doi.org/10.1016/S0955-2219(00)00295-8.

[3] M.-C. Corbeil, J.-P. Charland, E.A. Moffat, The characterization of cobalt violet pigments, Stud. Conserv. 47 (2002) 237–249, https://doi.org/10.1179/sic.2002.47.4.237.

[4] F. Du, B. Su, Further study of sources of the imported cobalt-blue pigment used on Jingdezhen porcelain from late 13 to early 15 centuries, Sci. China, Ser. E: Technol. Sci. 51 (2008) 249–259, https://doi.org/10.1007/s11431-008-0013-0.

[5] A. Tajne, T.V.K. Gupta, H. Ramani, Y. Joshi, A critical review on the machinability aspects of nickel and cobalt based superalloys in turning operation used for aerospace applications, Adv. Mater. Process. Technol. (2023) 1–34, https://doi.org/10.1080/2374068X.2023.2185850.

[6] H.A. Zaman, S. Sharif, M.H. Idris, A. Kamarudin, Metallic biomaterials for medical implant applications: a review, Appl. Mech. Mater. 735 (2015) 19–25, https://doi.org/10.4028/www.scientific.net/AMM.735.19.

[7] S.L.-F. Chan, T.L. Lam, C. Yang, S.-C. Yan, N.M. Cheng, A robust and efficient cobalt molecular catalyst for CO₂ reduction, Chem. Commun. 51 (2015) 7799–7801, https://doi.org/10.1039/C5CC00566C.

[8] Z. Gholami, Z. Tîsler, V. Rubàs, Recent advances in Fischer-Tropsch synthesis using cobalt-based catalysts: a review on supports, promoters, and reactors, Catal. Rev. 63 (2021) 512–595, https://doi.org/10.1080/01614940.2020.1762367.

[9] O. Gutfleisch, High-temperature samarium cobalt permanent magnets, in: Nanoscale Magnetic Materials and Applications, Springer US, Boston, MA, 2009, pp. 337–372, https://doi.org/10.1007/978-0-387-85600-1-12.

[10] X. Liu, Y. Tan, W. Wang, C. Li, Z.W. Seh, L. Wang, Y. Sun, Conformal prelithiation nanoshell on LiCoO₂ enabling high-energy lithium-ion batteries, Nano Lett. 20 (2020) 4558–4565, https://doi.org/10.1021/acs.nanolett.0c01413.

[11] S. Luo, K. Wang, J. Wang, K. Jiang, Q. Li, S. Fan, Binder-free LiCoO₂/carbon nanotube cathodes for high-performance Lithium ion batteries, Adv. Mater. 24 (2012) 2294–2298, https://doi.org/10.1002/adma.201104720.

[12] Cobalt Institute, Cobalt Mining. https://www.cobaltinstitute.org/about-cobalt/cobalt-life-cycle/cobalt-mining/, (Accessed 27 September 2023).

[13] M. Kaya, S. Hussaini, S. Kursunoglu, Critical review on secondary zinc resources and their recycling technologies, Hydrometallurgy 195 (2020) 105362, https://doi.org/10.1016/j.hydromet.2020.105362.

[14] V. Gunaratne, E.A. Rajapaksha, M. Vithanage, D.S. Alessi, R. Selvasembian, Mu. Naushad, S. You, P. Oleszczuk, Y.S. Ok, Hydrometallurgical processes for heavy metals recovery from industrial sludges, Crit. Rev. Environ. Sci. Technol. 52 (2022) 1022–1062, https://doi.org/10.1080/10643389.2019.1847949.

[15] B.S. Boyanov, V.V. Konareva, N.K. Kolev, Purification of zinc sulfate solutions from cobalt and nickel through activated cementation, Hydrometallurgy 73 (2004) 163–168, https://doi.org/10.1016/j.hydromet.2003.09.002.

[16] L. Zhang, L. Ji, L. Li, D. Shi, T. Xu, X. Peng, X. Song, Recovery of Co, Ni, and Li from solutions by solvent extraction with β-diketone system, Hydrometallurgy 204 (2021) 105718, https://doi.org/10.1016/j.hydromet.2021.105718.

[17] J.J. Eksteen, E.A. Oraby, V. Nguyen, Leaching, ion exchange based recovery of nickel and cobalt from a low grade, serpentine-rich sulfide ore using an alkaline glycine lixiviant system, Miner. Eng. 145 (2020) 106073, https://doi.org/10.1016/j.mineng.2019.106073.

[18] M.H. Dehghani, K. Yetilmezsoy, M. Salari, Z. Heidarinejad, M. Yousefi, M. Sillanpää, Adsorptive removal of cobalt(II) from aqueous solutions using multi-walled carbon nanotubes and γ-alumina as novel adsorbents: modelling and optimization based on response surface methodology and artificial neural network, J. Mol. Liq. 299 (2020) 112154, https://doi.org/10.1016/j.molliq.2019.112154.

[19] D.S. Flett, Solvent extraction in hydrometallurgy: the role of organophosphorus extractants, J. Organomet. Chem. 690 (2005) 2426–2438, https://doi.org/10.1016/j.jorganchem.2004.11.037.

[20] X. Chen, T. Zhou, J. Kong, H. Fang, Y. Chen, Separation and recovery of metal values from leach liquor of waste lithium nickel cobalt manganese oxide based cathodes, Sep. Purif. Technol. 141 (2015) 76–83, https://doi.org/10.1016/j.seppur.2014.11.039.

[21] A.B. Botelho Junior, D.B. Dreisinger, D.C.R. Espinosa, A review of nickel, copper, and cobalt recovery by chelating ion exchange resins from mining processes and mining tailings, Min. Metall. Explor. 36 (2019) 199–213, https://doi.org/10.1007/s42461-018-0016-8.

[22] S. Zarco-Fernández, M.J. Mancheño, R. Muñoz-Olivas, C. Cámara, A new specific polymeric material for mercury speciation: application to environmental and food

samples, Anal. Chim. Acta 897 (2015) 109–115, https://doi.org/10.1016/j.aca.2015.09.016.

[23] A. Bhaskarapillai, N.V. Sevilimedu, B. Sellergren, Synthesis and characterization of imprinted polymers for radioactive waste reduction, Ind. Eng. Chem. Res. 48 (2009) 3730–3737, https://doi.org/10.1021/ie806140b.

[24] W. Meouche, C. Branger, I. Burroies, R. Denoyel, A. Margaillan, Inverse suspension polymerization as a new tool for the synthesis of ion-imprinted polymers, Macromol. Rapid Commun. 33 (2012) 928–932, https://doi.org/10.1002/marc.201200039.

[25] J.K. Romary, J.D. Barger, J.E. Bunds, New multidentate .alpha.-pyridyl ligand. Coordination of bis(2-pyridylmethyl)amine with transition metal ions, Inorg. Chem. 7 (1968) 1142–1145, https://doi.org/10.1021/ic50064a08.

[26] Z. Liu, L. Wang, Y. Lv, X. Xu, C. Zhu, F. Liu, A. Li, Impactful modulation of micro-structures of acid-resistant picolylamine-based chelate resins for efficient separation of heavy metal cations from strongly acidic media, Chem. Eng. J. 420 (2021) 129684, https://doi.org/10.1016/j.cej.2021.129684.

[27] K.C. Sole, M.B. Mooiman, E. Hardwick, Ion exchange in hydrometallurgical processing: an overview and selected applications, Sep. Purif. Rev. 47 (2018) 159–178, https://doi.org/10.1080/15422119.2017.1354304.

[28] S. Brunauer, P.H. Emmett, E. Teller, Adsorption of gases in multimolecular layers, J. Am. Chem. Soc. 60 (1938) 309–319, https://doi.org/10.1021/ja01269a023.

[29] E.P. Barrett, L.G. Joyner, P.P. Halenda, The determination of pore volume and area distributions in porous substances. I. Computations from nitrogen isotherms, J. Am. Chem. Soc. 73 (1951) 373–380, https://doi.org/10.1021/ja01145a126.

[30] K. Laatikainen, C. Branger, B. Coulomb, V. Lenoble, T. Sainio, In situ complexation versus complex isolation in synthesis of ion imprinted polymers, React. Funct. Polym. 122 (2018) 1–8, https://doi.org/10.1016/j.reactfunctpolym.2017.10.022.

[31] M. Laatikainen, K. Laatikainen, S.-P. Reinikainen, H. Hyvönen, C. Branger, H. Siren, T. Sainio, Complexation of nickel with 2-(Aminomethyl)pyridine at high zinc concentrations or in a nonaqueous solvent mixture, J. Chem. Eng. Data 59 (2014) 2207–2214, https://doi.org/10.1021/je500164h.

[32] P. Gans, A. Sabatini, A. Vacca, SUPERQUAD: an improved general program for computation of formation constants from potentiometric data, J. Chem. Soc. (1985) 1195–1200, https://doi.org/10.1039/dt9850001195.

[33] I. Langmuir, The adsorption of gases on plane surfaces of glass, mica and platinum, J. Am. Chem. Soc. 345 (1918) 1361–1403.

[34] H. Freundlich, Über die Adsorption in Lösungen, Z. Phys. Chem. 57 (1906) 385–470.

[35] U.S. Remalow, S. Pingili, Synthesis of ethylene glycol Dimethacrylate-methyl methacrylate copolymers, determination of their reactivity ratios, and a study of dopant and temperature effects on their conductivities, Polymers (Basel) 2 (2010) 265–285, https://doi.org/10.3390/polym2030265.

[36] W. Meouche, K. Laatikainen, A. Margaillan, T. Silvonen, H. Siren, T. Sainio, I. Burroies, R. Denoyel, C. Branger, Effect of porogen solvent on the properties of nickel ion imprinted polymer materials prepared by inverse suspension polymerization, Eur. Polym. J. 87 (2017) 124–135, https://doi.org/10.1016/j.eurpolymj.2016.12.022.

[37] W. René, V. Lenoble, K. Laatikainen, B. Viguier, C. Branger, Influence of the synthesis parameters on the efficiency of fluorescent ion-imprinted polymers for lead detection, React. Funct. Polym. 170 (2022) 105134, https://doi.org/10.1016/j.reactfunctpolym.2021.105134.

[38] J. Chen, C. Zhao, H. Huang, M. Wang, X. Ge, Highly crosslinked poly (ethyleneglycol dimethacrylate)-based microspheres via solvothermal precipitation polymerization in alcohol-water system, Polymer (Guildf) 83 (2016) 214–222, https://doi.org/10.1016/j.polymer.2015.12.028.

[39] K.S.W. Sing, Reporting physisorption data for gas/solid systems with special reference to the determination of surface area and porosity (Recommendations 1984), Pure Appl. Chem. 57 (1985) 603–619, https://doi.org/10.1351/pac198557040603.

[40] R. Mu, B. Liu, X. Chen, N. Wang, J. Yang, Adsorption of Cu (II) and Co (II) from aqueous solution using lignosulfonate/chitosan adsorbent, Int. J. Biol. Macromol. 163 (2020) 120–127, https://doi.org/10.1016/j.ijbiomac.2020.06.260.

[41] Md.R. Awual, Md.M. Hasan, A. Islam, A.M. Asiri, M.M. Rahman, Optimization of an innovative composited material for effective monitoring and removal of cobalt (II) from wastewater, J. Mol. Liq. 298 (2020) 112035, https://doi.org/10.1016/j.molliq.2019.112035.

[42] E. Repo, J.K. Warchol, T.A. Kurniawan, M.E.T. Sillanpää, Adsorption of Co(II) and Ni(II) by EDTA- and/or DTPA-modified chitosan: kinetic and equilibrium modeling, Chem. Eng. J. 161 (2010) 73–82, https://doi.org/10.1016/j.cej.2010.04.030.

[43] M. Monier, D.M. Ayad, Y. Wei, A.A. Sarhan, Adsorption of Cu(II), Co(II), and Ni(II) ions by modified magnetic chitosan chelating resin, J. Hazard. Mater. 177 (2010) 962–970, https://doi.org/10.1016/j.jhazmat.2010.01.012.

[44] E. Ajenifuja, J.A. Ajao, E.O.B. Ajayi, Equilibrium adsorption isotherm studies of Cu (II) and Co (II) in high concentration aqueous solutions on Ag-TiO2-modified kaolinite ceramic adsorbents, Appl Water Sci 7 (2017) 2279–2286, https://doi.org/10.1007/s13201-016-0403-6.

[45] L.P. Lingdaminne, J.R. Koduru, H. Roh, Y.-L. Choi, Y.-Y. Chang, J.-K. Yang, Adsorption removal of Co(II) from waste-water using graphene oxide, Hydrometallurgy 165 (2016) 90–96, https://doi.org/10.1016/j.hydromet.2015.10.021.

[46] S.A. Al-Jlil, Adsorption of cobalt ions from waste water on activated Saudi clays, Appl Water Sci 7 (2017) 383–391, https://doi.org/10.1007/s13201-014-0253-z.