Friederike Kriese,1Stephan Lassen,1and Bernd Niemeyer1Show more
Academic Editor: Muhammad Raziq Rahimi Kooh
Effective and sustainable separation processes for critical metals, especially for the physicochemically similar elements nickel and cobalt in battery recycling, are of great interest in the future. Selective adsorption represents a highly potential process for this purpose. In this publication, a silica adsorbent functionalized with an amino-polycarboxylate derivate (HSU331) was investigated regarding the selective adsorption of Ni(II) in the presence of Co(II) in acidic solution (pH range at equilibrium 1.8–2.3) at elevated temperature. Comparable maximum equilibrium loadings () for Ni(II) and Co(II) of 0.59 μmol(Ni(II)) · μmol(Ligand)-1 (18.3 mg(Ni(II)) · g(Adsorbent)-1), and 0.52 μmol(Co(II)) · μmol(Ligand)-1 (16.0 mg(Co(II)) · g(Adsorbent)-1), respectively, were achieved at T = 50°C in single-component experiments. Under competitive conditions, the Ni(II) loading remained constant at 0.60 μmol(Ni(II)) · μmol(Ligand)-1 (18.4 mg(Ni(II)) · g(Adsorbent)-1), while the Co(II) loading drastically decreased to 0.09 μmol(Co(II)) · μmol(Ligand)-1 (2.7 mg(Co(II)) · g(Adsorbent)-1) in an equimolar dual-component system. Calculated stability constants of 3 · 103 and 0.7 · 103 L · mol-1, respectively, for the formed metal ion complexes of Ni(II) and Co(II) onto the adsorbent HSU331, clarify the clear selectivity of the adsorbent towards Ni(II) in the presence of Co(II) even at elevated temperature (T = 50°C).
Rising global warming requires an energy transition away from fossil fuel-based power generation to a sustainable generation from wind and solar energy. For the success of energy transition, electricity storage systems for wind power and photovoltaic systems or electric vehicles are mandatory. As a result, state-of-the-art lithium-ion batteries (LIB), whose mixed oxide cathodes consist of lithium (Li) and certain transition metals such as cobalt (Co) and nickel (Ni), will be increasingly demanded in the future. In 2015, 49% of the globally produced refined Co was already used in the rechargeable battery market  and the European Union (EU) classified this metal already as critical raw material . According to estimates, the application of Ni in batteries for electric vehicles will grow by 39% annually until 2025. At similar growth rates, this corresponds to a Ni requirement in 2030 of more than 50% of the current global Ni production .
From 2030 onwards, the EU thus demands material recovery rates from batteries, i.a. for Ni and Co of 95% each . Klimenko et al. postulate that an improvement of current global Co recycling rates from 30 to 50% is imperative to address an otherwise inevitable Co shortage by the middle of the century . Consequently, accelerated development of effective and sustainable recycling approaches for end-of-life Li batteries becomes an essential technological task.
Today, LIB recycling e.g. based on the Batrec, and Duesenfeld process, respectively, which combine mechanical and thermal treatments with hydrometallurgical methods, mainly acidic leaching in the presence of reductants [6, 7]. After leaching, Ni and Co normally exist as Ni(II) and Co(II) in an acidic aqueous phase, from which they are subsequently recovered by solvent extraction, and precipitation, respectively. Depending on the leaching agent employed, the recovery rates for Co(II) and Ni(II) range from approximately 80 to 99% [6, 8, 9]. Thus, considerable amounts of up to 20% of these valuable metals get lost via wastewater. In view of the required economic and ecological sustainability of industrial processes, further treatment of such sewage effluents are expedient. Another typical industrial process is electroplating [10–12], where Ni(II) and Co(II) are removed from acidic wastewater during plating or at the end of the process [10–13].
Common separation technologies for bivalent metal ions from aqueous solution are based on adsorption methods, membrane techniques, or electrochemical methods . Especially, Ni(II) and Co(II) separation processes from aqueous solution include complexation by chelating agents [10, 15] combined with electrodialysis [16, 17], adsorption [11, 13, 18, 19], and precipitation [20, 21]. Nevertheless, a selective separation of Ni(II) from Co(II) is challenging because of their similar physicochemical properties [7, 20].
Particularly, adsorption represents an advantageous elimination method for heavy metal ions in aqueous phase because of low operational costs, efficient separation at low concentrations, and an adjustable selectivity towards a specific target component [14, 18, 22, 23]. Adsorption selectivity can be achieved by surface modification of an adsorbent matrix, like porous silica gels, with target-directed ligands [24, 25]. These matrix materials offer adjustable porosities according to the selected process conditions, defined structures, and immense possibilities of specific surface functionalization [25, 26].
Generally, amino-polycarboxylic acids in a non-immobilized form show high complex stabilities with Ni(II) and Co(II)  and are consequently suitable for functionalization of adsorbents for heavy metal recovery. For instance, Repo et al.  showed high adsorption capacities for Ni(II) and Co(II) using silica gels functionalized with amino-polycarboxylic acids. A selectivity towards Ni(II) was suggested when utilizing chitosan [29, 30] as matrix matter. Several other matrices, e.g. acrylonitrile-divinylbenzene copolymer (AN-DVB)  or silica polyamine composites , were functionalized with amino-polycarboxylic acid derivatives which showed high adsorption capacities for Ni(II) and Co(II).
In this article, we demonstrate the outstanding performance of a mesoporous silica gel functionalized with an amino-polycarboxylate derivate for the selective adsorption of Ni(II) from Co(II) from an acidic model laboratory solution (–4) at elevated temperature (°C), which represents characteristic conditions for Ni(II) and Co(II) containing industrial process waters [16, 33, 34]. These aspects complicate the search of suitable adsorbents for integrated or attached recycling steps in industrial processes because adsorption is usually an exothermic process, which implicates that higher temperatures support desorption . Moreover, low pH values favor desorption of cations, here Ni(II) and Co(II) .
Equilibrium batch experiments were performed in order to investigate its adsorption behavior in single- and dual-component systems and the influence of different experimental parameters (temperature, pH, and various molar ratios) on the selective Ni(II) adsorption. Furthermore, a desorption method at 20°C is presented.
This study provides substantial information for the development of a selective adsorption-based separation process as a basis for a sustainable Ni(II) and Co(II) recovery in lithium-ion battery recycling and for a process-integrated Ni(II) and Co(II) recovery step in electroplating.
2. Materials and Methods
For the selective adsorption experiments, a silica-based adsorbent, designated as HSU331, with the following characteristics was applied: specific surface area of 510 m2·g-1, pore diameter of 58 Å, pore volume of 0.74 mL·g-1. It consisted of an irregular silica matrix (particle size: 40–63 μm, pore diameter: 60 Å), functionalized with an amino-polycarboxylate ligand. Ultrapure water of type 1 was utilized in all experiments and generated with the water purification system B30 Integrity (AQUAlab, Höhr-Grenzhausen, Germany). Cobalt(II) nitrate hexahydrate (Co(NO3)2 · (H2O)6, %), purchased from Carl Roth (Karlsruhe, Germany), and Nickel(II) nitrate hexahydrate (Ni(NO3)2 · (H2O)6, purity 99%) as well as 65 wt% Suprapur® nitric acid, delivered from Merck (Darmstadt, Germany), were applied for sample preparation.
2.2. Elemental Analysis of the Functionalized Silica Gel
The amount of functionalized ligands onto the silica surface (surface coverage ) was obtained by elemental analysis using a Vario EL Cube Elemental Analyzer (Elementar Analysensysteme, Langenselbold, Germany) with helium as carrier gas and sulphanilic acid (p.a., Merck, Darmstadt, Germany) as calibration standard. Each sample consisted of 10 mg adsorbent and was analyzed in triplicate.
2.3. Metal Ion Quantification
Ni(II) and Co(II) concentrations in the experimental samples were determined by Inductively Coupled Plasma Mass Spectrometry (ICP-MS) with an Agilent 7800 system (Agilent Technologies, Waldbronn, Germany) according to Kriese et al. .
2.4. Adsorption Experiments
Discontinuous adsorption experiments with 50 mg HSU331 in differently concentrated aqueous metal samples ( mL) were performed in 15 mL centrifuge tubes (VWR International, Darmstadt, Germany) at °C and °C in triplicate (). Metal samples were prepared from corresponding metal stock solutions ( mmol · L−1) by dilution with nitric acid (0.1 wt%). In single-component samples, the initial Ni(II) and Co(II) concentrations () comprised between 0.030 and 17 mmol · L−1 (Table 1). In the case of equimolarity (1 : 1) in dual-component samples, the adjusted concentrations of each metal ion ranged between 1.0 and 15 mmol · L-1. For the molar ratio of 1 : 3, the concentrations of the surplus component (Co(II)) also varied between 1.0 and 15 mmol · L-1, and those of the minor component (Ni(II)) between 0.33 and 5.0 mmol · L-1 (Table 1).
Initial concentrations of metal ions in the prepared single- and dual-component systems (molar ratio Ni(II) : Co(II) of 1 : 1 and 1 : 3) for the different adsorption experiments.
Initially, the pH value of each sample solution was adjusted to 3.5 with 65 wt% nitric acid. For equilibrium achievement, the centrifuge tubes rotated in an overhead rotator (Sunlab, Mannheim, Germany) with 40 rpm for 24 h at 20°C. Subsequently, an analogous series of experiments at °C proceeded in a temperature-controlled heating cabinet. After 24 h, aliquots ( mL) of each sample solution were diluted with 5 mL 0.65 wt% of nitric acid and analyzed with the ICP-MS. The equilibrium pH value (pHeq) in all sample solutions was measured in order to determine the ratio of released protons ((H+, released)) per adsorbed metal ion (n(M(II), adsorbed)) during adsorption. This ratio represents the number of ligand dentates that coordinate one metal ion, which is a measure of the resulting complex structure and was calculated as follows:where constitutes the initial and the equilibrium concentration in the sample solution and pH0 gives the initial pH value.
Based on the measured equilibrium concentrations compared to the initial ones, specific dynamic and thermodynamic equilibrium parameters (equilibrium loading, stability constant, enthalpy, entropy, and free enthalpy of formation) were calculated to clarify the underlying adsorption mechanism and to verify the selectivity of HSU331 (expressed by the selectivity coefficient and reaction engineering selectivity).
According to the measured experimental concentrations in μmol · L-1, each corresponding molar equilibrium loading in μmol(M(II)) · μmol(Ligand)-1 yields as:where represents the sample volume in L, the adsorbent mass in g, and the functionalization degree in μmol(Ligand)·g(Adsorbent)-1 (determined according to Section 2.2).
Usually, the adsorption of bivalent metal ions on functionalized surfaces proceeds according to the following equilibrium reaction:in which the M(II) ions and the adsorbent ligand L form ion ligand complexes. Considering the equilibrium concentrations (μmol · L-1) of the formed complexes , non-adsorbed metal ions , and free ligands , the stability constant () of the metal ion ligand complex was determined according to
The enthalpy () and entropy () were derived from the linear van’t Hoff equation , which describes the equilibrium position of a chemical reaction as a function of temperature:
By plotting (ordinate) against (abscissa)  resulted from the slope, and the intercept of the linear curve, respectively.
Subsequently, the free enthalpy of formation (ΔG0) according to the Gibbs-Helmholtz Equation (Equation (6)) substantiates the endergonic or exergonic character of the adsorption :
Two characteristic parameters, (1) the selectivity coefficient () and (2) the reaction engineering selectivity related to the formed Ni/HSU331 complex (), were chosen for discussing the selectivity of the adsorbent HSU331 towards Ni(II) and calculated from the results of the dual-component experiments as ratio of the stability constants of the formed Ni(II) and Co(II) complexes (1):and as ratio of the amount of formed Ni/HSU331 complexes () at equilibrium and the difference between the initial amount of the adsorbent’s ligands () and the amount of free ligands () at equilibrium (2), which means that the denominator describes the total occupied ligands :
2.5. Desorption Experiments
Prior to the desorption investigations, corresponding adsorption experiments (°C, ) were conducted as described in Section 2.4. Supernatants were discarded after 24 h and replaced by 5 mL of nitric acid with different pH values (1.0, 0.5, 0.0, and -0.3, respectively) for disclosing the optimal desorption conditions.
After equilibration (24 h) and rejection of the acidic supernatants, two washing steps ( mL) with ultrapure water took place in rotating sample containers for 15 min at 40 rpm, followed by a second adsorption step. Aliquots of every adsorption, desorption, and washing step were sampled, diluted, and analyzed for dissolved Ni(II), and Co(II), respectively, according to Section 2.3.
The desorption efficiency (ηDE) was calculated appropriate to the following equation:where represents the desorbed and the adsorbed amount of the respective transition metal ion.
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