Thermodynamic Characteristics of the Ion-Exchange Process Involving REMs of the Light Group

Abstract

Rare earth metals (REMs) are vital for high-tech industries, but their extraction from secondary sources is challenging due to environmental and technical constraints. This study investigates the ion-exchange extraction of light REMs (neodymium, praseodymium, and samarium) from sulfuric and phosphoric acid solutions, modeling industrial leachates from apatite concentrates and phosphogypsum. The study considers the use of anion- and cation-exchange resins with different functional groups for efficient and environmentally safe REM separation. Experimental sorption isotherms were obtained under static conditions at 298 K and analyzed using a thermodynamic model based on the linearization of the mass action equation. Equilibrium constants and Gibbs energy were calculated, which reveals the spontaneity of the processes. Cation-exchange resins demonstrated high selectivity towards individual REMs, while anion-exchange resins were suitable for group extraction. Infrared spectral analysis confirmed the presence of sulfate and phosphate complexes in the resin matrix, clarifying the ion-exchange mechanisms. Thermal effect measurements indicated exothermic sorption on anion-exchange resins with negative entropy and endothermic sorption on cation-exchange resins with positive entropy. The findings highlight the potential of ion-exchange resins for selective and sustainable REM recovery, offering a safer alternative to liquid extraction and enabling the valorization of industrial wastes like phosphogypsum for resource recovery.

1. Introduction

Rare earth metals (REMs) are critical for high-tech industries, including defense, laser technology, electronics, medicine, and steel alloying [1].

Primary REM sources are minerals such as loparite, monazite, and bastnasite. However, increasing attention is being given to the possibility of involving production wastes in processing, corresponding to the principles of green chemistry, aimed at minimizing the environmental impact of the chemical industry and preventing the formation of waste [2,3]. The use of secondary sources of REMs, such as red mud [4], phosphogypsum [5], nickel-metal hydride and lithium-ion batteries [6,7], will not only reduce the load on the environment, but also provide the output of byproducts, contributing to the sustainable use of resources and rational utilization of industrial waste [8,9].

Methods for extracting metals, including REMs, from process solutions include precipitation [10,11], solvent extraction [12], flotation-extraction [13] and sorption using various sorbents [14,15]. Solvent extraction and solid-phase extraction [16] are highly selective but primarily recover heavy REMs, leaving light REMs in solution. Moreover, solvent extraction uses toxic, volatile organic solvents, posing fire and explosion risks. Sorption processes, such as ion-exchange methods, offer an alternative approach for REM extraction.

Adsorption processes are widely used for metal ion recovery from process solutions, due to their high sorption capacity, extraction efficiency and selectivity toward target components [17]. This approach eliminates the necessity of using toxic solvents, overcoming a key limitation of solvent extraction and enabling sustainable REM recovery.

1.1. Sorption on Cation-Exchange Resins

The extraction of REMs by means of cation-exchange resins from acidic solutions of various compositions has been adequately addressed in the extant literature, due to their versality and effectiveness. Cation-exchange resins demonstrate significant sorption capacities and high REM extraction efficiencies.

Araucz et al. [18] investigated the potential application of Purolite S957 and Diphonix Resin^® cation-exchange resins for extracting rare earth metals from aqueous solutions obtained during Ni-MH battery reprocessing in the presence of citric acid. The effects of metal and citric acid concentrations, solution pH, solid-to-liquid ratio, contact time, and temperature were investigated. Langmuir and Freundlich isotherm models were used to describe the sorption process. The maximum adsorption capacity obtained from the Langmuir isotherm was 46.63 mg/g for Ni (II) and 60.75 mg/g for La (III) on Purolite S957, and 46.55 mg/g for Ni (II) and 60.12. mg/g for La (III) on Diphonix Resin^®. The adsorption character was defined as spontaneous and endothermic.

A series of experiments on La (III) sorption on macroporous resins containing iminodiacetic acid (Lewatit TP 207) and aminomethylphosphonic acid (Lewatit TP 260) groups were carried out in [19]. The parameters investigated included initial La (III) concentration, pH, temperature, and contact time. In an experiment with 0.1 g of resin and 5 mL of a solution with a concentration of 0.5 × 10⁻³ mol/L La (III), equilibrium was reached in 30 min. The optimum pH was 1.5–4.6 for Lewatit 207 and approximately 5.2 for Lewatit TP 260. The sorption capacities of Lewatit TP 207 and Lewatit TP 260 resins were 114.7 and 106.7 mg/g, respectively. The linear forms of adsorption isotherms were plotted using Langmuir and Freundlich models. It was found that the sorption process of La (III) on Lewatit TP 207 resin agrees with the Langmuir adsorption model, while the Freundlich model reliably describes the sorption on Lewatit TP 260. Thermodynamic analysis indicated an endothermic and spontaneous process, with decreasing Gibbs free energy at higher temperatures, suggesting improved sorption efficiency.

The adsorption and desorption process of Er(III) ions on D113-III resin was investigated in [20]. The resin exhibited a maximum sorption capacity of 250 mg/g at T = 298 K and pH = 6.04. Negative Gibbs free energy values, calculated using the Langmuir model, indicated a spontaneous and endothermic sorption process. It was found that Er(III) ions can be extracted from the solid phase using a 4.0 mol/L HCl solution.

Botelho Junior et al. [21] investigated the kinetics and thermodynamics of lanthanum and cerium sorption using chelating resins. The study was carried out for three chelating resins Dowex M1495, Lewatit TP 207 and Dowex XUS43605 with different functional groups. The effect of solution pH and solid-to-liquid ratio was evaluated. Thermodynamic analysis indicated a spontaneous and endothermic sorption process for lanthanum and cerium.

The regeneration of ion-exchange resins can be accomplished with both conventional acid solutions and environmentally friendly complexing agents. For instance, Kurkinen et al. [22] developed a “resin-in-leach” (RIL) method to extract REMs from phosphogypsum using Purolite S940 resin in dilute H₂SO₄ solutions. The effectiveness of alkaline solutions of the chelating agents, including N,N-dicarboxymethylglutamic acid (GLDA), methylglycindiacetic acid (MGDA), oxalic acid, citric acid and iminodiacetate, was evaluated. The chelating agent MGDA provided the production of REM compounds with high purity (up to 99.01%).

Despite the extensive research conducted on the extraction of REMs by cation-exchange resins, the existing literature lacks sufficient data on the thermodynamic parameters (equilibrium constants, Gibbs energy change, enthalpy, and entropy) of sorption. A notable oversight is the inadequate consideration of the potential separation of neighboring rare earth metals of the light group from concentrated phosphoric acid solutions, a subject of both scientific interest and practical significance.

1.2. Sorption on Anion-Exchange Resins

Anion-exchange resins have not been extensively utilized in REM extraction processes; however, their efficacy has been demonstrated in the separation of thorium, uranium, chromium, and certain other metals.

Uranium adsorption on the strongly basic anion-exchange resin Purolite A400 was studied by the authors of [23]. The effects of initial uranium concentration, pH, phase contact time and temperature were evaluated. Purolite A400 exhibited a sorption capacity of 117.6 mg/g under optimal conditions. Thermodynamic data best fit the Langmuir isotherm model.

Stroganova et al. [24] analyzed the sorption of Ge(IV) and Cu(II) ions on weakly basic anion-exchange resin AN-31 from chloride solutions. The dependence of equilibrium parameters of sorption of Ge(IV) and Cu(II) ions on the concentration of chloride ions was shown. Chloride concentration affected sorption capacity and extracted the complex structure, as determined by diffuse reflectance spectroscopy, sorption isotherms, and isomolar ion distribution diagrams. The synergistic effect of sorption of Cu(II) ions in the presence of Ge(IV) ions was established. Selective sorption of Cu(II) towards Ge(IV) ions was achieved in chloride solutions of 1 mol/L concentration at molar ratios of Ge(IV):Cu(II) from 1:3 to 1:1.

The study on the extraction of heavy metal ions—copper (II), nickel (II), cobalt (II) and zinc (II)—from model sulfate solutions by a new anion exchanger based on epoxidized vinyl ester of monoethanolamine, allylglycidyl ether and polyethyleneamine was carried out in [25]. The influence of metal salt concentration, solution pH, and contact time on the resin’s sorption capacity was evaluated. Optimal pH values for maximum sorption were 4.1 for Cu(II), 6.1 for Ni(II), 0.8 for Zn(II), and 5.4 for Co(II). It was found that the sorption capacity under optimal conditions reaches the following values: 705.2 mg/g for Cu(II), 598.8 mg/g for Ni(II), 536.4 mg/g for Zn(II), 436.0 for Co(II). Metal recovery rates of 75.6–80.7% confirm the possibility of using the resin for group sorption of heavy metal ions in hydrometallurgical processes.

Bekchanov at al. developed an anion-exchange resin from plasticized polyvinyl chloride and polyethylene polyamine to remove Cr(VI) from aqueous solutions [26]. X-ray diffraction, FT-IR spectroscopy, and scanning electron microscopy revealed the resin’s weakly basic amino groups and macroporous structure. Thermodynamic studies showed that the sorption process fit the Freundlich isotherm model. The resin’s maximum sorption capacity for Cr(VI) was 218.4 mg/g. Thermodynamic calculations indicated a spontaneous and endothermic sorption process.

The study [27] examined the sorption of perrhenate ions (ReO₄⁻) on an anion-exchange resin synthesized from epichlorohydrin oligomer and 4-vinylpyridine. The effects of rhenium concentration (0.102–1.024 g/L), solution pH, and contact time on the sorption characteristics of the synthesized material were studied. At optimal pH (5.1), the resin achieved a sorption capacity of 371.6 mg/g and 99% perrhenate sorption efficiency.

However, in conjunction with the more thoroughly studied positively charged complexes, REMs have the capacity to form negatively charged complexes with both organic and inorganic ligands. For instance, Hubicka et al. investigated the use of strongly basic gel and macroporous polystyrene anion-exchange resins for the sorption and separation of REM complexes with iminodiacetic acid [28]. The complexes were identified as Ln(IMDA)⁻, and the resins were converted to acetate form for optimal separation efficiency. Due to the different affinity of lanthanum and neodymium complexes with imidoacetic acid to different types of anion exchangers, the separation of these lanthanides on gel and macroporous anion-exchange resin is possible. Separation efficiency followed the order: strongly basic gel anion-exchange resins > strongly basic macroporous resins > weakly basic macroporous resins.

1.3. Problem Statement

The separation of REMs is a challenging scientific task due to their similar chemical properties [29]. Their similar chemical behavior complicates lanthanide separation from multicomponent mineral and industrial sources [30,31].

To address this challenge, the subtle structural differences between lanthanides, especially in physical properties such as ionic radii, must be exploited.

These differences are manifested in ionic radii and electronic structure, primarily 4f-orbitals [32]. Chemical bonding in REM complexes is predominantly ionic due to large lanthanide ion sizes, fully occupied 5s²5p⁶ shells resembling noble gas configurations, and shielded 4f-orbitals. However, available 4f-orbitals enable covalent bonding with donor ligands, resulting in different coordination numbers in REM compounds.

REM compounds with inorganic ligands are typically unstable in aqueous solutions due to high instability constants. However, in acidic leaching solutions from mineral and industrial sources, lanthanides are capable of forming relatively stable anionic complexes. Thus, in sulfuric acid solutions from phosphogypsum leaching, lanthanides form sulfate complexes, whose stage of coordination is directly determined by the concentration of sulfuric acid.

At an excess of sulfate ions, stable anionic sulfate complexes of the second coordination step are formed: [Ln(SO₄)₂]⁻. In low-pH phosphoric acid solutions, formed during apatite processing for fertilizer production, REMs form stable hydrogen phosphate complexes, leading to their high solubility (~0.1 wt %).

The determination of the composition of complex compounds and their stability in aqueous solutions was given special attention at the initial stages of the development of REM coordination chemistry, which is confirmed by a large number of works devoted to this subject. However, the necessity to create effective methods of REM separation has led to the development of thermochemical studies of lanthanide complex compounds, in order to clarify the comprehensive set of thermodynamic characteristics.

Thermodynamic differences in ion-exchange adsorption provide a foundation for the effective REM separation. Considering the characteristics of sulfuric and phosphoric acid leaching solutions of apatite raw materials, the thermodynamic properties of the ion-exchange process of light-group REM extraction using cation- and anion-exchange resins were examined.

2. Materials and Methods

In the present work, the sorption of sulfate and phosphate complexes of Pr(III), Nd(III) and Sm(III) from sulfuric and phosphoric acid solutions was studied using ion-exchange resins with characteristics listed in

Table 1. Characteristics of ion-exchange resins.

Ion-Exchange Resin	Functional Group	Matrix
Anion-exchange resin AN-31 (Russia)	secondary and tertiary aliphatic amino groups	gel, styrene-divinylbenzene
Anion-exchange resin Rosek-21 (Russia)	weakly basic amino groups, carboxyl groups	acryl- divinylbenzene-styrene
Anion-exchange resin PurometTM MTA6002PF (Purolite Ltd., Moscow, Russia)	quaternary amino groups	gel, styrene-divinylbenzene
Chelating cation-exchange resin PurometTM MTS9500 (Purolite Ltd., Moscow, Russia)	aminophosphonic groups	macroporous, styrene-divinylbenzene
Bifunctional cation-exchange resin PurometTM MTS9570 (Purolite Ltd., Moscow, Russia)	phosphonic and sulfonic groups	macroporous, styrene-divinylbenzene
Cation-exchange resin SupergelTM SGC650 (Purolite Ltd., Moscow, Russia)	sulfonic groups	gel, styrene-divinylbenzene
Cation-exchange resin PurometTM MTS9560 (Purolite Ltd., Moscow, Russia)	phosphonic groups	macroporous, styrene-divinylbenzene
Cation-exchange resin PurometTM MTS9580 (Purolite Ltd., Moscow, Russia)	phosphonic groups	macroporous, styrene-divinylbenzene

. Resin selection was based on prior data demonstrating high REM sorption capacities by ion-exchange resin matrices.

To study the sorption of REM disulfate and hydrogen phosphate complexes, anion-exchange resins were converted to the SO₄²⁻ form and cation-exchange resins to the H⁺ form. For this purpose, the anion-exchange resin was treated in 2 mol/L sodium sulfate solution for 24 h, followed by treatment with 2 mol/L sulfuric acid for 2 h after phase separation. The resin converted to the exchangeable form SO₄²⁻ was washed with distilled water until neutral medium was reached. Preliminary preparation of cation-exchange resins was carried out in a similar way, using sulfuric acid with a concentration of 2 mol/L as the eluent solution.

Experimental Study of the Sorption Process Under Static Conditions

The selection of acidic solutions containing sulfate and phosphate ions is explained by the composition of phosphoric acid and sulfuric acid leaching solutions both directly from apatite concentrate and products of its processing.

The composition of the phosphoric acid solutions used simulates the basic composition of the technological phosphoric acid solution containing H₃PO₄ 4.5 mol/L, H₂SO₄ 0.19 mol/L and REM from 3.8 to 200.7 mmol/L, which are in the form of cationic complexes of the composition [Ln(H₂PO₄)]²⁺ [33].

Sulfate solutions with pH = 2 contained H₂SO₄, 1 mol/L MgSO₄ and REM from 10.4 to 321.1 mmol/L in the form of anionic complexes of the composition: [Ln(SO₄)₂]⁻ [34].

The sorption process of Pr, Nd and Sm was studied by the method of variable concentrations under static conditions at a constant temperature of 298 K and a stirring speed of 120 rpm. Constant experimental conditions were ensured using a thermostated GFL Shaking Incubator 3032 mixing cabinet.

The ion-exchange resins were in contact with the solutions for a minimum of six hours, which ensured that equilibrium was reached in the heterogeneous system.

The extraction of REMs from sulfate solutions was performed using anion-exchange resins Rosek-21 and Puromet^TM MTA6002PF, from phosphoric acid media using cation-exchange resins Puromet^TM MTS9500, Puromet^TM MTS9570, Supergel^TM SGC650, Puromet^TM MTS9560 and Puromet^TM MTS9580.

The solid-to-liquid phase ratios were determined to be 1:1 for sulfate solutions and 1:5 for phosphoric acid solutions.

Resin sorption capacity (q, mol/kg) and sorption efficiency (α, %) were calculated using Formula (1) and presented in

Table 2. Results of Pr sorption from sulfate and phosphoric acid solutions.

Ion-Exchange Resin	q, mol/kg	α, %
AN-31	0.0101	38.2
Rosek-21	0.0084	44.2
PurometTM MTA6002PF	0.0077	47.9
PurometTM MTS9500	0.0656	90.6
PurometTM MTS9570	0.0694	95.6
SupergelTM SGC650	0.0615	94.8
PurometTM MTS9560	0.0092	12.5
PurometTM MTS9580	0.0073	9.4

.

$$\mathrm{q}={\displaystyle \frac{({\mathrm{C}}^0-{\mathrm{C}}^{\infty })·V}{m}}$$

(1)

Results showed that anion-exchange resin capacity was significantly lower than cation-exchange resin capacity. However, anion-exchange resins remained suitable for REM sorption from sulfuric acid leaching solutions. Among the cation-exchange resins, MTS9500, MTS9570 and SGC650 exhibited the highest recovery rates. Consequently, subsequent experimental investigations were conducted using anion-exchange resins Rosek-21 and Puromet^TM MTA6002PF (MTA6002PF) and cation-exchange resins Puromet^TM MTS9500 (MTS9500), Puromet^TM MTS9570 (MTS9570) and Supergel^TM (SGC650).

3. Results and Discussion

3.1. Sorption of REMs on Anion-Exchange Resins

The sorption isotherms of Pr, Nd and Sm on Rosek-21 and PurometTM MTA6002PF anion-exchange resins are presented in Figure 1.

The equivalent exchange of SO₄²⁻ functional groups of the polymer matrix for REM complex ions from sulfate solutions was expressed by the following equation [34]:

$$\overline{3{\mathrm{R}}_2\left[{\mathrm{S}\mathrm{O}}_4\right]}+2{\left[\mathrm{L}\mathrm{n}{\left({\mathrm{S}\mathrm{O}}_4\right)}_2\right]}^{-}\leftrightarrow \overline{2{\mathrm{R}}_3\left[\mathrm{L}\mathrm{n}{\left({\mathrm{S}\mathrm{O}}_4\right)}_3\right]}+{\mathrm{S}\mathrm{O}}_4^{2-}$$

(2)

where $\overline{3{\mathrm{R}}_2\left[{\mathrm{S}\mathrm{O}}_4\right]}$ and $\overline{2{\mathrm{R}}_3\left[\mathrm{L}\mathrm{n}{\left({\mathrm{S}\mathrm{O}}_4\right)}_3\right]}$] determine the concentrations of ${\mathrm{S}\mathrm{O}}_4^{2-}$ and ${\left[\mathrm{L}\mathrm{n}{\left({\mathrm{S}\mathrm{O}}_4\right)}_2\right]}^{-}$ in the solid matrix of polymer resin.

The experimental ion-exchange isotherms were described through the application of the fundamental law of thermodynamics, specifically the mass action equation [35], under the assumption that the solid phase is ideal and the values of activity coefficients in it are equal to one:

$$\mathrm{K}={\displaystyle \frac{{\mathrm{q}}_{{\left[\mathrm{L}\mathrm{n}{\left({\mathrm{S}\mathrm{O}}_4\right)}_3\right]}^{3-}}^2·a_{{\mathrm{S}\mathrm{O}}_4^{2-}}}{{\mathrm{q}}_{{\mathrm{S}\mathrm{O}}_4^{2-}}^3·a_{{\left[\mathrm{L}\mathrm{n}\right({\mathrm{S}\mathrm{O}}_4)}_2^{-}]}^2}}$$

(3)

$$\mathrm{K}={\displaystyle \frac{{\mathrm{q}}_{{\left[\mathrm{L}\mathrm{n}{\left({\mathrm{S}\mathrm{O}}_4\right)}_3\right]}^{3-}}^2·[{\mathrm{S}\mathrm{O}}_4^{2-}]·{\mathsf{\gamma }}_{\pm {\mathrm{M}\mathrm{g}\mathrm{S}\mathrm{O}}_4}^2}{{\mathrm{q}}_{{\mathrm{S}\mathrm{O}}_4^{2-}}^3·{\left[{\mathrm{L}\mathrm{n}\left({\mathrm{S}\mathrm{O}}_4\right)}_2^{-}\right]}^2·{\mathsf{\gamma }}_{\pm {\mathrm{M}\mathrm{g}\left[\mathrm{L}\mathrm{n}\right(\mathrm{S}\mathrm{O}}_4{)}_2{]}_2}^3}}$$

(4)

Linearization of the mass action equation and application of mathematical transformations allowed for obtaining a modified ion-exchange equation similar to the Langmuir equation, which describes a specific exchange process involving complex REM ions:

$${\displaystyle \frac{1}{{\mathrm{q}}_{\left[\mathrm{L}\mathrm{n}\right({\mathrm{S}\mathrm{O}}_4{)}_2{]}^{-}}}}={\displaystyle \frac{3}{{\mathrm{q}}_{\mathrm{\infty }}}}+{\displaystyle \frac{1}{{\mathrm{q}}_{\mathrm{\infty }}·{\mathrm{K}}^{\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$3$}\right.}}}·\left({\displaystyle \frac{2[{\mathrm{S}\mathrm{O}}_4^{2-}{]}^{\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$3$}\right.}·{\mathsf{\gamma }}_{\pm {\mathrm{M}\mathrm{g}\mathrm{S}\mathrm{O}}_4}^{\raisebox{1ex}{$2$}\!\left/ \!\raisebox{-1ex}{$3$}\right.}}{{\mathrm{q}}_{{\left[\mathrm{L}\mathrm{n}{\left({\mathrm{S}\mathrm{O}}_4\right)}_3\right]}^{3-}}^{\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$3$}\right.}·{\left[{\mathrm{L}\mathrm{n}\left({\mathrm{S}\mathrm{O}}_4\right)}_2^{-}\right]}^{\raisebox{1ex}{$2$}\!\left/ \!\raisebox{-1ex}{$3$}\right.}·{\mathsf{\gamma }}_{{\pm \mathrm{M}\mathrm{g}\left[\mathrm{L}\mathrm{n}\right(\mathrm{S}\mathrm{O}}_4{)}_2{]}_2}}}\right)$$

(5)

where the value of the total sorbent capacity (q_∞) is expressed through the equilibrium ion concentrations in the solid phase of the resin:

$${\mathrm{q}}_{\mathrm{\infty }}=2{\mathrm{q}}_{{\mathrm{S}\mathrm{O}}_4^{2-}}+3{\mathrm{q}}_{{\left[\mathrm{L}\mathrm{n}{\left({\mathrm{S}\mathrm{O}}_4\right)}_3\right]}^{3-}}$$

(6)

Considering the weak dependence of activity coefficients on the nature of ions and the strong dependence on charge, the average ionic activity coefficient was calculated using reference values [36].

In order to determine the ion-exchange equilibrium constant, a graphical dependence of the reverse sorption value or equilibrium concentration of REM ions in the solid phase ${\displaystyle \frac{1}{q_{\left[Ln\right({SO}_4{)}_2{]}^{-}}}}$ on the concentration argument $\mathrm{f}\left(\mathrm{c}\right)={\displaystyle \frac{2[{\mathrm{S}\mathrm{O}}_4^{2-}{]}^{\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$3$}\right.}·{\gamma }_{\pm {\mathrm{M}\mathrm{g}\mathrm{S}\mathrm{O}}_4}^{\raisebox{1ex}{$2$}\!\left/ \!\raisebox{-1ex}{$3$}\right.}}{q_{{\left[\mathrm{L}\mathrm{n}{\left({\mathrm{S}\mathrm{O}}_4\right)}_3\right]}^{3-}}^{\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$3$}\right.}·{[{\mathrm{L}\mathrm{n}\left({\mathrm{S}\mathrm{O}}_4\right)}_2^{-}]}^{\raisebox{1ex}{$2$}\!\left/ \!\raisebox{-1ex}{$3$}\right.}·{\gamma }_{{\pm \mathrm{M}\mathrm{g}\left[\mathrm{L}\mathrm{n}\right(\mathrm{S}\mathrm{O}}_4{)}_2{]}_2}}}$ was plotted (Figure 2).

The constants and Gibbs energies (∆G₂₉₈) of the ion-exchange equilibrium were calculated from the tangent of the slope angle of the linear forms of the sorption isotherms (

Table 3. Thermodynamic description of REM sorption on anion-exchange resins.

Ion-Exchange Resin	Complex Ion	K	−∆G298, kJ/mol
Rosek-21	${\left[\mathrm{P}\mathrm{r}{\left({\mathrm{S}\mathrm{O}}_4\right)}_2\right]}^{-}$	1.06 ± 0.05	0.14 ± 0.01
	${\left[\mathrm{N}\mathrm{d}{\left({\mathrm{S}\mathrm{O}}_4\right)}_2\right]}^{-}$	1.13 ± 0.06	0.30 ± 0.02
	${\left[\mathrm{S}\mathrm{m}{\left({\mathrm{S}\mathrm{O}}_4\right)}_2\right]}^{-}$	1.34 ± 0.07	0.72 ± 0.04
PurometTM MTA6002PF	${\left[\mathrm{P}\mathrm{r}{\left({\mathrm{S}\mathrm{O}}_4\right)}_2\right]}^{-}$	1.49 ± 0.07	0.99 ± 0.05
	${\left[\mathrm{N}\mathrm{d}{\left({\mathrm{S}\mathrm{O}}_4\right)}_2\right]}^{-}$	2.21 ± 0.11	1.96 ± 0.10
	${\left[\mathrm{S}\mathrm{m}{\left({\mathrm{S}\mathrm{O}}_4\right)}_2\right]}^{-}$	3.92 ± 0.20	3.38 ± 0.17

).

According to the calculated values of the effective equilibrium constants of the sorption process, the selectivity series of Rosek-21 and MTA6002PF anion-exchange resins follows the order: [Sm(SO₄)₂]⁻ > [Nd(SO₄)₂]⁻ > [Pr(SO₄)₂]⁻. The values of effective equilibrium constants, which are almost equal to one for the anion-exchange resin Rosek-21, are consistent with the low values of REM sorption values.

3.2. Sorption of REMs on Cation-Exchange Resins

The sorption isotherms of Pr, Nd and Sm on Puromet^TM MTS9500, Puromet^TM MTS9570 and Supergel^TM SGC650 cation-exchange resins are presented in Figure 3.

For the process of ion exchange from phosphoric acid solutions involving cation-exchange functional H⁺ groups of the resin, expressed by Equation (7),

$$\overline{2\mathrm{H}\mathrm{R}}+{\left[\mathrm{L}\mathrm{n}\left({\mathrm{H}}_2{\mathrm{P}\mathrm{O}}_4\right)\right]}^{2+}=\overline{\left[\mathrm{L}\mathrm{n}\left({\mathrm{H}}_2{\mathrm{P}\mathrm{O}}_4\right)\right]{\mathrm{R}}_2}+2{\mathrm{H}}^{+}$$

(7)

ion-exchange equilibrium constant takes the following form:

$$\mathrm{K}={\displaystyle \frac{{\mathrm{q}}_{{\left[\mathrm{L}\mathrm{n}\left({\mathrm{H}}_2{\mathrm{P}\mathrm{O}}_4\right)\right]}^{2+}}·a_{{\mathrm{H}}^{+}}^2}{{\mathrm{q}}_{{\mathrm{H}}^{+}}^2·a_{{\left[\mathrm{L}\mathrm{n}\left({\mathrm{H}}_2{\mathrm{P}\mathrm{O}}_4\right)\right]}^{2+}}}}$$

(8)

Linearizing this equation taking into account the ideality of the solid phase and transforming the total sorption capacity through the value of the sum of the equivalent concentrations of hydrogen ions and dihydrophosphate ions of REMs in the solid phase of the ion-exchange resin,

$${\mathrm{q}}_{\mathrm{\infty }}={\mathrm{q}}_{{\mathrm{H}}^{+}}+2{\mathrm{q}}_{{\left[\mathrm{L}\mathrm{n}\left({\mathrm{H}}_2{\mathrm{P}\mathrm{O}}_4\right)\right]}^{2+}}$$

(9)

the following expression was obtained:

$${\displaystyle \frac{1}{{\mathrm{q}}_{{\left[\mathrm{L}\mathrm{n}\left({\mathrm{H}}_2{\mathrm{P}\mathrm{O}}_4\right)\right]}^{2+}}}}={\displaystyle \frac{2}{{\mathrm{q}}_{\mathrm{\infty }}}}+{\displaystyle \frac{1}{{\mathrm{q}}_{\mathrm{\infty }}·{\mathrm{K}}^{\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$2$}\right.}}}·\left({\displaystyle \frac{a_{H^{+}}}{q_{{\left[Ln\left(H_2{PO}_4\right)\right]}^{2+}}^{\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$2$}\right.}·a_{{\left[Ln\left(H_2{PO}_4\right)\right]}^{2+}}^{\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$2$}\right.}}}\right)$$

(10)

The values of activity coefficients were calculated by the Lebedev–Kulyak equation for the solution ionic strength range of 0–2.5 mol/kg.

Graphical forms of linear sorption isotherms are presented in Figure 4, using which the values of ion-exchange equilibrium constants were calculated from the dependence of the value of reverse sorption ${\displaystyle \frac{1}{{\mathrm{q}}_{{\left[\mathrm{L}\mathrm{n}\left({\mathrm{H}}_2{\mathrm{P}\mathrm{O}}_4\right)\right]}^{2+}}}}$ on the concentration argument $\mathrm{f}\left(\mathrm{c}\right)=\left({\displaystyle \frac{a_{H^{+}}}{q_{{\left[Ln\left(H_2{PO}_4\right)\right]}^{2+}}^{\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$2$}\right.}·a_{{\left[Ln\left(H_2{PO}_4\right)\right]}^{2+}}^{\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$2$}\right.}}}\right)$.

The values of effective equilibrium constant and Gibbs energies of ion exchange on cation-exchange resins are presented in

Table 4. Thermodynamic description of REM sorption on cation-exchange resins.

Ion-Exchange Resin	Complex Ion	K	−∆G298, kJ/mol
PurometTM MTS9500	${\left[\mathrm{P}\mathrm{r}\left({\mathrm{H}}_2{\mathrm{P}\mathrm{O}}_4\right)\right]}^{2+}$	18.62 ± 0.93	7.25 ± 0.36
	${\left[\mathrm{N}\mathrm{d}\left({\mathrm{H}}_2{\mathrm{P}\mathrm{O}}_4\right)\right]}^{2+}$	19.84 ± 0.99	7.40 ± 0.37
	${\left[\mathrm{S}\mathrm{m}\left({\mathrm{H}}_2{\mathrm{P}\mathrm{O}}_4\right)\right]}^{2+}$	39.21 ± 1.96	9.09 ± 0.45
PurometTM MTS9570	${\left[\mathrm{P}\mathrm{r}\left({\mathrm{H}}_2{\mathrm{P}\mathrm{O}}_4\right)\right]}^{2+}$	34.39 ± 1.72	8.77 ± 0.44
	${\left[\mathrm{N}\mathrm{d}\left({\mathrm{H}}_2{\mathrm{P}\mathrm{O}}_4\right)\right]}^{2+}$	39.19 ± 1.96	9.09 ± 0.45
	${\left[\mathrm{S}\mathrm{m}\left({\mathrm{H}}_2{\mathrm{P}\mathrm{O}}_4\right)\right]}^{2+}$	115.29 ± 5.76	11.76 ± 0.59
SupergelTM SGC650	${\left[\mathrm{P}\mathrm{r}\left({\mathrm{H}}_2{\mathrm{P}\mathrm{O}}_4\right)\right]}^{2+}$	12.77 ± 0.64	6.31 ± 0.32
	${\left[\mathrm{N}\mathrm{d}\left({\mathrm{H}}_2{\mathrm{P}\mathrm{O}}_4\right)\right]}^{2+}$	25.15 ± 1.26	7.99 ± 0.40
	${\left[\mathrm{S}\mathrm{m}\left({\mathrm{H}}_2{\mathrm{P}\mathrm{O}}_4\right)\right]}^{2+}$	83.42 ± 4.17	10.96 ± 0.55

.

The process of ion exchange with the participation of dihydrophosphate complexes of REM is characterized by negative values of Gibbs energy, which is consistent with the shift of equilibrium in the direction of displacement of hydrogen ions from the solid matrix of the resin into the aqueous solution of phosphoric acid and high values of REM sorption degree. According to the calculated values of apparent equilibrium constants, the selectivity series of cation-exchange resins follows this order: [Sm(H₂PO₄)]²⁺ > [Nd(H₂PO₄)]²⁺ > [Pr(H₂PO₄)]²⁺. The process of REM extraction on cation-exchange resin MTS9570 is characterized by the highest differences in the values of thermodynamic parameters, which indicates the possibility of REM separation within the group.

3.3. Spectral Characteristics of Ion-Exchange Resins

The IR spectra of ion-exchange resins before and after REM extraction from sulfate and phosphoric acid solutions were studied in this work.

In the lumen geometry, the samples are opaque, even with press thinning. The transmittance region starts at 3600 cm⁻¹, so the detection of changes by this technique is uninformative. The ATR technique shows differences between the original and modified samples. Figure 5 show the IR spectra of anion- and cation-exchange resin samples before and after praseodymium sorption.

The spectra of anion-exchange resins before and after praseodymium sorption were identical up to the characteristic frequencies of 1200 and above 1400 cm⁻¹, indicating an unchanged styrene-divinylbenzene polymer matrix.

In the range of 1318–1350 cm⁻¹, in the IR spectrum of the samples after sorption, a broad characteristic absorption band of medium intensity is observed, which is characteristic of the SO₄²⁻ ion with Td-symmetry (tetrahedral). The preservation of Td-symmetry in the sulfate ion, which is a ligand in the composition of the REM complex ion, characterizes the chemical bonding in the complex, very close to the ionic type.

Thus, the results of spectral analysis confirm the presence of the sulfate ion and, consequently, the REM complex ion, in the ion-exchange resin phase.

The IR spectra of the cation-exchange resins before praseodymium sorption show several characteristic frequencies inherent to the phosphonic group R₂(HO)P = O (where R is an aromatic radical): 1138, 1086 and 925, 980 cm⁻¹ in the spectrum of the MTS9500 cation-exchange resin and 1146, 1082 and 925, 980 cm⁻¹ on the spectrum of the MTS9570 cation-exchange resin. The absorbance in the region of 1021 cm⁻¹ of cation-exchange resin MTS9500 indicates the presence of R(HO)₂P = O groups in the sample.

It is shown that after sorption in the spectrum of the MTS9500 sample, the values of characteristic frequencies are sustained, accompanied by a slight decrease in intensity and an increase in band width. Conversely, in the spectrum of the MTS9570 cation-exchange resin, the characteristic frequencies are no longer discernible, indicating the formation of a stronger bond between the ion-exchange resin matrix and the lanthanide ion.

3.4. Thermal Effect and Entropy of Ion-Exchange Process

The thermal effect of the sorption process was studied on complex neodymium ions in sulfate and phosphoric acid solutions. The obtained values of enthalpy, Gibbs energy of ion exchange, and the calculated entropy values are presented in

Table 5. Thermodynamic parameters of neodymium sorption on ion-exchange resins.

Ion-Exchange Resin	∆H298, J/mol	−∆G298, J/mol	∆S298, J/mol·K
Rosek-21	−13,326.19	298.93	−43.72
PurometTM MTA6002PF	−28,147.43	1958.49	−87.88
PurometTM MTS9500	−2050.15	7401.94	17.96
PurometTM MTS9570	2633.95	7990.09	36.65
SupergelTM SGC650	3180.05	9088.51	41.17

.

The REM adsorption process is spontaneous, exothermic, and accompanied by a decrease in entropy for anion-exchange resins.

The positive values of entropy and enthalpy in the REM sorption process of cationic resins can be explained by the loss of the hydrate shell of complex ions and by differences in the structure and functional groups of the resins.

4. Conclusions

The utilization of secondary sources and waste materials, such as extraction phosphoric acid and phosphogypsum, which are byproducts of apatite concentrate processing, offers a promising approach to the recovery of rare earth metals (REMs).

The process of ion exchange of the light group of REMs (Nd, Pr, Sm) from sulfate and phosphoric acid solutions, modeling the process of sulfuric acid leaching solutions from apatite concentrate and phosphogypsum, was studied. In concentrated phosphoric acid solutions, REMs are present in the form of cationic complexes [Ln(H₂PO₄)]²⁺, for the extraction of which cation-exchange resins (Puromet^TM MTS9500, Puromet^TM MTS9570, Supergel^TM SGC650) are used. In sulfate solutions, REMs can form anionic complexes [Ln(SO₄)₂]⁻, which requires the use of anion-exchange resins (Rosek-21, Puromet^TM MTA6002PF).

The experimental sorption isotherms were described by a thermodynamic model based on linearization of the mass action law equation obtained for the corresponding ion-exchange equations and mathematically transformed into a linear form.

Low values of the equilibrium constants (in the range of 1.06 to 3.92), and consequently of the Gibbs energy changes (in the range of −0.14 to −3.38 kJ/mol), indicate a low selectivity of anion-exchange resins with respect to individual REMs, but do not exclude the use of anion-exchange resins for the extraction of the sum of the light group of REMs.

The process of light-group REM extraction on cation-exchange resins from solutions modeling the composition of extraction phosphoric acid is characterized by significant differences in the values of equilibrium constants (in the range of 12.77 to 115.29) and Gibbs energy changes (in the range of −6.31 to −11.76 kJ/mol). The highest values of equilibrium constants were obtained for the cation-exchange resin MTS9570 (34.39 for praseodymium, 39.19 for neodymium and 115.29 for samarium), which allows a more complete extraction of REMs. However, the most significant discrepancies in the values of ion-exchange equilibrium constants were observed for SGC650 resin, which suggests that this resin is particularly effective for the separation of REMs into individual lanthanides.

The low values of equilibrium constants and Gibbs energies also indicate the possibility of regeneration of ion-exchange resins by sulfuric acid solution due to the shift in ion-exchange reaction equilibrium.

Spectral analysis of ion-exchange resins before and after sorption of REMs confirmed the presence of sulfate and phosphate complexes in the resin matrix and clarified the mechanism of ion exchange.

Based on the values of the Gibbs energy values and the experimentally determined enthalpy of the ion-exchange process, entropy values were calculated. The process of REM extraction on anion-exchange resins is characterized by negative enthalpy and entropy values. REM sorption on cation-exchange resins MTS9570 and SGC650 is endothermic with positive entropy values, which may be due to differences in the structure of the ion-exchange resins and the destruction of hydrate shells of REM complex ions.

The obtained results emphasize the potential of ion-exchange resins for REM extraction from sulfate and phosphoric acid media, offering an environmentally friendly alternative to liquid extraction. This approach enables the elimination of organic solvents, which are frequently utilized in substantial quantities during liquid extraction processes. The utilization of these solvents can result in environmental contamination and the formation of potentially explosive mixtures. Therefore, the method of ion-exchange extraction of REMs facilitates the efficient extraction of target components from process solutions while concomitantly contributing to sustainable development and the reduction in the environmental impact of the chemical industry.