One-Pot Strategies for Lithium Recovery from Beta-Spodumene and LTA-Type Zeolite Synthesis

Abstract

This study presents a groundbreaking method for extracting lithium from beta-spodumene while simultaneously achieving the sustainable synthesis of LTA-type zeolite, designated as LPM-15, without relying on organic solvents or calcination. Lithium extraction was efficiently performed using sodium salts, accompanied by the recycling of the mother liquor, with lithium content in the supernatant precisely quantified via atomic absorption spectroscopy (AAS). The optimized synthesis route enables the concurrent production of Li₂CO₃ and LPM-15, distinguished by a powdered appearance without a well-defined geometric framework and a unique cubic morphology with spherical facets, respectively. To gain deeper insights into the process, density functional theory (DFT) simulations were conducted to analyze how different cation exchanges (Na⁺ replacing Al³⁺, NH₄⁺ replacing Al³⁺, and Ca²⁺ replacing Al³⁺) influence the structural stability and diffusion dynamics within the zeolitic pores of LPM-15. Additionally, cation-exchange capacity (CEC) measurements further assessed ion mobility within the LPM-15 framework. This integrative approach not only sheds light on the fundamental mechanisms underpinning LTA-type zeolite synthesis but also demonstrates their versatile applications, with particular emphasis on water purification technologies.

1. Introduction

The extraction of lithium from lithium-bearing aluminosilicates (e.g., beta-spodumene—β-LiAlSi₂O₆) has become increasingly significant due to the growing demand for lithium in energy storage systems and other advanced technologies [1].

Current lithium extraction methods include calcination and roasting, which are efficient but energy-intensive, and co-precipitation of sulfates, which is cost-effective but prone to impurities. Ion-exchange and sorption processes offer high specificity but are expensive, while leaching and froth flotation are versatile yet environmentally damaging or less effective in complex matrices. Chemical treatments with chlorides or fluorides and molten salt electrolysis achieve high purity but face limitations due to corrosive reagents or high costs [2,3,4,5,6].

The processing of beta-spodumene often generates complex sodium–lithium aluminosilicate by-products, significantly increasing extraction costs. The high reliance on organic solvents and actinide bases poses additional challenges [7]. These by-products not only reduce lithium recovery yields but also produce lithium-poor liquors with impurities that are difficult to separate. The reuse of aluminosilicate mineral residues from industrial processes is currently one of the major topics in the development of crystalline microporous materials like zeolites [8,9]. This area has garnered significant attention due to the dual benefits of waste valorization and the development of cost-effective materials. For example, aluminosilicate residues from mining and metallurgical industries have been explored for their potential in creating zeolitic frameworks with tailored properties [10].

Zeolites are well-known for their microporous structure, which allows for the selective classification of molecules based on size and physical properties. Among these, LTA-type zeolites have attracted attention due to their low cost, high thermal stability, and promising applications in separation processes and catalysis [11,12]. The unique combination of their physical and chemical properties offers opportunities for optimizing lithium recovery processes while leveraging their selective ion-exchange capacity.

The use of zeolites as molecular sieves and their potential applications in lithium recovery have been investigated by various researchers [13,14,15]. For instance, studies on LTA-type zeolites have demonstrated their effectiveness in cation exchange, yielding high purity lithium carbonate under optimized hydrothermal conditions [16]. Recent contributions in computational simulations have provided insights into structural modifications of zeolites through cation exchange, enabling predictions of adsorption behavior. Such approaches integrate experimental data with density functional theory (DFT) to explore molecular interactions and structural transformations [17,18].

This study investigates the use of aluminosilicate residue as a sustainable source of silicon and aluminum for zeolite production. Our approach focuses on optimizing conditions for solubilizing beta-spodumene to produce lithium carbonate without calcination, refining hydrothermal synthesis for LTA-type zeolite formation, evaluating lithium carbonate precipitation efficiency and purity, and studying cation-exchange dynamics in LTA-type zeolites. By combining experimental data with computational simulations, we aim to advance zeolite synthesis and its role in lithium extraction.

This innovative dual-product strategy, which produces both Li₂CO₃ and LTA-type zeolite from beta-spodumene, enhances cost efficiency and addresses environmental concerns, offering a breakthrough in sustainable lithium recovery and byproduct synthesis.

2. Materials and Methods

2.1. Raw Materials

Beta-spodumene, supplied by Companhia Brasileira de Lítio (CBL, Minas Gerais, Brazil), was used in this study. Its chemical composition is detailed in

Table 1. The chemical compositions of the beta-spodumene sample (wt. %).

Component	SiO2	Al2O3	Li2O	Fe2O3	K2O	Others
Composition (%)	68.97	22.31	6.43	0.92	0.42	<0.40

, with a Si/Al molar ratio of 2.6. The sample lithium concentration was 3.73% Li, corresponding to the maximum lithium content typically found in spodumene minerals [19]. All additional reagents were of analytical grade and used without further purification.

2.2. Lithium Extraction and Obtaining LTA-Type Zeolite Procedure

The procedure was adapted from literature [20]. Initially, 1.0 g of sodium hydroxide (98+%, Sigma) was dissolved (demineralised water) to solubilize 10.0 g of beta-spodumene in a reflux system maintained at 353 K for 120 min. The resulting solution was then evenly split into two portions: (i) one portion was combined with sodium aluminate (99%, Sigma), and (ii) the other portion was treated with sodium carbonate (99.5%, Dinâmica) in an autoclave. The temperature for this treatment started at 368 K and was gradually raised to 523 K over 720 min. Portions (i) and (ii) were subsequently recombined and subjected to hydrothermal processing, which began at 368 K and was progressively increased to 383 K over a span of 360 min. The warm solution was filtered, and the mother liquor was reused for further solubilization with a fresh sodium hydroxide solution of similar concentration (up to three repetitions). Then, ammonium bicarbonate (99+%, Synth) was added until complete dissolution of lithium carbonate was achieved within 60 min of stirring, followed by filtration. The solid residue was collected and dried overnight, while the supernatant was evaporated to facilitate the precipitation of NH₃ and Li₂CO₃.

2.3. Evaluation of Zeolite Adsorptive Properties

2.3.1. Cation-Exchange Capacity (CEC)

The methodology involved a simple ion-exchange process, where the zeolite sample synthesized in the previous section was brought into contact with the target ion solution under constant agitation until equilibrium was reached [21]. The concentrations of ions in the solution, both before and after interaction with the zeolite, were analyzed to assess the exchange capacity. The ability of the zeolites to exchange cations was evaluated by replacing Na⁺ ions within the structure with NH₄⁺ and Ca²⁺ ions using 0.1 mol/L solutions of NH₄Cl and CaCl₂, respectively, with 1.0 g of the zeolite sample. The suspension was stirred at room temperature for 30 min, then filtered, and the trapped material was rinsed with 1.0 L of deionized water and dried in an oven overnight. The contact time for ion exchange was fixed at 30 min.

The dried zeolite sample was weighed and stirred at 7000 rpm for 10 min with 100 mL of ammonium chloride solution. A 0.5 mL aliquot of the supernatant was collected to measure ammonia absorption (q), calculated as the amount of ammonia removed per unit mass of zeolite (mgNH₃/gZ) using Equation (1), where C_o and C_f represent the initial and final ammonia concentrations, V is the volume of solution in liters, and m is the mass of the zeolite sample in grams [21].

$$q={\displaystyle \frac{\left(C_o-C_f\right)V}{m}}$$

(1)

For the determination of Ca²⁺ ion concentrations, a portion of the supernatant obtained from the calcium chloride solution (100 mL) was extracted, diluted, filtered, and analyzed through ion-exchange chromatography. Sodium ion concentrations in the LTA-type zeolite were quantified using inductively coupled plasma optical emission spectrometry (ICP-OES), while NH₄⁺ exchange measurements were performed with an atomic absorption spectrophotometer (Shimadzu, Kyoto, Japan). All cation-exchange values were converted to milliequivalents per gram (meq/g) for consistency.

2.3.2. Density Functional Theory (DFT)

Figure A1a (refer to Appendix A for all supplementary figures) illustrates the unit cell structure obtained from the Database of Zeolite Structures (IZA), visualized using GaussView 5.0.8 software. The displayed structure corresponds to the configuration with the lowest mean energy for the LTA-type zeolite [22]. Within the LTA simulation cell, there were 24 aluminum atoms [23]. A systematic evaluation of conformational isomers for each substituent was carried out. The reference conformation (Figure A1b) was employed as the simplest initial structure, where each Al³⁺ ion was systematically replaced with cations (Na⁺, NH₄⁺, and Ca²⁺), placed either at the central site or edge of the framework. A “star” pattern was applied as a structural constraint to avoid simultaneous isomer overlap [23]. No exhaustive conformational search was performed for the proposed structures, focusing instead on the molecular coordinates.

Computational calculations were performed using Gaussian 09W software at the density functional theory level. The M06-2X functional, which incorporates twice the amount of nonlocal exchange (2X), was employed with the Pople-style 6-311++G(d,p) basis set [22,24]. Frequency analyses were conducted to identify stationary points for the various conformations and confirm their stability. Structural geometry analysis was performed to assess molecular planarity and evaluate how planarity variations influenced connection energies [24].

Atomic charges were calculated using the Qeq method to achieve charge balancing, with the Ewald algorithm employed to account for weak interactions, such as electrostatic forces, hydrogen bonding, and van der Waals interactions [25]. To optimize computational efficiency, spin-polarization (UKS configuration) was deactivated, ensuring that the alpha and beta orbitals remained equivalent during DFT calculations [22].

2.4. Characterisations

The characterization of the materials utilized a combination of analytical techniques. X-ray diffraction (XRD) patterns of the powder samples were acquired using a PANalytical X’pert diffractometer(Malvern Panalytical, Malvern, UK) with CuKα radiation (λ = 1.54 Å) and a nickel filter. The system operated at a current of 10 mA and a voltage of 30 kV, with measurements taken at a step size of 0.02° and an acquisition time of 0.1 s, employing a Lynxeye detector. Crystallinity was assessed by analyzing peak intensities at specific 2θ values (7.1°, 10.1°, 12.4°, 16.1°, 21.6°, 23.9°, 27.1°, and 34.1°), with a highly crystalline LTA zeolite (Si/Al molar ratio ~1) serving as the reference.

Morphological characterization was performed using a JEOL JSM-6300 scanning electron microscope (SEM) (Akishima, Tokyo, Japan) equipped with a conventional thermionic emitter and Link-Isis imaging. The micrographs, primarily obtained through secondary electrons, were acquired with the sample positioned 7–15 mm from the lens. Backscattered electron (BSE) imaging was employed to guide energy-dispersive X-ray spectroscopy (EDS) analyses. EDS analyses, conducted using an Ultim Max detector (Oxford Instruments, High Wycombe, UK) and Aztec Version 4.0 software, were carried out at 20 kV.

To evaluate the chemical composition and structural features of beta-spodumene and the resulting LTA-type zeolite, Fourier-transform infrared (FT-IR) spectroscopy and atomic absorption spectroscopy (AAS) were employed. FT-IR spectra were obtained using a Nicolet 710 FT-IR spectrometer (Thermo Fisher Scientific, Waltham, MA, USA), offering insights into the structural framework of the materials. Li₂CO₃ qualification was analyzed using ICP-OES with a PerkinElmer Optima 2000 DV instrument (PerkinElmer, Waltham, MA, USA).

3. Results and Discussion

3.1. Recovery of Lithium as Li₂CO₃ and By-Product Formation of LTA-Type Zeolite

Lithium is primarily situated within the tetrahedral framework of silicon and aluminum in the beta-spodumene structure, forming a structure devoid of molecular-sized channels or voids. This structure limits chemical exchange and extraction possibilities [26]. This structural feature poses a challenge for lithium extraction.

The recovery process yields lithium in the form of Li₂CO₃ while simultaneously producing an LTA-type zeolite as a secondary product. Initial XRD analysis (Figure 1) of beta-spodumene reveals the presence of various crystalline phases that are notably rich in silicon and aluminum. These crystalline phases exhibit significant stability, which can impede the extraction process.

The introduction of NaOH serves two primary functions: it promotes the dissolution of the stable crystalline phases and enables a selective ion-exchange mechanism. This chemoselective process facilitates the exchange of lithium ions (Li⁺) in beta-LiAlSi₂O₆ with sodium ions (Na⁺) from Na₂CO₃ [26,27,28,29]. Furthermore, NaOH acts as a mineralizing agent, enabling the extraction of silicon and aluminum species from the mother liquor that remains after the leaching process. This process leads to the formation of Na₂SiO₃ and NaAlO₃, as described in Reactions (2) and (3) [30].

$$7\mathrm{NaOH}(\mathrm{aq})+2\mathrm{LiAl}{\mathrm{Si}}_2{\mathrm{O}}_6(\mathrm{aq})\stackrel{\Delta }{\to }{\mathrm{NaAlO}}_3(\mathrm{s})+3{\mathrm{Na}}_2{\mathrm{SiO}}_3(\mathrm{s})+{\mathrm{LiAlSiO}}_3(\mathrm{aq})+\mathrm{LiOH}(\mathrm{aq})+3{\mathrm{H}}_2\mathrm{O}$$

(2)

$$2{\mathrm{LiAlSiO}}_3(\mathrm{aq})+{\mathrm{Na}}_2{\mathrm{CO}}_3(\mathrm{s})\stackrel{\Delta }{\to }{\mathrm{Li}}_2{\mathrm{CO}}_3(\mathrm{aq})+2{\mathrm{NaAlSiO}}_3(\mathrm{s})$$

(3)

Through a series of experiments in which the NaOH concentration in the leaching liquor was varied, it was identified that approximately 7 g of NaOH was necessary for achieving optimal recovery of the solid phase, which is a critical parameter for Reaction (2). As shown in Figure 2, lithium dissolution demonstrated rapid kinetics at a Li₂O:2Na₂CO₃ mass ratio, achieving an extraction efficiency of 72% within 120 min. Beyond 240 min, the extraction rate leveled off, except when the process was conducted at 513 K. Notably, decreasing the temperature from 453 K to 393 K resulted in an approximately 10% improvement in lithium yield over a 240 min duration.

Lithium extraction from the solution, following the precipitation of silicon and aluminum, can be performed using any stablished method [27]. In this study, lithium was precipitated using an ammonium bicarbonate (NH₄HCO₃) solution, as described by Reaction (4). The dissolution behavior of Li₂CO₃ in the NH₄HCO₃–water system was found to be directly influenced by the partial pressure of CO₂ in a closed environment, while it was inversely affected by either an increase in the concentration of solids or the formation of insoluble Li₂CO₃. After heating the system to 95 °C, lithium was recovered in the form of lithium carbonate, accompanied by the gradual removal of H₂O and NH₃. The precipitated lithium carbonate was subsequently filtered, thoroughly rinsed with water, and dried to complete the recovery process.

$$\mathrm{LiOH}\left(\mathrm{aq}\right)+{\mathrm{NH}}_4{\mathrm{HCO}}_3\left(\mathrm{s}\right)\to {\mathrm{LiHCO}}_3\left(\mathrm{aq}\right)+{\mathrm{NH}}_3\left(\mathrm{aq}\right)+{\mathrm{H}}_2\mathrm{O}\stackrel{\Delta }{\to }{\mathrm{Li}}_2{\mathrm{CO}}_3\left(\mathrm{s}\right)+{\mathrm{H}}_2\mathrm{O}\uparrow +{\mathrm{CO}}_2\left(\mathrm{g}\right)\uparrow +{\mathrm{NH}}_3\left(\mathrm{g}\right)\uparrow$$

(4)

Reaction (4) allows CO₂ recovery by vacuum heating the LiHCO₃ solution, enhancing dissolution. Ambient temperature is preferred due to decreased Li₂CO₃ solubility at higher temperatures, as shown in

Table A1. Li₂CO₃ solubility in the NH₄HCO₃–water system (g/100 g H₂O).

Temperature (K)	373	353	333	323	298
Solubility	0.72	0.85	1.01	1.08	1.27

(refer to Appendix B for all supplementary tables). The XRD patterns (Figure 3) and ICP-OES analysis (

Table 2. Qualification of the formed Li₂CO₃.

Li2CO3 Purity (%)	Content of Impurities
	Na	Si	Mg	K	Al	Others
95.4	1.83	1.12	0.88	0.53	0.03	0.21

) confirmed an approximate Li₂O recovery of 82%. The composition was determined using ICP-OES, a method suitable for analyzing inorganic compounds, with the results processed and normalized to 100%. To surpass the 90% Li₂O (wt. %) yield threshold, the addition of approximately 10 g Li/L of Li₂CO₃ solution during precipitation was required.

When sodium aluminate was introduced to the mother liquor, a gel-like material with Si/Al ratios resembling those of LTA-type zeolite was formed. XRD patterns (Figure 4) were used to correlate Li₂O:8Al₂O₃ molar ratios with various autoclaving periods.

Following a 360 min hydrothermal treatment, the resulting product was free of impurities typically associated with analcime and sodalite phases (2θ = 13.9°, 19.8°, 28.1°, and 31.5°) [21]. This observation indicates that NaAlO₃ and Na₂SiO₃ species were effectively dissolved due to the presence of excess NaOH during the extended crystallization process. However, products from the 120 and 240 min treatments displayed low-intensity reflections at 2theta = 7.02°, 10.13°, 12.21°, and 16.44°, suggesting incomplete formation of a pure zeolite phase despite adjusting the Si/Al ratio. The addition of aluminate resulted in an Al excess due to non-dissolution of quartz under initial conditions. This led to the formation of a gel layer rich in zeolitic nuclei, transforming amorphous particles into zeolite crystals upon appropriate crystallization. Thus, the growth mechanism, occurring subsequent or parallel to primary particle agglomeration and densification [23], was observed in the 360 min reaction. The degree of crystallinity (DC) was calculated using Equation (A1) and detailed in

Table A2. Crystallinity degree by XRD, calculated based on the relative intensity of selected peaks.

Reference	360 min	240 min	120 min	30 min
LTA zeolite (IZA)	0.89	0.76	0.56	0.44

(refer to Appendix B for both items).

The hydrothermal treatment conducted at low temperatures (368 K) and the leaching process maintained within a temperature range of 298–303 K played a pivotal role in fostering crystal growth while suppressing the formation of competing phases, like GIS- and SOD-types [21].

The peaks visible in the diffractogram (Figure A2) were significantly influenced by particle size, where the full width at half maximum (FWHM) was inversely related to crystallite size. As the crystallite size increased, the FWHM narrowed, leading to a proportional enhancement in peak intensity to ensure the total peak area remained consistent.

One potential factor contributing to the observed similarity between the diffractograms in Figure 4 and the variation in crystallinity reported in Table A2 could be the configuration and settings of the diffractometer, along with other influencing variables. Figure A2 presents SEM-EDS layered images of LTA-type zeolite crystals derived from irregular beta-spodumene particles. Specifically, Figure A2a showcases crystals with cubic morphology and spherical facets [28], which are typical features of LTA-type zeolites. These characteristics suggest an equilibrium between the dissolution and crystallization process following a 360 min treatment. Conversely, Figure A2b,c display irregularly shaped crystals deposited beneath quartz crystals, suggesting mild synthesis conditions.

The conversion of beta-spodumene to the LTA-type zeolite is further confirmed by the infrared (IR) spectra recorded in the 4000–400 cm⁻¹ region (Figure A3). Distinct structural changes are evident in key bands, such as those near 1000 cm⁻¹, which correspond to Si-O-Al bonds in tetrahedral TO₄ units. Bands at 675, 562, and 465 cm⁻¹ reflect the crystallinity structure of LTA-type zeolite [18,21]. The transformation also results in the disappearance of certain bands, leaving a broad asymmetric band centered at 1220 cm⁻¹ [29]. Additional shifts in Si-O stretching bands and the absence of the Si-O-Al band near 735 cm⁻¹ suggest distortions in both the tetrahedral and octahedral frameworks [26]. The band at 570 cm⁻¹ is linked to the vibration of the TO₄ double ring (D4R), which is a key component of the secondary building unit for the LTA-type zeolite [21]. Vibrations at 3500 and 1650 cm⁻¹ are associated with -OH functional groups, which are indicative of zeolitic water [27].

Theoretical analysis of D6R units in the zeolite structure provided additional insights, enabling the assignment of specific bands and enhancing the understanding of the material’s structural characteristics (

Table A3. Structural diversity by FT-IR analysis of LTA (D6R) zeolites.

Wavenumber (cm−1)	Denotation
1008	Asymmetric stretching vibrations of bridge bonds—νs Si-O(Si) and νs Si-O(Al).
675	Symmetric stretching of bridge bonds—νs Si-O-Si and νs Si-O-Al.
562	Symmetric stretching of Si-O-Si bonds and bending vibrations—δ O-Si-O.
465	Bending vibrations—δ O-Si-O in “antiphase”, and δ O-Al-O.

).

Based on these findings, a schematic representation of the synthesis pathway was developed. This diagram illustrates the process by which Li₂CO₃ is produced as the primary product, alongside LTA-type zeolite as a secondary by-product, during lithium extraction from beta-spodumene. The schematic representation is shown in Figure 5, and the synthesized LTA-type zeolite has been designated as LPM-15.

3.2. Cation-Exchange Capacity (CEC) Test

Table S1 (Supplementary Material) shows the CEC of zeolitic materials obtained, acquired at different durations of hydrothermal treatment, in accordance with the outlined synthesis conditions. These values are compared with those of the synthesized NaP1 and 4A zeolites [21,29], showcasing their CEC for ammonium (NH₄⁺) and calcium ion (Ca²⁺), respectively.

The CEC of the synthesized NaP1 zeolite was measured as 2.5 meqNH₄⁺/g, aligning with the range of 2.5–4.0 meq/g reported by Cardoso and co-workers [29]. For zeolites produced at reaction times of 30, 120, and 360 min, the corresponding CEC values were 0.3, 0.8, and 2.2 meq/g, respectively. Extended reaction durations facilitated the dissolution of the vitreous matrix and quartz, whereas shorter reaction periods resulted in materials containing greater levels of impurities, such as mullite and quartz, or the formation of less desirable phases like analcime and sodalite.

The synthesized 4A zeolite exhibited a CEC of 4.5 meqCa⁺²/g, which is slightly lower than the reference value of 5.4 meq/g from the literature [29]. For calcium, CEC tests conducted at various reaction times (30, 240, and 360 min) yielded values of 2.0, 3.6, and 4.1 meq/g, respectively. The 4.1 meq/g value obtained after 360 min is particularly significant, as it highlights the production of a high-quality product suitable for use in the detergent industry [29]. This outcome underscores the efficiency of the synthesis method employed and demonstrates the potential of the synthesized 4A zeolite by-product in optimizing lithium extraction from beta-spodumene.

3.3. DFT Study of x/Al³⁺ Substitution in LPM-15 (x = Na⁺, NH₄⁺ or Ca²⁺)

The substitution of Na⁺ for Al³⁺ was examined by analyzing the positions of the extra-framework cation to determine the relative stability of its geometries (central or edge positions). Computational simulations and energy evaluations (

Table A4. Influence of x/Al³⁺ (where x = Na⁺, NH₄⁺, or Ca²⁺) substitutions on the structural geometry of LPM-15 material.

x/Al3+	RPCS 1	LP 2	APV 3	POD 4	E 5	ΔE 6
x = Na+	Center	10.7	688.6	5.5	<0.01	77.5
	Edge	12.8	695.1	6.0	0.04	101.2
x = N${\mathrm{H}}_4^{+}$	Center	11.9	3338.7	10.7	<0.01	106.1
	Edge	11.7	2338.7	9.8	0.25	307.3
x = Ca2+	Center	11.9	876.2	5.8	<0.01	115.4
	Edge	12.0	863.3	5.9	0.14	188.5

¹ Relative position of cations in the structure; ² lattice parameter (Å); ³ accessible pore volume (Å3); ⁴ pore opening diameter (Å); ⁵ absolute energies in Hartrees; ⁶ relative energies between the isomers (kJ/mol).

) showed that the Na⁺ cation preferentially resides at the central position, closely interacting with O²⁻ ions due to strong ionic attraction forces (Figure 6).

Unlike extra-framework hydrogen (H⁺), which tends to form O-H bonds at the edges, the Na⁺ cation is more likely to remain in proximity to oxygen atoms in LPM-15, contributing to structural stability at central sites where it exhibits greater energetic favorability [30]. The ionic radius of sodium is approximately 1.07 Å. For clarity, the values in this study were converted from Hartree to electron-volt (eV) and kilojoule per mole (kJ/mol) using the following conversion: 1 Hartree = 27.2 eV = 2630 kJ/mol.

As shown in Table B.4, cation substitution was found to correlate directly with the accessible pore volume of the zeolite. The introduction of larger cations, such as Na⁺ and Ca²⁺, into LPM-15 leads to an increase in physical space occupation, effectively reducing the internal pore volume. This causes repulsive forces to surpass the attractive forces exerted by O²⁻ ions, driving Na⁺ cations toward the center of the unit cell [22].

For the Ca²⁺/Al³⁺ substitution, the configuration where Ca²⁺ is surrounded by the O²⁻ ions (Ca-center configuration) was observed to be highly stable due to the strong attractive interactions (Figure 7a) [31]. In contrast, the Ca-edge configuration (Figure 7b), involving interactions with only four oxygen ions (O1, O2, O3, and O4), introduces distortions in the structural framework. These distortions reduce O-O interatomic distances and result in interference from the Ca-ring structure, leading to a Δlength of approximately 4.51 Å. This interference explains the higher energy values (ΔE) associated with the Ca-edge configuration compared to the Na-edge and Na-center geometries, as shown in Table A4 [32].

Adjustments to the lattice parameter (a3) could potentially alleviate the repulsive interactions within the ring structures [31]. However, such adjustments may also expand the overall volume, increasing the zeolite’s accessible pore size.

The position of substituents significantly affects the Al-Na bond distance. Larger groups, such as methyl and ethyl, tend to experience strong repulsive forces [31]. For instance, when comparing the pore-opening diameter between oxygen atoms, the distance to the Na-center was measured at 5.512 Å, whereas the distance to the Na-edge was slightly larger at 6.020 Å. The similar accessible pore volumes of Na⁺ and Ca²⁺ can be attributed to the similarity in their ionic radii (1.02 Å and 1.0 Å, respectively) [31,32]. In computational simulations, the difference observed was due to the UKS-off setting, which limited orbital interactions by preventing the “star” symmetry from utilizing all available orbitals.

The relative energy values indicate the most stable geometries for cation substitution. For the first series of substitutions (center-position), the stability order was Na⁺ > NH₄⁺ > Ca²⁺, whereas for the second series (edge-position), the order was Na⁺ > Ca²⁺ > NH₄⁺. Astala and co-workers [24] reported that ion exchange between Na⁺ and Ca²⁺ is commonly seen during the softening of hard water. Table A4 shows that if LPM-15-Na were exposed to water containing calcium, ion exchange would reduce the pore accessibility. This is evidenced by a decrease in the pore-opening diameter from 6.045 Å to 5.903 Å, indicating the replacement of sodium by calcium, which slows the process.

In the NH₄⁺/Al³⁺ exchange, the ammonium ion exhibits strong interactions due to the involvement of auxiliary aluminum atoms. These atoms facilitate hydrogen bonding between the oxygen atoms in the Si-O-Al bonds and the hydrogen atoms of the NH₄⁺ ion. This interaction stabilizes the remaining hydrogen atoms in the NH₄⁺ ions, aided by confinement effects created by the oxygen atoms in the LPM-15 lattice. The process contributes to the development of acidic properties in the material due to the formation of surface hydroxyl groups (Brønsted acid sites). These groups form because the NH₃ molecule lacks adsorptive affinity for Lewis acid sites, preventing strong hydrogen bonding [33].

The formation of active sites in the pentasil units of the zeolite structure is consistent with computational data, which indicate that the center position is the preferred site for H⁺/Al³⁺ substitution, as the center exhibits lower relative energy compared to the edge position. Simulations of simultaneous x/Al³⁺ (x = Na⁺, NH₄⁺, or Ca²⁺) substitutions were not performed because introducing multiple extra-framework cations would destabilize the LPM-15 structure, lowering the Si/Al molar ratio. According to the Lowenstein rule, Al-O-Al linkages are not allowed in zeolite frameworks, which sets a minimum Si/Al molar ratio of one for cationic aluminum substitution [33].

A study by the Lima group examined the impact of these cations on NH₄⁺ retention in wastewater [34]. The observed retention sequences was Na⁺ > Ca²⁺, with Na⁺ exhibiting stronger adsorption despite having an ionic radius comparable to that of Ca²⁺. The influence of Ca²⁺ on NH₄⁺ retention was minimal, which aligns with the preferred cation adsorption sequence in the zeolite framework.

4. Conclusions

This study highlights a novel and efficient approach for synthesizing zeolites as a secondary product from the lithium extraction process using beta-spodumene. By maintaining a Si/Al ratio close to that of the Li–aluminosilicate feedstock, the process minimizes pretreatment steps while the low-mass-percentage compounds in beta-spodumene exhibit negligible effects on the final zeolite composition. Prolonged crystallization of up to 2 h enhances material stability and refines particle size, resulting in well-defined pores and channels. The synthesized zeolite primarily comprised the NaP1 phase, with quartz as an impurity removable via decantation or flotation. Reaction time played a crucial role in lithium extraction and precipitation rates. Notably, over 80% lithium recovery was achieved after 4 h of solubilization without calcination, with residual lithium likely incorporated into the zeolite framework channels, creating counter ions for ion exchange. The fine-tuning of synthesis parameters enables the concurrent production of an LTA-type zeolite (LPM-15), characterized by high adsorption efficiency, as indicated by CEC. Computational modeling revealed structural stability via Na⁺/Al³⁺ exchange, enhancing planarity and reducing repulsion effects. The geometric parameters of LPM-15, including pore volume and opening diameter, establish a strong foundation for future research, particularly for applications in water purification and effluent treatment.

5. Patents

The work described in this manuscript has been patented under registration code BR102018016312-4 B1.