Conversion of Natural Clay into Na-A (LTA) Zeolite Adsorbent for Efficient Heavy Metals Adsorption from Aqueous Solution: Kinetic and Isotherm Studies

Abstract

In this work, zeolite LTA (Linde Type A) was synthesised from natural clay as a novel adsorbent for copper and lead ions removal from water effluents. The applied process allowed the reuse of kaolin, as natural clay, for the production of zeolite LTA through a stepwise process, which involved the formation of metakaolin. The results of characterisation showed the formation of crystalline cubic crystals of zeolite with a mean dimension of 2–3 microns, indicating the successful nucleation and development of the LTA zeolite phase. Batch adsorption studies were carried out to study the removal ability of zeolite LTA by testing Cu²⁺ and Pb²⁺ ions. Effects of contact time, pH, and adsorbent dosage were investigated. At pH > 5, the removal efficiency for both metals exceeded 95%. As the zeolite dosage increases from 2 to 10 g/L, the removal effectiveness for both metals markedly enhances (>95% at 10 g/L for lead ions and >90% at 10 g/L for copper ions). The adsorbent showed a higher adsorption capacity in removing lead compared to copper (Q_m = 81.5 mg/g for Pb²⁺ and 67.5 mg/g for Cu²⁺). The adsorption process was well described by the pseudo-second-order kinetic model, while the Langmuir isotherm adequately depicted the equilibrium behavior. Notably, the kinetics revealed distinct contributions from chemisorption and physisorption, with the AOAS model effectively quantifying their respective roles in metal ion removal. The findings revealed that prepared zeolite LTA acts as an efficient adsorbent to remove heavy metals.

1. Introduction

The developing world is recently facing the scarcity of pure and clean water due to an immense increase in population and industrialization [1]. The contamination of wastewater with heavy metals, resulting from rapid industrialisation and intensive human activities such as mineral processing, petrochemical operations, metal plating, mining, smelting, and battery manufacturing, has become a major global concern due to its significant environmental and health risks [2,3]. The concentration of heavy metals in soil, groundwater, and various water bodies is exceeding the acceptable limits in many countries as a result of unplanned urbanisation, industrialisation, population growth, and poor management policies, leading to a hidden threat to the human food chain [4]. Heavy metals like lead and copper, commonly found in industrial effluents, are non-biodegradable and tend to accumulate in the environment, leading to serious health issues in humans, including stunted growth, neurological impairments, organ damage, and cancer, as well as causing severe harm to ecosystems [5,6]. If these toxic heavy metals in the aquatic environment are transferred and amplified by aquatic animals, they may pose severe health risks to humans [7]. To address these challenges, a variety of techniques have been developed for the removal of heavy metal ions from wastewater, including chemical precipitation, ion exchange, chemical coagulation, membrane processes, and adsorption [3,8]. However, many treatment methods have some drawbacks, such as high energy requirements, complex operation, and limited conditions [9,10,11]. Among these, adsorption stands out as one of the simplest and most effective methods [12], owing to its low initial cost, straightforward design, and ease of operation, especially when using adsorbents with a high affinity for target metals [13]. Zeolites, in particular, are highly promising adsorbents due to their unique properties such as ion exchange capacity, strong affinity for heavy metal cations, and proven chemical and mechanical stability, making them excellent candidates for the treatment of metal-contaminated wastewater [14,15,16].

Copper (Cu) and lead (Pb) are among the most problematic heavy metals found in industrial and municipal wastewater due to their persistence, toxicity, and cumulative effects in biological systems [17]. Chronic exposure to Cu²⁺ can trigger hepatic and renal dysfunctions, gastrointestinal distress, and neurological damage [18], while Pb²⁺ is even more hazardous, causing severe neurotoxicity, hematological disorders, developmental delays in children, and irreversible organ damage [19]. Both metals resist natural degradation, accumulate in aquatic environments, and readily enter the food chain, amplifying risks for humans and ecosystems. While various adsorbents have been tested for their ability to capture heavy metals, most, including activated carbon, natural clays, and biosorbents, show significant limitations. These include a lack of selectivity for specific ions, rapid saturation, poor regeneration performance, and inefficiency in the presence of competing pollutants or fluctuating pH conditions [20,21,22]. Ashraff Aziz Marhoon et al. synthesized high-purity single-phase zeolite X from treated coal fly ash using a fusion-hydrothermal method, achieving efficient removal of nickel and vanadium ions from aqueous solutions with high adsorption capacity and excellent crystallinity [20]. Prepilková et al. investigated the adsorption efficiency of cadmium and manganese using natural sorbents, including bentonite, zeolite, and stabilized digested dewatered sludge, highlighting the superior metal removal capacity of the sludge in neutral mine drainage conditions, a less studied environment compared to acid mine drainage [23]. In contrast, synthetic zeolites such as LTA stand out for their well-defined, uniform microporous framework and high cation exchange capacity, which confer strong selectivity for heavy metal ions like Cu²⁺ and Pb²⁺.

Zeolites are crystalline hydrated aluminosilicates characterized by distinct three-dimensional microporous frameworks, synthesized via processes including hydrothermal treatment (preferred for its simplicity), alkali fusion, microwave irradiation, and ultrasonic procedures. In recent years, sustainable synthesis methods utilizing natural resources such as kaolin (calcined at 600–900 °C and activated with alkali), smectite clays (hydrothermal conversion), and diatomite (alkali fusion with Na₂CO₃) have facilitated the environmentally friendly production of zeolites, including types A, X, and Na-P1, achieving cost reductions of up to 40% while valorizing mineral waste [24,25,26,27]. These naturally sourced zeolites are gaining significance in environmental applications, such as heavy metal adsorption (up to 200 mg/g for Pb²⁺), wastewater treatment (98% elimination of NH₄⁺ in 15 min), CO₂ capture (4.2 mmol/g), and soil remediation [28,29]. Their porous architecture and elevated cation exchange capacity (2.8 meq/g for Na-P1) render them advantageous in low-carbon cements and solar energy storage systems, establishing natural zeolites as essential elements in the circular economy, with the global market anticipated to attain USD 2.8 billion by 2030. The characteristics and properties of zeolites have attracted attention for their efficiency in adsorbing heavy metals [30,31]. Their adsorption properties are unique since the ion exchange property allows cation exchange with other ions like heavy metals [32].

Zeolite Na-A (LTA) is a highly adaptable and extensively utilized synthetic aluminosilicate characterized by the formula Na₁₂[(AlO₂)₁₂(SiO₂)₁₂].27H₂O. It possesses a distinctive three-dimensional structure composed of interlinked double sodalite cages, forming extensive alpha cavities and a network of microporous channels. This structure features three categories of exchangeable cation sites: located on the six-membered rings of the sodalite cages, adjacent to the centers of the eight-membered rings encircling the alpha cavities, and on the four-membered rings, which accommodate 8, 3, and 12 cations per cavity, respectively. These attributes confer upon Na-A exceptional molecular sieving capabilities and significant hydrophilicity, rendering it advantageous in catalysis and separation operations. Na-A is commercially synthesized hydrothermally from aluminosilicate hydrogels employing silica and aluminum sources; however, there is growing interest in sustainable synthesis methods that utilize abundant and inexpensive raw materials, such as kaolin clay, slags, fly ash, and mining byproducts [33]. Kaolin is significant because its Si/Al ratio closely resembles that of zeolite A, facilitating the synthesis of highly crystalline Na-A by calcination and alkaline activation. By modifying synthesis parameters—such as the transformation temperature of kaolin to reactive metakaolin, the molar ratios of SiO₂/Al₂O₃, Na₂O/SiO₂, and H₂O/Na₂O, along with agitation and crystallisation conditions—the structural and chemical properties of the zeolite can be precisely optimized for targeted industrial and environmental applications, including wastewater treatment, gas separation, and CO₂ capture [34]. This study focuses on the synthesis of LTA zeolite from kaolin for the efficient removal of lead (Pb²⁺) and copper (Cu²⁺) ions from water effluents. The effects of various process parameters on the adsorption performance of the synthesized adsorbent were systematically investigated, aiming to develop a highly effective and durable material for mitigating heavy metal contamination in wastewater.

2. Materials and Methods

2.1. Materials

Sodium hydroxide (NaOH) and kaolin (designated as SZWMK1) were utilized for the production of zeolite A. The kaolin, provided by the ALGERIAN BENTONITES COMPANY EPE/SPA (Maghnia-Tlemcen, northwestern Algeria), is described in

Table 1. Chemical Composition of Kaolin.

Compound	Value (%)
SiO2	49.4
Al2O3	35.5
Fe2O3	0.9
TiO2	<0.3
K2O	1.55
Na2O	<0.1
CaO	<0.1
MgO	<0.4
LOI	11.8

with its chemical composition. The Loss on Ignition (LOI) represents the percentage of weight lost by the kaolin sample when heated to a high temperature; this loss corresponds mainly to the removal of moisture, organic matter, and other volatile substances during the heating process. The chemical composition of the kaolin is expressed in molar percentages, as detailed in Table S1 (see the Supplementary Data). Before synthesis, the kaolin was carefully dried at 110 °C for 24 h in a Pyrex beaker to remove residual moisture. High-purity sodium hydroxide (99.99%) was obtained from Sigma-Aldrich. Copper(II) nitrate trihydrate (Cu(NO₃)₂·3H₂O, 99.9%, Sigma-Aldrich) and lead(II) nitrate (Pb(NO₃)₂, 99.95%, Sigma-Aldrich) were used as metal ion sources for adsorption studies. All chemicals utilized were of exceptional purity. Deionised water (Milli-Q Millipore, 18.20 MΩ·cm) was consistently employed throughout the synthesis process.

2.2. Experimental Methodology

An amalgamated fusion/hydrothermal methodology, as described in previous studies [14], was employed to produce zeolite A from kaolin. Based on the results of the XRF analysis, the SiO₂/Al₂O₃ molar ratio of the raw kaolin was determined to be 2.362 (Table 1). NaOH was used by considering the NaOH/kaolin ratio of 0.9. Fused kaolin was created by combining 10 g of kaolin with 9 g of NaOH, grinding the mixture in a mortar for 15 min, and then fusing it for two hours at 550 °C. The combined product was ground up, dissolved in 100 mL of deionised water, then stirred for 5 h, and left to stir at room temperature for five hours. The mixture was put in a Teflon-coated stainless-steel autoclave, and the heating temperature and crystallization time were adjusted to 90 °C and 20 h, respectively. After the heated product was filtered through a suction device, it was repeatedly cleaned with distilled water to eliminate any remaining NaOH, and it was left to dry overnight at 80 °C in an oven. Figure 1 illustrates the synthesis process steps of LTA zeolite.

2.3. Batch Adsorption Studies

The preliminary adsorption performance of Cu²⁺ (C₀ = 80 mg/L) and Pb²⁺ (C₀ = 80 mg/L) using LTA zeolite was evaluated through batch experiments by separately testing the heavy metal ion solutions. To systematically investigate the effects of zeolite dosage (2–10 g/L), pH (1–11), and contact time (0–250 min) on the adsorption performance, experiments were conducted using 100 mL of solution in 250 mL conical flasks. These flasks were agitated at 150 rpm in shaking incubators (Zhicheng ZWYR-2102C, China). This approach enabled the controlled assessment of how each factor influenced the zeolite’s ability to adsorb Cu²⁺ and Pb²⁺ from aqueous solutions. Solutions were prepared from a stock solution, and their pH was adjusted with HNO₃ or NaOH. All experiments were performed at 25 °C, and solution separation from the adsorbent was achieved by filtration through 0.45 μm membrane filters. Heavy metal ion concentrations were then determined using atomic absorption spectrometry (AAS, PerkinElmer PinAAcle 900T, USA). The adsorption capacity (Q_t) was evaluated through Equation (1), while the removal efficiency (R%) was calculated by using Equation (2):

$${\mathrm{Q}}_{\mathrm{t}}={\displaystyle \frac{(C_0-{\mathrm{C}}_{\mathrm{t}})}{\mathrm{m}}}\mathrm{V}$$

(1)

$$\mathrm{R}\mathrm{\%}={\displaystyle \frac{({\mathrm{C}}_0-{\mathrm{C}}_{\mathrm{t}})}{{\mathrm{C}}_0}}\mathrm{\ast }100$$

(2)

where C₀ represents the initial concentration, C_t is the concentration at time t, V is the volume of the solution, and m is the mass of the adsorbent [12]. To increase the accuracy of the data, each experiment was repeated 3 times.

3. Results and Discussion

3.1. Transformation Pathway from Kaolin to Zeolite LTA

The transformation of kaolin into LTA zeolite typically follows a multi-step process, which can be described and refined as follows (Figure 2). Initially, kaolin was combined with NaOH in a 10:9 mass ratio and subjected to calcination at 550 °C, yielding metakaolin by dihydroxylation and structural reconfiguration, wherein aluminium transitions to tetrahedral/pentahedral coordination. In the second phase, hydrothermal synthesis is conducted by including water at a 1:10 ratio to the precursor mixture, followed by heating at 90 °C. This improved alkaline activation enhances the crystallization of pure LTA zeolite, reducing amorphous byproducts in contrast to traditional techniques. Meticulous regulation of these ratios improves the reactivity of metakaolin and the purity of the end product. The resultant LTA zeolites, characterised by their cubic architecture and elevated surface area, exhibit significant potential for heavy metal adsorption and other advanced applications.

The transformation of kaolin into LTA zeolite entails a stepwise procedure of thermal activation followed by alkaline hydrothermal synthesis, each facilitating critical structural and phase changes. This technique facilitates the synthesis of highly reactive and functional materials for sophisticated industrial applications. The thermal Activation Pathway involves:

• Key process: Dehydroxylation (550–650 °C).
• Structural change: Al coordination shifts from octahedral to tetrahedral/pentahedral.
• Industrial significance: Enhanced pozzolanic activity for cement/concrete.

Followed by the aluminosilicate rearrangement (Metakaolin as a Precursor for Zeolite LTA Synthesis) based on:

• Critical step: Alkaline activation (NaOH solutions or fusion at 550 °C).
• Phase evolution: Amorphous → crystalline LTA frameworks via hydrothermal treatment.
• Morphological features: Cubic crystals (2 µm avg. size) with intergrown twins.

Hierarchical Transformation Process

The synthesis of zeolite LTA from kaolin involves a series of controlled thermal and chemical treatments that facilitate the transformation of the raw clay mineral into a highly crystalline zeolite structure. Each major stage in this process is characterized by distinct reaction conditions and leads to specific phase developments, as outlined below in

Table 2. Process conditions and phase development in the synthesis of zeolite LTA from kaolin.

Stage	Conditions	Outcome
Kaolin → Metakaolin	550–650 °C calcination	Amorphous Al-Si structure
Metakaolin → LTA	Hydrothermal (90–110 °C, NaOH)	Cubic zeolite LTA + sodalite

:

3.2. XRD Analysis

The X-ray diffraction patterns presented in Figure 3 compare the crystalline structures of kaolin, metakaolin, and LTA zeolite. From Figure 3, the kaolin sample exhibits sharp and well-defined peaks characteristic of its highly ordered layered silicate structure. The result, suggesting the existence of several phases, such as illite (K₂Al₄Si₈O₂₄) and amorphous phases of quartz (SiO₂), in addition to kaolin (Al₂Si₂O₅(OH)₄) [35,36]. Upon calcination, the transformation to metakaolin was evidenced by the broadening and reduction in intensity of the diffraction peaks, indicating a loss of long-range order and the formation of an amorphous aluminosilicate phase [37,38]. Following hydrothermal synthesis, the XRD pattern of the LTA zeolite displays new, intense peaks distinct from those of kaolin and metakaolin, confirming the successful crystallisation of the LTA zeolite framework. The XRD pattern of LTA zeolite exhibits characteristic peaks at specific angles (2θ°) corresponding to its distinct crystal planes. The main peaks generally appear around 2θ° of 7.2°, 10.2°, 12.4°, 16.2°, 21.6°, 24°, and 27.1°. This pattern completely matches the JCPDS no. 71-0784 standard LTA (Na) zeolite diffraction pattern [39,40,41]. The presence and intensity of these peaks indicate a good crystallinity of LTA zeolite. The absence of additional peaks indicates that the sample is pure, without secondary phases.

3.3. SEM and EDS Measurement

Figure 4 presents the SEM images of the prepared LTA zeolite. At lower magnifications (40 μm and 20 μm scale bars), the images display aggregates of diminutive, irregularly shaped particles, indicating the preliminary phases of crystallisation and the presence of precursor material. With further magnification (5 μm scale bars), distinct cubic crystals became apparent inside the aggregates, indicating the successful nucleation and development of the LTA zeolite phase. These cubic crystals are indicative of LTA zeolite and exhibit a significant degree of homogeneity in both morphology and dimensions [42,43]. At maximum magnification (4 μm scale bars), distinct cubic crystals are observable, displaying smooth surfaces and sharp edges. The crystals measure roughly 2–3 μm, validating the successful conversion of kaolin into high-quality LTA zeolite. The uniform cubic morphology and limited size distribution shown in the photos suggest effective crystallization and the establishment of distinct zeolite structures, crucial for enhanced adsorption characteristics and efficacy in heavy metal remediation.

Energy Dispersive Spectroscopy (EDS) provides detailed information about the elements present in the sample, such as silicon (Si), aluminium (Al), oxygen (O), sodium (Na), and other elements. Figure 5 shows EDS spectra of the prepared LTA zeolite. The SEM-EDS study validates the effective hydrothermal synthesis of zeolite LTA (Linde Type A) from kaolin precursors, as demonstrated by the distinctive element composition and morphological characteristics. The elemental mapping indicates a uniform distribution of framework components: oxygen (43.64 wt%, 55.68 at%) as the primary bridging element, silicon (20.18 wt%, 14.67 at%) and aluminum (18.86 wt%, 14.27 at%) constituting the tetrahedral framework, with a Si/Al molar ratio of approximately 1.03, aligning with the theoretical LTA structure that necessitates Si/Al ≈ 1.0 for optimal ion-exchange characteristics [44,45]. The red zone in the elemental mapping corresponds to regions where the concentration of the analyzed element is highest, indicating a uniform and locally intense distribution of key components such as oxygen, silicon, and aluminum within the LTA zeolite structure. The substantial sodium concentration (17.32 wt%, 15.38 at%) signifies the existence of charge-balancing Na⁺ cations within the zeolitic cavities, crucial for the cation exchange process that facilitates heavy metal adsorption applications. The SEM micrographs illustrate the development of distinct cubic crystallites characteristic of LTA topology, exhibiting a uniform particle size distribution and the absence of amorphous phases, thereby affirming the complete conversion of the clay aluminosilicate precursor into the intended zeolitic framework under alkaline hydrothermal conditions, essential for attaining elevated adsorption capacity and selectivity in wastewater treatment processes.

3.4. Zeta Potential Measurement

The zeta potential measurement of LTA zeolite as a function of pH is illustrated in Figure S1 (see the Supplementary Data). From the figure, a clear transition from low (near-neutral) to highly negative values as the pH increases from 2 to 12. At acidic pH (2–4), the zeta potential is close to zero, indicating protonation of surface silanol (Si–OH) and aluminol (Al–OH) groups, which neutralize surface charges. As the pH increases, deprotonation of these surface hydroxyl groups occurs, resulting in the formation of negatively charged sites (Si–O⁻, Al–O⁻) that dominate at neutral and basic conditions [46]. This shift leads to strongly negative zeta potentials (up to −54 mV in this study), enhancing electrostatic dispersion and colloidal stability. These changes in surface charge directly affect the adsorption behavior of zeolite toward cationic contaminants, as electrostatic attraction is maximized above the point of zero charge (pH_pzc), typically in the pH range 6–12 for zeolite A [46].

3.5. Adsorption Study

3.5.1. Impact of pH and Adsorbent Dosage

Solution pH, one key factor that significantly affected the sorption of heavy metals, determined the metal speciation and surface charge of the adsorbent [47]. Figure 6 depicts the influence of pH on the efficacy of Pb²⁺ and Cu²⁺ ion removal by synthetic zeolite. At a pH of 1, the removal efficiency for both metals was negligible, with values under 10%. As the pH rises, removal performance significantly enhances for Pb²⁺, efficiency surpasses 90% at pH 4, while, for Cu²⁺, it nears comparable values by pH 5. Beginning at a pH of 5, both metals are extracted with exceptional efficiency, maintaining a recovery rate of 95% or higher up to a pH of 11. This phenomenon can be elucidated by the conflict between hydronium ions (H₃O⁺) and metal ions at low pH, which restricts the adsorption of Pb²⁺ and Cu²⁺ onto the zeolite [48]. At low pH, the zeolite surface exhibits a higher positive charge, and the prevalence of H₃O⁺ ions further inhibits the metal adsorption. The main Pb²⁺ species were presented as Pb²⁺ at pH ≤ 5, and the competition from H⁺ decreased the Pb²⁺ removal via ion exchange [28]. As the pH rises, the dissociation of surface hydroxyl groups (Si-OH and Al-OH) generates additional negatively charged sites, hence augmenting the attraction and binding of metal cations. At elevated pH levels (above 8), the elimination of metals may furthermore entail the precipitation of metal hydroxides; however, within the specified pH range (2–11), the substantial removal efficacy is predominantly ascribed to adsorption, particularly beneath the precipitation threshold. As the pH rises, the dissociation of surface hydroxyl groups (Si-OH and Al-OH) on the zeolite A framework generates additional negatively charged sites, thereby enhancing the attraction and binding of metal cations such as Cu²⁺ and Pb²⁺. At elevated pH levels (above 8), metal removal can also involve precipitation of metal hydroxides; however, within the studied pH range (2–11), the dominant removal mechanism is adsorption, particularly below the precipitation threshold. The crystalline microporous structure of zeolite A, composed of negatively charged aluminosilicate frameworks balanced by exchangeable cations, provides numerous active sites for ion exchange and adsorption of heavy metals. Studies have demonstrated that increasing pH favors deprotonation of hydroxyl groups, thus increasing adsorption capacities for Cu²⁺ and Pb²⁺ ions. Additionally, the zeolite’s ion-exchange properties facilitate selective adsorption, making it effective for treating metal-contaminated water [49,50,51,52]. The data indicate that the best heavy metal removal using zeolite occurs at neutral to slightly alkaline pH, with markedly reduced efficacy in severely acidic environments.

Figure 6b depicts the impact of LTA zeolite dose on the efficacy of Pb²⁺ and Cu²⁺ ion removal from aqueous solution. Increasing the amount of the adsorbent within a certain range can achieve the desired removal effect and reduce costs. As the zeolite dosage escalates from 2 to 10 g/L, the removal effectiveness for both metals markedly enhances. The removal efficiency for Pb²⁺ increases from approximately 65% at 2 g/L to above 95% at 10 g/L. Likewise, with Cu²⁺, the efficiency escalates from roughly 45% to over 90% with increasing dosage. This pattern suggests that increased zeolite dosages offer additional adsorption sites, hence improving the extraction of heavy metals from the solution [53]. Moreover, at every dosage level, Pb²⁺ is eliminated more effectively than Cu²⁺, perhaps due to variances in their attraction for the zeolite surface. The findings indicate that augmenting the quantity of LTA zeolite effectively enhances the elimination of Pb²⁺ and Cu²⁺ ions from water.

3.5.2. Kinetic Studies

Figure 7 presents the adsorption kinetics of Pb²⁺ and Cu²⁺ ions onto the synthesized zeolite. The adsorption process for both metals occurs rapidly within the first 60 min, followed by a slower phase until equilibrium is reached at around 250 min. The experimental maximum adsorption capacity is 63 mg/g for Pb²⁺ and 56.2 mg/g for Cu²⁺. This indicates that most of the adsorption takes place quickly, and the equilibrium is established within four hours. The contact duration between the metal ions and the nanosorbent plays an important role in the cost-effective treatment of wastewater. By increasing the duration of contact between contaminants and adsorbent, the adsorption efficiency significantly enhances because of the increased interaction between metals and chelation active sites. Typically, at the start of the adsorption process, the removal efficiency starts off fast and then builds up progressively. This happens as a result of the accessibility of the initial free active sites available for adsorption, which are progressively taken up by chelated metals over time [54]. Adsorption kinetic modelling evaluates the effectiveness of the adsorbent through the adsorption rate and mechanism [55]. The kinetic data were modelled by applying three kinetic models: pseudo-first-order, pseudo-second-order, and Elovich. Equations of kinetic models and the model parameters of each of the isotherm models, with the value of R², are presented in Table S1. The pseudo-first-order kinetic model is normally applied at the beginning stage of the adsorption process. Pseudo-second-order kinetic model predicts the chemisorption mechanism over time until the equilibrium state, while the Elovich model assumes the heterogeneous-based chemisorption process with the pure assessment of the kinetic behaviours [56]. The kinetic curves show that the pseudo-second-order model provides the best fit for both Pb²⁺ and Cu²⁺ adsorption, as evidenced by the close agreement between the experimental data (points) and the model predictions (R² = 0.995 for Cu²⁺ and 0.994 for Pb²⁺). This suggests that chemisorption is the dominant mechanism governing the adsorption process [3]. The pseudo-first-order and Elovich models fit the data less accurately, particularly at later stages of adsorption. Overall, the figure demonstrates that the adsorption of Pb²⁺ and Cu²⁺ ions onto zeolite is characterised by a rapid initial uptake, followed by slower adsorption as equilibrium approaches, and that the process is best described by the pseudo-second-order kinetic model, indicating the importance of chemical interactions between the metal ions and the zeolite surface, involving valence forces and potential electron sharing or exchange between the adsorbate and adsorbent.

Further, the Boyd kinetic model was used to determine the actual rate-controlling step involved in the heavy metals adsorption onto the zeolite LTA. The Boyd kinetics equation is represented by Equation (3) [57]:

$${\mathrm{B}}_{\mathrm{t}}=-0.4977-\ln \left(1-\mathrm{F}\right)$$

(3)

where F = Q_t/Q_e. The plot of B_t vs. time is called a Boyd plot (Figure 8), which is employed to distinguish between external transport (film diffusion) and intraparticle diffusion. If the plot gives a straight line passing through the origin, then the adsorption process is governed by an intraparticle diffusion mechanism; otherwise, they are governed by film diffusion or external mass transport [58]. The curves of the Boyd model plot do not pass through the origin, which indicates that external mass transport mainly governs the rate-limiting process of adsorption of Cu²⁺ and Pb²⁺ on zeolite LTA.

3.5.3. Phenomenological Mass Transfer Kinetics: Chemisorption and Physisorption

The adsorption process is known to be complex and can be governed by multiple processes, such as chemisorption and physisorption. The rates of physisorption and chemisorption can be evaluated by applying the adsorption onto the active sites (AOAS) model [59] expressed by Equation (4):

$${\displaystyle \frac{\mathrm{d}{\mathrm{q}}_{\mathrm{t}}}{\mathrm{d}\mathrm{t}}}={\mathrm{k}}_{\mathrm{c}\mathrm{h}\mathrm{e}}\mathrm{\ast }{\left({\mathrm{q}}_{\mathrm{e}}-{\mathrm{q}}_{\mathrm{t}}\right)}^2+{\mathrm{k}}_{\mathrm{p}\mathrm{h}\mathrm{y}}\mathrm{\ast }\left({\mathrm{q}}_{\mathrm{e}}-{\mathrm{q}}_{\mathrm{t}}\right)$$

(4)

where k_che [g/(mg·min)] and k_phy (1/min) represent the chemisorption and physisorption rate coefficients, respectively. The left side term (dQ_t/dt) can be estimated by applying the second-order kinetic model (Equation (S2) in Supplementary Information) to obtain the first derivative of Q_t with respect to time t. The rates of physisorption and chemisorption can be evaluated through Equations (5) and (6) (Figure 9):

$${\mathsf{\upsilon }}_{\mathrm{c}\mathrm{h}\mathrm{e}}={\mathrm{k}}_{\mathrm{c}\mathrm{h}\mathrm{e}}\mathrm{\ast }{\left({\mathrm{q}}_{\mathrm{e}}-{\mathrm{q}}_{\mathrm{t}}\right)}^2$$

(5)

$${\mathsf{\upsilon }}_{\mathrm{p}\mathrm{h}\mathrm{y}}={\mathrm{k}}_{\mathrm{p}\mathrm{h}\mathrm{y}}\mathrm{\ast }({\mathrm{q}}_{\mathrm{e}}-{\mathrm{q}}_{\mathrm{t}})$$

(6)

The AOAS model greatly describes the process with an R² > 0.995. The fastest chemisorption and physisorption rates during the copper and lead removal were reached at the initial stage, before reducing sharply for a longer contact time until reaching equilibrium. As far as the Pb²⁺ removal is concerned, the chemisorption rate dominates the adsorption process in all the investigated time range since its contribution is higher than the physisorption rate.

3.5.4. Isothermal Studies

To define these adsorption behaviours, three conventional nonlinear isotherm models were employed: the Langmuir and the Freundlich models, in addition to the Temkin model for comparative analysis (Figure 10). The Langmuir isotherm posits monolayer adsorption on a uniform surface with equivalent adsorption sites and no interactions among adsorbed species, whereas the Freundlich isotherm is an empirical model that characterizes adsorption on heterogeneous surfaces with sites exhibiting diverse adsorption energies. The adsorption isotherms depicted in Figure 8 demonstrate the equilibrium relationship between the concentration of heavy metal ions (Pb²⁺ and Cu²⁺) in solution (C_e) and their respective adsorption capacity (Q_e) on the produced LTA zeolite adsorbent. The maximum adsorption capacity values are 69.9 mg/g for Pb²⁺ and 59.2 mg/g for Cu²⁺. The experimental data points exhibit classic Type I isotherm behaviour, marked by swift initial absorption at low concentrations, succeeded by the establishment of a plateau at elevated equilibrium values. The fitted curves indicate that both the Langmuir and the Freundlich models adequately describe the experimental data; however, the Langmuir model demonstrates a superior fit quality, as evidenced by the smooth curve progression, signifying its superior fit to the metal adsorption data and affirming the surface homogeneity of the adsorbent. This observation aligns with literature that reports higher correlation coefficients (R²) for Langmuir isotherms in zeolite-based heavy metal adsorption systems [21,60]. The comparison indicates that Pb²⁺ attains a greater maximum adsorption capacity (Q_m) than Cu²⁺, approximately 81.5 mg/g versus 67.5 mg/g for Cu²⁺, suggesting a preferential adsorption of lead ions, likely attributable to variations in ionic radius, electronegativity, and hydration energy that affect the interaction strength with the zeolite framework [61]. Particularly, the low-hydrated ionic radius of Pb²⁺ (4.01 Å) compared to the Cu²⁺ one (4.19 Å) may justify the reported data. Likewise, the Pauling electronegativity values followed as Pb²⁺ > Cu²⁺, reflecting that Pb²⁺ possessed the greatest ionic potential [61]. The effective fitting of these models indicates that the synthesized LTA zeolite demonstrates advantageous adsorption properties for heavy metal removal, with the Langmuir model implying a primary monolayer covering mechanism on the zeolite surface. The value of 1/n obtained from the Freundlich adsorption isotherm is < 1 and also indicates a favorable condition for adsorption [62].

The separation factor (R_L) is then calculated using Equation (7):

$${\mathrm{R}}_{\mathrm{L}}={\displaystyle \frac{1}{1+{\mathrm{K}}_{\mathrm{L}}\mathrm{\ast }{\mathrm{C}}_0}}$$

(7)

R_L values were found to be 0.037 for Pb²⁺ ions and 0.029 for Cu²⁺ ions. The values range from 0 to 1, confirming favorable conditions [63] for the adsorption of Pb²⁺ and Cu²⁺ ions on the surface of zeolite LTA.

3.5.5. Adsorption Performance in Binary System (Cu²⁺/Pb²⁺)

In order to further evaluate the practical applicability of the synthesized LTA zeolite under realistic wastewater conditions, adsorption experiments were conducted using binary-metal systems. The kinetics of Cu²⁺ and Pb²⁺ ion adsorption are illustrated in Figure S2 (Supplementary Data). As shown in the figure, the adsorption capacity (Q_t) for Pb²⁺ rapidly increases, reaching approximately 28.4 mg/g at equilibrium (after ~50 min), while the maximum adsorption for Cu²⁺ is significantly lower, plateauing around 19 mg/g under the same conditions. By comparison, in the single-ion systems, the adsorption capacities were significantly higher: 63 mg/g for Pb²⁺ and 56.16 mg/g for Cu²⁺. The presence of Pb²⁺ caused a pronounced decrease of about 65% in Cu²⁺ uptake, whereas Pb²⁺ removal was only moderately reduced (by roughly 55%) under competitive conditions. This distinct difference demonstrates competitive adsorption: the presence of Pb²⁺ markedly decreases the capacity for Cu²⁺ uptake, whereas Pb²⁺ removal efficiency remains high and only slightly reduced compared to single-ion conditions. These results indicate that the synthesized zeolite exhibits preferential adsorption of Pb²⁺ over Cu²⁺, attributed to the lower hydration energy of Pb²⁺ (−1481 kJ/mol vs. −1760 kJ/mol for Cu²⁺) and its higher polarizability [32,60,64]. Such properties enhance the ion exchange and surface complexation mechanisms, making Pb²⁺ more competitive for active sites. Previous reports confirm this selectivity, with zeolites showing Pb²⁺ > Cu²⁺ in adsorption order in multi-component solutions. The selectivity for Pb²⁺ ions in the presence of Cu²⁺ provides significant benefits for practical wastewater treatment, where multiple heavy metal contaminants frequently coexist. This preferential adsorption leads to higher removal efficiency for Pb²⁺ compared to other metals and enables effective purification even in competitive conditions. Mechanistic studies consistently highlight that factors such as hydrated ionic radius and hydration energy contribute to zeolite selectivity, with similar competitive performance and mechanistic explanations reported in the literature [65,66].

3.5.6. Regeneration and Reusability of the Adsorbent

The regeneration and reusability of zeolite A for the removal of Cu²⁺ and Pb²⁺ ions were evaluated under the following conditions: initial metal concentration ([Cu²⁺]₀ = [Pb²⁺]₀ = 80 mg/L), zeolite dosage of 10 g/L, pH 5, and room temperature. Figure 11 shows the reusability performance of zeolite A for Cu²⁺ and Pb²⁺ removal over four consecutive cycles. The adsorbent underwent multiple adsorption–desorption cycles, with metal desorption achieved using an acid solution (HNO₃), allowing efficient recovery of adsorbed metals and restoration of active sites. This approach demonstrates the feasibility of chemical regeneration, enabling zeolite A to be reused effectively over successive cycles. After each cycle, the removal efficiency remained above 83% for both Cu²⁺ and Pb²⁺ ions, indicating high stability and resilience of the adsorbent. The gradual reduction in efficiency after four cycles reflects partial saturation or minor loss in available exchange sites, yet the material continues to exhibit practical applicability [28,29,67].

3.5.7. Adsorption Mechanism

Because of their chemical nature, the dominant adsorption mechanism for natural zeolites is ion exchange since zeolite cations can exchange with other cations in liquid solutions [31,68]. Besides, other mechanisms may be considered, such as electrostatic attraction, intrapore adsorption, surface complexation, and surface precipitation [28] (Figure 12).

Particularly, heavy metal ions may exchange with Na⁺ ions or react with the OH groups through surface complexation phenomena. Besides, due to the structure of zeolite-type materials, pore diffusion into existing cages may be considered. The adsorption kinetics and equilibrium isotherms confirmed that the removal of Cu²⁺ and Pb²⁺ ions by zeolite A is governed by physico-chemical adsorption mechanisms, predominantly ion-exchange processes. Detailed insights into the adsorption mechanism were obtained from the SEM and EDS analyses following sorption of the metal ions. SEM micrographs of ZeoliteA@Cu and ZeoliteA@Pb (see Figure S3 in the Supplementary Data) retained the characteristic crystalline morphology, indicating that the structural integrity of the zeolite remained stable after adsorption–desorption cycles. EDS elemental mapping demonstrated a significant reduction in the Na content concurrently with the appearance of strong Cu and Pb signals (see Figure S4 in the Supplementary Data), confirming the effective cationic exchange between the framework sodium ions and the adsorbed metal ions. The persistence of Al and Si peaks in the EDS spectra suggests that the zeolitic framework is preserved throughout repeated adsorption cycles. Moreover, after acid regeneration (desorption), the zeolite exhibited comparable SEM and EDS profiles to the initial material, signifying the reversibility of the exchange process and the resilience of the crystalline network. These results reveal that the adsorption and desorption of Cu²⁺ and Pb²⁺ do not compromise the chemical composition or microstructure of zeolite A, supporting its role as an effective and reusable adsorbent for the removal of toxic metals from aqueous solutions in sustainable water treatment applications.

3.6. Comparative Study

The performance of LTA zeolite synthesized from kaolin in adsorbing copper (Cu²⁺) and lead (Pb²⁺) ions from aqueous solutions was compared with various inorganic and composite adsorbents previously reported in the literature. Two primary considerations are emphasized: maximum adsorption capacity (Q_m, mg/g) and preparation cost, both crucial for large-scale wastewater treatment.

Table 3. Q_m value of adsorbents related to Cu²⁺ and Pb²⁺ removal.

Adsorbent	Qm, mg/g		Reference
	Cu2+	Pb2+
Fe3O4@SiO2-(-NH2/-COOH)	-	121.95	[3]
PAN-NaY-zeolite	44	74	[69]
Ni1-xMxFe2O4	-	24.25	[62]
Alginate–bentonite composite	2.884	41.152	[70]
NTA-silica gel	63.5	53.14	[71]
Diatomite/calcium alginate	9.930	17.331	[72]
AAC CH composite	56.82	78.12	[73]
Resacetophenone-loaded silica gel	12.31	14.55	[74]
MGAl-NPs	34.48	-	[75]
Amanita rubescens biomass	-	38.40	[76]
Banana Peel	29.26	39.32	[77]
CuO-Fe3O4/Zeolite nanocomposites	-	56.02	[78]
Fe–Mn oxide/Zeolite composites	53.35	-	[79]
MMA-Na-Y-Zeolite	37.97	65.29	[80]
Zeolite-NaX	-	14.22	[81]
Expanded perlite	8.62	13.39	[82]
LTA zeolite from kaolin	67.50	81.50	Present work

presents the maximum adsorption capacities of these adsorbents. Advanced materials such as Fe₃O₄@SiO₂ functionalized nanocomposites [3] and PAN-NaY-zeolite [69] frequently show high Q_m values, particularly for Pb²⁺. Natural adsorbents like banana peel and diatomite composites offer ecological and cost advantages but generally deliver lower performance. Remarkably, the LTA zeolite derived from kaolin in this work demonstrates excellent adsorption capacities, with Q_m values reaching 81.5 mg/g for Pb²⁺ and 67.5 mg/g for Cu²⁺. These results are competitive with or superior to many other materials in the table, including several complex nanocomposites, highlighting the practical efficiency of kaolin-based LTA zeolite for heavy metal removal. The synthesis of LTA zeolite from kaolin is also notable for its simplicity, scalability, and low cost, relying on abundant raw materials and straightforward calcination and hydrothermal steps. Unlike some high-capacity engineered adsorbents that require costly functionalization and multi-step processes, kaolin-derived LTA zeolite offers an optimal balance between performance and economic feasibility. With its high adsorption capacity Q_m = 81.5 mg/g for Pb²⁺ and 67.5 mg/g for Cu²⁺ and cost-effective synthesis, LTA zeolite from kaolin emerges as a strong candidate for large-scale environmental remediation applications, especially for treating water contaminated by heavy metals. This new data reinforces its position as a practical alternative to both expensive engineered nanomaterials and less efficient natural adsorbents.

4. Conclusions

Zeolite LTA cubic crystals were synthesised for the removal of Cu²⁺ and Pb²⁺ ions from wastewater, adopting a simple process based on the reutilization of natural clay such as kaolin. The results of characterisation analyses showed that a crystalline adsorbent with a mean crystal dimension of 2 μm. The adsorption process appears to be favoured at pH > 5 and higher adsorbent dosage. The zeolite LTA showed great adsorption capacity in the order Pb²⁺ > Cu²⁺, as proved by the Langmuir model. Then, the ability of zeolite LTA to adsorb the selected heavy metals could be better fitted by a pseudo-second-order model, indicating a chemisorption process. The effective chemical regeneration of zeolite A via acid desorption ensures its sustained adsorption capacity and operational durability for heavy metal removal. This reusability highlights the material’s potential as a cost-effective and environmentally friendly adsorbent for water treatment applications. Thus, according to the obtained results, the prepared adsorbent is able to reduce the concentrations of heavy metals in wastewater, opening the route to the application of novel, cheap materials for water remediation. Further research is needed to investigate the storage conditions and explore the feasibility of scaling up nanoparticle synthesis for commercial applications in pollutant removal.