Preparation and Characterization of Hydroxyapatite-Modified Natural Zeolite: Application as Adsorbent for Ni²⁺ and Cr³⁺ Ion Removal from Aqueous Solutions

Abstract

Natural zeolites (NatZ) are widely available, porous, crystalline aluminosilicate minerals that are commonly used as cost-effective adsorbents in water treatment processes. Despite their efficiency in removing various heavy metal ions from wastewater, NatZ show relatively low affinity toward Ni²⁺ and Cr³⁺ ions. This study aimed to develop composite adsorbents based on NatZ and hydroxyapatite using two methods, hydrothermal and mechanochemical, and their adsorption properties for the removal of Ni²⁺ and Cr³⁺ ions from aqueous solutions were investigated. X-ray powder diffraction and scanning electron microscopy analyses confirmed that under hydrothermal conditions, needle-like hydroxyapatite crystals were formed on the surface of NatZ, while the zeolite structure remained unchanged. Compared to the mechanochemically prepared sample, this adsorbent showed higher efficiency, binding 6.91 mg Ni²⁺/g and 16.95 mg Cr³⁺/g. Adsorption kinetics of the tested cations in both cases can be described by a pseudo-second-order model (R² is higher than 0.95 for all adsorbents). It is concluded that the presence of hydroxyapatite on the zeolite surface significantly improves the adsorption performance of NatZ, demonstrating its potential for the removal of heavy metal ions in wastewater treatment.

1. Introduction

In 2023, the global exploitation of natural zeolites (NatZ) was approximately 1.1 million tons [1]. NatZ are crystalline, aluminosilicate, porous minerals formed under conditions of moderate temperature and pressure. The most common NatZ are clinoptilolite, mordenite, and chabazite [2]. NatZ deposits are typically found at shallow depths, which allows cost-effective mining and enables their broader application. Deposits of clinoptilolite-rich tuffs contain a minimum of 50% clinoptilolite, with some exceeding 80% [3].

NatZ are materials with distinct physical and chemical properties, making them suitable for a wide range of applications. They have shown potential for gas adsorption and hydrogen storage [4,5]. Diogenes et al. investigated the use of clinoptilolite for CO₂ capture and reported its high selectivity for CO₂, showing significant separation between the gases in the separation scenarios of biogas upgrading and CO₂ capture from flue gas [6]. Fajdek-Bieda showed that clinoptilolite can be used as a catalyst in the chemical transformations of geraniol, achieving a conversion of 98 mol% [7]. Mohadesi et al. used KOH/clinoptilolite as a heterogeneous catalyst to accelerate biodiesel production from waste cooking oil and methanol in a microreactor, obtaining biodiesel with a purity of 97.45% under optimized reaction conditions [8].

Zeolites have been widely applied in wastewater treatment processes for the removal of harmful contaminants [9]. Industrial activities produce wastewater containing various pollutants, requiring the development of effective and sustainable treatment methods. A wide range of techniques have been developed for the removal of pollutants, including heavy metals, from wastewater. These methods include chemical precipitation [10], photocatalysis [11], membrane filtration [12], electrochemical treatment [13], and adsorption [14]. Among the available techniques, adsorption is one of the most widely used due to its cost-effectiveness and easy operation [15]. NatZ are promising adsorbents, as they are cheap, sustainable, environmentally friendly, and easily accessible [15]. Their crystalline lattice is negatively charged with exchangeable metal cations located in the lattice cavities. This structure enables NatZ to be used for the removal of harmful cations from wastewater.

Clinoptilolite has shown effective adsorption of radionuclides from aqueous solutions [16]. In the case of ammonium ion removal, clinoptilolite achieved an efficiency exceeding 99% [17]. Furthermore, zeolites have shown efficacy in removing toxic heavy metal cations. The adsorption kinetics typically follows a pseudo-second-order model, indicating that ion exchange is the dominant mechanism for cation removal. For instance, the commercial zeolite NaY exhibited excellent adsorption capacities for Sr²⁺ (260 mg/g), Pb²⁺ (220 mg/g), and Cu²⁺ (161 mg/g) [18]. Clinoptilolite was reported to achieve complete adsorption of zinc and cadmium ions at an initial metal concentration of 10 mg/dm³ [19]. Raw clinoptilolite, without modification, from the city of Balıkesir in Turkey adsorbed 80.93 mg Pb²⁺/g [20]. NatZ of Saudi origin adsorbed 2.72 mg Cr³⁺/g [21], while zeolite from the Zlatokop deposit (Serbia) showed an adsorption capacity of 1.9 mg Ni²⁺/g [22]. Clinoptilolite exhibits high selectivity for heavy metal ions, with selectivity following the order Pb²⁺ > Cd²⁺ > Cu²⁺ > Co²⁺ > Cr³⁺ > Zn²⁺ > Ni²⁺ > Hg²⁺ [23].

According to the Substance Priority List published by the Agency for Toxic Substances and Disease Registry, heavy metals comprise one of the most toxic types water pollutants and present significant risks to human health [24]. Therefore, the development of improved adsorbents for metal ion removal from wastewater before its discharge into recipient water is essential.

To enhance the adsorption properties of NatZ, various composite materials have been synthesized [25]. Some inorganic materials, such as hydroxyapatite, have been used for these purposes. Calcium hydroxyapatite (Ca₁₀(PO₄)₆(OH)₂) is a mineral commonly found in bones. Stoichiometric hydroxyapatite, with a molar ratio Ca/P = 1.67, has thermal stability up to 1200 °C. The two most widely used methods of hydroxyapatite synthesis are precipitation and hydrothermal synthesis. Hydroxyapatitesynthesized by the hydrothermal method typically have a higher degree of crystallinity and a smaller specific surface area compared to those obtained by precipitation [26]. By adjusting the parameters of hydrothermal synthesis, hydroxyapatite powders with varying characteristics can be obtained. Nanorod hydroxyapatite adsorbent synthesized from fish scales exhibited a Ni²⁺ adsorption capacity of approximately 134.3 mg Ni²⁺/g, whereas calcium hydroxyapatite synthesized by the precipitation method adsorbed 36.25 mg Ni²⁺/g [27,28]. Alagarsamy et al. investigated the use of magnesium-doped hydroxyapatite–zeolite nanocomposite porous polyacrylonitrile polymer beads for the adsorption of heavy metal ions from multi-ion contaminated groundwater [29]. Among the various composite beads, those containing both zeolite and hydroxyapatite showed higher multi-ion removal efficiency compared to beads containing zeolite only [29]. Li et al. synthesized a hydroxyapatite–zeolite composite material using blast furnace slag, which was used for the adsorption of ammonium and manganese ions [30]. Gupta et al. prepared a zeolite–hydroxyapatite composite by the precipitation method and reported higher Co²⁺ removal efficiency compared to raw zeolite [31].

In this study, NatZ was used instead of synthetic zeolite due to its wide availability, low cost, and environmentally friendly properties. Urea was selected for hydroxyapatite synthesis as it leads to more homogeneous precipitation. Also, the adsorption properties of zeolite–hydroxyapatite materials have been investigated before for the removal of dyes, antibiotics, bacteria, and certain heavy metals, including copper, lead, cadmium, zinc, and cobalt.

Ni²⁺ and Cr³⁺ ions are among the most toxic heavy metal ions according to the Substance Priority List published by the Agency for Toxic Substances and Disease Registry [24]. Exposure to water contaminated with Cr³⁺ can cause adverse health effects, including diarrhea, change in body weight, hypoactivity, lacrimation, and mydriasis [32]. Drinking water contaminated with Ni²⁺ may lead to nausea, vomiting, diarrhea, giddiness, lassitude, headache, and shortness of breath [33]. Therefore, their effective removal from contaminated wastewater is of great importance.

Ni²⁺ and Cr³⁺ may be found in various types of industrial wastewater, with electroplating and metal finishing processes being among the most significant sources of contamination. According to the literature, the concentrations of Ni²⁺ and Cr³⁺ in wastewater from these industries can range from trace levels to over 1 g/dm³ [34,35]. Akbal et al. analyzed three different metal plating wastewater samples, reporting nickel concentrations between 394 and 526 mg/dm³ and chromium concentrations ranging from 44.5 to 193 mg/dm³ [36]. Similarly, Al-Shannag investigated metal plating wastewater containing 57.6 mg/dm³ of nickel and 93.2 mg/dm³ of chromium [37]. According to all of these studies, there is a need for the preparation of adsorbents with a high affinity toward these two ions. Unfortunately, clinoptilolite itself shows low affinity toward Ni²⁺ and Cr³⁺ ions.

To the best of our knowledge, zeolite–hydroxyapatite composites have not yet been studied for the removal of Ni²⁺ and Cr³⁺ ions from aqueous solutions. Due to that, the aim of this study was to prepare a composite material based on the natural zeolite–clinoptilolite and calcium hydroxyapatite. The prepared composites were characterized in detail, and their adsorption properties for the removal of Ni²⁺ and Cr³⁺ ions from aqueous solutions were examined.

2. Materials and Methods

2.1. Materials

Natural zeolitic tuff from the Slanci deposit (Belgrade, Serbia) was used in this study. Materials used for the synthesis of calcium hydroxyapatite included calcium chloride dihydrate (CaCl₂·2H₂O, p.a., Merck, Rahway, NJ, USA), ammonium hydrogen phosphate ((NH₄)₂HPO₄, p.a., Merck, Rahway, NJ, USA), sodium ethylenediaminetetraacetate dihydrate (C₁₀H₁₄Na₂O₈·2H₂O, p.a., Superlab, Belgrade, Serbia), and urea (CH₄N₂O, p.a., Kemika, Ovada, Italy). For adsorption experiments, aqueous solutions of Ni²⁺ and Cr³⁺ ions were prepared using nickel(II) sulfate heptahydrate (NiSO₄·7H₂O, p.a., Fluka, Brussel, Belgium) and chromium(III) chloride hexahydrate (CrCl₃·6H₂O, p.a., Acros Organics, Geel, Belgium), respectively.

2.2. Preparation of the Adsorbents

Natural zeolitic tuff (NZ) with particle size 63–125 μm was used as reference adsorbent. The second adsorbent obtained by hydrothermal synthesis of stoichiometric calcium hydroxyapatite on the zeolite surface was denoted as HT. The preparation procedure of HT adsorbent was as follows:

• Calcium chloride dihydrate, sodium ethylenediaminetetraacetate dihydrate, ammonium hydrogen phosphate, and urea were added to 20 cm³ of distilled water (masses listed in

  Table 1. Masses of reactants for calcium hydroxyapatite synthesis.

  	CaCl2·2H2O	C10H14Na2O8·2H2O	(NH4)2HPO4	CH4N2O
  mass, g	0.2491	0.1744	0.1340	0.1414

  ). The molar ratio Ca/P in the reaction mixture was 1.67;
• The reaction suspension was homogenized using a magnetic stirrer at 400 rpm for 30 min at 25 °C;
• NZ (0.9 g) was added to the reaction mixture, followed by sonication (Sonopuls mini20, Bandelin, Berlin, Germany) for 6 min in 3 cycles to break the aggregates;
• The suspension was homogenized for an additional 30 min at 400 rpm before being placed in a stainless steel autoclave;
• Crystallization of calcium hydroxyapatite was carried out under hydrothermal conditions at 160 °C and autogenous pressure for 4 h;
• The resulting product was separated by vacuum filtration, washed with distilled water, and dried at 105 °C overnight.

Similar preparation methods for hydroxyapatite synthesis on a zeolite surface were reported previously [38,39].

The third and fourth adsorbents, MC10 and MC20, were prepared by mechanochemical modification of NZ in two steps.

Hydrothermal synthesis of calcium hydroxyapatite

Hydroxyapatite was synthesized according to a procedure previously published [40]. Briefly, the HT was obtained as follows:

• Calcium chloride dihydrate (0.3973 g) and ammonium hydrogen phosphate (0.2138 g) were added to 20 cm³ of distilled water;
• The pH of the reaction mixture was adjusted to 9 using 0.1 mol dm⁻³ NaOH solution;
• The mixture was homogenized at 400 rpm for 30 min before being placed in a stainless steel autoclave;
• Crystallization of calcium hydroxyapatite was carried out under hydrothermal conditions at 160 °C and autogenous pressure for 4 h;
• The resulting calcium hydroxyapatite crystals were sonicated for 6 min in 3 cycles to break the aggregates;
• At the end, calcium hydroxyapatite crystals were separated by vacuum filtration, washed with distilled water, and dried at 105 °C overnight.

Similar synthesis methods were previously reported [26].

Mechanochemical synthesis of MC10 and MC20

Mechanochemical synthesis was performed using a planetary mill (PM100, Retsch, Haan, Germany). The powder mixtures consisted of 90 wt% clinoptilolite and 10 wt% hydroxyapatite (2.25 g of clinoptilolite and 0.25 g of hydroxyapatite) for the MC10 adsorbent and 80 wt% clinoptilolite and 20 wt% hydroxyapatite (2.0 g of clinoptilolite and 0.5 g of hydroxyapatite) for the MC20 adsorbent. Milling was performed under the following optimized parameters: speed 250 rpm, milling time 15 min, and ball/powder mass ratio of 15:1.

At the end, four adsorbents—natural zeolitic tuff (NZ), hydrothermally synthesized calcium hydroxyapatite on the zeolite surface (HT), and calcium hydroxyapatite-modified zeolite obtained mechanochemically, MC10 and MC20—were used in the adsorption experiments.

2.3. Adsorption Experiments

Adsorption kinetics was performed at a solid/liquid phase ratio of 1:200 (g:cm³), as 1.0 g of the adsorbent was suspended in 200 cm³ of the solution. The initial concentration of metal ions in monocomponent aqueous solutions was 100 mg/dm³. The suspensions were shaken in a thermostatic water bath (WNB 22 Memmert, Nürnberg, Germany) at 25 °C for 24 h at 105 rpm. Aliquot portions (3 cm³) were taken from the suspension at predetermined time intervals: 30 min and then 1, 2, 4, 6, and 24 h. Each aliquot was filtered through a membrane filter with a pore size of 0.45 µm. At the end, the suspensions were separated by vacuum filtration using medium-porosity filter paper. The resulting filtrates were analyzed to determine the residual concentrations of Ni²⁺ and Cr³⁺ ions in the solution using atomic absorption spectroscopy and to calculate the adsorption capacities (1) and adsorption efficiency (2) according to the following equations:

$$q_t={\displaystyle \frac{(c_0-c)}{V}}\ast m$$

(1)

$$E={\displaystyle \frac{c_0-c}{c_0}}\ast 100$$

(2)

where c₀ (mg/dm³) is the initial concentration of metal ion in the solution, c (mg/dm³) is the concentration in the solution after the adsorption, m (g) is the mass of the adsorbent, and V (dm³) is volume of the suspension.

2.4. Adsorption Kinetics Experiments

The adsorption kinetics of Ni²⁺ and Cr³⁺ ions on NZ, HT, MC10, and MC20 adsorbents was investigated using non-linear kinetic models: pseudo-first-order (PFO), pseudo-second-order (PSO), and intraparticle diffusion (IPD). Mathematical expressions of the PFO, PSO, and IPD models are presented as Equations (3)–(5):

$$q_t=q_{e,1}·(1-e^{{-k}_1·t})$$

(3)

$$q_t={\displaystyle \frac{{q_{e,2}}^2·k_2·t}{1+q_{e,2}·k_2·t}}$$

(4)

$$q_t=k_{ipd}·t^{0.5}$$

(5)

where q_t is the adsorption capacity (mg/g) at time t (h), and q_e,1 and q_e,2 (mg/g) are the equilibrium adsorption capacities for PFO and PSO, respectively. Then, k₁ (h⁻¹), k₂ (g/(mg·h)), and k_ipd (mg/(g·h^0.5)) are the PFO, PSO, and IPD rate constants, respectively.

An Excel solver add-in and the Runge-Kutta method were used to solve the non-linear kinetic models and determine the kinetics constants, as described and provided in the paper by Wang and Guo (2020) [41]. The correlation coefficient (R²) was applied to evaluate the fitting results and to compare the kinetic models.

2.5. Adsorption Isotherms Experiments

Adsorption isotherm experiments were performed at a solid/liquid phase ratio of 1:200 (g:cm³), as 0.5 g of the adsorbent was suspended in 100 cm³ of the solution. The initial concentration of metal ions in monocomponent aqueous solutions ranged from 20 to 130 mg/dm³. The suspensions were shaken in a thermostatic water bath (WNB 22, Memmert, Nürnberg, Germany) at 25 °C for 24 h at 105 rpm. At the end, the suspensions were separated by vacuum filtration using medium-porosity filter paper. The resulting filtrates were analyzed to determine the residual concentrations of Ni²⁺ and Cr³⁺ ions in the solution using atomic absorption spectroscopy.

The obtained experimental results were analyzed using Langmuir (Equation (6)) and Freundlich (Equation (7)) isotherm models:

$$q_e={\displaystyle \frac{q_m{K}_L{C}_e}{1+K_L{C}_e}}$$

(6)

$$q_e=K_F{C}_e^{{\displaystyle \frac{1}{n}}}$$

(7)

where q_e (mg/g) is the adsorption capacity; C_e (mg/dm³) is the concentration in the equilibrium solution; q_m (mg/g) is the maximum adsorption capacity of the adsorbent; K_L is the adsorption equilibrium constant related to the heat of adsorption (dm³/mg); K_F is the constant of the magnitude of the adsorption capacity (dm³/mg); and n is the Freundlich isotherm adsorption constant.

2.6. Desorption Experiments

Desorption experiments were conducted to test the release of adsorbed Ni²⁺ and Cr³⁺ ions from the saturated adsorbents. Desorption from the saturated adsorbents was tested using distilled water, NaCl solution (2.0 mol dm⁻³), and NaEDTA solution (0.05 mol dm⁻³). Saturated adsorbents were suspended in water media, with a solid/liquid phase ratio of 1:100 (g:cm³)—1.0 g of the adsorbent was suspended in 100 cm³ of water media. Desorption experiments followed the same procedure as the adsorption experiments: the suspensions were shaken in a thermostatic water bath (WNB 22, Memmert, Nürnberg, Germany) at 25 °C for 24 h at 105 rpm. After 24 h, the suspensions were separated by vacuum filtration. The concentrations of Ni²⁺ and Cr³⁺ ions leached in the distilled water were measured using atomic absorption spectroscopy.

2.7. Characterization of the Adsorbents

The X-ray powder diffraction method (XRPD) was used to determine the phase composition. The XRPD pattern was obtained using a PW 1710/1820 automated diffractometer (Philips, Eindhoven, The Netherlands) using a Cu tube operated at 40 kV and 30 mA. The instrument was equipped with a diffracted beam curved graphite monochromator and a Xe-filled proportional counter. The diffraction data were collected over the range 4–45, 2θ, counting for 1 s per 0.020 step. The divergence and receiving slits were fixed at 1 and 0.1, respectively. All the XRPD measurements were performed at room temperature in a stationary sample holder. Quantitative mineralogical refinement was obtained by the Rietveld technique using SIROQUANT V4.0 software. The thermal properties of the adsorbents were tested using thermogravimetric analysis in a synthetic air atmosphere with a flow rate of 100 cm³ min^–1. The heating rate was 10 °C min^–1, and the temperature was increased up to 800 °C. Measurements were conducted using an SDT Q600 instrument (TA Instruments, New Castle, DE, USA). The morphology of the prepared adsorbents and the crystal size of hydroxyapatite were analyzed by scanning electron microscopy (SEM) using a TESCAN MIRA 3 XMU (Brno, Czech Republic) electron microscope operating at 20 keV with a secondary electron detector. Before SEM analysis, the samples were dried overnight at 105 °C, deposited on a carbon conductive tape, and coated with a thin layer of gold using a Polaron SC503 sputter coater (Fisons Instruments, Ipswich, UK). The specific surface area and pore size distribution of the adsorbents were determined by nitrogen adsorption analysis at −196 °C using a Micromeritics ASAP 2020 analyzer (Norcross, GA, USA). Before measurement, the samples were degassed overnight at 100 °C. The specific surface area of the materials was calculated based on their gas adsorption capacity using the BET method (Brunauer, Emmett, and Teller), while the volume and surface area of mesopores were determined using the BJH method (Barret–Joyner–Halenda). The initial concentrations of Ni²⁺ and Cr³⁺ ions in the solution and their residual concentrations after adsorption were determined in triplicate using a atomic adsorption spectrophotometer (Spectra AA 55B, Varian, Australia). The relative standard deviation of the instrument was found to be <2%. The pH values of the initial as well as the equilibrium solutions were measured using a pH meter FG2 (Mettler toledo, Schwerzenbach, Switzerland).

3. Results and Discussion

3.1. Structural Analysis

The phase compositions of the adsorbents were determined through XRPD analysis. Figure 1 shows the XRPD patterns of the prepared adsorbents. The analysis indicates that the major mineral phase in all prepared adsorbents is well-crystalized clinoptilolite. Characteristic peaks corresponding to the clinoptilolite phase were observed at diffraction angles (2θ) 9.82°, 11.14°, 17.30°, 18.98°, 22.43°, 26.04°, 26.15°, 29.90°, and 31.98°. Minor phases identified are quartz, plagioclase, and calcite. The literature reported similar patterns [42,43,44]. The diffractogram of the NZ confirmed that clinoptilolite is the dominant component, comprising 90.7 wt% of the sample. Similarly, the diffractogram of the HT showed the same peaks as the NZ sample, with additional peaks attributed to calcium hydroxyapatite (at diffraction angles (2θ) 31.72° and 32.78°). This suggests that the hydrothermal synthesis of hydroxyapatite did not affect the crystallinity of the clinoptilolite, whose crystal lattice remained intact. The percentage of calcium hydroxyapatite in the HT sample was determined to be 8.9 wt%. Structural analysis of the MC10 and MC20 showed that clinoptilolite remained the predominant phase in both cases. The hydroxyapatite content was 9.5 wt% and 19.4 wt% for MC10 and MC20, respectively. The diffractograms of MC10 and MC20 showed broader and less intense peaks compared to those of NZ and HT, suggesting a slightly decrease in crystallinity. This decline could be a consequence of composite preparation in the planetary mill. The quantitative mineralogical analysis of crystal phases obtained by the Rietveld refinement method is summarized in

Table 2. Quantitative mineralogical analysis of crystal phases obtained by the Rietveld refinement method of all adsorbents.

	Weight%
	Clinoptilolite	Quartz	Plagioclases	Calcite	Micas	Hydroxyapatite
NZ	90.7	4.7	3.5	1.1	/	/
HT	81.1	4.9	3.4	0.9	0.8	8.9
MC10	81.4	4.6	3.5	1	/	9.5
MC20	71.3	5.1	3.3	0.8	/	19.4

. All other crystal phases were below detection limits.

3.2. Thermal Analysis

Figure 2 illustrates the TG/DTG curves of the synthesized adsorbents. The total mass loss observed in all samples ranges from 12 wt% to 15 wt% and can be attributed to dehydration. Calcium hydroxyapatite shows thermal stability up to 800 °C without undergoing any phase transformation. According to previously published studies, the dehydroxylation of stoichiometric hydroxyapatite begins at temperatures above 900 °C, while the destruction of the crystal lattice and the formation of α-tricalcium phosphate and tetracalcium phosphate occur above 1350 °C [45]. Clinoptilolite is also thermally stable, with lattice destruction occurring above 800–900 °C, leading to amorphization or phase transitions [46]. Although the adsorbents were dried overnight at 105 °C after the synthesis, subsequent exposure to atmospheric conditions led to rehydration and binding of water molecules [47]. Several peaks are observed in the DTG curves of all adsorbents, indicating that water loss occurs in multiple stages. The first peak at approximately 60 °C corresponds to the loss of surface-bound water. This water is weakly associated with the crystal lattice. Even at temperatures below 100 °C, these bonds are broken and surface dehydration occurs. Higher-temperature peaks correspond to the in-pore dehydration, when water molecules trapped inside the channels and cavities of the zeolite structure are released.

3.3. Scanning Electron Microscopy

SEM analysis showed the morphology and structure of the adsorbent particles. Figure 3a shows the morphology of the NZ sample. Heterogeneous grains of clinoptilolite containing a small amount of impurities are observed. Micrographs indicate that clinoptilolite exhibits high crystallinity, which was also proved by XRPD. Clinoptilolite occurs in the form of plate-shaped crystals. Thin plates of clinoptilolite (some smaller than 1 μm) rarely exist as individual crystal grains but typically form aggregates with a lamellar structure. This zeolite structure aligns with previous findings, which attribute the split zeolite structure to hydrothermal solution filtration [48]. Additionally, clinoptilolite cleavage was identified as the main cause of mesoporosity [49]. The observed morphology of clinoptilolite from the Slanci deposit is consistent with the previously characterized clinoptilolite samples from Ukrainian, Slovakian [50], and Greek deposits [51].

Morphologies of HT, MC10, and MC20 adsorbents are also presented in Figure 3b–d. The micrographs show that in addition to lamellar clinoptilolite aggregates, needle-like crystals are also present. This indicates that the hydrothermal treatment did not affect the crystallinity of clinoptilolite, and the zeolite structure remained unchanged. The micrographs of the HT adsorbent show that needle-like crystals formed on the zeolite surface during hydrothermal synthesis (Figure 3b). The morphology of these crystals corresponds to hydroxyapatite synthesized by the hydrothermal process [52]. Similar observations were reported in earlier studies where needle-like crystals of hydroxyapatite were formed on the zeolite surface during synthesis [39,53,54]. Furthermore, spherical agglomerates of hydroxyapatite particles, approximately 5 μm in size, formed by agglomeration of needle-like crystals. These agglomerates are present due to the high concentration of ions forming hydroxyapatite in the precursor solution [26]. Micrographs of MC10 and MC20 adsorbents exhibit lamellar clinoptilolite aggregates and unevenly distributed needle-like crystals of hydroxyapatite. It can be assumed that this morphology is a consequence of the preparation method, where hydroxyapatite was first synthesized hydrothermally and subsequently combined with clinoptilolite via milling. This process likely prevents the formation of a hydroxyapatite layer on the zeolite surface, resulting in its uneven distribution within the composite.

3.4. Specific Surface Area and Pore Distribution

The textural parameters of the adsorbents are given in

Table 3. Textural parameters of the adsorbents.

Sample	SSA (m2 g−1)	Vtot (cm3 g−1)	Vmeso (cm3 g−1)	Daver (nm)
NZ	24.763	0.101	0.098	18.573
HT	37.442	0.096	0.091	11.566
MC10	29.900	0.110	0.105	17.028
MC20	33.782	0.135	0.131	16.981

. The specific surface area of the adsorbents ranges from 24.7 to 37.5 m² g⁻¹, with the lowest value observed for raw clinoptilolite. These results are consistent with data reported in the literature [26,50]. The hydrothermal synthesis of hydroxyapatite resulted in an increase in the specific surface area, with the effect being most pronounced in the HT sample. This suggests that hydrothermal synthesis of hydroxyapatite and crystallization on the zeolite surface had a more significant impact on the specific surface area compared to mechanochemical modification of zeolite with hydroxyapatite using a planetary mill. Furthermore, the higher hydroxyapatite content in the MC20 sample led to a more significant increase in the specific surface area compared to the MC10 sample. The total pore volume was approximately 0.1 cm³ g⁻¹, and the average pore diameter ranged from 11.5 to 18.6 nm. The surface area of an adsorbent material is an important characteristic, as adsorption is a predominantly surface-based phenomenon. Materials with higher surface areas and more porous structures generally exhibit superior adsorption performance [55]. Therefore, it can be expected that the synthesized composites will achieve better adsorption results compared to raw clinoptilolite.

The nitrogen adsorption and desorption isotherms of the adsorbents are presented in Figure 4. These isotherms correspond to Type IV behavior, which is characteristic of mesoporous materials according to the IUPAC classification [56]. This indicates that the initial monolayer–multilayer adsorption on the mesopore walls is followed by pore condensation. The results also reveal that capillary condensation is accompanied by hysteresis, a phenomenon commonly observed in mesoporous systems. Similar results were previously reported [57].

3.5. Adsorption Studies

The adsorption properties of the prepared composites toward Ni²⁺ and Cr³⁺ ions were tested. All experiments were performed at the initial pH value (5.10 and 2.81 for Ni²⁺ and Cr³⁺ solution, respectively). It is seen from Figure 5 that the amount of adsorbed metal ions increases over time for all adsorbents and both ions. The amount of adsorbed metal ions obtained by using HT is higher than NZ, MC10, and MC20 for both examined cations. For example, 6.91 mg Ni²⁺/g was adsorbed onto the HT. Compared with this, on other investigated adsorbents, an amount of Ni²⁺ ions around three times smaller was adsorbed (2.56, 2.16, and 2.28 mg Ni²⁺/g onto the NZ, MC10, and MC20, respectively). The adsorption of Ni²⁺ onto the synthesized adsorbents can be divided into two segments—first, a sharp step in the first 6 h, and then an equilibrium step from 6 to 24 h, when a plateau was reached. In the case of Cr³⁺ adsorption, HT showed the highest adsorbed amount (16.95 mg Cr³⁺/g), followed by MC20 (13.09 mg Cr³⁺/g), MC10 (8.19 mg Cr³⁺/g), and NZ (3.14 mg Cr³⁺/g). Here also, the equilibrium concentration of the adsorbed ions was achieved after 6 h of contact for MC10 and MC20. The concentration of the adsorbed Cr³⁺ onto the NZ and HT grew exponentially, and the plateau was not achieved after 24 h.

The adsorption capacities of similar adsorbents are shown in

Table 4. Adsorption capacities of different adsorbents.

Metal Ion	Adsorbent	Adsorption Capacity, mg/g	References
Ni2+	Mexican clinoptilolite	0.31	[58]
	Cellulose acetate–zeolite composite	16.95	[59]
	Greek clinoptilolite	1.98	[60]
	Chitosan–PVA–zeolite nanofibrous membrane	1.79	[61]
	NZ	2.56	This study
	HT	6.91	This study
	MC10	2.16	This study
	MC20	2.28	This study
Cr3+	Zeolite A–activated carbon composite	4.9	[62]
	Greek clinoptilolite	4.12	[60]
	Mordenite	3.6	[63]
	Kaolin	2.6	[63]
	NZ	3.14	This study
	HT	16.95	This study
	MC10	8.19	This study
	MC20	13.09	This study

. The results indicate that the modification performed in this study enhances the adsorption capacity of NZ and that the composite materials obtained here show higher adsorption capacities compared to those reported in the literature.

The results of the adsorption efficiency of Ni²⁺ and Cr³⁺ ions after 24 h are shown in Figure 6. For the removal of Ni²⁺, it is obvious that the adsorbents NZ, MC10, and MC20 exhibited similar adsorption behavior (the efficiency is 15.1, 12.2, and 13.5% for NZ, MC10, and MC20, respectively). The adsorbent HT showed the highest efficiency toward this cation, 40.8%. According to the presented results, it can be concluded that the mechanochemical modification of zeolite with hydroxyapatite did not significantly influence the adsorption properties, while hydrothermal synthesis of the HT sample had a positive effect on the adsorption process. The efficiency of the removal of Cr³⁺ was lowest onto the NZ, 17.5%, followed by the MC10 and MC20 at 45.7% and 73.1%, respectively. The adsorbent HT also exhibited the highest efficiency toward Cr³⁺ ions—almost 95%.

According to the presented results, HT achieved the highest adsorption efficiency, suggesting that hydrothermal synthesis had a greater influence on adsorption compared to mechanochemical modification. Mechanochemical modification of zeolite with calcium hydroxyapatite did not influence the adsorption of Ni²⁺, but it had a positive impact on Cr³⁺ adsorption. Furthermore, MC20 exhibited higher Cr³⁺ adsorption efficiency than MC10, indicating that an increased hydroxyapatite content enhances Cr³⁺ adsorption. Also, the content of the hydroxyapatite in the HT and MC10 is similar (8.9 and 9.5 wt.% for HT and MC10, respectively), but the performance of the hydrothermally synthesized sample is incomparably better for both investigated adsorption processes, especially in the case of Cr³⁺. This indicates that the process of hydrothermal synthesis is more appropriate for the preparation of new adsorbents based on natural zeolite and hydroxyapatite for the removal of Ni²⁺ and Cr³⁺ ions.

3.6. Adsorption Kinetics

The study of adsorption kinetics is essential for designing and scaling adsorption systems. It offers insights into the rate of pollutant removal, the mechanisms of the adsorption process, and the performance of the adsorbent utilized. Among the various proposed kinetics models, the PFO, PSO, and IPD models are the most commonly applied to analyze adsorption kinetics [64]. The PFO and PSO models suggest that the rate of metal adsorption onto the surface of sorbents is proportional to the number of available sites. The kinetics of PFO is influenced by the physical adsorption process, while PSO kinetics is governed by chemical processes involving the valence forces through the sharing or exchange of electrons between the sorbent and the adsorbate. Furthermore, the IPD model indicates that if the sorption mechanism occurs through intraparticle diffusion, a plot showing q_t versus t^0.5 would be linear [41].

The adsorption kinetic constants for the adsorption of Ni²⁺ and Cr³⁺ ions onto the NZ, HT, MC10, and MC20 obtained by using kinetics models are shown in

Table 5. Adsorption kinetic constants for the adsorption of Ni²⁺ and Cr³⁺ on the adsorbents NZ, HT, MC10, and MC20.

	Ads.	PFO			PSO			IPD
		k1, h−1	qe,1 mg/g	R2	k2, g/(mg·h)	qe,2 mg/g	R2	kipd, mg/(g·h0.5)	R2
Ni2+	NZ	0.991	3.03	0.97	0.27	2.59	0.981	0.626	0.828
	HT	0.0498	8.54	0.905	0.377	7.39	0.956	1.9	0.678
	MC10	0.222	2.56	0.951	0.467	2.25	0.988	0.102	0.69
	MC20	0.187	2.73	0.937	0.427	2.4	0.978	0.638	0.692
Cr3+	NZ	7.49 × 10−4	173.6	0.985	6.87 × 10−5	4.47	0.997	0.492	0.672
	HT	0.101	18.6	0.995	0.003	25.9	0.996	3.3	0.763
	MC10	0.033	9.58	0.982	0.286	8.18	0.993	2.01	0.827
	MC20	0.0216	15.5	0.975	0.302	13.3	0.995	3.29	0.8

. Results for the correlation factor, R², higher than 0.90 for both the pseudo-first-order (PFO) and pseudo-second-order (PSO) models show that both models describe all the tested systems well. Given that the R² values for the PSO model in all adsorption systems are closer to 1, it can be concluded that adsorption follows second-order kinetics, indicating that ion exchange is the dominant mechanism. Significantly lower values for R² were obtained using the IPD model. This indicates that intraparticle diffusion is not the sole rate-limiting step in the adsorption mechanism [65].

3.7. Adsorption Isotherms

The adsorption isotherm study was obtained on NZ, a reference adsorbent—raw clinoptilolite—and HT, which showed the highest adsorption capacity among all tested adsorbents. An adsorption study of the tested NZ and HT adsorbents toward Ni²⁺ and Cr³⁺ where the initial concentration ranged from 20 to 130 mg/dm³ was performed (

Table 6. Mean equilibrium adsorption capacity (q_e) and initial and equilibrium pH values.

Metal	Initial Concentration, mg/dm3	Initial pH	Equilibrium Adsorption Capacity (qe, mg/g)		Equilibrium pH
			NZ	HT	NZ	HT
Ni2+	20	5.00	0.988	3.8332	6.14	7.02
	40	5.06	1.534	5.736	6.06	6.96
	90	5.05	2.426	7.14	6.25	6.73
	100	5.10	2.58	6.91	6.23	6.70
	130	5.14	2.68	7.156	6.24	6.62
Cr3+	20	3.37	3.064	4.478	3.87	5.30
	40	3.10	3.202	8.636	3.49	5.14
	90	2.85	3.284	16.118	3.12	3.24
	100	2.81	3.14	16.91	3.07	3.11
	130	2.72	3.204	17.858	2.95	2.96

and Figure 7).

In this study, Langmuir and Freundlich adsorption isotherms were built to predict the maximum mass of metal ions (Ni²⁺ and Cr³⁺) that could be bound to the NZ and HT sorbents. The Langmuir isotherm assumes a monolayer surface of sorbent with uniform active sites, while the Freundlich isotherm assumes a heterogeneous surface of a sorbent, applicable to either monolayer (chemisorption) or multilayer (van der Waals) adsorption [66].

Adsorption isotherm constants for the adsorption of Ni²⁺ and Cr³⁺ on NZ and HT, obtained using the isotherms, are shown in

Table 7. Fitting parameters of Langmuir and Freundlich isotherm models.

Sample	Ions	Langmuir Model			Freundlich Model
		qm	KL	R2	KF	n	R2
NZ	Ni2+	3.987	0.019	0.996	0.258	1.987	0.987
	Cr3+	3.227	2.71	0.998	2.999	65.050	0.997
HT	Ni2+	7.293	0.360	0.995	3.496	6.017	0.986
	Cr3+	17.611	1.494	0.973	9.223	5.044	0.961

, and the fitted curves are plotted in Figure 7. Both Freundlich and Langmuir adsorption isotherm models effectively describe the adsorption process of Ni²⁺ and Cr³⁺ on NZ, even though the agreement with the Langmuir isotherm is better. For HT, the Langmuir model provides a better fit for the experimental data for both metal ions. Based on the assumptions of the Langmuir model, Ni²⁺ and Cr³⁺ ions are adsorbed at homogeneous sites of the adsorbents by monolayer adsorption. This indicates that the adsorption process will stop once a monolayer of both NZ and HT forms on the adsorbent surface, which has a finite number of adsorption sites. According to the available literature, the Langmuir adsorption isotherm fitted the adsorption data better than the Freundlich adsorption isotherm for the adsorption of Ni²⁺ and Cr³⁺ ions on natural zeolites and different hydroxyapatites [28,67,68,69].

The maximum calculated adsorption capacities of NZ for Ni²⁺ and Cr³⁺ were 3.987 and 3.227 mg/g, respectively. Furthermore, the Langmuir model indicated that its maximum adsorption capacity for Ni²⁺ and Cr³⁺ on HT exceeded that on NZ, with values of 7.293 and 17.661 mg/g, respectively.

According to the data presented in Table 7, the results of the adsorption isotherms are in accordance with the kinetic parameters (Table 5), where the adsorption capacities of NZ and HT are higher toward Cr³⁺ than toward Ni²⁺, emphasizing the more pronounced effect of HT than NZ.

The maximum amount of adsorbed Cr³⁺ ions on NZ and HT predicted by the Langmuir isotherm (3.2 and 17.6 mg/g, respectively) underestimated the q_e,2 calculated by the PSO model (4.47 and 25.9 mg/g, respectively). In the case of Ni²⁺ ion adsorption on NZ, the isotherm predicted a slightly higher value (3.99 mg/g) than the PSO model (2.59 mg/g), while for Ni²⁺ adsorption on HT, these values showed very good agreement, 7.29 mg/g for q_m and 7.39 mg/g for q_e,2. The results obtained for the HT adsorbent can be understood by considering the nature of the sorbent materials. The strong agreement with the Langmuir model indicates that there is a monolayer interaction between both metals ion species and the homogenous surface of the HT sorbent. This behavior is also confirmed by the proposed adsorption mechanisms for both Ni²⁺ and Cr³⁺ ions that follow the PSO adsorption kinetics, which involve ion-exchange interactions between the metal ions and the functional groups of the HT adsorbent.

3.8. Desorption from the Saturated Adsorbents

For desorption experiments, saturated adsorbents were suspended in water media at a solid/liquid ratio of 1:100 (g:cm³). The desorption rate in distilled water is the highest from NZ for both ions. The leached amount of Ni²⁺ ranged from 3 to 6%, while Cr³⁺ desorption was below 0.5% for all tested adsorbents (Figure 8).

For desorption tests in NaCl and NaEDTA solutions, only saturated HT samples were used, as the HT sample showed the highest adsorption capacity for Ni²⁺ and Cr³⁺ ions. Desorption was more pronounced in the NaEDTA solution, with 9.17% of Ni²⁺ and 3.44% of Cr³⁺ leached (Figure 9). Desorption in the NaCl solution was lower, with only 4.42% of Ni²⁺ and 0.88% of Cr³⁺ leached (Figure 9). In both tested solutions, the desorption rate remained below 10% for both metal ions.

The low desorption rates in distilled water and NaCl and NaEDTA solutions indicate that the prepared adsorbents exhibit strong binding affinities for Ni²⁺ and Cr³⁺ ions, making them suitable for metal removal from aqueous solutions with minimal risk of leaching. This suggests that they can be effectively used for the permanent removal and long-term storage of heavy metals. According to this, potentially, those saturated adsorbents can be used as binders in construction materials [70], as antibacterial agents [71], or for removing borate ions [72].

4. Conclusions

In this work, composites based on natural zeolite and calcium hydroxyapatite were synthesized hydrothermally and mechanochemically. The characterization of the synthesized composites confirmed that neither hydrothermal nor mechanochemical modification influence the crystallinity and the chemical properties of the zeolite structure. The synthesized composites were investigated as adsorbents of the heavy metal ions Ni²⁺ and Cr³⁺. The results presented in this study indicate that the hydrothermal modification of the zeolite positively influences the adsorption process of the investigated cations. The leaching of both cations in water media from the all materials was low (below 10% for all investigated solutions). According to results presented in this study, it can be concluded that hydrothermal modification, as one simple procedure, with a small amount of hydroxyapatite can be recommended for the preparation of the new adsorbents based on natural zeolite for Ni²⁺ and Cr³⁺ removal. The influence of other parameters (such as competing ions and pH) on adsorption, as well as the use of the adsorbents for the removal of heavy metal ions from real wastewater in both batch and flow systems, will be the subject of detailed investigation in future studies.