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Article

Uptake of Copper and Zinc Ions by Georgian Natural Heulandite and Resulting Changes in Its Chemical Composition and Structure

by
Vladimer Tsitsishvili
1,
Marinela Panayotova
2,*,
Nato Mirdzveli
3,
Vladko Panayotov
4,
Nanuli Dolaberidze
3,
Manana Nijaradze
3,
Zurab Amiridze
3 and
Bela Khutsishvili
3
1
Department of Chemistry and Chemical Technologies, Georgian National Academy of Sciences, Tbilisi 0108, Georgia
2
Department of Chemistry, University of Mining and Geology, “St. Ivan Rilski”, 1700 Sofia, Bulgaria
3
Petre Melikishvili Institute of Physical and Organic Chemistry, Tbilisi State University, Tbilisi 0186, Georgia
4
Engineering Sciences Unit, Bulgarian Academy of Sciences, 1000 Sofia, Bulgaria
*
Author to whom correspondence should be addressed.
Minerals 2025, 15(9), 902; https://doi.org/10.3390/min15090902
Submission received: 28 July 2025 / Revised: 20 August 2025 / Accepted: 22 August 2025 / Published: 25 August 2025
(This article belongs to the Section Clays and Engineered Mineral Materials)
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Abstract

Extraction of metal ions from polluted waters and immobilization of metals in contaminated soils can be conducted using zeolites—porous aluminosilicate ion exchangers. The uptake of copper and zinc ions by the Georgian natural heulandite was studied under conditions of interaction of the zeolite with solutions (“liquid-phase” ion exchange) and powders (“solid-state” ion exchange) of the corresponding salts. The aim of the study was to compare the effect of the two procedures on the chemical composition and structure of the zeolite. It was found that the “liquid-phase” procedure provides a higher degree of uptake, particularly of zinc ions. Ion-exchange causes slight dealumination without decationization. Uptake of divalent ions occurs mainly through the leaching of sodium ions. According to X-ray data of ion-exchanged samples, the uptake of copper and zinc does not change the crystal structure of the zeolite framework, but nitrogen adsorption measurements show that ion exchange affects the mesoporous structure: solution treatment reduces the specific total pore volume and leads to the appearance of pores with a diameter of 4 nm. The “solid-state” procedure leads to an increase in specific total pore volume mainly due to an increase in the number of relatively small nanosized pores.

1. Introduction

The ever-increasing environmental pollution, especially by heavy toxic metals [1], has led to an increased demand for the sustainable use of natural materials for decontamination on an industrial scale [2]. Heavy metals can be released by both natural and anthropogenic processes and end up in the air, water, and soil. Natural processes include volcanic eruptions, soil erosion, and weathering of rocks and minerals; however, human activities such as mining, metal smelting, fossil fuel use, transportation, waste incineration, and agricultural practices are more significant contributors to heavy metal pollution. Although heavy metals are present in the environment in small quantities, they can cause skin, liver, lung, and bladder cancer, brain and reproductive organ damage, and kidney dysfunction; they can also affect the immune system [3,4].
Natural zeolites, porous crystalline aluminosilicates Mn[SixAlnO2(n+x)].mH2O (M+ = Na+, K+, ½Ca2+, ½Mg2+, …), have recently attracted considerable attention due to their natural occurrence, unique physical and chemical properties, low cost, non-toxicity, and effectiveness in adsorbing many common pollutants [5,6]. Natural zeolites, mainly heulandite–clinoptilolites, are particularly suitable for the adsorption of heavy metals [7,8,9] such as chromium [10,11,12,13,14,15,16,17], manganese [11,18,19,20,21,22], iron [20,21], cobalt [23,24,25], nickel [10,26,27], copper [10,28,29,30,31,32], zinc [11,29,33,34,35], cadmium [11,13,29,30,36,37], lead [29,30,37,38,39,40], mercury [41,42,43,44], and others [7,45,46]. Studies on the sorption of heavy metal ions have also been conducted using synthetic zeolites [24,34,39,47,48,49,50,51,52,53,54,55,56,57,58,59,60,61] with a high aluminum content, which ensures their high ion-exchange capacity. Despite the very good results, there is little prospect of their widespread implementation due to the complexity of their preparation and the high cost of such ion exchangers.
Zeolites can be chemically or physically modified to improve their adsorption and ion exchange properties [62,63,64]. Zeolite modification technique includes ion exchange reactions (for example, sodium enrichment of Algerian zeolite increased nickel uptake [27], and enrichment of heulandite–clinoptilolite with copper resulted in increased chromium absorption [65]), surface functionalization (e.g., with surfactants [66]), treatment with acids, bases, or other chemicals to change surface properties or expand the porous structure of zeolites [67], introduction or impregnation of additional materials (e.g., zero-valent iron nanoparticles (nZVI) [68]), mixture formation (e.g., mixtures of Japanese natural heulandite–clinoptilolite with steel slag in different proportions were used for the simultaneous removal of cadmium and lead from binary solutions [69]), steam treatment, and calcination [70]. The main results of the application of modified zeolite in the remediation of heavy metals in contaminated soil are presented in a brief review [71]. The results of using zeolites in wastewater treatment are reviewed in [72]. Challenges, recent advances, and perspectives of zeolite modification are reviewed in [73].
Along with modification, the formation of composite materials based on natural or synthetic zeolites is of interest. Firstly, these are composites of activated carbon with synthetic zeolites for capturing nickel, copper, cadmium, and lead [74], chromium [75], copper [76], and lead [77], as well as manganese-oxide-coated zeolite for removal of manganese [19], chitosan/zeolite-A hybrid structures for absorption of cadmium and arsenic [78], synthetic zeolite-supported nZVI and nickel bimetallic composites for the removal of a wide range of heavy metals from industrial effluents [79], magnetite-containing (Fe3O4) magnetic composites [80], combinations of natural zeolite with Mg/Al-layered double hydroxides [81], biochar and bentonite [82], biochar and other additives [83].
Zeolites can adsorb heavy metals from contaminated waters by physisorption, as well as by chemisorption or ion exchange [84]. In addition, natural zeolites have been successfully used for the remediation of contaminated soils by immobilizing and decreasing the mobility of metals [85,86,87], thereby reducing the likelihood of heavy metal transfer to plant roots without significantly altering the natural functions of soils [88,89,90].
The efficiency of environmental purification depends on the type, chemical composition, adsorption, and ion exchange capacity and porosity of the natural zeolites used, as well as on the particle size and initial concentration of heavy metals, pH and ionic strength of the liquid phase, the contact time of the zeolite with the metal carrier, temperature, and the presence of other organic and inorganic pollutants [90].
The sorption of metal ions by zeolites occurs during the interaction of the zeolite with a solution of the salt of the corresponding metal (“liquid-phase” ion exchange, LIE), or during contact between highly dispersed powders of zeolite and salt (“solid-state” ion exchange, SIE). LIE procedures are usually used to obtain antimicrobial metal-containing zeolites [91,92,93], whereas SIE procedures are used in solvent-free mechanochemical synthesis of Metal–Organic Frameworks [93] and Zeolitic Imidazolate Frameworks [94,95]. The LIE procedure can be considered a model of the interaction between an ion exchanger with a dissolved pollutant, while the SIE procedure can serve as a model of such interactions in soil.
Comparing the efficiencies of the LIE and SIE procedures, as well as determining their influence on the chemical composition and the structure and properties of the ion exchanger immobilizing pollutants, is of interest. The aim of this study was to determine the influence of these two procedures involving copper and zinc ions on the chemical composition, crystalline and porous structure, and ion-exchange and sorption properties of natural zeolite belonging to the HEU group with an elementary cell of |M8(H2O)24|[Al8Si28O72] composition [96]. Natural varieties of the HEU group of zeolites, called heulandite–clinoptilolites, have Si/Al ratios ranging from 2.7 to 5.5 and varying cationic compositions [97]. Heulandite–clinoptilolites are the only natural zeolites recognized as safe based on EU legislation [98].
Copper is one of the most common pollutants in water and soil surrounding industrial facilities involved in mining, metal processing, electroplating, fertilizer manufacturing, and painting [99]. Excess copper can accumulate in the human body through the water cycle, causing nausea, vomiting, and possibly motor and neurological impairment [32]. Zinc is an essential micronutrient with a well-studied role in living organisms [34,100] and behavior in the environment, for example, in soil [101]. However, zinc ranks high among environmental pollutants due to its widespread occurrence and toxicity [102], as it tends to accumulate in living organisms and ecosystems [101] and has adverse effects on living organisms at elevated concentrations [102].

2. Materials and Methods

Samples of zeolite-bearing tuff were collected in the southern part of the Tedzami-Dzegvi deposit, Eastern Georgia, which has approved industrial reserves of over 36 million tons [103,104]. The tuff was crushed in a standard cone crusher and a Pulverisette 7 planetary mill (Fritsch Laboratory Instruments, Idar-Oberstein, Germany), fractionated to the particle size < 0.063 mm or 240 US mesh using a set of sieves, washed with distilled water to remove clay impurities, and dried in air at a temperature of 95–100 °C. According to the results of our recent study [67], the tuff contains up to ≈90% of zeolite phase consisting of high-silica sodium–calcium heulandite with chabazite admixture in an amount of up to 10%, empirical formula (Na0.25K0.06Ca0.19Mg0.15) [AlSi3.6O9.2]·3H2O, framework impurity Ti0.005, and mineral impurities Fe0.2 and Ca0.14 per one aluminum atom.
The LIE procedures were carried out in 1.0 mol/L solutions of copper and zinc chlorides at a solid-to-liquid ratio of 1:6 at 75–80 °C with stirring in a shaking water bath (OLS26 Aqua Pro, Grant Instruments, Pasadena, TX, USA) for 3 h. The SIE procedures were carried out by mixing zeolite and salt powders (weight ratios of 1:2 and 1:2.6 for copper chloride dihydrate and anhydrous zinc chloride, respectively, corresponding to one metal atom per one aluminum atom) and grinding the mixture in an agate mortar for 10 min. The prepared samples were transferred to a filter and washed with distilled water until all the Cl− anions were removed, and then dried at 100–105 °C. Copper(II) chloride dihydrate, CuCl2.2H2O, and zinc(II) chloride, ZnCl2 (p.a., obtained from Merck KGaA—Darmstadt, Germany), were used without any additional purification.
The Si/Al ratio and cationic composition of samples were calculated from the X-ray energy dispersive (XR-ED) spectra obtained from a high-performance scanning electron microscope JSM-6490LV (Jeol, Tokyo, Japan) equipped with INCA Energy 350 XRED analyzer (Oxford, UK). The XR-ED spectra were obtained from four 50 μm × 100 μm sections of the corresponding SEM image, and the resulting specific atomic abundances were averaged to obtain the final data with standard deviations and standard errors. The relative error of determination was ±1.4% for silicon, ±2.0% for aluminum, ±2.4% for sodium, ±1.1% for potassium, ±1.7% for calcium, ±2.3% for magnesium, ±2.6% for copper, and ±3.2% for zinc.
Powder X-ray diffraction (XRD) patterns were obtained from a diffractometer D8 Endeavor (Bruker, Mannheim, Germany) employing the Cu-Kα1 line (λ = 0.154056 nm). The XRD patterns were scanned in the 2Θ range of 8° to 64° with a 0.02° step at a scanning speed of 1°/min; the powder XRD pattern of the original sample with peak assignment and the X-ray data for the ion-exchanged samples are presented in Appendix A. Sorption of water and benzene vapors was measured under static conditions at room temperature and constant pressure. The measurement in each case was carried out on three samples weighing approximately 1 g. The results were averaged, with the relative error in measuring the adsorption of water vapor at p/po = 0.4 being 3%, and at p/po = 1.0 being 4.5%. The relative measurement error for benzene adsorption was 3.5%. Nitrogen adsorption/desorption isotherms were measured at 77 K using the ASAP 2020 Plus analyzer (Micromeritics, Norcross, GA, USA) on samples that were vacuum-degassed at 350 °C. Data analysis was conducted using Micromeritics software and involved applying the Brunauer–Emmett–Teller (BET) and Barrett–Joyner–Halenda (BJH) models (details are provided in Appendix B).

3. Results and Discussion

The original, untreated sample is designated HEU; samples obtained by processing in copper and zinc chloride solutions are designated Cu-HEU(LIE) and Zn-HEU(LIE), respectively; samples obtained via the SIE procedure using solid copper and zinc chlorides are designated Cu-HEU(SIE) and Zn-HEU(SIE), respectively.

3.1. Chemical Composition of the Ion-Exchanged Samples

Results of chemical analysis, including averaged empirical formulas of dehydrated original and ion-exchanged zeolites, calculated for 72 oxygen atoms in the unit cell, as well as Si/Al molar ratios and total charge of compensating cations per aluminum atom M+/Al, are given in Table 1. Changes in the number of “natural” compensating cations—Na+, K+, Ca2+, and Mg2+—relative to the initial cationic composition are shown in Figure 1.
The results obtained show that ion exchange reactions, regardless of the method of their implementation, result in the leaching of no more than 30% of aluminum from the zeolite framework, which is significantly lower than the result of acid treatment of the same zeolite, where more than 40% of aluminum is leached even at a low hydrochloric acid concentration (≤0.5 mol/L). Zeolite decationization is also insignificant, since the minimum relative total charge of the cations Me+/Al is 0.96 ± 0.05, whereas, as a result of acid treatment, even with dilute solutions, the negative charge of the aluminum atom is compensated by cations by 80% [67].
The diffusion coefficient of copper and zinc ions in solutions is much higher than in a solid state, but the uptake of copper by zeolite using the LIE procedure is only about 10% higher than using the SIE procedure (atomic ratio Cu/Al 0.16 and 0.15, respectively); however, in terms of zinc uptake by zeolite, LIE is more than twice as effective as SIE (atomic ratio Zn/Al 0.22 and 0.10, respectively). The obtained values of copper and zinc sorption are higher than the 0.13 value for both Cu/Al and Zn/Al achieved for the synthetic zeolite of the LTA type (Si/Al = 1) with high ion-exchange capacity [61]. This can be explained by the fact that the LIE procedures were carried out at low salt concentrations (up to 0.05 mol/L) to reduce the destruction of the LTA crystal structure.
The course of ion exchange reactions on zeolites depends on the nature of the exchanged ions and the structure of the zeolite framework [105]. The zeolite under study contains alkali (Na, K) and alkaline earth (Ca, Mg) metal cations as compensating cations. In aqueous solutions, the hydrated ions of these metals have the following geometry: the sodium ion [Na(H2O)6]+ is considered octahedral with a calculated ionic radius of 1.09 Å; the potassium ion [K(H2O)8]+ is square antiprismatic with an ionic radius of 1.50 Å, calculated from the structure in the solid state; the calcium ion [Ca(H2O)8]2+ is square antiprismatic with a calculated ionic radius of 1.12 Å; and the magnesium ion [Mg(H2O)6]2+ is octahedral with a calculated ionic radius of 0.76 Å [106]. As for the adsorbed ions, the hydrated copper(II) ion [Cu(H2O)6]2+ has a Jahn–Teller distorted octahedral configuration with calculated ionic radius of 0.62 Å, and the hydrated zinc(II) ion [Zn(H2O)6]2+ has a regular octahedral configuration with a calculated ionic radius of 0.74 Å, although Shannon’s classic work [107] indicates similar ionic radii for copper and zinc of 0.73 and 0.74 Å, respectively.
As Figure 1 shows, the uptake of relatively small copper and zinc ions occurs mainly due to the leaching of larger sodium ions, as well as the leaching of magnesium ions, which have the smallest radius among the “natural” cations. The relatively large calcium ion is leached to a lesser extent, and the largest potassium ion hardly participates in ion exchange reactions. In general, the same pattern of change in the leaching of “natural” cations Na+ > Mg2+ > Ca2+ > K+ was discovered in the case of acid treatment of the zeolite under study [67]; the exception is the case of Zn-HEU(LIE), when the leaching of potassium slightly exceeds the leaching of calcium.

3.2. Crystal Structure of the Ion-Exchanged Samples

Possible structural changes in the studied samples can be determined by powder XRD patterns. The powder XRD pattern of the original heulandite sample with peak assignments is presented in Figure 2; assignment details and X-ray data for the ion-exchanged samples are given in Appendix A. A comparison of the experimental XRD pattern of the original sample with the simulated pattern of the HEU-type zeolite from the collection [108] shows almost complete coincidence of the 2Θ positions of both the low-angle resolved peaks from such characteristic reflections as (200), (020), etc., and the intense overlapping peaks in the 22° < 2Θ < 34° region (see Table A1). However, the peak intensities differ, which can be explained by the different chemical composition (Si/Al ratio and compensating M+ ions) of the studied sample and the structure used to simulate the XRD pattern [109].
The positions of the peaks shown in Figure 2 do not change as a result of ion exchange reactions. Changes in the cation composition only affect the peak intensities (see Table A2). It is important to note the preservation of the positions of the peaks from the (200) and (020) reflections, which are sensitive to compression and expansion along the crystallographic axes a and b, respectively. For the synthetic LTA-type zeolite, significant broadening of all peaks was observed upon copper and zinc sorption, clearly indicating a decrease in the crystallinity of the zeolite. In addition, for samples with high zinc content, a shift in peaks towards higher 2Θ values was also observed in the XRD patterns [91]. Oheix et al. noted that it remains unclear whether this shift is associated with the structural damage to the zeolite framework.
The uptake of copper and zinc causes a decrease in the intensity of the resolved peaks from reflections (200), (−201), (−222) and (−132), as well as overlapping peaks at 2Θ = 30° (from reflections (−351), (151), (350), and (112)) and 31.8° (from reflections (530) and (−621)). The intensity of the remaining peaks changes slightly or non-monotonically. When studying the acid resistance of this Georgian heulandite [67], a decrease in the intensity of the peak from reflection (−201), as well as the overlapping peak from reflections (−261) and (061) was noted, up to the complete disappearance of these peaks with an increase in the concentration of hydrochloric acid. The greatest changes in peak intensities are observed as a result of copper sorption during the SIE procedure: the total intensity of overlapping peaks from reflections (131), (400), (330), (−421), and (240), as well as the intensities of resolved peaks from reflections (−402), (002), and (−422) increase significantly. An increase in the intensity of overlapping peaks at 2Θ = 22.5° was noted when heulandite was treated with hydrochloric acid solutions with a concentration of up to 1 mol/L. At a higher acid concentration, the intensity of these peaks decreased, while for the peak from reflection (−422) at 2Θ = 28.06°, the opposite effect occurred: its intensity decreased at low acid concentrations, but increased sharply at high concentrations [67].

3.3. Water Adsorption

Water vapor adsorption is used as a measure of the micropore volume of high-aluminum zeolites due to their hydrophilicity [110] and the small kinetic diameter of the water molecule (0.266 nm), which allows it to freely pass into the micropores through the “entrance windows” of heulandite, which has a diameter greater than 0.3 nm [96].
The results of water vapor adsorption measurements on the initial and ion-exchanged heulandite samples are shown in Figure 3. The data at a relative pressure of p/po = 0.4, corresponding to almost complete filling of the zeolite micropores, reflect the volume of the micropores, whereas the data at a saturated water vapor pressure (p/po = 1.0) reflect the total volume of all pores [111].
According to the obtained results, the sorption of copper and zinc ions by the porous heulandite structure has little effect on the filling of micropores with water molecules; the maximum effect is observed as a result of the introduction of zinc ions into the zeolite framework, where adsorption through micropores increases by 8%. At the same time, the number of water molecules adsorbed via larger pores at saturated water vapor pressure increases significantly, regardless of the method used for ion exchange reactions. This effect is associated both with the replacement of two sodium(I) ions [Na(H2O)6]+ with an ionic radius of 1.09 Å with one ion of divalent copper(II) [Cu(H2O)6]2+ or zinc(II) [Zn(H2O)6]2+, with ionic radii of 0.73 and 0.74 Å, respectively, and with changes in the mesopore system described below based on measurements of nitrogen adsorption–desorption isotherms.

3.4. Benzene Adsorption

Benzene (C6H6) is a large molecule with a kinetic diameter of 0.585 nm. It cannot pass through the “entrance windows” of heulandites into cavities and channels, and the adsorption of benzene can be considered a relative measure of the surface area and its hydrophobicity [67]. The adsorption of benzene on the outer surface of zeolite is influenced by factors such as the chemical composition and structure of zeolite, as well as the presence of other molecules [112]. Benzene molecules interact with zeolites through their π-electron system, forming complexes with active centers such as transition metal cations [113]. The adsorption of benzene on synthetic copper-exchanged zeolites of the FAU and ZSM types was studied using Fourier transform infrared spectroscopy and quantum mechanical calculations [114,115], and it was shown that benzene forms a stable complex via its π-electrons with the copper ion.
As shown in Figure 4, the results obtained indicate that the adsorption of benzene on copper- and zinc-containing heulandites depends on the method of conducting the ion exchange reaction, but changes insignificantly. The maximum increase in benzene adsorption is observed for copper-containing heulandite obtained by the LIE procedure, at 17 ± 4%, which is insufficient to indicate the formation of π-complexes.
The increase in benzene adsorption as a result of the copper–LIE procedure is approximately the same as after the treatment of heulandite with a hydrochloric acid solution with a concentration of 0.5 mol/L, when more than 40% of aluminum atoms are washed out and “hydroxyl nests” are formed on the surface [67]. As a result of ion-exchange reactions, dealumination is insignificant, and decationization does not occur; however, the hydrophobicity of the heulandite surface changes: it increases after the copper–LIE and zinc–SIE procedures and decreases after the copper–SIE and zinc–LIE procedures, albeit to a much lesser extent than as a result of heulandite calcination [116], when dehydration and amorphization occurs.

3.5. Nitrogen Adsorption

The kinetic diameter of the nitrogen molecule is 0.364 nm, which allows it to pass through only one “entrance window” of the heulandite micropores—that formed by an 8-member ring with dimensions of 0.46 × 0.36 nm. The N2 molecule cannot pass through the windows formed by the second 8-member ring (0.47 × 0.28 nm) and a 10-member ring (0.75 × 0.31 nm); thus, the amount of nitrogen molecules adsorbed to the micropores of narrow-pore zeolite of the HEU type can be very low compared to the amount of nitrogen molecules adsorbed to the mesopores and macropores.
According to the Brunauer–Deming–Deming–Teller classification [117], nitrogen adsorption–desorption isotherms measured on the original, copper- and zinc-exchanged heulandite samples (Figures S1 and S2, respectively) belong to type IV, which describes the complete filling and emptying of all pores. At low relative pressures (p/po < 0.4), the adsorption isotherms reflect the filling of micropores, and at higher pressures, a hysteresis loop is observed, which, according to the classification of Sing et al. [118], is a combination of loop H3 in the region of medium pressures (0.4 < p/po < 0.9), corresponding to the filling and emptying of relatively narrow slit-shaped pores, and loop H1 in the region of high relative pressures (0.9 < p/po < 1.0), reflecting the filling and emptying of conventionally cylindrical channels.
The porosity characteristics of natural Georgian heulandite and its ion-exchanged forms (calculated on the basis of nitrogen adsorption–desorption isotherms (Figures S1 and S2) using the Brunauer–Emmett–Teller (BET) model [119], Rouquerol’s consistency criteria for BET area calculation [120], and Barrett–Joyner–Halenda (BJH) model [121] for analysis of the mesoporous system; details are given in Appendix B) are presented in Table 2. Pore size distribution curves volume (V) vs. pore diameter (D) and differential dV/dD vs. D calculated using the BJH model from desorption isotherms are illustrated in Figure 5 and Figure 6, respectively.
The micropore volume Vm accessible to nitrogen molecules, as well as the surface area SBET increase for all ion-exchanged samples; the greatest effect is observed after treatment in a copper chloride solution; however, this is significantly weaker than the effect of an acidic environment—Vm increases by 12 times and SBET by 10 times after treatment in a 0.5 mol/L hydrochloric acid solution [67]. The surface area (SBET) increased from ≈16 to 28 m2/g after treatment of Indonesian clinoptilolite with an FeCl3 solution, and to ≈34 m2/g after the introduction of zero-valent iron nanoparticles [68]. Heating Iranian zeolite (a mixture of clinoptilolite with relatively wide-pore mordenite) led to an increase in SBET from ≈26 to 147 m2/g, although the surface area decreased sharply with increasing degree of amorphization [70]. In a wide-pore synthetic zeolite of the FAU type with an “entrance window” of 0.74 nm in diameter, which allows free passage of nitrogen molecules, the volume of micropores and the BET surface area decreased after the sorption of metals from solutions of copper acetate and zinc nitrate [122].
The volume of micropores Vm accessible to nitrogen molecules increases after both ion exchange procedures. The total volume of pores with a diameter of up to 180 nm, Vp, decreases after the LIE procedure and increases after the SIE procedure, as clearly indicated in the pore size distribution curves (see Figure 5). In the initial sample, the volume of micropores Vm is 7.5% of the total pore volume, Vp, and after the LIE procedure, the proportion of the micropore volume increases to 19.2% and 14.7% with the uptake of copper and zinc, respectively. In contrast, after the SIE procedure, this increase is not so significant—up to 8.2 and 8.5% for copper and zinc uptake, respectively.
Differential curves of the distribution of pore sizes with a diameter of less than 180 nm (see Figure 6) make it possible to describe the changes in the mesoporous system of heulandite as follows: (i) like acid treatment [73], the LIE procedure leads to the disappearance of the maximum at 12 nm characteristic of the initial sample and the appearance of a sharp maximum at 4 nm, (ii) as a result of the SIE procedure, the number of mesopores with a diameter of less than 4 nm increases, while the number of mesopores with a diameter of 4–7 nm decreases and the number of larger mesopores increases with the uptake of copper, so that the distribution maximum shifts to ≈ 18 nm, and the number of all mesopores with a diameter of up to 12 nm increases with the uptake of zinc.

4. Conclusions

Based on the results of the effect of ion exchange reactions on the chemical composition, structure, and properties of natural heulandites from the Tedzami-Dzegvi deposit, the following conclusions can be drawn:
  • Ion exchange reactions, regardless of the method of their implementation, cause slight dealumination of the zeolite framework without the formation of “hydroxyl nests”.
  • The degree of copper uptake by heulandite does not depend on the ion exchange procedure; however, zinc uptake is more than twice as high as unexchanged zeolite when the zeolite interacts with the liquid phase.
  • The uptake of transition metal ions occurs mainly due to the leaching of sodium and magnesium ions. Calcium ions are leached to a lesser extent, and potassium ions barely participate in ion exchange processes; that is, based on the same scheme as during decationization under acidic conditions.
  • The crystalline framework of heulandite does not change as a result of ion exchange reactions; changes in the peak intensities in powder XRD patterns are due to changes in cationic composition.
  • The change in the adsorption capacity of micropores for water molecules after the uptake of transition metals is insignificant; the adsorption of benzene molecules indicates only minor changes in the hydrophobicity of the outer surface of heulandites.
  • The volume of micropores accessible to nitrogen molecules and the BET surface area are increased by all ion exchange procedures, but to a much lesser extent than by acid treatment. The LIE procedure decreases the volume of mesopores, and pores with a diameter of 4 nm become predominant. The SIE procedure increases the volume of nano-sized mesopores, and the distribution of pore sizes depends on the nature of the immobilized metal.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/min15090902/s1, Figure S1: N2 adsorption–desorption isotherms on Georgian heulandite and its copper-exchanged forms; Figure S2: N2 adsorption–desorption isotherms on Georgian heulandite and its zinc-exchanged forms.

Author Contributions

Conceptualization, V.T.; investigation, N.M., M.N., Z.A., and B.K.; methodology, M.P., V.P., and N.D.; project administration, N.M. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the International Science and Technology Center (ISTC), grant number GE-2506 “Scientific substantiation of the possibility of creating new bactericidal zeolite filter materials for purification-decontamination of water from various sources”.

Data Availability Statement

Data is contained within the article and Supplementary Materials.

Conflicts of Interest

The authors declare no conflicts of interest.

Abbreviations

The following abbreviations are used in this manuscript:
XR-EDX-Ray Energy Dispersion
XRDX-Ray Diffraction
LIELiquid-phase Ion Exchange
SIESolid-state Ion Exchange
BETBrunauer–Emmett–Teller model
BJHBarrett–Joyner–Halenda model
STPStandard Temperature and Pressure: 273.15 K and 101.325 kPa
ZVIZero-Valent Iron

Appendix A

In the powder XRD pattern [108] simulated for potassium heulandite |K8.48(H2O)18| [Si26.64Al9.36O72] [109], the three most pronounced low-angle reflections, (020), (200), and (131), which are recommended for identifying HEU-type zeolites, show peaks at 2Θ = 9.85, 11.07, and 22.22° with relative intensities I/Io of 100, 64, and 60%, respectively. However, if the peaks from reflections (020) and (200) do not overlap with other peaks, then the peak of reflection (131) is superimposed by peaks of reflections (400), (300), (−421), and (240) with comparatively high intensity (I/Io > 20%), and in the experimental XRD patterns these five peaks merge into one broad peak with a full width at half maximum of 0.5°, a maximum at 2Θ = 22.5°, and an effective intensity of 115%, as shown in Table A1. Similarly, reflections (−351), (151), (350), and (112) show one broad peak at 2Θ = 30°, while reflections (−530) and (−621), and (−261) and (061) show narrow peaks at 2Θ = 31.80 and 32.76°, respectively.
Table A1. X-ray data of intense peaks (I/Io > 10%) in the simulated pattern and the experimental powder XRD pattern of natural heulandites.
Table A1. X-ray data of intense peaks (I/Io > 10%) in the simulated pattern and the experimental powder XRD pattern of natural heulandites.
Simulated Pattern [64]Experimental Pattern
Miller Indices hkld (Å)2Θ (°)I/Io (%)2Θ (°)I/Io (%)
020 *8.9799.851009.82100
200 *7.98911.0764.011.0448
−2016.79213.0317.813.0031
−3115.25816.8611.316.9032
1115.15717.2932.017.2429
−1314.66119.0420.319.0633
131 *
400
330
−421
240		4.003
3.995
3.979
3.931
3.914		22.21
22.25
22.34
22.62
22.72		60.2
34.7
53.8
21.9
29.4		22.5115
−3123.56324.9929.425.0025
−2223.43325.9650.826.0047
−4023.39626.2418.526.2039
0023.34126.6819.026.7029
−4223.17628.0945.128.0655
−4413.13228.4934.128.5628
−1323.08628.9322.828.8821
−351
151
350
112		3.007
2.988
2.978
2.975		29.71
29.90
30.01
30.03		15.0
41.6
27.8
12.7		30.059
530
−621		2.819
2.810		31.74
31.85		43.6
14.0		31.8041
−261
061		2.739
2.732		32.70
32.79		10.6
12.8		32.7620
* Three most pronounced low-angle reflections.
Following from Table A1, the positions of non-overlapping intense peaks in the experimental XRD pattern of the original heulandite correspond to the Bragg angle values 2Θ of the simulated pattern with an accuracy of ±0.04°, but the experimental and calculated intensities for many peaks differ significantly. A similar result was obtained from the analysis of the XRD patterns of ion-exchanged samples: the positions of non-overlapping peaks in the patterns of ion-exchanged samples coincide with the values of 2Θ in the pattern of the original heulandite with an accuracy of ±0.02°; the intensities of the peaks in the XRD patterns of the studied samples are provided in Table A2.
Table A2. Relative intensity (I/Io, %) of peaks in experimental powder XRD patterns of natural and ion-exchanged heulandites.
Table A2. Relative intensity (I/Io, %) of peaks in experimental powder XRD patterns of natural and ion-exchanged heulandites.
Miller Indices hklHEUCu-HEU(LIE)Cu-HEU(SIE)Zn-HEU(LIE)Zn-HEU(SIE)
020 *100100100100100
200 *4838333137
−2013124232221
−3113227353326
1112932282328
−1313330303228
131 *, 400, 330, −421, 240115104121109102
−3122524262220
−2224742454040
−4023938544438
0022927352927
−4225550735349
−4412824262223
−1322120201311
−351, 151, 350, 1125948494444
530, −6214136323333
−261, 0612018221716
* Three most pronounced low-angle reflections.

Appendix B

Nitrogen adsorption isotherms measured on initial and ion-exchanged heulandite samples (Figures S1 and S2) in the range of low relative pressures up to (p/po) ≈ 2 (the saturation pressure of nitrogen po at 77 K is 101.3 kPa) are well described by the Brunauer–Emmett–Teller (BET, [119]) equation:
{W[(po/p) − 1]}−1 = (WmCBET)−1 + {[(CBET − 1)/WmCBET)] (p/po)},
where W (cm3/g STP) is the specific volume of adsorbed nitrogen at relative pressure (p/po), Wm (cm3/g STP) is the specific volume of monolayer on the adsorbent surface, and the dimensionless constant CBET reflects the ratio of the adsorption equilibrium constant in the first layer to the condensation constant.
A comparison of the calculated BET isotherm plot and the experimental adsorption isotherm for the initial and typical ion-exchanged heulandite samples is shown in Figure A1, which also shows the linearization of the experimental data in the low relative pressure region in the BET equation coordinates {W[(po/p) − 1]}−1 vs. (p/po). The results of the calculations performed by the Micromeritics software are presented in Table A3.
Minerals 15 00902 g0a1
Figure A1. Adsorption isotherm plot (lower, solid line—calculation using the BET equation, crosses—measured values) and linearization of the experimental data in the BET equation coordinates (upper) for: (a) initial heulandite; (b) copper-LIE-exchanged heulandite.
Figure A1. Adsorption isotherm plot (lower, solid line—calculation using the BET equation, crosses—measured values) and linearization of the experimental data in the BET equation coordinates (upper) for: (a) initial heulandite; (b) copper-LIE-exchanged heulandite.
Minerals 15 00902 g0a1
Table A3. Parameters of the linearized BET isotherm plot.
Table A3. Parameters of the linearized BET isotherm plot.
SampleSlope (g/cm3 STP)Y-Intercept (g/cm3 STP)Correlation CoefficientWm (cm3/g STP)
HEU0.204521 ± 0.0006610.000582 ± 0.0000640.99994264.8756 ± 0.0158
Cu-HEU(LIE)0.145230 ± 0.0005460.000374 ± 0.0000420.99993646.8680 ± 0.0256
Cu-HEU(SIE)0.221737 ± 0.0009490.000379 ± 0.0000820.99990844.5021 ± 0.0193
Zn-HEU(LIE)0.197662 ± 0.0007910.000362 ± 0.0000690.99991995.0499 ± 0.0202
Zn-HEU(SIE)0.220244 ± 0.0006500.000942 ± 0.0000700.99994774.5211 ± 0.0133
Correct calculation of the surface area using the BET model is possible if all four Rouquerol’s consistency criteria [120] are met: (i) W[(1 − (p/po)] increases monotonically up to (p/po)max; (ii) the CBET value is positive; (iii) the relative pressure (p/po)mono corresponding to the calculated monolayer volume Wm falls within the linear region (p/po)mono ≤ (p/po)max; (iv) the relative pressure (p/po)mono-cal corresponding to the theoretical volume of the monolayer Wm-cal differs from (p/po)mono by no more than 20%. The values of (p/po)max were determined from the graphs W[(1 − (p/po)] vs. (p/po); the CBET constant value was calculated by the Micromeritics ASAP 2020 Plus 1.03 software, the relative pressure (p/po)mono values were selected from the tabular reports of the adsorption isotherms; the relative pressure corresponding to Wm-cal was calculated as (p/po)mono-cal = [1 + CBET0.5]−1. According to the results summarized in Table A4, all four criteria are met, and the surface area can be calculated as SBET = WmNAAx/M, where NA is Avogadro’s constant (6.023 × 1023 mol−1), Ax is the effective cross-sectional area of the N2 molecule (0.162 nm2), and M is the molar volume (22.414 L/mol).
Table A4. Parameters used in Rouquerol criteria calculated from adsorption isotherms of Georgian heulandite and its ion-exchanged forms.
Table A4. Parameters used in Rouquerol criteria calculated from adsorption isotherms of Georgian heulandite and its ion-exchanged forms.
Sample(p/po)maxCBET(p/po)mono(p/po)mono-calΔ (%) *
HEU0.14369 ± 640.0550.0509.1
Cu-HEU(LIE)0.10389 ± 440.0410.04817.1
Cu-HEU(SIE)0.10585 ± 1270.0430.0399.3
Zn-HEU(LIE)0.08547 ± 100.0460.04110.9
Zn-HEU(SIE)0.09234 ± 170.0640.0614.7
* Δ = 100|(p/po)mono − (p/po)mono-cal|/(p/po)mono.

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Figure 1. Fractions of the cations Na+, K+, Ca2+, and Mg2+ per Al atom retained after ion exchange reactions.
Figure 1. Fractions of the cations Na+, K+, Ca2+, and Mg2+ per Al atom retained after ion exchange reactions.
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Figure 2. Powder XRD pattern of the original sample (a). Numbers in parentheses are Miller hkl indices for heulandite (*—peaks of chabazite) compared with the simulated pattern from collection [108] (b).
Figure 2. Powder XRD pattern of the original sample (a). Numbers in parentheses are Miller hkl indices for heulandite (*—peaks of chabazite) compared with the simulated pattern from collection [108] (b).
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Figure 3. Water vapor adsorption on initial and ion-exchanged heulandites at relative pressures: (a) p/po = 0.4; (b) p/po = 1.0.
Figure 3. Water vapor adsorption on initial and ion-exchanged heulandites at relative pressures: (a) p/po = 0.4; (b) p/po = 1.0.
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Figure 4. Benzene adsorption at p/po = 1 on initial and ion-exchanged heulandites.
Figure 4. Benzene adsorption at p/po = 1 on initial and ion-exchanged heulandites.
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Figure 5. Pore size distribution curves of volume V vs. pore diameter D calculated by the BJH model from desorption isotherms measured on: (a) copper-exchanged heulandites and (b) zinc-exchanged heulandites.
Figure 5. Pore size distribution curves of volume V vs. pore diameter D calculated by the BJH model from desorption isotherms measured on: (a) copper-exchanged heulandites and (b) zinc-exchanged heulandites.
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Figure 6. Pore size distribution differential curves calculated by the BJH model from desorption isotherms measured on: (a) copper-exchanged heulandites and (b) zinc-exchanged heulandites.
Figure 6. Pore size distribution differential curves calculated by the BJH model from desorption isotherms measured on: (a) copper-exchanged heulandites and (b) zinc-exchanged heulandites.
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Table 1. Chemical analysis data.
Table 1. Chemical analysis data.
SampleEmpirical FormulaSi/AlM+/Al
HEU(Na1.96K0.47Ca1.49Mg1.17)[Al7.8Si28.2O72]3.62 ± 0.121.00 ± 0.03
Cu-HEU(LIE)Cu1.1(Na0.30K0.40Ca1.2Mg0.60)[Al6.7Si29.3O72]4.37 ± 0.150.97 ± 0.04
Cu-HEU(SIE)Cu0.95(Na0.42K0.40Ca1.15Mg0.60)[Al6.3Si29.7O72]4.71 ± 0.160.98 ± 0.04
Zn-HEU(LIE)Zn1.4(Na0.20K0.30Ca1.05Mg0.48)[Al6.4Si29.6O72]4.62 ± 0.160.99 ± 0.05
Zn-HEU(SIE)Zn0.67(Na0.75K0.45Ca1.3Mg0.65)[Al6.7Si29.3O72]4.37 ± 0.150.96 ± 0.05
Table 2. Porosity characteristics of Georgian heulandite and its ion-exchanged forms.
Table 2. Porosity characteristics of Georgian heulandite and its ion-exchanged forms.
Porosity ParameterHEUCu-HEU(LIE)Cu-HEU(SIE)Zn-HEU(LIE)Zn-HEU(SIE)
Specific volume of micropores Vm (cm3/g)0.006730.01410.009770.01060.0103
Surface area SBET (m2/g)12.829.919.622.019.7
Specific total pore volume Vp (cm3/g) * 0.08950.07300.11880.07200.120
Average pore diameter DBJH (nm)17.218.121.517.919.1
* The total volume of pores with a diameter less than 180 nm.
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Tsitsishvili, V.; Panayotova, M.; Mirdzveli, N.; Panayotov, V.; Dolaberidze, N.; Nijaradze, M.; Amiridze, Z.; Khutsishvili, B. Uptake of Copper and Zinc Ions by Georgian Natural Heulandite and Resulting Changes in Its Chemical Composition and Structure. Minerals 2025, 15, 902. https://doi.org/10.3390/min15090902

AMA Style

Tsitsishvili V, Panayotova M, Mirdzveli N, Panayotov V, Dolaberidze N, Nijaradze M, Amiridze Z, Khutsishvili B. Uptake of Copper and Zinc Ions by Georgian Natural Heulandite and Resulting Changes in Its Chemical Composition and Structure. Minerals. 2025; 15(9):902. https://doi.org/10.3390/min15090902

Chicago/Turabian Style

Tsitsishvili, Vladimer, Marinela Panayotova, Nato Mirdzveli, Vladko Panayotov, Nanuli Dolaberidze, Manana Nijaradze, Zurab Amiridze, and Bela Khutsishvili. 2025. "Uptake of Copper and Zinc Ions by Georgian Natural Heulandite and Resulting Changes in Its Chemical Composition and Structure" Minerals 15, no. 9: 902. https://doi.org/10.3390/min15090902

APA Style

Tsitsishvili, V., Panayotova, M., Mirdzveli, N., Panayotov, V., Dolaberidze, N., Nijaradze, M., Amiridze, Z., & Khutsishvili, B. (2025). Uptake of Copper and Zinc Ions by Georgian Natural Heulandite and Resulting Changes in Its Chemical Composition and Structure. Minerals, 15(9), 902. https://doi.org/10.3390/min15090902

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