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Article

Evaluating Zeolites of Different Origin for Eutrophication Control of Freshwater Bodies

by
Irene Biliani
,
Eirini Papadopoulou
and
Ierotheos Zacharias
*
Laboratory of Environmental Engineering, Department of Civil Engineering, University of Patras, 26500 Rio, Greece
*
Author to whom correspondence should be addressed.
Sustainability 2025, 17(15), 7120; https://doi.org/10.3390/su17157120
Submission received: 12 June 2025 / Revised: 4 July 2025 / Accepted: 1 August 2025 / Published: 6 August 2025
(This article belongs to the Section Sustainable Water Management)
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Abstract

Eutrophication has become the primary water quality issue for most of the freshwater and coastal marine ecosystems in the world. Caused by excessive nitrogen (N) and phosphorus (P) inputs, it has a significant impact on aquatic ecosystems, resulting in algal blooms, oxygen depletion, and biodiversity loss. Zeolites have been identified as effective adsorbents for removal of these pollutants, improving water quality and ecosystem health. Kinetic and isotherm adsorption experiments were conducted to examine the adsorption efficiency of four zeolites of various origins (Greek, Slovakian, Turkish, and Bulgarian) and a specific modification (ZeoPhos) to determine the most effective material for N and P removal. The aim of the study is to discover the best zeolite for chemical adsorption in eutrophic waters by comparing their adsorption capacities and pollutant removal efficiencies along with SEM, TEM, and X-RD spectrographs. Slovakian ZeoPhos has been identified as the best-performing material for long-term and efficient water treatment systems for eutrophication management.

1. Introduction

Eutrophication is an environmentally, socially, and economically threatening process globally [1], generating harmful algal blooms [2], diminishing water bodies’ oxygen levels [3], and distorting the ecosystem’s ecological balance [4], affecting human health [5]. As global human populations and agriculture increase [6], nutrient control and the prevention of pollution from extensive concentrations of nitrogen and phosphorus are considered the cornerstone of long-lived nutrient treatment; this has led researchers to explore remediation approaches that are particularly sustainable and low-cost, using zeolites or naturally occurring aluminosilicates as a remediation option because of their high efficiencies of adsorption capacities and their abundant supply [7,8,9]. In parallel, orthophosphate ions bind with the benthic environment of the water body, creating a nutrient reservoir, described as “phosphate legacy” [10], which can prolong eutrophication effects. Consequently, the reduction in only the inputs of these nutrients must be combined with measures that bind phosphorus in order to control heavily polluted benthic environments.
Natural zeolite is a hydrated, microporous aluminosilicate mineral, consisting mainly of aluminum (Al), silicon (Si), and alkalis or alkaline elements such as Na, K, Ca, and Mg. The charge is neutralized by cations (±Ba, ±Sr, and ±Mg) within a three-dimensional framework that can exchange cations in aqueous solutions. Its characteristic crystal structure creates a system of channels and cavities that facilitates the free passage and trapping of ions or small molecules [11]. Zeolite has strong ion exchange [12] and adsorption capacities [13], which are greatly exploited in nutrient removal applications. The phenomenon of chemical adsorption [14] on zeolitic materials involves physicochemical mechanisms along with the development of chemical bonds with adsorbed ions [15]. This makes natural zeolite effective in a variety of pollution conditions, with a wide range of applications, from drinking water treatment to wastewater remediation. Its application has been studied mainly in environments with high nutrient pollutant loads [16], it exhibits remarkable stability [17] and can be reused after appropriate regeneration. Zeolite’s natural origin and inherent recyclability make it a highly sustainable material for restoring eutrophic waters by effectively removing ammonium and orthophosphate ions without introducing harmful chemicals [18,19].
Zeolites—particularly natural and modified clinoptilolite—have been shown to provide high ammonium and phosphate removal from eutrophic water bodies, achieving consistent NH4+-N removal capacities of 17–33 mg/g and PO43−-P capacities up to 27 mg/g in recent studies [20,21,22,23], with optimal removal using Fe3+, La3+, Al3+, or Ca2+ modification at near-neutral pH. It is clear that the potential of zeolites for dual ammonium and phosphate adsorption is further enhanced through chemical modifications, after enhancing ion exchange [17] or after metal impregnation [24], along with advances in material engineering [25].
Adsorption studies have reviewed natural and modified zeolites [26,27]. Other studies have assessed the effects of various modification methods (i.e., calcination, microwave treatment, and metal-ion loading) on clinoptilolite’s ability to simultaneously adsorb NH4+ and PO43− in aqueous batch experiments [21,28]. Some studies present the influence of a dual-nutrient uptake, using calcium- and iron-modified zeolites [23,29], while other studies focus on evaluating wastewater treatment rather than freshwater bodies for clinoptilolite in both the US and the UK. Generally, the comparative aspect has mostly been limited to ammonium removal. Most studies conducted were based on a single-origin zeolite and do not provide sufficient information to allow for comparative ranking based on the origin of NH4+ and PO43− removal in batch conditions. Moreover, there are very few comparative data that allow direct performance ranking based on geological source, especially with respect to simultaneous N and P removal and using both natural and modified forms.
This study presents a comparative evaluation of the interaction of zeolite geological origin—mineral composition, regional diagenesis, and inherent physicochemical properties—with nutrient removal performance, since origin-based factors may significantly affect adsorption performance. The goal of this study was to assess nutrient removal efficiencies of zeolites from four different environments/regions/origins in order to restore eutrophic water bodies and, secondly, to determine the feasibility of zeolite applications in water quality restoration approaches and water bodies’ management.

2. Materials and Methods

2.1. Zeolite Sources and Geological Background

Τhe zeolites used for the analysis originate from Greece (N-Z, GR), from Slovakia (N-Z, SK), from Bulgaria (N-Z BG), and from Turkey (N-S TR). All the zeolites used belong to the clinoptilolite classification. Typically, clinoptilolite zeolites have a crystalline, hydrated aluminosilicate of alkali structure [30], with a large surface [31] area since they are typically hydrophilic materials. According to Boles [32], the typical Si/Al ratio for clinoptilolite zeolites is greater than 4 [33] which influences their ion exchange capacity.
The Greek zeolite originates from Thrace, situated in north-east Greece, with more than 75% of clinoptilolite present in its structure [34,35]. Deposits of the Greek Thracean zeolite are present in altered volcanic rocks in large deposits in Thrace, of the Eocene–Oligocene age, rich in heulandites/clinoptilolite and formed in an open hydrological system [33].
The Slovakian zeolite derives from the Nižný Hrabovec quarry site located in east Slovakia, which is one of the largest and most significant sources of natural clinoptilolite zeolite worldwide formed by acidic volcanic ash of alkaline fluids at elevated temperatures. Its geological background is defined by its volcanic genesis in the Miocene era, its distinct mineralogy, and conditions that resulted in the development of high-purity zeolite tuff. High silica content (74.3–77.6% SiO2) and a high-potassium, calc-alkaline affinity characterize the rhyolitic composition of the tuff [36]. Clinoptilolite-Ca is the sole zeolite phase in the deposit, resulting in an exceptionally homogenous and monomineralic structure.
The natural Bulgarian zeolite was formed from the alteration in volcanic tuffs that resulted in lake environments during the Paleogene and Early Oligocene [37]. They are located in the north-east Rhodopes within the Bulgarian territory, creating zoning medial sections obtaining high-cation-exchange-capacity zeolite [38].
Finally, the natural zeolite from Turkey has been extensively studied by researchers [39] as an adsorbent either for nutrients or toxic components of wastewater. The clinoptilolite zeolite originates from the Bayburt area located in north-east Turkey, created after the Miocene and Eocene volcanic sedimentary basin [40] by alkaline and saline waters at low-to-moderate temperature.

2.2. ZeoPhos Modification Protocol

ZeoPhos was created after consecutive immersion in CaCl2, FeCl3, and humic acid solutions. The detailed protocol of the creation of the ZeoPhos modification is described in a previous publication [29].

2.3. Material Characterization

Qualitative and quantitative microanalysis of the composition of natural and modified zeolites was made possible by Scanning Electron Microscopy (using a type JEOL 6300 microscope of the JEOL Ltd., Croissy-sur-Seine, France) coupled with an energy-dispersive X-ray spectrometer (Link ISIS 300, Oxford Instruments, Abingdon, UK). A 15.0 nm thick layer of gold was applied to the samples to improve image quality without altering the internal structure of the zeolite. The instrument operated at 15 kV; four analyses for each sample were performed, scanning the material to avoid dehydration and cation migration in the zeolite.

2.4. Adsorption Experiments

Natural and modified materials were washed, galvanized, sieved, and dehydrated prior to application, ensuring that any impurities were removed; they had a common particle size equal to 0.105 nm, and no humidity altering the weighed quantity of zeolites was present.

2.4.1. Kinetic Adsorption Experiment Set Up

Ammonium and orthophosphate adsorption kinetics were studied to determine the adsorption efficiency of both the natural and modified zeolites. The concentration of the selected pollutants tested for ammonium (381.0 mg NH4Cl at 1 L extra-pure de-ionized water) and orthophosphate (441.93 mg KH2PO4 at 1 L extra-pure de-ionized water) ions was 1 mg/L for both ions in separate solutions.
To perform the kinetic adsorption trials, 0.01 g of each natural and modified material was weighed on a high-precision balance (Mettler AE 200) and transferred to conical flasks containing 100 mL of ammonium ion solution (NH4+-N), and 0.015 g was weighed and placed in 150 mL of orthophosphate ion solution (PO43−-P). The adsorption experiment was performed collecting samples at the following 10 times: 0, 15 min, 30 min, 1 h, 2 h, 4 h, 8 h, 12 h, 24, and 48 h. Two 50 mL sample tubes were collected for each of the above kinetic times. The conical flasks containing the material were covered with foil to prevent interference from microorganisms in the atmosphere. The materials were vacuum-filtered using the Life Sciences Vacuum Pressure Pump and 0.45 μm gridded filters. Room temperature and a neutral pH level were assured during the adsorption experiments.
The experimental procedure for ammonium ion adsorption was performed according to the modified indophenol blue method [41], and the experimental procedure for orthophosphate ion adsorption was performed according to the Standard Methods of Water and Wastewater Treatment, specifically Method 4500-P F [42]. All experiments were replicated twice. Room temperature and a neutral pH level were assured during the adsorption experiments. Both analyses are analytical chromatographic and were conducted using a Lambda 25 UV–Vis spectrophotometer (Perkin–Elmer, Burladingen, Germany).

2.4.2. Kinetic Adsorption Studies

Pseudo-second-order kinetics were used to describe the kinetic adsorption results obtained from the kinetic experiment analysis due to zeolite’s chemisorption adsorption mechanism [18,29,43,44,45,46], which involves ion exchange in parallel with chemical interactions. The equations of pseudo-second-order kinetics can be found in other publications [29,47].

2.4.3. Isotherm Adsorption Experiment Set Up

The batch isotherm adsorption experimental set up aimed to quantify the adsorption capacities as a function of ammonium and orthophosphate aqueous solutions when 0.1 g of zeolite was weighed and added to 100 mL volumetric flasks with aqueous solutions of 0.1, 0.5, 1, 5, 10, 50, and 100 mg NH4+-N/L. Similarly, 0.15 g of adsorbent was weighed and added to 150 mL volumetric flasks with aqueous solutions of 0.1, 0.5, 1, 5, 10, 50, and 100 mg PO43−-P/L. The materials were covered with foil during the 24 h of treatment. The materials were then again vacuum-filtered using the Life Sciences Vacuum Pressure Pump and 0.47 nm gridded filters. Room temperature and a neutral pH level were assured during the adsorption experiments.

2.4.4. Isotherm Adsorption Studies

The Langmuir isotherm model was used to describe the adsorption mechanism of different zeolites for ammonium and orthophosphate ion adsorption, indicating that the adsorption process occurs as a monolayer [19,31,44] on a homogenous surface with finite sites. The equations of pseudo-second-order kinetics can be found in other publications [12,29].

3. Results

3.1. Zeolite Characterization Results by Origin

SEM analysis results are shown in Figure 1 and Table 1, which provide significant insights into the surface of the zeolites assessed and assist in optically identifying the key features of zeolites of different origins and different preparation protocols. Also, Figure 2 presents the spectrographs of the natural zeolites of different origins and their ZeoPhos modifications. The characterization enables us to identify notable morphological distinctions that are linked to their geological formation processes and result in different adsorption abilities.
The Greek zeolite from the Thrace region, originating in the Eocene–Oligocene epoch through the modification of volcanic rocks in an open hydrological system, displays remarkably well-defined crystalline structures with sharp angular facets typical of high-purity clinoptilolite production. The SEM pictures reveal a homogeneous distribution of particles measuring between 2 and 17 μm, exhibiting pristine crystal faces indicative of a stable formation environment and a composition of above 75% clinoptilolite content.
The Slovakian zeolite from the Nižený Hrabovec quarry exhibits the most uniform microstructure among all samples, indicative of its distinct monomineralic clinoptilolite-Ca composition originating from Miocene acidic volcanic ash with an extraordinarily high silica content of 74.3 to 77.6%. The SEM study demonstrates exceptionally homogeneous particle morphology with a consistent crystal habit and size distribution, signifying the meticulously regulated formation circumstances inside the acidic volcanoclastic environment. The surface characteristics exhibit crisp, well-delineated edges and clean crystal faces with no surface roughness, indicating ideal crystallization conditions that yielded a structurally flawless zeolite framework.
The Bulgarian zeolite displays the most varied microstructure, indicative of its genesis in volcanic tuffs in lacustrine environments during the Paleogene and Early Oligocene epochs. The SEM study indicates considerable morphological variety among various particles, with some exhibiting well-defined crystal faces and others presenting more rounded ones. This heterogeneity produces a complex surface topology characterized by different levels of crystallinity and roughness, yielding a material with several adsorption sites and processes.
The Turkish zeolite exhibits intermediate morphological traits and moderate crystallinity, with crystal faces that are less distinct than those of the Slovakian material yet more defined than those of the Bulgarian sample. The surface characteristics indicate the impact of the alkaline–saline formation environment, which was created by slightly rounded crystal edges and moderate surface roughness, which improved the material’s suitability for ion exchange while preserving structural integrity.

3.2. Adsorption Efficiency Results

Figure 3 illustrates the adsorption efficiency of different zeolites in ammonium (left) and orthophosphate (right) solutions. All materials reviewed were studied under the same conditions and showed a clear preference for adsorbing ammonium ions compared to orthophosphate ones. The natural and modified zeolites achieved a high ammonium adsorption efficiency ranging from 70 to almost 80%. The modification of ZeoPhos significantly altered the orthophosphate ion adsorption efficiency. Among all tested materials, the Greek and the Slovakian zeolites achieved higher ammonium and orthophosphate ion adsorption efficiency compared to the zeolites of other origins, portraying the strongest affinity for the targeted nutrients. The immersion in calcium, iron, and humic acid significantly assisted the orthophosphate ion adsorption by almost 20%. In the Slovakian zeolite, the presence of these elements assisted phosphorus adsorption by 30%. In terms of orthophosphate ion adsorption, the Bulgarian modified zeolite demonstrated lower efficiency but still remained effective. This could be attributed to its different pore structure or surface area.
Generally, when zeolite adsorbents are applied, the ammonium removal efficiency is considerably enhanced when compared with orthophosphate. It appears that zeolites efficiency in ammonium removal mainly occurs via ion exchange, when ammonium ions replace the “exchangeable cations” stored inside the zeolite. On the other hand, orthophosphate removal efficacy is greatly affected by precipitation mechanisms with metal ions (such as Ca2+, La3+) at high pH levels. Also, it is affected by adsorption onto metal oxide- or aluminum-based surfaces [48].

3.3. Adsorption Kinetic Results

The adsorption kinetic results indicate a steep correlation with pseudo-second-order kinetic models. Natural and modified zeolites exhibited superior conformity to the pseudo-second-order kinetic model. The experimental kinetic data fit the pseudo-second-order kinetic model at values above 99% for all tested materials, natural and modified, for ammonium and orthophosphate ion adsorption. The high correlation verifies that for all natural zeolites reviewed and for the ZeoPhos modified zeolites, chemisorption governs the adsorption mechanism [14,49] and is the rate-limiting step [50]. A full description of the pseudo-second-order kinetic results can be drawn from Figure 4.
Table 2 presents the pseudo-second-order kinetic parameters of the natural and modified zeolites reviewed. All materials validate the dependability of the pseudo-second-order kinetics of the zeolite-based materials. The optimal ammonium pseudo-second-order kinetic adsorption (“qe”) shows the Greek and Slovakian natural zeolite and the Slovakian ZeoPhos at capacities reaching 8.88, 8.65 mg/g, and 8.16 mg/g, respectively. Regarding the optimal orthophosphate pseudo-second-order kinetic adsorption, it was found that the Greek and Slovakian ZeoPhos reached 7.7 and 7.18 mg/g, respectively.
Regarding the materials’ kinetic rate constant (“k2p”), the Bulgarian natural zeolite exhibited the fastest kinetics in ammonium ion adsorption, reaching over 1 g/mg·min, and the Turkish ZeoPhos was twice as fast in orthophosphate ion adsorption compared to other materials. Generally, medium-sized particles (GR—Granular) often showed an optimal balance between capacity and kinetics.
The comparison of the ammonium ions’ linearized pseudo-second-order kinetic results denotes that natural and ZeoPhos modified zeolites of the same origin do not differ in terms of their ammonium kinetic adsorption results. Only Slovakian zeolite presents enhanced ammonium adsorption after the adoption of the ZeoPhos modification protocol. The comparison among orthophosphate pseudo-second-order distributions shows that the ZeoPhos modification favors the adsorption process. As expected, the same observation was verified by evaluating the adsorption efficiency results presented in the previous chapter.

3.4. Adsorption Isotherm Results

Figure 5 presents the Langmuir isotherm results for the ammonium uptake of 0–100 mg NH4+-N/L pollutant and 0–100 mg PO43−-P/L and assesses the adsorption efficacy of 10 zeolite-based materials in the removal of ammonium and orthophosphate ions.
The ammonium ion adsorption capabilities varied from 21.64 to 39.55 mg/g, with a mean of 30.71 mg/g. ZeoPhos GR had superior performance at 39.55 mg/g, followed by ZeoPhos SL at 36.87 mg/g and N-Z SL at 35.25 mg/g as described in Table 3. Natural zeolites had an average capacity of 31.56 mg/g, whereas ZeoPhos materials averaged 29.86 mg/g, demonstrating equal efficacy between the two material types. The ammonium Langmuir constant values for ammonium varied between 0.07 and 0.24 L/mg, with elevated values often signifying a greater binding affinity. All materials demonstrated exceptional model fit, with R2 values over 96%, thereby validating the Langmuir isotherm modeling.
The orthophosphate adsorption capabilities were typically inferior to those for ammonium, varying from 13.65 to 36.88 mg/g. ZeoPhos SL attained the maximum capacity of 36.88 mg/g, markedly surpassing alternative materials. ZeoPhos materials exhibited enhanced phosphate removal, achieving an average capacity of 30.64 mg/g, in contrast to 15.67 mg/g for natural zeolites, indicating a 95% enhancement. The phosphate KL values vary from 0.08 to 0.46 L/mg, with N-Z SL exhibiting a notably high binding affinity of 0.46 L/mg. This indicates that although natural zeolites may possess a reduced overall capacity for phosphate, they can demonstrate robust binding during adsorption.
ZeoPhos SL showed the best performance for simultaneous nutrient removal, with good capacities for both ions (36.87 mg/g NH4+, 36.88 mg/g PO43−). ZeoPhos GR showed the best capacity specifically for ammonium removal (39.55 mg/g). Natural zeolites continue to offer a cost-effective alternative for removing ammonium, where N-Z SL performs similarly to the modified materials. Studies on natural Chinese zeolite have shown 26.94 mg/g adsorption capacity at 5 g/L [14].
The values reported in this study are exceptionally interesting for water body restoration, but such high removal is likely due to synergistic benefits from the modification protocol of ZeoPhos as well as the characteristics of the pure zeolites used, and more specifically due to specific surface area, and would need further testing under conditions that mimic natural eutrophic systems.

4. Discussion

4.1. Discussion of Characterization Results

SEM analysis revealed a monomineralic composition of the Slovakian material, yielding a very uniform and predictable structure. The development of the Greek zeolite’s open hydrological system results in remarkable crystallinity and structural stability, while the Bulgarian material’s intricate formation history yields a variety of adsorption sites. The Turkish zeolite exhibits an optimal equilibrium between crystallinity and accessibility, indicative of its intermediate production conditions. The morphological variations seen in the SEM analysis results are directly linked with the materials’ effectiveness in ammonium adsorption applications. The Greek and Slovakian zeolites provide exceptional structural stability and dependable long-term performance. The heterogeneous structure of the Bulgarian zeolite facilitates varied adsorption methods, whereas the Turkish zeolite delivers consistent intermediate performance appropriate for diverse applications. The SEM characterization offers essential insights into the structure–property interactions that dictate zeolite efficacy in environmental remediation applications.
A potential limitation of this study is that comparisons were based on single batches/sacs of zeolite obtained from each supplier, which may not fully capture the inherent variability among different batches/sacs of the same material. Future investigations incorporating multiple batches from each source would help to strengthen the statistical basis of such comparisons and better account for possible heterogeneity.

4.2. Parameters Affecting the Adsorption Efficiency

The adsorption performance of natural and modified zeolites is important when evaluating them as sorbents for the nutrient control of aquatic water bodies, such as for ammonium and orthophosphate. Natural zeolites are highly effective in ammonium removal. The natural Chinese zeolite effectively removed 60–90% of ammonium ions when the adsorbent dosage varied from 8 to 56 g/L [43]. The efficiency of natural zeolites in orthophosphate removal is modest and is possibly attributed to precipitation mechanisms. Modified zeolites, particularly those doped with lanthanum or iron, offer distinct binding sites for phosphate, facilitating effective removal even at neutral pH [50,51,52,53]. Other studies of natural Chinese zeolites remark enhanced orthophosphate removal in the presence of ammonium, as the ion exchange process liberates Ca2+, leading to phosphate precipitation and achieving removal rates of up to 93% at neutral pH [14]. Also, Turkish zeolites, particularly from Yildizeli and Dogantepe [54] exhibit significant efficacy in ammonium removal, with performance contingent upon pH, contact duration, and adsorbent quantity; however, precise data about orthophosphate is absent. Finally, Romanian clinoptilolite demonstrates effective ammonium adsorption, with efficacy enhancing with elevated pH and temperature [19].
It is concluded that the removal efficiency is significantly affected by pH and temperature levels. Another significant parameter that plays a crucial role in nutrient removal efficiency is the co-presence of competing ions, in parallel with the particular alteration implemented on the zeolite. The efficiencies of ammonium and orthophosphate removal diminish in seawater due to competing ions and increased ionic strength, in natural and modified clay-based zeolites, such calcium- or iron-modified zeolites [18]. The presence of additional cations (Na+, K+, Ca2+, Mg2+) and anions (carbonate, chloride, sulfate, phosphate) can influence ammonium adsorption, with sodium and carbonate exerting the most substantial effect [43]. Other studies show that elevated temperatures (e.g., 35 °C) can improve ammonium removal efficiency, as demonstrated with modified Yemeni zeolite [55]. Also, studies which compared pH levels proved that for natural zeolite at a lower pH (pH < 4.0), the orthophosphate ion removal efficiency greatly rises, whereas at pH levels above 4, the orthophosphate adsorption efficiency gradually declines [44]. Finally, sodium activation, lanthanum impregnation, and salt/acid treatments enhance the surface area and cation exchange capacity and provide particular adsorption sites, resulting in improved removal efficiencies for both ammonium and orthophosphate [44,56,57,58].
Overall, the ammonium removal efficiency is greatly favored compared to that of orthophosphate when applying zeolite adsorbents. As the zeolites cation exchange capacity increases, the ammonium removal efficiency also increases, whereas orthophosphate removal efficacy is greatly affected by precipitation mechanisms with metal ions.

4.3. The Importance of the Kinetic Analysis

The kinetic analysis demonstrates that both natural zeolites and ZeoPhos materials are very efficient in nutrient removal applications, each offering unique benefits for various target ions. ZeoPhos materials exhibit distinct superiority in phosphate removal while sustaining competitive efficacy in ammonium removal.
Zeolites that possess both a high pseudo-second-order rate constant and high qe values are preferable for instances that require rapid removal of nutrients for effective implementation, known as “tea-bag” applications [59]. The main benefit of these applications is the relatively short “hydraulic retention time” for the treatment of eutrophic water bodies [60,61]. In the case of ammonium removal, natural zeolites (like clinoptilolite) can remove ammonium; however, acid treatment or loading with metal ions can greatly improve the capacity of natural zeolites to remove ammonium [13]. Modified zeolites can especially remove orthophosphate more efficiently, with the removal efficiency being attributed to improved surface basicity or incorporation of functional groups with binding sites for phosphorus [62].

4.4. Literature Review of Ammonium and Orthophosphate Adsorption Capacity

Reviewing the scientific literature, the ammonium and phosphate sorption capacities were significantly enhanced for zeolites that underwent controlled modifications, particularly with metal or acid treatments. Studies of the Chinese Fe(III)-modified clinoptilolite in synthetic eutrophic solutions showed a phosphate uptake capacity of 3.4 mg/g [20], whereas the ammonium capacity declined slightly to 27 mg/g [14]. The advantage of increased phosphate ion capacity comes at the cost of decreased ammonium ion capacity, which is primarily due to the introduction of Fe-OH groups, which increase the affinity for phosphate by inner-sphere complexation but may partially block cation exchange sites. Regeneration studies of Fe-modified zeolites indicate that there is some loss of phosphate removal efficiency over cycling, reiterating that durability and reusability need to be captured in future frameworks.
Yemenian zeolites synthesized with aluminum-pillared natural zeolites under sodium and aluminum chloride activation have also been investigated with batch testing utilizing solutions containing both ammonium and orthophosphate ions. The maximum ammonium and phosphate values reached approximately 40 mg/g and 10 mg/g for ammonium and phosphate, respectively, which are some of the highest values that have been consistently reported by researchers using natural zeolite modifications at near-neutral pH [21]. The improvements in ammonium and phosphate capacity have largely been linked to the production of Al-OH sites, which selectively adsorb phosphate while permitting ammonium exchange to continue to be effective at the cost of phosphate adsorption. The modification of ZeoPhos can achieve the same desired ammonium adsorption efficiency in the presence of the Greek zeolite and enhanced results in phosphate adsorption, outperforming the results of the Yemenian modified zeolite.

5. Conclusions

The comparative evaluation of zeolites from diverse origins highlights their differentiated potential to support sustainable eutrophication control strategies. Zeolites offer robust ammonium removal with minimal processing, while modified and waste-derived zeolites demonstrate superior phosphate uptake and promising reuse options as nutrient-enriched soil amendments. Selecting the appropriate zeolite based on local availability, treatment goals, and life-cycle impacts can significantly enhance the environmental, economic, and social sustainability of nutrient management interventions. Integrating such evidence-based material selection into aquatic ecosystem restoration efforts contributes meaningfully to SDG targets related to clean water, resource efficiency, and circular economy development.
When addressing the remediation of eutrophic freshwater bodies, the origin and modification of zeolite are an important part of removing ammonium (NH4+) and orthophosphate (PO43−) ions. Natural zeolites are effective at ammonium removal in freshwater, with the zeolites from Greece and Slovakia presenting ammonium removal efficiencies reaching up to 90%.
The ZeoPhos materials exhibited improved phosphate removal compared to natural zeolites, due to surface modifications, which improved the phosphate-binding sites. Generally, the structure of natural zeolites is likely conducive to cation exchange; therefore, both natural and ZeoPhos modified materials have the same adsorption behavior, since they are expressed by pseudo-second-order kinetic models and Langmuir isotherm models. ZeoPhos SL exhibited superior performance in simultaneous nutrient removal, with substantial capabilities for both ions (36.87 mg/g NH4+, 36.88 mg/g PO43−). ZeoPhos GR demonstrated the highest capacity for ammonium removal, at 39.55 mg/g. Natural zeolites remain a cost-efficient option for ammonium removal, with N-Z SL performing comparably to the modified materials.
Overall, the proven variations in the origin and genesis of natural zeolites contribute to differences in their structural characteristics and, consequently, in their nutrient adsorption capacity. These variations are found to be significant when addressing eutrophication with zeolite treatment, either in its natural or modified form. The study offers a valuable assessment of the adsorbents’ material crystal structure with the achieved ammonium and orthophosphate ion removal efficiency, allowing researchers to identify directions for optimum modifications that ensure enhanced nutrient removal efficiency.
Future optimization must concentrate on surface modification techniques that enhance phosphate binding while preserving ammonium capacity, as well as particle size modification to increase surface area and accessibility of binding sites. Future studies should combine zeolite technology with support for circular economy principles aiming to restore eutrophic waters with minimum environmental cost. Management actions should prevent future pollution and secure clean and safe water for long-term sustainability. Zeolites present a viable and sustainable foundation to harness these outcomes, supporting clean water resources and water ecosystem resilience.

Author Contributions

Investigation, I.B. and E.P.; writing—original draft, I.B.; writing—review and editing, I.Z.; funding acquisition, I.Z. All authors have read and agreed to the published version of the manuscript.

Funding

This work was funded by EEA and Norway Grants 2014–2021 through the project “BLUE-GREENWAY: Innovative solutions for improving the environmental status of eutrophic and anoxic coastal ecosystems” (project number 2018-1-0284, Support for Regional Cooperation).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data presented in this study are available on request from the corresponding author.

Acknowledgments

We would like to thank (i) Zeocem (from Bystré, Slovakia) (https://www.zeocem.com/en/, accessed on 27 May 2025) and Andrea Cseteová and Vassiliadis George for supplying the thermally activated zeolite material and, in particular, the product ZeoAqua, which is 100% natural clinoptilolite zeolite (+94% clinoptilolite), and (ii) Zeolife (from Thrace, Greece) (https://zeolife.gr/en/, accessed on 27 May 2025).

Conflicts of Interest

The authors declare no conflicts of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.

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Figure 1. SEM microscopic analysis at 20 kV and 1500 resolution of the natural zeolites (N–Z) on the left (a1,b1,c1,d1) and ZeoPhos zeolites on the right (a2,b2,c2,d2) as follows: Greek (a1,a2), Slovakian (b1,b2), Bulgarian (c1,c2), and Turkish zeolite (d1,d2). The samples were coated with gold—thickness (nm): 15.0; density (g/cm3): 19.32. The analysis was performed at the Laboratory of Electronic Microscope and Microanalysis of the University of Patras.
Figure 1. SEM microscopic analysis at 20 kV and 1500 resolution of the natural zeolites (N–Z) on the left (a1,b1,c1,d1) and ZeoPhos zeolites on the right (a2,b2,c2,d2) as follows: Greek (a1,a2), Slovakian (b1,b2), Bulgarian (c1,c2), and Turkish zeolite (d1,d2). The samples were coated with gold—thickness (nm): 15.0; density (g/cm3): 19.32. The analysis was performed at the Laboratory of Electronic Microscope and Microanalysis of the University of Patras.
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Figure 2. X-RD spectrograph at 20 kV and 1500 resolution of the natural zeolites on the left ((a1) N-Z GR, (b1) N-Z SL, (c1) N-Z BG, (d1) N-Z TK) and their modifications according to the ZeoPhos protocol on the right ((a2) ZeoPhos GR, (b2) ZeoPhos SL, (c2) ZeoPhos BG, (d2) ZeoPhos TK). The samples were coated with gold—thickness (nm): 15.0; density (g/cm3): 19.32. The analysis was performed at the Laboratory of the University of Patras.
Figure 2. X-RD spectrograph at 20 kV and 1500 resolution of the natural zeolites on the left ((a1) N-Z GR, (b1) N-Z SL, (c1) N-Z BG, (d1) N-Z TK) and their modifications according to the ZeoPhos protocol on the right ((a2) ZeoPhos GR, (b2) ZeoPhos SL, (c2) ZeoPhos BG, (d2) ZeoPhos TK). The samples were coated with gold—thickness (nm): 15.0; density (g/cm3): 19.32. The analysis was performed at the Laboratory of the University of Patras.
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Figure 3. Ammonium (a1,b1,c1,d1) and orthophosphate (a2,b2,c2,d2) ion adsorption efficiency results for 100 mg/L adsorbent material, Greek (a1,a2), Slovakian (b1,b2), Bulgarian (c1,c2), and Turkish zeolite (d1,d2), in separate aqueous solutions of 1 mg NH4+-N/L and 1 mg PO43−-P/L. Results presenting natural zeolites’ adsorption efficiency are shown in black, and results of preparation according to the ZeoPhos protocol are found in red. Graph visualization was performed in Origin Lab (version 9).
Figure 3. Ammonium (a1,b1,c1,d1) and orthophosphate (a2,b2,c2,d2) ion adsorption efficiency results for 100 mg/L adsorbent material, Greek (a1,a2), Slovakian (b1,b2), Bulgarian (c1,c2), and Turkish zeolite (d1,d2), in separate aqueous solutions of 1 mg NH4+-N/L and 1 mg PO43−-P/L. Results presenting natural zeolites’ adsorption efficiency are shown in black, and results of preparation according to the ZeoPhos protocol are found in red. Graph visualization was performed in Origin Lab (version 9).
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Figure 4. Pseudo-second-order ammonium (left) and orthophosphate (right) ion adsorption kinetic model result for 100 mg/L of adsorbent materials, Greek (a1,a2), Slovakian (b1,b2), Bulgarian (c1,c2), and Turkish zeolite (d1,d2), in 100 mL aqueous solutions at room temperature of 25 °C and pH level equal to 7. Natural zeolites are presented in black and ZeoPhos modified zeolites are presented in red. Graph visualization was performed in Origin Lab (version 9).
Figure 4. Pseudo-second-order ammonium (left) and orthophosphate (right) ion adsorption kinetic model result for 100 mg/L of adsorbent materials, Greek (a1,a2), Slovakian (b1,b2), Bulgarian (c1,c2), and Turkish zeolite (d1,d2), in 100 mL aqueous solutions at room temperature of 25 °C and pH level equal to 7. Natural zeolites are presented in black and ZeoPhos modified zeolites are presented in red. Graph visualization was performed in Origin Lab (version 9).
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Figure 5. Ammonium (left) and orthophosphate (right) adsorption efficiency results for natural and modified zeolites from Greece (a1,a2), Bulgaria (b1,b2), Slovakia (c1,c2), and Turkey (d1,d2) with 0.1 g of adsorbent in 100 mL ammonium solution with concentrations of 0.1, 0.5, 1, 5, 10, 50, and 100 mg NH4+/L and 0.15 g of adsorbent in 150 mL orthophosphate solution with concentrations of 0.1, 0.5, 1, 5, 10, 50, and 100 mg PO43−/L. Natural zeolites are presented in black and ZeoPhos modified zeolites are presented in red. Graph visualization was performed in Origin Lab (version 9).
Figure 5. Ammonium (left) and orthophosphate (right) adsorption efficiency results for natural and modified zeolites from Greece (a1,a2), Bulgaria (b1,b2), Slovakia (c1,c2), and Turkey (d1,d2) with 0.1 g of adsorbent in 100 mL ammonium solution with concentrations of 0.1, 0.5, 1, 5, 10, 50, and 100 mg NH4+/L and 0.15 g of adsorbent in 150 mL orthophosphate solution with concentrations of 0.1, 0.5, 1, 5, 10, 50, and 100 mg PO43−/L. Natural zeolites are presented in black and ZeoPhos modified zeolites are presented in red. Graph visualization was performed in Origin Lab (version 9).
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Table 1. Quantitative chemical composition results of the eight materials in terms of Weight Percentage. The analysis was conducted by the Laboratory of Electronic Microscopy and Microanalysis of the University of Patras (https://electronmicroscopylab.upatras.gr/, accessed on 27 May 2025).
Table 1. Quantitative chemical composition results of the eight materials in terms of Weight Percentage. The analysis was conducted by the Laboratory of Electronic Microscopy and Microanalysis of the University of Patras (https://electronmicroscopylab.upatras.gr/, accessed on 27 May 2025).
MaterialO (%)Na (%)Mg (%)Al (%)Si (%)K (%)Ca (%)Fe (%)Cu (%)C (%)
N-Z GR49.480.000.006.3337.020.000.632.193.281.07
N-Z SL48.870.430.445.2932.220.002.461.571.360.52
N-Z BG49.230.000.326.6938.400.002.012.000.540.80
N-Z TK49.750.000.666.9837.630.001.551.840.730.88
ZeoPhos GR42.150.000.115.6540.470.370.972.086.981.23
ZeoPhos SL42.580.000.005.6531.280.321.571.706.810.14
ZeoPhos BG53.260.000.454.9230.790.531.291.136.900.69
ZeoPhos TK49.130.190.536.1034.160.162.671.094.911.07
Table 2. Pseudo-second-order ammonium and orthophosphate adsorption kinetic model parameters result for 100 mg/L of adsorbent materials (N-Z GR, N-Z SL, N-Z BG, N-Z TK, ZeoPhos GR, ZeoPhos SL, ZeoPhos BG, ZeoPhos TK) at room temperature of 25 °C and pH level equal to 7.
Table 2. Pseudo-second-order ammonium and orthophosphate adsorption kinetic model parameters result for 100 mg/L of adsorbent materials (N-Z GR, N-Z SL, N-Z BG, N-Z TK, ZeoPhos GR, ZeoPhos SL, ZeoPhos BG, ZeoPhos TK) at room temperature of 25 °C and pH level equal to 7.
MaterialsAmmonium Ion ResultsOrthophosphate Ion Results
qe
(mg/g)		k2p
(g/mg·min)		R2 (%)qe (mg/g)k2p
(g/mg·min)		R2 (%)
N-Z GR8.880.0899.845.860.1999.94
N-Z SL6.920.1399.346.410.2099.92
N-Z BG6.321.0399.416.040.1599.92
N-Z TK6.710.3399.445.230.1799.81
ZeoPhos GR8.650.0999.887.700.1799.94
ZeoPhos SL8.160.0799.647.180.1399.92
ZeoPhos BG6.510.6899.336.850.0999.86
ZeoPhos TK6.610.2899.445.830.4399.54
Table 3. Langmuir ammonium and orthophosphate adsorption isotherm parameters result for 0.1 g of adsorbent materials in 100 mL ammonium solution with concentrations of 0.1, 0.5, 1, 5, 10, 50, and 100 mg NH4+/L and 0.15 g of adsorbent materials in 150 mL orthophosphate solution with concentrations of 0.1, 0.5, 1, 5, 10, 50, and 100 mg PO43−/L (N-Z GR, N-Z SL, N-Z TK, ZeoPhos GR, ZeoPhos SL, ZeoPhos TK) at room temperature of 25 °C and pH level equal to 7.
Table 3. Langmuir ammonium and orthophosphate adsorption isotherm parameters result for 0.1 g of adsorbent materials in 100 mL ammonium solution with concentrations of 0.1, 0.5, 1, 5, 10, 50, and 100 mg NH4+/L and 0.15 g of adsorbent materials in 150 mL orthophosphate solution with concentrations of 0.1, 0.5, 1, 5, 10, 50, and 100 mg PO43−/L (N-Z GR, N-Z SL, N-Z TK, ZeoPhos GR, ZeoPhos SL, ZeoPhos TK) at room temperature of 25 °C and pH level equal to 7.
MaterialsAmmonium Ion ResultsOrthophosphate Ion Results
Qm
(mg/g)		KL
(L/mg)		R2 (%)Qm
(mg/g)		KL
(L/mg)		R2 (%)
N-Z GR34.350.0999.4817.780.0998.83
N-Z SL35.250.1499.3818.010.4699.08
N-Z BG30.940.1096.6413.650.2899.86
N-Z TK31.640.2296.3815.670.1499.37
ZeoPhos GR39.550.0799.3131.910.0899.89
ZeoPhos SL36.870.1299.4136.880.1599.34
ZeoPhos BG26.430.1198.8829.690.2299.61
ZeoPhos TK24.830.2499.5226.850.0999.96
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Biliani, I.; Papadopoulou, E.; Zacharias, I. Evaluating Zeolites of Different Origin for Eutrophication Control of Freshwater Bodies. Sustainability 2025, 17, 7120. https://doi.org/10.3390/su17157120

AMA Style

Biliani I, Papadopoulou E, Zacharias I. Evaluating Zeolites of Different Origin for Eutrophication Control of Freshwater Bodies. Sustainability. 2025; 17(15):7120. https://doi.org/10.3390/su17157120

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Biliani, Irene, Eirini Papadopoulou, and Ierotheos Zacharias. 2025. "Evaluating Zeolites of Different Origin for Eutrophication Control of Freshwater Bodies" Sustainability 17, no. 15: 7120. https://doi.org/10.3390/su17157120

APA Style

Biliani, I., Papadopoulou, E., & Zacharias, I. (2025). Evaluating Zeolites of Different Origin for Eutrophication Control of Freshwater Bodies. Sustainability, 17(15), 7120. https://doi.org/10.3390/su17157120

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