Preparation of Sorbents Containing Straetlingite Phase from Zeolitic By-Product and Their Performance for Ammonium Ion Removal

Abstract

In this study, straetlingite-based sorbents were used for NH₄⁺ ion removal from a synthetic aqueous solution and from the wastewater of an open recirculation African catfish farming system. This study was performed using column experiments with four different filtration rates (2, 5, 10, and 15 mL/min). It was determined that breakthrough points and sorption capacity could be affected by several parameters such as flow rate and mineral composition of sorption materials. In the synthetic aqueous solution, NH₄⁺ removal reached the highest sorption capacity, i.e., 0.341 mg/g with the S30 sorbent at a filtration rate of 10 mL/min and an initial concentration of 10 mg/L of NH₄⁺ ions. It is important to emphasize that, in this case, the Ce/C0 ratio of 0.9 was not reached after 420 min of sorption. It was also determined that the NH₄⁺ sorption capacity was influenced by phosphorus. In the wastewater, the NH₄⁺ sorption capacity was almost seven times lower than that in the synthetic aqueous solution. However, it should be highlighted that the P sorption capacity reached 0.512 mg/g. According to these results, it can be concluded that straetlingite-based sorbents can be used for NH₄⁺ ion removal from a synthetic aqueous solution, as well as for both NH₄⁺ and P removal from industrial wastewater. In the wastewater, a significantly higher sorption capacity of the investigated sorbents was detected for P than for NH₄⁺.

1. Introduction

Natural and synthetic zeolitic materials have been used for NH₄⁺ removal from water, and there have been no data indicating that zeolite negatively affects fish life [1]. In recent years, there have been many studies related to the removal of NH₄⁺ via an ion exchange process using clinoptilolite. Wijesinghe et al. [2] used natural zeolites for NH₄⁺ adsorption, and determined that the NaCl treatment of natural zeolites improved the NH₄⁺ sorption capacity by 25%; the maximal sorption capacity increased from 9.48 mg-N/g for untreated zeolites to 11.83 mg-N/g for Na⁺-treated zeolites. Aziz et al. [3] stated that thermally activated natural zeolite at 150 °C improved the NH₄⁺ sorption capacity from 73.8% to 46.3% as compared with untreated zeolite. Sarioglu et al. [4] studied NH₄⁺ removal by using natural zeolite based on 45% clinoptilolite, 35% mordenite, and 15% feldspar. The experiments were carried out by using untreated zeolite, as well as zeolite that was chemically treated with acid. The results showed that the highest sorption capacity with untreated zeolite was 1.08 mg NH₄⁺-N/g when the pH was four, and the highest sorption capacity for acid-treated zeolite was 1.32 mg NH₄⁺-N/g.

To date, many studies have been conducted using different synthetic zeolites for the removal of NH₄⁺ from wastewater. The waste of coal fly ash biomass has been used to produce sodalite via a microwave and ultrasound irradiation method, as reported by Makgabutlane et al. [5]. New synthesized sodalite exhibited higher NH₄⁺ removal efficiency (up 82%) from urine as compared with natural zeolite (clinoptilolite). In another study by [6], synthetic zeolite was made from electrolytic manganese residue by using fusion. In this case, NH₄⁺ sorption capacity reached up to 27.89 mg/g when the initial NH₄⁺ concentration was 200 mg/L at 35 °C. Zhang et al. [7] used the same fusion method to convert fly ash into a faujassite-type zeolite. This synthetic zeolite was used for NH₄⁺ removal with 2.79 meq/g of cation exchange capacity.

Several studies have been conducted on NH₄⁺ ion exchange using the presentation of different cations and anions. Nitrogen and P are usually the leading causes of eutrophication in the fish farming industry; therefore, the removal of NH₄⁺ and P from wastewater has been the focus of many studies. Huang et al. [8] stated that other cations and anions had a significant influence on the removal of NH₄⁺ using zeolites, and determined their sorption capacity order; the order of the cation solutions for removal of NH₄⁺ was Na⁺ > K⁺ > Ca²⁺ > Mg²⁺, and the order for anion solutions was carbonate > chloride > sulfate > phosphate, at identical mass concentrations of NH₄⁺ ions. Wu et al. [9] stated that zeolite synthesized from fly ash could be used for NH₄⁺ and phosphate removal, and also reported that the potential of its removal capacity was not reduced at low concentrations of NH₄⁺ and phosphate. Similar results were published by Drenkova-Tuhtan et al. [10]. They stated that the competition of P species (phosphate, phosphonates) and various metal ions such as Ca²⁺, Pb²⁺ and Cu²⁺ had significant influence on the sorption of phosphorous compounds.

NH₄⁺ can be removed from wastewater in various ways under static (bath methods) and dynamic (column method) conditions. Sprynskyy et al. [11] investigated NH₄⁺ ion removal by using clinoptilolite under dynamic conditions. They determined that clinoptilolite could dominate external diffusion for NH₄⁺ sorption. Ershov et al. [12] determined that the cation exchange capacity of mordenite for NH₄⁺ ions was 1.64 meq/g by using an initial NH₄⁺ concentration of 1000 mg/L. In our previous study, the efficiency of NH₄⁺ ion removal was determined for zeolitic by-product under static conditions [13].

By analyzing scientific articles related to the sorption of NH₄⁺ ions using zeolites, we did not find information about the sorption capacity of sorbents containing straetlingite phase. These sorbents will expand the base of sorbents which is used for the sorption of ammonium ions. In this study, we aim to investigate using four different sorption materials containing straetlingite phase for NH₄⁺ ion removal from NH₄⁺ polluted wastewater prepared in a laboratory, and wastewater from a fish farm, and to evaluate the effects of filtration rate in synthetic solutions and wastewater on NH₄⁺ and P removal.

2. Results and Discussions

2.1. Mineral Composition of Sorption Materials

The mineral composition of sorption materials was evaluated using X-ray diffraction (XRD) analysis (Figure 1). The amount of CaO used had a significant effect on the mineral composition of the sorption materials. The cementitious compounds, i.e., straetlingite and calcite, formed in the S10 sorbent, with a similar amount of faujasite, were left unreacted. When a larger amount of CaO (S15 sorbent) was incorporated into the system, the peak intensities assigned to straetlingite were slightly increased. Additionally, calcite, faujasite, and portlandite were detected. By increasing CaO to 20% (S20 sorbent), the cementitious compound straetlingite had significantly higher peak intensities, and small amounts of calcite, faujasite, and portlandite were found. So, it is possible to state that the highest amount of straetlingite formed when the mixture was composed of 20% CaO. CaO of 30% completely changed the mineral composition of the sorption material of the S30 sorbent. In this case, cementitious compounds calcium silicate hydrate, aragonite, and calcite were formed after 7 days of reaction. The zeolitic by-product was a pozzolanic material, since faujasite and portlandite were also detected.

Our findings are in agreement with the findings of other studies on the formation of straetlingite, including the following: Heikal et al. [14] reported that straetlingite (Ca₂Al((AlSi)_1.11O₂)(OH)₁₂(H₂O)_2.25) formed in blends of calcium aluminate cement and ground granulated blast furnace slag in the presence of moisture, and that the optimal temperature for this reaction was 20–30 °C; Yaman et al. [15] determined that the formation of straetlingite positively influenced the compressive strength of sorbents; Xu et al. [16] proposed that the formation of straetlingite in Portland-based concrete with the addition of fly ash could explain the improved mechanical properties; Straetlingite has been formed in a pozzolanic admixture based on metakaolin and Ca(OH)₂ at curing temperature of 20 °C after 3 days [17]; Frıas et al. [18] stated that the main phases which form during the pozzolanic reaction between metakaolin and lime at an ambient temperature are calcium silicate hydrate, calcium aluminum hydrate, and calcium aluminum silicate hydrate (straetlingite).

2.2. Microstructure of Sorption Materials

The microstructures of the sorption materials formed during the chemical reactions between the solid compounds zeolitic by-product and CaO, with the addition of water, are shown in Figure 2. The mineral compositions of the sorption materials were confirmed by SEM analysis. The morphology of the hydration products had a strong influence on the amounts of solid materials such as zeolitic by-product and CaO.

In S10 sorbent, typical hexagonal crystals of straetlingite were dominant [19]. Moreover, spherical particles of faujasite coated with hydration products such as amorphous calcium silicate hydrate were also detected [20].

During hydration, the microstructure of the sorption material made from higher amounts of CaO (15%) and zeolitic by-product (S20 sorbent) consisted of spherical particles of faujasite coated with hydration products, column aggregate shaped portlandite [21], and hexagonal particles of straetlingite.

Ma et al. [22] stated that straetlingite (C₂ASH₈) is also known as hydrated gehlenite. In S20 sorbent, hexagonal platelets were dominant in the hydration products of zeolitic by-product and lime. This morphology of the hydrates could be assigned to straetlingite [23].

During hydration, the S30 sorbent, which was made from 70% zeolitic by-product and 30% CaO, produced calcium silicate hydrate (CSH) semi-amorphous conglomerate [24]. In these CSH semi-amorphous conglomerates, hexagonal platelets of straetlingite crystals were incorporated during the hydration reactions.

Therefore, in all investigated sorption materials, straetlingite formed typical hexagonal crystals, and the morphology of CSH was semi-amorphous conglomerates in the system of hydration products.

2.3. The Sorption of NH₄⁺ Ions by Straetlingite-Based Sorbents in a Synthetic Aqueous Solution

In the first part of this study, a synthetic aqueous solution was used for the evaluation of NH₄⁺ ion removal using four types of straetlingite-based sorbents (sorbents S10, S15, S20, and S30). Figure 3 shows the results of NH₄⁺ breakthrough curves at four filtration rates (2, 5, 10, and 15 mL/min). The initial NH₄⁺ ion concentration was the same in all experiments (10 mg/L). The experiments were continued until the sorbents reached saturation value (i.e., C_E/C₀ = 1). The obtained breakthrough curves are shown in Figure 4. The breakthrough point times were at 420, 280, 120, and 20 min for 2, 5, 10, and 15 mL/min, respectively (S10 sorption material). For the S15, S20, and S30 sorbents, the breaking point times were longer than 420 min, because after 420 min the C_E/C₀ ratio was less than 0.9 (Figure 3). Therefore, when the filtration rate increased, the breakthrough times for all investigated sorption materials were reduced, as shown in

Table 1. The influence of filtration rate on the breakthrough point time (min) and NH₄⁺ sorption capacity (mg/g).

Sorption Material	Filtration Rate, mL/min
	2	5	10	15	2	5	10	15
	Breakthrough Point, min				Sorption Capacity, mg/g
S10	420	280	120	20	0.135	0.129	0.103	0.0446
S15	>420 *	200	60	28	0.123 **	0.104	0.0694	0.0484
S20	>420 *	>420 *	52	22	0.137 **	0.178 **	0.0469	0.0404
S30	>420 *	>420 *	>420 *	60	0.169 **	0.215 **	0.341 **	0.0841

* After 420 min of sorption, a C_E/C₀ ratio of 0.9 was not reached. ** The sorption capacity was evaluated after 420 min of sorption.

. Temel et al. [25] reported a similar finding and detected a breakthrough point time (90 min) with a filtration rate of 10 mL/min. The reduction in breakthrough point time could have been related to the contact time of water and sorption material; by increasing the filtration time, the contact time was decreased [26].

The filtration rate through the column had a significant influence on the NH₄⁺ sorption capacity. Higher NH₄⁺ sorption was obtained for the experiment with a lower flow rate, i.e., 2 mL/min. When the filtration rate through the sorption materials increased, the NH₄⁺ sorption capacity gradually decreased (Table 1). This relationship was determined for all sorption materials that were used in this experiment. A similar relationship has been reported by Li et al. [26].

The sorption capacity and the values of breakthrough points are closely related to the filtration rate and also to the mineral composition of sorption materials. The highest sorption capacities were determined for the S30 sorbent at all of the filtration rates, and the two dominant minerals in the sorption material which acted as sorbent were straetlingite and calcium silicate hydrate. The synergetic effect of these two compounds could be the reason for the highest sorption capacity reached, which was 0.341 mg/g. In addition, Li et al. [27] reported that CSH and zeolite had significant impacts on NH₄⁺ and P removal.

2.4. The Sorption of NH₄⁺ Ions by Straetlingite-Based Sorbents in Wastewater from an Open Recirculation African Catfish Farming System

In the second part of this study, wastewater from an open recirculation African catfish farming system was used. Real wastewaters are complex solutions, as stated by Karapınar et al. [28]; instead of NH₄⁺ ions, P was detected in the wastewater. Two of the best sorption materials, i.e., S20 and S30, were selected for these experiments on NH₄⁺ and P removal, according to the sorption experiments using a synthetic aqueous solution (Section 2.3).

The relationship of flow rates on the breakthrough curve of NH₄⁺ ion sorption is shown in Figure 4 and

Table 2. The influence of filtration rates on breakthrough point times (min) and NH₄⁺ sorption capacities (mg/g).

Sorption Material	Filtration Rate, mL/min
	2	5	10	15	2	5	10	15
	Breakthrough Point, min				Sorption Capacity, mg/g
S20	100	60	52	20	0.0241	0.0313	0.0489	0.0273
S30	180	60	40	5	0.0255	0.0325	0.0452	0.0225

. P had a negative impact on NH₄⁺ removal using straetlingite-based sorbents; in this case, the breakthrough point times became significantly shorter as compared with the breakthrough point times in the synthetic aqueous solution without phosphorus. The NH₄⁺ sorption capacity decreased almost seven times when this wastewater instead of synthetic NH₄⁺ solutions was filtered through the column.

According to Hedström et al. [29], the NH₄⁺ sorption capacity of wastewater at breakthrough point time was about 50% lower as compared with the sorption capacity of synthetic solutions, and a similar tendency was determined by Huang et al. [8]. This decrease in NH₄⁺ ion sorption capacity in the wastewater could be related to the completion effect of different compounds (phosphorus) found in the wastewater. Li et al. [6] determined that NH₄⁺ sorption capacity was decreased by increasing PO₄³⁻ anions in a system; the NH₄⁺ sorption capacity decreased from 16.62 to 10.98 mg/g by increasing the phosphate ions from 50 mg/L to 250 mg/L. Karapınar et al. [28] used natural zeolite (Type C, Zeolith, Germany) for the removal of NH₄⁺ and for the precipitation of calcium phosphate. Their results showed that the sorption capacity of NH₄⁺ using zeolites decreased in the presence of phosphorus. Mazloomi et al. [30] reported that NH₄⁺ sorption capacity decreased from about 96% to 66.5% when P anions were added into a system. This decreased NH₄⁺ sorption capacity could be related to the presence of competing ions in the system [7]. The P anions may increase the surface tension of the aqueous phase, thereby reducing NH₄⁺ access to the micropores and macropores of the zeolite [30].

In this study, straetlingite-based sorbents for the removal of total P were also evaluated. Figure 5 shows the breakthrough curves of P ion sorption.

Similar characteristics for the breakthrough curves of NH₄⁺ ions were determined for P ions, i.e., by increasing the sorption rate, the breakthrough point times became shorter for both S20 and S30 sorption materials (

Table 3. The influence of filtration rates on breakthrough point times (min) and P sorption capacities (mg/g).

Sorption Material	Filtration Rate, mL/min
	2	5	10	15	2	5	10	15
	Breakthrough Point, min				Sorption Capacity, mg/g
S20	>420 *	250	262	120	0.264 **	0.2	0.341	0.181
S30	>420 *	>420 *	>420 *	250	0.215 **	0.403 **	0.512 **	0.383

* After 420 min of sorption, a C_E/C₀ ratio of 0.9 was not reached. ** The sorption capacity was evaluated after 420 min of sorption.

). However, a C_E/C₀ ratio of 0.9 was not attained after 420 min of the column experiments, see Figure 5.

A longer breakthrough point time occurred with the S30 sorption material than the S20 sorption material, due to the differences in mineral compositions of the sorption materials. Straetlingite prevailed in the S20 sorbent. Meanwhile, two types of minerals, i.e., straetlingite and calcium silicate hydrate (which could act as sorption compounds) dominated in the S30 sorbent. The maximal amount of NH₄⁺ adsorbed by the straetlingite-based sorbents was 0.0452 mg/g in the presence of phosphorus, in which the sorption capacity reached 0.512 mg/g when the initial concentration of NH₄⁺ was 11.2 mg/L, and the sorption capacity of P was 47.0 mg/L in the wastewater from an open recirculation African catfish farming system.

The P removal was improved by using a filtration rate of 10 mL/min and the S30 sorption material based on straetlingite and calcium silicate hydrate. In this case, the sorption capacity of P reached 0.512 mg/g. Our sorption capacity values were mostly in agreement with the values reported in related studies by Chen et al. [31] and Jiang et al. [32]. Taken together, on the basis of these findings, it can be concluded that straetlingite-based sorbents are suitable for the removal of NH₄⁺ from aqueous solutions, and especially from wastewater in which P is present.

3. Materials and Methods

3.1. Materials

For this study, reagent CaO (Chempur, Piekary Śląskie, Poland) was used with a purity of 99.0%.

Spent fluid catalytic cracking catalyst (zeolitic by-product) was received from a petroleum plant. Zeolites are commonly used in the process of fluid catalytic cracking. During a catalytic cracking process zeolite loses its catalytical properties, becomes degraded and becomes waste (by-product). The composition of these catalysts depends on the manufacturer and on the process that is going to be used. The chemical composition of the zeolitic by-product is shown in

Table 4. Oxide composition of the zeolitic by-product (%), according to X-ray fluorescence.

SiO2	Al2O3	Fe2O3	La2O3	TiO2	MgO	CaO	Na2O	Cl	P2O5	SO3	Other
35.4	48.77	1.02	1.63	3.57	0.44	0.37	0.31	2.57	0.08	0.07	5.77

. This material can be classified as aluminosilicate material because it is based on 84% SiO₂ + Al₂O₃.

X-ray diffraction analysis was applied to determine the mineral composition of the zeolitic by-product (Figure 6a); faujasite-type zeolite was the dominant component in the by-product [33].

The microstructure of zeolitic by-product was evaluated by SEM analysis (Figure 6a) and revealed that round shaped particles were dominant in the zeolitic by-product (Figure 6b); dos Santos et al. [34] determined a similar shape of zeolitic by-product particles.

Figure 6b shows the particle size distribution curves of the zeolitic by-product used in this work, forming a peak from 16 to 280 µm, with a mean diameter of 78.39 µm.

3.2. The Preparation of Sorption Materials

The sorption material granules were prepared from zeolitic by-product powder (Figure 7a) and CaO.

First, mixtures of zeolitic by-product and CaO were prepared. The compositions of the initial sorption materials based on straetlingite are shown in

Table 5. The composition of the initial material mixtures.

Mixture	Zeolitic By-Product (wt. %)	Reagent CaO (wt. %)	Water and Solid Materials Ratio (W/S)
S10	90	10	0.73
S15	85	15	0.70
S20	80	20	0.70
S30	70	30	0.70

. There were four mixtures of sorption materials consisting of different ratios of zeolitic by-product to reagent CaO. Then, water was added, and the mixtures were mixed thoroughly. The ratio of water and solid materials in all of the sorbent samples was almost the same. Finally, the mixtures were left for 7 days of hydration; the mixtures were covered with plastic material to protect from water evaporation.

During hydration, reactions between solid compounds of zeolitic by-product and CaO, as well as water, occurred. These chemical reactions led to setting and hardening of the zeolitic by-product/ CaO /water mixtures. After 7 days, the mixtures were crushed and sifted through a sieve. For the sorption experiment, 2–4 mm sized particles were used (Figure 7b).

Two types of initial solutions were used. The first NH₄⁺ ion initial solution was prepared by using the salt of NH₄⁺ chloride (synthetic aqueous solution) and deionized water. The initial NH₄⁺ ion concentration was 10 mg/L. The second initial solution (wastewater) was taken from an open recirculation African catfish farming system; in this wastewater, the NH₄⁺ ion concentration was 11.2 mg/L and the total P concentration reached 47.0 mg/L (

Table 6. Physical and chemical parameters of the wastewater from an open recirculation African catfish farming system.

Parameters and Unites	Values	Parameters and Unites	Values
Oxygen saturation	>40	NH4+, mg/L	11.2
pH	7.6	Nitrite, mg/L	<1.1
Temperature, °C	22	Nitrate, mg/L	<61
Free CO2, mg CO2/L	25	Iron, Fe, mg/L	<1.1
Total nitrogen, mg/L	<1.1	Total phosphorus, mg/L	47.0

).

The amount of NH₄⁺ or P in the solid phase (Q, mg/g) was calculated according to Equation (1) as follows:

$$\mathrm{Q}=\mathsf{\vartheta }\mathrm{A}{{\displaystyle \int }}_0^{\mathrm{t}}\left({\mathrm{C}}_0-{\mathrm{C}}_{\mathrm{E}}\right)\mathrm{tdt},$$

(1)

where C₀ and C_E are the influent and effluent NH₄⁺ ion concentrations, respectively (at breakthrough point t) (mg/L); ν represents the solution filtration rate through a sorption material (m/h); A is the cross-sectional area of the column (m²); and t is time (h).

The breakthrough point was assessed to be when the ratio between the initial concentration and the final concentration in the inflow was 0.9 (C_E/C₀ = 0.9) [26]. The sorption of NH₄⁺ at the breakthrough point was defined as its sorption capacity.

During NH₄⁺ ion sorption from the synthetic aqueous solution, straetlingite attracts NH₄⁺ ions from the solutions and releases calcium cations to the solutions. The ion exchange mechanism between straetlingite and the NH₄⁺ ions from water solutions can be represented by Equation (2):

straetlingite − Ca²⁺ + nNH₄⁺ ⇆ straetlingite − nNH₄⁺ + Ca²⁺

(2)

This equation is similar for all zeolites (straetlingite) that have been used for the NH₄⁺ removal [35].

3.3. Experimental Techniques

The chemical composition of the zeolitic by-product was determined by X-ray fluorescence using a Bruker X-ray S8 Tiger WD spectrometer (Karlsruhe, Germany), with a rhodium (Rh) tube, an anode voltage Ua up to 60 kV, and electric current I up to 130 mA. The pressed sorbent samples were measured in a helium atmosphere. Measurements were performed following the SPECTRA Plus QUANT EXPRESS method [36].

The X-ray diffraction analysis (XRD) of the materials was performed using a D8 Advance diffractometer (Bruker AXS, Karlsruhe, Germany) operating at a tube voltage of 40 kV and tube current of 40 mA. The X-ray beam was filtered with Ni 0.02 mm filter to select the Cu Kα wavelength. The sorbent samples were scanned over the range 2θ = 3–70° at a scanning speed of 6° min⁻¹ using a coupled two theta/theta scan type [37].

The particle size distribution of the zeolitic by-product was determined by using a particle size laser analyzer (CILAS 1090 LD, Orleans, France). The distribution of solid particles in the air stream was 12–15 wt.%; compressed air (2500 mbar) was used as a dispersing phase; and the measuring time was 15 s [38].

The microstructures of tree-type zeolitic by-product and hardened cement pastes were studied by scanning electron microscopy (SEM) using a high-resolution scanning electron microscope (ZEISS EVO MA10, Edmonton, Canada) [39].

The solution pH was determined using a Hanna ISE pH meter (Nușfalău, Romania).

Column tests were carried out using glass columns with a 3 cm inner diameter and 40 cm height. The zeolite bed was 6.5 cm high with a volume of 45.9 cm³. The experimental scheme of sorption in dynamic conditions is shown in Figure 8. The glass column was filled with different types of straetlingite-based zeolite, each with a mass of 20 g for the column tests.

A peristaltic volumetric DF-12M infusion pump (Viltechmeda, Vilnius Lithuania) was applied for the column solutions with sorption materials, at four different flow rates, i.e., 2, 5, 10 and 15 mL/min. During the sorption process, the exit solutions (effluent) were analyzed periodically. The NH₄⁺ concentration was determined following the Nessler method [40]. The total amount of P in the solutions was evaluated according to the vanadomolybdophosphoric acid method. An HI83399 multiparameter photometer (Hanna Instruments, Nușfalău, Romania) was used for the evaluation of NH₄⁺ and total P in the solutions. The experiment for the amount of NH₄⁺ or phosphorous was repeated at least three times. The mean value of the triple analysis was used to calculate the amount of NH₄⁺ or phosphorous in solution, and the limit of error for the samples was lower than 5%.

4. Conclusions

According to the XRD analysis, hydration reactions between zeolitic by-product and CaO resulted in the formation of straetlingite (Ca₂Al((AlSi)_1.11O₂)(OH)₁₂(H₂O)_2.25) after seven days. We determined that the NH₄⁺ sorption capacities depended on certain factors such as the filtration flow rate and the mineral composition of sorbents. The NH₄⁺ removal efficiency breakthrough times of all investigated straetlingite-based sorbents were reduced when the filtration rate was increased from 2 mL/min to 10 mL/min. The highest sorption capacity in a synthetic aqueous solution (i.e., 0.341 mg/g) was obtained by using the S30 sorbent with a filtration rate of 10 mL/min and an initial NH₄⁺ ion concentration of 10 mg/L; it is important to emphasize that, in this case, a C_E/C₀ ratio of 0.9 was not reached after 420 min of sorption. The results for NH₄⁺ sorption in wastewater showed an almost seven times lower sorption capacity than that in the synthetic aqueous solution; however, it should be emphasized that a P sorption capacity of 0.512 mg/g was reached. The NH₄⁺ removal experimental results indicate that NH₄⁺ ions can be removed from a synthetic aqueous solution, and also from the wastewater. In the case of wastewater, the P sorption capacity of straetlingite-based sorbents was significantly higher than their NH₄⁺ sorption capacity.