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Abstract

In this study, 4A zeolite was prepared from opal waste rock by hydrothermal method and applied in ammonium ion adsorption. To optimize synthesis conditions, the effect of crystallization time (1–8 h), crystallization temperature (65–115°C), Na₂O/SiO₂ (0.6–2.0), H₂O/Na₂O (20–70), and SiO₂/Al₂O₃ (1.0–3.5) was investigated. X-ray diffraction, scanning electron microscope imaging, cation exchange capacity, static water adsorption, Fourier transform infrared spectroscopy, and N₂ adsorption–desorption isotherm were used for assessing properties of 4A zeolite. Adsorption experiments were performed by 1.0 g l⁻¹ 4A zeolite with NH₄⁺ solution (5–300 mg l⁻¹) for 4 h at room temperature. The experiment results revealed with a crystallization time of 3 h, a crystallization temperature of 85°C, Na₂O/SiO₂=1.0, H₂O/Na₂O = 40, and SiO₂/Al₂O₃=2.0, the 4A zeolite synthesized had excellent performance with cation exchange capacity of 2.93 mmol (g dry zeolite)⁻¹ and static water adsorption of 22.3%. The adsorption process was described by Freundlich model (R²>0.99) and the maximum adsorption capacity could reach to 53.11 mg g⁻¹. The experimental results provided a novel approach for the utilization of opal waste rock, which is produced during the mining of opal-rich palygorskite, and for the synthesis of 4A zeolite and the removal of ammonium ion.

Keywords

Opal waste rock hydrothermal synthesis 4A zeolite palygorskite ammonium ion removal

Introduction

The increase of ammonium ion pollution in waters has drawn extensive concern owing to its harmful effects, such as the eutrophication of lakes and rivers, low dissolved oxygen, harm for aquatic life, and the corrosion acceleration of soil materials (Cheng et al., 2017; Huang et al., 2015). Ammonium ion in wastewater is mainly released from municipal, agricultural, or industrial effluents, as well as the utilization of fertilizers, including NO_x, NH₄⁺, NO₃⁻, and nitrogen containing organics (Liu et al., 2013a; Malekian et al., 2011). Therefore, the dispose of ammonium ion pollution in water counts a great deal. Conventional removal methods for ammonium ion from wastewater comprise steam stripping, adsorption, ion exchange, and biological nitrification–denitrification (Sun et al., 2017a; Zadinelo et al., 2015). Adsorption has achieved wide-ranging attention for its simplicity and high efficiency for elimination of ammonium ion (He et al., 2016). Amongst adsorption materials for NH₄⁺ ions of aqueous solutions and wastewater, zeolite is especially promising because of the considerable ions exchange capacity and intense affinity for NH₄⁺ compared with other adsorbents (Mazloomi and Jalali, 2016).

4A zeolite, whose ideal chemical structure formula is Na₁₂[(AlO₂)₁₂(SiO₂)₁₂]·27H₂O, is a type of crystalline aluminium-containing silicate with homogeneous microchannels (García et al., 2015; Shen et al., 2017). Negative charges are formed via the replacement of Si with Al in the tetrahedral framework and are compensated by Na⁺ ions (Ojuva et al., 2015a). As is well known, 4A zeolite has extensive use in fields of chemical industry, environmental conservation, metallurgy, medicine, and so on because of its excellent performance as ion exchangers, adsorbents, and catalysts (Ayele et al., 2015; Müller et al., 2015). The synthesis of 4A zeolite conforming to industry standard of China based on inexpensive and widely available alumina-silica materials under mild condition is of great importance for industrial applications.

Many materials have been explored to synthesize 4A zeolite, including kaolin, coal gangue, fly ash, and palygorskite. (Aldahri et al., 2016; Ayele et al., 2016; Cardoso et al., 2015; Jiang et al., 2014; Maia et al., 2015; Qian and Li, 2015; Tang et al., 2017; Wu et al., 2017; Zhou et al., 2013). A report by Aldahri et al. (2016) put forward that fly ash could be utilized to synthesize Na–P zeolite by using microwave-assisted hydrothermal treatment. Additionally, Zhou et al. (2013) synthesized ZSM-5 zeolite by palygorskite and found that the product has excellent catalytic performance. Sun et al. (2017b) also synthesized 4A zeolite from palygorskite at crystallization temperature of 90°C for 12 h. And in this paper, 4A zeolite was synthesized at 85°C for 3 h with opal as the raw material. A lens body and strip of opal waste rock (OWR) commonly existed in palygorskite mines, with approximately 100,000 t of OWR produced during the palygorskite mine exploitation only in Xuyi County every year (Chen et al., 2003). A previous study showed that the main component of OWR in a palygorskite mine is opal-CT, which can react with alkaline easily (Wilson, 2014). Moreover, a small quantity of palygorskite and dolomite along with scarcely any quartz or illite existed in the OWR. Based on the advantages above, high quality 4A zeolite was expected to be synthesized from OWR.

The aim of this work was to synthesize 4A zeolite with OWR as raw material and to determine the optimum synthesis parameters. The resulting 4A zeolite was then applied to investigate its capacity to remove ammonium ion. As it is known, a study about the synthesis of 4A zeolite from OWR has not been reported. This work provides a new material for synthesis of 4A zeolite and a new method for the removal of ammonium ion. Importantly, a novel method to utilize OWR producing during palygorskite mine exploitation was achieved.

Experimental

Material preparation

4A zeolite was synthesized based on hydrothermal method by using OWR of palygorskite mine from Xuyi County, Jiangsu Province, China. Natural OWR underwent extrusion, crushing, and screening to obtain OWR particles of 200 mesh (0.075 mm). OWR powder was added to 20% hydrochloric acid solution under 70°C with medium-speed stirring for 4 h to eliminate impurities, such as palygorskite and dolomite. The mixed solution then was centrifuged and dried (110°C), resulting in purified OWR samples for further investigation. Based on given molar ratio of SiO₂/Al₂O₃, Na₂O/SiO₂, and H₂O/Na₂O, OWR sample (3 g) after acid leaching, NaAlO₂, NaOH, and deionized water were blended. The admixture was transferred into a flip mixer and heated to the desired temperature. After the reaction finished, the admixture was washed, centrifuged, and dried overnight to get final sample for further tests.

Material characterization

The chemical composition of OWR sample before and after acid leaching was analysed on Shimadzu XRF-1800 instrument by Rh radiation. The mineralogy of the material and products was examined via X-ray diffraction (XRD) patterns by Dandonghaoyuan 2700 diffractometer with scan range of 3–70° and scan speed of 4°/min. Scanning electron microscope (SEM) imaging was performed on a JSM-6490LV instrument. Fourier transform infrared spectroscopy (FTIR) spectra of the product were obtained by using a spectrophotometer (VERTEX70, Japan) based on transmission technique under normal temperature and pressure with digital detector. And the measuring range was from 400 to 4000 cm⁻¹ with KBr and samples powder mixed at a mass ratio of 100:1. The N₂ adsorption–desorption isotherm and pore size distribution analyses of 4A zeolite were carried by Novawin 3000e Surface Area and Pore size Analyzer.

Material testing

Static water adsorption (SWA)

SWA was measured according to the procedure in the National Standard of GB/6287-86 of China; 1.5 g of synthetic product after annealing at 550°C for 1 h was used to adsorb water at 35 ± 1°C for 24 h. All weights were surveyed by electronic balance with the precision of 0.1 mg. SWA can be calculated using the following formula

X % = (m_{3} - m_{2}) / (m_{2} - m_{1}) \times 100 %

(1)

where X (%) represents SWA percentage and m₁, m₂, and m₃ (g) are the weights of the weighing bottle, the annealed 4A zeolite and weighing bottle, and the adsorbed water sample and weighing bottle, respectively.

Cation exchange capacity (CEC)

CEC was judged according to the procedure in the National Light Industry Standard of QB 1768-2003 of China. By adding little bit of HCl or NaOH, CaCl₂ solutions with original concentration of 5 mmol l⁻¹ regulated pH to 10. The aforementioned solution with 1.0 g l⁻¹ synthetic product was shaken under vigorous stirring conditions for 20 min at 35 ± 1°C. After the adsorption process and after centrifugation, the supernatant was assessed by an atomic absorption spectrometer (Wayee WYS 2200, China). Next, the CEC was calculated from the residual Ca²⁺ concentration compared with original Ca²⁺ concentration. The CEC was tabbed with mmol (g dry zeolite)⁻¹.

Adsorption experiments

Series of experiments were proceeded with three materials and OWR of 1.0 g l⁻¹ with NH₄⁺ solution of specified concentrations (5, 10, 20, 50, 100, 150, 200, and 300 mg l⁻¹). The NH₄⁺ solution pH was adjusted to 7–8 by tiny amounts of NaOH and HCl of different concentration and then mixture was placed on flip mixer for specific time at room temperature. After sorption experiments, the mixture was centrifuged for 5 min by centrifuge with the speed of 3500 r min⁻¹. The concentration of NH₄⁺ in liquid supernatant was determined by spectrophotometry. The removal degree of NH₄⁺ was figured out from the gap between original concentration and remnant concentration of NH₄⁺. The removal capacity (Qe, mg g⁻¹) could be calculated using following formula

Q e = V \times (C_{0} - C_{e}) / m

(2)

where C₀ (mg l⁻¹) and Ce (mg l⁻¹) are several original concentration and equilibrium concentration of NH₄⁺. V (ml) is volume of suspension and m (g) is the weight of 4A zeolite. To characterize adsorption equilibrium of ammonium ion by 4A zeolite, the Langmuir and Freundlich models were used. The linear forms (Jin et al., 2015; Shirzadi and Nezamzadeh-Ejhieh, 2017) are expressed in equations (3) and (4)

C e / Q e = 1 / (K_{L} \cdot q_{m}) + C e / q_{m}

(3)

\ln Q e = (1 / n) \ln C e + \ln K_{F}

(4)

where q_m (mg g⁻¹) refers to the maximum removal capacity of 4A zeolite, K_L (l mg⁻¹) denotes the constant of Langmuir model, 1/n represents the inhomogeneity of sorption sites, and K_F is the equilibrium coefficient (Li et al., 2017).

Results and discussion

Characterization of OWR

Table 1 shows the chemical composition of OWR before and after acid leaching. It is observed that, compared with natural OWR, the SiO₂ mass of OWR after acid leaching increases from 83.15 to 89.26% and impurities, such as MgO, Fe₂O₃, CaO, also clearly decrease. In other words, the impact of impurities for synthesis of 4A zeolite is diminished. Figure 1 presents XRD patterns of raw OWR and purified OWR. These reflections at 2θ = 20.9° and 35.7° are found and identified as tridymite, the reflection of 2θ = 21.6° is identified as cristobalite, the reflection at 2θ = 8.3° is identified as palygorskite, and the reflection at 2θ = 30.9° is identified as dolomite according to the comparison with standard pattern. The pattern indicates that the OWR is mainly composed of tridymite and cristobalite with lower crystallinity. Thus, the material in this study can be reacted with alkaline more easily. Moreover, from XRD pattern of purified OWR, the reflection of palygorskite becomes weaker, and the reflection of dolomite vanishes when compared with raw OWR, i.e. the structure of palygorskite is destroyed and the dolomite is removed. More importantly, SEM images of OWR (Figure 2(a) and (b)) show the surface pores of raw OWR are mostly blocked by clay minerals, and obvious porous structure can be observed on the surface of purified OWR, indicating that impurities of OWR were removed after acid leaching, thereby enhancing reactivity.

Table 1.

Chemical composition of OWR before and after acid leaching (%).

Acid leaching	SiO₂	MgO	Fe₂O₃	CaO	Al₂O₃	TiO₂	K₂O	Else	Loss on igniting
Before	83.15	5.41	2.17	1.17	1.91	0.28	0.18	0.18	5.55
After	89.26	2.88	1.54	0.01	1.39	0.30	0.13	0.07	4.42

OWR: opal waste rock.

Figure 1.

XRD patterns of OWR before and after acid leaching. OWR: opal waste rock.

Figure 2.

SEM images of raw OWR (a) and purified OWR (b), 4A zeolite crystallized for 1 h (c), 3 h (d, e, f), 5 h (g), 6 h (h), and EDS analysis (i) of 4A zeolite crystal (dotted square in (e)).

XRD analysis

In this paper, the influences of crystallization time, crystallization temperature, Na₂O/SiO₂, H₂O/Na₂O, SiO₂/Al₂O₃ on the synthesis products were investigated. The conditions for synthesis 4A zeolite based on OWR are shown in Table 2. The XRD patterns of synthetic products synthesized at different times are presented in Figure 3(a). Regardless of the different crystallization times, the peaks of 4A zeolite can be obviously seen in all products, and the crystallinity of products increase and then decrease with extending crystallization time. The crystallinity is most improved when crystallization time is just 3 h. The crystallization process is different from the results of the report by Hu et al. (2014), where merely some faint peaks of 4A zeolite appear when crystallization time is 3 h.

Table 2.

Experimental parameters for the synthesis of 4A zeolite from OWR.

A	B	C	D	E
Crystallization time (h)	Na₂O/SiO₂	Crystallization temperature (°C)	H₂O/Na₂O	SiO₂/Al₂O₃	No.
1	1.4	95	40	2.0	A₁
3	1.4	95	40	2.0	A₃
5	1.4	95	40	2.0	A₅
6	1.4	95	40	2.0	A₆
7	1.4	95	40	2.0	A₇
8	1.4	95	40	2.0	A₈
3	0.6	95	40	2.0	B_0.6
3	1.0	95	40	2.0	B_1.0
3	1.2	95	40	2.0	B_1.2
3	1.4	95	40	2.0	B_1.4
3	2.0	95	40	2.0	B_2.0
3	1.0	65	40	2.0	C₆₅
3	1.0	75	40	2.0	C₇₅
3	1.0	85	40	2.0	C₈₅
3	1.0	95	40	2.0	C₉₅
3	1.0	105	40	2.0	C₁₀₅
3	1.0	115	40	2.0	C₁₁₅
3	1.0	85	20	2.0	D₂₀
3	1.0	85	30	2.0	D₃₀
3	1.0	85	40	2.0	D₄₀
3	1.0	85	50	2.0	D₅₀
3	1.0	85	60	2.0	D₆₀
3	1.0	85	70	2.0	D₇₀
3	1.0	85	40	1.0	E_1.0
3	1.0	85	40	1.5	E_1.5
3	1.0	85	40	2.0	E_2.0
3	1.0	85	40	2.5	E_2.5
3	1.0	85	40	3.0	E_3.0
3	1.0	85	40	3.5	E_3.5

OWR: opal waste rock.

Figure 3.

XRD patterns of synthetic products synthesized at different conditions.

The XRD patterns of the 4A zeolite with different Na₂O/SiO₂ (0.6, 1.0, 1.2, 1.4, 2.0) are displayed in Figure 3(b). When 4A zeolite is synthesized with Na₂O/SiO₂ of 0.6, small peaks of 4A zeolite appear suggesting the generation of a small quantity of 4A zeolite. The crystallinity of product achieves a rapid growth when Na₂O/SiO₂ increases from 0.6 to 1.0. Scarcely any changes are observed in XRD patterns when Na₂O/SiO₂ is more than 1.0. These results indicate that the value of Na₂O/SiO₂ of 1.0 is adequate for synthesis of 4A zeolite crystals with high crystallinity, in accordance with the results of a published report (Ismail et al., 2010). Regarding 4A zeolite samples at different crystallization temperatures, the XRD patterns are presented in Figure 3(c). When synthesis temperature is 65°C, the master peak is weak cristobalite, showing that OWR cannot form 4A zeolite at 65°C. When crystallization temperature is increased to 75°C, the peak of cristobalite is replaced by faint peaks of 4A zeolite indicating the opening formation of 4A zeolite. With temperature increased to 85°C, the clear enhancement of 4A zeolite peaks can be observed. The results should be assigned to that the increase of temperature will accelerate both nucleation and crystal growth (Liu and Wang, 2017). Compared with that of 85°C, the peaks of 4A zeolite of 95, 105, and 115°C do not exhibit much change.

As shown in Figure 3(d), the crystalline degree of products increases with the decrease of H₂O/Na₂O. However, when H₂O/Na₂O decreases to 20, the reflection of sodalite is observed, conforming to the results of published reports (Ma et al., 2010), in which the decrease of H₂O/Na₂O, that is to say, the raise of alkali dosage, will lead to the generation of sodalite. By changing the dosage of NaAlO₂, the effect of SiO₂/Al₂O₃ on property of 4A zeolite is also studied. XRD patterns of 4A zeolite synthesized at different SiO₂/Al₂O₃ for 3 h are exhibited in Figure 3(e). Just when SiO₂/Al₂O₃ molar ratios vary from 1.0 to 3.0, the peaks of 4A zeolite emerge revealing that 4A zeolite will be produced at a narrow value zone of SiO₂/Al₂O₃. Similar results are also observed in the study made by Liu et al. (2013b). When SiO₂/Al₂O₃ molar ratio is 1.0, the peaks of gibbsite are observed in the XRD pattern, implying that OWR just translates into aluminium hydroxides when existing in the solution of excess aluminium and alkaline. The amorphous peaks when the SiO₂/Al₂O₃ value increases to 3.5 display the absence of 4A zeolite.

SEM analysis

Spherical particles coupled with some incomplete unformed crystals are observed in Figure 2(c) when crystallized for 1 h, indicating that 0–1 h is the nucleation stage, and 4A zeolite has not begun to crystallize. The sample crystallized for 3 h in Figure 2(d) shows cubic shape, suggesting that 4A zeolite has been formed; these results agree with former studies (Zayed et al., 2017). The crystals of 4A zeolite synthesized for 3 h show homogeneous cubic morphology as presented in Figure 2(e) and (f) revealing SEM image under lower magnification. As displayed in Figure 2(g) and (h), when crystallization time is 5 and 6 h, the crystal size of 4A zeolite do not have great change compared with 4A zeolite of 3 h. Figure 2(i) presents the energy dispersive spectroscopy (EDS) patterns of 4A zeolite crystal, i.e. the region delimited by the rectangle in Figure 2(e), and the O, Na, Si, and Al are consistent with 4A zeolite (Wang et al., 2015).

CEC and SWA analysis

The CEC and SWA results of all 4A zeolite are shown in Figure 4. As exhibited in Figure 4(a), CEC and SWA get a substantial decrease with crystallization time less than 3 h. When the crystallization time is over 3 h, the SWA of products do not have much change, whereas CEC decreases, except at the point of 5 h. The reason is that with the extension of crystallization time, 4A zeolite would translate into sodalite which had inferior exchange capacity leading to CEC decrease. In addition, in Figure 4(b) and (c), when Na₂O/SiO₂ and temperature are lower than 1.0 and 85°C, CEC and SWA undergo a significant decrease. That decrease hints at the fact that Na₂O/SiO₂ and the temperature will influence the performance of synthetic 4A zeolite, consistent with XRD results displayed in Figure 3(b) and (c). As presented in Figure 4(d) and (e), with the raise of H₂O/Na₂O and SiO₂/Al₂O₃, CEC and SWA first rise and then drop. CEC and SWA achieve the maximum value of 2.92 mmol (g dry zeolite)⁻¹ and 20.3% when H₂O/Na₂O is 40 (Figure 4(d)) and SiO₂/Al₂O₃ is 2.0 (Figure 4(e)), respectively. 4A zeolite based on OWR without acid leaching was also synthesized at Na₂O/SiO₂ of 1.0, H₂O/Na₂O of 40, SiO₂/Al₂O₃ of 2.0, crystallization temperature of 95°C for 6 h; its CEC and SWA results are 2.34 mmol (g dry zeolite)⁻¹ and 19.7%, respectively.

Figure 4.

CEC and SWA of synthesis product synthesized at different conditions ((a) Na₂O/SiO₂=1.4, T = 95°C, H₂O/Na₂O = 40, SiO₂/Al₂O₃=2.0; (b) t = 3 h, T = 95°C, H₂O/Na₂O = 40, SiO₂/Al₂O₃=2.0; (c) t = 3 h, Na₂O/SiO₂=1.0, H₂O/Na₂O = 40, SiO₂/Al₂O₃=2.0; (d) t = 3 h, Na₂O/SiO₂=1.0, T = 85°C, SiO₂/Al₂O₃=2.0; (e) t = 3 h, Na₂O/SiO₂=1.0, T = 85°C, H₂O/Na₂O = 40).

FTIR analysis

FTIR spectra of 4A zeolite crystallized at 95°C for 0, 1, 3, 5 and 6 h are shown in Figure 5. Three bands at 475, 785, and 1090 cm⁻¹, ascribed to OWR (Eckert et al., 2015), can be obviously seen when crystallization time is 0 h. The bands at 785 and 1090 cm⁻¹ are, respectively, ascribed to symmetrical and asymmetrical stretching vibration of the Si–O–Si, corresponding to amorphous silica (Jiang et al., 2012). All the 4A zeolite exhibit absorption bands at approximately 3435 and 1651 cm⁻¹ both ascribed to O–H vibrations of the absorbed water (Su et al., 2016). When the mixture crystallized for 1 h, the bands at approximately 991, 700, and 447 cm⁻¹ appear in the product and correspond to sodium aluminosilicate gels (Jiang et al., 2012). In addition, when crystallization time is 1 h, the weak band at 554 cm⁻¹ emerges which is assigned to D4R vibration, the second major frame of 4A zeolite (Wang et al., 2014), indicating the partial formation of 4A zeolite. Four bands at 462, 559, 670, and 1001 cm⁻¹, assigned to 4A zeolite (Sapawe et al., 2013), appear distinctly with crystallization time increasing to 3 h. The band at 667 cm⁻¹ is attributed to the symmetrical stretching vibration of T–O bond (T represents Si or Al) and 1001 cm⁻¹ attributed to asymmetrical stretching vibration of internal tetrahedron (Ni et al., 2014). All 4A vibrational bands decrease slightly with crystallization time extending to 6 h; this result is in accord with the XRD result as shown in Figure 3(a).

Figure 5.

FTIR spectra of the products crystallized under different times.

N₂ adsorption–desorption isotherm analysis

The N₂ adsorption–desorption isotherm and pore size distribution of 4A zeolite were presented in Figure 6. That N₂ adsorption–desorption isotherm displayed the typical IV-type isotherms according to the IUPAC standard (Thommes et al., 2015). And from the pore size distribution curves, the pore size of 4A zeolite showed a distribution of multipeaks but mainly mesopores. After calculation, the N₂-BET, the total pore volume, and the average pore diameter of 4A zeolite were 26.9 m² g⁻¹, 7.64 × 10⁻² cc g⁻¹, and 11.3 nm.

Figure 6.

N₂ adsorption–desorption isotherm and pore size distribution of 4A zeolite (t = 3 h, Na₂O/SiO₂=1.0, T = 85°C, H₂O/Na₂O = 40, SiO₂/Al₂O₃=2.0).

Ammonium ion removal experiments

The effect of time on ammonium ion sorption onto 4A zeolite was performed in Table 3. The removal amounts of ammonium ion raised with the prolongation of the adsorption time and the equilibrium was obtained very rapidly within 90 min. A slight increase was observed when the adsorption time increased from 90 to 180 min. We set the adsorption time to 4 h in the adsorption experiment of this work to guarantee the adsorption equilibrium.

Table 3.

Effect of adsorption time on removal of ammonium ion.

Time (min)	0	10	20	40	60	90	120	180
Ce (mg l⁻¹)	48.23	40.08	34.99	29.70	26.87	26.12	25.86	25.84
Qe (mg g⁻¹)	0	8.15	13.24	18.53	21.36	22.11	22.37	22.39

Experimental conditions: C₀ = 50 mg l⁻¹, m/V = 1.0 g l⁻¹, I = 1 mmol l⁻¹ NaCl, T = 298 K, pH near to neutral; 4A zeolite: t = 3 h, Na₂O/SiO₂=1.0, T = 85°C, H₂O/Na₂O = 40, SiO₂/Al₂O₃=2.0.

Figure 7(a) displays the adsorption isotherms of ammonium ion on materials a, b, c and OWR. a, b, and c represent 4A zeolite samples under t = 3 h, Na₂O/SiO₂=1.0, T = 85°C, H₂O/Na₂O = 40, SiO₂/Al₂O₃=2.0; t = 3 h, Na₂O/SiO₂=1.4, T = 95°C, H₂O/Na₂O = 40, SiO₂/Al₂O₃=2.0; t = 3 h, Na₂O/SiO₂=1.0, T = 85°C, H₂O/Na₂O = 40, SiO₂/Al₂O₃=2.5. From Figure 7(a), the removal capacities of three materials for ammonium ion increase in the sequence of a>b>c, in agreement with the trend of CEC, as presented in Figure 4. Moreover, the sorption capacity of OWR is far worse than materials a, b, c (4A zeolite).

Figure 7.

(a) Adsorption isotherms of ammonium ion on three materials and OWR, (b) linear plot of Langmuir isotherm, and (c) linear plot of Freundlich isotherm (Qe (mg g⁻¹): the removal capacity for NH₄⁺; Ce (mg l⁻¹): the equilibrium concentration of NH₄⁺).

Figure 7(b) and (c) shows linear plots of Langmuir and Freundlich models; relative parameters of two models are calculated out and listed in Table 4. Although both Langmuir and Freundlich models fit the experimental adsorption data, Freundlich model has higher R² (0.998, 0.999, 0.994) than Langmuir model (0.954, 0.938, 0.947) and lower SE according to Table 4. Therefore, Freundlich model is more suitable to fit experiment data. Moreover, the maximum adsorption capacity of ammonium ion on material a is 53.11 mg g⁻¹. The maximum adsorption capacity of some commonly used adsorbents is listed in Table 5.

Table 4.

Relative parameters based on the Langmuir and Freundlich models.

Material	Langmuir parameters					Freundlich parameters
	q_m			SE					SE
	(mg g⁻¹)	K_L	R²	Intercept	Slope	K_F	1/n	R²	Intercept	Slope
a	53.11	0.028	0.954	0.180	0.0028	3.95	0.4665	0.998	0.028	0.0022
b	47.73	0.024	0.938	0.241	0.0027	3.41	0.4636	0.999	0.007	0.0023
c	41.65	0.021	0.947	0.254	0.0029	2.63	0.4818	0.994	0.057	0.0015

Table 5.

A summary of the maximum adsorption capacity of some commonly used adsorbents for ammonium ion removal.

Adsorbent	Size (mm)	Time (h)	C₀ (mg l⁻¹)	Dose (g l⁻¹)	q_max (mg g)⁻¹	References
4A zeolite	0.075	4	5–300	1.0	53.11	This paper
Natural Iranian zeolite	–	2	90–3610	33.3	11.52	Malekian et al. (2011)
Zeolite clinoptilolite	0.2–1.0	1	175	10.0	12.29	Vassileva and Voikova (2009)
Na-rich birnessite	0.15	0.5	2–50	0.5	22.61	Cheng et al. (2017)
Zeolite synthesized from fly ash	0.15	1.25	10–300	4.0	24.30	Zhang et al. (2011)
limestone and rice husk carbon	1.18–2.36	1.75	1890.95	1.0	32.89	Halim et al. (2010)
Sepiolite	2.05	0.9	0.15–4	28.6	49.00	Balci and Dinçel (2002)
4A-molecular sieve from palygorskite	0.075	3	1–100	1.0	68.04	Sun et al. (2017a)

As we all know, 4A zeolite had superior ability of ion exchange and in the uptake process of vast majority cation, ion exchange was the main mechanism (Ojuva et al., 2015b; Zavareh et al., 2018). The exchange process for NH₄⁺ removal by 4A zeolite could be depicted as following

N H_{4}^{+} (s) + N a^{+} (z) = N H_{4}^{+} (z) + N a^{+} (s)

(5)

where (s) and (z) stand for solution and 4A zeolite, respectively.

Conclusions

In this study, 4A zeolite was successfully synthesized from OWR via acid pretreatment, followed by hydrothermal treating under temperate condition. Based on results of XRD, SEM, CEC, SWA, and FTIR, 4A zeolite has great performance when synthesized under Na₂O/SiO₂ = 1.0, H₂O/Na₂O = 40, SiO₂/Al₂O₃ = 2.0, crystallization time of 3 h, and crystallization temperature of 85°C. Under those conditions, 4A zeolite was synthesized with CEC of 2.93 mmol (g dry zeolite)⁻¹, SWA of 22.3%, and NH₄⁺ ion adsorption capacity up to 53.11 mg g⁻¹. The special pattern and pore structure of OWR of the palygorskite mine and the open channels via acid pretreatment all contribute to the fast translation of OWR into 4A zeolite. This research provides not only a new material for synthesis of 4A zeolite but also a new application of OWR. The adsorption properties of 4A zeolite as heavy metal adsorbent will be further studied in the following work.

Footnotes

Declaration of Conflicting Interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: The authors thank for the financial support from the National Natural Science Foundation of China (NSFC) (41572028), The Xuyi Palygorskite Application Technology development and Industrialization Center of Chinese Academy of Sciences, China (201503), Engineering Research Center of non-metallic minerals of Zhejiang Province and Key Laboratory of Clay Minerals, Ministry of Land and Resources for support this study.

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Preparation,characterization,and performance of 4A zeolite based on opal waste rock for removal of ammonium ion

Abstract

Keywords

Introduction

Experimental

Material preparation

Material characterization

Material testing

Static water adsorption (SWA)

Cation exchange capacity (CEC)

Adsorption experiments

Results and discussion

Characterization of OWR

XRD analysis

SEM analysis

CEC and SWA analysis

FTIR analysis

N2 adsorption–desorption isotherm analysis

Ammonium ion removal experiments

Conclusions

Footnotes

Declaration of Conflicting Interests

Funding

References

N₂ adsorption–desorption isotherm analysis