International Journal of Chemical Engineering
Volume 2020 (2020), Article ID 8818455, 14 pages
https://doi.org/10.1155/2020/8818455

Research Article

Removal of Ammonium from Aqueous Solutions Using Zeolite Synthesized from Electrolytic Manganese Residue

Changxin Li,¹ Yuan Yu,¹ Qingwu Zhang,¹ Hong Zhong,² and Shuai Wang²

¹College of Safety Science and Engineering, Nanjing Tech University, Nanjing 211816, Jiangsu, China
²College of Chemistry and Chemical Engineering, Central South University, Changsha 410083, Hunan, China

Correspondence should be addressed to Changxin Li; lichangxin2010@njtech.edu.cn

Received 13 June 2020; Revised 18 August 2020; Accepted 3 September 2020; Published 14 September 2020

Academic Editor: Andreas Bück

Copyright © 2020 Changxin Li et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

This paper carried out the study on removal of ammonium from aqueous solutions by zeolite derived from electrolytic manganese residue (EMR) via a fusion method. The variables of pH, contact time, EMRZ (EMR-based zeolite) dosage, initial ammonium concentration, and competitive cations and anions on the ammonium uptake capacity were systematically investigated in an attempt to illustrate adsorption performance of EMRZ. The results show that these influence factors had a remarkable impact on the ammonium uptake capacity of EMRZ. Maximum ammonium uptake capacity was achieved at pH value 8.0, EMRZ dosage 0.2 g/100 mL, contact time 100 min, initial ammonium concentration 200 mg/L, and temperature 35°C. Under optimized conditions, ammonium uptake capacity onto EMRZ was up to 27.89 mg/g. The competitive degree of cations in ammonium adsorption process follows the sequence of Na⁺>K⁺>Ca²⁺>Mg²⁺, and the sequence of anion effect on ammonium removal onto EMRZ is CO₃²⁻ > Cl⁻ > SO₄²⁻ > PO₄³⁻. The adsorption kinetic was explored and best represented by pseudo-second-order kinetic model. And the adsorption isotherm experimental data had best fitness with the Freundlich and Koble–Corrigan model, suggesting that heterogeneous uptake was the principal mechanism adopted in the process of ammonium adsorption. Moreover, calculation of thermodynamic parameters such as change in free energy (ΔG), enthalpy (ΔH), and entropy (ΔS) was carried out and it was determined to be −15.77∼−14.03 kJ·mol⁻¹, +37.66 kJ·mol⁻¹, and +173.38 J·mol⁻¹·K⁻¹, respectively. These parameters confirmed that ammonium uptake onto EMRZ was an endothermic and spontaneous process. Moreover, no obvious deterioration tendency was observed for the regenerated EMRZ compared with fresh EMRZ. These results indicate that EMRZ has wide application prospects in removing ammonium from wastewater.

1. Introduction

It is known that the accumulation of ammonium causes eutrophication in lakes, ponds, and reservoirs; what is more is that it creates a larger demand of oxygen by aquatic life and exposes them to toxicity at the same time [1–6]. With people’s understanding of the detrimental influences of ammonium getting to a more profound level nowadays, authorities have established stringent regulations and laws to restrict ammonium discharges. Thus, treatment on wastewater with high ammonia concentration is required prior to discharge into the receiving water. Hence, efficient removal techniques for ammonia in the wastewater are now required.

Ammonia can be removed from wastewater in various ways, including nitrification and denitrification, ammonia stripping, ion exchange, and breakpoint chlorination. Among the proposed processes, ion exchange with zeolite material has been acknowledged to be efficient and cost-effective on account of its low cost and easy operation [7–9]. Generally, the zeolites with open and solid three-dimensional honeycomb structure have a high CEC (cation exchange capacity) and great affinity for ammonia [10–12]. Therefore, the application of ammonium removal using zeolite has caught lots of eyeballs these days.

As we all know, the domestic production capacity of electrolytic manganese metal (EMM) is over 2.11 Mt/a, which covers around 98% of the global EMM production per year. Electrolytic manganese residue (EMR) is a byproduct during the process of producing electrolytic metal manganese (EMM) [13]. At present, for every ton of EMM produced, 10∼12 tons of EMR will be produced and discharged into the landfill sites. It is foreseen that there is a rise in generation rate of EMR as the grade of manganese ore is decreasing. Nowadays in China, almost all EMR are piled up into nearby landfill sites with no treatment except for a small proportion being used as building material [14, 15]. Under the condition of long-term weathering, the pollutants in EMR will gradually migrate and penetrate into the surrounding soil and water, causing environmental pollution problems. In addition, disposal of tons of EMR will also bring great economic burden to the enterprise due to high maintenance and new storage site costs. Thus, considering environmental protection and sustainable development, new recycling techniques for EMR have to be developed. Lately, our research group discovered that the zeolite material can be prepared from the silicon aluminum component in the EMR, which is an eco-friendly way to utilize EMR. Moreover, utilization of solid wastes generated from various industries to prepare related products was not only economical but also in line with the concept of green development [16–18].

At present, there are plenty of studies concerning removal of ammonium from aqueous solutions with different kinds of zeolites available [19–21]. However, investigation on the performance of EMRZ for ammonium removal has rarely been conducted. Accordingly, this work aims to examine the potential of EMRZ for removing ammonia ions from aqueous solutions under various conditions. Moreover, the adsorption characteristics of ammonium ion including equilibrium isotherms, thermodynamic parameters, and adsorption kinetics by EMRZ were also systematically investigated.

2. Materials and Methods

2.1. Materials

The EMR used in this study was obtained from an EMM plant located in Hunan Province, China. The main chemical composition of EMR was determined and listed hereafter, in mass%: Na₂O = 2.7, SiO₂ = 24.6, Al₂O₃ = 12.2, MgO = 1.7, K₂O = 2.4, CaO = 8.6, MnO = 4.6 and Fe₂O₃ = 7.9. The EMRZ was prepared using the silicon aluminum component in the EMR via a fusion method. The detailed preparation and characterization process of EMRZ is as described in our previous related studies [22]. From the x-ray diffraction (XRD) data (see Figure 1), zeolite A was identified as a major phase component of the synthesized EMRZ, with a small part of zeolite P. The morphology and particle size of EMRZ observed by field emission scanning electron microscopy (FE-SEM) confirmed that highly ordered cubes of zeolite A as well as a small amount of round crystals (zeolite P) were presented (see Figure 2) in EMRZ. Meanwhile, the CEC (cation exchange capacity) and SSA (specific surface area) were 3.45 meq/g and 39.38 m²/g, respectively. And the CEC of EMRZ (i.e., 3.45 meq/g) is far higher than the relevant zeolites reported in the literature [9, 20, 21], which implies that the EMRZ can be used as a cation exchange material for ammonium adsorption.

Figure 1: The XRD patterns of EMRZ samples.

Figure 2: FE-SEM images of EMRZ.

All inorganic chemicals in the study, such as NH₄Cl (purity 99.8%), NaCl (purity 99.5%), KCl (purity 99.8%), CaCl₂ (purity 96.0%), MgCl₂ (purity 99.0%), Na₂SO₄ (purity 99.0%), Na₂CO₃ (purity 99.8%), and Na₃PO₄ 12H₂O (purity 98.0%), were of analytical grade and supplied by Sinopharm Chemical Reagent Co., Ltd. Ammonium stock solution (500 mg NH₄⁺-N/L) was prepared using anhydrous salts of NH₄Cl supplied by freshly double-distilled water.

2.2. Adsorption Studies

Batch adsorption analysis was conducted in a thermostatic shaker, which was capable of controlling the temperature at the pre-set value. For each adsorption experiment, 0.2 g of EMRZ was firstly added to a 250 mL conical flask having 100 mL of a predetermined initial ammonium concentration solution at desirable pH value (adjusted using 0.1 mol/L HCl or 0.1 mol/L NaOH solution). Then, the flask was placed in a thermostatic shaker at the controlled temperature and shaken at 200 rpm for a given period. After reaching the predetermined time, the suspension was finally separated by filtration via 0.45 μm filter and the ammonia concentration in the filtrate was analyzed by the method described in characterization section. The ammonium adsorbed on EMRZ was evaluated and calculated according to

where C₀ represents the initial ammonium concentration (mg/L) and Ce represents the equilibrium ammonium concentration (mg/L), qe (mg/g) is ammonium uptake capacity of EMRZ (mg/g), V is the solution volume (L), and m is the mass of EMRZ used (g).

For determining the most favorable process parameters for ammonium removal by EMRZ, batch ammonium adsorption studies have been executed to study the influence of different factors such as contact time, pH, EMRZ dosage, initial ammonium concentration, and competitive cations (K⁺, Ca²⁺, Na⁺, and Mg²⁺) and anions (PO₄³⁻, SO₄²⁻, Cl⁻, and CO₃²⁻) on ammonium adsorption capacity.

2.3. EMRZ Regeneration

In this study, the used EMRZ was regenerated by NaCl solution (1 mol/L) treatment, since the “NaCl regeneration” method stood out in all methods [23]. In regeneration process, the used EMRZ was mixed with 1 mol/L NaCl solution at L/S ratio 15 : 1. Then, the mixture was stirred at 200 rpm at 25°C for 12 h and the regeneration was performed two times. Finally, the solid samples were filtered, washed, dried, and sieved through a 100 mesh for further analysis. And comparison was made between the ammonium uptake capacity obtained using regenerated samples and those obtained using fresh EMRZ ones.

2.4. Characterization

X-ray diffractometer (D8 Discover, Bruker, Germany) was used to study the phase composition of different samples. The chemical composition of different samples was determined by XRF (x-ray fluorescence spectrometer) (Axios, PANalytical, Holland). The morphological structure of the synthesized EMRZ was analyzed by field emission scanning electron microscopy (FE-SEM) (Mira3, Tescan, Czech Republic). BET (Brunauer–Emmett–Teller) equation by N₂ adsorption method on ASAP2020 (Micromeritics, USA) was used to determine the SSA. The study of Zhang et al. was used to measure CEC of EMRZ [24]. The ammonium concentration in the resulting solution was determined by the conventional Nesslerization method.

3. Results and Discussion

3.1. Effect of Contact Time

Results of ammonium uptake capacity of EMRZ versus contact time in ammonium solutions are given in Figure 3. As displayed in Figure 3, in the first 100 min, the ammonium uptake capacity of EMRZ was fast; thereafter, the uptake amount of ammonium onto EMRZ became almost stable for all the concentrations studied. This behavior is likely to be induced by the fact that almost all adsorption sites of EMRZ were empty and the concentration gradient of ammonium was high in the beginning. Therefore, the rapid utilization of the most easily available adsorption sites of EMRZ made high-speed diffusion and realization of rapid equilibrium possible. Namely, the quick utilization of the most readily available adsorbing sites of the EMRZ leads to fast diffusion and attainment of rapid equilibrium [25–27]. Based on these results and analysis, the contact time of 100 min was used in the following experiment.

Figure 3: Effect of contact time on ammonium adsorption (EMRZ dosage = 0.2 g/100 mL; pH value = 6.0; temperature = 25°C).

3.2. Effect of pH

To probe into the influence of pH value on ammonium uptake capacity of EMRZ, ammonium removal by EMRZ under diverse pH values ranging from 3.0 to 12.0 was investigated. As shown in Figure 4, the amount of ammonium uptake was raised from 3.24 to 18.2 mg/g, when pH value of solution changed from 3.0 to 8.0. And when the pH was 8.0, ammonium uptake reached the maximum amount. Afterwards, a substantial decrease occurred in the amount of ammonium uptake when pH increased to 12.0. This trend of ammonium uptake is in agreement with the observations stated in literature published earlier [25, 28, 29]. This behavior is caused by the fact that at higher pH (pH > 8.0) part of the ammonium ions are transformed to aqueous ammonia, which cannot be removed by EMRZ via ion exchange. And the lower ammonium uptake obtained at lower pH (pH < 8.0) can be explained by the fact that along with the decrease of pH value the concentration of hydrogen ion rises and the ammonium ions turn into competition mode against hydrogen ions among the exchange sites. Meanwhile, it is also likely that EMRZ starts to break down or dissolve, particularly when the pH is below 4.0 [24].

Figure 4: Effect of pH on ammonium adsorption (EMRZ dosage = 0.2 g/100 mL; initial ammonium concentration = 100 mg/L; temperature = 25°C; contact time = 100 min).

3.3. Effect of EMRZ Dosage

Figure 5 displays the results of experiments in which the effects of EMRZ dosage on ammonium uptake capacity are determined. As seen in Figure 5, the ammonium uptake capacity reduced from 18.2 to 3.38 mg/g while EMRZ dosage increased from 0.2 to 2.0 g/100 mL. Considering that the initial ammonium concentration stays constant despite changes in EMRZ dosage, it is only natural that the ammonium concentration gradient per unit mass of EMRZ decreases when EMRZ dosage increases. Thus, the driving force for ammonium adsorption generated from high concentration gradient inevitably decreases under this condition. That is to say, when the ammonium exchange with cations in the EMRZ was fully completed at a specific dosage, the ammonium removal reached equilibrium and much more exchangeable sites of the EMRZ remained unexchanged [24, 30, 31]. On the other hand, the development of aggregates or particle precipitate can been caused under high EMRZ dosage, leading to total surface area of the EMRZ getting reduced and diffusional path length getting increased [32, 33].

Figure 5: Effect of EMRZ dosage on ammonium adsorption (initial ammonium concentration = 100 mg/L; temperature = 25°C; contact time = 100 min; pH value = 8.0).

3.4. Effect of Initial Ammonium Concentration

For the purpose of analyzing the effect of the initial ammonium concentration, ammonium uptake capacity of EMRZ was investigated at various initial ammonium concentrations. As seen in Figure 6, the adsorption data clearly indicated that the ammonium uptake capacity of EMRZ increases along with the increase in initial ammonium concentration. It can be attributed to the fact that higher initial ammonium concentration supplied a larger mass transfer driving force resulting from concentration gradient [34, 35]. Consequently, the ammonium ions were able to move from outer surface to inner micropores of EMRZ to take the place of cations on the surface of EMRZ within certain contact time. Meanwhile, as temperature rises, the ammonium uptake capacity onto EMRZ increases. This may be contributed by the fact that uptake of ammonium onto EMRZ is controlled by an endothermic process. That is to say, the higher temperature is favorable to endothermic reaction once equilibrium is achieved. Therefore, ammonium ions can easily adsorb from the bulk phase by EMRZ with an increase in the temperature of the solution.

Figure 6: Effect of initial ammonium concentration on ammonium adsorption (EMRZ dosage = 0.2 g/100 mL; contact time = 100 min; pH value = 8.0).

3.5. Effect of Competitive Cations

Figure 7 shows the experimental results for ammonium removal by EMRZ in the presence of each competitive cation including K⁺, Ca²⁺, Na⁺, and Mg²⁺ ions. As seen in Figure 7, there is a reduction on ammonium adsorption to some extent in the presence of competitive cations in each case. Because K⁺, Ca²⁺, Na⁺, and Mg²⁺ ions present in the solution can compete with the ammonium for the available ion-exchange sites on the EMRZ, resulting in a decrease in the uptake amount of ammonium onto EMRZ [36–38], with increasing the concentrations of Mg²⁺, Ca²⁺, K⁺, and Na⁺ from 0 mg/L to 250 mg/L, the uptake amount of ammonium onto EMRZ decreased from 16.62 to 12.98, 11.25, 9.06, and 8.52 mg/g, respectively. At the identical concentration level of competitive cations, the reduction follows the sequence of Na⁺>K⁺>Ca²⁺>Mg²⁺, indicating the competitive degree of cations in the process of adsorbing ammonium followed the same sequence of Na⁺>K⁺>Ca²⁺>Mg²⁺. Comparable results can be found in other researches published earlier [21, 39, 40]. However, there are studies that show different results for the competitive degree of zeolite toward those cations. Zhang et al. [24] reported that the competitive degree for cations on the fly ash-made zeolite was K⁺ >Ca²⁺>Na⁺>Mg²⁺. Karadag et al. [41] demonstrated the competitive degree of four cations on ammonium adsorption by natural clinoptilolite as K⁺>Na⁺> Ca²⁺>Mg²⁺. The fact that various zeolites possess different characteristics and therefore various competitive degrees for cations can serve as the explanation for these unaligned results.

Figure 7: Effect of cations concentration on ammonium adsorption (EMRZ dosage = 0.2 g/100 mL; contact time = 100 min; pH value = 8.0; initial ammonium concentration = 100 mg/L; temperature = 25°C).

3.6. Effect of Anions

Figure 8 demonstrates the impact on ammonium removal by EMRZ triggered by anion species as a function of different anions concentrations. It is interesting to notice from Figure 8 that the ammonium uptake is negatively impacted by anions; i.e., the ammonium adsorption capacity went down while anions concentration went up. When the concentration of carbonate, phosphate, sulfate, and chloride ion increased from 50 mg/L to 250 mg/L in the ammonium solution, the ammonium uptake amount fell from 16.62 to 5.52, 10.98, 8.25, and 7.06 mg/g, respectively, which is an indication that the order of anion effect on ammonium adsorption onto EMRZ is CO₃²⁻ > Cl⁻ > SO₄²⁻ > PO₄³⁻. The explanation mechanism of the phenomenon was plausible as the existence of the anion ions might increase the surface tension of the aqueous phase to the degree of reducing access of ammonium to the micropores and macropores of the EMRZ [24, 42].

Figure 8: Effect of anions concentration on ammonium adsorption (EMRZ dosage = 0.2 g/100 mL; contact time = 100 min; pH value = 8.0; initial ammonium concentration = 100 mg/L; temperature = 25°C).

3.7. Adsorption Kinetics

The adsorption kinetics of ammonium is extremely important to clarify the adsorption process and the rate-controlling mechanism of ammonium uptake onto EMRZ. In this study, (A) pseudo-first-order kinetic model, (B) pseudo-second-order kinetic model, (C) Elovich equation kinetic model, and (D) intra-particle diffusion kinetic model were employed to test the investigational data [43–46]. And the above adsorption kinetic models can be summarized as follows:

where qe and qt represent the ammonium adsorption capacity of EMRZ (mg·g⁻¹) at equilibrium and at time t, respectively, k₁ (min⁻¹) and k₂ (g·mg⁻¹·min⁻¹) are the rate constants of pseudo-first-order kinetic and pseudo-second-order kinetic models, respectively, α is the initial adsorption rate (mg·g⁻¹·min⁻¹) and β is associated with the extent of surface coverage and activation energy (g·mg⁻¹), kid is the intra-particle diffusion rate constant (mg·min−1/2·g⁻¹), and C stands for the intercept.

Table 1 and Figure 9 demonstrate the linearized kinetic data using the above models, as well as the calculated constants. Comparing the correlation coefficients (R²) displayed in Figure 9 and Table 1, the correlation coefficients (R² = 0.9925, 0.9982, 0.9962, and 0.9949) obtained under different initial ammonium concentration (50, 100, 150, and 200 mg/L) for the pseudo-second-order kinetic model were much higher than the ones obtained for the pseudo-first-order kinetic model (R² = 0.7714, 0.8708, 0.7760, and 0.8102), Elovich equation model (R² = 0.8327, 0.8420, 0.8496, and 0.9438), and intra-particle diffusion model (R² = 0.6740, 0.6826, 0.8331, and 0.8719). As shown in Table 1, the calculated qe is also within close range of the experimental value (qe (exp)) at 50, 100, 150, and 200 mg/L initial ammonium concentrations. Consequently, it is obvious that the pseudo-second-order kinetic model is of higher accuracy during the adsorption process than pseudo-first-order kinetic model under different initial ammonium concentration.

Table 1: Kinetic parameters for ammonia removal using various kinetic models.

Ammonium concentration (mg·L⁻¹)
qe (exp) (mg·g⁻¹)
Pseudo-first-order model
Pseudo-second-order model
Elovich equation model
Intra-particle diffusion model

qe (mg·g⁻¹)
k₁ (min⁻¹)
α (mg·g⁻¹·min⁻¹)
qe (mg·g⁻¹)
k₂ (g·mg⁻¹·min⁻¹)
R²
α (mg·g⁻¹·min⁻¹)
β (g·mg⁻¹)
R²
kid (mg·min−1/2·g⁻¹)
R²

50
12.09
4.43
0.02594
0.1728
13.41
0.004335
0.9925
0.1728
0.3747
0.8327
0.5983
0.6740

100
16.70
7.32
0.03551
0.08929
17.79
0.005451
0.9982
0.08929
0.3595
0.8420
0.6239
0.6826

150
21.02
12.85
0.03274
0.04958
23.43
0.002274
0.9962
0.04958
0.2269
0.8496
1.0277
0.8331

200
24.28
16.75
0.03211
0.05952
27.17
0.001841
0.9949
0.05952
0.2005
0.9438
1.1810
0.8719

qe (exp) and qe are the experimental and calculated values of qe, respectively.

(a)

(b)

(c)

(d)

Figure 9: (a) pseudo-first-order, (b) pseudo-second-order, (c) Elovich equation model, and (d) intra-particle diffusion model kinetic plots for ammonia removal by EMRZ at different initial ammonium concentrations (EMRZ dosage = 0.2 g/100 mL; pH value = 6.0; temperature = 25°C).

3.8. Adsorption Isotherm

The equilibrium adsorption isotherm is a helpful factor when it comes to the description of the mutually active behavior between solutes and adsorbent, and it is also essential to lay the foundational ground for the design and operation process. The experimental data was matched with the five most widely used isotherm models, Langmuir, Freundlich, Koble–Corrigan, Temkin, and Dubinin–Radushkevich equations, which have been proposed and applied in the literature, to identify the most suitable model [24, 32, 47–49].

3.8.1. Langmuir Isotherm

The Langmuir isotherm model is used to depict the adsorption process occurring at a specific homogeneous location within the EMRZ. The linear form of the Langmuir isotherm model can be calculated by the following formula:

where KL is the adsorption equilibrium constant (L·mg⁻¹), q₀ is the maximum monolayer adsorption capacity, qe is the ammonium adsorption capacity at equilibrium time (mg·g⁻¹), and Ce is the equilibrium concentration (mg·L⁻¹).

As shown in Figure 10, Ce/qe versus Ce were plotted using the experimental data collected in Figure 6. In addition, Table 2 shows the q₀ and KL values, which were, respectively, calculated according to the linear plot. The values of q₀ at 25°C, 30°C, and 35°C were obtained as 31.08, 32.05, and 32.88 mg/g, respectively, and were higher than the experimental data (24.14 mg/g at 25°C, 25.28 mg/g at 30°C, and 27.89 mg/g at 35°C). The values of KL were 0.0205 L/mg at 25°C, 0.02653 L/mg at 30°C, and 0.03354 L/mg at 35°C, which show an increasing trend with temperature increase. It can be inferred that ammonium adsorption is an endothermic process. And the values of correlation coefficients (R²) were 0.9699, 0.9856, and 0.9886 at 25°C, 30°C, and 35°C, respectively.

Figure 10: Langmuir isotherm plots for the uptake of ammonium on the EMRZ.

Table 2: Isotherm constants and regression data of adsorption isotherms of ammonium on EMRZ at different temperatures (EMRZ dosage = 0.2 g/100 mL; contact time = 100 min; pH value = 8.0).

Adsorption isotherm
Temperature (°C)

25
30
35

Langmuir isotherm
KL = 0.0205
KL = 0.02653
KL = 0.03354

q₀ = 31.08
q₀ = 32.05
q₀ = 32.88

R² = 0.9699
R² = 0.9856
R² = 0.9886

Freundlich isotherm
KF = 3.2318
KF = 4.1460
KF = 5.4319

1/n = 0.3978
1/n = 0.3602
1/n = 0.3288

R² = 0.9916
R² = 0.9996
R² = 0.9988

Koble–Corrigan isotherm
a = 3.6079
a = 4.3106
a = 5.3322

b = −0.2311
b = −0.0246
b = 0.0161

n = 0.1927
n = 0.3261
n = 0.3498

R² = 0.9945
R² = 0.9990
R² = 0.9974

Temkin isotherm
AT = 0.2058
AT = 0.2891
AT = 0.4187

BT = 6.799
BT = 6.559
BT = 6.668

R² = 0.9582
R² = 0.9840
R² = 0.9805

Dubinin–Radushkevich isotherm
Qm = 21.5176
Qm = 22.8728
Qm = 25.2824

K = 6.9309 × 10⁻⁵
K = 5.4558 × 10⁻⁵
K = 3.8141 × 10⁻⁵

R² = 0.7579
R² = 0.8200
R² = 0.8297

Moreover, the basic characteristics of Langmuir isotherms can be expressed by dimensionless separation factors (RL). And RL is denoted as follows [25]:

where C₀ is the highest initial solute concentration (mg·L⁻¹) and KL is the Langmuir’s adsorption constant (L·mg⁻¹). The RL value suggests that the adsorption is unfavorable (RL > 1), linear (RL = 1), favorable (0<RL < 1), and irreversible (RL = 0). The values of RL in the present investigation have been found in the range 0.13∼0.20 depicting that the adsorption of ammonium at all temperatures used is very favorable.

3.8.2. Freundlich Isotherm

The Freundlich isotherm is an empirical equation used to describe heterogeneous systems. Here is how it is expressed:

where KF ((mg·g⁻¹) (L·g⁻¹)n) and 1/n are constants, representing adsorption capacity and adsorption intensity, respectively, and Ce and qe are of the same nature as those in the Langmuir isotherm model.

Freundlich isothermal plots and constants are presented and listed in Figure 11 and Table 2, respectively. The obtained KF was in the range of 3.2318∼5.4319 and showed an increasing trend with temperature increase, again confirming the endothermic nature of ammonium uptake by EMRZ. The values of 1/n were also found in the range of 0.3288∼0.3978 at all temperatures used, signifying again that adsorption is favorable. As also seen in Table 2, the correlation coefficients (R² between 0.9916 and 0.9996) in the Freundlich isotherm were higher than those in the Langmuir isotherm (R² between 0.9699 and 0.9886) for all investigated temperatures. The good adaptability of Freundlich isotherm to experimental data means that heterogeneous surface exists in EMRZ and that the adsorption of ammonia onto EMRZ takes place as a multilayer system [25, 50].

Figure 11: Freundlich isotherm plots for the uptake of ammonium on the EMRZ.

3.8.3. Koble–Corrigan Model

The Koble–Corrigan model is a combination of Langmuir and Freundlich isotherm models. It is expressed in the form below:

where a, b, and n are the Koble–Corrigan parameters and Ce and qe are of the same nature as those in the Langmuir isotherm model.

The experimental and predicted equilibrium data obtained using the Koble–Corrigan model are plotted in Figure 12. Table 2 lists out the obtained Koble–Corrigan parameters, a, b, and n. The correlation coefficients (R²) for the Koble–Corrigan isotherm model were 0.9945, 0.9990, and 0.9974 at 25°C, 30°C, and 35°C, respectively. In comparison with the Freundlich model, the correlation coefficients (R²) obtained in Koble–Corrigan model are very similar to those of the Freundlich model. Given the correlation coefficients (R²) of the Koble–Corrigan and Freundlich models, it was deduced that the main mechanism for the ammonium adsorption process was heterogeneous uptake.

(a)

(b)

(c)

Figure 12: Koble–Corrigan isotherm plots for the uptake of ammonium on the EMRZ.

3.8.4. Temkin Isotherm

Temkin isotherm model considers the interaction between adsorbent-adsorbate and the influence of the heat of adsorption of all molecules in the layer. The characteristic of this adsorption is the uniform distribution of binding energies. The Temkin isotherm model can be expressed in the form below:

where BT = RT/b and b is associated with the heat of adsorption, AT is the equilibrium binding constant (L/mol), BT is associated with the heat of adsorption, and Ce and qe are of the same nature as those in the Langmuir isotherm model.

Figure 13 displays a plot of qe versus lnCe at different temperatures. And the Temkin isotherm model parameters and correlation coefficient (R²) are calculated and listed in Table 2. AT were 0.2058, 0.2891, and 0.4187 L/mol at 25°C, 30°C, and 35°C, respectively, while BT were 6.799, 6.559, and 6.668 at those temperatures. And the correlation coefficient (R²) for Temkin isotherm model was 0.9582 (25°C), 0.9840 (30°C), and 0.9805 (35°C), respectively. As for the correlation coefficient (R²), the Temkin isotherm model was obviously not so competitive as the Freundlich and Koble–Corrigan isotherm models.

Figure 13: Temkin isotherm plots for the uptake of ammonium on the EMRZ.

3.8.5. Dubinin–Radushkevich (D–R) Isotherm

The D–R isotherm generally describes the microporous adsorbents adsorption process. And the data were also fitted by D–R isotherm to settle the adsorption type (physical or chemical) [24, 33]. The D–R isotherm model and mean free energy of adsorption (E) can be expressed in the form below:

where K is the adsorption energy (mol²/J²), Qm is the theoretical monolayer adsorption capacity (mg/g), and Ce and qe are of the same nature as those in the Langmuir isotherm model.

Figure 14 and Table 2 show the plots between lnqe and ε² at different temperatures and D–R isotherm model parameters. The Qm values were found to be 21.5176 mg/g (25°C), 22.8728 mg/g (30°C), and 25.2824 mg/g (35°C), respectively. And values of K were calculated to be 6.9309 × 10⁻⁵, 5.4558 × 10⁻⁵, and 3.8141 × 10⁻⁵ at 25°C, 30°C, and 35°C, respectively. Using equation (10), the E value was calculated as 0.08496, 0.09573, and 0.1311 kJ/mol at 25°C, 30°C, and 35°C, respectively. The values of E were found all smaller than 16 kJ/mol, suggesting that the sorption type of ammonium onto EMRZ was physical uptake process. On the other hand, the obtained correlation coefficient (R²) at 25°C, 30°C, and 35°C was 0.7579, 0.8200, and 0.8297, respectively, which are considerably smaller than those obtained from the other isotherm models as displayed in Table 2. Namely, the D–R model is not suitable for describing the experimental equilibrium data.

Figure 14: D–R isotherm plots for the uptake of ammonium on the EMRZ.

3.8.6. Summary of the Adsorption Isotherm Studies

To sum up, the above analyses were sufficient in proving that the experimental data are best fitted by the Freundlich and Koble–Corrigan model due to the much higher R² (see Table 2). These results are consistent with those reported by other researchers [24, 32, 51].

Table 3 compares ammonium uptake capacities among various zeolites. For the reason that the tests were carried out under different experimental conditions, the comparison between ammonium uptake capacities of EMRZ in this study and those of previously tested zeolites is not a practical one. However, as shown in Table 3, the EMRZ analyzed in this work is with a greater capacity for ammonia adsorption and it shows huge potential in serving as an effective adsorbent for ammonia. Moreover, utilizing EMR to prepare EMRZ not only contributes to reduction of waste generation but also boosts cost efficiency. Thus, the obtained EMRZ, as an adsorbent for ammonium removal, is both readily accessible and highly efficient.

Table 3: Comparison of maximum adsorption capacity of ammonium on various zeolites.

Adsorbent
Maximum adsorption capacity (mg/g)
Reference

EMRZ
31.08∼32.88
This work

Natural Chinese zeolite
5.87∼6.02
[52]

Natural Chinese clinoptilolite
10.53
[53]

Natural Iranian zeolite
8.51–10.39
[42]

Natural Iranian zeolite
43.47
[54]

Pretreated natural zeolites
32.73
[55]

NaCl-modified zeolite
14.26
[48]

NaOH-activated zeolite
21.2
[46]

Natural Turkish zeolite
5.76–7.36
[56]

Rice husk ash-synthesized zeolite
42.37
[25]

Fly ash derived zeolite
18.19
[57]

Fly ash derived zeolite
3.94
[58]

3.9. Adsorption Thermodynamics

Thermodynamic parameters, like enthalpy (ΔH°), Gibbs energy (ΔG°), and entropy (ΔS°), for the adsorption of ammonium on EMRZ were calculated from Langmuir constant at different temperatures (25°C, 30°C, and 35°C) using the following equations:

The values of ΔH°, ΔG°, and ΔS° parameters are summarized in Table 4. The free energy changes (ΔG) obtained were −14.03, −14.90, and −15.77 kJ mol⁻¹ at 25°C, 30°C, and 35°C, respectively. The negative values of ΔG suggest ammonium uptake by EMRZ is spontaneous. And the greater the negative value of ΔG goes, the more energetically favorable the adsorption becomes. The enthalpy (ΔH) was +37.66 kJ mol⁻¹; therefore, ammonium uptake by EMRZ is an endothermic process. This confirms the adsorption isotherm studies in Section 3.8. Also, the positive value of entropy (ΔS = +173.38 J·mol⁻¹·K⁻¹) indicates that the randomness of the solid-solution interface increases during the adsorption of the ammonium to the EMRZ.

Table 4: Thermodynamic parameters for the adsorption of ammonium onto EMRZ.

Adsorbent
ΔG (kJ·mol⁻¹)
ΔH (kJ·mol⁻¹)
ΔS (J·mol⁻¹·K⁻¹)

25°C
30°C
35°C

EMRZ
−14.03
−14.90
−15.77
+37.66
+173.38

3.10. Reuse Performance of EMRZ

In order to evaluate the reuse performance of EMRZ on the ammonium uptake performance, the ammonium uptake capacity of regenerated EMRZ was investigated and compared with fresh EMRZ under different pH values and cycle times [59]. The ammonium uptake capacity by the regenerated EMRZ with different pH values and cycle times is shown in Figure 15. As seen in Figure 15(a), the maximum uptake capacity of the first regenerated EMRZ also occurred at pH 8.0, and a small fall of 0.69 mg/g on the ammonium uptake capacity was observed comparing the regenerated and fresh EMRZ. On the other hand, it is obvious that the ammonium uptake capacity decreases moderately with the increase of cycle times from 18.20 mg/g for the fresh EMRZ to 14.43 mg/g for the fifth cycle (see Figure 15(b)). Therefore, these results still show that there was no clear deterioration observed for the regenerated EMRZ. And this finding was consistent with the report of Li et al. [60], Huang et al. [61], and Alshameri et al. [35]. Moreover, the efficient utilization of EMR as ingredients of EMRZ may be advantageous in reduction of not only waste generation but also manufacturing costs.

(a)

(b)

Figure 15: Comparison of ammonium uptake capacity by fresh and regenerated EMRZ. (a) EMRZ dosage = 0.2 g/100 mL; initial ammonium concentration = 100 mg/L; temperature = 25°C; contact time = 100 min. (b) EMRZ dosage = 0.2 g/100 mL; initial ammonium concentration = 100 mg/L; temperature = 25°C; contact time = 100 min; pH value = 8.0.

4. Conclusions

EMR can be converted into EMR-based zeolite (EMRZ) via a fusion method, as confirmed by XRD and SEM analyses. And the major crystalline component of the synthesized EMRZ was detected as zeolite A. The performance for ammonium uptake of EMRZ is dependent upon the contact time, EMRZ dosage, initial ammonium concentration, and pH value. The most favorable pH for ammonium uptake by the EMRZ was obtained at 8.0, and the equilibrium adsorption was rapidly attained within 100 min. The presence of both competitive cations and anions also affected ammonium uptake. The orders Na⁺>K⁺>Ca²⁺>Mg²⁺ for cations and CO₃²⁻ > Cl⁻ > SO₄²⁻ > PO₄³⁻ for anions were observed. The adsorption kinetics is best approximated by the pseudo-second-order model. The Freundlich and Koble–Corrigan adsorption isotherm model are best fitted by the equilibrium data, suggesting that heterogeneous uptake was the principal mechanism adopted in the process of ammonium adsorption. This study identified thermodynamic parameters as well. The negative values of ΔG indicate that ammonium uptake onto EMRZ is spontaneous process. The enthalpy (ΔH) and the entropy changes (ΔS) were both positive meaning endothermic adsorption process and increasing the randomness of the solid-solution interface during the adsorption of the ammonium to the EMRZ. The reuse of EMRZ could achieve a good ammonium uptake capacity, and there was no clear deterioration observed for the regenerated EMRZ. This study demonstrates that the EMRZ can be successfully used as a suitable and efficient adsorbent for removing ammonium from aqueous solutions.

Data Availability

The data used to support the findings of this study are available from the corresponding author upon request.

Conflicts of Interest

The authors declare that they have no conflicts of interest.

Acknowledgments

The authors are grateful for the project funded by China Postdoctoral Science Foundation (No. 2017M611799) and Jiangsu Provincial Basic Research Program (Natural Science Foundation)-Youth Foundation Project (No. BK20190690) and Hunan Provincial Science and Technology Plan Project, China (No. 2016TP1007), by offering the research fund.

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