A Further Investigation of NH 4 + Removal Mechanisms by Using Natural and Synthetic Zeolites in Different Concentrations and Temperatures

We investigated the ammonium removal abilities of natural and synthetic zeolites with distinct Si/Al ratios and various surface areas to study how adsorption and ion exchange processes in zeolites perform under different ammonium concentrations and different temperatures. Five zeolites—natural mordenite, chabazite, erionite, clinoptilolite, and synthetic merlinoite—were immersed in 20, 50, and 100 mg/kg ammonium solutions. The results demonstrate that zeolites under high ammonium concentrations (100 mg/kg) possess higher physical adsorption capacity (0.398–0.468 meq/g), whereas those under lower ammonium concentrations (20 mg/kg) possess greater ion exchange properties (64–99%). The ion exchange ability of zeolites is extremely dependent on the cation content of the zeolites, and the cation content is affected by the Si/Al ratio. The surface area of zeolites also has a partial influence on its physical adsorption ability. When the surface area is less than 100 m2/g, the adsorption ability of zeolite increases obviously with surface area; however, adsorption ability is saturated as the surface area becomes larger than this critical value of 100 m2/g. When we placed the zeolites in 50 mg/kg ammonium concentration at different temperatures (5–50 ◦C), we found that the zeolites exhibited the highest ammonium removal ability at 30 ◦C and the potassium release was enhanced at 30–40 ◦C.


Introduction
Natural zeolites are microporous aluminosilicate minerals with high cation exchange capacities and high ammonium selectivity properties [1].More than 50 different species of this mineral have been identified.Previous researchers have studied the ammonium removal ability of zeolites in the purification of livestock wastewater, landfill wastewater, and aquaculture water [2,3].In polluted systems, toxic NH 3 is transformed to NH 4 + under NH 4 + -reducing conditions.Zeolites can effectively remove NH 4 + in water and waste products in order to weaken the virulence of NH 3 .Natural zeolites, including clinoptilolite [1], mordenite [4], erionite [5], and chabazite [6] have been widely utilized in water purification.Previous studies revealed the mechanisms of NH 4 + removal through ion exchange and adsorption in different zeolites [7,8].Mordenite and clinoptilolite have frequently been employed to remove ammonium [9].Many previous investigations have been primarily concerned with equilibrium isothermal and kinetic models of ammonium adsorption on zeolites at different temperatures, pH, or initial concentrations [10][11][12][13][14][15].In this study, we wanted to compare a wider range of Si/Al ratios in zeolites.In order to understand the effect on adsorption and ion exchange capacities, we also measured the cation concentration of the reaction solutions.Moreover, we investigated the concentration and temperature effect on the ion exchange and adsorption mechanism.Cation release ability should be regarded as an important issue that can be advantageously applied in ecosystems and environmental treatments.

Materials
Different natural zeolites often grow together or coexist with other minerals; hence, the identification of zeolite composition is imperative.The natural zeolites that were used in this study include chabazites and erionites from Arizona, USA; clinoptilolites acquired from China; and mordenites from Japan.Furthermore, we deliberately synthesized pure merlinoite to manually create a zeolite composition with a lower Si/Al ratio than those of natural zeolites.The synthetic method was designed based on our previous studies by employing glass materials [16,17].The five samples were identified by X-ray Diffraction (XRD, MAC, Science Mxp 3, Yokohama, Japan) using a Cu target with the Cu Kα wavelength through a 1 • divergent slit.Operating conditions were established as 45 kV and 40 mA, the scanning step size was 0.0083556 • /s, and a step of 1.27 s was used in the 2θ range of 3 • -80 • .Mineral identifications by XRD are shown in Figure 1.Most of the zeolites were very pure, and only the chabazite contained a small amount (~5%) of quartz.
equilibrium isothermal and kinetic models of ammonium adsorption on zeolites at different temperatures, pH, or initial concentrations [10][11][12][13][14][15].In this study, we wanted to compare a wider range of Si/Al ratios in zeolites.In order to understand the effect on adsorption and ion exchange capacities, we also measured the cation concentration of the reaction solutions.Moreover, we investigated the concentration and temperature effect on the ion exchange and adsorption mechanism.Cation release ability should be regarded as an important issue that can be advantageously applied in ecosystems and environmental treatments.

Materials
Different natural zeolites often grow together or coexist with other minerals; hence, the identification of zeolite composition is imperative.The natural zeolites that were used in this study include chabazites and erionites from Arizona, USA; clinoptilolites acquired from China; and mordenites from Japan.Furthermore, we deliberately synthesized pure merlinoite to manually create a zeolite composition with a lower Si/Al ratio than those of natural zeolites.The synthetic method was designed based on our previous studies by employing glass materials [16,17].The five samples were identified by X-ray Diffraction (XRD, MAC, Science Mxp 3, Yokohama, Japan) using a Cu target with the Cu Kα wavelength through a 1° divergent slit.Operating conditions were established as 45 kV and 40 mA, the scanning step size was 0.0083556°/s, and a step of 1.27 s was used in the 2θ range of 3°-80°.Mineral identifications by XRD are shown in Figure 1.Most of the zeolites were very pure, and only the chabazite contained a small amount (~5%) of quartz.The morphology of the zeolites (shown in Figure 2) was identified by Scanning Electron Microscopy (SEM, Hitachi, model S-4800, Tokyo, Japan).All of the samples revealed good crystal forms and crystallinity.Chemical analyses were performed by way of Electron Probe Microanalyses (EPMA, JEOL, model JXA-8900 R, Tokyo, Japan) as a method for analyzing each grain of zeolite [18].The results of the chemical composition analyses of the zeolites are shown in Table 1.Operating conditions were established with an accelerating voltage of 15 kV and the electronic current was set at 5 nA.The two electron beam sizes were less than 5 and 10 µm, and the detection times were set at 10 and 15 s, respectively.Albite and silicate glass were chosen as the chemical standards.Average data for each sample were acquired from more than four individual grains, and each grain was repeatedly tested three times.Table 1 presents the averaged weight percentage of the chemical composition, the Si/Al molar ratios, and the moles of 2Ca + 2Mg + Na + K based on 96 oxygen molecules for each zeolite.The Si/Al ratios of these zeolites range between 2.19 and 5.73.
The morphology of the zeolites (shown in Figure 2) was identified by Scanning Electron Microscopy (SEM, Hitachi, model S-4800, Tokyo, Japan).All of the samples revealed good crystal forms and crystallinity.Chemical analyses were performed by way of Electron Probe Microanalyses (EPMA, JEOL, model JXA-8900 R, Tokyo, Japan) as a method for analyzing each grain of zeolite [18].The results of the chemical composition analyses of the zeolites are shown in Table 1.Operating conditions were established with an accelerating voltage of 15 kV and the electronic current was set at 5 nA.The two electron beam sizes were less than 5 and 10 μm, and the detection times were set at 10 and 15 s, respectively.Albite and silicate glass were chosen as the chemical standards.Average data for each sample were acquired from more than four individual grains, and each grain was repeatedly tested three times.Table 1 presents the averaged weight percentage of the chemical composition, the Si/Al molar ratios, and the moles of 2Ca + 2Mg + Na + K based on 96 oxygen molecules for each zeolite.The Si/Al ratios of these zeolites range between 2.19 and 5.73.

Experimental Methods
In order to understand the ion exchange and adsorption proportions of zeolites, we first removed excess impurities in the zeolites.The zeolites were washed in pure water and were ultrasonically cleaned three times to remove salts and excess adsorbed cations, since many of our zeolite samples were collected from natural saline lakes [19].The zeolites were then oven-dried at 110 After commencing the experiments, 5 mL of solution (test samples) was removed from the container every 6 h over a seven-day time span to measure the cation concentrations.Each test sample was diluted to measure the concentrations of NH 4 + , Na + , K + , Ca 2+ , and Mg 2+ .Released cations Na + , K + , Ca 2+ , and Mg 2+ were determined using Inductively Coupled Plasma Atomic Emission Spectrometry (ICP-AES, Horiba Scientific, Ultima II type, Kyoto, Japan).Ammonium concentrations were measured using a UV-vis Spectrophotometer (Metretech Inc., SP 8001 type, Taipei, Taiwan) at a transparent wavelength of 640 ± 1 nm.The equilibrium concentration of each cation was collected within the seven-day-long experiment, and the final result was determined.
The specific surface area of each zeolite sample was measured using a Specific Surface Area and Pore Size Distribution Analyzer (SSA, Micromeritics, model ASAP2020, Merit Science Corporation, Norcross, GA, USA).The samples were pretreated in a vacuum environment at 393.15 K overnight to clean their surface and were then analyzed at 77 K (liquid nitrogen boiling temperature).The surface area value was determined by using the BET equation to fit the relative pressure curve that ranges from 0.06 to 0.2.The test results were calculated by averaging the values of the six measurements (Table 1).Moreover, we determined the grain size of treatment zeolites using a laser particle analyzer (Malven Masterizer 2000 type, Malven Analysitcal, Malven, UK), which is capable of detecting particle sizes from 0.02 to 2000 µm.

Percentage of Ion Exchange and Adsorption in Different Concentrations
As one cation invades a zeolite structure, it can be adsorbed on the external surface of the channels or exchanged with the original cation that occupies the zeolitic cages.According to the fundamentals of ion exchange between solid and liquid phases [7,8], ion exchange between a zeolite frame and the aqueous ammonium solution can be expressed by the following Equation (1): where Z and M represent the zeolite and bonding cations in the zeolite, respectively; and, n is the number of the electric charge.Accordingly, we assume that total ammonium removal (NH 4 (2) Based on Equation ( 1), we assume that the released cations are equal to the ion exchange capacity (NH 4 + ion ) [8,20], which can be determined by Equation (3).Finally, the adsorption capacity of zeolite can be derived from Equations ( 2) and ( 3).The adsorption capacity (NH 4 + ads ) is given by Equation ( 4): As shown in Table 2, when we changed the ammonium concentration, we observed that the proportions of ion exchange and adsorption capacities for each zeolite were altered.In the 20 mg/kg ammonium solution, most zeolites have higher ion exchange capacities, and more than 90% of ammonium removal is dependent on the ion exchange mechanism.Only mordenite and erionite show lower than 70% ammonium removal by ion exchange.The results of Table 2 imply that a higher ammonium concentration leads to a higher proportion of adsorption by zeolites.The percentage of ion exchange capacity ranges between 64% and 99% at an initial concentration of 20 mg/kg, and it declines to 54-85% at a higher concentration of 50 mg/kg.In the highest ammonium concentration of 100 mg/kg, this percentage decreases to 52-70%.This phenomenon has also been proven in other studies [21].Comparing the ammonium removal capacity of clinoptilolite between ours and that found by Liu et al. [8] under the same conditions for 100 mg/kg, we found that the total ammonium removal capacity for their clinoptilolite mixed with heulandite and quartz was approximately 0.3 meq/g and the ion exchange capacity was approximately 0.2 meq/g [8]; in this study, the total ammonium removal capacity of our zeolites is higher than 0.4 meq/g, and the ion exchange capacity is higher than 0.2 meq/g (Table 2).

The Relationship of Ion Exchange and Adsorption with Zeolite Composition
Each zeolite possesses a different structure and composition.Theoretically, the number of exchangeable cations in a zeolite will influence its ion exchange capacity, and the channel pore size of a zeolite will impact its adsorption capacity.The pore sizes and numbers of channels in zeolites are somewhat dependent on the Si/Al ratio [19].Substitution of silicon by aluminum atoms in the crystal framework leads to additional negative charge, requiring balancing by IA and IIA cations (e.g., Na + , K + , Ca 2+ , and Mg 2+ ) [19].Therefore, we must compare the cation numbers and Si/Al ratios of zeolites with their ammonium removal ability.
Figure 3a presents the relationship between the ammonium ion exchange capacity and the Si/Al ratio of zeolites under initial NH 4 + concentrations of 20, 50, and 100 mg/kg.The trend indicates that zeolites with lower Si/Al ratios (merlinoite, chabazite) have slightly higher ion exchange capacities (NH 4 + ion ).This may be somewhat influenced by the more exchangeable IA and IIA cations in zeolites with lower Si/Al ratio [16,17].On the other hand, the adsorption capacity of a zeolite is primarily controlled by the zeolite structure.Therefore, we calculated the ammonium adsorption capacity and then compared it with the zeolite Si/Al ratio (Figure 3b).We discovered that zeolites with higher Si/Al ratios have slightly higher ammonium adsorption capacities (NH 4 + ads ), but that of erionite is especially higher.Leyva-Ramos et al. [10,22] compared the ion exchange capacities of chabazite and clinoptilolite and also reported that natural chabazite has a higher ion exchange capacity, which is similar to our result shown in Figure 3a.
Minerals 2018, 8, x FOR PEER REVIEW 6 of 13 of a zeolite will impact its adsorption capacity.The pore sizes and numbers of channels in zeolites are somewhat dependent on the Si/Al ratio [19].Substitution of silicon by aluminum atoms in the crystal framework leads to additional negative charge, requiring balancing by IA and IIA cations (e.g., Na + , K + , Ca 2+ , and Mg 2+ ) [19].Therefore, we must compare the cation numbers and Si/Al ratios of zeolites with their ammonium removal ability.Figure 3a presents the relationship between the ammonium ion exchange capacity and the Si/Al ratio of zeolites under initial NH4 + concentrations of 20, 50, and 100 mg/kg.The trend indicates that zeolites with lower Si/Al ratios (merlinoite, chabazite) have slightly higher ion exchange capacities (NH4 + ion).This may be somewhat influenced by the more exchangeable IA and IIA cations in zeolites with lower Si/Al ratio [16,17].On the other hand, the adsorption capacity of a zeolite is primarily controlled by the zeolite structure.Therefore, we calculated the ammonium adsorption capacity and then compared it with the zeolite Si/Al ratio (Figure 3b).We discovered that zeolites with higher Si/Al ratios have slightly higher ammonium adsorption capacities (NH4 + ads), but that of erionite is especially higher.Leyva-Ramos et al. [10,22] compared the ion exchange capacities of chabazite and clinoptilolite and also reported that natural chabazite has a higher ion exchange capacity, which is similar to our result shown in Figure 3a.We then compared the adsorption capacity (NH4 + ads) and the ion exchange capacity (NH4 + ion) with the sum total of IA and IIA cations in zeolites.Mordenite, clinoptilolite, and erionite have lower ion exchange capacities due to their lower cation contents (Figure 4), while chabazite and synthetic merlinoite with higher cation contents have higher ion exchange capacities (Figure 5).The results indicate a positive correlation between the cation content and the ion exchange capacity in all concentrations (Figure 5).
Low Si/Al zeolites (merlinoite, chabazite, and erionite) have eight-ring channels and a smaller aperture size, but high Si/Al zeolites (clinoptilolite and mordenite) have eight-ring, 10-ring, and 12ring channels and a larger aperture size.The adsorption capacity of zeolites is affected by the aperture size of channels because larger apertures facilitate the passing through of cations.Figure 3b shows that the higher Si/Al zeolites have slightly higher adsorption capacities than do the others.When we only consider the behavior of ion exchange, the diameter of NH4 + is approximately 2.96 Å [21], which passes through zeolite channels very easily.On the other hand, the diameter of a hydrated NH4 + ion is approximately 6.62 Å [19], which is larger than the aperture of most zeolites; thus, the hydrated ion cannot freely enter such small pores.Although the aperture size of a zeolite only reflects the average result of an ideal crystal, the specific surface area of zeolites may also affect the adsorption results.with the sum total of IA and IIA cations in zeolites.Mordenite, clinoptilolite, and erionite have lower ion exchange capacities due to their lower cation contents (Figure 4), while chabazite and synthetic merlinoite with higher cation contents have higher ion exchange capacities (Figure 5).The results indicate a positive correlation between the cation content and the ion exchange capacity in all concentrations (Figure 5).
Low Si/Al zeolites (merlinoite, chabazite, and erionite) have eight-ring channels and a smaller aperture size, but high Si/Al zeolites (clinoptilolite and mordenite) have eight-ring, 10-ring, and 12-ring channels and a larger aperture size.The adsorption capacity of zeolites is affected by the aperture size of channels because larger apertures facilitate the passing through of cations.Figure 3b shows that the higher Si/Al zeolites have slightly higher adsorption capacities than do the others.When we only consider the behavior of ion exchange, the diameter of NH 4 + is approximately 2.96 Å [21], which passes through zeolite channels very easily.On the other hand, the diameter of a hydrated NH 4 + ion is approximately 6.62 Å [19], which is larger than the aperture of most zeolites; thus, the hydrated ion cannot freely enter such small pores.Although the aperture size of a zeolite only reflects the average result of an ideal crystal, the specific surface area of zeolites may also affect the adsorption results.

Specific Surface Area of Zeolites
We attempted to measure the specific surface area (SSA) of zeolites in order to investigate its role in adsorption and ion exchange capacities.Erionite and chabazite have larger SSAs of 395.26 and 378.55 m 2 /g, respectively (Table 1), while mordenite and clinoptilolite show SSAs of 85.15 and 22.05 m 2 /g, respectively.Synthetic merlinite has an SSA of only 3.46 m 2 /g, which constitutes the smallest SSA.
SSA reveals no relationship with ion exchange capacity in any concentration (Figure 6b,d,f).The SSA results from Figure 6a,c,e show that higher-SSA zeolites (i.e., erionite, chabazite, mordenite, and clinoptilolite) have higher adsorption capacity than synthetic merlinoite, especially at high concentrations of 50 mg/kg and 100 mg/kg (Figure 6c,e), with a nonlinear relationship.Up to 100 m 2 /g, the adsorption capacity exhibits an obviously positive trend with the SSA.Interestingly, however, as the SSA rises above the critical value of 100 m 2 /g, the adsorption capacity no longer increases.This implies that the adsorption capacity has become saturated.

Specific Surface Area of Zeolites
We attempted to measure the specific surface area (SSA) of zeolites in order to investigate its role in adsorption and ion exchange capacities.Erionite and chabazite have larger SSAs of 395.26 and 378.55 m 2 /g, respectively (Table 1), while mordenite and clinoptilolite show SSAs of 85.15 and 22.05 m 2 /g, respectively.Synthetic merlinite has an SSA of only 3.46 m 2 /g, which constitutes the smallest SSA.
SSA reveals no relationship with ion exchange capacity in any concentration (Figure 6b,d,f).The SSA results from Figure 6a,c,e show that higher-SSA zeolites (i.e., erionite, chabazite, mordenite, and clinoptilolite) have higher adsorption capacity than synthetic merlinoite, especially at high concentrations of 50 mg/kg and 100 mg/kg (Figure 6c,e), with a nonlinear relationship.Up to 100 m 2 /g, the adsorption capacity exhibits an obviously positive trend with the SSA.Interestingly, however, as the SSA rises above the critical value of 100 m 2 /g, the adsorption capacity no longer increases.This implies that the adsorption capacity has become saturated.In order to examine the effect of temperature on zeolite ammonium removal, we selected three of the more effective ammonium-adsorbing zeolites (i.e., mordenite, chabazite, and erionite) to immerse in 50 mg/kg NH 4 + solution at 5, 15, 30, 40, and 50 • C. We calculated the percentage of total NH 4 + removal under these different temperatures (Figure 7).This experiment revealed that the percentage of NH 4 + removal reaches its highest level at 30 • C for these zeolites.The percentages of NH 4 + removal were 77.91% for mordenite, 81.96% for chabazite, and 75.94% for erionite at 30 • C.However, when the temperature is higher or lower than 30 • C, the ammonium removal ability decreases rapidly.This result agrees well with previous research [10,11,15,[23][24][25], which also demonstrated that the highest ammonium removal efficiency occurs at room temperature (25-35 • C).At low temperatures of 5 and 15 • C, chabazite and erionite have better removal abilities than mordenite (Figure 7).It is very interesting that the ammonium removal ability of mordenite shows an extreme decrease as the temperature reaches values lower than 20 • C. We know the rate of ion exchange should decrease as the temperature drops and the adsorption ability should decrease as the temperature goes up.In order to examine the effect of temperature on zeolite ammonium removal, we selected three of the more effective ammonium-adsorbing zeolites (i.e., mordenite, chabazite, and erionite) to immerse in 50 mg/kg NH4 + solution at 5, 15, 30, 40, and 50 °C.We calculated the percentage of total NH4 + removal under these different temperatures (Figure 7).This experiment revealed that the percentage of NH4 + removal reaches its highest level at 30 °C for these zeolites.The percentages of NH4 + removal were 77.91% for mordenite, 81.96% for chabazite, and 75.94% for erionite at 30 °C.However, when the temperature is higher or lower than 30 °C, the ammonium removal ability decreases rapidly.This result agrees well with previous research [10,11,15,[23][24][25], which also demonstrated that the highest ammonium removal efficiency occurs at room temperature (25-35 °C).At low temperatures of 5 and 15 °C, chabazite and erionite have better removal abilities than mordenite (Figure 7).It is very interesting that the ammonium removal ability of mordenite shows an extreme decrease as the temperature reaches values lower than 20 °C.We know the rate of ion exchange should decrease as the temperature drops and the adsorption ability should decrease as the temperature goes up.

Zeolite Selectivity at Different Temperatures
In order to compare ion exchange mechanisms, it is necessary to understand the cation selectivity of zeolites.We calculated the content of cations in solution and in zeolites at equilibrium.The equilibrium constant Ki of element i follows Equation ( 5): where (Ci)s is the concentration of element i in the solution and (Ci)z represents the concentration of element i in the zeolite.A high Ki value indicates that cations tend to be sorbed easier into the zeolite at equilibrium.Therefore, the reciprocal 1/Ki can be regarded as the cation release ability of element i from the zeolite.Table 3 presents each cation's release ability among the zeolites at different temperatures.Interestingly, our results demonstrate that the release ability (1/Ki) of all elements changes with temperature.This implies that zeolite selectivity changes with temperature and zeolite type.Moreover, most conditions reveal that the ion release priority follows Na > Ca > K for mordenite, chabazite, and erionite; however, when the temperature approached 30-40 °C, the release ability of all elements rose, and the release ability of K increased markedly in mordenite and erionite.This result reveals that the ion exchange ability of zeolite reaches a maximum at 40 °C (Table 3).The irregular release of Mg may be due to the zeolite structure controlling the release sequence of Mg.Mordenite possesses a lower Mg content than chabazite and erionite (Table 1), but the release ability

Zeolite Selectivity at Different Temperatures
In order to compare ion exchange mechanisms, it is necessary to understand the cation selectivity of zeolites.We calculated the content of cations in solution and in zeolites at equilibrium.The equilibrium constant K i of element i follows Equation ( 5): where (C i )s is the concentration of element i in the solution and (C i )z represents the concentration of element i in the zeolite.A high K i value indicates that cations tend to be sorbed easier into the zeolite at equilibrium.Therefore, the reciprocal 1/K i can be regarded as the cation release ability of element i from the zeolite.Table 3 presents each cation's release ability among the zeolites at different temperatures.Interestingly, our results demonstrate that the release ability (1/K i ) of all elements changes with temperature.This implies that zeolite selectivity changes with temperature and zeolite type.Moreover, most conditions reveal that the ion release priority follows Na > Ca > K for mordenite, chabazite, and erionite; however, when the temperature approached 30-40 • C, the release ability of all elements rose, and the release ability of K increased markedly in mordenite and erionite.This result reveals that the ion exchange ability of zeolite reaches a maximum at 40 • C (Table 3).The irregular release of Mg may be due to the zeolite structure controlling the release sequence of Mg.Mordenite possesses a lower Mg content than chabazite and erionite (Table 1), but the release ability of Mg in mordenite has higher priority than Mg in chabazite and erionite (Table 3).Although chabazite has higher potassium content (Table 1), its potassium was never released into the solution in our experiments (Table 3).This indicates that the zeolite type also influences the ion release ability.Previous studies often use clinoptilolite to explore the ranking of cation selectivity [23,26,27].Their results suggest the order K > Na > Ca > Mg.However, Lin et al. [8] found a different cation selectivity of clinoptilolite following the sequence Mg > K > Ca > Na.This result is congruent with our results.Our result also shows K > Ca > Na under most conditions, but the selectivity sequence of K and Ca changes at 40 • C for mordenite and erionite.This phenomenon gives us a hint that a higher temperature promotes the exchange of large cations with ammonium ion.Regular selectivity may directly reflect the ionic diameters of cations.The ionic diameter of K (2.66 Å) is larger than that of Ca (1.98 Å); that of Ca is larger than that of Na (1.9 Å); and, that of Na is larger than that of Mg (1.3 Å) [19].This result indicates that efforts to determine the release ability of cations should consider the cation size and the effect of the positions within the cage.

Kinetic Diffusion of Ammonium
The kinetic diffusion of ammonium in zeolites is another crucial mechanism controlling the mass transport and should be considered in analysis.When mass transport occurs in the solid phase, we need to consider the time-varied diffusion coefficient.The diffusion rate is higher at the early stage but it decreases when approaching the equilibrium condition.In the case of 20 mg/g ammonium concentration, all zeolites reached equilibrium conditions after 100 h (Figure 8).Previous research defined the mass transport mechanisms for film diffusion (D f ) and effective particle diffusion (D p ) in a homogenous particle diffusion model [28,29].The sorption on the spherical particles can be calculated through the effective particle diffusivity using Equation (6).As the rate of sorption is controlled by the liquid film diffusion, this rate can be expressed, as shown in Equation (7): where q t and q e are the solute loadings on the adsorbent phase at time t and equilibrium (mg•g −1 ), respectively; C s (mg•L −1 ) and C z (mg•kg −1 ) are the ion concentrations in solution and in the zeolite, respectively; r is the average radius of the zeolite particles obtained from our particle analysis (Table 4); t is the contact time (min); and h is the thickness of film around the zeolite particle (1 × 10 −5 m for a poorly stirred solution) [30].The coefficients of film diffusion (D f ) for all types of zeolites at different ammonium concentrations are significantly higher than those of particle diffusion (D p ) (Table 4).This result is consistent with previous works reporting ammonium sorption on natural zeolites at low initial ammonium concentrations [28][29][30] and it indicates that film diffusion should be the main mechanism controlling the mass transport.After comparing the diffusion data, we find that erionite has the maximal mass transport rate (Table 4).
where qt and qe are the solute loadings on the adsorbent phase at time t and equilibrium (mg•g −1 ), respectively; Cs (mg•L −1 ) and Cz (mg•kg −1 ) are the ion concentrations in solution and in the zeolite, respectively; r is the average radius of the zeolite particles obtained from our particle analysis (Table 4); t is the contact time (min); and h is the thickness of film around the zeolite particle (1 × 10 −5 m for a poorly stirred solution) [30].The coefficients of film diffusion (Df) for all types of zeolites at different ammonium concentrations are significantly higher than those of particle diffusion (Dp) (Table 4).This result is consistent with previous works reporting ammonium sorption on natural zeolites at low initial ammonium concentrations [28][29][30] and it indicates that film diffusion should be the main mechanism controlling the mass transport.After comparing the diffusion data, we find that erionite has the maximal mass transport rate (Table 4).

Conclusions
In this study, we controlled several experimental factors, including the initial ammonium concentration, the zeolite composition, and the temperature.We also discussed the mechanisms governing ammonium removal by zeolites to study zeolite adsorption and ion exchange capacity.The results of applying various initial concentrations of NH 4 + show that zeolites possess greater adsorption capacity under higher ammonium concentrations.In addition, it is found that ion exchange capacity is mainly controlled by the cation content in zeolites, while the adsorption capacity is mainly influenced by the Si/Al ratio and the SSA of zeolites.We discovered that mordenite has the highest adsorption capacity due to its sufficient SSA of nearly 100 m 2 /g and due to having the largest Si/Al ratio.On the other hand, synthetic merlinoite possesses the lowest Si/Al ratio and the highest 2Ca + 2Mg + Na + K content, and it is also accompanied by a higher ion exchange capacity.Moreover, when we performed removal experiments at 5, 15, 30, 40, and 50 • C, we found that 30 • C most benefits ammonium adsorption among all zeolites.Furthermore, the cation selectivity of mordenite, chabazite, and erionite follows the sequence of K > Ca > Na.This phenomenon involves the diameters of cations.The release of Mg is primarily controlled by the zeolite structure, and there is no specific trend among different zeolites.Finally, it was found that temperature changes the ion exchange ability of all zeolites and it slightly affects the release sequence of cations.The ammonium adsorption is mainly controlled by film diffusion, and the mass transportation rate of erionite is higher than those of the other zeolites examined.

Figure 3 .
Figure 3. Relationships of (a) ion exchange and (b) adsorption capacities with the Si/Al ratios of zeolites for different ammonium solutions.

Figure 3 .
Figure 3. Relationships of (a) ion exchange and (b) adsorption capacities with the Si/Al ratios of zeolites for different ammonium solutions.We then compared the adsorption capacity (NH 4 + ads ) and the ion exchange capacity (NH 4 + ion ) with the sum total of IA and IIA cations in zeolites.Mordenite, clinoptilolite, and erionite have lower ion exchange capacities due to their lower cation contents (Figure4), while chabazite and synthetic merlinoite with higher cation contents have higher ion exchange capacities (Figure5).The results indicate a positive correlation between the cation content and the ion exchange capacity in all concentrations (Figure5).Low Si/Al zeolites (merlinoite, chabazite, and erionite) have eight-ring channels and a smaller aperture size, but high Si/Al zeolites (clinoptilolite and mordenite) have eight-ring, 10-ring, and 12-ring channels and a larger aperture size.The adsorption capacity of zeolites is affected by the aperture size of channels because larger apertures facilitate the passing through of cations.Figure3bshows that the higher Si/Al zeolites have slightly higher adsorption capacities than do the others.When we only consider the behavior of ion exchange, the diameter of NH 4+ is approximately 2.96 Å[21],

Figure 4 .
Figure 4. Relationship between the Si/Al ratio and IA and IIA cation content in zeolite compositions.

Figure 5 .
Figure 5. Relationship between IA and IIA cation content and ion exchange capacity in initial concentrations of (a) 20 mg/kg, (b) 50 mg/kg, and (c) 100 mg/kg.

Figure 4 .
Figure 4. Relationship between the Si/Al ratio and IA and IIA cation content in zeolite compositions.

Figure 4 .
Figure 4. Relationship between the Si/Al ratio and IA and IIA cation content in zeolite compositions.

Figure 5 .
Figure 5. Relationship between IA and IIA cation content and ion exchange capacity in initial concentrations of (a) 20 mg/kg, (b) 50 mg/kg, and (c) 100 mg/kg.

Figure 5 .
Figure 5. Relationship between IA and IIA cation content and ion exchange capacity in initial concentrations of (a) 20 mg/kg, (b) 50 mg/kg, and (c) 100 mg/kg.

Figure 6 .
Figure 6.Relationship between specific surface area (SSA) and (a) adsorption capacity in a concentration of 20 mg/kg; (b) ion exchange capacity in a concentration of 20 mg/kg; (c) adsorption capacity in a concentration of 50 mg/kg; (d) ion exchange capacity in a concentration of 50 mg/ kg; (e) adsorption capacity in a concentration of 100 mg/kg; and, (f) ion exchange capacity in a concentration of 100 mg/kg.

Figure 6 .
Figure 6.Relationship between specific surface area (SSA) and (a) adsorption capacity in a concentration of 20 mg/kg; (b) ion exchange capacity in a concentration of 20 mg/kg; (c) adsorption capacity in a concentration of 50 mg/kg; (d) ion exchange capacity in a concentration of 50 mg/ kg; (e) adsorption capacity in a concentration of 100 mg/kg; and, (f) ion exchange capacity in a concentration of 100 mg/kg.

Figure 7 .
Figure 7. Percentage of total ammonium removal for zeolites at an initial NH4 + concentration of 50 mg/kg at different temperatures.

Figure 7 .
Figure 7. Percentage of total ammonium removal for zeolites at an initial NH 4 + concentration of 50 mg/kg at different temperatures.

Figure 8 .
Figure 8. Evolution of ammonium sorption uptake versus time in 20 mg/g ammonium concentration at 30 °C.

Table 1 .
Weight percentages of the chemical composition of zeolites by EPMA (n: analytic grain numbers; Si/Al and 2Ca + Mg + Na + K based on 96 oxygen in molar ratio).Specific surface area was determined by the BET method.

Table 2 .
Results of zeolite removal of NH 4 + and the proportions of ion exchange and adsorption in

Table 3 .
Release ability of each cation in zeolites and zeolite selectivity under different temperatures.

Table 4 .
Diffusion parameters of zeolites in different ammonium concentrations at 30 °C.Evolution of ammonium sorption uptake versus time in 20 mg/g ammonium concentration at 30 • C.

Table 4 .
Diffusion parameters of zeolites in different ammonium concentrations at 30 • C.