Kinetics of Struvite Crystals Formation through Adsorption and Crystallization Using Zeolite Modification

This study observed the formation of struvite crystals in wastewater using natural zeolite activated with Mg 2+ ions. Mg 2+ ions released from natural zeolite would react with PO 4 3-and NH 4 + ions from in wastewater to form struvite crystals. The results showed that at pH 8.5, the removal of PO 4 3-and NH 4 + ions was more effective using the modified zeolite than the natural zeolite. Adding 40 g/L Zeo-Mg (1) produced the best results, with PO4 3-(93.32%) and NH 4 (40%) adsorption. Meanwhile, 40g/L Zeo-Mg (2) adsorbed 81% PO 4 3-ions and 27.12% NH 4 + ions. The eqilibrium time of all the treatments was 40 min. The results of observation through SEM, EDX, and FTIR showed that struvite crystals were formed on the zeolite surface. Kinetic models indicated that the mechanism of struvite formation on the surface of natural and modified zeolite was mainly chemisorption. Further calculations showed that the adsorption of PO 43-and NH 4+ ion followed the pseudo-first-order kinetic


Introduction
Because of the anaerobic digestion of organic waste in the wastewater treatment plant, magnesium (Mg 2+ ), ammonium (NH4 + ), and phosphate (PO4 3-) ions are released.These ions can react to produce struvite crystals under specific conditions, such as pH 8.0-9.5 and ion concentrations that exceed those of the supersaturated solution [1][2][3][4][5].Many chemical compounds, such as solid or liquid organic pollutants, inorganic fertilizer, heavy metals, or hazardous materials, have entered wastewater due to industrial or human activities.Over time, those compounds may precipitate to form solid material and eventually degrade in a wastewater treatment plant.However, under specific physicochemical conditions, the dissolved Mg 2+ , NH4 + , and PO4 3-ions in wastewater can react to form struvite, causing hard-scale pipe surfaces or pipe blockage that inhibits the flow process.Consequently, it increases pumping costs, reduces plant capacity [6,7], and further affects the working processes of centrifuges, heat exchangers, and aerators.The struvite crystal is formed under alkaline conditions based on reaction Eq.(1) [8,9]  Although struvite crystal formation has long been the main problem, struvite crystal is a potential slow-release fertilizer, so wastewater treatment companies attempt to recover the wastewater by removing PO4 3-and NH4 + ions [9].According to Mijangos et al. [10], PO4 3-and NH4 + ion concentrations can be removed using natural zeolite activated with Mg 2+ ions to form struvite crystals.The struvite crystals are formed through a two-step mechanism: ion exchange and struvite formation.In this mechanism, physical adsorption of NH 4+ on the zeolite surface occurs in this mechanism, and then Mg 2+ reacts with PO4 3-and NH4 + ions to form struvite crystal, which adheres to the zeolite surface [10,11].It is essential to study the struvite formation in natural zeolite and modified zeolite, especially the adsorption mechanism.Previous studies reported that the benefits of removing the concentrations of NH4 + ions from solutions using zeolite include low cost, a simple process, and an easy procedure [12].Zeolites are crystalline, microporous aluminosilicate minerals consisting of three-dimensional frameworks of SiO4 4-and AlO4 5-tetrahedral linked through shared oxygen atoms [13,14].Zeolite with a low or intermediate ratio of Si/Al can be hydrophilic and act as a proper ion exchanger to remove heavy metals and radioactive compounds [15].Zeolite is frequently employed as an ion exchanger in domestic and commercial water purification, softening, and other applications because of its high selectivity for ammonium and metallic ions [16,17].Although natural zeolite can be easily applied in the adsorption process, it is necessary to improve the adsorption capacity [11] through treatments using acids, alkalis, and salts [18,19], calcination [20], microwave pre-treatment [21], and electrochemical treatment [22].Research by Lin et al. [17] on natural zeolite modified with NaCl solution showed that NaCl-modified zeolite adsorbed more NH4 than natural zeolite.Accordingly, despite the capacity to remove PO4 3-and NH4 + ions simultaneously, the efficiency rate was relatively low [11,23].Another method for the simultaneous removal of NH4 + ions is a combination of ion exchange and struvite formation.The rate of NH4 + ion adsorption on zeolite is strongly influenced by the pH of the solution because of the competing actions of H + and the fact that pH affects the ionization of the functional groups on the sorbent material's surface [24][25][26].The optimum pH of 6-7 [27], 7 [24], and 8 [25,28], for removing NH4 + ions using zeolite has been reported.Previous studies mentioned above have suggested the following Equation (2) that mainly performs the removal of NH4 + ions by zeolite [28]: Equation 2 illustrates the mechanism of ion exchange between Mg 2+ and NH4 + ions in zeolite.The Mg 2+ ions released will react with the PO4 3-and NH4 + ions in the solution.Mg 2+ ions are crucial in the formation of struvite crystals in zeolite because the more Mg 2+ ions are present in zeolite, the more sources of magnesium it must form struvite crystals [11].Furthermore, Equation 2shows that struvite crystals can be formed in solutions with pH greater than 7.The aim of this study is to determine the effect of pH, reaction time, and reagent dosage of the natural and modified zeolites on the removal of PO4 3-and NH4 + ions.We also referred to batch adsorption studies on natural and modified zeolites to identify the reaction kinetics and mechanism of adsorption of PO4 3-and NH4 + ions.

Materials 2.1.1. Preparing zeolite modification
The zeolite used in this study was derived from CV. Minatama, Bandar Lampung, Lampung Province, Indonesia, with a Si/Al ratio of 4/1 (% w/w).The samples of natural zeolite were pulverized to a 100-mesh particle size.Impurities such as salinity, sand specimens, and ash from natural zeolite were removed by washing it with distilled water.Then, the clean natural zeolite was dried at 100 ± 1 °C for 12 hours.The natural zeolite was modified at two temperatures: at 80 °C, named Zeo-Mg (1), and at room temperature, named Zeo-Mg (2).The preparation of zeolite Zeo-Mg(1) followed the research method described by Saputra et al. [29].One hundred grams of pulverized natural zeolite were placed into MgCl2 solutions (1 L of 2 mol/L).The mixture was heated to 80 °C and stirred for 6 hours in a refluxing flask, after which the zeolite was filtered.The mixture was mixed three times for maximum ion exchange before being oven-dried at 100 °C for 12 hours.In the preparation of Zeo-Mg (2), after the dispersion of natural zeolite was complete, the mixture was left to stir for 6 hours at room temperature and repeated three times.

Solution preparation
The synthetic wastewater solution was prepared by dissolving analytical-grade KHPO4 and NH4Cl into distilled water as sources of PO4 3-and NH4 + ions, respectively.The concentration was simulated with PO4 3-and NH4 + ion concentrations of 5 and 35 mM, respectively.

Experimental methods
Natural and modified zeolites of Zeo-Mg (1) and Zeo-Mg (2) were used in experiments to examine the PO4 3-and NH4 + ions removal from solutions at various pH levels and adsorbent dosages.The experiments were subsequently performed to achieve optimum conditions and identify the pH, adsorbent dosage, and equilibrium at which time the optimum conditions occur.PO4 3-and NH4 + ion solutions were mixed in a beaker glass at pH 7.8.In order to reach the desired pH, solutions were added with NaOH and HCl.The pH of the solution was determined using a digital pH meter and adjusted to the desired value using a stock solution of NaOH (0.05 M) and HCl (0.05 M).Pouring synthetic wastewater (500 mL) containing PO4 3-and NH4 + ions into a 1,000 mL beaker while stirring at 100 rpm was used to conduct the experiment.Then a specific dose of adsorbent was added to the synthetic wastewater.Synthetic wastewater (2 mL) was taken at a specific time to analyze PO4 3-and NH4 + ion removal using spectrophotometry (Hanna HI83300-02).The synthetic wastewater was filtered through a 0.2 µm filter membrane when the contact time was over.The adsorbent obtained was dried in an oven at 35 °C for 24 hours.The elemental morphology composition of the adsorbent was then examined using a scanning electron microscope with energy dispersive X-rays (SEM-EDX).The composition of the natural and modified zeolites was analyzed by FT-IR spectroscopy.A nitrogen gas adsorption analyzer determined the specific surface area, pore volume, and pore radius of natural and modified zeolites.The amount of adsorbate adsorbed at time t, and the percentage of removal of PO4 3-and NH4 + ions into natural zeolite and modified zeolite were calculated using Equations 4 and 5, respectively [30].
where Co is the initial adsorbate concentration (mg/L), Ct is the concentration of adsorbate at any time t, V is the volume of the adsorbate solution, and m is the mass of the adsorbent in (g).

Characterization of the solid adsorbent
The morphology of natural and modified zeolites identified using Scanning Electron Microscope (SEM) is illustrated in Figure 1. Figure 1a shows the morphology of natural zeolites after the activation process using Mg 2+ ions.Figure 1b shows that the Zeo-Mg (1) resulting from the modification of natural zeolite using Mg 2+ ions at a temperature of 80 o C, and Figure 1c shows that the Zeo-Mg (2) resulting from the modification of natural zeolite using Mg 2+ ions at room temperature.After modification, some changes occurred to the structure of zeolite.
Mg 2+ ions covered the surface of zeolite, on which pores appeared.Figure 1b shows more pores than in Figure 1c, indicating that heat during the activation process helped boost the ion exchange between the chemical components of zeolite and the Mg 2+ ions of the solution.The characteristics of natural zeolites, Zeo-Mg(1) and Zeo-Mg(2), were analyzed using an FT-IR analyzer in the range of 600-4000 cm -1 (Figure 2).Three peaks of adsorption bands were achieved at a significant rate after Mg 2+ ion modification.The peak of the Mg-Zeo(1) adsorption band is higher than that of Mg-Zeo(2), which indicates that the temperature activated in Mg-Zeo(1) has successfully increased the number of Mg 2+ ions at the surface of hydroxyl groups of in zeolite.According to Sadrara et al. [31].The bonds between the 600 and 700 cm -1 regions are generally related to the specific fingerprint of secondary building units of framework structures in zeolites.The adsorption bands at 775 cm -1 confirmed the presence of the symmetric stretching of Si-C [32,56].For natural zeolites, Zeo-Mg(1) and Zeo-Mg(2), Si-O-Si asymmetric stretching vibrations were observed at 1038, 1041, and 1045 cm -1 , respectively [33,34]. Figure 2 indicates that the adsorption bands at 3399, 3421, and 3455 cm -1 for natural zeolite, Zeo-Mg(1), and Zeo-Mg(2), respectively, were associated with the stretching bands of the hydrogen-bonded hydroxyl groups [35].It was noticed that the absorbance of these peaks was increasing after the modification of zeolite with Mg 2+ ions.
These results confirm the successful grafting of Mg 2+ ions onto the surface hydroxyl groups of natural zeolite [32,56].The stretching band at 3645-3600 cm -1 indicates a hydroxyl group within the solid sample layer [35].According to Figure 2, the hydroxyl group of the experimental results is at 3622, 3623, and 3624 cm -1 for natural zeolites, Zeo-Mg(1) and Zeo-Mg(2), respectively.The summarized BET analysis on surface area, pore volume, and pore radius of natural and modified zeolites can be seen in Table 1, which shows that the effect of zeolite modification can increase BET surface area, pore volume, and pore radius.The increase of surface area, pore volume, and pore radius is indicative of Mg 2+ ion exchange from MgCl solution to K + , Ca 2+ , Fe + , and Na + ions in zeolite [36,55].Table 2 shows an increase in surface area, pore volume, and pore radius from Zeo-Mg(1) compared to those in Zeo-Mg(2), which could be due to the release of K + , Ca 2+ , Fe + , and Na + ions as a result of heating when zeolite is modified with Mg 2+ ions [37,38].Chemical element identification in natural zeolite and modified zeolite was identified using EDX.Table 2 shows an indication of a decrease in the content of K + , Ca 2+ , Fe + , and Na + ions after the natural zeolite modification process.On the other hand, Mg 2+ ions increased on Zeo-Mg(1) and Zeo-Mg (2).It shows the occurrence of mass transfer in the zeolite modification process.At 80 °C, the mass transfer of Mg 2+ ions on the Zeo-Mg(1) process was greater than at room temperature.This observation suggests that the K + , Ca 2+ , Fe + , and Na + in natural zeolite were replaced by Mg 2+ ions, so the Mg 2+ ions in natural zeolite increased [11].The precipitates that formed at the optimal reaction conditions were collected from the experiment, and the solid was then analyzed by SEM-EDX and FT-IR to confirm our studies.The SEM images (Figures 3a, 3b, and 3c) illustrate the presence of struvite on the surface of zeolite.The Mg, P, and O peaks (major elements in struvite) were seen in the EDX spectrum [2].In Figure 4, FT-IR absorption spectra of struvite crystallized on natural zeolite and modified zeolite are shown.The first peaks were formed at 1006, 1002, and 1004 cm -1 for natural zeolite, Zeo-Mg(1), and Zeo-Mg(2), respectively.The second peak occurred at 1059, 1078, and 1088 for natural zeolite, Zeo-Mg(1), and Zeo-Mg(2), respectively.These peaks indicated the presence of phosphate ions [35].The peak became weak and broad at 1617, 1625, and 1618 cm -1 for natural zeolite, Zeo-Mg(1), and Zeo-Mg(2), respectively, due to the H-N-H or H-O-H scissoring effect [39].The peaks for natural zeolite, Zeo-Mg(1), and Zeo-Mg(2) that occurred at 1435, 1423, and 1424, respectively, can be attributed to the N-H bending vibrations [35].

Effect of pH on PO4 3-and NH4 + ions removal
The pH of the solution is critical in the adsorption process of PO4 3-and NH4 + ions on zeolites [40].The adsorption of NH4 + ions on the surface of zeolite was influenced by the competitive effect of H + [24].According to Chen et al. [41] and Hamdi and Srasra [42], increasing the pH of the solution in the range 4-9 decreases PO4 3-ions adsorption on the zeolite.Adsorption of PO4 3-ions was effective in an acidic pH range of 4-6 [42].In this study, the removal of NH4 + and PO4 3-ions was investigated at a pH range of 7-10 using natural and modified zeolites.The results are presented in Figures 5a and 5b. Figure 5a illustrates that more PO4 3-ions were removed from the solution using modified zeolite, Zeo-Mg(1) and Zeo-Mg(2), between pH 7 and 8.The removal of PO4 3-ions was greatest at pH 8.5 and decreased in the pH range of 9-10.The modified zeolite of Zeo-Mg(1) removed 92.32% of the PO4 3-ions, which was higher than the removal of PO4 3-ions by the modified zeolite of Zeo-Mg(2) and the natural zeolite.In other words, Zeo-Mg(1) modified zeolite increases the level of Mg2 + ions in zeolite, promoting the formation of struvite crystals on the zeolite surface.Furthermore, increasing the pH to 7-10 increased the removal of PO4 3-ions.These findings may be attributed to the formation of amorphous calcium phosphate compounds [23].Figure 5b shows that at a pH of 7-8, more NH4 was removed from the solution using natural zeolites and modified zeolites -Zeo-Mg(1) and Zeo-Mg (2).The maximum level of removal occurred at pH 8.5, namely 42.45, 31.82, and 20.91% for modified zeolites Zeo-Mg(1), Zeo-Mg(2), and natural zeolites, respectively.Physical adsorption of NH4 + ions into the zeolite surface and electrostatic pull-in [43], which was attributed to ion exchange between Na+ and Mg2 + ions, could be the cause.Mg2 + ions react with PO4 3-and NH4 + ions to form struvite, which attaches to the surface of the zeolite.This study found that a high concentration of Mg + ions in Zeo-Mg(1) resulted in a decrease in high levels of PO4 3-and NH4 + ions when compared to natural zeolite or Zeo-Mg(2).At pH > 8.5, less NH4 + was removed from solution, indicating that pH > 8.5 may have converted NH4 into NH3 gas, preventing zeolite from adsorbing NH3 and resulting in less NH4 removal [44,45].From our findings, the optimum pH for the removal of PO4 3-and NH4 + ions was 8.5 for all adsorbents.It indicates that struvite formed, Mg 2+ ions just adsorbed on the surface of zeolite, Mg 2+ ions entered the zeolite crystal lattice.Struvite crystallization results from the reaction of Mg 2+ , PO4 3-, and NH4 + ions.The reaction is significantly controlled over a pH range of 7-11.While struvite crystallization begins at pH 7.5 and peaks at pH 8.5 [46], Huang et al. [11] reported that struvite formation at pH 8-9.5 contributed to removing nutrients using modified zeolite.The removal of PO4 3-and NH4 + ions by Zeo-Mg(1) modified zeolite at pH 8.5 was 1000 1500 2000 2500 3000 3500 4000 Natural Zeolite 92.39% and 42.45%, respectively.Compared to Huang et al. [11].Compared to Huang et al. [11], this study used less zeolite (80g/L), but resulted in more removal of PO4 3-and NH4 + ions.

Effect of zeolite dosage on PO4 3-and NH4 + ions removal
Figure 6 shows that the percentage of PO4 3-and NH4 + ions removal increased with the dosage of adsorbent from 8 to 100 g/L for different types of zeolite modification.The removal rate of PO4 3-and NH4 + ions is higher in Zeo-Mg(1) modification than in Zeo-Mg(2) and natural zeolite.For Zeo-Mg(1), removing PO4 3-and NH4 + ions increased from 19.31 to 40.82 % and 50.00 to 92.90%, respectively, in the range of zeolite dosage.The equilibrium point was reached when more than 40 g/L adsorbent was added to modify Zeo-Mg(1) and Zeo-Mg(2), but a higher dosage would not affect the percentage of PO4 3-and NH4 + ions removal.This indicates that there are insufficient PO4 3-and NH4 + ions ions in the solution to form struvite crystals.The equilibrium points of PO4 3-and NH4 + ions removal occurred with the addition of 80 g/L natural zeolite.According to Huang et al. [11], increasing zeolite dosage tends to increase the magnesium concentration, while decreasing the ammonium concentration in the solution causes the decreased ammonium adsorption's driving force to be zeolite.According to Equation 1, the solution could be employed as a magnesium source for struvite crystallization by increasing the Mg 2+ ion content.The results may also be responsible for forming aggregates at a higher solid-liquid ratio of particular precipitation [25].Previous research, such as Huang et al. [11], which focused on the addition of Mg-zeolite modified with Mg2+ ions, found that the reaction of struvite crystals was capable of removing 90% of PO4 3- and 81.2% of NH4 + ions when added with 110 g/L adsorbent at pH 8.5.Peng Xia et al. [47] investigated the effect of diatomite activation with MgO and found that when combined with 0.3 g/L adsorbent at pH 7, it removed 160,9 mg/g PO4 3-and 77.05 mg/g NH4 + ions.In the current study, both PO4 3-and NH4 + ions were efficiently removed when 40g/L of modified zeolite was added to the wastewater. .

Effect of contact time
Figures 7a and 7b show the removal of PO4 3-and NH4 + ions nutrients at pH 8.5 using 80 g/L adsorbent at different contact times, with the percentage of removal increasing with contact time, especially in the first 30 minutes.Figures 7a and 7b show that the modified zeolite affected the removed PO4 3-and NH4 + ions.The results show that Zeo-Mg(1) modified zeolites removed more PO4 3-and NH4 + ions than Zeo-Mg(2) modified zeolites and natural zeolites.According to Figure 7, the removal of PO4 3-and NH4 + ions by the modified zeolite of Zeo-Mg (1) was 92.61% and 39.91%, respectively.This indicated that the activation method using Mg 2+ ions at 80 °C was very effective in the ion exchange of K + , Ca 2+ , Fe + and Na + ions from the zeolite, which replaced their positions with Mg 2+ ions as an adsorbing site.This resulted in fast diffusion and the rapid release of Mg 2+ ions from the zeolite [48].Furthermore, magnesium released from the zeolite reacts with PO4 3-and NH4 + ions to form struvite.It can be seen in Figure 7 that the removal of PO4 3-and NH4 + ions by natural zeolite quickly reached equilibrium within 20 and 40 minutes, respectively.There was a slight increase in the removal of PO4 3-ions, which was mainly due to the slow ion exchange of Mg 2+ ions from the natural zeolite.In other words, without any modification, natural zeolite is not suitable for struvite formation.Figure 7 shows that the equilibrium times across the three types of zeolites are not different, indicating that the amount of Mg ions in the zeolite did not significantly affect the equilibrium times.Similarly, Liu et al. [49] reported that a higher ratio of Mg/PO4 did not significantly increase the amount of struvite formed from the reaction.

Kinetic Study of Modified Zeolite on the Removal of PO4 and NH4
The experimental data were used to estimate the kinetic mechanism by applying pseudofirst-order and pseudo-second-order kinetic models as presented in the following Equations 4 and 5, respectively [50] Where qt (mg/g) is the amount of adsorbate adsorbed at time t, qe (mg/g) is the adsorption capacity at equilibrium, kf (min.), and ks (g/mg.min.) is the pseudo-first-order and pseudosecond-order rate constants, respectively.Pseudo-first-order and pseudo-second-order kinetic model parameters and values of linear regression coefficients are obtained from the respective fitted plots (Figure 8), which are tabulated in Tables 2 and 3.The experimental kinetic data were plotted as log (qeqt) against time (t), which generates a straight line with different slopes, as shown in Figures 8a and 8c, respectively.Further, the struvite crystallization is well exemplified by the pseudo-first-order kinetic model with a high correlation coefficient (R 2 ) above 0.977 for the removal of PO4 3-and NH4 + ions using natural zeolites and modified zeolites.On the other hand, the pseudo-second-order kinetic model predicted by plotting t/qe versus t showed that the experimental research data did not fit in a straight line (Figures 8b and 8c).The calculated (qe) values for the Pseudo-first order model for natural and modified zeolites were almost equivalent to the experimental (qe) values.For the removal of PO4 3-ions (Table 2), the estimated rate constants were 0.0834, 0.1140, and 0.0827/min for natural zeolite, Zeo-Mg(1) and Zeo-Mg(2), respectively.Meanwhile, the estimated rate constants of NH4 + ions removal (Table 3) were 0.0617, 0.1069, and 0.0389 minutes for natural zeolite, Zeo-Mg(1) and Zeo-Mg(2), respectively.The pseudo-first-order model indicates that the struvite crystallization on natural and modified zeolite may be controlled by a physical adsorption process [51].It is assumed that PO4 3-and NH4 + ions are adsorbed onto the surface during the formation of crystal nuclei, thus preventing them from growing beyond the critical size.Further data analysis identified the mechanisms involved in the adsorption system.The most common technique to identify the adsorption mechanism is the intra-particle diffusion model (Weber and J. C. Morris, 1963) as described in Equation 6 [52,53].
=    0.5 + C Where kid (mg/g.min 0.5 ) is the rate constant of intra-particle diffusion, t 0,5 is the half-life time in minutes, and C (mg/g) is a constant associated with the boundary layer thickness.The rate constant of kid was determined from the slope of the straight line obtained by fitting the plot of qt against t 0.5 as described in Equation 6for the experimental data in Figure 9. (a) (b) Figure 9: Intraparticle diffusion for removal of PO4 3-and NH4 + ions Figure 9 depicts plots with a straight line indicating the simultaneous occurrence of film diffusion and intra-particle diffusion, and that film diffusion may control struvite adsorption and crystallization on natural and modified zeolites at an earlier stage [54].Essentially, there was a mass transfer of PO4 3-and NH4 + ions from the bulk solution and Mg 2+ ions released from the zeolite, and they react to form struvite and adsorb into the zeolite's surface via the boundary layer, then diffuse into the zeolite's interior.The results of plotting the data in Figure 9 can be seen in Table 3.It was found that the zeolite modification could increase the Kid and C constants for PO4 3-and NH4 + ions removal.A modification of Zeo-Mg(1) resulted in a higher diffusion rate constant (Kid) than Zeo-Mg(2) for both PO4 3-and NH4 + ions removal.Further investigation revealed that the highest constant C for PO4 3-ion removal occurred on Zeo-Mg(2), whereas NH4 + ion adsorption occurred on Zeo-Mg (1).A high value of C indicates that the rate of the adsorption mechanism is influenced by the limiting boundary layer, suggesting a higher contribution of the surface sorption in the rate-limiting stage [55,57].

Conclusions
The simultaneous removal of PO4 3-and NH4 + ions from the solution was investigated using natural and modified zeolite as the adsorbent materials in struvite crystallization.The results showed that the removal of PO4 3-and NH4 + ions from the solution mainly depends on modified zeolite and the crystallization of struvite.The highest removal of PO4 3-and NH4 + ions was 93.32% and 42.45%, respectively, achieved at pH 8.5 with a dosage of modified zeolite Zeo-Mg (l) by 80 g/L.The pseudo-first-order kinetic model agreed well with the data for the adsorption of PO4 3-and NH4 + ions, and the SEM-EDS and FT-IR of natural and modified zeolite confirmed the presence of struvite.

Figure 3 :
SEM image and EDX spectrogram of the solid from the precipitates collected using natural and modified zeolites

Figure 6 :
Effect of zeolite dosage on the removal of PO4 (a) and NH4 (b) from Solution (pH = 8.5; 100 rpm; contact time 40 minutes)

Figure 8 :
Pseudo-first order and pseudo-second order for removal of PO4 3-ions (a) (b) and NH4 + ions (c) (d) using natural and modified zeolites

Table 1 :
The results of BET analysis

Table 2 :
Chemical composition of natural zeolite and modified zeolite by EDX : log(  −   ) = log   −

Table 2 :
Kinetic parameters for removal of PO4 3-and NH4 + ions for various kinetic models using natural and modified zeolites