Separation Science and Technology, 41: 3485–3498, 2006 Copyright # Taylor & Francis Group, LLC ISSN 0149-6395 print/1520-5754 online DOI: 10.1080/01496390600854636
Mechanism of the Adsorption of Ammonium Ions from Aqueous Solution by a Chinese Natural Zeolite Donghui Wen, Yuh-Shan Ho, Shuguang Xie, and Xiaoyan Tang Department of Environmental Sciences, College of Environmental Sciences, Peking University, Beijing, People’s Republic of China
Abstract: The adsorption of ammonium ions onto a Chinese natural zeolite in an agitated batch adsorber was studied. A trial-and-error non-linear method was developed to examine two widely used isotherms, the Langmuir and Freundlich. The data gained from the adsorption system fitted the Freundlich isotherm better. An ion exchange model, describing the relationship among the total metal ions in the solution, NHþ 4 removed from the solution, and ions initially released from the zeolite, was developed for the adsorption system. In addition, a parameter of the ion exchange potential was defined to describe the adsorption mechanism. Ion exchange was the main mechanism that accounted for the adsorption of ammonium ions onto the Chinese natural zeolite. Keywords: Natural zeolite, adsorption, isotherm, ion exchange
INTRODUCTION Since the 1970s, natural zeolites have been valued as low-cost adsorbents and ion-exchangers for water pollution control (1, 2). Indeed, zeolite-based systems have been advocated as potential solutions to a wide range of problems. Previous researchers have applied natural zeolites for the removal of ammonium from domestic wastewater (3, 4) as well as from industrial Received 21 February 2006, Accepted 16 May 2006 Address correspondence to Xiaoyan Tang, Department of Environmental Sciences, College of Environmental Sciences, Peking University, Beijing 100871, People’s Republic of China. Tel./Fax: 86 10 6275 1925; E-mail:
[email protected] 3485
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wastewaters such as tannery wastewater (5), aquaculture wastewater (6), and piggery wastewater (7, 8). In addition, a number of studies have focused on the application of natural zeolites to treat heavy-metal-contaminated wastewaters for the removal of lead (9, 10), cadmium (11, 12), and other heavy metals (13, 14). Good performance of a zeolite in pollution control is based on its physical and chemical traits. Natural zeolite is a porous mineral described chemically as aluminosilicate. Zeolites have large variance of specific surface area. The specific surface of most of Chinese natural zeolites range from 70 to 340 m2/g (15). Natural zeolites also possess special ion exchange property due to their crystal structure (16). Differing from the regular structure of silicate, a few crystal lattices of zeolite are occupied by aluminium ions, so an additional surplus charge is generated. The charge is balanced by ions of alkali or alkaline-earth metals, which are reversibly fixed in the cavities of the structure and can easily be exchanged by other cations. According to theirs composition, natural zeolites are of different sorts, of which clinoptilolite was regarded as the best ion-exchanger for ammonium (17 –19). The ion exchanging selectivity of clinoptilolite is as follows: Csþ . Rbþ . 2þ Kþ . NHþ . Agþ . Ba2þ . Naþ . Sr2þ . Ca2þ . Liþ . 4 . Pb 2þ 2þ 2þ Cd . Cu . Zn (16). For removing ammonium from aqueous solution using zeolite, it was suggested that both physical adsorption and ion exchange play roles (19). Physical adsorption of zeolite is essentially the same as other porous materials by dispersive force. While the process of ion exchange in the zeolite-solution system is quite similar to the physical adsorption process except that the ion exchange process is highly selective (20, 21). In the research of removing ammonium from wastewater by a zeolite, the microcosmic mechanism can be ignored and all forces of the zeolite effecting on NHþ 4 in the aqueous solution can be regarded wholly as “adsorption” (3, 18, 22 –25). To fit the data of NHþ 4 variation in the zeolite-solution system, it was feasible to use adsorption isothermal equations, for example, the Langmuir isotherm (4, 23, 25 –27), the Freundlich isotherm (22) or both isotherms (28 –30). However, for the selection of zeolite material, optimization of the operational parameters and regeneration conditions, it is better to distinguish the relationship of the ion exchange from the physical adsorption in wastewater treatment. In this study, the mechanism of the adsorption of NHþ 4 in aqueous solution by a Chinese natural zeolite was studied. A non-linear method was applied to compare two widely used isotherms, the Langmuir and Freundlich isotherms, in a mathematical fitting of the experimental data. A trial-and-error procedure was used for the non-linear method using the solver add-in with Microsoft’s spreadsheet, Microsoft Excel. In addition, an ion exchange model was presented to describe the relationship among the total metal ions in the solution, NHþ 4 removed from the solution, and ions initially released from the zeolite.
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MATERIALS AND METHODS Zeolite Material A kind of natrual zeolite, mainly clinoptilolite combined with mordenite and heulandite produced in Jinyun, Zhejiang Province, China, was used as the experimental material. Jinyun clinoptilolite ore has higher levels of Na and Ca, so it shall be categorized as Na-Ca-type or Na-type zeolite which is a rare source in China (31). In the experiment, two different sizes of the zeolite particles are screened, 1.0– 3.2 mm and 8 – 15 mm. With an electron microscope, the internal structure of the zeolite can be observed (Fig. 1). The physical properties and chemical ingredients of the zeolite are listed in Tables 1 and 2 respectively. The cavity structure of the zeolite mainly consists of mesopores and macropores, resulting in a lower specific surface area. Therefore, it is disadvantageous for physical adsorption, but advantageous for ion exchange since the diffusion resistance within the pores is reduced.
Figure 1.
Natural clinoptilolite produced in Jinyun, Zhejiang Province, China.
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D. Wen et al. Table 1. Major physical properties of the zeolite produced in Jinyun, Zhejiang Province, China Item Density Hardness Silicaon/aluminum ratio Thermal stability Specific surface area Cavity volumn Average diameter of pores
Value 2.16 3–4 4.25– 5.25 7508C 6.95 m2/g 0.0191 ml/g ˚ 112.44 A
EXPERIMENTAL METHOD Static condition was adopted as the following procedure: prepare NH4Cl solution by deionized water, in which only NHþ 4 cation exists; add zeolite, m (g), and NH4Cl solution, V (dm3), into a 250 ml flask; seal the flask and put it on an orbital shaker; set the rotation rate and temperature of the shaker and let the zeolite-solution system contact sufficiently until equilibrium is reached; examine the equilibrium concentration of NHþ 4 as well as the concentrations of other cations, i.e. Naþ, Kþ, Ca2þ, and Mg2þ in the solution. The monitoring method of NHþ 4 is by spectrophotometry for NH3-N with Shimadzu 2401 UV-VIS spectrophotometer; and method for detecting cations of Naþ, Kþ, Ca2þ, and Mg2þ is by inductively coupled plasma – atomic emission spectroscope (ICP-AES) with Leeman-Profile ICP spectrometer.
RESULTS AND DISCUSSION Adsorption Isotherm Under a static condition, we changed the initial concentration of NHþ 4 , Ci, and conducted the adsorption experiment under a constant temperature (258C). In the zeolite and ammonium brine system, the adsorption process reached a balance under the equilibrium concentration of NHþ 4 , Ce, in the solution and the equilibrium quantity of NHþ adsorbed on the zeolite, qe. A set of Ce 4 and qe data were acquired and fitted to the Langmuir and Freundlich isotherms, the widely used adsorption isotherms. Linear regression is frequently used to determine the best-fitting isotherm, however, non-linear regression is rather dependable than linear regression after several comparative studies were made (32). In the case of the non-linear method, a trialand-error procedure, which is applicable to computer operation, was
Major chemical ingredients of the zeolite produced in Jinyun, Zhejiang Province, China Chemical ingredienta (%)
Sample 1# 2# 3# a
Zeolite concentration
SiO2
TiO2
Al2O3
Fe2O3
FeO
MnO
MgO
CaO
Na2O
K2O
H2O
Ignition loss
65% 70% 71%
66.21 69.58 69.50
0.13 0.14 0.14
10.99 12.20 11.05
0.96 0.87 0.08
— 0.11 0.11
0.04 0.07 0.08
0.53 0.13 0.13
2.98 2.59 2.59
2.22 2.59 2.95
0.92 1.13 1.13
6.45 11.9 —
13.83 — 11.00
Besides, the zeolite also contains microelements such as Cu, Pb, As, Be, Zr, Ni, P, Mo, Sn, Ga, Cr, V, Yb, Y, Nb, La, etc.
Ammonium Ions Adsorption by a Chinese Natural Zeolite
Table 2.
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developed to determine the isotherm parameters using an optimization routine to maximize the coefficient of determination between the experimental data and isotherms in the solver add-in with Microsoft’s spreadsheet, Microsoft Excel. The abilities of two commonly used isotherms, the Langmuir and Freundlich isotherms, to model the equilibrium adsorption data were examined. Table 3 lists the values of the parameters and the coefficient of determinations, r2, of the two isotherms using the non-linear method. The coefficient of determinations, r2, indicates that the adsorption of NHþ 4 onto zeolite follows the Freundlich isotherm better. Figure 2 shows the nonlinear Langmuir and Freundlich isotherms with the experimental data for the adsorption of NHþ 4 onto the zeolite with two particle sizes.
Ion Exchange Model Within the zeolite-ammonium brine system, the ion exchange process can be expressed as the following chemical transfer: Z n M nþ þ nNH4þ !Z n nNH4þ þ M nþ
ð1Þ
where Z is zeolite, M is metal ions in the zeolite, for example Naþ, Ca2þ, Kþ, and Mg2þ, and n is the number of electric charge. If the ion exchange predominates over the process of adsorption in the liquid-solid system, the electric charge shall be balanced between the number of NHþ 4 adsorbed onto the zeolite and the total number of the metal ions emitted from the zeolite. By the non-linear Freundlich equation, Fig. 3 shows the relationship of the equivalent concentrations between the total metal ions emitted from the zeolite and NHþ 4 remained in the aquatic solution, Ce, with two particle sizes. It is clear that ion exchange increased with decreasing particle size of the zeolite. The total ion exchange fits the Freundlich model well, implying that the adsorption of the ammonium from the solution onto zeolite depends on ion exchange to a great extent. An ion exchange model can be set up to describe the relationship among the total emitted metal ions in the solution, NHþ 4 removed from the solution, and ions initially released from the zeolite
Table 3.
Isotherm parameters obtained by using the non-linear method Langmuir
1.0– 3.2 mm 8 –15 mm
Freundlich
qm (meq/g)
Ka (dm3/mg)
r2
1/n
KF (meq/g) (dm3/mg)1/n
r2
108 97.1
1.30 1.02
0.986 0.986
0.445 0.450
54.1 44.0
0.995 0.993
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Figure 2. Adsorption of NHþ 4 onto the zeolite fitted by the Langmuir and Freundlich isotherms.
due to re-distribution between the liquid and solid phases. There is a linear relationship with a high coefficient of determination as shown in Fig. 4. The model can be expressed as: IT ¼ IE þ IR
ð2Þ
IE ¼ PRN
ð3Þ
IT ¼ PRN þ IR
ð4Þ
where IT is the concentration of total metal ions (here refers Naþ, Kþ, Mg2þ, and Ca2þ) emitted from the zeolite, meq/dm3; IE is the metal ions exchanged from the zeolite in IT, meq/dm3; and IR is the metal ions initially released from the zeolite in IT, meq/dm3; P is an indicator constant; and RN is the concen3 tration of NHþ 4 removed from the solution, meq/dm . Table 4 lists the parameters (P and IR) of the ion exchange model. It can be seen from Fig. 4 and Table 4 that more ions released from the zeolite when the smaller particle size was used as adsorbent (intercept, IR is
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Figure 3. isotherm.
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Desorption of total metal ions from the zeolite fitted by the Freundlich
55.0 meq/dm3 for the smaller zeolite, and 7.63 meq/dm3 for the larger zeolite). This is to be expected because, for a fixed adsorbent dose, decreasing adsorbent particle size provides greater surface area so that more ions can be released from the liquid-solid interface at the initial stage. The indicator of the ion exchange potential, P, is a measure of how much ion exchange occurred þ þ 2þ between NHþ 4 in the aqueous solution and metal ions (i.e. Na , K , Mg , 2þ and Ca ) in the zeolite. In general, ion exchange shall be an equivalent process to keep electric neutralization of the system, i.e. when the adsorption reaches equilibrium, P shall be 1 if ion exchange is the sole adsorption mechanism for the zeolite effecting on aquatic NHþ 4 . This is confirmed by the case of the smaller particle size (dp ¼ 1.0 – 3.2 mm, P ¼ 1.03). But in the case of the larger particle size, ions in the zeolite exchanged less with NHþ 4 in the solution (dp ¼ 8.0– 15 mm, P ¼ 0.916). The reason shall not be attributed to physical adsorption of NHþ 4 since no physical adsorption mechanism occurred even in the smaller particle size of the same zeolite. It is also due to different particle sizes of the adsorbent that affected the ion exchange potential.
Ammonium Ions Adsorption by a Chinese Natural Zeolite
Figure 4.
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Ion exchange model for the zeolite-ammonium brine system.
Adsorption is a dual-rate process comprised of two phases, i.e. the rapid diffusion phase and the slow diffusion phase (rate-restricted phase). For a larger particle size, after the rapid diffusion phase, the ions in the zeolite encounter much greater resistance for further diffusing from the pore-canals of the crystal. When the adsorption does not reach the ultimate equilibrium, less ions have been diffused from the larger particle size of the zeolite while equilibrated quantity of NHþ 4 have already been adsorbed onto the zeolite as the result. But if more time is allowed the, P value should be improved to approach 1 for the larger particle size of the zeolite. Analyzing the possibility of the adsorption mechanism in the zeoliteammonium brine system, it can be predicted from the ion exchange model
Table 4.
Parameters of the ion exchange model
dp mm
P
IR (meq/dm3)
r2
1.0– 3.2 8 – 15
1.03 0.916
55.0 7.63
0.989 0.984
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D. Wen et al. Analysis of the zeolite adsorption mechanism by the ion exchange model
Model equation: IT ¼ PRN þ IR
Type of adsorption (when equilibrium is reached)
Values of P
P.1 P ¼1 0,P,1 P¼0
(impossible) Ion exchange only Ion exchange plus physical adsorption Non-ion exchange, only physical adsorption
Values of IR
IR , 0
Used zeolite releases NHþ 4 (often happens in the desorption or regeneration of zeolite) No ions release from zeolite (reaches ultimate equilibrium) Fresh or unbalanced zeolite releases metal ions (often happens in the adsorption of zeolite)
IR ¼ 0 IR . 0
as shown in Table 5. For the natural zeolite produced in Junyun, Zhejiang Province, the adsorption of ammonium from the aquatic solution is predominated by ion exchange, and physical adsorption has little influence on the process. For distinguishing the contribution from different metal ions in the total ion exchange, Fig. 5 describes the case of the smaller zeolite particle showing equivalent concentrations of Mg2þ, Ca2þ, Kþ, and Naþ, respectively under various equilibrium concentrations of NHþ 4 , and Fig. 6 describes the case of the larger zeolite particle. The cases of Mg2þ and Kþ are similar for both particle sizes. Concentration of Mg2þ is the lowest and has little emission from the zeolite in equilibrium. Thus Mg2þ is hardly involved in the ion exchange. Concentration of Kþ is lower in the solution, but increases a little in equilibrium. The reason shall be the re-distribution of Kþ between the liquid and solid phases instead of ion exchange because Kþ is prior to NHþ 4 in the ion exchange sequence of clinoptilolite (16). Ion exchange occurs mainly between Naþ and Ca2þ in the zeolite and NHþ 4 in the aquatic solution. In the cases of Naþ and Ca2þ in Figs. 5 and 6, it is found that the ion exchange is affected by different particle sizes. For the smaller zeolite particle, exchanging-degree is approximately similar for Naþ and þ Ca2þ replaced by NHþ 4 . However, the tendency indicates that Na is prior to þ be exchanged at lower concentration of NH4 (Ce , 80 meq/dm3), while more Ca2þ is selected for ion exchange at higher concentration of NHþ 4 (Ce . 80 meq/dm3). For the larger zeolite particle, Ca2þ is much more preferable to be exchanged when Ce is more than 15 meq/dm3. The ion size of Naþ is smaller than that of Ca2þ, so Naþ is easier to emit from zeolite, especially from the zeolite with smaller particle size under lower concentration difference, while higher concentration difference is necessary for Ca2þ emitting from the porecanals of zeolite. However, calcium ion has two electric charges whereas
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Figure 5. Equivalent concentrations of different metal ions under various equilibrium concentrations of NHþ 4 for the smaller zeolite particle (dp ¼ 1.0 – 3.2 mm).
sodium ion has only one electric charge, the equivalent concentration of Ca2þ increases rapidly with the increasing concentration pressure, and will definitely exceed the equivalent concentration of Naþ in the solution.
CONCLUSIONS For a Chinese natural zeolite, the mechanism of its adsorption of NHþ 4 from aqueous solution was studied. Based on the experimental data of either the adsorbed ammonium ion or the total emitted metal ions, the adsorption fits the Freundlich isotherm better. An ion exchange model, describing the relationship among the total metal ions in the solution, NHþ 4 removed from the solution, and ions initially released from the zeolite, can reveal the adsorption mechanism. The parameter of the ion exchange potential, P, is about 1, indicating that the adsorption of ammonium from the aquatic solution by the natural zeolite is predominated by ion exchange whereas physical adsorption has little influence. More ions are released from the zeolite with the
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Figure 6. Equivalent concentrations of different metal ions under various equilibrium concentrations of NHþ 4 for the larger zeolite particle (dp ¼ 8 – 15 mm).
decreasing of the particle size at the initial stage, as reflected from the parameter IR. Tracking the different ions, ion exchange mainly occurs þ between Naþ and Ca2þ in the zeolite and NHþ 4 in the aquatic solution. Na 2þ is prior to be exchanged at lower concentration difference, but Ca is much more preferable to be exchanged under higher pressure of concentration difference. Mg2þ is hardly involved in the ion exchange and Kþ is released from the zeolite by the re-distribution between the liquid and solid phases.
ACKNOWLEDGEMENTS This study is a part of work of the Project, Technology of Non-point Source Pollution Control in the Dianchi Watershed (Approval Number: K99-05-3502), financially supported by the Chinese Ministry of Science & Technology. The authors thank Dr. Wenqi Li, Dr. Jun Wang, Dr. Weizhong Wu, and Dr. Xi Zhang, who were involved in the project and helped a lot in the laboratory work.
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