STUDIES OF MODIFICATION OF ZEOLITE BY TANDEM ACID-BASE TREATMENTS AND ITS ADSORPTIONS PERFORMANCE TOWARDS THORIUM
Gustri Nurliati1*, Yuni K. Krisnandi2, Riwandi Sihombing2, Zainus Salimin1
1Center of Radioactive Waste Management-National Atomic Agency, Serpong 15310, Indonesia
2Department of Chemistry -University of Indonesia, Depok 16424, Indonesia
A R T I C L E I N F O A B S T R A C T AIJ use only:
Received date Revised date Accepted date Keywords:
Natural Zeolite
Tandem Acid-Base Treatments Mesopore Materials
Thorium Adsorption
Hierarchical zeolite was prepared from natural zeolite using tandem acid-base treatments and applied as adsorbent in removal Th(IV). Natural zeolite occurred naturally to have micropore size, was modified with two familiar methods that mostly used to change its micropore size into hierarchical pores in which are dealumination and desilication. Extensive characterization of both natural and modified zeolite were conducted using XRD, BET, SEM-EDS, AAS. XRD Pattern of Raw Zeolite, Pre-treated Zeolite, NaZ, ZA1, ZA2, and ZA2B shows that the process to modify this material has not changed the crystallinity characteristic of this material. TheSi/Al ratio is increased from 6.688 to 11.401 for NaZ and ZA2B respectively. Surface area is increased from 125.4 m2/g (NaZ) to216.8 m2/g (ZA2B).
Application of these material as adsorbent were carried out using solution of 50 ppm Th4+. The UV-Vis result shows the modified zeolite (c.a. 10 mg) has higher adsorption capacity than the natural zeolite. The adsorption process is fit into Freundlich isotherm and the adsorption capacity of this material increase from 909 mg/g to 2000 mg/g for NaZ and ZA2B respectively.
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INTRODUCTION
In Indonesia a large amount of natural zeolite can be found easily. Zeolite is an aluminosilicate material which has three dimensional frameworks consists of SiO4 and AlO4 tetrahedra arranged so that each oxygen atom is shared between two tetrahedral [1]. The replacement of Si4+ with Al3+ in tetrahedral structure creates negative charge and counter balanced by exchangeable cations (e.g., Na+, K+, Ca2+, Mg2+) [2].
Zeolite structure consists of water moleculs and counter balanced cations that filled in cavities and chanels with dimensions from 0,2 to 1 nm [3].
Small ions and moleculs can pass through these channels but large ions and moleculs are exclude.
Due to its microporosity and relatively high surface area natural zeolites have been.widely used as
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adsorbents, ion exchanger, catalysts, and separation media.
Despite its wide applications, natural zeolite has limitations due to (1) undesired impurity in its structure, (2) its properties did not optimized by nature [3].
These limitation can be overcome by modification of natural zeolite. Dealumination and desilication are commonly employed to change the properties of natural zeolite such as Si/Al ratio, and pore size.
Dealumination is a process that can remove framework aluminum atoms without destroying the micropore structure. It can be achieved by hydrolysis of the Al-O-Si bonds by two common methods which are thermal treatment (commonly by steaming) [4] or acid leaching [5]. Dealumination change Si/Al ratio in zeolite, hence affected the surface and acidic properties of zeolite [6].
Desilication process follow the same pattern as
dealumination such as type of lattice defects and mesopore formation. The difference lies in the used of alkaline solution for leaching method [7-9].
Desilication can introduce mesoporosity to zeolite through alkaline treatment [10-12]. J C. groen in Huang (2014) suggested for mordenite with optimal Si/Al ratios (20–30), mesoporosity was introduced after alkaline treatment (0.2 M NaOH at 65ºC for 30 min).
Recently, combination of dealumination and desilication have been done in order to modified zeolite through mesopore formation [13-15]. Van Laak (2010) found that sequential acid-alkaline treatments of mordenite zeolites (Si/Al = 8–15) are effective for the mesopore formation. Christine has done modification of zeolite by combining acid-base treatments in introducing mesoporosity to the natural zeolite from Lampung-Indonesia.
Research to introduce mesoporosity to natural zeolite by combination of acid-base treatments is hardly found. So the aim of this research is to modified natural zeolite from Bayat- Klaten by tandem acid-base treatments and study the effect of modification on thorium adsorption.
Thorium is naturally occurring radioactive element widely distribute over the earth’s crust which has half life 1,39 x 1010 years. Thorium has been extensively used in various application such as industry light bulb elements, lantern mantles, welding electrodes and heat-resistant ceramics. Due to its stability at ambient temperature, direct toxicity of thorium is low. However when living organisms exposed by thorium nitrate, thorium precipitates in a hydroxide form and mainly localized in liver, spleen and marrow [1].
EXPERIMENTAL METHOD 2.1. Modification of Natural Zeolite
Modification of natural zeolite from Bayat- Klaten consists of physical activation, pre-treatment, and post modification by tandem acid-base treatments.
Activation of natural zeolite were performed by washing the zeolite with demineralized water (1:3 w/v) under stirring for 3 hours. The solid phase were dried at 300ºC.The aim of physical activation is to remove water moleculs from its voids and open the active sites of zeolite.
The pre-treatment process were conducted following Ming and Dickinson [2]. The activated natural zeolite were treated with 1 M NaOAc buffered to pH 5, 30% H202, and dithionite-citrate- bicarbonate to remove free carbonates, organic matter, and free iron oxides, respectively. After pre-
treatment, zeolite were converted into the sodium homoionic form using 0,5 M NaCl solution (10 g zeolite/100 ml solution) at 80ºC under stirring for 2x8 hours.
Post modification started with dealumination process were conducted by stirring Na-zeolite in 0,6 M HCl solution (10 g zeolite/100 ml solution) at 100ºC for 2 hours (under reflux condition). The solid phase were washed with demineralized water and dried at 65ºC. Desilication were conducted by stirring zeolite in 0,2 M NaOH solution (3,3 g zeolite/100 ml solution) at 65ºC for 30 minutes. Characterization of modificationed zeolite were conducted with XRD, FTIR, AAS, and BET.
2.2. Adsorption
Kinetic studies of adsorption were perform with 50 ppm Th (IV) solution. 0,1 g zeolite (NaZ and ZA2B each) were mixed with 10 ml solution and placed in shaker in different time interval. The adsorbent was finnaly removed by centrifugation and filtration, and thorium concentration was determined with UV-Vis spectrometer. The isotherm studies were conducted with the same process as in the case of kinetics by varying concentration of thorium solution in the range of 5- 100 ppm and shaked for optimum adsorption time obtained from kinetics.
RESULTS AND DISCUSSION
The natural zeolite and its derived sample modified with 0.6 M HCl, 0,2 M NaOH and tandem acid-base treatments were characterized with an X- ray diffraction (XRD). This analysist shows that the main compositions of natural zeolite from Bayat- Klaten are mordenite and heulandite (Fig. 3.1), and acid/base treatments or combination of both treatments does not affect the phases of the zeolite sample. Further characterization with Energy dispersive X-Ray Spectroscopy (EDS) (Table 3.1) shows that the dominant exchangeable cation in natural zeolite structure from Bayat-klaten is Ca. It shows that natural zeolite from Bayat-Klaten is Ca- Mordenite and Ca-Heulandite type.
Figure. 3.1. XRD spectrum of raw natural zeolite and modified zeolite from Bayat-Klaten
Table 3.1.Characterization of raw natural zeolite from Bayat- Klaten using EDS
Element C O Na Mg Al Si K
%mass 9.52 53.10 0.86 0.52 5.95 25.72 0.57
Post Modification Treatments
In this experiment, acid treatment causes dealumination of the zeolites, in which Al-O bonds are weakened by protons attack causing skeletal vacancies and defects. The vacancies and defects enlarge the pore mouth of the zeolite, increasing the surface area and adsorption ability [16].
The suggested reaction in dealumination process showed in equation (1) [17].
Characterization using FTIR shows stretching vibration of silanol groups at 3750 cm-1, OH stretching band at 3540-3650cm-1. Characteristic lattice vibrations of the zeolitic structure can be distinguished from these band: H- O-H bending band at 1630 cm-1, asymmetric stretching vibrations band at 950-1025 cm-1 and 1050-1250 cm-1, symmetric stretching vibrations bands at 750-820 cm-1 and 650-750 cm-1, double ring vibrations at 500-650 cm-1, T-O bending vibrations at 420-500 cm-1, and pore opening.vibrationsat 300-420 cm-1 [18, 19].
Figure 3.2.FTIR spectrum of natural zeolite and its modified form
Dealumination process causing wave number shifting in asymmetric stretching of the tetrahedral atoms band to the higher number. Figure 3.2 shows asymmetric stretching increase from 1048,72 cm-1and 1054,84 cm-1 to 1065,04 cm-1 for raw zeolite, NaZ, and dealuminated zeolite (ZA1) respectively. This shifting is caused by the leaching of Al from zeolite framework and change the bond strength and the Si-O-Si angle [20].
Figure. 3.3. Wave number shifting in natural zeolite and its modified form
Dealumination process increase the Si/Al ratio of modified zeolite. Characterization of natural zeolite and its modified form using Atomic Absorption Spectroscopy (Table 3.2) shows that Si/Al ratio after the first dealumination increase from 6.688 to 11.031 for NaZ and ZA1 respectively.
The second dealumination also increase the Si/Al ratio to 14.265 for ZA2.
Next step is alkaline treatments of natural zeolite. The aim of this treatments is to leach Si atom from zeolite structure and introduce mesoporousity. Alkaline treatments increase % mass of Si, but overall decrease Si/Al ratio of dealuminates natural zeolite (from 14.265 to 11.401
for ZA1 and ZA2B respectively and from 6.688 to 4.4715 for NaZ and ZB1 respectively).
Structural changes caused by desilication can be observed by FTIR (Fig. 3.2). The intensity of silanol band characteristic (at 3500 cm-1) decreased in desilicated zeolite. This IR characteristic known as defect sites in zeolite , so it can be deduced that alkaline attack mostly to this incurred defect sites.
Table 3.2. Si/Al ratio of
natural zeolite and its modified form analysist using AAS
Zeolit % massa
Na
% massa Si
% massa Al
Si/Al
Raw zeolite 0.015 0.791 0.071 11.164
Z-pre
treatment 0.019 0.632 0.068 9.260
Na-Za 0.037 0.730 0.109 6.688
ZA1b 0.039 0.762 0.069 11.031
ZA2c 0.012 0.723 0.051 14.265
ZA2Bd 0.016 0.741 0.065 11.401
ZB1e 0.040 0.715 0.152 4.715
asodium homoionic form of zeolite, bfirst dealuminated zeolite,
csecond dealuminated zeolite, dtandem acid-base treated zeolite,
ebase treated zeolite
The surface area, isotherm adsorption and pore distribution of zeolite were determined by Brunauer-Emmett-Teller (BET) method. Table 3.3 shows the enhancement of zeolite’s surface area from 125.4 mg2g-1 (NaZ) to 138.0 and 216.8 mg2g-1 for alkaline treated zeolite, ZB1 and ZA2B respectively. It indicated that alkaline treatments can introduce mesoporousity in natural zeolite.
Table 3.3.
BET characterization of raw zeolite and its modified treated forms
Zeolit Sexta
(m2g-1) Vmicrob
(cc g-1) Vtotalc (cc g-1)
Raw zeolite 140.0 0.05934 0.1357
NaZ 125.4 0.04642 0.1109
ZA1 120.6 0.04864 0.1395
ZB1 138.0 0.05773 0.1326
ZA2B 216.8 0.09795 0.1804
aMultipoint BET
bt-method
cat P/Po = 0.992594
dVtotal-Vmicro
eDA method pore radius (mode)
The N2 isotherm adsorption of zeolite and its modified zeolite forms in Figure 3.4 shows that there is hysterist loop in the isotherm adsoption curve of desilicated zeolite ZA2B and ZB1 (Fig. 3.4 d and e) . The curve is following IUPAC isothermal curve type four, which indicated there are kapiler condensation in mesopourous material [21]
a Raw zeolite, b. NaZ, c. ZA1, d. ZB1, e. ZA2B ( ):adsorption ( ): desorption
Figure 3.4. Nitrogen isotherm adsorption curve of natural zeolite and its modified forms
Barret-Joyner-Halenda (BJH) analysist shows us the pore distribution in zeolite. Figure 3.5 a shows that pore distribution in raw zeolite are in diameter < 2 nm as well as in NaZ (Fig. 3.5.b), ZA1 (Fig. 3.5.c) and ZB1 (Fig. 3.5.d). While in ZA2B (Fig. 3.5.e and enlarge in Fig. 3.6), pore distribution lies in d < 2 nm and 2-8 nm. The existence of 2-8 nm pore indicated that tandem acid-base treatments can introduce mesoporousity in zeolite.
a. Raw zeolite, b. NaZ, c. ZA1, d. ZB1, e. ZA2B Figure3.5. BJH desorption curve of natural zeolite and its modified forms
a b
c d
e
a b
c d
e
Figure 3.6. BJH desorption curve of ZA2B
Base on the above description, it can be deduced that introduction of mesoporousity of natural zeolite from Bayat-Klaten has been done successfully. Next step is adsorption of thorium (IV) using sodium homoionic form of zeolite (NAZ) compare to modified zeolite that has the largest surface area which is acid-base treated zeolite (ZA2B).
Adsoption Experiments
The aim of this experiment was to determined optimum adsorption time and adsoption capacity of natural zeolite. UV-vis spectrometer analysist (Fig. 3.7) shows that the optimum time for adsorption 50 ppm Th(NO3)4 is 120 minutes for NaZ as well as ZA2B.
Figure 3.7.UV-vis spectrometer analysist for determination of optimum contact time in adsorption of Th(IV) by NaZ ( ) ZA2B ( )
Determination of adsorption capacity of zeolite was conducted by varying concentration of thorium and contacted with zeolite for 120 minutes.
Figure 3.8 shows that adsorption capacity of acid- base treated zeolite (ZA2B) is higher than natrium homo ionic form (NAZ)
Figure 3.8. UV-Vis characteristic of Th(IV) adsorption by NaZ ( ) and ZA2B ( )
To determined adsorption capacity quantitatively, the isotherm data are fitted in different isotherm adsorption models. Adsorption isotherm describes the equilibrium of liquid adsorption on solid surface. Two commonly
adsorption isotherm models on solid surface are Langmuir and Freundlich. The Langmuir equation shown in eq (2) [22]:
Ceqis ion concentration in equilibrium solution (mg/L or mmol/L), q is adsorption per gram of adsorbent which is obtained be dividing the amount of adsorbate by the weight of the adsorbent (mg/g or mmol/g) and qm is adsorption capacity (mg/g or mmol/L). Hence, if a graph of Ceq/q is plotted against Ceq, it will be a straight line, and adsorption capacity is 1/slope.
Freundlich equation shown in eq (3):
n is a constant value related to adsorption energy. K is a constant value related to adsorbent capacity.
Figure 3.9.Linearization of Langmuir isotherm (above) and Freundlich isotherm (below) NaZ ( ) and ZA2B ( )
Figure 3.9 shows that R-square value of both Langmuir and Freundlich isotherm model are far from the expected value (c.a. 1). It means that the adsorption of Thorium in zeolite doesn’t fitted in both isotherm models.
To determined adsorption capacity, we use equation 4 that is modified from equations in Langmuir and Freundlich isotherm models [22].
If Ceq1/n/q is plotted against Ceq1/n it will be a straigth line, hence the adsorption capacity is 1/slope.
micropore
(Eq.4) (Eq.3)
Mesopore
(Eq.2)
(Eq.3)
(Eq.4)
Figure 3.10. Langmuir-Freundlich isotherm adsorption NaZ ( ) and ZA2B ( )
Adsorption capacity of natural zeolite (NaZ and ZA2B) in Table 3.4 shows that tandem acid- base treatments of natural zeolite from Bayat-Klaten can improve its adsorption capacity c.a. 120 % compared to its homoionic form (NaZ).
Table 3.4. Adsorption capacity of thorium
using natural zeolite
Zeolite* Slope Adsorption capacity
(mg/g)
NaZ 0,0011 909
ZA2B 0,0005 2000
NaZ: homoionic form zeolite
ZA2B: tandem acid-base treated zeolite
CONCLUSION
Tandem acid-base treatments improves adsorption capacity of natural zeolite from Bayat- Klaten through introduction of mesoporousity in zeolite framework.
Adsorption capacity of acid-base treatments zeolite (ZA2B) is 120% higher than Natrium homoionic form (NaZ).
ACKNOWLEDGMENT
We thank Mr. Bambang Sugeng, Dr.
Adel Fisli and Mr. Sugeng Purnomo S.ST for help in XRD, BET, and AAS analysists respec- tively.
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