Applied Clay Science 61 (2012) 8–13
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Preparation of analcime from local kaolin and rice husk ash A.Y. Atta a, B.Y. Jibril a,⁎, B.O. Aderemi b, S.S. Adeﬁla c a b c
Petroleum and Chemical Engineering Department, Sultan Qaboos University, PO Box 33, Al Khoud, PC 123, Muscat, Oman Chemical Engineering Department, Ahmadu Bello University, Zaria, Nigeria Department of Petroleum Engineering, Covenant University, Ota, Ogun State, Nigeria
a r t i c l e
i n f o
Article history: Received 11 November 2010 Received in revised form 23 February 2012 Accepted 27 February 2012 Available online 24 April 2012 Keywords: Zeolite Analcime Rice husk Kaolin Phenol Adsorption
a b s t r a c t Analcime zeolite was synthesized using a hydrothermal technique with rice husk ash and metakaolin as sources of silica and alumina respectively. Both the raw materials and the ﬁnal product were characterized using XRD, FTIR, SEM and atomic absorption spectroscopy. XRD diffractogram of the rice husk exhibited αquartz as a dominant and critobalite as a minor component. Metakaolin consisted of alumina, with minor amounts of α-quartz. Analcime was obtained after 72 h aging and 24 h reaction time at temperature of 180 °C. Longer reaction time led to a decrease in the amount of analcime phase with corresponding increase in zeolite-p. SEM of the analcime crystals showed formation of trapezohedral morphology of sizes ranging from 15 to 25 μm. The analcime was tested in the adsorption of phenol from its aqueous solutions (50–400 ppm). Among the adsorption isotherms tested, the Langmuir model was found to give better representation of the data. The ultimate amount of phenol adsorbed on analcime (12.5 mg/g) was lower compared with a commercial zeolite (33.1 mg/g) used as reference. However, in terms of adsorption per surface area, the analcime (3.88 mg/m2) exhibited better performance than the other sample (0.01 mg/m2). The value of the constant related to free energy of adsorption in the Langmuir model was used to evaluate a dimensionless equilibrium parameter, RL. At high phenol concentrations (100–400 ppm), the RL values for the commercial sample (0.73–0.40) were higher than for analcime (0.50–0.20). The results show that the readily available raw materials could be used to prepare analcime zeolite with a good and favourable adsorptive capacity. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Zeolites are three-dimensional crystalline microporous solids with corner-sharing AlO4 and SiO4 tetrahedra forming frameworks that lead to channels and cages in the structures. The accessibility of different compounds to these structures and adsorption on the surface make it possible to use them in processes such as petroleum upgrading and pollutants removal (Chen et al., 2009). The present energy challenges and increase in demand for certain chemicals has increased the demand for zeolites. This has spurred research interest towards improving the existing zeolites, synthesizing new ones and employing them in new applications. Analcime is a form of zeolite with irregular channels. It is found in nature. However, abundant supplies of the natural analcime are distributed in limited regions of the world. Therefore, this motivates recent research effort towards synthesizing it using different local sources of silica and alumina (Chandrasekhar and Premada, 1999; Hegazy et al., 2010; Liu et al., 2005). In a recent report, its possible applications in catalysis to
⁎ Corresponding author. Tel.: + 968 24142582; fax: + 968 24141320. E-mail address: [email protected]
(B.Y. Jibril). 0169-1317/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.clay.2012.02.018
improve diffusion and development of nanoelectronics were demonstrated (Liu et al., 2005). Traditionally, zeolites are synthesized from sodium silicate and aluminate (Barrer, 1982; Breck, 1974). However, other aluminosilicate rich materials such as kaolin, rice husk ash and ﬂy ash have also been used (Chareonpanich et al., 2004; Vempati et al., 2006; Wang et al., 2007). Rice husk and ﬂy ash have the added advantage of being inexpensive and utilizing them as raw materials alleviates environmental pollution challenges. Many researchers have concluded that rice husk is an excellent source of high-grade amorphous silica (Katsuki et al., 2005). It is usually produced from rice husk by chemical leaching and carbon burn off processes. The silica obtained after carbon burn off is commonly referred to as rice husk ash (RHA). Kaolin clays are alumina-silicate minerals with crystal structure comprised of weakly bound layers. The absence of strong layer–layer binding facilitates easy cleavage. Highly reactive kaolin phase, metakaolin, was reported to have been produced at 900 °C and used for synthesis of pure phase zeolites (Chandrasekhar and Premada, 1999). In other reports, analcime was synthesized by direct hydrothermal route using silica of chrysotile and rice husk ash as sources of silica (Petkowicz et al., 2008). The controlled extraction of silica from rice husk ash was used for the synthesis of zeolites such as analcime
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(Hamdan et al., 1997). A successful synthesis of analcime framework by conventional hydrothermal alkaline activation of natural clinker was also reported (Sandoval et al., 2009). In this study, we report a hydrothermal synthesis of analcime using rice husk ash and metakaolin as sources of silica and alumina respectively. This attempt explores the feasibility of preparing the zeolite from locally available resources and alleviating the environmental challenges associated with a large supply of rice husks waste. The effects of reactants aging, alkalinity and reactions times were investigated. A sample of the product was tested in adsorption of phenol from its aqueous solution. Its performance was compared with that of a commonly used commercial zeolite.
2.1. Raw materials preparation Rice husks obtained from a rice milling factory in Zaria, Nigeria, were thoroughly washed and dried at 90 °C for 3 days. About 105 g of the dried sample of rice husks was calcined at 800 °C for 3 h. About 14.8 g of the resultant rice husk ash (RHA) was dissolved in NaOH (4 M) followed by reﬂuxing at 90 °C. Precipitate was formed when concentrated HCl was added to the dissolved RHA. This was subsequently ﬁltered, washed and ﬁnally dried at 120 °C for 8 h. The kaolin was collected from Kankara, Nigeria and dried at 90 °C for 4 days. Dry beneﬁciation technique was used in processing of the raw kaolin into ﬁne form. The kaolin was screened using a 45 μm British Standard Speciﬁcation (B.S.S.) sieve. The − 45-μm fraction was calcined at 900 °C for 3 h.
2.2. Zeolite synthesis In the zeolite synthesis recipe (for example 1.3Na2O:Al2O3:7SiO2:132H2O), the kaolin was used as the only source of alumina and partly of silica while RHA was used to make up for the silica. The experiment was performed in a stainless steel autoclave. After the completion of the reaction at 180 °C, the product was cooled to room temperature, ﬁltered and washed with distilled water. The product was dried at 120 °C for 8 h. The studies were carried out by varying the reaction time (12, 24, 48, 72 and 96 h). Aging was done for 24 and 72 h. The effect of alkalinity was also explored by varying the Na2O/Al2O3 ratios from 1.3 to 5.2.
Fig. 2. FTIR spectra of rice husk ash and metakaolin.
2.3. Characterization Elemental composition of the samples was determined using atomic absorption spectroscopy (ASS) and ﬂame photometer. Prior to the analyses, the samples were fused with sodium carbonate at high temperatures (1000 °C) in a Mufﬂe Furnace. The bulk structures of the samples were examined using Philips Analytical X-ray diffractometer (PW1710). The X-ray diffraction (XRD) patterns were recorded from 3 to 70° at a step size of 0.020° using Cu radiation. Fourier transmission infrared (FTIR) spectra were collected using Perkin Elmer 2000 using Potassium Bromide (KBr) as the diluent and binder. A sample/KBr ratio of 1:5 was used to form pellets for the analysis. The intensity and the corresponding wavelength of resulting infrared adsorption by the samples were measured. The morphology of the samples was examined using SEM model: JOEL-2010.
2.4. Adsorption tests The adsorption studies were conducted in 50 ml polypropylene containers. A measured amount (0.2 g) of the synthesized analcime or commercial ZSM-5 was added to a measured amount (20 ml) of aqueous solution of phenol of different initial concentrations (50–400 ppm). The covered containers were continuously agitated at 240 rpm at room temperature (25 ± 1 °C). The concentrations of the adsorbents were monitored with time: initially rapidly (5, 10, 15, 30 min) and subsequently slowly (1, 2 and 5 h) until equilibrium was reached. In each case, 1 ml of the solution was withdrawn. The concentrations were determined using an ultraviolet–visible spectrometer (Lambda 25, PerkinElmer) at 268.4 nm absorbance.
Table 1 Elemental analysis (in wt.%) of the raw materials and the synthesized analcime zeolite.
Fig. 1. XRD patterns of rice husk ash and metakaolin.
Rice husk ash
SiO2 Al2O3 Fe2O3 CaO MgO Na2O K2O LOI
93.0 1.9 1.1 trace 1.5 0.9 0.2 1.3
50.9 44.8 0.4 0.1 0.9 1.6 0.2 0.9
56.7 24.1 0.9 0.02 0.3 9.3 0.1 8.5
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Fig. 3. XRD patterns showing the effect of time on As-synthesised zeolite.
3. Results and discussions 3.1. Samples characterizations The XRD patterns of the reactants—metakaolin and rice husk ash— are shown in Fig. 1. The metakaolin diffraction pattern shows a broad peak with few low intensity peak at the 2θ angle of 26.64 assigned to α-quartz (Si3O6). This indicates that the crystalline kaolin was transformed to amorphous metakaolin at 900 °C with traces of α-quartz present. The amorphous metakaolin phase is known to be more reactive than crystalline kaolin, as reported earlier (Chandrasekhar and Premada, 1999). The XRD pattern of rice husk ash (obtained at 800 °C) suggests peaks peculiar to α-quartz and cristobalite (SiO2) phases of silica at 2θ angles of 26.64 and 21.84 respectively. An earlier study reported cristobalite and tridymite phases at a higher reaction temperature (Kordatos et al., 2008). The FTIR spectra of the samples are shown in Fig. 2. Peaks at 1214 cm− 1 (shoulder) and 1116 cm− 1 are assigned to Si asymmetrical
stretching. The peak at 798 cm− 1 is due to Si symmetrical stretching bands for the rice husk ash, as observed earlier (Vempati et al., 2006). Similarly, broad bands at 1148 and 816 cm− 1 associated with the tetrahedral structure are observed in the spectra of the metakaolin. The elemental analyses of the rice husk ash and metakaolin as raw materials are presented in Table 1. After heat treatment at 800 °C for 3 h, the results suggest rice husk ash as a rich source of silica. The high silica content corroborates the predominant silica phase observed in the XRD result. Metakaolin shows a silica-alumina ratio of about 2 with a very low composition (less of 2%) of other oxides observed. Fig. 3 shows the XRD pattern of the as-synthesised zeolite samples (1.3Na2O:Al2O3:7SiO2:132H2O) using rice husk ash and metakaolin for different reaction times (12, 24, 48, 72, 96 hr), at reaction temperature of 180 °C and without aging. At about 12 h of reaction time, analcime and zeolite-p were identiﬁed from the diffraction pattern (Baerlocher et al., 2007). The peculiar peaks of analcime were reduced in intensity with the increase in reaction time up to 24 h, with corresponding increase in the amounts of zeolite-p
Fig. 4. XRD patterns of the effect of aging and alkaline concentration on As-synthesised zeolite [Z1 = Not aged, Z2 = Aged for 3 days, Z3 = Z2, with alkalinity changed].
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Fig. 7. Adsorption isotherms of phenol in analcime and ZSM-5. Fig. 5. FTIR spectrum of analcime and zeolite P phases.
(Liu et al., 2005; Robson and Lillerud, 2005). Further increase in reaction time to 72 and 98 h showed a shift to more intense peaks of zeolite-p and silica, with analcime zeolite noticed only in a trace amount. Analcime appears to be a metastable intermediate in the formation of zeolite-p. Alternatively, the differences in the source of silica (rice husk ash and kaolin) and the presence of metal oxides as impurities could have inﬂuenced formation of multiple zeolite phases. This was demonstrated by an earlier study which showed that different sources of silica led to the formation of different silica species which crystallized to different zeolites (Chandrasekhar and Premada, 2004). In an attempt to improve the phase purity and enhance the synthesis, the effects of aging and alkalinity were explored at 24 h of reaction time (Fig. 4). Without aging (sample, Z1), a signiﬁcantly greater amount of analcime phase was observed than for the samples aged for 3 days (Z2). For Z2, only the zeolite-p and silica were identiﬁed as crystalline phases (Baerlocher et al., 2007). The increase in intensity of the peaks of Z2 suggests that during the ageing process, species involved in nucleation were increased (Cundy and Cox, 2005). However, when the alkali concentration was increased (Na2O/Al2O3 from 1.3 to 2.6) with aging for 3 days, a single crystalline phase of analcime zeolite was observed (Z3) as shown in Fig. 4. The analcime zeolite phase formed indicates that the alkali (NaOH) enhanced the formation of nucleation species that involves depolymerisation of the silica from the raw material. The FTIR spectra of the as-synthesised samples Z2 and Z3 (Fig. 5) present bands peculiar to aluminosilicate materials. The result shows band at 1032 cm − 1 which is characteristic of asymmetric stretching of T\O linkage zeolites for the two samples (Sandoval et al., 2009). The band at
452 cm − 1 is associated with the T\O bending vibration of the SiO4 and AlO4. Table 1 shows the elemental analysis of the analcime (Z3). The silica-alumina mole ratio of the zeolite is about 4.0 which is similar to 4.1 for the synthetic analcime ﬁbers reported by Liu et al. (2005). Sodium oxide of 9.3 wt.% could be attributed to sodium ion required to balance the resulting net negative charge on the framework aluminium in the zeolite structure. Theoretically, about 14.6 wt.% of sodium ions (Robson and Lillerud, 2005) would be required to balance the net negative charge of the alumino-silicate framework structure. This balance (of 14.6 minus 9.3 wt.% of sodium ion) was most probably compensated by the other metal ions (Fe, Ca, Mg and K), observed in the sample. These provide additional balancing effects that decrease the amounts of sodium ions otherwise required. From the SEM analysis, the analcime (Z3) shows the formation of trapezohedral morphology and range in size from 15 to 25 μm (Fig. 6A). The wide range of the average particle sizes may be associated with the presence of metallic impurities. A recent study (Hegazy et al., 2010) has shown that additions of vanadium or titanium into a synthesis mixture in preparation of zeolite led to structural defects in the analcime produced. TiO6 or VO6 octahedra displaced AlO4 tetrahedra in the lattice structure. Incorporation of vanadium (as VO6) exhibited a signiﬁcant decrease in the crystal size. Thus, the impurities associated with the rice husk ask and kaolin may lead to the wide variation in the particle sizes as observed in the SEM. Some of the crystals are clean facetted but intergrowths and fractures were observed in others (Fig. 6B). Similar trapezohedral morphology of analcime was reported in hydrothermal synthesis using clinker (Sandoval et al., 2009). Using hydrothermal treatment of artiﬁcial glasses, it was demonstrated that the analcime crystal system varied from orthorhombic,
Fig. 6. SEM micrographs of Z3: A (scale bar 50 μm) and B (scale bar 10 μm).
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tetragonal to cubic symmetry at varying temperatures (Ghobarkar and Schaf, 1999). The nature of the silica sources and the impurities present were also reported to have signiﬁcant effects on the ﬁnal zeolite product morphology (Mintova and Valtchev, 2002). 3.2. Phenol adsorption isotherm models A sample of the synthesised analcime was further characterized by adsorption of phenol from its aqueous solutions. The equilibrium adsorption proﬁle of the analcime was compared with that of a commercial ZSM-5 are presented in Fig. 7. The proﬁles were obtained using aqueous solutions of phenol of different concentrations, Co (50–400 ppm). The adsorption isotherms show a similar shape indicating that on both samples there were no competitive adsorptions between water and phenol molecules (Giles et al., 1960). The amount of phenol ultimately adsorbed from water was much higher on the ZSM-5 (33.1 mg/g) than on analcime (12.5 mg/g). Equilibrium adsorption isotherms are useful in understanding sorption mechanisms and capacities. Therefore, for further comparison, the equilibrium data were modelled using two commonly used isotherm models. Its concentrations and the amount adsorbed at equilibrium Ce and qe were analyzed using the Langmuir isotherm: qe ¼
qm K L C e 1 þ K LCe
Isotherm Langmuir-1 KL (l mg− 1) qm (mg g− 1) r2 Langmuir-2 KL (l mg− 1) qm (mg g− 1) r2 Langmuir-3 KL (l mg− 1) qm (mg g− 1) r2 Langmuir-3 KL (l mg− 1) qm (mg g− 1) r2 Langmuir-3 KL (l mg− 1) qm (mg g− 1) r2 Freundlich n KF (mg1−1/n l1/n g−1) r2
0.00010 671.141 0.809
0.00504 285.714 0.996
0.05470 18.282 0.962
0.00290 60.241 0.993
0.00995 13.992 0.978
0.00375 51.045 0.995
0.00245 32.992 0.915
0.00038 26.745 0.941
0.00157 40.828 0.887
0.00042 340.024 0.951
1.244 0.137 0.861
1.967 0.2569 0.961
and the Freundlich isotherm: 1
qe ¼ K F C ne
The Langmuir isotherm was derived on the assumption of monolayer adsorption on a ﬁnite number of binding sites of uniform adsorption energies, with no transmigration on the surface. The KL or Langmuir constant is determined by the probability of adsorbate sticking on adsorbent and exponentially proportional to the heat of adsorption (Q). The Freundlich isotherm assumed an exponentially decaying function of site density with respect to Q (Yang, 2003). The values of constants may be evaluated using linear forms of the isotherm models. While the Freundlich isotherm is commonly linearized in one form, the Langmuir model can have different forms, as shown in Table 2. It has been demonstrated that a particular form used for the analysis is important due to wide differences in the errors associated with each form as studied in details recently (Hamdaoui and Naffrechoux, 2007). Using linear forms of the isotherms, the values of their adsorption parameters were obtained (Table 3). For both ZSM-5 and analcime, the Langmuir models give better representation of the data. This suggests that the adsorptive sites in the samples may be uniform and a single layer of phenol dominantly covers the sites. Again, for both samples, the Langmuir model 3 gives the best correlation. From the model, the maximum adsorption capacity of ZSM-5 (51.0 mg/g) is about three times that of analcime (14.0 mg/g) similar to the experimental observations. Furthermore, the value of the constant related to free energy of adsorption KL in the Langmuir model may be used to evaluate a dimensionless equilibrium parameter, RL. This parameter indicates the favourable nature of the adsorption on the samples. The type of isotherm may be Table 2 Linear forms of Langmuir and Freundlich models. Isotherm
1 qe Ce qe
Langmuir-3 Langmuir-4 Langnuir-5 Freundlich
Table 3 Parameters of Langmuir and Freundlich models for analcime and ZSM-5 ZEolites.
¼ K L q1 C e þ q1 m
¼ qC e þ q 1K L m m qe ¼ − KqL Ce e þ qm qe ¼ −K q L e þ K L qm Ce K L qm 1 C e ¼ qe −K L Inðqe Þ ¼ InðK F Þ þ 1n lnðC e Þ
irreversible (RL = 0), favourable (0 b RL b1), linear (RL = 1) or unfavourable (RL > 1). The values calculated indicate favourable adsorption on both samples. At high phenol concentrations (50–400 ppm), the RL values for ZSM-5 (0.73–0.40) are higher than for analcime (0.50–0.20).
4. Conclusions Analcime zeolite was synthesised from rice husk ash, metakaolin and sodium hydroxide (Na2O/Al2O3 = 2.6) after aging for 3 days and reaction time of 24 h at temperature of 180 °C. Longer reaction time led to lower amounts of analcime with corresponding increase in side products. Analcime formed was found to have trapezohedral morphology and range in size from 15 to 25 μm. The analcime was tested in adsorption of phenol from aqueous solution. The adsorption equilibrium was found to be described by the Langmuir isotherm model, with maximum adsorption capacity of 14.0 mg/g. Thus, local raw materials could be used to prepare an adsorbent with a good and favourable adsorptive capacity.
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