Translate this page into:
Synthesis and characterization of silica gel from siliceous sands of southern Tunisia
*Corresponding author at: Graduate School of Life and Environmental Sciences, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8572, Japan. Tel.: +8129 853 7206; fax: +8129 853 4605 alisdiri@gmail.com (Ali Sdiri)
-
Received: ,
Accepted: ,
This article was originally published by Elsevier and was migrated to Scientific Scholar after the change of Publisher.
Peer review under responsibility of King Saud University.
Available online 18 November 2010
Abstract
The present work aimed to achieve valorization of Albian sands for the preparation of sodium silicates that are commonly used as a precursor to prepare silica gel. A siliceous sand sample was mixed with sodium carbonate and heated at a high temperature (1060 °C) to prepare sodium silicates. The sodium silicates were dissolved in distilled water to obtain high quality sodium silicate solution. Hydrochloric acid was then slowly added to the hydrated sodium silicates to obtain silica gel. The collected raw siliceous sands, as well as the prepared silica gels, were characterized by different techniques, such as X-ray fluorescence (XRF), X-ray diffraction (XRD), scanning electron microscopy (SEM) and thermal analysis (DSC). XRF confirmed that the detrital sand deposits of southern Tunisia contain high amounts of silica, with content ranging from 88.8% to 97.5%. The internal porosity varied between 17% and 22%, and the specific surface area was less than 5 m2/g. After the treatment described above, it was observed that the porosity of the obtained silica gel reached 57% and the specific surface area exceeded 340 m2/g. Nitrogen adsorption isotherms showed that the prepared silica gels are microporous and mesoporous materials with high adsorption capacities. These results suggest that the obtained silica gels are promising materials for numerous environmental applications.
Keywords
Albian
Silica sands
Adsorption
Isotherm
Sol–gel process
1 Introduction
In southern Tunisia, the Dahar plateau extends from Tataouine in the north to the Tripolitain border in the south and is bordered to the east by the Jeffara coastal plain. The stratigraphic succession covers all Mesozoic periods and provides very important reserves of siliceous sands (Busson, 1967; Ben Ismail et al., 1989; Benton et al., 2000). These deposits, which are exposed along Dahar cliffs, are excavated for use in building. However, investments in these natural resources remain below the expected levels because of the lack of detailed studies encouraging developers to focus on the exploitation of silica sands.
For this reason, Tunisia is pursuing policies aimed at expanding the use of local materials, such as carbonates, clay and silica sands. Many works have been undertaken to deepen knowledge of facies, thickness variation and the physical and chemical characteristics of Albian sands of southern Tunisia (Bouaziz et al., 1989; Louhaichi, 1991; Bouaziz, 1995). High purity siliceous materials are used for glass making (Louhaichi, 1991) and, more recently, for various environmental applications (Besbes, 1999; Jesionowski, 2002; Marzouk et al., 2004).
Silica gels, which are known for their high specific surface area and their good gas adsorption capacities, could be produced from sodium silicate solutions by hydrothermal methods using sol-gel processes (Bouaziz et al., 1993; Marzouk et al., 2004).
This work aimed (1) to characterize a local material (silica sands), (2) to prepare a local adsorbent that would partially substitute for imported zeolite and active carbon and (3) to seek possible environmental uses for local silica sands.
2 Materials and experimental methods
2.1 Materials
For the purpose of this study, six sand samples were collected from the Albian continental formation outcropping in Douiret (S1), Oum diab (S2, S3), Ouni (S4) and Dehibat (S5, S6) in Tataouine district (southern Tunisia) (Fig. 1). The continental deposits of the Oum diab formation consist of 15 m thick sand layers alternating with shale. At the “bled Oum diab” location, outcropping beds contain up to 20 m of loose sands. Special attention was devoted to this location due to the promising physicochemical characteristics of the deposits.
Outcrops of the continental deposits of southern Tunisia (modified after Bouaziz et al., 1989) and locations of the collected sand samples (stars).
2.2 Physical and chemical characterization
Prior to analyses, samples were dried for 24 h at 105 °C under vacuum conditions. Grain size distribution was carried out by dry sieving. The chemical composition of the studied samples was determined using X-ray fluorescence with an ARL® 9800 XP spectrometer (Thermo electron corp., Germany). Differential scanning calorimetry (DSC) was performed with a Perkin Elmer thermal analysis system (PerkinElmer Inc., Germany). About 5 mg of <63 μm-sized samples were heated from room temperature to 600 °C at a heating rate of 10 °C/min under air atmosphere. The total pore volume and percentage of porosity were determined by pycnometry. X-ray diffraction patterns were obtained by an X-ray diffractometer (“PANalytical X’ Pert High Score Plus”, The Netherlands) equipped with a dual goniometer of Cuα (λ = 1.5406 μm) using an acceleration voltage of 40 kV. The diffraction angle was scanned from 5° to 70° 2θ, at a range of 2°/min. SEM images were obtained using a Philips XL-30 electron microscope (Philips Electronics corp., The Netherlands).
The specific surface area and pore size distribution were determined with a SORPTOMATIC 1990 (CE instrument, Germany) using the adsorption–desorption isotherms of nitrogen.
2.3 Synthesis of silica gels
The preparation of silica gel from the selected siliceous sand (S3) began with: (1) the preparation of metasilicate sol and (2) the destabilization of metasilicate sol with dropwise addition of 2 M hydrochloric acid under continuous stirring (Bouaziz et al., 1993; Besbes, 1999; Marzouk et al., 2004). Silicates were prepared by heating a mixture of sands and sodium carbonate of a known molar ratio SiO2/Na2O. The resulting vitreous compound was dissolved in hot water to prepare hydrated sodium silicate. The last step consisted of preparing silica gel particles. These experiments were carried out by dropwise addition of 2 M hydrochloric acid to diluted sodium silicate under continuous stirring until polymerization. The obtained silica gels were washed several times with distilled water to remove chloride anions, dried at 105 °C for 24 h and stored in desiccators for further analysis. Eight gel samples (G1–G8) were prepared at different pH and molar ratios.
2.4 BET adsorption isotherms
The degassed silica gel powder was kept inside a glass tube; nitrogen was used as an adsorbate at 77 K to form a monolayer on the sample (Benzina, 1990; Benzina and Bellagi, 1990). At each step, a certain volume (V) of nitrogen was adsorbed by the walls of the pores, and the corresponding change in partial pressure (p/p0) was recorded. The lower portion of the adsorption isotherm was used for the measurement of specific surface areas, whereas the entire adsorption-desorption isotherm was used for pore analyses (Jesionowski, 2002).
Physical gas adsorption is often the technique of choice for examining the pore characteristics of materials (Benzina, 1990; Benzina and Bellagi, 1990). The technique determines the amount of gas adsorbed on silica gel; this is a direct indication of the porous properties, and, therefore, the overall structure, of the material. The isotherm obtained from these adsorption measurements provides information about the surface area, pore volume and pore size distribution (Gregg and Sing, 1982; Groen et al., 2003). Silica gel samples were degassed at 150 °C for 180 min before measurements.
3 Results and discussions
3.1 Characterization of sands
3.1.1 Chemical composition
Chemical compositions of silica sands by X-ray fluorescence are shown in Table 1. The amount of SiO2 ranged from 88.80% to 97.40%. Silica content plays an important role in the production of pure sodium silicates from sands. Al2O3 and Fe2O3 reached 5.88% and 2.33%, respectively, together with small amounts of K and Mg oxides. Loss on ignition (LOI) varied between 0.12% and 1.27%. Based on chemical analyses, only the pure sand sample (S3) was selected for the preparation of high quality sodium silicates. LOI, loss on ignition.
Sample
Locality
Chemical composition (wt%)
Physical properties
SiO2
Al2O3
Fe2O3
K2O
MgO
LOI
ρabs (g/cm3)
ρapp (g/cm3)
Porosity (%)
Pore volume (cm3/g)
SBET (m2/g)
S1
Douiret
94.44
2.32
2.23
0.32
0.12
0.37
2.49
2.11
15.26
0.07
5.0
S2
Oum diab
88.80
5.88
2.01
2.68
0.20
1.27
2.40
1.98
17.21
0.09
4.5
S3
97.43
1.36
0.67
0.06
0.11
0.18
2.70
2.08
22.96
0.11
1.2
S4
Ouni
95.96
1.25
2.33
0.07
0.08
0.12
2.50
1.98
21.24
0.11
–
S5
Dehibat
91.27
4.29
2.15
1.37
0.18
0.51
2.63
2.19
16.81
0.08
3.0
S6
96.45
2.08
0.63
0.35
0.13
0.17
2.65
2.00
24.53
0.12
1.5
3.1.2 Grain size distribution and porosity measurements
The particle size distributions of the studied sand samples showed that these samples are fine to very fine. The S3 sample (collected in the Oum diab location) was coarser than the other samples (Fig. 2). All curves showed redressed sigmoid shapes indicating little variation in their granulometry (Bederina et al., 2005). In addition, a sorting index (SO) (defined as
) confirmed well-sorted sands, where q1 and q3 represent the grain-size corresponding to cumulative contents of 75% and 25%, respectively.
Granular curves of the representative sand samples.
The physical properties of the studied sands are given in Table 1. Results show that the porosity varies from 15% to 24%, and apparent density was between 1.98 and 2.19 g/cm3, whereas absolute density ranged between 2.49 and 2.65 g/cm3. These values may indicate that the studied material has a low specific surface area, as was confirmed by the pore volume of less than 0.123 cm3/g.
3.1.3 X-ray diffraction (XRD)
XRD patterns of studied sands are shown in Fig. 3. Those patterns indicate the characteristic peak of quartz prevailing at 3.34 Å. The other peaks appearing at 4.25, 3.24, 2.46, 2.28, 2.24, 2.13, 1.98, 1.82, 1.67 and 1.54 Å are also characteristics of quartz and confirm the siliceous nature of the Albian sands of southern Tunisia. Detailed mineralogy was identified by the characteristic reflections according to Moore and Reynolds (1989).
XRD patterns of the collected sand samples.
3.1.4 Differential scanning calorimetry (DSC)
According to the thermal analysis data (Fig. 4), each curve presented two peaks. The first peak (at 23 °C) corresponded to the energy provided to remove physically bound water, while the second one was ascribed to the transformation from SiO2α to SiO2ß near 570 °C. Some thermal curves also showed an additional endothermic hump at 500 °C, which was attributable to the dehydroxylation of clay minerals (Chakchouk and Samet, 2006; Sdiri et al., 2010).
Thermal curves of representative sands.
3.1.5 Scanning electron micrograph (SEM)
It can be seen from the SEM images that Albian sands exhibit smooth grains, confirming the textural data (surface area and porosity) (Fig. 5). It was obvious that the absence of micropores and mesopores in the studied siliceous sands of southern Tunisia provide such a texture. Therefore, it was expected that this material would show low specific surface area and internal porosity.
SEM photograph of representative sands.
3.2 Characterization of the prepared silica gels
3.2.1 Physical properties
Physical properties of the prepared silica gels are shown in Table 2. Porosity measurements by the pycnometric method show that the obtained silica gels are characterized by high porosity, varying between 33% and 57.06% for G1 and G8, respectively. Both absolute and apparent densities are lower than 1.3 g/cm3, showing that the prepared silica gels are classified as lightweight materials (Table 2). This property may be beneficial for the use of the prepared gel in many environmental and industrial applications.
Sample
pH
Molar ratio = n SiO2/Na2O
ρabs (g/cm3)
ρapp (g/cm3)
Porosity (%)
Pore volume (cm3/g)
Vm: monolayer volume (cm3/g)
SBET (m2/g)
G1
6
2.5
1.11
0.75
33.00
0.44
79.86
340
G2
3
2.5
1.72
1.03
40.25
0.39
142.40
668
G3
6
2.5
1.51
0.81
46.21
0.57
109.98
464
G4
9
2.5
1.51
0.89
41.34
0.47
86.82
371
G5
3
3.5
2.20
1.30
40.90
0.31
163.30
702
G6
6
1.5
2.08
1.08
49.20
0.46
125.85
590
G7
9
1.5
1.44
0.70
51.40
0.73
133.79
602
G8
6
3.5
1.77
0.76
57.06
0.75
87.98
374
3.2.2 Differential scanning calorimetry (DSC)
Thermal curves, measured from ambient to 600 °C, are shown in Fig. 6. The DSC curves showed endothermic peaks at 39, 86, 54 and 57 °C for G1, G2, G3 and G4, respectively. These reactions were due to the removal of physically bound water. This was further confirmed by enthalpy values ranging between 7.2 and 11.75 J/g. The water molecules bound to the G2 sample evaporated at a higher temperature of 86 °C. The G2 sample had the highest specific surface area among the studied samples (G1, G2, G3 and G4); this likely explains its higher water adsorption capacity. Therefore, the strongly bound water molecules were not easily removed from the surface. Between 100 and 600 °C, the thermal curves did not show significant variation.
Thermal curves of representative silica gels.
3.2.3 Scanning electron micrograph
Various SEM images of the obtained silica gels showed their porous structure (Fig. 7). In general, similarity in SEM morphologies accounts for the uniformity of the silica source (sands) that produced silica gels with comparable particle sizes (Tsai, 2004; Kima et al., 2005). The observation of pore sizes using SEM confirmed that the obtained silica gels are mesoporous, microporous materials.
SEM photographs of representative silica gels.
3.2.4 BET adsorption isotherms
Fig. 8 shows the nitrogen adsorption isotherms of the representative silica gels. It can be seen that this material has both microporous and mesoporous structures. These physical properties depend on pH and molar ratio (SiO2/Na2O), as described in the literature (Bouaziz et al., 1993; Besbes, 1999; Kwon and Park, 2004). G2 and G5 silica gel powders are microporous in structure, while G1 and G4 are mesoporous (Fig. 8). Adsorption on G2 and G5 resulted in type I isotherms in the BET classification. This is indicative of microporosity, along with very limited mesoporosity (Groen et al., 2003). In contrast, G1 and G4 gave type IV isotherms, indicating mesoporous materials. In these cases, capillary condensation occurs during adsorption via “cylindrical meniscus”, while capillary evaporation during desorption occurs via “hemispherical meniscus”, separating the vapor and capillary-condensed phases. This results in hysteresis loops (Fig. 8a and c) and confirms the mesoporous structure of the prepared silica gels (Groen et al., 2003).
Nitrogen adsorption isotherms at 77 K for representative silica gels.
Table 2 shows the parameters derived from BET nitrogen gas adsorption isotherms. It can be seen from these results that specific surface area was directly related to the type of isotherm, and consequently to the pore structure. Moreover, adsorption-desorption properties of the prepared silica gels are likely to be influenced by some network effects, such as the tensile strength effect (Groen et al., 2003; Dutta et al., 2005). Specific surface area (SBET) of the prepared silica gels ranged between 340 and 702 m2/g and provided high adsorptive capacity. This observation was further confirmed by the high volume of adsorbed nitrogen (Table 2). It was clearly observed that the specific surface area was higher for a pH of 3, proving the higher monolayer volume of adsorbed nitrogen. The effect of pH was shown to be an influencing factor on the structure of the silica gel obtained. However, the effect of the molar ratio (SiO2/Na2O) was likely to follow a random distribution.
4 Conclusions
This work was undertaken in order to exploit the potential local sands for the preparation of silica gel. It was concluded that the studied sands consisted of more than 89% silica. In addition to its current use as a building material, the continental deposit of southern Tunisia would provide an additional resource for manufacturing high quality glassware. Furthermore, the Albian sands at the Oum diab site were found to be suitable for the preparation of high quality sodium silicate, used as a precursor in the preparation of silica gel. The specific surface area of the prepared silica gels exceeded 340 m2/g, and the monolayer volume was higher than 79 cm3/g. The microporous and mesoporous natures of these products constitute promising characteristics for their use as adsorbents in environmental projects, such as heavy metal removal from wastewater and the adsorption of volatile organic compounds. The use of silica gels prepared from Tunisian sands as an adsorbent needs further examination.
Acknowledgements
The authors gratefully acknowledge the logistic and financial support provided by the laboratory of “Water, Energy and Environment” (LR3E), National School of Engineers of Sfax, Tunisia. Special thanks go to Mr. Nidhal Baccar, assistant researcher in ENIS, University of Sfax, for his kind guidance. The help of Professor Abdelmottaleb Ouederni and Mr. Hechmi Zekri from National School of Engineers of Gabes, Tunisia, is greatly acknowledged. The authors also extend their thanks to Mrs. Fatma Masmoudi and Mr. Moez Ben Brahim from Borj Cedria Science and Technology Park, Tunisia, for providing XRD data.
References
- Reuse of local sand: effect of limestone filler proportion on the rheological and mechanical properties of different sand concretes. Cem. Concr. Res.. 2005;35:1172-1179.
- [Google Scholar]
- données biostratigraphiques sur le Callovien et les faciès «Purbecko-wealdien» (Oxfordien à Vraconien) dans la région de Tataouine (Sud tunisien) Bull. Soc. Geol. France. 1989;8:353-360.
- [Google Scholar]
- Dinosaurs and other fossil vertebrates from fluvial deposits in the Lower Cretaceous of southern Tunisia. Palaeogeogr. Palaeoclimatol. Palaeoecol.. 2000;157:227-246.
- [Google Scholar]
- Benzina, M., 1990. Contribution à l’étude cinétique et thermodynamique de l’adsorption de vapeur organique sur des argiles locales. Modélisation d’un adsorbeur à lit fixe. Thèse Doct. es Sc. Phy., Univ. Tunis II, Fac. Sci., Tunis.
- Détermination des propriétés du réseau poreux de matériau argileux par les techniques d’adsorption d’azote et de porosimétrie au mercure en vue de leur utilisation pour la récupération des gaz. Annal. Chim. Fr.. 1990;15:315-335.
- [Google Scholar]
- Besbes, M., 1999. Adsorption du bleu de méthylène par un gel de silice obtenu à partir d’un sable tunisien. D.E.A., Univ. Sfax, Fac. Sci., Sfax.
- Synthèse et caractérisation de gels de silice obtenus à partir du sable tunisien. J. Soc. Chim. Tun.. 1993;6:411-419.
- [Google Scholar]
- Bouaziz, S., 1995. Evolution de la tectonique cassante dans la plate-forme et l’Atlas saharien (Tunisie méridionale): Evolution des paléochamps de contrainte et implications géodynamiques. Thèse Doct. Es Sc., Univ. Tunis II. Fac. Sci., Tunis.
- Bouaziz, S., Donze, P., Ghanmi, M., Zarbout, M., 1989. La série à dominante continentale (Oxfordien à Cénomanien) de la falaise du Dahar (Sud tunisien): son évolution du Tebaga de Mednine à la frontière tripolitaine. Géologie Méditerranéenne, 14.
- Busson, G., 1967. Le Mésozoïque saharien, 1ére partie: l’extrême Sud tunisien. Edition du C. N. R. S.
- Study on the potential use of Tunisian clays as pozzolanic material. Appl. Clay Sci.. 2006;33:79-88.
- [Google Scholar]
- Pore structure of silica gel: a comparative study through BET and PALS. Chem. Phys.. 2005;312:319-324.
- [Google Scholar]
- Adsorption, Surface Area and Porosity (second ed.). London: Academic Press; 1982.
- Pore size determination in modified micro- and mesoporous materials. Pitfalls and limitations in gas adsorption data analysis. Microporous Mesoporous Mater.. 2003;60:1-17.
- [Google Scholar]
- Characterization of silica precipitated from solution of sodium metasilicate and hydrochloric acid in emulsion medium. Powder Technol.. 2002;127:56-65.
- [Google Scholar]
- Formation of spherical hollow silica particles from sodium silicate solution by ultrasonic spray pyrolysis method. Colloids Surf. A. 2005;254:193-198.
- [Google Scholar]
- Synthesis of layered silicates from sodium silicate solution. Bull. Korean Chem. Soc.. 2004;25:25-26.
- [Google Scholar]
- Louhaichi, M., 1991. Inventaire des substances minérales utiles dans les régions du Dahar, Jeffara et de Matmata (Sud de la Tunisie). Off. Nat. Min., division des substances utiles.
- X-ray diffraction and the identification and analysis of clay minerals (second ed.). Oxford: Oxford Univ Press; 1989.
- Mineralogical and spectroscopic characterization, and potential environmental use of limestone from the Abiod formation, Tunisia. Environ. Earth Sci.. 2010;61:1275-1287.
- [Google Scholar]
- The study of formation colloidal silica via sodium silicate. Mater. Sci. Eng.. 2004;106:52-55.
- [Google Scholar]
