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95
*N. Frini-Srasra1,2, E. Srasra2
ADSORPTION OF QUINALIZARIN FROM NON AQUEOUS SOLUTION ONTO ACID ACTIVATED PALYGORSKITE
1Departement de chimie, Faculte des Sciences de Tunis, universite El Manar,
Campus Universitaire, Tunisie 2Unite Materiaux, Centre National de Recherche des Sciences des Materiaux, Technopole Borj Cedria, BP95- 2050 Hammam-Lif Tunisie Corresponding author. Tel.: +216 71 430 470 ; Fax: +216 71 430 934. nfrini@gmail.com
Introduction
Aromatic compound constitute a major class of pollutants in waste waters from many industries. These compounds must be removed from waste-waters before they are discharged into the environment because of their high toxicological potential. Quinalizarin (1,2,5,8-Tetrahydroxy-antraquinone) is among such compounds. The major use of quinalizarin is in manufacture of dyes and pigments. The treatment of textile waste, comprising of dyestuffs and other non-biodegradable organics, poses considerable problems in the waste water treatment industry. It is difficult to remove this kind of waste because they are stable to light, heat and oxidizing agents and biologically non-degradable [1].
Adsorption is known to be one of the most promising processes of removing pollutant dyes from waste [2, 27, 3]. Activated carbon is the most widely used adsorbent for the removal of colour from textile effluents due to its high surface area and high adsorption capacity [4-6]. However it has a high cost and needs regeneration after each adsorption experiment. Recently, clay materials either as such or after modification, have been increasingly paid attention because they are cheaper; they have high specific surface area, high chemical stability and a variety of surface and structural properties [7-10]. To our knowledge, little attention was paid to fibrous clays even more, to acid activated fibrous clay. A limited number of studies were achieved using sepiolite as adsorbent [11-13] and palygorskite [14, 15].
The aim of this work is to study the adsorption of quinalizarin from organic solution onto acid activated palygorskite. Palygorskite was classed as a 2:1 layer inverted structure. Because of the inversions in the silica tetrahedral sheet, the structure has parallel channels or holes throughout, give a high surface area and a high capacity to adsorb and absorb various materials. Appropriate acid treatments modify its structural and textural properties and make it beneficial for many industrial applications (Environment absorbent, pharmaceuticals, catalyst support, petroleum refining, mineral and vegetable oil refining...).
Materials and methods
Adsorbent
Red palygorskite from south Tunisia was used as starting material. The crude sample contains impurities mainly dolomite (8%, measured by calcimetry) and less than 5% of quartz. Crude palygorskite was firstly treated with dilute HCl solution in order to remove carbonate. Then, it was washed with distilled water by centrifugation and dialysis to remove excess chloride. After sedimentation, the purified fraction was dried and ground to less than 60 ^m. Referring to ASTM file (Reference code: 21-0957), Tunisian palygorskite has monoclinic structure. Chemical composition of purified sample is SiO2 53.5%, Al2O3 11.98%, MgO 12.05%, Fe2O3 5.9%, K2O 0.72%, Na2O 0.03%, CaO 0.09%, LOI 16% and its structural formula, calculated according to [16], is as follows:
(Si7.324Al0.676)(Mg2.459Al1.257Fe0.608)(K0.125Na0.008Cacl. 0!3)O20(OH)2(H2O)4 4(H2O)
The cation exchange capacity (CEC) determined by Kjeldahl method is 23 meq/100g and its specific surface area is 59.7 m2g-1. The purified palygorskite sample was subjected to acid treatment with 2M hydrochloric acid using solid/liquid ratio of 1/10 (w/w) at boiling temperature under reflux for different periods. The resulting solids were washed with distilled water until chloride free and constant pH.
© Frini-Srasra N., Srasra E., Электронная обработка материалов, 2009, № 4, С. 62-68.
Adsorbate
The organic molecule dye used as adsorbate in this study is the 1,2,5,8-Tetrahydroxy- antraquinone (quinalizarin). Its molecular structure is given by the following chemical formula:
Methods
Activated and no activated palygorskite was characterised by X ray diffraction (using Pan analytical X’Pert High Score Plus diffractometer equipped with a Cu anticathode), Chemical analysis [17] and specific surface area (using Micrometric Autosorb 2050 Analyser) was determined from the nitrogen adsorption isotherms at 77 K by the BET method and after out degassing the samples at 150°C with the residual pressure of 10-5 mm Hg. Acid-base properties of different samples were determined using potentiometric titration [18].
The adsorbate concentration were analysed spectrophotometrically with UV-Visible spectrophotometer (Perkin Elmer Lomda 90) at 492 nm).
Adsorption experiment
Adsorption tests were carried out by mixing 0.1g of each adsorbent with 50ml of quinalizarin dissolved in ethanol. The initial quinalizarin concentration is ranged between 2.5 10-4 and 2.5 10-5 mol L-1. The whole was shaken at 100 rpm at room temperature for 24h to ensure adsorption equilibrium. After the contact time was completed, the suspension was centrifuged at 4000 rpm for 15min. Quinalizarin remaining in the supernatant was analysed by spectophotometry at 492 nm. Adsorption isotherm is constructed by measuring the concentration of the adsorbate in the medium before and after adsorption using the following equa-
Q (C - C„)V
tion: Q ads = -------------
m
Where Qads is the amount of quinalizarin adsorbed on palygorskite sample at considered equilibrium concentration (mol g-1), C0 the initial concentration of quinalizarin solution (mol L-1), Ceq the equilibrium concentration of quinalizarin solution (mol L-1), V the volume of quinalizarin solution (L) and m the mass of adsorbent used (g).
Results and discussion
Characterisation of the adsorbents
Acid treatment caused a progressive decrease in octahedral cations (Al, Mg and Fe) and a residual enrichment in SiO2 (table 1). In the beginning of acid treatment, the decrease in octahedral cations is limited showing the resistance of palygorskite structure and the difficulties of the opening of the channels. Once the H+ access throughout the structure is facilitated, the dissolution becomes rapid and reaches its maximum at 35h. For longer activation times, we note an increase in the octahedral cations, which can be attributed to coprecipitation.
Table 1. Chemical compositions of untreated and acid treated samples expressed in oxide form/100g of calcined sample __________________________________________________
Samples SiO2 Fe2O3 MgO Al2O3
Paly 63.69 7.023 14.34 14.26
Paly 2M8h 71.4 5.96 11.89 11.09
Paly 2M14h 73.33 5.59 11.38 10.06
Paly 2M35h 88.08 0.66 4.60 7.08
Paly 2M50h 79.58 3.79 5.76 8.57
The X-ray diffractograms of purified and acid treated samples are presented in fig. 1. The untreated palygorskite has sharp strong reflexions at 10.5, 6.4, 5.4, 4.5, 3.68 and 3.23A. With increasing treating time, the crystallinity of palygorskite decreased, with a concomitant formation of amorphous utter. The latter can
be observed by the appearance and increase of a broad ham at 16-30° 20. After 35h treatment only this ham is observed indicating the complete destruction of the clay.
225-
100-
25-
0-
900
400
100
400
100-
0
400-
100
0
1600
900
400-
100-
0.
900
400-
100-
0
Fig. 1. XRD diffractograms of treated and untreated palygorskite
BET specific surface area gradually increases with treating time up to 35h (from 59.7 to 437 m2g-1) decreasing thereafter (table 2). Such behaviour has been observed in the past [19-21]. The initial increase of SBET is attributed to the opening of the structure channels following dissolution of the octahedral sheet and amorphous silica formation; whereas the decrease can be attributed to the condensation of the silanol groups [26]. This confirms that silica contributes to the total amount of the surface area. Moreover, activation increases the total pore volume, from 0.3494 to 1.039 cm3g-1, due to development of mesoporosity.
Table. 2. Surface parameters of different palygorskite samples and maximum amount of adsorbed quinaliza-rin
Sample Sbet (m2g ') Vp (cm3 g-1) Hs (mmol g-1) Ds *(sites nm-2) Qmax (^mol g-1)
Paly 59.7 0.3494 0.518 5.22 1.8
Paly2M8h 161.6 0.5517 1.003 3.74 3.8
Paly2M14h 212,3 0.6118 1.012 2.87 3.56
Paly2M35h 437.0 1.0390 1.527 2.1 3.65
Paly2M50h 138.3 0.6631 1.425 6.2 2.5
(H, x Na )10-3 2 23 ,
* Ds =-18— (sites nm ); Where: NA is the Avogadro's number (6,02 .10 mol )
S BET X 10
The total surface site per solid weight (Hs) calculated from the potentiometric data by using the Gran plot in a previous work [22] is given on table 2. As expected, the Hs value increases with the period of acid activation. It passes from 0.518 mmolg-1 in starting material to 1.53 mmolg-1 in Paly 2M35h. Beyond this activation time, the total surface site decreases slightly.
The site density values (Ds) show that no correlation exists between the total surface site (Hs) and the surface site area SBET (table 2).
Adsorption isotherms
To identify the mechanism of the adsorption process, the adsorption of quinalizarin onto palygorskite is determined as a function of equilibrium quinalizarin concentration (Ce) and the corresponding adsorption
Intensity
Amourphous silica
Paly 2M35H
I ' Paly 2M14H
I Paly 2M8H
Purified Paly
10
5 A
I. I________LlL______i .i lli
5 |10 15 20 25 30 35
Ref.code: 00 021-0958
Crude Paly
l l l l l l | l l l l l l l l l | l l l l l l l l l | l l l l
10
20 30
20
40
isotherms are plotted in fig. 2. According to the classification of [23], the isotherm seems to be of the L type, indicating that the quinalizarin has a high affinity for the palygorskite surface. All isotherms are similar in shape and they show two clear plateaus. Adsorption isotherms with two plateaus were also observed in some studies concerning the adsorption of organic molecules on silica surface [24] and a sulphate adsorption on a volcanic ash soil [25].
According to the results of the quinalizarin adsorption isotherm experiments, the modified palygor-skites had higher adsorption capacities than the natural sample. It was believed that the surface structural changes of the material, play the most important role in the adsorption capacities of the quinalizarin. When the natural palygorskite sample was treated with 2M HCl, the surface structural of the material was changed due to the opening of channels, the creation of mesocroporosity and the increase of the surface acidity. However, no correlation of Qmax was observed nor with specific surface area, nor with the total surface site (Hs) (table 2).
Fig. 2. Adsorption isotherms of quinalizarin onto palygorskite before and after acid activation
Modelling adsorption isotherms
The equilibrium adsorption isotherms are one of the most important data to understand the mechanism of the adsorption systems. Several isotherm equations can be used to explain solid/liquid adsorption system. The most commonly used are Langmiur and Freundlich isotherms. The Langmuir adsorption isotherm assumes that adsorption occurs at specific homogeneous sites within the adsorbent and has found successful application in many studies of monolayer adsorption. The linear form of the Langmuir isotherm equation is written as:
1
Ceq Qad KQn
- + -^- Ceq Qm
Where Qad is the equilibrium capacity of adsorbate on adsorbent (mol g-1), Ceq is the equilibrium concentration of adsorbate solution (mol L-1), Qm is the monolayer adsorption capacity of the adsorbent (mol g-1), and K is the Langmuir adsorption constant (L mol-1) and is related to the free energy of adsorption.
The Freundlich isotherm is an empirical equation employed to describe heterogeneous adsorption surface with sites that have different energies of adsorption and are not equally available. The Freundlich isotherm is more widely used but provides no information on the monolayer adsorption capacity, in contrast to the Langmuir model. Its linearized form can be written as follows: LnQad = LnKf + nLnCeq
Where: Kf (L g-1) and n (dimensionless) are the Freundlich adsorption isotherm constants, being indicative of the extent of adsorption and the degree of nonlinearity between solution concentration and adsorption, respectively.
The correlation coefficients, calculated from the linear Langmuir and Freundlich equations, show that the measured adsorption could not be fitted with these models (table 3).
Table 3. The correlation coefficients calculated^ from the linear Langmuir and Freundlich
Langmuir Freundlich
Paly 0,896 0,987
Paly 2M8h 0,32 0,837
Paly 2M14h 0,67 0,675
Paly 2M35h 0,892 0,840
Paly 2M50h 0,52 0,851
Based on this result and the isotherms shape we can suggest that the adsorption sites aren’t homogenous but there are two kinds of adsorption sites: the stronger and the weaker sites. In order to check this hypothesis, we modified the Langmuir model as follows:
„ , Qm, K1 Ceq Qm2 K2 Ceq
Qad = —-------------- + 2
1 + KCeq
1 + K 2Ceq
where Qm1 , K1 et Qm2 , K2 are respectively the Langmuir parameters for the stronger and weaker sites.
The modified Langmuir parameters for the adsorption of quinalizarin are listed in table 4. Results show regression coefficients close on the unity (0.941-0.998). The maxima of quinalizarin adsorption at the stronger (Qm1) and the weaker adsorption sites (Qm2) increase with the acid activation time up to 35h and decrease thereafter. The experimental data produced perfect fit with the modified Langmuir isotherm showing that the surface of palygorskite was non-uniform in nature (fig. 3).
Qad. mol-g-1
Paly
Qad5 mol-g-1 Paly 2M8h
Qad? mol g 1
Paly 2M14h
Qad' mol-g-1 Paly 2M 35h
Qad, mol-g-1
Paly 2M50h
Fig. 3. The measured quinalizarin adsorption isotherms and the calculated Langmuir isotherms
Table 4. Langmuir isotherm constants and correlation coefficients. (1)for the stronger and (2) for weaker
adsorption sites
Paly Paly 2M8h Paly 2M14h Paly 2M 35h Paly 2M50h
R2i 0.974 0.988 0.998 0.995 0.941
Qm1 jumol g-1 1.23 1.25 1.54 2.32 1.23
Ki L mol-1 372136 160482 350391 139605 68072
2 2 Pi 0.998 0.987 0.992 0.997 0.995
Qm2 umol g-1 1.00 2.73 2.12 2 .25 2.55
K2 L mol-1 16085 11723 181800 7232 5602
(Qm1+Qm2)umol g-1 2.23 3.98 3.66 4.57 3.78
The thin lines were obtained using the simple Langmuir equation at the stronger adsorption sites .The thick lines were obtained using the modified Langmuir equation.
Conclusion
Acid activation increases the adsorption capacity of palygorskite. Under the same conditions the Qmax value for HCl activated palygorskite was about twice larger than for the non treated palygorskite. This increase is the result of structural surface changes.
Using the measured and theoretical isotherms, two kinds of adsorption sites was suggested: stronger and weaker adsorption sites. This heterogeneity was checked by using the modified Langmuir model.
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Received 15.04.09
Summary
In this study, the adsorption of quinalizarin by natural and mesoporous acid activated palygorskite has been examinated in order to remove coloured textile dyes from non aqueous solutions. The adsorption of quanilizarin from ethanol solution onto acid activated palygorskite was followed by UV-Visible measurement. The adsorption capacity was largely improved when the clay was activated by HCl. The high adsorption extents were observed from 2M8h to 2M 35h acid treated palygorskite samples. Adsorption isotherms of quinalizarin on acid activated palygorskite were determined and correlated with common isotherm equations such as Langmiur and Freundlich models. It was found that i/the adsorption isotherm exhibits two plateaus ii/ no model fit the isotherm data. In order to explain the adsorption behaviour of quinalizarin from ethanol onto palygorskite, the presence of stronger and weaker adsorption sites was suggested and checked by using the modified Langmuir model.
