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Removal of methyl violet from aqueous solution by perlite
Journal of Colloid and Interface Science 267 (2003) 32–41 www.elsevier.com/locate/jcis
Removal of methyl violet from aqueous solution by perlite Mehmet Do˘gan and Mahir Alkan ∗ Balıkesir University, Faculty of Science and Literature, Department of Chemistry, 10100 Balıkesir, Turkey Received 5 November 2002; accepted 22 May 2003
Abstract The use of perlite for the removal of methyl violet from aqueous solutions at different concentration, pH, and temperature has been investigated. Adsorption equilibrium is reached within 1 h. The capacity of perlite samples for the adsorption of methyl violet was found to increase with increasing pH and temperature and decrease with expansion and increasing acid-activation. The adsorption isotherms are described by means of the Langmuir and Freundlich isotherms. The adsorption isotherm was measured experimentally at different conditions and the experimental data were correlated reasonably well by the adsorption isotherm of Langmuir. The order of heat of adsorption corresponds to a physical reaction. It is concluded that the methyl violet is physically adsorbed onto the perlite. The removal efficiency (P ) and dimensionless separation factor (R) have shown that perlite can be used for removal of methyl violet from aqueous solutions, but unexpanded perlite is more effective. 2003 Elsevier Inc. All rights reserved. Keywords: Adsorption isotherms; Methyl violet; Perlite; Dye
1. Introduction Many industries routinely use dyes or pigments to color their products. A number of these dyes or pigments are inevitably left in the industrial waste, which could be a hazard to the environmental [1]. Textile industry waters are generally processed in biological treatment units for removal of biodegradable organic compounds. These biological processes typically accomplish very little towards color removal while handling these wastewaters [2]. The removal of color from textile wastewaters is one of the major environmental problems because of the difficulty of treating such water by conventional treatment methods [3]. In our day various physical–chemical techniques have been studied to assess their applicability for the treatment of this type of industrial discharges. Among these processes may be included coagulation, precipitation, flocculation, ozonation, reverse osmosis, ion exchange, and activated carbon adsorption [2,4]. Various types of materials have been used as adsorbents, such as activated carbon, manganese oxide, silica gel, fly ash, wollastonite, lignite, peat, soil, alumina, rutil, geothite, hematit, bentonit, sphalerit, anatase, red mud, mica, illite, kaolinite, and clays [4]. * Corresponding author.
E-mail address: [email protected] (M. Alkan). 0021-9797/$ – see front matter 2003 Elsevier Inc. All rights reserved. doi:10.1016/S0021-9797(03)00579-4
Perlite is a glassy volcanic rock that expands to about 20 times its original volume upon heating within its softening temperature range of 760 to 1100 ◦ C [5]. The uses of expanded perlite are many and varied and are based primarily upon its physical and chemical properties. As most perlites have a high silica content, usually greater than 70%, and are adsorptive, they are chemically inert in many environments and hence are excellent filter aids and fillers in various processes and materials. Furthermore, perlite is also used as a catalyst in chemical reaction [6]. Along the Aegean Coast, Turkey possesses about 70% (70 × 109 tons) of the world’s known perlite reserves [7]. The main consumption of perlite is in construction related fields. In our previous works, we investigated the electrokinetic properties of perlite [8], surface titrations of perlite suspensions [9], and adsorption of copper (II) onto perlite [4]. The present work is aimed to study a convenient and economic method for methyl violet removal from water by adsorption on a low-cost and abundantly available adsorbent on which no work could be found in the literature. The effects of expansion, solution pH, acid-activation, and temperature on methyl violet adsorption have been investigated. Furthermore, the results obtained have been applied to a batch design for the removal of methyl violet from aqueous media using perlite samples.
M. Do˘gan, M. Alkan / Journal of Colloid and Interface Science 267 (2003) 32–41
2. Materials and methods
33
solution was allowed to stand for 1–2 days until the absorbance of the solutions remained unchanged [1].
2.1. Materials 2.3. Method The unexpanded and expanded perlite samples were obtained from the Cumaovası Perlite Processing Plants of Etibank (˙Izmir, Turkey). The chemical composition of the perlite found in Turkey in the literature is given in Table 1 [7]. Treatment of the unexpanded and expanded perlite samples before use in experiments is described elsewhere [8]. In order to obtain the acid-activated perlite samples, H2 SO4 solutions were used [10]. The cation exchange capacity (CEC) of the various perlite samples was determined by the ammonium acetate method, density by pycnometry. The specific surface area of the samples of expanded (EP), acid-activated expanded perlite (EHP(0.6)), unexpanded (UP), and acid-activated unexpanded perlite (UHP(0.6)) were measured by BET N2 adsorption [10]. The results are summarized in Table 2. All chemicals were obtained from Merck. 2.2. Preparation of methyl violet solutions Methyl violet was dried at 70–80 ◦ C for 4 h to remove moisture and then was dissolved in distilled water. Since methyl violet is difficult to dissolve in water, methyl violet Table 1 Chemical composition of perlite Constituent
Percentage present
SiO2 Al2 O3 Na2 O K2 O CaO Fe2 O3 MgO TiO2 MnO2 SO3 FeO Ba PbO Cr
71–75 12.5–18 2.9–4.0 4.0–5.0 0.5–2.0 0.1–1.5 0.03–0.5 0.03–0.2 0.0–0.1 0.0–0.1 0.0–0.1 0.0–0.1 0.0–0.5 0.0–0.1
Adsorption experiments were carried out by shaking 0.5-g perlite samples with 50-ml aqueous solutions of methyl violet of desired concentrations at various pHs and temperatures for 1 h. Prior to adsorption experiments the solution was kept under N2 for 10 min. A preliminary experiment revealed that about 1 h is required for methyl violet to reach the equilibrium concentration. A thermostated shaker bath was used to keep the temperature constant. The initial concentration of methyl violet solutes, C0 , was varied in the range 5 × 10−7 –3 × 10−4 mol l−1 for unexpanded and expanded perlite samples. All adsorption experiments were performed at 30 ◦ C and pH 11 except those in which the effects of temperature and pH of methyl violet solution were investigated. The pH of the solution was adjusted with NaOH or HNO3 solution by using an Orion 920A pH meter equipped with a combined pH electrode. The pH meter was standardized with NBS buffers before every measurement. At the end of the adsorption period, the solution was centrifuged for 15 min at 3000 rpm and then the concentration of the residual methyl violet, Ce , was determined with the aid of a Cary |1E| UV-visible spectrophotometer (Varian). The measurements were made at the wavelength λ = 584 nm, which corresponds to maximum absorbance. Blanks containing no methyl violet were used for each series of experiments. The amounts of methyl violet adsorbed were calculated from the concentrations in solutions before and after adsorption. Each experimental point was an average of three independent adsorption tests [11].
3. Results and discussion 3.1. Adsorption isotherms Figure 1 shows the adsorption isotherms of methyl violet on the unexpanded and expanded perlite samples. The adsorbed amount of methyl violet for unexpanded perlite is greater than that for expanded perlite. The decrease in
Table 2 Some physicochemical properties of perlite samples used in the study Sample
Nomenclature
Expanded, purified in water Expanded, 0.2-M acid-activated Expanded, 0.4-M acid-activated Expanded, 0.6-M acid-activated Unexpanded, purified in water Unexpanded, 0.2 M acid-activated Unexpanded, 0.4-M acid-activated Unexpanded, 0.6-M acid-activated
EP EHP(0.2) EHP(0.4) EHP(0.6) UP UHP(0.2) UHP(0.4) UHP(0.6)
CEC (meg 100 g−1 )
Density (g ml−1 )
Specific surface area (m2 g−1 )
Zeta potential (mV)
33.30 38.20 43.38 54.24 25.97 32.79 35.00 36.56
2.24 2.10 2.04 1.93 2.30 2.32 2.38 2.46
2.30 – – 2.33 1.22 – – 1.99
−46.8 −47.1 −46.3 −44.0 −23.5 −21.8 −22.0 −21.1
34
M. Do˘gan, M. Alkan / Journal of Colloid and Interface Science 267 (2003) 32–41
Fig. 1. The effect of thermal treatment on the adsorption of methyl violet on perlite.
the amount of adsorption by expansion may be a result of events occurring during the calcination: (i) the decrease in the amount of hydroxyl groups and (ii) the removal of most of the micropores due to heating the sample. Infrared spectra of the unexpanded and expanded perlite samples show that the number of hydroxyl groups is decreased by the thermal treatment in the production of expanded perlite from unexpanded perlite. The decrease in the amount of hydroxyl groups of the adsorbent, which are mainly effective sites for adsorption, during the expansion of perlite is thought to cause a decrease in adsorption capacity, although expanded perlite has greater values of cation exchange capacity (CEC), zeta potential (ZP), and specific surface area than unexpanded perlite [12]. The effect of acid activation on the adsorption of methyl violet onto perlite samples is given in Fig. 2, indicating that the adsorbed amounts of methyl violet slightly decrease with the concentration of H2 SO4 used for the acid activation for both of the perlite samples. This decrease observed may be due to the partial destruction of perlite structure as shown by Gonzàlez-Pradas et al. [13] and López-Gonzàlez and Gonzàlez-Garcià [14] for bentonite and may be due to the decrease in OH groups in perlite during the activation process, as was shown by Do˘gan using IR-spectra. Furthermore, the increase in CEC with acid activation, as seen in Table 2, may cause a decrease in adsorption capacity of dye
as well, which is an indicator of charged replacable sites on the surface [12]. To study the influence of pH on the adsorption capacity of perlite samples for methyl violet, experiments were performed using various initial solution pH values, changing from 3 to 11 (Fig. 3). The removal of methyl violet by perlite samples has been increased when the pH of the dye solution was increased. In the discussion of the effect of pH, it is necessary to discuss the pKa -values of the S–OH groups of perlite samples. Surface charge will develop via the amphoteric ionization of the surface hydroxyl groups according to the reactions SOH+ 2
Kaint
SOH + H+ s , 1
(1)
Kaint
SOH SO− + H+ s , 2
(2)
where the subscript s denotes the surface and the equilibrium constant for Eqs. (1) and (2): = Kaint 1
[SOH][H+ s ] , + [SOH2 ]
(3)
Kaint = 2
[SO− ][H+ s ] . [SOH]
(4)
The formation of surface species, [SOH+ 2 and SOH], is the principal mechanism by which protons are released (or con-
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Fig. 2. The effect of acid-activation on the adsorption of methyl violet on perlite.
sumed) by many oxide surface in aqueous electrolyte solutions. For oxides in general, as pKaint and pKaint increase, 1 2 and S–OH, dethe acidity of both surface species, S–OH+ 2 int creases. In this sense, pKa values are considered as a meavalues for expanded and unexsure of surface acidity. pKaint 2 panded perlite samples were, respectively, determined as 2.7 and 3.0 with NaCl as an electrolyte [4]. It can be said that
the surface hydroxyl groups on perlite are acidic. When considered together with the fact that the surface is negatively charged through the entire range of the studied pH values (i.e., pH 3–10), this result shows that the reaction (2) is in favor of the right-hand side at low concentrations. Therefore, as the pH of the dye solution becomes higher (Eq. (2)), the association of dye cations with negatively charged perlite
36
M. Do˘gan, M. Alkan / Journal of Colloid and Interface Science 267 (2003) 32–41
Fig. 3. The effect of pH of the solution on the adsorption of methyl violet on perlite.
surface can more easily take place through an electrostatic interaction as follows (Eq. (5)): ≡SO− + Dye+ S–O− −+ Dye.
(5)
A study of the temperature dependence of adsorption reactions gives valuable information about the enthalpy change during adsorption. The effect of temperature on the adsorption isotherm was studied by carrying out a series of isotherms at 30, 40, 50, and 60 ◦ C for both of the perlite sam-
ples (unexpanded perlite and expanded perlite) and shown in Fig. 4. Results indicate that the adsorption capacity of unexpanded and expanded perlite for adsorption of methyl violet increases with increasing temperature, which is typical for the adsorption of most organics from their solution. The effect of temperature is fairly common and increasing the temperature must increase the mobility of the large dye cation. Furthermore, increasing temperature may produce a
M. Do˘gan, M. Alkan / Journal of Colloid and Interface Science 267 (2003) 32–41
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Fig. 4. The effect of temperature on the adsorption of methyl violet on perlite.
swelling effect within the internal structure of the perlite enabling large dyes to penetrate further. 3.2. Isotherm analysis The purpose of the adsorption isotherms is to relate the adsorbate concentration in the bulk and the adsorbed amount at the interface [15]. The analysis of the isotherm data is
important to develop an equation which accurately represents the results and which could be used for design purposes [16]. Several isotherm equations are available. Two of them have been selected in this study: Langmuir and Freundlich isotherms. The Langmuir isotherm assumes that all the adsorption sites are equivalent [17] and that there is no interaction between adsorbed species [16]. The linear form of the Lang-
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M. Do˘gan, M. Alkan / Journal of Colloid and Interface Science 267 (2003) 32–41
muir isotherm for adsorption onto a single site solid surface (a homogeneous surface having one type of site) has frequently been applied as Ce Ce 1 + = , Qe Qm K Qm
(6)
where Qe is equilibrium dye concentration on adsorbent (mol g−1 ), Qm is monolayer capacity of the adsorbent (mol g−1 ), K is adsorption constant (l mol−1 ), and Ce is equilibrium dye concentration in solution (mol l−1 ). According to the Eq. (6), a plot of Ce /Qe versus Ce should be a straight line with slope 1/Qm and intercept 1/Qm K when adsorption follows the Langmuir equation. The Freundlich equation in logarithmic form can be written as log Qe = log KF +
1 log Ce , n
(7)
where KF and n are empirical Freundlich constants, being indicative of the extent of the adsorption and the degree of nonlinearity between solution concentration and adsorption, respectively. The value of n is usually between 2 to 10. If Eq. (7) applies, a plot of log Qe against log Ce will give a straight line, of slope 1/n and intercept log KF [18]. Adsorption isotherms were obtained in terms of Eqs. (6) and (7) using experimental adsorption results in these equations. The Langmuir equation represents the adsorption process very well; the r-values were almost all higher than 0.99, indicating a very good mathematical fit. The fact that the Langmuir isotherm fits the experimental data very well may be due to homogenous distribution of active sites on the perlite surface, since the Langmuir equation assumes that the surface is homogenous [17,19]. Furthermore, r-values for fitting of the experimental data to the Freundlich isotherm were in the range 0.61–0.97. The removal efficiency, P , is given as [17] P=
C0 − Ce × 100. C0
(8)
The removal efficiency rose from 98.0–80.0% at 30 ◦ C up to 99.8–70.0% at 60 ◦ C for unexpanded perlite and from 98.0– 55.0% at 30 ◦ C up to 99.8–60.0% at 60 ◦ C for expanded perlite. The shape of the isotherm may also be considered with a view to predicting if an adsorption system is “favorable” or “unfavorable.” The essential characteristics of a Langmuir isotherm can be expressed in terms of a dimensionless separation factor or equilibrium parameter R [20], which is defined by 1 R= . 1 + KCe
(9)
According to the value of R the isotherm shape may be interpreted as follows:
Value of R
Type of adsorption
R > 1.0 R = 1.0 0 < R < 1.0 R=0
unfavorable linear favorable irreversible
The fact that all the R-values for the adsorption of methyl violet on the perlite are in the range 0.004–0.999 shows that the adsorption process is favorable. This indicates that the adsorption process becomes more favorable with increasing temperature [15]. The enthalpy of adsorption, Hads , as a function of coverage fraction (θ = Qe /Qm ) can be estimated from van’t Hoff isochore using the adsorption data at various temperatures for methyl violet [18,20]. The subscript θ means that the equilibrium constant at each temperature is measured at constant coverage. The values of Hads at θ = 0.5 were calculated as 13.4 kcal mol−1 for expanded perlite and 16.5 kcal mol−1 for unexpanded perlite from the data given in Fig. 5. The heat of physical adsorption, which involves only relatively weak intermolecular forces such as van der Waals and electrostatic interactions, is low compared to that of chemisorption, which involves essentially the formation of a chemical bond between the sorbate and molecule and the surface of the adsorbent. The upper limit for physical adsorption may be higher than 20 kcal mol−1 for adsorption on adsorbents. The heat of chemisorption ranges from over 100 kcal mol−1 to less than 20 kcal mol−1 [21]. The results above show that the interaction between surface and adsorbate molecules is a physical interaction. 3.3. Designing batch adsorption from isotherm data Adsorption isotherms can be used to predict the design of single-stage batch adsorption systems [17]. A schematic diagram is shown in Fig. 6 where the effluent contains V l of water and an initial methyl violet concentration C0 , which is to be reduced to C1 in the adsorption process. In the treatment stage W g perlite (dye-free) is added to solution and the dye concentration on the solid changes from Q0 = 0 (initially) to Q1 . The mass balance that equates the dye removed from the liquid effluent to that accumulated by the solid is V (C0 − C1 ) = W (Q1 − Q0 ) = W Q1 .
(10)
In the case of the adsorption of methyl violet on unexpanded and expanded perlite samples the Langmuir isotherm gives the best fit to experimental data. Consequently equation can be best substituted for Q1 in the rearranged form of Eq. (10) giving adsorbent/solution ratios for this particular system, C0 − C1 W C0 − Ce = ≡ Q KC . m e V Qe
(11)
1+KCe
Figures 7a and 7b show a series of plots derived from Eq. (11) for the adsorption of methyl violet on unexpanded and expanded perlite. An initial dye concentration of 5.0 ×
M. Do˘gan, M. Alkan / Journal of Colloid and Interface Science 267 (2003) 32–41
39
Fig. 5. Plot of − ln Ce versus 1/T for methyl violet adsorption on perlite.
Fig. 6. Single-stage batch adsorber.
10−5 mol l−1 at 30 ◦ C and pH 11 is assumed and figures show the amount of effluent which can be treated to reduce the methyl violet content by 50, 60, 70, 80, and 90% using various masses of adsorbent. 3.4. Conclusions The following points may be mentioned as the results of this study. 1. The experimental data were correlated reasonably well by the Langmuir adsorption isotherm.
2. The adsorbed amount of methyl violet slightly decreased with increasing concentration of H2 SO4 used for the acid activation for both of perlite samples. 3. The adsorbed amounts of methyl violet increased with increasing pH for both of perlite samples. 4. The adsorbed amount of methyl violet increased with increase in temperature for both of perlite samples. 5. The dimensionless separation factor (R) showed that perlite can be used for removal of methyl violet from aqueous solutions, but unexpanded perlite is more effective. Its adsorption capacity is greater than that of expanded perlite.
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M. Do˘gan, M. Alkan / Journal of Colloid and Interface Science 267 (2003) 32–41
Fig. 7. Volume of effluent (V ) treated against adsorbent mass (W ) for different percentages of methyl violet removal.
6. The values of Hads for unexpanded and expanded perlite samples were calculated as 16.5 and 13.4 kcal mol−1 , respectively. 7. As a result, it can be said that perlite has considerable potential as an adsorbent of dyes in a commercial system because it is cheap.
Acknowledgment The work was financially supported by the Balıkesir University Research Fund (Project 2000/1).
References [1] M. Dai, J. Colloid Interface Sci. 164 (1994) 223–228. [2] I. Arvanitooyannis, I. Eleftheriadis, E. Tsatsaroni, Chemosphere 18 (9/10) (1989) 1707–1711. [3] S.K. Khare, K.K. Panday, R.M. Srivastava, V.N. Singh, J. Chem. Technol. Biotechnol. 38 (1987) 99–104. [4] M. Alkan, M. Do˘gan, J. Colloid Interface Sci. 243 (2001) 280–291. [5] P.W. Harben, R.L. Bates, Industrial Minerals: Geology and World Deposits, Metal Bulletin Inc., London, 1990. [6] C.W. Chesterman, Industrial Minerals and Rocks, AIME, New York, 1975. [7] S.S. Uluatam, J. Am. Water Works Assoc. 83 (6) (1991) 70–71. [8] M. Do˘gan, M. Alkan, Ü. Çakir, J. Colloid Interface Sci. 192 (1997) 114–118. [9] M. Alkan, M. Do˘gan, J. Colloid Interface Sci. 207 (1998) 90–96.
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[10] O. Inel, Turkish J. Chem. 19 (4) (1995) 323–330. [11] M. Do˘gan, M. Alkan, Y. Onganer, Water Air Soil Pollut. 120 (2000) 229–248. [12] M. Do˘gan, MSc Thesis, University of Balıkesir, Department of Chemistry, Balıkesir, Turkey, 1997 (in Turkish). [13] E. Gonzàlez-Pradas, M. Villafranca-Sànchez, A. Valverde-Garcìa, M. Socias-Viciana, J. Chem. Technol. Biotechnol. 42 (1988) 105–111. [14] J.D. López-Gonzàlez, S. Gonzàlez-Garcìa, An. Fis. Quim. B 50 (1954) 465–470. [15] H.M. Asfour, O.A. Fadali, M.M. Nassar, M.S. El-Geundi, J. Chem. Technol. Biotechnol. 35 (1) (1985) 21–27.
41
[16] J. Eastoe, J.S. Dalton, Adv. Colloid Interface Sci. 85 (2000) 103–144. [17] G. McKay, M.S. Otterburn, A.J. Aga, Water Air Soil Pollut. 24 (3) (1985) 307–322. [18] K.J. Laidler, J.H. Meiser, Physical Chemistry, Houghton Mifflin, New York, 1999. [19] A. Gürses, S. Bayrakçeken, M.S. ¸ Gülabo˘glu, Colloids Surf. 64 (1) (1992) 7–13. [20] G. McKay, V.J.P. Poots, J. Chem. Technol. Biotechnol. 30 (6) (1980) 279–292. [21] K.E. Noll, V. Gounaris, W.S. Hou, Adsorption Technology for Air and Water Pollution Control, Lewis Publishers, 1992, pp. 21–22.
Removal of methyl violet from aqueous solution by perlite Mehmet Do˘gan and Mahir Alkan ∗ Balıkesir University, Faculty of Science and Literature, Department of Chemistry, 10100 Balıkesir, Turkey Received 5 November 2002; accepted 22 May 2003
Abstract The use of perlite for the removal of methyl violet from aqueous solutions at different concentration, pH, and temperature has been investigated. Adsorption equilibrium is reached within 1 h. The capacity of perlite samples for the adsorption of methyl violet was found to increase with increasing pH and temperature and decrease with expansion and increasing acid-activation. The adsorption isotherms are described by means of the Langmuir and Freundlich isotherms. The adsorption isotherm was measured experimentally at different conditions and the experimental data were correlated reasonably well by the adsorption isotherm of Langmuir. The order of heat of adsorption corresponds to a physical reaction. It is concluded that the methyl violet is physically adsorbed onto the perlite. The removal efficiency (P ) and dimensionless separation factor (R) have shown that perlite can be used for removal of methyl violet from aqueous solutions, but unexpanded perlite is more effective. 2003 Elsevier Inc. All rights reserved. Keywords: Adsorption isotherms; Methyl violet; Perlite; Dye
1. Introduction Many industries routinely use dyes or pigments to color their products. A number of these dyes or pigments are inevitably left in the industrial waste, which could be a hazard to the environmental [1]. Textile industry waters are generally processed in biological treatment units for removal of biodegradable organic compounds. These biological processes typically accomplish very little towards color removal while handling these wastewaters [2]. The removal of color from textile wastewaters is one of the major environmental problems because of the difficulty of treating such water by conventional treatment methods [3]. In our day various physical–chemical techniques have been studied to assess their applicability for the treatment of this type of industrial discharges. Among these processes may be included coagulation, precipitation, flocculation, ozonation, reverse osmosis, ion exchange, and activated carbon adsorption [2,4]. Various types of materials have been used as adsorbents, such as activated carbon, manganese oxide, silica gel, fly ash, wollastonite, lignite, peat, soil, alumina, rutil, geothite, hematit, bentonit, sphalerit, anatase, red mud, mica, illite, kaolinite, and clays [4]. * Corresponding author.
E-mail address: [email protected] (M. Alkan). 0021-9797/$ – see front matter 2003 Elsevier Inc. All rights reserved. doi:10.1016/S0021-9797(03)00579-4
Perlite is a glassy volcanic rock that expands to about 20 times its original volume upon heating within its softening temperature range of 760 to 1100 ◦ C [5]. The uses of expanded perlite are many and varied and are based primarily upon its physical and chemical properties. As most perlites have a high silica content, usually greater than 70%, and are adsorptive, they are chemically inert in many environments and hence are excellent filter aids and fillers in various processes and materials. Furthermore, perlite is also used as a catalyst in chemical reaction [6]. Along the Aegean Coast, Turkey possesses about 70% (70 × 109 tons) of the world’s known perlite reserves [7]. The main consumption of perlite is in construction related fields. In our previous works, we investigated the electrokinetic properties of perlite [8], surface titrations of perlite suspensions [9], and adsorption of copper (II) onto perlite [4]. The present work is aimed to study a convenient and economic method for methyl violet removal from water by adsorption on a low-cost and abundantly available adsorbent on which no work could be found in the literature. The effects of expansion, solution pH, acid-activation, and temperature on methyl violet adsorption have been investigated. Furthermore, the results obtained have been applied to a batch design for the removal of methyl violet from aqueous media using perlite samples.
M. Do˘gan, M. Alkan / Journal of Colloid and Interface Science 267 (2003) 32–41
2. Materials and methods
33
solution was allowed to stand for 1–2 days until the absorbance of the solutions remained unchanged [1].
2.1. Materials 2.3. Method The unexpanded and expanded perlite samples were obtained from the Cumaovası Perlite Processing Plants of Etibank (˙Izmir, Turkey). The chemical composition of the perlite found in Turkey in the literature is given in Table 1 [7]. Treatment of the unexpanded and expanded perlite samples before use in experiments is described elsewhere [8]. In order to obtain the acid-activated perlite samples, H2 SO4 solutions were used [10]. The cation exchange capacity (CEC) of the various perlite samples was determined by the ammonium acetate method, density by pycnometry. The specific surface area of the samples of expanded (EP), acid-activated expanded perlite (EHP(0.6)), unexpanded (UP), and acid-activated unexpanded perlite (UHP(0.6)) were measured by BET N2 adsorption [10]. The results are summarized in Table 2. All chemicals were obtained from Merck. 2.2. Preparation of methyl violet solutions Methyl violet was dried at 70–80 ◦ C for 4 h to remove moisture and then was dissolved in distilled water. Since methyl violet is difficult to dissolve in water, methyl violet Table 1 Chemical composition of perlite Constituent
Percentage present
SiO2 Al2 O3 Na2 O K2 O CaO Fe2 O3 MgO TiO2 MnO2 SO3 FeO Ba PbO Cr
71–75 12.5–18 2.9–4.0 4.0–5.0 0.5–2.0 0.1–1.5 0.03–0.5 0.03–0.2 0.0–0.1 0.0–0.1 0.0–0.1 0.0–0.1 0.0–0.5 0.0–0.1
Adsorption experiments were carried out by shaking 0.5-g perlite samples with 50-ml aqueous solutions of methyl violet of desired concentrations at various pHs and temperatures for 1 h. Prior to adsorption experiments the solution was kept under N2 for 10 min. A preliminary experiment revealed that about 1 h is required for methyl violet to reach the equilibrium concentration. A thermostated shaker bath was used to keep the temperature constant. The initial concentration of methyl violet solutes, C0 , was varied in the range 5 × 10−7 –3 × 10−4 mol l−1 for unexpanded and expanded perlite samples. All adsorption experiments were performed at 30 ◦ C and pH 11 except those in which the effects of temperature and pH of methyl violet solution were investigated. The pH of the solution was adjusted with NaOH or HNO3 solution by using an Orion 920A pH meter equipped with a combined pH electrode. The pH meter was standardized with NBS buffers before every measurement. At the end of the adsorption period, the solution was centrifuged for 15 min at 3000 rpm and then the concentration of the residual methyl violet, Ce , was determined with the aid of a Cary |1E| UV-visible spectrophotometer (Varian). The measurements were made at the wavelength λ = 584 nm, which corresponds to maximum absorbance. Blanks containing no methyl violet were used for each series of experiments. The amounts of methyl violet adsorbed were calculated from the concentrations in solutions before and after adsorption. Each experimental point was an average of three independent adsorption tests [11].
3. Results and discussion 3.1. Adsorption isotherms Figure 1 shows the adsorption isotherms of methyl violet on the unexpanded and expanded perlite samples. The adsorbed amount of methyl violet for unexpanded perlite is greater than that for expanded perlite. The decrease in
Table 2 Some physicochemical properties of perlite samples used in the study Sample
Nomenclature
Expanded, purified in water Expanded, 0.2-M acid-activated Expanded, 0.4-M acid-activated Expanded, 0.6-M acid-activated Unexpanded, purified in water Unexpanded, 0.2 M acid-activated Unexpanded, 0.4-M acid-activated Unexpanded, 0.6-M acid-activated
EP EHP(0.2) EHP(0.4) EHP(0.6) UP UHP(0.2) UHP(0.4) UHP(0.6)
CEC (meg 100 g−1 )
Density (g ml−1 )
Specific surface area (m2 g−1 )
Zeta potential (mV)
33.30 38.20 43.38 54.24 25.97 32.79 35.00 36.56
2.24 2.10 2.04 1.93 2.30 2.32 2.38 2.46
2.30 – – 2.33 1.22 – – 1.99
−46.8 −47.1 −46.3 −44.0 −23.5 −21.8 −22.0 −21.1
34
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Fig. 1. The effect of thermal treatment on the adsorption of methyl violet on perlite.
the amount of adsorption by expansion may be a result of events occurring during the calcination: (i) the decrease in the amount of hydroxyl groups and (ii) the removal of most of the micropores due to heating the sample. Infrared spectra of the unexpanded and expanded perlite samples show that the number of hydroxyl groups is decreased by the thermal treatment in the production of expanded perlite from unexpanded perlite. The decrease in the amount of hydroxyl groups of the adsorbent, which are mainly effective sites for adsorption, during the expansion of perlite is thought to cause a decrease in adsorption capacity, although expanded perlite has greater values of cation exchange capacity (CEC), zeta potential (ZP), and specific surface area than unexpanded perlite [12]. The effect of acid activation on the adsorption of methyl violet onto perlite samples is given in Fig. 2, indicating that the adsorbed amounts of methyl violet slightly decrease with the concentration of H2 SO4 used for the acid activation for both of the perlite samples. This decrease observed may be due to the partial destruction of perlite structure as shown by Gonzàlez-Pradas et al. [13] and López-Gonzàlez and Gonzàlez-Garcià [14] for bentonite and may be due to the decrease in OH groups in perlite during the activation process, as was shown by Do˘gan using IR-spectra. Furthermore, the increase in CEC with acid activation, as seen in Table 2, may cause a decrease in adsorption capacity of dye
as well, which is an indicator of charged replacable sites on the surface [12]. To study the influence of pH on the adsorption capacity of perlite samples for methyl violet, experiments were performed using various initial solution pH values, changing from 3 to 11 (Fig. 3). The removal of methyl violet by perlite samples has been increased when the pH of the dye solution was increased. In the discussion of the effect of pH, it is necessary to discuss the pKa -values of the S–OH groups of perlite samples. Surface charge will develop via the amphoteric ionization of the surface hydroxyl groups according to the reactions SOH+ 2
Kaint
SOH + H+ s , 1
(1)
Kaint
SOH SO− + H+ s , 2
(2)
where the subscript s denotes the surface and the equilibrium constant for Eqs. (1) and (2): = Kaint 1
[SOH][H+ s ] , + [SOH2 ]
(3)
Kaint = 2
[SO− ][H+ s ] . [SOH]
(4)
The formation of surface species, [SOH+ 2 and SOH], is the principal mechanism by which protons are released (or con-
M. Do˘gan, M. Alkan / Journal of Colloid and Interface Science 267 (2003) 32–41
35
Fig. 2. The effect of acid-activation on the adsorption of methyl violet on perlite.
sumed) by many oxide surface in aqueous electrolyte solutions. For oxides in general, as pKaint and pKaint increase, 1 2 and S–OH, dethe acidity of both surface species, S–OH+ 2 int creases. In this sense, pKa values are considered as a meavalues for expanded and unexsure of surface acidity. pKaint 2 panded perlite samples were, respectively, determined as 2.7 and 3.0 with NaCl as an electrolyte [4]. It can be said that
the surface hydroxyl groups on perlite are acidic. When considered together with the fact that the surface is negatively charged through the entire range of the studied pH values (i.e., pH 3–10), this result shows that the reaction (2) is in favor of the right-hand side at low concentrations. Therefore, as the pH of the dye solution becomes higher (Eq. (2)), the association of dye cations with negatively charged perlite
36
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Fig. 3. The effect of pH of the solution on the adsorption of methyl violet on perlite.
surface can more easily take place through an electrostatic interaction as follows (Eq. (5)): ≡SO− + Dye+ S–O− −+ Dye.
(5)
A study of the temperature dependence of adsorption reactions gives valuable information about the enthalpy change during adsorption. The effect of temperature on the adsorption isotherm was studied by carrying out a series of isotherms at 30, 40, 50, and 60 ◦ C for both of the perlite sam-
ples (unexpanded perlite and expanded perlite) and shown in Fig. 4. Results indicate that the adsorption capacity of unexpanded and expanded perlite for adsorption of methyl violet increases with increasing temperature, which is typical for the adsorption of most organics from their solution. The effect of temperature is fairly common and increasing the temperature must increase the mobility of the large dye cation. Furthermore, increasing temperature may produce a
M. Do˘gan, M. Alkan / Journal of Colloid and Interface Science 267 (2003) 32–41
37
Fig. 4. The effect of temperature on the adsorption of methyl violet on perlite.
swelling effect within the internal structure of the perlite enabling large dyes to penetrate further. 3.2. Isotherm analysis The purpose of the adsorption isotherms is to relate the adsorbate concentration in the bulk and the adsorbed amount at the interface [15]. The analysis of the isotherm data is
important to develop an equation which accurately represents the results and which could be used for design purposes [16]. Several isotherm equations are available. Two of them have been selected in this study: Langmuir and Freundlich isotherms. The Langmuir isotherm assumes that all the adsorption sites are equivalent [17] and that there is no interaction between adsorbed species [16]. The linear form of the Lang-
38
M. Do˘gan, M. Alkan / Journal of Colloid and Interface Science 267 (2003) 32–41
muir isotherm for adsorption onto a single site solid surface (a homogeneous surface having one type of site) has frequently been applied as Ce Ce 1 + = , Qe Qm K Qm
(6)
where Qe is equilibrium dye concentration on adsorbent (mol g−1 ), Qm is monolayer capacity of the adsorbent (mol g−1 ), K is adsorption constant (l mol−1 ), and Ce is equilibrium dye concentration in solution (mol l−1 ). According to the Eq. (6), a plot of Ce /Qe versus Ce should be a straight line with slope 1/Qm and intercept 1/Qm K when adsorption follows the Langmuir equation. The Freundlich equation in logarithmic form can be written as log Qe = log KF +
1 log Ce , n
(7)
where KF and n are empirical Freundlich constants, being indicative of the extent of the adsorption and the degree of nonlinearity between solution concentration and adsorption, respectively. The value of n is usually between 2 to 10. If Eq. (7) applies, a plot of log Qe against log Ce will give a straight line, of slope 1/n and intercept log KF [18]. Adsorption isotherms were obtained in terms of Eqs. (6) and (7) using experimental adsorption results in these equations. The Langmuir equation represents the adsorption process very well; the r-values were almost all higher than 0.99, indicating a very good mathematical fit. The fact that the Langmuir isotherm fits the experimental data very well may be due to homogenous distribution of active sites on the perlite surface, since the Langmuir equation assumes that the surface is homogenous [17,19]. Furthermore, r-values for fitting of the experimental data to the Freundlich isotherm were in the range 0.61–0.97. The removal efficiency, P , is given as [17] P=
C0 − Ce × 100. C0
(8)
The removal efficiency rose from 98.0–80.0% at 30 ◦ C up to 99.8–70.0% at 60 ◦ C for unexpanded perlite and from 98.0– 55.0% at 30 ◦ C up to 99.8–60.0% at 60 ◦ C for expanded perlite. The shape of the isotherm may also be considered with a view to predicting if an adsorption system is “favorable” or “unfavorable.” The essential characteristics of a Langmuir isotherm can be expressed in terms of a dimensionless separation factor or equilibrium parameter R [20], which is defined by 1 R= . 1 + KCe
(9)
According to the value of R the isotherm shape may be interpreted as follows:
Value of R
Type of adsorption
R > 1.0 R = 1.0 0 < R < 1.0 R=0
unfavorable linear favorable irreversible
The fact that all the R-values for the adsorption of methyl violet on the perlite are in the range 0.004–0.999 shows that the adsorption process is favorable. This indicates that the adsorption process becomes more favorable with increasing temperature [15]. The enthalpy of adsorption, Hads , as a function of coverage fraction (θ = Qe /Qm ) can be estimated from van’t Hoff isochore using the adsorption data at various temperatures for methyl violet [18,20]. The subscript θ means that the equilibrium constant at each temperature is measured at constant coverage. The values of Hads at θ = 0.5 were calculated as 13.4 kcal mol−1 for expanded perlite and 16.5 kcal mol−1 for unexpanded perlite from the data given in Fig. 5. The heat of physical adsorption, which involves only relatively weak intermolecular forces such as van der Waals and electrostatic interactions, is low compared to that of chemisorption, which involves essentially the formation of a chemical bond between the sorbate and molecule and the surface of the adsorbent. The upper limit for physical adsorption may be higher than 20 kcal mol−1 for adsorption on adsorbents. The heat of chemisorption ranges from over 100 kcal mol−1 to less than 20 kcal mol−1 [21]. The results above show that the interaction between surface and adsorbate molecules is a physical interaction. 3.3. Designing batch adsorption from isotherm data Adsorption isotherms can be used to predict the design of single-stage batch adsorption systems [17]. A schematic diagram is shown in Fig. 6 where the effluent contains V l of water and an initial methyl violet concentration C0 , which is to be reduced to C1 in the adsorption process. In the treatment stage W g perlite (dye-free) is added to solution and the dye concentration on the solid changes from Q0 = 0 (initially) to Q1 . The mass balance that equates the dye removed from the liquid effluent to that accumulated by the solid is V (C0 − C1 ) = W (Q1 − Q0 ) = W Q1 .
(10)
In the case of the adsorption of methyl violet on unexpanded and expanded perlite samples the Langmuir isotherm gives the best fit to experimental data. Consequently equation can be best substituted for Q1 in the rearranged form of Eq. (10) giving adsorbent/solution ratios for this particular system, C0 − C1 W C0 − Ce = ≡ Q KC . m e V Qe
(11)
1+KCe
Figures 7a and 7b show a series of plots derived from Eq. (11) for the adsorption of methyl violet on unexpanded and expanded perlite. An initial dye concentration of 5.0 ×
M. Do˘gan, M. Alkan / Journal of Colloid and Interface Science 267 (2003) 32–41
39
Fig. 5. Plot of − ln Ce versus 1/T for methyl violet adsorption on perlite.
Fig. 6. Single-stage batch adsorber.
10−5 mol l−1 at 30 ◦ C and pH 11 is assumed and figures show the amount of effluent which can be treated to reduce the methyl violet content by 50, 60, 70, 80, and 90% using various masses of adsorbent. 3.4. Conclusions The following points may be mentioned as the results of this study. 1. The experimental data were correlated reasonably well by the Langmuir adsorption isotherm.
2. The adsorbed amount of methyl violet slightly decreased with increasing concentration of H2 SO4 used for the acid activation for both of perlite samples. 3. The adsorbed amounts of methyl violet increased with increasing pH for both of perlite samples. 4. The adsorbed amount of methyl violet increased with increase in temperature for both of perlite samples. 5. The dimensionless separation factor (R) showed that perlite can be used for removal of methyl violet from aqueous solutions, but unexpanded perlite is more effective. Its adsorption capacity is greater than that of expanded perlite.
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Fig. 7. Volume of effluent (V ) treated against adsorbent mass (W ) for different percentages of methyl violet removal.
6. The values of Hads for unexpanded and expanded perlite samples were calculated as 16.5 and 13.4 kcal mol−1 , respectively. 7. As a result, it can be said that perlite has considerable potential as an adsorbent of dyes in a commercial system because it is cheap.
Acknowledgment The work was financially supported by the Balıkesir University Research Fund (Project 2000/1).
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