EXTERNAL MASS TRANSPORT PROCESS DURING THE ADSORPTION OF METHYL VIOLET ONTO SODIC BENTONITE

Transcription

1 Journal of the University of Chemical S. Guiza, Technology M. Bagane and Metallurgy, 47, 3, 2012, EXTERNAL MASS TRANSPORT PROCESS DURING THE ADSORPTION OF METHYL VIOLET ONTO SODIC BENTONITE S. Guiza, M. Bagane National Engineering School of Gabés Department of Chemical and Process Engineering Tunisia Received 28 February 2012 Accepted 15 May 2012 ABSTRACT The kinetics and mechanism of methyl violet (MV) adsorption onto sodic bentonite has been studied in an agitation batch adsorber. The effect of agitation, initial dye concentration, clay mass and clay particle size have been determined with the experimental data mathematically described using empirical external transfer. The experimental data show conformity with an adsorption process, with the removal rate heavily dependent on both external mass transfers. The mass transfer coefficient has been correlated with the system variable. Keywords: methyl violet, sodic bentonite, adsorption kineticks, external mass transfer. INTRODUCTION Clays are hydrous aluminosilicates broadly defined as those minerals that make up a colloid fraction (< 2 µm) of soils, sediments, rocks and water [1] and may be composed of mixtures of fine grained clay minerals and clay-sized crystals of other minerals such as quartz, carbonate and metal oxides. The clays invariably contain exchangeable ions on their surface and play an important role in the environment by acting as a natural scavenger of pollutants by taking up cations and/or anions either through ion exchange or adsorption or both. The prominent ions found on clay surface are Ca 2+, Mg 2+,Al 3+,Si 4+, H +, K +, NH 4+, Na +, and SO 4 2-, Cl -, PO 43, NO 3-. These ions can be exchanged with other ions easily without affecting the structure of the clay mineral [2]. Natural clay minerals are well known and familiar to mankind from the earliest days of civilization. Because of their low cost, abundance in most continents of the world, high sorption properties and a potential for ion exchange, clay materials are strong adsorbents. They possess layered structure and are considered as host materials for the adsorbates and counter ions. Vermiculite clay has the largest surface area and the highest cation exchange capacity. Its current market price (about US$ /kg) is considered to be about 20 times cheaper than that of activated carbon [3]. In recent years, there has been an increasing interest in utilizing clay minerals such as bentonite, kaolonite, diatomite and fuller s earth for their capacity to adsorb not only inorganic ions but also organic molecules. In particular, interactions between MV and clay particles have been extensively studied [4]. The clay minerals exhibit a strong affinity for MV [5 ]. The adsorption of MV on clay minerals is mainly dominated by ion-exchange processes. This means that the sorption capacity can vary strongly with ph. Al-Ghouti et al. [6] showed that the mechanism of adsorption of dye onto diatomite is due to physical sorption (depending on the particle size) and the presence of electrostatic interactions (depending on the ph used). Almeida et al. [7] studied the removal of methylene blue from synthetic wastewater by using a montmorillonite and described it as an efficient adsorbent where the equilibrium was attained in less than 30 min. The adsorption of dyes on kaolinite was also studied by Ghosh et al. [8]. They reported that its adsorption capacity can be improved by purification and treatment with NaOH solution. The adsorption capacity of other adsorbents for MV obtained by some other investigators is pre- 283

2 Journal of the University of Chemical Technology and Metallurgy, 47, 3, 2012 sented in Table 1. Relatively good removal capabilities of clay to uptake MV has been demonstrated by many researchers. Bagane and Guiza [8] reported an adsorption capacity of 526 mg/g and suggest that clay is a good adsorbent for MV removal due to its high surface area. The purpose of this work was to study the adsorption kinetics of methyl violet on natural clay. EXPERIMENTAL Adsorbent The Tunisian clay used is essentially sodic or it can be described as a sodic bentonite. It contains about % of sodic smectite, which is the most valuable type of smectite. The adsorbent was not subjected to any form of pretreatment, except that the clay was washed with distillated water to remove the adhering impurities and dried at 160 o C. Clay characteristics are shown in Table 2. Dye Methyl violet (MV) labeled Merck (C.I ), was retained as a cationic dye and contains a secondary amino group (see Fig.1). It has a maximum absorbency at wavelength 580 nm on a UV vis spectrophotometrer. The chemical formula and molecular mass of MV is C 24 H 28 ClN 3 and g/mol, respectively. ANALYSIS OF ADSORPTION KINETICS External mass transfer model A commonly used empirical model was used in this work to calculate the external mass transfer coefficient, [21]. In a well agitated batch adsorber mixing in the liquid is rapid, hence, the concentration of the adsorbate and the concentration, m s, of the particles are assumed to be uniform throughout the vessel. Consequently, m s is determined from the measured mass of adsorbent (W) and the volume of the particle free liquid (V) as shown in Eq.(1) below Fig. 1. Chemical structure of the methyl violet dye. W ms = (1) V The change in the fluid phase concentration, C, with respect to time, t, is related to the fluid phase concentration at the external surface, C s and the external mass transfer coefficient,, by dct = kf Ap( Ct Cs) (2) dt Ct = C at t = 0 (3) 0 Assuming smooth spherical particles the surface area for mass transfer to the particle can be obtained from m s using 6ms Ap = d ρ (1 ε ) (4) p p p where d p is mean particle diameter, å p is particle void age and ñ p is particle density. The differential mass balance of the solute within the particle has been determined by previous authors [20], so as C t C 0, Eq. (2) becomes C d t C dt 0 t = 0 = k f A p Hence, at t=0 a plot of C t /C 0 will yield a slop of - A p from which can be determined. The values can be correlated by the following equation: k = A(var iable) B f or in the logarithmic form: Ln( k ) ( ) (var ) B f = Ln A + BLn iable where the variable can represent agitation speed, initial dye concentration, clay mass or clay particle size. RESULTS AND DISCUSSION Effect of agitation Fig. 2 shows the experimental results obtained from a series of contact time studies for the adsorption of (MV) on clay in which the degree of agitation, N, was varied. The curves represent concentration-time profiles for moderate contact times. The experiments were conducted over an 1 h period. The results indicate that the rate of adsorption of MV dye is controlled by the degree of agitation. Thus, (5) 284

3 S. Guiza, M. Bagane Table 1. Comparison of sorption capacities of various adsorbents for methyl violet. Adsorbent MV capacity (mg/g) Ref. Spepiolite 92,58 9 Halloysite nanotubes 113,64 10 Granular activated carbon 95,00 11 Agricultural waste 92,00 12 Bagasse fly ash 26,24 13 Activated carbon Mansonia wood sawdust 16,11 15 Sepliolite 10,24 16 Ghassoul Table 2. Clay characteristics [17-19]. Fig. 2. Effect of agitation. C 0 = 20 mg/l, ph = 8,T = 20 C, m = 0.1g/l, dp=0.45 mm. Physical analysis Cation exchange capacity 80 eq/100 g Particle density (g/cm 3 ) 2.05 True density (g/cm 3 ) 2.91 Particle porosity (%) 36 Total pore volume (cm 3 /g) Total surface(n 2 BET method) (m 2 /g) 71 Mean particle diameter (ìm) 140 Particle shape spherical Chemical analysis SiO 2 49,22 Al 2 O 3 13,75 Fe 2 O 3 6,08 CaO 6,34 MgO 2,22 K 2 O 0,56 Na 2 O 1,16 P 2 O 5 0,43 P.F 17,75 Fig. 3. Plot of ln ( ) versus ln (N). the boundary layer resistance to mass transfer is reduced by increasing agitation. The external mass transfer coefficients,, have been determined using equation (1) and were plotted as ln ( ) versus ln(n) as shown in Fig. 3. From this figure, a linear dependence can be observed. The data indicates that the external adsorption of dye on clay controlled by the degree of increasing agitation diminishes the boundary layer resistance to mass transfer and increase the mobility of the system. From the values shown in Table 2 we found: A= and B = Effect of initial dye concentration The initial dye concentration of an effluent is important since a given mass of clay can only adsorb a Fig. 4. Effect of initial dye concentration ( m = 0.1g/l, ph = 8.5, T = 20 C, N = 461rpm, dp=0.45 mm). certain amount of dye. Therefore, the more concentration an effluent, the smaller is the volume of effluent that a fixed mass of clay can purify. The influence of initial dye concentration was studied and the range of concentrations was varied depending on the amount of colouring matter in the commercial salt. Experimental results are shown in Fig. 4 for the adsorption of MV on clay. At high concentrations the lines lie close together and the fractional adsorption is low. However, for low 285

4 Journal of the University of Chemical Technology and Metallurgy, 47, 3, 2012 Fig. 5. Plot of ln ( ) versus ln (C 0 ). concentrations the initial uptake of dye is rapid, indicating a rapid surface reaction. Consequently, the concentration of dye in the effluent will greatly affect the extent and rate of dye uptakes on clay. The external mass transfer coefficients,, were determined as before and were plotted as ln( )versus ln(c 0 ) as shown in Fig. 5. Linear dependence was observed. The results are presented in Table 2.The external mass transfer coefficients are increasing as C 0 decreases for the dye MV being adsorbed onto clay. The values of A and B are respectively equal to 11.63x10-5 and Effect of particle size range The influence of contact time on dye ranges of particle size of clay was investigated using the size ranges listed in Table 2.The experimental results are shown in Fig. 6. The data shows an increase in the rate of dye uptake as the mean diameter of clay decreases. This observation is in agreement with the proposed mechanism, as the large external surface area removes more dye in the initial stages of the adsorption process than large particles. The external mass transfer coefficients,, were determined as before and are listed in Table 2. Fig. 7 shows the ln( ) versus ln (dp) correlation of dye MV, and a linear graph was obtained indicating that varies with dp in a definite logarithmic manner. It is observed that increasing the mean particle diameter, dp, results in a decrease of the external mass transfer coefficient,. The values of A and B are respectively equal to 6.47 x 10-5 and Effect of clay mass The effect of clay mass was studied on the dye MV and experimental conditions maintained constant. Fig. 6. Effect of particle size range (C 0 = 20 mg/l, ph = 8.5, T = 20 C, m = 0.1g/l, N = 461 rpm. Fig. 7. Plot of ln ( ) versus ln (dp). Fig. 8. Effect of clay mass (C 0 = 20 mg/l, ph = 8.5, T = 20 C, N = 461rpm, dp=0.45mm). In all cases the rate of dye uptake increased with increasing clay mass; the results are shown in Fig. 8 as plot of C t /C 0 versus time. The external mass transfer coefficient depends on the driving force per unit area, and in this case, since C 0 is constant, increasing the mass of clay increases the surface area for adsorption and hence the rate of dye removal is increased. Since the particle size range is constant, the surface area will 286

5 S. Guiza, M. Bagane Table 4. Comparison of the external mass transfer coefficient. Adsorbent Dyes (m/s) Ref Activated AB [ 22 ] carbon AY Activated Polyethylene [23] carbon glycol Carbon Silica Phenol 10± [24] BY21 3± BY ± Fig. 9. Plot of ln ( ) versus ln (m). Table 3. External mass transfer coefficient. Effect of k (10-5 m/s) N (tr/min) C 0 (mg/l) m (g) dp (mm) be directly proportional to the mass of clay in the system. From Fig. 9 we found: A = 7.57x10-5 and B = The results of the analyses of the external mass transfer coefficient are illustrated in Table 3. Comparison with various adsorbents The comparison of the external mass transfer coefficient of Tunisian sodic bentonite clay with various adsorbents is given in Table 4. Even the studied couple s diversity, the external mass transfer coefficient,, remains in the range m/s. CONCLUSIONS The external mass transport phenomena which influence the extent and initial rate of uptake of methyl Bone char Cd Cu [25] Zn Dolomitic Reactive dye 3, N 0.14 [26] Sodic bentonite MV C N m 0.28 This work dp violet MV on clay have been studied. A general model has been proposed which enables the external mass transfer coefficient to be determined. The external mass transfer coefficient has been found to vary linearly with agitation, initial dye concentration, clay particle size, mass of clay, according to the general equation: = A (variable) B. The constants A and B have been determined for each variable. It appears that the rate of dye removal increases with the agitation speed and mass of adsorbent but it decreases with the initial concentration and the particle size. The external mass transfer coefficient,, and the results show that this coefficient is in the range of m/s. REFERENCES 1. R. Mohd, S. Othman, R. Hasmim, A. Ahmad, Adsorption of methylen blue on low-cost adsorbents: A review, J. Hazard. Mat., 177, 2010, K.G. Bhattacharyya, S.S. Gupta, Adsorption of a few heavy metals on natural and modified kalinite and montmorillonite: a review, Adv. Colloid Interface Sci., 140, 2008, S. Babel, T.A. Kurrniawan, Low-cost adsorbents for heavy metals uptake from contaminated water: areview, J.Hazard.Mat., B97, 2003, A. Gurses, S. Karaca, C. Dogar, R. Bayrak, 287

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