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Studies on the adsorption of dyes into clinoptilolite
Desalination 243 (2009) 286–292
Studies on the adsorption of dyes into clinoptilolite Muqing Qiua*, Chen Qianb, Jun Xub, Jianmin Wub, Genxuan Wangb a
College of Life Sciences, Shaoxing University, Shaoxing 312000, PR China Fax: +86 (575) 8507 1649; email: [email protected] b Institute of Agro-Ecology and Eco-Engineering, College of Life Sciences, Zhejiang University, Hangzhou 310058, PR China Received 19 November 2006; Accepted 18 April 2008
Abstract Adsorption techniques are widely used to remove certain classes of pollutants from waters, especially those that are not easily biodegradable. Dyes represent one of the problematic groups. Natural zeolite was used as a low-cost adsorbent to evaluate its ability to remove color from effluents. In this paper, the adsorption of dyes into clinoptilolite was studied by a series of experiments. The influence of sorbent concentration, adsorption time, initial dye concentration, and pH has been analyzed in detail with Amido Black 10B and Safranine T respectively. The results indicate that clinoptilolite has a limited adsorption capacity for Amido Black 10B and has a good adsorption capacity for Safranine T. The adsorption isotherm has also been determined using the adsorption data. It was found that the adsorption isotherms for Safranine T dye-natural zeolite system and Amido Black 10B dye-natural zeolite system at 80 mg/L of solid concentration, pH 7.0 and 2EC both fitted to the Langmuir isotherm. For the Safranine T dyenatural zeolite system, the fitted parameters qm and bL are 0.05513 mg/g and 1.9623 L/mg respectively, and for the Amido Black 10B dye-natural zeolite system, the fitted parameters qm and bL are 0.0112 mg/g and 0.5962 L/mg respectively. Keywords: Adsorption; Clinoptilolite; Dyes; Isotherm
1. Introduction Many industries such as dyestuffs, textile, paper and plastics, use dyes in order to color their products while in the same time consume substantial volumes of water. As a result, they *Corresponding author.
generate a considerable amount of colored wastewater. The presence of very small amounts of dyes in water is highly visible and undesirable [1–3]. In addition, many of these dyes are also toxic and even carcinogenic and this poses a serious hazard to aquatic living organisms. They, therefore, need to be removed or decolorized before the wastewater can be discharged. How-
0011-9164/09/$– See front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2008.04.029
M. Qiu et al. / Desalination 243 (2009) 286–292
ever, wastewater containing dyes is very difficult to treat since the dyes are recalcitrant organic molecules, resistant to aerobic digestion, and are stable to light, heat and oxidizing agents [4]. During the past three decades, several physical, chemical and biological decolorization methods have been reported [5–7]. Few, however, have been accepted by the industries.Among the numerous techniques of dye removal, adsorption is the procedure of choice and gives the best results as it can be used to remove different types of coloring materials. In addition, adsorption has been found to be superior to other techniques for water reused in terms of initial cost, simplicity of design, ease of operation and insensitivity to toxic [8]. Most commercial systems currently use activated carbon as sorbent to remove dyes in wastewater, which is an expensive material. Recently, in order to decrease the cost of treatment, attempts have been made to find inexpensive alternative adsorbents. Many non-conentional low-cost adsorbents, including natural materials, biosorbents, and waste materials from industry and agriculture, have been proposed by several workers. These materials could be used as sorbents for the removal of dyes from solution. Some of the reported sorbents include clay materials, zeolites, siliceous material, agricultural wastes, industrial waste products, biosorbents and others [9–15]. Adsorption of dyes on various materials have been extensively investigated, and activated carbon has proved the most effective because of its high specific surface area, high adsorption capacity and low selectivity for both ionic and nonionic dyes [16]. Unfortunately, activated carbon adsorption is an expensive method due to its high price and the difficulties involved in its regeneration for reuse. In recent years, many synthetic adsorbents such as hydrotalcite, low cost and easily obtainable natural materials such as montmorillonite, zeolite and pyrophyllite, biomaterials produced from agricultural byproducts, and industrial solid wastes such as fly
287
ash, as adsorbents, have been tested for pollutant removal [17]. Zeolites are highly porous aluminosilicates with different cavity structures. Their structures consist of a three dimensional framework, having a negatively charged lattice. The negative charge is balanced by cations which are exchangeable with certain cations in solutions. Zeolites consist of a wide variety of species, more than 40 natural species. However, the most abundant and frequently studied zeolite is clinoptilolite, a mineral of the heulandite group. Its characteristic tabular morphology shows an open reticular structure of easy access, formed by open channels of 8–10 membered rings. Clinoptilolite has been shown to have high selectivity for certain pollutants [18,19]. High ion-exchange capacity and relatively high specific surface areas, and more importantly their relatively low prices, make zeolites attractive adsorbents [20,21]. Therefore, an inexpensive residual management technology is needed for the disposal or beneficial uses of this material. In this paper, the adsorption of Amido Black 10B and Safranine T into clinoptilolite was studied by a series of experiments. The influence of sorbent concentration, adsorption time, initial dye concentration, and pH has been analyzed in detail with Amido Black 10B and Safranine T respectively for the purpose of understanding the adsorption mechanism of dyes on zeolite. 2. Experimental 2.1. Material 2.1.1. Adsorbents The clinoptiolite sample used in these experiments was obtained from the Zeolite Company in the People’s Republic of China. Chemical composition of the sample is: 78% SiO2, 1.25% CaO, 3.24% K2O, 0.03% SO3, 12.3% Al2O3, 1.05% MgO, 0.02% TiO2, 0.05% P2O5, 1.35% Fe2O3, and 0.35% Na2O(wt). Properties of the sample
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of solid concentration (solid/liquid), adsorption time, initial dye concentration, and pH have been analyzed with Amido Black 10B and Safranine T respectively. The value of pH in the solution was adjusted by (1+1) HCl or (1+1) NaOH. 2.3. Analytical determinations
Fig. 1. Chemical structure of Amido Black 10B.
The concentration of dyes was measured with a UV-1600 spectrophotometer at a wavelength corresponding to the maximum absorbance for each dye: 618 nm and 514 nm for Amido Black 10B and Safranine T respectively. The value of pH was measured with a pH probe. The solid phase loading was calculated by the following formula:
qe
Fig. 2. Chemical structure of Safranine T.
are: particle size of 0.5–1.0 mm, porosity of 0.425, solid density of 2.32g/cm3, and particle density of 1.25g/cm3. 2.1.2 Adsorbates Two dyes were chosen for investigation, Amido Black 10B and Safranine T, which were botained from the Chemical Company in Tianjin. Their purity is of analytical degree. Their chemical structures are presented in Figs. 1 and 2.
Ci Ce *V 1000* m
(1)
where qe is amount of dye adsorbed per gram of adsorbent in mg/g, Ci is the initial dye concentration in mg/L, Ce is the equilibrium (residual) dye concentration in mg/L, V is the volume of the solution in ml and m is mass of adsorbent in g. The removal efficiency was calculated by the following formula:
Q
A0 Ai *100% A0
(2)
where Q is the removal efficiency, A0 is the absorbency of initial dye, and Ai is the absorbency of equilibrium dye. 2.4. Statistical analyses of data
2.2. Method All adsorption experiments were conducted with 150 ml flasks containing 50 ml of solution. The flasks were placed stilly at 2EC. Distilled water was used in all experiments. The influence
All experiments were performed at three times. The data of results are the mean. The value of the SD can be calculated by Excell software.
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3. Results and discussion
3.2. Effect of sorbent concentration
3.1. Effect of adsorption time
Adsorption experiments were carried out against sorbent concentration (10, 20, 30, 40, 80 120 and 160 mg/L) for 360 min at 4 mg/l of initial dye concentration, pH 7.0 and 2EC. The results of effect of sorbent concentration on the adsorption of dyes into the natural zeolite are shown in Fig. 4. Fig. 4(a) shows that the solid phase loading decreases very quickly with increasing solid concentration, and the solid phase loading remains almost unchange after a soild concentrantion of 80 mg/L. However, from Fig. 4(b) it can be seen that the removal efficiency increases very quickly with increasing solid concentration and also remains almost constant after a soild concentration of 80 mg/L. Adding more clinoptilolite results in a little further adsorption. The data
A series of experiments at a constant dye concentration of 4 mg/L, pH 7.0, solid concentration of 40 mg/L and 2EC were carried out to find the effect of adsorption time on the adsorption of dyes into clinoptilolite. The results were determined at different time (30, 60, 120, 180, 240, 300, 360, 420, 480, 540 and 600 min). The results of tests are given in Fig. 3. As shown in Fig. 3, when the adsorption time increases, the amount of adsorption of dyes into the natural zeolite increases significantly. It is evident that most of the adsorption occurs within the first 360 min; 360 min later, the solid phase loading of both Amiod Black 10B and Safranine T increases very slowly. That is, the effect of the adsorption time on the adsorption of dyes into clinoptilolite is evident within the first 360 min. The increasing time up to 360 min led to a parallel increase in the amount of adsorbed dyes. The adsorption process reached over its equilibrium time. The amount of adsorbed dyes did not show significant change after 360 min.
Fig. 3. Effect of adsorption time on the adsorption of dyes into zeolite.
Fig. 4. Effect of sorbent concentration on the adsorption of dyes into the natural zeolite.
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obtained show that Amido Black 10B and Safranine T are adsorbed into the natural zeolite with a removal efficiency of 81.2% and 16.3% respectively. It also indicated that clinoptilolite has a limited adsorption capacity for Amido Black 10B. This result is consistent with the reported paper [20].
surface charge density decreases with an increase in the solution pH, the electrostatic repulsion between the positively charged dye and the surface of the adsorbent is lowered, which may result in an increase in the extent of adsorption. 3.4. Effect of initial dye concentration
Fig. 5 shows the adsorption of the natural zeolite at five different initial pH values for 360 min at an initial dye concentration of 4 mg/l, soild concentration of 80 mg/L and 2EC. It is seen that the amount of adsorption for Safranine T increases as the pH is increasing. When the pH is changed from 2.0 to 11.0, the adsorption will increase from 0.025 mg/g to 0.066 mg/g. Several investigations have also shown that the natural zeolite will have higher adsorption at higher pH values [14,15,22]. However, for Amido Black 10B, when the pH is increasing, the amount of adsorption decreases slowly. For cationic dyes, lower adsorption of the natural zeolite at acidic pH is probably due to the presence of excess H+ ions competing with the cation groups on the dye for adsorption sites. As
The experiments were undertaken to study the effect of varying the initial dye concentration on the rate of dye removal from solution. Amido Black 10B and Safranine T are adsorbed into the natural zeolite for 360 min of adsorption time at 80 mg/L of solid concentration, pH 7.0 and 2EC. Fig. 6 shows the curves for the adsorption of dyes on natural zeolite using different initial dye concentrations. As seen from Fig. 6, when the initial dye concentration was increased, the removal efficiency decreased in Safranine T and Amido Black 10B. That is, there is a positive adsorption on natural zeolite for Safranine T and Amido Black 10B, when the initial dyes concentration increases. It also shows how the adsorbate molecules partition are between the liquid and solid phases when the adsorption process reaches equilibrium conditions.
Fig. 5. Effect of pH on the adsorption of dyes on natural zeolite.
Fig. 6. Effect of the initial dye concentration on the adsorption of dyes on natural zeolite.
3.3. Effect of pH
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3.5. Analysis of the adsorption isotherms The adsorption data are usually fitted to different adsorption models.The most frequently used for dilute solutions are the Langmuir isotherm [23,24] represented by Eq. (3):
qe bc L e qm 1 bL ce
(3)
where qe is equibrium the solid-phase concentration in mg/g, qm is maximal adsorption concentration in mg/g, and bL is equibrium constant in L/mg, and ce is equilibrium liquid-phase concentration in mg/L. From Fig. 3, after a very rapid biosorption, dye uptake capacities increased with time and reached equilibrium values at about 360 min. After that, dye uptake became much less significant. Thus, it is thought the equilibrium time is 360 min. According to Eq. (3) and experimental data in Fig. 4, parameters in the equilibrium isotherms for the Safranine T dye-natural zeolite and Amido Black 10B dye-natural zeolite systems can be obtained. Experimental results are compared with theoretical results, and their R2 and SD results can also be obtained (Table1) through Excel2003 software. According to the R2 and SD results, it can be shown that for the Safranine T dye-natural zeolite and Amido Black 10B dye-natural zeolite Table 1 Parameters for the Safranine T dye-natural zeolite and Amido Black 10B dye-natural zeolite systems at 80 mg/L of solid concentration, pH 7.0 and 2EC Langmuir isotherm constants
Safranine T dye- Amido Black 10B natural zeolite dye-natural zeolite
qm ( mg/g) bL ( L/mg) R2 SD
0.05513 1.9623 0.996 0.06
0.0112 0.5962 0.995 0.04
291
systems at 80 mg/l of solid concentration, pH 7.0 and 2EC, the correlation between experimental data and theoretical data are both very good. For the Safranine T dye-natural zeolite system at 80 mg/L of solid concentration, pH 7.0 and 2EC, the fitted parameters qm and bL are 0.05513 mg/g and 1.9623 L/mg respectively, and for the Amido Black 10B dye-natural zeolite system at 80 mg/L of solid concentration, pH 7.0 and 2EC, the fitted parameters qm and bL are 0.0112 mg/g and 0.5962 L/mg respectively. Thus, the adsorption isotherm for Safranine T dye-natural zeolite and Amido Black 10B dye-natural zeolite systems at 80 mg/L of solid concentration, pH 7.0 and 2EC are both fitted to the Langmuir isotherm.
4. Conclusions In conclusion, we have shown that the optimum sorbent concentration and the optimum adsorption time are 80 mg/L and 360 min for the Safranine T dye-natural zeolite and Amido Black 10B dye-natural zeolite systems. Amido Black 10B and Safranine T are adsorbed into natural zeolite with a removal effecience of 81.2% and 16.3% respectively for 360 min of adsorption time at 4 mg/L of initial dye concentration, 80 mg/L of solid concentration, pH 7.0 and 2EC. It was indicated that clinoptilolite has a limited adsorption capacity for Amido Black 10B. The amount of adsorption for Safranine T increases as pH increases, but is the contrary for Amido Black 10B. According to the adsorption data, the adsorption isotherms for Safranine T dye-natural zeolite and Amido Black 10B dye-natural zeolite systems at 80 mg/L of solid concentration, pH 7.0 and 2EC are both fitted to the Langmuir isotherm.
Acknowledgements The study was supported by the Key Item of Zhejiang Province (2005C23082).
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References [1] G. Crini, Non-conventional low-cost adsorbents for dye removal: A review. Bioresource Technol., 97 (2006) 1061–1085. [2] I.M. Banat, P. Nigam, D. Singh and R. Marchant, Microbial decolorization of textile-dye-containing effluents: a review. Bioresource Technol., 58 (1996) 217–227. [3] T. Robinson, G. McMullan, R. Marchant and P. Nigam, Remediation of dyes in textile effluent: a critical review on current treatment technologies with a proposed alternative. Bioresource Technol., 77 (2001) 247–255. [4] Q. Sun and L. Yang, The adsorption of basic dyes from aqueous solution on modified peat-resin particle. Water Resource, 37 (2003) 1535–1544. [5] S.S. Tahir and N. Rauf, Removal of a cationic dye from aqueous solutions by adsorption onto bentonite clay. Chemosphere, 63 (2006) 1842–1848. [6] J.X. Chen and L.H. Zhu, Catalytic degradation of Orange II by UV-Fenton with hydroxyl-Fe-pillared bentonite in water. Chemosphere, 65 (2006) 1249– 1255. [7] F.L. Fu, Y. Xiong, B.P. Xie and R.M. Chen, Adsorption of Acid Red 73 on copper dithiocarbamate precipitate-type solid wastes. Chemosphere, 66 (2007) 1–7. [8] A.K. Jain, V.K. Gupta, A. Bhatnagar and A. Suhas, Utilization of industrial waste products as adsorbents for the removal of dyes. J. Haz. Mat. B, 101 (2003) 31–34. [9] L.T. Teng, E. Khor, T.K. Tan, L.Y. Lim and S.C. Tan, Concurrent production of chitin from shrimp shells and fugi. Carbohydrase Res., 332 (2001) 305– 316. [10] A. Shukla, Y.H. Zhang, P. Dubey, J.L. Margrave and S.S. Shukla, The role of sawdust in the removal of unwanted materials from water. J. Haz. Mat. B, 95 (2002) 137–152. [11] S.J. Allen, G. McKay and J.F. Porter, Adsorption isotherm models for basic dye adsorption by peat in single and binary component systems. J. Coll. Internat. Sci., 280 (2004) 322–333. [12] Z. Aksu, Application of biosorption for the removal of organic pollutants: a review. Proc. Biochem., 40 (2005) 997–1026.
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Studies on the adsorption of dyes into clinoptilolite Muqing Qiua*, Chen Qianb, Jun Xub, Jianmin Wub, Genxuan Wangb a
College of Life Sciences, Shaoxing University, Shaoxing 312000, PR China Fax: +86 (575) 8507 1649; email: [email protected] b Institute of Agro-Ecology and Eco-Engineering, College of Life Sciences, Zhejiang University, Hangzhou 310058, PR China Received 19 November 2006; Accepted 18 April 2008
Abstract Adsorption techniques are widely used to remove certain classes of pollutants from waters, especially those that are not easily biodegradable. Dyes represent one of the problematic groups. Natural zeolite was used as a low-cost adsorbent to evaluate its ability to remove color from effluents. In this paper, the adsorption of dyes into clinoptilolite was studied by a series of experiments. The influence of sorbent concentration, adsorption time, initial dye concentration, and pH has been analyzed in detail with Amido Black 10B and Safranine T respectively. The results indicate that clinoptilolite has a limited adsorption capacity for Amido Black 10B and has a good adsorption capacity for Safranine T. The adsorption isotherm has also been determined using the adsorption data. It was found that the adsorption isotherms for Safranine T dye-natural zeolite system and Amido Black 10B dye-natural zeolite system at 80 mg/L of solid concentration, pH 7.0 and 2EC both fitted to the Langmuir isotherm. For the Safranine T dyenatural zeolite system, the fitted parameters qm and bL are 0.05513 mg/g and 1.9623 L/mg respectively, and for the Amido Black 10B dye-natural zeolite system, the fitted parameters qm and bL are 0.0112 mg/g and 0.5962 L/mg respectively. Keywords: Adsorption; Clinoptilolite; Dyes; Isotherm
1. Introduction Many industries such as dyestuffs, textile, paper and plastics, use dyes in order to color their products while in the same time consume substantial volumes of water. As a result, they *Corresponding author.
generate a considerable amount of colored wastewater. The presence of very small amounts of dyes in water is highly visible and undesirable [1–3]. In addition, many of these dyes are also toxic and even carcinogenic and this poses a serious hazard to aquatic living organisms. They, therefore, need to be removed or decolorized before the wastewater can be discharged. How-
0011-9164/09/$– See front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2008.04.029
M. Qiu et al. / Desalination 243 (2009) 286–292
ever, wastewater containing dyes is very difficult to treat since the dyes are recalcitrant organic molecules, resistant to aerobic digestion, and are stable to light, heat and oxidizing agents [4]. During the past three decades, several physical, chemical and biological decolorization methods have been reported [5–7]. Few, however, have been accepted by the industries.Among the numerous techniques of dye removal, adsorption is the procedure of choice and gives the best results as it can be used to remove different types of coloring materials. In addition, adsorption has been found to be superior to other techniques for water reused in terms of initial cost, simplicity of design, ease of operation and insensitivity to toxic [8]. Most commercial systems currently use activated carbon as sorbent to remove dyes in wastewater, which is an expensive material. Recently, in order to decrease the cost of treatment, attempts have been made to find inexpensive alternative adsorbents. Many non-conentional low-cost adsorbents, including natural materials, biosorbents, and waste materials from industry and agriculture, have been proposed by several workers. These materials could be used as sorbents for the removal of dyes from solution. Some of the reported sorbents include clay materials, zeolites, siliceous material, agricultural wastes, industrial waste products, biosorbents and others [9–15]. Adsorption of dyes on various materials have been extensively investigated, and activated carbon has proved the most effective because of its high specific surface area, high adsorption capacity and low selectivity for both ionic and nonionic dyes [16]. Unfortunately, activated carbon adsorption is an expensive method due to its high price and the difficulties involved in its regeneration for reuse. In recent years, many synthetic adsorbents such as hydrotalcite, low cost and easily obtainable natural materials such as montmorillonite, zeolite and pyrophyllite, biomaterials produced from agricultural byproducts, and industrial solid wastes such as fly
287
ash, as adsorbents, have been tested for pollutant removal [17]. Zeolites are highly porous aluminosilicates with different cavity structures. Their structures consist of a three dimensional framework, having a negatively charged lattice. The negative charge is balanced by cations which are exchangeable with certain cations in solutions. Zeolites consist of a wide variety of species, more than 40 natural species. However, the most abundant and frequently studied zeolite is clinoptilolite, a mineral of the heulandite group. Its characteristic tabular morphology shows an open reticular structure of easy access, formed by open channels of 8–10 membered rings. Clinoptilolite has been shown to have high selectivity for certain pollutants [18,19]. High ion-exchange capacity and relatively high specific surface areas, and more importantly their relatively low prices, make zeolites attractive adsorbents [20,21]. Therefore, an inexpensive residual management technology is needed for the disposal or beneficial uses of this material. In this paper, the adsorption of Amido Black 10B and Safranine T into clinoptilolite was studied by a series of experiments. The influence of sorbent concentration, adsorption time, initial dye concentration, and pH has been analyzed in detail with Amido Black 10B and Safranine T respectively for the purpose of understanding the adsorption mechanism of dyes on zeolite. 2. Experimental 2.1. Material 2.1.1. Adsorbents The clinoptiolite sample used in these experiments was obtained from the Zeolite Company in the People’s Republic of China. Chemical composition of the sample is: 78% SiO2, 1.25% CaO, 3.24% K2O, 0.03% SO3, 12.3% Al2O3, 1.05% MgO, 0.02% TiO2, 0.05% P2O5, 1.35% Fe2O3, and 0.35% Na2O(wt). Properties of the sample
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of solid concentration (solid/liquid), adsorption time, initial dye concentration, and pH have been analyzed with Amido Black 10B and Safranine T respectively. The value of pH in the solution was adjusted by (1+1) HCl or (1+1) NaOH. 2.3. Analytical determinations
Fig. 1. Chemical structure of Amido Black 10B.
The concentration of dyes was measured with a UV-1600 spectrophotometer at a wavelength corresponding to the maximum absorbance for each dye: 618 nm and 514 nm for Amido Black 10B and Safranine T respectively. The value of pH was measured with a pH probe. The solid phase loading was calculated by the following formula:
qe
Fig. 2. Chemical structure of Safranine T.
are: particle size of 0.5–1.0 mm, porosity of 0.425, solid density of 2.32g/cm3, and particle density of 1.25g/cm3. 2.1.2 Adsorbates Two dyes were chosen for investigation, Amido Black 10B and Safranine T, which were botained from the Chemical Company in Tianjin. Their purity is of analytical degree. Their chemical structures are presented in Figs. 1 and 2.
Ci Ce *V 1000* m
(1)
where qe is amount of dye adsorbed per gram of adsorbent in mg/g, Ci is the initial dye concentration in mg/L, Ce is the equilibrium (residual) dye concentration in mg/L, V is the volume of the solution in ml and m is mass of adsorbent in g. The removal efficiency was calculated by the following formula:
Q
A0 Ai *100% A0
(2)
where Q is the removal efficiency, A0 is the absorbency of initial dye, and Ai is the absorbency of equilibrium dye. 2.4. Statistical analyses of data
2.2. Method All adsorption experiments were conducted with 150 ml flasks containing 50 ml of solution. The flasks were placed stilly at 2EC. Distilled water was used in all experiments. The influence
All experiments were performed at three times. The data of results are the mean. The value of the SD can be calculated by Excell software.
M. Qiu et al. / Desalination 243 (2009) 286–292
289
3. Results and discussion
3.2. Effect of sorbent concentration
3.1. Effect of adsorption time
Adsorption experiments were carried out against sorbent concentration (10, 20, 30, 40, 80 120 and 160 mg/L) for 360 min at 4 mg/l of initial dye concentration, pH 7.0 and 2EC. The results of effect of sorbent concentration on the adsorption of dyes into the natural zeolite are shown in Fig. 4. Fig. 4(a) shows that the solid phase loading decreases very quickly with increasing solid concentration, and the solid phase loading remains almost unchange after a soild concentrantion of 80 mg/L. However, from Fig. 4(b) it can be seen that the removal efficiency increases very quickly with increasing solid concentration and also remains almost constant after a soild concentration of 80 mg/L. Adding more clinoptilolite results in a little further adsorption. The data
A series of experiments at a constant dye concentration of 4 mg/L, pH 7.0, solid concentration of 40 mg/L and 2EC were carried out to find the effect of adsorption time on the adsorption of dyes into clinoptilolite. The results were determined at different time (30, 60, 120, 180, 240, 300, 360, 420, 480, 540 and 600 min). The results of tests are given in Fig. 3. As shown in Fig. 3, when the adsorption time increases, the amount of adsorption of dyes into the natural zeolite increases significantly. It is evident that most of the adsorption occurs within the first 360 min; 360 min later, the solid phase loading of both Amiod Black 10B and Safranine T increases very slowly. That is, the effect of the adsorption time on the adsorption of dyes into clinoptilolite is evident within the first 360 min. The increasing time up to 360 min led to a parallel increase in the amount of adsorbed dyes. The adsorption process reached over its equilibrium time. The amount of adsorbed dyes did not show significant change after 360 min.
Fig. 3. Effect of adsorption time on the adsorption of dyes into zeolite.
Fig. 4. Effect of sorbent concentration on the adsorption of dyes into the natural zeolite.
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obtained show that Amido Black 10B and Safranine T are adsorbed into the natural zeolite with a removal efficiency of 81.2% and 16.3% respectively. It also indicated that clinoptilolite has a limited adsorption capacity for Amido Black 10B. This result is consistent with the reported paper [20].
surface charge density decreases with an increase in the solution pH, the electrostatic repulsion between the positively charged dye and the surface of the adsorbent is lowered, which may result in an increase in the extent of adsorption. 3.4. Effect of initial dye concentration
Fig. 5 shows the adsorption of the natural zeolite at five different initial pH values for 360 min at an initial dye concentration of 4 mg/l, soild concentration of 80 mg/L and 2EC. It is seen that the amount of adsorption for Safranine T increases as the pH is increasing. When the pH is changed from 2.0 to 11.0, the adsorption will increase from 0.025 mg/g to 0.066 mg/g. Several investigations have also shown that the natural zeolite will have higher adsorption at higher pH values [14,15,22]. However, for Amido Black 10B, when the pH is increasing, the amount of adsorption decreases slowly. For cationic dyes, lower adsorption of the natural zeolite at acidic pH is probably due to the presence of excess H+ ions competing with the cation groups on the dye for adsorption sites. As
The experiments were undertaken to study the effect of varying the initial dye concentration on the rate of dye removal from solution. Amido Black 10B and Safranine T are adsorbed into the natural zeolite for 360 min of adsorption time at 80 mg/L of solid concentration, pH 7.0 and 2EC. Fig. 6 shows the curves for the adsorption of dyes on natural zeolite using different initial dye concentrations. As seen from Fig. 6, when the initial dye concentration was increased, the removal efficiency decreased in Safranine T and Amido Black 10B. That is, there is a positive adsorption on natural zeolite for Safranine T and Amido Black 10B, when the initial dyes concentration increases. It also shows how the adsorbate molecules partition are between the liquid and solid phases when the adsorption process reaches equilibrium conditions.
Fig. 5. Effect of pH on the adsorption of dyes on natural zeolite.
Fig. 6. Effect of the initial dye concentration on the adsorption of dyes on natural zeolite.
3.3. Effect of pH
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3.5. Analysis of the adsorption isotherms The adsorption data are usually fitted to different adsorption models.The most frequently used for dilute solutions are the Langmuir isotherm [23,24] represented by Eq. (3):
qe bc L e qm 1 bL ce
(3)
where qe is equibrium the solid-phase concentration in mg/g, qm is maximal adsorption concentration in mg/g, and bL is equibrium constant in L/mg, and ce is equilibrium liquid-phase concentration in mg/L. From Fig. 3, after a very rapid biosorption, dye uptake capacities increased with time and reached equilibrium values at about 360 min. After that, dye uptake became much less significant. Thus, it is thought the equilibrium time is 360 min. According to Eq. (3) and experimental data in Fig. 4, parameters in the equilibrium isotherms for the Safranine T dye-natural zeolite and Amido Black 10B dye-natural zeolite systems can be obtained. Experimental results are compared with theoretical results, and their R2 and SD results can also be obtained (Table1) through Excel2003 software. According to the R2 and SD results, it can be shown that for the Safranine T dye-natural zeolite and Amido Black 10B dye-natural zeolite Table 1 Parameters for the Safranine T dye-natural zeolite and Amido Black 10B dye-natural zeolite systems at 80 mg/L of solid concentration, pH 7.0 and 2EC Langmuir isotherm constants
Safranine T dye- Amido Black 10B natural zeolite dye-natural zeolite
qm ( mg/g) bL ( L/mg) R2 SD
0.05513 1.9623 0.996 0.06
0.0112 0.5962 0.995 0.04
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systems at 80 mg/l of solid concentration, pH 7.0 and 2EC, the correlation between experimental data and theoretical data are both very good. For the Safranine T dye-natural zeolite system at 80 mg/L of solid concentration, pH 7.0 and 2EC, the fitted parameters qm and bL are 0.05513 mg/g and 1.9623 L/mg respectively, and for the Amido Black 10B dye-natural zeolite system at 80 mg/L of solid concentration, pH 7.0 and 2EC, the fitted parameters qm and bL are 0.0112 mg/g and 0.5962 L/mg respectively. Thus, the adsorption isotherm for Safranine T dye-natural zeolite and Amido Black 10B dye-natural zeolite systems at 80 mg/L of solid concentration, pH 7.0 and 2EC are both fitted to the Langmuir isotherm.
4. Conclusions In conclusion, we have shown that the optimum sorbent concentration and the optimum adsorption time are 80 mg/L and 360 min for the Safranine T dye-natural zeolite and Amido Black 10B dye-natural zeolite systems. Amido Black 10B and Safranine T are adsorbed into natural zeolite with a removal effecience of 81.2% and 16.3% respectively for 360 min of adsorption time at 4 mg/L of initial dye concentration, 80 mg/L of solid concentration, pH 7.0 and 2EC. It was indicated that clinoptilolite has a limited adsorption capacity for Amido Black 10B. The amount of adsorption for Safranine T increases as pH increases, but is the contrary for Amido Black 10B. According to the adsorption data, the adsorption isotherms for Safranine T dye-natural zeolite and Amido Black 10B dye-natural zeolite systems at 80 mg/L of solid concentration, pH 7.0 and 2EC are both fitted to the Langmuir isotherm.
Acknowledgements The study was supported by the Key Item of Zhejiang Province (2005C23082).
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