Application of kinetic models to the sorption of disperse dyes onto alunite

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1 Colloids and Surfaces A: Physicochem. Eng. Aspects 242 (24) Application of kinetic models to the sorption of disperse dyes onto alunite Mahmut Özacar a,, İ. Ayhan Şengil b a Department of Chemistry, Science and Arts Faculty, Sakarya University, 54 Sakarya, Turkey b Department of Environmental Engineering, Engineering Faculty, Sakarya University, 54 Sakarya, Turkey Received 17 September 23; accepted 3 March 24 Available online 17 June 24 Abstract The sorption of three disperse dyes, namely, Disperse Blue 56 (DB56), Disperse Red 74 (DR74) and Disperse Yellow 119 (DY119), onto alunite has been studied in terms of pseudo-first- and second-order sorptions and intraparticle diffusion processes thus comparing chemical sorption and diffusion sorption processes. The pseudo-second-order model provided a high degree of correlation with the experimental data for the sorption processes. There was a small discrepancy at the beginning of the experiments (5 3 min) which suggested that intraparticle diffusion may be involved up to 3 min of the sorption process. The kinetics of sorption, based on the sorption capacities of disperse dyes on alunite, were followed at various time intervals. Results show that the intraparticle diffusion may be rate-limiting, followed by the pseudo-second-order kinetic model in the sorption of disperse dyes onto alunite during agitated batch contact time experiments. The rate constant, the equilibrium sorption capacity and the initial sorption rate were calculated as a function of the effect of alunite particle size, alunite dose, initial dye concentration and ph of the solution. 24 Elsevier B.V. All rights reserved. Keywords: Disperse dye; Alunite; Sorption; Kinetics; Pseudo-second-order; Intraparticle diffusion 1. Introduction The effluents of wastewater in some industries such as dyestuff, textiles, leather, paper, printing, plastic and food, etc. contain various types of synthetic dyestuffs. The treatment of textile wastewater comprising dyestuffs and other non-biodegradable organics and inorganics poses considerable problems in the wastewater treatment industry. Most studies have focussed on the development of a technique and a method for the treatment of dye wastewater. In general, there are several methods of reducing color in textile effluent streams: coagulation flocculation, biological treatment, oxidation ozonation, adsorption and membrane processes. The advantages and disadvantages of each technique have been extensively reviewed. Of these methods, adsorption has been found to be an efficient and economic process to remove dyes, pigments and other colorants [1,2]. Corresponding author. Fax: address: mozacar@hotmail.com (M. Özacar). The most widely used adsorbent for industrial applications is activated carbon. However, it is an expensive material unless regeneration is relatively easy, but this adds to the operational costs [3]. The use of activated carbon dose effectively removes color although it is ineffective against disperse and vat dyes [1]. However, the disperse dye group is one of the most important groups of dyes used in the textile dyeing industries [4]. In recent years, many cheap and widely-available materials have been identified as suitable adsorbents for the removal of color from wastewaters. The sorption of various dyes onto peat, pith, wood, sawdust, chitin, other various agricultural wastes, Fuller s earth, clay, slag, perlite and alunite has been studied [2,3,5 19]. The purpose of this work is to study the kinetics of adsorption of disperse dyes on alunite. Alunite, KAl 3 (OH) 6 (SO 4 ) 2, is a mineral not soluble in water in its original form. It is formed by the hydrothermal treatment of tuff. Therefore, it contains approximately 5% SiO 2. In our previous works, we investigated the adsorption of acid dyes [17], reactive dyes [18] and phosphate [2 22] onto alunite samples /$ see front matter 24 Elsevier B.V. All rights reserved. doi:1.116/j.colsurfa

2 16 M. Özacar, İ.A. Şengil / Colloids and Surfaces A: Physicochem. Eng. Aspects 242 (24) In order to design adsorption systems, a knowledge of the kinetic and mass transfer processes is desirable. Therefore, in the present paper three kinetic/mass transfer models are described and tested. These are a pseudo-first-order, a pseudo-second-order and an intraparticle diffusion model. This present work applies the batch sorption results of three dyes, namely, Disperse Blue 56 (DB56), Disperse Red 74 (DR74) and Disperse Yellow 119 (DY119) onto alunite, to investigate the influence of intraparticle diffusion and a pseudo-second-order mechanism. It compares the rate parameter, k int, of intraparticle diffusion, the rate parameter, k 2, of the pseudo-second-order and k 1, the rate parameter for the pseudo-first-order mechanisms. 2. Materials and methods The alunite sample was obtained from Dostel Aluminium Sulphate Ltd. in Kütahya-Şphane, Turkey. The composition of alunite ore is given in Table 1 [17]. Before using it in the experiments, the alunite sample was treated as follows [17]: Alunite was prepared by grinding it in a laboratory ball-mill. The alunite sample was calcined in muffle furnace at the temperature from 5 to 8 C for 6 min. The adsorption experiments with these calcined alunites, using dye solution having a concentration mg/l, were run to determine the effect of thermal treatment on the adsorption of disperse dyes on alunite. Then alunite was calcined at 6 C for different times (15 12 min), and same experiments were reperformed to determine the effect of calcination time on the adsorption of disperse dyes on alunite. The optimum calcination temperature and time were found to be 3 min at 6 C from these experiments (not shown). The thermal decomposition reactions and products of alunite are reported in our previous papers [17,18,21,22]. It was then sieved to give different particle size fractions using ASTM standard sieves. The BET specific surface areas were determined based on N 2 adsorption isotherms with a sorptiometer (Quantachrome Co., NOVA 2). The specific surface areas of different particle sizes are given in Table 2 [17]. Three disperse dyes were used as adsorbates. The dyes used in the experiments were Setapers Blue FBL (C.I. Dis- Table 1 Chemical composition of original alunite ore [17] Component Composition (wt. %) Al 2 O SiO SO Fe 2 O 3.7 TiO 2.17 CaO.17 MgO.1 K 2 O 4.59 Na 2 O.6 H 2 O 7.89 Table 2 BET specific surface areas for the different alunite particle size ranges [17] Calcination Temperature ( C) Time (min) Size range ( m) BET-specific surface area (m 2 /g) perse Blue 56 (DB56) λ max (wavelength at which maximum absorbance occurs) = 316 nm), Setapers Scarlet 2BLS (C.I. Disperse Red 74 (DR74) λ max = 565 nm) and Setapers Yellow 5GLH (C.I. Disperse Yellow 119 (DY119) λ max = 495 nm). The dyes were supplied by Setaş Chemical Co., Turkey. All these dyes are commercial grade and were used without further purification. All other chemicals used in the studies were obtained from Merck Chemical Co. Studies of the kinetics of disperse dyes adsorption onto alunite were carried out from their solutions. The dye concentration was mg/l, except those in which the effect of concentration was investigated. For the experiments of adsorption kinetics, 1 g of calcined alunite sample, except those in which the effect of adsorbent mass was investigated, was added into 1 L of dye solution at the desired particle size, concentration and ph. The ph of the solution was adjusted with NaOH or HCl solution and checked by ph meter. The mixture was continuously agitated by a magnetic stirrer at 298 K and 5 rpm for 18 min. A constant temperature bath was used to keep the temperature constant. The samples at predecided intervals of time were pipetted from the reactor by the aid of the very thin point micropipette, which prevented the transition to solution of alunite samples. The solution was centrifuged for 15 min at 5 rpm. The dye concentration in the clear supernatant at any time, t, was determined by spectrophotometric method. All measurements were recorded at the wavelength corresponding to maximum absorbance, λ max, using spectrophotometer (Shimadzu UV-15-2). The amount of adsorbed dye at any time, q t (mg/g), was calculated as follows: q e = (C C t )V/W (1) where C and C t are the initial and time t concentrations (mg/l), respectively, of dyes in solution; V is the volume of the solution (L) and W is the weight (g) of the dry adsorbent. 3. Results and discussion 3.1. Sorption kinetics Sorption of disperse dyes on alunite may involve a chemical sorption which may control the rate. In order to investigate the mechanism of sorption and potential rate-controlling

3 M. Özacar, İ.A. Şengil / Colloids and Surfaces A: Physicochem. Eng. Aspects 242 (24) steps such as mass transport and chemical reaction processes, several kinetic models were tested including the intraparticle diffusion, a pseudo-first- and second-order equations. The sorption kinetics may be described by a pseudo-first-order equation [2,3,5,1 12,17,18,2 23]. The differential equation is the following: dq t = k 1 (q 1 q t ) (2) dt After integration by applying the boundary conditions q t = at t = and q t = q t at t = t, Eq. (2) becomes: ( ) q1 log = k 1 q 1 q t 2.33 t (3) Eq. (3) can be rearranged to obtain a linear form: log(q 1 q t ) = logq 1 k t (4) where q 1 and q t are the amounts of dyes sorbed at equilibrium and at time t (mg/g), respectively, and k 1 is the equilibrium rate constant of pseudo-first-order sorption (1/min). q t (mg/g) d p (µm) 9-15 Experimen Experimen Experimen Experimen. dp (µm) 9-15 Experimen Experimen Experimen Experimen. dp (µm) 9-15 Experimen Experimen Experimen Experimen Fig. 1. Effect of alunite particle size on sorption of disperse dyes onto alunite. DB56, DR74 and DY119. The rate constant for pseudo-second-order sorption may be obtained from the following analysis [6,1 12,18,23 27]: dq t = k 2 (q 2 q t ) 2 (5) dt Separating the variables in Eq. (5) gives: dq t = k 2 t (6) q 2 q t Integrating Eq. (6) for the boundary conditions t = tot = t and q t = toq t = q t, gives: 1 = 1 + k 2 t (7) q 2 q t q 2 where k 2 is the equilibrium rate constant of pseudo-secondorder sorption (g/mg min). Eq. (7) can be rearranged to obtain a linear form: t = 1 q t k 2 q t (8) q 2 t/q t (g min/mg) d p (µm) d p (µm) d p (µm) Fig. 2. Pseudo-second-order kinetic plot for sorption of disperse dyes onto alunite at various alunite particle size. DB56, DR74 and DY119.

4 18 M. Özacar, İ.A. Şengil / Colloids and Surfaces A: Physicochem. Eng. Aspects 242 (24) Table 3 Parameters for the effects of alunite particle size, alunite dose, initial dye concentration and ph on sorption of DB56 onto alunite Parameters q e,exp (mg/g) Second-order kinetic model First-order kinetic model Intraparticle diffusion r 2 2 q 2 (mg/g) k 2 (g/(mg min)) 1 4 h (mg/(g min)) r 2 1 q 1 (mg/g) k 1 (1/min) 1 2 r 2 int k int (mg/(g min 1/2 )) Alunite particle size ( m) Alunite dose (g/l) Initial dye concertation (mg/l) ph Table 4 Parameters for the effects of alunite particle size, alunite dose, initial dye concentration and ph on sorption of DR74 onto alunite Parameters q e,exp (mg/g) Second-order kinetic model First-order kinetic model Intraparticle diffusion r 2 2 q 2 (mg/g) k 2 (g/(mg min)) 1 4 h (mg/(g min)) r 2 1 q 1 (mg/g) k 1 (1/min) 1 2 r 2 int k int (mg/(g min 1/2 )) Alunite paricle size ( m) Alunite dose (g/l) Initial dye concertation (mg/l) ph

5 M. Özacar, İ.A. Şengil / Colloids and Surfaces A: Physicochem. Eng. Aspects 242 (24) Table 5 Parameters for the effects of alunite particle size, alunite dose, initial dye concentration and ph on sorption of DY119 onto alunite Parameters q e,exp (mg/g) Second-order kinetic model First-order kinetic model Intraparticle diffusion r 2 2 q 2 (mg/g) k 2 (g/(mg min)) 1 4 h (mg/(g min)) r 2 1 q 1 (mg/g) k 1 (1/min) 1 2 r 2 int k int (mg/(g min 1/2 )) Alunite particle size ( m) Alunite dose (g/l) Initial dye concentration (mg/l) ph and h = k 2 q2 2 (9) where h is the initial sorption rate (mg/g min). The rate parameter of intraparticle diffusion [1,13,2, 28,29] can be defined as q t = k int t 1/2 (1) where k int is the intraparticle diffusion rate constant (mg/g min 1/2 ). Such plots may present a multilinearity [1,13,2], indicating that two or more steps are taking place. The first, sharper portion is the external surface sorption or instantaneous sorption stage. The second portion is the gradual sorption stage, where intraparticle diffusion is rate-controlled. The third portion is the final equilibrium stage where intraparticle diffusion starts to slow down due to extremely low adsorbate concentrations in the solution Effect of particle size The results of the effect of calcined alunite particle size on experiments carried out using the same initial dye concentration mg/l with alunite dose 1 g/l for three disperse dyes are shown in Fig. 1. The results are also shown in Fig. 2 as a plot of t/q t against t for sorption of disperse dyes for the pseudo-second-order model. Results from a series of kinetics experiments at four particle size ranges from 9 15 to 5 71 m for pseudo-second-order, pseudo-first-order and intraparticle diffusion rate models are shown in Tables 3 5. The data for each disperse dyes show a good compliance with the pseudo-second-order equation and the regression coefficients, r2 2, for the linear plots were all high (>.99). It was found that the equilibrium sorption of dye, q 2, the rate constant, k 2, and the initial sorption rate, h, are a function of alunite particle size. The initial sorption rate increased with a decrease in the alunite particle size. The initial sorption rates varied from 5.49 to mg/g min for DB56, from 6.65 to 9.54 mg/g min for DR74 and from 5.8 to 19.8 mg/g min for DY119, while the alunite particle size was varied from 5 71 to 9 15 m. This could be attributed to the increase in the outer surface area of adsorbent of smaller particle size, which would result in a more rapid uptake of dye molecules. Meanwhile, reducing the particle size would make the inner pores more accessible, but it would not change the internal structure of the alunite particles and the overall adsorbent capacity [3]. Inthe case of larger particles, the dye molecules have a longer intraparticle diffusion path than that of smaller particles [31]. The correlation coefficients, r1 2, the first-order rate parameters, k 1, and sorption capacity, q 1, are shown in Tables 3 5 and compared with r2 2, the pseudo-second-order rate

6 11 M. Özacar, İ.A. Şengil / Colloids and Surfaces A: Physicochem. Eng. Aspects 242 (24) g Experimen. 1. g Experimen. g Experimen. 2. g Experimen. g Experimen g 1. g g 2. g g q t (mg/g) g Experimen. 1. g Experimen. g Experimen. 2. g Experimen. g Experimen. log (q1 - qt) g 1. g g 2. g g g Experimen. 1. g Experimen. g Experimen. 2. g Experimen. g Experimen Fig. 3. Effect of alunite dose on sorption of disperse dyes onto alunite. DB56, DR74 and DY119. parameters, k 2, and sorption capacity, q 2. The values of r2 2, r1 2 and the intraparticle diffusion coefficients are all high (>.941). Although, based on the high correlation coefficients, it is difficult to distinguish between all three models, for the sorption of disperse dyes the results can be well represented by three kinetic models. However, the equilibrium sorption capacity for second-order is more reasonable than that for the first-order when comparing predicted results with experimental data because all of the equilibrium sorption capacities, q 1, are lower than experimental results. In the case of intraparticle diffusion model, the external surface sorption (stage 1) is absent because of completion before 5 min. All the plots have same general features, initial linear portion (stage 2, 5 3 min) followed by a plateau (stage 3, after 3 min). The initial linear portion is attributed to the intraparticle diffusion. Hence, it is concluded that the intraparticle diffusion model fits the experimental data well for an initial period (stage 2, 5 3 min) of the sorption processes only, whereas the pseudo-second-order kinetic model provides the best correlation for all of the sorption processes g 1. g g 2. g g t (min) Fig. 4. Pseudo-first-order kinetic plot for sorption of disperse dyes onto alunite at various alunite doses. DB56, DR74 and DY Effect of alunite dose The effect of alunite dose was studied on the three disperse dyes. Fig. 3 shows a series of contact time curves with alunite dose varying from.5 to g/l. The results are also shown in Fig. 4 as a plot of log (q 1 q t ) against t for sorption of disperse dyes for the pseudo-first-order model. The computed results obtained from the three kinetic models together with the correlation coefficients are listed in Tables 3 5. The values of r2 2, r2 1 and r2 int are all high (>.941) but in all cases the r2 2 values are higher than the others. In addition, all of the equilibrium sorption capacities, q 1, are much lower than experimental results. Again, the pseudo-second-order model provides the best correlation for all of the sorption processes while the intraparticle diffusion model fits the experimental data well for an initial period of the sorption processes only. The overall rate of disperse dyes sorption processes appear to be controlled by the chemical process in this case in accordance with the pseudo-second-order reaction mechanism.

7 M. Özacar, İ.A. Şengil / Colloids and Surfaces A: Physicochem. Eng. Aspects 242 (24) mg/l mg/l 15 mg/l 2 mg/l 25 mg/l 3 mg/l 15 q t (mg/g) mg/l Experimen. mg/l Experimen. 15 mg/l Experimen. 2 mg/l Experimen. 25 mg/l Experimen. 3 mg/l Experimen. Fig. 5. Effect of initial dye concentration on sorption of disperse dyes onto alunite. DB56, DR74, and DY119. The initial sorption rate, h, increases with an increase in the alunite dose. Tables 3 5 show that h varies from 9.41 to mg/g min for DB56, h varies from 7.16 to 14.3 mg/g min for DR74 and h varies from 9.41 to mg/g min for DY119, respectively, an alunite dose variation from.5 to g/l. This indicates that an increase the alunite dose increases the surface area for sorption and hence the rate of dye sorption is increased. Since the particle size range is constant. the surface area will be directly proportional to the alunite dose in the system [23,32] Effect of initial dye concentration The experimental results of the sorption of disperse dyes on alunite at various concentrations are shown in Fig. 5. The sorption capacities at equilibrium, q 2, increase from 5.8 to mg/g for DB56, from 51.6 to mg/g for DR74 and from 5.8 to mg/g for DY119 with an increase in the initial dye concentration from 5 to 3 mg/l with an alunite dose 1 g/l. Fig. 6 shows a plot of q t against the square root of t based on an intraparticle diffusion mechanism for Sorption capacity, qt, (mg/g) mg/l mg/l 15 mg/l 2 mg/l 25 mg/l 3 mg/l 5 mg/l mg/l 15 mg/l 2 mg/l 25 mg/l 3 mg/l Time.5, (min.5 ) Fig. 6. Intraparticle diffusion kinetics of disperse dyes onto alunite at various initial concentration. DB56, DR74, and DY119. the sorption of disperse dyes on alunite. The kinetic parameters for three kinetic models together with correlation coefficients are listed in Tables 3 5. For disperse dyes the values of r2 2, r2 1 and r2 int are all extremely high (>.942). However, the r2 2 values are the highest for all of disperse dyes. Again the equilibrium sorption capacities for second-order correlation are much more reasonable when compared with experimental results of the first-order system. Since most of the first-order q 1 values deviate highly from the experimental values, it suggest that the sorption of disperse dyes onto alunite follows the pseudo-second-order model Effect of ph The ph dependence of sorption was studied with a constant initial dye concentration of mg/l and alunite dose 1 g/l at with various initial ph values (Fig. 7). Fig. 8 shows good compliance with the pseudo-second-order equation. The experimental points are shown together with the

8 112 M. Özacar, İ.A. Şengil / Colloids and Surfaces A: Physicochem. Eng. Aspects 242 (24) ph 3 Experimen. ph 5 Experimen. ph 7 Experimen. ph 9 Experimen. ph 11 Experimen ph 3 ph 5 ph 7 ph 9 ph q t (mg/g) ph 3 Experimen. ph 5 Experimen. ph 7 Experimen. ph 9 Experimen. ph 11 Experimen. t/q t (g min/mg) ph 3 ph 5 ph 7 ph 9 ph ph 3 Experimen. ph 5 Experimen. 2 ph 7 Experimen. ph 9 Experimen. ph 11 Experimen ph 3 ph 5 ph 7 ph 9 ph 11 Fig. 7. Effect of ph on sorption of disperse dyes onto alunite. DB56, DR74, and DY119. theoretically generated curves. The agreement between the sets of data reflect the extremely high correlation coefficients obtained and shown in Tables 3 5. The correlation coefficients, r1 2, the first-order rate parameters, k 1, and sorption capacity, q 1, are also shown in Tables 3 5 and compared with r2 2, the pseudo-second-order rate parameters, k 2, and sorption capacity, q 2. The values of r2 2 are all higher than r1 2 values. Again, the equilibrium sorption capacities for pseudo-second-order are much more reasonable than those predicted by the pseudo-first-order kinetics when compared with experimental results because all of the equilibrium sorption capacities, q 1, are lower than the experimental equilibrium results. The intraparticle coefficients, rint 2, are lower than r2 2. These results suggest that the sorption of disperse dyes onto alunite follow the pseudo-second-order model. The initial sorption rate, h, increases with a decrease in the ph. Tables 3 5 show that h varies from 8.14 to mg/g min for DB56, from 7.29 to mg/g min for DR74 and from to mg/g min for DY119, respectively, for a ph variation from 11 to 3. On decreasing the ph from 11 to 3, the specific sorption at equilibrium, q 2, increased from to 12 mg/g for DB56, from 8 to t (min) Fig. 8. Pseudo-second-order kinetic plot for sorption of disperse dyes onto alunite at various ph. DB56, DR7, and DY mg/g for DR74 and from 99 to 11 mg/g for DY119. The increase in the equilibrium sorption of dye with decreasing ph indicates that a low ph favors dye removal by sorption on the alunite. Any oxide surface creates a charge (positive or negative) on its surface. This charge is proportional to the ph of the solution which surrounds the oxide particles [17,18,21]. The chief constituents of alunite are metal oxides mainly Al and Si (Table 1). These metal oxides mainly form metal hydroxide complexes in solution and the subsequent acidic or basic dissociation of these complexes at the solid solution interface leads to development of a positive or negative charge on the surface. A greater sorption of acidic and disperse dyes occurred at low ph values [33 35]. Lower adsorption of disperse dyes, anionic dyes, at alkaline ph is probable due to the presence of excess of OH ions competing with the dye anions for the adsorption sites. When the ph of the system decreases, the surface becomes positively charged and the sorption capacity for

9 M. Özacar, İ.A. Şengil / Colloids and Surfaces A: Physicochem. Eng. Aspects 242 (24) disperse dyes increases. Because positively charged surface sites on the alunite favor the sorption of disperse dyes due to electrostatic attraction. 4. Conclusion The kinetics of sorption of DB56, DR74 and DY119 on alunite were studied by using pseudo-first-order and pseudo-second-order equations and intraparticle diffusion kinetic model. For three dye/alunite systems, the pseudo-first-order chemical reaction does not provide the best fit models overall in the rate controlling step. The sorption of disperse dyes onto alunite can be described by the intraparticle diffusion model up to 3 min. The intraparticle diffusion model indicates that the external surface sorption (stage 1) is absent due to completing before 5 min, and final equilibrium sorption (stage 3) is started after 3 min. The disperse dyes are slowly transported via intraparticle diffusion into the particles and are finally retained in micropores. The sorption of disperse dyes onto alunite is best described by the pseudo-second-order chemical reaction kinetics which provides the best correlation of the data in all cases. The sorption of disperse dyes seems to follow a pseudo-second-order sorption reaction mechanism when the effects of alunite particle size, alunite mass, initial dye concentration and initial solution ph are investigated. References [1] G.M. Walker, L. Hansen, J.-A. Hanna, S.J. Allen, Water Res. 37 (23) [2] M. Doğan, M. Alkan, Chemosphere 5 (23) [3] Y.S. Ho, G. McKay, Conservation Recycl. 25 (1999) [4] S.H. Lin, J. Chem. Tech. Biotechnol. 57 (1993) [5] Y.S. Ho, G. McKay, Trans. IChemE 76B (1998) [6] M.-S. Chiou, H.-Y. Li, J. Hazard. Mater. 93 (22) [7] M.-S. Chiou, H.-Y. Li, Chemosphere 5 (23) [8] V.K. Garg, R. Gupta, A.B. Yadav, R. Kumar, Bioresour. Technol. 89 (23) [9] M.G. Neumann, F. Gessner, C.C. Schmitt, R. Sartori, J Colloid Interface 255 (22) [1] F.-C. Wu, R.-L. Tseng, R.-S. Juang, Environ. Technol. 22 (21) [11] F.-C. Wu, R.-L. Tseng, R.-S. Juang, Environ. Technol. 22 (21) [12] F.-C. Wu, R.-L. Tseng, R.-S. Juang, Water Res. 35 (21) [13] G. Annadurai, R.-S. Juang, D.-J. Lee, J. Hazard. Mater. 92 (22) [14] G. Annadurai, M.R.V. Krishnan, Indian J. Chem. Tech. 4 (1997) [15] O. Demirbaş, M. Alkan, M. Doğan, Adsorption 8 (22) [16] M. Doğan, M. Alkan, Y. Onguner, Water Air Soil Pollut. 12 (2) [17] M. Özacar, İ.A. Şengil, Adsorption 8 (22) [18] M. Özacar, İ.A. Şengil, J. Hazard. Mater. B98 (23) [19] Y. Guo, S. Yang, W. Fu, J. Qi, R. Li, Z. Wang, H. Xu, Dyes Pigments 56 (23) [2] M. Özacar, Adsorption 9 (23) [21] M. Özacar, Chemosphere 51 (23) [22] M. Özacar, Cem. Concr. Res. 33 (23) [23] Y.S. Ho, C.C. Chiang, Adsorption 7 (21) [24] Y.S. Ho, G. McKay, Process. Biochem. 34 (1999) [25] Y.S. Ho, G. McKay, D.A.J. Wase, C.F. Foster, Ads. Sci. Technol. 18 (2) [26] C. Namasivayam, D. Kavitha, Dyes Pigments 54 (22) [27] Y.S. Ho, G. McKay, Trans. IChemE 76B (1998) [28] Y.S. Ho, G. McKay, Adsorption 5 (1999) [29] P.K. Malik, Dyes Pigments 56 (23) [3] X.-Y. Yang, B. Al-Duri, Chem. Eng. J. 83 (21) [31] C.-C. Lin, H.-S. Liu, Indian Eng. Chem. Res. 39 (2) [32] C. Namasivayam, R. Radhika, S. Suba, Waste Manage. 21 (21) [33] M. Özacar, İ.A. Şengil, Environ. Geol., in press. [34] K.R. Ramakrishna, T. Viraraghavan, Waste Manage. 17 (1997) [35] K.R. Ramakrishna, T. Viraraghavan, Water Sci. Technol. 36 (1997)