Research Journal of Chemical Sciences ___ ______________________________ ______ ____ ___ IS SN 22 31 - 606X Vol. 3 ( 2 ), 65 - 72 , February (201 3 ) Res. J. Chem. Sci. International Science Congress Association 65 Adsorption Kinetics of Methylene Blue onto Clay Fractionated from Bijoypur Soil , Bangladesh Y. Zaker, M. A. Hossain* and T. S. A. Islam Department of Chemistry, University of Dhaka, Dhaka - 100, BANGLADESH Available online at: www.isca.in Received 21 st December 201 2 , revised 14th January 201 3 , accepted 27 th January 201 3 Abstract The adsorption kinetics of methylene blue (MB) from aqueous solution onto clay fractionated from Bijoypur (Netrokona) soil based on particle size (≤53µm) has been investigated. Batch studies were carried out to investigate the effect of contact time, initial dye concentration and temperature on adsorption kinetics. Kinetic studies showed a rapid adsorption during the first thirty minutes. Appl ication of pseudo first order, pseudo second order and intra particle diffusion model equations showed that the experimental results are well expressed by pseudo second order kinetic equation. Adsorption isotherm was constructed from the pseudo second orde r kinetic data. Maximum adsorption capacity, calculated from well fitted Langmuir equation, is 6.93 mg/g which increased with increase in temperature. The positive value of enthalpy change ( ∆H o =20.98 kJ/mol) and negative free energy change (∆G o ) indicated that the adsorption of MB on clay is endothermic and involve chemical process. Verification of intra - particle diffusion model showed that intra - particle diffusion could be one of the rate de termining steps but pseudo second order mechanism is predominant. Overall adsorption process appears to be controlled by more than one step. Keywords: Adsorption kinetics, clay, batch study, pseudo first order, pseudo second order, thermodynamic paramete rs, intra particle diffusion and step - wise adsorption. Introduction Wastewaters from textile industries commonly contain moderate concentrations (10 - 200 mg/L) of dyestuffs, contributing significantly to the pollution of aquatic ecosystem 1 . Various physi cochemical and biological techniques can be employed to remove dyes from wastewaters. In comparison with other techniques adsorption is superior in simplicity of design, initial cost, ease of operation and insensitivity to toxic substances. The removal of dye from wastewater by activated carbon 2 - 4 , polymeric resins 5 - 6 , sugarcane baggase 7 , clay minerals 8 and several biosorbents 9 - 10 has been reported widely. A survey of literature revealed that methylene blue (MB) has been used particularly for adsorption stu dies, not only because of its environmental concern but also for the fact that it has been recognized as a model adsorbate for adsorption of organic because of it known strong adsorption to clay minerals 11 . Despite of many investigations to study adsorptio n of MB on clay, specific mechanism by which the adsorption of MB takes place on clay is still ambiguous 12 . Kinetics is concern fundamentally with the details of the process whereby a system gets from an initial state to final state and the time required f or the transition, hence it gives ideal about the mechanism of adsorption. The availability of the kinetic model equations for the study of adsorption process on activated carbon permits a rational approach to study the mechanism of the adsorption process. It has been reported that over 25 kinetic models has been referenced in available literature, all attempting to describe quantitatively the kinetic behavior during the adsorption process. Each adsorption kinetic model has its own limitation and derived ac cording to certain initial conditions based on certain experimental and theoretical assumptions 13 . Two of the most used empirical equations worth mentioning are the pseudo first and second order. The aim of this present study is to establish the mechanism of adsorption of MB onto the clay fractionated from Bijoypur clay minerals from the kinetic study and evaluate the capacity of a low cost adsorbent to remove MB. Batch studies were carried out involving process parameters such as the initial dye concentrat ion, solution temperature and contact time. Equilibrium and kinetic analysis were conducted to understand adsorption process and optimization of various parameters. Material and Methods Adsorbent: Soil sample collected from Bijoypur (Netrokona) was fracti onated conducting standard and well known hydrometer method 14 based on particle size to three different fractions such as sand (≥140µm), silt (53 - 140µm) and clay (≤53µm). Smallest sized fraction is clay which without further treatment was characterized 15 by SEM, LIBS, FT - IR and XRD. This clay was used as an adsorben t. Adsorbate: Methylene blue (CI: 52015) is a heterocyclic aromatic chemical compound with the molecular formula C 16 H 18 N 3 SCl. The IUPAC name of methylene blue is 3,7 - bis(Dimethylamino) - phenothiazin - 5 - ium chloride and structural formula is shown in figure - 1. Its CAS No. is 61 - 73 - 4 and molar Research Journal of Chemical Sciences ___ _ _ _______________________________ ______________ _ ________ ISSN 22 31 - 606X Vol. 3 ( 2 ), 65 - 72 , February (201 3 ) Res. J. Chem. Sci. International Science Congress Association 66 mass 319.85 g/mol. Methylene blue is highly soluble in water and t he hydrated form has 3 molecules of water per molecule of methylene blue. Laboratory grade methylene blue (MB) supplied by Janssen Chemical, Belgium was used without further purification for the preparation of aqueous solution. Figure - 1 Structural formula of M ethylene blue Adsorption experiments: The stock solution was prepared by dissolving 0.016 g MB in 500 mL of distilled water. Serial dilutions were made to obtain the required lower concentrations of MB in the range of 3.5 to 28.5 mg/L. The pH of each MB s olution was maintained at 7.0. For each of the kinetic experiment, 40 mL of MB solution of known initial concentration and 0.1 g of clay were taken in a 60 mL reagent bottles (Pyrex glass, England) with air tight stopper. This mixture was agitated in a tem perature controlled water bath shaker (HAAKE SWB20, Fissions Ltd., Germany) at 30ºC with a constant shaking speed of 110 rpm. The flasks were agitated for a time interval of 15, 30, 45, 60, 75 and 90 minutes and the clay was separated from the mixture by c entrifuge. The concentration of MB in the supernatants was determined by measuring the absorbance at λ max of 663 nm using UV - visible spectrophotometer (UV - 1650 PC Shimadzu, Japan). The amount adsorbed ( q e ) were calculated in equation (1): (1) where, C o is the initial concentration of MB (mg/L) and C e is the equilibrium concentration of MB (mg/L), m is the weight of clay used for the adsorption studies (g) and V is the volume of MB solution ( L). Similar kinetic experiments were also performed at 40 and 50 o C to determine the effect of temperature on adsorption kinetics. Results and Discussion Effect of contact time and initial concentration: Effect of contact time on the adsorption is the fund amental basis of the adsorption kinetics. Figure - 2 shows the variation of amount adsorbed of MB on clay with contact time. It shows that the amount adsorbed increased at first thirty minutes to a near constant value with increase of contact time. Again, mo st of the adsorption occurred within first 60 minutes for different initial concentrations and adsorption became very slow at later. This may be attributed to lack of available active sites required for the high initial concentration of dye 16 . The amount a dsorbed increased from 1.29 to 6.93 mg/g within first 60 minutes of contact of MB with clay as the initial concentration increased from 3.5 to 28.5 mg/L. Figure - 2 Effect of contact time and initial concentration for a dsorption of MB on clay Different kinetic model equations were applied to the above observation to verify the nature of adsorption kinetics. The conformity between experimental data and the model - predicted values was expressed by the correlation coefficie nts ( R 2 , values close or equal to 1). Largergren pseudo first order equation: Pseudo - first order rate equation is commonly used to the adsorption of liquid/solid system based on adsorbent capacity 17 . According to this model, one adsorbate species reacts with one active site on surface. The differential form of the equation is generally expressed as, (2) where, q e and q t are the adsorption capacity at equilibrium and at t ime t , respectively (mg/g), k 1 is the rate constant of pseudo first order equation (L/min). Integrating equation (2) for the boundary conditions t = 0 – t and q t = 0 – q t gives: (3) Equation (3) can be rearranged to obtain the following linear form: (4) In order to obtain the rate constants, the values of ln( q e − q t ) were linearly correlated with t by plot of ln( q e − q t ) versus t to give a linear relationship from which k 1 and predicted q e can be determined from the slope and intercept of the plot, respectively 18 . The variation in the rate should be proportional to the first power of concentration of adsorbate. However, the relationship between initial solute concentration and rate of adsorption will not be linear when pore diffusion limits the adsorption process. The applicability of the pseudo - first order equation to experimental data generally, differs in two ways; the parameter does not represent the number of available sites and the parameter ln q e is an adjustable parameter and often found Research Journal of Chemical Sciences ___ _ _ _______________________________ ______________ _ ________ ISSN 22 31 - 606X Vol. 3 ( 2 ), 65 - 72 , February (201 3 ) Res. J. Chem. Sci. International Science Congress Association 67 not equal to the intercept of the plot ln( q e – q t ) versus t , whereas in t rue first order, ln q e should be equal to the intercept 19 . Figure - 3 shows the weak fitness of pseudo - first order plots at different initial concentrations. Values of correlation coefficients and different parameters for different concentration of MB are giv en in table - 1. The results showed that the pseudo first order rate constant, k 1 is irregularly changes with concentration i.e. k 1 independent of initial concentration. Similar result has been presented in literatures 11,18,20 . However, the experimental adso rption capacity was observed to increase with increase in initial concentration. Figure - 3 Pseudo first order kinetic model plots for adsorption of MB on clay at different concentrations Pseudo second order kinetics: P seudo - second order rate equation was applied to the adsorption kinetics of the present system 21 . The model was derived on the basis of the concentration of the adsorbate in the adsorbent phase 21 . The pseudo second order rate equation is expressed (5) as: (5) where, k 2 is the rate constant of the pseudo second order adsorption (g/mg·min). For the boundary conditions t = 0 to t = t and q t = 0 to q t = q e the integrated form of the equation becomes (6) Equation (6) can be rearranged to the linear form as below ( equation 7): (7) If the initial adsorption rate (mg/g·min), h = k 2 q e , then the equation (7) becomes (8) The plot of t/q t versus t of equation (7) s hould give a linear relationship from which q e and k 2 can be determined from the slope and intercept of the plot, respectively. T he linear plots of t / q t against t, as shown in figure - 4 , represent the good fit ( R 2 = 0.999) over the whole range of initial co ncentration of MB and contact time. Pseudo - second order rate equation was derived by assuming that two surface sites could be occupied by one divalent adsorbate ion. Thus the equation (7) would be expected to be applicable for the adsorption of methylene b lue ions due to the existence of divalent form of methylene blue (MB + ) 2 and/or MBH 2+ in aqueous solution 22 . The pseudo - second order mechanism is found to be predominant during the adsorption kinetics of MB onto clay 23 and Polyalthia Longifolia seed powder 24 . The pseudo - second order rate constants, initial rate constant and equilibrium amount adsorbed for different initial concentrations were calculated from the linear plots of t / q t vs . t and are shown in figure - 4 and presented in table - 2. The value of pseu do second order rate constant k 2 varied from 0.043 to 0.083 as the initial concentration increased from 3.5 to 28.5 mg/L. The equilibrium adsorption capacity calculated from pseudo second order rate equation, ( q e ) cal increased from 1.48 to 7.23 as the init ial concentration was increased from 3.5 to 28.5 mg/L. Figure - 4 Pseudo second order kinetic plots for adsorption of MB on clay at different concentrations Test for the fitness of kinetic models: The sum of square of error (SSE, %) is one method which has been used in literature to test the validity of each model that has been used. The sum of square of error is, (9) Research Journal of Chemical Sciences ___ _ _ _______________________________ ______________ _ ________ ISSN 22 31 - 606X Vol. 3 ( 2 ), 65 - 72 , February (201 3 ) Res. J. Chem. Sci. International Science Congress Association 68 Table - 1 Different parameters of pseudo first - order kinetic model for the adsorption of MB on clay at 30 o C C o (mg/L) k 1 (1/min) q e (expt) (mg/g) q e (cal) (mg/g) R 2 SSE (%) 3.5 0.058 1.348 1.704 0.971 1.774 6.7 0.073 2.614 2.647 0.988 9.3 0.066 3.694 3.369 0.946 13.0 0.067 5.048 3.264 0.937 14.6 0.079 5.7 99 3.108 0.964 19.0 0.084 6.685 4.963 0.992 22.4 0.086 6.853 4.716 0.976 25.5 0.075 6.991 4.419 0.973 28.5 0.079 6.992 4.464 0.965 Table - 2 The constant parameters of the pseudo second - order kinetic model for the adsorption of MB on clay at 30 o C C o (mg/L) k 2 ( g/mg·min) q e(exp) (mg/g) q e (cal) (mg/g) h (mg/g · min) R 2 SSE (%) 3.5 0.080 1.348 1.480 0.118 0.9996 0.2118 6.7 0.058 2.614 2.849 0.140 0.9991 9.3 0.063 3.694 3.863 0.243 0.9997 13.0 0.049 5.048 5.258 0.305 0.9995 14.6 0.083 5.799 5. 945 0.493 0.9998 19.0 0.043 6.685 6.979 0.300 0.9994 22.4 0.060 6.853 7.057 0.423 0.9998 25.5 0.050 6.991 7.225 0.361 0.9998 28.5 0.056 6.992 7.205 0.404 0.9993 where, N is the number of data points. The values of SSE (%) for the pseudo first a nd pseudo second kinetic models are given in table - 1 and 2, respectively. It can be seemed that the SSE (%) value is lower for the second order kinetic model (0.2118) than that for the pseudo order first model (1.774). This conform a better applicability o f the pseudo second order kinetic model. The correlation coefficient for the pseudo first order ranged between 0.93 and 0.99 whereas the values for the second order are closest to 1 (0.999). The higher the correlation coefficient and the lower the SSE (%) value, the better the fitness to the model. The correlation coefficient indicates that the experimental data best fitted into the pseudo second order suggesting that the process of adsorption follows pseudo second order kinetics. Tables - 1 and 2 also showed the ( q e ) expt and the ( q e ) cal for the two models. It can be observed that the ( q e ) expt differs significantly from ( q e ) cal for the first order model, whereas the values are much closer for the pseudo second order model. This again, indicated that the exper imental data follows the pseudo second order model. Similar reports have been presented in literature for adsorption of MB onto bamboo based activated carbon 11, 25 . Adsorption isotherm: The adsorption isotherm was determined from the amount adsorbed and t he equilibrium concentration calculated from well fitted pseudo second order kinetic studies for different concentrations. Figure - 5 shows the comparison of the Langmuir type adsorption isotherms of MB on clay constructed based on experimental result and ca lculated values. Figure - 5 Adsorption isotherm of MB on clay at 30 o C constructed from pseudo second order kinetic model Effect of temperature on adsorption kinetics: Adsorption kinetic experiments were performed at different temperatures for a fixed co ncentration of 13 mg/L of MB solution. Well fitted pseudo second order kinetic equation was applied to the experimental data as shown in figure - 6. Different parameters of pseudo - second order kinetic equation at different temperatures are presented in table - 3. The effect of temperature on the adsorption of MB onto clay shows that the amount adsorbed increased with increase in temperature indicating endothermic Research Journal of Chemical Sciences ___ _ _ _______________________________ ______________ _ ________ ISSN 22 31 - 606X Vol. 3 ( 2 ), 65 - 72 , February (201 3 ) Res. J. Chem. Sci. International Science Congress Association 69 nature of adsorption. The endothermicity of the adsorption was also confirmed by the increasing of pseudo second rate constant with increase of temperature. Increase in the value of k 2 with temperature is due to increased mobility and enhanced of MB diffusion at higher temperatures. The decrease in rate of MB adsorption with increasing temperature may b e due to the association of MB molecules at the boundary surface. Figure - 6 Pseudo second order kinetic plots for adsorption of MB on clay at different temperatures Table - 3 The constant parameters of the pseudo second - order kinetic model for the adsorption of MB on clay at different temperatures T (K) k 2 ( g/mg·min) q e (expt) (mg/g) q e (cal) (mg/g) h (mg/g · min) R 2 303 0.049 5.048 5.258 0.305 0.9995 313 0.051 5.191 5.408 0.276 0.9999 323 0.052 5.208 5.435 0.283 0.9998 The adsorption isotherms at different temperatures were also constructed from the amount adsorbed and equilibrium concentration calculated from well fitted pseudo second order kinetic studies for different concentrations as shown in figure - 7. The applica bility of Langmuir equation (10) for the adsorption of MB onto clay at different temperatures was verified. Figure - 7 Adsorption isotherm of MB on clay at different temperatures constructed from pseudo second order kinetic studies (10) where, q e = x/m = amount adsorbed at equilibrium time (mg/g), C e = equilibrium concentration of adsorbate in solution (mg/L), q m = maximum adsorption capacity (mg/g) and b = adsorbed intensity (L/mol or L/mg). Figure - 8 shows the line ar plots of C e / q e versus C e to evaluate the applicability of Langmuir model equation, for the adsorption of MB on clay at different temperatures. The calculated Langmuir constant and their corresponding linear regression correlation coefficient values ( R 2 ) from experimental results at different temperatures are given in table - 4. The results show that the adsorption isotherms for different temperatures are well fitted into Langmuir model. The maximum adsorption capacity, q m obtained from Langmuir isotherm is 6.93 mg/g at 30 o C which is increased to 7.14 mg/g with increasing temperature to 50 o C. Again, the adsorption intensity constant, b increased with increase in temperature. Generally, in case of chemical interaction - the amount adsorbed increases with incr easing temperature and the adsorption intensity also increases with increasing temperature. Figure - 8 Langmuir isotherm of MB adsorption on clay at different temperatures Table - 4 Langmuir constants for adsorption of MB on clay at different temperatures T ( o C) Langmuir parameters q m (mg/g) b (L/mg) R 2 30 6.925 9.756 0.9994 40 7.097 12.690 0.9997 50 7.143 11.669 0.9996 Adsorption thermodynamics : Thermodynamic parameters, namely the change of free energy (∆ G o ), enthalpy (∆ H o ) and entropy (∆ S o ) have an important role to determine spontaneity and heat change for the adsorption process. Assuming that the activity coefficients are unity at low concentr ations (the Henry's law sense), thermodynamic parameters were calculated from the Research Journal of Chemical Sciences ___ _ _ _______________________________ ______________ _ ________ ISSN 22 31 - 606X Vol. 3 ( 2 ), 65 - 72 , February (201 3 ) Res. J. Chem. Sci. International Science Congress Association 70 apparent equilibrium constant values ( K c ) (eq. 11) at different temperatures using the following equations (12 and 13) 25 - 27 : K c = C a / C e (11) ∆ G o = - RT ln K c (12) ln K c  (∆ S o / R ) - (∆ H o / RT ) (13) where, K c indicates the apparent equilibrium constant, C a and C e are the equilibrium concentration of MB on the clay (mg/L) and in the solution (mg/L), respectively. R is the universal gas constant (8.314 J/mol·K) and T is the temperature (K). From equation (12), ∆ G o were calculated using ln K c values for different temperatures. The values of ∆ H o and ∆ S o were calculated from the slope and intercept of the linear plot of ln K c vs. 1/ T , respectively, as shown in figure - 9. The ∆ G o values of MB adsorption on clay under different temperatures as well as ∆ H o and ∆ S o values are presented in table - 5. The positive value of ∆ H o (20.98 kJ/mol) indicated that the adsorption of MB on clay was endothermic and chemical in nature. The endothermicity of adsorption of MB was also observed during the adsorption onto clay at temperature from 40 to 60 o C, however b elow this temperature, the exothermic nature was observed 23 . T he heat of adsorption varies between 20 and 400 kJ/mol indicating the chemisorptions process 28 . Again, the negative values of ∆ G o decreasing with the increase of temperature indicated more effic ient adsorption at higher temperature. Positive ∆ S o values of MB adsorption on clay indicates an irregular increase of the randomness at the clay - solution interface during the adsorption which might be due to the fragmentation of MB molecules or/and struct ural change on clay surface or surface migration of adsorbed MB molecules 29 - 30 . Table - 5 Equilibrium constant and thermodynamic parameters of MB adsorption on clay at different temperatures T (K) K c ( - ) ln K c ∆ G o (kJ/mol) ∆ H o (kJ/mol) ∆ S o (kJ/mol  K) 303 149.54 5.008 - 12.615 20.98 0.111 313 223.14 5.408 - 14.073 323 253.90 5.534 - 14.861 Figure - 9 A plot of ln K c versus 1/ T for determination of enthalpy and entropy of MB adsorption on clay Intra - particle diffusion model: The possibility of in tra - particle diffusion of MB onto the clay was investigated using the intra - particle diffusion model (eq uation 14) 16 . q t = k p t 1/2 +C (14) where, q t is the amount of dye adsorbed (mg/g) at time t , C is the bounda ry layer thickness and k p is the intra - particle diffusion rate constant (mg/g· min 1/2 ). The plots of the amount adsorbed, q t versus t 1/2 at different initial concentrations is shown in figure - 10. The intra - particle diffusion constants at different initial concentrations are shown in table - 6. The correlation coefficients are high but when compared with that observed from pseudo second order kinetic model, the R 2 values of later were found to be much higher than the former. These suggest that pseudo second or der kinetic mechanism is predominant and the overall rate of the dye adsorption processes appear to be controlled by chemical interact ion . This is agreement with the investigation. The high correlation coefficient indicates the presence of intra - particle d iffusion as the rate determining step. The correlation coefficient ranged from 0.72 to 0.96 as the initial concentration varied from 3.5 to 28.5 mg/L. Figure - 10 Intra particle diffusion plots for adsorption of MB on cl ay at different initial concentrations at 30 o C, pH= 7.0 and dose = 2.5 g/L Table - 6 Intra particle diffusion constants of MB adsorption on clay at different initial concentrations C o (mg/L) k p (mg/g - min 1/2 ) C R 2 3.5 0.072 0.719 0.8766 6.7 0.131 1.492 0. 8347 9.3 0.150 2.767 0.9601 13.0 0.104 3.780 0.7329 14.6 0.113 4.843 0.7453 19.0 0.204 4.976 0.7297 22.4 0.151 5.578 0.7494 25.5 0.169 5.543 0.7913 28.5 0.159 5.638 0.7537 Research Journal of Chemical Sciences ___ _ _ _______________________________ ______________ _ ________ ISSN 22 31 - 606X Vol. 3 ( 2 ), 65 - 72 , February (201 3 ) Res. J. Chem. Sci. International Science Congress Association 71 Conclusion Clay fractionated from Bijoypur soil is suitable for adsorption o f MB from aqueous solution. The adsorption process is well expressed by pseudo - second order kinetic model. 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