A kinetics and thermodynamics study of methylene blue adsorption on wheat shells

Desalination 194 (2006) 259–267A kinetics and thermodynamics study of methylene blue adsorption on wheat shellsYasemin Bulut*, Haluk AydınDepartment of Chemistry, Faculty of Science and Arts, University of Dicle, Diyarbakır 21280, Turkey Fax: +90 (412) 248-8039; email: ybulut@.tr Received 27 July 2005; accepted 4 October 2005Abstract The use of wheat shells (WHS) for the removal of methylene blue from aqueous solutions at different contact times, temperatures, pH, adsorbent doses and initial dye concentration was investigated. The adsorption process attains equilibrium within 60 min. The extent of dye removal decreased with increasing adsorbent dosage and also increased with increasing contact time, temperature, and in solution concentration. The adsorption isotherms are described by means of the Langmuir and Freundlich isotherms. It was found that the Langmuir equation fit better than the Freundlich equation. Maximum adsorption capacity (Qm) was calculated at different temperatures (303, 313, and 323 K) 16.56, 20.83, and 21.50 mg/g, respectively. The adsorption kinetics of methylene blue could be described by the pseudosecond-order reaction model. The activation energy of system (Ea) was calculated as 16.62 kJ/mol. Free energy of adsorption (∆G0), enthalpy (∆H0), and entropy (∆S0) changes were calculated to predict the nature of adsorption. The estimated values for ∆G0 were !22.64, !24.11, and !26.35 kJ/mol at 303, 313, and 323 K, respectively, which are rather low, indicating that a spontaneous process occurred. The enthalpy changes and entropy of adsorption were 33.41 kJ/mol and 185 J/mol K, respectively. The obtained results are: (1) high levels of color removal (>95%) were achieved with low contact times adsorbent/dye (less than 90-min contact ); and (2) the whole WHS can be successfully used as adsorbent of methylene blue in aqueous solutions. WHS, an inexpensive and easily available material, can be an alternative for more costly adsorbents used for dye removal in wastewater treatment processes. Keywords: Adsorption; Isotherm; Methylene blue; Wheat shells; Dye adsorption; Kinetics of adsorption; Thermodynamics1. Introduction Dyes are used in many industries such as food, paper, carpets, rubbers, plastics, cosmetics, and*Corresponding author.textiles in order to color their products. The discharge of colored wastewater from these industries into natural streams has caused many significant problems such as increasing the toxicity and chemical oxygen demand (COD) of0011-9164/06/$– See front matter © 2006 Elsevier B.V. All rights reserveddoi:10.1016/j.desal.2005.10.032260Y. Bulut, H. Aydın / Desalination 194 (2006) 259–267the effluent, and also reducing light penetration, which has a derogatory effect on photosynthetic phenomena. From the aesthetic point of view, the presence of dyes, in particular carcinogenic compounds, in surface and underground waters is not safe, pleasant, or welcomed. Recently, governments have been under severe pressure by their constituents to stop this type of effluent from entering public watercourses, unless it is treated properly [1]. The most commonly used methods for color removal are biological and chemical precipitation. However, these processes are effective and economic only in cases where solute concentrations are relatively high [2]. There are advantages and disadvantages of various methods of dye removal from wastewaters [3]. Many physicochemical methods have been tested, but only that of adsorption is considered to be superior to other techniques. This is attributed to its low cost, easy availability, simplicity of design, high efficiency, ease of operation, biodegradability, and ability to treat dyes in more concentrated forms [4,5]. Activated carbons are widely used because of their high adsorption abilities for a large number of organic compounds. However, the price of activated carbons is relatively high, which limits their usage [6]. This has led many researchers to search for cheaper substitutes such as coal, fly ash, silica gel, wool wastes, agricultural wastes, wood wastes, and clay materials. They have been applied with varying success in dye removal [7]. Previously, some researchers have proved the success of several low-cost materials such as rice husk [2], Indian rosewood sawdust [3], neem leaf powder [6], pumice powder [7], pyrophyllite [8], perlite [9], jute processing wastes [10], eggshells [11], and fly ash [12] for the removal of methylene blue from aqueous solutions. In the present study, wheat shell (WHS), an agro-based waste material, was used as adsorbent for the removal of methylene blue from an aqueous solution. Methylene blue was selected as a model compound in order to evaluate the capacityof WHS for the removal of dyes from aqueous solutions. Though methylene blue is not strongly hazardous, it can cause some harmful effects. Acute exposure to methylene blue causes increased heart rate, vomiting, shock, cyanosis, jaundice, quadriplegia, and tissue necrosis in humans [2]. The aim of this study was to investigate the adsorption of methylene blue onto WHS, which is a low-cost adsorbent for the removal of dyes. The effects of contact time, adsorbent dose, initial dye concentration, pH and temperature on methylene blue adsorption were evaluated. Thermodynamic parameters, the kinetics and the factors controlling the adsorption process were also calculated and discussed. 2. Materials and methods 2.1. Materials WHS were obtained from local wheat mills. The collected materials (the last hard layer of wheat) were then washed several times with distilled water to remove all dirt particles. The washing process was continued till the wash water contained no color. The washed materials were then dried in a hot oven at 50EC for 12 h. The dried materials were then ground, using a steel mill. The adsorbent was sieved through 0.6mm sieve and used as such without any pretreatment. Characteristics of the WHS were determined, and the results are summarized in Table 1. An elemental analysis (C and H content) was carried out with an EA 1108 Fisons instruments [13]. Methylene blue was chosen for this study because of its known strong adsorption onto solids. Methylene blue (CI = 52015; chemical formula: C16H18ClN3S; molecular weight = 319.86 g/mol; maximum wavelength = 662 nm) supplied by Merck was used as adsorbate and was not purified prior to use. A stock solution of methylene blue of 1000 mg/L was prepared, which was diluted to the required initial concen-Y. Bulut, H. Aydın / Desalination 194 (2006) 259–267 Table 1 Physical and chemical properties of adsorbent used in the experiments Characteristics Chemical: Moisture content, % Water soluble components, % Insoluble components, % Ash, % Total loss of ignition, % C content, % H content, % pH Physical: Surface area (BET), m2 /g, Bulk density, g/mL Particle size, mm Results of Boehm titration [mmol/g] of adsorbent: Carboxylic Lactonic Phenolic Basic Acidic Value 6.40 20.89 72.34 2.58 88.45 44.59 6.56 6.05 0.67 0.36 0.602610.146 0.020 0.842 0.000 1.008solution was adjusted with 0.1 N HCl or 0.1 N NaOH by using a Jenway 3010 pH-meter with a combined pH electrode. The pH-meter was standardized with NBS buffers before every measurement. A constant bath was used to keep the temperature constant. At the end of the adsorption period, the solution was centrifuged for 10 min at 3000 rpm. After centrifugation, the dye concentration in the supernatant solution was analyzed using a UV spectrophotometer (Shimadzu UV-160) by monitoring the absorbance changes at a wavelength of maximum absorbance (662 nm). The samples were pipetted from the medium reaction by the aid of a very thin point micropipette, which prevented the transition to the solution of the WHS samples. Preliminary experiments showed that the effect of the separation time on the amount of adsorbed dye was negligible. The amount of dye adsorbed was calculated using the equation qeq = (Ci-Ceq) V/M (1)trations. Other chemicals were obtained from Merck. 2.2. Method Methylene blue was dried at 110EC for 2 h before use. All of the methylene blue solution was prepared with distilled water. The effect of contact time on the amount of dye adsorbed was investigated at 200 mg/L initial concentration of dye and at different temperatures (303, 313 and 323 K). A weighed sample of WHS (0.1 g) was mixed with a 200 mL dye solution. For the other adsorption experiments, 50 mL of dye solution of known initial concentration was shaken with a certain amount of adsorbent at desired pH and temperature at 150 rpm for 75 min. The pH of thewhere qeq is the amount adsorbed per gram of adsorbent, and Ceq is the equilibrium concentration of dye (mg/L). 3. Results and discussion 3.1. Kinetics studies The applicability of the pseudo-first-order and pseudo-second-order model was tested for the adsorption of methylene blue onto WHS particles. The best-fit model was selected based on the linear regression correlation coefficient, r2, values. Fig. 1 shows a plot of the amount of dye adsorbed (mg/g) vs. contact time for different temperatures (303, 313, and 323 K). It was observed that the amount of dye uptake, qt (mg/g), increased with contact time at all temperatures. Further, the amount of dye adsorbed increased with increase in temperature. It was also observed262Y. Bulut, H. Aydın / Desalination 194 (2006) 259–267model in predicting the kinetics of the methylene blue adsorption onto WHS particles. The kinetic data were further analyzed using Ho’s pseudo-second-order kinetics, represented by [2]2 + t /qeq t/qt = 1 / kps qeq(3)Fig. 1. Effect of contact time on the adsorption of methylen blue onto WHS (T = 303 K, Co = 200 mg/L, adsorbent = 1 g, V = 0.2 L).that dye uptake is rapid for the first 25 min and thereafter it proceeds at a slower rate and finally attains saturation. These observations show that the temperature has no effect on equilibrium time. The transient behavior of the dye adsorption process was analyzed by using the pseudo-firstorder, pseudo-second-order, and intraparticle diffusion. The pseudo-first-order kinetic model has been widely used to predict dye adsorption kinetics. A linear form of pseudo-first-order model was described by Lagergren [14]: log (qeq!qt) = log qeq!kpf t /2.303 (2)where qt is the amount adsorbed at time t (mg/g), and kpf is the equilibrium rate constant of pseudofirst-order adsorption (min!1). The values of log (qeq!qt) were calculated from the kinetic data (Fig. 1). The calculated qeq, kpf, and the corresponding 2 values linear regression correlation coefficient r1 are shown in Table 2. It was observed that the rate constant kpf increased with an increase in temperature. It was also observed that correlation coefficients were lower for all temperatures. This shows no applicability of the pseudo-first-orderwhere kps is the rate constant of second-order adsorption (g/mol min). If the second-order kinetics is applicable, then the plot of t/qt v. t should show a linear relationship. The calculated qeq, kps, and the corresponding 2 values linear regression correlation coefficient r2 are summarized in Table 2. The linear plots of t/qt vs. t show good agreement between experimental and calculated qeq values (Table 2). The correlation coefficients for the second-order kinetics 2 ) are greater than 0.99, indicating the model (r2 applicability of this kinetics equation and the second-order nature of the adsorption process of methylene blue onto WHS. Similar phenomena have been observed in direct dyes onto orange peel [1], the sorption of methylene blue onto rice husk [2], adsorption of basic dyes onto pumice powder [7], and removal of methylene blue from perlit [9]. As seen in Table 2, the values of both qeq and kps increase with increasing temperature. The adsorbate species are most probably transported from the bulk of the solution into the solid phase through an intraparticle diffusion process, which is often the rate-limiting step in many adsorption processes. The possiblity of intraparticle diffusion was explored by using the intraparticle diffusion model [9]: qt = kid t 1/2 + C (4)where C is the intercept and kid is the intraparticle diffusion rate constant (mol/g min1/2). The values of qt were found to be linearly correlated with the values of t1/2 (Fig. 2). The values kid, C, and the corresponding linear 2 regression correlation coefficient r3 values areY. Bulut, H. Aydın / Desalination 194 (2006) 259–267263effect [15]. The intraparticle rate constants calculated from Fig. 2 are 0.054, 0.045, and 0.044 mg/g min1/2 at 303, 313, and 323 K, respectively. From Table 2, it is observed that kid decreased with increasing temperature. The activation energy of adsorption was also calculated from the linearized Arrhenius equation: ln kps = ln ko!Ea /RT (5)Fig. 2. Intraparticle diffusion plots for methylene blue onto WHS (T = 303 K, Co = 200 mg/L, adsorbent = 1 g, V = 0.2 L). Table 2 Adsorption kinetic parameters of methylene blue onto WHS T, K 303 qea, mg/g Pseudo-first-order: kpf , min!1 qeb, mg/g r12 Pseudo-second-order: kps, g/mg min qeb, mg/g r22 Intraparticle diffusion: kid, mg/g min1/2 C r32 Ea, kJ/molawhere kps is the rate constant second-order of adsorption (g/mol min), ko is the independent temperature factor (g/mol min), Ea is the activation energy of adsorption (kJ/mol), R is the gas costant (J/molK) and T is the solution temperature (K). A plot of ln kps vs. 1/T gives a straight line, and the corresponding activation energy was determined from the slope of the linear plot. The result obtained was 16.62 kJ/mol for the adsorption of methylene blue onto WHS. 3.2. Effect of adsorbent dosage Adsorbent dosage is an important parameter because this factor determines the capacity of an adsorbent for a given initial concentration of the adsorbate. The effect of adsorbent dosage (adsorbent prepared in different batch) was studied on methylene blue removal by keeping all other experimental conditions constant (Fig. 3). The results show that as the adsorbent concentration increased, the percentage of adsorption also increased, but the amount adsorbed per unit mass of the adsorbent decreased considerably (Fig. 3). The decrease in unit adsorption with increasing dose of adsorbent is basically due to adsorption sites remaining unsaturated during the adsorption reaction [16]. 3.3. Effect of initial dye concentration The equilibrium adsorption capacity of the adsorbent for methylene blue increased with313 23.24 0.034 13.80 0.9070 0.0055 24.27 0.9973 0.045 17.76 0.9713 16.62323 31.32 0.047 21.66 0.7740 0.0060 32.26 0.9984 0.044 26.02 0.898416.34 0.035 13.70 0.9641 0.0040 17.92 0.9968 0.054 10.10 0.9528Calculated; bExperimental.given in Table 2. The values of intercept give an idea about the boundary layer thickness, i. e., the larger the intercept, the greater the boundary layer264Y. Bulut, H. Aydın / Desalination 194 (2006) 259–267Fig. 4. Effect of pH on the adsorption of methylene blue onto WHS (T = 303 K, Co = 100 mg/L, adsorbent = 0.5 g, V = 0.05 L, contact time = 75 min).Fig. 3. Effect of adsorbent dosage and initial dye concentration on adsorption of methylene blue onto WHS (T = 303 K, V = 0.1 L, contact time = 75 min).increasing initial dye concentration, as is also shown in Fig. 3. Dye removal is highly concentration dependent. The increase in loading capacity of the adsorbent with relation to dye ions is probably due to a high driving force for mass transfer. In fact, the more concentrated the solution, the better the adsorption. At 303 K, when the initial dye concentration was increased from 100 to 400 mg/L, the loading capacity of dried adsorbent increased from 76.06 to 113.31 mg of methylene blue per gram of WHS (Fig. 3). 3.4. Effect of pH The pH of the dye solution plays an important role in the whole adsorption process, particularly on adsorption capacity [12]. The effect of pH on the adsorption of methylene blue by WHS is shown in Fig. 4. It was observed that the solution pH affects the amount of dye adsorbed. The dye uptake was found to increase with increasing pH, and it increased from 5.76 to 9.57 mg/g for an increase in pH from 2 to 9. The methylene blue adsorption by WHS was significantly affected over the pH range of 2–4. At higher pH values(5–9), the dye adsorption was almost constant. The surface of WHS may contain a large number of active sites, and the solute (dye ions) uptake can be related to the active sites and also to the chemistry of the solute in the solution. At higher pH, the surface of WHS particles may become negatively charged, which enhances the positively charged methylene blue cations through electrostatic forces of attraction [2]. This, however, did not explain the constant adsorption by WHS at all the studied pH values. There might be another mode of adsorption, e.g., ion exchange. A similar trend was observed for adsorption of malachite green onto treated sawdust [17]. 3.5. Effect of temperature A study of the temperature dependence of adsorption reactions gave valuable knowledge about the enthalpy and entropy changes during adsorption. The removal of methylene blue onto WHS was studied at 303, 313, and 323 K to determine the adsorption isotherms and thermodynamic parameters. As seen in Fig. 1, adsorption ability of WHS for methylene blue increased with increasing temperature. The removal of methylene blue by adsorption onto WHS increased from 16.56 to 21.50 mg/g by increasing theY. Bulut, H. Aydın / Desalination 194 (2006) 259–267 Table 3 Langmuir and Freundlich isotherm constants for methylene blue onto WHS at different temperatures T, K Langmuir isotherm constants Qm (mg/g) 303 313 323 16.56 20.83 21.50 b (L/mg) 0.025 0.033 0.057 b (L/mol) 7,997 10,555 18,232 r2 0.990 0.992 0.990 Freundlich constants kF 1.46 0.53 0.61 n 2.74 1.90 2.42 r2 0.840 0.970 0.856 RL2650.12–0.80 0.09–0.75 0.06–0.64temperature of the solution from 303 to 323 K (Table 3), indicating that the process is endothermic. 3.6. Isotherm studies The equilibrium adsorption isotherm is of importance in the design of adsorption systems [12]. In general, the adsorption isotherm describes how adsorbates interact with adsorbents and thus is critical in optimizing the use of adsorbents. Several isotherm equations are available, and two important isotherms were selected for this study: the Langmuir and Freundlich isotherms. The Langmuir adsorption isotherm assumes that adsorption takes place at specific homogeneous sites within the adsorbent, and it has been used successfully for many adsorption processes of monolayer adsorption. The linearized Langmuir equation is represented as follows [2]: Ceq /qeq = 1 /b Qm + Ceq / Qm (6)tion used to describe heterogeneous systems. The Freundlich isotherm is given in logaritmic form as [2] log qeq = log kF + (1/n) log Ceq (7)where b is the equilibrium constant or Langmuir constant related to the affinity of binding sites (L/mg) or (L/mol) and Qm represents a partical limiting adsorption capacity when the surface is fully covered with dye molecules that assists in the comparison of adsorption performance. Qm and b were calculated from the slope and intercept of the straight lines of the plot Ceq / qeq vs. Ceq. The Freundlich isotherm is an emprical equa-where kF is roughly an indication of the adsorption and (1/n) of the adsorption intensity. kF and (1/n) can be determined from the linear plot of log qeq vs. log Ceq. Adsorption isotherms were obtained in terms of Eqs. (6) and (7) by using experimental adsorption results in these equations. The values for Qm, b, kF, and n are summarized in Table 3. The isotherm data were calculated from the least square method and the related correlation coefficients (r2 values) are given in the same table. As seen from Table 3, the Langmuir equation represents the adsorption process very well; the r2 values were all higher than 0.99, indicating a very good mathematical fit. The fact that the Langmuir isotherm fits the experimental data very well may be due to the homogeneous distribution of active sites onto the WHS surface, since the Langmuir equation assumes that the surface is homogenous [12]. As seen in Table 3, the maximum adsorption capacities for methylene blue onto WHS at 303, 313, and 323 K were found to be 16.56, 20.83, and 21.50 mg/g, respectively. Maximum adsorption capacities of WHS increased with increasing temperature. kF is a Freundlich constant that shows the adsorption capacity of an adsorbent, and is a constant which shows the strength of the relation-266Y. Bulut, H. Aydın / Desalination 194 (2006) 259–267 Table 4 Previously reported adsorption capacities of various adsorbents for methylene blue Adsorbent Indian rosewood sawdust Rice husk Neem leaf Pyrophyllite Jute processing waste Eggshell and eggshell membrane Fly ash Activated carbon Wheat shells Qm, mg/g 11.8–51.4 40.58 8.76–19.61 70.42 22.47 0.80–0.24 13.42 435 16.56–21.50 Ref. 3 2 6 8 10 11 12 20 This studyship between adsorbate and adsorbent [18]. The value kF of WHS for methylene blue at 303, 313, and 323 K are on the order 1.46, 0.53, and 0.61, respectively. It is generally stated that the values of n in the range of 1 to 10 represent good adsorption [18]. In the present work, the exponent was 1 <n <10, indicating favorable adsorption. The essential features of the Langmuir isotherm can be expressed in terms of a dimensionless constant separation factor (RL), which is defined by the following relationship [18]: RL = 1 / (1 + b Ci) (8)According to the value of RL, the isotherm shape may be interpreted as follows: Unfavorable C RL >1.0 Linear C RL = 1.0 Favorable C 1 >RL >0 Irreversible C RL = 0 The results given in Table 3 show that the adsorption of methylene blue onto WHS is favorable. As also seen in Table 3, the adsorption capacities of WHS for methylene blue varied in the range of 16.56 to 21.50 mg/g. Previously some researchers investigated several adsorbents such as rice husk [2], Indian rosewood sawdust [3], neem leaf powder [6], pumice powder [7], pyrophyllite [8], jute processing wastes [10], eggshells [11], fly ash [12], and activated carbon [20] for the removal of methylene blue from aqueous solutions. By comparison of the results obtained in this study with those in the previously reported works (Table 4) on adsorption capacities of various low-cost adsorbent and activated carbon in aqueous solution for methylene blue, it can be stated that our findings are extremly good. 3.7. Estimation of thermodynamic parameters The free energy of adsorption (∆G0) can be related with the equilibrium constant K (L/mol) corresponding to the reciprocal of the LangmuirTable 5 Values of thermodynamic parameters (kJ/mol) for the adsorption of dyes onto WHS T, K 303 313 323 ∆G0 !22.64 !24.11 !26.35 ∆H0 33.41 ∆S0 185constant, b, by the following equation [19]: ∆G0 = !RT ln b (9)where R is the gas universal constant (8.314 J/ mol K) and T is the absolute temperature. Also enthalpy (∆H0) and entropy (∆S0) changes can be estimated by the following equation: ln b = ∆S0 /R!∆H0 /R T (10)Thus, a plot of ln b vs. 1/T should be a straight line. ∆H0 and ∆S0 values were obtained from the slope and intercept of this plot, respectively. ∆G0, ∆H0 and ∆S0 obtained from Eqs. (9) and (10) are given in Table 5. The estimated values of ∆G0 for adsorption methylene blue onto WHS were !22.64, !24.11, and !26.35 kJ/mol, respectively, at 303, 313, andY. Bulut, H. Aydın / Desalination 194 (2006) 259–267267323 K, which are rather low. These findings indicate that a spontaneous physiosorption process occurred [19]. The enthalpy changes (∆H0) and entropy (∆S0) of adsorption were 33.41 kJ/mol and 185 J/molK, respectively. The positive value for ∆H0 indicates that the adsorption of methylene blue onto WHS is an endothermic process. The positive value of entropy reflects the affinity of the adsorbent for methylene blue. 4. Conclusions 1. The present study shows that WHS, an agro-based waste biomaterial, can be used as an adsorbent for the removal of methylene blue from aqueous solutions. 2. The amount of dye adsorbed was found to vary with adsorbent dosage, initial dye concentration, pH, and temperature. 3. The amount of dye uptake (mg/g) was found to increase with increase in contact time, temperature, and in initial dye concentration, but decreased with an increase in adsorbent dosage. 4. The rate of adsorption was found to conform to pseudo-second-order kinetics with a good correlation. 5. Equilibrium data fitted very well in the Langmuir isotherm equation, confirming the monolayer adsorption capacity of methylene blue onto WHS with a monolayer adsorption capacity of 21.50 mg/g. 6. The dimensionless separation factor (RL) showed that WHS can be used for removal of methylene blue from aqueous solutions. 7. The data obtained from adsorption isotherms at different temperatures were used to calculate thermodynamic quantities such as ∆G0, ∆H0, and ∆S0 of adsorption. The results indicate that methylene blue adsorption onto WHS is spontaneous and physical in nature. 8. WHS, an inexpensive and easily available material, can be an alternative for more costly adsorbents used for dye removal in wastewater treatment processes.Acknowledgements The authors are grateful to Dicle University, Research and Project Coordination, for its valuable support. References[1] M. Arami, N.Y. 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