Faculty of Sciences, University of Tlemcen, P.O. Box Tlemcen - ALGERIA Tel./Fax: 00 (213) : yahoo.

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1 INFLUENCE OF TEMPERATURE ON METHYLENE BLUE SORPTION FROM SYNTHETIQUE AQUEOUS SOLUTIONS USING ALMOND PEEL (HARD VARIETY): EXPERIMENTAL, THERMODYNAMIQUE AND MODELLING STUDIES H. Benaïssa* 1, M. Boumediene 1,2, B. George 3, S. Molina 3, A. Merlin 3 *1 Laboratory of Sorbent Materials and Water Treatment, Department of Chemistry- Faculty of Sciences, University of Tlemcen, P.O. Box Tlemcen - ALGERIA Tel./Fax: 00 (213) : yahoo.fr 2 Department of Hydraulics, Faculty of Technology, University of Tlemcen, P.O. Box Tlemcen - ALGERIA 3 LERMAB E4 4370, Faculty of Sciences and Techniques, University of Nancy, Boulevard of aiguillettes, P.O. Box Vandoeuvre-les-Nancy, France ABSTRACT In the present work, the effect of temperature on the kinetics and equilibrium of methylene blue sorption, as a model of basic dyes, from synthetic aqueous solution by an agricultural waste material: almond peel (hard variety), was studied in batch conditions. Temperature played an important part in sorption phenomenon: the methylene blue sorption increased as temperature increased. The pseudo second-order model was selected to describe the experimental data of methylene blue sorption kinetics. The results showed that the process follows a pseudo second order kinetics. Equilibrium data were mathematically fitted to Langmuir and Freundlich isotherm equations. Langmuir model gave an acceptable fit to the experimental data over the whole equilibrium dye concentrations range. A high dye sorption was observed by this sorbent material: a maximum sorption capacity about mg/g was achieved depending on temperature tested. The thermodynamic parameters obtained with system indicated spontaneous and endothermic process. Keywords: removal, methylene blue, hard almond peel, kinetics, isotherm, effect of temperature. INTRODUCTION Nowadays, water pollution control has become one of the major environmental problems. Synthetic dyes are extensively used by industries including dyehouses, paper printers, textile dyers, colour photography and as additives in petroleum products [1,2]. The effluents of these industries are highly coloured and disposal of these wastes into the environment can be extremely deleterious [1,3]. Among the various types of dyes, various cationic dyes, including methylene blue, are used in dye, paint production and in wool dyeing [4]. Though methylene blue is not strongly hazardous, it can cause some harmful effects (eye burns, nausea, vomiting, etc [5,6]. Thus, the treatment of effluent containing such dye is of major scientific interest, decolourization of dye-containing effluent becoming an obligation both -1-

2 environmentally and for water re-use. Adsorption processes using suitable adsorbents are proved to be an effective and attractive technique for removal of various dye types from wastewater. In recent years, it has led to the search of alternative low cost adsorbents including biosorbents, waste from agriculture/forest or industry and natural materials. Almond peel waste can be an alternative and favourable sorbent material for pollutants such dyes. To date, except the work carried out in our laboratory, no information are available for dyes sorption from aqueous solutions by almond peel in the literature. In this research, as a continuation to our previous work [7], the effect of temperature on the kinetics and equilibrium of methylene blue sorption, as a model of basic dyes, from synthetic aqueous solution by an agricultural waste material: almond peel (hard variety), was studied in batch conditions. The experimental data of dye sorption kinetics by this material tested were fitted by two current models namely: first-order and pseudo- second-order models. Those of equilibrium were analyzed by Langmuir and Freundlich models. Thermodynamic parameters such as free energy change ( G 0 ), enthalpy change ( H 0 ) and entropy change ( S 0 ) were also determined. MATERIALS AND METHODS 1. Sorbent material and dye Almond peel (hard variety) used as a sorbent is cheap and highly available lignocellulosic wastes in Algeria. It was collected from the region of Bensekrane (Tlemcen Algeria), crop of the summer 2006 in the form of large flakes. The waste was sun/air dried at ambient temperature during many days, crushed with a Moulinex Easy power R68 mill and sieved with an Automatic Sieve Shaker D403 device (Controlab) to keep only the size range mm. To be used as sorbent materials, this materials was washed thoroughly with distilled water to remove all the adhering dirt particles until constant ph and no colour observed, filtered and then oven-dried at 85 ± 5 C for 24 h. Methylene blue MB (C16H18CIN3S.3H2O with chemical structure given in Fig. 1, and, purchased from Janssen Chimica) was used as such without further purification, in single component aqueous solutions mg/l stock solutions of methylene blue were prepared in distilled water. All working solutions of the desired concentration were prepared by successive dilutions. 2- Uptake kinetics The initial solution methylene blue concentration was 100 mg/l for all experiments to examine the effect of temperature on the kinetics of methylene blue sorption by hard almond peel. In each sorption experiment, 1 g of hard almond peel was contacted with 1 L of dye solutions in a beaker agitated vigorously by a magnetic stirrer using a water bath maintained at a constant temperature, in the range C. In all cases, the working ph was that of the original solution and was not adjusted. At appropriate time intervals, samples from the clear supernatant were carefully pipeted, diluted and analyzed using UV-visible spectrophotometer (UV-2550 Shimadzu) at max 664 nm. The dye uptake qt (mg dye/g sorbent) was determined -2-

3 as follows: qt = (C0 Ct).V/m (1) where Co and Ct are the initial and time dye concentration (mg/l), respectively, V is the volume of solution (L), and m is the sorbent weight (g) in dry form. Dye sorption losses to the container walls were negligible. 3. Uptake equilibrium The dye equilibrium isotherms were determined by contacting a constant mass g of each sorbent material with equal solution volumes L of different initial dye concentrations: mg/l, in a series of conical flasks. The mixture obtained was agitated for a period of 24 h which was more than sufficient to reach equilibrium at a constant temperature in the range C. The mixture ph was not controlled during the experiments. At equilibrium, the final ph was measured and the equilibrium concentration of free dye in solution was determined with a UV-visible spectrophotometer. The amount of dye sorbed at equilibrium, qe (mg dye/g. sorbent), was determined by difference between concentrations: initial and at equilibrium, respectively. Duplicate tests showed that the maximum standard deviation of the results was 5 %. Preliminary experiments had shown that dye sorption losses to the container walls were negligible. RESULTS AND DISCUSSION 1 Sorption kinetics: Different parameters related to the sorbent, to dye and the medium can influence the kinetics of methylene blue sorption by hard almond peel. In this context, the influence of temperature (25, 35 and 45 C) on the kinetics of methylene blue sorption has been studied. The results obtained and presented on the Figure 1, indicate that the obtained curves have the same shape characterized by a strong increase of the amount of dye sorbed by hard almond peel during the first minutes of contact solution sorbent material, follow-up of a slow increase until to reach a state of equilibrium. There also indicate that an increase of the temperature in the interval C deals to an increase in the amount of dye sorbed at equilibrium: mg/g (T = 25 C) to mg/g (T= 45 C). In general, the necessary time to reach equilibrium is in the range of 4-5 h, and, an increase of sorption time to 8-10 h did not show notable effects. This increase of methylene blue sorption in this interval of temperature, means that the process of dye sorption by hard almond peel is endothermic. -3-

4 q t (mg/g) C 35 C 45 C Time (min) Figure 1. Effect of temperature on methylene blue sorption kinetics by hard almond peel. (Initial dye concentration = 100 mg/l, sorbent dose = 1 g/l, particles size = mm, natural initial ph of solution, agitation speed = 400 rpm) During the dye sorption experiments (Fig. no shown here), a decrease in the initial ph value of solution for all temperatures studied was observed between the initial and equilibrium time. The ph value at equilibrium was in the range which slightly decreased with the temperature increase. This phenomenon can be interpreted as a possible release of H3O + ions from the almond peel to solution due to some acidic groups present on its surface or by a possible fixation of OH - ions by the positive groups present on the sorbent surface. To analyze the sorption kinetic data of methylene blue by hard almond peel obtained at different temperatures, two common models from the literature, namely: the first-order rate model of Lagergren [8] and the pseudo second-order rate model [9,10] were chosen and shown below as Eqs. (2) and (3) in their linear forms: log (qe-qt) = log qe - klt/2.3 (2) t/qt = 1/ k.qe 2 + t / qe (3) where: kl is the Lagergren rate constant of sorption (min -1 ) and k the pseudo second-order rate constant of sorption (g.mg -1.min -1 ); qe and qt are the amounts of dye sorbed (mg.g -1 ) at equilibrium and at time t, respectively. For all temperatures tested, the different values of constants from the slope and intercept of linear plots of log (qe-qt) vs. t and t/qt vs. t ( Fig. not presented here) are summarized in the Table

5 Table 1: Effect of temperature on models rate constants for methylene blue sorption kinetics by hard almond peel. Initial methylene blue concentration = 100 mg/l First-order kinetics model Pseudo second order kinetics model T(K) qeexp. qecal. kl.10 3 R 2 qecal k.10 4 R 2 (mg/g) (mg/g ) (min -1 ) (mg/g) (g.mg -1.min -1 ) For all temperatures tested here, the results obtained confirm that the kinetic data obtained follow the pseudo second-order kinetic model. Compared to the first-order rate model (R 2 = ), the pseudo second order rate model acceptably described the kinetics of dye sorption with acceptable correlation coefficients (R 2 = ). In general, except one experimental point, the equilibrium ye sorption capacity, qe, increases with the increase in the initial dye concentration. The values of qe obtained from the fitting to the pseudo secondorder rate model are approximately similar to the experimental values obtained from the sorption kinetics at equilibrium. 2 Sorption equilibrium: Figs. 2 shows the sorption isotherms of methylene blue by hard almond peel obtained at different temperatures. For all isotherms, the amount of dye sorbed increases initially with the dye concentration at equilibrium but then reaches saturation. These isotherms obtained are of L type (Langmuir type) according to the classification of isotherms of Giles et al. [11]. The results obtained indicate that an increase of the temperature in the interval C deals to an increase in the maximum amount of dye sorbed: about 202 mg/g (T = 25 C) and 230 mg/g (T= 45 C). The increase of methylene blue sorption in the interval of temperature C means that the process of dye sorption by almond peel is endothermic. -5-

6 T = 298 K T = 308 K T = 318 K 200 q e (mg/g) C e (mg/l) Figure 2. Effect of temperature on methylene blue sorption isotherm by hard almond peel. (m = 1 g/l, dp = mm, natural initial ph of solution, agitation speed = 400 rpm) During all experiments of dye sorption equilibrium (results not presented here), it was observed that the initial ph of the solution decreased, and, the equilibrium ph varied with the initial concentration of dyes and the temperature tested. To analyze the sorption equilibrium data of methylene blue by hard almond peel obtained, two isotherm equations, commonly used to fit experimental data when solute uptake occurs by a monolayer sorption, have been tested in the present study, namely, Langmuir [12] and Freundlich [13]. The quality of the isotherm fit to the experimental data is typically assessed based on the magnitude of the correlation coefficient for the regression; i.e. the isotherm giving an R 2 value closest to unity is deemed to provide the best fit. a) The linearised Langmuir model has the following form: Ce/qe = 1/KL.qm +Ce/qm (4) with: qe: amount of dye sorbed at equilibrium per g of sorbent (mg/g), qm: maximal dye sorption capacity of sorbent material (mg/g), Ce: equilibrium dye concentration in the solution (mg/l) and KL: constant of equilibrium (L/mg) depending on temperature and the applied conditions. If the model of Langmuir is valid to describe the experimental results, it must verify the linearised shape of the basis equation, in system of coordinates Ce/qe = f (Ce), that will permit to obtain the constants qm and KL from the intercepts and slopes. b) The linearised Freundlich model has the following form: Ln qe = Ln KF + n Ln Ce (5) where: qe and Ce are as described before, KF and n: positive constants depending on the nature of system solute- sorbent and temperature, n <1. If this equation is verified with data of sorption equilibrium, a straight line should be obtained in the system of coordinates Ln qe = f (Ln Ce), the slope and the intercept to the origin give the values of constants n and KF respectively. -6-

7 Table 2. Effect of temperature on Langmuir and Freundlich models constants for methylene blue sorption isotherm by hard almond peel. Langmuir model Freundlich model T (K) qmax KL R 2 K F n R 2 (mg/g) (L/mg) From results obtained (Figs. not shown here) and the model parameters determined by least squares fit of the experimental sorption data presented in Table 2, it appears that the Langmuir model reasonably fits the experimental results over the experimental dye concentration range with acceptable coefficients of regression (R 2 = ). According to coefficients of correlation (R 2 = ) obtained, the model of Freundlich is not adequate for modelling the isotherms of dyes sorption by date pedicel in all the domain of studied concentrations. 3- Thermodynamic parameters of sorption In environmental engineering practice, both energy and entropy factors must be considered in order to determine which process will occur spontaneously. The Gibbs free energy change, G, is the fundamental criterion of spontaneity. Reactions occur spontaneously at a given temperature if G is a negative quantity. To estimate the effect of temperature on the methylene blue sorption by hard almond peel, the free energy change ( G 0 ), enthalpy change ( H 0 ) and entropy change ( S 0 ) were determined. The free energy of the dye sorption, considering the sorption equilibrium constant, KL, is given by the following equation: G = -RT. ln KL (6) where: G is the standard free energy change (J), R the universal gas constant, J/ mol. K and T the absolute temperature (K). The Gibbs free energy change, G, can be represented as follows: G = H 0 T S (7) As shown in Fig. 3, a plot of G versus T is linear. Enthalpy change, H, and entropy change, S, were determined from the slope and intercept of the plots. The thermodynamic parameters obtained for the dye sorption are shown in Table

8 -7000 Experimental Linear fit G 0 (J/mol) T (K) Figure 3: A plot of G versus T for Methylene blue sorption by hard almond peel. Table 3. Thermodynamic parameters of Methylene blue sorption by hard almond peel. T (K) : G 0 (KJ/Mol) ,018 H 0 (KJ/Mol) S 0 (KJ/Mol/K) R 2 = The negative G 0 values of methylene blue sorption by hard almond peel at various temperatures, confirmed the feasibility of the process and the spontaneous nature of sorption. The positive value of H 0 confirms the endothermic character of dye sorption, whereas the positive S 0 value confirms a certain increased randomness at the solid-solute interface during sorption. The low value of S 0 also indicates that no remarkable change on entropy occurs. CONCLUSION This work shows the interest of a concept based on the waste to treat another waste or to resolve an environmental problem. The results obtained confirm that hard almond peel can remove methylene blue from aqueous solution. Temperature plays an important part in methylene blue sorption by hard almond peel: the methylene blue sorption increased as temperature increased. The results also showed that the process follows a pseudo second order kinetics. Langmuir model gave an acceptable fit to the experimental data over the whole equilibrium dye concentrations range. The thermodynamics of the system pointed out the system was spontaneous and endothermic. -8-

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