APPLICATION OF ADSORPTION MODEL FOR DYE …：对染料的吸附模型的应用….doc

ADSORPTION OF METHYLENE BLUE ON TO LOW COST ADSORBENTS: KINETIC AND THERMODYNAMIC ANALYSISLokeshwari Navalgund* Corresponding author: E-mail address: ; Phone No: Fax: Amrutha Prabhu, Chitra Hegde, Sindhu Bhavi, Archana DanawadeDepartment of Chemical Engineering, S.D.M. College of Engineering and Technology, Dharwad- 580001, Karnataka, IndiaAbstractAdsorption is as an alternative technology to remove colour from wastewater. In this study, low cost adsorbent (Rice husk and Papaya seeds) has been utilized as the adsorbent for the removal of basic dyes from aqueous solution. A basic dye, Methylene Blue (MB) has been used as the adsorbate. Effect of pH, adsorbent dose, initial concentration of dye and contact time was studied. Colour was effectively removed at all selected pH, and the increase in dosage showed an increase in the removal percentage. The adsorption equilibrium for colour was reached after 90 minutes of contact time. The adsorption followed both Langmuir and Freundlich isotherms. Gibbs free energy values obtained confirmed that the process was feasible and spontaneous (G = -24.44, -23.90, and -22.75 kJ/mol). The value of H (-49.12 kJ/mol) indicated that an exothermic chemisorption had taken place. The value of S (-0.084 kJ/mol) suggested that the randomness decreased after adsorption. The experimental data were fitted into Lagergren pseudo second order, the chemisorption. The colour removal efficiency for Rice husk and Papaya seeds were 85% and 70% respectively for batch studies.Keywords: Methylene blue (MB), UV-Vis spectrophotometer, Lagergren pseudo order, Gibbs free energy.1. IntroductionColoured compounds are the most easily recognizable pollutants in the environment because of their appearance. Most of the industries such as textile, paper, carpet, and printing use dyes and pigments to colour their products. Due to their good solubility, synthetic dyes are common water pollutants and they may frequently be found in trace quantities in industrial wastewater (Abdullah et al., 2005). Colour is one of the characteristics of an effluent which is easily detected and readily traced back to source. Dyes are usually present in trace quantities in the treated effluents of many industries. Dye wastewaters discharged from textile and dyestuff industries have to be treated due to their impact on growing public concern due to their toxicity and carcinogenicity. A lot of cases throughout the world are reported about the role of dyes in connection with variety of skin, lung, and other respiratory disorders (Mckay et al., 1981). These wastewaters cause damages to the ecological system of the receiving surface water and create a lot of disturbance to the groundwater resources. Among the different pollutants of aquatic ecosystems, dyes are large and important group of industrial chemicals. Most dyes usually have synthetic origins and complex aromatic molecular structures and designed to be resistant to environmental conditions like light, effects of pH and microbial attack. Most dyes used in textile industries are stable to light and are not biodegradable. In order to reduce the risk of environmental pollution from such wastes, it is necessary to treat them before discharging to the receiving environment. Many physical and chemical methods including adsorption, coagulation, precipitation, filtration and oxidation have been used for the treatment of dye-containing effluent. Adsorption has become one of the most effective and comparatively low cost methods for the decolourization of textile wastewater (Khaled et al., 2009). Activated carbon is the most widely used adsorbent for this purpose because it has a high capacity for adsorption of organic matter, but its use is limited because of its high cost. The effectiveness of adsorption for dye removal from wastewaters has made it an ideal alternative to other expensive treatment methods. The major advantage of an adsorption system for water pollution control are less investment in terms of both initial cost and land, simple design and easy operation, no effect by toxic substance, and superior for the removal of organic waste constituents as compared to the conventional biological treatment processes. At the present, there is a growing interest in using low-cost, commercially available materials for the adsorption of dyes and heavy metal. A wide variety of materials such as biomass (Abudulla et al., 2005), Organe peel (Benaissa, 2005), Palm kernel coat (Oladeja et al., 2008), Cotton fibers (Rasheed et al., 2005), biosolids (Saryoghu et al., 2006), Calymperes Erosum (Adesola et al., 2008), Olive cake (Zahid et al., 2008), Bakers yeast (Saifuddin et al., 2007) have been studied. The use of alternative and cheaper materials is attractive. Review of Allen et al. (2005) evaluates number of adsorbents and dyes.Many industries such as textile, leather, paper and pulp and food industries consume dyes extensively. Among them, textile industry ranks first in the usage of dyes for colouration of fibre. Synthetic dyes and pigments are extensively used for dyeing and printing in industry over 7*105 tons and approximately 10000 dyes (Nigam et al., 2007) are produced annually worldwide, of which 10% is lost in the industrial effluents. Dyes can be divided into several categories, based on their chemical nature whether anionic or cationic dye, and basic or reactive dyes. Removal of direct dyes direct yellow 50 (DY50), direct red 80 (DR80) and direct blue 71(DB71) from an aqueous solution by different adsorbents such as activated carbon, raw kaolinite and montmorillonite was investigated by Yavuz et al. (2006). Methylene blue (MB) was chosen for this study because of its known strong adsorption onto solids. MB is the most commonly used material for dying cotton, wood, and silk with molecular weight 373.9 corresponds to methylene blue hydrochlorine with three groups of water. MB a cationic dye is not regarded as acutely toxic, but it has various harmful effects. On inhalation, it can give rise to short periods of rapid or difficult breathing, while ingestion produces a burning sensation (Bhattacharyya et al., 2005). In order to decrease the cost of treatment, numerous approaches have been made by various researchers to develop cheaper and effective adsorbents to remove dyes from waste. Thus present study was undertaken to evaluate and compare the efficiency of adsorbent (Rice husk and Papaya seed) for the removal of dye in aqueous solution. 2. Material and MethodsThe Rice husk powder was taken from the mill and washed with fresh water and then with distilled water to remove the dust particles. This was sun dried for a day. Then sieve analysis was carried out and 44mesh numbered sample have been used for the experiment. The papaya seeds were washed with distilled water and sun dried. The characteristics of the adsorbents are given in Table 1. The UV-Vis Spectrophotometer was used for analysis.Table 1. The characteristics of Rice husk and Papaya seedsPropertyRise huskPapaya seedsMean particle size856 m3mmApparent density389 kg/m3456 kg/m3Porosity0.640.44Sphericity0.490.892.1. ExperimentationThe experimentation includes the determination of the optimum conditions with which further batch studies were carried out with those optimum values. They include Optimum pH, Optimum dosage, Optimum concentration, Optimum temperature, Optimum RPM and Equilibrium studies. Analysis of Isotherm data, Thermodynamic parameters and Kinetic studies of the adsorption were carried out.3. Results and Discussion3.1. pH plotThe pH of the solution is perhaps the most important parameter in the adsorption of MB dye. The charge of the adsorbate and the adsorbent often depends on the pH of the solution. To understand the adsorption mechanism, the adsorption of dye as a function of pH was measured and the result is as shown in Fig. 1. The removal of MB was found to be highly dependent on hydrogen ion concentration of solution. The change in pH will affect the nature of the species of dye present, which in turn will affect the extent of adsorption. As methylene blue changes its colour in basic medium, effect of pH was studied by varying from pH 4-11. The optimum pH obtained was 8. It was found that pH of 8.9, 8.1, 8.5, 8.3, 8.2, 9.0 and 7.9 were optimal for brick kiln ash, cement kiln ash, cow dung charcoal, groundnut shells charcoal, pea shells charcoal, used tea leaves charcoal and wheat straw charcoal materials, respectively by Sumanjit et al. (2007).Fig. 1: pH Studies: Rice Hus Papaya seeds3.2. Concentration plotAdsorption experiments with rice husk and papaya seeds were conducted for solutions containing 2-20 ppm as initial concentration. As seen in Fig. 2 at lower concentrations of dye, rate of adsorption was fast but at higher concentrations it took longer time. At lower concentrations, all dye ions present in the solution would interact with the binding sites and thus facilitated 100% adsorption. At higher concentrations, more dye ions are left unabsorbed in solution due to the saturation of binding sites. This appears to be due to the increase in the number of ions competing for the available binding sites in the adsorbents. Influencing parameters examined were, dye concentration, nature of diluents, equilibration time, salt concentration, loading of dye in benzoic acid by Muthuraman et al. (2008). Reusability of solvents has been studied. Fig. 2. Concentration Studies: Rice Husk Papaya seeds3.3 Dosage plotAdsorbent dosage is an important parameter because this determines the capacity of an adsorbent for a given initial concentration of the adsorbate at the operating conditions. The concentration of both the dye ions and the adsorbent is a significant factor to be considered for effective adsorption. MB dye uptake rose with increase in adsorbent concentration from 50% at 1g rice husk to 55% at 2gm of rice husk. This appears to be due to the increase in the available binding sites in the adsorbent for the complication of MB dye. However, the dye uptake decreased gradually when the adsorbent concentration exceeded 2gm, which was optimum value as shown in Fig. 3. It is evident that adsorption increases with increase in the mass of the adsorbent. This is because at higher dosage of sorbent, more adsorption sites are available due to increased surface area (Karthikeyan et al., 2008)Fig. 3. Dosage Studies: Rice Husk Papaya seeds3.4 Temperature studiesThermodynamic parameters, like heat of adsorption and energy of activation play an important role in predicting the adsorption behaviour and both are strongly dependent on temperature. Changing the temperature will change the equilibrium capacity of the adsorbent for particular adsorbate. Temperature rise affects the solubility and chemical potential of the adsorbate, the latter being a controlling factor for adsorption. It has been reported that if solubility of the adsorbate increases with increase in temperature, then chemical potential decreases and both of these effects work in the same direction, causing a decrease in adsorption and vice versa. Optimum temperature was found to be 30oC for rice husk and 50oC for papaya seeds as shown in Fig. 4.Fig. 4. Temperature Studies: Rice Husk Papaya seeds3.5. RPM studyRpm study was conducted by adjusting different rotation using a magnetic stirrer in a beaker. The percentage of adsorption in RPM studies is effected by energy of activation and other conditions of dosage, temperature etc as shown in Fig.5.Fig. 5. RPM Studies: Rice Husk Papaya seeds3.6. Thermodynamic studiesThe adsorption process can be regarded as a heterogeneous and reversible process at equilibrium. The apparent equilibrium constant for the process has been shown to be . The change in Gibbs free energy of the adsorption process is thus given as Where Go is the standard Gibbs free energy change for the biosorption (Jmol-1), R the universal gas constant (8.314 Jmol-1K-1) while T is the temperature (K). Thermodynamic parameters were obtained by varying temperature conditions over the range of 303-333oK by keeping other variables constant. Adsorption being spontaneous process, the free energy change is negative. So the enthalpy change for adsorption must be negative and hence the process is exothermic. From thermodynamics, a plot of T against Go gives a straight line with slope So and an intercept of Ho. In Fig. 6 and Fig. 7, the slope is -3.913 and -7.145 Jmol-1 K-1 of papaya seed and rice husk while the intercept is 3098 and 5428 Jmol-1 of papaya seed and rice husk. Therefore, the values of the entropy and enthalpy are 3.913 and 7.145 Jmol-1K-1 and 3098 and 5428 Jmol-1, respectively. The decrease in the value of the free energy with increase in temperature indicates that the adsorption process is endothermic and it is thereby favoured with increase in temperature. The free energy values decrease with increase in temperature. This implies that the spontaneity of the adsorption process increased with increase in temperature. The free energy change (Go) obtained for the adsorption of MB dye at 310oK, initial dye concentration of 2ppm and pH 8 for papaya seeds and rice husk is -3252.17 and -2144.06 Jkmol-1. The large negative value of Go obtained for the adsorption of dye shows spontaneity of the adsorption process at that temperature. The large positive value of So suggested the increase in randomness at the solid/solution inter phase during the adsorption of dye on the adsorbents.Fig. 6. Thermodynamic studies for Papaya seedsFig. 7. Thermodynamic studies for Rice Husk3.7 Adsorption Isotherm3.71 Langmuir IsothermThe linear plots of 1/Ce versus 1/Xe suggest the applicability of the Langmuir isotherms. The values of Xm and were determined from slope and intercepts of the plots and are presented from the results in Table 2. It is clear that the value of adsorption efficiency Xm and adsorption energy of the adsorbent increases on increasing the temperature. From the values we can conclude that the maximum adsorption corresponds to a saturated monolayer of adsorbate molecules on adsorbent surface with constant energy and no transmission of adsorbate in the plane of the adsorbent surface. The observed values shows that the adsorbent prefers to bind basic ions and that speciation predominates on sorbent characteristics, when ion exchange is the predominant mechanism taking place in the adsorption. The regression value (R) was calculated and presented in the plots (Fig. 8, 9, 10 and 11). The values were found to be between 0 and 1 and confirm that the ongoing adsorption process is favourable.Fig. 8. Langmuir Isotherm for Papaya seedsFig. 9. Langmuir Isotherm for Rice Husk3.7.2. Freundlich IsothermThe Freundlich equation is an empirical equation based on adsorption on a heterogeneous surface is given by Equation, where Ceq is the equilibrium concentration (mg/l), qeq is the amount of dye ion bound to per gram of the adsorbent at equilibrium (mg/g) and KF and n are the Freundlich constants related to the sorption capacity and sorption intensity of the sorbent, respectively. The equation can be linearized in logarithmic form and Freundlich constants can be determined.Linear plots of ln Ce versus ln Xe showed that the Freundlich isotherm was also representative for the dye adsorption by the adsorbent tested. K and n were calculated from the slopes of the Freundlich plots and are shown in Table 2. The magnitude of K and n shows easy separation of dye ion from wastewater and high adsorption capacity. The value of n, which is related to the distribution of bonded ions on the sorbent surface, represents beneficial adsorption if it is between 1 and 2. The molecular weight, size and radii either limit or increase the possibility of the adsorption of the dye onto adsorbent. The n value for the adsorbent used was found to be greater than one, indicating that adsorption of dye is favourable (Arivoli et al., 2009).Fig. 10. Freundlich Isotherm for Papaya seedsFig. 11. Freundlich Isotherm for Rice HuskTable 2. Freundlich and Langmuir Isothermal Adsorption parametersAdsorbentFreundlich IsothermLangmuir IsothermRice Huskn= 1.848 and K=2.205= 3.472 and Xm= 2.141Papaya seedsn= 1.464 and K=1.983= 0.644 and Xm= 0.9293.8. Adsorption KineticsExpression for the Pseudo-second order kinetic model iswhere k2ad (g/mg min) is the rate constant of the Pseudo-second order adsorption. The integrated linear form of equation is. If the experimental data fits (linear relationship) the plot of t/q versus t, the Pseudo-second-order kinetic model is valid.The correlation coefficients of the Pseudo-first-order kinetic model obtained are very low. The theoretical qeq values found from the Pseudo-first-order kinetic model did not give acceptable values. It is probable therefore, that this adsorption system is not a Pseudo-first-order reaction. The Pseudo-second-order rate constant (k2ad) and qeq values were also determined from the slope and intercept of the plots t/q versus t as shown in Fig. 12 and Fig. 13. The values of the parameters (k2ad), qeq are calculated. The results indicate that the theoretical qeq values found from Pseudo second- order kinetic model was found to be very closer to the qeq experimental values with higher correlation coefficient. These results suggest that the sorption system is not a Pseudo-first-order reaction and that it is the Pseudo-Second-order model. The kinetics data show that the sorption was rapid and this could be useful in the design of column sorption process. The pseudo second-order reaction rate model adequately described the kinetics of dyes sorption with high correlation coefficien