Ind. Eng. Chem. Res. 2008, 47, 5433–5440
5433
Biosorption of Commercial Dyes on Azadirachta indica Leaf Powder: A Case Study with a Basic Dye Rhodamine B Jyotirekha Sarma,† Arunima Sarma,‡ and Krishna G. Bhattacharyya*,† Department of Chemistry, Gauhati UniVersity, Guwahati 781014, Assam, India, and Department of Chemistry, Morigaon College, Morigaon 782105, Assam, India
Biosorbents, collected and prepared from nature, are most widely used for this purpose. In the present work, removal of a basic dye called Rhodamine B from aqueous solution by adsorption onto a biosorbent, Azadirachta indica (neem) leaf powder (AILP), was investigated. Removal was tested in a batch process with concentration of dye solution, AILP load, pH, temperature, and contact time as the working variables. The adsorption was favored by an acidic pH range and was best described by a second-order rate equation. The experimental data were verified by fitting into both Freundlich and Langmuir isotherms. Thermodynamically, the process was found to be exothermic accompanied by a decrease in entropy and increase in Gibbs energy as the temperature of adsorption was increased from 303 to 333 K. The effect of solution temperature, and the determination of the thermodynamic parameters of adsorption of Rhodamine B (RB) on AILP enthalpy of activation, entropy of activation, and free energy of activation, on the adsorption rates are important in understanding the adsorption mechanism. The rate and the transport/kinetic processes of dye adsorption onto the adsorbents are described by applying various kinetic adsorption models. This would lead to a better understanding of the mechanisms controlling the adsorption rate. The pseudo-second-order model was the best choice among all the kinetic models to describe the adsorption behavior of RB onto AILP, suggesting that the adsorption mechanism might be a chemisorption process. The negative value of the enthalpy change suggested that the rise in the solution temperature did not favor RB adsorption onto AILP. 1. Introduction Industries like textiles, leather, cosmetics, paper, printing, plastics, etc., use many synthetic dyes to color their products. More than 9000 dyes are known to be used by different industries.1–3 Many dyes are suspected to be carcinogenic, mutagenic, and toxic that might affect aquatic biota and also humans2,4 and usually the highest toxicity is found among the basic and diazo direct dyes.2,5,6 The waste effluents, discharged by such industries, become colored due to the residual dyes and when they are disposed to the natural water sources, they pollute water. Colored effluents are known to cause high oxygen demand, fluctuating pH, large solid load, and resistance to biological oxidation.2,4,7 Even a very small amount of dye in water is visible6 and it impedes light penetration and thus reduces photosynthesis in aquatic plants, affects their growth, and decreases gas solubility, interfering with the natural purification process.2 Use of dyes has led to pollution load increase, and the industries have been required to use certain techniques to decolorize the effluents before disposal into the surface water or to the land. Many physical and chemical methods, such as coagulation, floatation, chemical oxidation, solvent extraction, hyperfiltration, etc., have been tried in order to remove color from wastewater, but they have not been very successful since dyes are stable to light and oxidizing agents, and they involve high operational cost and aerobic digestion.2,8 Adsorption has been found to be a superior technique compared to other methods of wastewater treatment in terms of cost, simplicity of design and operation, availability, effectiveness, and their insensitivity to toxic substances.2,8,9 * To whom correspondence should be addressed. Fax: +91 361 2570599. E-mail:
[email protected]. † Gauhati University. ‡ Morigaon College.
Adsorption arises due to interactions between the individual atoms, ions, or molecules of an adsorbate and those present in the adsorbent surface. Depending on the nature of the interactions, ionic species may be held to the surface through electrostatic attraction to sites of opposite charge at the surface or physisorbed due to action of van der Waals forces, or chemisorbed involving strong adsorbate-adsorbent bonding, which may also lead to attachment of adsorbate molecules at specific functional groups on the adsorbent surface. It is obvious that choice of adsorbent plays a very important role. Granular activated carbon (GAC) is widely used as an adsorbent and is favorite for removal of many dyes and other pollutants.2,4,8 The advantages of GAC are its high surface area and high adsorption capacity8 but it is cost prohibitive and both regeneration and disposal of used carbon is not straightforward.2,4,8,10,11 This has necessitated development of newer materials for use as adsorbents. Low cost, locally available, easy to prepare materials with no secondary sludge formation and full biodegradability, as has been the case with materials obtained from natural sources,12 have received particular attention. Such adsorbents have given satisfactory performance in laboratory scale for treatment of colored effluents.13,14 Biosorbents collected from biological sources, viz., water hyacinth roots, banana peel, coir pith, orange peel,7 etc., have been shown to give satisfactory results in removal of commercial dyes from aqueous solutions. In the present work, a fine powder made from the leaves of Azadirachta indica was used to remove the water-soluble basic dye, Rhodamine B. Biosorption was carried out in the batch process with a number of working variables such as pH, concentration of the dye, biosorbent load, time, and temperature of interaction, etc. The neem is a large evergreen tree of the mahogany family. The beneficial properties of the neem tree have been part of Indian folklore for thousands of years. The tree is dubbed “the village pharmacy” and has numerous medicinal properties,
10.1021/ie071266i CCC: $40.75 2008 American Chemical Society Published on Web 07/02/2008
5434 Ind. Eng. Chem. Res., Vol. 47, No. 15, 2008
ranging from controlling digestive disorders to diabetes and high cholesterol to cancer. Since ancient times, leaves of the neem tree have been used for antiviral, antimalarial, antistress, and anti-inflammatory treatment.15 The neem tree contains more than 100 bioactive ingredients and is rich in proteins. Its bitter taste is due to an array of complex compounds called “limonoids”. The most important bioactive principle is azadirachtin (repellent) along with gedunin (antimalarial), nimbin (anti-inflammatory, antipyretic), nimbidin (antibacterial), nimbidol (antimalarial, antipyretic), quercentin (antimalarial), salannin (repellent), and sodium nimbinate (spermicide). Young neem leaves contain 60% water, 23% carbohydrates, 7% proteins, more than 3% minerals, and 1% fat. Rhodamine B (Basic Violet 10) is a water-soluble organic dye, widely used in industrial purposes. It is a basic dye, which causes eye burn, irritation to the skin, the gastrointestinal tract, and the respiratory tract. The color of the dye is bright reddish violet. This dye is used for dying cotton, silk paper, bamboo, weed, leather, etc., preparing carbon paper, ball pen, stamp pad inks, paints, etc. It is a stable compound of molecular formula C28H31N20O3Cl. The dye is known to be harmful if swallowed and may cause irritation to skin, eyes, and respiratory tract. It also causes phototoxic and photoallergic reactions. 2. Experimental Section All the chemicals used were of analytical grade and were used without further purification. 2.1. Preparation of Adsorbent. The neem tree is widely distributed in the entire northeastern region of India. Mature neem leaves were collected from a number of tall trees and washed repeatedly with clean water to remove dust and other insoluble impurities. The neem leaves were allowed to dry in shade and then in an air oven at 333 K for several hours till they became crisp. These were then crushed in a grinder to obtain Azadirachta indica leaf powder (AILP). The powder passing through a 200 mesh (74 µm) sieve was preserved in a clean glass bottle for use as biosorbent. 2.2. Adsorption Experiments. The dye Rhodamine B (Sigma Aldrich) was used without further purification. A stock solution of strength 1000 mg/L was made by dissolving 1000 mg in 1 L double distilled water. The pH of the dye solution was 7.2. All other solutions of various concentrations were made from this solution. The UV-visible spectrophotometer (Hitachi 3210) was calibrated with a set of standard solutions from 10 to 100 mg/L by measuring absorbance (λ max ) 543 nm). The adsorption experiments were done in a batch process. The batch experiments were carried out in 100 mL conical flasks by mixing a preweighed amount of AILP and 50 mL of aqueous dye solution of fixed concentration. The flasks were then kept in a thermostated water bath shaker and were agitated for a predetermined time interval at a constant temperature. The parameters such as pH, time of contact, adsorbent amount, dye concentration, and temperature were varied during different sets of batch experiments. The pH was maintained at different values by addition of a few drops of dilute nitric acid or sodium hydroxide. After adsorption, the mixtures were allowed to settle, and portions of supernatant liquids were centrifuged and absorbance was measured as before to determine the concentration of the unadsorbed dye. 3. Results and Discussion 3.1. Effects of pH. It was observed that λmax for the dye changed very little in the pH range 2-11. Adsorption experi-
Figure 1. Influence of pH on adsorption (%) of Rhodamine B on AILP at different concentrations of the aqueous dye solution (AILP 4 g/L, temperature 303 K, time 3 h; dye concentration at the bottom as legend).
ments done in this pH range showed that pH of the aqueous dye solution had significant influence on uptake of the dye by AILP (Figure 1). The adsorption showed decrease as the pH was increased from 2.0 to 11.0, but the magnitude of decrease depended on the concentration of the aqueous dye solution. When the concentration was relatively high, the decrease was not much, but with decreasing concentration, the influence of pH became more pronounced. Thus, for dye solution of concentration 100 mg/L, the adsorption decreased from 96.5 to 93.5% only in the pH range 2.0-11.0. When the dye concentration was 20 mg/L, the decrease in the same pH range was from ∼95.0 to 80.0%. In any case, the decrease was clearly observed up to a pH of 9.0 after which the adsorption maintained an almost constant value. The natural pH of the aqueous Rhodamine B was 7.2 and adsorption of ∼82.5 to 96% could be achieved around this pH, the subsequent adsorption experiments were carried out without further adjustment of pH of the dye solutions. The results indicate that adsorption of the dye on AILP was favored by an acidic pH, but the adsorption was also appreciable at pH just below 7.0. In strongly acidic solution, the number of H+ ions would be large and they are likely to neutralize some of the anionic functional groups present on the surface of the AILP making the surface either positively charged or less negative. Under these conditions, the dye molecules are likely to attach to the AILP surface through COO- and Cl- groups resulting in higher adsorption of the anionic dye molecules. The surface charge increases negatively with increasing solution pH16–18 and the uptake of the anionic dye molecules comes down. Rhodamine B is known to exist in two principal forms, viz., lactone (RBL) and salt form (RB). In polar solvents, the colorless RBL form is transformed into the violet colored zwitterion (RB+-). A proton can be added to the carboxyl group formed by the opening of the lactone ring of RBL, forming RBH+ that has nearly the same absorption spectrum as RB+-. Anions may form ion pairs in solution with the Rhodamine cation, and the ion pairs also have been shown to yield the same absorption spectra as the free cations.19,20 The adsorption of the dye is therefore more likely at lower pH when RBH+ cations are formed and these attach to anions on AILP surface. 3.2. Effect of Agitation Time and Kinetic Study. The study of adsorption kinetics in wastewater treatment is significant as it provides valuable insight into the reaction pathways and into the mechanism of the reactions. Any adsorption process is
Ind. Eng. Chem. Res., Vol. 47, No. 15, 2008 5435
Figure 2. Variation of the amount of Rhodamine B adsorbed per unit mass of AILP (qt in mg/g) with time at different concentrations of the dye solution (AILP 4 g/L, temperature 303 K, pH 7.2; dye concentration at the bottom as legend).
normally controlled by three diffusion steps: (i) transport of the solute from bulk solution to the film surrounding the adsorbent, (ii) transport of the solute from the film to the adsorbent surface, and (iii) transport of the solute from the surface to the internal sites followed by binding of the adsorbate molecules to the active sites. The slowest of these steps determines the overall rate of the adsorption process and usually step (ii) leads to adsorption on the exterior surface and step (iii) leads to intraparticle diffusion resulting in adsorption on interior sites. It is generally acknowledged that the dominant rate-controlling step is not the actual physical attachment of adsorbate to adsorbent but rather intraparticle transport of the solute within the porous structure of the adsorbent. Interparticle transport from bulk fluid to the external surface of the porous adsorbent may also have an effect on the overall rate of adsorption under some circumstances. A series of experiments with different interaction times were carried out with a constant amount of AILP and different concentrations of the dye solution at 303 K. Figure 2 shows that the time necessary for Rhodamine B to reach saturation on the AILP surface was just over 120 min. Around 82.5 to 93.0% of the dye was removed at the equilibrium time as shown in Figure 3. The distribution of the dye in the liquid-solid interface at equilibrium is important to establish the adsorption capacity of AILP for the dyestuff.21 The relatively short equilibrium time of 120 min and a high percentage removal indicates that AILP possessed a high degree of affinity for the dye Rhodamine B. For different AILP amounts, the time required for saturation is the same. The equilibrium time also depends on other factors such as the molecular weight and the structural complexity of the dye molecules. Desai et al.22 have shown in their study of adsorption of acid dyes by neutral alumina that complex molecular structures adsorb less and take more time to attain equilibrium. In other words, the equilibrium adsorption and desorption kinetics is dependent upon the molecular dimensions of the dyes. The kinetics of Rhodamine B adsorption on AILP is verified with respect to the pseudo-first-order rate equation of Lagergren: log(qe-qt) ) -(k1/2.303)t + log qe
(1)
Figure 3. Influence of agitation time on percentage adsorption of Rhodamine B on AILP at different concentrations of the dye solution (AILP 4 g/L, temperature 303 K, pH 7.2; dye concentrations as legend).
where qt and qe are the amounts adsorbed per unit mass at time t and equilibrium time, and k1 is the first-order rate coefficient. Plot of log (qe - qt) vs t, which should ideally give a straight line is used to obtain k1. The first-order kinetics is considered to be valid when comparable values of qe from the intercept of the Lagergren plot23–25 and from experiments are obtained. If the two do not match, it is necessary to test the validity of the second-order kinetics given by the equation26 t/qt)1/qet + k2qe2
(2)
which again should yield straight line plot for t/qt vs t, allowing computation of qe and k2. The test of validity could be administered by comparing the experimental qe value with that obtained from the second-order plots. The first-order and the second-order kinetic plots [log (qe qt) vs t, and t/qt vs t] for a constant amount of AILP (4 g/L) and different dye concentrations are shown respectively in Figures 4 and 5. All the plots have very good linearity with the regression coefficient of ∼0.99. The values of the rate coefficients found from these plots as well as the linear regression coefficients are given in Table 1. A comparison of qe values obtained experimentally with those obtained from the first- and the second-order plots along with their percent deviations are given in Table 2. The pseudo-first-order Lagergren plots yielded first-order rate coefficient of 1.38 × 10-2 to 4.14 × 10 -2 min-1 indicating an appreciably fast reaction. However, it was seen that the experimentally obtained qe values did not match those determined from the Lagergren plots and very large deviation existed. Therefore, the validity of pseudo-first-order model of kinetics of adsorption of Rhodamine B on AILP was not good31 and could not be established with certainty. The second-order rate coefficients had widely different values (0.008 × 10-2 to 10.2 × 10-2 g/mg/min, mean 1.78 × 10-2 g/mg/min), but the experimental and theoretical qe values now match each other with small deviations. It is therefore more likely that the adsorption of Rhodamine B on AILP might take place through a second-order mechanism.
5436 Ind. Eng. Chem. Res., Vol. 47, No. 15, 2008 Table 2. Differences between Experimental qe and Those Obtained from the Plots of First- and Second-Order Kinetics for Adsorption of Rhodamine B on AILP at 303 K (AILP 4 g/L, pH 7.2) first-order plots
second-order plots
dye (mg/L)
qe(expt) (mg/g)
qe (mg/g)
deviation (%)
qe (mg/g)
deviation (%)
20 40 60 80 100 120
4.1 9.0 13.5 18.6 23.2 25.8
1.2 1.8 1.5 1.9 2.9 4.1
-70.7 -80.0 -88.8 -89.8 -87.5 -84.1
4.3 9.1 14.3 20.0 25.0 25.0
4.8 1.1 5.9 7.5 7.7 -3.1
Table 3. Diffusion Rate Coefficients for Adsorption of Rhodamine B on AILP for Different Concentrations of the Dye Solutions at 303 K (AILP 4 g/L, pH 7.2) intraparticle diffusion dye (mg/L)
Figure 4. Pseudo-first-order kinetic plots for adsorption of Rhodamine B on AILP at different concentrations of the aqueous dye solution (AILP 4 g/L, temperature 303 K, pH 7.2; dye concentrations as legend).
20 40 60 80 100 120
ki (min0.5) -2
6.0 × 10 8.0 × 10-2 9.0 × 10-2 11.0 × 10-2 17.0 × 10-2 14.0 × 10-2
r 0.93 0.88 0.92 0.89 0.87 0.77
Table 1. Rate Coefficients and Regression Coefficients for First- and Second-Order Kinetics for Adsorption of Rhodamine B on AILP for Different Concentrations of Dye Solutions at 303 K (AILP 4 g/L, pH 7.2) dye (mg/L) 20 40 60 80 100 120 mean
k1 (min-1) -2
1.38 × 10 2.76 × 10-2 2.07 × 10-2 1.84 × 10-2 2.30 × 10-2 4.14 × 10-2 2.42 × 10-2
r 0.99 0.99 0.99 0.98 0.96 0.98
k2 (g/mg/min) -2
10.2 × 10 0.35 × 10-2 0.13 × 10-2 0.03 × 10-2 0.009 × 10-2 0.008 × 10-2 1.78 × 10-2
r 0.99 0.99 0.99 0.99 0.99 1.00
Earlier also it has been shown by Allen et al.27 that the sorption kinetics of the basic dyes are described by a pseudosecond-order chemical reaction and that this reaction is signifi-
Kfd
r -2
-1.6 × 10 -2.4 × 10-2 -1.9 × 10-2 -1.6 × 10-2 -1.6 × 10-2 -1.6 × 10-2
-0.98 -0.99 -0.99 -0.98 -0.98 -0.98
cant in the rate-controlling step. These authors have also shown that physical adsorption and chemisorption may be indistinguishable in certain situations, and in some cases, both types of bonding can be present, as with covalent bonds between two atoms having some degree of ionic character and vice versa. The basic dyes ionize in solution to form positive ions. The adsorbent “kudzu” used by Allen et al.27 is cellulose based similar to AILP in this work, and it has been shown that the cellulose surface develops negative charge in contact with water. This negatively charged surface attracts the cations formed by the ionization of the basic dyes, and the chemical interactions between the two oppositely charges species appears to be the main rate-determining factor in the adsorption process. The intraparticle diffusion model, when adsorption takes place inside the pores by a diffusion mechanism, is also tested with the help of the equation qt)kit0.5
Figure 5. Second-order kinetic plots for adsorption of Rhodamine B on AILP at different concentrations of the dye solution (AILP 4 g/L, temperature 303 K, pH 7.2; dye concentrations as legend).
liquid film diffusion
(3)
where ki is the intraparticle diffusion rate coefficient. When this relation is valid, the plot of qt vs t0.5 gives a straight line with zero intercept and a slope equal to ki. The rate coefficient (ki) for intraparticle diffusion obtained from the plots23,28 of qt vs t0.5 was in the range of 6 × 10-2 to 17 × 10-2 mg/g/min0.5 (Table 3). These plots, despite having good linearity, had nonzero intercepts and therefore the intraparticle diffusion could not have a major role in deciding the uptake of Rhodamine B by AILP. Transport from the bulk phase to the adsorbent surface and diffusion to the interior through pores are two processes which may be independent of each other or may be operating simultaneously. If the steps are independent, the plots have normally two or more intersecting lines, depending on the mechanism, the first line representing the surface adsorption and the second line the intraparticle diffusion. No such features were observed in the plots of the present work, which indicated that the two steps were indistinguishable from each other. Still it would not give sufficient indication about which of the two steps is the rate-determining step. The plots do not have a zero intercept, indicating that the diffusion of Rhodamine B species into the pores of AILP is not the dominating factor controlling
Ind. Eng. Chem. Res., Vol. 47, No. 15, 2008 5437 Table 4. Variation in the Amount Adsorbed Per Unit Mass (qe) at Equilibrium with AILP Amount and Rhodamine B Concentration at 303 K (pH 7.2) qe (mg/g) for AILP amount (g/L) of concn 20 40 60 80 100 120
0.5
1.0
1.5
2.0
3.0
4.0
30.0 59.0 86.0 115.0 135.0 163.0
17 33 49 65 80 96
11.7 22.7 33.0 43.6 54.0 64.7
8.8 17.5 25.8 33.0 41.5 49.5
6.2 12.0 17.7 22.7 28.2 33.7
4.8 9.3 13.6 17.4 21.6 25.8
the mechanism of the process. Diffusion is also likely to be restricted due to the bulky nature of the dye molecules. Allen et al.29 have earlier evaluated intraparticle rate parameters for the adsorption of three basic dyes (basic red 22, basic yellow 21, and basic blue 3) on peanut hulls. The overall rate of adsorption has been found to be controlled by intraparticle diffusion, surface adsorption, or both. The rate-controlling mechanism was shown to vary during the course of the sorption process, and at least two possible mechanisms have been suggested: (i) an external surface mass transfer or film diffusion process in the very early stages of adsorption, controlling the initial rate of the sorption process, and (ii) a diffusion stage or stages where the adsorption process slows considerably within the different pores of the adsorbent. A similar mechanism might be operating for adsorption of Rhodamine B on AILP, but the second step does not appear to be dominating. Application the liquid film diffusion model involving diffusion from the bulk liquid phase to the surface of the adsorbent yields similar conclusions. When the mass transfer of the dye molecules from the liquid bulk phase to the adsorbent surface is likely to be the predominant process, the liquid film diffusion model23,30 could be applied with advantage: ln(1 - F) ) -kfdt
Figure 6. Influence of AILP loadings on amount of Rhodamine B adsorbed at equilibrium per unit mass at different dye concentrations (temperature 303 K, pH 7.2, time 3 h; dye concentrations as legend).
(4)
where F is the fractional attainment of equilibrium () qt/qe), kfd is the film diffusion rate coefficient. A linear plot of -ln(1 - F) vs t with zero intercept indicates that the adsorption process is controlled by bulk mass transfer processes. In the present study, the plots of -ln(1 - F) vs t were linear (r ) 0.98 to 0.99) with intercepts of -1.5 to -2.1. The curves did not pass through the origin as required by the model, but very small intercepts indicate that diffusion of the dye molecules from the liquid phase to the adsorbent surface might be having some role in deciding the rate processes. The film diffusion rate coefficient was in the range of -1.65 × 10-2 to -2.39 × 10-2 min-1 (Table 3). From the above results, it appears that the kinetics of adsorption of Rhodamine B on AILP has been mostly controlled by mass transfer from solid-liquid interface to the surface of the AILP particles although some roles for transfer from bulk liquid phase to the solid-liquid interface and transfer from surface to the pores cannot be entirely ruled out. 3.3. Effect of Adsorbate and Adsorbent Mass. Rhodamine B adsorption was influenced by the amount of AILP. With dye concentration of 50 mg/L, the adsorption increased from 84.0 to 97.0% in the AILP range of 0.5 to 8 g/L for a constant agitation time of 120 min at 300 K. On the other hand, for AILP amount of 4 g/L, the adsorption decreased from 93 to 63% when Rhodamine B concentration increased from 20 to 180 mg/L with the same agitation time and adsorption temperature. The amount of Rhodamine B adsorbed per unit mass (qe) increased gradually with increase in Rhodamine B concentration for any AILP amount (Table 4). This may be due to an increase in the number
Figure 7. Langmuir isotherm plots for adsorption of Rhodamine B on different amounts of AILP at 303 K (time 3 h; AILP loadings shown as legend at the bottom; dye concentrations of 20, 40, 60, 80, 100, and 129 mg/L for each AILP loading).
of Rhodamine B molecules per unit mass of AILP leading to higher uptake of the dye. The qe values decreased with the increase in the adsorbent amount for any concentration of dye solution due to availability of less number of Rhodamine B molecules per adsorbent unit mass (Figure 6). At higher concentration of Rhodamine B, the aqueous solution will have a large number of RB+- and RBH+, and under such conditions, the dye has been known to dimerize,20,31 which may also lead to an apparent decrease in adsorption of the dye on AILP surface still giving higher values of amount adsorbed per unit mass. Formation of the dimers involves dye-dye interactions and diminishes monomer solvation. However, such dimer formation is not likely to be overwhelming in water but may be considerable in nonaqueous, low-dielectric constant solvents.32,33
5438 Ind. Eng. Chem. Res., Vol. 47, No. 15, 2008 Table 5. Freundlich and Langmuir Coefficients for Adsorption of Rhodamine B on Different AILP Amounts and Different Concentration of the Dye Solutions at 303 K for Equilibrium Adsorption Time of 180 min AILP amount (g/L) parameters
0.5
1
r n Kf (L/g)
0.99 0.80 8.71
0.99 0.82 6.92
r qm (mg/g) b (L/mg) RL
0.95 500.0 0.013 0.78
0.94 250.0 0.023 0.76
1.5
2
3
4
0.99 0.77 4.68
0.99 0.65 4.89
0.99 0.47 6.16
0.95 111.1 0.035 0.71
0.96 55.6 0.072 0.58
0.94 30.3 0.183 0.39
Freundlich Isotherm 0.99 0.76 5.75
Langmuir Isotherm
Figure 8. Freundlich isotherm plots for adsorption of Rhodamine B on AILP at 303 K (time 3 h; AILP loadings shown as legend at the bottom; dye concentrations of 20, 40, 60, 80, 100, and 129 mg/L for each AILP loading).
3.4. The Adsorption Isotherms. The Langmuir isotherm equation Ce/qe)qm/b + Ce/qm
(5)
describes the relationship between Ce, the liquid phase concentration of the dye, and qe, the solid phase concentration of the dye (i.e., the amount adsorbed per unit mass), both at equilibrium. The Langmuir coefficient, qm, is defined as the amount adsorbed to form a monolayer on unit mass of the adsorbent (monolayer capacity) and b is the other Langmuir coefficient related to the adsorbate-adsorbent equilibrium. A plot of Ce/ qe vs Ce gives a straight line with 1/qm as the slope and qm/b as the intercept from which both the Langmuir coefficients can be found. Another coefficient, RL, known as the separation factor such that 0 < RL < 1 for favorable adsorption, is found from the expression RL ) 1/(1 + bCe)
(6)
The empirical isotherm equation given by Freundlich and useful in describing nonspecific adsorption is qe)KfCen
(7)
log qe)n log Ce+log Kf
(8)
or
where qe and Ce have similar meaning as earlier with Kf and n being the Freundlich coefficients. When eq 8 is obeyed, the plot of log qe vs log Ce yields a straight line and both the Freundlich coefficients could be obtained from the slope and the intercept of the plot. Rhodamine B adsorption on AILP followed both Langmuir isotherm (Figure 7) and Freundlich isotherm (Figure 8). The adsorption coefficients and the correlation coefficients obtained from the isotherms are given in Table 5. The Langmuir plots have good linearity (r ) 0.94 to 0.96), and the Langmuir
0.94 142.0 0.032 0.71
monolayer adsorption capacity (qm) decreased from 500.0 to 30.3 mg/g for AILP amount varying from 0.5 to 4.0 g/L. The adsorption equilibrium parameter, b, varied from 0.013 to 0.183 L/mg with the increase in AILP amount. In order to predict whether the adsorption process by AILP is favorable or unfavorable for the Langmuir type adsorption process, the isotherm shape can be classified by a term “RL”, a dimensionless constant separation factor.21,34,35 The dimensionless parameter, RL, had values in the range of 0.39-0.78, consistent with the requirement for a favorable adsorption process defined by 0 < RL < 1. The Freundlich plots have slightly better linearity (r ) 0.99). The adsorption affinity, n, remained between 0.47 and 0.82 satisfying the condition n < 1 for favorable adsorption. The adsorption capacity, Kf, showed a decrease from 4.68 to 8.71 L/g with increase in AILP amount from 0.5 to 2 g/L. The values of the coefficients are shown in Table 5. It is to be noted that the applicability of Langmuir equation is limited to the formation of a chemisorbed monolayer on a homogeneous surface of identical sites that are equally available and energetically equivalent such that each site carries equal number of molecules, which do not interact with one another36 (no adsorbate-adsorbate interactions). On the other hand, the Freundlich equation describes adsorption (possibly multilayer in nature) on a highly heterogeneous surface consisting of nonidentical and energetically nonuniform sites. In the present work, although both the equations are obeyed, the Freundlich isotherms have a slightly better correlation coefficient indicating that the AILP surface is heterogeneous in the long-range, but may have short-range uniformity. 3.5. Effect of Temperature and Thermodynamic Parameters. When the adsorption was carried out at six different temperatures from 303 to 333 K, the extent of adsorption decreased (Figure 9), showing Rhodamine B adsorption on AILP to be an exothermic process. Gibbs free energy, enthalpy, and entropy (∆G, ∆H, ∆S) for the adsorption process could be obtained from experiments carried out at different temperatures using the equations37 log(qe/Ce) ) ∆S/(2.303R) - ∆H/(2.303RT)
(9)
∆G ) ∆H - T∆S (10) where qe/Ce is known as the adsorption affinity. The values of ∆H and ∆S could be determined from the slope and intercept of the linear plots of log(qe/Ce) vs 1/T and eq 10 could be utilized to obtain ∆G. These plots are shown in Figure 10, and the values of the thermodynamic parameters for the adsorption process at equilibrium are given in Table 6 for a constant AILP loading of 4 g/L and a fixed interaction time of 3 h at different dye concentrations. All the plots show good linearity (regression
Ind. Eng. Chem. Res., Vol. 47, No. 15, 2008 5439 Table 7
Figure 9. Influence of temperature on percentage adsorption of Rhodamine B on AILP at different concentrations of the dye solution (AILP 4 g/L, time 3 h; dye concentrations shown as legend at the bottom).
dye
∆H (kJ/mol)
∆S (J/mol/K)
∆G (kJ/mol)
methylene blue Cibacron Reactive Black Cibacron Reactive Golden Yellow
97.20 - 9.40 53.99
446.7 271.5 -111.9
- 33.68 - 88.96 - 40.25
The adsorption process was accompanied by decrease in entropy of the system in the range of 160-180 J/K/mol (mean decrease 171.7 J/K/mol) that stabilized the dye-AILP adsorption complexes. Adsorption Gibbs energy, ∆G, had negative values in the temperature range of 303-313 K, indicating that the AILP particles spontaneously took up the dye molecules. The decrease was, however, in a narrow range with the mean values changing from 0.65 to 1.81 kJ/mol in the temperature range of 303-313 K in accordance with the exothermic nature of the adsorption process. Gibbs energy started to increase from 323 K onward and thus the spontaneity of the interactions would be lost if the adsorption was carried out at higher temperatures. The thermodynamic activation parameters ∆H, ∆S, and ∆G for adsorption of methylene blue, Cibacron Reactive Black, and Cibacron Reactive Golden Yellow dyes onto manganese oxides modified diatomite have been recently reported by Al-Ghouti et al.38 as shown in Table 7. The temperature thus has significant influence on adsorption of the dyes, and the process could be both endothermic and exothermic depending on the dye. In the present work, the uptake of the dye, Rhodamine B on AILP, was observed to be exothermic supported by appropriate values of ∆H, ∆S, and ∆G. 4. Conclusion
Figure 10. van’t Hoff plots for different concentrations of the dye solution (AILP 4 g/L, time 3 h; dye concentrations shown as legend). Table 6. Thermodynamic Parameters for Adsorption of Rhodamine B on AILP for Different Concentrations of the Dye Solutions at 303 K (pH 7.2) ∆G (kJ/mol) at dye ∆H ∆S (mg/L) (kJ/mol) (kJ/K/mol) 303 K 308 K 313 K 323 K 328 K 333 K 20 40 60 80 100 120
-56.6 -57.5 -57.1 -52.1 -49.5 -50.1
-0.18 -0.18 -0.18 -0.17 -0.16 -0.16
-2.06 -2.96 -2.60 -0.59 -1.02 -1.62
-1.16 -0.26 -2.06 -1.16 -1.66 -0.76 0.26 1.11 -0.22 0.58 -0.82 0.02
1.65 0.64 1.04 2.81 2.18 1.58
2.44 1.54 1.94 3.66 2.98 2.38
3.34 2.44 2.84 4.51 3.78 3.18
coefficient ∼0.98). Adsorption enthalpy, ∆H varied in the range of -49.5 to -57.5 kJ/mol with a mean value of -53.8 kJ/mol. These values indicate sufficiently strong chemical forces, which bind the dye molecules to the AILP particles.
It is found from this study that the powder made from mature, dried leaves of the tree Azadirachta indica could be a useful biosorbent for removal of dyes from aqueous medium. The results indicate the following: (i) Biosorption of the dye was favored in acidic pH range. Still, removal of up to 96% of the dye could be achieved at the natural pH (7.2) of the aqueous dye solution. (ii) Adsorption of the dye reached equilibrium at 120 min. Second-order kinetics was found to be most suitable in describing the biosorption process with mean rate coefficient of 1.78 × 10-2 g/mg/min. Intraparticle diffusion and liquid film diffusion might have some influence in controlling the biosorption process. (iii) Rhodamine B adsorption on AILP agreed with both Langmuir and Freundlich isotherms. The isotherm plots showed that the Freundlich equation gave slightly better linearity than the Langmuir equation (R ) 0.94 to 0.96 for Langmuir plots; 0.99 for Freundlich plots) indicating the AILP surface to be heterogeneous in the long range, but having some amount of uniformity locally. Langmuir monolayer adsorption capacity (qm) decreased from 500.0 to 30.3 mg/g for AILP amount varying from 0.5 to 4.0 g/L. The adsorption equilibrium parameter, b, varied from 0.013 to 0.183 L/mg. The Freundlich coefficients had values of 0.47-0.82 for n, and 4.68-8.71 L/g for Kf. The values of n being
