Equilibrium adsorption of rhodamine B on used black tea leaves from acidic aqueous solution.
Removal of dyes from textiles effluents is very important for their strong aromatic forms containing trace alkali, acid and carcinogenic metals. Rhodamine B (Rh-B) is a fluorone dye which is used as a dye and as a dye laser gain medium . It is often used as a tracer dye within water to determine the rate and direction of flow and transport. Rh-B is suspected to be carcinogenic and thus products containing it must contain a warning on its label . Thus the existence of Rh-B in waste water leads to a serious
Different methods of color removal from industrial effluents are biological treatment, coagulation, floation, adsorption, oxidation and filtration etc . Among the treatment options, adsorption appears to have considerable potential for the removal of color from industrial effluents due to its fewer amounts of sludge and clean operation. The removal of color by various adsorbents has been the subject of several recent researches. Activated carbon is perhaps the most widely used adsorbent for the removal of many organic contaminates which are biologically resistant. But activated carbon is prohibitively expensive. Consequently, the high cost of activated carbon, coupled with the problems associated with regeneration, has necessitated the search for alternative adsorbents. Agricultural waste materials such as baggage pith, saw dust, pine bark, maize cob, rice hull, coconut husk fibers, nut shells, soyabean and cotton seed hulls [3, 4] have been evaluated as low cost adsorbents for removal of dyes and other toxic heavy metals. But their adsorption capacities (less than 40 mg/g-adsorbent), however, are far smaller than activated carbon. Therefore, there is a need for the development of low cost, highly efficient and easily available materials, which can adsorb high amount of dye from aqueous solution.
Used black tea leaves (UBTL) is considered as a low cost adsorbents for removal process because of its high adsorption capacity . It has been observed that for the removal of Cr(VI) from aqueous solution at low pH (1.5-2.0), the maximum adsorption capacity (454 mg/g) of UBTL, is nearly equal to that of activated carbons (350- 460 mg/g) . Again, used tea leaves are available as a byproduct of tea industry. Another important fact is that after adsorption, UTBL can easily be destroyed and the adsorbed adsorbate can be recollected from solution . As a result, no secondary pollutant is produced. That is why in our laboratory, UBTL was used previously as an adsorbent for removal of Cr(VI) and Pb(II) from aqueous solutions [5, 7]. In the present study we were interested to remove Rh-B from aqueous acidic solution using used black tea leaves (UBTL) as a low cost adsorbent. Keeping this in mind, the adsorptive characteristics of Rh-B on UBTL was studied by investigating optimum pH, equilibrium time, and the effects of concentration, temperature and solution pH on adsorption.
Material and Methods
All chemicals used in this study were of analytical grade. Double distilled water was used for preparing different reagents. Commercial grade Rhodamine B (Rh-B) was collected from local market in Dhaka. The chemical formula of the Rh-B is [C.sub.28][H.sub.31][N.sub.2][O.sub.3]Cl and molecular mass is 479.02 g. IUPAC name of Rh-B is [9-(2-carboxyphenyl)-6- diethylamino-3-xanthenylidene]-diethyl ammonium chloride and CAS number is 81- 88-9. Synonyms of Rh-B are Rhodamine O, Tetraethyl rhodamine and Rhodamine 610. C. I. Name and number of Rh-B are Basic Violet 10 and 45170, respectively. The structural formula of Rh-B is shown in Figure 1. Rh-B is an amphoteric dye and is highly soluble in acidic media. Since Rh-B adsorb to plastics, solutions were kept in glass bottles . Fresh black tea leaves were collected from Islampur market, Dhaka, Bangladesh.
[FIGURE 1 OMITTED]
Preparation of adsorbent
About 200 g of fresh black tea leaves were boiled in 700 mL of distilled water for 2 hours. Boiled tea leaves were washed 3 times with hot distilled water followed by cold distilled water in several times until the tea liquor was completely disappeared. After washing, tea leaves were initially dried at room temperature and then were dried in an oven (NDO-450, EYELA, Japan) at 103[degrees]C for 10 hours. Prepared used black tea leaves (UBTL) were sieved through the metallic sieve with different particle sizes and different particle sizes was kept in an air-tight bottle. The surface morphology of prepared UBTL (425-500 [micro]m) was investigated by a Scanning Electron Microscope (JSM- 6490LA, JEOL, Japan), at 25 kV acceleration voltages. Before analysis, UBTL was platinum coated using a Pt-coated Auto system (JFC-1600, JEOL, Japan) to improve electron conductivity and image quality. Figure 2 shows the SEM micrograph of UBTL with 2000x image magnification.
[FIGURE 2 OMITTED]
Analysis of adsorbate
A stock solution of 500 mg/L Rh-B solution was prepared by dissolving required amount of commercial grade Rhodamine-B in distilled water. Quantitative analysis of RhB in solutions was performed using UV-visible spectrophotometer (Model-1650 PC, Shimadzu, Japan). For construction of calibration curve, a series of different concentrated Rh-B solutions were prepared by required dilution of stock solution and the pH of each solution was adjusted at a definite value of 2.0 using 0.1 mol/L HN[O.sub.3] or 0.1 mol/L NaOH solution. The absorbance of different concentrated Rh-B solutions at pH 2.0 was measured at the predetermined wavelength of absorption maxima ([[lambda].sub.max]) of 554.0 nm. Beer-Lambert law was verified by plotting the measure absorbance against the concentration of Rh-B within the range of 0.06 to 9 mg/L (figure not shown). The calibration curve at pH 2.0 was used to determine the concentration of Rh-B in different solutions of before and after adsorption.
Selection of solution pH
For the selection of solution pH, 0.1 g of UBTL was taken in each of the six reagent bottles. 50 mL of 9.1 mg/L Rh-B solutions was taken in each of 6 bottles. The bottles were placed in a thermostatic mechanical shaker (SW B-20, Fisons Ltd. Germany), maintained at 30 [degrees]C and were shaking continuously for four hours. The reagent bottles were withdrawn from the shaker and the solutions in bottles were separated from UBTL by centrifuged. After centrifuge, the pH of supernatants was measured. The difference of pH from the initial values was estimated as ApH. A plot of ApH vs initial pH produces a curve, which intersects the X-axis at two points as shown in Figure 3.
Determination of equilibrium time
To determine the equilibrium time, 0.1g of UBTL was taken in 50 mL of 9.5 mg/L Rh-B solution in each of the series of adsorption bottles. Before addition of UBTL, pH of Rh-B solution was adjusted at selected pH 2.0 by drop-wise addition of 0.1 mol/L HN[O.sub.3] or 0.1 mol/L NaOH solution. All bottles were shaken in a thermostated mechanical shaker (SW B-20, Fions Ltd. Germany). The bottles were individually taken out from the shaker after different time of interval such as 15, 45, 60, 120, 240 and 480 minutes of shaking. After adsorption, the UBTL was separated from solutions and the final pH of each solutions was checked, which showed that the pH of the solution did not change. The separated solutions were diluted as required concentration in calibration limit and pH of the diluted solutions were adjusted at 2.0 before measuring their absorbance at 554.0 nm, due to the construction of calibration curve at pH 2.0. The experiment was performed at the temperature of 30[degrees]C. The amounts of Rh-B adsorbed on to UBTL at different contact times were calculated using the following Eq: (1)
[q.sub.t] = ([C.sub.i] - [C.sub.t]) x [V/W] (1)
where [C.sub.i] and [C.sub.t] are the concentrations of Rh-B (mg/L) at zero time and at time t, respectively. V is the volume of solution in liter and W is the mass of the dry UBTL in g. The amount adsorbed is plotted against the contact time as shown in Figure 4.
Adsorption isotherms at different temperatures
For the determination of adsorption isotherm at pH 2.0, 7 adsorption bottles containing each of 0.1 g UBTL into 50 mL Rh-B solution of various concentrations ranging from 10 to 500 mg/L have been shaken at 30 + 0.5[degrees]C for 240 minutes which was equilibrium time as described in last section. After equilibrium adsorption, the residual concentrations of Rh-B in each bottles, were analyzed by UV-vis spectrophotometric method. The equilibrium concentrations were determined for different initial concentrations of Rh-B, and respective amounts adsorbed were calculated from adsorption data. To obtain adsorption isotherms at different temperatures, similar experiments were repeated at temperatures 40 and 50[degrees]C. Each temperature was controlled within + 0.5[degrees]C. Adsorption isotherms at different temperatures are shown in Figure 5. Standard thermodynamic parameters for the adsorption of Rh-B on UBTL were determined from the adsorption isotherms at different temperatures.
Effect of pH on adsorption
To determine the effect of pH on the adsorption of Rh-B on UBTL, 0.1g UBTL was taken in each of the four adsorption bottles. 50 mL of about 100 mg/L Rh-B solution, whose pH was maintained at 2.0, 4.0, 6.0 and 8.0 by addition of 0.1 mol/L HN[O.sub.3] or 0.1 mol/L NaOH solution, was added to each of the four bottles. All bottles were shaken in a thermostated mechanical shaker at 30[degrees]C for 4 hours. After adsorption, the UBTL was separated from each solution. The separated solutions were diluted as required concentration within calibration limits and pH of the diluted solutions were adjusted at 2.0 before measuring their absorbance at 554.0 nm, due to the construction of calibration curve at pH 2.0. In similar way, the concentrations of initial solutions prepared at different pH were determined by measuring the absorbance at pH 2.0 with proper dilution. The amounts adsorbed of Rh-B on UBTL for different initial pH of solutions were determined using equation (1). The amounts adsorbed are plotted against the initial pH of solutions.
Results and Discussion
Characteristics of adsorbent
Used black tea leaves (UBTL) was selected as a low cost adsorbent for the adsorption of Rhodamine B. The constituents of black tea leaves are polyphones, flavones, polysaccharides, cellulose and hemicelluloses, protein, lipids, lignin, caffeine, etc . The continuous treatment of black tea leaves by boiling water brings a considerable change in composition while preparing used black tea leaves (UBTL). Cellulose, hemicelluloses and lignin are the main composition of prepared UBTL . The SEM microgram of UBTL shows in Figure 2 is a heterogeneous surface indicating the possibility to adsorb Rh-B on different parts of UBTL surface.
Selection of solution pH
It is very important to select the solution pH at a suitable value to use for adsorption experiments. Since adsorption is a surface phenomenon, pH of the solution should be change due to the protonation or deprotonation of surface in acidic or basic media. Again, the nature of adsorbate also changes with solution pH. Therefore, the initial pH of solution should be change during adsorption process. Minimum change of pH is required for investigating the adsorption mechanism. Figure 3, a plot of change of pH as a function initial pH, shows the minimum change of pH is at 2.0 and 6.0, which are considered as the optimum pH for this adsorption. Previous studies had been reported that the UBTL is highly stable and acts as good adsorbent in acidic media . For this reason, pH 2.0 was selected for the adsorption Rh-B from aqueous solution on UBTL.
[FIGURE 3 OMITTED]
Estimation of equilibrium time
Equilibrium time is another important parameter for adsorption study which is indicated the adsorption and desorption became steady. For the determination of equilibrium time, the adsorption kinetic experiment was carried out for four hours at 30 [degrees]C. Maximum concentration of Rh-B solution was considered to be 500 mg/L at pH 2.0 and the amount of UBTL was taken for this experiment was 0.1 g for 50 mL Rh-B solution. Under these conditions the equilibrium time for the adsorption of Rh-B on UBTL was not found to be four hours (Figure 4). This was because after four hours the change of concentration with time was small but not steady. To confirm the fact that Rh-B adsorbed on UBTL after four hours, the adsorption experiment was continued for eight hours (Figure 4) but still equilibrium has not been achieved. It has been reported that the equilibrium time for the adsorption of Cr (VI) on UBTL is 15 days . So for the present system four hours was taken as contact time where most of adsorption taken place, which was considered as pre-equilibrium time.
[FIGURE 4 OMITTED]
Analysis of adsorption isotherms at different temperatures
Adsorption isotherm is the most important criteria for understanding an adsorption process. The adsorption isotherm was determined using four hours of equilibrium time and solution pH was at 2.0. Figure 5 shows the adsorption isotherms of Rh-B on UBTL at three different temperatures of 30, 40 and 50[degrees]C. In all cases, the amount adsorbed increases with the increase of equilibrium concentration. To analyze the adsorption isotherms, the most commonly used equilibrium relations, Freundlich and Langmuir equations, were applied to the equilibrium date at different temperatures [11, 12]. The linearized form of Freundlich (2) and Langmuir (3) isotherm can be written as follows:
log [x/m] = log [k.sub.F] + [1/n] log [C.sub.e] (2)
[C.sub.e]/[q.sub.t] = [1/[q.sub.m]b] + [[C.sub.e]/[q.sub.m]] (3)
where, [q.sub.e] = x/m = amount adsorbed at equilibrium time (mg/g), [k.sub.F] = proportionality constant, [C.sub.e] = equilibrium concentration of adsorbate in solution (mg/L) and n = Freundlich constant, referred to the intensity of adsorption (for favorable adsorption, n value falling in the range of 1-10), [q.sub.m] (mg/g) and b (L/mol or L/mg) are the Langmuir constants related to the complete monolayer or maximum adsorption capacity and the energy (or enthalpy) of adsorption, respectively . Figures 6 and 7 show the linear plots of log(x/m) versus log [C.sub.e] and [C.sub.e]/[q.sub.e] versus [C.sub.e] to evaluate the applicability of Freundlich and Langmuir model equations, respectively, for the adsorption of Rh- B on UBTL at different temperatures. The calculated Freundlich and Langmuir constants and their corresponding linear regression correlation coefficient values ([R.sup.2]) from experimental results at different temperatures are given in Table 1.
The results show that the adsorption isotherms at pH 2.0 for different temperatures are well fit with Freundlich model compared with the Langmuir one within the used concentration range. The maximum adsorption capacity, [q.sub.m] obtained from Langmuir isotherm is 72.46 mg/g at 30[degrees]C which is increased to 185.2 mg/g with increasing temperature to 50[degrees]C but the adsorption intensity constant, b decreases with increasing adsorption temperature. Generally, in case of chemical interaction: the amount adsorbed increases with increasing temperature and the adsorption intensity also increase with increasing temperature. But the experimental value of b decreases with increasing adsorption temperature which indicated other than chemical interaction might be occurred in the process. However, the higher adsorption efficiencies at increased temperature indicate that the adsorption of Rh-B dye molecules onto the UBTL is endothermic in nature [13-15].
[FIGURE 5 OMITTED]
[FIGURE 6 OMITTED]
[FIGURE 7 OMITTED]
The analysis of adsorption isotherms at different temperatures by Freundlich and Langmuir models do not give any idea about the adsorption mechanism. Therefore, to predict the adsorption mechanism of the Rh-B dye molecules on the UBTL as chemical or physical, the equilibrium data were also tested with the Dubinin-Raduskevich (D- R) isotherm model . The mathematical expression of the linear form of D-R equation is (4):
ln [q.sub.e] = ln [q.sub.m] - [beta][[epsilon].sup.2] (4)
where [q.sub.e] is the amount adsorbed of dye per unit weight of adsorbent (mol/g), [q.sub.m] the maximum adsorption capacity (mol/g), [C.sub.e] is the equilibrium concentration of dye in aqueous solution (mol/L), R is the molar gas constant and T is the temperature (K), [beta] is the activity coefficient related to the mean free energy of adsorption ([mol.sup.2]/[kJ.sup.2]) and [epsilon] is the Polanyi potential ([epsilon] = RT ln(1 + 1/[C.sub.e]) . The D-R isotherm constants, [beta] and [q.sub.m] can be determined from the slope and intercept, respectively, of the plot of ln [q.sub.e] against [[epsilon].sup.2] as shown in Figure 8. The mean free energy of adsorption, E, defined as the free energy change when 1 mole of ion is transferred to the surface of the solid from infinity in solution can be calculated from the [beta] value as given equation (5):
E = 1/[square root of 2[beta]] = (5)
The E value gives information about the adsorption type, chemical ion exchange (E = 816 kJ/mol) or physical adsorption (E < 8 kJ/mol) [15, 16]. The mean free energy of adsorption, E was recorded between 0.1120-0.1166 kJ/mol. This finding implies that physical adsorption may play a role in this removal process.
[FIGURE 8 OMITTED]
In order to determine whether the adsorption is favorable or unfavorable, the separation factor, [R.sub.L] has been calculated using the following equation (6):
[R.sub.L] = 1/[1 + b[C.sub.o]] (6)
where [C.sub.o] is the initial concentration of Rh-B (mg/L) and b is the Langmuir constant. The values of [R.sub.L] indicate the nature of the adsorption process to be favorable (0 < [R.sub.L] < 1), unfavorable ([R.sub.L] > 1), liner ([R.sub.L] = 1) or irreversible ([R.sub.L] = 0) [17-18]. The calculated values of RL for different initial concentrations and at different temperatures were found to be in the range of 0.1 to 0.9 which is in the favorable range (0-1) of adsorption.
Figure 9 shows the variation of separation factor with initial concentration and processing temperature. The gradual decreasing of separation factor with the increase of initial concentration indicating the favorable adsorption at low concentration of Rh-B, and the nature changed from reversible to irreversible with the increasing of dye concentration. Again, the increased of [R.sub.L] values with increase of temperature suggesting the less favorable adsorption at high temperature.
[FIGURE 9 OMITTED]
Thermodynamic parameters ([DELTA]G[degrees], [DELTA]H[degrees] and [DELTA]S[degrees]) of the adsorption process were calculated from the results of the effect of temperature on adsorption and using following equations (7-8) , (9) .
[(d ln [C.sub.e]/d(1/T)).sub.[theta]] = [DELTA]H[degrees]/R (7)
ln b = [DELTA]S[degrees]/R - [DELTA]H[degrees]/RT (8)
[DELTA]G[degrees] = -RT ln b (9)
where, [theta] indicates the fraction of surface coverage, [DELTA]H[degrees] is the enthalpy of adsorption, b is Langmuir constant and R is the molar gas constant. For a particular amount adsorbed (70 mg/g), the change of equilibrium concentration with temperature has been calculated from Figure 5. Differential heat of adsorption, [DELTA]H[degrees] was calculated from the slope (= [DELTA]H[degrees]/R) of the linear plot ln[C.sub.e] vs 1/T as shown in Figure 10. The positive value of [DELTA]H[degrees] (103.8 kJ/mol) for the removal of Rh-B by UBTL from aqueous solution at pH 2.0 shows the endothermic nature of adsorption.
The change of entropy ([DELTA]S[degrees]) and free energy ([DELTA]G[degrees]) of adsorption at different temperatures were calculated from Eq. (8) and (9), respectively and are presented in Table 2. Positive values of entropy change at different temperatures indicate that the increased randomness at the solid/liquid interface during the adsorption process and suggests good affinity of the Rh-B dye towards the UBTL. Negative values of [DELTA]G[degrees] indicate that the adsorption process was spontaneous in nature and confirm the affinity of the adsorbent towards the Rh-B dye at all temperatures studied. Similar thermodynamic findings have also been reported in the literature [19-21].
[FIGURE 10 OMITTED]
Effect of pH and adsorption mechanism
The effect of temperature and the values of thermodynamic parameters of the adsorption did not give its concrete mechanism of the process; endothermic nature of adsorption suggested the process is chemical but the values of mean free energy of adsorption at different temperatures obtained from D-R isotherm indicated the process is physisorption. Therefore, to elucidate the actual mechanism of the adsorption of Rh-B on UBTL, further study was carried out to investigate the effect of solution pH on the adsorption. The pH of solution is one of the most important factors controlling the adsorption of solid-liquid interface, because both adsorbed molecules and adsorbent particles may have functional groups which are affected by the concentration of hydronium ions ([H.sub.3][O.sup.+]) in the solution and which are involved in the molecular adsorption process at the active sites of adsorbent. Figure 11 shows that the adsorption of Rh-B on UBTL surface decreases with increase the solution pH. This effect can be explained from the viewpoint of surface characteristics of UBTL. The estimated zero point charge pH, [pH.sub.ZPC] of UBTL is 4.2 . Since Rh-B is an amphoteric dye, having number of benzene rings and act as an anionic species in acidic solution, when the pH was increased more then 4.2 the surface of UBTL became negative and because of this there was a repulsive force generated between electron rich negative species of Rh-B and UBTL surface. But as the solution pH was decreased the surface became positive  and the electrostatic force of attraction leading the high amount of adsorption. Figure 12 shows the schematic diagram of the adsorption mechanism of Rh-B on UBTL in acidic media. Again, the endothermic nature of the process is might be due to the fragmentation of large Rh-B molecules during adsorption on UBTL.
[FIGURE 11 OMITTED]
[FIGURE 12 OMITTED]
The adsorption characteristics of used black tea leaves for Rhodamine B were investigated by equilibrium study. The adsorption capacity was evaluated by constructing adsorption isotherm at different temperatures using four hours of pre- equilibrium time where maximum adsorption occurred. All isotherms obtained at different temperatures using pre-equilibrium time are well expressed by Dubinin-Raduskevich (D-R) equation as compared with Freundlich and Langmuir models. Freundlich equation in more applicable for the adsorption isotherm at low temperature compared with high temperature. The amount adsorbed obtained from Langmuir isotherm is decreases with the increase of temperature which indicated endothermic nature of chemisorption. Again, the small value of mean free energy of adsorption, (E = 0.112 kJ/mol) calculated from D-R isotherm suggested that physical adsorption may play a role in this removal process. Finally, the high amount adsorbed at low pH i.e. acidic solution is suitable for adsorption of Rh-B on UBTL, suggesting specific adsorption of the process. Thermodynamic parameters imply the favorability of the process.
The authors would like to thank to the Chairman of Chemistry Department of Dhaka University for providing different facilities during the study. We are also grateful to the Bangladesh Ministry of National Science and Technology for financial support (NST 2008-2009/739/1(21)ES-15) to perform the research.
References and Notes
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M. Abul Hossain * and M. Atiqur Rahman
Department of Chemistry, University of Dhaka, Dhaka-1000, Bangladesh.
Received: 03 August 2012; revised: 24 August 2012; accepted: 30 August 2012. Available online: 14 September 2012.
* Corresponding author. E-mail: email@example.com environmental problem.
Table 1 Freundlich, Langmuir and Dubinin-Radushkevich parameters for the adsorption of Rh-B on UBTL at pH 2.0 for different temperatures T Freundlich parameters ([degrees]C) [k.sub.f] 1/n [R.sup.2] 30 4.41 0.455 0.992 40 4.88 0.492 0.968 50 6.95 0.592 0.977 T Langmuir parameters ([degrees]C) [q.sub.m] b [R.sup.2] (mg/g) (L/mg) 30 72.46 0.0138 0.956 40 92.59 0.0108 0.970 50 185.18 0.0054 0.971 T Dubinin-Radushkevich (D-R) Parameters ([degrees]C) [q.sub.m] x [beta] E [R.sup.2] [10.sup.4] ([mol.sup.2]/ (kJ/mol) (mol/g) [kJ.sup.2]) 30 4.14 36.8 0.1166 0.997 40 6.36 37.4 0.1156 0.984 50 7.82 39.9 0.1120 0.993 Table 2. Thermodynamic parameters for the adsorption of Rh-B on UBTL at different temperatures T (K) b x [DELTA] [DELTA] [DELTA] [10.sup.3] H[degrees] S[degrees] G[degrees] (L/mol) (kJ/mol) (J/mol K) (kJ/mol) 303.16 6.609 + 0.416 -22.17 313.16 5.172 + 103.8 + 0.403 -22.264 323.16 2.586 + 0.387 -21.113
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|Author:||Hossain, M. Abul; Rahman, M. Atiqur|
|Publication:||Orbital: The Electronic Journal of Chemistry|
|Date:||Jul 1, 2012|
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