This study is concerned with the adsorption â desorption phenomenon of congo red from solution on the surface of cellulose fibers at different conditions of pH, ...
Adsorptive Removal of Congo red from Aqueous Solution by Cellulose fibers: Desorption Equilibrium and Kinetics
Laith S. Jassim -Assistant lecture / Chemical department/University of Al-Qadisiya.
Abstract This study is concerned with the adsorption – desorption phenomenon of congo red from solution on the surface of cellulose fibers at different conditions of pH, ionic strength and temperature. Desorption studies showed that 47.1% of dye desorbed by washing with distilled water. This indicated that chemical activation migh be taken place in the adsorption process. The desorption process of dye was reached complete equilibrium within 30min. kinetic studies showed that the intraparticle diffusion plays a significant role in the desorption process. The Freundlich equation is determined to best represent the equilibrium sorption data. The amount of dye removal by adsorbent was found to decrease with increasing temperature. Thermodynamic studies revealed that the adsorption of congo red on cellulose fibers is exothermic in nature. The amount of dye removal from solution by cellulose surface at different pH values showed an increase in the following order: 7.8 < 8.9 < 11.7 The adsorption process is affected by the electrolyte concentration. The results indicated an increase in adsorption of congo red in the presence of sodium chloride.
إزالة صبغة الكونغو الحمراء من محلولها المائي على سطح الياف السليلوز :حركية واتزان االبتزاز
ليث سمير جاسم /قسم الكيمياء /كلية التربية /جامعة القادسية
الخالصة: يعنىىه اىىلب ب بدىىس ب سليلو ب اتوف
مدلير ،وعن ظ و
أن بمتزب صب ب تزب ب صب
ب ى ظىىرا
ب كون ىىو ب داى ب مىىم مدلو هىىر ب اىىر ي علىىه ىىل ب يىىر
أمتىىزب -ب تىىزب صىىب
متبرين مم بألس ب هي وجيني ود ج ب د ب
وب قو بأليوني ادلول بألمتزب .
ب كون و ب دا ب عله ل ب سليلو ياكم أن يتم مم خالل بألمتىزب ب انشى واىلب مىر أك تى د ب ى
إذب ينت ب نتر ج رن % 47.1مم ب صب
ي رد ب سل
إن ب ىىزمم ب ىىال د دى وا بفتىىزبن فىىي عاليى ب تىىزب ب صىىب
ر سل ب اتك . نتىىر ج ب د كيى ىىرن بفنتشىىر
اىىو 03دقيقى ،كاىىر أظهى
ب ضاني ل قر ق يلعب دو بً يسير ً في عالي بف تزب . ينت ب نتر ج إن عالي بألمتزب تتبع بيزوثي د ف ين ش المتزب .وق وج إن كاي ب صب يرد د ج ب د ب ،والب يتفق مع مرتم ب توصل إ ي مم ب
ب ازبحى مىم ب ادلىول تقىل مىع
ب ث مودينرميكي إذ تبىيم ىرن عاليى بمتىزب ب صىب
مم ب نوع ب برعس لد ب . وج إن كاي بمتزب ب صب
ب ازبح مىم ب ادلىول وب ىل
ىل ب سىليلو عنى قىيم مةتلفى مىم بألس ب هيى وجيني
يزدبد وفقر ً لت تيب بألتي : 11.7 > 8.9 > 7.8 إن أمتزب صب
ب كون و ب دا ب يتأث ر قو بأليوني لادلول .فق ب دبد كاي ب صب
كلو ي ب صوديود.
ب ااتز في ب ادلول وجود
Introduction The removal of color from dye – bearing effluents is a major problem because of the difficulty in treating such wastewaters by conventional treatment method(1,2). Most dye–containing effluents from various industrial branches, mainly dye manufacturing and textile finishing, are discharged into river streams. Even in low concentrations, dyes affect aquatic life and the food web. Because many organic dyes are harmful to human beings, the removal of color from process or waste effluents is environmentally important. Various physicochemical and biological techniques can be employed to remove dyes from wastewaters. They include the membrane filtration(3), adsorption(4-6), ionic exchange(7-9), advanced oxidation (chlorination, ozonation)(10,11), chemical reduction(12) and biological treatment (bacterial and fungal biosorption, biodegradation in aerobic or anaerobic conditions(13,14). The technical and economic feasibility of each technique is determined by several factors (dye type, wastewater composition, operation costs and generated waste products). In comparation with other techniques adsorption is superior in simplicity of design, initial cost, ease of operation and insensitivity to toxic substances. This technique uses a large number of suitable sorbents as activated carbon(15,16), polymeric resins(17,18) or various low – cost adsorbents such as coal fly ash, wood, agricultural wastes, clays and cotton wastes(5,19-22). Identifications of a potential dye sorbent must be in good agreement with its dye binding capacity, its regeneration properties, its requirements and limitations with respect to environmental conditions.
The Aim of Present Work: The aim of this work into investigates the efficacy of cellulose fibers to removal congo red from its aqueous solution in different conditions of temperature, pH, ionic strength, and to study the mechanism of binding of dye to surface.
Materials and Methods Instruments: 1- Visible spectrophotometer. 2- Dunboff metabolic shaking Incubater GCA/ precision Scientific. 3- Hettich Universal (D-7200), Centrifuge tubes. 4- Electronic Balance, Sartorius Lab. L420 B, +0.0001. 5- pH-Meter, HM-73, TDA Electronic Ltd.
Materials: Congo red (Figure (A)), sodium hydroxide and sodium chloride were supplied by Fluka. Cellulose fibers were obtained from "Al-diwaniya textile factory".
Figure (A): The chemical structure of congo red
Methodology Cellulose fibers were used without further treatment. Wavelength of maximum absorbency (max) was recorded for Congo red dissolved in aqueous media and found 495nm (Figure (B)). This value was utilized for estimation of quantity of dye adsorbed. Solutions of different concentrations were prepared by serial dilution at 495nm and plotted against concentration values. The calibration
curve in the concentration range that falls in the region of applicability of Beer-Lambert's law was employed.
Figure (B) Visible - spectra of congo red Adsorption Isotherm The adsorption isotherms were determined by shaking 0.1g of cellulose fibers into 5ml dye solutions, having concentrations ranging from 1x10-5-7x10-5M at pH 7.8. After 30min. of shaking, the suspensions were centrifuged at 3000 rpm for 10 min. The dye concentration was determined spectrophotometrically. 4
The quantity of congo red adsorbed was calculated according to the following equation(23):-
Qe or x V (Co Ce ) m m
x : the quantity adsorbed. m : weight of adsorbent (g). Co : initial concentration (mg/L). Ce : equilibrium concentration (mg/ L). V : volume of solution (L).
Desorption Kinetics Cellulose fibers that was loaded with congo red (Co=7x10-5M) was mixed with 5ml of the solvent at 20oC with a thermostatic shaker. After different time internals, the mixture was centrifuge and the supernate was analyzed for the dye concentration and the amount of dye desorbed was determined.
Desorption Experiments Desorption experiments were carried out in two different methods. 1- The adsorbent utilized for the adsorption of an initial dye concentration (1x10-5-7x10-5M) was separated from the dye solution. Then, 5ml of eluent was added; after shaking for 30min., the suspensions were centrifuged. The clear supernate was decanted and dye content determined. 2- A solution of fixed concentration of dye (7x10-5M) was prepared in distilled water and the desorption experiment was performed as described above. The elution process was repeated until no significant amount of dye was recovered after several washings
Effect of Temperature Adsorption experiment was repeated in the same manner at temperatures of 20, 35 and 550C to estimate the basic thermodynamic functions.
Effect of pH on the removal of dye Adsorption experiment was carried out as mentioned previously as a function of pH using a fixed concentration of congo red. Sodium hydroxide was used to adjust the pH. The pH of the suspensions at the commencement of the adsorption was measured as well as at the end of experiment using pH-meter.
Effect of Ionic Strength The effect of (0.1, 0.2 and 0.4M) sodium chloride solutions containing different concentrations of congo red in electrolyte solution were added to flasks containing 0.1 gm of cellulose fibers. The procedure described for the adsorption experiment was followed.
Results and Discussion Effect of initial concentration and temperature The effect of initial concentration and temperature on the equilibrium uptake of congo red by cellulose fibers was studied, and a plot of the removal of congo red vs. Co with temperature as a parameter is given in Figure (1). It is evident that the sorption of congo red and, hence, the congo red removal from the solution increases as Co increase and temperature decrease. However, the cellulose fibers has a high affinity to dye molecules and that the dye removal is (88%) at the lowest initial concentration (6.9 mg/l) and temperature (20oC). Congo red removal is affected much by an increase in initial concentration. As Co increases and or/T decreases, the dye loading onto cellulose fibers (i.e., Qe) increases. This means that the congo red sorption capacity of the cellulose fibers increases significantly with a increase in Co, this is due to the increase in the mass-transfer driving force with increasing initial concentration. It is also reflects that the diffusion of congo red molecules into the pores is very fast(24).
of Removal of Congo Red %%
40 20oC 35oC
0 0 ``````````````````````````
Figure (1): % Removal of Congo red at different temperatures The adsorption capacity of cellulose fibers for congo red was determined by measuring equilibrium isotherm. The adsorption isotherm of cellulose fibers for the dye at temperature (20˚C) and at pH 7.8 is shown in Figure (2), where the quantities adsorbed on cellulose fibers are plotted as a function of equilibrium concentration.
Figure (2): Adsorption isotherm of congo red on cellulose fibers at pH 7.8 and constant temperature (20 oC)
The results showed an increase in adsorptive capacities of cellulose fibers as the concentration of congo red increased. The capacity of adsorption depends on several parameters such as the specific surface area, the expansible character, the mobility of dye molecules in the liquid phase and in the interior of the solid, and the forces of attraction between the surface of solid and the molecules of dye (25,26). The coulombic forces between dye species and negatively charged cellulose in water are the major interactions which affect the adsorption of dyes on the cellulose. The shapes of congo red adsorption isotherms were found to coincide with the S-type isotherm reported by Giles et al.(27). The isotherm S-shaped, probably corresponds to electrostatic adsorption of one layer, followed by adsorption of second layer by van der waals attraction. The S-type isotherm depends upon the Freundlich assumption about the heterogeneity of the surface. The presence of various planes, as fibers leads to heterogeneous adsorption behaviour(27).
Desorption kinetics Figure (3) shows the effect of time on the desorption of congo red by distilled water. It can be seen from Figure (3) that the desorption of congo red increased rapidly within (15) min. and then changed very little and no further desorption occurred after 30min.
amount The amount desorbed Thedesobate
Figure (3): Desorption kinetics of congo red – cellulose system The rate constant of the dye removal from the surface by distilled water was determined using the following equation(28).
ln Ce = ln CE + kt
Where k is the rate constant (mg.l.min-1), CE and Ce are the concentrations of the desorbing solution at equilibrium and at time t (min). Value of rate constant was obtained from the slope of the plot of lnCe vs. t (Figure(4)). 16
Figure (4): Linear plot of lnCe vs. t. The rate constant for the desorption process and the correlation coefficient are 31.65 and 0.988 respectively. These results indicated that desorption process, like adsorption, is diffusion – controlled.
Desorption studies Desorption studies help elucidating the mechanism of adsorption and recover of dye and adsorbent. This may make the treatment process economical. The extent of desorption of congo red from celllose fibers increasing as the concentration of dye increased (Figure(5)). This result may refer to the difficulty of desorption of dye at low concentration, which reflects a relatively higher adsorbate – adsorbent interaction and the heterogeneity of surface(29). 60
Q e (mg/g)
Figure (5): Desorption of congo red from cellulose fibers as a function of amount adsorbed Results of Figure (6) clarify he adsorption of congo red on the cellulose fibers is of physico – chemical type characterized by van der waals forces. Nearly half amount of the dye adsorbed was recovered after four washes by distilled water. This indicates that chemical activation might be taken place between active sites of surface and functional group of congo red. 50
No. of washing Elutions No.
Figure (6): Percent desorption of congo red from cellulose by repeated elution with distilled water 9
Freundlich model The adsorption of congo red on cellulose fibers, followed the linearized Freundlich model as shown in Figure (7). The relation between the dye uptake capacity of the adsorbent, residual dye concentration at equilibrium, Ce (mg/l), is given by:
x 1 log k log Ce m n
x (mg/g) and the m
Where n and k are constants for the given adsorbent and solute. -0.1
Figure (7): Linear form of Freundlich isotherm of congo red on cellulose fibers Figure (7) shown the linear relationship of log Qe versus log Ce. The values of Freundlich constants as well as the correlation coefficient are presented in Table (1). The isotherm data fit the Freundlich model well as indicated from the value of correlation coefficient. The fact that the value of 1/n is less than 1 indicates a favorable adsorption.
Table (1): Freundlich constants for the adsorption of congo red on cellulose fibers n
Thermodynamic parameters One way to elucidate dyes adsorption mechanism on the surfaces is to calculate apparent thermodynamic parameters. The general shapes of congo red adsorption isotherms at three different temperatures are given in Figure (8).
35o C 55 oC 0.3
Figure (8): Adsorption isotherms of congo red on cellulose fibers at pH 7.8 and different temperatures The results reveal that the dye uptake decreased with increasing temperature. The fact that the adsorption decreased as the temperature rise indicates that this system is exothermic; this could be interpreted as a result of weaking of attractive forces between the dye molecules and the surface with increasing temperature. The basic thermodynamic quantities of adsorption of congo red on cellulose fibers were estimated through calculating Xm values at different temperatures. The heat of adsorption (ΔH), the change in free energy (ΔG) and the change in entropy (ΔS) could be determined according to equations 2-4. Table (2) and Figure (9) demonstrate these calculations. ln X m
H constant RT
G RT ln K ΔS =
Table (2): Effect of temperature on the maximum adsorbed quantity for adsorption of congo red on cellulose fibers T (K)
Ce = 36.88 293 038 028
3.413 3.247 3.349
0.594 0.454 0.350
-0.520 -0.789 -1.049
Figure (9): Plot of lnXm against reciprocal absolute temperature for adsorption of congo red on cellulose fibers The values of basic thermodynamic functions of adsorption of congo red on cellulose fibers are given in Table (3).
Table (3): Values of thermodynamic functions of adsorption process of congo red on Cellulose fibers at 35 oC Cellulose fibers Adsorbate Congo red
The negative (ΔH) value confirms the exothermic nature of the overall sorption process. The adsorption process in the solid-liquid system is a combination of two processes: (a) the desorption of the molecules of solvent (water) previously adsorbed, and (b) the adsorption of the adsorbate species(30). ΔG value was positive, which indicates that the sorption process let to an increase in Gibbs free energy. A positive ΔG value indicates the nonspontaneity of adsorption process. The negative value of ΔS shows the decreased randomness at the solid – solution interface during adsorption of dye on cellulose fibers.
Effect of pH on the removal of dye The effect of pH on the dye removal by cellulose fibers was studied over a pH range of (7.8-11.7). in this study , blank studies for congo red were done at various pHs. The results obtained show that the molecular form of congo red in solution medium changes markedly in the acidic pH and at a high alkaline pH (pH over 12). Figure (10) depicts that the pH significantly affects the amount of dye removal by cellulose fibers and an increase in the amount removed with increasing pH was observed. 12
% Removal of dye
Figure (10): Effect of pH on the removal of congo red by cellulose fibers at 20 oC The nature of the solid surface as well as that of the solute and solvent, may be altered by change in pH. The solution pH affects the surface charge of the adsorbent and, therefore, the adsorption process through dissociation of functional groups, viz, surface oxygen complexes of acidic character (such as carboxyl and phenolic groups) or of basic character on the active sites of the adsorbent(30). Congo red is an acidic dye and contains negatively charged sulfonated group (-SO3-Na+). The surface charge of cellulose is negative which hinders the adsorption by electrostatic force of repulsion between the negatively charged dye molecule and adsorbent; but appreciable amount of adsorption in this pH range suggests strong involvement of physical forces such as hydrogen bonding, van der Waals force in the adsorption process
Effect of ionic strength The results obtained from the adsorption of congo red on cellulose fibers showed an increase in adsorption quantities of the dye with increasing the concentration of sodium chloride (Figure (11)). 0.4 M
Figure (11): Adsorption isotherms of congo red on cellulose fibers in the presence of different concentrations of sodium chloride 10
An overall increase in dye uptake with increasing electrolyte concentration (and hence the ionic strength), may be due to the reduction in adsorbate solubility as a result of higher interaction of electrolyte ions with the aqueous solvent. The solubility of ionic salts in aqueous media is normally higher than that of organic dye molecules(31). Therefore, a competition between them to interact with the solvent molecules lead to an increase in the attraction between the cellulose surface and the dye molecules(32).
CONCLUSIONS: 1. The cellulose fibers could be employed as adsorbent in wastewater treatment for the removal of congo red dye. 2. The adsorption of dye onto cellulose fibers was reversible to a limited extent. 3. the process of desorption is relatively fast and the kinetic desorption data indicating an intraparticle diffusion mechanism. 4. The adsorption isotherms of congo red on Cellulose fibers obeyed Freundlich isotherm. 5. Thermodynamic parameters show that the adsorption process is exothermic and nonspontaneous, which implies decreased sorption at higher temperatures. 6. adsorption of the dye on the surface was pH dependent. 7. There was a positive correlation between the amounts of congo red adsorbed and the ionic strength of solution.
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