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Adsorption of methylene blue on kaolinite
Applied Clay Science 20 (2002) 295 – 300 www.elsevier.com/locate/clay
Adsorption of methylene blue on kaolinite Dipa Ghosh, Krishna G. Bhattacharyya * Department of Chemistry, Gauhati University, Guwahati 781014, Assam, India Received 29 May 2000; received in revised form 1 February 2001; received in revised form 2 July 2001; accepted 9 July 2001
Abstract Methylene blue was adsorbed on kaolin from a local deposit. The raw kaolin itself was a relatively good adsorbent. The adsorption capacity was improved by purification and by treatment with NaOH solution. Calcination of the kaolin reduced the adsorption capacity. The adsorption data could be fitted by the Freundlich and Langmuir equations. Also, the thermodynamic parameters such as DH0, DS0 and DG0 were determined. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Kaolinite; Adsorption; Methylene blue
1. Introduction Kaolinite is found as a common constituent of soils and sediments. When different types of pollutants in domestic sewage, industrial effluents, sludge and other solid wastes are dumped on the earth surface, the soil particles including clay minerals can interact with the pollutants. The clay minerals in soil may therefore play a role in scavenging pollutants from the environment. Kaolinite has a low CEC of the order of 3 to 15 meq/100 g and therefore it is not expected to be an ion-exchanger of high order. The small number of exchange sites is located on the surface of kaolinite and it has no interlayer exchange sites (Raymahashay, 1987). Nevertheless, the small CEC and the adsorption properties may play an effective role in scavenging inorganic and organic pollutants from water.
*
Corresponding author. Fax: +91-361-570133. E-mail address: [email protected] (K.G. Bhattacharyya).
In industrial water pollution, the colour produced by minute amounts of organic dyes in water is considered very important because, besides having possible harmful effects, the colour in water is aesthetically unpleasant. The clays in soil can act as a natural scavenger in removing colour from the contaminated water. In the present study, aqueous solutions of a basic dye, methylene blue, were used as a model compound in an attempt to use kaolinite as an adsorbent. Although not strongly hazardous, methylene blue can have various harmful effects. On inhalation, it can give rise to short periods of rapid or difficult breathing while ingestion through the mouth produces a burning sensation and may cause nausea, vomiting, diarrhea, and gastritis. Accidental large dose creates abdominal and chest pain, severe headache, profuse sweating, mental confusion, painful micturation, and methemoglobinemia. The adsorption of methylene blue on clay minerals is likely to be dominated by ion-exchange processes. In the present work, the influence of (a) purification
0169-1317/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 1 3 1 7 ( 0 1 ) 0 0 0 8 1 - 3
296
D. Ghosh, K.G. Bhattacharyya / Applied Clay Science 20 (2002) 295–300
and calcination, (b) the adsorbent dose, (c) concentration of methylene blue, (d) pH of the medium and (e) temperature of adsorption is investigated. The effect of treating the kaolin with an aqueous NaOH solution is also studied.
2. Experimental 2.1. Materials The kaolin, in bright white lumps, was collected from Silonijan in the district of Karbi Anglong in Assam (India). After collection, stones and other heavy particles were removed from the samples, which were then crushed, ground and sieved through a 230-mesh sieve to remove the larger non-clay fractions for obtaining raw kaolin. A part of the raw kaolin was kept suspended in double distilled water in a 1-l beaker for several hours and then the mixture was stirred with the addition of a small amount of 30% hydrogen peroxide solution to remove organic substances till all effervescence ceased. The mixture was kept standing overnight and then, after decanting the clear liquid from the top, more water was added, stirred, and allowed to settle down for 3 –4 h. The clear liquid at the top was again decanted and the process was repeated several times to get rid of excess hydrogen peroxide solution. Finally, after adding more water, the mixture was agitated vigorously for some time and the suspended kaolin was taken in several wide glass plates, which were kept in an air oven at around 343 K for slow evaporation to dryness for obtaining pure kaolin (Moore and Reynolds, 1989). XRD and IR measurements showed the pure kaolin to be of very high purity with only trace amounts of feldspar. The pure kaolin was found to be similar to a kaolin sample (KGa-1b) obtained from the University of MissouriColumbia, Source Clay Minerals Repository, Columbia. Samples of both raw and pure kaolin were calcined at 770 K for 6 h. Amounts of 6 g of raw and pure kaolin were also shaken with 100 ml 1 M NaOH for 4 h, left overnight, filtered, and washed with water. The residue was dried at 350 K. Methylene blue (microscopic grade, Glaxo India, Mumbai) was used without any further purification.
2.2. Adsorption studies Adsorption of methylene blue was carried out in a batch process by varying kaolin dose, adsorptive concentration, pH of medium and temperature. A weighed sample of kaolin was mixed with 25 ml methylene blue solution of known concentration. The mixture, in a 100-ml conical flask, was shaken in the water bath of a thermostat at a particular temperature for 3 h. In all cases, adsorption equilibrium was reached within 2 h. If necessary, the pH was adjusted by adding a few drops of dilute NaOH or HCl before shaking. The mixture was allowed to settle and was centrifuged. The methylene blue concentration in the supernatant was determined with a spectrophotometer (Hitachi model 3210). 2.3. Adsorption isotherms The adsorption data from the experiments were fitted with: ðaÞ Freundlich Isotherm : x=m ¼ KCen
ð1Þ
where x/m is the amount of methylene blue adsorbed on the unit mass of the adsorbent, Ce is the equilibrium adsorbate concentration in aqueous phase, n and K are the Freundlich’s constants. ðbÞ Langmuir Isotherm : Ce =ðx=mÞ ¼ ð1=abÞ þ ð1=bÞCe
ð2Þ
where a and b are the Langmuir constants. The constants were obtained from the plots of the linearized equations. Another factor, RL, which is considered as a more reliable indicator of adsorption (Vermeulan et al., 1966; McConvey and McKay, 1985) was computed from: RL ¼ 1=ð1 þ aCÞ
ð3Þ
where a is Langmuir’s constant and C is any adsorbate concentration at which the adsorption is carried out. Favourable adsorption is indicated by 0 < RL < 1. 2.4. Thermodynamic parameters The thermodynamic parameters of the adsorption process are obtained from experiments at various
D. Ghosh, K.G. Bhattacharyya / Applied Clay Science 20 (2002) 295–300
297
The cation exchange capacity (CEC) of the kaolins was estimated by the conventional BaCl2 method and was found to be 2.62 (Cl), 2.85 (C2), 1.94 (C3), 3.08 (C4), 2.17 (C5) and 2.42 (C6) meq/100 g. The pure kaolin and its calcined sample have higher CEC compared to the other samples. Calcination improved the CEC of the pure kaolin while that of the raw kaolin decreased. The treatment with NaOH decreased the CEC of the raw and pure kaolin. 3.1. Effect of adsorbent and adsorptive doses
Fig. 1. Adsorption of methylene blue as a function of the initial methylene blue concentration. Contact time 3 h, kaolin dose 0.8 g/l, temperature 300 K.
The adsorption of methylene blue at a fixed kaolin dose of 0.8 g/l at 300 K is shown in Fig. 1. The sample C6 showed the maximum adsorption. The adsorption decreased in the order C 6 > C5 > C2 > C1 > C4 > C3. The raw kaolin (C1) showed a considerable adsorption, which was reduced after calcination. A similar influence was evident for the pure kaolin. Treatment with NaOH increased the adsorption. The NaOH-treated pure kaolin (C6) adsorbed
temperatures using the following equations (Khan et al., 1995): ðaÞ logKd ¼ DS 0 =ð2:303RÞ DH 0 =ð2:303RT Þ
ð4Þ
ðbÞ DG0 ¼ DH 0 T DS 0
ð5Þ
ðcÞ Kd ¼ ðx=mÞ=ðy=uÞ
ð6Þ
where Kd is the distribution coefficient for the adsorptive and is equal to the ratio of the amount adsorbed (x/m in mg/g) to the adsorptive concentration ( y/u in mg/dm3). The values of DH 0 and DS 0 were determined from the slope and intercept of the linear plot of log Kd vs. 1/T.
3. Results and discussion In all adsorption experiments, six different adsorbents were used: Cl—raw kaolin, C2—pure kaolin, C3—calcined raw kaolin, C4—calcined pure kaolin, C5—NaOH-treated raw kaolin, and C6—NaOH-treated pure kaolin.
Fig. 2. Variation of the methylene blue adsorption with pH. Contact time 3 h, initial methylene blue concentration 15 mg/l, kaolin dose 0.8 g/l.
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3.2. Effect of pH The methylene blue adsorption showed a minimum around pH 4 (Fig. 2). The increased adsorption at basic conditions may be related to preference of the dye cations for basic sites. In aqueous medium, the exchangeable alkali and other metal cations on the surface and in the interlayer region of the clay undergo hydration creating a hydrophilic environment (Lawrence et al., 1998). Increasing the pH of the adsorbing medium modifies the clay mineral surface and this might be responsible for the gradually increasing uptake of methylene blue at pH values above 4.0. Raymahashay (1987) in his comparative study of methylene blue adsorption on kaolin and some other adsorbents also found that the uptake of the dye increased with increasing pH. This is normally attributed to enhanced association of the dye cations, produced by the dissociation of methylene blue, with the negatively charged kaolin sur-
Fig. 3. Freundlich plots for the adsorption of methylene blue at room temperature.
nearly 100% methylene blue from a 12-ppm solution. The amount of the dye adsorbed showed a continuous decrease with an increase in the doses of kaolin for all the six adsorbents. This is because the equilibrium concentration of the dye decreased with increasing amount of kaolin such that amount adsorbed diminished. The above results indicate that the raw kaolin showed a substantial methylene blue adsorption. Higher amounts were adsorbed by the pure kaolin with a further enhancement in adsorption if this kaolin was treated with NaOH. The purification of the raw kaolin by treating with hydrogen peroxide solution (for removing organic matter) must have resulted in removal of non-clay matter and in generation of some basic or anionic sites for binding methylene blue cations. The treatment with NaOH creates additional which improve the adsorption. Calcination of the samples apparently removed some active sites.
Fig. 4. Langmuir plots for the adsorption of methylene blue at room temperature.
D. Ghosh, K.G. Bhattacharyya / Applied Clay Science 20 (2002) 295–300
299
Table 1 Adsorption constants for methylene blue adsorption on kaolins at room temperature Adsorbents
C1 C2 C3 C4 C5 C6
Freundlich constants
Langmuir constants
n
K (dm3 g 1)
a (dm3 g 1)
b (mg g 1)
RL
0.151 0.070 0.075 0.061 0.047 0.098
14.85 16.39 6.57 9.01 17.04 23.05
27.49 91.87 13.44 56.31 204.00 122.01
13.99 15.55 7.59 8.88 16.34 20.49
0.0021 0.0006 0.0042 0.0010 0.0028 0.0005
face at higher and higher pH (Singh and Srivastava, 1999). However, the tendency for increasing adsorption at pH values below 4.0 is difficult to explain, but a similar result was earlier observed by workers for adsorption of some other dyes on kaolin (Ganjidoust et al., 1995). 3.3. Adsorption isotherms Methylene blue adsorption on the kaolins can be described by the Freundlich and also Langmuir equations (Figs. 3 and 4, Table 1). The Freundlich exponent n between 0.047 and 0.151 indicates favourable
adsorption for which 0 < n < 1. The Freundlich constant K is appreciable for all the six adsorbents in general agreement with strong adsorption. The NaOHtreated pure kaolin has the largest value of K followed by the NaOH-treated raw kaolin. The calcined sample has the lowest K-value. The values of Langmuir constant b is in the order of C6 > C5 > C2 > C1 > C4 > C3 which is highest for the NaOH-treated pure kaolin and lowest for the calcined sample. The values of the Langmuir constant a also reflect an almost similar trend. The RL-values between 0.0005 and 0.0042 (Table 1) show favourable adsorption of methylene blue on kaolin.
Fig. 5. Log Kd vs. 1/T for six different doses of raw kaolin.
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Table 2 Thermodynamic parameters for adsorption of methylene blue on kaolins Adsorbent
DH 0 (kJ mol 1)
DS 0 (J mol 1 K 1)
DG 0 (kJ mol 1)
C1 C2 C3 C4 C5 C6
9.40 7.54 13.53 11.84 7.92 6.03
78.39 73.53 88.16 84.32 74.99 69.69
14.94 15.29 13.85 14.34 15.37 15.61
4. Conclusion The adsorption experiments reveal that kaolinite clay may be quite effective in removing a basic dye like methylene blue in relatively low concentrations from the aqueous medium. Although the experiments have been done with the raw kaolin and five modified forms, the raw kaolin itself has a relatively large adsorption capacity.
Acknowledgements 3.4. Thermodynamic parameters A typical log Kd vs. 1/T plot is shown in Fig. 5. The values demonstrate a spontaneous and favourable adsorption process. The standard enthalpy change (DH0) for the adsorption process is positive indicating that the process is endothermic in nature. The endothermic nature of adsorption of methylene blue was earlier observed by Mall and Upadhyay (1995,1998) with boiler bottom ash and fly ash as the adsorbents, and also by De and Basu (1999) with a sawdust-based adsorbent. Singh and Srivastava (1999) obtained positive DH and DS, and negative DG values for adsorption of methylene blue and a few other basic dyes on plant leaves in agreement with the present work. The values of standard entropy change (DS0) are not very large and indicate an increase due to adsorption. Normally, adsorption of gases leads to a decrease in entropy due to orderly arrangement of the gas molecules on a solid surface. However, the same may not be true for the complicated system of adsorption from solution on a non-uniform kaolin surface. Entropy increase was also observed by other workers as is pointed out above. The negative values of standard Gibbs energy change (DG0) in all the cases are indicative of the spontaneous nature of the interaction without requiring large activation energies of adsorption. Each of the thermodynamic parameters is in a narrow range of values for all the six clay adsorbents and therefore, it may be concluded that the thermodynamic processes involved in the clay –methylene blue interaction for all the adsorbents are more or less uniform in nature (Table 2).
The authors are grateful to the Assam Science, Technology and Environment Council (ASTEC) for providing a financial grant for this work.
References De, D.S., Basu, J.K., 1999. Adsorption of methylene blue on to a low cost adsorbent developed from sawdust. Indian J. Environ. Prot. 19, 416 – 421. Ganjidoust, H., Samadian, M., Deyhoal, F., 1995. Removal of dyes by sorption on soil from textile industries. Proc. of Third Intl. Conf. on Appropriate Waste Management Technology for Developing Countries, NEERI, Nagpur, pp. 523 – 530. Khan, S.A., Rehman, R., Khan, M.A., 1995. Adsorption of Cr (III), Cr (VI) and Ag (I) on bentonite. Waste Manage. 15, 271 – 282. Lawrence, M.A.M., Kukkadapu, R.K., Boyd, S.A., 1998. Adsorption of phenol and chlorinated phenols from aqueous solution by tetramethylammonium- and tetramethylphosphonium-exchanged montmorillonite. Appl. Clay Sci. 13, 13 – 20. Mall, I.D., Upadhyay, S.N., 1995. Removal of basic dyes from wastewater using boiler bottom ash. Indian J. Environ. Health 37, 1 – 10. Mall, I.D., Upadhyay, S.N., 1998. Studies on treatment of basic dyes bearing wastewater by adsorption treatment using fly ash. Indian J. Environ. Health 40, 177 – 188. McConvey, I.F., McKay, G., 1985. Chem. Eng. Process. 19, 267 – 275. Moore, D.M., Reynolds Jr., R.C. 1989. X-ray Diffraction and the Identification and Analysis of Clay Minerals. Oxford Univ. Press, Oxford, pp. 179 – 201. Raymahashay, B.C., 1987. A comparative study of clay minerals for pollution control. J. Geol. Soc. India 30, 408 – 413. Singh, D.K., Srivastava, B., 1999. Removal of basic dyes from aqueous solutions by chemically treated Psidium Guyava leaves. Indian J. Environ. Health 41, 333 – 345. Vermeulan, T.H., Hall, K.R., Eggleton, L.C., Acrivos, A., 1966. Ind. Eng. Chem. Fundam. 5, 212 – 223.
Adsorption of methylene blue on kaolinite Dipa Ghosh, Krishna G. Bhattacharyya * Department of Chemistry, Gauhati University, Guwahati 781014, Assam, India Received 29 May 2000; received in revised form 1 February 2001; received in revised form 2 July 2001; accepted 9 July 2001
Abstract Methylene blue was adsorbed on kaolin from a local deposit. The raw kaolin itself was a relatively good adsorbent. The adsorption capacity was improved by purification and by treatment with NaOH solution. Calcination of the kaolin reduced the adsorption capacity. The adsorption data could be fitted by the Freundlich and Langmuir equations. Also, the thermodynamic parameters such as DH0, DS0 and DG0 were determined. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Kaolinite; Adsorption; Methylene blue
1. Introduction Kaolinite is found as a common constituent of soils and sediments. When different types of pollutants in domestic sewage, industrial effluents, sludge and other solid wastes are dumped on the earth surface, the soil particles including clay minerals can interact with the pollutants. The clay minerals in soil may therefore play a role in scavenging pollutants from the environment. Kaolinite has a low CEC of the order of 3 to 15 meq/100 g and therefore it is not expected to be an ion-exchanger of high order. The small number of exchange sites is located on the surface of kaolinite and it has no interlayer exchange sites (Raymahashay, 1987). Nevertheless, the small CEC and the adsorption properties may play an effective role in scavenging inorganic and organic pollutants from water.
*
Corresponding author. Fax: +91-361-570133. E-mail address: [email protected] (K.G. Bhattacharyya).
In industrial water pollution, the colour produced by minute amounts of organic dyes in water is considered very important because, besides having possible harmful effects, the colour in water is aesthetically unpleasant. The clays in soil can act as a natural scavenger in removing colour from the contaminated water. In the present study, aqueous solutions of a basic dye, methylene blue, were used as a model compound in an attempt to use kaolinite as an adsorbent. Although not strongly hazardous, methylene blue can have various harmful effects. On inhalation, it can give rise to short periods of rapid or difficult breathing while ingestion through the mouth produces a burning sensation and may cause nausea, vomiting, diarrhea, and gastritis. Accidental large dose creates abdominal and chest pain, severe headache, profuse sweating, mental confusion, painful micturation, and methemoglobinemia. The adsorption of methylene blue on clay minerals is likely to be dominated by ion-exchange processes. In the present work, the influence of (a) purification
0169-1317/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 1 3 1 7 ( 0 1 ) 0 0 0 8 1 - 3
296
D. Ghosh, K.G. Bhattacharyya / Applied Clay Science 20 (2002) 295–300
and calcination, (b) the adsorbent dose, (c) concentration of methylene blue, (d) pH of the medium and (e) temperature of adsorption is investigated. The effect of treating the kaolin with an aqueous NaOH solution is also studied.
2. Experimental 2.1. Materials The kaolin, in bright white lumps, was collected from Silonijan in the district of Karbi Anglong in Assam (India). After collection, stones and other heavy particles were removed from the samples, which were then crushed, ground and sieved through a 230-mesh sieve to remove the larger non-clay fractions for obtaining raw kaolin. A part of the raw kaolin was kept suspended in double distilled water in a 1-l beaker for several hours and then the mixture was stirred with the addition of a small amount of 30% hydrogen peroxide solution to remove organic substances till all effervescence ceased. The mixture was kept standing overnight and then, after decanting the clear liquid from the top, more water was added, stirred, and allowed to settle down for 3 –4 h. The clear liquid at the top was again decanted and the process was repeated several times to get rid of excess hydrogen peroxide solution. Finally, after adding more water, the mixture was agitated vigorously for some time and the suspended kaolin was taken in several wide glass plates, which were kept in an air oven at around 343 K for slow evaporation to dryness for obtaining pure kaolin (Moore and Reynolds, 1989). XRD and IR measurements showed the pure kaolin to be of very high purity with only trace amounts of feldspar. The pure kaolin was found to be similar to a kaolin sample (KGa-1b) obtained from the University of MissouriColumbia, Source Clay Minerals Repository, Columbia. Samples of both raw and pure kaolin were calcined at 770 K for 6 h. Amounts of 6 g of raw and pure kaolin were also shaken with 100 ml 1 M NaOH for 4 h, left overnight, filtered, and washed with water. The residue was dried at 350 K. Methylene blue (microscopic grade, Glaxo India, Mumbai) was used without any further purification.
2.2. Adsorption studies Adsorption of methylene blue was carried out in a batch process by varying kaolin dose, adsorptive concentration, pH of medium and temperature. A weighed sample of kaolin was mixed with 25 ml methylene blue solution of known concentration. The mixture, in a 100-ml conical flask, was shaken in the water bath of a thermostat at a particular temperature for 3 h. In all cases, adsorption equilibrium was reached within 2 h. If necessary, the pH was adjusted by adding a few drops of dilute NaOH or HCl before shaking. The mixture was allowed to settle and was centrifuged. The methylene blue concentration in the supernatant was determined with a spectrophotometer (Hitachi model 3210). 2.3. Adsorption isotherms The adsorption data from the experiments were fitted with: ðaÞ Freundlich Isotherm : x=m ¼ KCen
ð1Þ
where x/m is the amount of methylene blue adsorbed on the unit mass of the adsorbent, Ce is the equilibrium adsorbate concentration in aqueous phase, n and K are the Freundlich’s constants. ðbÞ Langmuir Isotherm : Ce =ðx=mÞ ¼ ð1=abÞ þ ð1=bÞCe
ð2Þ
where a and b are the Langmuir constants. The constants were obtained from the plots of the linearized equations. Another factor, RL, which is considered as a more reliable indicator of adsorption (Vermeulan et al., 1966; McConvey and McKay, 1985) was computed from: RL ¼ 1=ð1 þ aCÞ
ð3Þ
where a is Langmuir’s constant and C is any adsorbate concentration at which the adsorption is carried out. Favourable adsorption is indicated by 0 < RL < 1. 2.4. Thermodynamic parameters The thermodynamic parameters of the adsorption process are obtained from experiments at various
D. Ghosh, K.G. Bhattacharyya / Applied Clay Science 20 (2002) 295–300
297
The cation exchange capacity (CEC) of the kaolins was estimated by the conventional BaCl2 method and was found to be 2.62 (Cl), 2.85 (C2), 1.94 (C3), 3.08 (C4), 2.17 (C5) and 2.42 (C6) meq/100 g. The pure kaolin and its calcined sample have higher CEC compared to the other samples. Calcination improved the CEC of the pure kaolin while that of the raw kaolin decreased. The treatment with NaOH decreased the CEC of the raw and pure kaolin. 3.1. Effect of adsorbent and adsorptive doses
Fig. 1. Adsorption of methylene blue as a function of the initial methylene blue concentration. Contact time 3 h, kaolin dose 0.8 g/l, temperature 300 K.
The adsorption of methylene blue at a fixed kaolin dose of 0.8 g/l at 300 K is shown in Fig. 1. The sample C6 showed the maximum adsorption. The adsorption decreased in the order C 6 > C5 > C2 > C1 > C4 > C3. The raw kaolin (C1) showed a considerable adsorption, which was reduced after calcination. A similar influence was evident for the pure kaolin. Treatment with NaOH increased the adsorption. The NaOH-treated pure kaolin (C6) adsorbed
temperatures using the following equations (Khan et al., 1995): ðaÞ logKd ¼ DS 0 =ð2:303RÞ DH 0 =ð2:303RT Þ
ð4Þ
ðbÞ DG0 ¼ DH 0 T DS 0
ð5Þ
ðcÞ Kd ¼ ðx=mÞ=ðy=uÞ
ð6Þ
where Kd is the distribution coefficient for the adsorptive and is equal to the ratio of the amount adsorbed (x/m in mg/g) to the adsorptive concentration ( y/u in mg/dm3). The values of DH 0 and DS 0 were determined from the slope and intercept of the linear plot of log Kd vs. 1/T.
3. Results and discussion In all adsorption experiments, six different adsorbents were used: Cl—raw kaolin, C2—pure kaolin, C3—calcined raw kaolin, C4—calcined pure kaolin, C5—NaOH-treated raw kaolin, and C6—NaOH-treated pure kaolin.
Fig. 2. Variation of the methylene blue adsorption with pH. Contact time 3 h, initial methylene blue concentration 15 mg/l, kaolin dose 0.8 g/l.
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3.2. Effect of pH The methylene blue adsorption showed a minimum around pH 4 (Fig. 2). The increased adsorption at basic conditions may be related to preference of the dye cations for basic sites. In aqueous medium, the exchangeable alkali and other metal cations on the surface and in the interlayer region of the clay undergo hydration creating a hydrophilic environment (Lawrence et al., 1998). Increasing the pH of the adsorbing medium modifies the clay mineral surface and this might be responsible for the gradually increasing uptake of methylene blue at pH values above 4.0. Raymahashay (1987) in his comparative study of methylene blue adsorption on kaolin and some other adsorbents also found that the uptake of the dye increased with increasing pH. This is normally attributed to enhanced association of the dye cations, produced by the dissociation of methylene blue, with the negatively charged kaolin sur-
Fig. 3. Freundlich plots for the adsorption of methylene blue at room temperature.
nearly 100% methylene blue from a 12-ppm solution. The amount of the dye adsorbed showed a continuous decrease with an increase in the doses of kaolin for all the six adsorbents. This is because the equilibrium concentration of the dye decreased with increasing amount of kaolin such that amount adsorbed diminished. The above results indicate that the raw kaolin showed a substantial methylene blue adsorption. Higher amounts were adsorbed by the pure kaolin with a further enhancement in adsorption if this kaolin was treated with NaOH. The purification of the raw kaolin by treating with hydrogen peroxide solution (for removing organic matter) must have resulted in removal of non-clay matter and in generation of some basic or anionic sites for binding methylene blue cations. The treatment with NaOH creates additional which improve the adsorption. Calcination of the samples apparently removed some active sites.
Fig. 4. Langmuir plots for the adsorption of methylene blue at room temperature.
D. Ghosh, K.G. Bhattacharyya / Applied Clay Science 20 (2002) 295–300
299
Table 1 Adsorption constants for methylene blue adsorption on kaolins at room temperature Adsorbents
C1 C2 C3 C4 C5 C6
Freundlich constants
Langmuir constants
n
K (dm3 g 1)
a (dm3 g 1)
b (mg g 1)
RL
0.151 0.070 0.075 0.061 0.047 0.098
14.85 16.39 6.57 9.01 17.04 23.05
27.49 91.87 13.44 56.31 204.00 122.01
13.99 15.55 7.59 8.88 16.34 20.49
0.0021 0.0006 0.0042 0.0010 0.0028 0.0005
face at higher and higher pH (Singh and Srivastava, 1999). However, the tendency for increasing adsorption at pH values below 4.0 is difficult to explain, but a similar result was earlier observed by workers for adsorption of some other dyes on kaolin (Ganjidoust et al., 1995). 3.3. Adsorption isotherms Methylene blue adsorption on the kaolins can be described by the Freundlich and also Langmuir equations (Figs. 3 and 4, Table 1). The Freundlich exponent n between 0.047 and 0.151 indicates favourable
adsorption for which 0 < n < 1. The Freundlich constant K is appreciable for all the six adsorbents in general agreement with strong adsorption. The NaOHtreated pure kaolin has the largest value of K followed by the NaOH-treated raw kaolin. The calcined sample has the lowest K-value. The values of Langmuir constant b is in the order of C6 > C5 > C2 > C1 > C4 > C3 which is highest for the NaOH-treated pure kaolin and lowest for the calcined sample. The values of the Langmuir constant a also reflect an almost similar trend. The RL-values between 0.0005 and 0.0042 (Table 1) show favourable adsorption of methylene blue on kaolin.
Fig. 5. Log Kd vs. 1/T for six different doses of raw kaolin.
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Table 2 Thermodynamic parameters for adsorption of methylene blue on kaolins Adsorbent
DH 0 (kJ mol 1)
DS 0 (J mol 1 K 1)
DG 0 (kJ mol 1)
C1 C2 C3 C4 C5 C6
9.40 7.54 13.53 11.84 7.92 6.03
78.39 73.53 88.16 84.32 74.99 69.69
14.94 15.29 13.85 14.34 15.37 15.61
4. Conclusion The adsorption experiments reveal that kaolinite clay may be quite effective in removing a basic dye like methylene blue in relatively low concentrations from the aqueous medium. Although the experiments have been done with the raw kaolin and five modified forms, the raw kaolin itself has a relatively large adsorption capacity.
Acknowledgements 3.4. Thermodynamic parameters A typical log Kd vs. 1/T plot is shown in Fig. 5. The values demonstrate a spontaneous and favourable adsorption process. The standard enthalpy change (DH0) for the adsorption process is positive indicating that the process is endothermic in nature. The endothermic nature of adsorption of methylene blue was earlier observed by Mall and Upadhyay (1995,1998) with boiler bottom ash and fly ash as the adsorbents, and also by De and Basu (1999) with a sawdust-based adsorbent. Singh and Srivastava (1999) obtained positive DH and DS, and negative DG values for adsorption of methylene blue and a few other basic dyes on plant leaves in agreement with the present work. The values of standard entropy change (DS0) are not very large and indicate an increase due to adsorption. Normally, adsorption of gases leads to a decrease in entropy due to orderly arrangement of the gas molecules on a solid surface. However, the same may not be true for the complicated system of adsorption from solution on a non-uniform kaolin surface. Entropy increase was also observed by other workers as is pointed out above. The negative values of standard Gibbs energy change (DG0) in all the cases are indicative of the spontaneous nature of the interaction without requiring large activation energies of adsorption. Each of the thermodynamic parameters is in a narrow range of values for all the six clay adsorbents and therefore, it may be concluded that the thermodynamic processes involved in the clay –methylene blue interaction for all the adsorbents are more or less uniform in nature (Table 2).
The authors are grateful to the Assam Science, Technology and Environment Council (ASTEC) for providing a financial grant for this work.
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