Adsorption of model textile dyes from aqueous ...

Int. J. Environment and Pollution, Vol. 34, Nos. 1/2/3/4, 2008

Adsorption of model textile dyes from aqueous solutions using agricultural wastes as adsorbents: equilibrium, kinetics and fixed bed column study Sampa Chakrabarti* and Basab Chaudhuri Department of Chemical Engineering, University of Calcutta, 92, Acharya P.C. Road, Kolkata 700 009, India E-mail: [email protected] E-mail: [email protected] *Corresponding author

Binay K. Dutta Department of Chemical Engineering, University of Technology Petronas, Malaysia, E-mail: [email protected] Abstract: Gram husk and groundnut shell are cheap and abundantly available agricultural waste materials in India. A systematic study on the adsorption of some model dyes from aqueous solution on these low-cost adsorbents has been attempted. Model dyes used were Methylene Blue, Rhodamine B, Congo Red, Eosine Y and Metanil Yellow. Equilibrium, kinetics and column experiments were performed. Effects of different process variables have been studied. Equilibrium data fitted well in Langmuir and Freundlich isotherm equations. A model using Freundlich equation has been developed for interpretation of kinetic data. Other models such as Lagergren equation and pseudo second order equation were also used. Of these models, pseudo second order equation was found to be the most satisfactory. Column experiments were carried out with gram-husk and Rhodamine B. The data could be correlated well with BDST model. Keywords: gram husk; groundnut shell; Lagergren; pseudo second order kinetics; BDST model. Reference to this paper should be made as follows: Chakrabarti, S., Chaudhuri, B. and Dutta, B.K. (2008) ‘Adsorption of model textile dyes from aqueous solutions using agricultural wastes as adsorbents: equilibrium, kinetics and fixed bed column study’, Int. J. Environment and Pollution, Vol. 34, Nos. 1/2/3/4, pp.261–274. Biographical notes: Sampa Chakrabarti received her BTech (1987), MTech (1989) and PhD (2006) in Chemical Engineering from the University of Calcutta. She has about 11 years of experience in reputed consultancy organisations and about eight years of teaching and research experience. She is presently a Reader in the Department of Chemical Engineering, University of Calcutta. She teaches heat and mass transfer and environmental engineering. Her research interest is in the area of environmental remediation. She has several publications in highly rated international journals. Basab Chaudhuri obtained his BTech (1984, University of Calcutta) and PhD (1989, University of Mumbai) in Chemical Engineering. Engaged in teaching Copyright © 2008 Inderscience Enterprises Ltd.

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S. Chakrabarti et al. and research since 1991, he is currently a Professor of Chemical Engineering and the Registrar of the University of Calcutta. He had worked in the University of Groningen, the Netherlands for two years. He was an INSA visiting scientist in the Chung – Buk National University, South Korea. He has published over thirty papers in noted journals. He has specialisation in separation process, catalysis and wastewater treatment. Binay K. Dutta (MTech., Calcutta University; PhD, IIT, Kharagpur) is a Professor in the Department of Chemical Engineering, Universiti Teknologi PETRONAS, Malaysia. He taught in the University of Calcutta during 1974–2005, was a Professor of Chemical Engineering during 1987–2006, and the Director, Academic Staff College, during 2000–2005. He was a visiting scientist at NIST, Boulder, Colorado, and at Stevens Institute of Technology, New Jersey; a Senior Associate of NRC at USEPA, Cincinnati, Ohio; and a Visiting Professor, University of Alberta, Canada. He authored two books (Heat Transfer – Principles and Applications, and Principles of Mass Transfer and Separation Processes) and published about 80 papers in reputed journals. He was the President of Indian Institute of Chemical Engineers in 2005.

1

Introduction

Wastewater from textile, paper, leather and some other industries contains different types of residual dyes. This wastewater must be treated before being discharged on to the surface because colours lower the aesthetic value of water and interfere with the photosynthesis of the aquatic plants. As the dye pollutants are not easily biodegradable, Adsorption and chemical coagulation are the two common techniques used for treatment of such effluents. Of these, adsorption is a more effective as well as a clean technique to remove dyes from wastewater. In most cases, the adsorbent is activated carbon. However, adsorption by active carbon is an expensive process, and regeneration is not always effective (McKay et al., 1987). In search of a low cost and commercially available adsorbent, agricultural wastes have been identified as suitable alternatives of activated carbon. Besides the low cost and easy availability, the spent material can be disposed of by burning as fuel. Studies have been reported on the use of hardwood sawdust (Asfour et al., 1985), maize-cob (El-Geundi, 1991), palm-fruit bunch particles (Nassar et al., 1999), bagasse pith (McKay et al., 1987), sunflower stalks (Sun and Xu, 1997) and sagaun sawdust (Khattri and Singh, 1999) among others as adsorbents for various textile dyes. Almost all of the above studies show that the adsorbents have higher affinity towards basic dyes compared to acid dyes. The isotherms generally follow Langmuir and Freundlich models. Lower adsorbent particle-size, lower initial dye concentration, higher speed of agitation and higher temperature favour rates of adsorption. Models have been developed to determine the effect of experimental parameters on mass transfer and on effective internal diffusion coefficients (McKay, 1984). Gram is a common pulse used in India, and groundnut is a source of edible oil. These are also produced in a large scale. Gram husk and groundnut shells are low density, high volume agricultural wastes, which are cheap and are easily available. The husk and the shells, in part, are used in animal feed formulation or burnt as fuel. In order to add value to these agricultural wastes, attention was focused on their utilisation as adsorbents.

Adsorption of model textile dyes from aqueous solutions

263

Though a report is available for the use of treated groundnut shell as adsorbent for toxic metals from aqueous solutions (Chamarthy et al., 2001), no studies have been reported for the similar use of gram husk. The objective of the present work is to evaluate the efficacy of gram husk and groundnut shell for the removal of selected textile dyes from aqueous solutions. Model dyes used with gram husk were Methylene Blue, Rhodamine B (basic dyes), Congo Red (direct dye) and Eosin Y (acid dye). Dyes used with groundnut shell were Rhodamine B (basic dye) and Metanil Yellow (acid dye). Effects of more important parameters like temperature, pH, addition of salt, loading of adsorbent and initial dye concentration on both equilibrium and rate of adsorption were studied. Column studies were undertaken for gram husk and Rhodamine B system. Bed Depth Service Time (BDST) model was examined to explore its applicability of interpreting column adsorption data. In this work, the adsorbents were used without any extensive chemical or thermal pretreatment that might have incurred extra cost on the process. Therefore, the results of this study may be useful for a cost-effective treatment of the wastewater from textile industries containing hazardous dyes.

2

Materials and methods

2.1 Adsorbents Groundnuts were procured locally and the shells were separated. The shells were then coarsely ground. Gram husk was procured from a local husking mill. These were washed repeatedly with warm distilled water till colourless wash water was obtained. The wet substances were dried in a hot air oven at a temperature of 70 to 80°C till a constant weight was obtained and then used as adsorbent. Characterisation of adsorbents: The adsorbent materials being in the form of thin flakes, it was not straightforward to characterise them with respect to particle size distribution, porosity or surface area by conventional techniques. The pore volume was determined by soaking the sample in water, removing the surface moisture and by weighing the moist solid. For gram husk, the value was determined as 1.35 ml g–1 whereas for groundnut shell, it was 1 ml g–1. Maximum available external surface area were obtained for both the adsorbents by measuring the area of whole-gram and whole-groundnut and then by weighing the husk/hull separated from the seed. The external surface area for gram husk was determined as 128 cm2 g–1 and that of groundnut shell was 62 cm2 g–1. Approximate particle size for gram husk was determined as 2.3 mm for 80 wt% and that for groundnut shell was 3 mm for 80 wt%.

2.2 Dyes The model dyes used were Methylene Blue (C.I.No. 52015), Rhodamine B (C.I.No. 45170), Congo Red (C.I.No. 22210), Metanil Yellow (C.I.No. 13065) and Eosin Y (C.I.No. 45380). All were suitable for microscopy and were from LOBA Chemie, India. Freshly prepared double-distilled water was used throughout, for preparation of solution.

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2.3 Analytical methods Dye samples were analysed for the concentration of a dye remaining in a solution using a Shimadzu UV-160A UV-visible spectrophotometer against standard calibration curve at proper dilution.

2.4 Experimental procedure For equilibrium studies, dye solutions of different concentrations were taken, in batches of 50 ml solutions, in 125 ml airtight Corning glass bottles. Accurately weighed adsorbents were added to each bottle and the contents were allowed to attain equilibrium at a constant temperature. After ensuring that equilibrium has been reached, the supernatant liquid was analysed spectrophotometrically using a 1 cm light path cell. In case of batch contact time study, separate sets of 50 ml dye solutions with weighed quantity of adsorbents in stopper bottles were shaken in a constant temperature shaker bath. Solutions were analysed from time to time for the remaining dye concentration; separate sets were used to minimise interruption and change in volume owing to withdrawal of samples. For column study, borosilicate-glass columns of 31.75 mm diameter and variable depths of the adsorbent-bed were used. Dye solutions were fed continuously from the bottom with a peristaltic pump. Samples were withdrawn from time to time and analysed by spectrophotometer as described before. All the experiments were replicated several times in identical conditions and the average coefficient of variation was within 3%.

3

Results and discussions

3.1 Isotherms Depending upon the functional groups and ionisation in aqueous medium, different dye molecules behaved differently towards the adsorbents. With both gram husk and groundnut shell adsorbents, adsorption capacities of the basic dyes were found to be much higher than those of direct or acid dyes. The negatively charged, cellulose based, adsorbents like groundnut shell and gram husk have a comparatively higher affinity for the basic/cationic dyes resulting in a higher extent of adsorption, but Metanil Yellow and Eosin Y with coloured anions have a lesser affinity for the adsorbents (McKay et al., 1987). Congo Red had an intermediate degree of affinity. The isotherms, in general, are smooth and concave to the concentration axis indicating monolayer formation. The adsorption processes are generally ‘favourable’. The equilibrium data were satisfactorily fitted by Langmuir (1918) and Freundlich (1906) isotherm equations, which can be represented as follows: qe =

q0 K L C e 1 + K L Ce

qe = K F Ce1 n

(1) (2)

Adsorption of model textile dyes from aqueous solutions q e: Ce: q0 : KL: KF: n:

265

dye adsorbed in mg g–1 of adsorbent at equilibrium concentration of the solution in mg l–1 at equilibrium maximum adsorption capacity (Langmuir) in mg g–1 of adsorbent Langmuir constant in l mg–1 Freundlich isotherm equation constant Freundlich index indicating non linearity of the system.

Temperature is an important parameter in case of adsorption. The increase in temperature of the system affects the solubility and the chemical potential of the dyes. Chemical potential is one of the controlling factors for adsorption. The effect of temperature on adsorption of all the dyes is less in almost all dye-adsorbent systems except for Methylene Blue-gram husk in the temperature range of 30–50°C and Congo Red-gram husk beyond 40°C. Table 1 shows the effect of temperature on the Langmuir and Freundlich constants of various dye-adsorbent systems. The decrease in the maximum adsorption capacity with increase in temperature indicates that physical adsorption process predominates within that particular temperature range. On the other hand, increase in percentage removal and maximum adsorption capacity with temperature indicates chemisorption, whereby more numbers of dye molecules acquire the necessary energy and get adsorbed (McKay et al., 1980, 1987; Asfour et al., 1985; Lin and Chen, 1997; Sankar et al., 1999; Hulscher and Cornellissen, 1996). The increase in adsorption also suggests that the number of active sites available for adsorption increased with increasing temperature (Mohan et al., 2002). An adsorption process involves both physical and chemical adsorption mechanisms. Chemisorption may also be of two types, activated and non-activated. It is likely that more than one of the above mechanisms are operating simultaneously for the adsorbate-adsorbent pairs in this study (Smith, 1970). In a particular temperature range, for a particular solid-solute pair, a particular mechanism predominates over the others. This predominance changes over with change in temperature and accordingly, the influence of temperature is manifested. This may macroscopically explain the variation of equilibrium adsorption characteristics with temperature. Table 1

Effect of temperature – Langmuir and Freundlich constants

Dye adsorbent system Methylene Blue on gram husk Rhodamine B on gram husk Congo Red on gram husk Rhodamine B on groundnut shell

Temperature o C 30 40 50 30 40 50 30 40 50 31 41 51

q0 mg. g–1 43.10 30.76 31.54 16.28 18.83 32.48 1.75 2.87 3.59 12.33 12.71 11.87

KL L.mg–1 0.3536 1.3728 0.8708 0.0237 0.0179 0.0088 0.0119 0.0072 0.0077 0.0262 0.0211 0.0347

R2 0.9934 0.9798 0.9872 0.9958 0.9872 0.9988 0.9880 0.9674 0.9588 0.9966 0.9929 0.9850

n 1.7349 2.2603 2.3046 1.3700 1.3118 1.2048 1.2479 1.1669 1.0224 1.8122 1.4407 1.5302

KF 9.9950 13.567 11.735 0.5487 0.4665 0.3669 0.0285 0.0267 0.0248 0.7635 0.4391 0.6404

R2 0.9701 0.9739 0.9769 0.9933 0.9854 0.9957 0.9642 0.9394 0.9714 0.9924 0.9788 0.9671

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Wastewater from textile industries often contains an appreciable amount of acid or alkali depending upon the process of dyeing resulting in varying pH, which is an important parameter in adsorption because it may alter the surface-charge properties of the adsorbent as well as of the dye molecules. For Methylene Blue, using 1% w/v gram husk adsorbent at 30°C, the percentage removal decreased for increase of pH from 5.8 to 8.5. At lower pH, positively charged coloured cations were attracted easily to the negatively charged surface. In case of Rhodamine B using 0.4% w/v gram husk at 35°C, decrease in percentage removal was rather low for the same change of the pH values. The influence of change in pH on the adsorption of Rhodamine B on gram husk is little. Variation of pH from 6.5 to 8.4 had a very little effect also on the adsorption-isotherm of Rhodamine B on groundnut shell. For Metanil Yellow, percentage removal of dye as well as maximum adsorption capacity decreased as the pH increased from 6.8 to 8.2, using 1% w/v adsorbent and 31°C temperature. Metanil Yellow, being an acid dye with coloured anions, was repelled by the negatively charged surface at alkaline pH. In an acidic pH, the negative charge density on the surface was reduced and the coloured anions were better adsorbed (Singh and Srivastava, 2001). Wastewater from textile industries frequently contains a large amount of salt. For example, Glauber’s salt is often used in the dye bath for dyeing cotton with direct dyes. It was found that presence of 500 mg l–1 of sodium sulphate increased the adsorption of Congo Red dye from 22.2 to 37.7% (Figure 1). The adsorption capacity increased from 0.078 mg g–1 to 0.4586 mg g–1 because of the presence of salt. Congo Red was an anionic dye and gram husk became negatively charged in aqueous medium. As expected, the Coulombic repulsion inhibited adsorption. Perhaps the presence of Na2SO4 neutralised the surface negative charge of the adsorbent with Na+ ions, thus facilitating the approach of the dye anions onto the surface of the adsorbent. Figure 1

Effect of salt addition on the adsorption isotherm of Congo Red on gram husk

Adsorbent loading: 1% w/v, Temperature: 35°C, Salt concentration: 50 mg. L–1 Na2SO4.


267

3.2 Adsorption kinetics In order to find the rate of uptake of the adsorbates from solution onto the adsorbents, kinetic experiments were performed. Ct/Ci against time was plotted for all experiments to study the time-concentration profiles. We have formulated following model to determine the rate parameters: The driving force for adsorption at any time t is basically the difference between the bulk concentration and the equilibrium concentration at that instant. On this basis, the rate equation can be written as follows: dCt ′ (Ct − Ce ) = −kad dt

(3)

′ is the adsorption rate parameter in h–1. The equilibrium concentration, Ce, can be kad eliminated by using equation (2) and after integrating, it becomes

∫

Ci − Ct

0

t dCt ′ ∫ dt = −kad 0 [Ct − A(Ci − Ct ) n ]

 V  A=   mK F 

m: V: Ct: Ci:

(4)

n

mass of adsorbent taken in g volume of solution taken in m3 dye solution concentration at any time t in mg l–1 Initial dye solution concentration in mg l–1.

The LHS of equation (4), designated as ‘Function’ in Figure 2, was numerically ′ as the integrated and plotted against t. The straight line passing through origin had kad slope. For the adsorption of dyes on gram husk, the kinetic data were fitted into equation (4) and hence, the rate constants were determined. Rate constants, thus obtained are given in Table 2. Figure 2 shows a sample of the plots made. The fitting was pretty good but the curves do not exactly pass through the origin. This intercept may be attributed to the initial wetting of the adsorbent. Kinetic data for groundnut shell-dye system, however, could not be fitted well in this model. The well-known Lagergren equation (Lagergren, 1898), given by equation (5) is an empirical equation in which the first order adsorption rate constant lumps together the liquid phase mass transfer and solid phase diffusion phenomena. It has been widely used for its simplicity and applicability to many systems (Ho, 2004). log(qe − qt ) = log qe −

k t 2.303

qe: dye adsorbed in mg.g–1 of groundnut shell at equilibrium qt: dye adsorbed in mg.g–1 of groundnut shell at any time t k: Lagergren adsorption rate parameter in h–1 (lumped kinetic parameter).

(5)

268 Figure 2

S. Chakrabarti et al. Determination of the rate constant for adsorption of Rhodamine B on gram husk at various temperatures using equation (4)

Adsorbent loading: 1% w/v, pH: 6.5, Initial dye concentration: 50 mg.l-1 (approx.). Table 2

Effect of temperature and initial concentration on rate constant

Dye adsorbent system

Parameter

Methylene Blue on gram husk Rhodamine B on gram husk

Temperature (°C)

Methylene Blue on gram husk Rhodamine B on gram husk

Initial concentration mg.l–1 (approx.)

Values

′ h–1 kad

R2

k2 g.mg–1.h–1

R2

30

1.932

0.9949

0.0274

0.9973

40

1.692

0.9853

0.0269

0.9923

50

1.596

0.9670

0.0622

35

0.114

0.9691

0.9927 –3

0.9977

–3

0.9990

–3

1.97 × 10

45

0.168

0.9797

55

0.216

0.9859

3.73 × 10

0.9994

200

3.738

0.9946

0.2074

0.9978

400

2.298

0.9915

0.0590

0.9989

500

1.932

0.9949

0.0274

0.9973

25

0.1260

0.9649

50

0.1140

0.9550

100

0.0840

0.9585

2.16 × 10

3.33×10

–3

0.9961

1.97×10

–3

0.9977

1.10×10

–3

0.9957

Adsorbent loading is different for different dyes.

k was obtained from the slope of the straight line when log(qe – qt) was plotted against t. The kinetic data for the adsorption of dyes on groundnut shell were interpreted using Lagergren equation and the rate constants were determined. The rate parameters are given in Tables 3 and 4. It has been observed that values of rate parameter obtained from Lagergren equation vary in a non-linear manner with increase in values of certain parameters such as adsorbent loading and initial dye concentration. Any kinetic or mass-transfer representation is likely to be global, a lumped analysis of kinetic data is therefore sufficient for practical purposes.


269

While studying adsorption of divalent metal ions onto sphagnum moss peat, Ho and McKay (1999) proposed a pseudo-second order kinetic equation based on the equilibrium adsorption as follows: dqt = k2 (qe − qt ) 2 dt

(6)

where k2 is the pseudo-second order rate parameter in g.mg–1 h–1. Integrating equation (6) and applying initial conditions, we get: t 1 1 = + t qt k2 qe2 qe

(7)

k2 was obtained from the plot of t/qt against t. qe is a function of initial dye concentration, adsorbent loading and nature of solute. Tables 2, 3 and 4 show the values. The experimental data could be fitted better with the above equation as indicated by higher correlation coefficient (>0.98). Furthermore, the values have consistent trends of variation with the change in relevant process parameters. The assumption behind the pseudo second order kinetic model was that the rate-limiting step might be chemisorption involving valency forces through sharing or exchange of electrons between adsorbent and adsorbate (Ho and McKay, 2000). As the correlation coefficients are very high, it may also be assumed that adsorption of dye occurs through chemisorption where dye-molecules or ions act as bidentate ligands, so that pseudo second order scheme holds good. Ho and McKay (1999, 2000) suggested a similar mechanism for the adsorption of divalent metal ion on peat. In this case also, we assumed that the adsorption process involved valency forces between adsorbate and adsorbent. Table 3

Effect of initial dye concentration on rate constants

Dye-adsorbent system Rhodamine B-groundnut shell Metanil Yellow-groundnut shell

Initial conc. mg.l–1 25

k h–1 0.3730

R2 0.9326

k2 g.mg–1.h–1 0.3472

R2 0.9864

50 100 25 50 100

0.3454 0.3592 0.6356 0.7323 0.4145

0.9602 0.9617 0.9814 0.9814 0.9752

0.2057 0.1303 17.79 × 10–3 15.82 × 10–3 6.975 × 10–3

0.9921 0.9928 0.9907 0.9835 0.9750

Adsorbent loading is different for different dyes. Table 4

Effect of adsorbent loading on rate constants

Dye-adsorbent system Rhodamine B-groundnut shell Metanil Yellow-groundnut shell

Adsorbent loading % w/v 0.4

k h–1 0.8152

R2 0.9262

k2 g.mg–1.h–1 2.218 × 10

R2 0.9942

1.0 2.0 1.0 2.0 3.0

0.3454 0.5389 0.5389 0.7322 0.7047

0.9602 0.9305 0.9710 0.9814 0.9900

3.49 × 10–3 14.56 × 10–3 2.73 × 10–3 15.82 × 10–3 66.27 × 10–3

0.9921 0.9991 0.9956 0.9835 0.9964

–3

270


As expected from the equilibrium study, the rates of uptake of various dyes were different depending on their structure and ionisation. With gram husk, the rates of adsorption were in the decreasing order as Methylene Blue, Rhodamine B, Congo Red and Eosin Y under identical conditions. Similarly, with groundnut shell, under the same experimental conditions, the rate of adsorption of Rhodamine B was much more than that of Metanil Yellow. The difference in the rate of adsorption may be attributed to the difference in the Coulombic force of attraction or repulsion between the different dyes and the adsorbent surface. Percent removal of dye from aqueous solution was expected to increase with increasing loading of the adsorbent under otherwise uniform conditions as an increase in adsorbent loading generally means availability of more adsorption sites. Rate of dye adsorption as well as percent removal increased with increase in adsorbent loading for all the dye-adsorbent systems. Rate constant values of groundnut shell adsorbent are given in Table 4. The rate of adsorption for all the dyes on gram husk adsorbent increased marginally with increase in temperature, in the range from 30 to 55°C. The results are tabulated in Table 2. This increase in rate constant is suggestive of chemisorption. The effect of temperature on the rate of adsorption of dyes on groundnut shell is almost insignificant within the temperature range of 35°C to 55°C. For adsorption of Metanil Yellow on groundnut shell, with 2% w/v adsorbent loading, the rate constant increased in the range of 35°C to 45°C, and then slightly decreased in the range of 45°C to 55°C. This is suggestive of chemisorption in the former range of temperature and physical adsorption in the later. A fixed amount of adsorbent can remove only a fixed amount of dye from solution. If the initial dye concentration of the solution is more, less will be the volume of solution that can be treated with a particular amount of adsorbent. Initial dye concentration was varied from 200 mg l–1 to 500 mg l–1 for Methylene Blue to study its effect. For other dyes, the initial concentration was varied from 25 mg l–1 to 100 mg l–1. The rate constants are given in Table 2 and 3. In case of adsorption of Rhodamine B on groundnut shell at various concentrations, Figure 3 indicates the related qt vs. t plot. For all the dye-adsorbent systems, other experimental conditions remaining the same, values of rate constants decreased as the initial dye concentration increased. The decrease in rate constant might be owing to the crowding of molecules near adsorption sites, resulting in a distorted orientation of a particular molecule to be adsorbed on the surface. Such a phenomenon makes the adsorption process difficult. Like isotherm, adsorption kinetics is also expected to be influenced by the change of pH of the solution. During adsorption on gram husk, rate constant of Methylene Blue decreased a little as the pH increased from 5.8 to 7.8, other experimental conditions remaining the same at 30°C, 0.6% w/v gram husk and 500 mg l–1 initial concentration. For adsorption of Rhodamine B on gram husk, the rate constant using equation (7) increased from 1.97 × 10–3 h–1 to 18.48 × 10–3 h–1 as the pH increased from 6.4 to 10.9 with 1% w/v gram husk and 50 mg.l–1 initial solution concentration. At higher pH, the surface of the cellulose-based adsorbent was negatively charged more easily and the coloured cation of the basic dye was better attracted. This is indicative of an ion-exchange process accompanying chemisorption.

Adsorption of model textile dyes from aqueous solutions Figure 3

271

Plot of amount adsorbed vs. time for adsorption of Rhodamine B on groundnut shell at various initial dye concentrations

Adsorbent loading: 1% w/v; pH: 6.5; Temperature: 35°C.

For groundnut shell as adsorbent, percent removal after six hours decreased with increase in pH from 6.4 to 8.5 for both Rhodamine B and Metanil Yellow. This observation agrees with the results obtained in case of isotherm study reported before. Presence of salts may also affect the kinetic adsorption process when dyes are removed from wastewater by adsorption. In presence of 500 mg.l–1 Na2SO4 salt, the rate constant for adsorption of Congo Red on gram husk increased from 0.018 h–1 to 0.114 h–1 with 2% w/v adsorbent and 50 mg.l–1 initial solution at 35°C. Addition of Na2SO4 might have enabled the dye anions to approach the surface, by compressing the electrical double layer on the surface by its Na+ ion.

3.3 Column studies The objective of the fixed bed operations is to reduce the concentration of solute in the fluid phase so that it does not exceed a predetermined breakthrough value (Cb). The original work on the BDST (Bed Depth Service Time) model was carried out by Bohart and Adams in 1920 (Ko et al., 2002). They proposed a relationship between bed depth Z in cm and the time taken tb in minutes for the breakthrough to occur. tb and Z are correlated with process parameters and initial solute concentration, solution-flow-rate and adsorption capacity. The BDST equation thus is as follows: tb = mxZ – Cx with slope mx = Nt/Civ in minutes.cm–1 and intercept Cx in minutes. Cx = −

Nt: Ci: k: v:

C  1 ln  i − 1 kCi  Cb 

volumetric adsorption capacity of bed in mg.l–1 initial solute concentration in mg.l–1 kinetic rate parameter in l.mg–1.hr–1 linear velocity in cm.sec–1.

(8)

272


The results of column experiments with Rhodamine B and gram husk are given Figure 4. Half-breakthrough time was found to be linearly proportional to the bed-depths; BDST model is applicable to the system, therefore. Figure 4

BDST plot for Rhodamine B on gram husk

Flow rate: 25 ml.minute–1; pH: 7.5.

Half-breakthrough time in minutes, tb/2 decreased from 6.52 to 2.79 as the solution flow rate increased from 25 ml min–1 to 40.4 ml min–1 with constant adsorbent bed depth. The decrease is because of the decrease in contact time between the dye and the adsorbent for increase in solution flow rate. It was observed that with increase in bed depth from 5.80 cm to 15.4 cm, half-breakthrough time in minutes, tb/2 increased from 1.33 to 6.52. This is because the time of contact between the adsorbent and the adsorbate increased with increase in the bed depth. Figure 4 is the BDST plot with different bed-depths and Table 5 tabulates the results. Table 5

Effect of bed depths in fixed bed adsorption column – Rhodamine B on gram husk Run I

II

III

IV

15.40

13.10

10.85

5.80

19.2892

16.6895

14.2

7.5398

25

25

25

25

Parameter Bed depth (cm) Weight of adsorbent (g) Flow rates. (ml min–1) pH

7.84

7.6

7.7

7.9

Half-breakthrough time (minutes)

6.52

4.32

3.73

1.33

The BDST equation for Rhodamine B on gram husk therefore becomes: tb/2 = 0.5090Z – 1.7706

(9) 2

With a regression coefficient (R ) value of 0.9593.


4

273

Conclusions

Based on the above studies, it may be concluded that the agricultural wastes, viz. gram husk and groundnut shell are reasonably effective to remove various textile dyes from aqueous solutions by adsorption. The isotherms followed Langmuir and Freundlich equations. The adsorption systems are ‘favourable’. The adsorbents have larger affinity towards the basic dyes as compared to the acidic dyes. The kinetic data for adsorption of dyes onto gram husk were interpreted using a kinetic model based on Freundlich equation as well as by using a pseudo second order equation. The rate parameter for the adsorption of dyes on groundnut shell was determined by classical Lagergren equation and pseudo second order kinetic model. Of these, pseudo second order model predicted the experimental results most satisfactorily. Effect of temperature on the equilibrium and rate of adsorption has been explained by change over of mechanisms predominating over different temperature ranges. Adsorption of Methylene Blue and Rhodamine B on gram husk was found to be independent of pH. Adsorption of Congo Red, a direct dye, on gram husk was facilitated in presence of 500 mg/l sodium sulphate solution. The value of the adsorption rate parameters was higher at lower initial dye concentration for all dye adsorbent systems. For packed bed adsorption column using Rhodamine B and gram husk, BDST model was found to hold good.

Acknowledgement Financial grant from All India Council of Technical Education (Grant No. 8018/RDII/BOR (157)/99-2000 dated 24/03/2000) is gratefully acknowledged. The authors are thankful to Professor S. Bhattacharjee for some useful discussions. SC thanks Development Consultants Pvt. Ltd. for the study leave granted to her.

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