The Kinetic and Equilibrium Studies on Adsorption of Rhodamine-B Dye From Aqueous Solution onto Rice Husk Carbon

 

Sarita Yadav1 , D.K. Tyagi1 and O.P. Yadav2*

1Department of Chemistry, D.A.V.P.G. College, Dehradun- 248001, Uttrakhand, India.

2Chemistry Department, CCS Haryana Agricultural University, Hisar-125001, India.

*Corresponding Author E-mail: opyadav02@rediffmail.com

 

ABSTRACT:

Kinetic and equilibrium studies for the adsorption of Rhodamine-B dye from aqueous solution onto activated rice husk carbon have been reported. The effect of parameters such as: contact time, dye initial concentration, temperature, adsorbent’s amount and particle size on the color removing efficiency of the activated rice husk carbon has been investigated. Kinetics of adsorption of dye at adsorbent in aqueous solution was monitored spectrophotometrically. At the given initial dye concentration, its percent adsorption increases with increase in temperature as well as the adsorbate-adsorbent contact period and the equilibrium was established after 120 minutes. However, at the given temperature, % adsorption of the dye decreases with the increase in its initial concentration. The curves representing adsorption isotherms are single and continuous leading to saturation, suggesting monolayer coverage of the dye on the adsorbent surface. The observed adsorption data was analysed in the light of Langmuir and Freundlich adsorption isotherms. Dye adsorption rate constant (kad) follows the first order kinetics. Activation energy (Ea), intra-particle diffusion rate constant and thermodynamic quantities of dye adsorption on the rice-husk carbon have been evaluated and interpreted.

 

KEYWORDS: equilibrium, Rhodamine-B, adsorption, activation energy, thermodynamic

 

 


1. INTRODUCTION:

Dye manufacturing, distilleries, textile, paper and pulp mills, tanneries, electroplating and food processing industries discharge colored wastewater into the environment. The colored effluents after mixing with surface and ground water system, also contaminate the drinking water. Color is a visible pollutant and the water contaminated with colors is neither suitable for drinking purpose nor it is fit for agricultural use because it inhibits photosynthetic process in plants. Dyes are carcinogenic, skin irritant and these can cause allergic dermatitis and mutation1. The usual methods adopted for the removal of color from industrial effluents include: coagulation by chemicals, biological treatment under aerobic or anaerobic conditions, photocatalytic degradation and adsorption2-5. The conventional method for the removal of dyes from water by using chemicals such as: Ferric chloride, alum, lime etc. is not cost effective.

 

Anaerobaic/aerobic biological treatment, generally used for the reduction of BOD and COD of polluted water, is also not an effective method for color removal. Rao and Viraraghavan6 reported that brown color present in distillery wastewater is further intensified during its anerobic/aerobic treatment. Further, photo-catalytic degradation of colored organic pollutants in water has its limitation because of low efficiency of semiconductor material used as a photocatalyst7.

 

Adsorption method may be an efficient, economic and environment friendly technique with considerable potential for the color removal from the contaminated water8-11. Adsorbents that have earlier been used for dye removal from contaminated water include: fly ash12-15 wood powder16-17, hull carbon18 coconut husk (coir pith)19 clay minerals20-21 (Nigam et al 2000; Apak et al 2001) lignin22 , peat23, jackfruit peal24, and bark25.  For the removal of dyes and biologically resistant organic pollutants, activated carbon is a widely used adsorbent5,26.

 

For designing the adsorption treatment systems, knowledge of kinetic and mass transfer processes is essential. Accumulation of rice husk solid in and around rice processing industries is a big problem for its disposal. Using rice husk for carbon synthesis may provide a solution to solid waste management, besides it will provide an adsorbent for the effective removal of water pollutants such as organic dyes. Rhodamine-B is widely used in several textile processing and paper industries and its presence in the industrial discharge water is a cause of environmental pollution. The adsorption and thermodynamic investigations of dyes in aqueous system may provide valuable information regarding the nature of intermolecular interaction involved at solid-liquid interface. The present work reports the kinetic and equilibrium studies on adsorption of Rhodamine-B from aqueous solution onto activated rice husk carbon. The effect of parameters such as: contact time, dye initial concentration, particle size, amount of adsorbent and temperature on the efficiency of the adsorbent for dye removal from aqueous solution have been investigated.

 

2. MATERIALS AND METHODS:

2.1. Materials:

Rice Husk was washed with distilled water and dried at 600C for 24 hours. The dried material was mixed with equal volume of conc. H2SO4 and kept, overnight, at room temperature. Excess of the acid was removed by washing the solid residue with distilled water and then dried the product at room temperature. The carbon thus prepared was kept in hot air oven at 120ºC for 10 hours and then  transferred to a muffle furnace kept at 500ºC for an hour. The activated carbon thus obtained was ground to yield a fine powder and  fractionated into different mesh sizes. The carbon thus prepared was analyzed by physico-chemical methods. The characteristic parameters of the rice husk carbon are presented in Table-1.

 

Rhodamine-B (C28H31ClN2O3) (MERCK) (molecular mass: 479.02 )- an amphoteric dye, contains in its molecule two –NH2 and one –COOH functional groups. It shows maximum absorbance at wavelength 543 nm. The molecular structure of Rhodamine-B is given in Fig. 1. Sulphuric acid (A.R. grade) was from SD fine chemicals. Doubly distilled water was used for preparing various solutions of dyes.

 

Fig.1. Molecular structure of Rhodamine-B.

 

2.2. Methods:

2.2.1 Characterization of Rice Husk Carbon:

2.2.1.1. XRD Spectra

XRD spectra of activated rice husk carbon was obtained at SAIF Panjab University Chandigarh using X-Ray diffractometer (Model: XRDML) (X-Ray wave length:  1.54060 ºA., 2θ range: 5-7º., step size:  2θ=0.0170º., step time: 30.368 seconds.)

 

2.2.1.2 TEM Analysis

Transmission Electron Microscopic (TEM) analysis of activated rice husk carbon (adsorbent) was carried out at SAIF Punjab University, Chandigarh, India. TEM picture was obtained in imaging mode using HV=80 KV and at magnification 3x105.

 

2.2.1.2. Specific Surface Area of   Carbon

Surface area per gm of the activated rice husk carbon was obtained using Sears method (Sear 1956; Mekhamer 2010). 1.5 g of carbon sample was mixed with 100 ml of water and 30 g NaCl. The mixture was stirred for five minutes. To this was added 0.1 N HCl to make final volume 150 ml and pH = 4.0. It was then titrated against 0.1N NaOH. The volume (V ml) of 0.1N NaOH required to raise the pH from 4.0 to 9.0 was noted. The specific area (i.e. area per gm) was obtained using the formula:

 

               A = 32V-25    -----------------------(2.2.1)

 

Where, A = Surface area of carbon per gm ( in m2/gm); V =  volume  of 0.1N NaOH required to raise the pH from 4.0 to 9.0.

 

2.2.3 Kinetics of Adsorption study:

A specified amount (0.5 or 1.0 g) of the adsorbent (rice husk carbon) was agitated with 100 ml of  dye solution of desired initial concentration in a  conical flask.  The flask was kept in a water-thermostat maintained at a specified temperature. Concentration of dye solution, free of suspended carbon, was measured at a regular interval of time using a UV-Visible digital spectrophotometer ( SIC Model 301).

 

3. RESULTS AND DISCUSSION:

3.1. Characterization of Rice Husk Carbon:

3.1.1. XRD Spectra

XRD spectra of activated rice husk carbon ( Fig. 3.1 ) show a single hump ranging from 2θ = 160 to 290 indicating amorphous disordered structure. It indicates that the X-Ray diffraction is mainly due to amorphous silica particles. As the atomic scattering factor of Carbon is very small, intervening carbon atoms seems to contribute little with respect to scattering from silica.

 

Fig. 3.1. XRD spectra of activated rice husk carbon

 

3.1.2. TEM Analysis

Transmission Electron Microscopic (TEM) analysis of activated rice husk carbon adsorbent is presented in Fig. 3.2. The average size of carbon particles is 45 nm.

 

Fig. 3.2  Transmission Electron Microscopic (TEM) image of activated  rice husk carbon adsorbent

 

3.1.3. Specific Surface Area of rice Husk carbon

The specific surface area of the activated rice husk carbon was obtained by the method described in the experimental part (Section: 2.2.1.2.). The volume of 0.1N NaOH required to raise the pH of the aqueous carbon suspension from pH = 4 to 9 was found to be 24.0 ml. Therefore, specific surface area calculated using equation (2.2.1) was found as: 518 m2/g

 

3.2. Effects of contact time, dye initial concentration and Temperature on Dye adsorption:

Values of % dye adsorbed as a function of time and temperature, using 10 mg/dm3 initial Rhodamine-B dye  concentration are given in Table-2. Percent adsorption values of the dye as a function of initial dye concentration are presented in Figures 2-4.  It is evident that at the specified initial dye concentration, its % adsorption  increases with increase in temperature as well as with the increase in the adsorbate-adsorbent contact period and the equilibrium was established after 120 minutes. Further, the curves are single and continuous leading to saturation, suggesting monolayer coverage of the dye on the adsorbent surface.

 

At the given temperature, the percentage adsorption of Rhodamine-B decreases with the increase in the initial dye concentration. It may be because at higher initial concentration of the adsorbate (dye), the number of available active sites per adsorbate molecule at the adsorbent surface become fewer, resulting in a decrease in the  adsorption of dye27a, 27b. However, net amount of the dye adsorbed per unit mass of the adsorbent, increases with the increase in the dye initial concentration.

 

Table:2. Values of % Rhodamine-B dye adsorbed as a function of  time  and temperature

(Dye initial concentration: 10 mg/dm3; Amount of adsorbent: 1.0 g/100ml; Mesh No. 60)

Time (min)

298.15K

308.15K

318.15K

15

81.4

83.2

87.1

30

82.9

84.4

88.4

45

84.1

85.9

89.7

60

85.7

87.2

91.1

75

86.5

88.7

92.7

90

87.9

90.0

93.9

105

88.5

91.4

94.8

120

88.7

91.6

94.9

 

Fig. 2. Plots of % Rhodamine-B dye adsorbed as a function of time using various initial dye concentrations (Series-1: 10 mg/ dm3; Series-2: 30 mg/ dm3; Series-3: 50 mg/ dm3; Series-4: 75 mg/ dm3) at 298.15.

 

Fig. 3. Plot of % Rhodamine-B dye adsorbed as a function of time using various initial dye concentrations (Series-1: 10 mg/ dm3; Series-2: 30 mg/ dm3; Series-3: 50 mg/dm3;Series-4: 75 mg/ dm3) at 308.15K.

 

Fig. 4.  Plot of % Rhodamine-B dye adsorbed as a function of time using variousinitial dye concentrations (Series-1: 10 mg/ dm3; Series-2: 30 mg/ dm3; Series-3: 50 mg/dm3;Series-4: 75 mg/ dm3) at 318.15K.

 

3.3. Adsorption Isotherms:

The values of equilibrium concentration, Ce, (mg/L) and the amount of dye adsorbed at equilibrium,Qe, (mg/g)  at different temperatures for Rhodamine –B dye are presented in Fig.5. Adsorption isotherms show the trend of leveling at higher adsorbate concentrations suggesting that these are of the Langmuir type and there is flat position of the adsorbate molecules at the adsorbent surface infering monolayer coverage28 of adsorbent surface with adsorbate. The observed data was analyzed using the linear form of Langmuir and Freundlich isotherms.

 

Fig. 5.  Plot of amount of Rhodamine-B dye adsorbed at equilibrium, Qe,(mg/g) as a function of equilibrium concentration Ce /(mg/L) of  dye  at different temperatures (upper curve: 318.15K; middle curve: 308.15K; lower curve: 298.15K).

 

3.3.1. Langmuir Adsorption Isotherm:

The Langmuir adsorption  isotherm is represented by the  equation29 -

               Ce/Qe  =  1/(Q0b)  +  Ce/Q0 -------------------(3.1)

Where, Ce is the equilibrium concentration (mg/L) of adsorbate in the bulk; Qe is the amount of adsorbate(mg/g) adsorbed at equilibrium. Q0 and ‘b’ are Langmuir constants related to adsorption efficiency and energy of adsorption, respectively. Linear plots of Ce / Qe as a function of Ce for Rhodamine-B dye (Fig.6) at different temperatures suggest the applicability of the Langmuir isotherms. The values of Q0 and ‘b’ are obtained from the slope and intercept of the plot, respectively, and these are presented in table-3. The observed Q0 value, an index of adsorption efficiency of the adsorbate ( dye) on the adsorbent  is positive. The observed positive values of Langmuir parameter ‘b’, that increases with increase of temperature, indicates the endothermic nature of the adsorption process, suggesting the absence of any chemical interaction in the adsorbate-adsorbent system.

 

Fig..6.  Plot of Ce/Qe (g/L) as a function of Ce (mg/L) at different temperatures for Rhodamine-B dye (Upper Curve:  298.15K; Middle Curve: 308.15K: Lower Curve:  318.15K).

 

Table-3. Parameters of Langmuir and Freundlich  adsorption Isotherms for Rhodamine-B dye on Rice Husk Carbon at different temperatures

Tempt.

(K)

Langmuir isotherm

Freundlich Isotherm

Parameters    

Parameters

S.D

Q0 (mg/g)

B (dm3/g)

S.D

Kf

n

298.15

0.021

96.92

8.99

0.019

11.22

1.405

308.15

0.010

204.08

16.30

0.024

15.85

1.600

318.15

0.014

212.76

28.95

0.031

31.62

2.667

 

3.3.2. Freundlich Adsorption Isotherm:

Freundlich   adsorption isotherm is given by the relation30

              

               logQe =logKf + 1/n LogCe  -------------------(3.2.)

 

Where, Qe and Ce have their usual meanings and the constants ‘Kf’  and ‘n’ are measures of adsorption capacity and  intensity of adsorption, respectively. Plots of log Qe as a function of logCe at the studied temperatures are linear (Fig.7.) for Rhodamine -B suggesting that the adsorption of the dye on the adsorbent (rice husk carbon) follows the Freundlich isotherm. The parameters ‘Kf’ and ‘n’ were obtained from the intercept and slope, respectively, of log Qe versus log Ce  linear plots and these are also recorded in Table-3. The value of ‘Kf’, a measure of adsorption capacity of the adsorbate on the adsorbent surface, increases with increasing temperature. This may be attributed to the enhanced rate of transfer of dye molecules from bulk to the adsorbant surface at higher temperature. Also,  higher temperature may produce a swelling effect within the internal structure of the adsorbent enabling more number of dye molecules to penetrate further31. The values of ‘n’, a measure of intensity of adsorption, also increases with the rise of temperature. The observed values of ‘n’, greater than unity, suggests the feasibility of the process of adsorption in present adsorbate-adsorbent systems.

 

Fig.7 . Plots of Log (C0/Ct)  as a function of time of contact, t, (minutes) at 298.15K for Rhodamine-B dye (Dye initial concentrations: Series-1: 10 mg/L; Series-2: 30 mg/L; Series-3: 50 mg/L; Series-4: 75 mg/L.)

 

3.4. Kinetics of adsorption:

The rate constant (kad) of adsorption, was determined using the following first order kinetic equation-

 

               Log (C0/Ct) = (kad/2.303).t -----------------( 3.3.1.)

 

Where, C0 is the dye initial concentration and Ct is its concentration of dye at time ‘t’. The plots of Log (C0/Ct) as a function of time, t (minutes), at 298.15 K for Rhodamine-B are presented in Fig-8. Adsorption rate constant, kad at varying dye initial concentrations and temperatures for the studied dye systems are given in table-4. At the given temperature, the adsorption rate constant (kad) decreases with the increase in initial dye concentration. This may be because at higher dye initial concentration, the number of available binding sites, at the adsorbent surface, per adsorbate molecule, decreases. However, at a constant initial dye concentration,  kad value increases on increasing the temperature for the dye systems. This may be due to enhanced intra-particle diffusion as well as larger pore size at the adsorbent surface at higher temperatures.

 

Fig.8 . Plots of Log (C0/Ct)  as a function of time of contact, t, (minutes) at 298.15K for Rhodamine-B dye (Dye initial concentrations: Series-1: 10 mg/L; Series-2: 30 mg/L; Series-3: 50 mg/L; Series-4: 75 mg/L.)

 

Table-4 Values of adsorption rate constant (kad x102/min)at varying initial dye concentration (C0) and temperature (K) for Rhodamine-B.

Adsorption rate constant (kad x102/min)

Temperature  (K)

C0=10  mgdm-3

C0=30 mgdm-3

C0=50 mgdm-3

C0=75 mgdm-3

298.15

0.6502

0.3754

0.2642

0.2051

308.15

1.1338

0.6649

0.5121

0.3332

318.15

2.0209

1.2404

0.9134

0.5506

 

3.4.  Activation energy (Ea)  of adsorption

Activation energy (Ea)  of adsorption  as a function of dye initial concentration (C0) were obtained from the observed adsorption rate constants (kad) values at different temperatures using Arrhenius equation-

               kad = A. e (-Ea /RT)  ----------------------------(3.4.1 )

               or  

               Log kad  = - Ea /(2.303RT) + logA    ------- (3.4.2 )

 

Where, A = Arrhenius factor;  R = Gas constant (8.314 J/K/Mol); T = Temp. in Kelvin.

 

Activation energy (Ea) of adsorption were obtained from the slope of the linear plot between Log kad and 1/T . The values of activation energy (Ea) as a function of dye initial concentration (C0) are given in table-5. The magnitude of Ea increases with the increase of initial dye concentration. It may be due to the involvement of stronger solute-solute as well as solute-solvent interactions at higher solute ( dye) concentrations which obstructs closer approach of adsorbate (dye) molecules to the adsorbent surface.. Further, the values of activation energy are low suggesting that adsorption process may be controlled by intra-particle diffusion32 .

 

Table5- Values of Energy of Activation of adsorption (Ea/KJmole-1) at varying  initial concentration (C0) for Rhodamine-B.

C0(mg/L)→

10

30

50

75

Ea/KJmole-1

44.81

47.42

49.15

51.20

 

 

3.5. Intra-particle diffusion

Possibility of intra-particle diffusion was explored by using the relation:

               Qt = kdif.(t)0.5 + C       ---------------( (3.5.1 )

 

Where, Qt (molg-1) is the amount of dye adsorbed at time ‘t’. Intra-particle diffusion rate constant kdif. (mol.min-0.5.g-1) is obtained from the slope of plot between Qt and (t)0.5. The magnitude of the intercept, C, is a measure of the thickness of adsorbed layer. Larger the intercept greater is the boundary layer effect33. The values of Qt  versus (t)0.5  at 298.15K at different Rhodamine-B initial concentration are presented in Fig. 9. The values of intra-particle diffusion rate constant, kdif, and intercept, C, as a function of dye initial concentration (C0) and temperature for the studied dye systems are given in table-6. It is found that at the given temperature diffusion rate constant, kdif, as well as intercept, C, increases with the increase of dye initial concentration. The increase in diffusion rate constant at higher dye initial concentration may be attributed to larger adsorbent (dye) concentration gradient between the surface and the bulk solution. An increase in the intercept (C), a measure of thickness of the adsorbate layer, at higher dye initial concentration, is obvious as the amount of dye adsorbed then would be more. It is also seen that at the given dye initial concentration, on increasing the temperature the diffusion rate falls. It is because at higher temperature, due to more thermal agitation, the concentration gradient of the adsorbate between adsorbent surface and the bulk vanishes  resulting in the lowering  of the diffusion rate. Further, the observed higher value of the intercept (an indicator of more adsorbate thickness at the adsorbent surface) at higher temperature, may be due to enhanced adsorption at higher temperature, described earlier.

 

Fig.9  Amount of Rhodamin-B dye adsorbed (Qt)x105 (molg-1) as a function of t0.5 /(min)0.5  at 298.15K :- Initial dye concentration of dye,C0, are:- Series-1: 10mg/dm3; Series-2: 30mg/dm3 ;Series-3:50mg/dm3; Series-4: 75mg/dm3)

Table: 6. The values of Intra-particle diffusion constant (kdif,) and C for Rhodamine-B dye as a function of initial dye concentration (C0) and temperature

T(K)

C0 (mg/dm3)

kdifx103(mol.min-0.5.g-1)

Intercept, C (mol/gm)

298.15

10

0.762

0.15

298.15

30

0.913

0.44

298.15

50

2.711

0.82

298.15

75

3.281

1.39

308.15

10

0.532

1.12

318.15

10

0.325

1.76

 

3.6. Equilibrium constant and Thermodynamic Parameters of adsorption:

Equilibrium constant, K0, for the adsorption of adsorbate (dye) at the adsorbent surface  was calculated using the relation-

               K0= Cad/Csol        -------------------------(3.6.1 )

 

Where,   Cad and Csol  represent the concentrations of the adsorbate (dye) on the solid adsorbent  and in solution phase, respectively. Gibbs free energy of adsorption ( ∆G0) values  were obtained using the relation-

 

∆G0= -RT. ln K0          -------------------(3.6.2 )

 

Where,  R= 8.314 JK-1mol-1 and T= temperature in Kelvin. Entropy of adsorption (∆S0 ) was obtained from the relation-

 

               ∆S0= -d(∆G0) /dT  ---------------- ---(3.6.3 )

 

Enthalpy of adsorption (∆H0) was obtained using Gibbs Helmholtz equation-

 

               ∆H0 =   ∆G0 + T. ∆S0         -----------( 3.6.4)

 

The values of equilibrium constant (K0) and thermodynamic parameters (∆G0, ∆H0 and ∆S0 ) thus obtained for Rhodamine-B dye systems are given in table-7. Equilibrium constant (K0) value decreases at higher dye initial concentration but increases at higher temperature. The observed negative values of ∆G0 for the studied adsorbate-adsorbent systems, indicates that adsorption process is spontaneous irrespective of initial dye concentrations as well as temperatures. At the given temperature, ∆G0 values increase (becomes less negative) with the increase of initial dye concentration. This may be due to decrease in intra-particle diffusion rate at higher concentrations of the dye. However, at a fixed initial dye concentration, ∆G0 decreases (becomes more negative) with the increase of temperature. It may be attributed to (a) the enhanced diffusion rate of the adsorbate molecules which facilitates their approach to the active sites at the adsorbent surface and (b) the larger pore-size of the adsorbent at higher temperature. The values of ∆H0 are positive indicating endothermic nature of adsorption process suggesting that the uptake of the adsorbate (dye) by the adsorbent is through physi-sorption27a,27b.


Table 7.  Equilibrium constant (K0) & thermodynamic parameters (DG0 , DH0 and TDS0 ) for adsorption of dye (Rhodamine-B) onto activated carbon. K0 - DG0 (KJ/mol); DH0(KJ/mol);   TDS0 (KJ/mol)

Dye (mg /dm3)

250C

350C

450C

250C

350C

450C

250C

350C

450C

250C

350C

450C

10

9.87

15.13

49.00

5.67

6.96

12.15

90.88

92.88

115.11

96.55

99.84

127.26

30

8.71

12.33

31.26

5.36

6.43

9.10

50.36

51.53

35.44

55.72

57.96

44.54

50

8.35

10.63

15.67

5.26

6.06

7.28

24.85

25.06

27.72

30.11

31.12

35.00

75

7.62

9.87

13.15

5.04

5.87

6.82

21.49

21.57

22.77

26.53

27.44

29.59

 

Table: 8. Effects of temperature, particle size (Mesh No.) and  amount of adsorbent on equilibrium constant ( Ko ), % adsorption  and thermodynamic parameters for adsorption: (Initial concentration, C0, of Rhodamine-B: 1.0x10-4 M)

Temp.

(K)

Mesh No. of adsorbent

Amount of Adsorbent

(g/100ml)

K0

% adsorption

-∆Go   (KJ/mol)

∆ Ho /(KJ/mol)

T∆ So (KJ/mol)

298.15

30

0.50

6.18

86.07

4.52

40.95

45.47

298.15

60

0.50

10.83

91.55

5.91

37.02

42.93

298.15

30

1.00

6.95

87.42

4.81

40.66

45.47

298.15

60

1.00

14.70

95.17

6.66

26.58

33.24

308.15

30

0.50

12.70

92.7

6.51

40.48

46.99

308.15

60

0.50

19.79

95.15

7.65

36.72

44.37

308.15

30

1.00

15.64

93.99

7.05

39.94

46.99

308.15

60

1.00

23.44

97.75

8.06

26.30

34.36

318.15

30

0.50

17.48

94.59

7.57

40.94

48.51

318.15

60

0.50

26.67

96.02

8.79

37.02

45.81

318.15

30

1.00

19.52

97.45

7.86

40.66

48.52

318.15

60

1.00

28.78

98.24

8.89

26.56

35.45

 

 


The observed positive ∆S0 indicates the increase of disorder and randomness at the adsorbent-adsorbate interface. This may be due the displacement of larger number of water molecules by the adsorbate (dye) species from the adsorbent surface, resulting in a gain of more translational entropy than that lost by the adsorption of adsorbate (dye) molecules27a,27b. At the given concentration, equilibrium constant (K0) and % adsorption of dye increase and ∆G0 values decrease on raising the temperature. This may be attributed to the enlargement of pore size and more activation of the adsorbent surface at higher temperature27a,27b.

 

3.7 Effects of the size and the amount of the adsorbent on adsorption:

Effects of the size and the amount of the adsorbent (Rhodamine-B dye) on % adsorption at equilibrium, equilibrium constant and thermodynamic parameters of adsorption are given in table-8. At the given temperature, percent adsorption of dye increases and Gibb’s free energy of adsorption (∆G0) decreases with the increase in the amount as well as the mesh number of the adsorbent particles. These observations may be explained in terms of the fact that on increasing the amount as well as mesh number of the adsorbent, the number of available active binding sites per adsorbate (dye) molecule at the adsorbent surface are raised. The magnitudes of endothermic enthalpy and entropy of adsorption increase on increasing amount as well as the adsorbent particle mesh number. It is obvious since at higher load of adsorbent and with finer adsorbent particles the release of water molecules by the adsorbing substrate molecules will be much higher leading to enhanced endothermicity as well as randomness in the system.

4. CONCLUSIONS:

The present work reports the kinetic and equilibrium studies on adsorption of Rhodamine-B dye from aqueous solution onto activated rice husk carbon. The effect of parameters such as : contact time, dye initial concentration, temperature, adsorbentparticle size and amount of adsorbent on % adsorption of Rhodamine-B on the activated rice husk carbon has been investigated. At the specified initial dye concentration, percent adsorption increases with increase in temperature as well as with the increase in the adsorbate-adsorbent contact period. However, the total amount of the dye adsorbed per unit mass of the adsorbent, increases with the increase in the dye initial concentration. The observed adsorption data was analysed in the light of Langmuir and Freundlich adsorption isotherms. From the evaluated thermodynamic parameters of adsorption it is inferred that the adsorption of Rhodamine- B dye at the rice husk carbon is endothermic and predominately controlled by entropy gain. The observed positive entropy of adsorption may be due to a net gain of more translational entropy due to the desorption of water molecules than that lost by the adsorption of dye molecules at the adsorbent surface.. At the given temperature, percent adsorption of dye increases and Gibb’s free energy of adsorption (∆G0) decreases with the increase in the amount as well as the mesh number of the adsorbent particles. These observations may be explained in terms of the fact that on increasing the amount and decreasing the particle size of adsorbent , the number of available active binding sites per adsorbate (dye) molecule at the adsorbent surface are raised.

 

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Received on 30.03.2011        Modified on 07.04.2011

Accepted on 11.04.2011        © AJRC All right reserved

Asian J. Research Chem. 4(6): June, 2011; Page 917-924