Removal of basic and acidic dyes from aqueous solution by ... - NOPR

Indian Journal of Chemical Technology Vol. 15, March 2008, pp. 130-139

Removal of basic and acidic dyes from aqueous solution by adsorption on a low cost activated carbon: Kinetic and thermodynamic study S Arivoli, M Sundaravadivelu & K P Elango* Department of Chemistry, Gandhigram Rural University, Gandhigram 624 302, India Email: [email protected] Received 31 May 2007; revised 1 January 2008 Batch experiments were carried out for the sorption of Congo red (CR), Malachite green (MG), Rhodamine B (RDB) and Rose Bengal (RB) dyes onto acid activated carbon prepared from a plant material, Aloe barbadensis Mill. The operating variables studied were initial dye concentration, pH, temperature and contact time. Equilibrium data were fitted to the Langmuir and Freundlich isotherm equations. From this, adsorption efficiency, adsorption energy, adsorption capacity, intensity of adsorption and dimensionless separation factor were calculated. The amounts of CR, MG, RDB and RB removed from a 60 mg L−1 of the dye solution at 30°C are 21.17, 26.19, 8.50 and 3.62 mg g−1, respectively. The results of these studies indicate that the adsorption is favourable. From the kinetic studies, the rate constant values for the adsorption process were calculated. The thermodynamic parameters like ∆G°, ∆H°, and ∆S° indicate that the adsorption process is endothermic and spontaneous in nature. The mechanism of the adsorption of dyes onto the adsorbent has been investigated by using the experimental results and confirmed by FT IR, XRD and SEM images. Keywords: Adsorption, Dyes, Kinetic, Thermodynamic, Activated carbon

Colour is a visible pollutant. Textile companies, dye manufacturing industries, paper and pulp mills, tanneries, electroplating factories, distilleries, food companies etc. discharge coloured wastewater into the environment. The discharge of coloured effluents, although frequently less toxic than many colourless effluents, is resented by the public on the ground, that colour is an indicator of pollution. However, studies have indicated that coloured dye wastes frequently contain a spectrum of heavy metals and other toxic pollutants1. The methods of colour removal from industrial effluents include biological treatment, coagulation, flotation, and adsorption. However, adsorption appears to have considerable potential for the removal of colour from industrial effluents2-15. Owen16 after surveying 13 textile industries has reported that adsorption using granular activated carbon has emerged as a practical and economical process for the removal of colour from textile effluents. Weber and Morris17 have identified many advantages of adsorption over several other conventional treatment methods for wastewater treatment. Activated carbon is perhaps the most widely used adsorbent for the removal of many organic contaminants, which are biologically resistant. The present study is undertaken to evaluate the efficiency of an activated carbon prepared from Aloe

barbadensis Mill., a fibrous medicinal plant which is distributed almost throughout India, for the removal of dyes. In order to design the adsorption treatment systems, knowledge of kinetic and mass transfer processes is essential. Hence, a systematic kinetic and thermodynamic study on the adsorption of CR, MG, RDB and RB onto the activated carbon has been undertaken. Experimental Procedure Dried leaves of Aloe barbadensis Mill. were carbonized with concentrated sulphuric acid in the ratio of 1:1(w/v). The carbonization was completed by heating for 12 h in a furnace at 400°C. The resulting carbon was washed with distilled water until pH of the slurry became constant. Then, the carbon was dried for 4 h at 100°C in a hot air oven. The dried material was ground well to a fine powder and sieved. The activated carbon prepared was characterized by conventional chemical and analytical methods as described earlier18 and the important characteristics are given in Table 1. All the chemicals including the dyes used were of high purity, commercially available Analytical grade (Merck or Sd-fine, India). Stock solutions of 1000 mg/L of the dyes were prepared using double distilled water. The equilibrium and kinetic studies on the adsorption of these dyes onto the chosen

ARIVOLI et al.: REMOVAL OF BASIC AND ACIDIC DYES BY ACTIVATED CARBON

adsorbent were carried out as described earlier19. In a typical experiment, adsorption was performed by agitating a given dose of the adsorbent with 50 mL of dye solution of desired concentration at 30±0.5°C in different stoppard bottles in a shaking thermostat machine. The shaking speed was 120 strokes/min throughout the study. At the end of predetermined time intervals, the sorbet was filtered and the concentration of dye was determined. All experiments were carried out twice and the adsorbed dye Table 1 — Characteristics of the adsorbent Properties Particle size (mm) Density (g/cc) Moisture content (%) Loss on ignition (%) Acid insoluble matter (%) Water soluble matter (%) pH of aqueous solution pHZPC Surface groups (m equiv/g) i) Carboxylic acid ii) Lactone, lactol, carboxyl iii) Phenolic iv) Basic (pyrones and chromenes)

Activated carbon ABC 0.11 0.4848 1.60 82.4 8.5 0.15 7.1 6.7 0.249 0.056 0.077 0.031

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concentrations given were the means of duplicate results. Desorption studies were carried out using the spent carbon. The carbon loaded with dye was separated by filtration and gently washed with distilled water to remove any unadsorbed dye. The dye-laden carbons were agitated with 50 mL each of sulphuric acid, hydrochloric acid, nitric acid, sodium chloride solutions (each 0.2 M) and water separately for 60 min and analyzed. The FT-IR spectra of the activated carbons before and after adsorption have been recorded using JASCO FT-IR 460 Plus spectrometer. The XRD patterns of the adsorbents were recorded at the National Institute for Interdisciplinary Science & Technology, Thiruvananthapuram, India. The SEM images were taken at Madurai Kamaraj University, Madurai, India. Results and Discussion Effect of contact time and initial dye concentration

The results of adsorptions of CR, MG, RDB and RB dyes on the activated carbon are given in Table 2. The results indicate that percent adsorption decreased with increase in initial dye concentration, but the

Table 2 — Equilibrium parameters for the adsorption of dyes onto activated carbon Ce (mg L−1) 40o 50o

[Dye]o (mg L−1)

30

10 20 30 40 50 60

1.56 3.52 5.75 8.91 12.78 17.67

1.52 3.40 5.55 8.52 12.20 17.24

1.49 3.28 5.36 8.17 11.65 16.42

1.45 3.16 5.16 7.78 11.06 15.67

4.22 8.24 12.13 15.54 18.61 21.17

10 20 30 40 50 60

0.51 1.18 2.24 3.62 5.77 7.63

0.50 1.14 2.19 3.52 5.63 7.56

0.48 1.11 2.13 3.42 5.49 7.39

10 20 30 40 50 60

1.38 2.98 5.57 8.95 13.81 17.49

1.30 2.83 5.47 8.40 12.69 16.40

10 20 30 40 50 60

3.88 5.46 8.75 12.08 17.44 23.84

3.23 4.97 8.32 11.17 17.04 23.28

o

o

Qe (mg g−1) 40o 50o

Dye removed (%) 40o 50o

60o

30o

4.26 8.36 12.32 15.91 19.18 21.79

4.28 8.42 12.42 16.11 19.47 22.16

84.3 82.4 80.8 77.7 74.4 70.6

84.8 83.0 81.5 78.7 75.6 71.3

85.1 83.6 82.0 79.6 76.7 72.6

85.5 84.1 82.8 80.5 77.9 73.9

0.46 1.07 2.08 3.32 5.35 7.21

Malachite green 4.74 4.75 4.76 9.41 9.43 9.45 13.88 13.91 13.93 18.19 18.24 18.29 22.11 22.19 22.26 26.19 26.22 26.31

4.77 9.46 13.96 18.34 22.32 26.39

94.8 94.1 92.5 91.0 88.5 87.3

95.0 94.3 92.7 91.2 88.7 87.4

95.2 94.5 92.9 91.4 89.0 87.7

95.4 94.7 93.1 91.7 89.3 88.0

1.01 2.69 5.23 7.80 11.87 16.03

0.96 2.22 4.92 7.24 11.09 14.97

1.76 3.40 4.89 6.21 7.24 8.50

Rhodamine B 1.74 3.43 4.91 6.32 7.46 8.72

1.80 3.46 4.95 6.44 7.63 8.79

1.81 3.56 5.02 6.55 7.78 9.01

88.2 85.1 81.4 77.6 72.3 70.8

87.0 85.8 81.8 79.0 74.6 72.7

89.8 86.6 82. 6 80.5 76.3 73.3

90.4 88.9 83.6 81.9 77.8 75.1

2.98 4.44 7.66 10.61 16.51 22.91

2.79 4.09 7.17 10.21 15.95 22.37

0.61 1.45 2.12 2.79 3.26 3.62

Rose Bengal 0.68 1.50 2.17 2.88 3.30 3.67

0.70 1.56 2.23 2.94 3.35 3.77

0.72 1.59 2.28 2.98 3.40 3.76

61.2 72.6 70.8 69.8 65.1 60.2

67.7 75.2 72.3 72.1 66.0 61.2

70.2 77.8 74.4 73.4 66.9 61.8

72.1 79.5 76.1 74.5 68.1 62.7

60

o

30

Congo red 4.24 8.30 12.22 15.74 18.90 21.38

60o

INDIAN J. CHEM. TECHNOL., MARCH 2008

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actual amount of dyes adsorbed per unit mass of carbon increased with increase in dyes concentration. This shows that the adsorption is highly dependent on initial concentration of dye. It is because that at lower concentration, the ratio of the initial number of dye molecules to the available surface area is low, subsequently the fractional adsorption becomes independent of initial concentration. However, at high concentration the available sites of adsorption becomes fewer, and hence the percentage removal of dye gets decreased with increase in initial concentration18-20. The effect of contact time between the adsorbent and adsorbate is depicted in Fig. 1. As seen from the figure, equilibrium was established after 50 min for all concentrations. Further, in Fig. 1, the curves are single and continuous, leading to saturation, suggesting the possible monolayer coverage of the dyes on the carbon surface18.

isotherms show the trend of leveling out at higher adsorbate concentrations and thus are of the Langmuir-type. The L type of adsorption isotherm is connected with flat position of the adsorbate molecule towards the adsorbent surface and refers to the monolayer coverage3. The experimental data were analyzed according to the linear form of the Langmuir22 and Freundlich23 isotherms. The Langmuir isotherm is represented by the following equation. Ce/Qe = 1/Qob + Ce/Qo

Effect of carbon concentration

The adsorption of the dyes on the adsorbent was studied by varying the carbon concentration (100-1000 mg/50 mL). The percent adsorption increased with increase in the carbon concentration (Fig. 2). This has been attributed to increased carbon surface area and availability of more adsorption sites with increase in dosage21. Adsorption isotherm

Figure 3 shows the adsorption isotherms for the dyes onto the activated carbon. All adsorption

Fig. 1 — Effect of contact time on the adsorption of dyes onto ABC [Dye] = 20 mg L−1; Temp = 30oC; pH = 7

Fig. 2 — Effect of adsorbent dose on the adsorption of dyes onto ABC [Dye] = 20 mg L−1; Temp = 30oC; pH = 7; Contact time = 60 min

Fig. 3 — Adsorption isotherms [Dye] = 10 mg L−1; Temp = 30oC; pH = 7; Contact time = 60 min


where Ce is the equilibrium concentration (mg/L), Qe is the amount adsorbed at equilibrium (mg/g) and Qo and b are Langmuir constants related to adsorption efficiency and energy of adsorption, respectively. The linear plots of Ce /Qe versus Ce suggest the applicability of the Langmuir isotherms and a representative plot is given in Fig. 4. The values of Qo and b were determined from the slope and intercepts of the plots and are presented in Table 3. From the

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results, it is clear that the value of adsorption efficiency Qo and adsorption energy, b, of the carbon, in general, increases on increasing the temperature. The observed Qo values are in the order CR>MG>RDB>RB. This may be due to the fact that the higher molecular weight, large size and radii of the RB and RDB limit the possibility of the adsorption of these dyes onto the adsorbent and hence a lower value of adsorption efficiency24. Further, the results indicate that the maximum adsorption corresponds to a saturated monolayer of adsorbate molecules on adsorbent surface with constant energy and no transmission of adsorbate in the plane of the adsorbent surface25. The observed b values suggest the endothermic nature of the process involved in the system. To confirm the favourability of the adsorption process, the separation factor (RL=1/1+bC0) has been calculated and the values were found to be between 0 and 1 which confirms that the ongoing adsorption process is favourable26. The Freundlich isotherm, in the following form, was also employed for the adsorption of the dyes on the adsorbent. logQe = logKf + 1/n logCe where Qe and Ce have usual meanings and K and n are constants incorporating all factors affecting the adsorption capacity and intensity of adsorption, respectively. Linear plots of logQe versus logCe show

Fig. 4 — Langmuir isotherms for the adsorption of dyes onto ABC Temp = 30oC; pH = 7

Table 3 — Isotherm results Dye

Temp. (°C) r

Langmuir isotherm Statistical parameters/constants S.D. Q0 b (mg g−1) (dm3 g−1)

r

Freundlich isotherm Statistical parameters/constants S.D. K

n

Congo red

30 40 50 60

0.999 0.999 0.999 0.999

0.02 0.02 0.02 0.02

35.71 38.46 39.37 41.84

0.086 0.082 0.083 0.080

0.995 0.995 0.996 0.996

0.03 0.02 0.02 0.02

2.04 2.07 2.09 2.12

1.40 1.38 1.36 1.33

Malachite green

30 40 50 60

0.999 0.999 0.999 0.999

0.02 0.02 0.02 0.02

34.58 34.36 34.25 34.25

0.313 0.323 0.337 0.352

0.992 0.993 0.993 0.993

0.03 0.03 0.03 0.03

1.89 1.89 1.88 1.88

1.57 1.57 1.58 1.58

Rhodamine B

30 40 50 60

0.999 0.998 0.994 0.992

0.01 0.02 0.02 0.03

10.31 11.77 11.24 11.21

0.168 0.157 0.170 0.193

0.994 0.991 0.998 0.992

0.02 0.03 0.01 0.02

1.78 1.89 1.81 1.80

1.73 1.58 1.69 1.70

Rose Bengal

30 40 50 60

0.976 0.969 0.984 0.990

0.04 0.05 0.03 0.03

7.69 6.67 6.25 5.88

0.440 0.060 0.078 0.093

0.948 0.943 0.966 0.976

0.60 0.65 0.40 0.35

1.33 1.41 1.49 1.52

3.46 2.94 2.52 2.36


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that the adsorption of dyes follows the Freundlich isotherm and a representative plot is given in Fig. 5. The statistical data of the plots and the constants are collected in Table 3. The results indicate that the values of adsorption capacity (K) decreases in the order MG>CR>RDB>RB. This may be due to the fact that the higher molecular weight, large size and radii of the RB and RDB limit the possibility of the adsorption of these dyes on to the adsorbent and hence a lower value of adsorption capacity24. The values of intensity of adsorption, n, are greater than one indicating that the adsorption is favourable27. Kinetics of adsorption

Kinetics of sorption describes the solute uptake rate, which in turn governs the residence time of sorption reaction. It is one of the important characteristics in defining the efficiency of sorption. In the present study, the kinetics of the dyes removal was carried out to understand the behaviour of this low cost carbon adsorbent. The adsorption of dyes from an aqueous solution follows reversible first order kinetics, when a single species is considered on a heterogeneous surface. The heterogeneous equilibrium between the dyes solution and the activated carbon may be expressed as: k1 A⇔B k2

where, k1 is the forward rate constant and k2 is the backward rate constant. A represents dyes remaining in the aqueous solution and B represents dyes adsorbed on the surface of activated carbon. Since the reaction in both directions is of first order, the rate constant of adsorption, kad, was determined using the following rate expression as described earlier28. log(Co/Ct) = (kad/2.303) t where Co and Ct are the concentration in mg L−1 of dye present initially and at time t respectively. The forward (k1) and backward (k2) rate constants were calculated using the following relations28 and are collected in Table 4. kad = k1 + k2 = k1 + (k1/Ko) = k1[1+1/Ko] Ko = k1/k2 where Ko is the equilibrium constant It is evident from the results that the forward rate constant is much higher than the backward rate constant suggesting that the rate of adsorption is clearly dominant. The rate constants kad (Table 4) decreases with increase in the concentration of dyes. In cases of strict surface adsorption a variation of rate should be proportional to the first power of concentration. However, when pore diffusion limits the adsorption process, the relationship between initial dye concentration and rate of reaction will not be linear. It shows that pore diffusion limits the overall rate of dye adsorption19. Intra-particle diffusion studies

Fig. 5 — Freundlich isotherms for the adsorption of dyes onto ABC Temp = 30oC; pH = 7

In adsorption studies, it is necessary to determine the rate-determining step. Therefore, the results obtained from the experiments were used to study the rate-limiting step. Since the particles were vigorously agitated during the experiment, it is reasonable to assume that the mass transfer from the bulk liquid to the particle external surface did not limit the rate. One might, then, postulate that the rate-limiting step might be film or intra-particle diffusion17. That is why, in this study, possibility of existence of intra-particle diffusion was tested by plotting the graph between amount of dye adsorbed and square root of time (Fig. 6). The double nature of these plots may be explained as: the initial curve portions are attributed to boundary layer diffusion effect, while the final linear portions are due to intra-particle diffusion effect. The rate constant for intra-particle diffusion coefficient kp, for the dyes was determined from


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Table 4 — Rate constants for the adsorption of dyes (103 kad, min−1) and rate constants for the forward (103 k1, min−1) and reverse (103 k2, min−1) processes [Dye]o (mg L−1)

o

30

o

40

30o

kad

o

50

60

o

k1

10 20 30 40 50 60

8.90 8.01 7.42 4.94 4.62 3.69

9.10 8.25 7.65 5.16 4.82 5.14

9.32 8.52 7.92 5.36 4.97 4.07

9.53 8.80 8.18 5.61 5.28 4.25

7.51 6.60 6.00 3.84 3.44 1.30

10 20 30 40 50 60

6.57 5.79 4.65 4.50 4.40 4.57

8.07 5.97 4.76 4.93 4.64 4.56

7.03 6.16 4.88 5.07 5.07 4.67

7.28 6.36 5.01 5.26 4.85 4.67

6.23 5.45 4.30 4.09 3.89 3.99

10 20 30 40 50 60

7.77 3.45 2.24 2.18 1.22 1.63

7.37 3.55 2.28 2.73 2.81 3.62

8.12 4.38 2.28 3.05 2.84 3.26

7.89 5.56 2.67 3.45 2.40 4.02

6.86 2.94 1.82 1.69 0.88 1.16

10 20 30 40 50 60

7.54 8.28 9.35 9.53 8.36 7.29

9.25 9.20 11.87 9.71 8.18 7.37

9.93 9.94 10.96 9.32 8.53 7.48

9.77 10.54 10.79 10.76 8.64 7.51

4.61 5.95 6.63 6.66 5.44 4.39

Temperature (°C) 40o k2 k1

Congo red 1.39 1.41 1.42 1.10 1.18 1.09

50o

60o

k2

k1

k2

k1

k2

7.71 6.85 6.23 4.06 3.64 3.67

1.39 1.40 1.41 1.10 1.18 1.48

7.93 7.12 6.51 4.26 3.81 2.95

1.38 1.40 1.41 1.09 1.16 1.11

8.16 7.40 6.77 4.52 4.11 3.14

1.38 1.39 1.41 1.10 1.17 1.11

7.67 5.63 4.41 4.49 4.11 3.98

0.40 0.34 0.35 0.43 0.52 0.57

6.69 5.82 4.54 4.63 4.51 4.10

0.34 0.34 0.35 0.43 0.56 0.58

6.94 6.02 4.66 4.82 4.33 4.10

0.34 0.34 0.35 0.44 0.52 0.56

6.55 3.04 1.86 2.15 2.09 2.63

0.83 0.50 0.42 0.57 0.71 0.99

7.30 3.79 1.88 2.46 2.16 2.39

0.82 0.59 0.40 0.60 0.67 0.87

7.14 4.94 2.22 2.82 1.87 3.01

0.75 0.62 0.44 0.62 0.53 1.01

6.26 6.92 8.58 7.00 5.39 4.51

2.99 2.28 3.29 2.71 2.79 2.86

6.97 7.74 8.16 6.85 5.71 4.62

2.96 2.21 2.80 2.47 2.82 2.85

7.04 8.38 8.21 8.01 5.88 4.71

2.73 2.16 2.58 2.75 2.76 2.80

Malachite green 0.34 0.34 0.35 0.41 0.51 0.58 Rhodamine B 0.91 0.52 0.42 0.49 0.34 0.48 Rose Bengal 2.93 2.23 2.73 2.88 2.92 2.89

slopes of linear portion of the respective plots. The values of kp (in mg g−1 min−0.5) are 0.223, 0.293, 0.099 and 0.042 for CR, MG, RDB and RB respectively. This may be due to the fact that the higher molecular weight, large size and radii of the RB and RDB limit the possibility of the intra-particle diffusion and hence a lower value of kp24. The amount of the dyes adsorbed (Table 2) was observed to be in the same order. The observed values of kp, in the present study, were comparable with those reported for these dyes elsewhere5,8,11. Further, the linear portions of the curves do not pass the origin in Fig. 5. This indicates that mechanism of dyes removal on the adsorbent is complex and both the surface adsorption as well as intra-particle diffusion contribute to the ratedetermining step26. Effect of temperature Fig. 6 — Intra-particle diffusion plot for the adsorption of dyes onto ABC [Dye] = 20 mg L−1; Temp = 30oC; pH = 7

The equilibrium constant, Ko, for the adsorption equilibrium was calculated according to the relation: Ko=Cad/Csol where Cad and Csol are the concentration of


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than is lost by the adsorbate molecules, thus allowing the prevalence of randomness in the system20. From the results, it could be concluded that the adsorption of the dyes is through physisorption process and enhancement of adsorption capacity of the activated carbon at higher temperatures may be attributed to the enlargement of pore size and activation of the adsorbent surface19.

the dye on solid phase and in solution phase respectivly4. The equilibrium constant values at different temperatures are collected in Table 5. Thermodynamic parameters such as change in free energy (∆G°), enthalpy (∆H°) and entropy (∆S°) were determined using the equilibrium constants through van’t Hoff equation and are also given in Table 5. The values of ∆H° are positive indicating endothermic nature of the adsorption process. Further, the magnitude of the enthalpy change suggests that the uptake of the dyes by the adsorbent is through physisorption20. The negative values of ∆G° (Table 5) indicate that the adsorption for all the four dyes was spontaneous. The positive values of ∆S° (Table 5) suggest the increased disorder and randomness at the solidsolution interface of the dyes and the adsorbent. This may be due to the fact that the adsorbed water molecules, which have been displaced by the adsorbate species, gain more translational entropy

Effect of pH

The experiments carried out at different initial pH show that there was a change in the per cent removal of dye with change in pH (Fig. 7). This indicates the strong force of interaction between the dye and the activated carbon that is either H+ or OH- ions could influence the adsorption capacity. This behaviour can be explained on the basis of zero point charge of the adsorbent (pHZPC = 6.7). At higher pH above this point, the OH- ions compete effectively with acidic dyes viz. CR and RB causing a decrease in percentage

Table 5 — Equilibrium constants and thermodynamic parameters for the adsorption of dyes on to ABC [Dye]o (mg L−1)

−∆Go

Ko 30

40

50

o

60 C

30

∆Ho

∆So

o

40

50

60 C

4.46 4.12 3.85 3.40 2.94 2.36

4.68 4.37 4.10 3.65 3.20 2.62

4.92 4.62 4.35 3.93 3.48 2.87

2.39 3.28 3.42 4.38 4.86 4.04

21.9 23.6 23.1 24.7 24.8 20.5

7.67 7.29 6.61 6.08 5.31 5.03

8.02 7.62 6.90 6.36 5.62 5.27

8.38 7.95 7.19 6.65 5.87 5.51

3.08 2.71 2.16 2.38 2.26 1.43

34.3 31.9 28.8 27.0 24.4 20.7

5.38 4.69 3.90 3.44 2.80 2.54

5.86 5.00 4.18 3.81 3.13 2.71

6.22 5.76 4.51 4.18 3.48 3.05

6.51 4.84 3.04 6.83 8.06 4.80

38.1 30.4 22.2 32.8 34.1 23.3

1.93 2.88 2.49 2.46 1.71 1.19

2.30 3.37 2.87 2.73 1.89 1.29

2.62 3.75 3.20 2.96 2.09 1.44

15.97 10.89 7.18 7.13 3.27 2.59

56.6 44.0 31.0 30.5 15.9 11.9

Congo red 10 20 30 40 50 60

5.40 4.68 4.22 3.49 2.91 2.40

5.56 4.88 4.40 3.69 3.10 2.48

5.73 5.09 4.60 3.90 3.29 2.65

5.91 5.32 4.81 4.15 3.52 2.83

10 20 30 40 50 60

18.36 15.94 12.38 10.06 7.66 6.86

19.08 16.48 12.72 10.35 7.88 6.93

19.85 17.07 13.08 10.69 8.11 7.12

20.68 17.69 13.45 11.04 8.34 7.32

4.24 3.89 3.63 3.14 2.69 2.20 Malachite green 7.33 6.97 6.33 5.82 5.13 4.85 Rhodamine B

10 20 30 40 50 60

7.51 5.70 4.39 3.47 2.62 2.43

7.90 6.07 4.48 3.75 2.94 2.66

8.86 6.44 4.74 4.13 3.21 2.74

9.46 8.00 5.09 4.52 3.51 3.01

5.08 4.38 3.73 3.13 2.43 2.24 Rose Bengal

10 20 30 40 50 60

1.58 2.66 2.43 2.31 1.87 1.52

2.10 3.03 2.61 2.58 1.93 1.58

2.36 3.51 2.91 2.77 2.03 1.62

∆Go kJ mol−1; ∆Ho kJ mol−1; ∆So J K−1 mol−1

2.58 3.88 3.18 2.92 2.13 1.68

1.15 2.47 2.23 2.11 1.57 1.05


Fig. 7 — Effect of pH on the adsorption of dyes onto ABC [Dye] = 20 mg L−1; Temp = 30oC; Contact time = 60 min

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Fig. 8 — FT-IR spectra of ABC (a) before adsorption, (b) loaded with CR

of these dyes removal. At a pH below this zero point charge, the surface of the adsorbent gets positively charged, which enhances the adsorption of negatively charged dye anions through electrostatic force of attraction18. As expected the basic dyes MG and RDB show opposite behaviour. Analytical evidences

A representative FT-IR spectrum of the raw activated carbon and after adsorption of dyes is shown in Fig. 8. It could be seen from the spectra that almost there is no change in the spectral pattern before and after adsorption however there is a slight reduction of stretching vibration of the predominant absorption bands. This clearly indicates that the adsorption of dyes onto the adsorbent is by physical forces and not by chemical combination which would have modified the chemical nature of the surface and consequently the FT-IR pattern of the adsorbent29. The X-ray diffraction (XRD) pattern of the activated carbon and malachite green loaded carbon is shown in Fig. 9, as a representative case. The presence of an intense main peak indicates the presence of some highly organized crystalline structure of raw activated carbon, after the adsorption of dyes, the intensity of the highly organized peaks are diminished appreciably. This can be attributed to the adsorption of dyes on the upper layer of the crystalline structure of the carbon surface by means of physisorption18. Representative SEMs of raw activated carbon and malachite green adsorbed

Fig. 9 — XRD pattern of ABC (a) before adsorption, (b) loaded with MG

activated carbon are shown in Fig. 10. The bright spots, show the presence of tiny holes on the crystalline structure of raw activated carbon, after treatment with the dye the bright spots became black showing the adsorption of the dye on the surface of the carbon29.


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Fig. 11 — Effect of calcium ions on the adsorption of dyes onto ABC [Dye] = 20 mg L−1; Temp = 30oC; pH = 7; Contact time = 60 min

Fig. 10 — SEM images of ABC (a) before and (b) after adsorption of MG Effect of other ions

The effect of added calcium and chloride ions, separately, on the adsorption process was studied at different concentrations of these ions. The ions were added to 20 mg/L of dye solution and the contents were agitated for 60 min at 30°C and from the residual concentration values the percentage of adsorption were calculated. The results, as seen from Figs 11 and 12, indicate that addition of chloride ions decreases the adsorption of acidic dyes namely CR and RB while calcium ions decreases the adsorption of basic dyes, MG and RDB. This may be due to the fact that with increase in the concentration of these ions the interference at available surface sites of the sorbent through competitive adsorption increases and hence the percentage of dye adsorption decreases. The interference was more in the presence of Ca2+ compared with Cl− ion. This may be due to the fact that ions with smaller hydrated radii decrease the swelling pressure within the sorbent and increase the affinity of the sorbent for such ions30.

Fig. 12 — Effect of added chloride ions on the adsorption of dyes onto ABC [Dye] = 20 mg L−1; Temp = 30oC; pH = 7; Contact time = 60 min Desorption studies

Desorption studies help to elucidate the nature of adsorption and recycling of the spent adsorbent and the dye. If the adsorbed dyes can be desorbed using water or dilute acids then the attachment of the dye of the adsorbent is by weak bonds18. The effect of various reagents (each 0.2 M) including water used for desorption studies is shown in Fig. 13. The results indicate that hydrochloric acid is a better reagent for desorption, because one could get more than 65%


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Education, Chennai and the Principal, M.R. Government Arts College, Mannargudi, for permitting him to do this work. References 1 2 3 4 5 6 7 8 9

Fig. 13 — Regeneration studies

removal of adsorbed dye. Desorption of the dyes by dilute mineral acid medium indicates that the dyes were adsorbed onto the activated carbon through physisorption mechanism. Conclusions The activated carbon prepared from Aloe barbadensis Mill., was found effective in removing CR, MG, RDB and RB dyes from aqueous solutions. The intra particle diffusion is found to be the rate limiting step in the adsorption process. The experimental data correlated reasonably well by the Langmuir and Freundlich adsorption isotherms. The temperature variation study showed that the dyes adsorption is endothermic and spontaneous. The percentage adsorption of these dyes varies significantly with pH and presence of other ions. The mechanism of adsorption of these dyes onto the activated carbon was mainly through physical forces as it is evidenced from enthalpy data and analytical measurements such as FT IR, XRD and SEM. Acknowledgement S. Arivoli thanks the University Grants Commission, New Delhi, Director of Collegiate

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