2.1 Preparation of adsorbents

Adsorbents ground nut shells and Eichhornia were collected respectively from the local shop and from a pond located in Amritsar, India. The materials were washed with tap water and finally with double distilled water to remove the suspended impurities, dust and soil and then dried in oven. Charcoal was prepared in a very economical way by just burning the material in the absence of free excess of air. Charcoal was sieved through sieves having mesh size 240–200, to remove coarse particles, and the corresponding particle size of 70–75 μ was obtained for both GNC and EC (Lachman and Lieberman, 2009).

2.2 Dye solution preparation

The basic dye, BB9 (Fig. 1) was obtained from S.D fine Chemical, Mumbai, India. An accurately weighed quantity of dye was dissolved in double distilled water to prepare the stock solution (14 mg/L). Experimental solutions of desired concentration were obtained by successive dilutions with double distilled water.

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Figure 1

Structure of Basic Blue 9 (BB9).

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2.3 Batch model adsorption studies

Batch adsorption experiments were carried out by shaking 0.1 g of the adsorbent with 100 mL of dye solution of desired concentration in different glass bottles using Metrex water bath shaker. After agitation, samples were withdrawn from the shaker and dye solutions were separated from the adsorbent using Whatman filter paper No. 42. Dye concentration in the supernatant solutions was estimated by measuring absorbance at maximum wavelength (665 nm) of dye with UV–Visible spectrophotometer (1800, Shimadzu, Japan) and computing from calibration curve. The calibration curve is drawn by making serial dilutions and then plotting the absorbance at a wavelength maximum against concentration. The amount of dye adsorbed by the adsorbent was calculated using the following equation. $$\%\text{Removal of dye}=\frac{\text{Co}-\text{Ce}}{\text{Co}}\times 100$$ where Co and Ce (mg/L) are the initial and equilibrium concentrations of the dye respectively.

3.1 FTIR spectroscopy studies

FTIR analysis Fig. 2(a,b) shows the spectrophotometric observations of the dye BB9 and the adsorbents (GNC, EC) in the range of 400–4000 cm⁻¹. Dye BB9 has sharp peaks at 1136 and 1244 cm⁻¹ which can be assigned to C⚌S and C–N bonds respectively. However a sharp absorption band at 1591 and 2357 cm⁻¹ is due to C⚌N and tertiary amine, respectively.

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Figure 2

(a) FTIR spectra of GNC, BB9 and dye loaded GNC. (b) FTIR spectra of EC, BB9 and dye loaded EC.

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Figure 2

(a) FTIR spectra of GNC, BB9 and dye loaded GNC. (b) FTIR spectra of EC, BB9 and dye loaded EC.

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FTIR spectra of adsorbents giving the broad band of OH stretching vibrations at 3100–3700 cm⁻¹ comprise both free and hydrogen bonded hydroxyl groups. On adsorption of the dye by adsorbent, the intensity of sharp peaks corresponding to C⚌N decreases and shifted from 1591 to 1516 cm⁻¹ and intensity of the peak decreases for the bond C⚌S. In EC the band at 1252 cm⁻¹ indicates the presence of carboxylic group. FTIR spectra of BB9 + GNC and BB9 + EC show that peaks in the low frequency region (<1000 cm⁻¹) which are present in the dye alone are not observed in the dye loaded adsorbents because of the adsorption of dye BB9 on the adsorbent surface. Decreased intensity of sharp peaks concluded that the dye has been functionalized by both the adsorbents.

3.2 X-ray diffraction studies

X-ray diffraction (XRD) technique is a powerful technique to analyze the crystalline and amorphous nature of the material under investigation. In crystalline material, well defined peaks are observed whereas in non crystalline or amorphous material shows broad peaks instead of sharp peaks. Fig. 3(a,b) shows that well defined peaks are observed in the adsorbent EC as compared to GNC which indicates that EC is more crystalline as compared to GNC. When the adsorbent gets loaded by the dye molecules the crystalline nature of the dye was changed into amorphous nature. It has been concluded that the dye molecules diffused into the micro and macro pores of the adsorbent molecules. XRD studies show change in crystallinity of the adsorbent due to adsorption.

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Figure 3

(a) XRD spectra of GNC, BB9 and dye loaded GNC. (b) XRD spectra of EC, BB9 and dye loaded EC.

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3.3 Scanning electronic micrograph studies (SEM)

SEM is widely used to study the morphological feature and surface characteristics of the adsorbent material. In the present study, SEM images of adsorbent before and after adsorption of dye BB9 reveal the porosity and surface texture. In Fig. 4, the surface of dye-loaded adsorbents shows that the surface of GNC and EC is covered with dye molecules.

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Figure 4

Scanning electron microscope (SEM) images (a) Ground nut shells charcoal (GNC) (b) dye adsorbed GNC (c) Eichhornia charcoal (EC) and (d) dye adsorbed EC.

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3.4 Effect of contact time

The equilibrium time is the time needed when maximum adsorption takes place. Fig. 5 shows the effect of contact time on the adsorption of dye BB9 onto GNC and EC at different temperatures. The nature of adsorption process will depend on the physical and chemical characteristics of the adsorbents and also on the system conditions. The amount of dye adsorbed increases with increase in contact time. The contact time curve shows rapid adsorption of BB9 in first 15 min by both the adsorbents followed by the gradual increase of adsorption rate and ultimately reaches saturation. The initial rapid phase may be due to increased number of available vacant sites of the adsorbent at the initial stage. The equilibrium time for EC is 30, 25, 15 min and for GNC 45, 20, 18 min at 303, 308 and 313 K, respectively.

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Figure 5

Effect of contact time on the adsorption of BB9 on GNC and EC (Co = 14 mg/L).

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3.5 Effect of adsorbent dose

Fig. 6 shows the effect of adsorbent dose on the percentage removal of BB9 by GNC and EC as adsorbents. With increase in the adsorbent dose of GNC and EC from 0.1 to 1.0 g the percentage removal increases up to 96.4% and 95.8% respectively. A large mass of the adsorbent could adsorb large amount of dye due to the availability of more adsorption sites and more surface area of adsorbent.

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Figure 6

Effect of adsorbent dose on percentage dye removal (Co = 14 mg/L).

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3.6 Effect of temperature

Temperature is an important factor for the adsorption process. To examine the effect of temperature, an adsorption study of BB9 was performed at three different temperatures, and is illustrated in Fig. 5. It has been believed that temperature generally has two major effects on the adsorption process. Increasing the temperature will increase the rate of diffusion of the adsorbate molecule across the external boundary layer and in the internal pores of the adsorbent particles. In addition, changing the temperature will change the equilibrium capacity of particular adsorbate (Wang and Li, 2007). Normally adsorption is exothermic in nature, so it was expected that with increase in temperature, adsorption capacity of adsorbents must be decreased. However if adsorption process is controlled by diffusion process (intra-particle transport diffusion), the sorption capacity will increases with increase in temperature due to endothermicity of diffusion process. An increase in temperature results in an increased mobility of the adsorbate and a decrease in the retarding forces acting on the diffusing adsorbate. Increase in adsorption capacity with increase in temperature may be partly attributed to the chemisorptions (Lakshmi et al., 2009).

3.7 Effect of pH

The pH of the solution affects the surface charge of the adsorbents as well as the degree of ionization of the materials present in the solution. The hydrogen ions and hydroxyl ions are adsorbed quite strongly, and therefore, the adsorption of other ions is affected by the pH of the solution. The change of pH affects the adsorptive process through the dissociation of functional groups on the active site of the adsorbent. This subsequently leads to the shift in reaction kinetics and equilibrium characteristics of the adsorption process .It is a common observation that the surface adsorbs anion favorably at lower pH due to the presence of H⁺ ions, whereas, the surface is active for the adsorption of cations at higher pH due to the deposition of OH⁻ ions (Mall et al., 2005).

Fig. 7 shows the influence of pH on the adsorption capacity of the adsorbents for the removal of BB9. The pH range of the solution considered for this investigation was 2 to 12. The variation of adsorption with pH can be explained by considering the difference in the structures of dyes, as well as nature of the adsorbent. At lower pH the surface has high positive charge density, and under these conditions the uptake of positively charged BB9 would be low due to electrostatic repulsion. With increasing pH, the negative charge density on the surface of adsorbent increases, resulting in an enhancement in the removal of BB9. Also lower adsorption of BB9 at acidic pH is due to the presence of excess H⁺ ions competing with the dye cation for the adsorption. For this reason basic pH (10–12) was selected for subsequent experimental work. Similar trend was observed by other workers for the adsorption of basic dye on bagasse ash (Mall et al., 2005).

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Figure 7

Effect of pH on adsorption of dye BB9 on GNC and EC (Co = 14 mg/L adsorbent dosage = 1 g/L).

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In order to confirm these results, the pH of point of zero charge (pH_pzc) of samples was carried out by the procedure described by Rivera-Utrilla et al. (2001). The pH_pzc is defined by the point where the curve pH_initial vs pH_final crosses the line pH_initial = pH_final. The pH_pzc was determined and the values obtained for GNC and EC were at pH 9.1 and pH 7.8 respectively. The pH above pH_pzc, the surface of the adsorbent is negative and there is strong electrostatic attraction between the surface group and the dye BB9 (Saka and Sahin, 2011).

3.8 Effect of initial concentration

A study of the effect of change in the amount adsorbed with initial concentration of the dye was carried out. From the Fig. 8 it is evident that BB9 percentage removal decreases with increase in Co, but the actual amount of the dye adsorbed for both the adsorbents increased with increase in adsorbate concentration. It is due to decrease in the resistance to the uptake of BB9 from the solution which increases the diffusion of the dye.

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Figure 8

Effect of initial concentration of dye BB9 on GNC and EC. Adsorbent dosage = 1 g/L (□-% color removal, ●-q_e (mg/g)).

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3.9 Effect of ionic strength

In dye processing, NaCl, KCl and Na₂SO₄ salts are used to enhance the bath exhaustion (Karadag et al., 2007). Different concentrations of NaCl and KCl (0.01–0.05 N) were added to the aqueous solutions of dye to investigate the effect of ionic strength on dye adsorption. Fig. 9 shows that increase in ionic strength causes decrease in adsorption of dye BB9 on both the adsorbents. In the literature, same effect has been reported for some cationic dyes (Karadag et al., 2007). It has been reported that an increase in ionic strength leads to decrease of the thickness of the electrical double layer and finally decreases the adsorption capacity of cationic dyes.

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Figure 9

Effect. of ionic strength on adsorption of BB9 on GNC and EC. 1 – NaCl, 2 – KCl (Co = 14 mg/L, adsorbent dosage = 1 g/L).

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3.10 Adsorption kinetic study

3.10.1

3.10.1 Pseudo first order and pseudo second order models

The pseudo-first-order equation is given as (Lagergren, 1898).

(1)

$$\frac{{\text{dq}}_{\text{t}}}{\text{dt}}=k_{\text{f}}(q_{\text{e}}-q_{\text{t}})$$ where q_t is the amount of dye adsorbed at time t (mg/g). q_e is the adsorption capacity at equilibrium(mg/g), k_f (min⁻¹) is the pseudo first orders rate constant, and t is the contact time (min). The integration of Eq. (1) with initial condition (q_t = 0 at t = 0) leads to following equation:

(2)

$$\log (q_{\text{e}}-q_{\text{t}})=\log q_{\text{e}}-\frac{k_{\text{f}}}{2.303}\text{t}$$ The values of k_f for the adsorption of dye BB9 on GNC and EC which are determined by the linear plot of log (q_e−q_t) vs t are given in Table 1., Fig. 10.

Table 1 Kinetic parameters for removal of BB9 on GNC and EC.

Equations	Parameters	Adsorbents
		GNC			EC
		303 K	308 K	313 K	303 K	308 K	313 K
Pseudo first order	qe exp (mg/g)	10.13	11.52	11.58	11.20	11.31	11.72
	qe cal (mg/g)	2.57	1.65	1.47	4.28	11.32	11.57
	kf (min−1)	0.09	0.12	0.08	0.20	0.33	0.46
	R2	0.97	0.98	0.98	0.96	0.90	0.92
Pseudo-second-order	qe exp (mg/g)	10.13	11.52	11.58	11.20	11.31	11.72
	qe cal (mg/g)	10.44	11.32	11.58	11.43	11.58	12.12
	ks (g/mg min)	0.066	0.180	0.188	0.123	0.116	0.127
	R2	0.99	0.99	0.99	0.99	0.99	0.99
Intra-particle diffusion	kid (mg/g min1/2)	0.29	0.39	0.48	0.43	0.47	0.64
	C (mg/g)	8.23	9.76	9.39	9.04	9.08	9.18
	R2	0.98	0.99	0.98	0.97	0.99	0.98
Bangham	ko (g)	58.93	73.77	71.16	108.9	104.4	102.4
	α	0.064	0.040	0.060	0.176	0.138	0.133
	R2	0.97	0.98	0.98	0.98	0.98	0.98
Elovich	a	2.3 × 105	8.5 × 107	3.5 × 106	7.2 × 105	4.5 × 105	0.6 × 105
	b	1.65	1.94	1.63	1.54	1.47	1.20
	R2	0.97	0.98	0.98	0.98	0.98	0.98

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Figure 10

Pseudo-first-order kinetic plots for the removal of BB9 on GNC and EC (Co = 14, mg/L, adsorbent dosage = 1 g/L).

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The pseudo-second-order model is represented as (Ho and Mckay 2000).

(3)

$$\frac{{\text{dq}}_{\text{t}}}{\text{dt}}=k_{\text{s}}{(q_{\text{e}}-q_{\text{t}})}^2$$ where k_s is the pseudo second order rate constant (g mg⁻¹ min⁻¹). Integrating Eq. (3) (q_t = 0 at t = 0) gives:

(4)

$$\frac{t}{q_{\text{t}}}=\frac{1}{k_{\text{s}}{q}_{\text{e}}^2}+\frac{1}{q_{\text{e}}}t$$ Fig. 11 shows the plot of t/q_t versus t. The equilibrium adsorption capacity, q_e is obtained from the slope and k_s is obtained from the intercept. The values are given in Table 1. The q_e experimental and the q_e calculated values from the pseudo second-order kinetic model are very close to each other. The calculated correlation coefficients (R² = 0.99) are also close to unity for pseudo-second order kinetic than that for the pseudo-first order kinetic model. Therefore, the sorption can be approximated more appropriately by pseudo second order kinetic model than the first-order kinetic model for both the studied adsorbents.

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Figure 11

Pseudo-second order kinetic plots for the removal of BB9 on GNC and EC (Co = 14, mg/L, adsorbent dosage = 1 g/L).

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3.10.2

3.10.2 Bangham's equation

Kinetic data were further used to know about the slow step occurring in the present adsorption system using Bangham's equation (Tutem et al., 1998).

(5)

$$\text{Log}(C_{\text{o}}/C_{\text{o}}-q_{\text{t}}m)=\mathit{log}(k_{\text{o}}m/2.303V)+\alpha \text{log}t$$ where C_o is the initial concentration of dye in solution (mg/L), V is the volume of the solution (mL), m is the weight of adsorbent per liter of solution (g/L), q_t (mg/g) is the amount of dye adsorbed at time t and α(<1) and k_o are constants. The double logarithmic plots are shown in Fig. 12. Linear plot shows that the diffusion of adsorbate into the pores of adsorbents is not the only rate controlling step (Tutem et al., 1998)

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Figure 12

Bangham's plots for the removal of BB9 on GNC and EC (Co = 14 mg/L, adsorbent, dosage = 1 g/L).

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3.10.3

3.10.3 Intra-particle diffusion study

An empirically found functional relationship common to most adsorption process, is that the uptake varies almost proportionally with t_½, the Weber–Morris plot, rather than with the contact time, t (Weber and Morris, 1963).

(6)

$$q_{\text{t}}=k_{\text{id}}{t}_{1/2}+C$$ where k_id is the intra-particle diffusion rate constant. According to Eq. (6), a plot of q_t versus t_½ should be a straight line with a slope k_id and intercept C when adsorption mechanism follows the intra-particle diffusion process. Values of the intercept gives an idea about the thickness of boundary layer i.e. larger the intercept the greater is the boundary layer effect (Kannan and Sundaram, 2001). Plot of q_t vs t_1/2 is illustrated in Fig. 13. The linearity is attributed to the macropore diffusion, which is an accessible site of adsorption. This is attributed to the instantaneous utilization of the most readily available adsorbing sites on the adsorbent surface .The values of k_id obtained from the slope of plots are listed in Table 1.

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Figure 13

Weber and Morris intra-particle diffusion plots for the adsorption of BB9 on GNC, and EC (Co = 14 mg/L, adsorbent dosage = 1 g/L).

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3.10.4

3.10.4 Elovich model

The Elovich equation generally used is expressed as (Wang et al., 2009).

(7)

$$q_{\text{t}}=\frac{1}{b}\text{ln}(\mathit{ab})+\frac{1}{b}\text{lnt}$$ where q_t (mg/g) is the amount of dye adsorbed at time t (min) and a and b are the constants. The chemical significance of these constants has not been clearly resolved (Wang et al., 2009). Fig. 14, shows the plot q_t versus ln t having slope 1/b and intercept [(1/b) ln(ab)].The values of a and b are given in Table 1.

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Figure 14

Elovich kinetic plots for the removal of BB9 on GNC and EC (Co = 14 mg/L, adsorbent dosage = 1 g/L).

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3.11 Adsorption Isotherms

Various isotherm equations have been used to describe the equilibrium nature of adsorption.

3.11.1

3.11.1 Langmuir isotherm

The linear form of Langmuir Isotherm is represented by the following equation.

(8)

$$C_{\text{e}}/q_{\text{e}}=C_{\text{e}}/C_{\text{m}}+1K_{\text{L}}{C}_{\text{m}}$$ where C_e is the concentration of dye solution (mg/L) at equilibrium. The constant C_m signifies the adsorption capacity (mg/g) and K_L (L/mg) is related to the energy of adsorption. Langmuir plot is shown in Fig. 15. The values of K_L and C_m (monolayer concentration) were calculated from the intercept and slope of the plots are given in Table 2. Maximum adsorption capacity increased with an increase in the temperature revealing the endothermic nature of the adsorption process. In the present work the maximum adsorption capacity was found to be 16.4 and 21.8 mg/g for GNC and EC respectively for the studied dye BB9. However activated clay shows the capacity of 57.8 mg/g for acid blue 9 (Gupta and Suhas, 2009) and mixture of activated clay and chitosan enhances the adsorption capacity to 330 mg/g for the dye BB9 (Wan Ngah et al., 2011).

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Figure 15

Langmuir isotherm plots for the removal of BB9 on GNC and EC (Co = 14 mg/L, adsorbent dosage = 1 g/L).

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Table 2 Isotherms parameters for removal of BB9 on GNC and EC.

Equations	Parameters	Adsorbents
		GNC			EC
		303 K	308 K	313 K	303 K	308 K	313 K
Freundlich	KF (mg/g)(L/mg)1/n	6.09	6.85	7.74	6.56	6.75	7.42
	N	3.21	2.56	2.04	2.22	2.06	2.06
	R2	0.98	0.98	0.99	0.97	0.99	0.99
Langmuir	KL (L/mg)	1.06	1.02	0.93	0.73	0.66	0.53
	Cm (mg/g)	11.29	13.45	16.36	15.21	16.79	21.76
	R2	0.99	0.98	0.98	0.99	0.99	0.96
Dubinin Radushkevih	qs (mg/g)	9.38	10.55	11.36	11.39	11.66	13.19
	E (kJ/mol)	1.69	1.88	2.21	1.56	1.65	1.73
	R2	0.97	0.98	0.98	0.99	0.98	0.99
Temkin	KT (L/mg)	12.67	8.60	7.38	5.56	5.03	4.37
	B1	2.35	3.17	3.94	3.75	4.11	5.14
	R2	0.98	0.97	0.99	0.98	0.99	0.98
Generalized	N	0.99	0.97	0.99	1.01	0.99	0.89
	KG (mg/L)	6.20	3.01	1.03	1.37	1.52	1.79
	R2	0.99	0.99	0.99	0.98	0.99	0.99

To identify the feasibility and favorability of the adsorption process, an approach recommended by Weber and Chakrabarti (Weber and Chakrabarti, 1974) was adopted as dimensionless constant, separation factor (R_L) (Hall et al., 1966) was calculated in each case using the following equation:

(9)

$$R_{\text{L}}=1/(1+K_{\text{L}}{C}_{\text{o}})$$ where C_o is the initial dye concentration (mg/L). The values of ‘R_L' were found to be less than unity for both the studied adsorbents (Table 3) and imply highly favorable adsorption for the dye BB9.

Table 3 Values of R_L and thermodynamic parameters for the adsorption of BB9.

Adsorbents	Temp. (K)	RL values	−ΔG (kJ mol−1)	ΔH (kJ mol−1)	ΔS (JK−1 mol−1)	R2
GNC	303	0.063	32.06	10.29	71.97	0.97
	308	0.065	32.49
	313	0.071	32.79
EC	303	0.089	31.14	25.19	19.78	0.97
	308	0.097	31.94
	313	0.112	31.33

3.11.2

3.11.2 Freundlich isotherm

This isotherm is an empirical equation employed to describe the heterogeneous system (Crini and Peindy, 2006). Freundlich isotherm is also applied to plot the equilibrium data of the adsorption.

(10)

$$\text{log}x/m=\log K_{\text{F}}+1/n\log C_{\text{e}}$$ where x is the amount of dye adsorbed (mg), m is the weight of the adsorbent used (g), C_e is the equilibrium concentration of the dye in solution (mg/L) and K_F and n are Freundlich constants, n is heterogeneity factor (adsorption intensity) and K_F denotes the adsorption capacity. The values of n > 1, reflecting the favorable adsorption conditions (Crini and Peindy, 2006). Freundlich plots are shown in Fig. 16 and linearity suggests that fit is well for the adsorption system under the studied concentration range. The values of n and K_F are calculated from the slopes and intercepts of the linear plots are recorded in Table 2. Increase in adsorption capacity (K_F) with temperature substantiated the endothermic adsorption.

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Figure 16

Freundlich isotherm plots for the removal of BB9 on GNC and EC (Co = 14 mg/L, adsorbent dosage = 1 g/L).

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3.11.3

3.11.3 Dubinin and Radushkevich (D–R) isotherm

This isotherm expressed as follows (Dubinin, 1960):

(11)

$$q_{\text{e}}=q_{\text{s}}\exp (-B{∊}^2)$$ where q_s is D–R constant and ∊ can be correlated as

(12)

$$∊=\text{RT}\ln (1+1/C_{\text{e}})$$ The constant B gives the mean free energy E of the adsorption per molecule of adsorbate when it is transferred into the surface of the solid from infinity in the solution and can be calculated using the following equation:

(13)

$$E=1/{(2B)}^{1/2}$$ The D–R isotherm plotted against experimental values is shown in Fig. 17. The calculated D–R constant is given in Table 2. It is clear that adsorption energy value increased with increase in the temperature for the adsorption of BB9 on GNC and EC intimating endothermic behavior.

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Figure 17

D-R isotherm plots for the removal of BB9 on GNC and EC (Co = 14 mg/L, adsorbent dosage = 1 g/L).

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3.11.4

3.11.4 Temkin isotherm

The Temkin isotherm equation assumes that the fall in the heat of adsorption of all the molecules in the layer decreases linearly with coverage due to adsorbent–adsorbate interactions, and that the adsorption is characterized by a uniform distribution of the binding energies up to some maximum binding energy (Aharoni and Ungarish, 1977). The Temkin isotherm has been applied in the following form.

(14)

$$q_{\text{e}}=B_1\ln K_{\text{T}}+B_1\ln C)e$$ where B₁ = RT/b.

A plot of q_e verses ln C_e enables the determination of the isotherm constant K_T and B₁. K_T is the equilibrium binding constant (L/mg) corresponding to the maximum binding energy and constant B₁ is related to the heat of adsorption. The Temkin isotherm plotted against the experimental values are shown in Fig. 18 and the values of the constants are given in Table 2.

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Figure 18

Temkin isotherm plots for the removal of BB9 on GNC and EC (Co = 14 mg/L, adsorbent dosage = 1 g/L).

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3.11.5

3.11.5 Generalized isotherm

The generalized isotherm is given as (Kargi and Ozminci, 2004).

(15)

$$\ln [(q_{\max }/q_{\mathrm{e}})-1]=\ln K_{\mathrm{G}}-N\ln C_{\mathrm{e}}$$ where K_G is the saturation constant (mg/L), N is the cooperative binding constant, q_max is the maximum adsorption capacity of the adsorbent (mg/g). q_e (mg/g) and C_e (mg/L) are the equilibrium dye concentrations in the solid and liquid phase, respectively. A plot of ln[q_max/q_e−1] verses ln C_e is shown in Fig. 19. The values of K_G and N are calculated from the slope and intercept of the plot and these values are given in Table 2.

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Figure 19

Generalized isotherm plots for the removal of BB9 on GNC and EC (Co = 14 mg/L, adsorbent dosage = 1 g/L).

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3.12 Thermodynamic parameters

Thermodynamic parameters (changes in standard Gibb's free energy ΔG, enthalpy change ΔH, and entropy change ΔS) were calculated using the following equations: