Adsorption kinetic and isotherm studies of Azure A on various activated carbons derived from agricultural wastes

2.1 Chemicals, reagents, adsorbent preparation and activation

Azure A (AA) dye supplied by Loba Chemie, India is used as an Adsorbate and λ_max value is 625–634 nm. All the other chemicals used in this study were of analytical grade and used as received.

The adsorbents required for this study are prepared from Indian almond shell, ground nut shell, areca nut shell, tamarind shell and cashew nut shell. They were collected locally and carbonized at 400 °C. Activation was done by treatment with acid, washed and heated at 120 °C for one hour in an air oven. Finally they were sieved to discrete particle sizes. All the solutions and reagents were prepared by double distilled water.

2.2 Instrumentation study

The SEM images of activated carbons were taken from scanning electron microscope (Hitachi S-3400 N). The FT-IR spectra of activated carbons were recorded by a Shimadzu Spectrometer. Hanna pen pH metre was used to study the effect of pH. The activated carbons were separated into different particle sizes with the help of a mechanical siever (Jayant brand, India). The optical density (OD) of dye solution was measured by using an Elico UV–visible spectrophotometer (model: SL207).

2.3 Batch mode adsorption studies

Exactly 50 ml of dye AA solutions of different initial concentrations was prepared in a standard measuring flask from the stock solution. A standard graph is drawn by connecting OD against concentration. The concentration of dye solution before and after adsorption is calculated from the standard graph. The batch mode experiments were conducted at room temperature (30 ± 1 °C). The dye AA solutions of different initial concentrations (C_i = 5–100 ppm) were prepared. Exactly 50 ml of AA solutions of various initial concentrations was taken and treated with dose of adsorbents (cac and all activated carbons = 2.0 g L⁻¹) of fixed particle size (90 micron) at initial pH (7.1) and shaken in a thermostatic mechanical shaker for 30 min with 200 rpm at 30 ± 1 °C. The effect of contact time was studied by keeping initial concentration, dose, pH and particle size constant except time (5–60 min). The effect of dose variation was studied by kept initial concentration, contact time, pH and particle size constant except dose (70–130 mg for all activated carbons and cac). The effect of pH was studied by keeping initial concentration, contact time, dose and particle size constant except pH (2–11 for all activated carbons and cac). The effect of particle size was studied by kept initial concentration, contact time, dose and pH constant except particle size (90–250 micron for all activated carbons). The percentage of AA removal is calculated using the following relationship

The amount adsorbed in (mg g⁻¹) is calculated using the following relationship.

C_i and C_f be the initial and final concentrations (mg L⁻¹) of dye respectively and ‘m’ be the mass of activated carbon (mg L⁻¹).

3.1 Effect of initial concentration

The batch type adsorption studies on the extent of removal of dye AA on activated carbons (IASC, GSC, ASC, TSC and CSC) and CAC were carried out at different initial concentrations (AA on activated carbons = 5–100 ppm and AA on CAC = 350–600 ppm) with a fixed dose (2 g L⁻¹) and contact time 30 min at room temperature (30 ± 1 °C). The percentage removals of dye AA on all the carbons tend to decrease with an increase in the initial concentration (Fig. 1). This may be due to lack of available active sites required for the high initial concentration of AA. At lower concentrations a substantial amount of AA got adsorbed leaving behind more number of active sites on the adsorbents and at higher concentrations lesser number of active sites remained available. Similar results have been reported in the literature on the extent of removal of dyes (McKay, 1983) and metal ions (Kannan 1991).

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Figure 1

Effect of initial concentration of AA on various activated carbons.

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3.2 Effect of dose of adsorbents

The effect of dose of adsorbents on the extent of removal of dye AA on activated carbons (IASC, GSC, ASC, TSC and CSC) and CAC was studied. The percent removal of AA by activated carbons and CAC increases with an increase in dose of adsorbents (Fig. 2). This may be due to an increase in the availability of surface active sites.

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Figure 2

Effect of dose variation of AA on various activated carbons.

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3.3 Effect of pH

The effect of pH on adsorption process for AA on activated Carbons (IASC, GSC, ASC, TSC and CSC) and CAC was studied at different pH values (2–11). At low pH, the surfaces of activated carbons may become positively charged due to excess of H⁺ ions in solution. The electrostatic repulsion between AA and surfaces of adsorbents decreases the amount of AA adsorbed. At high pH, the surfaces of activated carbons may become negatively charged due to excess of OH^- ions in solution. Consequently electrostatic attraction between cationic dye and anionic surfaces of activated carbons increases the amount of AA adsorbed (Fig. 3). The maximum percentage removal of AA occurs at pH 10–11.

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Figure 3

Effect of pH variation of AA on various activated carbons.

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3.4 Effect of particle size

The effect of particle size of activated carbons (IASC, GSC, ASC, TSC and CSC) to remove AA by adsorption was studied by varying the particle size as 90, 125, 150, 212 and 250 micron. The amount of AA adsorbed increases with a decrease in particle size of the activated carbons (Fig. 4). When the particle size decreases, the surface area increases.

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Figure 4

Effect of particle size variation of AA on various activated carbons.

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3.5 Adsorption isotherm

The data obtained from the study of AA on Activated carbons (IASC, GSC, ASC, TSC and CSC) and CAC by batch adsorption were applied with Freundlich, Langmuir, Temkin, Redlich–Peterson and Dubinin–Radushkevich isotherms (Table 1).

3.5.1

3.5.1 Freundlich isotherm

(3)

$$\text{Freundlich isotherm}\left(\log \frac{x}{m}\right)=\log K+\left(\frac{1}{n}\right)\log C_e$$

In the above equation log K value is the intercept and (1/n) value is the slope (Freundlich,1906). The plot of log C_e against log x/m gives straight line (Fig. 5) and the correlation coefficient (r value) indicates the applicability of this isotherm. These values are shown in Table 1. The value of 1/n lower than unity shows that AA is adsorbed by the activated carbons (IASC, GSC, ASC, TSC and CSC) and CAC.

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Figure 5

Freundlich isotherm plot for adsorption of AA on various activated carbons.

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3.5.2

3.5.2 Langmuir isotherm

(4)

$$\text{Langmuir isotherm}\left(\frac{C_{\text{e}}}{q_{\text{e}}}\right)=\left(\frac{1}{Q_{0}b}\right)+\left(\frac{C_{\text{e}}}{Q_0}\right)$$

q_e – Amount of dye adsorbed per unit mass of adsorbent (mg g⁻¹).

x – Weight of dye adsorbed.

m – Weight of adsorbent.

Q₀ – Monolayer adsorption capacity (mg g⁻¹).

C_e – Equilibrium concentration (ppm).

b – Langmuir constant.

The Langmuir isotherm plot for AA on activated carbons (IASC, GSC, ASC, TSC and CSC) and CAC is found to be linear (Fig. 6) with correlation coefficient (r) closer to unity indicating that the adsorption data are best fitted to this isotherm (Langmuir 1918). The feasibility of the process is expressed in terms of separation factor R_L which is given by the equation.

(5)

$$R_{\text{L}}=\frac{1}{(1+{\mathit{bC}}_i)}$$

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Figure 6

Langmuir isotherm plot for adsorption of AA on various activated carbons.

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b – Langmuir constant.

C_i – Initial concentration of dye.

The separation factor R_L indicates the nature of isotherm and the feasibility of adsorption process as favourable. The R_L values are found to be in the range of 0–1, indicating that the adsorption process is favourable (Senthilkumar et al., 2005). R_L values for AA on activated carbons are 0.040–0.092 and AA on CAC is 0.013 which indicates that the adsorption process is favourable. The order of adsorption capacities (Q₀) of all carbons is as follows. CAC > IASC > GSC > ASC > TSC > CSC.

3.5.4

3.5.4 Redlich–Peterson isotherm

The linearised form of Redlich–Peterson equation is given below

(7)

$$\log \left[\left(\frac{K_{\text{R}}{C}_{\text{e}}}{q_{\text{e}}}\right)-1\right]=\beta \log C_{\text{e}}+\log (\alpha R)$$ $$K=Q_{0}b(\text{L}{\text{g}}^{-1})$$

Q₀ – Langmuir monolayer adsorption capacity (mg g⁻¹); b – Langmuir constant (L mg⁻¹); (αR) – R–P isotherm constant; β – Exponent of R–P isotherm; C_e – equilibrium/liquid phase concentration of solute (mg L⁻¹), m – mass of adsorbent (g L⁻¹); x – amount of solute adsorbed by solid adsorbent (mg L⁻¹); q_e – equilibrium amount of solute adsorbed (mg g⁻¹). By plotting the values of log [(K_RC_e/q_e) − 1] against log C_e, the values of (αR) and β can be determined from the intercept [log (αR)] and the slope (β) respectively (Fig. 7). The adsorption data perfectly fitted to this isotherm.

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Figure 7

Redlich–Peterson isotherm plot for adsorption of AA on various activated carbons.

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3.6 Kinetics of adsorption

The following equations were used to study the kinetics of adsorption of AA on all carbons and the results are given in Table 2.

First order equation

Natarajan and Khalaf equation

Bhattacharya and Venkobachar equation

k_ad (min⁻¹) is the rate constant of the adsorption and q_t and q_e are the amount adsorbed at time t and at equilibrium respectively. The R² values of all carbons indicate the applicability of this equation. The calculated values q_e found from Pseudo first order equation are lesser than the experimental q_e for all carbons (Table 3). The experimental data are tested for pseudo second order equation (Ho and McKay, 1999). The plot of t/q_t against time gives a straight line (Fig. 8). Pseudo second order equation:

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Figure 8

Pseudo second order plot for adsorption of AA on various activated carbons.

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k₂ is the rate constant of sorption (g mg⁻¹ min⁻¹), q_e is the amount of adsorbate at equilibrium (mg g⁻¹), q_t is the amount of adsorbate on the surface of the adsorbent at any time, t (mg g⁻¹).

The R² values of all carbons are found close to unity which obviously indicates that the adsorption of AA on activated carbons and CAC follows Pseudo second order. A normalized standard deviation, Δq (%) is calculated to compare the efficiency of adsorption isotherms

q_t^exp is the experimental value of amount adsorbed at different time t and q_t^cal is the calculated value of amount adsorbed at different time t n is the number of observations. The Δq (%) values of pseudo second order are small and it shows that the adsorption of AA on activated carbons and CAC follows pseudo second order kinetics. The calculated values q_e found from pseudo second order equation agreed well with experimental q_e for all carbons (Table 3). This suggests that adsorption of AA on activated carbons (IASC, GSC, TSC, ASC and CSC) and CAC obeys pseudo second order kinetics more efficiently than other kinetic models.

3.7 Intra-particle diffusion model

The intra-particle diffusion is tested by applying the intra-particle diffusion model.

q_t is the amount of dye adsorbed at time t. C is the intercept and K_p is the intra-particle diffusion rate constant (mg g⁻¹ min^½). The correlation coefficient (r) values of all carbons indicate the existence of intra-particle diffusion process. The intercepts give an idea about boundary layer thickness. The larger the intercept, the greater is the boundary layer effect (McKay, 1983). The values of intra-particle diffusion rate constant (K_p) are calculated from the slope of the curve. The values of correlation coefficient (r), K_p and intercept are shown in Table 2.

3.8 SEM analysis

The surface morphology of the best two activated carbons IASC and GSC before and after adsorption of AA dye is shown in Fig. 9. Before adsorption of AA both the carbons appeared as fine structures (Fig. 9A and C). In contrast, after adsorption of AA, the porous structures of both the carbons disappeared (Fig. 9B and D). This indicates that AA dye molecules are strongly adsorbed by the adsorbents IASC and GSC.

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Figure 9

SEM images of IASC and GSC before (A, C) and after adsorption (B, D).

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3.9 FT-IR study

The Fourier Transform Infra-Red (FT-IR) spectra of AA loaded IASC and unloaded IASC are shown in Fig. 10. The peak of unloaded IASC at 752.2 may be due to N-H bending. The peak at 1122.5 may be due to C⚌S stretching and peak at 1672.2 may be due to C⚌N stretching. The FT-IR spectra of AA loaded IASC indicate that there is a slight shift in the frequency of peaks at 754.1, 1120.6, and 1670.2 which may be due to adsorption of AA on the surface of IASC. The functional groups present on the surface of IASC are responsible for adsorption of AA (Fig. 11).

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Figure 10

FT-IR spectrum of AA on IASC and IASC.

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Figure 11

Structure of Azure A dye.

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