Cucumis sativus used as adsorbent for the removal of dyes from aqueous solution

2.1 Materials

The commercial grade CV and RH B, models of the cationic dye with molecular formula of C₂₅H₃₀ClN₃ and C₂₈H₃₁ClN₂O₃ were used in the present study. Crystal violet (color index No. 42555) with molecular weight 407.99 and λ_max 584 nm are obtained from Thomas baker (chemicals) Ltd., Mumbai, India and rhodamine B (color index No. 45170) with molecular weight 479.02 and λ_max 554 nm are obtained from Qualigens fine chemicals Mumbai, India. The molecular structures are illustrated in Fig. 1. All the chemicals used throughout this study were of analytical-grade reagents and the adsorption experiments were carried out at room temperature (27 ± 2 °C).

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Figure 1

Structure of (a) Crystal violet dye (b) Rhodamine B.

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2.2 Adsorbent preparation

The fruit peel of C. Sativus used for the present study was collected from Palamudhir Nilyam, Coimbatore. The fruit peel was crushed and dried in an oven and used as a raw adsorbent (RCS). The oven dried peel of the adsorbent was treated with conc. H₂SO₄ for 12 h and was washed thoroughly with distilled water till it attained neutral pH and soaked in 2% NaHCO₃, for overnight in order to remove any excess of acid present. Then the material was washed with distilled water and dried. The material thus obtained was designated into activated carbon (CCS). The materials were sieved to get different geometrical sizes such as 75–125, 125–250 and 250–500 mμm.

2.3 Characterization of prepared adsorbents

Determination of zero point charge (pH_zpc) was done to investigate the surface charge of both chemically and microwave activated adsorbents at different solution pH.

2.4 Dye uptake experiments

The adsorption experiments were carried out in a batch process to evaluate the effect of pH, contact time, adsorbent dose, adsorption kinetics, adsorption isotherm of CV and RH B onto RCS and CCS. For each experiment, a series of flasks were prepared with 50 mL of dye solution (25–200 mg/L, respectively) and the pH was adjusted from 2 to 8 using a pH meter (Deluxe pH meter, model-101 E). About 0.2 g of the sorbent was added, and the flasks were agitated at 160 rpm. The sorbent was removed by centrifugation and the supernatant was analyzed using a Systronic Spectrophotometer-104 at wavelength of 584 nm.

The amount of dye adsorbed at equilibrium onto carbon, q_e (mg g⁻¹) was calculated by the following mass balance relationship.

3.1 Characterization of RCS and CCS

Table 1 shows the physico-chemical characteristics of the adsorbents. The moisture content was 18.74% for RCS whereas only 11.62 were observed for CCS. Moisture content not only decreases the adsorption sites but also causes additional weight (Sajjad et al., 2011). The ash content might produce defect (dangling carbon) in the elementary structure of charcoal leading to high adsorption capacity.

3.2 Batch adsorption studies

These studies enabled in determining the optimum operating conditions and to find out the effects of various parameters (like, the initial dye concentration, the solution pH and the contact time) on the percentage removal of dyes. The randomness is increased during adsorption process, resulting in better contact between the adsorbate and adsorbent. Consequently, this led to the enhanced rate of adsorption. The amount of dye adsorbed (mg/g) increased with increasing agitation time and reached equilibrium after 90 min for crystal violet and 80 min for rhodamine B for the initial dye concentration (25–200 mg/L) onto RCS and CCS.

The increase in uptake capacity of the adsorbent with increasing dye concentration may be due to the increase of sorbate quantity. At lower initial dye concentration, sufficient adsorption sites are available for the adsorption of dye ions. Conversely, the numbers of dye ions at higher initial concentration are relatively more as compared to the available adsorption sites. Similar trends are reported by Atmani et al. (2009) for CV onto almond waste and Ali and Muhammad (2008) for CV onto Calotropis procera.

The effect of solution pH (range of 2–8) on the amount of removal at a fixed adsorbent dosage (0.2 g) is shown in Fig. 2 for both dyes under consideration. From Fig. 2, it is evident that the maximum removal of color is observed at pH 7 for CV and RH B onto RCS, pH 6 for CV and RH B onto CCS. The percent removal of dyes increases with an increase in the pH and reaches the maximum up to 72.27% for CV onto RCS, 84.98% for CV onto CCS, 81.69% for RH B onto RCS and 90.26% for RH B onto CCS. This may be attributed to the hydrophobic nature of the developed carbon which led to absorb hydrogen ions (H⁺) onto the surface of the carbon when immersed in water and make it positively charged. Low pH value (1.0–3.0) leads to an increase in H⁺ ion concentration in the system and the surface of the activated carbon acquires positive charge by absorbing H⁺ ions. On the other hand, an increase in the pH value led to the increase in the number of negatively charged sites. As the adsorbent surface is negatively charged at high pH, a significantly strong electrostatic attraction appears between the negatively charged carbon surface and the cationic dye molecule leading to maximum adsorption of CV and RH B from waste water.

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Figure 2

Effect of solution pH for the removal of CV and RH B onto RCS and CCS.

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The lowest adsorption occurred at pH 2.0 and the greatest adsorption occurred at pH ∼6.0 and 7.0. Adsorbent surface would be positively charged up to pH < 4, and heterogeneous in the pH range 4–5. Thereafter, it should be negatively charged. Moreover, the increase in the adsorption of dye with the increasing pH value is also due to the attraction between cationic dye and excess OH⁻ ions in the solution. The adsorption capacity and rate constant have the tendency to increase as initial pH of the solution increases. This is due to the pH_zpc of the adsorbents which has an acidic value; this is favorable for cation adsorption. It is a commonly known fact that the anions are favorably adsorbed by the adsorbent at lower pH values due to the presence of H⁺ ions. At high pH values, cations are adsorbed due to the negatively charged surface sites of the adsorbents. The optimum pH value of CV and RH B onto RCS was pH 7 and CV and RH B onto CCS was pH 6.

3.3 Adsorption

In order to optimize the design of an adsorption system to remove the dye, it is important to establish the most appropriate correlations of the equilibrium data of each system (Altinisik et al., 2010). Equilibrium isotherm equations are used to describe the experimental adsorption data. The parameters obtained from the different models provide important information on the adsorption mechanisms, surface properties and affinities of the adsorbent (Bulut et al., 2008). The correlation with the amount of adsorption and the liquid-phase concentration was tested with the Langmuir, Freundlich, Dubinin Radushkevich and Temkin adsorption isotherm equations. Linear regression is frequently used to determine the best-fitting isotherm, and the applicability of isotherm equations is compared by judging the correlation coefficients.

The Langmuir isotherm theory assumes monolayer coverage of adsorbate over a homogeneous adsorbent surface (Langmuir, 1918). Once a metal molecule occupies a site, no further adsorption can take place at that site. It is commonly expressed as follows:

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Figure 3

Langmuir isotherm plot for the adsorption of CV and RH B onto RCS and CCS.

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The results obtained from the Langmuir model are shown in Table 2. The sorption capacities of RCS and CCS for the selected dyes are followed the order: 40.81 mg/g (RH B onto RCS) > 35.34 mg/g (CV onto CCS) > 34.01 mg/g (RH B onto CCS) > 33.22 mg/g (CV onto RCS). Sorption capacities of low cost adsorbents for various dyes are reported by other researchers and are presented in Table 3, for comparison. From the values, we conclude that the maximum adsorption corresponds to a saturated monolayer of adsorbate molecules. The correlation coefficient (R² > 0.9880) showed strong positive evidence on the adsorption which follows the Langmuir isotherm.

The Freundlich isotherm (Freundlich, 1960) can be applied to non-ideal adsorption on heterogeneous surfaces as well as multilayer sorption. The Freundlich isotherm can be derived assuming a logarithmic decrease in the enthalpy of adsorption with the increase in the fraction of occupied sites and is commonly given by the following non-linear equation:

A plot of log q_e versus log C_e enables to determine the constants K_f and 1/n. K_f is roughly an indicator of the adsorption capacity, related to the bond energy, and 1/n is the adsorption intensity of dyes onto the adsorbent of surface heterogeneity. The magnitude of the exponent, 1/n, gives an indication of the favorability of adsorption. A value for 1/n below one indicates a normal Langmuir isotherm, while 1/n above one is indicative of cooperative adsorption (Fytianos et al., 2000). It is lower than the experimental amounts corresponding to the adsorption isotherm plateau, which is unacceptable.

The D–R isotherm was also applied to estimate the porosity apparent free energy and the characteristics of adsorption (Dubinin, 1960; Radushkevich, 1949). It can be used to describe adsorption on both homogenous and heterogeneous surfaces (Shahwan and Erten, 2004). The D–R equation can be defined by the following equation.

Temkin and Pyzhey (1940) considered the effects of some indirect adsorbate or adsorbate interactions on adsorption isotherms and suggested that because of these interactions the heat of adsorption of all the molecules in the layer would decrease linearly with coverage. The Temkin isotherm has commonly been applied in the following form:

The constant β is related to the heat of adsorption (Pearce et al., 2003). The adsorption data were analyzed according to the linear form of Temkin isotherm Eq. (10). The linear isotherm constants and coefficients of determination are presented in Table 2. The correlation coefficients R² obtained from Temkin model were comparable to that obtained for Langmuir and Freundlich and D–R equations, which explain the applicability of Temkin model to the adsorption of CV and RH B onto RCS and CCS.

It can be seen that the Langmuir isotherm fits the data better than Freundlich, D–R, and Temkin isotherms. The fact that it may be due to the predominantly homogeneous distribution of active sites on the activated carbons surfaces; since the Langmuir equation assumes that the adsorbent surface is energetically homogeneous. This is also confirmed by the high value of R² in case of Langmuir. The maximum sorption capacity (Q_m) of the adsorption of CV and RH B onto RCS and CCS was compared with those reported in the literature (Table 3).

3.4 Adsorption kinetics

The study of adsorption kinetics describes the solute uptake rate and evidently this rate controls the residence time of the adsorbate at the solid solution interface. The rate of sorption can be computed from the kinetic study. In order to investigate the mechanism of sorption and potential rate controlling steps, several models have been developed. Thus, the kinetics were analyzed using pseudo-first-order (Lagergren, 1898), pseudo-second-order (Ho et al., 2000), Elovich (Chien and Clayton, 1980; Zeldowitsch, 1934) and intraparticle diffusion (Weber and Morris, 1963) kinetic models. The conformity between experimental data and the model-predicted values was expressed by the correlation coefficients (R², values close or equal to 1).

The pseudo first-order equation of Lagergren is generally expressed as follows:

The pseudo-second order equation is generally given as follows:

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Figure 4

Pseudo second-order kinetics at different initial concentrations for (a) CV onto RCS and CCS (b) RH B onto RCS and CCS.

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The Elovich equation is another rate equation based on the adsorption capacity generally expressed as follows.

It is simplified by assuming A_EB_Et ≫ t and by applying the boundary conditions q_t = 0 at t = 0 and q_t = q_t at t = t Eq. (18) becomes:

When the diffusion (internal surface and pore diffusion) of dye molecule inside the adsorbent is the rate-limiting step, then adsorption data can be presented by the following equation:

The fitted linear regression plots showed that the experimental data had its best fit with the pseudo-second-order model for all four investigated concentrations with higher determination coefficients (R² > 0.995) than those of the pseudo-first-order model and Elovich model. Therefore, it is found that the rate of CV and RH B adsorption onto RCS and CCS is of pseudo-second-order, suggesting that the adsorption of dye onto adsorbents is influenced by both the dye and the adsorbent concentrations under the investigated conditions (Abramian and El-Rassy, 2009). The fitted linear regression plots of pseudo-second-order model are shown in Fig. 5. In addition, the experimental adsorption capacity (q_{e (exp)}) values were very close to the model-calculated adsorption capacity (q_{e (cal)}) data (Table 4), verifying the high correlation of adsorption to the pseudo-second-order model.

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Figure 5

Intra particle diffusion plot at different initial dye concentrations for CV and RH B onto RCS and CCS.

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Intraparticle diffusion was analyzed using Eq. (20). The plots of intraparticle diffusion of CV and RHB onto RCS and CCS at the initial concentrations of 50, 100, 150, 200 mg/L are illustrated in Fig. 5. Referring to Fig. 5, the linearity of the plots demonstrated that intraparticle diffusion played a significant role in the uptake of CV and RH B. However, there is no sufficient indication about it. The author Ho (Ho, 2003) has shown that if the intraparticle diffusion is the sole rate-limiting step, it is essential for the qt versus t^1/2 plots to pass through the origin, which is not the case in Fig. 5, it may be concluded that boundary layer diffusion and intraparticle diffusion were concurrently operating during the adsorbate and adsorbent interactions.