A new alternative adsorbent for the removal of cationic dyes from aqueous solution

2.1 Adsorbent (CAS)

A. squmosa seed was thoroughly washed with tap water to remove dirt and grime then rinsed a few times with distilled water and air-dried. A. squmosa seed was dried, ground and sieved to different particle sizes and preserved in desiccator until use.

Chemical activation using H₂SO₄ at moderate temperatures produces a high surface area and high degree of micro-porosity. The dried biomass (1.0 kg) was added to 1000 mL of 98% H₂SO₄ and kept for 12 h at room temperature, washed thoroughly with distilled water until it attained the neutral pH and soaked in 2% NaHCO₃ solution overnight in order to remove any excess acid present. The material was then washed with distilled water until it reached neutral pH and dried at 110 ± 2 °C. The dry carbon was crushed into granules, sieved to different particle sizes and then preserved in desiccator until use.

Scanning electron microscopy (SEM) (Philips 501), BET (a continuous flow Nelson Eggertson type surface area analyzer), X-ray diffraction (XRD) (Arhensdurg Germany (Model No. ID 3000)), Elemental analysis was carried out with a C.H.N. 1106 Carlo Erba MicroAnalysing device equipped with inductive furnace analyzer and FT-IR spectra (Shimadzu, Model FTIR-8201PC) analysis was carried out on the CAS to study its surface texture and functional groups responsible for the adsorption of dyes.

2.2 Adsorbate

A stock solution of 500 mg/L was prepared by dissolving the appropriate amount of Malachite green (MG) and Methylene blue (MB) (obtained from S.D. Fine Chemicals, Mumbai, India) in 1000 mL with distilled water. Different concentrations ranged between 25 and 200 mg/L of MG and MB were prepared from the stock solution. All the chemicals used throughout this study were of analytical-grade reagents. Double-distilled water was used for preparing all of the solutions and reagents. The initial pH is adjusted with 0.1 M HCl or 0.1 M NaOH. All the adsorption experiments were carried out at room temperature (27 ± 2 °C).

2.3 Batch adsorption studies

Batch experiments were conducted to investigate the parametric effects of pH, adsorbent particle size, adsorbent dose, initial adsorbate concentration and adsorption time for MG and MB adsorption on the prepared carbon. Dye samples were prepared by dissolving a known quantity in distilled water and used as a stock solution and diluted to the required initial concentration (range: 25–200 mg/L). Batch adsorption experiments were carried out in a rotary shaker at 150 rpm using 250 mL shaking flasks at room temperature. The adsorbent and adsorbate solutions were separated by centrifuging at 3000 rpm for 5 min. The concentration of dye in solution was measured by using Systronic UV–vis Spectrophotometer-104. All the experiments were duplicated and only the mean values are reported. The maximum deviation observed was less than ± 4%.

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

For kinetic studies, 200 mg of CAS was contacted with 50, 100, 150 and 200 mg/L dye solution (50 mL) using a shaker at 150 rpm at room temperature. At predetermined time intervals, the amount of uptake of dye was evaluated spectrophotometrically.

3.1 Effect of pH

pH is one of the most important factors affecting the adsorption process. In order to investigate the influence of pH on the MG and MB removal by CAS, experiments were carried out over a pH range of 2.0–9.0 at dye concentration 50 mg/ L, adsorbent dosage 0.2 g/50 mL. As shown in Fig. 1, the maximum removal efficiency, approximately 75.66% for MG and 24.33% for MB were achieved around pH 6.0.

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Figure 1

Effect of pH on the removal of MG and MB onto CAS (CAS dosage 200 mg/50mL, MG 100 mg/L).

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Several parameters such as adsorption capacity of adsorbent, surface charges and active sites might be attributed to the adsorption behavior of the adsorbent at various pHs. The surface of CAS contains a large number of active sites. The dye uptake can be related to the active sites and also to the chemistry of the dye in the solution. Theoretically, at pH < isoelectrical point, the surface gets positively charged, which enhances the adsorption of the negatively charged dye anions through electrostatic forces of attraction. At pH > isoelectrical point, the surface of CAS particles gets negatively charged, which favors the adsorption of cationic dye (Ahmad and Kumar, 2010). As the pH of dye solution becomes higher, the association of dye cations with negatively charged functional groups in the adsorbent surface could more easily take place as follows:

3.2 Effect of adsorbate concentrations and equilibrium time

The removal of MG and MB by adsorption on CAS was found to increase with time and attained a maximum value at 100 min (Fig. 2). On changing the initial concentration of MG and MB solution from 25 to 200 mg/L, the amount adsorbed increased from 5.5356 to 29.0550 mg/g and percentage removal decreased from 88.57 to 58.11 with increased concentration of dye from 25 to 200 mg/L for MG and the amount adsorbed increased from 2.3075 to 9.3700 mg/g and percentage removal decreased from 36.92 to 18.74 with increased concentration of dye from 25 to 200 mg/L for MB .

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Figure 2

Effect of contact time on the removal of different initial concentrations of MG and MB using CAS (0.2 g/50 mL) at pH 6.0.

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This may be due to the fact that at lower concentrations almost all the dye molecules were adsorbed very quickly on the outer surface, but further increase in initial dye concentrations led to fast saturation of adsorbent surface, and thus most of the dye adsorption took place slowly inside the pores (Hameed and EI-Khaiary, 2008).

3.3 Effect of adsorbent particle size

The batch adsorption experiments were carried out using adsorbent with different particle sizes (75–125 μm to 250–500 μm) at pH 6.0, and initial concentration of 100 mg/L. The removal of MG and MB increased with the decrease in particle size (Fig. 3). The relatively higher adsorption with smaller adsorbent particle may be attributed to the fact that smaller particles yield large surface areas (Liew Abdullah et al., 2005).

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Figure 3

Effect of adsorbent particle size on MG and MB removals (C₀: 100 mg L⁻¹, pH 6.0, agitation speed: 150 rpm, temperature: 27 ± 2 °C).

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3.4 Adsorption dynamics

Lagergren’s pseudo-order equation is widely used to investigate the dynamics of the adsorption process from aqueous solution (Allen et al., 2005). In this study, pseudo-first-order equation and pseudo-second-order equation were separately used to describe the adsorption process of MG and MB onto CAS.

Log (q_e − q) was calculated using the first-order Lagergren equation, as shown in Eq. (2).

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Figure 4

Pseudo-first-order kinetics for MG and MB adsorption onto CAS. Conditions: adsorbent dosage 0.2 g/50 mL, pH 6.0, temperature 27 ± 2 °C.

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Figure 5

Pseudo-second- order kinetics for MG and MB adsorption onto CAS. Conditions: adsorbent dosage 0.2 g/50 mL, pH 760, temperature 27 ± 2 °C.

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The values of q_t were found to be linearly correlated with values of t^1/2 (Fig 6) and the rate constant K_dif directly evaluated from the slope of the regression line (Table 2). The values of intercept C (Table 2) provide information about the thickness of the boundary layer, the resistance to the external mass transfer increase as the intercept increase. The constant C was found to increase with increase of dye concentration from 50 to 200 mg L⁻¹, which indicating the increase of the thickness of the boundary layer and decrease of the chance of the external mass transfer and hence increase of the chance of internal mass transfer. The R² values given in Table 2 are close to unity indicating the application of this model. This may confirm that the rate-limiting step is the intraparticle diffusion process. The linearity of the plots demonstrated that intraparticle diffusion played a significant role in the uptake of the adsorbate by adsorbent. However, as still there is no sufficient indication about it, Ho (2003) has shown that if the intraparticle diffusion is the sole rate-limiting step, it is essential for the q_t versus t^1/2 plots to pass through the origin, which is not the case in Fig. 6, it may be concluded that surface adsorption and intraparticle diffusion were concurrently operating during the dye and CAS interactions.

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Figure 6

Intraparticle diffusion plot for MG and MB adsorption onto CAS. Conditions: adsorbent dosage 0.2 g/50 mL, pH 6.0, temperature 27 ± 2 °C.

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3.5 Adsorption isotherm

Langmuir isotherm models have been applied to describe the adsorption of dyes by different materials (Vimonses et al., 2009). In this study, the values of C_eq_e⁻¹ and C_e were calculated using Eq. (8).

The Freundlich isotherm model can be used for non-ideal sorption that involves heterogeneous adsorption. 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:

The D–R isotherm dose not assumes a homogeneous surface or constant adsorption potential. The D–R model has commonly been applied as

3.6 Comparison of various low cost adsorbents

Tables 4a and 4b compares the adsorption capacity of different types of adsorbents used for removal of MG and MB. The most important parameter to compare is the Langmuir Q_m value since it is a measure of adsorption capacity of the adsorbent. The value of Q_m in this study is larger than those in most of previous works. This suggests that MG, MB could be easily adsorbed on CAS.

3.7 Characterization of CAS (SEM, BET and FTIR of CAS)

The surface structure of CAS was analyzed by scanning electronic microscopy (SEM) (Fig. 7 It is clear that the adsorbents have considerable number of heterogeneous pores, a cave- like, uneven and rough surface morphology, where there is a good possibility for dye to be trapped and adsorbed (Hameed and EI-Khaiary, 2008).

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Figure 7

SEM photographs for CAS (2.0 ku × 1000).

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The adsorbents have heterogeneous surface, micro pores and mesopores as seen from its surface micrographs. The reported Brunauer–Emmett–Teeller (BET) surface area of the adsorbents is 20.54 m²/g for CAS.

The FTIR spectrum of CAS shows distinct peaks at 3922–3445 cm⁻¹ (OH stretch), 2928.47 cm⁻¹ (CH stretch shift), 2361, 2341, 2290 cm⁻¹ (NH stretch), 1747.36 cm⁻¹ (C⚌O stretch disappeared), 1498 cm⁻¹ (phenyl), and 1020 cm⁻¹ (C–O–C stretch), It is clear that the adsorbent displays a number of adsorption peaks, reflecting the complex nature of the adsorbent.

The adsorbents can be crystallographically characterized by means of X-ray diffraction (XRD). The XRD pattern (Fig. 8) of the adsorbents show typical spectrum having main and secondary peaks at 2Θ of 22° and 37° respectively. The main peak is taken as indicative of the presence of highly organized crystalline cellulose while the secondary peak is a measure of polysaccharides structure (Sonawane and Shrivastava, 2009). The results showed that the adsorbents have more pores and crystalline structure. The availability of more surface area is due to the typical lateral rack type of structure of the adsorbents as was clearly witnessed in the SEM micrographs.

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Figure 8

XRD photograph for CAS.

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The percentages of total carbon, nitrogen and hydrogen in CAS are 49.11, 06.11, and 01.12 which shows that the adsorbents contain mainly carbon with less amounts of nitrogen and hydrogen. A relatively larger percentage of carbon and hydrogen in comparison to nitrogen compounds indicates that carbon-hydrogen groups might be available for adsorption of dyes (Sonawane and Shrivastava, 2009).

3.8 Adsorption mechanism

Activated carbons are materials which have amphoteric characteristics, so the pH on its surface always changes depending on initial pH of the solution. Generally, 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 (Prahas et al., 2008). The FTIR spectrum of the adsorbent indicates that carboxyl and hydroxyl groups are present in abundance. The sorption of MB on the adsorbent may be due to the electrostatic attraction between these groups and the cationic dye molecules (MG⁺). At pH above 4, the carboxylic groups are deprotonated and negatively charged carboxylate ligands (–COO–) bind to the positively charged MG, MB molecules. This confirms that the sorption of MG and MB by adsorbent was an ion exchange mechanism between the negatively charged groups present in adsorbent and the cationic dye molecule (Saeed et al., 2010). These positively charged ions in the presence of dye solution could then be exchanged with dye cations as follows.

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