Equilibrium, kinetics and thermodynamics of Cadmium (II) biosorption on to composite chitosan biosorbent

2.1 Materials

Medium molecular weight chitosan (75–85% deacetylated), oxalic acid dehydrate, and 150 mesh-activated alumina were obtained from Sigma–Aldrich Chemicals (St. Louis, MO, USA). All the necessary chemicals used were of analytical grade and obtained from M/S Qualigens Fine Chemicals, Mumbai, India. Stock solution (1000 mg L⁻¹) was prepared by dissolving Cd(NO₃)₂.4H₂O. This was further diluted to obtain the desired concentration for practical use. The pH of the solution was measured with a Digisun electronics digital pH meter using a solid electrode calibrated with a standard buffer solution. A flame atomic absorption spectrophotometer (Shimadzu AA-6300, Japan) with deuterium background corrector was used. All measurements were carried out in an air/acetylene flame. A 10 cm long slot burner head, a lamp and an air-acetylene flame were used. The operating parameters for working element were set as recommended by the manufacturer. The FTIR spectra were recorded using Thermo-Nicolet FTIR, Nicolet IR- 200 series, Germany. Scanning Electron Microscopy (Model Evo15, CarlZeiss, England) has been used to study the surface morphology of the biosorbent. The synthetic solutions were prepared by diluting Cd(II) standard stock solutions (concentration 1000 ± 3 mg L⁻¹). Fresh dilutions were used in each experiment.

2.2 Synthesis of composite chitosan biosorbent

CCB was synthesized following the procedure described by Boddu et al. (2003). Ceramic alumina with 150 mesh sizes was dried in oven for 4 h at 110 °C followed by stirring with oxalic acid for 4 h at room temperature, filtering, washing twice with DI water, and drying in an oven at 70 °C under vacuum for 24 h. About 50 g of medium molecular weight chitosan was slowly added to 1000 mL of 10 wt.% oxalic acid solution with stirring. The acid and chitosan formed a viscous mixture (gel), which was heated to 40–50 °C to facilitate mixing. Approximately 500 mL of the chitosan gel was diluted with water and heated to 40–50 °C while stirring vigorously. About 500 g of the acid treated alumina was slowly added to the diluted gel and stirred for about 36 h at constant stirring rate. The contents are allowed to settle, and the clear liquid was filtered out under vacuum with Whatman no 41 filter paper. The composite biosorbent was washed twice with DI water and dried in the oven at 55 °C under vacuum for 24 h. The coating process was then repeated on the once coated biosorbent to increase loading of chitosan onto the alumina. Excess oxalic acid in the composite biosorbent was neutralized by treatment with aqueous NaOH. The mixture was then filtered with Whatman No. 41 filter paper and washed with DI water to remove excess NaOH. Twice-coated biosorbent was then allowed to dry in the oven under vacuum at 55 °C for about 48 h. The biosorbent was sieved and the beads between −32 and +60 mesh were used for the experimental work.

2.3 Batch studies

Batch adsorption experiments were carried out in Erlenmeyer flasks by adding 0.1 g of CCB in 100 mL of aqueous Cd(II) solution at desired initial pH, metal ion concentration and temperature. The initial pH was adjusted with solutions of 0.1 M HCl or 0.1 M NaOH. The flasks were gently agitated in a temperature controlled water bath shaker at 200 rpm for a period of 3 h. All the experiments were performed in triplicates at the desired initial conditions and the concurrent value was taken. The contents of flask were separated from the biosorbent by filtration, using Whatman No. 42 and the filtrate was analyzed for remaining metal concentration in the sample using atomic absorption spectrophotometer. The amount of metal ion sorbed per unit mass of the biosorbent (mg g⁻¹) was evaluated by using the following equation:

3.1 Characterization of biosorbent

3.1.3

3.1.3 FTIR analysis

FTIR spectral analysis is important in identifying the different functional groups present on the surface of biosorbent which are responsible for the binding of metal ions. FTIR spectra in the range of 4000–500 cm⁻¹ for the CCB before and after biosorption of Cd(II) are shown in Fig. 1(a) and (b) respectively. The FTIR spectrum of CCB in Fig. 1(a) indicates the presence of predominant peaks at 3379.8 cm⁻¹ (–OH and –NH stretching), 2924.87 cm⁻¹ (–CH stretching), 1661.0 cm⁻¹ (–NH bending in –NH₂), 1393.3 cm⁻¹ (–NH deformation vibration in –NH₂) and 1065.1 cm⁻¹ (–C–O–C– stretching). This reveals that all functional groups present in chitosan are intact even after coating on alumina and are available for interaction with the metal ions. The intensity of transmittance of peaks is relatively greater in the case of CCB loaded with metal ions compared to the virgin CCB. This may be attributed to the presence of fewer free functional groups in the CCB loaded with metal ion. This observation provides evidence that the functional groups such as –NH₂, and –OH are involved in binding the cadmium metal ion to CCB. The proposed model for the formation of a complex between chitosan and cadmium metal ion is shown in Supplementary Figure 1.

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Figure 1

FTIR spectra of (a) CCB beads before biosorption (b) Loaded with Cd(II).

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3.1.4

3.1.4 Scanning electron microscopic studies

Scanning electron micrographs (SEM), recorded using software controlled digital scanning electron microscope, are given in Fig. 2(a) and (b). The SEM of CCB beads shows no particular crystalline structure and it rather appears spherical. The SEM micrographs of CCB beads illustrate the surface texture and porosity of CCB beads with holes and small openings on the surface, thereby increasing the contact area, which facilitates the pore diffusion during adsorption. The porous nature is clearly evident from these micrographs.

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Figure 2

Scanning electron micrographs of (a) scanning electron micrographs of the CCB at 100× (b) scanning electron micrographs of the CCB at 800×.

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3.1.5

3.1.5 EDAX analysis

The sample composition and element contents were analyzed by using the energy dispersive analysis system of X-ray (EDAX) (EDAX, Ltd., USA). EDAX analysis was conducted to evaluate the adsorption of cadmium on CCB beads. The EDAX spectrum of CCB beads indicated the presence of C, O, Al, K, Na and Si in the structure (Fig. 3(a)) but did not show the characteristic signal of Cd ions on the surface of pure CCB beads. An EDAX spectrum was also recorded for CCB loaded with Cd(II) ions (Fig. 3(b)), which gives the characteristic peak for Cd at 3.20 keV. This confirms the binding of the cadmium ions to CCB surface.

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Figure 3

Energy dispersive X-ray (EDAX) analysis of (a) CCB beads before biosorption (b) After biosorption of Cd(II).

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3.1.6

3.1.6 X-ray diffraction studies

Powder X-ray diffraction patterns were recorded on a Bruker D8-Advance diffractometer using graphite monochromatic CuK_α1 (1.5406 Å) and K_α2 (1.54439 Å) radiations. Fig. 4 (a) and (b) shows the X-ray diffraction patterns of CCB biosorbent before and after adsorption of Cd(II). In Fig. 4(a) three broad peaks are centered at 2 theta = 19.20°, 29.95° and 41.61° and also half broad peak at 5°, which indicate the amorphous nature of the biosorbent, so the metal ion could more easily penetrate through the biosorbent surface. In Fig. 4(b), one major broad amorphous peak at 20–38°, one minor broad amorphous peak at around 42.17° and one broad hump at 6.52–10.33° also amorphous.

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Figure 4

XRD pattern of (a) CCB beads before biosorption (b) Loaded with Cd(II).

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3.2 Effect of pH

In order to evaluate the influence of this parameter on the adsorption of Cd(II) the experiments are carried out in the pH range of pH 2.0–8.0. The experiments could not be conducted at pH less than 2.0 to avoid dissolution of CCB beads and could not be conducted above pH at 8.0 to avoid the precipitation of Cd(II) ions. The variation of adsorption capacity of CCB beads with pH is graphically represented in Supplementary Fig 2. The maximum uptake of Cd(II) ions takes place at pH 6.0 and decreases in adsorption capacity on either side of pH 6.0. The free amino groups (–NH₂) in chitosan (Ch-) exist in equilibrium with the protanated amino group in the presence of acidic aqueous solution. $$\text{Ch}-{\text{NH}}_2+{\text{H}}_2\text{O}\leftrightarrow \text{Ch}-{\text{NH}}_3^{+}+{\text{OH}}^{-}$$

The amino groups are protophillic and become NH₃⁺ in an acidic medium. At low pH values, protons occupy most of the sorption sites on the biosorbent surface resulting in adsorption of smaller amounts of cadmium ions because of electrostatic repulsion. With the increase of pH, the amino groups become free from protanation due to a decrease in the concentration of hydrogen ions leading to the decrease in competition of H⁺ with metal ions for sorption sites and thus the adsorption capacity increases. However the precipitation of Cd(II) ions could lead to the decrease of adsorption capacity above pH 8.0.

Cadmium in an aqueous solution is hydrolyzed with the formation of various species, depending on the solution pH. Moreover, Cd²⁺, which is the main hydrolyzed cadmium species in the pH range 5.0–7.0 appears in the form of Cd(OH)⁺, Cd(OH)_2, and Cd(OH)₃⁻. Among them Cd²⁺ is the predominant species in the solution within this pH range. The fraction of negatively charged hydrolysis products in the solution increases as pH increases. Chitosan can form chelates with cadmium ions with the release of H⁺ ions. A chelate formation may require the involvement of the two or more complexion groups from the molecule. The Cd(II) ion may seek two or more amine groups from chitosan to form the complex. This should normally reduce the solution pH. In the case of chitosan, the protanation of NH₂ groups occurs at a rather low pH range. The fact that the pH of the solution increased as the adsorption progressed suggests that Cd(II) formed a covalent bond with the NH₂ group. The two NH₂ groups could come from two different glucosamine residues of the same molecule, or from two different molecules of chitosan. Jha et al. (1988) compared the stability constants for ammonia and amino complexes with those for chloro complexes of cadmium and noted that the formation of covalent bond with amine nitrogen is the more preferred reaction.

3.3 Effect of adsorbent dosage

To understand the effect of amount of adsorbent on adsorption of Cd(II), the experiments were performed at pH 6.0 with 100 ml of sorbate solution by varying the amount of adsorbent from 0.05 to 0.5 g. The percent removal of Cd(II) ions increases with increase of amount of CCB due to the greater availability of binding sites of the biosorbent. The maximum removal efficiency is with 0.4 g of CCB and the maximum percent removal is about 93.76% (Fig. 5).

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Figure 5

Effect of biosorbent dose on percent removal of Cd(II) onto CCB beads. (Experimental conditions: initial conc. 100 mg/L, contact time 3 h, pH 6.0.).

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3.4 Effect of contact time and initial metal ion concentration

The extent of adsorption increases with time and attained equilibrium for all the concentrations Cd(II) studied (100–400 mg L⁻¹) at 180 min. The plot was drawn between time and adsorption capacity (Supplementary Figure 3). In the initial stages the removal efficiencies of the biosorbent increased rapidly due to the abundant availability of active binding sites on the biomass, and with gradual occupancy of these sites, the sorption became less efficient in the later stages.

3.5 Adsorption kinetics

In order to investigate the mechanism and to determine the rate controlling step of adsorption of Cd(II) on CCB, kinetic models were used. The rate constants were calculated using pseudo-first-order and pseudo-second-order kinetic models (Siva Kumar et al., 2012) and the rate controlling step was determined by intra-particle diffusion model. The pseudo-first-order is represented by:

The intra-particle diffusion model is used to investigate the diffusion controlled adsorption system. The probability of the intra-particle diffusion was explored by using the following equation (Weber and Morris, 1964):

Boyd model is applied to check whether sorption proceeds via film diffusion or intraparticle diffusion mechanism. The model can be expressed in the following form (Boyd et al., 1947):

The Elovich equation has been applied satisfactorily to some chemisorption processes. The Elovich equation can be written in the following form (Kellner et al., 1998):

3.6 Adsorption isotherm modeling

Isotherms are the equilibrium relations between the concentration of adsorbate on the solid phase and its concentration in the liquid phase. From the isotherms the maximum adsorption capacity can be obtained. These data provide information on the capacity of the adsorbent or the amount required to remove a unit mass of pollutant under the system conditions. Langmuir and Freundlich isotherms are the most common isotherms describing the solid–liquid adsorption system. The Langmuir model assumes that adsorption takes place at specific homogeneous sites on the surface of the adsorbent and also, when a site is occupied by an adsorbate molecule, no further adsorption can take place at this site. The Langmuir equation is expressed by the following expression (Langmuir, 1916):

The essential characteristics of Langmuir isotherm can be expressed by a dimensionless constant called separation factor (or equilibrium parameter), R_L, which is defined by Weber and Chakkravorti as (Weber and Chakkravorti, 1974):

The Freundlich model assumes a heterogeneous surface with the involvement of sites of different energies in adsorption process. The Freundlich model is given as (Freundlich, 1906):

The equilibrium data were examined by using Dubinin–Radushkevich isotherm (Dubinin and Radushkevich, 1947) in order to determine the nature of the biosorption process as physical or chemical. The D–R sorption isotherm is more general than the Langmuir isotherm as its derivation is not based on ideal assumptions such as equipotent of the sorption sites, absence of steric hindrance between sorbed and incoming particles and surface homogeneity on microscopic level. The linear presentation of D–R isotherm equation is expressed by (Dubinin and Radushkevich, 1947):

Temkin isotherm (Temkin and Pyzhev, 1940) contains a factor that explicitly takes into account the adsorbent–adsorbate interactions. The heat of adsorption of all the molecules in the layer would decrease linearly with coverage due to adsorbent–adsorbate interactions. The adsorption is characterized by a uniform distribution of binding energies, up to some maximum binding energy. The linear form of the Temkin isotherm is expressed as:

3.7 Thermodynamic analysis

The thermodynamic parameters such as enthalpy change (ΔH°), the entropy change (ΔS°) and the Gibbs free energy change (ΔG°) for the sorption process were calculated from the variation of Langmuir constant (K_L) with temperature (T) using well known relations,