Adsorptive removal of cadmium from aqueous solution by cork biomass: Equilibrium, dynamic and thermodynamic studies

2.1 Materials

The cork powder used was supplied by the Algerian company “EPE/ JIJEL Etancheités SPA”. Particle size fractions were obtained by sieving and the experiments were carried out with the size fractions with a particle diameters of (d₁ < 0.08; 0.08 < d₂ < 0.1; 0.1 < d₃ < 0.16 and 0.16 < d₄ < 0.2 mm). Atomic absorption spectrometer AAS (Shimadzu AA-6200) with hallow cathode lamp and air acetylene flame was used for determining cadmium concentration. A HANNA instrument (pH 211) pH meter was used for pH measurements. A magnetic stirrer (Heidolph-RZR-2020) was used to agitating the samples.

2.2 Preparation of solution

All the chemicals used were of analytical reagent grade. Distilled water was used throughout the experimental studies. A stock cadmium solution for desired concentration was prepared by dissolving cadmium nitrate Cd(NO₃)·4H₂O in distilled water. Working standard was prepared by progressive dilution of stock cadmium solutions using distilled water. ACS reagent grade HCl, NaOH and buffer solutions (E. Merck) were used to adjust the solution pH.

2.3 Biosorption studies

The adsorption experiments were carried out by batch process. 0.05 g of biosorbent was placed in Erlenmeyer flasks with 50 mL solution of metal ions of desired concentration. The entirety was agitated during a certain time at 250 rpm. Once the operation time had elapsed, the liquid phase was taken out, and then filtered through filter paper (Double Boxing rings 102) and final concentration of metal ion was determined in the filtrate by atomic absorption spectrometry (AAS). The amount of metal ions adsorbed at equilibrium per unit mass of biosorbent was determined according to the following equation:

For the calculation of cadmium rate adsorption (R), the following expression was used:

2.4 Effect of contact time

Batch biosorption tests were done at different contact time at the initial metal ion concentration of 10 mg/L, and biosorbent concentration of 0.05 g in 50 mL solution, at 25 °C. The solid was kept in contact with the solution for different time periods (10; 20; 30; 40; 50; 60; 70; 80; 90 and 100 min), the solution of the specified flask was taken out and filtered. The concentration of metal was determined by AAS.

2.5 Effect of particles size

The effect of the biosorbent particle size (between 0.05 and 0.2 mm) on the adsorption capacity of cork biomass was investigated at 25 °C using 0.05 g of adsorbent, 50 mL of solution and cadmium concentration of 10 mg/L. The mixtures were shaken, and after equilibrium, the solutions were filtered. The concentration of metal ions was then determined.

2.6 Effect of pH

The effect of pH on the sorption of cadmium was studied by batch process as follows: 100 mL of metal solution was taken in beaker. The desired pH of solution was adjusted by adding dilute solution of HCl and NaOH. Then, 50 mL of these solutions were taken in Erlenmeyer flasks and mixed with 0.05 g of adsorbent. After equilibrium, the final concentration of metal ions was determined and the rate of metal adsorbed was calculated.

2.7 Equilibrium studies

Equilibrium isotherm studies were conducted for 1 h at 20, 30 and 40 °C using a constant amount of biosorbent (0.05 g) in 50 mL solution of pH 6 and for initial metal concentrations between 10 and 100 mg/L. The samples were then filtered and analysis was performed as mentioned before.

3.1 Effect of contact time

The results obtained from the biosorption rate tests are shown in Fig. 1. It revealed that the rate of adsorption increased with increasing contact time. It was noticed that the maximum rate of cadmium elimination equals 29.9%. The extent of sorption increased rapidly during the initial stage and then became slower at later stage till the equilibrium is attained. Equilibrium time for the sorption of cadmium was found to be 60 min. These data are important because equilibrium time is one of the key parameters for economical wastewater treatment application (Kadirvelu and Namasivayam, 2003). Thus, the agitation time was fixed at 100 min for the rest of the batch experiments to make sure that equilibrium was reached. This equilibrium time is shorter than those usually employed for the adsorption of cadmium by other adsorbent materials. Indeed, times of 24 h were proposed for adsorption systems involving chitin (Benguella and Benaissa, 2002), chitosan (Becker et al., 2000), Baker’s yeast (Vasudevan et al., 2003) or Pinus pinaster bark (Vazquez et al., 2002). Furthermore, other sorbents needed even longer time to reach the equilibrium state like 72 h for some carbonaceous material (Hanzlik et al., 2004) and even 3–5 days for an activated carbon (Leyva-Ramos et al., 1997).

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Figure 1

Effect of contact time cadmium ions sorption onto cork biomass (C₀ = 10 mg/L, m = 0.05 g, T = 25 °C, agitation rate = 250 rpm, d₁ < 0.08 mm).

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3.2 Effect of particles size

The contact surface between any sorbent and the liquid phase plays an important role in the phenomena of sorption. According to Fig. 2, the results showed that the rate of cadmium biosorption increases with the reduction in the diameter of the particles, which is probably due to the increase of the number of active sites and thus the increase of the biosorbent surface. The maximum rate elimination of cadmium (i.e. 30.19%) was reached using the fine particle (diameter less than 0.08 mm). Similar results have been reported for the adsorption of cadmium using manganese nodule residue (Agrawal and Sahu, 2006) and vegetable wastes (Azouaou et al., 2008).

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Figure 2

Effect of biosorbent particle size on cadmium biosorption onto cork biomass (C₀ = 10 mg/L, m = 0.05 g, T = 25 °C, agitation rate = 250 rpm).

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3.3 Effect of pH

The pH of the aqueous solution is an important controlling parameter in the adsorption process. The binding of metal ions with surface functional groups was strongly pH dependent (Lee and Davis, 2001). The percentage sorption of cadmium on cork powder increased with the increase of pH, attaining a maximum at pH 6 (Fig. 3). Availability of negatively charged groups on the adsorbent surface is necessary for the sorption of metals (Ajmal et al., 2005). Sorption at pH 2 was limited since the sorbent surface potentially gained a global positive charge due to the presence of H⁺ and H₃O⁺. In such conditions, hydrogen and hydronium ions compete with metal ions (Luef et al., 1991) resulting in active sites to become protonated to the virtual exclusion of metal binding on the sorbent surface (Low et al., 1993). Hence, at higher H⁺ concentration, the sorbent surface becomes more positively charged thus reducing the attraction between sorbent and metal cations (Aldov et al., 1995). In contrast, as the pH increases, more negatively charged surface becomes available thus facilitating greater metal uptake and therefore metal sorption tends to increase significantly by increasing pH.

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Figure 3

Effect of initial pH on cadmium biosorption by cork powder (C₀ = 10 mg/L, m = 0.05 g, T = 25 °C, agitation rate = 250 rpm, d₁ < 0.08 mm).

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3.4 Biosorption isotherms

The analysis of equilibrium data to monitor the adsorption process is quite important for design purposes. Adsorption isotherms express the mathematical relationship between the quantity of adsorbate and equilibrium concentration of adsorbate remaining in the solution at a constant temperature. Sorption data for wide range of adsorbate concentration are most conveniently described by sorption isotherms. The adsorption studies were carried out at 20, 30 and 40 °C to determine the adsorption isotherms and the isotherm parameters were evaluated using non-linear Langmuir and Freundlich models.

Langmuir adsorption isotherm is often used to describe the maximum adsorption capacity of adsorbent, which is a most important parameter for an adsorption system. The Langmuir equation (Langmuir, 1918) is written as follows:

The empirical Freundlich isotherm is based on adsorption on a heterogeneous surface. It is expressed by the following equation (Freundlich, 1906):

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Figure 4

Adsorption isotherms modelling related to the biosorption of cadmium onto cork powder at 20 °C (A), 30 °C (B) and 40 °C (C) [C₀ = 10–100 mg/L, m = 0.05 g, d₁ < 0.08 mm, pH 6].

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The experimental results are well fitted by the Langmuir model then the Freundlich model considering the values of regression coefficients presented in Table 2 which are higher than 0.99. The values of q_max and K_L calculated from Langmuir plots were found to be 9.65 mg/g and 0.20 L/mg for the experiments carried out at 20 °C. The values of both q_max and K_L increased with a rise in the solution temperature. The values of q_max increased from 9.65 to 14.77 mg/g, when the solution temperature increased from 20 to 40 °C. The increasing trend was observed for the values of K_L depending upon the temperature of solution. The increase in the values of q_max and K_L with temperature indicates that the cadmium ions are favourably adsorbed by cork powder at higher temperatures, which shows that the cadmium adsorption phenomenon is endothermic.

The essential feature of the Langmuir isotherm can be expressed in terms of dimensionless separation constant or equilibrium parameter (R_L), which is defined as:

3.5 Thermodynamics studies

In the environmental engineering practice, thermodynamic parameters including standard enthalpy change (ΔH°), standard entropy change (ΔS°) and standard free energy change (ΔG°) should be considered in order to determine if the adsorptive removal process will occur spontaneously. Thus, (ΔH°) and (ΔS°) were obtained from the Van't Hoff equation:

The very useful relationship between standard free energy change and equilibrium constant is given by the following equation:

The positive values of ΔH° (Table 4) suggest endothermic nature of adsorption and cadmium ions are adsorbed more efficiently on cork powder at higher temperatures. The standard free energy change (ΔG°) of the process decreases with increase in temperature, which indicates that the process is spontaneous and spontaneity increases with increase in temperature. The positive values of standard entropy change (ΔS°) shows the increase in randomness at the solid/solution interface during the adsorption of cadmium.

3.6 Kinetic studies

A quantitative understanding of the sorption is possible with the help of kinetic models. The pseudo-first-order kinetic model, as expressed by Lagergren (1898), can be written as:

Pseudo-second-order sorption rate equation (Ho et al. 1996) may be expressed as follows

The results have been analyzed using Eqs. (11) and (12). The experimental data fitted well with both equations. The values of q_e(theo) calculated from these models were compared with experimental values q_e(exp) (Table 5). It was found that for pseudo-first-order kinetic model the values of q_e(theo) and q_e(exp) differed appreciably. On the other hand, values of q_e(theo) are very close to q_e(exp) for pseudo-second-order model. The values of correlation coefficients (r²) are very high for pseudo-second-order model as compared to pseudo-first-order rate model (0.99 and 0.93, respectively). It is therefore confirmed that sorption of cadmium on cork follows the pseudo-second-order rate equation.