Effect of Initial pH and Its Optimisation
The initial pH of the aqueous solution is an important controlling parameter in the heavy metal sorption process. In order to determine the effect of initial pH on the biosorption of Cr(VI) ions onto bark biomass, the biosorption experiments were carried out with initial Cr(VI) concentration of 200 mg L−1 and the bark concentration of 5.0 g L−1 (bark particle size: 150–355 µm) by varying the initial pH values in the range of 1.0–8.0.
The bark surface has net electrical neutrality at its pHpzc value. At pH > pHpzc, the surface charge of bark is negative, whereas at pH < pHpzc the surface charge of bark is positive. As can be seen in Figure 2, the biosorption amount was higher at lower pH values (pH < pHpzc). By increasing the initial pH values from 2.0 to 6.0, the amount of biosorbed Cr(VI) ions decreased from 20.50 to 2.27 mg g−1. These observations can be explained by the facts that the most prevalent forms of Cr(VI) ions in aqueous solutions are acid chromates (HCrO4−), chromates (CrO42−), dichromates (Cr2O72−) and other oxyanions. At lower pH values, acid chromate ions are the dominant species. As the initial pH of the solution was decreased, the surface of the bark biomass may get positively charged as a result of hydrogenation from hydronium ions, and, thus, the increasing electrostatic attraction between the negative chromate species and the bark surface would drive the Cr(VI) biosorption more favourable at lower pH values. In contrast, when the initial pH value was increased (pH > pHpzc), the bark surface became more negatively charged. The competition between OH− and chromate ions, which is the dominant species at higher pH values, and also the electrostatic repulsion between the chromate ions and the bark surface sites increased and, hence, the Cr(VI) uptake decreased at higher pH values. As a result, for the biosorption of Cr(VI) onto bark biomass, the initial pH was optimised as 2.0.
Figure 2. Effect of solution pH on Cr(VI) biosorption onto bark (initial Cr(VI) concentration: 200 mg L−1; bark concentration: 5.0 g L−1).
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Effect of Contact Time and Biosorption Kinetics
The time dependent behaviour of Cr(VI) biosorption onto bark biomass was studied by varying the contact time in the range of 1–480 min. The initial concentration of Cr(VI) was kept as 100 mg L−1, while the amount of bark suspension was 5.0 g L−1. The mixtures were agitated at 400 rpm. The samples were taken at predetermined time intervals and filtered immediately through 0.45 µm nitrocellulose filter paper. The supernatant was analysed for the Cr(VI) level. The data showed that the Cr(VI) biosorption amount increased rapidly at initial stages of the biosorption because of the utilization of the readily available active sorption sites on the bark surface. Thereafter it continued at a slower rate and finally reached to equilibrium as a result of saturation of bark surface sites. A larger amount of Cr(VI) was removed in the first 30 min of contact time, and the Cr(VI) uptake became almost constant after 60 min, which can be considered as equilibrium time of Cr(VI) biosorption. However, to make sure that the sufficient contact time is provided for biosorption, further experiments were carried out for 120 min of contact time.
Different kinetic models such as pseudo-first order, pseudo-second order and intraparticle diffusion models have been developed in order to understand the mechanisms of the developed biosorption process and evaluate the performance of the biosorbents for metal removal.
The pseudo-first order model is expressed as;
where qt (mg g−1) is the amount of the metal ions biosorbed at time t, qe is the amount of the metal ions biosorbed at equilibrium (mg g−1), and k1 is the rate constant of the model (min−1).
After definite integration by applying the conditions qt = 0 at t = 0 and qt = qt at t = t the Equation (1) becomes the following,
A straight line of ln(qe − qt) versus t suggests the applicability of this model, and qe and k1 can be determined from the intercept and slope of the plot, respectively.
The pseudo-second order kinetic model is expressed as;
where k2 (g mg−1 min−1) is the rate constant of the second order equation; qt (mg g−1) is the amount of biosorption at time t (min), and qe (mg g−1) is the amount of biosorption at equilibrium.
After definite integration by applying the conditions qt = 0 at t = 0 and qt = qt at t = t the Equation (3) becomes the following,
The plot of t/qt versus t should give a straight line if second order kinetics is applicable, and qe and k2 can be determined from the slope and intercept of the plot, respectively.
In order to investigate the biosorption kinetics of Cr(VI) onto P. brutia bark, the pseudo-first order and the pseudo-second order kinetic models were used to fit the experimental data. By testing the plots of ln(qe − qt) versus t (for pseudo-first order) and t/qt versus t (for pseudo-second order), the rate constants k1 and k2 and the corresponding correlation coefficients were calculated. The value of correlation coefficient obtained from the pseudo-first order kinetic model (Table 2) is not satisfactory, and also qe cal determined from the model is not in a good agreement with the experimental value of qe exp. These results indicated that the biosorption of Cr(VI) onto bark biomass does not fit the pseudo-first order kinetic model. However the value of the correlation coefficient for the pseudo-second order model is relatively high, and the biosorption capacity (qe cal) calculated by the model is close to the experimental value (qe exp). Therefore, it has been concluded that the pseudo-second order model is more suitable to describe the biosorption of Cr(VI) onto P. brutia bark. The results indicated that the biosorption rate of Cr(VI) depends on the concentration of ions on the bark surface, and the behaviour of biosorption is in agreement with the chemical biosorption being the rate controlling step.The intraparticle diffusion model is expressed as;
where qt (mg g−1) is the amount of biosorption at time t (min), and kid (mg g−1 min−1/2) is the rate constant of intraparticle diffusion. The magnitude of C gives an idea about the thickness of the boundary layer. The multilinearity of the plot of qt versus t1/2 indicates that any biosorption process takes place in three main steps. The first stage is film diffusion which is attributed to the transport of biosorbate molecules from the bulk solution to the biosorbent external surface by diffusion. The second stage is pore or intraparticle diffusion in which the biosorbate molecules diffuse from the external surface into the pores of the biosorbent. The last step, which is related to the biosorption of the biosorbate on the active sites on the internal surface of the pores, occurs rapidly and hence it can be said that a biosorption process should be controlled by either film or pore diffusion, or a combination of both. If C value obtained from the intercept of the plot of qt versus t1/2 is zero, the pore diffusion is the only rate limiting step; if not, it is considered that the biosorption process is controlled by a combination of both film and pore diffusion.[41, 42]
Table 2. Parameters of pseudo-first order, pseudo-second order and intraparticle diffusion models
|qe exp (mg g−1)||Pseudo-first order||Pseudo-second order||Intraparticle diffusion|
|k1 (min−1)||qe cal (mg g−1)||R2||k2 (g mg−1 min−1)||qe (mg g−1)||R2||kid,1 (mg g−1 min−1/2)||R2||kid,2||R2||C|
|13.01||−1.51 × 10−2||3.87||0.788||1.25 × 10−2||13.23||0.999||1.619||0.954||0.012||0.982||5.86|
The intraparticle mass transfer curve of Cr(VI) biosorption followed two distinct phases, which were film diffusion (first stage) and intraparticle diffusion (second step). The intraparticle rate constants for the first phase (kid,1) and second phase (kid,2) and C parameters were obtained from the plot of qt versus t1/2 (Table 2). The lower value of kid,2 than kid,1 indicated that the rate limiting step is intraparticle diffusion, and the C value is not zero. Hence it can be concluded that the biosorption of Cr(VI) onto bark biomass is a complex process, and both intraparticle and film diffusion contribute to the rate-limiting step.
Biosorption Isotherms and the Effect of Bark and Initial Cr(VI) Concentrations
The equilibrium biosorption isotherms are one of the most important means in order to describe the interaction between the metal ions and biosorbents. Although different isotherm models can be used for that purpose, the Langmuir and Freundlich isotherm models are the most widely used models due to their simplicity.
The Langmuir isotherm is feasible for the biosorption on homogeneous surfaces and based on the assumption that the biosorption occurs at specific homogeneous sites on the biosorbent and the biosorption energy is always constant. The model is presented by;
where qe is the equilibrium metal ion concentration on the biosorbent (mg g−1), Ce is the equilibrium metal ion concentration in the solution (mg L−1), qmax is the Langmuir constant related to the maximum monolayer biosorption capacity (mg g−1), and b is related to the free energy or net enthalpy of the biosorption (L mg−1). The Langmuir model in linear form is;
The essential characteristics of the Langmuir isotherm can be expressed by means of ‘RL’, a dimensionless constant called the separation factor or equilibrium parameter. RL can be calculated using the following equation ;
where C0 (mg L−1) is the initial amount of biosorbate, and b (L mg−1) is the Langmuir constant described above.
The RL parameter is considered as more reliable indicator of the sorption process. There are four probabilities for the RL value: (i) for favourable sorption 0 < RL < 1, (ii) for unfavourable sorption RL > 1, (iii) for linear sorption RL = 1 and (iv) for irreversible sorption RL = 0.
The Freundlich isotherm model assumes that the biosorption takes place on heterogeneous surfaces which have different sorption energies and provides no information about the monolayer biosorption capacity. The Freundlich model has the form;
where Kf is a constant related to the biosorption capacity (mg g−1), and 1/n is an empirical parameter related to the biosorption intensity. The Freundlich model in linear form is;
In order to analyse the effects of bark and Cr(VI) concentrations on the uptake of this metal, the biosorption process was carried out with initial Cr(VI) concentrations between 50 and 1000 mg L−1 and various bark concentrations in the range of 1.0–20.0 g L−1. At equilibrium (120 min of contact time) the Cr(VI) concentration in each system was measured, and the Langmuir and Freundlich isotherms were plotted as a function of bark concentration, as displayed in Figure 3a and b, respectively. At a constant bark concentration, as the initial Cr(VI) concentration increased, the amount of Cr(VI) biosorbed (mg) per gram mass of the bark (g) increased, whereas at a constant Cr(VI) concentration, as the bark concentration increased, the amount of Cr(VI) biosorbed (mg) per gram mass of the bark (g) decreased.
Figure 3. Relationship between equilibrium Cr(VI) concentration and its uptake at various bark concentrations using (a) Langmuir isotherm model (b) Freundlich isotherm model (bark particle size: 150–355 µm; initial pH: 2.0).
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The isotherm constants and correlation coefficients were calculated from the linear Langmuir and Freundlich plots by plotting Ce/qe versus Ce (Figure 3a) and ln qe versus ln Ce (Figure 3b). The Langmuir constants qmax and b were obtained from the slope and intercept of the linear plots of Ce/qe versus Ce, respectively, and Freundlich constants Kf and 1/n were determined from the intercept and slope of the linear plots of ln qe versus ln Ce, respectively (Table 3). In all cases correlation coefficients were higher than 0.97, which strongly supports the fact that the biosorption of Cr(VI) onto P. brutia bark perfectly fits both Langmuir and Freundlich isotherm models. Furthermore, the values of 1/n were smaller than 1 indicating that the present biosorption process was favorable under studied conditions. Also the RL values calculated for initial Cr(VI) concentration range of 50–1000 mg L−1 were in the range of 0.315 and 0.902, at constant bark concentration (5.0 g L−1). This result also supports the fact that the biosorption of Cr(VI) onto P. brutia bark was favourable. In the view of these results it can be said that the surface of P. brutia bark is made up of both homogeneous and heterogeneous biosorption parts.
Table 3. Langmuir and Freundlich isotherm constants for the biosorption of Cr(VI) ions onto bark biomass at various bark concentrations at pH 2.0
|Bark conc. (g L−1)||Langmuir constants||Freundlich constants|
|qmax (mg g−1)||b (L mg−1)||R2||Kf (mg g−1)||n||R2|
The maximum adsorption capacity (qmax) of P. brutia bark was obtained as 140.8 mg g−1 at 1.0 g L−1 bark suspension. Table 4 lists the maximum adsorption capacity of different adsorbents reported in the literature for the adsorption of Cr(VI) ions. In general P. brutia bark exhibited comparable adsorption capacity in comparison with other adsorbents.
Table 4. Comparison of the maximum adsorption capacity of P. brutia bark with other reported adsorbents
|Adsorbent||Adsorption capacity (mg g−1)||Refs.|
|Hydrolyzed keratin/polyamide 6 blend nanofibres||59.9||Aluigi et al.|
|Acacia mangium wood carbon||37.16||Danish et al.|
|Phoenix dactylifera L. stone carbon||32.76||Danish et al.|
|Anion exchanger based nanosized ferric oxyhydroxide hybrid adsorbent||123||Ren et al.|
|Prunus serotina bark||93.61||Netzahuatl-Muñoz et al.|
|carnation flowers waste||6.25||Vargas et al.|
|Polyaniline/polystyrene nanocomposite||19.0||Lashkenari et al.|
|P. brutia bark||140.8||This work|
The Effect of Bark Particle Size
In order to evaluate the effect of the bark particle size on the biosorption of Cr(VI), the bark biomass with sizes in the range of <150, 150–355 and 355–710 µm were treated with a series of Cr(VI) solutions in the initial concentration range of 50–1000 mg L−1. At equilibrium Langmuir and Freundlich isotherms were obtained as a function of particle size, and the results are depicted in Figure 4a and b, respectively. The results indicated that the particle size affected the biosorption process, and uptake of Cr(VI) by bark biomass increased with decreasing the particle size of bark. This is an expected result because as the particle size of bark biomass decreases, the number of active biosorption sites on the surface of bark increases, and these particles attach more Cr(VI) ions to their surfaces. The linear Langmuir and Freundlich isotherm models were fitted to the experimental data, and the isotherm constants and correlation coefficients for each particle size are shown in Table 5.
Figure 4. Relationship between equilibrium Cr(VI) concentration and its uptake by bark at various bark particle sizes using (a) Langmuir isotherm model (b) Freundlich isotherm model (bark concentration: 5.0 g L−1; initial pH: 2.0).
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Table 5. Langmuir and Freundlich constants at different bark particle sizes at pH 2.0 and 5.0 g L−1 of bark concentration
|Particle size (µm)||Langmuir constants||Freundlich constants|
|qmax (mg g−1)||b (L mg−1)||R2||Kf (mg g−1)||n||R2|
The Effect of Temperature and Thermodynamic Parameters of Biosorption Process
In order to determine the effect of temperature on the biosorption of Cr(VI) onto bark, the biosorption experiments were conducted at different temperatures in the range of 0–40°C with initial Cr(VI) concentrations of 100 mg L−1 at pH 2.0. The degree of biosorption increased from 10.23 mg g−1 (51.2% removal) to 14.45 mg g−1 (72.2% removal) when the temperature was increased from 0 to 40°C, which may be due to the increase of the mobility of Cr(VI) ions and availability of more active biosorption sites on the surface of the bark at higher temperatures.
Thermodynamic parameters including the changes in free energy (ΔG), enthalpy (ΔH) and entropy (ΔS) were calculated from the following equations;
where R is the universal gas constant (8.314 J mol−1 K−1), T is the temperature (K), and Kd is the distribution coefficient. The Kd value was calculated using the following equation ;
where qe and Ce are the equilibrium concentration of metal ions on the biosorbent (mg L−1) and in the solution (mg L−1), respectively. The enthalpy (ΔH) and entropy (ΔS) changes of the biosorption were estimated from the following equation;
This equation can be written as;
The thermodynamic parameters of ΔH and ΔS were obtained from the slope and intercept of the plot of ln Kd versus 1/T, respectively. The Gibbs free energy changes (ΔG) were calculated from Equation (11), and the values of ΔG, ΔH and ΔS for the biosorption of Cr(VI) onto bark were given in Table 6. The negative values of ΔG in the temperature range of 10–40°C indicated that the biosorption process is spontaneous. And also the increase in ΔG values with increase in temperature shows the feasibility of the biosorption process at higher temperatures. The positive value of ΔH suggests the endothermic nature of the biosorption process. The magnitude of ΔH gives an idea about the type of the sorption. Two main types of biosorption may occur, physical and chemical. In physical biosorption the equilibrium is usually rapidly attained and easily reversible, because the energy requirements are small. The enthalpy for physical biosorption is usually no more than 1 kcal mol−1 (4.2 kJ mol−1) since the interactions are weak. The chemical biosorption involves interactions much stronger than in physical biosorption, and the enthalpy for chemical biosorption is more than 5 kcal mol−1 (21 kJ mol−1), so it seems that the biosorption of Cr(VI) ions onto bark is almost a chemical process. Finally, the positive value of ΔS suggested an increase in randomness at the solid/solution interface during the biosorption of Cr(VI) ions onto bark.
Table 6. Thermodynamic parameters for the Cr(VI) biosorption onto bark at different temperatures
|T (°C)||Thermodynamic equilibrium constant (Kd)||ΔG (kJ mol−1)||ΔS (J mol−1 K−1)a||ΔH (kJ mol−1)|
|0||0.92||0.18|| || |
|10||1.18||−0.40|| || |
|30||1.74||−1.40|| || |
|40||2.04||−1.86|| || |
Desorption of Cr(VI) ions
Desorption tests were also carried out by batch technique. The recovery of Cr(VI) ions from the bark was tested with HCI and NaOH solutions as desorbing agent. For that purpose 50 mg of bark was added to 100 mg L−1 of Cr(VI) solution at pH 2.0, and the system was agitated on a shaker for 120 min. After reaching equilibrium, the bark was separated by filtration and the filtrate was analysed by FAAS. The bark loaded with Cr(VI) ions was washed with deionized water for three times to remove the surface residual Cr(VI) ions and then dried in air for 1 day. The bark loaded with Cr(VI) ions was treated with 10 mL of HCI solution (in the concentration range of 0.01–0.5 M) and 10 mL of NaOH solution (in the concentration range of 0.01–3.0 M), separately for 120 min. The regeneration efficiency reached from 10% to 43% when the concentration of NaOH solution was increased from 0.01 to 3.0 M, and from 12% to 34% when the concentration of HCI solution was increased from 0.01 to 0.05 M. It is clear that, both eluents could not achieve the complete desorption of the biosorbed Cr(VI) ions from the bark. This may be due to the strong interactions between the Cr(VI) ions and the functional groups on the surface of the bark biomass.
The Effect of Foreign ions over the Biosorption Yield of Cr(VI) Ions
The foreign ions such as Na+, K+, Mg2+ and Ca2+ always exist in natural waters and industrial wastewaters, which may interfere the uptake of heavy metals by a biomass. Thus, the effect of these ions on the biosorption of Cr(VI) ions onto bark should be studied. For that purpose, the biosorption studies were carried out by adding 100 mg L−1 of Na+, K+, Mg2+ and Ca2+ individually, and the mixture of these ions in 100 mg L−1 of Cr(VI) solution containing 5.0 g L−1 of bark suspension. The presented biosorption procedure described above was applied to these solutions. The results are given in Figure 6a. It is clear that all of these ions partially depressed the uptake of Cr(VI) ions by bark, and also all of them exhibited approximately the same inhibition.
Figure 6. (a) Effect of foreign ions on Cr(VI) uptake by bark (initial Cr(VI) and foreign ions concentrations: 100 mg L−1 of each) (b) Effect of foreign ions concentrations on Cr(VI) uptake by bark (initial Cr(VI) concentration: 100 mg L−1).
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In order to investigate the effect of concentration of foreign ions on the biosorption of Cr(VI) ions onto bark, the biosorption experiments were carried out by adding foreign ions in the concentration range of 100–500 mg L−1, individually in 100 mg L−1 of Cr(VI) solution containing 5.0 g L−1 of bark suspension. The results indicated that as the concentration of these foreign ions was increased in the range of 100–500 mg L–1, the uptake of Cr(VI) ions by bark biomass decreased (Figure 6b).