Batch experiments were conducted to study the adsorption of hazardous cadmium onto low-cost algae biomass in aqueous solution with respect to concentration of adsorbate, adsorbent dosage, contact time, solution pH and temperature. Langmuir and Freundlich adsorption models were applied to describe the equilibrium isotherms and the isotherm constants were determined. The activation energy of adsorption was also evaluated for the adsorption of cadmium onto Ulva lactuca biomass. Experimental data were tested in terms of biosorption kinetics using pseudo-first-order and pseudo-second-order kinetic models. The results showed that the biosorption processes of Cd(II) followed well pseudo-second-order kinetics. Langmuir and Freundlich models were applied to describe the biosorption isotherm of the metal ions by Ulva lactuca biomass. Langmuir model fitted the equilibrium data better than the Freundlich isotherm. The biosorption capacity of Ulva lactuca biomass for cadmium was found to be 3.02 mg/g at pH 5.60 min equilibrium time and 20 °C. The mean free energy which was calculated was 6.24 kJ/mol for Cd(II) biosorption, which shows that the adsorption is physical. The calculated thermodynamic parameters (ΔG0, ΔH0 and ΔS0) showed that the biosorption of Cd(II) onto Ulva lactuca biomass was feasible, spontaneous and exothermic under examined conditions. The results indicate that algae Ulva lactuca could be employed as a low-cost material for the removal of metal ions from aqueous solution.

INTRODUCTION

Cadmium is known to be highly toxic and is among the heavy metals which are potentially very dangerous for human beings and the environment. It comes from various industries such as tanneries, ink, paints, etc., and is toxic even at low concentration and must imperatively be removed. The cadmium toxicity is mainly induced from Cd(II). It is toxic to humans, animals and even to plants (El hani et al. 2011). It can cause lung, kidney and liver cancers, as well as gastric damages. Its concentration should not exceed 0.01 mg/L in drinking water (Sheng et al. 2007). Toxicological studies have also shown that long-term effects of cadmium(II) poisoning include kidney damage and changes to the constitution of the bone, liver, and blood. Short-term effects include nausea, vomiting, diarrhea, and cramps. Therefore, minimizing the production of hazardous waste and heavy metals is regarded as one of the most important environmental challenges that the world faces today (Figueira et al. 2000; Davis et al. 2003). To eliminate these heavy metals present in industrial effluents, various processes can be applied, such as precipitation, adsorption, electrodepositing, electrocoagulation, membrane separation, liquid extraction, ion exchanges, etc. (Pavasant et al. 2006; Sud et al. 2008).

Algae have been used as biomonitors for heavy metal in the water because of their high cation exchange capacity (Ahmet & Tuzen 2008a, 2008b; Ahmet et al. 2008) and economic considerations which impel us to look for natural sorbents, which are absorbent, abundant in nature and exploitable in the raw state or after a simple treatment.

The present work focuses on the potential use of Ulva lactuca biomass for removal of Cd(II) ion from aqueous solution. Experimental parameters affecting the biosorption process such as pH, contact time and temperature were studied. The equilibrium biosorption data were evaluated by pseudo-first-order and pseudo-second-order kinetic models and Langmuir, Freundlich and isotherm models. The biosorption mechanisms were also investigated in terms of thermodynamics and kinetics.

Equilibrium modeling

Isotherm modeling

Equilibrium data, commonly known as adsorption isotherms, are basic requirements for the design of adsorption systems. Classical adsorption models (Langmuir and Freundlich) were used to describe the equilibrium between adsorbed metal ions on the algal cell (Qeq) and metal ions in solution (Ceq) at a constant temperature. The Langmuir equation validating the monolayer sorption in a surface of a finite number of identical sites is given by Equation (1). 
formula
1
where Qm (mg.g−1) is the maximum amount of metal ion per unit weight of alga to form a complete monolayer on the surface bound at high Ct, and KL (L.mg−1) is a constant related to the affinity of the binding sites. Qm represents a practical limiting adsorption capacity when the surface is fully covered with metal ions and assists in the comparison of adsorption performance, particularly in cases where the sorbent did not reach its full saturation in experiments. Qm and KL can be determined from the linear plot of Ct/Qt versus Ct (Langmuir 1918; Freundlich 1926).
The empirical Freundlich equation based on sorption on a heterogeneous surface is given below by Equation (2). 
formula
2
where KF and n are the Freundlich constants characteristic of the system. KF and n are indicators of adsorption capacity and adsorption intensity, respectively. Equation (2) can be linearized in logarithmic form and Freundlich constants can be determined. The Freundlich isotherm is also more widely used but provides no information on the monolayer adsorption capacity, in contrast to the Langmuir model (Langmuir 1918; Freundlich 1926).

Kinetic modeling

In order to investigate the mechanism of biosorption and potential rate controlling step such as mass transport and chemical reaction processes, kinetic models have been used to test experimental data. When the biomass is employed as a free cell suspension in a well-agitated batch system, all the cell wall binding sites are made readily available for metal uptake so the effect of external film diffusion on biosorption rate can be assumed not significant and ignored in any engineering analysis (Pavasant et al. 2006; Ozgür et al. 2008). The kinetic models included the pseudo-first-order and pseudo-second-order equations which can be used in this case assuming that measured concentrations are equal to cell surface concentrations. The first-order rate expression of Lagergren (Arica & Bayramoglu 2005) based on solid capacity is generally expressed as follows: 
formula
3
where Qm and Qt are the amounts of adsorbed metal ions on the biosorbent at equilibrium and at time t, respectively (mg.g−1) and k1,ad is the rate constant of first-order biosorption (min−1). After integration and applying boundary conditions, t =0 to t = tmax and Qt = 0 to Qt = Qm; the integrated form of Equation (3) becomes 
formula
4
A straight line of ln(Qm–Qt) versus t suggests the applicability of this kinetic model (Ho et al. 1996; Unnithan & Anirudhan 2001; Chen & Wang 2007). In order to fit Equation (4) to experimental data, the equilibrium sorption capacity, Qm, must be known. In many cases Qm is unknown and as adsorption tends to become immeasurably slow, the amount sorbed is still significantly smaller than the equilibrium amount. For this reason it is necessary to obtain the real equilibrium sorption capacity, Qm, by extrapolating the experimental data to t = tmax or by using a trial and error method. Furthermore in most cases the first-order equation of Lagergren does not fit well for the whole range of contact time and is generally applicable over the initial 20–30 min of the sorption process. The pseudo-second-order equation is also based on the sorption capacity of the solid phase. Contrary to the other model it predicts the behavior over the whole range of adsorption and is in agreement with an adsorption mechanism being the rate controlling step (Tunali et al. 2006, Ahmet & Tuzen 2008a, 2008b). If the rate of sorption is a second-order mechanism, the pseudo-second-order chemisorption kinetic rate equation is expressed as: 
formula
5
where k2,ad is the rate constant of second-order biosorption (g mg−1min−1). For the boundary which is the integrated rate law for a second-order reaction. Equation (6) can be rearranged to obtain 
formula
6

If second-order kinetics are applicable, the plot of t/Qt against t of Equation (6) should be a linear relationship, from which Qm and k2,ad can be determined from the slope and intercept of the plot and there is no need to know any parameter beforehand (Ho et al. 1996; Unnithan & Anirudhan 2001; Chen & Wang 2007).

Activation energy of biosorption

Activation energy of sorption reaction can be found using the Arrhenius equation providing a linear relationship between the rate constant and temperature. Although a large number of publications have recently suggested using living and non-living algae for accumulation and removing heavy metals from polluted water, there seems to be a study which reports both the equilibrium and kinetic modelling of cadmium(II) biosorption by dried Ulva lactuca, in a batch system in a wide range of cadmium(II) concentrations. The binding capacity of Ulva lactuca for cadmium(II) was shown as a function of initial pH, temperature and initial cadmium(II) concentration in this study. The sorption phenomena were expressed by the Langmuir and Freundlich adsorption models and the effect of temperature on the model constants was investigated. A limited number of studies have so far been focused on the kinetic analysis of biosorption of metal ions. In the literature, the experimental data were also analyzed using the first-order and second-order adsorption kinetic models and kinetic constants were calculated depending on temperature. Finally, the model which defined the biosorption accurately was decided.

EXPERIMENTAL

Raw materials

Table 1 presents the chemical products used in this work and their origin.

Table 1

The chemical products used in this work and their origin

Chemical products Company Country 
HCl VWR Prolabo chemicals India 
NaOH Loba Chemie (laboratory reagents and fine chemicals) France 
Cd(NO3)2 Acros Organics USA 
Chemical products Company Country 
HCl VWR Prolabo chemicals India 
NaOH Loba Chemie (laboratory reagents and fine chemicals) France 
Cd(NO3)2 Acros Organics USA 

Harvest and preparation of algae

Marine green algae (Ulva lactuca) were collected with peaches to plankton net in October 2013 at room temperature 22 °C (on) at the Moroccan Atlantic coast at the level of the beach of Rabat (34° 03′ North 6° 46′ West at an altitude of 79 m). These algae have been rinsed in sea water and polyethylene plastic bags previously rinsed with distilled water acidified to pure nitric acid, upon the arrival at the laboratory, the algae are again rinsed with distilled water.

These algae were dried in an oven at a temperature of 70 °C for 48 hours until the weight of the fibers became constant; then they were crushed. The resulting particles serve for the remainder of our study.

Cadmium solutions

The cadmium solutions were prepared by dissolving mass m = 0.75 g of Cd(NO3)2, in demineralized water. The initial solution of concentration [Cd] = 151.224 mg/L was diluted to obtain other solutions of different concentrations. The solution pH was adjusted by means of HCl and NaOH solutions (both of concentrations 0.1 M and 1 M), using a pH meter.

Batch experiments

Batch sorption experiments were performed suspending 2 g of biomass, or its equivalent fixed in agar, in 50 ml of Cd(II) solutions. The metal concentration C0 = 151.224 mg/L. Suspensions were kept in constant agitation at desired and constant pH = 5. After 10, 20, 30, 40, 60, 80, 100 and 120 min, the mixture (algal biomass + metal) was filtrated with membrane filters (0.45 μm). Afterwards, the final metals concentrations in the remaining solution were determined and metal uptake (Qt) was calculated using the following mass balance equation (Bai & Abraham 2001). 
formula
7
where C0 is the initial metal concentration (mg/L), Ct is the equilibrium metal concentration (mg/L), Vl is the solution volume (L), and Ms is the dry alga weight (g).

RESULTS AND DISCUSSION

pH effect

As known, the initial solution pH has a great influence on the elimination of metallic ions by adsorption phenomenon. Therefore its effect was considered for ion Cd(II) by varying its value as follows: pH 2, 3, 4, 5, 6,7 and 8.

The obtained results are shown in Figure 1.

The dependence of metal biosorption on pH is related to both the surface functional groups on the cell walls of the biosorbent and the metal chemistry in solution (Sheng et al. 2004; Arıca & Bayramoglu 2005). The medium pH affects the solubility of metals and the ionization state of the functional groups (i.e., carboxylate, phosphate, and amino groups) on the algal cell wall. The carboxylate and phosphate groups carry negative charges which allow the microbial cells to be potent scavengers of cations. The experimental results are presented in Figure 1. In all cases, the maximum heavy metal ions biosorption occurred between pH 5.0. The ability of adsorbed heavy metal ion (Cd(II) 21.14 mg/L) on the immobilized algal at pH 5.0 were 0.48 mg/g. There was an increase in the ability to adsorption in metal ion with pH from 0.42 to 0.48 mg/g. It seemed to level off at pH greater than 5.0. At acidic pH (pH ≈ 2), protonation of the cell wall component adversely affecting the biosorption capacity of the algal. But its effect becomes minor with increasing pH in the medium (Bayramoğlu et al. 2003). Maximum ability to adsorption was obtained for a pH of 5.0 and the interaction of the heavy metal ions with the algal could be primarily with the carboxylate and phosphate groups of the cell wall component. During the biosorption equilibrium experiments with immobilized-algae, no significant changes in pH of the medium were observed. Several researchers have investigated the effect of pH on biosorption of heavy metals by using different kinds of microbial biomass. For example, the biosorption of Cd(II) and Pb(II) by marine algae, was pH dependent and maximum biosorption was obtained in the pH range of 5–6 obtained in Ruhan et al. (2009).

Figure 1

Effect of pH on the ability of cadmium biosorption by algae Ulva lactuca.

Figure 1

Effect of pH on the ability of cadmium biosorption by algae Ulva lactuca.

Effect of initial cadmium(II) ion concentration on biosorption

The adsorption rates of Cd(II) ion on the biomass algal were obtained by following the decrease of the concentration of metal ions within the adsorption medium with time. As can be seen from Figure 2, the Cd(II) adsorption rate was high at the beginning of adsorption and saturation levels were completely reached at about 60 min. After this equilibrium period, the amount of adsorbed metal ions on the biosorbents did not significantly change with time. This trend in binding of metal ions suggests that the binding may be through interactions with functional groups located on the surface of the biosorbents. Feng and Aldrich studied Cu(II), Pb(II) and Cd(II) biosorption on marine algae Ecklonia maxima and the biosorption equilibrium was established about 60 min (Feng & Aldrich 2004). Note that there are several parameters, which determine the adsorption rate such as stirring rate of the aqueous phase, structural properties of both the support and the biosorbent. It could be said that relatively rapid biosorption rates were obtained using the biomass algal preparation in this study.

Figure 2

Effect of initial concentration on biosorption of cadmium(II) onto biomass Ulva lactuca, pH = 5.4, t = 20°C, Malgae = 2 g.

Figure 2

Effect of initial concentration on biosorption of cadmium(II) onto biomass Ulva lactuca, pH = 5.4, t = 20°C, Malgae = 2 g.

Biosorption kinetic modeling

In order to examine the controlling mechanism of biosorption process such as mass transfer and chemical reaction, kinetic models were used to test the experimental data. The Lagergren first-order and the Ritchie second-order kinetic models (Chen & Wang 2007) were applied to the biosorption of Cd(II) ion on the algal biomass. The comparison of experimental biosorption capacities and the theoretical values estimated from the above two equations and are presented in Table 2. According to the values in Table 2, the correlation coefficients for the linear plots of t/Qt against t for the second-order equation were greater than 0.999 for the algal biomass for a contact time of 120 min (Figure 3(b)). The theoretical Qm values for all the studied metal ions were very close to the experimental Qm values in the case of second-order kinetic model. These results suggest that the second-order mechanism is predominant and the biosorptions may be the rate limiting step that controls the biosorption process (Tunali et al. 2006). Conversely, the theoretical Qm values estimated from the first-order kinetic model give significantly different values compared to experimental values, and the correlation coefficients were also found to be slightly lower. These results showed that the biosorption systems were not well described by the first-order kinetic model.

Table 2

A comparison of the first-order and second-order adsorption rate constants at different concentrations of Cd(II)

   
 
First-order kinetic model
 
Second-order kinetic model
 
Metal ion C0 (mg/l) Qm,exp (mg/g) Qm (mg/g) K1,ad R2 Qm (mg/g) K2,ad R2 
Cd(II) 151.22 3.02 0.26 0.055 0.686 3.03 0.29 0.999 
42.87 0.89 0.05 0.018 0.523 0.87 3.28 0.999 
21.14 0.48 0.05 0.025 0.574 0.47 1.93 0.999 
   
 
First-order kinetic model
 
Second-order kinetic model
 
Metal ion C0 (mg/l) Qm,exp (mg/g) Qm (mg/g) K1,ad R2 Qm (mg/g) K2,ad R2 
Cd(II) 151.22 3.02 0.26 0.055 0.686 3.03 0.29 0.999 
42.87 0.89 0.05 0.018 0.523 0.87 3.28 0.999 
21.14 0.48 0.05 0.025 0.574 0.47 1.93 0.999 
Figure 3

(a) The first-order ln(QmQt) vs. t plot for biosorption of Cd(II) and Cr(VI) ions on the biosorbents. (b) The second-order (t/Qt) vs. t plot for biosorption of Cd(II) and Cr(VI) ions on the biosorbents.

Figure 3

(a) The first-order ln(QmQt) vs. t plot for biosorption of Cd(II) and Cr(VI) ions on the biosorbents. (b) The second-order (t/Qt) vs. t plot for biosorption of Cd(II) and Cr(VI) ions on the biosorbents.

Effect of temperature on cadmium(II) biosorption

Temperature is one of the important parameters for successful biosorption application. Figure 4 shows the biosorption efficiency of Cd(II) ion by Ulva lactuca as a function of contact time and temperature. As can be seen from Figure 4, the biosorption efficiency increases with rise in contact time up to 60 min at 20–40 °C and after it is almost constant. Therefore, the optimum contact time was selected as 60 min for further experiments. Increasing temperature from 20 to 40 °C during a 60 min contact time. This result indicated the exothermic nature of Cd(II) biosorption onto Ulva lactuca biomass. A decrease in the biosorption of Cd(II) ion with the rise in temperature may be due to either the damage of active binding sites in the biomass (Tunali et al. 2006) or increasing tendency to desorbed metal ions from the interface to the solution (Tunali et al. 2006). The optimum solution temperature was selected as 20 °C for further biosorption experiments.

Figure 4

Temperature on biosorption of Cd(II) onto Ulva lactuca biomass (metal concentration:151,224 mg/L; M = 2 g; pH = 5).

Figure 4

Temperature on biosorption of Cd(II) onto Ulva lactuca biomass (metal concentration:151,224 mg/L; M = 2 g; pH = 5).

Analysis of equilibrium data is important for developing an equation which can be used to design purposes. Several isotherm equations have been used for the equilibrium modelling of biosorption systems. Out of these isotherm equations, two have been applied for this study, the Freundlich and Langmuir isotherms.

The linearized Freundlich and Langmuir adsorption isotherms of cadmium(II) ions obtained at the temperatures of 20, 33 and 40 °C are given in Figures 5(a) and 5(b). The Freundlich and Langmuir adsorption constants evaluated from the isotherms at different temperatures with the correlation coefficients are presented in Table 3. As seen from the tables, there is a very high regression.4

Table 3

A comparison of the Freundlich and Langmuir adsorption constants obtained from the Freundlich and Langmuir adsorption isotherms of cadmium(II) ions at different temperatures

t( °C) Langmuir
 
Freundlich
 
 Linear expression Model parameters Linear expression Model parameters 
20 y = 0.34x3.02 KL = 0.113 y = −0.278x + 2.04 KF = 0.13 
Qm = 2.94 (mg.g−1nF = 3.65 > 1 
R2 = 0.995 R2 = 0.993 
30 y = 0.36x4.25 KL = 0.085 y = −0.330x + 2.24 KF = 0.11 
Qm = 2.77(mg.g−1nF = 3.03 > 1 
R2 = 0.999 R2 = 0.998 
40 y = 0.367x4.57 KL = 0.080 y = −0.355x + 2.33 KF = 0.10 
Qm = 2.72 (mg.g−1nF = 2.82 > 1 
R2 = 0.999 R2 = 0.998 
t( °C) Langmuir
 
Freundlich
 
 Linear expression Model parameters Linear expression Model parameters 
20 y = 0.34x3.02 KL = 0.113 y = −0.278x + 2.04 KF = 0.13 
Qm = 2.94 (mg.g−1nF = 3.65 > 1 
R2 = 0.995 R2 = 0.993 
30 y = 0.36x4.25 KL = 0.085 y = −0.330x + 2.24 KF = 0.11 
Qm = 2.77(mg.g−1nF = 3.03 > 1 
R2 = 0.999 R2 = 0.998 
40 y = 0.367x4.57 KL = 0.080 y = −0.355x + 2.33 KF = 0.10 
Qm = 2.72 (mg.g−1nF = 2.82 > 1 
R2 = 0.999 R2 = 0.998 
Table 4

Thermodynamic parameters calculated with the pseudo-second rate constant for Cd(II) adsorbed onto biomass Ulva lactuca

Temperature (K) k2 Ea (kJ.mol−1ΔH0 (kJ.mol−1ΔS0 (J.mol−1 K−1ΔG0 (J.mol−1.K−1
293 0.23    −3.32 
306 0.29 6.24 −9.45 −0.02 −3.05 
313 0.34    −2.90 
Temperature (K) k2 Ea (kJ.mol−1ΔH0 (kJ.mol−1ΔS0 (J.mol−1 K−1ΔG0 (J.mol−1.K−1
293 0.23    −3.32 
306 0.29 6.24 −9.45 −0.02 −3.05 
313 0.34    −2.90 
Figure 5

(a) The linearized Langmuir adsorption isotherm of Cd(II) ion obtained at different temperatures. (b) The linearized Freundlich adsorption isotherm of Cd(II) ion obtained at different temperatures.

Figure 5

(a) The linearized Langmuir adsorption isotherm of Cd(II) ion obtained at different temperatures. (b) The linearized Freundlich adsorption isotherm of Cd(II) ion obtained at different temperatures.

Correlation coefficients (>0.99) were found at the all temperatures studied. The higher correlation coefficients show that both the Freundlich and Langmuir models are very suitable for describing the biosorption equilibrium of cadmium(II) by the algal cells in the studied concentration range. An adsorption isotherm is characterized by certain constants the values of which express the surface properties and affinity of the sorbent and can also be used to find the adsorptive capacity of biomass for the cadmium(II). From Table 3, the magnitude of KF and n, the Freundlich constants, showed easy uptake of cadmium(II) from wastewater with a high adsorptive capacity of the dried Ulva lactuca, especially at 20 °C. The highest KF and n values were found as 0.13 and 3.65, respectively, at this temperature value. Table 3 also indicates that n is greater than unity, indicating that cadmium(II) ions are favorably adsorbed by dried Ulva lactuca at all the temperatures studied. The values of KF and n determined from the Freundlich plots decreased with the raise in temperature. Values of Qm and KL for different temperatures have been calculated from the Langmuir plots in Figure 5(b) and the results are also tabulated in Table 3. The maximum capacity Qm determined from the Langmuir isotherm defines the total capacity of the biosorbent for cadmium(II) (2.94 mg/g). The adsorption capacity of cadmium(II) also decreased with the increasing temperature. The value of Qeq obtained at 20 °C (i.e. maximum uptake) appears to be equal in comparison with the uptakes obtained at the other temperatures (Figures 5(a) and 5(b)).

Figure 6

Arrhenius plot for adsorption of cadmium by biomass Ulva lactuca.

Figure 6

Arrhenius plot for adsorption of cadmium by biomass Ulva lactuca.

Figure 7

Plot of ln(kL) against 1/t. The activation energy for the biosorption system of cadmium(II).

Figure 7

Plot of ln(kL) against 1/t. The activation energy for the biosorption system of cadmium(II).

Thermodynamic parameters

The second-order rate constant can be expressed as a function of temperature by the Arrhenius equation and the activation energy (Ea) can be determined as below (Ozgür et al. 2008): 
formula
8
 
formula
9
where A is the temperature-independent factor (g/(mg.min)), Ea is the apparent activation energy of sorption (kJ/mol), [R = 8.314 kJ/(mol K)] the gas constant, and T is the adsorption temperature in Kelvin. Figure 6 shows a linear plot of ln k2 as a function of 103/T for cadmium adsorption at 20 ± 1 to 40 ± 1 °C. The apparent activation energy calculated from the slope of the plot was 6.24 kJ/mol. The negative activation energy indicates that the biosorption of Cd(II) by Ulva lactuca was a physical adsorption.
The effect of temperature (T) on the adsorption of Cd(II) was studied within the range of 20–40 °C and the results are shown in Table 4. In order to determine the feasibility and rate of the adsorption, thermodynamic parameters such as equilibrium constant KL (Qe/Ce), the standard free energy (ΔG0), and enthalpy change (ΔH0), and entropy change (ΔS0) is calculated by using the equations: 
formula
10
 
formula
11
where kL is the constant (ΔH0 is the enthalpy of biosorption (kJ mol−1), ΔS0 is the entropy change, ΔG0 is the free energy, R is the gas constant (8.314 J.mol−1.K−1) and T is the solution temperature (K).

Increase in temperature shows a decrease in feasibility of biosorption at higher temperatures. Figure 7 shows a linear plot of ln(kL) as a function of (1/T). The enthalpy of biosorption (ΔH0) was found to −9.45 kJ/mol for Cd(II) biosorption. The negative ΔH0 is an indicator of exothermic nature of the biosorption and its magnitude gives information on the type of biosorption, which can be either physical or chemical. The enthalpy or heat of biosorption, ranging from 0.5 to 5 kcal/mol (2.1–20.9 kJ/mol) corresponds to a physical sorption as it ranges from 20.9 to 418.4 kJ/mol in the case of a chemical sorption (Alkan et al. 2008). The biosorption heats of Cd(II) ions fall into the heat range of physisorption (activation energy equal 14.20 kJ/mol). Therefore, the ΔH0 values showed that the biosorption processes of Cd(II) ion onto Ulva lactuca biomass were taken place via physisorption. The mean biosorption energy values obtained from the D–R model also confirm this result. The ΔS0 parameter was found to be − 0.02J/mol.K for Cd(II) biosorption. The negative ΔS0 value suggests a decrease in the randomness at the solid/solution interface during the biosorption process.

CONCLUSION

The present study clearly demonstrated that marine algae (Ulva lactuca) are an effective adsorbent for the removal of cadmium from aqueous solution and polluted water. The high adsorption capacity of metal ions of solutions (pH = 5.4) is due to the strong electrostatic interactions between its adsorption site and ions. As for the application of Langmuir and Freundlich equations, the experimental results show that the Freundlich model was the best. The low value of Ea suggests that the adsorption is physical. The kinetic data tend to fit very well in the pseudo-second-order kinetics model with high correlation coefficients. The ΔG0 values were negative. Therefore, the adsorption was spontaneous. The negative value of ΔH0 reveals that the adsorption process was exothermic in nature and a physical adsorption. The negative value of ΔS0 suggests the increase of orderliness at the solid–solution interface during the adsorption. Finally, the adsorbent biomass displayed the main advantages of excellent dispersion in aqueous solution, separation convenience and high adsorption capacity, which implied their application potentials for effective removal of other hazardous pollutants from aqueous solution.

REFERENCES

REFERENCES
Alkan
M.
Dogan
M.
Turhan
Y.
Demirbas
O.
Turan
P.
2008
Adsorption kinetics and mechanism of maxilon blue 5G dye on sepiolite from aqueous solutions
.
Chemical Engineering Journal
139
,
213
223
.
Davis
T. A.
Volesky
B.
Vieira
R. H. S. F.
2003
Sargassum seaweed as biosorbent for heavy metals
.
Water Research
34
,
4270
4278
.
El hani
S.
rifi
E. H.
Agaoui
H.
Mokht
N.
Bengueddour
R.
2011
Decontamination d'une solution d'un cadmium par des broyats d'algues de Ulva lactuca et de Gracilaria multipartita
.
Biological Research
1
,
1994
5108
.
Figueira
M. M.
Volesky
B.
Ciminelli
V. S. T.
Roddick
F. A.
2000
Biosorption of metals in brown seaweed biomass
.
Water Research
34
,
196
204
.
Freundlich
H.
1926
Colloid and Capillary Chemistry
.
Translated from the 3rd German edn by H. S. Hatfield
,
Methuen
,
London
.
Ho
Y. S.
Wase
D. A. J.
Forster
C. F.
1996
Kinetic studies of competitive heavy metal adsorption by sphagnum moss peat
.
Environ. Technol.
17
,
71
77
.
Pavasant
P.
Apiratikul
R.
Sungkhum
V.
Suthiparinyanont
P.
Wattanachira
S.
Marhaba
T. F.
2006
Biosorption de Cu2+, Cd2+, Pb2+et Zn2+en utilisant séché macroalgue vert marine Caulerpa lentillifera
.
Bioresour. Technol.
97
,
2321
2329
.
Tunali
S.
Akar
T.
Özcan
A. S.
Kiran
I.
Özcan
A.
2006a
Equilibrium and kinetics of biosorption of lead(II) from aqueous solutions by Cephalosporium aphidicola
.
Sep. Purif. Technol.
47
,
105
112
.
Unnithan
M. R.
Anirudhan
T. S.
2001
The kinetics and thermodynamics of sorption of chromium(VI) onto the iron(III) complex of a carboxylated polyacrylamide-grafted sawdust
.
Ind. J. Eng. Chem.
40
,
2693
2701
.