Abstract

The marine biomass Ulva compressa L. (ECL) was used as a low-cost biosorbent for the removal of Cr(VI) from contaminated aqueous solutions. The operating variables were optimized: pH ∼ 2, initial concentration of 25 mg/L, solid/liquid ratio of 6 g/L and a temperature of 50 °C, leading to an uptake elimination of 96%. A full factorial experimental design technique enabled us to obtain a mathematical model describing the Cr(VI) biosorption and to study the main effects and interactions among operational parameters. The equilibrium isotherm was analyzed by the Langmuir, Freundlich and Dubinin–Radushkevich (D-R) models; it has been found that the adsorption process follows well the Langmuir model. Kinetic studies showed that the pseudo-second order model describes suitably the experimental data. The thermodynamic parameters indicated an endothermic heat and a spontaneity of the Cr(VI) biosorption onto ECL.

INTRODUCTION

The pollution by toxic metals has become one of the most serious environmental problems; the contamination by heavy metals originates from the waste released directly or indirectly into the aquatic environment by several industrial activities. The release of hexavalent chromium is an environmental source of pollution. It is discharged from textile industries and tanneries (Mekatel et al. 2012) and is highly hazardous because of its potential carcinogenicity. Accordingly, several treatment methods have been developed to eliminate chromate from industrial effluents, such as chemical precipitation (Esalah et al. 2000; Rengaraj et al. 2003), reverse osmosis (Mohsen et al. 2007) and adsorption on a commercially prepared activated carbon (Barkat et al. 2009). The ion exchange and adsorption onto activated carbons are expensive processes especially when they are applied to large-scale effluents. In contrast, the adsorption on biological supports has been one of the favorite and most used methods for the removal of metals. Due to its efficiency and simplicity, this technique is considered as attractive and cost-effective for the water depollution.

The use of seaweed as biosorbents in aqueous solutions has been studied in many works for its bio-accumulative qualities toward heavy metals (Antunes et al. 2003; Tabaraki & Nateghi 2014). In this context, an array of marine algae have been tested for the retention of Cr(VI). Srivastava et al. (2014) studied the biosorption of Cr(VI) from aqueous solutions by green algae. Chaker Ncibi et al. (2008) have studied the Cr(VI) adsorption by algal biomass type Posidonia oceanica while Farzadkia et al. (2012) have investigated the Cr(VI) biosorption on brown seaweed Sargassum. This work aims to study mainly the biosorption capacity of Cr(VI) by Ulva compressa L. (ECL). A model describing the phenomenon was established by using a full factorial design at two levels in order to evaluate the effect of pH, Cr(VI) concentration, solid/liquid ratio and temperature. Equilibrium, kinetic and thermodynamic studies were also undertaken.

EXPERIMENTAL

Preparation of ECL

The fronds of seaweed ECL were collected several times in the region of Tipaza, a coastal town of Algeria. Identification of species was based on the classic morphological features. The seaweed was washed with tap water to remove debris, epiphytes, and traces of sand and salt adhering to its fronds and thoroughly rinsed with distilled water, dried overnight and then in an air oven at 80 °C (48 h). The seaweed was ground cryogenically using liquid nitrogen (powder fraction <0.2 μm). A stock solution of Cr(VI) (1 g/L) was prepared by dissolving K2Cr2O7 (Merck, 99.5%) in distilled water. The pH of the diluted solutions was adjusted by HCl or NaOH (0.1 M).

Characterization of ECL

The surface morphology of the algae was observed with a Quanta 600 W FEI scanning electronic microscope (SEM) equipped with a comprehensive system of microanalysis energy dispersive spectroscopy (EDS) (Figure 1).

Figure 1

(a) SEM micrograph of ECL, (b) EDS spectrum of ECL and (c) elemental composition microanalysis of ECL.

Figure 1

(a) SEM micrograph of ECL, (b) EDS spectrum of ECL and (c) elemental composition microanalysis of ECL.

The analysis of functional groups responsible for the metal fixation on the algae was performed with Fourier transform infrared (FT-IR) spectroscopy using a Nicolet model 6700 using the KBr pellet method (400–4,000 cm−1). The thermal analysis (thermogravimetric/derivative thermogravimetric analysis (TG/DTG)) was performed with Perkin Elmer STA 6000 equipment under air flow at a heating rate of 10 °C/min using ∼30 mg of the sample.

Biosorption experiments

The point of zero charge (pHPZC) of ECL was measured for understanding the biosorption mechanism of Cr(VI) uptake on the algae surface. The method reported by Kaouah et al. (2013) was used for this purpose. The pHPZC of ECL is the point where the curve of pHfinal versus pHinitial crosses the axis of the bisectors; a value of 7.10 was found (Figure 2). At low pHs (<7.10), the ECL surface was positively charged, while for higher pHs, the negative charge predominated.

Figure 2

pHfinal vs pHinitial plot for determination of pHPZC of ECL.

Figure 2

pHfinal vs pHinitial plot for determination of pHPZC of ECL.

The biosorption experiments of Cr(VI) on ECL were carried out in batch system with synthetic solutions. Different amounts of biosorbent were contacted with 25 mL of solutions of Cr(VI) at different concentrations and various pHs, and the mixtures were stirred for 120 min.

Analytic procedures

K2Cr2O7 (99%), Ni(NO3)2·6H2O (99%) and Co(NO3)2·6H2O (99%) were used as starting reagents. The concentrations of Co2+, Ni2+ and Cr(VI) before and after equilibrium were determined according to the standard colorimetric method. The samples were filtered on 0.45 μm (Millipore) filters and immediately acidified to form colored complexes, using thiocyanate, dimethylglyoxime and diphenyl carbazide respectively for Co2+ (Tebani et al. 2017) Ni2+ (Fedailaine et al. 2016) and Cr(VI) (Mekatel et al. 2012), which absorb in the visible region (λmax = 619, 465 and 540 nm respectively) with a detection limit of ±0.2 mg/L (Marczenko & Balcerzak 2000). A UV-visible spectrophotometer (Optizen 2120 UV) was used for analytic procedures. The biosorption of Cr(VI) and the absorption capacity (qe) are given by:  
formula
(1)
 
formula
(2)
where Co and Ce are the initial and equilibrium concentrations (mg/L) respectively, V the volume of the solution (L) and W the biosorbent mass (g).

RESULTS AND DISCUSSION

Characterization of ECL

The observation of ECL by SEM image before contact with the metal solution shows that the alga has typical aggregates with needle form (Figure 1(a)). The composition analysis of ECL reveals the existence of the main chemical elements (Figure 1(c)).

Figure 3 presents the infrared absorption spectra of ECL, Cr(VI)-ECL, Co2+-ECL and Ni2+-ECL. The peaks located at 2,919 and 1,327 cm−1 are attributed to the elongation vibration of C-H; the peaks centered at 1,444 and 1,266 cm−1 are assigned to stretching vibrations of C-O and C = O bands respectively and other bands of hydroxyl group –OH. The bands 3,438 and 1,652 cm−1 are due to the symmetric stretching vibrations of -OH with hydrogen bond for the binding of metal ions (Zivorad 2004) and water molecules (Nibou et al. 2010; Meitei & Prasad 2014). The authors have reported that the peaks 3,391, 2,920, 1,638, 1,534, 1,320, 1,161 and 1,046 cm−1 are responsible of the interaction of metal ions with the functional groups on the biomass material during the biosorption. Other bands at 1,050, 848, 793, 725, 606 and 534 cm−1 are attributed probably to stretching vibrations of inorganic alpha quartz phase SiO2 (Si-O, Si-O-Si), MgCa(CO3)2 (Mg-Ca, Mg-O, Ca-O), CaCO3, NaCl, KCl and Si-O-Me where Me is the exchangeable ion metal species in the biomass. The peaks at 2,327, 1,652, 1,327 and 1,050 cm−1 are also due to the vibration bands of -C ≡ N, ›C = N-, -NO2 and ›S = O bonds; these elements were confirmed by EDS analysis.

Figure 3

FT-IR spectrum of ECL in the region 4,000–400 cm−1.

Figure 3

FT-IR spectrum of ECL in the region 4,000–400 cm−1.

The thermal characterization of ECL (Figure 4) shows the TG/DTG curves of ECL. Four transformations during the heating are observed in the regions 50–150 °C, 200–290 °C (desorption of water), 300–400 °C (oxidation of organic matter) and 450–600 °C (ECL decomposition) respectively. The total ECL loss accounting for 80% occurs at 900 °C.

Figure 4

DTG and TG curves of ECL.

Figure 4

DTG and TG curves of ECL.

Modeling biosorption

The full factorial design with two 2 k-levels is most useful and widely used (Barkat et al. 2009; Houhoune et al. 2013; Farooq et al. 2017; Regti et al. 2017). This design type has a limited number of two levels for each factor, a low level indicating the lower limit selected by the experimenter and a high level indicating the upper limit. Thus, a full 24 factorial design was designed to optimize the conditions for the Cr(VI) biosorption on ECL and to examine which main factors and interactions influence the process. The regression equation of the four factors and their interactions is as follows:  
formula
(3)
where Y(%) is the objective function (biosorption uptake) which depends on the parameters obtained for each test through Equation (3); Ao, A1…A15 are the coefficients of interaction. X1, X2, X3 and X4 are coded factors for the selected parameters: pH, initial concentration (Co), solid/liquid ratio (R) and the temperature (T) respectively.
The real and coded variables and the measured responses are gathered in Table 1; the values of 16 coefficients were determined using JMP8 software. Replacing each coefficient by its value, we obtain the model describing the biosorption of Cr(VI) on ECL:  
formula
(4)
Student's t-test was used at 5% probability i.e. 95% confidence. It compares the ratio t obtained by the software JMP8 for each factor to the critical value of Student's t-test read on the Student table at 5% probability level. The critical value of the t-distribution is 2.306 and all effects and interactions with a probability smaller than the critical value of Student's t (2.306) are not significant.
Table 1

Experimental design 24 in coded and real variables

Experiment pH X1 Co (mg/L) X2 R (g/L) X3 T (°C) X4 Y(%)exp 
−1 25 −1 −1 25 −1 82.92 
+1 25 −1 −1 25 −1 26.50 
−1 125 +1 −1 25 −1 48.14 
+1 125 +1 −1 25 −1 22.80 
−1 25 −1 +1 25 −1 92.74 
+1 25 −1 +1 25 −1 36.99 
−1 125 +1 +1 25 −1 75.60 
+1 125 +1 +1 25 −1 35.36 
−1 25 −1 −1 50 +1 81.96 
10 +1 25 −1 −1 50 +1 22.81 
11 −1 125 +1 −1 50 +1 53.01 
12 +1 125 +1 −1 50 +1 30.57 
13 −1 25 −1 +1 50 +1 96.29 
14 +1 25 −1 +1 50 +1 53.88 
15 −1 125 +1 +1 50 +1 86.33 
16 +1 125 +1 +1 50 +1 79.71 
Experiment pH X1 Co (mg/L) X2 R (g/L) X3 T (°C) X4 Y(%)exp 
−1 25 −1 −1 25 −1 82.92 
+1 25 −1 −1 25 −1 26.50 
−1 125 +1 −1 25 −1 48.14 
+1 125 +1 −1 25 −1 22.80 
−1 25 −1 +1 25 −1 92.74 
+1 25 −1 +1 25 −1 36.99 
−1 125 +1 +1 25 −1 75.60 
+1 125 +1 +1 25 −1 35.36 
−1 25 −1 −1 50 +1 81.96 
10 +1 25 −1 −1 50 +1 22.81 
11 −1 125 +1 −1 50 +1 53.01 
12 +1 125 +1 −1 50 +1 30.57 
13 −1 25 −1 +1 50 +1 96.29 
14 +1 25 −1 +1 50 +1 53.88 
15 −1 125 +1 +1 50 +1 86.33 
16 +1 125 +1 +1 50 +1 79.71 

The analysis of variance shows that the relative value F (103.93335) is significantly greater than the critical value read on the Fisher–Snedecor table and is equal to 3.3130. This indicates that the regression is globally significant.

The results show that the studied factors (pH, Co, R and T), the second-order interactions (pH-Co, pH-T, Co-R, Co-T and R-T), and the third-order interactions (pH-Co-T and pH-R-T) are statistically significant for the adsorption uptake. The insignificant terms are neglected in Equation (5) and the regression equation describing the biosorption of Cr(VI) on the ECL becomes:  
formula
(5)

Parametric study

Effect of pH

The Cr(VI) biosorption was evaluated by studying the pH between 2 and 7 (Co = 25 mg/L, T = 298 K and R = 6 g/L). Figure 5 shows that the best adsorption uptake of Cr(VI) occurs at pH 2 with 95.64% and the rate decreases proportionally with increasing pH up to 7.

Figure 5

Effect of pH on the biosorption of Cr(VI) from aqueous solution onto ECL. Experimental conditions: Co = 25 mg/L, R = 6 g/L and T = 298 K.

Figure 5

Effect of pH on the biosorption of Cr(VI) from aqueous solution onto ECL. Experimental conditions: Co = 25 mg/L, R = 6 g/L and T = 298 K.

The ionic forms of chromate in acidic medium are anionic (HCrO4, Cr2O72−, Cr3O102− and Cr4O132−) within the pH range 1.5–4 (Tewari et al. 2005; Ucun et al. 2002). The functional groups are present on the surface of ECL tales (hydroxyl OH, with hydrogen and aromatic bonds) under high protonation, which confer an overall positive charge with electrostatic attractions. Indeed, the higher the pH, the greater the concentration of hydroxyl radical that may compete with the anionic species of Cr(VI) on active sites available on the ECL surface. The adsorption of negatively charged chromate ions were facilitated at low pHs (<pHPZC = 7.1), where the surface charge is positive, thus favoring the electrostatic attraction of HCrO4 ions (predominant species).

Effect of solid/liquid ratio, initial concentration and temperature

The biosorbent dose in solution also affects the adsorption process, since it determines the availability of active sites. To study the effect of the biosorbent mass on the removal of Cr(VI) (Co = 25 mg/L), we have varied R from 1 to 6 g/L at pH ∼ 2 and 298 K. Figure 6 shows that the biosorption increases significantly with increasing R, because of the availability of more sites for the same solution volume. At low biomass concentrations, a high amount of sorbate is readily available to be captured by the available and accessible sites.

Figure 6

Influence of biosorbent dose on the biosorption of Cr(VI) on ECL. Experimental conditions: pH ∼ 2, Co = 25 mg/L and T = 298 K.

Figure 6

Influence of biosorbent dose on the biosorption of Cr(VI) on ECL. Experimental conditions: pH ∼ 2, Co = 25 mg/L and T = 298 K.

In real effluents, the chromate concentration can reach 50 mg/L or more and it is interesting to study the concentration effect. The Co was varied in the range 25–125 mg/L (T = 298 K, pH ∼ 2, R = 6 g/L). However, Figure 7 shows that in the present case, Co has not a large effect on the Cr(VI) biosorption. The chromate uptake decreases with increasing Co, indicating that fewer favorable sites become involved at high concentrations. However, the adsorption capacity increases with increasing Co. The modeling results show that the concentration Co had a negative effect on the Cr(VI) biosorption.

Figure 7

Influence of initial concentration on the biosorption of Cr(VI) on ECL. Experimental conditions: pH ∼ 2, R = 6 g/L and T = 298 K.

Figure 7

Influence of initial concentration on the biosorption of Cr(VI) on ECL. Experimental conditions: pH ∼ 2, R = 6 g/L and T = 298 K.

The effect of temperature on the biosorption of Cr(VI) by ECL was studied within the range 298–363 K keeping the other parameters constant (Co = 25 mg/L, pH 2 and R = 6 g/L). Figure 8 reveals that the biosorption rate increases with raising temperature, due to improved mobility of Cr(VI) in solution, which facilitates the occupation of active sites hardly accessible. So, one can conclude that the results of modeling showed that the temperature has a positive effect on Cr(VI) biosorption.

Figure 8

Effect of the temperature on the biosorption of Cr(VI) on ECL. Experimental conditions: pH ∼ 2, Co = 25 mg/L and R = 6 g/L.

Figure 8

Effect of the temperature on the biosorption of Cr(VI) on ECL. Experimental conditions: pH ∼ 2, Co = 25 mg/L and R = 6 g/L.

Adsorption isotherms

The models available in the literature, such as those of Langmuir, Freundlich and Dubinin–Radushkevich (D-R) (Nibou et al. 2010; Dada et al. 2012; Barkat et al. 2015), describe the experimental adsorption data. The Langmuir and D-R models were applied to establish the relationship between the amount of Cr(VI) adsorbed onto ECL and the equilibrium concentration Ce (mg/L) in aqueous solution, given by Equations (6) and (7) respectively:  
formula
(6)
where qe is the amount of Cr(VI) adsorbed per gram of adsorbent at equilibrium (mg/g), Qo the saturated monolayer sorption capacity; KL the Langmuir isotherm constant. The essential characteristics of the Langmuir isotherm can be expressed in terms of the dimensionless constant RL, known as the separation factor, given by:  
formula
(7)
The Freundlich isotherm is commonly used for the adsorption characteristics of the heterogeneous surface and it is given by:  
formula
(8)
where KF is the Freundlich isotherm constant (mg/g) and n the adsorption intensity. The D-R isotherm is generally applied to express the adsorption mechanism with a Gaussian energy distribution onto a heterogeneous surface; the linear form is given by:  
formula
(9)
where K is a constant related to the adsorption energy and qm is the theoretical saturation capacity; ɛ is the Polanyi potential and Eads the free adsorption energy.  
formula
(10)
 
formula
(11)
where R is the universal gas constant, T is the absolute temperature and Eads is the free adsorption energy.

The constants of the isotherms are gathered in Table 2. Analytically, the adequacy of the models toward our experimental results shows that the Langmuir model is more adequate to describe the adsorption, with a high correlation coefficient (R2 = 0.9634), compared to that of D-R model (R2 = 0.6689). In addition, the Langmuir model is valid for monolayer adsorption onto a surface containing a finite number of identical adsorption sites (Mekatel et al. 2015). The separation factor (RL = 0.011), which characterizes the Langmuir isotherm, lies between 0 and 1 and indicates that the biosorption of Cr(VI) on ECL is more favorable (0 < RL < 1). All the arguments relating to this phenomenon indicate that the Langmuir model fits suitably the Cr(VI) biosorption.

Table 2

The isotherm constants and kinetic parameters for the biosorption of Cr(VI) onto ECL

Isotherm constants 
 Langmuir isotherm Qo (mg/g) KL (L/mg) R2 
21.66 0.89 0.9640 
 Freundlich isotherm KF (mg/g) 1/n R2 
1.96 0.267 0.7124 
 D-R isotherm qm (mg/g) K (mol2/kJ2R2 
24.04 2.88 0.6689 
Kinetic parameters 
 Pseudo-first order K1 (min−1qe,cal (mg/g) R2 
0.053 2.983 0.842 
 Pseudo-second order K2 (g/(mg·min)) qe,cal (mg/g) R2 
 18.182  0.009 0.996 
Isotherm constants 
 Langmuir isotherm Qo (mg/g) KL (L/mg) R2 
21.66 0.89 0.9640 
 Freundlich isotherm KF (mg/g) 1/n R2 
1.96 0.267 0.7124 
 D-R isotherm qm (mg/g) K (mol2/kJ2R2 
24.04 2.88 0.6689 
Kinetic parameters 
 Pseudo-first order K1 (min−1qe,cal (mg/g) R2 
0.053 2.983 0.842 
 Pseudo-second order K2 (g/(mg·min)) qe,cal (mg/g) R2 
 18.182  0.009 0.996 

The saturation adsorption capacity (qm) obtained from the D-R isotherm model for Cr(VI)biosorption onto ECL (24.04 mg/g) (Table 3) is close to that obtained from the Langmuir model (21.66 mg/g). The magnitude of Eads is useful for estimating the type of adsorption. The Eads value between 8 and 16 kJ/mol corresponds to the ion exchange process while values smaller than 8 kJ/mol give a physisorption (Nibou et al. 2011). The experimental value (0.42 kJ/mol) indicates a physical biosorption.

Table 3

Thermodynamic parameters for the biosorption of Cr(VI) onto ECL

  ΔH° (kJ/mol) ΔS° (J/(mol·K)) ΔG° (kJ/mol) 
T (K)   298 303 313 323 
Cr(VI) 17.657 108.132 −14.116 −15.008 −16.169 −17.251 
  ΔH° (kJ/mol) ΔS° (J/(mol·K)) ΔG° (kJ/mol) 
T (K)   298 303 313 323 
Cr(VI) 17.657 108.132 −14.116 −15.008 −16.169 −17.251 

Dynamic adsorption

To characterize the order of the adsorption of Cr(VI) onto ECL, the pseudo-first order (rate constant: K1) and pseudo-second order (rate constant: K2) models were used, whose relations are given respectively by:  
formula
(12)
 
formula
(13)
where qt is biosorption capacity at time t.

The graphical representations of ln (qe − qt) and t/qt versus time result in straight lines from which the theoretical values of K1 and K2 are determined from the slopes and intercepts, respectively (Table 2). The adsorption capacity qe,cal (2.98 mg/g) is smaller than the experimental one qe,exp (17.24 mg/g, R2 = 0.842) for the pseudo-first order model. Conversely, the adsorption capacity qe,cal (18.18 mg/g) of the pseudo-second order model is close to the experimental value qe,exp (17.24 mg/g, R2 = 0.996). Therefore, the biosorption of Cr(VI) onto ECL follows a linear variation according to the pseudo-second order.

Thermodynamic study

The change in Gibbs free energy (ΔG°), enthalpy (ΔH°) and entropy (ΔS°) help to understand the biosorption mechanism of Cr(VI) on ECL. They can be evaluated using the distribution coefficient KD (mL/g) (Equation (14)) at different temperatures (Mekatel et al. 2012; Nibou et al. 2010):  
formula
(14)
ΔH° and ΔS° (Table 3) were obtained by a linear regression from the slope and intercept of the linear plot ln KD versus 1/T (Figure 9):  
formula
(15)
Figure 9

Graphical determination of the enthalpy (ΔH°) and entropy (ΔS°).

Figure 9

Graphical determination of the enthalpy (ΔH°) and entropy (ΔS°).

ΔG° is calculated from the relation:  
formula
(16)

The positive ΔS° shows that the molecules at the solid–liquid interface are disordered while the negative ΔG° (Table 3) indicates the spontaneity and feasibility of the Cr(VI) biosorption with a high affinity Cr(VI)/ECL surface (Voudrias et al. 2002). Moreover, the decrease of ΔG° with raising temperature highlights the spontaneity of the biosorption while the positive ΔH° shows an endothermic nature (Demir et al. 2008).

According to the literature (Lian et al. 2009; Srivastava et al. 2016), the change of the adsorption enthalpy for the physisorption process is in the range 20–40 kJ/mol, while for the chemisorption, the enthalpy is between −400 and −80 kJ/mol. In our case, the positive ΔH° (17.657 kJ/mol) clearly indicates a physical biosorption.

ECL affinity

It is worthwhile to report, for a comparative purpose, the biosorption of other metals. Figure 10 shows the affinity of ECL for Cr(VI), Co2+ and Ni2+ ions. The initial concentration for each metal varies in the range 25–125 mg/L at T = 298 K, R = 6, pH(Cr(VI)) = 2, pH(Co2+) = 6 and pH(Ni2+) = 5. The performance of ECL biosorption of Cr(VI) is higher than that of Co2+ and Ni2+ ions. The affinity varies as follows: Cr(VI) > Co2+ > Ni2+. The analysis of spectra illustrated in Figure 3 confirms the ECL affinity toward Cr(VI) and Co2+ when closely examined; both spectra are different from that of ECL.

Figure 10

Affinity of ECL biosorption of Cr(VI), Co2+ and Ni2+ ions.

Figure 10

Affinity of ECL biosorption of Cr(VI), Co2+ and Ni2+ ions.

Comparative adsorption capacities onto other material adsorbents

The maximum capacity for the adsorption of Cr(VI) according to the Langmuir model shows that ECL biomass has a high potential compared to other biosorbents tested in several research works on Cr(VI) biosorption according to the Langmuir model. A comparison of the maximum biosorption capacity of the ions on various adsorbents is presented in Table 4.

Table 4

Comparison of maximum adsorption capacities, qm, of Cr(VI) by various adsorbents

Adsorbents qm (mg/g) Reference 
Green algae 3.50 Srivastava et al. (2014)  
Brown seaweed Sargassum 4.21 Farzadkia et al. (2012)  
Wool, sawdust, pine needles, almond hulls and cactus leaves 7.08 Dakiky et al. (2002)  
Activated carbon 8.02 Barkat et al. (2009)  
γ-Fe2O3/mesoporous 0.30 Wang & Lo (2009)  
Montmorillonite 0.15 Yuan et al. (2009)  
ECL 21.66 This work 
Adsorbents qm (mg/g) Reference 
Green algae 3.50 Srivastava et al. (2014)  
Brown seaweed Sargassum 4.21 Farzadkia et al. (2012)  
Wool, sawdust, pine needles, almond hulls and cactus leaves 7.08 Dakiky et al. (2002)  
Activated carbon 8.02 Barkat et al. (2009)  
γ-Fe2O3/mesoporous 0.30 Wang & Lo (2009)  
Montmorillonite 0.15 Yuan et al. (2009)  
ECL 21.66 This work 

It is clear that ECL biosorbent shows a considerably better performance than the other ones. Therefore, it appears that the abundance of this marine alga and its ability to remove the Cr(VI) ions with high performances are beneficial for this biological material to be used as an effective biosorbent.

CONCLUSION

In the present work, algal biomass ECL was used for the recovery of Cr(VI) ions from aqueous solution. The FT-IR analysis of natural ECL showed rich functional groups, responsible for the Cr(VI) biosorption. The operating parameters, i.e. pH, initial HCrO4 concentration, solid/liquid ratio and temperature were optimized, leading to an elimination rate of 96%. It has been shown that the experimental design method is effective to identify influential factors and interactions on the Cr(VI) biosorption. The pH is the crucial parameter influencing the phenomenon followed by the solid/liquid ratio, the concentration and finally the temperature. The results revealed that the Langmuir model describes satisfactorily the biosorption process. The biosorption data were modeled using kinetic equations of pseudo-first order and pseudo-second order. The latter describes suitably the kinetics of Cr(VI) uptake. The thermodynamic study showed that the adsorption of Cr(VI) on the algae is a spontaneous and endothermic process. The biosorption uptakes of the Cr(VI), Co2+ and Ni2+ ions onto ECL follow the sequence: Cr(VI) > Co2+ > Ni2+. In view of the results obtained in this study, the available cheap algal biomass Ulva compressa (L.) is a promising biological material, used as an effective bio-adsorbent for recovery of Cr(VI) ions.

ACKNOWLEDGEMENTS

This work was supported by the Laboratoire de Technologie des Matériaux, Algiers. The authors are indebted to Dr N. Boudechiche for his helpful discussion.

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