Physico-chemical characteristics and lead biosorption properties of Enteromorpha prolifera

Colloids and Surfaces B: Biointerfaces 85 (2011) 316–322

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Physico-chemical characteristics and lead biosorption properties of Enteromorpha prolifera Yan-Hui Li a,∗ , Qiuju Du a , Xianjia Peng b , Dechang Wang a , Zonghua Wang a , Yanzhi Xia a,∗ , Bingqing Wei c a b c

Shandong Provincial Key Laboratory of Fiber Materials and Modern Textile, the Growing Base for State Key Laboratory, Qingdao University, Qingdao 266071, China State Key Laboratory of Environmental Aquatic Chemistry, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, China Department of Mechanical Engineering, University of Delaware, Newark, DE 19716, USA

a r t i c l e

i n f o

Article history: Received 12 November 2010 Received in revised form 2 March 2011 Accepted 2 March 2011 Available online 10 March 2011 Keywords: Biosorption Enteromorpha prolifera Lead Pseudo second-order kinetic model Thermodynamics

a b s t r a c t Biosorption of lead ions onto Enteromorpha prolifera has been investigated. The physico-chemical properties of the biosorbent were characterized by thermal stability, zeta potential, and Boehm titration methods. Batch adsorption experiments were carried out to examine the effect of various parameters such as initial pH, particle size, adsorbent dosage, ionic strength, time, and temperature on biosorption. The kinetic studies showed that the adsorption process was very fast and equilibrium was reached after about 60 min of contact. The pseudo-first-order Lagergren equation, pseudo second-order rate equation, and second-order rate equation were used to describe the kinetic adsorption process. Thermodynamic parameters were determined at three different temperatures. The negative values of free energy change indicated the spontaneous nature of adsorption process. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Pollution of the environment by heavy metals such as lead, mercury, copper, zinc, and cadmium is serious problem because of their toxic effects on human health and living organism [1]. One of them, lead is non-biodegradable and tends to bioaccumulate in bones, brain, kidney and muscles. Long term consumption of water containing excessive lead can cause serious chronic or acute diseases [2]. Therefore, it must be removed to a permissible level from the wastewater before being discharged into drainage system. Many methods such as chemical precipitation [3], electrolysis [4], ion exchange [5], reverse osmosis, and membrane separation [6] have been developed to remove lead from wastewater. However, these technologies are either expensive due to the disposal of the secondary toxic metal sludge or ineffective when lead is present in the wastewater at low concentrations [7]. Recently, biosorption has gained increased concern in removing trace amounts of toxic metals from dilute aqueous solutions due to its high efficiency, low operating cost, minimization of chemical sludge, and the possibility of metal recovery [8,9]. The heavy metal sorption capabilities of biosorbents have mainly been attributed to the cell wall structure containing functional groups such as amino, hydroxyl, carboxyl and sulphate, which can act as binding sites for metals via both elec-

∗ Corresponding authors. Fax: +86 532 85951842. E-mail address: [email protected] (Y.-H. Li). 0927-7765/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfb.2011.03.003

trostatic attraction and complexation [10]. Various biomass types such as fungi [11], plant [12], yeast [13], and algae [14] have been screened and studied extensively in recent years with the aim of identifying highly efficient lead removal biosorbents. Among them, algae, a widely available marine biomaterial, have proved to be economic, eco-friendly, and effective heavy metal adsorbents. Brown algae such as Fucus spiralis [15], Sargassum hystrix [16], and Laminaria japonica [17], red algae such as Gelidium [18], and green algae such as Spirogyra [19], Cladophora fascicularis [20], and Chlorella vulgaris [21] were used for lead removal and had shown different lead biosorption capabilities. However, Enteromorpha prolifera (E. prolifera), a green alga, has seldom been used as biosorbent in removing the heavy metals from aqueous solutions. There were only few studies on using E. prolifera as biosorbent to remove copper [22] and nickel [23] from wastewater. So far, no information is available for lead removal by E. prolifera. Moreover, previous studies mainly focused on analyzing the effect of influencing parameters such as pH, temperature, time, initial metal ion and biosorbent dosage on metal ion adsorption behaviors. There are very few publications giving a detailed illustration to the effect of physico-chemical properties of E. prolifera on heavy metal removal characterization. The purpose of this study was to investigate the lead biosorption properties by E. prolifera. The physico-chemical properties of E. prolifera, such as thermal stability, zeta potential, and surface functional groups, were characterized. The effect of various experimental parameters on the biosorption process, such as pH, adsorbent particle size and metal ion concentration, were inves-

Y.-H. Li et al. / Colloids and Surfaces B: Biointerfaces 85 (2011) 316–322

tigated. The equilibrium and kinetic models were also applied to describe and analyze the experimental data. 2. Materials and methods

317

The effect of biosorbent dosage on the Pb2+ removal was conducted by adding different amounts (0.02–0.17 g) of adsorbent in size of 89 ␮m into 100 mL solutions with Pb2+ concentrations of 20, 40, and 60 mg/L, respectively, and shaken for 12 h at pH 5.0 and at a temperature of 25 ◦ C.

2.1. Sample preparation and characterization E. prolifera, collected from the Yellow Sea coast in Qingdao, China, was washed several times using tap water to remove salts. It was dried in an oven at 100 ◦ C for 24 h until all the moisture evaporated. The dried E. prolifera was ground by ball milling (XQM0.4L, Nanjing Daran group company, China) to 89, 124 and 250 ␮m, respectively, and then stored in a desiccator. The thermogravimetric analysis of E. prolifera was carried out with a heating rate of 10 ◦ C/min from 30 ◦ C to 800 ◦ C in an atmosphere of air flowing at 50 mL/min using a Mettler TGA/STDA 851 thermogravimetric analyzer. Zeta potential of E. prolifera was measured by a Malvern zetameter (Zetasizer 2000, Malvern Instruments Ltd, UK). The pH values of E. prolifera solution were adjusted from 2.0 to 8.0 by adding 0.1 M hydrochloric acid or sodium hydroxide solution to the glass beaker at 25 ◦ C. By measuring the zeta potential as function of pH, the acidity or basicity of E. prolifera surface and the isoelectric point (IEP) can be determined. Boehm titration method [24] was used to determine the amount of the acidic surface groups on the adsorbents. The experimental procedure is to disperse 0.2 g of E. prolifera in 50 mL deionized water. The suspension was then mixed with 10 mL of 0.1 M base solutions of sodium hydroxide, sodium hydrogen carbonate, and sodium carbonate and stirred in a sealed vessel for 48 h. After this period, the suspension was filtrated and 20 mL filtrate was added to 15 mL of 0.1 M hydrochloric acid, which neutralized the unreacted bases. The solution was then back-titrated with 0.1 M sodium hydroxide, and the volume of sodium hydroxide used to titrate the solution pH to 7.0 was recorded. 2.2. Batch biosorption tests Stock solution of Pb2+ ions (1000 mg/L) was prepared by dissolving the analytical reagent grade lead nitrate (Rgent, Tianjin, China) in deionized water. The stock solution was further diluted to the required concentrations before use. Concentration of lead was measured by atomic adsorption spectrometer (Shimadzu model AA-670). Amount of lead adsorbed at equilibrium was calculated using the following equation: qe =

C − C  e 0 m

V

2.3. Kinetic study Adsorption kinetics studies were conducted by adding 0.05 g of adsorbents into the 100 mL solutions with different Pb2+ concentrations (20, 40, and 60 mg/L) in a 250 mL flask at 25 ◦ C. The solution pH was controlled at 5.0. At predetermined time intervals, samples were collected utilizing a 0.45 ␮m membrane filter and then analyzed by an atomic adsorption spectrometer. The amounts of Pb2+ adsorbed were calculated using the Eq. (1).

2.4. Thermodynamic study To evaluate the thermodynamic properties, 0.05 g adsorbents were added into 100 mL solutions with pH of 5.0 and initial Pb2+ concentration ranging from 10 to 60 mg/L in a step size of 10 mg/L. These samples were shaken continuously for 12 h at three different temperatures (5 ◦ C, 25 ◦ C, and 35 ◦ C).

3. Results and discussion 3.1. Biosorbent characterization 3.1.1. Thermal stability analysis The TGA/DTA plots of E. prolifera (Fig. 1) exhibit a stepwise degradation pathway corresponding to the thermal decomposition of the different organic components. The weight loss shows three distinctive successive regions. The first stage is a process of dehydration and desiccation. The release of absorbed water molecules leads to a weight loss of 13% at a temperature range of 30–180 ◦ C and an endothermic peak of 73 ◦ C is found in the DTA curve. During the second stage, the decomposition of organics such as proteins and carbohydrates makes the weight losses from 87% to 36% at the temperature from 180 ◦ C to 500 ◦ C. The third stage is a combustion process of char. The burning of the char can give off lots of heat, which leads to an obvious exothermic peak at 559 ◦ C in the DTA curve [25]. The weight reduction is 22% due to the combustion of the char.

(1)

where C0 and Ce were initial and equilibrium concentrations of metal ion (mg/L), respectively, m was the mass of adsorbent (g) and V was volume of the solution (L). The effect of pH on the Pb2+ adsorption was studied in a pH range of 2.0–6.5. The pH of the 100 mL solution with Pb2+ concentrations of 20, 40 and 60 mg/L, respectively, was adjusted to the required pH using appropriate concentrations of HNO3 or NaOH (Rgent, Tianjin, China) solutions. The particle size of E. prolifera was 89 ␮m, the dosage was 0.05 g and the adsorption time was 12 h. The influence of particle size on the Pb2+ adsorption was carried out by putting 0.05 g adsorbents with different sizes into 100 mL solution with Pb2+ concentrations from 10 to 60 mg/L in an increment of 10 mg/L and agitated for 12 h at pH 5.0 and at a temperature of 25 ◦ C. The same experimental conditions were adopted to study the influence of ionic strength on Pb2+ adsorption. The particle size of E. prolifera was 89 ␮m, and the ionic strengths of the solutions were regulated at 0.01, 0.05, and 0.1 M using NaNO3 solution.

Fig. 1. TGA/DTA profiles of E. prolifera (heating rate: 10 ◦ C/min, air flow rate: 50 mL/min).

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3.1.2. Zeta potential Zeta potential is a physical parameter to quantify the electrical potential of the solid particle surface [26]. The pH value at which the zeta potential equals zero is called the isoelectric point (IEP), and it is used to qualitatively assess the adsorbent surface charge [27]. The IEP of E. prolifera is 4.4, suggesting that E. prolifera possesses an acidic surface. At pH < 4.4, E. prolifera has positive surface charge and it may act as anion exchanger. While at pH > 4.4, the surface charge of E. prolifera is negative, consequently, coulombic attraction can readily take place due to the interaction between cations and biosorbent. The acidic surface of E. prolifera may be attributed to the dissociation of carboxyl groups in alginic acid as well as some contribution from asparitic acid and glutamic acid in algal proteins [28]. 3.1.3. Boehm titration The functional groups can be quantitatively calculated by the Boehm titration method [26]. The method is based on the fact that sodium hydrogen carbonate only neutralizes carboxyl groups, sodium carbonate neutralizes carboxyl groups and lactones, and sodium hydroxide can neutralize carboxyl groups, lactones and phenols. So the different kinds of functional groups can be calculated through the known volume of used acid and bases. The concentrations of carboxyls, phenols, and lactones determined for E. prolifera are 0.87, 0.65, and 0.75 mmol/g, respectively. The large amount of the functional groups on E. prolifera benefits for improving the biosorption capacities of heavy metal ions [29]. 3.2. Biosorption of Pb2+ 3.2.1. Effect of initial pH It is well know that pH could affect the ionization of the functional groups on the biosorbents as well as the metal ion chemistry [30]. The effect of initial solution pH on Pb2+ removal by E. prolifera is shown in Fig. 2. At pH 2.0, the surface of adsorbent surrounded by hydronium ions [30] causes the competition of H+ and Pb2+ for the same adsorption active sites [29], which leads to the lower Pb2+ adsorption capacity. Moreover, at this pH the positive surface charge (IEP = 4.4) repels cations due to the electrostatic force of repulsion. With increase in pH, the amount of Pb2+ adsorbed increases gradually. At pH > 4.4, the Pb2+ adsorption capability increases rapidly with increment of the negative charged density on the biosorbent surfaces due to ionization of COOH groups present in E. prolifera [29]. The pH dependence of Pb2+ adsorbed by E. prolifera suggests that ion exchange, electrostatic interaction, complexation,

100

60 ppm 40 ppm 20 ppm

qe (mg/g)

80

60

40

20 2

3

4

5

6

7

pH Fig. 2. Effect of pH on Pb(II) biosorption by E. prolifera (temperature: 25 ◦ C, dosage: 0.5 g/L, contact time: 12 h, particle size: 89 ␮m).

120

100

80

qe (mg/g)

318

60

89 µm 124 µm 250 µm

40

20 0

5

10

15

20

25

30

Ce (mg/L) Fig. 3. Particle size effect on Pb(II) biosorption by E. prolifera (temperature: 25 ◦ C, dosage: 0.5 g/L, contact time: 12 h, pH: 5.0).

and chelation may be involved in the binding mechanism [31]. Because the speciation diagram of lead shows that at pH > 6.0 the species such as [Pb(OH)]+ , [Pb3 (OH)4 ]+ , and [Pb(OH)2 ] will be produced [32], in order to guarantee to truly examine the adsorption property of E. prolifera as well as to avoid precipitation of Pb2+ , all the following experiments were conducted at pH 5.0. 3.2.2. Influence of particle size Fig. 3 shows the effect of different adsorbent particle sizes on the Pb2+ adsorption capability by E. prolifera. It can be seen that the Pb2+ adsorption capacities decrease from 87.44 to 38.42 mg/g with the increased particle size from 89 to 250 ␮m at an equilibrium concentration of 5 mg/L. Decrease in the particle size would lead to an increase in surface area [33], and the increase in the binding opportunities between Pb2+ and the functional groups at the surface of the adsorbent. On the other hand, the small particle size shows benefits for the decrease of the diffusion resistance in mass transfer during the intra-particle diffusion of the adsorbate in the pores of the adsorbent. Therefore, the Pb2+ adsorption capacity of E. prolifera has a great improvement with decreasing the particle size. 3.2.3. Effect of dosage The effect of adsorbent dosage on the equilibrium adsorption capacity and the percentage removal of Pb2+ by E. prolifera at initial pH 5.0 is shown in Fig. 4. It is apparent that the Pb2+ removal percentage increases with increasing E. prolifera dosage. This may be due to the greater amount of adsorbent surface and pore volume available at higher adsorbent dosage providing more functional groups and active adsorption sites that result in a higher Pb2+ removal percentage [34]. After a rapid increase in the removal percentage of Pb2+ with increasing in adsorbent dosage from 0.02 to 0.08 g, it only has a slight change as the adsorbent dosage increases from 0.08 to 0.17 g. The minute rise in the Pb2+ removal percentage may be attributed to the attainment of equilibrium between adsorbate and adsorbent under the experimental conditions [35]. At the same E. prolifera dosage, the removal percentage of Pb2+ decreases with increasing Pb2+ concentration from 20 to 60 mg/L. Fig. 4 also shows that the equilibrium adsorption capacity of Pb2+ by E. prolifera decreases with increasing dosage. At low E. prolifera dosage, all active sites are entirely exposed and the adsorption on the surface is saturated faster, resulting in a higher qe value. While at higher E. prolifera dosage, only parts of active sites are occupied by Pb2+ , leading to lower qe values [19]. At the same E. prolifera dosage, higher Pb2+ concentration increases the diffusion

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200

100

90

Removal (%)

150

20 ppm 40 ppm 60 ppm

80

100 70

50

60

50 0.00

0.03

0.06

0.09

0.12

0.15

0.18

Dosage (g) Fig. 4. Dosage effect on Pb(II) biosorption by E. prolifera (temperature: 25 ◦ C, contact time: 12 h, pH: 5.0, particle size: 89 ␮m).

3.3. Kinetic studies The effect of contact time on Pb2+ adsorbed by E. prolifera was studied and the kinetic profile (Fig. 6a) shows a distinct two-step adsorption behavior. The adsorption rate of Pb2+ is faster during the first 10 min and then gradually decreases with time until saturation is reached in about 1 h. The first fast stage is due to that the higher driving force makes metal ions fast transfer to the surface of adsorbent particles, thus the active sites with higher affinity are first occupied. Subsequently, with the decrease of the driving force, the remaining active sites with lower affinities are occupied slowly. Moreover, metal ions will take a longer time to diffuse into the intra-particle pores of the adsorbent to reach the equilibrium [39]. In order to predict the kinetic mechanism that determines the adsorption process, the pseudo-first-order Lagergren equation, pseudo second-order rate equation, and second-order rate equation [40] were applied to analyze the data, and their corresponding Eqs. (2), (3) and (4) are listed as follows: log(qe − qt ) = log qe −

120 NaNO3=0.00 M NaNO3=0.01 M

90

NaNO3=0.05 M

qe (mg/g)

NaNO3=0.10 M

60

30

0

5

10

15

20

25

30

Ce (mg/L) Fig. 5. Ionic strength effect on Pb(II) biosorption by E. prolifera (temperature: 25 ◦ C, dosage: 0.5 g/L, contact time: 12 h, pH: 5.0, particle size: 89 ␮m).

driving force of Pb2+ to transfer in the pores in the adsorbent and raises the contact probability of metal ions between the aqueous and solid phases [35], which results in higher equilibrium adsorption capacity. 3.2.4. Effect of ionic strength Fig. 5 shows that increasing the ionic strength leads to a significant decrease in Pb2+ adsorption by E. prolifera. The adsorption capacity of Pb2+ by E. prolifera is 93.2 mg/g at a Pb2+ equilibrium concentration of 5 mg/L as no NaNO3 was added to the test solutions (ionic strength = 0.00 M). It has a slight decrease and reaches 85.4 mg/g at ionic strength of 0.01 M. When ionic strength increases to 0.1 M, the adsorption capacity decreases sharply and is only 31.9 mg/g. The effect of ionic strength on Pb2+ adsorption may be attributed to that at high ionic strength, adsorption sites will be surrounded by Na+ ; Na+ could compete with the Pb2+ and reduce the adsorption capability of E. prolifera [36]; on the other hand, the negative charged groups come from deprotonated free carboxyl and other functional groups will partially lose their charge due to the high Na+ concentration, and this weakens the binding force by an electrostatic interaction [37]. The ionic strength dependence suggests that ion exchange is the main mechanism for Pb2+ adsorbed by E. prolifera [38].

319

k1 t 2.303

(2)

t 1 t = + qt qe 2k2 q2e

(3)

1 1 = + k3 t qe − qt qe

(4)

where kl is the Lagergren rate constant of adsorption (1/min), k2 is the pseudo second-order rate constant of adsorption (g/mg min) and k3 the rate constant (g/mg min). Linear plots of log (qe − qt ) vs. t, t/qt vs. t and 1/(qe − qt ) vs. t are shown in Fig. 6b, c, and d, respectively. All constants are calculated from the slopes and intercepts and are listed in Table 1. The correlation coefficients, r2 , of the pseudo-first-order Lagergren and second-order rate models are less than 0.9789 and 0.6314, respectively, which suggests that both models are inapplicable to fit the kinetic experimental data. The correlation coefficient of the pseudo-second-order rate model is very close to 1, and the calculated qe values also agree very well with the experimental data. These indicate that the kinetic modeling of Pb2+ adsorbed by E. prolifera belongs to the second-order kinetic model. 3.4. Thermodynamic study The effect of temperature on Pb2+ adsorbed by E. prolifera has been investigated at three different temperatures and is given in Fig. 7a. The adsorption capacity increases from 57.2 to 107.2 mg/g at a Pb2+ equilibrium concentration of 5 mg/L as temperature rises from 5 to 35 ◦ C. The increase in adsorption capacity with increasing temperature indicates endothermic nature of the adsorption proTable 1 Kinetic parameters for the adsorption of Pb2+ onto E. prolifera. Lead concentration (mg/L)

20

40

Pseudo-first-order k1 (×10−3 ) (1/min) qe (mg/g) r2

60

69.13 24.52 0.9767

87.75 52.24 0.9789

96.74 84.58 0.9749

Pseduo-second-order k2 (×103 ) (g/mg min) qe (mg/g) r2

10.26 39.37 0.9998

74.52 75.70 0.9998

210.18 110.25 0.9996

Second-order k3 (g/mg min) qe (×10−3 ) (mg/g) r2

2.22 55.80 0.6195

4.28 28.69 0.6109

6.55 21.09 0.6314

320

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120

2.0

a

b 20 mg/L 40 mg/L 60 mg/L

1.6 1.2

log(qe-qt)

qe (mg/g)

90

60

0.8 0.4

30

20 mg/L 40 mg/L 60 mg/L

0.0

0 0

20

40

60

80

100 120 140 160 180 200

0

10

20

5

c

40

50

d

120

20 mg/L 40 mg/L 60 mg/L

100

1/(qe-qt)

4

30

Time (min)

Time (min)

t/qt

3

2

80 60 40

1

20 mg/L 40 mg/L 60 mg/L

20

0

0 0

30

60

90

120

150

180

0

40

80

120

160

Time (min)

Time (min)

Fig. 6. (a) Effect of contact time on Pb(II) adsorption rate by E. prolifera, (b) pseudo-first-order Lagergren kinetic plot, (c) pseudo second-order kinetic plot, (d) second-order kinetic plot.

120

308 K 298 K 278 K

4.0 100 3.5

80 lnCs/Ce

qe (mg/g)

where as is the activity of adsorbed Pb2+ , ae is the activity of Pb2+ in solution at equilibrium, Cs is the amount of Pb2+ adsorbed by per mass of E. prolifera (mmol/g), vs is the activity coefficient of the adsorbed Pb2+ and ve is the activity coefficient of Pb2+ in solution. As Pb2+ concentration in the solution decreases and approaches zero, K0 can be obtained by plotting ln(Cs /Ce ) vs. Cs and extrapolating Cs to zero (Fig. 7b) [42]. A linear regression analysis finds that the straight line fits the data well, the values of K0 are obtained from the straight line intercept with the vertical axis. The calculated values of K0 at temperatures of 5, 25, and 35 ◦ C are 3.044, 3.843, and 4.151, respectively. The average standard enthalpy change (H0 ) is obtained from Van’t Hoff equation:

b

a

60 308 K 298 K 278 K

40

3.0

2.5

20 2.0 0

3

6

9

Ce (mg/L)

12

0.4

0.8

1.2 Cs

1.6

2.0

Fig. 7. (a) Temperature effect on Pb(II) adsorbed by E. prolifera, (b) plots of ln Cs /Ce vs. Cs for estimation of thermodynamic parameters.

cess. The phenomenon is similar to the previous reports on Pb2+ adsorbed by different algae [19]. The influence may be attributed to the formation of some new active sites, or to an increase in the diffusion rate of metal ions from the bulk solution to the surface of biosorbent [41]. Thermodynamic parameters can be calculated from the variation of the thermodynamic equilibrium constant K0 with the change in temperature [42]. For adsorption reactions, K0 is defined as follows: K0 =

vs Cs as = ae ve Ce

(5)

ln K0 (T3 ) − ln K0 (T1 ) =

−H 0 R

1

T3

−

1 T1



(6)

where T3 and T1 are two different temperatures. The calculated value of H0 is 1759.08 cal/mol. The adsorption standard free energy changes (G0 ) can be calculated according to: G0 = −RT ln K0

(7)

where R is the universal gas constant (1.987 cal/mol K) and T is the temperature in Kelvin. The standard entropy change (S0 ) can be obtained by: S 0 = −

G0 − H 0 T

(8)

The calculated values of G0 at temperatures of 5, 25, and 35 ◦ C are −871.08, −797.15, and −614.90 cal/mol, respectively. The calculated values of S0 at temperatures of 5, 25, and 35 ◦ C are 9.46, 8.58, and 7.71 cal/mol•K, respectively. Positive values of H0

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321

Table 2 Parameters of Langmuir and Freundlich adsorption isotherm models for Pb2+ adsorbed by E. prolifera. Temperature (◦ C)

5 25 35

Langmuir

Freundlich 2

qmax mg/g)

kL (L/mg)

r

n

kF (mg/g)

r2

127.1 134.8 174.2

0.16 0.35 0.34

0.9936 0.9683 0.9725

1.51 1.55 1.51

18.42 32.14 40.13

0.9918 0.9719 0.9830

Table 3 Adsorption capacities of Pb2+ by various adsorbents. Adsorbents

Experimental conditions

Acorn waste Marine algae Azolla filiculoides Mucorrouxil Sago waste Carbon nanotubes Activated carbon E. prolifera

Ref.

Dosage (g/L)

pH

Temp. (◦ C)

10.0 2.0 5.0 0.2 >3.0 0.5 2.0 0.5

5.0 4.5 3.5–4.5 6.0 4.5–5.5 5.0 5.68 5.0

25 30 25

30 25

3.5. Adsorption isotherms In order to investigate how Pb2+ interacts with adsorbents, the Langmuir and Freundlich models were applied to describe the isotherm data obtained at three temperatures. The Langmuir isotherm assumes a surface with homogeneous binding sites, equivalent sorption energies, and no interaction between adsorbed species. Its mathematical form is written as: (9)

where qmax is the maximum adsorption capacity corresponding to complete monolayer coverage (mg/g) and kL is a constant indirectly related to the energy of adsorption (L/mg), which characterizes the affinity of the adsorbate towards the adsorbent. A straight line is obtained when Ce /qe was plotted against Ce and qmax and kL could be calculated from the slopes and intercepts (Table 2). Although the correlation coefficients r2 are not high (>0.9683), the Langmuir equation can be used to roughly fit the adsorption data. The increase of bonding energy coefficient (kL ) with increasing temperature indicates that the Pb2+ affinity of E. prolifera increases with temperature [44]. The maximum adsorption capacity calculated at 25 ◦ C is 134.8 mg/g, it reaches 174.2 mg/g as temperature increases to 35 ◦ C. Table 3 compares maximum adsorption capacities obtained in this study with some other values reported in the literature. The comparison shows that E. prolifera has high Pb2+ adsorption capacity at similar experimental conditions, suggesting that it is a promising adsorbent to remove heavy metals from aqueous solutions. The Freundlich isotherm is an empirical equation based on an exponential distribution of adsorption sites and energies. It is represented as lnqe = ln kF +

1 ln ce n

100 10–200 10–400 10 50–100 2–14 10–50 10–60

25

suggest that the interaction of Pb2+ adsorbed by E. prolifera is endothermic process, which supported by the increasing adsorption of Pb 2+ with the increase in temperature. The negative values of G0 reveal the fact that adsorption process was spontaneous. The positive values of S0 indicate increased randomness at the adsorbent/solution interface during the adsorption of Pb 2+ onto E. prolifera [43].

Ce Ce 1 = + qe qmax qmax kL

Concentration (mg/L)

(10)

where kF (mg/g) and n are Freundlich constants related to adsorption capacity and adsorption intensity, respectively. From

qmax (mg/g) 96.8 126.5 93.0 53.8 13.9 17.4 43.85 134.8

[2] [16] [45] [46] [47] [48] [49] This study

the slope and intercept of the straight portion of the linear plot obtained by plotting ln qe against ln Ce , the values of Freundlich parameters were calculated and listed in Table 2. From the higher values of correlation coefficients r2 , it can be seen that the adsorption data fit Freundlich isotherm models. The Freundlich coefficient, kF , is defined as an adsorption or distribution coefficient used to describe the amount of Pb2+ adsorbed onto the adsorbents for the unit equilibrium concentration. The higher values of kF in Table 2 indicate that E. prolifera has a higher Pb2+ adsorption capacity. 4. Conclusions The biosorption behavior of lead (II) onto E. prolifera was investigated in the batch experiments. The adsorption was found to be dependent on pH, adsorbent dosage, particle size, and ionic strength. The rate of adsorption was rapid and it only took about 60 min to reach equilibrium. Isotherm analysis of the data showed that the adsorption of Pb2+ onto E. prolifera followed the Langmuir model. Using the Langmuir model equation, the maximum Pb2+ adsorption capacity of E. prolifera was found to be 134.8 mg/g at pH 5.0 and at a temperature of 25 ◦ C. The pseudo-second-order rate equation described the kinetic data best. Thermodynamic parameters depicted the endothermic nature of adsorption and the process was favorable. Acknowledgements This work was supported by the National Natural Science Foundation of China (50802045 and 20975056), SRF for ROCS, SEM, the Middle-aged and Youth Scientist Incentive Foundation of Shandong Province (BS09018) and the Taishan Scholar Program of Shandong Province, China. References [1] D. Sud, G. Mahajan, M.P. Kaur, Agricultural waste material as potential adsorbent for sequestering heavy metal ions from aqueous solutions—a review, Biores. Technol. 99 (2008) 6017–6027. [2] A. Örnek, M. Özacar, I˙ .A. S¸engil, Adsorption of lead onto formaldehyde or sulphuric acid treated acorn waste: equilibrium and kinetic studies, Biochem. Eng. J. 37 (2007) 192–200. [3] Z. Djedidi, M. Bouda, M.A. Souissi, R.B. Cheikh, G. Mercier, R.D. Tyagi, J.F. Blais, Metals removal from soil, fly ash and sewage sludge leachates by precipitation and dewatering properties of the generated sludge, J. Hazard. Mater. 172 (2009) 1372–1382.

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