Cd (II) removal from aqueous solution by Eleocharis acicularis biomass, equilibrium and kinetic studies

Bioresource Technology 101 (2010) 2637–2642

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Cd (II) removal from aqueous solution by Eleocharis acicularis biomass, equilibrium and kinetic studies Patricia Miretzky *, Carolina Muñoz, Alejandro Carrillo-Chavez Centro de Geociencias-Universidad Nacional Autónoma de México, Campus Juriquilla, Boulevard Juriquilla 3001, Queretaro 76230, Mexico

a r t i c l e

i n f o

Article history: Received 28 July 2009 Received in revised form 1 October 2009 Accepted 25 October 2009 Available online 24 November 2009 Keywords: Biosorption Macrophyte biomass Cadmium Isotherms Kinetic models

a b s t r a c t Batch experiments were carried out to determine the capacity of Eleocharis acicularis biomass to adsorb Cd2+ ions from contaminated solutions with respect to pH, initial Cd2+ concentration, contact time, solution ionic strength and biomass dose. The experimental data were modeled by Langmuir, Freundlich and Dubinin–Radushkevich (D–R) isotherm models. Freundlich and D–R models resulted in the best fit of the adsorption data. The maximum adsorption capacity for Cd2+ was 0.299 mmol g1 (33.71 mg g1) with efficiency higher than 80% (pH 6.0 and 5 g L1 biomass dose). The mean adsorption free energy value derived from the D–R model (8.058 kJ mol1) indicated that adsorption was governed by an ionic exchange process. The pseudo-first order, pseudo-second order, Elovich kinetic models and the intraparticle diffusion models were used to describe the kinetic data and to evaluate rate constants. The best correlation was provided by the second-order kinetic model, implying that chemical sorption was the rate-limiting step, although intra-particle diffusion could not be ignored. The practical implication of this study is the development of an effective and economic technology for Cd2+ removal from contaminated waters. The macrophyte biomass used in this study did not undergo any chemical or physical pre-treatment, which added to macrophyte abundance and its low cost makes it a good option for Cd2+ removal from waste water. Ó 2009 Elsevier Ltd. All rights reserved.

1. Introduction Water contamination with heavy metals is a very important problem all over the world. Cadmium is one of the most toxic heavy metals, it has been reported to cause renal dysfunction, hypertension, lung insufficiency, bone lesions, cancer, etc. (Kazi et al., 2008). The drinking water guideline value recommended for this element by the WHO and American Water Works Association (AWWA) is 0.005 mg L1 (Mohan and Singh, 2002). The principal industrial sources of Cd in the environment are electroplating, smelting, alloy manufacturing, pigments, plastic, battery, mining and refining processes. A number of methods including ion exchange, reverse osmosis, precipitation, solvent extraction, membrane technologies, electrochemical treatment, sorption, are available to remove toxic metals from water. The latter is by far the most versatile and widely used method, and activated carbon is the most commonly used sorbent. However, the use of activated carbon is expensive, so there has been considerable interest in the use of other sorbent materials, particularly biosorbents. Cd can be removed by inexpensive biological materials such as algae, fungi and bacteria and also low cost agrowastes (Kaewsarn

* Corresponding author. Tel.: +52 442 2381104x132; fax: +52 442 2381100. E-mail address: [email protected] (P. Miretzky). 0960-8524/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2009.10.067

and Yu, 2001; Tangaromsuk et al., 2002; Cruz et al., 2004; Lodeiro et al., 2006; Anayurt et al., 2009; Sari and Tuzen, 2009). Certain dried plants have been shown to be highly efficient in detoxifying dilute effluents. Such biosorbents minimize the volume of chemical and/or biological sludge, require no nutrients, are of low cost, renewable and relatively easy to transport and handle (Veglio and Beolchini, 1997). There are few studies on Cd biosorption by dead macrophytes (Elifantz and Tel-Or, 2002; Miretzky et al., 2006; Rakhshaee et al., 2006; Bunluesin et al., 2007; Verma et al., 2008), among them is Eleocharis acicularis, a submerged aquatic rhizomatous sedge widely distributed throughout the world (Morton and Keeley, 1990). The aim of the present study was to investigate the Cd removal from aqueous solution byE. acicularis biomass under different experimental conditions in order to optimize the efficiency of the adsorption process. Equilibrium isotherm models and kinetic models were conducted for a better understanding of the adsorption process.

2. Methods 2.1. Adsorbent material Eleocharis acicularis macrophytes from the Laguna de San Bartolome (Queretaro, Mexico) (20° 320 3000 N, 100° 320 4600 W) were col-

P. Miretzky et al. / Bioresource Technology 101 (2010) 2637–2642

2.2. Reagents All chemicals used were of analytical-reagent grade. Ultrapure quality deionised water (Nanopure, Infinity) was used throughout. Cd2+ solutions were prepared by dilution of 1000 mg L1 PE Pure Standard (Perkin–Elmer). HNO3 and NaOH (J.T. Baker) solutions (0.01 M) were prepared by dilution of concentrated acid and base. Working solutions of 0.1, 0.01, 0.001 M NaCl (J.T. Baker) were prepared by dissolving respective salts in nano pure water. All the glassware used for dilution, storage and experiments was cleaned with Extran detergent, thoroughly rinsed with tap water, soaked overnight in a 20% HNO3 solution and finally rinsed with ultrapure quality water before use. 2.3. Experimental design Batch equilibrium experiments were conducted by suspending the macrophyte biomass (0.15 g) in the Cd2+ metal solution (0.030 L) during 60 min in a rotary shaker at 140 rpm. Preliminary experiments of adsorption kinetics indicated that a period of 60 min was sufficient to attain equilibrium. At the end of the agitation period, samples were centrifuged at 3000 rpm for 5 min and then filtered using 0.45 micron acetate cellulose membrane (Micro Separations Inc., MSI). Different initial metal concentrations were used (from 0.089 up to 0.89 mM). The experiments were performed at different pH (4.0, 5.0 and 6.0). The initial and final concentration of Cd2+ in the water solution was determined and also the initial and final pH. Blanks were performed under the same conditions but in the absence of metals. The influence of biomass dose was studied in batch experiments by suspending macrophyte biomass (0.30 g) in the Cd2+ metal solution (0.030 L) for 60 min in a rotary shaker at 140 rpm at pH 6.0 and proceeding as before. To determine the effect on Cd2+ adsorption by macrophyte biomass in solutions of different ionic strengths, batch experiments were carried out by suspending the macrophyte biomass (0.15 g) in the Cd2+/NaCl solution (0.030 L) during 60 min in a rotary shaker at 140 rpm and proceeding as mentioned before. Three different NaCl solutions (0.1, 0.01 and 0.001 M) were tested. Kinetic studies were carried out by suspending macrophyte biomass (0.15 g) in 0.030 L of 0.178, 0.267 and 0.356 mM Cd2+ solutions for 0, 1.0, 2.0, 3.0, 4.0, 5.0, 10.0, 20.0, 30.0, 40.0, 50.0, 60.0 and 80.0 min. The results were corrected by subtraction of the results obtained from control experiments performed under the same conditions but in the absence of the biosorbent. Results represent the average from two replicate experiments. Cd concentrations were determined by flame atomic absorption spectrophotometry (FAAS) (Perkin Elmer, Aanalyst 300) according to APHA, 1993 at k 228.8 nm. The Cd detection limit was 5 lg L1. All determinations were performed in triplicate. The relative error was <1.0%. The amount of metal adsorbed Qe (mmol g1) was calculated according to Eq. (1)

Q e ¼ ðC 0  C e ÞV=W

ð1Þ

where C0 (mM) is the initial Cd2+ concentration, Ce (mM) is the equilibrium concentration after the adsorption has taken place, W is the dried macrophyte biomass (g) and V the solution volume (L). The efficiency of biosorption (%) was calculated using Eq. (2)

% ¼ ðC 0  C e Þ100=C 0

ð2Þ

3. Results and discussion 3.1. Isotherm studies 3.1.1. Effect of initial solution pH on metal adsorption The effect of initial solution pH on Cd2+ removal by E. acicularis biomass is shown in Fig. 1 The amount of Cd2+ adsorbed by the macrophyte biomass ranged from 0.013 to 0.136 mmol g1 at pH 4.0, from 0.015 to 0.139 mmol g1 at pH 5.0 and from 0.015 to 0.136 mmol g1 at pH 6.0, for the range of initial metal concentration studied (0.089–0.89 mM). The biosorption efficiency ranged from ca 80 to 85% for initial Cd2+ concentration from 0.890 to 0.089 mM at pH 6.0, similar results were obtained at pH 4.0 and 5.0. So, further biosorption studies were performed at pH 6.0. Similar results were reported by Bunluesin et al. (2007) studying Cd removal by Hydrilla verticillata biomass at pH 3.0–9.0. Cd speciation (V MINTEQ, Cd 0.45 mM) showed that Cd2+ was the principal specie prior to the adsorption experiments (>99.8%) at pH 4.0, 5.0 and 6.0. When performing the adsorption experiments the pH increased from the initial value of pH 4.0 and 5.0 and 6.0 to 4.76, 5.46 and 6.57 (macrophyte biomass 5.0 g L1, Cd2+ initial concentration 0.145 mM), respectively at sorption equilibrium. Since no Cd2+ precipitation as hydroxide should theoretically occur, the increase of pH must have been due to the binding of H+ to the macrophyte biomass, competing with Cd2+ ions. The precipitation of Cd(II) as hydroxide was found to occur at pH 9.2 (Ajmal et al. 2006) although Módenes et al. (2009) reported that metal precipitation began to occur at pH 5.5. The main functional groups present in plant cell walls constituents are carboxylic groups (pKa ca 3.5) (Schneider and Rubio 1999) that present negative charge at the pH range in study and so are able to absorb Cd2+ ions.

0.160

0.140

0.120

0.100 -1

lected and washed with ultrapure quality deionised water (Nanopure, Infinity) to eliminate the remains of lake sediments and particulate matter, dried at 60 °C, then crushed, milled and sieved through a 0.5 mm sieve (N° 35 mesh).

Qe (mmol g )

2638

0.080

0.060 pH 6 pH 4 pH 5

0.040

0.020

0.000 0.00

0.05

0.10

0.15

0.20

Ce (mM) Fig. 1. Influence of pH on Cd2+ adsorption by macrophyte biomass.

0.25

2639

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Sorption isotherms were used to describe the equilibrium between Cd2+ ions and the macrophyte biomass.

infinite surface coverage is predicted, indicating multilayer sorption on the surface.

3.1.1.1. Langmuir model. The equilibrium adsorption data from the batch experiments were modeled using Langmuir model, representative of monolayer adsorption occurring on an energetically uniform surface on which the adsorbed molecules are not interactive. Equilibrium is attained once the monolayer is completely saturated. The Langmuir model (Langmuir 1918) is described by Eq. (3)

3.1.1.3. Dubinin–Radushkevich model. The experimental data were fitted to Dubinin–Radushkevich (D–R) model (Dubinin and Radushkevich, 1947) in order to determine if the adsorption occurred by a physical or chemical process. The D–R equation is more general than Langmuir model because it does not assume a homogeneous surface or a constant sorption potential or absence of steric hindrance between sorbed and incoming particles (Malik et al., 2005; Shah et al., 2009). The lineal form of this model is represented by Eq. (7)

Q e ¼ bQ max C e =ð1 þ bC e Þ

ð3Þ

1

where Qmax (mmol g ) is the maximum sorption capacity, b (L mmol1) the Langmuir constant related to the binding strength, Ce the solution metal ion concentration at equilibrium (mM) and Qe the metal ion concentration at equilibrium in the macrophyte biomass (mmol g1). Linear regression analysis was used for isotherm data treatment. The linear form of the Langmuir isotherm used was

C e =Q e ¼ C e =Q max þ 1=ðQ max bÞ

ð4Þ

The values of Qmax and b were obtained from the slope and intercept of the plot between Ce/Qe and Ce. Table 1 shows the isotherm fitting (constant model values and correlation coefficient r2 values) for Cd2+ adsorption onto macrophyte biomass at different pH values. Langmuir Qmax values for Cd2+ were similar at the different pH in study (Qmax = 0.299 mmol g1). The equilibrium dimensionless parameter RL, which is defined as RL = 1/(1 + b C0), were C0 (mM) is the initial metal concentration and b (L mmol1) the Langmuir constant, was calculated. The mean RL value was 0.254 for the range of metal initial concentrations in study (0.089–0.89 mM) and no significant difference was shown with pH. An RL value ranging between 0 < RL < 1, reflects a favorable adsorption process (Susmita et al. 2006). 3.1.1.2. Freundlich model. Freundlich model (Freundlich 1907), described by Eq. (5) describes the adsorption on an energetically heterogeneous surface on which the adsorbed molecules are interactive.

Qe ¼

K F C 1=n e

ð5Þ

where KF is the Freundlich constant or adsorption capacity (mmol g1 (mmol L1)1/n) and 1/n stands for adsorption intensity. Linear regression analysis was used for isotherm data treatment. The linear form of the Freundlich isotherm used was

ln Q e ¼ ln K F þ 1=n ln C e

ð6Þ

The values of KF and 1/n were calculated from the intercept and slope of the plot between ln Qe vs ln Ce. Freundlich isotherms fitted better than Langmuir the experimental sorption data. As expected the sorption capacity constant KF increased slightly from pH 4.0 to pH 6.0. This isotherm does not predict any saturation of the sorbent by the sorbate. Instead,

ln Q e ¼ ln Q m  be20

ð7Þ 2+

1

where Qe (mmol g ) is the amount of Cd adsorbent per g of biomass and Qm (mmol g1) represents the maximum sorption capacity of the adsorbent, b (mol2 kJ2) is a constant related to sorption energy while e is the Polanyi sorption potential (Polanyi 1932) calculated by Eq. (8)

e ¼ RT lnð1 þ 1=C e Þ

ð8Þ 1

1

where R is the gas constant 8.314 J mol K , T is the temperature in Kelvin and Ce (M) is the metal equilibrium concentration. The Polanyi sorption approach assumes a fixed volume of sorption space close to the sorbent surface and the existence of sorption potential over these spaces. The sorption space in the vicinity of a solid surface is characterized by a series of equipotential surfaces having the same sorption potential. This sorption potential is independent of the temperature but varies according to the nature of sorbent and sorbate. The values of b and Qm were calculated from the slope and intercept of the plot ln Q e vs e20 (Fig. 2). The mean free energy of sorption E (kJ mol1) required to transfer one mole of ion from the infinity in the solution to the surface biomass can be determined by the following equation

E ¼ ð2bÞ1=2

ð9Þ

The sorption mean free energy calculated in the D–R model was 8.058 kJ mol1. It is a well known fact that when the E value lies between 8 and 16 kJ mol1, the adsorption process takes place by chemical ion exchange while E < 8 kJ mol1 implies that the adsorption process is physical (Argun et al., 2007), so the E obtained indicated the Cd2+ adsorption was a chemical process. The sorption capacity (Qm) was 4.70 mmol g1 for Cd2+ which is higher than the sorption capacity observed at the Langmuir region. This may be attributed to different assumptions taken into consideration while formulating the isotherms. Moreover, these isotherms were devised to explain the sorption of gases on solid surfaces and have further been extended to sorption of metal ions from aqueous solution to solid sorbents (Shah et al., 2009). From the correlation coefficient values, it was concluded that sorption of Cd2+ ions on E. acicularis biomass followed Freundlich and D–R isotherm models.

Table 1 Fitting parameters for the Freundlich, Langmuir and Dubinin–Radushkevich equations. Biomass g L1

5.0 5.0 5.0 10.0 5.0 5.0 5.0

pH

4.0 5.0 6.0 6.0 6.0 6.0 6.0

I (NaCl) M

0.001 0.010 0.100

Freundlich Q e ¼ K F C 1=n e

Dubinin–Radushkevich ln Qe = ln Qm  b e0 2

Langmuir Qe = Qmax b Ce/(1 + bCe)

KF

1/n

r2

Qm (mmol g1)

b (L mmol1)

r2

Qm (mmol g1)

b

E (kJ mol1)

r2

0.952 1.068 1.421 0.372 1.382 1.190 0.317

0.959 0.970 1.074 0.898 1.022 0.968 0.806

0.978 0.985 0.998 0.992 0.996 0.999 0.997

0.291 0.299 0.299 0.155 0.266 0.225 0.125

4.774 4.785 4.799 3.909 4.556 3.998 0.998

0.907 0.961 0.872 0.856 0.832 0.789 0.724

4.069 4.618 4.703 1.660 4.519 0.497 0.152

0.0074 0.0075 0.0077 0.0076 0.0075 0.0072 0.0065

8.220 8.165 8.058 8.112 8.165 8.324 8.549

0.979 0.987 0.985 0.996 0.999 0.999 0.996

P. Miretzky et al. / Bioresource Technology 101 (2010) 2637–2642

-8.00 -8.50400

450

500

550

600

650

700

750 pH 4.0

-9.00

pH 6.0

-9.50

ln Qe

800

pH 5.0

-10.00 -10.50 -11.00 -11.50 -12.00 ε° 2 (J 2 mol -2)

Fig. 2. Dubinin–Radushkevich equilibrium isotherm ðln Q e ¼ ln Q m  be20 Þ.

3.1.2. Effect of solution ionic strength on metal adsorption Wastewater normally contains a certain amount of electrolytes with a variety of ionic species. The presence of salt or co-ions in solution can affect the sorption of metal ions. The effect of ionic strength solution on the amount of cadmium sorbed by E. acicularis biomass was analyzed with 0.001, 0.01 and 0.1 M NaCl solutions at pH 6.0. Table 1 shows the fitting of sorption data at different ionic strengths to the three isotherm models. The data fitted better Freundlich and D–R isotherms. Although the fitting to Langmuir isotherm was not good enough, results showed that an increase in ionic strength led to a decrease in the maximum adsorption capacity and the adsorption affinity. The Qmax values obtained were 0.266, 0.225 and 0.125 mmol g1 for NaCl 0.001, 0.01 and 0.1 M. The amount of Cd adsorbed was 88.42%, 87.24% and 71.85% for 0.001, 0.01 and 0.1 M NaCl solutions respectively. Theoretical speciation and different aqueous complexes formed (visual MINTEQ) are shown in Table 2. It seems that the Cd adsorption on the biomass is directed linked to Cd2+ species percentage: 18.9%, 61.4% and 92.8% of total Cd in NaCl 0.1, 0.01 and 0.001 M, respectively (Table 2). These results indicated that Cd2+ was the principal charged species that contribute to the Cd adsorption on the biomass. So it was concluded that metal adsorption was a function of free-metal activity rather than total metal concentration. The observed variations in Cd sorption with ionic strength suggest that Cd exhibits non-specific or outer-sphere adsorption, as independence of sorption with background electrolyte concentration has been interpreted to indicate that the sorption process is primarily non electrostatic in nature (Stumm and Morgan, 1996). Increasing ionic strength could result in a decrease in the thickness (1/j) of the electric double layer (EDL), leading to a decrease in adsorption. Also, a decrease in 1/j should increase the amount of indifferent ions approaching the biomass surface, leading to competition between Cd2+ ions and Na+ explaining the results obtained.

5.0 to 10.0 g L1 resulted in a decrease of the Qe from 0.0748 to 0.0364 mmol g1 (Cd2+ = 0.442 mM, pH 6.0). The Cd% removal decreased slightly from 84.5% to 82.3% (Cd2+ = 0.442 mM, pH 6.0) when biomass dose increased 100%. The decrease of Qe with increase of biomass dose might be due to the formation of aggregates between the biomass particles at high biomass doses, reducing the effective active area. Similar results were obtained for Cu biosorption onto microorganism Sphaerotilus natans (Esposito et al., 2001) and for Cd on fungus Aspergillus niger (Barros Junior et al., 2003) and for Pb on nopal (Opuntia Streptacantha) (Miretzky et al., 2008). Experimental isotherms for 5.0 and 10.0 g L1 biomass doses at pH 6.0 are shown in Table 1. Freundlich isotherms models resulted in best fitting of the experimental data, KF and Qmax values were higher when the lower biomass dose was used. Better results will probably be achieved using lower biomass doses. 3.1.4. Effect of initial metal concentration on metal adsorption The efficiency of Cd2+ adsorption by the biomass at different initial Cd2+ concentrations (from 0.088 up to 0.89 mM) was investigated by carrying out adsorption experiments at the best experimental conditions: pH 6.0 and biomass concentration 5.0 g L1. An increase in the initial Cd2+ concentration from 0.088 to 0.89 mM, resulted in an increase in Qe of one order of magnitude, from 0.0146 to 0.113 mmol g1, whereas% efficiency decreased only from 83.6% to 80.6% (Fig. 3). Therefore, E. acicularis biomass seems to be an efficient material in the Cd2+ removal of Cd2+ contaminated solutions up to 0.89 mM. 3.2. Kinetic studies The extent of biosorption is dependent only on the initial and final equilibrium state, whereas the rate of biosorption is dependent on the way that leads from the initial to the final step. Sorption solid–liquid kinetics may be controlled by several independent processes, which normally act in conjunction, and involve transport phenomenon and chemical reactions. In porous media these include four steps (Lodeiro et al., 2006): transport of the sorbate (cadmium) in the bulk solution, film diffusion from the bulk solution through the boundary layer of fluid immediately adjacent to the external surface of the biosorbent particle, diffusion through the particle, and the chemical binding reaction of the sorbate. Normally, the transport process in the bulk solution and the film diffusion through the boundary layer of bioadsorbent are considered rapid processes, so, intra-particle diffusion or chemical binding reaction control the sorption kinetic mechanism. Kinetic studies were performed in order to get an insight of the Cd2+ rate of adsorption onto E. acicularis biomass and to determine the rate-limiting step of the transport mechanism. The models that

90

0.12

Cd2+ (%)

CdCl+ (%)

CdCl2

0.1 0.01 0.001

18.89 61.38 92.81

65.33 37.28 6.95

15.76 1.22 0.03

(aq)

(%)

þ

CdNO3 ð%Þ 0.02 0.11 0.21

-1

70 60

0.08

50

0.06

40 30

0.04 0.02

Table 2 Cd speciation at different ionic strength solution (Cd 0.45 mM, pH 6.0). NaCl M

80

0.10

Qe (mmol g )

3.1.3. Effect of biomass dose on metal adsorption Biomass dose is an important parameter due to its effect on efficiency (%) and on the amount of metal removed per unit weight of biomass (Qe). An increase in biomass dose generally increases the amount of adsorbed metal because of an increase in adsorption surface area, but in our study, an increase in the biomass dose from

0.00 0.00

0.20

0.40

0.60

Qe

20

%

10

0.80

Cd removal efficiency (%)

2640

0 1.00

C° (mM) Fig. 3. Adsorption capacity (Qe) and efficiency removal (%) of the adsorption of Cd2+ by macrophyte biomass.

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P. Miretzky et al. / Bioresource Technology 101 (2010) 2637–2642 Table 3 Fitting parameters for the pseudo-second order, Elovich kinetic model and intra-particle diffusion model.

0.0295 0.0444 0.0602

Pseudo-second-order kinetics t/Q = 1/v0 + t1/Qe

Elovich Qt = 1/b ln (ab) + 1/b ln t

v0(min1 mmol g1)

k2 (g mmol1 min1)

Qe (mmol g1)

r2

0.4550 0.4470 0.4259

564.10 225.74 117.10

0.0284 0.0445 0.0603

1.000 439.39 1.000 162.05 1.000 55.67

consider that metal biosorbent reaction is the rate-limiting step imply that diffusion, both in the bulk solution and in the biosorbent are faster than the chemical reaction between metal and sorption sites in the surface biomass. Three models were tested; the pseudo-first-order model, described by Lagergren (1898), the pseudo-second-order model by Ho and McKay (1999) and the Elovich kinetic model (Elovich and Larionov, 1962). 3.2.1. Pseudo-first-order model The first order rate equation of Lagergren is usually expressed as

log10 ðQ e  Q t Þ ¼ log10 Q e  kt=2:303

ð10Þ

where Qe is the amount of metal ion adsorbed at equilibrium by the biomass (mmol g1), Qt is the amount of metal ion adsorbed at any time t (mmol g1) and k is the adsorption rate constant (min1). Qe (mmol g1) values were obtained from the intercept of the plot of Eq. (5) and the overall rate constant k (min1) was calculated from the slope. However, the Qe values obtained differed from the experimental Qe values: 0.0298, 0.0444 and 0.0602 mmol g1 for initial Cd2+ concentration: 0.178, 0.267 and 0.356 mM, respectively. As a consequence, the adsorption reaction could not be considered as pseudo-first order although the correlation coefficients were high. 3.2.2. Pseudo-second-order model As a result of the non-applicability of pseudo-first-order model, the kinetics for the adsorption of Cd2+ on macrophyte biomass was tested with the pseudo-second-order model. In the pseudo-second-order model the rate of occupation of adsorption sites is proportional to the square of the number of unoccupied sites and the number of occupied sites is proportional to the fraction of the metal ion adsorbed. The kinetic rate equation is (Ho and McKay (1999)

dQ =dt ¼ k2 ðQ e  Q Þ2

ð11Þ

where k2 (g mmol1 min1) is the rate of pseudo-second-order adsorption and Qe and Q, are the amount of Cd2+ ion adsorbed at equilibrium by the biomass (mmol g1) and the amount adsorbed at any time t (mmol g1), respectively. The sorption rate v 0 ¼ k2 Q 2e (mmol g1 min1) can be regarded as the initial sorption rate as t approaches 0. Integrating and rearranging Eq. (10)

t=Q ¼ 1=v 0 þ t1=Q e

ð12Þ

v0 and Qe can be determined by plotting t/Q against t. Experimental adsorption data fitted the pseudo-second-order model (Table 3) (r2 values from 0.999 to 1.000). The Qe values determined from Fig. 4 were 0.0284, 0.0445 and 0.0603 mmol g1 for Cd2+ initial concentration: 0.178, 0.267 and 0.356 mM respectively, in good agreement with experimental values Qe values (0.0298, 0.0444 and 0.0602 mmol g1). As expected the initial removal rate (v0) was higher the for lower initial Cd2+ concentration.

a

b (min1 mmol g1) (g mmol1) 500.0 285.7 185.2

intra-particle diffusion Qt = kd t½ r2

r2

kd (mmol g1 min1/2)

0.861 0.0034 0.903 0.006 0.900 0.0073

0.9577 0.972 0.9524

3000 -1

0.178 0.267 0.356

Qe experim (mmol g1)

t/Q (min g mmol )

C (mM)

2500 2000 1500 1000

Cd = 0.356 mM

500

Cd = 0.178 mM Cd = 0.267 mM

0 0

10

20

30

40

50

60

70

80

90

time (min) Fig. 4. Pseudo-second-order kinetic model (t/Q = 1/v + t 1/Qe).

3.2.3. Elovich kinetic model Elovich equation is a rate equated based on the adsorption capacity

dQ t =dt ¼ a expðbQ t Þ

ð13Þ

where a (mmol g1 min1) is the initial adsorption rate and b (g mmol1) is the desorption constant related to the extent of the surface coverage and activation energy for chemisorption. Eq. (13) can be simplified by assuming ab >> t and by applying the boundary conditions Qt = 0 at t = 0 and Qt = Qt at t = t, then

Q t ¼ 1=b lnðabÞ þ 1=b ln t

ð14Þ

The slope and intercept of the plot of Qt vs ln t result in the determination of the kinetic constants a and b. Experimental adsorption data fitted the Elovich kinetic model (Table 3) (r2 values from 0.861 to 0.903). The fitting to Elovich model seems to be not as good as to pseudo-second-order kinetic model. As expected the initial adsorption rate (a) was higher the for lower initial Cd2+ concentration. As seen in Table 3, the r2 values for the pseudo-second order are the higher ones for all the values of initial Cd2+ concentration, so the kinetics of Cd2+ adsorption using E. acicularis biomass as an adsorbent can be better explained by the pseudo-second-order model suggesting that adsorption rate was proportional to the number of unoccupied sites. The pseudo-second-order model is based on the assumption that the rate-limiting step is chemisorption involving sharing or exchange of electrons between adsorbent and adsorbate. The existence of other processes such as intra-particle diffusion, mass transfer or ion interaction is not taken in account. Although experimental data yield a good fit to this simplified model, one must bear in mind that the model assumes that all adsorption sites are homogeneous, and does not consider the heterogeneous nature of macrophyte biomass. 3.2.4. Intra-particle diffusion model When agitation speed is high enough, the thickness of the boundary layer surrounding the particle should be minimal and boundary layer resistance or film diffusion should not be a major rate-controlling factor. Then, the intra-particle diffusion is the

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rate-limiting step and the uptake of the adsorbate varies with the square root of time (Weber and Morris, 1962)

Q t ¼ kd t 1=2

ð15Þ 1

1/2

where kd is the internal diffusion coefficient (mmol g min ), Qt the amount of metal adsorbed at time t (mmol g1) and t (min) the sorption time. Experimental data were fitted through this model only in the first 5 min, which indicates partial intra-particle diffusion. Although the correlation coefficients were high (r2 values ranged from 0.952 to 0.978), the straight lines obtained when fitting experimental data did not pass through the origin, also indicating that pore diffusion was not the only controlling step. The correlation coefficients for the pseudo-second-order kinetic model are higher than those for the intra-particle diffusion model (Table 3), suggesting a chemical reaction mechanism (Ho and McKay, 2003). This would indicate that the sorption of Cd2+ is a complex mix of surface chemisorption occurring in the boundary layer of the macrophyte particle and intra-particle diffusion. The pseudo-second-order model has been successfully applied to the description of Cd2+ biosorption by algae, wheat bran, fungi, weed, etc. (Cruz et al., 2004; Nouri et al., 2007; Ajmal et al., 2006; Anayurt et al., 2009; Sarı and Tuzen, 2009). 4. Conclusions Eleocharis acicularis biomass was demonstrated to be an efficient material in the Cd2+ removal (>80%) from contaminated solutions up to 0.89 mM. The equilibrium sorption data fitted Freundlich and Dubinin–Radushkevich isotherms. The adsorption process was governed by ionic exchange. Cd maximum sorption capacity was 0.299 mmol g1 (33.71 mg g1), similar to results reported by Schneider and Rubio (19990, Miretzky et al. (2006) and Rakhshaee et al. (2006). The adsorption kinetics was rapid, showing that 85% of biosorption capacity was achieved in the first 5 min of contact and followed second-order chemical reaction kinetics. The rapid kinetics of the adsorption process has significant practical importance ensuring efficiency and economy. It is important to note that macrophyte biomass used in this study did not undergo any chemical or physical pre-treatment, which added to macrophyte abundance and its low cost makes it a good option for Cd2+ removal from waste water. The practical implication of this study is the development of an effective and economic technology for Cd removal from contaminated waters. References Ajmal, M., Khan Rao, R.A., Ahmad, R., Khan, M.A., 2006. Adsorption studies on Parthenium hysterophorous weed: removal and recovery of Cd(II) from wastewater. J. Hazard. Mater. B135, 242–248. Anayurt, R.A., Sari, A., Tuzen, M., 2009. Equilibrium, thermodynamic and kinetic studies on biosorption of Pb(II) and Cd(II) from aqueous solution by macrofungus (Lactarius scrobiculatus) biomass. Chem. Eng. J. 151, 255–261. APHA, 1993. Standard Methods for the Examination of Water and Wastewaters. American Public Health Association, Washington DC, USA, 874pp. Argun, M.E., Dursun, S., Ozdemir, C., Karatas, M., 2007. Heavy metal adsorption by modified oak sawdust: thermodynamics and kinetics. J. Hazard. Mater. 141, 77– 85. Barros Junior, L., Macedo, G., Duarte, M., Silva, E., Lobato, A., 2003. Biosorption of cadmium using the fungus Aspergillus niger. Braz. J. Chem. Eng. 20, 229–239. Bunluesin, S., Kruatrachue, M., Pokethitiyook, P., Upatham, S., Lanza, G.R., 2007. Batch and continuous packed column studies of cadmium biosorption by Hydrilla verticillata biomass. J. Biosci. Bioeng. 103, 509–513. Cruz, C.C.V., da Costa, A.C.A., Henriques, C.A., Luna, A.S., 2004. Kinetic modeling and equilibrium studies during cadmium biosorption by dead Sargassum sp. Biomass. Bioresour. Technol. 91, 249–257.

Dubinin, M.M., Radushkevich, L.V., 1947. Proc. Acad. Sci. USSR, Phys. Chem. Sec 55, p. 331. Elifantz, H., Tel-Or, E., 2002. Heavy metal biosorption by plant biomass of the macrophyte Ludwigia Stolonifera. Water Air Soil Pollut. 141, 207–218. Elovich, S.Yu., Larionov, O.G., 1962. Theory of adsorption from nonelectrolyte solutions on solid adsorbents. 1. Simplified analysis of the equation of the adsorption isotherm from solutions. Bull. Acad. Sci. USSR Div. Chem. Sci. 11 (2), 191–197. Esposito, A., Pagnanelli, F., Lodi, A., Solivio, C., Veglio, F., 2001. Biosorption of heavy metals by Sphaerotitus natans: an equilibrium study at different pH a biomass concentrations. Hydrometallurgy 60, 129–141. Freundlich, H., 1907. Ueber die adsorption in Loesungen. Z. Phys. Chem. 57, 385– 470. Ho, Y., McKay, G., 1999. Pseudo-second order model for sorption process. Process Biochem. 34, 451–465. Ho, Y., McKay, G., 2003. Sorption of dyes and copper ions onto biosorbents. Process Biochem. 38, 1047–1061. Kaewsarn, P., Yu, Q., 2001. Cadmium (II) removal from aqueous solutions by pretreated biomass of marine alga Padina sp.. Environ. Pollut. 112, 209–213. Kazi, T.G., Jalbani, N., Kazi, N., Jamali, M.K., Arain, M.B., Afridi, H.I., Kandehro, G.A., Pirzado, Z., 2008. Evaluation of toxic metals in blood and urine samples of chronic renal failure patients, before and after dialysis. Renal Failure 30, 737– 745. Langmuir, I., 1918. The adsorption of gases on plane surfaces of glass, mica and platinum. J. Am. Chem. Soc. 40, 1361–1403. Lagergren, S., 1898. Zur theorie der sogenannten adsorption geloster stoffe. Kungliga Svenska Vetenskapsakademiens, Handlingar 24, 1–39. Lodeiro, P., Herrero, R., Sastre de Vicente, M.E., 2006. Thermodynamic and kinetic aspects on the biosorption of cadmium by low cost materials: a review. Environ. Chem. 3, 400–418. Malik, U.R., Hasany, S.M., Subhani, M.S., 2005. Sorptive potential of sunflower stem for Cr (III) ions from aqueous solutions and its kinetic and thermodynamic profile. Talanta 66, 166–173. Miretzky, P., Muñoz, C., Carrillo-Chavez, A., 2008. Experimental binding of lead to a low cost on biosorbent: nopal (Opuntia streptacantha). Bioresource Technol. 99, 1211–1217. Miretzky, P., Saralegui, A., Fernández Cirelli, A., 2006. Simultaneous heavy metal removal mechanism by dead macrophytes. Chemosphere 62, 247–254. Módenes, A.N., de Abreu Pietrobelli, J.M.T., Espinoza-Quiñones, F.R., 2009. biosorption by non-living aquatic macrophytes Egeria densa. Water Sci. Technol. 60, 293–300. Mohan, D., Singh, K.P., 2002. Single multi-component adsorption of cadmium and zinc using activated carbon derived from bagasse – an agricultural waste. Water Res. 36, 2304–2318. Morton, B.A., Keeley, J.E., 1990. C 4 Acid fixation in photosynthesis of the submerged aquatic Eleocharis acicularis (L.) R. & S. Aquat. Bot. 36, 379–388. Nouri, L., Ghodbane, I., Hamdaoui, O., Chiha, M., 2007. Batch sorption dynamics and equilibrium for the removal of cadmium ions from aqueous phase using wheat bran. J. Hazard. Mater. 149, 115–125. Polanyi, M., 1932. Theories of the adsorption of gases. A general survey and some additional remarks. Trans. Faraday Soc. 28, 316–332. Rakhshaee, R., Khosravi, M., Ganji, M.T., 2006. Kinetic modeling and thermodynamic study to remove Pb(II), Cd(II), Ni(II) and Zn(II) from aqueous solution using dead and living Azolla filiculoides. J. Hazard. Mater. B134, 120–129. Sarı, A., Tuzen, M., 2009. Kinetic and equilibrium studies of biosorption of Pb(II) and Cd(II) from aqueous solution by macrofungus (Amanita rubescens) biomass. J. Hazard. Mater. 164, 1004–1011. Schneider, I.A.H., Rubio, J., 1999. Sorption of heavy metal ions by the non-living biomass of freshwater macrophytes. Environ. Sci. Technol. 33, 2213–2217. Shah, B.A., Shah, A.V., Singh, R.R., 2009. Sorption isotherms and kinetics of chromium uptake from wastewater using natural sorbent material. Int. J. Environ. Sci. Technol. 6, 77–90. Stumm, W., Morgan, J., 1996. Aquatic Chemistry. John Wiley and Sons Inc., New York. Susmita, S.G., Krishna, G., Bhattacharyya, K.G., 2006. Adsorption of Ni(II) on clays. J. Colloid Interface Sci. 295, 21–32. Tangaromsuk, J., Pokethitiyook, P., Kruatrachue, M., Upatham, E.S., 2002. Cadmium biosorption by Sphingomonas paucimobilis biomass. Bioresour. Technol. 85, 103– 105. Veglio, F., Beolchini, F., 1997. Removal of metals by biosorption: a review. Hydrometallurgy 44, 301–316. Verma, V.K., Tewari, S., Rai, J.P.N., 2008. Ion exchange during heavy metal biosorption from aqueous solution by dried biomass of macrophytes. Bioresour. Technol. 99, 1932–1938. Weber, W.J., Morris, J.C., 1962. Advances in water pollution research: removal of biologically resistant pollutants from waste waters by adsorption. In: Proceedings of International Conference on Water Pollution Symposium, vol. 2, pp. 231–266.