Adsorption optimization of lead (II) using Saccharum bengalense as a non-conventional low cost biosorbent: isotherm and thermodynamics modeling.

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Adsorption Optimization of Lead (II) Using Saccharum Bengalense as a NonConventional Low Cost Biosorbent: Isotherm and Thermodynamics Modeling Muhammad Imran Din

a b

b

c

, Zaib Hussain , Muhammad Latif Mirza ,

d

Asma Tufail Shah & Muhammad Makshoof Athar

b

a

Department of Chemistry , The Islamia University of Bahawalpur , Bahawalpur , Pakistan b

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Institute of Chemistry , University of the Punjab , Lahore , Pakistan c

University of Sargodha , Sargodha , Pakistan

d

IRCBM, Comsats Institute of Information Technology , Lahore , Pakistan Accepted author version posted online: 03 Jul 2013.Published online: 23 Jan 2014.

To cite this article: Muhammad Imran Din , Zaib Hussain , Muhammad Latif Mirza , Asma Tufail Shah & Muhammad Makshoof Athar (2014) Adsorption Optimization of Lead (II) Using Saccharum Bengalense as a Non-Conventional Low Cost Biosorbent: Isotherm and Thermodynamics Modeling, International Journal of Phytoremediation, 16:9, 889-908, DOI: 10.1080/15226514.2013.803025 To link to this article: http://dx.doi.org/10.1080/15226514.2013.803025

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International Journal of Phytoremediation, 16:889–908, 2014 C Taylor & Francis Group, LLC Copyright ISSN: 1522-6514 print / 1549-7879 online DOI: 10.1080/15226514.2013.803025


ADSORPTION OPTIMIZATION OF LEAD (II) USING SACCHARUM BENGALENSE AS A NON-CONVENTIONAL LOW COST BIOSORBENT: ISOTHERM AND THERMODYNAMICS MODELING Muhammad Imran Din,1,2 Zaib Hussain,2 Muhammad Latif Mirza,3 Asma Tufail Shah,4 and Muhammad Makshoof Athar2 1

Department of Chemistry, The Islamia University of Bahawalpur, Bahawalpur, Pakistan 2 Institute of Chemistry, University of the Punjab, Lahore, Pakistan 3 University of Sargodha, Sargodha, Pakistan 4 IRCBM, Comsats Institute of Information Technology, Lahore, Pakistan In the present study a novel biomass, derived from the pulp of Saccharum bengalense, was used as an adsorbent material for the removal of Pb (II) ions from aqueous solution. After 50 minutes contact time, almost 92% lead removal was possible at pH 6.0 under batch test conditions. The experimental data was analyzed using Langmuir, Freundlich, Timken and Dubinin-Radushkevich two parameters isotherm model, three parameters Redlich—Peterson, Sip and Toth models and four parameters Fritz Schlunder isotherm models. Langmuir, Redlich—Peterson and Fritz-Schlunder models were found to be the best fit models. Kinetic studies revealed that the sorption process was well explained with pseudo second-order kinetic model. Thermodynamic parameters including free energy change (G◦ ), enthalpy change (H◦ ) and entropy change (S◦ ) have been calculated and reveal the spontaneous, endothermic and feasible nature of the adsorption process. The thermodynamic parameters of activation (G#, H# and S#) were calculated from the pseudo-second order rate constant by using the Eyring equation. Results showed that Pb (II) adsorption onto SB is an associated mechanism and the reorientation step is entropy controlled. KEY WORDS: biosorption, kinetics, lead, thermodynamics

INTRODUCTION Water pollution is a major problem faced by modern society which causes ecological disequilibrium and health hazards. Heavy metals are one of the main contributors of aqueous pollution and may cause cellular damage to living cells (Aksu 2001). Lead is detrimental to living organisms and accountable for many major environmental health problems. Input of lead into the bodies of living organisms is primarily through drinking water and harvest irrigation (Wang et al. 2013). Lead toxicity can affect organs

Address correspondence to Zaib Hussain, Institute of Chemistry, University of the Punjab, Lahore-54590, Pakistan. E-mail: [email protected] Color versions of one or more of the figures in the article can be found online at www.tandfonline/bijp. 889


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and tissues in the body including the reproductive and nervous systems. The severity of lead poisoning can lead to hypertension, nephritis, abdominal pain, constipation, cramps, nausea, vomiting and behavioral changes (Wadanambi et al. 2008). Several methods have been developed for the management of heavy metals. The most commonly used procedures for removing these metals from waterways include chemical precipitation, lime coagulation, ion exchange, reverse osmosis and solvent extraction (Din and Mirza 2013). Traditional metal removal methods have certain disadvantages such as incomplete metal removal, high reagent and energy requirements and, generation of toxic sludge or other waste products which require careful disposal (Pino et al. 2006). Hence, it is of fundamental importance to develop a cost-effective treatment/method which is capable of removing heavy metals from aqueous effluents. Previous literature has demonstrated that different adsorbent systems can be developed from a range of industrial waste materials for the exclusion of heavy metals. However, development of a low cost and efficient adsorbent for the removal of toxic metals from water is of great concern. In the present work, investigations have been carried out to develop a nonconventional low cost biosorbent system for the removal of Pb (II) from aqueous solution using biomass derived from the pulp of Saccharum bengalense. The cell wall structure of this adsorbent consists of an assembly of different functional groups such as amino, hydroxyl, carboxyl and sulphate which can act as binding sites for the removal of Pb (II) ions via electrostatic attraction, ion exchange and complexation. The mechanism of biosorption has been explained by application of Langmuir, Freundlich, Timken, D-R equation and Harkins-Jura adsorption isotherms. Efficiency of adsorption has been studied in terms of pseudo-first-order, pseudo-second-order and intra-particle diffusion kinetic models. Thermodynamical parameters were also calculated to find spontaneity and feasibility of the sorption process. EXPERIMENTAL Saccharum Bengalense—Biosorbent Biosorbent Saccharum bengalense, as shown in Fig. 1a, was collected from the banks of The River Satluj Bahawalpur, Pakistan (Din and Mirza 2013). Large parts of Saccharum bengalense were collected, washed to remove any dust or other foreign particles, dried and finally milled with a laboratory knife mill. The dried plants were ground and sieved through a 60–80 μm mesh (ASTM standards). Finally, the biosorbent was oven dried (333 K) to constant mass and stored in air tight plastic bottles labeled as SB (Imran Din et al. 2013). Lead Synthetic Wastewater—Adsorbate Analytical reagent grade Pb (NO3 )2 , supplied by Merck (Darmstadt, Germany), was used for preparation of the synthetic wastewater. A stock solution of Lead (1000 mg/L) was prepared by dissolving 1.578 g of Pb (NO3 )2 in one liter of deionized water. The pH of the solution was adjusted by addition of 0.1 M HNO3 or 0.1 M NaOH solution, respectively. Adsorption Studies Batch adsorption experiments were carried out in a series of Erlenmeyer flasks of 250 ml capacity covered with aluminum foil to prevent contamination. The effects of adsorbent dose (0.1–2 g), contact time (10–80 min), pH (1.0–10), and temperature (10◦ C–70◦ C)


ADSORPTION OF LEAD (II) ON SACCHARUM BENGALENSE

Figure 1 (a) Photograph of Saccharum bengalense plant; (b) FTIR of SB and Pb-loaded SB.

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were studied. For each experiment 50 mL of Pb containing solution, of known concentration, was used. For adsorption isotherms, Pb containing solutions of different known concentrations C0 (mg/L) were agitated with a known amount of adsorbent until equilibrium was achieved. The kinetics of adsorption was determined by analyzing adsorptive uptake (qt mg/g) of the Pb (II) ions from the aqueous solutions at different time intervals. All experiments were performed in triplicate and an average of three values reported. The deviation was found to be 2% of the average value. After prescribed contact times, the solutions were filtered and the concentrations of metal ions were determined by atomic absorption spectrometry. A Perkin-Elmer 2380 Atomic Absorption Spectrophotometer (Massachusetts,USA), with air-acetylene flame, was used for the determination of lead concentration at 283.3 nm. The percentage removal efficiency of adsorbent (% R), was calculated as follows: %Removal (R%) =

C o − Ce × 100. Co

(1)

Equilibrium adsorption studies were conducted by using 50 mL of Pb solution of different initial concentration (10, 20, 30, 40, 50, 60 and 70 mg/L) with 0.5 g of SB powder in 100 mL conical flasks for a period of 2 h to achieve equilibrium. The adsorption capacity at equilibrium qe (mg/g) was calculated as follows: qe (mg/g) = (Co − Ce ) ×

V m

(2)

where Co is the initial metal ions concentration (mg/L), Ce is the concentration of Pb metal at equilibrium (mg/L), V is the volume of solution in liters (L) and m is the mass of SB powder (g). RESULTS AND DISCUSSIONS Characterization of the Biosorbent Physicochemical analysis of saccharum bengalense. Elemental analysis was accomplished by using a Perkin-Elmer 2400 Series II CHNS/O Elemental Analyzer (Massachusetts, USA) using sulfanilamide as the standard. SB comprised of 41.23% Carbon, 4.2% Nitrogen and 0.9% Hydrogen as shown in Table 1. BET surface area and single point surface area of SB was determined from the N2 adsorption isotherm at 77 K in the range of relative pressure 10−6 to 1.0 with a surface area and pore size analyzer (Autosorb 1, Quantachrome Instruments). Before measurement, the sample was degassed at 300◦ C for 2 h. The 9.43 m2/g BET surface area and 5.78 m2/g single point surface area of SB was measured (Din and Mirza 2013). Fourier transform infrared (FT-IR) analysis. Functional groups in SB were characterized by FTIR analysis. A potassium bromide (KBr) disc method was used to scan the FTIR spectra by using FTIR spectrophotometer (Tensor 27, Bruker Germany). The FTIR analysis of SB, before and after adsorption of Pb (II), was performed to determine the vibrational frequency changes in functional groups within the range 4000–400 cm−1 (Fig. 1B). The existence of free and intermolecular bonded hydroxyl groups in SB are indicated by an absorption peak near 3356 cm−1. The peaks observed at 2909 cm−1 can be assigned to stretching vibration of the C–H group; the presence of a strong C O band at 1039 cm−1 is due to the –OCH3 group. In addition, the peaks at 662 and 898 cm−1 are related to bending



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modes of aromatic compounds confirming the presence of the lignin structure (Din and Mirza 2013; Imran Din et al. 2013). From the Pb (II) loaded FT-IR spectra, it was found that oxygen containing functional groups viz, carboxy –COOH, methoxy –OCH3 and phenolic –OH groups are affected after the Pb uptake process. Potentiometric titration. Acidic and basic sites on SB were determined by Boehm’ titration. The procedure is discussed elsewhere (Boehm 1994; Ofomaja et al. 2009), briefly, the total acid sites (carboxylic and phenolic sites were neutralized using a 0.1 M NaOH solution while the basic sites were neutralized with a 0.1 M HCl solution. The carboxylic sites were determined with a 0.1 M NaHCO3 solution, and the phenolic sites were estimated by method of difference. The acidic and basic sites were determined by mixing 1 g of SB powder with 50 mL of 0.1 M titrating solution in a 50 mL volumetric flask at a constant temperature 20◦ C in water bath for 5 days. A sample of 10 mL was titrated with 0.1 M HCl or NaOH solution (Ofomaja et al. 2009). Biomaterial SB generally considered as structures built by cellulose macromolecules and surrounded by hemicellulosic materials (glucomannans, galactans, and arabogalactans), lignin and pectin, which contains polar functional groups such as carboxylic, phenolic, alcohols, aldehydes, ketones and ether groups (Din and Mirza 2013). Results reveal that SB contains 0.92 mmolg−1 of carboxylic group, 1.21 mmolg−1 of phenolic group. The total acidity and total basicity was determined of 2.91 and 4.64 mmolg−1 respectively. Scanning electron micrograph (SEM). The surface morphology of SB and Pb-SB was studied using a SEM Hitachi S-4700 microscope operated at 205 kV in a working range of 5 mm. Prior to analysis, the SB and modified-SB biosorbent samples were interspersed onto Al or C tapes. These tapes are adhesive and supported on metallic discs coated with Au. Images of the biomaterials surfaces were recorded at magnifications and different areas. Figure 1C (a-b) shows rigid and compact morphology of the untreated biosorbent SB. No fragmentation could be seen in the SEM pictures. The surface is irregular and porous thus facilitating the adsorption of metal ions on SB. The pores are large enough to allow the penetration of metal ions.

Effect of Adsorbent Dosage Effect of amount of SB on Pb (II) biosorption is shown in Figure 2a. The percentage removal of lead increased to 90.5% when the dosage of adsorbent material was increased from 0.1 to 0.5 g/50 mL. A further increase in adsorbent material dosage (2.0 g/50 mL) decreased the percentage removal of lead. Minimum adsorption was 85.3% at 0.1 g/50 mL and maximum adsorption was 90.5% at 0.5 g/50 mL. Hence, for further experiments 0.5 g/50 mL of adsorbent material was selected as the optimum dosage for Pb (II) removal. When initial metal ion concentration and volume of solution remained constant, removal of metal ions was enhanced due to the increase in number of active sites. However, after establishing equilibrium increase in adsorbent dosage has no effect (Gundogdu et al. 2009). At higher dosage of SB, all the Pb2+ in the solution could interact with the binding sites of SB as result percentage removal of lead increased. But the biosorption capacity qe (mg/g) decreased by increasing amount of SB biosorbent. This is due to the split in the concentration gradient between the adsorbate concentration in the solution and the one in the surface of the adsorbent. Similar results have been reported earlier (Deng and Bai 2004; Jiang et al. 2009; Mart´ın-Lara et al. 2010).


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Figure 2 (a–b) General view of SB showing the fiber surfaces.

Effect of Contact Time For Pb (II), the minimum adsorption was 72% at 10 minutes and maximum adsorption 90% at 50 minutes, which indicates availability of the adsorption sites. At extended contact time (80 minutes), the adsorption was constant after which an increase in contact time decreased the removal of Pb (II) as shown in Figure 2c. The fast adsorption rate in beginning is due to fact that at the initial stage more binding sites are available, and the lead ions can interact easily with the these sites, as a result a higher adsorption rate is achieved. In later stage, the slow adsorption rate is due to slower diffusion of adsorbate into the interior of the adsorbent (Jiang et al. 2009). Therefore, further adsorption experiments for lead were carried out for a contact time of 50 minutes. Similar results have been reported earlier on different biomasses, studying metal biosorption (Gundogdu et al. 2009; Ibrahim et al. 2012; Melek et al. 2006). Effect of pH Adsorption of heavy metal ions is strictly pH dependent. The distribution of Pb (II) ions in aqueous water is mainly dependent on pH conditions. The uptake of Pb (II) by adsorbent material was found to increase with increasing pH (1.0–6.0). Maximum removal efficiency was 91.33% at pH 6. It is evident from Figure 2b, very low and high pH values (above 6.0) were avoided for the biosorption of Pb (II) ions. At higher values of pH,


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biosorption decreased because the Pb metal ion was precipitated as hydroxide. At lower pH values, the surface charge of the SB became too positive and adsorption of Pb+2 ions was unfavorable. Moreover, at low pH values H+ ions could compete strongly with Pb+2 ions (Melek et al. 2006). At low pH (1–2), there was little or no biosorption of Pb+2 ions on SB and with the increase of pH, the amount of lead uptake by SB increased and a sharp increase in percentage removal of Pb was observed in the pH range (3–6). A plateau was observed near pH 6. Hence, for further experimental work, pH 6.0 was selected as an optimum pH value (Alomá et al. 2012). Similar results have been reported by a number of earlier researchers studying lead metal biosorption on different biomasses (Gundogdu et al. 2009; Ibrahim et al. 2012; Melek et al. 2006; Mohapatra et al. 2009; Wang et al. 2013).

Adsorption Isotherm Modeling Adsorption isotherms are used to determine the relationship between the mass of the solute adsorbed per unit mass of adsorbent qe (mg/g) and the solute concentration in the solution at equilibrium Ce (mg/L). Isotherm studies provide information about the capacity of the adsorbent material or the amount required to remove a unit mass of pollutants. The experimental data was evaluated using different isotherm models: Langmuir, Freundlich, Timken and Dubinin-Radushkevich (two parameters), Redlich–Peterson, Toth and Sips (three parameters) and Fritz- Schlunder (four parameters). To gain a better understanding of the mechanism implicated with the use of different sorbate-sorbent systems, and to find the best fit isotherm model, correlation coefficients (R2) and chi square test (χ 2) were carried out for linear and non-linear methods. Linear method. The linear method assumes that the distribution error is the same at every value of the abscissa and the scattered points around the line follow a Gaussian distribution (Zakhama et al. 2011). Equilibrium isotherms. Langmuir adsorption isotherm. The Langmuir adsorption isotherm model assumes that adsorption takes place at specific homogeneous sites within the adsorbent. Each site can hold one adsorbate molecule. The non-linear equation of the Langmuir isotherm model is qe =

KL qm Ce 1 + KL Ce

(3)

where qe (mg/g) is adsorption capacity at equilibrium; Ce (mg/L) is metal concentration at equilibrium, qm is monolayer adsorption capacity of the adsorbent (mg/g); KL is Langmuir constant (L/mg) related to the free energy of adsorption. One of the simplest linear forms of the Langmuir isotherm can be represented as: 1 1 1 = + qe KL qm Ce qm

(4a)

From Figure 4a it is indicated from the high value of R2 that the adsorption of lead on SB followed the Langmuir isotherm. The Langmuir constants for Pb were calculated and the results are presented in Table 2.

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Figure 3 (a) Effect of adsorbent dose, (b) Effect of contact time (c) Effect of pH; for biosorption of Pb (II) ions on SB at 60◦ C; size of SB < 125 μm; speed of agitation 125 rpm.

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Figure 4 Linear adsorption Isotherm; (a) Langmuir Isotherm; (b) Freundlich isotherm (c); Dubbin–Radushkevich (D–R) at 60◦ C; size of SB < 125 μm; speed of agitation 125 rpm and pH 6.0; SB dose 0.5 g/50 mL.

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M. IMRAN DIN ET AL. Table 1 FTIR and Physico-chemical analysis of Saccharum bengalense

Elemental analysis (%wt.) C H N

Physical analysis BET surface area (m2/g) Porosity Mean Size(μm)

41.2 4.2 0.9

9.43 0.043 16.02


Table 2(a) Linear Adsorption Isotherms parameters and RL values for sorption of Pb (II) on SB Adsorption isotherm

Error function R2

Constant parameters

Langmuir Isotherm

qm (mg/g) 5.73

Freundlich Isotherm Freundlich Isotherm (D–R) Isotherm

KF 0.302 β 4.00E-06

KL (L/mg) 0.034

Ce (mg/L) 10 20 30 40 50

RL = 1+K1L Co 0.901 0.820 0.752 0.694 0.645 n 1.509

qDR (mg/g) 2.5

0.992

0.930

Es (kJ/mol) 0.354

0.87

Table 2(b) Non-Linear Adsorption Isotherms parameters for sorption of Pb (II) on SB Name Langmuir Freundlich D-R Redlich-Petrson

Hill

Toth

Sips

Fritz-Schlunder

qm KL KF n qs β KR aR g qH nH Kd KT aT t Ks β a A α B β

Parameters

SD±

χ2

4.431 0.055 0.485 1.969 2.98 5.1 × 10−5 0.149 4.84 1.1 3.29 1.68 53.13 1.23 8.12 1.39 0.061 1.68 0.01 23.6 50.7 1.44 1.76

0.141 0.0208 0.175 0.132 0.232 1.52 × 10–5 0.0125 0.012 0.281 0.468 0.514 12.9 0.77 6.54 0.31 0.067 0.115 0.018 0.74 4.11 0.14 0.32

0.366 0.675 0.360 0.092

0.233

0.463

0.283

0.250


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Specific surface area. The specific surface area of SB for monolayer adsorption of Pb (II) was determined using the following equation:


S=

qm A.No M

(4b)

where qm is the biosorption capacity (mg/g), N0 is Avogadro’s Number (6.022 × 1023), A is the cross sectional area of metal ion (A2) and M the molecular mass of metal ion. The molecular mass of Pb (II) is 207 and the cross-sectional area is 5.56 (A◦ ) 2 (the radius of Pb (II) ions for close packed monolayer is 1.33 Å). The specific surface area for Pb (II) ions was found to be 9.26 m2/g. Freundlich Isotherm. Freundlich isotherm is an empirical equation and can be used to describe heterogeneous systems. The different forms of Freundlich adsorption model are (Ncibi 2008; Zakhama et al. 2011) qe = Kf .Ce1/n ln qe = lnKF +

(5) 1 ln Ce n

(6)

where K F and n are the Freundlich adsorption isotherm constants, being indicative of the extent of the adsorption and the degree of non-linearity, respectively. The plot of lnCe versus lnqe for the adsorption was employed to generate the intercept value of K F and the slope value of n, respectively. When lnqe was plotted against lnCe, straight lines with slopes ‘1/n’ were obtained for lead (Fig. 3b) which indicates that the adsorption of lead did not follow the Freundlich isotherm. The Freundlich constants were calculated and values of n are shown in Table 2. Dubinin–Radushkevich (D–R) Isotherm. The Dubinin–Radushkevich (D–R) isotherm model is more general than the Langmuir isotherm model due to the fact that it does not assume a homogeneous surface or constant adsorption potential (Imran Din et al. 2013; Dubinin and Radushkevich 1947). It was applied to distinguish between physical and chemical adsorption (Foo and Hameed 2010). The linear equation of (D–R) isotherm model can be represented as qe = qDR e−βε2

(7)

ln qe = ln qm − βε 1 ε = RT ln 1 + Ce

(8)

2

(9)

where β is a constant connected with the mean free energy of adsorption per mole of the adsorbate, qm is the theoretical saturation capacity (mg/g), ε (J2 /mol2) is the Polanyi potential, R (J mol−1 K−1) is the gas constant and T (K) is the absolute temperature. Hence, by plotting lnqe versus ε2 it is possible to generate the value of qm from the intercept and the value of β from the slope. The D–R isotherm model was found to fit well with the equilibrium data since the R2 value was found to be 0.847 for Pb (II) metal ions (Fig. 3c). The adsorption mean free energy (Es ) is as follows (Foo and Hameed 2010). 1 Es = √ 2β

(10)

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Biosorption mean free energy (Es, kJ/mol) values were found to be 0.354 kJ/mol for Pb (II) (Table. 2). Non-linear method. The non-linear method, based on a trial and error procedure, uses the raw experimental data for model parameter determination. For isotherm model parameters determination, the non-linear method is preferred (Dhaouadi and M’Henni 2009). Non-linear chi-square test (χ 2). The non-linear chi-square test (χ 2) model is represented as


χ2 =

n qe , Cal − qe means qe means 2 n=1

(11)

The large value of χ 2 indicates deviation while the smaller value shows understanding of the experimental data (Din and Mirza 2013; Foo and Hameed 2010; Ncibi 2008). Non-linear modeling. All the model parameters were evaluated by non-linear regression using Excel 2007 software (Microsoft, USA). The optimization procedure requires a trial and error procedure to be defined in order to be able to evaluate fit of equation to the experimental data. The non-linear plot for biosorption of Pb (II) on SB is presented in Figure 4. It can be observed that among two parameter isotherms the adsorption of Pb (II) on SB biosorbent follows Langmuir and D-R isotherms indicating that the uptake occurs on a homogenous surface by monolayer adsorption. The Freundlich isotherm has high chi-square test (χ 2) values. The Redlich–Peterson model has the lowest value of chi-square (χ 2 = 0.043) for Ni (II) ions on SB biomass. The β value is close to unity indicating that the Redlich–Peterson model fits well for the expression of adsorption of Pb (II) on SB (Brdar et al. 2012; Din and Mirza 2013). On the basis of chi-square (χ 2) values the Sip isotherm appears slightly preferable than that of the Toth isotherm. The Fritz–Schlunder isotherm model gave the best fit to the experimental data. The results obtained using the three-parameter and four-parameter equations showed that the best-fit adsorption isotherm models were in the order: Fritz–Schlunder > Redlich Peterson > Sip.

Kinetic Modeling Three typical kinetic models namely pseudo first order, pseudo second order and Elovich equations were applied to interpret the kinetic data. To investigate the rate determining step, the intra- particle diffusion model was applied on the experimental data. Each kinetic model with their linear forms is given in Table 3. The pseudo-first order and pseudosecond order kinetic models have been applied for the experimental data to investigate the kinetics of Pb (II) ions on SB. The linear forms are shown as under ln (qe − qt ) = ln qe − kf t

(12)

t 1 t = + qt ks qe2 qe

(13)


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Table 3 Kinetics parameters for biosorption of Pb (II) on SB Kinetics Model


1. Pseudo-First-order ln (qe − qt ) = ln qe – kf t 2. Pseudo-second-order 1 t t qt = ks q e2 + qe 3. Intra- particle diffusion Model √ qt = Kid t + C

Parameters qe (cal.) mg/g 4.5 qe (cal.) mg/g 4.5 C mg/g

Kf (min.−1) 0.042 Ks (gmg−1 min−1) 0.029 Kid (mg g−1min−1/2)

qe (exp) mg/g 2.651 qe (exp) mg/g 4.49 qe (exp) mg/g

R2 0.577 R2 0.945 R2

4.5

0.450

1.003

0.779

where qe (mg/g) and qt (mg/g) are amounts of Pb (II) ions adsorbed on SB at equilibrium and at time t (min) respectively, kf (min−1) and ks (mg/g min) are the rate constants for pseudo-first order and pseudo-second order kinetics. The plots for pseudo-first order and pseudo-second order kinetic models are shown in Figures 3a and 3b respectively. The kinetic parameters for the adsorption of Pb (II) ions onto SB were evaluated from Figures 5a and 5b, and summarized in Table 3. For pseudo-first order kinetics, there is a large difference between calculated and experimental qe values. This inference is also confirmed by the low value of R2. For pseudo-second order, the R2 values are close to 0.98. In addition, the calculated qe values are close to the experimental qe values. Hence, it can be concluded that the biosorption of Pb (II) by SB followed the pseudo-second order kinetic model. Several authors who studied the biosorption of divalent metal ions on various sorbents reported that the biosorption generally followed pseudosecond order kinetics (Farooq et al. 2010; Lawal et al. 2010; Riaz et al. 2009; Sarı et al. 2007; Tunali et al. 2012). The intra particle diffusion model was applied to investigate the rate determining step (Farooq et al. 2011; Witek-Krowiak 2011). The linear form is shown as follows: √ (14) qt = Kid t + C where kId is the intra particle diffusion rate constant (mg/g min1/2) and C is the intercept. The rate determing step can be determined by plotting a graph between qt and t0.5. The values of intra particle diffusion parameters were evaluated from Figure 5c are presented in Table 3. From Figure 5c, a dual nature of the curves has been observed due to the varying extent of biosorption in the initial and final stages of the process for Pb (II) ions biosorption on SB. It indicated that more than one type of diffusion processes is involved in the biosorption. The initial part of the curves, having an increasing slope, represent the boundary layer diffusion whereas the linear parts at the later stages of the curves indicate the intra particle diffusion. As intercept ‘C’ is not zero, the curves are non-linear and do not pass through the origin. The boundary layer diffusion was found to be the rate determining step for the biosorption of Pb (II) on SB. Thermodynamic Studies Effect of temperature. For lead solutions maximum adsorption was observed at 50◦ C and removal efficiency was 91%. In the case of cadmium solutions, maximum adsorption was obtained at 60◦ C and removal efficiency was 83%. Once equilibrium had been established adsorption remained constant or decreased. At elevated temperatures,


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Figure 5 Non-Linear Langmuir, Freundlich, D–R, R-P, Toth, Sips and Fritz-Schlunder adsorption Isotherm for biosorption of Pb (II) ions on SB at 60◦ C; size of SB < 125 μm; speed of agitation 125 rpm and pH 6.0; SB dose 0.5 g/50 mL.

kinetic energy increased which resulted in increased metal adsorption. Increased adsorption at higher temperatures indicated the endothermic nature of the adsorption process. To confirm the thermodynamic feasibility and nature of the adsorption process, thermodynamic parameters were calculated. In order to describe thermodynamic properties of the adsorption of Pb onto adsorbent material, enthalpy change (H ◦ ), Gibbs free energy change (G◦ ) and entropy change (S◦ ) were calculated using the following equations: G◦ = H◦ − TS◦ G◦ = −RT lnKD −G◦ −H◦ S◦ ln KD = = + RT RT R KD = qe /C e

(15) (16) (17) (18)

where R is the universal gas constant (8.314 J/mol K), T is temperature (K) and K D (qe /Ce ) is the distribution coefficient. The Gibbs free energy indicates the degree of spontaneity of the sorption process and a higher negative value reflects a more energetically favorable sorption. To calculate thermodynamic parameters, a graph was plotted between ln KD and 1/T (Fig. 6a). The values of G◦ for Pb are given in Table 4. The values of free energy change ◦ (G ) increased with an increase in temperature thereby indicating an endothermic nature of the adsorption process.The enthalpy changes (H ◦ ) were observed to be 0.084 kJ mol−1 for Pb. An increase in temperature favors the removal process and the negative values of G◦ confirm the thermodynamic feasibility of the process and spontaneous nature of

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Figure 6 (a) Pseudo-First-order kinetics model (b) Pseudo-second-order kinetics model, (c) Intra-particle diffusion Kinetic model, for biosorption of Pb (II) ions on SB at 60◦ C; size of SB < 125 μm; speed of agitation 125 rpm and pH 6.0; SB dose 0.5 g/50 mL.

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Table 4 Thermodynamic parameters for sorption of Pb (II) on SB 1/T (K−1)

ln KD

G◦ (kJ mol−1)

Parameters

0.0035 0.0034 0.0033 0.0032 0.0031 0.0030

2.882 3.273 3.522 3.691 3.985 3.892

−6.78 −7.97 −8.87 −9.60 −10.70 −10.78

H ◦ (kJ mol−1) S◦ (kJ mol−1K−1) G# (kJ mol−1) H # (kJ mol−1) S# (kJ mol−1K−1) Ea (kJ mol−1)

+0.084 +16.66 +60.500 +2.498 −197.962 +5.054

adsorption (Farooq et al. 2011). The entropy change (S◦ ) was observed to be 16.66 kJ K−1 mol−1 for Pb. The positive value of S◦ reflected the affinity of the adsorbent material for metal Pb (II) (Imran Din et al. 2013). Activation parameters. The activation energy Ea , Gibbs free energy change (G#), changes in the enthalpy (H #) and entropy (S#) of activation for Pb (II) adsorption were calculated by applying the Arrhenius, Eyring and Gibb equations at different temperatures (Eqs. 19–20). lnk2 = ln k0 −

ks ln T

Ea RT

kB S = H = = ln + − h R RT G# = H# − TGS#

(19)

(20)

(21)

where ks is the rate constant (pseudo-second order) of adsorption (g mg−1 min−1), k0 is the independent temperature factor (g mg−1 min−1), R is the gas constant (J mol−1 K−1), T is the solution temperature (K), kB is the Boltzmann constant, h is Planck’s constant, T is the absolute temperature and R is the general gas constant. A plot of ln k2 versus 1/T gives a straight line and the corresponding activation energy was determined from the slope of the linear plot shown in Figure 6b. The magnitude of activation energy gives an idea about the type of adsorption which is mainly physical or chemical. Low activation energies (5–50 kJ mol−1) are characteristics for physical adsorption, while higher activation energies (60–800 kJ mol−1) suggest chemical adsorption. The activation energy for the biosorption of Pb (II) on SB was found to be 5.054 kJ mol−1. This relatively low activation energy value indicated that the adsorption of Pb (II) on the SB surface is rapid. A plot of ln (k2 /T) versus 1/T gives a straight line and the changes in enthalpy (H #) and entropy (S#) of activation were determined from the intercept and slope of the linear plot as shown in Figure 6c. The activation parameters were calculated from Figure 6c and presented in Table 4. The values for (G#) were found to be positive at all temperatures indicating the existence of an energy barrier for the Pb (II) adsorption. It further indicates that an input of energy is required for reactant molecules to have enough kinetic energy to overcome the energy barrier and the chemical reaction to take place.

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Figure 7 (a) Thermodynamical Parameters (b) Activation energy from Arrhenius plot (c) thermodynamic parameters of activation,for biosorption of Pb (II) ions on SB at different temperatures; size of SB < 125 μm; speed of agitation 125 rpm and pH 6.0; SB dose 0.5 g/50 mL.

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M. IMRAN DIN ET AL. Table 5 Comparison of biosorption performance of SB with some other biosorbents

Biosorbent


1. Symphoricarpus albus 2. Porphyridium purpureum 3. Oak stem 4. Phaeodactylum tricornutum 5. Calophyllum inophyllum 6. Olive stone 7. Saccharum bengalense

qmax (mg/g)

Equilibrium Model

Ref.

0.0621 0.32 0.75 1.49 4.86 5.25 5.78

Langmuir Langmuir — Langmuir Langmuir Langmuir and Sips Langmuir and Hill

S.T. Akar et al. D. Schmitt et al. M.N.V. Prasad et al. D. Schmitt et al. O.S. Lawal et al. G. Blázquez et al. Present work

The positive values of (H #) reveal that considerable energy is required for the exchange of the lead metal ions. The value of S# gives an indication whether the adsorption process is an associative or dissociative mechanism (Mohapatra et al. 2009). From Table 4 it is clear that (S#) has a negative value indicating that the movement of the lead metal ions in the SB is more restricted as compared to the outgoing ions. The negative (S#) value also shows that Pb (II) adsorption onto SB is an associated mechanism (Mahmood et al. 2011). Comparison of SB with other biosorbents SB has been compared in terms of biosorption capacity qm (mg/g), and equilibrium model (Table 5). The data indicates that the biosorption capacity of SB is much greater as compared to a number of other biosorbents (Akar et al. 2009; Blázquez et al. 2010; Cimino et al. 2000; Lawal et al. 2010; Prasad and Freitas 2000; Schmitt et al. 2001). CONCLUSION It may be concluded that Saccharum bengalense is an effective and low cost biosorbent for the removal of Pb (II) ions from aqueous solution. Results obtained indicate that a non-linear method could be an enhanced way to obtain the parameters as compared to the linear method. The precision of adsorption isotherm models to support experimental data progresses with degree of freedom. The Langmuir isotherm model, Redlich-Peterson model and Fritz-Schlunder model were found to be the best fit models amongst the twoparameter, three parameter and four parameter isotherm models, respectively. From values of G◦ , H ◦ and S◦ the adsorption process was found to be spontaneous, feasible and endothermic in nature. The positive values of (G∗ ) indicate the existence of an energy barrier for the Pb (II) biosorption onto SB. The increase in the uptake of Pb (II) metal ions, with temperature, is attributed to the appearance of some active sites on SB. The negative value of (S◦ ) indicates that the Pb (II) adsorption onto the SB is an associated mechanism. Batch studies proved the high efficiency (92.5%) of SB biosorbent as an efficient adsorbent for removal of toxic metal ions, in particular Pb (II) ions from contaminated water bodies. REFERENCES Akar ST, Gorgulu A, Anilan B, Kaynak Z, Akar T. 2009. Investigation of the biosorption characteristics of lead(II) ions onto Symphoricarpus albus: Batch and dynamic flow studies. J Hazard Mater 165:126–133.



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