Equilibrium, kinetic and thermodynamic studies on biosorption ... - NOPR

Indian Journal of Chemical Technology Vol. 20, January 2013, pp. 57-69

Equilibrium, kinetic and thermodynamic studies on biosorption of lead(II) and cadmium(II) from aqueous solution by polypores biomass M Suguna1 & N Siva Kumar 2,* 1

Biopolymers and Thermophysical Laboratories, Department of Chemistry, Sri Venkateswara University, Tirupati 517 502, India 2 Department of Safety Environmental System Engineering, Dongguk University, Gyeongju 780714, Republic of Korea Received 23 April 2012; accepted 18 July 2012 Polypores (woody pore fungi) biomass has been used as an biosorbent for the removal of Pb(II) and Cd(II) from aqueous solution under batch equilibrium experimental conditions. The resulting biosorbent is characterized by SEM and FTIR techniques. The effect of experimental parameters, such as solution pH, biomass dosage, contact time, initial metal ion concentration and temperature of Pb(II) and Cd(II) have been evaluated on the metal uptake performance. The pH for optimum biosorption is found to be 5.0 for Cd(II) (92.5% removal) and Pb(II) (96.4% removal). The adsorption kinetic data are best described by pseudo-second-order kinetic model with good correlation coefficient and low error function values. The experimental results indicate that the Langmuir isotherm describes the biosorption of Cd(II) and Pb(II) ions onto the biomass better than the Freundlich, Dubinin-Radushkevich (D-R) and Temkin models at all the temperatures studied. The calculated thermodynamic parameters (∆Go, ∆Ho and ∆So) show that the biosorption of both metal ions is feasible, spontaneous and exothermic in nature at 303–313 K. Based on the results obtained such as good uptake capacity and its low cost, polypores biomass appears to be a promising biosorbent material for the removal of Cd(II) and Pb(II) ions from aqueous media. Keywords: Adsorption kinetics, Biosorption, Polypores, Isotherms, Thermodynamic study

Heavy metal pollution has become one of the most serious environmental problems today. Pb(II) and Cd(II) are commonly present in different types of industrial effluents and are responsible for environmental pollution. They are non-biodegradable and hence are persistent in all parts of the environment. Pollution due to these metal ions represents an important problem due to their toxic effect and accumulation throughout the food chain which leads to serious ecological and health problems1,2. The reduction of these metals in effluents to a permissible limit before discharging them into streams and rivers is very important with respect to human health, environmental and economical considerations3. Lead is a particularly hazardous heavy metal because once it gets into human body it disperses throughout the body immediately and causes harmful effects wherever it lands. For example, it can damage the red blood cells and limit their ability to carry oxygen to the organs and tissues. It can also affect the nervous system, kidneys and hearing4. In particular, __________________ * Corresponding author. E-mail: [email protected], [email protected]

unborn babies and young children are at risk of health problems from lead poisoning because their smaller bodies make them more susceptible to absorbing lead ions. Lead compounds are known as metabolic poison and enzyme inhibitor5. Different industrial processes, such as battery manufacturing, printing and pigment, metal plating and finishing, soldering material, ceramic and glass industries, iron and steel manufacturing units are major sources of lead contamination in wastewater6,7. Cadmium is also a dangerous pollutant that is released into aquatic medium from electroplating industries, batteries, phosphate fertilizers, mining, pigments, stabilizers and alloys, cadmium finds its way to the water streams through wastewaters8,9. Cadmium toxicity may be observed by a variety of syndromes and effects including renal dysfunction, hypertension, hepatic injury, lung damage and teratogenic effects10,11. The main techniques that have been used to separate the heavy metal content from effluents include chemical precipitation, ion-exchange, adsorption onto activated carbon, membrane processes and electrolytic methods, each one having merits and shortcomings12. Most of these conventional methods suffer from some drawbacks,

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INDIAN J. CHEM. TECHNOL., JANUARY 2013

such as high capital and operational cost, the disposal of the residual metal sludge and un-suitable for small scale industries13. Therefore, it is essential to develop efficient and environmentally compatible techniques capable of removing or detoxifying heavy metals in an economical way14. Adsorption is a well-defined technique and widely used for the removal of poisonous metals in waste solutions. This technique is nowadays recognized as an effective and economic method for wastewater purification. Adsorption on activated carbon is an effective method, but both the preparation and regeneration are extremely expensive, limiting its wide application15. A number of low cost adsorbents have been used earlier for the removal of organic pollutants and toxic metal ions from wastewater16-28. Biosorption plays an important role in the elimination of metal ions from aqueous solutions in water pollution control7,8. Biosorption, based mainly on the affinity between biosorbent and sorbate, has been suggested as a potential alternative to the exiting physico-chemical technologies for the detoxification and the recovery of toxic and valuable metals from wastewater29-31. The biosorption process offers the advantages of reusability of biomaterial, low operating cost, reduced amount of chemical or biological sludge to be disposed off, and high efficiency in decontaminating effluents32. Studies on the mechanism of biosorption of heavy metals on fungi show that polysaccharides, proteins and lipids with many functional groups present in fungal cell walls are responsible for the binding of metals33,34. Fungal biomasses are considered to be good biosorbents for heavy metals because of their advantages such as low cost, environmental friendliness, ease of regeneration35. Several fungal biosorbents such as macrofungus36,37, Aspergillus niger38, Mucor rouxii39, Penicillium40, Lentinus sajor-caju41, Penicillium simplicissimum42, Trametes versicolor43, Rhizopus oryzae, Aspergillus oryzae44, Rhizopus arrhizus45, Spirogyra species46 and Mucor rouxii47 are found to be capable of efficiently accumulating heavy metal ions. Polypores biomass is natural and readily available in abundance. This fungus is inedible and so the use of this biosorbent for the removal of Pb(II) and Cd(II) from aqueous solution is important from point of environment and human beings health. It provides a cost effective solution for industrial and natural water management. This new material has been chosen as biosorbent in this study, as it is natural, easily available, and a low-cost biomass for dissolved metal ions.

The aim of the present work is to investigate the possible use of polypores biomass as an alternative biosorbent material for removal of Cd(II) and Pb(II) ions from aqueous solutions. Experimental parameters affecting the biosorption process such as pH, initial metal ions concentration, biomass dosage, contact time and temperature have been studied. The experimental equilibrium adsorption data are analyzed by Langmuir, Freundlich, Dubinin-Radushkevich and Temkin isotherm models. The kinetic models and thermodynamic parameters such as ∆Go, ∆Ho and ∆So for biosorption process are also evaluated. Experimental Procedure Biomass preparation

The polypores (woody pore fungi) biomass was used as biosorbent for the biosorption of Cd(II) and Pb(II) ions. Samples of the biomass were collected from Tirumala Tirupati hills, Andhra Pradesh, India. The samples were washed with tap water, followed by de-ionized water to remove dust and other impurities. It was then sun-dried, followed by drying in an oven at 60oC for 24 h. Finally, the dried biomass was chopped, sieved and the particles with an average size of 0.5 mm were used for biosorption experiments. Reagents and equipments

All chemicals used were of analytical reagent grade and used without further purification. Double deionized water (Milli-Q Millipore 18.2 MΩ cm-1 conductivity) was used for all dilutions. The pH of the solution was measured with a Digisun electronics digital pH meter using solid electrode calibrated with standard buffer solutions. A flame atomic absorption spectrophotometer (Shimadzu AA-6300, Japan) with deuterium background corrector was used. All measurements were carried out in an air/acetylene flame. A 10 cm long slot burner head, a lamp and an air-acetylene flame were used. The operating parameters for working elements were set as recommended by the manufacturer. Fourier transform infrared (FTIR) spectra of dried unloaded biomass and Pb(II) and Cd(II) loaded biomass prepared as KBr discs were recorded at 400–4000 cm-1 wavenumber range using a FTIR (Thermonicolet-200series, Germany) spectrometer. Scanning electron microscopy (Model Evo15, CarlZeiss, England) has been used to study the surface morphology of the biosorbent.

SUGUNA & SIVA KUMAR: LEAD (II) & CADMIUM (II) BIOSORPTION ONTO POLYPORES BIOMASS

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Batch studies

Determination of point of zero charge

Batch adsorption experiments were carried out in Erlenmeyer flasks by adding 0.1 g of polypores biomass in 100 mL of aqueous metal solution at desired initial pH, metal ion concentration and temperature. Initial solutions with different concentration of Pb(II) and Cd(II) were prepared by proper dilution from stock 1000 mg/L Pb(II) and Cd(II) standards. The initial pH was adjusted with solutions of 0.1M HCl or 0.1M NaOH. The flasks were gently agitated in a temperature controlled water bath shaker at 200 rpm for a period of 4 h. The adsorption on the glassware is found to be negligible and is determined by running blank experiments. All the experiments were performed in triplicates at the desired initial conditions and the concurrent value was taken. The content of flask was separated from biosorbent by filtration, using Whatman filter paper No. 42 and the filtrate was analyzed for remaining metal concentration in the sample using atomic adsorption spectrophotometer. The amount of metal ion sorbed per unit mass of the biosorbent (mg/g) was evaluated by using the following equation:

The point of zero charge (PZC), for polypores biomass was determined by the solid addition method48. 100 mL of solutions was transferred into a series of 200 mL conical flasks with pH varying from 1 to 12. The initial pHi values of the solutions were roughly adjusted by adding either 0.1 M hydrochloric acid or sodium hydroxide. The pHi of the solutions was then accurately noted and 0.1 g of polypores biomass was added to each flask, which was capped immediately. The suspensions were then manually shaken and allowed to equilibrate for 48 h, after which the pHf value of the supernatant for each flask was noted. The difference between the initial and final pH values (∆pH = pHi - pHf) was plotted against the pHi and the point of intersection of the resulting curve at which ∆pH = 0 gives the pHPZC value.

 C − Ce  Qe =  i  v m  

Statistical evaluation of kinetic parameters

The Marquardt’s49 percent standard deviation (MPSD) error function is employed in this study for finding out suitable kinetic model to represent the experimental data. The MPSD error function has been used previously by a number of researchers in the field, as shown below: p

… (1)

F er r o r ( % ) = 1 0 0 ×

∑ i

 q i m o d el − q i ex p  q i ex p 

2

  1  .      p −1

… (2) where Qe (mg/g) is the adsorption capacity at equilibrium; Ci and Ce, the initial and equilibrium concentrations of metal ion (mg/L) respectively; V (L), the volume of adsorbate in liter; and m (g), the amount of adsorbent in gram. To study the effect of initial pH on metal ion uptake by polypores biomass, sorption experiments were performed by using 100 mL of solution with initial metal ion concentration of 100 mg/L and adsorbent dose of 0.1 g at temperature 303 K by varying the pH of the solution. The effect of adsorbent dose on adsorption of metal ions was studied by agitating 100 mL of 100 mg/L metal solution with different amounts of adsorbent. Effect of initial metal ion concentration was studied with an initial metal ion concentrations of 100, 200, 300 and 400 mg/L and polypores biomass of 0.1 g; pH was kept at 5.0 for Cd(II) and Pb(II). The time required for reaching the equilibrium conditions was estimated by drawing samples at regular intervals of time till equilibrium was reached.

where qi model is the each value of q predicted by the fitted model; qi exp, the each value of q measured experimentally; and p, the number of experiments performed. Results and Discussion characterization of biosorbent

Presence of functional groups on a biomass can be identified by FTIR analysis50. FTIR spectra of polypores biomass, before and after adsorption of metal ions, are shown in Fig. 1. The spectrum of polypores biomass [Fig. 1 (a)] shows the presence of amide, carboxyl, hydroxyl and phosphate groups on it. The broad and strong peak at 3380.9 cm-1 is due to the bounded hydroxyl (-OH) or amine (-NH) stretching vibrations. The peak at 2922.6 cm-1 can be assigned to the –CH groups of the biomass. The peak at 1640.3 cm-1 is due to primary and secondary amide bands. The bands observed at 1044.9 cm-1 are due to the presence of P-O-C and P-OH links of the organic phosphorus groups in the biomass.

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Fig. 1FTIR spectra of (a) polypores before adsorption, (b) loaded with Cd(II), and (c) loaded with Pb(II)

It is observed that the asymmetrical stretching vibration at 3380.9 cm-1 is shifted to 3367.4 and 3363.3 cm-1 after the biosorption of Cd(II) and Pb(II) respectively. The changes in -OH absorption peak indicate that the hydroxyl group has been changed from multimer to monopolymer or even dissociative state. It offers more opportunity for Cd(II) and Pb(II) to be bound to the hydroxyl or amine groups51. The amide peak at 1640.3 cm-1 is shifted to 1644.7 and 1643.9 cm-1 for Cd(II) and Pb(II)-loaded biomass. This is related to the N-H and C-H bending vibration, indicating involvement of amide group C=O and carboxylate anion in metal adsorption. The shifts in the absorption peaks generally observed indicate the existence of a metal binding process taking place on the surface of the biomass. SEM analysis

Scanning electron micrographs (SEM), recorded by using software controlled digital scanning electron microscope, are given in Fig. 2. The SEM of pure

polypores biomass shows a well defined rod like clusters in mat format with uneven surface texture. The figure also illustrates the surface texture and porosity of polypores biomass with holes and small openings on the surface, which facilitates the pore diffusion during adsorption. Figures 2(b) and (c) describe the surface characteristics and morphology of the polypores biomass after their exposure to metal ion solutions. It can be observed that there is a change in the morphology of the surface after adsorption. Surface morphological studies reveal that the process of metal adsorption on polypores biomass is predominantly a surface phenomenon and this is confirmed by the SEM images. Effect of pH

The pH has been identified as one of the most important parameters that influence the metal sorption. The effect of pH on the biosorption of Pb(II) and Cd(II) onto polypores biomass is studied in the pH range 2-8, employing 0.1 g of polypores biomass


with 100 mL of a 100 mg/L adsorbate solution at 303 K. The results are given in Fig. 3(a). The biosorption efficiency is found to be 92.5% for Cd(II) and 96.4% for Pb(II) at pH 5.0. Therefore, the remaining biosorption experiments are conducted at this pH value. The main factors influencing the effect of pH on biosorption process are metal ion species and functional groups present on the adsorbent surface. At highly acidic pH, the overall surface charge on the active binding sites becomes positive and metal

Fig. 2Scanning electron micrographs of (a) polypores biomass, (b) loaded with Cd(II), and (c) loaded with Pb(II)

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cations and protons compete for binding sites on cell wall, which results in lower uptake of metal. As the pH increases, there are few protons present in the solution, consequently less competition with metal ions for binding sites. This results in the increasing biosorption amount of Pb(II) and Cd(II). Point of zero charge determination of the biosorbent is important in elucidating the biosorption mechanism. The pHpzc of polypores biomass is 4.8 [Fig. 3(b)] and the sorption of positively charged species will be favored only at pH> pHpzc52. For the pH at PZC, the surface charge of biosorbents is neutral and the electrostatic forces between metal ion and surface of adsorbents are balanced. This balance is disturbed when pH is deviated from pHPZC. At pHpHPZC, the surface charge of the biosorbent becomes negative and Pb(II) and Cd(II) ions in solution are attracted to their surface. Maximum sorption is likely to occur at pH values greater than pHPZC when adsorbents have a net negative charge. Decrease in biosorption capacity at

Fig. 3 (a) Effect of pH on the biosorption of Cd(II) and Pb(II) onto polypores biomass (initial conc 100 mg/L, contact time 4h, agitation speed 200rpm, and biosorbent dose 0.1 g) (b) Determination of point of zero charge, error bars represents ± S D

62


higher pH (pH > 5.0) is not only related to the formation of soluble hydroxilated complexes of the metal ions [(lead ions in the form of Pb(OH)2, and cadmium ions in form of Cd(OH)2)] but also to the ionized nature of the cell wall surface of the biomass under the studied pH53. The pH values of 5–6 have also been reported in literature for biosorption of Cd(II) and Pb(II) ions by using different kinds of biosorbents54-58. Therefore, for both metal ions the optimum pH value is fixed at 5.0 throughout this work.

with the amount of metal ion desorbed from the biosorbent. The equilibrium time for Cd(II) and Pb(II) is found to be 150 min and 180 min respectively. From Figs 4(a) and (b), it is observed that Cd(II) and Pb(II) removal increases with the increase in initial metal ion concentration. The equilibrium biosorption amount of Cd(II) and Pb(II) are 35.37 & 54.84 mg/g, 73.07 & 98.27 mg/g, 52.06 & 92.52 mg/g and 127.2 & 159.14 mg/g respectively at an initial concentrations of 100, 200, 300 and 400 mg/L.

Effect of contact time and initial concentration

Effect of biosorbent dosage

In order to determine the equilibrium time for maximum uptake of Cd(II) and Pb(II) on polypores biomass, experiments were performed with initial metal ion concentrations ranging from 100 mg/L to 400 mg/L at different contact times. The sorption data for the uptake of cadmium and lead versus contact time as a function of initial concentration are presented in Figs 4(a) and (b). It can be observed that the adsorption capacity increases with time and at certain point of time, it attains a constant value. At this point, the amount of metal ion being adsorbed by the biosorbent is in a state of dynamic equilibrium

The sorption of Cd(II) and Pb(II) ions by polypores biomass was studied by changing the quantity of sorbent from 0.05 g to 0.5 g, while maintaining the initial metal ion concentration of 100 mg/L, contact time of 150 min for Cd (II) and 180 min for Pb (II), temperature of 303 K, stirring speed of 200 rpm and pH of 5.0. Figures 5(a) and (b) show the effect of adsorbent dosage on the % removal of Cd(II) and Pb(II) ions respectively. The amount of Cd(II) and Pb(II) ions sorbed at equilibrium decreases from 44.78 mg/g to 17.58 mg/g and from 73.25 mg/g to

Fig. 4Effect of contact time on the biosorption of (a) Cd(II) and (b) Pb(II) onto polypores biomass (biosorbent dose 0.1g and pH 5.0), error bars represent ± S D

Fig. 5Effect of adsorbent dose on the biosorption of (a) Cd(II) and (b) Pb(II) onto polypores biomass (initial concentration 100 mg/L, contact time 4h and pH 5.0)


17.79 mg/g respectively with the increase in adsorbent dosage. The increase in sorbent dose at constant metal concentration and volume will lead to unsaturation of sorption sites, through the sorption process. The adsorption capacity decrease may also be due to overlapping or aggregation of sorption sites resulting in decrease in total sorbent surface area available to metal ions. On the other hand, an increase in the biosorbent dosage from 0.05 g to 0.5g increases the percentage of Cd(II) and Pb(II) removal from 23.65 to 89.15 and from 39.23 to 91.31 respectively. This may be due to increased sorbent surface area and availability of more sorption sites. Sorption kinetics

In order to predict the sorption kinetic models of Pb(II) and Cd(II), pseudo-first-order, pseudo-secondorder kinetic models were applied to the data. Pseudo-first-order equation can be represented as59

 K  log(qe − qt ) = log qe −  1  t  2.303 

… (3)

where K1 (min-1) is the pseudo-first-order rate constant; and qe and qt, the amounts of the metal ions adsorbed (mg/g) at equilibrium and time t (min)

63

respectively. The sorption rate constants (K1) can be determined experimentally by plotting log (qe-qt) versus t. The values of K1, R2 and calculated uptake capacity qe along with the Marquardt’s per cent standard deviation (MPSD) error function values for both the metal ions Cd(II) and Pb(II) are included in Tables 1 and 2 respectively. R2 and MPSD values in Tables 1 and 2 indicate that the biosorption of Cd(II) and Pb(II) metal ions onto polypores biomass does not follow the pseudo-first-order kinetic model. Pseudo-second-order model can be represented as60

t 1 1 = + t 2 qt K 2 q e q e

… (4)

where K2 (g/mg/min) is the rate constant of the second-order equation. The values of K2, R2, MPSD and qe for Cd(II) and Pb(II) are given in Tables 1 and 2. The R2 values are close to unity and the MPSD error function values are lower compared to that of pseudo-first-order kinetic model for the biosorption of both the metal ions. In view of these observations, it may be concluded that the pseudo-second-order kinetic model provides a good fit for the biosorption of Cd(II) and Pb(II) ions onto polypores in comparison to the pseudo-first-order kinetics.

Table 1Parameters of pseudo-first-order, pseudo-second-order, Weber-Morris and Elovich models for the biosorption of Cd(II) onto polypores biomass Kinetic model

Parameter

100 mg/L

200 mg/L

300 mg/L

400 mg/L

Pseudo-first-order

qe, cal mg/g K1×10-2 R2 MPSD,%

27.37±1.01 1.4±0.13 0.942 5.24

40.04±3.51 1.6±0.13 0.938 18.77

45.14±3.47 1.7±0.31 0.920 15.37

44.28±3.14 1.2±0.31 0.959 21.61

Pseudo-second-order


33.02±0.54 4.00±1.00 0.991 6.63

53.06±1.58 5.13±0.29 0.996 3.25

70.83±2.31 6.24±0.38 0.997 3.06

95.86±1.16 5.70±1.01 0.997 2.45

Weber-Morris

qe, cal mg/g kid, mg/g/min-1/2 R2 MPSD,%

32.74±0.58 2.15±0.33 0.958 7.43

52.02±1.35 2.59±0.12 0.947 5.14

70.01±2.47 2.51±0.61 0.951 4.18

94.66±3.35 2.69±0.48 0.953 3.68

Elovich

qe, cal mg/g α, mg/g/min β×10-2, g/mg R2 MPSD,%

32.98±1.87 1.78±0.16 9.9±0.2 0.967 2.81

52.48±3.33 5.81±0.24 8.1±0.6 0.978 4.32

70.63±2.12 3.41±0.18 8.4±0.4 0.968 3.33

97.45±4.25 167.0±0.2.43 0.080±0.2 0.939 0.83

100, 200, 300 and 400 mg/L are the initial concentration of Cd (II).


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Table 2Parameters of pseudo-first-order, pseudo-second-order, Weber-Morris and Elovich models for the biosorption of Pb(II) onto polypores biomass Kinetic model

Parameters

100 mg/L

200 mg/L

300 mg/L

400 mg/L

Pseudo-first-order


37.29±3.91 1.51±0.18 0.984 22.47

39.26±2.21 1.73±0.30 0.982 41.02

42.21±1.83 1.21±0.15 0.974 47.36

43.29±1.35 1.02±0.16 0.967 73.10

Pseudo-second-order


52.06±2.73 5.11±0.66 0.993 0.20

100±1.78 6.21±0.75 0.999 2.96

127±1.83 4.34±0.71 0.996 0.66

166.6±1.50 5.33±0.45 0.997 3.37

Weber-Morris

qe, cal mg/g kid, mg/g/min-1/2 R2 MPSD,%

55.94±2.01 2.44±0.66 0.987 5.4

93.06±0.54 2.68±0.12 0.965 3.45

127.72±1.66 3.51±0.61 0.986 4.09

159.42±1.94 3.28±0.48 0.958 2.96

Elovich

qe, cal mg/g a, mg/g/min-1 β×10-2, g/mg R2 MPSD,%

52.22±1.51 10.54±1.81 8.8±0.6 0.997 0.3

91.53±2.13 98±20.49 7.9±0.3 0.977 0.57

123.89±2.57 190.57±77.67 6.2±0.1 0.998 2.6

154.87±3.14 260.17±120.46 6.7±0.1 0.990 1.2

Weber-Morris model

Since neither the pseudo-first-order nor the pseudo-second-order models identify the diffusion mechanism, the kinetic results were analyzed by the intraparticle diffusion model. During batch mode of operation, there is a possibility of intraparticle pore diffusion of Cd(II) and Pb(II) ions, which is often the rate-limiting step. The Weber-Morris intraparticle diffusion model is expressed as61

Qt = k id t 1 / 2 + C

… (5)

where C is the intercept; and kid (mg/g min-1/2), the intraparticle diffusion rate constant calculated from the slope of the plot qt versus t1/2. The dual nature of the curves is due to the varying extent of sorption in the intial and final stages of the experiment. In the initial stages, sorption is due to boundary layer diffusion effect, whereas in the final stages it is due to the intraparticle diffusion effect. But the plots do not pass through the origin, so intraparticle diffusion is not the only rate controlling step. Boyd model

Boyd model is applied to check whether sorption proceeds via film diffusion or intraparticle diffusion mechanism. The model can be expressed in the following form62:

F = (1 -

6 Π2

) exp(- Bt )

… (6)

where F is equal to qt/qe; qe, the amount of metal ions adsorbed at equilibrium (mg/g); qt, the amount of ions adsorbed at any time t (min); and Bt, a mathematical function of F. Equation (6) can be rearranged by taking the natural logarithm to obtain the following equation:

Bt = −0.4977 − 1n(1 - F )

… (7)

The plots of Bt vs t at different initial concentrations for both Cd(II) and Pb(II) are linear with correlation coefficient (R2) greater than 0.987. The results suggest that the adsorption process is controlled by film diffusion. Elovich equation

The Elovich equation has been applied satisfactorily to some chemisorption processes. The Elovich equation can be written in the following form63: 1 1 Qt =   ln(αβ) +   ln(t ) β β 

… (8)

where α is the adsorption rate (mg/g/min); and β is related to the extent of surface coverage and the active


energy involved in chemisorption (g/min). The values of α, β and error function are given in Tables 1 and 2. The plots are linear with good correlation coefficients and low error function values. This suggests that the sorption process follows the pseudo-second-order kinetic model based on the assumption that the rate determining step may be chemisorption, involving valence forces through sharing or exchange of electrons between adsorbent and adsorbate. Sorption isotherms

The isotherms were obtained by using a batch adsorption data at different metal ion concentrations. Different isotherm models have been utilized for describing sorption equilibrium. Langmuir, Freundlich, Dubinin–Radushkevich and Temkin isotherm equations were used for the present work. The Langmuir sorption isotherm describes that the uptake occurs on a homogeneous surface by monolayer sorption without interaction between sorbed molecules. The Langmuir isotherm may be represented as qe =

qm ax b C e 1 + bCe

… (9)

where qe is the equilibrium adsorption capacity (mg/g); Ce (mg/L), the equilibrium metal ion concentration in the solution (mg/L); b, related to affinity of the binding site; and qmax, the monolayer adsorption capacity of the adsorbent. The linearized form of Eq (9) is  1  1 1 1 =   + qe q m ax b  C e  qm ax

… (10)

The isotherms show good fit to the experimental data with good correlation coefficients (>0.997). The sorption capacity of polypores biomass for Cd(II) and Pb(II) decreases with increasing temperature from 303K to 313 K (Table 3). This confirms the exothermic nature of the biosorbent. The essential characteristics of the Langmuir isotherm can be expressed in terms of dimensionless parameter (RL), which is defined as RL =

1 1 + K LCi

… (11)

where KL (qmax×b) is the Langmuir constant; and Ci, the initial concentration of metal (mg/L). The value of

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RL indicates the type of isotherm i.e. unfavorable (RL>1), linear (RL=1) and favourable (0< RL