Accepted Manuscript Design of a new integrated Chitosan-PAMAM Dendrimer biosorbent for heavy metals removing and study of its adsorption kinetics and thermodynamics Zabihullah Zarghami, Ahmad Akbari, Ali Mohammad Latifi, Mohammad Ali Amani PII: DOI: Reference:
S0960-8524(16)30028-1 http://dx.doi.org/10.1016/j.biortech.2016.01.052 BITE 15963
To appear in:
Received Date: Revised Date: Accepted Date:
10 December 2015 12 January 2016 14 January 2016
Please cite this article as: Zarghami, Z., Akbari, A., Latifi, A.M., Amani, M.A., Design of a new integrated ChitosanPAMAM Dendrimer biosorbent for heavy metals removing and study of its adsorption kinetics and thermodynamics, Bioresource Technology (2016), doi: http://dx.doi.org/10.1016/j.biortech.2016.01.052
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Design of a new integrated Chitosan-PAMAM Dendrimer biosorbent for heavy metals removing and study of its adsorption kinetics and thermodynamics Zabihullah Zarghami a, Ahmad Akbari a, Ali Mohammad Latifi b,*, Mohammad Ali Amani b a
Institute of Nano Science and Nano Technology, University of Kashan, Kashan, Iran.
Applied Biotechnology Research Center, Baqiyatallah University of Medical Science, Tehran, Iran.
Abstract In this research, different generations of PAMAM-grafted chitosan as an integrated biosorbent were successfully synthesized via step by step divergent growth approach of dendrimer. The synthesized products were utilized as adsorbents for heavy metals (Pb2+ in this study) removing from aqueous solution and its reactive Pb2+ removal potential was evaluated. The results showed that as-synthesized products with higher generations of dendrimer, have more adsorption capacity compared to products with lower generations of dendrimer and sole chitosan. Adsorption capacity of as-prepared product with generation 3 of dendrimer is 18 times more than sole chitosan. Thermodynamic and kinetic studies were performed for understanding equilibrium data of the uptake capacity and kinetic rate uptake, respectively. Thermodynamic and kinetic studies showed that Langmuir isotherm model and pseudo second order kinetic model are more compatible for describing equilibrium data of the uptake capacity and kinetic rate of the Pb2+ uptake, respectively. Keywords: Adsorption isotherm. Dendrimer-grafted. PAMAM. Biosorbent. Chitosan.
Corresponding authors. Tel: +98 21 88617712; fax: +98 21 82482549. E-mail address: [email protected]
At present, the release of toxic and heavy metals such as Pb2+ from several industries, for example, paints, metal plating, electroplating, mining, photograph development, batteries, and alloy industries, results in a number of environmental problems (Arancibia-Miranda et al., 2016; Momcilovic et al., 2011; Machida et al., 2012). The element lead, Pb2+ is a neurotoxin that accumulates both in soft tissues and bones and leads to serious damage to the nervous system. In addition, an excessive amount of Pb2+ causes blood or brain disorders in mammals. Therefore, it is necessary to carefully eliminate such highly toxic ions from the environment. Due to environmentally friendly reasons, many attempts have been made to use low cost biomaterials for removing heavy metal ions from wastewater (Yargıç et al., 2015; Luo et al., 2015). Biotechnology as a clean technology has great potential to resolve the environmental concerns. In this regard, bioremediation technique as a crucial method in environmental biotechnology has attracted great deal of researcher’s attention. The tools of bioremediation technology include microorganisms, plants and products derived from such as enzymes and various biopolymers. The application of biopolymers such as bacterial exopolymers, alginate, collagen, chitin and chitosan is one of the emerging adsorption methods for the removal of dyes and heavy metal ions such as U, Cd, Pb, Hg, Cs even at low concentrations (Crini, 2006; Latifi et al., 2012; Newsome et al., 2014; Wanga and Chen, 2014) due to the presence of amino and hydroxyl groups, which can serve as the active sites (Wu et al., 2001). The presence of a large number of amine groups on the chitosan chain increases the adsorption capacity of chitosan (Evans et al., 2002). Chitosan, a nitrogenous polysaccharide composed mainly of poly(β-1-4)-2-amino-2-deoxy-d-glucopyranose, is produced through the deacetylation of chitin, which is widely spread among marine and terrestrial invertebrates and in lower forms of the plant kingdom (Ngah and Fatinathan, 2006; Chatterjee et al., 2005). In order to improve the chemical and mechanical properties, chitosan can be physically or chemically modified. Chemical modification is the application of chemical treatment on chitosan, which improves its stability in acidic media and enhances the selectivity for metal
adsorption. A variety of processes for chemical modification such as crosslinking, grafting, blending, sulfonation, and carboxymethylation can be utilized (Sun and Wang, 2006). In this study, grafting of dendrimer onto chitosan was investigated. Dendrimers are a group of highly branched polymer compounds having different functional groups where they can react with different functional entities of other molecules in nanometer scale. Compared with polymers, dendrimers possess especial physical properties such as viscosity, flexibility, and density distribution (Shimei et al., 2006). For the particular envisaged properties; the distribution of functional groups in the dendrimer skeleton plays a significant role (Astruc et al., 2010; Klajnet and Bryszewska, 2001). Generally, two diverse synthetic strategies are currently explored to synthesize structurally well-defined dendritic polymers, namely the divergent and convergent growth approaches (Tomalia and Frechet, 2002; Newkome and Shreiner, 2008). Herein, different generations of polyamidoamine (PAMAM) dendrimer were successfully grafted to chitosan via step by step divergent growth approach of dendrimer. For investigation of Pb2+ adsorption capacity of products, Polarographic method was used due to its high sensitivity to low metal ion concentrations. In addition, effect of temperature, pH, solid/liquid ratio, ion initial concentration and other important parameters on adsorption capacity were investigated. Thermodynamic and kinetic studies were performed for understanding equilibrium data of the uptake capacity and kinetic rate uptake, respectively. The equilibrium data were analyzed by applying Langmuir, Freundlich and Temkin isotherm models. The kinetics were evaluated using pseudo-first order, pseudo-second order and intraparticle diffusion equations.
2. Experimental 2.1. Materials
Chitosan with a degree of deacetylation 81.69%, Methyl acrylate (MA), ethylenediamine (EDA), Pb(NO3), acetic acid glacial (HOAc) and 37% HCl were purchased from Aldrich. All of the other reagents used were of analytical reagent grade. For purifying of chitosan powder a typical method was utilized as follow: 4 gr of Chitosan was dissolved in 20 ml of 0.1 M HOAc. After stirring for 24 h, the solution was filtered through a medium-pore-size sintered glass funnel to remove insoluble substances. Then, 10 ml of 0.1 M NaOH was added drop wise to solution to precipitate chitosan powder. Afterwards, the chitosan powder was separated through a medium-pore-size sintered glass funnel and washed several times with methanol and dried at 50 °C for 24 h. Methyl acrylate (MA) and ethylenediamine (EDA) were refluxed over sodium and distilled just before use. 2.2. Characterization The crystal structural and compositional properties of products were recorded by X-ray diffraction (PhilipsX’PertPro) and FT-IR (Magna-IR, spectrometer 550 Nicolet with 0.125 cm-1 resolution in KBr pellets in the range of 400-4000 cm-1). Pb2+ ions concentration in aqueous medium was determined by polarographic method using a Metrohm 797 VA Computrace. Zeta potentials and isoelectric point (point of zero charge) determination of obtained products performed using a Malvern Zetasizer Nano ZS model of zeta potential apparatus. A 0.1 wt % adsorbent suspensions were prepared and pH adjustments made using HCl or NaOH. After the pH stabilized, required amount of these solutions transferred to the measuring cell and about 3 values of zeta potential were measured at room temperature. The stock solutions of Pb(II) was prepared by dissolving 1.6 g of lead nitrate in 1000 mL of double distilled water such that each 1 mL of the solution contains 1 mg of divalent metal. The exact concentration of Pb(II) solution was calculated on mass basis and expressed in terms of mg.L1- (ppm). The required lower concentrations were prepared by dilution of the stock solution. All precautions were taken to minimize the loss due to evaporation during the preparation of
solutions and subsequent measurements. The stock solutions were prepared fresh for each experiment as the concentration of the stock solution may change on long standing. 2.3. Preparation of PAMAM-grafted chitosan Grafting reaction and propagation of PAMAM dendrimer from purified chitosan powder surface was achieved by two steps, (1) Michael addition of MA to amino groups on the surface, and (2) amidation of terminal groups with EDA (Tsubokawa et al., 1998). Michael addition was carried out as follows: Into a 500 ml flask that contained 4 g chitosan powder, 150 ml of methanol and 1 ml of MA were added. The flask was sealed under N2 gas and the mixture was stirred with a magnetic stirrer at 50°C. After 24 h, the resulting powder was precipitated by centrifugation (5 x 103 rpm for 20 min) and washed with methanol repeatedly. The obtained product was called CS-G0.5. The amidation of terminal ester groups was carried out as follows: Into a 500 ml flask that contained the silica, 150 ml of methanol and 10 ml of EDA were added. The flask was sealed under N2 gas, and the mixture was stirred with a magnetic stirrer at 50°C. After 24 h, the resulting powder was precipitated by centrifugation (5 x 103 rpm for 20 min) and washed with methanol repeatedly (CS-G1). The propagation for 2nd generation was carried out as follows: Into a 500 ml flask, the chitosan powder obtained from the above reactions was treated with 2 ml of MA (twice the volume of the preceding reaction) in 150 ml of methanol (CS-G1.5). After the reaction, the resulting powder was treated with 20 ml of EDA (twice the volume of the preceding reaction) in 150 ml of methanol (CS-G2). Both the Michael addition and the amidation reactions were repeated to propagate the dendrimer from the CS powder surface (CS-G2.5 and CS-G3). The synthesis terms are listed in Table 1. 2.4. Adsorption Study A static adsorption experiment was employed to determine the adsorption capability and thermodynamic and kinetic parameters of samples. A typical way was that a dose of desired amount of the Pb2+ solution was
added to a 50-mL Pyrex glass tube, and then the glass tube was placed in a thermostated shaking assembly. A known amount of sample was charged, and the mixture was mechanically shaken at room temperature for 3 h. The solution in the tube was then separated from the adsorbent, and the concentration of metal ion was detected by means of polarographic method. To determine kinetic parameters, a dose of desired amount of the Pb2+ solution was added to a 10 ml syringe, and then the syringe was placed in a thermostated shaking assembly and at different times, 0.1 ml of filtered solution was separated and the concentrations of metal ion at different times were detected by means of polarographic method. The adsorbed amount was calculated according to the equation as follows:
Where Q is the adsorption amount (mg/L); C0, the initial concentrations of metal ions (mg/L); C, the final concentrations of metal ions (mg/L); V, volume (L); W, the dry weight of adsorbent (g).
3. Results and discussion 3.1 Chemical Characterization of PAMAM-grafted chitosan The chemical structure of different generations of PAMAM-grafted chitosan was confirmed by FT-IR. The FT-IR spectra of sole chitosan (sample CS) and different generations of PAMAM-grafted chitosan (samples CS-G0.5, CS-G1, CS-G1.5, CS-G3 and CS-G3 after Pb2+ adsorption) were recorded to understand the structural changes during different steps of biosorbents preparation (Figs. S1a-f). Sole chitosan (sample CS) shows the absorption band at 1653 cm-1 related to C=O stretching of amide I. The absorption band at 1592 cm-1 is assigned for -NH2 bending. The broad peak at 3437 is assigned to the stretching vibrations of the OH groups of the hydroxyls groups. Two bands at 2927 and 2858 cm-1 correspond to –CH2 asymmetric
and symmetric stretching modes. The peak located at 1034 cm-1 is assigned to C-O stretching, the absorptions at 1425 and 1381 cm-1 that are assigned as CH2 and CH bending bands, respectively. Also the peak
cm–1 is attributed to -CN axial deformation of amino groups (Fig. S1a). In ester-terminated dendrimer polymer grafting samples CS-G0.5 and CS-G1.5 (Figs. S1b and d), the absorption at about 1718-1723 cm-1 suggested the presence of ester bonds (–CO2CH3). A broad band in the region of 3454 cm–1, which is attributed to the stretching vibrations of the OH groups of the hydroxyls groups of the structure, in addition to physiosorbed water of the biopolymer. Moreover, this also involves the absorption of the NH groups from acetylated units of the biopolymer (Sakkayawong et al., 2005). The characteristic absorption peaks of ester bonds disappeared in the corresponding amino-terminated samples CTS-G1.0 and CTS-G3.0 (Figs. S1c and e) and amide characteristic peak at about 1645-1654 cm–1 appeared indicating that the ester bonds were almost inverted into amino-terminated products. Based on the above analysis, it was concluded that esterand amino-terminated hyperbranched PAMAM were grafted successfully on the CS surface via the step by step divergent approach. Moreover, for sample CS-G3 after exposing to Pb2+ (Fig. S1f), a new peak at 428 cm-1 emerged that can be assigned as Pb-N stretching vibration and this finding shows the proper adsorption of Pb2+ by amino groups. For further structural assessments, XRD patterns of different generations of PAMAM-grafted chitosan (samples CS, CS-G0.5, CS-G1 and CS-G1.5) were also recorded (Figs. S2a-d). Compared with CS, the intensity of the maximum peak at 2θ = 20.52° in the diffractogram of the sample CS-G0.5 was slightly increased and the peak at 2θ = 10.98° apparently decreased, which means that the introduction of grafted groups (ester groups here) affected the crystallinity of the main chain of CS. From the diffractogram of the sample CS-G1.0, it could be seen that the intensity of the peak at 2θ = 11° increased compared with that of CS-G0.5, indicating that the conjugation between the grafted groups and main chain of CS increased when
the ester-terminated groups were converted into the amino-terminated groups. This fact could be interpreted as that amino groups were more easily available than ester groups to form hydrogen-bonds with the main chain of CS. The intensity of the peak at 2θ = 20.4° in the diffractogram of sample CS-G1.0 apparently increased compared with CS and CS-G0.5, demonstrating that the introduction of polar amino groups increased the crystallinity of CS. It should be noted that the intensity of the peak at 2θ = 20.4° of CS-G1.5 evidently decreased. Its intensity also decreased compared with CS-G1.0, which demonstrates that the crystallinity of CS-G1.0 decreased when the polar amino groups were reconverted into weakly polar ester groups. The above-mentioned fact revealed that introducing weakly polar groups (e.g., ester groups) into CS would result in a decrease in the crystallinity of derivatives of CS, and introducing strongly polar groups (e.g., amino groups) would result in an increase in the crystallinity of the derivatives of CS. 3.2 Zeta Potential Analysis The variation of the zeta potential of samples of CS, CS-G1, CS-G2 and CS-G3 with respect to pH was studied in order to determine the surface charge of these biopolymers. The results of the zeta potential variation as a function of pH are shown in Fig. 1. The point of zero charge (PZC) of CS, CS-G1, CS-G2 and CS-G3 were obtained about 6.7, 7.1, 7.4 and 8, respectively confirming successful grafting of different generations of PAMAM on chitosan surface. Because by increasing the amino group of adsorbents, the negative charge of surface will be increased and as a result, slipping plane will be increased and finally, the point of zero charge (PZC) shifts to higher value. Over a wide range of pH up to about PZC; the biopolymers exhibit a net positive zeta potential attributed to the protonation of the amino groups onto biosorbents surface. Above the PZC, the mentioned adsorbents demonstrate a net negative zeta potential due to the lack of protonation and attachment of OH- ions. So, for cation adsorption like Pb2+, the pH above PZC is appropriate because below PZC, the repulsion between positive charge of surface and cation can decrease the uptake capacity.
3.3 Effect of pH at different temperatures The effect of pH on the adsorption of Pb2+ ions were studied at different pH values using CS-G3 sample at constant metal ion concentration (100 mg L-1) and amount of biosorbent (0.1 g) at different temperatures. The obtained results are presented in Fig. 2a. The initial pH of the metal ion solutions were changed between 2 and 8. As seen in Fig. 2a, adsorption of Pb2+ ions increases with an increase in pH where the charge of surface decreases also within the studied pH range, the best adsorption was obtained at pH values close to PZC of CS-G3. On the other hand, According to the distribution of lead species as a function of pH (Nurchi and Villaescusa, 2011), precipitation of Pb2+ ion at pH ˃ 6 will be significantly increased in aqueous medium and as a result, the obtained absorption capacity for adsorbent will not be accurate and a positive error will occur and so the optimum pH for adsorption of lead ions should be near 6. The low level of metal ion uptake by the biosorbent at lower pH values could be attributed to the increased concentration of hydrogen (H+) ions which compete along with Pb2+ ions for binding sites on the biomass. As the pH is lowered, the overall surface charge on the biosorbents become positive, which will inhibit the approach of positively charged metal cations. However there is still adsorption even though there is a repulsion between surfaces and metal ions. This might indicate a limited contribution of chemical adsorption that is caused by the unpaired electrons of nitrogen atoms at amino functional groups of PAMAM-grafted chitosan. At pH values above the isoelectric point, there is a net negative charge on the surface and the ionic point of ligands such as carboxyl, hydroxyl and amino groups are free so as to promote interaction with the metal cations. This would lead to electrostatic attractions between positively charged lead cations and negatively charged binding sites. 3.4 Effect of Solid Liquid (S/L) Ratio and Initial Concentration of Pb2+ The effect of solid/liquid (adsorbent/adsorbate solution) ratio (g/ml) for Pb2+ ion adsorption on PAMAMgrafted chitosan n was examined at constant operational parameters (T=298 K, pH=6, C0=100 mg/L and
t=24 h). The results are presented in Fig. 2b (blue line). Generally, it is observed that heavy metal ion removal increases with increasing solid/liquid ratios for PAMAM-grafted chitosan. Increasing solid/liquid ratio leads to an increase in the number of active sites available for adsorption and thus fixation of a large amount of solute ions as long as an enough number of these ions is available in solution in contact with solid. Relation between initial concentration of lead ions and adsorption capacity were evaluated by performing several experiments and altering the initial concentration of Pb2+ ions between 10-1000 ppm. The experiments terms are listed in Table 1. The results are shown in Fig. 2b (red line) and as can be seen, the adsorption capacity was increased by increasing initial concentration and after concentration near 250 ppm, the adsorption capacity was constant. In other words, the maximum amount of Pb2+ weight is adsorbed on CS-PAMAM G3 surface at 250 ppm and the adsorption sites are completely occupied by Pb2+ ions and active sites for adsorption are unavailable. So, at a constant weight of adsorbent, specific amount of Pb2+ ions will be adsorbed. 3.5 Adsorption Isotherms The adsorption isotherm expresses the relation between the mass of the heavy metal ions adsorbed at a particular temperature, the pH, and liquid phase of the heavy metal ion concentration. Several isotherms such as Langmuir, Freundlich, and Temkin isotherms were investigated. Langmuir isotherm is one of the most widely-used equations in describing the adsorption of heavy metals. The equation is based on the following assumptions: the adsorbent has a fixed amount of individual sites that can only occupy one solute per site, all sites have the same level of energy, and accumulation of the adsorbed solute is up to monolayer coverage (Febrianto et al., 2009). The Langmuir equation can be written as follows Eq. (2) (Langergren and Svenska, 1898; Ozcan and Ozcan, 2005):
where, qe, Ce, KL, and Q0 are the amount of heavy metal ion adsorbed on PAMAM-grafted chitosan at equilibrium (mg/g), the equilibrium concentration of heavy metal ion solution (mg/L), Langmuir constant (L/g) and the maximum adsorption capacity (mg/g), respectively. The linear form of Langmuir equation is indicated by Eq. (3):
Also, isotherm data were tested with Freundlich isotherm. The Freundlich model is applicable to a solid with energetically heterogeneous surfaces in monolayer coverage (Ngah and Fatinathan, 2008) and it can be expressed by Eq. (4) (Langergren and Svenska, 1898; Ozcan and Ozcan, 2005):
Where, KF is the adsorption capacity at unit concentration and 1/n is adsorption intensity. 1/n values indicate the type of isotherm to be irreversible (1/n ˂ 0), favorable (0 < 1/n < 1) and unfavorable (1/n > 1). Eq. (4) can be rearranged to a linear form, Eq. (5):
log = log + log
Where K and n are Freundlich constants that refer to the adsorption capacity (mg g-1) and adsorption intensity of the solute on the adsorbent, respectively (Kamari and Ngah, 2009). The Temkin isotherm is given by Eq. (6): =
Which can be linearized as indicated by Eq. (7):
= $ ln + $ ln
Where Eq. (8): $ =
The Temkin isotherm equation assumes that the heat of adsorption of all the molecules in layer decreases linearly with coverage due to adsorbent–adsorbate interactions, and that the adsorption is characterized by an uniform distribution of the bonding energies, up to some maximum binding energy (Senthilkumaar et al., 2006). A plot of qe versus ln Ce enables the determination of the isotherm constants B1 and KT from the slope and the intercept, respectively. KT is the equilibrium binding constant (Lmol-1) corresponding to the maximum binding energy, and constant B1 is related to the heat of adsorption. Also, T, R, and b are the absolute temperature (K), the universal gas constant (8.314 J/mol K) and a constant related to heat adsorption, respectively. The isotherm parameters for the Langmuir, Freundlich, and Temkin isotherms are determined from the linear plots of 1/qe versus 1/Ce, log qe versus log Ce and qe versus ln Ce, respectively (Figs. 3a-c). The values of KL, KF, KT, R12, R22, R32, Q0, n and B1 were calculated and shown in Table 3. The data of Table 3 indicate that the Langmuir isotherm is the most appropriate to describe the adsorption of Pb2+ ions on CSPAMAM samples. The CS-G3 has the biggest value of R12 (Longmuir isotherm) and by decreasing the dendrimer generation the R12 decreases and the R22 (Freundlich isotherm) increases. From obtained results, it can be found that grafted G3 dendrimer on chitosan as an adsorbent has adsorption sites with the same level of energy adsorption and by decreasing the generation of dendrimers, the adsorption energy of sites will be heterogeneous. Comparison of Q0 values for the four adsorbent indicate that the CS-PAMAM G3 has more capacity for elimination of Pb2+ in aqueous solution. In general, Comparison of Q0 values of the three adsorbents showed that they follow the order of CS-G3 > CS-G2 > CS-G1 > CS. The binding sites of
PAMAM-grafted chitosan are contributed by chitosan and PAMAM. The binding sites on PAMAM-grafted chitosan are the hydroxyl group (-OH) and amine group (-NH2). The oxygen of the hydroxyl group and the nitrogen of the amine group have a pair of electrons that can be shared with a proton or cation. However, the oxygen in the hydroxyl group has a stronger attraction to its electron pair, which makes nitrogen to have a better tendency in donating its electron pair to a cation (Rangel-Mendez et al., 2009). The metal ions, M(II) act as a Lewis acid, where it has empty orbitals that makes it capable of accepting an electron pair. Meanwhile, -NH2 and -OH- groups act as Lewis bases, ready to donate their available electron pairs. AS noted before, Based on the isotherm studies, Pb(II) best fits Langmuir isotherm. It implies that Pb(II) adsorption results in a monolayer coverage by attaching to energetically homogenous sites, which means the occurrence of a single site mechanism. Pb(II) is adsorbed involving only one type of binding site, which is the amine group (-NH ) as provided by the Eq. (9) below: RNH2 + Pb2+
Effect of temperature on adsorption capacity was evaluated and the results are shown in Fig. 3d. As can be seen, the increase of temperature from 298 K to 318 K led to increase the adsorption capacity. This can be related to that by increasing the temperature, the segmental motion of chitosan chains will be more freely and interwoven structures of chains will be untwined. As a result, the adsorption sites comes to interface of chitosan-aqueous medium and the available sites for adsorption increases that it leads to enhancement of adsorption capacity. 3.6 Kinetic study To determine the kinetic rate uptake of Pb(II), at the solid-liquid interphase of CS-PAMAM samples, three kinetic models such as pseudo-first order or Lagergren, pseudo-second order and intraparticle diffusion
model were applied to the experimental data. The pseudo-first order or Lagergren equation is one of the most popular kinetic equation and this model is expressed as Eq. (10) (Langergren and Svenska, 1898):
( log( − ' ) = log − ().+,+ )-
Where k1 is the pseudo-first order rate constant (min-1) of adsorption and qt and qt (mg/g) are the amounts of metal ion adsorbed at equilibrium and at time t (min), respectively. On the other hand, the pseudo-second order model is expressed as Eq. (11) (Ho, 2001): '
+ / 0
Where K2 is the pseudo-second order rate constant of adsorption (g mmol-1 min-1). The possibility of intraparticle diffusion resistance affecting adsorption was explored by using the intraparticle diffusion model as indicated by Eq. (12) (Ho and McKay, 1999): ' = 12 - ,.3 + 4
Where kp is the intraparticle diffusion rate constant, and I is intercept. Values of I give an idea about the thickness of the boundary layer, i.e., the larger intercept results in the boundary layer effect will be greater. According to this model, the plot of uptake should be linear if intraparticle diffusion is involved in the adsorption process and if these lines pass through the origin then intraparticle diffusion is the rate controlling step (Ozcan and Ozcan, 2005). When the plots do not pass through the origin, this is indicative of some degree of boundary-layer control and this further show that the intraparticle diffusion is not the only rate limiting step, but also other kinetic models may control the rate of adsorption, all of which may be operating simultaneously.
The variations of adsorption capacity as a function of time at initial concentration of 100 ppm and pH 6 is shown in Fig. 4a. As shown, by increasing time, the adsorption capacity increases and at a certain time (240 min) reaches to its maximum values that by more increasing time its value is almost constant. This time is optimum amount for adsorption on these kind of adsorbents. Moreover, the comparison of maximum amount of adsorption capacity for each sample at 298 k, initial concentration of 100 ppm and pH 6 are demonstrated in Fig. 4b and as can be seen, by increasing the generation of grafted PAMAM on chitosan, the maximum of adsorption capacity has been increased and the adsorption capacity of CS-G3 is almost 18 times more than sole chitosan (CS) which indicating the significant effect of generation increasing on adsorption capacity. The kinetic parameters are also extractable from Fig. 4a. The kinetic parameters for the pseudo-first, pseudo-second and intraparticle diffusion models are determined from the linear plots of log (qe-qt) versus t, (t/qt) versus t and qt versus t0.5, respectively (Fig. 5a-c). The values of k1, k2, kp, R12, R22, R32 and I were calculated and shown in Table 4. Adsorption kinetics of Pb2+ ions was studied and the rates of sorption were found to be conforming to the pseudo-second order kinetics with excellent correlation. By moving from G3 to G0, pseudo-second order kinetic fitting was decreased and intraparticle diffusion model fitting was increased. This shows that by grafting PAMAM dendrimers to chitosan, kinetic of sorption and consequently, the mechanism of sorption will be changed and adding new amine sites to chitosan have significant effect on mechanism of sorption. As shown in Table 4, the value of qe is more comparable to the experimental one in the case of pseudo-second order model. On the other hand, the R22 value indicates more fitness for pseudo-second order. This indicates that the pseudo-second order model is more valid to describe the kinetics of the undergoing adsorption process. The adsorption takes place through multi-step mechanism including: i. external film diffusion; ii. intraparticle diffusion and
iii. interaction between adsorbate and active site (Ji et al., 2009). The first step is almost excluded by shaking the solution. So the rate determining step is one of the two other steps. The following kinetic of adsorption from pseudo-second order equation implies that the ratelimiting step is chemisorption, where the process involves the formation of covalent bonds through the sharing or exchange of electrons between the CS-PAMAM biosorbents and metal ions.
4. Conclusion In summary, different generations of PAMAM-grafted chitosan as a new biosorbents were successfully synthesized via divergent growth approach. The prepared products were utilized as adsorbents for heavy metals, Pb2+ in this research, removing from aqueous medium. The results showed that as-synthesized products with higher generations of dendrimer, have more adsorption capacity compared to products with lower generations of dendrimer and sole chitosan. Thermodynamic and kinetic studies showed that Langmuir isotherm model and pseudo second order kinetic model are more compatible for describing equilibrium data of the uptake capacity and kinetic rate of the Pb2+ uptake, respectively.
Acknowledgments Authors are grateful to council of applied biotechnology research center-Baqiyatallah Medical Science University and University of Kashan for providing financial support to undertake this work.
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Figure caption Fig. 1. Variations of zeta potential value versus pH. Fig. 2. (a) Effect of pH on adsorption capacity at different temperatures; (b) Effect of solid/Liquid (S/L) ratio on adsorption capacity; and (c) Effect of initial concentration of Lead (II) on adsorption capacity. Fig. 3. (a) Langmuir isotherm; (b) Freundlich isotherm; (c) Temkin isotherm plots of samples CS, CS-G1, CS-G2 and CS-G3; and (d) Adsorption isotherm of Pb2+ on sample CS-G3 at different temperatures. Fig. 4. (a) Effect of time on adsorption of Pb2+ on CS, CS-G1, CS-G2 and CS-G3; and (b) comparison of maximum amount of adsorption capacities. Fig. 5. (a) Pseudo-first; (b) pseudo-second; and (c) intraparticle diffusion kinetic models of samples CS, CS-G1, CS-G2 and CS-G3. Table 1. The synthesis terms of different generations of PAMAM-grafted chitosan. Table 2. The experiments terms of investigation of initial concentration of Pb2+ on adsorption capacity. Table 3. The isotherm parameters of Pb2+ removal for the Langmuir, Freundlich, and Temkin isotherms. Table 4. The kinetic constants of Pb2+ removal for the Pseudo-First, the Pseudo-Second Order and the Intraparticle Diffusion Model.
V methanol (ml)
Chitosan weight (gr)
Table 3 Langmuir isotherm Adsorbent
Table 4 Pseudo-first order Adsorbent
Chitosan-PAMAM Dendrimer as a new integrated biosorbent were successfully prepared. The prepared samples were used as adsorbents for Pb2+ removing from aqueous solution. Effect of different parameters on adsorption capacity of biosorbents were evaluated. Adsorption capacity of sample CS-G3 was 18 times more than sole chitosan. Thermodynamic and kinetic models were studied for understanding equilibrium data.