Langmuir 2002, 18, 9765-9770
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Mechanisms of Lead Adsorption on Chitosan/PVA Hydrogel Beads Li Jin and Renbi Bai* Department of Chemical and Environmental Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260 Received May 7, 2002. In Final Form: August 30, 2002 Removal of lead from aqueous solution with chitosan/PVA (poly(vinyl alcohol)) hydrogel beads was studied in batch adsorption experiments at various solution pH values (2-7.6). Lead adsorption on chitosan/ PVA beads was found to be strongly pH-dependent and displayed a maximum uptake capacity at pH around 4 and a minimum at pH about 6.4. ζ-Potential study indicated that chitosan/PVA beads possessed positive ζ-potentials at pH < 6.3 and negative ζ-potentials at pH > 6.3. Hence, adsorption occurred even though the interaction between lead and chitosan/PVA beads was electrostatically repulsive at pH < 6.3. Complexation, ion exchange, and electrostatic interaction are all believed to play a role in lead adsorption on chitosan/PVA beads, but the relative importance of each of these mechanisms varies with solution pH values. Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy spectra suggested that lead adsorption was mainly through interactions with the N atoms in chitosan in the pH range studied.
Introduction Effective removal of heavy metal ions from aqueous solution is important in the protection of environmental quality and public health. Because heavy metal ions are not biodegradable, they are therefore usually physically or chemically removed from the contaminated water through processes such as chemical precipitation, membrane filtration, ion exchange, and adsorption. One of the new developments in recent years to accumulate precious metals or remove toxic metals from dilute solutions is the use of adsorbents of biological origin, including alginate, dead and living biomass, chitosan, lignin, and so on.1 Chitosan is produced from N-deacetylation of chitin, a major component of crustacean shells and fungal biomass, and is readily available from seafood processing wastes. The chemical structures of chitin and chitosan are shown in Figure 1. Chitosan has increasingly been studied for the adsorption of various metal ions from dilute solution or wastewater.2-5 Chitosan is often used in the form of flakes or powder in metal adsorption. Progress has been made to produce chitosan hydrogel beads so that they can be regenerated after metal adsorption and be reused in subsequent adsorption operations. The production of chitosan hydrogel beads involves dissolution of chitosan flakes in an acetic acid solution followed by a precipitation process of injecting the chitosan solution in droplets into a dilute sodium hydroxide solution.6 A major material limitation of the hydrogel beads is their poor chemical resistance and * To whom correspondence should be addressed. Tel: (65)6874 4532. Fax: (65)6779 1936. E-mail:
[email protected]. (1) Bailey, S. E.; Olin, T. J.; Bricka, R. M.; Adrian, D. D. Water Res. 1999, 33, 2469-2479. (2) Dambies, L.; Guimon, C.; Yiacoumi, S.; Guibal, E. Colloid Surf., A 2001, 177, 203-214. (3) Lasko, C. L.; Hurst, M. P. Environ. Sci. Technol. 1999, 33, 36223626. (4) Burke, A.; Yilmaz, E.; Hasirci, N.; Yilmaz, O. J. Appl. Polym. Sci. 2002, 84, 1185-1192. (5) Bassi, R.; Prasher, S. O.; Simpson, B. K. Sep. Sci. Technol. 2000, 35, 547-560. (6) Rorrer, G. L.; Hsien, T. Y.; Way, J. D. Ind. Eng. Chem. Res. 1993, 32, 2170-2178.
Figure 1. Structures of (a) chitin and (b) chitosan.
mechanical strength. To improve these properties, crosslinking of chitosan beads in glutaraldehyde (GA), epichlorhydrine, or ethylene glycol glycidyl ether (EGDE) has commonly been used,7-9 but cross-linking has been found to decrease the adsorption capacity of chitosan.9,10 In addition, the cross-linking agents such as GA and EGDE are not preferred due to their physiological toxicity. In recent years, polymer blending has become a method for providing polymeric materials with desirable properties for practical applications. In particular, chitosan blended with poly(vinyl alcohol) (PVA) has been reported to have good mechanical and chemical properties and, as a topic of great interest, has been extensively studied in the biomedical field.11-13 The enhanced property has been (7) Ruiz, M.; Sastre, A. M.; Guibal, E. React. Funct. Polym. 2000, 45, 155-173. (8) Ngah, W. S. W.; Endud, C. S.; Mayanar, R. React. Funct. Polym. 2002, 50, 181-190. (9) Hsien, T. Y.; Rorrer, G. L. Ind. Eng. Chem. Res. 1997, 36, 36313638. (10) Hsien, T. Y.; Rorrer, G. L. Sep. Sci. Technol. 1995, 30, 24552475. (11) Zheng, H.; Du, Y. M.; Yu, J. H.; Huang, R. H.; Zhang, L. J. Appl. Polym. Sci. 2001, 80, 2558-2565.
10.1021/la025917l CCC: $22.00 © 2002 American Chemical Society Published on Web 10/30/2002
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Figure 2. Proposed structure of chitosan/PVA beads showing the interactions between chitosan and PVA.
attributed to the interactions between chitosan and PVA in the blend through hydrophobic side chain aggregation and intermolecular and intramolecular hydrogen bonds, as shown in Figure 2.14 The adsorption properties and mechanisms of the chitosan/PVA blend for metal removal have however seldom been studied. In this work, we report the removal of lead ions from aqueous solution with chitosan/PVA hydrogel beads. The aim is to understand the mechanisms of lead binding to chitosan/PVA beads. Batch adsorption experiments were conducted under various solution pH values. ζ-Potential analysis, Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) spectra were used to identify the adsorption mechanisms.15-17 The results suggest that complexation, ion exchange, and electrostatic interactions may all play a role in lead adsorption on chitosan/PVA hydrogel beads. Experimental Section Materials. Chitosan flakes (85% deacetylated) were purchased from Sigma Co. PVA (99% hydrolyzed, average molecular weight 89 000-98 000) was obtained from Aldrich Chemical Co. Acetic acid was provided by Merck, and lead chloride (98% w/w) was obtained from Nacalai Tesque Inc., Japan. Production of Chitosan/PVA Hydrogel Beads. A 4.26 g amount of chitosan flakes was dissolved in 100 mL of dilute acetic acid (2% w/w) at 60 °C in a flask placed in a thermostatic water bath shaker with a stirring speed of 148 rpm for 5 h. An 8.51 g amount of PVA was dissolved in 100 mL of deionized (DI) water in a beaker and agitated on a magnetic stirrer (at about 800 rpm) at 80 °C for 5 h. The two solutions were then blended together with stirring on the magnetic stirrer (at about 800 rpm) at 70 °C for 48 h, followed by another 48 h under room temperature (21-23 °C) to obtain a homogeneous gel blend. The gel blend was filtered and then introduced in droplets into a NaOH-methanol solution (0.1 mol NaOH per liter methanol) through a vibration nozzle unit (Nisco Engineering AG, Switzerland) to form hydrogel beads with chitosan (2% w/w) and PVA (4% w/w). The hydrogel beads were thoroughly rinsed with DI water until neutral pH and were stored in DI water under room temperature prior to adsorption experiments. The beads were in reasonably good spherical shape, and their diameters were in the range of 3.71 ( 0.48 mm, determined by measuring 30 beads with a vernier caliper. A drying test indicated that the water content of the hydrogel beads was at 93%. A scanning electron microscopy (SEM) image study verified that the addition of PVA indeed made the hydrogel beads become much denser, as shown in Figure 3. (12) Miya, M.; Iwamoto, R.; Mima, S. J. Polym. Sci. 1984, 22, 11491151. (13) Nakatsuka, S.; Andrady, A. L. J. Appl. Polym. Sci. 1992, 44, 17-28. (14) Cho, Y. W.; Han, S. S.; Ko, S. W. Polymer 2000, 41, 2033-2039. (15) Zhang, X.; Bai, R. B. Langmuir 2002, 18, 3459-3465. (16) Bai, R. B.; Zhang, X. J. Colloid Interface Sci. 2001, 243, 52-60. (17) Zhang, X.; Bai, R. B. J. Mater. Chem. 2002, 12, 2733-2739.
Figure 3. SEM images showing the surface morphologies of the chitosan and chitosan/PVA hydrogel beads. ζ-Potential Measurements. A 0.2 g amount of dry powder of PVA, chitosan, or chitosan/PVA beads was added into 200 mL of DI water, and the mixture was stirred for 24 h. The pH of the suspension was adjusted with 0.1 M NaOH or 0.1 M HCl solutions. The suspension was then settled for 1 h, and the supernatant with small fragments in it was used for ζ-potential measurements by a Zeta-Plus4 instrument (Brookhaven Corp., U.S.A.). Batch Adsorption Experiments. All experiments were carried out in 250 mL conical flasks. A certain amount of chitosan/ PVA hydrogel beads was taken from the stock and placed on a filter paper for a few minutes to remove the attached surface water. Then, about 5 g of the beads was weighted and added into a 200 mL solution with a desired pH value (adjusted with 0.1 M NaOH or 0.1 M HCl) and a known lead concentration (made by addition of lead chloride solution). The mixture was shaken in an orbital shaker at a speed of 148 rpm under room temperature. Samples were taken at predetermined time intervals for the analysis of lead concentrations in the solution until adsorption equilibrium was reached. Lead concentrations were analyzed by an Inductively Coupled Plasma-Optical Emission Spectrometer (ICP-OES, Perkin-Elmer Optima 3000 DV). The initial concentrations of lead in all of the test solutions were 30 mg L-1, but the initial pH values of the solutions were different, ranging from 2 to about 8 (pH was not controlled during adsorption as this would be closer to the conditions for actual applications, and also, the addition of HCl or NaOH solutions to control pH would change the adsorption conditions). The adsorbed amount of lead per unit weight of hydrogel beads at time ti, q(ti) (mg g-1), was calculated from the mass balance equation as n
∑ (C
q(ti) )
i)1
ti-1
- Cti)Vti-1
m
(1)
where Ct0 () C0) and Cti (mg L-1) are the initial lead concentration and the lead concentrations at time ti, respectively; Vti is the volume of the solution at time ti (samples taken for lead
Lead Adsorption on Chitosan/PVA Hydrogel Beads
Figure 4. ζ-Potentials of chitosan/PVA beads, as compared with those of chitosan and PVA, respectively. concentration analysis were not returned to the flask); and m is the weight of the hydrogel beads. FTIR Spectroscopy. Infrared spectra of chitosan/PVA beads with or without adsorbed lead were obtained by a FTS3500 FTIR Spectrometer. Pressed pellets were prepared by grinding the powder specimens with IR grade KBr in an agate mortar. XPS Spectroscopy. XPS analyses of chitosan/PVA beads before and after lead adsorption were made on a VG ESCALAB MKII spectrometer with an Al KR X-ray source (1486.6 eV of photons). The XPS spectra peaks were decomposed into subcomponents by fixing a 0% Lorentzian-Gaussian curve-fitting program with a linear background through an XPSpeak 4.1 software package. The full width half-maximum was maintained at 1.4. The calibration of the binding energy (BE) of the spectra was performed with the C 1s peak of the aliphatic carbons at 284.6 eV.15
Results and Discussion ζ-Potentials. The ζ-potentials of chitosan, PVA, and chitosan/PVA beads under different solution pH values are shown in Figure 4. Chitosan has positive ζ-potentials in an acidic condition and negative ζ-potentials in a basic condition, with a point of zero ζ-potential at about pH 6.6, which is close to the pKa values of 6.3-6.6 for the amino group in chitosan reported by others.18-20 The ζ-potentials of chitosan/PVA beads appear to be dominated by that of chitosan although the blending of PVA with chitosan indeed slightly lowers the magnitude of ζ-potentials of the chitosan/PVA beads. The point of zero ζ-potential for the chitosan/PVA beads prepared in this study is at around pH 6.3. Lead, as a prevalent contaminant in many industrial effluents, is known to exist in different forms in aqueous solution at different pH values. At pH below 6, Pb2+ is the major species, and with the increase of pH from 6 to 9, PbOH+ dominates.21 Hence, from an electrostatic interaction point of view, the adsorption of lead on chitosan/PVA beads can be enhanced at pH > 6.3, due to the negative ζ-potentials of chitosan/PVA beads attracting the positively charged lead or lead hydroxides, and on the other hand, lead transport and attachment to chitosan/PVA beads may be affected at pH < 6.3, due to the positive ζ-potentials possessed by both chitosan/PVA beads and lead species. Adsorption Behavior. Figure 5 shows the experimental results for the amounts of lead removal by chitosan/ PVA hydrogel beads under various initial solution pH (18) Koyano, T.; Koshizaki, N.; Umehara, H.; Nagura, M.; Minoura, N. Polymer 2000, 41, 4461-4465. (19) Claesson, P. M.; Ninham, B. W. Langmuir 1992, 8, 1406-1412. (20) He, P.; Davis, S. S.; Illum, L. Int. J Pharm. 1998, 166, 75-88. (21) Kovacevic. D.; Pohlmeier, A.; Ozbas, G.; Narres, H. D.; Kallay, M. J. N. Colloid Surf., A 2000, 166, 225-233.
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Figure 5. Adsorption amounts of lead to chitosan/PVA beads at different initial solution pH values at an initial Pb2+ concentration of 30 mg L-1.
values at adsorption equilibriums (equilibrium was usually reached within 1 h, but the experiments were conducted for 6 h). At pH values greater than 6.4, the amounts of lead removal indeed increased with increasing solution pH values. This is in agreement with the expectation from the ζ-potential study of the chitosan/ PVA beads. For pH decreasing from 6.4 to 4, lead adsorption on chitosan/PVA beads was found to increase with the decrease of solution pH values. These results are opposite to the expectation from the ζ-potential study of the chitosan/PVA beads. From pH 4 to pH 2, the amount of lead adsorption to chitosan/PVA beads displayed the trend of decrease with decreasing solution pH values, which is again in agreement with the expectation from the ζ-potential study of the chitosan/PVA beads. Several important implications can therefore be drawn from the results in Figures 4 and 5: (i) lead adsorption to chitosan/ PVA beads is strongly pH-dependent; (ii) there is a maximum and a minimum adsorption of lead to chitosan/ PVA beads at pH ranging from 2 to about 8 (higher pH values were not studied because of the possibility for precipitation to occur); (iii) the trend of change in the amount of lead adsorption could not be explained solely on the basis of electrostatic interactions (attraction or repulsion); and thus (iv) there must be more than one mechanism responsible for lead removal on chitosan/PVA beads. Adsorption Mechanisms. Many studies using chitosan for metal adsorption have reported the existence of an optimum pH at which metal adsorption was greatest or adsorption amounts first increased and then decreased after reaching a maximum with the decrease of pH in the pH range below the point of zero ζ-potential of chitosan.3,22,23 However, the mechanisms responsible for this phenomenon have so far not been well-explained. For the chitosan/PVA hydrogel beads used in this study, it may be expected that the only adsorption sites for lead are at the nitrogen atoms of the amino groups in chitosan and the oxygen atoms of the hydroxyl groups in both chitosan and PVA. Both nitrogen and oxygen atoms have a lone pair or lone pairs of electrons that can bind a proton or a metal ion through an electron pair sharing to form a complex. Because of the stronger attraction of the lone pair of electrons to the nucleus in an oxygen atom than in a nitrogen atom, the nitrogen atoms would have a greater tendency to donate the lone pair of electrons for (22) Dantas, T. N. D.; Neto, A. A. D.; Moura, M. C. P.; Neto, E. L. B.; Telemaco, E. D. Langmuir 2001, 17, 4256-4260. (23) Jansson-Charrier, M.; Guibal, E.; Roussy, J.; Delanghe, B.; LeCloirec, P. Water Res. 1996, 30, 465-475.
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sharing with a metal ion to form a metal complex than the oxygen atoms. On this consideration, the following chemical reactions may be proposed to account for the adsorption of lead on chitosan/PVA beads:
R-NH2 + H+ T R-NH3+
(2)
R-NH2 + Pb2+ f R-NH2Pb2+
(3)
R-NH3+ + Pb2+ f R-NH2Pb2+ + H+
(4)
R-NH2Pb2+ + H2O T PbOH+ + R-NH3+
(5)
where R represents all other components except -NH2 in the chitosan/PVA beads. The reaction in eq 2 indicates the protonation and deprotonation of the amino groups in chitosan. At lower pH conditions, more amino groups are protonated and thus result in higher positive ζ-potentials of the chitosan/ PVA beads. At higher pH values, OH- ions may be adsorbed to the surface of chitosan/PVA beads through a hydrogen bond, which contributes to the negative ζ-potentials of the chitosan/PVA beads, i.e.
R-NH2 + OH- f R-NH2‚‚‚OH-
(6)
The negative ζ-potentials of the chitosan/PVA beads at higher pH values may also be attributed to the following reaction:
R′-OH + OH- T R′-O- + H2O
(7)
where R′ represents all other components except -OH in the chitosan/PVA beads. The equilibrium of the reaction in eq 2 was usually established according to the initial solution pH values. When lead ions were added into the solution, the reaction in eq 3 started, due to the sharing of the lone pair of electrons from the nitrogen atom with lead ion, with a similar mechanism to that of the reaction in eq 2. However, the binding of a lead ion to a nitrogen atom can be expected to be stronger than the binding of a H+ to a nitrogen atom (i.e., protonation of the amino group) since the electrical attraction force between the lone pair of electrons from the nitrogen atom and the divalent lead ion (Pb2+) would be stronger than that between the lone pair of electrons from the nitrogen atom and the monovalent proton (H+). This difference in the binding force drives the reaction in eq 4 to take place through a competitive adsorption of Pb2+ over H+ to the nitrogen atom, which may sometimes be considered as an ion exchange mechanism.24 The reaction in eq 4 however can be expected to be slower than the reaction in eq 3, attributed to the smaller attraction force between the N in R-NH3+ and Pb2+ as compared to the force between the N in R-NH2 and Pb2+. In addition, the complexes of R-NH2Pb2+ are subject to the reaction in eq 5, due to the greater binding force of Pb2+ with the OH- group from water than with the nitrogen of the amino group in chitosan. For pH below the point of zero ζ-potential of chitosan, the decrease of initial pH values increases the amount of protonated -NH3+ and reduces the amount of neutral -NH2. Lead adsorption through the reaction in eq 3 is therefore reduced with decreasing pH. On the other hand, lead adsorption through the reaction in eq 4 increases (24) Onsoyen, E.; Skaugrud, O. J. Chem. Technol. Biotechnol. 1990, 49, 395-404.
Table 1. Changes in Solution pH Values and the Amount of Lead Adsorption after Regeneration at an Acidic Condition pH conditions first time adsorption before after 4.06 6.19 6.95
5.09 5.81 6.09
second time adsorption before after 4.10 6.25 6.88
4.67 5.76 5.84
adsorbed amount (mg g-1) first time adsorption
second time adsorption
0.9 0.55 0.57
0.01 0.00 0.17
with decreasing pH due to the increased amount of -NH3+ present on the surface of the beads. Moreover, the reaction in eq 5, together with the reactions in eqs 2 and 4, favors Pb2+ to exist in the complex of R-NH2Pb2+ rather than to distribute into the solution (i.e., existing as Pb2+ or PbOH+) at a lower initial solution pH. The overall performance of lead adsorption on chitosan/PVA beads thus displays a trend of increase with decreasing initial solution pH values (from 6.4 to 4). However, lead adsorption to chitosan/PVA beads is also controlled by the transport of lead ions from the bulk solution to the beads’ surface before adsorption can take place. The transport of Pb2+ to the surface sites of R-NH3+ could be inhibited at very low pH as most of the R-NH2 are protonated and the high electrostatic repulsion force would prevent Pb2+ from approaching close enough to the surface for adsorption to occur. This could explain the observed decrease of lead adsorption in Figure 5 when pH was reduced further from 4 to 2. For the minimum adsorption at initial pH about 6.4, although the reaction in eq 3 favors lead adsorption, due to the high amount of R-NH2, the reaction in eq 5 causes Pb2+ to dissociate from the complex (reduction of R-NH3+ at high pH) and thus results in the lowest adsorption observed. At pH greater than 6.4, the increase of lead adsorption with increasing pH is expected mainly through nonspecific adsorption due to the increased electrostatic attraction between Pb2+ or PbOH+ and the R-NH2‚‚‚OHor deprotonated R′-OH- sites (see eqs 6 and 7). The contribution of specific adsorption through the reaction in eq 3 may be compromised by that in eq 5 at high solution pH values. Studies were carried out to regenerate chitosan/PVA beads (after lead adsorption at different initial pH values in the first set of adsorption experiments) in an acidic condition of pH 4, and then, the beads were reused in another set of adsorption operations. The results are shown in Table 1. In the first set of adsorption experiments, chitosan/PVA beads were taken from the stock and the initial pH values of the lead solutions were at 4.06, 6.19, and 6.95, respectively. After adsorption, the corresponding pH values of the solutions changed to 5.09, 5.81, and 6.09. For the two experiments with higher initial pH values (i.e., pH 6.19 and 6.95), the decrease of pH after adsorption is a clear indication of the existence of the reaction in eq 4 during adsorption of lead. For the experiment with a low initial pH (i.e., pH 4.06), the pH after adsorption did not decrease but rather increased to 5.09. This was later found to be due to the protonation of chitosan/PVA beads, which occurred initially when the beads from the stock were added into the solution. The beads in stock were kept in DI water of pH about 6.5. When they were transferred to a solution of pH about 4.06, the protonation of the amino group (the reaction in eq 2) consumed the H+ and caused the increase in solution pH. The effect of the reaction in eq 4 on solution pH was therefore hidden out. This is supported by the results from another adsorption experiment where the beads were preprotonated at the
Lead Adsorption on Chitosan/PVA Hydrogel Beads
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Figure 6. Typical FTIR spectra of (a) chitosan/PVA beads before adsorption and (b) chitosan/PVA beads with lead adsorbed (solution pH 6.28). Table 2. Assignments of Infrared Absorption Bands wavenumber (cm-1)
intensity shape
assignment
3600-3750 3400-3550 3100-3500 2500-3400 2700-2950 1400-1660 1280-1430 1160-1420 900-1350 900-1380 800-880
sharp sharp strong-broad weak-broad variable variable variable variable variable variable medium-strong
O-H stretching O-H stretching N-Hstretching O-H stretching C-H stretching N-H bending C-H bending O-H bending C-N stretching C-O stretching N-H and C-H rocking
same pH (about 4) as the adsorption experiment for a while, and no apparent pH increase was observed after the lead adsorption experiment. It is interesting to note that the results in Table 1 indicate the ineffectiveness of acid washing for the regeneration of the beads after adsorption at an initial solution pH of 4.06 or 6.19, as in the second set of adsorption experiments, where there was no lead adsorption taking place at all. This phenomenon is in fact suggested by the reactions in eqs 2 and 5, i.e., higher H+ concentrations favor Pb2+ to stay as R-NH2Pb2+ on the surface of chitosan/PVA beads. For the beads with an initial pH of 6.95 in the first set of adsorption experiments, acid regeneration however indeed recovered some of the adsorption capacity in the second set of adsorption experiments. As discussed earlier, at high pH, some lead ions were adsorbed through nonspecific electrostatic attraction. These ions can be more easily removed from the surfaces of the beads during acid regeneration than those that formed complexes on the surface. FTIR Spectra Study. FTIR spectra are a useful tool to identify the presence of certain functional groups in a molecule as each specific chemical bond often has a unique energy absorption band. To understand the nature of lead adsorption and identify the possible sites of lead binding to chitosan/PVA beads, FTIR spectra were obtained for chitosan/PVA beads before and after lead adsorption. Figure 6 shows the typical FTIR results with the corresponding band assignments given in Table 2. Although there is the possibility of overlapping between the N-H and the O-H stretching vibrations, the strong broad band at the wavenumber region of 3300-3500 cm-1 is characteristic of the N-H stretching vibration. The significant decrease of transmittance in this band region after lead adsorption indicates that the N-H vibration was affected
Figure 7. Typical XPS spectra of chitosan/PVA beads, showing lead adsorption on the surface.
due to lead adsorption. Other changes in the transmittances can be observed at the wavenumbers of 1659.5, 1597.8, 1428, 1099.9, and 852.95 cm-1, respectively. These wavenumbers are closely related to the N-H bending, C-N stretching, and N-H rocking bands. In other words, lead adsorption is found to affect all of the bonds with N atoms, indicating that nitrogen atoms are the main adsorption sites for lead adsorption on chitosan/PVA beads. In fact, the FTIR data showed that the N-H bending vibration wavenumbers at 1566.9 and 1551.5 cm-1 before lead adsorption were shifted to 1559.2 and 1543.8 cm-1 after lead adsorption, suggesting the attachment of lead to nitrogen atoms, which reduced the vibration intensity of the N-H bond due to the molecule weight becoming heavier after lead attachment. Another major change in the transmittance can also be observed at the wavenumber of 2944.8 cm-1 after lead adsorption. This band region may be assigned to both C-H (variable) and O-H (weak-broad) stretchings. As lead is unlikely to be attached to a carbon atom, the results may therefore suggest that oxygen atoms in the hydroxyls could also be involved in lead adsorption, but their effect appears to be much less significant than nitrogen atoms. XPS Spectra Study. XPS spectra are widely used to distinguish the different forms of the same element and to identify the existence of a particular element in a material.15-17 Figure 7 shows the typical results of XPS spectra for chitosan/PVA beads before and after lead adsorption. The presence of lead on the surface of chitosan/ PVA beads is obvious after a lead adsorption experiment. In Figure 8, the typical N 1s XPS spectra of chitosan/PVA beads with and without adsorbed lead are presented. Before lead adsorption, there is only one peak in the N 1s spectrum at a BE about 399.2 eV, see Figure 8a. This is attributed to the N atom in the R-NH2 group. Although, under different pH values, the R-NH2 group may undergo protonation to form R-NH3+ that could contribute to a
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This was due to the formation of R-NH2Pb2+ complexes in the reactions of eqs 3 and 4, in which a lone pair of electrons in the nitrogen atom was donated to the shared bond between N and Pb2+, and as a consequence, the electron cloud density of the nitrogen atom was reduced, resulting in a higher BE peak observed. Therefore, the XPS spectra provide evidence of lead binding to nitrogen atoms. O 1s XPS spectra however did not clearly show any changes of O 1s BEs before and after lead adsorption (results not shown). To confirm whether oxygen atoms are involved in lead adsorption, experiments were conducted by using pure chitosan beads and chitosan/PVA beads (containing the same amout of chitosan). It was found that the amount of lead adsorption on chitosan/ PVA beads at adsorption equilibrium could be increased by up to 28%, as compared to that of pure chitosan beads at pH 6.3, indicating that at least the -OH group in PVA may play a role in lead adsorption at this pH condition. Because FTIR and XPS spectra did not clearly show changes of chemical bonds associated with oxygen atoms, it may be speculated that the contribution of lead-oxygen interaction to lead adsorption may be mainly through a nonspecific electrostatic attraction (physical interaction). As a last point to be mentioned, the adsorption capacity of chitosan/PVA hydrogen beads is dependent on the experimental conditions, such as initial lead concentrations in the solution. The adsorption capacity can be several times higher if the initial lead concentrations in the solution are greater than 30 mg/L as reported in Figure 5. The adsorption capacity of lead in this paper is given by “mg lead per g chitosan/PVA hydrogel beads”. Because the hydrogel beads contained only about 2% (w/w) chitosan (93-94% water and about 4% PVA), the adsorption capacity would be much greater in terms of per gram of chitosan. For example, 2 mg lead/g chitosan/PVA hydrogel beads may convert to an adsorption capacity of 100 mg lead/g chitosan. Although the adsorption capacity of the hydrogel beads appears to be much lower than that reported in the literature using chitosan powder1 (possibly due to the lower surface area), chitosan/PVA beads provide the advantage of regeneration for potential reuse in adsorption operations. Conclusions Figure 8. Typical N 1s XPS spectra of chitosan/PVA beads (a) before lead adsorption, (b) with lead adsorption at pH 4.2, and (c) with lead adsorption at pH 6.28.
peak at a BE greater than 399.2 eV, the formation of R-NH3+ is in fact accomplished through a loose attachment of OH-, i.e., R-NH3+‚‚‚OH-, or Cl-, i.e., R-NH3+‚‚‚ Cl-. When the samples were dried before XPS analysis, H2O or HCl was lost due to evaporation and thus the complex of R-NH3+‚‚‚OH- or R-NH3+‚‚‚Cl- reduced to R-NH2 again. This explains the reason that even the beads were placed in different solution pH values, only one peak for the N atom in the R-NH2 group was detected. Also, the single peak of N 1s spectrum in Figure 8a suggests that chitosan flakes obtained from Sigma Co. were completely deacetylated during the production of chitosan/PVA beads. After lead adsorption, however, a new peak is observed at a BE greater than 399.2 eV, see Figure 8b,c. This indicates that in addition to the neutral form of the N atom in the R-NH2 groups, some N atoms existed in a more oxidized state due to lead adsorption.
Lead adsorption on chitosan/PVA hydrogel beads was found to be strongly pH-dependent and to display a maximum and a minimum uptake capacity in the pH range of 2-7.6. Chitosan/PVA beads had positive ζ-potentials at pH < 6.3, and the adsorption performance cannot be simply explained through an electrostatic interaction. Complexation, ion exchange, and electrostatic interactions are all identified to play a role in lead adsorption on chitosan/PVA beads, but their relative importance varies with solution pH values. FTIR and XPS spectra clearly show the binding of lead ions to nitrogen atoms in chitosan. The study also suggests that the oxygen atoms in the -OH groups may contribute to lead adsorption on chitosan/ PVA beads (possibly through a nonspecific electrostatic interaction), but the effect could be much less significant, as compared with that of the nitrogen atoms in chitosan. Acknowledgment. This work was supported by the Academic Research Funds, National University of Singapore. LA025917L
