Highly Enhanced Adsorption of Lead Ions on Chitosan Granules

Oct 13, 2006 - Cross-linked chitosan granules (denoted as CTS) were surface-functionalized with poly(acrylic acid) (PAAc) and examined for the adsorpt...
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Ind. Eng. Chem. Res. 2006, 45, 7897-7904

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Highly Enhanced Adsorption of Lead Ions on Chitosan Granules Functionalized with Poly(acrylic acid) Nan Li† and Renbi Bai*,‡ Department of Chemical and Biomolecular Engineering and DiVision of EnVironmental Science & Engineering, Faculty of Engineering, National UniVersity of Singapore, Singapore 117576

Cross-linked chitosan granules (denoted as CTS) were surface-functionalized with poly(acrylic acid) (PAAc) and examined for the adsorption of lead ions from aqueous solutions. PAAc was successfully grafted on CTS through a simple two-step reaction in a solution. The PAAc-functionalized chitosan granules (denoted as CTS-PAAc) showed significantly greater adsorption capacities for lead ions than CTS, and the performance improved with the increase of pH in the pH range of 1-6 examined. An adsorption isotherm and kinetic study conducted at pH 4 and room temperature showed a maximum adsorption capacity of 294.12 mg/g and an adsorption equilibrium time of less than 5 h for lead ions on CTS-PAAC, in contrast to only 95.15 mg/g and up to 8 h on CTS. Mechanism study revealed that the excellent adsorption performance of CTS-PAAc for lead ions was attributed to the many carboxyl groups grafted on CTS-PAAc. It was found that adsorbed lead ions on CTS-PAAc can be effectively desorbed and the regenerated CTS-PAAc can be reused almost without any loss of adsorption capacity. Introduction The presence of heavy metal ions in the environment has been of major concern due to their severe toxicity to human beings and other wildlife. Lead, one of the most common heavy metals, has been widely used in many important industrial applications, such as gasoline, electroplating, storage batteries, printing circuit board, pigments, photographic materials, explosives, and so on.1 As a consequence, lead ions are usually found in the wastewaters or effluents from lead mining, battery recycling plants, printed circuit board factories, electronics assembly plants, and military bases or facilities, as well as in landfill leachate and urban rainwater runoff, etc. This has presented great potential for lead pollution to soils, sediments, and various water resources. Lead is well-known to be a cumulative poison through water intake or food chains and can cause brain damage and dysfunction of the kidneys, the liver, and the central nervous system in human beings, especially in children.2 Hence, lead has been classified as priority pollutant by the US Environmental Protection Agency (EPA) and the maximum contaminant level (MCL) of lead ions in drinking water, for example, has been set at a low level of 0.015 mg/L by the EPA.3 The stringent regulation on lead ions has therefore placed a great challenge to conventional treatment technologies in many cases and made it necessary to develop more effective and efficient technologies for the removal of lead ions from various aqueous solutions, particularly lead-containing wastewater or industrial effluents. The concentrations of lead ions in various industrial wastewaters or effluents may range from a few to up to 100-150 mg/L,4 with the solution pH values usually in the highly acidic range. The conventional technologies that have been used to remove lead ions mainly include precipitation, ion exchange, or activated carbon adsorption. These technologies however either consume additional chemicals and generate extra waste that is difficult to handle or are ineffective in removing lead * To whom all correspondence should be addressed. Tel.: (65) 65164532. Fax: (65) 67744202. E-mail: [email protected]. † Department of Chemical and Biomolecular Engineering. ‡ Division of Environmental Science & Engineering.

ions from aqueous solutions at medium to low concentrations and/or at low solution pH values.5 One of the new developments for heavy metal removal in recent years is to use biosorption, with materials of biological origins as the adsorbents. This is largely attributed to the fact that these biological materials are naturally available, low cost, and environmentally benign.6-9 For example, chitosan, a derivative from N-deacetylation of chitinsa naturally occurring and abundant biopolymershas been found to be capable of adsorbing various heavy metal ions, including copper, lead, mercury, cadmium, chromium, and so on; this is largely attributed to the presence of the amine groups of chitosan that have strong affinity to and can form complexes with various heavy metal ions.10-14 Chitosan can be easily extracted from chitin which is widely and often freely available in large quantities from seafood processing waste, especially in many countries in Asia, such as China, Singapore, Vietnam, Thailand, etc. This has therefore attracted significant research and application interest in using chitosan as the adsorbent (in the natural flake form or more preferably in a processed bead or granular form) to remove heavy metal ions from various industrial or municipal wastewaters.15 However, chitosan is generally not chemically stable at solution pH values below 4 (it tends to dissolve).16 It has been a common practice to cross-link chitosan with various chemicals to extend its chemical stability in acidic solutions (possibly down to a pH of 1), but the cross-linked chitosan usually shows significantly reduced adsorption performance, especially at low solution pH, because of the consumption of the amine groups in chitosan during the cross-linking reaction.10 This has, to some extent, limited chitosan as an effective adsorbent to remove lead ions in many industrial applications where the effluents have relatively low solution pH values17 and the use of cross-linked chitosan may be necessary. In the past decade, the rapid development of surface modification technologies used as popular methods to provide materials with improved or desirable properties for practical applications has been observed.18 Among the many methods for surface modification, surface grafting has appeared to be a relatively simple and versatile approach to introduce various functional groups onto the surface of a material. The application

10.1021/ie060514s CCC: $33.50 © 2006 American Chemical Society Published on Web 10/13/2006

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of the surface grafting method to chitosan beads has also been explored to enhance their adsorption capacity or selectivity.17 In this study, surface modification of cross-linked chitosan granules through surface grafting of poly(acrylic acid) (PAAc) was investigated in order to achieve highly enhanced adsorption performance for lead ions under acidic solution conditions. This work is to serve as an expansion of the many studies reported in the literature using chitosan as an adsorbent for heavy metal removal. The choice of PAAc is due to its high content of carboxyl groups and its anionic polyelectrolyte feature,19 which can provide favorable or attractive electrostatic interaction for lead ion adsorption. The long chains of PAAc grafted onto the chitosan granules can also be expected to significantly increase the number of adsorption sites for lead ions and thus increase the adsorption capacity. Although PAAc has been widely used in filtration, drug delivery, and enzyme immobilization, very little research has been done so far on the grafting of PAAc on biomaterials to enhance the removal of heavy metal ions, especially on chitosan for lead ions.19 In addition, a simple twostep reaction process in a solution was used in this study for the grafting of PAAc on chitosan granules. This is in contrast with the commonly used methods for PAAc grafting through plasma discharge and UV irradiation20,21 that are complex and not well-applicable to granular materials. A series of batch adsorption experiments were conducted in the laboratory to evaluate the adsorption and desorption performance of lead ions on the granules, and Fourier transform infrared spectroscopy was used to elucidate the mechanisms of surface grafting and lead ion adsorption in the study. Materials and Methods Materials. Commercial chitosan flakes (about 85% deacetylated from crab shells) and acetic acid from Merck were used in this study to prepare chitosan granules. Poly(acrylic acid) (PAAc, 50%) from Acros was used as the functional polymer to be grafted on the surfaces of the cross-linked chitosan granules. Water soluble 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride (WSC), purchased from Dojindo Chemical Co., was used as the preactivation agent of PAAc in the grafting reaction. Ethylene glycol diglycidyl ether (EGDE), the most commonly used cross-linking chemical, and lead(II) standard solution (1000 mg/L) from Merck were used in the cross-linking reaction of chitosan granules and in the adsorption experiments for lead ion removal, respectively. Titriplex III (disodium salt-ethylenediamine-tetraacetic acid or EDTA, 0.1 M), nitric acid (70%), and phosphoric acid, supplied by Merck, were examined as the desorption agents in the desorption study. Preparation of PAAc-Functionalized Chitosan Granules. Chitosan granules were prepared and cross-linked according to the procedures described in detail elsewhere.22 In brief, a 2% (w/w) chitosan solution was first prepared by dissolving chitosan flakes in a dilute acetic acid solution at 70 °C. Then, the homogeneous chitosan solution was injected in droplets through a vibration nozzle into a 1 M NaOH solution to form hydrogel beads (the process can be continuous if a large quantity is needed). The beads were then cross-linked by placing 100 mL of them into an EGDE solution (100 mL of DI water plus 1.43 mL EGDE) at 70 °C for 6 h in a thermostatic water bath with continuous agitation. Finally, the cross-linked chitosan beads were washed in an ultrasonic bath with sufficient DI water until the pH of the solution became 6-6.5, then dried in a vacuum drier at room temperature (22-24 °C) for 48 h, and stored in a desiccator for further use. The product prepared in this way had an average size of about 1 mm and was denoted as CTS in the study.

To graft PAAc on CTS, a solution containing 20 g of PAAc and 1.33 g of WSC was first prepared with DI water in a flask to achieve a WSC concentration of 5 mg/mL. The contents of the flask were stirred at 4 °C for 1 h to allow a fraction of the carboxyl groups of PAAc to be preactivated by WSC. Then, a 10 g amount of CTS was added into the flask and the grafting of PAAc on CTS was allowed to proceed in the solution at 4 °C for 24 h. Finally, the granules were separated from the solution, thoroughly washed with DI water in an ultrasonic bath (to remove any unreacted or loosely bound PAAc), and then dried and stored in a vacuum desiccator for further use. The PAAc-functionalized CTS was denoted as CTS-PAAc in the study. Lead Adsorption Experiments. The adsorption performance for lead ions with CTS-PAAc and CTS was examined at various initial solution pH values. A 0.025 g amount of each type of the granules was added, respectively, into a number of flasks containing 50 mL of a lead ion solution with the same initial concentration (20 mg/L) but a different pH value (in the range of 1-6). The contents of the flasks were stirred at 200 rpm and room temperature for 24 h, and the final lead ion concentrations in the solutions were determined. Adsorption isotherm experiments were conducted with solutions of different initial lead ion concentrations in the range of 10-200 mg/L but of the same solution pH value of 4. A 0.025 g amount of CTS-PAAc was added respectively into 50 mL of a lead solution in a flask. The contents of the flasks were stirred at 200 rpm and room temperature for 24 h, and the final lead ion concentrations in the solutions were determined. For comparison, the same types of experiments were also conducted with CTS. Kinetic adsorption experiments with CTS-PAAc for lead ions were conducted at a solution pH value of 4 and an initial lead ion concentration of 200 mg/L. A 0.5 g amount of CTS-PAAc was added into 1000 mL of the lead solution in a flask. The contents of the flask were stirred at 200 rpm and room temperature for a period of up to 1440 min (24 h). Samples (5 mL) were taken at desired time intervals and analyzed for lead ion concentrations. The adsorbed amounts of lead ions per unit weight of the granules at time ti, q(ti) (mg/g), was calculated from the mass balance equation as n

(Ct ∑ i)1

i-1

q(ti) )

- Cti)Vti-1 m

(1)

where Ct0 () C0) and Cti (mg/L) are the initial lead ion concentration and the lead ion concentrations at time ti, respectively; Vti is the volume of the solution at time ti (samples taken for lead concentration analysis were not returned to the flask), n is the number of samples taken at time up to ti, and m is the weight of the granules added into the flask. Desorption Experiments. To examine the desorption behaviors of lead ions from CTS-PAAc, adsorption experiments were first conducted by placing a 0.125 g amount of CTS-PAAc in 250 mL of a lead ion solution (with an initial concentration of 20 mg/L and a solution pH value of 4) for 24 h, and the final lead ion concentration in the solution was analyzed. The CTS-PAAc beads adsorbed with lead ions were separated from the solution (by filtration with a filter paper on a vacuum Buchner funnel, which minimized the solution remaining on the beads) and then added into 50 mL of a 0.1 M EDTA, 1 M HNO3, or 1 M H3PO4 solution in a flask for lead ion desorption. The contents of the flask were stirred at 200 rpm and room

Ind. Eng. Chem. Res., Vol. 45, No. 23, 2006 7899 Scheme 1. Two-Step Process for Poly(acrylic acid) Grafting on Chitosan Beads

temperature for a time period of up to 4 h, and the lead ion concentrations in the solution at various times were analyzed. The CTS-PAAc beads were finally collected from the solution by filtration, washed with DI water, and then reused in the next cycle of adsorption experiments. The adsorption-desorption experiments were conducted for three cycles. In this study, the lead ion concentrations in all the samples were analyzed with an inductively coupled plasma-optical emission spectrometer (ICP-OES, Perkin-Elmer Optima 3000DV). SEM Observation. The surface morphologies of CTS-PAAc and CTS were also examined with a field emission scanning electron microscope (FESEM, JEOL JEM 6700) at 5 kV. Samples were vacuum-dried in a desiccator and were platinumcoated by a vacuum electric sputter coater (JEOLJFC-1300) to a thickness of about 500 Å before being glue-mounted onto the sample stud for the FESEM scan. Zeta Potential Measurement. The zeta potentials of CTSPAAc and CTS under various solution pH conditions were analyzed, following a similar procedure described elsewhere.10 A Zeta-Plus4 instrument (Brookhaven Corp., USA) was used to measure the zeta potentials of all the samples. FTIR Analyses. Reflectance Fourier transform infrared (FTIR) spectra were obtained for CTS-PAAc and CTS (with or without adsorbed lead ions) with a FTIR microscope spectrometer (Bio-Rad UMA500) to confirm the grafting of PAAc on CTS and to elucidate the adsorption mechanisms of lead ions on CTS-PAAc. Results and Discussion Grafting of PAAc on CTS. As described earlier, the grafting of PAAc on CTS was achieved through a two-step reaction process that is simple and can be easily applied to surface modification of granular materials. Similar methods were also used in the surface modification of microspheres for drug delivery.23 The reactions in the two steps may be seen in Scheme 1. The first step was to preactivate the carboxyl groups of PAAc with WSC before they can react with the amine groups of chitosan on CTS. The grafting process controlled the molar ratio

of carboxyl groups (COOH) to WSC at about 20:1 so that only a small portion of the carboxyl groups of PAAc were involved in the reaction to form amide bridges that connected amine groups of CTS with preactivated carboxyl groups of PAAc. In this way, most of the carboxyl groups of PAAc grafted on CTSPAAc were reserved and available as the functional groups for lead ion adsorption. The FESEM images of CTS and CTS-PAAc in Figure 1 show a clear change in the surface morphology, i.e., the more porous surface on CTS became a much denser one on CTS-PAAc after the surface grafting, suggesting that PAAc has been grafted on the surfaces. To support the FESEM observations and the reactions proposed in Scheme 1, FTIR spectra of CTS and CTS-PAAc were obtained, as shown in Figure 2. The major peaks for CTS in Figure 2a can be assigned as follows: 3507 cm-1 (sOH and sNH stretching vibrations), 2950 cm-1 (sCH stretching vibration in sCH and sCH2), 1676 cm-1 (sNH bending vibration in sNH2), 1479 cm-1 (sNH deformation vibration), 1386 cm-1 (sCH symmetric blending vibration in sCHOHs ), and 1327, 1162, and 1128 cm-1 (sCN stretching vibration).24 The FTIR spectrum for CTS-PAAc in Figure 2b however shows a few major changes. Two new peaks at the wavenumbers of 1575 and 1650 cm-1 appeared, and they can be assigned to the characteristic CsN and CdO stretching vibrations of the amide groups, which suggests that the grafting reaction took place between the NH2 groups of chitosan and the COOH groups of PAAc and formed the amide structures. Another new and very strong peak appeared at 1743 cm-1, and it can be assigned to the CdO stretching vibration of the COOH groups. This indicates that many COOH groups from PAAc existed or were grafted on the surface of CTS-PAAc (shown as the strong peak at 1743 cm-1). In addition, the disappearance of the peaks at 1676 and 1162 cm-1 for the primary amine groups of CTS after PAAc grafting can be a direct consequence of their conversion to the amide groups on CTS-PAAc (this resulted in the reduced intensities of the peaks at 1676 and 1162 cm-1 for the primary amine and the increased intensities of the peaks at 1575 and 1650 cm-1 for the amide groups on CTS-PAAc). Hence, all the FTIR results confirmed that PAAc was grafted on CTS-

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Figure 3. Zeta potentials of CTS and CTS-PAAc in solutions of different pH values.

Figure 4. Effect of solution pH values on the performance of lead ion adsorption on CTS and CTS-PAAc.

Figure 1. FESEM images of the surfaces: (a) CTS and (b) CTS-PAAc.

Figure 2. FTIR spectra of (a) CTS and (b) CTS-PAAc.

PAAc and the grafting was through forming amide connection between the NH2 groups of CTS and the COOH groups of PAAc, as suggested in Scheme 1. Zeta Potentials. Figure 3 shows the zeta potentials of CTS and CTS-PAAc as a function of the solution pH values. It can be observed that CTS had a zero point of zeta potential at about pH 7.8 and possessed negative zeta potentials only under the basic solution conditions (i.e., pH >7.8). In contrast, CTS-PAAc had a zero point of zeta potential at about pH 4 and possessed negative zeta potentials under weak acidic, neutral, and basic solution conditions (i.e., in a wider pH range of pH > 4). From the electrostatic interaction point of view, the negative zeta potentials of an adsorbent can be favorable for the adsorption of metal ions which carry positive zeta potentials in solutions.

Hence, it may be expected that lead ion adsorption on both CTS and CTS-PAAc can be enhanced with the increase of solution pH values but CTS-PAAc would show better adsorption performance for lead ions than CTS, due to the less repulsive or more attractive electrostatic interactions between CTS-PAAc and the lead ions to be adsorbed. Adsorption Performance at Different Solution pH Values. Figure 4 shows the adsorption performance of lead ions on CTS and CTS-PAAc as a function of solution pH values. The experiments were conducted in the pH range of 1-6 and at a relatively low initial lead ion concentration of 20 mg/L to avoid any possibility of lead precipitation in the pH range studied. As expected, the results in Figure 4 clearly show that the adsorption uptake of lead ions on both CTS and CTS-PAAc increased with the increase of the solution pH values and that CTS-PAAc always had significantly greater adsorption uptake for lead ions than CTS in all the cases. Even though lead ions were hardly adsorbed on CTS at solution pH values below 4, CTS-PAAc still showed much improved or enhanced adsorption of lead ions in this low solution pH range. For example, at pH 3, the uptake of lead ions on CTS-PAAc was about 12 mg/g but that on CTS was only about 0.8 mg/g. The adsorption performances in Figure 4 may be related to the nature of the zeta potentials of the two types of adsorbents discussed early. The carboxyl groups grafted from PAAc on CTS-PAAc made the adsorbent have much smaller positive zeta potentials or greater negative zeta potentials, and therefore, CTS-PAAc became more effective than CTS for lead ion adsorption. Thus, CTS-PAAc would be expected to have much greater potential than CTS for lead ion removal from various industrial effluents or wastewaters that usually have pH values in the highly acidic range. It is observed in Figure 4 that the adsorption uptake for lead ions on both CTS-PAAc and CTS increased sharply in the pH range of 3-4. For CTS-PAAc, this can be easily explained by the electrostatic interaction between CTS-PAAc and lead ions. CTS-PAAc had a zero point of zeta potential at a pH of about 4. When the solution pH value increased from 3 to 4,

Ind. Eng. Chem. Res., Vol. 45, No. 23, 2006 7901 Table 1. Calculated Isotherm Adsorption Equilibrium Constants for Lead Ions on CTS-PAAc (at pH 4 and Room Temperature) isotherm models

parameter

Pb

Langmuir model

qm (mg/g) b (L/mg) R2 KF (mg/g)(L/mg)bF bF R2 RT/b (mg/g) A (L/mg) R2

294.12 0.097 0.992 33.25 0.535 0.977 50.926 1.732 0.954

Freundlich model Temkin model

used in this work to fit the experimental isotherm data for lead ion adsorption on CTS-PAAc. The Langmuir isotherm model assumes a monolayer adsorption as well as equal sorption activation energy for all adsorption sites on the surface and can be given in its linearized form as

Ce 1 1 ) + C qe bqm qm e

Figure 5. Adsorption isotherms of lead ions on (a) CTS-PAAc and (b) CTS.

many of the -COOH groups on CTS-PAAc were dissociated into the -COO- groups. This made the electrostatic interaction between CTS-PAAc and lead ions become significantly favorable or attractive and hence resulted in the observed dramatic increase in lead ion uptake. For CTS, however, the adsorption mechanism for lead ions was much more complicated. Both the electrostatic interaction and surface complexation played a role and their combined effect contributed to the observed phenomenon.10,22,25 Adsorption Isotherms. Adsorption isotherms describe how adsorbates interact with adsorbents and are important in optimizing the use of adsorbents. The isotherm adsorption data of lead ions on CTS and CTS-PAAc are presented in Figure 5. In general, lead ion uptake on both CTS and CTS-PAAc increased with the initial or the equilibrium lead ion concentrations in the solutions, and the equilibrium uptake of lead ions on CTS-PAAc were always significantly greater than those on CTS; see Figure 5a and b, respectively. The lead ion uptake curves in Figure 5 may be considered to consist of two major parts: (1) a rapidly increasing phase at lower initial or equilibrium concentrationssthe adsorption uptake increased almost proportionally with the initial or equilibrium lead concentrations, suggesting that the adsorption sites on the adsorbent were sufficient and the amount of adsorption in these cases was dependent on the number of metal ions that were transported from the bulk solution to the surfaces of the adsorbentsand (2) a flattened phase at higher initial or equilibrium concentrationssthe adsorption uptake no longer increased proportionally with the lead ion concentrations in the solutions. This indicates that the number of available adsorption sites on the surfaces of the adsorbent has actually become the limiting factor that controls the amount of adsorption uptake in these cases. Because much more adsorption sites were available on CTS-PAAc, resulting from the grafted PAAc, the transition of the increasing phase to the flattened phase is found to cover a considerably wider concentration range for CTS-PAAc (C0 ) 10-100 mg/L) than for CTS (C0 ) 10-30 mg/L) in the cases studied. The Langmuir, Freundlich, and Temkin isotherm models have widely been used in adsorption isotherm studies and were also

(2)

where qe is the adsorption amount at equilibrium (mg/g), qm is the maximum amount of adsorption (mg/g), b is the adsorption equilibrium constant (L/mg), and Ce is the equilibrium concentration of the metal ions in the solution (mg/L). The Freundlich isotherm model assumes that as the adsorbate concentration increases so does the concentration of adsorbate on the adsorbent surface and can be given in its linearized form as

ln qe ) bF ln Ce + ln KF

(3)

where KF (mg/g)(L/mg) is a constant characterizing the adsorption capacity and bF (dimensionless) is a constant depicting the adsorption intensity. The Temkin isotherm model takes into consideration of some of the indirect adsorbate/adsorbate interactions in an adsorption process and assumes that the heat of adsorption of all the molecules in the adsorbing layer would decrease linearly with the coverage.26 The Temkin isotherm model has been used in the following form:

qe )

RT RT ln A + ln Ce b b

(4)

where A is the equilibrium binding constant corresponding to the maximum binding energy, b is the Temkin isotherm constant, T is the absolute temperature (K), and R is the ideal gas constant. The fittings of the Langmuir, Freundlich, and Temkin isotherm models to the experimental isotherm data for lead ion adsorption on CTS-PAAc are also shown in Figure 5a, and the corresponding parameter values from the data fitting calculation are given in Table 1. The results show that both the Langmuir and Freundlich isotherm models can give adequate fittings to the experimental results, although the Langmuir isotherm model appears to give a slightly better fitting than the Freundlich isotherm model. The results therefore suggest that the phenomenon of lead ion adsorption on CTS-PAAc was somewhat complex. From the Langmuir isotherm model fitting, the maximum adsorption capacity for lead ions on CTS-PAAc was calculated to be 294.12 mg/g at the solution pH value of 4 studied. As a comparison, the fitting of the Langmuir isotherm model to the case of CTS is also given in Figure 5b. The Langmuir isotherm model also appears to give an adequate fit to the experimental data, and the maximum adsorption capacity for

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Figure 7. Desorption kinetics of lead ions from CTS-PAAc in different solutions.

Figure 6. Kinetic adsorption results of lead ions on CTS-PAAc.

lead ions on CTS was calculated to be 95.15 mg/g. Therefore, the grafting of PAAc on CTS significantly enhanced the adsorption capacity of the adsorbent from about 95 mg/g (0.46 mmol/g) to 294 mg/g (1.42 mmol/g). In the literature, other researchers have reported lead ion adsorption on activated carbon as 0.14 mmol/g,27 on various algal biomasses as 0.451.46 mmol/g,9 on native and chemically modified alfalfa biomasses as 0.21 and 0.43 mmol/g, respectively,28 or on acetone-washed yeast biomass to be in the range of 0.0350.27 mmol/g.29 Hence, the CTS-PAAc developed in this study obviously have comparative and competitive advantages for lead ion removal from aqueous solutions. Adsorption Kinetics. Rapid interaction of the metal ions to be removed with the adsorbent is desirable and beneficial for practical adsorption applications. The kinetic results of lead ion adsorption on CTS-PAAc are shown in Figure 6. It can be observed that lead uptake on CTS-PAAc was a fast process. The amount of adsorption increased rapidly in the first 3 h, contributing to about 90% of the ultimate adsorption amount, and then augmented slowly. The adsorption equilibrium was achieved within about 5 h in this case. This is in contrast with about 8 h for cross-linked chitosan granules10 and with up to 10-20 h or even more for commercial activated carbon commonly used in the industry to reach adsorption equilibrium for metal ion adsorption.30 The pseudo-second-order kinetic model has often been used to fit the experimental kinetic adsorption data and determine whether an adsorption process is dominated by the chemical adsorption phenomenon. The linearized pseudo-second-order kinetic equation usually takes the following form

1 t t ) + q t k q 2 qe

(5)

2 e

where qt (mg/g) is the adsorption amount at time t (min), k2 (g/mg‚min) is the rate constant of the pseudo-second-order kinetic adsorption. The values of k2 and qe can be obtained from the intercept and slope of the plot of the experimental t/qt versus t. The fitting of eq 5 to the experimental kinetic data from the study is given in the insertion of Figure 6. It is observed that the adsorption kinetic data of lead ions on CTS-PAAc are indeed well-represented by the pseudo-second-order kinetic model, with the correlation coefficient R2 being almost unity (R2 ) 0.999). Therefore, it is reasonable to conclude that lead ion adsorption on CTS-PAAc was dominated by a chemical adsorption process that contributed to the fast adsorption kinetics (as compared to other conventional or physical adsorption processes). The parameter values of qe and k2 determined from the fitting of the kinetic model in eq 5 to the experimental data in Figure 6 are found to be 285.71 mg/g and 4920 g/mg‚min, respectively. The maximum adsorption capacity of 285.71 mg/g obtained from the pseudo-second-order kinetic model is very close to

Table 2. Adsorptiona and Desorption Behaviors of Lead Ions on CTS-PAAc cycle I desorption media 0.1 M EDTA 1 M HNO3 1 M H3PO4 a

cycle II

cycle III

uptake uptake uptake (mg/g) recovery (mg/g) recovery (mg/g) recovery 35.2 35.2 35.2

97.7% 95.8% 97.8%

34.6 34.1 34.7

97.3% 95.1% 97.3%

33.9 33.2 34.1

97.0% 94.6% 97.1%

Adsorption initial concentration was 20 mg/L and pH was 4.

that obtained from the Langmuir model earlier (294.12 mg/g). Again, the fast uptake rate and short adsorption equilibrium time provide evidence to support the idea that the surfaces of CTSPAAc had a high density of active adsorption sites for lead ions and the uptake of lead ions on CTS-PAAc may mainly take place on the surfaces. Desorption Study. Desorption experiments were conducted to regenerate the lead-adsorbed CTS-PAAc adsorbent in various solutions, including 0.1 M EDTA, 1 M HNO3, and 1 M H3PO4. The kinetic desorption results are shown in Figure 7. In general, all the three types of solutions can effectively desorb the lead ions from CTS-PAAc (about 97% of desorption at equilibrium). It is however observed that the desorption process was much faster in the 1 M nitric acid solution and it took only 10 min to reach about 95% of the desorption. In comparison, lead ion desorption in the 1 M H3PO4 or 0.1 M EDTA solution appeared to be slower. The adsorption and desorption processes were repeated to examine the potential of CTS-PAAc for practical applications. Table 2 shows the experimental results on the amounts of lead ions adsorbed and the percentages of desorption in three consecutive adsorption-desorption cycles. It is observed that desorption efficiency was generally high and the adsorption capacity was almost not affected by the cycles. In addition, since the volume of the desorption solution was much smaller than the volume of the lead ion solution used in the adsorption study (only one-fifth), the desorbed lead ions were present in a more concentrated solution, which may be of benefit for possible recovery of the lead ions, if desired. Adsorption Mechanism of Lead Ions on CTS-PAAc. To elucidate the mechanisms of lead ion adsorption on CTS-PAAc, the FTIR spectra of CTS-PAAc with or without lead ion adsorption under different solution pH conditions were obtained. The characteristic peaks and their corresponding changes are shown in Figure 8. For the CTS-PAAc without lead ion adsorption, it can be found that, with the increase of the solution pH values from 2 to 4 to 6, the peak at around 1749.4-1743.2 cm-1 for the Cd O stretching vibration of the carboxyl (sCOOH) groups was greatly reduced and the peak at around 1658.8-1666.5 cm-1 for the asymmetric vibration of the deprotonated carboxyl groups

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Figure 8. Characteristic peaks and corresponding changes of the FTIR spectra for CTS-PAAc with or without lead ion adsorption at pH 2, 4, and 6.

or carboxylate ion groups (sCOO-) significantly increased with the increase of the solution pH values; see Figure 8a, c, and e. The FTIR results confirm that more and more of the carboxyl groups were deprotonated into the carboxylate ion groups with the increase of the solution pH values. This conclusion is also supported by the changes of the CsO stretching vibration frequency (in the range of about 1303.9-1330.8 cm-1) observed in Figure 8a, c, and e. For pH values varying from 2 to 4 to 6, the peak for the CsO stretching vibration shifted to a higher vibration frequency, i.e., from 1303.9 cm-1 at pH 2 to 1327.0 cm-1 at pH 4 and to 1330.8 cm-1 at pH 6. These changes can be attributed to the deprotonation of the CsOH moiety in the carboxyl groups (i.e., CsOH became CsO-), which increased the vibration frequency of the CsO bond. If calculation is made on the basis of pKa ) 3.7 for the carboxyl groups, the deprotonation percentage of the carboxyl group or the amount of COO- groups can be predicted to be 2.0% at pH 2, 66.6% at pH 4, and 99.5% at pH 6. The significant changes in the amount of sCOO- groups with solution pH values from the calculation is clearly reflected by the FTIR results in Figure 8 (by the peak intensities for the COO- groups at 1658.8 cm-1 at pH 2, 1658.0 cm-1 at pH 4, and 1666.5 cm-1 at pH 6). After lead ion adsorption, the peaks at around 1658.8-1666.5 cm-1 for the COO- groups at each pH value were all shifted to a lower vibration frequency (1658.8 to 1655.0 cm-1 at pH 2, 1658.0 to 1656.0 cm-1 at pH 4, and 1666.5 to 1665.5 cm-1 at pH 6). This can be attributed to the formation of the coordinated -COO- and Pb2+ complexes (or -COO-‚‚Pb2+), which reduced the vibration frequency of the -COO- groups. Another shift is also observed for the peaks at around 1303.9-1330.8 cm-1 to higher vibration frequencies (1303.9 to 1354.0 cm-1 at pH 2, 1327.0 to 1338.6 cm-1 at pH 4, and 1330.8 to 1350.2 cm-1 at pH 6). This type of shift has been attributed to the complexation of a metal ion with the oxygen atom in the COOH groups.31 Therefore, all these FTIR results confirmed that the carboxyl groups of PAAc on CTS-PAAc were the active functional groups for lead ion uptake and that lead ion adsorption on CTS-PAAc was mainly through forming complexes with the

-COO- groups (i.e., forming -COO-‚‚Pb2+ complexes). It is also observed that the peak at 1655.0 cm-1 representing the coordinated -COO- and Pb2+ complexes at pH 2 is much weaker than that at 1656.0 cm-1 at pH 4 and again than that at 1665.5 cm-1 at pH 6, indicating that much less coordinated -COO- and Pb2+ complexes were formed at pH 2 than at pH 4, and again at pH 4 than at pH 6. This can be a direct consequence of the fact that less -COO- groups were available at a lower pH than at a higher pH for lead ion adsorption. The FTIR results thus clearly explain the experimentally observed adsorption phenomenon of lead ions on CTS-PAAc at different solution pH values (i.e., adsorption increased with solution pH), as shown in Figure 4. Another interesting change to be noted in Figure 8 is the intensity of the peak representing the -COO- groups after lead ion adsorption under each pH condition examined. The intensity of the peak for the -COO- groups after lead ion adsorption became much stronger than that for the -COO- groups before lead ion adsorption (comparing the peak at 1655.0 to 1658.8 cm-1 at pH 2, the peak at 1656.0 to 1658.0 cm-1 at pH 4, and the peak at 1665.5 to 1666.5 cm-1 at pH 6 in Figure 8). The results appear to indicate that more -COO- groups were generated with the adsorption of lead ions and, thus, increased the vibration intensities of the -COO- groups. The explanation for this phenomenon may be given as follows: The adsorption of Pb2+ with -COO- may have disrupted the protonationdeprotonation equilibrium established between the -COOH and -COO- groups before lead ion adsorption and driven more -COOH groups to be deprotonated. As a consequence, the intensity of the peak for the -COOH groups at each pH value is also observed to be reduced after lead ion adsorption (see the peaks at 1743.1 and 1749.4 cm-1 at pH 2, the peaks at 1739.8 and 1743.2 cm-1 at pH 4, and the peaks at 1735.8 and 1743.2 cm-1 at pH 6 in Figure 8). Conclusions Cross-linked chitosan granules (CTS) can be successfully functionalized with poly(acrylic acid) (PAAc) via a simple twostep reaction process in a solution. FTIR study confirmed that PAAc was grafted on CTS through the formation of an amide bridge between an amine group of CTS and a carboxyl group of PAAc. Zeta potential analysis indicated that the PAAcfunctionalized chitosan granules (CTS-PAAc) had less positive or more negative zeta potentials than CTS and favored the adsorption of lead ions in a wide solution pH range. Adsorption experiments showed that CTS-PAAc had highly enhanced adsorption capacity for lead ions, in comparison with CTS, in the solution pH range of 1-6 examined. Adsorption isotherm study suggested that lead ion adsorption on CTS-PAAc best followed the Langmuir isotherm model, and the maximum adsorption capacity of lead ions on CTS-PAAc was calculated to be as high as 294.12 mg/g in the case of pH 4 examined, showing obviously comparative or competitive advantages over CTS and many other adsorbents reported in the literature. Adsorption mechanism study confirmed that the high adsorption capacity of CTS-PAAc was attributed to the high content of the carboxyl groups grafted on CTS-PAAc. Desorption study showed that lead ions adsorbed on CTS-PAAc can be easily and effectively desorbed in a short time and the regenerated adsorbent can be reused almost without any loss of adsorption capacity. The work shows the potential of CTS-PAAc as a novel adsorbent to remove lead ions in water and wastewater treatment. For more practical consideration, further research work will be extended to investigate the competing adsorption effect

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ReceiVed for reView April 24, 2006 ReVised manuscript receiVed September 4, 2006 Accepted September 16, 2006 IE060514S