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Adsorption of Divalent Cadmium (Cd(II)) from Aqueous Solutions onto Chitosan-Coated Perlite Beads Shameem Hasan, Abburi Krishnaiah, Tushar K. Ghosh,* and Dabir S. Viswanath Nuclear Science and Engineering Institute, UniVersity of Missouri, Columbia, Missouri 65211
Veera M. Boddu and Edgar D. Smith US Army Engineering Research and DeVelopment Center (ERDC), Construction Engineering Research Laboratories, EnVironmental Process Branch (CN-E), Champaign, Illinois 61826
Chitosan-coated perlite beads were prepared in the laboratory via the phase inversion of a liquid slurry of chitosan dissolved in oxalic acid and perlite to an alkaline bath for better exposure of amine groups (NH2). The NH2 groups in chitosan are considered active sites for the adsorption of heavy metals. The beads were characterized by scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) microanalysis, which revealed their porous nature. The chitosan content of the beads was 32%, as determined using a thermogravimetric method. The adsorption of Cd2+ from an aqueous solutions on chitosan-coated perlite beads was studied under both equilibrium and dynamic conditions in the concentration range of 1005000 ppm. The pH of the solution was varied over a range of 2-8. The adsorption of Cd2+ on chitosan was determined to be pH-dependent, and the maximum adsorption capacity of chitosan-coated perlite beads was determined to be 178.6 mg/g of bead at 298 K when the Cd(II) concentration was 5000 mg/L and the pH of the solution was 6.0. On a chitosan basis, the capacity was 558 mg/g of chitosan. The XPS data suggests that cadmium was mainly adsorbed as Cd2+ and was attached to the NH2 group. The adsorption data could be fitted to a two-site Langmuir adsorption isotherm. The data obtained at various temperatures provided a single characteristic curve when correlated according to a modified Polanyi’s potential theory. The heat of adsorption data calculated at various loadings suggests that the adsorption was exothermic in nature. It was noted that a 0.1 N solution of HCl could remove the adsorbed cadmium from the beads, but a bed volume of approximately three times the bed volume of treated solution was required to completely remove Cd(II) from the beads. However, one bed volume of 0.5 M ethylenediamine tetra acetate (EDTA) solution can remove all of the adsorbed cadmium after the bed became saturated with Cd(II) during dynamic study with a solution containing 100 mg/L of cadmium. The diffusion coefficient of Cd(II) onto chitosan-coated beads was calculated from the breakthrough curve, using Rosen’s model, and was determined to be 8.0 × 10-13 m2/s. Introduction Cadmium plating is used for fasteners and other very-tighttolerance parts, because of the dual qualities of lubricity at minimal thickness and superior sacrificial corrosion protection to many chemicals at high temperatures. Long-term exposure to cadmium can cause damage to the kidneys, liver, bone, and blood. Therefore, cadmium should be removed from waste streams before discharge into the environment. The removal of cadmium from wastewater is generally accomplished by precipitation with a hydroxide, carbonate, or sulfide compound. Also, several adsorbentssincluding activated carbon, recycled iron, novel organo-ceramic, hydrous cerium oxide, and low-cost adsorbents such as rice husk, date pits, fly ash, aerobic granular sludge, sewage sludge, tunable biopolymers, alginated coated-loofa sponge disks, surfactant-modified zeolites, porous poly(methacrylate) beads, red mud, goethite, perlite, and duoliteshave been used for cadmium removal.1-17 The cadmium adsorption capacities of various adsorbents are summarized in Table 1. Several researchers18-22 have investigated chitosan as an adsorbent for removal of heavy metals, including cadmium from aqueous streams. Chitosan is a natural biopolymer, it is hydrophilic, and it has the ability to form complexes with metals. * To whom correspondence should be addressed. Tel.: 1-573-8829736. Fax: 1-573-884-4801. E-mail address:
[email protected].
It is also a nontoxic, biodegradable and biocompatible material. According to Rorrer et al.,23 chitosan flake or powder swells and crumbles, making it unsuitable for use in an adsorption column. Chitosan also has a tendency to agglomerate or form a gel in aqueous media. Although the amine and hydroxyl groups in chitosan are mainly responsible for adsorption of metal ions, these active binding sites are not readily available for sorption when it is in a gel or in its natural form.24 Guibal et al.25 noted that the maximum uptake of chitosan flakes was approximately half of that obtained with chitosan beads for molybdate. The adsorption capacity can be enhanced by spreading chitosan on physical supports that can increase the accessibility of the metal binding sites. Bodmeier et al.26 noted that freeze-drying of chitosan gel produced particles with a high internal surface area, which boosted the metal binding capacity. Several attempts have been made to modify the structure of chitosan chemically, and its performance has been evaluated through the adsorption of heavy-metal ions from aqueous solutions. The amine and hydroxyl groups in chitosan allow a variety of chemical modifications.27 Kawamura et al.28 prepared a porous polyaminated chitosan chelating resin by introducing poly(ethylene amine) onto the cross-linked chitosan beads. The resultant beads showed high capacity and high selectivity for the adsorption of metal ions including cadmium. Hsien and Rorrer29 investigated the adsorption of cadmium on porous magnetic chitosan beads. They found
10.1021/ie0402620 CCC: $33.50 © 2006 American Chemical Society Published on Web 06/15/2006
Ind. Eng. Chem. Res., Vol. 45, No. 14, 2006 5067 Table 1. Adsorption Capacity of Various Adsorbents for Cadmium adsorbent chitosan flakes magnetic chitosan beads 1 mm size 3 mm size non-cross-linked chitosan beads N-acylated chitosan beads cross-linked chitosan crown ethers chitosan aryl crown ethers glutaraldehyde cross-linked chitosan rice husk perlite diamine grafted chitosan crown ethers activated carbon novel organo-ceramic modified corncorbs Duolite GT-73 resin Amberlite IRC-718 resin Amberlite-200 resin alginate-coated loofa sponge disks porous poly(methyl methacrylate) beads chitin chitosan-coated perlite
pH
adsorption capacity (mg/g)
6.0
9.9
6.5 6.5
518 188 169 216 32.2 39.3 134.9 0.007 0.64 28.1 3.4 212.8 70.56 9.0 105.7 258.6 224.8 88 24.2 15.0 178.6
2.0-6.5 6.0 3.0 7.0 6.6-6.7 5.0 5.0 4.8 4.8 4.8 4.8 5.0 6.0 6.0 5.0
that the adsorption capacities for 1- and 3-mm beads were 518 and 188 mg Cd/g of adsorbent, respectively. They also investigated the effects of acylation and cross-linking of chitosan using gutaraldehyde.30 They later noted that, although the crosslinked chitosan beads became more resistant to acid, the capacity of cross-linked chitosan for cadmium decreased significantly, compared to non-cross-linked beads. The adsorption of metal ions on gallium(III)-templated oxine type of chemically modified chitosan was reported by Inoue et al.31 Chitosan cross-linked with crown ethers32,33 showed improved adsorption capacity for cadmium and high selectivity for Ag(I) or Cd(II) in the presence of Pb(II) and Cr(III). Becker et al.34 studied the adsorption characteristics of cadmium, along with other metals, on dialdehyde or tetracarboxylic acid cross-linked chitosan. Although several attempts have been made to enhance the adsorption capacity of chitosan for cadmium and other metal ions through the cross-linking of chitosan, using various chemicals, the sorption capacity for metal ions decreased after the cross-linking of chitosan. Hasan et al.24 noted that, by dispersing chitosan on an inert substrate (perlite) enhanced its adsorption capacity for Cr(VI). It is assumed that the active group, such as NH2, became more readily available. In this work, chitosan was dispersed on an inert substance to expose more-active sites for adsorption. Chitosan was coated on perlite, and the coated adsorbent was prepared as spherical beads. It is expected that the swelling and gel formation of chitosan can be reduced in this way, thus allowing regeneration and repeated use of the chitosan adsorbent in a column. The adsorbent was evaluated for cadmium by obtaining equilibrium adsorption data at different temperatures and pH. We also explored the regeneration of the adsorbent using dilute acid and the ethylenediamine tetra acetate (EDTA) solutions. Experimental Section Materials. Perlite (grade YM 27) was donated by Silbrico Corporation (Hodgkins, IL). Chitosan and cadmium chloride were obtained from Aldrich Chemical Corporation (Milwaukee, WI). The chitosan used in this study was 75%-85% deacetylated and had a molecular weight of ∼190 000-310 000, as determined from the viscosity data by Aldrich Chemical Corporation. All chemicals used in this study were of analytical grade. Oxalic acid, EDTA, and sodium hydroxide were pur-
reference Bassi et al.20 Rorrer et al.25 Rorrer et al.25 Hsien et al.26 Hsien et al.27 Peng et al.29 Yang et al.23 Becker et al.31 Khalid et al.5 Mathialagan and Viraghavan16 Yang et al.30 An et al.21 Gomez-Salazar et al.3 Vaughan et al.7 Vaughan et al.7 Vaughan et al.7 Vaughan et al.7 Iqbal and Edyvean11 Denizli et al.13 Benguella and Benaissa22 present work
chased from Fisher Scientific Co. (Fairlawn, NJ). A stock solution containing 5000 mg/L of Cd(II) was prepared in distilled, deionized water using CdCl2. The working solutions of various Cd(II) concentrations were obtained by diluting the stock solution with distilled water. Preparation and Characterization of Chitosan-Coated Perlite Beads. Perlite powder (35 mesh) was first soaked with 0.2 M oxalic acid for 4 h. It then was washed with distilled water and dried in an oven for 12 h. Sixty grams of acid-washed perlite was mixed with 30 g of chitosan flakes in a beaker that contained 1 L of 0.2 M oxalic acid. The mixture was stirred for 4 h while heating at 313-323 K (40-50 °C) to obtain a homogeneous mixture. The spherical beads of chitosan, coated on perlite, were prepared via dropwise addition of the mixture into a 0.7 M NaOH precipitation bath. The beads were washed with deionized water to a neutral pH and freeze-dried for subsequent use. A detailed description of the bead preparation method has been discussed by Hasan et al.24 The final diameter of the freeze-dried beads was ∼2 mm. The Brunauer-Emmett-Teller (BET) surface area of the beads was determined to be 25 m2/g, compared to 3 m2/g for the perlite particles. Most of the surface area of the beads is expected to be internal, because the external area is extremely small. The surface morphology of pure perlite changed significantly, following coating with chitosan, as indicated by scanning electron microscopy (SEM) micrographs of the beads. The SEM micrographs of the cross section of a bead also revealed the porous structure of the beads (Figure 1a). A transmission electron microscopy (TEM) micrograph (Figure 1b) of the beads showed that individual perlite particles were not necessarily coated with chitosan; rather, a group of particles were lumped together and coated by the chitosan film. Energy-dispersive X-ray spectrometry (EDS) microanalysis of the chitosan beads before and after their exposure to the cadmium solution confirmed the presence of Cd(II) in the interior of the beads. The EDS microanalysis shown in Figure 2a exhibited peaks for aluminum and silicon, which are two major constituents of perlite. A strong peak at ∼3.5 keV for Cd on beads that were exposed to Cd(II) can be observed in Figure 2b. In the EDS spectrum (Figure 2b), a small peak for chlorine was also observed. The chlorine peak may have resulted either from the adsorption of chlorine on chitosan from the solution or from
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Figure 3. Thermogravimetric analysis (TGA) of chitosan-coated perlite beads: (- ‚ - ‚ -) freeze-dried beads and (s) oven-dried beads.
773 K. The change of mass, as a function of temperature, is shown in Figure 3. Experimental Procedure
Figure 1. (a) Scanning electron microscopy (SEM) micrograph of the cross section of the chitosan-coated perlite beads. (b) Transmission electron microscopy (TEM) micrograph of chitosan-coated perlite beads. The black spots represent perlite particles.
Equilibrium batch adsorption studies were performed by exposing the beads to aqueous solutions that contained different concentrations of Cd(II) ions in 125-mL Erlenmeyer flasks to a predetermined temperature. Approximately 0.25 g of beads was added to 50 mL of solution. The pH of the solutions was adjusted by adding either 0.1 N sulfuric acid or 0.1 M sodium hydroxide. The flasks were placed in a constant temperature shaker bath for a specific time period. Following the exposure of the beads to Cd(II), the solutions were filtered and the filtrates were analyzed for Cd(II) via atomic absorption spectrometry (AAS). The adsorption isotherm at a particular temperature was obtained by varying the initial concentration of Cd ions. The amount of Cd(II) adsorbed per unit mass of adsorbent (Qe) was calculated using the following equation:
Qe )
(Ci - Ce)V M
(1)
Results and Discussion
Figure 2. (a) Energy-dispersive X-ray spectrometry (EDS) microanalysis of chitosan-coated perlite beads, showing the presence of silicon and aluminum (platinum and silver were from the sputter coating of the sample for electrical contact). (b) EDS microanalysis of chitosan-coated perlite beads following exposure to CdCl2.
the solution trapped inside the pores. However, it may be noted that some of the amine groups can undergo protonation in acidic solution, forming NH3+, which is capable of adsorbing anions such as chlorine. Thermogravimetric analysis (TGA) of the beads indicated that the chitosan content of the bead was ∼32%. Chitosan started to decompose at ∼473 K and was completely burned out at
Effect of Surface Charge and pH on the Adsorption of Cd(II). The surface charge of the bead was determined via a standard potentiometric titration method in the presence of a symmetric electrolyte (sodium nitrate). The magnitude and sign of the surface charge was measured with respect to the point of zero charge (PZC). The pH at which the net surface charge of the solid is zero at all electrolyte concentrations is called the PZC. The pH of the PZC for a given surface is dependent on the relative basic and acidic properties of the solid35 and allows estimation of the net uptake of H+ and OH- ions from the solution. The surface charge of the beads in the presence of 0.1 M NaNO3 was determined in the following manner.36 Four flasks, each containing 2 g of chitosan-coated perlite beads, were exposed to 100 mL of 0.1 M NaNO3 solutions. The flasks were placed in a shaker for 24 h at 125 rpm. Samples from one of the flasks were titrated directly with 0.1 M HNO3, and samples from another flask were titrated with 0.1 M NaOH. After the addition of the acid or base, the pH of the solution was recorded, after it was allowed to equilibrate. Titration was conducted over a pH range of 3-11. The beads were separated from the solutions of the remaining two flasks. The supernatants were titrated in a similar manner, but in the absence of beads. The net titration curve was obtained by subtracting the titration curve of the supernatant that was obtained without the presence of beads from the titration curve obtained with the beads. In the
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absence of specific chemical interaction between the electrolytes and the surface of the bead, the net titration curves usually meet at a point that is defined as the pHPZC. Similar experiments were conducted with a 0.05 M NaNO3 solution. No difference between the two titration curves obtained using two different ion strengths was observed. The surface charge was calculated from the following equation:37
σ0 (C/m2) )
(Ca - Cb + [OH-] - [H+])F Sa
(2)
[H+] and [OH-] were calculated from the pH of the solution, after appropriate correction was made, using the activity coefficient. The activity coefficient was calculated using the Davis equation:38
log γi ) -0.5109 I ) 0.5
(
xI - 0.3I I + xI
∑i Cizi2
)
Figure 4. Surface charge of chitosan-coated perlite beads in the presence of CdCl2: ([) 0.1 M NaNO3, (0) 0.05 M NaNO3, and (2) cadmium solution. Table 2. Cadmium Uptake at Equilibrium at Various Solution pH at 298 K
(3)
concentration at equilibrium of the liquid phase (mmol/L)
(4)
0.303 0.589 1.607 3.482 6.964
Initial pH of the Solution: 2 0.0285 0.0607 0.1250 0.1964 0.3214
2.1 2.1 2.4 2.5 2.6
0.089 0.402 1.250 3.036 6.696
Initial pH of the Solution: 4.5 0.036 0.098 0.196 0.286 0.424
4.5 4.6 4.6 5.0 4.9
0.225 0.402 1.160 2.679 6.160
Initial pH of the Solution: 6 0.080 0.143 0.268 0.375 0.536
6.2 6.2 6.3 6.5 6.6
0.089 0.320 1.160 2.770 5.940
Initial pH of the Solution: 8 0.089 0.161 0.321 0.446 0.580
8.0 8.1 8.2 8.3 8.6
The results are shown in Figure 4. The PZC value of the chitosan-coated perlite bead was determined to be 8.5, which was similar to that reported by Jha et al.19 for chitosan flake. However, Udaybhaskar et al.39 reported a PZC value in the range of 6.2-6.8 for pure chitosan. The surface charge of chitosancoated perlite bead was almost zero in the pH range of 6-8.5. It may be noted that the pKa value of perlite was determined to be ∼7. The protonation of the beads sharply increased at the pH range of 3-4.5, making the surface positive. At pH 7, cadmium started to precipitate
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Figure 5. Effect of pH on cadmium uptake by pure perlite (initial concentration of 1 mg/L; data from ref 16), pure chitosan (initial concentration of 2 mg/L; data from ref 19), and chitosan-coated perlite beads (initial concentration of 100 mg/L; data from this work).
out from the solution. Therefore, experiments were not conducted at pH >8.0. The increased capacity at pH >7 may be a combination of both adsorption and precipitation on the surface. It is concluded that the beads had a maximum adsorption capacity at a pH of ∼6, if the precipitated amount is not considered in the calculation. The amine group of the chitosan has a lone pair of electrons from nitrogen, which primarily act as an active site for the formation of chitosan-metal-ion complex. As mentioned previously, at lower pH values, the amine group of chitosan undergoes protonation, forming NH3+, which leads to an increased electrostatic attraction between NH3+ and the sorbate anion. Cadmium in an aqueous solution is hydrolyzed with the formation of various species, depending on the solution pH. Moreover, Cd2+, which is the main hydrolyzed cadmium species in the pH range of 5-7 appear in the form of Cd(OH)+, Cd(OH)20, and Cd(OH)3-. Among them, Cd2+ is the predominant species in the solution within this pH range. The fraction of negatively charged hydrolysis products in the solution increases as pH increases. Various hydrolysis reactions are given by Reed and Matsumoto40 and Baes and Mesmer.41 Chitosan can form chelates with cadmium ions (Cd2+) with the release of H+ ions. A chelate formation may require the involvement of two or more complexing groups from the molecule. The Cd ion may seek two or more amine groups from chitosan to form the complex. This should normally reduce the pH of the solution. Kaminiski and Modrzejewska42 suggested that the increase in pH may be attributed to the exchange of released H+ ions between the surface of the bead and the solution. In the case of chitosan, the protonation of NH2 groups occurs at a rather low pH range. The fact that the pH of the solution increased as the adsorption progressed suggests that Cd(II) formed a covalent bond with the NH2 group. The two NH2 groups could come from two different glucosamine residues of the same molecule, or from two different molecules of chitosan. Jha et al.19 compared the stability constants for ammonia and amino complexes with those for chloro complexes of cadmium and noted that the formation of covalent bond with amine nitrogen is the more-preferred reaction. It was noted that the present adsorbent can adsorb 4.98 mmol Cd/g of chitosan at 298 K when the Cd(II) concentration was 5000 mg/L and the pH of the solution was 6.0. The NH2 groups are the main active sites for cadmium adsorption. As can be seen from Figure 8 (presented later in this paper), two NH2 groups will be
Figure 6. X-ray photoelectron spectroscopy (XPS) survey scans for chitosan flakes (top spectrum) and chitosan-coated perlite beads (bottom spectrum). The inset in top spectrum indicates the C 1s position in the chitosan flake, whereas the inset in bottom spectrum indicates the C 1s position in chitosancoated perlite beads.
necessary for the adsorption of one Cd ion, because the concentration of NH2 on chitosan is ∼6.9 mmol/g. The maximum capacity for cadmium should be ∼3 mmol/g. Because the maximum capacity obtained experimentally is 4.98 mmol/ g, other sites such as CH2OH, OH, or O groups are also involved in adsorbing cadmium. At pH
