Adsorption of Chromium(VI) from Aqueous Solutions Using Cross

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SEPARATIONS Adsorption of Chromium(VI) from Aqueous Solutions Using Cross-Linked Magnetic Chitosan Beads Guolin Huang,†,‡ Hongyan Zhang,† Jeffrey X. Shi,‡ and Tim A. G. Langrish*,‡ Department of Chemical Engineering and Technology, East China Institute of Technology, Jiangxi, 344000, China, and School of Chemical and Biomolecular Engineering, The UniVersity of Sydney, 2006, Australia

The performance of a cross-linked magnetic chitosan (CMC), which has been coated with magnetic fluids and cross-linked with epichlorohydrin, has been investigated for the adsorption of chromium(VI) from aqueous solutions. Infrared spectra of chitosan before and after modification show that it is a successful coating and that the cross-linking process has been effective. The influences of the pH of solution and the contact time on the adsorption amounts have been discussed, and the appropriate process conditions for the adsorption of Cr(VI) have been obtained. Scanning electron micrographs of the CMC before and after adsorption indicate that the reduction in the number of pores on the surface was due to the adsorption of Cr(VI). The adsorption experimental equilibrium data was found to fit the Langmuir model well, and the uptake of Cr(VI) was 69.4 mg/g. The experimental data for the kinetics of adsorption correlated well with the first-order Langergren rate equation, and Langergren rate constants have been determined. The adsorbed Cr(VI) ions can be removed from the exhausted CMC effectively by being regenerated with a 0.1 mol/L hydrochloric acid solution. 1. Introduction Considerable quantities of industrial waste effluents in China today contain significant amounts of heavy metals and/or other toxic species. Among the many methods available for the removal of heavy metals from aqueous solution are electrochemical precipitation,1 ion exchange,2 ultrafiltration,3 and reverse osmosis.4 Adsorption techniques have been shown to be a feasible option, both technically and economically.5 Chitosan has been reported to have high potential for adsorption of chromium(VI).6-8 The amine groups and hydroxyl groups on the chitosan chain can act as chelation sites for Cr(VI) or other metal ions. Recent research interest has been focused on the modification of chitosan for enhanced adsorption performance.5,9-12 Coating chitosan with magnetic fluids is a typical modification method, and it has been reported that it can improve the surface area for adsorption and reduce the required dosage for the adsorption of metal ions.13,14 Hsien and Rorrer15 studied N-acylated chitosan by casting it into beads and then crosslinking the beads with glutaric dialdehyde in order to reduce its solubility for adsorption applications. Furthermore, glutaraldehyde, epichlorohydrin, and ethylene glycol diglycidyl ether have been used to cross-link the magnetic chitosan to improve the adsorption behavior, since the crosslinking process can change the crystalline nature of chitosan and enhance the adsorption ability. 12,16 Hsien and Rorrer17 have reported the production of highly porous chitosan beads by dropwise adding an acidic chitosan solution into a precipitation bath of sodium hydroxide solution. The gelled chitosan beads were cross-linked with glutaraldehyde and then freeze-dried. Well-mixed batch adsorption experiments revealed that the cross-linked chitosan had good adsorption for both metal and hydronium ions by a chelation mechanism. Li et al.18 have * To whom correspondence should be addressed. Tel.: +61-293514568. Fax: +61-2-93512854. E-mail: [email protected]. † East China Institute of Technology. ‡ The University of Sydney.

reported a magnetic type of chitosan prepared by coating chitosan with magnetite particles during the coprecipitation of Fe3+ and Fe2+ in alkaline solutions, and then cross-linking the resulting material with glutaraldehyde. The adsorption capacities of La3+, Nd3+, Eu3+, and Lu3+ ions on the magnetic chitosan were studied, and the adsorption behavior was fitted using the Langmuir equation. A cross-linked magnetic chitosan (CMC) adsorbent has been prepared in this paper. Before and after the modification, infrared (IR) techniques and scanning electron microscopy (SEM) have been used to study the change in the surface properties of the CMC. The Cr(VI) adsorption behavior on the prepared CMC has been studied in a batch reactor for various adsorption parameters, including the pH value of the solution and the adsorption contact time. The adsorption process has then been optimized. The Cr(VI) adsorption isotherm and kinetics have been measured and discussed. 2. Experimental Details 2.1. Material. Chitosan was purchased from China Chemical Agent Co. as a flaked material, with a deacetylation percentage of approximately 90%. All other reagents used in this study were analytical grade, and distilled or double distilled water was used in the preparation of all solutions. The Cr(VI) stock solution was prepared by dissolving 0.7071 ((0.0001) g of K2Cr2O7 dried at 105 °C for two hours in a 1000 mL volumetric flask with deionized water to form a Cr(VI) stock solution with concentration of 250 mg/L. Our experimental solution was prepared at 50, 100, and 125 mg/L by serial dilution from the stock solution of 250 mg/L. 2.2. Preparation of Cross-Linked Magnetic Chitosan Beads. A known volume of gelatin solution, 150 mL of double distilled water, 11.25 mL of 0.5 mol/L ferric sulfate solution and 10.00 mL of 1 mol/L Fe(NO3)3 solution were added in a four-neck rounded bottom flask, with a dropper, a thermometer, a magnetic stirrer, and a N2 purge gas connected to the reaction

10.1021/ie800814h CCC: $40.75  2009 American Chemical Society Published on Web 02/05/2009

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flask. The mixture solution was purged with nitrogen and stirred in a water bath at 60 °C for 30 min. The solution’s pH was adjusted to maintain a level of 8.0-9.0 by adding 25% (v/v) ammonium hydroxide solution during the reaction. A magnetic fluid was obtained for use in the chitosan coating. A 0.5 g chitosan flake was dissolved in a 50 mL, 0.05 mol/L hydrochloric acid solution to give a final concentration of 1% (w/v). The chitosan solution was then dropped into 50 mL of the obtained magnetic fluid in the flask through a dropper. After this step, 13 mL of pure epichlorohydrin was added into reaction flask to mix with the solution and stirred at 85 °C for 3 h, before the flask was cooled down to room temperature. The precipitate was washed with distilled water until there was no Cl- (AgNO3 method) existing in the effluents. The precipitate was then washed with ethanol and ether and dried in a 50 °C vacuum oven. The obtained product appears to be a deep yellow powder and is described in this paper as a cross-linked magnetic chitosan (CMC). 2.3. Cross-Linked Magnetic Chitosan Bead Characterization. A swelling study for the CMC was carried out in distilled water at 25 °C for a period of 24 h, and the amino content of the CMC was analyzed by the following procedure: 1 g of dry CMC was dissolved in a 20 mL, 0.3 mol/L hydrochloric acid solution. A sodium hydroxide (0.3 mol/L) solution was used for titration with methyl orange as indicator. Both the percentage swelling and the amino content of the prepared CMC were calculated by using the following equations: percentage of swelling )

Ws - W × 100% W

(1)

percentage of amino (NH2) ) (CHClVHCl - CNaOHVNaOH) × 16 × 100% (2) W × 1000 Here Ws and W are the weight of swollen and dry beads in grams, respectively, CHCl and VHCl are the concentration and volume of hydrochloric acid standard solution added, in moles per liter and milliliters, respectively, and CNaOH and VNaOH are the concentration and volume of sodium hydroxide standard solution used in the titration process, in moles per liter and milliliters, respectively. Infrared spectra of the chitosan flake were recorded for the sample before and after the modification on a Vertex 7.0 IR spectrum (Bruker Co., Switzerland) spectrophotometer by using pressed KBr pellets. The FTIR spectra of Cr(VI) loaded CMC by Nicolet 6700 FT-IR spectrometer (Thermo Fisher Scientific. America) by using pressed KBr pellets. A scanning electron microscope (JSM-5900) photograph of the CMC samples was obtained before and after adsorption of chromium(VI) sample. The sample was coated with carbon, and the SEM was operated at 10 keV. 2.4. Adsorption Experiments. Batch adsorption experiments were carried out by using the CMC as the adsorbent. A series of conical flasks containing Cr(VI) solutions with initial concentrations of 100 mg/L and a known dosage of the CMC (0.6, 0.8, 1.0, 1.2, 1.4, 1.6 g/mL) were shaken in a SHA-C shaker (Changzhou, China) with a shaker speed of 100 rpm until the system reached equilibrium. Equilibrium was considered to be achieved when two consecutive Cr(VI) concentrations in solution that were measured over a time period of 20 min between samples showed no significant difference in concentration. Our experiments indicated that 3 h was adequate at ambient temperatures to ensure that equilibrium was reached. Before shaking, the pH value of the solution was adjusted with 0.5 mol/L H2SO4 or 0.5 mol/L NaOH to cover a range from 2.0 to

8.0, which was measured using a PHS-3C pH Meter (Hangzhou, China). After filtration, the concentration of Cr(VI) in the supernatant was analyzed by a standard spectrophotometer method (China National Standards, QB/T7467). Three replicates were used for each adsorption datum, and variation among these replicates found to be less than 0.5%. The adsorption amount was calculated based on the difference in the Cr(VI) concentration in the aqueous solution before and after adsorption, according to the following equation: qe )

(C0 - Ce)V W

(3)

Where, C0 and Ce are the initial and equilibrium concentrations of Cr(VI) in milligrams per liter, respectively, V is the volume of Cr(VI) solution, in liters, and W is the weight of the CMC used, in grams. Several workers in the literature have reported that a low initial Cr(VI) concentration is preferred for adsorption.19,20 Accordingly, the Cr(VI) solution with initial concentrations of 100 mg/L was studied for adsorption process, and three Cr(VI) solutions with initial concentrations of 50, 100, and 125 mg/L were studied for adsorption kinetics in our experiments. 2.5. Regeneration Technique. For the purpose of reducing the operating cost and minimizing waste disposal, a practical recycling CMC method was developed. A 0.2 g Cr(VI) saturated CMC was immersed in a 100 mL beaker with 20 mL of 0.1 mol/L hydrochloric acid solution for 24 h at room temperature. After filtration, the residual was washed with distilled water until there was no Cr(VI) in the effluent. The CMC was then dried under vacuum. It was recycled to adsorb Cr(VI) from 100 mL of solution with an initial concentration of 100 mg/L under the same adsorption conditions, as described previously. 3. Results and Discussion 3.1. Characterization of the CMC. The CMC obtained in our study was found to be insoluble in acidic and alkaline media as well as distilled water. Compared with the original chitosan flake’s swelling behavior of 12.4% and its amino content of 10.20%, the swelling behavior of the modified CMC and its amino content were improved to 6.80% and 18.80%, respectively. All the results were analyzed with one independent replicate. According to the literature, the amino group has been recognized as an active binding site for the adsorption of heavy metal ions,21 and low swelling percentages have been recognized as an important for use in a continuous adsorption column.22 Infrared spectra of chitosan flake samples before and after the modification are shown in Figure 1. Figure 1(1) shows the basic characteristics of chitosan: 3436 cm-1 is typical for OsH bond stretching and NsH bond stretching, 1657 cm-1 (CdO of sNHsCdO bond stretching), 1083 cm-1, and 1023 cm-1 (CsOH bond stretching), according to Yan et al.23 Compared with Figure 1(1), the spectra of the CMC shown in Figure 1(2) indicates that a shift occurs from 1083 to 1068 cm-1, and the peak of 1068 cm-1 becomes wider, meanwhile, for CMC, the shoulder peak of 1023 cm-1 has disappered. These results demonstrate that the sNH2 group and sOH group of chitosan have become involved in the cross-linked reaction.23,24 Further, the peaks at 744 and 1528 cm-1 that appear in CMC are absorption peaks of CsCl bonds and NsH bonds, which show that chitosan is linked with epichlorohydrin. In CMC, a slight shift from 3436 to 3399 cm-1 occurs, and the peak value of this wavenumber becomes wider, suggesting an enhanced hydrogen bond. 25 From these results, we conclude that the chitosan sample has been successfully cross-linked.

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Figure 1. IR spectrum of chitosan (1) and CMC (2).

Figure 2. Effect of pH on the adsorption capacity.

3.2. Effect of pH on the Adsorption Process. The effect of the solution pH on the adsorption process has been investigated over the range from 2.0 to 8.0, and the results are shown in Figure 2. Selecting an optimum pH is very important for the adsorption process, since pH affects not only the surface charge of adsorbent, but also the degree of ionization and the speciation of the adsorbate during the reaction.20 As indicated in Figure 2, the uptake capacity of Cr(VI) increased when the solution pH was raised from 2.0 to 4.0. This could be explained by the fact that, at a lower pH, the amine groups on CMC surfaces can easily protonate, inducing an electrostatic repulsion of Cr(VI) ions.12,26 The maximum adsorption capacity occurred at a pH of 4.0 for the concentration range studied. Above a pH of 4.0, the adsorption capacity of Cr(VI) began to decrease and showed a significant decrease from 5.0 to 8.0. Similar observations have been reported by Sankararamakrishnan et al.,27 Juang and Shao,21 and Schmuhl et al.28 The optimal pH of 4.0 found here has been selected for further study in the experiments. 3.3. SEM Scans and Energy Dispersive (X-ray) Spectroscopy (EDS) Analysis of the CMC before and after Adsorption. The changes in the surface structure of the CMC before and after Cr(VI) adsorption are shown in Figure 3. The SEM image of the CMC before Cr(VI) adsorption (left) shows the existence of many open loose pores on the surface.

Such loose pore structures are beneficial for mass transfer during Cr(VI) adsorption onto the CMC. The SEM image after Cr(VI) adsorption shows a significant reduction in the porous structure. This result indicates that some pores may have been filled by Cr(VI) in the adsorption process. On the other hand, JSM-6360 LV scanning electron microscope with Falon EDS analysis has been used to confirm the adsorbed Cr(VI), and the results are shown in Figure 4. The EDS analysis of CMC after (right) adsorption in Figure 4 showed Cr(VI) have been adsorbed successfully onto the CMC. Possible mechanisms of adsorption in our work could be the chelation of the -NH2 group and hydroxyl groups to Cr(VI) ions. It is known that amino (mainly) and hydroxyl groups on CMC are active binding sites for Cr(VI) ions.21 The FTIR spectra of Cr(VI) loaded CMC is shown in Figure 5. It can be seen from Figure 5 that N-Cr bonds were formed between nitrogen atoms of the -NH2 group and Cr(VI), since the peak at a wavenumber of 3380 cm-1 has become narrower and the peak at a wavenumber of 2900 cm-1 has almost disappered. Meanwhile, a new peak at a wavenumber of 720 cm-1, attributed to the Cr-O bond from the Cr(VI) species, suggesting that Cr(VI) was adsorbed on the surface and nitrogen atoms on the CMC was involved in the adsorption. 3.4. Adsorption Isotherm. The equilibrium adsorption isotherm is fundamental for describing the interactive behavior between the solution and the adsorbent and is important in designing an adsorption system. The widely used Langmuir model has been found to fit the process successfully.5,7,27 The equation can be expressed as qe )

ambCe 1 + bCe

Ce 1 1 ) C + qe am e amb

(4)

(5)

Here qe is the amount of adsorbed Cr(VI) per gram of the CMC and Ce is the equilibrium concentration of Cr(VI) in the bulk of the solution, in milligrams per gram and milligrams per liter.

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Figure 3. SEM appearance of CMC before (left) and after (right) adsorption with magnification factor of 400.

Figure 4. EDS analysis of CMC before (left) and after (right) adsorption. Table 1. Comparision of the First-Order and Second-Order Adsorption Rate Parameters Langergren adsorption and constants for second-order rate reaction

Langergren constants for first-order

initial Cr concentration (mg/L)

qe,exp(mg/g)

K1 (min-1)

qe,cal (mg/g)

R2

SSEa(%)

K2 (g/mg · min)

qe,cal(mg/g)

R2

SSEa(%)

50 100 125

53.4 64.5 69.6

0.059 0.056 0.053

64.6 77.6 83.2

0.9983 0.9922 0.9948

3.99 4.64 4.81

6.16 × 10-4 4.41 × 10-4 4.13 × 10-4

73.4 84.7 90.9

0.9776 0.9912 0.9952

7.07 7.14 7.53

a

The number of data points is 8.

Figure 5. FTIR spectra of Cr(VI) loaded CMC.

The constants am and b are characteristic parameters of the Langmuir equation, in milligrams per gram and liters per milligram, respectively.

The Langmuir isotherm (Ce/qe versus Ce) for the adsorption of Cr(VI) with a concentration of 100 mg/L from aqueous solutions onto CMC has been plotted in Figure 6. The values of am and b were found to be 69.4 mg/g and 1.24 L/g by leastsquares fitting. It has been reported that the modification by cross-linking only reduces the adsorption capacity,5,6,12,22 but the modification of chitosan by coated with magnetic fluids then cross-linked with epichlorohydrin seems not to reduce the overall adsorption capacity from these experiments. The goodness of fit is excellent for the CMC over the concentration range studied, with a correlation coefficient (R2) of 0.9985 for the initial Cr(VI) concentrations of 100 mg/L. Furthermore, the essential characteristics of the Langmuir isotherm can be described by a separation factor, which is defined by the following equation:27,29

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Figure 6. Langmuir plot of the CMC for Cr(VI).

Figure 8. Langergren sorption diagram of Cr(VI) on CMC, first-order equation.

Figure 7. Effects of contact time on the adsorption behavior.

1 RL ) 1 + bC0

Figure 9. Langergren sorption diagram of Cr(VI) on CMC, second-order equation.

(6)

The value of RL indicates the shape of the Langmuir isotherm and the nature of the adsorption process. It is considered to be a favorable process when the value is within the range 0-1. In our study, the values of RL calculated for the initial Cr(VI) concentrations was 0.89. Since the result is within the range of 0-1, the adsorption of Cr(VI) onto CMC appears to be a favorable process. 3.5. Adsorption Kinetics. The effect of the contact time on the amount of adsorption has been investigated over the range from 10 to 80 min with three initial Cr(VI) concentrations of 50, 100, and 125 mg/L, and the results are shown in Figure 7. As shown in Figure 7, the uptake capacity of Cr(VI) increases with longer contact time and attains equilibrium at about 60 min for the three initial Cr(VI) concentrations, implying that equilibrium has been reached. Therefore, the optimum contact time for adsorption of Cr(VI) appears to be 60 min. In order to examine the controlling mechanism for the adsorption process, kinetic models have been used to assess the experimental data. The first order rate equation of Langergren is one of the most widely used for the adsorption of a solute from a liquid solution.30-33 The first order rate equation may be represented as follows: log(qe - qt) ) -

K1t + log qe 2.303

(7)

Similarly, the second-order rate equation is given by 1 1 t ) t+ qt qe Kq2

(8)

2 e

Here qe and qt are the amounts of Cr(VI) adsorbed at equilibrium and at time t, in milligrams per gram, K1 and K2 are rate constants, in inverse minutes and grams per milligram minute, respectively.

Figure 10. Effect of recycling CMC on Cr(VI) adsorption.

The plots of log(qe - qt) versus t and t/qt versus t are shown in Figures 8 and 9, giving K1, K2, qe, and R2 for the Langergren adsorption and second-order rate equation, and the calculated results are listed in Table 1. The results of the kinetic parameters for Cr(VI) adsorption are also shown in Table 1. On the basis of the correlation coefficient and SSE, the adsorption of Cr(VI) appears to be well described by the first-order rate equation of Langergren. Slight differences in rate constants are observed among the three initial Cr(VI) concentrations. It seems that the Langergren rate constants for the adsorption of Cr(VI) are influenced slightly by the initial concentrations. The linearity is shown from the results in Figure 9 for the second-order equation, and from the higher values of the SEE(%) ) [(∑(qe,exp - qe,cal)2)/N]1/2 (7.07%, 7.14%, 7.53%) in Table 1 for the second-order equation, it appears that the adsorption process is less likely to be a secondorder one. Figure 8, however, shows more linear behavior. 3.6. CMC Regeneration Ability. The effect of recycling times of the CMC on the adsorption process was repeated 10 times, and the results are shown in Figure 10. It is shown in Figure 10 that the uptake capacity of Cr(VI) on the CMC decreased slowly with increasing cycle number. The percentage adsorption remained steady at 90% in the first

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four cycles, and then the uptake capacity of Cr(VI) decreased from the fifth cycle to the tenth cycle. At the end of ten regeneration cycles, the adsorption remained steady at 60% of the initial value. These results show that loaded CMC can be recycled for Cr(VI) adsorption with 0.1 mol/L hydrochloric acid solution, and the sorbent can be reused. The probable mechanism of the regeneration could be that in the first four cycles, both electrostatic and complexation reaction occurred between the hydrochloric acid solution and the metal ion; however, after four cycles, only electrostatic interaction prevailed. 4. Conclusions Cross-linked magnetic chitosan coated by magnetic fluids and cross-linked with epichlorohydrin was prepared and characterized in this work. The Cr(VI) adsorption behavior on the prepared CMC has been studied under various conditions of different solution pH values and adsorption contact times. The optimal adsorption conditions of Cr(VI) on CMC have been found to be as follows: pH of solution 4.0 and adsorption time of 60 min. Scanning electron microscope images of the CMC show that the reducing porosity on the surface after adsorption was linked to the adsorption of Cr(VI) into these pores. The Langmuir model was found to fit the experimental equilibrium data well, with correlation coefficients (R2) of 0.9985 for the initial Cr(VI) concentrations of 100 mg/L. The experimental data for the kinetics of adsorption were correlated well by the first-order Langergren rate equation. The loaded CMC could be regenerated with 0.1 mol/L hydrochloric acid solution and reused repeatedly for Cr(VI) adsorption for as many as ten cycles. Literature Cited (1) Meunier, N.; Drogui, P.; Montane, C.; Hausler, R.; Mercier, G.; Blais, J. Comparison between Electrocoagulation and Chemical Precipitation for Metals Removal from Acidic Soil Leachate. J. Hazard. Mater. 2006, 137, 581–590. (2) Rengaraj, S.; Yeon, K. H.; Moon, S. H. Removal of Chromium from Water and Wastewater by Ion Exchange Resins. J. Hazard. Mater 2001, B 87, 273–287. (3) Kamble, S. B.; Marathe, K. V. Membrane Characteristics and Fouling Study in MEUF for the Removal of Chromate Anions from Aqueous Streams. Sep. Sci. Technol. 2005, 40, 3051–3070. (4) Wang, F.; Xie, Z.; Xie, L. Preparation of Polyamide Microcapsules and Their Application in Treating Industrial Wastewater Containing Cr6+. Chem. Res. 2002, 13, 36–38 (in Chinese with English abstract). (5) Ngah, W. S.; Ghani, S. A.; Kamari, A. Adsorption Behaviour of Fe (II) and Fe (III) Ions in Aqueous Solution on Chitosan and Cross-linked Chitosan Beads. Bioresour. Technol. 2005, 96, 443–450. (6) Modrzejewska, Z.; Sujka, W.; Dorabialska, M.; Zarzycki, R. Adsorption of Cr(VI) on Cross-linked Chitosan Beads. Sep. Sci. Technol. 2006, 41, 111–122. (7) Jaros, K.; Kaminski, W.; Albinska, J.; Nowak, U. Removal of Heavy Metal Ions: Copper, Zinc and Chromium from Water on Chitosan Beads. EnViron. Prot. Eng. 2005, 31, 153–162. (8) Zarzycki, R.; Sujka, W.; Dorabialska, M.; Modrzejewska, Z. Adsorption of Cr(VI) on Chitosan Beads. Chem. Inz. Ekol. 2002, 9, 1561– 1570. (9) Baba, Y.; Aoya, Y.; Ohe, K.; Nakamura, S.; Ohshima, T. Adsorptive Removal of Copper(II) on N-methylene Phosphonic Chitosan Derivative. J. Chem. Eng. 2005, 38, 887–893. (10) Cestari, A. R.; Vieira, E. F. S.; Oliveira, I. A.; Bruns, R. E. The Removal of Cu (II) and Co (II) from Aqueous Solutions Using Cross-linked Chitosan-Evaluation by the Factorial Design Methodology. J. Hazard. Mater. 2007, 143, 8–16. (11) Mallika, P.; Himabindu, A.; Shailaja, D. Modification of Chitosan Towards a Biomaterial with Improved Physico-chemical Properties. J. Appl. Polym. Sci. 2006, 101, 63–69.

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ReceiVed for reView May 22, 2008 ReVised manuscript receiVed November 28, 2008 Accepted December 4, 2008 IE800814H