Nanoporous Magnetic Cellulose–Chitosan Composite Microspheres

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Nanoporous Magnetic Cellulose−Chitosan Composite Microspheres: Preparation, Characterization, and Application for Cu(II) Adsorption Shuai Peng,† Hecheng Meng,‡ Yong Ouyang,† and Jie Chang*,† †

The Key Laboratory of Fuel Cell Technology of Guangdong Province, School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, China ‡ College of Light Industry and Food Sciences, South China University of Technology, Guangzhou 510640, China ABSTRACT: Novel nanoporous magnetic cellulose−chitosan composite microspheres (NMCMs) were prepared by sol−gel transition method using ionic liquids as solvent for the sorption of Cu(II). The composite microspheres were studied by scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), X-ray diffraction (XRD), and vibrating sample magnetometry (VSM). Subsequelty, the adsorption of Cu(II) to NMCMs was investigated systematically with varried parameters such as pH, contact time, and initial concentration. Results revealed that the composite microspheres exhibited efficient adsorption capacity of Cu(II) from aqueous solution, due to their favorable chelating groups in structure. The adsorption process was best described by a pseudo-second-order kinetic model, while isotherm modeling revealed that the Langmuir equation better describe the adsorption of Cu(II) on NMCMs as compared to Freundlich model. Moreover, the loaded NMCMs can be easily regenerated with HCl and reused repeatedly for Cu(II) adsorption up to five cycles. The environmental friendly microspheres were expected to be a promising candidate for future practical use in heavy metal ions removal. adsorption.7 Chitosan, poly[β-(1-4)-linked-2-amino-2-deoxy-Dglucose], is the N-deacetylated product of chitin, which is a major component of arthropod and crustacean shells.8 Chitosan has been reported to have the highest sorption capacity for Cu(II) or other metal ions, with the abundant amino and hydroxyl groups in chitosan acting as the chelation sites.9 However, chitosan still presents considerable disadvantages, such as weak mechanical strength, poor chemical resistance, and high crystallinity, which have limited its use as an effective sorbent.10 In order to overcome these limitations and other difficulties, we hereby explore a novel nanoporous magnetic composite microsphere by using cellulose as a blending polymer for chitosan, without further surface modifications of the base microsphere. This newly developed bioadsorbent not only has high adsorption capacity for effective removal of Cu(II) but also possesses large surface area, desired mechanical strength, and chemical stability, minimized secondary wastes, and easy recovery from aqueous solutions by applying a magnet. Until now, nanoporous magnetic cellulose−chitosan composite microspheres (NMCMs) fabricated via an environmentfriendly process have been reported scarcely. In this research, NMCMs are prepared by sol−gel transition method using ionic liquids as solvent for the sorption of Cu(II). Subsequently, the removal Cu(II) from aqueous solution with various effecting parameters such as contact time, initial concentration, and pH

1. INTRODUCTION During the past few years, environmental contamination with heavy metal ions has been a public concern owing to their longterm risk to ecosystems and humans. Heavy metals are highly toxic even at very low concentrations and can accumulate in living organisms, causing several disorders and diseases.1 It is well-known that copper compounds are widely used in many industrial processes, such as the manufacturing of fungicides, metal plating, wood pulp production, antifouling paints, electric devices, and so on.2 The industrial waste effluents contain a large amount of copper pollutants, which are harmful to ecological systems and human health. It has been reported that ingestion of copper at certain amount is responsible for temporary stomach and intestinal disorders, kidney or liver damage, and even cancer.3 Therefore, many techniques have been developed for heavy metal ions removal, including precipitation, coagulation, ion exchange, solvent extraction, and membrane filtration.4 However, these methods are either too costly or inefficient in removing heavy metal ions from dilute solutions.5 Recently, the application of magnetic bioadsorbent technology to solve environmental problems has received considerable attention, because of their easy availability, low cost, presence of a variety of functionalities, and nontoxic nature. Moreover, the bioadsorbent can be separated from the system by a simple magnetic separation after the adsorption process.6 Among various biopolymers available, cellulose and chitosan are wellknown for their attractive properties such as biocompatibility, biodegradability, and thermal and chemical stability. Cellulose, a linear 1,4-β-glucan, is the most abundant natural renewable polymer and exhibits unique physical and mechanical properties that are useful in many applications. Cellulose-based beads, films, and resins have been widely applied in heavy metals © 2014 American Chemical Society

Received: Revised: Accepted: Published: 2106

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mL Cu(II) solutions and 50 mg NMCMs were shaken at room temperature for some time. The pH of the solution was adjusted with 1 mol/L HCl solution. The initial and final solutions through magnetic separation after adsorption were analyzed by the atomic absorption spectrophotometer (Type Z5000, Hitachi, Japan) to get the concentration of Cu(II). Three replicates were used for each adsorption datum, and variation among these replicates found to be less than 0.5%. The amount of adsorption (q) was defined as the following equation:

were investigated to determine adsorption behavior and performance of NMCMs.

2. EXPERIMENTAL SECTION 2.1. Materials. Cellulose was provided by State Key Laboratory of Pulp and Paper Engineering, South China University of Technology. Chitosan (with a deacetylation degree of 90%) was purchased from Sinopharm Chemical Reagent. 1-Butyl-3-methylimidazolium chloride ([BMIM]Cl, ionic liquid) was purchased from SiYu Co. Ltd. (Lanzhou, China). Sodium sulfate and copper sulfate were obtained from Guanghua Sci-Techno Co. Ltd. (Guangdong, China). FeCl2· 4H2O and FeCl3·6H2O were purchased from Yuqiao Chemical Company (Guangdong, China). 2.2. Preparation of NMCMs. NMCMs were obtained by an adapted method.11 Briefly, Fe3O4 nanoparticles were firstly synthesized by chemical coprecipitation method under alkaline conditions.12 Cellulose and chitosan were dissolved in [BMIM] Cl at 100 °C for 30 min to obtain a 7 wt % (composition ratio of cellulose: chitosan was 1:2) solution. Then, magnetic fluid was immediately added to the solution by vigorous agitation for 15 min. Subsequently, 20 mL well-mixed solution was emulsified with a solution containing 80 mL vacuum pump oil and 4 mL Tween80 by stirring at 1000 rpm in an 100 °C oil bath. Another 100 mL vacuum pump oil with 5 mL Tween80 was emulsified with 80 mL 0.4 mol L−1 sodium sulfate solution, and this mixture was added to the previous cellulose−chitosan emulsion. By slowly decreasing the temperature, the composite microspheres were obtained. The final NMCMs were washed three times with deionized water followed by washing thoroughly with ethanol. Finally, the products were then stored in DI water for further use and the yield of the microspheres production was above 95%. 2.3. Characterization. NMCMs were freeze-dried by using a lyophilizer (Labogene CoolSafe 55-4, Denmark). Fouriertransform infrared (FTIR) spectra of the microspheres were recorded on an Vector33 FTIR spectrophotometer (Bruker Co. Ltd., Germany). The samples were prepared by using the KBrdisk method. XRD patterns were recorded on the X-ray diffractometer (D8-Advance, Bruker, Germany) with Cu Kα radiation (λ = 1.5418 nm) at 40 kv and 40 mA in the range 5− 60° at room temperature. The morphology and structure of the microspheres was determined with scanning electron microscopy (SEM, S-3700, Hitachi, Japan and LEO 1530 VP, LEO. Ltd., Germany) with an accelerating voltage of 10 kV. The definite size distribution of NMCMs was determined with a Mastersizer 2000 laser particle size analyzer (Malvern, U.K.). The porous properties of the microspheres were investigated with a BET analyzer (Micromeritics Instrument, Model Tristar II 3020). Thermal analysis (thermogravimetric analysis; TGA) was conducted on Instruments TGA (Q500, American). Samples were heated from room temperature to 700 °C at a rate of 10 °C/min under constant nitrogen purging. The magnetic properties of the composite microspheres were measured via vibrating sample magnetometry (VSM, Lake Shore, 7410, U.S.A.) at room temperature. The surface elemental composition of NMCMs were analyzed by X-ray photoelectron spectrometer (XPS, Axis Ultra DLD, Kratos, Britain). A standard Al Kα excitation source (1.2 kV, 10 mA) was employed. 2.4. Adsorption of Cu(II) on NMCMs. Batch adsorption experiments were carried out to evaluate adsorption capacities of NMCMs. A series of conical flasks (25 mL) containing 10

q = (C0 − Ce)

V M

(1)

where q is the amount of Cu(II) adsorbed onto the bioadsorbents (mg/g), C0 and Ce are the initial and equilibrium concentrations of Cu(II) (mg/L), respectively, V is the volume of Cu(II) solution (L), and M is the weight of the bioadsorbents (g). The scheme of the preparation for NMCMs and application for Cu(II) adsorption is shown in Scheme 1. Scheme 1. Schematic Illustration of the Preparation for NMCMs and Application for Cu(II) Adsorption

2.5. Desorption and Reusability. For desorption studies, the bioadsorbents loaded with Cu(II) were placed in the hydrochloric acid with varying pH values and shaken at room temperature for 24 h. The unabsorbed Cu(II) after magnetic separation was removed by gentle washing with distilled water. The desorption ratio (Ds) was defined as the following equation: DS =

Ce′ 100% (C0 − Ce)

(2)

where C0 and Ce are the initial and equilibrium concentrations of Cu(II) (mg/L), Ce′ is the equilibrium concentration of Cu(II) (mg/L) in the elution medium. To investigate the reusability of the NMCMs on Cu(II) adsorption, the regenerated NMCMs were subjected to further cycles of adsorption and desorption processes, and the adsorption amounts and desorption ratios were calculated. 2107

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Figure 1. SEM image of NMCMs at lower magnification (a) and cross-section image (b).

Figure 2. Paticle size distribution (a) and pore size distribution (b) of NMCMs.

3. RESULTS AND DISCUSSION 3.1. Structure and Properties of NMCMs. 3.1.1. SEM. Figure 1 shows the SEM images showing the surface

Figure 4. XRD spectra of (a) naked Fe3O4, (b) regenerated chitosan− cellulose microspheres, and (c) NMCMs.

Figure 3. FTIR spectra of (a) cellulose, (b) chitosan, (c) naked Fe3O4, and (d) NMCMs.

morphology (a) and interior structure (b) of the obtained NMCMs. The SEM photographs of the composite microspheres at lower magnification exhibited irregular spheres structure with the mean diameter of about 10 μm (Figure 1a). Cellulose-based support fabricated by Luo and co-workers,13 which, composed of just cellulose and magnetic nanoparticles without chitosan, exhibited an excellent spherical shape with a smooth surface. Therefore, the displayed rough surface of NMCM is due to the addition of chitosan.14 The cross-section structure of NMCM shown in Figure 1b clearly exhibited the interior structure of NMCM, in which nanoporous structure

Figure 5. Hysteresis loop of (a) naked Fe3O4 and (b) NMMCs.

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corresponded to the stretching vibration of hydroxyl groups. The absorption bands at 1655 and 1590 cm−1 corresponded to the carbonyl stretch and NH bending in amides (HN CO) of chitosan, respectively (Figure 3b). The new absorption peak at around 580 cm−1 in the spectrum of NMCM (Figure 3d) was assigned to the characteristic absorption peak of Fe3O4. The weakened band at 1590 cm−1 was due to the strong hydrogen bonding among Fe3O4, chitosan, and cellulose.16 That high stability is of great importance for the applications of the NMCM. 3.1.3. XRD. The powder X-ray diffraction (XRD) patterns of (a) naked Fe3O4, (b) regenerated chitosan−cellulose microspheres, and (c) NMCM are presented in Figure 4. The characteristic XRD peaks for Fe3O4 (2θ = 30.1°, 35.5°, 43.1°, 53.4°, and 57.0°), marked by their indices ((220), (311), (400), (422), (511), and (440)) were observed for both Fe3O4 (Figure 4a) and NMCM (Figure 4c). These peaks were consistent with the database in JCPDS file (PDF No. 65-3107), which was the standard pattern for crystalline magnetite with spinel structure.17 The typical diffraction pattern of regenerated chitosan−cellulose microspheres still existed, as seen in Figure 4c, only with exhibited lower intensity. It can be explained that the crystallization of chitosan was suppressed due to the formation of hydrogen bonds between chitosan and cellulose and the miscibility between two polymers. The analogous results have also been observed by other investigators.16 3.1.4. VSM. The magnetic hysteresis loop of NMCMs is demonstrated in Figure 5b. As can be seen, the saturation magnetization of NMCMs was 30.1 emu g−1. The result indicated that the composite microspheres possessed a sensitive magnetic responsiveness, which is important for their recovery.

Figure 6. TGA curves of (a) cellulose, (b) chitosan, (c) regenerated chitosan−cellulose microspheres, (d) NMCMs, and (e) naked Fe3O4.

and Fe3O4 nanoparticles (10−20 nm) were observed. It was also proved that the nanoporous magnetic cellulose−chitosan composite microspheres have been fabricated successfully. The size distributions of the microspheres were also measured (Figure 2). It was found that the mean particle size of the microspheres prepared was with a narrow size distribution. It is well-known that for smaller particles the larger surface-to-volume ratios arouse larger adsorption capacity.15 Pore size distributions were calculated to further elucidate the structural characteristics of NMCM. Figure 2b shows the pore size of NMCMs ranging from 10 to 200 nm with peak value of the pore diameter of 80 nm. The SBET and pore volume of the dry NMCMs are 102.3 m2/g and 4.1 cm3/g, respectively. 3.1.2. FTIR. FTIR spectra of the original (a) cellulose, (b) chitosan, (c) naked Fe3O4, and (d)NMCM were examined and are shown in Figure 3. The peaks at 3300−3450 cm−1

Figure 7. (a) Effect of pH on adsorption. Conditions: initial concentration, 150 mg/L; contact time, 24 h; room temperature. (b) Effect of contact time on adsorption. Conditions: initial concentration, 140 mg/L; pH 5.0; room temperature. (c) Effect of initial concentration on adsorption. Conditions: pH 5.0; contact time, 20 h; room temperature. 2109

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Figure 8. Model fit of kinetic data. (a) pseudo-first-order model, (b) pseudo-second-order model, and (c) intraparticle diffusion model.

Table 1. Best-Fit Model Parameters for Copper Adsorption on NMCMs param. 1 first-order model second-order model intraparticle model Langmuir Freundlich

param. 2 −1

R2

qe(cal)(mg/g) = 34.43

k1 (min ) = 0.002303

0.959

qe(cal)(mg/g) = 71.42

k2 (g/(mg min)) = 0.0001949 kp (mg/g min0.5) = 1.278 b = 0.05856 n = 3.29

0.999

qm (mg/g) = 75.82 KF (mg1−(1/n) L1/n g−1) = 15.06

0.854 0.999 0.945

3.1.5. TGA. The TG curves of (a) cellulose, (b) chitosan, (c) regenerated chitosan−cellulose microspheres, (d) NMCMs, and (e) naked Fe3O4 are shown in Figure 6. It is seen that regenerated chitosan−cellulose microspheres have a lower onset temperature of decomposition than that of pure cellulose and chitosan but give a higher char yield, indicating a higher residual masses after the decomposition. Similar observations have been reported by Swatloski et al.18 The different TGA

Figure 10. Desorption of Cu(II) by HCl.

results may be caused by the changes of degree of crystallinity and morphologies of cellulose or chitosan before and after regeneration. Moreover, the NMCMs exhibited an onset of weight loss at around 230 °C, which was higher than that of the regenerated chitosan−cellulose microspheres. This means that

Figure 9. (a) Linear plot of Langmuir adsorption isotherm and (b) Linear plot of Freundlich adsorption isotherm. 2110

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the most part of the incremental residual components in NMCMs, indicating Fe3O4 particles have been embedded in NMCMs. It was in agreement with the above-mentioned discussion on the SEM, FTIR, XRD, and VSM analyses. 3.2. Adsorption of Cu(II) on NMCMs. 3.2.1. Effect of pH. The removal of copper from the aqueous solution was highly dependent on the pH of solution. The pH values selected in the experiments were in the range 1.0−6.0. As shown in Figure 7a, the uptake capacity of Cu(II) increased when the solution pH was raised from 1.0 to 5.0. A maximum adsorption capacity was achieved at pH 5.0, at which the adsorbed quantity was 65.8 mg/g and decreased to 61.4 mg/g when pH was increased to 6.0. This can be explained by the fact that, at a lower pH, the amine groups on NMCMs surfaces can easily protonate, inducing an electrostatic repulsion of Cu(II), while Cu(II) can be hydrolyzed in the pH range 6.0−7.0.19 3.2.2. Effect of Contact Time. Adsorption time is an important parameter because it can reflect the adsorption kinetics of an adsorbent. Figure 7b shows the contact time profile of Cu(II) removal with NMCMs. The adsorption amount of Cu(II) increased sharply within the first 1.5 h of contact time, after which the increase slowed down and the adsorption amount reached saturation after 20 h. The explanation may be that the adsorption sites were void and that adsorbates easily interacted with these sites during the initial stage. With time increasing, the remaining vacant sites are not easily occupied, until the system reached equilibrium.20 3.2.3. Effect of Initial Concentration of Cu(II). Adsorption experiments were performed at different initial Cu(II) concentration ranging from 10 to 150 mg/L with fixed contact time (20 h) and pH 5.0 (Figure 7c). Obviously, the uptake of Cu(II) gradually increased as the initial concentration increased until the saturation point was reached at 150 mg/L, thereafter reaching the plateau. The high adsorption capacity can be attributed to the structure of the adsorbents. 3.3. Adsorption Kinetics. In order to investigate the controlling mechanism of adsorption processes, the adsorption/time datas obtained were applied to three kinetic models, including pseudo-first-order model, pseudo-second-order model, and intraparticle diffusion model.21,22 The pseudo-first-order equation was represented by

Figure 11. Effect of recycling NMCMs on Cu(II) adsorption.

Figure 12. Effect of common ions (150 and 300 mg/L) on the adsorption of Cu(II) (initial concentration 150 mg/L; 10 mL; pH 5.0; room temperature) by NMCMs (50 mg).

Table 2. Comparison of the Maximum Cu(II) Removal Capacity and Adsorption Time with Some Chitosan-Based Adsorbents in References adsorbent

qmax(mg/g)

t

refs

chitosan−sand chitosan−cellulose composite cellulose−chitin beads chitosan−cellulose acetate membranes chitosan−cellulose beads

8.18 26.5 16.54 48.2 53.3

4h 10 h 5h 70 min 15 h

28 29 30 31 14

log(qe − qt) = log qe −

k1 t 2.303

(3)

where qe and qt (mg/g) are the amount of Cu(II) adsorbed on NMCMs at equilibrium and at a given time t, respectively; k1 is

NMCMs have a higher thermal stability due to the incorporation of Fe3O4 nanoparticles. Obviously, Fe3O4 consits

Figure 13. XPS spectra of the NMCMs before (a) and after (b) Cu(II) adsorption. 2111

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rate constant (min−1) of Lagergren-first-order kinetic model for adsorption. The pseudo-second-order equation can be expressed as t 1 t = + qt qe k 2qe2

application value of the bioadsorbent.27 From Figure 11, the adsorption capacities for Cu(II) did not show any decrease after five cycles. Therefore, the reuse study of adsorbent indicated that NMCMs were efficient and stable adsorbent for Cu(II) removal. 3.6. Effect of Common Coexisting Ions. The common ions that are normally present in water, viz., Na+, Ca2+, K+, Cl−, NO3−, and HCO3− ions, were applied in the coexisting ions investigation. It is shown in Figure 12 that the influences of Na+, Ca2+, and K+ on the adsorption of Cu(II) were rather insignificant. Similarly, the competitive influence of Cl−, NO3−, and HCO3− on Cu(II) adsorption can be ignored. Thus, the common coexisting ions almost had no negative effect on the Cu(II) adsorption of NMCMs, which also decided the possible of practical application. 3.7. Comparison with Other Adsorbents. A comparison of the maximum Cu(II) removal capacity and adsorption time with some chitosan-based adsorbents reported in the literature was presented in Table 2. Clearly, NMCMs possess an enhanced adsorption capacity for Cu(II). Furthermore, NMCMs showed a fast Cu(II) removal performance to reach 61% of the complete removal within 1.5 h, which was much quicker than some other adsorbents. In addition, the used NMCMs could be regenerated easily and can be simply separated from aqueous solution by a magnetic field. Thus, because of the good adsorption performance and the facile method of regeneration, the asprepared NMCMs bioadsorbent is a highly promising material that can be used for application in heavy-metal removal. 3.8. Adsorption Mechanisms. To further investigate the adsorption mechanism, XPS spectra were used to analyze the bioadsorbents before and after adsorbing Cu(II). Figure 13 shows the typical results of XPS spectra for NMCMs before (a) and after (b) Cu(II) adsorption. It is clear that a new peak at the binging energy (BE) of 933.2 eV appeared after Cu(II) adsorption, which represented the oxidation state +2 for the Cu 2p3/2 orbital. There was one peak in the N 1s spectrum at a BE of 396.9 eV (Figure 13a), and a peak was observed at a BE of 404.8 eV (Figure 13b). The shifts might be explained as a lone pair of electrons in the nitrogen atom was donated to the shared bond between N and Cu(II). Therefore, the XPS spectra provide direct evidence of Cu(II) binding to nitrogen atoms. The O 1s BEs did not show significant changes before and after Cu(II) adsorption (less than 0.5 eV was considered to be insignificant). Therefore, it may be speculated that the hydroxyl groups were not involved in the chemical adsorption of Cu(II).

(4)

where k2 (g/(mg min)) is the adsorption rate constant of pseudo-second-order. The intraparticle diffusion model was given by qe = k pt 1/2 + C

(5) 0.5

where kp (mg/g min ) is the intraparticle diffusion rate constant and C of adsorption constant is the intercept. The results of the kinetics were obtained by analyzing Figure 8. Based on the obtained correlation coefficients, the pseudosecond-order kinetic equation was feasible to describe the adsorption process, as shown in Table 1, suggesting that the overall process seemed to be controlled by chemisorption and no involvement of a mass transfer in solution.21 3.4. Adsorption Isotherms. For interpretation of the adsorption data, Langmuir and Freundlich adsorption isotherm models were used to describe the equilibrium adsorption for Cu(II) on NMCMs. Linearized Langmuir equation is represented as follows:23 ce c 1 = + e qe bqm qm (6) where qe and ce are the amount adsorbed (mg/g) and the adsorbate concentration in solution (mg/L), both at equilibrium. b (L/mg) is a constant related to the heat of adsorption and qm (mg/g) is the maximum adsorption capacity for monolayer formation on adsorbent. The Freundlich isotherm is an empirical equation employed to describe heterogeneous systems24 ln qe = ln KF +

1 ln Ce n

(7)

where KF is Freundlich constant and n is the heterogeneity factor. The theoretical parameters of adsorption isotherms along with regression coefficients are summarized in Table 1. For the two studied systems (Figure 9), the Langmuir isotherm correlated better (R2 > 0.999) than Freundlich with the experimental data from adsorption equilibrium of Cu(II) by NMCMs, suggested a monolayer adsorption. The maximum adsorption values was in good accordance with experimentally obtained values. The fact that the Langmuir isotherm model assumes monolayer adsorption on a surface with a finite number of identical sites, that all sites are energetically equivalent and that there is no interaction between adsorbed molecules. It has been reported that the values of n in the range 1−10 represent good adsorption.25 In the present work, the exponent was 1 < n < 10, predicting the adsorption system is “favorable”. 3.5. Desorption and Reusability. To evaluate the property of desorption, solutions with different pH were used as eluents. The results (Figure 10) showed that the solution with pH 1 can efficiently remove the adsorbed Cu(II), suggesting that NMCMs exhibited poor adsorption capacity in an acidic environment, which is in accordance with the similar result reported by other investigators.26 The regeneration character was an important factor for evaluating potential

4. CONCLUSIONS In this study, novel magnetic cellulose−chitosan composite microspheres were synthesized by a method combining the emulsification procedure and cellulose−chitosan regeneration from ionic liquid without further surface modifications. The composite microspheres exhibited porous structure, large surface area, and affinity on metals, leading to the efficient uptake capacity of Cu(II). The maximum adsorption capacity for Cu(II) reached 65.8 mg/g when the initial concentration of Cu(II) was 150 mg/L within 20 h. Additionally, the common coexisting ions almost had no negative effect on the Cu(II) adsorption of NMCMs. The kinetics adsorption data were well fitted to the pseudo-second-order kinetic model, and the adsorption equilibrium could be well described by Langmuir adsorption isotherms, namely monolayer adsorption. Further2112

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more, the magnetic composite microspheres can be easily regenerated by hydrochloric acid solution without any loss in adsorption capacity after five cycles. The biocompatible material, from the environment friendly preparation and component, is proposed to have promising potential for removal of heavy metals from aqueous solution.

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AUTHOR INFORMATION

Corresponding Author

*Tel/Fax: +86 20 87112448. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the financial support from the National Basic Research Program of China (2013CB228104) and National High-tech R&D Program (863 Program) (2012AA051801).

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REFERENCES

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dx.doi.org/10.1021/ie402855t | Ind. Eng. Chem. Res. 2014, 53, 2106−2113