Facile Preparation of Ion-Imprinted Chitosan ... - ACS Publications

Research Article pubs.acs.org/journal/ascecg

Facile Preparation of Ion-Imprinted Chitosan Microspheres Enwrapping Fe3O4 and Graphene Oxide by Inverse Suspension Cross-Linking for Highly Selective Removal of Copper(II) Delong Kong, Nian Wang, Ning Qiao, Qi Wang, Zhuo Wang, Zhiyong Zhou,* and Zhongqi Ren* College of Chemical Engineering, Beijing University of Chemical Technology, No. 15, North 3rd Ring Road East, Beijing 100029, P.R. China S Supporting Information *

ABSTRACT: A novel magnetic copper imprinted chitosan/ graphene oxide composite biomaterial was prepared by the combination of ion imprinting and inverse suspension crosslinking for selective adsorption of Cu(II) from aqueous solution. High adsorption capacity for copper was obtained with a low level cross-linking and the addition of graphene oxide and triglycine in the preparing process. The prepared ion-imprinted magnetic chitosan polymer microsphere (CSIIP) was characterized by FT-IR, TGA, SEM, and EDX. The results indicated that the CS-IIP was prepared successfully and showed good thermostability. Effects of different experimental conditions like pH value, contact time, and Cu(II) concentration on the adsorption capacity were investigated. The adsorption process follows the Freundlich isotherm equation and the pseudo-second-order kinetic model. The highest adsorption capacity of CS-IIP was 132 mg g−1. The calculation of selective factors and relative selectivity factors of CS-IIP for Cu2+/M2+ (M = Zn, Ni, Co, or Cd) was studied. Moreover, the reusability and stability of CS-IIP were investigated too. KEYWORDS: Cu(II), Ion-imprinted, Graphene oxide, Triglycine, Chitosan

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INTRODUCTION

high adsorption capacity, good selectivity, and excellent separation property. Graphene is a fascinating carbon material with excellent mechanical and physicochemical properties. Since graphene oxide (GO) as a kind of important derivative of graphene10 has high surface area and an abundance of oxygen-containing functional groups,11 it has been recognized as a promising material for efficient removal of heavy metal ions. Chitosan-GO composites combining the excellent adsorption property of chitosan and the inherent property of GO have been used as bioadsorbent and biosensor through self-assembly, controlled surface deposition, and direct covalent attachment methods.12,13 The combination of GO with chitosan not only improves the adsorption capacity and mechanical property of chitosan but also solves the difficulty of recovering GO. Ion-imprinted polymer (IIP) is a kind of promising adsorbents for removal and enrichment of heavy metal ions because of their good selectivity.14−16 Recently, a great deal of attention has been focused on the application of IIPs for the preparation of bioadsorbents.17,18 Different polymeric biomaterials such as chitosan and sodium alginate have been

The problem of heavy metal pollution has become a big threat to our ecosystem and human health.1 Various technologies like chemical precipitation,2 electrodepositions, liquid−liquid extraction,3 and membrane separation4,5 have been widely used to remove heavy metal ions. However, these methods are usually ineffective or expensive. Adsorption is attracting increasing attention owing to easy operation and high efficiency. Bioadsorption based on biomass has drawn increasing attention because of biocompatibility, biodegradability, and renewability.6 In particular, polysaccharides such as chitosan (CS) have attracted significant interest as effective adsorbents for removal of heavy metal ions, whereas some disadvantages for CS significantly limit its practical applications.6,7 First, the poor mechanical performance and dissolution of chitosan in acidic conditions result in a bad regeneration ability. Though cross-linking can effectively improve the mechanical strength and decrease the solubility of chitosan in acid solution, it decreases the adsorption capacity.8 Furthermore, the recovery of powdery adsorbents is difficult, and these traditional materials are not suitable for packing in a column for solid phase extraction.9 In addition, most of these materials cannot selectively adsorb target metal. Therefore, it is very urgent to synthesize a novel chitosan-based adsorbent that can provide © 2017 American Chemical Society

Received: June 2, 2017 Revised: June 21, 2017 Published: July 1, 2017 7401

DOI: 10.1021/acssuschemeng.7b01761 ACS Sustainable Chem. Eng. 2017, 5, 7401−7409

Research Article

ACS Sustainable Chemistry & Engineering

Figure 1. Scheme of the whole experimental process and corresponding mechanism.

successfully used to prepare IIPs that can uptake different target metal ions.19,20 Additionally, a kind of separation technologies called magnetic assisted separation21−24 is considered as an effective method to realize the separation of solid−liquid mixtures. The novel adsorbents prepared by the combination of the ion imprinting method and magnetic separation technology can solve the difficulty of recovering conventional materials from the adsorption solution. In this study, a copper-ion-imprinted biomaterial composed of chitosan enwrapping nanosized Fe3O4 and graphene oxide was prepared for the first time by a novel technique combining ion imprinting and inverse suspension cross-linking. In this case, no radical polymerization and no extra grafting reaction were needed, and a simple cross-linking step could be finished by the condensation reaction between amino group and aldehyde group. Cross-linking and grafting reactions were performed at the same time, that is, the triglycine was grafted to the surface of chitosan in the cross-linking imprinted process.

To the best of our knowledge, bioadsorbents obtained by grafting triglycine on CS have rarely been reported. The goal of this work was to synthesize the ion-imprinted magnetic chitosan polymer microsphere (CS-IIP) with high adsorption capacity, excellent separation properties, and good adsorption selectivity for copper. FT-IR, SEM, EDX, and TGA were used for the characterization of the CS-IIPs. Effects of initial pH, initial Cu2+ concentration, and contact time on the adsorption capacity were investigated. Different kinetic and isotherm models were used to study the thermodynamics and kinetics behaviors of the adsorption process. The reusability of the prepared sorbent was also studied. This facile method also provides much more technological support for preparing linear or branched polymer cross-linking imprinted microspheres.

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EXPERIMENTAL SECTION

Materials. Powdery chitosan (CS) was obtained from the Sinopharm Group (Beijing, China) and used without further 7402


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ACS Sustainable Chemistry & Engineering purification. GO (1−3 μm) was purchased from JC Nano Co. Ltd. (Jiangsu, China). Magnetic Fe3O4 (20−30 nm outer diameter; 98% purity) was purchased from Tong Ren Wei Ye Technology Co. Ltd. (Shijiazhuang, China). Glutaraldehyde was obtained from Tianjian Fine Chemical Industry Co. Ltd. (Tianjin, China). (CuNO3·3H2O) and other reagents were of analytical grade. Characterization. FT-IR spectra were conducted on an FT-IR spectrometer (UV-8400, Shimadzu, Japan) at room temperature. The particle morphology was examined using SEM (S-4700, Hitachi Ltd.) TGA was performed using a thermal analyzer (TG209C, Netzsch, Germany) in a dynamic N2 atmosphere from 25 to 800 °C. Energydispersive X-ray spectroscopy (EDX; S-4700 SEM-EDS) was used to detect copper in the IIP before and after elution. Preparation Process of CS-IIP. The entire process and corresponding mechanism are shown in Figure 1. The detail preparation process is shown in the Supporting Information.

chelation between copper and the N atom; compared to CS-IIP after elution with CS-NIP, the NH2 absorption peak appeared. The FT-IR analysis indicated that the composite ion-imprinted materials were prepared successfully. SEM. SEM was used to evaluate the morphology of CS-IIP. Figures 3(a), 3(b), and 3(c) show that CS-IIP is an almost spherical particle, the size of the particles is about in the range of 5−15 μm, and it can be suitable for packing in a column for applications. Moreover, the irregular fold and lamellar structure surface of the CS-IIP (Figures 3(c) and 3(d)) could be observed, due to the formation of the cross-linking network on the surface and GO enwrapped in microspheres. Degree fold could be observed on the surface of CS-IIP, which would benefit the increase of adsorption ability. EDX. The composition of CS-IIP was determined by EDX analysis. The results of EDX spectrometry are shown in Figure 4. C, N, Fe, O, and Cu were present in the structure at 43.24, 12.46, 7.22, 34.16, and 2.93 wt %, respectively. Although these values were not accurate owing to the presence of C in the tape and holder of the sample, Cu was present in IIP before elution and was not present in IIP after elution, which showed that the Cu(II) was eluted successfully with 0.2 M HCl solution. In addition, the high content of O and N could provide more binding sites and could bring a good adsorption capacity for copper. TGA. The weight loss of CS-IIPs is shown in Figure 5. An approximately 10% weight loss from 24 to 160 °C could be observed, which corresponds to the loss of water in CS-IIPs. A huge weight loss of 60% from 160 to 780 °C corresponds to the decomposition of the biomass and triglycine. The TGA indicated that the composite materials were thermally stable. Effect of an Additional Amount of the Cross-Linking Agent on the Adsorption Performance of CS-IIP. As we all know, the cross-linking agent plays an important role in the preparation process of IIPs. Several properties of chitosan-like mechanical strength will be greatly improved after cross-linking with glutaraldehyde. Therefore, the effect of an additional amount of glutaraldehyde on the adsorption performance of CS-IIPs was studied. Preparation conditions: GO 60 mg, Cu(II) 250 mg, a certain concentration of glutaraldehyde solution containing 0.1 g of triglycine. In this study, the different concentrations of the glutaraldehyde solution containing triglycine (5 mL) were used, and the results are given in this section. Figure 6 shows that the absorption capacity of the prepared CS-IIP increases with increasing the concentration of glutaraldehyde from 2 to 4 wt %. When the concentration of glutaraldehyde solution is higher than 4 wt %, the adsorption capacity of the CS-IIPs decreases. The maximum adsorption capacity was obtained using 4 wt % glutaraldehyde solution. This could be explained by the fact that the too low cross-linking degree would not allow CS-IIP to maintain stable cavity configurations. However, the stiffness of the network would be increased, and the number of recognition sites per unit mass of CS-IIP would be reduced with an excessive amount of cross-linkers. Therefore, wt % was selected as the optimal additional amount of the cross-linking agent. Effect of an Additional Amount of Cu(II) on the Adsorption Performance of the CS-IIP. The addition of a template ion will be favorable for the protection of the adsorption functional group, such as the active amine and hydroxyl groups, and can also modify the structure during the cross-linking process. Preparation conditions: GO 60 mg, 4 wt % glutaraldehyde solution containing 0.1 g of triglycine. The

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RESULTS AND DISCUSSION FT-IR. FT-IR spectroscopy analyses were used to evaluate the preparation process of composite ion-imprinted materials

Figure 2. FT-IR spectra of different materials.

and the insertion of Fe3O4 and GO in the composite imprinted materials. The FT-IR results of GO, Fe3O4, CS, CS-IIP, and CS-NIP are shown in Figure 2. The adsorption peaks at 1596, 1157, and 1079 cm−1 in the spectrum of CS correspond to −NH2 bending, asymmetric stretching of C−O−C, and stretching vibration of C−O. The absorption peak at 3436 cm−1 was ascribed to the superposition of O−H axial stretching vibration with N−H stretching vibration, which confirms the presence of amino and hydroxyl groups, providing the coordinating ability with the ion. However, significant differences in FT-IR spectrum of composite ion-imprinted material could be found. First, the 1596 cm−1 peak assigned to the N−H bending vibration became weaker, which demonstrates that the cross-linking took place with atom N in chitosan and triglycine. The adsorption peak at 1638 cm−1 (stretching vibration of C O) shifted and became stronger, due to the superposition of the CO peak in triglycine. Second, the absorption peak of C−O moved from 1079 to 1065 cm−1 and became stronger. Third, the appearance of the sharp peak at 565 cm−1 ascribed to Fe−O was attributed to the insertion of Fe3O4 in the cross-linked chitosan. For CS-IIP before elution, the NH2 absorption peak intensity of the imprinted materials weakened because of the 7403


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Figure 3. SEM images of CS-IIP.

Figure 5. TGA curve of CS-IIPs.

effect of an additional amount of Cu2+ on the adsorption performance of CS-IIPs was investigated. Figure 7 shows that the adsorption capacity of CS-IIP increases with increasing the amount of the template ion from 0 to 150 mg, due to the formation of much specific binding cavities in IIPs as a result of the imprinting effect and the strong chelation interactions between the functional group of the polymer and Cu2+. The adsorption capacity of the prepared CS-IIP reaches a maximum value between 100 and 150 mg and decreases with an increasing amount of Cu2+. This is because of the influence of the excessive amount of copper ions on the cross-linking reaction, which are not favorable for the production of imprinted cavity. Effect of an Additional Amount of GO on the Adsorption Performance of CS-IIP. GO acts as a key role

Figure 4. (a) EDX spectrum of CS-IIPs before elution. (b) EDX spectrum of CS-IIPs after elution. 7404


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Figure 9. Effect of an additional amount of triglycine on the adsorption capacity of CS-IIP (20 mg of adsorbent, 20 mL of 100 mg L−1 copper solution, pH 6.0).

Figure 6. Effect of concentration of the glutaraldehyde solution on the adsorption capacity of CS-IIP (20 mg of adsorbent, 20 mL of 100 mg L−1 copper solution, pH 6.0).

Figure 7. Effect of an additional amount of copper on the adsorption capacity of CS-IIP (20 mg of adsorbent, 20 mL of 100 mg L−1 copper solution, pH 6.0). Figure 10. Effect of pH on the adsorption capacity (10 mg of adsorbent, 20 mL of 50 mg L−1 copper solution).

Figure 8. Effect of an additional amount of GO on the adsorption capacity of CS-IIP (20 mg of adsorbent, 20 mL of 100 mg L−1 copper solution, pH 6.0). Figure 11. Effect of time on the adsorption capacity (10 mg of adsorbent, 20 mL of 100 mg L−1 copper solution, pH 6.0).

in the adsorption process of IIPs. Therefore, the effect of an additional amount of GO on the adsorption capacity of CS-IIP was investigated. Preparation conditions: Cu2+ 100 mg, 4 wt % glutaraldehyde solution containing 0.1 g of triglycine, 20−80 mg of GO. Figure 8 shows that the adsorption capacity of CS-IIP first increases and then slightly decreases with the increase of an additional amount of GO from 0 to 80 mg. The maximum adsorption capacity of 47.42 mg g−1 was obtained when the additional amount of GO was 60 mg. The big number of oxygen-containing functional groups in GO would be favorable for the adsorption of Cu. However, the carboxyl group in GO could also cross-link with the N group in CS, which would reduce the adsorption sites number and decrease the

adsorption capacity. Therefore, 60 mg of GO was selected as the optimal amount in this study. Effect of an Additional Amount of Triglycine on the Adsorption Performance of CS-IIP. Grafting modification is a very important method for improving the adsorption performance. In this work, triglycine was selected to modify the CS, because it has NH- and COO-functional groups, which are favorable for the enhancement of the adsorption performance. Preparation conditions: GO 60 mg, Cu(II) 100 mg, 4 wt % glutaraldehyde solution containing triglycine. 7405


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ACS Sustainable Chemistry & Engineering Table 1. Comparison of Pseudo-First-Order and Pseudo-Second-Order Adsorption Rate Constants pseudo-first-order kinetic models

pseudo-second-order kinetic models

qexp (mg·g−1)

qe (mg·g−1)

k1

R2

qexp (mg·g−1)

qe (mg·g−1)

k2

R2

68.56

26.86

0.0237

0.9823

68.56

70.43

0.0023

0.9997

chain. Therefore, the effect of pH ranging from 2 to 6 on adsorption performances was investigated. Figure 10 shows that the adsorption capacity increases significantly with the increase of the pH value. The N atoms and COO− group are highly protonated at low pH, which reduces the chelating effect of the ions, leading to the decrease of the adsorption capacity. The adsorption capacity significantly increases with increasing the pH from 4 to 6. The protonation level of amino groups decreases with an increasing pH, resulting in the increase of the adsorption capacity. Effect of Contact Time on the Adsorption Performance of CS-IIPs. Kinetics property is very important for the adsorption process. Therefore, the study of the adsorption rate was conducted at room temperature. Figure 11 shows that the adsorption capacity first increases sharply with increasing contact time from 0 to 100 min. However, nearly no change of the adsorption capacity could be observed when the contact time is larger than 120 min, demonstrating the achievement of adsorption equilibrium within 120 min. The adsorption capacity at 5 min is approximately half of that at the equilibrium time, and the adsorption rate was fast, due to the strong chelating interactions between the N atom, COO− group, and Cu2+ ion. The kinetic models, such as the pseudo-first-order model and pseudo-second-order model, were employed to investigate adsorption kinetics. Pseudo-first-order kinetic model:25

Figure 12. Effect of the initial Cu(II) concentration on the adsorption capacity (10 mg of adsorbent, 20 mL of copper solution, pH 6.0).

Table 2. Comparison of the Different Copper(II)-Imprinted Polymers adsorbent

qmax (mg·g−1)

ref

Cu-ICH Cu-IIP(MAA/4-VP) IIP-TSC/Cu IIP-AQ/Cu Cu-IIP(CS chain) Cu-IIP(TEPA) IIP (MAA/diethylenetriamine) Cu-IIM carbon-Cu-IIP magnetic-Cu-IIP (Cu-PEI)-imprinted hydrogel IIEP-Cu CS/GO/Fe3O4-IIP

14.8 15 38.8 4.7 109 33.33 76.5 25 26.71 42.2 40.00 84 132

27 28 29 30 31 32 33 34 35 36 37 38 this work

⎛ k ⎞ log(qe − qt ) = log qe − ⎜ 1 ⎟t ⎝ 2.303 ⎠

(1) 26

Pseudo-second-order kinetic model: t 1 t = + qt qe k 2qe 2

Figure 9 shows that the adsorption capacity of CS-IIP first increases with increasing the amount of triglycine from 0 to 100 mg and then decreases with increasing the amount of triglycine from 100 to 200 mg. The triglycine and cross-linking agent were added to the system simultaneously. Therefore, when the amount of triglycine is beyond the range from 100 to 150 mg, a large amount of triglycine is cross-linked together, which is not favorable for the grafting to CS. Therefore, the adsorption capacity decreases when the additional amount of triglycine is above 100 mg. Effect of pH on the Adsorption Performance of CSIIPs. The pH value affects the number of ions and protonated nitrogens at the bonding sites of IIPs, which shows a significant influence on the adsorption performance. The binding properties of the IIPs toward its template ion are dependent on the internal relationship between the template ion and functional group. Moreover, the CS is a pH-sensitive polymer

(2)

As shown in Figure S1, plots were drawn to show the linear relationship with log (qe−qt) or t/q on the y-axis versus t on the x-axis, respectively. The calculating rate constants and correlation coefficient (R2) are listed in Table 1. The correlation coefficient R2 for the pseudo-first-order equation (0.9823) is smaller than that for the pseudo-second-order equation (0.9997), indicating that the experimental data basically follows the pseudo-second-order equation. In addition, the calculated qe (70.43 mg g−1) is very close to the experimental value (68.56 mg g−1). Hence, the adsorption behavior of CS-IIPs could be described by a pseudo-secondorder kinetic model. Effect of Cu2+ Initial Concentration on the Adsorption Performance of CS-IIP. The initial concentration of the template ion has significant influence on the mass transfer

Table 3. Comparison of Langmuir and Freundlich Isotherm Constants Langmuir −1

−1

Freundlich 2

Qexp (mg·g )

Qmax (mg·g )

b

R

132

142.86

0.0195

0.925 7406

KF

n

R2

6.86

2.19

0.978


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Table 4. Adsorption Selectivity of Cu(II) onto the CS-IIP and CS-NIP Adsorbents in the Presence of Competitive Metal Ions CS-IIP Cu/Zn Cu/Co Cu/Cd Cu/Ni

CS-NIP

Kd(Cu(II))

Kd(ion(II))

k

Kd(Cu(II))

Kd(ion(II))

k

k′

1.345 0.73 1.053 1.114

0.0296 0.0084 0.0353 0.0131

45.44 86.90 29.83 85.04

0.818 0.447 0.687 0.964

0.038 0.022 0.055 0.018

21.35 20.48 12.45 54.47

2.128 4.244 2.397 1.561

were used as interfering ions. The selectivity coefficients k for each binary system were obtained by eq S3 and listed in Table 4. The selectivity factors of Cu2+/M2+ (M = Zn, Ni, Co, and Cd) were 45.44, 85.04, 86.90, and 29.83, respectively. Furthermore, the relative selectivity factors of Cu2+/M2+ (M = Zn, Ni, Co, and Cd) were 2.128, 1.561, 4.244, and 2.397, respectively. All the relative selectivity factors are higher than one because of the imprinting effect. The results reveal that Cu2+ could be selectively removed by CS-IIP from aqueous solution. Reusability and Stability. The regeneration ability of adsorption materials is a key factor for industrial application. 0.2 M HCl solution was used as elute solution. The adsorption− desorption experiment was conducted and repeated five times with the same adsorbents to study the reusability of CS-IIPs. Figure 13 shows that the adsorption capacity of CS-IIPs decreases slightly after the fifth cycle, indicating that the elution process has nearly no influence on cavity structure and chemical property of CS-IIP. The results show that CS-IIPs have good stability and could be used repeatedly.

Figure 13. Regeneration of CS-IIP (10 mg of adsorbent, 20 mL of 100 mg L−1 copper solution, pH 6.0).

driving force during the adsorption procedure, which affects the adsorption capacity of IIP. Figure 12 shows that the adsorption isotherms of CS-IIPs were investigated at different Cu2+ initial concentrations. The adsorption capacity increases with the increase of Cu2+ initial concentration. When the Cu2+ initial concentration was 500 mg L−1, the maximum adsorption capacity of the CS-IIP (132 mg g−1) was obtained. The adsorption capacity for Cu2+ by prepared CS-IIP is much higher than that by all the other adsorption materials published previously, as listed in Table 2. The thermodynamic models like Langmuir and Freundlich isotherm models were employed to investigate adsorption thermodynamic performances. Langmuir isotherm model: Ce/qe = Ce/qmax + 1/(qmax ·b)

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CONCLUSIONS A new Cu2+ ion-imprinted magnetic biomaterial composed of CS enwrapping nanosized Fe3O4 and GO was synthesized for selective removal of Cu2+. The simple cross-linking process could be realized by the condensation reaction between the amino group and aldehyde group with no need of radical polymerization. The triglycine used as modifier was grafted to the surface of chitosan in the cross-linking process. The prepared IIPs were characterized by FT-IR, SEM, TGA, and EDX. Effects of preparation conditions, such as an additional amount of cross-linking agent, template ion, GO and modifier, and operation conditions like pH, Cu2+ concentration, and adsorption time on the adsorption capacity were studied. The adsorption process followed a pseudo-second-order kinetic equation and the Freundlich isotherm adsorption model. The maximum adsorption capacity of the CS-IIP (132 mg g−1) was obtained with 500 mg L−1 initial Cu2+ concentration at pH 6.0. The selectivity factors of Cu2+/M2+ (M = Zn, Ni, Co, and Cd) were 45.44, 85.04, 86.90, and 29.83, respectively. The good selectivity performances are attributed to the imprinting effect. Cu2+ could be selectively removed by CS-IIP from aqueous solution. The Cu(II)-IIPs show good stability and regeneration property.

(3)

Freundlich isotherm model: ln qe = ln KF +

⎛1⎞ ⎜ ⎟ln C e ⎝n⎠

(4)

The curves of Ce/qe or ln qe on the y-axis versus Ce or ln Ce on the x-axis are shown in Figure S2. The calculated results are listed in Table 3. It can be seen that the correlation coefficient (R2) for the Freundlich equation (0.978) is much larger than that for the Langmuir equation (0.925), indicating that the Freundlich isotherm model is more suitable for describing adsorption behavior by CS-IIP than the Langmuir isotherm model. However, the Langmuir corresponding theoretical value of qmax (142.86 mg g−1) is also close to the experimental result (132 mg g−1), indicating that the adsorption of Cu2+ with prepared CS-IIP was probably a monolayer and multiple-layer mixed adsorption process. Adsorption Selectivity. In order to evaluate the selective behavior of CS-IIP and CS-NIP, binary mixtures with the same concentration were prepared. The binary mixtures contained two kinds of metal ions, Cu2+/M2+ (M = Zn, Ni, Co, or Cd). Since the above ions are always to coexist with Cu2+ in real wastewater and have similar chemical properties to Cu2+, they

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b01761. Descriptions of preparation process of CS-IIP, adsorption experiments and analysis method, kinetic fitting data and plots of pseudo-first-order equation, pseudo-second7407


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order equation, fitting data and plots of Freundlich isotherm and Langmuir isotherm models (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*Phone: +86-10-64433872. Fax: +86-10-64423089. E-mail: [email protected] (Z.R.). *E-mail: [email protected] (Z.Z.). ORCID

Zhongqi Ren: 0000-0002-2571-5702 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (21576010 and U1607107), by the Beijing Natural Science Foundation (2172043), the Fundamental Research Funds for the Central Universities (BUCTRC201515), and Higher Education and High-quality and Worldclass Universities (PY201607). The authors gratefully acknowledge these grants.

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ABBREVIATIONS chitosan graphene oxide ion-imprinted polymers ion-imprinted magnetic chitosan polymer microsphere

CS GO IIPs CSIIP nonimprinted magnetic chitosan polymer microsphere C S NIP Fourier transform infrared spectroscopy FT-IR scanning electron microscopy SEM thermogravimetric analyzer TGA energy-dispersive X-ray spectroscopy EDX other competitive metal ions M2+ adsorption capacity (mg·g−1) Q initial Cu(II) concentration (mg·L−1) C0 Cu(II) concentration (mg·L−1) C equilibrium Cu(II) concentration (mg·L−1) Ce solution volume (mL) V adsorbents dosage (g) m equilibrium adsorption capacity (mg·g−1) qe adsorption capacity at time t (mg·g−1) qt maximum adsorption capacity (mg·g−1) qmax distribution coefficient Kd selectivity factor k relative selectivity factor k′ pseudo-first-order rate constant (min−1) k1 pseudo-second-order rate constant (g·mg−1·min−1) k2 Freundlich constants KF and n Langmuir constants b

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REFERENCES

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