Highly efficient selective adsorption of anionic dyes by modified β-cyclodextrin polymers

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Highly efficient selective adsorption of anionic dyes by modified b-cyclodextrin polymers Meng-Ya Xua, Hong-Liu Jianga,*, Ze-Wu Xieb, Zeng-Tian Lib, Di Xua, Fu-An Heb,* a b

School of Materials Science and Engineering, Nanchang Hangkong University, Nanchang 330069, China School of Materials Science and Engineering, Guangdong University of Petrochemical Technology, Maoming 525000, China

A R T I C L E

I N F O

Article History: Received 14 August 2019 Revised 25 December 2019 Accepted 10 January 2020 Available online xxx Keywords: b-cyclodextrin based polymer Modification Anionic dye adsorption Selective adsorption Recycle ability

A B S T R A C T

The nitrile groups in the tetrafluoroterephthalonitrile-crosslinked b-cyclodextrin (b-CD) were modified by (1) the reaction with the diethanolamine resulting in the hydroxyalkylaminoalkylamide groups or the hydroxyalkylaminoimide groups (2) the reduction of the borane-tetrahydrofuran complex resulting in the amide groups to form two novel b-CD based polymers (CDPs), namely the CDP-DEA and the CDP-NH2, correspondingly. Furthermore, the dye adsorption abilities of these two CDPs in the aqueous solution were investigated using the anionic dyes including the Congo red (CR) and the orange G (OG). For both the CDP-DEA and the CDP-NH2, the pseudo second-order model fitted well with their adsorption dynamics and their equilibrium adsorption process followed the Langmuir isotherm model. The maximum adsorption capacities of the CDP-DEA for CR at pH = 5 and OG at pH = 3 were 813 mg/g and 442 mg/g, respectively, while the corresponding values of the CDP-NH2 were 40 mg/g and 446 mg/g, respectively. It was found that the electrostatic attraction played an important role in the anionic dye adsorption of the CDP-DEA and the CDP-NH2. © 2020 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

1. Introduction The rapid development in the industrial and agricultural production have led to the significant deterioration of water quality, and more and more surface water and groundwater have been polluted. The pollution of the water resources has become one of the major environmental problems that needs to be solved urgently. Various dyes are commonly found in the wastewater from the textile industry and many other industries such as cosmetics, paper printing, and plastics [1,2]. The disadvantages of the dye-contaminated water include high toxicity, complex component, wide pH-value range, dark colority, difficulty in biochemical degradation, and so on. These harmful factors seriously affect not only the growth of aquatic organisms and microorganisms in water but also the self-purification of water. The dye contaminants are difficult to be removed from wastewater resulting in toxic and carcinogenic problems to human beings [3]. Therefore, the removal of various dyes from the wastewater is an urgent and important environment task. There are many treatment technologies, such as ion exchange, membrane filtration, adsorption, coagulation-flocculation-sedimentation, and so on, have been developed and successfully applied to remove the dye contaminants from the wastewater [4
7]. Among these strategies, adsorption is one of the most widely-used method

* Corresponding authors. E-mail addresses: [email protected] (H.-L. Jiang), [email protected] (F.-A. He).

because it is easy and safe to be handled with the advantages of high efficiency, low cost, and generating no dangerous by-products [8,9]. Therefore, the preparation of the regenerable adsorbents with rapid adsorption rate and high adsorption capacity are very critical. b-cyclodextrin (b-CD) produced by the enzymatic hydrolysis of starch is a cyclic oligosaccharide consisting of a seven-a-linked-D-glucopyranose-unit structure. The b-CD exhibits the hydrophilicity on its outer wall and the hydrophobicity inside its lumen demonstrating the great potential for separating organic contaminants from the wastewater [10,11]. However, the high solubility of b-CD in water is an obstacle to its independence as an adsorbent. The porous b-CD based polymer (P-CDP) synthesized by Alsbaiee et al. was derived from the nucleophilic aromatic substitution of the hydroxyl group of the b-CD with the fluoride on tetrafluoroterephthalonitrile (TFPN), which could quickly isolate many organic contaminants in trace amount [12]. It is possible that the nitrile group in the P-CDP can be converted into a variety of other chemical structures including amidoxime and hydroxyalkylaminoalkylamide [14]. For example, it has been reported that the hydrolysis of the nitrile group of the b-CD based polymer (CDP) could produce a mixture of the amide and carboxylated functional groups with the affinity for the dyes of different natures [14]. On the other hand, the chemical modification of the nitrile group in the CDP by the ethanolamine or the diethanolamine mainly produced a hydroxyalkylaminoalkylamide structure, leading to good selective adsorption ability for anionic dyes [14]. The anionic dyes occupy a great proportion in the discharged dyes that can seriously damage the ecology and survival environment. For

https://doi.org/10.1016/j.jtice.2020.01.005 1876-1070/© 2020 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

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instance, the widely-applied anionic Congo red (CR) dye may lead to many harmful effects such as. a human carcinogen. Hence, it is still highly desirable to develop a high-efficient b-CD-based absorbent for the removal of the anionic dyes from wastewater. Since the anionic dyes containing the negative charges can be strongly absorbed by the materials containing the positive charges resulting from the electrostatic interaction, we supposed that the introduction of a lot of the positive functional groups on the b-CD-based absorbents may greatly contribute to the adsorption for the anionic dyes [15]. In this study, for the purpose of the effective adsorption of the anionic dyes from water, the hydroxyl groups of the b-cyclodextrin were subjected to the nucleophilic aromatic substitution with the cross-linking agent TFPN to form CDPs containing nitrile groups (CDP-CN), and then the nitrile groups in the CDP-CN were modified by (1) the reaction with the diethanolamine resulting in the positive hydroxyalkylaminoalkylamide groups or the hydroxyalkylaminoimide groups (2) the reduction of the borane-tetrahydrofuran (THF) complex resulting in the positive amide groups to form two novel CDPs, namely the CDP-DEA and the CDP-NH2, correspondingly. Furthermore, the dye adsorption abilities of these two CDPs in the aqueous solution were investigated using the anionic dyes including CR and orange G (OG) as the target pollutants.

2. Experimental 2.1. Materials

b-CD (>98%), tetrahydrofuran (THF, 99.9%, dried with molecular sieves, stabilized with BHT, water
30 ppm), N, N-dimethylformamide (DMF, 99.9%, dried with molecular sieves, water
30 ppm), diethanolamine (DEA, 99%) were purchased from Shanghai Saen Chemical Technology Co., Ltd., China. TFPN (>98%) was purchased from Shanghai Macklin Biochemical Co., Ltd., China. Borane-THF complex solution (1.0 M in THF, with <0.005 M, sodium borohydride stabilizer) was purchased from Sigma-Aldrich. Anhydrous potassium carbonate (K2CO3), methanol (CH3OH, AR), and dichloromethane (CH2Cl2, AR) were purchased from Xilong Chemical Co., Ltd., China. Three dyes, as shown in Fig. 1, were used to study dye adsorption in aqueous solution. Anionic CR (C32H22N6Na2O6S2, molecular weight: 696.68), anionic OG (C16H10N2Na2O7S2, molecular weight: 452.37), and cationic methylene blue (MB) (C16H18ClN3S, molecular weight 319.85), which were purchased from Aladdin Chemical Technology Co., Ltd., China. In addition, the structural changes of the CR and the OG under different acid-base conditions were also shown in Fig. 1.

Fig. 1. Chemical structures of dyes used in the present study and structural changes of CR and OG under different acid-base conditions.

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3

Fig. 2. Synthetic route of (a) CDP-CN, (b) CDP-DEA, and (c) CDP-NH2 (the reaction sites of the hydroxyl groups, the fluoride group, and the nitrile group were marked with the red asterisk). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

2.2. Synthesis of CDP-CN The synthetic route of the CDP-CN is shown in Fig. 2a. b-CD (0.94 g), TFPN(0.5 g), K2CO3 (1.43 g), THF (36 mL) and DMF (4 mL) were added into a dried 100-mL flask under N2 protection, and then the resultant mixture in the 100-mL flask was placed in an oil bath (85 °C) and magnetically stirred at 500 rpm for 2 days. The resultant orange suspension was cooled and then filtered, and the residual K2CO3 was removed by washing the solid on the filter paper with 1 N HCl until the CO2 evolution stopped. The resultant light yellow solid was activated by soaking in H2O for 15 min, THF (2 £ 100 mL) for 30 min, and CH2Cl2 (1 £ 50 mL) for 15 min in sequence, followed by being dried at 60 °C for 12 h to obtain the CDP-CN. 2.3. Synthesis of CDP-DEA and CDP-NH2 The synthesis procedure of the CDP-DEA was as follow (see Fig.2b). The CDP-CN powder (230 mg) was placed in a two-necked flask equipped with a reflux condenser, and then DEA (20 mL) was added under N2 atmosphere. The reaction was carried out at 150 °C for 48 h by magnetic stirring. After the end of the reaction, it was cooled for 15 min, and 100 mL of ethanol was poured into the reaction product, followed by magnetic stirring for 15 min. The resulting mixture was filtered and washed with water (500 mL). To remove any residual DEA, the product was dispersed in ethanol and stirred magnetically for 12 h, then filtered, washed with water, and dried in a vacuum oven at 80 °C for 12 h to obtain the CDP-DEA. The synthesis procedure of the CDP-NH2 was as follow (see Fig. 2c). CDP-CN powder was dissolved in 65 mL anhydrous THF by magnetic stirring under N2 protection. Next, the mixture was cooled to 0 °C and borane-THF (1.0 M, 48 mL) complex was dropwisely

added into it. After refluxing and magnetic stirring at 80 ° C for 12 h followed by cooling, 80 mL ethanol was dropwisely added to the resulting mixture to remove excess borane at 65 °C. After cooling, the light yellow solid in the resultant mixture was collected by filtration and stirred 12 h, methanolic HCl (4.0 M), and then separated by filtration and stirred in 100 mL 5% aqueous NaOH solution for 3 h. The solid in the resultant mixture was collected by filtration and washed repeatedly with water until neutral follow by being dried at 120 °C under vacuum 12 h to obtain the CDP-NH2. 2.4. Adsorption experiments In the adsorption experiment, the anionic CR and OG dyes were selected as the target pollutants, and a high concentration of the CR and OG stock solutions were prepared with the deionized water. The CR and OG solutions for investigation were obtained by diluting the high-concentration stock solution to the predetermined concentrations. The experiment temperature and the rotational speed were controlled by a shaking tank with a constant-temperature water bath, and the adsorption experiment was carried out in a batch mode. To investigate the effect of contact time on the adsorption equilibrium and adsorption kinetic experiments, 10 mg adsorbent was added to a 100-mL Erlenmeyer flask containing 25 mL dye solution of the CR or the OG. The resultant mixture was shaken with the rotation speed of 150 rpm at 30 °C for the predetermined time, and then filtrated by a 0.22 mm-pore-size polytetrafluoroethylene (PTFE) membrane. The remaining dye concentration of the filtrate was determined by the UV
Vis spectrophotometer (Shimadzu, UV-2550). The maximum absorption wavelengths of the CR and the OG are 697 nm and 475 nm, respectively. The equilibrium adsorption

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capacity qe (mg/g) and the removing percentage (%) were calculated by the Eqs. (1) and (2), correspondingly [16,17]. qe ¼

ðC0
Ce Þ V m

Removing percentage ð%Þ ¼

ð1Þ C0
Ce  100 C0

ð2Þ

where C0 (mg/L) and Ce (mg/L) were the initial and final equilibrium concentrations of the dye solution, V (mL) was the volume of the solution, and m (mg) was the mass of the adsorbent. To investigate the effect of the pH value on the dye adsorption, 10 mg adsorbent and 25 mL CR or OG solution were added into a 100-mL Erlenmeyer flask. The pH value of the dye solution was adjusted from 3 to 10 using a 0.1 mol/L HCl or NaOH solution. The resultant mixtures in the Erlenmeyer flasks were shaken on a shaker at 150 rpm at 30 °C for 24 h, and then filtrated by a 0.22 mm-poresize PTFE membrane. The remaining dye concentration of the filtrate was determined by the UV
Vis spectrophotometer. In the adsorption isotherm experiment, the pH values of the CR and the OG were 5 and 3, respectively. The CR concentration ranges of the adsorption by the CDP-DEA and the CDP-NH2 were 75
400 mg/L (concentration gradients: 25 mg/L for 75
200 mg/L and 50 mg/L for 200
400 mg/L) and 20
160 mg/L (concentration gradient: 20 mg/L), respectively, while the OG concentration range of the adsorption by the CDP-DEA and the CDP-NH2 concentration was 25
200 mg/L (concentration gradient: 25 mg/L). 2.5. Regeneration experiments For the purpose of the dye desorption, the adsorbed adsorbent was washed about 7
8 times using 100 mL of 0.1 mol/L NaOH solution for about 1 min and 200 mL ethanol for about 1 min as the filtration eluents in sequence until the filtrate was colorless. Next, the filter cake was washed three times using the distilled water and once using the ethanol, and then dried at 60 °C under vacuum for the next adsorption cycle. Five consecutive adsorption-desorption cycles were performed to investigate the adsorbent regeneration. After each cycle, the filtrate was collected to calculate the dye removing efficiency by the UV
Vis spectrophotometer. 2.6. Characterization Fourier transform infrared spectroscopy (FTIR) was carried out in the diffusing reflectance mode ranging from 4000 cm-1 to 400 cm-1. Zeta potential measurement was carried out by a zeta potential meter (Malvern ZS90) at 298 K, in which the sample was dispersed in the aqueous ethanol with the concentration of 1 mg/mL and the pH value was adjusted in the range of 3
10. Thermogravimetric analysis (TGA) was carried out by heating the sample using a thermogravimetric analysis/differential scanning calorimetry (TGA/DSC) analyzer (Perkin Elmer, USA) at a heating rate of 20 °C/min under a nitrogen flow of 200 mL/min. 13C NMR experiments were carried out on a JNM-ECZ600R spectrometer. The nitrogen adsorption/desorption isotherm was measured using a Mike ASAP2460 absorber at 77 K. A Hitachi/SU8010 scanning electron microscope (SEM) and a JEOL/JEM2010 transmission electron microscope (TEM) were used to study the structures of the CDP-CN, the CDP-DEA, and the CDP-NH2. The X-ray photoelectron spectroscopy (XPS) experiments were carried out the Thermal scientific Escalab 250Xi equipment. 3. Result and discussion 3.1. Characterization of CDP-DEA and CDP-NH2 Fig. 3a shows the FTIR spectra of the b-CD, the TFPN, and the CDPCN. The FTIR spectrum of the TFPN had the absorbance bands at

Fig. 3. FT-IR spectra of (a) b-CD, TFPN as well as CDP-CN, and (b) CDP-CN, CDP-DEA as well as CDP-NH2.

2248 cm-1 and 1268 cm-1 originating from the nitrile group and the C-F group, correspondingly [12]. The FTIR spectrum of the b-CD had the absorbance bands at 2925 cm-1, 1031 cm-1, and 3367 cm-1 originating from the aliphatic C

H group, the CHO group, and the hydroxyl group, correspondingly [18
21]. The characteristic absorbance bands of both the TFPN and the b-CD also could be found in the FTIR spectrum of the CDP-CN confirming the successful crosslinking of the b-CD by the TFPN [12]. Moreover, the XPS result showed that the CDP-CN contained different elements including C (66.2 atomic%), N (7.16 atomic%), O (20.01 atomic%), and F (6.63 atomic%). This indicated that each TFPN moiety in the CDP-CN has been averagely substituted by 2.15 alkoxides, which was consistent with the result reported by Alsbaiee et al. [12]. As shown in Fig. 3b, the characteristic FTIR absorbance bands at 2244 cm-1 of the nitrile group for both the CDP-DEA and the CDP-NH2 significantly weakened in comparison with that of the CDP-CN, indicating the possible conversions of the nitrile group to the hydroxyalkylaminoalkylamide group (or the hydroxyalkylaminoimide groups) and the amide group, correspondingly. The FTIR spectrum of the CDP-NH2 had the characteristic absorbance bands of the amide group at 3588 cm-1, 1594 cm-1, and 836 cm-1, indicating that the nitrile group of CDP-CN was successfully reduced to be the amine group by the borane-THF complex [22]. On the other hand, as reported by Satilmis et al., the nitrile group of the CDP-DEA could react with one DEA molecule to form the hydroxyalkylaminoimide group or two DEA molecules to form the hydroxyalkylaminoalkylamide group [14]. Hence, the newly-formed carbonyl group CHO (or imine group CHN) at 1624 cm-1 in the FTIR spectrum of the CDP-DEA implied the attachment of the hydroxyalkylaminoalkylamide group (or the hydroxyalkylaminoimide group) [14]. The XPS result (see Fig. S1) showed that the CDP-DEA contained different elements including C (60.44 atomic%), N (7.23 atomic%), O (28.22 atomic%), and F (4.11 atomic%). This result indicated that the CDP-DEA contained 37 atomic% nitrile group and 63 atomic% functional group of the hydroxyalkylaminoimide and the hydroxyalkylaminoalkylamide. Fig. 4 gives the 13C NMR results of the b-CD, the CDP-CN, the CDPDEA, and the CDP-NH2. The b-CD had six characteristic 13C NMR peaks at 103 ppm (C1), 74 ppm (C2, C3, and C5), 84 ppm (C4), and 60 ppm (C6), respectively. These peaks still remained in the 13C NMR spectrum of the CDP-CN. Compared to the b-CD, three new 13C NMR peaks in relation to the C6’ of the alkoxy group between the b-CD moiety and the TFPN moiety, the C7 in the TFPN moiety, and the carbon atom of the nitrile group in the TFPN moiety were found at 99 ppm, 143 ppm, and 110 ppm, respectively, indicating the successful crosslinking reaction [12]. Similar

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Fig. 4.

13

5

C NMR results of CDP-CN, CDP-DEA, and CDP-NH2.

to the FTIR result, the carbon atom of the nitrile group in the TFPN moiety almost disappeared in the 13C NMR spectrum of the CDP-DEA and the CDP-NH2, implying its conversions to the hydroxyalkylaminoalkylamide group and the amide group, correspondingly. In comparison with CDP-CN, a series of new peaks including C8 (the aromatic carbon atom attaching to the hydroxyalkylaminoalkylamide group), C9 (163 ppm, the carbon atom in the newly formed carbonyl group), C10 (60 ppm, the carbon atom attaching to the nitrogen atom in the hydroxyalkylaminoalkylamide group), and C11 (50 ppm, the carbon atom attaching to the hydroxyl group in the hydroxyalkylaminoalkylamide group) could be found in the 13C NMR spectrum of the CDP-DEA, confirming the formation of the hydroxyalkylaminoalkylamide group [14]. On the other hand,

for the CDP-NH2, the 13C NMR peaks at 118 ppm labeled as C12 and 34 ppm labeled as C13 originated from the aromatic carbon atom and the carbon atom in the benzylamine group, correspondingly, proving the existence of the amide group [22]. In order to investigate the degree of protonation, the zeta potentials of the CDP-CN, the CDP-DEA, and the CDP-NH2 at different pH values was measured, as shown in Fig. 5. It could be found that the zeta potential of the CDP-CN was negative in the whole investigated pH value range and the modification of the CDP-CN was able to increase the zeta potentials of the resultant CDP-DEA and the CDPNH2. As the pH value increased from 3 to 10, the zeta potentials of both the CDP-DEA and the CDP-NH2 decreased from positivity to

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Fig. 5. Zeta potentials of CDP-CN, CDP-DEA, and CDP-NH2.

negativity. The CDP-DEA and CDP-NH2 were protonated and positively charged under the acidic condition, which was beneficial to the adsorption of anionic dyes. On the other hand, the zeta potential of the CDP-DEA was higher than that of the CDP-NH2, which was possibly due to the existence of a lot of carbonyl groups and tertiary amine groups in the CDP-DEA.

As shown in Fig. 6, for the CDP-CN, the mass loss of 8.0% at 110 °C was attributed to the evaporation of adsorbed water [22
25]. The TGA curve remained stable at 110
200 °C indicating that the CDP-CN was thermally stable in this temperature range. When the temperature exceeded 200 °C, the mass loss of the CDP-CN was 43.7% due to the decompositions of the b-CD unit and the TFPN unit. At a temperature higher than 500 °C, there was a mass loss of 18.5% probably caused by the decomposition of the carbon chain [26]. The TGA curve of the CDP-DEA showed a weight loss below 150 oC owing to the removing of the moisture, and a decomposition of the hydroxyalkylaminoalkylamide group (or the hydroxyalkylaminoimide group), the b-CD unit as well as the TFPN unit at 200
450 °C. At a temperature higher than 500 °C, there was a mass loss of 12.1%, which might also be due to the decomposition of the carbon chain in the CDP-DEA. Similarly, the weight loss of the CDP-NH2 below 150 °C was due to the removing of the moisture. On the other hand, a weight loss of 7.1% in the temperature range of 350
450 oC could be attributed to the loss of the NH3 from the decomposition of the benzylamine group in the CDP-NH2 [13]. The nitrogen adsorption-desorption isotherm result (see Fig. 7a) shown that the BET surface area of CDP-CN was 72.5 m2/g with an average pore diameter of 3.54 nm, which confirmed that there were many nano-pores in the CDP-CN. On the other hand, according to the SEM results (see Fig. 7a, b, and c) the morphologies of the CDP-DEA and the CDP-NH2 were similar to that of the CDP-CN with the particle morphology, indicating that the original macrostructure of the CDPCN still remained after the modification. Furthermore, the microstructures of the CDP-DEA and the CDP-NH2 were investigated by the TEM

Fig. 6. TGA results of (a) CDP-CN, (b) CDP-DEA, and (c) CDP-NH2.

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Fig. 7. (a) Nitrogen adsorption-desorption isotherms of CDP-CN [inset: the curve of dV/dw (pore volume) as a function of pore width], SEM images of (b) CDP-CN, (c) CDP-DEA, and (d) CDP-NH2.

(see Fig. S2). It could be found that the CDP-DEA and the CDP-NH2 exhibited the crosslinked particle morphology containing a lot of nano-pores, which provided the possibility to capture the small CR and OG dye molecules. 3.2. Batch experiment results 3.2.1. Selective adsorption experiment It was expected that the CDP-DEA and the CDP-NH2 could effectively absorb the anionic dyes but repel the cationic dyes because there was a large amount of positive charges on their surface under the suitable low-pH-value condition. In order to confirm this hypothesis, the selective uptake experiments of the CDP-DEA for the anionic OG in the mixed aqueous solution containing the OG and the cationic MB were studied. As shown in Fig. 8a, after the adsorption by the CDP-DEA for 4 h, the characteristic UV
Vis adsorption peak of the OG for the mixed aqueous OG/MB solution almost disappeared but the adsorption peak of the MB still remained strong suggesting that the adsorption ability of the CDPDEA for the MO was significantly weaker than that for the OG. Fig. 8d and e shows photos of the mixed OG/MB aqueous solution before and

after adsorption by the CDP-DEA, correspondingly. It could be observed that the color of the MB/OG dye mixture after the adsorption changed from dark green to blue, which was almost the same color as the aqueous solution of pure MB (see Fig. 8c). This indicated that most OG dyes in the orange color (see Fig. 8b) were removed from the mixed OG/MB aqueous solution by the CDP-DEA, and the selective separation of the OG from the dye mixture of the MB and the OG was achieved. 3.2.2. Effect of pH on adsorption The effects of the pH value of the dye aqueous solution on the adsorption capacities of the CDP-DEA and the CDP-NH2 for the CR and the OG were shown in Fig. 9 The lower adsorption efficiency of the CDP-DEA and the CDP-NH2 for the CR and the OG could be found in both low- and high-pH-value ranges. Under the acidic condition with low pH value, both the CR and the OG molecules were present in the cationic form, which could repel the positive CDP-DEA and CDP-NH2 [27
29]. Similarly, under the alkaline condition with high pH value, both the CR and the OG molecules were present in the anionic form, which could also repel the negative CDP-DEA and CDPNH2. Therefore, the maximum adsorption capacities of the CDP-DEA

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Fig. 8. (a) UV
Vis spectra mixed OG/MB solution before and after adsorption by CDP-DEA, the photos of (b) OG solution, (c) MB solution, mixed OG/MB solution (d) before and (e) after adsorption by CDP-DEA.

Fig. 9. Effects of pH on CR adsorption by (a) CDP-DEA as well as (c) CDP-NH2 and on OG adsorption by (b) CDP-DEA as well as (d) CDP-NH2. (adsorption time = 12 h, temperature = 303 K, solution volume = 25 mL, absorbent dosage = 10 mg).

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9

Fig. 10. Adsorption capacities as a function of time (pH of CR solution = 5, pH of OG solution = 3, temperature = 303 K, solution volume = 25 mL, absorbent dosage = 10 mg): (a) CR as well as (b) OG by CDP-DEA, and (c) CR as well as (d) OG by CDP-NH2.

and CDP-NH2 mainly contributed by the electrostatic interaction only could be obtained in the suitable low-pH-value ranges of 3
5 for the CR and 2
3 for the OG, correspondingly. On the other hand, the reason for the relatively low adsorption capacity of the CDP-NH2 for the CR was not clear yet. Since both the CR molecule and the CDP-NH2 had the same amide group that could attract the sulfonate group of the CR molecule by electrostatic interaction, it was possible that many CR molecules absorbed on the CDP-NH2 were removed by the CR molecules in the solution resulting in the relatively low adsorption capacity.

the removing rate could reach 80
90% for about 2 h basing on different adsorption systems. As the contact time between the adsorbent and the dye increased, the adsorption equilibrium could be slowly reached within 3 h with the removal rate of 95
99% for different adsorption systems. To further evaluate the changes in the adsorption process with the contact time, the experimental data basing on Fig. 10 were simulated by the pseudo-primary and pseudo-secondary models, as shown in Fig. S3 and Fig. 11, correspondingly. The pseudo-first-order dynamic and pseudo-second-order dynamic models are generally expressed by the Eqs. (3) and (4), correspondingly [30]:

3.3. Adsorption kinetic

lnðqe
qt Þ ¼ lnðqe Þ
K1 t

ð3Þ

As shown in Fig. 10, the adsorption of the CR and the OG in the aqueous solution by the CDP-DEA and the CDP-NH2 was quite rapid. Although the adsorption equilibrium didn't reach in the early stage,

t 1 t þ ¼ qt ðK2 qe 2 Þ qe

ð4Þ

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Fig. 11. Pseudo second-order kinetic fitting of CR adsorption by (a) CDP-DEA as well as (c) CDP-NH2 and of OG adsorption by (c) CDP-DEA as well as (d) CDP-NH2. (pH of CR solution = 5, pH of OG solution = 3, temperature = 303 K, solution volume = 25 mL, absorbent dosage = 10 mg).

Table 1 Fitting parameters of adsorption kinetics for CDP-DEA and CDP-NH2. Adsorbents

CDP-DEA

Adsorbates

CR OG

CDP-NH2

CR OG

C0 (mg/g)

150 200 100 125 50 75 100 125

Pseudo-first-order model -1

2

qe (mg/g)

K1 (min )

R

503 493 89 110 169 36 143 106

0.0284 0.0295 0.0251 0.0247 0.01328 0.01393 0.02264 0.0231

0.927 0.970 0.968 0.968 0.879 0.919 0.937 0.960

where t is the contact time; K1 (min-1), K2 [g/(mg min)] are the rate constants of the pseudo first-order dynamic and pseudo-secondorder dynamic models, correspondingly; qe (mg/g) and qt (mg/g) are the adsorption capacities of the CDP-DEA or the CDP-NH2 at equilibrium and the contact time of t moment, correspondingly. The kinetic parameters of the CR and the OG adsorption by the CDP-DEA and the

Pseudo-second-order model qe (mg/g)

K2 [g/(mg min)]

R2

285 552 253 302 39 34 258 320

7.25 £ 10
5 4.15 £ 10
5 5.87 £ 10
4 4.78 £ 10
4 2.60 £ 10
4 5.06 £ 10
4 4.60 £ 10
4 4.78 £ 10
4

0.996 0.999 0.999 0.999 0.987 0.987 0.999 0.999

CDP-NH2 are shown in Table 1. It could be found that the correlation coefficients (R2) of the pseudo second-order dynamic model could reach about 0.99 and were much greater than that of the pseudo first-order dynamic model indicating that the adsorption kinetics of the CR and the OG adsorption by the CDP-DEA and the CDP-NH2 favored the pseudo second-order process.

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11

Fig. 12. Linear Langmuir fitting of CR adsorption by (a) CDP-DEA as well as (b) CDP-NH2 and of OG adsorption by (c) CDP-DEA as well as (d) CDP-NH2. (adsorption time = 12 h pH of CR solution = 5, pH of OG solution = 3, temperature = 303 K, solution volume = 25 mL, absorbent dosage = 10 mg). Table 2 Fitting parameters of linear Langmuir model and non-linear Langmuir model parameters for CR and OG adsorption by CDP-DEA and CDP-NH2. Adsorbents

CDP-DEA

Adsorbates

CR

OG

CDP-NH2

CR

OG

T (K)

303 313 323 303 313 323 303 313 323 303 313 323

Linear Langmuir model

Non-linear Langmuir Model

qmax (mg/g)

KL (L/mg)

R2

a

B

R2

813 854 862 442 444 446 40 47 52 446 450 454

0.085 0.008 0.097 0.55 1.153 2.000 3.86 0.692 0.745 1.66 0.404 0.306

0.996 0.987 0.991 0.997 0.996 0.999 0.999 0.997 0.999 0.998 0.999 0.999

820.20 821.87 828.86 440.86 450.26 465.35 46.91. 47.75 50.36 428.13 432.70 438.75

0.09 0.92 0.11 0.63 1.38 1.99 9.59 7.17
1.99 2.90 4.12 4.83

0.943 0.956 0.966 0.943 0.871 0.947 0.647 0.459 0.888 0.867 0.918 0.957

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of about 0.99. By the judgment on the values of the R2, the linear Langmuir isotherm model belonging to the single layer adsorption mechanism was better to explain the adsorption process of the CR and the OG by the CDP-DEA and the CDP-NH2 in comparison with the other three isotherm models [32,33]. According to the linear Langmuir isotherm model, the maximum adsorption capacities of the CDP-DEA for the CR at pH = 5 and the OG at pH = 3 were 813 mg/g and 442 mg/g, respectively, while the corresponding values of the CDP-NH2 were 40 mg/g and 446 mg/g, respectively. For the purpose of comparison, the maximum adsorption capacities of different reported adsorbents are listed in Table 3. Obviously, the adsorption capacities of the CDP-DEA for the adsorption of the CR and the OG was much higher than the other listed adsorbents.

Table 3 Comparison of adsorption capacities of various adsorbents for CR and OG. Adsorbents Sphere-like Mn2O3 micro-particles ZnO(AC)/MWCNT composite Fe3O4/Chitosan composite b-CD/Viologen composite b-CD/NH2-HBP/cotton fibers Magnetic GO/PEI composite Exfoliated graphite Terbium luminescent MOF CeO2 nanotubes CDP-DEA CDP-NH2 Carbon mesoporous material Octadecylamine modified nanoclay Vinyl-pyrrolidone modified chitosan Activated carbon Magnetic silica Fly ash Fe3O4/MIL-101 composite CDP-DEA CDP-NH2

Adsorbates

qmax (mg/g)

Reference

CR CR CR CR CR CR CR CR CR CR CR OG OG OG OG OG OG OG OG OG

136 250 700 323 351 575 196 637 362 813 40 189 44 64 9 66 14 200 442 446

[34] [35] [36] [37] [38] [39] [40] [41] [42] This work This work [43] [44] [45] [46] [47] [48] [49] This Work This Work

3.5. Adsorption thermodynamics The parameters of the adsorption thermodynamics including the Gibbs free energy change (DG), the entropy change ( DS), and the enthalpy change (DH) for the absorption process of the CR and the OG by the CDP-DEA and the CDP-NH2 were calculated basing on the Eqs. (9)
(11):

DG ¼
RT lnKd 3.4. Adsorption isotherm lnKd ¼ In this study, two commonly used adsorption isothermal models, namely the linear Langmuir isotherm model (see Eq. (5)), the linear Freundlich isotherm model (see Eq. (6)), the non-linear Langmuir isotherm model (see Eq. (7)), the non-linear Freundlich isotherm model (see Eq. (8)) were used for data analysis, which can be expressed as follows: Ce 1 1 ¼ Ce þ q max KL qe q max lnqe ¼ qe ¼

Kd ¼

ð6Þ

aL bL Ce 1 þ b L Ce

ð7Þ

qe ¼ aF CebF

DS DH R




ð10Þ

RT

qe Ce

ð11Þ

where R (8.314 J mol-1K-1) is the gas constant, T (K) is the absolute temperature, and Kd is the equilibrium constant. The parameters of the adsorption thermodynamics were obtained from the data fitting (see Fig. S8) and listed in Table 4. The (DG) values for the absorption process of the CR by the CDP-DEA as well as of the OG by the CDPDEA and the CDP-NH2 were negative, demonstrating the spontaneous nature of these adsorption in the investigated temperature range [50,51], while the positive (DG) value for the absorption process of the CR by the CDP-NH2 implied the great absorption difficulty resulting in low absorption capacity. On the other hand, the decreasing (DG) values with the increasing temperature suggested that all the adsorption proceedings for different adsorption system was more favorable at high temperature. Meanwhile, the positive (DH) values and the positive (DS) values indicated the endothermic nature and the increasing randomness at the solid-solution interface in the adsorption process, correspondingly [50,51].

ð5Þ

1 lnCe þ lnKF n

ð9Þ

ð8Þ

where Ce (mg/L) is the concentration of the CR or the OG at equilibrium; qe (mg/g) is the adsorption capacity of the CDP-DEA or the CDP-NH2 at equilibrium; qmax (mg/g) is the maximum adsorption capacity of the CDP-DEA or the CDP-NH2 basing on the linear Langmuir isotherm model; KL (L/mg) and KF (mg/g) are the equilibrium constants of the linear Langmuir isotherm model and the linear Freundlich isotherm model, correspondingly; and n is the exponent of the linear Freundlich isotherm model. The aL (mg/g) and bL (L/mg) as well as the aF [(mg/g)(mg/L)n] and bF are the equilibrium constants of the non-linear Langmuir isotherm model and the non-linear Freundlich isotherm model, correspondingly. The data fittings according to the relation between the Ce and the qe (see Fig. S4) by the isothermal models were studied (see Figs. 12, S5
S.7) and the model parameters of these four models were calculated (see Tables 2 and S1). It could be found that the fitting results of the linear Langmuir isotherm model possessed the highest correlation coefficients (R2) values

3.6. Adsorption mechanisms The hypothetical adsorption mechanism was explained using the adsorption of CR by the CDP-DEA as an example (see Fig. 13), which included three adsorption contributions [31,52]. (1) The b-CD moiety of the CDP-DEA had a non-polar hydrophobic cavity containing an oxygen bridge and hydrogen atoms, which could hold a CR molecule with a hydrazone structure by a complex action. (2) Similar to that reported by Alsbaiee et al., the three-dimensional porous hydrophilic network structure of the CDP-DEA was constructed by the cross-

Table 4 Thermodynamic parameters for CR and OG adsorption by CDP-DEA and CDP-NH2. Adsorbents

Adsorbates

Thermodynamic parameter

DH (KJ/mol) 0

CDP-DEA CDP-NH2

CR OG CR OG

5.633 15.886 6.819 9.154

DS (J/mol K)

DG(KJ/mol)

0

36.66 73.08 14.22 54.29

298K

303K

313K

323K


5.29
5.91 1.95
7.02


5.47
6.28 1.88
7.30


5.84
7.01 1.74
7.84


6.21
7.74 1.60
8.38

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Fig. 13. Hypothetical CR adsorption mechanism by CDP-DEA.

Fig. 14. Removing efficiency of OG by regenerated CDP-DEA and CDP-NH2.

linking of TFPN and the subsequent introduction of hydroxyalkylaminoalkylamide groups (or the hydroxyalkylaminoimide group) on the b-CD [12], leading to the porous network capture ability of the CDPDEA for the CR dyes. (3) Under the suitable acidic condition with the pH value between 4 and 6, the positive CDP-DEA easily attracted the negative -SO3- group of the CR molecule by the mutual electrostatic interaction [53]. It was found that the maximum adsorption capacity of the CDP-CN for the CR at pH = 5 was only 177 mg/g (see Fig. S9). This value was much lower than that of the CDP-DEA (813 mg/g), indicating that the mutual electrostatic interaction played the most important role among these three adsorption contributions. 3.7. Recyclable ability Fig. 14 shows the result of the repeated regeneration experiments of the CDP-DEA and the CDP-NH2 for the OG adsorption. It was found that the adsorption effects of both the CDP-DEA and the CDP-NH2

were good, and the adsorption removing efficiency in the first round could reach above 99.5% while the removing efficiency after four repetitions could still maintain above 98%. This result indicated that both the CDP-DEA and the CDP-NH2 had good regenerability for the adsorption of OG. 4. Conclusions The chemical modification of the b-CD in this study could realize the preparation of the polymers including CDP-DEA and CDP-NH2, which exhibited good adsorption ability and high selectivity for the anionic dyes relative to the cationic dyes from aqueous media. This result was mainly attributed to the strong electrostatic interaction between the anionic dyes and the positive hydroxyalkylaminoalkylamide groups (or the hydroxyalkylaminoimide group) and amide groups introduced of the b-CD-based absorbents. In the relatively low pH value range, The CDP-DEA possessed the good adsorption ability

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for the CR and the OG, which was superior to those of the CDP-NH2 and many other adsorbents reported in the literature. In addition, both the CDP-DEA and the CDP-NH2 could maintain high removing efficiency of the CR and the OG in the recyclability. We believe that these new type absorbents including the CDP-DEA and the CDP-NH2 should have great potential in the treatment of dye-containing wastewater. Declaration of Competing Interest The authors declare that there are no conflicts of interest. Acknowledgments This work was financially supported by NSFC (No. 51462024), the Open Fund of Guangdong Provincial Key Laboratory of Petrochemical Pollution Process and Control for the Guangdong University of Petrochemical Technology, China (No. 2018B030322017), the College Student Training Program of Guangdong University of Petrochemical Technology (No. 733402 and No. 733192), the Special Innovation Project Foundation for Students of Guangdong Common University, China (No. PDJH2019b0327), and the National Innovation and Entrepreneurship Training Program for College Students, China (No. 733286). Supplementary materials Supplementary material associated with this article can be found in the online version at doi:10.1016/j.jtice.2020.01.005. References [1] Albadarin AB, Mo J, Glocheux Y, Allen S, Walker G, Mangwandi C. Preliminary investigation of mixed adsorbents for the removal of copper and methylene blue from aqueous solutions. Chem Eng 2014;255:525–34. [2] Adeyemo AA, Adeoye IO, Bello OS. Adsorption of dyes using different types of clay: a review. Appl Water Sci 2017;7:543–68. [3] Yaseen D, Scholz M. Treatment of synthetic textile wastewater containing dye mixtures with microcosms. Environ Sci Pollut Res 2018;25:1980–97. [4] Mahmoud A, Hoadley AFA. An evaluation of a hybrid ion exchange electrodialysis process in the recovery of heavy metals from simulated dilute industrial wastewater. Water Res 2012;46:3364–76. [5] Gao J, Sun SP, Zhu WP. Chelating polymer modified P84 nanofiltration (NF) hollow fiber membranes for high efficient heavy metal removal. Water Res 2014;63:252–61. [6] He J, Chen K, Cai X. A biocompatible and novelly-defined Al.-HAP adsorption membrane for highly effective removal of fluoride from drinking water. J Coll Interface Sci 2017;490:97–107. ras F. Chemical coagulation of combined sewer over[7] Samrani AGE, Lartiges BS, Villie flow: heavy metal removal and treatment optimization. Water Res 2008;42:951–60. [8] Liu L, Gao ZY, Su XP. Adsorption removal of dyes from single and binary solutions using a cellulose.-based bioadsorbent. ACS Sustain Chem Eng 2015;3:432–42. [9] Gupta VK, Ali I, Saleh TA. Chemical treatment technologies for waste.-water recycling.-an overview. RSC Adv 2012;2:6380–8. [10] Huang W, Hu Y, Li Y. Citric acid-crosslinked b-cyclodextrin for simultaneous removal of bisphenol A, methylene blue and copper: the roles of cavity and surface functional groups. J Taiwan Inst Chem Eng 2018;82:189–97. [11] Li H, Meng B, Chai SH. Hyper-crosslinked b-cyclodextrin porous polymer: an adsorption-facilitated molecular catalyst support for transformation of water-soluble aromatic molecules. Chem Sci 2016;7:905–9. [12] Alsbaiee A, Smith BJ, Xiao L, Alsbaiee A, Smith BJ, Xiao L, Ling Y, Helbling DE, Dichtel WR. Rapid removal of organic micropollutants from water by a porous b-cyclodextrin polymer. Nature 2015;529(7585):190–4. [13] Mason CR, Maynard-Atem L, Heard KWJ, Satilmis B, Budd PM, Friess K, Lanc M, Bernardo P, Clarizia G, Jansen JC. Enhancement of CO2 affinity in a polymer of intrinsic microporosity by amine modification. Macromolecules 2014;47:1021–9. [14] Satilmis B, Alnajrani MN, Budd PM. Hydroxyalkylaminoalkylamide PIMs: selective adsorption by ethanolamine and diethanolamine-modified PIM-1. Macromolecules 2015;48:5563–9. [15] Satilmis B, Budd PM. Selective dye adsorption by chemically-modified and thermally.-treated polymers of intrinsic microporosity. J Colloid Interface Sci 2017;492:81–91. [16] Lo SF, Wang SY, Tsai MJ. Adsorption capacity and removal efficiency of heavy metal ions by Moso and Ma bamboo activated carbons. Chem Eng Res Des 2012;90:1397–406.

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