One-Step Synthesis of Cationic Hydrogel for Efficient Dye Adsorption

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One-Step Synthesis of Cationic Hydrogel for Efficient Dye Adsorption and Its Second Use for Emulsified Oil Separation Daikun Li, Qing Li, Ningning Bai, Hongzhou Dong, and Daoyong Mao ACS Sustainable Chem. Eng., Just Accepted Manuscript • Publication Date (Web): 03 May 2017 Downloaded from http://pubs.acs.org on May 5, 2017

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ACS Sustainable Chemistry & Engineering

One-Step Synthesis of Cationic Hydrogel for Efficient Dye Adsorption and Its Second Use for Emulsified Oil Separation Daikun Li, Qing Li,* Ningning Bai, Hongzhou Dong, Daoyong Mao School of Chemistry and Chemical Engineering, Southwest University, Tiansheng Road No. 2, Chongqing 400715, P. R. China

ABSTRACT: Removal of toxic dyes and insoluble oil from wastewater is a hot topic in both academic and industrial fields. Herein, we report a cationic absorbent poly-epichlorohydrin-ethylenediamine hydrogel (PEE-Gel) by a simple one-step copolymerization and it can be a successful use for removal of toxic dyes and insoluble oil from wastewater. The adsorption towards anionic dyes shows high efficiency and high selectivity at a wide pH range from 2-12. The adsorption capacity at low equilibrium concentration (10 mg.L-1) is as high as 1411.4 mg.g-1 which is very close to the maximum adsorption capacity (1540.19 mg.g-1). The adsorption of dye molecules onto PEE-Gel is very steady and can hard be regenerated. The PEE-Gel with full dye adsorption (PEE-Gel-Dye) shows superoleophobicity under water, and PEE-Gel-Dye can be reused to separate toluene-water emulsion with high efficiency and durability. These results suggest that PEE-Gel is a promising and competitive candidate for water purification. Keywords:

Cationic

hydrogel,

selectively

adsorption,

dyes,

under-water

superoleophobicity, emulsion oil.

Introduction Water contamination with soluble dyes and insoluble oil is one of a major global environmental issue caused by industrial development.1-5 When those pollutants are discharged as untreated sewage into the environment, they affect aquatic life, food chain, and also bring a serious challenge to human.6-9 Due to the differences of physical and chemical properties of soluble dyes and insoluble oil, one approach can usually effectively remove one of the two types of pollutants.2,10 Therefore, it is of *Email address of the corresponding author: [email protected]. (Qing Li)

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great practical significance to develop a versatile material which can remove both soluble dyes and insoluble oil from water. Nowadays, commercially dyes are widely used in dyestuffs, textile, paper, plastics, cosmetics, tannery and paints,11,12 and over 50,000 tons of dyes are discharged into the environment annually.13,14 Owing to dyes being recalcitrant, resistant to aerobic digestion, stable to oxidizing agents, and insoluble low concentration, treatment of dyeing wastewater is a big challenge.15 Many treatment methods, including biodegradation, chemical oxidation, membrane separation, coagulation-flocculation, photocatalysis, and adsorption have been proposed to remove dyes from wastewater.16,17 Among these approaches, adsorption is considered as the most attractive technology due to its high efficiency, economic feasibility, simplicity of operation as well as the wide suitability for diverse dyes.18-20 An ideal adsorbent is expected to show high adsorption capacity, fast adsorption rate, high selectivity and low cost, simultaneously. Up to now, various kinds of materials have been reported as high performance adsorbents, such as porous carbon materials,21 metal-organic framework (MOF),22 graphene oxide,23 polymer,24 natural mineral,25 and so on. However, most of those materials show high adsorption capacity in high concentrated dye wastewater, but poor efficiency in low concentrated dye wastewater. For example, Zhao et al. reports a β-cyclodextrin-based adsorbent, the maximum adsorption capacity of this adsorbent is as high as 826.45 mg.g-1. However, the equilibrium adsorption capacity is less than 100 mg.g-1 when the equilibrium concentration was 10 mg.L-1.26 In addition, the high cost is also a main barrier which limits the practical application of present adsorbents.12 So it is still a major challenge to develop cheap and efficient adsorbents. Quaternary ammonium (R4N+) based materials have been reported as outstanding adsorbents to remove anionic dyes, radioactive technetium and AuCl4- from water with high selectivity, high capacity and fast adsorption rate.27-29 The zeta potential of R4N+ based materials is positive in both acid and alkali conditions which make them better for adapting to complicated effluent treatment.27 However, the preparation processes of present reports are quite complex which lead to high cost. In addition, the ACS Paragon Plus Environment

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adsorption towards organic pollutant molecules is quite steady and hard to desorption. The regeneration in the present reports is usually achieved by immersing in high concentration of salt solution which is costly and unsustainable. So far, there are no feasible approaches which can overcome the drawbacks of the R4N+ based adsorbents. In this work, we prepare a quaternary ammonium hydrogel via a simple one-step copolymerization by common reagents, and as an efficient adsorbent to remove dyes from water. The adsorption study towards soluble dyes indicates that PEE-Gel can adsorb anionic dyes with high selectivity and high capacity at a wide range of pH values. The adsorption capacity at low equilibrium concentration is very close to the maximum adsorption capacity, indicating that PEE-Gel is a competitive candidate to treat practical dye wastewater. The adsorption of dyes onto PEE-Gel is very steady and can hard be regenerated. However, The PEE-Gel-Dye shows superhydrophilicity in air and superoleophobicity under water, which makes PEE-Gel-Dye to be reutilized to separate span 80 stabilized toluene-water emulsion with high efficiency and high durability. This original research not only develops a simple, cheap and efficient adsorbent, but also provides a sustainable approach for oil/water separation.

Materials and methods Materials Ethylenediamine (EDA), epichlorohydrin (EPI), tetrabutyl ammonium bromide (TBAB), sodium hydroxide, hydrochloric acid and toluene were purchased from Chongqing Chuandong Chemical (Group) Co., Ltd. Acid red 1 (AR1), acid yellow 36 (AY36), direct red 23 (DR23), acid orange 7 (AO7), direct blue 15 (DB15), basic violet 10 (BV10), basic violet 3 (BV3), basic red 14 (BR14), basic blue 7 (BB7), and basic yellow 2 (BY2) were purchased from Tianjin Hengxing Chemical Reagent Co., Ltd. All reagents were obtained from commercial sources and used without further purification. Synthesis of the PEE-Gel A mixture of EDA (6.0 g, 0.1 mol), TBAB (0.2 g), NaOH (16.0 g, 0.4 mol) was dissolved in deionized water (150 ml), and then EPI (34.7 g, 0.5 mol) was slowly



added with continuous stirring in ice-water bath until the solution forms gel. Then the gel was ageing at room temperature for 2 days. The final product was thoroughly washed with deionized water and then dried at 60 oC. Finally, PEE-Gel was ground into powder for better adsorption. Dye adsorption, desorption and separation For adsorption of dyes, the PEE-Gel (10 mg) was added into aqueous dye solution (50 ml) at 40 oC, followed by magnetic stirring. The pH was adjusted by adding a negligible amount of 1 mol·L-1 HCl and 1 mol·L-1 NaOH to examine the effect of different pH values on adsorption capacity. Other adsorption experiments were processed without adjusting pH. After adsorption, the solution was separated through filter paper and the concentration was calculated by comparing the UV-vis absorbance (TU-1901 UV-vis spectrophotometer; Beijing Purkinje General Instrument Co., Ltd.) to the appropriate calibration curve. The removal efficiency of dye was calculated by the following equation:

(1) where co and ct (mg·L-1) are concentrations of dye in aqueous solution at the beginning, t-time, respectively.30 The adsorption capacity qt (dye removal per gram of PEE-Gel of at time, mg·g-1) and qe (dye adsorbed per gram of PEE-Gel at equilibrium, mg·g-1) were calculated according to the following equations:

qt =

(co - ct).V m (2)

qe =

(co - ce).V m (3)

where ce (mg·L-1) represents concentration of dye in aqueous solution at equilibrium; m (g) is the mass of PEE-Gel and V (L) is the volume of dye solution.31,32 Desorption experiment was processed with fresh, completely DR23 filled PEE-Gel by immersing in 0.1 mol·L-1 HCl, 0.1 mol·L-1 NaOH and 0.1 mol·L-1 NaCl solutions and stirred for 24 h. The concentration of DR23 was determined by UV-vis ACS Paragon Plus Environment

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adsorbance. The separation of dye mixture was also investigated by adding 10 mg of PEE-Gel into 100 ml of the mixture solution. The initial concentrations of both the dyes in the mixture were 5×10-5 mol·L-1. After being stirring at 40 oC for 60 min, the concentration of mixture solution was calculated by comparing the UV-vis absorbance. Separation of toluene-in-water emulsion Into a mixture of 1 wt% toluene in water, 0.02 wt% of Tween 80 was added as a surfactant.33 The mixture was mechanically stirred for 30 min to form a full emulsion solution with an average toluene-droplets size of about 405.8 nm. PEE-Gel-DR23 powder (1.5 g) stuck uniformly in the middle of two pieces of filter paper (pore radius = 15-20 µm) with a radius of 3.8 cm was fixed between two glass tubes. Another two pieces of filter paper without PEE-Gel-DR23 was prepared as contrast. The toluene-in-water emulsion (50 ml) was poured into the glass tube. The separation process was driven by vacuum pump with 0.1 MPa pressure. The toluene content in water was measured by an adsorption method using a UV-vis spectrometer. The separation efficiency for dye mixture solutions and toluene-in-water emulsion were quantitatively calculated according to equations (4): Separation efficiency (%) = ቀ1-

C1 C0

ቁ ×100% (4)

where c0 and c1 (mg·L-1) are the concentrations of dye or toluene before and after separation, respectively.34,35 Characterization The Fourier transform infrared (FTIR) spectrum was recorded on an IR-10300 (PerkinElmer, America) from 4000-400 cm-1 using powder-pressed KBr pellet at room temperature. The surface visualization of PEE-Gel was performed by scanning electron microscopy (SEM; HITACHI S-4800). Surface chemical characterizations were carried out by X-ray photoelectron spectroscopy (XPS; Thermo ESCALAB 250). The thermo stability of the PEE-Gel sample was obtained by Pyris 1 thermo gravimetic analyzer (TGA; Perkin-Elmer, USA) in the range 40-800 °C at a heating



rate of 10 °C·min-1 under a nitrogen atmosphere. The structure of the sample was examined using X-ray powder diffraction (XRD; Beijing Purkinje General Instrument Co., Ltd., Beijing, China). The nitrogen physisorption isotherms were measured at 75K on an AutoSorb iQ-C TCD analyzer. Specific surface areas were calculated according to the Brunauer-Emmett-Teller (BET) method. Pore size and total pore volume were calculated on the basis of the Brrett-Joyner-Halenda (BJH) method. The Zeta potential of PEE-Gel was measured by a Zetasizer nano potential analyzer (ZEN 3600, Britain). The contact angle (CA) was measured with a contact angle meter (JC2000C1, China). Hydrodynamic diameters of emulsions were measured by dynamic light scattering (DLS, Malvern Zen 1690) and optical microscopy (Olympus BX51).

Results and discussion Characterization of PEE-Gel PEE-Gel was synthesized by crossing linked copolymerization of EPI and EDA. TBAB worked as surfactant to disperse EPI and NaOH was added to neutralize HCl produced during the reaction process. The reaction mechanism was amine hydroxylation and ring opening polymerization of epoxy group (Shown in Scheme S1).36 Figure 1 was the summary of the synthesis process and possible chemical structure of PEE-Gel. The solution formed hydrogel after adding EPI and the color of PEE-Gel changed from milky white to brown yellow after aging for 48h. EDA and EPI were highly cross-linked to form complex net-like structure which leaded to form hydrogel. Meanwhile, the as-synthesized material was examined insoluble in common organic solvents, such as dimethylsulfoxide (DMSO), dimethyl formamide (DMF), acetone and alcohol.


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Figure 1. Synthesis route and one possible chemical structure of PEE-Gel.

The FTIR spectrum of PEE-Gel was shown in Figure S1a. The absorption at 3412 cm-1 was attributed to the O-H stretching vibration.37 The peaks at 2924 cm-1 and 2877 cm-1 were assigned to the -CH2- units.38 The characteristic band at 1105 cm-1 was caused by the special vibration of quaternary ammonium groups (R4N+).39 All of those detected groups could be well consistent with PEE-Gel. The XRD pattern of PEE-Gel (Figure S1b) showed that there was only one wide and blunt peak at 2θ = 21.14 o, which indicated that PEE-Gel had poor crystallinity. The TGA spectrum of PEE-Gel in Figure S1c showed that the weight decreased mainly from 225 to 500 oC with approximately

80%

and

only

7.3%

solid

residue

was

left.

The

N2

adsorption-desorption isotherm of PEE-Gel powder was shown in the Figure S1d and the corresponding results were summarized in Table 1. According to the BET model, the specific surface area was calculated to be 19.77 m2·g-1. The total pore volume and pore radius were 0.022 cm3·g-1 and 27.93 nm, respectively. Compared with commercial activated carbon (surface area: 500-2000 m2·g-1),40 the surface area of PEE-Gel powder was quite small, which suggested that surface area was not the major



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contribution of the high adsorption capacity. The chemical composition of PEE-Gel was investigated by XPS (Figure S1e), confirming that the product was only consisted of the elements C, N, O, Cl and H, without any undesirable elements being detected. Moreover, the S 2p peak (168.1 eV) appeared in the spectrum of PEE-Gel after adsorption DR23. At the same time, the Cl 2p peak (197.1 eV) disappeared. This observation indicated that the adsorption of DR23 altered the chemical composition of the PEE-Gel, indicating that the essence of the adsorption was anion exchange between chloride ions and anionic dye molecules. In order to understand the extent of reaction, high-resolution XPS N1s spectrum of PEE-Gel was carried out and shown in Figure S1f. The N1s spectrum was deconvoluted into two peaks which were assigned to free amino groups (-NHR2/-NH2R/-NH3) at 399.2 eV and quaternary ammonium groups around 401.8 eV, respectively. As shown in Table S2, 58.3 percent of amino groups in PEE-Gel had been transformed to quaternary ammonium groups (N+R4). The SEM shown in Figure S1g exhibited that PEE-Gel powder was irregular particulate matter. And it inflated and the surface became more roughness after adsorption DR23 (Figure S1h) which mainly caused by adsorbing large number of dye molecules. Table 1. BET analysis of PEE-Gel powder. Sample

Surface area (m2·g-1)

Pore volume (cm3·g-1)

Pore radius (nm)

PEE-Gel

19.77

0.022

27.93

Dye adsorption The pH values of the solution had big influence on both ionization degree of dye molecules and surface charge of the adsorbent which would lead to huge adsorption difference.41 To study the effect of pH, one anionic dye DR23 (pH range: 2-12) and one cationic dye BB7 (pH range: 2-10, higher pH values would lead BB7 to form sediment) were chosen as soluble pollutants to investigate the adsorption capacity of PEE-Gel, as shown in Figure 3a. It was found that the adsorption capacity of DR23 onto PEE-Gel was about 240 mg·g-1 and had no significant change with change of pH values from 2 to 12. On the other hand, the adsorption of BB7 was quite low in pH


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range of 2-10 and the highest capacity was only 5.42 mg·g-1. These results indicated that PEE-Gel could selectively adsorb DR23 in wide pH range and barely adsorb BB7. The Zeta potential of PEE-Gel in different pH solution had been presented in Figure S2. The PEE-Gel was electropositive at a wide pH range from 2-12 which was consistent with a previous quaternary ammonium based material.27 On account of this property, PEE-Gel could selectively adsorb DR23 from water by electrostatic attraction. The main reason was that quaternary ammonium group (R4N+) could be completely ionized in water in both acid and alkali conditions. So PEE-Gel could efficiently capture anionic dye molecules but hardly adsorb cationic dye molecules in wide range of pH according to electrostatic attraction and repulsion (shown in Figure 2). This advanced property made PEE-Gel a potential candidate for separation of dye mixture.

Figure 2. Schematic of selective adsorption and separation process of dyes.

The adsorption behavior of PEE-Gel towards different water-solution dyes was investigated to verify that the above conclusion had universality. Ten dyes, including both anionic dyes (DR23, AR1, AO7, AY36, and DB15) and cationic dyes (BR14, BV10, BY2, BB7, and BV3) were employed, and their chemical structures were summarized in Table S1. Figure 3b showed the adsorption efficiencies of PEE-Gel to ten different dyes. The removal rates of DR23, AR1, AO7, AY36, and DB15 reached 99.1%, 99.8%, 98.0%, 98.8% and 100%, respectively. While the corresponding color



removal rates of BR14, BV10, BY2, BB7, and BV3 were 1.02%, 0.54%, 2.4%, 1.2% and 0.96%, respectively. The UV-vis adsorption spectra and the digital images of the corresponding color change of the ten dyes solution before and after adsorption were shown in Figure S3 and Figure S4. It was clear that after adsorption by PEE-Gel, the solutions of anionic dye almost became colorless. Meanwhile, the adsorption peaks of anionic dyes disappeared, while the spectra and colors of cationic dyes had almost no change compared with the initial solutions. Those results indicated that PEE-Gel could selectively adsorb various kinds of anionic dyes.

Figure 3. (a) Effect of pH on the adsorption of DR23 and BB7 onto PEE-Gel at 40 oC for 60 min. (b) The removal of anionic dyes and cationic dyes at 40 oC for 120 min. (c) UV-vis adsorption spectra of DR23 after different processing time at 40 oC. (d) UV-vis adsorption spectra of 100 mg·L-1 DR23 solution and after desorption of PEE-Gel-DR23 in 0.1 mol·L-1 HCl, NaOH, NaCl for 24 hours at 40 oC.

The spectroscopic investigation in Figure 3c of the supernatants showed that the concentration of DR23 in water significantly decreased with the time increased. The


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curve of spectrum became a straight line paralleling to the X axis after adsorption for 80 min which indicated that PEE could completely remove soluble pollutants from water. Desorption experiment was investigated with fresh, completely DR23 filled PEE-Gel after washed twice with distilled water. The UV-vis adsorption spectra shown in Figure 3d were almost straight lines paralleling to the X axis which suggested the adsorption of dye molecules onto PEE-Gel was hard to desorption. This character made PEE-Gel a candidate for practical using for industrial effluents which usually possess high salinity and high pH value.

Separation of Dye Mixture High selective adsorbents had prospective application in reclamation of valuable chemicals, chemical and biological sensing, and smart separation.42 The selective adsorption capacity of the PEE-Gel toward anionic dyes made it a promising candidate for separation dye mixtures. To exhibit this favorable ability, verification tests were carried out. A mixture solution of an anionic (DB15) and cationic (BR14) dye was made with a molar ratio about 1:1 (5×10-5 mol·L-1). The UV-vis spectra showed that the maximum peaks of pure DB15 and BR14 were at 598 nm and 519 nm, respectively, while the mixture solution only had one peak maximum at 519 nm. The reason may be attributed to the combination of BR14 and DB15 through electrostatic attraction in water solution. After added into PEE-Gel, the spectra of the mixture and the pure BR14 solutions appeared to overlap (Figure 4a). At the same time, the color of two individual aqueous dye solutions and the dye mixture of BR14 and DB15 before and after adsorption by PEE-Gel were recorded by digital images, as displayed in Figure 4c. It was clear that the color of the dye mixture solution changes from purple to red after adsorption, which was almost as same as that of the pure BR14 aqueous solution. This suggested that the DB15 was selectively adsorbed from dye mixture by PEE-Gel. Subsequently, the separation experiment for mixed dyes of anionic dye (AY36) and zwitterionic dye (VB10) was conducted and the separation result was shown in Figure 4bd. It could be clearly seen that the color of the dye mixture became the same with the pure VB10 solution after separation (Figure 4d). Besides, the UV-vis spectrum of ACS Paragon Plus Environment


the separated dye mixture revealed the same shape as the pure VB10, at the same time, the AY36 peak was disappeared. These results indicated that PEE-Gel could allow anionic dye (AY36) to get adsorbed and had strong repulsion to zwitterionic dye (VB10). The possible reasons were as follows: under the separation conditions, VB10 had a permanent positive charge and carboxyl group was partly deprotonated, which resulted in a negative charge as well.43 However, the positive charge of VB10 was stronger than its negative charge. That was, the electrostatic repulsion-force between PEE-Gel and VB10 was greater than their electrostatic adsorption-force. The obtained separation efficiency for BR14/DB15 and VB10/AY36 were as high as 99.62% and 97.13%, separately. Indicating that PEE-Gel could efficiently separate anionic and cationic dye mixture solutions.

Figure 4. (a) The UV-vis spectra of dye separation of BR14 form BR14/DB15. (b) The UV-vis spectra of dye separation of VB10 from VB10/AY36. (c) The digital images of separation process of (a). (d) The digital images of the corresponding separation process of (b).

Adsorption isotherm Adsorption isotherm models are used to fit the experimental data and describe the adsorption equilibrium.44 The experimental isotherm data obtained from the adsorption studies were fitted through the most commonly used Freundlich (Figure 5c) and Langmuir models (Figure 5d).45


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The linear form of the Langmuir isotherm models is expressed as: ce ce 1 qe = bqm + qm (5)

where ce is the equilibrium liquid-phase solid-phase of concentration respectively; qe (mg·g-1) are the equilibrium amount of dye adsorption. qm (mg·g-1) is the monolayer capacity of the adsorbent; b (L·mg-1) is Langmuir constant related to the energy of adsorption, representing the affinity between adsorbent and adsorbate.46

Figure 5. (a) Effect of adsorbent dosage (conditions: temperature, 40 oC; processing time: 30 min; DR23 concentration: 100 mg·L-1). (b) Adsorption capacity of PEE-Gel with regard to DR23 (conditions: adsorbent dose, 200 mg·L-1; temperature: 40 oC; processing time: 24 h; DR23 concentrations: 100 to 440 mg·L-1). (c) Freundlich isotherm and (d) Langmuir adsorption isotherm plots for adsorption of DR23 by PEE-Gel.

The Langmuir isotherm can be expressed in term of a dimensionless equilibrium parameter RL: RL =

1 1+bco (6)



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where co (mg·L-1) is the initial concentration of DR23, RL value indicates that the type of the isotherm to be either unfavorable (RL > 1), linear (RL = 1), favorable (0 < RL < 1) or irreversible (RL = 0).44 The Freundlich model is assumed that adsorption occurs on the surface of non-homogeneous medium by adsorption sites and exponential distribution of energy. The Freundlich model is expressed as: 1 ln qe = ln kF + n ln ce (7)

where kF (L·mg-1) and n are Freundlich constants, representing adsorption capacity and adsorption intensity respectively, which are calculated from the plots of lnqe versus lnce. Table 2. Kinetic parameters for adsorption of DR23. Experimental

Freundlich

-1

Langmuir 2

qmax(mg·g )

kF

1/n

R

1541.64

853.9

0.143

0.8853

-1

qm(mg·g )

b (L·mg-1)

1540.19

1.096

RL

R2

0..002-0.009

0.9995

Table 3. Comparison of adsorption capacities with various adsorbents. qe (mg.g-1)

qmax (mg.g-1)

References

≈ 700

782

18

Methyl orange

≈ 240

310.65

30

Graphene-based hybrid aerogels

Methyl orange

≈ 220

560

31

Polymer networks

Methylene blue

≈ 400

1000

43

AgBr-Based Nanomaterials

Indigo carmine

≈ 50

139.28

47

Magnetic graphene oxide

Methylene blue

≈ 39

270.94

48

Cellulose-based Bioadsorbent

Methylene blue

≈ 674

1814

49

Anionic MOFs

Methyl viologen

--

160

50

Boron nitride nanosheets

Lysozyme

≈ 240

312

51

δ-MnO2 nanoparticles

Methyl orange

≈ 11

427

52

ZnAl-LDH-3

Methylene blue

≈ 467

1153

53

PEE-Gel

Direct red 23

1411.4

1541.64

This work

Adsorbent

Adsorbate

Boron nitride nanosheets

Congo red

Cr-doped ZnO

ce = 10 mg .L-1

As shown in Table 2, the determination coefficient R2 (0.9995) of Langmuir model was much higher than that R2 (0.8853) of the Freundlich model, indicating that the Langmuir equation could fit the experimental adsorption data better. The value of RL (0.002-0.009) was found to be between zero and one, indicating favorable adsorption


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of DR23 by PEE-Gel. The calculated maximum adsorption capacity of dye was 1540.19 mg·g-1 according to the Langmuir which was close to the experimental data (1541.64 mg·g-1). The maximum adsorption capacity was higher than most of the available adsorbents (Shown in Table 3) reported in present literatures. Moreover, the adsorption capacity at low equilibrium concentration was as high as 1411.4 mg·g-1, about 91.6 percent of the maximum adsorption capacity. However, most of the corresponding adsorption capacity of other adsorbents is less than 40 percent of the corresponding maximum adsorption capacity, indicating that PEE-Gel was a competitive adsorbent for wastewater treatment. Adsorption kinetic studies

Figure 6. (a) Effect of contact time on the adsorption of DR23 by PEE-Gel at different temperature (adsorbent dose: 200 mg·L-1; DR23 concentration: 100 mg·L-1). (b) Fit of kinetic data to pseudo first order model. (c) Fit of kinetic data to pseudo second order model. (d) Fit of kinetic data to intra-particle diffusion model.

Kinetic studies on dye adsorption provide important information about the mechanism of the adsorption process. The kinetic data for DR23 adsorption by PEE-Gel were ACS Paragon Plus Environment


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fitted according to the pseudo-fist order (eqn 8), pseudo-second order (eqn 9),54 and the intra-particle diffusion (eqn 10) models,55 as expressed in the following equations: k 1t

log (qe - qt) = logqe -

2.303 (8)

t 1 t = + 2 qt qe k 2 qe

(9)

q t = kit 0.5 + C(10)

where qe and qt are the amount of adsorbed dye (mg·g-1) at equilibrium and time, t is the time (min), and k1 is the adsorption rate constant (min). k2 (g·mg-1·min-1) is the pseudo-second order rate constant. ki is the intra-particle diffusion rate (mg·g-1·min -0.5) and c (mg·g-1) is a constant related to the thickness of the boundary layer. Table 4. Kinetic parameters of adsorption of DR23 onto PEE-Gel at different temperature. qe,exp

Pseudo-fist-order model

(mg·g-1)

qe(mg·g-1)

k1 (min-1)

R2

qe(mg·g-1)

k2 (g·mg-1·min-1)

R2

30

498.5

310.4

0.01283

0.8680

446.4

1.82×10-4

0.9977

40

499.4

494.3

0.06782

0.9359

534.8

2.38×10-4

0.9991

50

499.8

223.4

0.07655

0.8823

518.1

5.11×10-4

0.9987

Temperature (oC)

Pseudo-second-order model

Table 5. Parameters of the intraparticle diffusion model for adsorption of DR23 onto PEE-Gel. Temperature (oC)

30

Parameters

First part

Second part

Third part

C (mg·g-1)

33.05

160.30

223.37

ki (mg g-1 min -0.5)

56.10

27.36

17.72

0.9977

0.9938

0.9637

68.25

330.64

494.20

67.14

18.79

0.04386

0.9783

0.9727

0.9102

31.47

355.87

0.2883

93.74

20.14

491.95

0.9764

0.9537

0.9381

R

2 -1

C (mg·g ) 40

-1

ki (mg g min R

-0.5

)

2 -1

C (mg·g ) 50

-1

ki (mg g min R

2

-0.5

)

The effect of temperature on the adsorption process shown in Figure 6a suggested that the initial adsorption rate increased with the increasing temperature in this adsorption system. The fitting results to the experimental data using pseudo-first order


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and pseudo-second order (shown in Figure 6b and Figure 6c) were summarized in Table 4. The determination coefficient of the pseudo-second-order kinetic model (R 2) was much higher than that of the pseudo-first-order kinetic model, which indicated that the adsorption of DR23 onto PEE-Gel fit the pseudo-second-order model well at these temperatures. The plots of qt versus t0.5 for the adsorption of DR23 onto PEE-Gel at different temperature were shown in Figure 6d (see values at Table 5). It was observed that the plot of qt vs.t0.5 is not a straight line containing three linear parts and does not pass through the origin, indicating that intra-particle diffusion displayed a significant role but not the only rate-controlling factor at different temperatures.55

Separation performance of the PEE-Gel-DR23 for toluene-in-water emulsions

Figure 7. (a) The photograph of the setup for separating toluene-in-water emulsion. The photograph and microscopic images and DLS data of toluene-in-water emulsion before (b, c) and after (d, e) filtration.

PEE-Gel was an outstanding adsorbent which could efficiently remove anionic contaminant molecules from water. On the other hand, the steady adsorption of contaminant molecules onto PEE-Gel made it hard to regenerate. Subsequent research found that PEE-Gel-Dye exhibited robust superhydrophilicity, superlipophilicity in air, and superoleophobicity under-water which mean that PEE-Gel-Dye could be used as a potential oil/water separation material. Figure S5ab shown the wettability of



PEE-Gel-DR23 to water and oil in air, when a water or oil droplet contacted with the surface of PEE-Gel-DR23, it immediately spread out and permeated into PEE-Gel-Dye, and the contact angles were both nearly 0o. However, in the oil/water/solid three-phase system, the PEE-Gel-DR23 could not be wetted by oil under water with an oil contact angle of 152.6o (Figure S5c).

Figure 8. The separation efficiency of toluene-in-water emulsion versus the recycle numbers.

The separation test of span 80 stabilized toluene-water emulsion was carried out at 0.1 MPa with a vacuum-driven (shown in Figure 7a). Optical micrograph of the original emulsified oil was showed in Figure 7b, where dense oil droplets were clearly observed. DLS data showed that the average size of the oil droplets was about 400 nm (Figure 7c). Separation of such an emulsion in this composition was pretty critical due to the very low oil content (1 vol%) and small droplet size of toluene.56 No toluene droplets were observed in the entire view after separation by PEE-Gel-Dye (Figure 7de), while there was no apparent change after filtered with two pieces of filter paper (Figure S5d), indicating that the outstanding separation properties of PEE-Gel-DR23. In addition, the solution was colorless after separation, which suggested that the dye molecules did not discharge from PEE-Gel-DR23 during the filtration. The content of the oil was calculated by measuring the toluene weight percentage in the filtrate by UV-vis adsorption spectra. The normalized line was fitted to show the linear relationship between adsorption intensity and oil concentration (Figure S6). It was found that the removal efficiency of toluene in the filtrate was about 98.96% after the


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first separation. After five separation processes, the toluene-water separation efficiency remained stable and the removal efficiency was still as high as 98.14% (Figure 8). Those results indicated that PEE-Gel-DR23 could separate toluene-water with high efficiency and high durability.

Conclusion In summary, this work has demonstrated a facile and simple method to prepare an organic cationic absorbent which can successfully remove both toxic dyes and insoluble oil from water. This material possesses abundant R4N+ groups and shows effective and selective adsorption anionic dyes. The removal efficiency towards anionic dyes exceeds 99% and the maximum adsorption capacity is 1540.19 mg·g-1, which is higher than most other available adsorbent. Moreover, the adsorption exhibits high efficiency at low equilibrium concentration which has big significance to treat practical wastewater. On the other hand, PEE-Gel can hardly adsorb cationic dyes. On account of this special property, PEE-Gel can efficiently and quickly adsorb anionic dyes over dye mixture solutions. In addition, PEE-Gel-Dye can be reused to successfully separate surfactant-stabilized toluene-water emulsion with high efficiency and high durability. This original research not only develops a simple, cheap and real efficient adsorbent, but also provides a cheap and sustainable approach for oil/water separation. We expect that this original research would lead to the development of advanced water purification techniques.

Supporting Information Chemical structures of dyes; Reaction mechanism of PEE-Gel; FTIR spectrum, XRD, TGA, BET results, XPS and SEM graphs of PEE-Gel; Zeta potentials of PEE-Gel at different pH solution; UV-vis adsorption spectra and the corresponding photographs of different dye solutions; Wettability of PEE-Gel-DR23; Wettability and emulsified oil separation of PEE-Gel.

AUTHOR INFORMATION Corresponding Author



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*Tel: +86 023 68252360. Fax: +86 023 68254000. E-mail: [email protected].

ACKNOWLEDGMENTS The authors specially thank for the ﬁnancial support of this work from the National Natural Science Foundation of China (51103120).

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One-Step Synthesis of Cationic Hydrogel for Efficient Dye Adsorption and Its Second Use for Emulsified Oil Separation Daikun Li, Qing Li,* Ningning Bai, Hongzhou Dong, Daoyong Mao

A brief synopsis: PEE-Gel was prepared by a simple one-step copolymerization and it could selectively remove anionic dyes from water. Moreover, PEE-Gel-Dye could be reused as a sustainable material for span 80 stabilized toluenewater emulsion separation.

503x297mm (72 x 72 DPI)


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One-Step Synthesis of Cationic Hydrogel for Efficient Dye Adsorption

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