Bidirectionally pH-Responsive Zwitterionic Polymer Hydrogels with

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Materials and Interfaces

Bi-directionally pH-responsive zwitterionic polymer hydrogels with switchable selective adsorption capacities for anionic and cationic dyes Ran Wei, Wanying Song, Fan Yang, Jukai Zhou, Man Zhang, Xiang Zhang, Weifeng Zhao, and Changsheng Zhao Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b01027 • Publication Date (Web): 31 May 2018 Downloaded from http://pubs.acs.org on May 31, 2018

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Industrial & Engineering Chemistry Research

Bi-directionally pH-responsive zwitterionic polymer hydrogels with switchable selective adsorption capacities for anionic and cationic dyes

Ran Weia, Wanying Songa, Fan Yanga, Jukai Zhoua, Man Zhanga, Xiang Zhanga*, Weifeng Zhao and Changsheng Zhaoa* a

College of Polymer Science and Engineering, State Key Laboratory of Polymer

Materials Engineering, Sichuan University, Chengdu 610065, People’s Republic of China

* Corresponding author. Tel: +86-28-85400453, Fax: +86-28-85405402, E-mail: [email protected]

or

[email protected]

[email protected] (X. Zhang).

1

ACS Paragon Plus Environment

(C.S.

Zhao);

Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT: Solution pH is an important factor in the adsorptive behavior of ionizable dyes in chemical and biotechnology industries. In this study, novel pH-responsive zwitterionic hydrogels were prepared by copolymerization of acrylic acid and methylacryloyloxyethyl trimethylammonium chloride. The results indicated that the hydrogels showed bi-directionally pH-responsive behaviors: the swelling ratio was small in neutral pH value, and increased with the increase or decrease of pH values. Meanwhile, the selective adsorption capacities of the hydrogels for anionic or cationic dyes could be controlled by adjusting the pH values. The hydrogels might be applied in many fields such as adsorption/separation and water treatment. KEYWORDS: Zwitterionic hydrogels; pH-responsive; selective adsorption; swelling behavior; cationic/anionic dyes

2


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1. INTRODUCTION The rapid development of chemical, metallurgy and biotechnology industries benefits mankind but brings serious water pollution 1. The water contaminations mainly come from industry, sanitary, agricultural sewage, and so on

2, 3

. Recently,

dyes have become the indispensable chemicals for industrial processes, including dyes manufactures, textile, printing industry, food, cosmetics and others 4. According to the charge types, dyes can be divided into two categories, i.e., anionic dyes and cationic dyes 5. Because of the incontinent use, inevitable leakage and unbridled emission, water pollution caused by dyes has become a serious problem and how to remove them is becoming extremely significant for water treatment and ecological environment protection. Among various physical and chemical techniques for dyes removal, adsorption method is attractive and admired due to its low-cost and high efficiency 6, 7. Many polymer hydrogels and supramolecular hydrogels have been reported as adsorbents for water treatment, including dyes removal

8-11

. These researches

indicated that the hydrogels could adsorb dyes or other kind of pollutants efficiently, and various adsorption mechanisms were demonstrated, including specific and nonspecific adsorptions, such as electrostatic interaction. For instance, cationic dyes such as methylene blue (MB) and methyl violet (MV) could be adsorbed by negative charged hydrogels which contained carboxylic and/or sulfonic groups

12

; while

positive charged hydrogels which contained amino or pyridine could uptake the anionic dyes such as acid red and amaranth (AR) 13. However, due to the intrinsic charge properties, the conventional hydrogels

14, 15

could only remove the dyes with

given charge type, and the fabrications of hydrogels with selective adsorption capacities for cationic or anionic dyes were rarely studied. The emergence and utilization of polyampholyte made it possible for the preparation of novel hydrogels 16

. Polyampholyte and zwitterionic polymers are ionic polymers which contained

zwitterionic groups and/or a mixture of anionic and cationic groups 3


17, 18

. The


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electrostatic interaction between a positive charge and a negative charge within the same molecular chain results in the charge neutrality of the polymers several types of zwitterionic polymers such as carboxybetain oxygen ethyl dimethyl amine and sulfobetaine

21

19

Till now,

20

, methyl acryloyl

have been synthesized successfully

and their properties have been investigated in detail. For instance, zwitterionic polymer of poly(sulfobetaine methacrylate) provided antifouling properties and had 22

admirable biocompatibility

; block copolymer of poly(ε-caprolactone) and

poly(allylethylene phosphate) was used for enhanced drug delivery to tumor 23; Luo et al. prepared tough and self-healing hydrogels by using the oppositely charged polyelectrolytes of 3-(methacryloylamino) propyl-trimethylammoniumchloride sodium p-styrenesulfonate

and

24

, the network of these hydrogels which consisted of

oppositely charged polyelectrolytes increased the strength and toughness of the material. By now, zwitterionic polymers were rarely considered as accessible absorbent materials because their charge neutrality always resisted electrostatic adherence

19

.

However, some researches toward the swelling property of zwitterionic hydrogels indicated that the charge strength and type of the hydrogels would change when responding to pH values

25-27

: the hydrogels could show given type of charges in

given pH values. This is to say, zwitterionic hydrogels will bring negative charge or positive charge at different pH values, and thus show selective adsorption for cationic or anionic dyes. Herein, we provided a facile method to prepare pH-responsive hydrogels with selective adsorption capacities by random cross-linking copolymerization of positively charged methylacryloyloxyethyl

trimethylammonium chloride (DMC)

28

and negatively charged acrylic acid (AA) in aqueous solution. The chemical cross-linking and the electrostatic interactions between the opposite charges increased the stability of the hydrogels. The carboxyl groups in the PAA segments could be protonated at low pH, which brought out the increase of the positive charges of the hydrogels; when the carboxyl groups were deprotonated at high pH, the negative charges of the hydrogels increased due to the existence of PDMC segments. The 4


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physical and chemical properties of the hydrogels were characterized, and the swelling properties were studied under different pH values. Meanwhile, Anionic dyes of AR and quinoline yellow (QU), and cationic dyes of MB and MV, were chosen to verified the pH-responsive selective adsorption capacities, and the adsorption kinetics, isotherms, and desorption behaviors were investigated and discussed. 2. EXPERIMENTAL 2.1 Materials Methylacryloyloxyethyl trimethyl ammonium chloride (DMC, 75 wt. % in H2O) and acrylic acid (AA) were purchased from Sigma-Aldrich (Shanghai, China), N, N’-methylenebisacrylamide

(MBA,

cross-linker,

AR),

2,

2’-azobis

(2-methylproplonamldine) dihydrochloride (AIBA, initiator, 97 wt. %), maranth (AR), quinoline yellow (QY), methylene blue (MB) and methyl violet (MV) were supplied by Aladdin reagent Co. Ltd. (China). Deionized water (DI water) was used to prepare all the aqueous solutions throughout the studies. Unless otherwise stated, the reagents were analytical grade and received from Chengdu Best Reagent Co. Ltd. (China). 2.2 Fabrication of the zwitterionic polymeric hydrogels Zwitterionic polymeric hydrogels (ZPHs) were fabricated as follows: firstly, given amounts of DMC, AA, MBA and DI water were added to a 50 mL glass flask; the total concentrations of the monomers were 10 wt. % (mass ratio with respect to the whole solution), MBA was 7 % (molar ratio with respect to the monomers). Then, the resulting solutions were stirred until the monomers were completely dissolved, and then purged with nitrogen for 10 min to remove dissolved oxygen. After that, the initiator AIBA (1 % molar ratio with respect to the monomers) was added and the resulting solutions were stirred again for 30 minutes. At last, given amount of the solutions were put into special molds (about 400 µL solutions in each molds), and placed into an oven with 70 ºC for 24 hours. After the reaction, the prepared hydrogels were taken out from the molds and dipped in fresh DI water to remove residual solvent and unreacted chemicals. The yield of each hydrogel is calculated using the 5



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following equation: Yield (%) =

Wdry Wmon

(1)

× 100%

where Wdry is the weight of the dried hydrogel, Wmon is the weight of the monomers in each hydrogel. The details of the hydrogels names and their chemical dosages are shown in Table 1 (for example, “G-1-2” represented the monomer molar ratio of DMC to AA was 1:2). For the sake of comparison, single component hydrogels of PAA and PDMC were also prepared by the same method. Table 1.The chemical dosages and sample names for the hydrogels. Sample

DMC (g)

AA (g)

MBA (g)

AIBA (g)

DI water (g)

Yield (%)

G-AA

0

1

0.150

0.038

10.000

96.2

G-DMC

1.250

0

0.052

0.013

9.330

85.8

G-1-2

1.250

0.692

0.155

0.039

16.730

91.6

G-1-3

1.250

1.041

0.207

0.052

20.450

93.6

G-1-4

1.250

1.383

0.259

0.066

24.110

94.5

2.3 Characterization of the hydrogels The morphologies of the hydrogels were observed by scanning electron microscopy (SEM); the chemical compositions of the hydrogels were determined by Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron (XPS), zeta potential and energy dispersive spectrometer (EDS). The details of the instruments and characterization processes are shown in the Supporting information. 2.4 Study on the pH-responsive swelling behaviors The pH-responsive swelling behaviors of the hydrogels were investigated at different pH values (from 2 to 12). The pH values were adjusted by 0.1 M HCl or NaOH solution, and the swelling ratios of the hydrogels were determined by a gravimetric method 29: The dried hydrogels were weighted firstly, and then immersed into DI water at given pH values for 24 h (The swelling equilibriums of all the 6


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hydrogels were reached within 24 hours). It should be mentioned that the solution pH would be influenced by the hydrogel, so we adjusted the solution pH every 6 hours after the hydrogel was applied in to maintain the solution pH. After that, the wet hydrogels were taken out of the water, wiped by tissues and weighted at different time intervals, until the wet hydrogels sufficient swelled (the weights of them no longer increased). The swelling ratio (SR) was calculated using equation (2):

Swelling Ratio

=

Wswell-Wdry Wdry

(2)

×100%

where Swell and Wdry are the weights of the swollen and dried hydrogels, respectively. 2.5 Study on the pH-responsive adsorption Anionic dyes of amaranth (AR) and quinoline yellow (QY), and cationic dyes of methylene blue (MB) and methyl violet (MV) were chosen to investigate the pH-responsive selective adsorption behaviors of the hydrogels. All the dyes were dissolved in DI water with the concentration of 200 µmol/L, and the pH values of the solutions were adjusted to the range of 2-12 by using 0.1M HCI or NaOH solution. All the adsorption experiments were carried out in conical flasks that contained 30 mL dye solutions and given one piece hydrogel fabricated above (the hydrogels were swelled sufficiently in DI water with corresponding pH values firstly). The conical flasks were placed in a water bath oscillator at 25 ℃ with continuous shaking. The adsorption equilibriums were reached after about 24 hours, the concentrations of each dye solutions were measured by a UV–vis spectrophototmeter 756 PC at the wavelength of 662 nm, 580 nm, 520 nm and 416 nm for MB, MV, AR and QY, respectively. The adsorption amounts (Q) and removal ratio (R) are determined using equation (3) and (4): Q=

(q0 - qt ) V

(3)

m

7



R=

q0 -qt q0

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(4)

× 100 %

where q0 is the initial concentration of the solution (µmol/L); qt is the concentration at the time t (µmol/L); V is the volume of the solution (L) and m is the mass of the dried hydrogels (g). It should be mentioned that for the above fabrication of the hydrogels, about 400 µL resulting solutions were injected in each molds, thus every piece of hydrogels have nearly equal mass in each groups. The average mass of one piece of G-DMC, G-AA, G-1-2, G-1-3, G-1-4 was 34.3 mg, 38.5 mg, 36.7 mg, 37.5 mg and 37.8 mg, respectively. For all the adsorption studies in our work, we used one piece hydrogel in each experiment, and the results were determined by calculating the mean value of each experiment. 2.6 Study on the influences of adsorption time and initial dye concentrations The influences of adsorption time and initial dye concentrations on the adsorption capacities of the hydrogels were investigated. For the influence of adsorption time, the hydrogels were applied in 500 µmol/L MB solutions with the pH value of 12 and 500 µmol/L QY solutions with the pH value of 2, respectively. The concentration of each solution was measured at different time intervals from 0.5 to 24 hours. For the influence of initial dye concentrations, the hydrogels were applied in MB (pH=12) and QY (pH=2) solutions for 48 hrs with different concentrations (from 200 to 800 µmol/L), respectively. The concentration of each solution was measured after 48 hours. 2.7 Desorption and resorption experiments The G-1-3 gels were chosen to investigate the repeatability of the hydrogels. After the adsorptions of MB (500 µmol/L, pH=12) and QY (500 µmol/L, pH=2), the G-1-3 gels were transferred into ethanol solution at 25 ℃ for 48 h to remove the adsorbed dyes. After that, the hydrogels were washed by DI water thoroughly for several times and applied to the adsorption procedures again

30

. The desorption amounts were

8


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determined by measuring the concentrations of the desorbed dyes in ethanol. The desorption and resorption ratios were calculated by comparing the desorption and resorption amounts to the adsorption amounts of the fresh prepared hydrogels, respectively. The adsorption and desorption cycle was repeated for three times following the same process to estimate the reuse abilities of the hydrogels.

3. RESULTS AND DISCUSSION 3.1 Fabrication and characterization of zwitterionic polymeric hydrogels (ZPHs) In this study, zwitterionic polymeric hydrogels (ZPHs) were fabricated by free radical copolymerization. In order to build the zwitterionic system, acrylic acid (AA) was chosen as the negatively charged segment, while methylacryloyloxyethyl trimethyl ammonium chloride (DMC) was chosen as the positively charged segment (the chemical formulas of the two monomers are shown in Figure 1). As the reactivity ratio of DMC was higher than AA when the polymerization was initiated by AIBA in aqueous solution, the moles of AA for the copolymerization were higher than DMC 31, 32

. In this study, three kinds of gels were prepared, and the mole ratios of DMC to AA

were 1:2, 1:3 and 1:4, respectively; and termed G-1-2, G-1-3 and G-1-4, respectively.

9



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Figure 1. The schematic illustration for preparing DMC and AA polymeric hydrogels via in situ polymerization.

Figure 2. (A) SEM images of the cross-section views of the ZPH. (B) Pore-size distributions of ZPH with various amounts of PAA and PDMC. The SEM images of the ZPHs are shown in Figure 2A. All the prepared hydrogels showed typical uniform porous structures, the similar structures could also be observed in an earlier research in which the hydrogels were also crosslinked by MBA 33

. Meanwhile, with the increase of the AA amounts, the densities of the pores

increased while the pore sizes decreased. The pore sizes and size distributions of the hydrogels were calculated and the results are shown in Figure 2B. The pore sizes for the G-1-2 (about 60 µm) were the largest, and those for the G-1-3 and G-1-4 gradually decreased (about 40 µm for the G-1-3 and 20 µm for the G-1-4). The pore morphology of hydrogel was determined by the chemical composition, the preparation

process,

the

degree

of

cross-linking

and

other

molecular

association/interaction 34. In this study, the introduction of PAA reduced the pore sizes 10


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of the hydrogels. This phenomenon could be ascribed to the electrostatic interaction between the PAA and PDMC segments. The polyion complexes formed by electrostatic interaction among the oppositely charged groups could be regard as physical cross-linking 24. With the increase in the amounts of the PAA segments, the electrostatic interaction became stronger, leading to the increase of the pore density and the decrease of the pore size

35-37

.The mechanical properties were enhanced for

the ZPHs, as shown in Figure S5 and Table S1.

Figure 3. (A) The FTIR spectra of the hydrogels. (B) The XPS spectra of the hydrogels. Table 2.The atom molar ratios and chemical compositions for the hydrogels from the XPS results. C (%)

N (%)

O (%)

PAA (%)

PDMC (%)

G-AA

66.42

3.88

29.69

100

0

G-DMC

79.42

7.46

13.12

0

100

G-1-2

73.28

5.62

21.10

32.53

67.47

G-1-3

72.48

4.94

22.57

40.69

59.31

G-1-4

71.77

4.37

23.86

47.54

52.46

The chemical compositions of the hydrogels were characterized by Fourier transform infrared spectroscopy (FTIR) and the results are shown in Figure 3A. For the FTIR spectrum of the G-DMC, two distinct peaks appeared at 1030 cm-1 and 950 cm-1, which were assigned to the C-N and N+ vibrations in PDMC segments, 11



respectively. For the G-AA, a peak at 1723 cm-1 could be observed, which was associated with the asymmetrical stretching vibration of the C=O. Then, for the spectra of the ZPHs, all the peaks in G-DMC and G-AA spectra were observed. The intensities of the peaks at 1030 cm-1 and 950 cm-1 (which belonged to the PDMC segments) were gradually decreased from G-1-2 to G-1-4, this was because that the relative contents of the PDMC segments in the ZPHs decreased due to the increasing of the PAA segments. The FTIR results indicated that the ZPHs were prepared successfully. To further determine the chemical compositions of the hydrogels quantificationally, XPS analysis was carried out and the atom molar ratios were calculated. The full spectra of all the hydrogels are shown in Figure 3B, and the atom molar ratios are shown in Table 2. The C 1s, N 1s and O 1s were observed for all hydrogels. For the C 1s spectra (Figure S3), the C-C bond (284 eV) and C=O bond (287.5 eV) were observed for all hydrogels, AA and DMC both offered the C-C bond and C=O bond. The C-N+ bond (285.2 eV) which belonged to the DMC could be only observed in G-DMC, G-1-2, G-1-3 and G-1-4. For the N 1s spectra (Figure S4), C-N bond (399.4 eV, which belonged to DMC and MBA) was observed for all hydrogels, while C-N+ bond (404.2 eV, which belonged to DMC) could not be observed in G-AA. Meanwhile, the atom molar ratios of the prepared hydrogels were calculated from XPS spectra 38, and the chemical compositions were also calculated. The G-AA have the highest molar ratios of C, O and have the lowest molar ratio of N. The G-DMC have the highest molar ratio of N and have the lowest molar ratios of C, O. For the chemical compositions in ZPHs, the molar ratios of PAA in hydrogels increased with the increase of AA feed ratios: 32.53 %, 40.69 % and 47.54 % for the G-1-2, G-1-3 and G-1-4, respectively. The results indicated that the relative compositions of the copolymers in hydrogels agreed well with the feed ratios of the monomers. 3.2 Study on the pH responsive swelling behaviors

12


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Figure 4. (A) The images of the prepared hydrogels in dried state (hydrogels were air-dried) and swollen state. (B) The average diameters of the prepared hydrogels in dried state and swollen state. Figure 4 shows the digital photos and the diameters of the prepared hydrogels in dried state and swollen state. Since the feeding amount for every hydrogel was equivalent, all the hydrogels showed same sizes in the dried state. In swollen state, due to the electrostatic interaction between the negatively and positively charged segments, the sizes of the ZPHs were smaller than those of the G-AA and G-DMC. In addition, with the increase of the AA amounts, the diameters of the hydrogels decreased gradually due to the enhanced electrostatic interaction. Meanwhile, the color and transparency of the G-PAA in air-dried and swelling states were different from most researches, this might be caused by the different dosage and species of initiator, cross-linker and the methods of polymerization and drying. 34, 39

13



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Figure 5. (A) The zeta potential of the hydrogels. (B) The pH-responsive swelling property for the hydrogels. (C) The repeatable swelling property for the PAA and the G-1-3. (D) The image of the G-1-3 in the pH range of 2-12. (E) The image of the G-1-3 in repeatable swelling test. For the G-DMC, an “upside-down-U type” relationship curves between the zeta potential/swelling ratios and pH values were observed: in the pH range of 2-7, the swelling ratios/the positive value of the zeta potential increased with the increase of the pH values; the swelling ratios/the positive value of the zeta potential were highest at neutral pH values (more than 6000 %/35 mV in the pH range of 7-8); and in the pH range of 8-12, the swelling ratios/the positive value of the zeta potential decreased with the increase of the pH values. The electrostatic repulsions among the positive charges N+ in G-DMC increased the swelling ratio/the positive value of the zeta potential. However, the positive charges in G-DMC were shielded by the high concentration of Cl- or OH- at acidic condition or at alkaline condition (because in this study, the pH values of the solutions were adjusted by HCl and NaOH). While for the G-AA, the swelling ratio showed classic positive pH-responsive behavior: it increased (from 50 % to 4000 %) with the increase of the pH value (from 2 to 10)

40, 41

, the

increased swelling ratio was determined by the ionization of the carboxylic group from 4 to 7, which was the pKa region of carboxylate groups

42

. The carboxylic

groups were fully deprotonated at pH>10, and the carboxylate ions were shielded by 14


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the high concentration of Na+ when further increasing the pH value, thus, the swelling ratio slightly decreased in the pH range of 10-12. The zeta potential of the G-AA showed opposite change: decreased from 2 to 10 and increased in the pH range of 10-12. The swelling ratios of the ZPHs were very different from those of the G-AA and G-DMC, and “U type” curves were observed. As shown in Figure 5B, at pH 2, the ZPHs showed the highest swelling ratios (about 3000 %). When the pH values increased in the range of 2-6, the swelling ratios of the ZPHs decreased; the ZPHs showed the lowest swelling ratios (lower than 1000 %) in the pH range of 6-8. However, when increasing the pH values in the range of 8-12, the hydrogel swelling ratios increased and gradually reached the highest swelling ratios at pH 12 (about 3000 %). A mechanism of this bi-directionally pH-sensitive swelling behavior was proposed as: When the pH values were in the range of 2-4, the negative charged carboxylic groups in the PAA segments were protonated by high concentration of H+. Hence, the electrostatic interaction between the PDMC and PAA segments was weak; the repulsion between the positive charges (N+) in the PDMC segments began to strengthen and played a dominant role, even though it had been weakened by the chloride ions (Cl-). At neutral pH values (5-9), due to the strong electrostatic interaction between the positive charged PDMC and the negative charged PAA, the polymer chains twisted together and the hydrogels showed lower swelling ratios. When the pH values increased from 9 to 12, the carboxylic groups in the PAA segments were fully deprotonated. In the same time, the positive charges (N+) in the PDMC segments were shielded by the high concentration of OH-. As a result, the repulsion between the negative charges (COO-) in the PAA segments began to play a dominant role, and thus the swelling ratios increased. In summary, the swelling ratios of the ZPHs were sensitive to pH values and this bi-directionally pH-sensitive behavior was special. The pH-responsive swelling mechanism for the ZPHs could also be confirmed by Figure 5A, the positive value of the zeta potential at low pH was attributed to the positive charge (N+). When increasing the pH value, the positive value of the zeta 15



potential decreased to zero and then the negative value of the zeta potential increased due to the deprotonation of the carboxylic groups. The carboxylic groups were fully deprotonated at pH>10, and the positive charge (N+) was shielded by the high concentration of OH-, thus, the negative potential of the zeta potential increased continuously. The point of zero electric charge (PZCs) were at pH 6.2, 4.6 and 4.0 for the G-1-2, the G-1-3 and the G-1-4, respectively. The PZC decreased with the increase in the amounts of the PAA segments, since the pKa region for carboxylate groups are around 4-7. In addition, the repeatable swelling abilities of the hydrogels were studied by repeatedly immersing the hydrogels in aqueous solutions with pH values of 2, 7 and 12, respectively. All the hydrogels showed excellent repeatable swelling abilities except the G-DMC, which cracked and broke after one or two cycles. The repeatable swelling abilities for the G-AA and the G-1-3 are shown in Figure 5C (due to the similarity, the results of G-1-2 and G-1-4 were not given). The morphologies and pH-sensitive behaviors of the ZPHs maintained well after three cycles of pH variations. The results indicated that the pH-sensitive behaviors of the hydrogels were repeatable. As a result, when the hydrogels were used as adsorbent materials, they might be reusable in aqueous environments at a wide range of pH values. 3.3 Study on the pH-responsive selective adsorption The above results indicated that the swelling ratios of the ZPHs could response to the pH change and the driving forces were the electrostatic interactions of the positively and/or negatively charged segments in the hydrogels; in other words, the charge types of the hydrogels could be adjusted by the external pH values. It is known that the hydrogels with electric charges could effectively remove some charged pollutants, such as cationic and anionic dyes

43, 44

. Therefore, we speculated that by

responding to the pH values, the adsorption capacities of the ZPHs for cationic and anionic dyes could be controlled. In this study, cationic dyes of methylene blue (MB) and methyl violet (MV), and anionic dyes of amaranth (AR) and quinolone yellow (QY), were chosen as model 16


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pollutants. The adsorption abilities of the G-AA and G-DMC at different pH values were tested and the removal ratios are shown in Figure 6. For the positively charged G-DMC, the removal ratios for MB and MV were below 10 % in the pH range of 2-12; however, those for AR and QY were higher than 95 % in the pH range of 2-12; and the adsorption behaviors showed no response to the change of the pH values. For the negatively charged G-AA, the removal ratios for AR and QY were below 10 % in the pH range of 2-12. And the removal ratios for the cationic dyes MB and MV showed positive pH-responsive behaviors: the removal ratios were very low at low pH values (the removal ratios were about 10 % at pH 2), and the removal ratios increased with the increase of the pH values; when the pH values were higher than about 4.5 (closed to the pKa of AA), the removal ratios reached the highest value (about 95 %).

Figure 6. (A) The removal ratios for MB, MV, AR and QY of the G-DMC. (B) The removal ratios for MB, MV, AR and QY of the G-AA. The removal ratios of the ZPHs for the dyes are shown in Figure 7, and the adsorption behaviors in the pH range of 2-12 were quite different from those of the G-AA and G-DMC. As shown in Figure 7A-C, in the pH range of 2-3, the removal ratios of the ZPHs for QY and AR were higher than 90 %. When the pH value increased from 3 to 7, the removal ratios decreased rapidly. Then in the pH range of 7-12, the ZPHs showed very low removal ratios for AR and QY (lower than 20%). While for the cationic dyes MB and MV, the ZPHs showed low removal ratios at pH 2 (lower than 10 %) and high removal ratios at pH=11-12 (higher than 90 %). The removal ratios increased rapidly when the pH value increased from 2 to 10. In 17



addition, the removal ratios increased faster with the increase of the PAA segments in the ZPHs (G-1-4>G-1-3>G-1-2). The digital photos of G-1-3 before and after the adsorption are shown in Figure 7D, the adsorption behaviors of the hydrogels for the dyes could be observed visually. Meanwhile, the hydrogels showed low removal ratios for both anionic and cationic dyes at particular pH values (the pH values of 6, 4.9 and 4.3 for the G-1-2, G-1-3 and G-1-4, respectively). That was to say, the ZPHs were electrically neutral at these pH values. These pH values were closed to the PZCs of the hydrogels in Figure 5A, it was proved that the hydrogel showed equal removal ratio at the PZC.

Fig. 7. The removal ratios of the G-1-2 (A), G-1-3 (B) and G-1-4 (C) for cationic dyes MB, MV and anionic dyes AR, QY. (D) The digital photographs of the adsorption results of the G-1-3 for QY, AR, MB, and MV. The chemical structure of the monomers, polymers and dyes are shown in Figure 8A. The aliphatic backbones of polymers had weak affinity with the hydrophobic benzene rings of the dyes 45, 46, thus, the electrostatic interaction was the main driving 18


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force for the adsorption, and the results verified the pH-responsive adsorption behaviors of the hydrogels. The mechanism of this pH-responsive adsorption behavior was the same as the pH-responsive swelling behavior as discussed above (Figure 8B): at low pH values, the negatively charged carboxylic groups in the PAA segments were protonated by high concentration of H+, since the removal ratios of the ZPHs for cationic dyes were very low. In the same time, the positively charged PDMC segments played the significant effect in anionic dye removal, so the ZPHs showed high removal ratios for anionic dyes. With the increase of the pH values, the carboxylic groups in the PAA segments were deprotonated gradually. Therefore, the ZPHs had more negatively charged PAA segments, the removal ratios for the cationic dyes increased. While the negatively charged PAA segments would shield the positively charged PDMC segments, and thus the removal ratios for the cationic dyes of the hydrogels decreased. The pH-responsive adsorption mechanism was also confirmed by the zeta potential test for the hydrogels, and the results are shown in Figure S6 (Supporting information).

19



Figure 8. (A) The chemical structure of monomers, polymers and dyes. (B) The illustration of the pH-responsive adsorption. 3.4 Study on the influences of adsorption time and initial dye concentration In order to reveal the adsorption capacities and mechanism of the adsorption processes, the influences of adsorption time and initial dye concentration were studied; then adsorption kinetics (the pseudo-first-order, pseudo-second-order and intraparticle diffusion equation) and isotherm equations (the Langmuir and Friedrich isotherm) were applied for further discussion. Meanwhile, since the ZPHs could show effective adsorption abilities for the cationic/anionic dyes in the acid/basic aqueous environment, the adsroption capacities for MB with the pH value of 12 and for QY with the pH value of 2 were studied. The adsorption capacities for MB with the pH value of 2 and for QY with the pH value of 12 were also studied, the results are shown 20


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in Figure S7 (Supporting information).

Figure 9. The adsorption amounts for MB(A) (pH=12) and QY(B) (pH=2) at different time intervals. The adsorption amounts for MB (C) (pH=12) and QY (D (pH=2) with different initial concentrations. The influence of adsorption time on the adsorption amounts is shown in Figure 9. For the adsorption of MB as shown in Figure 9A, the adsorption amounts increased very fast in the first 4 hours and then slowed down; the adsorption equilibriums were reached in 10 hours, and the equilibrium adsorption amounts of G-1-2, G-1-3, G-1-4 for MB were 243.2 mg/g, 270.9 mg/g, 342.9 mg/g, respectively. It was the same for the adsorption of QY in Figure 9B, and the equilibrium adsorption amounts of G-1-2, G-1-3, G-1-4 were 475.2 mg/g, 347.4 mg/g, 232.1 mg/g, respectively The results indicated that with the increase of the AA amounts, the adsorption capacities for cationic dyes were increased; while for anionic dyes the adsorption capacities were reduced. The adsorption behaviors of the adsorption processes were investigated by the 21



Pseudo-first-model, the Pseudo-second-model and the intraparticle diffusion model. The result indicated the adsorption processes were well fitted with the pseudo-second-order model and the intraparticle diffusion model, but not fitted with the pseudo-first-order, data were shown in Figure S8 (Supporting information). The effect of initial dye concentrations on the equilibrium adsorption amounts of the hydrogels (G-1-3) for MB in acid environment and for QY in basic environment are shown in Figure 9. For the removal of MB (in Figure 9C), the adsorption amounts increased with the increase of the initial concentrations, and the adsorption amounts could reach the maximums when the initial concentrations were higher than 600 µmol/L. The highest adsorption amounts of G-1-2, G-1-3, G-1-4 were 272.4 mg/g, 303.9 mg/g, 386.4 mg/g, respectively. It was same for the adsorption of QY (in Figure 9D), and the highest adsorption amounts of G-1-2, G-1-3, G-1-4 were 529.2 mg/g, 388.5 mg/g, 259.7 mg/g, respectively. The adsorption isotherms of the adsorption processes were investigated by the Langmuir and the Freundlich adsorption isotherms models. The result indicated the adsorption processes were well fitted with the Langmuir model, but not the Freundlich model, data were shown in Figure S9 (Supporting information). 3.6 Study on the adsorption repeatability The G-1-3 hydrogels were chosen to investigate the adsorption repeatability. After the adsorptions for MB in acid environment (pH=2) (or for QY in basic environment, pH=12), the hydrogels were transferred into ethanol solution for desorption. After the desorption, the hydrogels were applied to MB solution (or QY solution) again. This adsorption-desorption process was repeated for three times and the results are shown in Figure 10. In Figure 10A, the hydrogels showed high desorption ratios in ethanol (nearly 90% in the first desorption experiment and had a slightly decrease in next adsorption-desorption cycles). The hydrogels also had high resorption ratios (nearly 80% in the amounts of the original hydrogels after three adsorption-desorption cycles). In Figure 10B, although the resorption amounts reduced gradually with the increase of the repeating times, after three adsorption-desorption cycles the resorption amounts 22


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could still reached about 282.2 mg/g and 203.1 mg/g for MB and QY, respectively. These results demonstrated that the hydrogels could re-adsorb anions and/or cations more than once without decreasing the adsorption efficiency.

Figure 10. (A) Desorption and resorption curves for MB and QY. (B) Adsorption amounts for MB and QY in different cycles. 4. CONCLUSION In this work, novel zwitterionic hydrogels were successfully prepared by simple co-polymerization of DMC and AA. The relation curves of the hydrogel swelling ratios and the pH values showed a “U type”, indicating the special bi-directionally pH-responsive behaviors due to the synergistic electrostatic interactions. The adsorption tests indicated that by responding to pH values, the adsorption capacities of the hydrogels for cationic dyes showed positive responsive behaviors; while the adsorption capacities for anionic dyes showed negative responsive behaviors. This is to say, the selective adsorption capacities for anionic and cationic dyes could be switched by adjusting the pH values. The adsorption processes fitted the pseudo-second-order kinetic model and Langmuir isotherm well. In addition, the adsorption-desorption experiment proved that the hydrogels could be reused for dyes removal. We hope that this research could provide new ideas for the development of smart materials, especially the bio-medical and water treatment fields which involved more pH-responsive applications.

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ASSOCIATED CONTENT Supporting Information This material is available free of charge via the Internet at http://pubs.acs.org. Characterization for hydrogels, the EDS results, the XPS results, the mechanical property, the mechanism for adsorption, the adsorption kinetics results, the adsorption isotherm results, adsorption experiment using a dye adsorbed hydrogel. AUTHOR INFORMATION Corresponding Authors *E-mail:

[email protected]

or

[email protected]

(C.S.

Zhao);

[email protected] (X. Zhang). *Tel.: +86-28-85400453, Fax: +86-28-85405402 Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This work was financially sponsored by the National Natural Science Foundation of China (Nos. 51433007 ， 51503125 and 51673125), the State Key Research Development Programme of China (2016YFC1103000 and 2016YFC1103001), and the Younth Science and Technology Innovation Team of Sichuan Province (Grant No. 2015TD0001). We should also thank our laboratory members for their generous help.

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graphical abstract 67x28mm (300 x 300 DPI)


Bidirectionally pH-Responsive Zwitterionic Polymer Hydrogels with

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