Giant Microgels with CO2-induced On-Off, Selective and Recyclable

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Energy, Environmental, and Catalysis Applications

Giant Microgels with CO2-induced On-Off, Selective and Recyclable Adsorption for Anionic Dyes Zanru Guo, Qiang Chen, Hongjian Gu, Zhanfeng He, Wenyuan Xu, Jiali Zhang, Yongxin Liu, Leyan Xiong, Longzhen Zheng, and Yujun Feng ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b13448 • Publication Date (Web): 17 Oct 2018 Downloaded from http://pubs.acs.org on October 18, 2018

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ACS Applied Materials & Interfaces

Giant Microgels with CO2-induced On-Off, Selective and Recyclable Adsorption for Anionic Dyes Zanru Guo,a* Qiang Chen,a Hongjian Gu,a Zhanfeng He,*b Wenyuan Xu,a Jiali Zhang,a Yongxin Liu,a Leyan Xiong,a Longzhen Zhenga and Yujun Feng*c a

Department of Polymer Materials and Chemical Engineering, School of Materials

Science and Engineering, East China Jiaotong University, Nanchang, Jiangxi 330013, P. R. China. b

State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Southwest

Petroleum University, Chengdu 610500, P. R. China. c

Polymer Research Institute, State Key Laboratory of Polymer Materials Engineering,

Sichuan University, Chengdu 610065, P. R. China.

KEYWORDS: microgel; CO2 stimulation; on-off adsorption; selective sorption; anionic dyes ABSTRACT: Adsorbents that are capable of controllable pollutants adsorption and release without secondary pollution are attractive in water treatment. Here, we propose eco-friendly CO2 as a trigger to switch the charge states and collapse-expansion transition of giant microgels consisted of hydrophilic acrylamide (AM) and hydrophobic 2-(diethylamino)-ethyl methacrylate (DEA), and demonstrated the on-off, selective and recyclable adsorption of anionic dyes on microgels under CO2 stimulation. Apart from easy-handling separation from the water by a simple 1

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filtration process, the maximum adsorption capacity is as high as 821 mg·g-1, and the adsorption

isotherms

and

kinetics

obeyed

Langmuir

isotherm

and

the

pseudo-second-order kinetics models, respectively. The anionic dye also can be separated from the mixture solution using CO2-treated microgels. Moreover, a wastewater treatment prototype with microgel-packed column was fabricated. Under continuous flow condition, the dye was removed and recovered by alternative bubbling CO2 and flushing with aqueous alkali (pH 12). Thus this type of microgels with CO2-induced protonation-deprotonation transition can serve as a cost-effective, environmentally friendly and efficient absorbent for water purification applications.

INTRODUCTION

Currently, environmental pollution related with dyes has become an issue of global concern and is gaining significant attention,1 as it were widely used in many industrial fields2,3 and ca.15% of the used dyes enter the environment as effluent.4 Most dyes not only are poor degradable5 and toxic6,7 but also exhibit carcinogenic and teratogenic properties,6-8 which causes major risk to human health and aquatic-to-terrestrial ecosystems. Therefore, removal of these dyes from effluents is of great importance from the point of view of environmental remediation. Up to day, several

treatment

techniques

such

as

coagulation,9

membrane

filtration,10

photocatalysis,11 advance oxidation,12 adsorption13 etc., have been employed to remove the dyes from wastewater.9 Among these treatment procedures, adsorption technology is especially attractive because it is economic, easy operation, high 2


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efficiency, and does not result in the formation of harmful substances.13,14 In recent years, various materials such as activated carbon,15 clays,16 mesoporous silica,17 polymeric reins and hybrids18 have been employed as adsorbents for removal dyes. However, most of them usually require harsher regeneration conditions (e.g. steam, oxidation, strong acids or bases treatments, organic solvent extraction),19,20 and showed low selectivity.21 Adsorbent with selective adsorption and eco-friendly regeneration may separate dyes from their mixture during the removal of dyes from the polluted water, and then provide possibility in recovery of adsorbate without secondary pollution, which is crucial in terms of environmental and economical concern. Recently, smart adsorbents with trigger-controlled adsorption shown selective adsorption and eco-friendly regeneration.22-27 They used UV or temperature trigger to open/close the pores of mesoporous silica grafted with photo-responsive moieties or thermal-responsive polymers to switch the accessibility to the reactive adsorption sites.22-27 But its adsorption capacities are relatively low (lower than 50 mg g-1),22-27 which may be attributed to the absence of abundant functional groups interacted with adsorbates. Alternative smart adsorbents with high adsorption capacity are still required for wastewater treatment. To enhance the adsorption performance, functionalization of adsorbent with abundant functional groups represents one of the choices to satisfy such requirements.28 Polymeric hydrogels have 3D cross-linked network structure and can carry many interactive moieties, implying that it shown great potential as alternative adsorbents.29,30 Some hydrogels have exhibited the selective adsorption to dyes based 3



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on the chemisorption or electrostatic interaction between hydrogel and dyes.29,30 Nevertheless, hydrogels often showed big block morphostructure and possess low aspect ratios, which lead to that the dye’s adsorption mainly happened on the surface of gel and the absorption efficiency consequently decreased.31 In this regard, microgels have small particle size and high specific surface area while hydrogel’s intrinsic structure and properties are preserved.32 The advantages offered by microgels will improve the absorption efficiency. To obtain the smart adsorption behavior similar to that of smart adsorbents, stimuli-responsive microgels have been recently employed.

Serpe’s

group33,34

synthesized

serial

different

size

poly(N-isopropylacrylamide)-co-acrylic acid (PNIPAM-co-AAc) microgels, and found the dye’s uptake efficiency increased at elevated temperature based on the thermal-responsive property of PNIPAM. Li et al.35 demonstrated a multi-responsive microgel of hyperbranched poly(ether amine) (hPEA-mGel) for the selective adsorption and separation of fluorescein dyes. These microgels exhibited switchable and selective adsorption mainly via hydrophobic interaction, but their regeneration and the adsorbate recovery were overlooked, and the adsorption process was high energy consumption. Apart from hydrophobic interaction, electrostatic interaction was adopted to selectively sorb ionic dyes.30 When net charge of microgels can be switched by triggers, microgels may exhibit intelligent adsorption ability and smart regeneration duo to the switchable electrostatic interaction between microgels and adsorbates. Very recently, we and others have employed eco-friendly CO2 and inert gas such as 4


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nitrogen to switch between ionization and deionization of polymers with amidine or amino groups, and then control the hydrophobic-hydrophilic transition of corresponding polymers.36-39 Compared with acids/bases stimulation, the CO2 switching process was robust and repeatable over many cycles without any chemical contamination and salt accumulation.36-39 Therefore, CO2 would be a “green” trigger to switch the charge states for on-off, selective and recyclable adsorption. As for microgels, it was demonstrated that CO2 could react with the CO2 responsive components and tuned their charge properties.40-44 However, the overwhelming majority of CO2-responsive microgels reported so far are confined to gas-induced swelling and deswelling.40-44 To the best of our knowledge, there have been few reports of using CO2 to tune the adsorption behaviors of smart microgel adsorbents. Besides, the size of the reported CO2-responsive microgels is less than one micron, making that their separation reckoned on energy-consuming centrifugation rather than simple filtration.40-43 As such, it is anticipated that CO2-sensitive microgel possessing suitable size would be a smart adsorbent with controllable adsorption and easy-handling separation. In this report, resorting to inverse-suspension polymerization technique, just one simple step, a series of giant CO2-sensitive microgels were synthesized by using 2-(diethylamino)-ethyl methacrylate (DEA) and acrylamide (AM) as monomers. The microgels can undergo switchable collapse-expansion transition via the protonation and deprotonation of amine groups upon CO2 trigger. Based on their smart characters, the microgels exhibited on-off, selective and recyclable adsorption for anionic dyes by 5



reversible electrostatic interactions. The adsorption isotherms and kinetics were fitted with different models. After adsorption, the microgels could easily be separated from water by a simple filtration process. The anionic dye also can be separated from the mixture solution using CO2-treated microgels. Moreover, a wastewater treatment prototype with the microgel-packed column was constructed. Under continuous flow condition, the dye was removed and recovered by alternative bubbling CO2 and flushing with aqueous alkali (pH 12). Based on these desirable characteristics, this type of CO2-sensitive giant microgels may find practical applications in the large-scale water purifcation.

EXPERIMENTAL SECTION

Materials. 2-(Diethylamino)ethyl methacrylate (DEA, 99%, Aladdin Co., Ltd) was distilled under reduced pressure prior to use. Acrylamide (AM, 99%, Aladdin Co., Ltd) and N,N′-methylenebis(acrylamide) (MBA, ≥99%, Aladdin Co., Ltd) was purified by recrystallization. Sorbitan monooleate (Span 60, 98%), Alizarin red (AR, 96%), Allura Red AC (ARAC, 96%), Methyl blue (MB, 98%), Methylene blue (MEB, 98%), Fluorescin (FR, 97%), were purchased from Aladdin Co., Ltd and used as received. Preparation of Microgels. A series of microgels with different feed ratios of DEA were synthesized by inverse-suspension polymerization. The obtained microgels were coded as Mx, where M represents microgel, and x is the DEA molar ratio with respect to AM and MBA. Note that in M100, the DEA molar ratio is 98 mol%. A typical procedure for the preparation of M80 is as follows: 70 mL of cyclohexane and 0.50 g 6


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of Span 60 were added into a 250 mL three-necked round-bottomed flask, following by vigorously mechanical stirring at 37 oC for 30 min. Then, a mixture containing 9.0324 g of DEA, 12 mL of 4 M HCl, 0.7798 g of AM, 0.1880 g of MBA and 0.100 g of potassium peroxydisulfate (KPS) was gradually added to the flask while stirring at 400 rpm. The solution was deoxygenated by bubbling with nitrogen for at least 30 min. After polymerization at 70 oC for 24 h, the microgel was separated from cyclohexane by filtration. Then, the microgel was transferred into serials of NaOH solution with pH 7.5, pH 8.0, pH 9.0, pH 10.0 and pH 12 in sequence to deprotonate the positively charged amine group of DEA. After washing with pure water and drying at 60 oC for 24 h, M80 was obtained. The other four microgels were prepared by a similar procedure, and the detailed parameters of microgels are given in Table 1. Determination of Equilibrium Swelling Ratio of Microgels. The swelling ratio of microgels were determined by the gravimetric method, in which 0.40g of the original microgels was kept in a tea bag whose pore diameter were less than microgels and immersed in a beaker which covered with plastic wrap and contained 250 mL of pure water for swelling. The swelling ratio of microgels with CO2 or N2 was realized by bubbling CO2 or N2 into beaker at 200 mL min-1 for 2 h. The bag contained microgels was removed at different times, and excess water was drained and wiped. Then the bag was weighed and then returned to the respective beakers for further swelling. The swelling ratio (S, g g-1) of the microgels was defined with respect to the weights of tea bag (Wt), original microgels (Wd) and tea bag containing swollen microgels (Ws) as S=

7


(1)


Page 8 of 41

Selective Adsorption of Dyes on Microgels. Dried microgels (40 mg) swelled in pure water under N2 atmosphere or with the aeration of CO2 at 200 mL min-1 for 30 min, obtaining N2-treated and CO2-treated microgels. This two kind of microgels were packed in syringe tube, respectively, forming the columns with N2-treated or CO2-treated microgels. 10 mL of dyes solution passed through the columns under the force of gravity. The filtrates were analyzed on UV−vis spectrophotometer. Based on the absorbance value at the maximum absorption, the concentration of unabsorbed dyes in filtrate was calculated by its predetermined calibration curve. Adsorption of Dyes on Microgels. 50.0 mg of dried microgels swelled in pure water following CO2 aeration at 200 mL min-1 for 30 min, and was added into 50 mL of dye solutions at 25 oC. After separation of microgels, the concentration of unabsorbed dye in the medium was measured by an UV−vis spectrophotometer. For equilibrium adsorption experiments, the adsorption was carried out for 48 h to ensure the adsorption process has reached equilibrium. The equilibrium adsorption capacity (Qe, mg g-1) was evaluated by the following formula: Q =

( )

(2)

where C0 and Ce are the initial and the final dye concentrations (mg L-1), respectively. V is the volume of the solution (L) and m is the mass of microgels (g). Qe increased with the increasing of C0 and will remain the same when C0 is large enough. The maximum equilibrium adsorption capacity was labeled as the maximum adsorption capacity (Qm). For kinetic adsorption studies, the unadsorbed dye concentration (Ct) at various 8


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times was measured. The adsorbed dye amounts were calculated from Equation (3). Q =

( )

(3)

where Qt (mg g-1) is the amount of dye adsorbed for one gram of adsorbent at time t, C0 (mg L-1) and C (mg L-1) are the initial concentration of dye and the concentration at time t. Reusability of Microgels. Anionic dye ARAC was used to research the cyclic adsorption/desorption. Swelled M80 (50mg in dried state) treated with CO2 was placed in a 50 mL ARAC (800 mg L-1) solution for 48 h, then the adsorption capacity of ARAC was calculated using Equation 2. After separation by filtration, the microgel was soaked in NaOH solution with pH 12 for 24 h to desorb the dye. After collection, the microgel was rinsed with pure water and treated with CO2 at 200 mL min-1 for 30 min for next adsorption process. The adsorption-desorption procedure was repeated 10 times. Characterization. Infrared spectra were recorded on a Fourier transform infrared (FTIR) spectrometer (Perkin-Elmer Spectrum Two) from 4000–400 cm-1 using the KBr pellet method. An elemental analyzer (AA280 Duo (280FS+280Z), Varian Company, USA) was used to determine the relative percentage of each element in the microgels. The optical microscopy images were recorded on an SG50-3A43L-A microscope (Suzhou Shenying Optical Co.Ltd) connected to a computer to observe the morphology of microgels. With total 100 microgels counted by an image analysis software (Nano Measure) on diameter distribution. 9



Scanning electron microscope (SEM) observation was done on a field-emission SEM (FEI Czech Republic Limited; Nova NanoSEM 450) equipped with an Oxford EDS X-Max 20. Microgels samples were frozen by immersing into liquid nitrogen, and then lyophilized, sputter-coated with gold before analysis. The zeta potential values of microgels were measured with zetameter ZetaPALS (Brookhaven, USA). Each test was carried out for five times and the average values were taken as the final results. RESULTS AND DISCUSSION Synthesis and structural characterization. Considering the merit of easy-handling separation of large size microgels, inverse-suspension polymerization was employed here to prepare the giant microgels. To impart CO2-sensitive property on microgels, CO2-responsive component is required. Based on previous reports,36-44 polymers containing amidine or tertiary amine groups exhibited CO2-responsiveness. However, as for amidine-based CO2-responsive polymers, heating is usually needed to expel the captured CO2.36 In contrast, polymer containing tertiary amine groups could realize reversible responsiveness in the same condition upon alternate treatment of CO2 and N2.45 DEA was an inexpensive, commercially available and typical monomer with tertiary amine group and was selected as the monomer. As DEA is hydrophobic, a common water-soluble monomer acrylamide (AM) was chosen to tune the hydrophilicity of microgels. Scheme 1a shows the general synthetic route used for the preparation of giant microgels which cross-linked by N,N′-methylenebis(acrylamide) (MBA). To examine the effect of poly[2-(diethylamino)ethyl methacrylate] (PDEA) 10


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content on the responsive and adsorption performance, a series of giant microgels were synthesized by varying the feed ratios of DEA, as listed in Table 1. Note that the hydrophobic DEA was acidized with aqueous hydrochloric acid solution so as to copolymerize with hydrophilic AM, and microgels were obtained after neutralization. In such way, just one simple step was needed to obtain the targeted giant microgels. Scheme 1 (a) The synthesis strategy for microgels. (b) CO2-responsive mechanism of microgels

Table 1 Synthesis and characterization data for microgels prepared by reversed phase suspension polymerization of DEA/AM/MBA in cyclohexane/water mixture with Span 60 as the stabilizer under different reaction conditions. Sample

M0

AM

MBA

Oil/Water

dN2a

dCO2b

(mol %)

(mol %)

(mol %)

(v/v)

(µm)

(µm)

0

98

2

3:1

126±39

127±38

1.01

M30

30

68

2

3:1

115±28

126±36

1.10

M50

50

48

2

3:1

58±24

123±30

2.12

M80

80

18

2

3:1

48±14

173±37

3.60

d

3.42

M100 a

RSDc

DEA

98

0

2

3:1

d

103±75

352±32

dN2 is the diameter of microgels in N2 atmosphere and calculated from OM images.

b

dCO2 is the diameter of microgels after treating with CO2 and calculated from OM

11



images. cRSD is the relative swelling diameter which is determined by dN2/dCO2. dThe diameter of M100 was only counted from that with regular shape. FT-IR spectroscopy was employed to confirm the component of the resultant microgels. As shown in Figure 1a and Figure S1, the characteristic stretching vibrations of C=O and bending vibrations of N-H from AM and MBA appeared at 1640 cm-1 and 3450 cm-1,44 respectively, suggesting that M0 consisted of AM through cross-linking by MBA. After copolymerization with DEA, such as in M30, new peaks appeared at 1130 cm-1 and 1730 cm-1, which are assigned to the stretching vibrations of N-C and C=O of PDEA,45 indicating that DEA was introduced into microgel. Moreover, the strength of the peaks from PDEA increased with an increasing DEA ratio from 30 mol% to 98 mol%, implying that the component of PDEA in microgels increased. To further confirm the relative amount of PDEA in microgels, elemental analysis was carried out. It was found that the measurement values were very close to that of theoretical values (Table S1), suggesting that the composition of as-fabricated microgels coincided with the designed ratios. However, it should be pointed out that few of amide groups of microgels might be hydrolyzed into carboxyl groups during polymerization, though the characteristic peak at 1,560 cm−1 corresponding to C = O on carboxyl group46 was not clearly observed (Figure S1). Thus the effect of carboxyl groups on microgel’s properties was not considered in this work.

12


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Figure 1 (a) FT-IR spectra of microgels. (b, c) Optical microscope (OM) image and the size distribution histogram of M80 in pure water. (d) Swelling ratio of microgels in pure water. To visualize the morphology of microgels, optical microscope (OM) was taken. Figure 1b and Figure S2a-e showed OM images of the microgels suspended in pure water. M0 display spherical shape and smooth surfaces. With over 100 microgels counted by image analysis software (Nano Measure) on diameter distribution, we found that M0 had an average size of 126±39 µm with relatively broad distribution. With the adding of DEA, microgels with smooth surfaces were still obtained (Figure 1b and Figure S2b-e). Extraordinarily, their size and size distribution gradually became smaller with increasing DEA ratio from 30 mol% to 80 mol%. Among the samples, M80 possessed a smallest size (48±14µm) and a relatively narrow size distribution (Figure 1c). This may be mainly attributed to the fact that the hydrophobicity of microgels gradually enhanced as increasing the content of hydrophobic PDEA and the microgels could not swell, though their preparation 13



conditions were held constant. For size distribution, the protonated DEA in synthesis process might inhibit the coalescence of suspended droplets because of electrostatic repulsion between the positively charged amine groups, resulting in narrower distributions. However, when the ratio of PDEA in the microgel was further increased such as in M100, the microgel showed irregular shape including large size and cracked microgels and fragment hydrogels (Figure S2e), which may be ascribed to that stabilizer Span 60 could not provide colloidal stability for the system with high electrostatic repulsion. Since the size of the obtained microgels is over 10 to 100 times bigger than that of classical microgels,41-43 we called them as giant microgels. As mentioned above, the microgel contained more DEA and had higher hydrophobicity. To confirm this, the swelling ratios of dried microgel in pure water were tested, as hydrophilic microgel is facile upon water absorption. As illustrated in Figure 1d, the swelling ratios gradually decreased with an increasing DEA ratio from 0 mol% to 100 mol% with respect to AM, indicating that hydrophilicity of microgel decreased. Thus the hydrophilicity of microgel can be tailored by varying the monomer ratios. In addition, the stability of the microgels was evaluated by sonication. As given in Figure S3, the microgels could suffer from sonication for 1 h without rupture, indicating that microgels are stable. CO2-Switchable behavior of microgels. We found previously that the PDEA itself can undergo a hydrophobic–hydrophilic transition by the stimulation of CO2,38,45 and reported microgels containing PDEA exhibited size expansion and collapse upon cyclic purging with CO2/N2.40-44 It was anticipated that the as-fabricated giant 14


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microgels are still gas sensitiveness. To confirm this speculation, CO2 gas was introduced into the suspensions of giant microgels. Figure S4a shows the swelling ratio variation of the microgels suspended in water with time of bubbling CO2. All the microgels absorbed water and swelled at a higher rate in the beginning. After a certain period of time, the water uptake became constant, and the microgels achieved their equilibrium swelling ratio. Especially for M80 (0.026 g mL-1), a liquid-like suspension became a self-supporting solid-like material after being treated with CO2 (Figure S5), causing a CO2-triggered jamming transition.47 The maximum swelling ratio increased between M0 and M80 before slightly decreasing in M100. Compared with that of without bubbling CO2, the maximum swelling ratio of microgels exhibited an inverse tendency (Figure S4b), i.e., microgels with high DEA displayed a high water swelling under the treatment of CO2. The net swelling ratio could be calculated by deducting the water absorption in pure water without CO2. One can find that the net swelling ratio of M0 was close to 0, while microgels with DEA, especially for M80, have obvious net increase in swelling ratio (Figure 2a). Nevertheless, after removing CO2 by purging N2, the microgels desorbed water, which further led to the deswelling (Figure 2b). Such a procedure is still effective beyond three cycles of bubbling CO2 and N2 (Figure 2b), suggesting that the swelling/deswelling state of microgels can be switchablely controlled.

15



Figure 2 (a) Net swelling ratio and the relative swelling volume (RSV) of each microgel. (b) Net swelling ratio evolution of M80 in water after alternate aeration of CO2 and N2 at 200 mL min-1. (c) OM images of M80 during treating with CO2 and N2. (d, e) SEM images of M80 treated with N2 and CO2. (f) Zeta potential of M80 during cycling CO2/N2 stimulation. In order to get direct information on the swelling/deswelling state of microgels during the alternative treating with CO2 and N2, OM observation was performed. After purging CO2, obvious expansion states of the microgels can be observed except for M0 (Figure S2f-j), which are in good agreement with the results of swelling ratio. Based on the change in diameter (Table 1), the relative swelling volume (RSV) which defined as the volume of the swollen microgels over that of the collapsed ones44 was calculated (Figure 2a). The variation of RSV values was consistent with those 16


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observed in the net swelling ratio. Possessing the highest net swelling ratio (ca. 22 g g-1), M80 also exhibited the highest RSV (46.9). The RSV of M80 is higher than that of the reported microgels44 though most of the previous papers did not provide this data. However, when N2 is bubbled into the suspensions to expel CO2, the microgels reinstated in the initial size (Figure S2k-o). During the responsive process, the expansion and collapse of one microgel was directly recorded by OM. Figure 2c clearly showed the microgel fast swelled within one minute under the stimulation of CO2 and the microgel gradually reverted back to the initial size when CO2 was blown away, indicative of the reversiable collapse-expansion transition. Since M80 has the best morphology and most obvious CO2 responsiveness, we would mainly focus on M80 in the following experiments unless otherwise specified. To further visulize micromorphology of microgels at the swelling and collapse state, SEM which is very often used to study morphological details of the gels was employed to observe their microstructure. As depicted in Figure 2d, M80 exhibited a compact collapse structure under N2 atmosphere and the diamater is ca. 50 µm which is consistent with the OM observation. In contrast, an expansion morphology with porous network, similar to the structure of hydrogels, was found for CO2-treated microgel (Figure 2e), suggesting that water is inclined to penetrate into microgel to expand its volume under the uptake of CO2. Thus microgel undergo a compact collapse to porous expansion transition upon the trigger of CO2. This phenomenon should be attributed to the CO2-responsiveness of PDEA component, as shown in Scheme 1b. After purging CO2 into suspension, the tertiary 17



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amine groups of PDEA reacted with CO2 and water to form ammonium bicarbonates and

transferred

into

cationic

repeat

units,38,45

leading

to

the

hydrophobic-to-hydrophilic conversion for PDEA.38,45 During the aeration of CO2, pH of the suspension was monitored and dropped from 8.1 to 4.5. According to the 7.3 of pKa value of PDEA homopolymer and protonation degree equation δ = 1/(1 + 10pH˗ pKa 44

),

the protonation degree of PDEA increased from 13.7% to 99.8%. As the ions

concentration inside the microgel is higher than the outside, the osmotic pressure formed and drove water to permeate into hydrophilic network of microgel, causing microgel to swell. The slightly decreasing of the swelling ratio of M100 in comparison with that of M80 may ascribe to the hydrophobicity of unprotonated DEA which could restrain water absorption. After the removal of CO2, an opposite deprotonation effect occurred in microgel. Hence the osmotic pressure disappeared and the hydrophilicity decreased, further leading to the deswelling of the microgel. To further confirm that abovementioned responsive mechanism happened in microgels, zeta potential of the suspension had been measured. The zeta potential for the microgel treated with CO2 reached +45.0 mV, as exhibited in Figure 2f, supporting the formation of positive ammonium ions in microgel. After removing CO2 by purging N2, the zeta potential decreases to nearly zero (ca. 0.6 mV) (Figure 2f), suggesting that positive ammonium ions of the microgel were mostly deprotonated. When repeating this sequence several times, the zeta potential of the suspension was recovered each time; moreover, the final zeta potential without CO2 was also reproducibly obtained. These results amply demonstrate that the response of the 18


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suspension to CO2 was fully reversible and reproducible. Application in adsorption of dyes. Following confirming the CO2-induced switchable charge states of giant microgel, the use of CO2 as a trigger to tune the adsorption may be attained. We chose three types of water-soluble model dyes including cationic (Methylene blue, MEB), nonionic (Fluorescin, FR) and anionic (Alizarin red (AR), Allura Red AC (ARAC), Methyl blue (MB)) dyes, to evaluate the adsorption behaviors of the obtained microgels, and their structure is illustrated in Figure S6. Possessing highest RSV, M80 was used to fabricate microgel columns for removing dyes, as adsorption column is convenient for observation. Firstly, the dye solutions passed through the columns packed with N2-treated microgel (Figure 3a and Figure S7a-d). It was found that the filtrates still exhibited the color of corresponding dyes (Figure 3a and Figure S7a-d), indicating that microgel could not adsorb the dyes under the treatment of N2. To evaluate the adsorption efficiency, UV–Vis spectroscopy was used to monitor the absorbance of dye solutions before and after filtration, as the spectra are known to be very sensitive to the concentration of dyes. Compared with that of original dye solutions, only a small decreasing in absorbance for the filtrates (Figure 3b and Figure S7e-h), which verified that the dyes could not be effectively removed by N2-treated microgel, i.e, the microgel was in “off” state and exhibited dyes repellency.

19



Figure 3 (a, b) and (c, d) Images and UV-Vis spectra of the ARAC solution before and after passing through the column with M80 treated with N2 and CO2. The initial concentration was 100 mg L-1. The final concentration after filtrating by M80 treated with N2 and CO2 were 89 mg L-1 and 0.06 mg L-1. However, when the dyes solution passed through the column with CO2-treated microgel, one can find that the filtrates of cationic and nonionic dyes did not show an obvious change in color (Figure S9a, b), while all three kinds of anionic dye solutions tranferred into colorless after filtration (Figure 3c and Figure S9c, d), suggesting that microgel treated with CO2 selectively adsorbed anionic dyes. Similarly, absorbance of dyes were recorded by UV–Vis spectra. After filtration, cationic and nonionic dyes showed high absorption peaks (Figure S9e, f), while the absorbance of anionic dye solutions decreases to nearly zero (Figure 3d and Figure S9g, h), indicating that anionic dyes were selectively and completely removed. In other words, microgels turned to “on” state for adsorption of anionic dyes following the uptake of CO2.

20


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The above results show that microgels exhibited an on-off, selective sorption behavior for anionic dyes by alternative bubbling CO2 and N2, which should be ascribed to CO2-responsive characteristics. As confirmed by SEM (Figure 2d), under N2 atmosphere, M80 microgels were bias to hydrophobicity and displayed a compact collapse morphology because of the hydrophobic PDEA component (Figure 2d), which may inhibit the dyes to penetrate into microgels. When microgels contacted with CO2 in water, microgels expanded and had porous network structure caused by the formation of protonated PDEA·H+ (Figure 2e), which facilitate the dyes to diffuse into microgel network. However, microgels with positive charge would show electrostatic repulsion and weak interaction with cationic and nonionic dyes, resulting in a very small adsorption. In contrast, electrostatic attraction could form between the positively charged amine groups of microgel and the negatively charged anionic dyes, making the dye preferable to be adsorbed. Thus, anionic dyes were selectively absorbed, whereas the cationic and nonionic dyes were excluded. To validate the selective adsorption depended on electrostatic attraction, the maximum adsorption capacity (Qm), an important parameter for adsorbents in practical applications,30 was calculated. As shown in Figure 4a, M80 with CO2 exhibited a very low Qm to cationic MEB and nonionic FR caused by physical diffusion. However, Qm for anionic dye AR, ARAC, and MB were 821 mg g-1, 500 mg g-1, 302 mg g-1, respectively, indicative a high adsorption for anionic dyes. The large difference in Qm suggests that microgels treated with CO2 exhibited the selective adsorption to the anionic dyes based on the electrostatic attraction, as the positively 21



charged microgels did show more adsorption of anionic dye such as AR, ARAC, and MB than cationic and nonionic dyes. Interestingly, Qm of the three kinds of anionic dyes followed the order AR > ARAC > MB. Though the size of dye has effect on its adsorption, the primary cause of this variation may be the number of sulfonic acid groups in dye molecule.13 The molecule of AR, ARAC and MB has one, two and three sulfonic acid groups, respectively. The dye with more sulfonic acid groups would consume more cationic groups when it was adsorbed, leading to a smaller Qm. Based on their Qm, we can calculate that each dye molecule of AR, ARAC, and MB interacted with approximately 2.13, 4.80 and 12.83 positively charged amine groups of microgel (see Supporting Information), respectively, which consistent with the abovementioned analysis. Here it further indicated that the electrostatic attraction is the driving force for selective adsorption.

Figure 4 (a) Qm of different dyes on M80 treated with CO2. (b) Qm of dye ARAC on different microgels with CO2. (c, d) Langmuir and Freundlich adsorption isotherm models of ARAC on M80 treated with CO2.

22


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To provide additional evidence that electrostatic attraction between microgels and the guest dyes is the predominant factor to Qm. The microgels with different DEA content were employed to adsorb ARAC. As shown in Figure 4b, Qm gradually increased with the increasing of positively charged amine groups of microgels, i.e., the more cationic groups the microgel has, the more anionic dye was adsorbed, providing another proof that electrostatic attraction is the principal factor for selective adsorption. Though Qm of M100 was higher than that of M80, M100 has a bad formation in shape and then was ignored in following adsorption studies. The easy-handling separation from the water after adsorption is another salient point of our giant microgels. Expectively, after the adsorption, the giant microgels could be separated and collected by filtration (Figure S10). In addition, the microgels could be placed in water permeable bag like tea bag for adsorption (Figure S10), which provided a convenient way for separation. To understand the adsorption behavior, i.e., how adsorbates interact with adsorbents, adsorption isotherms for the anionic dyes were investigated. The equilibrium isotherms for the adsorptions of AR, ARAC, and MB were shown in Figure S11. Usually, the experimental data was correlated with adsorption isotherm models to elaborate the characteristic of adsorbent.14,30,31 Two classic isotherm models, Langmuir48 and Freundlich49, have been widely used to fit adsorption isotherms. The Langmuir and Freundlich models assume that a monolayer adsorption takes place on a homogeneous surface with adsorption sites of uniform adsorption energies48 and

23



Page 24 of 41

multilayered adsorption exist on heterogeneous surfaces,49 respectively. Their equations are expressed as follows, Langmuir model:

=

+

(4)

Freundlich model: InQ = InC + InK

(5)

where, Qe (mg g-1) is the equilibrium adsorption capacity; Ce (mg L-1) is the equilibrium concentration of adsorbate in the solution; Qm (mg g-1) is the maximum amount of the dyes adsorbed per unit weight of the adsorbent; KL (L g-1 ) is the Langmuir adsorption equilibrium constant; KF (mg g-1) and n are Freundlich constants indicating the adsorption capacity and adsorption intensity, respectively. In Langmuir model, separation factor (RL) is used to predict if an adsorption system is “favorable” or “unfavorable”.50 RL >1, RL = 1, 0 < RL < 1, and RL = 0 indicate the type of isotherm to be unfavorable, linear, favorable, and irreversible, respectively.49 RL is defined by the following equation:

R = !"

(6)

where C0 is the initial dye concentration. According to Langmuir and Freundlich models, the linearized adsorption isotherms for the corresponding adsorption curves were exhibited in Figure 4c and d, from which the correlation coefficients and model parameters were calculated, as presented in Table 2. Comparison of the correlation coefficients (R2), the Langmuir adsorption isotherm has a better fit than the Freundlich model. The maximum adsorption capacities (Qm,cal) for the dyes were calculated from Langmuir model, which matched the experimental data (Qm,exp) with small deviations (Table 2). This result further 24


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confirmed that the adsorption obeys the Langmuir model, i.e., the monolayer dyes were adsorbed on the network through ionic interaction. In addition, RL values are in the range 0 < RL < 1, indicating that the micorgel was favorable adsorbents for removal of dye from aqueous solution. Table 2 Isotherm parameters for the adsorption of AR, ARAC, MB on M80. Dyes

Langmuir

Qm,exp -1

Freundlich

(mg g )

Qm,cal (mg g-1)

KL (L mg-1)

RL

R2

KF

n

R2

AR

821

840

0.0910

0.0119

0.99931

70

2.1583

0.76625

ARAC

500

493

0.1649

0.0146

0.99665

231

7.6775

0.84873

MB

302

307

0.1196

0.0173

0.99928

73

4.0056

0.86341

To evaluate the mass transfer process in the dye adsorption into microgel, adsorption kinetics of anionic dyes were investigated (Figure S12). The experimental data was fitted with kinetic models for revealing the mechanism of adsorption and the rate-determining step involved. The pseudo-first-order,51 pseudo-second-order,52 and intraparticle diffusion53 models were employed here to analyze the experimental results. The linearized form of the three modes can be expressed by the following equation:

In(Q − q ) = InQ − k t

Pseudo-first-order model:

Pseudo-second-order model:

=

' '

+

(7) (8)

*

Intraparticle diffusion model:

Q = C + k () t '

(9)

where Qt is the amount of dyes removed (mg g-1) at time t; Qe is the equilibrium adsorption capacity (mg g-1); k1 (min-1), k2 (g mg-1 min-1) and kid (mg g-1 h-0.5) are the

25



Page 26 of 41

rate constant of pseudo-first-order, pseudo-second-order, and intraparticle diffusion modes, respectively; C indicates the thickness of boundary layer.

Figure 5 (a) Pseudo-first-order kinetics, (b) pseudo-second-order kinetics and (c) intraparticle diffusion model of ARAC on M80. As shown in the Figure 5, it was observed that the adsorption kinetics of dyes on microgels followed the pseudo-second-order model better than that of the pseudo-first-order and intraparticle diffusion models, since the pseudo-second-order model fit has very high correlation coefficients (>0.99) (Table 3). Moreover, the maximum

equilibrium

adsorption

values

(Qe2,cal)

26


calculated

by

the

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pseudo-second-order model are closed to that of experimental adsorption results (Qe,exp), which further confirmed that the pseudo second-order model was more suitable to describe the adsorption kinetics of the three anionic dyes on microgel. This means that rate controlling mechanism may be chemisorption caused by binding forces through exchange or sharing of electrons between the dye and protonated amino groups.50 Table 3 Kinetic parameters for the adsorption of AR,ARAC,MB by M80 Qm,e Dyes

xp

pseudo-first-order model

k2/10-5

k 1/10-3

Qe1,cal

-1

(min-1)

(mg g-1)

AR

821

0.73

508

ARAC

500

1.21

MB

302

0.87

(mg g )

pseudo-second -order model

R

2

-1

(g mg

Weber-Morris model

Qe2,cal

Kid C

R2

150

150.19

0.7653

0.99972

162

162.11

0.8699

0.99921

61

60.973

0.8415

(mg

R

2

-1

-1

min )

g )

0.95926

3.06

855

0.99924

274

0.94656

6.97

495

203

0.93524

1.25

315

(mg g-1 0.5

min )

Interestingly, the experimental data in intraparticle diffusion model could be fitted into two separated straight lines with high R2 (Figure 5c). The first linear portion relates to the boundary layer diffusion, while the second linear portion indicates an intraparticle diffusion step.14 In addition, the plots had nonzero intercepts, the intraparticle diffusion was not the sole rate-determining step and the external mass transfer had some role in deciding the adsorption rates.14 Therefore, both intraparticle diffusion and external mass transfer occurred simultaneously in the whole adsorption process. Separation of the mixture of dyes. Motivated by the abovementioned unique selective adsorption for anionic dyes, the obtained microgels would be applied into the practical application in separation. Compared with the color of ARAC and MEB, 27



ARAC/MEB mixture solution exhibited a garnet color (Figure 6a), where the initially concentration ratio of ARAC and MEB ([ARAC]0/[ MEB]0) in solution was 3.95. When M80 treated with CO2 was added into the mixture solution of ARAC and MEB, the color of mixture solution turned from garnet to blue with the contact time. After 7 h, the color of the solution became blue of MEB, while the microgels in solution turned red of ARAC, suggesting that M80 adsorbed ARAC selectively from the mixture solution and MEB stayed in solution. During the adsorption process, UV−vis spectra were carried out to trace the variation of dye’s concentration in solution. As shown in Figure 6b, the concentration of MEB had no obvious change with the increasing contact time, while the concentration of ARAC in solution decreased abruptly from the initial 145 mg L-1 to 1.1 mg L-1. After separation for 7 h, the concentration ratio of ARAC and MEB in solution ([ARAC]eq/[MEB]eq) decreased to 0.03 (Figure 6c), indicating that ARAC was mostly separated from the aqueous solution of ARAC and MEB.

Figure 6 (a) Photograph of ARAC and MEB mixture before and after separation for 7 h by M80 treated with CO2 in comparsion with ARAC and MEB. (b) UV-vis spectra 28


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of ARAC and MEB mixture during separation experiment. (c) the concentration ARAC and MEB and its ratio ([ARAC]/[MEB]) variation during separation. Reusability of microgels. As for adsorbents, reusability is crucial for their practical application based on economic consideration.14,30 In addition, since dyes are useful in a wide range of industries, the recovered dyes can be used as raw materials and may also prohibit the secondary contamination. Having demonstrated that the dyes were adsorbed by electrostatic attraction, the microgels would release the dyes through deprotonation. Thus we tried to bubble N2 into the suspension to examine the release of

model

dye

ARAC

from

microgel

because

of

the

reversible

protonation-deprotonation transition under cycling CO2/N2 treatment. However, the release was not observed even by bubbling N2 over 10 h. This may be attributed to the strong electrostatic interaction as evidenced by adsorption kinetics analysis, i.e., chemisorption, leading to that the reversible reactions existed in the CO2 aqueous solutions (CO2 + H2O ↔H2CO3↔ H+ + HCO3−) were inhibited. Besides, the microgel became collapse and its size decreased to ca. 70 µm after adsorbing dye duo to the decrease of osmotically active sites caused by the less net positive charge groups (Figure S13), which also affect the dyes to escape from microgel. Alternatively, a series of alkaline solution with different pH values was used to deprotonate the positive charge amine groups (Figure 7a). One can find that the desorption efficiency approached ~100% at pH 12 (Figure 7a). Though the desorption was realized by alkaline solution, the concentration is much lower than that of acid or alkali solution

29



used in previous reports,14,50 and this way also avoid the utilization of organic solvents.30 In addition, the desorbed dye could be recovered from the alkali solution.

Figure 7 (a) Effect of pH on the desorption of ARAC on M80. (b) Regeneration cycles for M80 treated with CO2. After desorption, the microgel was regenerated with CO2 and swelled again, and 10 cycles of adsorption-desorption were performed (Figure 7b). The recyclability results showed that the adsorption capacity retained more than 80% after 10 cycles compared with the original adsorption capacity (Figure 7b), suggesting that the microgels could be regenerated and reused in wastewater treatment. The decrease in performance may be ascribed to part of pre-adsorbed ARAC trapped inside the microgels because of the shrinkage after deprotonation, reducing the total available positively-charged amine groups for subsequent adsorption cycles.

30


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Apart from the batch-by-batch dye’s adsorption and recovery, a wastewater treatment prototype with a continuous flow of dye solution was constructed (Figure 8a and Moive S1). The microgel M80 was packed within a glass column (Φ=2 cm, h=20.0 cm) and CO2 was bubbled into the colume for 10 min. When the dye solution (500 mL, 3.4723 mg L-1) with a fow rate (2.5 mL min-1) passed through the glass column under the aid of peristaltic pump, the lower part of the column became red, while the treatment water tuned to transparent (Moive S1), indicating that microgel absorbed dye in the flowing aqueous solution. After tracing with UV-Vis spectra (Figure 8b), the dye in the aqueous solution was completely removed up to the limit of detection. Moreover, when 500 mL of aqueous solution with pH 12 passed through the glass column, a red color solution was collected, suggesting that the dye was released from the microgels. According to the concentration of released dye soluton (1.7882 mg L-1), it was calculated that about 50% of dye was washed out. After bubbling CO2 in the column, the microgels would adsorb dye again. This results showed the microgels can remove pollutions with continuous flowing stream manner, which is suitable for the practical application of water purication.

31



Page 32 of 41

Figure 8 (a) Demonstration of a dye absorption/desorption test using a column packed with M80. The inset is schematic explanation of the wastewater treatment prototype. (b) Absorption spectra of the ARAC solution before and after testing.

CONCLUSIONS

In summary, a series of giant microgels were synthsized through one-step inverse-suspension polymerization of hydrophilic PAM and hydrophobic DEA. The microgels with high DEA content can undergo switchable collapse-expansion transition upon alternative stimuli of CO2 and N2. Based on the CO2-induced protonation-deprotonation and collapse-expansion transition, the microgels exhibited on-off, selective and recyclable adsorption for anionic dyes by reversible electrostatic interactions. Apart from easy-handling separation from the water by a simple filtration process, the maximum adsorption capacity is as high as 821 mg·g-1, and the adsorption

isotherms

and

kinetics

obeyed

Langmuir

isotherm

and

the

pseudo-second-order kinetics models, respectively. The anionic dye also can be separated from the mixture solution using CO2-treated microgels. Moreover, the microgels was fabricated as a column of a wastewater treatment prototype with a continuous flowing stream manner, and the dye was removed and recovered by alternative bubbling CO2 and flushing with aqueous alkali (pH 12). Thus this type of microgels may serve as a cost-effective, environmentally friendly and efficient absorbent for water purification applications. In addition, the CO2-switchable property based on reversible protonation and deprotonation may be suitable for the recyclable 32


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removal of heavy metal ions through complexing action. If their size can be diminished, future generation of these microgels may find application in biomedicines. ASSOCIATED CONTENT

Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI:

Additional data related to: OM images of microgels, the data of elemental analysis and swelling ratio, calculations and pictures of adsorption (PDF). AUTHOR INFORMATION Corresponding Author E-mail addresses: [email protected] (Z. Guo); [email protected] (Z He); [email protected] (Y. Feng) Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT

This project is supported by Open Fund (PLN1508) of State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation (Southwest Petroleum University), National Natural Science Foundation of China (51563009, 21465011, 21464006) and Natural Science Foundation of Jiangxi Province, China (20181BAB213006). 33



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Giant Microgels with CO2-induced On-Off, Selective and Recyclable

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