Reinforced-concrete-structured Hydrogel Microspheres with Ultrahigh

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Reinforced-concrete-structured Hydrogel Microspheres with Ultrahigh Mechanical Strength, Restricted Water Uptake and Superior Adsorption Capacity Haifeng Ji, Xin Song, Zhen-Qiang Shi, Chengqiang Tang, Lian Xiong, Weifeng Zhao, and Changsheng Zhao ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b04323 • Publication Date (Web): 11 Apr 2018 Downloaded from http://pubs.acs.org on April 11, 2018

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ACS Sustainable Chemistry & Engineering

Reinforced-concrete Structured Hydrogel Microspheres with Ultrahigh Mechanical Strength, Restricted Water Uptake and Superior Adsorption Capacity Haifeng Ji, †,§ Xin Song, †,§ Zhenqiang Shi, † Chengqiang Tang, † Lian Xiong, † Weifeng Zhao, †,‡,* and Changsheng Zhao. †,* † College of Polymer Science and Engineering, State Key Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu, 610065, People’s Republic of China ‡ State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, Donghua University, Shanghai, 201620, People’s Republic of China *Corresponding author. E-mail: [email protected] (W-F. Zhao*); [email protected] (C-S. Zhao**) Tel.: +86-28-85400453; Fax: +86-28-85405402. §These authors contributed equally to this work.

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KEYWORDS: Reinforced-concrete structured, Polyacrylic acid hydrogel, Ultrahigh mechanical strength, Restricted water uptake, Superior adsorption capacity

ABSTRACT: Functional hydrogels own superior advantages in wastewater purification thanks to their abundant functional groups and porous structure. However, the hydrogels cannot be used as stable adsorbents due to their insufficient mechanical property and exorbitant swelling ratio. Inspired by the strong mechanical property of the reinforced-concrete structure, the novel reinforced-concrete structured hydrogel microspheres were prepared, and the structure endowed the hydrogel with superior mechanical property (the compressive stress was 27.4 MPa with a strain of 80%) and restricted swelling ratio (the water uptake was less than 2.2 g/g under pH=7), but caused no obviously negative effect on the originally outstanding adsorption ability, and the adsorption column filled with the concrete-structured hydrogel microspheres reached stable and efficient removal of the cationic dyes and heavy metal salts (1246 mg/g for MB, 491 mg/g for MV, 1253 mg/g for Cu(II), 564 mg/g for Ni(II), 640 mg/g for Cd(II), and 252 mg/g for Pb(II)). These results indicated the microspheres were qualified for the work as stable and efficient adsorbents, more importantly, the reinforced-concrete structure might accelerate the actual application of the hydrogels in diverse fields to a great extent.

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INTRODUCTION Water pollution increases the risks to the water supply of growing populations and also poses fatal threats to living species.1, 2 Organic dyes and heavy metal ions have become the major pollutants in water resources due to the heavy emissions from many industries.3, 4 For removing the contaminants in wastewater, a mass of methods such as precipitation, filtration, adsorption, oxidation, etc. have been applied.5-8 Among these methods, adsorption has been widely used in environmental remediation for its cost-effective property.9, 10 Conventional adsorbents, such as activated carbon, clays and silica,11-16 usually do not possess sufficient adsorption capacity or collectability. Compared with these adsorbents, traditional polymeric materials own advantages in adsorption capacity, but always show disadvantages in adsorption rate, which restrict their applications in environmental remediation.17,

18

Therefore, we

expected a novel material possesses both adsorption capacity and rapid adsorption rate.

Hydrogels, one kind of the most high-profile polymeric materials, have outstanding adsorption capacity owing to their high density of functional groups.19 Moreover, due to the high specific surface area, the hydrogels exhibit rapid adsorption rate to target molecules.20 Hydrogels exhibit enormous advantages in environmental remediation, but their insufficient mechanical strength and high water uptake seriously restrict their applications. Many researches have been carried out to enhance

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the mechanical strength or decrease the water uptake of hydrogels by means of multivalent host-guest interactions, hydrogen bonds, aromatic stacking, hydrophobic interactions, etc.21-26 However, the enhancement of the mechanical strength of these hydrogels commonly sacrifices high functional group content, and the fabrication processes are commonly complicated. For the moment, making functional hydrogels with sufficient mechanical strength is still challenging.27,

28

Thus, it is crucial to

develop novel hydrogel materials with high mechanical strength, restricted water uptake as well as superior adsorption capacity.

It is well known that reinforced concrete has been widely used in construction for its excellent mechanical property. But either rebar or concrete, as the constituents of the reinforced concrete, is brittle and does not possess sufficient mechanical property. In the application of the reinforced concrete, the rebar performs the effect as the skeleton, and the concrete fills the interval between the skeleton. When the reinforced concrete suffers from stress, the skeleton can effectively protect the concrete.29 Inspired by the reinforced concrete, we attempted to fill the hydrogels into hydrophobic skeletons, which was supposed to enhance the mechanical strength and restrict the water uptake of the hydrogels. Additionally, we also expected that the skeleton structure could be controlled, ensuring that it would not suppress the superior adsorption capacity of the hydrogels.

Herein, reinforced-concrete structured poly(acrylic acid) (PAA) hydrogel

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microspheres were prepared, the “concrete” (PAA hydrogel) was filled into the “rebar” (polyethersulfone (PES)), since PAA hydrogel shows excellent adsorption capacity on heavy metal ions and cationic dyes,30 and AA is a kind of economical material, PES is chosen as the skeleton matrix because it can be prepared into porous particles with fine surface morphology by means of a facile liquid-liquid phase separation technique,31 and PES also shows outstanding oxidative, thermal hydrolytic stability, as well as good mechanical property.32 In order to prepare microspheres with controlled sizes for practical application, the PES skeleton was first fabricated via electrospraying. Then, the PAA hydrogels were in situ polymerized in the skeleton. We anticipated this structure could be used for enhancing the mechanical strength and restricting the water uptake of the functional hydrogels, and we further expected that this method could make the functional hydrogels closer to practical applications.

EXPERIMENTAL

Different kinds of the skeleton solutions were synthesized via an in situ crosslinking polymerization. Typically, a certain amount of polyethersulfone (PES), N-vinyl-2-pyrrolidone

(VP),

2-hydroxyethyl

methacrylate

(HEMA)

and

N,

N'-methylidenebis (acrylamide) (MBA) were dissolved in a 100 mL three-necked round flask with N, N'-dimethylacetamide (DMAc) to obtain a homogeneous solution. The copolymerization was initiated by adding 2, 2-azobisisobutyronitrile (AIBN) under nitrogen with mechanical stirring (600 rpm) at 120 °C for 36 h (Supplementary

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Table S1).

The skeleton solutions were dropped into reaction solution containing 120 g DI water, 280 g AA, 6 g MBA, 1.2 g ammonium persulfate (APS) and 1 g sodium dodecyl sulfate (SDS) via electrospraying, and the reaction solution would replace the DMAc in the skeleton solution during the phase inversion. Then the skeletons with the reaction solution were heated and kept at 80 °C for 30 min to fabricate the reinforced-concrete structured hydrogel microspheres (RHMs).

The compressive strengths of the RHMs and hydrogel microspheres (HMs) were measured by a universal tensile testing machine (SANS CMT4000). For the swelling tests, to exclude the adsorbed water by the skeletons, the equilibrium water uptake (EWU) was defined as eq 1:

EWU (%) =

ሺௐ೐భ ିௐ೐మ ሻିሺௐ೏భ ିௐ೏మ ሻ ሺௐ೏భ ିௐ೏మ ሻ

× 100

(1)

where We1 and We2 are the weights of the wet RHMs and skeletons, Wd1 and Wd2 are the weights of the dry RHMs and skeletons.

Ion exchange capacity (IEC) of the RHMs was calculated by eq 2:

IEC (mequiv/g) =

ሺெಹ಴೗ ௏ಹ಴೗ ሻିሺெಿೌೀಹ ௏ಿೌೀಹ ሻ ௠

× 1000

(2)

where MHCl is the molar concentration of the HCl solution; VHCl is the volume of the HCl solution; MNaOH is the molar concentration of the NaOH solution; VNaOH is the 6


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volume of the NaOH titrated by residual HCl solution; m is the dried weight of the RHMs.

The adsorption experiments were carried out in 50 mL centrifuge tubes containing various amounts of dye or metal salt solutions. After the samples were agitated using flash shaker (Shanghai sile instrument Co. Ltd) at room temperature for 48 h, the adsorption capacities of the RHMs were measured. For detecting dye concentration, UV-vis spectrometer (UV-1750, Shimadzu Co. Ltd, Japan) was used (MB: 631 nm; MV: 589 nm). For detecting metal salt concentration, atomic absorption spectroscopy (Shimadzu SPCA-626D, Japan) was used.

1 g of the microspheres were placed into a needle tubing (10 mL) to prepare an adsorption column. For exploring the selective filtration capacity of the adsorption column, a mixed solution containing methylene blue (MB) (250 µmol) and methyl orange (MO) (250 µmol) was applied to the column with a streaming rate of 4 mL/min. The separation efficiency (R) was calculated by eq 3:

R (%) = (1-Cm/C0) × 100

(3)

where Cm and C0 are the concentrations of cationic dyes in the original mixed solution and in the solution after adsorption, respectively.

To verify whether the column owned powerful adsorption capacity in practical application, the solutions with a variety of pollutants including MB (74.78 ppm), 7



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methyl violet (MV, 79.64 ppm), CuSO4 (1000 ppm) and CdSO4 (1000 ppm) were applied to the adsorption column, and the streaming rate was controlled at 4 mL/min.

To investigate the breakthrough behaviour of the RHMs, we studied dynamic adsorption of the microspheres to evaluate MB breakthrough behaviour against the adsorption column. In brief, MB solution (500 µmol/L) was filtered by the column with different streaming rates of 2.0 mL/min and 4.0 mL/min, the concentration of the effluent was measured using an UV-vis spectrophotometer. Breakthrough curves were obtained by plotting Ct/C0 against V (mL), where Ct (µmol/L) is the effluent dye concentration, C0 (µmol/L) is the influent dye concentration and V is the influent dye volume. The adsorbed amount of the adsorption column was calculated by eq 4:

qe =

஼బ ×௏೟೚೟ೌ೗ ×ெ ଵ଴ల ×௠బ

ೡసೡ

×

‫׬‬ೡసబ ೟೚೟ೌ೗ሺ஼బ ି஼೟ ሻௗ௩

(4)

ೡసೡ

‫׬‬ೡసబ ೟೚೟ೌ೗ ஼బ ௗ௩

where qe is the adsorbed amount of the adsorption column (mg/g); M and m0 represent the relative molecular mass of MB (g/mol) and the dry weight of the adsorption column (g), respectively; Vtotal is the influent dye volume (mL).

RESULTS AND DISCUSSION Firstly, by means of an in situ crosslinking polymerization, various PES skeleton solutions using DMAc as the solvent, were prepared to explore the influence of the skeletons’ hydrophilia and structure on the property of the RHMs. For instance, P8 contained 8 wt.% of PES and the detailed compositions of the skeleton solutions are summarized in Table S1 (Supplementary Information). Afterwards, for filling PAA

8


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hydrogels into the skeletons uniformly and controlling the sizes of the hydrogel microspheres effectively, the skeleton solutions were directly dropped into a reaction solution via electrospraying (Figure. 1A). Once the skeleton solution droplets were immersed into the reaction solution, phase inversion occurred: the skeletons were knitted uniformly with a spherical morphology, and the DMAc in the skeleton solutions were replaced by the reaction solution containing monomer, initiator and cross-linker. Then the spherical skeletons with the reaction solution were heated to initiate the crosslinking polymerization (Figure. 1C). The reinforced-concrete structured hydrogel microspheres with different skeletons of P8, P8/HEMA-VP and P4/HEMA-VP were named as P8/Gel, P8/HEMA-VP/Gel, and P4/HEMA-VP/Gel microspheres, respectively. The average diameter of the microspheres was about 550 µm, and all of the microspheres exhibited porous structure (Supplementary Figure. S1, the average diameter was determined by the ruler of an optical microscope). To explore the distribution of the PAA in the microspheres, Cu2+ ions were introduced into the microspheres, since Cu2+ ions can be adsorbed by the –COO- groups from the PAA but cannot be adsorbed by PES.33 The location of Cu2+ ions were observed by EDX mapping analysis. As shown in Figure. 1D, the Cu2+ ions were uniformly distributed in the microspheres, indicating the uniform distribution of the PAA macromolecules. Furthermore, S element distributed uniformly, which indicated that the PES skeleton was mixed with the PAA hydrogel well. Thus, it could be concluded that the PAA hydrogels coupled with the PES skeleton successfully formed the

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reinforced-concrete-mimicking structure. Then the presences of PAA, PHEMA and PVP in the microspheres were confirmed by Fourier transform infrared spectra. The peaks at 1710, 3420 and 1670 cm-1 were corresponding to the vibrations of the -C=O in PAA, the -OH in PHEMA, and the -C=O in PVP, respectively (Supplementary Figure. S2B and C). The components of the skeletons were also measured by thermo gravimetry analysis, the contents of P(HEMA-VP) in the microspheres of P8, P8/HEMA-VP and P4/HEMA-VP were 0, 6.25 and 5.72 wt. %, respectively; and the contents

of

the

PAA hydrogels

in

the

P8/Gel,

P8/HEMA-VP/Gel,

and

P4/HEMA-VP/Gel were 72.25, 73.96 and 84.09 wt. %, respectively. (Supplementary Figure. S2D and E).

Figure 1. (A) The process of the electrospraying. (B) The graphic symbols of (C) the preparation process of the RHMs. (D) EDX mapping analysis of the RHMs corresponding to the red circle.

The PES skeletons of the P8/Gel, P8/HEMA-VP/Gel and P4/HEMA-VP/Gel microspheres could be dissolved by DMAc, and the hydrogel microspheres (HMs) 10


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without

the

skeletons

were

named

Gel(P8),

Gel(P8/HEMA-VP)

and

Gel(P4/HEMA-VP), respectively. As shown in Figure S4A, the PES skeletons showed great mechanical strength, and the introduced hydrophilic polymers did not reduce the mechanical strength of the PES. Figure. 2A showed the compressive stress-strain curves of the RHMs and HMs. The stress of the P8/Gel reached up to 22.84 MPa with a strain of 80%; while that of the P8/HEMA-VP/Gel slightly decreased to 17.93 MPa with a strain of 60%, and the stress of the P4/HEMA-VP/Gel was 27.41 MPa with a strain of 80%. However, the stress of the HMs was less than 0.13 MPa (the stress of the HMs crosslinked by another common crosslinking agent (ethyleneglycol dimethacrylate) was less than 0.07 MPa, as shown in Figure S4B). In addition, the tensile strength of the reinforced-concrete structured PAA hydrogel (9.95 MPa with a strain of 30%) was enhanced more than about 100 times comparing with that of the PAA hydrogel without the skeleton (0.10 MPa with a strain of 10%) (Supplementary Figure. S5), which indicated that the PES and PES/HEMA-VP skeletons could effectively protect the PAA hydrogels and endow the RHMs with ultrahigh mechanical strength. However, the compressive and tensile stresses of most of the hydrogels reinforced by traditional double network were less than 10 MPa and 0.5 MPa, respectively.34,

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Furthermore, the RHMs also exhibited dimensional

recovery after compression, but the skeleton microspheres showed no dimensional recovery and HMs were cracked after compression (as shown in Figure. 2B). The enhanced mechanical strength of the hydrogel was visually verified by a “hammering”

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experiment, as shown in Figure. 2C (Video S1). After hammering, the HMs were cracked; however, the RHMs were not cracked but became “pancake”. When the pancaked gel was placed in DMAc, the PES skeleton was dissolved, then, the pancaked gel recovered spherical hydrogel, which indicated that the crosslinking points of the PAA hydrogel were not broken, and this phenomena further suggested that the PAA hydrogel only supported few of the load while the skeletons supported the majority.36 Thus, it could be concluded that the “reinforcing bars” (PES) effectively enhanced the mechanical strength of the “concrete” (PAA hydrogel).

Benefiting from the reinforced-concrete-mimicking structure, the swelling behavior of the PAA hydrogels were also strongly restricted by the skeletons. As shown in Figure. 2D and Figure. S6, the RHMs (P4/HEMA-VP/Gel) showed unconspicuous changes under dry, wet and alkaline conditions, while the HMs (Gel (P4/HEMA-VP)) exhibited obvious dimension changes when compared with the RHMs. The water uptakes are summarized in Figure. 2E, when compared the RHMs with PES skeletons, the RHMs with hydrophilia skeletons showed slightly higher water uptakes. Moreover, compared with HMs, the water uptakes of the RHMs were dramatically restricted. All the water uptakes of the samples increased with increasing the pH values due to the deprotonation under alkaline environment.37 Moreover, the water uptake of the Gel(P4/HEMA-VP) reached about 22.2 g/g under pH=7, which was about 10 times than that of the P4/HEMA-VP/Gel. These results indicated that the RHMs possessed high size stability for use. 12


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Figure 2. (A) The compressive stress-strain curves of the RHMs and the HMs. (B) The dimensional recovery ratios of the P8/Gel, PES, and Gel/P8 microspheres after the compression (h0 and h1 were the heights of the microspheres before and after compression, the microspheres are immersed into water for dimensional recovery). (C) Digital pictures of the RHMs and the HMs before and after hammering. (D) Digital pictures of the P4/HEMA-VP/Gel and the Gel(P4/HEMA-VP) microspheres dyed by methylene blue, after swelling (10 mg of the microspheres is used). (E) The water uptakes of the RHMs and the HMs.

Interestingly, the IECs of the reinforced PAA hydrogels (P8/HEMA-VP/Gel and P4/HEMA-VP/Gel) with hydrophilic skeleton were closer to the theoretical value, while the IEC of the P8/Gel was far away from the theoretical value, indicating that the RHMs with hydrophilic skeletons showed less negative effect on the IECs of the PAA hydrogels, as shown in Figure. 3A. The P4/HEMA-VP/Gel exhibited ultrahigh IEC of 10.88 mequiv/g, while the ion exchange particle/resin based on PAA derivative has an IEC around 3 mequiv/g in general.38 Benefiting from the ultrahigh IEC, the RHMs showed ultrahigh adsorption amounts of MB, but both of the PES and

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PES/HEMA-VP skeletons contributed little adsorption amounts of MB, which indicated PAA hydrogel mainly contributed to the adsorption capacity, what’s more, the adsorption amount of MB for the P4/HEMA-VP/Gel could reach up to 1233.06 mg/g (Figure. 3B), which was the highest adsorption amount for MB among the hydrogel materials.39,

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For further exploring the skeletons’ influences on the

adsorption ability of the PAA hydrogels, the MB adsorption kinetics of the microspheres were investigated. As shown in Figure. 3C, the adsorption rate of MB for the Gel(P8) was higher than that for the P8/Gel; and the equilibrium adsorption amount of MB for the Gel(P8) (960.17 mg/g) was slightly higher than that for the P8/Gel (835.26 mg/g). The results indicated that the hydrophobic PES skeletons affected the interactions between the adsorbed sites and MB molecules, resulting in the decrease in the adsorption rate and the equilibrium adsorption amount. Nevertheless, for the microspheres with hydrophilic skeletons, the equilibrium adsorption amounts of MB for the Gel (P8/HEMA-VP) and Gel (P4/HEMA-VP) were 948.21 and 1118.49 mg/g, and the adsorption rates and adsorption amounts of MB for the P8/HEMA-VP/Gel and P4/HEMA-VP/Gel were almost the same as those for the Gel (P8/HEMA-VP) and Gel (P4/HEMA-VP) (Figure. 3D and E), which indicated that the negative influences of the skeletons on the adsorption ability of the PAA hydrogels were suppressed by enhancing the hydrophilia of the skeletons.

Moreover, to investigate the adsorption mechanisms of the RHMs, different kinetic models (the pseudo-first-order kinetic model, the pseudo-second-order kinetic 14


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model and intraparticle diffusion kinetic model) and adsorption isotherms (Langmuir and Freundlich isotherms) were performed. As shown in Table S2, the adsorption process of the RHMs fitted intraparticle diffusion kinetic model well, and agreed with the pseudo-second-order kinetic model better than the pseudo-first-order kinetic model, indicated that there were three diffusion steps during the adsorption process,41 and the chemical process played a leading role during the adsorption process.42, 43 In additionally, it was reported that the adsorption mechanism of MB for the PES was physical absorption.44, 45 Both chemical and physical adsorptions existed during the adsorption of dyes in the RHMs. The adsorption processes included the external surface adsorption and the intraparticle diffusion. Electrostatic interaction was the driving force to promote the dye molecules diffusing into the particles, and the adsorption amounts of cationic dyes thus increased significantly. Furthermore, the adsorption process was a multilayer adsorption since the Freundlich isotherm (R2=0.9999) described it better than the Langmuir isotherm (R2=0.9907) ,46 as seen Table S3.

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Figure 3. (A) The ion exchange capacities (IECs) of the RHMs. (B) The adsorption amounts of MB per unit mass for the RHMs (P8/Gel, P8/HEMA-VP/Gel and P4/HEMA-VP/Gel), HMs (Gel(P8), Gel(P8/HEMA-VP) and Gel(P4/HEMA-VP)), and skeleton microspheres (P8, P8/HEMA-VP and P4/HEMA-VP). The adsorption amounts of MB for the different RHMs and HMs (C, D and E). (F) The adsorption amounts of MB for the RHMs. (G) Digital pictures of the MB solution after contacting with the RHMs (P4/HEMA-VP/Gel) at different time intervals (0, 15, 30, 60, 90 and 120 min).

Afterwards, the influences of pH values and MB concentrations on the adsorption capacities of the RHMs were also explored (the details were shown in Supplementary Figure. S7A and B). The RHMs also showed superior adsorption capacities to other cationic dye and heavy metal ions, the adsorption amounts of

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methyl violet, Cu(II), Pb(II), Cd(II) and Ni(II) for the P4/HEMA-VP/Gel were 491, 1253, 640, 564 and 252 mg/g, respectively (as shown in Supplementary Figure. S7C and D). The robust adsorption capacities of the RHMs were ascribed to the high specific surface area of the microspheres and the large amount of the negative and oxygen-containing functional groups.47 The outstanding adsorption capacities made the RHMs as ideal materials for the removal of organic dyes and heavy metal ions from wastewater.

Regenerable adsorbents are highly desired in the industrial wastewater treatment since the use-cost of materials can be greatly reduced. Thus, the P4/HEMA-VP/Gel microspheres, which showed the highest adsorption amounts, were used to test the recyclability of the microspheres in this study. As shown in Figure. S7E (Video S2), almost all of the adsorption and desorption ratios reached more than 90% in the adsorption-desorption cycles. These results demonstrated that the RHMs could be reused again by simply washing the microspheres with HCl solution.

For further exploring the practical application, the P4/HEMA-VP/Gel microspheres were filled into a needle tube (10 mL) for imitating the adsorption column (column height = 1.2 cm, column diameter = 1.4 cm, the weight of the RHMs in the column = 1 g, and test temperature = 25 °C). According to a previous report, PAA hydrogels can adsorb cationic dyes via electrostatic adsorption but cannot adsorb anionic dyes (Figure. 4A).48 Thus, for exploring the selective adsorption

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ability of the P4/HEMA-VP/Gel microspheres, a mixed solution containing MB (cationic dye) and methyl orange (MO, anionic dye) in a 1:1 molar ratio was filtered through the adsorption column. As shown in Figure. 4B (Video S3), the colour of the solution changed from mazarine to orange (the colour of MO) after adsorption. The UV-vis spectra showed that the absence of MB dramatically decreased after being adsorbed by the hydrogel microspheres (close to zero), and the calculated separation efficiency was 95.25%. These results demonstrated that the adsorption column could eliminate cationic toxins selectively from pollutants.

In practical industrial wastewater treatment, the wastewater usually contains many kinds of toxins, such as organic dyes and heavy metal ions. Thus, using a multifunctional adsorption column can improve the processing efficiency and reduce the use-cost. Previous reports indicated that PAA hydrogels could not only adsorb the cationic dyes by electrostatic adsorption, but also adsorb the heavy metal ions by electrostatic complexation (Figure. 4C).20,

48

To investigate the multifunctional

adsorption ability of the P4/HEMA-VP/Gel microspheres, different organic dyes (MB and MV) and different heavy metal ions (Cu (II) and Cd (II)) were mixed together, and the mixed solution was passed through the adsorption column. As shown in Figure. 4D, the colour of the solution changed from mazarine to colourless, and the concentrations of MB and MV greatly decreased as shown in the UV-vis spectra, indicating the excellent removal capacities of the adsorption column. As shown in Figure. 4E, the removal ratios of MB and MV reached more than 98%, while those of 18


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heavy metal ions reached over 70%. Thus, in practical industrial wastewater treatment, the wastewater contained both organic dyes and heavy metal ions could be effectively purified by passing through the adsorption column.

For further exploring the adsorption stability and efficiency of the adsorption column, the breakthrough behaviors of the adsorption column were investigated. After the breakthrough experiments, the colour of the RHMs in the column changed from white to mazarine, and there was no deformation for the RHMs due to the high mechanical property and restricted water uptake (Figure. 2 and S6 (Supplementary Information)), demonstrating that the RHMs adsorption column effectively adsorbed MB molecules and stood filtration process for a long time. In practical applications, the flow rate for an adsorption column is one of the main factors affecting the adsorption,49 therefore, we selected two flow rates of 2 mL/min and 4 mL/min to explore the effects of flow rates on the breakthrough behavior of the column. As shown in Figure. 4F (Video S4), high removal ratio (about 90%) could be maintained within a certain volume of MB solution at both of flow rates, followed by a decrease in the removal ratios as the volumes of the solutions increased. The reason was that the adsorption sites of the RHMs became gradually saturated with increasing the volumes of the solutions. The removal ratio under 2 mL/min was higher, but its breakthrough curve was steeper. In addition, the maximum adsorption capacities of the adsorption column were 1132.32 mg/g at the rate of 4 mL/min and 845.33 mg/g at the rate of 2 mL/min calculated from the breakthrough curve, which were close to the 19



static adsorption capacity of the RHMs at the same MB concentration (1233.06 mg/g). These results indicated that the adsorption column owned efficient and stable removal capacity in actual industrial applications.

Figure 4. (A) The illustration of the adsorption of MB for MB/MO mixed solution. (B) The UV-vis spectra of the mixed solution and filtered solution, as well as the digital picture of the selective adsorption experiment (conditions: initial MB concentration = 250 µM, MO concentration = 250 µM). (C) The illustration of the adsorption for heavy metal ions and cationic dyes. (D) Digital picture of the adsorption column experiment and the UV-vis spectra of the mixed solution and filtered solution. (E) The removal ratios of dyes and heavy metal ions by the adsorption column. (F) The breakthrough curves of the adsorption column and the digital pictures of the adsorption column before and after the adsorption (conditions: initial MB concentration = 500 µM). For further stating the advancements of this study, systematical comparisons of some previous studies were made, as shown in Table 1. Compared with traditional porous materials such as activated carbon, clays and silica,50, 51 the RHMs exhibited

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higher adsorption capacity due to their abundantly available functional groups. Compared with other interpenetrating polymer networked or double networked hydrogels with high mechanical properties and high adsorption capacities,20, 52 the RHMs exhibited improved dimensional stability due to their higher mechanical property and lower water uptake. Compared with other polymers with high dimensional stability,53,

54

the RHMs exhibited faster adsorption rate and higher

adsorption capacity thanks to their porous structure and abundantly available functional groups. What’s more, the RHMs were convenient to fill an adsorption column and showed excellent removal efficiency. Additionally, the raw materials to prepare the RHMs were cheap industrial chemicals (PES is quoted about 550 $ per ton, HEMA is quoted about 1200 $ per ton, VP is quoted about 4000 $ per ton, and AA is quoted about 800 $ per ton, the data come from Alibaba). Thus, the RHMs had great advantages in practical applications. Table 1. Systematical comparison of the results of the hydrogels used in wastewater treatment. Composites

Adsorption capacity

Mechanical property

Amino-functionalized

256.4 mg/g for Cd(II) (200 mg/L)

The compressive stress was 4.23

starch/PAA hydrogel 20 Chitosan/acrylamide hydrogel 52

MPa with a strain of 85% 38.96 mg/g for Cd(II) (55 mg/L) 48.31 mg/g for Pb(II) (42 mg/L)

The compressive stress was 3.50 MPa with a strain of 74%

38.05 mg/g for Cu(II) (20 mg/L) Cellulose/clay nano

782.9 mg/g for MB (100 mg/L)

21


The compressive stress was 4.70


KPa with a strain of 51%

-composite hydrogels 55 TiO2-rGO-PDMAA

92 mg/g for MB (5 mg/L)

nanocomposite hydrogel 56 Polyampholyte hydrogel 57

manganese

dioxide-poly(N-hydroxymeth

The

compressive

stress

was

559.91 KPa with a strain of 90% 38.12 mg/g for Cd(II) (40 mg/L) 38.21 mg/g for Pb(II) (40 mg/L)

Hydrous

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93.86 mg/g for Cd(II) (112 mg/L)

The compressive stress was about 95 KPa with a strain of 70% Not mentioned

186.39 mg/g for Pb(II) (207 mg/L)

yl acrylamide/2-hydroxyethyl

54.88 mg/g for Cu(II) (64 mg/L)

acrylate) 58

43.37 mg/g for Ni(II) (59 mg/L)

Hemin-functionalized

341 mg/g for rhodamine B (50 mg/L)

graphene gydrogel 59

Not mentioned

About 40 mg/g for MO (50 mg/L) 99.2 mg/g for MB (50 mg/L) About 180 mg/g for congo red (50 mg/L)

This study

1246 mg/g for MB (187 mg/L) 491 mg/g for MV (204 mg/L) 1253 mg/g for Cu(II) (2.52 g/L) 564 mg/g for Ni(II) (2.12 g/L) 640 mg/g for Cd(II) (4.49 g/L) 252 mg/g for Pb(II) (8.29 g/L)

CONCLUSION 22


The compressive stress was 27.4 MPa with a strain of 80%

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In summary, reinforced-concrete structured PAA hydrogel microspheres were designed and prepared via electrospraying followed by cross-linking polymerization. Comparing with pure PAA hydrogels, this structure effectively enhanced the compression strength of the hydrogels by more than 200 times and reduced the water uptake of the hydrogels by more than 10 times. Interestingly, this structure did not affect the high IECs of the hydrogels, and the microspheres showed ultrahigh adsorption capacities for both of cationic dyes and heavy metal ions. Moreover, the microspheres showed excellent recyclability. The adsorption column filled with the microspheres also exhibited good selective removal for cationic dyes and heavy metal ions. Additionally, the breakthrough behaviour indicated that the adsorption column owned stable and efficient filtration effect. What’s more, the proposed method was simple and universal to fabricate functional hydrogel microspheres with reinforced-concrete structure for diverse applications by simply altering the monomers in reaction solutions. For examples, the hydrogels microspheres preparing from the monomers with positive groups (such as acrylamide) can be used to adsorb the anionic dyes, and the hydrogel microspheres with sulfonic groups (such as 1-acrylanmido-2-methylpropanesulfonic acid) can be used in blood purification. Thus, it is believed that the reinforced-concrete structured PAA hydrogel microspheres with superior performances have strong advantages in cost-effective pollutant remediation systems.

ACKNOWLEDGMENTS 23



This work was financially sponsored by the National Natural Science Foundation of China (No. 51433007, 51503125 and 51673125), State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, Donghua University (LK1619) and the Youth Science and Technology Innovation Team of Sichuan Province (Grant No.2015TD0001). We should also thank our laboratory members for their generous help.

AUTHOR INFORMATION *Corresponding Author Tel.: +86-28-85400453; Fax: +86-28-85405402. E-mail: [email protected] (W-F. Zhao*); [email protected] (C-S. Zhao**) ORCID Weifeng Zhao: 0000-0003-2689-0251 Changsheng Zhao: 0000-0002-4619-3499 Author Contributions §These authors contributed equally. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest.

ASSOCIATED CONTENT Supporting Information

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The morphologies of the RHMs (P8/Gel) observed by an optical microscope. FTIR spectra for P8, P8/HEMA-VP and P4/HEMA-VP. FTIR spectra for P8/Gel, P8/HEMA-VP/Gel and P4/HEMA-VP/Gel. The TGA curves for P8, P8/HEMA-VP and P4/HEMA-VP. The mass fractions of PAA hydrogel, PES and P(HEMA-VP) in the RHMs. The surface and the cross-sectional morphologies of the RHMs observed by SEM. The cross-sectional morphologies of the PES and PAA hydrogel microspheres observed by SEM. The compressive stress-strain curves of the P8 and P8/HEMA-VP microspheres. The compressive stress-strain curves of the PAA hydrogel microspheres cross-linked by ethyleneglycol dimethacrylate. The tensile stress-strain curves of the P4/HEMA-VP/Gel, PES, and Gel(P4/HEMA/VP) (specimen size: 10×1×0.06 mm3). Digital pictures of the PES/Gel and the Gel(PES) microspheres under dried, wet and alkaline conditions. The effects of pH values and initial MB concentrations on the adsorption amounts of the RHMs. The adsorption amounts for dyes and toxic metal ionsof the RHMs. The recyclability of the P4/HEMA-VP/Gel microspheres. The kinetic models and the parameters for the adsorption of MB to the RHMs. Isotherm parameters for MB adsorption on the RHMs (P4/HEMA-VP/Gel). This material is available free of charge via the Internet at http://pubs.acs.org.

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Table of Contents graphic

Synopsis Reinforced-concrete structured hydrogel microspheres were designed with superior mechanical property, restricted swelling to remove the cationic dyes and heavy metals.

31



Reinforced-concrete structured hydrogel microspheres were designed with superior mechanical property, restricted swelling to remove the cationic dyes and heavy metals. 47x24mm (300 x 300 DPI)


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Figure 1. (A) The process of the electrospraying. (B) The graphic symbols of (C) the preparation process of the RHMs. (D) EDX mapping analysis of the RHMs corresponding to the red square. 76x32mm (300 x 300 DPI)



Figure 2. (A) The compressive stress-strain curves of the RHMs and the HMs. (B) The dimensional recovery ratios of the P8/Gel, PES, and Gel/P8 microspheres after the compression (h0 and h1 are the heights of the microspheres before and after compression, the micrespheres are immersed into water for dimensional recovery). (C) Digital pictures of the RHMs and the HMs before and after hammering. (D) Digital pictures of the P4/HEMA-VP/Gel and the Gel(P4/HEMA-VP) microspheres dyed by methylene blue, after swelling (10 mg of the microspheres is used). (E) The water uptakes of the RHMs and the HMs. 74x31mm (300 x 300 DPI)


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Figure 3. (A) The ion exchange capacities (IECs) of the RHMs. (B) The adsorption amounts of MB per unit mass of the RHMs (P8/Gel, P8/HEMA-VP/Gel and P4/HEMA-VP/Gel), HMs (Gel(P8), Gel(P8/HEMA-VP) and Gel(P4/HEMA-VP)), and skeleton microspheres (P8, P8/HEMA-VP and P4/HEMA-VP). The adsorption amounts of the different RHMs and HMs (C, D and E). (F) The adsorption amounts of the RHMs. (G) Digital pictures of the MB solution after contacting with the RHMs (P4/HEMA-VP/Gel) at different time intervals (0, 15, 30, 60, 90 and 120 min). 135x103mm (300 x 300 DPI)



Figure 4. (A) The illustration of the adsorption of MB for MB/MO mixed solution. (B) The UV-vis spectra of the mixed solution and filtered solution, as well as the digital picture of the selective adsorption experiment (conditions: initial MB concentration = 250 µM, MO concentration = 250 µM). (C) The illustration of the adsorption for heavy metal ions and cationic dyes. (D) Digital picture of the adsorption column experiment and the UV-vis spectra of the mixed solution and filtered solution. (E) The removal ratios of dyes and heavy metal ions by the adsorption column. (F) The breakthrough curves of the adsorption column and the digital pictures of the adsorption column before and after the adsorption (conditions: initial MB concentration = 500 µM). 113x73mm (300 x 300 DPI)


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Reinforced-concrete-structured Hydrogel Microspheres with Ultrahigh

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