Removal of Heavy Metal Ions by Poly(vinyl alcohol) - ACS Publications

Research Article pubs.acs.org/journal/ascecg

Removal of Heavy Metal Ions by Poly(vinyl alcohol) and Carboxymethyl Cellulose Composite Hydrogels Prepared by a Freeze−Thaw Method Liang-Yi Wang and Meng-Jiy Wang* Department of Chemical Engineering, National Taiwan University of Science and Technology, 43, Keelung Road, Sec. 4, Taipei 106, Taiwan S Supporting Information *

ABSTRACT: Environmentally friendly composites made of poly(vinyl alcohol) and carboxymethyl cellulose (PVA/CMC) hydrogels were proposed. The PVA/CMC hydrogels were prepared by freeze−thaw processes and applied to adsorb heavy metal ions including Ag+, Ni2+, Cu2+, and Zn2+. The phase separation of polymer solutions during freezing stages led to the formation of crystallites and resulted in insoluble hydrogels. The PVA/CMC hydrogels were effectively acquired, confirmed by at least 71% of insoluble gels found in the gel fraction. In addition, the gel fraction, physical properties, and swelling ratios were clearly modulated by the compositions of PVA and CMC among the samples of pure PVA, P2C1 (containing two-thirds of PVA and one-third of CMC), P1C1, and P1C2 hydrogels. For pure PVA hydrogels, the swelling ratio was 416%, while the P1C2 hydrogels exhibited a much higher swelling ratio (1437%). For the metal adsorption, the experimental results indicated that the P2C1 hydrogels presented not only the highest adsorption capacity toward Ag+ (8.4 mg per gram of hydrogel) but also the highest selectivity toward Ag+ in the mixed solutions. Interestingly, the experimental results revealed clearly that the adsorbed metal ions were further reduced on the P2C1 hydrogels. In summary, an insoluble hydrogel composed of PVA/CMC with excellent capacity of adsorbing metal ions was prepared in an economic and energy-saving manner. The prepared hydrogels showed potential applications for removal of heavy metal ions and wastewater treatment. KEYWORDS: Poly(vinyl alcohol), Carboxymethyl cellulose, Freeze−thaw, Metal ion recovery, Hydrogels

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INTRODUCTION Hydrogels are insoluble, cross-linked, three-dimensional hydrophilic polymeric networks which can swell and absorb 10% to thousands of times their dry weight of water.1,2 The threedimensional, cross-linked networks of hydrogels possess high sorption capacity which has been used in various fields. For instance, Pourjavadi and co-workers have prepared hydrogels based on carboxymethyl cellulose (CMC) and sodium alginate as a superabsorbent which showed the swelling capacity of 221.3 g water/g hydrogel at 85 °C.3 Similarly, the research group of Abou Taleb developed poly(vinyl alcohol) (PVA)/ CMC copolymer hydrogels for the removal of dyes. The authors discussed the effects of hydrogel composition, temperature, and pH on the adsorption. The results displayed high sorption capacities of the synthesized hydrogels which can adsorb around 140 mg dye/g hydrogel.4 For different applications, Nugent et al. prepared physically stable PVA/ NaOH hydrogels combining with poly(acrylic acid) films for drug delivery. The composite hydrogels were incorporated with theophylline which followed the release kinetic of Fickian model.5 Recently, hydrogels have been particularly utilized as © XXXX American Chemical Society

an adsorbent to remove heavy metal ions from wastewater. As the water diffused into hydrogels, the metal ions were carried into the polymer networks. Then, the functional groups of the hydrogels such as hydroxyl, amine, carboxyl, phosphate, and sulfhydryl can act as binding sites to uptake the metal ions that the removal of metal ions from aqueous solutions became possible.6 Hydrogels can be synthesized either by the formation of chemical or physical linkages. The physically formed hydrogels were synthesized by cross-linking due to the molecular entanglements, ionic interaction, hydrogen bonding, protein interaction, or crystallization.2,7,8 The hydrogels prepared by physical method allowed to prevent the usages of cross-linking agents or initiators, which are advantageous for biomedical and pharmaceutical applications.9 On the other hand, the methods for preparing hydrogels by high energy sources such as electron beam, X-rays, and γ-irradiation required expensive apparatus Received: February 18, 2016 Revised: March 30, 2016

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DOI: 10.1021/acssuschemeng.6b00336 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Research Article

ACS Sustainable Chemistry & Engineering Table 1. Summaries of the Results for Removal of Heavy Metals by PVA or CMC Based Hydrogels adsorption conditions samples

preparation method

CMC CMC/ECH

γ- irradiation chemical cross-linking

CMC-g-poly(NIPAm-coAA)/MMT

chemical cross-linking

CMC-g-PAA/APT chitosan/PVA chitosan/PVA PVA-EDTA

chemical cross-linking precipitation in NaOH drop into NaOH/methanol solution γ- irradiation

PVA/CMC

freeze−thaw

heavy metals

adsorption capacities (mg/g)

Cu Cu Ni Pb Cu Pb Pb Cu Pb

∼230 6.49c 4.06c 5.15c 2.23 (1.11)c,d 1.51 (0.41)c,d 952.38 47.85 ∼0.95a

Pb Cd Ag Ni Cu Zn

8.5 4.2 8.4 6.0 5.5 5.3

a

(5.6)d (4.3)d (7.7)d (2.7)d (3.9)d (3.9)d

initial concentration (ppm)

time (h)

pH

temperature (°C)

10 0−15b

120 72

7

NA RT

28 29

8 (4)b,d

48

4

RT

30

3000−4500 0−14 30

2 1.17 6

2−6.25 1−6 2−7

30 NA NA

29 31 32

100

72

NA

33

100

24

15

this work

b

∼5 1.6

ref

Read from the figures. bExpressed in millimole per liter. cExpressed in millimole per gram. dNumber in parentheses: results of competitive adsorption. NA: not available. a

in Table 1. Apart from the adsorption, the PVA possessed the functions of reducing and stabilizing the metal ions. For example, Ag+ was reduced on PVA to form PVA/Ag composite films or hydrogels.24,25 Furthermore, the hydroxyl and carboxyl groups in CMC were reported to reduce and stabilize both gold and silver ions to become Au or Ag nanoparticles on the polymers.26,27 In this study, PVA and CMC were chosen to prepare PVA/ CMC hydrogels by physical cross-linking via freeze−thaw processes. The effects of CMC content in PVA/CMC hydrogels on the physical properties including the crystallinity, gel fraction, and swelling ratio were investigated. Besides, the PVA/CMC hydrogels were applied to adsorb different heavy metal ions including Ag+, Ni2+, Cu2+, and Zn2+. The contribution of this developed method allowed preparing hydrogels that can be utilized to collect and recover heavy metals from industrial wastewater and other hazardous conditions in a facile and effective manner.

and the prepared hydrogels generally showed relatively poor mechanical strength.10,11 One of a physical method to prepare hydrogels was the crystallization which was first proposed by Peppas, who prepared hydrogels by freeze−thaw techniques. The hydrogels formed because of the phase separation in polymer solution after repeated freezing and thawing processes.12 Furthermore, the polymer crystallites formed when the polymer solutions were exposed under low temperature, which acted as junction sites and resulted in the insoluble polymer networks.12,13 The hydrogels prepared by freeze−thaw processes have drawn extensive attention because the better elastic properties than that of the hydrogels prepared by chemical methods.14 Furthermore, the mechanical properties of these freeze−thawed hydrogels can be modulated by altering the concentration of polymers, the number of freeze−thaw cycles, the freezing and thawing time period, and the freezing temperature.8,15 Additionally, the freeze−thaw cycles allow a porous structure to be created in hydrogels because of the space left from the melting ice crystals at thawing stages.16,17 PVA is a semicrystalline polymer synthesized from the hydrolysis of polyvinyl acetate. Due to its good solubility, biocompatibility, and biodegrablility, PVA has been used in many industrial applications including food, textiles, and pharmaceutics.18 In addition, polymeric networks among PVA polymer chains can be formed by chemical reactions or hydrogen bonding on the pendent OH groups of the repeating unit in PVA which resulted in cross-linked effects.19 On the other hand, CMC is a major derivative of cellulose in which carboxymethyl groups replace some of the hydrogen atoms of the hydroxyl groups on the cellulose backbone. CMC was reported to be biocompatible with characteristics of good water solubility and biodegrability.20 CMC was used in various fields such as thickening agents and emulsion agents in food and pharmaceutical industries.21 Moreover, CMC can act as an adsorbent to adsorb and stabilize metal ions by the hydroxyl and carboxyl groups on the structure.22,23 Previous studies reported that PVA or CMC based hydrogels showed the capabilities to adsorb heavy metals, as summarized

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EXPERIMENTAL SECTION

Preparation of PVA/CMC Composite Hydrogels. PVA/CMC hydrogels were prepared by repeating freeze−thaw processes. A 5 w/v % of PVA (Hayashi Pure Chemical Ind., Ltd., M.W. 88 000) aqueous solution was prepared by dissolving PVA in 90 °C DI water whereas a 1 w/v% of CMC (Sigma-Aldrich, M.W. 250,000) aqueous solution was prepared at room temperature. The PVA/CMC hydrogels were formed by repeating the freeze−thaw cycles where one cycle is facilitated by freezing the PVA and CMC solutions with different ratios at −20 °C for 6 h and followed by thawing at room temperature for 1 h. The freeze−thaw cycles were repeated five times. According to the ratios of added PVA/CMC, the as-prepared PVA/CMC composite hydrogels were named as PVA (1:0, PVA:CMC), P2C1 (2:1, PVA:CMC), P1C1 (1:1, PVA:CMC), and P1C2 (1:2, PVA:CMC), correspondingly. Gel Fraction and Swelling Behavior. The gel fraction and swelling behavior of the prepared PVA/CMC hydrogels were evaluated by gravimetric method. The samples with the size of 1 cm2 × 1 cm2 were dried at 37 °C until reached constant weight that was denoted as W0. The samples were then soaked in DI water at room temperature for 24 h to remove the soluble parts, followed by drying at 37 °C for another 24 h and to weight, and denoted as W1. B


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ACS Sustainable Chemistry & Engineering

Figure 1. Cross-sectional images of (a) PVA, (b) P2C1, (c) P1C1, and (d) P1C2 hydrogels (scale bar: 100 μm). The samples were named according to the ratios of added PVA/CMC: PVA (1:0, PVA:CMC), P2C1 (2:1, PVA:CMC), P1C1 (1:1, PVA:CMC), and P1C2 (1:2, PVA:CMC). The gel fraction was calculated as

gel fraction % = (W1/W0) × 100%

spectroscopy (ATR-FTIR, Bio-Rad FTS-3500) in the region of 600− 4000 cm−1. The infrared spectra were recorded at resolution of 8 cm−1 with 256 scans. The cross-sectional morphology of the PVA/CMC hydrogels was observed by a field-emission scanning electron microscope (FE-SEM, JEOL JSM-6500F). Prior to the FE-SEM experiment, the hydrogels were freeze-dried at −40 °C, 0.02 mbar for 24 h. The dried hydrogels were immersed in liquid nitrogen and snapped immediately, followed by platinum coating. The crystallinity of the hydrogels was examined by X-ray diffractometer (XRD, Bruker) with a Cu Kα radiation source and the data was collected from 2θ = 5−40° with a scanning rate of 1° min−1. The amount of the adsorbed metal ions by PVA/CMC hydrogels was quantified by inductively coupled plasma-atomic emission spectrometer (ICP-AES, JY 2000-2) and electron spectroscopy for chemical analysis (ESCA, VG ESCA Scientific Theta Probe). For ICP analyses, PVA/CMC hydrogels after adsorbing ions were dried at 37 °C, followed by weighting and immersing in 50 wt % of HNO3 for 24 h to dissolve the adsorbed metallic ions. The resultant solution was then diluted to 5 wt % and filtrated by a 0.22 μm filter. The wavelengths used for ICP were set at 328.068, 324.754, 206.200, and 231.604 nm for Ag, Ni, Cu, and Zn, respectively. In addition, ESCA analyses were applied to determine the surface chemical composition of PVA/CMC hydrogels after adsorption. The instrument used Al (1486.6 eV) as an excitation source with pass energy of 50 kV. Sample characterization was taken with the X-ray spot size of 400 nm and take off angle was 53°. Data processing including background subtraction, integration, and

(1)

The swelling ratio was measured by immersing the dried PVA/ CMC hydrogels (Wd) in excessive DI water under room temperature for 96 h until equilibrium. Then, the mass of wet hydrogels (Wt) was measured after carefully removing the surface water. The swelling ratio of hydrogels was calculated by the following equation:

swelling ratio % = (Wt − Wd)/Wd × 100%

(2)

Adsorption of Metal Ions by PVA/CMC Composite Hydrogels. The adsorption capacity of the prepared PVA/CMC hydrogels toward silver ions was evaluated by immersing the samples of 1 cm2 × 1 cm2 in 1 mL, 100 ppm Ag+ solution for 24 h at 15 °C. The sample with the reduced silver was named as sample/Ag. The sample with the highest adsorption capacity toward silver ions was then taken to perform the experiments on the adsorption toward different metal ion solutions including Ni2+, Cu2+, and Zn2+ (High-Purity Standards, 1000 ppm). Moreover, the competitive adsorption experiment was also carried out by adding hydrogel samples in the mixed metal ion solutions containing 100 ppm of Ag+, Ni2+, Cu2+, and Zn2+. It is noted that all the adsorption experiments were performed at 15 °C for 24 h. Material Characterizations. The chemical composition was examined by the attenuated reflection Fourier transform infrared C


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Figure 2. (a) ATR-FTIR spectra and (b) X-ray diffraction of of (i) PVA, (ii) P2C1, (iii) P1C1, (iv) P1C2 hydrogels, and (v) CMC films.

Table 2. Analyses for the Chemical Composition from ATR-FTIR Spectra for Prepared Hydrogels: PVA, CMC, and PVA/CMC wavenumber (cm−1) PVA

P2C1

P1C1

P1C2

CMC

assignment

ref

3000−3600 2943, 2908

3000−3740 2908 1593 1412 1327 1142 1092 1057 914 841

3000−3750 2916 1593 1412 1323 1142 1092 1057 914 849

3000−3800 2920 1593 1412 1323 1142 1092 1057

3000−3700 2916 1589 1416 1323

OH stretching C−H stretching −COO− stretching CH2 scissoring −OH bending crystalline band of PVA C−O stretching CH2−O−CH2 stretching CH2 vibration C−C stretching

36 36 38 36 36 36 36 9,38 36 31

1412 1327 1142 1092 918 837

830

1057

leaving porous structure in the hydrogels.34,35 The PVA/CMC hydrogels with different ratios were prepared: PVA (1:0, PVA:CMC), P2C1 (2:1, PVA:CMC), P1C1 (1:1, PVA:CMC), and P1C2 (1:2, PVA:CMC). It was noted that the pure CMC could not form hydrogel by the proposed freeze−thaw method. The cross-sectional images of the prepared hydrogels showed that all of the PVA/CMC hydrogels possessed sponge-like structure with high extent of porosity (Figure 1). The porous structure was created by the freeze−thaw processes that the ice crystals formed in the hydrogels. After thawing, the ice crystals melt, leaving the porous structure in the PVA/CMC hydrogels.35 The network morphology with irregular distribution of pores was found in the prepared PVA/CMC hydrogels. In particular, the porous structure in P2C1 hydrogels was more obvious among all the prepared hydrogels with the size around 30 μm (Figure 1b). ATR-FTIR spectroscopy revealed that PVA/CMC hydrogels showed characteristic functional groups of both PVA and CMC (Figure 2a, Table 2). Several peaks can be observed on all of the samples: the broad bands appeared at around 3000−3700 cm−1 corresponding to the −OH stretching; the peaks at about 2900 cm−1 referred to the C−H stretching. Finally, the adjacent peaks at 1412 and 1327 cm−1 were assigned to CH2 scissoring and −OH bending. Importantly, the absorption peaks at 1589 cm−1, attributed to COO− stretching from CMC, were only found on PVA/CMC hydrogels and were absent on PVA hydrogels. In addition, the peaks at 1142 cm−1, known as the crystallization-sensitive peak of PVA, were eliminated as the increase of the ratio of CMC in PVA/CMC hydrogels. Besides, both of the peaks at 1092 and 1057 cm−1, representing C−O

deconvolution was carried out using Peak-fit software. The peak position of all ESCA spectra was calibrated with C 1s at 284.6 eV. Statistic Analyses. Statistical analyses were performed by one-way analysis of variance (ANOVA) executed by original lab software. Tukey comparison tests were utilized to compare the difference between different samples. In all tests, the significance level was indicated by the asterisk mark (*): * for p-value < 0.05, ** for p-value < 0.01, and *** for p-value < 0.001.

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RESULTS AND DISCUSSION In this work, PVA/CMC hydrogels were prepared by freeze− thaw processes. The gel fraction and swelling behaviors of the PVA/CMC hydrogels were investigated. Besides, the prepared PVA/CMC hydrogels with different PVA and CMC ratios were employed to adsorb and reduce silver ions. In addition, PVA/ CMC hydrogels with the highest adsorption capacity of silver ions were used for further evaluation for the adsorption capacities toward different metallic ions in aqueous solutions. Moreover, the adsorption experiments were performed in both single and mixed metallic ions conditions. Preparation of PVA/CMC Hydrogels. PVA/CMC hydrogels were prepared by repeating the freeze and thaw cycles. During the freezing stages, the water in PVA and CMC mixed solution froze at low temperature which caused the phase separation into a polymer-rich phase and a water-rich phase. The adjacent polymer chains in the polymer-rich phase resulted in the formation of hydrogen bonding and crystallites. In addition, the thawing procedure facilitated the interactions and crystalline regions between the remained polymers, leading the formation of hydrogel networks.9,13 Furthermore, the ice crystals acted as porogen during the formation of hydrogels, D


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For pure PVA hydrogels, the gel fraction was about 96% which meant that the PVA polymer was cross-linked in very high degree. During the freeze−thaw process, the hydrogen bonding between PVA polymer chains increased which enhanced the cross-linking degree and the crystallinity in the hydrogels, leading to the high gel fraction.39 For P2C1 hydrogels, the gel fraction decreased to 90%. Moreover, the gel fraction further decreased to 71% as the CMC content increased to two-thirds, for P1C2 hydrogels. The presence of CMC decreased crosslinking reactions as well as the crystallinity between PVA polymer chains and therefore lowered the gel fraction of the PVA/CMC hydrogels.40 In addition, the addition of CMC into PVA resulted in the increase of the compliance of the composites that the elongation at break was found to be the lowest for PVA and increased for at least 15% on P1C2, P1C1, and P2C1 (Figure S1, the Supporting Information). The higher gel fraction of PVA/CMC hydrogels suggested that freeze−thaw processes can be a promising method to prepare hydrogels. Comparing to the previous studies which performed long freezing time (from 18 h up to 3 days) for the freeze−thawed methods to prepared PVA/CMC hydrogels, the proposed approach in this study allowing acquiring the hydrogels in a more efficient way.39,40 In contrast to gel fraction, the presence of CMC showed positive effects on the swelling ratio of PVA/CMC hydrogels. By 96 h of adsorption in water, the swelling ratios of PVA/ CMC hydrogels were 416, 743, 930, and 1437% for the PVA, P2C1, P1C1, and P1C2 hydrogels (Figure 3). It was found that all the PVA/CMC hydrogels possessed high water uptake ability. The higher swelling degree in PVA/CMC hydrogels was most probably due to the increasing hydrophilicity from CMC.34,39 Moreover, as the amount of PVA decreased, the physical cross-linked density in the hydrogel decreased which benefited to the permeation of water molecular, leading to the improvement of the swelling ability.14,40 The results of gel fraction and swelling ratio showed different trends that the PVA hydrogels possessed the highest gel fraction while P1C2 hydrogels showed the highest swelling ratio. For PVA and P2C1, the higher gel fraction represented higher cross-linked structure in the hydrogels. On the other hand, the highly cross-linked polymer networks reduced the water uptake by hindering the penetration of water that the hydrogels with higher gel fraction showed lower swell ratio.41,42 Adsorption of Metal Ions. Previous studies have demonstrated the possibility to use PVA or CMC to incorporate or to remove metal ions that the results were listed in Table 1. In this

and C−O−C stretching, can be observed on PVA/CMC hydrogels which only appeared on PVA and CMC individually. It was also found that as the content of CMC increased, the peaks at 918 and 837 cm−1 which were assigned to the rocking vibration of CH2 and C−C stretching were found to be diminished and shifted which might be due to the interactions between PVA and CMC.36 Furthermore, the OH stretching bands were broader in PVA/CMC hydrogels when compared to the pure PVA hydrogels, suggesting the hydrogen bonding between −OH and carboxymethyl groups in PVA and CMC.4,34,37 Therefore, by freeze−thaw processes, the physical cross-linking structure can be formed between PVA and CMC and confirmed the successful preparation of PVA/CMC hydrogels. The crystallinity of the prepared PVA/CMC hydrogels was examined by XRD. The pure PVA hydrogels exhibited the strong diffraction peak at 2θ = 19.5°, corresponding to the (101) crystal plane in PVA while CMC presented a broad peak at 2θ around 21°.21,34,35 It was observed that the characteristic peak of PVA became weaker in the PVA/CMC hydrogels due to the formation of hydrogen bonding and the interactions between PVA and CMC which decreased the crystallinity of the PVA/CMC hydrogels.11,14 Gel Fraction and Swelling Behavior. The influence of CMC content in PVA/CMC hydrogels on gel fraction and swelling behavior were investigated. The results showed that the presence of CMC in PVA/CMC hydrogels tended to decrease the gel fraction. As more CMC was added in PVA/ CMC hydrogels, the lower the gel fraction became (Figure 3).

Figure 3. Gel fraction and swelling ratio of PVA/CMC hydrogels: (−■−) gel fraction; (−△−) swelling ratio; N: 3.

Figure 4. Quantity of (a) adsorbed silver ions on PVA/CMC and (b) adsorbed metal ions on P2C1 hydrogels (concentration of metal ions: 100 ppm; N: 3; *, **, and *** represent p < 0.05, p < 0.01, and p < 0.001, respectively). E


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Figure 5. FE-SEM images of (a) P2C1 hydrogels and (b) the reduced silver particles on P2C1 hydrogels. (inset) Pictures of P2C1 and P2C1/Ag hydrogels; scale bar: (a) 100 μm, (b) 100 nm. (c) Chemical compositions and (d) high resolution ESCA spectra of (i) P2C1 and (ii) P2C1/Ag hydrogels (concentration of Ag+: 100 ppm, immersion time: 24 h).

networks easily, revealing faster adsorption rate and higher adsorption capacity.30 Due to the similar radius, similar adsorption capacity was found for nickel, copper, and zinc ions on P2C1 hydrogels. Beside the individual metallic ion solution, the adsorption experiments were also performed in the mixed metal ion solutions where the concentration of each metal ion was fixed as 100 ppm (total concentration of ions: 400 ppm). The results showed that the adsorption capacity decreased for all metal ions in mixed solutions when compared with the adsorption behavior in individual metal ion solution (Figure 4b). The adsorption capacity in the mixtures was 7.7, 2.7, 3.9, and 3.9 mg/g against Ag+, Ni2+, Cu2+, and Zn2+, which decreased about 8.0, 54, 28, and 26% compared with that in corresponding single metal ion solutions. The results revealed that the adsorption of Ni2+ was significantly affected by the existence of the other metal ions. Among the applied divalent metal ions (Ni2+, Cu2+, and Zn2+), the amount of the adsorbed Ni2+ showed the lowest value, owing to the weaker attraction between Ni2+ ions and oxygen atoms containing in PVA and CMC, which is consistent with the findings reported from previous studies.29,43,44 The morphological analyses showed that the as-prepared P2C1 hydrogels were translucent with rough surface (Figure 5a). By immersing P2C1 in silver ion solution, the apparently brown color can be clearly observed even by naked eyes (insert image, Figure 5b). Furthermore, the FE-SEM image clearly revealed the uniformly distributed particles on the hydrogels with irregular shape and the size around 80 nm. ESCA analyses showed that the pristine P2C1 hydrogels revealing the peaks of C 1s, O 1s, and Na 1s at 284.6, 531.0, and 1071.8 eV (Figure 5c).32 After immersing P2C1 in Ag+ solution, no peak of Na was observed on P2C1/Ag hydrogels. Moreover, the high resolution ESCA spectra showed two spin−orbit splitting peaks could be found at 367.3 and 373.3 eV, which corresponded to Ag 3d5/2 and Ag 3d3/2 (Figure 5c and d). The results confirmed

study, PVA/CMC hydrogels were applied to adsorb metal ions which showed that the adsorption capacity of PVA/CMC hydrogels related to the content of CMC. For pure PVA hydrogels, the adsorption capacity toward Ag+ was 4.7 mg/g (per gram of hydrogel), which increased to 8.4 mg/g in P2C1 hydrogels (Figure 4a). The advantageous characteristic of P2C1 might be due to the −OH groups presented in every repeating unit in PVA, as well as the abundant carboxylate and hydroxyl groups in CMC, that acted as binding sites for metal ions and resulted in the higher adsorption capacity to silver ions.22 However, for the hydrogels containing higher fraction of CMC, the quantity of adsorbed Ag+ decreased. For P1C1 and P1C2 hydrogels, the amount of the adsorbed Ag+ was 2.3 and 2.9 mg/ g, respectively. The lower adsorption capacity for P1C1 and P1C2 hydrogels may be due to the increasing hydrogen bonding between PVA and CMC which hindered the bonding sites available for adsorbing metal ions on the hydrogels. The silver adsorption results indicated that P2C1 hydrogels displayed the highest adsorption capacity toward Ag+ so that the P2C1 hydrogels were chosen for further adsorption experiments. Apart from silver, various heavy metals including Ni2+, Cu2+, and Zn2+ were often used and found in industrial wastes. Therefore, in this study the individual and mixed solutions containing the metallic ions of Ag+, Ni2+, Cu2+, and Zn2+ were prepared to discuss selectivity of P2C1 for the metal ions adsorption. For the specific metal ions adsorption, P2C1 hydrogels were immersed in individual metal ion solutions. The results showed that the adsorption capacity of P2C1 hydrogels to Ag+ was about 8.2 mg/g, whereas the adsorption capacity toward Ni2+, Cu2+, and Zn2+ was 6.0, 5.5, and 5.3 mg/g, respectively (Figure 4b). The highest adsorption capacity toward Ag+ might be due to the single valent of Ag+ when compared with the other divalent metal ions. It was known that the adsorption capacity was affected by the size of metal ions. The metal ions with smaller size could diffuse into polymer F



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the reduction of Ag.33,34 Furthermore, the chemical composition evaluated by ESCA revealed that the Ag content increased to 3.81% after adsorbing Ag and the content of Na decreased from 2.08% to 0.08% (Table 3). The mechanism of silver

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.6b00336. Mechanical properties of the prepared PVA/CMC composites (PDF)

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element (at %) sample

C 1s

O 1s

Na 1s

Ag 3d

284.6

530.4

1071.8

373.3

P2C1 hydrogel P2C1/Ag hydrogel

69.03 72.26

28.89 23.85

2.08 0.08

0 3.81

ASSOCIATED CONTENT

S Supporting Information *

Table 3. Chemical Composition of P2C1 Hydrogel and P2C1/Ag Hydrogel

peak position

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AUTHOR INFORMATION

Corresponding Author

*Tel.:+886-2-2730-1146. Fax: +886- 2-2737-6644. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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adsorption/reduction could be contributed to the ion exchange reactions that H+ and Ag+ replaced Na+ on the carboxylate groups of CMC, as indicated by the evolution of the surface chemical composition from ESCA analyses. In summary, the proposed freeze−thaw method allowed to prepare different ratio of PVA/CMC composite hydrogels with excellent chemical properties. The swelling ratio of the prepared hydrogels revealed the opposite tendency to gel fraction which could be modulated by the content between PVA and CMC. Moreover, the functionalities existed on the PVA/CMC composited revealed excellent capabilities to adsorb and reduce metallic ions including silvers, zincs, coppers, and nickels. With the development of this facile method which requires low energy consumption, the prepared PVA/CMC composites showed great potentials for the applications for the collections and recovery of heavy metal and for industrial wastewater treatment.

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ACKNOWLEDGMENTS The authors would like to express special appreciation to the Minister of Science and Technology, Taiwan (MOST: 1022221-E-011-115-MY3) and Taiwan Tech for their financial supports.

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REFERENCES

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CONCLUSION

This study applied a freeze−thaw method to prepare PVA/ CMC hydrogels with high degree of cross-linking which can be easily utilized in heavy metal adsorption and collection. SEM images revealed that the PVA/CMC hydrogels possessed porous structure which could contribute to high swelling ratio. The results of gel fraction and swelling ratio studies implied that the increase of CMC content directed toward lower gel fraction and higher swelling ratio for the PVA/CMC hydrogels with different CMC ratios. The prepared hydrogels were employed to adsorb metal ions in solution containing single or mixed metal ions to evaluate the adsorption behavior in noncompetitive and competitive conditions. Among all the PVA/CMC hydrogels, the P2C1 hydrogels showed the highest adsorption ability against Ag+. It was noted the P2C1 hydrogels revealed higher selectivity to Ag+ than to Ni2+, Cu2+, and Zn2+ in both competitive and noncompetitive conditions. On the other hand, the amount of adsorbed Ni2+ was the lowest due to the weaker attraction between Ni2+ and the functional groups on PVA/CMC hydrogels. In conclusion, the prepared PVA/ CMC composite hydrogels showed excellent adsorption capacity for the adsorption and reduction of heavy metal ions. Therefore, the PVA/CMC hydrogel can be a potential candidate for various applications including removal and recovery of heavy metal ions or synthesis of metal/PVA/ CMC composites for biomedical applications. G


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