Gelatin Hydrogel Particles for Removal of Multiple

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Materials and Interfaces

Spherical Chitosan/Gelatin Hydrogel Particles for Removal of Multiple Heavy Metal Ions from Wastewater Suguna Perumal, Raji Atchudan, Dong Ho Yoon, Jin Joo, and In Woo Cheong Ind. Eng. Chem. Res., Just Accepted Manuscript • Publication Date (Web): 22 May 2019 Downloaded from http://pubs.acs.org on May 28, 2019

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Industrial & Engineering Chemistry Research

Spherical Chitosan/Gelatin Hydrogel Particles for Removal of Multiple Heavy Metal Ions from Wastewater Suguna Perumal,a Raji Atchudan,b Dong Ho Yoon,c Jin Joo,a,* and In Woo Cheonga,* a

Department of Applied Chemistry, School of Engineering, Kyungpook National University,

Buk-gu, Daehak-ro 80, Daegu 41566, South Korea b

c

School of Chemical Engineering, Yeungnam University, Gyeongsan 38541, South Korea

R & D Center, Kuk-Il Paper Mfg. Co., Ltd., Baekok-daero 563, Cheoin-gu, Yongin 17128,

South Korea

AUTHOR INFORMATION Corresponding Authors *Email: [email protected] (I.W.C.), [email protected] (J.J.)

1

ACS Paragon Plus Environment

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

ABSTRACT: This article describes a simple preparation of chitosan/gelatin (CG) spherical hydrogel particles for the effective removal of multiple heavy metal ions. The CG hydrogel particles were prepared by inverse emulsion from the aqueous solutions of chitosan, gelatin, and glutaraldehyde. The oven-dried C2G1 hydrogel particles showed a maximum removal efficiency of 98% for Hg(II) ions in a single metal ion solution, and which was higher than C1G1 (85%) and C1G2 (58%) particles. This result was affected by the composition of the hydrogels rather than the pore size or degree of swelling. Remarkably, the removal efficiencies for the Pb(II), Cd(II), Hg(II), and Cr(III) ions reached about 73%–94% in a multiple metal ion solution. The results indicate that the CG hydrogel particles can be used to remove coexisting heavy metal ions from wastewater, providing a versatile method to remove multiple metal ions from natural or industrial wastes. Keywords: Heavy metals, Adsorption, Hydrogel particles, Chitosan, Gelatin

INTRODUCTION One of the major causes of water pollution is toxic industrial effluents containing heavy metals such as cadmium, chromium, mercury, copper, lead, and nickel.1,2 These toxic metals cause serious diseases such as cancer and brain, kidney, and DNA damage.3,4 Therefore, serious and immediate attention is required to remove toxic metal ions from groundwater. Various methods based on physical and chemical processes, such as ultrafiltration, electrochemical precipitation, ion exchange, and osmosis have been developed for this purpose.5-7 However, these methods have a limit of high operating cost.5-7 In recent years attention has been focused on removing toxic metals from wastewater by using low cost and environmentally-friendly biocompatible materials such as algae, yeast, dead microbial biomass, chitosan, gelatin, lignin, and others.4,8,9 In particularly, there have been many reports 2


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on the removal of heavy metal ions using crosslinked chitosan and gelatin hydrogels.3,10-16 Crosslinked magnetic chitosan-phenylthiourea resin was used for the adsorption of Hg(II), Cd(II), and Zn(II) ions;2,12 chitosan nanofibrils were used for Cu(II), Pb(II), and Cd(II) ions;3 chitosan and cellulose were used for various heavy metal ions;17 glutamic-chitosan hydrogels were used for Cu(II) and Ni(II) ions;18 and chitosan/PVA hydrogel beads were used for Pb(II) ions.11 Until now, the hydrogels used in heavy metal adsorption were in the form of monoliths, flakes, particles, or powders. Some of these forms have the advantage of being easy to recycle and reuse. Recently, we have published an article on gelatin-chitosan hydrogel flakes to remove Hg(II).10 Heavy metal ion absorption by the hydrogel flakes was first tracked by coffee ring effect. The flakes were obtained by grinding the prepared monolithic hydrogel and the different sizes of flakes were then separated using sieving technique; however, this method is difficult to control the shape and size of the hydrogel. As a continuous work, we prepared spherical hydrogel particles using different combinations of biopolymers by changing the ratio of chitosan (C) to gelatin (G). The adsorption of the heavy metal ions—Pb(II), Cd(II), Cr(III), and Hg(II)—was performed using the prepared hydrogels (C1G1, C2G1, and C1G2) in a single metal ion solution as well as in a multiple metal ion solution. The effects of chitosan/gelatin (CG) hydrogel composition on the partition coefficient (Kd) and removal efficiency for heavy metal adsorption were analyzed and discussed.

EXPERIMENTAL SECTION Materials Chitosan powder was obtained from YB Bio (degree of deacetylation = 92%). Gelatin (porcine skin, gel strength = 300 g Bloom) was purchased from Merck. Span 85 was obtained 3



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from TCI Chemicals, and glutaraldehyde (50% aqueous solution) was purchased from Daejung. Glacial acetic acid, hexanes (extra pure grade), and mercuric chloride (HgCl2, extra pure grade) were provided by Duksan. Lead (II) chloride (PbCl2, purity: 98.0%), cadmium chloride hemipentahydrate (CdCl2·21/2H2O, purity: 98.0%), and chromium (III) chloride hexahydrate (CrCl3·6H2O, purity: 98.0%) were purchased from Samchun Chemicals. All the chemicals were used as received. Double distilled water was used throughout the experiments.

Table 1. Weights of reactants used in the preparation of spherical CG hydrogel particles and solid content of chitosan in the hydrogel particlesa

a

Sample Code

Span 85 (5 wt.%) (g)

Chitosan (3 wt.%) (g)

Gelatin (10 wt.%) (g)

Glutaraldehyde (1 wt.%) (g)

Solid content (wt.%) of Chitosan in the CG hydrogel particle

C1G1

75

25

25

25

23

C2G1

75

50

25

37.5

38

C1G2

75

25

50

37.5

13

Reactants used were all in the aqueous solution state and their concentrations (by weight%)

are noted.

Preparation of chitosan/gelatin hydrogel particles CG hydrogel particles were prepared by inverse suspension at different ratios of chitosan to gelatin aqueous solutions, as listed in Table 1. For example, C1G1 was prepared as follows: To a hexane solution of Span 85 (75 g, 5 wt.%) in double-jacketed reactor at 45 °C, chitosan and gelatin solutions were added and stirred at 550 rpm for 2 h. Chitosan (0.75 g, 3 wt%) was dissolved in a 0.1 M acetic acid aqueous solution (25 g) with stirring overnight at 50 °C. Gelatin (2.5 g, 10 wt.%) was dissolved in water (25 g) at 40 °C. After stirring for 2 h, chitosan and gelatin were crosslinked by adding an aqueous glutaraldehyde solution (0.25 g 4


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in 25 g of water, 1 wt.%) dropwise and stirred further for 30 min (Scheme 1). The spherical CG hydrogel particles obtained were washed with hexane five times to remove Span 85 and then washed with water until the pH of the washed solution reached 7. The resulting CG hydrogel particles were dried in a convection oven at 60 °C and in a freeze dryer and finally stored for further use. While preparing the CG hydrogel particles, the weight ratio of the chitosan solution to the gelatin solution was varied to 1:1, 2:1, 1:2, denoted by C1G1, C2G1, and C1G2, respectively (Table 1).

Scheme 1. Synthetic scheme for the preparation of CG hydrogel particles for heavy metal ion adsorption.

Swelling studies To determine the degree of swelling (DS%), the dried hydrogel particles (15 mg, C1G1, C2G1, and C1G2) was dispersed in water (15 mL) and rotated at 150 rpm for 24 h. DS% was calculated as follows: (1)

5



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where Ww and Wd are the weights of the hydrogel particles in wet and dry state, respectively.

Heavy metal ion adsorption studies using hydrogels For heavy metal adsorption, CG hydrogel particles (100 mg) were added to the heavy metal ion solutions (10 mL, metal ion concentration = 1 g/L) and stirred at 150 rpm for 24 h. The concentrations of heavy metal ions for Pb(II), Hg(II), Cd(II), and Cr(III) were 3.60, 3.68, 4.38, and 3.75 mM, respectively. After 24 h, the solution was decanted and the metal ion concentration was measured using inductively coupled plasma optical emission spectrometry (ICP-OES, Optima 7300 DV & Avio 500, PerkinElmer, USA). The distribution coefficient (Kd) and removal efficiency (%) were calculated as: (2) (3) where A is the initial metal ion concentration (mg/L) and B is the final metal ion concentration after adsorption (mg/L). The initial and final metal ion concentrations were measured using ICP-OES. V is the volume of the metal ion solution (mL), and Wh is the weight (g) of the hydrogel used. All the values reported here are obtained from two replicate experiments. Characterization The morphology of the spherical hydrogel particles was first visualized by using an optical microscope (OM, Eclipse LV 100D, Nikon, Japan) equipped with a video camera (TrueChrome II, Tucsen, China) and further using a field emission scanning electron microscope (FE-SEM, 15 kV, Hitachi SU8220, Japan). An attenuated total reflectance Fourier 6


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transform infrared spectrophotometer (8400S, Shimadzu, Japan) was used to examine the functional groups of the hydrogels and to confirm the crosslinking reaction. X-ray photoelectron spectroscopy (XPS) was performed on a Quantera SXM ULVAC–PHI spectrometer (Physical Electronics, Inc., USA) with a monochromated Al Kα source (13.00 eV). Narrow scans were carried out on hydrogel particles before and after heavy metal ions adsorption at C1s, N1s, O1s, Hg4f, Pb4f, Cd2p, and Cr3p energy levels.

RESULTS AND DISCUSSION Size and morphology of hydrogel particles Photographs and OM images of C1G1, C2G1, and C1G2 hydrogel particles are shown in Figs. S1–S3, and which represent photographic images of as-prepared (Fig. S1(a), S2(a), and S3(a)), oven-dried (Fig. S1(b), S2(b), and S3(b)), and freeze-dried (Fig. S1(c), S2(c), and S3(c)) particles, respectively. Both oven- and freeze-dried particles are spherical. Fig. S1(d– g), Fig. S2(d–g), and Fig. S3(d–g) show OM images of the oven- and freeze-dried particles, respectively. These images suggest that freeze-dried particles are bigger than oven-dried particles. Both the oven- and freeze-dried particles are transparent, and the pores inside the particles are clear in the image. Optical images of the hydrogel particles, C1G1, C2G1, and C1G2, reveal that the pore size of the oven-dried particles is larger than that of the freezedried particles. In addition, the freeze-dried particles have an abundant number of pores with cotton-like buffy structures than that of oven-dried particles. Figs. 1(a–c) show FE-SEM images of oven-dried C1G1, C2G1, and C1G2 particles, respectively, and Figs. 1(d–f) show FE-SEM images of freeze-dried C1G1, C2G1, and C1G2 particles, respectively. Cross-sectional images of the oven- and freeze-dried C1G1, C2G1, and C1G2 particles are shown as insets in Fig. 1. As with the OM image, the FE-SEM images 7



also confirm that the pore size of the oven-dried particles is larger than that of the freezedried particles.

Figure 1. FE-SEM images of (a–c) oven-dried and (d–f) freeze-dried particles C1G1, C2G1, and C1G2, respectively. The insets show the corresponding cross-sectional images.

After preparation, the hydrogel particles were dried in a convective oven at 60 °C, which caused evaporation of water and a subsequent formation of large cavities or pores in the particles, but the number of pores was found to be a few. In the freeze drying method, hydrogel particles were frozen before drying and the pores were maintained inside the particles even after drying. Fig. S4 and Table 2 present the size information pertaining to the oven- and freeze-dried hydrogel particles. The mean diameters of the freeze-dried hydrogel 8


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particles are found to be ca. 1000 μm from the OM and FE-SEM images, whereas the mean diameters of oven-dried hydrogel particles are found to be 200–700 μm from the OM and FESEM images. The size of the freeze-dried particles is nearly 2.5 times larger than that of the oven-dried particles. This is due to the difference in drying method. Particles were frozen before drying, which helps the particles to maintain their size during the drying process. In oven drying, the particles shrink due to the evaporation of water, and thus, they are small in size.

Table 2. Size measurements of oven- and freeze-dried CG hydrogel particles from OM and FE-SEM images Sample Code

Oven-dried Particles (μm)

Freeze-dried Particles (μm)

OM

FE-SEM

OM

FE-SEM

C1G1

272 ± 93

252 ± 92

720 ± 220

820 ± 240

C2G1

424 ± 125

439 ± 191

870 ± 410

1000 ± 310

C1G2

680 ± 108

700 ± 233

1330 ± 300

1190 ± 230

Swelling behavior of hydrogel particle The average DS% values of the hydrogel particles were obtained using Eq. (1), and the results are shown in Fig. 2, where (a) and (b) show the DS% graphs of the oven- and freezedried hydrogel particles, respectively. All the oven-dried hydrogel particles show similar DS% values. Among the freeze-dried particles, C2G1 shows the highest DS% value. The DS% values of freeze-dried C2G1, C1G1, and C1G2 particles are 6.8, 2.8, and 4.0 times higher than the corresponding DS% values of oven-dried particles, respectively. This is because there are more pores inside the freeze-dried particles than oven-dried particles. Therefore, freeze-dried particles absorb more water than oven-dried particles. 9



Figure 2. Average DS% values of (a) oven-dried and (b) freeze-dried hydrogel particles.

FTIR analyses of hydrogel particles The CG hydrogel particles were formed through the crosslinking reaction of chitosan and gelatin using glutaraldehyde, and the formation was confirmed by FTIR analysis. Fig. S5 shows the FTIR spectra of chitosan, glutaraldehyde, gelatin, and CG hydrogel particles (C1G1, C2G1, and C1G2). The spectra of chitosan show characteristic bands at 3366 and 3292 cm-1 attributable to O–H and N–H stretching, respectively.14,19 The bands at 2922 and 2862 cm-1 are attributed to the stretching of asymmetric and symmetric C–H group, respectively. The absorption band at 1655 cm-1 corresponds to C=O stretching and the band at 1589 cm-1 is attributable to the N–H deformation of amino groups. The presence of glucose units of the polysaccharide structure can be confirmed from the bands between 1152 and 897 cm-1. Gelatin shows the bands responsible for O–H and C–H stretching at 3290 and 2941 cm1

respectively.20 In addition, gelatin shows three prominent amide peaks: amide-I (C=O

stretching, 1627 cm-1), amide-II (N–H bending 1561 cm-1), and amide-III (in plane of vibrations C–N and N–H groups of bound amide or vibrations of CH2 groups of side chains, 1226 cm-1). Glutaraldehyde shows characteristic peaks at 3350, 2952, and 1638 cm-1 corresponding to O–H, C–H, and C=O stretching, respectively.20 10


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The O–H stretching (approximately 3360 cm-1) is more depressed in the prepared C1G1, C2G1, and C1G2 particles than in glutaraldehyde and chitosan, which indicates a successful crosslinking reaction between chitosan and gelation with glutaraldehyde.20 In addition, the absence of C=O stretching of glutaraldehyde at 1638 cm-1 and the N–H deformation of amino groups at 1589 cm-1 in C1G1, C2G1, and C1G2 further confirm that the aldehyde group of glutaraldehyde was crosslinked with the amine groups of chitosan and gelatin. Overall, FTIR analyses indicate the successful formation of C1G1, C2G1, and C1G2 hydrogel particles.

Single metal ion adsorption with oven-dried hydrogel particles The performance of oven-dried C1G1, C2G1, and C1G2 hydrogel particles with regard to the single metal ion adsorption of Pb(II), Hg(II), Cd(II), and Cr(III) aqueous solutions was tested. It should be noted that the pH of the metal ion solutions in the experiment was not adjusted; the initial pH values for Pb(II), Hg(II), Cd(II), and Cr(III) were 4, 5, 6, and 4, respectively; however, these values remained the same even after the adsorption experiment was completed. In the case of C2G1, the concentration changes of Pb(II), Hg(II), Cd(II), and Cr(III) measured by ICP-OES before and after the adsorption experiment were 883834, 56112, 638622, and 224213 ppm, respectively. In the case of C1G1, the concentration changes of Pb(II), Hg(II), Cd(II), and Cr(III) were 883807, 56186, 638638, and 224213 ppm, respectively. In the case of C1G2, the concentration changes of Pb(II), Hg(II), Cd(II), and Cr(III) were 883830, 561237, 638638, and 224204 ppm, respectively. Based on these results, the Kd values (non-equilibrium distribution coefficients) were calculated using Eq. (2) and shown in Fig. 3.10,21 For C1G1, the Kd value decreases in the order of Hg(II) > Pb(II) > Cr(III) > Cd(II), and for C2G1 and C1G2, the Kd values decrease in the order of Hg(II) > Cr(III) > Pb(II) > Cd(II). The Kd value of Hg(II) adsorption by hydrogel 11



particles is 136–4450 mL/g. The Kd value of Cd(II) is nearly zero (0–2.55 mL/g). The Kd values of Pb(II) and Cr(III) are very low (6–12 mL/g). These suggest that the CG hydrogel particles adsorb Hg(II) more efficiently than other metal ions. In addition, C2G1 shows higher Hg(II) adsorption than C1G1 and C1G2. This is attributable to the higher chitosan content of C2G1 than C1G1 and C1G2. However, the adsorption of the other metal ions Pb(II), Cd(II), and Cr(III) is not dependent on the ratio of chitosan to gelatin. The high value of Kd for Hg(II) can be explained as follows. Metal ions coordinate to form complexes with ligands containing nitrogen, sulfur, or oxygen atoms.2,22,23 Hg–N bond is known to be 2–2.5 times stronger than Hg–O and Hg–C bonds.24 The stability of the Hg– COOH complexes are weak, but when the oxygen atom is replaced by a sulfur or nitrogen atom, the stability of the complexes increases.24 This supports the results obtained in the present work that the coordination of Hg(II) ion with a nitrogen atom in chitosan and gelatin forms a stable complex. Thus, the CG hydrogel particles exhibit Hg(II) removal efficiencies of at least 58% (Fig. S6). It was reported that the crosslinked magnetic chitosanphenylthiourea resin exhibited a removal efficiency of 83% at pH 5.0 and a temperature of 30 °C, which was attributed to the efficient coordination of thiourea with Hg(II) ion. In contrast, the unmodified resin shows a removal efficiency of only 33%.2 In this work, heavy metal adsorption was performed by using as-prepared hydrogel particles without pH adjustment at room temperature. Even under these conditions, the hydrogel particles exhibit a removal efficiency of 98%, which is greater than previously reported values. Pb(II) shows a high affinity for nitrogen atoms; however, the binding affinity varies with pH.11,24 It should be noted that the pH of Pb(II) was 4 and such a low pH value might inhibit the approach of Pb(II) ions to the surface of CG hydrogel particles because of the protonated amine groups in the chitosan-gelatin polymer.11 Moreover, Pb(II) ions approaching the 12


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particle surface might be restricted because of the electrostatic repulsion between the positive charge (NH3+) of the hydrogel particles and the Pb(II) ions in the solution.25 In addition, H+ in aqueous solutions can compete with cationic metal ions, thereby reducing the adsorption ability of the hydrogel particles. These effects may decrease the amount of Pb(II) ions approaching the surface and inside the hydrogel particle. Therefore, the removal efficiency values are very low for Pb(II) ions (< 10%, Fig. S6) compared to Hg(II) ions (> 57%). The CG hydrogel particles show low Kd values (< 10%) for Cr(III) (Fig. S6). This suggests that the approach of Cr(III) ions from the bulk solution to the particle surface at a low pH (~ 4) as in the case of Pb(II) can be inhibited by the protonated amino groups. The removal efficiencies of Cd(II) are very low (0 and 2%) as shown in Fig. S6. This indicates that Cd(II) does not coordinate with either nitrogen or oxygen atom in chitosan and gelatin. Consequently, in the single metal ion adsorption experiments using the prepared hydrogel particles, only Hg(II) ion can be effectively adsorbed. In addition to the above discussions, the high adsorption of Hg(II) ion using CG particles can be explained by considering the hard soft acid base (HSAB) concept, that soft acid Hg(II) ion will have a high binding affinity with –NH2 and –CN groups in hydrogel particles.10 Furthermore, the metal ion binding strength related to the electronegativity of ionic charge was calculated with different ligands and reported in the order of Hg(II) > Cr(III) > Pb(II) > Cd(II).26 This order is similar to that of the present work in that Kd and removal efficiency values are in the order of Hg(II) > Cr(III) > Pb(II) > Cd(II). Therefore, it can be concluded that the removal efficiency and Kd value of Hg (II) are higher than other ions because the ionic binding strength of Hg (II) is strong.

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Figure 3. Kd values for oven-dried hydrogel particles C1G1, C2G1, and C1G2 in single metal ion solutions of (a) Pb(II), (b) Hg(II), (c) Cd(II), and (d) Cr(III).

XPS analysis was also performed to investigate the binding between the functional groups of CG hydrogel particles and the metal ions. As shown in the ICP results, C2G1 exhibited higher adsorption for all metal ions than the other CG particles, C1G1 showed the least adsorption for Cd(II) ion. Thus, the XPS results were tested for C2G1 particles in the single and multiple metal ion solutions. In addition, the XPS results of C1G1 tested in the single metal ion solution of Cd(II) and are presented as a representative spectrum. The C1s peak of pristine C2G1 (before heavy metal adsorption) corresponds to the four components, C-H, C-C, C-O/C-N/C-OH, and O=C-OH, with binding energy peaks at 284, 285, 286, and 289 eV, respectively (Fig. S7). After the adsorption of metal ions, four components, C-H, C-C, C-O/C-N/C-OH, and O=C-OH, with binding energy peaks at 284, 285, 286, and 289 eV, respectively with no significant changes in C1s level were obtained. The C=N peak around 288 eV overlapped with O=C-OH peak and slightly shifted to lower energy (Fig. 4 and Fig. S8). The presence of C=N peak confirms the formation of hydrogel 14


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particles by crosslinking of chitosan and gelatin with glutaraldehyde. The C1s spectra for all hydrogel particles are almost the same (Fig. 4 and Fig. S8). Fig. S7 shows two N1s level peaks before metal adsorption by deconvolution at binding energies of 399.7 and 400 eV, respectively, belonging to amide (C-N) peaks from gelatin and amine (N-H) peaks from gelatin and chitosan. After adsorption of metal ions, N1s exhibits three peaks at binding energies 399, 400 and 402 eV attributed to amide (C-N), amine (N-H) and amine (NHg/Cd/Pb/Cr) groups coordinated to metal ions, respectively (Fig. 4 and Fig. S8). The additional peak at 402 eV confirms the nitrogen group for hydrogel particles coordinated to metal ions, Hg, Pb, Cd, and Cr.27 Hg(II) shows a peak of 402 eV, which is slightly bigger than other metal ions, confirming the higher adsorption of Hg(II) ions by hydrogel particles (Fig. 4 and Fig. S8). The broad peak at 402 eV in N1s level suggests that Cd(II) does not coordinate well with nitrogen atoms of C2G1 particle and thus it shows very low removal efficiencies and Kd values (Fig. 4). Fig. S7 shows the O1s high-resolution spectra of pristine C2G1 particles. Fig. 4 and Fig. S8 show the O1s high-resolution spectra of the C2G1 after metal ion adsorption. The O1s core level spectra of both pristine C2G1 and after metal ion adsorption indicate three peaks. The peaks at 531, 532, and 533 eV correspond to C=O, C-O, and O=C-OH, respectively. It was confirmed that the metal ions were well adsorbed on the hydrogel particle surface from the XPS peaks of Hg4f, Pb4f, Cd3d, and Cr2p in C2G1 particle (Fig. 4 and Fig. S8). The absence of Cd(II) peak in the survey scan confirms that C1G1 does not adsorb Cd(II) ions (Fig. S9(h)), which similarly reveals neither nitrogen nor oxygen coordinates with Cd(II) ion. This is in good agreement with the removal efficiency and Kd values of Cd(II) as shown in Fig. 3(c) and Fig. S6(c).

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Figure 4. XPS spectra of C2G1 after adsorption with Hg in single metal ion solution: (a) C1s, (b) N1s, (c) O1s, (d) Hg4f. XPS spectra of C2G1 after adsorption with Cd in single metal ion solution: (e) C1s, (f) N1s, (g) O1s, (h) Cd3d. 16


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Multiple metal ion adsorption with oven-dried hydrogel particles Since actual natural waste and industrial effluents contain mixtures of heavy metal ions, multiple metal ion adsorption experiments were also performed. Equivalent amounts of the metal ion solutions with the initial concentrations of 883, 561, 638 and 225 ppm for Hg(II), Pb(II), Cr(III), and Cd(II), respectively, were used to examine the competitive adsorption characteristics of metal ions. The pH of the metal ion solution before and after the adsorption experiments was 5. The distribution coefficient Kd and removal efficiency are shown in Fig. 5.

Figure 5. (a) Kd values and (b) removal efficiencies (%) of C1G1, C2G1, and C1G2 ovendried particles in multiple metal ion solution comprising Cd(II), Pb(II), Hg(II), and Cr(III).

Similar to the single metal ion adsorption, the multiple metal ion adsorption also shows high Kd values for Hg(II) ions, i.e., > 1000 mL/g; these values are higher than the other ions (< 500 mL/g). Fig. 5(a) indicates that the Kd values for the adsorption of Hg(II) decrease with an increase in the gelatin (C1G2) content of the hydrogel particles when compared to C1G1 particle. As shown in Fig. 5(b), the removal efficiencies of all the metal ions are above 70%, regardless of the composition of chitosan and gelatin. C1G1 particle shows slightly higher Kd and removal efficiency values than C2G1 and C1G2 particles. This suggests that in multiple 17



metal ion solution the C1G1 become more efficient. Whereas C2G1 particles become more efficient in a single metal ion solution. However, Kd and removal efficiency values are in error ranges for all hydrogel particles. The high removal efficiencies for all the metal ions might be attributed to the following factors. In the multiple metal ion solution, all the metal ions compete with each other for the adsorption, which might synergistically enhance the adsorption of all the metal ions. In addition, in the case of a single metal ion solution using CG hydrogel particles, only Hg(II) solution with pH 5 shows high Kd and removal efficiency values when compared to other metal ions. Thus, for the multiple metal ion solution, a mixture of all metal solution with pH 5 shows high Kd and removal efficiency values, suggesting pH 5 is a promising condition for obtaining maximum multiple metal ion adsorption. This suggests that the prepared hydrogel particles can be efficiently applied to a multi-component system rather than a single-component system, which is essential for the treatment of natural and industrial wastes. In addition, the removal efficiency is maintained regardless of the chitosan and gelatin content. Fig. 6 shows the XPS spectra of C2G1 after adsorption of multiple metal ions. The C1s, N1s, and O1s energy levels show similar deconvoluted peaks as that obtained for single metal ion solution (Fig. 4). The decomvolued C1s level shows four peaks and O1s level shows three peaks, respectively, as shown in Fig. 4(a) and Fig. 4(c). The deconvolued N1s level reveals three peaks and the peak at 401.5 eV corresponds to nitrogen atoms coordinated with metal ions from C2G1 (N–M in Fig. (b)). Compared to the single metal ion solution, the N– M peak in the multiple metal ion solution slightly shifts to lower (401.5 eV) binding energy. This can be due to the coordination of multiple metal ions with C2G1 particle.

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Figure 6. XPS spectra of multiple metal ion adsorption with C2G1: (a) C1s, (b) N1s, (c) O1s, (d) Hg4f, (e) Pb4f, (f) Cd2p, and (g) Cr3d.

The presence of metal ion peaks (Hg4f, Pb4f, Cd2p, and Cr3d) confirms the existence of multiple metal ions on C2G1 particle in the multiple metal ion solution. However, the 4f5/2 peak intensity of Hg4f is diminished in the multiple metal ion solution (Fig. 6(d)) when 19



compared to the 4f5/2 peak of Hg4f metal ion in the single metal ion solution (Fig. 4(d)). In addition, the 3d3/2 peak of Cr3d diminished in the multiple solution than that of in the single metal ion solution. This suggests that 4f5/2 of Hg4f and 3d3/2 of Cr3d might be effectively involved in coordination with nitrogen atoms. Consequently, the removal efficiency of Hg(II) and Cr(III) in the multiple metal ion solution is higher than Pb(II) and Cd(II) ions (Fig. 5). Thus, XPS results reveal the C2G1 hydrogel particles removed only Hg(II) efficiently in a single metal ion solution. In the multiple metal ion solution, all metal ions are removed efficiently, suggesting that all metal ions coordinate well with nitrogen atoms of the C2G1 hydrogel particles in the multiple metal ion solution. Since oven drying is more convenient than freeze drying, all of the adsorption results described above are measured using oven-dried hydrogel particles. However, hydrogels prepared by freeze drying exhibit higher porosity and better swelling than those by oven drying. Among the oven-dried particles, only C2G1 shows higher DS% and Kd values. Thus, in the case of freeze dried particles, only C2G1 was used to calculate the Kd and removal efficiency values in single and multiple metal ion solutions (refer to the descriptions on Figs. S10–S12 in supporting information). The DS% value of freeze-dried particles is higher than that of oven-dried hydrogel particles (Fig. 2), and which can be explained by that the freeze-dried particles have an abundant number of pores compared to oven-dried particles. However, there is no difference between oven drying and freeze drying in adsorption of metal ions. This is because both oven- and freeze-dried particles actually adsorb maximum metal ions in equilibrium with the metal ions. This confirms that oven drying is a convenient method for large scale production, unlike freeze drying. Comparison of metal ion adsorption by hydrogels 20


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Previous adsorption studies found in the literature were conducted under various experimental conditions such as different pH levels, initial metal ion concentrations, and contact times.2,4,28-30 Therefore, it is difficult to compare the adsorption results of these studies with the adsorption results obtained in this work. However, the removal efficiencies for single and multiple metal ion solutions are compared, as shown in Table S11-3,10,29 and Table 3,2,3,10,30 respectively. The efficiencies obtained in some previous studies are higher than that obtained in this work, especially for single metal ion solutions. For multiple metal ion solutions, the efficiency obtained in the present work is higher than those obtained in previous studies. Mostly the adsorption of heavy metal ions was reported in adsorption capacity and thus here we have compared the absorption capacity values of obtained with the reported ones (see Table S2).3,12,31

Table. 3 Comparison of removal efficiency of metals from aqueous solutions using different adsorbents in multiple metal ion solution Adsorbents

Removal Efficiency (%)

References

C1G1 Oven-dried

Hg(II) 93

Pb(II) 76.6

Cd(II) 73.44

Cr(III) 84.23

Present Work

C2G1 Oven-dried

90.87

75.38

73.20

79.84

Present Work

C1G2 Oven-dried

84.45

75.42

73.30

84.88

Present Work

C2G1 Freeze-dried

94.51

77.16

76.95

77.7

Present Work

97

12

2

24

10

Gelatin-Chitosan Hydrogel ChitosanPhenylthiourea Resin Chitosan Nanofibril Cellulose-based Hydrogel

79.4

55.7

55.4

2

79.1

3

80

30

21



CONCLUSION In this study, the CG hydrogel particles were successfully synthesized using biopolymers, chitosan, and gelatin followed by dried either by convective oven drying or freeze drying methods. Particle size, inner morphology, and swelling degree of the hydrogel particles were characterized, and their adsorption behavior was investigated in the single and multiple metal ion solutions comprising Pb(II), Cd(II), Hg(II), and Cr(III). The OM and FE-SEM results revealed that the oven-dried particles were smaller (< 700 µm) than the freeze-dried particles (> 700 µm); this difference was mainly attributed to the drying method. The freeze-dried particles had abundant pores, so the DS% values of the freeze-dried particles C2G1, C1G1, and C1G2 were 6.8, 2.8, and 4.0 times higher than those of the samples prepared by oven drying. Inner morphology and DS% of hydrogel particles depend on the drying method, but such differences did not affect Kd values and removal efficiency of heavy metal ions. In the single metal ion solution, CG hydrogel particles showed higher adsorption for Hg(II) ion (> 50%), while the other metal ions showed low removal efficiencies (< 10%). In the multiple metal ion solution, CG hydrogels particles showed high removal efficiencies for all metal ions (54–95%). This work suggests that oven drying is more convenient and efficient in absorbing heavy metal ions than freeze drying, and which can be quite useful for large-scale production of CG hydrogel particles for adsorption of multiple metal ions.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: XXXXXX. Photographic images of hydrogel particles; size measurements of hydrogel particles; FTIR 22


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spectra of starting materials and hydrogel particles; removal efficiencies of heavy metal ion using hydrogel particles; XPS spectra for pristine C2G1 and after metal ion adsorption; adsorption of single and multiple metal ions using freeze-dried hydrogel particles; comparison tables of removal efficiency and adsorption capacity with reported ones.

AUTHOR INFORMATION Corresponding Authors In Woo Cheong, Phone: +82-53-9507590. Fax: +82-53-9506594. E-mail: [email protected] Jin Joo, Phone: +82-53-9505585. Fax: +82-53-9506594. E-mail: [email protected] ORCID In Woo Cheong: 0000-0001-8901-4959 Jin Joo: 0000-0003-4781-4004 Notes The authors declare no competing financial interests. I.W.C. and D.H.Y. conceived and managed the research. S.P. performed hydrogel synthesis and characterization. I.W.C. and J.J. reviewed the results and revised the manuscript. S.P. and I.W.C. composed the original draft. R.A. conducted the XPS analysis of the manuscript revision.

ACKNOWLEDGMENT This work was supported by the Ministry of Trade, Industry and Energy (Grant No. 10070241).

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GRAPHICAL ABSTRACT

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Gelatin Hydrogel Particles for Removal of Multiple

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