Hemicellulose-Based

Article pubs.acs.org/jced

Cite This: J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Development of β‑Cyclodextrin-Cellulose/Hemicellulose-Based Hydrogels for the Removal of Cd(II) and Ni(II): Synthesis, Kinetics, and Adsorption Aspects Debashis Kundu, Supriyo Kumar Mondal, and Tamal Banerjee* Department of Chemical Engineering, Indian Institute of Technology Guwahati, Guwahati 781039, Assam, India

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S Supporting Information *

ABSTRACT: Carboxymethyl cellulose (CMC), microcrystalline cellulose (MCC), and xylan are cross-linked with β-cyclodextrin (βCD) using ethylene glycol diglycidyl ether cross-linker to produce hydrogels, namely, βCD-CMC, βCD-MCC, and βCD-xylan, in alkaline medium at 1:1 mole ratio. Additionally pure βCD gel is also prepared in alkaline medium. The synthesized hydrogels are characterized by Fourier transform infrared spectroscopy, and the swelling ratio, gel fraction, and the morphologies are observed by a microscope. The hydrogels are used to adsorb cadmium (Cd(II)) and nickel (Ni(II)) ions from aqueous solution. The adsorption studies are carried out by varying adsorbent dosage from 80 to 500 mg, concentration from 5 to 500 mg L−1, pH from 2 to 8, and temperature from 25 to 55 °C. The equilibrium adsorption data closely follow the Langmuir model, suggesting the monolayer adsorption of metal ions by the hydrogels. The adsorption kinetics are found to closely follow the pseudo-second-order model.

form biocompatible and pH-sensitive hydrogel.12 Hemicellulose is a heteropolysaccharide with various types of hexose and pentose sugar and is amorphous in nature. Xylan constitutes 25−35% of hemicellulose class13 and is found in hard wood species. It consists of β-1,4-D-xylopyranosyl unit as a backbone with a low degree of polymerization of 200.14 Further, it contains 10% 4-O-methyl-D-glucoronic acid and β1,4-D-xylopyranosyl units as backbone.15 Cyclodextrin (CD) is a toroid-shaped oligosaccharide with 6, 7, and 8 D-glucose units covalently connected by α-1,4-glucosidic linkages. Depending upon the number of units, it is classified as α- (6 units), β- (7 units), and γ- (8 units) CD. Primary hydroxyl groups of CD are located at the narrower, i.e., primary, face, and secondary hydroxyl groups are located at the wider, i.e., secondary, face.16 This structural formation makes the exterior of CD hydrophilic and interior hydrophobic. CDs have found extensive applications in pharmacy, textile, food, and packaging industries. CD-based hydrogel can be synthesized by facile functionalization or chemical cross-linking with primary or secondary hydroxyl moieties. Nickel (Ni(II)) and cadmium (Cd(II)) are toxic contaminants that can cause severe damage in human and aquatic life after being accumulated in high concentrations. Cadmium is commonly used in batteries, and coating and painting industries.17−19 It is carcinogenic, causes iron deficiency,20 and accumulates in liver and kidney.21 Nickel comes out as pollutant from electroplating, metal finishing, and battery

1. INTRODUCTION Hydrogel is three-dimensional network containing a large amount of water. Natural or synthetic biopolymers often initiate via physical or chemical cross-linking to create network gel. The components are often biocompatible, resulting in widespread use of hydrogel in tissue engineering,1 drug delivery,2 biomedical applications,3 etc. Apart from the conventional applications, hydrogels are used in sensors and actuators,4 metal-ion removal,5 and dye removal.6 Polysaccharides and oligosaccharides are considered as suitable materials for hydrogel because of abundance of hydroxyl groups, which can be either physically cross-linked via van der Waals or hydrogen-bonding interaction or functionalized for chemical cross-linking. Being naturally abundant, biocompatible, and biodegradable, polysaccharides hold advantages over synthetic polymers with respect to the preparation of hydrogel.7 Cellulose, homopolysaccharide in nature, exhibits low density, high strength, and high stiffness due to the inherent crystallinity.8 However, crystallinity of cellulose limits its application due to its insolubility in water and common organic solvents. Several solvents, namely, polar solvent, ionic liquid, deep eutectic solvent, and alkali aqueous system,9 have been employed to solubilize cellulose. This enables cellulosebased hydrogel to be prepared from cellulose solution via chemical or physical cross-linking process. Carboxymethyl cellulose (CMC) is a water-soluble derivative of cellulose and is a linear polymer. Low immunogenicity, high biocompatibility, and biodegradability make CMC an attractive material for biomedical and pharmaceutical applications.10,11 Crosslinked CMC is known to absorb a large amount of water to © XXXX American Chemical Society

Received: January 27, 2019 Accepted: May 17, 2019

A

DOI: 10.1021/acs.jced.9b00088 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

Article

Table 1. Details of the Chemicals Used in This Work name of the chemical

CAS registry number

purity (%)

supplier

carboxymethyl cellulose sodium salt beechwood xylan β-cyclodextrin potassium bromide microcrystalline cellulose nickel chloride hexahydrate cadmium chloride monohydrate hydrochloric acid sodium hydroxide urea ethylene glycol diglycidyl ether

9004-32-4 9014-63-5 7585-39-9 7758-02-3 9004-34-6 7791-20-0 654054-66-7 7647-01-0 1310-73-2 57-13-6 2224-15-9

≥99 ≥90 ≥97 ≥99 ≥99 ≥97 ≥98 36.5−38 ≥97 ≥99 ≥99

Sigma-Aldrich Sigma-Aldrich Sigma-Aldrich Sigma-Aldrich Merck Merck Merck Merck Merck Merck TCI Chemicals

purification method used used used used used used used used used used used

as as as as as as as as as as as

received received received received received received received received received received received

will be appropriate for the study of the adsorption of metal ions. In this work, we aim to synthesize βCD-cellulose/hemicellulose-based hydrogels, namely, βCD-xylan, βCD-CMC, βCD-MCC, and βCD gel, cross-linked by ethylene glycol diglycidyl ether (EGDE). EGDE is a difunctional cross-linker having two epoxide moieties at both ends.34 Due to the presence of high strain in the epoxide ring, this ether is more reactive than the other ethers. The opening of the epoxide ring and reaction with hydroxyl moieties of pyranose ring occur in high basic medium.35 However, for successful cross-linking, both the opening of the epoxide ring and cross-linking with polysaccharide/oligosaccharide must happen simultaneously. The synthesized hydrogels in this work are characterized by Fourier transform infrared (FT-IR) spectroscopy. Further, the physical characterization of hydrogels is performed by swelling study at varying pH and measurement of gel fraction. The morphologies of hydrogels are then visualized through optical microscopy and field emission scanning electron microscopy (FESEM) instruments. The synthesized hydrogels are used to adsorb Cd(II) and Ni(II) from aqueous solution. Atomic absorption spectroscopy (AAS) is used to measure the concentration of the metal-ion solution. The equilibrium concentration data will then reveal the nature of the adsorption by comparing the Langmuir and Freundlich isotherms. Timeresolved adsorption study will also reveal the kinetics of adsorption, i.e., elucidating the adsorption mechanism by hydrogel.

industry. Further, it causes problems in lungs and kidney. Adsorption is a preferable treatment method of removal of metal ions from water due to high efficiency, simplicity in design, and low cost. Biopolymers such as hydrogel are the newest entrant as adsorbent in wastewater treatment. Adsorption behavior depends on various parameters such as ionic concentration, pH, contact time, temperature, etc. In recent years, acrylate-,22 acrylamide-,23 alginate-,24 poly(vinyl alcohol)-,25 and chitosan26-based hydrogels are used to remove Ni(II) and Cd(II) from wastewater. Cellulose-based hydrogels are widely used for wastewater treatment.27 Microcrystalline cellulose (MCC)-based hydrogel loaded with MnO2 is used to remove lead from wastewater.28 Carboxymethyl cellulosebased hydrogels like poly(vinyl alcohol)−CMC25 and CMC− sodium alginate24 are used to remove Ni(II). Godiya et al. reported CMC/polyacrylamide-based hydrogel for the removal of Cd(II) and other metal ions.29 Unlike cellulose, limited studies consisting of O-acetyl galactoglucomannan-based hydrogels are available for the removal of heavy-metal ions such as Ni(II) and Cd(II).30 βCD-based hydrogels such as βCD-acrylic acid and silica-glycidyl methacrylate-βCD-acryloyl chloride are reported for the removal of heavy-metal ions, mainly Ni(II) and Cd(II).31 Huang et al. also reported βCDacrylic acid-based hydrogels for the removal of Cd(II) and other metal ions.32 The biggest advantage of CD-based hydrogels is the formation of inclusion complexes. The hydrophobic cavity attracts the guest molecules and polymers to separate them from solution. Further the hydrophilic periphery of CD and its derivatives participate in various forms of cross-linking reactions such as physical, covalent interactions, and sliding ring hydrogel.16 The dominating ionexchange process that facilitates the hydrogels for the formation of host−guest complex puts heavy-metal ions and available H+ ions of medium into competition for adsorption sites. The straight-chain polymeric hydrogels offer no difference between H+ and metal ions. However, the hydrophobic cavity prefers binding with metal ions. One prominent disadvantage of copolymerized βCD-based hydrogel is the aggregation of other polymer in the presence of external stimuli. Nozaki et al. identified the collapse of poly(Nisopropylacrylamide) chains above the lower critical solution temperature of the βCD-poly(N-isopropylacrylamide) gel.33 Further, the cyclic structure of CD hinders the swelling of the hydrogel, allowing lesser amount of entrapment within the network. Considering the literature, no detailed adsorption study based on the polysaccharide−cyclodextrin cross-linked hydrogels has been reported. Thus, we presume that βCDbased hydrogels cross-linked with cellulose and hemicellulose

2. MATERIALS AND METHODS 2.1. Materials. The chemicals, i.e., monomers, cross-linker, metal salts, and auxiliary chemicals, were of analytical grade and purchased from commercial sources. All of the chemicals were used as received. The CAS registry number, purity of chemicals, and suppliers are given in Table 1. Deionized (DI) water was supplied from in-house Millipore water synthesis unit (Millipore, model: ELix-3). 2.2. Preparation of Hydrogels. β-Cyclodextrin (βCD) was dissolved in 1.5 mol L−1 aqueous sodium hydroxide (NaOH) solution. Carboxymethyl cellulose sodium salt (CMC) and beechwood xylan (xylan) were separately dissolved in 1 mol L−1 aqueous NaOH solution. Microcrystalline cellulose (MCC) was dissolved in 60 wt % NaOH and 40 wt % urea solution, as reported by Zhou et al.36 After achieving a homogeneous mixture, βCD was added in all three cases in a 1:1 mole ratio. The mixtures were heated at 50 °C and stirred until a homogeneous precursor solution was obtained. EGDE was added dropwise into the solution. Pure βCD-based B



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Figure 1. Reaction scheme of βCD-CMC hydrogel formation.

hydrogel was prepared by adding EGDE into the homogeneous solution of βCD. The gelation was seen to complete within 30 min. The gels were washed with deionized (DI) water to remove unreacted monomers and cross-linker. Thereafter, the gels were neutralized with 0.1 M hydrochloric acid (HCl) solution and subsequently lyophilized (freeze drier; Martin Christ, model: α 2-4 LD). The structures of monomers and cross-linker are shown in Figure S1. The schematic diagram of βCD-CMC hydrogel is given in Figure 1 and others are explicitly given in Figures S2−S4 of the Supporting Information. 2.3. Characterization. FT-IR characterization was performed using an FT-IR spectrometer (Shimadzu, model: IRAffinity-1). Freeze-dried hydrogels were ground and mixed with dried potassium bromide (KBr). Thereafter, transparent pellets were formed in a hydraulic press. The FT-IR spectra of the pellets were recorded from 400 to 4000 cm−1 using 30 scans per sample with a resolution of 4 cm−1. The morphologies of the freeze-dried hydrogels were observed in a field emission scanning electron microscope (Carl Zeiss, model: GeminiSEM 300). The freeze-dried hydrogels were fixed on a specimen stub, which was later sputter-coated with gold. The observation was performed with a potential of 3 kV. Optical microscopy of the hydrogels was performed using a microscope (Carl Zeiss, model: Scope A1, AXIO, software: ZEN 2.3). The freeze-dried hydrogels were deposited on a glass slide and covered with a coverslip. We then observed the morphologies of Cd(II)- and Ni(II)-loaded hydrogels in the fluorescence mode using the same microscope at 475 nm wavelength light. The swelling ratio of hydrogel was recorded at pH = 2, 4, 6, and 8 and in deionized (DI) water. The freeze-dried hydrogels were immersed in various pH solutions and were allowed to reach equilibrium for 48 h at 25 °C. After 48 h, the swelled hydrogels were filtered and weighed. The percentage of swelling was measured by the following formula

SR (%) =

(Weq − Wi ) Wi

× 100%

(1)

where Wi is the weight of the freeze-dried hydrogel and Weq is the equilibrium weight of the hydrogel. All of the physical characterization data were triplicated and standard deviation was represented in terms of error bars. The pH of solution was adjusted by a digital pH meter (Eutech Instruments, model: EUTECH pH700). The gel fractions of hydrogels were calculated as described by Alshehri et al.37 Briefly, the hydrogels were dried at 60 °C for 24 h to get a constant weight (W0). Each sample was immersed in DI water and heated at 121 °C for 4 h. After 4 h, the hydrogels were again dried at 60 °C for 72 h and the weight was measured as W1. The gel fraction was calculated as ji W zy gel fraction (%) = jjj 1 zzz × 100% j W0 z k {

(2)

The point of zero charge (PZC) analysis was performed by the ΔpH method. The analysis was carried out to determine the surface functionality of the hydrogel with respect to the change of pH of the solution.38 The difference in pH was calculated by ΔpH = pH i − pH f

(3)

where pHi is the initial fixed pH maintained by HCl or NaOH solution. After immersing the hydrogel, the pH changes to pHf. Therefore, ΔpH was plotted against pHi. PZC of the hydrogel was identified by the point of intersection of the curve at the zero value of ΔpH. 2.4. Adsorption Experiments. The adsorption of metal ions using hydrogels was performed in aqueous solutions of Cd(II) and Ni(II). The respective solutions were prepared after dissolving a certain amount of salt in deionized water (DI). Cadmium chloride monohydrate (CdCl2·H2O, 1.7909 g) was dissolved in DI water in a 1 L volumetric flask. A wellshaken solution was made up to 1 L with DI water to make 1000 mg L−1, 1000 mL stock solution of Cd(II). Similarly, 4.0499 g of nickel chloride hexahydrate (NiCl2·6H2O) was C



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Figure 2. FT-IR spectra of (a) pure βCD, (b) βCD gel, (c) βCD-xylan, (d) βCD-MCC, (e) βCD-CMC.

equilibrium adsorption capacities (qe, mg g−1) and timeresolved adsorption capacities (qt, mg g−1) are calculated by the following equations

dissolved in DI water and the final volume was made up to 1 L to make 1000 mg L−1, 1000 mL Ni(II) stock solution. The adsorption experiments were carried out at varying dosage of adsorbent, concentration, pH of metal-ion solution, and temperature. The stock solutions were diluted according to the requirement to prepare various concentrations of metal-ion solutions. A measured amount of hydrogel was put into a 100 mL conical flask with 25 mL Cd(II) or Ni(II) solution at different concentrations. The flasks were kept into an orbital shaker (Daihan Labtech, model: LSI 3016R) at 25 °C and rotated at 120 rpm for 12 h to reach equilibrium. Then, the solutions were filtered and the filtrates were taken to AAS to measure the metal-ion concentration. The similar process was followed for pH-dependent adsorption and temperaturedependent adsorption. The removal efficiency (%removal) is calculated as below ij C − Ce yz zz × 100% removal (%) = jjj 0 j C0 zz k {

iV y qe = (C0 − Ce) × jjj zzz km{ iV y qt = (C0 − C t) × jjj zzz km{

(5)

(6)

where C0, Ct, and Ce are initial, time-resolved, and equilibrium concentrations of metal ion in solution (mg L−1), respectively; V is the volume of metal-ion solution taken for adsorption (in L); and m is the dry weight of hydrogel (g). All of the measurements are triplicated. The concentration of metal ions in the solution during adsorption was measured by atomic absorption spectroscopy (AAS) (Varian Australia, model: AA240FS) using flame mode. Both cadmium and nickel were measured separately in a cadmium lamp of wavelength 228.8 nm and a nickel lamp of wavelength 232.0 nm.

(4)

where C0 and Ce (in mg L−1) are the initial and equilibrium concentrations of metal ion in solution, respectively. The D



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Figure 3. FESEM images of hydrogels: (a) βCD-xylan, (b) βCD-CMC, (c) βCD-MCC, and (d) βCD gel.

Figure 4. Optical microscopy images of hydrogels (no metal-ion adsorption): (a) βCD-xylan, (b) βCD-CMC, (c) βCD-MCC, and (d) βCD gel. Fluorescent microscopy images of Cd(II)-loaded hydrogels (e−h) and Ni(II)-loaded hydrogels (i−l). (e, i) βCD-xylan, (f, j) βCD-CMC, (g−k) βCD-MCC, and (h−l) βCD gel.

peaks at 2912−2918 and 2877−2878 cm−1 are assigned to C− H stretching and bending, respectively. The asymmetric stretching of COO− is generally observed within the range of 1600−1620 cm−1.12 However, in all FT-IR spectra, a peak at 1645−1647 cm−1 is observed. Due to the hygroscopic nature of the gels as well as the precursors, this is primarily described as the HOH bending of the physically absorbed water.40,41 Therefore, the asymmetric stretching of COO− in CMC is merged with this spectra. The symmetric −CH2 bending is observed at 1424−1452 cm−1.42 However, in close proximity to this, the symmetric stretching vibration of COO− moiety is found at 1423−1446 cm−1.35 Hence, the two are merged in a broad peak at 1456 cm−1 and assigned to the symmetric bending of −CH2 and symmetric stretching vibration of COO−. The −CH2 moiety here is present in the aliphatic chain of βCD, MCC, and CMC, as well as in EGDE cross-linker. Hence, this is observed in all FT-IR spectra. The skeletal vibrations of C−O and C−C in the pyranose ring are observed at 1354 and 1338 cm−1, respectively. A broad peak is observed at 1086−1088 cm−1, which is considered as a merger of several peaks. The antisymmetric β-(1 → 4) glycosidic linkage is

3. RESULTS AND DISCUSSION 3.1. FT-IR Spectra. The synthesized hydrogels are characterized by FT-IR spectra, which are given in Figure 2. The polysaccharides are composed of pyranose ring. Two pyranose rings are connected by either α-(1 → 4) glycosidic linkage (βCD) or β-(1 → 4) glycosidic linkage (MCC, CMC, and xylan). Figure 2a presents FT-IR spectra of pure βCD in pregel condition. The O−H stretching is observed as a broad peak at 3450 cm−1. The C−H stretching is obtained at 2922 cm−1. The peak at 1456 cm−1 is assigned for the bending of −CH2. Skeletal vibrations are also observed at 1338 cm−1. The stretching vibration of α-(1 → 4) glycosidic linkage is observed at 1159 cm−1, and the vibration of the same linkage is observed at 937 cm−1.39 The bending of C−C is assigned at 1082 cm−1, while the stretching of C−O, bonded with the two pyranose rings, is assigned at 1028 cm−1. The glycosidic deformation of C−H with ring vibration is assigned to 856 cm−1. The FT-IR characterizations of all four gels are given in Figure 2b−e. The characteristic vibrations are mostly in similar positions and assigned as follows. The O−H stretching is assigned as a broad peak from 3417 to 3437 cm−1. The doublet E



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binding with hydroxyl and carboxylate ions, the coarse structure will not obstruct the process. The hydrogels appear as a glossy structure in optical microscopy (Figure 4a−d). Except βCD-MCC gel, others are visible at isolated assemblies, whereas βCD-MCC gel is an interconnected gel. Figure 4e−l represent the fluorescent microscopy images of the metal-ionadsorbed hydrogel. Further, concentrated metal-ion spots are observed in the images. These may refer to the trapped Ni(II) and Cd(II) ions inside the network. 3.3. Swelling Ratio and Gel Fraction. Swelling represents the amount of water a hydrogel can absorb. It is a fundamental physical property of hydrogel which implies that a higher swelling is desired. We present the swelling of four hydrogels at pH 2, 4, 6, and 8 and DI water, as given in Figure 5a. The toroid shape of βCD limits its swelling. The swelling of βCD gel initiates through the flexibility of cross-linker. Therefore, it shows least swelling in the pH medium and DI water. The cellulose and hemicellulose consist of pyranose ring and are linear in nature. It should be noted that the gelation procedure creates a network structure where the flexibility of network is more than the βCD gel network. Here, the individual cellulose and hemicellulose chains can expand, allowing more water to be trapped in the structure. Further, we observe an increase in the swelling ratio for all four hydrogels with the increment in pH. The highest swelling of 362.53% is observed for βCD-CMC gel at pH 8. The swelling data with the increment of pH can be explained with the electrostatic repulsion of the hydrogel network. The dominant charge species of the synthesized hydrogels are unprotonated hydroxyl (MCC, xylan, βCD) and carboxylate anions (CMC). The electrostatic repulsion of these charged moieties causes swelling. Compare to other three hydrogels (βCD gel, βCD-xylan, and βCD-MCC), βCD-CMC has a longer side chain attached to the pyranose ring. Hence, it is more flexible to swell. At lower pH, the unprotonated anions get protonated, thereby reducing the ionic repulsions. There is lower swelling at pH = 2. Near the neutral (pH = 6) or alkaline

Figure 5. (a) Swelling ratio of hydrogels in DI water and pH 2, 4, 6, and 8. (b) Gel fraction of hydrogels.

reported at 1090 cm−1, 43 while the bending of C−C is reported at 1064 cm−1.35 Therefore, the broad peak at 1086− 1088 cm−1 is collectively assigned to the C−C bending and antisymmetric β-(1 → 4) glycosidic linkage. The α-(1 → 4) glycosidic linkage of βCD is again observed at 937−942 cm−1. As before, the glycosidic deformation of C−H with ring vibration is shifted to 872−874 cm−1. 3.2. Morphology. The morphologies of the synthesized hydrogels were visualized through FESEM and optical microscopy, and are given in Figures 3 and 4, respectively. The coarse nature of the gels is visualized in the FESEM images (Figure 3). Small discontinuous elongations are visible in the surface, which can be presumed as possible binding sites for metal ions. However, coarse gels are also reported to adsorb Cd(II).23 Since adsorption of metal ions happens by

Figure 6. Equilibrium adsorption capacity (qe) and removal of pollutant at various dosages of adsorbent at 50 mg L−1 initial metal-ion concentration. (a) qe of Cd(II), (b) qe of Ni(II), (c) removal of Cd(II), and (d) removal of Ni(II). F



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Figure 7. Equilibrium adsorption capacity (qe) and removal of pollutant at various initial metal-ion concentrations: (a) qe of Cd(II), (b) qe of Ni(II), (c) removal of Cd(II), and (d) removal of Ni(II).

solution. The equilibrium adsorption capacities and removal efficiencies are given in Figure 6. From Figure 6, the highest adsorption capacity and removal efficiency are observed for 80 mg adsorbent dosage. With increase in the amount of adsorbent dosage, the binding sites for adsorption increase; hence, it can bind with a larger number of metal ions. Hence, for higher adsorbent dosage, the number of vacant sites are more. Further, in the expression of adsorbent capacity (eq 5), mass of adsorbent is at denominator. Hence, with the increment of dosage, adsorption capacity is decreasing. However, at 80 mg dosage of hydrogel, the removal efficiencies of Cd(II) and Ni(II) are higher than 80 and 50%, respectively. Hence, the 80 mg adsorbent dosage is considered as optimum dosage for further study. 3.5.2. Effect of Concentration. Figure 7 represents the concentration variation adsorption study of both the metal ions. The initial metal-ion concentrations are varied from 5, 15, 25, 50, 100, 300, and 500 mg L−1 and are performed at 25 °C in DI water. The removal efficiency of the hydrogel decreases over the increase in concentration, which also increases the adsorption capacity. This implies, at lower concentration, that all of the adsorption sites are not occupied with metal ions. However, at 100 mg L−1, a significant change in the nature of capacity curve for the adsorption of Ni(II) is observed (Figure 7b). Here, after 100 mg L−1 initial concentration, the adsorption capacity does not change significantly and it reaches near saturation adsorption by the gels. For the adsorption of Cd(II), the rate of adsorption decreases after 100 mg L−1 initial concentration solution, and thereafter, negligible change is observed after 300 mg L−1. Among the four hydrogels, βCD-CMC hydrogel has an adsorption capacity of 41.06 mg g−1 for Cd(II) and 18.44 mg g−1 for Ni(II) for 500 mg L−1 initial concentration. The order of adsorption capacity for both ions is βCD-CMC > βCD-xylan > βCD-MCC > βCD gel. Metal ions are being adsorbed in surface as well as in hydrophobic cavity of βCD. In addition to hydroxyl moieties, CMC-based hydrogels have −COO− moiety, which can easily bind with the metal

(pH = 8) regions, the unprotonated anionic moieties repel more, causing higher swelling.25,35 Figure 5b represents the gel fraction of four hydrogels calculated from eq 2. βCD gel has the highest gel fraction of 96.54%, and βCD-CMC has the least gel fraction of 47.59% among four hydrogels. The trend of gel fraction follows the opposite trend of swelling behavior.25 Higher gel fraction represents a densely cross-linked structure. The densely crosslinked structure has less flexibility to expand and thus lower water uptake capability, i.e., lower swelling. However, the low gel fraction of βCD-CMC allows free carboxymethyl moieties to expand and increase water uptake. 3.4. Point Zero Charge. The PZC of four hydrogels lie within pH 2.25−3.0 (PZCβCD‑xylan = 2.66, PZCβCD‑CMC = 2.25, PZCβCD‑MCC = 2.33, PZCβCD gel = 3.00). The net charge on the surface of hydrogel turns positive below the PZC. This implies the protonated binding sites on the surface of the hydrogel. However, at pH higher than the PZC, the net surface charge of the material is negative, i.e., the carboxylate/hydroxyl moieties of βCD-based gels are deprotonated. The deprotonated binding sites readily adsorb the metal ions from the aqueous solution. The cellulose/hemicellulose copolymers add more number of anionic binding sites with the βCD, thus reducing the PZC of the copolymerized hydrogels. 3.5. Removal of Cd(II) and Ni(II). The synthesized hydrogels are used to adsorb Cd(II) and Ni(II) ions from aqueous solutions. The parameters of adsorption are optimized with variation of adsorbent dosage, initial concentration of metal ions in solutions, pH of solutions, and temperature. The removal efficiency is calculated from eq 4, and the equilibrium adsorbent capacity is determined from eq 5. All of the adsorption experiment data are triplicated and standard deviation is reported in the plots. 3.5.1. Effect of Adsorbent Dosage. The effect of adsorbent dosage on the adsorbent capacity and removal of the metal ions is determined at 50 mg L−1 initial metal-ion concentration solution in DI water at 25 °C. The amount of adsorbent varies from 80 to 500 mg, which is taken in a 25 mL metal-ion G



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Figure 8. Equilibrium adsorption capacity (qe) (a−d) and removal of pollutant (e−h) at various pH values for the initial metal-ion concentrations of 5 and 100 mg L−1. (a, c) qe of Cd(II), (b, d) qe of Ni(II), (e, g) removal of Cd(II), and (f, h) removal of Ni(II).

ions. Surface adsorption is the likely occurrence in βCD-xylan and βCD-MCC hydrogels due to the ionic bonding of metal ions with hydroxyl moiety. However, at lower initial concentration, the difference in removal or adsorption capacity is not visible. At 5 mg L−1, all of the hydrogels show efficient removal of more than 90% for cadmium and 80% for nickel. The removal efficiency of hydrogel decreases with the increase in initial metal-ion concentrations. At 100 mg L−1 initial concentration, the removal efficiencies are 78.76 and 50.85% for Cd(II) and Ni(II), respectively, with βCD-CMC gel. Therefore, we have used these two initial concentrations to measure the effect on adsorption with various pH values. 3.5.3. Effect of pH. The effect of pH on the adsorption capacity and removal efficiency of the metal ions is determined at pH = 2, 4, 6, and 8 for 5 and 100 mg L−1, and are reported in Figure 8. The removal efficiency is the lowest at pH = 2 for all of the cases. The removal efficiency increases with increase

in pH, and the highest is obtained at pH = 6 for all four hydrogels. In the strong acidic condition, H+ ions are freely available. Hence, H+ and metal ions compete with each other to bind with available carboxylate or hydroxyl moieties of hydrogel resulting in poor removal rate. The removal efficiency lies in the order βCD-CMC > βCDxylan > βCD-MCC > βCD gel for both 5 and 100 mg L−1 initial concentrations. The toroid structure of βCD provides less flexibility to swell and hence the lowest rate of removal of metal ions. However, a decrease in removal efficiency is observed at pH = 8. The reason is attributed to the metal hydroxide formation in alkaline condition. The formation of Cd(OH) 2 and Ni(OH) 2 begins at pH = 10 and 7, respectively.30 The adsorption capacities follow a similar trend of removal. At pH = 6 and 100 mg L−1 initial concentration, the adsorbent capacities of βCD-CMC are 24.66 and 15.93 mg g−1 for Cd(II) and Ni(II), respectively. All H



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Figure 9. Equilibrium adsorption capacity (qe) and removal of pollutant at various temperatures: (a) qe of Cd(II), (b) qe of Ni(II), (c) removal of Cd(II), (d) removal of Ni(II).

hydrogels swell more, allowing a large exposure of surface hidden under the top layer. The newly exposed surfaces tend to have more binding sites to adsorb the metal ions. The variation in adsorption capacity with all four hydrogels is relatively small for Cd(II) than Ni(II). For both the metal ions, the adsorption capacities of βCD-xylan and βCD-MCC lie within the range of βCD-CMC and βCD gel. The increment in adsorption capacity is attributed to the endothermic nature of adsorption, i.e., energy is being consumed during the uptake. The removal efficiency for temperature variation follows a similar trend of adsorption capacity. However, as the increment of capacity is very small, we take 25 °C as the optimum adsorption temperature for the model fitting of adsorption isotherm and kinetics. 3.5.5. Adsorption of Cd(II) and Ni(II) from Mixed Feed. After optimizing the adsorption parameters of single metal-ion adsorption, we move to the selective adsorption of Cd(II) and Ni(II) from mixed feed. However, the mixed feed stream varies with a possible number of combination of concentration of metal ions, leading to a huge number of data set, which is beyond the scope of this work. Hence, we take 5 mg L−1 individual concentration of Cd(II) and Ni(II) at 80 mg adsorbent dosage, pH = 6, and T = 25 °C. Such lower concentration of metal ions for the mixed feed study is also reported in the literature.44 It is evident from Figure 10 that Cd(II) adsorbs at a higher rate than Ni(II) for all hydrogels studied. However, the order of adsorption for hydrogels is consistent, which implies that the highest adsorption capacity is obtained with βCD-CMC gel and the least with βCD gel. Due to the selective adsorption, less amounts of Cd(II) and Ni(II) are adsorbed than the adsorption of individual metal ions. The equilibrium adsorption capacity of each metal ion is then calculated with respect to its individual initial concentration. 3.5.6. Equilibrium Adsorption Isotherm. The adsorption isotherm is based on the assumption that every adsorption site is equivalent and that the ability of a particle to bind at adsorption site is independent of whether or not the site is

Figure 10. Equilibrium adsorption capacity (qe) and removal of Cd(II) and Ni(II) from mix feed: (a) qe of metal ions and (b) removal of metal ions.

four hydrogels adsorb considerably less amount of Ni(II) than Cd(II) ions making the adsorption process favorable for Cd(II) removal using βCD-cellulose/hemicellulose-based hydrogels. 3.5.4. Effect of Temperature. The effect of temperature of adsorption process is measured by varying the temperature at T = 25, 35, 45, and 55 °C, and is reported at Figure 9. All temperature variation adsorption experiments are carried out at pH = 6 and 100 mg L−1 initial metal-ion concentration. There is a mild increase in the adsorption capacities as well as removal efficiencies with increase in temperature. At higher temperature also, βCD-CMC uptakes higher amount of metal ions than the other three. Thus, the order of adsorption capacity and removal efficiency becomes βCD-CMC > βCDxylan > βCD-MCC > βCD gel. At higher temperature, I



Article

Table 2. Parameters of the Langmuir and Freundlich Isotherms Cd(II) qm (mg g−1) Langmuir isotherm

b (L mg−1)

βCD-xylan

βCD-CMC

βCD-MCC

βCD gel

βCD-xylan

βCD-CMC

βCD-MCC

βCD gel

42.0168

41.8410

39.3701

37.4532

0.0867

0.1162

0.0762

0.0689

Kf Freundlich isotherm

nf

βCD-xylan

βCD-CMC

βCD-MCC

βCD gel

βCD-xylan

βCD-CMC

βCD-MCC

βCD gel

4.4300

5.2674

3.6208

3.5083

2.3159

2.4845

2.1697

2.2247

Ni(II) qm (mg g−1) Langmuir isotherm

b (L mg−1)

βCD-xylan

βCD-CMC

βCD-MCC

βCD gel

βCD-xylan

βCD-CMC

βCD-MCC

βCD gel

16.6667

18.6220

12.2100

10.1729

0.1067

0.1587

0.1084

0.0814

Kf Freundlich isotherm

nf

βCD-xylan

βCD-CMC

βCD-MCC

βCD gel

βCD-xylan

βCD-CMC

βCD-MCC

βCD gel

2.8041

3.8107

2.1652

1.8733

2.9560

3.2744

3.0544

3.1939

equilibrium is reached. To explore the adsorption mechanism of Ni(II) and Cd(II) by four hydrogels, Langmuir and Freundlich isotherm models shall be adopted. The equilibrium concentrations and equilibrium adsorbent capacities are measured at T = 25 °C, pH = 6 for the corresponding metal-ion concentrations of 5, 15, 25, 50, 100, 300, and 500 mg L−1. 3.5.6.1. Langmuir Isotherm. The Langmuir isotherm model assumes monolayer adsorption of solutes on a homogeneous surface.46 The homogeneous surface refers to a finite number of identical sites having homogeneous adsorption energy. Further, the identical and equivalent number of localized sites are having monolayer thickness. Thus, once the binding site is occupied with adsorbate, no further adsorption can happen at that site.45 Therefore, the site poses equal affinity for the adsorbates with uniform energies of ion exchange.47,48 The Langmuir isotherm is given by Ce 1 1 = + Ce qe qmb qm

Figure 11. Langmuir isotherm fitting with the experimental equilibrium concentration of metal ions: (a) Cd(II) and (b) Ni(II).

(7)

where Ce (in mg L−1) is the equilibrium concentration of metal ions in solution, qe (in mg g−1) is the equilibrium adsorption capacity, qm (in mg g−1) is the maximum adsorption capacity, and b is the Langmuir constant (in L mg−1). b is related to the energy of adsorption and affinity to binding sites. The dimensionless constant equilibrium parameter (RL) determines the essential characteristic of Langmuir isotherm. A favorable isotherm is obtained for the values of RL between 0 and 1, and an unfavorable isotherm happens for RL greater than 1. RL = 1

occupied.45 The isotherms describe the nature of interaction between adsorbate and adsorbent. Further, the analysis of data determines the maximum adsorption capacity of the adsorbent, which in turn optimizes the use of adsorbent. The equilibrium solution concentration is obtained when a dynamic balance of concentration of adsorbate is established with bulk solution to that of liquid−adsorbent interface. In this scenario, adsorption

Table 3. RL Values for the Adsorption of Cd(II) and Ni(II) on Langmuir Model RL Cd(II)

Ni(II)

initial metal-ion concentration (mg L−1)

βCD-xylan

βCD-CMC

βCD-MCC

βCD gel

βCD-xylan

βCD-CMC

βCD-MCC

βCD gel

5 15 25 50 100 300 500

0.6976 0.4347 0.3157 0.1874 0.1034 0.0370 0.0225

0.6325 0.3646 0.2561 0.1469 0.0792 0.0279 0.0169

0.7242 0.4667 0.3443 0.2079 0.1160 0.0419 0.0256

0.7437 0.4917 0.3672 0.2249 0.1267 0.0461 0.0282

0.6520 0.3844 0.2726 0.1578 0.0857 0.0303 0.0184

0.5576 0.2958 0.2013 0.1119 0.0593 0.0206 0.0124

0.6486 0.3809 0.2696 0.1558 0.0845 0.0298 0.0181

0.7108 0.4503 0.3295 0.1973 0.1094 0.0393 0.0240

J



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Figure 12. Equilibrium adsorption capacity (qe) and removal of pollutant at various time intervals: (a) qt of Cd(II), (b) qt of Ni(II), (c) removal of Cd(II), and (d) removal of Ni(II).

both the ions are shown in Figure 11, and rest are given in Figures S5 and S6 of the Supporting Information. For both the metal ions, the R2 values are greater than 0.99 for all hydrogels, which indicate the monolayer adsorption phenomenon for metal-ion adsorption. Further, we obtain the highest energy of adsorption (b) for βCD-CMC-based hydrogel and least for βCD gel. Therefore, the metal ions prefer to bind more on carboxylate (βCD-CMC gel) and hydroxyl (for all gels) than the inner cavity of βCD. Table 3 presents the dimensionless constant RL for all Langmuir isotherm fittings and is within 0−1 for all concentrations, and the decreasing trend of RL is obtained with the increasing concentration. Thus, the adsorption is favorable for both the metal ions with all four hydrogels. The lower values of RL (approaching 0) indicate an irreversible nature of adsorption. At higher concentration, metal ions readily binds with adsorption sites leaving no site vacant to adsorb further attachment. The maximum adsorption capacity (qm) of 18.6 mg g−1 is obtained with βCD-CMC hydrogel for Ni(II) removal, whereas βCD-xylan gel produces the highest qm of 42.0 mg g−1 for Cd(II) removal. 3.5.6.2. Freundlich Isotherm. The Freundlich isotherm is used to describe the multilayer adsorption on a heterogeneous adsorbent surface. The surface is assumed to have no uniform distribution of heat of adsorption. The linear form of the Freundlich isotherm is expressed by

Figure 13. Pseudo-second-order model fitting with the experimental kinetic data of adsorption of metal ions by βCD-CMC hydrogel: (a) Cd(II) and (b) Ni(II).

indicates irreversible isotherm, and RL = 0 indicates the linear nature. Mathematically, it is expressed as 1 RL = 1 + bC0 (8)

log qe =

where C0 (in mg L−1) is the initial concentration of metal-ion solution. Table 2 presents the model parameters applying the Langmuir and Freundlich models for the experimental data of removal of metal ions using four hydrogels. Table S1 (Supporting Information) represents the corresponding model-fitted isotherm equations for each case. The goodness of fit of experimental data on the isotherm model is expressed as a correlation coefficient value (R2) and are reported in Table S1. Further, the model-fitted isotherms for one hydrogel of

1 log Ce + log K f nf

(9)

where Ce and qe are the same as defined previously, and Kf and nf are Freundlich constants relating the adsorption capacity and adsorption intensity, respectively. Here, nf indicates the favorability of adsorption. Value of nf < 1 implies poor adsorption; moderate adsorption characteristic lies for a value of nf between 1 and 2, while a good adsorption lies between 2 and 10. Freundlich model-fitted equations and R2 values are also reported in Table S1, and model parameter values are given in Table 2. The R2 values for adsorption of Cd(II) lie K


1.51 × 10−2 6.12 4.25 × 10−3 8.87

between 0.92 and 0.95, suggesting that the adsorption is not a nonideal phenomenon. Further, it rejects multilayer adsorption of metal ions on heterogeneous surface since metal ions prefer to bind only with carboxylate and hydroxyl moieties. The correlations are even worse for Ni(II) adsorption as R2 lies between 0.85 and 0.91, although nf lies in the adsorption region (except for Ni(II) removal with βCD gel). Comparing the two isotherms, we conclude that the experimental data appropriately fit with Langmuir isotherm, reflecting the monolayer isotherm. The hydrogel surfaces behave like a homogeneous surface. The results are understandable as H+ is not lost during the synthesis or adsorption conditions. However, the carboxylate and hydroxyl moieties donate proton in the solution, making vacant site accessible for metal ions to adsorb. 3.5.7. Adsorption Kinetics. The time-resolved adsorbent capacities and removal rate of Cd(II) and Ni(II) with all four hydrogels are given in Figure 12. For the adsorption of Cd(II), we observe fast adsorption kinetics up to 120 min (Figure 13a). More than 75% of Cd(II) is adsorbed within this period by βCD-CMC hydrogel, and more than 60% is adsorbed with other hydrogels. Further, the initial adsorption capacity of hydrogel is very similar for βCD-CMC and βCD-xylan hydrogels. For Cd(II), the adsorption capacities of all four hydrogels up to 100 min are very close reflecting strong affinity of Cd(II) toward the hydrogel moieties. For the adsorption of Ni(II), we observe a prolonged adsorption period of 200 min. Unlike the adsorption kinetics of Cd(II), time-resolved adsorption capacities of Ni(II) are distinctively placed. However, the rate of adsorption is higher for the first 100 min and decreases gradually between 100 and 200 min. The rate of solute uptake by the adsorbent is described by the kinetics of adsorption. The analysis provides valuable information regarding binding pathways and the mechanism. Here we employ pseudo-first-order and pseudo-second-order models to describe the kinetic behavior of models. Like the previous isotherm fit, goodness of fit is also expressed by R2. The pseudo-first-order model assumes that the rate of adsorption is proportional to the number of unoccupied sites by the solutes and is expressed by the Lagergren equation.49 The model is expressed as

βCD-CMC βCD-xylan

1.18 × 10−2 11.45 1.52 × 10−3 16.00 1.41 × 10−2 11.12 3.00 × 10−3 22.83

βCD gel βCD-MCC

1.12 × 10−2 10.31 2.50 × 10−3 24.33

βCD-CMC

1.84 × 10−2 13.13 2.63 × 10−3 25.84

βCD-xylan

1.62 × 10−2 13.28 2.59 × 10−3 25.06 (min−1) (mg g−1) (g mg−1 min−1) (mg g−1)

ln(qe − qt) = ln qe − k1t

where qe (in mg g ) is the equilibrium adsorption capacity, qt (in mg g−1) is the time-resolved adsorption capacity, and k1 is the pseudo-first-order rate constant (in min−1). The model is primarily used for solid or liquid adsorption based on the adsorbent capacity. It is based on direct proportional relationship between the difference of saturation uptake and time-resolved uptake with the time of adsorption. The linear equations for each case are given in Table S2 of the Supporting Information, and the R2 values are less than 0.99. Further, the model-fitted kinetics for one hydrogel of both the ions are shown in Figure 13 and rest are given in Figures S7 and S8 of the Supporting Information. The lower regression values suggest the incapability of the pseudo-first-order model in describing the adsorption of metal ions on the hydrogel surface. The model is particularly suited for very rapid adsorption process. Further, for entire adsorption kinetics, the pseudo-first-order model may not be suitable to describe the total period of adsorption.50 The pseudo-second-order model assumes that the rate of occupation of solute at the adsorption site is proportional to

pseudo-second-order model

pseudo-first-order model

(10)

−1

k1 qe k2 qe

calculated constants

Cd(II)

Table 4. Parameters for Pseudo-First-Order and Pseudo-Second-Order Models at 298 K

9.70 × 10−3 8.22 2.21 × 10−3 16.78

Ni(II)

βCD-MCC

Article

1.11 × 10−2 5.90 3.36 × 10−3 10.99

βCD gel


L



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Table 5. ΔG and ΔH of Adsorption Process Cd(II) −1

ΔG (kJ mol )

ΔHT1→T2 (kJ mol−1)

Ni(II)

temperature (K)

βCD-xylan

βCD-CMC

βCD-MCC

βCD gel

βCD-xylan

βCD-CMC

βCD-MCC

βCD gel

298 308 318 328 298 → 308 308 → 318 318 → 328

−0.65 −0.58 −0.54 −0.55 2.67 1.87 0.18

−0.59 −0.49 −0.45 −0.46 3.34 1.97 0.10

−0.69 −0.64 −0.62 −0.63 2.04 1.19 0.23

−0.85 −0.78 −0.75 −0.76 2.84 1.89 0.43

−1.89 −1.82 −1.80 −1.85 4.00 2.53 0.23

−1.67 −1.64 −1.58 −1.61 2.60 3.39 0.55

−2.73 −2.55 −2.50 −2.52 8.03 4.27 1.90

−3.30 −3.10 −3.05 −3.07 8.98 4.90 2.42

spontaneous nature of the adsorption process. However, with the increase in temperature, the ΔG value decreases up to 318 K. This implies that the adsorption becomes less spontaneous upon increasing temperature and external mechanical shaking is required. However, at 328 K, the increasing trend of ΔG is again observed. We view this as the loosening of the crosslinking structure at this temperature, resulting in higher removal. Therefore, it is recommended to perform the adsorption process at 298 K considering the spontaneity of the process. The calculated change in enthalpy between two successive temperature increments is positive. The endothermic nature of the adsorption of metal ion on the binding site is established by the positive change of value in enthalpy over the said temperature range.51 The interaction among the adsorbent sites and adsorbate can be inferred from the lower value of ΔH. However, the lower values further suggest a physisorption process in adsorption as chemisorption requires an enthalpy change greater than 40 kJ mol−1.54 Further, at higher temperature, the enthalpy change reduces significantly, implying the possibility of desorption increase at higher temperature. This ensures weak interaction between metal ions and host moieties of hydrogel matrix. 3.5.9. Proposed Adsorption Mechanism. There are several interactions, such as ion exchange, complexation, electrostatic interaction, and chelation, possible among the binding sites and adsorbate, making adsorption a complicated process. The mechanism of adsorption can be formulated from the thermodynamics of the system or kinetics of the adsorption. However, the thermodynamic data are derived from the longterm equilibrium data set, whereas the kinetic data set is derived from the time-dependent adsorption study. Based on our thermodynamic calculations, we conclude that the adsorption of Cd(II) and Ni(II) in hydrogel is a physisorption process. The lower values of ΔH further support this. On the contrary, the kinetic data follow the pseudo-second-order rate equation, which is based on the chemisorption process. The hydrophobic cavity of βCD is capable of hosting Cd(II) and Ni(II) ions inside, making the complexation phenomenon feasible. Therefore, it can be concluded that the metal ions first attract to the cavities of βCD, which leads to the compliance of pseudo-second-order model. However, the formation of complex with the metal ions and cavity of βCD is realistic since pure βCD gel adsorbs a significant amount of metal ions. Once the cavity is saturated, the metal ions form complex with the hydroxyl/carboxyl moieties of the cellulose/hemicellulose. A combination of good adsorption capacity and feasible desorption of pollutant from the adsorbent makes the adsorbent efficient in large-scale use. However, the desorption technique must be of low cost and not damage the adsorbent. Hence, we prepare a 0.1 M HCl solution for the desorption study. The desorption process using 0.1 M HCl has been

Table 6. Regeneration Efficiency (%) Using 0.1 M HCl Solution βCD-xylan βCD-CMC βCD-MCC βCD gel

Cd(II)

Ni(II)

59.03 61.44 30.18 23.26

49.86 55.85 30.30 19.65

the square number of unoccupied sites.51 Further, the concentration of binding sites on the surface of adsorbent determines the rate of adsorption. It is expressed as t 1 t = + 2 qt qe k 2qe (11) where k2 is the pseudo-second-order rate constant (in g mg−1 min−1). The R2 values (Table S2 of the Supporting Information) are greater than 0.99 for all cases. For that, the pseudo-second-order model agrees well for the adsorption of Cd(II) and Ni(II) on the hydrogel surface compared to the pseudo-first-order model. The values of the parameters for both the pseudo-first-order and pseudo-second-order models are given in Table 4. 3.5.8. Thermodynamic Parameters. The Gibbs free energy and change in enthalpy of the adsorption process are calculated from the van’t Hoff equation and is given by52,53 d ln kad ΔH = (12) dT RT 2 where R is the ideal gas law constant (8.314 J mol−1 K−1); kd is the equilibrium constant at constant temperature, which is calculated by kad = Cad/C0, where C0 and Cad are the initial and adsorbed metal-ion concentrations, respectively; and ΔH is the change in enthalpy (in kJ mol−1). Therefore, integrating eq 12 from T1 to T2 temperature and assuming that the change in enthalpy during this period is ΔHT1→T2, we obtain

ΔHT1→ T2 = R

kad, T2 T2T1 ln T2 − T1 kad, T1

(13) −1

However, the Gibbs free energy (ΔG, in kJ mol ) is calculated from the following equation ΔG = −RT ln kad

(14)

Here, the negative sign is given to express the spontaneity of the process. The calculated values for ΔG and ΔHT1→T2 are reported in Table 5 at 80 mg adsorbent dosage, 100 mg L−1 initial concentration of metal ion, pH = 6 of solution, and over a temperature range of 298−328 K. For all of the adsorption studies, we observe negative values for ΔG, which implies the M


29

32

discussed in the literature.6,55 Cd(II)- and Ni(II)-loaded gels were immersed in freshly prepared HCL and inside the orbital shaker for a period of 6 h. Thereafter, the samples were filtered and the concentration of metal ions are measured in AAS. The HCl medium recovered some amount of metal ions, which is measured by the regeneration efficiency and given in Table 6. The regeneration efficiency is given by

Langmuir isotherm 98.88 mg g−1 at pH = 5 for Cd(II) removal. Freundlich isotherm 139.8 mg g−1 at pH = 5.5 and 25 °C for Cd(II) removal Langmuir isotherm 24.66 mg g−1 and 15.93 mg g−1 Cd(II) and Ni(II), respectively, at pH 6, 25 °C Langmuir with 100 mg L−1 initial metal-ion concentration isotherm

regeneration efficiency (%) amount of metal ion released = × 100 amount of metal ion adsorbed

βCD, MCC, CMC, and xylan

CMC, acrylamide

βCD, acrylic acid

Huang et al. Godiya et al. present study

O-acetyl galactoglucomannan hemicellulose, acrylamido-2-methyl-1-propanesulfonic acid MCC, methyl acrylate

CMC, poly(vinyl alcohol)

(15)

However, this simplistic approach has a drawback of removing the metal ions bounded inside the cavity of βCD. This will reduce the regeneration efficiency of hydrogels for subsequent cycles. There are other chemical treatments such as ethylene diamine tetraacetic acid disodium,29 nitric acid,56 ethanol,57 and methanol23 reported for the desorption study. Therefore, a dedicated research is required for the complete regeneration and subsequent reusability of hydrogels, which will be our key focus in future research. 3.5.10. Comparison of Metal-Ion Removal Study with the Literature. Here, we have compared the performance of the adsorption of Cd(II) and Ni(II) using βCD-cellulose/hemicellulose-based hydrogels with a few literature reports. The comparison is summarized in Table 7. On a closer look at the table, it includes at least one work of either cadmium or nickel removal using each of our precursors. To optimize the adsorption process, a robust parameter optimization is considered and the performances of four hydrogels are evaluated in the adsorption process. However, βCD-, cellulose-, and hemicellulose-based hydrogels are individually being reported in the literature. Here, we have synthesized a new set of hydrogels, which are successfully applied for the removal of Cd(II) and Ni(II). The removal of Cd(II) (24.66 mg g−1) is considerably higher than that reported in the literature. The adsorbed capacity of Ni(II) is 15.93 mg g−1, which is comparable to that reported in the literature. Typically, the most favorable adsorption occurs involving hydrogel in the pH range of 4−6 because of less competition from H+ ions. In the aqueous medium, the hydrogel swells, making the removal of metal ions by the entrapment in the network less likely. This leaves the only option to remove the metal ions either the host−guest complex formation or ionic bond formation between the metal ions with the active binding site of hydrogel. For a polymeric structure, the binding site for hydrogel repeat unit is limited and mostly lie with the side chain of backbone. Therefore, a shorter-chain repeat unit can attract more number of metal ions over the same weight for a longer-chain repeat unit. On a closer look at Table 7, both Godiya et al.29 and Huang et al.32 reported higher amount of adsorption of Cd(II) compared to our work. Similarly, Jia et al.22 reported a higher amount of adsorption of Ni(II) compared to our work. All three of them used the lowermolecular-weight-based repeat units such as acrylamide, acrylic acid, and methyl acrylate, respectively, which provide more number of binding sites than the same weight of polysaccharide-based hydrogels. However, the cavity of βCD is perceived to adsorb more metal ions, which is visible from the desorption studies.

140 mg g−1 at pH 4.5 and 28 °C for Ni(II) removal

15.20 mg g at pH 5.5 and 25 °C for Cd(II) removal, 11.69 mg g and 25 °C for Ni(II) removal 6 mg g−1 at 15 °C, DI water for Ni(II) removal

19 mg g−1 and 23.7 mg g−1 Cd(II) and Ni(II), respectively, at pH 3, 25 °C

at pH 5.5 Langmuir isotherm

pseudo-second-order kinetic mechanism pseudo-second-order kinetic mechanism pseudo-second-order kinetic mechanism pseudo-second-order kinetic mechanism

22

30

25

31 pseudo-second-order kinetic mechanism

Article

Cd(II), Ni(II), and others Ni(II) and others Cd(II), Ni(II), and others Ni(II) and others Cd(II) and others Cd(II) and others Cd(II), Ni(II) βCD-acryloyl chloride, silica-glycidyl methacrylate

Wang et al. Wang et al. Elgueta et al. Jia et al.

metal ions adsorbed monomers article

Table 7. Comparison of Metal-Ion Removal Study with the Literature

−1

maximum adsorption capacities

−1

fitted isotherm

fitted kinetic mechanism

reference


N



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4. CONCLUSIONS βCD-based four hydrogels are synthesized from βCD, CMC, xylan, and MCC oligosaccharides, using EGDE cross-linker in alkaline condition. The swelling study of the synthesized hydrogels are carried out at pH = 2, 4, 6, and 8 and using DI water, whose absorbance increases with pH. The highest swelling ratio of 362.53% is observed with βCD-CMC gel at pH = 8. All of the hydrogels are shown to have better adsorption abilities toward Cd(II) than Ni(II). Overall, the βCD-CMC hydrogel has proven to be a better candidate for adsorbing Cd(II) and Ni(II) among all of the studied hydrogels. The parameters of adsorption are studied with the variation of adsorbent dosage, concentration of metal-ion solution, pH, and temperature. The order of adsorption capacity for both the metal ions is βCD-CMC > βCD-xylan > βCD-MCC > βCD gel with all studied parameters. For Cd(II), βCD-CMC-based hydrogel gives adsorbent capacities of 13.81 mg g−1 at 80 mg adsorbent dosage; 24.61 mg g−1 at 100 mg L−1 initial metal-ion concentration; 24.66 mg g−1 at pH 6 of the metal-ion solution; and 26.42 mg g−1 at 55 °C temperature. Similarly, for Ni(II), βCD-CMC-based hydrogel gives adsorbent capacities of 10.58 mg g−1 at 80 mg adsorbent dosage; 15.89 mg g−1 at 100 mg L−1 initial metal-ion concentration; 15.93 mg g−1 at pH 6 of the metal-ion solution; and 17.29 mg g−1 at 55 °C temperature. The surface adsorption of metal ions is confirmed by the Langmuir isotherm model with RL values less than 1 for all cases, reflecting favorable condition for adsorption. The pseudosecond-order model is proved to be the better-fitted model for the adsorption process.

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Ct, time-resolved concentration of metal ion in solution (mg L−1) Cad, concentration of adsorbed metal ion inside hydrogel matrix (mg L−1) V, volume of metal-ion solution (L) m, dry weight of hydrogel (g) removal, removal efficiency (%) regeneration efficiency, regeneration efficiency (%) qe, equilibrium adsorption capacity (mg g−1) qt, time-resolved adsorption capacity (mg g−1) qm, maximum adsorption capacity (mg g−1) b, energy of adsorption and affinity to binding sites (L mg−1) RL, equilibrium parameter for Langmuir isotherm (dimensionless constant) Kf, Freundlich constants related to the adsorption capacity nf, Freundlich constants related to the adsorption intensity R, ideal gas constant (8.314 J mol−1 K−1) T, temperature (K) k1, pseudo-first-order rate constant (L min−1) k2, pseudo-second-order rate constant (g mg−1 min−1) kad, equilibrium constant ΔH, change in enthalpy (kJ mol−1) ΔG, Gibbs free energy (kJ mol−1) ΔHT1→T2, change in enthalpy from T1 to T2 temperature (kJ mol−1)

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(1) Khan, F.; Tare, R. S.; Oreffo, R. O. C.; Bradley, M. Versatile biocompatible polymer hydrogels: Scaffolds for cell Growth. Angew. Chem., Int. Ed. 2009, 48, 978−982. (2) Bajpai, A. K.; Shukla, S. K.; Bhanu, S.; Kankane, S. Responsive polymers in controlled drug delivery. Prog. Polym. Sci. 2008, 33, 1088−1118. (3) Katsoulos, C.; Karageorgiadis, L.; Vasileiou, N.; Mousafeiropoulos, T.; Asimellis, G. Customized hydrogel contact lenses for keratoconus incorporating correction for vertical coma aberration. Ophthalmic Physiol. Opt. 2009, 29, 321−329. (4) Lee, Y.-J.; Braun, P. V. Tunable inverse opal hydrogel pH sensors. Adv. Mater. 2003, 15, 563−566. (5) Muya, F. N.; Sunday, C. E.; Baker, P.; Iwuoha, E. Environmental remediation of heavy metal ions from aqueous solution through hydrogel adsorption: a critical review. Water Sci. Technol. 2015, 73, 983−992. (6) Kurdtabar, M.; Kermani, Z. P.; Marandi, G. B. Synthesis and characterization of collagen-based hydrogel nanocomposites for adsorption of Cd2+, Pb2+, methylene green and crystal violet. Iran. Polym. J. 2015, 24, 791−803. (7) Coviello, T.; Matricardi, P.; Marianecci, C.; Alhaique, F. Polysaccharide hydrogels for modified release formulations. J. Controlled Release 2007, 119, 5−24. (8) Zhang, X.; Wu, X.; Gao, D.; Xia, K. Bulk cellulose plastic materials from processing cellulose powder using back pressure-equal channel angular pressing. Carbohydr. Polym. 2012, 87, 2470−2476. (9) Shen, X.; Shamshina, J. L.; Berton, P.; Gurau, G.; Rogers, R. D. Hydrogels based on cellulose and chitin: fabrication, properties, and applications. Green Chem. 2016, 18, 53−75. (10) Salama, A.; Abou-Zeid, R. E.; El-Sakhawy, M.; El-Gendy, A. Carboxymethyl cellulose/silica hybrids as templates for calcium phosphate biomimetic mineralization. Int. J. Biol. Macromol. 2015, 74, 155−161. (11) Salama, A.; El-Sakhawy, M. Preparation of polyelectrolyte/ calcium phosphate hybrids for drug delivery application. Carbohydr. Polym. 2014, 113, 500−506.

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jced.9b00088.

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REFERENCES

Structure of precursors, cross-linking reaction scheme, isotherm and kinetic equations, and fitting diagrams (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: +91 361 2582291. Tel: +91 361 2582266. ORCID

Tamal Banerjee: 0000-0001-8624-6586 Notes

The authors declare no competing financial interest.

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NOMENCLATURE SR, swelling ratio (%) Weq, equilibrium weight of hydrogel (g) Wi, weight of freeze-dried hydrogel (g) W0, constant weight of hydrogel (g) W1, measured weight of hydrogel after 72 h, 60 °C (g) ΔpH, change in pH pHi, initial fixed pH pHf, final pH C0, initial concentration of metal ion in solution (mg L−1) Ce, equilibrium concentration of metal ion in solution (mg L−1) O



Article

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