Three-Dimensional Rice Straw-Structured Magnetic Nanoclay

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Three-Dimensional Rice Straw Structured Magnetic Nanoclay Decorated Tri-polymeric Nanohydrogels as Superabsorbent of Dye Pollutants Niladri Sarkar, Gyanaranjan Sahoo, Rashmita Das, and Sarat K Swain ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.7b00358 • Publication Date (Web): 27 Feb 2018 Downloaded from http://pubs.acs.org on February 28, 2018

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ACS Applied Nano Materials

Three-Dimensional Rice Straw Structured Magnetic Nanoclay Decorated Tripolymeric Nanohydrogels as Superabsorbent of Dye Pollutants Niladri Sarkar, Gyanaranjan Sahoo, Rashmita Das and Sarat K. Swain*

Department of Chemistry, Veer Surendra Sai University of Technology, Burla, Sambalpur768018, Odisha, India *Corresponding author: Email: [email protected]. Phone: 91-9937082348, Fax: 91-6632430204 ABSTRACT Herein, a simple eco-friendly route was adopted to synthesize 3D rice straw structured polyvinyl alcohol (PVA)/ chitosan (Cs)/agar-agar (Agr), tri-polymer based magnetic nanohydrogels with incorporations of Cloisite®30B and magnetite (Fe3O4) nanoparticles via sulfate (SO42-) induced quick gelation approach. The 3D rice straw structures of nanohydrogels were evolved due to the rolling of exfoliated clay platelets during transformation of PVA/Cs/Agr hydrogel network, whereas; the situation exfoliation was achieved because of long time ultrasonication and strong repulsive interactions between positively charged tri-polymeric chain and interstitial quaternary ammonium ions of nanoclays. Synthesized nanohydrogels were characterized with FTIR, XRD, SEM, TEM, XPS, AFM and TGA/DTG along with their rheological measurements to interpret the viscoelastic behaviors. The suitability of these nanohydrogels was investigated in detail for removal of rhodamine B (RhB) and methylene blue (MB) dyes from aqueous solution with variation of contact time, initial dye concentration, adsorbent dosage, temperature and pH, which were observed to be fitted well with pseudo 2nd order kinetic and Langmuir isotherm models with revealing endothermic physisorption nature of adsorption. The low cost, large-scale production, high adsorption capacities (qmax@RhB: 780 mg.g-1; qmax@MB: 800 mg.g-1), easiness of regeneration, biocompatibility and high magnetization value (38.16 emu.g-1) may explore the synthesized PVA/Cs/Agr@Clay/Fe3O4 nanohydrogels as potential scavenger of organic dyes. KEYWORDS: Nanohydrogels, Cloisite®30B, XPS, Adsorption, Dye removal. 1

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1. INTRODUCTION Water contamination with potent synthetic dyes from various industrial sectors1-3 has recently earned a global recognition as harmful threats to the aquatic organisms and human health. It is because of their non-biodegradable4, carcinogenic and mutagenic attributes5. Therefore, researchers are highly motivated6 to develop cost-effective simple technique7 to treat those toxic pollutants. Among different removal strategies, adsorption is considered as the simple and best because of its low cost, ease of operation, high efficiency, regeneration and insensitivity to toxic substances

8,9

over other techniques such as ozonation,10 coagulation-flocculation,11

advance oxidation,12 photochemical degradation,13 and membrane technology.14 Apart from the choice of removal strategies, designing of low cost, eco-friendly and more efficient adsorbent is another insight of water remediation. Recently, soft and wet polymeric hydrogels16 with architectural preferences (2D/3D)15 are gained huge attention in sewage treatment along with their other applications in implantable artificial organs,17 cell encapsulation,18 drug delivery

19

and biosensors.20 It is because of their

biocompatibility, high water holding ability, stimuli responsiveness and ultralow surface frictions21 Depending on the macromolecular interactions during formation of networking structures, hydrogels are categorized as physical

22,23

and chemical gels. Chemical gels are quite

stable due to formation of irreversible covalent bonding between polymeric chains and crosslinkers24. However, the removal of unused toxic cross-linkers from the hydrogel structure is a serious issue that limits its further use in environmental aspects. Recently, anion specific gelation has grabbed the attention as an interesting way to modulate the property of hydrogels.25 Polyvinyl alcohol (PVA) is a low-cost, biodegradable and biocompatible synthetic polymer; widely known to form superabsorbent hydrogels having high elasticity and chemical resistance properties through chemical26-29/physical30 protocols. Although PVA hydrogels are 2


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frequently used in artificial organ implantation,31 wound dressings,32 making of drug delivery devices,33 textile industry

34

and coating application

35

, but their use in water remediation is

limited because of its frozen macromolecular chains.36 Hence, macromolecular PVA was grafted with κ-carrageenan to fabricate κ-carrageenan/PVA nanocomposite hydrogels for removal of cationic dyes.37 Amphiphilic graphene oxide was incorporated within PVA to remove methylene blue from aqueous environment.36 Functional PVA hydrogel for dye removal application was fabricated through the incorporation of iron oxide (Fe3O4) nanoparticles and graphene oxides.38 Polyvinyl alcohol is currently explored to form hydrogel with addition of sodium salt of sulfate (SO42-), phosphate (PO43-) and citrate (C6H8O73-) via anion induced gelation route and used for metal ion speciation.39 In particular, Na2SO4 salts are low-cost, non-toxic materials with wide applicability as kosmotropic agent in wet spinning of vinylon fibers.40 Being blessed with non-cytoxicity, high visco-elasticity, porous network, cartilage mimicking attitude, PVA-sulfate hydrogels were used in knee meniscus application41 and microorganism immobilization.42 The formation of PVA-sulfate hydrogel was demonstrated via two completely distinct approaches. The first one correlated the gelling phenomenon of PVA-sulfate as “salting out” route and the assynthesized PVA-sulfate was assigned as physically cross-linked gels,

42

whereas; in another

approach was linked with the cross-linking of SO42- and –OH groups of PVA and well supported by EDS analysis.43 Due to the presence of chemically cross-linked SO42- in PVA-sulphate hydrogel network, these can be a better choice of dye adsorbent. Herein, we introduce another two cheap and abundant biopolymers; namely chitosan and agar-agar with PVA to develop highly cross-linked hydrogel structure via SO42- induced gelation with aqueous solution of Na2SO4. Chitosan is structurally defined as copolymer of (1, 4) linked D-glucosamine and Nacetyl D-glucosamine44 and offers potential adsorbency to anionic pollutants

45, 46

in acidic

environment. Because of the high-water solubility in low pH, chitosan is modified with poly 3



acrylic acid

47

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and carboxymethyl chitosan.48 Although, the porous chitosan-sulfate bead was

demonstrated for loading and release of theophylline,49 but it is not used in water remediation. On the other hand, agar-agar (MW: 120 kDa) is a thermoreversible50 hetero-polysaccharide, comprising with agarose and agaropectin subunits, and well-known for sewage treatment as physically cross-linked hydrogel51 and with bimetallic nanoparticles,52 graphene oxide

53

etc.

Agarose subunit is distinguished with (3-6) linked-β-D-galactose and (1,4)-linked 3,6-anhydro-αL-galactose54 and show distinct gelling behavior55 in presence of SO42- ions. Therefore; for the first time, we are aiming to fabricate SO42- cross-linked tri-polymeric PVA/Cs/Agr hydrogel with single coagulating agent, Na2SO4 and explore this material for water remediation purpose. Moreover, this quick gelation approach was used to entrap two different nanostructures. The first one is organically modified cloisite®30B which is incorporated to enhance the removal efficiency of the hydrogel as these nanostructures are highly acknowledged because of their unique layered dimension56-58, low cost, high surface area (up to 750-800 m2/g) and high cation exchange capacity59. Nanoclays are widely used with polymers like cellulose,60 chitosan,61 poly acrylic acid (PAA)/polyethylene glycol (PEG) hydrogels

62

and polyvinylidene fluoride

(PVDF)/chitosan63 to remove cationic pollutants. On the other hand, magnetite nanoparticles are incorporated in PVA/Cs/Agr hydrogel structure to ensure easy removal of the adsorbent after adsorption

64,65

as these are low cost, easy to fabricate, low-toxic and strongly super-

paramagnetic66 over other magnetic spinels [M(II)Fe2O4, where, M(II) = Ni, Co, Cu & Zn]. The present article mainly focuses on the designing of biocompatible multi-component PVA/Cs/Agr@Clay/Fe3O4 nanohybrid hydrogel for the first time in aqueous phase via simple one pot synthetic protocol. Nanohydrogels are structurally characterized to correlate with their 3D rice straw patterned morphology. The prepared nanoclay decorated tri-polymeric hydrogels

4


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(PVA/Cs/Agr@Clay/Fe3O4) have behaved as the excellent superabsorbent for removal of dye pollutants.

2. EXPERIMENTAL SECTION 2.1. Materials The polyvinyl alcohol (PVA) was procured from Loba Chemie (India), whereas; chitosan powder (DA

90 %) was purchased from Fischer Scientific (Qualigens), Mumbai, India. Agar-

agar (molecular formula: (C12H18O9)n, gel strength: 716.8g/cm2,) was obtained from Sigma Aldrich Chemical Pvt. Ltd., India. Quaternary ammonium salt modified natural montmorillonite, Cloisite®30B was purchased from Southern Clay Products (Austin TX). Salts like FeCl3.6H2O (AR, 98 %), FeCl2.4H2O (AR, 99 %), and Na2SO4 (AR, 99 %) were purchased from Loba Chemie, Mumbai, India. Other reagents, like NaOH, NH3, glacial acetic acid were of analytical grade and used as such. All solutions were prepared with double distilled water. 2.2. Preparation of Magnetic Fe3O4 Nanoparticles According to the chemical co-precipitation method, 1.731 g of FeCl3.6H2O [0.06 mol/L] and 0.626 g of FeCl2.4H2O [0.06 mol/L] were dissolve in 100 mL of doubled distilled water under a nitrogen gas flow with vigorous stirring at 90oC for 1.5 h. After that 15 mL of 25 wt % aqueous solution of NH4OH was drop wise added to the reaction vessel till the color of the solution turned into brown orange to black. The prepared Fe3O4 nanoparticles were collected through a permanent magnet. The collected Fe3O4 nanoparticles were washed with distilled water for several times and dried in an oven at 80 oC for 24 h. After drying, the as-prepared Fe3O4 nanoparticles were restored in a glass container for further use. 5



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2.3. Synthesis of PVA/Cs/Agr@Fe3O4/Clay Tri-Polymeric Nanohybrids As per the procedure, 50 mL aqueous solutions of polyvinyl alcohol (PVA) (5%), chitosan (Cs) (2 %) and agar-agar (1 %) were prepared as per their solubility criterion. PVA solution was prepared with constant stirring of 2.5 g of PVA granules in 50 mL of hot distilled water, whereas; chitosan (1 g) solution was prepared with 1 % acetic acid in 50 mL double distilled water to facilitate the dissolution process. Agar (0.5 g) solution was formulated with constant stirring at 90 oC. Then the agar solution was allowed to come at 50oC, not at room temperature (< 35 oC) to avoid the helix transformation. After that 10 mL of each solution was homogeneously mixed with constant stirring (600 rpm) for 30 min to prepare 5:2:1 (w/v %) blend of PVA, Cs and Agr. On the other hand, 0.05 wt % aqueous suspension of cloisite®30B was prepared in 10 mL of double distilled water through magnetic stirring (800 rpm) for 30 minutes followed by ultrasonication (120 W/60 kHz) for 30 min. Then the as-prepared suspension of cloisite®30B was added to the tri-polymeric PVA/Cs/Agr blend and allowed to stir (600 rpm) constantly for 30 min followed by ultrasonication (120 W/60 kHz) for 1 hr. To the nanoclay dispersed tripolymeric (PVA/Cs/Agr@clay) blend, 10 mL of the ultrasonically dispersed nano Fe3O4 [0.005 (w/v) %] was added slowly and the resultant mixture was treated with ultrasonication for 1 hr to disperse the Fe3O4 nanoparticles uniformly. Just after that 2.5 (w/v) % sodium sulfate (Na2SO4) solution was rapidly mixed to the clay @Fe3O4 hybrid nano phase dispersed tri-polymeric (PVA/Cs/Agr) blend under sonication to entrap the clay platelets, as well as Fe3O4 nanoparticles within the evolved hydrogel network. After the complete formation of PVA/Cs/Agr@clay/Fe3O4 polymeric nanohybrids, these were separated out from the parent solution and washed with water for several times to remove the unreacted salts. After washing, PVA/Cs/Agr@clay/Fe3O4 polymeric nanohybrids were transferred into a hot air oven and dried for 24 h at 60 oC. With the 6


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same synthetic protocol, PVA/Cs/Agr@Fe3O4 polymeric nanohybrids were designed only without

the

addition

of

nano-clays

(cloisite®30B).

The

whole

preparation

of

PVA/Cs/Agr@Fe3O4 is shown in Scheme 1.

Scheme 1. Synthesis protocol of PVA/Cs/Agr@Clay/Fe3O4 nanohydrogels [Digital image of (a) dispersed nanoclay; (b) dispersed clay/Fe3O4 in PVA/Cs/Agr phase; (c) settled and (d) magnetically separated PVA/Cs/Agr@Clay/Fe3O4 nanohydrogels].

2.4. Standard Techniques Used The surface morphology of the PVA/Cs/Agr@Clay/Fe3O4 nanohydrogel was examined with the help of scanning electron microscope (SEM) (Hitachi SU3500, Japan, 5kV). The distribution of clay platelets and Fe3O4 nanoparticles within PVA/Cs/Agr hydrogel was examined using transmission electron microscope (TEM) [JEOL JEM 6700, 120 kV) has resulted in burning of soft polymeric phase within the tubular architectures (Figure S1). To provide direct 10


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evidence with elemental composition of PVA/Cs/Agr@Clay/Fe3O4 nanohydrogels, EDS study of the tubular phase as well as polymer wrapped particle phase is analyzed during TEM observation. The selected portion of tubular phase (Figure S2) is found to contain with C (43 %), O (44 %), Al (0.4 %), Si (1.0 %), S (3.6 %), Cu (8 %), whereas; particle phase is obtained with C (41 %), O (39 %), Fe (2.0 %), S ( 3 %), Cu (15 %). The presence of Al and Si in tubular phase is due to the presence of nanoclay and supports the fact of rolling of clay platelets during SO42induced quick gelation.

Similar type observation is reported by Liu et al.68 for chitosan-

halloysite nanocomposite. The presence of Cu in the elemental composition is due to the carbon coated copper grid, used as support of sample during TEM observation. The selected area electron diffraction (SAED) pattern of PVA/Cs/Agr@Clay/Fe3O4 nanohydrogel shows the diffraction spots related to the (220), (311), (400), (511) and (440) crystal planes of Fe3O4 NPs [Figure-2(f)]. To interpret the morphological evolution of PVA/Cs/Agr@Clay/Fe3O4 nanohydrogel as patterned rice straw, the scanning electron micrographs of PVA/Cs/Agr hydrogel is illustrated as Figure S3. The surface morphology of PVA/Cs/Agr hydrogels is appeared as porous architecture with varying pore sizes (nm to µm). The presence of extensive pores throughout the PVA/Cs/Agr hydrogel structure is an indication of effective cross-linking with sulfate ions to explore 3-D cross-linked polymeric structures. The absence of rice straw pattern in PVA/Cs/Agr hydrogel again signifies the fact that those “rice straw” structures may be attributed from the incorporation of clay platelets (cloisite®30B) or the synergistic contribution of nanoclay and Fe3O4 nanoparticles in PVA/Cs/Agr phase. To elucidate the contribution of Fe3O4 nanostructures on the rice straw structured, electron (SEM/TEM) micrographs of PVA/Cs/Agr@Fe3O4 nanohydrogels are further investigated. But the incorporation of Fe3O4 nanostructures is found to have the same characteristics as we observed in the bottom part of the PVA/Cs/Agr@clay/Fe3O4 nanohydrogels [Figure S4] and strongly correlates with the result of 11



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TEM-EDS (Figure S2). But the nanostructural Fe3O4 may have the influence to develop such rice straw architecture of nanoclay in PVA/Cs/Agr hydrogel network and to verify the fact SEM images (Figure S5) of PVA/Cs/Agr@Clay nanohydrogel are also analyzed. Interestingly, it is found that PVA/Cs/Agr@Clay nanohydrogel reveals a bundle shape of nanoclay platelets, but the structure is different from rice straw pattern as observed for PVA/Cs/Agr@Clay/Fe3O4 nanohydrogel. Therefore, nanostructural Fe3O4 may act to develop the internal hollow channel of tubular architecture of 3D rice straw pattern and then coming out at the bottom part during gelation. Although the rice straw pattern of clay platelets in tri-polymeric hydrogel network is reported for the first time, but the effects of clay platelets in evolution of various morphology of polymer nanocomposite is reported in recent literatures. The preferential location of silicate layers of cloisite®30B in polymeric blend of polylactic acid (PLA) and polybutylene succinate (PBS) and their synergistic interactions to develop different surface morphologies with respect to their fabrication techniques (melt blending, solution casting and spin casting) was evidenced by Ray and his co-workers.69 Vijayalakshmi et al.70 also reported the unique contribution of 1-alkyl3-methylimidazolium MMT clay in shish-kebab structures with poly(vinylidene fluoride) (PVDF).The possible mechanistic pathway of structural evolution of “rice straw” pattern of PVA/Cs/Agr@clay/Fe3O4 nanohydrogels is illustrated in Scheme S1. During ultrasonication, positively charged tri-polymeric (PVA/Cs/Agr) chains are interpenetrated to the gallery space of swelled clay platelets via replacing the positively charged quaternary ammonium ions from the interlayer and therefore, leading to the situation of extreme exfoliation. Then the addition of Na2SO4 solution, induce a quick gelation in the tri-polymeric (PVA/Cs/Agr) phase where, clay platelets are already in the condition of extreme exfoliation, leading to the squeezing or rolling of clay platelets in the “rice straw” pattern and we get the bundle of 3D rice straw structures of PVA/Cs/Agr@Clay/Fe3O4 nanohydrogels. At the time of gelation, homogeneously dispersed 12


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Fe3O4 nanostructures are entrapped in cross-linked hydrogel network and because of the gravitational pull, these phases are appeared at the bottom of “rice straw” architecture with a thin coverage of SO42- cross-linked PVA/Cs/Agr phase.

Figure 1. SEM micrographs (a-e) of PVA/Cs/Agr@Clay/Fe3O4 nanohydrogels with different magnification.

13



Figure 2.

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TEM micrographs (a-b) of PVA/Cs/Agr@Clay/Fe3O4 nanohydrogels; TEM

micrographs (c-e) of PVA/Cs/Agr containing Fe3O4 nanostructures of PVA/Cs/Agr@Clay/Fe3O4 nanohydrogels; SAED pattern of PVA/Cs/Agr@Clay/Fe3O4 nanohydrogels.

3.2. Structural Analysis To interpret the chemical interactions within PVA/Cs/Agr@Clay/Fe3O4 nanohydrogels, FTIR spectra of PVA, chitosan, agar-agar, cloisite®30B, nano Fe3O4 and fabricated PVA/Cs/Agr@Clay/Fe3O4 nanohydrogels are analyzed [Figure-S6 (a)]. FTIR spectrum of pure PVA shows a broad band at 3284 cm-1 due to the presence of hydrogen bonded –OH groups,71 whereas; the peak at 2930cm-1 is assigned to the C-H stretching mode. The FTIR peak at 1726 cm-1 is due to >C=O groups, whereas; the peak 1238 cm-1 is related with the C-H wagging mode.72 The C-O stretching of PVA is revealed at 1020 cm-1. Moreover; the semicrystalline nature of PVA is revealed through the peak at 1083 cm-1. In case of chitosan, broad band around 3412 cm-1 is assigned as the characteristic amino saccharide due to overlapping of -OH and -NH2 stretching vibration.73 FTIR peaks around 938 cm-1 and 1193 cm-1are highly correlated with the saccharide structure. FTIR peak at 1298 cm-1 and 1636 cm-1 of chitosan are assigned as amide I (chitin component) and amide II (chitosan component) vibrational mode, whereas; peak at 1069 cm-1 indicates the C-O stretching vibration.74 For agar-agar, the peak at 3330 cm-1 is assigned to OH groups, whereas; peak at 2933 cm-1 is assigned to the C-H stretching mode of agar. The FTIR peak at 1633 cm-1 corresponds to the stretching vibration of the conjugated peptide bond, formed by the -NH2 and acetone group in agar.75 The FTIR peak at 1369 cm-1 is due to the ester sulfate group of agar. The FTIR peaks at 1036 cm-1 and 931 cm-1 is due to the >C=O stretching group of 3,6- hydro-galactose. In FTIR spectrum of PVA/Cs/Agr@Clay/Fe3O4 nanohydrogels [Figure-S6(b)], the broad band centered at 3312 cm-1 may be ascribed as the strong interactions of -OH/-NH2 groups, contributed from PVA, chitosan and agar-agar and cloisite®30B. In FTIR 14


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spectrum of pure cloisite®30B [Figure-S6(b)], bands at 3620 cm-1 and 3395 cm-1 are assigned as the -OH stretching of the silicate and water molecules respectively.76 The peak of 3620 cm-1 is also appeared in the spectrum of PVA/Cs/Agr@Clay/Fe3O4 nanohydrogels with shifting to 3600 cm-1, implying that these Si-OH groups are in strong chemical interactions with tri-polymeric (PVA/Cs/Agr) phase and therefore, may lead to the morphological change in stacked layer conformation of nanoclay, as observed in SEM images of PVA/Cs/Agr@Clay/Fe3O4 nanohydrogels. Moreover, the Si-O bending vibrations of cloisite®30B is appeared at 1001cm-1 in the FTIR spectrum of nanohydrogels. Furthermore, the FTIR peak at 1114 cm-1 of PVA/Cs/Agr@Clay/Fe3O4 nanohydrogel supports the formation of sulphate cross-linkages with in PVA/Cs/Agr hydrogel structure. The other FTIR peaks at 2922cm-1, 2844cm-1, 1633cm-1 and 1470 cm-1of pure cloisite®30B are assigning the presence of asymmetric and symmetric stretching mode of >CH2 groups, bending vibration of H-O-H lattice water molecules and deformation vibration of ternary amine group, whereas; the peaks at 916 cm-1, 883cm-1, 796 cm-1 are related to the Al-OH and A(Mg)O groups of nanoclay. In FTIR spectrum of PVA/Cs/Agr@Clay/Fe3O4 nanohydrogel, peaks below the wavenumber of 1005 cm-1 is not clearly visible and it may be due to the rolling of nanoclay layers with tri-polymeric matrix. The presence of Fe3O4 NPs in PVA/Cs/Agr@Clay/Fe3O4 nanohydrogels are also encountered with the peak at ~560 cm-1 due to the presence of Fe-O stretching vibrations [Figure-S6 (b)].77 Therefore, the FTIR spectrum of PVA/Cs/Agr@Clay/Fe3O4 nanohydrogel assures the strong chemical interaction between PVA, chitosan and agar-agar as well as with cloisite®30B and Fe3O4 NPs in SO42- cross-linked hydrogel structure. Figure 3 (a) illustrates the XRD patterns of three polymers, PVA, chitosan and agar-agar which are allowed to interact with each other in their aqueous phase as blend and used to entrap 15



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the nanostructural cloisite®30B and magnetite (Fe3O4) in SO42- cross-linked PVA/Cs/Agr phase. According to the XRD patterns, all three polymers are assigned to have the semicrystalline structures. The prime component, PVA shows one sharp diffraction peak at 2θ value of 19.2o and another shoulder at 2θ value of 21.7o due to (101) (101) and (200) reflections respectively.36 It also depicts the atactic nature of the macromolecular chains. Ricciardi et al.78 demonstrated the structural orientation of crystallite and amorphous PVA in its physically cross-linked hydrogel structure; where, PVA crystallites were acted as the knot of the gel network with ensuring high dimensional stability and elasticity. The XRD pattern of raw chitosan shows the characteristics crystalline peaks at 2θ values of 10.1o and 20o, revealing α and β crystal form of chitosan,79 whereas; another biopolymer, agar-agar shows two distinct diffraction peaks at 2θ values of 11.27o and 23.01o, demonstrating the semicrystalline nature.80 In the XRD pattern of fabricated PVA/Cs/Agr@Clay/Fe3O4 nanohydrogels [Figure-3(b)], crystalline peaks are appeared at 2θ values of 35.25o,43.18o, 57.18o and 62.72o along with a semicrystalline phase at 2θ value of 19.70o. The crystalline phases are due to the Fe3O4 nanostructures within PVA/Cs/Agr phase. Nanostructural Fe3O4 [inset of Figure-3(b)], displays eight distinct diffraction peaks at 2θ values of 18.2o, 30.1o, 35.25o,43.18o,53.63o 57.18o, 62.72o and 74.09o, corresponding to (111), (220), (311), (400), (422), (511) (400) and (533) crystal planes, respectively of pure cubic spinel crystal structure

(JCPDS:19-0629).81

The

semicrystalline

phase

(2θ=19.70o)

of

PVA/Cs/Agr@Clay/Fe3O4 nanohydrogel may be ascribed as the combined phase of nanoclay decorated SO42- cross-linked tri-polymeric (PVA/Cs/Agr) matrix. Figure 3-(c) reveals the low angle XRD pattern of pure cloisite®30B and PVA/Cs/Agr@Clay/Fe3O4 nanohydrogels, whereas; inset of Figure 3(b) shows the wide angle diffraction patterns of pure cloisite®30B. Nanostructural cloisite®30B shows the XRD pattern similar to that of montmorillonite 18A [Na0.3(Al,Mg)2Si4O10OH2.6H2O](PDF000120219) along with another highly crystalline phases 16


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at 2θ value of 4.8o[Figure-3(c)], indicating the presence of quaternary ammonium salt with basal spacing of 1.85 nm.82 The distinct peak at 2θ value of 4.8o of pure cloisite®30B phase is not observed in case of PVA/Cs/Agr@Clay/Fe3O4 nanohydrogels, indicating the complete exfoliation or delamination of

clay platelets through interpenetration of tri-polymeric

PVA/Cs/Agr matrix followed by formation of 3-D rice straw structured within the nanohydrogel. Similar type of result is observed in our earlier studies.83 Therefore, the fabricated PVA/Cs/Agr@Clay/Fe3O4 nanohydrogels are found to have both, polymer wrapped Fe3O4 along with rice straw structured polymer linked cloisite®30B.

Figure 3. XRD patterns of (a) tri-polymeric components (PVA, chitosan and agar-agar) and (b) (WADX) PVA/Cs/Agr@Clay/Fe3O4 nanohydrogels [insets are pure nanostructural phases (cloisite®30B and Fe3O4 NPs)]; XRD pattern (SAXS) of (c) PVA/Cs/Agr@Clay/Fe3O4 nanohydrogels along with pure phase of cloisite®30B]; XPS spectrum of (d) PVA/Cs/Agr hydrogel and PVA/Cs/Agr@Clay/Fe3O4 nanohydrogels; (e) deconvoluted XPS spectrum of Fe (2p3/2) and Fe (2p1/2) peaks of PVA/Cs/Agr@Clay/Fe3O4 nanohydrogels. 17



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The X-ray photoelectron spectroscopy (XPS) is used to interpret the elemental analysis of prepared PVA/Cs/Agr@Clay/Fe3O4 nanohydrogels. In XPS survey of PVA/Cs/Agr hydrogels and PVA/Cs/Agr@Clay/Fe3O4 nanohydrogels [Figure-3(d)], the presence of S (2p) at binding energy (Eb) of 163.8 eV implies the S-O binding mode

84

of SO42- in the cross-linked state with

tri-polymeric PVA/Cs/Agr phase. The XPS peak at 532.3 eV is due to the presence of O(1s), signifies the presence of –OH functionality of PVA, chitosan and gar-agar in PVA/Cs/Agr hydrogel. The O (1s) peak for PVA/Cs/Agr@Clay/Fe3O4 nanohydrogel indicates the presence of -OH groups as discussed for PVA/Cs/Agr hydrogel as well as the –OH and metal oxides of alumino silicate (cloisite®30B).85 The photoelectron peaks of Si (2p) [Eb=102.4 eV] and Al (2p) [Eb= 74.8 eV] are precisely observed in PVA/Cs/Agr@Clay/Fe3O4 nanohydrogel due to the presence of alumino-silicate frame-work of cloisite®30B.

The N (1s) at 399.8 eV of

PVA/Cs/Agr hydrogel may be ascribed as the amino phase of chitosan in PVA/Cs/Agr phase; whereas; for PVA/Cs/Agr@Clay/Fe3O4nanohydrogel, N (1s) peak is related to the quaternary ammonium ions of cloisite®30B as well as the amino phase of chitosan. The XPS peak at 285.6 eV for both hydrogel and nanohydrogel ascribing the presence of C-C and C-O bonds. The PVA/Cs/Agr@Clay/Fe3O4 nanohydrogel also shows the signature XPS peaks of Fe3O4 nanostructural phase at 710.72 eV and 724.2 eV assigning Fe (2p3/2) and Fe (2p1/2) respectively. The presence of Fe2+ and Fe3+ in Fe3O4 is further evidenced by the deconvolution of Fe (2p3/2) into two peaks at 710.78 (Fe2+) and 713.18 (Fe3+) [Figure-3(e)].86 3.3. AFM Analysis The

surface

roughness

of

PVA/Cs/Agr@Fe3O4

and

PVA/Cs/Agr@Clay/Fe3O4

nanohydrogels are further examined through atomic force microscopy (AFM). The surface 18


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roughness values of the nanohydrogels are estimated from the height of the AFM peaks in out of plane (z) direction via AFM software. From the 3D AFM images [Figure-S7], the surfaces of the prepared PVA/Cs/Agr@Clay/Fe3O4 nanohydrogels [Figure-S7(e)] are appeared with more elevated peaks as compared to PVA/Cs/Agr hydrogels [Figure-S7(a)] and PVA/Cs/Agr@Fe3O4 nanohydrogels [Figure-S7(c)] and it may be due to the rice straw pattern of the rolled nanoclay containing sulfate cross-linked PVA/Cs/Agr matrix. Figure S7-(b), (d) and (f) are the respective roughness

profiles

of

PVA/Cs/Agr

hydrogel,

PVA/Cs/Agr@Fe3O4

and

PVA/Cs/Agr@Clay/Fe3O4 nanohydrogels. The software processed roughness for PVA/Cs/Agr hydrogel and PVA/Cs/Agr@Fe3O4, PVA/Cs/Agr@Clay/Fe3O4 nanohydrogels are evaluated as 74.8 nm, 136.2 nm and 212.5 nm respectively. The enhanced surface roughness of both the nanohydrogels as compared to the PVA/Cs/Agr hydrogel can be described as the increase in surface area, leading to high adsorption capacity. Similar result has been reported in earlier report

87

during the study of swelling characteristics of chitosan and carboxymethyl chitosan

(CMC) grafted sodium acrylate-co-acrylamide with reinforcement of nanoclay towards dye removal applications. The enhancement in surface roughness of PVA/Cs/Agr@Clay/Fe3O4 nanohydrogels reveals the strong interfacial interactions between tri-polymeric matrix and nanostructured fillers which in turn reflect the high thermal stability of nanohydrogels (Shown in Figure S8) as compared to individual polymers. 3.4. Rheology Measurements Rheological measurements of the hydrogel and nanohydrogels have been performed to interpret their visco-elastic phenomenon. The variation of storage modulus (G/) and loss modulus (G//)

of

PVA/Cs/Agr

hydrogels,

PVA/Cs/Agr@Fe3O4

and

PVA/Cs/Agr@Clay/Fe3O4

nanohydrogels are measured in frequency sweep experiments and shown in Figure 4-(a) and (b). 19



Page 20 of 49

Figure-4 (a) and (b) depicts the higher values of G/ as compared to G// over the entire frequency range for all three sulfate (SO42-) cross-linked polymeric hydrogel and nanohydrogels. Moreover, higher values of storage modulus for PVA/Cs/Agr@Fe3O4 and PVA/Cs/Agr@Clay/Fe3O4 nanohydrogels as compared to PVA/Cs/Agr hydrogels are due to the incorporations of layered nanoclays and Fe3O4 NPs in hydrogel networks. This signifies the more elastic response with more compact gel network. The frequency dependency of G/ for SO42- cross-linked PVA/CS/Agr hydrogel is similar to that of PVA-sulfate hydrogels

39

of earlier reports. Strong frequency

dependence in storage modulus plot of PVA/CS/Agr hydrogel reveals the low tolerance by external factors.88 With incorporation of nanoclay and Fe3O4 NPs in PVA/Cs/Agr hydrogel network, the frequency dependency is lowered with an indication of stable hydrogels, having high tolerance of external factors. The loss factor (tanδ) is considered as the ratio of dissipated energy (G//) to stored energy (G/) during the stress deformation of gel materials. It depicts the damping behavior of the material and inversely proportional to the gel network. In fact, for a perfect chemical network structure, the loss factor should approach zero. The finite value of tanδ is related to the elastic defects due to the random existence of looped chains and dangling chains in hydrogel structure.89 Figure-4 (c) represents the lower value of tanδ [0.25-0.45] over the entire frequency range and thereby, indicating a chemically cross-linked gel network. The complex viscosity (η*) is also plotted as a function of frequency [Figure -4(d)]. For an ideal chemical gel, logη* is a linear function of logω by the slope of -1, indicating an infinite relaxation time as compared with the experimental time frame.89 According to the complex viscosity vs frequency plots [Figure 4(d)] of PVA/Cs/Agr hydrogel, PVA/Cs/Agr@Clay and PVA/Cs/Agr@Clay/Fe3O4 nanohydrogels, it is clearly observed that variation in complex viscosity approaches to almost linear with incorporation of layered nanoclay and Clay@Fe3O4 nanostructures in hydrogel

20


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network,

revealing

more

cross-linked

gel

structures

of

PVA/Cs/Agr@Clay

and

PVA/Cs/Agr@Clay/Fe3O4 nanohydrogels as compared to PVA/Cs/Agr hydrogel.

Figure 4. Storage modulus (a), loss modulus (b), loss factors (c) and complex viscosity (d) of PVA/Cs/Agr hydrogel, PVA/Cs/Agr@clay and PVA/Cs/Agr@Clay/Fe3O4 nanohydrogels 3.5. Kinetic Study of Dye Removal Efficient adsorbents are defined with their excellent adsorption capacities in short period time, resulting from their rapid interactions with model dye pollutants and capability of working in severe conditions. The study of kinetic behavior is an essential path to realize the adsorption interactions. The variation of adsorption capacity (qt, mg/g) of PVA/Cs/Agr@Fe3O4 and PVA/Cs/Agr@Clay/Fe3O4 nanohydrogels (Adsorbent dosage: 0.01 g) towards model dye pollutants (RhB/MB, Co~10 mgL-1) at 298 K (pH 7.0) are shown in Figure-5 (a) and (b) along with their ability to % of dye removals as insets of Figure-5 (a) and (b). It reflects the high dye (RhB/MB) adsorption capacity as well as high % of dye removal for fabricated 21



Page 22 of 49

PVA/Cs/Agr@Clay/Fe3O4 nanohydrogels as compared to PVA/Cs/Agr@ Fe3O4 nanohydrogels. Moreover, with incorporation of cloisite®30B the equilibrium time is found to be shortened as compared to PVA/Cs/Agr@ Fe3O4 nanohydrogels. The fast removal of RhB and MB by PVA/Cs/Agr@Clay/Fe3O4 nanohydrogels is illustrated in Figure 5 (c) and (d). Moreover, the adsorption capacities of PVA/Cs@Clay/Fe3O4 and PVA/Agr@Clay/Fe3O4 nanohydrogels are compared with PVA/Cs/Agr@ Clay/Fe3O4 nanohydrogel for the removal of rhodamine B [Figure S9 (a)] and methylene blue [Figure S9 (b)] with initial dye concentration of 10 mg/L, adsorbent dosage 0.01 g and pH of 7.0. It is interestingly observed that the adsorption capacity of PVA/Agr@Clay/Fe3O4 nanohydrogel is higher as compared to PVA/Cs@Clay/Fe3O4 nanohydrogel and it is because of the more surface functionality towards active dyes. In case of chitosan-sulfate containing hydrogel, i.e. PVA/Cs@Clay/Fe3O4 nanohydrogel the negative polarity of the cross-linker SO42- ions are ionically engaged with positively charged amine functionality of chitosan and therefore, less available surface functionality to the cationic dyes. The variation of adsorption capacity (qt, mg/g) of PVA/Cs/Agr@Clay/Fe3O4 nanohydrogels towards model dye pollutants (RhB/MB) with contact time (t, min) reveals two step adsorption process. The adsorption removal is very fast at initial 10 min and 5 min for RhB and MB respectively and it removes almost 58% and 63.5% of the dyes [Co(RhB/MB) ~10 mgL1

; adsorbent dosage 0.01 g]. The higher adsorption rate of initial step is probably due to the

presence of more active sites of 3-D rice straw structured PVA/Cs/Agr@Clay/Fe3O4 nanohydrogels to bind with the dye molecules. In second step of adsorption, the removal efficiency with respect to contact time (min) is increased slowly and finally approaches to the equilibrium phase within 30 to 40 min. This fact is attributed to the decreasing in the number of

22


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free adsorption sites on the surface of PVA/Cs/Agr@Clay/Fe3O4 nanohydrogels for incoming dye pollutants with progression of time.90

Figure 5. Variation of adsorption capacity with contact time for removal of (a) RhB [10 mg/L] and (b) MB [10 mg/L] by 0.01 g of PVA/Cs/Agr@Fe3O4 and PVA/Cs/Agr@Clay/Fe3O4 nanohydrogels [pH:7.0] (insets are % of dye removals (RhB/MB) by 0.01 g of PVA/Cs/Agr@Fe3O4 and PVA/Cs/Agr@Clay/Fe3O4 nanohydrogels); UV-visible spectrum of (c) PVA/Cs/Agr@Clay/Fe3O4 nanohydrogels (0.01 g) for removal RhB[10 mg/L] and (d) MB [10 mg/L] at pH 7.0 and temperature 298 K.

23



Page 24 of 49

Figure 6. Variation of adsorption capacity of PVA/Cs/Agr@Clay/Fe3O4 nanohydrogels with contact time for (a) RhB and (b) MB, having different initial concentrations (10 mg/L, 20 mg/L, 30 mg/L, 40 mg/L and 50 mg/L); % of dye removals (Rh/MB) with respect to (c) initial dye concentration and (d) adsorbent dosage.

The effect of initial dye concentration (RhB/MB) on the adsorption behavior of PVA/Cs/Agr@Clay/Fe3O4 nanohydrogels is elucidated in Figure 6-(a) and (b). At 298 K, 0.01 gm of PVA/Cs/Agr@Clay/Fe3O4nanohydrogelis used in adsorption experiment with 200 mL of RhB/MB having concentrations 10 mgL-1, 20 mgL-1, 30 mgL-1, 40 mgL-1and 50mgL-1 respectively. As shown in Figure 6-(c), although the % of dye removals are found to decrease with increasing the initial concentrations, the adsorption capacity of PVA/Cs/Agr@Clay/Fe3O4 nanohydrogels is increased and it is due to the increase in the driving force of mass transport and 24


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also the probability of collisions between dye pollutants and active sites on the surface of PVA/Cs/Agr@Clay/Fe3O4 nanohydrogels.91 Figure-6(d) shows the % of dye removals [C0 (RhB/MB) ~50 mgL-1] with different adsorbent dosage [0.01g, 0.03g, 0.05g and 0.1g], revealing the increase in removal % with increasing the adsorbent dosage, but the insight analysis signifies the decrease in adsorption capacity with respect to the adsorbent dosage. This is due to the decrease in the exposed surface area of the adsorbent with increasing the adsorbent dosage and therefore, leading to the decrease in the value of adsorption capacity. To further investigate the adsorption mechanism and its potential rate controlling steps, such as mass transfer, diffusion control and chemical reactions, the experimental kinetic data of adsorption process are fitted with different kinetic models including pseudo first order (PFO) [Figure-S10 (a) and Figure-S11 (a)], pseudo second order(PSO) [Figure-S10 (b) and Figure-S11 (b)], and Elovich models [Figure-S10 (c) and Figure-S11 (c)]92 with linear expressions given as

( − ) = − ………..(3)

=

+

…………(4)

= ( ) + ( )……..(5)

Where, qe and qt are the adsorption capacity (mg/g) of the model dyes on to the prepared nanohydrogels at equilibrium and a given time t (min). The k1 (1/min) is the pseudo first order rate constant of the adsorption process, k2 (g/mg.min) is the rate constant for 2nd order reaction process. The value of k1 and k2 and qe is determined from the slope and intercept of pseudo first order and pseudo 2nd order models. For Elovich kinetic model equation, α is the initial adsorption 25



Page 26 of 49

rate (mg.g-1.min-2) and β is the desorption constant (g mg-1.min-1) related to the extent of surface coverage and activation of chemisorptions. The value of α and β can be calculated from the plot of qt vs lnt. The summary of estimated kinetic parameters along with regression coefficient (R2) is provided in Table S1 and Table S2. A significant difference in calculated and experimental values of equilibrium adsorption capacity (qe) as well as the low co-relation coefficients (R2~0.93 to 0.96) of the linear regressions for both the dye adsorption rely on the poor fitting of pseudo first order model with the present investigation [Figure-S10 (a) and Figure S11 (a)]. Among different kinetic models, adsorptions of RhB/MB on to the surface of PVA/Cs/Agr@Clay/Fe3O4 nanohydrogels are in well correlation with the pseudo second order (PSO) kinetic model [Figure S10 (b) and Figure S11 (c)] with respect to the coefficient of regression (R2~ 0.999) as well as calculated qe (mg/g). It reveals that the rate of the adsorption process is limited by the diffusion of model dye pollutants (RhB/MB) within the macro-porous architecture of straw shaped cloisite®30B, entrapped within the SO42- cross-linked PVA/Cs/Agr tri-polymeric phase, where strong electrostatic interaction are taking place between the adsorbent and adsorbate. Similar type of result is also reported during removal of dye molecules with chitin hydrogels93 and reduced graphene oxide.94 For adsorption of MB onto the surface PVA/Cs/Agr@Clay/Fe3O4 nanohydrogels, Elovich model is slightly favor over with R2~ 0.98 to 0.99 due to stronger chemical interactions with adsorbent and molecular cation of MB over RhB. Intra-particle diffusion (IPD) model of Weber-Morris is further evaluated two understand the external diffusion process deeply. Model is assumed to have three stages, diffusion to boundary layers, inter particle diffusion and dye adsorption is compared with the experimental values. The overall diffusion can be described as 52

= !" / + ! ……….(6) 26


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Where, Kid is the intra-particle diffusion constant (mg/g.min1/2); qt is the adsorption capacity (mg/g) at time t (min) and Ci is the intercept at stage i. As shown in Figure S10-d and S11-d, the plots of adsorption capacity (qt) with square root of contact time (t0.5) show at least two steps involved during adsorption before reaching to equilibrium phase as the fitted plots are appeared with two linear sections with co-efficient of determinations, R2~0.98 to 0.99 for RhB and R2~0.95 to 0.97 for MB. As shown in Figure-S10 (d) and Figure S11 (d), in first step, dye molecules

are

penetrating

through

the

macro-pores

of

3D

straw

structured

PVA/Cs/Agr@Clay/Fe3O4 nanohydrogels and saturate the external surface (inner and outer layer of the tubes) until to the near equilibrium phase and then the pores are diminished resulting in the decrease of the diffusion rate of the dyes. As the fitted lines of IPD model are not originated from origin, this model fails to describe the adsorption mechanism solely. Figure S-12 (a), (b), (c) and (d) represent the comparative PFO, PSO, Elovich and intra-particle diffusion model for RhB and MB. However, the present adsorption may be ascribed as the pseudo 2nd order process predominantly along with the slight contribution from the other two models, Elovich and intraparticle diffusion model. The present observation is in accordance with the earlier report.95 3.6. Adsorption Isotherms of Dye Removal To interpret the molecular level distribution of model dye pollutants (RhB/MB) onto the surface of the fabricated PVA/Cs/Agr@Clay/Fe3O4 and also to depict the interface interactions experimental results are analyzed with fitting of four different adsorption isotherms, namely Langmuir, Freundlich, Temkin and Elovich adsorption models. The mathematical expressions of Langmuir, Freundlich, Temkin and Elovich adsorptions 96, 97 are shown in their linear forms with equation 7, 8, 9 and 10 respectively.

27



=

%$+

$% &

Page 28 of 49

……………..(7)

= ' + …….. (8)

= ( + ( … … … . (*) ( ) … … … . . () + , - = ( . ) − Where, Ce (mg/L) is the equilibrium dye concentration in solution, qe (mg/g) is the equilibrium adsorption capacity. qmax (mg/g) is the maximum adsorption capacity. The constants KL(L/mg), KF((mg/g)(L/mg)1/n), Kt (L/mg) and KE are isotherm constants, related to Langmuir, Frieundlich, Temkin and Elovich adsorption models, respectively. The factor 1/n is known as heterogeneity factor in Frieundlich adsorption, whereas; another constant B of Temkin adsorption model is related to the heat of adsorption. The characteristics of different adsorption models are discussed in section S2 of supporting information. For a better clarity and comparison, different adsorption parameters like qmax (mg/g), KL(L/mg), KF((mg/g)(L/mg)1/n), Kt (L/mg) and KE are calculated from Figure S13-(a), Figure S13-(b), Figure S13-(c), Figure S13(d) and enlisted in Table S3. Figure S13 (a) represents the plot of Ce/qevs Ce related to Langmuir adsorption isotherm, whereas; the plot of lnqevs lnCe is drawn for Frieundlich isotherm fitting [Figure S13 (b)]. Figure S13 (c) and Figure S13 (d) are the plots of qe vs lnCe and ln (qe/Ce) vs qe, related to Temkin and Elovich adsorption models. As shown in Figure S13, the experimental results of our present work are in good agreement with Langmuir adsorption isotherm as compared to other adsorption models because of high correlation co-efficient (R2) values of 0.996/0.998 and therefore, depicts the monolayer 28


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coverage of model dye pollutants (RhB/MB) with maximum adsorption capacities of more than 800 mg/g for RhB and MB dyes onto the surface of PVA/Cs/Agr@Clay/Fe3O4 nanohydrogels. For a better insight to the adsorption ability of PVA/Cs/Agr@Clay/Fe3O4 nanohydrogels, we compare our results with some previously reported adsorbents that have been used for removal of RhB and MB along with maximum adsorption capacity and other optimal conditions are listed in Table 1. In compare to other adsorbents, our PVA/Cs/Agr@Clay/Fe3O4 nanohydrogels are observed to have higher values of adsorption capacity and efficiency in removal of cationic dyes. Our synthesized PVA/Cs/Agr@Clay/Fe3O4 nanohydrogel is formed due to the SO42cross-linking in PVA, chitosan and agar agar in their tri-polymeric blend. PVA macromolecules are covalently linked with SO42- and provide a negatively charged environment, whereas; cationic chitosan is ionically linked with the SO42- ions and provide oxygen functionality for Hbonding interactions with the cationic dyes. Additionally, the agar-agar counter-part in tripolymeric blend has undergone to “salting out” process in presence of kosmotropic salt, Na2SO4 and precipitated as self-assembled hydrogel in tri-polymeric network and provides huge –OH functionality for hydrogen bonding with upcoming dyes, i.e. each polymeric component in SO42cross-linked condition provide better platform for dye adsorption. Moreover, the nanostructural clay, cloisite®30B are well known for their high cation exchange capacity and widely used for removal of cationic dyes. Therefore, each component of the nanohydrogel has higher affinity for cationic dyes and may synergistically involve for higher dye adsorption behavior of PVA/Cs/Agr@Clay/Fe3O4 nanohydrogel over other reported nanohydrogels

29



Page 30 of 49

Table 1 Comparison of PVA/Cs/Agr@Clay/Fe3O4 Nanohydrogels with Recent Reported Adsorbents, Used for Removal of RhB and MB.

Adsorbent

Dye

Time

pH

Reference

Rh-B

Adsorption Capacity(mg/g) 258.76

Fe-MMT

-- hr

4

98

Gy-cl-P(AA-co-AAm)/Fe3O4

Rh-B

654.87

24 hr

6.5

99

G/ Fe3O4 nanocomposites hydrogel

Rh-B

Three-Dimensional Rice Straw-Structured Magnetic Nanoclay

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