Carboxymethyl

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Materials and Interfaces

Layer-Structured POSS-modified Fe-Aminoclay/Carboxymethyl Cellulose Composite as a Superior Adsorbent for the Removal of Radioactive Cesium and Cationic Dyes Muruganantham Rethinasabapathy, Sung-Min Kang, Ilsong Lee, Go Woon Lee, Seung-Kyu Hwang, ChangHyun Roh, and Yun Suk Huh Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b02764 • Publication Date (Web): 25 Sep 2018 Downloaded from http://pubs.acs.org on September 25, 2018

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Industrial & Engineering Chemistry Research

Layer-Structured

POSS-modified

Fe-Aminoclay/Carboxymethyl

Cellulose Composite as a Superior Adsorbent for the Removal of Radioactive Cesium and Cationic Dyes Muruganantham Rethinasabapathy,a Sung-Min Kang,a Ilsong Lee,a Go-Woon Lee,a,b Seung Kyu Hwang,a Changhyun Roh,c,d,* and Yun Suk Huha,e,* a

Department of Biological Engineering, Biohybrid Systems Research Center (BSRC), Inha University,

100 Inha-ro, Incheon, 22212, Republic of Korea. b

R&D Platform Center, Korea Institute of Energy Research (KIER), 152 Gajeong-ro, Daejeon, 34129,

Republic of Korea. c

Biotechnology Research Division, Advanced Radiation Technology Institute (ARTI), Korea Atomic

Energy Research Institute (KAERI), 29 Geumgu-gil, Jeongeup-si, Jeonbuk, 56212, Republic of Korea d

Radiation Biotechnology and Applied Radioisotope Science, University of Science and Technology

(UST), 217 Gajeong-ro, Daejeon 34113, Republic of Korea. e

WCSL of Integrated Human Airway-on-a-Chip, Inha University, 100 Inha-ro, Incheon, 22212, Republic

of Korea.

* Corresponding authors. E-mail: [email protected] (Y. S. Huh) E-mail: [email protected] (C. Roh)

ACS Paragon Plus Environment

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

ABSTRACT In this work, multifunctional Fe-aminoclay (FeAC)/carboxymethyl cellulose (CMC)/ polyhedral oligomeric silsesquioxane (POSS) composite (FeAC/CMC/POSS) with layered structure was successfully synthesized and utilized as adsorbent for the removal of cesium ions (Cs+) and cationic dyes methylene blue (MB) and chrysoidine G (CG) from aqueous solutions. The FeAC/CMC/POSS exhibit excellent adsorption capacities for Cs+ ions, MB and CG of 152, 438 and 791 mg g-1, respectively. The adsorption capacities for Cs+ ions, MB and CG are substantially greater than those of many previously reported adsorbents due to (i) the layered morphology of the composite and abundance of amino (–NH2) groups on clay surface; (ii) existence of carboxylate (–COO-) and hydroxyl (–OH-) groups on the CMC backbone, which contribute to the adsorption of large number of Cs+ ions and dye molecules through electrostatic attraction and ion exchange process. More importantly, the incorporation of POSS increases the interlayer spacing of Fe-aminoclay by intercalation providing room for the encapsulation of Cs+ ions and dye molecules. Owing to its unprecedented adsorption capacity, the devised FeAC/CMC/POSS composite could be a promising organic-inorganic material used to costeffectively remove the multitude of environmental pollutants.

Keywords: Fe-aminoclay; Cationic dyes; Cesium adsorption; Intercalation; Layered structure.


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1. Introduction

Environmental pollution is one of the greatest problems which causes irreparable damage to the earth.1 In recent years, water pollution caused by heavy metal ions, organic dyes and radionuclides has become a serious environmental hazard. In order to address global warming and to satisfy the increased demand for energy, nuclear power is being considered as a primary source of power due to its carbon-free energy footprint. However, despite its advantages, the uncontrolled release of radionuclides from nuclear power plants is perceived as a major threat to the environment. Of these radionuclides, cesium (137Cs) poses threat to humans and ecosystem due to its long half-life (30.2 years) and high water solubility.2-5 The other major environmental issue concerns the short- and long-term ecologic damages caused by the non-degradable complex-structured aromatic dye molecules such as methylene blue (MB) and chrysoidine G (CG) due to their mutagenic, carcinogenic and toxic properties.6-8 Methylene blue, belongs to the thiazine class of metachromic cationic dye, which degrade the aesthetic nature of water even at a low concentration (1 mg L-1) and inflict life threatening disease.9-11 CG is another toxic synthetic azo-dye widely used in the textile industry which has been recognized as carcinogen and the usage of which in food has been banned in many countries.12-14 Various techniques such as chemical oxidation, electrochemical processes, biodegradation, and membrane separation have been employed to remove hazardous inorganic (heavy metals)/organic (dyes) and radioactive nuclides from the environment and of those techniques, adsorption is the most efficient found to date, as it allows physiochemical separation with low-energy requirements, high efficiency, and ease of operation.9,15 In recent years, many adsorbent materials such as metal oxides, activated carbons, zeolites, natural clay minerals and zeolites have been investigated for the removal of dyes and radionuclides from waste water but their adsorption capacities are low with limitation



to adsorb some specified contaminants.16,17 So, the researchers have been developing some novel adsorbent materials with high adsorption capacity and potential to capture range of pollutants from the environment as a primary need. Recently, the clay minerals like montmorillonite and bentonite are emerging as novel environmental remediation adsorbents due to their low-cost, high adsorption capacity, abundancy and eco-friendliness.18-20 Among the reported clay materials, Fe-aminoclay (FeAC) comply with striking features of specific layered structure, less toxicity, high chelating capacity with metal, high cation-exchange capacity, excellent swelling ability, and ease of modification.21 More importantly, the Fe-aminoclay allows much more molecules to intercalate into the interlayer space due to its expandable properties and expected to exhibit high adsorption capacity. But it is indispensable to exploit some modifications in clay materials in order to improve their adsorption capacity and affinity towards specific adsorbates which is possible by designing claypolymer composites.8,22,23 The polymer-clay composites are cheap and have good dispersion of layered clay structure within the polymer matrix along with large adsorption capacity, mechanical stability and thermal property. Carboxymethyl cellulose (CMC) is a biodegradable artificial-natural polymer which is derived from cellulose and with excellent biocompatibility. Upon integrating clay materials with CMC, the carboxylate ions (COO-Na+) present in the backbone of CMC act as ion-exchange sites and involves in the adsorption of Cs+ ions, heavy metals and cationic dyes through cationic exchange with Na+ ions.2 We can further tailor-made the clay-polymer composites with organo-modifiers to achieve dispersion and exfoliation of clay in the polymer matrix. The modification expands the interlayer space of the clays which facilitates the entry of polymer molecules, dyes, heavy metals and radionuclides. Polyhedral oligomeric silsesquioxane (POSS) is one such nanostructured


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inorganic-organic molecules comprised of an inorganic silica-like core (Si8O12) surrounded by eight corner groups, which can be amines, esters, alcohols, isocyanates, and silanes.24-27 Cubic structured POSS is biocompatible, thermally stable (>400 °C) and has excellent recyclability. Thus, the POSS-modified clay-polymer composite is expected to have excellent adsorption capacity due to (i) the increase in inter-lamellar spaces which allows more Cs+ ions and dye molecules to be encapsulated; and (ii) increase of surface area by good dispersion of clay particles. In this study, the adsorption capacities of layer-structured FeAC/CMC/POSS composite towards Cs+ ions, MB and CG are investigated along with kinetics of adsorption in detail. The materials were characterized by XRD, FTIR, SEM, TEM, XPS and TGA. Adsorption capacities were determined using the Langmuir and Freundlich adsorption models, and adsorption kinetic order were determined.

2. Materials and methods 2.1. Chemicals 3-Aminopropyltriethoxysilane (APTES, 99%), iron (III) chloride, sodium carboxymethyl cellulose (CMC, MW, 90000) and cesium chloride (CsCl) were purchased from Sigma-Aldrich (USA) and used without further purification. The cationic dyes methylene blue (MB; C16H18ClN3S; molecular weight: 319.85 g mol-1; λmax = 664 nm) and chrysoidine G (CG or basic orange 2; C12H12N4·HCl; molecular weight: 248.71; λmax = 449 nm) were also obtained from Sigma-Aldrich (USA) and their structures are provided as Figure S1a, b. Stock solutions of MB and CG were prepared by dissolving 2000 mg MB or CG in a liter of water. The organic modifier, aminopropylisooctyl polyhedral oligomeric silsesquioxane (POSS), was supplied by



Sigma-Aldrich (USA). Sodium hydroxide and hydrochloric acid obtained from Merck (Darmstadt, Germany) were used to adjust solution pH values. All chemicals used in this study were of analytical grade.

2.2. Characterization XRD (X-ray diffraction) patterns were obtained using a Bruker D2 PHASER (Germany) diffractometer and Cu Kα radiation. XPS (X-ray photoelectron spectroscopy) was performed using a Thermo Scientific, K-Alpha electron spectrometer equipped with an Al X-ray source (XPS peaks were fitted with CASA XPS software and binding energies were obtained with respect to C 1s at 284.6 eV). FTIR spectra were obtained using a Jasco FT/IR-6600 unit using KBr pellets from 4000 to 400 cm-1. SEM images which were used to investigate the morphologies of samples were obtained using an S-4800SE microscope at an acceleration voltage of 15 kV. TEM was performed using a Tecnai G2 (FEI, the Netherlands) unit equipped with EDS, at an accelerating voltage of 200 kV. TGA was performed using a Tarsus TG 209 F3 from room temperature to 1000 °C at a heating rate of 10 °C/min in a nitrogen atmosphere. ICPMS (inductively coupled plasma mass spectrometry) was carried out using a PerkinElmer ELAN6100 and was used to measure metals. pH values were measured using a JENWAY3510 pH Meter.

2.3. Adsorption experiments Cesium ions (Cs+) and two cationic dyes (MB and CG) were used to evaluate the adsorption characteristics of FeAC/CMC/POSS composite. Adsorption isotherms were obtained by performing batch experiments at different adsorbate and solution pH values. Initial Cs


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concentrations were varied from 1 to 500 mg L-1. A fixed amount of adsorbent (20 mg) was added to 20 mL of an aqueous cesium solution and shaken at 40 rpm using a rotary shaker for 24 h when the equilibrium had been achieved. Adsorbents were then withdrawn, separated by filtration, and cesium concentrations were analyzed by ICP-MS. For dye adsorption experiments, 20 mg of the FeAC/CMC/POSS was dispersed in 20 mL of MB or CG solutions of concentrations 1 to 3000 mg L-1 with mild stirring. Adsorption capacities at equilibrium were calculated using the following equation (Eq. 1)

= ( − )

(1)

Where qe is equilibrium adsorption capacity (mg g-1) at t min, C0 and Ct are the concentrations of adsorbate (mg L-1) after 0 and t min, respectively, V is solution volume (mL), and m is adsorbent mass (g). The experiments were conducted by varying pH, dose rate and contact time. The pH values were varied between 5 and 11 using dilute HCl and NaOH for adjustment.

2.4. Kinetics experiments Kinetic tests were conducted to evaluate the effects of time on adsorptions and to quantify adsorption rates. The adsorption kinetics of FeAC/CMC/POSS for Cs+ ions, MB and CG were investigated at room temperature in a thermostatic shaker. Briefly, 20 mg of FeAC/CMC/POSS powder was added to 10 mL of Cs (50 mg L-1), MB (100 mg L-1), or CG solution (100 mg L-1). Analytical samples were extracted at pre-set times (1, 2, 5, 10, 20, 30, 60, 120, 180, 240, 300, 360, 420, and 480 min) and separated by centrifugation. Cs+ ions concentrations in water were measured by ICP-MS, whereas those of the cationic dyes (MB and CG) were measured using heights of maximum adsorption peaks of MB (664 nm) and CG (449



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nm) using a UV/vis spectrophotometer. The amount of adsorbate adsorbed at time t was calculated using the following equation:

= ( − )

(2)

Where C0 and Ct (mg L-1) are initial adsorbate concentration and adsorbate concentration at time t, respectively, V (L) is adsorbate solution volume and m (g) is the mass of adsorbent used.

2.5. Synthesis of Fe-aminoclay (FeAC) 3-Aminopropyl-functionalized iron phyllosilicate was synthesized using previously described procedure.21,28,29 Briefly, FeCl3·6H2O (24 g) was dissolved in 250 mL of ethanol in a 500 mL beaker under constant stirring for 30 min. 3-Aminopropyltriethoxysilane (32 mL) was added and the molar ratio of FeCl3·6H2O to C9H23NO3Si was adjusted to 0.7:1. The mixture was stirred for 12 h to attain equilibrium, and the precipitated AIP clay so obtained was washed repeatedly with ethanol followed by water and then centrifuged at 8000 rpm for 20 min. FeAC was obtained by heating the washed clay at 50 °C for 24 h. The unit structure FeAC consisted of a central octahedral brucite-like Fe(OH)2 structure with its top and bottom overlaid with tetrahedral silica, followed by capping with vertical layers of flexible –(CH2)3NH2 groups. The structure of FeAC clay is provided as Figure S1c.30

2.6. Syntheses of FeAC/CMC and FeAC/CMC/POSS FeAC (1 g) was dispersed in 50 mL of deionized water and stirred at 85 °C for 8 h. Separately, sodium carboxymethyl cellulose (2.0 g) was slowly added to 100 mL of deionized water in a beaker and stirred at 85 °C for 12 h. The FeAC solution was then added slowly to the


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CMC solution with stirring, which was maintained for 12 h at 85 °C. Amino POSS (0.1 g) was added slowly to the brick-red colloidal solution obtained with continuous stirring, which was continued for 12 h at 85 °C. The weight ratio of amino POSS to FeAC in the mix was maintained at 10:1. The mixture obtained was then heated to 150 °C to obtain a cake-like final product, which was calcined by heating at 250 °C in a muffle furnace, ground, washed with ethanol and distilled water several time and dried at 60 °C to produce the final product (FeAC/CMC/POSS). FeAC/CMC was synthesized in an identical manner without the amino POSS addition. The schematic of the synthesis of FeAC/CMC/POSS is shown as Scheme 1.

3. Results and discussion The textures, structures and the interlayer spacings of the as-prepared materials were determined by XRD. Figure 1a shows the X-ray diffraction patterns of CMC, FeAC, FeAC/CMC and FeAC/CMC/POSS. The XRD of CMC shows a semi-crystalline nature with peak at 2θ = 19.81°.31 The FeAC showed broad in-plane reflection peaks at 2θ vales of 6.87°, 11.31°, 22.23°, 35.56° and 59.93°corresponding to (001), (002), (020, 110), (130, 200), and (060, 330) planes. The inter planar spacings of the as-synthesized FeAC were calculated as d001 = 12.83 Å , d002 = 0.78 nm, d020, 110 = 0.39 nm, d130, 200 = 0.25 nm, and d060,330 = 0.15 nm. The low-angle inerlayer d001 reflection at 2θ = 6.87° (d001 = 12.83 Å) and the low intensity smectite reflection (060 plane) at 2θ = 60.03° was ascribed to an organic-inorganic hybrid layered clay structure of 1:1 layered phyllosilicates.21,28,32,33 The layered structure of FeAC clay was further confirmed by HR-TEM. The formation of FeAC/CMC clay-polymer composite was evident from the shifting of characteristic peaks of FeAC towards lower 2θ values after the addition of CMC. The characteristic peaks of FeAC/CMC/POSS showed distinct peak shifts toward lower 2θ values as



compared with FeAC/CMC due to the addition of POSS (Figure S2). This shifting of peak showed POSS molecules were intercalated into the inter-lamellar of FeAC and expanded interlayer spaces. After POSS modification, the basal spacing (d001 value) of FeAC increased from 12.83 Å to 14.53 Å as evidenced by shifting of its characteristic from 6.87° to 6.08°. In spectra of FeAC/CMC and FeAC/CMC/POSS, the strong diffraction peaks located at 31.6°, 45.32°, and 56.43° may be due to the formation trace amounts of crystalline Si and FeSi2 during the calcination process.34 The FT-IR spectra of CMC, FeAC, FeAC/CMC and FeAC/CMC/POSS are shown in Figure 1b. The main functional groups of CMC were characterized by peaks at 3483, 2878, and 1602 cm-1 corresponding to the hydroxyl (–OH), methylene (–CH2) and carboxylate (COO–) groups, respectively.35 The main characteristic peaks of FeAC were –OH (3451 cm-1), –CH2 (3026 cm-1), –NH3+ (2010 cm-1), –NH2 (1605 cm-1), Si–O–Si (1102 cm-1), Fe–O–Si (692 cm-1) and Si–O (478 cm-1) which are in good agreement with the previous reports.30,33 Bands due to – OH stretching vibrations for FeAC/CMC and FeAC/CMC/POSS shows variations in intensity and peaks are shifted as compared with those of FeAC and CMC due to the effect of –OH stretching vibrations of CMC. The intensity of NH3+ peak (2010 cm-1) in FeAC is found to decrease in FeAC/CMC and FeAC/CMC/POSS, indicating that the amine groups of FeAC were involved in the reaction of FeAC with CMC. Also, the addition of CMC with FeAC causes a pair of intense peaks around 2878 and 2953 cm-1 in both FeAC/CMC and FeAC/CMC/POSS which may be attributed to the symmetric and antisymmetric vibrations of the –CH2 and –CH3 by the addition of CMC.7,8,36 Absorption peaks at 1611 and 1630 cm−1 in the spectra of FeAC/CMC and FeAC/CMC/POSS were putatively attributed to carboxamide as a result of reaction between the carboxylic groups of CMC and –NH2 groups on the surface of FeAC. The intensity of


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carboxamide and Si-O-Si stretching vibrations are higher in FeAC/CMC/POSS than that of FeAC/CMC due to the presence additional –NH2 and Si-O-Si groups in POSS. This confirms the presence of POSS in the FeAC/CMC/POSS composite. SEM and TEM characterizations were used to observe the morphologies of the FeAC and its composites (FeAC/CMC and FeAC/CMC/POSS). Figure 2a shows an SEM image of FeAC. The morphology of FeAC/CMC composite shows pores (Figure 2b) at certain investigated area due to the addition of CMC with FeAC, but the majority of the portions were seen as the agglomerated sharp flakes (Figure 2c). Whereas SEM images of FeAC/CMC/POSS (Figure 2d and e) show fine layers of non-agglomerated flakes caused by the addition of POSS. The incorporation of POSS by intercalation resulted in the exfoliation of FeAC. Elemental mapping (Figure 2f) confirmed the presence of Si, N and Fe in the FeAC/CMC/POSS composite. HR-TEM images of FeAC at different magnifications are provided as Figure S3a-c. The high magnification TEM image in Figure S3c shows a typical clay structure with dense packing of exfoliated clay sheets. FeAC/CMC shows appreciable levels of exfoliation caused by CMC addition and randomly distributed and uniformly dispersed clay layers (Figure S3d-f). The FeAC/CMC had a layer structure and exhibited exfoliated layers of FeAC. HR-TEM images of FeAC/CMC showed large numbers of exfoliated clay layers and a few intercalated stacks of FeAC.37-39 Figure 3a-e shows HR-TEM images of FeAC/CMC/POSS. The incorporation of POSS resulted in better dispersion of modified clays in CMC matrix and appreciable clay exfoliation and clay platelets (Figure 3b).25 Edge view (Figure 3d and e) of the FeAC/CMC/POSS shows a clear layered structure and that highly intercalated clay sheets were arranged in regular, and uniform manner.40 The elemental mapping (Figure 3f) confirms the presence of Si, Fe, O and C in the composite.



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The chemical composition and the oxidation states of elements present in FeAC/CMC/POSS were determined by XPS. Sharp peaks in the full scan survey spectrum (Figure 4a) revealed the presence of C, O, Si, Fe, and N on the surface of FeAC/CMC/POSS composite. Six different peaks centered at binding energies of 530.39, 531.14, 532.04, 532.73, 532.86, and 535.43 eV were observed in the O 1s spectrum (Figure 4b) of FeAC/CMC/POSS and these correspond to Si–O–Si, O=C–O, SiO2, Si–OH, C–OH, and respectively.41,42

C–O–Si groups,

The C 1s spectrum of FeAC/CMC/POSS (Figure 4c) was deconvoluted to

five peaks with binding energies of 283.88, 285.55, 284.67, 286.23, and 288.0 eV, which were attributed to SiC, CN, C–C, C–O, and O–C=O groups, respectively.43 The main peaks of the Si 2p spectrum of FeAC/CMC/POSS (Figure 4d) were deconvoluted into five components at 100.73, 101.822, 102.77, 103.78 and 105.15 eV signifying the existence of FeSi2, Si–O–Si, Si– OH, SiO2 and Si–O–C, respectively.44 The Fe 2p spectrum (Figure 4e) showed peaks corresponding to Fe 2p3/2 and Fe 2p1/2 at 711.55 and 724.39 eV, respectively, which revealing the formation of Fe3O4.41,42 In the N 1s spectra (Figure 4f) of FeAC/CMC/POSS, the peaks at 399.6 eV was assigned amine (–NH2) nitrogen and that at 401.78 eV to NH3+ groups.45 The XPS spectra of FeAC/CMC is provided in Figure S4. The high-resolution O 1s and Si 2p spectra of FeAC/CMC/POSS showed significant Si–O–Si enhancement peak versus FeAC/CMC caused by the presence of large numbers of Si-O-Si groups on the surface of POSS, which confirmed the incorporation of POSS into FeAC/CMC. Also, the higher atomic percentage of Si in FeAC/CMC/POSS as compared with FeAC/CMC (Table S1) confirmed the incorporation of POSS into FeAC/CMC. The results of XPS results were in-line with our previous FTIR and XRD findings.


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TG curves of FeAC, FeAC/CMC and FeAC/CMC/POSS are given as Figure S5. Multidecomposition patterns were observed for all the three systems. Decomposition courses of all the systems showed the first major weight loss below 200 °C presumed to be due to the evaporation of adsorbed water and dehydration of water molecules absorbed in pores and between the silicate layers.46 In FeAC, the second (240 - 450 °C) and third (450 - 600 °C) stage weight loss were attributed to the degradation of aminoproyl functionalities and dehydroxylation of silanol groups, respectively.47,48 In FeAC/CMC, the incorporation of CMC slightly increases the thermal stability of FeAC and the second (270 to 400 °C) and third (> 400 °C) correspond to the thermal degradation of carboxymethyl cellulose and the evaporation of tightly bound water molecules with carboxylate groups (polymer backbone) through polar interactions, respectively.45 Interestingly, the incorporation of clay into the polymer matrix was found to enhance the thermal stability. In FeAC/CMC/POSS, the mass loss between 300 and 450 °C is attributed to the decomposition of the incorporated POSS modifier and the weight loss above 450 °C may be assigned to the decomposition of structural hydroxyl groups.25,46,49 FeAC/CMC/POSS showed much higher thermal stability than FeAC/CMC, presumably due to the molecular rigidity of POSS and the thermal stability of Si-O from POSS.25

3.1. Effect of pH The pH can influence the charge transfer between liquid and solid interface.7,50 The adsorption of Cs+ ions, MB and CG depends on the pH of the solution. The adsorption activity of FeAC/CMC/POSS was evaluated in Cs+ ions, MB and CG solutions of varying pH from 5 to 11. The FeAC/CMC/POSS shows negligible difference in the adsorption capacity of Cs+ ions over the pH range between 5 and 11. So, the Cs+ ion adsorption studies were conducted at neutral pH



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in pure water which implies that the adsorbent is capable of adsorbing Cs+ ions over a wide pH range. Besides, the FeAC/CMC/POSS composite exhibited higher adsorption towards cationic dyes (MB and CG) at higher pH value of 10 (Figure S6). This may be due to the fact that at low pH, the protonation of amino groups (present at the surface of FeAC), hydroxyl and carboxyl functional

groups

(present

at

the backbone of CMC) causes

electropositivity in

FeAC/CMC/POSS which hinders the adsorption of positively charged cationic MB and CG dyes. In contrast, at higher pH, the –NH2 on the FeAC, –OH and –COOH groups on CMC gets deprotonated and acquire net negative charge (Figure S7) which resulted in the high adsorption of positively charged cationic MB and CG dyes. Thus we conducted the dye adsorption experiments at high pH of 10.

3.2. Adsorption isotherm studies Adsorption equilibrium isotherms largely determine the distributions of adsorbate molecules between solid and liquid phases and the nature of the equilibrium state.51 The classical Langmuir52 and Freundlich53 isotherm models were used to define adsorption isotherms, and the constants afforded by these isotherms were used to predict the adsorption capacities. Langmuir isotherm provides a model of monolayer adsorption. According to this model, adsorption takes place on homogeneous adsorbent surfaces at identical sites, having equivalent energy, and equal numbers of molecules are attached to each site and adsorbate molecules do not interact.54 The linear and non-linear forms of the Langmuir isotherm30,55 are represented by

= +

(3)

=


(4)

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Where qe and qmax represent the equilibrium adsorption capacity and monolayer maximum adsorption capacity (mg g-1), respectively, and the constant KL account for affinity between the adsorbent and adsorbate. The Freundlich adsorption isotherm model is based on an empirical equation that predicts multilayer adsorption on a heterogeneous adsorption surface with unequal available sites of different adsorption energies.56 The Freundlich model is described by the following equations: ln = ln +

!" # $

% &

=

(5) (6)

KF is related to the multilayer adsorption capacity of the adsorbent, and n determines the strength of adsorption. In the present study, adsorption experiments were carried out to determine the adsorption capacities of FeAC/CMC/POSS as a function of different concentrations of Cs, MB, and CG. These adsorption experiments showed that adsorption capacity of FeAC/CMC/POSS increased with increasing Cs, MB, and CG concentrations due the availability of large numbers of active sites. Experimental data were fitted to the Langmuir and Freundlich isotherm models (Figure 5). According to the experimental data for Cs adsorption (Figure 5a), the Langmuir model provided a better fit than the Freundlich model with an R2 value of 0.99. The calculated values of the constants KF and n were 0.65 and 1.48, using Freundlich model (Table 1). The linear plots of Langmuir and Freundlich isotherm are provided as Figure S8a and d. The heterogeneity factor “n” value of 1.48 (> 1) indicated the favorable adsorption. Experimental data and isotherm analyses showed the FeAC/CMC/POSS composite had an equilibrium Cs+ ions adsorption capacity (qe) of 152.16 mg g-1 which is higher than those previously reported (Table S2).



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The components of FeAC/CMC/POSS each played a role in the adsorption of Cs+ ions through electrostatic attraction, ion-exchange and surface complexation. Cs+ ions are attracted by deprotonated amines (–NH2 on the FeAC) or hydroxyl groups (Eqs. 7 and 8) and attach to the clay by forming coordination bonds with nitrogen or oxygen atoms. The carboxymethyl cellulose present in FeAC/CMC/POSS possesses large numbers of carboxylate (COO-Na+) and hydroxyl groups (-OH) on its backbone (Eqs. 9 and 10), and these enable the adsorption of more Cs+ ions by ion-exchange and electrostatic attraction, respectively. More importantly, the unprecedented amount of Cs+ ions adsorption observed may have been caused by Cs+ ions encapsulation between FeAC interlayers due to the incorporation of POSS (Eq. 11), as the intercalation of POSS molecules into the inter-lamellar spaces expanded these spaces from 12.83 Å to 14.53 Å which is evident from the XRD analyses. These interlayer spaces provide sufficient room for the encapsulation of Cs+ ions (Eq. 11), which has a hydrated radius (0.226 nm) much smaller than the interlayer spaces of FeAC/CMC/POSS. This possibility explains why FeAC/CMC/POSS had a high adsorption capacity (152.16 mg g-1) for Cs+ ions. The mechanism of cesium adsorption is shown in Figure 6a. Fe-aminoclay–NH2 + Cs+ → Fe-aminoclay–NH-–Cs+ + H+ Fe-aminoclay–OH + Cs+ → Fe-aminoclay–O-–Cs+ + H+ CMC–COO-Na+ + Cs+ → CMC–COO-–Cs+ + Na+

(7) (8) (9)

CMC–OH + Cs+ → CMC–O-–Cs+ + H+

(10)

Fe-clay + POSS → Exfoliated Fe-clay–Cs+ encapsulation

(11)

Adsorption studies on the cationic dye adsorption studies, MB (Figure 5b) and CG (Figure 5c) showed the Langmuir isotherm model exhibited better fit with R2 values of 0.96 and 0.96, respectively. Using the Freundlich model, calculated KF and n values were 61.03 and 3.15,


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


respectively, for MB system and 98.04 and 3.55 for CG (Table 1). The Langmuir model showed FeAC/CMC/POSS had an impressive qmax values of 438.34 and 791.91 mg g-1, for MB and CG, respectively. The linear Langmuir and Freundlich isotherm models of MB and CG are provided as Figure S8b,e and S8c,f, respectively. MG and CG molecules had high affinities for FeAC/CMC/POSS and were readily adsorbed by clay suspensions. At high pH, the amino groups (–NH2) at the surface of the clays are deprotonated and acquire a net negative surface charge. These negatively charged clay surfaces attracts positively charged cationic dyes electrostatically. Furthermore, the enhanced dye adsorption capacity of FeAC/CMC/POSS composite may have been due to the conversion of siloxane in clay to negatively charged silanol group at high pH 10, and these negatively charged silanol groups would increase adsorption capacity by hydrogen bonding with the amine group of the cationic dyes. Also, at elevated pH, ionization of carboxyl groups (–COOH) in the CMC backbone would interact electrostatically with MB and CG (Figure 6a). Also, the interlayer spacing of organo-clays is also an important parameter of adsorption capacity. The intercalation of POSS molecules in FeAC/CMC/POSS composite substantially increased the interlayer spacing of FeAC as evidenced by XRD (Figure 1a) and TEM (Figure 3), and thus, enables relatively large number of MB and CG molecules to be accommodated. Thus, it appears the high adsorption capacity of FeAC/CMC/POSS may be attributed to (i) electrostatic attraction between surface functional groups (deprotonated amine and silanol groups at high pH) and cationic dyes; (ii) electrostatic attraction between cationic dyes and abundant surface carboxyl and hydroxyl groups on CMC, (iii) and additional trapping of dye molecules in the POSS-expanded interlayer spaces of FeAC (Figure 6a).



Page 18 of 39

3.3. Adsorption Kinetics studies Adsorption kinetics (Figure S9) provide an important means of determining the adsorption mechanisms and the abilities of adsorbents to removal pollutants.57,58 The pseudofirst-order kinetics significantly explains the rate controlling steps like mass transport and chemical reaction which is described by the following equation: '( '

= ) ( − )

(12)

Where qe and qt are adsorption capacities (mg g-1) at equilibrium and at time t, respectively, and k1 is the pseudo first-order rate constant (g mg-1 min-1). By integrating and applying the boundary conditions t = 0 to t and qt = 0 to qt, this equation may be written as follows: ln( − ) = ln − ) *

(13)

A straight-line plots of ln(qe-qt) versus t indicates pseudo-first-order (Figure S10), whereas a straight line plot of t/qt against t indicates pseudo-second-order (Figure 6b). Experimentally determined kinetic parameters were calculated using these plots (Figure 6b) and are shown in Table 2. Pseudo-first-order kinetic plots (Figure S10) did not fit well with the experimental results. Low R2 values were obtained for pseudo-first-order kinetics and qe values calculated based on pseudo-first-order kinetics were considerably lower than experimentally determined values. Pseudo-second-order kinetics indicate that the overall rate of adsorption is determined by chemisorption of adsorbate on adsorbent. Pseudo-second-order kinetics can be represented as follows: '( '

= )+ ( − )+


(14)

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(

= ,-

. . #

/ + * #

(15)

qe and k2 (second-order rate constant, g mg-1 min-1) values were calculated by plotting ln (qe-qt) versus t (Figure S8). The pseudo-second-order kinetics of Cs adsorption had qe and k2 values of 4.5188 mg g-1 and 0.13290 g mg-1 min-1, respectively (Table 2). The results showed the second-order model provided a better fit than pseudo-first-order model with an R2 value of 0.9999, and show FeAC/CMC/POSS adsorption rates are dependent on active sites and not on solution cesium concentration. Second-order kinetic analyses for MB and CG produced qe and k2 values of 48.5437 mg g-1 and 0.0146 g mg-1 min-1, respectively, for MB and 45.7247 mg g-1 and 0.0222 g mg-1 min-1, respectively, for CG. These results showed that the second-order kinetics provided a better fit with R2 values of 0.9999 and 0.9999 for MB and CG, respectively. In particular, these R2 values were much higher than those obtained assuming pseudo-first-order kinetics, and calculated qe values also agreed well with experimental data. Therefore, pseudosecond-order kinetics best described the adsorption behaviors of Cs, MB, and CG onto FeAC/CMC/POSS.

4. Conclusion In summary, layer-structured FeAC/CMC/POSS composite was successfully synthesized and its Cs+ ions, MB and CG adsorption capacities from aqueous solution were determined using adsorption experiments. The synthesized FeAC/CMC/POSS was characterized by FTIR, XPS, XRD, HR-SEM, HR-TEM and TGA analyses. XRD confirmed POSS intercalation increased the interlayer spacing of FeAC. FT-IR and XPS confirmed the presence of functional groups and provided the elemental make-up of FeAC/CMC/POSS. The layered morphology of FeAC/CMC/POSS and the exfoliated, intercalated structure of the clay material were visualized



by HR-TEM. Adsorption experimental data were analyzed using isotherm models and equilibrium results well fitted Langmuir isotherm predictions. FeAC/CMC/POSS composite showed maximum adsorption capacities for Cs+ ions, MB and CG of 152, 438 and 791 mg g-1, respectively. These adsorption capacities for Cs+ ions, MB and CG are substantially greater than previously reported values. The high adsorption capacities of FeAC/CMC/POSS are believed to be due to the combined effects of electrostatic attraction, ion-exchange and surface complexation attributed to its elemental and molecular make-up caused by (i) the abundance of –NH2 group on the layered clay material, (ii) the existence of –COO- and –OH groups on CMC backbone, and more importantly (iii) the incorporation POSS into the clay increased the interlayer spacing by intercalation which facilitate room for the encapsulation of large number of Cs+ ions, MB and CG. Furthermore, the results obtained from during the present study suggest a means of removing Cs+ ions and cationic dyes in an ecofriendly process. Supporting Information Chemical structures of Fe-aminoclay, MB and CG; XRD of POSS; TEM and XPS analyses of FeAC/CMC; TGA and Zeta potential analysess of FeAC/CMC/POSS; Langmuir and Freundlich linear isotherm plots of Cs, MB, and CG; Pseudo-first-order kinetic model fits for the adsorption of Cs, MB and CG; atomic percentages of various elements (Si, Fe, N, O, C, and Cs) and their peak binding energies from XPS analyses; adsorption capacities of Cs+ ions, MB and CG on FeAC/CMC/POSS and other reported materials.

Acknowledgement


Page 20 of 39

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


This

work

was

supported

by

the

Radiation

Fusion

Technology

Program

(2015M2A2A6A02045262(3)) from Nuclear Research R&D Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (MSIP), Republic of Korea. Muruganantham Rethinasabapathy gratefully acknowledges financial support from the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (NRF2017R1D1A1B03034714). Corresponding Author Correspondence to Changhyun Roh and Yun Suk Huh. *E-mail: [email protected] (Y.S. Huh) *E-mail: [email protected] (C. Roh)

Notes Radiation Fusion Technology Program (2015M2A2A6A02045262(3)) from Nuclear Research R&D Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (MSIP), Republic of Korea.

National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (NRF-2017R1D1A1B03034714)



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Scheme 1. Schematic of FeAC/CMC/POSS formation.


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Figure 1. (a) XRD and (b) FT-IR analyses of FeAC, CMC, FeAC/CMC, and FeAC/CMC/POSS.



Figure 2. SEM morphologies of (a) FeAC, (b and c) FeAC/CMC, (d, e) FeAC/CMC/POSS and (f) elemental mapping of FeAC/CMC/POSS showing the presence of Fe, Si and N (inset: scale bar information added).


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Figure 3. TEM images of (a-e) FeAC/CMC/POSS at various magnification and (f) elemental mapping of FeAC/CMC/POSS showing the presence of Fe, Si and N.



Figure 4. XPS analysis of FeAC/CMC/POSS showing (a) a survey scan spectrum and spectra of (b) O 1s, (c) C 1s, (d) Si 2p, (e) Fe 2p and (f) N 1s.


Page 34 of 39

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Figure 5. Langmuir and Freundlich isotherm models for the adsorptions of (a) Cs, (b) MB and (c) CG.



Figure 6. (a) Adsorption mechanism of Cs, MB and CG. (b) Pseudo-second-order kinetic model fits for the adsorption of (i) Cs, (ii) MB and (iii) CG.


Page 36 of 39

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Table 1. Parameters of the Langmuir and Freundlich models for the adsorptions of Cs, MB, and CG by FeAC/CMC/POSS.

Langmuir parameters Adsorbate KL (L/mg)

qmax g-1)

Cs

0.0009

MB CG

(mg

Freundlich parameters R2

KF (L/mg) n

R2

152.16

0.99

0.65

1.48

0.98

0.0352

438.34

0.96

61.03

3.15

0.79

0.0104

791.91

0.96

98.04

3.56

0.81



Page 38 of 39

Table 2. Kinetic parameters of pseudo-first-order and pseudo-second-order models for Cs, MB, and CG adsorption by FeAC/CMC/POSS.

Pseudo-first-order model

Pseudo-second-order model

Initial concentration C0 (mg L-1)

k1 (min-1)

qe (mg g-1)

R2

k2 (g mg-1 min-1)

qe (mg g-1)

R2

Cs – 50

0.0081

0.4736

0.6162

0.1329

4.5188

0.9999

MB - 100

0.0116

4.6037

0.7148

0.0146

48.5437

0.9999

CG - 100

0.0105

3.7553

0.7874

0.0222

45.7247

0.9999


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TOC


Carboxymethyl

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