Microwave Initiated Facile Formation of Sepiolite-Poly

Research Article pubs.acs.org/journal/ascecg

Microwave Initiated Facile Formation of SepiolitePoly(dimethylsiloxane) Nanohybrid for Effective Removal of Congo Red Dye from Aqueous Solution Shappur Vahidhabanu,*,† Adeogun Abideen Idowu,†,‡ D. Karuppasamy,† B. Ramesh Babu,† and M. Vineetha† †

CSIR-Central Electrochemical Research Institute, Pollution Control Division, Karaikudi-630003, Tamil Nadu, India Chemistry Department, Federal University of Agriculture, Abeokuta, Nigeria

‡

S Supporting Information *

ABSTRACT: A single pot microwave assisted process was employed for the synthesis of sepiolite-poly(dimethylsiloxane) (SP-PDMS) nanohybrid for removal of Congo red (CR) dye from contaminated water in a batch process. The synthesized nanohybrids were characterized by X-ray diffraction (XRD) and Fourier transform infrared (FTIR) spectroscopy, confirming the nanohybrids formation. The structures of the nanohybrids were confirmed by scanning electron microscopy (SEM), field emission scanning electron microscopy (FESEM), and transmission electron microscopy (TEM) while the specific surface area and pore size were determined by the Brunauer−Emmett− Teller (BET) method using N2 adsorption isotherms. The dye removal process was subjected to kinetic and equilibrium studies, the results of which showed that pseudo-first-order kinetics dominated the adsorption of CR onto the nanohybrid. The equilibrium data were analyzed using Freundlich, Langmuir, Tempkin, Dubinin−Radushkevich, and generalized isotherm models. The nanohybrids demonstrated favorable adsorption properties toward CR with the isotherm models fit in the order Generalized > Langmuir > Freundlich isotherm > Tempkin > Dubinin−Radushkevich. The maximum adsorption capacities (Qmax) of 132.22, 167.45, and 85.33 mg g−1 were obtained for SPPDMS1, SP-PDMS2, and SP-PDMS3, respectively. The removal efficiency was found to be 100% up to the fifth cycle. It was found that the synthesized nanohybrid can effectively remove 98% of CR dye from a given aqueous solution. The proposed synthesis is a simple and easily scalable process in the production of SP-PDMS nanohybrid for efficient removal of CR and water purification. KEYWORDS: Adsorption, Wastewater treatment, Isotherms, Congo red dye, Sepiolite, Nanohybrid

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INTRODUCTION It is a daunting task curtailing the contamination of water bodies through leaching and direct disposal of dye effluents from various industries such as textile, paper, cosmetic, food, rubber, plastics, pesticide, pharmaceutical, leather, and so on1−3 into the water body. Dyes pollutants are undesirable in the environment, owing to their toxicity and carcinogenic effects even at very small concentrations.4 Dye colors impart persistent visible pollution over long distances in streams with attendant decreased reaeration capacity of the stream and reduced photosynthesis activity. The treatment of dyeing wastewater poses several problems due to the stability (of the dyes used in industries) to light, heat, and oxidation. Also, they are not biodegradable, they display resistant to aerobic digestion, and when degraded or digested, they produced toxic and hazardous products.5,6 Therefore, effective removal of dye pollutant is necessary for water conservation and the health of the living things in the environment. © XXXX American Chemical Society

Numerous techniques are available for dye wastewater treatment;7,8 they include coagulation, flocculation, chemical oxidation,9,10 membrane separation,11 catalytic degradation,12 biodegradation,13 and adsorption.14 Compared with all other methods, the adsorption method gives the best results; it is easy to perform, insensitive to toxic substances, and effective for different types of pollutants. In industrial wastewater treatment systems, the most commonly used adsorbent for the adsorption process is activated carbon due to its large specific surface area.14 However, the application of activated carbon for a largescale wastewater treatment is limited because of its high cost.15 Therefore, research has been continued for inexpensive alternative adsorbents having reasonable adsorption efficiencies.14 Received: July 18, 2017 Revised: August 22, 2017

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DOI: 10.1021/acssuschemeng.7b02364 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering Scheme 1. Synthesis and Application of SP-PDMS Nanohybrid for CR Dye Removal

Raw as well as cations modified clays have been explored for wastewater treatment;16 however, modified clays have been reported to render higher adsorption efficiency.13,17 Nanohybrid based adsorbent has also been explored for wastewater treatments,18 Riaz et al.,18 used bentonite-poly(o-toluidine) nanohybrid for the removal of Malachite Green with excellent adsorption capacity. Similarly, Chen et al.19 reported Cr(VI) removal from an aqueous solution using polyaniline/montmorillonite clay nanocomposites. Organic/inorganic nanohybrid containing polyaniline and α-zirconium phosphate have been shown to exhibit excellent affinity toward the adsorption removal of methyl orange from aqueous solution.20 We have recently demonstrated the removal of Congo red dye with alginate modified sepiolite with improved adsorption capacity over and above the adsorbents in a similar category.21 Although nanohybrid adsorbents have wide application, the drawbacks in their usage include cumbersome synthesis process, separation, and recycling after use. The synthesis process of many nanohybrids involves the use of chemicals with high purity and long processes which can also elevate their costs. This study therefore, presents a single step microwave assisted process for the synthesis of clay−polymer

nanohybrid based on an SP-PDMS system through in situ intercalation followed by polymerization. The expectation is to combine the good sorptivity and rheological and catalytic properties and high thermal and chemical stabilities of the sepiolite with very good optical and thermal properties as well as inertness and nonflammability and nontoxic properties of poly(dimethylsiloxane) (PDMS). The nanohybrid was then characterized using XRD, SEM, FESEM, TEM, and FTIR, while the specific surface area was determined using N2 adsorption/desorption isotherms. Adsorption of Congo red from aqueous solution was studied in a batch process using the nanohybrids as the adsorbents; adsorption data were subjected to kinetics analysis, while equilibrium data were analyzed with the isotherm models.

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MATERIALS AND METHODS

Materials. Sepiolite clay (SP) (fine particle), dimethylsiloxane (DMS), and ferric chloride (AR grade) were products of SigmaAldrich; they were used without further purifications. Other reagents were analar grade and were prepared with Milli-Q water. Synthesis of SP-PDMS nanohybrid. About 10 g of sepiolite clay was washed several times and dispersed in Milli-Q water, the B


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ACS Sustainable Chemistry & Engineering dispersion was allowed to settle, the supernatant was discarded, and the sediment was dried. Initially, 5 g each of dried sepiolite and dimethylsiloxane were transferred into a 100 mL glass beaker, about 30 mL of Milli-Q water was added, and the mixture was homogenized by constant stirring for 2 h on a magnetic stirrer. FeCl3 was added to the mixture to initiate polymerization, and the beaker was quickly transferred into a microwave oven, where the mixture was irradiated for 6 min. The residue in the flask was washed several times with MilliQ water and methanol followed by drying at 70 °C in a vacuum oven to remove the solvents. The synthesis and application of the sepiolitePDMS are summarized by Scheme 1. Degree of FeCl3 induced polymerization. The degree of FeCl3 induced polymerization of dimethylsiloxane (DMS) was investigated by repeating the synthesis with varying ratios of SP to DMS. Three samples were prepared in the weight ratio of 1:0.25, 1:0.5, and 1:1 and labeled SP-PDMS1, SP-PDMS2, and SP-PDMS3, respectively. Characterization of the adsorbent. XRD data were collected using an X-ray diffractometer with Cu Kγ radiation (α = 1.5418 Å) from 10−50° at 5°/min. Field emission scanning electron microscopy (FE-SEM) using Carl Zeiss AG (Supra 55VP) with an accelerated voltage 5−30 kV was used to characterize the Au sputtered sample. Transmission electron microscopy (TEM) images were obtained with a Philips CM 200 transmission electron microscope operating at an accelerating voltage of 200 kV; the samples were coated over a copper grid for TEM studies. The average pore diameter and specific surface area, [BET (Brunauer−Emmett−Teller) surface and pore volume], were measured on a Quantochrome NOVA 1000. Fourier transform infrared (FT-IR) spectra of samples were recorded with KBr pellets in the range 4000−400 cm−1 using a Nexus 670 Nicolet FT-IR spectrometer. The UV−visible (UV−vis) absorption spectra were recorded in a Hitachi (model U-4100) UV−vis−NIR spectrophotometer equipped with a 1 cm quartz cuvette holder (for liquid samples). Batch adsorption studies. Congo red used in this study is a sodium salt of 3,3′-([1,1′-biphenyl]-4,4′-diyl)bis(4-aminonaphthalene1-sulfonic acid) a Direct Red 28; 573-58-0 (Figure 1). An analar grade

Qe =

(Co − Ce)V W

(2)

where C0 (mg/L) is the initial concentration and Ct (mg/L) is the concentration of the dye at time t in the liquid phase. Ce (mg/L) is the concentration of the dye at equilibrium in the liquid-phase V is the volume of the solution (L), and W (g) is the mass of adsorbent. In order to investigate the mechanisms of the adsorption process, pseudo-first-order, pseudo-second-order, Elovich kinetic, and intraparticle diffusion models22−25 (eqs 3−6) were applied to describe the kinetics of adsorption of CR to SP-PDMS nanohybrids.

Q t = Q e(1 − e−k1t ) Qt =

(3)

k 2Q e 2t 1 + k 2Q et

(4)

Q t = 1/βel ln(αβel *t )

(5)

Q t = K id × t 0.5 + Ci

(6) −1

Qe is the amount of CR in mg g adsorbed at equilibrium while k1 (min−1) and k2 (g mg−1 min−1) are the pseudo-first-order and secondorder rate constants, respectively. The Kid (mg g−1 min−0.5) is the intraparticle diffusion constant; Ci (mg g−1) is a measure of the thickness of the surface prior to the adsorption while α and βel in the Elovich equation represent the initial adsorption rate (mg g−1 min−1) and the desorption constant (g mg−1), respectively.25 Similarly, the Langmuir, Freundlich, Tempkin, Dubinin−Radushkevic, and Generalized adsorption isotherm models26−30 (represented by eqs 7 − 11) were employed for the analysis of the equilibrium data from the adsorption studies. Qe =

Q obCe 1 + bCe

(7)

Q e = KFCe1/ n

Qe =

Qe =

with dye content of >85% impurity from Central Drug House, Delhi, India, 1000 mg/L aqueous solution of CR, was prepared with deionized water as the stock solution and was further diluted with deionized water to obtain the working standard solutions. The dye solution was adjusted to the desired pH value with an aliquot of HCl or NaOH (1.0 mol L−1) prior to the adsorption study. The adsorption processes were conducted by contacting 25 mL of CR (50, 75, 100, 150, and 200 mg/L) in a 200 mL flask with 10 mg of SP-PDMS nanohybrids at 30 ± 1 °C and agitating on an orbital shaker at a speed of 100 rpm. The samples were collected at time intervals of 0, 5, 10, 15, 30, 60, and 120 min, and the adsorbent was separated by filtration. The concentrations of the dye in the solutions were estimated at 496 nm using a spectrophotometer (Hitachi model U4100 UV−vis−NIR). Similarly, the adsorption isotherms were performed in a set of 30 flasks (200 mL) where solutions of dye (25 mL) with different initial concentrations (10−100 mg/L) at pH 7 were contacted with 10 mg of SP-PDMS. The solutions were kept in an isothermal shaker (30 ± 1 °C) for 48 h to reach equilibrium of the solid−solution mixture. The flasks were then removed from the shaker, and the final concentration of dye in the solution was analyzed. The amounts of dye removed at time t, Qt (mg g−1) and at equilibrium Qe (mg g−1) were calculated using eqs 1 and 2 below:

(Co − Ct )V W

RT ln aT Ce bT

(9)

Q e = Q s exp(− βε 2)

Figure 1. Structure of Congo red dye.

Qt =

(8)

Q maxCe

(10)

m

KG + Ce m

(11) −1

−1

The Langmuir equation parameters, Qo (mg g ) and b (L mg ), are the constants related to the adsorption capacity and energy of adsorption, respectively, the KF ((mol g−1)(mol L−1)−1/n) and n of the Freundlich isotherm model are the isotherm parameters characterizing the adsorption capacity and intensity, respectively. The Tempkin isotherm assumed that the heat of adsorption decreased linearly with increasing coverage; its parameters aT and bT are the equilibrium binding constant (L/g) and heat of adsorption (J/mol), respectively. The Dubinin−Radushkevic (D-R) isotherm model proposed surface inhomogeneity with adsorption energy distribution. Qs is the theoretical saturation capacity (mol g−1), β is a constant related to the mean free energy of adsorption per mole of the adsorbate (mol2 J−2), and ε (J mol−1) is the Polanyi potential given by the relation; ε = RT ln(1 + 1/Ce), R (J mol−1 K−1) is the gas constant, and T (K) is the absolute temperature of the equilibrium experiment. The constant β gives an idea about the mean free energy E (kJ mol−1) of adsorption per molecule of the adsorbate when it is transferred to the surface of the solid from the relationship E = −(2β)−0.5. If the magnitude of E is between 8 and 16 kJ mol−1, the process is chemisorption, while values of E < 8 kJ mol−1 suggest a physical process. In the Generalized adsorption isotherm, KG (mg/L) represents the saturation constant, m is the cooperative binding constant, while Qmax (mg g−1) is the adsorption capacity.30

(1) C


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ACS Sustainable Chemistry & Engineering Statistical test. The acceptability and the best fit of a model are mostly based on the square of the correlation coefficients, R2, which may be appropriate for linearized models. However, since the leastsquare fit was employed for the data fit, there is a need to compare the error distribution, because choosing an error function may be indispensable. Therefore, in this study, three error functions were used to validate the fit kinetic models, which include the sum square error function (SSE), root-mean-square error (RMSE), and composite fractional error (HYBRD) (eqs 12 − 14).31

amorphous nature of the nanohybrid caused by the addition of the PDMS moiety. The results of the surface morphological studies are shown by the TEM and FE-SEM images in Figure 3. Pure sepiolite

N

SSE =

∑ (Q (exp) − Q (cal))2

(12)

i=1 N

RMSE =

HYBRD =

∑i (Q (exp) − Q (cal))2 (13)

N 100 N−P

N

∑ i

(Q (exp) − Q (cal)) Q (exp)

(14)

where N represents the number of data points and P are the numbers of the parameters in the model. The higher the value of R2 and the lower the value of SSE, RMSE, and HYBRD errors, the more acceptable the model.

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RESULTS AND DISCUSSION Characterization of sepiolite−PDMS nanohybrid. The XRD patterns of pure SP and SP/DMS nanohybrids with different DMS contents are shown in Figure 2. Pure sepiolite

Figure 2. XRD patterns of sepiolite−PDMS nanohybrids.

Figure 3. TEM images of (a) neat sepiolite, (b) SP-PDMS1 (1:0.25), (c) SP-PDMS2 (1:0.5), and (d) SP-PDMS3 (1:1) and FE-SEM images of (e) sepiolite and (f) SP-PDMS1.

exhibits crystalline peaks s at 2θ = 12.3°, 17.7°, 19.8°, 25.4°, 26.4°, and 35.0° corresponding to (1 3 0), (1 5 0), (0 6 0), (2 4 1), (0 8 0), and (3 7 1) crystalline planes.32 In the nanohybrid system, the addition of a small quantity of PDMS altered the peak intensity and crystalline nature of sepiolite clay. Reduced intensity is noted in the crystal plane (1 5 0) in SP-PDMS1 alone, while reduced intensity is noted at the crystal planes (0 6 0) in all the nanohybrids. Increase in DMS leads to the disappearance of planes (1 5 0) in SP-PDMS2 while SPPDMS3 with the highest amount of DMS lost the crystal planes (1 5 0), (2 4 1), and (0 8 0) along with all other peaks below 20° to the higher loading of PDMS into the clay galleries. Furthermore, some of the peaks were broadened due to the

exhibits fibrous-like needle structure with a diameter of about 22 nm as shown in Figure 3a. The interaction of individual needle-like particles led to the formation of the aggregate depicted in the image.32 Figures 3(b−d) displayed the structure of individual needle-like fibers after coating with PDMS; the intercalation and in situ polymerization of PDMS in sepiolite led to the formation of fiber bundles of larger width with different diameters.33 The needles diameter after coating with the PDMS moiety increased to between 94 and 140 nm. The coating of PDMS over sepiolite greatly changed the diameter of the needle structure. In addition, FE-SEM images (Figures 3e and f) of sepiolite-PDMS nanohybrid also confirmed the coating of PDMS over sepiolite clay. D


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Figure 4. BET analysis: (a) N2 adsorption isotherms; (b) pore size distribution of SP-PDMS1, SP-PDMS2, and SP-PDMS3.

exhibited a H3-type hysteresis loop commonly observed for loosely packed aggregates. The table showed that SP-PDMS3 has the lowest specific surface area, while the highest was recorded for SP-PDMS2. The elemental compositions of the nanohybrids were also confirmed by the EDX analysis, the result of which is presented in Figure 5. The elemental compositions of the pure sepiolite and the nanohybrids are shown in Table 2. The presence of Mg and Si in all the nanohybrids suggests the presence of sepiolite, although a decrease in weight percent is noted as the polymer/ sepiolite ratio increases in the nanohybrids. FT-IR studies of the adsorbent. The FTIR spectra of Congo red dye and nanohybrids before and after adsorption

Figure 4 shows the N2 adsorption isotherms at 77 K while the summary of the BET data calculated from adsorption isotherms is presented in Table 1, respectively. The sample Table 1. BET Analysis of SP-PDMS Nanohybrid Sample

Specific Surface Area (m2/g)

Pore Volume (cm3/g)

Average Pore Diameter (nm)

SP-PDMS 1 SP-PDMS 2 SP-PDMS 3

105.4 180.0 77.3

0.0222 0.0299 0.0114

8.43 6.64 5.90

Figure 5. Elemental composition of (a) pure sepiolite, (b) SP-PDMS1, (c) SP-PDMS2, and (d) SP-PDMS3. E


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ACS Sustainable Chemistry & Engineering Table 2. EDX Analysis of Pure Sepiolite and Nanoybrids Pure sepiolite

SP-PDMS1

SP-PDMS2

SP-PDMS3

Element

Weight%

Atomic%

Weight%

Atomic%

Weight%

Atomic%

Weight%

Atomic%

C O Mg Al Si Cl K Fe

4.43 47.04 12.14 0.74 23.86 0.22 0.32 11.24

7.53 59.99 10.18 0.56 17.33 0.13 0.17 4.11

9.49 51.81 10.05 0.80 20.45 3.51 0.28 3.61

14.72 60.28 7.70 0.56 13.56 1.85 0.13 1.20

12.73 56.29 8.35 0.62 18.95 1.01 0.18 1.89

18.64 61.88 6.04 0.40 11.86 0.50 0.08 0.59

14.79 6.64 1.01 0.32 2.26 2.54 1.36 71.08

38.97 13.14 1.32 0.37 2.55 2.27 1.10 40.28

were recorded in the region 4000−400 cm−1 and presented in Figure 6. The FT-IR spectrum of CR dye gives all the

PDMS1, SP-PDMS2, and SP-PDMS3, respectively. These figures showed that the quantity of dye adsorbed (Qt) with time increases with increase in initial dye concentrations. The adsorbed quantity increased rapidly for the first 30 min of the experiment and became slower with time as equilibrium was approached. This may be as a result of the empty sites, which are occupied as the adsorption progressed and become saturated at equilibrium. Concentration dependent on the adsorbed quantity also depicted a higher adsorbed quantity as the concentration increases with maximum with SP-PDMS2 showing the maximum adsorbed quantity. Kinetics and mechanism of adsorption. The kinetics of adsorption of CR on the synthesized nanohybrids was studied in order to estimate the optimum operational conditions for full-scale processes. The study of kinetic of adsorption is important because it is one of the important criteria in elucidating the efficiency as well as the mechanism of the adsorption process. Figures S1, S2, and S3 (Supporting Information) give the plots of four different kinetic models used to explain the adsorption of CR by SP-PDMS1, SPPDMS2, and SP-PDMS3, respectively. These figures were obtained from the kinetic models given by eqs 3−6, and the parameters for the fits are presented in Table S1 (Supporting Information). The kinetic parameters in Table S1 showed the first-order kinetic model best fitted the kinetic data. Although the average values of R2 obtained for both pseudo-first-order and pseudo-second-order kinetic models are almost the same, first order was adjudged as the best fit owing to the values of Qe obtained (Qe calc), which are consistent with the experimental values (Qe exp) when compared with the second order parameters. The Elovich model’s parameter in Table S1 (Supporting Information) showed an increase in adsorption rate as the initial concentration of the dye increases as shown by the values of α (adsorption rate); this is due to an increase in the concentration gradient across the surface; the decrease in desorption rate (β) with increased initial concentration is an indication of formation of a chemical bond between the dye and functional groups present on the nanohybrids. The mechanisms of the adsorption of CR to nanohybrids were investigated using an intraparticulate diffusion model with the parameters reported in Table S2 (Supporting Information). The Kid values increase as the initial concentration of the dye increases. This could be attributed to the resistance of the surface boundary to the increased driving force with the concentration gradient as the dye molecules access the available sites in the hydrogel. The values of the intercepts C1 obtained showed that the initial stage of adsorption was characterized by both the intraparticle diffusion and external mass transfer, with the later playing a significant role.36

Figure 6. FT-IR spectra of (a) CR, (b) nanohybrid before adsorption, and (c) nanohybrid after adsorption.

characteristic peaks viz., N−H, NN, SO (sulfonic acid), and C−H (aromatic ring) stretching vibrations at 3441, 1651, 1383, and 695 cm−1, respectively. In the case of nanocomposites sample alone, it exhibits a broad peak centered at around 3395 cm−1, representing the O−H groups present in the sepiolite clay minerals. The peaks above 3600 cm−1 were assigned to oxides of Mg present in clay mineral34 while peaks at 1051 and 793 cm−1 correspond to the Si−O−Si stretching vibrations and O−Si−O vibration from the siloxane moiety, respectively. These confirmed the presence of both sepiolite and PDMS moieties in the nanohybrid system. After the adsorption of CR dye, some changes in the absorption wavenumbers were observed and also the intensities of some characteristic peaks were increased. A broad absorption peak in the region 3300−3600 cm−1 represents the overlapping of bonded O−H and N−H peaks (overlapping of the water peak may be considered for CR alone and its nanohybrid mixture). Furthermore, slight shifts in the peak positions were observed in the regions 1651−1649 and 1383−1381 cm−1. This is evidence that the adsorption of CR over nanohybrid is governed by the interaction of hydrogen atoms in the N−H groups of the dye molecule with the S−O group present in the nanohybrid.35 Effect of contact time and initial dye concentration on adsorption. Figures 7(a−c) showed the influence of contact time on the quantity of the CR adsorbed by SPF


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Figure 7. Effect of contact time and initial concentration on adsorption of Congo red dye on (a) SP-PDMS1, (b) SP-PDMS2, or (c) SP-PDMS3. (Temp: 30 °C, pH: 7.0, and adsorbent dosage: 4 g/L.)

Figure 8. Isotherm fits for the adsorption of Congo red dye onto (a) SP-PDMS1, (b) SP-PDMS2, or (c) SP-PDMS3. (Initial dye conc: 10−100 mg/ L, Temp: 30 °C, pH: 7.0, and adsorbent dosage: 4 g/L.)

Adsorption isotherms. The adsorption isotherm relates the amount of a substance adsorbed into the solid phase of

adsorbent to the concentration of the substance in bulk solution at a particular temperature in equilibrium.21 The plots G


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ACS Sustainable Chemistry & Engineering Table 3. Isotherm Parameters for the Adsorption of CR onto the Nanohybrids Isotherms Langmuir

Freundlich

Tempkin

Dubinin−Radushkevich

Generalized Isotherm

Parameter

SP-PDMS 1

SP-PDMS 2

SP-PDMS 3

Qmax (mg g−1) b (L mg−1) RL R2 KF × 102 ((mol g−1) (mol L−1)−1/n n R2 aT (L mg−1) bT R2 Qs (mg g−1) β × 106 (mol J−1)2 E (kJ mol−1) R2 Qmax (mg g−1) KG (mg/L) M R2

132.223 0.053 0.350 0.998 11.478 0.583 0.997 109.679 0.953 0.993 79.139 7.077 0.266 0.989 129.544 1.017 19.032 0.998

167.452 0.210 0.154 0.996 34.688 0.522 0.995 86.980 4.208 0.990 110.086 6.599 0.870 0.983 211.986 0.823 5.724 0.997

85.325 0.071 0.301 0.995 13.317 0.419 0.984 148.340 0.941 0.991 64.614 1.107 0.213 0.994 70.731 1.585 41.572 0.996

of five different isotherm models used to gain insight into the equilibrium adsorption of CR into the nanohybrids are shown in Figure 8(a−c) using eqs 7−11. The parameters obtained from the least-squares fits of the models are shown in Table 3. The Langmuir isotherm, which proposed a monolayer adsorption on surfaces with identically homogeneous sites, gives the fit in Figure S1 of the Supporting Information. It is obvious from the plot that the adsorption capacity increases initially, owing to the presence of active sites which got saturated as the dye concentration increases. Maximum adsorption capacities (Qmax) of 132.22, 167.45, and 85.33 mg g−1 were obtained for SP-PDMS1, SP-PDMS2, and SP-PDMS3, respectively. These trends are in accordance with the specific surface area and the pore volume obtained for the adsorbent. SP-PDMS has the highest specific surface area and corresponding pore volume and, hence, a higher affinity for the CR due to ease of diffusion into its pore volume. Table S3 (Supporting Information) showed that the adsorbents compared favorably with other synthetic and modified adsorbents. For the RL and Freundlich isotherm parameters, 1/n values of less than 1 obtained for all the adsorbents are an indication of favorable adsorption processes. The Tempkin isotherm parameters obtained showed that SP-PDMS1 has the highest binding constant and hence the least value of maximum adsorption capacity obtained as shown by Langmuir and Generalized adsorption isotherm parameters. The Dubinin− Radushkevich model gave theoretical saturation capacity values (Qs of 79.14, 110.08, and 64.61 mg g−1, respectively, for the adsorbents) which are less than the maximum adsorption capacity. The maximum adsorption energies, E, range between 0.21 and 0.87 kJ mol−1, showing that the process is a physisorption dominated process. The overall comparison of the isotherms using the average values of R2 shows that the isotherm fits are in the order Generalized > Langmuir > Freundlich isotherm > Tempkin > Dubinin−Radushkevich, although it is widely reported in the literature that the Langmuir model exhibited the best fitting with the experimental isotherm data for the adsorptive removal of dyes by magnetic carbon nanotubes.37,38”. Regeneration study. The most economically viable option to minimize the cost of the adsorption process is the reusability

of adsorbent. In this regard, desorption studies were performed as follows: Initially, 0.05 g of sepiolite−PDMS nanohybrid was contacted with 50 mL of CR dye (90 mg/L) at pH 7 over the duration of 6 h. The desorption studies of CR dye loaded sepiolite−PDMS nanohybrid were performed by using 50 mL of 0.1 M NaOH solution. Thereafter, the regeneration of active sorption sites was achieved by treating with 2 M HCl solution. The reusability of sepiolite−PDMS nanohybrid was scrutinized by using regenerated adsorbents for seven consecutive adsorption−desorption cycles. Figure 9 shows the efficiency

Figure 9. Congo red removal efficiency of regenerated sepiolitePDMS.

of the regenerated adsorbent. The removal efficiency of 100% was obtained for the nanohybrid for the first five cycles. After five cycles, there was a gradual decrease in removal efficiencies until the seventh cycle, when 59.3% efficiency was obtained. This is due to the fact that the nanohybrid surface becomes saturated after the fifth cycle. Hence, we observe that the sepiolite−PDMS nanohybrid can be successfully reused for the five adsorption cycles with no loss of removal efficiency.

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CONCLUSION Three sepiolite−PDMS nanohybrids were successfully synthesized in a one-pot, microwave assisted route by varying SP to DMS ratio during synthesis. Characterization of the nanoH


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hybrids revealed that the intercalation of the PDMS moiety into the clay galleries greatly affects the crystallinity of clay and also leads to breakdown of particles with accompanying surface enhancement. The nanohybrids demonstrated favorable adsorption properties toward CR. The adsorption kinetic data fitted well with a pseudo-first-order kinetics model while the isotherms were best fitted by the Generalized isotherm. The maximum adsorption capacities (Qo) from the Langmuir isotherm showed that SP-PDMS2 has the highest adsorption capacity. The removal efficiency was found to be 100% up to the fifth cycle.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b02364. (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; [email protected]; Tel.: +91-4565-241441; Fax: +91-4565-227779. ORCID

Shappur Vahidhabanu: 0000-0002-4361-9618 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS One of the authors (S. Vahidhabanu) is thankful to Department of Science and Technology, India, for financial support through DST-INSPIRE fellowship.

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REFERENCES

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