Layered double hydroxide – poly(diallyldimethylammonium chloride) nanocomposites: synthesis, characterization and adsorption studies

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ScienceDirect Materials Today: Proceedings 18 (2019) 1044–1053

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ICN3I-2017

Layered double hydroxide – poly(diallyldimethylammonium chloride) nanocomposites: synthesis, characterization and adsorption studies Sujata Mandal*, Sandhya K Centralised Sophoisticated Instruments Facility (CSIF), Inorganic and Physical Chemistry Laboratory, CSIR-Central Leather Research Institute, Chennai, India

Abstract The present study reports synthesis of layered double hydroxide (LDH) – poly(diallyldimethylammonium chloride) (PDDA) nanocomposites, their characterization and application for adsorptive removal of an azo-dye from water. LDH-PDDA nanocomposites with different PDDA concentrations were synthesized through a facile method. The physico-chemical characteristics of the nanocomposites were studied by TGA, XRD, FTIR, FESEM, HRTEM, zetasizer and surface area/porosity analyzer. The adsorption property of the LDH-PDDA nanocomposites was investigated for the removal of orange II dye from water. Adsorption kinetics, equilibrium and isotherm experiments were performed to study the adsorption characteristics of the LDH-PDDA nanocomposites. The dye adsorption capacity and the rate of adsorption of the nanocomposites were found to be influenced by the polymer content in the composite. The orange II adsorption capacity of the LDH-PDDA composite is approximately two times that of the pristine LDH. © 2019 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of International Conference on Nanotechnology: Ideas, Innovations & Initiatives-2017 (ICN:3i2017). Keywords: poly- diallyldimethylammonium chloride; layered double hydroxide; adsorption; dye; orange II

1. Introduction Layered double hydroxides (LDHs), also named as hydrotalcite-like compounds, are a family of inorganic layered materials with positively charged metal hydroxide layers and interlayer balancing anions [1]. Due to the presence of large interlayer spaces, tunable chemical composition and the reasonable number of exchangeable anions, they found 2214-7853 © 2019 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of International Conference on Nanotechnology: Ideas, Innovations & Initiatives-2017 (ICN:3i2017).

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Nomenclature C0 Ce k2 t v w qe qt Vm b k n

initial adsorbate (orange II) concentration (mg/l) equilibrium adsorbate concentration (mg/l) second order rate constant (g/mg.min) time (min) volume of solution (ml) weight of the adsorbent (g) amount of adsorbate per unit gram of the adsorbent at equilibrium (mg/g) amount of adsorbate per unit gram of the adsorbent at time t (mg/g) Langmuir isotherm constant representing monolayer adsorption capacity (mg/g) Langmuir isotherm constant representing adsorption bond energy (l/mg) Freundlich isotherm constant representing adsorption capacity Freundlich isotherm constant representing adsorption intensity

potential applications as ion-exchangers, adsorbents, catalysts support, inorganic fillers and in pharmaceuticals [24]. In recent years, LDH - polymer composites and nanocomposites have received considerable attention worldwide [5] due to the superior properties they exhibit compared to their precursors (LDH or the polymer). Besides, the physico-chemical properties of the composites can be tuned based on intended application. In the recent decade, increasing numbers of reports on synthesis and application of various LDH-polymer (nano)-composites show that these materials have promising future. Synthetic organic dyes are extensively used in paper-pulp, textile, cosmetic, plastic, leather, food, and several other industries. Effluents from these industries containing various organic dyes impart environmental hazards. They are not only aesthetic pollutants, but coloration of water by the dyes interferes with light penetration affecting aquatic ecosystems [6]. Most of these dyes are resistant to microbial degradation and hence their removal from the waste streams is an environmental priority. Among the various methods available for removal of dyes/colorants from water, the adsorption method is considered as the most effective and low-cost method. Review of literature shows that wide variety of composites/nanocomposite adsorbents with different clay - polymer combinations have been synthesized and used for water/wastewater treatment [7, 8]. However LDH - polymer composite adsorbents for the removal of toxic contaminants from water/wastewater have not been explored extensively except few [9, 10]. The present communication reports synthesis of LDH – poly(diallyldimethylammonium chloride) (PDDA) nanocomposites through a facile method, their characterization using various characterization techniques and application as adsorbent for the removal of an azo-dye from water. 2. Materials and Methods 2.1. Chemicals and Reagents Magnesium nitrate hexahydrate and aluminium nitrate nonahydrate were procured from MERCK Chemicals. Sodium hydroxide pellets (AR grade) were purchased from Himedia laboratories. The polymer poly(diallyldimethylammonium chloride) [PDDA, molecular formula (C8H16NCl)n, average molecular weight 100 – 200 K, 20 wt.% in water] was procured from Sigma-Aldrich. Orange II Sodium Salt (CAS No. 633-95-5, empirical formula: C16H11N2NaO4S, molecular weight 350.32, dye content  85%) was procured from Sigma-Aldrich. Double distilled water was used in all the experiments and for the preparation of standard solutions.

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(a)

(b)

Scheme 1. Chemical structure of (a) Poly(diallyldimethylammonium chloride) and (b) Orange II dye.

2.2. Synthesis of the LDH The precursor LDH was prepared via co-precipitation method [11]. A solution containing the mixture of magnesium and aluminium nitrates (Mg:Al molar ratio 2) was co-precipitated in an aqueous medium using 2M NaOH solution at 60 C and at a pH of 10±0.5. The precipitate thus formed was aged for 16 h under continuous stirring after which the precipitated clay was separated by centrifugation. The clay thus obtained was washed thoroughly with distilled water to remove any excess alkali and dried in an oven at 60 C. The dried solid (Mg/Al LDH) was powdered and preserved for further studies. 2.3. Synthesis of the LDH-PDDA nanocomposite A fixed amount of the synthesized Mg/Al LDH was reacted with an aqueous solution of the polydiallyldimethylammonium chloride (PDDA), at 50 C for 4 h. The solid suspension was then separated by centrifugation and dried in an air oven at 50 C. A series of LDH-PDDA composites were prepared by varying the initial PDDA concentration between 0 - 50% (w/v). The LDH-PDDA composites obtained by using 5, 15, 25 and 50% PDDA solution were named respectively as PDDA-5, PDDA-15, PDDA-25 and PDDA-50. All the LDHPDDA composites were powdered and preserved for characterization and adsorption studies. 2.4. Characterization techniques The thermogravimetric analyses of the adsorbents were performed using Q50 TGA from TA Instruments, USA. The phase confirmation and crystallinity of the samples were determined by X-ray diffraction studies using Cu-K radiation (PW-3040, Philips Analytical). FT-IR spectra of the pristine LDH and the LDH-PDDA composites were recorded by KBr pellet method using Cary 600 FT-IR spectrometer, Agilent Technologies Pvt. Ltd, USA. FT-IR spectrum of the polymer PDDA (liquid) was recorded in ATR mode. High resolution transmission electron microscopic (HRTEM) images were obtained using JEOL JEM 3010 instrument with a UHR polepiece. The scanning electron microscopic images were taken in Environmental Scanning Electron Microscope (FEI Quanta 200). Concentrations of orange II in the solution were measured at 483 nm wavelength using UV-visible spectrophotometer (CARY 100, Agilent Technologies, USA). The zeta potential () of the LDH and LDH-PDDA nanocomposites was measured using Zetasizer Nano ZS of Malvern Instruments, after dispersing the particles in water. BET surface area and pore structures were measured by nitrogen adsorption–desorption technique at 77 K using surface area analyzer (Autosorb 1 from Quantachrome Instruments, USA). 2.5. Batch adsorption experiments The batch adsorption experiments were performed under isothermal conditions in a thermostatic shaking water bath (Julabo SW23). Generally, 100 ml aqueous solution of orange II of known concentration was contacted with 0.05 g of the adsorbent in glass conical flask for 2 h in the thermostatic shaker. The solution was filtered and the

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residual dye concentration was measured by UV-visible spectrophotometer. The adsorption capacity was estimated using the formula [9]: ,

⁄

(1)

The percentage adsorption was calculated using the formula [7]: %

(2)

The adsorption isotherm experiments were performed at three different temperatures, 30, 40 and 50 C, the initial dye concentration was varied between 0 - 800 mg/l. Experiments to study the adsorption kinetics were conducted at 30 C for a time period of 0–6 h with an initial AO-II concentration of 200 mg/l and adsorbent dose of 0.5 g/l. Sample solutions were withdrawn at fixed time intervals and measured for residual dye (orange II) concentrations. 3. Results and Discussions 3.1. Characterization The TGA thermograms of the Mg/Al LDH and the LDH-PDDA composites are presented in Fig. 1a. The excess weight loss in LDH-PDDA composites with respect to that of LDH is due to the decomposition of PDDA in the composites at higher temperature. Therefore, the difference in weight loss between the LDH and the LDH-PDDA composite indicates the amount of PDDA loading in the composite.

Fig. 1. (a) TGA thermogram and (b) FT-IR spectra of the Mg/Al LDH and the LDH-PDDA composites.

Fig. 1a show that on increasing the PDDA concentration, the PDDA loading on the LDH initially increases. However, on increasing the PDDA concentration beyond 15% there is a decrease in PDDA loading on the LDH as indicated by the percentage weight of residue of the TGA. The lowest PDDA loading on LDH is obtained when high PDDA concentration (50%) is used during synthesis. The PDDA loading in the composites, as indicated by the percentage weight loss, are 13.9%, 27.14%, 23.97% and 0.06% for PDDA-5, PDDA-15, PDDA-25 and PDDA-50 respectively. Nevertheless, the thermal stability of the LDH-PDDA composites is not significantly affected by the PDDA loading in the composite. The zeta potential values of the LDH, PDDA-5, PDDA-15, PDDA-25 and PDDA50 are 25.7±1.6, 35.0±1.4, 49.5±2.8, 33.6±2.3, 29.1±2.1 mV respectively. The pristine LDH has a positive surface charge which increased with increasing PDDA content in the composites.

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The FT-IR spectra of the LDH, LDH-PDDA composites and the polymer poly(diallyldimethylammonium chloride) are presented in Fig. 1b. Due to the very low concentration of PDDA in the composites, the FT-IR spectra of the LDH-PDDA composites and LDH are almost alike. In addition to the typical vibrational bands of Mg/Al/NO3 LDH, the characteristic bands corresponding to PDDA are also found in the FT-IR spectra of the PDDA-5 and PDDA-15. The asymmetric and symmetric stretching vibrations of CH bond at 3063 cm−1 and the CH2 deformation vibration at 1448 cm−1 are observed in the FT-IR spectra of PDDA-5 and PDDA-15 [12]. The strong and sharp band at 1620 cm-1 observed in the PDDA may be attributed to C=C stretching vibration. The strong and broad band with the peak maxima at 3450 cm-1, which predominates the spectra of Mg/Al LDH, PDDA-5 and PDDA-15, are due to the O-H stretching vibrations from the surface O-H group and interlayer water molecule. The intensity and nature of this peak changed in PDDA-25 and PDDA-50 indicating absence of hydrogen bonded O-H and decrease in the number of interlayer water molecules. The decrease in water molecule is also revealed by the decrease in the absorption band intensity at 1612 cm-1 due to angular deformation of water molecule. Shifting of the absorption band due to angular deformation of water molecule from 1612 cm-1 in the LDH to 1577 cm-1 in PDDA-15 indicates there is interaction between the O-H group and the PDDA in the composite.The additional band with absorption maxima at 3580 cm-1 observed in PDDA-5 and PDDA-15, may be attributed to the O-H stretching vibration from the free O-H group in the LDH-PDDA composite. On the basis of TGA and FT-IR results, PDDA-15, the composite with highest PDDA loading, is selected for detail characterization and its characteristics are compared with those of the precursor LDH. The FESEM and HRTEM images of the pristine LDH and PDDA-15 are presented in Fig. 2. Both SEM and TEM images show nanoflakes having an average diameter of 50 - 100 nm for both pristine LDH and PDDA-15. However, the particles of LDH are stacked over one another forming agglomerate while those of PDDA-15 are well dispersed with delamellar nano-flakes.

Fig. 2. FESEM image of (a) Mg/Al LDH; (b) PDDA-15 and HRTEM images of (c, d) Mg/Al LDH; (e, f) PDDA-15.

The XRD patterns of the PLDH and PDDA-15 (Fig. 3a) are identical and show the 003, 006, 012, 015, 018, 110 and 113 diffraction planes typical for synthetic layered double hydroxides [5]. The sharp diffraction peaks in PDDA-15 indicate its higher crystallinity as compared to its precursor LDH. The lattice parameters calculated from the XRD data are presented in the inset of Fig. 3a. The shifting of the peak corresponding to the 003 Bragg reflections from 10.78 in LDH to 11.43 in PDDA-15 indicates decrease in interlayer distance from 0.833 nm (LDH) to 0.767 nm (PDDA-15). The change in interlayer distance and hence the c dimension of the hexagonal lattice of LDH indicates change in interlayer anion in PDDA-15. The nitrate ions present in the interlayer region of the pristine LDH is replaced by the chloride ion of the Poly(diallyldimethylammonium chloride) in PDDA-15. The values of interlayer distance are well matching with those reported in the literature for Mg/Al LDHs with nitrate (0.833 nm) and chloride (0.767 nm) intercalated anions [13]. The average crystallite sizes calculated from the XRDdata using Debye-Scherrer equation are 52.8 and 82.3 nm for the LDH and PDDA-15 respectively.

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The nitrogen adsorption-desorption isotherms and the BJH desorption pore size distributions (inset) of the LDH and PDDA-15 are presented in Fig. 3b. The specific surface area of the LDH and PDDA-15 is 105.1 and 101.6 m2/g respectively, while the total pore volume of both remains the same (0.22 cc/g). The shapes of the hysteresis loop for the LDH and the PDDA-15 are different indicating a change in pore structure of the pristine LDH on interacting with PDDA. According to the IUPAC classification, the N2 adsorption isotherms for both LDH and PDDA-15 are typical Type IV pattern, which is characteristic of the mesoporous materials [14, 15]. However, the hysteresis loop of LDH is H3 type that indicates aggregated particles forming slit-like pores. The hysteresis loop of PDDA-15 is H1 type, which indicates uniform arrangement of particles with cylindrical pore geometry [16]. The H1 type hysteresis loop also indicates relatively high pore size uniformity and facile pore connectivity. The high pore size uniformity of the PDDA-15 is demonstrated in the BJH desorption pore size distribution curve presented in the inset of Fig. 3b.

Fig. 3. (a) X-ray diffraction patterns and (b) N2 adsorption –desorption and pore size distribution of the LDH and the PDDA-15.

3.2. Adsorption performance test of the composites The adsorption performance of the LDH-PDDA nanocomposites was tested for adsorption of orange II dye in water. Fig. 4 shows the adsorption capacity of the Mg/Al LDH and the LDH-PDDA composites for the dye orange II in aqueous medium. Adsorption capacities of all the composites are much higher than that of the Mg/Al LDH. However among all the LDH-PDDA nanocomposites, the maximum adsorption capacity is shown by PDDA-15 where PDDA loading is the highest (indicated by TGA in Figure 1a).

Fig. 4. Orange II adsorption capacity of the Mg/Al LDH and the LDH-PDDA composites (initial dye conc. 200 mg/l, adsorbent dose: 0.5 g/l, Temperature: 30 C; contact time for adsorption capacity study: 2 h).

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The orange II adsorption kinetics of the LDH and LDH-PDDA composites were studied by investigating the change in orange II concentration as a function of time (Fig. 5). Fig. 5 shows that the rates of decrease in orange II concentrations for the LDH-PDDA composites are significantly faster than that for the Mg/Al LDH. Moreover, among the LDH-PDDA composites the rate is largely influence by the amount of PDDA present in the composite. This can be vindicated by the fact that within 30 minutes, the composites PDDA-5, PDDA-15, PDDA-25 and PDDA-50 showed 75, 90, 73 and 62% decrease in orange II concentration while the LDH showed 42% decrease.

Fig. 5. Change in orange II concentration as a function of time (initial dye conc. 200 mg/l, adsorbent dose: 0.5 g/l, temperature: 30 C).

The kinetic data were fitted to a pseudo second-order kinetic model (eq 3) proposed by Ho and McKay [17].

t t 1   2 qt k .q e qe

(3)

The values of the rate constant k were calculated from slope and intercept of the linearized plot of the eq 3 and presented in Table 1 along with their correlation coefficients. The R2 values indicate that reasonably good fitting was obtained for all the adsorbents. The amount of PDDA in the composite was found to influence the rate of adsorption. The rate of orange II adsorption on PDDA-15 is more than 3 time faster than that of the other three LDH-PDDA composites. The rate of orange II adsorption on the LDH-PDDA nanocomposites follows the order PDDA-15 > PDDA-5 = PDDA-25 > PDDA-50, which is also the order of PDDA loading on the composites. Table 1. Pseudo second-order rate constants and correlation coefficients. Adsorbents

Rate constant (k2104) g/(mg.min)

Correlation coefficient (R2)

LDH PDDA-5 PDDA-15 PDDA-25 PDDA-50

1.53

0.998

1.97

1.0

7.02

1.0

1.97

1.0

1.35

0.991

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Because of the fact that the adsorption capacity and rate of adsorption of the PDDA-15 nanocomposite is significantly higher than those of the other adsorbents studied, hence further investigations were continued with the PDDA-15 nanocomposite. The influence of initial dye concentration and reaction temperature on the adsorption capacity was studied by varying the initial dye concentration between 0 and 800 mg/l and the reaction temperature between 30 and 50 C. The orange II adsorption isotherms of PDDA-15 at three different temperatures are presented in Fig. 6. Fig. 6 shows that the orange II adsorption capacity of PDDA-15 increases on increasing the reaction temperature indicating endothermic nature of the adsorption process. The equilibrium adsorption capacities are 624.65, 755.67 and 836.34 mg/g at 30, 40 and 50 C respectively.

Fig. 6. Orange II adsorption isotherms of PDDA-15 (initial dye conc. 0- 800 mg/l, adsorbent dose: 0.5 g/l, contact time: 2 h).

The equilibrium adsorption data were fitted to the standard Langmuir [18] and Freundlich [19] isotherm models. The isotherm equations are presented in Eq. 4 and 5. Langmuir isotherm model:

Ce Ce 1   qe Vm bVm

(4)

Freundlich isotherm model:

ln q e  ln k F 

1 ln C e n

(5)

The isotherm constants, calculated from slope and intercept of the linearized plot of the respective isotherm equations are presented in Table 2 along with their correlation coefficients (R2). Table 2. The Langmuir and Freundlich isotherm parameters for orange II adsorption on PDDA-15. Temperature C 30 40 50

Langmuir parameters b Vm R2 0.0879 625 0.99 0.0977 769.23 0.99 0.1690 833.33 0.99

Freundlich parameters n k R2 5.73 217.48 0.98 6.93 315.13 0.99 7.01 372.82 0.98

The values of Langmuir isotherm constant, Vm, representing monolayer adsorption capacity matches reasonably well with those of the experimental equilibrium adsorption capacity values. The values of Langmuir constant, b, that

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represents adsorption bond energy, are very low (less than one), which is indicative of physical nature of the adsorption. The values of the Freundlich isotherm constant, n, lie within 5 to 7 at all the three experimental temperatures, which indicates feasibility of the adsorption process. Though the values of correlation coefficients (R2) for both Langmuir and Freundlich models represent excellent fitting of the experimental data with both the isotherm models however, the Langmuir model explains the present adsorbent/adsorbate process more appropriately. To evaluate the orange II adsorption capacity of the LDH/PDDA composite, the adsorption capacity value from the present study is compared to that of the other reported adsorbents in Table 3. The data in Table 3 show that LDH/PDDA composite adsorbent exhibits reasonably high adsorption capacity as compared to that by the other adsorbents reported in the literature. Table 3. Comparison of orange II adsorption capacities of various adsorbents. Adsorbent Zr(IV) doped chitosan/perlite composite Copper(II) Complex of Dithiocarbamate-Modified Starch Banana Peel activated carbon Titania aerogel zirconium-based chitosan microcomposite PDDA-15, LDH/PDDA composite

Adsorption capacity mg/g 476.2

Reference

114.6

[21]

333 420 926

[22] [23] [24]

624.6

Present study

[20]

4. Conclusion Layered double hydroxide – poly(diallyldimethylammonium chloride) (PDDA) nanocomposites with varying PDDA contents were synthesized successfully by a facile method. The TGA thermogram, FT-IR spectra and zeta potential studies confirm the formation of LDH-PDDA composites. The FESEM and HRTEM images confirm the nano-size of the composites. X-ray diffraction studies revealed that the typical two-dimensional layered structure of the LDH was retained and the crystallinity was improved after the composite formation with PDDA. Also, the nitrate interlayer anion of the Mg/Al LDH was exchanged by the chloride ions in the LDH-PDDA composites. The specific surface area and pore volume of the composites were very close to the precursor LDH. However, there was change in pore structure of the LDH after composite formation. The LDH has slit-like pores while the LDH-PDDA nanocomposite showed cylindrical pore geometry. All the LDH-PDDA nanocomposites demonstrated very high adsorption capacities as well as high rate of adsorption for the azo dye, orange II. The orange II adsorption capacities and adsorption rates follow the order PDDA-15 > PDDA-5 ~ PDDA-25 > PDDA-50 > LDH. The adsorption kinetics could be explained using a pseudo second-order kinetic model. The adsorption isotherm studies indicated endothermic nature of the adsorption of orange II on PDDA-15. The Langmuir isotherm model was found to be the most suitable model to explain the orange II adsorption on LDH-PDDA nanocomposites. Acknowledgements Authors wish to thank Science and Engineering Research Board, Department of Science and Technology, Govt. of India, for financial support (SR/FT/CS-145/2011).

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