Fabrication of poly(β-cyclodextrin-co-citric acid)/bentonite clay nanocomposite hydrogel: thermal and absorption properties

Abstract

A β-cyclodextrin (β-CD)/bentonite clay (BNC) nanocomposite hydrogel was prepared through combining in situ intercalative polymerization and melt intercalation methods. A suspension solution of β-CD with different amounts of BNC was prepared and then copolymerized with citric acid (CT) to prepare CDP-CT/BNC nanocomposite hydrogels. Their structures were characterized by ATR-FTIR, XRD, SEM, EDAX, TEM, TGA, DTG, DSC and DDSC spectroscopy. The results are indicative that BNC was grafted onto CDP-CT macromolecules, and this led to good linkers in the nanocomposite hydrogels and consequently a significant improvement of their thermal properties. The swelling ratio and absorption properties toward methylene blue (MB) of the nanocomposite hydrogels were studied.

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1. Introduction

Hydrogels are composed of hydrophilic polymers, either natural (such as collagen, chitosan and dextran) or synthetic (such as polyethylene glycol),¹ that are formed by cross-linking polymers through covalent bonds or noncovalent interactions into 3D structures. It is well known that natural polymers have higher biocompatibility and less toxicity than most of the synthetic polymer hydrogels, thus pure natural polymer hydrogels would be more used in industry.²

Nowadays, several research groups have attempted to create various macromolecules based on polysaccharides in large quantities which are harmless to nature.^3–5 There are many reports in the literature on the macromolecules consisting of polysaccharides, which the current exclusive attention has been given to β-CD as natural molecule derived from cellulose or starch. The materials involved β-CD have been increased the sorption values in comparison to the compounds which contain cellulose or starch. These compounds play an important role in the sorption process due to their inclusion complex which occurred via host–guest phenomena.^6,7 This and the other properties of β-CD and its derivatives are in special non-toxic to human body and easily degraded in the nature which are the main reason to be used in a great variety research, industrial, and environmental applications.⁷ Recently the new CD-based materials have been developed in practical uses in the prevention of environmental pollution such as removal of pollutants from water and wastewater. So, the growing interests can be observed through the increasing number of papers published each year on this subject. Due to their solubility in water, therefore CD derivatives cannot be used directly for separation and purification purposes. Thus it needs to produce insoluble CD-based materials via polymerization reactions or supporting CD molecules on a solid face by grafting.^7,8

The most researches have been focused on the water-soluble β-CD polymers (β-CDP)^9,10 and few topics are related to the water-insoluble β-CD, such as hydrogels. Water-insoluble β-CDPs, by chemical cross-linker, are used as a new type of adsorbent for removal of organic pollutants and heavy metals from water.^2,6,11–16 This kind of polymers are commonly synthesized using cross-linking the hydroxyl groups of β-CD with bi- or multi-functional molecules such as epichlorohydrin,^6,17–20 diisocyanates,^21–23 polycarboxylic acids^11,24,25 and anhydrides.^23,26,27 β-CDPs have been identified as a more efficient adsorbent toward some ions, basic dyes and aromatic amines when modified by carboxyl/carboxylate groups (–COOH/–COO⁻). On the other hand, CT has been known as a polycarboxylic acid with low toxicity and friendly to the environment. CDP-CT was directly synthesized using direct melt polycondensation between β-CD and CT that was performed at a temperature lower than 200 °C and in the absence of organic solvents and harmful additives.²⁸ Also, CDP-CT, which contains β-CD and carboxylic groups, is a hydrogel that has been useful to adsorb organic and inorganic pollutants.^13,16

Polymer nanocomposite gels are a class of hybrid materials which can achieve by good mechanical properties due to the multiple noncovalent effects between organic polymer and inorganic nanoparticles.^26,29–31 The macroscopic properties of the organic polymers can markedly change due to the presence of nanoparticle with large surface area, even at a very low concentration, and in addition it can contribute many new properties to the polymer, such as increased heat resistance, decreased gas permeability and flammability. Hydrogel nanocomposites can generally be prepared by four different methods including (1) in situ template synthesis, (2) solution intercalation, (3) in situ intercalative polymerization and (4) melt intercalation. Recently, the in situ intercalative polymerization and melt intercalation methods have received much attention. In the in situ intercalative polymerization, the layered silicate is swollen within the liquid monomer (or a monomer solution) so the polymer formation can take place between the intercalated sheets. Polymerization can be started by heat, the diffusion of a suitable initiator, or an organic initiator. In addition, the melt intercalation method involves mixing the layered silicate with the polymer matrix in the molten state. Under these conditions, the polymer can crawl into the interlayer space and form an intercalated or an exfoliated nanocomposite. This method has great advantages over other methods due to environmentally benign as the absence of organic solvents and compatible with current industrial process.^10,15,32

Among all of the potential nanocomposite fillers, clay materials is natural, abundant, and low-cost that has high mechanical strength and chemical resistance therefore nanocomposite based on clay has been more broadly investigated.^30,33–35 Clay minerals, such as bentonite,³⁶ attapulgite, mica³⁷ and kaolinite,³⁸ can be found in nature from different sources and have been used in different purposes. BNC, a natural clay mineral, is available in many places of the world which is mainly composed of montmorillonite and may also contain feldspar, biotite, kaolinite, illite, cristobalite, pyroxene, zircon, and crystalline quartz (Table 1).²⁵ BNC with crystalline structure involves an alumina octahedral layer which is between two tetrahedral layers of silica.³⁹ It was selected due to its large swelling capacity which cause the polymer molecule was adsorbed onto both the particle surface as well as the interlayer surfaces. Therefore polymer include BNC has used in a wide variety on various industries like wastewater treatment, ceramics, cement and etc.^40–43

Table 1 Chemical composition of BNC (manufacturer information)

Component	% Component
SiO2	63.02
Al2O3	21.08
Fe2O3	3.25
FeO	0.35
CaO	0.65
MgO	2.67
Na2O	2.57
Trace	0.72
LOI	5.64

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The CDP and BNC are known as biocompatible and bioabsorbable materials. Thus, in this work we propose to synthesize water-insoluble β-CD nanocomposite hydrogel by using CT as a cross-linker and BNC as a nanofiller. Here, this nanocomposite was prepared with combine of in situ intercalative polymerization and melt intercalation methods. We thought that modification of CDP-CT with BNC could be an important route for modification of some properties of CDP such as thermal and surface properties. The effect of amount of bentonite in CDP-CT/BNC nanocomposite hydrogels on the swelling behaviour and physical properties was investigated. Therefore, the combination of CDP and BNC will develop potential applications in various fields include industrial, and environmental applications.

2. Experimental

2.1. Materials

BNC was obtained from Acros and β-CD was purchased from SDFCL, Mumbai. Sodium phosphate dibasic dodecahydrate (Na₂HPO₄·12H₂O), CT monohydrate, methanol and MB were purchased from Merck.

2.2. Preparation of CDP-CT/BNC nanocomposite

To a solution of 1.5 g of CT, 0.9 g of Na₂HPO₄ and 1.5 g of β-CD in 15 mL of water different amounts of the BNC (1, 5, 10 and 15 wt%) were added. The obtained solution was sonicated for 30 min and then dried at 100 °C. The mixture transferred into a culture dish and heated in an electric thermostatic oven (Electrolux, Sweden) at 180 °C for 45 min. Let the reaction cool to the ambient temperature, then the crude product (recorded as m₀) was purified by soaking and washing with water and methanol for several times, then freeze dried for 7 days (recorded as m₁). The insoluble polymer fraction (gel fraction, Table 2) was calculated as follows:

2.3. Preparation of CDP-CT

For comparison, CDP-CT (β-CD polymer without clay) was also prepared as previously described.²⁸ In a solution of 1.5 g of CT and 0.9 g of Na₂HPO₄ in 15 mL of water, 1.5 g of β-CD was dissolved, and the obtained solution was dried at 100 °C. The powdered mixture heated in an electric thermostatic oven (Electrolux, Sweden) at 180 °C for 45 min. Then, the solid was purified using the procedure described for CDP-CT/BNC.

Table 2 Compositions used to prepare hydrogels

Sample code	Molar ratio of β-CD to CT	Amount of clay (wt%)	Temp. (°C)	Y g	Time (min)
CDP-CT	1 : 6	0	180	73.13	45
CDP-CT/BNC 1	1 : 6	1	180	74.85	45
CDP-CT/BNC 5	1 : 6	5	180	76.37	45
CDP-CT/BNC 10	1 : 6	10	180	68.42	45
CDP-CT/BNC 15	1 : 6	15	180	72.64	45

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2.4. Characterization

ATR-FT-IR. The Fourier-transform infrared (FTIR) (Bruker Tensor 27) spectra were measured at room temperature using attenuated total reflection (ATR) mode in the range of 4000–400 cm⁻¹.

Thermal analysis. Differential scanning calorimetry (DSC), differential of differential scanning calorimetry (DDSC), thermogravimetric measurements (TGA) and differential thermogravimetric analysis (DTG) were acquired on a Netzsch (STA 409 PC/PG) instrument. About 10 mg of the samples was heated from 50 °C to 500 °C at a heating rate of 10 °C min⁻¹ under nitrogen atmosphere (flow rate of 50 mL min⁻¹).

XRD. Powder X-ray diffraction (XRD) analysis of the BNC and the synthesized products was determined on a Philips X'Pert PRO equipped with Cu Kα radiation (λ = 1.54178 Å) having a scanning range of 10–80° Bragg's angle. In addition, the XRD peaks from BNC, CDP-CT/clay 1 and CDP-CT/clay 15 was determined on a stone equipped with Cu Kα radiation having a scanning range of 0–10° Bragg's angle.

SEM and EDAX. The hydrogels were immersed in distilled water at ambient temperature to reach equilibrium. The equilibrated hydrogels were lyophilized with a Zirbus Freeze Drying System to avoid collapse of the porous structure until all of the solvent had been sublimed. The surface morphology of hydrogels were examined by field emission-scanning electron microscopy (FE-SEM, Sigma, Zeiss). To prepare samples for FE-SEM, all samples were fixed on a brass stub using double-sided tape and then gold coated in vacuum by a sputter coater. The pictures were taken at an excitation voltage of 15 kV. The elemental composition of the nanocomposites was determined by energy dispersive spectroscopy (EDS) analysis with an EDAX Oxford instruments attached to the SEM column.

TEM. Transmission electron microscopy (TEM) analysis was performed at a Zeiss EM10C (Germany) instrument using an acceleration voltage of 100 kV, to observe the dispersion of clay platelets within the polymer matrix. For this measurement, hydrogels were firstly sonicated, and the samples were prepared by dropping diluted hydrogels on the Lacey support grid and dried in a vacuum oven an ambient temperature.

2.5. Swelling properties

The classical gravimetric method was adopted for following the progress of the swelling process. The dried CDP-CT/BNC nanocomposite (ca. 0.10 g) was immersed in water at ambient temperature for regular time intervals. The samples were then removed from the water with centrifuged and weighed after the water on the surface was carefully and quickly wiped out with wet filter paper. The swelling ratio (g g⁻¹) of the nanocomposite was calculated as follows:
where W was the swelling ratio, W_s and W_d are the weight of the hydrogels before and after drying respectively. The reported swelling results are the average of three measurements.

2.6. Adsorption experiments

The adsorption studies of MB were conducted by batch equilibrium method. CDP-CT and CDP-CT/BNC (0.1 g) were mixed with 50 mL of MB in flask under stirring. The suspension was incubated at 25 °C for 4.5 h and then the adsorbent was separated from solution by centrifuged for 15 min. After adsorption equilibrium, the concentration of MB in the solution was recorded using a UV-visible spectrophotometer (Cary 50; Varian, Australia) at 664 nm. The equilibrium concentration of MB was calculated referring to the calibration curve of MB. The equilibrium sorption capacity (q_e) was calculated using the following equation:
where C₀ and C_e were initial and equilibrium concentrations of MB (mg L⁻¹), respectively, m was the mass of adsorbent (g) and V was the volume of the solution (L).

To exhibit the removal efficiency directly, 0.01 g of hydrogels were mixed with 5 mL of MB (1500 ppm for CDP-CT and CDP-CT/clay 1). After the mixtures were stirred for 1 h and centrifuged, each supernatant was put into a vessel for photography. Deionized water was taken as the reference.

3. Results and discussion

A schematic representation of the overall polymerization is given in Scheme 1. Nanocomposite hydrogel was prepared with combine the in situ intercalative polymerization and melt intercalation methods. Here, the BNC was dispersed within the β-CD solution by the use of sonication, which was absorbed the β-CD by the pore/layers of BNC. The β-CD was formed as a layer over the clay particles and caused stability of the dispersed compound in water.⁴⁴ β-CD as a monomer, CT as a cross-linker and Na₂HPO₄ as an initiator were mixed with clay under magnetic stirring, and these stirring was continued until all of the water was evaporated. In the meantime, the presence of β-CD may cause be the exfoliation of the clay layers. Polymerization of β-CD in this mixture was carried out with melt procedure in the presence of CT and Na₂HPO₄ at 180 °C, which formed a nearly uniform layer over the exfoliated layers of the BNC. This confirms the formation of nanocomposite particles.

Scheme 1 An illustration for the polymerization and binding of CD onto BNC.

3.1. Characterization

ATR-FTIR. The ATR-FTIR spectra of BNC, CDP-CT and CDP-CT/BNC are shown in Fig. 1. In spectra of the BNC, the peak at 3625 and 3440 cm⁻¹ corresponds to OH stretching of Si–OH and water, and 1637 cm⁻¹ corresponds to deformation vibrations of the interlayer water in the clay. The peak presented at 1062 cm⁻¹ is related to the Si–O stretching vibration of silicate layer, and 527 and 470 cm⁻¹ are due to the stretching of Al–O and bending of Si–O, respectively.^5,19 The spectra of the CDP-CT/BNCs revealed the O–H stretching vibration at 3467 cm⁻¹, the C–H stretching vibration at 2936 cm⁻¹, the C=O stretching vibration at 1740 cm⁻¹ and other adsorption peaks in the CDP-CT also appeared nearly at the same wavenumbers and peaks of BNC in nanocomposite was disappeared. These results indicating that the nanocomposites were prepared very well and these are fabricated chiefly by CDP-CT units.

Fig. 1 ATR-FTIR spectra of BNC, CDP-CT, and CDP-CT/BNC 1, 5, 10, and 15.

XRD analysis. XRD was used to investigate the change in the interlayer spacing of clay particles upon intercalation. For the XRD pattern of nanocomposite hydrogels (shown in Fig. 2a–c), there are no obvious differences between the pure CDP-CT and all of the CDP-CT/BNC, which suggests that the clay does not have any effect on the degree of crystallinity of the CDP-CT. The BNC (Fig. 2a) shows a diffraction peak at 2θ = 7° corresponds to the 12.63 Å basal spacing, 001 reflection of the montmorillonite and the peak around 2θ = 28.6°, assigned to the impurities be presented in clay (mostly quartz and feldspar). Fig. 2b and c shows the hydrogels containing BNC do not present the montmorillonite peaks, which suggesting the existence of exfoliated montmorillonite layers throughout the polymer matrix. The peak around 2θ = 28.6° is disappear in XRD patterns of CDP-CT/BNC 1 and 5, conversely, a small shoulder corresponding to the peak around 2θ = 28.6° is present in XRD patterns of CDP-CT/BNC 10 and 15. This indicates that a small part of the clay formed aggregates during film formation with increased of amount of BNC. For the CDP-CT/BNC hydrogels, the XRD patterns indicate a very good dispersion of the BNC in the polymer matrix, since there is no significant peak evident around the expected 2θ angle. The achieved results imply that the montmorillonite in BNC was exfoliated all over of the nanocomposites, while only a small amount of clay platelets namely in CDP-CT/BNC 10 and 15 were still remained as aggregates.

Fig. 2 XRD pattern of (a) BNC (2θ = 1–80), (b) CDP-CT/BNC 1 and 15 (2θ = 1–10), and (c) CDP-CT, and CDP-CT/BNC 1, 5, 10 and 15 (2θ = 10–80).

SEM and EDAX analyses. The surface morphology of CDP-CT and CDP-CT/BNC 1 and 15, and to evaluate the dispersion of the BNC in the CDP-CT/BNC nanocomposite was studied by FESEM analysis (Fig. 3). As can be seen, all the hydrogels have similar porous structures. For the FESEM image of CDP-CT/BNC 1 (shown in Fig. 3), there are no obvious differences in surface, which proposes that a few amount of clay does not have any effect on morphology of product and it was completely covered. Furthermore, in the CDP-CT/clay 15 (Fig. 3) was found that the layers of BNC are well distributed and homogenized in the composite and the stacked layers are delaminated indicating perhaps an exfoliation morphology. This revealed that in the CDP-CT/BNC 15, BNC and CDP-CT was formed nanocomposite; hence, this was proved a greater amount of clay would result in an improved composite level or a decreased average pore size. The CDP-CT/BNC 15 nanocomposite yield a decreased in pore and functional group of CDP-CT due to the interaction between polymer matrixes with clay nanolayer and are believed to decrease absorptivity.

Fig. 3 FESEM images of CDP-CT, CDP-CT/BNC 1 and CDP-CT/BNC 15.

The composition of CDP-CT, CDP-CT/BNC 1 and CDP-CT/BNC 15 were estimated from EDAX measurements. As shown in Fig. 4, only the element of C (60.5%) and O (39.5%) are existing in pure CDP-CT. The EDAX performed on CDP-CT/BNC 1 also shows only the presence of C (54.7%) and O (45.3%), and hence confirm the complete covered of BNC during the polymerization. Compared with the EDAX spectrum of CDP-CT and CDP-CT/BNC 1, presence of Si, Al, Fe and Mg peaks and decrease in percent of C (site 1 = 57.6 and site 2 = 43.7 wt%) and O (site 1 = 31.1 and site 2 = 30.8 wt%) elements were confirmed the existence of BNC nanofiller in the CDP-CT/BNC 15.

Fig. 4 EDAX spectra of the CDP-CT, CDP-CT/BNC 1 and CDP-CT/BNC 15 (two sites).

TEM analysis. For further investigation of the clay morphology inside the polymers and to evaluate the degree of dispersion, we were considered TEM studies. The TEM micrographs of nanocomposite samples with different loadings amount of BNC (1 and 15%) have been shown in Fig. 5. In these nanocomposites, the clay particles are well dispersed throughout the polymer matrix so that an exfoliated structure is obtained. The extracted data from TEM are agree with results of XRD and FE-SEM.

Fig. 5 TEM images of CDP-CT/BNC 1 and CDP-CT/BNC 15.

Thermogravimetric analysis. Fig. 6 shows the TGA plot of the materials, along with their derivatives (i.e., so-called differential thermogravimetry (DTG) plot). TGA and DTG are used to determine the thermal stability of CDP molecules after composited with BNC. Pure CDP-CT curve shows two main steps of weight loss. The first step ranging from 50 to 180 °C is due to the loss (6%) of different types of water molecules depending up on their interaction with the CDP-CT. The second weight loss (47.5%) ranging from 190 to 400 °C is assigned to the decomposed of CDP-CT. In addition, the DTG curve of CDP-CT exhibits two notable peaks centered at 270 and 340 °C. These two thermal events are related to mass losses from the decomposition of the copolymer as follows; (i) decomposition of CT portion at 270 °C, and (ii) mass losses due to β-CD decomposition at 340 °C. The incorporation of BNC in CDP-CT decreased the thermal stability of the CDP-CT/BNC 1 and increased the thermal stability of the CDP-CT/BNC 10 and 15 nanocomposite. It is notable that the clay acts protectively against the polymer thermal degradation, at temperatures above 350 °C, where the CDP-CT/clay 10 and 15 retains almost 50% and 44% (respectively) of its initial mass until 500 °C, while pure CDP-CT and CDP-CT/clay 1 retains 40% and 39% (respectively) of its initial mass at the same temperatures stated above. This enhancement in the thermal stability is because of the interaction between the clay nanolayer and the polymer matrix in CDP-CT/clay 10 and 15, which can act as barriers maximizing the heat insulation and minimizing the permeability of volatile degradation products to the material. Decreased thermal stability in the presence of 1% of BNC has also been observed for CDP-CT/clay 1. In this compound, clay may be treated as an impurity which caused decreased in thermal stability.

Fig. 6 TGA (upper) and DTG (bottom) curves for CDP-CT and CDP-CT/BNC 1, 10 and 15.

DSC studies. The differential heat flow curves, together with its derivative (i.e., so-called differential of differential heat flow (DDSC) plot), of pure CDP-CT, CDP-CT/BNC 1 and 15 are presented in Fig. 7. The DSC data of these compounds are shown an endothermic event occurring about 100 °C, due to vaporization of residual water.⁴⁵ It is well documented that the DSC curve of β-CD, a crystalline compound, exhibits two endothermic peaks which these two peaks are appeared at its decomposition at around 320 °C.⁷ In the DSC thermogram of CDP-CT, an endothermic peak was appeared at about 315 °C that confirmed formation of CDP-CT. DSC curve of CDP-CT/BNC 1 was revealed the exothermic peak of the polymer decomposition without any noticeable changes, which showed the nanocomposites were prepared chiefly by CDP-CT units. Based on DSC curve of CDP-CT/BNC 15, the peak of the decomposition of polymer units were shifted after mixing with clay, and revealed lower intensity than CDP-CT and CDP-CT/BNC 1. This phenomenon should be due to the preparation of nanocomposite hydrogels of interaction between CDP-CT and BNC. The DDSC curve of hydrogels, shown in Fig. 6 are consistent with the results which were obtained from DSC.

Fig. 7 DSC (upper) and DDSC (bottom) thermograms for the CDP-CT and CDP-CT/BNC.

3.2. Swelling properties

To determine the influence of BNC on the swelling behavior of the hydrogels, the swelling ratios of the CDP-CT and CDP-CT/BNC nanocomposites were tested and the results are presented in Fig. 8. CDP-CT (contain numerous –OH and –COOH groups) are hydrophilic, which can form hydrogen bonds with water to combine with more water molecules. The CDP-CT revealed an equilibrium swelling of 397% which was increased with the incorporation of 1% BNC to 448%. Although this result is not clear, but we propose it can due to act of both functional groups of CDP-CT and BNC for water absorption in CDP-CT/BNC 1. However, the swelling ratio was decreased with the incorporation of 5 and 10% BNC to 371 and 350 (respectively) in the nanocomposite hydrogels.

Fig. 8 Swelling ratio of the dried CDP-CT and CDP-CT/BNC with different amounts of clay (0–15 wt%).

The CDP-CT/BNC with 10% clay has the lowest minimum swelling ratio. However, the equilibrium swelling ratio of the CDP-CT/BNC hydrogels decreased when clay was increased from 1 to 5 and 5 to 10 wt%. This is probably because of the decrease of the hydrophilic content, the –COOH and –OH groups on the CDP-CT due to the formation of hydrogen bonding with the BNC, and also much denser networks formed in the hydrogels. On the other hand, the increase of swelling ratio at CDP-CT/BNC 15 (370%) in compared to CDP-CT/BNC 10 is due to the presence of free functional groups of BNC.

3.3. Investigation of removal MB from water

Nowadays, a number of technologies such as membrane separation, filtration, coagulation, ion exchange, foam flotation and adsorption are available to purify wastewater.^32,40,46 However, adsorption is strongly favoured over the others due to its simplicity, low cost and effective for removing pollution from wastewater. In recent years, considerable attention has been paid to the organic environmental pollutants such as organic dyes. Dyeing, widely used in the synthesis, textile, printing, food and cosmetic industries, have been released into the environment in large quantities.⁴⁷ Some dyes and their degradation products are not easily biodegradable and have a toxic or carcinogenic properties on human beings.⁴⁸ Therefore, it is necessary to remove dyes prior to their discharge. Numerous adsorbents, such as BNC,²⁴ graphene,⁴⁹ graphene oxide,⁵⁰ garlic peel,⁵¹ activated clay,⁵² carbon nanotubes²⁰ and activated desert plant³⁹ have been studied for adsorption of dyes from aqueous solutions. The major of these researches are based on the availability of raw materials such as clays and agricultural waste.

In this work, adsorption of MB onto β-CD hydrogel and its nanocomposites were performed using monitored spectrophotometrically. For comparison the adsorption behaviours of these nanocomposite hydrogels, the amount of BNC and initial dye concentration were as operating variables for the adsorption study. In particular, the pH (a constant value of 6), mass of hydrogels, temperature and time for all the experiments were kept constant.

As was illustrated in Fig. 9, the photograph directly displays the successful decoloration of MB by hydrogels. When initial concentration of MB is 1500 ppm for CDP-CT and CDP-CT/BNC 1, the decolorized solution is transparent and approximately colorless. The removal efficiencies of the CDP-CT/BNC 1 is slightly higher than the CDP-CT.

Fig. 9 Photographs of the MB solution before (MB) and after the CDP-CT and CDP-CT/clay 1 treatment at 25 °C.

The absorption isotherm of MB by hydrogels are shown in Fig. 10. At low C_e values, the absorption capacities (q_e) increase quickly and almost linearly when C_e was lower than 20 mg L⁻¹ for all hydrogels. A platform was gradually formed when C_e was higher than 20 mg L⁻¹, showing that the adsorption capacity of β-CDP was approximately saturated. When initial MB concentrations are lower than 1500, 1500, 1300, 1000 and 1000 mg L⁻¹, the removal efficiencies are higher than 99.6, 99.4, 99.8, 99.9 and 99.8% for CDP-CT, CDP-CT/BNC 1 to 15, respectively. Because they could remove efficiencies of MB solution in a wide concentration range, these hydrogels might be excitedly applied in treating not only the industrial effluent but also the contaminated natural water.

Fig. 10 Adsorption isotherms of MB on CDP-CT and its nanocomposites hydrogels at 25 °C.

For describing equilibrium, studies of the adsorption dyes on materials have been used through mathematical models. Adsorption isotherms describe how dye interact with sorbent materials. Here, the experimental data are utilized to fit with both the Freundlich and Langmuir models.

The Langmuir model assumes that the adsorption takes place on a homogenous surface without any interaction between adsorbents in the plane of the surface. The equation of the Langmuir isotherm can be expressed as follows:¹⁸

where C_e (mg L⁻¹) and q_e (mg g⁻¹) are the liquid phase concentration and solid phase concentration of adsorbent at equilibrium, q_m (mg g⁻¹) is the maximum adsorption capacity reflected to a complete monolayer coverage, K_L (L g⁻¹) is a Langmuir constant related to the strength or affinity of the binding sites and energy of adsorption. A straight line was obtained when C_e/q_e was plotted against C_e (Fig. 11), and q_m and K_L could be calculated from the slope and intercept of this straight line. Table 3 lists the values of q_m and K_L for the all hydrogels.

Fig. 11 The Langmuir isotherm plots for MB adsorption by hydrogels at pH 6.

Table 3 Adsorption isotherm parameters for MB adsorption on hydrogels

Adsorbents (amount of clay)	Langmuir isotherm
	q m (mg g^−1)	K L (L g^−1)	R ^2	R L
0	744.7	1.741	0.9999	0.0004
1%	798.45	1.442	0.9999	0.0004
5%	737.83	1.763	0.9999	0.0003
10%	661.32	1.912	0.9996	0.0004
15%	623.28	2.065	0.9998	0.0003

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Additional parameter R_L, called the separation factor, which is defined as follows:⁵³

where K_L (L g⁻¹) is the Langmuir constant and C₀ (mg L⁻¹) is the highest initial dye concentration. This parameter shows the isotherm is unfavorable (R_L > 1), favorable (R_L < 1), linear (R_L = 1), or irreversible (R_L = 0).⁴⁸Table 3 shows R_L values between 0 and 1, which indicates the adsorption of MB onto CDP-CT and its nanocomposites are favorable.

The Freundlich equation is an equation based on adsorption on a heterogeneous surface. The equation is usually expressed as follows:³⁷

A straight line was obtained when ln q_e was plotted against ln C_e, n and k_F could be calculated from the slope and intercept. Apparently, this plot demonstrated that equilibrium adsorption data of MB were not described by the Freundlich isotherm.

As it can be seen from Table 3, both q_m and K_L depend on the amount of BNC. Comparing the adsorption capacity of the four nanocomposite adsorbents, the q_m decreased with increase of the BNC amount. But with increasing 1% of BNC in CDP-CT, the values of q_m and K_L increase, which indicates that the CDP-CT with 1% of BNC, the more advantageous for the adsorption of MB onto nanocomposite.

Chemically, there are plenty of oxygen atoms on β-CD in the forms of ether and hydroxyl groups. The oxygen atoms are highly friendly to positively charged molecules due to strong electrostatic interactions.^21,54 We have reported that β-CD interact with Cu²⁺ intensely.⁸ Numerous studies on β-CD molecules have demonstrated the strong interaction of oxygen atoms with cations.⁵⁵ Therefore, β-CD in the polymer is very important in the adsorption toward guest molecules through at least two aspects. One is that the cavity of β-CD in the polymer can directly adsorb guest materials with the molecular size suitable to the cavity of β-CD. The other is that the fixed structure of β-CD increase the brittleness of the polymer, so that the polymer is easy to be grinded into fine granulae and the functional groups are well exposed to outside.²³ The results of previous studies was showed that cyclodextrin polymer cross-linked with epichlorohydrin (CDP-EP), include ether and hydroxyl groups, was inefficient in adsorption toward MB,¹⁶ probably because MB was quite hydrophilic and difficult to be included by β-CD and the electrostatic interaction between the polymer and the MB molecules, MB molecules are positively charged in nature, is not efficient. Alternatively, CDP-CT included oxygen atoms in the forms of ether, hydroxyl, ester and carboxyl groups were efficient in adsorption toward MB (Table 3) that due to the electrostatic interaction between the hydrogel and the MB molecules is the primary binding strength. Here, these indicated that the sorption on CDP-CT was chiefly dependent on the presence of carboxylic groups.

On the other hand, BNC with anionic and cationic ingredient exhibit a strong affinity for carboxylic anionic CDP-CT and heteroaromatic cationic dyes.²⁴ The highest adsorption in CDP-CT/BNC 1 compared to CDP-CT are due to the presence of both functional groups of CDP-CT and BNC. Nevertheless, decrease in adsorption capacity of nanocomposite with increase amount of clay is inferred that the carboxyl groups of CDP-CT were effectively connected to the groups of BNC.

The absorption capacity of CDP-CT and the best its nanocomposites (CDP-CT/BNC 1) with other absorbents, using the data by Langmuir model, which is commonly adopted in the studies of other absorbents, were compared. The q_m value of CDP-CT and CDP-CT/BNC is highest of other β-CD or clay-based absorbents, such as BNC, CD containing carboxymethylcellulose (CD/CMC), CDP-CT-PVA and clay, also, is competitive with that of other excellent absorbents, such as graphene oxide (GO). Table 4 summarizes the adsorption capacity of various absorbents of MB.

Table 4 Comparison of the maximum adsorption capacities of MB on different absorbents

Adsorbents	q m (mg g^−1)	References
Graphene oxide	714	Yang, 2011 (ref. 50)
G–CNT hybrid	81.97	Ai, 2012 (ref. 57)
Bamboo-based active carbon	454.2	Hameed, 2007 (ref. 36)
Filtrasorb 400	476	El Qada, 2008 (ref. 58)
Peat	323.7	Fernandes, 2007 (ref. 12)
Powdered activated carbon	91	Yener, 2008 (ref. 14)
Carbon nanotubes	46.2	Yao, 2010 (ref. 20)
Silica nano-sheets	12.66	Zhao, 2008 (ref. 27)
Clay	6	Gürses, 2004 (ref. 59)
Clay	300	Freundlich, 1906 (ref. 60)
BNC	172	Hong, 2009 (ref. 24)
CD/CMC	56.5	Crini and Peindy, 2006 (ref. 56)
CDP-CT-PVA	105	Zhao, 2009 (ref. 23)
CDP-CT	752.15	This work
CDP-CT/clay 1	806.45	This work

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Crini et al. reported an CDP-EP contained carboxyl groups which had an adsorption capability of 56.5 mg g⁻¹ toward C.I. Basic Blue 9 (i.e. MB).⁵⁶ Zhao et al. reported a β-CDP cross-linked by CT contained PVA, which had an adsorption capability of 105 mg g⁻¹ toward MB. They showed that the adsorption was dependent on the attendance of carboxyl groups. Our result was very well, probably due to β-CDP cross-linked by CT included a larger number of acidic groups and CDP-CT/BNC contained a larger number of acidic groups of CDP-CT and anionic groups of BNC.

4. Conclusions

The results showed that the nanocomposite hydrogels containing β-CD, –COOH and BNC (5, 10 and 15%) nanofiller was superior to polymer containing only β-CD, –COOH or BNC (1%) in mechanically properties, probably due to the formation of good composited between CDP-CT and BNC nanofiller. However, the nanocomposite hydrogels containing β-CD and –COOH with 1% of BNC was superior to hydrogel containing only β-CD, –COOH adsorption toward MB, possibly due to the presence of free carboxylic groups of polymer and anionic groups of BNC. The nanocomposite hydrogels was obtained by exfoliated of BNC using CD and then polymerization from nontoxic materials and through environment of friendly procedures, so it was probable to become an efficient and green adsorbent for water purification. It is also attractive to find for these hydrogels various potential applications such as drug delivery system and biocompatible materials.

Acknowledgements

The authors express appreciation to the Shahid Bahonar University of Kerman Faculty Research Committee for its support of this investigation.

Notes and references

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