One-step synthesis of super-absorbent nanocomposite hydrogel based on bentonite

The nanoparticles of bentonite can be easily synthesized by hydrothermal treatment with microwave-assisted method. The products were characterized by XRD, FTIR, BET, SEM, EDS, and DLS. Results have revealed that the nano-bentonite was in nanometer scale with particles size of 200–300 nm. The SEM analysis of the nano-bentonite dispersion showed that a decrease in agglomeration and surface roughness. The nano-bentonite possessed high surface area (152.4 m2.g−1) and pore volume (0.25 cm3.g−1). Additionally, nanocomposite hydrogels (NB/PAA) were synthesized through the free-radical polymerization of acrylic acid (PAA) in the presence of the as-synthesized nano-bentonite (NB). The presence of NB in hydrogel matrix may improve the tensile strength of hydrogel. The tensile strength of the hydrogel increases from 0.294 to 1.609 MPa when NB content was 1.0% (NB10/PAA). Under the optimum conditions, the nanocomposite hydrogels (NB10/PAA) exhibited high water absorption capacity of 420.0 gwater.ghydrogel−1. These findings contribute to the sustainable development and green chemistry.


Introduction
Sodium and calcium bentonite are a natural clay mineral with montmorillonite (MMT) as the main component. In the chemical composition of bentonite, beside the two main elements which are aluminum (Al) and silicon (Si), several elements such as Fe, Ca, Mg, Ti, K, and Na are present [1,2]. The influence of Al 2 O 3 /SiO 2 ratio on the MMT ranges from 0.25 to 0.50 [3]. The network structure of MMT, isomorphic substitution of cations often occurs in (1) the Al 3+ octahedral network replaced by Mg 2+ , Fe 2+ , Zn 2+ , or Li 2+ or (2) the Si 4+ tetrahedral network replaced by Al 3+ or Fe 3+ [4]. The conformational replacement of high valence cations with low valence cations results in positive charge deficiency in the shale structure with negative and positive charges appeared at the MMT surface (SiO 2 ) and the edges (Al 2 O 3 ), respectively [5] . Nano-bentonite (denoted as NB) refer to bentonite with detachable MMT layers that are less than 100 nm in size [6]. The highest surface area and the power to modify the polarity, zeta potential, acidity, cation exchange capacity (CEC), capillary size, and other properties of clay minerals are two major advantages of NB. The NB have been synthesized by various methods, including centrifugation, freezing, ultrasound, the use of surfactants, and concentrated acids [7][8][9]. Ali Morsali has developed a straightforward process to produce fibrous NB by using high-intensity ultrasound method and ethanol over a 0.5 to 4.0 h period [10]. However, the as-synthesized NB has a lower surface are (11.08 m 2 .g −1 ) than the the raw bentonite (24.08 m 2 .g −1 ). By activation bentonite with 8.0 M HNO 3 at 60°C for 24 h, KengYuen Foo has produced NB with Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. pesticide adsorption capacity of 2.032 mmol.g −1 [11]. The NB with a high surface area (493.97-868 m 2 .g −1 ) was also synthesized using a denaturation method with strong acids (e.g. HCl, HNO 3 , and H 2 SO 4 ) for 2 h at 70°C. The NB exhibited high adsorption capacity for Pb 2+ , Cu 2+ , and Co 2+ ions in aqueous solution [12]. The benefits of acidification include simple filtration, enhanced material's surface qualities, removal of undesired components, and development of acidic adsorption sites that promote adsorption capacity, as compared to the raw bentonite [11]. However, the use of acidification method requires time-consuming synthesis and high concentrations, which cause negative impacts to the environment. Therefore, innovative technologies (e.g. ultrasonic and microwave-assisted methods) are employed to enhance the production effectiveness, while reducing the acid concentration and shortening the synthesis time.
The NB is widely used in numerous industries, including biomedicine [13], environmental remediation [14], building [15] and polymer [16]. Hydrogels, which is referred to a three-dimensional network structure of hydrophilic functional groups with a high water absorption and retention capacity, has been used to contain NB in the studies of polymers. There are numerous uses for hydrogel in adsorption, biosensors, biomedicine, agriculture, etc [17][18][19]. The use of NB is increasingly favorable to enhance the properties of hydrogels due to its mechanical and thermal strength [20][21][22][23]. A nanocomposite hydrogel was formed through the free-radical polymerization of acrylic acid and acrylamide in the presence of inorganic clay (hectorite) with a 1500.0% of mechanical strength [24]. In this paper, NB nanoparticles were synthesized from the raw bentonite (denoted as Ben) deposits of Binh Thuan province (Vietnam) by a simple method and application to the synthesis of superabsorbent materials. Additionally, our peculiar attention is to investigate the effect of NB size on the water adsorption and mechanical properties of the nanocomposite hydrogels.

Synthesis of NB
Briefly, a total of 200 g of Ben was initially dried in a drying cabinet at 85°C for 12 h. Next, 100 g of pretreatment bentonite was introduced gradually into a reactor containing 300 ml of the solution with 2.0 M H 2 SO 4 and 2.0 M HNO 3 (1:1 volume ratio) and was continuously stirred at 250 rpm with a mechanical stirrer for 30 min at room temperature. The mixture was then introduced into a Teflon lined autoclave, and placed in microwave reactor (Model MWO-G20SA, power 700 W) over a period of 20 min. The soil product was rinsed twice with 0.1 M NaOH to reach pH 4.0 and five times with distilled water and ethanol to reach a constant pH at approximately 7.0. Finally, the product was dried at 100°C for 12 h and calcinated at 600°C in air to obtain NB. The NB samples synthesized at 5, 10 and 20 min were denoted as NB5, NB10 and NB20.

Synthesis of nanocomposite hydrogels
A nanocomposite hydrogels synthesized based on AA and NB graft polymerization in presence of MBA crosslinking agent using a KPS initiator was carried out according to the following procedure with slight modification [25]. Briefly, a 250 ml three-necked flack was fixed with a magnetic stirrer. Oxygen gas was removed by nitrogen gas to allow the polymerization reaction for 10 min. The reaction was heated up to the desired temperature using the Julabo Thermostat (temperature precision ±0.5°C). For the NB suspension solution, NB was sonicated for 10 min until the mixture became clear using CPX500 ultrasonography (500 W, 20 KHz, USA). Subsequently, NB and a quantity of AA (30.0 percentage by mass) that had been cold-neutralized with NaOH were gradually added to the reactor until the total volume reached V = 40 ml. Under an inert atmosphere, MBA (32 mg) and KPS (160 mg) were added. For the duration of the reaction, a 400 rpm stirring rate and a N 2 atmosphere was maintained. The reaction mixture was raised to a pre-polymerization temperature of 50°C over a period of 15 min. Finally, the hydrogel was formed at 70°C for 3 h.
After the reaction, the mixture of hydrogel and AA monomer was precipitated, filtered, and washed many times with ethanol to eliminate the remaining monomer and obtain the pure hydrogel. After being washed, the product was ground into uniform-sized particles and dried to constant weight at 60°C in a vacuum drying cabinet. The hydrogel samples that contain Ben and NB are designated as Ben/PAA, NB5/PAA, NB10/PAA, and NB20/PAA, while those without NB are identified as PAA.

Characterization methods
The phase compositions of the obtained samples were characterized by the x-ray Diffraction (XRD) method using D8 Advance bucker equipped with graphite monochromatized Cu K α radiation (λ = 1.5418 Å) and Fourier transform infra-red spectroscopy (FT-IR) spectrometry (Effect 410 -Nicolte, USA). The particle morphologies of the materials were observed by using Scanning electron microscope (SEM) analysis (Hitachi S4800, Japan). The chemical composition of the materials was analyzed by x-ray fluorescence (XRF) method (S4-Bruker, Germany). The surface area and pore size distribution of the samples were analyzed by N 2 adsorption-desorption isotherms at 77 K (BET) method on TristarII-3030 system (Micromeritics, USA) with N 2 absorption at 77 K. The particle size distribution was determined by Dynamic light-scattering (DLS) method on Malvern Zetasizer Nano ZS system (Malvern Instrument, UK).

Water absoption capacity (Q ads ) of hydrogel nanocomposites
Prior to the experiment, all samples for the equilibrium swelling were dried under vacuum at 60°C to a net weight. The gravimetric method was used to determine the water absorption capacity (Q ads ). A total of about 0.2 ± 0.001 g of dry hydrogel was weighed in a filter bag, then submerged in distilled water (or 0.9% salt) at 25°C for 24 h. Once the achieve equilibrium was achieved, the remaining water was filtered through a 100-mesh standard sieve (100 mesh ASTM BS410, China). The hydrogel mass was calculated after swelling. The experiment was performed in triplicates and the average value was represent as mean ± standard deviation (Error bars). The following formula was used to calculate the water absorption equilibrium.
Where m 2 (swelled) is the weight of the nanocomposite hydrogel after water absorption and m 1 (dried) is the weight of the dry gel.

XRD patterns
The XRD diagrams of the as-prepared Ben, NB5, NB10 and NB20 were presented in figure 1.
In the figure 1, the crystalline peaks at 2θ value of 5.72°in purified MMT sample, corresponding to the periodicity d = 13.04 Å in direction of (001) of the purified bentonite. The peak at 2θ of 23.5°and 26.64°are quartz coordinates (quartz). Dolomite (CaMg(CO 3 ) 2 ) is represented by the values of 2θ = 31°. The peak at 2θ value of 29.5°stand for calcite (CaCO 3 ) [26,27]. Due to the purification process, NB has less quartz, feldspar, and calcite and more clay minerals (montmorillonite and kaolinite) than Ben. The NB samples show characteristic peaks at 2θ = 5.72°, 20.8°, 35.9°, corresponding to the direction (001), (110) and (124) of MMT, respectively [26]. The basic distance d 001 is 13.04 Å when the compensating cation is Ca 2+ or Mg 2+ in the central layer of bentonite. When Al 3+ is replaced by H + for the lower bentonite layer spacing and Ca 2+ or Mg 2+ cations are replaced by Na + , this value was lowered to 9.9 Å, resulting in a microporous of bentonite [27,28]. Results from the XRD diagram ( figure 1(b)) also show that the peak characteristic for d 110 of the NB5 has a greater intensity than the peak d 110 of the NB10 and NB20 samples. It shows that a smaller particle size is responsible for the broadened diffraction peak. The crystalline diameter of NB samples were calculated according to the Scherrer's equation [29,30]. The crystalline diameters are 20.07 nm, 26.17 nm and 32.71 nm for NB10, NB20 and NB5, respectively. This result indicates that the treatment of acid mixture enables the removal of impurity phases and decreasing particle size. The acid treatment reduced d 001 by dispersing the principal bentonite particles into smaller ones and flattened the solvate shell of the cations between layers. Figure 2 shows the FTIR spectra of Ben, NB5, NB10 and NB20 samples. The sharp absorption peak at 1028 cm −1 to 1040 cm −1 characterized the valence oscillation of the Si-O bond in the tetrahedron, while the peaks at 471 cm −1 and 526 cm −1 are due to an Si-O bending vibration [27] . The peak at 532 cm −1 characterized the replacement of Al 3+ by Mg 2+ in the tetrahedron. The absence of the bands at 1450 cm −1 to 1550 cm −1 and 850 cm −1 to 890 cm −1 is frequently linked to variations in the CO 3 2bond in bentonite and show the purity of the bentonite after high denaturation [5]. As H + protons attacked the -OH groups during the alteration of bentonite, the -OH vibration and octahedral cations were altered. Concurrently with the removal of Al 3+ from the tetrahedral plates, the primary particles that had a three-dimensional spatial structure and were smaller than the starting material were produced [31]. Figure 3 shows the N 2 adsorption isotherms of the Ben and NB samples. The textual characteristics of Ben and NB samples are given in table 1. According to de Boer's classification [32], figure 3(a) depicts the isotherms of the two samples with a D-type mean capillary hysteresis loop. This hysteresis pattern reveals the samples with capillaries with a slit form. The arrangement of Ben particles into plates causes these capillaries to develop [32,33]. When P/P o ∼ 1, the isotherm grows abruptly in a vertical direction, thereby indicating the presence of relatively big capillaries. The general pattern of the adsorption isotherm matches with bentonite structure. Figure 3(b) shows that all samples have typical capillaries that form in-between the bentonite grains with an average diameter of 2.0 to 16.0 nm. In comparison to the Ben, more capillaries are present in NB with a diameter of 5.0-6.0 nm. As shown in table 1, NB10 sample had the highest surface area (152.4 m 2 .g −1 ) and pore volume (0.25 cm 3 .g −1 ) of the examined samples, while Ben demonstrated the lowest surface area (33.9 m 2 .g −1 ) and pore volume (0.071cm 3 .g −1 ). The pores become more open in the presence of soluble solvents due to acid interactions that increase the surface area of NB.

N 2 adsorption-desorption isotherms at 77 K
The following equation can be used to determine the number of structural layers (n) in a bentonite bead, given the bentonite surface area (S BET = 801.3·n −1 + 5.13) [34]. Bentonite has 5.0 layers (NB) and 27.0 layers (Ben) in its structural layers before and after modification, respectively. Therefore, the surface area increases with decreasing bentonite particle size. Results have indicated that after purification, NB has a microcapillary structure, a large surface area, and contains very minute particles. These findings are in line with the results from XRD, SEM, and DLS analyses below.

XRF analysis
The chemical compositions of NB samples are demonstrated in table 2.
The XRF analysis (table 2) was conducted to identity the chemical composition of Ben and NB samples. The main constituents of Ben and NB samples are silica and aluminium oxides. In the NB samples, the weight   According to the EDX results, O, Si, Al, and C are the chemical elements that are most abundant in Ben (figure S1 and table S1). Additionally, S and N elements are activated during the Ben activation process using an H 2 SO 4 and HNO 3 solution. As a result, it is clear that the Ben and NB are a 2:1 type of smectite clay that mainly consists of MMT, followed by a minor quantity of nontronite, and low iron forms of goethite that co-exist with MMT. Following denaturation, the amount of Fe and Ca elements in NB was decreased, while the Si/Al ratio increased to 3.7.

Morphology of Ben and NB samples
To determine the size and shape of the Ben and NB particles, the samples were recorded in SEM image (figure 4) and TEM image (figure S2). Figure 4(a) illustrates Ben agglomeration with a rough surface, indicating numerous impurities adhered to the surface. In contrast, NB showed lowered agglomeration and a cleaner surface (figures 4(c) and (d)). As shown in TEM images of NB samples, the nanoparticles of the NB samples were in nanometer scale with distribution between 214.35 nm to 283.56 nm ( figure S2). The size of NB particles decreased in the following order: NB5 > NB20 > NB10. The reaction of acid (H 2 SO 4 and HNO 3 ) by using microwave reduces the particle diameter as it removes the contaminants from bentonite particles and destroys the particles, hence reducing the particle size.

Dynamic light-scattering (DLS) analysis
The particle size of Ben and NB were determined using DLS measurements ( figure 5). Figure 5 has shown that Ben is has nano-sized scale with large particle size distribution (800.0 nm), as a result of aggregation of bentonite particles when water was employed to suspension them [35]. As shown in figure 5 and table 3, the lowest size (216.0 nm) and the highest percentage (96%) was obtained between 200.0 and 290.0 nm, which imply that the microwave-assisted acid treatment is an effective method for decreasing particle size. Due to the agglomeration of nanoparticles caused by the precipitation of bentonite during suspension in the solution, DLS measurements have shown a wide particle size distribution.

Investigation of effective factors on nanocomposite hydrogel water absorption
The synthetic hydrogel from AA with MBA crosslinking displayed water absorption of 307 g.g −1 ( figure 6), which is similar to the previous publications [36]. The hydrogel samples were then added to 1.0% of the NB content which changed the speed as well as the water absorption capacity of the material.
In the presence of NB, the absorption were 420 g.g −1 , 395 g.g −1 and 369 g.g −1 for NB10/PAA, NB20/ PAA and NB5/PAA, respectively. According to previous studies, the water absorption potential of hydrogels increases with its degree of structure [36]. In addition, the -OH functional groups of NB contributes to weaken the hydrogen bonds, which allow the water molecule to penetrate into the substance. These observations are in close agreement with several studies by Zhang et al [37], Wang et al [38] and S B. Shruthi et al [39].

Effect of NB content (%wt) and size (nm)
The effect of NB content on water absorption was investigated with various NB10 concentrations from 0.5 to 1.5 mass % ( figure 7(a)). The equilibrium water absorption increases proportionally with the addition of NB to the hydrogel nanocomposite to a concentration of 1.0%, yet declined as the NB concentrations increased to 1.5%. This can be explained that the presence of NB10 in the network has acted as a crosslink point. The high amount of NB10 would increase the crosslinking density, which in turns limits the elasticity and the spatial lattice spacing for water retention, as well as the porous size for water absorption, hence a reduced absorption capacity [40,41]. As shown in figure 7(b), the increasing particle size of NB reduced the water absorption capacity for all hydrogels. This result is similar to Q Y Lv et al [42] in which the smaller nanoparticles can effectively promote the cross-linking density of the hydrogel, thereby increasing the water absorption capacity of the material. In addition, smaller cross-linking agents are more beneficial in dissipating the stress of the polymer chain, thus ensuring that the hydrogel stability upon swelling [36]. Therefore, NB10/PAA with an average NB size of 216 nm was selected for further research. Figure 8(a) illustrates the differences in water absorption for hydrogels at different pH. The equilibrium water absorption increased to a constant value as the pH increased from 1.07 to 7.16. This can be explained by the fact that at pH < 7.0, hydrogen bond formation between surface functional groups on NB and carboxyl limits the flexibility of the polymer chain. The carboxylic groups (COOH) of acrylic acid become ionized as the pH rises, forming carboxylate ions (COO-) which increase the electrostatic repulsion between these groups and promote water molecule diffusion, hence improving water absorption [43]. Figure 8(b) shows the equilibrium water absorption of hydrogel in the presence of NaCl and CaCl 2 salts (0.9%wt). The results show that the water absorption of the materials decreased in the presence of Na + and Ca 2+ ions. This is attributed to the excessive presence of cations that create a charge shielding effect, reducing the osmotic pressure difference between the polymer network and the solution and resulting in poor water absorption performance. It is interesting that all of the hydrogels were unable to attain a suitable swelling capacity in the CaCl 2 solution at the same concentration, which is due to the synthesis of calcium carboxylates that are insoluble in water [44].

The stability and tensile strength of nanocomposite hydrogel
The hydrogels had a maximum Q ads expansion at 25°C, and then were recovered and placed in a drying cabinet to remove moisture at 80°C. The resulted dried hydrogel was used for subsequent experiments. It can be seen that the Q ads of the hydrogel decreased as the reuse cycle increased (figure 9). The drying and swelling process may exert the main effect on disrupting the cross-linked structure. Even so, Q ads of the hydrogel can still reach 350 g.g −1 after 5 cycles higher than PAA reaching 150 g.g −1 after 3 reuses.
Additionally, a universal testing device (Gotech AI-7000M, Taiwan) was used to evaluate the tensile strength of the NB10/PAA and PAA hydrogel. The results (figure S3) indicated that the tensile strength of the hydrogel increases from 0.294 to 1.609 MPa when NB content is 1.0%. This demonstrates the cross-linked solid skeletal structure exists within the hydrogel in the presence of NB. The water absorption property of this NB10/PAA hydrogel was compared with other super-absorbent hydrogels in table 4. This value of Q ads is relatively higher than that of recently reported materials [21,23,39].

Conclusion
NB have been synthesized from Ben from Binh Thuan District, Viet Nam by hydrothermal treatment with a microwave assisted method. All the results obtained from SEM images, TEM, XRD, DLS, EDX and FT-IR analyses confirm the production of uniform NB. Additionally, the nanocomposite hydrogels were created using the polymerization method in the presence of the as-synthesized NB. The water absorption capacity of nanocomposite hydrogels were then investigated under different environments and results have shown that this new absorption exhibited excellent swelling capacity (420.0 g water .g hydrogel −1 ), high strength, good tolerance against salt (NaCl and CaCl 2 solution) and pH between 5.0 and 11.0.