Synthesis and Evaluation of the Ability of Poly(Methacrylic Acid-co-acrylamide)/nanoclay Composite Hydrogel in the Adsorption of Methylene Blue Dye

13 The performance of poly(methacrylic acid-co-acrylamide/nanoclay composite (poly(MAA-co- 14 AAm)/NCC) hydrogel to adsorb methylene blue (MB) dye from aqueous solutions was investigated and 15 the adsorption efficiency was improved by incorporating Cloisite 30B nanoclays in the adsorbent structure. 16 The hydrogels were analyzed using FTIR, XRD, TGA, and SEM analysis. The effect of adsorbent dose, 17 temperature, initial dye concentration, contact time, and pH on the efficiency of the adsorption process was 18 investigated. Adsorption efficiencies of 98.57 and 97.65% were obtained for poly(MAA-co-AAm)/NCC 19 and poly(MAA-co-AAm) hydrogels, respectively. Kinetic study revealed that the adsorption process 20 followed pseudo-first-order kinetic model and α -parameter values of 6.558 and 1.113 mg/g.min were 21 obtained for poly(MAA-co-AAm)/NCC and poly(MAA-co-AAm) hydrogels, presence of Cloisite 30B clay nanoparticles in its structure. In addition, results of R L , n, and E parameters 33 showed that the adsorption process was performed optimally and physically.

higher ability of nanocomposite hydrogel in adsorbing MB-dye. In addition, the results of the intra-particle 23 diffusion model showed that various mechanisms such as intra-particle diffusion and liquid film penetration 24 are important in the adsorption. The Gibbs free energy parameter of adsorption process showed negative 25 values of -256.52 and -84.071 J/mol.K for poly(MAA-co-AAm)/NCC and poly(MAA-co-AAm) hydrogels 26 indicating spontaneous nature of the adsorption. The results of enthalpy and entropy showed that the 27 adsorption process was exothermic and random collisions were reduced during the adsorption. The 28 equilibrium data for the adsorption process using poly(MAA-co-AAm)/NCC and poly(MAA-co-AAm) 29 hydrogels followed Freundlich and Langmuir isotherm models, respectively. The maximum adsorption 30 capacity values of 32.83 and 21.92 mg/g were obtained for poly(MAA-co-AAm)/NCC and poly  AAm) hydrogels, respectively. Higher adsorption capacity of nanocomposite hydrogel was attributed to the 32

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Dyes are used as an important industrial raw material in many applications such as paper, textile 38 and plastic industries [1]. The improper discharge of dye containing wastewater streams into the 39 environment that can lead to many environmental problems [2]. In general, dyes can be classified 40 into cationic, anionic, and nonionic categories [3]. Various studies have shown that among the 41 various dyes used in industry, cationic dyes are more harmful because they easily bind to the cell 42 cytoplasm [4,5]. Methylene blue (MB) is one of the synthetic cationic dyes that is extensively 43 used in industries such as textile, paper, pesticides, cosmetics, and printing due to its high solubility 44 and stability in water [6]. Prolonged exposure to MB dye can cause various diseases such as 45 vomiting, nausea, anemia, hypertension, respiratory failure, eye damage, local burns, increased 46 transpiration, and mental disorders [7]. Therefore, it is important to eliminate contaminants such 47 as dyes from industrial wastewater streams. 48 However, the refining of industrial effluents containing dyes is very difficult due to the presence 49 of complex compounds with poor biodegradation, high pH and turbidity [8]. Various methods have 50 been proposed to eliminate contaminants from wastewaters, however, most of them possess 51 disadvantages such as high operating and maintenance costs, low performance, and production of 52 toxic sludge [9]. Surface adsorption method has received much attention by many researchers due 53 to the advantages of easy performance, high efficiency, no secondary sludge production, and 54 environmental friendliness [10]. Various adsorbents such as different types of clay soils, natural 55 fibers, carbon nanotubes, polymeric materials, zeolites, active carbon, and magnetic composites 56 have been used for dye removal from aqueous solutions [11]. Clay soils show a high potential to 57 adsorb heavy metals and organic compounds due to their suitable photochemical properties such 58 as multilayer structure, high surface area, and high cation exchange capacity. Among the clay soils 59 used, expandable layer silicates such as montmorillonite (MMT) have been recognized and used 60 as adsorbents [12]. MMT clay is one of the low-cost adsorbents with a high cation exchange 61 capacity, which has the ability to absorb cationic pollutants due to high negative charges. Cloisite 62 30B is a natural MMT clay whose surface is modified by quaternary ammonium salts [13].

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Polymeric hydrogels have received much attention due to their advantages such as ease of 64 preparation, high tendency to absorb water, high porosity, ease of operation, and diverse 65 morphology [14]. Hydrophilic cross-linked polymers that form a three-dimensional network with 66 the ability to absorb large amounts of water and aqueous fluids form hydrogels [15]. These 67 hydrogel adsorbents respond to process conditions such as pH, redox, ionic strength, light, electric 68 field, etc. [16,17]. In the adsorption process, the hydrogels absorb contaminants to their surface 69 until they are saturated. After the complete saturation of hydrogels, they can be disposed of as 70 agricultural fertilizer or compost due to their biodegradability [18]. Among the hydrogels used in 71 the adsorption process, those produced based on poly(methacrylic acid) show good absorption 72 properties due to inclusion of many functional groups in their structure. These functional groups 73 provide active centers for the adsorption of dye molecules upon swelling of the hydrogel system 74 [19].

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In the present study, it was hypothesized that fabricating nanocomposite hydrogels using Cloisite 76 30B clay nanoparticles could increase the efficiency of the adsorbent to remove MB-dye. Thus, 77 poly(methacrylic acid-co-acrylamide/nanoclay composite (PMAc-co-Ac/NCC) hydrogels were 78 synthesized and characterized. Moreover, the effect of clay concentration, pH, temperature, 79 contact time, adsorbent dose, and initial MB dye concentration on the efficiency of the adsorption 80 process was investigated. After determining the optimal value of the desired parameters, 81 thermodynamic, equilibrium, and kinetic studies of the process were examined.   with different nanoclay concentrations was dispersed in distilled water (50 mL) at room 107 temperature and kept stirred overnight. Then, the hydrogels were separated from the aqueous 108 medium and the swelling per cent was calculated using Eq 1. (1) 110 where Ws and Wi are the weights of swollen and initial hydrogels, respectively. (R) and adsorbent adsorption capacity (qe) were determined using the following Eqs: 293 Pseudo -First -Order : q = 2 q 2 t 1 + k 2 q t (7) 294 Elovich : q = 1 ln(α β t)

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Where qt and qe are dye adsorption capacity per grams of dry adsorbent determined from 296 experimental results and kinetic models (mg/g), k1 is the constant rate of adsorption (min -1 ), k2 is 297 the constant rate of pseudo-second-order kinetic model (g/mg.min), α is the initial absorbance 298 (mg/g.min), and β is the desorption constant (g/mg).

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Nonlinear relationship of PFO, PSO and Elovich kinetic models for MB-dye adsorption process 300 using both types of adsorbents (at 10 %wt. of clay nanoparticles) are shown in Fig. 6b, c and the 301 variables determined using these models are reported in Table 2. Results showed that the kinetic 302 behavior of the adsorption process followed the PFO model, since this model provides a higher 303 correlation coefficient compared to other models and its root-mean-square error (RMSE) is lower.

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The PFO kinetic model showed that chemical reactions do not play an effective role in the MB-305 dye adsorption process. In addition, α parameters for poly(MAA-co-AAm) hydrogel and 306 poly(MAA-co-AAm/NCC hydrogel were determined to be 1.113 and 6.558 mg/g.min, 307 respectively indicating the higher performance of NCC adsorbent to adsorb MB-dye [41].

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The intra-particle diffusion model assumes that the capacity of dye adsorption changes with the 309 expression t 1/2 , which has been used in previous research to study the kinetics of the adsorption 310 [42]. The linear relationship of the intra-particle diffusion model is shown in Eq. 9:

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Where Kint (mg/g.min 1/2 ) is the parameter of intra-particle diffusion rate, which is determined from 313 the slope of the qt diagram vs. t 1/2 (Fig. 6d). According to this model, if the qt plot vs. t 1/2 is a 314 straight line, the intra-particle diffusion is the limiting step in the adsorption process, and if the 315 plot is not a straight line, the liquid film diffusion is the limiting adsorption process [43]. The 316 constants and correlation coefficient (R 2 ) of the intra-particle diffusion model are reported in Table   317 2. According to Fig. 6d, the qt versus t 1/2 diagram for the MB-dye adsorption process is not a 318 straight line, indicating that the adsorption process using these adsorbents is complicated process 319 and includes adsorption and intra-particle diffusion [48].      process. In addition, the value of KL parameter showed that the ability of poly(MAA-co-

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AAm/NCC hydrogel was higher than that of poly(MAA-co-AAm) hydrogel in MB-dye 357 adsorption. The values of the RL and n parameters showed that the adsorption process is desirable.

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The value of parameter ε for MB-dye adsorption using poly(MAA-co-AAm) hydrogel and

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In the present study, synthesized poly(MAA-co-AAm) and poly(MAA-co-AAm)/NCC hydrogels 375 by radical polymerization method was used to adsorb MB-dye. FTIR, TGA, XRD and SEM 376 analysis showed that the synthesized adsorbents were successfully developed and the silicate 377 layers of Cloisite 30B nanoclay were successfully placed between nanocomposite copolymeric 378 chains. Poly(MAA-co-AAm)/NCC hydrogels adsorbent exhibited superior adsorption efficiency 379 compared to poly(MAA-co-AAm hydrogel. Meanwhile such adsorbent had an advantage of 380 application at lower concentration dose using low contact time. Thermodynamic study revealed 381 exothermic and spontaneous nature of adsorption process using both types of adsorbents. Also, the 382 equilibrium behavior of poly(MAA-co-AAm) and poly(MAA-co-AAm)/NCC followed the 383 Langmuir and Freundlich models, respectively. The values of RL, n and ε parameters showed that 384 the adsorption process is desirable and physical. The kinetic behavior of the adsorption process 385 followed the pseudo-first-order kinetic model, which shows the low effect of chemical events in 386 the adsorption process. In addition, the intra-particle diffusion model showed that various 387 mechanisms such as intra-particle diffusion and liquid film penetration are effective in the 388 adsorption process. Therefore, based on the mentioned results, it can be stated that the use of