The Utility of Novel Superabsorbent Core Shell Magnetic Nanocomposites for Efficient Removal of Basic Dyes from Aqueous Solutions

A series of novel superabsorbents Fe3O4/poly(Acrylamide-co-sodium acrylate) core shell magnetic nanogel based on crosslinked poly (sodium acrylate-co-acrylamide) are investigated for the removal of methylene blue from aqueous solutions. The superabsorbents are characterized by FTIR, HR-TEM and SEM. The adsorbents exhibited high sorption capacities toward basic dyes, viz.: 2167.802-2765.893 mg g-1. The adsorption kinetics followed a pseudo-second order equation. Equilibrium isotherm data are analyzed according to Langmuir and Freundlich equations. The thermodynamic parameters including ΔG°, ΔH° and ΔS° for the adsorption processes of MB on the nanocomposite hydrogel were also calculated, and the negative value of ΔG° indicated the spontaneous nature of adsorption.


Introduction
AIn recent years, pollution from dye wastewater has become a serious environmental problem due to the vast and increasing uses of a variety of dyes. [1] Methylene blue (MB) is the most commonly used substance for dying cotton, wood and silk. It may cause some hazardous problems to human and animal health [2]. Amongst the numerous techniques of dye removal [3][4][5][6][7][8][9][10], adsorption is a wellknown equilibrium separation process and is an effective method for water decontamination applications [11,12].
Recently, there has been an increasing interest in the design and fabrication of magnetic polymeric particles consisting of one or more magnetic nanoparticles as a core with a coating matrix of polymer as a shell. The main advantage of magnetic nanocomposite particles (MCPs) over conventional polymer nanocomposite particles is that, because of their magnetic properties, they can be rapidly separated from the mixtures by magnetic extraction [13]. They have been tried extensively in various fields including industrial water treatment [14,15].
To our best knowledge, there is no literature focusing on the use of Fe3O4/ poly(sodium acrylate co acrylamide) core-shell magnetic nanogel for the removal of MB from aqueous solutions. The objective of this work is to develop crosslinked p(Am-co-Na Ac) nanocomposite hydrogel that contain different amounts of Fe 3 O 4 /p(Am-co-Na Ac) core-shell magnetic nanogels as potential effective adsorbents to remove MB from aqueous solution attributing to their following unique properties: (i) Fe 3 O 4 increase mechanic strength to enhance the usable life. (ii) The p(Am-co-Na Ac) shells with nano-scaled size provide much larger specific surface area. (iii) The raw materials are low-cost and the synthetic approach is simple, which make these nanoparticles potentially commercializable. (iv) These nanoparticles do not cause environmental pollution during the course of treatment.

Experimental Materials
All chemicals were of analytical grade and used as received, Iron (III) chloride hexahydrate (FeCl 3 .6H 2 O>99%), Iron (II) chloride tetrahydrate (FeCl 2 .4H 2 O>99%) were purchased from Merck. Acrylic acid (AA), acrylamide (AM) monomers and N,N-methylene-bis-(acrylamide)(MBA) as a crosslinker, Potassium peroxydisulfate (KPS) as initiator and N,N,N`,N`-tetramethylethylenediamine (TEMED) as an accelerator were purchased from Aldrich Chemical Company. MB was purchased from Sigma-Aldrich. Doubly Distilled Water (DDW) was used throughout the present work. solution of KPS (2 wt% with respect to total monomer amount) and 150 µl of TEMED were added to the solution to start polymerization. The reaction was carried out at room temperature under N 2 gas for 15 hour. After preparation of stable nanogel dispersion, the resultant polymer coated magnetic nanoparticles were collected with the aid of an external magnetic field, washed with DDW several times and finally dispersed in 45 ml DDW.
Sodium acrylate(Na-Ac) was prepared as following:12.01 g (0.3003 mol) NaOH and 10 ml water were added to a 50 ml Erlenmeyer flask equipped with a magnetic stirring bar, and the mixture was stirred to dissolution. The NaOH solution was carefully added to a 250 ml beaker containing 28.03 g (0.3893 mol) acrylic acid (exothermic reaction), with continuous stirring. The mixture was allowed to cool, 50 ml of acetone were added, and the precipitate vacuum filtered. The wet sodium acrylate was first air dried and subsequently dried in an oven at 55 C-60 C for 12-15 hours to obtain 27.31 g (96.8% yield) of sodium acrylate.
Superabsorbents nanocomposite hydrogels (Ac blank, Ac4% and Ac8%) containing 0, 4, 8 wt% of nanogel, respectively, were prepared by in situ free radical copolymerization of acrylamide and sodium acrylate in an aqueous dispersion of nanogel particles using MBA as crosslinker and KPS as the initiator. The total mass percentage of both monomers in the reaction mixture was 30%. In brief, to prepare a sample(S), 2.15 g of acrylamide, 2.84 g of sodium acrylate, 0.05 g of crosslinker MBA (1 wt % based on total monomer amount) were dissolved in water in an ice cooled bath under nitrogen atmosphere. Then, aqueous dispersion of magnetic nanogel particles was added to the mixture under vigorous stirring. Afterwards, 3 ml aqueous solution of initiator (0.05 g KPS, 1wt% with respect to total monomer amount) was added and the whole reaction mixture was transferred into a test tube (internal diameter 10 mm) and kept in an electric oven at 50°C for 10 hours. The hydrogel codes and complete compositions are listed in Table 1.
The formed nanocomposite hydrogels were recovered carefully by breaking the test tubes and cut into small disks of 3 mm thickness and 10 mm diameter. These hydrogel discs were safely transferred into DDW and allowed to equilibrate over a period of 5 days (water was repeatedly changed every 10 h) to remove unreacted monomers, crosslinker, initiator and soluble or un-crosslinked polymers, etc. Finally, the sample was dried in air followed by overnight drying in an electric oven to get completely dried gels and stored in airtight plastic bags.

Characterization
FTIR spectra were analyzed with a Nicolet FTIR spectrophotometer using KBr in a wavenumber range of 4000-5000 cm -1 with a resolution accuracy of 4 cm -1 . The images for magnetite and Fe 3 O 4 /p(Amco-Na Ac) nanocomposites were recorded using High resolution transmission electron microscopy (HR-TEM) (JEM-2100F, JEOL, Japan) at an acceleration voltage of 120 kV. The surface morphologies of nanocomposite hydrogels were investigated using scanning electron microscopy (SEM, model JSM-T 220A, JEOL, Japan) equipped with an EDX detector. Zeta potential measurements of nanocomposite hydrogels were measured at different pH values using Laser Zeta meter Malvern Instruments Model Zetasizer 2000.

Adsorption studies
Batch sorption experiments were done by shaking 0.02 g of nanocomposite with 50 ml aqueous solution of MB in 250 ml-Erlenmeyer flasks placed in a temperature controlled shaking water bath at different concentrations (between 100 and 1600 mg/l), pHs (between 2 and 12), ionic strength (between 0.005 and 0.3 mole/l), temperatures (between 25°C and 45°C) and sorbent doses (between 0.01 and 0.05 g) at a constant shaking rate of 125 rpm. The concentration of MB in the residual solution was analyzed spectrophotometrically at 662 nm. The amounts of dye removed by superabsorbents (qe) and the percent of extraction (%E) are calculated using the following equations: Where q e is the amount of dye adsorbed (mg/g). C o and C e are the initial and equilibrium liquid-phase concentrations of dye (mg/g), respectively. V is the volume of the solution (l), and m is the weight of the sorbent used (g).

Characterization of adsorbents
The chemical structure of the prepared magnetite nanoparticles was confirmed by FT-IR analysis. The adsorption of AM, AA-Na and MBA as a ligand onto Fe 3 O 4 colloidal nanoparticles clusters has been studied by means of FTIR. Figure 1 shows the FT-IR spectra of Fe 3 O 4 nanoparticles, Ac blank, Ac 4% and Ac 8% nanocomposites. The FT-IR spectrum of Fe 3 O 4 ( Figure 1a) shows characteristic absorption band at 586 cm -1 corresponding to the Fe-O bond. Also, the presence of a band in the region of 3250-3600 Cm -1 indicates the presence of water of crystallization in the magnetite sample. The small peaks in the region 1500-1400 cm -1 can also be correlated to the different modes of bonded water molecules existing in the ferrofluid. In FT-IR spectrum of Ac 4% and Ac 8% nanocomposites (Figures 1b and 1c) there is shifting in ferrite peak by 15 Cm -1 . The shifting as well as broadening of the peaks  can be attributed to interaction of 3d orbit of ferrite with N atom in AM to form coordinate bond [13]. In the FT-IR spectra of Ac blank, Ac 4% and Ac 8% (Figures 1b-d), the peaks found at 3400, 1650, and 1600 cm -1 indicate N-H stretching, C=O stretching and N-H bending of the amide bonds, respectively, which are characteristics of the-CONH group in the AM. In addition, the peak at 1411 cm -1 is due to the-C-N stretching, and the 765-710 cm -1 band represents the out of plane bending of the weak bond N-H. Based on the above observations, the Fe 3 O 4 nanoparticles are located in the polymer shell.
The HR-TEM micrograph of the magnetite nanoparticles, Figure  2a, suggests that Fe 3 O 4 nanoparticles are spherical in shape with mean diameter of 8 nm. Figure 2b confirms that the magnetic Fe 3 O 4 nanoparticles coated with poly(Acrylamide-co-sodium acrylate) polymer are nanocapsules of spherical shape with an average diameter of 11 nm. On encapsulation with polymer, the size of the nanocapsules is increased from 8 to 11 nm. This is an indirect confirmation of the encapsulation of Fe 3 O 4 magnetic core with a 3 nm thick polymer shell.
The morphologies of Ac blank, Ac 4%, Ac 8% nanocomposite, as observed by SEM, are shown in Figure 3. The pulverized Ac blank, obtained from the radical chain polymerization in aqueous solution, was essentially a smooth sheet free of any significant particulate nature ( Figure 3a). The magnetic nanogel loadings at 4 and 8 %wt were immobilized in the Ac 4% and Ac 8% nanocomposite showed the nanogel particles to be well dispersed and so were found to reside on the surface and the inner depths (Figures 3b and 3c). The nanogel particle distribution increased with increasing nanogel content. The figures reveal that small spherical particles of 30 to 50 nm are imbedded within the polymer matrix with absence of phase separation or agglomerations of magnetic particles on the surface.
Zeta potential of Ac blank, Ac 4 % and Ac 8% has been investigated; the results are shown in Figure 4. It could be clearly seen that, with increasing the amount of nanogel on nanocomposite adsorbents the zeta potential shift to more negative values. So, Ac blank has less negative potential whereas Ac 8% has the highest negative potential. The presence of negatively charged carboxylate groups on the surface of nanogel is the main reason for shift in zeta potential to more negative values. This implies that the adsorption capacity of adsorbents toward positively charged MB will increase in the order Ac 8% Ac 4% Ac blank due to presence of more negatively charged active sites in that order. Also, the pH causes a dramatic change in zeta potential. As pH increase from pH 4 to pH 8, zeta potential increase from -20.7 to -29.7 mv, from -21.9 to -31.9 mv and from -22.8 to -36.1 mv for Ac blank, Ac 4% and Ac 8% respectively. With increasing the pH, zeta potential become more negative due to ionization of carboxylic group of acrylic acid. In contrast to that, as pH decrease, zeta potential decrease due to a b

Adsorption studies
Effect of pH: Figure 5 shows the effect of pH on the removal of MB onto Ac blank, Ac 4% and Ac 8% nanocomposites from aqueous solutions. The adsorption capacity of MB onto nanocomposite hydrogels increased significantly with increasing pH. The high adsorption capacity is due to the strong electrostatic interaction between the nanocomposite hydrogels and cationic dye molecules. Moreover, when pH value is increased, the carboxylic groups of the nanocomposites become ionized and the electrostatic repulsion between the molecular chains is predominated which leads to more expansion of the network and high adsorption of MB. When the pH increases from pH=3 to pH=4, The amount of MB adsorbed at equilibrium (q e ) increase from 570.38 to 1389.89 mg/g, from 590.64 to 1396.41 mg/g and from 614.68 to 1443.31 mg/g, for Ac blank, Ac 4% and Ac 8% nanocomposites, respectively. At acidic pH, the lower dye removal is probably due to the presence of excess H+ ions competing with positively charged MB molecules for the sorption sites of sorbents.

Effect of initial dye concentration:
The effect of initial concentration on the removal of MB by Ac blank, Ac 4% and Ac 8% nanocomposites is shown in Figure 6. The adsorption capacities of the sorbents increase with an increase in initial dye concentration and the adsorption at different concentrations is rapid in the initial stages and gradually decreases with the progress of adsorption until the equilibrium is reached. This may be attributed to the fact that, the higher the initial MB concentration, the greater the driving force of the concentration gradient at solid-liquid interface which cause an increase of the amount of MB adsorbed on the adsorbent [16]. When the initial concentration of MB increases from 100 to 1000 mg/l at 25°C, the amount of MB adsorbed at equilibrium (q e ), increase from 244.54 to 2153.33 mg/g, from 244.87 to 2374.87 mg/g and from 248.23 to 2729.16 mg/g for Ac blank, Ac 4% and Ac 8%, nanocomposites, respectively. At higher MB concentrations adsorption capacity reached a plateau indicating saturation of the available binding sites on the adsorbent.

Effect of contact time:
The adsorption capacity of Ac blank, Ac 4% and Ac 8% nanocomposites increased rapidly with the increase of contact time from 0 to 20 min ( Figure 7) and more than 90% of the equilibrium adsorption capacity for MB occurred within 15 min. After 60 min, the adsorption capacity became constant and the adsorption reached equilibrium. Therefore, 60 min was selected as the contact time for the adsorption of MB onto the composites under our experimental conditions. Effect of adsorbent dosage: When the sorbent dose increases from 0.01 to 0.02 g (Figure 8), the percent dye removals by Ac blank, Ac 4% and Ac 8% nanocomposites increase from 43.33% to 86.06%, from 47.30% to 95.21% and from 55.01% to 99.01%, respectively. This can be simply attributed to the increased sorbent surface area and availability of more sorption sites [17]. The decrease in adsorption capacity with increase in adsorbent dosage is due to the high number of unsaturated adsorption sites.
Effect of ionic strength: Figure 9 presents the effect of ionic strength on the uptake of MB. The adsorption capacity decreased with the increase in ionic strength. As the concentration of NaCl ions  Effect of temperature: The adsorption capacity of the nanocomposites increased with increasing of the temperature from 25 to 45°C (Table 2). This is attributed to the increase in the kinetic energy of MB with increasing temperature and the flexibility of the hydrogel increases as well. Consequently, the rate of diffusion of MB molecules across the external boundary layer and in the internal pores in the nanocomposites increases with temperature [18], and the amount of MB uptake increases. When the temperature increased from 25°C to 45°C, the maximum amounts of MB removed by Ac blank, Ac 4% and Ac 8% nanocomposites are found to be increased from 2167.802 to 2176.677 mg/g, from 2377.078 to 2390.351 mg/g and from 2759.505 to 2765.893 mg/g, respectively.

Adsorption isotherms
Adsorption is usually described through isotherms, that is, the amount of adsorbate on the adsorbent as a function of its pressure (if gas) or concentration (if liquid) at constant temperature. The two most common isotherm equations namely, Langmuir and Freundlich are tested here [19]. The Langmuir equation can be written as follows: e 0 l e l e q Q K C /1 K C = + Where qe is the amount of dye adsorbed on nanocomposites at equilibrium (mg/g), Q 0 is the maximum adsorption capacity (mg/g), K l is the Langmuir constant, and C e is the concentration of dye solution at equilibrium (mg/L). The linear form of Langmuir equation is: The monolayer adsorption capacity, Q and the Langmuir constant K l can be obtained from the linear plot of C e /q e against C e . An essential characteristic of Langmuir isotherm can be expressed by a dimensionless constant called equilibrium parameter, R L [20], defined by: Where C 0 is the highest initial dye concentration (mg/L), R L values indicate the type of isotherm to be either unfavorable (R L >1), linear (R l =1), favorable (0<R L <1) or irreversible (R L =0) [21].
The Freundlich equation is given by [19]: 1/n e f e q K C = (6) Where K F is the Freundlich constant which is a comparative measure of the adsorption capacity of the adsorbent, and n is an empirical constant which gives valuable information about the isotherm shape. 1/n values indicate the type of isotherm to be irreversible (1/n=0), favorable (0<1/n<1) and unfavorable (1/n>1).
The Freundlich parameters be obtained from the following linearized equation: By linear plotting ln q e as the function of ln C e , the values of K f and n can be obtained from the slope and the intercept of the plot. Ac blank Ac 8% Ac 4% NaCI (mol/I) adsorption capacity (mg/g) Figure 9: Effect of ionic strength on the removal of MB by Ac blank, Ac 4% and Ac 8% nanocomposites. (Conditions: T=25°C; adsorbent dose=0.02 g/50 ml; Cₒ=1000 mg/l; time=2 h).   Langmuir model are all the same as the experimental data. Also, R L values obtained are in the range of 0.1195-0.2534, thereby confirming that the adsorption is a favorable process. It can be concluded that the monolayer Langmuir adsorption isotherm is more suitable to explain the adsorption of MB onto the proposed nanocomposites.

Adsorption kinetics
The pseudo-first order kinetic model was suggested by Lagergren its linear form can be formulated as Where q e is the adsorption uptake of MB at time t (mol g -1 ) and k1 (min -1 ) is the rate constant of the pseudo-first-order adsorption. A plot of log (q e -q t ) versus t should be linear; the parameters k 1 and R 2 (correlation coefficient) calculated from the data ( Figure 12) are listed in Table 5. The large differences between the experimental q e values (q e,exp ) and the calculated q e values (q e,ca l) indicate that the pseudo-first order kinetic model was poor fit for the adsorption processes of Ac blank, Ac 4% and Ac 8% nanocomposites for MB.
The Ho's pseudo-second-order kinetic model [22] can be expressed as: t / q 1/ K q t / q = + Where k 2 (g mol -1 min -1 ) is the rate constant of pseudo-secondorder adsorption. Figure 13 shows the plot of t/q versus t for MB onto Ac blank, Ac 4% and Ac 8% nanocomposites. The correlation coefficients(R 2 ), Table 6, for the pseudo-second order kinetic model are all over 0.9999, moreover, the qe,cal values for the pseudo-second order kinetic model are all consistent with the q e,exp values. These findings suggest that the adsorption processes of Ac blank, Ac 4% and Ac 8% nanocomposites for MB can be well described by the pseudo-second order kinetic model.
The intra-particle diffusion parameter, K p (mmol.g -1 h -0.5 ) is defined by equation: Where k p is the intra-particle diffusion rate constant (mmol.g -1 h -0.5 )   log(q e -q Figure 12: Pseudo-first-order kinetic model.    and C is a constant. According to this model q t versus t 0.5 should be linear if intra-particle diffusion is involved in the adsorption process [23]. From Eq. (5), if pore diffusion is the rate limiting step, then a plot of q t against t 0.5 must give a straight line with a slope that equals k p and the intercept value c represents the resistance to mass transfer in the external liquid film. Figure 14 shows the plot of q t against t 0.5 of Ac blank, Ac 4% and Ac 8% nanocomposites for MB at three different initial concentrations; the plots are multi-linear, containing at least three linear segments which indicate that three steps occur during adsorption process. The first sharper portion is the transport of dye molecules from the bulk solution to the adsorbent external surface by diffusion through the boundary layer (film diffusion). The second portion is the diffusion of the dye molecules from the external surface into the pores of the adsorbent. The third portion is the final equilibrium stage, where the dye molecules were adsorbed on the active sites on the internal surface of the pores and the intra-particle diffusion starts to slow down due to the solute concentration getting lower and lower in solution [24]. The linear portions of curves did not pass through the origin, suggesting that pore diffusion is not the step controlling the overall rate of mass transfer at beginning of adsorption.

Thermodynamic studies
The data obtained from the temperature studies were used for thermodynamic analysis. The Gibbs free energy is presented in Eq. (12). The Gibbs free energy can also be expressed using enthalpy and entropy at a constant temperature. The linearized form of Eqs. (11) and (12) results in Eq. (13), which is the Van't Hoff equation [25]: where ΔG° (kJ/mol) is the change in Gibbs Free Energy, ΔH° (kJ/mol) is the enthalpy change of MB adsorption, ΔS° (J/mol K) is the entropy change of MB adsorption, R is the universal gas constant (8.314J/mol K), T is the absolute temperature (K), and K l is the Langmuir constant.
By plotting a graph of ln K c versus 1/T (Figure 15), the values of ΔH° and ΔS° can be estimated from the slope and intercept of Van't Hoff plots, respectively. The thermodynamic parameters are given in Table 7    and 10). Moreover, the positive values of ΔS° point out the increased randomness at the solid/liquid interface during the sorption of MB on Ac blank, Ac 4% and Ac 8% nanocomposites.

Conclusion
The results of the present study reveals that Fe 3 O 4 /poly (Acrylamideco-sodium acrylate) core shell magnetic nanogel (Fe 3 O 4 /p(Am-co-Na Ac)) based on crosslinked poly (sodium acrylate-co-acrylamide) may be an extremely viable adsorbent for application in the treatment of water and industrial wastewater contaminated with dyes. The amount of MB adsorbed was found to increase with increasing the wt% of nanogel from 4% to 8%. Moreover, the adsorption of MB was dependent on initial concentration, reaction temperature and pH. The MB adsorption capacity increased with the increase of pH in the range of 3-8, where ionization of carboxylic group occurs. The adsorption of MB onto the nanocomposites reached equilibrium within about 60 min. The adsorbents exhibited high sorption capacities toward basic dyes, viz.: 2167.802-2765.893 mg g -1 . The adsorption equilibrium could be well described by Langmuir adsorption isotherms, namely monolayer adsorption on a homogenous surface. The adsorption kinetics followed a pseudo-second order kinetic model and intra-particle diffusion was involved in the adsorption process. Thermodynamic results indicated that the adsorption process was spontaneous and endothermic in nature.