Acrylamide based hydrogels in swelling and uptake of methylene blue from aqueous solutions

Abstract

Several kinds of acrylamide-poly(ethylene glycol) hydrogels were synthesized at room temperature (25°C) by adding N,N,N’,N’-tetramethylethylenediamine crosslinker to AAm aqueous solutions containing 0, 0.20, 0.40, 0.60, 0.80 and 1 mg of PEG₆₀₀₀. Water uptake and dye-sorption properties of the synthesized poly(acrylamide) and acrylamide-poly(ethylene glycol) hydrogels were investigated as a function of chemical composition of the hydrogels. Cationic dye (methylene blue) has been used in sorption studies. It was determined that poly(ethylene glycol) used for pore forming in synthesizing the hydrogel has an influence on the adsorption of methylene blue as in the case of adsorption phonemia. Adsorption equilibrium was investigated by using Langmuir and Freundlich isotherm models. Two kinetic models, (i) pseudo-first-order and (ii) pseudo-second-order kinetic models were applied to test the experimental data. Freundlich isotherm model represented the experimental data very well. Methylene blue adsorption followed the pseudo-second-order kinetic model which indicated chemical adsorption.

Keywords

Acrylamide hydrogel absorption dye sorption kinetic model

1 Introduction

Researchers over the years defined hydrogels in many different ways. Hydrogels are three dimensional cross-linked hydrophilic reticulated polymers. Network structure composing hydrogel consists of polyelectrolyte chains linked each other by cross-links. In these chains carboxylic acid groups are usually found as substituted groups. Negative charges on the polymer chains push each other and expand. The carboxyl groups also interact by forming hydrogen bonds with water [1, 2].

Cross linked polymers are insoluble, they can be swell when keeping in the suitable solvents for a length of time. Polymeric structures which swellable in this manner are called gel. In other words; cross linked reticulated, homo or copolymers which have swellable ability by taking solvent inside it is called as xerogel [3, 4].

Hydrogels are classified as neutral, anionic, cationic, amphoteric hydrogels according to the carried loads; affine reticulated and combined (phantom) reticulated according to the mechanics and structural characteristics; homopolymer, copolymer, multi polymer, interwoven reticulated polymer according to the method of preparation; amorphous, quasicrystal, hydrogen bound, hydro colloidal batch, super molecular network according to the physical structure. Another classification is in the way that “stimuli-responsive” gels [5].

In recent years, natural hydrogels were gradually replaced by synthetic hydrogels which has long service life, high capacity of water absorption, and high gel strength. Fortunately synthetic polymers usually have well-defined structures that can be modified to yield tailorable degradability and functionality [6].

Because of these properties hydrogels have many application areas. The use of polymer hydrogels for removing of heavy metals, dyes and other toxic organics from wastewater or aqueous solutions has been continued to attract considerable attention in recent years [7 –9].

Hydrogels are considerably appropriate polymers for adsorption except for water retention characteristic. For this reason as well as in the water purification, removing of heavy metal/dye, ion exchanging, removing of water from petrol and fat content industrial waste practices benefits from hydrogels. Numerously studies were done in order to decontamination of industrial wastewater which contains most especially heavy metal or dye from this kind of contaminating species. Adsorption using different polymeric materials and synthetic resins is the method of choice in many wastewater treatment processes for removing of heavy metal ions, dyes and other hazardous materials from chemical process industries in certain developed countries [10 –13]. Therefore the hydrogels may be used as an alternative adsorbent for the removal of cationic dyes from aqueous solution.

In this study; acrylamide-poly(ethylene glycol) (AAm/PEG₆₀₀₀) based hydrogels were synthesized. With reference to literature data in the synthesizing of acrylamide-poly(ethylene glycol) based hydrogels PEG₆₀₀₀ which has a molecular weight 6000 was chosen. In the synthesizing of acrylamide-poly(ethylene glycol) hydrogels PEG₆₀₀₀ with six different mass percent was used. Swelling capacity of synthesized all hydrogels was determined. Adsorption kinetics and equilibrium isotherms for methylene blue dye onto acrylamide-poly(ethylene glycol) (AAm/PEG₆₀₀₀) hydrogels were investigated to illustrate the adsorption mechanism of cooperative AAm and PEG₆₀₀₀.

2 Experimental section

2.1 Materials

Acrylamide (AAm) as a monomer, poly(ethylene glycol) (PEG, Mw = 6000), were supplied from Merck, Schuchardt, Germany. PEG was dried by azeotropic distillation with anhydrous toluene under a dry nitrogen atmosphere and used immediately. The N,N’-methylenebisacrylamide (MBA, Fluka, analytical grade) was used as a crosslinking agent. The redox initiator system comprised of ammonium persulfate initiator (APS, Merck, Darmstadt, Germany, analytical grade) and N,N,N’,N’-tetramethylethylenediamine co-initiator (TMEDA, Merck, Schuchardt, analytical grade). Cationic dye, methylene blue (MB) was used in sorption studies, was purchased from Fluka, Steinheim, Germany. During experiments, a digital balance of model BB3000 Metler-Toledo (Switzerland), a vacuum oven of model BINDER (Tuttlingen, Germany) and a shaking water bath of model JSR were used.

2.2 Synthesis of crosslinked acrylamide based hydrogels

In this study, crosslinked polyacrylamide P(AAm) and acrylamide-poly(ethylene glycol; PEG) based hydrogels were synthesized by free radical solution polymerization in aqueous solution.

A series of hydrogels were prepared by the following procedure.

N,N’-methylenebisacrylamide was used as crosslinker in the hydrogel synthesis. For the synthesis of AAm based hydrogel, 1 g of AAm monomer was dissolved in 8 mL distilled water, then 0.0777 mmol of N,N’-methylenebisacrylamide was added, which is 1% of the monomer. 1 mL/0.219 mmol of APS was added from the 5 g/100 mL APS solution and 1 mL/0.086 mmol of TMEDA was added from the 1% TMEDA solution. The prepared solution was poured into a 3 mm-diameter glass tube and was held for the gelling under a nitrogen atmosphere. To prepare AAm//PEG₆₀₀₀ hydrogels same method was used as mentioned above with addition of 0.20, 0.40, 0.60, 0.80 and 1.00 g PEG₆₀₀₀ to aqueous monomer solution per 1.0 g of AAm. The polymerization reaction was performed at 30°C in a shaking water bath for 24 hours. Synthesized hydrogels were removed from the glass tube and were put into distilled water for the washing process. After washing the hydrogels were primarily dried at room temperature then in the vacuum oven 1 hour at a temperature of 40°C. The experiments were replicated thrice to obtain a reasonable average.

2.3 Adsorption experiments

In the adsorption studies, synthesized polyacrylamide and acrylamide-polyethylene glycol based hydrogels were used as an adsorbent for removing of methylene blue from aqueous solution by adsorption technique. Equilibrium isotherms were obtained with 0.5 g of adsorbent contained in glass bottles in contact with 50 mL of methylene blue-solution at concentrations of 1.5×10^–2, 3×10^–2, 6×10^–2, 7.5×10^–2 and 9×10^–2 g×L^–1. The pH of the initial solutions was adjusted to 7.0.

In the experimental studies initial dye concentrations of methylene blue solutions were prepared by diluting of 500 mg/L stock methylene blue dye solutions as 15, 30, 60, 75 and 90 mg/L. 0.1 g of hydrogel and 50 mL of methylene blue solution were put into the Erlenmayers. Sealed Erlenmayers were placed into the shaker for each test series. Adsorption experiment was performed by mixing 42 hours (experiments came to equilibrium in 42 hours) at 25°C temperature and 250 rpm of constant mixing speed. The samples were taken at regular time intervals and filtered through the ordinary filter paper. Adsorption values of samples were determined by Shimadzu spectrophotometer at 665 nm and adsorption experiments were continued until obtaining unchanging absorbance values.

3 Result and discussion

3.1 Dynamic swelling characterization

Experimentally synthesized polyacrylamide P(AAm) and acrylamide-poly(ethylene glycol) (AAm/PEG₆₀₀₀) hydrogels reached a constant water sorption value in the swelling experiments by coming into balance. This value is called as the equilibrium swelling value. The water absorbency, Q (g H₂O/g sample) was calculated using Equation 1: $Q = [(m - m_{0}) / m_{0}]$ (1) the water absorbency is indicated as amount of the water in the swollen samples per amount of dry absorbent. In this equation, m and m₀ are denoted as the weight of the gel swollen by water and the weight of the absorbent, respectively. By the aid of obtained data adsorption capacity-time graphic was drawn and it is given in Fig. 1.

Fig.1

Adsorption capacity versus time of acrylamide-poly(ethylene glycol) hydrogels with different PEG₆₀₀₀ amount.

In Fig. 1, it is seen that acrylamide-poly(ethylene glycol) hydrogels synthesized by using pore forming PEG₆₀₀₀ absorb water faster than polyacrylamide hydrogel. The reason is that macro and super porous structures make easier transfer of water molecules between hydrogel matrix and outer aqueous phase. In other words increasing of porousness and broad porosities affect swelling ratios. Thus hydrogel samples which have broad porosities in its structure have more swelling content [14]. Swelling content of hydrogels is at the maximum level in the first 210 minutes afterwards it goes to the slowdown tendency. As seen also in the experiments hydrogels were reached the equilibrium swelling value in the 650 minutes (results are not given in the graphic). From Fig. 1, for the acrylamide-poly(ethylene glycol) hydrogels synthesized by increasing pore forming PEG₆₀₀₀ ratio, swelling content of hydrogel also increases. Similar studies were done also in the literature and with the increasing of molecular weight and percentage of polyethylene glycol, swelling content of synthesized hydrogel was also increased and this result shows similarity with our study [15].

3.2 Effect of initial methylene blue concentration of solution on adsorption

The purpose of surveying effect of initial dye concentration of solution on adsorption is search the accuracy of data obtained as a result of swelling experiments. For this reason at this section effect of initial dye concentration of solution on the removal of methylene blue was investigated only for polyacrylamide and acrylamide-poly(ethylene glycol) hydrogels which have the highest swelling content.

Adsorption experiments were performed to study the effect of initial concentrations of methylene blue in the range of 15 to 90 mg×L^–1. At time t = 0 and equilibrium, the methylene blue concentrations of flasks were measured and the amount of equilibrium adsorption, q_e (mg×L^–1) was calculated by Equation 2: $qe = \frac{(C_{0} - C_{e}) V}{W}$ (2) where, C_o and C_e (mg×L^–1): liquid phase concentrations of dye at initial and at equilibrium, respectively, V: volume of the solution (mL) and W: mass of dry hydrogel used (g).

The experiments were carried out at a fixed adsorbent dose (0.5 g) and at different initial methylene blue concentrations such as 15, 30, 60, 75 and 90 mg×L^–1 for different time intervals at 25°C as shown in Fig. 2.

Fig.2

Adsorption of methylene blue dye with changing initial concentrations on polyacrylamide P(AAm) A. and acrylamide-poly(ethylene glycol) AAmPEG₆₀₀₀ hydrogels B.

Figure 1 shows the removal of methylene blue for different initial concentrations as a function of contact time, at pH 7.0 and 25°C. It was observed that methylene blue uptake is rapid for the first 7 hours and thereafter it proceeds at a slower rate and finally attains saturation. In other words adsorption generally increases with the increasing dye beginning concentration for polyacrylamide and acrylamide-poly(ethylene glycol) hydrogels. As also shown in Fig. 2, adsorbing capacity was showed an increase till 21 hour. Decelerated adsorbing capacity after the 21 hour was reached the equilibrium at the 28 hour. At the equilibrium state; maximum adsorbing capacity of acrylamide-poly(ethylene glycol) hydrogel, q_emax value was observed at 90 mg/L and was calculated as 3.52 mg/g. q_emax value of polyacrylamide P(AAm) hydrogel was calculated as 1.42 mg/g. Increasing of q_e values with the increase of initial dye concentration is basically related to amount of dye ions trapped to active places on the surface of adsorbent. Similar trend was also observed by Basava et al. [16] and Aroguz et al. [17], for fly ash adsorbent of methylene blue. The monolayer adsorption capacity of Amitraz (AZ) was found as 28.53 mg×g^–1 [18], the maximum methylene blue (MB) mono adsorption capacity was found to be 5.13 mg×g^–1 obtained by Langmuir model [19], methylene blue adsorption capacity of acrylamide/N-vinylpyrrolidone/3-(2-hydroxyethyl carbamoyl) acrylic acid was found as 5.50 mg/g [20].

3.3 Adsorption isotherm studies

The equilibrium experimental data for adsorbed methylene blue on hydrogels were compared by using the adsorption isotherm equations, namely Langmuir Equation 3 and Freundlich [21]: $Langmuir equation : Ce / qe = 1 / qmaxb + Ce / qmax$ (3) where, C_e is the equilibrium concentration of methylene blue in solution (mg/L), q_e is the equilibrium concentration of methylene blue on the adsorbent (mg/g), q_max is the Langmuir constant expresses adsorption capacity (mg/g) and b is the Langmuir constant expresses adsorption energy (L/mg). The Langmuir constants q_max and b are determined from the intercept and the slope of the linear plot of q_e vs C_e (Fig. 3).

Fig.3

Langmuir isotherm for adsorption of methylene blue on P(AAm) and AAmPEG₆₀₀₀ hydrogels.

Freundlich isotherm which corresponds to the heterogeneous adsorbent surfaces was also applied to the experimental data as following Equation 4. $Freundlich equation : qe=KF {Ce}^{\frac{1}{n}}$ (4) hereby; C_e and q_e express same terms in the Langmuir isotherm, K_F (mg/g) and n are the Freundlich isotherm constants and indicate the adsorption capacity of the adsorbent and the adsorption affinity for the adsorbate, respectively. To make linear by taking both sides logarithm of Freundlich Equation 4, Equation 5 is obtained; (Fig. 4): $\log qe = \log KF + 1 / n \log Ce$ (5)

The obtained data were fitted to the models and their corresponding constants were calculated using Statistica 6.0 program software (Statsoft Inc., Tulsa, OK). t-test was used to compare the population of the two groups. All results were analyzed statistically from Student’s t-test (p < 0.05). The results obtained from Student’s t-test (p < 0.05) illustrate that adsorption isotherms followed both Langmuir and Freundlich models (Table 1). Since n > 1, the adsorption is favorable.

As can seen in Table 1, due to exceeding of R² values of polyacrylamide and acrylamide-poly(ethylene glycol) hydrogels over 0.980 for the Freundlich isotherm type, it can be readily said that adsorption is in accord with the Freundlich isotherm. The R² value of Langmuir isotherm is lower than Freundlich isotherm. q_max values were found as 2.32 and 12.92 mg/g for polyacrylamide and acrylamide-poly(ethylene glycol) hydrogels, respectively. b values obtained in the study were found as low. This situation is a net proof that hydrogels used in this study are good adsorbents. Due to the fact that 0 < R_L < 1, it can be said that adsorption is desired adsorption [17].

Table 1

Langmuir and Freundlich isotherm constants of polyacrylamide and acrylamide-poly(ethylene glycol) hydrogels

Hydrogels	Langmuir Isotherm	Constants of Langmuir Isotherm			Freundlich Isotherm	Constants of Freundlich Isotherm
	R²	b(L/mg)	q_max(mg/g)	R_L	R²	K_F(mg/g)	n
AAm/PEG₆₀₀₀	0.980	0.0043	12.92	0.72	0.989	0.84	1.2
P(AAm)	0.986	0.0170	2.32	0.40	0.994	0.97	1.7

¹These results were analyzed statistically from Student’s t-test (p < 0.05).

n value is related to occurrence of adsorption and if this value is between 1 and 10 that denotes a good adsorption. In other words, the value of Freundlich exponent n (1.2 and 1.7) is in the mentioned interval indicating a favorable adsorption [19, 22]. K_F values from Freundlich constants obtained as a result of adsorption experiments are given in Table 1. High values of K_F indicates that greatness of adsorbent pore size this means that it is an important indicator of being a good adsorbent. High K_F values of adsorbents used in other studies is supported with the literature [23].

Fig.4

Freundlich isotherm for adsorption of methylene blue on P(AAm) and AAmPEG₆₀₀₀ hydrogels.

3.4 Adsorption kinetics

The study of adsorption kinetics describes the solute uptake rate, and evidently this rate controls the residence time of adsorbate uptake at the solid-solution interface. The kinetics of methylene blue adsorption on the hydrogel was analyzed using pseudo-first-order [24] and pseudo-second-order [25] kinetic models. The most consistent model was found by calculating least square method, the coefficient of determination (R²) values. Furthermore; by calculating rate constants, k₁, k₂ and q_e values for various parameters applied in experiments, calculated q_e values were compared with the experimental q_e values.

The pseudo-first-order kinetic model can be expressed by Equation 6: $In (qe - qt) = In qe - k 1 t$ (6) here, q _e (mg/g) is the amount of adsorbed substance per gram adsorbent at the equilibrium moment, q_t (mg/g) is the amount of adsorbed material per gram adsorbent, k₁ (min^–1) is the pseudo-first-order rate constant, t is the contact time.

The values of k₁ and q_e were determined from the slope and intercept of the plot of log(q_e–q_t) versus t, respectively.

The pseudo-second-order rate equation is given by Equation 7 [25]: $\frac{t}{qt} = \frac{1}{{k 2 q}_{e}^{2}} + \frac{1}{qe} t$ (7) where k₂ (g/mg min) is the pseudo-second-order rate constant. The values of q_e and k₂ were determined from the slope and intercept of the plot of (t/q_t) versus t, respectively.

The coefficient of determination (R²) values were used to express the conformity between the experimental and theoretical data.

The rate constants, the correlation coefficients and the calculated q_e for the two kinetic models of P(AAm) and AAm/PEG₆₀₀₀ are shown in Table 2.

Table 2

Kinetic parameters of methylene blue adsorption

Hydrogels	Pseudo first order			Pseudo second order
	R²	k₁(h^–1)	q_e,cal (mg/g)	R²	k₂ (h^–1)	q_e,cal (mg/g)	q_e_,exp (mg/g)
AAm/PEG₆₀₀₀	0.857	0.148	2.11	0.999	0.199	3.66	3.52
P(AAm)	0.998	0.204	1.36	0.998	0.241	1.57	1.42

Adsorption kinetics was evaluated using pseudo-first-order and pseudo-second-order kinetic models. In order to examine conformity of both models and experimental results, the linear plots of ln(q_e–q_t)-t and (t/q_t)–t was used for pseudo-first-order and pseudo-second-order kinetic models, respectively. As seen in Table 2, the coefficient of determinations (R ² ) of the pseudo-first-order model were 0.998 and 0.857 for P(AAm) and AAm/PEG₆₀₀₀ hydrogels, respectively. For the pseudo-second-order model the coefficient of determinations (R²) were 0.999 and 0.998 for P(AAm) and AAm/PEG₆₀₀₀, respectively. As a result the correlation coefficients of pseudo-first-order model indicate the poor correlation of methylene blue adsorption onto hydrogel. Application of a pseudo-second-order model provides much better correlation coefficients. In addition, the calculated q_e values of the pseudo-first-order model for the adsorption of methylene blue by P(AAm) and AAm/PEG₆₀₀₀ were 1.36 and 2.11 mg/g, respectively. For the pseudo-second-order model the calculated q_e values of P(AAm) and AAm/PEG₆₀₀₀ were 1.57 and 3.66 mg/g, respectively. Obviously the calculated q_e values of P(AAm) and AAm/PEG₆₀₀₀ hydrogel agreed with the experimental data (1.42 and 3.52 mg/g) in the case of the pseudo-second-order model. It is clear that methylene blue adsorption onto hydrogel followed the pseudo-second-order kinetic model. Similarly, several researchers have been used the pseudo-second-order kinetic model in order to express methylene blue adsorption onto different hydrogels [26, 27].

4 Conclusions

In this study; polyacrylamide and acrylamide-poly(ethylene glycol) hydrogels were synthesized by free radical solution polymerization in aqueous solution. Swelling tests were applied to synthesized hydrogels, as a result of obtained data it was observed that macroporous hydrogels synthesized by PEG₆₀₀₀ have the highest swelling content, polyacrylamide P(AAm) hydrogels have the least swelling content. Increasing the amount of PEG₆₀₀₀ in the synthesis of hydrogel also increased the swelling content of the hydrogel. Adsorption tests of methylene blue dye on the P(AAm) and AAm/PEG₆₀₀₀ hydrogels were carried out and the obtained results showed that methylene blue adsorption process is dependent on PEG₆₀₀₀ content. The adsorption processes of the methylene blue dye on P(AAm) and AAm/PEG₆₀₀₀ hydrogels followed the pseudo-second-order model and the Freundlich isotherm, respectively. By introducing 100 wt % PEG₆₀₀₀ into acrylamide polymeric network the obtaining hydrogel composite showed the highest adsorption capacity for methylene blue. Therefore AAm/PEG₆₀₀₀ hydrogels can be used as an effective adsorbent for the removal of methylene blue dye from wastewaters.

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