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Water and dye sorption studies of novel semi IPNs: acrylamide/4-styrenesulfonic acid sodium salt/PEG hydrogels.

INTRODUCTION

Materials with the ability to absorb water in high amounts are again under investigation because of their potential applications in bioengineering, biomedicine, food industry, communication technology, building industry, chromatography, water purification, separation processes, and agriculture. Networks of hydrophilic polymers (hydrogels) have the capability of absorbing large amounts of water, without losing their three-dimensional structure. Although many naturally occurring polymers may be used to produce this type of materials, the structural versatility available in synthetic hydrogels has given them distinctive properties, which in turn have enhanced their practical utility. The ability of polymer gels to undergo substantial swelling and collapsing, up to 1000 times in volume, as a function of their environment is one of the most notable properties of these materials. These polymers are often called polymer hydrogels and generally they are low crosslinked hydrophilic electrolytes. In order to keep the spatial structure, the polymer chains are usually physically or chemically crosslinked. Hydrogels are a unique class of polymeric materials, which imbibe enormous amount of water when left in a water reservoir for long times. The underlying property for this unusual behavior of hydrogels is their transition from a glassy to a rubbery state when contacted with thermodynamically compatible solvents (1-6).

Water pollution due to organic compounds such as dye molecules and heavy metals remains a many serious environmental and health problem in living media (7-10). Dyes and aromatic molecules such as phenolic derivatives and polycyclic aromatic compounds are often found in the environment as a result of their wide industrial processing. Water pollutants in wastewater are known to be toxic and carcinogenic. Therefore, their presence in the environment, in particular in water should be controlled. Adsorption procedures are a way of one of the most widely used for pollutants such as dyes and organic compounds from industrial effluents. Adsorption is a well-known equilibrium separation process. Recently, new effective, efficient and economic methods for water decontamination applications and for separation analytical purposes have been investigated (7-10).

Hydrogel--hydrogel composite semi-interpenetrating polymer networks (semi IPNs) can be prepared by using "new" hydrophilic comonomers. Materials formed from IPNs share properties characteristic of each network (10-12). To reduce costs and improve the comprehensive water-absorbing or other related physical properties of superabsorbent materials based on acrylic monomers, grafting acrylic monomers onto clay and fabricating a composite consisting of a polymer and clay can be priority. In many previous studies, several kinds of superabsorbent composites based on some clay or synthetic polymers were prepared, and these superabsorbents composites showed high water absorbency and water retention, good salt--resistance, and low production costs in comparison with pure organic superabsorbent polymers under the same preparation condition (10-12).

Polyacrylamide-based hydrogels have received considerable attention because of their use in many applications (as specific sorbent, etc.). In our previous studies, copolymeric hydrogels of acrylamide with some acidic monomers were prepared by free radical solution polymerization and used in separation and adsorption of some dye molecules (8), (10), (11), (13), (14).

It was reported that a lot of 4-styrenesullonic acid sodium salt, (NaSS)-based studies by free radical polymerization (15-18). In these studies, it was described that strongly charged property of NaSS at some polyelectrolyte copolymers (15-18).

Poly (ethylene glycol) (PEG) is of great interest in numerous biomedical applications for several purposes. PEG is water-soluble and is nontoxic for body immune system. PEG based hydrogels have good biocompatibility. PEG-based hydrogel systems have been used at many biotechnological applications (10), (11), (19), (20).

In this study, it was of interest to increase or decrease the water and dye sorption capacity of AAm hydrogels with highly hydrophilic functional groups containing chemical reagents such as NaSS with PEG via free radical solution polymerization method. So, AAm/NaSS hydrogels and AAm/NaSS/PEG semi IPNs can be prepared by free radical solution polymerization. AAm is a highly hydrophilic monomer, NaSS is anionic and strongly charged monomer and PEG is linear polymer. The main purpose of this study was to combine both monomers and a polymer in a new polymeric system. In this respect, a series of AAm-based copolymeric hydrogels were synthesized by changing the content of NaSS and PEG. For characterization, first, for structural characterization. FTIR analysis can be made. Then, some swelling, and some diffusional properties of AAm/NaSS hydrogels and AAm/NaSS/PEG semi IPNs were studied in water by dynamic swelling studies for swelling characterization. The other stage of the study is dye sorption. For this, batch sorption studies can be applied in all sorption experiments. Water uptake and dye sorption properties of AAm/NaSS hydrogels and AAm/NaSS/PEG semi IPNs were investigated as a function of chemical composition of the hydrogels.

EXPERIMENTAL

Materials

Acrylamide (AAm), the initiator, ammonium persulfate (APS), supplied by Merck, (Darmstad, Germany), the activator N, N, N', N'-tetramethylethylenediamine (TEMED) were supplied by Merck (Schuchardt, Germany). Anionic comonomer, NaSS, and poly(ethylene glycol) (PEG, [M.sub.w] = 4000), were supplied by Fluka Chemical Co., (Steinheim, Germany). Poly(ethylene glycol)dimethacrylate (PEGDMA, [M.sub.n] = 330) as crosslinker were supplied from Aldrich, (Steinheim, Germany).

Cationic dye, Union Green B, (Janus Green B, UGB) used in sorption studies, was purchased From Fluka. Some properties of UGB were presented at Table 1. All chemicals were used as received.

TABLE 1. Some properties of Union Green B.

Name             Chemical  Molar mass (g     [[lambda].  C.I. Nr.
                  formula   mo[l.sup.-1]  sub.max] (nm)

Union Green                       511.07            660     11050
B. (UGB)
(Janus Green B)


Preparation of AAm/NaSS Hydrogels and AAm/NaSS/PEG Semi IPNs

To prepare AAm/NaSS hydrogel systems, AAm weighing 1.0 g (14.07 mmol) was dissolved in 1.0 mL water. Then 0 mg, 10 mg/0.0485 mmol, 20 mg/0.0970 mmol, 30 mg/0.145 mmol. 40 mg/0.194 mmol, 50 mg/0.242 mmol, 60 mg/0.291 mmol, 70 mg/0.339 mmol, 80 mg/0.388 mmol of NaSS were added to each AAm solutions, respectively. After these additions, for the synthesis, 0.040 mL/0.133 mmol of PEGDMA and 0.2 mL/0.0438 mmol aqueous solutions of APS (5.0 g APS/0.022 mol/100 mL water) and 0.25 mL/0.0167 mmol 1% concentration of TEMED were added these aqueous solutions. The solutions were placed in PVC straws of 3 mm diameter. After gelation, fresh hydrogels obtained in long cylindrical shapes were cut into pieces of 3-4 mm in length. They were washed 4 clays in distilled water to remove unreacted materials, blotted dry with filter paper, dried in air and vacuum, and stored for swelling and sorption studies.

To prepare highly swollen AAm/NaSS/PEG semi IPNs, same method was used as mentioned above with addition of 0.50 g PEG to aqueous monomer solution per 1.0 g of AAm. For investigation of PEG effect onto swelling properties of the semi-IPN systems, the samples containing 1.0 g AAm and 60 mg NaSS have been synthesized by using of 0.25, 0.50, 0.75, and 1.00 g PEG, and used at characterization.

For swelling studies. AArn/NaSS hydrogels and AAm/NaSS/PEG semi IPNs were accurately weighted and transferred into water. Water uptake with respect to time was obtained by periodically removing the samples from water; quickly blot drying, and reweighing. The measurements were conducted at 25 [+ or -] 0.1[degrees]C in a water bath.

FTIR Analysis of AAm/NaSS Hydrogels and AAm/NaSSI PEG Semi IPNs

For structural characterization. FTIR analysis was made. Spectra were taken on KBr discs by using VARIAN FTS 800 FTIR spectrophotometer.

Sorption Studies of AAm/NaSS Hydrogels and AAm/NaSSI PEG Semi IPNs

Batch sorption studies were applied in all sorption experiments. Cationic dye, Union Green B. (UGB) used in sorption studies and some properties of UGB were given in Table I. Solutions of UGB concentration range 1.25 x [10.sup.-3] M to 2.00 x [10.sup.-3] M in distilled water were prepared. AAm/NaSS hydrogels and AAm/NaSS/PEG semi IPNs containing 60 mg NaSS was used in a known volume of dye solution until equilibrium was reached. For NaSS effect on the dye sorption, 2.00 x [10.sup.-3] M aqueous solution of UGB was used. After sorption, dye solution was separated by decantation from the hydrogels. Spectrophotometric method was applied to dye solutions. Spectrophotometric measurements were carried out using a SHIMADZU UV 1601 model UV-VIS spectrophotometer at ambient temperature. The absorbance of these solutions was read at 660 nm for UGB (11). Distilled water was chosen as the reference. The equilibrium concentrations of the cationic dye solutions were determined by means of precalibrated scales.

RESULTS AND DISCUSSION

Dried AAm/NaSS hydrogels and AAm/NaSS/PEG semi IPNs are glassy and very hard, but swollen gels are soft. NaSS received attention in recent years due to its strongly ionizable charged group.

FTIR Analysis

AAm/NaSS hydrogels and AAin/NaSS/PEG semi IPNs were prepared by free radical solution polymerization. The synthesis of AAm/NaSS hydrogels and AAm/NaSS/PEG semi IPNs via radical chain polymerization is a well-established procedure. Representative chemical structures of monomers and possible binding of AAm/NaSS copolymer have been presented at Fig. 1. To understand binding and crosslinking of AAm/NaSS hydrogels and AAm/NaSS/PEG semi IPNs during polymerization, FTIR spectra of the hydrogel systems were evaluated and are presented in Fig. 2. In the FTIR spectra of the hydrogels, the bands at about 1700 and 3100-3500 c[m.sup.-1] are important. The bands at 1600-1700 c[m.sup.-1] could be attributed to a shift in stretching vibration associated with hydrogen that is bonded directly to an overtone of the strong carbonyl absorption. The peak at 1650-1660 c[m.sup.-1] is the carbonyl group and related to amide groups and at 1500-1600 c[m.sup.-1] is the N-H bonding vibration. The much broader absorption peaks in the regions of 3100 c[m.sup.-1] and 3500 c[m.sup.-1] are N-H bands and related to "polymeric" bands. In Fig. 2, there is a group of absorption peaks between 3400 and 3600 c[m.sup.-1], which is due to stretching bands of the OH groups, and the band at 1640 c[m.sup.-1] also corresponds to the OH groups. The broad peak 3500 is characteristic peak of primary amine. On the other hand, it is thought that the peaks at 1200 c[m.sup.-1] are C-N bands, and the peaks at 2850 c[m.sup.-1], and 1400 c[m.sup.-1] show -C[H.sub.2]- groups on the polymeric chain. The characteristic absorption peak of NaSS units is shown at 1040 c[m.sup.-1] due to S=0 group (18), (21).

The characteristic absorption peak of PEG units is shown at 1100-1200 [cm.sup.-1] due to aliphatic ether bonds (21). The peaks observed in the FTIR spectra confirm the presence of AAm, NaSS, and PEG.

Equilibrium Swelling Similes

A fundamental relationship exists between the swelling of a polymer in a solvent and the nature of the polymer and the solvent. The percentage swelling (S%) of the hydrogels in distilled water was calculated from the following relation,

S% = [[m.sub.1] - [m.sub.0]] / [m.sub.0] x 100 (1)

where nit is the mass of the swollen gel at time t and mo is the mass of the dry gel at time 0.

The water intake of initially dry hydrogels was followed for a period of time, gravimetrically. Swelling isotherms of the hydrogels were constructed and representative swelling curve is shown in Figs. 3 and 4.

Figures 3 and 4 show that swelling increase with time up to certain level, and then levels off. This value of swelling may be called the "equilibrium percentage swelling" ([S.sub.eq]%). The values of [S.sub.eq]% of AAm/NaSS hydrogels and AAm/NaSS/PEG semi IPNs are used for the calculation of network characterization parameters. The values of [S.sub.eq]% of AAm/NaSS hydrogels and AAm/NaSS/PEG semi IPNs are given Table 2. Table 2 shows that Seq% of AAm hydrogels is 660%, but [S.sub.eq]% of AAm/NaSS are 690-1330% with the incorporation of NaSS groups into AAm hydrogels, and [S.sub.eq]% of AAm/NaSS/PEG hydrogels are 720-1310% with the incorporation of PEG groups into AAm/NaSS hydrogels, while equilibrium swelling percent value of AAm/PEG hydrogels is 580%. In Table 2, [S.sub.eq]% of the hydrogels increased with the NaSS content in the copolymers. [S.sub.eq]% of AAm/NaSS hydrogels is higher than [S.sub.eq]% of AAm hydrogels. Table 2 also shows that Seq% values of AAm/NaSS/PEG semi IPNs lower than [S.sub.eq]% values of AAm/NaSS hydrogels. One of the reasons of these results is decreasing of hydrophilic character at crosslinked polymeric systems. Additionally, the PEG chains are located in the free space of crosslinked polymer networks; therefore water diffusion is prevented by the PEG chains.

TABLE 2. Values of the equilibrium percentage swelling
([S.sub.eq]%) of AAm/NaSS hydrogels and AAm/NaSS/PEG
(containing 0.5 g PEG) semi IPNs.

NaSS (mg)   0    10    20    30     40     50     60     70     80

          Equilibrium percentage swelling (S%)

          660   690   780   840    950   1070   1190   1220   1330

PEG       580   720   870   960   1020   1160   1200   1250   1310


It is well known that the swelling of a hydrogel is induced by electrostatic repulsion of the ionic charges of its network. The ionic charge content is important. NaSS contains many ionic groups (--S[O.sub.3]Na) (Fig. l). The swelling increase is due to an increase in the anionic units. The salt group is almost completely ionized, and a large number of hydrophilic groups occur. The hydrophilic group numbers of AAm/NaSS hydrogels and AAm/NaSS/PEG semi IPNs are higher than those of AAm, and so the swelling values of AAm/NaSS hydrogels and AAm/NaSS/PEG semi IPNs are greater than that of AAm swelling values. Upon swelling the hydrogels were strong enough to retain their shape.

Diffusion

When a glassy hydrogel is brought into contact with water, water diffuses into the hydrogel and the network expands resulting in swelling of the hydrogel. Diffusion involves migration of water into pre-existing or dynamically formed spaces between hydrogel chains. Swelling of the hydrogel involves larger segmental motion resulting, ultimately, in increased separation between hydrogel chains.

Analysis of the mechanisms of water diffusion into swellable polymeric systems has received considerable attention in recent years, because of important applications of swellable polymers in biomedical, pharmaceutical, environmental, and agricultural engineering.

The following equation is used to determine the nature of diffusion of water into hydrogels (22-24).

F = [M/sub.t] / [M.sub.s] = k[t.sub.n] (2)

Here, F is the fractional uptake at time t. Here. [M.sub.t] and Ms are the mass uptake of the solvent at time [M.sub.8] and the equilibrium, respectively. Equation 3 is valid for the first 60% of the fractional uptake. Fickian diffusion and Case II transport are defined by n values of 0.5 and 1.0, respectively. Anomalous transport behavior (non-Fickian diffusion) is intermediate between Fickian and Case II. That is reflected by n between 0.5 and 1.0 (24-26). The values or (n) and (k) were calculated from the slope and the intercept of the plot of In F against In t, respectively.

For AAm/NaSS hydrogel and AAm/NaSS/PEG semi IPNs. In F vs. In t graphs are plotted, and representative results are shown in Fig. 5 for AAm/NaSS/PEG semi IPNs. Diffusional exponents (n) and diffusion constant (k) are calculated and listed in Table 3.

Table 3 shows that the number determining the type of diffusion (n) is over 0.50. Hence the diffusion of water into the super water-retainer hydrogels is generally found to have a non-Fickian character. When the diffusion type is anomalous behavior, the relaxation and diffusion time are of the same order of magnitude. As solvent diffuses into the hydrogel, rearrangement of chains does not occur immediately.

TABLE 3. Some diffusion parameters of AAm/NaSS hydrogels
and AAm/NaSS/PEG (containing 0.5 g PEG) semi IPNs.

NaSS      0     10     20     30     40     50     60     70     80
(mg)

Diffusion exponent (n)

      0.599  0.609  0.611  0.614  0.621  0.604  0.610  0.589  0.624

PEG   0.567  0.580  0.581  0.589  0.582  0.584  0.638  0.625  0.609

Diffusion constant (k x [10.sup.2]

       2.83   2.83   2.89   2.69   3.05   2.70   2.36   2.69   2.38

PEG    3.96   3.37   3.10   2.77   2.71   2.98   2.43   2.31   2.57

Diffusion coefficient (D x [10.sup.6], [cm.sup.2] [min.sup.-1]

         97    109    150    149    189    127    129   1 19    155

PEG      75     64     85     94     80    120    175    145    134


The study of diffusion phenomena of water in hydrogels is of value in that it clarifies polymer behavior. For hydrogel characterization, the diffusion coefficients can be calculated by various methods. The diffusion coefficient (D) of the water was calculated using the following equation (25), (26).

D = [pi][n.sup.2][(k/4).sup.1/n] (3)

Here, D is in [cm.sup.2], r is the radius of a cylindrical polymer sample, 00 is the diffusional exponent and (k) is a constant incorporating characteristic of the macromolecular network system and the penetrant. The values of diffusion coefficient determined for AAm/NaSS hydrogels and AAm/NaSS/PEG semi IPNs are listed in Table 3. Table 3 shows that the values of the diffusion coefficient of AAm/NaSS hydrogels and AAm/NaSS/PEG semi IPNs vary from 64.0 x [10.sup.-6] [cm.sup.2] [min.sup.-1] to 189.0 x [10.sup.-6] [cm.sup.2] [min.sup.-1]. There is no good correlation between the values of the diffusion coefficient of AAm/NaSS hydrogels and AAm/NaSS/PEG semi IPNs.

PEG Effect on the Swelling and Diffusion

For investigation of the effect of mass/content of PEG on the swelling properties of AAm/NaSS/PEG semi IPNs, the related swelling isotherms of AAm/NaSS/PEG hydrogels were constructed and representative swelling isotherms and the plot of In F vs. In t are plotted and representative results are shown in Figs. 6 and 7. PEG effect on some swelling and diffusion parameters of AAm/NaSS/PEG semi IPN systems containing 60 mg NaSS are tabulated in Table 4.

TABLE 4. Some swelling and diffusion parameters of
AAm/NaSS/PEG semi IPNs with various contents of PEG
and containing 60 mg NaSS.

0 g PEG  0.25 g PEG  0.50 g PEG  0.75 g PEG  1.00 g PEG

Equilibrium percentage swelling ([S.sub.eq]%)

1190           1190        1200        1170        1070

Diffusion exponent (n)

0.610         0.614       0.638       0.567       0.556

Diffusion constant (k x [10.sup.2]

2.36           2.44        2.43        3.26        3.58

Diffusion coefficient (D x [10.sup.6], [cm.sup.2] [min.sup.-1]

129             130         176          92          89


It was shown that a decreasing of the equilibrium percentage swelling, equilibrium water contents, diffusion constant, diffusion exponent, and diffusion coefficient of AAm/NaSS/PEG semi IPN systems when PEG has been added to the hydrogel systems. Incorporation of PEG into the copolymer network leads to lower degrees of swelling. On the other hand, also, it is seen that an increasing of diffusion constant (k) of AAm/NaSS/PEG hydrogels with increasing content or PEG in hydrogel systems from Table 4. Here, it was said that PEG chains was placed in the crosslinked polymeric systems, in stead of crosslinked AAm and NASS molecules, it was seen that decreasing of the value of the equilibrium swelling percent and related parameters, because of decreasing of hydrophilic character at crosslinked polymeric systems. In addition of this phenomenon, the PEG chains are located in the free space of crosslinked polymer networks; water diffusion is prevented by the PEG chains. This is also caused of decreasing of the equilibrium percentage swelling and related parameters.

Equilibrium Sorption

Studies To observe the sorption of UGB, AAm/NaSS hydrogels and AAm/NaSS/PEG semi IPNs were placed in aqueous solutions of UGB and allowed to equilibrate for four days at 25[degrees]C. At the end of this period AAm/NaSS hydrogels and AAm/NaSS/PEG semi IPNs in the UGH solutions showed the dark coloration. But acrylamide hydrogel did not sorb any dye from solution. In the sorption system at equilibrium, the total solute (dye) concentration: [C.sub.o] is following equation (11), (27), (28).

[C.sub.o] = [C.sub.b] + C (4)

Here, [C.sub.b] is the equilibrium concentration of the solute (dye) on the sorbent per liter solution (bound solute concentration) and C is the equilibrium concentration of the solute in the solution (free solute concentration). The value of the bound concentration may be obtained by using Eq. 4. For a fixed free solute concentration, Ch is proportional to the polymer concentration on the binding system: the amount bound can therefore be conveniently expressed as the binding ratio r, defined by

r = [C.sub.b] / P (5)

Thus, with in and 12 is base mol (moles of monomer units) per liter solute represents the average number of molecules of solute bound each monomer unit at that free solute concentration. To determine the sorption/binding kinetics of UGB into AAm/NaSS hydrogels and AAm/NaSS/PEG semi IPNs, binding characterization parameters can be investigated.

The binding data were interpreted on the basis of the uniform site-binding (u.s.b.) model, which in statistical-thermodynamic terms corresponds to a formation of an ideal localized one-dimensional monolayer of solute on the polymer chains. This leads to the hyperbolic (Langmuir) from of the binding isotherm, which applies to many polymer/solute (dye molecules) binding system (11), (27), (28).

r = [n.sub.8] KC / 1 + KC (6)

K is the binding constant, i.e. the equilibrium constant for the attachment of a molecule of dye onto a site by a specific combination of non-covalent forces. Here, [N.sub.8] is the site density (the limiting value of [N.sub.8] for monolayer coverage) which is therefore of density of the sites along the polymer chain. To reciprocal of [N.sub.8] is the site-size, u, which may be taken to represent either average number of monomer units occupied by the bound solute molecule, more generally the average spacing of solute molecules when the chain is saturated. The initial binding constant, [K.sub.i] is the initial slope of the binding isotherm, and therefore the average binding strength of a solute molecule by a single monomer unit on an occupied chain. [K.sub.i] is equal to the product [n.sub.s]K.

To get the best values for the binding parameters from the experimental data, the linearization methods of Eq. 7 have been developed by some researches as Klotz. Scatchard and Langmuir (11), (27), (28).

Langmuir Method

Mathematical analysis of Eq. 6 gives the bottom equation.

C/r = 1/[n.sub.s]K + C/[n.sub.s] (7)

So that here a plot of C/r vs. C should be a straight line of slope 1/[n.sub.s], ordinate intercept 1/[n.sub.s]K.

The Langmuir plots of AAm/NaSS hydrogels and AAm/NaSS/PEG semi IPNs are shown in Figs. 8 and 9, respectively, and the binding parameters for are calculated from the intercept and slopes of the binding isotherm methods.

The binding parameters [K.sub.i], K, [n.sub.s], and u are listed in Table 5 for AAm/NaSS hydrogels and AAm/NaSS/PEG semi IPNs. In Table 5, the final column contains the derived values of the [^.0], the maximum fractional occupancy attained experimentally, calculated from the definition of fractional occupancy [^.0]:

TABLE 5. Some binding parameters of AAm/NaSS hydrogels
and AAm/NaSS/PEG semi IPNs with UGB.

                [K.sub.i] x            K x
                [10.sup.-4]    [10.sup.-4]

                         (L             (L     n      u  [^.O]
              mo[l.sup.-1])  mo[l.sup.-1])

AAm/NaSS               4.95           1.31  3.78  0.265   1.05

AAm/NaSS/PEG           7.94           1.18  6.74  0.148  0.866


[^O] = r/[n.sub.s] (8)

Using the value of r at the maximum experimental free dye concentration and with the site-density obtained for the (u.s.b.) model.

The binding parameters [K.sub.i]. K, n, u and [^0] are listed in Table 5 for AAm/NaSS hydrogels and AArn/NaSS/PEG semi IPNs-dye binding systems. There has been shown good accordance to the linear regression of the experimental data (Figs. 8 and 9).Then, it was said that related linearization method such as Langmuir can be used in the hyperbolic binding system for other dye-polymers binding processes.

To observe the sorption of Union Green B (UGB), AAm/NaSS hydrogels and AAm/NaSS/PEG semi IPNs were placed in aqueous solutions of UGB and allowed to equilibrate for 4 days at 25[degrees]C. At the end of this period AAm/NaSS hydrogels and AAm/NaSS/PEG semi IPNs in the UGB solutions showed the dark coloration. But acrylamide hydrogel did not sorb any dye from solution.

For equilibrium sorption studies, the sorption capacity (q) (mass amount as "mol" of sorption per unit mass (as gram) of the adsorbent, adsorption percentage (Ads%), and partition coefficient ([K.sub.d]) can be investigated.

The sorption capacity (q) of AAm/NaSS hydrogels and AAm/NaSS/PEG semi IPNs were evaluated by using the following equation:

q = ([C.sub.0] - C)v/m (9)

where q is the sorption capacity of AAm/NaSS hydrogels and AAm/NaSS/PEG semi IPN's (mol [g.sup.1], [C.sub.0] and C are the concentration of the dye in the initial solution and the aqueous phase after treatment for a certain period time, respectively (mol [L.sub.1], v is the volume of the aqueous phase (L), m is the amount of dry AAm/NaSS hydrogels and AAm/NaSS/PEG semi IPN's.

Adsorption percentage (Ads%) of AAm/NaSS hydrogels and AAm/NaSS/PEG semi IPNs was calculated by following equation.

Ads% = [C.sub.0] - C/[C.sub.0] x 100 (10)

Here, [C.sub.0] and C were defined earlier.

The effect of NaSS contents onto sorption capacity was measured. The sorption capacity, i.e., the amount of dyes sorbed onto unit dry mass of the gel was calculated for uptake of dye within the hydrogel in 2.00 x [10.sup.-3] mol UGB in L of aqueous solutions, and presented in Table 6. Table 6 presents that the sorption capacity of AAm/NaSS hydrogels and AAm/NaSS/PEG semi IPN's (1.24 x [10.sup.-4] to 4.05 x [10.sup.-4] mol [g.sup.-1]) and adsorption percentage of these (18-67%) both are increased with together. The sorption capacity and adsorption percentage of AAm/NaSS hydrogels and AAm/NaSS/PEG semi IPNs gradually increased with increasing of NaSS content in hydrogels and semi IPNs.

TABLE 6. Some adsorption parameters of AAm/NaSS hydrogels
and AAin/NaSS/PEG (containing 60 mg NaSS and 0.5 g PEG) semi
IPNs in aqueous solutions of UGB.

NaSS    10    20    30    40    50    60    70    80
(mg)
                 Sorption capacity (q x [10.sup.4])

      1.24  1.80  2.31  2.90  3.10  3.32  3.62  3.77

PEG   1.51  2.06  2.50  2.75  3.40  3.64  3.64  4.05

                  Adsorption percentage (Ads%)

        18    25    38    39    48    54    56    67

PEG     19    25    31    39    49    53    61    62

                  Partition coefficient ([K.sub.d])

      0.22  0.34  0.62  0.64  0.91  1.16  1.30  2.02

PEG   0.24  0.34  0.44  0.64  0.96  1.13  1.57  1.65


Equilibrium UGB sorption isotherms of AAm/NaSS/PEG semi IPNs is presented in Fig. 10. To Fig. 10, the sorption capacity of the hydrogel systems is increased with the increasing concentration of UGB. This is expected result.

Partitioning of dissolved constituents between an aqueous phase and adsorbents in waters and sediments has commonly been described by an empirical partition coefficient that simply relates the total concentration of a dissolved species to the total concentration of the adsorbed species. For this parameter, the given equation at below can be used (29), (30).

[K.sub.d] = [C.sub.0] - C/[C.sub.0] (11)

Here, [K.sub.d] is empirical partition coefficient at equilibrium. [C.sub.0]., and C were defined earlier. Partition coefficients of UGB between dye solution and hydrogels were calculated, and are shown in Table 6. In Table 6, [K.sub.d] values of AAm/NaSS hydrogels is 0.22-2.02, but [K.sub.d] values of AAm/NaSS/PEG semi IPNs is 0.24-1.65 with the incorporation of PEG groups into the hydrogels.

Here, [K.sub.d] values of AAm/NaSS hydrogels and AAm/NaSS/PEG semi IPNs (having 60 mg and more than this NaSS content) are higher than 1.0. So, it can be said that synthesized crosslinked AAm/NaSS hydrogels and AAm/NaSS/PEG semi IPNs (having 60 mg and more than this NaSS content) could be used as potential adsorbent (29), (30).

PEG Effect on the Sorption of UGB

For investigation of the effect of mass/content of PEG on the sorption properties of AAm/NaSS/PEG semi IPNs, some sorption parameters such as sorption capacity, adsorption percentage and partition coefficient of AAm/NaSS/PEG semi IPNs systems containing 60 mg NaSS are tabulated in Table 7.

TABLE 7. PEG effect on some adsorption parameters of
AAm/NaSS/PEG semi IPNs with various contents of PEG and
containing 60 mg NaSS in aqueous solutions of UGB.

0 g PEG  0.25 g PEG  0.50 g PEG  0.75 g PEG  1.00 g PEG

                  Sorption capacity (q x [10.sup.4])

3.32           3.54        3.64        3.87        3.67

                      Adsorption percentage (Ads%)

54               53          53          59          48

                    Partition coefficient ([K.sub.d])

1.16           1.13        1.13        1.42        0.93


It was shown that chancing of adsorption percentage (48-59%) and partition coefficient (0.93-1.42) of AAm/NaSS/PEG semi IPN systems when PEG has been added to the hydrogel systems. Incorporation of PEG into the copolymer network leads to lower values of adsorption percentage and partition coefficient of AAm/NaSS/PEG semi IPN systems. Here, [K.sub.d] values of AAm/NaSS/PEG semi IPNs including of 0 g, 0.25 g, 0.5 g, and 0.75 g of PEG are higher than 1.0 (1.16-1.42). So, it can be said that synthesized crosslinked AAm/NaSS/PEG semi IPNs (having 60 mg NaSS and 0.0 g0.75 g PEG content) could be used as potential adsorbent (29), (30). On the other hand, it can be said that there is no important changing of the sorption capacity of AAm/NaSS/PEG semi IPN systems when PEG has been added to the hydrogel systems (3.32 X [10.sup.-4] to 3.87 x [10.sup.-4] mol [g.sup.-1]).

Here, it was said that PEG chains was placed in the crosslinked polymeric systems, in stead of crosslinked AAm and NaSS molecules, it was seen that decreasing of the adsorption percentage, because of decreasing of hydrophilic character at crosslinked polymeric systems. In addition of this phenomenon, the PEG chains are located in the free space of crosslinked polymer networks; UGB sorption is prevented by the PEG chains. This is also caused of partition decreasing of the adsorption percentage.

The ionic charge content in the polymeric structure is important. NaSS contains ionic units (--S[O.sub.3]Na). The swelling degrees of the hydrogels increase due to increasing of the hydrophilic units on hydrogel structure (Fig. 1). Therefore AAm/NaSS hydrogels and AAm/NaSS/PEG semi IPNs have many ionic groups that can increase interaction between the cationic dye molecules and anionic groups of hydrogels. The results of swelling studies are parallel character to the results of sorption studies. Both of them, it can be seen that swelling or sorption capability of AAm/NaSS hydrogels and AAm/NaSS/PEG semi IPNs are increased with increasing NaSS content in copolymeric structure. The most important effect is hydrophilicity of copolymeric gels. Hydrophilicity of AAm/NaSS and AAm/NaSS/PEG copolymers becomes greater than that of AAm, when addition of NaSS to the copolymeric structure.

For good binding analysis, Table 8 can be arranged. There can be many reasons for noncovalent interactions in the binding of UGB by AAm/NaSS hydrogels and AAm/NaSS/PEG semi IPNs. The main interactions between the hydrogel and the monovalent cationic dye may be hydrophobic and hydrogen bonding. Specially, hydrogen bonding will be expected to occur between

amine groups and nitrogen atoms on the dye molecules and the amine and carbonyl groups on the monomer unit of crosslinked polymer. Hydrophobic effects are especially aqueous solutions interactions which in the present case will involve those aromatic rings on the dye molecules and the methine and methyl groups on the gel. There can be some other interactions such as dipole--dipole and dipole-induced dipole interactions between the dye molecules and the hydrogel chains.

TABLE 8. Possible non-covalent interactions in the binding
of UGB by AAm/NaSS hydrogels and AAm/NaSS/PEG semi IPNs
systems.

Copolymer  Union Green  Interaction type  Copolymer  Dye
           B (UGB)

                        Hydrogen bonding  N and 0    H atom
                                          atom = 0.  Alkyl Amine
                                          methine.
                                          Methyl

                       Hydrophobic        Hydrogen   Benzene ring
                                          atom

                       Dipole--dipole     Amide or   Benzene ring
                                          sulfonic
                                          groups

                       Dipole-induced     Amide and  Polarizable
                       dipole             sulfonic   aromatic group
                                          groups


CONCLUSION

Incorporation of hydrophilic group containing chemicals such as NaSS and a polymer such as PEG in AAm hydrogels can be obtained successively by free radical solution polymerization method. Multifunctional crosslinker such as PEGDMA used at the polymerization process. AAm/NaSS hydrogels and AAm/NaSS/PEG semi 1131% showed high water absorbency. The equilibrium percentage swelling ranges are 690-1330% for AAm/NaSS hydrogels and 720-1310% for AAm/NaSS/PEG semi IPNs. It was seen that swelling of AAm/NaSS hydrogels and AAm/NaSS/PEG semi IPNs increased with the increasing of content of NaSS. But, it was seen that a decreasing of values of Seq% from 1200% to 1070% when the adding of PEG for containing of 60 mg of NaSS.

To determine the sorption characteristics of UGB into AAm/NaSS hydrogels and AAm/NaSS/PEG semi IPNs, some sorption parameters have been investigated. For equilibrium sorption studies, sorption capacity, adsorption percentage, and partition coefficient of AAm/NaSS hydrogels and AAm/NaSS/PEG semi IPNs system have been investigated. Consequently, AAm/NaSS hydrogels and AAm/NaSS/PEG semi IPNs developed in this study may serve as a potential device for water and dye sorbent. The utilization of these types of materials, in pharmaceuticals, agriculture, biotechnology, environment, separation, purification, and immobilization makes hydrogels more popular.

ACKNOWLEDGMENTS

The work was supported by Adnan Menderes University Research Fund, under project number FEF 08 034.

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Correspondence to: Erdener Karadag; e-mail: ekaradag@adu.edu.tr

DOI 10.1002/pen.23378

Published online in Wiley Online Library (wileyonlinelibrary.com).

[c] 2012 Society of Plastics Engineers

Omer Baris Uzum, Erdener Karadag

Department of Chemistry, Fen-Edebiyat Faculty, Adnan Menderes University, Aydin, Turkey
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