Synthesis of Potato Starch-Acrylic-Acid Hydrogels by Gamma Radiation and Their Application in Dye Adsorption

1Nuclear and Radiation Chemistry Division, Institute of Nuclear Science and Technology, Bangladesh Atomic Energy Commission, G. P. O. Box 3787, Dhaka, Bangladesh 2Department of Chemistry, Shahjalal University of Science & Technology, Sylhet 3114, Bangladesh 3Department of Applied Quantum Physics and Nuclear Engineering, Faculty of Engineering, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan


Introduction
Hydrogels can be defined as hydrophilic polymer networks that can retain a significant amount of water within their structures and can swell without dissolving in water.The hydrogels which can absorb very large amount of water and hold it even under pressure are called superabsorbent hydrogels [1].So far, two general classes of hydrogel are mentioned in the literature.One is physical gels or pseudo gels and the other is chemical gels.Physical gels are nonpermanent and possible to convert in polymer solution by heating, while chemical gels are permanent and usually composed of covalently bonded cross-linking network.Many applications of hydrogels are already mentioned in the literature.The most successful and promising use is in the field of medicine, pharmacy, and tissue engineering [2,3].Due to their high water absorption capacity and biocompatibility properties, they have been used in wound dressing, drug delivery, agriculture, and dye adsorption, as well as a number of commercially available products: contact sanitary pads, transdermal systems, dental materials, implants, injectable polymeric systems, ophthalmic applications, and hybrid-type organs (encapsulated living cells) [4][5][6][7].Recently, many researchers have focused on modifying hydrogels with a view to enhance their water absorbency, network strength, and absorption rate [8,9].Starch, a specimen in the present study, can be a good 2 International Journal of Polymer Science natural-polymer superabsorbent because of containing a large number of hydrophilic groups (−OH) and having adequate properties for practical use (such as renewable, very cheap, and biodegradable) [10].
Radiation-induced processes, in which radiation-energyabsorbed backbone polymer generally initiates a free radical process, have many advantages over other conventional methods [2,9].In the radiation synthesis of hydrogel from polymer solution, there is no necessity to add catalysts, initiators, cross-linkers, and so forth which are possibly harmful and difficult to remove.In addition, these processes discharge almost no waste and can be carried out with relatively low costs.Swelling behavior of acrylic-acid-grafted hydrogel can be improved by treatment with alkaline solution [11].By these reasons, the authors adopted an irradiation synthesis method for synthesizing hydrogels in the present study.So far, the radiation grafting of acrylamide, acrylonitrile, methacrylonitrile, alkyl methacrylate, and vinyl ketones 2dimethylaminoethylmethacrylate onto starch was investigated intensively [12]; however, there are almost no investigations on starch's property change by the grafting of acrylic acid (AAc) by low-dose gamma radiation.
Water pollution is a serious problem nowadays.Different bioadsorbents are being used to remove the water soluble dyes from water [13].Hydrogel is one of the best candidates for removing soluble dyes from water.
In the first part of the present study, the authors investigated how the gel fraction is influenced by the gammaradiation dose and starch concentration in the preparation process of AAc-grafted starch (starch/AAc) hydrogels initiated by applying low-dose gamma radiation.Secondly, the swelling properties of the starch/AAc hydrogels were examined including the effect of the hydrolysis with sodium hydroxide (NaOH).Thirdly, the authors also investigated the methylene blue (MB) adsorption efficiency of NaOH-treated starch/AAc hydrogel in order to explore the possibility of the hydrogels' application for removing dyes from aqueous solution.The pseudo-first-order and pseudo-second-order models are used to correlate the adsorption kinetics data of MB onto synthesized hydrogels.
MB is a cationic dye and is most commonly used for coloring paper, dyeing cottons, wools, and so forth.In addition, this dye is generally used to test the adsorption capacity of various adsorbents, which permits quantitative comparisons between the adsorption capacities of various adsorbents [14].Other than these, adsorption of MB is still important from an environmental point of view because acute exposure to MB will cause increased heart rate, vomiting, shock, Heinz body formation, cyanosis, jaundice, quadriplegia, and tissue necrosis in human.

Preparation of Starch/AAc
Hydrogel.An aqueous solution of potato starch was prepared by dissolving it in the distilled water in a three-neck flask under vigorous stirring at 200 rpm and with heating at 80 ∘ C for 30 minutes.After the dissolution, the starch solution was left at room temperature for a while for cooling.Then, AAc was added to the cooled solution with stirring.The composition of three starch/AAc solutions wasAfter becoming homogeneous, the starch/AAc solution was taken in several test tubes, which were sealed and allowed to be irradiated with 5, 10, 15, 20, and 25 kGy of 60 CO gammaray-radiation dose at 6.8 kGy/h of radiation-dose rate.Finally, after being cut into small pieces, the starch/AAc hydrogels were dried in the air and then in an oven until becoming of constant weights.

Determination of Gel
Fraction.For 24 hrs, the dried gel samples had been kept in beakers containing 50 ∘ C distilled water in order to remove soluble fraction.Then, the gel samples were taken out from the beakers and dried to constant weights in an oven at 60 ∘ C. The gel fractions of samples were calculated as follows: where  0 and  1 are the dried gel weights before and after the rinsing, respectively.

Determination of Swelling Properties.
After immersing the rinsed-and-dried hydrogel samples in distilled water at room temperature, the samples were periodically weighed with blotting excess surface water with filter papers.The water absorption of the samples was calculated as follows: Water absorption where  1 and   are the gel weights before and after immersing in the water, respectively.

Improvement of Swelling Properties.
After the immersion of rinsed-and-dried hydrogel samples in 0.45 M NaOH aqueous solutions for 24 hrs, the swollen samples were dried in the air and then in an oven (60 ∘ C) until becoming of constant weights.The swelling properties of the NaOH-treated hydrogels were investigated and analyzed in the same way as described in Section 2.4.

Determination of FTIR Spectroscopy.
The FTIR spectra of the several pellets of the powder mixture of the rinsed-anddried starch/AAc gels and KBr were obtained from a Fourier Transform Infrared Spectrometer (IP Prestige-21, Shimadzu).

Differential Scanning Calorimetry
. By utilizing a differential scanning calorimeter (Q100, TA Instruments), thermograms of untreated and NaOH-treated dried hydrogels having composition 5% starch/15% AAc were obtained with elevating temperature at 10 ∘ C/min in a range from 30 to 580 ∘ C and sample weight 28.400 mg.During the DSC measurements, inert atmosphere was maintained by purging nitrogen  at 50 mL/min of flow rate.An empty pan was used as a reference.

SEM Photograph.
The SEM micrograph of the untreated and NaOH-treated dried hydrogels having composition 5% starch/15% AAc was observed by using an electron microscope (JSM-6490, JEOL) operated at 20 kV.

Determination of Dye Adsorption.
The hydrogels of different composition of starch and AAc synthesized by applying 15 kGy gamma radiation were immersed in aqueous MB solutions (250 mL, initial dye concentration: 80 mg/L), and the dye-concentration evolution of the surrounding solutions was measured with utilizing a UV-visible spectrometer (UV-1800 Series, Shimadzu).The adsorption capacity after certain period  during the treatment,   (mg/g), is given by [12] where  0 and   are the dye concentration (mg/L) of the initial solution and that at , respectively, while  is the volume of the solution (L) and  is the dry weight of the hydrogel (g).

The Mechanism of the Reaction.
The cross-linking mechanism of polymers in a solution by gamma irradiation was reported in [2,[15][16][17][18][19].There are three main reactive species formed in water upon irradiation: hydrated electron, hydroxyl radicals, and hydrogen radicals.Electrons exhibit low reactivity toward simple, hydrophilic hydrogel-forming polymers.This is an expected behavior since they do not contain functional groups being efficient scavengers of hydrated electrons.Reaction of hydrogen radicals with polymers is rarely studied in detail.In acidic media, their contribution to the total yield of macroradicals does not exceed 20%.Two hydrogen radicals can recombine causing the evolution of hydrogen gas.When the aqueous blend of starch and AAc is irradiated by gamma radiation, the hydroxyl radicals are produced from water.The hydroxyl radicals abstract hydrogen atoms from macromolecules and thus macroradicals are formed (Figure 1) [2,20].

Gel Fraction.
Figure 2 shows the effects of radiation dose and starch concentration on gel fraction of the starch/AAc hydrogel.The gel fraction of the starch/AAc hydrogel increased with increase in radiation dose and attained a maximum value at 15 kGy radiation dose.At higher radiation doses macromolecules undergo degradation [21].Above 15 kGy radiation dose, the gel fraction of hydrogel showed a decreasing trend.Since starch is a radiation degradable polymer, when starch/AAc solution is subjected to irradiation, starch molecule may degrade to lower molecular weight compound above 15 kGy radiation dose [16,22,23].Above this radiation dose, the gel fraction of hydrogel showed a decreasing trend.Since starch is a radiation degradable polymer [15], when starch/AAc solution is subjected to irradiation, starch molecule may degrade to lower molecular weight compound above 15 kGy radiation dose.From Figure 2, it is also found  that, in a broad way, the gel fraction of hydrogel increased with increased concentration of starch in starch/AAc solution.When the aqueous solution of starch/AAc is subjected to irradiation with gamma ray, free radicals are generated on starch and AAc.Random reactions of these radicals lead to formation of the graft copolymer of starch and AAc.When the radiation dose has been increased beyond a certain value, the polymer chains cross-link and then a gel-like material is obtained.For the formation of cross-linked macromolecules, the subsistence of two radicals on neighboring chains and their subsequent combination are required.At the higher concentration of polymer, the macromolecules come close and that makes it easy to form cross-linking.

Swelling Properties.
Swelling properties of the hydrogels are influenced by the network cross-linking feature.With the increment of cross-linkage density, the swelling ratio usually decreases because of the decrement of the slack in the network polymer necessary for expanding the network on the swelling of the hydrogels.Figures 3(a)-3(e) show the effects of starch concentration and gamma-radiation dose on the water absorption of the starch/AAc hydrogel.The water content reached equilibrium around 24 hrs.It was observed that the water absorption capacity decreases with the starch content, which indicates the cross-link-density increase with the starch content.

Swelling Properties after Treatment of NaOH.
Figure 4 shows the effects of starch concentration and gammaradiation doses on the water absorption of the NaOH-treated starch/AAc hydrogels.It was found that the water absorption capacities of NaOH-treated starch/AAc hydrogels are higher than those of the corresponding starch/AAc hydrogels without NaOH treatment (Figure 3).This may come from the transformation of AAc in the starch/AAc hydrogel into sodium carboxylate caused by NaOH.The hydrophilicity of carboxylate may be higher than that of carboxylic group.The maximum water absorbance capacity was found for 5% starch/15% AAc composite with 15 kGy gamma-radiation dose.Figure 5 shows an NaOH-treated hydrogel before and after swelling in water.
3.5.FTIR Analysis.Figure 6 presents the infrared spectra of potato starch and the starch/AAc gel.The peaks observed at 3423 cm −1 and 1022-1161 cm −1 can be ascribed to the O-H stretching and C-O-C skeletal vibration of starch.The absorption peak at 2945 cm −1 corresponds to C-H stretching of the acrylate unit and starch, while the bands at 1412 and at 1321 cm −1 correspond to the angular deformation of C-H.The peak at 1714 cm −1 can be assigned to the C=O stretching of carbonyl group of AAc, which is absent in the spectra of starch.This result indicates that AAc interacts with/entraps starch.

Thermal Analysis of Untreated and NaOH-Treated Hydrogel.
Figure 7 shows the DSC thermal properties of the untreated and NaOH-treated hydrogels with 5% starch and 15% AAc composition measured at 10 ∘ C/min of heating rate under the flow of nitrogen gas.
The DSC curves were obtained for starch-acrylic-acid hydrogels before and after alkali (NaOH) treatment.The thermogram of starch-acrylic-acid hydrogel before alkali treatment reveals endothermic glass transition temperature at ∼225 ∘ C and exothermic peak at ∼276 ∘ C due to decomposition of hydrogel.Thermogram of alkali treated starch-acrylic-acid hydrogel reveals glass transition temperature at ∼135 ∘ C and decomposition temperature at ∼256 ∘ C. It can be concluded that the glass transition temperature and decomposition temperature of starch-acrylic acid hydrogel decrease due to alkali treatment.

Scanning Electron Microscopy (SEM)
. SEM micrographs of the starch/AAc hydrogels before and after the NaOH treatment are shown in Figure 8.As can be seen in the figure, the NaOH treatment also induces a significant change in the surface structure of the starch/AAc hydrogel.Before the NaOH treatment, the surface shows a compact structure in SEM image (Figure 8(a)) which may be due to hydrogen-bond attraction between the grafted -COOH chains.On the other hand, the surface shows a more expanded structure after the NaOH treatment (Figure 8(b)), which is thought to come from the repulsion between the grafted chains with the larger electrostatic force among -COO − groups.This expansion can facilitate the NaOH-treated hydrogel to MB adsorption.

Application of Prepared Hydrogels in Dye Removal from
Wastewater.Other than the above-described basic properties, the important purpose in the present study was to investigate the availability of the starch/AAc hydrogels for       the removal of hazardous dyes from the waste fluids (adopting MB as a model material).In the MB-capturing experiments, the starch/AAc hydrogels prepared by 15 kGy gammaradiation dose were utilized because their gel fractions showed maxima at this dose as shown in Figure 2, indicating the most preferable condition for forming gel network.Figure 9 shows MB-adsorption evolution by the starch/ AAc hydrogels with different compositions prepared by irradiating 15 kGy gamma-radiation dose.Their MB adsorptions increased with time and, at 72 hrs, attained their respective maximum values around 449, 576, and 360 mg/g for 5% starch, 7.5% starch, and 15% starch/15% AAc hydrogels, respectively.The capturing mechanism of MB by the starch/AAc hydrogel is forming an ionic complex between the imine group of MB and the ionized carboxyl group of starch/AAc hydrogel.As shown in Figure 9, 7.5% starch/15% AAc hydrogel shows the highest MB-capturing efficiency, which may come from more beneficial conditions than those of other starch/AAc hydrogels for forming the complex between the end groups of MB and starch/AAc hydrogel.
Someone can notice, from Figure 9, that MB-capturing rates of all the starch/AAc hydrogels are higher in the beginning than in the later stage.These features can be explained by two mechanisms with the MB-adsorption progress: one can be the decrease in unoccupied -COO − end groups in starch/AAc hydrogel and the other will be stiffening of the hydrogel network preventing the -COO − swinging for searching for the MB's imine group to form their complex.
3.9.Adsorption Kinetics of MB Adsorption.Two simplified kinetic models were applied to fit the adsorption by the alkalimodified AAc-starch hydrogels.The pseudo-first-order and pseudo-second-order equations [24] are expressed as follows: where   and   are the amount of MB adsorbed (mg/g) at time  and equilibrium time, respectively,  1 was the rate constant (hr −1 ) of first-order adsorption, and  2 is the rate   constant (g hr −1 mg −1 ) of second-order adsorption.Figure 10 shows log(  −  ) versus  straight line plots for the adsorption of MB on hydrogels of different starch composition.The  1 ,  ,cal (calculated equilibrium concentration), and correlation coefficients  2 were calculated for these plots and listed in Table 1 along with experimental   values.The correlation coefficients  2 listed in Table 1 for first-order kinetic model were 0.958, 0.976, and 0.996 for 5%, 7.5%, and 15% starch concentrations, respectively.
Figure 11 shows plots of t/  versus  for second-order kinetic model.The  2 and  ,cal values from (5) and correlation coefficient  2 values from the plots of second-order kinetic model were also listed in Table 1. 2 values for this model were 0.999, 0.994, and 0.992 for 5%, 7.5%, and 15% starch concentrations, respectively, which were higher than the  2 values obtained for the pseudo-first-order model.It can be seen that the difference between the experimental   values and the   values calculated from first-order kinetic model was very high, while the difference was lower for second-order kinetic model.It can be seen that the difference between the experimental   and calculated values for pseudo-first-order kinetic model was very high while the difference was lower for pseudo-second-order kinetic model.
Based on the intraparticle diffusion model, the diffusion rate of the dye molecule in particle,   , can be calculated by the following equation [25,26]: where   is the amount of dye adsorbed in time  (mg/g),   is the intraparticle diffusion rate constant, and  (mg/g) is a constant that characterizes boundary layer thickness.In Figure 12, the plots of   against  1/2 represented multilinearity  containing at least three linear segments.It can be seen that the order of adsorption rate was  1 >  2 >  3 and the diffusion rate constant was the highest for 7.5% starch composition.The first stage was attributed to the rapid transfer of adsorbate onto the adsorbent surface by diffusion through the boundary layer.In the second stage, the MB molecules diffused from the external surface into the pores of the hydrogel adsorbent and this was the rate determining stage which actually represents adsorption due to intraparticle diffusion.The third stage was the final stage where the MB molecules were adsorbed on the active sites on the internal surface of the pores and the equilibrium was attained.Therefore, the change of diffusion rate constant values can be considered to the adsorption stages of the external surface, internal surface, and equilibrium, respectively.
So, the pseudo-second-order kinetic model fits with the experimental kinetic data which indicates that intraparticle diffusion process was the rate limiting step of the adsorption.3.10.UV-Visible Spectroscopic Analysis of Dye Adsorption.By using Beer-Lambert law, concentration of methylene blue (MB) was calculated at 663 nm [27].
Figure 13 shows the UV-visible absorbance spectrum of the initial MB solution (80 mg/L) and three UV spectra of the aqueous phases surrounding the respective starch/AAc hydrogels after attaining the equilibrium MB adsorptions (at 72 hrs).This result indicates that the methylene blue is absorbed by hydrogel.

Conclusion.
From the present investigations, it can be concluded that the preparation of starch/AAc hydrogel can be satisfactorily performed by utilizing 60 Co gamma radiation.With respect to the gel fraction, 15 kGy radiation dose is considered as a required radiation dose for the preparation of the starch/AAc hydrogel.Swelling behaviors of the starch/AAc hydrogel can be improved by the treatment with NaOH.The highest swelling ratio of the 5% starch/15% AAc hydrogel

Figure 1 :
Figure 1: Possible grafting and cross-linking reaction mechanism of starch and acrylic acid.

Figure 2 :
Figure 2: Effects of radiation dose and starch concentration on gel fraction of starch/AAc hydrogel.

Figure 3 :
Figure 3: Effects of radiation dose and starch concentration on the swelling behavior of starch/AAc hydrogel.

Figure 4 :
Figure 4: Effects of radiation dose and starch concentration on water absorption of starch/AAc hydrogel after being treated with NaOH.

Figure 5 :Figure 6 :
Figure 5: Volume change of an NaOH-treated starch/AAc hydrogel before and after swelling.

Figure 9 :
Figure 9: Effect of standing time and concentration of starch/AAc hydrogel on dye adsorption.

Figure 11 :
Figure 11: Pseudo-second-order kinetic model for adsorption of MB on starch-acrylic-acid hydrogel.
Potato starch was obtained from Sigma Aldrich and methylene blue (MB, C.I. number 52015) and acrylic acid were obtained from Merk.Chemical formula of methylene blue is C 16 H 18 ClN 3 S (molecular weight: 319.85,  max : 663 nm).Distilled water was used as a solvent. 2.1.Materials.

Table 1 :
Adsorption kinetic parameters for the adsorption of MB on hydrogels.