1. Introduction

The water pollution due to the industrial aqueous waste discharged to the water body is one of the most important issues nowadays. Waste water containing organic dyes is harmful to the humans as well as aquatic animals. The textile,dyestuff industries and pulp mills discharge highly colored waste water to the water streams,which has provoked serious environmental concerns ^{[1, 2]}. The organic dyes mixed in water,which affects the nature of the water,inhibit the sunlight penetration into the stream and reduce the photosynthetic reactions. Some dyes are carcinogenic and toxic for the humans ^{[3, 4]}. Recent studies indicate that the current adsorption methods are invariably successful to decolorize waste water. Moreover,the adsorption of the organic dyes by using superabsorbent hydrogels is more effective in decolorizing the waste water with higher capacity.- Polymeric adsorbents,due to their wide variations in porosity and surface chemistry,especially their regenerability on site and reuse for continuous processes,have been increasingly used to remove and recover organic pollutants from waste streams ^{[5, 6, 7]}. Superabsorbent hydrogels are able to absorb water greater than 100 times of their own dry weight. Because of the presence of different anionic and cationic functional groups,they are able to remove both anionic and cationic organic dyes from water. Previously we have reported that the hydrogels had been successfully utilized to remove dyes from water ^{[8, 9]}. The results showed that the allyl group containing monomer DAPB was successfully used to form the hydrogel network with other monomers and utilized to remove cationic dyes from aqueous solution ^[10]. We have developed a new superabsorbent hydrogels using DAPB,DMAAm and AASS monomers. This superabsorbent hydrogels have the higher water absorption capacity compared to the previously reported DAPB containing hydrogels ^[10] and therefore higher dye adsorption capacity. The poly[DAPB-co- DMAAm-co-AASS] superabsorbent hydrogel is successfully utilized to remove two anionic dyes,e.g.,Reactive Red 5B and Reactive Orange M2R,from water. The SAH DDA₆ (poly[DAPB-co-DMAAmco- AASS]SAH) can efficiently remove 100 mg/L of anionic dyes from water as compared to lower capacity of other adsorbents such as bacteria,waste red mud and fly ash ^{[11, 12, 13]}.

2. Experimental

The acrylic acid sodium salt was prepared by adding 40 mL of 40% sodium hydroxide (w/v) into 60 mL of magnetically stirred acrylic acid with ice cooling. Preparation of superabsorbent hydrogels with different proportion of monomers is described as follows: DAPB (1.5,1,0.5,1,1,1 mol),DMAm (1,0.5,1.5,1,1, 1 mol) and AASS (1,1,0.5,1,1.5,2 mol) were mixed in 10 mL of double distilled water in a 100 mL round bottom flask under magnetic stirring. Then 3.2 mmol of MBAm was added. Finally after addition of 1.8 mmol of KPS,the solution was put in a microwave oven and irradiated at 280Wfor 1 min. Thereafter the product in the form of gel was transferred in a 100 mL beaker and put into an electric oven at 60 8C for 3 h for drying. The dry hydrogels were immersed in double distilled water at room temperature overnight. Water was replaced several times to reduce the residual monomer content in the hydrogels. After swelling of the hydrogels,the swollen hydrogels were immersed in acetone for 4 h. In order to dry the products more efficiently, acetone was replaced four times. Finally the hydrogels were dried in an air forced oven at 60 8C overnight.

3. Results and discussion The % swelling has been described in our previous paper ^[10]. The characterization and other related analysis of SAHs are given in Supporting information. The adsorption of RR5B and ROM2R dyes from water was investigated in batch experiments. The effect of pH of the adsorption medium,adsorption rate,adsorbent dosage and initial dye concentrations on adsorption was investigated. Through the entire adsorption experiments,the stock solutions of 1000 mg/ L of anionic dyes were diluted to 100 mg/L concentration with distilled water. The poly[DAPB-co-DMAAm-co-AASS]SAH containing 2 mol of AASS was used in all adsorption studies. The detailed adsorption procedures were described in our previous paper ^[10].

To evaluate the effect of pH on the adsorption,the adsorption experimentswere carriedout at differentpH(1-11). Fig.1 showsthe effect of pH on anionic dye adsorption. The adsorption capacity of SAH DDA6 increased as the pH value of the adsorption medium increased,reaching the highest level at pH value of 9 (243.63 mg/g for RR5B and 238.51 mg/g for ROM2R). In basic pH,dye adsorption occurs via strong interactions between positively charged -NMe3 group in dimethyl acrylamide and the quaternary nitrogen in DAPB of the crosslinked hydrogel chain and negatively charged -SO3Na group of the dye molecules. Due to the basic pH of the dye solution, the hydrogel functional groups become less protonated and are therefore responsible for the hydrophobic interactions with the dye molecules and give the better adsorption. In an acidic pH,the ionic interactions are comparativelyweakdue toanioniccharacters of dye molecules ^[9]. So it is hypothesized that the higher swelling degree of hydrogel at basic pH was responsible for the diffusion of the dyes into the hydrogel matrix and increased their adsorption capacity.

In this study,we have measured the adsorption capacity of SAH DDA6 at different time interval between 0 and 60 min. From Fig. 2,it can be concluded that,as the adsorption rate increases; there is an increase in the adsorption capacity. The high adsorption rate was observed at the beginning (approximately within 10 min) and then plateau values were gradually reached within 30 min. This initial rate suggests that the adsorption occurs mainly on the polymer surface. The adsorption behavior of dyes was directly related to some experimental factors such as the pH value of the solution,the characteristics of adsorbent and the dimensions of the dye molecules. Various polymeric sorbents having different properties adsorbed different dyes from aqueous solutions within the range of 2 h-5 days ^{[14, 15, 16, 17]}. It is reasonable to claimthat the adsorption rate of the SAHDDA6 for both RR5B andROM2R dyeswas very fast. These higher adsorption rate and higher adsorption capacity of SAH DDA₆ and the faster equilibrium will provide several advantages in using this hydrogel for dye adsorption process.

To investigate the effect of the adsorbent dose on the adsorption,different amount of adsorbent was tested. The adsorbent dose was varying from 4 mg to 20 mg. The adsorption capacity dropped from 468.78 mg/g to 244.72 mg/g for RR5B and 461.72 mg/g to 242.28 mg/g for ROM2R dyes by increasing the adsorbent dosage. The decrease in adsorption capacity by increasing the adsorbent dosage is basically due to the fact that the sites remain unsaturated during the adsorption process.

The adsorption capacity of the SAH DDA₆ was also determined with different initial dye concentrations of RR5B and ROM2R dyes. The concentration of anionic dyes was varying from 60 mg/L to 140 mg/L. It was found that an increase in RR5B and ROM2R dye concentration led to an increase in adsorption capacity as shown in Fig. 3. This increase in adsorption capacity in relation to the RR5B and ROM2R dye concentration could be explained with the high driving force of mass transfer.

The common models used to investigate the adsorption isotherm are Langmuir and Freundlich equations ^{[18, 19]} applied to the experimental data. The Langmuir and Freundlich isotherm equations are given below.

where q_e is the equilibrium adsorption capacity of dye on the adsorbent (mg/g); C_e,the equilibrium dye concentration in solution (mg/L); q_m,the maximum capacity of the adsorbent (mg/g); and K_L,the Langmuir adsorption constant (L/mg) related to free energy. By using the linear form of the isotherm,the plot of 1/ qe versus 1/C_e gives a line with a slope of 1/qm and an intercept of 1/ K_L,which is shown in Fig. 4.

The linear form of the Freundlich equation can be represented as follows:

where q_e and C_e are defined as above,K_F is the Freundlich constant (L/mg),which indicates the relative adsorption capacity of the adsorbent related to the binding energy,and n is the heterogeneityfactor representing the deviation from linearity of adsorption and is also known as Freundlich coefficient. According to Eq. (2),the plot of the log q_e versus log C_e gives a straight line and the K_F and n values can be calculated from the intercept and slope of the straight line,respectively. The Langmuir isotherm model fits better to the obtained equilibrium with a regression coefficient value of 0.9999 (RR5B) and 0.9999 (ROM2R) than the Freundlich isotherm model (Table 1).

4. Conclusion

The results of this study show that the poly[DAPB-co-DMAAmco- AASS] SAH DDA₆ is successfully utilized for the removal of RR5B and ROM2R dyes from water. The optimum conditions for adsorption were determined and the most efficient condition for RR5B and ROM2R dye removal were achieved at pH 9. The increase in concentration of RR5B and ROM2R leads to an increase of the adsorption capacity of the SAH. The Langmuir isotherm model is a better linear fit to the obtained equilibrium data compared to the Freundlich isotherm model. Results show that SAH DDA₆ can successfully remove 97% of RR5B and ROM2R dyes from water under optimum experimental conditions.

Acknowledgment

The authors gratefully acknowledge the head,Department of Chemistry,Sardar Patel University for providing research facilities. Author is grateful to the University Grant Commission,New Delhi for providing the Research Fellowship. Appreciation is expressed for the studies in the SICART,Vallabh Vidyanagar for FTIR analysis. Authors are also thankful to head,Department of Materials Science,Sardar Patel University for SEM and TGA analysis. Appendix A. Supplementary data

Supplementary data associated with this article can be found,in the online version,at http://dx.doi.org/10.1016/j.cclet.2013.06.009.