Adsorption of Dyes onto Sodium Alginate Graft Poly(Acrylic Acidco-2-Acrylamide-2-Methyl

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1 Adsorption of Dyes onto Sodium Alginate Graft Poly(Acrylic Acid-co-2-Acrylamide-2-Methyl Propane Sulfonic Acid)/ Kaolin Hydrogel Composite Adsorption of Dyes onto Sodium Alginate Graft Poly(Acrylic Acidco-2-Acrylamide-2-Methyl Propane Sulfonic Acid)/ Kaolin Hydrogel Composite Linhui Zhu *, Chengdong Guan, Bin Zhou, Zhentao Zhang, Rui Yang, Yaoji Tang, and Jing Yang College of Chemical and Environmental Engineering, Shandong University of Science and Technology, Qingdao, Shandong, , PR China Received: 5 July 2015, Accepted: 25 November 2016 SUMMARY Adsorption of four dyes, namely methyl violet (MV), rhodamine 6G (R6G), acid chrome blue K (AK) and xylenol orange (XO) onto sodium alginate graft poly(acrylic acid-co-2-acrylamide-2-methyl-1-propanesulfonic acid)/kaolin (SA-g-P(AA-co-AMPS)/KL) hydrogel composite is studied. The factors influencing the adsorption capacities, including the initial concentrations of dye solutions, contact time, initial ph values and dosage of the adsorbent as well as ionic strength of the solution are discussed. It is more effective for the composite to adsorb cationic dyes such as MV and R6G rather than anionic dyes AK and XO. The maximum adsorption capacities of MV, R6G, AK and XO are , , and mg/g, respectively. The adsorption thermodynamics for the four dyes are in accordance with both the Freudlich and Redlich-Peterson equations. It is shown that adsorption of the cationic dyes R6G and MV is spontaneous, while that of anionic dyes AK and XO is not. The kinetics studies show that the adsorption of the four dyes fitted a pseudo-second-order equation. Keywords: Adsorption, Dyes, Composite hydrogel 1. INTRODUCTION Dye wastewater shares a large proportion of the industrial wastewater with the development of printing and dyeing industry. As there are many kinds of aromatic compounds which are poisonous and nondegradable in dye wastewater, it is important to remove the organic dyes from the effluent 1,2. The methods commonly used are flocculation, biodegradation, electrochemistry and adsorption, of which adsorption shows the best prospect due to its low cost but high efficiency 3. Polymer/clay hydrogel composites are important candidates in wastewater treatment. With the three-dimensional crosslinked networks and the functional * Corresponding author: Linhui Zhu; zhulinhui@yeah.net. Tel: ; Fax: Smithers Information Ltd., 2017 groups such as -COOH, -CONHand -OH, they are able to remove dyes through adsorption. Li et al. 4 prepared polyacrylamide/laponite nanocomposite (PAAm/Lap NC) and investigated its adsorption of cationic dyes such as basic blue 12 (BB 12), basic blue 9 (BB 9) and basic violet 1 (BV 1). It indicated that the adsorption rates of the three dyes increased in the following order, BB 9 >BB 12>BV 1, which was corresponding to the swelling results of the PAAm/Lap NC in these dye solutions. Yatin et al. 5 synthesised a novel cationic hydrogel, poly(acrylamide/ N-vinylpyrrolidone/ N,N-diallylpyrrolidinium bromide) [P(AAm/NVP/DAPB)], and investigated its sorption behaviour of some textile anionic dyes. It showed that the sorption of dyes increased in the following order, reactive golden yellow>orange- II>acid yellow. The adsorption process satisfied both Langmuir and Freundlich isotherms. Kaplan et al. 6 synthesised (acrylamide-itaconic acid sodium salt-montmorillonite) (AAm-IANa- MMT) nanocomposite hydrogel to remove brilliant cresyl blue (BCB) from aqueous solutions. It was found that the adsorption reached equilibrium (458.1 mg/g) after 30 min. In recent years Ilgin et al. 7 have prepared a novel superabsorbent polymer, poly(2,3-epoxypropyl methacrylate-co-2-(methacryloyloxy) ethyltrimethylammonium chloride). The polymer was used for the removal of anionic dyes from aqueous environment. It revealed that the adsorption was correlated by both Langmuir and Freundlich isotherm models. The kinetic parameters followed the pseudo-second-order model well. Mahida et al. 8 have studied the removal of cationic and anionic dyes from Polymers & Polymer Composites, Vol. 25, No. 8,

2 Linhui Zhu, Chengdong Guan, Bin Zhou, Zhentao Zhang, Rui Yang, Yaoji Tang, and Jing Yang aqueous solutions by poly(nipaam/ N,N-diallylpyrrolidinium bromide/ AA) superabsorbent amphoteric nanohydrogels. It showed that adsorption obeyed the pseudo-second order kinetic model. Tang et al. 9 have done a detailed study on the sorption behaviour of Rhodamine B onto sodium alginate graft poly(acrylic acid-co-acrylamide)/kaolin composite hydrogel. However, adsorption of a series of dyes onto sodium alginate graft poly(acrylic acid-co-2-acrylamide- 2-methyl-1-propanesulfonic acid)/ kaolin (SA-g-P(AA-co-AMPS)/ KL) hydrogel composite has rarely been studied. Since there are many kinds of dyes including both cationic and anionic ones in the industrial wastewater, it is very important to study the sorption behaviour of dye mixtures onto the adsorbents. In the present work, SA-g-P(AAco-AMPS)/KL hydrogel composite was synthesised and used to remove four different dyes, namely methyl violet (MV, a), rhodamine 6G (R6G, b), acid chrome blue K (AK, c) and xylenol orange (XO, d; see Figure 1), from aqueous solutions. The factors influencing the adsorption capacities including initial concentration of dye solutions, contact time, initial ph values and dosage of the adsorbent as well as ionic strength of the solution were studied in detail. Possible mechanisms of adsorption were considered. The sorption kinetics as well as thermodynamics were investigated. 2. MATERIALS AND METHODS 2.1 Chemicals Acrylic acid (AA) was provided by Sinopharm Chemical Reagent Co., Ltd.(Shanghai, China). 2-acrylamide- 2-methyl propanesulfonic acid (AMPS) was obtained from Union Petrochemical Co., Ltd. (Shouguang, China). Sodium alginate (SA) was from Fine Chemical Institute (Tianjin, China). Kaolin (KL) was purchased from Chemical Industry Co., Ltd. (Shantou, China). R6G was from the Gracia Chemical Technology Co., Ltd. (Chengdu, China). MV, AK and XO were from Guangcheng Chemical Reagents Co., Ltd. (Tianjin, China). AA, SA and KL were chemically pure, while other chemicals were analytical reagent grades. 2.2 Preparation of SA-g- P(AA-co-AMPS)/KL Hydrogel Composite SA-g-P(AA-co-AMPS)/KL hydrogel composite was prepared as follows 10 : AA (7.2 g) was neutralised to a degree of 60% with NaOH solution (6.25 mol/l) for future use. SA (0.36 g) and distilled water (30 ml) were put into a four-necked flask equipped with a mechanical agitator, a condenser, a thermometer and a nitrogen line. After stirring for 20 min at room temperature, the mixture was slowly heated to 50 C on a water bath. Then APS solution (0.1 g /ml, 0.36 ml), the abovementioned neutralised AA solution, AMPS (0.72 g) and KL (0.42 g) were added to the flask consecutively. After stirring vigorously for 30 min, MBA solution (0.01g /ml, 0.36 ml) was added dropwise into the mixture. The reaction mixture was heated to 80 C till the rubbery gel was formed. The gel was dried in an oven at 60 C to constant weight and put in a dryer before use. Figure 1. Chemical structures of methyl violet (MV, a), rhodamine 6G (R6G, b), acid chrome blue K(AK, c) and xylenol orange (XO, d) (a) (b) (c) (d) 628 Polymers & Polymer Composites, Vol. 25, No. 8, 2017

3 Adsorption of Dyes onto Sodium Alginate Graft Poly(Acrylic Acid-co-2-Acrylamide-2-Methyl Propane Sulfonic Acid)/ Kaolin Hydrogel Composite 2.3 Adsorption Experiments Four series of dye solutions, namely MV, R6G, AK and XO solutions, with different concentrations were prepared, respectively. Each solution was added with a certain mass of SA-g-P(AAco-AMPS)/KL hydrogel composite. After adsorption for a period of time at room temperature, the mixtures were filtered and the filtrates were obtained. A visible spectrophotometer (WFG-7200, Unico instrument Co., Ltd., Shanghai, China) was used to test the absorbency of the filtrates at the maximum absorption wavelengths of each kind of dye solution, using deionised water as the reference. The amount of dyes adsorbed on the composite was calculated according to Eq. (1). Table 1. Equations and correlation coefficients (R 2 ) of the standard curves Dye Equation Correlation Coefficient (R 2 ) MV R6G AK XO A=0.0707c A=0.1697c A=0.0373c A=0.0216c Figure 2. Standard curves of: MV (a), R6G (b), AK (c) and XO (d) (1) where c 0 (mg L 1 ) is the initial concentration of dye solution, c t (mg L 1 ) is the concentration of dye solution after adsorption; V 0 (L) is the initial volume of dye solution, V t (L) is the volume of dye solution after adsorption; m (g) is the mass of the composite and q (mg g 1 ) is the amount of adsorption. 3. RESULTS AND DISCUSSION Figure 3. Influence of initial concentration of dye solutions on adsorption (adsorbent dosage 0.05 g, contact time 24 h, ph value 7.0, temperature 25 C) 3.1 Standard Curves The standard curves of the four dye solutions were measured at the maximum absorption wavelengths (MV 585 nm, R6G 526 nm, AK 530 nm and XO 430 nm), respectively. The curves are shown in Figure 2 and the corresponding parameters are listed in Table The Influence of Initial Concentration of Dye Solutions on Adsorption The influence of initial concentration of dye solutions on the adsorption amounts is shown in Figure 3. Polymers & Polymer Composites, Vol. 25, No. 8,

4 Linhui Zhu, Chengdong Guan, Bin Zhou, Zhentao Zhang, Rui Yang, Yaoji Tang, and Jing Yang As shown in Figure 3, adsorption amounts increased and reached equilibrium gradually with the enhancement of dye concentrations. For the cationic dyes such as MV and R6G, the adsorption amounts increased significantly as the dye concentrations increased from 0 to 1000 mg/l. However, for anionic dyes such as AK and XO, the increase of adsorption amounts was relatively slow. The adsorption approached equilibrium Figure 4. Influence of contact time on adsorption (adsorbent dosage 0.05 g, initial concentration of each dye solution 1000 mg/l, ph value 7.0, temperature 25 C) when the dye concentrations were 600 mg/l. It is known that increase of the dye concentration accelerates the diffusion of dye molecules from the solution to the surface of the adsorbent. Thus, it is advantageous to adsorption to raise the dye concentrations 11. For cationic dyes, there are strong electrostatic forces between the negatively charged groups such as -COO - and -SO 3 - on the composite and the positively charged groups on the dye molecules. Thus, the adsorption amounts are enhanced greatly as the dye concentrations increase. Figure 5. Influence of initial ph value on adsorption (adsorbent dosage 0.05 g, initial concentration of each dye solution 100 mg/l, contact time 24 h, temperature 25 C) 3.3 The Influence of Contact Time on Adsorption The influence of contact time on adsorption is shown in Figure 4. It is obvious that with the elapse of time, adsorption amounts of cationic dyes increased rapidly within the first 2 h and reached equilibrium in about 12 h. For the anionic dyes AK and XO, adsorption amounts increased and reached a maximum in the first 4 h, and then decreased slightly. This was due to the fast spread of the networks of the composite at the early stage. As a result, it was much easier for dye molecules to penetrate into the inside of the resin and combine with the adsorption sites 12. With the extension of time, the active groups on the surface of the composite became less and the adsorption approached equilibrium The Influence of Initial ph Value on Adsorption The influence of initial ph values of dye solutions on adsorption is shown in Figure 5. It is clear that the adsorption amounts of the four dyes increase first and then decrease slowly with the rise of ph values. Maximal adsorption amounts are achieved as ph value reaches 7. In strong acidic media (ph<3) the shrinkage of the resin s network opposes the adsorption of dyes. As the ph rises from 3 to 7, the carboxyl and sulfonic acid groups on the composite exist as -COOH 630 Polymers & Polymer Composites, Vol. 25, No. 8, 2017

5 Adsorption of Dyes onto Sodium Alginate Graft Poly(Acrylic Acid-co-2-Acrylamide-2-Methyl Propane Sulfonic Acid)/ Kaolin Hydrogel Composite and -SO 3 H, which is better for the formation of hydrogen bonds between the composite and dye molecules, as shown in Figure 6. If the ph values are higher than 7.0, the weakly alkaline media are propitious to the electrostatic interactions between the composite and cationic dye molecules (Figure 6a). However, as ph exceeds 10, adsorption amounts of both cationic and anionic dyes decrease because of the repulsion of anionic groups on the composite and the competitive adsorption of Na Influence of Adsorbent Dosage on Adsorption The influence of adsorbent dosage on adsorption is shown in Figure 7. Adsorption amounts of the four dyes reduce rapidly as the dosage of the adsorbent increases from 0.02 to 0.08 g. Dye molecule adsorption onto the surface of the resin are reduced in consequence of the increase of the adsorbent dosage. Besides, an excessive amount of adsorbent brings about overlap of the adsorption sites 13. Therefore, the adsorption amounts decrease accordingly Influence of Ionic Strength on Adsorption The influence of ionic strength on adsorption was investigated by introducing NaCl into the dye solutions. The results are presented in Figure 8, which shows that with the increase of NaCl concentration the adsorption amounts of the four dyes decreased constantly. High ionic strength is Figure 6. Possible mechanisms of adsorption: (a) adsorption of cationic dyes, (b) adsorption of anionic dyes Polymers & Polymer Composites, Vol. 25, No. 8,

6 Linhui Zhu, Chengdong Guan, Bin Zhou, Zhentao Zhang, Rui Yang, Yaoji Tang, and Jing Yang detrimental to the swelling of the resin, which thereby hinders the full contact of dye molecules with the adsorbent. Besides, salt screens the electrostatic interaction between the adsorbent and dye molecules Adsorption Isotherms Adsorption isotherms are used to study the interactions between the adsorbates and adsorbents on the surface. Three models are commonly used, which are the Langmuir equation (Eq. (2)), Freudlich equation (Eq. (3)) and Redlich-Peterson equation (Eq. (4)), respectively. (2) Figure 7. Influence of adsorbent dosage on adsorption (initial concentration of each dye solution 1000 mg/l, ph value 7, contact time 24 h, temperature 25 C) (3) (4) In these equations, C e (mg L 1 ) is the mass concentration of solute at adsorption equilibrium, q e (mg g 1 ) is the equilibrium adsorption capacity, q m (mg g 1 ) is the maximum adsorption capacity, K l is the Langmuir adsorption constant. K f is the Freudlich constant, and n is a constant indicating the intensity of adsorption. It indicates that adsorption is easy to be performed if 1 < n < 2. Otherwise, if n<0.5, it is difficult. A and B are the Redlich- Peterson constants. g is an empirical constant ranging from 0 to1. The data of Figure 3 are fitted with Langmuir, Freudlich and Redlich- Peterson equations, respectively. The results are shown in Table 2. Figure 8. Influence of ionic strength on adsorption amount (adsorbent dosage 0.05 g, initial concentration of dye solution 100 mg/l, ph value 7, contact time 24 h, temperature 25 C) As shown by the R 2 values in Table 2, adsorption of MV, R6G, AK and XO are in accordance with both Freundlich and Redlich-Peterson equations. The values of n for adsorption of MV, R6G and AK are more than 1, which indicates that the processes are smooth and mainly at the surface or heterogeneous interspaces. 3.8 The Thermodynamic Parameters The standard Gibbs free energy change ( G θ ) during adsorption is calculated with the standard equilibrium constant (K θ ) obtained from the Freundlich equation. The value of K can be calculated by substituting K f and n into the equation K θ =(K f ) 1/n. Thus, G θ is obtained by substituting R=8.314(J mol -1 K-1 ), T= (K) and K θ into Eq. (5): 632 Polymers & Polymer Composites, Vol. 25, No. 8, 2017

7 Adsorption of Dyes onto Sodium Alginate Graft Poly(Acrylic Acid-co-2-Acrylamide-2-Methyl Propane Sulfonic Acid)/ Kaolin Hydrogel Composite Table 2. The calculated constants and correlation coefficients (R 2 ) for adsorption of dyes onto SA-g-P(AA-co-AMPS)/KL hydrogel composite Models AK XO MV R6G q m Langmuir K l R Freundlich n K f R Redlich- A Peterson B g R Table 3. Kinetic parameters for adsorption of dyes onto SA-g-P(AA-co-AMPS)/ KL hydrogel composite Model Parameter AK XO MV R6G Pseudo first-order k q e R Pseudo second-order k q e R G θ = -RT.lnK θ (5) The results are obtained as follows: G θ MV = (kJ mol-1 )<0, G θ R6G = (kJ mol-1 )<0, G θ AK =0.489(kJ mol-1 )>0, G θ XO =3.843 (kj mol -1 )>0. Obviously, the adsorption of the cationic dyes MV and R6G onto SA-g-P(AA-co-AMPS)/KL hydrogel composite is spontaneous, while that of anionic dyes AK and XO is not. 3.9 Adsorption Kinetics Kinetic studies are important to predict the adsorption efficiency at different stages. The equations commonly used include the pseudo first-order kinetic model (Eq. (6)) and pseudo secondorder kinetic model (Eq. (7)). (6) (7) where q e (mg g -1 ) is the equilibrium adsorption capacity, q t (mg g -1 ) is the adsorption capacity at time t. k 1 (h -1 ) and k 2 (g mg -1 h -1 ) are the adsorption rate constant of the pseudo first-order kinetic model and pseudo second-order kinetic model, respectively. The data from Figure 4 are fitted according to Eq. (6) and Eq. (7), respectively. The results are shown in Table 3. The data show that the adsorptions of the four dyes satisfy the pseudo second-order kinetic model. Chemical adsorption controls the rate, and the initial concentrations of dye solutions greatly affect the adsorption rate CONCLUSIONS Adsorption performances of the four dyes, namely methyl violet, rhodamine 6G, acid chrome blue K and xylenol orange, onto SA-g-P(AAco-AMPS)/KL hydrogel composite were investigated in this paper. It was found that the adsorption capacities of cationic dyes were significantly higher than those of anionic dyes at room temperature. For the anionic dyes AK and XO, hydrogen bonding played an important role during adsorption, while for the cationic dyes MV and R6G, there were electrostatic interactions between the hydrogel composite and dye molecules as well as hydrogen bonding. The adsorptions of R6G, MV, AK and XO were all in accordance with both Freundlich and Redlich- Peterson equations, and the kinetics conformed to the pseudo second-order kinetic model. It was also found that the adsorption of the cationic dyes R6G and MV was spontaneous, while that of AK and XO was not. ACKNOWLEDGEMENTS The authors are thankful to a Project of Shandong Province Higher Educational Science and Technology Program (J13LD07), the Specialised Research Fund for the Doctoral Program of Higher Education of China ( ) and the Project of Coal Joint Funds of the National Natural Science Foundation of China (U ) REFERENCES 1. Ngah W.S.W., Teong L.C., and Hanafiah M.A.K.M., Adsorption of dyes and heavy metal ions by chitosan composites: A review. Carbohydrate Polymers, 83 (2010) Liu Q.Q., and Pan C. A novel route to treat wastewater containing cationic dyes. Separation Science and Technology, 47 (2012), Desmet G., Taka cs, E., Wojna R.L., et al. Cellulose functionalization via high-energy irradiation-initiated grafting of glycidyl methacrylate and cyclodextrin immobilization. Radiation Physics and Chemistry, 80 (2011) Li P., Siddaramaiah; Nam H.K. et al. Poly(acrylamide/laponite) Polymers & Polymer Composites, Vol. 25, No. 8,

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