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poly(N,N-Diethylaminoethyl methacrylate) hydrogel and its use in dye removal from aqueous solutions
Reactive & Functional Polymers 73 (2013) 1531–1536
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Reactive & Functional Polymers journal homepage: www.elsevier.com/locate/react
Preparation of starch/poly(N,N-Diethylaminoethyl methacrylate) hydrogel and its use in dye removal from aqueous solutions E.S. Abdel-Halim ⇑ Chemistry Department, College of Science, King Saud University, P.O. Box 2455, Riyadh 11451, Saudi Arabia
a r t i c l e
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Article history: Received 2 May 2013 Received in revised form 29 July 2013 Accepted 9 August 2013 Available online 22 August 2013 Keywords: Starch Graft copolymerization Hydrogel Adsorption Anionic dye
a b s t r a c t Starch/poly(N,N-Diethylaminoethyl methacrylate) graft copolymer was synthesized by graft polymerizing N,N-Diethylaminoethyl methacrylate monomers onto cooked starch using ceric ammonium nitrate/nitric acid mixture as an initiator. After extracting the graft copolymer from the homopolymer and evaluating the graft yield, the graft copolymer was subjected to crosslinking treatment using epichlorohydrin in alkaline medium to convert it to hydrogel. The so prepared hydrogel was evaluated for its swelling ratio which was found to be 100. The hydrogel with its cationic functionality was tried for removing the anionic dye Direct Red 81 from its aqueous solution and all factors affecting the hydrogel adsorption capacity towards the anionic dye, like the hydrogel graft yield, the adsorbate pH, the immersion time and the hydrogel dose were studied. The data obtained from the adsorption results were found to fit well to the Langmuir adsorption model. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Removing color and other pollutants from industrial wastewater by different adsorption techniques has attracted considerable interest and has become a feasible procedure for pollution control. During the last two decades, many research work has been carried out and many papers reported adsorption equilibria and adsorption kinetics of dye removal processes by use of various types of adsorbents [1,2]. There are many types of commercially available adsorbents, among which activated carbons [3,4], chitin and chitosan based adsorbents [5], cellulosics-based adsorbents [5– 8], silica gel [9], adsorbents based on lignocellulosic materials and by-products [10], saw dust [11–13], peat residues [14], different carbohydrates other than cellulose [15–17] and many other synthetic polymeric adsorbents [18–21]. Among all the above mentioned adsorbents, the most important one and the most widely used is the activated carbon, which is characterized by high degree of porosity, which makes it highly efficient in adsorbing a wide range of different types of adsorbates [22]. Synthetic dyes which are nowadays widely used in textile industry are known to be bulky molecules composed of high number of fused aromatic rings, that is why removing these dyes by use of conventional activated carbons does not give satisfactory results. Polymeric-based adsorbents and chemically modified hydrogel
⇑ Address: National Research Center, Dokki, Cairo, Egypt. Tel.: +20 108113477. E-mail address: [email protected] 1381-5148/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.reactfunctpolym.2013.08.003
systems are characterized by many advantages in this field and proved to be highly efficient in adsorbing and removing such bulky dye molecules. The most simple definition of the term hydrogels can be stated as polymeric materials having threedimensional network structures, which give them the ability to absorb huge amounts of liquid to swell several hundred times of their original weight. Due to the unique properties of these hydrogels, they attracted the attention of many researchers and found considerable applications in different industrial and medical areas. Many factors and parameters during the preparation of the hydrogels strongly affect their behavior in the swollen state, like for example the density of crosslinking and the mesh size. One of the most important applications of hydrogels is the preparation of membranes used in water purification [23–25], manufacturing catheters [26] and contact lenses [27] and they are also applied in food industry [28] and have many other medical and biotechnological applications [29]. Functionalized hydrogels containing wide variety of functional groups like hydroxyl groups, amino groups or carboxyl groups could be synthesized successfully and applied in various predetermined purposes. Functionalization of polymers (introducing desirable functional groups) can be achieved by different chemical techniques like for example graft copolymerization of monomers having the desired functional groups onto another synthetic polymer either by chemical or irradiation initiation techniques. Textile wastewater is a complicated mixture of different components like alkali, bleaching agents, surfactants, dyes and auxiliaries. This complicated nature of such wastewater makes it
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2.2. Grafting of N,N-Diethylaminoethyl methacrylate onto starch
Scheme 1. Chemical structure of the anionic dye Direct Red 81 (DR 81).
very hard to remove color from such waters by conventional methods. Some research groups could use successfully various kinds of adsorbents for removing different acidic and basic dyes from their aqueous solutions [2]. From biological point of view, it is well established that residual dyes produced during textile coloration, either through dyeing or printing processes are classified as toxic and hazardous materials and can cause direct destruction of aquatic life [30–32]. To achieve an economical and feasible way for removing color from industrial wastewater, recent studies were directed to develop cheap and effective adsorbents [33,34]. Hydrogels based on crosslinked synthetic polymers or chemically modified natural polymers are hydrophilic in nature and when in contact with aqueous solutions, they are capable of absorbing water and swelling to thousands times of their original weights. Due to their broad spectrum applications in dealing with complicated environmental, industrial and medical problems, these polymeric network structures (hydrogels) attracted more and more attention. One of the most sounding applications of these polymeric hydrogels is the removal of dyes with high molecular weights. When hydrogel is in contact with the dye solution they absorb the dye solution and swell. The dye molecules will be trapped inside the hydrogel due to interaction between the ionic functional groups of the dyes and the ionic functional groups of the hydrogel [35,36]. In this study, an adsorbent having cationic functionality was prepared by the graft copolymerization of N,N-Diethylaminoethyl methacrylate onto starch to form starch/poly(N,N-Diethylaminoethyl methacrylate) graft copolymer. The so prepared graft copolymer was further crosslinked using epichlorohydrin to give hydrogel [37]. The so prepared hdrogel is expected to be more effective in removing anionic dyes, compared to traditional adsorbents prepared by other research groups based on the following facts. The cationic character of the substituted amino group in the N,N-Diethylaminoethyl methacrylate moiety of the hydrogel is expected to be very effective in binding to the negatively charged Direct Red 81 anions and thus removing them from their aqueous solutions. When the so prepared hydrogel was tried to remove the anionic dye Direct Red 81 from its aqueous solution, the adsorption study showed good removal results and the adsorption data was found to obey Langumier adsorption isotherm (see Scheme 1).
Slurry of 20 g maize starch in 100 mL distilled water was stirred well and the temperature was raised to 90 °C for 15 min to completely cook the starch. The cooked starch was then cooled to reach the desired polymerization temperature 60 °C. When the desired reaction temperature was attained, 20 g of N,N-Diethylaminoethyl methacrylate were added to the reaction medium together with 1 g of ceric ammonium nitrate and 20 mL of 0.1 M nitric acid, and the mixture was stirred for 1 h at the predetermined polymerization temperature, 60 °C. At the end of the predetermined polymerization duration, 5 mL hydroquinone aqueous solution (1%) was added to terminate the polymerization reaction. The formed starch/poly(N,N-Diethylaminoethyl methacrylate) graft copolymer was precipitated in ethanol and separated from the homopolymer, the unreacted reaction components and initiator and purified by use of water dialysis in a cellophane tube and finally air dried [38]. Percentage graft yield (G.Y.) was calculated as follows:
%G:Y: ¼
W2 W1 100 W1
where W1 is the dry weight of original sample and W2 is the dry weight of the grafted sample. 2.3. Preparation of hydrogels Starch/poly(N,N-Diethylaminoethyl methacrylate) graft copolymer (10 g) was ground well and treated with 100 mL sodium hydroxide solution (0.1 N). Upon continuous mechanical stirring, the graft copolymer is dissolved in the alkaline solution. Epichlorohydrin (5 g) is added to the graft copolymer solution drop wise with keeping continuous and vigorous stirring. At the end of the cross-linking treatment, the formed paste is dried overnight in an air oven at 50 °C and the dry paste was cured at 150 °C for 2 min [39]. 2.4. Swelling measurements To study the swelling properties of the prepared hydrogel, the simple traditional gravimetric procedure was used for this purpose [40,41]. Accurately weighed hydrogel samples (on dry basis) are immersed in distilled water for about 24 h at room temperature until the sample swell as much as possible. The swollen hydrogels are removed from the water and wiped carefully and gently with filter paper without pressing them in order to remove the excess water from their surfaces and then weighed accurately to determine the swelling ratio (SR) according to the following equation:
SR ¼ W s =W d where Wd is the dried sample weight (gm) and Ws is the swollen sample weight (gm). The SR was found to be 100 and this means that each one gram from the dry hydrogel is capable of absorbing 0.1 l from distilled water.
2. Experimental
2.5. Adsorption of Direct Red 81 by the hydrogel
2.1. Materials
Adsorption experiments were carried out in batch conditions at 30 °C using cylindrical glass vessels. The temperature of 30 °C was chosen to carry out the adsorption study at which because this temperature was found to be the average temperature of textile mill effluent, which is a mixture of hot dyeing bath effluent, hot soaping bath effluent and cold rinsing water. Definite weights from the hydrogel samples were immersed separately, each in 50 mL of 25–250 mg/l dye solution. After predetermined exhaustion time, the residual amount of dye in the adsorbate solution was
Maize starch used as a substrate in this study was supplied by LOBA CHEMIE PVT. Ltd. The anionic Direct Dye (DR 81), having the IUPAC name Disodium 7-benzamido-4-hydroxy-3-[[4-[(4-sulphonatophenyl)azo]phenyl]azo]naphthalene-2-sulfonate was supplied by Aldrich. N,N-Diethylaminoethyl methacrylate, epichlorohydrin (ECH) ceric ammonium nitrate (CAN) and nitric acid were analytical-grade reagents.
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determined spectrophotometrically. All of the experiments were triplicated to assure accuracy of the results. 3. Results and discussion 3.1. Factors affecting the dye adsorption onto the hydrogel
3.1.2. Effect of adsorbate pH The pH of the adsorbate solution affects to great extent the adsorption process as a whole, as pH plays an important role in determining the ionization degree of the adsorbate, in addition to affecting the surface charge of the adsorbent particles. To study the effect of the adsorbate initial pH on the efficiency of the adsorbent in the dye removal process, different adsorbate solutions having various initial pH values were prepared. The concentrations of these solutions were the same but the initial pH was changed in the range of 1–10 and the adsorption efficiency was determined in each case. The results of the dye removal efficiency, expressed in terms of percent dye removal are presented in Fig. 2 which shows the change in the percent of dye removal as a function of pH. It is clear from the results presented in Fig. 2 that maximum dye removal percent (95.65%) is attained at pH 1. As the pH values get higher, the percent dye removal shows decrement in its value until it reaches 0% at pH 10. The enhancement in percent dye removal in the strongly acidic medium and its decreament as the medium turns alkaline can be understood in terms of the surface charge of the adsorbent due to the protonation of the amino group involved in its structure. The first stage of the suggested ion exchange mechanism is the protonation of the hydrogel amino, while the second step is the attachment of the anionic dye molecule from its negative center to the protonated amino groups. The zero percent removal at pH 10 can be attributed to the fact that at pH 10, the high concentration of OH groups means that there is no protonation to the amino groups and accordingly they will not be available as ion exchange
Fig. 2. Effect of adsorbate medium pH on the percent dye removal.
100
Dye removal (%)
3.1.1. Effect of graft yield Graft yield of the starch/poly(N,N-Diethylaminoethyl methacrylate) graft copolymer has great effect on the adsorption capacity of the crosslinked hydrogel as more graft yield means more amino groups available for interaction with the anionic dye. Fig. 1 shows the effect of graft yield percent on the adsorption capacity of the hydrogel towards the anionic dye. The data indicates that the amount of the adsorbed dye increases continuously by increasing the graft yield percent. The increase in adsorption capacity of the graft copolymer by increasing the graft yield is understood because increasing the poly(N,N-Diethylaminoethyl methacrylate component in the starch/poly(N,N-Diethylaminoethyl methacrylate graft copolymer will increase the amount of available amino groups of the modified starch and this means increment in the active sites available for ion-exchange or adsorption.
80 60 40 20 0
10
20
30
40
50
60
70
80
90
Time (minutes) Fig. 3. Effect of immersion time on percent dye removal.
sites to attract the negatively charged dye anions. Under such conditions the dye does not exchange and remain in the solution. Also as the pH value increases from 1 to 10, the protonation of the amino groups decreases and as a result, the adsorption capacity decreases until it reaches its minimum value at pH 10. 3.1.3. Effect of immersion time The time required for equilibrium between the dye molecules in the adsorbate medium and the hydrogel depends on the nature of the adsorbate (the dye solution) and the nature of the adsorbent (the hydrogel). To examine the relation between the immersion time of the hydrogel and percent removal of the dye from the adsorbate solution, the following experiments were carried out. Different hydrogel samples were immersed, individually, in adsorbate solutions having the same dye concentration for various time intervals ranging from 10 min to 90 min at fixed temperature (30 °C). The results of percent dye removal at different time intervals are given in Fig. 3. It is clear from this figure that the percent dye removal improves from 47% after 10 min to reach 95% after 50 min. The percent dye removal becomes almost constant and
Fig. 1. Effect of hydrogel graft yield on the percent dye removal.
E.S. Abdel-Halim / Reactive & Functional Polymers 73 (2013) 1531–1536
100
120
80
100
qe (mg / g)
Dye removal (%)
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60 40
80 60 40
20 20 0 0
1
2
3
4
0
5
0
5
10
Adsorbent concentration (g/l)
15
20
25
30
35
Ce (mg / l)
Fig. 4. Effect of hydrogel concentration on percent dye removal.
Fig. 5. Adsorption isotherm of Direct Red 81 onto the hydrogel.
no more removal takes place after 50 min, This means that the adsorbent surface becomes completely saturated by dye molecules. At this point (after 50 min immersion time), there exist an equilibrium state between the dye molecules in the adsorbate solution and the dye molecules adsorbed onto the hydrogel surface.
in thermodynamic equilibrium state, is very important factor to examine the affinity of the hydrogel for adsorbing the dye under investigation. The thermodynamic equilibrium state for the dye under investigation is quite clear in Fig. 5.
3.1.4. Effect of adsorbent dose To investigate the effect of varying the hydrogel concentration in the adsorption medium on the percent of the dye removed, the concentration of hydrogel used as adsorbent was varied in the range of 0.5–5 g/l and the adsorption process was carried out using the batch technique by immersing the hydrogel separately, in the adsorbate solution of fixed concentration and at the same temperature. The results of the percent dye removal using different adsorbent doses are presented in Fig. 4. It is clear from the data given in Fig. 4 that, the percent dye removed on using hydrogel concentration of 0.5 g/1 is about 31.42%. This percent dye removal increases by increasing the hydrogel concentration in the adsorbate solution to reach 100% on using hydrogel concentration of 2.5 g/1. Upon using hydrogel concentrations above 2.5 g/l, the percent dye removal remains almost constant, indicating that an adsorbent dose of 2.5 g/l is sufficient for the optimum removal of the dye. The increase in percent dye removal with increasing adsorbent concentration in the first stage could be attributed to the greater availability of the exchangeable sites of the adsorbent. The leveling of adsorption capacity at higher adsorbent concentration could be attributed to the complete exhaustion of the dye upon using 2.5 g/l adsorbent dose.
According to the Langmuir treatment, maximum adsorption of the adsorbate molecules onto the adsorbent surface takes place when the surface is saturated with a monolayer of the adsorbate molecules. In this case it is assumed that the energy of adsorption becomes constant and no transmigration of the adsorbate molecules from the adsorbent surface to the bulk of the adsorbate solution nor from the bulk of the adsorbate solution to the surface of the adsorbent through the plane of the surface can take place. Langmuir isotherm is represented by the following equation [42].
Adsorption data measured for a wide range of adsorbate concentrations including low medium and high values are commonly treated by different adsorption isotherms, like for example Langmuir and Freundhich isotherms. Langmuir isotherm, for example represents mathematical relation between the adsorption capacities of the adsorbent, referred to as qe (heavy metal or dye removed per unit weight of the adsorbent) and the adsorbate concentration at the equilibrium state (in the bulk of the fluid phase), which is referred to as Ce. Fig. 5 represents the adsorption isotherm of the dye Direct Red 81, adsorbed onto the hydrogel samples at fixed temperature. Different concentrations from the dye solution ranging from 25 mg/l to 250 mg/l were prepared and accurately weighed hydrogel samples were immersed separately in the same volume from each dye concentration. The hydrogel capacity for adsorbing the dye under investigation can be estimated by recording the adsorption data and establishing its equilibrium isotherms. The distribution of the adsorbate molecules (in this case Direct Red 81) between the bulk of the adsorbate solution and the surface of the adsorbent (in this case the hydrogel), when the system is
Ce Ce 1 ¼ þ qe Q o Q o b
ð1Þ
where the term Ce represents the equilibrium concentration of the adsorbate molecules (mg/l), the term qe represents the amount of dye molecules adsorbed onto the adsorbent surface (mg/g adsorbent) and the two parameters Qo and b are two Langmuir constants related to the maximum adsorption capacity (mg/g) and the energy of adsorption (l/mg), respectively. The Langmuir equilibrium constant, KL can be calculated from the following equation
KL ¼ Q o b
ð2Þ
When the values (Ce/qe) are plotted against the values of Ce, for different initial adsorbate concentrations, a straight line should result if the adsorption data fit well to Langmuir isotherm. The slop of such straight line is the reciprocal of (Qo) 1/Qo and its intercept is equal to 1/(Qo * b). Fig. 6 represents the Langmuir adsorption isotherm of Direct Red 81 adsorbed onto the hydrogel. As stated above, the Langmuir constants, Qo and b are to be calculated from the equation representing the straight line and after obtaining these
0.3 0.25
Ce/qe (g/l)
3.2. Adsorption isotherms
3.3. Langmuir isotherm
0.2 0.15 y = 0.0089 x + 0.0058 R† = 0.9979
0.1 0.05 0 0
10
20
30
40
Ce (mg / l) Fig. 6. Langmuir isotherm for Direct Red adsorption onto the hydrogel.
E.S. Abdel-Halim / Reactive & Functional Polymers 73 (2013) 1531–1536 Table 1 Relation between the separation factor, RL and the shape of the isotherm. RL value
Type of isotherm
RL > 1 RL = 1 0 < RL < 1 RL = 0
Unfavorable Linear Favorable Irreversible
Table 2 RL values calculated for the adsorption of Direct Red 81 onto the hydrogel, based on Langmuir equation. Dye initial concentration (mg/l)
RL values
25 50 75 100 125 150 200 250
0.0384 0.0196 0.0131 0.0099 0.0079 0.0066 0.0048 0.004
values, one can substitute in Eq. 2 to calculate the value of the constant KL. The calculated values of the three Langmuir constants calculated for the adsorption of Direct Red 81 onto the hydrogel are as follows: Qo = 112 mg/g, b = 1.050 l/mg and KL = 117 l/g. The R2 value calculated from Langumir equation was found to equal 0.9979. To describe the type of isotherm, a dimensionless constant, which is known as the separation factor or the equilibrium parameter, RL, can be calculated using Langmuir equation [43] and this separation factor is defined by the following equation:
RL ¼ 1=ð1 þ bC o Þ
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the presence of amino group in its structure, the hydrogel was expected to be effective in removing dyes having anionic functionality like Direct Red 81 from its aqueous solutions. Complete adsorption study was carried out and all factors affecting the adsorption process, like the graft yield of crosslinked hydrogel, the initial pH of Direct Red 81 aqueous solution, the immersion time of the hydrogel and the hydrogel dose were studied. The results obtained from the adsorption systematic study showed a continuous increament in the amount of the adsorbed dye by increasing the hydrogel graft yield percent. The initial pH of the adsorbate solution was found to have an important role in the adsorption efficiency, through affecting the ionization degree of the adsorbate and the surface charge of the adsorbent particles. Maximum dye removal percent (95.65%) was attained at pH 1 and the percent dye removal shows decreament in its value by increasing the adsorbate initial pH until it reaches 0% at pH 10. Regarding the effect of immersion time, it was found that the percent dye removal improves from 47% after 10 min to reach 95% after 50 min. The results of the percent dye removal using different adsorbent doses showed that, the percent dye removal on using hydrogel concentration of 0.5 g/1 is about 31.42%. This percent was found to increase by increasing the hydrogel concentration to reach 100% on using hydrogel dose of 2.5 g/1. In general, the data obtained from the adsorption study was found to fit well to the Langmuir adsorption model. Acknowledgement This project was supported by King Saud University, Deanship of Scientific Research, College of Science Research Center. References
ð3Þ
where the term b is the Langmuir constant (l/mg) and the term Co is the initial concentration of dye solution (mg/l). After evaluating the separation factor, RL one can predict the shape of the isotherm by comparing the calculated values with those given in Table 1. Table 2 presents the values of RL calculated for the adsorption of different initial concentrations of Direct Red 81 onto the hydrogel. According to the calculated values of RL for adsorption of Direct Red 81 onto the hydrogel, which are listed in Table 2, it is clear that the values of RL fall in the range 0.0384 and 0.004. All calculated RL values are between 0 and 1, which indicates that the adsorption of Direct Red 81 onto the hydrogel is favorable. Also the correlation coefficient, value (R2) calculated from Langmuir equation for the adsorption of Direct Red 81 onto the hydrogel was found to be 0.9979, which means that this adsorption process obeys the Langmuir isotherm. 4. Conclusion In this work a hydrogel having cationic functionality was prepared by grafting the monomer N,N-Diethylaminoethyl methacrylate onto coocked maize starch and then crosslinking the so obtained starch/poly(N,N-Diethylaminoethyl methacrylate) graft copolymer using epichlorohydrin. The graft copolymerization reaction was initiated using ceric ammonium nitrate/nitric acid initiation system and the graft copolymer was separated and purified from the hompolymer and unreacted polymer to determine the graft yield. The crosslinking reaction was achieved by reacting starch/poly(N,N-Diethylaminoethyl methacrylate) graft copolymer with epichlorohydrin in alkaline medium to get a hydrogel which when evaluated was found to have a swelling ratio of 100. Based on the cationic functionality of the so prepared hydrogel, due to
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Reactive & Functional Polymers journal homepage: www.elsevier.com/locate/react
Preparation of starch/poly(N,N-Diethylaminoethyl methacrylate) hydrogel and its use in dye removal from aqueous solutions E.S. Abdel-Halim ⇑ Chemistry Department, College of Science, King Saud University, P.O. Box 2455, Riyadh 11451, Saudi Arabia
a r t i c l e
i n f o
Article history: Received 2 May 2013 Received in revised form 29 July 2013 Accepted 9 August 2013 Available online 22 August 2013 Keywords: Starch Graft copolymerization Hydrogel Adsorption Anionic dye
a b s t r a c t Starch/poly(N,N-Diethylaminoethyl methacrylate) graft copolymer was synthesized by graft polymerizing N,N-Diethylaminoethyl methacrylate monomers onto cooked starch using ceric ammonium nitrate/nitric acid mixture as an initiator. After extracting the graft copolymer from the homopolymer and evaluating the graft yield, the graft copolymer was subjected to crosslinking treatment using epichlorohydrin in alkaline medium to convert it to hydrogel. The so prepared hydrogel was evaluated for its swelling ratio which was found to be 100. The hydrogel with its cationic functionality was tried for removing the anionic dye Direct Red 81 from its aqueous solution and all factors affecting the hydrogel adsorption capacity towards the anionic dye, like the hydrogel graft yield, the adsorbate pH, the immersion time and the hydrogel dose were studied. The data obtained from the adsorption results were found to fit well to the Langmuir adsorption model. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Removing color and other pollutants from industrial wastewater by different adsorption techniques has attracted considerable interest and has become a feasible procedure for pollution control. During the last two decades, many research work has been carried out and many papers reported adsorption equilibria and adsorption kinetics of dye removal processes by use of various types of adsorbents [1,2]. There are many types of commercially available adsorbents, among which activated carbons [3,4], chitin and chitosan based adsorbents [5], cellulosics-based adsorbents [5– 8], silica gel [9], adsorbents based on lignocellulosic materials and by-products [10], saw dust [11–13], peat residues [14], different carbohydrates other than cellulose [15–17] and many other synthetic polymeric adsorbents [18–21]. Among all the above mentioned adsorbents, the most important one and the most widely used is the activated carbon, which is characterized by high degree of porosity, which makes it highly efficient in adsorbing a wide range of different types of adsorbates [22]. Synthetic dyes which are nowadays widely used in textile industry are known to be bulky molecules composed of high number of fused aromatic rings, that is why removing these dyes by use of conventional activated carbons does not give satisfactory results. Polymeric-based adsorbents and chemically modified hydrogel
⇑ Address: National Research Center, Dokki, Cairo, Egypt. Tel.: +20 108113477. E-mail address: [email protected] 1381-5148/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.reactfunctpolym.2013.08.003
systems are characterized by many advantages in this field and proved to be highly efficient in adsorbing and removing such bulky dye molecules. The most simple definition of the term hydrogels can be stated as polymeric materials having threedimensional network structures, which give them the ability to absorb huge amounts of liquid to swell several hundred times of their original weight. Due to the unique properties of these hydrogels, they attracted the attention of many researchers and found considerable applications in different industrial and medical areas. Many factors and parameters during the preparation of the hydrogels strongly affect their behavior in the swollen state, like for example the density of crosslinking and the mesh size. One of the most important applications of hydrogels is the preparation of membranes used in water purification [23–25], manufacturing catheters [26] and contact lenses [27] and they are also applied in food industry [28] and have many other medical and biotechnological applications [29]. Functionalized hydrogels containing wide variety of functional groups like hydroxyl groups, amino groups or carboxyl groups could be synthesized successfully and applied in various predetermined purposes. Functionalization of polymers (introducing desirable functional groups) can be achieved by different chemical techniques like for example graft copolymerization of monomers having the desired functional groups onto another synthetic polymer either by chemical or irradiation initiation techniques. Textile wastewater is a complicated mixture of different components like alkali, bleaching agents, surfactants, dyes and auxiliaries. This complicated nature of such wastewater makes it
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2.2. Grafting of N,N-Diethylaminoethyl methacrylate onto starch
Scheme 1. Chemical structure of the anionic dye Direct Red 81 (DR 81).
very hard to remove color from such waters by conventional methods. Some research groups could use successfully various kinds of adsorbents for removing different acidic and basic dyes from their aqueous solutions [2]. From biological point of view, it is well established that residual dyes produced during textile coloration, either through dyeing or printing processes are classified as toxic and hazardous materials and can cause direct destruction of aquatic life [30–32]. To achieve an economical and feasible way for removing color from industrial wastewater, recent studies were directed to develop cheap and effective adsorbents [33,34]. Hydrogels based on crosslinked synthetic polymers or chemically modified natural polymers are hydrophilic in nature and when in contact with aqueous solutions, they are capable of absorbing water and swelling to thousands times of their original weights. Due to their broad spectrum applications in dealing with complicated environmental, industrial and medical problems, these polymeric network structures (hydrogels) attracted more and more attention. One of the most sounding applications of these polymeric hydrogels is the removal of dyes with high molecular weights. When hydrogel is in contact with the dye solution they absorb the dye solution and swell. The dye molecules will be trapped inside the hydrogel due to interaction between the ionic functional groups of the dyes and the ionic functional groups of the hydrogel [35,36]. In this study, an adsorbent having cationic functionality was prepared by the graft copolymerization of N,N-Diethylaminoethyl methacrylate onto starch to form starch/poly(N,N-Diethylaminoethyl methacrylate) graft copolymer. The so prepared graft copolymer was further crosslinked using epichlorohydrin to give hydrogel [37]. The so prepared hdrogel is expected to be more effective in removing anionic dyes, compared to traditional adsorbents prepared by other research groups based on the following facts. The cationic character of the substituted amino group in the N,N-Diethylaminoethyl methacrylate moiety of the hydrogel is expected to be very effective in binding to the negatively charged Direct Red 81 anions and thus removing them from their aqueous solutions. When the so prepared hydrogel was tried to remove the anionic dye Direct Red 81 from its aqueous solution, the adsorption study showed good removal results and the adsorption data was found to obey Langumier adsorption isotherm (see Scheme 1).
Slurry of 20 g maize starch in 100 mL distilled water was stirred well and the temperature was raised to 90 °C for 15 min to completely cook the starch. The cooked starch was then cooled to reach the desired polymerization temperature 60 °C. When the desired reaction temperature was attained, 20 g of N,N-Diethylaminoethyl methacrylate were added to the reaction medium together with 1 g of ceric ammonium nitrate and 20 mL of 0.1 M nitric acid, and the mixture was stirred for 1 h at the predetermined polymerization temperature, 60 °C. At the end of the predetermined polymerization duration, 5 mL hydroquinone aqueous solution (1%) was added to terminate the polymerization reaction. The formed starch/poly(N,N-Diethylaminoethyl methacrylate) graft copolymer was precipitated in ethanol and separated from the homopolymer, the unreacted reaction components and initiator and purified by use of water dialysis in a cellophane tube and finally air dried [38]. Percentage graft yield (G.Y.) was calculated as follows:
%G:Y: ¼
W2 W1 100 W1
where W1 is the dry weight of original sample and W2 is the dry weight of the grafted sample. 2.3. Preparation of hydrogels Starch/poly(N,N-Diethylaminoethyl methacrylate) graft copolymer (10 g) was ground well and treated with 100 mL sodium hydroxide solution (0.1 N). Upon continuous mechanical stirring, the graft copolymer is dissolved in the alkaline solution. Epichlorohydrin (5 g) is added to the graft copolymer solution drop wise with keeping continuous and vigorous stirring. At the end of the cross-linking treatment, the formed paste is dried overnight in an air oven at 50 °C and the dry paste was cured at 150 °C for 2 min [39]. 2.4. Swelling measurements To study the swelling properties of the prepared hydrogel, the simple traditional gravimetric procedure was used for this purpose [40,41]. Accurately weighed hydrogel samples (on dry basis) are immersed in distilled water for about 24 h at room temperature until the sample swell as much as possible. The swollen hydrogels are removed from the water and wiped carefully and gently with filter paper without pressing them in order to remove the excess water from their surfaces and then weighed accurately to determine the swelling ratio (SR) according to the following equation:
SR ¼ W s =W d where Wd is the dried sample weight (gm) and Ws is the swollen sample weight (gm). The SR was found to be 100 and this means that each one gram from the dry hydrogel is capable of absorbing 0.1 l from distilled water.
2. Experimental
2.5. Adsorption of Direct Red 81 by the hydrogel
2.1. Materials
Adsorption experiments were carried out in batch conditions at 30 °C using cylindrical glass vessels. The temperature of 30 °C was chosen to carry out the adsorption study at which because this temperature was found to be the average temperature of textile mill effluent, which is a mixture of hot dyeing bath effluent, hot soaping bath effluent and cold rinsing water. Definite weights from the hydrogel samples were immersed separately, each in 50 mL of 25–250 mg/l dye solution. After predetermined exhaustion time, the residual amount of dye in the adsorbate solution was
Maize starch used as a substrate in this study was supplied by LOBA CHEMIE PVT. Ltd. The anionic Direct Dye (DR 81), having the IUPAC name Disodium 7-benzamido-4-hydroxy-3-[[4-[(4-sulphonatophenyl)azo]phenyl]azo]naphthalene-2-sulfonate was supplied by Aldrich. N,N-Diethylaminoethyl methacrylate, epichlorohydrin (ECH) ceric ammonium nitrate (CAN) and nitric acid were analytical-grade reagents.
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determined spectrophotometrically. All of the experiments were triplicated to assure accuracy of the results. 3. Results and discussion 3.1. Factors affecting the dye adsorption onto the hydrogel
3.1.2. Effect of adsorbate pH The pH of the adsorbate solution affects to great extent the adsorption process as a whole, as pH plays an important role in determining the ionization degree of the adsorbate, in addition to affecting the surface charge of the adsorbent particles. To study the effect of the adsorbate initial pH on the efficiency of the adsorbent in the dye removal process, different adsorbate solutions having various initial pH values were prepared. The concentrations of these solutions were the same but the initial pH was changed in the range of 1–10 and the adsorption efficiency was determined in each case. The results of the dye removal efficiency, expressed in terms of percent dye removal are presented in Fig. 2 which shows the change in the percent of dye removal as a function of pH. It is clear from the results presented in Fig. 2 that maximum dye removal percent (95.65%) is attained at pH 1. As the pH values get higher, the percent dye removal shows decrement in its value until it reaches 0% at pH 10. The enhancement in percent dye removal in the strongly acidic medium and its decreament as the medium turns alkaline can be understood in terms of the surface charge of the adsorbent due to the protonation of the amino group involved in its structure. The first stage of the suggested ion exchange mechanism is the protonation of the hydrogel amino, while the second step is the attachment of the anionic dye molecule from its negative center to the protonated amino groups. The zero percent removal at pH 10 can be attributed to the fact that at pH 10, the high concentration of OH groups means that there is no protonation to the amino groups and accordingly they will not be available as ion exchange
Fig. 2. Effect of adsorbate medium pH on the percent dye removal.
100
Dye removal (%)
3.1.1. Effect of graft yield Graft yield of the starch/poly(N,N-Diethylaminoethyl methacrylate) graft copolymer has great effect on the adsorption capacity of the crosslinked hydrogel as more graft yield means more amino groups available for interaction with the anionic dye. Fig. 1 shows the effect of graft yield percent on the adsorption capacity of the hydrogel towards the anionic dye. The data indicates that the amount of the adsorbed dye increases continuously by increasing the graft yield percent. The increase in adsorption capacity of the graft copolymer by increasing the graft yield is understood because increasing the poly(N,N-Diethylaminoethyl methacrylate component in the starch/poly(N,N-Diethylaminoethyl methacrylate graft copolymer will increase the amount of available amino groups of the modified starch and this means increment in the active sites available for ion-exchange or adsorption.
80 60 40 20 0
10
20
30
40
50
60
70
80
90
Time (minutes) Fig. 3. Effect of immersion time on percent dye removal.
sites to attract the negatively charged dye anions. Under such conditions the dye does not exchange and remain in the solution. Also as the pH value increases from 1 to 10, the protonation of the amino groups decreases and as a result, the adsorption capacity decreases until it reaches its minimum value at pH 10. 3.1.3. Effect of immersion time The time required for equilibrium between the dye molecules in the adsorbate medium and the hydrogel depends on the nature of the adsorbate (the dye solution) and the nature of the adsorbent (the hydrogel). To examine the relation between the immersion time of the hydrogel and percent removal of the dye from the adsorbate solution, the following experiments were carried out. Different hydrogel samples were immersed, individually, in adsorbate solutions having the same dye concentration for various time intervals ranging from 10 min to 90 min at fixed temperature (30 °C). The results of percent dye removal at different time intervals are given in Fig. 3. It is clear from this figure that the percent dye removal improves from 47% after 10 min to reach 95% after 50 min. The percent dye removal becomes almost constant and
Fig. 1. Effect of hydrogel graft yield on the percent dye removal.
E.S. Abdel-Halim / Reactive & Functional Polymers 73 (2013) 1531–1536
100
120
80
100
qe (mg / g)
Dye removal (%)
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60 40
80 60 40
20 20 0 0
1
2
3
4
0
5
0
5
10
Adsorbent concentration (g/l)
15
20
25
30
35
Ce (mg / l)
Fig. 4. Effect of hydrogel concentration on percent dye removal.
Fig. 5. Adsorption isotherm of Direct Red 81 onto the hydrogel.
no more removal takes place after 50 min, This means that the adsorbent surface becomes completely saturated by dye molecules. At this point (after 50 min immersion time), there exist an equilibrium state between the dye molecules in the adsorbate solution and the dye molecules adsorbed onto the hydrogel surface.
in thermodynamic equilibrium state, is very important factor to examine the affinity of the hydrogel for adsorbing the dye under investigation. The thermodynamic equilibrium state for the dye under investigation is quite clear in Fig. 5.
3.1.4. Effect of adsorbent dose To investigate the effect of varying the hydrogel concentration in the adsorption medium on the percent of the dye removed, the concentration of hydrogel used as adsorbent was varied in the range of 0.5–5 g/l and the adsorption process was carried out using the batch technique by immersing the hydrogel separately, in the adsorbate solution of fixed concentration and at the same temperature. The results of the percent dye removal using different adsorbent doses are presented in Fig. 4. It is clear from the data given in Fig. 4 that, the percent dye removed on using hydrogel concentration of 0.5 g/1 is about 31.42%. This percent dye removal increases by increasing the hydrogel concentration in the adsorbate solution to reach 100% on using hydrogel concentration of 2.5 g/1. Upon using hydrogel concentrations above 2.5 g/l, the percent dye removal remains almost constant, indicating that an adsorbent dose of 2.5 g/l is sufficient for the optimum removal of the dye. The increase in percent dye removal with increasing adsorbent concentration in the first stage could be attributed to the greater availability of the exchangeable sites of the adsorbent. The leveling of adsorption capacity at higher adsorbent concentration could be attributed to the complete exhaustion of the dye upon using 2.5 g/l adsorbent dose.
According to the Langmuir treatment, maximum adsorption of the adsorbate molecules onto the adsorbent surface takes place when the surface is saturated with a monolayer of the adsorbate molecules. In this case it is assumed that the energy of adsorption becomes constant and no transmigration of the adsorbate molecules from the adsorbent surface to the bulk of the adsorbate solution nor from the bulk of the adsorbate solution to the surface of the adsorbent through the plane of the surface can take place. Langmuir isotherm is represented by the following equation [42].
Adsorption data measured for a wide range of adsorbate concentrations including low medium and high values are commonly treated by different adsorption isotherms, like for example Langmuir and Freundhich isotherms. Langmuir isotherm, for example represents mathematical relation between the adsorption capacities of the adsorbent, referred to as qe (heavy metal or dye removed per unit weight of the adsorbent) and the adsorbate concentration at the equilibrium state (in the bulk of the fluid phase), which is referred to as Ce. Fig. 5 represents the adsorption isotherm of the dye Direct Red 81, adsorbed onto the hydrogel samples at fixed temperature. Different concentrations from the dye solution ranging from 25 mg/l to 250 mg/l were prepared and accurately weighed hydrogel samples were immersed separately in the same volume from each dye concentration. The hydrogel capacity for adsorbing the dye under investigation can be estimated by recording the adsorption data and establishing its equilibrium isotherms. The distribution of the adsorbate molecules (in this case Direct Red 81) between the bulk of the adsorbate solution and the surface of the adsorbent (in this case the hydrogel), when the system is
Ce Ce 1 ¼ þ qe Q o Q o b
ð1Þ
where the term Ce represents the equilibrium concentration of the adsorbate molecules (mg/l), the term qe represents the amount of dye molecules adsorbed onto the adsorbent surface (mg/g adsorbent) and the two parameters Qo and b are two Langmuir constants related to the maximum adsorption capacity (mg/g) and the energy of adsorption (l/mg), respectively. The Langmuir equilibrium constant, KL can be calculated from the following equation
KL ¼ Q o b
ð2Þ
When the values (Ce/qe) are plotted against the values of Ce, for different initial adsorbate concentrations, a straight line should result if the adsorption data fit well to Langmuir isotherm. The slop of such straight line is the reciprocal of (Qo) 1/Qo and its intercept is equal to 1/(Qo * b). Fig. 6 represents the Langmuir adsorption isotherm of Direct Red 81 adsorbed onto the hydrogel. As stated above, the Langmuir constants, Qo and b are to be calculated from the equation representing the straight line and after obtaining these
0.3 0.25
Ce/qe (g/l)
3.2. Adsorption isotherms
3.3. Langmuir isotherm
0.2 0.15 y = 0.0089 x + 0.0058 R† = 0.9979
0.1 0.05 0 0
10
20
30
40
Ce (mg / l) Fig. 6. Langmuir isotherm for Direct Red adsorption onto the hydrogel.
E.S. Abdel-Halim / Reactive & Functional Polymers 73 (2013) 1531–1536 Table 1 Relation between the separation factor, RL and the shape of the isotherm. RL value
Type of isotherm
RL > 1 RL = 1 0 < RL < 1 RL = 0
Unfavorable Linear Favorable Irreversible
Table 2 RL values calculated for the adsorption of Direct Red 81 onto the hydrogel, based on Langmuir equation. Dye initial concentration (mg/l)
RL values
25 50 75 100 125 150 200 250
0.0384 0.0196 0.0131 0.0099 0.0079 0.0066 0.0048 0.004
values, one can substitute in Eq. 2 to calculate the value of the constant KL. The calculated values of the three Langmuir constants calculated for the adsorption of Direct Red 81 onto the hydrogel are as follows: Qo = 112 mg/g, b = 1.050 l/mg and KL = 117 l/g. The R2 value calculated from Langumir equation was found to equal 0.9979. To describe the type of isotherm, a dimensionless constant, which is known as the separation factor or the equilibrium parameter, RL, can be calculated using Langmuir equation [43] and this separation factor is defined by the following equation:
RL ¼ 1=ð1 þ bC o Þ
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the presence of amino group in its structure, the hydrogel was expected to be effective in removing dyes having anionic functionality like Direct Red 81 from its aqueous solutions. Complete adsorption study was carried out and all factors affecting the adsorption process, like the graft yield of crosslinked hydrogel, the initial pH of Direct Red 81 aqueous solution, the immersion time of the hydrogel and the hydrogel dose were studied. The results obtained from the adsorption systematic study showed a continuous increament in the amount of the adsorbed dye by increasing the hydrogel graft yield percent. The initial pH of the adsorbate solution was found to have an important role in the adsorption efficiency, through affecting the ionization degree of the adsorbate and the surface charge of the adsorbent particles. Maximum dye removal percent (95.65%) was attained at pH 1 and the percent dye removal shows decreament in its value by increasing the adsorbate initial pH until it reaches 0% at pH 10. Regarding the effect of immersion time, it was found that the percent dye removal improves from 47% after 10 min to reach 95% after 50 min. The results of the percent dye removal using different adsorbent doses showed that, the percent dye removal on using hydrogel concentration of 0.5 g/1 is about 31.42%. This percent was found to increase by increasing the hydrogel concentration to reach 100% on using hydrogel dose of 2.5 g/1. In general, the data obtained from the adsorption study was found to fit well to the Langmuir adsorption model. Acknowledgement This project was supported by King Saud University, Deanship of Scientific Research, College of Science Research Center. References
ð3Þ
where the term b is the Langmuir constant (l/mg) and the term Co is the initial concentration of dye solution (mg/l). After evaluating the separation factor, RL one can predict the shape of the isotherm by comparing the calculated values with those given in Table 1. Table 2 presents the values of RL calculated for the adsorption of different initial concentrations of Direct Red 81 onto the hydrogel. According to the calculated values of RL for adsorption of Direct Red 81 onto the hydrogel, which are listed in Table 2, it is clear that the values of RL fall in the range 0.0384 and 0.004. All calculated RL values are between 0 and 1, which indicates that the adsorption of Direct Red 81 onto the hydrogel is favorable. Also the correlation coefficient, value (R2) calculated from Langmuir equation for the adsorption of Direct Red 81 onto the hydrogel was found to be 0.9979, which means that this adsorption process obeys the Langmuir isotherm. 4. Conclusion In this work a hydrogel having cationic functionality was prepared by grafting the monomer N,N-Diethylaminoethyl methacrylate onto coocked maize starch and then crosslinking the so obtained starch/poly(N,N-Diethylaminoethyl methacrylate) graft copolymer using epichlorohydrin. The graft copolymerization reaction was initiated using ceric ammonium nitrate/nitric acid initiation system and the graft copolymer was separated and purified from the hompolymer and unreacted polymer to determine the graft yield. The crosslinking reaction was achieved by reacting starch/poly(N,N-Diethylaminoethyl methacrylate) graft copolymer with epichlorohydrin in alkaline medium to get a hydrogel which when evaluated was found to have a swelling ratio of 100. Based on the cationic functionality of the so prepared hydrogel, due to
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