Adsorption Kinetics Studies of Polymeric Nanocomposite Coagulants

Transcription

1 Adsorption Kinetics Studies of Polymeric Nanocomposite Coagulants Adsorption Kinetics Studies of Polymeric Nanocomposite Coagulants Seher Uzunsakal a1, Serhat Zeytinci a, Ömer L. Uyanık b, and Nurseli Uyanık c a Ak-Kim Kimya San. ve Tic. A.S., Research and Development Department, Yalova, Turkey b Bahçeşehir University, Environmental Engineering Department, Beşiktaş, Istanbul, Turkey c Istanbul Technical University, Chemistry Department, Maslak, Istanbul, Turkey SUMMARY The aim of this study is to investigate the adsorption characteristics of dicyandiamide-formaldehyde resin (DCD-F) nanocomposites. For this purpose, 2%, 4%, and 8% unmodified layered silicate clay (Na-Montmorillonite) containing DCD-F nanocomposites (DCD-F- MMT) were prepared. The synthesized samples were structurally defined by XRD, SEM-EDX, and TGA characterizations. The adsorption performance of nanocomposites was tested by using two anionic dyes. Adsorption capacities of pure resin and three nanocomposites were followed against time and the nanocomposite containing 8% Na-MMT (DCD-F-MMT8%) was found to have the highest equilibrium adsorption capacity (q e ). Therefore, the effects of dye concentration, ph, and temperature on q e were investigated using DCD-F-MMT8%. The kinetic and thermodynamic parameters of adsorption were obtained based on the experimental data. Keywords: Resin nanocomposite, Montmorillonite, Anionic dye, Adsorption kinetics, Adsorption thermodynamics 1. INTRODUCTION Water soluble polymers include a wide range of products of natural and synthetic origin. They have many applications in various fields, including food technology, agriculture, ceramics technology, paper and ink technologies, textile industries, and wastewater treatment 1. They may be nonionic or ionic, but most of the water soluble polymers are ionic that are also called polyelectrolytes. Polyelectrolytes are classified as anionic and cationic, based on their structures. Some of the polyelectrolytes are used as coagulants, i.e., to facilitate the precipitation of suspended particles in water. Color in water discharges of textile industries result from textile ionic dyes. Decolorization should be applied to colored wastewaters before discharging them into watercourses in 1 Corresponding Author, seher.uzunsakal@akkim.com.tr Smithers Rapra Technology, 2013 order to satisfy legally allowed color limits. Colored wastewaters can be decolorized by coagulation processes in which coagulants are used for neutralizing and thus precipitating colloidal particles by bringing them together. Hydrophobic colloids in wastewaters are generally negatively charged 2. The addition of dicyandiamideformaldehyde resin (DCD-F) to wastewater increases the number of cations in the medium because of the cationic nature of the resin. Increase in the number of cations causes a decrease in the thickness of the electrical double layer surrounding the colloidal particles and thus zeta potential becomes closer to zero. Zeta potential surrounds anionic contaminants and stabilizes them in suspended form resulting in high values of parameters such as color, turbidity, and suspended solids. Eliminating this zeta potential causes the suspended particles to aggregate forming flocks and finally to precipitate. The use of clays as a decolorizing agent to aid the coagulation in wastewater treatment is a well established technology due to their layered structures and high specific surface area 3. Recently, nanocomposite polymeric materials that are prepared by combining the polymer with clay are used extensively in adsorption and coagulation 4. Polyamides which are used as organic coagulants can be classified into three groups, namely dicyandiamide/ formaldehyde, epichlorohydrin/amine, and dicyandiamide/amine derivatives 5. In this study, Na-Montmorillonite (MMT) containing dicyandiamideformaldehyde resin (DCD-F) nanocomposites (DCD-F-MMT) were prepared. After their XRD, SEM- EDX and TGA characterizations were completed, they were used as adsorbent Polymers & Polymer Composites, Vol. 21, No. 3,

2 Seher Uzunsakal, Serhat Zeytinci, Ömer L. Uyanık, and Nurseli Uyanık for the removal of color imparted by two kinds of anionic dyes. Color removal is a result of two mechanisms, namely adsorption and ion exchange 6. The adsorption mechanism influenced by the physicochemical parameters such as dye-adsorbent interaction, temperature, ph, and specific surface area of adsorbent was investigated. Studies on adsorption kinetics are expressed and evaluated by mathematical models. These studies allow the determination of effective contact time between adsorbent and adsorbate, and also the rate determining step in the overall adsorption process. From the results of the adsorption experiments, kinetic parameters, namely adsorption rate constant and activation energy of adsorption were calculated. Also, the experimental data obtained at different temperatures allowed the evaluation of thermodynamic parameters including enthalpy change of adsorption, entropy change of adsorption, and free energy change of adsorption. 2. EXPERIMENTAL 2.1 Materials The Cloisite Nanoclay (Na-MMT) was supplied from Southern Clay Inc. Its cation exchange capacity (CEC) is 92.6 meq/100 g and its specific surface area is greater than 750 m 2 /g. Dicyandiamideformaldehyde (DCD-F) resin was a Wuxi Lansen Chemicals Co Ltd. product. It is a colorless sticky liquid having a viscosity of mpa.s at 20 C. Rosso Kemaset 2B (Acid Red) and Giallo Yellow 2R (Acid Yellow) were used as anionic textile dyestuffs which are acid dye mixtures. Their maximum absorbance wavelength (λ max ) values are 507 nm for Acid Red and 408 nm for Acid Yellow in UV/Vis Spectrophotometry. They are the products of Kemcolor and were supplied from Ersur Textile Company. 2.2 Equipment ph Meter The ph measurements were made for adjusting the ph values of the adsorption experiments using Hach Lange HQ Jar Tester This test was carried out using Stuart Flocculator SW6 to determine the chemicals, dosages, and conditions required to achieve optimum results, and allowing the use of six or less number of different samples simultaneously with a maximum of 250 rpm mixing rate Centrifuge During the adsorption experiments, the adsorbent in the dye solutions is centrifuged using Electromag that has a maximum revolution speed of 3000 rpm X-Ray Diffraction Analyzer (XRD) X-Ray diffraction analysis of unmodified clay and nanocomposites were made by Panalytical X Pert Powder XRD. X-Ray diffraction (XRD) patterns of the samples were recorded by monitoring the diffraction angles (2θ) from 1.5 to 30 on the diffractometer, using CuKα radiation. The wavelength used was λ= Å Scanning Electron Microscopy-Energy Dispersive Spectroscopy (SEM-EDX) Field-emission Scanning Electron Microscopy (FE-SEM)(SUPRA 35VP, LEO-Gemini, GmBH, Germany) was employed to examine the morphology of the fractured samples by applying carbon coatings. The distribution of Na in DCD-F-MMT8% sample was investigated by the mapping results obtained from SEM-EDX analysis Thermal Gravimetric Analysis Thermal gravimetric analysis was made using Mettler-Toledo TGA/ DSC1 Star System, to obtain weight loss versus temperature with a heating rate of 10 C/min starting from room temperature to 900 C under N 2 atmosphere Ultraviolet-Visible Spectrophotometer (UV-Vis) Shimadzu UV-1601, UV-Vis spectrophotometer was used for measuring the wavelength of maximum absorbance for each dyestuff Colorimeter Hach Lange DR 2800 was used for color measurements. The operational wavelength of the device was adjusted to the maximum wavelength for each dyestuff, obtained from the UV-Vis spectrophotometer. 2.3 Preparation of Nanocomposites In this study, the adsorbent was Na- Montmorillonite (MMT) containing dicyandiamide-formaldehyde resin (DCD-F) nanocomposite (DCD- F-MMT). Nanocomposite samples were prepared by the addition and vigorous mixing of MMT swelled in deionized water to the polymer solution. In the preparation of these nanocomposite samples, Na-MMT having cation exchange capacity (CEC) of 92.6 meq/100 g was used in the proportions of 2%, 4%, and 8%. 2.4 Adsorption Experiments The adsorption performances of nanocomposites were tested using water colored with anionic acid dyes (Acid Red and Acid Yellow). Coagulation for color removal was carried out by jar tester according to ASTM D Standard Practice for Coagulation- Flocculation Jar Test of Water. A certain amount of adsorbent was added to a known volume of colored water at a definite dye concentration and the solution was mixed at a considerably high rpm. At every 20 minutes, the solution was settled for some time and then centrifuged at 2500 rpm 162 Polymers & Polymer Composites, Vol. 21, No. 3, 2013

3 Adsorption Kinetics Studies of Polymeric Nanocomposite Coagulants for 10 minutes. The absorbance of supernatant liquid was measured using colorimeter at the wavelength giving the maximum absorbance for that dye previously determined from UV-Vis spectrophotometer (507 nm for Acid Red and 428 nm for Acid Yellow). The dye concentration in the solution corresponding to the measured absorbance was obtained by the help of calibration equation (absorbance versus concentration) for that dye. From the difference between initial dye concentration and the dye concentration at that instant, the adsorbed dye concentration was calculated. Na atoms proves that the clay layers are distributed in exfoliated form supporting the XRD pattern. The weight loss (wt.%) of the pure resin and nanocomposite samples were presented as a function of temperature in Figure 3. The residue weight percentages at 900 C are 2.23%, 4.39% and 8.19% of DCD-F-MMT2%, DCD-F-MMT4%, DCD-F-MMT8%, respectively. These thermograms showed that the amounts of clay remaining in the nanocomposite samples were in agreement with the actual values. 3.2 Equilibrium Adsorption Capacities The variation of adsorption capacity with time for DCD-F, DCD-F that contains 2% MMT (DCD-F-MMT2%), DCD-F that contains 4% MMT (DCD- F-MMT4%), and DCD-F that contains 8% MMT (DCD-F-MMT8%) were plotted using each dye separately. (Figures 4a, 4b) All adsorption capacity experiments were done with a dye concentration of 40 mg/l, mass of adsorbent of 0.15 g, and at a temperature of 30 C. ph s are taken as the original values, 9.41 for Acid Red and 9.52 for Acid Yellow. Adsorption capacity, q t, which is defined as mg dye adsorbed per g of adsorbent was calculated as a function of time at a certain dye concentration, ph, and temperature with a certain mass of adsorbent. Figure 1. XRD Results of pure clay (Na-MMT) and DCD-F-MMT8% nanocomposite 3. RESULTS AND DISCUSSION 3.1 Characterization of Nanocomposites The structures of the synthesized samples were characterized by XRD, SEM-EDX, and TGA characterizations. The results of the XRD measurements of the unmodified clay (NA-MMT) and DCD-F-MMT8% nanocomposite were given in Figure 1. From the diffraction values, the distance between clay layers (d 001 ) in original clay and in the nanocomposite sample were calculated from the Bragg equation. The distance between clay layers was increased in the nanocomposite sample and the exfoliation of clay layers were achieved. Figure 2. Na mapping result of DCD-F-MMT8% nanocomposite on the SEM- EDX image The elemental compositions of individual Na particles and agglomerates were studied using SEM- EDX mapping for DCD-F-MMT8%. The results were given in Figure 2. In this image, the perfect dispersion of Polymers & Polymer Composites, Vol. 21, No. 3,

4 Seher Uzunsakal, Serhat Zeytinci, Ömer L. Uyanık, and Nurseli Uyanık Figure 3. TGA results for DCD-F and DCD-F-MMT samples On each graph, the maximum value of the adsorption capacity was observed at equilibrium which was reached when sufficiently long time has elapsed. The maximum value of the adsorption capacity is termed equilibrium adsorption capacity, q e. Equilibrium adsorption capacities of DCD-F, DCD-F-MMT2%, DCD-F-MMT4% ve DCD-F-MMT8% samples were found as 49 mg/g, 52 mg/g, 61 mg/g and 77 mg/g, respectively, for Acid Red dye adsorption and as 10 mg/g, 14 mg/g, 26 mg/g and 30 mg/g, respectively, for Acid Yellow dye adsorption. Increasing the percentage of MMT in the nanocomposite samples resulted in greater equilibrium adsorption capacities than the host polymer. The effects of other parameters, namely, dye concentration, ph, and temperature, on equilibrium adsorption capacities were examined by using DCD-F- MMT8% due to its better equilibrium adsorption capacity than those of the other samples. Adsorption capacity versus time graph was plotted for each case (Figures 5a, 5b, 6a, 6b, 7a, 7b). The effect of initial dye concentration on equilibrium adsorption capacity is observed in Figures 5a, 5b. It is seen that equilibrium adsorption capacity increases with increasing dye concentration for both dyes. Adsorption experiments using 0.15 g Figure 4. (a) Adsorption capacity versus time for DCD-F and DCD-F-MMT samples in Acid Red solution, (b) Adsorption capacity versus time for DCD-F and DCD-F-MMT samples in Acid Yellow solution 164 Polymers & Polymer Composites, Vol. 21, No. 3, 2013

5 Adsorption Kinetics Studies of Polymeric Nanocomposite Coagulants DCD-F-MMT8% samples in 40, 80, 120 mg/l dye solutions were carried out at 30 C and at the original ph of the dye (9.41 for Acid Red and 9.52 for Acid Yellow). By increasing the concentration of dyes from 40 mg/l to 120 mg/l, equilibrium adsorption capacities increase from 77 mg/g to 187 mg/g for Acid Red and from 30 mg/g to 139 mg/g for Acid Yellow (Figure 5a, 5b). The effect of ph on equilibrium adsorption capacity is observed in Figures 6a, 6b. It is seen that equilibrium adsorption capacity increases with decreasing ph for both dyes. Adsorption experiments were carried out at 30 C, using 0.15 g DCD-F-MMT 8% samples in 40 mg/l of dye solutions, at different ph values and the results are shown in Figure 6a, 6b. Investigation of these graphs show that, adsorption capacities increase with decreasing ph of dye solutions. A decrease in ph from 9.5 to 5.5 results in an increase in the equilibrium adsorption capacity from 77 mg/g to 89 mg/g for Acid Red and from 30 mg/g to 84 mg/g for Acid Yellow. The effect of ph is more significant for the adsorption capacities in Acid Yellow than those in the Acid Red. The effect of temperature on equilibrium adsorption capacity is observed in Figures 7a, 7b. It is seen that equilibrium adsorption capacity increases with increasing temperature for both dyes. Adsorption experiments were carried out using 0.15 g DCD- F-MMT8% sample in 40 mg/l dye solution, at 20 C, 30 C, 40 C and at the original ph of the dye (9.41 for Acid Red and 9.52 for Acid Yellow). The equilibrium adsorption capacities are 51 mg/g for Acid Red and 21 mg/g for Acid Yellow at 20 C, 77 mg/g for Acid Red and 30 mg/g for Acid Yellow at 30 C and 134 mg/g for Acid Red and 44 mg/g for Acid Yellow at 40 C. The increase in equilibrium adsorption capacity with increasing temperature can be attributed to the increase in the ability of the molecules to be adsorbed Figure 5a. Adsorption capacity versus time for different Acid Red concentrations for DCD-F-MMT8% sample Figure 5b. Adsorption capacity versus time for different Acid Yellow concentrations for DCD-F-MMT8% sample Figure 6a. Adsorption capacity versus time for different ph values in Acid Red solutions for DCD-F-MMT8% sample Polymers & Polymer Composites, Vol. 21, No. 3,

6 Seher Uzunsakal, Serhat Zeytinci, Ömer L. Uyanık, and Nurseli Uyanık Figure 6b. Adsorption capacity versus time for different ph values in Acid Yellow solutions for DCD-F-MMT8% sample Figure 7a. Adsorption capacity versus time at different temperatures in Acid Red solutions for DCD-F-MMT8% sample Figure 7b. Adsorption capacity versus time at different temperatures in Acid Yellow solutions for DCD-F-MMT8% sample onto the surface due to increase in their kinetic energies (Figure 7a, 7b). 3.3 Adsorption Kinetics Adsorption mechanism depends on physical and chemical properties of adsorbent. Therefore the kinetic models can be developed and used for the determination of mechanism and rate of adsorption processes. The applicability of adsorption data on various rate equations was investigated and it was found that the data fitted pseudo-second order rate equation 7,8 : t 1 = 2 + t q t k 2.q e q e (1) where q t is the adsorption capacity at time t, q e is the equilibrium adsorption capacity, and k 2 is the pseudo secondorder rate constant. The suitability of this rate equation to the adsorption data was verified by plotting t/q t values obtained from Figures 8a, 8b against t for four different adsorbents at 30 C and obtaining straight lines for each plot. The slope of each line is 1/q e and the intercept is 1/k 2.q e2. From the slopes and the intercepts of the straight lines at 30 C, q e and k 2 values were obtained and the results were given in Tables 1, 2 together with the correlation factor for each straight line. Another important kinetic parameter is the activation energy, E a, for adsorption. It was calculated using Arrhenius equation: k = A exp (-E a /RT) (2) where k is the rate constant (pseudosecond order rate constant k 2 for adsorption), A is the frequency factor, R is the gas constant, and T is the absolute temperature. For DCD-F-MMT8%, t/q t values obtained at 20 C, 30 C, and 40 C from Figures 9a, 9b were plotted against t and q e and k 2 values were obtained from the slopes and intercepts of the straight lines. These results were tabulated in Tables 3, Polymers & Polymer Composites, Vol. 21, No. 3, 2013

7 Adsorption Kinetics Studies of Polymeric Nanocomposite Coagulants ln k 2 values were calculated for each T, and E a was obtained from the slope of the ln k 2 versus 1/T plot. E a for the adsorption of Acid Red dye on DCD- F-MMT8% = 29.4 kj/k.mol, and E a for the adsorption of Acid Yellow dye on DCD-F-MMT8% = 18.0 kj/k.mol. Figure 8a. Pseudo-second order rate equation fit to data for the adsorption of Acid Red on DCD-F and DCD-F-MMT samples at 30 C 3.4 Adsorption Thermodynamics Adsorption can be examined thermodynamically to determine the enthalpy change ( H ), the entropy change ( S ), and the free energy change ( G ), of adsorption 9,10. Equilibrium constant for adsorption (K c ) at each temperature was calculated first. K c is the equilibrium constant for the adsorption of dye X: Figure 8b. Pseudo-second order rate equation fit to data for the adsorption of Acid Yellow on DCD-F and DCD-F-MMT samples at 30 C X (aqueous) X (adsorbed) ; K c = C a / C e (3) where C a and C e are the equilibrium concentrations of the adsorbed dye and the dye in solution, respectively. G value at each temperature was calculated from the following equation: G = R T ln K c (4) The values of K c at three different temperatures were applied to van t Hoff equation: ln K c = (ΔS /R) (ΔH /RT) (5) A plot of ln K c versus 1/T is a straight line with slope ( H /R) and intercept ( S /R). H was calculated from the slope and S from the intercept. The thermodynamic parameters for the adsorption of Acid Red and Acid Yellow on DCD-F-MMT8% were tabulated in Tables 5, CONCLUSIONS The objective of this study was to investigate removal of dyes from water by adsorption method using an adsorbent which is a cationic, water soluble polymer. DCD-F and its Table 1. Kinetic parameters for the adsorption of Acid Red on DCD-F resins (Acid Red concentration=40 mg/l, mass of adsorbent=0.15 g, ph=9.41, t=30 C) Sample k 2 (g/mg.min) q e (mg/g) R 2 DCD-F x DCD-F-MMT2% x DCD-F-MMT4% x DCD-F-MMT8% x Table 2. Kinetic parameters for the adsorption of Acid Yellow on DCD-F resins (Acid Yellow concentration=40 mg/l, mass of adsorbent=0.15 g, ph=9.52, t=30 C) Sample k 2 (g/mg.min) q e (mg/g) R 2 DCD-F x DCD-F-MMT2% x DCD-F-MMT4% x DCD-F-MMT8% x Polymers & Polymer Composites, Vol. 21, No. 3,

8 Seher Uzunsakal, Serhat Zeytinci, Ömer L. Uyanık, and Nurseli Uyanık Figure 9a. Pseudo-second order rate equation fit to data for the adsorption results of DCD-F-MMT8% at different temperatures for Acid Red Figure 9b. Pseudo-second order rate equation fit to data for the adsorption results of DCD-F-MMT8% at different temperatures for Acid Yellow nanocomposites were used as adsorbent for two types of anionic dyes. Na Montmorillonite (MMT) was used to produce the nanocomposite resins in 2%, 4% and 8% percentages. The characterization results showed that the perfect exfoliation was achieved for the DCD-F-MMT8% nanocomposite sample (the sample which has the highest adsorption capacity). Adsorption capacities of pure resin and its nanocomposites were investigated. The effects of dye concentration, ph, and temperature on equilibrium adsorption capacity were investigated for the 8% MMT containing sample (DCD- F-MMT8%) which has the highest adsorption capacity. For both dyes, the results of the experimental work showed that equilibrium adsorption capacity increases with decreasing ph, increasing dye concentration, and increasing temperature. Table 3. Kinetic parameters for the adsorption of Acid Red on DCD-F- MMT8% at different temperatures (Acid Red concentration=40 mg/l, mass of adsorbent=0.15 g DCD-F-MMT8%, ph=9.41) T( C) k 2 (g/mg.min) q e (mg/g) R x x x Table 4. Kinetic parameters for the adsorption of Acid Yellow on DCD-F- MMT8% at different temperatures (Acid Yellow concentration=40 mg/l, mass of adsorbent=0.15 g DCD-F-MMT8%, ph=9.52) T( C) k 2 (g/mg.min) q e (mg/g) R x x x The data obtained from adsorption experiments were applied to the adsorption kinetics and thermodynamics. The kinetics study indicated that, the data fitted to pseudosecond order rate equation and the rate constants and activation energy for adsorption were calculated from this study. From the experimental results obtained at three different temperatures, thermodynamic parameters ( G 0, H 0, S 0 ) were determined by using van t Hoff equation. The thermodynamic calculations showed that the type of adsorption was physical adsorption and ΔG 0, ΔH 0, and ΔS 0 are positive. In other words, the adsorption is nonspontaneous and the process is endothermic. In conclusion, the presence of MMT increased the color removal efficiency of DCD-F resin due to its better adsorption properties. Hence, the DCD-F-MMT nanocomposites, especially the ones having high MMT contents can be used for wastewater treatment, especially from textile effluents, instead of DCD-F resin to provide reduction in the consumption of chemicals. 168 Polymers & Polymer Composites, Vol. 21, No. 3, 2013

9 Adsorption Kinetics Studies of Polymeric Nanocomposite Coagulants Table 5. Thermodynamic parameters of DCD-F-MMT8% on Acid Red T(K) K C G (kj/mol) H (kj/mol) S (kj/k.mol) Table 6. Thermodynamic parameters of DCD-F-MMT8% on Acid Yellow T(K) K C G (kj/mol) H (kj/mol) S (kj/k.mol) Acknowledgement The authors thank to Southern Clay Products Inc. for supplying the Cloisite Nanoclay, to Eczacıbaşı ESAN for their help in taking the XRD measurements, to Sabancı University for their help in taking SEM-EDX measurements and to Ak-Kim for their support throughout the study. REFERENCES 1. Williams P.A., Handbook of Industrial Water Soluble Polymers, Wiley & Sons, UK, (2007). 2. Henze M., Harremoes P., Jansen J.C., and Arvin E., Wastewater Treatment: Biological and Chemical Processes, Springer, New York (2001). 3. Liu P. and Zhang L., Sep. Purif. Technol, 58, (2007), Hararah M.A., Ibrahim K.A., Al-Muhtaseb H.A., Yousef R.I., Abu-Surrah A., and Qatatsheh A., J. Appl. Polym. Sci., 117, (2010), Reife A. and Freeman S.H., Environmental Chemistry of Dyes and Pigments, Wiley & Sons., New York, (1996). 6. Slokar Y.M. and Marechal M.L., Dyes and Pigments, 37, (1998), McKay G., Use of Adsorbents for the Removal of Pollutants from Wastewaters, CRC Pres, Inc., U.S.A. (1996). 8. Ho Y-S., Adsorption, 10, (2004), Christidis G.E., Scott P.W., and Dunham A.C., Appl. Clay Sci., (1997), Qiuhong H., Zhiping X., Qiao S., Haghseresht F., Wilson M., and Qing G.L., J. Colloid Interf. Sci., (2007), 191. Polymers & Polymer Composites, Vol. 21, No. 3,

10 Seher Uzunsakal, Serhat Zeytinci, Ömer L. Uyanık, and Nurseli Uyanık 170 Polymers & Polymer Composites, Vol. 21, No. 3, 2013