Songklanakarin Journal of Science and Technology SJST R2 BAMROONGWONGDEE

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1 Adsorption of Congo red from aqueous solution by surfactant-modified rice husk: kinetic, isotherm and thermodynamic analysis Journal: Songklanakarin Journal of Science and Technology Manuscript ID SJST--.R Manuscript Type: Original Article Date Submitted by the Author: -May- Complete List of Authors: BAMROONGWONGDEE, Chanut; King Mongkut's University of Technology North Bangkok, Industrial Chemistry Suwannee, Songsiri; King Mongkut's University of Technology North Bangkok, Industrial Chemistry Kongsomsaksiri, Manit; King Mongkut's University of Technology North Bangkok, Industrial Chemistry Keyword: Congo red, Modified rice husk, Kinetics, Thermodynamics, Adsorption

2 Page of Original Article Adsorption of Congo red from aqueous solution by surfactant-modified rice husk: kinetic, isotherm and thermodynamic analysis Chanut Bamroongwongdee*, Songsiri Suwannee, and Manit Kongsomsaksiri Department of Industrial Chemistry, Faculty of Applied Science, King Mongkuts University of Technology North Bangkok, Bangkok,, Thailand. * Corresponding author, address: chanut.b@sci.kmutnb.ac.th Abstract In this present work, organo-modified rice husk (SMRH) adsorbent was prepared by using cetyltrimethyl ammonium bromide (CTAB) for removing Congo red (CR), a model anionic dye, from aqueous solution. The FTIR analysis indicated that CTAB was adsorbed onto the surface of rice husk. The adsorption was found to be favored at a lower ph. The amount of CR uptake was found to increase with increasing the initial CR concentration and contact time but decrease with increasing the amount of adsorbent. The equilibrium adsorption data were better described by the Langmuir model. The adsorption kinetics showed that CR adsorption onto SMRH followed the pseudo-second-order model. The intra-particle diffusion was applied to experimental data to examine the rate-limiting step. It was found that the intra-particle diffusion is not the only rate-determining step. Thermodynamic studies revealed that CR adsorption onto SMRH was spontaneous and endothermic in nature. Keywords: Adsorption, Congo red, Modified rice husk, Kinetics, Thermodynamics.

3 Page of. Introduction One of the major pollution problems is the release of large volumes of wastewater from various industries such as textile, paper, printing, cosmetics and pharmaceuticals due to the presence of a large number of synthetic dyestuffs (Roy & Mondal, ; Xi et al., ). These colored compounds have several negative affects not only on the aquatic life due to reduction in light penetration and photosynthetic activity but also on human beings because of their toxicity and carcinogenic nature (Munagapati & Kim, ). Congo red (CR) is a benzidine-based anionic diazo dye, commonly used in the textile, paper, printing and other industries (Mane & Babu, ). This synthetic dye is highly water-soluble and hardly biodegraded and photodegraded due to its structural stability (Zenasni, Meroufel, Merlin, & George, ). It is also known to metabolize to carcinogenic products and causes irritation to the skin, eyes and gastrointestinal tract (Roy & Mondal, ). Therefore, the dye removal from wastewater becomes an important issue for researchers. Several physicochemical and biological methods such as coagulation-flocculation, ozonation, advanced oxidation, liquid-liquid extraction, biosorption and adsorption have been widely employed for the decolorization of dye containing wastewater (Zenasni, Meroufel, Merlin, & George, ). Among these techniques, the adsorption process, a well-known separation technique, provides an effective and attractive treatment method for the removal of soluble organic pollutants like dyes from wastewater. This process offers several advantages including simple design, easy operation and handling, insensitive to toxic substances and low operation cost (Mane & Babu, ). Activated carbon is highly efficient adsorbent for the removal of various dissolved organic pollutants from wastewater; however, the high

4 Page of cost of commercial activated carbon limited its use in the wastewater treatment. Therefore, there is a need to search for low cost and effective adsorbents. Recently, agricultural by-products and wastes such as Eucalyptus wood saw dust (Mane & Babu, ), peanut husk (Sadaf & Bhatti, ), wheat straw (Zhang et al., ), and magnolia leaf (Yu et al., ) have been used as an alternative to remove various dyes from aqueous solution. Rice husk is an abundant low-cost agricultural by-product in the rice milling industry. Chemical compositions of rice husk is similar to those of organic fibers, consisting of cellulose, lignin, hemicellulose, silicon dioxide (Ludueña, Fasce, Alvarez, & Stefani, ) and has been used as an adsorbent for the removal of several dyes from aqueous solution (Safa, & Bhatti, ). However, it is well known that natural agricultural by-products have a low adsorption capacity, which can be enhanced through physical and chemical modification (Munagapati & Kim, ). Some researchers used a cationic surfactant to modify an agricultural by-product to remove anionic ions. Cetyltrimethyl ammonium bromide (CTAB) was used to modify some agricultural byproducts such as silkworm exuviae (Chen, Zhao, Wu, & Dai, ), sawdust (Ansari, Seyghali, Mohammad-khah, & Zanjanchi, ) and wheat straw (Zhang et al., ), which has shown promising results for the removal of anionic dyes. However, fewer researches are published using rice husk modified by CTAB for dye adsorption. Thus, this study was conducted to evaluated CTAB modified rice husk for the removal of CR from an aqueous solution. The effects of various parameters on the adsorption such as solution ph, contact time and initial dye concentration were investigated. The adsorption isotherm, kinetics and thermodynamic study have been carried out to determine the mechanism of CR adsorption onto modified rice husk.

5 Page of. Materials and Methods. Materials Congo red (M.W. =. g/mol) was provided by Ajax Finechem Pyt Ltd. and used without further purification. The stock dye solution of. g/l was prepared by dissolving the required amount of CR with distilled water. The other solutions of desired concentration were prepared by diluting from the stock solution. Cetyltrimethyl ammonium bromide (M.W. =. g/mol) was purchased from Ajax Finechem Pty Ltd. All reagents used in this study were of analytical grade and all of the solutions were prepared with distilled water.. Biosorbent preparation Rice husk (RH) collected from a local rice mill was washed repeatedly with tap water for several times to remove the adhering dirt. Then it was washed with distilled water several times and dried in an oven at C for h. Dry rice husk was ground and then sieved to obtain particle size in the range of - µm ( to mesh). To modify rice husks, g of the dried rice husk was mixed in ml of CTAB solution (. wt%). The suspension was then shaken in shaker machine with rpm at room temperature for h. The mixture was filtered and washed several times with distilled water until no bromide ion was detected by adding a.m AgNO solution to the filtrate. The sample was dried in an oven at C for h and stored in an air-tight container. The obtained product was labelled as surfactant-modified rice husk (SMRH).. Characterization of SMRH

6 Page of FTIR spectra of RH and SMRH were analyzed by a Fourier transform infrared absorption spectrophotometer (FTIR-, Perkin Elmer) using a KBr disc method. The sample spectra were collected in the range of - cm with a resolution of cm. The ph-zero point of charge (ph PZC ) was measured by preparing. M NaCl solution at ph -.. g of the adsorbents was added to ml of each solution. After a contact time of h, the final ph was measured. The difference between the initial and final ph value was plotted against the initial ph. The ph PZC is the point where ph final ph initial =. (Kaur, Rani, & Mahajan, ).. Batch adsorption experiments The adsorption experiments of SMRH for CR were carried out by batch technique at,,, and K. The adsorption experiments were conducted for four different initial concentrations at,,, and mg/l of CR.. g of SMRH was added to ml of CR solution of known concentration in -ml Erlenmeyer flask. The suspension was then shaken in a water-bath shaker at a speed of rpm. The dye solutions were separated from the adsorbent using a centrifugation after withdrawing at predetermined time intervals. The supernatant concentration was determined using a UV-vis Spectrophotometer at the maximum absorbance wavelength of nm. The effect of ph on the dye adsorption was studied by adjusting ph from to using.m HCl or.m NaOH solution. The amount of CR adsorbed at time t and the percentage removal of CR by SMRH were calculated using the following equations: q t ( C C ) V = t () W C C % CR Re moval= t () C

7 Page of where C and C t (mg/l) are the CR concentration at initial and a given time t, respectively. V (L) is the volume of dye solution, and W (g) is the weight of the adsorbent.. Adsorption kinetic studies Adsorption kinetics is necessary for understanding the adsorption mechanism and rate-determining step. In order to investigate the mechanism of the dye adsorption from different concentration in an aqueous solution, various kinetic models were fitted to the experimental data. In the present work, the pseudo-first-order, pseudo-second-order, Elovich, Bangham s and intra-particle diffusion kinetic models were tested for their applicability for the CR adsorption onto SMRH. The suitability of the kinetic models was evaluated base on the linear regression correlation coefficient (R ) values.... pseudo-first-order kinetic model The linearized form of the pseudo-first-order model is obtained (Roy, & Mondal, ): ln( q q ) = lnq k t () e t e where q e (mg/g) is the amount of CR adsorbed at equilibrium, q t (mg/g) is the amount of CR adsorbed at any time t (min) and k (/min) is the pseudo-first-order rate constant. A plot of ln(q e q t ) versus t yields a straight line whose slope and y-intercept can be used to determine the pseudo-first-order rate constant (k ) and the equilibrium adsorption density (q e ) respectively.... pseudo-second-order kinetic model The linearized form of the pseudo-second-order model is obtained (Lin et al., ):

8 Page of t = + t () q k q q t e e where k (g/mg min) is the pseudo-second-order rate constant. A plot of t/q t against t gives a straight line from which q e and k can be determined from the slope and y-intercept.... Elovich equation The simple Elovich model equation is generally expressed by the following equation (Tamilselvi, Asaithambi, & Sivakumar, ): q t b b = ln( ab) + ln t () where a (mg/g min) is the initial adsorption rate of the Elovich equation and b (g/mg) is an another constant related to the extent of surface coverage and activation energy for chemisorption. The slope and y-intercept of the linear plot of q t versus ln t were used to determine the values of the constants a and b.... Intra-particle diffusion model The intra-particle diffusion model has been applied to elucidate the diffusion mechanism and its formula is expressed as (Weber Jr., & Morris, ): q = k t + C t id where k id (mg/g min / ) is the intra-particle diffusion rate constant and C (mg/g) is the constant which describes the width of the boundary layer. According to this model, the plot of q t versus t / should be linear and pass through the origin if the intra-particle diffusion is the only rate-controlling step (Tamilselvi, Asaithambi, & Sivakumar, ).. Thermodynamic studies ()

9 Page of To further understand the adsorption process taking place between CR and SMRH, thermodynamic parameters such as standard Gibbs free energy change ( G ), standard enthalpy change ( H ), and standard entropy change ( S ) were determined. These parameters were determined using the following equations (Baek, Ijagbemi, O, & Kim, ; Tamilselvi, Asaithambi, & Sivakumar, ): G = RT ln K () C C C C K = a = e = () C C C e e e G = H T S () G H S ln K = = + () RT R T R where R (. J/mol.K) is the universal gas constant, T (K) is the absolute temperature, K is the equilibrium constant, and C e (mg/l) is the equilibrium CR concentration. The values of H and S were determined from the slope and intercept of the van t Hoff plot (i.e. plot of ln K versus /T],respectively.. Results and Discussion. Characterization The FTIR spectra of RH and SMRH were represented in Figure. The spectra of the adsorbent display many adsorption peaks, reflecting the complex nature of the materials. The board and strong band located at cm was attributed to the OH stretching vibrations, indicating the presence of OH groups presented in cellulose, hemicellulose and lignin (Sadaf, & Bhatti, ). The peak at cm contributed to stretching vibrations of C H bond of the methylene ( CH ) group was

10 Page of (Pathania, Sharma, & Siddiqi, ). The peaks located at and cm were assigned to C=O bond in the carboxylic groups (Munagapati & Kim, ). The peak at cm could be assigned to stretching vibrations of C OH bond of cellulose. A higher intensity at cm was probably due to the superposition of C OH stretching vibrations in the - cm range and Si O stretching vibrations. The peak at cm was due to stretching vibrations of Si O bond and the peak at cm was belonged to bending vibrations of siloxane bonds (Si O) (Tavlieva, Genieva, Georgieva, & Vlaev, ). The FTIR comparison of RH with that of SMRH shows that the adsorption band of CH at cm was stronger while the shoulder peak of CH at cm was slightly resolved. This was due to an increase in the aliphatic carbon contents (from CTAB) in SMRH, which indicated the existence of CTAB on the SMRH surface.. Batch biosorption studies... Effect of initial solution ph Figure showed that the maximum CR sorption was occurred at ph. followed by significant decrease in CR sorption with increasing the ph. The maximum sorption capacity of CR was found to be. mg/g at its optimum ph. The CR removal for SMRH decreased from. to.% at ph.-.. At low ph, the SMRH surface are positively charged resulted in increasing CR removal due to increased electrostatic attraction between the positively charged surface of SMRH and anionic CR molecule. On the other hand, lower CR adsorption at higher ph may be due to the presence of excess of hydroxide ions competing with the sulfonate groups ( SO ) of CR for biosorption sites. A similar result has been observed for the CR adsorption as reported in literatures (Xi et al., ). In order not to keep the error created by ph

11 Page of adjustment, CR solutions without ph adjustment were used. The CR solution had ph of.-.. The trend of the data in these experiments could be further confirmed on the basis of ph PZC, indicating the type of surface active centers and the adsorption ability of the surface. The ph PZC of SMRH was. as shown in Figure. This indicates that at ph <., SMRH surface are positively charge and enhances the anionic dye removal due to stronger electrostatic attraction between anionic dyes and SMRH surface whereas at ph >., the SMRH surface becomes negatively charged resulting in electrostatic repulsion with the CR molecule. The anionic dye adsorption is favored at ph < ph PZC, where the SMRH surface becomes positively charged.... Effect of adsorbent dosage The percentage removal of CR was increased from.% to.% with increasing the SMRH dose from to g/l (Figure ). It could be due to the fact that the higher SMRH dose provided more available adsorption sites on the surface to which the dye molecules can adsorb (Dawood & Sen, ). However, the adsorption uptake of CR decreased from. mg/g to. mg/g with increasing in adsorbent dose. This is probably due to a split in the concentration gradient between adsorbate and adsorbent. Therefore, the amount of dye adsorbed onto unit weight of adsorbent decreased with increasing the adsorbent mass, hence causing a decrease in q e value with increasing the adsorbent mass concentration. (Dawood & Sen, ; Roy & Mondal, ).... Effect of initial dye concentration As shown in Figure, the percentage of CR removal decreases with increasing the initial CR concentration in the biosorption process. This result is consistent with previous studies reported in the literature (Pathania, Sharma, & Siddiqi, ; Xi et al.,

12 Page of ). This can be attributed to the fact that there is a limited number of vacant adsorption sites on SMRH, which become saturated at a certain dye concentration (Xi et al., ). Moreover, the adsorption capacity increased rapidly with increasing the initial dye concentration which may be due to an increase in the concentration gradient (Mane & Babu, ).... Effect of contact time Figure shows that the adsorption capacity increased with increasing the contact time at all initial dye concentrations and the equilibrium was attained within min. The equilibrium time increased from to min with increasing the initial dye concentration from to mg/l. This is probably due to the higher competition for the surface active sites at higher concentration and the adsorption process becoming slow down. This explains the more adsorption time for higher concentration (Nandi et al., ). In addition, the adsorption capacity of SMRH rapidly increased at the beginning and gradually became slower until the equilibrium was reached. An increase of adsorption capacity at the initial stage is probably due to the availability of the active sites on the adsorbent surface. After a period, the remaining active sites cannot be occupied because of the repulsive force between the dye molecule on the adsorbent surface and in the bulk phase (Roy & Mondal, ). Another reason is that the biosorbent surface for dye adsorption was large in the initial stage and then dye molecules on the surface slowly diffused into the interior sites of the adsorbent particles (Pathania, Sharma, & Siddiqi, ).. Adsorption kinetics

13 Page of Adsorption kinetic models are important for understanding the kinetic behaviour and rate-limiting step. The pseudo-first-order, pseudo-second-order, Elovich, and intraparticle diffusion models were applied for the CR adsorption onto SMRH. The values of k, R, q e,exp and q e,cal for the pseudo-first-order model at different CR concentrations are evaluated from Figure a and presented in Table. This table shows that the experimental data did not agree with the pseudo-first-order model due to the low values of R ranging from. to.. Moreover, the theoretical q e,cal values for the pseudo-first-order model are significantly lower than the experimental q e,exp values. The experimental kinetic data were then analysed using the pseudo-second-order model as shown in Figure b. The R values from the pseudo-second-order model are significantly higher than those from the pseudo-first-order model and the theoretical q e,cal and experimental q e,exp values are very close to each other. Hence, CR adsorption onto SMRH can be well described by the pseudo-second-order model (Pathania, Sharma, & Siddiqi, ). The k values decreased from. to. min with increasing the initial CR concentration. This phenomenon may be due to lower competition for the adsorption sites at lower initial CR concentration. At higher initial CR concentration, the competition for the adsorption sites will be stronger, and the adsorption rates were consequently decreased (Dawood & Sen, ). The Elovich equation was applied to describe the chemisorption mechanism in nature. The initial adsorption rate (a) decreased from. to. mg/g min with increasing the initial CR concentration. This variation is consistent with the results obtained from the pseudo-second-order model. The R values were between. to

14 Page of., indicating that the CR adsorption onto SMRH would not follow the Elovich equation. The movement of dyes from aqueous solution to biosorption surface may be controlled by film diffusion, intra-particle diffusion or a mixture of both mechanisms. If the diffusion (internal surface and pore diffusion) of dye molecules inside the adsorbent is the rate-controlling step, then the Weber-Morris plot of q t versus t / should show a linear relationship with the experimental data. Figure d shows that these curves are not linear over the whole time range and there are multi-linearity regions. This phenomenon indicated that CR adsorption onto SMRH was controlled by more than one type of diffusion process. This plot can be separate into two linear regions. In the first linear region, the dye molecules diffused from bulk solution to external surface of SMRH and their rates were very fast. This could be attributed to the boundary layer diffusion. While the second stage of adsorption was very slow, where the intra-particle diffusion of CR into pores is the rate-limiting step (Mane & Babu, ). Moreover, the plots do not give straight lines passing through the origin (i.e., C ), indicating that the intraparticle diffusion involved in the adsorption process but that was not the only ratelimiting step (Dawood & Sen, ). From Table, the values of k id and C i increase with increasing the initial CR concentration. An increase in the initial CR concentration will produce a higher concentration gradient resulting in faster diffusion and quicker adsorption (Nassar, Daifullah, Kelany, & Farah, ).. Adsorption isotherms The adsorption isotherm is important for the description of the equilibrium nature of adsorption. It describes how the adsorbate molecules interact with the adsorbent and gives an idea of the adsorption capacity of the adsorbent (Dawood & Sen,

15 Page of ). In this work, two well-known models, the Langmuir and Freundlich isotherms, were chosen to investigate the adsorption behavior. Langmuir, Freundlich, C e + Ce qe qm K L qm = () log qe log K F + logce n = () where q m (mg/g) is the maximum monolayer adsorption capacity and K L (L/mg) is the Langmuir isotherm constant relate to the free adsorption energy. The values of q m and K L were calculated from the slope and intercept of a linear plot of C e /q e versus C e, respectively. Similarly the Freundlich constants K F and /n were determined from the intercept and slope of a linear plot of log q e versus log C e, respectively. Langmuir and Freundlich models fitted the experimental adsorption data with the R values of. and., respectively (Table ). These conclude that Langmuir isotherm had a better fitting model than Freundlich isotherm model. The significant characteristics of the Langmuir isotherm can be classified by the dimensionless constant called separation factor, R L, which is defined by (Sadaf, & Bhatti, ): RL = () + K LC The values R L.,.,. and. at four different initial CR concentrations were all in the range indicating that the adsorption process was favorable (Lin et al., ). From Table, /n value calculated from Freundlich model was in the range - indicated that CR adsorption onto SMRH was favorable (Munagapati, & Kim, ).. Adsorption thermodynamics

16 Page of From Table, the negative values of G were observed, indicating that CR adsorption onto SMRH is thermodynamically spontaneous in nature. The H value is positive (. kj/mol), indicated that CR adsorption onto SMRH is endothermic in nature. Further, the positive value of S (. J/mol K) showed the strong affinity of SMRH for CR and increasing randomness at the solid-solution interface during CR adsorption onto SMRH.. Conclusions This study revealed that CTAB modified rice husk is an effective adsorbent for CR removal from aqueous solution. CR removal was strongly dependent on ph of solution, adsorbent dose, initial CR concentration and contact time. Equilibrium studies show that Langmuir isotherm gave better fitting with experimental data than Freundlich isotherm. The adsorption capacity of CR on SMRH was obtained as. mg/g at K. The kinetic studies revealed that CR adsorption process followed the pseudo-secondorder model. Thermodynamic studies demonstrate that CR adsorption onto SMRH was spontaneous and endothermic in nature. Acknowledgments The authors gratefully acknowledge the Department of Industrial Chemistry at King Mongkut s University of Technology North Bangkok, Thailand for providing research facilities. References

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21 Page of q t (mg/g) % Transmittance RH MRH Wavenumber (cm - ) Figure FTIR spectra of (a) RH and (b) SMRH. qt (mg/g) %CR Removal... ph Figure Effect of initial ph on CR adsorption onto SMRH (C : mg/l, adsorption dosage: g/l, ph: -, Temp: K, contact time: min). % CR Removal

22 Page of q t (mg/g) ph final - ph initial RH SMRH ph initial Figure Point of zero charge of RH and SMRH. qt (mg/g) %CR Removal Adsorbent dosage (g/l) Figure Effect of adsorbent dosage on CR adsorption onto SMRH (C : mg/l, adsorption dosage: - g/l, ph:.-., Temp: K, contact time: min). %CR Removal

23 Page of q t (mg/g) qt (mg/g) %CR Removal..... Initial dye concentration (mg/l) Figure Effect of initial concentration on CR adsorption onto SMRH (C : mg/l, adsorption dosage: g/l, ph:.-., Temp: K, contact time: min). q t (mg/g) mg/l mg/l mg/l mg/l Time, min Figure Effect of contact time on CR adsorption onto SMRH (C : mg/l, adsorption dosage: g/l, ph:.-., Temp: K, contact time: min). %CR Removal

24 Page of (a) log (q e -q t ) (c) q t (mg/g) mg/l mg/l mg/l mg/l Time, min mg/l mg/l mg/l mg/l ln t Figure (a) Pseudo-first-order kinetics, (b) Pseudo-second-order kinetics, (c) Elovich kinetic model, and (d) Intra-particle diffusion kinetic model for CR removal by SMRH (C : mg/l, adsorption dosage: g/l, ph:.-., t/q t (g min/mg) q t (mg/g) Temp: K, contact time: min). mg/l mg/l mg/l mg/l (b) Time, min mg/l mg/l mg/l mg/l (d) t /

25 Page of Pseudo-first-order C (mg/l) q e,exp (mg/g) q e, calc (mg/g) k (min ) R Pseudo-second-order C (mg/l) q e, calc (mg/g) k (g/mg min ) R Elovich C (mg/l) a (mg/g min) b (g/mg) R Intra-particle diffusion C (mg/l) k id, (mg/g min / ) C (mg/g) R

26 Page of... Intra-particle diffusion C (mg/l) k id, (mg/g min / ) C (mg/g) R Table Kinetic parameters for the removal of CR by SMRH (C : mg/l, adsorption dosage: g/l, ph:.-., Temp: K, contact time: min). Isotherm model Parameter Values R Langmuir q m (mg/g ).. K L ( L/mg ). R L. Freundlich /n.. K F (mg/g). Table Isotherm parameters for the adsorption of CR onto SMRH (C : mg/l, adsorption dosage: g/l, ph:.-., Temp: K, contact time: min).

27 Page of T (K ) G (kj/mol) H (kj/mol) S (J/mol.K) Table Thermodynamic parameters for the adsorption of CR onto SMRH (C : mg/l, adsorption dosage: g/l, ph:.-., Temp: - K, contact time: min).