KINETIC MODELING OF LIQUID-PHASE ADSORPTION OF REACTIVE DYES AND METAL IONS ON CHITOSAN

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1 PII: S (00) Wat. Res. Vol. 35, No. 3, pp , 2001 # 2001 Elsevier Science Ltd. All rights reserved Printed in Great Britain /01/$ - see front matter KINETIC MODELING OF LIQUID-PHASE ADSORPTION OF REACTIVE DYES AND METAL IONS ON CHITOSAN FENG-CHIN WU 1, RU-LING TSENG 2 and RUEY-SHIN JUANG 3 * M 1 Department of Chemical Engineering, Lien-Ho Institute of Technology, Miao-Li 360, Taiwan; 2 Department of Safety, Health and Environmental Engineering, Lien-Ho Institute of Technology, Miao-Li 360, Taiwan and 3 Department of Chemical Engineering, Yuan Ze University, Chung-Li 320, Taiwan (First received 25 February 2000; accepted in revised form 9 June 2000) Abstract}The rates of adsorption of three commercial reactive dyes and Cu(II) from water in the absence and presence of complexing agents using chitosan were measured at 308C. Three simplified kinetic models, i.e., pseudo-first-order, pseudo-second-order, and intraparticle diffusion, were tested to investigate the adsorption mechanisms. It was shown that the adsorption of reactive dyes and Cu(II) in the absence of complexing agents could be best described by the intraparticle diffusion model, whereas that of Cu(II) in the absence of complexing agents such as EDTA, citric acid, and tartaric acid by the pseudo-second-order equation. Kinetic parameters of the three models and the normalized standard deviations between the measured and predicted results were also calculated and discussed. # 2001 Elsevier Science Ltd. All rights reserved Key words}kinetic modeling, liquid-phase adsorption, chitosan, reactive dyes, metal ions, complexing agents NOMENCLATURE C t solute concentration in the aqueous phase at time t, g/m 3 or mol/m 3 C 0 initial solute concentration in the aqueous phase, g/m 3 or mol/m 3 D diffusivity of solute in the particle, m 2 /s D p average diameter of chitosan particle, mm k 1 rate constant of pseudo-first-order adsorption defined in equation (3), 1/min k 2 rate constant of pseudo-second-order adsorption defined in equation (5), kg/g min k p;2 rate parameter of intraparticle diffusion model, g/kg min 1/2 m s specific dosage of dry chitosan, kg/m 3 q e amount of adsorption at equilibrium, g/kg or mol/kg q t amount of adsorption at time t, g/kg or mol/kg Dq normalized standard deviation defined in equation (9), % r radius of the particle, m t time, min INTRODUCTION Chitosan is a partially acetylated glucosamine biopolymer existing in the cell wall of some fungi such as the Mucorales strains. However, it mainly *Author to whom all correspondence should be addressed. Tel.: ext 555; fax: ; cejuang@ce.yzu.edu.tw results from deacetylation of chitin (Muzzarelli, 1983). Chitosan has many useful features such as hydrophilicity, biocompatibility, biodegradability, and anti-bacterial property. This biopolymer is also a known sorbent, effective in the sorption of transition metal ions because the amino ( NH 2 ) and hydroxy ( OH) groups on chitosan chains can serve as coordination and electrostatic interaction sites, respectively (Monteiro and Airoldi, 1999; Guibal et al., 1994; Onsoyen and Skaugrud, 1990). In this regard, chitosan appears to be more economically attractive than activated carbons for adsorption removal of colors, organic and inorganic matters from process/waste effluents because chitin is the second abundant polymer in nature next to cellulose (Bailey et al., 1999; El-Geundi, 1997). In contrast to chitin, adsorption ability of metals using chitosan is superior due to its higher content of amino functional groups (Yang and Zall, 1984; Muzzarelli, 1983). In our laboratory, the excellent capability of chitosan for adsorption of reactive dyes and metal ions from aqueous solutions, in the absence or presence of strong complexing agents such as EDTA (ethylenediaminetetraacetic acid), citric and tartaric acids, has been presented (Juang et al., 1997, 1999; Wu et al., 1999; Tseng et al., 1999). The aim of this paper is to study the mechanism of such adsorption processes. Although several rigid models such as the homogeneous surface diffusion model, pore diffusion 613

2 614 Feng-Chin Wu et al. model, and heterogeneous diffusion model (also known as pore and diffusion model) have been extensively applied in batch reactors to successfully describe the transport of molecules inside the adsorbent particles (Chatzopoulos et al., 1993; Eichenmuller et al., 1997; Vidic et al., 1994; Zhou and Martin, 1995), the mathematical complexity of these models make them rather inconvenient for practical use (Ho and McKay, 1998; Guibal et al., 1998; Raven et al., 1998). Here, the concentrationtime profiles of batch adsorption of three commercial reactive dyes and Cu(II) from water on chitosan, and those of Cu(II) in the presence of EDTA, citric and tartaric acids, were measured. Three simplified kinetic models such as pseudo-first- and secondorder equations, and the intraparticle diffusion model were selected. This information will be useful for further applications of system design in the treatment of practical waste effluents. MATERIALS AND METHODS Adsorbent, reagents, and solutions Chitosan produced from lobster shell wastes was offered as flakes from Ying-Huah Co., Kaohsiung, Taiwan, without further purification. Prior to use as adsorbents, the raw flakes were ground and sieved into three particle size ranges, , , and mm, respectively. They were represented as the average diameter d p of 0.335, 0.505, and mm. The BET surface areas were found to be 13.5, 13.0, and 12.3 m 2 /g, respectively, based on N 2 adsorption isotherms using sorptometer (Porous Materials Inc., Model BET-202A). The degree of deacetylation of chitosan was obtained to be 80 mol% following the method of Guibal et al. (1994). The molar mass of chitosan was obtained to be by the Mark Houwink equation from viscosity measurements of solutions containing different amounts of chitosan in 100 mol/m 3 acetic acid and 200 mol/m 3 NaCl (Roberts and Domszy, 1982). The commercial-grade reactive dyes were purchased from Sumitomo Chemical Co. Ltd., Japan and used as received. They were Sumifix Super Scarlet 2 GF (Reactive Red 222, RR222), Sumifix Super Yellow 3 RF (Reactive Yellow 145, RY145), and Sumifix Super Navy Blue BF (Reactive Blue 222, RB222). They all had monochlorotriazine and vinyl sulfone bifunctional groups (Juang et al., 1997). CuSO 4, NaOH, EDTA, tartaric and citric acids were the products of Merck Co. as analytical-grade reagents. The aqueous solutions were prepared by dissolving the solutes in deionized water (Millipore Milli-Q) without ph adjustment. In the presence of complexing agents, the aim of the addition of NaOH was to increase the aqueous solubility of complexing agents, especially for EDTA. In this work, a molar concentration ratio of Cu(II), complexing agent, and OH of 1:0.5:1 was selected due to its relatively high adsorption ability of Cu(II) on chitosan (Tseng et al., 1999). Experimental procedures The batch contact-time experiments were made in a Pyrex glass vessel of 100 mm ID and 130 mm high, fitted with four glass baffles, 10 mm wide. An aqueous solution (0.6 dm 3 ) was poured and agitated by using a Cole Parmer Servodyne agitator with six blades, flat-bladed impeller (12 mm high, 40 mm wide). The maximum stirring speed was 500 rpm because above this the agitation has little effect on adsorption process. A known amount of dry chitosan was then added into the vessel and the timing was started. The vessel was immersed in a water bath controlled at 308C (Haake Model K-F3). At preset time intervals, the aqueous samples (5 cm 3 ) were taken and the concentration was analyzed. The concentrations of dyes were measured with an UV/visible spectrophotometer (Hitachi Model U-2000) and that of Cu(II) was analyzed with an atomic absorption spectrophotometer (Shimadzu Model AA68). Each experiment was at least duplicated under identical conditions. The amount of adsorption at time t, q t (g/kg or mol/kg), was obtained as follows: q t ¼ðC 0 C t Þ=m s ð1þ where C 0 and C t (g/m 3 or mol/m 3 ) are the liquid-phase concentrations of solutes at initial and any time t, respectively, m s is the dosage of adsorbent in the solution (kg/m 3 ). KINETIC MODELS OF ADSORPTION Pseudo-first- and second-order equations In order to examine the controlling mechanism of adsorption processes such as mass transfer and chemical reaction, several kinetic models are used to test experimental data. The large number and array of different chemical groups on chitosan chains (e.g., NH2, OH) imply that there are many types of chitosan solute interactions (Monteiro and Airoldi, 1999; Muzzarelli, 1983). It is probable that any kinetic or mass transfer representation is likely to be global. From a system design viewpoint, a lumped analysis of adsorption rates is thus sufficient to practical operation. A simple kinetic analysis of adsorption is the pseudo-first-order equation in the form (Ho and McKay, 1998, 1999): dq t =dt ¼ k 1 ðq e q t Þ ð2þ where k 1 is the rate constant of pseudo-first-order adsorption and q e denotes the amount of adsorption at equilibrium. After definite integration by applying the initial conditions q t ¼ 0att¼0 and q t ¼ q t at t ¼ t, equation (2) becomes logðq e q t Þ ¼ log q e k 1 2:303 t ð3þ In addition, a pseudo-second-order equation based on adsorption equilibrium capacity may be expressed in the form (Ho and McKay, 1998, 2000): dq t =dt ¼ k 2 ðq e q t Þ 2 ð4þ where k 2 is the rate constant of pseudo-second-order adsorption. Integrating equation (4) and applying the initial conditions, we have 1 ðq e q t Þ ¼ 1 þ k 2 t ð5þ q e or equivalently, t ¼ 1 q t k 2 q 2 þ 1 t ð6þ e q e It should be noted that, compared to equation (5), equation (6) has an advantage that k 2 and q e can be

3 Kinetic modeling of solute adsorption on chitosan 615 obtained from the intercept and slope of the plot of ðt=q t Þ vs. t and there is no need to know any parameter beforehand. Intraparticle diffusion model Because the above two equations cannot give definite mechanisms, another simplified model is tested. Intraparticle diffusion model used here refers to the theory proposed by Weber and Morris (1962). The fractional approach to equilibrium changes according to a function of ðdt=r 2 Þ 1=2, where r is the particle radius and D the diffusivity of solute within the particle. The initial rates of intraparticle diffusion can be obtained by linearization of the curve q t ¼ f ðt 1=2 Þ (Daifullah et al., 1997; Guibal et al., 1998). Previous studies showed that such plots may present a multi-linearity (McKay et al., 1980; McKay, 1983), which indicates that two or more steps occur. The first, sharper portion is the external surface adsorption or instantaneous adsorption stage. The second portion is the gradual adsorption stage, where the intraparticle diffusion is ratecontrolled. The third portion is final equilibrium stage where the intraparticle diffusion starts to slow down due to extremely low solute concentrations in the solution. A good correlation of the kinetic data in this model can justify the adsorption mechanisms (Jansson-Charrier et al., 1996). adsorption by about 15 30% after 240-min contact under comparable initial solute concentrations. When the aqueous phase contains metals and strong complexing agents, most metals especially transition metals readily form stable 1:1 complexes. Thus, the half of Cu(II) is complexed and the half is free ions under the solution condition of [Cu 2+ ]: [complexing agent]:[oh ]=1:0.5:1 encountered in this work. It is known that the uptake of transition metals on chitosan (RNH 2 ) is mainly effected via coordination with their unprotonated amino groups (Monteiro and Airoldi, 1999; Onsoyen and Skaugrud, 1990). Two OH groups and one NH 2 group are grabbed by Cu(II) and the fourth site is probably occupied by a water molecule or the OH group on the third carbon atom. Cu 2þ þ RNH 2, CuðRNH 2 Þ 2þ ð7þ On the other hand, the uptake of the complexed anions such as CuEDTA 2 on chitosan occurs via RESULTS AND DISCUSSION Time profiles of solute adsorption Figures 1 3 typically show the time profiles of liquid-phase concentrations ðc t =C 0 Þ for adsorption of three kinds of solutes, i.e., dye RR222, free Cu(II) ions, and Cu(II) ions in the presence of tartaric acid, respectively. Similar results are also observed for adsorption of other solutes (not shown). It is seen from Figs. 2 and 3 that the presence of tartaric acid in the aqueous phase enhances the amount of Cu(II) Fig. 2. Time profiles of liquid-phase concentrations of Cu(II) at different initial solute concentrations. Fig. 1. Time profiles of liquid-phase concentrations of RR222 at different dosages of chitosan. Fig. 3. Time profiles of liquid-phase concentrations of Cu(II) in the presence of tartaric acid at different initial solute concentrations.

4 616 Feng-Chin Wu et al. electrostatic interaction with their protonated amino groups (Wu et al., 1999). CuEDTA 2 þ 2RNH þ 3, CuEDTAðRNH 3Þ 2 ð8þ The different mechanisms of equations (7) and (8) can be experimentally supported from ph changes after the adsorption: the solution ph increases to a larger extent in the presence of complexing agents than their absence under comparable conditions such as initial ph and initial Cu(II) concentration (Tseng et al., 1999; Wu et al., 1999). Test of kinetic models As indicated above, the fitting validity of these models can be checked by each linear plot of log ðq e q t Þ vs. t, ðt=q t Þ vs. t, and q t vs. t 1=2, respectively. The values of q e used to fit the pseudo-first-order equation are taken from previous studies (Juang et al., 1997, 1999; Wu et al., 1999; Tseng et al., 1999). In order to quantitatively compare the applicability of each model, a normalized standard deviation Dq is calculated. sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi P 2 qt;exp q t;cal =qt;exp Dqð%Þ ¼100 ð9þ ðn 1Þ where n is the number of data points. Tables 1 and 2 list the calculated results. In addition, Figs 4 6 show the best-fitting results. It is found that the adsorption of reactive dyes RR222, RY145, and RB222, as well as free Cu(II) ions, can be best described by the intraparticle diffusion model. On the other hand, the adsorption of Cu(II) in the presence of EDTA, citric and tartaric acids follows the pseudo-second-order equation, although the fitting by intraparticle diffusion model is only slightly worse. Under the ranges studied, the values of Dq for the best-fit models are found to be less than 2.7%. Figure 7 typically illustrates the comparison between the calculated and measured results for adsorption of reactive dyes. It is seen that the pseudo-first-order equation overestimates and the pseudo-second-order equation underestimates at the initial stage of adsorption (about min), and conversely at the later stage. Comments on adsorption mechanisms The intraparticle diffusion model (Figs. 4 and 5) illustrates that the external surface adsorption (Stage 1) is less apparent for the adsorption of reactive dyes and uncomplexed Cu(II) ions. In general, the slope of the lines in each stage is termed as the rate parameter Table 1. Kinetic parameters and normalized standard deviations for adsorption of reactive dyes on chitosans at 308C (C 0 ¼ 200 g/m 3 ) Solute m s d p First order Second order Intraparticle diffusion (kg/m 3 ) (mm) k 1 (1/min) Dq (%) k 2 (kg/g min) q e (g/kg) Dq (%) k p;2 (g/kg min 1/2 ) Dq (%) RR RY RB Table 2. Kinetic parameters and normalized standard deviations for Cu(II) adsorption on chitosans at 308C (m s ¼ 1:0 kg/m 3 ) Solute C 0 First order Second order Intraparticle diffusion (mol/m 3 ) k 1 (1/min) Dq (%) k 2 (kg/g min) q e (g/kg) Dq (%) k p;2 (g/kg min 1/2 ) Dq (%) Cu(II) Cu(II)+Citrate Cu(II)+Tartrate Cu(II)+EDTA

5 Kinetic modeling of solute adsorption on chitosan 617 Fig. 4. Test of intraparticle diffusion model for adsorption of RR222 on different sizes of chitosan particles. Fig. 6. Test of pseudo second-order equation for adsorption of Cu(II) in the presence of citric acid at different initial solute concentrations. Fig. 5. Test of intraparticle diffusion model for adsorption of Cu(II) at different initial solute concentrations. Fig. 7. Typical plot of comparison between the measured and modeled time profiles for adsorption of reactive dyes. k p;i ði¼ stage numberþ. Rate parameter of the intraparticle diffusion control stage (that is, Stage 2), k p;2, for both types of solutes is also listed in Tables 1 and 2. As expected, k p;2 for adsorption of RR222 decreases with increase in the size of chitosan particles. In addition, k p;2 for adsorption of free Cu(II) ions increases with increasing C 0. On the other hand, literature review has shown that most adsorption studies reported can be represented as pseudo-first-order rate mechanism (Ho and McKay, 1998; Nassar, 1997). Of all the systems studied here, this model is restricted only to a limited range of reactions (not shown), as found earlier also (Sarkar and Chattoraj, 1993). For pseudo-second-order equation, a two-step linear relation is obtained although the range of first step is much shorter (Fig. 6). Because this equation is based on the adsorption capacity ðq e Þ, it predicts the behavior over the whole range of studies strongly supporting the validity, and agrees with chemisorption (chemical reaction) as rate-controlling mechanism (Ho and McKay, 1998, 2000). Unlike the absence of complexing agents, the reaction-controlling mechanism of Cu(II) adsorption in the presence of EDTA, citric or trataric acid on chitosan may be explained below. The amino groups of chitosan may be protonated according to RNH 2 þ H þ, RNH þ 3 ; K p ð10þ The protonation constant was reported to be log K p ¼ 6:3 (Muzzarelli, 1977). The amino groups of chitosan tend to be protonated in the present systems because the initial solution ph is about 3.43 (citric acid), 3.93 (tartaric acid), and 5.36 (EDTA), and is about 5.32 in the absence of complexing agents. This means that the two reactions of equations (7) and (10) compete (parallel reactions) for adsorption of free Cu(II) ions, but the two reactions of equations (10) and (8) occur consecutively (series reactions) for adsorption of the complexed anions. Although the common reactions caused by electrostatic interactions proceed very rapidly, the heterogeneous reactions between the complexed anions and the protonated amino sites on

6 618 Feng-Chin Wu et al. chitosan particle may be retarded within more hindered pore environments due to the steric and structural effects (Guibal et al., 1998). CONCLUSIONS Kinetics and mechanism of adsorption of reactive dyes RR222, RY145, and RB222, as well as Cu(II) ions in the absence and presence of EDTA, citric and tartaric acids from aqueous solutions on chitosan are studied at 308C. The following results are obtained: 1. Within the concentration ranges studied (dye, 200 g/m 3 ; Cu(II), 5.05 mol/m 3 ), the adsorption of RR222, RY145, and RB222, as well as free Cu(II), is best described by the intraparticle diffusion model. However, the adsorption of Cu(II) in the presence of complexing agents is best fitted by the pseudo-second-order equation. 2. The intraparticle diffusion model shows that the rate parameter k p,2 for adsorption of RR222 decreases with increasing the size of chitosan particles. Furthermore, k p,2 for adsorption of free Cu(II) increases with increasing initial solute concentration. 3. The reaction mechanism of Cu(II) adsorption in the presence of complexing agents may be due to the retarded electrostatic interactions occurring between the complexed Cu(II) anions and the protonated amino sites on chitosan within the more hindered pore environments. Acknowledgements}Support for this work by the National Science Council, ROC, under the Grant No. NSC E is gratefully acknowledged. REFERENCES Bailey S. E., Olin T. J., Bricka R. M. and Adrian D. D. (1999) A review of potentially low-cost sorbents for heavy metals. Water Res. 33, Chatzopoulos D., Varma A. and Irvine R. L. (1993) Activated carbon adsorption and desorption of toluene in the aqueous phase. AIChE J. 39, Daifullah A. E., El-Reefy S. and Gad H. (1997) Adsorption of p-nitrophenol on Inshas incinerator ash and on pyrolysis residue of animal bones. Adsorpt. Sci. Technol. 15, Eichenmuller B., Bunke G., Behrend K., Buchholz R. and Gotz P. (1997) Adsorption of acenaphthene on porous organic polymers. J. Environ. Eng. ASCE 123, El-Geundi M. S. (1997) Adsorbents for industrial pollution control. Adsorpt. Sci. Technol. 15, Guibal E., Milot C. and Tobin J. M. (1998) Metal-anion sorption by chitosan beads: equilibrium and kinetic studies. Ind. Eng. Chem. Res. 37, Guibal E., Saucedo I., Jansson-Charrier M., Delanghe B. and Le Cloirec P. (1994) Uranium and vanadium sorption by chitosan and derivatives. Water Sci. Technol. 30(9), Ho Y. S. and McKay G. (1998) A comparison of chemisorption kinetic models applied to pollutant removal on various sorbents. Trans. Inst. ChemEng. 76B, Ho Y. S. and McKay G. (1999) Comparative sorption kinetic studies of dyes and aromatic compounds onto fly ash. J. Environ. Sci. Health A34, Ho Y. S. and McKay G. (2000) The kinetics of sorption of divalent metal ions onto sphagnum moss peat. Water Res. 34, Jansson-Charrier M., Guibal E., Roussy J., Delanghe B. and Le Cloirec P. (1996) Vanadium(IV) sorption by chitosan: kinetics and equilibrium. Water Res. 30, Juang R. S., Tseng R. L., Wu F. C. and Lee S. H. (1997) Adsorption behavior of reactive dyes from aqueous streams on chitosan. J. Chem. Technol. Biotechnol. 70, Juang R. S., Wu F. C. and Tseng R. L. (1999) Adsorption removal of Cu using chitosan from a simulated rinse solution containing chelating agents. Water Res. 33, McKay G. (1983) Adsorption of dyestuffs from aqueous solutions using activated carbon. J. Chem. Technol. Biotechnol. 33A, McKay G., Otterburn M. S. and Sweeney A. G. (1980) The removal of color from effluent using various adsorbents- III. Silica: rate processes. Water Res. 14, Monteiro Jr. O. A. C. and Airoldi C. (1999) Some thermodynamic data on copper-chitin and copper-chitosan biopolymer interactions. J. Colloid Interface Sci. 212, Muzzarelli, R. A. A. (1977) Chitin, pp Pergamon Press, New York. Muzzarelli R. A. A. (1983) Chitin and its derivatives: new trends of applied research. Carbohydr. Polym. 3, Nassar M. M. (1997) The kinetics of basic dye removal using palm-fruit bunch. Adsorpt. Sci. Technol. 15, Onsoyen E. and Skaugrud O. (1990) Metal recovery using chitosan. J. Chem. Technol. Biotechnol. 49, Raven K. P., Jain A. and Loeppert R. H. (1998) Arsenite and arsenate adsorption on ferrihydrite: kinetics, equilibrium, and adsorption envelopes. Environ. Sci. Technol. 32, Roberts G. A. F. and Domszy J. G. (1982) Determination of viscometric constants for chitosan. Int. J. Macromol. 4, Sarkar D. and Chattoraj D. K. (1993) Activation parameters for kinetics of protein adsorption at silica water interface. J. Colloid Interface Sci. 157, Tseng R. L., Wu F. C. and Juang R. S. (1999) Effect of complexing agents on liquid-phase adsorption and desorption of copper(ii) using chitosan. J. Chem. Technol. Biotechnol. 74, Vidic R. D., Suidan M. T. and Brenner R. C. (1994) Impact of oxygen mediated oxidative coupling on adsorption kinetics. Water Res. 28, Weber, W. J. and Morris, J. C. (1962) Advances in water pollution research: removal of biologically resistant pollutants from waste waters by adsorption. In Proceedings of International Conference on Water Pollution Symposium. Vol. 2, pp Pergamon Press, Oxford. Wu F. C., Tseng R. L. and Juang R. S. (1999) Role of ph in metal adsorption from aqueous solutions containing chelating agents onto chitosan. Ind. Eng. Chem. Res. 38, Yang T. C. and Zall R. R. (1984) Adsorption of metals by natural polymers generated from seafood processing wastes. Ind. Eng. Chem. Prod. Res. Dev. 23, Zhou M. L. and Martin G. (1995) Adsorption kinetics modeling in batch reactor onto activated carbon by the model HSDM. Environ. Technol. 16,