Kinetics and Mechanism of Tartrazine Adsorption onto Chitin and

Apr 27, 2012 - ABSTRACT: In this work the kinetics and mechanisms of tartrazine adsorption onto chitin and chitosan were studied at different pH value...
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Kinetics and Mechanism of Tartrazine Adsorption onto Chitin and Chitosan Guilherme L. Dotto,† Mery L. G. Vieira,† and Luiz A. A. Pinto*,† †

Unit Operation Laboratory, School of Chemistry and Food, Federal University of Rio Grande − FURG, Rio Grande, RS, Brazil ABSTRACT: In this work the kinetics and mechanisms of tartrazine adsorption onto chitin and chitosan were studied at different pH values (3, 5, 7, 9 and 11). Five models were employed to elucidate the adsorption kinetics. The adsorption mechanisms were verified by use of models based on mass transfer and elemental analysis. The adsorption capacity was increased with pH decrease, reaching maximum values at pH 3. The maximum values were 30 mg·g−1 and 350 mg·g−1 for chitin and chitosan, respectively. For both adsorbents a fast kinetic was observed, and the Avrami model was the more adequate to fitting the experimental data. Tartrazine adsorption onto chitin occurred only by film diffusion. In the chitosan case, adsorption occurred by film and intraparticle diffusion, however, intraparticle diffusion was the rate-limiting step. At pH 3, tartrazine adsorption onto chitin and chitosan occurred by chemical interactions.

1. INTRODUCTION The production of commercial dyes is estimated at 7 × 105 to 1 × 106 tons per year, and about 15% of the used dyes enter the environment through wastes.1 Dyes are considered to be particularly dangerous organic compounds for the environment because even a small quantity of dye in water can be toxic and highly visible.2 The food azo dye tartrazine appears to cause the most allergic and intolerance reactions of all the azo dyes. It is considered toxic and can act as a catalyst in hyperactivity and other behavioral problems; it may also cause asthma, migraines, eczema, thyroid cancer, and lupus.3−5 Thus, treatment of wastewater containing dyes is an important area of research.2,6−8 Dye wastewaters are very difficult to treat, since the dyes are recalcitrant molecules, resistant to aerobic digestion, and stable to oxidation agents.5,6 Dye wastewaters are usually treated by flocculation combined with flotation, electroflocculation, membrane filtration, electrokinetic coagulation, electrochemical destruction, ion-exchange, irradiation, precipitation, ozonation, and katox treatment method involving the use of activated carbon and air mixtures. However, these technologies are generally ineffective in color removal, expensive, and less adaptable to a wide range of dye wastewaters.2,6−9 In this context, adsorption is an alternative for color removal from dye wastewaters.2,6,7,9,10 The adsorption process has been reported as a good method to remove textile dyes from aqueous solutions,11−13 however, the removal of food dyes is little investigated. Many adsorbents have been used to remove dyes, such as activated carbon,14 solid wastes,13 modified activated clay,11 Spirulina platensis,15 and others.2,7,9 In recent years, chitin and chitosan have been used to remove dye from aqueous solutions.6,10,16−19 According to Wan Ngah et al.,8 the application of these biopolymers is one of the emergent adsorption methods for dye removal. These biopolymers are low-cost materials obtained from natural resources and their use as biosorbents show a good cost− efficiency ratio. In addition, their adsorption rates are high.6 In dye adsorption onto chitin and chitosan the kinetics and mechanisms are important for the process control because they © 2012 American Chemical Society

lead to information on the factors that affect the reaction rate, and the interactions that occur between the adsorbent and adsorbate.17,19−21 The pH is an important factor in adsorption mechanism affecting the surface charge of the adsorbent and the ionization degree of the material in the solution.6 According to Ruthven,22 an adsorption process can be described by the following four consecutive steps: transport of adsorbate in the bulk solution; external mass transfer across the film surrounding the adsorbent particles (film diffusion); migration of adsorbate within the adsorbent (pore diffusion and/or surface diffusion); and chemical reaction. This mechanism for the tartrazine adsorption onto chitin and chitosan is not found in the literature. This work aims to elucidate the kinetics and mechanism of tartrazine adsorption onto chitin and chitosan at different pH values (3, 5, 7, 9 and 11). Pseudo-first-order, pseudo-secondorder, Elovich, Avrami, and Bangham models were employed to analyze the kinetic behavior. The adsorption mechanism was studied by mass transfer models and energy dispersive spectroscopy (EDS) analysis.

2. EXPERIMENTAL SECTION 2.1. Production and Characterization of Chitin and Chitosan. Chitin and chitosan were obtained from shrimp (Penaeus brasiliensis) wastes. First, chitin was obtained by demineralization, deproteinization, deodorization, and drying steps. Chitosan in paste form was obtained by alkaline deacetylation of chitin followed by purification.23 Chitosan paste was dried to obtain a chitosan powder.24 The samples were ground (Wiley Mill Standard model 03, USA) and sieved until the discrete particle size ranged from 68 to 75 μm. The adsorbent samples were characterized according to deacetylation degree (infrared analysis) (Prestige 21, The Received: Revised: Accepted: Published: 6862

December 30, 2011 April 24, 2012 April 27, 2012 April 27, 2012 dx.doi.org/10.1021/ie2030757 | Ind. Eng. Chem. Res. 2012, 51, 6862−6868

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210045, Japan),25 molecular weight,25 scanning electron microscopy (SEM) (Jeol, JSM-6060, Japan),26 specific surface area, pore volume and average pore radius (BET method) (Quantachrome, Nova station A, USA).27 2.2. Adsorbate. The adsorbate used was the food azo dye tartrazine (FD&C yellow 5, color index 19140, molecular weight 534.4 g·mol−1, λmax 425 nm, pKa 9.4) (Plury chemical Ltd.a, Brazil). The chemical structure of the dye was optimized using ChemBio 3D 11.0.1 software. The chemical structure and optimized three-dimensional structural formula of tartrazine are shown in Figure 1a and b, respectively.. Distilled water was used to prepare all solutions. All reagents utilized were of analyticalgrade

where C0 is the initial dye concentration in liquid phase (mg·L−1), Ct is the dye concentration in liquid phase at time t (mg·L−1), m is adsorbent amount (g), and V is the volume of solution (L). 2.4. Kinetic Models. To investigate the adsorption kinetics of tartrazine onto chitin and chitosan, the models nominated pseudo-first-order, pseudo-second-order, Elovich, Avrami, and Bangham were fitted to the experimental data. The kinetic models of pseudo-first-order28 and pseudosecond-order29 are presented in eqs 2 and 3, respectively: qt = q1(1 − exp( − k1t ))

(2)

t (1/k 2q2 2) + (t /q2)

(3)

qt =

where qt is the adsorbate amount adsorbed at time t (mg·g−1), k1 and k2 are the rate constants of pseudo-first-order and pseudo-second-order models, respectively, in (min−1) and (g·mg−1·min−1), q1 and q2 are the theoretical values for the adsorption capacity (mg·g−1), and t is the time (min). The Elovich kinetic model can be described according to the eq 4:30

qt =

1 ln(1 + abt ) a

(4)

where “a” is the initial velocity due to dq/dt with qt = 0 (mg·g−1·min−1) and “b” is the desorption constant of the Elovich model (g·mg−1). An alternative kinetic equation was proposed by Avrami,31 based on the thermal decomposition, as shown in eq 5: qt = qAV (1 − exp( − kAVt )n )

Figure 1. (a) Chemical structure and (b) optimized three− dimensional structural formula of tartrazine.

where kAV is the Avrami kinetic constant (min−1), qAV is the Avrami theoretical values for the adsorption capacity (mg·g−1), and n is a fractionary reaction order which can be related to the adsorption mechanism. Kinetic data can be used to check whether pore diffusion is the only rate-controller step or not in the adsorption system through the Bangham equation (eq 6)32

2.3. Adsorption Experiments. The adsorbents (250 mg, dry basis) (preliminary tests showed that the adsorption of tartrazine was linear between 250 mg and 1 g) were added in 0.80 L of distilled water and had the pH corrected through the addition of 50 mL of buffer (pH 3, 5, 7, 9 and 11) which did not present interaction with the dye.16 The solutions were agitated for 30 min so that the pH reached equilibrium, and it was measured before and after the adsorption process (Mars, MB10, Brazil). Fifty mL of a solution containing 2 g·L−1 of dye was added to each solution, being completed to 1 L with distilled water, thus, the initial dye concentration was approximately 100 mg·L−1.17,18 The experimental runs were carried out in a jar test (Nova Ética, 218 MBD, Brazil), under constant agitation (100 rpm) and ambient temperature (298 ± 1 K). Aliquots were removed in preset time intervals (2, 4, 6, 8, 10, 15, 20, 25, 30, 40, 50, 60, 80, 100, 120, 180, 240, and 300 min) through filtration with Whatmann Filter Paper 40, which did not present interaction with the dye,16 and the dye concentration was determined by spectrophotometry (Quimis, Q108 DRM, Brazil) at 425 nm. All experiments were carried out in triplicate and blanks were performed. The adsorption capacity at time “t” (qt) was determined by eq 1 qt =

C0 − Ct V m

(5)

⎡ ⎛ ⎞⎤ ⎛ km ⎞ C0 ⎟⎟⎥ = log⎜ 0 ⎟ + α log(t ) log⎢log⎜⎜ ⎝ 2.303V ⎠ ⎢⎣ ⎝ C0 − qt m ⎠⎥⎦

(6)

where k0 (L·g−1) and α are Bangham constants. 2.5. Mechanism Study. In chitin/chitosan−dyes systems, the mechanism can be described in three consecutive steps: film diffusion, intraparticle diffusion, and chemical reaction.6,17,19 In this work, film diffusion and intraparticle diffusion were studied according to Crank equations.33 The chitin/chitosan−dye interactions were elucidated (in the more adequate condition) by energy dispersive spectroscopy (EDS) (Jeol, 5800, Japan).18 Crank,33 based on Fick law showed film diffusion and intraparticle diffusion models as presented in eqs 7 and 8 ⎛ Df ⎞0.5 = 6⎜ 2 ⎟ t 0.5 qe ⎝ πR P ⎠ qt

qt qe

(1) 6863

=1−

⎛ D π 2t ⎞ 6 exp⎜ − P 2 ⎟ 2 RP ⎠ π ⎝

(7)

(8)

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Figure 2. SEM photographs: (a) chitin sample ( ×250; 100 μm); (b) chitin sample ( ×1000; 10 μm); (c) chitosan sample ( ×500; 50 μm); and (d) chitosan sample ( ×3000; 5 μm).

where Df and DP are the film and intraparticle diffusivities (m2·min−1), Rp is the average radius of the adsorbent particle (m), and qe is the equilibrium adsorption capacity (mg·g−1). 2.6. Regression Analysis. The kinetic and mass transfer coefficients were determined by the fit of the models with the experimental data through nonlinear regression. The calculations were carried out by the Quasi-Newton estimation method using the Statistic 7.0 software (Statsoft, USA). The fit quality was measured through coefficient of determination (R2) and average relative error (ARE).

Table 1. Characteristics of Chitin and Chitosan Particles characteristics deacetylation degree (%) particle size (μm) surface area (m2·g−1) average pore radius (Å) pore volume (mm3·g−1) a

chitina 45 72 3.6 14.3 4.9

± ± ± ± ±

1 3 0.1 0.2 0.1

chitosana 85 72 4.3 33.1 9.5

± ± ± ± ±

1 3 0.1 0.2 0.1

Mean ± standard error in triplicate.

curves of adsorption capacity as a function of time. Figure 3a, b shows the tartrazine adsorption capacity onto chitin and chitosan. In Figure 3a, it can be observed that in all pH values, the adsorption of tartrazine onto chitin was a very fast process, reaching the maximum adsorption capacity in about 10−15 min. In the same way, the adsorption of tartrazine onto chitosan was fast, reaching about 95% of saturation in 80 min. After, the adsorption rate decreased considerably. Fast kinetics is characteristic in dye−chitin/chitosan systems and is desirable in wastewater treatment because high adsorption capacities are reached in short times. Similar behavior was obtained by Dotto and Pinto,19 in the adsorption of acid blue 9 and food yellow 3 onto chitosan. They found about 85% of saturation in 60 min. Akkaya et al.21 in the adsorption of reactive yellow 2 and reactive black 5 onto chitin found about 90% of saturation in 50 min.

3. RESULTS AND DISCUSSION 3.1. Characterizations of Chitin, Chitosan, and Dye. The chitin SEM photographs are shown in Figure 2a, b, and chitosan SEM photographs are shown in Figure 2c, d. In Figure 2a, b, it can be observed that chitin presented a typically rigid and nonporous surface. On the other hand, Figure 2c, d show that the surface of chitosan was a typically wrinkled polymeric network with irregular pores. In addition, a heterogeneous surface area and porous internal structure were observed. Table 1 shows the characteristics of chitin and chitosan particles. It can be observed in Table 1 that chitosan presented higher values of surface area, pore radius, and pore volume in relation to chitin. Chitin and chitosan presented molecular weight values of 150 ± 5 kDa. 3.2. Adsorption Kinetics. The adsorption kinetics of tartrazine onto chitin and chitosan were measured through the 6864

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adsorption capacity values ten times superior for chitosan in relation to chitin. The pseudo-first-order, pseudo-second-order, Elovich, Avrami, and Bangham models were employed to elucidate the adsorption kinetics and to obtain information about the mechanism of tartrazine adsorption onto chitin and chitosan. Kinetic parameters of tartrazine adsorption onto chitin and chitosan are shown in Table 2. It was observed in Table 2 that in the tartrazine adsorption onto chitin, the pseudo-first-order model showed good fit with the experimental data (R2 > 0.98 and ARE < 7.00%). Pseudofirst-order model assumes that the adsorption occurs due to a concentration difference between adsorbate surface and solution, and this occurs only when external mass transfer coefficient controls the process. 4 Then, the tartrazine adsorption onto chitin was controlled by external mass transfer coefficient. In spite of a good fit of pseudo-first-order model, it was observed that the Avrami model showed best fit with experimental data (R2 > 0.99 and ARE < 5.00%). The n values of Avrami model (near 1) indicated that the tartrazine adsorption onto chitin is more approximated to the firstorder kinetic. The low values of R2 to the Bangham model (Table 2) confirmed that the tartrazine adsorption onto chitin was not controlled by pore diffusion. In relation to the tartrazine adsorption onto chitosan, the pseudo-second-order model presented good fit with the experimental data (R2 > 0.98 and ARE < 6.00%) (Table 2). Pseudo-second-order model had the same equation for internal and external mass transfer mechanism.29 This suggests that tartrazine adsorption onto chitosan occurred by internal and external mass transfer mechanisms. Avrami model showed best fit with experimental data (R2 > 0.99 and ARE < 3.00%), but the n values (near 2) indicated that tartrazine adsorption onto chitosan is more approximated to the second-order kinetics. The low values of R2 to the Bangham model (Table 2) demonstrated that tartrazine adsorption onto chitosan was not controlled only by pore diffusion. Because the Avrami model was the best to represent the adsorption experimental data for chitin and chitosan (Table 2), the values of kAV were compared. It was observed that the kAV values for chitin were higher than kAV values for chitosan. This shows that the adsorption onto chitin was faster than adsorption onto chitosan. 3.3. Adsorption Mechanism. To identify the mass transfer steps in the tartrazine adsorption onto chitin and chitosan, the adsorption capacities were plotted as a function of square root of time.35 According to Weber and Morris,35 the plot qt versus t1/2 shows multilinearity, and each portion represents a distinct mass transfer mechanism. Figure 4 shows the Weber and Morris plots for tartrazine adsorption onto chitin and chitosan. In the tartrazine adsorption onto chitin (Figure 4a), an initial portion relative to the boundary layer diffusion (film diffusion) was observed until 15−20 min. After this time, the equilibrium was achieved. Then the mass transfer step in tartrazine adsorption onto chitin was the film diffusion.35 This occurred because chitin showed a rigid and nonporous surface (Figure 2a) and low values of surface area, average pore radius, and pore volume (Table 1). In addition, the dye molecular size is higher than average pore radius of chitin (Figure 1), inhibiting the internal diffusion. For tartrazine adsorption onto chitosan (Figure 4b), linear forms can be observed with two distinct phases. The initial portion relates to the boundary layer diffusion (film diffusion),

Figure 3. Kinetic curves for tartrazine adsorption onto (a) chitin and (b) chitosan (● pH 3; ■ pH 5; ◆ pH 7; ▲ pH 9; * pH 11).

The pH decrease from 11 to 3 caused an increase from 10 to 30 mg·g−1 in the adsorption capacity of tartrazine onto chitin (Figure 3a), and from 30 to 350 mg·g−1 in the adsorption capacity of tartrazine onto chitosan (Figure 3b). This occurred because, under acidic conditions, hydrogen atoms (H+) in the solution could protonate the amine groups (−NH2) of chitin and chitosan.34 In addition, tartrazine was dissolved and its sulfonate groups were dissociated, so the adsorption process then occurred due to the electrostatic interactions between dye sulfonated groups and chitin/chitosan protonated amino groups. The pH decrease caused protonation of more chitin/ chitosan amino groups, increasing adsorption sites, and consequently increasing the adsorption capacity. Similar effect was found by Piccin et al.16 in the adsorption of FD&C Red 40 onto chitosan. In their work, the pH decrease from 7.4 to 5.7 caused an increase in the adsorption capacity from 90 to 360 mg·g−1. In Figure 3a and b it can be observed that chitosan showed higher values of adsorption capacity in relation to chitin. This occurred because chitosan presented more basic nitrogen centers than chitin (Table 1), increasing the adsorption sites, and consequently increasing the adsorption capacity. In addition, chitosan presented higher values of surface area, pore radius, and pore volume in relation to chitin (Table 1). This behavior was observed by Prado et al.20 in the adsorption of indigo carmine onto chitin and chitosan. They found 6865

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Table 2. Kinetic Parameters of Tartrazine Adsorption onto Chitin and Chitosan chitin pH pseudo-first-order q1 (mg·g−1) k1 (min−1) R2 ARE (%) pseudo-second-order q2 (mg·g−1) k2 × 103 (g·mg−1·min−1) R2 ARE (%) Elovich a (g·mg−1) b (mg·g−1·min−1) R2 ARE (%) Avrami qAV (mg·g−1) kAV (min−1) n R2 ARE (%) Bangham k0 (L·g−1) A R2 ARE (%)

3

5

7

30.5 0.297 0.994 1.56

20.3 0.223 0.986 6.16

17.1 0.279 0.996 1.07

31.8 17.6 0.951 5.69

21.4 16.9 0.931 11.39

0.45 24096.1 0.831 11.29

chitosan 9

11

3

5

7

9

11

14.2 0.319 0.988 3.48

10.1 0.237 0.983 2.76

325.1 0.170 0.882 10.55

222.3 0.231 0.873 9.88

184.4 0.228 0.945 6.21

66.2 0.094 0.934 7.59

32.2 0.088 0.914 7.35

17.8 28.9 0.956 4.11

14.9 39.9 0.925 7.51

10.6 41.5 0.961 3.99

350.6 0.7 0.987 5.37

236.6 1.6 0.987 5.24

196.3 1.8 0.997 1.68

72.3 1.8 0.997 3.36

35.3 3.6 0.993 3.44

0.48 484.2 0.782 20.57

0.74 5336.1 0.862 10.14

1.00 22287.1 0.801 12.56

1.12 907.9 0.876 9.90

0.02 1161.5 0.981 3.47

0.04 3787.6 0.978 3.91

0.05 2612.1 0.969 5.69

0.09 44.8 0.941 9.43

0.19 18.6 0.943 10.11

30.4 0.43 1.17 0.999 1.27

20.0 0.42 1.23 0.999 4.36

17.1 0.41 0.92 0.999 0.97

14.1 0.46 1.32 0.999 1.21

10.2 0.54 0.77 0.999 2.61

369.4 0.41 1.64 0.999 1.55

247.1 0.33 1.55 0.999 2.40

196.0 0.37 1.58 0.999 2.40

69.2 0.18 1.68 0.999 2.62

33.5 0.17 1.70 0.999 2.66

0.020 0.07 0.822 11.63

0.012 0.10 0.755 22.22

0.011 0.08 0.856 10.79

0.010 0.07 0.788 12.90

0.006 0.09 0.852 10.81

0.161 0.14 0.965 5.51

0.127 0.11 0.962 5.16

0.105 0.11 0.942 7.07

0.025 0.19 0.892 14.08

0.012 0.20 0.892 15.25

chitin/chitosan amino groups were easily protonated, consequently, the electrostatic attraction between these groups was increased, facilitating the mass transfer in the boundary layer. Similar values of film diffusivity were obtained by Dotto and Pinto19 in the adsorption of acid blue 9 and food yellow 3 onto chitosan. In the tartrazine adsorption onto chitosan, the pH decrease caused an increase in the intraparticle diffusivity (Dp) (Table 3). This behavior shows that at low pH values, the dye molecules are diffused more easily in chitosan polymeric network. This behavior can be explained based in chitosan swelling effect.6 In basic conditions, a limited swelling of the chitosan particles occurs, inhibiting the diffusion of dyes. On the other hand, in acidic conditions, the swelling effect is more accentuated, causing expansion of the porous structure of the network, and facilitating the dye molecules diffusion. Similar behavior was obtained by Chiou and Li36 in the adsorption of reactive red 189 onto cross-linked chitosan beads. Comparing the film diffusivities with the intraparticle diffusivities (Table 3) it can be observed that in all pH values, the process was controlled by intraparticle diffusion (Df > DP). The results of EDS analysis from an average of scanned surfaces of chitin and chitosan before and after adsorption process are shown in Table 4. It can be observed in Table 4 that the major elements of chitin and chitosan before adsorption process are C, N, and O. After adsorption process a low increase in C and the appearance of S can be observed. This occurred due to the trapped dye molecules which contain aromatic rings and sulfonic groups, indicating the chemical interactions chitin−dye and chitosan−dye. Similar behavior was observed by others researchers.6,10,16,18

and the second portion describes the gradual adsorption step, where the intraparticle diffusion is the rate-limiting step.35 Until 300 min the equilibrium was not achieved. Figure 4b shows that film diffusion and intraparticle diffusion were simultaneously occurring during the adsorption process, because chitosan presented a porous internal structure (Figure 2b) and higher values of area, average pore radius, and pore volume than chitin (Table 1). In addition, the dye molecular size is lower than average pore radius of chitosan (Figure 1), and it facilitates the dye molecules diffusion within the particle. To estimate the film diffusivity (Df) and the intraparticle diffusivity (DP), experimental data relative to first portion of Weber−Morris plot were fitted with the film diffusion model (eq 6), and experimental data of second portion were fitted with eq 7. Table 3 shows film and intraparticle diffusivity values, coefficients of determination (R2), and average relative error (ARE). In Table 3 it can be observed that the film diffusion model showed a good fit with experimental data relative to the first portion of the Weber−Morris plot, and the intraparticle model showed a good fit with experimental data of second portion (R2 > 0.97 and ARE < 5.00%). Thus, film and intraparticle diffusivities were estimated. For chitin, the intraparticle diffusivity was not estimated, because the process occurred only by external mass transfer. Table 3 shows that in the tartrazine adsorption onto chitin and chitosan, the pH decrease caused an increase in the film diffusivity (Df). This suggests that the pH decrease led to an increase in adsorption velocity, and consequently, the contribution of external mass transfer mechanism is decreased. This occurred because at low pH values, the tartrazine sulfonated groups were more rapidly dissociated. In addition, 6866

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Table 4. EDS Analysis of Chitin and Chitosan Before and After Adsorption element (% weight)a chitin chitosan chitin−dye chitosan−dye a

C

N

O

S

73.15 67.35 75.45 73.85

17.10 25.45 11.65 8.55

9.75 7.20 10.65 13.05

0.00 0.00 2.25 4.55

Mean values obtained from analysis of five surfaces.

mechanism study showed that the tartrazine adsorption onto chitin occurred only by film diffusion. In the chitosan case, adsorption occurred by film and intraparticle diffusion, however, intraparticle diffusion was the rate-limiting step. The pH decrease caused an increase in the film diffusivity (Df) and intraparticle diffusivity (Dp). The EDS analysis suggested that the tartrazine adsorption onto chitin and chitosan occurred by chemical interactions.

■

AUTHOR INFORMATION

Corresponding Author

*Tel.: +55 53 3233 8648. Fax: +55 53 3233 8745. E-mail: [email protected]. Mail: FURG, 475 Engenheiro Alfredo Huch Street, 96203-900, Rio Grande, RS, Brazil. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS We thank CAPES (Brazilian Agency for Improvement of Graduate Personnel) and CNPq (National Council of Science and Technological Development) for the financial support.

■

Figure 4. Weber-Morris plots for tartrazine adsorption onto (a) chitin and (b) chitosan (● pH 3; ■ pH 5; ◆ pH 7; ▲ pH 9; * pH 11).

REFERENCES

(1) Koprivanac, N.; Kusic, H. Hazardous Organic Pollutants in Colored Wastewaters; New Science Publishers: New York, 2009. (2) Salleh, M. A. M.; Mahmoud, D. K.; Abdul Karim, W. A. W.; Idris, A. Cationic and Anionic Dye Adsorption by Agricultural Solid Wastes: A Comprehensive Review. Desalination 2011, 280, 1. (3) Mittal, A.; Mittal, J.; Kurup, L. Adsorption Isotherms, Kinetics and Column Operations for the Removal of Hazardous Dye, Tartrazine from Aqueous Solutions using Waste Materials: Bottom Ash and De-Oiled Soya as Adsorbents. J. Hazard. Mater. 2006, B136, 567. (4) Amin, K. A.; Abdel Hameid, H., II; Abd Elsttar, A. H. Effect of food azo dyes tartrazine and carmoisine on biochemical parameters related to renal, hepatic function and oxidative stress biomarkers in young male rats. Food Chem. Toxicol. 2010, 48, 2994.

4. CONCLUSION Tartrazine adsorption onto chitin and chitosan was a fast process reaching high adsorption capacities in short times. For both biopolymers the pH decrease caused an increase in adsorption capacity, being that the maximum values were 30 and 350 mg·g−1 for chitin and chitosan, respectively, obtained at pH 3. Tartrazine adsorption onto chitin occurred by external mass transfer and tartrazine adsorption onto chitosan occurred by external and internal mass transfer. The Avrami model was the most adequate to represent tartrazine adsorption onto chitin and chitosan (R2 > 0.99 and ARE < 5.00%). The

Table 3. Mass Transfer Parameters of the Tartrazine Adsorption onto Chitin and Chitosan film diffusion model

intraparticle diffusion model

adsorbent

pH

Df × 1013 (m2·min−1)

R2

ARE (%)

DP × 1013 (m2·min−1)

R2

ARE (%)

chitin

3 5 7 9 11 3 5 7 9 11

172.04 146.68 88.72 70.75 54.73 9.22 7.26 4.17 2.88 2.32

0.987 0.975 0.988 0.995 0.993 0.975 0.984 0.992 0.983 0.973

2.12 4.76 2.98 0.95 3.45 1.23 4.86 2.99 3.89 4.13

1.21 1.12 0.94 0.76 0.56

0.994 0.997 0.999 0.993 0.999

1.43 4.34 2.89 3.95 4.12

chitosan

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dx.doi.org/10.1021/ie2030757 | Ind. Eng. Chem. Res. 2012, 51, 6862−6868