ARTICLE pubs.acs.org/IECR
Removal Mechanisms of Toluene from Aqueous Solutions by Chitin and Chitosan Maryam Mohamed* and Sabeha Ouki* Centre for Environmental Health Engineering, University of Surrey, Guildford, Surrey, GU2 7XH, United Kingdom ABSTRACT: Low-cost materials of natural origin such as chitin and chitosan were found to effectively remove toluene from aqueous solutions. Batch adsorption experiments for the removal of toluene (5200 mg/L) were conducted to obtain the isotherm profile. The effects of various parameters such as initial concentrations, adsorbent dose, and contact time on the removal performance of toluene were evaluated. Chitosan shows better removal efficiency, compared to chitin, both in terms of the level of toluene adsorbed per unit mass of adsorbent and in terms of the adsorption kinetics. Adsorption capacity in the range of 33%58% efficiency was obtained when the chitosan materials were used. The adsorption data on toluene were found to best fit the Langmuir and RedlichPeterson isotherm models. The kinetic studies revealed that the adsorption of toluene followed the pseudo-secondorder rate model. Overall, the study demonstrated that chitosan is a potential adsorbent for the removal of toluene at concentrations as high as 200 mg/L.
1. INTRODUCTION Among the volatile organic compounds (VOCs), toluene is harmful to human health and the environment at very low concentrations. Toluene is mainly found in wastewater from petroleum refining industries and petrochemical manufacturing plants. Because of the substantial uses of petroleum products, this compound represents a hazard to the public health. Low concentrations of toluene were found in groundwater and surface water in a few micrograms per liter; however, accidental emissions may lead to higher concentrations.1 The presence of toluene contamination related to petroleum products is due to their high concentration in gasolines: 1.5% (v/v) of toluene, 1% (v/v) of benzene, 10 mg/mL.
dX ¼ Ks ðXe XÞ2 dt
ð12Þ
where Ks is the overall pseudo-second-order rate constant. The integration of eq 12 with the initial conditions of X = 0 at t = 0 leads to t 1 1 ¼ + t ð13Þ X Ks X e 2 Xe The initial sorption rate (h) at time t f 0 is defined as h ¼ Ks Xe 2
ð14Þ
The plot of t/X against t is presented in Figure 3. The adsorption capacity (Xe) is obtained from the slope of the line and the initial sorption rate (h), and Ks is obtained from the intercept. The R2 correlation coefficients that are presented in Table 2 are closer to unity for the pseudo-second-order rate equation than that for the pseudo-first-order rate equation; hence, the pseudo-secondorder rate model provides an accurate description for the sorption kinetic of toluene by both chitin and chitosan. The values of Ks and h are calculated from the plot in Figure 3 and are shown in Table 2. It can be observed that the value of the initial sorption rate h for the adsorption of toluene on chitosan is the highest. 9561
dx.doi.org/10.1021/ie200110t |Ind. Eng. Chem. Res. 2011, 50, 9557–9563
Industrial & Engineering Chemistry Research
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Table 2. Parameters of Kinetic Models for the Adsorption of Toluene by Chitin and Chitosana Pseudo-First-Order Rate Constant Model R2 Kf (h1)
adsorbent
linear
nonlinear
chitin
0.4333
0.7635
0.9891
chitosan
0.6128
0.9353
0.9823
Pseudo-Second-Order Rate Constant Model R2 adsorbent h (mg/(g h)) Ks (g/(mg h)) Xe (mg)
linear
nonlinear
chitin
1.4436
1.5411
0.9679
0.9993
0.9999
chitosan
2.1594
1.6620
1.1399
0.9991
0.9993
Bangham Model R2 adsorbent
k0 (g)
R
linear
nonlinear
chitin
0.0012
0.1616
0.9907
0.9626
chitosan
0.0015
0.1828
0.9308
0.9918
Intraparticle Diffusion Model
Figure 4. Bangham model for the removal of toluene by chitin and chitosan. (Conditions: temperature = 22 ( 1 °C, initial concentration of toluene = 50 mg/L, and adsorbent dosage = 15 g/L.)
where m is the amount of adsorbent used per liter of solution, V is the volume of solution, and k0 and R are constants (R < 1). The applicability of this model is tested to demonstrate whether it can simulate toluene sorption by chitin and chitosan. The linearity of the double logarithmic plot of eq 15 (see Figure 4) indicates that the diffusion of toluene into the pores is a controlling step in the sorption process. Values of the constants, k0 and R, are shown in Table 2. 3.3.4. Intraparticle Diffusion Model. The applicability of intraparticle diffusion model as a single rate-controlling step was tested using the Weber and Morris method:29
R2 kid (mg/(g h1/2))
I (mg/g)
linear
nonlinear
chitin
0.1606
1.3083
0.9670
0.9893
chitosan
0.1862
1.5931
0.8623
0.9792
adsorbent
a Conditions: temperature = 22 ( 1 °C, initial concentration of toluene = 50 mg/L, and adsorbent dosage = 15 g/L.
Figure 3. Pseudo-second-order rate model for the removal of toluene by chitin and chitosan. (Conditions: temperature = 22 ( 1 °C, initial concentration of toluene = 50 mg/L, and adsorbent dosage = 15 g/L.)
3.3.3. The Bangham Equation. Since the initial rapid adsorption reaction is followed by a slow stage of adsorption, the Bangham equation suggests that C0 k0 m + R log t log log ¼ log 2:303V C0 ð X=M Þm ð15Þ
X ¼ kid t 1=2 + I M
ð16Þ
where kid and I are constants. A plot of X/M against t1/2 gives a straight line (not shown), which means that the intraparticle diffusion is an appropriate model to describe the adsorption mechanism. The values of the intercept I are given in Table 2 and present the thickness of the boundary layer. Elevated intercept values indicate significant effects of the boundary layer. The straight lines obtained did not pass through the origin, indicating that the pore diffusion is not the only rate-controlling step in the removal of toluene, especially in the initial stages of the adsorption process. This is particularly true for toluene sorption by chitosan for which the intercept values are higher and the linearity coefficients are lower than those by chitin.
4. CONCLUSIONS In this study, chitin and chitosan, which are natural fishery waste materials, can be used as effective adsorbents for the removal of toluene from aqueous solutions. Higher toluene removal by chitin and chitosan is possible when the initial concentration of toluene was low in the aqueous solution. The Langmuir and RedlichPeterson isotherms presented the best fit of the equilibrium data. The optimum parameters of the adsorption process were determined. The results showed that the chitin and chitosan have considerable potential for toluene removal from aqueous solutions over a wide range of initial concentrations. The optimum adsorbent dose for removal of toluene by chitin and chitosan were determined to be 27 and 15 mg/mL, respectively, with an initial toluene concentration of 50 mg/L. The kinetics of the adsorption process indicates that a minimum contact time of 8 h is required to attain equilibrium 9562
dx.doi.org/10.1021/ie200110t |Ind. Eng. Chem. Res. 2011, 50, 9557–9563
Industrial & Engineering Chemistry Research conditions. Adsorption kinetics is best represented by a secondorder rate model. The adsorption mechanism favor chitosan to be used for the effective removal of toluene from wastewater.
’ AUTHOR INFORMATION Corresponding Author
*Tel.: +44 1483 686633. Fax: +44 1483 682135. E-mail addresses:
[email protected] (S.O.),
[email protected] (M.M.).
’ NOMENCLATURE a = Langmuir constant or the maximum adsorbent-phase concentration of adsorbate when surface sites are saturated with adsorbate (mg adsorbate/g adsorbent) aR = RedlichPeterson isotherm constant (L/mg)1/β b = Langmuir constant, or constant that is related to the free energy of adsorption B1 = Temkin isotherm constant BD = DubininRadushkevich isotherm constant bT = Temkin isotherm constant C0 = initial adsorbate concentration (mg/L) Ce = residual adsorbate concentration at equilibrium (mg/L) E = mean free energy of adsorption (kJ/g) h = initial sorption rate (mg/(g h)) I = intercept of intraparticle diffusion equation (mg/g) k0 = constant of the Bangham equation (g) kid = intraparticle diffusion rate constant (mg/(g h1/2)) K = Freundlich constant (mg/g)(mg/L)1/n) Kf = overall pseudo-first-order rate constant (h1) KR = RedlichPeterson isotherm constant (l/g) KT = Temkin isotherm constant (L/mg) Ks = overall pseudo-second-order rate constant (g/(mg h)) m = amount of adsorbent used per liter of solution (g/L) M = amount of solid adsorbent material used (g) n = Freundlich constant that indicates the intensity of the adsorption qd = DubininRadushkevich isotherm constant (mg/g) R = universal gas constant; R = 8.314 J/(mol K) R2 = correlation coefficient RL = dimensionless separation factor T = absolute temperature (K) t = time (h) V = volume of adsorbate solution (mL) X = amount of adsorbate adsorbed at time t (mg) Greek Letters
β = exponent in the RedlichPeterson equation R = constant in the Bangham equation
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