Use of cellulose-based wastes for adsorption of dyes from aqueous solutions

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Use of cellulose-based wastes for adsorption

of dyes from aqueous solutions

Gurusamy Annadurai

a

, Ruey-Shin Juang

b,∗

, Duu-Jong Lee

a

a_{Department of Chemical Engineering, National Taiwan University, Taipei 106, Taiwan, ROC} b_{Department of Chemical Engineering, Yuan Ze University, Chung-Li 320, Taiwan, ROC} Received 26 October 2001; received in revised form 16 January 2002; accepted 17 January 2002

Abstract

Low-cost banana and orange peels were prepared as adsorbents for the adsorption of dyes from aqueous solutions. Dye concentration and pH were varied. The adsorption capacities for both peels decreased in the order methyl orange (MO) > methylene blue (MB) > Rhodamine B (RB) > Congo red (CR) > methyl violet (MV) > amido black 10B (AB). The isotherm data could be well described by the Freundlich and Langmuir equations in the concentration range of 10–120 mg/l. An alkaline pH was favorable for the adsorption of dyes. Based on the adsorption capacity, it was shown that banana peel was more effective than orange peel. Kinetic parameters of adsorption such as the Langergren rate constant and the intraparticle diffusion rate constant were determined. For the present adsorption process intraparticle diffusion of dyes within the particle was identified to be rate limiting. Both peel wastes were shown to be promising materials for adsorption removal of dyes from aqueous solutions. © 2002 Elsevier Science B.V. All rights reserved.

Keywords: Adsorption; Dyes; Banana and orange peels; Isotherms; Kinetics

1. Introduction

Many industries often use dyes and pigments to color their products. Most dyes are inert and non-toxic at the concentration discharged into the receiving water, however, they impart color undesirable to the water user. Color removal from textile effluents is a major environmental problem because of the difficulty to treating such streams by conventional physicochemical and biological treatment methods [1]. Liquid-phase adsorption has been shown to be an effective way for removing suspended solids, odors, organic matter, and oil from aqueous solutions. Nassar and El-Geundi [2] evaluated the cost of dye removal

∗_{Corresponding author. Tel.:}_{+886-3-4636800x555; fax: +886-3-4559373.}

E-mail address: [email protected] (R.-S. Juang).

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2.1. Adsorbents and dyes

Banana and orange peels were obtained from a local fruit stall at Chung-Li, Taiwan. The peels were dried, crushed, and washed thoroughly with deionized water (Milli-Q, Millipore) to remove the adhering dirt. They were air dried in an oven at 100–120◦C for 24 h. After drying, the adsorbent was sieved through a 5 mm mesh size. The densities of banana and orange peels were 1.72 and 1.47 g/ml, respectively. The BET surface areas of both peels were in the range 20.6–23.5 m2/g obtained from N2adsorption isotherms by sorptiometer (Quantachrome Co., NOVA 2000). All dyes were obtained from Merck Co. The solution pH was adjusted by adding a small amount of 0.1 M HCl or NaOH.

2.2. Adsorption studies

Adsorption equilibrium experiments were carried out by adding the dried adsorbent (0.1 g) in 100 ml of dye solution with desired concentration and pH at 30◦C in a shaker at 180 rpm. After 24 h the suspension was filtered and the final concentration of dye in solution was measured using an UV–VIS spectrophotometer (Hitachi U-2000). The amount of dye adsorbed onto the peels, qe(mg/g), was calculated by a mass balance relationship (Eq. (1)).

qe= (C0− Ce)V

W (1)

where C0 and Ce are the initial and equilibrium liquid-phase concentrations of dyes, respectively (mg/l), V the volume of the solution (L), and W the weight of the dry peel used (g).

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The procedures of kinetic experiments were basically identical to those of equilibrium tests. The aqueous samples were taken at preset time intervals, and the concentrations of dyes were similarly measured.

3. Results and discussion 3.1. Adsorption isotherms

Figs. 1 and 2 show the adsorption isotherms of different dyes (qeversus Ce) using banana and orange peels, respectively. Two isotherm equations are tested in this work. One is the Langmuir equation (Eq. (2)), which has been successful applied to many adsorption processes [10–15]. 1 qe = 1 qmon + 1 KLqmon 1 Ce (2) where KLis the Langmuir constant and qmonthe amount of dye adsorbed when the saturation is attained. A plot of 1/qeversus 1/Cegives KLand qmonif the isotherm follows the Langmuir equation. Table 1 lists the parameters calculated. The present adsorption capacities (qmon) are smaller than those obtained using activated carbons, mainly because their BET surface areas are significantly different (21–24 m2/g versus 600–1000 m2/g) [1,2].

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Fig. 2. Adsorption isotherms of dyes using orange peel.

The Freundlich equation [14,15], which is also often used for heterogeneous surface energy systems. lnqe= ln KF+ 1 n lnCe (3)

The intercept KFobtained from the plot of log qeversus log Ceis roughly a measure of the sorption capacity and the slope (1/n) of the sorption intensity (Table 2). It was indicated by that magnitude of the term (1/n) gives an indication of the favorability and capacity of the adsorbent/adsorbate systems [15].

Table 1

Parameters obtained for the Langmuir equation

Dye Banana peel Orange peel

qmon(mg/g) KL(l/mg) R qmon(mg/g) KL(l/mg) R MO 21.0 11.4 0.9340 20.5 16.5 0.9541 MB 20.8 16.5 0.9524 18.6 19.9 0.9659 RB 20.6 32.6 0.9572 14.3 32.0 0.9377 CR 18.2 37.7 0.9480 14.0 46.8 0.9739 MV 12.2 29.1 0.9616 11.5 56.7 0.9794 AB 6.5 17.4 0.9632 7.9 59.2 0.9747

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Table 2

Parameters obtained for the Freundlich equationa

KF 1/n R KF 1/n R MO 1.73 0.26 0.9861 2.20 0.17 0.9401 MB 1.34 0.33 0.9851 1.75 0.26 0.9519 RB 0.39 0.47 0.9644 1.01 0.39 0.9399 CR 0.05 0.54 0.9589 0.65 0.44 0.9820 MV 1.08 0.73 0.9867 0.24 0.59 0.9566 AB 1.26 0.66 0.9687 0.65 0.99 0.9470 a_{Unit of KF}_{was (mg/g)(mg/l)}n_.

Based on the correlation coefficient (R) shown in Tables 1 and 2, the adsorption isotherms with banana peel can be slightly better described by the Freundlich equation, and by the Langmuir equation in the case of organic peel. The fit of the data to the Freundlich equation may indicate the heterogeneity of the adsorbent surface.

3.2. Effect of solution pH on adsorption capacity

The effect of solution pH on the adsorption of dyes by banana and orange peels was shown in Figs. 3 and 4. For banana peel, the amount of adsorption increases when the pH

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Fig. 4. Effect of pH on adsorption of dyes using orange peel (C0= 100 mg/l)

is increased. The adsorption capability reaches maximum at pH 6–7, but decreases when pH is increased further. Similar results of pH effect were also reported for the adsorption of dyes on banana peel [16] and biological waste slurry [17], and even of Ni2+on orange peel [18]. In the case of orange peel (Fig. 4), however, the amount of dye adsorbed reaches a plateau at pH> 7. Under the conditions tested in Figs. 3 and 4, the highest amounts of adsorption are 17.2 (MO), 15.9 (MB), 13.2 (RB), 11.2 (CR), 7.9 (MV), and 7.9 (AB) mg/g using banana peel, as well as 15.8 (MO), 13.9 (MB), 9.1 (RB), 7.9 (CR), 6.1 (MV), and −3.8 (AB) mg/g using orange peel.

Solution pH would affect both aqueous chemistry and surface binding-sites of the ad-sorbents. At lower pH, H+ may compete with dye ions for the adsorption sites of both peel wastes, thereby inhibiting the adsorption of dyes. Actually, the zeta potentials of the adsorbent suspensions were measured (Malvern Zetasizer 3000 system). From pH 2.0 to 8.0, the zeta potentials of original banana and orange peels decline from−8.2 to −62.4 mV and from−3.2 to −44.5 mV, respectively (not shown). The sharper change in zeta potential for banana peel than orange peel may explain their different pH trends of dye adsorption.

The SEM images of original banana and orange peels (Fig. 5a and b) show that the pores within the peel particles are highly heterogeneous. However, this is not the case after adsorption (Fig. 5c and d). After dye adsorption, a significant change is observed in structure of the peels. The peels appear to have a rough surface with crater-like pores because they are partially covered by dye molecules.

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Fig. 5. SEM images for banana and orange peels: (a) original banana peel; (b) original orange peel; (c) banana peel after adsorption and (d) orange peel after adsorption.

3.3. Adsorption kinetics

Figs. 6 and 7 show that the amount of dye adsorption increases with time and it remains constant after a contact time of about 65 min (i.e. the equilibrium time). The equilibrium time is independent of initial dye concentration. The time profile of dye uptake is a single, smooth, and continuous curve leading to saturation, suggesting the possible monolayer coverage of dyes on the surface of the adsorbent.The Langergren equation, a pseudo-first-order equation, describes the kinetics of adsorption process as follows [4,19].

log(qe− q_t) = logqe−

k1 2.303

t (4)

where q_tis the amount of dye adsorbed (mg/g) at any time, and k1is the rate constant. The plot of log (qe−qt) versus t indicates that such first-order rate expression is not so valid to the present systems (not shown, correlation coefficient<0.91). However, the values of k1 calculated are still listed in Table 3 for reference.

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Fig. 6. Time profiles of solid-phase concentrations of dyes using banana peel (C0= 50 mg/l).

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Table 3

Parameters obtained for different dyes

k1(min−1) kp(mg/(g min1/2)) k1(min−1) kp(mg/(g min1/2))

MO 0.39 2.42 0.40 1.68

MB 0.35 2.12 0.29 1.20

RB 0.19 1.70 0.21 0.81

The data of solid-phase dye concentrations against time at an initial dye concentration of 20 mg/l were further processed for testing the role of diffusion (as the rate controlling step) in the adsorption process. Adsorption process incorporates the transport of adsorbate from bulk solution to the interior surface of the pores [20]. There is a possibility that the transport of dyes from the solution into the pores of the adsorbent is rate controlling in batch experiments with rapid stirring. The rate parameters for intraparticle diffusion (kp) for different dyes are determined using the following equation [21].

qt = kpt1/2 (5)

where kpis the intraparticle diffusion rate constant.

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Fig. 9. Test of intraparticle diffusion model for adsorption of dyes using orange peel.

Due to mass transfer effects, the shape of q_t versus t1/2 plot is curved at a small time limit [20]. All the plots have the same general features, initial curved portion followed by linear portion and a plateau. The initial curved portion is attributed to the bulk diffusion and the linear portion to the intraparticle diffusion. These phenomena have been reported for the adsorption of Cd2+on waste Fe(III)/Cr(III) hydroxide [22] and on thermally activated electroplating sludge [23]. At a certain time limit (about 20 min), the curves reveal a linear characteristic that the intraparticle diffusion controls the adsorption process (Figs. 8 and 9). The values of kpare obtained from the slope of the straight lines, and are listed in Table 3. According to the acceptable adsorption capacity (based on per specific surface area) and rate, the present low-cost peel wastes are expected to be attractive for adsorption removal of color from aqueous streams. Unlike activated carbons, however, thermal regeneration of the peel wastes is not feasible. Thus, the spent adsorbents may be post-treated with other solid wastes such as activated sludge.

4. Conclusion

Equilibrium and kinetic studies were made for the adsorption of dyes from aqueous solutions onto banana and orange peels in the concentration range 10–120 mg/l at 30◦C. The Freundlich equation showed a somewhat better fit than does the Langmuir equation for

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adsorption of dyes using banana peel, but exactly reversed using orange peels. Under the conditions tested (C0 = 100 mg/l, dosage of adsorbent 1 g/l), the amounts of adsorption were maximized at pH 6–7 up to 17.2 (MO), 15.9 (MB), 13.2 (RB), 11.2 (CR), 7.9 (MV), and 7.9 (AB) mg/g using banana peel, and at pH> 7 up to 15.8 (MO), 13.9 (MB), 9.1 (RB), 7.9 (CR), 6.1 (MV), and−3.8 (AB) mg/g using orange peel. The intraparticle diffusion of dye molecules within the particles was found to be rate controlling in these adsorption processes after 20 min contact. The present work revealed that the waste banana and orange peels were the promising materials for the removal of dyes from aqueous solutions.

Acknowledgements

Financial support for this work by the National Science Council, ROC, under Grant NSC 89-2211-E-002-008, is gratefully appreciated.

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