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Latin American applied research

Print version ISSN 0327-0793On-line version ISSN 1851-8796

Lat. Am. appl. res. vol.43 no.1 Bahía Blanca Jan. 2013

 

Use of rice hull as a biomass sorbent for organic pollutant removal: adsorption isotherms and thermodynamic

E. L. Foletto, D.S. Paz, A. Cancelier, G.C. Collazzo, M.A. Mazutti and S. L. Jahn

Chemical Eng. Department, Federal University of Santa Maria, Santa Maria-RS, 97105-900, Brazil. E-mail: efoletto@gmail.com

Abstract— The present study describes the adsorption of textile dye on rice hull. The adsorption parameters were obtained from Langmuir and Freundlich isotherms utilizing linear regression plots to fit the data. The results showed that Langmuir equation was more suitable than the Freundlich one to predict the experimental adsorption isotherms, indicating that the adsorption of dye molecules is localized in a monolayer on the surface of the adsorbent. Thermodynamic parameters such as ΔHa, ΔSa and ΔGa were determined in this work. The heat evolved during adsorption is endothermic. The maximum sorption capacity of rice husk was satisfactory when compared to capacities of other low-cost adsorbent materials.

Keywords— Adsorption; Dye; Rice Hull; Isotherm; Thermodynamic.

I. INTRODUCTION

The discharge of dyes in aquatic environment generates severe pollution problems by inhibiting photosynthetic activity (Reife and Fremann, 1996). Conventionally, dye wastewater discharged by the textile industry is treated with various chemical and physical methods, such as chemical coagulation, biological treatment, adsorption, ultrafiltration and advanced oxidation processes (Papic et al., 2000; Cassano and Alfano, 2000; Karadag et al., 2006; Mohorcic et al., 2006; Bielska and Prochaska, 2007; Alfano and Cassano, 2009; Foletto et al., 2010). Among several chemical and physical methods, the adsorption has been found to be superior compared to other techniques for wastewater treatment in terms of its capability for efficiently adsorbing a broad range of adsorbates and its simplicity of design (Tan et al., 2008). Natural materials that are available in large amounts, or certain waste products from industrial or agricultural operations, may have potential as inexpensive sorbents. Removal of dye by agricultural by-products and other low-cost sorbents such as cotton fiber (Pal and Esteve, 1959), kaolin (Nandi et al., 2009), coir pith (Namasivayam et al., 2001), carbon (Pereira et al., 2003), clay (choonheydt and Heughebaert, 1992), oxide (Foletto et al., 2009), and rice hull (Kumar and Sivanesan, 2007) has been investigated.

In the Rio Grande do Sul state (Brazil), the greatest national rice producer, about 8,900 million ton of rice were produced in harvest 2010/2011 (Rice Riograndense Institute, 2011). Knowing that the hulls represent 20% of this value, the annual production of this residue is about 1.782 million ton. In the present study rice hulls has been used as adsorbent for the removal of red dye from its aqueous solution. Thus, the use of biomass sorbent may offer promising benefits for commercial purpose in the future.

Adsorption of dyes in aqueous solution is a surface phenomenon involving the removal of the coloring matter (the adsorbate) by means of a material (the adsorbent). Adsorption process has been usually classified into physical adsorption (physiosorption) and chemisorption. In the first type, the forces are the weak van der Walls, and the heat evolved during adsorption is less than 40 kJ.mol-1. In chemisorption the molecules are chemically bound to the adsorbent surface. The heat evolved during this type of adsorption is comparable to that of a chemical reaction (more than 80 kJ.mol-1) (Wedler, 1976).

"Various" types of adsorption isotherms have been reported in the literature to describe the adsorption of colored compounds on adsorbents, however the most commonly used are the Langmuir and Freundlich adsorption isotherms. The applicability of adsorption isotherms in the study of dyes removal from aqueous solutions has already been reported in the literature (Walker and Weatherley, 2000; Wong et al., 2004; Allen et al., 2004; Kumar and Sivanesan, 2007; Tanyildizi, 2011).

The objectives of the present study were to investigate the equilibrium of adsorption of reactive red dye in aqueous solution and to elucidate the forces between the rice hull and the dye molecules. The Langmuir and Freundlich models were used to fit the equilibrium isotherms, and thermodynamic parameters were evaluated to elucidate the extent of adsorption data.

II. METHODS

A. Materials

Procion red H-E7B dye (CI Reactive Red 141), azo dye extensively used in the textile industry, was used as model compound. The chemical structure of dye is given in Fig. 1. Dye was provided by a textile industry located in the Santa Catarina State (Brazil). The rice hull in the present investigation was obtained from a local rice mill. The whole rice hulls (seen image in Fig. 2) were washed thoroughly with distilled water to remove all the dirt particles. The material were then dried at 60 °C for 24 h and stored in plastic bottles for use.


Fig. 1. Chemical structure of azo dye.


Fig. 2. Image of rice hulls used in the adsorption experiments.

B. Batch sorption experiments

For adsorption tests, different initial concentrations of rice hull (0.5, 1.0, 1.5 and 2.0 g) were added to 300 mL of the aqueous solution of dye at initial concentration of 30 mg.L-1 and at an optimum pH 2.5 (showed in Fig.3). The pH was adjusted by using dilute H2SO4. The resulting solution was then magnetically stirred at 100 rpm in a thermostatic bath at constant temperature (293, 303, 313 and 323°K) in the dark to achieve the adsorption equilibrium of dye on the solid. During the runs, 5 mL samples of solution were withdrawn at regular intervals and were filtered through a membrane to completely remove solid particles. All samples were analyzed using a Shimadzu 1650C UV spectrophotometer at λmax.= 543 nm. The adsorption capacity of the dye was calculated as follows:

(1)


Fig. 3. Influence of pH on the adsorption of dye onto rice hull. Experimental conditions: madsorbent=0.5 g, Co = 30 mg.L-1, T = 293K.

where qe (mg.g-1) is the amount of dye adsorbed in the equilibrium, Co (mg.L-1) is the initial dye concentration, Ce (mg.L-1) is the concentration of dye in the aqueous phase at equilibrium, W (g) is the adsorbent amount in the solution, and V (L) it the volume of the solution.

III. RESULTS AND DISCUSSION

The effect of pH on the adsorption of the dye was studied in the range 2.5-8.5 (Fig. 3). From the figure, it was observed that the adsorption was high in acid medium (pH 2.5). In pH 5.3 (the point of zero charge, pHpzc), the surface of rice hull had neutral charge (Costa et al., 2009). However, at pH < pHpzc the solid surface present positive charge, favoring the adsorption of anionic molecules of the dye that is formed by several sulphonated groups charged negatively (Fig. 1). Similar trend of pH effect was observed for the adsorption of direct and acid dye onto soy meal hull, with maximum adsorption in pH 2.0 (Arami et al., 2006), and for the adsorption of anionic dye onto babassu coconut mesocarpo (Vieira et al., 2009).

The amount of dye adsorbed at equilibrium on rice hull at different amounts (0.5, 1.0, 1.5 and 2.0 g) and temperatures (293, 303, 313 and 323°K), at pH 2.5, are given in Table 1. The contact time (not shown here) required to attain equilibrium was about 40 min for all the experimental conditions used in this work. It is observed that the adsorption capacity increased with increasing temperature from 293 to 323 K, showing that this process is endothermic. Similar result was reported for the adsorption of cationic dye by activated carbon (Karaca et al., 2008). But, it is noted that the adsorption capacity decreased with increasing the amount of rice hull. Similar result was observed for the adsorption of Acid Red 114 dye using activated carbon prepared from seed shells (Thinakaran et al., 2008). This may be attributed to the aggregation of adsorbent particles at high dosage, which reduces the total surface area of the adsorbent and results in an increase in the diffusion path length (Shukla et al., 2002).

Table 1. Adsorption capacity of dye (q e) in the equilibrium on rice hull.

The adsorption data were analyzed according to the two frequently used isotherm models: Langmuir and Freundlich. The isotherms for adsorption from solutions are given by equations:
Langmuir:

(2)

Freundlich:

(3)

where qo is the maximum amount of adsorption corresponding to complete monolayer coverage on the surface (mg.g-1), KL is the Langmuir constant related to the energy of adsorption (L.mg-1), Ce is the concentration of dye in the aqueous phase at equilibrium and KF is the adsorption capacity (mg.g-1) and n is a value that varies between 1 and 10 and is a measure the intensity of adsorption (Haghseresht and Lu, 1998; Tsai et al., 2004).

The Langmuir constants qo and KL were calculated from the slope of plot between 1/qe versus 1/Ce (Fig. 4), while the values of Freundlich constants, n and KF were obtained from of plot log qe versus log Ce (Fig. 5). Parameters of the isotherms and correlation coefficients are given in Table 2. As seen from Table 2, the increase of qo and n values with rise in temperature suggest that higher temperature creates more adsorption sites.


Fig. 4. Langmuir isotherms for the adsorption of dye on rice hull at different temperatures.


Fig. 5. Fleundlich isotherms for the adsorption of dye on rice hull at different temperatures.

Table 2. The values of constants evaluated from the isotherms at different temperatures with the correlation coefficients.

Comparing Fig. 4 and 5, it observed that the best fit was obtained by Langmuir isotherms, and this can be analyzed through of values of correlation coefficients (R2) shown in Table 2. Langmuir model is more efficient than Freundlich equation to describe the phenomenon of dye adsorption on rice hull surface. The Langmuir model assumes that there is no interaction between the sorbate molecules and that the adsorption is localized in a monolayer. The values of n calculated from the Freundlich equation, are within the range 1-10 generally reported in the literature (Haghseresht and Lu, 1998; Tsai et al., 2004), suggesting that the adsorption was favorable.

Table 3 gives the maximum adsorption capacity (qo) values of dyes adsorption on vegetable waste adsorbents investigated by us and other researchers.

Table 3. The maximum adsorption capacity (qo) of dyes on various low-cost solid wastes.

From Table 3, it is observed that the maximum sorption capacity of rice husk is satisfactory when compared to capacities of other low-cost adsorbent materials. Furthermore, as an agricultural by-product, rice husk is vast and cheap, so it shows potential for the removal of Reactive Red 141 from the solution.

The data obtained from adsorption isotherms were used to determine thermodynamic parameters such enthalpy of adsorption (ΔHa), Gibbs free energy of adsorption (ΔGa) and adsorption entropy (ΔSa).

The enthalpy of adsorption (ΔHa) and adsorption entropy (ΔSa) were estimated by modified Vant Hoff equation:

(3)

where T is the temperature in Kelvin, R is the universal gas constant (8.31414 J. mol-1.K-1) and KD (qe/Ce) is the linear adsorption distribution coefficient.

The Gibbs free energy of adsorption (ΔGa) was evaluated using following equation:

(4)

The values of ΔH° and ΔS° were calculated from the slope and intercept of the plot between ln KD versus 1/T (Fig. 6), and are given in Table 3. The values of ΔGa are also reported in Table 3. The positive values of ΔHa indicate that the adsorption process is endothermic in nature. Bulut and Aydin (2006) found a value of 33.41 kJ.mol-1 for the heat (ΔHa) involved in the adsorption of methylene blue dye on wheat shells, and Prado et al. (2004) found a value of 29.25 kJ.mol-1 for the heat involved in the adsorption of indigo carmine dye on chitin and chitosan. Values of (ΔHa) varying between 11 and 45 kJ.mol-1 were found for the adsorption of methylene blue dye on activated carbon (Tan et al., 2007). The negative values of ΔGa show spontaneous nature of adsorption process and the positive values of ΔSa indicate the affinity of the adsorbent for dye. Considering the total adsorption energy, the entropic contribution T. ΔSa is much larger than the enthalpic contribution. This indicates that the adsorption process of dye on the adsorbent is entropically governed.


Fig. 6. Plot of ln KΔ vs. 1/T for dye adsorption on rice hull at various dosages.

Table 3. Thermodynamic parameters for adsorption of dye on rice hull with different dosages of adsorbent at different temperatures.

IV. CONCLUSIONS

In the present study, rice hull was used as adsorbent for removal of textile dye from the aqueous solutions. Adsorption of dye was studied by batch technique. The experimental data show that the Langmuir model was more applicable than the Freundlich model. The thermodynamic parameters were determined. Considering the total adsorption energy, the entropic contribution T.ΔSa is much larger than the enthalpic contribution. This indicates that the adsorption process of dye on adsorbent is entropically governed.

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Received: July 17, 2011.
Accepted: November 24, 2011.
Recommended by Subject Editor Orlando Alfano.

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