Kinetics and thermodynamics of β-carotene and chlorophyll adsorption onto acid-activated bentonite from Xinjiang in xylene solution

doi:10.1016/j.jhazmat.2009.06.047

Journal of Hazardous Materials

Volume 171, Issues 1–3, 15 November 2009, Pages 582-587

https://doi.org/10.1016/j.jhazmat.2009.06.047 Get rights and content

Abstract

The kinetics and thermodynamics of β-carotene and chlorophyll adsorption from xylene solution onto acid-activated bentonite (AAB) within the temperature range 65–95 °C were investigated. Adsorption of β-carotene was described well with the Langmuir isotherm, whereas chlorophyll adsorption was determined well with the Freundlich isotherm, and the experimental data on chlorophyll adsorption were also fitted by the Langmuir isotherm to a certain extent, as reflected by correlation coefficients (R²) over 0.9865. In addition, the adsorption of β-carotene and chlorophyll onto AAB are favorable. The pseudo-second-order model was found to explain the kinetics of adsorption of both pigments more effectively. Increase of temperature enhanced the adsorption rate and equilibrium adsorption capacity of β-carotene and chlorophyll on AAB. The activation energy for the sorption of β-carotene and chlorophyll on AAB was 19.808 kJ/mol and 16.475 kJ/mol, respectively. The thermodynamic parameters ΔH^θ, ΔS^θ and ΔG^θ, computed from K_F of the adsorption isotherm constant, were 21.766 kJ/mol, 92.244 J/K mol and −9.554 kJ/mol respectively for the adsorption of β-carotene on AAB at 65 °C, and for adsorption of chlorophyll on AAB at 65 °C were 31.051 kJ/mol, 93.549 J/K mol and −0.729 kJ/mol, respectively. The adsorption of β-carotene and chlorophyll in xylene solution on AAB was a spontaneous and endothermic process with increasing in the randomness at the solid–solution interface.

Introduction

Vegetable oils contain numerous pigments, mainly including chlorophyll, β-carotene, xanthophylls and their derivatives, which are removed to give the oil a color that is acceptable to the consumer [1]. Activated clays, activated carbon and silica-based products are adsorbents commonly used in the edible oil decolorization process. Bentonite is highly valued for its sorption properties, which stem from its high specific surface area, swelling capacity and cation exchange capacity (CEC). Some of these properties are related to the crystal isomorphic substitution [2], [3]. In addition, the specific surface area, porosity and acid sites are improved with mineral acid activation to yield acid-activated bentonite (AAB) [4]. AAB is widely used in various applications such as catalysts, catalyst supports and a component of carbonless copying papers [5]. Moreover, AAB is the most popular adsorbent for purification, decolorization and stabilization of edible oils as compared to the activated carbon and silica-based products as it is less expensive than activated carbon [6], [7], [8], [9], [10]. Also, it removes undesirable compounds such as β-carotene, chlorophyll and other color substances, and reduces traces of Cu²⁺and Fe³⁺, phospholipids, soap and some undesirable oxidation products during bleaching [11].

Most of the previous studies were focused on the removal of β-carotene and chlorophyll from various types of oils by AAB [6], [7], [8], [9], [10], [12]. Only a few attempts were made to describe the adsorption of β-carotene on acid-activated montmorillonite at low temperature (20–40 °C), using acetone or benzene as solvents [13], [14], while for chlorophyll, using hexane or acetone were used as solvents [15], [16], [17], due to their high volatility of the solvents. Such temperatures are low as compared to those employed during oil bleaching [6].

Studies on the optimum conditions of preparing AAB from the bentonite of Xinjiang in China used for cotton oil bleaching were carried out recently [18]. The bleaching performance of the AAB was good and the application prospect of the product is wide [19]. In addition, adsorption isotherms and knowledge of kinetics and thermodynamic parameters are the main requirements considered for the design of adsorption experiments. In the present study, the experimental data of adsorption of β-carotene and chlorophyll from xylene solution by AAB were fitted by Langmuir and Freundlich equations. The pseudo-first-order and pseudo-second-order models were applied to determine the best fitting of experimental data. Furthermore, the kinetics and thermodynamic parameters for the adsorption of the pigments on AAB from xylene solution were calculated to study the adsorption behavior at various temperatures.

Section snippets

Materials

The raw bentonite from the deposit of Xiazijie was donated by Xinjiang Xiazijie Bentonite Co. (China). Our previous studies indicated that the sample consisted of predominantly montmorillonite, substantial amounts of quartz and feldspar impurities, in addition to minor amounts of illite and kaolinite [18]. Anhydrous xylene of analytical grade used was obtained from Xi’an Chemicals (China). β-Carotene of HPLC grade (>98%), purchased from Sigma Co. (America), was used to prepare the calibration

Equilibrium studies

The equilibrium data for adsorption of β-carotene and chlorophyll on AAB were analyzed using the Langmuir and Freundlich isotherms. The Langmuir adsorption Eq. (2), predicts the existence of monolayer coverage of the adsorbate at the outer surface of the adsorbent [24]. $\frac{C_{e}}{Q_{e}} = \frac{1}{q_{m} K_{L}} + \frac{C_{e}}{q_{m}}$ where C_e and Q_e are as defined in Eq. (1), q_m is the adsorption maximum (mg/g); K_L is the sorption equilibrium constant (L/mg). The Freundlich isotherm [24] is an empirical equation, which can be applied to

Conclusions

The use of an AAB from Xinjiang in China as adsorbent of β-carotene and chlorophyll from xylene solution at temperatures 65–95 °C has been examined. β-Carotene adsorption was described well by the Langmuir model, whereas the Freundlich model fitted better with experimental data on chlorophyll adsorption. Higher temperature was favor of enhancing the adsorption of β-carotene and chlorophyll on AAB. The kinetics results indicated that the adsorption of β-carotene and chlorophyll was described by

Acknowledgements

This work was financially supported by the Ministry of Education “Chunhui Plan” International Cooperation Project (Z2006-1-83018), Xinjiang Bingtuan Key Science and Technology Industry Project (2008GG24) and Shihezi University’ High Level Talent Start Fund Project (RCZX200636) P. R. China.

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Kinetics and thermodynamics of β-carotene and chlorophyll adsorption onto acid-activated bentonite from Xinjiang in xylene solution

Abstract

Introduction

Section snippets

Materials

Equilibrium studies

Conclusions

Acknowledgements

J. Biosci. Bioeng.

Chin. J. Chem. Eng.

React. Funct. Polym.

Appl. Clay Sci.

Appl. Clay Sci.

Chin. J. Chem. Eng.

Chin. J. Chem. Eng.

J. Hazard. Mater.

Food Chem.

Smectites: the relationship between their properties and isomorphic substitution

J. Mater. Sci.

Evolution of porosity and surface acidity in montmorillonite clay on acid activation

Ind. Eng. Chem. Res.

Mechanism of palm oil bleaching by montmorillonite clay activated at various acid concentrations

J. Am. Oil Chem. Soc.

Decolorisation of crude palm oil by acid-activated spent bleaching earth

J. Chem. Technol. Biotechnol.