Review
Textural characteristics, surface chemistry and activation of bleaching earth: A review

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Abstract

The unique properties of clay and clay minerals had made them valuable in the wide range of industrial applications. Low cost, local availability and effectiveness are the prevailing factors that have made clay and clay mineral to be used extensively as adsorbent in the purification of vegetable oils. The textural characteristics and surface chemistry play important roles in the bleaching earth performance. These two factors can be modified by various techniques including acid, basic, organic, thermal and pillaring activation. In these reviews, a comprehensive list of literatures on chemical and physical modification techniques of the bleaching earth was compiled and reviewed in relation to its effect on the structure, surface chemistry and adsorption capacity.

Introduction

Refining process is an essential step for the production of vegetable oils and fats. There are basically two types of refining process available in the vegetable oils industry, namely; chemical and physical refining [1], [2]. These processes differ in the type of chemicals used and the mode of removing the free fatty acid (FFA) [3]. Refining process comprises of several stages such as degumming, neutralization, bleaching and deodorization [4]. Among the four stages, bleaching is the most critical stage since it helps to improve the appearance, flavor, taste and stability of the final oil products [1], [5], [6], [7]. During bleaching process, the oil is brought into contact with a surface-active substance that adsorbs undesired particles. The adsorbent and the adsorbed particles are filtered off, and the oil leaves the plant with the desired colour [8]. Generally, there are three types of bleaching methods being used in the bleaching process namely, heat bleaching, chemical oxidation and adsorption. The most commonly used method is bleaching by adsorption [3]. The process involves the removal of colouring materials which are either dissolved or colloidally dispersed in the oil [4].

The most extensively used adsorbent in the bleaching process is bleaching earth [9], [10], [11], [12], [13]. Other adsorbents that have been tested for the removal of pigment and impurities from vegetable oils included activated carbon and silica-based products [12], [14]. However, bleaching earth is the most preferred due to its relatively high absorption performance for coloured materials and low purchase cost [14], [15]. Bleaching earth improves the quality of the oil and reduces the tint of any coloured oil to a lighter shade by changing the basic colour units in the oil without altering the chemical properties of the oil [16]. It is also responsible for the removal of pigments and other impurities, such as soap, trace metals, phospholipids, oxidation products and polyaromatics [1], [10], [17], [18], [19], [20], [21]. Bleaching earth has been used in the refinement of vegetable oils since the end of the 18th century [22]. Even today, bleaching earth or bleaching clay is also known as fuller's earth, and it has been most extensively used in the bleaching process [23], [24].

Natural bleaching earth and activated bleaching earth are the two basic types of commercial bleaching earth [25], [26], [27], [28], [29]. The latter is the most preferred since it possesses a higher adsorption capacity than natural bleaching earth [25], [27], [30]. In order to enhance the chemical and physical properties of bleaching earth, activation by acid, alkaline or organic are the most commonly used techniques [31], [32]. During these treatments, the structure and textural properties of the clay and clay mineral are altered in a controlled way to enhance the specific properties of bleaching earth [2]. Valenzuela-Diaz and Souza-Santos [2] indicated that the increase in the specific surface area and pore volume improves the adsorption capacity of metal impurities, phosphatides and colour bodies. Therefore, the relationship between porous morphology and surface chemistry plays an important role in bleaching earth performance.

Application of bleaching earth for the purification of vegetable oils has led to several problems such as oil retention, filtration and environmental effect. If the amount of bleaching earth used is higher than the required value, oil losses will be greater due to the oil retention properties of bleaching earth [8]. The types of clays and their particle sizes influence the filtration efficiency. Clays made up of very fine particles are more compact and needs a longer filtration time to separate the clays from the oils [33]. In addition, the excessive use of activated bleaching earth can cause environmental problems and increase the land-fill disposal costs [32]. Due to these problems, numerous researches attempting to improve the effectiveness of bleaching earth have been carried out in the recent years.

The surface chemistry and modification methods of bleaching earth have been reviewed by some researchers such as reviews by Schoonheydt and Johnston [34]; Bergaya et al. [35]; Heller-Kallai [36]; Komadel and Madejova [37]; Lagaly et al. [38]. These recent information were mainly focused on the fundamental structural and surface properties of clay minerals, their industrial and environmental applications as well as analytical techniques. In this paper, the textural characteristics, surface chemistry and modification techniques on bleaching earth for the enhancement of the adsorption capacity were reviewed. The parameters that influence the activation treatment of bleaching earth were figured out.

Section snippets

Clays and clay minerals

There are numerous books and reviews about clay and clay minerals, as well as their applications. They are essential to our current understanding of how and why clay minerals have such an extensive industrial utilization [39]. This chapter discusses general description of the clay and clay minerals as well as some of the important applications related to the bleaching earth.

The term clay and clay mineral are considered very different in its context. Because of that, the Association

Conclusions

In this review, textural characteristics, surface chemistry and various activation techniques of bleaching earth have been presented. However, despite numerous papers published on bleaching earth, there is still diminutive information on a full study pertaining to the relationship between porous morphology and surface chemistry of bleaching earth. The literature review shows that there is a need for a more detailed systematic study on the surface chemistry, focusing mainly on the active sites

References (156)

  • W.T. Tsai et al.

    Adsorption of acid dyes from aqueous solution on activated bleaching earth

    J. Colloid Interface Sci.

    (2004)
  • H. Babaki et al.

    Kinetic model for the isothermal activation of bentonite by sulfuric acid

    Mater. Chem. Phys.

    (2008)
  • N.S. Gunawan et al.

    Bentonites modified with anionic and cationic surfactants for bleaching of crude palm oil

    Appl. Clay Sci.

    (2010)
  • R.A. Schoonheydt et al.

    Developments in clay science: handbook of clay science

  • F. Bergaya et al.

    Developments in clay science: handbook of clay science

  • L. Heller-Kallai

    Developments in clay science: handbook of clay science

  • P. Komadel et al.

    Developments in clay science: handbook of clay science

  • G. Lagaly et al.

    Developments in clay science: handbook of clay science

  • J. Madejova

    FTIR technique in clay mineral studies (Review)

    Vib. Spectrosc.

    (2003)
  • M.F. Brigatti et al.

    Developments in clay science: handbook of clay science

  • B. Tyagi et al.

    Determination of structural modification in acid activated montmorillonite clay by FT-IR spectroscopy

    Spectrochim. Acta A

    (2006)
  • M. Hassan et al.

    Glauconitic clay of El Gidida, Egypt: evaluation and surface modification

    Appl. Clay Sci.

    (2004)
  • G.E. Christidis et al.

    Acid activation and bleaching capacity of bentonites from the islands of Milos and Chios, Aegean, Greece

    Appl. Clay Sci.

    (1997)
  • E. Srasra et al.

    Textural properties of acid activated glauconite

    Appl. Clay Sci.

    (2000)
  • H. Noyan et al.

    The effect of sulphuric acid activation on the crystallinity, surface area, porosity, surface acidity and bleaching power of a bentonite

    Food Chem.

    (2007)
  • J.C. Dai et al.

    Surface modification of clays and clay-rubber composite

    Appl. Clay Sci.

    (1999)
  • M.E. Schrader et al.

    Wettability of clay minerals

    J. Colloid Interface Sci.

    (1990)
  • A. Steudel et al.

    Alteration of swelling clay minerals by acid activation

    Appl. Clay Sci.

    (2009)
  • G.M. Habashy et al.

    Characterization of some Egyptian clays to be used as bleaching agents

    Surf. Tech.

    (1982)
  • L. Rozic et al.

    Modeling and optimization process parameters of acid activation of Bentonite by response surface methodology

    Appl. Clay Sci.

    (2010)
  • F. Bergaya et al.

    CEC of clays: Measurement by adsorption of a copperethylendiamine complex

    Appl. Clay Sci.

    (1997)
  • P Komadel et al.

    Structural Fe(III) reduction in smectites

    Appl. Clay Sci.

    (2006)
  • C. Fernandes et al.

    Catalytic conversion of limonene over acid activated Serra de Dentro (SD) bentonite

    Appl. Catalysis A: Gen.

    (2007)
  • C. Breen et al.

    Correlation of catalytic activity with infra-red, 29Si MAS NMR and acidity data for HCl-treated fine fractions of montmorillonites

    Appl. Clay Sci.

    (1995)
  • S.R. Chitnis et al.

    Industrial applications of acid-treated clays as catalysts

    React. Funct. Polym.

    (1997)
  • M.A. Didi et al.

    Colza oil bleaching through optimized acid activation of bentonite, a comparative study

    Appl. Clay Sci.

    (2009)
  • W.A. Allo et al.

    Mineralogy, chemistry and potential applications of a white bentonite in San Juan Province, Argentina

    Appl. Clay Sci.

    (2004)
  • T. Novakovic et al.

    Synthesis and characterization of acid activated Serbian smectite clays obtained by statistically designed experiments

    Chem. Eng. J.

    (2008)
  • J. Konta

    Clay and man: clay raw materials in the service of man

    Appl. Clay Sci.

    (1995)
  • W.P. Gates et al.

    Mineralogy of a bentonite from Miles, Queensland, Australia and characterization of its acid activation products

    Appl. Clay Sci.

    (2002)
  • A.O. Oboh et al.

    Laboratory trials on bleaching palm oil with selected acid-activated Nigerian clays

    Food Chem.

    (1988)
  • M.A. Vicente Rodriguez et al.

    Preparation of microporous solids by acid treatment of a Saponite

    Micropor. Mater.

    (1995)
  • M. Önal et al.

    Preparation and characterization of acid-activated bentonite powders

    Powder Tech.

    (2007)
  • F.R. Valenzuela-Diaz et al.

    Studies on the acid activation of Brazilian smectite clays

    Quim. Nova.

    (2001)
  • N.A. Morad, M.K.A. Aziz, R.M. Zain, Process design in degumming and bleaching of palm oil, Master Thesis, Universiti...
  • E.L. Foletto et al.

    Clarification of cottonseed oil: how structural properties of treated bentonites by acid affect bleaching efficiency

    Lat. Am. Appl. Res.

    (2006)
  • M. Bockish

    Fats and Oils Handbook

    (1998)
  • Y. Bayrak

    Adsorption isotherms in bleaching hazelnut oil

    J. Am. Oil. Chem. Soc.

    (2003)
  • E. Gulsah Kirali et al.

    Statistical modelling of acid activation on cotton oil bleaching by Turkish bentonite

    J. Food Eng.

    (2006)
  • N.A.B. Joy et al.

    Adsorption of palm oil carotene and free fatty acids onto acid activated Cameroonian clays

    J. Appl. Sci.

    (2007)
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