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