Chemical Engineering Journal 170 (2011) 90–106
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Chemical Engineering Journal
journal homepage: www.elsevier.com/locate/cej
Review
Textural characteristics, surface chemistry and activation of bleaching earth:
A review
Farihahusnah Hussin, Mohamed Kheireddine Aroua ∗ , Wan Mohd Ashri Wan Daud
Department of Chemical Engineering, Faculty of Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia
a r t i c l e
i n f o
Article history:
Received 18 October 2010
Received in revised form 14 March 2011
Accepted 14 March 2011
Keywords:
Bleaching earth
Textural characteristics
Surface chemistry
Activation
a b s t r a c t
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.
© 2011 Published by Elsevier B.V.
1. 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
[1,5–7]. 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 [9–13]. 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
∗ Corresponding author. Tel.: +60 03 79674615; fax: +60 03 79675319.
E-mail address: mk [email protected] (M.K. Aroua).
1385-8947/$ – see front matter © 2011 Published by Elsevier B.V.
doi:10.1016/j.cej.2011.03.065
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
[1,10,17–21]. 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 [25–29]. 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]. ValenzuelaDiaz 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
F. Hussin et al. / Chemical Engineering Journal 170 (2011) 90–106
91
Nomenclature
Al3+
BP
Ca2+
Fe2+
t
K+
N
Mg2+
Na+
nm
dp
SBET
Si4+
Vp
P
aluminium ion
bleaching power
calcium ion
iron ion
time (hour)
potassium ion
acid concentration (normality)
magnesium ion
sodium ion
surface acidity (10−4 /mol g)
pore diameter in nanometers
BET surface area (m2 g−1 )
silicon ion
total pore volume ( cm3 g−1 )
power in watt
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.
2. 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 Internationale
pour l’Etude des Argiles (AIPEA) nomenclature committee for 2006
summarized some recommendations of nomenclature for minerals, phyllosilicates, clays and clay minerals. The AIPEA has defined
clay as “a naturally occurring material composed primarily of finegrained minerals, which is generally plastic at appropriate water
contents and will harden when dried or fired” [40]. The definition of
clay minerals according to the AIPEA refers to “phyllosilicate minerals and minerals which impart plasticity to clay which harden
upon drying or firing” [40]. Clays and clay minerals have been used
extensively as low-cost adsorbents for diverse applications, such
as adsorbents in the bleaching of edible oils, medications, catalyst
beds, in the preparation of pillared clays, carbonless copy paper,
textiles, petroleum refining, etc. [31,39]. Table 1 shows some of the
industrial uses of clay minerals.
The most important factor contributing to the properties of different type of clay minerals is their molecular structure. When one
octahedral sheet is linked to one tetrahedral sheet, the clay layer is
called 1:1 layer type, such as kaolin subgroup and serpentine subgroup [41] (Fig. 1). Within the 1:1 layer type, there are different
Fig. 1. The clay mineral structure of 1:1 and 2:1 layer type. Where, Oa, Ob and Ooct
refer to tetrahedral basal, tetrahedral apical and octahedral anionic position, respectively. M and T show the octahedral and tetrahedral cation, respectively (adapted
from Brigatti et al. [42]).
mineral species, for example, dioctahedral (e.g. kaolinite, dickite,
nacrite and halloysite) with a general composition of Al2 Si2 O5 (OH)4
and trioctahedral (e.g. lizardite, antigorite and chrysotile) with an
ideal composition of Mg3 Si2 O5 (OH)4 [41,42]. The composition of
the kaolinite, dickite, nacrite and holloysite are characterized by a
predominance of Al3+ in octahedral sites, although some isomorphous substitution of Mg2+ , Fe3+ , Ti4+ , and V3+ for Al3+ can occur
[42]. However for lizaedite, antigorite and chrysolite, it is trioctahedral layer minerals and it contains mainly Mg2+ as central atom
in the octahedral sites [41].
The structure created from two tetrahedral sheets sandwiching on octahedral sheet is called 2:1 layer type, such as smectite
subgroup, micas subgroup, pyrophyllite subgroup, vermiculite subgroup and chlorite subgroup [41,42] (Fig. 1). Among these clay
minerals subgroup, the most commonly used in various branches
of industry is smectite subgroup. This is due to their high specific
surface area, high adsorption capacity, chemical and mechanical
stability, high cation exchange capacity and swelling ability [41].
The most important species of dioctahedral smectite is montmorillonite, beidellite, nontronite and volkonskoite [42]. Among these
species, the most common dioctahedral smectite are montmorillonite where two-thirds of the octahedral sites are occupied mainly
by trivalent cations (e.g. Al3+ ) but partly substituted with (e.g. Fe3+
and Mg2+ ). The tetrahedral sites of montmorillonite normally contain Si4+ as central atoms with some Al3+ substitution [43]. The
silica tetrahedra (T) (Si4+ in tetrahedral coordination with O2− ) and
alumina octahedra (O) (Al3+ in octahedral coordination with O2− )
are interconnected by the sharing of O2− at polyhedral corners and
edges in such a way that a sheet of alumina octahedral is sandwiched between two sheets of silica tetrahedral. It is thus referred
to as a three-layered clay mineral with T–O–T layers making up the
structural unit [44]. Moreover, hectorite, saponite and sauconite
represent the trioctahedral species of the smectites. Though Mg2+
dominates the octahedral sites in both of these minerals, a partial
substitution of Li+ for Mg2+ occurs in hectorite and saponite have a
pronounced Al3+ for Si4+ substitution in the tetrahedral sheet [41].
The classifications of clay minerals are presented in Table 2.
2.1. Textural characteristic
2.1.1. Specific surface area and porous volume
The textural characteristics such as surface area and pore volume play a significant role in the bleaching earth performance [45].
As mentioned earlier, these two factors can be enhanced by modifying the clay minerals with various techniques; including acid,
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F. Hussin et al. / Chemical Engineering Journal 170 (2011) 90–106
Table 1
Industrial uses of clay minerals (adapted from Murray [39]).
Clay minerals
Physical characteristics
Industrial applications
Kaolin
White or near-white in colour
Very limited substitutions
Minimal charge on the layer
Very low cation exchange capacity
Relatively low surface area
Low absorption capacity
Smectite
Variable colour, usually tan or greenish-gray
Considerable lattice substitutions
High layer charge
Medium to high cation exchange capacity
Very high surface area
High absorption capacity
Palygorskite–serpentine
Light tan, brown, cream, or bluish green colour
Some lattice substitutions
Moderate layer charge
Medium base exchange capacity
High surface area
High absorption capacity
Adhesives, cement, ceramic, cosmetics, cracking catalysts,
crayon and chalk, enamels, extender pigment in ink, extender
pigment in paint, fertilizers, fiberglass, filler in plastics, filler in
rubber, foundries, fruit and vegetable protection, insecticide
and pesticide carriers, medicines and pharmaceuticals, paper
coating, paper filling, pencil lead, polishing compounds, roofing
granules, soaps and detergents, tanning leather, textiles
Adhesives, aerosols, agricultural carriers, animal feel bonds,
barrier clays, bleaching clays, cat litter, catalysts, cement,
ceramics, cosmetics, crayons, de-inking of paper, desiccants,
detergents, drilling mud, emulsion stabilizer, food additives,
industrial oil adsorbents, insecticide and pesticide carrier, iron
ore pelletizing, medicines, nanoclays, organoclays, paint,
paper, pencil leads, pharmaceuticals, pillared clays,
plasticizers, rubber filler, sealan, slurry trench stabilization,
tape joint compounds, water clarification
Adhesives, agricultural carriers, animal feed binders,
anti-craking agent, barrier sealants, bleaching clays, catalyst
support and carrier, ceramics, cosmetics, drilling fluids,
environmental adsorbent, industrial floor adsorbents, laundry
washing powders, medicines and pharmaceuticals, oil refining,
paint, paper, polishes, suspension fertilizer, tape-joint
compounds
basic, organic, thermal and pillaring treatment [31,32]. According
to Murray [39], the physical and chemical properties of a particular
clay mineral are dependent on its structure and composition. For
instance, the structure and composition of kaolins, smectites, and
palygorskite–sepiolite are very different even though the fundamental building blocks, i.e. the tetrahedral and octahedral sheets,
are similar. Smectites and palygorskite–sepiolite are widely used
as adsorbent in the purification of vegetable oil. This is due to their
fine particle size, high surface area and medium exchange capacity which give smectite, palygorskite and sepiolite a high capacity
to absorb and adsorb various liquids, making them very useful
in many industrial applications [39]. On the other hand, kaolinite exhibits low absorption and adsorption properties, which are
directly related to the low surface charge on the particle. They
also have a relatively low surface area (8–15 m2 g−1 ). In addition,
the base exchange capacity of kaolinite is typically in the range of
1–5 meq/100 g [46]. As such, no report has been found on examining the possibility of using this clay mineral to bleach the vegetable
oil.
Zschau [47] reported that, natural montmorillonite clay has a
specific surface area between 40 m2 g−1 and 160 m2 g−1 , whereas
the specific surface area of acid activated is in the range of
150–350 m2 g−1 . Valenzuela-Diaz and Souza-Santos [2], reviewed
on the acid activation of brazilian smectitic clays. The authors
revealed the works of many researchers to conclude that the spe-
cific surface area can be increased greatly with acid treatment
compared to natural clay. The authors also reported that natural
smectites have a specific surface area ranging from 16 m2 g−1 to
97 m2 g−1 and shows a lower adsorption capacity to absorb colouring matter and other impurities in oils and solutions. In addition
to that, the surface area of smectites is several times larger in
acid treated clay, reaching values of 200–400 m2 g−1 . Compared to
untreated clay, acid activated clay shows a much higher activity
[2]. The specific surface area of the raw and activated clay and clay
minerals are summarized in Table 3.
Falaras et al. [29], investigated the effect of acid activated
montmorillonite with sulfuric acid (H2 SO4 ) for the bleaching of
cottonseed oil. The results reveal that a linear dependence of
the bleaching efficiency on the clay surface area and acidity was
observed. Moreover, it showed that medium activation of the clay
was most effective in bleaching the cottonseed oil. They also stated
that the removal of colour and peroxide value appeared higher in
the clay sample with the greatest surface area. The largest surface
area of acid activated montmorillonite is around 107 m2 g−1 . The
surface area and pore volume influence the bleaching efficiency of
acid activated montmorillonite. They also concluded that enhanced
Bronsted surface acidity and surface area are the deciding factors in
the bleaching efficiency of acid activated montmorillonite [29]. This
effect was also reported by Hassan and El-shall [45] where large
surface area and pore volume play significant roles in the bleaching
Table 2
Classification scheme of phyllosilicate clay minerals.
Layer type
Group ( = layer charge)
Subgroup
Species
1:1
Kaolin–serpentine
∼0
Smectite or montmorillonite-saponite
∼ 0.2–0.6
Kaolinites
Serpentines
Dioctahedral smectites or
montmorillonite
Trioctahedral smectites or saponites
Dioctahedral vermiculite
Trioctahedral vermiculite
Dioctahedral vermiculite
Trioctahedral vermiculite
Dioctahedral brittle micas
Trioctahedral brittle micas
Dioctahedral chlorites
Trioctahedral chlorites
Kaolinite, dickite, nacrite, halloysite
Chrysolite, lizardite, amesite
Montmorillonite, nontronite, beidellite
Saponite, hectorite, sauconite
2:1
2:1:1
Vermiculite
∼ 0.6–0.9
Mica
∼1
Brittle mica
∼2
Chlorite
∼2
Adapted from Bailey [156].
Dioctahedral vermiculite
Trioctahedral vermiculite
Muscovite, paragonite
Biotite, phlogopite
Marganite
Seybertite, xanthophyllite, brandisite
Pennine, dinochlore, prochlorite
F. Hussin et al. / Chemical Engineering Journal 170 (2011) 90–106
Table 3
The specific surface area of the raw and activated clay and clay minerals.
Adsorbents
Raw claySBET (m2 g−1 ) Activated clay* SBET (m2 g−1 ) Reference
Glauconite
Bentonite
Smectite
Montmorillonite
Sepiolite
Montmorillonite
Vermiculite
Palygorskite
Glauconite
Saponite
Sepiolite
Phlogopite
Vermiculite
Griffithite
Palygorskite
49
69
63
93
150
19
3.5
186
78
35
293
4
8
35
208
*
326
238
238
107
263
188
275
286
324
367
549
91
162
306
265
[51]
[65]
[90]
[29]
[155]
[102]
[102]
[107]
[64]
[98]
[108]
[103]
[103]
[109]
[110]
In Table 3, display the maximum value of activated surface area.
and adsorption capacity of clay minerals [45].
Working on sunflower oil, Lomić et al. [48] attempted to
correlate the textural properties of raw purified acid activated
and Al-pillared samples with its bleaching capacities. The author
observed that BET surface area and micropore surface area are not
crucial factors for adsorption capacities of clay and clay derivates.
They revealed that the bleaching capacity of clay and its derivates is
determined by using sample total pore volume, instead of the sample surface area. This is due to the fact that samples with higher
pore volumes offer the best adsorption capacities in respect of
different pigments in raw sunflower oil. They also suggested that
pigments with higher molecules can penetrate only into the pores
with higher dimensions, thus micropores do not take part in pigment adsorption during bleaching processes of raw sunflower oil
[48].
This effect was also reported by Christidis et al. [49] where
the increase in the surface area is associated with changes in
the smectite structure. They stated that, although surface area is
an important characteristic of the activated materials, maximum
bleaching capacity is not associated with maximum surface area.
This observation was supported by Lin and Yoo [21], based
on their research work, which they also claimed that the highest removal of iron, phosphorus and secondary oxidation products
were not associated with the highest surface area. They stated that
although attapulgite clay had lower surface area, 139 m2 g−1 and
157 m2 g−1 respectively, they adsorbed more iron, copper, phosphorus and secondary oxidation products than the acid activated
bentonite which possessed a higher surface area of 234 m2 g−1 . The
possible reason for this is that the reduction of phosphorus was proportionate to the dosage of clay used and dosage of phosphoric acid.
Kheok and Lim [50] reported that the mechanism for phosphorus
reduction was due to the adsorption of the phosphorus ions on the
lattice structure of the clay.
On the contrary, Srasra and Trabelsi-Ayedi [51] studied the textural properties of acid activated glauconite in the bleaching of
rapeseed oil. Generally, glauconite is a dioctahedral iron illite. The
results show that medium activation of glauconite was most effective in bleaching the rapeseed oil. The best bleaching performance
correlates with the maximum of the external specific surface area,
but not with the specific surface area and total pore volume. The
fairly diverse results may be due to different types of clay used and
the different preparation methods of this adsorbent. Noyan et al.
[52] reported the effect of H2 SO4 activation on the crystallinity,
surface area, porosity, surface acidity, and bleaching power of a bentonite. They stated that the bleaching power is controlled more by
the pore size distribution rather than other adsorptive properties of
the bleaching earth. Although the bleaching power increases with
93
increasing surface area, porosity and surface acidity, the maximum
bleaching power depends more on the mesopore size distribution
of the bleaching earth [52].
The adsorption capacity for a specific clay minerals depends on
the accessibility of the molecules into pores and consequently on
the size of pores and of clay minerals [31]. According to Gregg
and Sing [53] pore sizes can be classified into three types. The
pores of widths below 2 nm are called micropores, those with
widths between 2 nm and 50 nm are called mesopores and those
larger than 50 nm are called macropores [53]. Babaki et al. [31]
reported that, the physical and chemical properties of bentonites
such as adsorption and catalytic activity depend extensively on the
micro and mesopores. Although the micropores and mesopores
are located within the particles, and the macropores are located
between the particles, nevertheless the effect of the macropores on
the adsorptive properties of a solid is at negligible level compared
to that of the micropores and mesopores. The bentonites clay is the
most important among solids that contain natural mesopores and
also, contain a small amount of micropores [31].
2.2. Surface chemistry
In order to identify the adsorbing mechanism of clay minerals,
clay surface studies (both outside and interlamellar) have been
investigated by Schoonheydt and Johnston [34], which help to
clarify clay’s surface, such as adsorbed species’ location on the
adsorbent and etc. Generally, clay minerals have many attractive
features in its structure, namely; active sites such as hydroxyl
groups, Lewis and Bronsted acidity, exchangeable interlayer cations
and the difference of SiO4 tetrahedral sheet and Al2 (OH)6 octahedral sheet in chemical stability [54]. Studies of clay mineral’s
surfaces to date have included active sites and the functional
groups. Active sites or surface functional group is very important,
because they determine the chemical reactions. The active site is an
atom or a group of atoms that belongs to the solid and can participate in reactions with the species in the surrounding media [55].
When the protonation–deprotonation reactions at the surface of
phyllosilicates are considered, the focus is directed to surface oxygens instead of metal ions, because they are the atoms that can
become protonated or deprotonated [34].
The reactivity of surface oxygen depends on the type and the
spatial distribution of atoms surrounding it; thus a surface functional group is usually written as a single oxygen together with
the atoms that are directly attached to it. Therefore, different surface planes exhibit different surface functional groups. They can
also be described on the basis of their location (edge or basal
surface), geometric arrangement of surface atoms, chemical composition and accessibility [34]. Generally, there are six kinds of
active sites which are important in explaining the interactions of
organic molecules to the clay surfaces. They are isomorphic substitution sites, hydrophobic sites, broken edge sites/hydroxyl surface,
neutral siloxane surface, metal cations occupying cation exchange
sites and water molecules surrounding the exchangeable cations.
Among these active sites, the first four sites are the most important
and will be discussed in greater details in this chapter [34].
2.2.1. Isomorphic substitutions
Surfaces of clay mineral particles are classified as having either
permanent charge or pH-dependent charge. A permanent charge
is derived from isomorphic substitutions in the mineral structure
and is indifferent to the conditions surrounding the mineral [56].
Isomorphic substitution is the most important active sites on the
clay mineral surface which occurs within the tetrahedral and/or
octahedral sheets and generates negative charge on the layers [43].
Generally, it involves replacement of constituent metal ions of the
lattice by cations of lower charge, generating a net negative charge
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F. Hussin et al. / Chemical Engineering Journal 170 (2011) 90–106
Sposito et al. [55] reported that, when isomorphic substitution occurs in the octahedral sheet, the negative charge is more
delocalized and the Lewis base character of the siloxane surface
is enhanced. Depending on the extent of isomorphic substitution,
these negatively charged sites are separated by distances ranging
from 0.7 nm to 2 nm on the basal surface. The negative charge that
results from isomorphic substitution is balanced by the presence of
exchangeable cations which are Ca2+ , Mg2+ , K+ and Na+ ions [55].
The excess negative charge smeared over all the superficial oxygen
and makes the clay surface highly acidic. As for the sorption of polar
and charged organic materials, the location of isomorphic substitution sites in 2:1 layer silicates for example on octahedral or on
tetrahedral will have a great impact. Moreover, the extent and type
of isomorphic substitution occurring in layered silicates can highly
influence the selectivity for inorganic and organic cations, degree
of swelling and accessibility of the interlamellar region [34].
Fig. 2. Representative of 2:1 phyllosilicate minerals indicating charge sites and
different locations of substitutions (adapted from Matocha [56]).
on the mineral, which is compensated by the positive charge of
adsorbed (exchangeable) cations. The substitution can occur during
or after crystal formation [56].
In 1:1 phyllosilicate minerals, normally, no isomorphic substitution occurs in either sheet, thus kaolinite and serpentine have no
significant permanent negative structural charge on basal sites and
little cation exchange capacity [55]. Isomorphic substitution can
create kind of active site in the clay lattice of 2:1 layer silicates. This
phenomenon happens on the basal surfaces of charged 2:1 layers
silicates including smectite, vermiculites and micas. The number of
cation positions occupied within the octahedral sheet determines
whether a clay mineral is di- or trioctahedral. If Al3+ is present in
the octahedral sheet, then the mineral is dioctahedral, and trioctahedral if Mg2+ is present. Within the smectite group of minerals,
dioctahedral montmorillonites have the majority of structural negative charge. These are derived primarily from octahedral site
substitutions of Fe2+ or Mg2+ for Al3+ with a structural formula
of Si8 IV M0.66 H2 OAl3.34 VI (Fe2+ , Mg2+ )0.66 O20 (OH)4 , where M refers
to the exchangeable cation balancing the permanent charge produced by substitution of Fe2+ or Mg2+ for Al3+ . The octahedral layer
charge for montmorillonite, or the number of moles of excess electron charge per formula unit, would be −0.66, balanced by M0.66
exchangeable cations adsorbed near the basal plane of the tetrahedral sheet. In dioctahedral beidellites, tetrahedral substitutions
of Al3+ for Si4+ exceed substitutions in the octahedral sheet. Nontronite is similar to beidellite except it has redox-active Fe3+ in
octahedral coordination (Fig. 2) [56]. The vermiculite group differs
from the smectite group because the primary source of negative
charge arises from a large degree of Al3+ for Si4+ substitution in the
tetrahedral layer, giving rise to a layer charge greater than that in
smectites, ranging from 1.2 to 1.8 [56].
2.2.2. Hydrophobicity and hydrophilicity
Surfaces that attract water are termed hydrophilic, whereas surfaces that repel water are termed hydrophobic. Polar liquids like
water and alcohols interact more strongly with hydrophilic surfaces. Similarly, nonpolar liquids such as petroleum-based solvents
interact more strongly with hydrophobic surfaces. Neutral-layer
silicate clays such as talc and pyrophyllite are hydrophobic
whereas, clay with an octahedral such as montmorillonite and
hectorite and/or tetrahedral such as vermiculite charge deficit is
hydrophilic [57]. Schrader and Yariv [58] reported that the clay
minerals, (e.g. talc and pyrophyllite), are hydrophobic and are not
as readily wetted by water as the hydrophilic clay mineral vermiculite. They explained the hydrophobic and hydrophilic properties
of these minerals based on the electronic properties of the clearage planes. They argued that talc and pyrophyllite are hydrophobic
because the oxygens on the cleavage surfaces (siloxane surface) are
not good electron donors (Lewis bases) and consequently do not
form effective hydrogen bonds to water. In contrast, vermiculite
is hydrophilic and they attributed this hydrophilicity to two factors: (1) hydration of exchangeable ions on the clearage faces, and
(2) the substitution of Al for Si in the clearage faces of vermiculite
making the oxygens good electron donors (effective Lewis bases)
which form hydrogen bond to water [59].
Sorption of organic molecules on the clay surfaces can impart
a hydrophobic nature to the clay surface [34]. Jaynes and Boyd
[60], prepared a series of montmorillonite with reduced-charge
and hence, reduced exchangeable cation content. These reducedcharge clays were then exchanged with small organic cations. It can
be seen that, the adsorption of aromatic hydrocarbon molecules
from water to samples of these reduced-charge organoclays substantially increased with greater charge reduction and hence, lower
organic cation content. Hydrocarbon sorption was found to be proportional to the surface area. This suggests that the hydrocarbons
were adsorbed onto the siloxane surface and that this surface is
hydrophobic [60].
2.2.3. Hydroxyl surfaces
Another type of active sites that clay minerals and related
hydroxides possess is hydroxyl surface such as Si–OH and Al–OH
or Mg–OH. However, hydroxyl groups located at broken edges,
steps and related defects of clay minerals and oxides are different, and are called “terminal OH groups”. These OH groups are
under-coordinated and carry either a positive or negative charge
depending on the type of metal ion and the pH of the ambient aqueous solution [34]. Hydroxyl groups are strongly dependent on pH
value as they became protonated or deprotonated according to the
pH value of the solution. At low pH, these sites will develop a positive charge due to the adsorption of protons, so organic acids and
oxyanions can interact with these positive charged sites strongly.
F. Hussin et al. / Chemical Engineering Journal 170 (2011) 90–106
With the increase of the pH, these sites will develop a neutral charge
sites and finally will develop a negative charge when taken to very
high pH. At the same time, the contribution of these edge sites to
the cation exchange capacity (CEC) depends on the size and shape
of the clay particles. If the particles are large, the CEC from these
sites are minimal. On the contrary, if the particles are small, their
contribution will become greater [34].
2.2.4. Neutral siloxane surface
In addition, the neutral siloxane surface which is considered
to have a predominantly hydrophobic character due to the strong
bond formed between the Si and O atoms, the siloxane surface on
the clay minerals is the least reactive. This surface has interesting
sorption properties and is capable of polarizing water molecules
surrounding the exchangeable cations. The uncoordinated surface
atoms which can donate protons to adjacent organic solutes are
also important active sites for clay minerals. In general, the origins
of charges on the clay mineral lattice are believed to be primarily
due to isomorphic substitution, lattice imperfections, broken bonds
at the edges of the particles and exposed structural hydroxyls [34].
2.3. Analysis methods
There are various experimental techniques been used to characterize the clay minerals such as X-ray fluorescence (XRF), X-ray
diffraction (XRD), Cation exchange capacity, Fourier transforms
infrared spectroscopy (FTIR), Scanning electron microscopy (SEM),
Differential thermal analysis (DTA), Thermal gravimetric analysis
(TGA), BET surface area and Pore size distribution [36,44,61,62].
The XRF method is widely used to measure the elemental and
chemical compositions of materials before and after modification.
This method is fast and non destructive to the sample; therefore it is
the method of choice for bleaching earth applications and industrial
production for control of material [62]. XRF analysis can be performed on a MagiXPRO spectrometer from Philips equipped with
a rhodium X-ray tube (stimulation power: 3.2 kW) using molten
pellets. The loss of ignition was determined separately by storing a
sample in an oven at 1000 ◦ C for 2 h [62].
XRD is the most widely used technique to identify the crystalline
phases present in the clay samples [19,63,64]. XRD patterns are
obtained on a D8 advance X-ray Diffractometer-Bruker AXS with
Cu-K␣ monochromatized radiation (40 kV and 40 mA). The powder
patterns were recorded between 3◦ and 70◦ 2 at a scanning speed
of 1◦ 2/min [49,65].
FTIR spectroscopy is used to analyze the surface chemistry, especially different oxides in materials [41,44]. The equipment can also
be used to distinguish between different types of clay minerals and
to derive information concerning their structure, composition and
structural changes during acid activation. FTIR analysis of the clay
samples was conducted on a Perkin-Elmer spectrum 400. The KBr
pressed disc technique is used. FTIR spectra were recorded within
a range of 400–4000 cm−1 at resolution of 4 cm−1 [41].
The surface morphology of the clay samples are analyzed by SEM
(Hitachi S520 SEM) instrument operating at 20 kV [49].
The BET surface area and pore size distribution are determined
from nitrogen adsorption isotherm at 77 K using a Quantachrome
Autosorb-1MP instrument. Prior to analysis, the samples are outgassed 24 h under vacuum at 95 ◦ C. The specific surface areas are
determined according to the BET method at the relative pressure
in the range of 0.05–0.32. The total pore volumes of micro and
meso pores were directly determined from the nitrogen adsorption at P/PO = 0.98. The pore size distribution is calculated using the
Barret–Joyner–Haleda (BJH) method using the adsorption isotherm
[19,65].
The cation exchange capacity (CEC) is one of the basic properties of clay minerals. Several methods to determine the CEC
95
have been developed. Mehlich [66] determined the CEC by saturating the clay with one cation, then washing out excess salt and
finally replacing the cation by several exchange/washing cycles
with another cation. The collected solutions were employed for the
determination of the amount of replaced cation. Another method is
involves leaching with ammonium acetate and subsequent replacement of the NH4 + ions from exchange sites [66–69]. However, this
method is time-consuming because the exchange with NH4 + ions
involves five to six steps. Janek and Lagaly [70] proposed methods by using cationic surfactants. The advantage of this method is
that it can be carried out with simple materials. The preparation
of the samples is easy and fast to carry out. However, the general
problem when using surfactants is that an excess will adsorb on
the clay, requiring determination of the point when the equivalent
amount is adsorbed. Lastly, metal–organic complexes are employed
as exchange cations. The affinity of the clay minerals towards this
type of cation is high, so that complete exchange can be achieved in
one single treatment step [71]. An excess of the complex is added
to the clay dispersion and one has only to determine the remaining
concentration after the exchange reaction. Cobalthexamine [72],
silverthiourea [73], copper bisethylenediamine [74] or copper triethylenetetramine [75] can be used for this purpose.
Layer charge is a basic criterion for the characterization and classification of clay minerals [76]. The layer charge can be determined
by the alkylammonium method [62–78]. However, the method is
time-consuming and requires a number of X-ray diffractograms for
one sample. The techniques, such as optical properties of organic
dyes can also be used to characterize the layer charge. An alternative way to calculate the charge differences between tetrahedral
and octahedral sheets is to determine the cation exchange capacity
(CEC) before and after Li fixation [78].
Thermal analysis is a very useful device to characterize clay minerals. The thermal analysis measurements give information on the
release of water, phase transformations, decomposition and recrystallisation. Clay minerals contain various kinds of water, which are
released at different temperature ranges. First dehydration occurs
up to temperatures of around 300 ◦ C. During dehydration adsorbed
water from the mineral surfaces and interlayer water from inner
surfaces are released in endothermic reactions. The dynamic mass
loss during the dehydration is equal to the static determined water
content at 200 ◦ C. Dehydroxylation occurs between 330 ◦ C and
850 ◦ C. During dehydroxylation structural water associated with
hydroxyl groups migrates out of the mineral structure. The dehydroxylation temperature gives information about the stability of
clay minerals and is associated with the structure of the octahedral
layer of dioctahedral 2:1 clay minerals. Thermal analysis is a combination of thermogravimetry (TG) with differential thermal analysis
(DTA) or differential scanning calorimetry (DSC). The samples were
measured at a defined temperature–time program with a defined
heating rate [62].
2.4. Activation techniques
Activation is defined as a chemical or physical treatment applied
to certain types of clays to develop a capacity to absorb colouring
matter and other impurities in oils and solution [2,31]. Physical
and chemical behavior of clay minerals is studied by numerous
researches because of its relation to their adsorbing and catalytic
properties. This behavior is governed by the extent and nature of
their external surface which can be modified by suitable treatments [79]. The treatments or modification of clay minerals can
be categorized to (1) physical modification (thermal or microwave
treatment) which involves alteration of chemical composition and
crystalline structure by the effect of high temperature, (2) chemical
modification is usually alteration of structure and surface functional groups and (3) pillaring treatment which involves chemical
96
F. Hussin et al. / Chemical Engineering Journal 170 (2011) 90–106
Modification of Bleaching Earth
Chemical Modification
Acid Treatment
Basic Treatment
Physical Modification
Organic Treatment
Pillaring/intercalation
Microwave Treatment
Heat Treatment
Fig. 3. Categorized of activation treatment.
and physical restricting of the clay mineral structure to increase
capacity for adsorption or to create spaces that encourage adsorption of specific ions (Fig. 3).
2.5. Modification of chemical characteristics
2.5.1. Acid treatment
Acid treatment is one of the most common chemical modifications of bleaching earth [80]. In general, acid activation process
consists of two steps; (1) the substitution of the exchangeable
cations by protons and (2) the dissolution of metals ions from the
clay structure, by depopulating the octahedral sheet (Fig. 4) [81,82].
A similar description was made by Madejova et al. [43] where
during the acid attack the interlayer cations are replaced by protons
and the octahedral cations (Al3+ , Mg2+ , Fe2+ , Fe3+ ) are dissolved. The
products are X-ray amorphous, of high SiO2 content, and show a
high specific surface area and micro- and mesopores. The bleaching
power depends on the activation procedures (acid concentration,
temperature, time). The composition of the clay layers substantially
affects their stability against the acid attack, with trioctahedral layers dissolve much faster than dioctahedral layers. Increased Mg
and/or Fe for Al substitution in dioctahedral smectites, increases
their dissolution rate in acids [43,83]. As reported by Agnello [84] in
Fig. 4. Mechanism of acid activation [82].
further treatment, the acid penetrates accordingly from the crystal
surface deeper and deeper into the crystal structure of the individual layers, which causes the inner surface of these crystal platelets
to increase in size and active acid centers to be formed [84]. Thus
acid activation promotes catalytic activity by increasing the number of Bronsted and potential Lewis acid sites [85]. Srasra and
Trabelsi-Ayebi [51] studied the effect of bleaching treatment on the
textural properties of acid activated glauconite. The results showed
that, the colour pigment and impurities in the bleaching process can
be eliminated by acid treatment. They found out that adsorption
of carotenoid can be catalysed by the Bronsted and Lewis acidity
as shown in Fig. 5. ␤-carotene attaches to the clay surface in the
form of carbonium ions either by forming coordination bonds with
Lewis sites or by forming hydrogen bonds with Bronsted sites of
the activated clay mineral [86]. The acid treatment has been used
to increase the specific surface area, modify the surface functional
group, to obtain solids with high porosity and to increase a number of acidic centers. The most important role is to enhance the
adsorption capacity for the removal impurities and colour pigment
from the oil. Fig. 6 is a simplified flow diagram of the process in the
production of acid activated bleaching earths.
Numerous studies have been reported on the acid treatment of clays, especially on bentonite-smectite or montmorillonite [6,14,16,24,36,44,49,86–101] vermiculite [102,103]; kaolin
[104–106]; palygorskite-sepiolite [85,107–110] and glauconite
[45,51,64]. Among all of these, bentonite is the most commonly
used for the production of acid activated clays in the purification of
vegetable oils [87,111–113]. Bentonite has always had a multitude
of markets and acid activated bentonite was a traditional product
for many decades. It is usually a Ca-bentonite that was treated with
organic or inorganic acid solutions to replace divalent calcium ions
with monovalent hydrogen ions and to leach out ferric, ferrous, aluminium and magnesium ions thus altering the layers of smectite
and increasing the specific surface area and porosity [37].
There are various types of acids used for acid treatment including inorganic acid such as hydrochloric, sulfuric and nitric and
organic acid such as acetic, citric, oxalic and lactic. Among all of
these, hydrochloric acid (HCl) and sulfuric acid (H2 SO4 ) are probably the most widely used in acid activation, because it shows strong
affection by the processes parameters and good results in specific
surface area, porosity and adsorption capacity [2,114].
2.5.1.1. Inorganic acid. Komadel et al. [115] have investigated the
effect of acid type and concentration on the reaction rate and
products of dissolution of hectorite in inorganic acids. The author
reported that the rate of hectorite dissolution decreased in the
order HCl ≥ HNO3 > H2 SO4 at the same molar concentration. The
F. Hussin et al. / Chemical Engineering Journal 170 (2011) 90–106
Fig. 5. The mechanism of carotenoid adsorption on Bronsted and Lewis acid sites
[51].
dissolution rates of Li+ was distinctly greater than that of Mg2+ in
all acids at low acid concentrations. The result indicated that at
lesser acid concentrations, protons are preferentially attacked to
the Li+ (in the octahedral sheet), where the local negative charges
are located. However, when the acid concentration is increased,
the respective dissolution rates for Li+ and Mg2+ in any given acid
apparently converged to a similar value [115]. Van Rompaey et al.
[116] reported that the dissolution rates of Li+ was slightly greater
than that of Mg, the results is similar with the conclusion was
made by Komadel et al. [115]. On the contrary, Van Rompaey et al.
97
[116] concluded that the sequence for the dissolution rate of hectorite decreased in the order H2 SO4 ≥ HNO3 > HCl for equal molar
concentration. The order of dissolution is different of that found
by Komadel et al. [115]. The differences in the results are clearly
demonstrated by the dissolved fractions. For example; at 1 M H2 SO4
for 8 h, more than 70% of all Li+ dissolved, whereas at the same molar
concentrations, HNO3 and HCl dissolved 58% and 53%, respectively
[116].
Hongping et al. [117] reported that when Ca-montmorillonite
samples are activated by H2 SO4 , there was progressive dissolution of the octahedral layers. The dissolving order of cations
in the octahedral sheet of montmorillonites observed was
MgO > Fe2 O3 > Al2 O3 . The result revealed that MgO is the most easiest to be removed and Al2 O3 is the most difficult to be dissolved.
The content of Al2 O3 decreased significantly due to the dissolution of octahedral Al, whereas the proportion of SiO2 increased
relatively [117]. This effect was also reported by Suarez Barrios
et al. [107], with increasing acid concentration, the content of
SiO2 increases and the content of octahedral cations Al2 O3 , MgO
and Fe2 O3 decreases. Treatment of palygorskite with mineral acid
resulted in the increase in the percentage of SiO2 and the decrease
in Al2 O3 , MgO and Fe2 O3 are related to the progressive dissolution
of palygorskite. The BET surface area of the original palygorskite
increased from 186 m2 g−1 to 286 m2 g−1 , after which it decreased.
The reasons for the increase are mainly due to the elimination
of exchangeable cations and generation of silica. A substantial
increase in the specific surface area is also observed by the treatment of clay with mineral acids. It has also been observed by several
other researchers, for instance: Vicente Rodriguez et al. [98]; Salem
and Karimi [118], Suresh et al. [119], Mills et al. [120] and Myriam
et al. [110]. On the other hand, in the treatment of clay with mineral acids at high acid concentration (e.g. 7N HCl) the structure of
the palygorskite was completely destroyed [107]. Consequently the
BET surface area of the clay is diminished, which in turn diminished
the decolourizing power [121]. Their work does not contradict that
of Franus et al. [64] which stated that the layer structure of the
glauconite is preserved with mineral acid.
Taylor and Ungermann [122] patented the procedures for making acid activated bleaching earth using high susceptibility source
clay and novel bleaching earth product. The process involves treating mixtures of palygorskite clay (attapulgite or sepiolite) and
bentonite with H2 SO4 in the amount corresponding to an acid
dosage in the range of 1–10% by weight. The results indicated that
several advantages can be achieved from the invention including lower acid costs/unit mass of clay treated, lower production
costs (does not involves washing, filtering or waste treatment process) and environmental friendly (no harmful environmental waste
products are produced).
Steudel et al. [62], studied on the dissolution behavior of six bentonites with a high content of dioctahedral smectites, a hectorite
and vermiculite. They used two types of inorganic acid such as HCl
and H2 SO4 at the same molar concentration. The results revealed
that the dissolution of the octahedral cations like Al, Mg and Fe
were most effective with 10N H2 SO4 than compared to any con-
Activated with inorganic
acid (HCl or H2SO4)
Mining
Crushing
Sizing
Grinding
Acid treatment
Drying
Washing acid
free
Filtering
Fig. 6. Simplified flow diagram of the process of acid activated bleaching earth [26].
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F. Hussin et al. / Chemical Engineering Journal 170 (2011) 90–106
centration of HCl. Most of the octahedral cations were dissolved by
the treatment with 10N H2 SO4 . At the same molarity, the reactivity of HCl is weaker than H2 SO4 due to the lower concentration of
protons. Vermiculite and hectorite showed similar behavior against
both acids. They were much more stable against acid attack than
dioctahedral smectites, whose stability was affected by the content
of Fe and Mg in the octahedral sheet [62].
In a research article by Salawudeen et al. [123] showed that acid
treatment increased the bleaching capacity to remove colour from
oil up to 94.28% in HCl activated clay and 89.46% bleaching capacity was achieved with H2 SO4 activated clay. The results indicated
that the use HCl activated clays proved to be more effective for
the removal the colour from oil than H2 SO4 [123]. Al-zahrani [121]
compared the activation of the clay by inorganic acid and calcination for corn oil bleaching. The result revealed that the highest
percentage of bleaching for natural clay activation using H2 SO4
at compositions of 25%, 35% and 55% by weight, was obtained at
33%, 38% and 40%, respectively. The maximums of the percentage
of bleaching for natural clay activation in HCl at compositions of 4%
and 10% by weight, were 33% and 43%. The author concluded HCl
is more effective in the activation of the local clay than H2 SO4 . On
the other hand, acid activation of the clay after calcination using
35% H2 SO4 gave good results in the percentage of bleaching up to
83% and it showed that H2 SO4 is more effective in activation of the
local clay after clacination than HCl [121].
Sabah and Majdan [24], chemically modified a sepiolite by
mixing it with HNO3 , in order to investigate the adsorption of phosphorus (P) from rapeseed oil. The initial concentration of P in the oil
is 29.7 mg/kg. The adsorption capacity of P per unit weight of acid
activated sepiolite (AAS) decreased from 2.50 mg/kg to 1.84 mg/kg
at 80 ◦ C and from 2.98 mg/kg to 1.91 mg/kg at 100 ◦ C, corresponding to 0.5–1.5% AAS dosages, respectively. There is evident decrease
in the adsorption of P with AAS dosage for the both temperatures, which results from the fact that the adsorption sites remain
unsaturated during the bleaching process. At low AAS/oil ratios
the removal of phosphorus is more efficient, since most active
adsorption centers are occupied by phosphorus, whereas for higher
AAS/oil ratios competition between phosphorus and colour bodies
(carotenoids and chlorophylls) in the adsorption process occurs. A
similar observation was made by Sabah [23], where they reported
that sepiolite treated with 1N HNO3 is a superior adsorbent for the
removal of chlorophyll-a; under the optimum conditions of 100 ◦ C
bleaching temperature and 1.5% sepiolite dosage respectively. At
this condition, chlorophyll-a level in the rapeseed oil was reduced
from 4.85 mg/kg to 0.16 mg/kg.
2.5.1.2. Organic acid. Treatment of clay with mineral acids such as
H2 SO4 or HC1 may generate new surface acid sites quickly, but they
may also cause extensive leaching of A12 O3 from the clay mineral,
thereby leading to structure collapse [106,107,124]. In contrast,
organic acids may preserve the clay structure, but are less effective
in generating surface acid sites [106]. Moreover, organic acids dissolve less of Al2 O3 and thus its activation power is much lesser than
mineral acids [121]. The influence of inorganic and organic acids on
the de-tert-butylation activity of Jordanian bentonite and kaolin
were studied by Mahmoud and Saleh [106]. The result indicated
that 1N HCl is the best acid solution for the activation of bentonite
and kaolin samples; after 0.5 h of reaction, 90% and 92% conversions
were obtained for both samples, respectively. This is due to the fact
that HCl is a strong acid, which completely dissociates in water to
H+ and Cl− ions. Also, the metal chlorides produced as a result of
this acid treatment are completely soluble in water, and removed
by the pretreatment process. On the other hand, acetic acid is a
weak acid and hence is far less effective at activation of the clays
compared to HCl of the same concentration. A conversion of 66%
after 0.5 h reaction was observed for both samples when treated
with 1N acetic acid. Moreover, phosphoric acid was more effective
than acetic acid and was comparable to HCl. This may be attributed
to the phosphoric acid being stronger than acetic acid, but is still
weaker than HCl [106].
Sutheimer et al. [125] compared the surface characteristics and
dissolution behavior of well crystallized (KGa-1b) and poorly crystallized kaolinite (KGa-2). The author reported particles of KGa-1b
generally have hexagonal micromorphology and crystallographically controlled microtopographic features. Particles of KGa-2
are also hexagonal, but their micromorphology tends to be more
rounded and basal-plane surfaces are more irregular with fewer
clearly crystallographically controlled features. KGa-1b particles
tend to be larger in diameter and thicker than KGa-2 particles.
Dissolution experiments were conducted in oxalic acid and inorganic acids at pH 3 at 22 ◦ C. Dissolution rates in 1 mM oxalic
acid was approximately twice as fast for KGa-2 as for KGa-1b,
while it is similar in HNO3 . The comparable dissolution rates for
these two sedimentary kaolinites suggest that the fundamental
structure of kaolinite has a greater influence on the dissolution
kinetics rather than specific surface details. For dissolution in 1 mM
oxalate, Al–oxalates complexes are observed almost exclusively in
agreement with results of equilibrium speciation calculations. For
dissolution in HNO3 , uncomplexed Al species are identified. It can
be seen that, Si release is similar for inorganic vs. organic acids for
KGa-1b. However, Si release is substantially greater in organic than
inorganic solution for KGa-2 [125].
In a separate study, Samsuri and Irawan [126] investigated the
removal of iron from Sabah montmorillonite by extracting with
three organic acids (citric, gluconic and oxalic acid). They believed
that, the bleaching earth also can be treated with organic acid.
Among all of these, only oxalic acid is suitable for use on a large
scale. It is capable of removing the iron oxides from montmorillonite, but extraction is effective only in a strongly acid medium
(pH ≤ 2), at this condition, oxalic acid dissolves a great quantity of
iron, forming a soluble complex [126].
Oboh and Aworh [97] demonstrated the use of organic and
inorganic acid for palm oil bleaching. They used hydrochloric, sulfuric and acetic acid at different molar concentration. The result
indicated that bleaching power of clays treated with HCl and
H2 SO4 increased with the concentration of acid to a maximum
and declined thereafter. In contrast, clays treated with acetic acid
showed lower bleaching power which was equivalent to 45.59%,
when the concentration of acid was at a maximum. Clays treated
with HCl and H2 SO4 were more effective in bleaching palm oil than
those treated with acetic acid [97].
However, the criteria for choice, either organic or inorganic acid
in the industrial process, it depends on the nature and type of clay,
concentration of the acid, temperature and the time of treatment
during acid activation [2]. Several acid treatments experiments
under different operating conditions have been reported in literature in particular the choice of acid used, types of clay, temperature,
the concentration of acid, and activation time [6]. Table 4 summarized the advantages and disadvantages of inorganic and organic
acid in acid activation.
2.5.2. Basic treatment
There were studies carried out to determine the effects of
basic treatment of bleaching earth on the adsorption capacity. In
a research article by Novikova and Korsakov [127] after treating
the clay with 0.5N sodium hydroxide (NaOH) solution, the content
of the active centers lowered, and the surface acidity decreased
from 1.2 to 0.33. The specific surface area also decreased from
19.3 m2 g−1 to 15.2 m2 g−1 after the base treatment at 20 ◦ C. At the
same time, the content of the sorption centers decreased, although
it was slightly greater after treatment at 105 ◦ C. This is probably due to the precipitation of amorphous and finely crystalline
F. Hussin et al. / Chemical Engineering Journal 170 (2011) 90–106
99
Table 4
Summary of advantages and disadvantages of inorganic and organic acid in acid activation.
Type of acid
Advantages
Inorganic acid
Inorganic acid
Increases BET surface area and pore volume
Fastest in dissolution of octahedral cation such as Fe, Mg
and Al
More effective in bleaching the palm oil
May generate new surface acid sites quickly
Inorganic acid
Inorganic acid
Inorganic acid
Inorganic acid
Organic acid
Organic acid
Organic acid
Disadvantages
May destroys the clay structure
Removal high content of free fatty acid, peroxide value and
phosphorus
Preserve the clay structure
Dissolves a great quantity of iron
Organic acid
Organic acid
Organic acid
Organic acid
silicates and aluminosilicates. After the treatment with NaOH solution at 20 ◦ C, the total content of the active centers increased
from 0.19 mg mol/100 g to 0.31 mg mol/100 g. After further treatment with 5.0N NaOH solution, the content of the Lewis acid and
base centers increased, and the content of the Bronsted centers
decreased. The specific surface area after the base treatment at
105 ◦ C substantially decreased to 10.5 m2 g−1 . It was found that by
using the base treatment, it shows a lower adsorption capacity to
absorb colouring matter and other impurities in oils and solutions
[2,127].
Okwara and Osoka [16] studied the influence of caustic activation of local clays for palm oil bleaching. Their findings indicated
that the adsorption capacity for bleaching earth increased up to 79%
at the optimum concentration of 1.0N NaOH. Compared to the base
treatment, acid treatment shows a much higher adsorption activity increase up to 99%. Even though there are a few studies for the
base activation effect on the adsorption capacity of bleaching earth,
they believe that base activation is also suitable for use in palm oil
bleaching. This is because the cost of sodium hydroxide (NaOH) is
much lesser than that of inorganic acid such as H2 SO4 and HCl [16].
2.5.3. Organic treatment
According to Gunawan et al. [32] the use of acid activated bentonite for the purification of palm oil has led to several problems
such as corrosion of the process vessels, increasing of free fatty
acids (FFA) and peroxide value (PV) in oil products and can cause
environmental problems. Therefore, in order to reduce these problems, bentonite was modified with cationic and anionic surfactants
for the removal colour pigment, free fatty acids and peroxide value
from bleaching palm oil. They used cetyl trimethylammonium bromide (CTAB) and linear alkylbenzene sulfonate (LAS) as surfactant
and the process was conducted under conventional heating and
microwave irradiation technique. The results showed that, the
highest colour removal was observed with LAS under microwave
irradiation technique. It indicated that LAS was more effective as
bleaching agent than CTAB [32].
2.5.4. Intercalation/pillaring
Pillaring is a type of modification methods which alters the
properties of bleaching earth by using combination of chemical
and physical treatment. Pillaring is defined as a process by which
a layered compound is transformed in a thermally stable microand/or mesoporous material with retention of the layer struc-
Reference
[107]
[107]
May decrease BET surface area and
pore volume
Less effective in generating surface
acid sites
Dissolves less of Al2 O3
Removal high content of free fatty acid,
peroxide value and phosphorus
Lower bleaching power and less
effective in bleaching palm oil
[97]
[106]
[121]
[29]
[106]
[126]
[121]
[106]
[121]
[63]
[97]
ture [81,106,128,129,130]. Pillared clay minerals have attracted
increasing attention, particularly from the industry since the 1970s,
because of their microporous nature and catalytic potential. However, pillared clay minerals have not been used as commercial
adsorbents and catalysts. This is because the treatment was developed in the laboratory; it is difficult to extend to an industrial scale
[35].
Generally, the pillaring treatment is presented as a way of
increasing the accessibility of the clay layers, but an alternative
viewpoint is that it stabilizes ‘oxide’ particles of nanometre dimension that prevent aggregation by interaction with the layers. Thus,
the surface groups exposed by these ultra-dispersed ‘oxides’ and
probably play a key role in determining the promising adsorptive
and catalytic properties of pillared clays [131]. The clays pillared
with metal oxides materials are of great importance because of
their high thermal stability, high specific surface area, and high
catalytic activity. These materials are usually prepared by ionexchanging cations in the interlayer region of swelling clays with
bulky alkylammonium ions, polynuclear complex ions bearing
inorganic ligands (hydroxo ligand or chloro ligand) and large metal
complex ions bearing organic ligands. The intercalated species are
capable of preventing the collapse of the interlayer spaces, propping open the layers as pillars, and forming interlayer space. At
high temperature, the intercalated inorganic species are converted
to metal oxide clusters, generating a stable microporous structure
with a high surface area [131]. Pillared clays having large surface
areas with high catalytic activity have been reported by numerous
researches and these higher surface area (>200 m2 g−1 ) clays could
be intended for use in bleaching vegetable oils. Because of their
enhanced acidity and high surface area, pillared clays are extensively used as adsorbents, catalysts, catalytic supports, membranes
and bleaching media [131].
Falaras et al. [132] investigated the physical and chemical properties of the pillared materials in order to determine the effect of the
capacity for adsorbing coloured species from vegetable oils and the
bleaching efficiency in the processing of Greek cottonseed oil. They
reported that, the bleaching efficiency of the final pillared product depends on the level of the acid activation. The results showed
that, the pillared materials have high bleaching efficiencies, comparable to that obtained with a commercial industrial bleaching earth
(Tonsil) and more than twice that of the untreated clay. Davies et al.
[133] in his patent (US 5486499) described the procedures for making pillared clay as effective adsorbents for use in the purification
of vegetable oils and found that pillared clay showed the highest
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F. Hussin et al. / Chemical Engineering Journal 170 (2011) 90–106
bleaching capacity to remove chlorophyll from the oils at 70% and
even up to about 99% in some instances.
In an article by Bergaya et al. [35] numerous studies have been
reported on the history, synthesis, properties and applications of
the pillared clays. However, there are a few studies on a new
method and the effective process of pillared clay. Bergaya et al.
[35] suggested that, to be of useful interest, these pillared clays
must be used in some industrial or environmental applications and
researches have to pay more attention to the fundamental point of
view towards the pillaring processing methods.
2.6. Modification of physical characteristics
2.6.1. Heat treatment
Apart from chemical modification, the physical modifications
using thermal treatment also play an important role for bleaching earth. The most common physical modification is thermal
treatment. The structure and composition of clay minerals can be
modified by heating at high temperature [36]. A similar description was made by Valenzuela-Diaz and Souza-Santos [2] stated
that thermal treatment is a method of alteration of chemical composition and/or crystalline structure by the effect of temperature.
Some physicochemical properties of bentonites such as swelling,
strength, cation exchange capacity, particle size, specific surface
area, surface acidity and catalytic activity as well as mineralogy can change considerably depending on the thermal treatment
[100,134]. Many studies have been reported on the adsorptive and
catalytic properties of acid activation of clays [7,12,23,50,132,135].
However, there are only few studies for the thermal effect on these
properties [134,136].
In a research article by Sarikaya et al. [134] the results of the thermal analysis, showed that dehydration is a physical event where
water is lost without structural loss, whereas the dehydroxylation
is a chemical event that invokes decomposition. As the temperature
increases to 400 ◦ C, cation exchange capacity decreased slightly
because of reversible dehydration. As the temperature increases
above 500 ◦ C, the space between the crystal layers was reduced
and swelling decreased gradually because of irreversible dehydration and dehydroxylation [134]. It was shown by Önal [101] that
the crystallinity and porosity of smectites found in bentonites as
major clay mineral reduce greatly by the thermal treatment. The
results revealed that the change by dehydration up to 400 ◦ C is
reversible. Such properties after heating above 600 ◦ C reduced irreversibly. Moreover the crystal structure of the smectite collapsed
irreversibly at 900 ◦ C. The specific micropore volume increased
rapidly to a maximum of 0.045 cm3 g−1 while the heating temperature rises to 400 ◦ C by the effect of dehydration and then
decreased rapidly by the effect of dehydroxylation and reached to
zero at 900 ◦ C by the effect of decrystallization. The maximum of
the specific mesopore volume was attained at 400 ◦ C, with the values of 0.115 cm3 g−1 . The specific mesopore volume did not change
considerably up to 800 ◦ C and then decreased rapidly while the
temperature increasing from 900 ◦ C to 1300 ◦ C. This is due to the
collapsing of the crystal structure of the smectite and interparticle sintering. The specific surface area of smectite increased to its
maximum of 90 m2 g−1 at 400 ◦ C, after which is decreased to zero
at 1300 ◦ C [101].
Mathers et al. [137] investigated the effect of acid and heat treatment on montmorillonoids. The results showed that heating Hand Al-montmorillonites at 100 ◦ C and 200 ◦ C for 12 h, caused considerable reduction in the amounts of exchange acidity, but did
not reduce water sorption. In contrast, the Li-montmorillonite suffered almost complete loss of cation exchange capacity and became
non-expanding on heating to 200 ◦ C. Heating at 300 ◦ C for 12 h
caused large loss in cation exchange capacity and produced irrevisible dehydration of both H- and Al-montmorillonites. This was
evidenced by the water sorption and the 0 0 1 spacing. The H- and
Al-nontronite, however, remained expanding and retained its lattice charge at 300 ◦ C. At 400 ◦ C, lattice hydroxyls had been expelled
from the nontronite, though some regularity in the C-direction persisted. In addition, montmorillonites with largely octahedral charge
had smaller cation exchange capacity after heating to 500 ◦ C than
did montmorillonites with appreciable tetrahedral charge [137].
Ilic et al. [138] stated the main changes revealed by heat treatment were initially observed at temperatures below about 200 ◦ C
where the release of water absorbed in pores and on the surfaces
occurs. Between 200 ◦ C and 450 ◦ C, mass loss attributed to the predehydration process takes place, as a result of the reorganization in
the octahedral layer. In the temperature range 450–650 ◦ C, dehydroxylation of kaolinite and formation of metakaolinite takes place,
while at about 1000 ◦ C, mullite was formed, as indicated by an
exothermic peak. The observed endothermic peak with a maximum at 552 ◦ C may be attributed to dehydroxylation process. The
results showed that at calcination temperatures of 550 ◦ C, 600 ◦ C
and 650 ◦ C, mass loss increases up to 2 h, while prolonged heating has a negligible effect on the mass loss. For all applied heating
times at temperature 700 ◦ C, the obtained values for mass loss are
nearly the same ∼12%. It can be seen that at calcination temperature 650 ◦ C and heating time 2 h, mass loss is almost identical with
the values obtained at 700 ◦ C. Therefore, the optimal parameters
for calcination are temperature of 650 ◦ C and heating time of 2 h
[138].
As reported by Štubňa et al. [139] dehydroxylation begins at
the temperature 420 ◦ C. When kaolinite is heated beyond the temperature of the dehydroxylation endotherm, a new phase called
a metakaolinite is formed. During this reaction, as XRD showed,
the higher-order reflections lost their intensity and vanished in the
XRD background. This result led to the opinion, that the matakaolinite can be amorphous, now a conception of the short-range order
crystalline structure of metakaolinite predominates. The loss of
high-order reflections indicates that dehydroxylation results in
structural disturbances through the breaking of unstable bonds. As
a result, the degree of ordering became lower than that in kaolinite as dehydroxylation progressed. The metakaolinite does not
collapse but, rather, retains a layered structure. In addition, the
result of this is decreasing of the specific mass of the kaolinite from
2.64 g cm−3 to 2.51 g cm−3 , increasing the porosity of the kaolin
from 45% to 50% and as well as increasing its specific surface area
to 5 m2 g−1 [139]. A similar observation was made by Varga [140]
the results of isothermal firing showed that the dehydroxylation
begins at 420 ◦ C. During dehydroxylation process, octahedral layer
is likely to be changed more than the tetrahedral silica layer.
Habashy et al. [63] reported increasing the heat treatment
temperature from 110 ◦ C to 150 ◦ C invariably involves a marked
increase in the specific surface area and total pore volume of the
treated clays. This is due to the loss of mechanically held water,
followed by the initial loss of lattice water of hydration occurring
at the first low stage, from the montmorillonitic structure present
in the tested clays. However, with a further increase in the temperature of treatment to 250 ◦ C, although the percentage loss still
increases because of the increased loss of the water content, it leads
to decrease in the specific surface area and the total pore volume of
the Maadi clay C (M). Similarly to the Alexandria-Cairo road clay C
(A), increasing the heat temperature, specific surface area and total
pore volume obtained also increased, after which it decreased [63].
Al-zahrani [121] compared the bleaching power of clay samples
by calcination and without prior calcination. The results indicated
that the maximum bleaching power of the local clay activated by
calcination or by acid treatment alone is low and rarely exceeds
50%. However, acid activation of the clay after calcination at 400 ◦ C
gives good results in the bleaching power, which values equivalent
to 83% [121].
F. Hussin et al. / Chemical Engineering Journal 170 (2011) 90–106
2.6.2. Microwave treatment
In addition to the physical modifications, microwave-heating
treatment also plays an important role for bleaching earth.
Microwave-heating techniques are now widely used in various
engineering applications including organic and inorganic synthesis [44,141]. Korichi et al. [141] studied the effect of acid activation
under microwave irradiation on the textural and structural properties of a smectite from Maghnia (Algeria). The results showed
that, the specific surface area, micropore and mesopore of smectite increased with increasing irradiation time. Therefore, the
irradiation power affects the textural characteristics and surface
chemistry of smectite. Korichi et al. [141] also reported that the
temperature and time required by the microwave-heating method
for preparing adsorbents are far shorter than by the conventional thermal activation method. Their findings indicated that,
the method is simple, economic, time saving and energy efficient.
Clark et al. [142] have discussed the advantages associated with
the microwave processing to be: (a) rapid and uniform heating; (b)
selective heating of microwave absorbing phases and components
in materials and products; (c) decreased sintering temperatures;
(d) enhanced physical and mechanical properties; (e) efficient processing of complex shapes; and (f) controlled rates and extent
of processing. These advantages are not observed in the conventional processes. The author have suggested the further research
and fundamental point of view towards the microwave processing
techniques will lead to a more effective and efficient manufacturing
of a wide range of applications and products [142].
Deng and Lin [143] stated more studies on the fundamental aspects of the microwave-heating method for solid dispersion
are needed in order to optimize the synthesis process. Such
a studies should include investigation of the effects of heating
atmosphere, microwave power and different casket materials on
the heating curves, ending temperature and dispersion of the
active species. Sorbents or catalysts with better properties could
be prepared when the microwave synthesis process is further
optimized. Baldassari et al. [144] synthesized organoclays using
both conventional-hydrothermal and microwave-hydrothermal
processes, in order to compare the results obtained with two different heating methods under the same reaction conditions. The
author conducted several experiments on an organic derivatization with the octadecyl-trimethylammonium cation. The results
showed that the intercalation is almost complete after 2 h irradiation in the microwave oven. However, with conventional heating,
the intercalation was completed only after a 6-h treatment. This
proved that the microwave-hydrothermal procedure is distinctly
superior to the conventional-hydrothermal processing, as a complete conversion of the original clay can be achieved in shorter
reaction times [144]. Yapar [145] stated the use of microwave for
drying purposes will cause a further reduction in the time required.
Application of microwave irradiation at 360 W for 5 min is enough
to achieve a 70% exchange efficiency which is comparable to the
organic cation exchange efficiencies reported using conventional
techniques. The author concluded that the use of microwave irradiation in the synthesis of organoclays is beneficial since almost
the same exchange efficiency was obtained in a very short time in
comparison to the conventional hydrothermal technique [145].
As reported by Zemanová et al. [146] the microwave heated
dispersions showed slightly higher layer charge reduction than
conventionally heated samples where no significant changes of
the layer charge were observed. On the other hand, a series of
solid powders with different interlayer lithium content treated
with microwaves revealed high layer charge reduction. The charge
reduction was much higher than for conventionally heated powders as the efficiency was enhanced by increasing interlayer lithium
content. It can be seen that the lithium migration into the structure of the montmorillonite, supported by microwaves through
101
ionic conduction is an extremely fast process, probably completed
within the first few minutes of the treatments. Moreover, the
infrared spectroscopy showed that the products obtained with
microwave treatments correspond well to the materials heated
conventionally for much longer periods of time. Hence, by applying microwave treatment, the time and energy required can be
significantly reduced [146].
Gunawan et al. [32] investigated the effect of organo-bentonite
for the purification of vegetable oils by using normal heating and
microwave irradiation method. They found that, by microwave
irradiation method, the amounts of free fatty acid (FFA) removed
during bleaching process are higher than by the conventional heating. However, a significant decrease in peroxide value (PV) was
observed for both methods, by microwave irradiation and conventional heating [32].
2.7. Optimum operating conditions
Despite numerous papers published on activation of bleaching
earth, there is still diminutive information on a full study pertaining
to the optimum and effective parameters for obtaining the highest bleaching capacity. Therefore, a technique known as design of
experiments (DOE) especially response surface methodology (RSM)
is a powerful and effective technique for process characterization,
optimization and modeling. The technique involves the process of
planning and designing an experiment. The collected data is subsequently analyzed and interpreted. The approach used by this
technique is to reduce the number of experiments, improves statistical interpretation and locate the optimum parameters for acid
activation [11,65,87]. As with most processing techniques, several variables affect the performance of bleaching earth. However,
a detailed study on the DOE technique cannot be presented due
to the lack of investigation on the variables using this technique.
Nevertheless, this study considers the effects of parameters on the
activation treatment of bleaching earth.
There are many factors influencing the bleaching performance
including type of adsorbent used, adsorbent dosage, effect of time
and bleaching temperature, effect of vacuum, effect of agitation, oil
retention, effect of moisture and effect of particle size. These factors
have been reviewed by some researchers such as reviews by Bockish [8]; Rich [25]; Richardson [26]; Patterson [27]; Morgan et al.
[33]; Zschau [17]; Zschau [47] and Brekke [147]. However, in this
chapter the most effective parameters on the activation treatment
are determined.
2.7.1. Parameters affecting the treatment techniques
2.7.1.1. The effect of acid concentration. This parameter has a significant effect in order to enhance the bleaching capacity in the
edible oil treatment. A study by Didi et al. [87] investigated the
effects of activation parameters, namely acid concentration, contact
time and solid/liquid ratio by using factorial experimental design
in order to enhance the bleaching capacity in the colza oil treatment. The bleaching capacity was chosen for the process response.
It could be seen that the acid concentration and activation time
influenced the bleaching capacity of bentonite greatly. The results
indicated that, the highest bleaching capacity for acid activation
of Algerian bentonite was 99% under optimum activation conditions when acid concentration used was 11.6N H2 SO4 . However,
an increase in the acid concentration beyond this limit would not
result in any significant change in the bleaching capacity. This is due
to the collapse of the clay mineral structure followed by a significant
decrease of the specific surface area. This, in turn, results in a reduction in the bleaching capacity [87]. Another study by Makhoukhi
et al. [148] using factorial experiment design, the highest bleaching
capacity for Algerian bentonite was found to be 99% under optimum acid concentration (12.8N H2 SO4 ). A similar observation was
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F. Hussin et al. / Chemical Engineering Journal 170 (2011) 90–106
made by Woumfo et al. [13] where with increasing acid concentration, the bleaching capacity increases to maximum, after which
it decreases. The results indicated that the concentrations below
than 0.7N HCl seemed insufficient for the attainment of maximum
efficiency within 4 h. Whereas, concentration of more than 1N HCl
become more and more ineffective due to the excess protons that
destroy the phyllosilicate of the material. Therefore, the maximum
efficiency of the smectite clay samples is obtained after activation
with 0.7N HCl and 1N HCl [13].
Myriam et al. [110] studied the structural and textural modifications of palygorskite and sepiolite under acid treatment. The
author reported that an increase in acid concentration resulted in a
progressive destruction of the structures of palygorskite and sepiolite. They also observed the greatest specific surface area obtained
for sepiolite was at 3N HCl and for palygorskite at 9N HCl. Suarez
Barrios et al. [107] showed that a significant increase in specific
surface area of the activated palygorskite was observed, with the
more dilute solution used (>1.0N HCl) [114]. Zhansheng et al. [149]
demonstrated the effects of the acid concentration in activation
on the performance of the acid activation clay in the bleaching
cottonseed oil. In their research, bentonite clay was treated with
5.6N, 7.5N, 9.4N, 11.3N, 15.0N and 16.9N H2 SO4 . They observed the
maximum bleaching capacity was obtained when bentonite clay
was treated with 9.4N H2 SO4 , which values equivalent to 70.3%,
73.1%, 83.2%, 81.8% and 88.9% respectively. Further increase in acid
concentration, would not result in any significant change in the
bleaching capacity [149].
Gonzalez et al. [150], reported with increasing acid concentration and reaction time, the condensation of the silanol groups
increased and, thus, the microporosity and the specific surface
area also decreased. The specific surface area of the magnesic
palygorskite reached a maximum, 399 m2 g−1 , three times that
of the original sample after treating with 2N HCl and decreased
smoothly with increasing acid concentration until, at 6N HC1 for
24 h, a drastic decrease of the specific surface, 197 m2 g−1 , was
observed. Moreover, the total volume of adsorbed N2 , increased
to 0.538 cm3 g−1 after the acid treatment until the concentration
reached 2N HC1 and remained constant with more severe acid
treatment [150].
In a research article by Rozic et al. [65] response surface methodology (RSM) method based on central composite rotatable design
(CCRD) was used to optimize the operating parameters of acid
activation of bentonite which include temperature, time, acid concentration, solid-to-liquid ratio and stirring rate. The results of
the acid activated bentonite were expressed in terms of the specific surface area, regarding as being the response function in
the investigated process. The author reported that an increase
in acid concentration greatly increased the specific surface area
of the activated bentonite clays from 63.4 m2 g−1 to 202 m2 g−1 .
They observed the maximum specific surface area of activated
bentonite was obtained when the temperature and acid concentration increased simultaneously. Their results showed that, textural
characteristics such as specific surface area and pore volume of
acid activated bentonite were strongly affected by the activation
parameters. They have suggested that an adsorbent with optimal
porosity and good adsorption properties can be produced by chemical treatment of the bentonite clay. A similar trend was observed
by Novakovic et al. [90] and Rozic et al. [91], the specific surface
areas increased greatly in activated bentonite clays with values up
to 238 m2 g−1 , three times higher than the surface area of natural
bentonite clays (63 m2 g−1 ). This phenomenon is corroborated by
the work of Vicente Rodriguez et al. [98] who observed an increase
in acid concentration resulted in a significantly increase in the
specific surface area of the activated saponite from 35 m2 g−1 to
367 m2 g−1 . A similar observation was made by Vicente Rodriguez
et al. [108] where an increase in acid concentration resulted in
Table 5
Acid concentration and corresponding optimum values.
Acid concentration
(N)
Optimum acid concentration (N)
Reference
1.0–8.0
1.0–7.0
3.0–6.0
2.0–7.0
1.0–3.0
9.0–13.5
10.5–13.5
5.6–16.9
0.4–6.5
1.0–6.0
2.0–20
4.0
5.0
6.0
4.5
1.0
11.6
12.8
9.4
0.4
2.0
20
[29]
[107]
[90]
[65]
[11]
[87]
[148]
[149]
[108]
[150]
[151]
a significant increase in the specific surface area of the activated
sepiolite.
Panda et al. [151] investigated the effect of the acid treatment on
the structural and chemical properties of kaoline clay. The results
indicated that as the acid concentration increases, the Al2 O3 , MgO
CaO and K2 O contents in the acid treated material decrease progressively. Simultaneously, SiO2 content increased with an increase
in the acid concentration, of which the Si/Al ratio increased from
0.65 to 8.09. In addition, the specific surface area and pore volume
are attained also increased when acid concentration increased. The
maximum specific surface area and pore volume were found to be
143 m2 g−1 and 1.18 c3 g−1 at 20N H2 SO4 [151].
On the other hand, Falaras et al. [29] observed that the specific
surface area of the activated Ca-montmorillonite increased from
93 m2 g−1 to its maximum, 107 m2 g−1 at 4N H2 SO4 and decreased
to 103 m2 g−1 at 8N H2 SO4 . Other figures reported by various
researchers are presented in Table 5.
2.7.1.2. Treatment time. In the work of Rozic et al. [91], the highest
leaching of MgO from Serbian smectite clay is determined using factorial and orthogonal central composite design methods. The result
indicated that the leaching yield of MgO depends significantly on
the acid concentration and reaction time. This is in agreement with
the work of Novakovic et al. [90], who asserted activation time
has a significant effect in the leaching yield of MgO from smectite clay. The results revealed that the highest leaching yield of
MgO was found to be 41.86% at 3 h activation time. Whereas, the
lowest leaching yield of MgO was obtained at 1 h activation time.
Foletto et al. [152] reported that the increase in treatment time
and acid concentration resulted in a more effective attack on the
clay structure, as observed by the decrease in the relative amount
of octahedral cations (Al, Mg and Fe). Hence, these parameters
resulted in an increase on the bleaching efficiency.
Vicente Rodriguez et al. [153] have investigated the structural
changes of sepiolite, palygorskite and two saponites occuring during the acid treatment using the FT-IR spectra. The results indicated
that 56.29% of the Mg(II) cations were removed when saponite was
treated with 0.4N HC1 for 6 h at 25 ◦ C. If the time of treatment
is extended to 48 h, 75.22% of the Mg(II) is removed. The sample
treated with 0.8N HC1 for 48 h has a maximum value of the BET
surface area, 392 m2 g−1 . The author showed that Mg(II) cations
removed were around 83.8% when griffithite was treated with 0.4N
HCl for 24 h and the amount increased to 100% if treated with 0.8N
HC1 for 24 h. Similarly with saponite, the maximum value of the
BET surface area was obtained at 0.8N HC1 for 48 h, of which value
is equivalent to 367 m2 g−1 . In addition, when treating the sepiolite sample with 0.8N HC1 for 6 h, 42.4% of the Mg(II) cations were
removed. Whereas, 100% of the Mg(II) cations were removed when
it was treated with 1.6N HCl for 48 h. The maximum in the BET surface area, 549 m2 g−1 , was reached after treatment with 0.4N HC1
for 48 h. For palygorskite, 50.3% of the Mg(II) cations were removed
F. Hussin et al. / Chemical Engineering Journal 170 (2011) 90–106
103
Table 7
Temperature and corresponding optimum values.
Table 6
Reaction time and corresponding optimum values.
Treatment time (h)
Optimum treatment time (h)
Reference
Temperature (◦ C)
Optimum temperature (◦ C)
Reference
1.0–7.0
0.5–7.0
2.0–6.0
6.0–12.0
2.0–3.5
1.0–3.0
1.0–3.0
4.0–8.0
2.0–48
2.0–48
2.0–48
7.0
4.0
6.0
10.0
2.0
3.0
3.0
7.0
48
24
48
[64]
[51]
[11]
[87]
[152]
[90]
[91]
[148]
[153]
[153]
[153]
100–900
70–90
70–90
63–97
60–120
96
22–120
100
90
90
90
90
96
120
[155]
[90]
[91]
[65]
[148]
[96]
[154]
after treatment with concentration (3N), activation time (1 h) and at
90 ◦ C temperature. Whereas, 99% is removed if 7N HC1 is employed.
The sample treated with 5N HC1 displayed a maximum value of
the surface area, 286 m2 g−1 . It could be seen that activation time
influences the dissolution rate of octahedral cation greatly [153].
Woumfo et al. [13] investigated the effect of activation parameters of Cameroonian smectite clays on its bleaching capacity. The
results indicated that by increasing the activation time from 2 to
8 h, the removal of carotene increases to a maximum, after which
it decreases. This is due to the progressive destruction of phyllosilicates and acid centers [13]. Gulsah Kirali and Lacin [11] studied
the effect of activation parameters of Turkish bentonite clay on its
bleaching capacity by using full factorial design. The highest bleaching capacity was obtained for clay activated for 6 h activation time,
whereas the lowest bleaching capacity was obtained for 2 h activation time. The results revealed that with increasing activation
time, the bleaching capacity increases to a maximum. Makhoukhi
et al. [148], studied the effect of three parameters namely; temperature, contact time and acid concentration on the bleaching capacity
of an Algerian bentonite in the colza oil processing. The author
reported that among the parameters investigated, acid concentration and activation time greatly influenced the bleaching capacity of
bentonite. Didi et al. [87] reported that a maximum bleaching effectiveness was obtained for an activation time of approximately 9 h.
Beyond these values, a significant reduction in bleaching activity
was observed and resulted in a significant decrease in the specific
surface area and structure porosity.
Srasra and Trabelsi-Ayedi [51] showed that the specific surface
area, the porosity and the microporosity of acid activated glauconite were strongly affected by the activation time. They observed
that with increasing activation time, the specific surface area, the
porosity and the microporosity obtained also increases. However,
the maximum bleaching efficiency of rapeseed oil is not associated with the maximum activation time. This result was further
supported by a research from Franus et al. [64] where textural
parameters of the glauconite increased with the activation time.
The result showed that the specific surface area increased from
78 m2 g−1 in natural glauconite to 329 m2 g−1 after longer activation time. This effect was also reported by Christidis et al. [49] where
the optimum conditions for activation can be achieved by varying
the combinations between acid concentration and activation time.
They concluded the optimum bleaching capacity of rapeseed oil
does not follow the variations of the specific surface area. The reaction time and corresponding optimum values are summarized in
Table 6.
2.7.1.3. Temperature effect. Novák and Číčel [96] showed the best
correlation was found between the decomposition rate expressed
as the halftime of dissolution and the octahedral substitution in
dioctahedral smectites. The decomposition rate of dioctahedral
smectites in 6N HCl at 96 ◦ C, increases with increasing substitu-
tion of Mg2+ and Fe3+ content in octahedral sheets. It can be seen
that temperature influences the dissolution rate of the octahedral
cations (e.g. Mg2+ and Fe3+ ).
Rozic et al. [91] revealed that temperature, leaching time and
acid concentration exert the strongest influence on the specific surface area of smectite. The author reported the highest leaching yield
of MgO is obtained at 90 ◦ C, whereas the lowest leaching yield of
MgO is attained at 70 ◦ C. However, the temperature does not seem
to significantly influence the leaching yield of MgO. Makhoukhi
et al. [148] showed that the temperature had no significant individual effect on the activation of bentonite. The results revealed
that the optimum bleaching capacity was obtained at temperature
(90 ◦ C). Therefore, between 90 ◦ C and 120 ◦ C, the phase temperature did not seem to significantly influence the bleaching capacity
of the bentonite. The work of Al-Zahrani et al. [154] stated that the
activation reaction does not proceed to any appreciable degree at
low temperatures. The results showed that the highest bleaching
power was found to be 101% after 0.25 h activation at 120 ◦ C and the
lowest bleaching power was attained after 1 h activation at 22 ◦ C,
which values equivalent to 8%.
Balci [155] reported that the maximum specific surface area
and textural properties of the sepiolite were obtained at 100 ◦ C
and decreased with further treatment of temperature to 200 ◦ C.
In contrast, Rozic et al. [65] reported that temperature and activation time had a highly significant effect on the specific surface
area and textural properties of the acid activated bentonite. The
author showed the maximum specific surface area of activated
bentonite was obtained when the temperature and acid concentration increased simultaneously. At high temperature phase and
low acid concentrations, high specific surface areas were achieved
[65]. When glauconite was activated by HCl at 60 ◦ C, there was progressive dissolution of the octahedral layer. Moreover, the specific
surface area and textural properties of activated glauconite depend
on the activation temperature. It was observed that the BET surface area was found to be 131 m2 g−1 after 3 h activation at 60 ◦ C
and 186.7 m2 g−1 , was obtained after 3 h activation at 100 ◦ C [64].
A similar trend was observed for the volume of micropores and
mesopores. The temperature and corresponding optimum values
are summarized in Table 7.
2.7.1.4. Solid/liquid ratio. When the solid to liquid ratio is increased
from 260 g/L to 340 g/L, it resulted in a slight increase in the bleaching capacity from 97.5% to 99%. However this parameter would not
result in any significant change in the bleaching capacity. Therefore,
this parameter is indicated as the weakest influence, as compared to
the other parameters [87]. A similar observation was made by Rozic
et al. [91] where solid to liquid ratio is not a strong influence on the
specific surface area of smectite, as compared to others parameters.
Al-zahrani et al. [154] studied the effect of solid to liquid ratio on
the bleaching power of the bentonite clay. The results revealed that
when the solid to liquid ratio increased to 0.05 g/L, the bleaching
power of activated clay also increased, whereas further increase to
0.07 g/L resulted in a decrease in the bleaching power. Therefore,
the best result of solid to liquid ratio was obtained at 0.05 g/L. At
low solid to liquid ratio, the contact between the acid and the clay
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F. Hussin et al. / Chemical Engineering Journal 170 (2011) 90–106
is lesser than when the ratio is high and the rate of reaction is slow
[154]. However, this parameter has a small effect on the bleaching power compared to the effects of acid concentration, activation
time and temperature.
Novakovic et al. [90] showed that an increase in the solid to
liquid ratio resulted in an increase of the leaching yield of MgO. This
effect was reported by Rozic et al. [65] where among the parameters
investigated, solid to liquid ratio, acid concentration and stirring
rate had significant effect. In contrast, Gulsah Kirali and Lacin [11]
observed that the highest bleaching capacity is attained at 500 g/L
solid to liquid ratio, whereas the lowest bleaching capacity was
obtained at 100 g/L solid to liquid ratio. The results indicated that
solid to liquid ratio has a significant effect on the bleaching capacity.
2.7.1.5. Moisture. According to Rich [25] bleaching earth normally
contains between 10% and 18% free moisture. If clay is dried prior
to bleaching, the layers collapse, resulting in impaired bleaching
power because of decrease in the specific surface area. However, in
the bleaching, the colour bodies cannot be adsorbed to the clay’s
maximum capacity until its free moisture has been removed by
heating.
Gulsah Kirali and Lacin [11] observed that the highest bleaching
capacity obtained for bentonite was at 4% moisture, whereas the
lowest bleaching capacity attained was also at 4% moisture. They
noted that the increase from 4% to 8% in the moisture of bentonite
had no significant effect on bleaching capacity of the cottonseed oil.
Oboh and Aworh, (1988) stated that with increasing acid concentration, the moisture content at which maximum percentage colour
reduction is obtained, decreases for clay activated with Sulfuric acid
(H2 SO4 ) and acetic acid (CH3 COOH). However, with increasing acid
concentration, the moisture content at which maximum percentage colour reduction is obtained, increases for clay activated with
HCl. The author showed that the highest colour reduction for clay
activated with H2 SO4 and HCl was attained at 11.02% and 13.37%
moisture, which is equivalent to 93.07% and 92.99%. Whereas the
highest colour removal was found to be 45.59% for clay activated
with CH3 COOH at 12.06% moisture. Therefore, the results revealed
that the highest colour removal is not associated with the maximum moisture content [97].
3. 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 or functional groups
and some technical improvements in the adsorption processes. In
addition, the performance of bleaching earth is also depending on
the activation treatment which results in structural modifications.
Therefore, it is very important to monitor the structural and textural modification of bleaching earth which occurs during acid, basic,
organic, thermal and pillaring treatment. However, there are several parameters that affect the effectiveness of these treatments
such as, type of acids used, acid concentration, temperature, reaction time, particle size and moisture content. Although much has
been accomplished in the areas of bleaching earth, a great deal of
work is necessary (i) to make in depth study on a new techniques
that are more environmental friendly (ii) to investigate a new alternative source for the bleaching earth (iii) to investigate an effective
technique for the bleaching earth that can be used for the purification of vegetable oils and (iv) to conduct a detailed study on the
most effective parameters of the bleaching earth using design of
experiments (DOE), where it is becoming a powerful tool for the
optimization of many processes.
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