Analysis, characterization and bleaching properties of

Bulletin of Pure and Applied Sciences Vol.35 F-Geology (No.1-2)2016:P.39-78

Print version ISSN 0970 4639 Online version ISSN 2320 3234

DOI 10.5958/2320-3234.2016.00004.4

ANALYSIS, CHARACTERIZATION AND BLEACHING

PROPERTIES OF SMECTITE FROM ILIRI ROAD

SWELLING CLAYS IN NORTH EASTERN UGANDA,

LEACHED IN DIFFERENT ACIDS

*Mukasa-Tebandeke, I.Z1 Wasajja-Navoyojo, Schumann, A2 and M. Ntale1

1Makerere University, Department of Chemistry, Box 7062, Kampala, Uganda

2Makerere University, Department of Geology and Petroleum geosciences, Box 7062,

Kampala, Uganda

* Person to whom correspondences can be made. ishamukasa <[email protected]

Recieved 01 March 2016 : Accepted 30 November 2016

ABSTRACT

Clay from Iliri was activated using hydrochloric and sulfuric acid of differing

concentrations. The chemical compositions of the raw clay and clay activated at

different acid concentrations were analyzed to investigate the extent of cation

dissolution. From the results, the clays were classified as sodium bentonite

mixed with feldspars, illite, kaolinite and plagioclase. When cotton and

sunflower oil was bleached at 90°C the optimal time was 25 minutes using1%

clay activated with 20% acid, the bleaching efficiency obtained were 90 and

80% for hydrochloric and sulfuric acid leached clays respectively. Clay leached

in 20% hydrochloric acid bleached to the same level as that leached in 30%

sulfuric acid. The equilibrium data was analyzed using Freundlich and Langmuir

adsorption isotherms and the former was found to provide a better fit for the

data. The values of the Freundlich constant, n, for cotton-seed oils were larger

than sunflower oils because the extent of bleaching differed. The values of k

increased with increase in strength of the acid used to the clay. Iliri clay

contained montmorillonite. The clay was made of 40-50% smectite, 20-30%

feldspars and plagioclase showing they formed from alkaline intrusive granitoids

in basic medium. The presence of iron, aluminium, and silicon in approximate

percentages of 11, 18 and 60% respectively indicated that the clay was

montmorillonite. The IR spectrum absorption peaks at 3600, 3454, 526 and

466cm-1 among others were due to Al-Al-OH, Mg-OH-Al, Si-O-Al and Si-O-Si

deformations respectively indicated presence of smectites in the clay. DTA

showed peaks at temperatures of 100, 250 and 650oC due to presence of

smectites.

Keywords: Clay from Iliri, Acidity, Isotherms, Activated clay, 1-aminobutane.

1.0 INTRODUCTION

1.1 Chemical analysis of clay

A variety of chemical analyses have been offered. Samples which can be taken into solution

for example in nitric acid are analyzed by ICP-MS or ICP-OES. Analysis of various materials

by X-ray fluorescence (XRF) can also be arranged. XRF is often more appropriate for many

Mukasa-Tebandeke, I.Z, Wasajja-Navoyojo, Schumann, A. and M. Ntale

40

geological materials where total elemental analysis is required, such as many rocks and soils.

These types of sample are often difficult to take in solution in a matrix suitable for analysis

by ICP, so XRF analysis is more appropriate when it is important to know true total

elemental concentrations.

Quantification of heavy and trace metal contamination in soil can be arduous, requiring the

use of lengthy and intricate extraction procedures which may or may not give reliable results.

Of the many procedures in publication, some are designed to operate within specific

parameters while others are designed for more broad application. Most procedures have been

modified since their inception which creates ambiguity as to which procedure is most

acceptable in a given situation.

Clays can be the reservoir for many harmful constituents, elemental and biological, including

heavy metals and trace metals. Total metal content of soils can be useful for many

geochemical applications but often the speciation (bioavailability) of these metals can be

more interesting agriculturally in terms of biologically extractability. Speciation is the

identification and quantification of the different, defined species, forms or phases in which an

element occurs. It is essentially a function of the mineralogy and chemistry of the soil sample

examined Quantification can be done using chemical solutions of varying, but specific,

strengths and reactivities to release metals from the different fractions of the examined soil

(Ryan et al, 2002). In terms of bioavailability, various species of metals may be more

biologically available than others. If bioavailability and the mobility of metals were related,

the higher the concentration of mobile toxic metals (Cu, Pb, Cd, and Al) in the soil column,

the higher the potential for plant uptake, and animal or / and human consumption (Ratuzny et

al, 2009).

1.2 Mineralogical analysis using X-ray diffraction

X-ray Powder Diffraction (XRD) is a versatile technique that can be used to identify any

crystalline substances, such as most minerals. It can also be used to quantify the proportions

of different minerals or indeed many other substances when they are present in a mixture.

Typically, clay mineral analysis involves the separation of a clay sized fraction (usually < 2

µm) from the sample. Once obtained the clay fraction is prepared by collecting it on a filter

and transferring the layer of clay to a glass slide substrate. This 'filter peel' method enhances

the preferred orientation of the platy clay particles which helps to obtain a good diffraction

signal from the diagnostic basal planes of the clay minerals. It is also the best way to make a

homogenous sample, essential is quantitative results are required. These oriented samples are

run on the diffractometer (air-dried) and then run again following various treatments such as

solvation with ethylene glycol, and heating to specified temperatures for specified times.

Other treatments may be appropriate for the identification of some clay minerals. Peak

positions, shapes and intensities and changes in these between treatments are diagnostic for

the identification of different clay minerals. Following identification, quantitative analysis

may be made by an intensity ratio method whereby the integrated intensity (peak area) of

selected clay mineral peaks is related to their weight fraction in the mixture by means of a

predetermined constant of proportionality termed mineral intensity factors (MIF) or more

generally known as Reference Intensity Ratios (RIR). These are most readily determined

from calculated one dimensional X-ray diffraction patterns, using for example the NEWMOD

program as described in Moore and Reynolds (1997). Precise identification of clay minerals

is the first step in any clay mineral analysis, and we base our identifications on years of

ANALYSIS, CHARACTERIZATION AND BLEACHING PROPERTIES OF SMECTITE FROM ILIRI

ROAD SWELLING CLAYS IN NORTH EASTERN UGANDA, LEACHED IN DIFFERENT ACIDS

41

experience of the interpretation of diffraction pattern. The clay powder x-ray diffraction

patterns were recorded on photographic films as series of lines in the form of arcs concentric

with the original x-ray beam and from the positions of lines, angles of diffraction and

separation spacing of planes producing them. XRD studies on montmorillonite (Mills et al,

1950) were used to show that clay exchanges oxygen in form of hydroxyl (SiOH) and (HOH)

with hydration of oxygen to surface exposing fresh oxygen previously in the body of the

oxide. XRD hot stage analysis was also used to demonstrate the presence of a more

thermally stable polymorph of the kaolinite (Ip et al, 2008; Mukasa-Tebandeke et al, 2015).

1.3 Structural studies using Infrared spectroscopic studies

Infrared spectroscopy (IR) gives a unique chemical overview of a sample, with all the

chemicals present contributing to the spectrum produced. The method allows the qualitative

analysis of a variety of samples ranging from biological samples to clay minerals.

Identification of the unknown is possible through interpretation of spectra, in conjunction

with the use of spectral libraries. The analysis done using IR is largely qualitative,

quantitative work is also possible.

The IR spectrum which represents a chemical fingerprint of the sample is a plot of

absorbance on the y-axis (zero absorbance = 100% Transmittance) at the top and total

absorbance (0%Transmittance) at the bottom versus frequency in the x-axis, the unit of

frequency is the wave number (cm-1). Absorption band intensities are determined by the

strength of the change in dipole moment involved in the particular inter-atomic vibration. The

stronger the dipole moment change the greater the band intensity.

Many samples analyzed using the FTIR spectrometer involve identification of components in

complex mixtures such as scales, deposits, drilling fluids and muds from the oil industry.

Interpretation of IR spectra can be achieved using functional group frequency tables,

computer search of in-house and commercial IR spectral libraries and recognition of

absorption band patterns gained by experience. A high match index does not necessarily

mean a successful identification hence close comparison of the unknown sample spectrum

and the search match spectrum by the analyst is highly recommended before a conclusion is

reached. It has been noted that all the bands have to match the reference mineral spectrum not

just one or two bands for a positive identification.

Different groups in clays show different stretching modes at different frequencies in the infra

red region of the electromagnetic spectrum. The IR absorption spectrum for the smectite-rich

natural clay was shown to have absorption band at 3640 cm-1 attributed to stretching

vibrations of the OH group while that at 3454 cm-1, the presence of interlayer water. The

amount of adsorbed water in clays is related to the deformation vibrations of the H–O–H

group (1664 cm-1). The bands at 1042 and 798 cm-1 are attributed to Si-O stretching

vibrations (Christidis et al, 1995). The bands at 526 and 466 cm-1 correspond to deformation

vibrations of Si–O–Al and Si–O–Si, respectively (Mukasa-Tebandeke et al, 2015; Russell,

1979; Volzone et al, 2003).

1.4 Structural studies using differential thermal Analysis (DTA)

DTA is a thermo analytic technique in which the material under study and an inert reference

are made to undergo identical thermal cycles, while recording any temperature difference


42

between sample and reference. This differential temperature is then plotted against time, or

against temperature (DTA curve or thermo gram). Changes in the sample, either exothermic

or endothermic, can be detected relative to the inert reference. Thus, a DTA curve provides

data on the transformations that have occurred, such as glass transitions, crystallization,

melting and sublimation. The area under a DTA peak is the enthalpy change and is not

affected by the heat capacity of the sample. Modified DTA is incorporated technology to a

thermo gravimetric analysis (TGA), which provides both mass loss and thermal information.

With today’s advancements in software, even these instruments are being replaced by true

TGA-DSC instruments that can provide the temperature and heat flow of the sample,

simultaneously with mass loss. A DTA consists of a sample holder comprising

thermocouples, sample containers, a ceramic or metallic furnace, a temperature programmer;

and a recording system. The basic configuration is the two thermocouples are connected in a

differential arrangement and connected to a high gain low noise differential amplifier. One

thermocouple is placed in an inert material such as Al2O3, while the other is placed in a

sample of the material under study. As the temperature is increased, there will be a brief

deflection of the voltage if the sample is undergoing a phase transition. This occurs because

the input of heat will raise the temperature of the inert substance, but be incorporated as latent

heat in the material changing phase. A DTA curve can be used only as a finger print for

identification purposes but usually the applications of this method are the determination of

phase diagrams, heat change measurements and decomposition in various atmospheres.

DTA may be used in cement chemistry, mineralogical research and in environmental studies

The different temperatures required to extract structural water from the lattices of clay

minerals reflected the types of minerals in the clay. The amount of water released when clay

was heated at high temperatures. The kaolinite minerals gave large endothermic peak on the

DTA curve with a maximum in the range 500-600oC which is more prominent than similar

peaks with the 550-700oC region produced by illite. The different temperatures required to

extract structural water from the lattices of clay minerals reflected the clay-mineral types.

The amount of water released when clay was heated at high temperatures (Mukasa-

Tebandeke et al, 2015; Stab et al, 2006). Pure crystalline kaolin shows a very sharp

exothermic peak at 980o- 1000oC. However, this peak is broader, smaller and occurs at lower

temperature for less ordered material (Stab et al, 2006).

1.5 Bleaching of oils

Bleaching of oils is a process whereby the clay adsorbent mixed with oil under specified

conditions removes unwanted color bodies and contaminants (Richardson, 1978). The

primary function of the bleaching process is to remove pigments, gums, trace metals,

peroxides and secondary oxidation products (Wiedermann, 1981). Four basic steps are used

to refine oil: neutralization and separation, bleaching and deodorizing (Siddiqui, 1968;

Zschau, 1983). Diatomaceous earth, clays, peroxides or carbon is added to bleach and

adsorb the dark colored impurities in the oil in order to give it a clear color (Adekeye and

Bale, 2008; Mukasa-Tebandeke et al, 2015; Naeem and Ahmad, 2008; Salawudeen et al,

2007; Usman et al, 2013). The bleaching process combines catalytic action such as peroxide

decomposition and equilibrium adsorption of pigments from oil (Reddy et al, 2001). The

peroxides decompose to volatile aldehydes and ketones due to further oxidation as shown

below in equation 1.2a-1.2c, then they get adsorbed onto clay (Subramanian et al, 1993).



43

-CH=CH-CH=CH-CH-CH2- activated clay -CH=CH-CH=CHCOCH2- +H2O ……..1.2a

OOH

(peroxide) (Secondary oxidation product)

-CH=CH-CH=CH-CH-CH2- activated clay -CH=CH-CH=CH-CH-CH=CH-+

l H2O……1.2b

OOH

-CH=CH-CH2-CHOH-CH2- Activated clay -CH=CH-CH=CH-CH2- + H2O

………..1.2c

Bleaching is carried out under steam, nitrogen or vacuum to minimize oxidation of oils by

oxygen at elevated temperatures (Reddy et al, 2001).

Clays bleach by interacting with the medium in which the materials or impurities to be

removed are present. A nutritive component of crude oils, B-carotene, decomposes on

exposure to light or oxygen. Decomposition of β-carotene is catalyzed metallic ions like Fe3+

or Cu2+ which may be present on clay surface. Metal ions catalyze the reaction or serve as

active sites for the chemisorption of β-carotene (Choo et al, 1993). Pigments dissolved or

dispersed in crude oil have olefinic groups which can be protonated by the acidic clay groups

(Hui, 1996). Adsorption of carotenoids can be catalyzed by Broensted and Lewis acidity as

shown in equations 1.3a and 1.3b below.

1.3a


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1.3b

β-carotenes attach to clay surface in form of carbocations either through formation of

coordinate bonds with Lewis sites or by formation of hydrogen bonds with Broensted sites of

the bleaching earths (Srasra and Trabelshi-Ayedi, 2000; Sarrier and Guller, 1989). β-

carotenoids behave as both electron donors and acceptors, so they react with acid centers of

bleaching earths surface by means of its seventh atom (Richardson, 1978). The pigment

molecules are held on the clay surface by electrostatic attraction (Patterson, 1992; Toro-

Vazquez, 1991).

In order to bleach cottonseed and sunflower seed oil, optimum conditions for sulfuric acid

activation of the raw bentonites were investigated, which were obtained by selecting various

acid strength, at 96-98C and activating for 4 h with 1:2 solid-liquid ratio. The acid

activation bentonites were suitable for decolorization of cottonseed oil through removing

carotene and chlorophyll. The high bleaching capabilities of different pigments with activated

bentonite was achieved after treating it with 25% sulfuric acid (Zhansheng et al, 2006).

High bleaching performance depends on activation conditions such as acid concentration,

contact time and the temperature of the activation process, all of which contribute to

structural modification (Farihahusnah et al, 2011). Many studies have reported successful

bleaching of oil using acid activated clays from across the world (Diaz and de Souza, 2001;

Makhoukhi et al, 2008; Mukasa-Tebandeke et al, 2015; Ujeneza et al, 2014; Usman et al,

2013). The distribution of color materials between the liquid phase and adsorbent is a

measure of equilibrium position in the sorption process and can generally be expressed by

one or more series of isotherms (Kaynak et al, 2004).



45

The removal of the main color pigments in neutralized sun flower oils with different type of

commercial bleaching earth materials was examined in a pilot system at temperature of

105ºC, pressure of 50 mmHg and varying bleaching earth doses (Topkafa et al, 2013).

The Freundlich isotherm model is given in equation 1

lnq = lnk +n lnC

………………………………………………………………………………..1

where q is the amount of the sorbed analyte per unit weight of the solid phase at the

equilibrium concentration, C; the Freundlich constant, k, is related to the sorption capacity;

and 1/n is related to the sorption intensity of a sorbent (Kara et al., 2008). The bleaching

capacity of an adsorbent and its characteristic manner of adsorption may be described,

respectively, by the k and n parameters defined by Freundlich (Rossi et al., 2003). The k

constant is a rough measure of the surface area of the adsorbent (Achife et al., 1989). The 1/n

value ranges between 0 and 1, and if the numerical value of 1/n is less than 1, it indicates a

favorable sorption (Gezici et al., 2007;Ahmaruzzaman et al., 2005). The Langmuir model

assumes uniform energies of sorption on the surface and no transmigration of sorbate in the

plane of the surface (Kara et al., 2008). The most important model of monolayer adsorption

came from the work of Langmuir (Tor et al., 2006; Langmuir, 1916). In this study, the

sorption characteristics of color bodies to the clays have been discussed on the basis of

Freundlich and Langmuir isotherms (Kaynak et al, 2004).

The decrease in absorbance with increase in temperature and concentration of acid used, has

been interpreted to show that the bleaching capacity of the clays increased (Rich, 1964;

Topallar, 1998). The increase in bleaching ability was caused by removal of octahedral ions

from the clay matrices by the acid and dissociation of water, to produce hydroxyl groups on

the clay surface (Jadambaa, 2006; Folleto et al, 2009). For sunflower and cotton seed oils,

the relative changes in pigment adsorbed, x, mass of clay used, m, and the residual relative

quantity in equilibrium, Xe, showed that effective bleaching occurred. The absorbance of

bleached cotton-seed oils decreased as the temperature of activation and concentration of the

acid in the leaching medium increased. This showed that the amount of pigment adsorbed, x,

increased and the residual relative amount at equilibrium Xe, decreased for the cotton-seed

oils which were bleached.

An isotherm is the expression of the relationship between the partial pressure of adsorbate

gas, or solute concentration in solution, and the surface coverage of the adsorbent at a

constant temperature (Gregg and Sing, 1997). The Langmuir isotherm has been used to

describe the oil pigment adsorption and adsorption of other minor oil solutes during oil

processing. A plot of p/n against p gives a linear relation (Yuksel, 2003). The Langmuir

isotherm has been applied to pigment adsorption from vegetable oil.

The Freundlich equation modified by Boki et al, (1994) was used to relate the quantity of

impurities adsorbed, x, and residual relative quantity of impurity remaining in the oil, Xe.


46

1.6 Activation of clays

Activation of clay is a chemical or physicochemical treatment applied to clay to develop

capacity to adsorb coloring matter and other impurities in vegetable, animal or petroleum oils

(Lamar, 1951). The activity of clay denotes surface chemical and physicochemical reactivity

leading to increase in surface area of solid (Gregg and Sing, 1982). Activation of clay aims at

producing modifications on surface of material for use as adsorbent for liquid, solid or gas or

obtain high rate of reaction or dissolution.. Thermal dehydroxilation occurs at high

temperature between 600 and 1000oC follows the chemical reaction shown below in equation

2,

2OH- H2O + O2- ……………………………………………….2

Acid activation involves removal of exchangeable interlayer ions as well as dissolution of

tetrahedral and octahedral ions from alumina and silica layers (Vanzuela-Diaz and Souza-

Santos, 2001) and replaces them with hydrogen causing structural deformations. The

composition and texture of clay changes when leached (Franus et al, 2004; Rozic et al, 2010).

Activation of clay or a clay mineral and activated clays cover a wide range of chemical

treatments, most of them aimed at producing modifications on the surface of the clay mineral

crystals. Besides the acid activated smectites or bentonites the following are some of the

treatments used: sodium exchanged smectites, quaternary ammonium exchanged smectites

(organophilic bentonites); organic clayed kaolinites (oleic acid; silanes; organic polymers);

surface modified kaolinites; intercalated inorganic polyhydroxications; pillared smectites;

thermally activated kaolinites and palygorskite-sepiolite; thermally activated bauxites

(crystalline aluminium hydroxides).

The classes of methods used to enhance or to activate the properties of a natural clay or clay

mineral for several industrial uses include physical alteration of particle size (specific surface

area), thermal alteration of chemical composition and/or crystalline structure by the effect of

temperature, chemical alteration which is usually limited to ionic exchange: therefore, it does

not include massive chemical destruction of the clay mineral structure and pillaring regarded

as using chemical and physical restructuring of the clay mineral structure to increase capacity

for adsorption or to make spaces that encourage adsorption of specific ions. Natural clays

need to undergo appropriate physical or chemical treatments such as acid activation, ion

exchange and heating in order to increase surface properties, adsorption capacity, and meet a

range of other application requirements (Rossi et al, 2003; Rožic et al, 2010; Foletto et al,

2011). It was observed that activated clay adsorb color pigments more than raw clays

(Mukasa-Tebandeke et al, 2006) and concluded that modified clay minerals have a high

potential of serving as an alternative to the most widely used and high-cost activated carbon

(Ajemba and Onukwuli, 2013). Some properties of clays are improved on mineral acid

activation include, surface area, porosity and acid sites (Kaviratna and Pinnavaia, 1994).

Acid activated bentonites are widely used as catalysts, catalyst supports in the chemical

industry (Zhou et al, 2004), and a component of carbonless copying papers. They are

importantly applied in the purification, decolorization and stabilization of vegetable oils in

term of market consumption. They are able to remove undesirable colors by decreasing the

levels of chlorophyll, carotene and other color bodies, to reduce traces of copper (II) , iron

(III), phospholipids and soap and to minimize the increasing of free fatty acid during



47

bleaching (Rossi et al, 2001; Srasra et al, 1989; Rossi et al, 2003; Sarrier and Guler, 1988;

Gonzalez et al, 1994).

1.8 Objectives of study

The purpose of this study has been to identify clay in the swelling deposits at Iliri and

investigate its capacity to bleach cotton and sunflower oils.

2.0 MATERIALS AND METHODS

, 2.1 Location

Iliri is located in Bokora Corridor Wildlife Reserve, which lies between Toror and Napak

mountains in North Eastern Uganda’s Karamoja region. The raw swelling clay from the

deposit in Iliri was collected from the roadside from Katakwi to Moroto from a ditch 35 cm

deep.

2.2 Preparation of clay

Composites of clays collected from Iliri were soaked in distilled water, sieved to pass through

a mesh of 5.3 x 10-4 m diameter, dried at 105 oC and pound using a porcelain mortar and

pestle, then sieved to eliminate sand.

2.3 Preparation of oils

Sunflower seeds and cotton seeds were collected from Kampala’s St Balikuddembe market

and Jinja oil mills, pressed to collect the different oils.

2.4 Chemical analyses

The chemical analyses of clays was done following the method of Hutchinson (1974). Silica

was determined by gravimetry and the other elements were analyzed using the Perkin-Elmer

3030 model Atomic absorption spectrometer after dissolution of the sample in the

hydrofluoric acid-perchloric acid digestion mixture.

2.5 X-ray Powder diffraction

The mineralogy of clays was determined using X-ray Powder diffraction (Philips

diffractometer with PW1710 control unit operating at 40kV and 30mA using the Ni-filtered

Cu K radiation). The diffractograms was automatically matched with JCPDS-cards in the

computerized XRD CD-rom. Bulk mineralogy was studied with randomly oriented air dried

samples (Reynolds and Moore, 1989).

2.6 Infrared spectroscopic studies

The clay powder (0.003 g) was mixed with potassium bromide (0.1g) ground to powder,

pressed into discs. The infrared spectra was run using the KBr discs using B10RD FT540

Fourier Transform IR spectrometer in the frequency range of 3700 – 400 cm-1 (Russell,

1979).

http://www.triposo.com/loc/Bokora_Corridor_Wildlife_Reserve

javascript:void(0)

http://www.triposo.com/loc/Uganda


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2.7 Differential Thermal Analysis

Clay sample (0.5 g, 1.2mmol) and thermally inert reference sample, calcined alumina, was

placed in cavities of a nickel block and heated to 1100o C at the rate of 12o C per minute. The

changes in mass and temperature of the sample in comparison to the reference was

automatically recorded on a DTA graph. DTA studies were conducted with differential

thermal analyzer.

2.8 Calcining of clay

Clay samples and de-oiled spent clay (200.0 g, 0.32 mol) was placed in a furnace operated at

the temperatures ranging from 450-500o C for two hours (Al-Zaharani and Alhamed, 1996;

Foletto et al, 2001).

2.9 Acid activation of clay

Clay powder (100.0 g, 0.25mol) was mixed separately with hydrochloric and sulfuric acid of

appropriate concentration (500.0 cm3) in a flask. The mixture was heated at 105o C for 4

hours; then cooled and filtered. The residue was washed to neutrality with distilled water;

then dried at 105o C in the thermo-stated oven. The dried leached powders was labeled and

stored for future use (Beneke and Lagaly, 2002; Didi et al, 2009; Fahn 1976; Nwabanne and

Ekwu, 2013).

2.10 Degumming of vegetable oils

Crude oil (100.0 g) was placed in a flask, 85% phosphoric acid (1.0 g, 0.1 mmol) added, the

mixture heated at 90o C while stirring at 900 revolutions per minute for 10 minutes under

nitrogen blanket. The oil was filtered under nitrogen (Sadia, 1992).

2.11 Neutralization of oil

Mixture of the degummed oil (200.0 g, 0.85 mol) and 01.M sodium hydroxide (10.0 cm3) was

placed in 250 cm3 Pyrex glass flasks, fitted with a magnetic stirrer. The mixture was stirred

vigorously for 10 minutes at room temperature and filtered. Neutralized cottonseed oil used

in this study was bought from local market. All chemicals used were analytical grade.

2.12 Bleaching of oil

Mixture of the degummed, neutralized oil (200.0 g, 0.85 mol) and appropriate clay powder

masses (2,4, 6, 8, or and 10.0 g,) was placed in 250 cm3 Pyrex glass flasks, fitted with a

magnetic stirrer. The flask was immersed in a thermo-stated iso-electric mantle at various

temperatures of 40, 50, 60, 70 …130 oC. The mixture was heated while stirring continuously

for a further two hours at the set temperature under high vacuum (Patterson, 1992). The hot

oil and clay mixture was filtered in nitrogen atmosphere using a vacuum suction pump. The

bleached oil was tested by measuring absorbance (Rožic et al, 2010).



49

2.13 Absorbance of bleached oils

The absorbance of bleached oil was determined using ultra violet-visible spectrophotometer,

Shimadzu, 1201. The absorbance of the bleached oils at 550 nm was determined for each oil

sample obtained at the different temperatures of activation.

3.0 RESULTS AND DISCUSSION

3.1 Clay analysis

The chemical analysis of sample was carried out and the data were presented in Table 1. In a

study involving clays, it is important to establish the elemental constitution of the solid

because surface and bleaching properties of clays and clay material depend on the elements

present. The averaged data obtained in the elemental analyses of Iliri clays is shown in Table

1.

Table 1 The average chemical composition of the raw clays

clay component SiO2 Al2O3 Fe2O3 MgO CaO Na2O K2O LOI

Iliri % 46.7 15.9 7.8 1.5 2.0 3.8 1.3 14.5

Iliri clay had high silica content showing presence of quartz in the mineral mixture. The high

quartz content suggests that the clay had been transported via erosion fro where it was mined.

The clays from magmatic sediments in oceans were reported to contain major elements with

relatively high magnesium oxide, ranging from 23.9 to 40.4 % and low silicon dioxide and

aluminium oxide, ranging from 12.8 to 29.0% (Herzig et al, 1998) . However, clays from

Iliri are associated with volcanic margins of Toror and Napak mountains. The loss on ignition

(LOI) of the selected clays used in this study is less than 6% showing that heating clays to

105oC did not results in loss of structural water from the clay (Ball, 1964). The reaction

occurring in the clay materials was dehydration of soil used. The ratio of Na2O to CaO was

1.2, a value greater than 1, which is indicative of the presence of swelling bentonite (Basim,

2011; Usman et al, 2012). Therefore, the clay was classified as sodium bentonite or

montmorillonite.

3.2 X-Ray Diffraction studies

The XRD patterns indicated that the sample consisted of predominantly montmorillonite,

substantial amounts of quartz and feldspar impurities, in addition to minor amounts of illite

and kaolinite. The mineralogical compositions, determined using the Reynold’s semi-

quantitative method for the Iliri clay was found to consist of k-feldspars 24.5%, kaolin 7.7%,

quartz 15.4%, illite 5.5%, plagioclase 4.2%, and smectite 41.2%. The dominant mineral in

Iliri clay is smectite and this coincided with studies on clays formed from volcanic sediments.

The presence of montmorillonite among volcanic sediments is documented in literature

(Christidis et al, 2001; Hamza, 1968; Mills, 1958; Nuttings, 1933). The main smectite present

in the Iliri road swelling clay is nontronite, which is a di-octahedral clay as shown by the X-

ray pattern in Figure 1. Powder XRD patterns can be used to characterize minerals and

elements present in solids. Clay powders can be distinguished in two broad classes: smectites

and kaolinites basing on XRD patterns. XRD analyses provide vital information in

elucidating clay structures (Reynolds and Moore, 1989). Figure 1 is the XRD pattern of the


50

natural Iliri bentonite. It shows the presence of smectite (S), kaolinite (K), feldspars (Kf),

plagioclase (Pc), and quartz (Q) (Brindley and Brown, 1980). The clay contained quartz

because it had been transported by erosion to valleys where it was mined. Quartz may have

represented unaltered clay

precursors.

Figure 1: XRD pattern of selected clay from Iliri

The presence of K-feldspars and plagioclase in these clays showed that the parent rocks were

volcanic ashes (Christidis et al, 1995). The presence of nontronite may be responsible for the

swelling nature of the clay at Iliri. Sodium smectites are known to have high swelling and

cation exchange capacities.

The percentages of iron, aluminium and silicon among bentonites were shown to be

approximately 11, 18, and 60% respectively (Al-Zaharani and Alhamed, 1996). Basing on

this, clays from Iliri ressembled bentonites as it contains 65% SiO2, 15% Al2O3, 5% Fe2O3,

2% MgO, 2%Na2O. On the basis of relative percentages of aluminium, silicon and alkaline

metals or alkaline earth metals (Gates, 2002), the clay studied have been found to satisfy the

formula (½Ca,Na)0.33(Mg,Fe+2)3(Si,Al)4O10(OH)2·4H2O4H2O and

Ca.5(Si7Al.8Fe.2)(Fe3.5Al.4Mg.1)O20 (OH)4 for montmorillonite and nontronite respectively

(Gates et al, 2002). Basing on the clay minerals present in the sample it can be concluded

c

o

u

n

t

s

http://en.wikipedia.org/wiki/Calcium

http://en.wikipedia.org/wiki/Sodium

http://en.wikipedia.org/wiki/Magnesium

http://en.wikipedia.org/wiki/Iron

http://en.wikipedia.org/wiki/Silicon

http://en.wikipedia.org/wiki/Aluminum

http://en.wikipedia.org/wiki/Water

http://en.wikipedia.org/wiki/Water

http://en.wikipedia.org/wiki/Calcium

http://en.wikipedia.org/wiki/Silicon

http://en.wikipedia.org/wiki/Magnesium



51

that smectites occur in Iliri clays lying in the Bokora wild reserve located between Toror and

Napak, well known volcanic mountains.

3.3 IR

Infrared spectroscopy may give chemical overview of a sample, with all the chemicals

present contributing to the spectrum produced. The method can be used to qualitatively

analyze a variety of samples ranging from biological samples to clay minerals. The spectrum

in Figure 2 is for clay from Iliri. It consists of several peaks at different frequencies.

Figure 2: IR spectrum for Iliri clay

The band at 1040 cm-1 has been assigned to (Si-O). The band at 3454 cm-1 was due to

adsorbed water, and that at 3640 cm-1 was due to (Al-Al-OH, Mg–OH–Al). The absorption

bands at 918 and 879cm-1 in were obscured by Si-O mode which broadened. These bands are

due to the bending mode of Al-Fe-OH bonds and smectites are also expected to show

absorption bands at 845cm-1 (Brigatti et al, 2006; Christidis et al, 1995). Some absorption

peaks were not well resolved because the different clay minerals were mixed together.

3.4 Differential thermal analyses (DTA)

DTA curves showed that clays from Central Uganda contained kaolinites as they showed

peaks at 150, 530 and 600oC. As kaolinites show minimal bleaching tendancy and surface

Si-O-Al

H-O-H

Si-O-Si

Si-O-H

MgOHAl

Al-AlOH

Al-FeOH

Si-O-Si


52

acidity, it can be concluded that DTA curves can be used to identify kaolinites and smectites.

Further more, on the basis that smectites gain bleaching capacity at faster rate than kaolinites,

it can be inferred that DTA curves can be used to identify clays that can be very useful as

bleaching earths. Generally, DTA curves showed a small change in the endothermic peak

after acid treatment, corresponding to the loss of adsorbed water (at about 150°C), and peaks

corresponding to the loss of structural hydroxyl groups (530 and 600°C) (Grim and Roland,

1958; Brindley and Brown, 1980; Lombardi et al, 2002). The endothermic peak in the range

of temperatures between 550 and 660oC is attributed to dehydration of the mineral leading to

formation of γ-aluminium oxide in kaolinite clays (Insley and Ewell, 1935). The halloysites

showed the same thermal reactions as kaolinites with additional sharp endothermic peak in

the temperature range of 100 and 150oC accompanying loss of water and transition to

metahalloysite (Hendricks, 1938). DTA curve showed no evidence of the difference between

halloysite and kaolinite except for the difference in the initial endothermic peaks representing

water loss at low energy levels (Grim and Rowland, 1958). The differential thermal analysis

data presented in Figure 3 showed that the clays lost inter layer water molecules at a

temperature range of 230oC and 210oC. The clay samples studied exhibited endothermic

peaks at 140o and 210oC due to loss of hydration water (Nyakairu et al, 2001). Smectite clays

showed exothermic peaks in the range of 900 – 1000oC.

Key: For the curve below: 1 is dehydration, 2 is dehydroxylation, 3 is calcinations and 4 is recrystallization.

Figure 3: Thermal differential analysis of Iliri clay

The thermal properties of the raw bentonite is shown in Figure.3, where the DTA curve has

two endothermic peaks at 120-125C and 680-700C, and exothermic peak at high

temperature near 1200C. Each endothermic peak indicates the loss of different forms

water(adsorption water molecules and chemically bound OH－ groups) from the raw

bentonite mineral. Exothermic peak is correspondingly due to the appearance of new

crystallization. Illite showed three endothermic peaks in the temperatures between 100 and

200 oC, 500 oC and 650 oC and at about 900 oC. This gives three endothermic peaks. The

second peak in the temperature range of 500 to 650 oC represents loss of most of the water

from the clay lattice. The third peak is associated with the break down of the crystal structure

(Grim and Bradley, 1940). Halloysite, kaolinite and illite show endothermic peak at

temperature between 500 and 600 oC attributed to loss of water (Grim and Rowland, 1958).



53

Montmorillonite showed initial endothermic peak at temperature between 100 and 250 oC due

to loss of water held between the basal planes of the lattice structure (Hendricks et al, 1938).

The initial peak is large and extended over a wide temperature range, indicating that the clay

possessed water molecules other than pore water which causes no thermal reaction above 100 oC. Lattice water is lost at temperatures higher than 100 oC (Grim, 1942; Hendricks and

Jefferson, 1941). The thermal stability of montmorillonite is related to crystalline structure

(Lombardi et al, 2002). The loss of hydroxide ions from clay material causes irreversible

modification of the crystal structure, producing the endothermic peak at temperatures

between 650 and 700 oC (Sudo and Shimada, 1970) which suggests the clay has high thermal

stability. The deformations in crystal structure of montmorillonite clay are initiated by

different isomorphic substitutions which cause a temperature shift of the peak (Siguin et al,

1994). The higher the degree of octahedral substitution, the lower the hydroxyl release

temperature (Lombardi et al, 2002). The high octahedral substitution of iron in

montmorillonite clay causes a shift of the endothermic peak from at a range of 544 oC to 600 oC, due to isomorphic substitutions of the 0.60 octahedral of the bentonite (Lombardi et al,

2002). As Figure 3 has peaks at 140o, 210, 650 and 900 oC, it has been asserted that there are

three different clay phases in the Iliri sample, namely kaolinite, illite and montmorillonite.

This showed that the selected clay has similar structure to montmorillonite.

3.5 Acid activation

The concentration of hydrochloric and sulphuric acid when varied between 0 and 30%, to

attack the interlayer cations and replace them with hydrogen ions (Didi et al, 2009). The

extent of removal of octahedral ions would influence the bleaching capacity of the product

formed. Retention of acid in the clay matrix is known to reduce bleaching power (Beneke and

Lagaly, 2002). Hence the bleaching efficiency of clays depends mainly on amount of

hydrated silica in the bleaching clay. Bleaching efficiency was shown to increase with

concentration of acid and temperature of activation by several authors (Motlagh et al., 2011.,

Mukasa-Tebandeke et al, 2015., Ujeneza et al, 2014). Reacting natural clays or silicates with

sufficiently strong acids leads to formation of silica which strongly enhances the clarifying

and bleaching effect. Silicates have to be reacted with acid until they are decomposed to silica

to a large extent. Acid-leaching of clays enhances bleaching power of silicates for oils,

waxes, paraffins by transforming silicic acid of natural silicates to hydrated silica (Beneke

and Lagaly, 2002). Similarly, a fine bentonite suspension put in contact with sulphuric acid

at various temperatures between 60 and 120 oC for durations between six and twelve hours is

known to yield bleaching clay on being washed to neutrality with distilled water (Didi et al,

2009; Kamal et al, 2011). Athi River bentonite was activated using sulphuric acid at various

acid concentrations, clay acid ratio, temperature and contact times and both cation exchange

capacity and the apparent bulk density were found to decrease with increase in acid

concentration yet bleaching efficiency increased (Ujeneza et al, 2014). Acid activation of

Iliri clay modified the structure of the clay and this was indicated by the change in the

chemical composition given in Table 2. Both hydrochloric and sulfuric acid destroyed the

structure of the clay to nearly the same extent.

Silicon dioxide content of leached Iliri clay rose from 46.71% to 71.8 % as concentration of

acid used was increased from 0 to 30%. The content of other elements decreased after

activation. The increase in Silica content and concentration decrease the for other oxides was

in agreement with the results reported by several authors (Ujeneza et al, 2014., Usman et al,

2013 and Arfaoui et al, 2010). The octahedral sheets got destroyed as the exchangeable


54

cations dissolved in the acid while the silica generated by the tetrahedral sheet remained in

the solid phase due to its insolubility (Diaz and Santos, 2001) so increasing silica content.

Table 2: Chemical composition of raw and acid leached Iliri Clay

3.6 Effect of concentration of acid on bleaching performance

The absorbance of cotton and sunflower oil decreased with increase in temperature of

activation and concentration of acid used to leach the clay. The representative changes in

absorbance of oils are shown in Figures 1a and 1b. The decrease in absorbance was greater at

temperatures between 40 and 90 oC than between100 and 120 oC.

Key: A10 refers oil bleached using clay leached in 10% hydrochloric acid, similarly A20 and A30 were leached

in 20 and 30% hydrochloric acid.

Figure 1a: Variation of absorbance of sunflower oil with temperature of clay activation

%

Oxide

Raw 10%H2SO4 20% H2SO4 30%H2SO4 10%HCl 20%HCl 30%HCl

SiO2 46.7 60.0 67.2 71.5 61.0 68.2 71.8

Al2O3 15.9 13.7 7.0 6.8 13.5 7.0 6.8

CaO 4.0 0.5 0.5 0.5 0.5 0.4 0.4

MgO 1.5 0.4 0.3 0.3 0.4 0.3 0.3

Na2O 3.8 0.9 0.7 0.7 0.9 0.7 0.7

K2O 1.3 1.1 1.0 1.1 1.1 1.0 1.1

Fe2O3 7.8 4.0 1.5 0.8 3.7 1.2 0.8

LOI 14.5 17.5 16.3 11.0 16.5 16.3 10.0



55

The initial rapid decrease in absorbance at temperatures between 40 and 90 oC is a result the

crystal structure of the clay remaining unaltered. So adsorption takes place to the optimum

expected before equilibrium is reached. The position of equilibrium depends on the

temperature of activation. Equilibriation is delayed when the temperature is increased.

Activation of clay at temperature above 90 oC may cause irreversible damage to the clay

structure which reduces the capacity of the clay to adsorb impurities from oils.

Key: B10 refers oil bleached using clay leached in 10% sulfuric acid, similarly B20 and B30 were leached in 20

and 30% sulfuric acid.

Figure 1b: Variation of absorbance of cotton oil with temperature of clay activation

The decrease in ability to adsorb coloring bodies from oils is reflected in gradual decrease in

absorbance of oils and eventually the absorbance increases. The increase in absorbance at

temperatures near 120 oC shows break down of the bleaching agent. Since absorbance of

bleached oils decreased as shown in Figures 1a and 1b above, it was necessary to determine

the percentage change in absorbance between the various temperatures at which the bleaching

of oils was carried out. A plot of percentage decrease in absorbance against the temperature

of activation gave Figures 2a and 2b. Figures 2a and 2b show that the percentage decrease in

absorbance of bleached oils increase with increase in temperature the eventually decreased

after a maximum value was reached. The steady increase in percentage decrease in

absorbance signifies that the bleaching efficiency of the clay was increasing with increasing

temperature. The most suitable temperature for activation of bleaching earths deducible from

these figures is 90 oC. In Figure 2a and 2b concentration of the leaching medium is also

varied. It is evident that the clay activated using 20% hydrochloric acid attained the highest

bleaching efficiency at 90 oC yet in Figure 2b it was clay activated using 30% sulfuric acid

that attained highest value.


56

Figure 2a: Variation of percentage decrease in absorbance of sunflower oil with temperature

of clay activation

Figure 2b: Variation of percentage change in absorbance cotton oil with temperature of clay

activation

The maximum bleaching efficiency for sunflower oil attained was 90% for A20, 90% for

A10, and 70% for A30 yet for cotton oil it was 60% for B30, 50% for B10 and 48% for B20.

The results are in agreement with those published else where (Berbesi, 2006; Ujeneza et al,

2014). The clay activated using 10% showed increase in percentage change in absorbance to

much higher temperatures than the rest. The fact that highest bleaching efficiency was

attained with A20 implied that clay leached in 20% hydrochloric acid would serve as the best

bleaching earth. The initial increase in bleaching performance with increasing sulfuric acid

concentration was probably due to formation of active sites on clay surface. As the acid

activation is increased, the surface area increased rapidly and reached a maximum and

dropped. There was rise in specific surface area resulting from of the unoccupied octahedral

spaces left when aluminium, iron(III) and magnesium ions dissolved in acid during leaching

of clay. Then as the activation progresses, the empty spaces grow larger and the micro pores

are transformed into mesopores and finally, because of the decomposition of the crystal



57

structure at some locations, some of the mesopores disappear, leading to a drop in specific

surface area (Onal et al., 2002; Motlagh et al., 2011). The maximum bleaching efficiency

may not correspond to the maximum surface area value. Subsequent decline in bleaching

performance could be due to passivation of the rest of the clay, thus protecting the clay layers

from further acid attack (Christidis et al, 1997). The increase in percentage decrease in

absorbance of bleached resulted from removal of pigments. The removal of pigments

increase with increase in temperature was probably because of increased kinetic energy of the

molecules leading to more adsorption. Adsorption reactions are generally exothermic (Walter

and Weber, 1974) hence subsequently bleaching efficiency decreases higher temperatures as

shown in Figures 2a and 2b.

The variation of the percentage decrease in absorbance of bleached oils with time of contact

with clay was investigated. The results shown in Figures 3a to 3b indicate that the efficiency

of clay increases with time period over which the clay is left in contact with the oils because

adsorptive bleaching involves chemical reactions which take energy and time to take place.

Key: B10 refers oil bleached using clay leached in 10% sulfuric acid, similarly B20 and B30 were leached in 20

and 30% sulfuric acid.

Fig: 3a


58

Fig: 3b

Fig:3c

Figures 3a to 3c: Representative variation of percentage decrease in absorbance oil with

contact time for clays activated at 90 oC.

As shown in Figures 3a to 3c, the percentage decrease in absorbance increased with increase

in contact time for all clays activated at 90 oC due to that fact that the reaction taking place

was similar and therefore suffered form the similar limitations. However, the increase in

percentage decrease in absorbance for clay leached in 20% acid is highest indicating that the

material attained the highest bleaching efficiency. Basing on this material it is further

observed that 90% efficiency was attained at 25 minutes. So if clay has to be used bleach

edible oils, it should be allowed to remain in contact with the oil for only 25 minutes before

filtering off the adsorbent used. This result indicates that the oil bleaching industries can



59

operate efficiently by allowing the bleaching process to occur within 25 minutes. In the 25

minute time, the observed bleaching efficiency is 90% for B20 85% for B30 and 70% for

B10. The rapid increase in percentage decrease in absorbance of oil in the first 5 to 20

minutes can be attributed to the increased availability of vacant surface sites at the initial

stages. The optimal contact time was a maximum at 25 minutes for the various acid

concentrations and this is in agreement with the results of other workers (Ujeneza et al, 2014;

Usman et al, 2013; Berbesi, 2006; Walter andWeber,1974) . After 25 minutes, the adsorption

was slow, implying equilibrium conditions had been attained.

3.6 Adsorptive bleaching of cotton and sunflower seed oil with calcined, acid-leached

Iliri clay

Iliri clay was activated using hydrochloric and sulfuric acid at various acid concentrations.

The equilibrium data was analyzed using Freundlich and Langmuir adsorption isotherms and

the former was found to provide a better fit for the data. Bleaching involves adsorption of

coloring matter on clay leading to decrease in absorbance of oils. Comparison between the

absorbance of bleached and unbleached oils is of great importance, illustrating the efficiency

of clay used. This can be best presented in form of adsorption isotherms. The isotherms

portray what takes place in the entire bleaching process at various temperatures. Adsorption

isotherms of pigments from alkali refined oils were presented and showed that Langmuir and

Freundlich equations can be used to elucidate adsorption characteristics of pigments on clays

and sepiolites (Boki et al, 1992).

The absorbance of bleached, unbleached sunflower-seed and cotton-seed oils have been used

to study the nature and extent of adsorption of the impurities in crude oils on clay materials

(Kamal et al, 2011; Mbah, 2005; Topallar, 1998). The decrease in absorbance of cotton and

sunflower seed oils on being bleached has been explained in a similar way to what Hui

(1996) noted when he observed the 5–20 % reduction in the red color of soybean, canola and

palm oils occurred during the bleaching with acid activated clays. It was noted that a

reduction occurs in chlorophyll and carotene concentrations occurred during bleaching. There

was an increase in adsorption activity of clays after acid treatment is due to the weakness of

the Si-O bonds in the clay structure (Hui, 1996). The capacity of a clay to bleach and

manner in which adsorption takes place may be described, respectively, by the k and n

Freundlich constants (Rossi et al., 2003). The k constant is a rough measure of the surface

area of the adsorbent (Achife and Ibemesi, 1989). The 1/n value ranges between 0 and 1, and

if the numerical value of 1/n is less than 1, it indicates a favorable sorption (Gezici et al.,

2007; Ahmaruzzaman and Sharmal., 2005). The amount of pigment adsorbed, x, increased

and the residual relative amount at equilibrium, Xe, decreased for both sunflower and cotton

seed oils bleached with Iliri clays. This is illustrated by Langmuir isotherms in Figures 4a to

4f. The decrease in absorbance with increase in temperature and concentration of acid used,

has been interpreted to show that the bleaching capacity of the clays increased (Rich, 1964;

Topallar, 1998). The increase in bleaching ability was caused by removal of octahedral ions

from the clay matrices by the acid and dissociation of water, to produce hydroxyl groups on

the clay surface (Jadambaa, 2006; Folleto et al, 2009). The Langmuir isotherm has been used

to describe the oil pigment adsorption and adsorption of other minor oil solutes during oil

processing. The decrease in both Xe and Xe /{x/m} was most rapid for clay leached in 20%

hydrochloric acid. This showed that the clay attained the highest bleaching capability

(Christidis and Kossairi, 2003). The Langmuir isotherms for clays were plotted as shown in


60

Figures 4a to 4f. Each figure shows the decrease in values of Langmuir constants a and b as

the temperature increased from 40 to 90oC. The linearity shown in these Langmuir isotherms

indicated the effectiveness of the adsorptive process for impurities on the clays increased

with increase in temperature used to activate the clays.

Key: A and B respectively refer to hydrochloric and sulfuric acid used to leach the clay. 10, 20 and 30

respectively designate the mass percent acid.

Fig: 4a

Fig: 4b



61

Fig: 4c

Fig: 4d


62

Fig: 4e

Fig: 4f

Figures 4a to 4f: Representative Langmuir isotherms for oils bleached using HCl leached (a to c )

and H2SO4 leached (d to f)



63

The observed Langmuir equations for the clays studied were as shown below;

Xe/(x/m) = 14.061Xe - 1.0043 R² = 0.9906 for B30 and a= 0.0711 b= 0.0714.

Xe/(x/m)= 18.504Xe - 3.4721 R² = 0.9842 for B20 and a= 0.0540 b= 0.1875.

Xe/(x/m) = 19.369Xe - 3.9051 R² = 0.9817 for B10 and a= 0.0516 b= 0.2015.

Xe/(x/m) = 7.9487Xe + 0.4617 R² = 0.976 for A30 and a= 0.1258 b= 0.0581.



As Figures 4a to 4f are highly linear Langmuir adsorption isotherms were obeyed when acid-

leached clays matrices adsorbed color bodies cotton and sunflower-seed oils. The extent of

adsorption increased with temperature and concentration of acid used to leach the clay. While

A20 and B30 showed highest linearity both A10 and B10 showed the lowest. Therefore clay

leached in 20% hydrochloric acid bleached to the same level as that leached in 30% sulfuric

acid. The coefficients of linearity, expressed as R2 ranged from as low as 0.976 + 0.017 to

as high as 0.9947 + 0.017 . This revealed that all clays used strongly bound the color bodies

in oils. Hence, their capacity to bind colored impurities was good because steric interferences

between the adsorbed and free impurities present in vegetable oils being bleached was low.

Further more, acid leached clays had many adsorption sites and could not easily get saturated

(Travis and Etnier, 1981; Olsen and Watanabe; 1957). Linearity would be extended to higher

concentration of impurity in oils. The linearity of the isotherms was a function of impurity

concentration, availability of surface adsorption sites and interactions between the adsorbed

impurities and impurities still remaining unadsorbed in the oil being bleached (Hundal,

1988). The tests conducted, where bleaching was optimized at different temperatures and

concentrations acid to activate clays, the slopes for the Langmuir isotherms were positive

showing that bleaching capacity and surface area increased with increase in both temperature

and concentration of acid used (Alemdaroglu et al, 2003). As long as the clay material did

not easily reach saturation with the adsorbed impurities, the Langmuir isotherms obtained

were highly linear. The degree of linearity reduced as the saturation point for the clay was

approached.

The deviation from linearity of Langmuir isotherms occurs in three regions, which is

attributed to existence of surface sites having multiple adsorption free energies (Srinivasan

and Fogler, 1990). The adsorption of impurities in oils may be governed by impurity-oil and

impurity-clay surface interactions (Travis and Etnier, 1981). Steric interferences hamper

adsorption.

The increased linearity as temperature increased for the clays tested confirmed that the

bleaching activity or decolorizing power of the clay matrices increased with increase in

temperature up to a maximum at about 90 oC. Although Langmuir behavior is observed when

a straight line is obtained when Xe/(x/m) is plotted against Xe, this is not definitive proof of a

simple, reversible monolayer, chemisorption mechanism. In other words, simply conforming

to the Langmuir equation does not constitute absolute proof of the mechanism (Gregg and

Sing, 1997).


64

It was noted that the clays studied bleached optimally at 90oC. This is an indication that at

this temperature the crystal structure of the clay materials are not greatly changed as crystal

water would not have been lost (Dandy, 1968). This evidence is also used to indicate that the

water or/and hydroxyl groups in the clay play a major role in the bleaching of vegetable oils

(Rich, 1964).

The gradients of the graphs in Figures 4a to 4f increased positively with increase in

concentration of the acid used to leach the clay because acid-activation of the clay greatly

enhanced the bleaching capacity of the clay. Acid-leaching removed octahedral ions like

sodium, magnesium, calcium and potassium ions from the clay matrix which retard bleaching

(Balaras et al, 1999). Removal of octahedral ions left a silica skeleton with enhanced

adsorptive properties (Boyd, 1988; Hassan, 2006; Madejova, 2007). Langmuir constants are

expected to decrease with increasing temperature for all adsorbent systems used. This shows

increasing availability of adsorption sites with elevated temperatures because all sites were

not energetically equal (Boki et al, 1992).

The Langmuir constants decreased with increasing concentration of acid used to leach the

clays as well as the temperature used to activate the clays. This indicated that acid-leaching

and heating up to temperatures below 90oC increased availability of adsorption sites because

all sites were not energetically equal (Boki et al, 1992). The values of the Langmuir constants

a and b decreased with increase in concentrations of the acid used to leach every clay studied

because leaching increased the number of adsorption sites per unit mass of clay. The values

of a and b decreased also with increase in the temperature at which the clay was thermally

activated. This was due to the heat of activation of the clay leads to increase in number of

adsorption sites per gram of clay matrix. However, the decrease in values of constants a and

b for Chelel clays were particularly higher than those for any other clay studied, as this clay

had a much higher smectite content than others used in this study. So its activated forms had

higher numbers of adsorptive sites for polar impurities in the cotton and sunflower seed oils.

Table 3a: A sample of Freundlich data on bleaching sunflower oils

Log Xe log(x/m) logXe Log(x/m) logXe log(x/m)

Temp oC A10 A20 A30

40 -0.284 -0.319 -0.420 -0.208 -0.215 -0.409

50 -0.347 -0.260 -0.444 -0.194 -0.260 -0.347

60 -0.456 -0.187 -0.523 -0.155 -0.301 -0.301

70 -0.523 -0.155 -0.824 -0.071 -0.523 -0.155

80 -0.602 -0.125 -0.658 -0.056 -0.620 -0.119

90 -0.658 -0.056 -1.097 -0.036 -0.699 -0.097

100 -0.620 -0.066 -1.000 -0.046 -0.469 -0.180

110 -0.638 -0.060 -0.959 -0.051 -0.456 -0.187

120 -0.921 -0.056 -0.959 -0.051 -0.456 -0.187



65

Table 3b: A sample of Freundlich data on cotton oils bleaching

The representative data in Tables 3a and 3b was plotted to yield Freundlich isotherms shown

in Figures 5a to 5f. The coefficients of linearity of the plots in Figures 5a to 5f showed that

Freundlich adsorption occurred during the bleaching of cotton and sunflower-seed oils. The

isotherms for both sunflower and cotton seed oils followed the Freundlich equation in a

manner similar to the adsorption isotherms of decolorization of maize oil. This indicated the

existence of heterogeneous adsorption sites on the solid’s surface (Christidis and Kossairi,

2003).

Key: A and B respectively refer to hydrochloric and sulfuric acid used to leach the clay. 10, 20 and 30

respectively designate the mass percent of acid used.

Fig: 5a

LogXe log(x/m)B10 logXe log(x/m)B20 LogXe log(x/m)B30

Temp oC

40 -0.167 -0.495 -0.180 -0.469 -0.208 -0.420

50 -0.201 -0.432 -0.215 -0.409 -0.237 -0.377

60 -0.252 -0.357 -0.268 -0.337 -0.310 -0.292

70 -0.260 -0.347 -0.276 -0.328 -0.387 -0.229

80 -0.301 -0.301 -0.319 -0.284 -0.409 -0.215

90 -0.310 -0.292 -0.328 -0.276 -0.409 -0.215

100 -0.319 -0.284 -0.292 -0.310 -0.398 -0.222

110 -0.328 -0.276 -0.276 -0.328 -0.398 -0.222

120 -0.319 -0.284 -0.284 -0.319 -0.398 -0.222

http://ccm.geoscienceworld.org/search?author1=George+E.+Christidis&sortspec=date&submit=Submit


66

Fig: 5b

Fig: 5c



67

Fig: 5d

Fig: 5e


68

Fig: 5f

Figure 5a to 5f: Freundlich isotherms for oil bleached using clay leached in HCl (a to c) and

and H2SO4 leached (d to f).

The observed Freundlich equations for the clays studied were as shown below;

Log (x/m) = -0.9903 logXe - 0.615; R² = 0.9897; n= -0.9903; k= - 0.615 for B30.



Log (x/m) = -0.6361 logXe - 0.5103; R² = 0.9538; n= -0.6361; k= - 0.5103 for A30.



The Figures 5a to 5f showed linearity in the temperature ranges between 40 and 110 oC

depicting that Freundlich isotherms are observed when adsorption occurs on clays if

thermally activated at temperatures below 110 oC (Topallar, 1998). The linearity of

Freundlich on isotherms expressed by the R2 values ranged from as low as 0.9538 + 0.002 to

0.9897+ 0.002 for the clays used. This showed that the clays did not have limitations

resulting from overcrowding, steric interference, thermodynamic instability at high impurity

concentrations (Hundal, 1988). The strongly adsorbing clays have been better described by

the Freundlich isotherms within lower concentration ranges (Kothawala et al, 2008).

As Freundlich adsorption isotherms assume monolayer adsorption capacity complications

arise when clays and clays minerals exhibit multi-layer adsorption tendencies and this causes



69

deviation from linearity of the isotherms. Clays with low adsorption capacities deviated from

Freundlich isotherms because they easily got saturated with impurities from oils leading to

steric interactions between the adsorbed and unadsorbed impurities (Nodvin et al, 1986). The

amount of impurities in vegetable oils that get adsorbed on clay matrices change with small

changes in the equilibrium bulk concentration and this is reflected in the deviation from

linearity of Freundlich adsorption isotherms. Commonly, if the clay matrix becomes saturated

with impurities, repulsion sets in and data obtained after the clay is saturated gives non linear

Freundlich isotherms (Tor et al, 2006). Smectite-rich clays showed high degree of linearity

of Freundlich adsorption isotherms and also have high increase in surface acidity and surface

area (Alemdaroglu et al, 2003).

The larger the value of Freundlich constant, k, the better the clay at removing impurities from

the vegetable oils. The values of k increased with increase in strength of the acid used to the

clay. However, leaching with more concentrated acids resulted in creation of more impurity

adsorption sites in the resulting clay matrix. The highest magnitude of ‘k’was obtained with

B10 so this clay had highest bleaching strength.

The Freundlich equation is valid for any colour measurement method, provided the units are

additive and proportional to the oil pigment concentration. Clays with higher k values bleach

better than those with smaller values of k. The value of the Freundlich constant n is used to

determine the range of decolorization within which the adsorbent is most effective. An

adsorbent with a higher value of n will be relatively effective at binding impurities in oils, but

inefficient at bleaching oil to a low colour value. The opposite is true for an adsorbent with a

low n value. A high n value is desirable but not at the expense of k (Achife and Ibemesi,

1989).

The values of Freundlich constant, n, decreased with increase in leaching acid concentration

due to proportionate increase in the bleaching strength of the clay (Gadzekpo et al, 1991).

The differences in bleaching efficiency of the clays used in this study appeared to be due to

differences in the surface properties of the bleaching media developed as a result of leaching

the different natural clays. And this is similar to what was advanced when cotton-seed oil was

bleached using acid-activated montmorillonite (Falaras et al, 1999).

The Freundlich constant k increased with increase in temperature for both oils, showing that

the formation of adsorptive sites on the clay rose. In this respect, the acid-activated clays

behaved like the Turkish clay used in bleaching hazelnut oil (Yuksel, 2003).

The values of the Freundlich constant, n, calculated using cotton-seed oils are much larger

than the values calculated using sunflower-seed oils because the extent of bleaching of cotton

seed oils was not as high as that for sunflower-seed oils. Generally, the concentration of

coloring materials in cotton-seed oils is higher compared to sunflower-seed oils. This is

reflected in the values of constants calculated. The values of Freundlich constants prove that

the acid-leached Chelel clay is as good as a bentonite, if activated under appropriate acid and

temperature conditions. Characteristically, the clay had greater number of adsorption sites

than any other clay and adsorbed more impurities beyond its monolayer capacity, so its value

of n was less than five.


70

The values of the Freundlich constants, k, were low for kaolinite-rich clays from central

Uganda showing that these clays had lower bleaching capacity than the smectite-rich clays

from volcanic sediments. The values of k for the kaolinite-rich clays were in the range of -

5x10-3 to -5.6x10-3 for cotton seed oils showing that the clays had less capacity to adsorb

impurities in cotton seed oils than for impurities in sunflower seed oils for which the k values

were found to be in the range –0.029 to –0.1132 (Topallar, 1998).

The clay matrices used to bleach the oils were heterogeneous because they have Broensted

and Lewis centers. In addition, they contain different phases of clays, such as kaolinite,

nontronite and illite layers, which also have active centers on their surfaces. Similarly,

heterogeneity of Cyprus bentonite was attributed both to different active centers on the

smectite surface (Broensted and Lewis centers), and to the different phases present in

bentonite, such as illitic layers and clinoptilolite, which also have active centers on their

surfaces as the clay was a mixture of illite and smectite (Christidis and Kossairi, 2003;

Falaras et al, 1999).

The obstacles to practical application of the Langmuir and Freundlich isotherm theories

include the following: (1) These isotherms do not effectively address adsorption versus

degradation and competitive adsorption; (2) the conclusions are not all inclusive. For

example adsorption constants and coefficients do not hold true in all cases within similar oil

types let alone across different oil types; (3) the process has so many variables that the

additive variance is commonly too great to prove any subtle difference between clays other

than a vastly different level of activity. This problem is especially true when using log vs.

log plots with incremental changes on the order of 0.1%); and (4) the adsorption constants

and coefficients have limited use for the oil refiner (Proctor and Snyder, 1987).

4.0 CONCLUSIONS

The following conclusions can be drawn from the results of this study:

Clay from iliri consists predominantly of a dioctahedral nontronite montmorillonite, which

also contains significant amounts of quartz and feldspar, as well as minor amounts of illite,

kaolinite and plagioclase. The mineralogy, chemical analysis and physical properties of the

raw bentonite indicated a predominance of sodium, which is allowed to characterize the

samples mainly as sodium bentonites and the montmorillonites content is 41.2%.

Acid activation could pose structural changes of the raw bentonites, while the area of the

interlayer water peak hardly changed and the main dehydroxylation peak became more

diffuse with increasing the acid strength activation. When more concentrated acids were used,

the crystallinity of the bentonite was decreased; as well the rate loss of hydroxyls was much

slower in the activated samples than the untreated sample.

Acid activated Iliri clay has potential of decolorizing cotton and sunflower-seed oils by

removing color bodies by over 80% and meets the requirement for application in oil

decolorization. The most efficient bleaching capability of bentonite was obtained by

activating with 25% hydrochloric or 30% sulfuric acid, towards different color bodies in

cotton and sunflower oils.

5.0 Recommendations

More clays from the Karamoja region should be tested for bleaching, elemental,

mineralogical and sructural properties.




71

Acknowledgement

We thank Mr. Rontgen R. Budigi for heating the clay samples studied in the furnace, Mr.

Moses Nkolongo (rest in peace) for availing us chance to use the spectrophotometers in the

analytical laboratories. Prof. B.T. Kiremire(rest in peace) for availing us space in

Laboratories with reagents and encouraging remarks which strongly enabled the compilation

of the manuscript.

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