cottonseed oil bleaching by acid-activated...

Clay Minerals (1999) 34, 221-232

Cottonseed oil bleaching by acid-activated montmorillonite

P. F A L A R A S , I. K O V A N I S , F. L E Z O U AND G. S E I R A G A K I S *

Institute o f Physical Chemistry, NCSR 'Demokritos " 153 l O A gh ia Paraskevi Attikis, and MINER VA S.A., Edible oils Enterprises', 31 Valaoritou St., 14452 Metamorphosis, Attica, Greece

(Received 24 September 1997; revised 16 June 1998)

ABSTRACT: A progressive decrease in calion exchange capcity (CEC) values was observed by treating Ca-montmorillonite with sulphuric acid solutions and this can be understood in terms of the layered structure of the clay. Elemental analysis showed that moderate activation occurred and only 25 30% of the octahedral cations were removed. At the same time the total surface area and the clay acidity increase. X-ray and FTIR data confirmed that acid activation affects both the octahedral and the tetrahedral sheets. The efficiency of acid-activated montmorillonite for the bleaching of cottonseed oil was investigated. The differences in bleaching efficiency appeared to be due to differences in the physical and chemical properties of the bleaching media. The oil acid value was not affected by the bleaching procedure but a slight shift in the absorption maximum of the bleached cottonseed oil was observed. Medium activation of the clay (treatment of Ca-montmorillonite with 4 N H2SO4) was the most effective in bleaching the cottonseed oil, resulting in the best colour index and the lowest peroxide value. A linear dependence of the bleaching efficiency on the clay surface area and acidity was observed. The role of the increased Bronsted acidity is also discussed.

In comparison with the other edible vegetable oils, cottonseed oil contains many more saturated substances, presents excellent frying resistance and has the advantage of being able to produce the [3 + crystal structure of margarine. The crude oil is a dark reddish brown colour and has a strong characteristic flavour and odour. It contains free fatty acids, triglycerides, numerous minor compo- nents such as gossypol, C30H3008 (which is a yellow toxic pigment), phospholipids, tocopherols, sterols, carbohydrates, hydrocarbons and other pigments (Padley et al . , 1996). The world production in 1989-1990 was -3 .6 • 106 tonnes with China, producing more than 20% of the total cottonseed oil (Mielke, 1991), the largest producer. In Greece, cottonseed oil is a traditional edible oil and its annual production is about 50 x 103 tonnes (Kellens, 1997), most of which is used in home consumption and only small quantities are exported.

Oil bleaching, which is performed in order to prepare a sufficiently light-coloured product of enhanced appearance and improved stability, is

usually achieved by treating the crude or the refined oil with powdered absorbent. The principle of bleaching is based on several adsorption mechanisms including physical adsorption through van der Waals' forces, chemical bonding via covalent or ionic bonds, ion exchange, molecular trapping and chemical decomposition (Pritchard, 1994). During the b leach ing process, co lour ing p igments (primarily carotenoids, chlorophylls, gossypol), peroxides and other impurities (such as soap, trace metals, phosphatides, sulphur, oxidation products) are removed from the cottonseed oil.

Both natural and acid-activated earths or clays are used as absorbents. Acid-treated clays are aluminosilicates chemically activated with mineral acids (Breen, 1991). They have already been used as solid acid catalysts and catalyst supports tbr a number of organic reactions of considerable industrial interest (Breen et al., 1995, 1997a,b; Bovey & Jones, 1995; Mokaya & Jones, 1995). They also present enhanced acidity which results in greater adsorptive power for pigments, and for that

.~ 1999 The Mineralogical Society

222 P. Falaras et al.

reason are particularly useful for dark oils. In fact, the unique efficiency of the acid-activated montmorillonite as bleaching earth in the processing of palm, palm kernel and coconut oils has already been demonstrated (Morgan et al . , 1985). Moreover, ferric additives are very efficient in improving the performance of acid-activated bentonite and kaolinite in the bleaching of palm oil (Hymore, 1996).

Acid-activated montmorillonites which are commercially used for the bleaching or decolour- izing of oils exhibit a wide range of chemical and physical properties depending on the extent of activation, which influences the efficiency of the bleaching process (Boki et al.,1992). The absorbent and catalytic properties of sepiolite have been elucidated via a detailed physicochemical characterization (Vicente et al., 1994). Acid activation of palygorskite revealed an important increase in the specific surface area (Vicente et al., 1995). An increase in the number of acid centres and destruction of the silicate structure was also observed during acid activation of a ferrous saponite (Vicente et al., 1996). These facts lead to the conclusion that, for a particular clay mineral, bleaching earth properties such as surface area and acidity should be optimized by properly controlling the activation conditions such as type and amount of acid, temperature and treatment time (Breen et al., 1997a). It is, therefore, important to know how differences in physicochemical properties of montmorillonite might affect the capacity for adsorbing coloured species from vegetable oils. In this work, the effect of treatment with sulphuric acid on the structure, cation exchange capacity (CEC), elemental composition, surface area, material morphology and acidity of a Ca-montmorillonite has been explored and an attempt to correlate the above properties with the bleaching efficiency has been made. As an example, the efficiency of acid- activated montmorillonites for the bleaching of Greek cottonseed oil is presented.

E X P E R I M E N T A L

Typical samples of overflowed neutral (neutralized with soda) cottonseed oil were obtained from Manos s.a. oil industry (Piraeus, Greece). This oil was difficult to bleach, probably because the cottonseeds had been exposed to elevated temperatures. The clay STx-1, a Ca-montmorillonite (CEC = 80 mEq/100 g) from Gonzales County, Texas, USA, was obtained

from the Clay Minerals Society. The raw material was purified using sedimentation and the <2 I.tm fraction was selected.

Fifty grams of this Ca-montmorillonite was ground and stirred magnetically with 250 ml of H2SO 4 (analytical grade Riedel-de Hahn) at 80~ for 2 h in a round-bottom flask. Three different acid-activated clays (designated as A2, A3 and A4 samples) were prepared by treating the Ca-montmorillonite (designated as AI sample) with sulphuric acid of concentration 1, 4 and 8 N, respectively. The slurry was cooled in air, centrifuged and washed twice with distilled water. The samples were then dialysed against deionized water until the pH was neutral and the conductivity was stable.

Bleaching experiments were conducted in open vessels containing a stirred dispersion of clay (2%) in cottonseed oil heated to 120~ for 5 min in a procedure analogous to that of the American Oil Chemical Society (AOCS) Official Method Cc 8a- 52. The colour of the oil was determined according to the AOCS Official Method Cc 13e-92 on a Lovinbond Automatic Tintometer (TYPE D) equipped with 1 inch cells. This method determines colour by comparison with glasses of known colour characteristics and it is applicable to all normal fats and oils, providing that no turbidity is present in the sample. Absorption spectra (350-1100 nm) were obtained using a U-2000 Hitachi double beam spectrophotometer. Peroxide value was determined in terms of milliequivalents of peroxide per 1000 g of sample, according to the AOCS Official Method Cd 8-53.

X-ray diffraction (XRD) patterns were obtained using a Siemens D-500 X-ray diffractometer, using Cu-Ket radiation ()v = 1.54050 A), with a secondary graphite monochromator. Infrared (IR) spectra were recorded with a Nicolet 550 FTIR spectrometer using KBr pellets. Cation exchange capacities were measured using the Co 2+ ion uptake method using a COSO4 solution. Elemental analysis was performed using a Perkin Elmer (OPTIMA 3000) ICP spectrometer. The clay morphology was investigated by atomic force microscopy (AFM Nanoscope III, Digital Instruments) in the contact mode. The acidity of the clays was determined using NaOH titrations (Kumar et aL, 1995) and the pH of the aqueous clay suspensions (10% w/w) was measured with a Griffin pH meter at room temperature. Thermogravimetric data were obtained on a model 2050 TGA Thermobalance using 20 mg samples

Cottonseed oil bleaching by acid clays 223

transferred directly from pyridine vapour to the instrument, flushing with dry N2 (20 cm3min 1) for 20 rain and then raising the temperature at a rate of 20~ min -1. The BET surface area values of the clay samples were determined from adsorption- desorption nitrogen isotherms, taken at 77 K using an Autosorb-1 (Quantachrome).

R E S U L T S A N D D I S C U S S I O N

As the oil was neutral (Free Fatty Acids, FFA = 0.09%), its acid value was not affected by the bleaching procedure. The bleaching efficiencies (Lovinbond Tintometer colours) of the natural and three acid-activated montmorillonites for pigments and peroxides in the poor quality cottonseed oil are presented in Table 1. These data clearly proved that sample A3 (corresponding to a medium activation of the clay) had the best decolouration effect (colour index: 2.5R-30Y). The determination of all the substances in the bleached oil able to oxidize potassium iodide (KI) confirmed that A3 also removed the largest amount of peroxides (-53%).

The clay efficiency for pigment adsorption was also measured. The untreated cottonseed oil presented three absorption maxima in the visible region; the most important at 412 nm (A = 2.1) and two others of lower intensity at 604 nm (A = 0.14) and 669 nm (A = 0.24). The latter two decreased upon treatment with clays and completely disap- peared when sample A3 was used. The results for the first absorption are presented in Table 1 in the form of the fractional degree of bleaching, FDB(%), at 412 rim. The FDB results demonstrated that the bleaching efficiencies of the various montmorillonites can be ranked in the order:

A3 (20%) > A4 (16%) - A2 (15%) > A1 (13%)

reinforcing the observation that A3 was the most effective bleaching agent. Note that, upon bleaching, the absorption maximum of the cottonseed oil shifted to shorter wavelengths (from 3 to 23 nm). The better the bleaching efficiency, the greater the blue shift.

The observed differences in bleaching efficiency of acid-activated montmorillonite in terms of s t ruc ture , CEC, e l e m e n t a l compos i t i on , morphology, acidity and specific surface area of the clay samples were considered. It is generally accepted that the rate of dissolution of tetrahedral cations is significantly lower than that of octahedral cations (Osthaus, 1956), and IR spectroscopy is very sensitive to the structural changes which occur in the clay upon acid treatment (Breen et al., 1995) and the IR data recorded herein suggest that both the octahedral and tetrahedral sheets were suscep- tible to acid attack. Figure 1 shows IR spectra of samples A1-A4 and Table 2 summarizes the main vibrations observed. As the treatment proceeds, the bands due to the different types of water in the 3200-3700 cm 1 region decreased in intensity. Similar behaviour was shown by the deformation band at 1638 cm -1, which may be used to indicate the amount of water in clay samples (Mortland & Raman, 1968). The band at 915 cm -1 corresponding to the AIA1OH bending deformation and that at 845 cm-1 (A1MgOH) in the octahedral sheet (Bukka et al., 1992) decrease and become very weak for samples A3 and A4, suggesting a significant depopulation of the octahedral sheet. The existence of the 521 cm 1 band, the most sensitive indicator of the presence/absence of

TABLE l. Cottonseed oil bleaching efficiencies of the acid-activated montmorillonites for pigments and peroxides.

Sample Colour Index Peroxide value FDB (%)1 (in Red-Yellow units) (mEq/kg)

cottonseed oil without clay oil treated with A 1 clay oil treated with A2 clay oil treated with A3 clay oil treated with A4 clay

8 . 1 R - 8 0 Y 1 2 . 4 -

3 . 0 R - 3 5 Y 11.8 13 3.0R-30Y 7.8 15 2.5R-30Y 5.8 20 2.6R-30Y 6.1 16

1Fractional degree of bleaching, FDB(%) = (Aunbleached -- Ableached)/Aunbleached X 100, where Aunbleached and Ableached a r e the absorbance of unbleached oil and bleached oil, respectively, at the absorbance maximum of the untreated cottonseed oil (412 nm).

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wavenumber (era -I) FIG. 1. FTIR spectra of untreated Ca-montmorillonite (a) and samples activated with 1 N (b), 4 N (c) and 8 N (d)

H2SO4.

octahedral A1 (Breen et al., 1997a), clearly demonstrates that the degradation of the octahedral sheet was not complete, even in sample A4.

The bands at 1042 and 521 cm -I corresponding to S i - O - S i stretching and S i -O-A1 bending vibrations show that the tetrahedral sheet of the clay was also affected, especially at higher acid concentration. The progressive decrease of the 1042 cm-1 band together with the increase of the

band at 794 cm ~ due to the free silica reflects a relative alteration in the tetrahedral sheet with increasing acid concentration (Moenke, 1974; Bovey & Jones, 1995; Breen et al., 1997a). A 29Si MAS NMR study (Tka6 et al., 1994) proved that the final reaction product of acid-treated montmorillonite is amorphous silica formed by a three-dimensional cross-linked SiO4 framework with Si atoms bearing OH groups. Thus these

TABLE 2. Characteristic FTIR bands for acid-activated montmorillonites.

Wavenumber (cm -I) Band assignment

3624, 3549, 3476, 3239 3416 1638, 1618 1089, 1042 915 845 793 626,521,467

OH stretching hydration, OH stretching hydration, HOH deformation SiO stretching OH deformation, linked to 2A13+ OH deformation, linked to A13+ and Mg 2§ silica phase SiO deformation and AIO stretching


FTIR results were more consistent with a partial preservation of the clay layered structure which supports the view that the acid treatment used did not completely destroy the clay.

The XRD patterns (Fig. 2) did not show any crystalline impurity trace and confirmed the observations reported above. The untreated sample exhibited a well defined and very intense 001 peak, two higher-order reflections (003 and 004) and two hk diffractions. When the acid concentration was increased, the crys ta l l in i ty o f the samples decreased. In fact, the line intensities of almost all reflections, particularly that for the strongest line 20 = 7.23 ~ corresponding to 12.21 A., decreased without significant changes in 20 values. The presence of the 12.21 A. spacing in the A 4 sample confirmed that the clay structure was partially preserved, even under strong activation conditions. This is supported by the presence of the 003 reflection at 20 = 21.53 ~ (4.13 A) which remained, even when the clay was treated with 8 N sulphuric

acid. Note that a non-basal diffraction line generally appears at -4.5 A. in montmorillonites (Brindley, 1980). The broad hump in the 15-30 ~ (20) region has been attributed to amorphous silica (Komadel et al., 1990). On the other hand, the non-basal hk two- dimensional reflections which arise from diffraction from randomly stacked layers were stronger in the acid-activated samples. Each is the summation of several hk index pairs (Chen et al., 1995). The diffraction peak at a 20 value of 19.6 ~ (d = 4.53 A) is the summation of hk indices of 02 and 11 and the diffraction at 20~35 ~ (d = 2.55 ,~) is the summation of 13 and 20 hk indices.

The CEC data given in Table 3 show that there is a sharp decrease in the CEC of 23% from sample A1 to A2. Clay A3 possessed -73% and clay A4 maintained only 64% of the initial CEC. This progressive decrease in CEC values upon treatment with sulphuric acid can be understood in terms of the depopulation of the octahedral sheet. In fact, it is well established that leaching of the octahedral

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TABLE 3. Cation exchange capacity, BET surface area, acidity and pH of acid-activated montmorillonites.

Sample CEC BET surface area Acidity pH (mEq/100 g) (m2/g) mEq/100 g)

As 88 (100%) 93 4 9.00 A2 68 (77%) 98 42 3.27 A3 64 (73%) 107 60 3.05 A 4 56 (64%) 103 36 3.00

cations (Mg 2+, Fe 2+) results in a reduction of the negative layer charge and therefore of the CEC (Breen et al. , 1995). Elemental analysis performed by ICPS (calculated on an ignited (0% H20) basis and reported on Table 4), provided clear evidence of the changes in the clay composition following acid activation. The net reduction in the CaO content indicated that the Ca 2+ exchange cations were replaced by hydrogen ions and/or polyvalent cations leached from the octahedral sheet. The decrease in the octahedral sheet oxides (A1203, MgO, Fe203) along with the concomitant increase in silica content proved that the original structure was altered. Although the results clearly show that the depletion of the octahedral sheet occurred in a controlled stepwise manner, the material seemed to have a high ability to resist acid treatment in so far as only 25-30% of the octahedral cations were leached from the clay, following acid activation by 8 y H2SO4. In parallel, the relative silica content increased progressively and reached 83% for the A 4

sample. These observations agree with the FTIR and XRD data and correlate well with the increase of the 794 cm -1 band for free silica and the concomitant appearance of the broad hump at the 20 region between 15-30 ~ . It is clear that a significant proportion of the alumina octahedra were resistant to acid attack whereas a number of the lower valent ions (e.g. Mg 2+) were removed and

finally some of the A1 ions can be dissolved from the tetrahedral silica sheets (Griffiths, 1990). The use of high acid concentration solutions (8 N) strains the above phenomena but the layered structure of the clay was not completely destroyed.

The bleaching performance may also depend on the surface adsorption characteristics and the size of the clay particles. Natural and acid-activated samples exhibited reversible type II isotherms, characteristic of non-porous or macroporous adsor- bents. In all cases, type H3 hysteresis loops (in the IUPAC classification) were observed, (Gregg & Sing, 1982; Theocharis, 1993) which did not exhibit any limiting adsorption at high relative pressures (p/p~ which is typical for aggregates of plate-like particles giving rise to slit-shaped pores (Sing et al.,

1985). The BET specific surface areas calculated in the

linear section of the isotherm (from 0.1 to 0.35 relative pressure) are given in Table 3. The untreated material gave a surface area of 93 m2g 1 which falls within typical surface area values for montmoriUonites, i.e. 50-120 m2g -1 (Morgan et al., 1985; Van Olphen & Fripiat, 1979). Following acid activation, the surface area showed a relative increase and the largest value was observed for the sample A3 (107 m2g 5). Table 3 shows that even though the clay surface area was only slightly affected by activation as were the CEC

TABLE 4. Elemental composition (% W/W) for untreated and acid-activated montmorillonites

Sample SiO2 A1203 MgO Fe203 CaO K20 TiO 2 MnO2 P205

Al 76.46 16.63 3.87 0.77 1.87 0.10 0.22 0.02 0.05 A 2 78.99 16.30 2.94 0.73 0.74 0.05 0.22 0.02 A3 81.27 13.99 3.00 0.60 0.84 0.06 0.22 0.02 - A4 83.06 12.94 2.59 0.53 0.63 0.03 0.20 0.02 -


and elemental analysis data, a linear dependence for the clay bleaching efficiency with surface area was observed. The FDB of the cottonseed oil varied linearly with the BET surface area presenting a slope of 0.46 and an error factor R 2 - 0.90 (Fig. 3a). Low surface area and linear FDB dependence may explain the relatively low bleaching efficiency values obtained with the acid-activated samples (20% for sample A3) because it has been shown that the pore volume may make an important contribution to the adsorption process (Mokaya et al., 1993). Unfortunately, attempts to perform pore-size distribution analyses were not successful which was not unexpected because type H3 hysteresis loops are considered unlikely to yield a reliable estimate of pore distribution, even for comparative purposes (Sing et al., 1985).

Efficient cottonseed oil processing requires both enhanced bleaching efficiency and short filtration times along with minimization of oil retention on the filter cake. Thus, clay texture, surface morphology and particle size are likely to influence

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FIG. 3. Dependence of bleaching efficiency (FDB values) on surface area (a) and acidity (b).

both the material, adsorbing ability and the filtration performance. Thus, clay texture could play a role in determining the bleaching efficiency by controlling the effective adsorption and access of substances through channels in the layers. Following Fitch and co-workers (Edens et al., 1991), a clay material is composed of individual sheets which present random and oriented domains. The face of a platelet consists of an array of hexagonal holes, while the edge surface consists of broken S i -O and A1-O bonds. Medium activation of the clay ensures that major structural collapse does not occur and leads to optimum physical, structural and chemical properties (Morgan et al., 1985). During acid activation, the major change in physical appearance occurs at the edges of the platelets. The clay film becomes disordered and the layers take on the appearance of bundles of used bank notes. Certainly, the structure remains intact toward the middle of each platelet with the layers separating toward their edges. These changes are supported by the XRD and FTIR observations and may partially explain the differences in bleaching activity between untreated and acid-activated materials. Thus, the access and the subsequent adsorption of colouring matter is easier after acid activation.

Observations by atomic force microscopy (AFM) were performed on thin films deposited on glass slides from aqueous clay suspensions (2 g/l) and the results are presented in Fig. 4. The three dimensional large-scale resolution images clearly show that the untreated material (Fig. 4a) is composed mainly of oriented domains of large blocks Nl.5 gm in size, probably resulting from a preferable deposition of clay platelets having a face-to-face interaction. The discontinuity in the film structure was evident from the presence of voids between the 'blocks'. The acid-activated materials (Fig. 4b,c,d), also exhibited some preferential orientation but the films presented a more textured and complex surface topography with a greater number of smaller sized features. Smaller clay particles are more efficient in the adsorption process and account for the increase in bleaching performance. The roughness analysis (Table 5) confirmed the observed trends and revealed that the standard deviation of the difference between the highest and lowest points (RMS) as well as the mean roughness (Ra) were significantly lower in the acid-treated montmorillonites. The geometric complexity of the clay surface was evaluated by a more detailed


F~G. 4. AFM images of untreated Ca-montmorillonite (a) and samples activated with l N (b), 4 N (c) and 8 N (d) H2S04.

fractal analysis (Table 5). Maximum fractal dimension value (2.224) corresponding to maximum surface development (Falaras, 1998; Falaras & Lezou, 1998) was observed for the material

T~,~LE 5. Fractal and roughness parameters on natural and acid-activated montmorillonites.

Fractal Rms Ra Sample dimension (nm) (nm)

A1 2A66 213 176 A2 2.196 183 147 A3 2.224 150 117 A4 2,204 113 89

treated with 4 y H2SO4, in good agreement with the BET surface area values. Harsher acid treatment did not further increase the fractal dimension parameter but showed a clear decrease (2.204) following acid activation with 8 N H2804, attributed to the partial destruction of the clay structure.

The acidity of the activated montmorillonites may also affect their bleaching properties. The total acidity of the clays (determined by sodium hydroxide titrations and expressed in mEq of NaOH used per 100 g of clay) is presented in Table 3. Sample A 3 exhibited the maximum acidity (60 mEq/100 g), in excellent agreement with its bleaching efficiency. The pH of the aqueous clay suspensions (Table 3) decreased as the acid treatment of the clay became harsher. The FDB values varied almost linearly with the clay


acidity (Fig. 3b) having a slope of 0.14 (error factor R 2 = 0.81).

The fact that the acidity of the clay is crucial for its bleaching performance was not surprising. In fact, acidity in clays arises from H § ions occupying exchange sites on the surface or by dissociation of the water hydrating the exchangeable metal cations:

[M(H20).~] ~'+ -+ [M(OH)(H20)x t] ~ ~+ + H §

Bronsted acid sites are generated by the exchange of interlamellar cations with protons and Lewis acid sites correspond to Mg 2+ and AI 3+ present at the edges of octahedral sheets (Kumar et al., 1995). Thermogravimetric data confirmed that the acidity on the montmorillonite clay following acid activation could mainly be attributed to Bronsted acid sites. Thermograms for pyridine desorption from the pure and acid-activated clays are shown in Fig. 5. Untreated and the acid-activated clays show an initial desorption maximum at 190~ indepen- dent of the extent of the activation. This desorption is due to pyridine H bonded to pyridinium ions in the interlayer (Breen, unpublished results). A second desorption maximum was observed at 340~ for the untreated (AI) material and at -355~ for the acid-activated (A2, A3, A4.) samples, due to Bronsted acid sites (Breen et aL, 1987; Breen, 1991). These results are supported by literature data. In fact, it has been observed that the number of acid centres significantly increases in acid-activated samples cmnpared with natural saponite (Vicente et al., 1996). Like inorganic acids, clay surfaces can be highly acidic. The Bronsted acidity stems from terminal hydroxyl groups and from bridging oxygens (Laszlo, 1987). An enhancement on the (Bronsted) acidity of the host matrix following acid treatment has been observed (Mokaya & Jones, 1994) and explained as the number of the matrix protons (on the clay sheets), not associated with the interlayer cations, which may be increased by acid treatment.

The higher Bronsted clay acidity may presumably be the origin of the enhancement in bleaching efficiency observed following acid activation. The fact that the acidity of the clay is crucial for its bleaching performance is not surprising. In fact, in the acid-activated montmorillonite, the surface hydroxyl groups on the broken alumina sheets can create a kind of adsorption sites following the reaction:

AI-OH + H30 + ~ A1-OH~ + H20

in a similar way as in the case of the alumina pillars of an alumina pillared clay in acidic solutions (Molinard et al., 1994). The protonated A1OH~ structure can serve as an effective binding site permitting the attachment of pigments and other cotouring matters contained in the cottonseed oil.

Such behaviour is not unexpected. II has been suggested that pigments such as ~-carotene can either be adsorbed directly onto a cation to form a chemisorbed complex which undergoes further reaction or react directly with the protonic centres present on the clay surface (Morgan et al., 1985). The removal of impurities (transition metals, soaps and phospholipids) occurs by a similar mechanism to that for pigment removal and is dependent on surface activity. Furthermore, studies on acid- activated organoclays (Breen et al., 1997b) attributed the observed catalytic activity to both the hydrophilic and hydrophobic character of the clay surface. In a similar way, one could suppose that the protonated clay framework, highly hy&o- philic in character, attracts the polar pigments. On the contrary, extensive leaching of cations from the octahedral sheet transforms the material to essen- tially hydrophobic silica which serves to adsorb the non-polar colouring matters contained in the cottonseed oil. Although it is generally accepted that the adsorption process in bleaching of vegetable oils is governed by the molecular sieving properties of the adsorbate involved as well as the electrostatic field strengths of the exchanged cations (Taylor et al., 1984), our results for the treatment of cottonseed oil prove that enhanced Bronsted surface acidity and surface area are the deciding factors in bleaching efficiency of acid-activated montmoril/onite.

C O N C L U S I O N S

Acid treatment of Ca-montmorillonite by 1 N, 4 N, and 8 rq H 2 S O 4 leads to clay samples whose structural, physical and chemical properties strongly depend on the extent of activation. Vibrational, X-ray and elemental analysis data confirm that acid activation affects both the octahedral alumina sheet as well as the tetrahedral silica sheet. The CEC progressively decreases while surface acidity and BET surface area are at a maximum for clay treated with 4 N H2804.

The acid activation has been found to affect the efficiency of the clay for the bleaching of cottonseed oil. The clay bleaching efficiency varies


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FIo. 5. DTA curves of untreated Ca-montmorillonite (a) and samples activated with 1 N (b), 4 N (C) and 8 N (d) It2SO4.

linearly with the surface area. The best results were obtained for the clay treated with 4 N H2804 as it had the greatest surface area and acidity and also the best colour index and the lowest peroxide value in the bleaching of the cottonseed oil. Increased Bronsted acidity corresponding to the protonation of the clay framework probably causes the enhanced bleaching efficiency of the acid-activated montmorillonite.

ACKNOWLEDGMENTS

Assistance from Dr D. Tsiourvas, Dr C. Trapalis, Dr M. Karakasides and T. Steriotis, helpful discussions with Dr A. Provata and financial support from the Greek General Research Secretariat are acknowledged.

REFERENCES

Boki K., Kudo M., Wada T. & Tamura T. (1992) Bleaching of alkali-refined vegetable oils with clay minerals. J. Am. Oil Chem. Soc. 69, 232-236.

Bovey J. & Jones W. (1995) Characterization of A1- pillared acid activated clay catalysts. J. Mater. Chem. 5, 2027 2035.

Breen C. (1991) Thermogravimetric study of the desorption of cyclohexylamine and pyridine from an acid-treated Wyoming bentonite. Clay Miner. 26, 473 -486.

Breeu C., Deane A.T. & Flynn J.J. (1987) The acidity of t r ivalent cation exchanged montmori l loni te . Temperature-programmed desorption and infrared studies of pyridine and n-butylamine. Clay Miner. 22, 169--178.

Breen C., Madejov/t J. & Komadel P. (1995) Characterization of moderately acid-treated, size-


fractionated montmorillonites using IR and MASNMR spectroscopy and thermal analysis. J. Mat. Chem. 5, 469-474.

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cottonseed oil bleaching by acid-activated...

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