The effectiveness of Ibusa kaolinite as an adsorbent in removing pigments from palm oil has been investigated in this study. Ibusa kaolinite was prepared as an adsorbent by treating it with hydrochloric acid. The surface area of the clay was found to increase with increase in acid dosage up to a maximum and then de-creased while its cation exchange capacity decreased with increase in acid dosage. The activated clay was used as an adsorbent for the removal of pigments from palm oil. The bleaching process was investigated by varying the clay dosage, acid concentration and temperature. The highest removal of pigments was recorded at 7 M HCl concentration, 4 g clay dosage and 100°C temperature, and about 97.4% pigments were removed in 80 minutes. Four isotherm models, three kinetic models, and the intra-particle diffusion model were applied to fit the experimental data. It was found that the equilibrium data were best represented by the Temkin isotherm model. The experimental data fitted well the pseudo-second-order kinetic model. Diffusion studies indicated that in-tra-particle diffusion is not the sole rate-controlling factor. The bleaching pro-cess was found to be spontaneous and endothermic, with increasing random-ness of adsorbed species.
Kaolinite, which is a hydrated aluminium silicate with the chemical formula Al2Si2O5(OH)4 is the most abundant true clay mineral. Raw clay material that consists primarily of kaolinite is called kaolin. Kaolinite occurs in nature in relatively thick beds made up of billions of these tiny kaolinite crystals, which typically measure about one micrometer, across the plate face by about 0.1 micrometer in thickness. Several structural variations of the fixed kaolinite formula exist, depending on differences in internal arrangement of the Al, Si, and O atoms in the crystal [
Vegetable oils in their crude form are deeply coloured. These colour impurities have to be removed to make the final product attractive and acceptable to the end user. Impurities in vegetable oil include pigments such as chlorophyll, tocopherol, xanthophylls, carotenoid, phosphatides, trace metals, traces of soap, peroxides and free fatty acids [
The impurity load in vegetable oils can be reduced considerably by bleaching which is an adsorption process that utilizes clay as adsorbent. This may be naturally active or activated clays. Naturally active clays possess some bleaching activity and show a high adsorption capacity due to their high surface area. However, activated bleaching clays show a much higher activity [
Adsorbents are activated by a mineral acid treatment resulting in the dealumination of the structure. A number of metal ions in the octahedral layer and impurities such as calcite are also removed by leaching with an inorganic acid at elevated temperature [
Ajemba and Onukwuli [
The present study focuses on the removal of pigments from palm oil using acid activated Ibusa clay. Different adsorption isotherms and kinetic models were fitted to the experimental data. In addition, thermodynamic parameters such as ΔGo, ΔHo, and ΔSo were estimated.
The kaolinite sample used in this research was sourced from Delta State of Nigeria-Ibusa (Lat 6'11''N, Long 6'38''E). At the point of mining, the clay was wet and the debris was manually separated. It was spread in the sun for 24 hours to dry. The crude palm oil (CPO) was bought from local oil mill at Ezema village, Ojoto in Idemili South local government area of Anambra State Nigeria. The crude palm oil was degummed with the use of phosphoric acid. It was then characterized with atomic absorption spectrophotometer (AAS). The picture of Ibusa kaolinite is shown in Plate 1.
Plate 1. Ibusa kaolinite.
The clay material was prepared for activation by drying it under the sun at an ambient temperature of 35˚C to make them amenable to grinding. The clay sample was then pulverized and sieved to a particle of 300 µm. 50 g of the clay sample was mixed with 250 ml of the prepared acid. The resulting suspension was heated on a magnetically stirred hot plate at a temperature of 98˚C for 2.0 hours. The clay residue was washed free of the acid several times with distilled water until a neutral point was obtained with pH meter. The clay was then dried at a temperature of 110˚C for 3 hours, then ground again using laboratory mortar and pestle, sieved with 75 µm sieve and stored in desiccators. Also, the effect of acid concentration on the physical properties of the clay samples was investigated by measuring the cation exchange capacity and surface area of the samples at each acid concentration used in the activation process.
An ARL 9400XP+ Wavelength-dispersive XRF Spectrometer with a Rh source was used for the X-ray fluorescence analyses of the samples. The NBSGSC fundamental parameter program was used for matrix correction of major elements, as well as Cl, Co, Cr, V, Sc, and S. The Rh Compton peak ratio method was used for the other trace elements. Samples were dried and fired at 1000˚C to determine the percentage loss on ignition; for the samples this was less than 2%. Major element analyses were carried out on fused beads. A pre-fired sample of 1 g and 6 g of lithium tetra-borate flux was mixed in a 5% Au/Pt crucible and fused at 1000˚C in a muffle furnace, with occasional swirling. The glass disk was transferred into preheated Pt/Au mould and the bottom surface was analyzed.
The infrared spectra were recorded in the mid-infrared region (400 - 4500 cm−1) in an evacuated chamber of Shimadzu FTIR-8400S spectrophotometer using potassium bromide (KBr) discs as matrices. A spectral resolution of 2 cm−1 was used and spectra were accumulated over 32 scans. The FTIR spectroscopy was applied to all samples. Only 2 mg of each sample was mixed with 100 mg of KBr and pressed under 6 tonnes for 2 minutes in making disk. At first the samples were crushed and ground before making the KBr pellets. The fitting of peaks and smoothing were done with OPUS 2000 software on the Shimadzu 8400 S over the working window, 400 - 4500 cm−1.
100 g of the refined unbleached palm oil was measured out into a 250 ml conical flask and 2 g of the sized activated clay samples were also added. The mixture of clay and oil was heated to a temperature of 80˚C for thirty minutes on a magnetically stirred hot plate. At the completion of the time, the hot oil and clay mixture was filtered under gravity using Whatman filter paper No.42 (15 cm diameter), before measuring the absorbance. The bleaching/adsorption efficiency of the activated clay samples was then determined by measuring the color of the bleached oil using UV-VIS Spectrophotometer (Model WFJ 525) at 450 nm [
Bleaching Efficiency ( % ) = A unbleached − A bleached A unbleached × 100 (1)
where Aunbleached and Ableached are absorbencies of unbleached and bleached palm oil respectively, at 450 nm.
To investigate the effect of process variables on bleaching efficiency of the activated clay sample, the above experimental procedure was carried out at different values of the parameters. The experiment was performed at different mass (concentration) of the adsorbent (activated clay) which was varied at 1, 2, 3, 4, and 5 grams. The temperature and time of heating were also varied at 70˚C, 80˚C, 90˚C, and 100˚C and 5, 10, 15, 20, 30, 40, 50, 60, 70 and 80 minutes, respectively. The effect of activation parameters on the bleaching/adsorption efficiency of the clay samples was studied by using the different samples activated with varying acid concentration in the bleaching process.
The result of XRF analysis of the clay shows that Alumina (Al2O3), Iron Oxide (Fe2O3) and Silicon Oxide (SiO2) are present in major quantities while other components are present in trace amounts. The following compositions were obtained: Al2O3 (17.5%), SiO3 (56.60%), Fe2O3 (19.29%), SO3 (1.52%), CaO (2.36%), TiO2 (2.36%), V2O5 (0.14%), Cr2O3 (0.09%), Mn2O3 (0.20%), P2O5 (0.43%), NiO (0.04%), CuO (0.03%), ZnO (0.06%), MoO3 (0.30%), Rh2O3 (1.10%), Ta2O5 (0.10%), Re2O7 (0.10%), IrO2 (0.27%), Se2SO3 (0.03%), CdO (0.60%).
The FTIR spectra of Ibusa clay is shown in
The physical properties of the raw and activated clay sample used as adsorbent are given in
Property | Ibusa clay | |
---|---|---|
Raw | Activated | |
Bulk density (g/cm3) | 954.6 | 753.9 |
Oil retention (%) | 22 | 45 |
Surface area (m2/g) | 72.4 | 259.2 |
Acidity | 0.01 | 0.02 |
pH | 6.6 | 4.2 |
CEC (meg/100g) | 81 | 68 |
Property | Crude palm oil | Bleached palm oil |
---|---|---|
Absorbance | 2.763 | - |
Moisture content % | 2.72 | 1.11 |
Peroxide value (meg/kg) | 0.867 | 0.001 |
Free fatty acid (FFA) % | 7.473 | 8.11 |
Deterioration of bleachability index (DOB) % | 2.56 | 1.61 |
Iron (ppm) | 5 | 3.1 |
Phosphorous (ppm) | 9.2 | 5.1 |
The variation of the surface area and the cation exchange capacity of the adsorbent used with the level of the acid activation were closely monitored as these properties of the adsorbent play an important role in determining their adsorption/bleaching efficiency for edible oil.
The effect of increasing the acid activation level on the cation exchange using HCl is shown in
The effect of increasing the acid dosage during activation on the surface area is given in
Different dosages of the clay/adsorbent material were used to adsorb colour
pigments from degummed palm oil. The relationships between the percentage colour reduction and the clay dosage variation using the clay sample is shown in
The influence of temperature on the bleaching efficiency of Ibusa clay activated with 7 M hydrochloric acid has been shown in
log ( q e − q t ) = log q e − k 1 2.303 t (Pseudo-first-order model) (2)
t q t = 1 k 2 q e 2 + t q e (Pseudo-second-order model) (3)
q t = 1 β ln ( a β ) + 1 β ln t (Elovich model) (4)
q t = K i d t 1 2 + c (Intra-particle diffusion model) (5)
The associated kinetic parameters have been evaluated from the slopes and intercepts of the respective linear plots of the kinetic equations, and the values are shown in
Comparison of the analyzed data based on the linear regression coefficient (R2) values as shown in
Kinetic models | Parameters | Temperature (K) | |||
---|---|---|---|---|---|
343 | 353 | 363 | 373 | ||
Pseudo-first-order | K1 (min−1) | 3.22 × 10−2 | 3.22 × 10−2 | 3.9 2× 10−2 | 4.15 × 10−2 |
qe (mg/g) | 0.701 | 0.986 | 1.183 | 1.040 | |
R2 | 0.947 | 0.950 | 0.947 | 0.959 | |
Pseudo-second-order | K2 (g/mg·min) | 3.59 × 10−3 | 3.41 × 10−3 | 1.19 × 10−2 | 1.40 × 10−2 |
qe (mg/g) | 1.727 | 2.165 | 1.473 | 1.550 | |
R2 | 0.998 | 0.997 | 0.997 | 0.998 | |
Elovich | Α (mg/g·min) | 3.21 × 10−2 | 4.69 × 10−2 | 6.33 × 10−2 | 7.87 × 10−2 |
Β (g/mg) | 0.500 | 3.559 | 3.497 | 3.125 | |
R2 | 0.932 | 0.939 | 0.972 | 0.980 | |
Intra-particle diffusion | Kid (mg/g∙min1/2) | 0.080 | 0.112 | 0.112 | 0.125 |
c | −0.158 | -0.210 | −0.121 | −0.096 | |
R2 | 0.995 | 0.996 | 0.997 | 0.992 |
kinetic equation is shown in
Langmuir considered adsorption to distribute molecules over the surface of the adsorbent in the form of a uni-molecular layer and for dynamic equilibrium between adsorbed and free molecules [
C e q e = 1 K a Q m − C e Q m (6)
where, C e is the equilibrium concentration of the pigment adsorbed (mg/L); q e is the amount of pigment adsorbed (mg/g), K a is the Langmuir adsorption constant (L/mg) and Q m is the theoretical maximum adsorption capacity (mg/g). Since the absorbance measurements are taken in all experiments for the bleaching process, the relative amount of pigment adsorbed (X) and the residual relative amount at equilibrium ( X e ) are obtained from Equations (7) and (8).
X = A o − A t A o (7)
X e = A t A o = 1 − X (8)
where, A o is the absorbance of unbleached (crude) palm oil and A t is the absorbance of bleached oil at time t. By writing X e instead of C e and X / m instead of q e , where m is the mass of the adsorbent, Langmuir isotherm takes a new form as shown in Equation (9).
X e X / m = 1 K a Q m − X e Q m (9)
The Freundlich isotherm is based on the multilayer adsorption (heterogeneous surface) [
ln q e = ln K F + 1 n ln C e (10)
where, q e is the amount of pigment adsorbed at equilibrium (mg/g); C e is the equilibrium concentration of the adsorbate (mg/L); K F (L/mg) and n are the Freundlich equilibrium coefficients. The value of n gives information on the favourability of adsorption process and K F is the adsorption capacity of the adsorbate. Putting C e as X e and q e as X / m , Freundlich isotherm takes a new form as shown in Equation (11).
ln X / m = ln K F + 1 n ln X e (11)
Temkin and Pyzhev considered the effect of the adsorbate interaction on adsorption and proposed the model known as Temkin isotherm [
q e = B 1 ln K T + B 1 ln C e (12)
where, B 1 = R T / b , T is the absolute temperature in K, R the universal gas constant, 8.314 J∙K−1∙mol−1, K T the equilibrium binding constant (L/mg) and B 1 is related to the heat of adsorption. Putting C e as X e and q e as X / m , Temkin isotherm takes a new form as shown in Equation (13).
X / m = B 1 ln K T + B 1 ln X e (13)
The linear form of Dubinin and Radushkevich (DR) isotherm equation is given in Equation (14) [
ln q e = ln Q m − β ε 2 (14)
where, Q m is the D-R monolayer capacity (mg/g), β is a constant related to adsorption energy, and ε is the Polanyi potential which is related to the equilibrium concentration as shown in Equation (15).
ε = R T ln ( 1 + 1 C e ) (15)
where, R is the gas constant (8.314 J/mol K) and T is the absolute temperature. The constant β gives the mean free energy, E of adsorption per molecule of the adsorbate when it is transferred to the surface of the solid from infinity in the solution, and can be computed using Equation (16).
E = 1 2 β (16)
The isotherm parameters estimated from the isotherm plots with the regression coefficients are listed in
Thermodynamic parameters, when properly examined could give detailed information regarding the intrinsic energy and structural changes after adsorption. In the practice of environmental engineering, both energy and entropy factors ought to be considered to determine the processes that occur spontaneously
Isotherm models | Parameters | Temperature (K) | |||
---|---|---|---|---|---|
343 | 353 | 363 | 373 | ||
Freundlich | KF (L/mg) | 0.096 | 0.241 | 0.222 | 0.335 |
n | −0.404 | −1.014 | −1.193 | −2.558 | |
R2 | 0.841 | 0.745 | 0.776 | 0.651 | |
Langmuir | Ka (L/mg) | −2.024 | −3.230 | −4.354 | −7.669 |
qm (mg/g) | 0.046 | 0.095 | 0.148 | 0.211 | |
R2 | 0.632 | 0.642 | 0.722 | 0.740 | |
Temkin | KT (L/mg) | 0.924 | 0.762 | 0.614 | 0.213 |
B1 | −0.649 | −0.468 | -0.386 | −0.219 | |
R2 | 0.988 | 0.959 | 0.940 | 0.829 | |
Dubinin and Radushkevich | β | -2 × 10−7 | −7 × 10−8 | −4 × 10−8 | −9 × 10−9 |
Qm (mg/g) | 0.045 | 0.161 | 0.254 | 0.433 | |
R2 | 0.761 | 0.640 | 0.618 | 0.432 |
[
Δ G o = − R T ln K c (17)
where R is the universal gas constant (8.314 × 10−3 J∙mol−1∙K−1), T is the absolute temperature and K c is the thermodynamic equilibrium constant. The thermodynamic equilibrium constant ( K c ) of the adsorption is defined as shown in Equation (18).
K c = q e C e (18)
The enthalpy (ΔHo) and entropy (ΔSo) values are estimated from the substitution of Equation (17) into Equation (19) which gives Equation (20).
Δ G o = Δ H o − T Δ S o (19)
and
ln K c = Δ S o R − Δ H o R T (20)
The values of ΔGo were calculated from Equation (19). The values of ΔHo and ΔSo were calculated from the slope and intercept of the plot of ln K c versus 1/T (not shown). The values of the thermodynamic parameters are shown in
Temperature (K) | ΔG (kJ/mol) | ΔH (kJ/mol) | ΔS (J/mol∙K) |
---|---|---|---|
343 | −0.500 | 110.39 | 323.33 |
353 | −3.730 | ||
363 | −6.970 | ||
373 | −10.200 |
higher temperatures. The value of ΔHo was positive, indicating the endothermic nature of the adsorption of pigments onto activated Ibusa clay in the temperature ranges of 343 - 373 K. The positive value of ΔSo suggested an increase in randomness at the solid/liquid interface during the adsorption.
Ibusa clay activated with hydrochloric acid has been identified as an efficient adsorbent for the removal of pigments from palm oil, with the removal reaching 97.4% at 373 K. Kinetic studies reveal that equilibrium was reached within 80 minutes and pseudo-second-order model fitted the experimental data better than other kinetic models. The adsorption isotherms suggest that Temkin isotherm better explained the experimental data for the bleaching of palm oil using activated Ibusa clay than Langmiur, Freundlich and Dubinin-Radushkevich isotherms. The bleaching process was more favourable at higher temperatures, and tends to be endothermic, with increasing randomness at the solid/solution interface.
The authors declare no conflicts of interest regarding the publication of this paper.
Okafor, V.N., Nnanwube, I.A., Obibuenyi, J.I., Onukwuli, O.D. and Ajemba, R.O. (2019) Removal of Pigments from Palm Oil Using Activated Ibusa Kaolinite: Equilibrium, Kinetic and Thermodynamic Studies. Journal of Minerals and Materials Characterization and Engineering, 7, 157-170. https://doi.org/10.4236/jmmce.2019.74012