Scholarly article on topic 'Adsorptive Separation Studies of 𝜷-Carotene from Methyl Ester Using Mesoporous Carbon Coated Monolith'

Adsorptive Separation Studies of 𝜷-Carotene from Methyl Ester Using Mesoporous Carbon Coated Monolith Academic research paper on "Chemical engineering"

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Academic research paper on topic "Adsorptive Separation Studies of 𝜷-Carotene from Methyl Ester Using Mesoporous Carbon Coated Monolith"

Hindawi Publishing Corporation

Journal of Chemistry

Volume 2013, Article ID 235836, 6 pages

Research Article

Adsorptive Separation Studies of ^-Carotene from Methyl Ester Using Mesoporous Carbon Coated Monolith

M. Muhammad,1 Moonis Ali Khan,2 and T. S. Y. Choong3,4

1 Department of Chemical Engineering, Faculty of Engineering, Malikussaleh University Aceh, Lhokseumawe, Indonesia Chemistry Department, College of Science, King Saud University, Riyadh 11451, Saudi Arabia INTROP, Universiti Putra Malaysia, Selangor, 43400 Serdang, Malaysia 4 Department of Chemical and Environmental Engineering, Faculty of Engineering, Universiti Putra Malaysia, Selangor, 43400 Serdang, Malaysia

Correspondence should be addressed to T. S. Y. Choong;

Received 10 January 2012; Revised 17 May 2012; Accepted 23 May 2012

Academic Editor: Saima Q. Memon

Copyright © 2013 M. Muhammad et al. "ttisisan open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Adsorption of ^-carotene on mesoporous carbon coated monolith (MCCM) from methyl ester as a solvent was investigated. Kinetics and thermodynamics parameters have been evaluated. Maximum ^-carotene adsorption capacity was 22.37 mg/g at 50 °C. Process followed Langmuir isotherm. "tte adsorption was endothermic and spontaneous. Contact time studies showed increase in adsorption capacity with increase in ^-carotene initial concentration and temperature. Pseudo-second-order model was applicable to the experimental data. "tte value of activation energy confirmed physical adsorption process.

1. Introduction

tte characteristic orange color of crude palm oil is due to the presence of carotenoids (a- and ^-carotenes). ttese carotenoids are of commercial importance as they are utilized as natural coloring agents in edible and pharmaceutical products. Transesterification of palm oil produces an ecofriendly diesel (or biodiesel) containing methyl ester as a major constituent. tte biodiesel (or methyl ester) contains a rather high concentration of carotenoids. tterefore, it is essential to develop a method to recover this valuable product. Separation of carotenoids from methyl ester by nanofiltration was reported by Darnoko and Cheryan [1].

tte utility of carbonaceous (powder and granular) materials in the form of fixed bed for separation is associated with high pressure drops, potential channeling, and many other demerits. Compared to carbonaceous material, mesoporous carbon coated monolith (MCCM) has large external surface area and a very less pressure drop across fixed bed MCCM column. High mechanical stability and thermal expansion coefficient are some of the other properties of MCCM. tte MCCM columns can also be placed in vertical or horizontal position and in mobile system without deforming shape and

is easier to be scaled up due to its simple design and uniform flow distribution.

In our previous studies, we had reported the adsorption and desorption of ^-carotene on MCCM using isopropyl alcohol and n-hexane as solvents [2, 3]. In this study we had utilized MCCM for adsorptive separation of ^-carotene form methyl ester in synthetic solution system. Various thermodynamics and kinetics parameters were studied.

2. Materials and Methods

2.1. Materials. Cordierite monoliths (channel width 1.02 ± 0.02 mm and wall thickness 0.25 ± 0.02 mm) were obtained from Beihai Huihuang Chemical Packing Co., Ltd, China. Others materials like ^-carotene was purchased from Sigma-Aldrich, Malaysia. tte stock solution of ^-carotene (500 mg/L) was prepared by dissolving required amount in solvent.

2.2. Chemical and Reagents. Methyl ester, a solvent for carotene was purchased from Sigma-Aldrich, Malaysia. Fur-furyl alcohol (FA), pyrrole, and poly(ethylene glycol) (PEG,

MW-8000) were purchased from Fluka, Malaysia. Nitric acid (HNO3) 65% was purchased from Fisher, Malaysia. All the chemicals used were of analytical grade.

2.3. Preparation of MCCM. tte polymerization of samples was carried out by mixing FA and PEG in percentage volume ratio of 40: 60. tte polymerization catalyst, HNO3, was added stepwise, at every 5min. After addition of the acid, the mixture was stirred for an hour while maintaining temperature at approximately 21-23°C. Detailed method of MCCM preparation was reported elsewhere [2].

2.4. Adsorption Equilibrium and Kinetics. Batch adsorption experiments were carried out under nitrogen atmosphere. ^-carotene of concentrations 50 to 500mg/L were taken in 250 mL conical stopper cork flasks. Methyl ester was used as a solvent. tte MCCM, 0.8 g, was added to each flask. tte flasks were wrapped with aluminium foil to minimize ^-carotene photo degradation. tte flasks were shaken at 150 rpm in a water bath shaker (Stuart SBS40) at desired temperatures (30, 40 and 50°C). At equilibrium, the samples were collected and were analyzed.

Kinetics studies were carried out under similar experimental conditions. tte MCCM, 3 g, was taken in 250 mL conical flasks for reaction with ^-carotene. Samples were collected at desired time intervals using a digital micropipette (Rainin Instrument, USA). tte samples were analyzed using a double beam UV/VIS spectrophotometer (ttermo Electron Corporation) at wavelength 446 nm.

tte concentration of solute adsorbed on the MCCM at equilibrium was calculated as

V (Co - Ce)

where qe is the solid phase concentration at the equilibrium phase (mg/g), C0 and Ce are the initial and equilibrium concentrations of the liquid phase (mg/L), V is the liquid volume (L), and m is the adsorbent mass (g).

3. Results and Discussion

3.1. Equilibrium Isotherms. Langmuir isotherm implies formation of monolayer coverage of adsorbate on the surface of the adsorbent. A linearized form is given as

Çe <?e

Krb + bCe'

where KL is Langmuir adsorption equilibrium constant (L/mg), and b is the monolayer capacity of the adsorbent (mg/g).

Freundlich isotherm describes equilibrium on heterogeneous surfaces where adsorption energies are not equal to all adsorption sites. Linear form is given as

log qe = log KF + l/nlogCe,

where KF is the Freundlich constant for a heterogeneous

Table 1: Isotherm parameters for ^-carotene adsorption on MCCM at different temperatures.

Isotherms Parameters 30°C 40° C 50°C

b 20 21.23 22.37

Langmuir KL 0.0053 0.0064 0.0079

0.28 0.24 0.20

R2 0.9803 0.9944 0.9919

KF 0.61 0.96 1.43

Freundlich 1/n 0.52 0.46 0.42

R2 0.9597 0.9842 0.9658

Table 2: Comparative mon olayer adsorption capacities (b) for fi-

carotene at 50° C.

Adsorbent b (mg/g) Solvent Reference

MCCM 62.12 Isopropyl alcohol [2]

Silica gel 25.32 n-hexane [5]

Florisil 86.21 n-hexane [5]

MCCM 22.37 Methyl ester "ttis study

tte coefficient of determination (R2) values for Langmuir model at 30, 40, and 50°C were higher compared to Freundlich model showing better applicability of Langmuir model (Table 1). ttese results were in good agreement with previously reported studies on ^-carotene adsorption on acid-activated montmorillonite [4] and on silica-based adsorbent [5]. However, for ^-carotene adsorption from crude maize and sunflower oil on acid-activated bentonite, applicability of Freundlich model was reported [6]. tte values of b and KL generally increased with increasing temperature. Table 2 compares ^-carotene maximum adsorption capacity (b) with literature.

tte separation factor (RL) is a dimensionless parameter. It is defined as

1 + KLC o

adsorbent (mg/g)(L/mg)1/n, and n is the heterogeneity factor.

tte Rl values for the present study were in range of favorable adsorption process (Table 1).

3.2. Effect of Temperature. tte ^-carotene adsorption increases with temperature (Figure 1) suggesting that the intraparticle diffusion rate of the adsorbate molecules into the pores increased with increase in temperature since diffusion is an endothermic process [7]. Physical adsorption is normally considered to be the dominant adsorption mechanism for temperature lower than 100°C and chemisorption for temperature higher than 100°C [8]. tte pigment is adsorbed only on the outer surface of the adsorbent at lower temperatures, and both on the outer surface and pore surface at higher temperatures [9]. However, at higher temperature destruction of ^-carotene may occur [5]. tterefore, the adsorption experiments were carried out up to 50° C.

Temperature (°C)

Figure 1: Effect of temperature on ^-carotene adsorption onto MCCM.

3.3. Estimation of Iermodynamic Parameters. tte data obtained from the Langmuir isotherm can be used to determine thermodynamic parameters such as Gibbs free energy change (AG), enthalpy change (AH), and entropy change (AS). tte Gibbs free energy change was calculated as

AG = -RT\nb,

where T is the absolute temperature (K) and R is the universal gas constant (8.314 J/mol-K). tte AH and AS values were determined from the following equation:

, , AS AH

In b =---—.

tte AG values at 30, 40, and 50°C were -7546.7, -7951.23, and -8345.7 J/mol, respectively. tte decrease in AG values with temperature suggests that more ^-carotene is adsorbed with increasing temperature [10]. ttis implies that the adsorption is favored at higher temperature. tte positive AH value (4560.31 J/mol) indicates that the adsorption is endothermic. tte positive AS value (39.96 J/mol-K) suggests increasing randomness at the solid/liquid interface during carotene adsorption on MCCM.

3.4. Effect of Contact Time. tte experiments were performed varying temperature (i.e., 30,40 and 50°C) at a fixed initial carotene concentration (500 mg/L). An increase in reaction temperature causes a decrease in solution viscosity leading to an increase in ^-carotene molecules rate of diffusion across the external boundary layer and into the internal pores of the adsorbent. In addition, an increase in temperature increases MCCM equilibrium capacity for ^-carotene. As shown in Figure 2, the recovery of ^-carotene increased with increase in temperature. ttis may be the result of increase in the carotene molecules movement with temperature. An increasing number of molecules may also acquire sufficient energy to undergo an interaction with active sites. As presented

100 150

Time (min)

30° C 40° C 50° C

Figure 2: Effect of contact time on ^-carotene adsorption on MCCM at different temperatures (initial ^-carotene concentra-tion—500 mg/L).

in Table 3 the ^-carotene adsorption capacity onto MCCM increased from 8.218 to 10.775 mg/g with an increase in reaction temperature from 30 to 50°C, indicating that the process is endothermic [11]. tte equilibration time at various temperatures was 200 min.

^-carotene adsorption on MCCM for various adsorbate concentrations was fast initially, thereafter, the adsorption rate decreased slowly as the available adsorption sites decreases gradually (Figure 3). tte equilibration time increases from 165 to 200 min while the adsorption capacity increases from 3.099 to 10.775 mg/g with increase in concentration from 50 to 500 mg/L (Table 3).

3.5. Adsorption Kinetics. Lagergren rate equation is one of the most widely used adsorption rate equations to describe the adsorption kinetics. Linearized form is expressed as [12]:

where qe and qt are the adsorbed amount at equilibrium and at time t and is the pseudo-first-order rate constant (1/min).

tte pseudo-second-order model in linearized form is expressed as [13]


qt k2 <û <?<

where k2 is the rate constant of pseudo-second-order sorption (g/mg-min).

tte values of R2 for pseudo-second-order model were comparatively higher. tte calculated adsorption capacity (q calc) values for pseudo-second-order model were much

Table 3: Kinetics data for jS-carotene adsorption on MCCM.

Temp (°C) Co (mg/L) <ie,exp. (mg/g) Pseudo-first-order iie,calc.(mg/g) (1/min) R2 Pseudo-second-order <ie, calc. (mg/g) fc2 (g/mg-min) 2

50 50 3.099 1.842 0.0221 0.9791 3.262 0.0249 0.9997

50 250 5.969 2.818 0.0235 0.9475 6.203 0.0187 0.9998

50 500 10.775 5.212 0.0237 0.9576 11.186 0.0105 0.9999

30 500 8.218 4.756 0.0196 0.9311 8.772 0.0073 0.9983

40 500 9.615 5.145 0.0216 0.9548 10.152 0.0081 0.9997

50 mg/L 250 mg/L 500 mg/L

100 150

Time (min)



Figure 3: Effect of contact time on j3-carotene adsorption on MCCM at different concentrations at 50° C.

T1/2 (min1/2)

30° C 40° C 50° C

Figure 4: Weber and Morris plot for j3-carotene adsorption at different temperatures (Initial jS-carotene concentration was 500 mg/L).

closer to experimental adsorption capacity (qe exp) values (Table 3). tterefore, it is concluded that the pseudo-second-order kinetics model better describes /^-carotene onto MCCM. Similar results were reported for /^-carotene adsorption on acid activated bentonite [10, 14] and florisil [5].

3.6. Adsorption Mechanism. tte rate-limiting step prediction is an important factor to be considered in sorption process. For solid-liquid sorption process, the solute transfer process was usually characterized by either external mass transfer (boundary layer diffusion) or intraparticle diffusion or both. tte mechanism for /^-carotene removal by adsorption may be assumed to involve three successive transport steps: (i) film diffusion, (ii) intraparticle or pore diffusion, and (iii) sorption onto interior sites. tte last step is considered negligible as it is assumed to be rapid. /^-carotene uptake on MCCM active sites can mainly be governed by either liquid phase mass transfer or intraparticle mass transfer rate.

tte most common method used to identify the mechanisms involved in the adsorption process is by fitting the

experimental data to the intraparticle diffusion plot. tte intraparticle diffusion equation can be expressed as [15]

It = fcid t

where fc^ is intraparticle diffusion rate constant (mg/g-

• 1/2\ min ).

tte Weber-Morris plots of qt versus f1/2 were presented in Figures 4 and 5, for the /^-carotene adsorption onto MCCM as a function of temperature and initial concentration. For the adsorption process to be intraparticle diffusion controlled, the plots of versus i1/2 should pass through the origin and the .R2 should be sufficiently close to unity. tte intraparticle diffusion parameters, fc^, for these regions were determined from the slope of the plots.

tte adsorption data for versus i1/2 for the initial period show curvature, attributed to boundary layer diffusion effects or external mass transfer effects [16]. As shown in Figures 4 and 5 the adsorption process followed two phases, suggesting that the adsorption process proceeded first by surface adsorption and then intraparticle diffusion. ttis demonstrated that, in the initial stages, adsorption was due

Table 4: Intraparticle diffusion parameters for ^-carotene adsorption on MCCM.

Temp. (°C) Conc. (mg/L) "W (mg/g) fcid,i (mg/g-min1/2) R2 hda (mg/g-min1/2) R2

50 50 3.099 0.2448 0.9505 0.0675 0.9160

50 250 5.969 0.3542 0.9439 0.0706 0.8241

50 500 10.775 0.7540 0.9445 0.1133 0.9537

30 500 8.218 1.0631 0.9490 0.1239 0.9204

40 500 9.615 0.8993 0.9372 0.1190 0.9530

В 6 -

tte relationship between the rate constants and solution temperature is expressed as

T1/2 (min1/2)

О 50 mg/L Л 250 mg/L □ 500 mg/L

Figure 5: Weber and Morris plot for ^-carotene adsorption at different initial concentrations and temperatures 50°C.

to the boundary layer diffusion effect and subsequently due to the intraparticle diffusion effect [17].

tte Weber-Morris plots did not pass through the origin (Figures 4 and 5), implying that the mechanism of adsorption was influenced by two or more steps of adsorption process. ttis also indicates that the intraparticle diffusion is not the sole rate-controlling step. tte values of rate parameters of intraparticle diffusion (fcid>1 and fcid>2) and correlation coefficients (R2) were presented in Table 4. tte intraparticle diffusion rate increases with increase in initial ^-carotene concentration and reaction temperature. tte driving force of diffusion was very important for adsorption processes. Generally driving force changes with ^-carotene concentration in bulk solution. tte increase in ^-carotene concentration and reaction temperature result in increase of the driving force, which in turn increases the diffusion rate of ^-carotene molecules in monolith pores.

3.7. Determination of Activation Energy. tte values of rate constant found from adsorption kinetics could be applied in the Arrhenius form to determine the activation energy.

^2 = kQ exp^-^^

where k0 is the temperature independent factor, Ea is the activation energy (kJ/mol), R is the gas constant (8.314 J/mol K), and T is the solution temperature (K). Equation (10) could be transformed into a linear form as

log^2 = log^Q -


tte values of Ea and k0 were obtained from the slope and intercept of the plot log k2 versus 1/T (figure not shown).

As shown in Table 3, the values of rate constant for pseudo-second-order (k2) were found to increase from 0.0073 to 0.0105 g/mg-min, with increasing solution temperature from 303.15 (30°C) to 323.15 K (50°C). tte magnitude of activation energy could provide information on type of adsorption, either physical or chemical. tte value of activation energy for ^-carotene adsorption was 14.73 kJ/mol. ttis value was <42.0 kJ/mol and is therefore consistent with physical adsorption process [18]. Adsorption of ^-carotene by an acid-activated bentonite [6], sorption of ^-carotene and chlorophyll onto acid-activated bentonite [10], and the sorptions of ^-carotene on tonsil [19] have been reported to be controlled by physical adsorption.

4. Conclusions

^-carotene adsorption studies onto MCCM from methyl ester solution were conducted. Langmuir was the best applicable isotherm model with maximum monolayer adsorption capacity 22.37 mg/g at 50°C. tte adsorption process was endothermic and followed physisorption mechanism. Kinetics studies showed applicabilityofpseudo-second-order kinetics model. tte activation energy was 14.73 kJ/mol, suggesting that ^-carotene adsorption onto MCCM is via physical adsorption.


tte authors would like to acknowledge Universiti Putra Malaysia for financial support of this project (partially via vot: 9199659).


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