Adsorptive Separation Studies of ββ-Carotene fromMethyl Ester UsingMesoporous Carbon CoatedMonolith

1 Department of Chemical Engineering, Faculty of Engineering, Malikussaleh University Aceh, Lhokseumawe, Indonesia 2 Chemistry Department, College of Science, King Saud University, Riyadh 11451, Saudi Arabia 3 INTROP, Universiti Putra Malaysia, Selangor, 43400 Serdang, Malaysia Department of Chemical and Environmental Engineering, Faculty of Engineering, Universiti Putra Malaysia, Selangor, 43400 Serdang, Malaysia


Introduction
e characteristic orange color of crude palm oil is due to the presence of carotenoids (-and -carotenes).ese carotenoids are of commercial importance as they are utilized as natural coloring agents in edible and pharmaceutical products.Transesteri�cation of palm oil produces an ecofriendly diesel (or biodiesel) containing methyl ester as a major constituent.e biodiesel (or methyl ester) contains a rather high concentration of carotenoids.erefore, it is essential to develop a method to recover this valuable product.Separation of carotenoids from methyl ester by nano�ltration was reported by Darnoko and Cheryan [1].
e utility of carbonaceous (powder and granular) materials in the form of �xed 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 �xed bed MCCM column.High mechanical stability and thermal expansion coefficient are some of the other properties of MCCM.e 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 �ow 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.

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.e stock solution of -carotene (500 mg/L) was prepared by dissolving required amount in solvent.

Chemical and Reagents.
Methyl ester, a solvent for carotene was purchased from Sigma-Aldrich, Malaysia.Furfuryl alcohol (FA), pyrrole, and poly(ethylene glycol) (PEG, MW-8000) were purchased from Fluka, Malaysia.Nitric acid (HNO 3 ) 65% was purchased from Fisher, Malaysia.All the chemicals used were of analytical grade.

Preparation of MCCM. e polymerization of samples
was carried out by mixing FA and PEG in percentage volume ratio of 40 : 60. e polymerization catalyst, HNO 3 , was added stepwise, at every 5 min.Aer 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].

Adsorption Equilibrium and Kinetics.
Batch adsorption experiments were carried out under nitrogen atmosphere.-carotene of concentrations 50 to 500 mg/L were taken in 250 mL conical stopper cork �asks.Methyl ester was used as a solvent.e MCCM, 0.8 g, was added to each �ask.e �asks were wrapped with aluminium foil to minimize -carotene photo degradation.e �asks 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.e MCCM, 3 g, was taken in 250 mL conical �asks for reaction with -carotene.Samples were collected at desired time intervals using a digital micropipette (Rainin Instrument, USA).e samples were analyzed using a double beam UV/VIS spectrophotometer (ermo Electron Corporation) at wavelength 446 nm.
e concentration of solute adsorbed on the MCCM at equilibrium was calculated as where   is the solid phase concentration at the equilibrium phase (mg/g),  0 and   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).

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 where   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 where   is the Freundlich constant for a heterogeneous adsorbent (mg/g)(L/mg)  1).ese 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 sun�ower oil on acid-activated bentonite, applicability of Freundlich model was reported [6].e values of b and   generally increased with increasing temperature.Table 2 compares -carotene maximum adsorption capacity (b) with literature.
e separation factor (  ) is a dimensionless parameter.It is de�ned as e   values for the present study were in range of favorable adsorption process (Table 1).

Effect of
Temperature.e -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]. e 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].erefore, the adsorption experiments were carried out up to 50 ∘ C.

Estimation of ermodynamic
Parameters. e data obtained from the Langmuir isotherm can be used to determine thermodynamic parameters such as Gibbs free energy change (ΔG), enthalpy change (ΔH), and entropy change (ΔS).e Gibbs free energy change was calculated as where T is the absolute temperature (K) and R is the universal gas constant (8.314J/mol-K).e ΔH and ΔS values were determined from the following equation: e ΔG values at 30, 40, and 50 ∘ C were 7546.7,7951.23,and 8345.7 J/mol, respectively.e decrease in ΔG values with temperature suggests that more -carotene is adsorbed with increasing temperature [10].is implies that the adsorption is favored at higher temperature.e positive ΔH value (4560.31J/mol) indicates that the adsorption is endothermic.e positive ΔS value (39.96J/mol-K) suggests increasing randomness at the solid/liquid interface during carotene adsorption on MCCM.

Effect of Contact Time
. e experiments were performed varying temperature (i.e., 30, 40 and 50 ∘ C) at a �xed 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.is 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 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].e equilibration time at various temperatures was 200 min.-carotene adsorption on MCCM for various adsorbate concentrations was fast initially, thereaer, the adsorption rate decreased slowly as the available adsorption sites decreases gradually (Figure 3).e 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).

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   and   are the adsorbed amount at equilibrium and at time t and  1 is the pseudo-�rst-order rate constant (1/min).e pseudo-second-order model in linearized form is expressed as [13] where  2 is the rate constant of pseudo-second-order sorption (g/mg-min).e values of  2 for pseudo-second-order model were comparatively higher.e calculated adsorption capacity ( calc ) values for pseudo-second-order model were much closer to experimental adsorption capacity (  ) values (Table 3).erefore, it is concluded that the pseudosecond-order kinetics model better describes -carotene onto MCCM.Similar results were reported for -carotene adsorption on acid activated bentonite [10,14] and �orisil [5].
3.6.Adsorption Mechanism.e 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.e mechanism for -carotene removal by adsorption may be assumed to involve three successive transport steps: (i) �lm diffusion, (ii) intraparticle or pore diffusion, and (iii) sorption onto interior sites.e 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.e most common method used to identify the mechanisms involved in the adsorption process is by �tting the experimental data to the intraparticle diffusion plot.e intraparticle diffusion equation can be expressed as [15]   =  id  1/2  ( where  id is intraparticle diffusion rate constant (mg/gmin 1/2 ).e Weber-Morris plots of   versus  1/2 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  1/2 should pass through the origin and the  2 should be sufficiently close to unity.e intraparticle diffusion parameters,  id , for these regions were determined from the slope of the plots.
e adsorption data for   versus  1/2 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 �rst by surface adsorption and then intraparticle diffusion.is demonstrated that, in the initial stages, adsorption was due to the boundary layer diffusion effect and subsequently due to the intraparticle diffusion effect [17].e Weber-Morris plots did not pass through the origin (Figures 4 and 5), implying that the mechanism of adsorption was in�uenced by two or more steps of adsorption process.is also indicates that the intraparticle diffusion is not the sole rate-controlling step.e values of rate parameters of intraparticle diffusion ( id1 and  id2 ) and correlation coefficients ( 2 ) were presented in Table 4. e intraparticle diffusion rate increases with increase in initial -carotene concentration and reaction temperature.e driving force of diffusion was very important for adsorption processes.Generally driving force changes with -carotene concentration in bulk solution.e 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.

Determination of Activation
Energy.e values of rate constant found from adsorption kinetics could be applied in the Arrhenius form to determine the activation energy.e relationship between the rate constants and solution temperature is expressed as where  0 is the temperature independent factor,   is the activation energy (kJ/mol), R is the gas constant (8.314J/mol K), and T is the solution temperature (K).Equation ( 10) could be transformed into a linear form as log  2 = log  0 −   2303  (11) e values of   and  0 were obtained from the slope and intercept of the plot log  2 versus 1/T (�gure not shown).
As shown in Table 3, the values of rate constant for pseudo-second-order ( 2 ) 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). e magnitude of activation energy could provide information on type of adsorption, either physical or chemical.e value of activation energy for -carotene adsorption was 14.73 kJ/mol.is 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.

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. e adsorption process was endothermic and followed physisorption mechanism.Kinetics studies showed applicability of pseudo-second-order kinetics model.e activation energy was 14.73 kJ/mol, suggesting that -carotene adsorption onto MCCM is via physical adsorption.

F 1 :
Effect of temperature on -carotene adsorption onto MCCM.

F 2 :
Effect of contact time on -carotene adsorption on MCCM at different temperatures (initial -carotene concentration-500 mg/L).

F 4 :
Weber and  Morris plot for -carotene adsorption at different temperatures (Initial -carotene concentration was 500 mg/L).
F 3: Effect of contact time on -carotene adsorption on MCCM at different concentrations at 50 ∘ C.