Article Cite This: J. Agric. Food Chem. 2017, 65, 9753-9762
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Adsorption Removal of Glycidyl Esters from Palm Oil and Oil Model Solution by Using Acid-Washed Oil Palm Wood-Based Activated Carbon: Kinetic and Mechanism Study Weiwei Cheng,† Guoqin Liu,*,†,‡,§ Xuede Wang,§ and Lipeng Han⊥ †
School of Food Science and Engineering and ‡Guangdong Province Key Laboratory for Green Processing of Natural Products and Product Safety, South China University of Technology, Guangzhou 510640, China § College of Food Science and Engineering, Henan University of Technology, Zhengzhou 450001, China ⊥ School of Chemistry and Chemical Engineering, Guangzhou University, Guangzhou 510006, China S Supporting Information *
ABSTRACT: Acid-washed oil palm wood-based activated carbon (OPAC) has been investigated for its potential application as a promising adsorbent in the removal of glycidyl esters (GEs) from both palm oil and oil model (hexadecane) solution. It was observed that the removal rate of GEs in palm oil was up to >95%, which was significantly higher than other adsorbents used in this study. In batch adsorption system, the adsorption efficiency and performance of acid-washed OPAC were evaluated as a function of several experimental parameters such as contact time, initial glycidyl palmitate (PGE) concentration, adsorbent dose, and temperature. The Langmuir, Freundlich, and Dubinin−Radushkevich models were used to describe the adsorption equilibrium isotherm, and the equilibrium data were fitted best by the Langmuir model. The maximum adsorption capacity of acid-washed OPAC was found to be 36.23 mg/g by using the Langmuir model. The thermodynamic analysis indicated that the adsorption of PGE on acid-washed OPAC was an endothermic and physical process in nature. The experimental data were fitted by using pseudo-first-order, pseudo-second-order, and intraparticle diffusion models. It was found that the kinetic of PGE adsorption onto acid-washed OPAC followed well the pseudo-second-order model for various initial PGE concentrations and the adsorption process was controlled by both film diffusion and intraparticle diffusion. The desorption test indicated the removal of GEs from palm oil was attributed to not only the adsorption of GEs on acid-washed OPAC, but also the degradation of GEs adsorbed at activated sites with acidic character. Furthermore, no significant difference between before and after PGE adsorption in oil quality was observed. KEYWORDS: adsorption, glycidyl esters, acid-washed oil palm wood-based activated carbon, thermodynamics, isotherms, kinetics
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INTRODUCTION Glycidyl esters (GEs) are a group of new oil-processing contaminants with various fatty acyl groups and a terminal epoxy group. They are mainly formed in the deodorization step during plant oil refining for high temperature exposure and thus were widely found in refined plant oil.1,2 Although there is no enough toxicologic evidence for the detriment of human and animal body, GEs can be ingested and hydrolyzed to their hydrolysate, glycidol, which has been identified as genotoxic carcinogen3 and categorized as “probably carcinogenic to humans’’ (group 2A) by International Agency for Research on Cancer (IARC).4 It was found that GE content in palm oil is largely higher than that in other plant oils,5 and the main pollution sources consisting of GEs are palm oil and palm oilbased food. To our knowledge, compared to other plant oils, palm oil possesses the largest production, assumption, and international trade until now and is widely used as food dressing, frying oil, industrial raw material, etc., for example, the production of palm oil-based biodiesel.6,7 Hence, because of the high toxicity and carcinogenic properties of GEs and wide application of palm oil, the removal of GEs from palm oil is crucial. In recent years, various treatment methods including inhibition and removal of precursors,8,9 that is, diacylglyceride © 2017 American Chemical Society
and monoacylglyceride, modification of GE formation conditions,10−12 and elimination of formed GEs,13,14 have been investigated for the removal of GEs in palm oil. Nevertheless, because of the unclear formation mechanisms of GEs, the first two methods are either inefficient or difficult to achieve in industrial production. Of the third method, the utilization of different adsorbent materials such as calcinated zeolite, silicon oxide, sodium aluminum silicate, synthetic calcium silicate, and synthetic magnesium silicate for GE removal from palm oil was studied by Strijowski et al.14 However, it was found that only calcinated zeolite and synthetic magnesium silicate could reduce the amount of GEs in a range of 1), linear (RL = 1), favorable (0 < RL < 1), and irreversible (RL = 0). It was observed that the values of RL was in the range of 0.06−0.74 for the various initial concentrations of PGE at four temperature points, which suggested that the Langmuir isotherm model was favorable in nature for PGE adsorption on acid-washed OPAC in this study. Freundlich Isotherm. It is well-known that the Freundlich isotherm model is also used to describe the adsorption process.37 However, unlike the Langmuir isotherm, it is valid for multilayer adsorption and is based on the assumption of a heterogeneous surface with nonuniform interaction between adsorbate molecules and varying energy distribution. On the basis of the Freundlich equation as follows, the adsorption energy exponentially decreases on the adsorption sites of an adsorbent during the adsorption process: qe = KFCe1/ n
E=
(11)
where qe (mg/g) is the PGE amount on unit mass of acidwashed OPAC at equilibrium, Ce (mg/L) is the equilibrium PGE concentration in solution, KF ((mg/g)(L/mg)1/n) is the Freundlich constant related to the bonding strength, and 1/n is the heterogeneity factor which is an empirical indicator of adsorption intensity and surface heterogeneity of an adsorbent. The value of 1/n below one illustrates that the adsorption is a physical process while that above 1 represents a chemical adsorption process. The adsorption data and fitting lines of ln qe against Ce at the temperature range of 30−60 °C are depicted in Figure S2b. The values of KF and 1/n were determined from the intercepts and slopes of the fitting lines and listed in Table 2. The values of 1/n were found to be 0.19− 0.54 at different temperatures, indicating a physical adsorption of PGE onto acid-washed OPAC. The R2 values ( Freundlich > D− R, indicating the monolayer coverage of PGE on acid-washed OPAC. Also, the calculated adsorption data from Langmuir model was be closest to the experimental data. The maximum adsorption capacity of PGE for acid-washed OPAC was found to be 36.23 mg/g at 30 °C. Adsorption Kinetic Modeling. To investigate the adsorption efficiency and controlling mechanisms involved in mass transfer and chemical reaction during PGE adsorption process by acid-washed OPAC, and also to provide useful knowledge for the design of industrial removal equipment, the pseudo-first-order and pseudo-second-order equation were used to test the kinetics of PGE adsorption, while the Weber and Morris intraparticle diffusion equation was further applied to elucidate the diffusion mechanisms of the adsorption system in the present study. Pseudo-First-Order Kinetic Model. First, the experimental data were modeled by the pseudo-first-order equation, which was proposed and proved by Langergren.39 The model is expressed as
(10)
1 ln Ce n
(14)
The plots of ln qe versus ε2 at various temperatures are shown in Figure S2c. The values of qs and B were calculated from the intercept and slope of the fitting plots, and the results are listed in Table 2. The R2 values were lower than that of Freundlich isotherm model, and B values were found to be 0.54−1.18 mol2/kJ2 from which the mean free energy of adsorption, E, could be calculated by means of the following formula:
Eq 10 can be converted into the following linear form after the logarithm at both sides of equal sign: ln qe = ln KF +
(13)
dq t dt
= k1(qe − qt )
(16)
After being integrated by the boundary conditions qt = 0 at t = 0 and qt = qt at t = t with a subsequent logarithm processing, it becomes as follows:
(12) 9758
DOI: 10.1021/acs.jafc.7b03121 J. Agric. Food Chem. 2017, 65, 9753−9762
Article
Journal of Agricultural and Food Chemistry
Table 3. Pseudo-First-Order, Pseudo-Second-Order, and Intraparticle Diffusion Model Kinetic Parameters for PGE Adsorption by Acid-Washed OPAC at 30 °C initial PGE concentration (mg/L) kinetic model pseudo-first-order kinetic model
pseudo-second-order kinetic model
intraparticle diffusion model
experimental data
ln(qe − qt ) = ln qe −
k1t 2.303
parameters
10
20
30
40
50
100
qe, cal (mg/g) k1 (min−1) R2 qe, cal (mg/g) k2 (×10−3 g/(mg min)) R2 h (mg/(g min)) kp (mg/(g min1/2)) C R2 qe, exp(mg/g)
40.29 0.073 0.987 35.52 2.57 0.998 3.24 2.84 6.43 0.865 31.83
130.69 0.105 0.919 71.53 1.09 0.997 14.76 5.36 14.76 0.871 62.80
136.14 0.076 0.978 112.61 0.47 0.996 5.60 8.61 14.55 0.923 94.07
112.43 0.098 0.949 139.28 0.74 0.992 5.94 9.42 43.76 0.788 125.04
224.65 0.132 0.991 165.56 0.92 0.998 14.36 10.12 66.27 0.795 153.81
192.69 0.143 0.973 309.60 1.23 0.999 117.92 13.34 187.85 0.705 300.99
model, which indicated that the adsorption of PGE onto acidwashed OPAC better obeyed the pseudo-second-order kinetic model for the entire adsorption period. Also from Table 3, k2 values decreased as the initial PGE concentrations increased from 10 to 30 mg/L, while the initial concentration exceeded 30 mg/L, k2 values increased. However, to our knowledge, the lower adsorbate amount may lead to the less competition for the active sites, and therefore presents the higher k2 value in the adsorption process.29 The reason for this adsorption behavior may be attributed to the ring-opening reaction of GEs on acidwashed OPAC. It has been reported that acidic condition can lead to the reduction of GE content during deodorization process of plant oil.5,9 Additionally, h values increased with the rising PGE concentration. The results are also obtained in several other adsorbate−adsorbent systems.42−44 Intraparticle Diffusion Model and Adsorption Mechanisms. The adsorption of PGE on acid-washed OPAC was observed to be rapid at the beginning stage and then become slow until finally attaining constant with increasing contact time (Figure 2). Therefore, on the basis of the previous report,45 the GE adsorption by acid-washed OPAC was speculated to cover three steps: (1) migration of GEs from the bulk solution to the surface of OPAC (film diffusion); (2) GE diffusion into the interior part of acid-washed OPAC particles (intraparticle diffusion); (3) GE adsorption onto the interior sites of acidwashed OPAC pores (adsorption). The overall rate of GE adsorption will be governed by the slowest step, that is, ratedetermining step. To our knowledge, the general adsorption process is performed by either the liquid mass transfer (external diffusion) or the intraparticle-diffusion mass transfer or both, and the third step is very fast. Therefore, the rate-determining step may be distributed between film diffusion and intraparticle diffusion. Although the present adsorption process was found to be well described by the pseudo-second-order kinetic model, it seemed not to provide enough useful information about the adsorption mechanism. Therefore, to further identify the mechanisms involved in GE adsorption on acid-washed OPAC in the present study, the adsorption data were modeled by the intraparticle diffusion kinetic equation, which was suggested by Weber and Morris.46 The equation is shown as follows:
(17)
where qe and qt (mg/g) are the mass amounts of PGE adsorbed on per gram of acid-washed OPAC at equilibrium and adsorption time t (min), respectively, and k1 (min−1) is the first-order adsorption rate constant. The linear plots of ln(qe − qt) against t for different concentrations of PGE adsorption onto acid-washed OPAC at 30 °C are presented in Figure S3a. k1 and qe were calculated from the slope and intercept of fitting plots, as well as their correlation coefficients (R2), listed in Table 3. The R2 values was found to be in the range of 0.919− 0.991. If the predicted qe value is not closed to the experimental qe value, the adsorption process is impossibly the first-order reaction even though the fitting plots have high R2 values.40 From Table 3, the great differences of qe values between predicated and experimental data for varied initial PGE concentrations were observed, suggesting that this adsorption process seemed to be poorly fitted by pseudo-first-order kinetic model. Pseudo-Second-Order Kinetic Model. Then the adsorption data were also modeled by Ho’s pseudo-second-order kinetic model,41 which is commonly expressed as follows: dqt dt
= k 2(qe − qt )2
(18)
After being integrated by using the boundary conditions depicted above, eq 18 becomes the linear form as follows: t 1 1 = + t qt qe k 2qe 2
(19)
where k2 (g/(mg min)) is the second-order adsorption rate constant. The qe and k2 values were calculated from the slope and intercept of linear plot of t/qt against t (Figure S3b). The k2 value was used to compute the initial adsorption rate h (mg/(g min)), which is expressed as follows: h = k 2qe 2
(20)
qe, k2, and h values under varied initial PGE concentrations for second-order kinetic model are listed in Table 3. Compared with the pseudo-first-order kinetic model, the predicted qe values were relatively closer to the corresponding experimental data for all initial PGE concentration. Moreover, the R2 values (>0.99) of the linear plots at all initial PGE concentration were found to be much higher than that of pseudo-first-order kinetic
qt = k pt 1/2 + C 9759
(21) DOI: 10.1021/acs.jafc.7b03121 J. Agric. Food Chem. 2017, 65, 9753−9762
Article
Journal of Agricultural and Food Chemistry where C (mg/g) is the intercept and kp (mg/(g min1/2)) is the intraparticle diffusion rate constant. The linear plots of qt against t1/2 for different initial PGE concentrations are presented in Figure S3c. C and kp values were the intercept and slope of those straight lines, respectively, and were listed in Table 3. kp values of PGE adsorption increased from 2.84 to 13.34 mg/(g min1/2) as the initial concentration rose from 10 to 100 mg/L. This may be because the higher initial adsorbent concentration leads to the greater driving force, which increases the diffusion rate of adsorbent in pore.15 Additionally, C value represents the boundary layer effect. The smaller C value, the less contribution of surface adsorption to the entire adsorption rate during PGE adsorption process onto OPAC. C = 0, that is, the fitting straight line passes the origin, indicates that the intraparticle diffusion is the sole rate-determining step.47 Conversely, the straight lines at all tested PGE concentrations did not pass the origin, which may be attributed to the difference of mass transfer rates between the initial and final period of adsorption. Therefore, this also explains the low R2 values (0.705−0.923) of the straight lines as plotted in Figure S3c. Thus, these results provided evidence that film diffusion and intraparticle diffusion were simultaneously performed in the present adsorption process. The adsorption of PGE onto acid-washed OPAC may be controlled by film diffusion at the early period, and then as the PGE molecules are adsorbed onto acid-washed OPAC, the process is controlled by intraparticle diffusion. Similar results have also been reported in the adsorption of anionic dye Congo red by acid-treated pine cone powder.29 Desorption Studies. In the present study, the desorption test was also carried out to further elucidate the mechanism of GE removal by acid-washed OPAC and recovery of adsorbent in batch adsorption system. Acid-washed OPAC adsorbing PGE was separated from hexadecane solution by centrifugation. The separated OPAC was agitated with 5 mL of n-hexane for 10−30 min and then removed by centrifugation. The desorbed PGE was analyzed. The desorption equilibrium was acquired at over 25 min for which the desorption data were not presented here. Furthermore, apart from PGE, monopalmitin, glycerin, and free fatty acid were also found in the desorption solution for which data were not presented here, suggesting that when PGE was loaded on the acid-washed OPAC, the ring-opened and hydrolysis reaction occurred and thus may be helpful to the removal of PGE. As depicted in Figure 1, the removal rate of GEs in palm oil by acid-washed OPAC was significantly higher than that by OPAC (P < 0.05). In fact, the observation that acid can promote the degradation and removal of GEs has been reported previous.9 Therefore, it may be found that the removal of GEs from palm oil is attributed to not only the adsorption of GEs on acid-washed OPAC, but also the degradation of GEs adsorbed at activated sites with acidic character (Table S1). Additionally, to fully evaluate the availability of acid-washed OPAC for the elimination of GEs in refined palm oil, the effects of GE adsorption by acid-washed OPAC on the quality of palm oil was also investigated in the present study, and the results are depicted in Table S2. It was found that when GE contents decreased from 3.77 mg/kg in untreated palm oil to 0.17 mg/ kg in adsorbed palm oil and the removal rate was up to 95.49% (Figure 1), the quality of palm oil, consisting of oxidation stability, gardner, and acid value, varied indistinctively (P > 0.05) between before and after GE adsorption. Thus, the adsorption of GEs onto acid-washed OPAC is a high-efficiency
method for GE removal in palm oil with no compromise in oil quality. In summary, the present study shows that acid-washed OPAC can be used as an adsorbent for the removal of GEs from palm oil. The maximum GE removal rate is up to >95%, which is significantly higher than other adsorbents. In batch adsorption system, the adsorption of PGE on acid-washed OPAC from hexadecane solution strongly depends on initial PGE concentration, temperature, and acid-washed OPAC dose. It was found that the amount of PGE adsorbed on OPAC increased with increasing initial PGE concentration and acidwashed OPAC dose but decreased with increase in temperature, suggesting that the adsorption between PGE and acidwashed OPAC was an exothermic process. The adsorption process was rapid and equilibrium was attained within 20−60 min for various initial PGE concentration (10−100 mg/L). The thermodynamics, equilibrium, and kinetics studies were conducted for the adsorption of PGE from hexadecane solution onto acid-washed OPAC. On the basis of the thermodynamic parameters such as ΔH, ΔS, and ΔG, calculated in this study, the adsorption process was found to be physical and exothermic in nature. The equilibrium data were analyzed using Langmuir, Freundlich, and D−R adsorption isotherm and were best described by the Langmuir isotherm model. The maximum adsorption capacity of PGE from Langmuir model was found to be 36.23 mg/g. The kinetic of PGE adsorption onto acidwashed OPAC followed well the pseudo-second-order model for various initial PGE concentrations. Moreover, film diffusion and intraparticle diffusion was simultaneously performed in the present adsorption process, which has been confirmed by intraparticle diffusion model. The desorption test indicated the removal of GEs from palm oil was attributed to not only the adsorption of GEs on acid-washed OPAC, but also the degradation of GEs adsorbed at activated sites with acidic character. No difference between before and after adsorption in oil quality was observed. Therefore, it can be concluded that acid-washed OPAC is an efficient and promising adsorbent for the removal of GEs from palm oil.
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jafc.7b03121. Physicochemical properties of OPW and acid-washed OPAC; effects of GEs adsorption by acid-washed OPAC on the quality of palm oil; thermodynamic study; adsorption isotherms; kinetic models of PGE onto acid-washed OPAC (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Phone: 8620-8711-4262. Fax: 8620-8711-3875. ORCID
Weiwei Cheng: 0000-0002-4907-7872 Guoqin Liu: 0000-0001-9513-1115 Funding
The work was financed by the National Key Research and Development Program of China (Project No. 2016YFD0400401−5), the National Natural Science Fund of China (Project Nos. 31471677, 31771895, 31271885, and 31401603), the National Hi-tech Research and Development 9760
DOI: 10.1021/acs.jafc.7b03121 J. Agric. Food Chem. 2017, 65, 9753−9762
Article
Journal of Agricultural and Food Chemistry
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Project of China (Project No. 2013AA102103), and the Public Welfare (Agriculture) Research Project (Project No. 201303072), the Fundamental Research Funds for the Central Universities of China, and the Open Project Foundation of Guangdong Province Key Laboratory for Green Processing of Natural Products and Product Safety (No. 201616). Notes
The authors declare no competing financial interest.
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ABBREVIATIONS USED GEs, glycidyl esters; 3-MPCD, 3-monochloropropane-1,2-diol; OPAC, oil palm wood-based activated carbon; OPW, oil palm wood; PGE, glycidyl palmitate; ΔG, Gibb’s free energy change; ΔH, enthalpy change; ΔS, entropy change; PBA, phenylboronic acid; MTBE, methyl tertiary butyl ether; MAGs, monoacylglycerols; AOCS, American Oil Chemists’ Society
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DOI: 10.1021/acs.jafc.7b03121 J. Agric. Food Chem. 2017, 65, 9753−9762
Article
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DOI: 10.1021/acs.jafc.7b03121 J. Agric. Food Chem. 2017, 65, 9753−9762
