Equilibrium, Kinetic, and Thermodynamic Studies for Crude Structured

Article pubs.acs.org/jced

Equilibrium, Kinetic, and Thermodynamic Studies for Crude Structured-Lipid Deacidification Using Strong-Base Anion Exchange Resin Wei-Liang Wu,†,‡,∥ Quan Zhou,§ Lu Yuan,∥ Xiao-Ling Deng,† Shun-Rong Long,‡ Cui-Li Huang,‡ Hai-Ping Cui,‡ Li-Si Huo,‡ and Jian-Xian Zheng*,∥ †

Institute of Nutrition and Food Safety, Guangdong Provincial Center for Disease Control and Prevention, Guangzhou 510300, Guangdong, China ‡ National Testing Center of Food Quality Supervision (Guangdong), Guangdong Testing Institute of Product Quality Supervision, Foshan 528300, China § Guangzhou Technical Service Center for Drug and Food Inspection and Review & Licensing, Guangzhou 510030, China ∥ School of Light Industry and Food Sciences, South China University of Technology, Guangzhou 510640, China ABSTRACT: This study presents the adsorption behavior of strongbase anion exchange D202 resin for the elimination of free fatty acids (FFAs) from structure-lipid in nonpolar solvent. The kinetics and thermodynamic behaviors that govern FFAs removal by D202 resin were investigated, and appropriate equilibrium isotherm and kinetics models for the removal in batch experiments were identified. Both the Freundlich and Langmuir adsorption isotherm models were fitted to the experimental results with the best fit obtained by the latter. The adsorption capacity of D202 resin was 235.69 mg·g−1, as calculated from the nonlinear model of the Langmuir adsorption. The experimental data from the batch adsorption processes were analyzed in terms of the parameters from three adsorption kinetics models, namely, the liquid-film diffusion, the intraparticle, and the chemical-reaction models, to estimate the fit for the adsorption in these systems. The outcomes indicate that the adsorption kinetics between the free fatty acid ions and resin are controlled by liquid-film diffusion as the rate-determining step. In addition, the activation energy (Ea) and the changes in the Gibbs free energy (ΔG), enthalpy (ΔH), and entropy (ΔS) obtained from the thermodynamic studies reveal that the ion exchange adsorption of FFAs on D202 resin is a spontaneous, endothermic, and entropy-driven process. groups on the polymeric resin backbone.13 Our preliminary study found that anion exchange D202 resin has capacity to bond to FFAs by means of ionic and coordinative mutual effects.5 Mass transfer within resin matrix can be complex, as adsorption is an inherently transient process involving some short-range diffusion in both the fluid and adsorbed phases.13 The features of adsorption behavior are generally inferred in terms of both adsorption kinetics and equilibrium isotherms.14 Substantial investigations on the adsorption mechanism of various resins in aqueous systems have been published.13,15−19 However, few studies have investigated the adsorption behavior of basic resin in nonpolar solutions. Consequently, the experiments in this work are conducts to study the adsorption behavior of basic resin D202 when it was employed for the deacidification of crude SL.

1. INTRODUCTION Transesterification has been frequently employed as an effective method for the modification of oils and fats.1 However, transesterification yields structured lipids (SL) with substantial free fatty acids (FFAs) contents. Thus, both conventional and novel deacidification processes have been applied for the removal of FFAs from crude edible oils which have been experienced transesterification, such as alkali neutralization,2 physical refinement,3 molecular distillation,4 and liquid−liquid extraction with strong-base anion exchange resins.5 Among these methods, liquid−liquid extraction with anion exchange resin is a promising approach for the elimination and recovery of FFAs from crude oils because of its low cost and regeneration.6 As absorbents, ion exchange resins are highly regarded and have been widely used in purification and separation.7−9 Because of their chemical stability and adsorption ability, ion exchange resins, especially basic resins, have been used for the adsorption of FFAs from nonaqueous systems.5,10−12 In the process of deacidification, the adsorption of FFAs on ion exchange resins depends on the multiple adjacent functional © 2016 American Chemical Society

Received: January 7, 2016 Accepted: April 19, 2016 Published: May 2, 2016 1876

DOI: 10.1021/acs.jced.6b00017 J. Chem. Eng. Data 2016, 61, 1876−1885

Journal of Chemical & Engineering Data

Article

Lipozyme TL IM started when the lipase and 3 Å molecular sieves were placed. The flask was incubated at 60.0 °C for 12 h with 120 rpm stirring. The crude product was obtained by filtration through glass-sintered filters with filter papers, which separated the lipase. The crude structured lipid was saved in a freezer at low temperature (−20 °C) before removal of FFAs by the anion exchange resin. 2.4. Adsorption Study. A series of equilibrium experiments at 30 °C was carried out by placing resin in contact with crude SL/hexane solutions containing different levels of FFAs (from 4 to 120 mg·mL−1). The flasks with the mixtures were agitated at a fixed stirring speed of 200 rpm for 6 h after the addition of the resin. At the end of the adsorption period, the fatty solutions were obtained using vacuum filtration to separate the resin from the mixtures. The SLs after deacidification were obtained by rotary evaporation to remove hexane for the acid value (AV) determination, and the concentrations of FFAs before and after interaction with the resin were calculated according to the AV. The amount of adsorption at equilibrium, qe (mg·g−1), was calculated as follows:

Adsorption isotherms at different temperatures, the parameters of Freundlich and Langmuir adsorption model, and thermodynamic parameters were obtained from batch interactions between crude SL/hexane solution and the D202 resin in nonaqueous system.

2. MATERIALS AND METHODS 2.1. Materials. Crude SL samples containing different levels of FFAs were prepared as described in our previous study.4 The commercially available strong-base anion exchange resin D202 was supplied by Shanghai Resin Factory Co., Ltd. (Shanghai, China). Table 1 exhibits the characteristics of the adsorbent. Table 1. Physical and Chemical Properties of Strong-Base Anion Exchange D202 Resin properties ionic form functional group matrix pore structure bead size (min >95%) (mm) moisture content (%) total exchange capacity (Cl−) surface area (m2·g−1) stability, temperature range (°C) stability, pH range

OH− N+(CH3)2C2H4OH styrene−divinylbenzene macroporous 0.315−1.250 47−57 ≥3.50 mmol·g−1 dry resin ≥122.5 mEq·g−1 30−40 0−80 1−14

qe =

(C0 − Ce) × v m

(1)

where qe is the adsorption quantity of dry resin at equilibrium (mg·g−1); C0 and Ce are the initial and equilibrium FFAs concentrations (mg·mL−1), respectively; v is the volume of the crude SL/hexane solution (mL); and m is the mass of anion exchange resin D202 used (g). 2.5. Kinetics Study. Batch-adsorption kinetics studies with the anion exchange resin D202 were conducted using a temperature-controlled magnetic stirrer for 500 mL of crude SL/hexane solutions with four different levels of FFAs concentrations (10, 15, 20, and 40 mg·mL−1) and a constant resin addition of 100 g. The agitation speed of the solution was fixed at 200 rpm for all of the batch experiments. Then, 50 mL aliquots were sequentially taken at regular intervals (from 5 to 360 min) and separated using vacuum filtration. The SL was obtained after hexane removal, and the AV was determined by titration. 2.6. Thermodynamics Study. First, two crude SL/hexane solutions (20 mL each) with different FFAs concentrations was separately mixed with anion exchange resin D202 (4 g) in round-bottomed flasks for various durations (from 30 to 360 min). The adsorptions were conducted at a fixed temperature (from 303 to 343 K) with magnetic stirring at 200 rpm. A condenser was used when the reaction was conducted above 313 K. The resin was separated from the product mixture using vacuum filtration after the adsorption equilibrium achieved. Hexane was evaporated and recovered via rotary evaporation, and the deacidified lipid was attained for AV measurement. 2.7. Acid Value. The AVs of the SL before and after interaction with anion exchange resin D202 were examined according to the AOCS official method Cd 3d-63.21 The AV was calculated as follows:

Ethyl alcohol, n-hexane, and ether were purchased from Sigma Chemical Co. (Shanghai, China). Cocoa butter was supplied by Shangke Food Co., Ltd. (Wuxi, China) with an acid value of 3.16 mg KOH·g−1. Lipozyme TL IM was purchased from Novozymes Biotechnology Co., Ltd. (Tianjin, China), which is a sn-1, 3 specific lipase derived from Aspergillus oryzae immobilized on a silica gel. Octanoic acid and docosanoic acid were obtained from Jiulin Trading Company (Shanghai, China) and Sipo Chemical Co., Ltd. (Sichuan, China), respectively. All chemicals used were of analytical grade, unless otherwise mentioned. 2.2. Resin Pretreatment. Anion exchange resin D202 was pretreated according to the procedures reported by Wu et al.:5 The deionized water was decanted after D202 resin immersion into it overnight; after that the resin was rinsed in 4 volumes of 95 vol % aqueous ethanol overnight and flushed with deionized water triplicate times. The D202 resin was then soaked in 4 volumes of 1 mol·L−1 sodium hydroxide overnight and washed with deionized water to neutral pH. Then, the adsorbent was soaked in 4 volumes of 1 mol·L−1 hydrochloric acid overnight and then washed with deionized water to neutral pH. Finally, after resin immersion in 5 volumes of 1 mol·L−1 sodium hydroxide with magnetic stirring overnight, it was soaked and flushed with substantial deionized water until the pH was neutral, followed by drying to a constant weight at room temperature. 2.3. Preparation of Crude SL. The procedure presented by Fomuso and Akoh20 with minor revisions was employed to prepare crude SL. Octanoic acid was gradually loaded into a round-bottomed flask when cocoa butter and docosanoic acid were placed in the container and completely melted in a water bath at 80 °C. Then, the substrates were agitated by magnetic stirrer at 60 °C for 0.5 h after the flask transferred into a thermostatic water bath. The transesterification catalyzed by

acid value (AV) =

56.11CV M

(2)

where C is the molar concentration of potassium hydroxide (mol·L−1), V is the volume of potassium hydroxide expended in the titration (mL), and M is the weight of the sample (g). 2.8. Statistical Analysis. Statistical analysis was conducted using analysis of variance (ANOVA) and Tukey test for 1877



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pairwise comparisons of the averages among different deacidification processes. The data with error bars in the figures are presented as the means ± standard deviations of triplicate measurements. All statistics were performed using PASW Statistics 18.0 (IBM, Armonk, New York, U.S.A.).

Equation 3 can be rearranged to obtain its linear form log qe = log K +

1 log Ce n

(4)

where qe and Ce are as previously defined for eq 1, and K and n are the Freundlich constants. The Langmuir isotherm can be expressed as

3. RESULTS AND DISCUSSION 3.1. Adsorption Equilibrium Study. Crude SL/hexane solutions with various initial FFAs concentrations were treated

qe =

CebQ 0 bCe + 1

(5)

and can also be rearranged into one of the linear forms Ce C 1 = e + qe Q0 Q 0b

(6) −1

where Q0 is the maximum sorption quantity (mg·g ) and b is the adsorption affinity constant. To determinate the best-fitting isotherm model, linear and nonlinear regression methods were respectively performed to the Freundlich and Langmuir equations to confirm the experimental data. The results are presented in Figure 2. The obtained slopes, intercepts, and values of the parameters for the theoretical models are summarized in Table 2. The Freundlich isotherm model is commonly used to describe the absorption behavior of a resin in dilute solution, and K and n are two of the most crucial parameters. The surface area of the resin, including the internal pores, is expressed by the parameter K and reflects the adsorption capacity, whereas the parameter n, which is dependent on temperature and the adsorbent−adsorbate system, is used to describe adsorption favorability. Commonly, a value of n ranging between 1 and 10 indicates that the adsorption process is favorable, whereas an n less than 0.5 means that the adsorbent is unable to adsorb the adsorbate well.6,16,19,26,27 The linear plot of log qe versus logCe provided K and n values of 3.074 and 1.16, respectively (Figure 2a, Table 2), whereas the values were 5.120 and 1.39 when the experimental data were fitted to the Freundlich theoretical model using the nonlinear method (Figure 2b, Table 2). Both the linear and nonlinear regression analyses afforded n values that were greater than 1, which suggests that the sorption of FFAs onto the resin in this nonaqueous system is favorable.26−30 Our observed results are similar to the results of Deboni et al., who obtained n values ranging from 1.76 to 2.40 when the resin (Amberlyst A26 OH) was used to adsorb FFAs from soybean oil at different temperatures. As mentioned above, n is dependent on the adsorbents and the systems in which they are employed. In aqueous systems, n varied from 0.12 to 4.09 for adsorbents, such as Amberlite IRA-910 resin, Amberlite IRA 400, valonia tannin resin, XAD-4 resin, Diphonix resin, and Lewatit S6328A, were used in different aqueous systems.13,14,19,26−29 In this investigation, the value of n is similar to these aqueous system values, which indicates that changing the system from aqueous to nonaqueous might not impair the resin capacity. The theoretical linear Langmuir model plot of 1/qe against 1/Ce, nonlinear Langmuir model and the experimental plots are illustrated in Figure 2c and d, respectively. The constants Q0 and b were calculated as 775.19 mg·g−1 and 0.003 from the slope and intercept, respectively. These values, as obtained from the nonlinear regression method, were 235.69 mg·g−1 and 0.0123, respectively. Although our results were poorer than those obtained by Doboni et al. in their study,30 the maximum capacity of D202 resin in the nonaqueous system was larger

Figure 1. Adsorption equilibrium curve for free fatty acids adsorbed on D202 resin.

with a fixed resin dose, keeping other experimental parameters constant. The adsorption results are presented in Figure 1. The quantity of FFAs adsorbed by anion exchange resin D202 increased linearly with the initial concentration of the FFAs increasing from 4 to 50 mg·mL−1, due to the transfer of most of the FFAs dissolved in the initial solutions to the solid phase.19 However, the amount adsorbed by D202 resin reaches an equilibrium value of 223.71 mg·g−1 when the concentration of FFAs in the solution is 75 mg·mL−1, suggesting that the functional groups in the resin have been completely saturated by FFA ions. However, the FFAs elimination, in terms of the percentage of the initial concentration, decreases as the FFAs concentration in the initial solution increases (P < 0.05). The results of our study are similar to the conclusions reported by Gode and Pehlivan.22 The Freundlich and Langmuir adsorption isotherms are frequently employed to express the dynamic balance between the quantity of adsorbate adsorbed on a resin and the concentration of the adsorbate dissolved in the liquid.17,23 The parameters of these isotherms express the different characteristics of resin, such as its affinity and surface property. In our current investigation, the Freundlich and Langmuir isotherms were applied to investigate the features of the FFAs elimination process using anion exchange resin D202 in a nonaqueous system. The Freundlich isotherm describes nonideal adsorption on heterogeneous surfaces as well as multilayer sorption. In contrast, the Langmuir model considers adsorption onto the heterogeneous surfaces of polymers as monolayer sorption.24,25 The Freundlich isotherm can be expressed as qe = K (Ce)1/ n

(3) 1878



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Figure 2. Freundlich and Langmuir adsorption isotherms of free fatty acids adsorbed on D202 resin: (a) linear regression of Freundlich; (b) nonlinear regression of Freundlich; (c) linear regression of Langmuir; (d) nonlinear regression of Langmuir.

As shown in Table 2, the R2 values of the linear and nonlinear Freundlich and Langmuir theoretical isotherms were 0.9063 and 0.9283, and 09676 and 0.9902, respectively; thus, the Langmuir isotherm better describes the adsorption behavior of FFAs onto anion exchange resin D202 in crude SL/hexane solution. Therefore, the ion exchange adsorption between D202 resin and the FFAs may be considered monolayer adsorption. The capacity of the D202 resin was compared with those of other resins under different conditions (Table 3).10,12,30,31 For the adsorption of FFAs in nonaqueous systems, the D202 resin performance is in the same range as those in the literature. However, the D202 resin was slightly inferior to other resins in term of resin characteristics and the composition of the system. This may be due to the complexity of the system composition, which contained various FFAs and may have affected the adsorption capacity of the resin. 3.2. Kinetic Study. The effects of the interaction duration and initial FFAs concentration on the adsorption of FFA ions by anion exchange resin D202 are shown in Figure 3. The quantity of FFAs adsorbed increased with increasing contact time (P < 0.05), reaching equilibrium after 4 h. The time to reach equilibrium is independent of the initial FFAs amount in the nonaqueous system. However, the quantity of adsorbate adsorbed by D202 resin rises with the increasing initial FFAs concentration (P < 0.05). Figure 3 also shows that the initial rate of sorption is greater for higher initial FFAs concentrations

Table 2. Comparison of the Isotherm Parameters for Anion Exchange D202 Resin Obtained from Freundlich and Langmuir Models by Linear and Nonlinear Regression model Freundlich

regression linear

nonlinear

Langmuir

linear

nonlinear

than those of eliminations in the absorbents area, functional

parameters

values

slope intercept n K R2 n K R2 slope intercept b Q0 (mg·g−1) R2 b Q0 (mg·g−1) R2

0.8597 0.4877 1.16 3.074 0.9063 1.39 5.120 0.9283 0.4336 0.0013 0.0030 775.19 0.9676 0.0123 235.69 0.9902

the various absorbents used in metal ion aqueous systems, which may be attributed to characteristics, including their specific surface group ionization, and pore size.19,26,28,29 1879



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Table 3. Comparison of Resin Characteristics, Adsorption Capacity, Time, and Adsorption System with Different Resins functional group

concentration of active sites

capacity (mg·g−1)

time (h) 6

D202

N+(CH3)2C2H4OH

≥ 122.5 eq·g−1

235.69

Amberlyst A21 MK-450 UPW Amberlite MB-150 Amberlyst A26 OH Amberlyst A26 OH

quaternary quaternary quaternary quaternary quaternary

≥ 1.30 eq·L−1

278.00 270.00 260.00 359.84 500.00

ammonium ammonium ammonium ammonium ammonium

≥ 0.80 eq·L−1 ≥ 0.80 eq·L−1

in the crude SL, as the mass transfer of FFA ions from the nonaqueous solution to the surface of the absorbent is affected by the FFA concentration. During ion exchange, the adsorbate ions in the solution interact with the functional group of the resin via the following steps: (1) the dissolved adsorbates reach the resin surface through liquid-film diffusion; (2) the adsorbates access the internal resin network and diffuse to active exchange center via the cross-linked resin matrix; (3) the adsorbate ions exchange with the active groups of the resin; and (4) the exchanged ions (OH−) combine with the hydrogen ions released from the free fatty acids to form H2O which is held by the internal matrix of the resin.25 The three possible rate-limiting steps of the ion exchange adsorption process, namely, film diffusion, particle diffusion, and the chemical reaction, can be expressed as follows: (7)

particle diffusion: qt = kt 0.5

(8)

chemical reaction: 1 − (1 − F )1/3 = kt

(9)

reference in this paper

4 5 6

linoleic acid + soybean oil + hexane oleic acid + sunflower oil

30 31

10 12

analyze the adsorption data. The three aforementioned equations were employed to generate linear regression fittings for the experimental data graphically shown in Figure 4. Kinetics models for the different rate-determining steps were obtained to describe the ion exchange process between D202 resin and FFA ions in crude SL/hexane solution (Figure 4). The linear regression fitting results for the ion exchange kinetics models of D202 resin are tabulated in Table 4. The adsorption kinetics models for D202 resin in contact with different FFAs concentrations (10, 15, 20, and 40 mg·mL−1) controlled by liquid-film diffusion are presented in Figure 4a. The correlation coefficients were greater than 0.90 (Table 4), suggesting that ion exchange between the active groups of the resin and FFAs is primarily controlled by liquid-film diffusion. The ion exchange rate constants are approximately 0.01 h−1 for the interaction between the resin and FFAs. The slopes of the liquid-film diffusion model plots indicate that the ion exchange rate is quite low. This is attributed to the structures of most of FFAs in the crude SL: as long-chain fatty acids, they suffer greater resistance to liquid film diffusion. Higher FFAs concentrations in the nonaqueous systems correspond to higher ion exchange rate constants. The increment of the ion exchange rate constant is ascribed to an improvement in the collision probability between the FFAs and resin, arising from the increase in the FFAs concentration, resulting from the acceleration of the FFAs molecular motion.32,33 However, intraparticle diffusion affects the ion exchange between the resin and FFAs, as shown by the values of the intercepts from the film-diffusion model, which are greater than the normal values.24 The correlation coefficients for the ion-exchange kinetics models obtained from the experimental data-fitting based on the Kannan-Sundaram intraparticle diffusion model,6,13,14,19,26−29 are less than 0.90 (Table 4), indicating that intramatrix diffusion is not the primary rate-limiting procedure for the interaction between D202 resin and the FFAs, but it did affect the ion exchange process. As shown in Figure 4b, the linear graphs obtained from the experimental data fitting do not pass through the origin, which further demonstrates that the ion exchange between the functional groups on D202 resin and the FFAs in the crude SL/hexane solution is partially affected by intraparticle diffusion.34 As shown in Table 4, the linear regression models acquired from the chemical-reaction-controlled model afford correlation coefficients between 0.85 and 0.90. These values suggest that the ion exchange process is not primarily limited by the chemical reaction, which was expected because the D202 resin is a macro-porous strong-base anion exchange type. When the FFAs diffuse to the resin’s surface through the liquid film and enter into the resin matrix, chemical reactions with the functional exchange centers occur immediately. However, the FFA ions, in the process of exchanging with the active centers

Figure 3. Kinetics curves for the adsorption of free fatty acids on D202 resin: ■, fatty solution with 40 mg·mL−1 free fatty acids; ●, fatty solution with 20 mg·mL−1 free fatty acids; ▲, fatty solution with 15 mg·mL−1 free fatty acids; ▼, fatty solution with 10 mg·mL−1 free fatty acids.

film diffusion: ln(1 − F ) = −kt

adsorption system FFAs + structured cocoa butter + hexane oleic acid + enthanol + water oleic acid + soybean oil

where k is the reaction rate constant and F = qt/qe. qt is the resin quantity at reaction time t (mg·g−1), and qe is the equilibrium adsorption quantity on the resin (mg·g−1). To study the sorption mechanisms concerning on the interaction between anion exchange resin D202 and FFA ions in the nonaqueous system, including mass transfer and chemical reaction, an appropriate kinetics model is needed to 1880



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Figure 4. Ion exchange adsorption kinetics as a function of the three different rate-determining steps: (a) liquid-film-diffusion control; (b) intraparticle-diffusion control; (c) chemical-reaction control.

Table 4. Regression Results of Kinetic Models for Anion Exchange D202 Resin rate-determining step film diffusion control

particle diffusion control

chemical reaction

free fatty acid concentration (mg·mL−1)

slope

intercept

R2

10 15 20 40 10 15 20 40 10 15 20 40

0.0112 0.0150 0.0161 0.0170 1.8005 2.3948 3.1375 5.3395 0.0019 0.0021 0.0015 0.0020

0.8303 0.8433 0.8228 1.0498 21.131 36.325 39.854 128.49 0.2596 0.2844 0.2495 0.3519

0.9421 0.9763 0.9167 0.9675 0.8417 0.8759 0.8101 0.7828 0.8581 0.9022 0.8460 0.8876

of the resin, negatively affect the ion exchange rate, as the high composition of long-chain fatty acids (over 50%) in the crude SL increases the steric hindrance in the resin matrix. Generally, the ion exchange between D202 resin and FFAs in nonaqueous systems is a liquid film rate-determining process. That is, the liquid film features FFAs concentration gradients in

the ion exchange process, whereas FFAs concentration gradients are not found in the inner resin particles. The reversible process of ion exchange between resin and adsorbate have been demonstrated by the previous kinetics studies, and thus the ion exchange rate constant is the sum of the forward and reverse reaction rate constants.22,25 The total 1881



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Table 5. Rate Constants for Free Fatty Acids Removal with Anion Exchange D202 Resin Using Film-Diffusion-Control Model free fatty acids concentration (mg·mL−1)

overall rate constant, k = k1 + k2 (h−1)

forward rate constant, k1 (h−1)

backward rate constant, k2 (h−1)

10 15 20 40

0.0112 0.0150 0.0161 0.0170

0.0110 0.0146 0.0155 0.0164

0.0002 0.0004 0.0006 0.0006

Figure 6. Linear regression for the calculation of activation energies for the adsorption of free fatty acids on D202 resin: ■, fatty solution with 15 mg·mL−1 free fatty acids; ●, fatty solution with 40 mg·mL−1 free fatty acids.

is remarkably larger than the amounts returned to the crude SL/hexane solution through the reverse process. 3.3. Thermodynamics Study. The quantity of FFAs adsorbed onto D202 resin from the fatty solution was determined at different temperatures to ascertain whether the ion exchange process was temperature dependent. As presented in Figure 5, the adsorption amount of the resin has a increment with temperature for the fatty solutions containing both low and high FFAs concentrations (P < 0.05), indicating that temperature positively affects the interaction between D202 resin and the FFAs in the crude SL/hexane solution. The reaction rate constants k for the different temperatures derived from the liquid-film diffusion model in expression eq 7 were calculated to further discuss the influence of temperature on sorption process of the resin. The results shown in Table 6 indicate that a rise in temperature increases the ion exchange rate constant. Thus, increasing the temperature can accelerate ion exchange. According to the results presented in Table 6, the activation energies for adsorption by the D202 resin can be calculated using the Arrhenius equation

Figure 5. Effects of temperature on free fatty acids adsorbed on D202 resin: (a) fatty solution with 15 mg·mL−1 free fatty acids; (b) fatty solution with 40 mg·mL−1 free fatty acids.

ion exchange rate constant k was plugged into the formula deduced by Rengaraj et al.35 to calculate the rate constants of the forward (k1) and reverse reactions (k2) of the ion exchange process between D202 resin and the FFA ions in a nonaqueous system controlled by liquid-film diffusion. The results are presented in Table 5. The reaction rate constant for FFAs removal (i.e., for the forward reaction) is significantly greater than the reverse reaction rate constant, suggesting that the amount of FFAs adsorbed by D202 resin through ion exchange

k = k 0e(Ea / RT )

(10)

where R is the universal gas constant (J·mol−1·K−1); T is the absolute temperature (K); k is the apparent adsorption rate

Table 6. Reaction Rate Constants Free Fatty Acids Removal with Anion Exchange D202 Resin at Different Temperatures temperature (K) −1

free fatty acids concentration (mg·mL )

303

313

323

333

343

15 40

0.0102 0.0111

0.0111 0.0121

0.0119 0.0126

0.0120 0.0129

0.0134 0.0173

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Table 7. Regression Results of Activation Energy of Adsorption for Free Fatty Acids Removal with Anion Exchange D202 Resin at Different of Free Fatty Acids Concentrations regression results free fatty acids concentration (mg·mL−1)

R2

slope

intercept

k0 (min−1)

activation energy (kJ·mol−1)

15 40

0.9511 0.7820

−0.6482 −0.9758

−2.4400 −1.3127

0.0872 0.2691

5.389 8.113

Table 8. Thermodynamic Parameters for the Adsorption of Free Fatty Acids onto Anion Exchange D202 Resin ΔG (KJ·mol−1) −1

−1

−1

−1

free fatty acids concentration (mg·mL )

ΔH (KJ·mol )

ΔS (J·K ·mol )

303 K

313 K

323 K

333 K

343 K

15 40

12.692 59.603

72.571 218.086

−8.851 −8.757

−10.986 −7.161

−10.551 −8.211

−11.205 −12.951

−12.484 −17.241

The relationship between Kc and the thermodynamics parameters ΔS and ΔH can be represented by the van’t Hoff equation log Kc =

(12)

ΔH and ΔS were calculated from the slopes and intercepts of the linear curves obtained by plotting log Kc versus 1/T (Figure 7), and are shown in Table 8. The positive values of ΔH suggest that the adsorption of FFAs onto D202 resin is endothermic. The ion exchange reaction between the resin and the FFAs is facilitated by increasing temperature. In addition, ΔH values of 12.692 and 59.603 kJ·mol−1 demonstrate that the adsorption of FFAs onto the resin surfaces is controlled by a physical rather than a chemical mechanism.16 The positive values of ΔS indicate increasing randomness at the solid/ solution interface during the adsorption process in the nonaqueous system,37 which suggests that the ion exchange adsorption is an entropy rather than an energy-driven process.30 The results from the thermodynamics study are consistent with those reported by Jamal et al.,38 who found that the physicochemical character of the Dowex monosphere MR-450 UPW resin employed in the adsorption of oleic acid from soybean oil resulted in a spontaneous and endothermic process, with positive entropy change. Similar behavior for FFAs adsorption on LSD-263 resin was also found by Du et al.6 However, Maddiker et al. observed completely the opposite behavior on physicochemical character, which may have been related to the type of adsorbent and the corresponding interactions between the FFAs and its active sites.39

Figure 7. Linear regression for the calculation of ΔH and ΔS for the ion exchange adsorption of free fatty acids on D202 resin: ■, fatty solution with 15 mg·mL−1 free fatty acids; ●, fatty solution with 40 mg·mL−1 free fatty acids.

constant of the pseudo-first-order kinetic equation (min−1); k0 is the pre-exponential factor (frequency factor); and Ea is the apparent activation energy (kJ·mol−1). Linear relationships (R2 = 0.9511 and 0.7820, respectively) are obtained when lnk is plotted versus 1/T for the sorption of FFAs from nonaqueous solution at two concentrations (Figure 6, Table 7). The Ea values, readily acquired from the slopes of the lines, are 5.389 and 8.113 kJ·mol−1, respectively. Thermodynamic parameters such as the Gibbs free energy change (ΔG), enthalpy change (ΔH), and entropy change (ΔS) are conventionally applied to describe the thermodynamic behavior of the adsorption of adsorbates onto adsorbents.13−16,19,36 The ΔG for ion exchange may be calculated using the following expression: ΔG = −RT ln Kc

ΔS ΔH − 2.303 2.303RT

4. CONCLUSIONS Equilibrium, kinetics, and thermodynamics studies were conducted for the ion exchange adsorption of FFAs from nonaqueous systems onto D202 resin in the concentration range from 10 to 40 mg·mL−1. The experimental equilibrium data were analyzed using the Langmuir and Freundlich isotherm models. The characteristic parameters for each model and the related correlation coefficients were determined. The FFAs removal was best fitted by the Langmuir adsorption isotherm, as indicated by the higher R2 for the Langmuir regression model. The adsorption capacity of D202 resin obtained from the Langmuir regression model was 235.69 mg· g−1. The kinetics of the ion exchange between D202 resin and the FFAs was investigated using liquid-film-diffusion, intraparticle-diffusion, and chemical-reaction models. For the nonaqueous system, the liquid-film-diffusion model provided

(11)

where Kc is the adsorption equilibrium constant can be calculated using Kc = Cse/Ce, where Cse is the equilibrium concentration in the solid phase (mg·mL−1); Ce is the equilibrium concentration in the liquid phase (mg·mL−1). The values of ΔG for the ion exchange between D202 resin and the FFAs at different temperatures are given in Table 8. Each value of ΔG is less than 0 irrespective of temperature, demonstrating that the FFAs tend to be adsorbed onto the functional groups of the resin from the crude SL/hexane solution through ion exchange. This process is spontaneous.13−16 1883



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the best fit for the rate-determining step, whereas the intraparticle diffusion model provided the worst fit. However, the kinetics studies revealed that intraparticle-diffusion and chemical-reaction processes also influenced the ion exchange behavior between the FFAs and the resin. Furthermore, the forward reaction rate constant for the ion exchange adsorption of the FFAs from nonaqueous solutions onto D202 resin was much greater than the reverse reaction rate constant. Thermodynamics studies showed that the ion exchange adsorption of the FFAs on D202 resin is a spontaneous, endothermic, and entropy-driven process, and the adsorption of the FFAs onto D202 resin in nonaqueous systems may be a result of physical rather than chemical mechanisms.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] or [email protected]. Tel./Fax: +86-20-87112278. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding

We express our thanks to the Technology Project of the General Administration of Quality Supervision, Inspection and Quarantine of the People’s Republic of China (2013QK277). Notes

The authors declare no competing financial interest.

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Equilibrium, Kinetic, and Thermodynamic Studies for Crude Structured

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