Adsorptive Removal of Saturated and Unsaturated Fatty Acids Using

Article pubs.acs.org/IECR

Adsorptive Removal of Saturated and Unsaturated Fatty Acids Using Ion-Exchange Resins Ganesh L. Maddikeri,† Aniruddha B. Pandit,† and Parag R. Gogate*,† †

Chemical Engineering Department, Institute of Chemical Technology, Matunga, Mumbai 40019, India ABSTRACT: One of the pretreatment approaches for decreasing the initial acid content of waste vegetable oil or nonedible oils with high initial free fatty acid content, with an objective of obtaining a suitable starting raw material for the production of biodiesel, is the adsorption of the free acids using ion-exchange resins. The present work deals with investigation of adsorption characteristics of saturated (stearic) and unsaturated (oleic) fatty acids on different ion exchange resins (polymeric strong (Indion 810) and weak (Indion 850 and Indion 860) anion exchange resins). The ion exchange resins contain tertiary or quaternary amino functional groups on the styrene-divinyl benzene copolymer matrix which can facilitate the adsorption of acids. Kinetic adsorption studies have been carried out initially to determine the contact time required to reach the adsorption equilibrium between fatty acid adsorbed on the resin and remaining fatty acid present in the oil. Equilibrium adsorption studies have been carried out at different temperatures viz. 293, 303, and 313 K. It has been observed that adsorption of saturated and unsaturated fatty acids increased with its concentration in the liquid at constant temperature and decreased with an increase in the temperature at constant concentration. Also, the adsorption equilibrium data have been found to be well described by the Freundlich type isotherm. The negative values of ΔG and ΔH showed that the adsorption of both saturated and unsaturated acids on the ion exchange resins was spontaneous and also exothermic. pretreatment of waste vegetable oil having high FFA content.11 Thus it is imperative to develop alternative processing methods to remove FFA that are less energy-intensive and consume less solvents, acids, and bases.12 One of the processing methods for reducing the acid content is based on the use of different types of adsorbents. Adsorption processes for the separation of FFA from waste vegetable oil have gained a wider attention as an efficient and low energy option. There are a few studies investigating the adsorption of fatty acids on adsorbents such as different types of clay. Bayrak13 investigated the adsorption of saturated fatty acids such as capric acid, lauric acid, myristic acid, palmitic acid, and stearic acid on montmorillonite clay. It has been reported that adsorption of fatty acids on montmorillonite adsorbent follows the Langmuir isotherm, and the obtained parameters indicate that the adsorption process is dominated by physical forces. Freitas et al.14 investigated the modification of the surface of the montomorillonite by treating it with hexadecyltrimethylammonium bromide and its subsequent application for the adsorption of organic acids. It has been reported that the modified montomorillonite has higher adsorption capacity as compared to the unmodified montomorillonite due to the presence of organic cation among the layers in the modified form. Similarly, sepiolite, celtek, kaolinite, cristoblite, and bentonite clay have also been used for the adsorption of saturated and unsaturated fatty acids.15−19 Some recent studies have also reported the use of rice husk, which is a waste product of the rice mill containing silica, for the adsorption of oleic and palmitic acid.20−22

1. INTRODUCTION Waste vegetable oil is relatively abundantly available at lower costs as compared to the fresh vegetable oils and hence can be effectively used as a raw material for the biodiesel production. Waste vegetable oil contains triglycerides and low amounts of diglycerides, monoglycerides, free fatty acids (FFA), and additional trace components, which might require a purification step before its processing to biodiesel. FFA content in the waste vegetable oil results from the breakage of the triglyceride ester bond.1 The FFA content is composed of either unsaturated or saturated fatty acids depending on its origin. FFA in waste vegetable oil usually reacts with the alkaline catalyst (kOH or NaOH) in the transesterification stage resulting in the formation of soap. The soap formation in the reaction prevents the effective glycerol separation, which drastically reduces the ester yield.2 Also, the formation of soap can create serious problems in the reactor operation. To increase the yield of biodiesel and also possibly reduce the processing time, it is essential to use pretreated waste vegetable oil (devoid of FFA) for transestrification. In the pretreatment stage, the FFA content is brought to less than 1%. The overall pretreatment of waste vegetable oil involves a series of steps in order to remove the undesirable contaminants; the most important step being the removal of FFA. Pretreatment can be carried out using either a physical or chemical methods. Physical methods involve distillation3 and membrane separation,4 and chemical methods involve esterification with strong acids,5,6 supercritical extraction,7 enzymatic treatment,8 crystallization,9 and saponification.10 The physical treatment is energy intensive, and the chemical treatment generates high amounts of soap with trapped triglyceride in the solidified soap matrix resulting in a material loss. The traditional refining methods are not appropriate for the © 2012 American Chemical Society

Received: Revised: Accepted: Published: 6869

January 9, 2012 April 3, 2012 April 27, 2012 April 27, 2012 dx.doi.org/10.1021/ie3000562 | Ind. Eng. Chem. Res. 2012, 51, 6869−6876

Industrial & Engineering Chemistry Research

Article

for the design of actual ion exchange based adsorption columns. Overall, the present work deals with the development of an effective treatment scheme to reduce the free fatty acid content in the sustainable raw materials (waste vegetable/cooking oils and nonedible oils) with high initial acid content, which usually limits its processing using the transesterification step due to the associated problems of soap formation.

The present work reports the use of ion exchange resins for the adsorption of fatty acids. Ion exchange resins are based on polymeric backbone, usually polystyrene, cross-linked with divinylbenzene or polyacrylate.23 Both acidic and basic ion exchange can be used to remove FFA from waste vegetable oil. The functional group on the polymeric matrix can be a secondary or tertiary base, forming weakly basic ion exchange resins, or a quaternary ammonium group forming strongly anion exchange resin. The presence of the sulfonic acid groups results in acidic ion exchange resins. In acidic ion-exchange resin systems, FFA removal is accomplished by the esterification of FFAs at the acidic functional site in the presence of methanol or ethanol. Esterification of FFAs on acidic ion exchange resins is a pretreatment option of increasing interest in biodiesel processing.24−27 In the basic ion-exchange resin systems, the mechanism of FFA removal appears to be the adsorption on the basic site, although no step-by-step evaluation of the reaction mechanism has been presented in the literature to the best of our knowledge. Fatty acids are long-chain monocarboxylic acids that can be considered as very weak electrolytes with partially dissociated carboxylic groups in polar solutions. The adsorption of fatty acids in anionic resins involves an ion exchange equilibrium in which the hydroxyl ions present initially in the resin are partially replaced by counterions generated from the dissociated fatty acids.28 The adsorption is due to the formation of weak hydrogen bond complex between the loan pair of electron on nitrogen of the amino group present on the resin matrix and acidic hydrogen of the fatty acid (Figure 1). The interaction

2. MATERIAL AND METHODS 2.1. Materials. Saturated and unsaturated fatty acids such as stearic and oleic acid have been obtained from Godrej Ind. Ltd. (Mumbai, India) as a gift sample. Refined sunflower oil (acid value = 0.01) for the preparation of fatty acid stock as a diluent was procured from Sahakari Bhandar Limited (Mumbai, India). Neutralized ethanol (99%) was procured from Merck (India) Ltd., Mumbai, whereas potassium hydroxide and phenolphthalein indicator were procured from SD. Fine-Chem Pvt. Ltd. (Mumbai, India). The weak anion-exchange resin (Indion 850 and Indion 860) and the strong anion-exchange resin (Indion 810) were obtained from Ion Exchange Ltd. (Mumbai, India) as a gift sample. All three resins have a common styrene-divinyl benzene copolymer matrix and a different functional group. The detailed physical properties of the different ion exchange resins have been depicted in Table 1. Table 1. Physical Properties of the Resins type of resin properties

Indion 810

type polymer matrix structure functional group

standard ionic form particle size mm surface area(m2/g) cross-linking (%) total exchange capacity mol/kg dry resin moisture holding capacity density kg/m3

Figure 1. Schematic representation of the interaction between acid and weak base resin.

which is expected between the loan pair of electrons on nitrogen of secondary and tertiary amino groups and acidic hydrogen can be treated as a Lewis acid−base interaction.29,30 Adsorption of fatty acids, either saturated or unsaturated, is generally carried out using a solvent such as isooctane,19 benzene,13 or ethanol11 as a diluent for the preparation of fatty acid stock solution. In the present work, refined sunflower oil having an acid value less than 0.2 has been used as a diluent instead of the commonly used organic solvents. The purpose of using sunflower oil as a diluent is to simulate a system which is similar or identical to the actual system to be encountered in an industrial practice in terms of its physicochemical properties as compared to the use of synthetic diluents. The objective of the present study is to investigate the adsorption behavior of saturated (stearic acid) and unsaturated (oleic acid) fatty acids on different polymeric anion exchange resin as a function of temperature and initial concentration of the adsorbent, through adsorption isotherms and determine their thermodynamic parameters such as enthalpy variation (ΔH), entropy (ΔS), and Gibbs free energy variation (ΔG). Once the effectiveness of the ion exchange resins is established, the results can be very well exploited in the actual commercial practice, and the design information generated in the present work would be very useful

Indion 860

Indion 850

strong basic styrene divinyl benzene copolymer benzyl trimethyl amine -N+R3

weak basic styrene divinyl benzene copolymer tertiary amine -NR2

weak basic styrene divinyl benzene copolymer

chloride 0.3−1.2 27.7 7−8 2.2

free base 0.3−1.2 35 7−8 4.2

quaternary ammonium -NR2 and -N+R3 free base 0.3−1.2 35 7−8 3.0

56−63%

52−56%

47−55%

650

640

640

2.2. Experimental Procedure. 2.2.1. Pretreatment of Ion Exchange Resin (IER). A known quantity of resin has been taken in a conical flask, and methanol (about 2−3 times the volume of resin) is added in the flask. The conical flask containing the resin and methanol is then kept in an orbital shaker at 295 K and shaken at 180 rpm for five hours. The treated resin is separated from methanol by using simple filtration and subsequently dried by keeping the resin in oven at 313 K for 12 h. Pretreatment is carried out to remove free charge on the resin and also activate the resin for adsorption study. The structure and properties of the resin are not affected by this pretreatment, as the conditions are maintained below the upper physical limits of the resins. 2.2.2. Sample Preparation for Adsorption Study. A known amount of fatty acid such as stearic and oleic acid was dissolved in refined sunflower oil to give fatty acid concentration of 66 mg/mL (6% by weight as the stock solution). This stock solution was used for all the experimental runs in the 6870

dx.doi.org/10.1021/ie3000562 | Ind. Eng. Chem. Res. 2012, 51, 6869−6876


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q = kFCen

adsorption study. Phenolphthalein indicator (2% by weight) was prepared by dissolving 2 g of phenolphthalein in 100 mL of neutralized ethanol and used for the analysis. An alkali solution was prepared by dissolving a known quantity of potassium hydroxide in ethanol. The solution was standardized with known normality of potassium hydrogen phthalate with phenolphthalein as an indicator. The solution was subsequently diluted using distilled water to obtain a normality of the standard solution as 0.003 N, which was done to achieve a desirable range of the titration readings with an objective of minimizing the experimental errors in deciding the end point. 2.2.3. Study of Adsorption Kinetics. Studies of adsorption kinetics were carried out to determine the time required to reach equilibrium between the amount of fatty acid adsorbed on IER and the amount of fatty acid remaining in the fatty acid stock solution. In adsorption kinetic studies, a known quantity of IER (Indion 860) was contacted with a known amount of fatty acid stock solution at 303 K, and sampling was carried out at definite time intervals to find the amount of fatty acid adsorbed per unit amount of IER using titration of the stock solution and subsequent material balance. After a certain time it has been observed that no further adsorption takes place as indicated by a constant titration reading, and this condition can be defined as the equilibrium adsorption. The time required for achieving this saturation was quantified and used for all the remaining investigations related to the batch adsorption studies. 2.2.4. Batch Adsorption Study. In batch adsorption investigations, pretreated IER over different loadings as 0.15, 0.30, 0.45, 0.60, 0.75, 0.90, 1.05, and 1.20 g of Indion 860 were transferred to a 250 mL conical flask provided with a screw cap. A 20 mL fatty acid stock of stearic acid and oleic acid was introduced into each conical flask using a pipet. The flasks were kept in an orbital shaker with a speed of 180 rpm and temperature of 303 K for the desired time required to reach the equilibrium conditions. The speed was selected in such a way that no mass transfer effects prevailed in the system, as any mixing effects are likely to interfere with the adsorption capacity measurements. Subsequently, the flasks were left undisturbed for 30 min until all IER settled to the bottom. A clear solution (5 mL) from each flask was taken for the analysis. From the equilibrium data analysis, comparison of the different ion exchange resins was performed to obtain the best resin giving maximum adsorption capacity. A similar adsorption procedure was then repeated with the desired resin and known loading to investigate the effect of temperature (from 293 to 313 k) on the adsorption characteristics. 2.2.5. Analysis. Analysis of the progress of adsorption has been carried out using titration. For the analysis, 5 mL of sample is transferred into a conical flask containing 20 mL of neutralized ethanol. The mixture was titrated with 0.003 N kOH using phenolphthalein as an indicator to determine the unadsorbed acid in the solution. The observed end point is colorless to pink. All the experiments were repeated at least two times to check the reproducibility of the obtained results. It has been observed that the experimental errors were within ±2%. 2.2.6. Adsorption Isotherms. Adsorption isotherm describes the relationship between the amount of a fatty acid adsorbed on the resins and the amount of fatty acid remaining in the stock solution at equilibrium. Fitting of a particular type of adsorption isotherm can be used to evaluate the mechanism of adsorption. In the present system, the Freundlich type adsorption isotherm has been employed to fit the obtained data. The Freundlich isotherm can be described as

(1)

The Freundlich maximum adsorption capacity can be calculated as per the procedure described by Halsey31 q = kFC0n

(2)

where q is the surface concentration of fatty acid (mg/g of the IER), Ce is the concentration of fatty acid in equilibrium with suspension expressed as mg/g of fatty acid stock solution, C0 is the initial concentration of fatty acid. kF (mg1‑n gn‑1) represents the adsorption capacity when the adsorbate equilibrium concentration equals 1, and finally n indicates the degree of adsorption. kF and n are the constants of the isotherm and can be determined from the intercept and slope of the straight line fitting of the data points using the least-squares fitting method. 2.2.7. Thermodynamic Parameters. Thermodynamic parameters (ΔG0ads, ΔS0ads, and ΔH0ads) have also been calculated from the variation in the thermodynamic equilibrium constant, K0 (or the thermodynamic distribution coefficient) with the change in temperature. K0 for the adsorption process can be defined as the ratio of the activity (concentration) of adsorbed fatty acid molecule on the resin (as) to the activity (concentration) of the fatty acid molecule which remains in a fatty acid stock solution at equilibrium (ae). Mathematically the equation can be written as

K0 =

as ν q = s ae νe Ce

(3)

where q is the surface concentration of fatty acid in mg/g of IER and Ce is concentration of fatty acid in mg/g of fatty acid stock solution at equilibrium, as is the activity coefficient of the adsorbed solute, and ae is the activity coefficient of the solute in the equilibrium solution. As the concentration of the solute (FFA) in stock solution approaches zero, the activity coefficient approaches unity. Equation 3 may then be rewritten as lim

CS → 0

q a = s = K0 Ce ae

(4)

The value of K0 is determined by plotting ln (q/Ce) versus Ce and extrapolating the data to set Ce equal to zero. A straight line is fitted to the points, and its intersection with the vertical axis gives the value of K0 as also confirmed from the earlier studies of Khan and Singh.32 The free energy of adsorption (ΔG0ads) was calculated from the K0 values using the following equation 0 ΔGads = −RT ln K 0

(5)

where R is the universal gas constant in kJ/kmol K, and T is the temperature in K. The values of the thermodynamic parameters, enthalpy variation (ΔH0ads) and entropy variation (ΔS0ads), were calculated from the curve relating the distribution coefficients (K0) as a function of the temperature using the equation ln K 0 =

0 0 ΔSads ΔHads − R RT

(6)

The ΔH0ads was obtained from the slope of the straight line, and ΔS0ads was determined32 from the intercept of the graph of ln (K0) vs 1/T. 6871



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3. RESULTS AND DISCUSSION 3.1. Adsorption Kinetics. The adsorption of oleic and stearic acid on the Indion 860 has been studied as a function of the contact time with an objective of determining the required time to reach the equilibrium conditions (Figure 2) and also to

where qe (mg/g) is the amount of fatty acid adsorbed at equilibrium, qt (mg/g) is the amount of fatty acid adsorbed at any time t, while kd (min−1) is the pseudo-first-order rate constant. Kinetic parameters of the model can be calculated from the slope and intercept of linear plots of log (qe − qt) vs t, which has been shown in Figure 3. From the figure, it can be seen that

Figure 2. Kinetic adsorption for determination of residence time stearic and oleic acid with Indion 860 at 303 K (filled symbols: stearic acid; hollow symbols: oleic acid; pseudo-first-order:  for stearic acid, ...... for oleic acid).

Figure 3. The linear plot of pseudo-first-order kinetic model for stearic and oleic acid with Indion 860 at 303 K (filled symbols: stearic acid; hollow symbols: oleic acid;  for stearic acid, ...... for oleic acid).

estimate the rate of adsorption. It can be seen from the figure that the amount of fatty acid adsorbed on the resin increases with an increase in the time initially and reaches a constant value indicating the attainment of equilibrium at which no further adsorption of fatty acid is possible on the resin. In other words, the concentration of fatty acid present in the solution decreases with respect to time due to an increased surface concentration of the adsorbed acid and its coverage on the IER. The adsorption takes place due to the interaction between the loan pair of electron on the nitrogen group of the resin and the hydrogen atom of the acid. Initially, enhanced quantum of loan pair electrons of the nitrogen group of resin is present for possible interaction with the hydrogen of the acid group. After 300 min, it appears that all the available loan pair electrons have interacted with hydrogen of the acid group, and hence no further adsorption takes place after this time. This is the stage of equilibrium between the fatty acid adsorbed per gram of resin and the amount of fatty acid remaining in the stock solution. The equilibrium condition is where maximum fatty acid adsorption occurs per gram of resin at a given temperature. Thus, it can be established that the time necessary to reach equilibrium was about 300 min based on the operating condition used in this work. A similar study has been carried out by Jamal et al.33 for the adsorption of oleic acid on a mixed bed of resin, and it has been reported that 240 min is required to reach the equilibrium. Based on the results of the kinetic studies, it can be finalized that 300 min is the contact time adequate for reaching the equilibrium and hence was used for all the remaining adsorption studies. The adsorption kinetics study is very important in determining the mechanism of the adsorption process. In the present work, a pseudo-first-order equation was used to describe the adsorption kinetics. The pseudo-first-order equation can be written as follows log(qe − qt ) = log qe −

kdt 2.303

the pseudo-first-order rate equation provides a good fit to the experimental kinetic data with coefficient of determination (R2) as 0.95 and 0.99 for stearic and oleic acid respectively. Calculated and experimental values of q and kd have been shown in Table 2. It can be seen that q value based on the experimental results agrees well with the calculated ones. Table 2. Kinetic Model Parameters for the Exchange of Oleic and Stearic Acid on Indion 860 for Pseudo-First-Order Mechanism compound

qe (exp) (mg/g)

qe (pre) (mg/g)

kd (min‑1)

R2

stearic acid oleic acid

393 282

391 304

0.0131 0.0103

0.95 0.99

3.2. Equilibrium Adsorption and Adsorption Isotherm. Equilibrium adsorption studies of stearic and oleic acid on different IER (such as Indion 810, Indion 850, and Indion 860) have been carried out at different operating temperatures (293, 303, and 313 K). The obtained data have been depicted in Figures 4 and 5. It has been observed that the amount of fatty acid adsorbed on different IER increases with an increase in the initial concentration of fatty acid. This is because as the concentration of fatty acid increases, a greater number of hydrogens of the fatty acid molecule are available per gram of the fatty acid to interact with the loan pair of electrons on the nitrogen group per gram of resin. Freitas et al.14 have studied the adsorption of organic acids from aqueous solutions by unmodified and modified montmorillonite clays. It has been reported that the adsorption capacity of the adsorbents increased with an increase in the initial concentration of the adsorbate due to the enhancement in the driving force, which allows the movement of organic acids molecules from the bulk solution to the adsorbent surface, thus overcoming the mass transfer resistance between phases and increasing the overall flux.

(7) 6872



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concentrations which is higher than expected from the monolayer coverage confirms the concurrency of the Freundlich model. The Freundlich isotherm constant and adsorption capacity have been calculated from eqs 1 and 2 as described earlier, and the obtained results have been depicted in Table 3. The lower values of the Freundlich constant, n (less than 1), indicate favorable interaction of the acid with the sorbent. Table 3. Adsorption Constant for Saturated and Unsaturated Fatty Acid Adsorbed on Different Ion Exchange Resin at Different Temperatures adsorbate oleic acid

Figure 4. Adsorption of stearic and oleic acid on different anion exchange resin at temperature of 303 K (hollow symbols: oleic acid; filled symbol: stearic acid; ⧫ or ◊: Indion 860; ▲ or △: Indion 850; ● or ○: Indion 810; Freundlich isotherm:  for stearic acid, ...... for oleic acid).

stearic acid

temp (K) 293 303

313 293 303

313

adsorbent

KF

n

qm (g/g) × 103

R2

860 810 850 860 860 860 810 850 860 860

142 2 52 94 44 182 7 68 130 64

0.63 2.44 0.98 0.78 0.98 0.54 1.84 0.95 0.64 0.91

491 251 357 441 305 532 254 441 456 384

0.99 0.96 0.93 0.97 0.93 0.98 0.97 1.00 0.96 0.99

3.3. Comparison of Adsorption Using Different IER. Adsorption of stearic and oleic acid on different IER (such as Indion 810, Indion 850, and Indion 860) has been investigated at 303 K. All three resins have a common styrene-divinyl benzene copolymer matrix and a different functional group. The fatty acid uptake by polymer resins is dependent on the specific interaction of the acid with the basic functional group in the resins, and the contribution of the styrene-divinylbenzene polymer matrix is expected to be negligible. The adsorption of the acid is expected due to the hydrogenbonding between the nitrogen of the amino group of the resins and the hydrogen from the hydroxyl group present in the fatty acid. The variation in the extent of adsorption with the type of resin for both stearic and oleic acid have been shown in Figure 4, and the reported constants for Freundlich isotherm fitting have been mentioned in Table 3. It can be seen from the reported data that Indion 860 has more adsorption capacity as compared to Indion 810 and 850. Indion 850 and Indion 860 resins have tert-amino functional groups, while Indion 810 has a quaternary ammonium functional group. The observed trends can be attributed to the fact that the controlling mechanism for adsorption is based on the Lewis acid base interaction between the lone pair of electrons on nitrogen of the amine group of IER and hydrogen of the fatty acid group (Figure 1). The interaction can be quantified in terms of the Lewis acidity or basicity (i.e., electron donor or electron acceptor mechanism). Indion 860 has more electron acceptor groups as compared to Indion 810 and Indion 850, because the styrene-divinyl benzene copolymer matrix of Indion 860 contains a tertiary amine group whereas Indion 810 contains a quaternary amine group and Indion 850 contains a mixture of quaternary and tertiary amine groups (Table 1). Also, the nitrogen of the functional tertiary amino group should be relatively easily accessible than the nitrogen of the quaternary group in a strongly basic resin. So uptake of the fatty acid is higher for the tertiary amine based basic resins as compared to the resins with

Figure 5. Adsorption of stearic and oleic acid on Indion 860 at different temperature (filled symbol: stearic acid; hollow symbols: oleic acid; ⧫ or ◊: 293 K; ▲ or △: 303 K; ● or ○: 313 K; Freundlich isotherm:  for stearic acid, ...... for oleic acid).

It has been also observed that the adsorption of stearic acid is more as compared to oleic acid on all the ion exchange resins used for adsorption study. This can be attributed to the unsaturation of oleic acid which introduces some affinity toward a triglyceride molecule so that they are not easily available for adsorption on the resin as compared to the saturated fatty acid i.e. stearic acid. Another possibility is that stearic acid has lower miscibility towards triglyceride as compared to the oleic acid due to higher melting point. Holman34 have investigated the separation of unsaturated fatty acid using Darco G 60 as an adsorbent, and it has been reported that stearic acid has a higher adsorption capacity as compared to oleic acid. The observed trend has been explained by the fact that for acids of equal carbon chain length, increasing the number of double bonds decreases its adsorption on IER. The adsorption process was evaluated quantitatively with the help of adsorption isotherms. It has been established that the Freudlich isotherm shows good fitting of the experimental data with a correlation coefficient (R2) in the range of 0.93−0.99 at different examined temperatures. The Freundlich model predicts that an adsorbent can have almost infinite capacity. In the present study as well, the obtained capacity at higher 6873



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Demirbas et al.19 studied adsorption of stearic acid on bentonite at 298, 308, and 318 K. Based on the determination of the thermodynamic parameters such as Gibb’s free energy, it has been reported that the sorption capacity of bentonite decreases with an increase in the temperature for the adsorption of stearic acid. 3.5. Thermodynamic Parameters. The thermodynamic distribution coefficient (K0) as a function of temperature for the adsorption process of stearic and oleic acid on Indion 860 has been shown in Figures 6 and 7 respectively. It was observed

quaternary groups because the latter should interact mainly with the ionized oleate or stereate ions through electrostatic forces but should show relatively poor attractive interaction with the molecular form of the acid. It may, however, induce a shift in the ionization of the acid locally. Further Indion 850 has higher adsorption capacity as compared to Indion 810. Indion 850 contains tertiary as well as quaternary ammonium groups, whereas Indion 810 contains benzyl trimethyl amine and hence the higher adsorption capacity for Indion 850 can be explained. Anasthas and Gaikar30 investigated the adsorption of acetic acid on IER such as Indion 810, Indion 850, and Pulsion A-8X MP, and it has been reported that Indion 850 has more adsorption capacity as compared to Indion 810. The authors also reported that the maximum uptake capacity for Indion 850 and Indion 810 is 11 and 5 mol per kg of adsorbent. In our case, the maximum uptake capacity for stearic acid at 303 K on Indion 850 and Indion 810 has been observed to be 1.553 mol/kg (441 mg/g) and 0.894 mol/kg (254 mg/g) respectively. The reduced molar adsorption capacity can be attributed to the fact that the adsorbate is a fatty acid which has much higher molecular weight as compared to acetic acid resulting in less interaction with the IER. Patil and Gaikar35 carried out purification and recovery of curcuminoids from Curcuma longa extract by reactive sorption using polymeric adsorbent carrying a tertiary amine functional group. It has been reported that the maximum uptake of curcumin from methanol solutions using Indion 860 as the adsorbent is 1.4 mol/kg adsorbent, which is only marginally lower than that obtained in the present work. In the present study, it has been observed that the adsorption capacity of stearic acid on Indion 860 is 1.873 mol/kg (532 mg/g), which can be explained by the fact that curcumin has a higher molecular weight and is structurally more complex as compared to stearic acid and hence gives a lower extent of adsorption. Based on the comparison of different ion exchange resins, only Indion 860 has been considered for further study to study the effect of temperature on the adsorption of fatty acid because Indion 860 gives higher adsorption capacity as compared to Indion 850 and 810 at 303 K. 3.4. Effect of Temperature on Adsorption. The experiments were carried out at 293, 303, and 313 K to investigate the effect of temperature on the adsorption of stearic and oleic acid on Indion 860. The obtained results have been given in Figure 5, and it can be seen that the amount of oleic and stearic acid adsorbed on Indion 860 decreased with an increase in the temperature, from 293 to 313 K. It is observed from the plots that adsorption behavior of the stearic and oleic acid on Indion 860 is in good agreement with the Freundlich isotherms with the correlation coefficient (R2) in the range of 0.93−0.99 at the investigated temperatures. The Freundlich parameters obtained from the plots in Figure 5 are summarized in Table 3. It can be noted that stearic and oleic acid were significantly adsorbed on Indion 860 with a qm value of 532 and 491 mg g−1 respectively at 293 K as compared to the operating temperature of 313 K where the qm value of 384 and 305 mg g−1 are obtained for the adsorption of stearic and oleic acid respectively. The values of the adsorption constant and IER capacity (Table 3) show that increase of the temperature negatively influenced the adsorption capacity of the adsorbents due to a possible desorption of the molecules at the interface (increase in the solubility of the fatty acids in the triglyceride solution with an increase in the system temperature).

Figure 6. Plot for determination of thermodynamic distribution coefficient for stearic acid adsorption on Indion 860 (⧫: 293 K; ▲: 303 K; ●: 313 K).

Figure 7. Plot for determination of thermodynamic distribution coefficient for oleic acid adsorption on Indion 860 (⧫: 293 K; ▲: 303 K; ●: 313 K).

that the thermodynamic distribution coefficient (Table 4) is positive which indicates that the amount of fatty acid present per amount of resin is high as compared to the amount of fatty acid present per unit weight of the stock solution. Further, the thermodynamic distribution coefficient increases with an increase in the temperature as shown in Table 4 which indicates that the amount of fatty acid adsorbed per g of resin in equilibrium with solution concentration is high at low temperature as compared to higher temperature which is consistent with the earlier experiments related to the temperature effect. Similar results have been reported by Demirbas et al.19 for the adsorption of stearic acid on bentonite and Khan and Singh32 for the adsorption of carbofuran on Sn (IV) arsenosilicate. 6874



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Table 4. Thermodynamic Distribution Coefficient, Standard Gibbs Free Energy, and Enthalpy and Entropy of Stearic and Oleic Acid Adsorption from Fatty Acid Stock onto Indion 860 adsorbate oleic acid

stearic acid

temp (K)

K0

ΔG0ads (kJ/mol)

ΔH0ads (kJ/mol)

ΔS0ads (kJ/mol)

293 303 313 293 303 313

179 109 42 241 164 68

−12.6 −11.8 −9.7 −13.4 −12.8 −11

−55

−0.144

−48

−0.118

The value of ΔG0ads calculated from eq 5 for both the compounds (stearic and oleic acid) have been observed to be negative at all the investigated temperatures, indicating working feasibility of the adsorption process of stearic and oleic acid on the Indion 860. Negative value of the Gibb’s free energy for adsorption indicates that adsorption of stearic and oleic acid on Indion 860 is spontaneous.21 Gibb’s free energy lies in the range of −9 to −14 kJ/mol for both stearic and oleic acid adsorption, suggesting that the adsorption of stearic and oleic acid on Indion 860 was dominated by the physical interaction.36 It can be also seen from Table 4, that as the temperature increases, the value of Gibb’s free energy decreases. This is because the interaction between the fatty acid and resin at higher temperatures is weaker with an increased solubility of acid in the triglyceride solvent and thus the adsorption is less favorable. Bayrak13 investigated the adsorption of stearic acid on the montmorillonite and reported that the ΔG0ads value lies in the range of −6.6 to −6.9 kJ/mol. Similarly, Demirbas et al.19 investigated the adsorption of stearic acid on bentonite and reported a ΔG0ads value in the range of −21.7 to −22.8 kJ/mol. In the present case of adsorption of stearic acid on ion exchange resins, ΔG0ads value has been obtained to be in the range of −11 to −13.4 kJ/mol. The observed difference in the values can be attributed to the dependence of the adsorption process on the interactions between fatty acid and the type of adsorbent. It also depends on the diluents used to prepare the fatty acid stock solution. In the two reported illustrations,13,19 isooctane has been used as diluent, whereas in the present case, sunflower oil has been used to prepare the fatty acid stock solution. Figure 8 shows the graph of ln K0 versus 1/T, and it can be seen from the figure that a linear variation is obtained. The value of enthalpy of adsorption, ΔH0ads, and the entropy of adsorption, ΔS0ads, for fatty acid adsorption on Indion 860 have been calculated using eqs 5 and 6, and the obtained values have been reported in Table 4. A negative value of ΔH0ads indicates that the process is exothermic i.e. a decrease in temperature favors adsorption.37 The entropy variation is related to the variations in the order−disorder in a system. The entropy will be lower when the system tends to be less random and vice versa. The negative values found for ΔS0ads indicate that the fatty acids molecules are in a less random orientation in the adsorbed state as expected. Zou et al.38 investigated the adsorption of 2,4-dichlorophenol from aqueous solution on the microwave modified activated carbon, and from the thermodynamic study it has been reported that negative values of entropy are obtained for the adsorption process. It has been established that the negative

Figure 8. Plot for determination of thermodynamic parameter such as ΔS and ΔH for saturated and unsaturated fatty acid adsorption on Indion 860 (⧫: stearic acid; ●: oleic acid).

values of entropy reveal the affinity of carbon for 2,4dichlorophenol and decreased randomness at the solid-solution interface during the adsorption process.

4. CONCLUSIONS The present work has evaluated the effectiveness of different types of ion exchange resins (Indion 810, Indion 850, and Indion 860) for the removal of stearic and oleic acid from a freshly prepared fatty acid solution using sunflower oil as a diluent. It has been observed that the adsorption process is controlled by the interaction between the hydrogen of the acid group and the loan pair of electrons of the amino group of the resins. Indion 860 has been shown to give higher adsorption capacity as compared to Indion 810 and Indion 850. Kinetics and adsorption isotherm models have been used to describe the adsorption process mechanistically. The kinetic studies revealed that 5 h of contact time is required for reaching the equilibrium for the stearic and oleic acid adsorption in the system. The kinetics of the adsorption process is found to follow the pseudo-first-order kinetic model. Among the two acids investigated in the work, it has been observed that stearic acid has a higher adsorption capacity as compared to oleic acid at all the investigated temperatures (293 K, 303 K, and 313 K). Freundlich isotherm equations were also found to fit the experimental data adequately. It has also been established that the extent of adsorption decreases with an increase in temperature. The adsorption process has been found to be exothermic in nature and based on the Gibbs energy calculations (negative values) it has been also established that the adsorption is physical in nature. Overall, the utility of the ion exchange resins for the removal of free fatty acids from the solution has been clearly established, and the design related thermodynamic information has been presented.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +91-22-33612024. Fax: +91-22-3361 1020. E-mail: pr. [email protected]. Notes

The authors declare no competing financial interest. 6875



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Article

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ACKNOWLEDGMENTS One of the authors G.L.M. acknowledges the funding of UGC for a junior research fellowship. All the authors would like to thank Ion Exchange Ltd., Mumbai, India for the supply of resins and Godrej Ind. Ltd., Mumbai, India for providing fatty acid samples.

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REFERENCES

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