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Effect of Temperature on Reactive Extraction of Gallic Acid Using Tri‑n‑butyl Phosphate, Tri‑n‑octylamine and Aliquat 336 Kalpana Rewatkar, Diwakar Z. Shende,* and Kailas L. Wasewar Department of Chemical Engineering, Visvesvaraya National Institute of Technology, Nagpur 440010, India S Supporting Information *

ABSTRACT: The present work is focused on the effect of temperature on extraction of gallic acid using Aliquat 336, tri-n-butyl phosphate and tri-n-octylamine in hexanol and octanol. Distribution coefficient, loading ratio, percent extraction, equilibrium complexation constant, water coextraction, enthalpy, and entropy change were evaluated for various acid−extractant− diluent combinations with varying temperature. The extraction of GA was found to be highly exothermic at lower molality of TBP in octanol with evaluated enthalpy change as −28 743 J/mol. The extraction of gallic acid process was favored at low temperature as the process was evaluated as an exothermic in nature. Maximum extraction efficiency of 97.3% was observed at higher molality of Aliquat 336 in hexanol at lower temperature. industries.15−17 In situ extractive separation keeps the acid content in fermentation broth at low level by continuous removal from the broth in order to suppress product inhibition. GA can be produced by fermentation at various temperatures (20 to 40 °C).10 Hence, it was considered significant to study the thermodynamic aspect of extractive recovery of GA. The thermodynamic study of reactive extraction of various acids such as succinic, lactic acid (at 0 to 75 and 10 to 40 °C),18,19 citric acid (at 25 to 60 °C),20 acrylic, and propionic and butyric acid (at 32 to 60 °C)21 have been reported. Harington et al.19 investigated that the performance of extraction of lactic acid from aqueous solution at 38 °C resulted in more extraction yield comparative to extraction at room temperature. Tamada et al.18 concluded that in the extraction of succinic acid, lactic acid decreases with increase in temperature. Keshav et al.21 observed that the partition coefficient increases with temperature, whereas dimerization coefficient decreases with increasing temperature for acrylic, propionic, and butyric acid. Water coextraction is the amount of water entered in the organic phase along with solute during reactive extraction. It is one of the important parameters, studied by the scientific community,18,22−26 as the coextracted water in the organic phase, has potential to affect the distribution of acid into organic phase by interacting with acid-extractant complex. The higher amount of coextracted water in the organic phase affects the overall process economy. The temperature affects the water coextraction in reactive extraction. Tamada27 stated that the water coextraction increases with temperature up to 40 to 50 °C and thereafter decreases in the reactive extraction of succinic acid using amine-based extractant.

1. INTRODUCTION Gallic acid (3,4,5-trihydroxy benzoic acid, GA) is a phenolic acid which is widely used in food, pharmaceutical, and cosmetic industries due to its antifungal, anticancer, and antiviral properties. It has also been used in synthesis of propyl gallate as a food preservative and of trimethoprim as an antibiotic and in treating uterine, nephritic and pulmonary hemorrhages.1,2 It is also employed as a source material for inks, paints, and color developers.3 GA is an organic acid found in a variety of foods and herbs such as blueberries, walnuts, apples, oak bark, and tea. Badhani et al.4 explored a detailed review on the applicability of GA and its various properties. Its worldwide annual demand is approximately 8000 tons. The chemical synthesis of GA is found to be difficult due to its complex structure and the stereo specificity.5 Also the direct extraction from plant secondary metabolites is difficult due to interference of the unknown compounds. The production of GA by acid hydrolysis of tannins, releases a large percentage of toxic compounds into environment that causes environmental hazards and suffers from high cost, low yield, and purity. Now a days, the biotechnology option is more economical as compared to conventional routes. Commercially, GA is produced by the fermentation of tannins using microbial enzyme tannase.1,6−9 The various methods for its recovery from fermentation broth and waste streams have been suggested viz. precipitation, solvent extraction,8,10 ultrafiltration,11,12 adsorption using resins,13 biofilteration,14 and so forth. The methods suffer from large volumes of waste, high cost, and low efficiency. The reactive extraction may potentially overcome many drawbacks to propose a promising technique for recovery of GA. It is economical, energy efficient, and a clean method which permits reuse of the extractant for separating GA from downstreams of pharma, food, and fermentation © XXXX American Chemical Society

Received: April 15, 2016 Accepted: July 25, 2016

A

DOI: 10.1021/acs.jced.6b00310 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

Article

The extractants in the range of 10−30% v/v were selected. The aqueous phase was analyzed using HPLC after settling and separation to find out aqueous phase acid content. The experiments were performed in duplicate to check the reproducibility. The equilibrium molality of the gallic acid in organic phase was determined by material balance with assumptions: (i) both phases have the same volume, (ii) neglecting the solubility of water in organic phase, and (iii) neglecting the solubility of extractants and diluents in water. 2.2. Sample Analysis. Equilibrium molality of GA in aqueous phase was determined by high-performance liquid chromatography (HPLC Agilent 1200 series, U.S.) equipped with a quaternary pump, a vacuum degasser, diode array detector, integrated chemstation software, and injection port with 20 μL sample loop. The C18 column (Zorbax Eclipse Plus; 4.6 mm ID × 250 mm, 5 μm; Agilent) was used with water: acetonitrile (90:10 v/v) as a mobile phase at a flow rate of 1 mL/min. The samples were filtered using syringe filter (0.2 μm PVDF filter whatman, USA). The GA was detected at the wavelength of 264 nm. The retention time of GA was found to be 3.9 min (see Supporting Information Figure S1). All samples were analyzed twice to report average value. The water content in the organic phase was determined by Karl Fischer titration (KF) using automatic potentiometric titrator (model AT 38 C, Spectra lab instruments Pvt. ltd.). 2.3. Uncertainty Analysis. All experimental measurements are bound to certain uncertainty due to instrumental error or radom variation. The deviation of the observation from the average value of the observation are used as experimental error. The experimental uncertainty (u) was determined by using eq 1

The complete design of the reactive extraction process requires equilibrium, kinetic, thermodynamic, and the water coextraction data. Reactive extraction with organophosphorus compounds, for example, tri-n-butyl phosphate (TBP), and amine-based extractants, for example, quaternary ammonium salt, Aliquat 336 and tertiary amine, tri-n-octylamine (TOA), are gaining much attraction for recovering acids from aqueous waste streams.15,16,28−30 The diluents are used to enhance the physical properties of the extractants like viscosity, density, surface tension etc. and to provide solvation to the acid−extractant complex formed. The polar diluents are more favorable than nonpolar low dielectric constant diluents. Hence, hexanol and octanol were used as diluents in the present work. The thermodynamics of GA extraction has not been reported in the literature. The present study focuses on the optimum extractant-diluent system for recovery of GA from fermentation broth or aqueous waste streams at various operating ranges. The effect of temperature on partition and dimerization coefficients, water coextraction, distribution coefficient and loading ratio has been studied.

2. MATERIALS AND METHOD Gallic acid (Sigma-Aldrich, Germany) was used to prepare the aqueous solution. The extractants tri-n-butyl phosphate (TBP), tri-n-octylamine (TOA), and Aliquat 336 were obtained from SD. Fine-Chem. Ltd., ACROS, and Himedia, respectively. The diluents 1-hexanol and 1-octanol were purchased from SD. FineChem. Ltd., India. The HPLC grade water, Acetonitrile were obtained from Merck, India. The details of the chemicals used are presented in Table 1. All the chemicals were used without any Table 1. Sample Description of Chemicals Used chemical name

source

gallic acid TBP TOA Aliquat 336 1-hexanol 1-octanol acetonitrile

Sigma-Aldrich SD. Fine-Chem. Ltd. ACROS Himedia SD. Fine-Chem. Ltd. SD. Fine-Chem. Ltd. Merck Specialties Pvt Ltd.

u=

initial mole final mole fraction purification fraction analysis purity method purity method 0.99 0.97 0.99 0.95 0.985 0.995 0.998

1 N−1

N

∑ (xi − x ̅ )2 i=1

(1)

where xi is the experimental observation, x̅ is sample mean of three observations, and N is the number of observations.

None None None None None None None

3. RESULT AND DISCUSSION The effect of temperature on extraction of GA by the diluents (physical extraction) and the extractant/diluent combinations (chemical extraction) was explored. 3.1. Physical Extraction. The physical extraction of GA was carried out by using the solvents 1-hexanol and 1-octanol. The extraction efficiency (η, %) was calculated using the formula as given as mGAorg η= × 100 mGA0,aq (2)

further purification. GA mother stock solution of 0.03 mol·kg−1 was prepared by dissolving appropriate amount of GA in double distilled water. Different molalities of the aqueous solution of GA were prepared by diluting the mother stock to appropriate volume. 2.1. Experimental Method. The batch extraction experiments were performed in an orbital shaking incubator (REMI instruments Ltd., India) by shaking equal volumes (20 mL) of aqueous and organic phases in a 150 mL conical flask. The molality of GA in aqueous phase was kept constant for 0.015 mol·kg−1 for all the experiments. A mixture of diluent and extractant constitutes the organic phase. The solution of aqueous and organic phase was stirred at 250 rpm for 2 h followed by 1 h settling to achieve the separation of phases. The optimum stirring time sufficient to achieve the equilibrium and the settling time for the separation of phases were decided based on preliminary experiments. The diluents (1-hexanol and 1-octanol) and the extractants (TBP, TOA, and Aliquat 336) were used to study the extraction efficiency, water coextraction, and thermodynamics of the system at a temperature range of (288 to 318) K.

where mGAorg denotes the equilibrium molality of gallic acid in organic phase (mol·kg−1), and mGA0,aq represents initial molality of gallic acid in aqueous phase (mol·kg−1). The distribution coefficient was calculated as30 mGAorg KD = mGAaq (3) where mGAaq is the equilibrium molality of GA in the aqueous phase. For a weak molality of the acid, relationship between distribution coefficient (KD), partition coefficient (P) and dimerization coefficient (D) can be represented as30 KD = P + 2DP 2mGAaq B

(4) DOI: 10.1021/acs.jced.6b00310 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

The partition coefficient (P) is a ratio of the molality of unionized solute in the organic phase and the aqueous phase at equilibrium, whereas the distribution coefficient (KD) is a ratio of molality of the solute in all forms (ionized, un-ionized, dimer) in both organic and the aqueous phase at equilibrium. The partition coefficient (P) and the distribution coefficient KD are identical for the solute which shows only one chemical form in each phase. If the solute exists in more than one chemical form (dimer, ionized, un-ionized) in either phase, then P and KD show different values. Table 2 shows the values of P and D as calculated by plotting KD Versus mGA0,aq. The partition coefficient, P increases with temperature for both the solvents 1-hexanol and 1-octanol. The dimerization coefficient, D was found to be decreased with temperature. The similar trends for KD, P and D values were observed by Keshav et al.21 for extraction of propionic acid using oleyl alcohol at the temperature range of (305 to 333) K. Figure 1

mGA0,aq, initial molality of the gallic acid in the aqueous phase (mol·kg−1); mGA,aq, equilibrium molality of the gallic acid in the aqueous phase (mol·kg−1). bStandard uncertainties u are u(mGA,aq) = 0.0001 mol·kg−1 and u(T) = 1 K.

5.5 0.5 13.9 0.36 47.8 0.24 octanol

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Figure 1. Distribution coefficient with varying initial molalities of gallic acid in hexanol at various temperature.

shows the effect of temperature on distribution coefficient for physical extraction of GA using hexanol. Temperature effect on KD using octanol is presented in Supporting Information Figure S2. The distribution coefficient increases with initial molality of the acid in aqueous phase and decreases with temperature of the system. The extraction was observed higher with 1-hexanol as compared to 1-octanol (Table 1). The gallic acid−diluent solvation is expected through hydrogen bonding (O···H) between O atom from −OH group of alcohol (diluent) and H atom from −COOH group of GA. The process of solvation is exothermic in nature. Therefore, the distribution coefficient decreases with the temperature. However, relative ability of these alcohols depends on the strength and number of hydrogen bonds with the acid molecule. The influence of the alcohols on the distribution coefficient corresponds to the solubility of alcohol in water and ability to form the hydrogen bond with acid molecule. 1-Hexanol is more soluble in water comparatively to 1-octanol. Hence, extraction of GA present in aqueous phase increases in the presence of hexanol comparatively to octanol in organic phase. 3.2. Chemical Extraction. The extraction accompanied by chemical reaction is a complex process and highly dependent on the composition of organic phase. Extractant (E) reacts with gallic acid (GA) to form acid-extractant complex (E:GA) by intermolecular hydrogen bonding or ion-pair formation and transfers to the organic phase as the complex is more soluble in organic phase. The extent of ion pair formation or the proton

a

1.5 0.79

1.5

D P

1.49

71.9 74.5 76.5 78.1 61.3 62.5 67.7 69.2

η KD

2.56 2.93 3.25 3.57 1.58 1.66 2.1 2.25 0.0017 0.00225 0.00276 0.00321 0.00228 0.00331 0.00379 0.00452

mGA,aq D

10.4 0.61

P η

73.8 75.6 78.8 81.6 61.9 64.8 67.9 72.1 2.81 3.1 3.724 4.437 1.63 1.84 2.11 2.58

KD mGA,aq

0.0015 0.0022 0.0025 0.0027 0.0022 0.0031 0.0038 0.0041

D

23.5

P

0.45

η

75.0 77.0 80.5 82.7 63.4 67.3 70.3 73.9 3.000 3.353 4.130 4.762 1.732 2.06 2.367 2.83

KD mGA,aq

0.0015 0.0020 0.0023 0.0026 0.0022 0.0029 0.0035 0.0038 45.8

D P

0.37

77.5 79.1 82.1 84.9 67.0 72.4 73.6 78.1

η KD

3.44 3.79 4.58 5.63 2.03 2.62 2.79 3.56 0.0059 0.0082 0.012 0.015 0.0059 0.0082 0.012 0.015 hexanol

mGA,aq mGA0,aq diluents

0.0013 0.0018 0.0021 0.0022 0.0019 0.0024 0.0031 0.0032

T = 318 K T = 308 K T = 298 K T = 288 K

Table 2. Distribution Coefficient, KD, Extraction Efficiency, η, Partition Coefficient, P, and Dimmerization Coefficient, D, Values for the Physical Extraction of Gallic Acid in Hexanol and Octanol at Various Temperatures, T, and Pressure = 0.0994 MPa with Uncertainty of 0.0001 MPaa,b


C



D

6.0 8.3 3.1 4.2 7.9 8.9 ALQ

TOA

a Ext, extractants; ALQ, Aliquat 336; ms, molality of the extractants in the organic phase (mol·kg−1); KE, equilibrium complexation constant (kg mol−1). bStandard uncertainties u are u(mGA,aq) = 0.0001 mol·kg−1, u(T) = 1 K, u(w) = 0.002.

KE

09.07 05.58 40.76 08.19 30.54 29.38 0.032 0.012 0.059 0.019 0.060 0.022

z η

79.6 87.6 91.6 87.2 88.6 95.8 03.9 07.0 10.9 06.8 07.7 22.7 0.00303 0.00185 0.00125 0.00190 0.00170 0.00063 6.9 8.4 3.2 4.9 9.6 10 10.45 7.44 63.09 9.98 34.11 33.8 0.033 0.012 0.061 0.019 0.061 0.022 81.8 90.4 94.4 89.2 89.6 96.3 4.50 9.39 16.9 8.29 8.65 26.1 0.0027 0.0014 0.0008 0.0016 0.0015 0.0005 6.1 6.9 2.3 4.9 8.3 8.6 15.7 08.69 75.88 13.8 38.13 36.8 0.035 0.012 0.062 0.020 0.061 0.022 87.1 91.6 95.3 92.0 90.6 96.6 06.7 10.9 20.3 11.5 09.7 28.4 0.00192 0.00124 0.00070 0.00119 0.00139 0.00051 4.6 4.8 2.3 4.5 9.2 9.0 27.27 13.59 102.2 18.25 49.46 46.07 0.037 0.013 0.063 0.020 0.063 0.022 92.1 94.5 96.5 93.8 92.6 97.3 11.7 17.1 27.3 15.2 12.5 35.5 0.442 1.275 0.281 0.844 0.267 0.786

0.0012 0.00082 0.00052 0.00092 0.0011 0.00041

T = 318 K

KD mGA,aq 100w KE z η

T = 308 K

KD mGA,aq 100w KE z

TBP

where mGAorg and mEo,org are the total molality of gallic acid in organic phase at equilibrium and the initial molality of extractant in the organic phase, respectively. Loading ratio, z helps to decide the formation of acid-extractant complex ratio as 1:1, 2:1, 3:1. Table 3 and 4 show the values of z in the range of 0.01 to 0.06, suggesting the formation of acid−extractant complex in the ratio21,34 of 1:1 in the selected temperature range (Figure 2). TBP, TOA, and Aliquat 336 were used as extractants in the present study. TBP carrying the donor group P = O with sufficient polarity having two lone electron pairs for donating electrons to the acid molecule to form hydrogen bonding (see Supporting Information, Figure S3a). The three oxygen atoms in TBP molecule are less electronegative due to ether linkages than the phosphoryl oxygen. Hence, the phosphoryl oxygen predominates in the hydrogen bonding. Aliquat 336, the quaternary ammonium salt contains electron deficient nitrogen causing the cation−anion displacement or ion-dipole interaction between extractant and acid (see Supporting Information, Figure S3b). TOA being the tertiary amine, has lone pair of electron on the nitrogen atom coordinated with the acid molecule (see Supporting Information, Figure S3c). Hence, the extractants bind the acid molecules with a dipole−dipole or ion-dipole type of intermolecular hydrogen bonding. The bond strength depends on the stability of the partially charged proton donor

η

where m(E:(GA)n)org and mEorg are the molalities of acid-extractant complex and the extractant in the organic phase, respectively. The extent to which an organic phase could be loaded with GA can be expressed as loading ratio, z33 mGA org z= m Eo,org (8)

T = 298 K

(7)

KD

n m Eorg mGA aq

mGA,aq

m(E:(GA)n )org

100w

KE(n :1) =

(6)

KE

KE(n :1)

nGA aq + Eorg ←⎯⎯→ (E: (GA)n )org

z

The equilibrium complexation constant (KE(n:1)) and the number of extractant molecules (n) reacting were computed by applying the mass action law.

η

(5)

T = 288 K

KE(n :1)

GA aq + (E: (GA)n − 1)org ←⎯⎯→ (E: (GA)n )org

KD

· · ·

mGA,aq

KE(2:1)

GA aq + (E: GA)org ←⎯⎯→ (E: (GA)2 )org

ms

KE(1:1)

GA aq + Eorg ←⎯⎯→ (E: GA)org

ext

Table 3. Distribution Coefficient, KD, Extraction Efficiency, η, Loading Ratio, z, Equilibrium Complexation Constant, KE, and Water Mass Fraction in the Organic Phase, w, Values for the Extraction of 0.015 mol·kg−1 Gallic Acid Using TBP, TOA, and Aliquat 336 in Hexanol at Various Temperatures, T, and Pressure = 0.0994 MPa with Uncertainty of 0.0001 MPaa,b

transfer between extractant and the acid, decides the measure of extractability in reactive extraction. GA is considered to be an efficient electron donor due to presence of easily ionizable carboxylic group.4,31 Diluents were used in extraction system to improve the physical properties (viscosity, density, surface tension at the interface) of the extractants and solvation of the acid−extractant complexes formed.30 In the present study, lower pH of the aqueous solution (3.2) as compared to pKa (= 4.4) of the gallic acid32 shows negligible effect of acid dissociation. Hence, the aqueous GA solution contains only undissociated form of acid.

100w

Article


Ext, extractants; ALQ, Aliquat 336; ms, molality of the extractants in the organic phase (mol·kg−1); KE, equilibrium complexation constant (kg mol−1). bStandard uncertainties u are u(mGA,aq) = 0.0001 mol·kg−1, u(T) = 1 K, u(w) = 0.002.

ALQ

TOA

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Figure 2. 1:1 Gallic acid−extractant complex structure: (a) GA:TBP complex, (b) GA:TOA complex, and (c) GA:Aliquat 336 complex.

and acceptor atoms. The electronegativity of the atoms decides the stability of its charges which is a measure of the tendency of an atom to pull a shared pair of electrons toward it. Higher electronegativity difference between hydrogen (2.1) and oxygen (3.5) creates stronger hydrogen bonding. Reactive extraction of gallic acid was performed by varying the temperature in the range of (288 to 318) K and extractant 10 to 30% v/v dissolved in hexanol and octanol. The results are presented in Tables 3 and 4 and shown graphically in Figure 3a,b. The figures illustrate that KD decreases with the temperature for all the extractants. KD was found to increase with the increase in molality of the extractants except for TOA. TOA is a relatively poor solvating medium for polar complex formed. Hence, increasing the amount of TOA in the organic phase, loading of the acid decreases due to less favorable solvent as a solvating medium. The diluents may also affect the basicity of the amine in solution or activity coefficients of reactant or the product.35 Keshav et. al28 observed that the extraction of propionic acid falls for the TOA above 40% v/v in 1-octanol. The extraction mechanism involves the transfer of proton from GA to nitrogen of TOA to form the ion pair. Aliquat 336 was found to be the superior over other extractants for higher molalities of the extractants (Figure 3b). The higher basicity of Aliquat 336 as compared to other extractants, results in more extraction of undissociated acid. Also, the ability of Aliquat 336 to extract both dissociated and undissociated form of acid improves the extraction efficiency. Effect of temperature on KD using different extractants in octanol is shown in Supporting Information Figure S4. Extraction was observed to be more in hexanol as compared to octanol (Tables 3 and 4). The alcohol solvates the GA molecule in the aqueous phase and stabilizes the acid-extractant-alcohol complex36 due to hydrogen bonding with the alcohol. The solvation of acid-extractant complex is promoted by increasing the polarity of the diluent. The properties like polarity, dielectric

a

4.5 5.0 4.9 5.1 7.5 7.5 06.77 04.40 34.63 07.60 19.03 24.27 0.030 0.011 0.058 0.019 0.056 0.021 74.2 84.6 90.2 86.2 82.7 94.9 2.88 5.50 9.15 6.25 4.77 18.5 0.00383 0.00228 0.00146 0.00205 0.00257 0.00076 5.0 5.2 5.2 5.2 8.5 8.0 08.54 06.16 46.19 08.57 23.29 25.70 78.4 88.5 92.4 87.6 85.4 95.2 3.62 7.70 12.2 7.05 5.83 19.6 11.58 06.88 64.39 11.88 25.23 32.99 0.034 0.012 0.061 0.020 0.059 0.022 83.1 89.6 94.4 90.7 86.3 96.2 04.9 08.6 16.9 09.8 06.3 25.2 0.00251 0.00155 0.00083 0.00138 0.00203 0.00057 4.3 4.1 5.1 5 6.0 6.5 21.34 11.21 79.73 14.19 33.45 39.18 0.037 0.013 0.062 0.020 0.061 0.022 90.0 93.3 95.5 92.1 89.3 96.8 9.0 14.0 21.0 11.7 8.4 29.9 0.436 1.262 0.277 0.835 0.263 0.777 TBP

0.00148 0.00099 0.00068 0.00117 0.00159 0.00048

KD mGA,aq 100w KE z

4.2 5.1 5.0 5.3 7.6 7.3

0.00321 0.00171 0.00113 0.00184 0.00217 0.00072

0.032 0.012 0.060 0.019 0.058 0.022

100w KE z η

T = 318 K

KD mGA,aq 100w KE z η

T = 308 K

KD mGA,aq 100w KE η η

z

T = 298 K T = 288 K

KD mGA,aq ms ext

Table 4. Distribution Coefficient, KD, Extraction Efficiency, η, Loading Ratio, z, Equilibrium Complexation Constant, KE, and Water Mass Fraction in the Organic Phase, w, Values for the Extraction of 0.015 mol·kg−1 Gallic Acid Using TBP, TOA, and Aliquat 336 in Octanol at Various Temperatures, T, and Pressure = 0.0994 MPa with Uncertainty of 0.0001 MPaa,b


E



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solvent. The smaller value of activity coefficient corresponds to better solubility of the compound.38 The activity coefficient of the GA in solvents increases with temperature as the hydrogen bonding with the solvent decreases with rise in temperature.39 Additional experiments were performed to compare the water coextraction which accompanies the GA extraction by the extractants and diluents at various temperature. Table 3 and 4 illustrate the water coextraction in terms of water mass fraction in the organic phase with variation of temperature and extractants in hexanol and octanol. The extent of water coextraction was found to be highest with Aliquat 336 as compared to TBP and TOA; further, it was observed higher with hexanol than octanol because of higher solubility of hexanol in water. Thus, GA with Aliquat 336 carries more water with them as compared to other extractants. The water coextraction increases in the trend similar to the increase in the solubility of the diluents in the water.18 However, TOA results more coextracted water in octanol than hexanol. This could be related to more solubility of the acidextractant-water complex in octanol than in hexanol and the low solvating ability of the TOA. No significant effect of temperature was observed on the water coextraction. In all the cases, the coextracted water is varying with ±10%, indicating the insignificant effect on the process viability as the water carried along with the organic phase is very less as compared to the amount of the water to be used during backextraction. It was observed that the coextracted water increases with the increased molality of extractant. The extractants at higher molalities coextracts more water as compared to that at lower molalites. This may be attributed to the more complexation due to higher contribution of the extractants. Although, the type of extractant affects the water coextraction Figure 3. Variation of distribution coefficient with temperature with different extractants in hexanol: (a) 10% v/v extractants; (b) 30% v/v extractants.

aGA + E + pH2 O ↔ [(GA)a : E: (H 2O)p ] (acid−extractant−water complex)

(9)

The complex formed due to hydrogen bonding among the molecule of GA, water, and extractant. The hexanol has good solubility for water and the acid−extractant−water complex as compared to octanol. The similar results were shown by Tamada and King18 in the extraction of lactic acid where coextracted water increases with TOA. 3.4. Thermodynamics of Extraction. Thermodynamic properties viz. enthalpy (H), entropy (S), and free energy (G) are found to be important in order to carry out industrial applications of the scientific research. The van’t Hoff equation is used in calculating the change in enthalpy and entropy of extraction. It is derived from Gibbs free-energy expression40

constant, solubility and aromaticity of diluents affect the extractability of the acid. Hence, the presence of more polar hexanol increases the extraction of GA as compared to octanol. The higher dielectric constant, ε (a dimensionless constant, measure of polarity) of hexanol (13.3) as compared to octanol (10.3) may be one of the reasons for higher value of equilibrium complexation constant in the presence of hexanol as compared to octanol. In Table 3, the higher value of KD as 27.3 confirm the excellency of TOA in hexanol at lower molality of extractant whereas the higher value of KD as 35.5 for Aliquat 336 reveals its excellency at higher molality in hexanol. 3.3. Water Coextraction. The solubility of water in organic phase results into water coextaction. It has been reported that the solubilities of some acids in the organic solvents increase in the presence of coextracted water in the organic phase. For example, the solubility of adipic acid in water-saturated methylcyclohexanone, water, and anhydrous methylcycolhexanone are 9.7, 1.4, and 1.6% (w/w), respectively, at 25 °C, that is, the solubility increases with a multiple of 6 in a solvent saturated with water as compared to the solvent alone.24,37 This may be attributed to the intermolecular interaction between solute and solvent, which increases in the presence of water molecules due to lower activity coefficient of solute in mixture of water and solvent than with water or solvent alone. The solubility of GA in some organic solvents increases markably in the presence of water content in the organic phase. Activity coefficient is an important thermodynamic parameter describing the tendency of compound to be solubilized into a

ΔGo = ΔH o − T ΔS o = −RT ln KE

(10)

ΔH o ΔS o + RT R

(11)

ln KE = −

The above eq 11 is used to get the values of enthalpy and the entropy of the reaction from the plot of ln KE vs 1/T. The values of ΔHo as a slope and ΔSo as an intercept of the line were calculated from Figure 4a,b for extractants at higher molalities in hexanol and octanol, respectively. The plot of ln KE vs 1/T for extractants at lower molalities in hexanol and octanol is shown in Supporting Information (Figure S5a,b). The values of ΔHo, ΔSo, and ΔGo are tabulated in Table 5. ΔGo values were calculated for lower temperature. The higher negative value of enthalpy change indicates more exothermic nature of the process. The exothermic nature of the process can be described in the order: TBP > TOA > F



Article

and extractant. The bond strength in O−H···O hydrogen bond is greater than that of O−H···N hydrogen bond41 due to approximately 180° bond angle and the shorter bond length. As oxygen is more electronegative (3.5) than nitrogen (3.0), two lone pairs of electrons on oxygen of phosphoryl group of TBP are closer to the oxygen nucleus than the single lone pair of electrons on nitrogen of amines to the nitrogen nucleus. Hence the hydrogen bond in O−H···O is shorter and stronger than the O− H···N hydrogen bond. Therefore, the presence of TBP increases the exothermic nature of the extraction process. The net enthalpy change (due to energy released by hydrogen bond formation in the acid-extractant complex less the energy spent for releasing the water molecule) for the extractants except TOA, is more negative in octanol as compared to hexanol (Table 5). Thus, it is clear from the table that the enthalpy and the entropy changes vary in a related and compensatory manner.

4. CONCLUSION The effect of temperature, extractants, and diluents on the extraction of GA were studied. The enthalpy and the entropy of extraction process were obtained as negative for all the extractants and diluent combinations. The negative entropy signifies that the system moved toward more ordered state as the acid extracted from aqueous to organic phase. The extraction process of GA is exothermic with negative enthalpy, hence favoring the extraction at low temperature. The extraction of GA was found to be highly exothermic at lower molality of TBP in octanol and hexanol with evaluated enthalpy change as −28 743 J/mol and −28 465 J/mol, respectively. A 1:1 acid−extractant complex was predicted in all cases of varying extractants. Maximum extraction yield of 97.26% was obtained at higher molality of Aliquat 336 in hexanol at lower temperature (288 K). The obtained data can be used as the basis for designing the extraction process of gallic acid.

Figure 4. Plot of ln KE vs 1/T for gallic acid extraction using different extractants at 30% v/v in (a) hexanol and (b) octanol.

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Aliquat 336 at a particular molality and diluent. The ΔG values were observed to be negative in the temperature range of (288 to 318) K, which suggests that the extraction of gallic acid was spontaneous process and the given temperature range can be considered for gallic acid extraction. The extraction of GA was favored at negative enthalpy and entropy change with all the extractants as illustrated by Table 5. The molecular complex formed by virtue of reaction between extractant and the acid, which makes the system more ordered and causes decrease in entropy. The value of enthalpy change for extraction in the presence of TBP was found to be more negative as compared to the other extractants which may be attributed to the formation of stronger hydrogen bond between acid

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jced.6b00310. HPLC chromatogram, distribution coefficient, structures, variation of distribution coefficient, and ln KE vs 1/T. (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +91-712-2801567. Fax: +91-712-2223969.

Table 5. Thermodynamic Parameters for Gallic Acid Extractiona diluents

extractants

ms (mol·kg−1)

ΔHo (J/mol)

ΔSo (J/mol. K)

ΔGo (J/mol), 288 K

R2

hexanol

TBP

0.442 1.275 0.281 0.844 0.267 0.786 0.436 1.262 0.277 0.835 0.263 0.777

−28441 −21577 −22324. −20799 −11929 −10959 −28715 −22274 −21525. −16759 −13535 −12895

−71.93 −53.65 −38.84 −48.09 −9.303 −6.42 −75.01 −57.72 −38.04 −36.05 −18.06 −14.34

−7724 −6127 −11138 −6948 −9250 −9111 −7111 −5651 −10568 −6378 −8335 −8764

0.955 0.97 0.967 0.995 0.96 0.972 0.96 0.952 0.989 0.974 0.963 0.96

TOA ALQ octanol

TBP TOA ALQ

a

ms, molality of the extractants in the organic phase; ALQ, Aliquat 336. G



Article

Notes

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The authors declare no competing financial interest.

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