Reactive Extraction of Gallic Acid Using Tributyl Phosphate in Different

Article Cite This: J. Chem. Eng. Data XXXX, XXX, XXX−XXX

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Reactive Extraction of Gallic Acid Using Tributyl Phosphate in Different Classes of Diluents Nishant Joshi, Amit Keshav,* and Anil. K. Poonia

J. Chem. Eng. Data Downloaded from pubs.acs.org by UNIV OF LOUISIANA AT LAFAYETTE on 05/01/19. For personal use only.

Department of Chemical Engineering, National Institute of Technology Raipur, Raipur, Chhattisgarh 492010, India ABSTRACT: Gallic acid is an important carboxylic acid used in pharmaceutical industries owing to its medicinal properties. The separation of gallic acid from fermentation broth and dilute aqueous streams is a challenging task. Therefore, in the present study the reactive extraction of gallic acid from an aqueous solution by using tributyl phosphate in octanol, ethyl acetate, n-hexane, and toluene as diluents, respectively, has been carried out at constant temperature (303 ± 1 K). The results were compared with those of the extraction using pure diluents in order to compute the intensification of extraction obtained. Experiments were performed to investigate the effect of initial acid concentration, extractant concentration, and diluent type. TBP + hexane was found to be most suitable solvent system resulting in the highest distribution coefficient (KD = 24) when TBP was employed at a concentration of 1.516 mol·kg−1. The equilibrium complexation constant (KE) was observed to be in the ranges of 3.18−8.8 for n-hexane, 0.91−4.54 for toluene, 2.295−4.546 for ethyl acetate, and 3.57−6.17 for octanol.

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INTRODUCTION Phenolic acids are a type of carboxylic acids derived from plants. Gallic acid, 3,4,5-trihydroxybenzoic acid (shown in Figure 1), is

The retrieval of carboxylic acids through precipitation has been found to produce a large amount of solid and slurry handling and a high quantity of sludge.7 Apart from that there are various other technologies being explored for carboxylic acid recovery. Extraction of acid and employing different solvents have not gained popularity owing to low yields leading to substantially low transfer of the acid into the solvent.8 The sorption process has limited capacity of solid surface for the carboxylic acid and is its major drawback.9 Membrane separation based processes, such as nanofiltration, electrodialysis, and reverse osmosis have been also tried in the literature, though none of the studies have gone above the development stages.10,11 Among the various techniques employed in the literature, reactive extraction promises to be an effective method for the recovery of carboxylic acids from fermentation broths and model solutions offering advantages such as acid re-extraction, solvent reuse, pH control, low cost along with higher recovery with greater purity.12−16 The inherent advantage of reactive extraction is that the acid complex can be transformed to pure acid by various available back extraction techniques, resulting in solvent recycle and leading to approach of green chemistry. Because of the massive commercial importance of gallic acid, some equilibrium studies from the past few years can be found in the literature. Athankar et al. (2015) studied the reactive extraction of gallic acid with tri-n-caprylamine dissolved in hexanol.17 Rewatkar and group focused on physical and chemical extraction studies on gallic acid using various extractants (Aliquat 336,

Figure 1. Structure of gallic acid (3,4,5-trihydroxybenzoic acid).

an important phenolic acid present in complex polyphenols, especially hydrolyzable tannins. Gallic acid is commonly found in food plants, such as, oleaeuropaea (olives), gall nuts, etc. The medicinal properties of gallic acid include antioxidant, antiinflammatory, antimutagenic, antiaging, anticancer, antiviral, and antimicrobial activities.1 There are three ways to obtain the acid: (a) direct recovery from plant sources, (b) chemical synthesis employing hydrolysis of tannic acid, and (c) bio-route involving a fermentation process. The direct recovery of gallic acid from the plant is quite troublesome as the free form occurrence is limited. Chemical synthesis results in environmental unfriendly byproduct formation; hence, bioconversion of gallic acid is considered more viable. Gallic acid can be obtained from plant sources by bioconversion of tannins using several microbes such as Aspergillus niger, Rhizopus oryzae, and Aspergillus fetidus.2−6 The separation of organic acids from waste streams and fermentation broth has always piqued the interest of researchers. © XXXX American Chemical Society

Received: February 26, 2019 Accepted: April 22, 2019

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DOI: 10.1021/acs.jced.9b00192 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

Article

Table 1. Chemicals Used in the Present Work and Their Properties chemical

molecular weight

supplier

molecular formula

purity (%)

CASRN

gallic acid tributyl phosphate octanol ethyl acetate n-hexane toluene

188.14 266.32 130.23 88.11 86.18 92.14

Molychem Loba Chemie Loba Chemie Loba Chemie Loba Chemie Molychem

C7H6O5.H2O C12H27O4P C8H18O C4H8O2 C6H14 C7H8

99.5 99 99 99.5 95 99

5995-86-8 126-73-8 111-87-5 141-78-6 110-54-3 108-88-3

TBP, and TOA) in alcohols (hexanol and octanol)18 and also presented few optimization studies (RSM18,19,21 and ANN20). Pandey et al., have shown the effect of polarity of solvents (MIBK, octanol, kerosene, decane) and type of extractant (TOA, TBP) on the extraction of gallic acid.22 The literature shows that, apart from that for gallic acid, few other reactive extraction studies on other phenolic acids such as benzoic acid23 and protocatechuic acid24−27 have been carried out. Tributyl phosphate (TBP) is an inexpensive extractant which has been used by various researchers because of its potential to extract acid from dilute solution, providing higher a extraction for various carboxylic acids.28−32 It is desirable to tailor various available extractant−diluent combinations to obtain the most suitable solvent system for gallic acid recovery; therefore, in the present study the physical as well as the reactive extraction of gallic acid from the aqueous phase using tributyl phosphate (TBP) in different diluents such as ethyl acetate, n-hexane, octanol, and toluene was studied. The aim is obtain a synergism effect which could result in a solvent system which is inexpensive and nontoxic, and could provide a significant extraction. The results of the work is presented in terms of distribution coefficient, separation efficiency, and loading ratio. For the complexation process, the equilibrium complexation constant and solvation number are also calculated using the mass action law model.

that there was negligible mutual solubility of extractant and diluents in water. The assumptions are fair as the solubility of tributyl phosphate in water is negligible. Water coextraction was observed to be negligible. The initial pH of gallic acid solutions in the chosen concentration range was low ( octanol > toluene > n-hexane. The average extraction efficiency is 60.03% for ethyl acetate, 58% for octanol, 8.5% for toluene, and 7.41% for n-hexane. It can be observed that the distribution coefficient increases with the increase in initial acid concentration, which can occur because at high initial acid concentration there is higher concentration gradient and therefore the driving force is greater. It was observed that the distribution coefficients are moderate for an active solvent such as ethyl acetate and octanol ( toluene > n-hexane. C



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Table 4. Determination of Distribution Coefficient (KD), Solvation Number (s), Equilibrium Extraction Complexation Constant (KE) and Loading Ratio (Z) for the Reactive Extraction of Gallic Acid Using TBP in Octanol at Temperature of 303 K and Atmospheric Pressure of 101.325 kPaa [HA] (mol·kg−1)a

[TBP] (mol·kg−1)a

[HA]aq (mol·kg−1)a

KD

s

KE

Z

R2

0.01

0.379 0.758 1.137 1.516 0.379 0.758 1.137 1.516 0.379 0.758 1.137 1.516 0.379 0.758 1.137 1.516 0.379 0.758 1.137 1.516 0.379 0.758 1.137 1.516

0.0046 0.0030 0.0020 0.0015 0.0075 0.0053 0.0035 0.0025 0.0100 0.0065 0.0046 0.0034 0.0125 0.0080 0.0057 0.0043 0.0155 0.0099 0.0069 0.0051 0.0180 0.0110 0.0077 0.0060

1.17 2.33 4.00 5.66 1.66 2.77 4.71 7.00 2.00 3.61 5.52 7.82 2.20 4.00 6.01 8.30 2.22 4.05 6.24 8.80 2.33 4.45 6.79 9.00

1.138

3.576

0.99

1.027

4.373

0.973

5.171

0.951

5.605

0.983

5.817

0.976

6.178

0.0147 0.0095 0.0073 0.0058 0.0342 0.0201 0.0150 0.0119 0.0547 0.0321 0.0231 0.0182 0.0753 0.0438 0.0313 0.0244 0.0944 0.0548 0.0393 0.0307 0.1150 0.0670 0.0477 0.0369

0.02

0.03

0.04

0.05

0.06

0.97

0.99

0.97

0.98

0.99

a Standard uncertainties, u, are u(T) = 1 K, u(p) = 1 kPa, u(HA) = 0.001 mol·kg−1, u(TBP) = 0.001 mol·kg−1, and u(HAaq) = 0.001 mol·kg−1. HA, initial molality of the gallic acid in the aqueous phase; HAaq, equilibrium molality of the gallic acid in the aqueous phase; TBP, initial molality of the extractant in organic phase.

Table 5. Determination of Distribution Coefficient (KD), Solvation Number (s), Equilibrium Extraction Complexation Constant (KE) and Loading Ratio (Z) for the Reactive Extraction of Gallic Acid Using TBP in Ethyl Acetate at Temperature of 303 K and Atmospheric Pressure of 101.325 kPaa [HA] (mol·kg−1)a



KD

s

KE

Z

R2

0.01

0.379 0.758 1.137 1.516 0.379 0.758 1.137 1.516 0.379 0.758 1.137 1.516 0.379 0.758 1.137 1.516 0.379 0.758 1.137 1.516 0.379 0.758 1.137 1.516

0.0030 0.0020 0.0014 0.0010 0.0050 0.0032 0.0024 0.0015 0.0065 0.0041 0.0029 0.0020 0.0080 0.0050 0.0035 0.0024 0.0091 0.0058 0.0040 0.0027 0.0105 0.0061 0.0041 0.003

2.33 4.00 5.84 9.00 3.00 5.25 7.33 11.90 3.61 6.31 9.34 14.00 4 7 10.42 15.66 4.46 7.62 11.5 17.51 4.71 8.83 13.63 19

0.944

5.755

0.98

0.950

7.454

0.953

9.055

0.961

10.090

0.961

11.163

0.999

12.606

0.0184 0.0105 0.0075 0.0059 0.0395 0.0221 0.0154 0.0122 0.0620 0.0341 0.0238 0.0184 0.0844 0.0461 0.0321 0.0248 0.1079 0.0583 0.0404 0.0312 0.1306 0.0737 0.0510 0.0390

0.02

0.03

0.04

0.05

0.06

0.97

0.98

0.98

0.98

0.97

a Standard uncertainties, u, are u(T) = 1 K, u(p) = 1 kPa, u(HA) = 0.001 mol·kg−1, u(TBP) = 0.001 mol·kg−1, and u(HAaq) = 0.001 mol·kg−1. HA, initial molality of the gallic acid in the aqueous phase. HAaq, equilibrium molality of the gallic acid in the aqueous phase. TBP, initial molality of the extractant in organic phase.

D



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Table 6. Determination of Distribution Coefficient (KD), Solvation Number (s), Equilibrium Extraction Complexation Constant (KE) and Loading Ratio (Z) for the Reactive Extraction of Gallic Acid Using TBP in n-Hexane at Temperature of 303 K and Atmospheric Pressure of 101.325 kPaa [HA] (mol·kg−1)a



KD

s

KE

Z

R2

0.01

0.379 0.758 1.137 1.516 0.379 0.758 1.137 1.516 0.379 0.758 1.137 1.516 0.379 0.758 1.137 1.516 0.379 0.758 1.137 1.516 0.379 0.758 1.137 1.516

0.0080 0.0040 0.0022 0.0010 0.0145 0.0059 0.0031 0.0013 0.0205 0.0080 0.0040 0.0016 0.0261 0.0095 0.0041 0.0020 0.0312 0.0110 0.0047 0.0022 0.0360 0.0122 0.0051 0.0024

0.25 1.50 3.54 9.00 0.37 2.38 5.45 14.38 0.46 2.75 6.50 17.75 0.53 3.21 8.75 19.00 0.60 3.54 9.63 21.72 0.66 3.91 10.76 24.00

2.532

3.187

0.99

2.559

5.004

2.563

6.007

2.568

7.092

2.568

7.943

2.570

8.806

0.0052 0.0079 0.0068 0.0059 0.0145 0.0193 0.0148 0.0123 0.0250 0.0290 0.0228 0.0187 0.0366 0.0402 0.0315 0.0250 0.0496 0.0514 0.0398 0.0315 0.0633 0.0630 0.0482 0.0379

0.02

0.03

0.04

0.05

0.06

0.98

0.98

0.97

0.97

0.98


Table 7. Determination of Distribution Coefficient (KD), Solvation Number (s), Equilibrium Extraction Complexation Constant (KE) and Loading Ratio (Z) for the Reactive Extraction of Gallic Acid Using TBP in Toluene at Temperature of 303 K and Atmospheric Pressure of 101.325 kPaa [HA] (mol·kg−1)a



KD

s

KE

Z

R2

0.01

0.379 0.758 1.137 1.516 0.379 0.758 1.137 1.516 0.379 0.758 1.137 1.516 0.379 0.758 1.137 1.516 0.379 0.758 1.137 1.516 0.379 0.758 1.137 1.516

0.0091 0.0072 0.0050 0.0031 0.0170 0.0112 0.0070 0.0040 0.0241 0.0151 0.0092 0.0043 0.0301 0.0179 0.0102 0.0044 0.0351 0.0203 0.0110 0.0042 0.0390 0.0211 0.0110 0.0041

0.11 0.42 1.00 2.33 0.17 0.81 1.85 4.00 0.25 1 2.33 5.97 0.33 1.23 3.00 8.09 0.42 1.50 3.54 10.90 0.53 1.85 4.45 14.00

2.151

0.912

0.98

2.220

1.631

2.2293

2.195

2.234

2.859

2.241

3.597

2.255

4.540

0.0023 0.0036 0.0043 0.0045 0.0079 0.0116 0.0114 0.0105 0.0155 0.0196 0.0182 0.0169 0.0261 0.0291 0.0262 0.0234 0.0393 0.0391 0.0343 0.0302 0.0554 0.0513 0.0430 0.0368

0.02

0.03

0.04

0.05

0.06

0.99

0.98

0.97

0.98

0.98


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3.2. Reactive Extraction. The value of separation efficiency was observed to be low in physical extraction, therefore the separation of gallic acid was carried out with an extractant, tributyl phosphate used in proportion varying from 10 to 40% (v/v) in different diluents. The presence of tributyl phosphate leads to the complex formation with acid molecules. The results for the reactive extraction are shown in Tables 4−7. The extraction efficiency increases from 82.35 to 95% for ethyl acetate + TBP, 70 to 90% for octanol + TBP, 35 to 93% for toluene + TBP, and 40 to 96% for n-hexane + TBP systems for an initial acid concentration of 0.06 mol·kg−1 and at a TBP concentration range of 0.379−1.516 mol·kg−1. The extraction equilibrium isotherms for gallic acid extraction by TBP in ethyl acetate, octanol, n-hexane, and toluene, are shown in Figures 3−6, respectively. The greatest intensification was observed for n-hexane. Figure 5. Reactive extraction equilibria for gallic acid using TBP in n-hexane.

Figure 3. Reactive extraction equilibria for gallic acid using TBP in ethyl acetate.

Figure 6. Reactive extraction equilibria for gallic acid using TBP in toluene.

Figure 4. Reactive extraction equilibria for gallic acid using TBP in octanol.

Figure 7. Effect of initial acid concentration on distribution coefficient (KD) for TBP + ethyl acetate at atmospheric pressure and 303 K.

3.3. Effect of the Initial Acid Concentration. At a fixed concentration of tributyl phosphate, the distribution coefficient (KD) and separation efficiency (%E) increases for all diluents with the increase in initial acid concentration from 0.01 to 0.06 mol·kg−1. The variations of the distribution coefficient

(KD) with an initial acid concentration of gallic acid for different diluents at different concentrations of tributyl phosphate (TBP) are presented in Figures 7−10. It was observed that for all TBP F



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Figure 10. Effect of initial acid concentration of gallic acid on distribution coefficient (KD) for TBP + n-hexane at atmospheric pressure and 303 K.

Figure 8. Effect of initial acid concentration of gallic acid on distribution coefficient (KD) for TBP + octanol at atmospheric pressure and 303 K.

Figure 11. KD for different diluents at the initial acid concentration of 0.06 mol·kg−1. Figure 9. Effect of initial acid concentration of gallic acid on distribution coefficient (KD) for TBP + toluene at atmospheric pressure and 303 K.

(n-hexane and toluene) provide higher distribution coefficient compared to the active solvents (octanol and ethyl acetate). Similar results have also been obtained by Pandey et al. (2018).22 Figure 12 shows the variation of KD for various diluents at different vol % TBP at 0.01 mol·kg−1 gallic acid concentration. At low TBP concentration (10% v/v) in different diluents, ethyl acetate remains the best diluent providing a distribution coefficient (KD) as high as 2.33. At a higher TBP concentration of 20% v/v and 30% v/v, n-hexane offers a greater distribution coefficient (KD) than toluene and octanol but at even higher concentrations of TBP, such as 40% v/v n-hexane offers a distribution ratio equal to that provided by the ethyl acetate (both providing a KD of 9). 3.5. Effect of Extractant Concentration. The concentration of extractant tributyl phosphate (TBP) has a significant effect on the distribution coefficient. The maximum separation efficiency of gallic acid is 96% with 40% v/v TBP in n-hexane, 93.3% with 40% v/v TBP in toluene,, 90% with 40% v/v TBP in octanol and 95% with TBP 40% v/v in ethyl acetate at 0.06 mol·kg−1 of the gallic acid initial concentration. It was observed that the

levels, KD increased with the increase in the initial concentration of gallic acid. The highest recovery efficiency (95% in ethyl acetate, 90% in octanol, 93.33% in toluene, and 96% in n-hexane) was obtained with 1.516 mol·kg−1TBP at 0.06 mol·kg−1 gallic acid concentration. 3.4. Effect of Diluents. To show the effect of diluents on the separation of gallic acid, two different cases were chosen (one at low initial acid concentration of 0.01 mol·kg−1 and the other at high initial acid concentration of 0.06 mol·kg−1) Figure 11 shows the variation of KD with various diluents at different vol % of TBP in diluents at 0.06 mol·kg−1 gallic acid concentration. The effect of diluents such as n-hexane and toluene varies with the TBP concentration. The presence of 10% TBP (v/v) in inactive diluents such as, toluene and n-hexane, resulted in very a low distribution coefficient as compared to that in ethyl acetate and octanol. However, at high concentration of TBP (40% v/v) in different diluents, the inactive solvents G



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aqueous phase to organic phase.20 In the mass action law the concentrations of the species are directly proportional to the activities of the species and the nonideal behavior is accounted in the equilibrium constant.33 The interaction of TBP and gallic acid (HA) resulting in the complex formation (HA·(TBP)s,org) at the interface of the aqueous and organic phase can be expressed as HA + s TBPorg ↔ HA· (TBP)s ,org

(5)

where, s represents the solvation number. The equilibrium complexation constant (KE) may be expressed in terms of amount complexed by KE =

[HA·(TBP)s ]org s [HA][TBP]org

(6)

The greater is the numerator term, the higher is the value of KE. The complexation is higher for either a strong acid or a solvent with higher solvation power. Thus, higher KE would provide the higher distribution of acid defined in the form of eq 6,

Figure 12. KD for different diluents at an initial acid concentration of 0.01 mol·kg−1.

separation efficiency and distribution coefficient depends directly on the concentration of TBP. The higher the amount of TBP used, the higher is the complex formation and therefore the higher is the separation efficiency obtained. 3.6. Complex Formation. The nature and type of complexes formed during the process of reactive extraction can be explained by the use of mass action law. The complexation is the process in which an acid molecule (gallic acid) interacts with extractant molecules (TBP) at the interface of the aqueous and organic phases and becomes attached by the formation of hydrogen bonds. Diffusion and solubilization are the primary mechanism for the transfer of gallic acid from

KD =

[HA]org [HA]aq

=

s [HA][TBP]org

[HA]aq + [A−]aq

(7)

and the two accounting terms for acid transfer may be expressed in dependence to each other and acid ionization as, KD =

KD =

s KE[HA]aq [TBP]org

[HA]aq + KHA[HA]aq /[H+]aq

(8)

s KE[TBP]org

1 + KHA /[H+]aq

(9)

Figure 13. Formation of different gallic acid: extractant complexes during the reactive extraction process. H



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Table 8. Literature Comparison of Reactive Separation of Gallic Acid Using Tributyl Phosphate system used TBP + MIBK TBP + octanol TBP + kerosene TBP + decane TBP + octanol

TBP + ethyl acetate TBP + octanol TBP + toluene TBP + n-hexane

operating conditions concentration of gallic acid, 0.0029−0.0588 kmol.m−3; TBP, −10 to 40% v/v; temp, 298 K concentration of gallic acid, −0.015 mol·kg−1; extractant concn, 0.436 and 1.262 mol·kg−1; temp, 288−318 K concentration of gallic acid, 0.01−0.06 mol·kg−1; TBP concentration, −10−40% v/v in diluent; temp, 303 K

distribution coefficient (KD) 0.8−24.5 1.3−9.33 0.25−21.18 0.44−21.35 9.0−14 (at 288 K) 4.9−8.6(at 298 K) 3.62−7.70(at 308 K) 2.88−5.5(at 318 K) 2.33−19 1.17−9 0.11−14 0.25−24

equilibrium extraction complexation constant (KE) 1.762−14.632 2.266−6.79 2.361−8.373 1.932−8.22 21.34−11.21(at 288 K) 11.58−6.88(at 298 K) 08.54- 06.16(at 308 K) 6.67−4.40(at 318 K) 5.755−12.606(KE overall) 2.295- 4.546(KE chem.) 3.576−6.178 0.912−4.540 3.187−8.806

(10)

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1.138−0.976 2.151−2.255 2.532−2.570

53.9−90 9.9−93.3 20−96

ref 22

18

present study

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected], [email protected]. Tel.: 09630058194. ORCID

Amit Keshav: 0000-0002-4911-8865 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank NIT Raipur for providing all necessary facilities. REFERENCES

(1) Badhani, B.; Sharma, N.; Kakkar, R. Gallic acid: a versatile antioxidant with promising therapeutic and industrial applications. RSC Adv. 2015, 5, 27540−27557. (2) Trevino-Cueto, B.; Luis, M.; Contreras-Esquivel, J. C.; Rodriguez, R.; Aguilera, A.; Aguilar, C. N. Gallic acid and tannase accumulation during fungal solid state culture of a tannin-rich desert plant (LarreatridentataCov.). Bioresour. Technol. 2007, 98, 721−724. (3) Seth, M.; Chand, S. Biosynthesis of tannase and hydrolysis of tannins to gallic acid by Aspergillus awamori - optimization of process parameters. Process Biochem. 2000, 36, 39−44. (4) Kar, B.; Banerjee, R.; Bhattacharyya, B. C. Microbial production of gallic acid by modified solid state fermentation. J. Ind. Microbiol. Biotechnol. 1999, 23, 173−177.

[HA]org [TBP]

0.944−0.999

44.56−96.09 56.46−90.32 20.07- 95.49 30.61−95.53 90−93.3 83.1−89.6 78.4−88.5 74.2−84.2 69.9−95

4. CONCLUSION The chemical and physical extraction of gallic acid with and without TBP in diluents; octanol, ethyl acetate, n-hexane, and toluene, have been presented, and resultsare reported in terms of separation efficiency, loading ratio, equilibrium extraction, complexation constant, and solvation number. The value of the highest KD for the TBP diluent combination have been report to follow the trend TBP + n-hexane (KD = 24) > TBP + ethyl acetate (KD = 19) > TBP + toluene (KD = 14) > TBP + octanol (KD = 9). KE was observed to follow a similar trend with the complexation observed to be 1:1 for gallic acid−TBP− octanol/ethyl acetate and 1:2 complexes for gallic acid−TBP−nhexane and 1:3 for gallic acid−TBP−toluene. The average loading ratio Z was observed to follow the order ethyl acetate (0.0425) > octanol (0.0378) > n-hexane (0.0302) > toluene (0.0244).

Thus, assessing the experimental information for KD versus initial TBP concentration (as the complexed TBP is very less as compared to initial TBP concentration), eq 10 leads to the evaluation of KE and s. From the Tables 4−7 it can be seen that the for ethyl acetate and octanol the value of solvation number (s) is very near to 1; therefore, 1:1 gallic acid/TBP complex is suggested. For toluene the value of s lies in the range of 2.15−2.25; therefore, a 1:2 gallic acid/TBP is suggested. For n-hexane the values of s are in the range of 2.53−2.57; therefore, the formation of both 1:2 and 1:3 gallic acid/TBP complexes are suggested. Also the values of KE are in the range of 3.18−8.8 for n-hexane, 0.91−4.54 for toluene, 5.75−12.60 for ethyl acetate, and 3.57−6.17 for octanol suggesting that complexation is relatively high for ethyl acetate, medium for octanol and n-hexane, and lower for toluene. The formation of gallic acid/TBP complexes is shown in Figure 13. In the case of ethyl acetate a significant role is played by diluent alone (as shown by the average physical extraction efficiency of 60%); therefore, neglecting the contribution by diluent, KE was observed to be in the range of 2.295- 4.546. Table 8 provides a comparison of different parameters such as distribution coefficient (KD), equilibrium extraction complexation constant (KE), solvation number (s), and extraction efficiency (E) obtained by various equilibrium studies by other authors for reactive separation of gallic acid using TBP. 3.7. Loading Factors. The loading ratio, Z is expressed as the extent to which the organic phase may be loaded with acid (mathematically it is the ratio of the acid concentration transferred in the organic phase to the initial TBP concentration) and is represented as Z=

0.669−1.238 0.434−0.88 2.077- 2.517 1.484−2.591 1:1 acid: TBP complex proposed

extraction efficiency (E %)

observed to follow the order ethyl acetate (0.0425) > octanol (0.0378) > n-hexane (0.0302) > toluene (0.0244).

under the similar conditions as stated earlier for chosen acid concentrations, neglecting the dissociation effect, we get, log KD = log KE + s log[TBP]org

solvation number (s)

(11)

The loading ratio values were calculated (presented in Tables 4−7). A higher loading ratio was obtained at lower concentration of TBP (0.379 mol·kg−1) as compared to when a higher concentration of TBP was used. Further, loading was observed to increase with an increase in acid concentrations for a fixed extractant concentration, for all the TBP + diluents combinations employed in the study. Average loading ratio Z was I



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