Extraction Equilibria of Pyruvic Acid Using Tri-n-butyl Phosphate

Article pubs.acs.org/jced

Extraction Equilibria of Pyruvic Acid Using Tri‑n‑butyl Phosphate: Influence of Diluents Dharm Pal* and Amit Keshav* Department of Chemical Engineering, National Institute of Technology, Raipur, Chhattisgarh 492010, India ABSTRACT: Pyruvic acid is extensively utilized in the chemical, pharmaceutical, and agrochemical industries. Novel biotechnological routes to pyruvic acid production receiving great attention and recent focus of research are toward its separation from fermentation broth or recovery from waste streams. Based on reversible complexation, extraction equilibria of the system (water + pyruvic acid + extractant/diluents) were measured at T = 308.2 K using various concentrations of pyruvic acid (0.1 kmol·m−3 to 0.5 kmol· m−3) and extractant (tri-n-butyl phosphate, TBP) (0.36 kmol·m−3 to 2.56 kmol·m−3) in different diluents (n-heptane, toluene, and methyl-iso-butyl ketone, MIBK). Different physiochemical properties such as dipole moment and ET parameter have been used for comparison. Results are given in terms of various parameters such as distribution coefficient, equilibrium complexation constant, loading ratio and extraction efficiency. Using expressions derived for extraction equilibria, equilibrium complexation constants (KE) were evaluated to be 0.369 m3·kmol−1, 0.482 m3·kmol−1, and 0.578 m3·kmol−1 for n-heptane, toluene, and MIBK, respectively, whereas solvation number of TBP (m) was estimated to be one for all the three diluents. Only (1:1) complex was proposed for all the three diluents with no overloading (Z < 0.5) in any case. Higher chemical extraction was observed in inactive diluents: n-heptane and toluene.

1. INTRODUCTION Pyruvic acid (2-oxopropanoic acid) is a valuable organic acid that contains both reactive ketonic and carboxyl groups. Hence, it serves as an important precursor for variety of chemicals and widely utilized in chemical, pharmaceuticals1,2 and agrochemical industries. Its role in central metabolism of living cells is well known.3 It is used as a weight control supplement4 because it has a fat reducing metabolic effect in the human body.5 It is also used in crop protection agents, polymers, cosmetics, as a nutraceutical6 and also as an antioxidant.7 For the production of pyruvic acid different chemical routes and recently few biotechnological routes have been reported. For commercial scale production of pyruvic acid, a classical chemical method is used, comprising dehydration followed by decarboxylation of tartaric acid.8 Though this process is simple, it is costly due to involvement of high energy consuming distillation steps. Therefore, despite numerous applications, pyruvic acid is rarely used to substitute other carboxylic acids due to high price. Due to recent developments in a strain that belongs to genus Torulopsis yielding an appreciable amount of pyruvate, biotechnological methods of pyruvic acid production is emerging as an efficient alternative, cost-effective at the same time environmentally benign. Biotechnological methods9 of the production of pyruvic acids includes (i) enzymatic methods,10 (ii) resting cell method,11 and (iii) the fermentation process.12 Although each method has some pros and cons, enzymatic and resting cells method has potential to produce pyruvic acid with good yield and selectivity and at the same time fewer byproducts.13 However, product recovery is still a big hurdle © 2014 American Chemical Society

in biotechnological processes. Hence, besides the production process, considerable attention must be put on product recovery as apart from its high contribution to the total process cost; it is also a crucial factor if the product is inhibitory.14 Traditionally, organic acids are separated from the fermentation broth by precipitating the acids as insoluble calcium salts after removing the microorganism. To get free acids, salts are further treated with sulfuric acid.15 This method is neither cost-effective nor ecofriendly due to generation of huge amount of solid wastes. Recently reverse osmosis and ion exchange are the reported method employed for recovery of pyruvic acid from the fermentation solutions.13,16,17 However, because of low exchange capacity and use of concentrated acids, the ion exchange method is not attractive or sustainable. The reverse osmosis membrane method is inefficient and expensive as it requires a set of special equipment. An efficient alternative approach for separation or recovery of carboxylic acids is reactive extraction based upon reversible complexation, which is highly selective and effective for separation of organic compounds having polar groups especially from the dilute solutions.13,17 This method offers the advantage of simplicity, energy efficiency, with high distribution coefficient at the same time it also preserve thermal stability of the product.15,18 Kertes and King15 classified extractants as (i) oxygen-bearing hydrocarbons (methyl-iso-butyl ketone (MIBK), octanol, etc.); (ii) phosphorus-bonded oxygen bearing Received: February 7, 2014 Accepted: August 4, 2014 Published: August 12, 2014 2709

dx.doi.org/10.1021/je500125j | J. Chem. Eng. Data 2014, 59, 2709−2716

Journal of Chemical & Engineering Data

Article

and MIBK). The extraction behavior was explained based on physical and chemical equilibrium with the help of various parameters.

extractants (tri-n-butyl phosphate(TBP), trioctylphosphineoxide (TOPO) etc.); and (iii) high-molecular-weight aliphatic amines (Aliquat 336, tri-n-octylamine (TOA), etc.). Solvents of class (i) and (ii) are non reactive and extraction proceeds due to donor bonds, whereas class (iii) solvents extract through acid−amine complexations with improved separation. In general, the physical state of extractants is either solid or viscous liquid. Therefore, to modify the physical properties (viscosity, surface tension, etc.) extractants are often used with diluents. Diluents also contribute in solvation to the acid− amine complexes formed.20 Extraction of carboxylic acid has long been studied using phosphorus based extractant such as TBP,19−26 (TOPO),27,28 and high molecular weight alkyl amines22,23,29−33 as an extractant. However, in literature only few research related to reactive extraction of pyruvic acid has been reported.13,17,27,29,32,33 A mini review has been presented by Li and co-workers on biotechnological production of pyruvic acid along with comparison of recovery methods and cost of different production methods.12 Production of pyruvic acid by fermentation was found to be a competitive one.12 Senol has performed an extensive study on effect of diluents on extraction of pyruvic acid using amine.32 Different diluents (proton donating and accepting, polar and inert types) was tried to find complex solvation strength. The maximum synergistic extraction efficiency was found for cyclic alcohol with amine.32 The results were correlated using modified Langmuir model and chemical model.32 In a recent work, Senol has reported extraction equilibria for a pyruvic acid−alcohol−alamine system in which the extraction parameters were correlated using solvation relation.33 Ma et al. in their work have tried to recover pyruvic acid of biotransformed solutions from lactic acid using reactive extraction with TOA.13 The effects of various diluents (ethyl acetate, butyl acetate, chloroform, n-hexane, and noctanol), stoichiometric ratio and pH was also investigated in that study. Recovery was reported up to 82 % for pyruvic acid with up to 92 % removal of lactic acid.13 They have also studied back extraction using NaOH and trimethyl amine (TMA).13 Hano et al.27 have extracted various mono-, di-, and tricarboxylic acids using TOPO. They found that salvation number is equal to the number of carboxylic group on each acid molecule. They also reported that association constant is dependent on polarity of the solvent, and therefore, the equilibrium constant for extraction depends on hydrophobicity of the acids rather than pKa.27 Morales et al.22 have studied the effect of extractant (TBP and TOA), diluents and modifiers on extraction behavior of acids with one carboxylic group, and it was observed that extraction constant and solvation number depends on organic phase composition and its method of determination. From literature reviews, it seems that effect of extractant and diluents on the extraction equilibrium is a crucial factor and must be analyzed quantitatively. TBP has been used successfully as an extractant by various researchers in the recent past for the extraction of metals and organic acids such as acrylic acid,20 propionic acid,23−26 and so forth. Reported results are encouraging and envisaged the possibility of commercial-scale production with economic viability. However, to the best of our knowledge, no study on reactive extraction of pyruvic acid using TBP as an extractant has been reported to date. In this background, extraction of pyruvic acid was performed with tri-n-butyl phosphate (TBP) dissolved in diluents with different chemical nature (n-heptane, toluene

2. MATERIALS AND METHODS 2.1. Materials. The pyruvic acid was diluted in deionized ultrapure water (Millipore) to prepare an aqueous phase. Extractant taken for this study was tri-n-butyl phosphate (TBP). Both TBP and pyruvic acid (98 % pure) was obtained from SRL Pvt. Ltd., Mumbai, India. Different diluents (n-heptane, toluene, and methyl-iso-butyl ketone (MIBK)) were obtained from Merck Specialist Pvt. Ltd., Mumbai, India. Sodium hydroxide (NaOH) was obtained from Ranbaxy, India, and used for titration. NaOH was standardized with 99.8 % purity oxalic acid supplied by S. D. Fine-Chem. Ltd., India. Indicator used was phenolphthalein (pH range 8.2 to 10.0) obtained from Ranbaxy, India. Analytical grade reagents without modification were utilized. The initial TBP concentrations range ([TBP]o) studied was 0.1 kmol·m−3 to 0.7 k mol·m−3. Low initial aqueous concentrations of pyruvic acid ([HA]o= 0.1 kmol·m−3 to 0.5 kmol·m−3) were used because in most of the biotechnological process the concentration of acid produced in the fermentation broth is below 10 %.9,12,13,28 Physiochemical properties of chemical used and properties of diluents are presented in Table 1 and 2, respectively. Table 1. Chemicals Used in Present Work chemicals

pyruvic acid tri-n-butyl phosphate (TBP) n-heptane toluene MIBK

molecular weight

Avg. Density at 20 °C

g/mol

(g/cm3) × 103

C3H4O3 (CH3CH2CH2CH2O)3PO

88.06 266.31

1.270 0.973

CH3(CH2)5CH2 C6H5CH3 (CH3)2CHCH2C(O)CH3

100.21 92.14 100.16

0.683 0.863 0.801

structure

Table 2. Properties of Diluents Used in Present Work diluent

dielectric constant

ET value

dipole moment debye

n-heptane toluene MIBK

1.93 2.38 13.11

31.1 33.9 39.4

0 0.31 4.20

2.2. Methods. Equilibrium experiments were carried out by shaking equal volumes (10 mL) of aqueous and organic phases in a 50 mL conical flask made of glass for 6 h (sufficient time to attain equilibrium) at constant conditions (T = 308.2 ± 1 K and at atmospheric pressure) at 190 rpm in a water bath shaker (REMI instruments (P) Ltd., India). Equilibrium mixture was kept for settling for at least 4 h at constant conditions. Aqueous phase was sampled by a pipet and the concentration of acid was measured by titration using fresh and standardized (with oxalic acid) NaOH (0.025 N). Organic phase acid concentration was calculated by mass balance. For weighing solid, AG Gottingen Germany, made weighing balance (Soritorious CP 2245) with an accuracy of 0.1 mg was used. 2.3. Experimental Uncertainty. In the present work uncertainty analysis was performed as per the National Institute of Standard and Technology (NIST) guidelines. Some of the 2710



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experiments were repeated in triplicate and consistency of replicated experiments was within ± 2 %. Standared uncertainty was evaluated to be ± 0.001 by using eq 1 N

μ(x) =

∑i = 1 (xi − x ̅ )2 (N − 1)

(1)

where xi = values of experimental observation, x̅ = sample mean of three observations, and N = number of observations. For further chemical calculations average of the replicated data was used.

3. RESULT AND DISCUSSION 3.1. Physical Extraction. Physical extraction refers to use of conventional organic solvents (only diluents with no Table 3. Physical Equilibria of Extraction of Pyruvic Acid Using Different Diluents at Temperature T = 308.2 Ka diluent

MIBK

toluene

n-heptane

[HA]0

[HA]aq

[HA]org

kmol· m−3

kmol· m−3

kmol· m−3

0.05 0.1 0.2 0.3 0.4 0.5 0.05 0.1 0.2 0.3 0.4 0.5 0.05 0.1 0.2 0.3 0.4 0.5

0.0389 0.0779 0.1549 0.2319 0.3081 0.3843 0.0475 0.0950 0.1875 0.3725 0.2800 0.4650 0.0492 0.0983 0.1959 0.2926 0.3893 0.4860

0.0111 0.0221 0.0451 0.0681 0.0919 0.1157 0.0025 0.0050 0.0125 0.0275 0.0200 0.0350 0.0008 0.0017 0.0041 0.0074 0.0107 0.0140

KD

P

Figure 1. Physical equilibrium isotherm of extraction of pyruvic acid [0.1 kmol·m−3 to 0.5 kmol·m−3] in different diluents: ◇, n-heptane; □, toluene; Δ, MIBK. Subscript: aq, aqueous phase; org, organic phase. Solid line (−) indicates the best fit of experimental data.

D m3/ kmol

0.2845 0.2845 0.2913 0.2936 0.2981 0.3009 0.0526 0.0526 0.0667 0.0738 0.0714 0.0753 0.0169 0.0169 0.0211 0.0254 0.0276 0.0289

0.282

0.31

where distribution coefficient (Kdiluent ) is defined as D

KDdiluent 0.051

11.34

0.015

66.67

[HA]aq

(3)

where [HA]aq denotes acid concentration in aqueous phase and [HA]diluent represents acid concentration in organic phase when org only diluents was used. Table 3 shows physical extraction data of pyruvic acid, using n-heptane, toluene, and MIBK. Physical extraction has been found to be very poor in all the diluents. In the present study, the KD values for pyruvic acid by diluents alone was found in the range of 0.2845 to 0.3009, 0.0526 to 0.0753, and 0.0169 to 0.0289 for MIBK, toluene, and n-heptane, respectively. A similar study was performed by Senol where distribution of pyruvic acid was investigated in various diluents. Remarkably small physical extraction by diluents alone was reported and distribution coefficient observed was less than 0.7 for hydrocarbons.32 In the same work Senol32 has proposed order of diluents extraction efficiency and complex formation ability as MIBK > toluene > n-heptane. The trend of distribution coefficient (Table 3) has been found similar to as suggested by King et al.,30 Senol,32 and Keshav et al.20 Keshav et al.25 have studied propionic acid extraction with petroleum ether and KD value reported was 0.25, which is much higher than present case except MIBK. In another study by Keshav and co-workers, the extraction of acrylic acid was investigated in different diluents.20 The physical extraction was found quite low for hydrocarbons.20 Keshav et al. explained this based on physiochemical parameters such as dipole moment (μ) and ET values. The dipole moment and ET value of toluene (μ, 0.31; ET, 33.9) and n-heptane (μ, 0; ET, 31.1) is lower than MIBK (μ, 4.2; ET, 39.4). The ET parameter also known as the Dimroth− Reichardt parameter is an empirical parameter.34 It is related with the absorption band of pyridinium-N-phenolbetaine in a given solvent. ET, called ET (30) by its originators, is generally expressed in kilocalories per mole. A solvent polarity is indicated by its ET value and it gauges solute solvation energy.34 More affinity of pyruvic acid for water seems to be responsible for low distribution coefficient of pyruvic acid by diluents alone in general and for corresponding low values

Standard uncertainties u are u(T) = ± 1 K, u(HA) = ± 0.001 kmol/ m3. a

extractant) such as aliphatic hydrocarbons, aromatic hydrocarbons, esters, ketones, alcohols, and so forth. The solvent extracts the acid based on the solubility of the acid in particular media and accommodate in itself based upon dispersion or ion−ion interaction. Weak diluents such as aliphatic and aromatic hydrocarbons involve the dimerization of acid due to poor solvation characteristic. On the other hand active diluents involve ion-pair or H bonding and dimerization is not predicted. Therefore, in the aqueous phase, the presence of acid is based on the pH of the solution. At pH < pKa (2.49) of acid, acid is present in ionic form, whereas at pH > pKa, the presence of acid in dimerized form is accounted by dimerization constant represented as the concentration of dimerized acid in organic phase to concentration of total acid in organic phase. Because pH < pKa of acid (neglecting ionization) the distribution coefficient in terms of dimerization constant (D) and partition coefficient (P) may be correlated as follows:15,20 KDdiluent = P + 2P 2D[HA]aq

=

[HA]diluent org

(2) 2711



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Table 4a. Chemical Equilibria of Extraction of Pyruvic Acid Using TBP in n-Heptane at Temperature T = 308.2 Ka [TBP]o

[HA]o

[HA]org

kmol·m−3

kmol·m−3

kmol·m−3

0.366

0.1 0.2 0.3 0.5 0.1 0.2 0.3 0.5 0.1 0.2 0.3 0.5 0.1 0.2 0.3 0.5 0.1 0.2 0.3 0.5

0.0025 0.0104 0.0229 0.0664 0.0075 0.0236 0.0470 0.1159 0.0162 0.0380 0.0792 0.1610 0.0288 0.0740 0.1216 0.2240 0.0450 0.1028 0.1608 0.2769

0.733

1.099

1.831

2.564

KD

z

E%

0.026 0.055 0.082 0.153 0.081 0.134 0.186 0.302 0.194 0.234 0.359 0.474 0.403 0.587 0.672 0.812 0.818 1.058 1.156 1.241

0.0068 0.0284 0.0624 0.1814 0.0102 0.0322 0.0641 0.1582 0.0148 0.0346 0.0721 0.1463 0.0157 0.0404 0.0658 0.1223 0.0175 0.0401 0.0627 0.1079

2.53 5.21 7.58 13.27 7.49 11.82 15.68 23.20 16.25 18.96 26.42 32.16 28.72 36.99 40.19 44.81 44.99 51.41 53.62 55.38


Table 4b. Chemical Equilibria of Extraction of Pyruvic Acid Using TBP in Toluene at Temperature T = 308.2 Ka [TBP]o

[HA]o

[HA]org

kmol·m−3

kmol·m−3

kmol·m−3

0.366

0.1 0.2 0.3 0.5 0.1 0.2 0.3 0.5 0.1 0.2 0.3 0.5 0.1 0.2 0.3 0.5 0.1 0.2 0.3 0.5

0.0075 0.0200 0.0412 0.0871 0.0150 0.0380 0.0677 0.1377 0.0237 0.0560 0.0976 0.1780 0.0350 0.0800 0.1367 0.2470 0.0475 0.0980 0.1643 0.2907

0.733

1.099

1.831 −3

Figure 2. Chemical equilibria of pyruvic acid [0.1 kmol·m to 0.5 kmol·m−3] using TBP [0.366 kmol·m−3 to 2.564 kmol·m−3] in different diluents (a) n-heptane (b) toluene (c) MIBK: □, 10 %TBP; Δ, 20 % TBP; × , 30 % TBP; ◇, 50 % TBP; *, 70 %TBP. Subscript: aq, aqueous phase. Solid line (−) indicates the best fit of experimental data.

2.564

KD

z

E%

0.081 0.111 0.159 0.211 0.176 0.234 0.291 0.380 0.311 0.389 0.482 0.553 0.538 0.667 0.837 0.976 0.905 0.961 1.211 1.389

0.0205 0.0546 0.1126 0.2379 0.0205 0.0518 0.0924 0.1880 0.0216 0.0510 0.0888 0.1620 0.0191 0.0437 0.0746 0.1349 0.0185 0.0382 0.0641 0.1134

7.49 9.99 13.72 17.42 14.97 18.96 22.54 27.54 23.72 28.01 32.52 35.61 34.98 40.01 45.56 49.39 47.51 49.01 54.77 58.14


compared to propionic acid in particular. Presence of additional proton accepting ketonic group in pyruvic acid promotes intermolecular hydrogen bonding. The partition coefficient (Table 3) was evaluated (by applying eq 1), for MIBK (P = 0.282), toluene (P = 0.051), and n-heptane (P = 0.015). Due to domination of dispersion forces in case of toluene and nheptane high dimerization is expected. However, in the case of MIBK, low dimerization is attributed to more solute−diluent interaction than solue−solute interaction. On the other hand, compared to MIBK (D = 0.31 m3·kmol−1) both n-heptane (D =

66.67 m3·kmol−1) and toluene (D = 11.34 m3·kmol−1) possess significant dimerization constant values (Table 3). Solvation by donar bonds and equilibrium solubility seems to be main factor for the removal of acid when diluents were used alone. Variation of distribution ratio in different diluents for the same acid emphasizes the importance of solvation for extraction of the carboxylic acid. Hence, the main objective of extraction 2712



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3.2. Chemical Extraction. Compared to C-bonded oxygen bearing extractant, organophosphorous compounds like TBP and TOPO are strongly solvating extractants and offer more distribution ratio, hence, being utilized for the recovery of inorganic acids and metals.35 Due to the hydrogen bond forming ability of phosphoryl group, acid molecules can be extracted by organophosphorous compounds.19−28 The presence of phosphoryl group in TBP makes it possible to behave as a strong Lewis base and, hence, show tendency for complexation.36 Reactive extraction equilibria of pyruvic acid (HA) with the solavating extractant, TBP proceeds as follows:27,37

Table 4c. Chemical Equilibria of Extraction of Pyruvic Acid Using TBP in MIBK at Temperature T = 308.2 Ka [TBP]o

[HA]o

[HA]org

kmol·m−3

kmol·m−3

kmol·m−3

0.366

0.1 0.2 0.3 0.5 0.1 0.2 0.3 0.5 0.1 0.2 0.3 0.5 0.1 0.2 0.3 0.5 0.1 0.2 0.3 0.5

0.0287 0.0620 0.0964 0.1745 0.0362 0.0788 0.1240 0.2263 0.0412 0.0884 0.1378 0.2585 0.0475 0.1016 0.1608 0.2884 0.0525 0.1124 0.1746 0.3171

0.733

1.099

1.831

2.564

KD

z

E%

0.403 0.449 0.474 0.536 0.568 0.650 0.705 0.827 0.702 0.792 0.850 1.070 0.905 1.032 1.156 1.363 1.105 1.283 1.393 1.734

0.0785 0.1692 0.2633 0.4765 0.0495 0.1076 0.1693 0.3089 0.0375 0.0804 0.1254 0.2352 0.0259 0.0555 0.0878 0.1575 0.0205 0.0438 0.0681 0.1237

28.72 30.99 32.16 34.90 36.22 39.39 41.35 45.27 41.25 44.20 45.95 51.69 47.51 50.79 53.62 57.68 52.49 56.20 58.21 63.42

HA + mTBPorg ↔ HA· (TBP)m ,org

(4)

The solvation number of TBP is denoted by m. In organic layer pyruvic acid is present as acid: TBP complex and, hence, a quantative interpretation of the acid-extractant equilibria by equilibrium complexation constant (KE) may be represented as KE =

[HA·(TBP)m ]org [HA][TBP]morg

(5)

KE is expected to depend on diluent’s solvation efficiency as well as on acid nature. The experimentally accessible distribution coefficient KD may be presented as


KD =

[HA]org [HA]aq

=

[HA][TBP]morg [HA]aq + [A−]aq

(6)

In aqueous solution, ionization of the acid [HA] is given by HA ↔ H+ + A− KHA =

[H+][A+] [HA]

(7)

(8)

Combining eq 6 with 8 KD =

=

m KE[HA]aq [TBP]org

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

(9)

KE[TBP]morg 1 + KHA /[H+]aq

(10)

In the organic phase, free TBP concentration is given by [TBP]org = [TBP]o,org − [HA· (TBP)m ]org

(11)

For negligible physical extraction and (1:m) acid:TBP complex formation,

Figure 3. Estimation of extraction equilibrium constant and apparent number of reacting molecules for extraction of pyruvic acid using TBP [0.366 kmol·m−3 to 2.564 kmol·m−3] at fixed initial acid concentration [0.3 kmol·m−3] in different diluents: ◇, MIBK; Δ, toluene; × , nheptane. Subscript: org, organic phase. Solid line (−) indicates the best fit of experimental data.

[HA·(TBP)m ]org = m[HA]org

(12)

and hence, [TBP]org = [TBP]o,org − m[HA]org

(13)

For dilute solutions of pyruvic acid, KHA/[H+]aq can be neglected and eq 10 can be expressed as

(high distribution and selectivity) is not possible to achieve by diluents alone. Furthermore, low extraction with conventional solvents requires high flow rate of solvent resulting further dilution of target acid. Figure 1 depicts the equilibrium isotherm performed at 35 °C in different diluents. Results reveal strong solute−diluent interaction by more polar diluents; MIBK compared to relatively very less polar toluene and nonpolar n-heptane.

log KD = log KE + m log[TBP]org

(14)

KE and m can be evaluated from the plot between log KD versus log [TBP]org that must give a straight line with a unit slope for (1:1) complex. The equations mentioned above are simplified versions of a more general and thermodynamically consistent 2713



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Table 5. Summary of Results of Reactive Extraction of Pyruvic Acid Using TBP and Comparison with Previous Similar Works acid

extractant

diluents

solvation no., m

KE

complex (acid: TBP)

loading ratio, Z

reference

(1:1) (1:1) (1:1) (1:1) (1:1)

Z < 0.5

present work

Z < 0.5

28 23 24

m3kmol−1

a

pyruvic

TBP

propionic propionic propionic

TOPO TBP TBP

lactic

TOPO

propionic acetic propionic butyric lactic propionic

TBP TBP

TBP

n-heptane toluene MIBK hexane n-hexanol MIBK 1-decanol toluene heptane hexane petroleum ether dodecane

dodecane

1.04 1.03 0.90 1

1

(0.992 to 1.19)

0.369 0.482 0.578 0.702 0.266 0.8491 0.591 1.84 1.67 1.17 1.91 0.58a 6.45a 0.344 2.546

1.28 1.08

(1:1) (1:1) (1:1) (1:1)

(1:1) (1:1)

27

Z < 0.5

25 21

22

Based on given intercepts (−0.24 to 0.81).

E% =

one allowing for the simultaneous formation of multiple solvates.15 To account for the physical extraction and acid dimerization, eq 9 requires additional terms in case of solvent stoichiometric deficiency. It has been established that solvation number of the aliphatic carboxylic acids is equal to the number of carboxylic group on each acid molecule.21,37 As pyruvic acid is a monocarboxylic acid, solvation number can be taken as one. The loading of the extractant, Z is found to be dependent on aqueous phase acid concentration and on its extractability. It is the ratio of total concentration of the acid (all form) in the organic phase, [HA]org to the total concentration of extractant (all form) in the organic phase, [TBP]total [HA]org [TBP]total

(16)

Figure 2 represents the effect of extractant concentration (% TBP) on chemical equilibrium. Due to less water coextraction (4.67 mass %) and poor solubility in the aqueous phase (0.039 mass %), TBP was chosen as extractant. Presence of a  P(O)OH group (containing both electron acceptor and donor) make it suitable for specific interaction and H bonding. The extraction efficiency of TBP is significant enough; however, it is generally used with diluents due to its high viscosity (3.56 × 10−3 Pa·s) and specific gravity (0.98).38 Extraction efficiency was found to increase with increase in concentration of TBP, which is in agreement with previous similar works.20,22,27 Separation of pyruvic acid using TBP (10 % to 70 %) in nheptane, toluene, and MIBK has confirmed to be efficient (Table 4a to 4c, Figure 2). Compared to extraction by diluents alone, a significant improvement was found in case of chemical extraction. For example using 1.831 kmol/m3 TBP, improvement in extraction achieved was 42.0 %, 42.2 % and 34.6 % when diluents used were n-heptane, toluene, and MIBK, respectively (Tables 3 and Table 4a to 4c). However, relatively low chemical extraction with TBP is observed when MIBK was used as diluents and this is attributed to large physical extraction owing to presence of proton accepting CO group. A similar trend was observed in the extraction of propionic acid and acrylic acid with TBP by Keshav et al.20,24 They concluded that inert diluents gives higher chemical extraction than butyl acetate when TBP was used as extractant.20 Formation of acid−TBP complex can be predicted by high distribution coefficients which also confirm higher chemical extraction compared to physical extraction. Significant enhancement in overall KD values is observed ranging from 0.026 to 1.241, 0.081 to 1.389, and 0.403 to 1.734 for nheptane, toluene, and MIBK, respectively. At 2.564 kmol/m3 TBP concentration, distribution coefficient is greater than one for all the diluents tested. Consequently, high complex formation and more extractions were achieved at high TBP concentration (Table 4a to 4c). Overall distribution coefficient follows the expected trends (TBP + MIBK) > (TBP + toluene)

Figure 4. Loading curves of extraction of pyruvic acid (0.05 kmol/m3 to 0.4 kmol/m3) using TBP (30 %) in different diluents: ◇, (TBP + nheptane); □, (TBP + toluene); Δ, (TBP + MIBK). Subscript: aq, aqueous phase. Solid line (−) indicates the best fit of experimental data.

Z=

KD × 100 (1 + KD)

(15)

The percent extraction (E %) is the ratio of organic phase acid concentration to total acid concentration (both aqueous and organic phase). In terms of KD, it can be expressed as 2714



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> (TBP + n-heptane) as Kdiluent decreases in the order of MIBK D > toluene > n-heptane. From the plot of log KD versus log [TBP]org, extractant constant (KE) and apparent reacting TBP molecules (m) were estimated from the intercept and slope, respectively (using eq 14). In Figure 3, straight lines with a unit slope were obtained and it was found that experimental results satisfied eq 14 and the assumption made in eq 12, for all the diluents examined. It is also observed that the acid associated with TBP is equal to the number of carboxylic groups (unity in present case). Similar behavior was observed in the previous study by Hano et al.21,27 Though the diluents contribution was duly considered in case of MIBK yet the solvation number of TBP observed was slightly below unity. This little deviation is due to high Kdiluent D for MIBK and also due to assumption made in eq 12. As MIBK is an active ketonic diluents, it has significant physical extraction (E % = 22.6) compared to inactive diluents toluene and nheptane, having negligible physical extraction of around 6.1 and 2.2 %, respectively (Table 3, Figure 1). Summary of the results and comparison with previous published similar works are presented in Table 5. Loading curves for pyruvic acid for 1.099 kmol/m3 TBP in all the three diluents used is presented in Figure 4. Values of loading ratio were found below 0.5 for all the diluents for entire concentration range of study. That indicates the formation of only 1:1, pyruvic acid:TBP complex. Also, higher loading was observed at high acid concentration for all extraction systems (Tables 4a and 4b and Figure 2).

Notes

The authors declare no competing financial interest.

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Subscripts

4. CONCLUSION The influence of diluents (n-heptane, toluene, and MIBK) and extractant (TBP; 0.36 k.mol m−3 to 0.56 k.mol m−3) on reactive extraction of pyruvic acid (0.1 kmol m−3 to 0.5 kmol m−3) was investigated. Physical extractions were found to be ineffective with very low distribution ratio and follows the order of MIBK > toluene > n-heptane. However, a significant improvement was noticed in case of chemical extraction with an observation that distribution coefficient (with all the diluents) increases with increase in TBP concentration. It was concluded that the solvation number was equal to the number of carboxylic group. However, equilibrium extraction constant depends on the diluent’s nature. Based on the chemical interaction between extractant and solute, the apparent reacting molecules of extractant was evaluated to be 0.90, 1.03, and 1.04 for MIBK, toluene, and nheptane, respectively. Only (1:1) complex formation between pyruvic acid and TBP was observed for all the diluents with loading ratio below 0.5 for the entire concentration range examined. By applying law of mass action, equilibrium complexation constants were estimated to be 0.369, 0.482, and 0.578 m3 kmol−1 for n-heptane, toluene, and MIBK, respectively. The influence of TBP was more profound for nonpolar diluents than polar one. The results obtained are in good agreement with previous similar studies (Table 5). The data obtained along with stoichiometry and equilibrium constant are useful for design of extractor to achieve specific objective.

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NOMENCLATURE KD = overall distribution coefficient of acid [−] Kdiluent = distribution coefficient of acid extracted by diluents D alone [−] E % = percent extraction [−] KE = extraction equilibrium constant [m3 kmol−1] [HA] = concentration of pyruvic acid [kmol m−1] [TBP]org = concentration of free TBP in organic phase [kmol m−3] [HA.(TBP)] = concentration of acid−TBP complex [kmol m−3] [HA]diluent = concentration of acid extracted by diluents org alone [kmol m−3] m = solvation number of the extractant (TBP) [−] Z = loading ratio [−] P = partition coefficient [−] D = dimerization constant [m3 kmol−1] TBP = tri-n-butyl phosphate [−] MIBK = methyl-iso-butyl ketone [−] TOPO = trioctylphosphineoxide [−] TOA = Tri-n-octylamine [−] ET = Dimroth-Reichardt parameter [k cal mol−1]

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aq = aqueous phase org = organic phase o = initial

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Extraction Equilibria of Pyruvic Acid Using Tri-n-butyl Phosphate

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