(242e) Extraction and Back-Extraction Studies of Picolinic Acid Using Tri-n-Octylamine Dissolved in 1-Decanol

Extraction and
Back-Extraction Studies of Picolinic Acid Using Tri-n-Octylamine
Dissolved in 1-Decanol

Dipaloy
Datta¹ and Sushil Kumar²

Department
of Chemical Engineering

Birla
Institute of Technology and Science (BITS), PILANI ? 333031 (Rajasthan), INDIA

¹
E-mail: dipaloy@bits-pilani.ac.in;
dipaloy@gmail.com

Homepage:
http://universe.bits-pilani.ac.in/pilani/dipaloy/profile

²E-mail: skumar@bits-pilani.ac.in; sushilk2006@gmail.com

Phone : +91-1596-245073 Ext 215; Fax: +91-1596-244183

Homepage:
http://universe.bits-pilani.ac.in/pilani/skumar/profile

Abstract

The chemical
industry has come under increasing pressure to make chemical production more
eco-friendly due to its reliance on fossil resources, its environmentally
damaging production processes and its toxic byproducts and waste. Within this
framework, bio-based chemistry and biotechnologies offer great prospects. Microbial
production of organic acids is a promising approach for obtaining
building-block chemicals from renewable carbon sources (Hatti-Kaul et al.,
2007). Picolinic acid acts as a bidentate ligand with two active groups such as
a carboxyl group (-COOH) and a pyridinic nitrogen atom (-N). The acid is well
known for its efficient chelating activity and can chelate metals like Cu, Fe,
Ni, Zn, Cd, Pb, Mn, Cr, and Mo in the human body (Suzuki
et al., 1957). 3-hydroxyanthranilic
acid oxygenase enzyme can oxidize 3-hydroxyanthranilic acid by the catabolism
of tryptophan through kynurenine to picolinic acid (Smith et al., 2007). A major drawback of fermentation is low acid product yield and
concentration which leads to the difficulty in recovery of the product from the
very dilute solution. To improve the biological production of picolinic
acid, it is necessary to develop a cheap and environment friendly recovery
method (Wang et al., 2005). Among the several separation processes,  reactive
extraction using a suitable extractant has been found to be a most promising
alternative to recover carboxylic acids from the fermentation broth and dilute aqueous
solution. Tertiary amines especially tri-n-octylamine (TOA) are found to
be effective extractant to recover different carboxylic acids by reactive
extraction (Kertes and King, 1986; Tamada et al., 1990;
Datta and Kumar, 2011).

Literature is widely available on the reactive extraction of
different carboxylic acids from aqueous streams, but work on reactive
extraction and back-extraction of picolinic acid is limited. Therefore, the present
work is aimed to intensify the recovery of picolinic acid (0.01
to 0.05 kmol×m^-3) using reactive extraction with TOA (= 0 to 0.344 kmol×m^-3)
using 1-decanol as active (polar) diluent. The experimental data are analyzed
by calculating distribution coefficient (K_D = C_org/C_aq),
degrees of extraction [E = K_D / (1 + K_D)]
and loading ratios (Z =). In
the experiment, equal volumes (20 ml) of organic and aqueous phases are
equilibrated by shaking for 8 h at 298 K in a constant temperature water bath
(Remi Labs, HS 250, India). After reaching equilibrium, the phases are allowed
to settle for 2 h to have a clear separation of the phases. The aqueous phase
is sampled by a pipette. Picolinic acid concentration in the aqueous phase (C_aq)
is measured by titration using NaOH solution of 0.01 N. Acid concentration in
the organic phase (C_org) is calculated by a mass balance. The
initial and equilibrium pH of aqueous solution are measured using a digital
pH-meter (ArmField Instruments, PCT-40, UK). The back-extraction of picolinic
acid is carried out by pure water (temperature swing regeneration) at 353 K with
1:4 volume ratio of phases.

The equilibrium isotherms are drawn between organic and aqueous
phase concentrations of picolinic acid at four initial acid concentrations and
three concentrations of TOA in Figure 1. It is observed that the
distribution coefficients ( << 1) of picolinic acid in 1-decanol alone are not
sufficiently high. Also the degree of extraction is very low. Physical
extraction of picolinic acid with 1-decanol has been found unsuitable. The
hydrophilic nature of picolinic acid [dipole moment, μ = 4.42 D (Kulkarni
et al., 1978); log P = -0.97 (Leo et al., 1971)] makes it poorly
extractable by common organic solvents. Therefore, the equilibrium chemical
extraction experiments for the recovery of picolinic acid are also carried out
with TOA (0.115 - 0.344 kmol·m^-3) dissolved
in 1-decanol and isotherms are shown in Figure 1. Improved extraction
efficiency in terms of K_D is observed when an extractant is
used with diluent as compared to that of diluent alone. The maximum value of K_D
in the chemical extraction is found to be 7 with TOA (0.344 kmol·m^-3)
at 0.01 kmol·m^-3 of picolinic acid initial
concentration. The distribution coefficients are found to be higher at lower
concentration of acid (0.01 kmol·m^-3). It
can be observed that at a fixed concentration of picolinic acid, the
distribution coefficient increases with an increase in the TOA (0.115 to 0.344
kmol·m^-3) concentration, whereas upon
varying the acid concentration for a fixed extractant concentration, K_D
values decrease (Figure 2). The maximum removal of picolinic acid is 87.5% with
TOA (0.344 kmol·m^-3) in 1-decanol at 0.01 kmol·m^-3 of picolinic acid initial concentration.
The distribution coefficient increases from 3 to 7 when the amount of TOA is
increased from 0.115 to 0.344 kmol·m^-3. The effect of loading ratio (Z) on the extraction efficiency is
shown in Figure 3. It can be seen that loading ratio decreases with increasing
concentration of TOA at a fixed concentration (0.01 kmol·m^-3)
of acid. The same trend is observed for other concentrations of acid. Also, at
higher acid concentration, the organic phase is more loaded with the acid
molecule compared to low acid concentration. The extraction mechanism of
picolinic acid also depends on the pH and the pK_a of acid. In
the present study, the values of equilibrium pH are found to be in the range of
3.55 to 4.11 which is in between the pK_a^'s
of the acid (pK_a1 = 1.01, pK_a2 = 5.29;
John, 1972).

Figure
1. Equilibrium isotherms of picolinic acid (0.01 to
0.05 kmol·m^-3) for different concentrations of TOA (0.115 to
0.344 kmol·m^-3) dissolved in
1-decanol

Figure
2. Effect of initial acid and extractant concentration on K_D

Figure
3. Effect of initial acid and extractant concentration on Z

An equation is
derived using mass action law to relate K_D with m, n
and equilibrium constant (K_E) of equilibrium reaction as:

                                                            (1)

The values of m, n and K_E are
estimated by optimizing the error between the experimental and predicted values
of K_D using Eq. (1). The estimated values of m are
found to be near about one with TOA imply that there are mainly formation of
(1:1) acid-TOA complex in the organic phase (Table 1). The higher values of K_E
are found at lower extractant concentration which shows faster mass transfer of
the solute into the organic phase.

Table
1. Values of m, n and K_E

In the back-extraction
of picolinic acid (Figure 4), it can be seen that with an increase in the
concentration of TOA, the slope of the isotherm decreases i.e. distribution
coefficient of back-extraction (K_D′ = C_aq/C_org)of picolinic acid is reduced. Though, at higher concentration of TOA may
provide higher extraction of acid but would make the regeneration process
difficult. The regeneration of the extractant loading with high concentration
of acid (C_in = 0.05 kmol·m^-3)
will be easier (Z = 0.297 at  =
0.115 kmol·m^-3) and higher distribution of
acid (K_D′ = 4.037) can be achieved. Less loading of the
extractant with the acid (Z = 0.122 at = 0.344
kmol·m^-3) results in lower distribution of
acid (K_D′ = 0.714) and incomplete regeneration of the
extracting agent.

Figure
4. Back-extraction isothermal curve of picolinic acid using TOA in 1-decanol

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