Reactive Extraction Results
The complex formation between acid and extractant molecules is the chief
difference between the reactive and solvent extractions.
Trioctylphosphine oxide, tributyl phosphate, and Cyanex 23 are the types
of organophosphorus extractants that have been broadly used to recover
carboxylic acids [5, 28, 33–36]. Organophosphorus extractants have
good chemical stability, they are effective in acid recovery and their
recycle is possible. Besides, they co-extract water in negligible
amounts. The phosphoryl group present in the organophosphorus extractant
and the carboxylic group of the carboxylic acid react with each other,
and so an acid-extractant complex forms.
Reactive extraction results are presented in terms of KDand E % in Table 3. Fig. 1a-b shows the change in KDand E % values versus the change in Ph3PO
concentration. The results in the presence of reactive extractant were
better compared to the conventional solvents alone. The physical
extraction efficiencies were 23, 21, 44, 54, and 48 % with oleyl
alcohol, dimethyl adipate, isobutanol, methyl isopropyl ketone, and
methyl ethyl ketone, respectively. Methyl isopropyl ketone and methyl
ethyl ketone showed the best performance in terms of extraction
efficiency, followed by isobutanol, dimethyl adipate, and oleyl alcohol.
As seen, inadequate extraction efficiencies were obtained in the case of
physical extraction. For the reactive extraction experiments,
Ph3PO was varied from 12 % to 44 % by volume in the
solvents to see its effect on the efficiency. Remarkably, in the
presence of Ph3PO, the efficiencies increased up to 61,
76, 86, 67, and 84 %, respectively. All the reactive extraction
efficiencies, even at the lowest TPPO amount, are quite higher than
those obtained in the physical extractions. These great
differences emphasize the importance of the reactive extractant. As seen
from Fig. 1 and Table 3 that KD values followed
identical sequences with E % values. It is clear from the reactive
extraction results that the acid-extractant complex was poorly solved by
oleyl alcohol and dimethyl adipate, and thus the use of them with
Ph3PO would not be suggested. Whereas, acid-extractant
complex was efficiently solved by the ketones and isobutanol, and
therefore the use of isobutanol, methyl isopropyl ketone, and methyl
ethyl ketone with Ph3PO would be suggested. Acid
recovery is highly dependent on the extractant concentration. However,
the extractants are expensive. For the sake of the economy, the
extractant amount should be optimized. Irrespective of cost, a further
increase in the Ph3PO amount can cause a third phase
formation if solvent amount would not be enough to solve the complex.
The solvent properties like dielectric constant, boiling point, density,
molecular weight, Dimroth-Reichardt ET parameter, and
dipole moment (µ) have been attempted to correlate with
KD [37]. ET parameter gives
information about the ionization power of the solvent. However, there
was no significant correlation found between KD and
them.
Z =\(\frac{{[HGA]}_{\text{org}}}{[Ph_{3}\text{PO}]}\)(4)
The loading factor (Z) is the ratio of organic phase acid concentration
to the organic phase extractant concentration. Basically, it is the
fraction that shows how many acid molecules are loaded on an extractant
molecule. The loading factors were calculated and written in Table 3. As
seen, the higher extractant concentrations were used in the solvents,
the less were the loading factor values, which can be expressed by the
definition of Z [37]. The loading factor values were calculated to
be less than 0.5, showing that there was no overload on the extractant
molecule. In other words, one extractant molecule reacted with one acid
molecule, showing 1:1 acid-extractant complex formation. The reaction
between carboxylic group and phosphoryl group is shown in Fig. 2. The
complexation constant KE gives information about the
type of the formed complex and estimated from the loading factors. If
loading factors are less than 0.5, complexation constant values are
calculated as in the below Eq. (5) [37, 38]:
\(\frac{Z}{1-Z}\) = KE [HGA]aq (5)
where KE is the experimental complexation constant of
1:1 acid-extractant complex formation, and it is found from the slope
when \(\frac{Z}{1-Z}\) is plotted versus [HGA]aq[39].
Mass Action Law Model explains the nature and type of the formed
complex. First, acid and extractant molecules interact with each other
at the interphase, then complexation occurs, and afterwards the formed
complex is solubilized in the solvent. Dipole-dipole interactions and
hydrogen bonding enable dissolution of the complex in the solvent.
Although according to the Mass Action Law model, activities of species
are proportional to the concentrations of species, a non-ideal behavior
can be observed for the equilibrium constant. The following equation
gives the relationship of distribution coefficient with Mass Action Law
model’s equilibrium constant [27, 37]:
Log KD = Log KE,MAL + s Log
[Ph3PO]org (6)
where KE,MAL is the equilibrium constant of the Mass
Action Law model, s is the solvation number.
[Ph3PO]org is the organic phase
Ph3PO concentration. KE,MAL might be
obtained from the intercept, when Log KD is plotted
against Log [Ph3PO]org. Higher
KE,MAL value means higher complexation. In this
circumstance, solvent would have strong ability to solve the complex and
the extractant would display high complexation potential with the acid
[27]. KE,MAL values were found to be dissimilar to
KE values.