Partition Studies in Biphasic Systems of Nitrophenols
IOANA DIACONU*, ELENA RUSE, EUGENIA EFTIMIE TOTU, GHEORGHE NECHIFOR
University Politehnica Bucharest, Faculty of Applied Chemistry and Material Science, Department of Analytical Chemistry and
Instrumental Analysis, 1-3 Polizu, 011061, Bucharest , Romania
The paper presents experimental data obtained in the study of liquid-liquid partition equilibria in biphasic
system in order to optimize the process of extraction of nitrophenols. The partition equilibria of some
nitrophenols using chloroform as extraction solvent were studied. The influence of the pH on the partition
equilibria was investigated. The extraction diagrams were realized and the repartition coefficient and the
pKa values of the studied nitrophenols were calculated in the system water-chloroform. The studied
compounds were: p-nitrophenol, o-nitrophenol and, 2,4-dinitrophenol. In optimum extraction conditions
(pH=2 for the aqueous phase) extraction efficiencies of 62-65% were obtained. The analytical control of the
process was realized using molecular absorption in the range ultraviolet-visible at their characteristic
wavelengths.
Keywords: nitrophenols, extraction, partition coefficients
In the last years a special attention was given to the
presence of nitroaromatic compounds in the environment
[1-4]. Nitrophenols are contaminants frequently
encountered in the environment as herbicides or
degradation products of some organophosphorus
insecticides [5-6]. Nitrophenols are encountered in
wastewater from industries such as: pharmaceuticals aspirin substitute acetaminophen-, the manufacture of
pesticides such as parathion, paraoxon and methyl
parathion, explosives, solvents and intermediates of dyes
[7-10]. A review related to this point of view has been
published [11].
Nitrophenols have a high toxic potential level with strong
effects on health of the human beings, animals and plants.
For example long exposure to high concentrations of paranitrophenol can cause blood disorder, kidney and liver
damage. It can also cause skin and eye irritation [12].
Nitrophenols are encountered in surface water as well as
in groundwater or in deep soils.
Due to the economic and environmental impact it is
important to separate and develop efficient separation
methods for nitrophenols.
In order to realize the removal and recovery of phenols
from the environment various researches were realized.
For the treatment of wastewaters containing nitrophenols
two main procedures were used: destructive procedures
such as biodegradation, thermal decomposition and
oxidation [13]; and recuperative procedures such as
adsorption, liquid membrane separation [14-16], liquid–
liquid extraction [18-19].
From the procedures mentioned methods solvent
extraction and membranary methods are two modalities
that are used to separate compounds in a mixture, being
intensively promoted. The solvent extraction permits
obtaining of efficient separations through simple
experimental procedures. Consequently solvent extraction
was the topic of several researches aimed to improve the
experimental techniques. Improvement of the solvent
extraction methods permitted the enrichment of the
separation techniques with new techniques using the
transport through membranes [20-22]. In the field of liquidliquid extraction it can be pointed out the separation of
orto-nitrophenol from para-nitrophenol in a triphasic
extraction system [19]. The triphasic system in this case
is formed from polyethylene glycol with different molecular
weight and simulated wastewater-containing isomers of
para-nitrophenol and orto-nitrophenol. The results showed
a great effect of the solution pH, extractant type on the
partition coefficient of the two isomers. At pH = 4.00, about
85% orto-nitrophenol and 90% para-nitrophenol partitioned
into the top organic phase and middle polymer-rich phase,
respectively. The separation factor decreased sharply with
the increase of the pH solution from 6.5 to 8.5. The
separation procedure based on liquid emulsion
membranes [23-25] and supported liquid membranes
techniques were applied [26-28].
The main objective of this study is to determine if the
liquid-liquid extraction can be used as nitrophenol
recuperative process. The extraction solvent used was
chloroform based on the excellent results obtained for
transport through bulk liquid membranes with chloroform
membrane [14]. The liquid-liquid extraction, characterized
by easiness to operate and convenience to scale up, has
received much attention in recent years [19].
Experimental part
Reagents
All the reagents used are of analytical grade. pnitrophenol, o-nitrophenol, 2,4-dinitrophenol were supplied
by Merck (Germany). Chloroform, previously saturated with
distilled water, is used as an organic solvent in the partition
equilibria was purchased from Merck (Germany).
Hydrochloric acid and sodium hydroxide were used for the
variation of the pH between 2 and 12. The distilled water
used in this study was also saturated with chloroform.
Apparatus
The pH was measured with a glass/AgCl, Ag combined
electrode using SevenMulti Metler Toledo pH-meter. The
nitrophenols content from the aqueous phases and in
solvent, respectively, was determined by molecular
spectrofotometry using a LAMBDA 750 spectrophotometer
(Perkin Elmer Co).
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Procedures
Liquid-liquid extraction experiments were carried out in
the range pH 2-12. The pH variation was obtained by adding
to a solution on nitrophenol (10-3 mol/L) in HCl 0.01 mol/L,
a solution with the same concentration of nitrophenol in
NaOH 0.01 mol/L. 10 mL of nitrophenol at different pH
values were extracted with the same volume of chloroform.
After an agitation of 15 min the two phases were let to rest
for 5-10 min and then were separated. All the experiments
were carried out at 25±10C, the room being climatizated.
The content of nitrophenol was analyzed by molecular
spectrometry in the ultraviolet-visible region, at the
characteristic wavelength for each compound: in aqueous
solutions p-nitrophenol at 317 nm (acid form) and 404 nm
(phenolate form), o-nitrophenol at 363nm (acid form) and
417nm (phenolate form), while 2,4 dinitrophenol at 358
nm (acid form) and 361 nm (phenolate form). In organic
phase, chloroform solutions, where there are present only
the acid form of the nitrophenol the characteristic
wavelength are: p-nitrophenol at 312 nm, o-nitrophenol at
321 nm and 2,4-dinitrophenol at 259 nm.
Results and discussions
Partition equilibria in biphasic system were studied in
order to determine the optimum conditions of the extraction
of the nitrophenols from aqueous solution in the organic
solvent, chloroform.
Nitrophenols (Ar-OH) are organic compounds with acid
character that distribute in a liquid-liquid biphasic system
according to the partition equilibrium:
Table 1
THE EXTRACTION PARAMETERS DETERMINED THROUGH LIQUIDLIQUID EXTRACTION IN THE SYSTEM WATER-CHLOROFORM
The equilibria mentioned above are summarized in
figure 1.
Fig. 1. Scheme of the partition equilibria in biphasic system
Accordingly, the results indicate that the pH of the
aqueous phase is an operational parameter that can
influence the repartition of the nitrophenols as shown by
the experimental data presented in figure 2.
(1)
where:
w = aqueous phase;
s = solvent
depending on how much its repartition coefficient R
permits:
(2)
In the aqueous phase the equilibrium with proton transfer
is established:
(3)
Equilibrium (3), controlled by pH, is characterized by the
acid dissociation constant Ka:
(4)
At pH < pKa Ar-OH molecular species extractable in
the organic phase predominates, while at pH > pKa Ar-Ohydrophilic ionic species, non-extractable predominates.
For a global quantitative assessment of the process the
distribution coefficient r is defined as:
Fig. 2. Extraction curve of nitrophenols in chloroform
Extrapolating the extraction graphs, the nitrophenol
repartition coefficients were determined in waterchloroform system and are presented in table 1. The pKa
values determined in biphasic system differ from the values
presented in literature for homogeneous systems [29]. In
the table there are presented also the acidity constants in
water and in water-chloroform system. As can be seen the
equilibrium (3) shifts to left, because of Ar-OH extraction
in organic phase. If the organic phase volume is equal to
aqueous phase one, the acidity constant of the couple ArOH/Ar-O- in biphasic system will be:
(7)
or:
(5)
and so:
(8)
(6)
From the extraction diagram of the three nitrophenols
some useful observation can be made regarding the
possibility of separation the monosubstitute nitrophenols
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Table 2
DISTRIBUTION OF
NITROPHENOLS IN BI AND
TRIPHASIC SYSTEM
(p-nitrophenol and o-nitrophenol) from the disubstituted
one, 2,4- dinitrophenol respectively. At pH < 4 all the three
nitrophenols are extractible in chloroform; at pH > 7 all
the three nitrophenols are in the aqueous phase in the form
of phenolates (Ar-O-). A possible separation can take place
at the pH 5.5 - 6 when the 2, 4-dinitrophenol is the aqueous
phase as phenolate (Ar-O-) while the monosubstituted
derivatives (p-nitrophenol and o-nitrophenol) are mainly in
the organic phase.
In table 2 it is realized a quantitative assay of the partition
of studied nitrophenols in biphasic system relatively to the
triphasic system, presented in a previous paper [14, 15].
It can be observed from table 2 that the extraction
efficiency obtained in optimum partition conditions in the
biphasic system water-chloroform (pH = 2 for the aqueous
phase) range between 60-65%.
The triphasic system that we are referring to is
characteristic for the bulk liquid membranes and it is
formed from a feed source with acid pH (nitrophenol
solution with pH = 2), a chloroform membrane and a
receiving phase (alkaline solution with pH = 12). In the
triphasic system the studied nitrophenols are transferred
from a feed source through a chloroform membrane into
an alkaline receiving phase with efficiencies higher than
90%.
Conclusions
The paper presents the results of the partition of
nitrophenols in biphasic system. The repartition data in
biphasic system allowed the determination of some
analytical constants in the water-chloroform system. The
extraction curves obtained from experimental data
provided the possibility of separating the mononitrophenols
(o-nitrophenol and p-nitrophenol) from 2,4-dinitrophenol.
When comparing the partition in biphasic system with
the triphasic system it clearly shows that the efficiencies
of the separation of the studied nitrophenols in triphasic
system are higher. Thus in triphasic system the studied
nitrophenols have been transferred from a feed source with
pH = 2 through a chloroform membrane into a receiving
phase with pH = 12 with efficiencies higher than 90%.
Aknowlegement: The work has been funded by the Sectorial
Operational Programme Human Resources Development 20072013 of the Romanian Ministry of Labour, Family and Social
Protection through the Financial Agreement POSDRU/6/1.5/S/19.
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