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Annals of West University of Timisoara
Series of Chemistry 18 (1) (2009) 73-80
THE UTILIZATION OF AQUEOUS PEG(1550) -
NANO3 TWO-PHASE SYSTEM FOR THE EFFICIENT
EXTRACTION OF PB(II) WITH IODIDE
EXTRACTANTS *
SUMMARY
Laura BULGARIU a , D. Bulgariu b,c*
a Technical University Gheorghe Asachi of Iasi, Faculty of Chemical Engineering and
Environmental Protection, Department of Environmental Engineering and
Management, D. Mangeron, 71A, Iasi, 700050, ROMANIA
b
University ”Al. I. Cuza” of Iasi, Faculty of Geography and Geology, Department of
Geology and Geochemistry, Bd. Carol I, 20A, 700506, Iasi, ROMANIA
c
Romanian Academy, Filial of Iasi, Collective of Geography, Bd. Carol I, 18A,
700506, Iasi, ROMANIA
The extraction of Pb(II) ions was studied in aqueous PEG(1550) - NaNO3 twophase
systems, in absence and in presence of iodide extractants. The influence of several
experimental parameters: initial Pb(II) concentration, salt stock solution pH and iodide
ions concentration added in extraction systems was also investigated, in order to establish
the optimum conditions of extraction process. The experimental results have show that in
absence of suitable extracting agent, the Pb(II) extraction percent is low (< 15 %), and
the variation of salt stock solution pH does not change too much the extraction efficiency.
The addition of iodide ions determined a quantitative partition of Pb(II) into PEG-rich
phase. The Pb(II) extraction efficiency is very little affected on the acidity of salt stock
solution, but depends on the iodide concentration added in extraction system. On the basis
of experimental results it can be stated that the extraction process of Pb(II) with iodide
extractants, in considered aqueous two-phase system, involve successive elementary
equilibriums, strictly dependent by the experimental conditions.
Keywords: Pb(II); iodide extractants; extraction; aqueous two-phase system
* This paper was presented at “Academic Days” Conference, XI edition
Timişoara, 28-29 May 2009
73

INTRODUCTION
74
B ULGARIU L., B ULGARIU D.
The presence of Pb(II) in the environment has been serious concern, especially
with the rapid industrialization, which created new uses for lead. The acute toxicity of lead
to aquatic life and humans, and the stringent effluent standard to be met by industries as
specified regulatory organizations has necessitated the development of innovative and
economical methods for treating of lead-bearing waste waters [1].
The aqueous two-phase systems have recently been showed to be a clean and
economical alternative to the traditional extraction systems in industrial separation as well
as environmental remediation applications [2, 3]. Aqueous two-phase systems, are formed
when a water-soluble organic polymer (ex. polyethylene glycol, PEG) is mixed with certain
inorganic salt (such as: Na2SO4, (NH4)2SO4, Na2CO3, NaNO3, K2HPO4), in specific
concentration. Such aqueous two-phase systems are composed by two immiscible aqueous
phases: a superior one - rich in PEG, which has the same role as organic phases from
traditional extraction systems, and an inferior one - rich in inorganic salt.
Beside, some evident advantages, like low toxicity, low flammability and low
volatility, resulted by the fact that the organic solvents are not required for to prepare these
extraction systems, another important one is that the quantitative extraction of metal ions
can be obtained by using inexpensive water-soluble extracting agents, as halide or pseudohalide
ions [3-6]. Thus, the utilization of aqueous two-phase systems for the extraction of
metal ions, make that the risk of environmental contamination to be relatively low.
The extraction of metal ions in aqueous two-phase systems mainly depend on the
two factors: (i) the characteristics of aqueous two-phase systems, determined by the nature
and concentration of inorganic salt and by molecular mass and concentration of PEG, and
(ii) the stability and hydration degree of metallic species formed in extraction system [7-9].
If the optimum characteristics of aqueous two-phase systems (stability, clear interface, time
for phases separation, etc.) can be obtained by suitable selection of phase-forming
components (PEG and inorganic salt, the second factor required the utilization of adequate
extracting agent.
According to studies from literature [10-15], in aqueous PEG-based two-phase
systems some toxic metal ions (Hg(II), Cd(II), Bi(III), Cu(II), etc.) can be quantitatively
extracted into PEG-rich phase using halide or pseudo-halide ions as extracting agents. It has
also showed that the values of extraction percents are higher with the stability of metallic
species formed in extraction system is higher, and the number of extractants associated with
the metal ion depends on the acidity of salt stock solution [16, 17].
In this study, the extraction of Pb(II) ions was studied in aqueous PEG(1550) -
NaNO3 two-phase system, in absence and in presence of iodide extractants. The influence of

AQUEOUS PEG(1550) - NANO3 TWO-PHASE SYSTEM
several experimental parameters: initial Pb(II) concentration, salt stock solution pH, iodide
ions concentration added in extraction system, was also investigated in order to establish the
optimum conditions of extraction process. The experimental results show that the Pb(II)
extraction efficiency is very little affected on the acidity of salt stock solution, but depends on
the iodide concentration added in extraction system. It can be also stated that the extraction of
Pb(II) using iodide extractants, in considered aqueous two-phase system, occurs by successive
elementary equilibriums, strictly dependent by the experimental conditions.
MATERIALS AND METHODS
Polyethylene glycol, PEG(1550), was purchased from Serva Feimbiochema Gmb
& Co and was used as received. The inorganic salts: NaNO3 and NaI (from Aldrich) were
reagents of analytical degree and were used without further purifications. The 40 % (w/w)
of PEG(1550) and NaNO3 aqueous stock solutions were prepared by dissolving an
appropriate mass of solid chemicals in twice distilled water. The different pH values (1.15,
2.50, 4.65 and 7.05) of NaNO3 stock solution were obtained by adding small volumes of
HNO3 concentrated solution (Reactivul Bucharest), which was considered in the total
solution mass. The stock solution of 2000 µg Pb(II)/ml (∼ 0.01 mol/l) was prepared by the
dissolving of lead nitrate (from Reactivul Bucharest) in twice distilled water, followed by
the solution standardization [18]. The extracting agent solution (1 mol/l NaI) was obtained
by dissolving halide salt and diluting to known volume with salt stock solution.
For each experiment an aqueous two-phase system was prepared by mixing equal
volumes of PEG(1550) stock solution and NaNO3 stock solution, with different pH, in a glass
centrifuge tube. The pH values of NaNO3 stock solution was measured with a Radelkis OK-
271 pH/ion-meter, equipped with a combined glass electrode. Then 0.3 ml of Pb(II) stock
solution and (0.1 - 0.7 ml) of 1 mol/l NaI solution were added. The phases were disengaged by
10 min of centrifugation at 2000 rpm and were carefully separated. Equal volumes from each
phase were measured for the spectrophotometric analysis of Pb(II) with Arsenazo III. The
Pb(II) concentration was determined in duplicate (Digital Spectrophotometer S 104 D, 1-cm
glass cell, λ = 600 nm) [19], using a prepared calibration graph.
The Pb(II) extraction efficiency was evaluated using experimental extraction
percent values (E %), calculated with the relation:
where: is the distribution coefficient, defined as the ratio of Pb(II) concentration in the
PEG-rich phase to that in the salt-rich phase, and v is the volume ratio of the salt-rich phase
and PEG-rich phase.
(1)
75

RESULTS AND DISCUSSIONS
76
B ULGARIU L., B ULGARIU D.
In general terms, the formation of aqueous PEG-based two-phase systems is
determined by the competition for hydration between PEG and inorganic salt [20, 21]. The
addition of inorganic salt increase the dehydration degree of polymer chains, due to its
salting-out effect, and in consequence the separation of the two phases occurs. This is the
main reason for which at the preparation of aqueous two-phase systems inorganic salts with
high and negative hydration free Gibbs energy, such as Na2SO4 or (NH4)2SO4, are
preferred.
Unfortunately in case of Pb(II) extraction, such inorganic salts are not adequate. In
presence of sulphate ions, the precipitation of Pb(II) in the salt-rich phase occurs, according
with the reaction:
Pb 2+ + SO4 2- � PbSO4
(2)
and this will increase the time of extraction process and will decrease its efficiency [22].
On the basis of these considerations, the NaNO3 was chose as inorganic salt for the
preparation of aqueous two-phase system, and the concentration of phase-forming
components (PEG and NaNO3) was selected that the system to remain two-phases
throughout all experiments [23].
In absence of iodide ions, Pb(II) remains predominantly in the lower, salt-rich
phase, and the variation of salt stock solution pH not improve too much the extraction
efficiency. The experimental value of Pb(II) extraction percent in function of salt stock
solution pH are given in Table I.
Table I. The Pb(II) extraction percent values obtained in absence
of iodide in considered aqueous two-phase system
pH 1.15 2.50 4.65 7.05
E, % 14.43 11.62 10.02 6.96
This is means that the extraction does not occur by chemical interactions between
Pb(II) ions and donor of PEG from PEG- rich phase. The lower E % values can by
explained by the high negative hydration free Gibbs energy of Pb(II) (ΔGhydr. = -1760 kJ/
mol) [24], which make that Pb(II) to be incompatible with the low hydrated environment of
PEG-rich phase of aqueous two-phase system. Under these conditions, for to increase the
extraction efficiency the Pb(II) ions must be transformed into stable and low hydrated
species, using suitable extracting agent.
The use of iodide ions as inorganic extracting agent was investigated by following
the Pb(II) extraction efficiency as a function of several experimental parameters: initial

AQUEOUS PEG(1550) - NANO3 TWO-PHASE SYSTEM
lead(II) concentration, salt stock solution pH and iodide concentration added in extraction
system.
The influence of initial lead(II) concentration was studied over the range 20 - 65 μg
Pb(II) /ml (Figure. 1). As shown in Figure 1, the Pb(II) extracted into the PEG-rich phase at
constant iodide concentration (0.07 mol/l) increases with increasing initial metal ion
concentration.
[Pb(II)]- PEG-rich phase, µg/ml
60
50
40
30
20
10
20 30 40 50 60 70
Initial lead(II) concentration, µg/ml
Figure 1. The effect of initial
lead(II) concentration on
extraction in presence of
iodide ions (0.07 mol/l), salt
stock solution pH = 4.65,
temperature 19 °C
On the basis of these observations and taking into account the limits of the
spectrophotometric method used for Pb(II) analysis, an initial concentration of 63.7 μg
Pb(II)/ml was selected for the further distribution studies.
Within the pH range 1-7, the phase forming components (PEG, Na+, NO3-) are
essentially uninvolved in secondary processes [23], indicating that stock salt solution pH
has little effect on phase formation and separation.
Experimental results (Figure 2) show that Pb(II) extraction efficiency is very little
affected by variation in stock salt solution pH; the extraction percentages are high (92- 98
%) for all pH values.
E, %
120
110
100
90
80
70
60
0 2 4
Salt stock solution pH
6 8
Figure 2. The effect of salt
stock solution pH on Pb(II)
extraction in the presence of
iodide ions (0.07 mol/l). The
initial Pb(II) concentration was
63.7 µg/ml, temperature=19°C
77

78
B ULGARIU L., B ULGARIU D.
In general, a decrease of salt stock solution pH, increases the PEG-rich phase
hydrophobicity and enhances metal ion extraction [21]. However, for Pb(II) extraction, in
considered aqueous two-phase system this tendency is not evident due to the high stability
of the lead-iodide species formed in extraction system. Thus, the extraction efficiency does
not increase in systems with higher acidity. In addition, the high Pb(II) extraction efficiency
at pH = 7.05 suggests that the protonation of PEG ether oxygen atoms is not necessary.
The use of iodide ions as inorganic extracting agent was investigated by studying
the extraction of Pb(II) as function of NaI concentration added in extraction system. A
comparison for all four media studied is given in Figure 3.
It can be observed from Figure 3 that in these systems Pb(II) is quantitatively
extracted for all four pH values. Maximum extraction occurs at lower iodide concentration
when the acidity is high. Thus, at pH = 1.15 and 2.05, the maximum extraction is obtained
at 0.04 mol iodide/l; for pH=4.65, the iodide necessary is 0.05 mol/l; and for the systems
with pH=7.05 it is higher yet (0.06 mol/l). This behaviour is attributed to the increased
PEG-rich phase hydrophobicity with decreased pH, which suggests that equilibriums
forming lead-iodide species are involved.
On the basis of these experimental observations, we can assume that the extraction
process of lead(II) with iodide, in considered aqueous two-phase system, involve a
succession of elementary equilibriums, which are strict dependent by the experimental
conditions (Figure 4).
E, %
100
80
60
40
20
0
pH=1.15
pH=2.50
pH=4.65
pH=7.05
0 0.02 0.04
NaI, mol/l
0.06 0. 08
Figure 3. The dependence of
Pb(II) extraction percent on
the NaI concentration added
in extraction system. The
initial Pb(II) concentration was
63.7 µg/ml, temperature=19°C
In considered aqueous two-phase system, the anionic species formed at the
interface cross into the PEG-rich phase, where they will interact, predominantly by ionic
forces, with PEG ether oxygen atoms. Electroneutrality of the two aqueous phases is
ensured by ionic transfer. Thus, during extraction of Pb(II) anionic complexes, the nitrate
ions which accompanied phase separation are expelled into the salt-rich phase and the
phases are kept electrically neutral.

AQUEOUS PEG(1550) - NANO3 TWO-PHASE SYSTEM
Figure 4. Schematic representation of elementary equilibriums involved in extraction process of Pb(II)
with iodide, in aqueous PEG(1550) - NaNO3 two-phase system
CONCLUSION
The extraction of Pb(II) ions in aqueous PEG(1550) - NaNO3 two-phase systems
was investigated as a function of initial lead(II) concentration, salt stock solution pH and
iodide ions concentration added in extraction system.
In absence of suitable extractants, the Pb(II) prefer to remain in the salt-rich phase
of extraction system and a poor extraction was observed for all four studied pH values of
salt stock solution.
The addition of iodide ions in the 0.01 - 0.07 mol/l concentration range determined
an enhancement of extraction efficiency, and Pb(II) is quantitatively extracted for all four
pH values. Maximum extraction is obtained at lower iodide concentration when the acidity
of salt stock solution is high. Thus, at pH = 1.15 and 2.05, the maximum extraction is
obtained at 0.04 mol iodide/l; for pH=4.65, the iodide necessary is 0.05 mol/l; and for the
systems with pH=7.05 it is higher yet (0.06 mol/l). This behaviour is attributed to the
increased PEG-rich phase hydrophobicity with decreased pH, which suggests that
equilibriums forming lead-iodide species are involved.
On the basis of the experimental results, it can be stated that the extraction process
of Pb(II) with iodide extractants, in considered aqueous two-phase system, involve a
succession of elementary equilibriums, which are strictly dependent by the experimental
conditions.
ACKNOWLEDGEMENTS
This research occurred by PNCDI 2 – D5 no. 51045-2007 for which the authors
acknowledge financing authority.
79

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