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Sustain. Environ. Res., 25(3), 171-176 (2015)
171
Technical Note
Preconcentration and determination of copper(II) and silver(I)
onto polyurethane foam functionalized with salycilate
Mohamed M. El Bouraie*
Central Laboratory for Environmental Quality Monitoring
National Water Research Centre
El-Qanater El-Khairiya 13621, Egypt
Key Words: Polyurethane foam, salicylate, sorbent, functionalized, preconcentration
ABSTRACT
Polyurethane foam was chemically functionalized with salicylate through -N=N- group generating
a stable chelating sorbent (PUFS) to adsorb copper(II) and silver(I). The synthesized sorbent was
characterized by Infrared Spectrometry measurement. Good stability towards various solvents was
noticed. The effect of pH and equilibration shaking time was studied for metal adsorption onto
functionalized foam. Extraction of copper and silver were accomplished in 25 and 19 min, respectively.
Cu and Ag at ppm level were absorbed as the salicylate complex on powered PUFS at pH about 7.0 and
8.0. Copper and silver extraction efficiency could be achieved at 86 and 82% from 500 mL Cu and Ag
solutions (1.72 and 0.65 mol mg-1, respectively) which shows the suitability of salycilate foam for preconcentration analysis.
INTRODUCTION
Polyurethane foam (PUF) is one of the most
important synthetic polymers, and it is synthesized
through a polyaddition reaction between a
polyisocyanate (a polymeric molecule with two or
more isocyanate groups, such as toluene diisocyanate
and methylene diphenyl diisocyanate) and a polyol (a
polymer with two or more reactive hydroxyl groups,
such as polyethylene adipate and poly(tetramethylene
ether)glycol). Both the polyisocyanates and the polyols
are currently derived from petroleum oil [1].
Polyurethane is a material with excellent
hydrodynamic characteristics and has been widely
exploited as a solid phase for extraction and preconcentration of inorganic and organic species from
different media by conventional methods. It can be
used either without pretreatment (unloaded PUF) or
as a solid support for organic reagents (loaded PUF).
While unloaded PUF can be used for the sorption of
more than 50 metals, with some restriction loaded
PUF with organic reagents provides the possibility of
*Corresponding author
Email: [email protected]
modifying it to improve their selectivity and sorption
proprieties [2,3]. Although the excellent proprieties
for preconcentration and separation of loaded PUF
for metal ions, the leaching of the loaded chelating
reagent during the extraction, limits the application
and will influence the extraction yield concerning the
reutilization and reproducibility, affecting the final
results [2]. The preparation of PUF using the chelating
reagent is a good alternative to avoid leaching.
Polyurethanes present terminal toluidine groups in its
structure, which make possible diazotization and azo
coupling reactions [4]. In the work described in this
paper PUF was functionalized with salicylate through
an -N=N- group in order to eliminate the problem of
ligand leaching. The sorption behavior of copper and
silver on to PUFSalicylate was studied to optimize the
best conditions for its removal and preconcentration
from water.
MATERIALS AND METHODS
1.Reagents and Materials
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El Bouraie, Sustain. Environ. Res., 25(3), 171-176 (2015)
All the chemicals were of analytical reagent grade
and used as received. All solutions were prepared
from CuCo3 (Ksp = 2.5 x 10-10) and AgCl (Ksp = 1.8
x 10-10) stock solution containing 1000 ppm (SigmaAldrich). 0.1 and 6 M HCl (Malinckrodt AR), 0.5 M
sodium nitrite (Merck) and 1% w/v sodium salicylate
(Merck) in 0.2 M sodium carbonate (Merck) solutions
by dissolving the appropriate amount were used in the
PUF functionalization [5]. Deionized water was used
throughout.
Commercial white of open-cell polyether (nominal
density 23 kg m-3) PUF was used. The PUF was cut
into small particles (less than 1 mm) in a blender with
doubly distilled water and purified by treating with a
large quantity of 0.1 M HCl for 30 min, then washed
with water up to distilled water pH, followed by
acetone (Merck), dried in air and then stored in darkglass jar.
A VKS-75 mechanical shaker throughout
operated at 120 rpm was used. All measurements were
performed using either a UV/Vis 918 and UV/Vis 911A
GBC spectrophotometer, a FTLA 2000 ABB Infrared
Spectrometer, a WTW Checker pH meter and a Rigaku,
model B3, wavelength dispersive X-Ray Fluorescence
Spectrometer, with a molybdenum anode X-ray tube
operated at 40 kV and 30 mA, with a LiF analyzer
crystal and a NaI-Tl scintillation detector [6,7]. The pH
of each solution was adjusted with NaOH or HCl. The
bottles are closed with lids and were agitated at 30 °C
by orbital shaker at fixed speed, 160 rpm for various
time intervals. The adsorbates were separated using
Whattman filter paper and filtrate was analyzed for
residual concentration of the metals by converting them
into colored complexes. Triplicate runs differing by
less than 1% of all the tests were achieved assuring the
reproducibility of the obtained data [8].
1.1. PUF structure and properties
The majority of the polymers manufactured in
industry have a fairly simple chemical structure since
they are synthesized from one or two monomers,
therefore leading to the formation of homopolymers
or copolymers. Examples of these polymers are
poly(styrene), poly(ethylene), poly(propylene),
poly(butadiene), etc. On the other hand, polyurethanes
possess more complex chemical structures that
typically comprise three monomers: a diisocyanate, a
macroglycol (which is an oligomeric macromonomer)
and a chain extender. Accordingly, the nature of the
polyurethane composition implies a wide diversity
of surface characteristics, which in turn, are of prime
importance when dealing with an eventual use of
PUFs. Basically, PUF is a sorbent that has been used
quite frequently in recent years for the determination of
trace amounts of different components. The structural
form of PUF allows the easy use of this sorbent in
automatic and on-line pre-concentration systems. In
this context, it has advantages over other sorbents, such
as active carbon, alumina and silica. PUF has attained
considerable attention because it is one of the most
interesting materials that have excellent hydrodynamic
and chemical properties making it a promising sorbent
in the area of solid phase extraction. This is also due to
the wide variety in chelating reagents available to react
chemically with the chemical groups on the surface of
PUF [9].
1.2. Advantages of PUFSalicylate as sorbent
The foam material has several advantages
over other solid phase sorbents. Among those, it is
commercially available, easy to prepare and handle.
PUF is an excellent sorbent material due to high
available surface area, cellular structure and extremely
low cost. In addition, it is stable in acids (except
concentrated nitric and sulfuric acids), bases and
organic solvents and also, it will not change its structure
when heated up to about 180 °C. Moreover, the PUFs
have been used in column techniques in off-line or flow
injection preconcentration system; it is advantageous
because it shows low resistance to passage of fluids
and does not show any overpressure nor swelling as
commonly occur when using other sorbents [10].
1.3. Synthesis of PUF functionalized with salicylate
(PUFSalicylate)
The purified blended PUF (5 g) was soaked in 200
mL 6 M HCl, stirred for 1 h to hydrolyze the terminal
urethane and isocyanate groups, then the HCl was
removed and 100 mL of 0.1 M HCl was added and
cooled in ice (5 °C). Sodium nitrite (0.5 M, 50 mL)
was then added drop by drop under vigorous stirring.
Sodium salicylate (50 mL) solution then was added and
left for overnight in the fridge as shown in Fig. 1. The
PUFSalicylate formed was washed with 0.1 M HCl,
followed by distilled water, then acetone, dried at room
temperature and stored in dark-glass jar [11].
2. General Procedure
2.1. Characterization of the PUFSalicylate
Terminal urethane (-NHCOO-) and isocyanate
(-NCO) groups in white open-cell polyether-type PUF
(density ≈ 23 kg m-3) were readily hydrolyzed with 6 M
HCl producing amino groups (-NH) distributed on the
foam [4]. The PUFSalicylate was then synthesized by
diazotization of the white polyurethane foam by use of
sodium nitrite then coupling with salicylate (C7H5O3;
MW 137 g mol-1). Results from UV/Vis and Infrared
El Bouraie, Sustain. Environ. Res., 25(3), 171-176 (2015)
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Fig. 1. Schematic representation of the synthesis and
structure of polyurethane foam functionalized
with salycilate.
Spectrometry spectroscopy were studied to characterize
the PUFSalicylate.
2.2. Sorption investigation
The sorption of Cu(II) and Ag(I) was carried out
by a batch technique at 20 °C. A mass of PUFSalycilate
foam was mixed with an aliquot of the copper and
silver solutions in a shaker at the desired time, then the
foam and the solution were separated under vacuum
through a filter paper (diameter 25 mm), washed and
the copper and silver concentrations were determined.
The copper and silver concentrations were
determined spectrophotometrically by X-Ray
Fluorescence [6,7]. The following equations were
used to calculate the distribution coefficient (Kd) and
percentage uptake (E):
(1)
%E =
(Co − C )100
Co
(2)
Where Co and C are respectively the initial and
final copper and silver concentrations, V is the volume
of solution, and w is the weight of PUFSalicylate used.
PUFSalicylate were soaked in copper and silver
solutions overnight. The PUFSalicylate was characterized by diffused reflectance spectroscopy. The
adsorbent PUFSalicylate particle size was magnified by
Scanning Electron Microscope (SEM) studies by using
JEOL 30-kV apparatus as shown in Fig. 2.
Fig. 2. SEM of PUFSalycilate coated with Cu (a, c and e)
and PUFSalycilate coated with Ag (b, d and f).
RESULTS AND DISCUSSION
1.Characterization of the PUFSalycilate
1.1.Effect of different solvents on washing out the
salycilate
The PUFSalycilate was tested with different
solvents in order to study the leaching of
salycilate from PUFSalicylate. 100 mg (± 1 mg)
of the PUFSalycilate was mixed with 25 mL of
the appropriated solvent in a shaker. After 1 h the
solvent was separated from PUFSalycilate and
measured spectrophotometrically. The salycilate was
not detected in the presence of ethanol, methanol,
isopropyl alcohol, acetone diethyl ether and 1-6 M of
HCl and NaOH, showing that the PUFSalycilate has
good chemical stability [12].
1.2.PUF and PUFSalycilate infrared spectra
Infrared spectra of the PUF and PUFSalycilate
were studied using the potassium bromide technique.
The results obtained show that in the PUFSalycilate
spectrum there was some modifications relative to PUF
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El Bouraie, Sustain. Environ. Res., 25(3), 171-176 (2015)
spectrum. There was a shift in the -NH absorption band
of urethane group (-NHCOO-) of PUF from 3354 to
3366 cm-1 after coupling with salycilate as shown in
Fig. 3. Other additional bands appeared near 1665 and
930 cm-1 indicating the salycilate PUF bonding [13].
charge similar to that of the ions. The adsorption at
low pH values may be due to limited contribution of
chemical adsorption that is caused by the unpaired
electrons of nitrogen at acetamido and amino functional
groups of PUFSalicylats.
2. Sorption Investigation
2.2. PUFSalycilate copper and silver sorption capacity
determination
2.1. Effect of pH on the sorption of copper and silver
The effect of sorption of copper and silver onto
PUFSalycilate was examined at different pH using the
batch equilibrium technique. 60 mg (± 1 mg) of the
PUFSalycilate was mixed with an appropriated pH
with 25 mL aliquot of the copper and silver solutions
(10 mg L-1) in a shaker for 60 min, then concentrations
of copper and silver extracted were determined and
the pH values were plotted against Log Kd as shown in
Figs. 4a and 4b, respectively [11,12]. The percentage
removal of copper increased with increase in the pH
from pH 5 to 7, after which there was decrease in
the percentage removal of copper with the increase
in the pH up to 8. In the case of silver, the maximum
removal of metal occurred at pH 8 after which there
is a decrease showing that the removal of metals from
solution depends on the pH of the medium [14]. The
effect of pH on heavy metal adsorption from aqueous
solutions has been reported in [13]. In general, the
removal of heavy metal ions was pH dependent with
amount adsorbed depending on the adsorbent type,
metal ion and/or initial concentration of metal ions.
The result showed that maximum uptake Cu(II)
and Ag(I) from the solutions were at pH value 6 to
8 and 7 to 9, respectably. The effect of pH on metal
ion adsorption by PUFSalicylats is due to zero charge
potential of PUFSalicylats of pH 5. At low pH values
adsorption is low where surfaces have strong positive
The copper capacity was obtained for 10 mg
L -1 copper solution, pH 7 ± 0.5. The solution was
equilibrated with 100 mg (± 1 mg) PUFSalycilate by
shaking for 1 h. The PUFSalycilate foam was filtered
and washed with the appropriate pH adjusted wash
solution (pH 7 ± 0.5) to remove the copper. Then
the copper fixed onto the foam was eluted from the
PUFSalycilate with 0.1 M HCl and determined spectrophotometrically. The adsorption capacity for Cu(II)
ions at pH 7 ± 0.5 was found to be 1.72 mol mg-1.
Similarly the adsorption capacity determined from 10
mg L-1 silver solution at pH 8 ± 0.5 was found to be
0.65 mol mg-1.
These results showed that adsorption capacity
sequence was in the order Cu(II) > Ag(I), i.e., capacity
of the PUFSalicylate calculated for Cu(II) and Ag(I)
using the batch technique depends on ionic size, the
structure of the complexing agent, and steric hindrance.
The corresponding molar ratios (M:PUFSalicylate)
for these ions are 2:2.7 and 2:1 for Cu(II) and
Ag(I), respectively. When the sorption capacity of
PUFSalicylate was compared with Amberlite XAD-2
functionalized with different reagents [15], it was found
the capacity of PUFSalicylate was better or comparable
for most metal ions.
2.3. Effect of shaking time
In order to study the shaking time on the extraction
efficiency, copper was extracted from 25 mL copper
solutions (10 mg L -1; pH 8 ± 0.5) on 100 mg (± 1
mg) PUFSalycilate by batch extraction technique
at different time intervals at 20 °C. The time values
were plotted against copper extracted as shown in Fig.
5a. The time required for sorption equilibrium for a
maximum extraction was achieved in 25 min. Similarly,
the sorption equilibrium time for silver (10 mg L-1; pH
7 ± 0.5) was 19 min (Fig. 5b).
2.4. Preconcentration of copper and silver on
PUFSalycilate
Fig. 3. The FTIR spectrum of the effect Salycilate to the
polyurethane foam.
The performance of the PUFSalycilate foam
in the preconcentration of 0.32 mg of copper from
500 mL at pH (7 ± 0.5) was studied using 150 mg
(± 1 mg) of PUFSalycilate at 20 °C for 30 min. The
extraction efficiency of copper could be achieved at
80% which shows the suitability of salycilate foam for
El Bouraie, Sustain. Environ. Res., 25(3), 171-176 (2015)
175
preconcentration analysis. The preconcentration of 0.33
mg of silver from 500 mL, at pH (8 ± 0.5) was studied
using 150 mg (± 1 mg) of PUFSalycilate at 20 °C for
22 min. The extraction of silver could be achieved at
83% which shows the suitability of salycilate foam for
preconcentration analysis.
CONCLUSIONS
This work deals with the preparation and
characterization of a new solid phase based onto
commercial PUF functionalized with salycilate. The
experimental showed that the reagent is not washed
out from the PUFSalycilate foam. PUFSalycilate can
be used to extract and preconcentrate copper and silver
from diluted solution. The capacity for Cu(II) and
Ag(I) ions at pH (7 ± 0.5) and (8 ± 0.5) were found
to be 1.72 and 0.65 mol mg -1. Copper and silver at
ppm level can be determined by preconcentration
in the PUFSalycilate. The above qualities make the
PUFSalycilate a promising sorbent for the separation
and preconcentration of copper and silver from
different matrixes.
ACKNOWLEDGEMENTS
The author would like to thank the staff of
Central Laboratory for Environmental Quality
Fig. 5. Effect of the shaking time on sorption onto
PUFSalycilate foam for (a) copper and (b) silver.
Monitoring for their cooperation during measurements.
Acknowledgement is also to the X-ray and the Infrared
Laboratory of Central Laboratory for Tanta University,
for providing every facility to accomplish the work.
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Fig. 4. Effect of pH on sorption onto PUFSalycilate foam
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Discussions of this paper may appear in the discussion section of a future issue. All discussions should
be submitted to the Editor-in-Chief within six months
of publication.
Manuscript Received: February 20, 2014
Revision Received: May 12, 2014
and Accepted: July 1, 2014