Mercury_JECE

Discrimination for inorganic and organic mercury species by cloud
point extraction of polyethylene glycol
Pallabi Samaddar, Kamalika Sen*
Department of Chemistry, University of Calcutta, 92 APC Road, Kolkata 700009, India
A R T I C L E I N F O
Article history:
Received 24 December 2015
Received in revised form 2 March 2016
Accepted 7 March 2016
Available online 8 March 2016
Keywords:
Cloud point extraction
Polyethylene glycol
Confocal microscopy
Zeta potential
Speciation
A B S T R A C T
Speciation and extraction of mercury has been studied using cloud point extraction of polyethylene glycol
(PEG). A solution of PEG itself is reported to form cloud at a considerably high temperature which is
unfavorable for the extraction and separation of volatile analytes. Different inorganic salt solutions
(Na2SO4, NaH2PO4, NaCl, NaOAc) when used as additives were found to lower the cloud point
temperature of PEG (#6000) effectively. The lowest temperature for cloud formation was observed at
35
C with 0.8 M Na2SO4. The cloud so formed was found suitable to discriminate between inorganic and
organic mercury species.1-(2-pyridylazo)-2-napthol (PAN) was used for spectrophotometric detection of
inorganic and organic mercury. Confocal microscopic images and zeta potential values reveal the actual
interactions of the inorganic Hg species with the organized PEG micelles in aqueous medium.
Environmental samples were also analyzed using the present method.
ã 2016 Elsevier Ltd. All rights reserved.
1. Introduction
Speciation of mercury has become significant as mercury bio
accumulation in different tissues mainly depends on its chemical
form and has important implications on tissue-specific toxicity.
The most prominent species of mercury are inorganic mercury
(Hg+
, Hg2+
) and organic mercury (monomethyl mercury, dimethyl
mercury) which enters the water bodies of the nature through
several routes. Mercury is exposed to the atmosphere from coal
fired power plants as it is highly volatile in nature. It is also released
in the air due to volcanic eruptions. Several organisms including a
class of seaweeds dwelling near industrial sites accumulate
mercury. Water currents of the sea as well as other aquatic
organisms broaden the distribution of mercury in natural water
bodies. Consequently, drinking water becomes one of the routes of
mercury invasion into other living organisms. Due to high
bioaccumulation in fish, mercury directly enters the food chain
e.g., predatory fish can have up to 106
times higher mercury
concentrations than ambient water [1]. This was the cause of the
well known event in Minamata Bay in Japan. The World Health
Organization (WHO) recommends a maximum intake of methyl-
mercury of 1.6 mg kgÀ1
body weight per week [2]. Therefore,
precise monitoring of each mercury species and understanding of
species transformations are essential for reliable risk deliberation.
Direct analysis of mercury from aqueous media is possible using
several sophisticated instruments such as inductively coupled
plasma optical emission spectrometry (ICP-OES) [3,4], inductively
coupled plasma mass spectrometry (ICP-MS) [5], cold vapor atomic
absorption spectrometry (CV-AAS) [6] and electrothermal atomic
absorption spectrometry (ETAAS) [7–9] have been developed to
detect Hg species after extraction by cloud point technique.
However, these instruments are very expensive to purchase and
operate which is further complicated by the exhaustive sample
preparation methods. Additionally, these instruments have already
inbuilt interferences like solvent dependent nebulization rate,
background interferences, etc.
Cloud point extraction is based on the fact that polymer/
surfactants in aqueous solutions form micelles and become opaque
when it is allowed to heat at a specific temperature (cloud point
temperature) or in the presence of an electrolyte. Most of the
previous works on cloud point extraction (CPE) method have been
done by using Triton X-100 and Triton X-114. Many metal species
had been successfully extracted in the Triton rich phase with
different chelating agents at different conditions [10–13]. Triton X-
100 is inherently toxic as revealed in several experiments [14,15].
Conversely, reports on cloud point extraction using polyethylene
glycol (PEG) are very limited [16–18]. PEG is mostly used as an
additive to maintain the viscosity of paint, as an additive in the
production of paper, to wrap the surfaces of different materials
[19], as ingredient of various commercial products like sanitizer,
shampoo etc., and even in pharmaceuticals and laxatives. These
polymers are commercially available over a wide range of
* Corresponding author.
E-mail address: kamalchem.roy@gmail.com (K. Sen).
http://dx.doi.org/10.1016/j.jece.2016.03.010
2213-3437/ã 2016 Elsevier Ltd. All rights reserved.
Journal of Environmental Chemical Engineering 4 (2016) 1862–1868
Contents lists available at ScienceDirect
Journal of Environmental Chemical Engineering
journal homepage: www.elsevier.com/locate/jece

molecular weights from 300 g/mol to 10,000,000 g/mol and form
various types of vesicular arrangement depending on the number
of repetitive polymeric chains present in it [20]. Hence there
remains a considerable possibility of extraction related study of
different species in a variety of vesicular arrangements of PEG
cloud. Moreover different mercury species were already found to
get extracted in PEG/salt aqueous biphasic systems [21] with no
possibility of speciation. However, to the best of our knowledge
extraction of mercury species in PEG cloud is still unexplored.
The article presents the first report on the speciation and
extraction behavior of organic and inorganic mercury using cloud
point extraction method in PEG. In the present work only PEG
solution (4%) is used for the formation of cloud as well as to form
complex with the Hg(II) ions in HCl medium. The results of cloud
point extraction of mercury species are compared with previously
reported aqueous biphasic extraction results. Certain salts were
used as additives to lower the cloud point temperature of PEG. For
spectral detection of Hg species we have chosen 1-(2-pyridylazo)-
2-napthol (PAN) indicator which efficiently quantify the extraction
percent at very low mercury concentration in the micellar
medium. Zeta potential data of the cloud containing solution
has been taken for better understanding of electrokinetic stability
of the system. Finally the effect on the structural modifications of
the cloud after CPE of Hg has been visualized using confocal
microscopy.
2. Experimental
2.1. Materials
All the inorganic salts HgCl2, Na2SO4, NaCl, NaH2PO4, Bi(NO3)3,
Pb(NO3)2, CdCO3, Na3AsO4, Na2SeO4, NaI, NaOAC, PEG#6000 were
purchased from Merck. CH3HgCl was obtained from Sigma-Aldrich.
All the solutions were made in triple-distilled water. The
complexing agent 1-(2-pyridylazo)-2-napthol (PAN) and all other
chemicals were of analytical grade. Fluorescein was procured from
Sigma Aldrich.
2.2. Apparatus
The absorption spectra were obtained using an Agilent 8453
diode array spectrophotometer. Mettler Toledo seven compact pH/
Ion meter S220 was used to measure and adjust the pH of different
solutions. Hermle microprocessor controlled universal refrigerated
high speed Table top centrifuge (model Z 36 K) with an adjustable
speed range of 200–30000 was used for centrifugation. Zeta
potentials were measured using Malvern Instrument zetasizer
nano (Zn).
2.3. The cloud point extraction (CPE):
Cloud point extraction of two different mercury species, HgCl2
and CH3HgCl were studied. We have taken PEG#6000 for cloud
formation. 0.5 mL of 0.5 mM HgCl2/1 mL of 1 mM CH3HgCl, 3 mL
sodium sulfate (varying pH and concentration), 1 mL of 20%(w/v)
PEG(#6000) were taken and the volumes were made upto 5 mL.
This results in a 4%(w/v) final concentration with respect to PEG
(#6000). pH (2.5–10.8) was adjusted using dilute HCl solution in
the acidic range and dilute NaOH solution in the basic range. The
solutions were heated in a water bath for 15 min at 35–40
C. The
solution appeared cloudy, which was then centrifuged at 3000 rpm
for 5 min. The centrifuge tubes were then cooled in ice water for
10 min. The surfactant rich phase appeared at the upper surface of
the solution and was separated out carefully. Then the separated
portion was dissolved in distilled water and taken for absorption,
zeta potential and confocal micrcoscopic studies. For absorption
studies, the cloud dissolved in water was treated with pH 9.1 borate
buffer followed by addition of 0.5 mM PAN. At this condition a new
complex of Hg-PAN was formed which was spectrophotometrically
estimated at its lmax 560 nm wavelength. 0.5 mM, 1 mM and
1.5 mM solutions of NaCl, NaH2PO4, Bi(NO3)3, Pb(NO3)2, CdCO3,
Na3AsO4, Na2SeO4 and NaI were prepared in triple distilled water
for interference studies. These ions were mixed with equimolar
concentrations of Hg solution and analyzed for their interference.
For confocal microscope imaging, the cloud dissolved solutions
were spiked with a small volume of diluted fluorescein solution. A
drop of this solution was placed on a glass slide, covered with a
cover-slip, observed under the confocal microscope and the images
were recorded.
2.4. Analysis of environmental samples
Water samples and river sediment were collected from Hooghly
river from Babughat, Kolkata, India and a pond water sample was
collected from Baruipur, West Bengal, India. The freshly collected
samples were concentrated by evaporation and filtered before
analysis. Concentrated HCl extract of the sediment was obtained
after keeping the sediment immersed in the acid for overnight and
then slow heating of the mixture for 30 min. The extract was cooled
and filtered through Whatman 40. The filtrate was collected,
evaporated to dryness and taken in small volume of distilled water
to be analyzed after CPE. Our proposed CPE method was also
applied to both the water samples. Standard addition method has
been used to find mercury concentration in these samples.
3. Results and discussion
PEG is chiefly a thermo separating polymer but cloud point
temperature of PEG in an aqueous binary system is very high
(above 373.15 K) [22]. The cloud point temperature of micellar
solutions can be controlled by addition of salts, alcohols, organic
compounds, etc. We have taken four different salts as additives to
lower the cloud point temperature of PEG. The variations of cloud
point temperatures versus concentration of four different salts e.g.
Na2SO4, NaCl, NaH2PO4, NaOAc are shown in Table 1. Among the
four salts, sodium sulfate efficiently reduces cloud point of PEG to
35
C. This observation can be explained on the basis of ionic
interactions. Generally, chaotropes (ClÀ
, IÀ
) are large singly
charged ions, with low charge density and they exhibit weaker
interactions with water molecules. Conversely, small or multiply-
Table 1
Variations of cloud point temperatures of PEG with concentration of four different
salts (NaCl, NaH2PO4, NaOAc, Na2SO4).
Concentration of PEG Concentration of salts Cloud point temperature
4% PEG 1.6 M NaCl 100
C
1.8 M NaCl 85
C
2.4 M NaCl 80
C
2.8 M NaCl 70
C
4% PEG 0.2 M NaH2PO4 100
C
0.4 M NaH2PO4 100
C
0.6 M NaH2PO4 80
C
0.8 M NaH2PO4 70
C
4% PEG 0.8 M NaOAc 100
C
1.2 M NaOAc 90
C
1.6 M NaOAc 85
C
2.0 M NaOAc 80
C
4% PEG 0.2 M Na2SO4 100
C
0.4 M Na2SO4 70
C
0.6 M Na2SO4 40
C
0.8 M Na2SO4 35
C
P. Samaddar, K. Sen / Journal of Environmental Chemical Engineering 4 (2016) 1862–1868 1863

charged ions, with high charge density, are kosmotropes (for
example, SO4
2À
, HPO4
2À
), always strongly interact with water
molecules and are capable to bring cloudy appearance faster at
relatively lower temperature. Moreover, the H-bond strength
between water molecules is ion specific, hence more kosmotropic
ions are able to form strong hydrogen bonded clusters with more
rigid conformation (i.e., low librational freedom). As the same
cation (Na+
) is present in all the four salts, difference in anionic
character is the key factor here. Kosmotropic nature of SO4
2À
is
responsible for decreasing PEG solvation in water resulting in
cloudy aggregation at low temperature. This incident can also be
explained on the basis of Hofmeister series. Extreme left portion in
Hofmeister series is occupied by kosmotropes which have very
high salting out ability. In our experiment we have used 0.8 M
Na2SO4 which is heavier than 4% PEG solution. That’s why resulting
cloud appeared at the upper surface of the solution and it was
easily taken out by simple pipetting followed by cooling.
From Fig. 1 we observe that Hg2+
species was completely
extracted in PEG cloud at lower pH (3.6) whereas CH3Hg+
was not
at all extracted in PEG cloud at any pH in the range 2.5–10.8.
Percentage efficiency of extraction of inorganic Hg was however
very high ($100%) in the concentration range 5–1 mg LÀ1
. Higher
concentrations are also possible to extract but then the detection
process needs to be modified with serial dilution of the CP
extracted PEG rich phase in suitable media. The method is suitable
for CPE based removal and preconcentration of Hg2+
from samples
containing upto $200 mg LÀ1
of inorganic mercury.
The analytical figures of merit for the CPE process is tabulated in
Table 2. The linear range of the analysis was found in the region 10–
100 mg LÀ1
. The slope, intercept and the r2
values of 0.479, 0.00 and
0.992 obtained from the calibration indicates that method is
appropriate for analysing Hg2+
in water samples. The% RSD values
were calculated using the equation
RSD = 100S/x (1)
where S is the standard deviation and x is the average value. Within
day and between day reproducibility were found to be 2.1% and
2.6% respectively at pH 3.6, 1.2% and 1.5% respectively at pH 1.2 and
1.5% and 1.8% respectively at pH 4.5. The limit of detection (LOD)
and limit of quantification (LOQ) were calculated using the blank
determination method as shown in the following equations [23]:
LOD = Xb1 + 3Sb1 (1)
LOQ = Xb1 + 10Sb1 (2)
where Xb1 is the mean concentration of the blank and Sb1 is the
standard deviation of the blank. The LOD and LOQ values so
obtained were 5 mg LÀ1
and 8 mg LÀ1
respectively. A comparison of
the results of recent reports together with the present method is
tabulated in Table 3.
The possible reason behind this behavior is that the pH in the
acidic range was adjusted using HCl which introduces more
chloride ions in the medium. Hg halides have reported interactions
with short polyethlylene glycol chains forming complexes which
have three dimensional aspects similar to crown ethers. Such
complexes also have axial halides coordinated to Hg which take
part in forming polymeric structures via chloride bridging [24].
Higher ClÀ
ion concentration at lower pH (3.6) enhances the
possibility of this chloride bridging and hence higher CPE of Hg
($100%). At even more acidic condition (pH 2.5) the cloud
formation is however affected due to the higher dissolution of
the polymer assemblies and hence CPE of Hg becomes lower
($95%). CPE of Hg at even lower pH was therefore avoided. In the
neutral and basic range of pH, absence of this excess chloride
restricts the chloride bridging of the polymer and hence extraction
of Hg is nullified. This observation is also strengthened by the fact
that upon adjusting the acidic pH using suitable buffers instead of
HCl (to exclude ClÀ
), the cloud formation and extraction efficiency
decreased drastically. This is solely because of the absence of
pH10.8pH8.5pH6.7pH5.5pH4.5pH3.6pH2.5
0
20
40
60
80
100
Hg
2+
MeHg
+
%Extraction
Sodium
sulfate
Fig. 1. Extraction profile for Hg2+
and CH3Hg+
in PEG cloud at varied pH condition.
Table 2
The basic analytical figures of merit.
Linear range 10–100 mg LÀ1
Slope 0.479
Intercept 0.00
Correlation coefficient (r2
) 0.992
Within day reproducibility in extraction (%) pH 2.5 (93.0 Æ 1.2)%
pH 3.6 (100.0 Æ 2.1)%
pH 4.5 (28.7 Æ 1.5)%
Between day reproducibility in extraction (%) pH 2.5 (93.0 Æ 1.8)%
pH 3.6 (100.0 Æ 2.6)%
pH 4.5 (28.7 Æ 1.8)%
LOD 5 mg LÀ1
LOQ 8 mg LÀ1
1864 P. Samaddar, K. Sen / Journal of Environmental Chemical Engineering 4 (2016) 1862–1868

bridging chloride ions which causes the self assembly of the
polymer to become weaker and hence much lesser extraction
percent is observed in each case.
In CH3HgCl, Hg is however already crowded with the methyl
group and hence is unable to get extracted in the cloud. Even the
addition of excess ClÀ
is unable to trap the bulky group in the
interstices of the polymeric assembly. To check the optimum PEG
concentration for effective CPE of mercury, the extractions were
carried out with varying concentrations of PEG with other factors
viz., pH, salt concentration and temperature remaining unaltered.
The results indicate that the 4% PEG concentration was optimum as
below this value, the percentage of Hg extraction falls and at higher
values (4%) the extraction of Hg remains unchanged (Fig. 2).
Moreover, the cloud that appeared at lower concentrations did not
allow easy phase separations and hence extraction of Hg was less.
The effect of interfering radicals in CPE of Hg2+
was studied in
presence of different cations and anions of different concentrations
(0.5 mM, 1 mM and 1.5 mM) individually (Fig. 3) at equimolar
concentrations with Hg2+
. The cations like, Cd2+
, Bi3+
, and Pb2+
were found to cause no interference in the Hg2+
extraction.
However, some of the anions like, AsO4
3À
, SeO4
2À
and PO4
3À
exert a
negative effect in the CPE of Hg2+
. This is due to possible
interactions of Hg2+
with the respective anions [25–27] in forming
stable compounds that restrict the Hg2+
-polymer interactions as
described above. So, it will be desirable to remove these anions
before CPE of Hg2+
.
Table 3
Comparison of the proposed CPE method with the previously reported methods.
Method Chelating agent Measurement
wavelength,
lmax
Linear
range
(mg LÀ1
)
Detection limit
(mg LÀ1
)
Preconcentration
factor
RSD% Media Reference
Spectrophotometry PAN 554 nm 10–
1000 mg LÀ1
1.65 mg LÀ1
33.3 4.18 (1:1) Triton X-114 [1]
TAR 389 nm 50–
2500 mg LÀ1
14.5 mg LÀ1
33.3 1.35 (1:1)
Cold vapor atomic
absorption
spectrometry
ammonium O,O-
diethyldithiophosphate
(DDTP)
– 0.117 mg/kg 4.6 Triton X-114 [3]
Inductively
Coupled Plasma
Optical Emission
Spectrometry
(ICP-OES)
3-nitro benzaldehyde
thiosemicarbazone (3-
NBT)
– 10–100 ng/
mL
1.1 ng/mL 28.6 3.2 Triton X-114 [28]
Electrothermal
Atomic
Absorption
Spectrometry
(ETAAS)
2-(5-bromo- 2-
pyridylazo)-5-
(diethylamino)-phenol
(5-Br-PADAP)
– – 0.01 mg/L – 4 Polyethyleneglycolmono-
p-nonyphenylether
(PONPE 7.5)
[9]
Inductively
Coupled Plasma
Mass
Spectrometry
(ICP-MS)
sodium
diethyldithiocarbamate
– 0.02–
2.00 mg LÀ1
13 ng LÀ1
(forMeHg+
),
8 ng LÀ1
(for
PhHg+
),
6 ng LÀ1
(for
Hg2+
)
5.3
(forMeHg+
),
2.3(for
PhHg+
), 4.4
(for Hg2+
)
Triton X-114 [29]
Inductively
Coupled Plasma
Mass
Spectrometry
(ICP-MS)
sodium
diethyldithiocarbamate
(DDTC)
– – 4–10 ng LÀ1
– – Triton X-114 [30]
Spectrophotometry PAN 560 nm 10–
100 mg LÀ1
9 mg LÀ1
(river
water),
20 mg LÀ1
(river
sediment)
– 2.6 Polyethylene glycol
(#6000)
Our work
0
20
40
60
80
100
1.6 2.4 3.2 4 6
ExtracƟon,%
PEG conc. (%w/v)
Fig. 2. Extraction proﬁle for Hg2+
at pH 3.6 with variation of PEG concentration during CPE.

The results were further justified by confocal microscopic
images and zeta potential values. The fluorescein treated cloud of
PEG (Fig. 4A) shows a ring like aggregation of polymeric chain with
a hollow space in between. Upon extraction of Hg2+
the polymeric
aggregation becomes stronger and more prominent with a larger
hollow space due to aforesaid complexation (Fig. 4B). The
aggregation resulting out of a bare cloud generates a smaller
hollow space ($25 mm diameter) in the ring shape whereas an Hg2
+
extracted cloud generates larger hollow space ($70 mm diame-
ter) and a more prominent ring arising due to ionic interactions of
Hg2+
with the micellar surface. In case of aqueous biphasic systems,
PEG phase efficiently extract different mercury species at lower pH
conditions whereas CPE using PEG offers selectivity for inorganic
mercury extraction. From this comparison we can easily assume
that vesicular arrangement of PEG cloud is obviously different from
PEG rich phase of aqueous biphasic system. Ring type aggregation
was absent in PEG phase taken out from a PEG/salt biphase [20]. As
a consequence, species dependent extraction is more suitable in
PEG cloud especially for mercury.
Higher absolute value of the zeta potential indicates higher
stability of the system. We obtained highest zeta potential value
(À2.29 mV) of Hg2+
extracted cloud indicating higher electrokinet-
ic stability. As methylmercury was not at all extracted in PEG cloud,
it shows a similar zeta potential value with bare PEG cloud which
were found to be À0.692 mV and À0.583 mV respectively. Negative
charge develops at the outer surface of the aggregation due to
higher availability of OHÀ
moiety of the PEG polymeric chain.
When counter ions (Hg2+
) are attracted by the negatively charged
moiety, potential difference is generated in the system and
therefore zeta value increases. This would be the probable reason
for different zeta potential values of the different Hg species
treated PEG clouds.
3.1. Analysis of environmental samples
The pondwatersample didnot showHgconcentrationwithin the
detection limit of our CPE method even after concentrating
it. The river water sample however was found to contain 9 Æ1
mg LÀ1
and the river sediment contained 20 Æ 2 mg LÀ1
inorganic Hg.
The results of recovery of Hg2+
from these samples by the proposed
method are tabulated in Table 4.
Fig. 3. Extraction of Hg2+
in presence of different interfering radicals at different concentrations.
Fig. 4. Confocal microscopic images of (A) bare PEG cloud and (B) Hg2+
extracted PEG cloud.

4. Conclusion
Our proposed method can be applied in the separation and
detection of trace Hg2+
species in various environmental and
industrial samples for our health concern. This spectrophotometric
approach based on CPE is simple and sensitive than conventional
methods. Interferences from similar heavy metal cations were not
significant although those from anions like arsenate, selenate and
phosphate are prominent which therefore need suitable pretreat-
ment before analysis. Non toxic PEG is also very cheap and largely
available in the market. Different behavior of inorganic and organic
mercury towards PEG cloud brings a new concept in speciation
study. Confocal microscopic images also show exceptionally
interesting ring like vesicular arrangement of PEG cloud. Moreover,
zeta potential values of cloud may enrich the understanding of
phase dispersion phenomena at cloud point condition. Environ-
mental samples of pond water, river water and river sediment
extract were also analyzed using the method using standard
addition technique.
Acknowledgements
We gratefully acknowledge Dr. Anupam Banerjee (Lyca instru-
ments) for confocal microscopic study. We also thank Dr. Abhijit
Saha, UGC-DAE Consortium for Scientific Research, Kolkata, India,
for zeta measurement. P. S. expresses sincere thanks to the UGC,
India [Memo no. UGC/1228C/Major Research (SC) 2012] for
providing necessary fellowship.
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Table 4
Recovery of Hg2+
added to environmental samples by the proposed method.
Samples Added Hg2+
(mg LÀ1
) Found Hg2+
(mg LÀ1
) Recovery (%)
River Water 0 9 Æ 1 –
16 25 Æ 3 100.0
20 30 Æ 5 103.4
23 33 Æ 5 103.1
27 36 Æ 5 100.0
30 39 Æ 6 100.0
33 43 Æ 7 102.4
Pond water 0 0 Æ 1 –
16 15 Æ 2 93.8
20 22 Æ 3 110.0
23 21 Æ 2 91.3
27 26 Æ 4 96.3
30 33 Æ 5 110.0
33 35 Æ 5 106.1
Sediment 0 20 Æ 3 –
16 36 Æ 5 100.0
20 42 Æ 6 105.0
23 42 Æ 6 97.7
27 46 Æ 7 97.9
30 51 Æ 7 102.0
33 53 Æ 7 100.0

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