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Highly efficient and selective transport of Hg2+ ions through a bulk liquid membrane containing Cyanex 301 as carrier.

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ASIA-PACIFIC JOURNAL OF CHEMICAL ENGINEERING
Asia-Pac. J. Chem. Eng. 2011; 6: 631–638
Published online 28 June 2010 in Wiley Online Library
(wileyonlinelibrary.com) DOI:10.1002/apj.463
Research article
Highly efficient and selective transport of Hg2+ ions
through a bulk liquid membrane containing Cyanex 301
as carrier
S. S. Madaeni,1 * N. Islami,2 M. K. Rofouei2 and M. Ghader3
1
Membrane Research Center, Chemical Engineering Department, Razi University, Kermanshah, Iran
Faculty of Chemistry, Tarbiat Moallem University, Tehran, Iran
3
Physicochemistry Department, Razi Vaccine & Serum Research Institute, Karaj, Iran
2
Received 13 October 2009; Revised 9 April 2010; Accepted 26 April 2010
ABSTRACT: Mercury is present in various forms in many industrial wastewaters, and in some natural water sources.
In this work, a novel bulk liquid membrane (BLM) system containing bis(2,4,4-trimethyl(pentyl)dithiophosphinic acid
(Cyanex 301) as carrier was evaluated for mercury transport and recovery of the ions from acidic nitrate solutions.
Cyanex 301 acts as an efficient and selective carrier for the uphill transport of Hg(II) through a chloroform bulk liquid
membrane. The selectivity and efficiency of Hg(II) transport from aqueous solutions containing different mixtures of
cations were investigated. The fundamental parameters influencing the transport of mercury ion such as concentration of
nitric acid in the feed solution, carrier concentrations in the membrane, type of organic solvent, nature and composition
of the receiving phase, stirring speed and transport time were determined. Most of the Hg(II) ions (89%) penetrated
through the bulk liquid membrane after 120 min using 0.04 M Cyanex 301 in the membrane phase and 5 M HNO3 in
the receiving phase. However, the maximum transport (93%) occurred after 140 min at a stirring speed of 200 rpm in
the feed and stripping solutions. The present study demonstrates the effectiveness of the bulk liquid membrane system
for separating Hg(II) ions in amalgam solutions by combining extraction and stripping operations in a single process.
 2010 Curtin University of Technology and John Wiley & Sons, Ltd.
KEYWORDS: mercury ion removal; nitric acid solutions; Cyanex 301 carrier; bulk liquid membrane
INTRODUCTION
Heavy metal pollution represents an important environmental problem due to the toxic effects of metals.
The accumulation of heavy metals along the food chain
leads to ecological and health problems.[1] Mercury
ions are generated in wastewater by several industrial
processes (e.g. chloro-alkali, pharmaceutical, cosmetics, combustion of fossil fuels, electrical and electronic, metal processing, metal plating, metal finishing,
pulp and paper, etc.) resulting in the contamination
of aquatic systems.[2 – 7] Mercury is present in natural water in different forms including elemental mercury (Hg0 ), ionic mercury (Hg+ , Hg2+ ) and methylated
mercury [CH3 Hg+ , (CH3 )2 Hg]. Monomethyl compound
(CH3 Hg+ ) is a neurotoxin which is commonly found
in aquatic environments. Due to the affinity for fatty
*Correspondence to: S. S. Madaeni, Membrane Research Center,
Chemical Engineering Department, Razi University, Kermanshah,
Iran. E-mail: smadaeni@yahoo.com
 2010 Curtin University of Technology and John Wiley & Sons, Ltd.
Curtin University is a trademark of Curtin University of Technology
tissue in animals, methylmercury tends to accumulate
more readily compared with other mercury species.[8,9]
Several technologies can be used to remove these
toxic metals from liquid effluents including precipitation, solvent extraction, ion exchange and liquid membrane (LM). The latter may be employed in separation,
concentration and analytical applications. The advantages of LM over conventional separation operations
include high selectivity, low energy utilization and ease
of installation.[10]
Various liquid membrane systems including bulk liquid membrane (BLM) have been designed for selective
separation of mercury from aqueous solutions containing different complexing agents such as polybutadiene,
bis(di(2-ethyl hexyloxy))thiophosphoryl) disulfide, triisobutyl phosphine sulfide (Cyanex 471X), triisobutyl
phosphate, cyclic polythioethers and benzoylthiourea
derivates.[11 – 16] The focus of BLM process is the design
of chemical system, i.e. composition of the source and
receiving phases.
Despite the increasing industrial usage and the hazardous effects,[17] the transport of Hg ion across liquid
632
S. S. MADAENI et al.
membrane is not comprehensively investigated.[16,18 – 20]
The development of efficient carrier for selective
removal of mercury from contaminated sources is
still a challenging task.[21] In 1980s, Cytec introduced two novel thiophosphinic extractant Cyanex
301 and Cyanex 302.[22] These reagents substituted
sulfur for oxygen in organophophorus acids (P S)
and provide various properties to metal extraction,
as predicted by the Hard–Soft Acid–Base (HSAB)
principle[23,24] In the last two decades, some alkylphosphines of the Cyanex series have gained prominence as metal ion extractants due to their low
aqueous solubility and resistance to hydrolysis. Concerning the two recently commercialized extractants
Cyanex 301 [whose active component is the bis(2,4,
4-trimethylpentyl)dithiophosphinic acid] and Cyanex
302 [with bis(2,4,4-trimethylpentyl)monothiophos
phinic acid as an active component] no comprehensive
information is available.[25] The presence of soft donor
atoms in ligands results in a considerable increase in
stability of their complexes with soft cations such as
mercury and silver ions while diminishing the stability of their alkali, alkaline earth metal ions and hard
transition metal ion complexes.[26,27] In this respect,
thio-containing compounds have attracted widespread
attention owing to the unique properties of these compounds and their use as carrier.[28] The expectation
in BLMs is increasing the selectivity toward mercury
and silver over alkali, alkaline earth metal and several other metal ions. Metal ions classified as soft
acids such as Hg(II), Au(III) and Ag(I) may effectively
interact with compounds containing sulfur as donor
atoms.[23]
There are several articles that report the employment of organophosphorous compounds for the extraction of silver ions from various systems.[29 – 32] . Solvent extraction of gold ions by Cyanex 301 and
Cyanex 302 as carrier has been discussed by some
authors.[33,34] There are few works describing the
separation of mercury from aqueous solution with
some thiophosphono compounds.[35] However, to the
best of our knowledge there is no report regarding employment of thiophosphinic acid compounds
as carrier for transport of mercury through liquid
membrane.
In the present article, the application of Cyanex 301
as a novel carrier for the extraction of mercury using
bulk liquid membrane was investigated. The focus was
based on developing a technique to recover mercury
from a nitrate solution. For this purpose, the effect of
concentration of HNO3 in the source phase, effect of
carrier concentration in the membrane phase, nature
and concentration of the strip phase, type of solvents
stirring speed and process time and selectivity of the
system for optimizing the mercury ion transport were
investigated.
 2010 Curtin University of Technology and John Wiley & Sons, Ltd.
Asia-Pacific Journal of Chemical Engineering
EXPERIMENTAL
Reagents
The extractant diisooctyldithiophosphinic acid [traditional name for bis(2,4,4-trimethylpentyl) dithiophosphinic acid, commercial name: Cyanex 301] was supplied by Cytec Industries Inc. (Hamburg, Germany).
This was used as the mobile carrier without further purification. Reagent grade chloroform and other
organic compounds (CH2 Cl2 , C2 H4 Cl2 and CCl4 ) from
Merck (Darmstadt, Germany) were used as the membrane organic solvent. Mercury (II) nitrate, nitrate salts
of other cations, HNO3 and other acids (HCl, H2 SO4 ,
H3 PO4 ) were of the highest available purity from Merck
and used without further purification. Samples for analytical application (as real samples), i.e. dental amalgam capsules of Non Gamma 2 amalgam (Cinalux
and Cinasilver) purchased from Dr. Faghihi Dental Co
(Tehran, Iran). Stock solutions were prepared with doubly distilled water.
Apparatus
The concentrations of mercury ions in the aqueous solutions (feed and strip phases) were measured by cold
vapor atomic absorption spectroscopy (CVAAS) using
SperctrAA 220 (Varian, Walnut Creek, CA, USA). All
measurements were carried out under recommended
conditions for each metal ion. A bulk type liquid membrane cell[36] was used in all experiments. The organic
phase was stirred at variable speeds magnetically by
magnetic stirrer (Heidolf, MR3001, Schwabach, Germany).
Procedure
All transport experiments were carried out at ambient temperature. A cylindrical glass cell (inside diameter 4.0 cm) holding a glass tube (inside diameter
2.0 cm) thus separating the two aqueous phases (Fig. 1)
was used. The chloroform solution (10 ml) containing
ionophore was located at the bottom of the glass cell and
two portions of aqueous donor (5 ml) and acceptor solutions (10 ml) were carefully added on top. As the aim
of the this work was to increase the rate of separation
of mercury from the source phase and accompanying
elements, a larger volume (i.e. 10 ml) was chosen as
the striping medium. The organic layer was stirred by
a Teflon-coated magnetic bar. Samples of both aqueous
phases were analyzed for metal content by AAS. A similar transport experiment was carried out in the absence
of the carrier for reference. The detailed experimental
conditions are included in the tables. The average standard deviations of data were obtained by running the
Asia-Pac. J. Chem. Eng. 2011; 6: 631–638
DOI: 10.1002/apj
EFFICIENT AND SELECTIVE TRANSPORT OF HG2+ IONS
Asia-Pacific Journal of Chemical Engineering
Figure 3.
Scheme of the counter-coupled transport
mechanism of Hg(II) ions using Cyanex 301 as carrier. HA
represents the monomeric carrier that exists in the organic
phase.
Figure 1. Liquid membrane apparatus, S: source phase; R: receiving
phase; M: liquid membrane.
experiments three times. The error bar of each point
was calculated using SigmaPolt 11 software.
RESULTS AND DISCUSSION
Mechanism of uphill transport
It is expected that Cyanex 301 shows high capability
for the extraction of mercury. The structure of the
conjugated base (Fig. 2) in Cyanex 301 results in the
distribution of the negative charge between two sulfur
atoms. This leads to a good charge distribution per unit
volume which provides high stability to the conjugated
base and hence accounts for the high relative acidity
of this organophosphorus acid compound. Accordingly,
by enhancing the acidity of the extractant, the selected
base metal ion such as mercury may be significantly
extracted. The general reaction for the extraction of a
metal cation by mono or dithioorganophosphorous acids
may be written as[37] :
Mn+ + (m + n)/p(HA)p
MAn(HA)m + nH+
(1)
where A is the ligand of interest; p is the degree of
association of the extractant and overlining represents
species in the organic phase. The extraction of most
metal ions by dithio-phosphoric or phosphinic acids is
characterized by the formation of metal complexes of
the general formula MAn , where n is the valence of the
extracted metal species. In general, the complexes do
not contain the molecular form (HA) of the ligand and
dithio acids are monomeric in solution, in contrast to
their analogs containing one or more oxygen atoms.[38]
Therefore, transport of mercury (II) ions using Cyanex
301 as carrier, obeys the facilitated counter-coupled
transport according to the mechanism represented in
Fig. 3 and the following equilibrium:
M2+(aq) + 2HA(org)
MA 2( org) + 2H+ (aq)
(2)
where M2+ is mercury ion and A is ligand.
Thus in order to form an uphill extraction, the extractant has to be acidic giving forward reaction. In this
case, the pH difference between feed and stripping liquid can be the driving force for the extraction. Therefore, the larger the acid strength of the extractant,
the easier is the direct reaction in Eqn (2). The mercury (II) ions present in the feed phase react with the
monomeric carrier HA on the feed-phase/organic membrane interface to form HgA2 which diffuses through the
membrane phase to the organic membrane/strip-phase
interface. On this interface, the complex reacts with the
hydrogen ions from the strip phase, according to the
backward reaction of Eqn (2), liberating Hg2+ ions into
the strip phase and regenerating the carrier, which, in
turn, diffuses back to the feed interface to react again
with the next mercury (II) ion. The net permeation is
the transport of mercury from the feed to the strip phase
and transfer of hydrogen ion in the opposite direction.
Effect of concentration of acid in the source
Figure 2. Structure of Cyanex 301 and resonance structure
of the conjugate base.
 2010 Curtin University of Technology and John Wiley & Sons, Ltd.
Cyanex 301 is a dialkyldithiophosphinic acid extractant.
This sulfur-containing compound is much stronger acid
Asia-Pac. J. Chem. Eng. 2011; 6: 631–638
DOI: 10.1002/apj
633
634
S. S. MADAENI et al.
Figure 4. Effect of HNO3 concentration in the source
phase on transport of mercury (II) ions through BLM: (ž)
transported; ( ) remaining. Conditions: source phase,
5 ml of 5.0 × 10−4 M Hg(II) and varying concentration of
HNO3 ; membrane phase, 10 ml of 5.0 × 10−2 M Cyanex
301 in chloroform; strip phase, 10 ml of 5 M HNO3 ; rate
of stirring, 300 rpm; transport time, 2 h. Data represent
the mean of three replications ±SD, and the error bar
represents the deviation of the obtained efficiency from
the average value.
compared with the analogous oxy-acid, Cyanex 272.
This is capable of extracting many metals at low
(<2) pH.[39] Accordingly, the presence of acid in the
source phase affects the transport phenomenon. The
source of mercury in the feed was Hg(NO3 )2 with the
concentration of 5.0 × 10−4 mol L−1 . Due to higher
solubility values of nitrate salts of mercury and avoiding
the interfering effect of other anions such as Cl− , SO4 2−
and PO4 3− only nitric acid was used.
The results (Fig. 4) revealed that the transport efficiency is increased with the addition of proton concentration up to 0.1 M HNO3 . At lower concentrations of
H+ , the mercury transport is lower probably due to the
uncompleted formation of HgA2 . However, at concentrations above 0.1 M HNO3 there is a decreasing effect
in the transport efficiency due to excessive presence of
H+ and competition with mercury ions and protonation
of ligand.
Effect of carrier concentration
The influence of the concentration of diisooctyl dithiophosphinic acid in the chloroform, as the organic phase,
on the transport was studied at eight different initial
concentrations. The results are presented in Fig. 5. The
transport of mercury through the bulk liquid membrane was improved with increasing the ionophore concentration up to a certain concentration. The maximum transport occurred at the carrier concentration of
around 4.0 × 10−2 M (Cyanex 301 in chloroform). This
 2010 Curtin University of Technology and John Wiley & Sons, Ltd.
Asia-Pacific Journal of Chemical Engineering
Figure 5. Effect of carrier concentration in the membrane
phase on transport of mercury (II) ions through BLM: ( )
transported; (ž) remaining. Conditions: source phase, 5 ml
of 5.0 × 10−4 M Hg(II) and 0.1 M HNO3 ; membrane phase,
10 ml of varying concentration of Cyanex 301 in chloroform;
strip phase, 10 ml of 5 M HNO3 ; rate of stirring, 300 rpm;
transport time, 2 h. Data represent the mean of three
replications ±SD, and the error bar represents the deviation
of the obtained efficiency from the average value.
may be explained by further formation of HgA2 complex leading to additional extraction into the membrane
phase. This was followed by a decline in transport efficiency by further increment in carrier concentration.
The transport was dropped from 80% (in ligand concentration of 0.040 M) to 71% (in ligand concentration
of 0.090). This is probably due to the difficult releasing
of the cations into the strip solutions as a result of firm
bounding to the ionophore and the increasing viscosity of the liquid membrane. In other word, the elevated
HgA2 complex formation (above a certain carrier concentration in membrane) causes two phenomena:
1. Mercury tends to remain in the complex form (in the
membrane phase) without getting stripped due to the
cation release difficulty into the strip solution, as a
result of firm bounding to the carrier.
2. Viscosity enhancement in the membrane phase
which limits the diffusivity of the ion-carrier complex in the membrane.
Moreover, an experiment was performed without
any carrier in the membrane. Transport of Hg(II)
ions through the liquid membrane was around 8.5%,
suggesting that the transfer of mercury ions through
BLM is fulfilled by the carrier.
Effect of organic solvent
The LM solvent is an important parameter for obtaining
a stable BLM. The characteristics of the diluents are
Asia-Pac. J. Chem. Eng. 2011; 6: 631–638
DOI: 10.1002/apj
EFFICIENT AND SELECTIVE TRANSPORT OF HG2+ IONS
Asia-Pacific Journal of Chemical Engineering
Table 1. Effect of organic membrane solvent on
transport of mercury (II) ions through BLMa .
Table 2. Effect of nature of receiving phase on
transport of Hg ions through BLMa .
Solvent
Remained (%)
Transported (%)
Stripping agent
Remained (%)
CH2 Cl2
C2 H4 Cl2
CHCl3
CCl4
13 ± 2.4
11 ± 1.8
8 ± 2.1
8.5 ± 2.7
32.5 ± 2.2
38 ± 1.5
80 ± 2.8
53 ± 2.3
HNO3 : 5 M
HCl: 5 M
H2 SO4 : 5 M
H3 PO4 : 5 M
EDTA: 0.1 M (pH = 4)
8 ± 1.6
23 ± 2.6
21 ± 2.0
21 ± 2.3
24.5 ± 1.7
Conditions: source phase, 5 ml of 5.0 × 10−4 M Hg(II) and 0.1 M
HNO3 ; membrane phase, 10 ml of 4.0 × 10−2 M Cyanex 301 in
different solvents; receiving phase, 10 ml of 5 M HNO3 ; stirring
speed, 300 rpm; transport time, 2.
a
Each standard deviation obtained with three measurements.
mainly responsible not for the life-time of the liquid
membrane but also for the solubility of the complex
formed and the penetration coefficient of solvent. The
penetration coefficient is an intrinsic property of the
solvent which mainly depends on the viscosity of the
solvent.[40] The effect of solvents on the transport
process was studied under the same conditions. The
obtained results for the transport of Hg(II) ions through
BLM in different solvents are presented in Table 1. The
maximum (80%) transport of mercury ions occurred
in the presence of chloroform as organic solvent. The
mercury transport efficiency was increased in the order
of CH2 Cl2 < CH2 Cl–CH2 Cl < CCl4 < CHCl3 .
The mercury transport in different solvents depends
on many factors including diffusion coefficient of the
mercury ions, interactions of formed complex with
solvent and penetration coefficient (and hence viscosity) of the solvent. When using CH2 Cl2 , as the solvent, the membrane volume is decreased over time
due to the low boiling point and high volatility of
dichloromethane. For longer transport time the two
phases (source and strip phase) are mixed. When using
CH2 Cl–CH2 Cl as the membrane phase, the organic
phase turned to a turbid state reducing the efficiency
of mercury transport. The transport of mercury ions
by CCl4 as solvent was less (53% vs 80%) compared
with CHCl3 . Inordinate viscosity of carbon tetrachloride compared to chloroform results in a decline in
the mercury transport. In summary, we demonstrated
that the characteristics of the membrane solvents are
one of the major factors in establishing the transport
efficiency.
Effect of nature and composition
of the receiving phase
The nature and composition of the receiving phase
exhibit a dramatic influence on mercury ion transport through BLM. The effects of various stripping
agents are presented in Table 2. The utilization of 5 M
HNO3 as scavenger caused an evident enhancement in
 2010 Curtin University of Technology and John Wiley & Sons, Ltd.
Transported (%)
80 ± 2.9
3.5 ± 2.4
15 ± 1.8
42 ± 1.5
9 ± 1.9
Conditions: source phase, 5 ml of 5.0 × 10−4 M Hg(II) and 0.1 M
HNO3 ; membrane phase, 10 ml of 4.0 × 10−2 M Cyanex 301 in
chloroform; receiving phase, 10 ml of different solutions; stirring
speed, 300 rpm; transport time, 2 h.
a
Each standard deviation obtained with three measurements.
the efficiency of mercury transport, whereas the presence of other stripping agents such as 5 M HCl, 5 M
H2 SO4 , 5 M H3 PO4 and 0.1 M EDTA (pH = 4) in the
receiving phase resulted in a pronounced decrease in
mercury transport efficiency. The transport of mercury
ions in the presence of 5 M H3 PO4 in the receiving
phase is low (42% vs 80%) compared with HNO3 .
This is probably due to the weakness of H3 PO4 in
protonation of complex in membrane/strip-phase interface. The efficiency of H2 SO4 (15%) was very low.
Highly increasing the ionic strength of the stripping
medium by H2 SO4 leading to low activity coefficient
of hydrogen ions[40] for decomposition of the HgA2
complex (A = carrier) resulting in a lower transport
of mercury ions compared with HNO3 . Although HCl
is a strong acid, the efficiency of mercury transport
is low (5%). This may be explained by the formation of some species such as Hg(Cl)(A), where A is
the anionic form of carrier, and mercury chloride complex (HgCl2 ) in organic membrane/strip-phase interface.
These compounds are excessively established at low
pH and/or high chloride concentrations[9] and remain in
non-polar solvent[41] with low entering into the receiving phase.
Effect of EDTA as stripping agent was investigated. The efficiency of mercury transport using EDTA
(0.1 M, pH = 4) was 9%. This may be attributed to low
formation constant (Kf ) of the Hg-EDTA (log Kf = 21.5
in aquatic condition)[42] complex compared with the
formation constant of HgA2 . This means EDTA is not
able to completely decompose the HgA2 complex and
release the total mercury ions in the strip phase. The
attempt for finding the formation constant of EDTA at
low acidic condition was unsuccessful. However, it is
believed that the formation constant in acidic solution is
low compared with aquatic condition. In general, sulfurcontaining ligands (e.g. cysteine, mercaptoacetate)[9]
establish stronger bind with mercury compared with
oxygen containing (e.g., acetate, citrate), EDTA, chloride, phosphate, sulfate, etc. The effect of HNO3 concentration in the strip phase on transport efficiency of
mercury ions through BLM is presented in Fig. 6. The
Asia-Pac. J. Chem. Eng. 2011; 6: 631–638
DOI: 10.1002/apj
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S. S. MADAENI et al.
Asia-Pacific Journal of Chemical Engineering
Table 3. Effect of stirring speed on transport of
mercury (II) ions through BLMa .
Stirring speed
100
200
300
400
500
Remained (%)
Transported (%)
12 ± 2.1
2±1
6 ± 2.2
11 ± 2.4
21 ± 2.7
67 ± 2.7
89 ± 2.2
80 ± 2.8
59 ± 2.9
47 ± 2.2
Conditions: source phase, 5 ml of 5.0 × 10−4 M Hg(II) and 0.1 M
HNO3 ; membrane phase, 10 ml of 4.0 × 10−2 M Cyanex 301 in
chloroform; receiving phase, 10 ml of 5 M HNO3 ; stirring speed,
variable; transport time, 2 h.
a
Each standard deviation obtained with three measurements.
Figure 6. Effect of HNO3 concentration in the receiving
phase on transport of mercury (II) ions through BLM:
(ž) transported; ( ) remaining. Condition: source phase,
5 ml of 5.0 × 10−4 M Hg(II) and 0.1 M HNO3 ; membrane
phase, 10 ml of 4.0 × 10−2 M Cyanex 301 in chloroform;
receiving phase, 10 ml of varying concentration of HNO3 ;
rate of stirring, 300 rpm; transport time, 2 h. Data
represent the mean of three replications ± SD, and
the error bar represents the deviation of the obtained
efficiency from the average value.
results indicate that an increase in HNO3 concentration
up to 5 M improves the efficiency of mercury transport. At high acid concentration, the decomposition rate
of complex containing Hg(II) ions and Cyanex 301 is
enhanced and mercury ions enter to the receiving phase
more readily. A further increase in HNO3 concentration (>5 M) decreases the efficiency to some extent.
The very high HNO3 concentration in the strip phase
leads to back extraction of Hg(II) ions. Moreover, the
very high H+ concentration diminishes the activity of
hydrogen ions for decomposition of HgA2 complex.
This means lower transport of mercury ions into the
receiving phase.
Figure 7. Transport of mercury (II) ions vs time through BLM:
(ž) transported; ( ) remaining. Condition: source phase,
5 ml of 5.0 × 10−4 M Hg(II) and 0.1 M HNO3 ; membrane
phase, 10 ml of 4.0 × 10−2 M Cyanex 301 in chloroform;
receiving phase, 10 ml of 5 M HNO3 ; rate of stirring,
200 rpm. Data represent the mean of three replications
±SD, and the error bar represents the deviation of the
obtained efficiency from the average value.
Effect of stirring speed
Effect of time
Stirring of feed and strip is necessary to minimize
concentration polarization in the feed side and greater
penetration of mercury ions into the strip. The effect
of stirring speed of organic phase in the range of
100–500 rpm on the efficiency of mercury transport
was elucidated. The results (see Table 3) indicate that
the maximum efficiency of mercury transport occurs
at moderate values (89% for 200 rpm). At low stirring
speed, concentration polarization results in greater resistance against passage of mercury ions through the liquid
membrane. At high stirring speeds, the possibility of
mixing of source and receiving phases is maximized
leading to lower net transport of Hg(II) ions through
the liquid membrane. The same trend can be found in
the previous researches.[21,43,44]
The concentration–time profile of mercury transport
under the optimum experimental conditions is depicted
in Fig. 7. The results show a rapid rise in metal
concentration in the receiving phase as well as a sharp
decrease in mercury concentration in the source phase.
After 120 min, 89% of the Hg(II) ions were transferred
into the strip phase and around 2% remained in the
source phase. In this condition, 9% of mercury remained
in the organic phase. After 140 min, 93% of mercury
(II) ions were moved from the source phase. No major
increment in transport efficiency was observed beyond
that time and concentrations of mercury in aqueous
phases were approximately constant. During time, the
driving forces, i.e. concentration gradient is diminished.
This leads to the equilibrium ending the transfer.
 2010 Curtin University of Technology and John Wiley & Sons, Ltd.
Asia-Pac. J. Chem. Eng. 2011; 6: 631–638
DOI: 10.1002/apj
EFFICIENT AND SELECTIVE TRANSPORT OF HG2+ IONS
Asia-Pacific Journal of Chemical Engineering
Table 4.
Transport of mercury (II) ions (for a
competitive experiment) through BLM from various
cation mixtures.
Cation mixtures
Cation
Transported
(%)
Na+ , K+ , Hg2+
Hg2+
Na+
K+
Hg2+
Ca2+
Ba2+
Hg2+
Al3+
Ga3+
Hg2+
Co2+
Ni2+
Zn2+
Hg2+
Cd2+
Ag+
87
0
0
86
0
0
87
0
0
89
0
0
28
89
23
0
2+
2+
Ca , Ba , Hg
2+
Al3+ , Ga3+ , Hg2+
Co2+ , Ni2+ , Zn2+ , Hg2+
Cd2+ , Ag+ , Hg2+
Remained
(%)
2
98
96
3
96
100
2
91
83
2
88
92
56
1
45
89
Conditions: source phase, 5 ml of 5.0 × 10−4 M of each cation and
0.1 M HNO3 ; membrane phase, 10 ml of 4.0 × 10−2 M Cyanex 301
in chloroform; receiving phase, 10 ml of 5 M HNO3 ; stirring speed,
200 rpm; transport time, 2 h.
Selectivity of bulk liquid membrane
The selectivity of the bulk liquid membrane system for
transport of mercury over other cations in equimolar
mixtures is illustrated in Table 4. Among different
cations tried, a low quantity of Ga3+ , Zn2+ and Cd2+
ions were transported with Hg2+ ions. This phenomenon
is in agreement with Pearson’s hard and soft acids and
bases principles and clarifies the affinity of Cyanex
301 as a soft donor base for interaction with most
soft cations. However, transport of these cations has
no serious effect on the efficiency of mercury transfer.
The system is very selective and none of the cations
studied effectively interferes with the proposed system.
Application
Two standard amalgam capsules (Cinalux and Cinasilver) were obtained from Dr. Faghihi Dental Co, Tehran,
Iran. One gram of each sample was dissolved in the
minimum amount of concentrated nitric acid upon mild
heating. The obtained solution was transferred into a
100-ml calibrated flask and diluted to volume with doubly distilled water. Aliquots of 2.00 ml were diluted
to 100 ml with water and were treated by the recommended procedure.[45] These solutions were then utilized for the determination of mercury. The experimental results (Table 5) indicate that the proposed BLM
 2010 Curtin University of Technology and John Wiley & Sons, Ltd.
Table 5. Recovery of mercury (II) ions in real samplesa .
Dental
amalgam
Hg
(II) Content
(mg l−1 )
Value
found (mg l−1 )
Recovery
(%)
Cinalux
Cinasilver
86.5 ± 2.3
87.6 ± 2.6
76.7 ± 2.4
77.8 ± 2.3
88.6
88.8
a
Each standard deviation obtained with three measurements.
Table 6. Selectivity of bulk liquid membrane system for
transport of mercury over other cations in amalgam.
Dental
amalgam
Amalgam
composition
(%)
Hg2+ (45)
Cu2+ (12.10)
Sn2+ (15.95)
Ag+ (26.95)
Cinasilver Hg2+ (45)
Cu2+ (7.15)
Sn2+ (14.85)
Ag+ (33)xsxs
Cinalux
Cation
Hg2+
Cu2+
Sn2+
Ag+
Hg2+
Cu2+
Sn2+
Ag+
Transported Remained
(%)
(%)
89
0
0
0
87
0
0
0
1
90
93
89
2
93
93
87
can be efficiently employed for separation of mercury
content in dental waste samples.
The selectivity of the bulk liquid membrane system
for transport of mercury over other cations in amalgam
solutions is illustrated in Table 6. The results indicate
that the available cations in amalgam composition do
not show any effect on the efficiency of mercury transfer. These experiments confirm that this system is highly
selective and none of the studied cations interferes with
the proposed system. In summary, the proposed BLM
can be efficiently employed for separation of mercury
in dental waste samples.
CONCLUSIONS
A novel bulk liquid membrane system containing
Cyanex 301 was prepared successfully for efficient
and selective extraction of mercury from a mixture of
cations. The extraction behavior of Cyanex 301, with
active component bis(2,4,4-trimethyl(penthyl)dithio
phosphinic acid was found as an efficient reagent for
recovery of mercury ions from acidic nitrate solutions.
The BLM contained 0.04 M carrier in chloroform as
organic solvent. The addition of 0.1 M HNO3 in the
feed solution and 5 M HNO3 in strip phase provided
dominant transport efficiency for BLM. Most of the
Hg(II) ions (89%) penetrated through the bulk liquid
membrane after 120 min. However, the maximum transport (93%) occurred after 140 min at a stirring speed
of 200 rpm in the feed and strip solutions. The present
Asia-Pac. J. Chem. Eng. 2011; 6: 631–638
DOI: 10.1002/apj
637
638
S. S. MADAENI et al.
Asia-Pacific Journal of Chemical Engineering
study demonstrates the effectiveness of the liquid membrane system for combining extraction and stripping
operations in a single process. The proposed system
with appropriate efficiency and selectivity demonstrates
the possibility for selective removal of mercury ion from
mixtures.
Acknowledgements
This work was partially supported by the Physicochemistry Department, Razi Vaccine & Serum Research
Institute in Karaj (Iran). Their support is gratefully
acknowledged. A special thanks go to Dr A. Eshaghi at
the same Institute.
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DOI: 10.1002/apj
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