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Electrowinning of silver from silver-calixarene complexes by
two-phase electrolysis
V. Stankovića*; I. Duob, Ch. Comninellisb and F. Zonnevijllec
aTechnical Faculty Bor, University of Belgrade; VJ 12; 19210 Bor, SCG;
bSchool of Basic Sciences; Swiss Federal Institute of Technology; CH-1015 Lausanne,
Switzerland
cHaute Ecole Valaisanne, Sion, Switzerland
2
Abstract
The work presents a new process for metal recovery from acidic solutions. In establishing
of the new process, the approach adopted is the integration of the solvent extraction and
the electrowinning in one step, making the whole process shorter – and in that manner
simpler than the conventional SX-EW technology. Calixarene-tetramide and its thio-
analogue are used for the extraction of silver ions from aqueous phase. Parameters
affecting the extraction, such as: extractant and silver concentration, pH value of the
aqueous phase, presence of strange ions are changed and the distribution data are
collected. Efficient solvent extraction of silver is achieved with both extractant.
Due to the impossibility of stripping the extracted metal in a conventional way, the
electro-reductive stripping of silver from the loaded organic phase is carried out in the
calixarene nitric acid two-phase system. The effect of current density or electrode
potential on the silver deposition process, current efficiency and cell voltage is also
investigated. Based on experimental data a new process for silver removal from very
dilute solutions is proposed. The proposed process has shown, besides a high extraction
degree of targeted ion from the aqueous phase (due to good features of applied
extractants), a high electrowinning degree of complexed metal from the loaded organic
phase. Renewed extractants can be recycled to the extraction step. High current efficiency
and reasonably low cell voltage are achieved in this process, as well.
Keywords: calixarene; electrowinning; electroreductive stripping; silver recovery;
solvent extraction;
3
1. Introduction
During the last third of the past century numerous processes for metal recovery from
various effluents were developed and offered to the world market. Solvent
extraction/stripping process (SX), followed by the electrowinning (EW) of metal from the
loaded stripping solution, have frequently been considered in these investigations as a
method for selective removal and recovery of some particular metal from water streams.
The intention was to find a proper extractant able to complex targeted ion selectively and
as much as possible quantitatively from the sources containing initially low concentration
of metals as rinse and wastewaters usually have. Noble metals have often been targeted
in these investigations, because of their high price and toxicity at the same time, but also
heavy metals as extremely dangerous pollutants for ground and underground waters [1].
Calixarenes and their derivatives have been attracting much attention as novel and
interesting extractants able to recognize and discriminate metal ions, making them
suitable as specific receptors [2-4]. In these studies, silver has been investigated more
frequently than the other metals [5-7], but also palladium and platinum and heavy metals
alone or in mixture with different noble metals [5,7]. It was found out that alkali and
alkaline earth metals will readily be co-extracted with most of calixarenes, sometimes in
significant amounts [2,4,7,8-10].
However, most of mentioned studies of calixarenes as extractants have had rather a
fundamental than an applicative nature or were performed for analytical purposes. This is
why not much data have been published concerning the stripping of metals complexed
with calixarenes. This problem has slightly been touched in only a few cases but
4
successful way for stripping of metal-calixarene complexes has not yet been found. The
process considered here has arisen from this inability to carry out successfully the
stripping stage in a conventional way. The idea was to apply the two-phase
electrowinning for stripping of the complexed metal from the loaded organic phase,
renewing it for the new extraction cycle. The application of two phase electrolysis in the
electrochemical synthesis of organic substances has been shown as an attractive method
having several technological advantages [11,12]. Some calixarenes are studied for the
electrochemical recognition of barium [13], calcium [14] but also silver [15] and the
other ions as is described by G. McMahon et al. [16] and this fact was encouraging to go
towards the electrochemical stripping of metal bonded by calixarene. Dichloromethane
used frequently as a solvent for calixarenes, is known as an electrochemically stable
compound having a wide potential window that has also directed our considerations
towards the electrochemical treatment of metal-calixarene complexes. Some of our
voltammetric experiments with silver-calixarene complexes were also promising,
indicating that it is possible to reduce silver-calixarene complex in the loaded organic
phase. Discouraging moment is that calixarene amides can easily be electrochemically
oxidized to a calixquinone form, losing their primary features [17,18].
This paper presents a new method for silver recovery from dilute acidic aqueous
solutions using solvent extraction followed by the two-phase electrochemical stripping of
extracted metal from the loaded organic phase with an aim of saving the extracting
capabilities of considered calixarenes during the electrochemical decomposition of silver-
calixarene complexes formed with the extracted metal.
5
2. Experimental
2.1 Chemicals and Solutions
Two calix[4]arene amide derivatives have been employed as extractants in research of the
silver solvent extraction/electrowinning process:
Calix[4]arene-tetramide, with an overall fully-named formula: 5,11,17,23-tetra-t-butyl-
25,26,27,28-tetrakis(N,N-diethylaminocarbonil)methoxocalix[4]arene –LBC; and its
thio-form: calix[4]arene-thiotetramide, having a formula: 5,11,17,23-tetra-t-butyl-
25,26,27,28-tetrakis(N,N- diethylaminothiocarbonil)methoxocalix[4]arene – THIO.
Structural formulae of these calixarenes are presented in Fig. 1
LBC THIO
Fig.1 Structural formulae of calixarenes used in the experiments
LBC and THIO have both been synthesized in the Laboratory of Organic Chemistry of
HEVs Sion, Switzerland, starting from p-t-butylcalix[4]arene, kindly provided by CAL-X
Group from Saxon, Switzerland. The calixarenes were dissolved in dichloromethane
6
previously wetted in 0.1 M of nitric acid aqueous solution. Concentration of calixarene in
dichloromethane in this starting solution was 1ּ10-2 moldm-3.
Transfer of silver ion from an acidic aqueous to the organic phase by calixarenes was
used as a model-system in this study. Aqueous solution of silver is prepared using
standard silver nitrate solution (0.1 moldm-3), supplied by Fluka. The initial concentration
of silver was 1·10-2 moldm-3. Depending on the experiment, these starting solutions were
further diluted by dichloromethane or by 0.1 moldm-3 nitric acid solution, respectively.
2.2 Solvent extraction experiments
Solvent extraction of silver from nitric acid aqueous solution has been carried out in a
classical way. That means, equal volumes of the chosen extractant and silver nitrate
solution were shaken for 5 minutes in a separating funnel and left for phases settling.
After phase separation, samples of the aqueous phase were taken and the concentration of
residual silver was determined by AAS (Shimadzu AAS 665-X). Based on mass balance
the concentration of silver transferred to the organic phase was then calculated. Series of
distribution experiments were performed varying the concentration of extractants,
changing pH, etc., with an aim to evaluate the extraction degree (ED), process
stoichiometry, process equilibrium and the other data relevant for the solvent extraction
process.
2.3 Electrochemical experiments
The electrochemical experiments were performed in a conventional glass three-electrode
cell, equipped by a magnetic stirrer. Platinum spiral served as an anode, while platinum
wire and plate as well as titanium rod (frontal part only was in contact with the
7
electrolyte) and plate electrode were used as a cathode. Silver wire, positioned close to
the cathode, served as a pseudo-reference electrode. Anode was immersed in the aqueous
phase while the cathode was placed into the organic phase, forming side-by-side
configuration as is shown in Fig. 2.
Fig. 2 Schematic presentation of three-electrode cell and the electrolytes
Placing the anode in the aqueous phase should prevent any exposition of the extractant to
a positive electrode potential, thus preventing its anodic destruction. In such a way the
reaction of silver reduction occurs in the organic phase while the reaction of oxygen
evolution will take place as an anodic reaction in the aqueous phase. Prior to immersion
of titanium electrode in the cell, its surface was deoxidized by sand blasting for a few
seconds and then cleaned in ultrasonic bath with a mixture of 2-propanol and distilled
water. Platinum electrodes were only washed in the ultrasonic bath.
W
R
Organic
Phase
Aqueous
Phase
Magnetic
Stirrer
C
8
As a power supplier a computer controlled potentiostat/galvanostat (Autolab PGstat 30,
Eco Chemie B.V.) has been used.
2.4 Experimental procedure and sample analysis
Solvent extraction of silver followed by the electrochemical experiments is carried out
using starting solutions (0.01 moldm-3), in a way as described previously. That means
adequate volumes of chosen extractant and silver nitrate solution (O:A=1:1 in case of
LBC and O:A=1:2 in case of THIO), were shaken in a separating funnel, left for phase
separation and then introduced into the cell, forming there two layers – the lower one
loaded with silver was used as a catholyte and the upper one serving as an anolyte. Tetra-
butyl-ammonium perchlorate serving as a supporting electrolyte was added to the
catholyte in the concentration of 0.01 moldm-3. Anode is immersed into anolyte and both
reference electrode and cathode were submerged into catholyte. Experiments were
performed at an ambient temperature that was varied from 19 to 21oC in the air-
conditioned laboratory.
3. Results
3.1. Solvent extraction and process stoichiometry
To obtain information on the extraction ability, process stoichiometry and the distribution
equilibrium of the extractants, expressed via the extent of extraction, distribution
experiments were carried out, keeping the concentration of silver constant and equal to
1ּ10-3 moldm-3and varying the concentration of extractants in the range from 1ּ10-4 to
2ּ10-3 moldm-3. It means that the molar concentration ratios of the extractant and the
metal to be extracted (CL: CM) were changed in the range of 0.1 to 2. The change of silver
9
concentration in the aqueous phase against the relative concentration of extractant is
presented in Figure 3.
0
0.0002
0.0004
0.0006
0.0008
0.001
0.0012
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2
C
L
mol/mol Ag
+
C
w,Ag+
/moldm
-3
Fig.3 Ability of LBC and THIO to extract silver – the effect of extractant concentration: Initial
concentration of silver 1ּ10-3 moldm-3; O: W = 1:1; ▲,× -THIO, ■,□ - LBC;
O: W represents the ratio between organic (O) and aqueous (W) volumes in the experiments.
It is clear that both LBC and THIO are efficient extractants for silver, achieving an
extraction degree, higher than 90%, with LBC, and even more than 99.9% with THIO,
when they are added in a stoichiometric amount. Higher extraction degree can be
achieved with LBC adding it in a surplus of about 40%. THIO extracts silver almost
quantitatively just adding an amount slightly exceeding the value of 0.5 moles per mole
of silver.
From the experimental data, (Fig. 3), it is also evident that the stoichiometry of silver
complexion with LBC and THIO is different. From the points of the linear parts
10
intersection on the graphs (dashed lines) and their projection on the x-axis [5], the
following stoichiometry of the process of silver complexion with THIO was estimated:
2Ag+w + 2NO-3,w + Lo ↔ [(Ag+)2L](NO3-)2,o (1)
For LBC, the relevant slope on the graph is close to unity, indicating a 1:1 molecular ratio
of silver and calixarene, and the following stoichiometric relation is proposed:
Ag+w +NO-3,w + Lo ↔ [Ag+L]NO3,o (2)
Here, L represents the calixarene as extractant and the subscript w and o refers to
the aqueous and the organic phase, respectively. The other useful data about the solvent
extraction are summarized in Table 1.
Table1. Solvent extraction of silver – working conditions and results
Property/Extractant THIO LBC
O:A* 1:1 1:1
Amount of calixarene
to achieve the highest
ED
0.6 mol/1mol Ag+ 1.4 -1.5 mol/1mol Ag+
ED of Silver >99% 95-97%
Co lour of the
complex
Brown Colourless
Transparency Slightly Turbid Transparent
Influence of pH No influence is
remarked
The higher pH the
lower ED
Influence of Na+
concentration on the
ED of silver
No effect in the
range of 0.001–0.2
moldm-3
ED decreases more
than tenfold in the
same range
* - Volume ratio of the organic and the aqueous phase; the initial concentration of Ag+ is
1 mmoldm-3.
Very good results concerning the extraction of silver are obtained with both investigated
extractants. It is worthy to notify that the loading capacity of THIO is twice higher than
11
of LBC. Also, THIO is equally effective in acid as well as in neutral solutions and is not
sensitive at all to alkali ions, making it more attractive to be employed as an efficient
extractant for selective removal of silver from sources containing alkali ions.
3.2. Cyclic voltammetry and chronoamperomety
Cyclic voltammetry and chronoamperometric experiments were carried out in a stagnant
electrolyte using either platinum wire or titanium rod (2.2 mm in diameter) as the
cathode. The frontal surface of the cathode was only exposed to the electrolyte while the
lateral one was insulated by a plastic resistant to dichloromethane. Representative results
are presented in Fig. 4.
Fig .4a illustrates the change in the cyclic voltammograms, for silver complexed with
THIO. Voltammograms are recorded at different scan rates in the cathode potential range
from 0 to -1.5V. Two cathodic peaks are observed at slower scan rates (v≤ 50 mVs-1).
The first reduction peak is well expressed and appears at a lower potential (750 mV) and
the second one, smaller and poorly defined, at 1 V which disappears at higher scan rates.
Reduction peaks are shifted towards more negative potential indicating an irreversible
reduction process.
12
-2.0E-03
-1.6E-03
-1.2E-03
-8.0E-04
-4.0E-04
0.0E+00
4.0E-04
-1.6-1.4-1.2-1.0-0.8-0.6-0.4-0.20.0
E, V vs. Ag+/Ag
i, A cm-2
4 - 200 mV/s
3 - 100 mV/s
2 - 50 mV/s
1 - 20 mV/s
4
3
1
2
0
0.4
0.8
1.2
1.6
2
0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5
v
0.5
i
p
/mAcm
-2
Fig. 4:.(a) Cyclic voltammograms of Ag-THIO complex recorded at different scan rates;
(b) Plot of the first current peaks against the v0.5
Cathodic peak currents plotted against the square root of the scanning rate are presented
in Fig. 4b. Linear relationship is obtained indicating a diffusion controlled reaction of the
silver complex reduction.
Chronoamperometric measurements are performed with the same electrodes and the same
composition of electrolyte as that used for cyclic voltammetry. The cathode potential was
kept constant and equal to that one corresponding to the first peak obtained at lowest scan
rate and the current change was recorded with time. By plotting the current density
against t-0.5, in accordance with the Cottrell equation, a linear relationship is obtained for
both extractants, as presented in Fig. 5, confirming a diffusion controlled process.
(a)
(b)
13
0
5
10
15
20
25
30
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
t
-1/2,
s
-1/2
I, mA
,,
Fig. 5 Linearization of the chronoamperogramms according to the Cottrell equation: ▲-Ag-THIO
complex; ▄ - Ag-LBC complex
From a phenomenological point of view, the process of silver solvent extraction /
electrowinning in a two-phase system, as we were used in this study, includes three major
steps [19]:
i. mass transfer inside the organic phase that comprises transport of silver-
calixarene complex from the bulk to the cathode; decomposition of the
complex on the cathode; silver deposition; transfer of the reaction products
(nitric ion and released calixarene molecule) away from the cathode to the
bulk;
ii. mass transfer in the aqueous phase, that includes protons transfer, generated in
water splitting reaction, away from the anode to the interface;
iii. ion discharge reaction at the interface that comprises transfer of NO3- ions
from the bulk to the interface, their discharge with H+ ions and mass transfer
of nitric acid in the aqueous phase away from the interface;
14
Being not able to predict which step the rate determining is, at this stage we just like to
keep the discussion at a qualitative level.
As for the stoichiometry of the process, the following reactions are supposed:
The anode reaction splits water generating oxygen and protons:
eHOOH 22
2
2
1
2++→ + (3)
For Ag-THIO complex, for example, the cathodic reaction could be described by the
following stoichiometric equation:
(
)
−
+
−++→+++↔ 33
2
32 22222 NOLAgeAgNOLNOLAg (4)
Similar equation could be written for Ag-LBC complex:
−
−+
−++→+++↔ 333 NOLAgeAgNOLAgLNO (4a)
Nitrate ions will form the flux towards the interface to be discharged by protons forming
nitric acid that will be transferred away from the interface to the bulk of the aqueous
phase:
33 HNONOH ↔+ −
+ (5)
3.3 Electrowinning of silver in two-phase system
While the cyclic and chronoamperometric experiments were performed without stirring
of the electrolyte, during the electrowinning experiments the organic phase was gently
stirred (100 rpm) in order to decrease the concentration polarization but with no
disruption of once established interface between two phases. Prior to switching the cell
on, the aqueous phase was sampled and analyzed by means of AAS to evaluate an actual
15
concentration of silver in the organic phase upon extraction. After completing the
experiment and switching the cell off, the aqueous phase was sampled again for
analyzing. Electrodeposited silver was dissolved in a known volume of a nitric acid
solution, then sampled and analyzed, too. Obtained results helped us to make a mass
balance in both solvent extraction and electrowinning stages. The electrowinning
experiments were performed in both galvanostatic (at different current densities) and
potentiostatic mode (at different cathode potentials). During each experiment, cell voltage
was monitored and changes in either current (under potentiostatic) or cathode potential
(under galvanostatic condition) were recorded, too. These data, together with the
distribution data of silver between phases, served to get a measure on the overall
electrowinning efficiency.
Data relating to the electrowinning conditions as well as some quantitative results about
the deposition of silver are summarized in Table 2.
Table 2. Two-phase electrowinning of silver – working conditions and results
Conditions/Extractant THIO LBC
Cathode Pt, Ti Pt, Ti, Expanded Cu
Anode Pt Pt
c.d. mA/cm2 0.5 – 2 0.25 – 2
Stirring Magnetic Magnetic
EW Feasibility Feasible Feasible
Deposit Structure Powdered Powdered; Dense on TDE
Deposit Co lour Black Black
Deposit Adhesion Poor Poor;
Recycling of Organics Very good Very good
EWD 50 – 80% 60 – 90 %
CE 60 – 90 % 60 – 80 %
AED 20 - 30 % ≈ 30 %
16
Based on these data one may point out that the deposition of silver from the organic
phase is feasible and efficient. Obtained deposit is powdery and poorly adhesive to the
cathode.
The other relevant data, also given in Table 2, such as:
- electrowinning degree (EWD), defined here as a ratio of the mass of deposited
silver and the total amount of silver in the catholyte;
- current efficiency (CE);
- additional extraction degree (AED), as a measure of how much silver was
additionally transferred from the aqueous to the organic phase during the electrowinning,
due to a disturbance of once established equilibrium between phases moving it towards
further transfer of residual silver ions from the aqueous to the organic phase- have high
values for both extractants. The most important fact is that both considered extractants
did not lose their extraction abilities during the electrowinning process. It means that
neither chemical nor electrochemical change of the organic phase has been observed
during the experiments. In other words, it is possible to carry out the electrodeposition of
silver from Ag-calixarene complex and to recycle in such a way the renewed extractant
back to the extraction stage. Five cycles were conducted consecutively with both
extractants, where each cycle implies one SX and one EW stage. A portion of these
results, i.e. the first and the fifth cycle, is presented in Fig. 6.
In the first cycle, silver from the feeding solution in contact with the fresh extractant, is
being transferred into the organic phase. Very high extraction degree is achieved in the
extraction stage. During the electrowinning it was slightly increased due to the AED,
reaching more than 99.6%.
17
Fig. 6 Distribution of silver in solvent extraction followed by two-phase electrolysis of silver
from the loaded organic phase: THIO is used as an extractant; potentiostatic mode of operation
In the electrowinning stage, the loaded organic phase was depleted on silver for almost
81%. Spent organic phase, still containing an amount of silver-calixarene complex, was
mixed with a fresh portion of the feeding solution in the second cycle. Reached extraction
degree in the second cycle is lower than in the first one due to the residual amount of
complexed silver not destroyed in the previous electrowinning stage. Thus, an ED in each
cycle will depend on the working conditions (working current density, or potential) in the
electrowinning stage of that cycle. In any case, a certain amount of silver will be captured
in the organic phase circulating through the process. But this is also the case with the
conventional solvent extraction - electrowinning process. Due to the AED effect, the
SX: ED=69.4%; Overall ED= 95%
EW: E=0.95V; CE=64%;
Feed5: 27.8 mg
Dep. 37mg
E5i:35.6mg
R1:14.2mg V. CYCLE
SX: ED=97%; Overall ED=99.6 %
Feed1: 43.2mg
E1o: 10.4
mg
R1: 1.33 mg
EW: E=0.5V; CE=84%; EWD=81%
S: 0 mg
To II. CYCLE
I. CYCLE
E1i=41.86 mg
To VI. CYCLE
Dep.:32.66.mg
E4: 22 mg; From IV. CYCLE
R2: 0.17 mg
R
2
:2.3 mg
18
overall ED (defined as a sum of ED and AED) does not change significantly and remains
close to that one achieved in the first cycle. It means that the ED in a certain cycle
strongly depends on the EWD in the previous cycle, while the loading capacity of the
extractant remains almost constant and independent of the number of cycling.
The same experiments as presented above we have done with LBC. Similar results are
also obtained with this extractant and given in Table 2, confirming that it is possible to
reuse successfully this extractant, too.
In all experiments the cell voltage did not exceed 3.5 V, so that one may expect moderate
specific energy consumption. Solving the problem of metal stripping by means of direct
electrowinning from the loaded organic phase, that allows the recycling of employed
calixarene many times, makes the described SX/EW process closer to an engineering
application.
3.4. Proposed process for two-phase electrowinning of silver
Conventional extraction/electrowinning process implies three stages: extraction, stripping
and electrowinning stage. In the proposed process, the stripping and the electrowinning
steps are joined into one as it is presented schematically in Fig. 7, making the whole
technology shorter and thus simpler. The proposed process consists only of two steps.
Ionic species, from a feeding solution, entering in the solvent extraction step, will be
extracted by the organic phase. Depleted aqueous phase - raffinate leaves the system as
an off-stream. Loaded organic phase enters into the electrowinning step to be depleted in
metal. After releasing of metal in the electrowinning stage, the spent extractant is
recycled back to the extraction step. Cathode, loaded with deposited metal, is being
19
periodically removed; metal would be stripped and further processed. Fresh cathode
would be introduced in the cell instead of the one loaded with deposited metal.
Fig. 7 Block diagram of the solvent extraction/two-phase electrowinning process
Several benefits come out from the electrowinning of metal complexed by calixarene
from the loaded organic phase. The process is shorter and simpler. That means the
investments will be lower as well as the operating and maintenance costs.
Another fact has arisen from these results, too. Thanking to the AED, it is possible to
perform a SX/EW process continuously in one unit, for example of the mixer/settler type,
Feeding solution
SX Step
Two-phase EW Step
Raffinate
Spent extractant
Loaded cathode out Fresh cathode in
DC
Pregnant extractant
20
as is illustrated in Fig. 8. In this mode, the aqueous phase passes through the unit in a
single pass, while the extractant forms a closed circuit.
Fig. 8 Schematic presentation of the electrodes configuration and phases flowing in an
extractor/cell of a mixer-settler type
These early results are the first but encouraging steps in establishing the new process for
metal recovering from wastewaters by means of the two-phase electrochemical reduction
of a metal-calixarene complex formed in the solvent extraction process. The
electrowinning of silver, complexed by calixarenes, is used as a model reaction system in
these investigations; but one can assume that the other metal complexes having similar
characteristics should behave similarly, widening the process applicability and opening a
new frontiers in electrochemical technology and electrochemical engineering.
4. Concluding remarks
Several conclusions may be drawn from the presented experimental results.
21
Both THIO and LBC are highly effective extractants for silver existing in Ag+-form in
solutions. Having an extremely high ED, higher than 99% in case of THIO, it is possible
to remove silver separately with this calixarene from acid or neutral solutions, achieving
very low concentrations, less than 1·10-7 moldm-3 without affecting the other ions present
in aqueous phase. THIO appears to be better than LBC, having twice-higher loading
capacity, reaching higher ED and keeping high extraction features independently to pH
values of the aqueous phase. By them, THIO does not complex sodium ions while LBC
has considerably high affinity to Na+, often present in solutions containing silver.
The electrowinning of silver from the organic phase, keeping the anode in the aqueous
phase and cathode in the organic phase appears as a novel, possible and effective mode
for metal recovery achieving a reasonably high current efficiency and electrowinning
degree with an acceptably low cell voltage and likewise acceptably low energy
consumption.
The main goal is that both considered calixarenes remain unchanged during the
electrowinning process and can be reused as many times as one wishes. Certain amount
of complexed silver will constantly be captured in the organic phase and recycled back to
the extraction stage. But this fact is well known and always present in conventional SX-
EW processes existing on industrial scale.
Acknowledgements
The first author would like to express his gratitude to the Department of Chemistry and
Chemical Engineering of the School of Basic Sciences of the Swiss Federal Institute of
Technology, Lausanne, Switzerland for giving him the opportunity to carry out the
experiments at their laboratories.
Many thanks also to L. Outtara, B. Correa and L. Menkari for their help in carrying out
the experiments.
22
References
1. V. Stankovic; Chemical and Biochemical Engineering Quarterly; 27(1)(2007)
2. C. D. Gutsche; Calixarenes; Royal Soc. of Chemistry; Cambridge, 1989
3. K. Ohto, E. Murakami, T. Shinohara, K. Shiratsuhi, K. Inoue, M. Iwasaki; Analytica
Chimica Acta; 341(1997) p.275
4. Danil de Namur A. F., M.L. Zapata-Ormachea and R.G. Hutcherson; J. Phys. Chem.
B; 102(1998), 7839.
5. A. F. Danil de Namur, M.T.Goitia, A.R.Casal, J.A.Villanueva-Salas; PCCP; (2001)
No3, 5242
6. M. Ouchi, Y. Shibutami, K. Yakobe, T. Shono, H. Shintani, A. Yoneda, T. Hakushi
and E. Weber; Bioorganic and Medicinal Chem. 7(1999) 1123
7. A. T. Yordanov, D.M. Roundhill and J.T. Mague; Inorganica chimica Acta; 240(1995)
441
8. Regnouf-de-Vains J. R. J.O. Dalbavie, R. Lamartine and B.Fenet; Tetrahedron Letters
42(2001) 2681.
9. K. Ohto, H. Jamaga, E. Murakami and K. Inoue; Talanta 44(1997) 1123
10. Yordanov A. T., D.M. Roundhill; Coordination Chem. Reviews 170(1998) 93
11. Y. C. Hoh, T.Y. Wei, Y.Y.W. T.M. Chiu; Hydrometallurgy 19(1987) 209
12. T. Raju, K. Kulangiappar, M.A. Kulandianthan, G.K. Shankar and A. Muthukumaran;
Electrochimica Acta; 51(2005) 356
13. T.D. Chung, J.Park, J. Kim, H. Lim, M-J. Choi, J.R. Kim, S. Chang and H. Kim;
Anal. Chem.; 73(2001) 3975
14. S.K. Kang, T.D. Chung and H. Kim; Electrochim. Acta 45(2000) 2939
23
15. T. Shinohara, H. Higuchi, Y. Senba, K. Ohto, K. Yoshizuka and K. Inoue; Analytical
Science; 17(2001) 889
16. G. McMahon, S. O’Malley, K. Nolan and D. Diamond; ARCHIVOC VII (2003) 23
17. R. Vataj, A. Louati, C. Jeunesse and D. Matt; Electrochemistry Communication
2(2000) 769
18. R. Vataj, H. Ridaoui,A. Louti, V. Gabelica, S. Steyer and. D. Matt; J. of
Electroanalytical Chemistry; 519(2002) 123
19. F. Reymond, D. Fermin, H. J. Leeand H. Girault; Electrochim. Acta 45(2000) 2647
24
Figure captions
Fig.1 Structural formulae of calixarenes used in the experiments
Fig. 2 Schematic presentation of three-electrode cell and the electrolytes
Fig.3 Ability of LBC and THIO to extract silver – the effect of extractant concentration:
Initial concentration of silver 1ּ10-3 moldm-3; O: W = 1:1; ▲,× -THIO, ■,□ - LBC;
O: W represents the ratio between organic (O) and aqueous (W) volumes in the
experiments.
Fig. 4:.(a) Cyclic voltammograms of Ag-THIO complex recorded at different scan rates;
(b) Plot of the first current peaks against the v0.5
Fig. 5 Linearization of the chronoamperogramms according to the Cottrell equation: ▲-
Ag-THIO complex; ▄ - Ag-LBC complex
Fig. 6 Distribution of silver in solvent extraction followed by two-phase electrolysis of
silver from the loaded organic phase: THIO is used as an extractant; potentiostatic mode
of operation
Fig. 7 Block diagram of the solvent extraction/two-phase electrowinning process
Fig. 8 Schematic presentation of the electrodes configuration and phases flowing in an
extractor/cell of a mixer-settler type
