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Silver electrowinning from silver(I)–calixarene complexes by two-phase electrolysis

  • Intex Diagnostika GmbH, Basel, Switzerland

Abstract and Figures

Results are reported, providing the basis for an elegant new process for metal recovery from acidic solutions, by integrating solvent extraction (SX) and electrowinning (EW) in a single process, rather than operating them separately, as in conventional SX–EW processes. Calixarene-tetramide and its thio-analogue were used for the extraction of silver(I) ions from aqueous phases into dichloromethane, both compounds achieving extraction efficiencies >90%. The effects were determined of extractant and silver(I) concentrations, aqueous phase pH, and the presence of sodium ions, on the distribution of Ag(I) between aqueous and organic phases. Due to the impossibility of stripping the extracted metal conventionally, electro-reductive stripping of silver(I) from the loaded organic phase was carried out in the calixarene/nitric acid two-phase system. The effects were also investigated of current density or electrode potential on silver deposition current efficiencies and cell voltages. Based on experimental data, a new process for silver(I) removal from very dilute solutions was proposed that showed high percentage extractions of silver(I) from the aqueous phase, coupled to direct EW of silver from the calixarene complex in the loaded organic phase, regenerating the extractant for recycle. Current efficiencies in the range from 60% to 90% and cell voltages <3.5V were achieved simultaneously in this process.
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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,
cHaute Ecole Valaisanne, Sion, Switzerland
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;
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
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.
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
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
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
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
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.
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
concentration in the aqueous phase against the relative concentration of extractant is
presented in Figure 3.
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2
mol/mol Ag
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
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
0.6 mol/1mol Ag+ 1.4 -1.5 mol/1mol Ag+
ED of Silver >99% 95-97%
Co lour of the
Brown Colourless
Transparency Slightly Turbid Transparent
Influence of pH No influence is
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
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
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.
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
0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5
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.
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
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
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;
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+++ (3)
For Ag-THIO complex, for example, the cathodic reaction could be described by the
following stoichiometric equation:
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
+ (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
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 %
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
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%.
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
R1:14.2mg V. CYCLE
SX: ED=97%; Overall ED=99.6 %
Feed1: 43.2mg
E1o: 10.4
R1: 1.33 mg
EW: E=0.5V; CE=84%; EWD=81%
S: 0 mg
E1i=41.86 mg
E4: 22 mg; From IV. CYCLE
R2: 0.17 mg
:2.3 mg
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
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
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
Spent extractant
Loaded cathode out Fresh cathode in
Pregnant extractant
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.
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
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.
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.
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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
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
... The application of two-phase electrolysis in the electrochemical synthesis of organic substances has been shown to be an attractive method having several technological advantages [21,23]. For metal recovery, this method has not yet been applied and this is the first attempt also described in [25]. ...
... As a power supplier a computer controlled potentiostat/galvanostat (Autolab PGstat 30, Eco Chemie B.V.) was employed. The electrowinning experiments were performed in both the potentiostatic and glavanostatic mode of operation [22,25]. ...
... To avoid any anodic destruction of the calixarenes, the electrochemical experiments were conducted using a previously proposed two-phase electrowinning method [21,23]. The experimental procedure and the obtained results are described in detail elsewhere [22,25]. Hence, only a brief description of the stripping process and the electrowinning results will be presented in this paper. ...
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Solvent extraction experiments were carried out to recover silver from nitric acid solutions using as the extractants calix[4]arene tetramide (LBC) and its thio-analogue (THIO) dissolved in dichloromethane. The stoichiometry of the formation of the silver-calixarene complex differs depending on the used extractant. It was shown that one silver ion reacts with a single molecule of LBC, while two silver ions are extracted with a single THIO molecule. Very high percent extractions (>90% with LBC and >99% with THIO) were achieved with both extractants by just adding surplus extractant (THIO: 0.6mol/mol Ag+ and LBC: 1.4–1.5mol/mol Ag+). Based on equilibrium data, the extraction isotherm relationship and the extraction constant Kex for each extractant were derived. The extraction results show that THIO is an efficient extractant extracting silver either from acid or from neutral solutions with the same efficiency. Nitric acid, in the concentration range from 0 to 0.1moldm−3, strongly affects the extraction capability of LBC in less acidic solutions, reducing the percent extraction down to 40%. LBC extracts both silver and sodium collectively and with the same efficiency. The selectivity coefficient has a value close to unity (β≈1). THIO has demonstrated extremely high selectivity for silver over sodium in a mixture. Stripping of silver was performed by two-phase electrowinning from the loaded organic phase, thereby achieving a high stripping degree (50–90%) and reasonably high current efficiency (60–90%). The calixarenes did not change their extraction features during the electrowinning process and can be reused for extraction many times.
... However, the low level of silver in the leaching solutions makes its recovery very challenging. For example, conventional electrowinning (EW) is widely used for silver recovery from concentrated solutions, [19][20][21][22][23] however, in the case of dilute silver solutions, the mass-transport limitation during electrowinning can lead to the dramatic increase of energy consumption and operation time. 24,25 Various approaches such as ion-exchange, 26 membrane separation, 27 adsorption 28 or solvent extraction [29][30][31] have been investigated as ways to obtain silver-rich solutions, nevertheless, these methods commonly require the utilization of organic-based substances or extra chemical additions that can result in environmental issues. ...
... A high silver recovery of 98% with 99% purity was achieved, albeit at a long recovery time of 128 h. Stanković et al. (2007) employed electrowinning on Ag(I) extracted with calixarine tetramide and its thiol analogue since traditional stripping results in low efficiencies. Around 60 to 90% of Ag(I) can be recovered from the thiol calixarene, which can be recirculated throughout the process. ...
Critical and precious metals are essential in many modern applications. While their natural sources are depleting, one must adapt to guarantee a reliable supply by developing new and optimizing existing techniques to recover the elements from unexplored material flows. The aquatic phase is of great meaning to this issue, as migration from solid to liquid streams is ubiquitous during industrial manipulation of the raw materials. The resulting (waste) waters are characterized by low concentrations and varying chemical composition. Hence, hydrometallurgical technologies should cope with such specific system conditions and physico-chemical properties of critical and precious metals when elaborating a recovery strategy. This review provides an overview of the present status and outlook on technologies used to recover critical metals from solution, including cementation, precipitation, reduction, ion exchange, solvent extraction, electrochemical methods and adsorption onto novel, sustainable materials. Special attention is given to adsorption technology, which is considered as one of the most promising metal recovery options owing to its facile implementation, low cost, high availability and high removal efficiencies even at low target metal concentrations. Key directions are suggested to tackle existing challenges in the field of resource recovery and improve the sustainability of future material cycling.
... Biphasic electrolysis has a distinct advantage over conventional homogenous electrolysis in practical electro-organic synthesis. [19][20][21][22] In this biphasic electrolysis process the extraction of product from the reaction mixture by an organic solvent is not necessary as the product concentration increases in the organic phase while the electrolysis proceeds. After completion of electrolysis the product containing the organic phase can be separated by simple phase separation and the aqueous electrolyte can be reused, which is an important aspect considered during the reaction scaleup, as this makes the process economical. ...
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A simple method of electrochemical bromination of a series of cyclic and acyclic enes (styrene and substituted styrenes, stilbene, indene and cyclooctene) in biphasic water-chloroform mixture mediated by bromide/bromine redox system is reported. Aqueous 25% NaBr/ H2SO4 is used as the electrolyte. Regio and stereo selective dibromination of enes is achieved. Moderate to excellent yields of the product (83-98%) is obtained depending on the substrate. Electrolyte reuse has also been demonstrated successfully using HBr in the dibromination of styrene.
... These solvents dissolve calixarenes quite well; these extractants have low viscosity and reasonably higher density than the aqueous phase, ensuring fast and efficient phase separation and could be acceptable in some especial cases as it is SX of precious metals. Recently, ionic liquids have been considered as diluents for some calixarene derivatives [10], what opens a new possibility to apply a direct electrowinning of extracted metals from organic phase [11]. ...
Conference Paper
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Rhodium has currently twice higher price than platinum and approximately seven times higher than palladium on the world market. Its price has had a very turbulent motion within last five years, going up and dawn following the world metal market instability. Currently, its price is rising slowly reaching at the end of 2010 the price of around 106.000 $US/kg. In this article a review on rhodium recycling processes from spent catalytic materials will be considered. Particular attention is paid to the solvent extraction and cementation of rhodium from acidic solutions. In this regard, the case-study will be presented, considering a solution containing rhodium, palladium and platinum ions in a real leach solution. Some experimental results on separation of these ions by solvent extraction with calixarenes and cementation with copper powder are also given and discussed. A new technology scheme for rhodium recycling is proposed. Keywords rhodium recycling; solvent extraction; PGMs; cementation
Phosphonium-based ionic liquids (IL), i.e., triethyl-n-pentyl, triethyl-n-octyl, and triethyl-n-dodecyl phosphonium bis(trifluoromethyl-sulfonyl)amide, [P222X][NTf2], (X=5, 8, and 12) were investigated for Au(III) extraction. The IL–Au complex was identified as [P2225][AuCl4] using UV–Vis–NIR and Raman spectroscopic analyses. Slope analyses with the concentration dependence of [P222X+] confirmed the anion-exchange mechanism of Au(III) extraction by [P222X+] (X=5, 8, and 12). The enthalpy, entropy, and Gibbs free energy for Au(III) extraction were determined using thermodynamic analysis, indicating that lower temperatures had a positive effect on the Au(III) extraction. Electrochemical analysis revealed that extracted Au(III) can be reduced in two steps: (i)Au(III) + 2e- → Au(I), (ii) Au(I) + e- → Au(0)]. The diffusion coefficients of the extracted Au(III) species in [P222X][NTf2] (X=5, 8, and 12) were evaluated from 323 to 373 K using semi-integral and semi-differential analyses. Because of the viscosity of the IL medium, the diffusion coefficient of the extracted Au(III) increases with increasing alkyl chain length. The 4f7/2 spectrum based on X-ray photoelectron spectroscopy revealed that the Au electrodeposits obtained after 10 cycles of continuous extraction and electrodeposition were in the metallic state.
A sustainable and effective sulfuric acid-based process with the combination of facile acid leaching and electrowinning has been developed for the recovery of valuable elements from spent silver oxide batteries. Results suggest that the dissolution of elementary Ag was markedly promoted by the presence of MnO2 in the spent silver oxide batteries. Also, H2O2 was added to support an improved Mn and Ag extraction after 240 min of leaching. In the leaching step, 97% of silver and over 99% of Mn and Zn could be extracted under the optimum conditions: 1 mol/L H2SO4, a leaching temperature of 70 °C, an S/L ratio of 50 g/L, addition of 3 v/v % H2O2 at 240 min, and a total leaching time of 270 min. Ultra-pure silver (Ag w/w % ≥ 99.98%) was further recovered from the pregnant leaching solution (PLS) by potentiostatic electrowinning. Under the optimum deposition potential of −0.10 V and after 4 h of electrowinning, the silver recovery reached 98.5% with a high energy efficiency of 98.7%. Simultaneously, 5.6% Mn was recovered on the anode in the form of MnO2. Overall, these promising results suggest feasibility in the recycling of silver oxide batteries in sulfuric acid media.
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The demand of silver is ever increasing with the advance of the industrialized world, whereas worldwide reserves of high grade silver ores are retreating. However, there exist large stashes of low and lean grade silver ores that are yet to be exploited. The main impression of this work was to draw attention to the most advance technologies in silver recovery and recycling from various sources. The state of the art in recovery of silver from different sources by hydrometallurgical and bio-metallurgical processing and varieties of leaching, cementing, reducing agents, peeling, electro-coagulants, adsorbents, electro-dialysis, solvent extraction, ion exchange resins and bio sorbents are highlighted in this article. It is shown that the major economic driver for recycling of depleted sources is for the recovery of silver. In order to develop an nature-friendly technique for the recovery of silver from diverse sources, a critical comparison of existing technologies is analyzed for both economic viability and environmental impact was made in this amendment and silver ion toxicity is highlighted.
The stripping of cerium (IV) from loaded organic solvent (30% TBP-70% dichloromethane) by valency change with two-phase electrolysis was studied experimentally. The results show that the average current efficiency and the electrical energy consumption are in the range of 50–70% and 1.0–2.0 kW/kg cerium, respectively. The effects of various parameters on cerium stripping were also studied. The selected parameters were agitation speed, rate of nitrogen bubbling, organic and aqueous acidities, surfactant addition and density ratio. The density ratio (defined as density of the organic phase to that of the aqueous phase) is a dominant factor for the process studied.
Distribution experiments involving silver (I) perchlorate in the mutually saturated water–dichloromethane solvent system at 298.15 K were carried out. The data were used to calculate the partition, Kp and the distribution, Kd constants in this solvent system and the ion-pair formation constant, Ka in the water saturated organic phase. Comparison of Kd, Kp and Ka values with corresponding data in the water–nitrobenzene solvent system clearly reflect the medium effect on these parameters. Distribution data in the presence of various calix(4)arene amine derivatives were measured. These data show that in the water–dichloromethane solvent system the stoichiometry of the extraction process (1 : 2; ligand : metal-cation) has been altered relative to that in water–nitrobenzene (1 : 1). This is corroborated by conductometric titrations of silver (I) and these macrocycles in water saturated dichloromethane. An equation for the derivation of the extraction constant, Kex and Kd from distribution data for a 1 : 2 (ligand : metal-cation) extraction process has been formulated and successfully applied. Good agreement is found between the Kd values derived from distribution data in the absence and in the presence of the macrocycle. The individual steps which contribute to the overall extraction of silver (I) from water to dichloromethane are discussed. Final conclusions are given.
The compound 5,11,17,23-tetra-tert-butyl-25,26,27,28-tetra-(2-bromoethoxy)calix[4]arene has been prepared by first converting 5,11,17,23-tetra-tert-butyl-25,26,27,28-tetra-(2-hydroxyethoxy)calix[4]arene into the tosylate, and then to the product by reaction with LiBr. The compound crystallizes in the trigonal space group P3221 with a = 13.160(2), c = 25.595(6) Å, a = 90.00(2), β = 90.00(1), γ = 120.000(9)0, Z = 3, ϱcalc = 1.40 g cm−3. The final R value for 2391 unique reflections was 0.061. The compound reacts with excess sodium N,N-dimethyldithiocarbamate to give 5,11,17,23-tetra-tert-butyl-25,26,27,28-tetra-(2-N,N-dimethyldithiocarbamoylethoxy)calix[4]arene. This compound is an effective extractant for transferring palladium(II) from an aqueous to a chloroform phase. No extraction of PtCl42− is observed under thermal conditions. Under photochemical conditions using a mixture of PtCl42− and PtCl62−, extraction of platinum into the chloroform layer is observed. An explanation for this observation is given.
Dibenzo-16-crown-4 (1) indicates high silver and thallium(I) ion selectivity over sodium, potassium, and rubidium ion evaluated from the solvent extraction of metal picrates, while its cation-binding ability is lower than those of dibenzo-18-crown-6 (2) and dibenzo-22-crown-6 (3). Taking account of the highest thallium(I) ion selectivity for 1 obtained from extraction experiments, PVC membrane thallium(I)-selective electrodes based on 1 are prepared. The electrode shows the best potentiometric selectivity coefficients for thallium(I) over potassium and rubidium than those of 2 and 3, and commercially available bis(crown ether)s (4).
The electrochemical reduction of nitrate ion was studied by cyclic voltammetry on Pt(111) and [n(111)×(111)] stepped Pt surfaces, where n (=14, 10, 7, 6, 5, 4, 3, 2) is the number of terrace atoms, in 0.1M HClO4+10mM KNO3. The electrocatalytic nitrate reduction was found to hardly proceed on Pt(111) in the hydrogen adsorption region, while the electrocatalytic activity was improved with the increase in the step density. Inactivation was observed in the presence of adsorbed hydrogen or nitrate-derived reduced adsorbate, i.e. adsorbed NO, on (111) step sites. It was, therefore, concluded that the electrocatalytically active NO3− species does not adsorb on the (111) terraces but on the (111) monoatomic steps. The nitrate reduction current increased with the step density in a non-linear relationship. The overall current density at 0.21V (RHE) corresponding to the peak potential of the main electrocatalytic nitrate reduction wave which was maximum at n=2, abruptly increased with short terraces, i.e. n
A simple method for the preparation of benzyl chloride from toluene by two-phase electrolysis is reported. The major product is benzyl chloride in contrast to chlorotoluenes in homogeneous electrolysis. Electrolysis was carried out at 30°C in chloroform solvent. Toluene conversion ranges from 80 to 85% and the selectivity is as high as 95%. The effect of different electrodes, temperatures, and current density are studied and the optimum condition is reported.
The reduction of volatile organic chlorides has been investigated on different electrode materials, in pure aprotic and mixed aqueous-aprotic solvents, by means of cyclic voltammetry (CV) and of preparative electrolyses. Silver resulted in all cases the most electrocatalytic material, but some AgBi and AgSn alloys can be considered as promising alternatives. The CV features of the reduction of trichloromethane show two main reduction steps, mainly governed by the accessibility of the electrode active sites. The reduction end products of preparative electrolyses are methane and traces of higher partially dehalogenated hydrocarbons. Oligomeric products sensibly lower the reaction rate in pure aprotic solvent, while totally disappear in presence of water. Production of methane, generally accompanied by chloromethane, is observed, although to a lower extent, also at working potentials much more positive than the ones corresponding to dichloromethane reduction.
The voltammetric study on a water-soluble calix[4]arene (calix[4]arene-triacid-monoquinone, CTA) in basic aqueous solution in the presence of Ca2+ ion provided important information about the unique electrochemical behavior of Ca2+–CTA complex. The redox behavior of CTA and voltammetric responses to Ca2+ ion are reminiscent of those of quinone-derivatized calix[4]arenes in aprotic media. Using CTA, Ca2+ ion in aqueous solution could be recognized quantitatively by voltammetric techniques.
The electrochemical oxidation of p-But-calix[4]arene-(OH)2-(OCH2CONEt2)21 has been investigated for the first time and was shown to result in the formation of the corresponding diquinone 3. The reaction proceeds via two successive two-electron irreversible oxidation steps both governed by an ECE mechanism. Alkali cations recognition can be realized by exhaustive oxidation of 1 in the presence of alkali salts.