model for metal extraction with basic extractants: a
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Model for Metal Extraction with Basic Extractants: aCoordination Chemistry Approach
Rayco Lommelen, Tom Hoogerstraete, Bieke Onghena, Isabelle Billard, KoenBinnemans
To cite this version:Rayco Lommelen, Tom Hoogerstraete, Bieke Onghena, Isabelle Billard, Koen Binnemans. Model forMetal Extraction with Basic Extractants: a Coordination Chemistry Approach. Inorganic Chemistry,American Chemical Society, In press, �10.1021/acs.inorgchem.9b01782�. �hal-02271223�
1
Model for Metal Extraction with Basic 1
Extractants: a Coordination Chemistry Approach 2
Rayco Lommelen,a Tom Vander Hoogerstraete,
a Bieke Onghena,
a Isabelle Billard
b and Koen 3
Binnemansa,*
4
aKU Leuven, Department of Chemistry, Celestijnenlaan 200F, P.O. box 2404, B
–3001 5
Leuven, (Belgium) 6
bUniversité Grenoble Alpes, CNRS, Grenoble INP, LEPMI, 38000 Grenoble (France) 7
8
2
ABSTRACT 1
2
Solvent extraction is a technique very often used for metal separation on industrial scale. 3
The extractants for solvent extraction can be subdivided into three categories: acidic, neutral 4
and basic extractants. The metal extraction mechanism of basic extractants is typically 5
described as an anion exchange process, but this mechanism does not correctly explain all 6
observations. This paper introduces a novel model for the extraction of metals by basic 7
extractants supported by experimental data on methyltrioctylammonium chloride and Aliquat 8
336 chloride systems. The hypothesis is that the metal species least stabilized in the aqueous 9
phase by hydration (i.e. the metal species with the lowest charge density) is extracted more 10
efficiently than the more stabilized species (i.e. species with higher charge densities). Once 11
transferred to the organic phase, the extracted species undergoes further Lewis acid-base 12
adduct formation reactions with the chloride anions available in the organic phase to form 13
negatively charged chloro complexes, which can associate with the organic cations. Salting-14
out agents have an influence on the extraction, most likely by decreasing the concentration of 15
free water molecules, which destabilizes the metal complex in the aqueous phase. The 16
evidence provided includes: (1) the link between extraction and transition metal speciation; 17
(2) the trend in extraction efficiency as a function of different salting-out agents, and (3) the 18
behavior of HCl in the extraction system. The proposed extraction model explains better the 19
experimental observations and allows predicting the optimal conditions for metal extractions 20
and separations a priori, by selecting the most suitable salting-out agent and its concentration. 21
22
23
KEYWORDS: Aliquat 336; Anion exchangers; Transition metals; Solvent extraction; 24
PCA-MCR-ALS. 25
3
1. INTRODUCTION 1
2
Solvent extraction is a technique very often used for metal separation on industrial scale, 3
because it can process large volumes in a controllable manner.1,2
In solvent extraction, a 4
metal-containing aqueous phase is contacted with an immiscible organic phase (solvent) 5
containing an extractant, a diluent and sometimes a modifier. The extractant is an organic 6
ligand designed to selectively coordinate to the target metal ions. The diluent is used to 7
increase the solubility of the metal complex in the organic phase and to reduce the viscosity, 8
but is in some cases omitted. The modifier is used to change some important physical 9
properties of the organic phase, for instance to prevent crud formation and to avoid third-10
phase formation.3 During the extraction, metal separation is achieved based on the difference 11
in affinity of the metal ions for the selected extractant. The extractants can be divided into 12
three main classes: (1) cation exchangers or acidic extractants (e.g. alkyl phosphorus acids or 13
carboxylic acids),4,5
(2) solvating extractants or neutral extractants (e.g. ketones or 14
organophosphorus esters),2,6
and (3) anion exchangers or basic extractants (e.g. protonated 15
amine or quaternary ammonium salts).7–10
16
The extraction mechanism of metals by basic extractants is typically described as an anion 17
exchange process in which a negatively charged metal complex present in the aqueous phase 18
is exchanged for anions in the organic phase: 19
20
(1) 21
22
In equation (1), M represents the metal, X a metal-coordinating anion (e.g. Cl–, NO3
–, 23
SCN–, …), Q is a cation (e.g. protonated amine, quaternary ammonium ion,…) and the 24
horizontal bars indicate species in the organic phase. 25
4
The extraction of metals by basic extractants is assumed to be facilitated by the formation 1
of the anionic
complex in the aqueous phase (equation 2).1,11,12
A sufficiently high 2
concentration of the metal-coordinating anion X– is thus required: 3
4
with – and (2) 5
6
Although many examples on the extraction of metals by basic extractants have been 7
reported in the literature, the extraction mechanism has not been fully elucidated yet, due to 8
the difficulties in describing all the equilibria involved in the extraction process.1 9
Several interactions occur between the amine in the organic phase and the metal–10
coordinating anion X–
in the aqueous phase. First, non-protonated amines extract acids via an 11
acid-base reaction in the form of , with A being an amine. Secondly, the extractants 12
might form oligomers in the nonpolar organic phase.13
Thirdly, the extractant is involved in 13
the extraction of the metal, and fourthly, all these equilibria depend on the nature of the 14
diluent. Moreover, the driving force behind extraction is the addition of salt to the aqueous 15
phase. This salt addition does not only change the metal speciation in the aqueous phase, but 16
it also changes the ionic strength and thus the activity of the species in the aqueous phase. 17
Hence, it is very difficult to determine stability constants and derive appropriate equations for 18
the extraction of metals by basic extractants. Moreover, additional equilibria occur at high 19
salt concentrations related to the association of the salting-out cation with the anion, reducing 20
the “free” anion concentration in the aqueous phase, (e.g. the formation of CaCl+ by 21
coordination of Cl– to Ca
2+. Finally, various anionic metal species are present in the organic 22
phase as well. 23
In this paper, an alternative model for the extraction of metals by basic extractants is 24
presented. This model better explains the experimental observations described in the 25
5
literature and therefore it is probably closer to the true extraction mechanism. Although the 1
term extraction mechanism is often referring to how a metal is transferred from the aqueous 2
into the organic phase, the current approach rather focuses on how the extraction equilibrium 3
state is reached and influenced by the aqueous and organic phase. After introduction of the 4
new model, it is experimentally tested by studying the influence of salting–out agents on the 5
distribution ratios of Cu(II), Co(II) and Zn(II), related to the ionic strength, speciation and 6
hydration of the metal complexes in the aqueous phase, and the speciation in the organic 7
phase. Also, the behavior of HCl in solvent extractions using basic extractants is considered 8
to investigate the supposed competition between HCl and metal extraction reported in the 9
literature.14–19
10
The difficulty in explaining all observations from metal extraction with basic extractants 11
has attracted attention in recent literature. Uchikoshi et al. investigated the aqueous 12
coordination chemistry of Cu(II) and Co(II) and tried to relate their findings to the absorption 13
of Cu(II) and Co(II) on anion-exchange resins. They concluded that an anion-exchange 14
mechanism for adsorption on resins was correct, although significant adsorption was already 15
found when only very small concentrations of anionic species were present.20,21
Also, no 16
explanation for the decreasing adsorption efficiency at high HCl concentrations could be 17
given. Onghena et al.22
and Vander Hoogerstraete et al.23
investigated the speciation of 18
trivalent lanthanide ions in solvent extraction systems and concluded that the speciation in the 19
aqueous and organic speciation is different. Deferm et al. studied the relation between solvent 20
extraction and speciation, but now of In(III).24
They already made a link between the aqueous 21
In(III) species and the extraction efficiency of In(III). Also, the extraction mechanism of 22
Fe(III) with Cyphos IL 101 has gotten some attention.25
The authors proposed that Fe(III) can 23
also be extracted as FeCl3 via an ion association mechanism together with the extraction of 24
FeCl4- via an anion exchange mechanism. 25
6
2. EXPERIMENTAL SECTION 1
2
2.1. Chemicals 3
An aqueous solution of hydrogen bis(trifluoromethylsulfonyl)imide (80%) was purchased 4
from Iolitec (Heilbronn, Germany). HNO3 (65 wt%), NH4Cl (99.99%), NaCl (99.99%), LiCl 5
(99.9%), NaOH (0.1 M), HCl (~37 wt%) and toluene were purchased from VWR (Leuven, 6
Belgium). CuO (99.99%), CuCl2 (99.995%), MgCl2·2H2O (>99%), CaCl2·2H2O (>99%), 7
AlCl3·6H2O (>99%) and Aliquat® 336 were purchased from Sigma-Aldrich (Overijse, 8
Belgium). The cesium, copper, cobalt, scandium, indium and zinc aqueous standards (1000 9
mg L–1
in 35% HNO3), Lanthanum organic standard (1000 µg g-1
in Standard matrix oil 55-10
65 mPas), CoCl2·6H2O (>98%), ZnCl2 (>98%) and KCl (>99.5%) were obtained from Chem 11
Lab (Zedelgem, Belgium). Methyltrioctylammonium chloride (98%) was purchased from 12
J&K Scientific (Lommel, Belgium). CsCl (>99.9%) was obtained from Carl Roth (Karlsruhe, 13
Germany). Ethanol (absolute, >99.8%) was purchased from Fisher Scientific (Merelbeke, 14
Belgium). Water was always of ultrapure quality, deionized to a resistivity of >18.2 MΩ cm 15
with a Sartorius Arium Pro ultrapure water system. All chemicals were used as received, 16
without any further purification. 17
18
2.2. Metal extraction and quantification 19
Metal extractions (Cu(II), Co(II) and Zn(II)) were performed with 1 mL of aqueous phase 20
and 1 mL of organic phase in glass vials with a volume of 4 mL. The metal concentration in 21
the aqueous phase was kept constant by adding a fixed volume of a metal stock solution in 22
water to an aliquot of a highly concentrated acid or salt solution (HCl, LiCl, NaCl, KCl, 23
CsCl, MgCl2, CaCl2, AlCl3 or NH4Cl), diluted with a certain volume of ultrapure water to a 24
total volume of 10 mL. The final concentrations of Cu(II), Co(II) and Zn(II) in the aqueous 25
7
samples was 0.5 g L-1
, 1.0 g L-1
and 2.0 g L-1
, respectively. A higher concentration of Zn(II) 1
was chosen as it is extracted much more efficiently, making it harder to measure accurately 2
the Zn(II) concentration in the aqueous phase after extraction. The salt solutions were 3
prepared by working very close to the solubility limit. The exact salt concentrations were 4
calculated based on the densities measured after preparing the solutions. In this way, 5
weighing errors due to the uptake of water by the hygroscopic salts were avoided.26
The 6
concentrations of the salt solutions of AlCl3 were calculated based on the weights because of 7
the unavailability of density–concentration data. HCl (0.5 mL, 37 wt%) was added to 25 mL 8
of the AlCl3 salt solution to avoid hydrolysis of Al(III) and 0.0225 mol L-1
HCl was added to 9
all Zn(II) aqueous phases to avoid hydrolysis of Zn(II) during extraction. 10
The organic phase was made by diluting methyltrioctylammonium chloride (TOMAC) 11
(dried overnight on a Schlenk line) in toluene to a concentration of 0.2 M. The extractions 12
were performed for 30 min at room temperature at 2000 rpm and phase separation was 13
accomplished by centrifugation for 2 min at 5000 rpm. 14
The metal concentration in the aqueous phase before and after extraction was measured 15
using ICP-OES and the aqueous HCl concentration was corrected for the loss of HCl to the 16
organic phase, based on HCl extraction experiments. Distribution ratios (D) were calculated 17
with the following formula: 18
19
(3) 20
21
where and are the equilibrium metal concentrations in the aqueous and 22
organic phase after extraction, respectively. The concentration of metal in the organic phase 23
was calculated via the mass balance. In case of an equal volume of organic and aqueous 24
phase, equation (3) can be rewritten as: 25
8
1
(4) 2
3
with being the initial metal concentration in the aqueous phase. The experimental 4
error was calculated based on triplicate measurements and was less than 5%. Error bars on 5
graphs were omitted for the sake of legibility. 6
7
2.3. HCl extraction and quantification 8
Extractions of HCl were performed on a slightly larger scale, i.e. 5 mL of aqueous and 5 9
mL of organic phase in centrifuge tubes of 15 mL. The aqueous phase consisted of HCl of 10
which the concentration was determined via its density. The organic phase was either water-11
saturated Aliquat 336, water-saturated TOMAC or 0.24 mol L-1
Aliquat 336 in toluene (0.2 12
mol L-1
quaternary compound; commercial Aliquat 336 has 80 wt% quaternary 13
compounds).27
All organic phases were presaturated with water to avoid large volume 14
changes. Extractions were performed using a Burrel wrist action shaker at room temperature 15
for 1 hour at 450 rpm. Afterwards the HCl concentration in the aqueous phase was 16
determined via its density and corrections were made for volume changes, determined 17
visually using a graduated cylinder. The organic water content was determined using Karl 18
Fischer titration. 19
20
2.4. Instrumentation and analysis methods 21
UV/VIS absorption spectra of the aqueous phases were measured with an Agilent Cary 22
6000i spectrophotometer and Cary WinUV software. Metal ion concentrations were 23
determined by inductively coupled plasma optical emission spectroscopy (ICP–OES), with a 24
Perkin Elmer Avio 500 spectrometer equipped with an axial/radial dual plasma view, a 25
9
GemCone High Solids nebulizer, a baffled cyclonic spray chamber and a demountable quartz 1
torch with a 2.0 mm internal diameter alumina injector. Samples, calibration solutions and 2
quality controls solutions were diluted with HNO3 (2 vol%). All ICP-OES spectra were 3
measured in triplicate. Calibration curves were made using a solution of 0.1, 1 and 10 mg L-1
4
of the corresponding metal from a standard solution. Quality checks were performed with 5 5
mg L–1
metal in different concentrations of the salt, equal to the matrix concentrations after 6
dilution. In(III) or Sc(III) were added and only applied as internal standards if the quality 7
checks failed because of matrix effects. 8
Densities of the acid and salt solutions were measured with an Anton Paar DMA 4500M 9
densitometer. Nemus Life Thermo Shakers TMS–200 were used for the extraction 10
experiments. A Heraeus Labofuge 200 centrifuge was used to accelerate phase separation. 11
The water content in organic phases was measured using a Mettler-Toledo V30S volumetric 12
Karl Fischer titrator. The HCl in the organic phase (2 mL) was neutralized prior to titration 13
using triethylamine (10 mL) in dry methanol (10 mL) to avoid interference in the 14
measurements. 15
16
2.5. Synthesis 17
Copper bis(trifluoromethylsulfonyl)imide (Cu(Tf2N)2) was synthesized by mixing Cu(II) 18
oxide CuO (1.5 g, 18.8 mmol) with hydrogen bis(trifluoromethylsulfonyl)imide (HTf2N, 5.53 19
g, 15.7 mmol) in water for 4 h at 80 °C in a sealed vial. Afterwards, excess of CuO was 20
removed by filtration. The obtained metal solution was measured by ICP–OES and used as 21
metal stock solution for the UV/VIS measurements of the aqueous phase from 0 to 2 mol L-1
22
HCl. 50 mL of the stock solution was removed and water was evaporated by a rotary 23
evaporator. Afterwards, the obtained Cu(Tf2N)2 salt was dissolved in 50 mL of ~37 wt% HCl 24
10
(12 mol L-1
) and used as metal stock solution for the UV/VIS measurements of the aqueous 1
phase between 2 and 12 mol L-1
HCl. 2
3
2.6. Calculations 4
The UV/VIS absorption spectra were analyzed following the Multivariate Curve 5
Resolution-Alternative Least Square (MCR-ALS) technique using toolboxes working in 6
Matlab R2018b software.28
Chemical constraints were fixed to a minimum: (1) the total 7
amount of the metal was normalized (2) the independent UV/VIS absorption spectra do not 8
have negative absorbance values and (3) the concentrations of the species cannot be negative. 9
Principal component analysis (PCA) (using the same toolbox in Matlab) was performed prior 10
to the MCR-ALS study to determine how many different species were present in the solutions 11
and the results were compared with the literature. 12
The results of the MCR-ALS analysis are displayed in the form of the UV/VIS absorption 13
spectra of the independent species present in all samples containing that metal ion and of the 14
mole fraction of all species in function of the acid or salt concentration. The mole fraction of 15
a metal species (M2+
) is expressed as follows: 16
17
(5) 18
19
with n being the maximum amount of chlorides coordinated to M2+
. 20
Note that this mathematical treatment of the data is not based on any chemical model. 21
Therefore, ascribing a UV/VIS absorption spectrum to a certain metal complex is done by 22
considering the change in the concentration percentages as a function of the HCl 23
concentration and/or by comparison with previously published UV/VIS absorption spectra.29
24
25
11
3. RESULTS AND DISCUSSION 1
2
3.1. Background 3
There are some generally accepted principles in the extraction of metals by basic 4
extractants.2,11
First, the extraction of metals by basic extractants is assumed to occur via the 5
exchange of an anionic metal complex in the aqueous phase by one or more negatively 6
charged anions in the organic phase (equation 1). Secondly, the distribution ratio is higher 7
when the counter-ion of the salting-out agent is a metal cation instead of a proton. For 8
instance, the distribution ratio for extraction of metal ions from an aqueous solution 9
containing 8 mol L-1
LiCl is higher than from 8 mol L-1
HCl (Figure 1, curve 1 and 2). 10
Thirdly, the extraction of metal ions from HCl media typically shows a maximum in the 11
graph of the distribution ratio as a function of the HCl concentration at intermediate HCl 12
concentrations, resulting in a “bell-shaped” curve (Figure 1, curve 2). This phenomenon has 13
been attributed to the presence of the HCl2– species, which competes with the anionic metal 14
complex for the extractant.19
Fourthly, metal ions forming strong chloro complexes have 15
maxima in the distribution ratios at lower HCl concentration compared to metal ions that 16
form weaker chloro complexes (Figure 1, curve 3). 17
12
1
Figure 1. Typical shape of curves showing the distribution ratios of a metal ion as function of the 2
salting-out concentration with (1) LiCl as salting–out agent, (2) HCl as salting–out agent or (3) HCl 3
as salting–out agent in case the metal forms strong chloride complexes. It is assumed that there is 4
only one species in the organic phase. 5
Historically, the term anion exchange originates from salt metathesis reactions, in which 6
the anion initially present in the organic phase is exchanged by another anion initially present 7
in the aqueous phase. For instance, anion exchange resins are based on this phenomenon, 8
which are solids supports that trap ions from a solution.30
The exchange between anions can 9
be predicted by the Hofmeister series for anions (equation 6).31–33
Hydrophilic ions, in 10
general anions with a high charge density, will preferentially distribute to the aqueous phase 11
(left side of the series), whereas hydrophobic anions, which generally have a low charge 12
density, will preferentially distribute to the organic phase (right side of the series). Another 13
factor influencing the position of an anion in the Hofmeister series are the intramolecular 14
interactions between the anion and the aqueous or organic phase. The tendency of the anion 15
to be transferred from the aqueous to the organic phase is: 16
17
Citrate3–
< SO42–
< HPO42–
< F– < Cl
– < Br
– < I
– < NO3
– < ClO4
– < SCN
– (6) 18
13
1
The term anion exchange has been taken over by practitioners in the field of solvent 2
extraction. However, this term does not completely fit with experimental observations, and 3
this for several reasons. For instance, lanthanides (Ln) are extracted from nitrate media as 4
complexes by basic extractants, but it has been shown that, even at very high 5
nitrate concentrations, only hydrated species exist in the aqueous phase and no 6
species.
22,34 The situation is even more striking in case of a split-anion 7
extraction where lanthanides are extracted from aqueous chloride media to a nitrate organic 8
phase in the form of complexes.
35 Also in chloride media, only positively 9
charged lanthanide complexes exist and efficient extraction to basic extractants is observed at 10
high LiCl concentrations.23
The extraction of In(III) is a second example for which an anion 11
exchange mechanism does not fit the observations. The anion exchange mechanism cannot 12
explain the change in speciation: In(III) is present in the aqueous phase as the octahedral 13
complex (x ≤ 3), while it is present in the organic phase as the tetrahedral 14
complex.
24 The experimental data of many reported metal extraction studies from 15
HCl media do not support the anion exchange hypothesis: a decrease in distribution ratio as a 16
function of the HCl concentration (Figure 1, curve 2 and 3) is not in accordance with Le 17
Châtelier's principle. The combination of equation (1) and (2) demonstrates that any increase 18
in chloride concentration should shift the equilibrium to the right, thus increasing the 19
extraction. The decrease in distribution ratio has often been attributed to the coextraction of 20
HCl as the anion species HCl2– into the organic phase, competing with the metal 21
extraction.14–19
However, no direct evidence for the competition between metal and HCl 22
extraction can be found in the literature. Furthermore, a decreasing distribution ratio at high 23
chloride concentrations to Aliquat 336 is observed for the extraction of Zn(II) from LiCl 24
media and decreasing distribution ratios of Zn(II) are also discovered for different chloride 25
14
salt solution – anion exchange resin systems.36,37
In this case, no competition between LiCl 1
and Zn(II) extraction occurs as LiCl is not extracted. Also, the decreasing distribution ratios 2
cannot be explained by a changing speciation in the organic phase or because of a changing 3
composition of the organic phase, as the composition of the organic phase is almost constant 4
over the whole LiCl range.38
5
6
3.2. New extraction model 7
The analysis of former extraction studies and the difficulty of explaining the whole 8
extraction process with currently accepted theories show that another model is required to 9
describe the extraction of metals by basic extractants. An alternative extraction model for the 10
extraction of metals with basic extractants is proposed here and experimentally tested as 11
described in the next sections. The new model is explained for a divalent metal ion (M2+
) and 12
its five different chloro complexes in the aqueous phase ([MClx]2-x
with 0 ≤ x ≤ 4), but can be 13
applied to metal ions with another charge and a different aqueous speciation, as well (Figure 14
2). 15
At low chloride concentrations, M2+
is hydrated in its first, second and even third 16
coordination sphere by a large number of water molecules (its hydration sphere). By 17
increasing the chloride concentration, there is a shift towards the formation of and 18
complexes. The charge density of the species is lower than that of M2+
, 19
decreasing the hydration sphere and hydration energy. The complex has no charge, and 20
the total number of hydrating water molecules in its hydration sphere is at a minimum. At 21
higher chloride concentrations, the species and
are formed, for which the 22
charge density increases again, resulting in larger hydration spheres and higher hydration 23
energies. According to the Hofmeister series, the species with the lowest hydration energy 24
(here MCl2) will preferentially distribute to the organic phase. The intermediate and short 25
15
living species present in the organic phase or at the interface reacts with one or two 1
molecules of the Lewis base in the organic phase to form or
, which 2
associates with the cation of the extractant. As a result, the maximum in the distribution ratio 3
is found close to the HCl concentration at which the fraction of metal species with the lowest 4
charge density (e.g. MCl2) in the aqueous phase is the highest. The smallest distribution ratios 5
will be found at those HCl concentrations at which the fraction of the metal species with the 6
highest charge densities (e.g. M2+
or )
is the highest in the aqueous phase. This 7
model is shown in Figure 2. 8
Not only the chloride concentration and linked metal speciation have an influence on the 9
hydration of the metal complex. Also the availability of water molecules that can hydrate the 10
metal complex, i.e. the water activity, has an influence on the extraction. The availability of 11
water molecules for the metal complex can be changed by the choice of cation of the salt 12
which provides the chlorides in the aqueous phase. A cation with a large charge density, such 13
as Li+, is strongly hydrated, making the water molecules less available for the metal complex 14
to be extracted. This decreases the hydration shell and the hydration energy of the [MClx]2-x
15
species in the aqueous phase at high ionic strength resulting in higher distribution ratios, 16
known as the salting-out effect. This is also visualized in Figure 2. A cation with a smaller 17
charge density (like K+) will not be hydrated to such an extent and more water molecules will 18
be available for binding to the metal complex. 19
16
1
Figure 2. Model for the extraction of a metal ion from weak (left) and strong (right) salting-out 2
agents. The metal ion is depicted in red and changes from positively charged at low salting-out agent 3
concentration to negatively charged by the complexation of salting-out anions. The salting-out cations 4
are depicted in green, the associated water molecules are blue and the organic cation is illustrated in 5
brown-yellow. 6
This extraction model can give new insights in the extraction of metals by basic 7
extractants, and it gives an explanation for poorly understood extraction phenomena. First, 8
lanthanide ions are extracted by basic extractants as positively charged and hydrated metal 9
complexes Ln(H2O)x3+
at high LiCl concentration, which cannot be explained by the anion 10
exchange mechanism.23
However, the new model states that the hydration (in all coordination 11
spheres) of the metal complexes determines the extraction efficiency. The Ln(H2O)x3+
12
complexes are less hydrated at very high LiCl concentrations due to the low water activity 13
and thus extracted more. Secondly, the generally accepted principle that the decrease in metal 14
extraction at higher HCl concentrations is due to competition between metal and HCl 15
extraction via the HCl2- species is refuted by the newly proposed model. The decrease in 16
distribution ratio at high chloride concentrations is related to the formation of stronger 17
hydrated metal complexes in the aqueous phase, instead of the competition between HCl and 18
metal extraction. Thirdly, The fraction of the metal species with the lowest charge density in 19
the aqueous phase (e.g. [MClx]0) is formed at lower chloride concentrations for metals 20
17
forming strong chloride complexes (e.g. Zn(II)).36
Therefore, these metal complexes have 1
their maximum in distribution ratio at lower HCl concentrations. The observation of a 2
maximum in the distribution ratio of Zn(II) in function of the LiCl concentration can also be 3
explained using the same concepts. Zn(II) forms negatively charged chloro complexes at low 4
chloride concentration, where the salting-out effect of LiCl is less pronounced, and there are 5
still enough free water molecules available to hydrate ZnCl3- and ZnCl4
2- slightly more than 6
ZnCl2. 7
The absolute distribution ratio of a metals is, of course, also dependent on the stability of 8
the chloro complexes in the organic phase. However, the effect of the stability of the metal 9
complexes in the organic phase on the distribution ratio is almost the same over the whole 10
aqueous chloride concentration range, as the speciation of the metal complex in the organic 11
phase is independent of the chloride concentration in the aqueous phase.22
The sole changes 12
in the organic phase that can influence the distribution ratio of a metal are the presence of 13
significant quantities of other compounds in the organic phase These include the presence of 14
HCl in the organic phase when high aqueous HCl concentrations are used and the presence of 15
the same or other extracted metals in the organic phase at high loadings. The latter, although 16
significant for industrial metal separations, is outside the scope of present paper and will be 17
the topic of future work. 18
19
3.3. Literature data on speciation and extraction 20
Linking speciation and extraction allows to investigate the way metals are extracted and 21
can be used to test the applicability of the newly proposed extraction model. A speciation 22
profile of metal chloride complex can be constructed using stability constants reported in the 23
literature. However, stability constants are given at standard condition (i.e. zero ionic 24
18
strength) and significant deviations the speciation profiles derived from these constants are 1
expected at high ionic strength. To correct for the deviations of chloride, an approximation of 2
the activity coefficient model by Helgeson et al. at 25 °C was used:39,40
3
4
(7) 5
6
with I the ionic strength and ZCl- the absolute charge of chloride. The complete model can 7
calculate activity coefficients quite accurately, but this approximation is more limited than 8
the complete model of Helgeson et al. The approximation was used as it is user-friendly and 9
an approximate image of the speciation of a metal in chloride solution is sufficient for these 10
preliminary literature studies. The stability constants of a metal forming strong chloride 11
complexes (Zn(II)) and of one forming weak chloride complexes (Ni(II)) were used as 12
examples. The resulting speciation profiles were linked to their corresponding extraction 13
profiles using diluted Aliquat 336 and HCl or LiCl in the aqueous phase, respectively (Figure 14
3 and Figure 4).36,41–43
15
The maximum in distribution ratio of Zn(II) from HCl media coincides very well with the 16
highest mole fraction of the Zn(II) species with the lowest charge density (ZnCl2). This 17
species is likely the least hydrated and thus the most extracted according to our model 18
presented above. The extraction is lower at chloride concentration were the mole fraction of 19
positively or negatively charged Zn(II) species is higher, but the reduction in distribution 20
ratio is less pronounced when the amount of negatively charged Zn(II) increases. This might 21
be due to the higher HCl concentration, which acts as a minor salting-out agent by hydrating 22
some water molecules and due to the smaller increase in charge density of the anionic 23
complexes, which is related to their larger volume. 24
19
The extraction of Zn(II) from HCl media is significantly higher compared to the extraction 1
of Ni(II) from HCl media over the whole chloride range, because Zn(II) forms chloride 2
complexes much easier. Despite the absence of negatively charged Ni(II) complexes in the 3
aqueous phase, Ni(II) is still extracted to some extent. This cannot be explained by an anion 4
exchange mechanism, which requires negatively charged species. However, the extraction of 5
Ni(II) is significant at LiCl concentrations were the mole fraction of neutral NiCl2 is highest, 6
which is in agreement with our model. 7
8
Figure 3. Extraction and speciation profile of Zn(II) from HCl towards Aliquat 336 in benzene using 9
literature data.42,43
10
20
1
Figure 4. Extraction and speciation profile of Ni(II) from LiCl towards Aliquat 336 in diethylbenzene 2
using literature data.36,41
3
4
3.4. Metal speciation 5
The speciation and extraction curves presented above are only approximate due to the 6
insufficient consideration of the influence of ionic strength. An experimental investigation of 7
the speciation of metal complexes at the same ionic strength as the actual extraction is much 8
more reliable. Therefore, the speciation of Cu(II) and Co(II) in HCl media was investigated 9
and linked to extraction experiments with the same aqueous solutions, because of the 10
particular trend of the curve of the distribution ratio of Cu(II) and Co(II) vs. HCl 11
concentration. Methyltrioctylammonium chloride (TOMAC) (0.2 M) dissolved in toluene 12
was used for the extractions instead of its industrial equivalent (Aliquat 336) to avoid the 13
influence of impurities and to exactly quantify and control the amount of quaternary 14
compounds in the organic phase. Aliquat 336 has only about 80 wt% quaternary compounds 15
21
(MeR3NCl with R a mixture of C8 and C10 hydrocarbons) while also having about 8 wt% 1
alcohols and other impurities not accounted for.27
2
The speciation of Cu(II) and Co(II) as a function of the HCl concentration was studied by 3
UV/VIS absorption spectroscopy. UV/VIS absorption spectra were recorded at different 4
aqueous solutions containing Cu(II) (0.5 g L–1
) or Co(II) (1.0 g L-1
) and different amounts of 5
HCl (0 to 11.9 mol L-1
Cl–). The Cu(II) solutions for the speciation measurements were made 6
from copper bis(trifluoromethylsulfonyl)imide (Cu(Tf2N)2) to avoid the presence of chlorides 7
from the metal salt at very low HCl concentrations. No TfN2 was present in the solutions for 8
the extraction experiments. The Tf2N anion does not associate with Cu(II) in solution, thus 9
resulting in a fully hydrated Cu(II) complex in water. The color of the Cu(II) solutions 10
changed gradually from blue towards green/yellow (Figure 5), because of the changes 11
between the five different Cu(II) species in the aqueous phase: Cu2+
, CuCl+, CuCl2, CuCl3
– 12
and CuCl42–
.44–46
13
14
Figure 5.Color change of 0.5 g L–1
Cu(II) (added as Cu(Tf2N)2 salt) as a function of the HCl 15
concentration (0 to 11 mol L-1
Cl–) in the aqueous phase. 16
It is known that hydrated octahedral copper ([Cu(H2O)6]2+
) is a weak absorber in the 17
UV/VIS spectral region investigated here (220 to 500 nm, Figure 6). No absorption is 18
observed in the measured wavelength range except for a small increase in absorbance below 19
250 nm.47
The presence of [CuCl(H2O)5]+ in solution is evident from the presence of an 20
22
absorption maximum at 250 nm.47
The individual absorption spectra of the [CuCl2(H2O)4] 1
and [CuCl3(H2O)]- species, having maxima at 270 and 283 nm, respectively, cannot be 2
directly observed in Figure 6 due to overlap of the absorption spectra of the different 3
species.48,49
Note that the geometry changes from octahedral [CuCl2(H2O)4] to tetrahedral 4
[CuCl3(H2O)]-.49
The octahedral [Cu(H2O)6]2+
, [CuCl(H2O)5]+ and [CuCl2(H2O)4] complexes 5
do not contribute to the absorption maximum around 380 nm, while the main contribution at 6
380 nm comes from the tetrahedral complex [CuCl4]2–
.50
The optical absorption spectra of 7
Cu(II) in the aqueous phase obtained in this study show large similarities with those reported 8
previously by other research groups.20,45,50,51
9
10
Figure 6. UV/VIS absorption spectra of 0.5 g L–1
Cu(II) (added as Cu(Tf2N)2 salt) as a function of the 11
HCl concentration (0 to 11.8 mol L-1
Cl–) in the aqueous phase. 12
It is not possible to derive the exact speciation of Cu(II) at a given HCl concentration 13
directly from the UV/VIS absorption spectra. Nevertheless, a full speciation profile and de 14
UV/VIS absorption spectra of each of the Cu(II) species can be deduced using a 15
statistical/mathematical technique called Principal component analysis (PCA) and 16
23
Multivariate Curve Resolution-Alternative Least Square (MCR-ALS).52,53
The mole fractions 1
of the different Cu(II) species as a function of the HCl concentrations, as obtained from the 2
PCA-MCR-ALS analysis of the UV/VIS absorption spectra, can be found in Figure 7. The 3
calculated spectra (See SI) and speciation profile are consistent with models, theoretical 4
calculations and UV/VIS absorption analysis reported by other authors.20,47–50,54
Also, the 5
distribution ratio of Cu(II) for extraction from HCl media towards 0.2 mol L-1
TOMAC in 6
toluene is given in Figure 7, to enable a comparison between the speciation and the extraction 7
of Cu(II). 8
9
Figure 7. The distribution ratio of Cu(II) from HCl media towards 0.2 mol L-1
TOMAC in toluene 10
(black) and the mole fraction of the different Cu(II) species as a function of the chloride concentration 11
in HCl medium (colored). 12
The lowest distribution ratios for Cu(II) extracted from HCl media are found at very low 13
(e.g. 0.5 mol L-1
) and very high HCl (e.g. 10 mol L-1
) concentrations where the mole fractions 14
of [Cu(H2O)6]2+
or [CuCl4]2–
are the highest. In other words, the results depicted in Figure 7 15
suggest that an increase in [CuCl4]2–
concentration lowers the distribution ratio, which is not 16
24
in accordance with the generally accepted equations (1 and 2) for the extraction of metals by 1
basic extractants via anion exchange. 2
The highest distribution ratio for Cu(II) is obtained at 5.6 mol L-1
HCl, which is close to 3
the maximum in the mole fraction of [CuCl2(H2O)4] found at 4.8 mol L-1
HCl. Also, the 4
general trend in extraction efficiency seems to follow the general trend in the speciation of 5
[CuCl2(H2O)4].The difference between the maximum of the distribution ratio and the 6
maximum of the mole fraction of [CuCl2(H2O)4] can be explained by the non-negligible 7
solubility of HCl in the organic phase (vide infra) and changes in the water activity due to 8
changes in the HCl concentration. The HCl concentration in the aqueous phase was corrected 9
for the loss of HCl to the organic phase, but no corrections were made for the increase in 10
chloride concentration in the organic phase. The latter was omitted due to the absence of a 11
quantitative relation between the extraction efficiency and the organic HCl concentration. 12
Next, the abovementioned methodology was used to investigate the relation between the 13
extraction behavior of Co(II) and its aqueous speciation in HCl media. Although Cu(II) was 14
added as Tf2N salt for the spectroscopic study, this lengthy procedure was not repeated for 15
the Co(II) speciation study as no significant influence on the speciation curve of Cu(II) was 16
detected compared to what has been reported in the literature.50,54
The UV/VIS absorption 17
spectra were recorded and two distinctive regions were visible (Figure 8). Octahedral Co(II) 18
complexes weakly absorb in the region between 400 and 550 nm, while tetrahedral Co(II) 19
complexes show a much more intense absorption between 550 nm and 750 nm.55,56
The 20
octahedral Co(II) complexes are mainly present at low HCl concentrations (up to 7.8 mol L-1
, 21
vide infra) while tetrahedral Co(II) complexes are formed at higher HCl concentrations (from 22
7.8 mol L-1
, vide infra). The same observations can be deduced from the color change of the 23
Co(II) solutions from pale pink, for [Co(H2O)6]2+
at low HCl concentrations, to dark blue, for 24
[CoCl4]2-
complexes at high HCl concentrations. 25
25
1
Figure 8. UV/VIS absorption spectra of 1.0 g L–1
Co(II) as a function of the HCl concentration (0.015 2
to 11.8 mol L-1
Cl–) in the aqueous phase. 3
The large difference in the molar absorption coefficient of the octahedral and tetrahedral 4
Co(II) species makes it more difficult to determine the amount of the different Co(II) species 5
present in solution. Presumably there are 5 different species present (i.e. CoClx2-x
with 0 x 6
4), but many studies on the speciation of Co(II) in aqueous media are inconsistent.21
An 7
extensive study on the speciation of Co(II) species in HCl media has been published recently 8
by Uchikoshi, who aimed to remove the ambiguity on the speciation of Co(II).21,57
The author 9
used UV/VIS and X-ray absorption spectroscopy to arrive at the conclusion that only three 10
species exist in aqueous HCl media, being octahedral hydrated [Co(H2O)6]2+
and 11
[CoCl(H2O)5]+, and tetrahedral [CoCl4]
2-. This conclusion is consistent with the PCA analysis 12
performed on the UV/VIS absorption spectra in the current study, which revealed that three 13
species were necessary to lower the residuals below the experimental error of 1%. Thus, the 14
MCR-ALS analysis of the Co(II) spectra was performed using three species and the resulting 15
spectra of the individual Co(II) species and the speciation profile of Co(II) in HCl can be 16
26
found in the SI and Figure 9, respectively. The speciation profile and individual spectra of 1
Co(II) are in good agreement with those reported in the literature.21,57
2
The distribution ratio of Co(II) from aqueous HCl media towards 0.2 mol L-1
TOMAC in 3
toluene are given in Figure 9 to allow for a comparison between its speciation in aqueous 4
media and extraction towards basic extractants. The maximum in distribution ratio of Co(II) 5
is located at 8.1 mol L-1
HCl and the highest mole fraction of the Co(II) species with the 6
lowest charge density (CoCl+) is at 7.6 mol L
-1 HCl. As for Cu(II), the similar HCl 7
concentration for the highest mole fraction of [CoCl(H2O)5]+ and the maximum in 8
distribution ratio of Co(II) suggest, together with the similar shape of the distribution curve 9
and speciation curve of [CoCl(H2O)5]+, that the species with the lowest charge density (and 10
lowest hydration) is extracted preferentially. 11
Three further comments can be made: (1) Again the maximum in distribution ratio is 12
located at slightly higher HCl concentration compared to the maximum in distribution ratio of 13
the species with the lowest charge density. This can be explained by the presence of HCl in 14
the organic phase or changes in the water activity as mentioned for the Cu(II) extraction; (2) 15
The species with the lowest charge density is not necessarily the neutral species. CoCl+ is the 16
species with the lowest charge density, as CoCl2 is not present in solution; (3) The absence of 17
a changing speciation of the metal complex in the organic phase for both Cu(II) and Co(II) 18
(see SI for details) shows the validity of linking the extraction as a function of HCl 19
concentration to the changes in speciation of the aqueous phase. 20
27
1
Figure 9. The distribution ratio of Co(II) from HCl media towards 0.2 mol L-1
TOMAC in toluene 2
(black) and the mole fraction of the different Co(II) species as a function of the chloride concentration 3
in HCl medium (colored). 4
5
3.5. Influence of salting-out agents 6
As mentioned in the introduction of the new extraction model, a cation of a salting-out 7
agent with a higher charge density would be more strongly hydrated and thus would decrease 8
the amount of free water present in solution. This would more efficiently in decreasing the 9
effective hydration of the metal complex that is extracted. To test qualitatively the effect of 10
slating-out agents and hydration on metal extraction with basic extractants, extraction 11
experiments were performed with Cu(II), Co(II) and Zn(II) from aqueous chloride media 12
towards 0.2 mol L-1
TOMAC in toluene. Figure 10 shows the distribution ratios of 0.5 g L-1
13
Cu(II) extracted by 0.2 mol L-1
TOMAC in toluene as a function of the concentration of 14
mono-, di- and trivalent salting-out agents in the aqueous phase. The extraction of 2.0 g L-1
15
Zn(II) by 0.2 mol L-1
TOMAC in toluene was studied in function of the concentration of 16
28
different monovalent salting-out agents to investigate the effects of salting-out agents in more 1
detail (Figure 11). Zn(II) was chosen, as it forms strong chloro complexes,42
resulting in a 2
maximum in distribution ratio at low chloride concentrations. This way, all alkali chlorides, 3
NH4Cl and HCl could be used as salting-out agents up to a concentration larger than the 4
chloride concentration linked with a maximum in distribution ratio of Zn(II). 5
6
Figure 10. Extraction of 0.5 g L-1
Cu(II) as a function of the initial chloride concentration to 0.2 mol 7
L-1
TOMAC dissolved in toluene. 8
9
Above 2 mol L-1
Cl–, the distribution ratios of Zn(II) increases in the following order of 10
monovalent salting-out cation: 11
12
Li+ > Na+ > H+ > K+ ≈ NH4+ > Cs+ (8) 13
14
This series is slightly different from the Hofmeister series.33
For instance, the similar 15
extraction efficiency for Zn(II) from KCl and NH4Cl can be explained by their similar ionic 16
29
radii.58,59
The positioning of HCl in the series (equation 8) is not that clear, as its presence in 1
the organic phase also influences the extraction of metals (vide supra). Nevertheless, the 2
similarities between the Hofmeister series and the general decrease in cationic charge density 3
from left to right in equation 8 suggest that the amount of free water molecules in the aqueous 4
phase has a large effect on the shape of the graph displaying the distribution ratio as a 5
function of the chloride concentration. 6
7
Figure 11. Extraction of 2.0 g L-1
Zn(II) as a function of the initial chloride concentration to 0.2 mol 8
L-1
TOMAC dissolved in toluene 9
The distribution of Cu(II) was performed with a few alkali chlorides (LiCl and NaCl), 10
NH4Cl, HCl and some divalent and trivalent chloride salts (CaCl2, MgCl2 and AlCl3) to check 11
the observations from the Zn(II) extraction experiments and to the effect of salting-out agent 12
with a cation with a higher charge density than Li+. Below 2 mol L
-1 chlorides, the 13
distribution ratios of Cu(II) are very similar for the different chloride salts. This is expected, 14
because the chloride activity in the aqueous phase and the amount of free water in the 15
30
aqueous phase would be rather similar in the different systems at these relatively low salt 1
concentrations. 2
Above a concentration of 2 mol L-1
chloride, the distribution ratios of Cu(II) from CaCl2, 3
MgCl2 or AlCl3 are higher than those of NH4Cl and HCl at equal initial chloride 4
concentrations, which is expected based on the Hofmeister series. Surprisingly, the 5
distribution ratios of Cu(II) for extraction from solution with salting-out agents of divalent 6
and trivalent cations are lower at equal initial chloride concentrations than the distribution 7
ratios with NaCl and LiCl as salting-out agent, even though the charge density of the cations 8
are higher. This can be explained by the association reaction between the salting-out cations 9
and anions in the aqueous phase resulting in a significant decrease in ionic strength of the 10
aqueous phase.60
For instance, the association/dissociation reaction of CaCl2 can be written 11
as: 12
13
(9) 14
15
The situation at the right side of equation (9) is found at low ionic strength, but the 16
association reactions between Ca2+
and Cl– cannot be neglected at higher ionic strengths 17
(from 1 mol L-1
). Equation (9) clearly shows that the charge of the ions is significantly 18
reduced by this association reaction: one +II charged metal ion (Ca2+
) and one –I charged 19
chloride ion (Cl–) are replaced by one calcium(II) monochloride cation (CaCl
+) with a +I 20
charge. This decrease in charge density lowers the ionic strength of the aqueous phase 21
significantly and might explain the decrease in distribution ratios of Cu(II) to values below 22
those found for extraction from LiCl media at similar initial chloride concentrations. The 23
degree of association cannot easily be predicted because of the changing ionic strengths. For 24
instance, Johnsen et al. calculated that about 46% of the Ca2+
and 50% of Mg2+
are not 25
31
bonded with Cl– in sea water, whereas 85% of Na
+ ions are still in its dissociated form under 1
the same conditions.60
2
Although the similarities between hydration effects, the charge density of the salting-out 3
agent, the Hofmeister series and the extraction are very clear, the experimental evidence 4
given is only an indirect proof. A direct observation of the hydration sphere or energy of 5
salting-out agents and transition metals in function of the salting-out concentrations would be 6
very interesting to study the proposed extraction model more in detail and make the model 7
quantitative. This will be the subject of further investigation. 8
Another way to explain the difference in metal extraction from solutions with different 9
salting-out agents would be via a change in metal speciation when changing the salting-out 10
agent. Different salting-out agents can influence the speciation of a metal because of two 11
reasons: (1) a change in the activity of all species in solution due to different interactions with 12
the cation of a different salting-out agent, and (2) a change in free chloride concentration due 13
to the association of mono-, di- and certainly trivalent salting-out agents. The speciation of 14
Co(II) in LiCl was determined via UV/VIS absorption spectroscopy and PCA-MCR-ALS, 15
similarly to the determination of Co(II) species in HCl. The two speciation profiles were 16
compared (Figure 12) to determine the difference in speciation due to a change in monovalent 17
salting-out agent. The association of both LiCl and HCl is similar and considerably lower 18
compared to that of divalent and trivalent salting-out agents. This allows to observe the effect 19
of activity on the speciation of Co(II), although effects of changing salting-out agent 20
association cannot be completely excluded.. 21
32
1
Figure 12. Speciation profile calculated with PCA-MCR-ALS of Co(II) in HCl (top) and LiCl (bottom) 2
as function of the total chloride concentration. 3
The speciation profile of Co(II) in LiCl is very similar to that of Co(II) in HCl. The curves 4
of [CoCl(H2O)5]+ and [CoCl4]
2- are shifted to higher chloride concentrations by 1 mol L
-1 5
unit, but this cannot explain the observed difference in extraction behavior of Co(II) from 6
HCl and LiCl towards basic extractants at high chloride concentrations. To clarify, the 7
distribution curve of Co(II) from HCl shows a maximum at 8 mol L-1
HCl, while the 8
distribution curve of Co(II) from LiCl keeps on increasing with increasing chloride 9
concentration.36
A mole fraction close to one of [CoCl4]2-
complexes is still formed at high 10
LiCl concentrations, which agrees with the observed blue color of the measured solutions. 11
The increase in mole fraction of species with a higher charge density would still decrease the 12
extraction efficiency due to an increased hydration of these Co(II) species. However, the high 13
LiCl content decreases the amount of free water molecules drastically, due to association of 14
water molecules with the Li(I) cation, which has a very high charge density. This results in a 15
33
net decrease in hydration of Co(II) in the aqueous phase at high chloride concentrations, so 1
that extraction is more efficient. 2
The speciation of metal complexes in divalent and trivalent salting-out agents might be 3
shifted more over the chloride concentration range due to the association of the salting-out 4
agents. However, the results presented above suggest that this will not change the 5
applicability of our extraction model. The determination of the speciation of metal complexes 6
in a wide range of mono-, di- and trivalent salting-out agents is of interest for a more 7
quantitative explanation of the observed extraction phenomena and will be investigated in the 8
future. 9
10
3.6. Extraction behavior of HCl 11
Apart from the metal extraction itself, also the behavior of HCl in the extraction systems 12
requires further investigation. A first impression of the HCl extraction behavior towards 13
basic extractants can be provided by performing extraction experiments on HCl itself 14
towards water-saturated Aliquat 336 and 0.24 mol L-1
Aliquat 336 in toluene (resulting in 15
0.2 mol L-1
quaternary compounds) (Figure 13). Aliquat 336 was chosen here, because 16
these results can be used directly in other works to correct for the HCl concentration in 17
both phases, as Aliquat 336 is an industrially relevant extractant. However, the metal 18
extractions were performed using TOMAC to increase the accuracy for investigating the 19
extraction mechanism of basic extractants. A smaller scale study was performed using 20
water saturated TOMAC as extractant for comparison with Aliquat 336 and the HCl 21
extraction followed the same trend (see SI). 22
Both the HCl concentration in the water-saturated Aliquat 336 and 0.24 mol L-1
Aliquat 23
336 phase increased linearly with the equilibrium HCl concentration in the aqueous phase, up 24
to a value above the stoichiometric concentration of basic extractant in the organic phase. 25
34
Volume changes of both phases were negligible (± 0.1 mL), because of a compensating linear 1
decrease in water content in the organic phase. Saturated Aliquat 336 initially contained 20.7 2
± 1.3 wt% of water, while 13.1 ± 0.9 wt% water was left in the Aliquat 336 phase after 3
contact with 11.4 mol L-1
HCl. The water content in the 0.24 mol L-1
Aliquat 336 in toluene 4
phase was evidently lower, due to the lower polarity of toluene. The Aliquat 336-toluene 5
phase had a water content of 2.00 ± 0.25 wt% before extraction, which linearly decreased to 6
0.65 ± 0.08 wt% after contact with 10 mol L-1
HCl. Based on the proposed extraction 7
mechanism for the extraction of HCl (equation 10), loading effects should be observed close 8
to the stoichiometric concentration and an organic HCl concentration higher than the 9
stoichiometric concentration should not be possible.17,18
10
11
(10) 12
13
The linear increase of the HCl concentration in the organic phase, even above a 14
stoichiometric concentration, suggests that the amount of basic extractant does not influence 15
the distribution of HCl and shows that the extraction mechanism for the formation of HCl2- 16
(equation 10) cannot explain the experimental observations. It can be argued that equation 10 17
is valid and HCl is also present due to other effects to obtain such a high HCl concentration in 18
the organic phase. However, the absence of any loading effects make this argument unlikely. 19
35
1
Figure 13. Extraction isotherm of HCl towards Aliquat 336 presaturated with water (▪) and towards 2
0.24 mol L-1
Aliquat 336 in toluene (●). The dotted lines represent the concentration of basic 3
extractant ([Quat extractant]) in water-saturated Aliquat 336 and in 0.24 mol L-1
Aliquat 336 in 4
toluene. 5
If competition between HCl extraction and metal extraction existed, this would not only be 6
visible in the extraction efficiency of the metal. The HCl extraction itself should also be 7
influenced by the amount of metal present in the system. Thus, the HCl concentration in the 8
organic phase at a fixed aqueous equilibrium HCl concentration should decrease when more 9
metal is present and extracted. The distribution of HCl between the aqueous phase and 10
water-saturated Aliquat 336 was determined via the density of the aqueous phase before and 11
after extraction (see SI for density-concentration conversion). Co(II) was added to the 12
aqueous phase at concentrations between 0.07 to 0.36 mol L-1
and the amount of extracted 13
Co(II) was quantified using ICP-OES. No influence of the amount of extracted Co(II) on the 14
extraction of HCl could be observed (Figure 14), while the extracted amount of Co(II) 15
significantly decreased the amount of unassociated Aliquat 336 due to the formation of 16
36
[NR4]2[CoCl4] complexes in the organic phase. For instance, the extraction of Co(II) from 1
0.36 mol L-1
CCo(II),0 at 8.7 mol L-1
HCl decreases the available Aliquat 336 concentration by 2
54% (complete data in SI). Figure 14 shows that the distribution of HCl is not influenced by 3
the extraction of metal ions; HCl distributes itself between the aqueous and organic phase in a 4
way that is independent of the amount of Co(II) extracted. 5
6
Figure 14. Distribution of HCl between an aqueous and water-saturated Aliquat 336 phase with 7
varying initial HCl and Co(II) concentrations. 8
From Figure 11 it is evident that the decrease in extraction efficiency at high chloride 9
concentrations is not unique for HCl. The curve of the extraction of Zn(II) from CsCl is 10
parallel to that of the extraction of HCl and a slight decrease is seen for the extraction of 11
Zn(II) from KCl and NH4Cl media above a chloride concentration of 3 mol L-1
. It should be 12
noted that some precipitation occurred in the aqueous phase before the start of extraction of 13
Zn(II) from CsCl due to the formation of the double salt Cs2ZnCl4.61
During extraction the 14
precipitation completely disappeared in all samples as the concentration of Zn(II) in the 15
aqueous phase decreased. Also, alkali metals are generally not extracted by basic extractants. 16
37
In fact, the extraction of Cs(I) in this study is negligible compared to the extraction of Zn(II) 1
and only decreases the available extractant concentration (if it associates with the TOMA 2
cation) by 1% at the highest CsCl concentration used in this study (details in SI). 3
The decrease of Zn(II) extraction from CsCl cannot be explained by competition between 4
Zn(II) and Cs(I) extraction. Thus, the decrease seems to be related to the increased charge 5
density and hydration of negatively charged Zn(II) species formed at higher CsCl 6
concentrations. Furthermore, Cs+ is a cation with a low charge density, which will not lower 7
the water activity significantly, making it an ineffective salting-out agent. The same 8
explanation can be given for the decrease in extraction efficiency for the extraction of Zn(II) 9
from high concentrations of KCl or NH4Cl. However, the downwards trends is much less 10
pronounced as the charge density of K+ and NH4
+ is higher than that of Cs
+ making them 11
more efficient salting-out agents. 12
13
14
38
4. CONCLUSIONS 1
2
A novel extraction model for the extraction of metals with basic extractants from chloride 3
media is proposed, because the commonly accepted anion exchange model cannot explain all 4
observed extraction phenomena using basic extractants. The novel model was experimentally 5
tested in several ways: (1) by comparison of speciation and extraction curves of transition 6
metals from the literature and experiments, (2) by studying the effect of different salting-out 7
agents on the metal and (3) by investigating the distribution of HCl between the aqueous and 8
organic phase. The proposed extraction model explains better the experimental observations 9
and allows predicting proper conditions for metal extractions and separations a priori, 10
without the need for changing the extractant concentration. If the distribution ratios are too 11
low for a specific chloride concentration, then it is recommended to select a salting-out cation 12
with a high charge density, such as Li+
to enhance the extraction. In contrast, the use of HCl 13
is recommended if one wants to extract metal ions that form strong chloride complexes. In 14
this case, very high or very low chloride concentrations give distribution ratios that are not 15
too high, so that removal of the metals from the organic phase (back-extraction) is possible. 16
This is a quantitative description and further optimization will be done to get a deeper insight 17
in the extraction behavior at high ionic strength, to allow for more quantitative predictions of 18
metal extractions and separations. 19
39
Corresponding Author 1
* Koen Binnemans, KU Leuven, Departement of Chemistry, B–3001 Heverlee, Belgium 2
Email: [email protected] 3
Phone: +32 16 32 74 46 4
ORCID 5
Rayco Lommelen: 0000-0001-8169-4566, Tom Vander Hoogerstraete: 0000–0002–1110–6
699X, Bieke Onghena: 0000-0002-8809-0842, Isabelle Billard: 0000–0002–2842–7706, 7
Koen Binnemans: 0000–0003–4768–3606. 8
Author Contributions 9
The manuscript was written through contributions of all authors. All authors have given 10
approval to the final version of the manuscript. 11
Funding Sources 12
The authors thank the KU Leuven (project GOA/13/008) and the FWO Flanders 13
(postdoctoral fellowship to TVDH and project G0B6918N) for financial support. The 14
research was supported by the European Research Council (ERC) under the European 15
Union’s Horizon 2020 Research and Innovation Programme: Grant Agreement 694078 — 16
Solvometallurgy for critical metals (SOLCRIMET). The contents of this publication are the 17
sole responsibility of the authors and do not necessarily reflect the opinion of the European 18
Union. 19
Notes 20
The authors declare no competing financial interest. 21
22
40
1
Acknowledgments 2
I. Billard is grateful to Dr. Ludovic Duponchel for their valuable discussions about the MCR-3
ALS method. 4
Supporting Information 5
Includes absorption spectra of Cu(II) and Co(II) species from PCA-MCR-ALS analysis, 6
UV/VIS absorption spectra of Cu(II) and Co(II) in the organic phase, HCl extraction data 7
with TOMAC, corrections on the HCl concentration from density measurements in Co(II) 8
matrix and data on the Cs(I) extraction. 9
41
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7
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TOC graphic and synopsis 1
2
3
4
5
Synopsis 6
A novel model for the extraction of metals by basic extractants is presented, based on the 7
concept that the metal species that is the least stabilized in the aqueous phase by hydration is 8
extracted more efficiently than the more stabilized species. Salting-out agents also have an 9
influence on the extraction, most likely by decreasing the concentration of free water 10
molecules, which reduces the stabilization of the metal complex. 11