Synergistic extraction of nickel(II) from a sulfate medium by a mixture of N-(2-hydroxybenzylidene)aniline and methyl–isobutyl ketone

Article abstract of DOI:10.1080/24701556.2017.1357612

The synergistic solvent extraction of nickel(II) from a sulfate medium with N-(2-hydroxybenzylidene)aniline in the absence and presence of Methyl–Isobutyl Ketone (MIBK) in chloroform at 25°C has been studied. A synergistic effect is observed for the metal

Full text of DOI:10.1080/24701556.2017.1357612

Inorganic and Nano-Metal Chemistry
ISSN: 2470-1556 (Print) 2470-1564 (Online) Journal homepage: http://www.tandfonline.com/loi/lsrt21
Synergistic Extraction of Nickel(II) from
Sulfate Medium by a Mixture of N -(2-
hydroxybenzylidene)aniline and Methyl-Isobutyl
Ketone
S. Almi, F. Adjel & D. Barkat
To cite this article: S. Almi, F. Adjel & D. Barkat (2017): Synergistic Extraction of Nickel(II) from
Sulfate Medium by a Mixture of N -(2-hydroxybenzylidene)aniline and Methyl-Isobutyl Ketone,
Inorganic and Nano-Metal Chemistry, DOI: 10.1080/24701556.2017.1357612
To link to this article: http://dx.doi.org/10.1080/24701556.2017.1357612
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Synergistic Extraction of Nickel(II) from Sulfate Medium by a Mixture of N -(2-
hydroxybenzylidene)aniline and Methyl-Isobutyl Ketone
S. ALMI*, F. Adjel and D. BARKAT
Laboratoire de chimie moléculaire et environnement, université de Biskra, Biskra, BP 145,
Algérie
*sbenakcha@yahoo.fr
The synergistic solvent extraction of Nickel(II) from sulfate medium with N-(2-
hydroxybenzylidene)aniline in the absence and presence of Methyl-Isobutyl Ketone (MIBK) in
chloroform at 25°C, has been studied. Synergistic effect is observed for the metal at low and high
(MIBK) concentration ratios, respectively. The synergistic extraction of nickel (II) with the
mixtures has been investigated with the analysis methods of slope. The extractable species has
been identified as NiL2 and NiL2(MIBK)2. The extraction constants are reported.
Keywords
Solvent extraction; nickel(II), N-(2-hydroxybenzylidene)aniline; Methyl-Isobutyl Ketone;
Synergism.
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1. Introduction
The key to an efficient separation process is the development of an extractant with a strong
preference for relevant chemical species. Examples of such extractants include macrocyclic
Schiff bases. Solvent extraction studies from our laboratory have shown that bidentate Schiff
bases such as N-salicylideneaniline were found to be powerful extractants for metal ions,
including transition metal [1-4]. These compounds are well known as ligands that are easy to be
synthesized and have structural rigidity [5-7]. They show interesting properties, e.g., their
catalytic activity in hydrogenation of olefins [8] and transfer of an amino group [9], photochromic
properties [10], and complexing ability towards toxic metals [11].
In the present work, we study the solvent extraction of Nickel (II) ions, which is a highly toxic
metal ion that is readily encountered in the environment [12–14]. The extraction of Ni(II) is
vulnerable to third phase formation [15], but the addition of Méthyl-Isobutyl Ketone (MIBK) to
the organic phase leads to a synergistic enhancement and improvement of the extraction of this
ion. The synergic extraction of metal chelates has been mostly explained by an increase in the
hydrophobicity of extracted chelates. By a replacement reaction of water molecules bound to the
central metal ion by basic neutral organic molecules: pyridine, Methyl-Isobutyl Ketone (MIBK),
trioctylphosphine oxide (TOPO), tri-n-butyl phosphate (TBP), etc. [16] or by hydrogen-bonding
between metal chelates and phenol derivatives in the organic phase [17].
2. Experimental
2.1.Chemicals and apparatus
All of the chemicals and solvents used for the synthesis of the ligand were of commercially
available reagent grade and they were used without purification. Chloroform was pre-equilibrated
with aqueous solution. NiSO4, 6H2O was purchased from Fluka. Infrared spectra were recorded
as KBr discs on a SHIMADZU, FTIR-8400S spectrophotometer in the 4000–400 cm-1. UV–Vis
spectra were recorded on a PYEUNICAM SP6-350 (PHILIPS) spectrophotometer in the range
400–1100 nm. The powder X-ray diffraction pattern of the complex was scanned in the range 3–
40°(h) with a D8 Advance-Brucker using a CuKα line at 0.1540 nm in steps of 0.03 at scan speed
2°/min.
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The calculation was carried out with GAUSSIAN 09W software package (19). The
optimization of the geometry was performed employing density functional theory (DFT) with
Becke three-parameter hybrid functional combined with Lee-Yang-Parr correlation functional
(B3LYP) (20) with the standard 6-311G(d,p)/basis sets.
2.2. Synthesis of Schiff base ligands
The N-(2-hydroxybenzylidene)aniline (HSA) was synthesized by literature procedure (21).
Refluxing equimolar quantities of salicylicaldehyde and aniline dissolved in minimum pure
ethanol (figure 1) is done inside a balloon tricol provided with a magnetic stirrer, a cooling agent,
and a graduated thermometer. The unit is maintained with backward flow and under agitation for
approximately 2 h. The mixture is cooled at room temperature and then concentrated by the
elimination of solvent using a rotary evaporator. The solid product (Yellow crystals) obtained is
filtered and then recrystallized in minimum pure ethanol with Yield = 80 (%).
2.3. Extraction and analytical procedure
An aqueous nickel solution (40 ml, [Ni2+] = 6.81×10-3mol dm-3 with [Na2SO4] = 0.33M) was
placed in thermostated vessels at 25°C with an equal volume of chloroform solutions, which pre-
equilibrated with aqueous solution contain Na2SO4 only, containing 0.4, 0.8 and 1.6 mol dm-3 of
HSA alone or a mixture of 0.4 mol dm-3 of HSA with different concentration of MIBK. The
mixture was shaken for 15min. this time was sufficient since the equilibrium was reached rapidly
and then both phases were separated by gravity. The pH was adjusted by adding a few drops of
0.2 moldm-3 NaOH. After the two phases were separated completely, the metal ion concentrations
were determined in aqueous phase photometrically at 720 nm using photoLab® 6100 VIS. The
metal ion concentrations in the organic phase were calculated from the difference between the
metal ion concentrations in the aqueous phase before and after extraction.
3. Results and discussion
3.1. General treatment of extraction equilibrium of nickel(II) with HSA
The overall reaction in the extraction of nickel(II) with chelating extractant (HSA), in
chloroform from sulphate aqueous medium of ionic strength I=1.0 mmol g-1 ([Na2SO4] = 0.33M),
may be expressed as
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K ex

N i2 

n  m H L
N iL n H L

n H




o rg
m
o rg
Where the subscript org correspond to the organic phase. The distribution ratio D of nickel is
defined as
2 


N i

 o rg
D 
(Eq1)
2 


N i


The extraction constant, Kex, is defined as equation (2) and it can be rewritten as equation (3)
by using the distribution ratio, D, of the metals.



 n


N iL n H L
H




m
 o rg

(Eq2)
K ex

n  m
2 


N i
H L



o rg
2 


N i

 o rg
lo g D  lo g
 lo g K

n  m lo g H L
 n p H
(Eq3)
o rg




ex
2 


N i


Equation (3) show that the function log D=f(pH) at a constant [HL]org or logD=f([HL]org) at a
constant pH, should yield a straight line with a slope of n and n+m, respectively.
The efficiency of extraction of metals E % in the case where the volumes of the two phases are
equal (A/O) = 1, is defined as follows (Eq 4)
D
E %  1 0 0
(Eq 4)
D  1
3.2. Extraction of nickel (II) with HSA alone:
Preliminary experiments led to the following observations. Virtually no nickel was extracted
from sulfate medium into MIBK alone. Therefore we will base, in our study, on the extraction of
the Ni (II) by HSA.
The extraction of nickel by HSA was accompanied by a slow development of green color in
the organic phase. The intensities of the colors were qualitatively observed to peak at six hours
over a wide pH range 6-8. When nickel (I1) were extracted at pH > 8, third phase was detected at
the boundary of the aqueous and organic phases. Extensive studies have been conducted on the
third phase formation systems, which have been reviewed in detail (21, 22). It is the splitting of
organic phase into two phases with the heavier one, rich in metal–solvate and the lighter phase
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rich in diluent (23). In various extraction systems the third phase formation is troublesome to the
operation of the reprocessing processes. For example, it may change the mass transfer and phase
separation, and reduce the extraction efficiency.
The formation of the third phase in our system, which decreases the extraction of Ni (II) by
HAS, may be due to the hydrolysis of NiSO4, which occurs in this pH range.
S.A. Berger (24) has studied the solvent extraction of nickel (II) with Chlorendic Acid. The
author has observed that the slopes of the plots of log D vs. pH, yield fractional values lower than
the normally expected integral values of 1.0 or 2.0 for ions of Nickel in the oxidation state of +2.
This value depends upon the existence of complexation in the aqueous phase.
Several authors have observed for the extraction of metals using carboxylic acid extractants,
the maximum efficiency occurs at pH values just below that for the precipitation of their metal
hydroxides (25-27). and it has attributed the lower of the extraction to the existence of a partially
hydrolyzed metal species or the presence of other anionic complexing agents which react with the
+1
metal in the aqueous phase (28,29) which would form such species as [M(II)(OH)]
[M(II)(X)] +1.
and
The stoichiometry of the extracted species in the pH rang 6-8 was determined by analyzing the
experimental data by the use of the slope analysis method.
Figure 2 show the results obtained for the extraction of nickel (II) with solutions of various
concentration of HSA.
The degree of extraction of Ni(II) (figure 3) increase with increase in pH and the concentration
of extractants. The plots of the logD versus pH for Ni cation and for various concentrations of
HSA are linear with a slope equal to 2 (n=2), this suggesting exchange of two protons between
the metal cation and the extractant by his phenolate group.
According to equation (3), the number of HSA molecules involved in the extracted species can
be determined from the slope of the plots of logD=f([HL]) at constant pH. The plots of logD
versus log [HL]org at constant pH values are also linear with slopes equal to 2 (as shown in figure
4).
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These results give n=2 and m= 0 which are readily replaced in equation (5). This means that
the complex extracted is NiL2 into chloroform.
N i2 

2 H L

N iL 2

2 H  (Eq 5)

org
The extraction constant obtained is logKex= -14.75. The structures of the extracted species may be
proposed as follows (figure 5).
The complex formed has a structure of an adduct, in which particular places in the
coordination sphere of nickel ion is occupied in total by two molecules of extractant.
3.2.1 Electronic spectra
Based on the electronic spectral data, nickel (II) complex, (Figure 6), has the octahedral
geometry with its characteristic features. The electronic spectrum of the organic phase showed in
addition to the band associated with the transfer of charge occurs at 420 nm which is
corresponding to the 3A2g→3T1g (P) transition, two author bands. One occurs at 620 nm, which is
3
corresponding to A2g → 3T1g (F) electronic transition and the other located at 950 nm
3
corresponding to the A2g→3T2g electronic transition. The three transitions are consistent with a
distorted octahedral geometry with two water molecules at an axial position of the complex. (30)
3.2.2 IR spectra and mode of bonding
In order to study the binding mode of the ligand with the metal ions, a comparison was made
between the IR spectra of the free ligands with that of the complex extracted in the organic phase
as showed in figure 7. The data (table 1) revealed that the complex formed showed a broad band
in the range 3440 cm−1 assignable to ν(OH) of the coordinated or uncoordinated water and/or
alcohol molecules associated with the complex. In addition, the band at 1615.27 cm−1 assigned to
ν(C=N) in the free ligand was shifted to lower wave number in the complex, indicating the
participation of the azomethine nitrogen in chelation. The band at 1453 cm−1 assigned to ν(C–O)
in the free ligand was shifted to 1445.47 cm−1 in the complexes and the intensity of the band has
decreased showing the involvement of the phenolic oxygen in coordination. The new bands
observed at 569.92 and 877.75 cm-1 are respectively assigned to ν(M-O) and ν(M-N) (Metal-
Ligands) stretching bands. (31-33)
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3.2.3 DFT studies
In order to gain further insights on the Ni-HSA bonding, we have carried out a theoretical
investigation with the density functional theory. The calculated geometrical parameters of the
complex found to be in good agreement with the experimental values obtained from the
electronic study (Table 2). According to those bond lengths, the structure shown a distorted
octahedral geometry involving two bidentate (SA-), The O-Ni and N-Ni distances are found to be
1.84021 Å and 1.9013 Å, respectively. The H2O-Ni distances are 3.1145 Å indicate the repulsion
between the Ni2+ and two water molecules situated at an axial position of the complex. The
association energy of this complex is 79.74 kcal/mol.
3.2.4 X-ray diffraction studies
Single crystal of the complex could not be prepared to get the XRD and hence the powder
diffraction data were obtained for structural characterization and In order to test the degree of
crystallinity of the synthesized metal complex. Powder X-ray diffractograms of free Schiff base
ligand and Ni(II) complex show sharp peaks which indicate their crystalline nature. By
comparing the X-ray diffractogram of free Schiff base ligand (HL) with its Ni(II) complex, as
show in figure 9, the Ni(II) complex shows few new peaks which suggest the formation of metal
chelates. X-ray diffraction analysis of the nickel (II) complex confirm the Monoclinic crystal
system with Space group P21. Complex has unit cell dimensions a = 7,3080 Å, b =11,0930 Å, c
= 12,2380 Å et α =90,0000°, β = 98,0970°, γ= 90,0000°. The crystal of the ligand retained its
orthorhombic crystal structure with lattice dimensions a = 27.971Å, b = 5.939Å, c =
12.879Å and α = β = γ = 90° with non-centro symmetric space group Fd2d. (34).
3.3. Synergistic extraction of nickel(II) with HSA and MIBK:
In the synergistic extraction of nickel (II) with a mixture of HSA and methyl-isobutyl ketone
(MIBK), the extraction equilibrium and the synergistic extraction constant Ksyn, can be expressed
by
n 

M

n  m
H L
 p M IB K
M L n H L
M IB K
 n H (Eq 6)

 







org
org
m , org
p , org
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n









M L n H L
M IB K
H




m ,o rg
P ,o rg
(Eq 7)
K syn





 p
n  m
n 


[M
]
H L
M IB K



o rg
o rg




The equation (7) shows that the plots of log D against pH at constant [(HL)org] and constant
[(MIBK)org] should consist of straight lines with slope 2. Typical plots for the extraction of
nickel(II) from 0.33M sodium sulfate solution by mixture of HSA and methyl-isobutyl ketone in
chloroform are shown in figure 6. The slopes obtained 2.00 confirm the expected second-order
dependence of metal distribution upon pH.
As illustrated in figure 6, the addition of methyl-isobutyl ketone causes an enhanced extraction
of nickel (II) for all MIBK concentration; this means that MIBK gives a synergistic effect for
these concentrations.
The addition of MIBK to the solvent leads to a synergistic enhancement and improvement of the
extraction of Ni(II). However, no formation of third phase was observed.
ΔpH1/2 Values was determined from extraction isotherms in figure 10, using equation (8) :
Δ pH 1/2 = pH 1/2 H S A - pH 1/2 (H S A + M IB K (Eq 8)


It can be seen from table 3, that ΔpH0.5 is significantly higher when using a mixture of HSA
and MIBK compared to HSA alone. The most selective extraction results can be achieved by
HSA to MIBK concentration ratio of 0.4 M/5M (HSA/ MIBK) in which the ΔpH1/2 increases
from 0.15 for 0.4 M HSA to more than 1. Figure 11 illustrates the effect of adding MIBK as
synergist on pH1/2. It is observed that pH1/2 values decreases with increasing MIBK concentration
added to HSA. In addition, it can be inferred from figure 11 that the extraction efficiency was
higher with MIBK concentration equal to 5M.
The number of MIBK molecules involved in the extracted species can be determined from the
slope of the graph log D against log[MIBK]org, at constant pH and [(HL)]. As figure 11 shows,
the plot yielded a straight line with a slope of 2.00. Thus, the number of MIBK involved in the
extracted complex is two. There is, therefore, a complex extraction of type NiL2(MIBK)2org. From
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above results, the equation for the synergistic extraction equilibrium of nickel(II) is considered to
be
N i2   2 H L
 2 M IB K
N iL 2 M IB K
 2 H 
2 org






org
org
(Eq 9)
The extraction constant obtained is logK syn= -13.51. The formation of NiL2(MIBK)2org is due to
the replacement by MIBK of two water molecules at an axial position of the Ni-complex (figure
12).
The synergistic enhancement coefficient SC, was calculated by the following equation (Eq.
10). [35]
S C = log
D
/
D
H S A + D M IB K
(Eq 10)




m ix
where DHSA, DMIBK and Dmix represents distribution coefficients of the nickel with the two
extractants HSA and MIBK obtained separately and with their mixture.
Table 4 shows the values of SC at different concentrations of MIBK for 6.99 pH value. At the
constant pH, SC was increased with increasing in MIBK concentrations. The largest synergistic
coefficient was 16.49 for (0.4 M HSA/5 M MIBK) system. Therefore, in the extractant mixture,
the mixed-ligand complex of Ni(II) with HSA and MIBK contributed to the increased extraction
of Nickel compared to the complex of Nickel with HSA alone.
K. K. Sahu et al. (36) reported the extraction of iron (III) from acid chloride solutions with
methyl isobutyl ketone (MIBK), tri-n-butyl phosphate (TBP) and di(2-ethylhexyl) phosphoric
acid (D2EHPA). On comparing the extraction of iron (III) with mixed and individual extractant,
it was found that both D2EHPA-MIBK and D2EHPA-TBP mixtures exhibit synergism, the latter
having better extraction ability. An increase in the concentration of MIBK and TBP in the mixed
organic resulted in higher synergistic coefficient. Stripping of iron (III) from D2EHPA-MIBK
loaded solvent is better than D2EHPA-TBP. The study shows the incorporation of one molecule
of MIBK in the iron (III) species extracted from D2EHPA-MIBK system, and the extracted
species was ClFeR2. L(org)
.
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According to the study of the electronic absorption spectral for the complex in the presence
and absence of MIBK (figure 14), the nickel extracts are octahedral coordination compounds.
The complex shows two weak bands in the regions 907–946 and 617–650 nm, in addition to
ligand bands (37). The positions of these bands suggest that these complexes have distorted
octahedral stereochemistry . T. ZHU (38) explains the existence of much large number of Ni(ll)
octahedral coordination compounds compared to its other configuration complexes. Moreover, it
has been confirmed by many authors that certain quantity of water always extracted with nickel
(39-41). Thus, nickel (II)-HSA complex in chloroform is characterized by distorted octahedral
structure involving two bidentate (SA-) with two water molecules at an axial position.
It is found by Preston [42] that during the extraction of nickel with organophosphorous acids,
the extraction was enhanced by the addition of isodecanol or nonylphenol to xylene diluent.
This is in agreement with the present study in that the diluent substitutes with a water molecule
in the complex.
The synergic extraction of metal chelates has been mostly explained by an increase in the
hydrophobicity of extracted chelates by this replacement reaction of water molecules bound to the
central metal ion by basic neutral organic molecules, such as Methyl-Isobutyl Ketone (MIBK).
4. Conclusion
The solvent extraction of nickel(II) from sulphate medium with N -(2-hydroxy-
benzylidene)aniline (HL) in chloroform in the absence and presence of methyl-isobutyl ketone
(MIBK) in chloroform was studied and the following information was obtained. In the absence of
the synergic agent MIBK, the extracted species is NiL2, and in its presence, the extracted species
is NiL2(HL)2(MIBK)2org. The addition of MIBK to the system of extraction of nickel leads to a
synergistic enhancement and no formation of third phase was observed. The increase in the
hydrophobicity of extracted chelates by the replacement of water molecules by MIBK, has been
taken into account to explain the synergic effect.
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Table 1: Vibration spectral data for free ligand (HSA) and the Ni/complex in KBr disk (cm-1).
O-H
3448. 2
3440
C=N
1615.27
1606
C-O
1453
M-O
-
M-N
-
Ligand seul
Nickel/ligand
1447.47
569.92
877.75
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Table 2: Selected bond lengths (angstroms) for nickel complex.
C-N1
C-O
M-N1
M-O
M-OH2(1)
3.1145
M-OH2(2)
3.1126
1.30354
1.31431
1.90133
1.84021
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Table 3: pH1/2 value in the extraction of nickel for different mixtures of HSA and MIBK at
25°C.
Molar ratio of HSA/MIBK
PH1/2
7.72
7.57
7.4
Δ PH1/2(HSA-mixture)
0.00
0,15
0,32
0,4
0.4
0.4/0.4
0.4/0.8
0.4/1.6
0.4/2
7.32
6.99
6.55
0.73
1.17
0.4/5
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Table 4: Values of the synergistic coefficients ([HSA]=0.4 M) for different MIBK
concentrations at pH value equal 6.99
[MIBK]
SC
0,26872
0,55219
0,69403
1,2598
0.4
0.8
1.6
2
2,10115
5
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Figure 1: Schematic of the synthesis reaction of the Schiff Bases.
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Figure 2: pH effect on the Ni2+ extraction with HSA, in chloroform.
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Figure 3: The efficiency of extraction of Ni(II) with HSA, in chloroform
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Figure 4: Concentration effect of HSA in chloroform on the Ni2+ extraction.
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Figure 5: Structures of the Ni-complexes of HAS
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Figure 6: UV-Vis spectra of Ni complex.
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Figure 7: IR spectra of Ni complex.
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Figure 8: Optimization of the geometry of Ni-complex performed with B3LYP 6-311G(d,p).
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Figure 9: XRD spectrum of HSA crystal and its nickel complex.
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Figure 10: Synergistic effect of MIBK in the extraction of nickel with HSA in chloroform.
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Figure 11: Effect of MIBK concentration on the pH1/2
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Figure 12: Determination of the number of MIBK’s involved in the extracted species.
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