Selective Extraction of Cu (II) Using Novel C=O–NOH-Contained Compound-Dodecyl Phenyl Ethyl Hydroxamic Acid and Its Extraction Mechanism

ABSTRACT Herein, a novel C=O–NOH-contained compound was synthesized and used in the extraction and recovery of copper from sulfate solution. Dodecyl phenyl ethyl hydroxamic acid (DPEHA) exhibited high selectivity to Cu(II) over the competing ions, and the separation factors (β) obtained were β Cu/Zn = 227.370, β Cu/Co = 199.094, β Cu/Ni = 188.889, respectively. The separation effect was stronger than that of 2-hydroxy-5-nonyl acetophenone oxime. Meanwhile, extraction capacity of 0.10 g Cu(II) per gram DPEHA was reached, and the extracted Cu(II) can be stripped effectively. The investigation of extraction mechanism revealed that O atoms in C=O and O-H groups of DPEHA operate as active sites, which could powerfully chelate with Cu(II) by forming stable five-membered rings. The high copper extraction efficiency and selectivity associated with a simple synthesis of DPEHA make it to serve as the potential selective Cu(II) extractant, which also provide a novel insight for the research and development of economical Cu(II) extractants. Graphical Abstract


Introduction
[15][16][17] As the components of many commercial copper extractants, 2-hydroxy-5-nonyl benzaldehyde oxime and 2-hydroxy-5-nonyl acetophenone oxime can extract Cu(II) with high selectivity. [18,19]2-Hydroxy-5-nonyl benzaldehyde oxime has strong extraction ability and fast extraction kinetics, but it is difficult to strip.2-Hydroxy-5-nonyl acetophenone oxime has the advantages of good phase separation performance and easy stripping, while its extraction ability is not as good as 2-hydroxy-5-nonyl benzaldehyde oxime.Although these compounds suffer from the drawbacks of the usage of expensive hydroxylamine as synthetic raw material, the extraction ability of these compounds towards Cu(II) has been extensively investigated because of the unique chemical properties of the hydroxyoxime group.Much work have been conducted on selective extraction of Cu(II) from leaching solutions containing other metal ions, [20][21][22] such as Zn(II), Co(II), and Ni(II).In the C=N-OH group of oxime compounds, both nitrogen and oxygen atoms have lone pair electrons, which make them have good coordination ability with Cu(II). [23,24]Therefore, novel Cu(II) extractants have been widely studied and synthesized on the basis of C=N-OH structure.Przemysław et al. [25] synthesized series more complex N, N-dialkyl-N'-hydroxypyridine -2-carboximidamides containing C=N-OH group to be used in the extraction of Cu(II) over Zn(II) from the Cu-Zn sulfate solutions. [26,27]Adeleye et al. [28] reported that a novel bidentate imidazole-oxime extractant, 1-octylimidazole-2-aldoxime, which was synthesized and applied in the separation of Cu(II) from Zn(II), Co(II), and Ni(II).
Similarly, hydroxamic acid and its derivatives can chelate with Cu(II) by forming stable five-membered rings using two O atoms on C=O-NOH group.Haron et al. [29] synthesized several hydroxamic acid derivatives, which can selectively extract Cu(II) from magnesium, aluminum, and nickel.Haraguchi et al. [30] reported that the extractions of some metal ions in nitrate solutions were conducted using N-hexanoy-and N-octanoyl-N-pheny hydroxy amines which could realize the selective extraction of Cu(II) from Zn(II), Co(II), and Ni(II) by regulating the pH of solution.Although the compound with C=O-NOH group possesses the potential of selective Cu(II) extraction, its application to selectively extract Cu(II) from sulfate solutions has not been reported.
Herein, a novel dodecyl phenyl ethyl hydroxamic acid (DPEHA, Figure 1) was synthesized and introduced as an extractant of Cu(II) from Cu-Zn-Co-Ni sulfate solutions.DPEHA can be used as a highly efficient extractant for selective separation of copper and avoid the use of expensive hydroxylamine as raw material.Extraction mechanism was systematically elucidated by slope and saturated extraction experiment analyses, Fourier transform infrared (FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS) analyses and density functional theory (DFT) calculation.

Materials
All of the reagents used in this study were of analytical grade and used directly without further purification.The water used in the experiment was distilled water.Dodecylbenzene (98%, Shanghai Aladdin Biochemical Technology Co., Ltd.) and acetyl chloride (90%, Sinopharm Chemical Reagent Co., Ltd.) were the raw materials for the synthesis of DPEHA.Other reagents needed include concentrated sulfuric acid, concentrated nitric acid, ammonium chloride, zinc powder (95%, Sinopharm Chemical Reagent Co., Ltd.), sodium bicarbonate, acetonitrile, tetrahydrofuran, methylene chloride, and petroleum ether.
In the extraction experiment, the synthesized DPEHA was dissolved in the diluent chloroform to make the organic phase solution, and the aqueous phase solutions were prepared with the metal(II) sulfate, copper(II) sulfate (CuSO

Synthesis of the extractant
[33] The procedure consisted of synthesis of dodecyl nitrobenzene, which next was reduced to dodecyl benzene hydroxylamine.Dodecyl phenyl ethyl hydroxamic acid was obtained from the reaction of acetyl chloride with the synthesized dodecyl benzene hydroxylamine in the tetrahydrofuran.The detailed synthesis processes and the yield have been described in the Supplementary Information.The synthesized dodecyl benzene hydroxylamine was characterized by NMR ( 1 H and 13 C) spectroscopy (see Table S1, Fig. S1 and Fig. S2 in the Supplementary Information for details).DPEHA was characterized by FT-IR and NMR ( 1 H and 13 C) spectroscopy (see Table S2, Fig. S3, Fig. S4 and Fig. S5 in the Supplementary Information for details).

Extraction procedure
The aqueous phase solutions used in extraction process were prepared by dissolving the corresponding metal sulfate in the distilled water, and the synthesized DPEHA was diluted with chloroform to obtain the organic phase solutions.The extraction experiments were carried out in the thermostatic oscillator.The temperature and oscillation speed are set to 298.15 K and 150 r/min, respectively.The concentration of DPEHA was 0.20 mol/L (except extractant concentration experiments) and that of metal ions in the aqueous phase was 0.01 mol/L.The aqueous phase (1.0 ml) and the organic phase (1.0 ml) were mixed at a volume ratio of 1:1 and oscillated for 15 min.After shaking for 15 min, the aqueous phase was separated for metal ion determination.The pH was adjusted to a required value by adding diluted sodium hydroxide (0.20 mol/L) or sulfuric acid solution (0.20 mol/L) and measured using digital pH meter.The metal ions concentration in the aqueous phase was determined by ICP-AES after dilution with 5% HNO 3 .The percentage extraction (E), distribution ratio (D) and separation factor (β) were calculated using the following equations: Where [M(II)] aq.init and [M(II)] aq.eq respectively represent initial concentrations of metal ions in the aqueous phase before extraction and equilibrium concentrations of metal ions in the aqueous phase after extraction.Where V aq and V org stand for the volumes of aqueous and organic phases, respectively.And M(II) 1 and M(II) 2 refer to Zn (II), Co (II), Ni (II) and Cu(II).

Cu(II) stripping
Of the four ions, only Cu(II) could be well extracted from the aqueous phase, so further consideration was given to the stripping extraction of the copperloaded organic phase.The stripping agent in the stripping experiments was the H 2 SO 4 solution.Stripping experiments were carried out in a thermostatic oscillator at 298.15 K, and the oscillation speed was set to 150 r/min.The copper-loaded organic phase and the H 2 SO 4 solution were mixed at a volume ratio of 1:1 and oscillated for 15 min.Then the aqueous phase was separated for metal ion determination.The stripping efficiency (SE) of Cu(II) were calculated using the following formula: Where [Cu(II)] aq.s , and [Cu(II)] org.s , are the concentrations of Cu(II) stripped into aqueous and initial concentrations of Cu(II) in the organic phase before stripping.

Determination of pH
The pH was measured using PHS-3C pH meter (Yidian Scientific Instruments Co., Ltd., Shanghai).The electrode of pH meter was calibrated pH standard buffer solutions by a three-point method.The three standard buffer solutions used were phthalate (pH = 4.00), phosphate (pH = 6.86), and borate (pH = 9.18), respectively.

FTIR analyses
The FTIR-740 infrared spectrometer (Thermo Nicolet, USA) was used to acquire FTIR spectra.The organic phase in the experiment was the concentration of 0.08 mol/L, which extracted the fresh Cu(II) solutions of 0.05 mol/L several times until the Cu(II) concentration of aqueous phase no longer decreased.And the aqueous phase (20 ml) and the organic phase (20 ml) were mixed at a volume ratio of 1:1.The obtained was the maximum copperloaded organic phase and subsequently evaporated until it was completely dried.Both the resulting sample and DPEHA were used for FTIR analysis with KBr disk pellets in a wavenumber range of 400 to 4000 cm −1 .

XPS measurements
XPS profiles of the samples were acquired by using a Thermo Fisher ESCLAB 250Xi photoelectron spectrometer (Thermo Fisher Scientific, USA), while the samples preparation method was consistent with that of the FTIR spectra analyses.The XPS measurement experiments were operated at 200 W using an Al Kα X-ray source with a 20 eV pass energy in a vacuum pressure range of 10 −10 to 10 −9 Torr.

DFT calculations
DFT, a widely used quantum mechanical method, can be employed to evaluate the chemical properties of DPEHA and predicate its interaction performances with metal ions.The initial molecular structure of DPEHA was drawn on the ChemBioDraw software and preliminarily optimized using the MM2 and PM3 methods. [34]The obtained structure was further optimized at the B3LYP/6-311+G(d, p) and SDD level by Gaussian 09 software and then the required energy was calculated at the basis set level of def2TZVP.In addition, the polarizable continuum model (IEF-PCM) with water as solvent was chosen in the calculation. [35,36]

Effect of initial pH
The effect of pH on the extraction of Cu(II), Zn(II), Co(II), and Ni(II) was examined by varying initial pH of aqueous phase from 1.0 to 5.5.The results are shown in Figure 2(a).The constructed graph shows that the Cu(II) extraction is rather limited in the more acidic solutions, while much better results can be obtained at higher pH.The results also reveal that regardless of initial pH of the aqueous phase the D values of Zn(II), Co(II) and Ni(II) were at a very low level, existing a large gap with Cu(II), which indicates that there is a possibility for the separation of Cu(II) and Zn(II), Co(II) and Ni(II).Meanwhile, the equilibrium pH of the aqueous phase after extraction was also determined.As shown in Figure 2(b), the equilibrium pH of Zn(II), Co(II) and Ni(II) gradually increases with the increase of the initial pH.However, there was no difference between the equilibrium pH and the initial pH.The equilibrium pH of Cu(II) first increases with the increase of the initial pH, and then gradually tends to a stable value.But the equilibrium pH of Cu(II) was significantly lower than the initial pH.This indicates that DPEHA extracts Cu(II) through a cation exchange mechanism, which results in H + releasing and transferring to the aqueous phase during the extraction. [29,37]Increasing of pH (decreasing of [H + ]) means that the equilibrium reaction is liable to move to the direction of extraction, which explains why the extraction efficiency is also high as the pH is higher.Moreover, at low values of pH, the accumulation of H + in solutions inhibited the release of H + from DPEHA molecules which is required for the chelation between DPEHA and Cu(II) for the complex formation. [38]

Effect of oscillating time
The effect of oscillating time on extraction of Cu(II) Zn(II), Co(II), and Ni(II) in sulfate solution was investigated.The obtained results are illustrated in Figure 3

Effect of extractant concentration
The effect of extractant concentration on the extraction was studied by contacting 0.01 mol/L sulfate solution of Cu(II), Zn(II), Co(II) and and Ni(II) has basically not changed after extraction, indicating that they were not extracted.However, the pH eq of Cu(II) first decreased and then gradually appears as a platform, indicating that the extraction effect of Cu(II) was enhanced with the increase of extractant concentration.

Separation of Cu(II)
][41] The previous experiment results showed that DPEHA may be for the separation of Cu(II) from Zn(II), Co(II) and Ni(II) in the sulfate solution.The selectivity of Cu(II) extraction over Zn(II), Co(II) and Ni(II) using DPEHA as the extractant was carried out using the mixture of both and four metals in concentration of 0.01 mol/L.The separation effects of commercial extractants 2-hydroxy-5-nonyl benzaldehyde oxime and 2-hydroxy-5-nonyl acetophenone oxime were compared.In the experiment, the three extractants were dissolved in chloroform and the concentration was 0.20 mol/L.The pH of the mixtures was between 4.7 and 5.0, phase ratio was 1:1 and oscillating time was 15 min.The results obtained are presented in Tables 1 and 2. It can be calculated that the separation factors (β) of DPEHA for the mixture of both ions are, β Cu/Zn = 227.370,β Cu/Co = 199.094,β Cu/Ni = 188.889.The order of the separation factor is as follows: β Cu/Zn > β Cu/Co > β Cu/Ni .Moreover, the decrease in the β values in term of the mixture of four metals are slightly.Meanwhile, the selective separation effect of DPEHA for Cu(II) is weaker than that of 2-hydroxy-5-nonyl benzaldehyde oxime, and stronger than that of 2-hydroxy-5-nonyl acetophenone oxime.The presented results indicate that DPEHA has ability to selectively extract Cu(II) from Cu-Zn-Co-Ni sulfate solution at the acceptable level.

Extraction capacity
Larger extraction capacity means that more Cu(II) can be extracted with the same amount of extractant, which makes for higher extraction efficiency.To evaluate the saturated Cu(II) extraction capacity of DPEHA, the organic solutions of the extractants with concentrations of 0.02, 0.04, and 0.08 mol/L respectively contact the fresh aqueous solution of Cu(II) with concentrations of 0.02, 0.04 and 0.04 mol/L several times until the aqueous concentration of Cu(II) no longer decreased.The accumulated concentration of Cu(II) in the organic phase was calculated by mass balance.Results are listed in Figure 4(a).The organic phase with different concentration continued to extract Cu(II) after loading Cu(II) more than half of its capacity by the second contact and the saturation was reached at the fifth contact.The saturated extraction capacity of DPEHA with concentrations of 0.02, 0.04 and 0.08 mol/L for Cu(II) is 0.01, 0.02 and 0.04 mol/L, respectively.The presented result has shown that the maximum load capacity of DPEHA to Cu(II) is 0.10 g/g.In addition, it inferred that one mole of Cu(II) was chelated by two moles of DEPHA.

Stripping of Cu(II)
The above results have shown that Cu(II) can be selectively extracted with DPEHA.Hence, it is necessary to further study the stripping performances of the copper-loaded organic phase.The stripping experiments were carried out using the Cu-loaded organic phases with concentration of 0.58 g/L.Various concentrations of H 2 SO 4 in the range of 0.05 mol/L −3.0 mol/L were used as stripping agents for the extracted Cu(II).As previously mentioned, increasing of [H + ] means that the equilibrium reaction is liable to move to the opposite direction of extraction.Thus, as presented in Figure 4(b), the loaded Cu(II) stripping efficiency with H 2 SO 4 increases from 74.31% to 97.41% with the increase in the H 2 SO 4 concentration from 0.05 to 3.0 mol/L.The results indicate that a relatively low acidity is sufficient for complete stripping of the loaded Cu(II).Using this copper-loaded organic phase the stripping with 0.2 mol/L H 2 SO 4 is enough.
To study the effect of oscillating time on the stripping, the H 2 SO 4 solutions of 0.2 mol/L were used to strip the Cu-loaded organic phases with concentration of 0.58 g/L.As displayed in Figure 4(c), the variation in the stripping time has shown that the stripping periods of 2 min is sufficient for the achieving of the stripping equilibrium.In general, the stripping performance of the DPEHA-Cu(II) system is satisfactory.

Slope analysis method
Based on the above results and the cationic exchange reaction, the probable Cu(II) extraction equilibrium reaction with DPEHA could be expressed as follows: According to Eq. ( 5) the extraction equilibrium and the distribution ratio (D) can be written as follows: Combing with Eqs. ( 6) and ( 7), the equation can be given: Equation ( 8), after logarithmic transformations, can be rewritten as Eq. ( 9): The values of n and m can be estimated by pH-lgD diagram and lg-[DPEHA]-lgD diagram obtained from Eq. ( 9).
As shown in Figure 5(a), the slopes of the lines obtained from the plot lgD versus equilibrium pH were close to 2, which indicates that 2 mole of H + was released into the aqueous phase on the formation of per mole of the complexes.The released H + resulted in an enhancement of the acidity of aqueous phase and if the transmission of H + is limited, the efficiency of Cu(II) extraction is also limited.The equilibrium data for the initial pH value of 1.4, was also plotted as relation lgD and lg[DPEHA], and presented in Figure 5(b).As observed, the obtained was straight line with slope close to 2, which indicates that 2 molecules of DPEHA were involved in the extracted Cu(II) complexes.

FTIR analyses
The FTIR spectra of the DPEHA and DPEHA-Cu complexes samples are presented in Figure 6(a).DPEHA exhibits a characteristic peak at around 3125.91 cm −1 , which is assigned to the stretching vibration of the O-H group, [42,43] and bands at 2916.36 and 2848.49cm −1 , which are attributed -CH 3 and -CH 2 stretching vibrations, respectively. [44,45]And the C=O peak of -C(=O)NOH group appeared at around 1637.04 cm −1 . [46]The peaks near 1441.98 cm −1 and 1394.70 cm −1 were assigned to the corresponding C-H symmetric bend for CH 2 /CH 3 and the O-H bend of N-O-H. [47]The band at 1507.94 cm −1 was due to the C-N vibration of -C(=O)NOH group and the O-H peak due to H 2 O emerged at 3448.52 cm −1 . [42]After interacting with Cu(II), the O-H peak at around 3125.91 cm −1 in DPEHA disappeared in the  DPEHA-Cu complexes. [48]And the new peaks at near 590.12 and 489.84 cm −1 are evident, which is assigned to the Cu-O stretching vibration along the (101) direction. [49,50]The FTIR spectra of the complexes displays a sharp and intense C=O stretching band at 1590.90 cm −1 , showing a shift to lower wavenumbers than in the DPEHA. [51]These undoubtedly confirm the involvement of the C=O and O-H groups in coordination of the Cu(II).

XPS analyses
To further study the bonding between DPEHA and copper, XPS analyses of DPEHA and copper-extracted DPEHA were conducted.As shown in Figure 6(b) and Table 3, the atomic concentration of copper increased, confirming the interaction between DPEHA and Cu(II) in extraction process.The high-resolution XPS spectra of O in DPEHA and DPEHA-Cu complexes are shown in Figure 6(c).For DPEHA, the O 1s XPS bands appeared at approximately 533.27 eV and 531.39 eV, which was assigned to the oxygen of C=O group and -OH group, respectively. [52]For DPEHA-Cu complexes, the O 1s XPS profile of DPEHA-Cu complexes consists of only one peak at about 531.56 eV.The emergence of a new peak and the disappearance of two initial peaks were primarily due to the interaction between the oxygen atom of the C=O-NOH group in DPEHA and Cu(II), revealing the similar bonding environment of the two O atoms in the C=O-NOH group after bind to Cu(II), [53,54] which indicated the possible formation of five-membered-ring structure of DPEHA-Cu complexes. [55]Furthermore, the probable chelation process of the DPEHA with Cu(II) is listed in Figure 6(d) based on all above analyses.

Charge population analysis
The molecular electrostatic potential (MEP) was employed to describe the molecular electron density and considered to be a useful means in predicating the active sites in target molecules. [56]The MEP diagram of DPEHA was displayed in Figure 7(a).The areas colored red and blue respectively represent the most positive and negative electrostatic potential regions.It is obvious that the negative regions of DPEHA are mostly centered on the C=O and O-H groups.This indicates that C=O and O-H are probably the active groups of DPEHA binding to metal ions, which is supported by the shift of C=O and

Binding ability evaluation
][59][60][61] In the XPS analyses of O 1s of DPEHA-Cu complexes, it can be known that DPEHA is likely to form a five-membered rings through the two oxygen atoms of C=O and O-H with Cu(II) coordination bond and covalent bond, respectively.Furthermore, the geometries of the complexes are optimized by means of DFT calculation and displayed in Figure 8.The calculated natural charges of complexes are presented in Table 4.The interaction between DPEHA and metal ions involved the transfer of electrons and the oxygen atoms donated the valence electrons to metal ions, which resulted in the decrease of metal ions charge (the valence charge of free ions is + 2).Meanwhile, the net charge of atoms on the C=O-NOH group also  changed.This result revealed that the decrease of Cu(II) charge is largest, indicating that the higher the number of electrons transfer and the electron cloud density were exhibited between Cu(II) and oxygen atoms compared to that of Zn(II), Co(II) and Ni(II).Furthermore, their binding energies were calculated as follows: where ΔE b refers to the binding energy and E c , E d and E m refer to the energies of the complexes, extractant anion and metal ions, respectively.The calculated results are shown in Table 5.The calculated binding energies are all negative values, indicating the spontaneous nature of the binding between DPEHA and metal ions.Moreover, the absolute values of △E b were arranged as follows: Cu(II) > Ni(II) > Co(II) > Zn(II), which also suggested the greater tendency of DPEHA to interact with Cu(II) over other ions and the separation possibility of Cu(II) from Zn (II), Co (II) and Ni (II).This is consistent with the order of the separation factor obtained from the extraction experiments.

Conclusion
In this study, DPEHA was firstly synthesized and tested as extractant of Cu(II) from sulfate solution.Expensive hydroxylamine was avoided in

Disclosure statement
No potential conflict of interest was reported by the author(s).
(a).The D values of Cu(II) increased with the increase of oscillation time in the first five minutes.Above this time the D Cu values hardly changed.The D values of Zn(II), Co(II) and Ni(II) were significantly lower than that of Cu(II) The equilibrium pH of the aqueous phase after extraction was also determined.The initial pH of the aqueous phase of Cu(II) Zn(II), Co(II) and Ni(II) in the experiments were 4.74, 5.82, 5.32 and 5.45, respectively.It can be seen that the pH of Zn(II), Co(II) and Ni(II) solution was basically not changed after extraction, but the pH of Cu(II) decreased significantly after extraction.The presented results indicate that only the Cu(II) has been well extracted with DPEHA.

Table 1 .
Calculated distribution ratio and separation factor of three extractants for the mixture of both ions.

Table 2 .
Calculated distribution ratio and separation factor of three extractants for the mixture of four ions.

Table 3 .
Elements content of DPEHA and DPEHA-Cu complexes.H in the infrared spectra of DPEHA-Cu complexes.The calculated natural charges of DPEHA and its anion are listed in Table4, the corresponding atomic numbers are displayed in Figure7(b).The results indicate that the net charge of N and O atoms is negative.A small amount of positive charge is distributed on the hydrogen atom of O-H group and the removal of the hydrogen atom also results in significant increase in the negative charge number of the O atoms.Moreover, the disappearance of O-H in DPEHA-Cu complexes infrared spectrum and the decrease of pH of Cu(II) solution after extraction indicate that O-H group is presumably dehydrogenated before interaction with metal ions.

Table 4 .
The net charge of atoms and C=O-NOH group for DPEHA, its ion and complexes with different metals.