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Insights into the saponification process of di(2-ethylhexyl) phosphoric acid extractant: Thermodynamics and structural aspects
Accepted Manuscript Insights into the saponification process of di(2-ethylhexyl) phosphoric acid extractant: Thermodynamics and structural aspects
Min Sun, Shijun Liu, Yunran Zhang, Mei Liu, Xin Yi, Jiugang Hu PII: DOI: Reference:
S0167-7322(18)35577-6 https://doi.org/10.1016/j.molliq.2019.02.025 MOLLIQ 10426
To appear in:
Journal of Molecular Liquids
Received date: Revised date: Accepted date:
28 October 2018 2 February 2019 5 February 2019
Please cite this article as: M. Sun, S. Liu, Y. Zhang, et al., Insights into the saponification process of di(2-ethylhexyl) phosphoric acid extractant: Thermodynamics and structural aspects, Journal of Molecular Liquids, https://doi.org/10.1016/j.molliq.2019.02.025
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ACCEPTED MANUSCRIPT Insights into the saponification process of di(2-ethylhexyl) phosphoric acid extractant: thermodynamics and structural aspects
a
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Min Suna, Shijun Liua,*, Yunran Zhanga, Mei Liub, Xin Yia, Jiugang Hua,c,*
School of Chemistry and Chemical Engineering, Central South University, Changsha
College of Chemistry, Chemical Engineering and Environmental Engineering,
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b
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410083,PR China.
Hunan Provincial Key Laboratory of Chemical Power Resources, Central South
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c
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Liaoning Shihua University, Fushun 113001, China.
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University, Changsha 410083, China.
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Corresponding authors: [email protected] (J. Hu);[email protected] (S. Liu).
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ACCEPTED MANUSCRIPT Abstract: The saponification process of di(2-ethylhexyl)phosphoric acid extractant (D2EHPA) was investigated by the isothermal titration calorimetry (ITC), attenuated total reflection infrared (ATR-IR), and small-angle X-ray scattering (SAXS) spectroscopies.
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The effects of the temperature, types of saponifier, and diluent on the saponification
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process of D2EHPA were disclosed by ITC method. The saponification reaction is an
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exothermic process, but the contribution of temperature on the maximum saponification degree of D2EHPA is slight. The theoretical maximum saponification
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degree of D2EHPA is about 40%. Moreover, the maximum effective saponification
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degree of D2EHPA is the same at the condition of different saponifiers because the two systems release the same amount of heat. The effective saponification degree of
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D2EHPA in octane is larger than that in toluene at the same content of saponifier. The
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IR spectra indicate that the structure of organic phases varies significantly with the increase of saponification degree. The most water-in-oil microemulsion in the organic
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phase is formed at the 40% theoretical saponification degree. The water content and
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SAXS data of organic phase further verify this phenomenon. Solvent extraction results show that the saponified extractant can obviously improve extraction efficiency of cobalt. These results will be helpful for optimizing the saponification process of D2EHPA during metal extraction. Keywords: di(2-ethylhexyl)phosphoric acid; saponification; interaction; calorimetry
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ACCEPTED MANUSCRIPT 1. Introduction Liquid-liquid extraction is an energy-saving technique to separate and concentrate the targeted materials in the fields of chemical engineering, hydrometallurgy, and environment [1, 2]. It has been widely applied as an efficient
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method for recovery of metal ions from various aqueous media with the advantages of
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low cost and recyclability of components [3]. Since tributylphosphate was used to
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extract uranium in the 1940s, many commercial extractants have been successfully developed for metal separation, such as organophosphorous [4], carboxylic [5], and
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quaternary amine extractants [6].
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In the extraction process, organophosphorous extractants [7] such as 2-ethylhexyl phosphoric acid-2-ethylhexyl ester (EHEHPA) [8], di(2-ethylhexyl) acid
(D2EHPA)
[9],
bis(2,4,4-trimethylpentyl)
phosphinic
acid
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phosphoric
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(Cyanex272) [10] have been comprehensively used for the extraction of various metal ions from aqueous solutions because of their high extraction efficiency and selectivity.
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As a cheap and stable extractant, D2EHPA is a cation exchanger to extract metal ions
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in the form of monomer and dimer [11], which has extensively used in hydrometallurgical processes for the separation and purification of transition metals such as copper, cobalt, zinc, iron [12, 13]. Mellah et al. investigated solvent extraction of zinc and cadmium from phosphoric acid solution using D2EHPA (denoting as HA) diluted in kerosene. The results were concluded that the extracted zinc was coordinated with 1.5 molecules of HA monomer and the cadmium was coordinated with 1.25 molecules of HA monomer [14]. Yang et al. investigated solvent extraction
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ACCEPTED MANUSCRIPT of Fe3+ from phosphoric acid media by HA in kerosene and found that the structure of extracted species was demonstrated to be FeClA2·4HA [15]. Because acid extractants are usually presented as the form of hydrogen-bonded dimer H2A2 in the non-polar diluents, these extractants need to be saponified by
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acid/base neutralization reaction to increase their extractability and avoid the increase
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of aqueous phase acidity induced by releasing hydrogen ions during extraction [16].
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After breaking the hydrogen bonds in dimers and associating with alkali metal cations, the saponified extractants easily form water-in-oil microemulsions in an organic phase,
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which will potentially affect the separation efficiency and extraction selectivity [17].
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Wu et al. studied the structural changes and the composition of extractants in the saponification process with different saponifiers [18, 19]. They focused on the
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formation of microemulsion by saponifying acid extractants and found that the region
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of the single and transparent microemulsions by NH4OH was larger than those of NaOH and KOH. They also investigated the extraction of rare earth or divalent metal
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ions by the saponified extractants and emphasized the importance of the role of water
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in the extraction process. Although saponification process of acidic extractants has been considered as a routine strategy during metal extraction, the effective saponification degree of extractants is still unclear and it is dependent on the various parameters including types of diluent and saponifier, temperature, and so on. Some literature [20, 21] indicated that there could be a limited saponification degree of acid extractants because the extraction efficiency of metal ions cannot be further improved after reaching a certain saponification degree. Therefore, insights into the
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ACCEPTED MANUSCRIPT saponification process of D2EHPA are helpful for improving extraction efficiency and understanding the extraction mechanism. In this work, the saponification process of di(2-ethylhexyl)phosphoric acid (D2EHPA) were explored using isothermal titration calorimetry (ITC), attenuated
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total refraction Infrared (ATR-IR) spectroscopy, and small-angle X-ray scattering
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(SAXS). The effects of temperature, diluents, and saponifiers on the saponification
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degree of D2EHPA were investigated. The water content and SAXS spectra of the saponified organic phases were determined to evaluate the saponification behaviors
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and interaction between D2EHPA and saponifiers.
2. Experimental
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2.1 Reagents and Solution Preparation
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The extractant di(2-ethylhexyl)phosphoric acid (D2EHPA, purity of 95%) was purchased from Sinopharm. Toluene and octane of analytical grade were used as
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diluents. The extractants and diluents were used without further purification. The
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molecular structures of D2EHPA monomer and dimer were given in Fig. 1. The diluents (toluene or octane) are used in the saponification process to decrease the viscosity of extractant and provide a suitable solvation environment [22-24]. Desired amounts of extractants were dissolved in the diluents to prepare the organic phases, and the concentration was adjusted to 0.2 mol·L-1. 2 mol·L-1 NaOH or NH4OH aqueous solution was prepared as the saponifiers. The concentrations of NaOH and NH4OH aqueous solutions were calibrated with a standard concentration of 0.1
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Fig. 1 Structures of D2EHPA monomer and its dimer.
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2.2 Calorimetric Measurements
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The isothermal titration calorimetry was used to study the saponification reaction of D2EHPA (0.2 mol·L-1) with NaOH or NH4OH solutions (2 mol·L-1) [25]. As shown
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in Fig. 2, D2EHPA solution was placed in the sample pool, and the corresponding
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diluent was placed in the reference pool. The saponifier solution was placed in a 5 mL syringe. The titration experiment consisted of 10 injections of 25 μL each with
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intervals of 20 min between injections. The equivalent NaOH or NH4OH solution was
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simultaneously injected into the sample pool and reference pool to deduct the dilution heat of titrant and titrate. The thermal power-time curve of the saponification process
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was determined under the different conditions. Raw measurement data consisting of
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downward or upward peaks indicate an overall exothermic or endothermic process, respectively. After baseline correction, the integrated area under the curve was calculated as the generated heat Q. The thermal effect was calculated as follows. 𝑄 = ∆𝑛∆𝑟 𝐻𝑚
(1)
where Q is the total heat released during saponification reaction (J), Δn is the amount of substance involved in the reaction (mol), ∆r Hm is the reaction heat of the saponification process (J·mol-1).
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Fig. 2 Schematic diagram of ITC instrument for saponification reaction. 2.3 ATR-IR Measurement
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For the measurement of ATR-IR spectra, the saponified D2EHPA with different
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saponification degrees were prepared previously. The ATR-IR spectra of saponified organic phases were recorded at room temperature with a Nicolet 6700 FT-IR
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spectrophotometer. The attenuated total reflection through cell equipped with a Ge
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crystal of 45º incident angle and 12 reflections was used. The IR spectra with a resolution of 4 cm-1 were collected by 320 scans in the range of 4000-800 cm-1. Each
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sample was measured three times in order to check the spectral repeatability, and the
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average spectra were used for further analysis. Data processing was carried out using the OMNIC 8.2 software package. 2.4 SAXS Measurements and Data Interpretation Small-angle X-ray scattering (SAXS) measurements are ideally suited for the investigation of the morphology (shape and size) of particles of the microemulsions or reverse micelles. SAXS data for the D2EHPA organic phase of different theoretical saponification degree were collected on the beamline BL16B1 at the Shanghai
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ACCEPTED MANUSCRIPT Synchrotron Radiation Facility with X-ray energy of 10 keV (SSRF, China). All solutions were measured using a single 2 mm diameter quartz capillary tube. The sample-to-detector distance was set to 1770 mm. Scattering was detected in the q ranges of 0.1–1.3 nm−1 in which q=(4πsinθ)/λ. FIT2D software was used to convert
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the one-dimensional (1D) data from the 2D scattering pattern. All the data were
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background-subtracted and normalized. In the absence of water-in-oil microemulsions,
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the organic phase produces weak scattering intensity that is barely discernible above the diluent background.
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2.5 Extraction Experiments
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The extraction experiments were performed by contacting 5 mL 0.2 mol·L-1 D2EHPA extractant with 5 mL 0.02 mol·L-1 CoSO4 aqueous solution for 30 min in a
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vibrating mixer at room temperature. Before the extraction experiment, the saponified
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organic phase was washed 4-5 times with the saturated sodium chloride and then centrifuged to remove the entrained aqueous solution in the organic phase. After
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extraction, the aqueous phase was separated by centrifugation at 5000 rpm for 5 min,
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and the concentration of Co(II) in the aqueous phase was measured by inductively coupled plasma-atomic emission spectrometry (ICP-AES, Perkin Elmer Optima 5300 DV) before and after extraction. The concentration of Co(II) in the loaded organic phase was calculated by mass balance. The extraction efficiency (E%) is the ratio of the equilibrium concentration of Co(II) in the loaded organic phase to the initial concentration of Co(II) in the aqueous phase.
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ACCEPTED MANUSCRIPT 3. Results and Discussion In order to elucidate the saponification behavior, the interaction between D2EHPA and saponifier was investigated using isothermal titration calorimetry and ATR-IR spectra. The saponification degree (sd) of D2EHPA in the experiment was
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adjusted by mixing with certain amounts of saponifier (NaOH or NH4OH). The
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calculation formula of the saponification degree was defined according to industrial
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calculation methods (equation 2). Herein, the saponification degree also refers to the theoretical saponification degree in order to distinguish the effective saponification
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degree of D2EHPA in this work, which is justified by the amount of heat released in
sd =
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the saponification process. 𝑛(saponifier) 𝑛(𝐷2𝐸𝐻𝑃𝐴)
× 100%
(2)
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D2EHPA, respectively.
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where n(saponifier) and n(D2EHPA) are the initial mole numbers of the saponifier and
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3.1 Calorimetric analysis
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The effects of temperature, saponifier, and diluent on the saponification of D2EHPA were studied separately by the calorimeter. Fig. 3a presents the thermal variation process of the saponification reaction at the different temperatures, which exhibited an exothermic process. The total heat released of the saponification reaction was calculated by the integral. As shown in Fig. 3b, the released heat during the saponification process decreases as rising temperature, indicating that the lower temperature is favorable for the saponification of D2EHPA. However, the change of
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ACCEPTED MANUSCRIPT temperature does not affect the theoretical maximum saponification degree of D2EHPA. There is a slight difference between the reaction heats at different temperatures when the theoretical saponification degree increases from 0% to 40%. Because temperature affects the extent of the saponification reaction, which in turn
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leads to the difference between the effective saponification degree and the theoretical
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maximum saponification degree.
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Fig. 3 (a) Calorimetric experimental data of saponification process of 0.2 mol·L-1 D2EHPA at different temperatures and (b) the corresponding total reaction heat at
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different saponification degrees. The diluents is octane; saponifier is NaOH.
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During the industrial application, NaOH and NH4OH are the common saponifiers, but their basicity difference will affect the theoretical maximum saponification degree of D2EHPA. As illustrated in Fig. 4, the theoretical maximum saponification degree of D2EHPA is ca. 40% and ca. 55% for NaOH and NH4OH, respectively. However, the saponification process releases almost the same heat for two systems when the added saponifiers exceed the theoretical maximum saponification degree. Because the essence of the saponification process is an
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ACCEPTED MANUSCRIPT acid-base neutralization reaction, when the heat released during the saponification process is the same, the amount of OH- participating in the saponification reaction should be the same. Since NH4OH is a weak base, more NH4OH molecules are needed to provide the same amount of OH- as NaOH. For instance, at the 30%
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saponification degree, the reaction heat for NaOH system is larger about 15.4 J than
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that of NH4OH, which means the effective saponification degree for NaOH system is
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greater than that of NH4OH at the same concentration of saponifier. On one hand, the sodium ions can coordinate with D2EHPA via coordination bond, but the ammonium
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ions are combined with D2EHPA by electrostatic interaction in the saponification
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process [12, 26]. On the other hand, when NH4OH participates in the saponification reaction, it needs to absorb heat to break the hydrogen bond between ammonia
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molecules. Moreover, NH4OH could be easier to enter the D2EHPA reverse micelles
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together with water to participate in the reaction than NaOH because of the hydrogen
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bonds.
Fig. 4 (a) Calorimetric experimental data of saponification process of D2EHPA using NaOH and NH4OH as saponifiers and (b) the corresponding total reaction heat at
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ACCEPTED MANUSCRIPT different saponification degrees. The diluent is octane; the temperature is 298K. The effect of diluents on the saponification process of D2EHPA was evaluated. As shown in Fig. 5, although the reaction heat was different, the theoretical maximum saponification degree of D2EHPA for both octane and toluene reaches 40%. For the
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specific reaction, the reaction heat ∆𝑟 𝐻𝑚 can be only influenced by temperature.
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According to the equation (1), when different diluents are used, the difference of
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reaction heat is related to the amount of substance involved in the saponification reaction. The value of relative polarity and the dielectric constant of octane and
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toluene are listed in Table 1. Because the polarity of a diluent is proportional to its
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dielectric constant [27], the interaction between D2EHPA and diluents becomes stronger when the diluent is changed from octane and toluene. Accordingly, the
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reaction of NaOH with D2EHPA in toluene is more difficult than D2EHPA in octane,
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which further affects the effective saponification degree of D2EHPA. Although the theoretical maximum saponification degree of D2EHPA for both octane and toluene
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are the same, the effective saponification degree of D2EHPA in octane is larger than
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that of D2EHPA in toluene.
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ACCEPTED MANUSCRIPT Fig. 5 (a) Calorimetric experimental data of saponification process of D2EHPA using toluene and octane as diluents and (b) the corresponding total reaction heat at different saponification degrees. The saponifier is NaOH; the temperature is 298K. Table 1 Dielectric constants of the diluents used in experiment[28]. Dielectric constant
Octane
0.02
1.95
Toluene
0.24
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Relative polarity a
2.36
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a
Diluent
The polarity value of water is consider to be 1.000.
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According to the calorimetric data, although there is a slight change in total
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reaction heat, the theoretical maximum saponification degree of D2EHPA can reach about 40% in each case. When continuously increasing the saponifier contents,
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D2EHPA molecules will be no more saponified whatever changing temperature or
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type of diluent. This phenomenon is consistent with the extraction experiments of metal ions in literature. Yin et al. studied the effect of different saponification degree
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on the extraction of Nd(III) using NaOH as the saponifier and found that the
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extraction efficiency was the best when the saponification degree was 40% at pH 4.0 [21]. Therefore, the results in this work will be helpful for optimizing the saponification process of D2EHPA during metal extraction.
3.2 Structural change of saponified organic phases From the calorimetric investigation, octane is the optimal diluent for improving the effective saponification degree of D2EHPA. In order to understand this
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ACCEPTED MANUSCRIPT phenomenon, the interaction between D2EHPA and NaOH is further investigated with ATR-IR spectroscopy [16]. The IR spectra of D2EHPA in octane are shown in Figs. 6 and 7 before and after saponification. In Fig. 6a, the characteristic band at 1035 cm-1 broad peak is identified as the superposition peak of P-O-C and P-O-H groups of
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D2EHPA. With the increase of the theoretical saponification degree, the peak position
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shifts to a low wavenumber of 1080 cm-1 and accompanies the appearance of the
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shoulder peak at 1030cm-1. The probable reason is that the P-O-H bond interacts with metal ion Na+ to form a coordination bond after saponification. After the theoretical
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saponification degree of D2EHPA reaches 40%, there will be a new peak at 1065 cm-1.
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This phenomenon could be induced by the interaction of P-O-C bond and water, thus water-in-oil microemulsions or reverse micelles may be formed in the saponified
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organic phase. In Fig. 6b, the broad characteristic band at 1225 cm-1 is identified as
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the P=O stretching vibration of D2EHPA. As the theoretical saponification degree increases, the stretching vibration peak of P=O has a decreasing trend due to the
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formation of a hydrogen bond between P=O and H2O. When the theoretical
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saponification degree reaches 40%, the stretching vibration peak of P=O obviously decreases and the peak position moves to 1205 cm-1. In Fig. 7, the characteristic peaks at 1648 cm-1 and 3412 cm-1 are the vibration absorption peak and the stretching vibration peak of the water in the organic phase after saponification, respectively. No characteristic absorption band of water could be observed in the IR spectra when the theoretical saponification degree was lower than 30%. After the theoretical saponification degree was over 30%, the vibration
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ACCEPTED MANUSCRIPT absorption peak and the stretching vibration peak of water appears [24]. The reason is that the hydrogen bond of the D2EHPA dimers was broken and water-in-oil
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microemulsions or reverse micelles were formed.
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Fig. 6 ATR FT-IR spectra of D2EHPA in octane at different saponification degrees. (a)
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the P-O peak and (b) P=O peak of D2EHPA. The saponifier is NaOH.
Fig. 7 ATR FT-IR spectra of D2EHPA in octane at different saponification degrees. (a) 15
ACCEPTED MANUSCRIPT vibration absorption peak and (b) the stretching vibration peak of water in the organic phase after saponification. The saponifier is NaOH. Fig. 8 is the IR spectra of D2EHPA saponified with different saponifiers in the organic phase. After saponification, the absorption frequency of the characteristic
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peaks of D2EHPA is reduced for both saponifiers. The stretching vibration peak of
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P=O of the unsaponified D2EHPA is 1236 cm-1. After saponified with NaOH, the
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absorption frequency of P=O bond moves to 1210 cm-1. However, when D2EHPA is saponified with NH4OH, the absorption frequency of the P=O bond is further reduced
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to 1200 cm-1. The reason is that the extractant anion (RO)2PO2- is coordinated with
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Na+, resulting in weakening of the P=O bond. But NH4OH is a polar molecule, and NH4+ has no empty 3d orbital and cannot form a coordination bond with (RO)2PO2-.
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Therefore, in addition to the electrostatic interaction between NH4+ with the anion
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(RO)2PO2-, NH4+ can also form hydrogen bonds with (RO)2PO2-, which causes the absorption frequency of the P=O bond to move toward lower wavenumbers. In
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addition, the water peak at 1645 cm-1 for NH4OH-saponified D2EHPA is wider than
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that of NaOH-saponified D2EHPA.
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ACCEPTED MANUSCRIPT Fig. 8 FT-IR spectra of D2EHPA saponified with different saponifiers. The saponification degree is 50%. HA is an abbreviation of D2EHPA. The water content of the saponified organic phases was further determined by Karl Fisher titrator method. Fig. 9 shows the intensity of the associated water peak in
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infrared spectra and the water content of organic phases under the different
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saponification degrees. It can be found that the stretching vibration peak of the
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associated water has a similar trend with the water content under different saponification degrees. When the theoretical saponification degrees are 0%, 10%, and
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20%, there is a weak stretching vibration peak of water because the water content in
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the organic phase is lower than 2 wt%. After the theoretical saponification degree reaches 30%, the stretching vibration peak of the associated water appears. And then
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the water content in the organic phase increases as increasing the amount of saponifier.
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When the theoretical saponification degree reaches 40%, the water content in the organic phase sharply increases to 10.8 wt%, accompanying the appearance of the
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maximum stretching vibration peak of the associated water. This phenomenon can be
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attributed to the formation of water-in-oil microemulsions or reverse micelles in the saponified organic phase when the saponification degree reaches a certain level, which causes a significant increase of the water content in the organic phase. However, after the 40% saponification degree, the water content in the organic phase has a downward trend. The possible reason is that when the water content in the microemulsions reaches a certain value, the water-in-oil microemulsions could be demulsified, resulting in a decrease in the water content.
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Fig. 9 The intensity of the associated water peak in infrared spectra and water content of organic phases under the different saponification degrees with NaOH as a
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saponifier.
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The SAXS measurements of organic phases were carried out in order to further understand the structure and interaction between D2EHPA and saponifiers under the
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different saponification degrees. Fig.10 (a) shows the variation trend between the
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scattering I(Q) and momentum transfer Q of the organic phase with D2EHPA in the different theoretical saponification degree, suggesting that there were no notable
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changes in the aggregate state because of the similar envelope. However, the different
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I(0) values indicate the size of extractant aggregates has an evident change. As shown in Fig.10 (b), the I(0) value sharply increases below the saponification degree of 40%, and then it decreases as increasing the theoretical saponification degree. This suggests that the water-in-oil microemulsions or reverse micelles begin to be formed when mixing with saponifier. The saponified extractant systems may form the largest micelles in the 40% saponification degree, which is the theoretical maximum saponification degree. However, when the amount of NaOH exceeds the theoretical
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ACCEPTED MANUSCRIPT saponification degree, the excess NaOH could act as a demulsifier. Thus, the formed microemulsions are easily flocculated and aggregated into large droplets. The structure of microemulsions will be destroyed, thereby resulting in a decrease of the water content in the organic phase. The sharp decrease of I(0) value also indicates that
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the structure of microemulsions has been destroyed. This phenomenon is consistent
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with the result of the water content.
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Fig. 10 (a) Log-log plot of the SAXS data for the D2EHPA in the different theoretical saponification degrees; (b) I(0) calculated from the entire scattering curves for the
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D2EHPA organic phase under the different theoretical saponification degrees.
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3.3 Extraction efficiency of saponified D2EHPA Fig. 11 shows the extraction behaviors of Co(II) with the saponified D2EHPA at different saponification degrees. It can be found that saponification of D2EHPA has a favorable effect on the extraction of cobalt ions, and the extraction efficiency raises with increasing saponification degree for both NaOH and NH4OH saponified D2EHPA. When the D2EHPA is not saponified, the extraction efficiency of cobalt is only 3.94%. However, when the theoretical saponification degree reaches 40% with 19
ACCEPTED MANUSCRIPT NaOH as a saponifier, the extraction efficiency increases to 98.46%. And then the extraction efficiency only has a slight increase. Notably, as seen from the insert of Fig.11, the saponification degree of D2EHPA has an obvious effect on the distribution coefficient (D) of cobalt, which is the ratio of cobalt in the organic phase to the
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aqueous phase. For NaOH system, the distribution coefficient of cobalt increases as
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saponification degree increases to 40%, and then it increases slowly. A similar trend
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can be found for NH4OH-saponified D2EHPA, but the improvement of which on extraction behavior of cobalt is significantly lower than the NaOH system. Below the
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effective saponification degree of D2EHPA, the D2EHPA dimers can be broken into
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the easily-dissociated D2EHPA monomer, which benefits for the extraction of Co(II). When the effective saponification degree of D2EHPA reaches the maximum (such as
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40% for NaOH system), previous results show that the water-in-oil microemulsions or
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reverse micelles in organic phase are formed, thereby the extraction of Co(II) can be enhanced. The acidic D2EHPA in the organic phase acts as a phase transfer agent
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allowing the metal ions Co2+ to pass through the oil-water interface into the
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microemulsions as CoA2. Therefore, the saponification of D2EHPA is necessary and beneficial for the metal extraction process.
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ACCEPTED MANUSCRIPT Fig. 11 The extraction efficiency of cobalt at different saponification degrees with NaOH or NH4OH as saponifiers. The insert is the
relationship between the
distribution coefficient of cobalt and the saponification degree of D2EHPA. Aqueous
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phase is 0.02 mol/L CoSO4 at pH 6. The diluent is octane.
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4. Conclusions
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The interaction between the acid extractant di(2-ethylhexyl)phosphoric acid (D2EHPA) and saponifiers (NaOH or NH4OH) in the saponification process were
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investigated from thermodynamics and structural aspects. The saponification reaction
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of D2EHPA is an exothermic process. Changing the temperature does not affect the theoretical maximum saponification degree of D2EHPA in the range of 15 to 45 °C,
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which is ca. 40%. Although the theoretical maximum saponification degree of
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D2EHPA is ca. 40% for NaOH and ca. 55% for NH4OH, the types of saponifier have almost no influence on the effective saponification degree because the two
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saponification systems release the same amount of heat. Moreover, the effective
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saponification degree of D2EHPA in octane is larger than that in toluene as more heat is released in octane system at the same content of saponifier. The IR and SAXS data shows that the structure of organic phases varies significantly with the theoretical saponification degree. Because of the formed water-in-oil microemulsions, the water content in the organic phase increases obviously and reaches the maximum value at the theoretical saponification degree of 40%. The microemulsion structure of saponified organic phases will be broken when the theoretical saponification degree is
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ACCEPTED MANUSCRIPT beyond 40%. The relationship between extraction efficiency and saponification degree can be concluded that the saponification of the extractant is beneficial for the metal extraction process.
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Acknowledgements
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This work was financially supported by the National Basic Research Program of
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China (No.2014CB643401), the Hunan Provincial Science and Technology Plan Project of China (No. 2016TP1007), and the Changsha Science and Technology
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Project (No. kq1801069) .
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ACCEPTED MANUSCRIPT [5] E. Jaaskelainen, E. Paatero, Characterisation of organic phase species in the extraction of nickel by pre-neutralised Versatic 10, Hydrometallurgy 55 (2000) 181. [6] K.L. Wasewar, D. Shende, A. Keshav, Reactive extraction of itaconic acid using quaternary amine aliquat 336 in ethyl acetate, toluene, hexane, and kerosene, Ind. Eng.
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ACCEPTED MANUSCRIPT from solutions with D2EHPA in three diluents: extraction equilibria and modeling, J. Chem. Technol. Biotechnol. 92 (2017) 133. [13] R.K. Biswas, M.A. Habib, M.N. Islam, Some physicochemical properties of (D2EHPA). 1. Distribution, dimerization, and acid dissociation constants of D2EHPA
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ACCEPTED MANUSCRIPT Highlights The saponification process of D2EHPA was studied by ITC, ATR-IR and SAXS methods. The effects of the temperature, types of saponifier, and diluent were evaluated.
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The theoretical maximum saponification degree of D2EHPA is about 40%.
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The most water-in-oil reverse micelles are formed at the 40% saponification degree.
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Min Sun, Shijun Liu, Yunran Zhang, Mei Liu, Xin Yi, Jiugang Hu PII: DOI: Reference:
S0167-7322(18)35577-6 https://doi.org/10.1016/j.molliq.2019.02.025 MOLLIQ 10426
To appear in:
Journal of Molecular Liquids
Received date: Revised date: Accepted date:
28 October 2018 2 February 2019 5 February 2019
Please cite this article as: M. Sun, S. Liu, Y. Zhang, et al., Insights into the saponification process of di(2-ethylhexyl) phosphoric acid extractant: Thermodynamics and structural aspects, Journal of Molecular Liquids, https://doi.org/10.1016/j.molliq.2019.02.025
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ACCEPTED MANUSCRIPT Insights into the saponification process of di(2-ethylhexyl) phosphoric acid extractant: thermodynamics and structural aspects
a
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Min Suna, Shijun Liua,*, Yunran Zhanga, Mei Liub, Xin Yia, Jiugang Hua,c,*
School of Chemistry and Chemical Engineering, Central South University, Changsha
College of Chemistry, Chemical Engineering and Environmental Engineering,
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b
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410083,PR China.
Hunan Provincial Key Laboratory of Chemical Power Resources, Central South
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c
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Liaoning Shihua University, Fushun 113001, China.
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University, Changsha 410083, China.
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Corresponding authors: [email protected] (J. Hu);[email protected] (S. Liu).
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ACCEPTED MANUSCRIPT Abstract: The saponification process of di(2-ethylhexyl)phosphoric acid extractant (D2EHPA) was investigated by the isothermal titration calorimetry (ITC), attenuated total reflection infrared (ATR-IR), and small-angle X-ray scattering (SAXS) spectroscopies.
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The effects of the temperature, types of saponifier, and diluent on the saponification
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process of D2EHPA were disclosed by ITC method. The saponification reaction is an
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exothermic process, but the contribution of temperature on the maximum saponification degree of D2EHPA is slight. The theoretical maximum saponification
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degree of D2EHPA is about 40%. Moreover, the maximum effective saponification
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degree of D2EHPA is the same at the condition of different saponifiers because the two systems release the same amount of heat. The effective saponification degree of
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D2EHPA in octane is larger than that in toluene at the same content of saponifier. The
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IR spectra indicate that the structure of organic phases varies significantly with the increase of saponification degree. The most water-in-oil microemulsion in the organic
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phase is formed at the 40% theoretical saponification degree. The water content and
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SAXS data of organic phase further verify this phenomenon. Solvent extraction results show that the saponified extractant can obviously improve extraction efficiency of cobalt. These results will be helpful for optimizing the saponification process of D2EHPA during metal extraction. Keywords: di(2-ethylhexyl)phosphoric acid; saponification; interaction; calorimetry
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ACCEPTED MANUSCRIPT 1. Introduction Liquid-liquid extraction is an energy-saving technique to separate and concentrate the targeted materials in the fields of chemical engineering, hydrometallurgy, and environment [1, 2]. It has been widely applied as an efficient
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method for recovery of metal ions from various aqueous media with the advantages of
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low cost and recyclability of components [3]. Since tributylphosphate was used to
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extract uranium in the 1940s, many commercial extractants have been successfully developed for metal separation, such as organophosphorous [4], carboxylic [5], and
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quaternary amine extractants [6].
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In the extraction process, organophosphorous extractants [7] such as 2-ethylhexyl phosphoric acid-2-ethylhexyl ester (EHEHPA) [8], di(2-ethylhexyl) acid
(D2EHPA)
[9],
bis(2,4,4-trimethylpentyl)
phosphinic
acid
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phosphoric
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(Cyanex272) [10] have been comprehensively used for the extraction of various metal ions from aqueous solutions because of their high extraction efficiency and selectivity.
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As a cheap and stable extractant, D2EHPA is a cation exchanger to extract metal ions
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in the form of monomer and dimer [11], which has extensively used in hydrometallurgical processes for the separation and purification of transition metals such as copper, cobalt, zinc, iron [12, 13]. Mellah et al. investigated solvent extraction of zinc and cadmium from phosphoric acid solution using D2EHPA (denoting as HA) diluted in kerosene. The results were concluded that the extracted zinc was coordinated with 1.5 molecules of HA monomer and the cadmium was coordinated with 1.25 molecules of HA monomer [14]. Yang et al. investigated solvent extraction
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ACCEPTED MANUSCRIPT of Fe3+ from phosphoric acid media by HA in kerosene and found that the structure of extracted species was demonstrated to be FeClA2·4HA [15]. Because acid extractants are usually presented as the form of hydrogen-bonded dimer H2A2 in the non-polar diluents, these extractants need to be saponified by
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acid/base neutralization reaction to increase their extractability and avoid the increase
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of aqueous phase acidity induced by releasing hydrogen ions during extraction [16].
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After breaking the hydrogen bonds in dimers and associating with alkali metal cations, the saponified extractants easily form water-in-oil microemulsions in an organic phase,
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which will potentially affect the separation efficiency and extraction selectivity [17].
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Wu et al. studied the structural changes and the composition of extractants in the saponification process with different saponifiers [18, 19]. They focused on the
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formation of microemulsion by saponifying acid extractants and found that the region
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of the single and transparent microemulsions by NH4OH was larger than those of NaOH and KOH. They also investigated the extraction of rare earth or divalent metal
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ions by the saponified extractants and emphasized the importance of the role of water
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in the extraction process. Although saponification process of acidic extractants has been considered as a routine strategy during metal extraction, the effective saponification degree of extractants is still unclear and it is dependent on the various parameters including types of diluent and saponifier, temperature, and so on. Some literature [20, 21] indicated that there could be a limited saponification degree of acid extractants because the extraction efficiency of metal ions cannot be further improved after reaching a certain saponification degree. Therefore, insights into the
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ACCEPTED MANUSCRIPT saponification process of D2EHPA are helpful for improving extraction efficiency and understanding the extraction mechanism. In this work, the saponification process of di(2-ethylhexyl)phosphoric acid (D2EHPA) were explored using isothermal titration calorimetry (ITC), attenuated
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total refraction Infrared (ATR-IR) spectroscopy, and small-angle X-ray scattering
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(SAXS). The effects of temperature, diluents, and saponifiers on the saponification
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degree of D2EHPA were investigated. The water content and SAXS spectra of the saponified organic phases were determined to evaluate the saponification behaviors
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and interaction between D2EHPA and saponifiers.
2. Experimental
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2.1 Reagents and Solution Preparation
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The extractant di(2-ethylhexyl)phosphoric acid (D2EHPA, purity of 95%) was purchased from Sinopharm. Toluene and octane of analytical grade were used as
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diluents. The extractants and diluents were used without further purification. The
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molecular structures of D2EHPA monomer and dimer were given in Fig. 1. The diluents (toluene or octane) are used in the saponification process to decrease the viscosity of extractant and provide a suitable solvation environment [22-24]. Desired amounts of extractants were dissolved in the diluents to prepare the organic phases, and the concentration was adjusted to 0.2 mol·L-1. 2 mol·L-1 NaOH or NH4OH aqueous solution was prepared as the saponifiers. The concentrations of NaOH and NH4OH aqueous solutions were calibrated with a standard concentration of 0.1
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ACCEPTED MANUSCRIPT mol·L-1 HCl.
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Fig. 1 Structures of D2EHPA monomer and its dimer.
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2.2 Calorimetric Measurements
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The isothermal titration calorimetry was used to study the saponification reaction of D2EHPA (0.2 mol·L-1) with NaOH or NH4OH solutions (2 mol·L-1) [25]. As shown
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in Fig. 2, D2EHPA solution was placed in the sample pool, and the corresponding
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diluent was placed in the reference pool. The saponifier solution was placed in a 5 mL syringe. The titration experiment consisted of 10 injections of 25 μL each with
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intervals of 20 min between injections. The equivalent NaOH or NH4OH solution was
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simultaneously injected into the sample pool and reference pool to deduct the dilution heat of titrant and titrate. The thermal power-time curve of the saponification process
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was determined under the different conditions. Raw measurement data consisting of
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downward or upward peaks indicate an overall exothermic or endothermic process, respectively. After baseline correction, the integrated area under the curve was calculated as the generated heat Q. The thermal effect was calculated as follows. 𝑄 = ∆𝑛∆𝑟 𝐻𝑚
(1)
where Q is the total heat released during saponification reaction (J), Δn is the amount of substance involved in the reaction (mol), ∆r Hm is the reaction heat of the saponification process (J·mol-1).
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Fig. 2 Schematic diagram of ITC instrument for saponification reaction. 2.3 ATR-IR Measurement
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For the measurement of ATR-IR spectra, the saponified D2EHPA with different
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saponification degrees were prepared previously. The ATR-IR spectra of saponified organic phases were recorded at room temperature with a Nicolet 6700 FT-IR
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spectrophotometer. The attenuated total reflection through cell equipped with a Ge
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crystal of 45º incident angle and 12 reflections was used. The IR spectra with a resolution of 4 cm-1 were collected by 320 scans in the range of 4000-800 cm-1. Each
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sample was measured three times in order to check the spectral repeatability, and the
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average spectra were used for further analysis. Data processing was carried out using the OMNIC 8.2 software package. 2.4 SAXS Measurements and Data Interpretation Small-angle X-ray scattering (SAXS) measurements are ideally suited for the investigation of the morphology (shape and size) of particles of the microemulsions or reverse micelles. SAXS data for the D2EHPA organic phase of different theoretical saponification degree were collected on the beamline BL16B1 at the Shanghai
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ACCEPTED MANUSCRIPT Synchrotron Radiation Facility with X-ray energy of 10 keV (SSRF, China). All solutions were measured using a single 2 mm diameter quartz capillary tube. The sample-to-detector distance was set to 1770 mm. Scattering was detected in the q ranges of 0.1–1.3 nm−1 in which q=(4πsinθ)/λ. FIT2D software was used to convert
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the one-dimensional (1D) data from the 2D scattering pattern. All the data were
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background-subtracted and normalized. In the absence of water-in-oil microemulsions,
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the organic phase produces weak scattering intensity that is barely discernible above the diluent background.
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2.5 Extraction Experiments
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The extraction experiments were performed by contacting 5 mL 0.2 mol·L-1 D2EHPA extractant with 5 mL 0.02 mol·L-1 CoSO4 aqueous solution for 30 min in a
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vibrating mixer at room temperature. Before the extraction experiment, the saponified
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organic phase was washed 4-5 times with the saturated sodium chloride and then centrifuged to remove the entrained aqueous solution in the organic phase. After
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extraction, the aqueous phase was separated by centrifugation at 5000 rpm for 5 min,
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and the concentration of Co(II) in the aqueous phase was measured by inductively coupled plasma-atomic emission spectrometry (ICP-AES, Perkin Elmer Optima 5300 DV) before and after extraction. The concentration of Co(II) in the loaded organic phase was calculated by mass balance. The extraction efficiency (E%) is the ratio of the equilibrium concentration of Co(II) in the loaded organic phase to the initial concentration of Co(II) in the aqueous phase.
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ACCEPTED MANUSCRIPT 3. Results and Discussion In order to elucidate the saponification behavior, the interaction between D2EHPA and saponifier was investigated using isothermal titration calorimetry and ATR-IR spectra. The saponification degree (sd) of D2EHPA in the experiment was
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adjusted by mixing with certain amounts of saponifier (NaOH or NH4OH). The
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calculation formula of the saponification degree was defined according to industrial
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calculation methods (equation 2). Herein, the saponification degree also refers to the theoretical saponification degree in order to distinguish the effective saponification
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degree of D2EHPA in this work, which is justified by the amount of heat released in
sd =
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the saponification process. 𝑛(saponifier) 𝑛(𝐷2𝐸𝐻𝑃𝐴)
× 100%
(2)
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D2EHPA, respectively.
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where n(saponifier) and n(D2EHPA) are the initial mole numbers of the saponifier and
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3.1 Calorimetric analysis
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The effects of temperature, saponifier, and diluent on the saponification of D2EHPA were studied separately by the calorimeter. Fig. 3a presents the thermal variation process of the saponification reaction at the different temperatures, which exhibited an exothermic process. The total heat released of the saponification reaction was calculated by the integral. As shown in Fig. 3b, the released heat during the saponification process decreases as rising temperature, indicating that the lower temperature is favorable for the saponification of D2EHPA. However, the change of
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ACCEPTED MANUSCRIPT temperature does not affect the theoretical maximum saponification degree of D2EHPA. There is a slight difference between the reaction heats at different temperatures when the theoretical saponification degree increases from 0% to 40%. Because temperature affects the extent of the saponification reaction, which in turn
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leads to the difference between the effective saponification degree and the theoretical
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maximum saponification degree.
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Fig. 3 (a) Calorimetric experimental data of saponification process of 0.2 mol·L-1 D2EHPA at different temperatures and (b) the corresponding total reaction heat at
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different saponification degrees. The diluents is octane; saponifier is NaOH.
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During the industrial application, NaOH and NH4OH are the common saponifiers, but their basicity difference will affect the theoretical maximum saponification degree of D2EHPA. As illustrated in Fig. 4, the theoretical maximum saponification degree of D2EHPA is ca. 40% and ca. 55% for NaOH and NH4OH, respectively. However, the saponification process releases almost the same heat for two systems when the added saponifiers exceed the theoretical maximum saponification degree. Because the essence of the saponification process is an
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ACCEPTED MANUSCRIPT acid-base neutralization reaction, when the heat released during the saponification process is the same, the amount of OH- participating in the saponification reaction should be the same. Since NH4OH is a weak base, more NH4OH molecules are needed to provide the same amount of OH- as NaOH. For instance, at the 30%
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saponification degree, the reaction heat for NaOH system is larger about 15.4 J than
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that of NH4OH, which means the effective saponification degree for NaOH system is
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greater than that of NH4OH at the same concentration of saponifier. On one hand, the sodium ions can coordinate with D2EHPA via coordination bond, but the ammonium
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ions are combined with D2EHPA by electrostatic interaction in the saponification
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process [12, 26]. On the other hand, when NH4OH participates in the saponification reaction, it needs to absorb heat to break the hydrogen bond between ammonia
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molecules. Moreover, NH4OH could be easier to enter the D2EHPA reverse micelles
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together with water to participate in the reaction than NaOH because of the hydrogen
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bonds.
Fig. 4 (a) Calorimetric experimental data of saponification process of D2EHPA using NaOH and NH4OH as saponifiers and (b) the corresponding total reaction heat at
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ACCEPTED MANUSCRIPT different saponification degrees. The diluent is octane; the temperature is 298K. The effect of diluents on the saponification process of D2EHPA was evaluated. As shown in Fig. 5, although the reaction heat was different, the theoretical maximum saponification degree of D2EHPA for both octane and toluene reaches 40%. For the
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specific reaction, the reaction heat ∆𝑟 𝐻𝑚 can be only influenced by temperature.
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According to the equation (1), when different diluents are used, the difference of
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reaction heat is related to the amount of substance involved in the saponification reaction. The value of relative polarity and the dielectric constant of octane and
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toluene are listed in Table 1. Because the polarity of a diluent is proportional to its
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dielectric constant [27], the interaction between D2EHPA and diluents becomes stronger when the diluent is changed from octane and toluene. Accordingly, the
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reaction of NaOH with D2EHPA in toluene is more difficult than D2EHPA in octane,
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which further affects the effective saponification degree of D2EHPA. Although the theoretical maximum saponification degree of D2EHPA for both octane and toluene
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are the same, the effective saponification degree of D2EHPA in octane is larger than
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that of D2EHPA in toluene.
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ACCEPTED MANUSCRIPT Fig. 5 (a) Calorimetric experimental data of saponification process of D2EHPA using toluene and octane as diluents and (b) the corresponding total reaction heat at different saponification degrees. The saponifier is NaOH; the temperature is 298K. Table 1 Dielectric constants of the diluents used in experiment[28]. Dielectric constant
Octane
0.02
1.95
Toluene
0.24
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Relative polarity a
2.36
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a
Diluent
The polarity value of water is consider to be 1.000.
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According to the calorimetric data, although there is a slight change in total
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reaction heat, the theoretical maximum saponification degree of D2EHPA can reach about 40% in each case. When continuously increasing the saponifier contents,
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D2EHPA molecules will be no more saponified whatever changing temperature or
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type of diluent. This phenomenon is consistent with the extraction experiments of metal ions in literature. Yin et al. studied the effect of different saponification degree
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on the extraction of Nd(III) using NaOH as the saponifier and found that the
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extraction efficiency was the best when the saponification degree was 40% at pH 4.0 [21]. Therefore, the results in this work will be helpful for optimizing the saponification process of D2EHPA during metal extraction.
3.2 Structural change of saponified organic phases From the calorimetric investigation, octane is the optimal diluent for improving the effective saponification degree of D2EHPA. In order to understand this
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ACCEPTED MANUSCRIPT phenomenon, the interaction between D2EHPA and NaOH is further investigated with ATR-IR spectroscopy [16]. The IR spectra of D2EHPA in octane are shown in Figs. 6 and 7 before and after saponification. In Fig. 6a, the characteristic band at 1035 cm-1 broad peak is identified as the superposition peak of P-O-C and P-O-H groups of
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D2EHPA. With the increase of the theoretical saponification degree, the peak position
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shifts to a low wavenumber of 1080 cm-1 and accompanies the appearance of the
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shoulder peak at 1030cm-1. The probable reason is that the P-O-H bond interacts with metal ion Na+ to form a coordination bond after saponification. After the theoretical
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saponification degree of D2EHPA reaches 40%, there will be a new peak at 1065 cm-1.
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This phenomenon could be induced by the interaction of P-O-C bond and water, thus water-in-oil microemulsions or reverse micelles may be formed in the saponified
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organic phase. In Fig. 6b, the broad characteristic band at 1225 cm-1 is identified as
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the P=O stretching vibration of D2EHPA. As the theoretical saponification degree increases, the stretching vibration peak of P=O has a decreasing trend due to the
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formation of a hydrogen bond between P=O and H2O. When the theoretical
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saponification degree reaches 40%, the stretching vibration peak of P=O obviously decreases and the peak position moves to 1205 cm-1. In Fig. 7, the characteristic peaks at 1648 cm-1 and 3412 cm-1 are the vibration absorption peak and the stretching vibration peak of the water in the organic phase after saponification, respectively. No characteristic absorption band of water could be observed in the IR spectra when the theoretical saponification degree was lower than 30%. After the theoretical saponification degree was over 30%, the vibration
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ACCEPTED MANUSCRIPT absorption peak and the stretching vibration peak of water appears [24]. The reason is that the hydrogen bond of the D2EHPA dimers was broken and water-in-oil
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microemulsions or reverse micelles were formed.
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Fig. 6 ATR FT-IR spectra of D2EHPA in octane at different saponification degrees. (a)
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the P-O peak and (b) P=O peak of D2EHPA. The saponifier is NaOH.
Fig. 7 ATR FT-IR spectra of D2EHPA in octane at different saponification degrees. (a) 15
ACCEPTED MANUSCRIPT vibration absorption peak and (b) the stretching vibration peak of water in the organic phase after saponification. The saponifier is NaOH. Fig. 8 is the IR spectra of D2EHPA saponified with different saponifiers in the organic phase. After saponification, the absorption frequency of the characteristic
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peaks of D2EHPA is reduced for both saponifiers. The stretching vibration peak of
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P=O of the unsaponified D2EHPA is 1236 cm-1. After saponified with NaOH, the
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absorption frequency of P=O bond moves to 1210 cm-1. However, when D2EHPA is saponified with NH4OH, the absorption frequency of the P=O bond is further reduced
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to 1200 cm-1. The reason is that the extractant anion (RO)2PO2- is coordinated with
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Na+, resulting in weakening of the P=O bond. But NH4OH is a polar molecule, and NH4+ has no empty 3d orbital and cannot form a coordination bond with (RO)2PO2-.
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Therefore, in addition to the electrostatic interaction between NH4+ with the anion
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(RO)2PO2-, NH4+ can also form hydrogen bonds with (RO)2PO2-, which causes the absorption frequency of the P=O bond to move toward lower wavenumbers. In
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addition, the water peak at 1645 cm-1 for NH4OH-saponified D2EHPA is wider than
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that of NaOH-saponified D2EHPA.
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ACCEPTED MANUSCRIPT Fig. 8 FT-IR spectra of D2EHPA saponified with different saponifiers. The saponification degree is 50%. HA is an abbreviation of D2EHPA. The water content of the saponified organic phases was further determined by Karl Fisher titrator method. Fig. 9 shows the intensity of the associated water peak in
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infrared spectra and the water content of organic phases under the different
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saponification degrees. It can be found that the stretching vibration peak of the
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associated water has a similar trend with the water content under different saponification degrees. When the theoretical saponification degrees are 0%, 10%, and
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20%, there is a weak stretching vibration peak of water because the water content in
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the organic phase is lower than 2 wt%. After the theoretical saponification degree reaches 30%, the stretching vibration peak of the associated water appears. And then
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the water content in the organic phase increases as increasing the amount of saponifier.
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When the theoretical saponification degree reaches 40%, the water content in the organic phase sharply increases to 10.8 wt%, accompanying the appearance of the
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maximum stretching vibration peak of the associated water. This phenomenon can be
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attributed to the formation of water-in-oil microemulsions or reverse micelles in the saponified organic phase when the saponification degree reaches a certain level, which causes a significant increase of the water content in the organic phase. However, after the 40% saponification degree, the water content in the organic phase has a downward trend. The possible reason is that when the water content in the microemulsions reaches a certain value, the water-in-oil microemulsions could be demulsified, resulting in a decrease in the water content.
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Fig. 9 The intensity of the associated water peak in infrared spectra and water content of organic phases under the different saponification degrees with NaOH as a
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saponifier.
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The SAXS measurements of organic phases were carried out in order to further understand the structure and interaction between D2EHPA and saponifiers under the
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different saponification degrees. Fig.10 (a) shows the variation trend between the
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scattering I(Q) and momentum transfer Q of the organic phase with D2EHPA in the different theoretical saponification degree, suggesting that there were no notable
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changes in the aggregate state because of the similar envelope. However, the different
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I(0) values indicate the size of extractant aggregates has an evident change. As shown in Fig.10 (b), the I(0) value sharply increases below the saponification degree of 40%, and then it decreases as increasing the theoretical saponification degree. This suggests that the water-in-oil microemulsions or reverse micelles begin to be formed when mixing with saponifier. The saponified extractant systems may form the largest micelles in the 40% saponification degree, which is the theoretical maximum saponification degree. However, when the amount of NaOH exceeds the theoretical
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ACCEPTED MANUSCRIPT saponification degree, the excess NaOH could act as a demulsifier. Thus, the formed microemulsions are easily flocculated and aggregated into large droplets. The structure of microemulsions will be destroyed, thereby resulting in a decrease of the water content in the organic phase. The sharp decrease of I(0) value also indicates that
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the structure of microemulsions has been destroyed. This phenomenon is consistent
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with the result of the water content.
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Fig. 10 (a) Log-log plot of the SAXS data for the D2EHPA in the different theoretical saponification degrees; (b) I(0) calculated from the entire scattering curves for the
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D2EHPA organic phase under the different theoretical saponification degrees.
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3.3 Extraction efficiency of saponified D2EHPA Fig. 11 shows the extraction behaviors of Co(II) with the saponified D2EHPA at different saponification degrees. It can be found that saponification of D2EHPA has a favorable effect on the extraction of cobalt ions, and the extraction efficiency raises with increasing saponification degree for both NaOH and NH4OH saponified D2EHPA. When the D2EHPA is not saponified, the extraction efficiency of cobalt is only 3.94%. However, when the theoretical saponification degree reaches 40% with 19
ACCEPTED MANUSCRIPT NaOH as a saponifier, the extraction efficiency increases to 98.46%. And then the extraction efficiency only has a slight increase. Notably, as seen from the insert of Fig.11, the saponification degree of D2EHPA has an obvious effect on the distribution coefficient (D) of cobalt, which is the ratio of cobalt in the organic phase to the
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aqueous phase. For NaOH system, the distribution coefficient of cobalt increases as
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saponification degree increases to 40%, and then it increases slowly. A similar trend
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can be found for NH4OH-saponified D2EHPA, but the improvement of which on extraction behavior of cobalt is significantly lower than the NaOH system. Below the
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effective saponification degree of D2EHPA, the D2EHPA dimers can be broken into
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the easily-dissociated D2EHPA monomer, which benefits for the extraction of Co(II). When the effective saponification degree of D2EHPA reaches the maximum (such as
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40% for NaOH system), previous results show that the water-in-oil microemulsions or
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reverse micelles in organic phase are formed, thereby the extraction of Co(II) can be enhanced. The acidic D2EHPA in the organic phase acts as a phase transfer agent
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allowing the metal ions Co2+ to pass through the oil-water interface into the
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microemulsions as CoA2. Therefore, the saponification of D2EHPA is necessary and beneficial for the metal extraction process.
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ACCEPTED MANUSCRIPT Fig. 11 The extraction efficiency of cobalt at different saponification degrees with NaOH or NH4OH as saponifiers. The insert is the
relationship between the
distribution coefficient of cobalt and the saponification degree of D2EHPA. Aqueous
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phase is 0.02 mol/L CoSO4 at pH 6. The diluent is octane.
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4. Conclusions
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The interaction between the acid extractant di(2-ethylhexyl)phosphoric acid (D2EHPA) and saponifiers (NaOH or NH4OH) in the saponification process were
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investigated from thermodynamics and structural aspects. The saponification reaction
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of D2EHPA is an exothermic process. Changing the temperature does not affect the theoretical maximum saponification degree of D2EHPA in the range of 15 to 45 °C,
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which is ca. 40%. Although the theoretical maximum saponification degree of
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D2EHPA is ca. 40% for NaOH and ca. 55% for NH4OH, the types of saponifier have almost no influence on the effective saponification degree because the two
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saponification systems release the same amount of heat. Moreover, the effective
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saponification degree of D2EHPA in octane is larger than that in toluene as more heat is released in octane system at the same content of saponifier. The IR and SAXS data shows that the structure of organic phases varies significantly with the theoretical saponification degree. Because of the formed water-in-oil microemulsions, the water content in the organic phase increases obviously and reaches the maximum value at the theoretical saponification degree of 40%. The microemulsion structure of saponified organic phases will be broken when the theoretical saponification degree is
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ACCEPTED MANUSCRIPT beyond 40%. The relationship between extraction efficiency and saponification degree can be concluded that the saponification of the extractant is beneficial for the metal extraction process.
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Acknowledgements
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This work was financially supported by the National Basic Research Program of
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China (No.2014CB643401), the Hunan Provincial Science and Technology Plan Project of China (No. 2016TP1007), and the Changsha Science and Technology
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Project (No. kq1801069) .
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ACCEPTED MANUSCRIPT Highlights The saponification process of D2EHPA was studied by ITC, ATR-IR and SAXS methods. The effects of the temperature, types of saponifier, and diluent were evaluated.
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The theoretical maximum saponification degree of D2EHPA is about 40%.
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The most water-in-oil reverse micelles are formed at the 40% saponification degree.
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