Extraction of Eu(III), Gd(III), and Tb(III) in aqueous two-phase systems based on polyethylene glycol 1500-NaNO3-H2O with the addition of extractants (D2EHPA, TBP, TOMAN)

Luminophores that used, for example, in fluorescent lamps, contain a large number of rare earth elements. Therefore, the processing of waste equipment containing luminophores is a rational approach to the obtaining of rare-earth metals, firstly, from the economics point of view, since they have a high cost, and secondly, from the ecological point of view, since environmental pollution will not occur. The cheapest way to extract rare earth elements from waste products is to dissolve them in strong acids and the following reprocessing by liquid extraction methods. In this case, neutral or ion exchange extractants (tributyl phosphate, di(2-ethylhexyl)phosphoric acid and quaternary ammonium salts) are used, which show high extraction ability and, in some cases, selectivity. Their applying is associated with the use of non-polar organic solvents, which contradicts the principles of «green» chemistry. A good and promising alternative to organic solvents can be aqueous two-phase systems, which have already proved themselves as low-toxic, but highly effective systems for the separation of a number of metals. Thus, in this work, we performed an experimental study of the interphase distribution of Eu(III), Gd(III), and Tb(III) in two-phase systems based on water-soluble polymers with or without the introduction of traditional organic extractants as an additive. The possibility of using such ATPS as a «green» solvent for traditional extragents for Eu(III), Gd(III), and Tb(III) extraction has been shown.


Introduction
Rare earth elements (REEs) are widely used, they can be found in smartphones and PCs, in energysaving lamps and displays, in permanent magnets and electric motors, in radars and lasers [1]. Since the demand for REE is growing, their production is also growing, as a result, prices for this already expensive raw material are growing. Consequently, the demand for alternative sources of these elements is increasing. This is e-waste, the increase in which is directly proportional to the growth in demand for electronic equipment. However, at the moment, only about 1% of the used REE is returned from secondary raw materials, which is explained by the difficulties associated with waste collection [2].
The traditional method based on the leaching of phosphor scrap using mineral acids (sulfuric, nitric, hydrochloric and their mixtures) with subsequent extraction and separation of REEs [4]. Researchers Tunsu and others [5] leached Y and Eu with a mixture of hydrochloric and nitric acids (within 24 hours), De Carolis and others [6] leached them with HCl (6 mol·L -1 ) for a day, Takahashi and others [7] succeeded in doing this using 1.5 mol·L -1 H2SO4 at 70°C per one hour.
Organophosphate acids are effective extractants of REE, among which D2EHPA is of the greatest practical application [8]. The extraction proceeds according to the cation-exchange mechanism and is characterized by high distribution coefficients. The work [9] studied the extraction of Y, Dy, and Nd.
Salts of quaternary ammonium bases (QAB) are extracted with REE by an anion exchange mechanism [10]. They have the highest selectivity among primary, secondary, tertiary amines, and make it possible to extract REE not only from nitrate, but also from thiocyanate solutions [11]. QAB salts can effectively extract REE from solutions of mineral acids (in the form of synergistic mixtures with other extractants) and from weakly acidic or neutral solutions (QAB salts with inorganic anions) [12].
The listed methods imply the use of strong acids, which entails strong economic losses due to the need for neutralization of reagents and disposal of waste water [13]. In addition, organic solvents are often used, which are not only toxic and environmentally unsafe, but also flammable and explosive [14]. Also, often, traditional extraction systems have low selectivity, which reduces the efficiency of the entire process [15].
These problems can be avoided by following the principles of "green" chemistry and making a partial or full transition to "green" solvents and reagents [16]. In addition, many modern approaches allow using and combining existing ineffective methods, increasing their effectiveness. A striking example is the hybrid method of liquid-liquid chromatography -this is one of the most promising methods for separating organic and inorganic substances, combining the ideology of liquid extraction and high efficiency of chromatography [17]. At present, using this method, the processes of separation of rareearth metals and a number of other metals [18] have been implemented, both with the use of column equipment and industrial extractors [19]. Another promising approach to replacing organic solvents with "green" reagents is the use of deep eutectic solvents (DES) [20]. Being a eutectic mixture of solids (Lewis and Bronsted acids and bases) linked by hydrogen bonds, DES are most often liquid at room temperature and show high efficiency for the extraction of rare earth metals and their compounds from various media [21]. Another approach is the use of aqueous two-phase systems, including those based on water-soluble polymers [22]. These heterogeneous systems are already used for the separation and extraction of organic [23,24] and inorganic compounds, including metals [25,26]. Replacing the organic solvent with non-toxic polymers will have a positive effect in the environmental aspect, as well as change the selectivity of the processes. In addition, in studies [27,28] it was found that the use of salts of quaternary ammonium bases has a positive effect on the extraction efficiency, and in some cases contributes to the selective extraction of certain metals [29]. On the contrary, the use of aqueous twophase systems is not a complete solution to the problem of extraction metals, since the system can have low selectivity and simultaneously extract a number of metals [30]. The initial solution medium is also important, for example, sulfate [31] or chloride. In addition, attention is paid to liquid-liquid equilibria of aqueous two-phase systems, which can subsequently be used for extraction processes [32].
In this regard, the aim of this study was the replacement of organic solvents with aqueous two-phase systems, in the processes of extraction processing of acidic solutions of leaching of phosphors. Promising luminophores containing Eu, Gd, and Tb were chosen as model solutions.
The metal ions extraction was carried out using nitrate two-phase aqueous systems, based on polyethylene glycol 1500 and with an initial metal concentration of 0.01mol·L -1 . Phase composition were chosen based on phase diagrams, PEG-1500 -NaNO3 [34]. The acid concentration in the salt phase was equal 0.01mol·L -1 (for the hydrolyze avoiding).
The metal ions Eu(III), Gd(III) and Tb(III) extraction was carried out at a temperature of 25°C in graduated plastic tubes in an Enviro-Genie thermostatically controlled shaker (Scientific Industries, Inc.) at a rotation speed of 30 rpm for it was chosen as optimal time for the effective extraction of both metals.
The complexometric titration using EDTA and xylenol orange at pH=6 was used to determine the metal ions concentration in the initial solutions and in the aqueous phases after extraction.
The presented experimental data are the result of a series of experiments and processed by methods of mathematical statistics. was motivated by the fact that the introduction of traditional extractants into the system intensifies the extraction process in the internal combustion engine. As a result, quantitative characteristics of extraction were obtained, such as distribution coefficients (Fig.1), degrees of extraction (Fig.2) and the number of theoretical stages (Fig.3) of extraction for metal ions Eu(III), Gd(III) and Tb(III). The number of theoretical stages of extraction was calculated by the formula (1):

Results and discussion
where are n the number of theoretical extraction steps; q is the ratio Xo / Xn; Xo is the initial concentration of the metal introduced into the system, mol·L -1 ; Xn is the final specified metal concentration, mol·L -1 ; λ is the ratio Vn / Vw; Vn -volume of the lower vase, ml; Vb -volume of the upper phase, ml; D -distribution coefficient. When using this formula, the following assumptions were made: • distribution coefficient D = const; • the volumes of the phases Vn and Vw are constant, therefore, their ratio (λ = const); • the original extractant does not contain the substance to be distributed. With the initial metal concentration Xo = 0.01 mol·L-1, the final metal concentration Xn = 1·10-6 mol·L-1, therefore, q = 10000, thus, the extraction of metal ions will occur with a degree of 99.99%.
As can be seen in Figures 1-3, with the addition of TOMAN ( Fig. 1-3, b), in comparison with the first experiment ( Fig. 1-3, a), the distribution coefficients increased: for Eu(III) by 1.78, Gd(III) by 1.15 and Tb(III) by 1.65. The number of theoretical stages of extraction has noticeably decreased (by about IOP Publishing doi:10.1088/1757-899X/1212/1/012012 4 2 times). Consequently, such an extraction system with the addition of TOMAN can be used in the extraction of Eu(III), Gd(III), and Tb(III), but is impractical due to low distribution coefficients.
With the addition of TBP ( Fig. 1-3, c) to the upper phase, in comparison with the first experiment ( Fig. 1-3, a), the distribution coefficients increased: for Eu(III) by 1.12, Gd(III) by 2.74 and Tb(III) at 1.96. The number of theoretical stages of extraction, as well as in the system with the addition of TOMAN, decreased by about 2 times. This extraction system can be used in the extraction of Eu(III), Gd(III) and Tb(III), however, it is impractical due to low distribution coefficients.
It is known from the literature [35] that the combined use of TBP and D2EHPA gives rise to a synergistic effect, and the extraction of Nd(III) from acidic media, the distribution coefficients increased several times. It was decided to conduct a series of experiments to find the most effective ratio of TBP and D2EHPA for extraction ( Fig. 1-3, d, f, e). The synergistic effect was calculated by the formula (2): (2) where are D the distribution coefficient for extraction with a mixture of extractants; D1, D2distribution coefficients for extraction with each extractant separately.
From the data obtained ( Figs. 1-3, d, e, f), a synergistic effect is observed only in the case of an extraction system with the addition of TBP and D2EHPA in a ratio of 1: 4 and only in the case of Eu(III): Sk(Eu) = 1.99, that is, this system can improve the extraction by almost two times. The distribution coefficients are high in all cases, which makes it possible to solve the problem of element extraction. Accordingly, such extraction systems with the addition of TBP and D2EHPA are excellent for use in the extraction of Eu(III), Gd(III) and Tb(III).
The system PEG-1500 (16.3 wt.%) -NaNO3 (36 wt.%) -H2O + TBP:D2EHPA (1:4) is the most effective for separating Eu(III) from its mixture with Gd (III) and Tb (III). We have calculated the separation coefficients based on experimental data by the formula (3): where, βEu/Me -is the separation coefficient of Eu(III) in relation to another metal; DEu -is the distribution coefficient of Eu(III); DMe -is the distribution coefficient of Eu(III).
Thus, βEu/Gd=2.97 and βEu/Tb=3. 36 are high values for the separation coefficients for metal [36], which suggests that this system is excellent for solving the problem of selective extraction of Eu(III) from this mixture of metals.

Conclusions
The extraction system based on PEG-1500 (16.3 wt.%) -NaNO3 (36 wt.%) -H2O with the addition of TBP and D2EHPA in ratios of 1:4, 2:3, and 4:1 is more efficient for the extraction of Eu(III), Gd(III) and Tb(III) compared to systems without the addition of this mixture of extractants or with the addition of TOMAN or TBP. It was found that in the system with the addition of TBP and D2EHPA in a ratio of 1:4, Eu(III) is extracted with high efficiency.

Acknowledgments
This work was supported by IGIC RAS state assignment.