from concentrated phosphoric acid by synergistic

Hydrometallurgy 129–130 (2012) 118–125

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Modeling of the extraction of uranium (VI) from concentrated phosphoric acid by synergistic mixtures of bis-(2-ethylhexyl)-phosphoric acid and tri-n-octylphosphine oxide Denis Beltrami a, b, c, Gérard Cote a, b, Hamid Mokhtari c, Bruno Courtaud c, Alexandre Chagnes a, b,⁎ a b c

Chimie ParisTech, Laboratoire d'Electrochimie, Chimie aux Interfaces et Modélisation pour l'Energie (LECIME), 11 Rue Pierre et Marie Curie, 75005 Paris, France CNRS, UMR 7575, 75005 Paris, France AREVA Mines, Service d'Etudes de Procédés et Analyses (SEPA), B.P. 71 87250 Bessines-sur-Gartempe, France

a r t i c l e

i n f o

Article history: Received 17 March 2012 Received in revised form 5 September 2012 Accepted 5 September 2012 Available online 18 September 2012 Keywords: Uranium Phosphoric acid D2EHPA TOPO Solvent extraction

a b s t r a c t A thermodynamic model has been developed to describe the extraction of uranium (VI) from 5.3 mol L−1 phosphoric acid (a typical concentration encountered in wet phosphoric acid) by mixtures of bis-(2-ethylhexyl)phosphoric acid (D2EHPA, also denoted hereafter HL) and tri-n-octylphosphine oxide (TOPO). A good agreement between experimental and calculated distribution coefficients of uranium (VI) was obtained by taking into account the following species in the organic phase: (HL)2TOPO, (HL)5TOPO, (HL)2(TOPO)2, UO2(HL2)2, UO2(HL2)2TOPO, and UO2L2TOPO where L is the deprotonated form of D2EHPA (HL). The speciation of uranyl, TOPO and D2EHPA in the organic phase was derived from this model as a function of the concentrations of the latter species. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Wet phosphoric acid (WPA) is a significant resource of uranium but its exploitation is difficult due to the high complexing power of phosphoric acid whose concentration varies between 3 and 6 mol L−1 depending on the process. In the 1980s, the synergistic mixture of bis-(2-ethylhexyl)-phosphoric acid (D2EHPA) and tri-n-octylphosphine oxide (TOPO) was implemented in solvent extraction processes to recover uranium (VI) from WPA (Hurst and Crouse, 1973; Hurst et al., 1972). Despite of numerous works on the physicochemistry involved in the extraction of uranium (VI) from concentrated phosphoric acid by D2EHPA/ TOPO, the speciation of uranium (VI) in feed phosphoric acid solutions and the nature of the extracted species are still subject to controversies. Uranium (VI) in phosphoric acid has been reported to exist as charged and uncharged phosphate complexes such as UO2H3PO42+, UO2H2PO4+, UO2(H2PO4)2, UO2(H2PO4)H3PO4+, UO2(H2PO4)2H3PO4, UO2(H2PO4)(H3PO4)2+, UO2(H2PO4)3− depending on pH and phosphoric acid concentration (Baes, 1956a; Baes et al., 1953; Elyahyaoui et al., 1987; Habashi, 1960; Issa et al., 1972; Marcus, 1958; Marković and Pavković, 1983; Mathur, 1991; Schereyer and Baes, 1954; Thamer, 1957).

⁎ Corresponding author at: Chimie ParisTech, Laboratoire d'Electrochimie, Chimie aux Interfaces et Modélisation pour l'Energie (LECIME), 11 Rue Pierre et Marie Curie, 75005 Paris, France. Tel./fax: +33 667146759. E-mail address: [email protected] (A. Chagnes). 0304-386X/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.hydromet.2012.09.005

In 0.1–4 mol L − 1 phosphoric acid, uranium (VI) was initially expected to exist as UO2H2PO4+, UO2(H2PO4)2 and UO2(H2PO4) H3PO4+ (Baes, 1956a, 1956b; Baes et al., 1953; Marcus, 1958). However, some authors reported the presence of UO2(HPO4) and even that of anionic species such as UO2(HPO4)22 − (Dongarra and Nagmuir, 1980; Moskvin et al., 1967). Nevertheless, the presence of UO2(HPO4)22 − was questioned by Tripathi (1982). More recently, Elyahyaoui (Elyahyaoui et al., 1987, 1988) reported the presence of UO2(H2PO4)2H3PO4, UO2(H2PO4)(H3PO4)2+, UO2(H2PO4)3− and UO2OH(H2PO4)32 − in 4 mol L − 1 phosphoric acid and concluded that the main species in industrial wet phosphoric acid might be UO2(H2PO4)(H3PO4)2+ and UO2(H2PO4)2H3PO4. Various papers are focused on the liquid–liquid equilibria involved in the recovery of uranium (VI) from concentrated phosphoric acid by mixtures of D2EHPA (HL) and TOPO, but there is not a consensus for a unique set of equations. This is partly due to the lack of reliable data on the speciation of uranium (VI) in phosphoric acid, but also because the liquid–liquid equilibria are complex and possibly dependent on the range of phosphoric acid concentration. For instance, Bunus et al. (1978) suggested that the recovery of uranium (VI) from 4.3 mol L −1 phosphoric acid by D2EHPA/TOPO is as the result of the formation of UO2(HL)2L2TOPO according to the following equation:

2þ þ UO2 þ 2 HL 2 þ TOPO ¼ UO2 ðHLÞ2 L2 TOPO þ 2H

ð1Þ

D. Beltrami et al. / Hydrometallurgy 129–130 (2012) 118–125

HL and L denote the monomeric protonated and deprotonated form of D2EHPA, respectively. Overbar refers to species in the organic phase and the absence of overbar denotes aqueous species. Girgin (Girgin et al., 2002) concluded that uranium (VI) is extracted as UO2L2(HL)4(H3PO4)2(TOPO)n when the extraction takes place between 1.5 mol L − 1 and 2.4 mol L − 1 H3PO4 (Eq. (2)), but as UO2L2(H3PO4)2(TOPO)n above 2.4 mol L − 1 H3PO4 (Eq. (3)): UO2 ðH2 PO4 Þ2 ðH3 PO4 Þ þ 3ðHLÞ2 þ nTOPO ¼ UO2 L2 ðHLÞ4 ðH3 PO4 Þ2 ðTOPOÞn þ H3 PO4

ð2Þ

UO2 ðH2 PO4 Þ2 þ ðHLÞ2 þ nTOPO ¼ UO2 L2 ðH3 PO4 Þ2 ðTOPOÞn

ð3Þ

119

2.3. Computing Computer simulations were carried out on a PC with the free mathematical software Scilab © (Scilab, 1989–2005). The set of mass balance equations for the calculation of the speciation were solved by using the Newton–Raphson method.

3. Results and discussions 3.1. Experimental data and modeling of the distribution of U(VI)

where n was not identified. In the present work, the distribution of uranium (VI) at equilibrium was investigated between 5.3 mol L −1 phosphoric acid and D2EHPA/TOPO diluted in Isane IP 185, an aliphatic diluent, as a function of D2EHPA concentration at constant concentration of TOPO (0.125 mol L −1) and as a function of TOPO concentration at constant D2EHPA concentration (0.2, 0.5, 0.8 and 1.4 mol L −1). The concentration of phosphoric acid was chosen equal to 5.3 mol L −1 corresponding to a typical value encountered in WPA. A thermodynamic model has been derived from these experimental data and the speciation of the extracted uranyl, D2EHPA and TOPO in the organic phase has been deduced from this model.

Fig. 1 shows the logarithm of the distribution coefficient of uranium (VI) as a function of the logarithm of initial D2EHPA concentration in the presence of 0.125 mol L −1 TOPO (Fig. 1a) and as a function of the logarithm of initial TOPO concentration in the presence of 0.5 mol L −1 D2EHPA (Fig. 1b). The logarithm of the distribution coefficient of uranium (VI) vs. the logarithm of initial D2EHPA concentration, at constant TOPO concentration, increases up to about 1 mol L−1 D2EHPA and then reaches a plateau region (Fig. 1a). The curve Log (DU(VI)) vs. Log ([TOPO]) plotted at constant concentration of D2EHPA exhibits a maximum at about [TOPO] = 0.2 mol L −1 (Fig. 1b). In order to explain the shape of these curves a thermodynamic model has been developed.

2. Experimental 2.1. Reagents Bis-(2-ethylhexyl)-phosphoric acid (Aldrich, purity >97%), trin-octylphosphine oxide (Fluka, purity > 90%) and Isane IP 185 (Total Fluides, a 100% aliphatic diluent with flash point and boiling point equal to 66 °C and 187 °C, respectively) were used as received. Phosphoric acid at 5.3 mol L −1 was prepared by diluting concentrated phosphoric acid (VWR, AnalanR Normapur, purity > 85%) with water (resistivity >18 M Ω cm) purified with a milli-Q Gradient system from Millipore Corporation. The aqueous feed solutions initially containing 340 mg L −1 uranium (VI) [1.4 10 −3 mol L −1] were prepared by diluting 0.1 mol L −1 uranyl nitrate (Fluka) in 5.3 mol L −1 phosphoric acid. The influence of the nitrates on the extraction equilibria was considered as negligible because the nitrate concentration in the feed solution is very low compared to that of phosphoric acid and the complexing power of nitrates towards uranium (VI) is weak (Grenthe et al., 1992). 2.2. Solvent extraction procedure Liquid–liquid extraction of uranium (VI) was carried out by mixing the feed solution with mixtures of bis-(2-ethyl-hexyl)-phosphoric acid (D2EHPA) and tri-n-octylphosphine oxide (TOPO) diluted in Isane IP 185 at various D2EHPA/TOPO molar ratios, for a phase volume ratio equal to 1. Both phases were stirred during 1 hour at (25.0± 0.2) °C with a mechanical stirring apparatus (Gherardt Laboshake) thermostated with a Gherardt Thermoshake, so that the equilibrium was reached. Uranium concentration in the aqueous solutions was determined by ICP-AES using a Varian Vista Pro spectrometer. Before analyses, the aqueous solutions were filtered to remove traces of organic phase with a hydrophilic filter (Minisart NML 16555 K, cellulose acetate, 0.45 μm, d = 28 mm). The concentration of uranium (VI) in organic phases was deduced from mass-balance calculation, i.e. from the difference between the uranium concentration in the aqueous phase before and after extraction. The experimental error on the distribution coefficient of uranium (VI) [DU(IV)] was estimated to be within 5%.

Fig. 1. Logarithm of the distribution coefficient of uranium (VI) at 25 °C between 5.3 mol L−1 phosphoric acid and D2EHPA/TOPO diluted in Isane IP 185 as a function of (a) the logarithm of initial D2EHPA concentration at constant initial TOPO concentration (0.125 mol L−1), (b) the logarithm of initial TOPO concentration at constant initial D2EHPA concentration (0.5 mol L−1). Initial concentration of uranium = 1.43 10−3 mol L−1, phase volume ratio O/A = 1. O and □: Experimental values, continuous lines ____: calculated curves with the thermodynamic model and the corresponding constants reported in Table 1.

120


The total uranium (VI) concentration (CU(VI)) in aqueous phase can be expressed as follows:

Furthermore, it is well known that D2EHPA and TOPO are involved in the following equilibria :

h h i i 2þ 2þ 1 þ ∑ f ð½i; βi ¼ UO2 α C UðVIÞ ¼ UO2

HL ¼ HL

ð8Þ

2HL ¼ HL 2

ð9Þ

i

ð4Þ

where [i] and βi denote the concentration of free inorganic ligand (i) that complexes uranium (VI) in the aqueous phase (i.e., i = H3PO4, H2PO4−, HPO42−, etc.) and the corresponding formation constant, respectively, whereas α refers to the complexing factor. Thus, the distribution coefficient of uranium (VI) [DU(VI)] between phosphoric acid and the organic phase can be expressed as: C UðVI Þ DUðVI Þ ¼ 2þ UO2 α

ð5Þ

where C UðVI Þ denotes the total concentration of uranium (VI) in the organic phase after equilibration. As the concentration of phosphoric acid is high (i.e., 5.3 mol L −1) compared to that of uranium (initially, 1.4 10−3 mol L −1), one can consider that, except for uranium which is extracted, the aqueous phase keeps a constant composition (i.e., constant concentrations of H3PO4, H2PO4−, HPO42−, etc.) and a constant pH in all the extraction experiments reported in the present work. In particular, it is of interest that the amount of hydrogen ions released in the aqueous phase in parallel to the extraction of uranium (VI) is at the most equal to 2.8 10−3 mol L −1 for a phase volume ratio O/A = 1, which is low compared to the concentration of free H + ions naturally present in 5.3 mol L −1 phosphoric acid, namely about 1.6 mol L −1 (Ennaassia et al., 2002; Menoyo et al., 2001; Stenström et al., 1986). As a result, it can be reasonably assumed that both the complexing factor (α) and the pH keep constant values, specific to the 5.3 mol L−1 phosphoric acid medium, whatever the yield of uranium extraction is. The existence of interactions between D2EHPA and TOPO in organic phases was put in evidence by isopiestic and infrared spectroscopic investigations (Baes, 1962; Baker and Baes, 1962; Brown et al., 1957; Staszak and Prochaska, 2008). Baker and Baes (1962). They suggested the existence of the two following equilibria:

−

þ

ð10Þ

TOPO ¼ TOPO

ð11Þ

HL ¼ L þ H

Finally, the extraction of uranium (VI) can be considered as the result of Eqs. (12)–(14): 2þ þ UO2 þ 2 HL 2 ¼ UO2 ðHL2 Þ2 þ 2H

ð12Þ

2þ þ UO2 þ 2 HL 2 þ TOPO ¼ UO2 ðHLÞ2 L2 TOPO þ 2H

ð13Þ

2þ þ UO2 þ HL 2 þ TOPO ¼ UO2 L2 TOPO þ 2H

ð14Þ

Eq. (13) was derived from literature (Bunus et al., 1978) and Eqs. (12) and (14) were implemented in the model so that the best quality of fit was obtained as reported in Fig. 1. From the chemical Eqs. (6) to (14) and considering that the phase volume ratio O/A is equal to 1, one can derive the mathematical Eqs. (15)–(17). h i h i2 1 1 ka B 1 þ P þ 2k2 HL þ P ½Hþ þ 2k21 k2 TOPO HL þ 2k22 k2 TOPO HL C C B h i 3 C B 3 C Þ TOPO HL C h i B C B 4 kex;1 k2 C UðVI Þ HL þ 4 kex;2 k2 UðVI D2EHPA ¼ HL B C 2 2 2 2 þ þ α α C B 0 ½H h ½H i C B C B A @ kex;3 C UðVI Þ TOPO HL þ2 k2 α ½Hþ 2 0

ð15Þ h

ðHLÞ2 þ TOPO ¼ ðHLÞ2 −TOPO

ð6Þ

ðHLÞ2 þ 2TOPO ¼ ðHLÞ2 −ðTOPOÞ2

ð7Þ

TOPO

i 0

0 4 1 h i 2 kex;2 2 C UðVI Þ HL 1 2 C k2 þ k21 k2 HL þ 2k22 k2 TOPO HL þ 1þ h i B B C kD;T α ½ Hþ 2 C ¼ TOPO B 2 B C C Þ HL k @ A UðVI ex;3 k2 þ 2 þ α ½H

ð16Þ

Table 1 Equilibria and corresponding equilibrium constants implemented in the model for the calculation of the distribution coefficient of U(VI) between 5.3 mol L−1 phosphoric acid and mixtures of D2EHPA and TOPO diluted in Isane IP 185. Equilibrium HL ¼ HL

Equilibrium constant ½HL P ¼ ½HL

2HL ¼ HL 2

k2 ¼

HL = L− + H+

½L ka ¼ ½H½HL

TOPO ¼ TOPO

kD;T ¼

ðHLÞ2 þ TOPO ¼ ðHLÞ2 −TOPO

k21 ¼

ðHLÞ2 þ 2TOPO ¼ ðHLÞ2 −ðTOPOÞ2

k22 ¼

HL þ 2 HL 2 þ TOPO ¼ ðHLÞ5 TOPO þ UO2þ 2 þ 2 HL 2 ¼ UO2 ðHL2 Þ2 þ 2H þ UO2þ 2 þ 2 HL 2 þ TOPO ¼ UO2 ðHL Þ2 L2 TOPO þ 2H þ UO2þ 2 þ HL 2 þ TOPO ¼ UO2 L2 TOPO þ 2H

½ðHLÞ 2 2 ½HL þ

−

Value 3.47 103 3.39 104

1.99 10−2

½TOPO

104

½ðHLÞ2 −TOPO ½TOPO ½ðHLÞ2

25(⁎)

½TOPO

½ðHLÞ2 −ðTOPOÞ2 2 ½ðHLÞ2 ½TOPO ð HL Þ TOPO ½ 5 k51 ¼ 2 ½TOPO ½ðHLÞ2 ½HL þ ½UO2 ðHL2 Þ2 ½H 2 kex;1 ¼ 2 ½ðHLÞ 2 ½UO2þ 2 ½UO2 ðHLÞ2 L2 TOPO ½Hþ 2 kex;2 ¼ 2 ½TOPO ½ðHLÞ 2 ½UO2þ 2 ½UO2 L2 TOPO ½Hþ 2 kex;3 ¼ ½TOPO ½ðHLÞ 2 ½UO2þ 2

1.5 102(⁎) 5.5 104 kex;1 α

= 12.2(⁎)

kex;2 α

= 8.1 104(⁎)

kex;3 α

= 7.1 102 \(⁎)

The values of P, k2, ka and kD,T were taken from the literature (Biswas et al., 2000). In all calculations, the concentration of [H+] was taken equal to 1.6 mol L−1 for 5.3 mol L−1 phosphoric acid (Ennaassia et al., 2002; Menoyo et al., 2001; Stenström et al., 1986). (*) Values deduced by minimization between experimental and calculated data of the distribution coefficient of uranium.


h i 1 0 4 4 TOPO HL k k HL ex;2 2 B 1 þ ex;1 k2 k þ þC C B α 2 ½H þ 2 α 2 ½H þ 2 C B h i ½U0 ¼ C UðVI Þ B C 2 C B TOPO HL A @ kex;3 k2 2 þ α ½H

121

ð17Þ

h i h i In these equations, D2EHPA , TOPO and [U]0 represent the 0

0

initial concentration of D2EHPA, TOPO and uranium (VI) whereas i h HL , TOPO , CU(VI)and [H +]denote the monomeric concentrations of D2EHPA and TOPO in the organic phase, the concentrations of uranium (VI) and H + in the aqueous phase at equilibrium, respectively. The definition of the various constants is given in Table 1. By using these equations, we tried to simulate the experimental data plotted in Fig. 1a and b. P, kD,T, k2, ka were taken from literature (Biswas k

k

k

ex;2 ex;3 et al., 2000), whereas ex;1 α , α , α , k21 and k22 were optimized to attempt to fit the theoretical curve to the experimental points. Unfortunately, no satisfying fitting could be obtained with the above set of equations. At this point, the interactions between D2EHPA and TOPO in the organic phase were reconsidered and it was found that considering the existence of higher aggregates between D2EHPA and TOPO drastically improved the quality of fits between the simulated curves and the experimental points. In particular, an excellent fitting was obtained by taking into account the formation of (HL)5TOPO according to the following equilibrium, in addition to Eqs. (6) and (7).

HL þ 2 HL 2 þ TOPO ¼ ðHLÞ5 TOPO

Fig. 2. Logarithm of the distribution coefficient of uranium (VI) at 25 °C between 5.3 mol L−1 phosphoric acid and D2EHPA/TOPO diluted in Isane IP 185 as a function of the logarithm of initial TOPO concentration for various concentrations of D2EHPA: (3) 0.2 mol L−1; (2) : 0.8 mol L−1; (1) 1.4 mol L−1. Initial concentration of uranium = 1.43 10−3 mol L−1, phase volume ratio O/A = 1. Δ, O and □: Experimental values, continuous lines ____: calculated curves with the thermodynamic model and the corresponding constants reported in Table 1 (series of data independent of the results reported in Fig. 1a and b).

ð18Þ

Considering the formation of (HL)5TOPO in the organic phase leads to an improved version of Eqs. (15) and (16) shown in Eqs. (19) and (20), respectively. The quality of fits between the calculated curves and the experimental points can be seen in Fig. 1a and b and the full set of corresponding optimized constant values is given in Table 1. Aggregation constants in the organic phase deduced from the model (Table 1) may change depending on the solvent composition but it can be expected that such a variation remains low in the investigated domain of concentrations of D2EHPA and TOPO, although such an effect cannot be excluded for highly concentrated organic solutions. 0

1 h i 1 k 1 þ þ 2k2 HL þ aþ þ 2k21 k2 TOPO HL B C P P ½ H B C i h i2 4 h B C B þ5k51 k2 2 HL TOPO þ 2k22 k2 TOPO HL þ C B C h i h i C B 3 3 B C C Þ TOPO HL D2EHPA ¼ HL B kex;1 2 C UðVI Þ HL kex;2 2 UðVI C 0 k2 k þ 4 B4 C 2 þ 2 þ 2 B C α α ½H h ½H i B C B C C Þ TOPO HL @ A UðVI kex;3 k2 þ2 α ½Hþ 2

ð19Þ 1

0

2 5 1 2 C B 1 þ k þ k21 k2 HL þ k51 k2 HL C B D;T 4 C B h i C h i h i B C Þ HL k C B þ2k k TOPO HL 2 þ ex;2 k2 UðVI TOPO ¼ TOPO B C ð20Þ 22 2 α 2 C B 0 ½Hþ 2 C B C UðVI Þ C B k HL A @ ex;3 þ 2 k2 2 α ½ Hþ

To assess the relative success of the model, a linear square regression analysis of the form: h

Log DUðVIÞ

i calc

h i ¼ u Log DUðVIÞ

exp

ð21Þ

Fig. 3. Influence of k2 on the distribution coefficients DU(VI) calculated with the thermodynamic model, the others parameters having the values given in Table 1. Log DU(VI) as a function of (a) the logarithm of initial D2EHPA concentration at constant initial TOPO concentration (0.125 mol L−1) and (b) Log of initial TOPO concentration at constant initial D2EHPA concentration (0.5 mol L−1). (1): k2 = 105; (2): k2 = 3 104; (3): k2 = 104.

122


Fig. 4. Influence of (a) k21, (b) k22 and (c) k51 on the distribution coefficients DU(VI) calculated with the thermodynamic model, the others parameters having the values given in Table 1. (1): k21 = 0; (2): k21 = 25; (3): k21 = 45; (4): k22 = 0; (5): k22 = 150; (6): k22 = 200; (7): k51 = 0; (8): k51 = 5.5 104; (9): k51 = 8 104.

was employed where [Log DU(VI)]calc and [Log DU(VI)]exp represent the calculated and experimental values of decimal logarithm of DU(VI), respectively. The resulting value of the slope u and the correlation coefficients (R 2) are 0.9927 and 0.9939 for D2EHPA–TOPO system at constant TOPO concentration and 1.0071 and 0.9902 for D2EHPA– TOPO system at constant D2EHPA concentration, respectively. In order to further test the model presented above, a series of independent extraction experiments covering an extended domain of D2EHPA and TOPO concentrations were performed as shown in Fig. 2. It is of interest that the thermodynamic model simulates perfectly this new set of experimental data with the constant values previously optimized (Table 1) from the experimental data reported in Fig. 1a and b, confirming the validity of our model.

3.2. Sensitivity of the model The influence of the parameters implemented in the model on the shape of the calculated curves Log (DU(VI)) = f(Log[D2EHPA]) or Log (DU(VI)) = f(Log[TOPO]) has been studied. There is no influence of P, pKa or kD,T when their values vary between 1000–6000, 0.5–3 and 101–10 6, respectively. Indeed, the values of P and kD,T are high enough to consider that D2EHPA and TOPO are mostly dissolved in the organic phase independently on D2EHPA or TOPO concentration. The other parameters (k2, k21, k22, k51, kex,1/α, kex,2/α and kex,3/α) influence the results of the model as depicted in Figs. 3–5. An increase of k2 is responsible for an increase of the distribution coefficient of uranium (VI), as illustrated in Fig. 3.


123

Fig. 5. Influence of (a) kex,1/α and (b) kex,3/α on the distribution coefficients DU(VI) calculated with the thermodynamic model, the others parameters having the values given in Table 1. (1): kex,1 = 0; (2): kex,1/α = 1.2; (3): kex,1/α= 5; (4): kex,3/α= 0; (5): kex,3/α= 70; (6): kex,3/α = 140.

Fig. 4(a–c) show that an increase of the association between TOPO and D2EHPA is responsible for a decrease of the distribution coefficient of uranium (VI) since less free extractant molecules are available to form uranium (VI) complexes. The shape of the curves is slightly modified by the values of the association coefficients. Indeed, a decrease of k21 and k22 lowers the slope of the linear part of the plot of Log (DU(VI)) vs. Log ([D2EHPA]) at low D2EHPA concentrations and shifts the maximum of the curves (Fig. 4a and b). Without considering the formation of (HL)5TOPO species [Equilibria (19)], i.e. k51 = 0, the curve Log (DU(VI)) vs. Log ([D2EHPA]) [Fig. 4c] does not exhibit a plateau region when Log ([D2EHPA]) is higher than 0, i.e., the curve continues to increase and thus does not fit the experimental data. Other supramolecular aggregates such as (HL)x–(TOPO) (x = 3, 4, 6 and 7) and (HL)5TOPO2 have been considered and tested. Among all these complexes, no species permit to obtain better fits than those obtained by considering only (HL)5TOPO, in addition of (HL)2TOPO and (HL)2TOPO2. Nevertheless, the same quality of fit can be obtained by considering the presence of a couple of supramolecular forms in the organic phase, as for instance (HL)4TOPO and (HL)6TOPO or (HL)3TOPO and (HL)7TOPO, instead of (HL)5TOPO. As stated above, such higher aggregates between D2EHPA and TOPO have not yet been reported in literature but their presence may be explained by intermolecular interactions arising from supramolecular assemblies (Cote, 2003). Similar aggregates were pointed out by Antonio et al. (Antonio et al., 2008) for solutions of di-n-hexylphosphoric acid (HDHP) and N,N′-dimethyl-N,N′dioctylhexylethoxymalonamide (DMDOHEMA) in n-alkane. More precisely, Antonio et al. pointed out the existence of a supramolecular complex composed of two HDHP and either four or five DMDOHEMA molecules. Furthermore, an increase of k51 decreases the distribution coefficient of uranium (VI) and lowers the slope of Log DU(VI) = f([D2EHPA]) at low

D2EHPA concentration where the curve can be assimilated to a straight line (Fig. 4c). Fig. 5a shows that the calculated values of Log (DU(VI)) vs. Log([TOPO]) do not match the experimental data at high D2EHPA concentration when the contribution of Eq. (12) (kex,1/α= 0) is not considered. A variation of kex,2/α does not influence the global shape of the curves Log DU(VI) = f([D2EHPA]) or Log DU(VI) = log ([TOPO]) but a decrease of kex,2/α lowers the distribution coefficient of uranium (VI). Fig. 5b shows that an increase of kex,3/α decreases the slope of the linear region of the curve Log DU(VI) = f(Log[D2EHPA]) at constant TOPO concentration and low D2EHPA concentration and flattens slightly the bell-shape curve at high TOPO concentration when D2EHPA remains constant. 3.3. Speciation in the extraction solvent The species concentration in the organic phase has been calculated from the present thermodynamic model as a function of the logarithm of D2EHPA concentration at 0.125 mol L −1 TOPO (Fig. 6a and b). At constant concentration of TOPO, Fig. 6a shows that D2EHPA exists mainly in the dimeric form (HL2) and associated with TOPO. At log ([D2EHPA]) b − 0.7, free TOPO is the predominant species in the organic phase. (HL)2TOPO complex and the dimer of D2EHPA (HL)2 are present at higher concentration than (HL)2–(TOPO)2, (HL)5TOPO species and free TOPO when − 0.6 b log ([DEHPA]) b − 0.1. At log ([DEHPA]) > − 0.1, the predominant species in the organic phase are the dimeric form of D2EHPA, the aggregates (HL)5TOPO and, at a lesser degree (HL)2TOPO. Calculations show that UO2(HL)2L2TOPO is the main uranyl species in the organic phase regardless D2EHPA concentration in the investigated range of concentration (Fig. 6b). At low D2EHPA concentration, nearly one third of uranium is extracted as UO2L2TOPO. However,

124


Fig. 6. Diagram of speciation for D2EHPA/TOPO diluted in Isane IP 185 vs. logarithm of initial D2EHPA concentration at initial [TOPO] = 0.125 mol L−1 after equilibration of the organic phase with 5.3 mol L−1 phosphoric acid containing 1.43 10−3 mol L−1 uranium (VI) (25 °C, pH = 0.3). Diagram calculated from the model developed in this work (Table 1). HL and L denote the monomeric protonated and deprotonated form of D2EHPA, respectively.

the concentration of this complex tends to decrease by increasing D2EHPA concentration. The low extraction ability of uranium (VI) by D2EHPA alone is confirmed by the low concentration of UO2L2(HL)2 in the D2EHPA/TOPO mixtures (Fig. 6b). The increase of TOPO concentration at 0.5 mol L −1 D2EHPA leads to the disappearance of the dimer of D2EHPA (HL)2 and the formation of mixed complexes of D2EHPA and TOPO in the organic phase such as (HL)2TOPO, (HL)2(TOPO)2 and (HL)5TOPO (Fig. 7a). As expected, an increase of TOPO concentration at constant D2EHPA concentration favors slightly the formation of UO2(HL)2L2TOPO and the disappearance of UO2(HL)2L2 in the extraction solvent (Fig. 7b). 4. Conclusion The extraction of uranium (VI) from 5.3 mol L − 1 phosphoric acid by D2EHPA–TOPO diluted in Isane IP 185 can be quantitatively described by a thermodynamic model whose characteristics are as follows. This model considers the presence of monomeric and dimeric D2EHPA in organic phase, the partitioning of D2EHPA and TOPO between phosphoric acid and Isane IP 185 and the association of TOPO and D2EHPA in organic phase leading to the formation of mixed species such as (HL)2TOPO, (HL)2(TOPO)2, as well as higher aggregates such as (HL)5TOPO or alternatively couples of aggregates as for instance {(HL)4TOPO and (HL)6TOPO} or {(HL)3TOPO and (HL)7TOPO}. In this model, the extraction of uranium from the aqueous phase into the organic phase is assumed to be mainly due to the formation of the mixed complex UO2(HL)2L2TOPO, and at a much lessser degree to that of UO2L2TOPO and UO2(HL)2L2, where HL and L denote the monomeric protonated and deprotonated form of D2EHPA, respectively. A good

Fig. 7. Diagram of speciation for D2EHPA/TOPO diluted in Isane IP 185 vs. logarithm of initial TOPO concentration at [D2EHPA] = 0.5 mol L−1 after equilibration of the organic phase with 5.3 mol L−1 phosphoric acid containing 1.43 10−3 mol L−1 uranium (VI) (25 °C). Diagram calculated from the model developed in this work (Table 1).

agreement between experimental and calculated distribution coefficients of uranium (VI) has been found only when all the complexes and aggregates cited above, especially (HL)5TOPO or alternatively couples of aggregates such as {(HL)4TOPO and (HL)6TOPO} or {(HL)3TOPO and (HL)7TOPO}, are taken into account. Presently, we have interpreted the extraction data by assuming the existence of higher aggregates of D2EHPA and TOPO than previously reported in the literature, but it is certainly possible to reach the same quality of fitting between calculated and experimental data by assuming deviation from ideality in the organic phase (variation of k2, k21; k22, etc.). Nevertheless recent results about the supramolecular speciation of organic phases obtained by SANS, SAXS, VPO, etc. (Antonio et al., 2008) lead us to conclude that the existence of higher aggregates such as (HL)5TOPO , assumed here, deserves to be considered, which does not exclude deviation from ideality effects. Further work is also necessary to understand and simulate the extraction of uranium (VI) from WPA in large range of H3PO4 concentration, especially as far as the speciation of uranium (VI) in aqueous phase and salting-out effects are concerned. References Antonio, M.R., Chiarizia, R., Gannaz, B., Berthon, L., Zorz, N., Hill, C., Cote, G., 2008. Aggregation in solvent extraction systems containing a malonamide, a dialkylphosphoric acid and their mixtures. Sep. Sci. Technol. 43, 2572–2605. Baes Jr., C.F., 1956a. A spectrophotometric investigation of uranyl phosphate complex formation in perchloric acid solution. J. Phys. Chem. 60, 878–883. Baes Jr., C.F., 1956b. The reduction of uranium(VI) by iron(II) in phosphoric acid solution. J. Phys. Chem. 60, 805–806. Baes Jr., C.F., 1962. An isopiestic investigation of di-(2-ethylhexyl)-phosphoric acid (DPA) and tri-n-octylphosphine oxide (TPO) in n-octane. J. Phys. Chem. 66, 1629–1634. Baes Jr., C.F., Schreyer, J.M., Lesser, J.M., 1953. The chemistry of uranium(VI) orthophosphate solutions: Part I. A spectrophotometric investigation of uranyl phosphate complex formation in perchloric acid solutions. AECD-3596. US Atomic Energy Commission, Oak Ridge, Tennessee, USA.

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