2.1. Chemicals and reagents

TBDGA (purity>98%) was synthesized in our lab according to the procedure reported in Ref.[16]. Its chemical structure is given in Fig.1. The rare earth oxides (>99.9%) was dissolved with concentrated HNO₃. The working solution was prepared by diluting. TBDGA solutions were obtained by dissolving TBDGA in toluene. Other chemical reagents employed, such as the toluene and HNO₃, were of AR grade.

Fig. 1.Full-screen View

Chemical structure of N,N,N′,N′-tetrabutyl-3-oxa-diglycolamide (TBDGA)

2.2. Extraction of HNO₃ with TBDGA

The aqueous phase and organic TBDGA solution with phase ratio of 1:1 were vibrated at frequency for ten minutes. After separation by centrifugation, the acidities of each phase were measured by NaOH titration. The organic solution was diluted in ethanol before titration.

2.3. Extraction of lanthanide ions

TBDGA solutions were employed after pre-equilibration with HNO₃ solutions at acidities the same as that of aqueous phase. The organic phase and aqueous phase with equal volumes were shaken for 30 min at 25°C. The two phases were then centrifuged and analyzed. The concentration of lanthanide ions in aqueous phase (C_e) was detected by spectrophotometric method (Arsenazo-III). The concentration of lanthanide ions in organic phase (C_o) was obtained by subtracting the C_e from the total initial aqueous concentration of lanthanide ions (C_i). The distribution ratio (D) was defined as

2.4. Preparation and characterization of the extracted species

0.20 mol·dm⁻³ TBDGA solution was shaken with saturated solution of gadolinium nitrate, centrifuged, and the organic phase separated. The samples were measured using liquid cells with KBr windows. FTIR spectrum was recorded with an FTS-165 spectrometer, over the wavenumber range of 400–4000 cm⁻¹, in 60 scan times and 2 cm⁻¹ resolution.

3.1. Extraction of nitric acid

The extraction of HNO₃ with TBDGA can be represented by,

where the subscript (o) denotes the organic phase. The conditional equilibrium constant (K_H) can be expressed as,

where [H⁺], [NO₃⁻] and [TBDGA]_(o) are the equilibrium concentrations ^[17]. In Eq. (3), the activity coefficients were neglected under the studied conditions ^[2,18]. The logarithmic expression of Eq. (3) is

where [H⁺]_(o) = [HNO₃·nTBDGA]_(o) and [H⁺] = [NO₃⁻].

A plot of lg[H⁺]_(o) −2lg[H⁺] vs. log[TBDGA]_(o) gives a straight line and the slope value is 1.06 (Fig.2). This shows that the stoichiometry of the main extracted specie is HNO₃·TBDGA. The K_H value was calculated at 0.35±0.02, being a little bigger than those of monoamide and diamide ^[16-20]. Pathak et al. studied the extraction of nitric acid with N,N-dialkyl amide and found that conditional acid uptake constant (K_H) values for 1:1 species in 0.5–3 mol·dm⁻³ HNO₃ were in the following order: D2EHPVA (0.06 ± 0.002)< D2EHIBA (0.15 ± 0.04) < D2EHPRA (0.20 ± 0.01)< D2EHAA (0.28 ± 0.03) ^[19]. In their study on the uptake of nitric acid with N,N-dihexyloctanamide (DHOA) in n-dodecane, Gupta et al. found K_H = 0.188 for the extracted species of HNO₃·DHOA^[20]. In extraction of HNO₃ with diamide N,N,N′,N′-dimethyl dibutyl tetradecyl malonamide, Nigond et al. suggested the competing formation of four adducts: L₂·HNO₃, L·HNO₃, L·(HNO₃)₂, and L·(HNO₃)₃, with their equilibrium constants for 0.72 mol·dm⁻³ amide in TPH being 0.167, 0.215, 5.19×10⁻³ and 3.70×10⁻⁴, respectively ^[21]. These show that the basicity of TBDGA, indicated by the equilibrium constant for HNO₃ extraction, is a little bit larger than those of monoamide and diamide. Based on HSAB (hard and soft acids and bases) theory, the enhanced basicity favors the coordination of TBDGA to lanthanide ions.

Fig. 2.Full-screen View

Extraction of nitric acid with TBDGA.

3.2. Extraction of lanthanide ions

3.2.1 Effect of nitric acid concentration on the extraction

The extraction ratios of Er(III), Dy(III), La(III), Ce(III), Nd(III), Sm(III), Eu(III), Gd(III), and Tb(III) ions from aqueous solution at different concentrations of HNO₃ are shown in Fig.3. The extraction distribution ratios of lanthanide ions increased with the HNO₃ concentration. This is in agreement with the result by C.Cuillerdier et al. ^[22]. NO₃⁻ ion plays an important role due to its salt effect and common ion effect.

Fig. 3.Full-screen View

Distribution of lanthanide ions as a function of HNO₃ concentration.

The order of extractability of TBDGA for the studied trivalent ions is La(III) <Ce(III) <Nd(III) <Sm(III) <Eu(III) <Gd(III) <Tb(III)< Dy(III)< Er(III). In other words, the extraction pattern gradually ascends across the lanthanide ions series. Narita et al. reported similar results using N,N′-dimethyl-N,N′-diphenyldiglycolamide as extractant ^[23]. The reason may be that the charge densities of Ln(III) ions increase with a decreasing ionic radius. Therefore, the electrostatic interaction of the heavier Ln(III) ions with ligands is stronger than that of the lighter Ln(III) ions. The interaction results in the enhanced affinity of TBDGA toward heavy lanthanide ions. This ascending pattern is opposite to those obtained using diamide in Refs. [24,25], where the degree of extractant basicity to dehydrate and the hydration energy of the Ln(III) ion were suggested to explain this phenomenon.

3.2.2 Effect of TBDGA concentration on distribution ratio

The reaction occurred in extracting lanthanide ions from aqueous nitrate medium by TBDGA in toluene can be expressed as

${\text{M}}^{\text{3}+}+{\text{3NO}}_{\text{3}}^{-}+n{\text{TBDGA}}_{\text{(o)}}={\text{M(NO}}_{\text{3}}{\text{)}}_{\text{3}}\cdot n{\text{TBDGA}}_{\text{(o)}},$ (5)

where M³⁺ is lanthanide ions. The slope analysis method was used to determine the “n” value of the predominant extracted species. Under the studied conditions, TBDGA is in large excess to lanthanide ions, so the TBDGA concentration can be regarded as constant. The distribution ratios as a function of the TBDGA concentration are shown in Fig.4. Solvate numbers of different lanthanide ions obtained by slope analysis are approaching 3, suggesting that lanthanide ions may be extracted as an extracted species of tri-solvates. This is similar to that obtained with TODGA, where the extracted complex was M(TODGA)₄(NO₃)₃ or M(TODGA)₃(NO₃)₃. The metal complex needs 3 or more TODGA molecules to keep stable in non-polar solvents ^[26].

Fig. 4.Full-screen View

Influence of TBDGA concentration on the distribution of lanthanide ions.

The extraction equilibrium constant, K_ex, is defined as

$K_{\text{ex}}=\frac{a_{{\text{M(NO}}_{\text{3}}{\text{)}}_{\text{3}}\cdot {\text{3TBDGA}}_{\text{(o)}}}}{a_{{\text{M}}^{\text{3}+}}{a}_{{\text{NO}}_{\text{3}}^{-}}^{\text{3}}{a}_{\text{TBDGA}}^{\text{3}}}=\frac{C_{{\text{M(NO}}_{\text{3}}{\text{)}}_{\text{3}}\cdot {\text{3TBDGA}}_{\text{(o)}}}}{{\gamma }_{{\text{M}}^{\text{3}+}}{C}_{{\text{M}}^{\text{3}+}}{\gamma }_{{\text{NO}}_{\text{3}}^{-}}^{\text{3}}{C}_{{\text{NO}}_{\text{3}}^{-}}^{\text{3}}{C}_{\text{TBDGA}}^{\text{3}}},$ (6)

where γ represents the activity coefficients of NO₃⁻ and lanthanide ions. The species activity in organic phase was suggested to be equal to its concentration due to the poor interaction in organic phase. The total concentration of ions in aqueous solution, C_M, can be written as,

$C_{\text{M}}=C_{{\text{M}}^{\text{3}+}}\text{(1}+{\beta }_{\text{1}}{C}_{{\text{NO}}_{\text{3}}^{-}}\text{)}.$ (7)

The γ values of NO₃⁻ and lanthanide ions, calculated by Debye-Hückel formula ^[27], were 0.9025 and 0.3974, respectively. β₁ is the first order stability constant of lanthanide ions with nitrate and the data, La³⁺(1.3±0.3), Ce³⁺(1.3±0.3), Nd³⁺(0.77±0.1), Sm³⁺(0.95), Eu³⁺(1.8±0.4), Gd³⁺(2.33±0.02), Tb³⁺(1.13±0.05), Er³⁺(0.54) were reported by other authors ^[28-30] (the γ value of Dy³⁺ has not been reported).

The distribution ratio D is

$D=\frac{C_{\text{M}\left(\text{o}\right)}}{C_{\text{M}}}=\frac{{\text{C}}_{\text{M}{\left({\text{NO}}_{\text{3}}\right)}_{\text{3}}\cdot \text{3TBDGA}\left(\text{o}\right)}}{C_{{\text{M}}^{3+}}\left(1+{\beta }_1{C}_{{\text{NO}}_{\text{3}}^{\text{-}}}\right)}.$ (8)

Therefore, K_ex can be written as

$K_{\text{ex}}=\frac{D\left(1+{\beta }_1{C}_{{\text{NO}}_{\text{3}}^{\text{-}}}\right)}{C_{{\text{NO}}_{\text{3}}^{\text{-}}}^3{C}_{\text{TBDGA}\left(\text{o}\right)}^3{\gamma }_{{\text{M}}^{\text{3+}}}{\gamma }_{{\text{NO}}_{\text{3}}^{\text{-}}}}=K\frac{1+{\beta }_1{C}_{{\text{NO}}_{\text{3}}^{\text{-}}}}{{\gamma }_{{\text{M}}^{\text{3+}}}{\gamma }_{{\text{NO}}_{\text{3}}^{\text{-}}}},$ (9)

where K is the conditional extraction constant defined as

$K=\frac{D}{C_{{\text{NO}}_{\text{3}}^{\text{-}}}^3{C}_{\text{TBDGA}\left(\text{o}\right)}^3}$ (10)

The calculated lgK_ex and lgK values are given in Table 1.

Table 1.Full-screen View

The lgK and lgK_ex values for extraction of lanthanide ions with TBDGA.

M3+	lgK	lgKex
La3+	2.52±0.11	3.27±0.11
Ce3+	2.79±0.20	3.54±0.20
Nd3+	2.97±0.17	3.65±0.17
Sm3+	3.96±0.22	4.63±0.22
Eu3+	4.16±0.16	4.98±0.16
Gd3+	4.23±0.08	5.10±0.08
Tb3+	4.69±0.07	5.42±0.07
Dy3+	5.03±0.08	……
Er3+	5.06±0.05	5.70±0.05

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3.2.3 The temperature effect on the distribution

As shown in Fig.5, the distribution ratios of lanthanide ions decrease with increasing temperatures, indicating that the extraction reaction is exothermic.

Fig. 5.Full-screen View

Extraction of lanthanide ions with TBDGA in toluene at different temperatures

The following thermodynamic equation was employed to calculate enthalpy change (Δ_rH_m^θ) of the extraction reaction ^[19].

${\left[\partial \left(\text{lgD}\right)/\partial \left(1/T\right)\right]}_{\text{p}}=-{\Delta }_r{H}_{\text{m}}{}^{\text{θ}}/\left(2.303R\right),$ (11)

where constant R＝8.314. The enthalpy changes (in kJ·mol⁻¹) of the extraction reactions of Ln(III) with TBDGA are: La³⁺,−69.34±5.82; Ce³⁺, −59.28±2.37; Nd³⁺,−69.87±2.34; Eu³⁺,−96.90±2.51; Gd³⁺, −93.00±3.12; Tb³⁺,−106.2±6.20; Dy³⁺,−97.86±3.06; and Er³⁺,−91.7±6.34. It indicates that the enthalpy change benefits extraction reaction ^[2]. This is in accordance with common reports for most extraction processes ^[18,31].

3.3. IR spectra study on the extracted species

The IR spectra of extractant and Gd(III) extracted species are shown in Fig.6. The C=O stretching vibration of free TBDGA appears at 1652 cm⁻¹. After extraction of Gd(III), the absorption band of C=O shifts to 1605 cm⁻¹. The shift of the C=O frequency for TBDGA before and after extraction is up to 47 cm⁻¹, suggesting a strong binding of TBDGA with Gd(III) in the extracted species. This indicates that lanthanide ions are coordinated to the oxygen of the carbonyl group of TBDGA. The band shift of the etheric bond is not obvious. Considering the distinct extractability of TBDGA for trivalent lanthanide ions, the etheric oxygen atom must be coordinated to lanthanide ions in the extracted species. That is to say, three oxygen atoms of each TBDGA molecule binding with lanthanide ion form two five-cycle chelate rings, hence the stability of the extracted species. The tridentate coordination of diglycolamide to the lanthanide ions has been established already by X-ray diffraction and molecular dynamics simulation studies ^[32]. It is therefore possible that the three ligands in one extracted species completely encloses the metal and forms a nine-coordinated complex that provides a hydrophobic exterior. An ion-pair mechanism involving the formation of [Ln(TBDGA)₃]³⁺ for lanthanide ions extraction seems to occur^[33].

Fig. 6.Full-screen View

IR spectra of TBDGA and Gd-TBDGA extracted species