A. Reagents and experimental equipment

All chemicals are of reagent grade. Carbon, hydrogen, and nitrogen were analyzed by a Perkin-Elmer PE2400C elemental analyzer. IR spectra were recorded at 4000–500 cm^-1 with KBr discs on a Bio-Rad FTS-165 spectrometer. The ¹H was obtained with a Bruker DPX-400 NMR. DMDDDGA was synthesized in the following three steps (Fig. 1).

Fig. 1.Full-screen View

Synthesis scheme of DMDDDGA.

B. Extraction procedure

Equal volumes of aqueous phase and organic phase were agitated for 30 min to obtain equilibrium at a constant temperature. The two phases were centrifuged and assayed by taking known aliquots (0.05–0.10 mL) from the aqueous phase. The M³⁺ content in aqueous phase was determined by Arsenazo-III visible spectrophotometric analysis, and the M³⁺ content in organic phase was obtained by subtracting the aqueous concentrations from the total initial aqueous concentration of M³⁺. The distribution ratio (D) was calculated as the ratio of the concentration of M³⁺ in the organic phase to that in the aqueous phase.

C. FT-IR characterization

Equal volumes of 0.20 mol/dm³ DMDDDGA solution and 0.05 mol/dm³ of trivalent lanthanide ions were agitated. FT-IR spectra of the organic phase were recorded using a potassium bromide window with scanning times of 16 and the resolution of 2 cm^-1.

A. Effect of HNO₃ content on distribution ratio

The extraction of lanthanides(III) from nitric acid medium was studied using DMDDDGA within the initial nitric acid concentration of 1.0–3.5 mol/dm³ (Fig. 2).

Fig. 2.Full-screen View

Effect of HNO₃ concentration on extraction of lanthanides. C_M3+=5.00×10^-3mol/dm³, C_DMDDDGA=0.05 mol/dm³, 298 K.

Nitric acid concentration has little effect on the distribution ratio in the range of 1.0–2.0 mol/dm³. The D value increases markedly with the nitric acid concentration above 2.0 mol/dm³, indicating that the anion NO₃^- plays an important co-ion role in this extraction system. Fig. 2 also shows that the extraction behaviour of DMDDDGA on the Gd(III), Dy(III), Ho(III) and Er(III) is a positive sequence with the atomic number. Shimojo et al. [19] also found similar phenomena that the heavier lanthanide cations were effectively extracted.

B. Effect of DMDDDGA content on extraction

The extraction of trivalent lanthanide elements (M³⁺) from HNO₃ solution by DMDDDGA can be expressed as:

The extraction equilibrium constant is

The conditional equilibrium constant is

where [M³⁺] is the aqueous concentration of M³⁺, and [M(NO₃)₃·nDMDDDGA]_(o) is the concentration of extracted species in the organic phase. With a constant [NO₃^-], Eq. (3) can be rearranged into Eq. (4) by taking the logarithm:

From Eq. (4), n can be estimated from the slope analysis of the relationship between log D and log [DMDDDGA]_(o). The effect of DMDDDGA concentration on the distribution ratio of lanthanides is shown in Fig. 3. It can be seen that log D increases with DMDDDGA concentration, and the slopes are n= 4.02, 3.74, 3.64 and 3.03 for Er³⁺, Ho³⁺, Dy³⁺ and Gd³⁺, respectively. So, the stoichiometries of the main extracted species can be Gd(NO₃)₃⋅3DMDDDGA and M(NO₃)₃⋅4DMDDDGA (M = Dy³⁺, Ho³⁺, Er³⁺). Similar variations in stoichiometry of the metal ligand complex were corroborated by EXAFS studies [20] of Er³⁺-DGA complex in ethanol, and it was suggested that only two of the four DGA molecules might be bonded in tridentate fashion with some water molecules in the inner coordination sphere.

Fig. 3.Full-screen View

Effect of DMDDDGA concentration on extraction of lanthanides $C_{HNO₃}$=2.5 mol/dm³, $C_{M³⁺}$=5×10^-3 mol/dm³, 298 K.

The extraction mechanism can be expressed as

$\begin{array}{ll}\hfill & {\text{M}}^{\text{3+}}{\text{+3NO}}_3^{-}{\text{+3DMDDDGA}}_{\text{(o)}}\hfill \\ \hfill & {\text{=M(NO}}_{\text{3}}{\text{)}}_{\text{3}}{\text{·3DMDDDGA}}_{\text{(o)}}\text{M=Gd,}\hfill \\ \hfill & \hfill \\ \hfill & {\text{M}}^{\text{3+}}{\text{+3NO}}_3^{-}{\text{+4DMDDDGA}}_{\text{(o)}}\hfill \\ \hfill & {\text{=M(NO}}_{\text{3}}{\text{)}}_{\text{3}}{\text{·4DMDDDGA}}_{\text{(o)}}\text{M=Dy,Er,Ho}\text{.}\hfill \end{array}$

The log K of extracting Gd³⁺, Dy³⁺, Ho³⁺ and Er³⁺ with DMDDDGA in chloroform from nitric acid calculated using Eq. (3) were 3.10±0.06, 4.84±0.06, 5.03±0.06 and 5.21±0.07, respectively. The asymmetric DMDDDGA has a better extraction effect than TBDGA in our previous work [21].

FT-IR spectra of the organic phase loading extracted species were recorded (Fig.4). The wave number of the stretching frequency of C=O band is shifted from 1660 to 1622 cm^-1. Sasaki et al. [22] also reported that the wave number of C=O is shifted to lower number after extraction. The FT-IR results indicated that the carbonyl groups of DMDDDGA molecules are coordinated to metal ions. The wave number of C^-O^-C is shifted from 1122 cm^-1 to 1126 cm^-1. The structural characterization of lanthanum(III) complex coordinated by diglycolamide (DGA) ligands shows that the ligand is tridentated [23].

Fig. 4.Full-screen View

FT-IR spectra of organic phase loaded Ln(III) with DMDDDGA in chloroform.

C. Temperature effect on extraction

Figure 5 shows the temperature effect on the extraction distribution ratio. The extraction rate decreases with increasing temperature. From the Van’t Hoff equations [24], the thermodynamic parameters can be calculated by [24]:

Fig. 5.Full-screen View

log D vs. 1/T for extraction of lanthanides(III) with DMDDDGA. $C_{{\text{M}}^{3+}}$=5×10^-3 mol/dm³, $C_{H}NO₃$=2.5 mol/dm³, C_DMDDDGA=0.05 mol/dm³.

where, R is the gas constant and C is a conditional constant. So, -ΔH/(2.303R) is the slopes in Fig. 5. From Eq. (5) and Fig. 5, we have ΔH = -36.57 kJ/mol, -38.87 kJ/mol, -42.70 kJ/mol and -46.34 kJ/mol for Gd³⁺, Dy³⁺, Ho³⁺ and Er³⁺, respectively, which indicates that the extraction reactions are exothermic.