Liquid–liquid anion exchange extraction studies of samarium(III) from salicylate media using high molecular weight amine

2.1 Apparatus

An Elico digital spectrophotometer Model SL-171 MINI SPECTRO with 1 cm quartz cells was used for absorbance measurements and pH adjustments were carried out using an Elico digital pH-meter Model LI-127. All weighing operations were done by using Tapson’s analytical single pan balance model 200 T having 0.001 g accuracy.

2.2 Reagents

2.3 Extraction and determination procedure for samarium(III)

The aqueous phase containing 50 μg samarium(III) was maintained at 0.01 mol dm⁻³ sodium salicylate and pH was adjusted to 8.0 with dil hydrochloric acid and sodium hydroxide solution by maintaining total dilution volume to 25 mL and transferred to 125 mL separatory funnel. Ten mL of 0.025 mol dm⁻³ 2-OAP in xylene was added into the separatory funnel and equilibrated for 4 min and the two phases were allowed to separate. Samarium(III) from the organic phase was stripped with 0.5 mol dm⁻³ nitric acid (3 × 10 mL) solution.

The stripped solution containing samarium(III) was evaporated to moist dryness and extracted into water. To this solution 1 mL of 0.05% Arsenazo-I was added, the pH was adjusted to 7.2 (±0.1) by adding 2 mL of triethanolamine buffer solution. The absorbance was measured at 575 nm wavelength using the reagent blank solution.

All procedures of the extraction were carried out at a room temperature of 298 K. In these experiments the percentage extraction (%E) was determined as follows: $$\text{Percentage extraction}(\%E)=\frac{[A_1-A]}{[A_1-A_0]}\times 100$$ where A₀ is the absorbance of arsenazo I solution without cation, A₁ is the absorbance of the arsenazo I solution containing the known concentration of the cation before the extraction and A is the absorbance of the arsenazo I solution containing the known concentration of cation after the extraction.

The distribution ratio (D) was calculated as, $$D=\frac{(V_{\text{w}}/V_{\text{o}})\times \%E}{100-\%E}\times 100$$ where the V_w is the volume of aqueous phase (25 mL) and V₀ is the volume of organic phase (10 mL).

The percentage stripping (%S) values were determined by using relation, $$\text{Percentage stripping}(\%S)=\frac{\text{Amount of metal stripped by strippant}}{\text{Amount of metal extracted}}\times 100$$

3.1 Effect of pH

The relationship between pH variation and samarium(III) extraction efficiency was analyzed and represented as extraction isotherm as shown in Fig. 1. The curve indicates that the maximal extraction of 50 μg samarium(III) takes place in 7.5–10.0 pH range from 0.01 mol dm⁻³ sodium salicylate solutions, which implies a favorable formation of the more stable ion-pair complex. Hence all the extractions were carried out at pH 8.0. There is a possibility of hydrolysis at these pH values since it is a particular tendency of the trivalent lanthanide elements. However, the physical phenomenon that characterized the hydrolysis was not observed in the stoichiometric study of the extracted metals species (Fig. 2).

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Figure 1

Effect of pH on the extraction of samarium(III).

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Figure 2

(A) Plot of log D_[Sm(III)] against log C_[salicylate] at 0.025 mol dm⁻³ 2-OAP. (B) Plot of log D_[Sm(III)] against log C_[2-OAP] at 0.01 mol dm⁻³ salicylate.

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3.2 Effect of reagent concentration

2-Octylaminopyridine dissolved in xylene with varying concentrations from 0.005 to 0.5 mol dm⁻³ were employed to optimize the extraction conditions of 50 μg of samarium(III) at pH 8.0. It was found that 10 mL of 0.020 mol dm⁻³ 2-OAP was sufficient for the quantitative extraction of 50 μg of samarium(III) from 0.01 mol dm⁻³ sodium salicylate solutions but in the actual experiment we used 10 mL of 0.025 mol dm⁻³ 2-OAP in xylene to ensure the complete extraction of metal ion. The excess of reagent concentration (>0.150 mol dm⁻³) affected the percentage extraction of metal ion (Table 1).

The slope of log–log linear relationships between concentrations of the extractant with the corresponding distribution ratios of samarium were found to be around one in the investigated system (Fig. 2(B)). This indicates the participation of one molecule of 2-OAP in the extracted metals species.

3.3 Effect of weak organic acid concentration

The extraction of samarium(III) was carried out at pH 8.0 with 0.025 mol dm⁻³ 2-OAP in xylene in the presence of varying concentrations of sodium malonate, sodium succinate, sodium salicylate and sodium oxalate as weak organic acid media in the range of 0.001 to 0.1 mol dm⁻³ (Table 2). The extraction of 50 μg samarium(III) was found to be quantitative in the range of 0.008–0.02 mol dm⁻³ for sodium salicylate and 0.005–0.007 mol dm⁻³ for sodium succinate. The extraction of samarium(III) was found to be incomplete in the malonate and oxalate media. Hence, in the recommended extraction procedure we used 0.01 mol dm⁻³ sodium salicylate media due to availability of large concentration range and it has low cost.

3.4 Effect of diluents

In this study, the extraction of 50 μg samarium(III) was carried out by using various organic diluents of different dielectric constants. The diluents containing 0.025 mol dm⁻³ 2-OAP were used for the extraction of 50 μg samarium from 0.01 mol dm⁻³ sodium salicylate solutions. It was found that the extraction of 50 μg samarium(III) was quantitative at pH 8.00 with xylene and carbon tetrachloride while there was incomplete extraction in toluene (83.9%), kerosene (59.3%), methyl isobutyl ketone (67.2%), n-butanol (73.7%), amyl alcohol (52.0%), chloroform (87.3%), 1,2-dichloroethane (70.7%) and amyl acetate (51.2%). Throughout the experiment xylene was used as solvent due to safety reasons.

We did not observe any relationship between the dielectric constant of the solvent used for the extraction and the extraction efficiency of samarium(III).

3.5 Effect of stripping agents

Samarium(III) from organic phase was stripped with three 10 mL portions of various stripping agents at different concentrations of mineral acids, buffer solutions and some bases. Samarium(III) was quantitatively stripped with nitric acid (0.3–1.0 mol dm⁻³), acetic acid (0.9–1.0 mol dm⁻³) and hydrochloric acid (0.1–0.3 mol dm⁻³) from the organic phase. However, the percentage recovery of samarium(III) from organic phase was found to be incomplete with strippants like sulfuric acid, perchloric acid, acetate buffer, ammonia, sodium hydroxide and potassium hydroxide. Inthe recommended procedure, three 10 mL portions of 0.5 mol dm⁻³ nitric acid were used for the complete stripping of the loaded organic phase (Table 3).

3.6 Variation of equilibration time

The period of equilibration was varied from 0.25 to 20 min. The extraction of samarium(III) was quantitative over a period of 1.5–7 min shaking of the solution but with prolonged shaking over 7 min there was a decrease in the percentage extraction of samarium(III). This may be due to the dissociation of ion-pair complex by agitation. Thus, the equilibration time for 50 μg samarium(III) extraction was kept at about 4 min throughout the study to ensure the complete extraction.

3.7 Extraction behavior of samarium(III) as a function of metal loading capacity

Varying concentrations of samarium(III) (25–2000 μg) were extracted with 10 mL portions of 0.025 mol dm⁻³ 2-octylaminopyridine in xylene from 0.01 mol dm⁻³ sodium salicylate media. It was observed that the extraction of samarium(III) was quantitative upto 800 μg and further increasing concentration, decreases the extraction efficiency of 10 mL of 0.025 mol dm⁻³ 2-OAP. This study clearly indicates that 10 mL of 0.025 mol dm⁻³ 2-OAP has 800 μg as a loading capacity.

3.8 Effect of aqueous to organic volume ratio

The effect of contacting the different volume ratios of aqueous to organic phase was studied. The results indicate that the preferred aqueous/organic (A/O) phase ratio in this study must be between 1:1 and 4:1. The extraction decreases beyond 4:1 ratio due to the less availability of reagent. However, in the recommended procedure, the phase ratio was maintained at 2.5:1.

3.9 Stoichiometry of extracted species

The stoichiometry of extracted species was determined by analyzing the experimental data. The conventional slope analysis method was used. The nature of the extracted species of samarium(III) by 2-OAP in xylene is whether mononuclear or polynuclear was investigated. By varying the initial samarium(III) concentration in the aqueous phase with the distribution ratio (D), it was observed that the distribution ratio was independent of samarium(III) concentration, which is a clear indication that the extracted species is mononuclear in the whole range of experimental study.

The plot of log D_[Sm(III)] versus log C_[Salicylate] at pH 6.0 and 7.0 with 0.025 mol dm⁻³ 2-OAP concentration was a linear graph with slopes of 1.74 and 1.77, respectively, (Fig. 2(A)) which revealed that two salicylate ions react with one mole of samarium(III), while the graph of log D_[Sm(III)] versus log C_[2-OAP] at pH 6.0 and 7.0 with 0.01 mol dm⁻³ sodium salicylate concentration was a linear graph with slopes of 1.15 and 1.25, respectively, (Fig. 2(B)) indicating that one 2-OAP molecule is used in the reaction process. Therefore, the probable extraction mechanism is given as,

Extraction mechanism:

The probable extracted species $[2\text{-}{\text{OAPH}}^{+}\text{Sm}{(\text{sal})}_2^{-}]$ formed in organic phase was stripped out by using three portions of 10 mL of 0.5 mol dm⁻³ nitric acid solution. Reactions occurring in stripping mechanism could be given as,

Stripping mechanism:

3.10 Temperature effect on extraction of samarium(III)

Under constant pH, salicylate concentration and 2-OAP concentration, samarium(III) extraction was enhanced by varying the temperature in the range of 308–321 K. Results show that with the increase in the temperature, log K_ex values also increase. The change in log K_ex values with the temperature is expressed by

The various thermodynamic parameters like ΔG and ΔS also were calculated using following relations,

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Figure 3

Effect of temperature on the extraction of samarium(III).

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The mean enthalpy change accompanying the ion-pair complexation was found to be 36.24 kJ mol⁻¹. Also negative values of free energy (ΔG) reveals that the extraction reaction is favorable for samarium(III) under the proposed extraction conditions (Table 4).

3.11 Effect of foreign ions on samarium(III) extraction system

The effect of various foreign ions was tested on the extraction of samarium(III) with 0.025 mol dm⁻³ 2-OAP in xylene as an extractant. The tolerance limit of individual foreign ions was defined as the amount required to cause ±2% error in the recovery of trivalent samarium using the proposed method. The results presented in the Table 5 indicate that many metal ions do not interfere but significant interference was caused due to anions like phosphate, tartrate, and EDTA.

4.1 Separation of samarium(III) from associated metal ions

Based on the partition data, the separation of samarium(III) from some commonly associated metal ions like Fe(III), Cr(VI), V(V), Co(II), Th(IV), U(VI), Zr(IV), Nb(V), Y(III), La(III), Ce(IV), Nd(III), and Gd(III) using 2-OAP in xylene can be achieved by taking advantage of the difference in the extraction conditions.

Under the optimum extraction conditions of samarium(III), metal ions like Fe(III), V(V), Cr(VI), Co(II), Zr(IV), Nb(V), Th(IV), and U(VI), remain quantitatively unextracted in the aqueous phase and these were determined by using the known methods (Marczenko, 1976; Vogel, 1972; Sandell, 1965). Samarium(III) extracted in organic phase was stripped out using 0.5 mol dm⁻³ HNO₃ and determined by using Arsenazo I method.

Since, some associated elements added, were found to be coextracted along with samarium(III), attempts were made to use different masking agents for the selective separation of the coextracted ions. Elements, such as Ce(IV) and Y(III) were masked with 5 mg of thiocyanate ions, under this condition samarium(III) was quantitatively extracted and determined as described in the general extraction procedure.

Samarium(III) was separated from La(III) and Gd(III) by using 3 mg of citrate ions as a masking agent while Nd(III) by 5 mg fluoride ions. The recoveries of samarium(III) and those of the added ions were obtained greater than 99.6% as shown in Table 6.

4.2 Separation of samarium(III) from synthetic mixtures

A ternary mixtures of Sm(III) containing transition metals like Fe(III), Cr(VI) Co(II), V(V) and some associated elements like Th(IV), U(VI), Zr(IV), Nb(V) were separated from an aqueous solution at pH 8.0 with 0.025 mol dm⁻³ 2-OAP in xylene, whereby samarium(III) is extracted while other elements remain unextracted in the aqueous phase. Then the extracted samarium(III) was stripped with 0.5 mol dm⁻³ nitric acid determined by arsenazo I at 575 nm wavelength (Table 7).