The effect of ionic strength on the extraction of thorium(IV) from perchlorate solution by didodecylphosphoric acid (HDDPA)

3.1 Effect of HDDPA concentration on D_c

The effect of didodecylphosphoric acid (HDDPA) concentration on the extraction of thorium(IV) ion was studied at ionic strengths (1.00, 3.00 and 5.00 M HClO₄ and NaClO₄) and constant pcH (−log Molar concentration of H⁺) 1.00 and 2.00 in perchlorate media. The D_c value increases with increase in the HDDPA concentration. The logarithms of the D_c values obtained were plotted versus the corresponding logarithm of the HDDPA concentration. A straight line was obtained with slopes as shown in Figs. 1 and 2 (1.75 at I = 1.00 M, 1.76 at I = 3.00 M and 1.76 at I = 5.00 M at pcH 1.00 and 1.72 at I = 1.0 M, 1.75 at I = 3.00 M and 1.71 at I = 5.00 M at pcH 2.00) approximately 1.75 in perchlorate media.

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Figure 1

Variation of Log D_c with Log [HDDPA] in perchlorate media (I = 1.00, 3.00 and 5.00 M, pcH 1.00, T = 30 °C).

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Figure 2

Variation of Log D_c with Log [HDDPA] in perchlorate media (I = 1.00, 3.00 and 5.00 M, pcH 2.00, T = 30 °C).

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This means that at least two complexes are formed in organic phase between Th(IV) and didodecylphosphoric acid from perchlorate media, 25% contain one molecule of HDDPA, and 75% contain two molecule of HDDPA, which involved in the formation of the thorium–HDDPA complex in perchlorate media.

3.2 Effect of pcH on the extraction of Th(IV) ion

The effect of pcH on the extraction of thorium(IV) ion from perchlorate media of ionic strength (I = 1.00 and 3.00 M) has been studied, the logarithm of the D_c values obtained were plotted against the corresponding pcH values. A straight line with a slope of about 0.17 and 0.20 was obtained at I = 1.00 and 3.00, respectively, as shown in Fig. 3. These values represent the number of hydrogen ions released during the formation of thorium–HDDPA complex.

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Figure 3

Variation of Log D_c with pcH in perchlorate media (T = 30 °C).

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3.3 Effect of ionic strength on D_c

The effect of ionic strength was studied at constant pcH, a plot of the D_c values versus the corresponding ionic strength values give a straight line with a slope of 0.03 and 0.06 at pcH 1.00 and 2.00, respectively. The data indicate that the D_c value increases with the increasing of ionic strength in perchlorate. This is explained by the increase of the thermodynamic activity of the metal salt extracted and decrease in the activity of water as the ionic strength increases (Kolarik, 1982), this is shown in Fig. 4, which means that perchlorate anion is functioning as a salting out agent.

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Figure 4

Variation of D_c with ionic strength at pcH 1.00 and 2.00 in perchlorate media, T = 30 °C.

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The effect of salt variation upon ${\text{UO}}_2^{2+}$ extraction by 3-phenyl-4-benzoyl-5-isoxazolone (HPBI) was studied. The distribution ratio increases with the increase in salt concentration of NaClO₄, or NaNO₃ in ternary extraction system. But it decreases with the increase of salt concentration of NaCl, and this is explained by the complexing ability of the anion which is in the order ${\text{Cl}}^{-}>{\text{NO}}_3^{-}\gg {\text{ClO}}_4^{-}$ (Bagawde et al., 1978).

By considering both the HDDPA concentration effect and the pcH effect on the D_c value, the extracted species can be written in this form Th(X)₄(HR₂)_Y, where $\text{X}={\text{ClO}}_4^{-}$ and Y = 1 or 2 in perchlorate media. This means that didodecylphosphoric acid in toluene as a carrier diluent extracted thorium(IV) ion from perchlorate solution by a solvation mechanism. This mechanism can be represented by the following equilibrium equations: $$\text{Th}(\text{X}){\hspace{0.17em}}_{4\text{aq}}+Y({\text{HR}}_2){\hspace{0.17em}}_{\text{org}}\leftrightarrow \text{Th}(\text{X}){\hspace{0.17em}}_4({\text{HR}}_2){\hspace{0.17em}}_{Y\text{org}}$$ where $\text{X}={\text{ClO}}_4^{-}$ , Y = 1.0 or 2.0, (HR₂) is the didodecylphosphoric acid (HDDPA) extractant dissolved in the organic phase which is toluene.

The equilibrium constant for the extraction process can be given by: $$K_{\text{ex}}=\frac{[\text{Th}(\text{X}){\hspace{0.17em}}_4({\text{HR}}_2){\hspace{0.17em}}_Y]{\hspace{0.17em}}_{\text{org}}}{[\text{Th}(\text{X}){\hspace{0.17em}}_4]{\hspace{0.17em}}_{\text{aq}}[{\text{HR}}_2]{\hspace{0.17em}}_{\text{org}}^Y}$$ $$K_{\text{ex}}=\frac{D_{\text{c}}}{[{\text{HR}}_2]{\hspace{0.17em}}_{\text{org}}^Y}$$ where $$D_{\text{c}}=\frac{[\text{Th}(\text{X}){\hspace{0.17em}}_4({\text{HR}}_2){\hspace{0.17em}}_Y]{\hspace{0.17em}}_{\text{org}}}{[\text{Th}(\text{X}){\hspace{0.17em}}_4]{\hspace{0.17em}}_{\text{aq}}}$$ Thorium was extracted quantitatively with the neutral ligand 1-phenyl-2,3-dimethyl-5-pyrazolone (Apy) in the presence of perchlorate $({\text{ClO}}_4^{-})$ , di- and tri-chloroacetates (DCA/TCA) at pH 2.5 into an organic solvent by solvation mechanism (Roopa et al., 2005).

The suggested mechanisms for the three complexes were as follows: $${\text{Th}}_{\text{aq}}^{4+}+2{\text{Apy}}_{\text{aq}}\leftrightarrow [\text{Th}(\text{Apy}){\hspace{0.17em}}_2({\text{ClO}}_4){\hspace{0.17em}}_4]{\hspace{0.17em}}_{\text{org}}$$ $${\text{Th}}_{\text{aq}}^{4+}+2{\text{Apy}}_{\text{aq}}+4{\text{TCA}}_{\text{aq}}\leftrightarrow [\text{Th}(\text{Apy}){\hspace{0.17em}}_2(\text{TCA}){\hspace{0.17em}}_4]{\hspace{0.17em}}_{\text{org}}$$ $${\text{Th}}_{\text{aq}}^{4+}+{\text{Apy}}_{\text{aq}}+4{\text{DCA}}_{\text{aq}}{\text{H}}_2\text{O}\leftrightarrow [\text{Th}(\text{Apy})({\text{H}}_2\text{O})(\text{DCA}){\hspace{0.17em}}_4]{\hspace{0.17em}}_{\text{org}}$$ The extraction of thorium(IV) ion from perchlorate medium by HDDPA in chloroform diluents followed the mixed ion exchange–solvation mechanism (Savvin, 1961). The equilibrium reaction was found to be: $$\text{Th}({\text{ClO}}_4){\hspace{0.17em}}_{4-n\text{aq}}^{n+}+2.5(\text{HR}){\hspace{0.17em}}_{2\text{org}}\leftrightarrow \text{Th}({\text{ClO}}_4){\hspace{0.17em}}_{4-n}{\text{R}}_n(\text{HR}){\hspace{0.17em}}_{5-n\text{org}}+n{\text{H}}_{\text{aq}}^{+}$$ where n is 1 or 2.

3.4 Effect of temperature on extraction of Th(IV) ion

Effect of temperature on the K_ex value was investigated at pcH 2.00. From the data shown in Fig. 5, it is clear that the K_ex value decreases by increasing temperature.

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Figure 5

Variation of Log K_ex with 1/T in perchlorate media (I = 1.00, 3.00 and 5.00 M).

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3.5 Calculation of the extraction equilibrium constant (K_ex)

The K_ex can be calculated based on the following proposed equations for the reactions: $$\text{Th}(\text{X}){\hspace{0.17em}}_{4\text{aq}}+Y({\text{HR}}_2){\hspace{0.17em}}_{\text{org}}\leftrightarrow [\text{Th}({\text{ClO}}_4){\hspace{0.17em}}_4({\text{HR}}_2){\hspace{0.17em}}_Y]{\hspace{0.17em}}_{\text{org}}$$

The HDDPA concentration in the organic phase at equilibrium [HR]_{equ org}, can be calculated from the difference between the initial HDDPA concentration and the amount used for complexation. Substituting the previous values in Eq. (I) to calculate K_ex as shown in Table 1. From this, we can see that K_ex increases with the increase of ionic strength in perchlorate solution. This could be explained by the increase in dehydration of Th(IV) ion upon formation of complex.

3.6 Calculation of the thermodynamic parameters

From the van’t Hoffs equation $$\frac{\mathrm{\Delta }\mathrm{Log}{D}_{\text{c}}}{\mathrm{\Delta }(1/T)}=\frac{-\mathrm{\Delta }H}{2.303R}$$ ΔH can be calculated by plotting Log D_c against 1/T. The change in free energy (ΔG) can by calculated at 303 K by using the equation ΔG = −RT ln K_ex, while ΔS can be calculated at 303 K from ΔG and ΔH. All the thermodynamic values are shown in Table 1.

From the data in Table 1, it is clear that the extraction at different ionic strengths from perchlorate solutions behave similarly. Since the enthalpy changes are exothermic, the extraction process is favored (spontaneous). In addition, the entropy changes decrease with increase in ionic strength (Yaftian et al., 2004).