Solvent Extraction Research and Development, Japan Vol. 24(2017) No. 2

The extraction characteristics of N-dodecyldiglycolamic acid (C₁₂DGAA), with a secondary amide group, for 56 metal ions have been investigated, and compared with those of N,N-dioctyldiglycolamic acid (DODGAA) with a tertiary amide group. C₁₂DGAA is capable of quantitative transfer for a variety of metal ions through a proton-exchange reaction, and the extraction behavior as a function of the aqueous-phase pH is similar for C₁₂DGAA and DODGAA. Compared with DODGAA, C₁₂DGAA has a poor extraction performance and separation ability for rare-earth metal ions, except for Sc(III). However, C₁₂DGAA tended to provide better extraction for relatively small-sized metal ions than DODGAA. In addition, it was found that C₁₂DGAA enables the selective removal of Hg(II) from aqueous solutions containing various divalent metal ions (Hg(II), Pb(II), Cu(II), Cd(II), Zn(II), Mn(II), Co(II), and Ni(II)).

Solvent extraction of valuable metals such as copper and nickel is often carried out from ammoniacal alkaline solutions with chelating extractants, during which coextraction of ammonia is inevitable. In the present study, as a basis of modeling of the whole ammoniacal extraction process, the equilibrium distribution ratios of ammonia between a 3 mol L^-1 ammonium nitrate solution and LIX84I (the active component of which is 2-hydroxy-5-nonylacetophenone oxime, HR) dissolved in a nonpolar diluent were measured as a function of pH and LIX84I concentration at 298 K. We successfully modeled the measurement results by assuming the formation of two associated species, (HR)_n･(NH₃) and (HR･NH₃)_n, the aggregation of HR, and the dissociation of NH₄⁺. Fourier transform infrared spectroscopy results suggested that ammonia was extracted via hydrogen bonding with HR, and these results were in accord with the proposed equilibrium model.

Thiol and dithioether derivatives of tripodal extraction reagents have been newly prepared and employed to investigate the extraction behavior of precious metals. Both sulfur-containing compounds did not exhibit a pH dependency for precious metal extraction. The thiol type derivative exhibited a high extraction ability for gold(III), silver and palladium(II), and a 1 : 3 (extractant : metal) stoichiometry for gold(III) and silver. The results showed that the thiol derivative exhibited little structural effect due to the strong functionality of thiol group. The dithioether derivative also extracted gold(III), silver and palladium(II), however the silver extraction is caused by the structural effect of the tripodal framework, while gold(III) and palladium(II) extraction is probably attributed to a partial structural effect. The tripodal derivative possesses a high extraction ability compared with the corresponding monopodal compound. Tripodal and monopodal derivatives showed 1 : 1 and 1 : 2 (extractant : metal) stoichiometry for silver. This means that the tripodal derivative exhibited the structural effect as a size effect, converging effect of the multi functionality, complementary effect. The coordination site of the tripodal derivative was confirmed by ¹H-NMR spectra before and after the silver loading. Stripping of the loaded silver from the tripodal derivative was also investigated. Finally, the stepwise separation of silver and palladium(II) was carried out and roughly achieved by using different eluents.

Recently, cyclopentyl methyl ether (CPME) has found use as a commercially available solvent for various applications. As CPME shows better properties such as more hydrophobicity, less solublity in water, less volatility, and more stablity compared with typical ethereal solvents, it could be used as a diluent or an extractant in liquid-liquid extraction systems. In the present study, CPME was found to be useful for the extraction of Au(III) in hydrochloric acid media. Extraction of Au(III) increased with as increase in the hydrochloric acid concentration. Au(III) was selectively extracted using CPME from other precious metal ions and base metal ions. From the result of the dependency of the Au(III) concentration, CPME can load at least 0.93 g/dm³ Au(III). Extracted Au(III) was quantitatively stripped from CPME using 0.1 M aqueous thiourea solution. As the solubility of water into CPME is much smaller than that into alcohols such as 1-hexanol, CPME is more favorable as an extractant for Au(III) in hydrochloric acid media.

Extraction and separation of Pt(IV) and Pd(II) from a hydrochloric acid solution was examined with a mixture of undiluted ionic liquids, 1-methyl-3-octylimidazolium bis(trifluoromethylsulfonyl)imide ([C₈mim][Tf₂N]) and trihexyltetradecylphosphonium chloride ([P_6,6,6,14][Cl]). Imidazolium-based [C₈mim][Tf₂N] shows a high selectivity for Pt(VI), whereas [P_6,6,6,14][Cl] has a high extraction ability for both Pt(IV) and Pd(II). The addition of [P_6,6,6,14][Cl] to [C₈mim][Tf₂N] improved the extraction efficiency for Pt and the separation factor between Pt and Pd also increased. This improvement was attributed to the presence of Cl⁻ being more hydrophilic than Tf₂N⁻ in the [C₈mim][Tf₂N] extraction phase. Stripping of the metals was possible with HNO₃ solution without any degradation of the ionic liquid extraction phase. The ionic liquid mixture was shown to be reusable for at least five extraction cycles.

The formation and aggregation of the Pd(II) complex with 2-(5-bromo-2-pyridylazo)-5-diethylaminophenol (HL) at the heptane-water interface was investigated by UV-vis absorption and resonance Raman spectrometry using the centrifugal liquid membrane (CLM) method. The interfacial aggregate had an absorption maximum wavelength at 488 nm, showing a hypsochromic shift from the absorption maximum wavelength at 580 nm of the complex monomer (PdLCl). The critical aggregation concentration of PdLCl was determined to be 2.6 × 10^-10 mol cm^-2. The observed UV-vis absorption and resonance Raman spectra implied that the ligand in the interfacial aggregate was mainly in the state of azo form, because the spectra were very similar to those of the PdLCl complex obtained in a low-polarity solvent such as toluene.

The solvent extraction of Se(IV), Zr(IV), Pd(II), and Cs(I) from nitric acid into 1-octanol (OC), or 1-octanol and n-dodecane has been performed. These elements include long-lived radionuclides in spent nuclear fuels, so a simple separation method is indispensable for the development of the treatment of high-level liquid radioactive waste. It was found that Se can be extracted using phenylenediamine, Zr(IV) can be extracted using tetraoctyl diglycolamide and di-2-ethylhexyl phosphoric acid, and Pd can be extracted using (methylimino)bis(N,N-dioctylacetamide) and N,N,N’,N’,N”,N”-hexaoctylnitrilotriacetamide. These elements can be recovered in over 90% yield by these extractants from nitric acid into OC. A distribution ratio of Cs(I) of greater than 1 can be obtained using di-t-butyldibenzo-18-crown-6. It is clear that 90% recovery of Cs(I) can be achieved using an extraction solvent with ten times the volume of the aqueous phase.

Extraction selectivity of In(III) and Ga(III) was investigated using 2-ethylhexyl thioglycolate (EHTG) with a thiol group and an oxygen atom to recover these metals from zinc refinery residue and waste solar panels. The extraction order with EHTG was Cu(II)>In(III)>Ga(III)>Zn(II), while Al(III) was hardly extracted at all. While In(III) and Ga(III) were not extracted at all with 1-dodecanethiol (DDT) with contains only a thiol group although Zn(II) and Cu(II) were extracted. This indicates that the oxygen atom in the EHTG ester plays an important role in the extraction of these metals. The mutual separation of In(III), Ga(III), Zn(II) and Cu(II) is possible in almost one step with EHTG. The extraction equilibria of In(III) and Ga(III) with EHTG are also discussed. Furthermore, the stripping of In(III) and Ga(III) extracted into the organic phase was achieved using appropriate concentrations of NaOH and acids.

Separation of cobalt and nickel was investigated using a 4-stage countercurrent mixer-settler cascade, employing 2-ethylhexyl phosphonic acid mono-2-ethylhexyl ester as the extractant. Extraction equilibrium formulations were established up to high loading ratios and extraction constants were determined, based on a batch extraction system in both single metal and binary metal systems. The separation of cobalt and nickel was then carried out using the 4-stage countercurrent mixer-settler cascade. Selective extraction of cobalt was achieved by the extraction step, while a small amount of nickel was also extracted. Nickel in the organic solution was removed by scrubbing with the solution containing the metals, while the recovery of cobalt was decreased. The concentration profiles of the metals with the countercurrent mixer-settler cascade were calculated based on the determined extraction equilibrium formulations and the material balance, although the calculation diverged when the pH was high especially at the 4th stage.

3-Hydroxypropionic acid (3-HP) is an isomer of 2-hydroxypropionic acid (2-HP) and an important platform molecule. A biological production route of 3-HP has become of interest as a sustainable alternative to the chemical route. However, its hydrophilic property makes the separation of 3-HP from the fermentation broth difficult. In this study, reactive solvent extraction and an aqueous two phase system (ATPS) were utilized to extract 3-HP from an aqueous solution. In the reactive extraction system, synergism, which was found in the reactive extraction of 2-HP, was not observed with mixed extractants of TBP and TOA diluted in heptane. TOA diluted in 1-octanol gave the highest extractability in the reactive extraction system. There was little effect on the extractability of 3-HP by the addition of salt to the system. In an ATPS composed of alcohols and salts, binodal curves and tie lines were measured, and then 3-HP was extracted with the ATPS. The highest extractability and distribution ratio were obtained by using an ATPS composed of t-BuOH and Na₂SO₄. The maximum distribution ratios in both systems were similar.