A new approach using water as the stripping phase for extraction and recovery of low-concentration Ga (III) from concentrated hydrochloric acid solutions by Aliquat 336-based supported liquid membrane (SLM) is suggested. Generally, the stripping of Ga (III) from the loaded organic phase during liquid-liquid two-phase stirring extraction using quaternary ammonium salt as an extractant is extremely difficult. However, in the present work, an efficient mass transfer of low-concentration Ga (III) through the SLM was observed by introducing a low acidity (pH=2) aqueous solution as the stripping phase of the SLM. It was revealed that dissociation of GaCl 4 - and HCl 2 - anions into Ga 3+ , Cl - and H + ions at the stripping/membrane phase interface play an important role in driving the mass transfer of GaCl 4 - from the concentrated hydrochloric acid solutions towards stripping phases. Kinetic analysis suggests that the permeability coefficient of Ga (III) ions through the SLM is dependent on the reaction temperature, extractant Aliquat 336 concentrations, and the difference in Cl - concentrations between the two sides of the liquid membrane phase , but independent of H + concentrations. In view of future’s applications for selective separation and recovery of low-concentration Ga (III) from the concentrated hydrochloric acid leaching solutions of various gallium-containing solid wastes, separation selectivity of Ga (III) against other coexisting metal impurity ions, such as Fe (II), Al (III), Mg (II) and Cu (II), was also evaluated.
The unique characteristics of the interface environment that distinguish bulk phases can promote or inhibit certain reactions and significantly change chemical reactivity. In this work, the separation of Rh (III) and Fe (III) on the surface of concentrated hydrochloric acid was studied by a thin-layer oil membrane (TOM) extraction based on laminar flow with 18-Crown-6-ether (18C6) as the extractant. A new phenomenon different from conventional extraction was found. The TOM extraction prefers to identify hydrated Rh (III) anions rather than H 3 O + ions at the interface due to a lower concentration of free hydrated hydrogen ions on the surface of concentrated hydrochloric acid. However, during the conventional extraction, the H 3 O + ions in the bulk phase are preferentially extracted by the 18C6, and then the hydrated rhodium (III) anions could be extracted by the protonated crown ether through the ion association mechanism. Besides, it was found that the TOM extraction could enhance extraction of hydrated rhodium (III) anions by taking advantage of the uniqueness of rhodium species on the interface, and by adjusting the thickness of the oil membrane and the flow rate of the aqueous phase . The decrease in the oil membrane thickness and the increase in the flow rate of the aqueous phase are conducive to the complexation of 18C6 with the hydrated rhodium (III) at the interface, which can better reflect the targeting and the controllability of extracting single species of hydrated rhodium (III) ions at the interface. Considering the applications of selective recovery of Rh (III) from concentrated hydrochloric acid leaching solution of waste automobile catalyst, the selective separability of Rh (III) to other coexisting metal impurity ions such as Fe (III), Cu (II), Co (III) and Ni (II), was also evaluated. This work provides a new strategy for preferential recovery and separation of rhodium in concentrated hydrochloric acid leach solutions containing other base metal ions for possible future's application.
This study proposes a new thin-layer oil membrane (TOM) extraction strategy for enhanced mutual separation of Pd(II) and Pt(IV) chlorocomplex anions in concentrated aqueous hydrochloric acid solutions using di- n -butyl sulfoxide (DBSO) as an extractant. It was discovered that Pd(II) could be preferentially extracted even in concentrated hydrochloric acid solutions. The separation coefficient between Pd(II) and Pt(IV) considerably increased than that between them during conventional extraction using a separating funnel. A new extractor was designed to perform TOM extraction by spreading a thin layer of organic extractant liquid membrane on the surface of aqueous feed solutions under laminar flow . A simplified TOM extraction model and controlled experiments were used to investigate the effect of H + and Cl − ion concentrations on the mutual separation between Pd(II) and Pt(IV). Molecular dynamics (MD) simulations were used to understand the microscopic separation mechanisms at the molecular level. Experimental results revealed that the competitive distribution of H + and Cl − ions in the boundary layer along the normal vertical direction of the organic-aqueous interface are crucial in enhancing the separation between Pd(II) and Pt(IV). The competitive adsorption and enrichment of Cl − ions at the interface prevent DBSO molecules from being protonated by H + ions, which is beneficial for the substituted coordination of DBSO molecules with PdCl 4 2− . However, the ionic association of protonated DBSO molecules with PtCl 6 2− remains dominant at the interface. As a result, even in concentrated hydrochloric acid solutions, the difference in mass transfer between PdCl 4 2− and PtCl 6 2− toward the interface was intensified.
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