Synergistic Extraction Equilibrium of Gallium(III) with Di(2-ethylhexyl)phosphoric Acid and 8-Quinolinol Derivatives into Supercritical Carbon Dioxide

Abstract

The synergistic effect of five 8-quinolinol derivatives (HQs) on the extraction of gallium(III) with di(2-ethylhexyl)-phosphoric acid (D2EHPA or HA) from weak nitric acid into supercritical carbon dioxide (SC-CO₂) was investigated. 8-Quinolinol (Hq), 5-ethoxymethyl-8-quinolinol (HO₂q), 5-hexyloxymethyl-8-quinolinol (HO₆q), 5-(2,2,2-trifluoroethoxymethyl)-8-quinolinol (HFO₂q), and 5-chloro-8-quinolinol (5-Cl-Hq) were used as synergists. Synergism was observed for all of the HQs. The synergistic effect increased in the following order: Hq < HO₂q < HFO₂q < 5-Cl-Hq ≈ HO₆q. The extent of synergism can be related to both the distribution constant and the acid dissociation constant of the HQ. The synergistic extraction equilibrium of gallium(III) with D2EHPA and 5-Cl-Hq into SC-CO₂ was investigated; the results suggest that the composition of the extracted complex is as GaA₂Q·HA. The synergistic extraction mechanism was the same for both SC-CO₂ and n-heptane. The synergistic extraction equilibrium constants in both cases were calculated based on the experimental results.