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

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 (HO(2)q), 5-hexyloxymethyl-8-quinolinol (HO(6)q), 5-(2,2,2-trifluoroethoxymethyl)-8-quinolinol (HFO(2)q), 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 < HO(2)q < HFO(2)q < 5-Cl-Hq approximate to HO(6)q. 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 GaA(2)Q center dot 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.