Supercritical carbon dioxide extraction equilibrium of gallium(III) with 2-methyl-8-quinolinol and 2-methyl-5-butyloxymethyl-8-quinolinol

This work performed fundamental studies for the extraction of gallium(III) with 2-methyl-8-quinolinol (HMQ) and 2-methyl-5-butyloxymethyl-8-quinolinol (HMO4Q) into supercritical carbon dioxide (SF-CO2) from a weakly acidic solution. The distribution constants of HMO4Q between aqueous and SF-CO2 phases were determined at 45 °C, 8.6–20.4 MPa and I=0.1 M (H, Na)NO3 (M=mol dm−3). At 45 °C and 15.7 MPa, gallium(III) was hardly extracted with HMQ into SF-CO2, but was quantitatively extracted with HMO4Q in the pH range of 2.20–2.84. The extraction constant, Kex, SF-CO2 (=[Ga(OH)(MO4Q)2]SF-CO2[H+]3[Ga3+]−1[HMO4Q]SF-CO2−2), of gallium(III) with HMO4Q was determined to be 10−2.6±0.1 at 45 °C, 15.7 MPa and I=0.1 M (H, Na)NO3, which was 63 times larger than that in heptane at 45 °C and 0.10 MPa. It was also found that the addition of 3,5-dichlorophenol as a synergist enhanced the extractability of gallium(III) with HMO4Q into SF-CO2.