Orthogonal array design for the optimization of stripping Sr(II) from ionic liquids using supercritical CO2

The strontium ions extracted from the aqueous phase into 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (C2mimNTf2) with dicyclohexyl-18-crown-6 (DCH18C6) was stripped effectively by supercritical CO2 (sc-CO2). Hexafluoroacetylacetone (HFAA)-acetonitrile was found to be an excellent modifier of sc-CO2 to enhance the stripping efficiency. In the orthogonal array design (OAD), OA25 (55) matrix was employed to optimize the stripping of Sr(II) from the DCH18C6-C2mimNTf2 system. Effects of five experimental factors: temperature, pressure, concentration of HFAA, static and dynamic extraction times as well as each factor at five-levels on the stripping of Sr(II) were optimized. The effects of these parameters were treated by the analysis of variance (ANOVA). The results showed that Sr(II) could be nearly 100% extracted from the IL phase at 308 K, 30 MPa, 40 min of dynamic extraction and 60 mmol·L− 1 HFAA in acetonitrile, respectively. Finally, the stripping mechanism was studied by ESI-MS.

Graphic AbstractThe strontium ions extracted from the aqueous phase into 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (C2mimNTf2) with dicyclohexyl-18-crown-6 (DCH18C6) was stripped effectively by supercritical CO2 (sc-CO2). Hexafluoroacetylacetone (HFAA)-acetonitrile was found to be an excellent modifier of sc-CO2 to enhance the stripping efficiency.95