Solution extraction of several lanthanides from nitric acid with isohexyl-BTP in [Cnmim][NTf2] ionic liquid

The extraction behavior of several lanthanides (La3+, Eu3+, Lu3+) from nitric acid (HNO3) solution was studied using a novel extraction system based on 2,6-bis(5,6-dihexyl-1,2,4-triazin-3-yl) pyridine (isohexyl-BTP) as extractant in 1-alkyl-3-methylimidazolium bis(trifluoromethylsufonyl)imide ([Cnmim][NTf2]) ionic liquid. Isohexyl-BTP in ionic liquids exhibited remarkably better extraction performance for lanthanides than that in octanol-dodecane (3:7 v/v) system. Lower HNO3 concentration and short alkyl chain length of [Cnmim]+ were more favourable for removal of lanthanides. Besides, it was confirmed that isohexyl-BTP in ILs formed a 1:3 complex [Ln(BTP)3(NO3)n](3–n)+ (n=0, 1) by slope analysis. In addition, [C2mim][NTf2] preferred to extract lanthanides via a cation exchange mechanism at lower acidity, which was proved via UV-Vis measurement. It was speculated that extraction mechanism shifted from cation exchange to neutral species extraction with increase in HNO3 concentration and alkyl chain length of [Cnmim]+ due to the H+ completion, NO3− inhibition and hydrophobicity of IL.

Graphical AbstractDependence of extractability of lanthanides on HNO3 concentration using isohexyl-BTP in [C2mim][NTf2] (closed symbols and solid line) or in octanol-dodecane (3:7 v/v.) (open symbols and broken line). Aqueous phase: [La3+](squares)=[Eu3+](circles)=[Lu3+](triangles)=2 mmol/L. Extracting phase: [isohexyl-BTP]=25 mmol/L when using [C2mim][NTf2] or [isohextyl-BTP]=100 mmol/L when using octanol-dodecane (3:7 v/v) (Extraction time 48 h, O:A=1:1, T=25 °C)Figure optionsDownload full-size imageDownload as PowerPoint slide