Fig. 1. Flow chart of three-stage continuous countercurrent extraction. The separatory funnel with numbers represents a single extraction. A(1-3) and O(1-3) represent the aqueous and organic phases after extraction to reach equilibrium, respectively.

2.4 Analysis

Metal ions concentration of aqueous phase was determined by inductively coupled plasma-atomic emission spectroscopy (ICP-AES) (ICAP6500 DUO; America Thermo Scientific). The concentrations of boric acid in aqueous phase were determined by titration[27], but the trace amounts of boric acid were measured by ICP-AES. The boron in the organic phase was stripped into the aqueous for analysis, so the accuracy of the analysis can be verified by the mass balance of the boron in the two phases. The pH of all liquid samples was measured by a pH meter (Seven excellence, Mettler Toledo). After extraction, the organic phases with and without boron complexes were measured by FTIR spectra (Thermo Nicolet Corporation 670 Spectrometer). Raman spectrometer (DXR; American Thermo Fisher Scientific Co., Ltd.) has been used to characterize the presence of boron in the aqueous phase.
In this study, the extraction efficiency (E ) and distribution ratio (D ) were calculated by the formulas[28]:
E =\(\frac{\text{\ \ \ \ }V_{\text{org\ }}\ {*\ \ \left[B\right]}_{\text{org\ \ \ }}}{\ V_{\text{aq\ }}\ {*\ \ \left[B\right]}_{\text{aq\ \ \ }}+\ \ V_{\text{org\ }}\ {*\ \ \left[B\right]}_{\text{org\ \ \ }}}\)*100% Eq. (1)
D =\(\frac{\text{\ \ \ }{\text{\ \ }\left[B\right]}_{\text{org\ \ \ \ \ }}}{{\text{\ \ \ \ \ }\left[B\right]}_{\text{aq\ \ \ \ \ }}}\)Eq. (2)
where [B]org and [B]aq are the equilibrium concentrations of boron in the organic and aqueous phases, respectively.