Fig. 10. Effect of temperature on boron extraction. [TMPD] = 0.4 mol/L; pHini = 4.6; O/A = 1.

3.2 Spectroscopic studies of the organic and aqueous phases

Raman spectroscopy and FTIR spectroscopy can provide specific information on chemical bonds in molecules by detecting the radiation absorption caused by vibrational patterns[30]. Raman is more suitable for studying the structural characteristics of molecules in aqueous solutions. In previous studies, Raman spectroscopy has been used for the characterization of B(OH)3 and [B(OH)4]-, whose main absorption peaks were assigned as 875 cm-1 and 750 cm-1, respectively[33,34]. Therefore, following aqueous phase trials using pH values ranging from 1 to 7 and a H3BO3 concentration of 0.2 mol/L (dissolved in saturated magnesium chloride solution), Raman spectra of aqueous samples were obtained and presented in Fig. 11. With the increase of pH, the intensity of the absorption peak of H3BO3 decreases gradually, and the absorption peak of [B(OH)4]-increases gradually, indicating that the existing form of boron in aqueous solution was gradually converted from B(OH)3 to [B(OH)4]-. Then, TMPD/CCl4 extraction system was used to extract these aqueous solutions. The experimental results are shown in Table 3. When pH ≤2, the formation of binary boric acid ester and the release of H+ reduce the pH in the solution. The results of extraction efficiency indicate that B(OH)3 is easier to complex with TMPD, so the extraction efficiency is higher under acidic conditions.
Fig. 11. Raman spectra of boron-containing brine at different pH values.
Table 3. Results of extraction of boron-containing saturated magnesium chloride solution at different pH values using TMPD.