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.