Aqueous geochemistry at gigapascal pressures: NMR spectroscopy of fluoroborate solutions

Abstract

Aqueous geochemistry could be extended considerably if nuclear-magnetic resonance (NMR) methods could be adapted to study solutions at elevated temperatures and pressures. We therefore designed an NMR probe that can be used to study aqueous solutions at gigapascal pressures. Fluoride solutions were chosen for study because 19 F couples to other nuclei in the solutions (31 P and 11 B) in ways that make peak assignments unequivocal. Correspondingly, NMR spectra of 19 F-and 11 B were collected on aqueous HBF 4-NH 4 PF 6 solutions to pressures up to 2.0 GPa. At pressure, peaks in the 19 F spectra were clear and assignable to the BF 4 À (aq), F À (aq) and BF 3 OH À (aq) ions, and these aqueous complexes varied in signal intensity with pressure and time, for each solution. Peaks in the 11 B spectra at pressure could be assigned to the BF 4 À (aq) and BF 3 OH À (aq) species. Additionally, there is a single peak that is assignable to H 3 BO 3 o (aq) and B(OH) 4 À (aq) in rapid-exchange equilibria. These peaks broaden and move with pressure in ways that suggest reversible interconversion of borate and fluoroborate species. The PF 6 À ion was found to provide a suitable 19 F shift and intensity standard for high-pressure spectra because it was chemically inert. The positions and intensities of the doublet peak also remains constant as a function of pressure and pH. Addition of electrolytes considerably distorts the phase diagram of water such that the stability region of the aqueous solution expands to well beyond the 0.8 GPa freezing pressure of pure water; some fluoroborate solutions remain liquid until almost 2.0 GPa.

Molecular identification guided process design for deep removal of fluoride from electroplating effluent

“…The peak at −150.7 ppm was ascribed to fluoroborate ions (BF − 4 ). 30 Moreover, the results in Fig. 2d indicated two main peaks with a chemical shift of 8.9 and −1.5 ppm 34,35 appearing in the 11 B NMR spectrum of the secondary effluent from LXWWT, corresponding to boric acid (H 3 BO 3 ) and BF − 4 , respectively.…”

“…2c and S3,† there were two main peaks (−121.3 and −150.7 ppm) in the wastewater sample from every section of LXWWT. Considering that the chemical shift of 19 F NMR was extremely sensitive to the surrounding chemistry such as complex configuration, solution pH, and even fluoride concentration, 30 the commercially available fluoride species including NaF and NaBF 4 were used as the references. Obviously, the peak at −121.3 ppm corresponded to the free fluoride ions, 31 some of which might integrate with the coexisting organic matter through electrostatic attraction or hydrogen bonding.…”

“…S3, there were two main peaks (-121.3 and -150.7 ppm) in the wastewater sample from every section of LXWWT. Considering that the chemical shift of 19 F NMR was extremely sensitive to the surrounding chemistry such as complex configuration, solution pH, and even fluoride concentration, 30 the commercially available fluoride species including NaF and NaBF 4 were used as the references.…”

“…25 The resonances of fluoride compounds did not overlap with those of 13 C and 1 H, making 19 F NMR very promising in analyzing the complex matrix like industrial wastewater. 25 Moreover, it has been widely reported that minor change in the chemical environment might cause significant chemical shifts of 19 F NMR spectra, 30,31 and thereby it is possible to identify all the fluoride compounds in the industrial effluents. In spite of those merits, the attempts at employing 19 F NMR for the molecular identification of fluoride contaminants were rarely reported.…”

Size- and shape-adaptable mixed-alkoxide-aggregates: encapsulation of tetrafluoroborate and butadiynediide

“…The high-resolution solution NMR of BF 4 @1 allows the observation of the 11 B– 19 F spin–spin coupling constants of 12 Hz. In aqueous solutions of NaBF 4 , the 11 B– 19 F splitting 14 is considerably smaller (2–5 Hz) and dependent on the concentration. 15 IR spectroscopy of BF 4 @1 reveals two absorption bands at 1092 cm −1 ( ν as , very strong) and 542 cm −1 ( δ , weak); both results are in accordance with the corresponding modes in solid NaBF 4 .…”