Hydroxide promotes ion pairing in the NaNO₂-NaOH-H₂O system

Area of Science:

• Solution Chemistry
• Nuclear Waste Management
• Materials Science

Background:

• Nitrite (NO2-) behavior in solution is complex, affecting nuclear waste treatment by influencing solubility in sodium hydroxide (NaOH)-rich environments.
• Understanding ion pairing is crucial for developing accurate thermodynamic models for nuclear waste processing.

Purpose of the Study:

• To investigate molecular-scale sodium nitrite (NaNO2) and NaOH ion-pairing processes.
• To characterize the solubility of NaNO2 in aqueous NaOH mixtures.
• To elucidate the impact of these interactions on water structure and solution properties.

Main Methods:

• High-field Nuclear Magnetic Resonance (NMR) spectroscopy (23Na, 15N, 1H).
• Raman spectroscopy.
• X-ray total scattering pairwise distribution function (X-ray PDF) analysis.
• Pulsed field gradient NMR for diffusion coefficient measurements.

Main Results:

• NMR studies indicated changes in Na+ and NO2- chemical environments, suggesting ion pairing even when Raman bands were insensitive.
• Ion pairing correlated with decreased diffusion coefficients of solution species.
• X-ray PDF and Raman analyses revealed disruption of the water hydrogen network, forming a contracted OO correlation structure in concentrated NaNO2 and NaOH mixtures.
• Nitrite exhibited greater ion pairing in concentrated NaNO2/NaOH mixtures compared to solutions with only NaNO2.

Conclusions:

• The study provides molecular-level understanding of NaNO2 and NaOH ion pairing, essential for thermodynamic modeling in nuclear waste treatment.
• Concentrated NaNO2 and NaOH mixtures alter water structure and enhance ion pairing.
• Findings are critical for managing solubility and predicting behavior in complex industrial waste streams.