PNNL

Abstract

Understanding ion pairing in concentrated alkaline electrolytes facilitates predicting and controlling chemical processes within nuclear waste. Whereas sodium ions are the major alkali in nuclear waste, cesium ions often exhibit distinctive bulk properties compared to solutions of other alkali cations. Aqueous mixtures of cesium hydroxide and sodium nitrite solutions were compared to alkali hydroxide analogues using small-angle X-ray scattering, which revealed that cesium is not compatible with the sodium nitrite electrolyte structure, forming cesium-rich domains. This is distinct from solution structures observed for the other mixed sodium nitrite - hydroxide systems. Raman spectroscopy supports cesium nitrite ion pairing, characterized by specific vibrational shifts distinct from sodium nitrite interactions. Multinuclear nuclear magnetic resonance spectroscopy confirmed spectroscopic signatures of cesium-nitrite and cesium-hydroxide ion pairing, revealing a disparate local environment for nitrite ions in cesium hydroxide solutions. These findings collectively demonstrate that cesium uniquely leads to a solution architecture dominated by specific ion pairing within segregated domains. This description of cesium's ion pairing phenomena and its consequent structural organization provide insight into potentially altered radical generation pathways in complex, high-ionic-strength alkaline media relevant to nuclear waste processing and management.