Cs sorption by "soluble" and "insoluble" iron hexacyanocobaltates probed by Mössbauer spectroscopy

Efficient sorbents of radiocesium are essential for purifying the radioactive contaminated wastewaters and soils. Cyanometallates analogous of Prussian Blue possess a very high sorption capacity and selectivity for Cs. Aiming to explore the mechanisms of Cs fixation in cobalticyanides our study focuses on the Mössbauer spectra in cesium-embedding ferrous hexacyanocobaltates prepared by three different routes: the precipitation of CsFe[Co(CN)6], ion exchange of K for Cs in "soluble" KFe[Cs(CN)6] and Cs intercalation into "insoluble" porous Fe[Cs(CN)6]2/3. The Cs capture is accompanied by removal of anionic vacancies and densifying the porous framework that is seen in Mössbauer spectra through increasing the population of the Fe2+ species enclosed into FeN6 octahedra. The number of water molecules coordinating to iron, on average, decreases. This leads to reduction of the asymmetry in distribution of the Fe2 +  valence electron over d-orbitals and reduces also the valence term of electric field gradient. An exception to this rule applies where recrystallization is prevented during the Cs intrusion into existing dense phase at low temperature.