An XAFS study of Cs adsorption by the precipitation bands of Mn–Fe-based Prussian blue analogues spontaneously formed in agarose gel

The adsorption of Cs+ ions by the precipitation bands of a Mn–Fe based Prussian blue analogue (Mn–Fe PBA) that form spontaneously in agarose gel was investigated by X-ray absorption fine structure spectroscopy coupled with scanning electron microscopy (SEM) and X-ray fluorescence (XRF) distribution analysis. Two gel samples were prepared by contacting a gel containing 0.05 M [Fe(CN)6]3− and 2.3 mass% agarose with a 0.50 M MnSO4 solution, into one of which a 0.10 M CsCl solution was introduced. The SEM images and the XRF intensity distributions reveal that Mn–Fe PBA forms cubic crystallites (approx. 3 × 3 × 3 μm in size) in the gels that trap Cs+ ions with considerably high affinity. Cs L3-edge and Mn K-edge X-ray absorption near-edge structure (XANES) spectra, which were analyzed with the aid of FEFF simulations, strongly suggest that Cs adsorption occurs at relatively large defect sites close to the sub-cube faces in the PBA. This suggestion is supported by Cs L3-edge extended X-ray absorption fine structure spectroscopy, which suggested that the first and second coordination shells around the Cs+ ions are at a Cs–O distance of 0.35 ± 0.02 nm and a Cs–N distance of 0.43 ± 0.01 nm, respectively, with coordination numbers of 1.5 ± 0.5 and 3.0 ± 0.5. The Mn K-edge XANES data also suggest that H2O molecules, which initially occupy many cubic centers in the Mn–Fe PBAs, are mostly displaced during Cs adsorption. These findings provide valuable insight toward fully understanding Cs adsorption by Mn–Fe PBA.