Proton-exchange mechanism of specific Cs+ adsorption via lattice defect sites of Prussian blue filled with coordination and crystallization water molecules

Abstract

We have revealed the fundamental mechanism of specific Cs⁺ adsorption into Prussian blue (PB) in order to develop high-performance PB-based Cs⁺ adsorbents in the wake of the Fukushima nuclear accident. We compared two types of PB nanoparticles with formulae of Fe^III ₄[Fe^II(CN)₆]₃·xH₂O (x = 10–15) (PB-1) and (NH₄)_0.70Fe^III _1.10[Fe^II(CN)₆]·1.7H₂O (PB-2) with respect to the Cs⁺ adsorption ability. The synthesised PB-1, by a common stoichiometric aqueous reaction between 4Fe³⁺ and 3[Fe^II(CN)₆]⁴⁻, showed much more efficient Cs⁺ adsorption ability than did the commercially available PB-2. A high value of the number of waters of crystallization, x, of PB-1 was caused by a lot of defect sites (vacant sites) of [Fe^II(CN)₆]⁴⁻ moieties that were filled with coordination and crystallization water molecules. Hydrated Cs⁺ ions were preferably adsorbed via the hydrophilic defect sites and accompanied by proton-elimination from the coordination water. The low number of hydrophilic sites of PB-2 was responsible for its insufficient Cs⁺ adsorption ability.