1.1. Structure

Prussian blue (PB) is a dark blue pigment synthesized by ferrous ferrocyanide salts with chemical formula Fe₇(CN)₁₈. It has a porous structure with the capacity to adsorb the ¹³⁷Cs⁺ ions into its pores and store them there. It is a metal organic framework (MOF) where the inorganic vertices, which donate electrons in the structure, are linked to each other via organic compounds. The complete chemical formula is Fe^III₄[Fe^II(CN)₆]₃·xH₂O. The compound has a face-centered cubic structure (FCC) structure (Figure 1) belonging to the Fm3̅m space group with a lattice parameter of 10.166 Å. Fe exists in two oxidation states within the structure: Fe³⁺ and Fe²⁺. These ions form two different FCC lattices displaced by half a lattice parameter with respect to each other. However, the bi and tri-valent Fe are coordinated differently. Furthermore, they are linked to each other via cyanide groups (C ≡ N) i.e., C groups are linked to Fe²⁺ and N groups to Fe³⁺ with high and low spins respectively, in octahedral configurations. The Fe²⁺ and Fe³⁺ ratio of 3:4 implies that in order to obtain a charge neutrality within the structure a 25% vacancy of [Fe⁺²(CN)₆]⁴⁻ molecules is necessary ^[1]. Coordinated water molecules occupy the resulting octahedral cavities created by such vacancies; six water molecules are linked to Fe²⁺. The other interstitial water molecules occupy the eight corners of the unit cell (¼, ¼, ¼) and are essential for the insertion of the ¹³⁷Cs⁺ ions in the structure.

Figure 1. Framework of Prussian blue analogues. Adapted with permission from ^[2], Copyright RSC, 2012.

Fe⁺² can be replaced by other transition metals with the same +2 oxidation states such as Ni, Mn, Cu and Co, coordinated exactly like Fe⁺² in the structure and are called PB analogs. However, there are reports of Cd and Zn with slightly larger atomic radii also being incorporated into the structure owing to their +2 oxidation state ^[3]. The aim in including different species into the structure is to provoke a distortion of the PB lattice by producing vacancies of the high spin state molecule along with distortions in the vacant cages, in order to facilitate the capture and sequestration of the ¹³⁷Cs^{+ [4]}.

1.2. ¹³⁷Cs⁺ Ion Capture Mechanism in Prussian Blue

The compound is insoluble in water and the basic mechanism consists of ion exchange of ¹³⁷Cs⁺ and H⁺ with the former occupying hydrophilic vacancies ^[5]. PB analogs have very different mechanisms of ion exchange or capture depending upon the anionic and alkali metal cation concentrations. Since PB and its analogs contain large amounts of interstitial and coordinated water, ¹³⁷Cs⁺ is captured by a defect created by a [Fe⁺²(CN)₆] vacancy, which creates a spherical cavity whose size is equivalent to the hydration radius of ¹³⁷Cs⁺. Nevertheless, recent calculations have demonstrated that a completely dehydrated ¹³⁷Cs⁺ ion can be incorporated into the structure with the release of a water molecule from the interstitial sites ^[6]. This is similar to certain clays, where on dehydrating the interlayers the ¹³⁷Cs⁺ selectivity increases ^[7]. On the other hand, water soluble analogs such as metal hexacyanoferrates (HCF) consisting of a alkali metal cation with a [Fe⁺²(CN)₆] anion, used for the extraction of ¹³⁷Cs⁺ have shown less efficiency. In such compounds Na⁺ or K⁺ are incorporated during the synthesis of the MOF in order to render them water-soluble ^[8]. In addition to ¹³⁷Cs⁺ capture mechanisms for non-soluble analogs; the water-soluble analogs mainly depend on the Na⁺ or K⁺ ion exchanges with Cs⁺. Takahashi et al., have studied the ¹³⁷Cs⁺ uptake in KCuHCF PB analog in order to understand their lower adsorption capacity ^[8]. Three main mechanisms governed the ¹³⁷Cs⁺ ion exchange according to them, with the ¹³⁷Cs⁺-K⁺ ion exchanges being predominant, as also stipulated by other research groups. In case of low anionic vacancies, the percolation of ¹³⁷Cs⁺ through the vacancies was prevalent. Finally, for low K⁺ incorporation in the structure, proton exchange between ¹³⁷Cs⁺ and K⁺ ions was evidenced. Ayrault et al., report a degradation in the crystal structure of the KCuHF soluble compound after ¹³⁷Cs⁺ adsorption which was not observed in the non-soluble counterpart ^[9].

1.3. Nanostructured Prussian Blue

¹³⁷Cs⁺ adsorption in PB crystals is a very slow process. Fujita et al., have demonstrated that after two weeks of adsorption experiments, the depth of ¹³⁷Cs⁺ adsorption was at most between 1–2 nm, irrespective of the crystal size ^[10]. This implies that most of the adsorption occurs on the surface of the crystallites. This low diffusion depth is mainly attributed to the blocking of the vacancies by captured ¹³⁷Cs⁺ ions, which in turn hinders further ¹³⁷Cs⁺ diffusion. Since the diffusion depth appears to be a constant, increasing the specific surface would therefore be a solution to increasing the ¹³⁷Cs⁺ uptake. One way of augmenting the specific surface is by synthesizing nanoparticles of PB. The surface to volume ratio of crystallites increases as their size decreases; therefore nanoparticles have an extremely large surface to volume ratio. For example, a 3 nm nanoparticle will have 50% of its atoms on its surface. This would also imply that in the case of PB nanocrystals most of the vacancies and sites responsible for ¹³⁷Cs⁺ adsorption would be available on the surface, thus enhancing its specific surface. To this end, different research groups have produced various PB analogs of type Metal(M)-Co, where the nature of M defines the efficiency of the uptake. Liu et al., have demonstrated that Zn assisted Fe-Co PB analogs present high ¹³⁷Cs⁺ uptake efficiency ^[11]. They also observed that the size of the PB analog particle reduced with the reduction of Fe in the structure; for pure Zn-Co analogs, a crystallite size of ~73 nm was calculated, displaying the highest ¹³⁷Cs⁺ adsorption as depicted in Figure 2.

Figure 2. Adsorption efficiency of Fe-Co and Zn-Co prussian blue (PB) analogs as a function of time. The Zn-Co PB analog exhibits a higher Q_t (Q_t is the adsorption capacity per unit gram of the sorbent at a given time t) owing to the reduction in size of the nanoparticles. Adapted from ^[11] under the Creative Commons agreement from RSC, 2017.

Considering that the ¹³⁷Cs⁺ adsorption depth is only about 1–2 nm, hollow PB nanoparticles may offer many more advantages. They not only have a very active surface area due to their large surface to volume ratio but their hollow interior is also capable of capturing and storing ¹³⁷Cs^{+ [12]}. A surfactant polyvinylpyrrolidone (PVP) was used to stabilize the nanoparticle and increase their dispersion in aqueous solutions. Figure 3 compares the efficiency of solid and hollow PB cubes of ~200 nm, in the capture of ¹³⁷Cs⁺. The elemental mapping of Figure 3B depicts a higher concentration of captured ¹³⁷Cs⁺ ions within the hollow structures than the filled ones in Figure 3A. Nevertheless, ¹³⁷Cs⁺ diffusion depth greater than 2 nm would require higher activation energy at room temperature. Other methods are required to determine the exact diffusion depth in such structures, as these results are mainly qualitative. Besides, it is well known that the use of a surfactant shields the active sites and prevents the capture of ¹³⁷Cs⁺. In order to avoid such shielding effects, PB could be coated onto support materials instead through hydroxyl bonds that anchor the PB to the support material. Carboxylic groups also tend to immobilize the PB particles in a sturdier manner. Wi et al., used a polyvinyl support surface functionalized with acrylic acid. This allowed converting the OH groups to COOH and providing a better adhesion of the PB ^[13]. The PB nanoparticles were immobilized on the PVP sponge and an increase in ¹³⁷Cs⁺ uptake efficiency by five times was reported, compared to the hydroxyl bond functionalization.

Figure 3. Elemental mapping images of solid (A) and hollow PB (B) nanoparticles of 190 nm in diameter. (a) Dark-field TEM image, (b) elemental mapping of both Fe and Cs (c) elemental mapping of Fe, and (d) elemental mapping of Cs. Adapted with permission from ^[12], Copyright RSC, 2012.