# Prussian Blue: A Safe Pigment with Zeolitic-Like Activity
## 4 Adsorption Properties of PB
### Adsorption of Cesium
Moreover, the almost totality of performed kinetic studies showed that the adsorption can be described by a pseudo-second-order kinetic model; this implies that the chemical adsorption process is the rate-limiting step, and that Cs\({}^{+}\) are adsorbed through chemisorption rather than physical adsorption [24]. However, [PERSON] et al. proposed that the external surface of PB crystal was the only bonding site for Cs\({}^{+}\) because of the extremely low intracellular diffusion coefficient (less than 3.3 \(\times\) 10\({}^{-22}\) m\({}^{2}\) s\({}^{-1}\)) [28]. Due to this slow adsorption rate, the Cs\({}^{+}\) could penetrate only 1-2 nm (1-2 units of the crystalline lattice) after 2 weeks at room temperature. [PERSON] et al. also demonstrated the high selectivity of PB; even when molar concentration of H\({}_{3}\)O\({}^{+}\) was more than 200 times higher or molar concentration of K\({}^{+}\) was more than 50,000 times higher than that of Cs\({}^{+}\) in the aqueous solution, the equilibrium adsorption amount was reduced by only approximately one-half to two-third of that the pure system [28]. Despite the high selectivity of PB and PBA toward Cs\({}^{+}\), when the sorbent is in the form of fine powder, it can produce problems at the moment of separating the sorbent from the purified solution, since fine particles can form a dense cake, which is difficult to filter. Moreover, the colloidal character of a PB suspension prevent easy separation after use. These shortcomings can be overcome by the synthesis of composites in which PB is encapsulated into a polymer matrix or located coating a support. In this way, PB or PBA were linked to a matrix formed by silica or glass [29]. The adsorbents were also incorporated into a crosslinked, bead-type copolymer resin composed of polystyrene (PS) and divinylbenzene (DVB) [30]. PB has also been immobilized by means of a linkage of polydopamine (PD) on the surface of nanofibers fabricated by electrospinningning of polyacrylamide (PAN) [31], on poly(vinyl alcohol) (PVA) sponge [32; 33], caged in a gel of sodium alginate [34], in the same colloid to develop an intestinal release delivery system [35], in spongiform adsorbents [36], bonded to sepiolite (hydrated magnesium silicate) [37], embedded into a binder matrix [38], in porous cellulose aerogel [39] or in microporous carboxymethyl cellulose nanofibrils membranes [40]. [PERSON] et al. found that PB/PBA precipitated spontaneously in agarose gels, and these materials were able to adsorb Cs\({}^{+}\) ions effectively [41]. [PERSON] et al. prepared a clay-based composite hydrogel containing PBA nanoparticles (potassium copper hexacyanoferrate). The hydrogel presented a maximum adsorption capacity in water of \(\sim\)173 mg/g [42]. A good strategy to separate the contaminate product from the water is to coat PB on superparamagnetic nanoparticles. The magnetic characteristics of this core/shell nanocomposite make suitable the use of an external magnet to separate the contaminated nanoparticles [43; 44; 45; 46; 47] In this way, [PERSON] et al. prepared magnetic nanoparticles in the presence of poly(diallydimethylammonium chloride) (PDDA). Then, the hexacyanoferrate (II) ions react with the ferric ions released from magnetic nanoparticles (single-precursor method) [48]. However, PDDA has no superparamagnetism and Cs affinity, and thus reduces the magnetic and adsorption ability of PB-coated magnetic adsorbents. [PERSON] et al. have reported the preparation of PB-functionalized magnetic nanoclusters. The magnetic nanoclusters were synthesized using the hydrothermal method and were then coated with PB by the single-precursor method [49]. With this method, [PERSON] et al. prepared PB-coated magnetic nanoparticles decorated with polyethylene glycol (PEG) to remove Cs from blood [50]. [PERSON] et al. have prepared magnetic microparticles of PB. The microparticles coated an electrode and were used as adsorption/desorption materials in the electrochemically switched ion exchange (ESIX) technique [51].