The Uptake Characteristics of Prussian-Blue Nanoparticles for Platinum-Group Metal and Molybdenum Ions in a Nitric Acid Solution Toward Application for the Recovery of Precious Metals from Nuclear and Electronic Wastes

We have examined the uptake mechanisms of platinum-group-metals (PGMs) and molybdenum (Mo) ions into PBNPs in a nitric acid solution for 24-h sorption test, using inductively coupled plasma atomic emission spectroscopy, powder XRD, and UV-Vis-NIR spectroscopy in combination with rst-principles calculations, and revealed that the Ru 4+ and Pd 2+ ions are incorporated into PBNPs by substitution with Fe 3+ and Fe 2+ ions of the PB framework, respectively, whereas the Rh 3+ ion is incorporated into PBNPs by substitution mainly with Fe 3+ and minorly with Fe 2+ ion, and Mo 6+ ion is incorporated into PBNPs by substitution with both Fe 2+ and Fe 3+ ions, with maintaining the crystal structure before and after the sorption test. Assuming that the amount of Fe elusion is equal to that of PGMs/Mo substitution, the substitution eciency is estimated to be 39.0% for Ru, 47.8% for Rh, 87% for Pd, and 17.1% for Mo 6+ . This implies that 0.13 g of Ru, 0.16 g of Rh, 0.30 g of Pd, and 0.107 g of Mo can be recovered by using 1g PBNPs with a chemical form of KFe(III)[Fe(II)(CN) 6 ]. distribution of the melter. Considering that Ru, Rh, and Pd with an amount of 2.09, 0.36, and 1.20 kg, respectively, are generated in 1 t of used nuclear fuels (burnup: 30,000 MWd/t, cooling period: 150 days) for light-water reactors, it is useful to recover PGMs from N-wastes not only for the vitrication processes but also for recycling of precious metals from our alternative perspective. In the present study, we have revealed the uptake mechanisms of PGMs and molybdenum (Mo) ions into PBNPs in a nitric acid solution, using inductively coupled plasma atomic emission spectroscopy, powder XRD, and UV-Vis-NIR spectroscopy in combination with rst-principles calculations, and demonstrated that 0.13 g of Ru, 0.16 g of Rh, 0.30 g of Pd, and 0.107 g of Mo can be recovered by using 1g PBNPs with a chemical form of KFe(III)[Fe(II)(CN) 6 ]. The present ndings indicate that PB (or its analogues) will be one of the hopeful candidates to develop the recycling of precious metals from E- and N-wastes.


Introduction
Recovery of precious metals from both nuclear wastes (N-wastes) and electronic wastes (E-wastes) plays one of key roles in solving energy and environmental issues in order to maintain our sustainable developing society. For the former one (N-wastes), the spent nuclear fuel generated from the power plants are vitri ed at the reprocessing plant. After separating uranium (U) and plutonium (Pu) from the spent fuels by using the PUREX (Plutonium Uranium Redox EXtraction) method in order to re-use them as new fuels, the high-level radioactive liquid wastes (HLLW) are vitri ed and geologically disposed. [1][2][3] In the vitri cation processes, platinum-group metals (PGMs: especially, ruthenium: Ru, rhodium: Rh, and palladium: Pd) and molybdenum (Mo) cause serious problems: (i) PGMs tend to settle on the sidewall surface of a glass melter, giving rise to an inhomogeneous thermal distribution of the melter, and (ii) Mo forms low-viscosity uid compounds so-called "yellow phase" in the vitri ed object. 4,5 These issues degrade the quality and stability of the vitri ed objects due to heterogeneity, and increase both disposal spaces and costs in conjunction with additional vitri ed rods produced by ushing the glass melter.
Considering that Ru, Rh, and Pd with an amount of 2.09, 0.36, and 1.20 kg, respectively, are generated in 1 t of used nuclear fuels (burnup: 30,000 MWd/t, cooling period: 150 days) for light-water reactors (those generated amounts will increase by 1.5-2 times for fast breeder reactors), it is useful to recover PGMs from HLLW not only for the disposal of N-wastes but also for the recycling of precious metals from our alternative perspective. It is, of course, noted that it takes few decades to reduce their radioactive levels below safety standard except 107 Pd long-lifetime nuclide (half-life: 6.5 million years) that is planned to be deleted using an extinction process with fast neutrons. In a similar manner to PGMs, Mo is also used both as Mo-Cu alloys and MoS 2 materials for electronic and car industries, respectively, and also used as an alternative material for tungsten (W) because of its lower price.
For the latter case (E-wastes), the abundance of precious metals (Ru, Rh, Pd, rhenium: Re, osmium: Os, iridium: Ir, platinum: Pt, gold: Au) is much smaller by 10 −6 -10 −9 times than that of rock-forming elements (sodium: Na, magnesium: Mg, aluminum: Al, silicon: Si, potassium: K, calcium: Ca, and iron: Fe). 6 Although the precious elements are not uniformly dotted over the world in nature, the much amount of them has, however, been stored in E-wastes so far. For example, the amount of Au contained in 1 t of mobile phones is 300-400 g, which is much higher by 10-80 times than that in 1 t of natural ore. The other elements have a similar situation to Au. Consequently, the recovery of those precious elements from E-wastes is much more effective and e cient when compared to their collections from natural ore.
The aim of the present study is to solve the environmental and energy issues by utilizing the nanospace of Prussian blue (PB), which is one of metal hexacyanoferrates (MHCFs), as a sorbent. 7,8 Since MHCFs have a simple cubic lattice structure like a jungle gym (inset in Fig. 2), in which divalent (M 2+ ) and trivalent (M 3+ ) metal cations are cross-linked with each other via cyano-group anion (CN − ), and exhibit many fascinating features, [9][10][11] they have been extensively investigated from viewpoints of both scienti c and industrial aspects. [12][13][14][15][16][17][18] Recently, PB (FeHCF) has been applied to remove radioactive cesium-134 ( 134 Cs) and 137 Cs elements from contaminated soils caused by Fukushima nuclear plant accident in 2011, [19][20][21][22][23] because PB has the jungle-gym cubic structure with a 0.5 nm-interstitial site (4c site in space group) that plays a role in trapping Cs ions e ciently. However, the uptake mechanisms of PB for multivalent metal ions have been still unclear so far, though three possible sorption processes can be considered: (i) surface adsorption, (ii) insertion or diffusion into interstitial sites (nanospace) in a similar manner to Cs ions, and (iii) substitution of Fe with metals in the framework of PBs. In order to develop high-performance MHCF sorbents for recovery of the precious metals from N-and E-wastes, it is necessary to unravel the uptake characteristics of PB for those metal ions.
More recently, we have investigated the uptake mechanism of Pd (one of the most important elements in industry such as catalyst) ion into PB nanoparticles (PBNPs) in a nitric acid solution, and revealed that the Pd 2+ ion is incorporated into PBNPs by substitution with Fe 2+ ion of the PB framework with maintaining the crystal structure before and after Pd sorption, and the substitution e ciency was estimated to be 87% per PB unit cell. 24 This implies that 0.30 g Pd can be recovered by using 1g PB with a chemical form of KFe(III)[Fe(II)(CN) 6 ].
In the present study, we report here on the uptake characteristics of PBNPs for PGMs/Mo ions in a nitric acid solution as well as Pd ion. The uptake e ciency of PGMs/Mo into PBNPs and the elution e ciency of Fe from PBNPs were measured using inductively coupled plasma atomic emission spectroscopy (ICP-AES) before and after 24 h sorption test. We also examined changes in the structural and electronic properties of PBNPs before and after the sorption test, using powder x-ray diffraction (XRD) and ultraviolet-visible-Near IR (UV-vis-NIR) spectroscopy, in combination with rst-principles calculations based on density functional theory. Furthermore, in order to understand the difference in the sorption e ciency among PGMs/Mo ions, we estimated the surface adsorption, diffusion, and substitution energies when PGMs/Mo ions are incorporated into PB unit cell, using the rst-principles calculations. formed were rinsed with ultrapure water after centrifugation (3000 rpm), which was performed for ve times. Thereafter, PBNPs were dried at 75°C for 12 h and thereafter kept in the vacuum desiccator for 3 h.

Experimental And
Sorption test of PGMs/Mo ion (1 mM) for PBNPs (500 mg) were carried out in 1.5 M nitric acid solution (10 mL) upon shaking for 24 h. Subsequently, the mixtures were centrifuged to separate the PBNPs from the solution, and the concentration (C) of PGMs/Mo ion in the supernatant liquid was measured before (C initial ) and after (C nal ) the test, using ICP-AES (ICPE-9000, SHIMADZU), in order to estimate the sorption e ciency, [(C initial -C nal )/C initial × 10 %], of PGMs/Mo ions into PBNPs. We also measured the concentration of Fe ion in the supernatant liquid after the sorption test, and estimated the elution e ciency of Fe ion when compared to the initial amount of PBNPs. Details of sorption test conditions have been described elsewhere. 25 Powder XRD patterns and UV-vis-NIR diffuse re ectance spectra of the pristine and PGMs/Mo-sorbed PBNPs were measured using Rigaku RINT2200 (Cu Ka) and Shimazu UV-2600 spectrometer, respectively.
The diffuse re ectance spectra thus obtained were converted to the corresponding absorption spectra in terms of the Kubelka-Munk conversion equation.

Theoretical methods
Theoretical absorption spectra of the pristine and PGMs/Mo-sorbed PBs were obtained using the relativistic con guration interaction (CI) method, 26,27 because the present method has been already con rmed to reproduce the experimental absorption spectra of Fe, Co, and Ni ferrocyanides quantitatively, using Fe(II)M(III)(CN -) 11 6-(M = Fe, Co, Ni) cluster model. 28 The multiplet energy levels and absorption spectra were calculated using Fe 2+ Fe 3+ (CN -)  Here, each term denotes the total energy of PGM/Mo-sorbed PBs, pristine PB, and PGMs/Mo themselves, respectively, which were calculated using CASTEP. The maximally-localized Wannier functions (MLWFs) of the C2p and N2p atomic orbitals (AOs) on the (100) surface of PB were obtained using Wannier90. 42 The diffusion energy of PGMs/Mo ions through the nanospace of PBNPs was estimated for the unit cell model by using the Nudged elastic band (NEB) method implemented in QUANTUM-ESPRESSO. A diffusion pathway for PGMs/Mo ions in the nanospace of PBNPs was considered to be a route from the center of the square made of the four Fe ions in the (100) plane to the same site of the adjacent (100) plane via the 4c site (see the inset of Fig. 4).
The substitution energy (E S ) of PGMs/Mo with Fe of PB was estimated for the unit cell model consisting of 60 atoms, which corresponds to 12.5% Fe ions substituted with PGMs/Mo ions. The E S was evaluated using the following equation, (2) Here, the rst and second terms denote the total energy of the PGMs/Mo-sorbed and pristine PB models, respectively, whereas the third and fourth terms denote the chemical potentials of Fe and PGMs/Mo, respectively. In the present study, we used the chemical potential of the oxide at the oxidation limit and of the neutral atom at the reduction limit. These calculations were performed by DFT+U method, 43 [44][45][46] All calculations in the present study were performed until the residual forces and stresses below 0.01 eV/Å and 0.02 GPa, respectively. A uniformed background charge, so called the Jellium model, was used to accommodate the non-neutral states.

UV-vis-NIR spectra
As shown in Table 1, the elution e ciency of Fe ion from PBNPs strongly supports that Ru, Rh, and Mo ions are also substituted with Fe ion of PBNPs when incorporated into PBNPs as well as Pd ion. In order to con rm the substitution of PGMs/Mo ions with Fe ion, we examined the UV-vis-NIR spectra of PBNPs before and after PGMs/Mo sorption in combination with rst-principles calculations of theoretical spectra for the simple PB cluster model (see Fig. 1) by PGMs/Mo substitution with both Fe 2+ and Fe 3+ sites. Figure 1 shows (a) the experimental spectra with respect to the absorption energy before (black) and after and Fe 3+ ions.
In summary, it is reasonably concluded from the results of Fig. 1 Figure 2 shows XRD patterns of the pristine (black) and PGM-sorbed (blue: Ru, pink: Rh, and red: Pd) PBNPs. Here, the inset shows the unit cell of PB crystal structure. The XRD pattern (black) shows that the pristine PBNPs have a face centered cubic (FCC) structure (space group: ), which is consistent with the previous results obtained using high-resolution transmission electron microscope. 24 Table 2 summarizes the lattice constant and crystallite size of the PBNPs before and after 24 h PGMs/Mo sorption test, which were estimated by tting the (200) diffraction peak with pseudo-Voigt function and by using the Scherrer equation 47 with a constant of 1.5, respectively. Here, the constant value means an area-weighted effective diameter along the direction of the diffraction vector. The results of Fig. 2 and Table 2

Elementary processes of PGMs sorption into PBNPs
To discuss the elementary processes of PGMs/Mo when incorporated into PBNPs, we next examined (i) the adsorption energy of PGMs/Mo ions on the PB surface, (ii) the diffusion energy of PGMs/Mo ions into the nanospace of PBNPs, and (iii) the substitution energy of PGMs/Mo ions with Fe ion, using rstprinciples calculations based on DFT.
We rst discuss (i) the adsorption energy. Figure 3 shows ( Fig. 3(a), the (100) surface model was optimized to be the zig-zag structure which is stabler by 1.4 eV than the bulk at surface structure. As shown in shown in the top view of Fig. 3(a), it is found that all PGMs/Mo ions (red circle) are stably adsorbed on the center of the square consisting of the four Fe ions in the (100) plane. This is because PGMs/Mo ions surrounded by the four CNanion groups became stable energetically due to the attractive Coulomb interactions. The optimized structural coordination was summarized in Tables S1-S5 (supporting information). Figure 3 (b) shows the plot of the E ad on the (100) surface with respect to the PGM elements. Here, a lager positive value of E ad implies a more di culty to be adsorbed on the surface. It is found that the E ad seems to increase non-linearly with the valence number of PGMs ions: E ad = 0 eV for Pd 2+ ion, ca. 0.2 eV for Rh 3+ ion, ca. 1.8 eV for Ru 4+ ion, and ca. 3.2 eV for Mo 6+ . This is partly because the repulsive Coulomb interactions between the adsorbed PGMs/Mo ions and their surrounding Fe 2+ /Fe 3+ ions become greater than the attractive ones between the adsorbed ions and their surrounding CNgroups as the valence number of the adsorbed PGM/Mo ions increases, and partly because the latter attractive interactions become greater to cause a large distortion of the lattice with increasing the valence number as well. In addition, as show in in Fig. 3 (c), since the 2p y and 2p z AOs of both C and N atoms are expanded within the (100) in-plane, the PGMs/Mo cations are easy to be trapped in the (100) in-plane. It is noted that the present study estimated the E ad in solid phase and in vacuum. For the practical HLLW system with metal ions in nitric acid solution, the metal ions are more easily transported to the PB surface via the solvent, and the electrostatic potential of the (100) surface should also be somewhat altered.
We next discuss (ii) the diffusion of PGMs ions in the nanospace of PB unit cell, whose energy was calculated using the NEB method. Figure 4 shows the plot of the relative energy with respect to the ve migration steps for PGMs/Mo. Inset schematically illustrates a snap shot at each migration step. The accurate structural parameters for each migration step were summarized in Tables S7.1 PGMs/Mo and Fe ions will take place.
We nally discuss the substitution energy (E S ) of PGMs/Mo ions with the Fe 2+ /Fe 3+ ions of PB, which was estimated using the unit cell model containing 60 atoms (inset of Fig. 2). Although it is supposed that the model corresponds to 12.5% Fe ions (one of eight) substituted with PGMs ions when applied to the solid state, we just focused on the substitution occurred locally using the unit cell. Figure 5 shows the E S of PGMs/Mo ions with the Fe ions of PB unit cell. When the E S is a negative value, the substitution takes place preferably. Since the E S was de ned by equation (2) described above, the chemical potentials were required. In the present study, we used the chemical potential of each metal oxide at the oxidation limit, and of neutral metal atom at the reduction limit. At the oxidation limit, Pd 2+  However, comparison between the experimental and theoretical results (Fig. 1) indicates that Mo 6+ is substituted not only with Fe 3+ ion but also with Fe 2+ ion, which is contradictory to that the E s of Fe 2+ with Mo 6+ is a large positive value of ca. 4.0 eV at the oxidation limit, as shown in Fig. 5. Since the present PB unit cell cluster model did not consider the Madelung potentials and the lattice relaxation, the E s may be overestimated when Fe 2+ /Fe 3+ is substituted with the higher valent Mo 6+ ion.
We examined the three elementary processes when PGMs ions are incorporated into PB: the adsorption on the (100) surface of PB, the diffusion in the nanospace of PB unit cell, and the substitution of PGMs ions with Fe ions. As summarized in Table 3, all the three processes are most easily to progress for Pd 2+ ion, and subsequently for Rh 3+ ion, whereas the surface adsorption process becomes the rate-determining step for Ru 4+ ion. This is consistent with the results of the sorption and substitution e ciencies for these metal ions, as shown in Table 1. On the other hand, despite the order of the elusion and substitution e ciencies for Mo ion can be well explained using the results of Table 3 as well as PGMs ions, the sorption e ciency of Mo ion was higher than that for PGMs ions. Since Mo ion tends to be formed as polynuclear Mo oxide complexes in a nitric acid solution, 29 a part of them was presumably precipitated in the solution, which was not involved in the 24-h sorption test.
The ndings in the present study provide us an important insight that these three processes mutually affect the sorption amount and the sorption e ciency, thus leading to develop a high-performance sorbent for recovery of rare metals not only from N-wastes but also from E-wastes.

The uptake amount of PGMs per 1g PBNPs
As shown in Furthermore, PBNPs were con rmed to exhibit several resistances against heat (up to 300°C), nitric acid (up to 8 M), and g-ray radiation (up to 1000 kGy), thus indicating that PBNPs are practically used not only for disposal processes of N-wastes but also for recycle processes of E-wastes.

Summary
We have examined the uptake mechanisms of platinum-group-metals (PGMs) and molybdenum (Mo) ions into PBNPs in a nitric acid solution for 24-h sorption test, using ICP-AES, powder XRD, and UV-Vis-NIR spectroscopy in combination with rst-principles calculations, and revealed that the Ru 4+ and Pd 2+ ions are incorporated into PBNPs by substitution with Fe 3+ and Fe 2+ ions of the PB framework, respectively, whereas the Rh 3+ ion is incorporated into PBNPs by substitution mainly with Fe 3+ and minorly with Fe 2+ ion, and Mo 6+ ion is incorporated into PBNPs by substitution with both Fe 2+ and Fe 3+ ions, with maintaining the crystal structure before and after the sorption test. Assuming that the amount of Fe elusion is equal to that of PGMs/Mo substitution, the substitution e ciency is estimated to be 39.0% for Figure 1 Theoretical spectra of the pristine (b) and PGMs/Mo-sorbed PB using the cluster models for Fe 2+ (c) and Fe 3+ (d) substitution, along with the experimental UV-vis-NIR spectra of PB before (black) and after (red) PGMs sorption (a).   Relative energy of PGMs/Mo ions into PB jungle-gym structure at each migration step for the diffusion model. Inset shows the snap shot of each migration step in the nanospace of PB unit cell. Figure 5