Composite Zn(II) Ferrocyanide/Polyethylenimine Cryogels for Point-of-Use Selective Removal of Cs-137 Radionuclides

Abstract

The feasibility of several approaches to the fabrication of monolith composite cryogels containing transition-metal ferrocyanides for Cs+ ion uptake has been evaluated. Although in the series of investigated metal ion precursors (Cu(II), Zn(II), Ni(II), and Co(II)), in situ formation of the sorption active phase in polyethyleneimine (PEI) cryogel was feasible only in the case of Zn(II) ferrocyanide, this approach has shown significant advantages over the immobilization of ex situ synthesized ferrocyanide nanoparticles. Nanoparticles of the mixed ferrocyanide Zn1.85K0.33[Fe(CN)6] formed in situ had an average size of 516 ± 146 nm and were homogeneously distributed in the monolith located at the polymer surface rather than embedded in the matrix. The Young modulus of the PEI cryogel increased after modification from 25 to 57 kPa, but composites maintained high permeability to the flow. Sorption of Cs+ ions has been investigated at superficial velocity up to 8 m/h. Steep breakthrough profiles and uptake efficiency of >99.5% until breakthrough point confirmed that a supermacroporous structure of the monolith composite assured good mass transfer, so that intraparticle diffusion was not the limiting stage of sorption kinetics. Application of the rate-constant distribution model (RCD model) to analyze the breakthrough curves of Cs+ sorption allowed the identification of two types of sorption sites with a difference in sorption rate constants of ~1 log unit. Most likely, sorption on “fast” sorption sites was governed by ion exchange between Cs+ ions in solution and K+ ions in the ferrocyanide lattice. Cs-137 radionuclide removal was investigated using the monolith composite columns of various geometries at superficial velocity up to the 6.6 m/h; specific gamma activity was reduced from 265 kBq/L to the background level, showing high potential of these materials for POU application.

Polyethylenimine as a Non-Innocent Ligand for Hexacyanoferrates Immobilization

“…To elucidate reasons leading to the drastic decrease of cesium sorption activity of hexacyanoferrates (II) (HCF(II)) after immobilization in PEI matrix [ 23 ], we have synthesized a series of materials using PEI cross-linked with 1,4-butanediol diglycidyl ether [ 25 ] as a support. In contrast to the earlier reported by our group monolithic composite materials [ 24 ], the inorganic phase was immobilized to the fine fraction of cross-linked PEI via successive contact steps with metal salts and K 3 [Fe(CN) 6 ] or K 4 [Fe(CN) 6 ] in solution under static conditions. This route is typical for the HCFs immobilization in polymer matrices or ion-exchange resins [ 14 ], when the content of inorganic phase in the composite is determined by the ion-exchange capacity of the matrix.…”

“…The maximal sorption activity was observed for Zn(II) HCFs, regardless of the anion precursor type—(K 3 [Fe(CN) 6 ] or K 4 [Fe(CN) 6 ]). Analysis of the elemental composition of the crystalline HCFs and related PEI–HCF hybrids ( Table 1 ) reveals the following features: (i) potassium was either not detected or present in PEI–HCF hybrids in very low content, although it was found in the PEI–Zn(II)HCF(II) monolith composite obtained earlier using another fabrication method [ 24 ]; (ii) Zn:Fe atomic ratios were close to 2:1 for both PEI–Zn(II)HCF(II) and PEI–Zn(II)HCF(III) hybrids suggesting that Fe(III) to Fe(II) reduction occurred during synthesis and PEI–Zn(II)HCF(II) hybrids were obtained using both anionic precursors; (iii) Cu:Fe atomic ratios in PEI–Cu(II)HCFs were higher than was expected for both HCF(II) and HCF(III), despite the excess of anionic precursors was added during synthesis. Furthermore, PEI–Cu(II)HCFs had, untypical for the crystalline analogs, dark blue color.…”

“…Dissolution of the crystalline HCFs in PEI solution was also observed for Zn(II)HCF(II) and Fe(III)HCF(III). Although weaker binding of Zn(II) to PEI allowed earlier fabrication of composites with crystalline Zn(II)HCF(II) phase [ 24 ], the much longer fabrication time used here, most likely, led to the partial dissolution of crystalline nanoparticles and, thus, to the lower sorption activity of the hybrid ( Table 1 ). The difference in fabrication conditions also affected the elemental composition: when monolith PEI cryogel was modified with Zn(II)HCF(II) under dynamic conditions mixed zinc/potassium hexacyanoferrate was formed [ 24 ].…”

“…Despite the large number of successful examples of polymer-hexacyanoferrates composite sorbents, we have surprisingly failed to fabricate composite sorbent using PEI cryogels as a matrix for Cu(II) hexacyanoferrate immobilization [ 23 ]. Later, we have shown that Zn(II) nanoparticles formed immediately after addition of K 4 [Fe(CN) 6 ] to the solution of PEI−Zn(II) complex were dissolved within a few minutes, so that the fabrication method had to be carefully adapted to obtain an efficient composite for cesium uptake [ 24 ].…”

Flow-Through Polyethylenimine/ZnS Supermacroporous Composite for Hg(II) Uptake at ppb Concentrations

“…The approach consisting in the sequential loading of the monolith PEI cryogel with transition metal ions and S 2– ions, similar to the fabrication of PEI/ferrocyanide composites, which we have recently reported for the uptake of cesium radionuclides, is beneficial in terms of both fabrication simplicity and low cost. Moreover, the much more effective performance of PEI for Hg­(II) uptake in comparison with GO and PVA used as a polymer matrix earlier , enables one to expect a stronger synergetic effect of different Hg­(II) sorption mechanisms in the case of composites formed by PEI and transition metal sulfides.…”