Incorporation of rhenium into heteropolyoxoanions: Towards separation and immobilization of technetium in radioactive waste

Several approaches towards incorporation of technetium into “rock”-type materials with rhenium as non-radioactive surrogate for technetium using heteropolyoxometalate systems were investigated, (1) reactions of ReIV and Re V complexes with lacunary Keggin-type heteropolyoxoanions produced complexes stable in acidic solution (pH = 2–6): [PReIV/VW ₁₁O₄₀]5−/4− and [(ReO)₃(AsW ₉O₃₃)₂]9−. These were characterized by IR, multinuclear NMR spectroscopy, elemental analysis and cyclic voltammetry. The results indicated that [PReIV/VW₁₁O₄₀] 5−/4− has the well known Keggin structure, with the overall symmetry of the anion reduced to C_s due to incorporation of rhenium. The anion incorporating ReIV was oxidized quickly in air to produce the ReV derivative.; According to 183W-NMR spectroscopy [(ReO)₃(AsW ₉O₃₃)₂]9− adopts a sandwich-type structure similar to the tungsten analog: [As₂W₂₁O ₆₉(H₂O)]6−.; Thermal decomposition of (NH₄)₉[(ReO)₃(AsW ₉O₃₃)₂] produced an oxide material with chemical composition: Re_1.9W₁₂O₄₁ indicating that ∼95% of the rhenium was incorporated in the oxide framework. Powder diffraction analysis showed the presence of two phases with tungsten-bronze-like structures. Similarity of ionic radii of ReV/VI and WVI suggested that rhenium atoms had partially replaced tungsten in these structures.; (2) The bridging oxygen atoms on the surfaces of the hexametalate anions: [M₆O₁₉]8− (M = Nb, Ta) were shown be basic enough to bind positively charged {lcub}M′(CO) ₃{rcub}+ (M′ = Re, Mn) moieties forming polyoxoanions: [{lcub}M′(CO)₃{rcub}_nM ₆O₁₉](8−n)−, (n = 1, 2) stable even in alkaline solution (pH = 4–10). The resulting species were characterized using IR and 17O-NMR spectroscopy, elemental analyses and X-ray crystallography. When two {lcub}M′(CO)₃{rcub} + moieties were bound, cis- and trans-isomers could be isolated.; (3) Since the surfaces of polyoxoanions greatly resemble the surfaces of oxide materials an attempt was made to make solid-state analogs of discrete [{lcub}M′(CO)₃{rcub}nM₆O₁₉] (8−n)− polyoxoanions. The {lcub}Re(CO₎₃{rcub} + moiety was intercalated into the layered structure of LiTaS ₂. Powder diffraction studies of the resulting material showed increased interlayer spacings that compared well with the estimated size of {lcub}Re(CO) ₃{rcub}+. The presence of the (100) reflection suggested no change in the structure of TaS₂ layer. IR spectroscopy confirmed facial arrangement of the three carbonyls, consistent with {lcub}Re(CO)₃{rcub} + groups inserted between TaS₂ layers.