Hydrothermal synthesis and ion exchange properties of the novel framework sodium and potassium niobium silicates
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Two novel metastable sodium niobium silicates of the empirical formula: Na(1+x-y)H(y)(Nb(1-x)Si(x))O3nH2O, where x=0.33-0.38, y<1+x, n=0.7-1.1 (NbSi-Na, 6.0 phase), and Na(3-x)H(x)Nb3Si2O13nH2O, where x<1.5, n=2.5-3.5 (NbSi-Na, 12.6 phase), and two novel potassium niobium silicates: K(4-x)H(x)Nb4Si5O22nH2O, where x<1, n=3.5-4.0 (NbSi-K, 10.0 phase), and K(1-x)H(x)NbSi4O11nH2O, where x<0.2, n=0.4-0.5 (NbSi-K, 6.05 phase), were synthesized in the homogeneous alkaline reaction system NbCl5 - SiO2 - NaOH (KOH) - H2O2 - H2O under mild hydrothermal conditions. The compounds were characterized by elemental analysis, FTIR, TGA, MAS 29Si NMR and X-ray diffraction. It was found that alkali metal niobium silicates have open framework structures. Their ion exchange affinity towards alkali, alkaline earth and some transition metal ions was studied. All alkali metal niobium silicates are moderately acidic ion exchangers. Both sodium niobium silicates show a distinct affinity for Cs+ ion among alkali metal ions, whereas potassium niobium silicate, the NbSi-K, 10.0 phase, exhibits affinity for Rb+ ion. The affinity of the sodium niobium silicate, NbSi-Na, 6.0 , toward strontium ion in neutral solutions is equal or superior to the best Sr-selective inorganic ion exchangers. The sodium niobium silicate (NbSi-Na, 12.6 phase) exhibits extremely high affinity for Pb2+ ion in acidic and neutral media, and both sodium niobium silicates also show a moderate affinity for Hg2+ ion in neutral and highly alkaline media. These exchangers could be promising for the treatment of some specific nuclear waste and contaminated environmental and biological liquors containing lead, mercury and radioactive strontium.
