Hydrothermal synthesis and ion exchange properties of the novel framework sodium and potassium niobium silicates

Two novel metastable sodium niobium silicates of the empirical formula: Na1 +x-yHy(Nb1-xSix)O-3. nH(2)O, where x=0.33-0.38, y<l+x, n=0.7-1.1 (NbSi-Na, 6.0 Angstrom phase), and Na3-xHxNb3Si2O13. nH(2)O, where x<1.5, n=2.5-3.5 ( NbSi-Na, 12.6 Angstrom phase), and two novel potassium niobium silicates: K 4-xHxNb4Si5O22. H2O, where x<1, n=3.5-4.0 (NbSi-K, 10.0 Angstrom phase), an d K1-xHxNbSi4O11. nH(2)O, where x<0.2, n=0.4-0.5 (NbSi-K, 6.05 Angstrom pha se), were synthesized in the homogeneous alkaline reaction system NbCl5 - S iO2 - NaOH (KOH) H2O2- H2O under mild hydrothermal conditions. The compound s were characterized by elemental analysis, FTIR, TGA, MAS Si-29 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 silica tes show a distinct affinity for Cs+ ion among alkali metal ions, whereas p otassium niobium silicate, the NbSi-K, 10.0 Angstrom phase, exhibits affini ty for Rb+ ion. The affinity of the sodium niobium silicate, NbSi-Na, 6.0 A ngstrom, 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 Angstrom phase) exhibits extremely high affinity for Pb2+ i on in acidic and neutral media, and both sodium niobium silicates also show a moderate affinity for Hg2+ ion in neutral and highly alkaline media. The se exchangers could be promising for the treatment of some specific nuclear waste and contaminated environmental and biological liquors containing lea d, mercury and radioactive strontium.