Effect of solution composition on the recrystallization of kaolinite to feldspathoids in hyperalkaline conditions: limitations of pertechnetate incorporation by ion competition effects

The incorporation of pertechnetate (TcO₄^-) into feldspathoids produced by alkaline alteration of aluminosilicate clays may offer a potential treatment route for ⁹⁹Tc-containing groundwater and liquors. Kaolinite was aged in NaOH to determine the effect of base concentration, temperature, and solution composition on mineral transformation and pertechnetate uptake. In all reactions, increased temperature and NaOH concentration increased the rate of kaolinite transformation to feldspathoid phases. In reactions containing only NaOH, sodalite was the dominant alteration product; however, small amounts (6-15%) of cancrinite also formed. In experiments containing NaOH/Cl and NaOH/NO₃ mixtures, sodalite and nitrate cancrinite were crystallized (at 70°C), with no reaction intermediates. The addition of SO₄^2- crystallized sulfatic sodalite at 40 and 50°C, but at higher temperatures (60 and 70°C) sulfatic sodalite transforms to vishnevite (sulfatic cancrinite). In experiments where a pertechnetate tracer was added (at ~1.5 μmol l^-1), only 3-5% of the ⁹⁹Tc was incorporated into the feldspathoid phases. This suggests that the larger pertechnetate anion was unable to compete as favourably for the internal vacancies with the smaller OH^-, NO₃^-, SO₄^2- or Cl^- anions in solution, making this method likely to be unsuitable for groundwater treatment.