Effect of solution composition on the recrystallisation 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 crystallised (at 70 oC), with no reaction intermediates. The addition of SO42- crystallised sulfatic sodalite at 40 & 50 oC, but at higher temperatures (60 and 70 oC) sulfatic sodalite transforms to vishnevite (sulfatic cancrinite). In experiments where a pertechnetate tracer was added (at ~1.5 μmols L-1), only 3-5 % of the 99Tc was incorporated to 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-, SO42- or Cl- anions in solution, making this method likely to be unsuitable for groundwater treatment.