11:30 AM - 11:45 AM
[HCG32-03] Formation of magnesium silicates and its geochemical modeling at high alkaline conditions
Keywords:magnesium silicate, layered double hydroxide
For formation of LDHs, 50 mM of each of Mg ion solution (Mg(NO3)2・6H2O), Al or Fe ion solution (Al(NO3)3・9H2O or Fe(NO3)3・9H2O ), and silicate anion solution(Na2SiO3) as an initial solution was mixed with different ratios. After the mixing, these mixed solutions were adjusted to pH 9 or pH 12 and left at 25 ℃ for 24 hours. The reason for paying attention to silicate ions is that they are concerned about dissolution from cement materials and vitrified bodies used in waste disposal and considered to be analogues of 79Se and 99Tc.
In Mg-Al system, at pH 9, in the case of adding above 15 mM silicate ions, M-S-H and amorphous aluminium (with Al ion), or M-S-H and smectite (without Al ion) were precipitated. On the other hand, in the case of below 15 mM silicate ions, LDH (Mg/Al ≥ 0.5) and gibbsite or boehmite (Mg/Al ≤ 0.5) were precipitated At pH 12, in the case without Al ion and with Mg ion, brucite was confirmed. Moreover, M-S-H (Mg/Al ≤ 0.1) or LDH (Mg/Al ≥ 0.3) instead of gibbsite and boehmite were generated. In other systems, no change was observed with the precipitates at pH 9.
In Mg-Fe system, formation of LDH was confirmed in a system containing less silicate ion and much Mg ion, but ferrihydrite or M-S-H was precipitated in other systems containing Fe ion.
In order to investigate these results thermodynamically, by calculating and incorporating M-S-H and LDH database based on previous study(Hase et al. 2017), we could represent thermodynamically precipitated mineral species.