Dependence of the dynamic behavior of supersaturated sillcic acid on the surface area of the solid phase

Cement is an essential material to construct a geological disposal system. Such a material (which is stable in high pH) may alter the groundwater up to 13 in pH. In Japan, the water table is predicted to be shallow compared to the depth level of the repository system (deeper than 300 m). The backfill will condition the groundwater entering the repository to high pH. Since the solubility of silicic acid is very large in pH>9, its mixing with natural groundwater would cause silicic acid supersaturation. So far, the authors have reported that the deposition layer (amorphous), resulting from the supersaturated silicic acid, strongly affect the sorption of RNs. The current study examined the precipitation rates of the silicic acid, in order to evaluate the area of altered surface surrounding the repository. In the results, when the initial supersaturated concentration was set to 2.5 mM or 5.6 mM, the initial precipitation rates of the silicic acid depended on the surface area of the solid phase. However, the kinds of the solid phase did not affect the precipitation rates. With the increment of temperature, the precipitation rates increased. Moreover, the precipitation rate constant, k, was evaluated in the initial precipitation rates. The value of k was around 2.0x10^-10 (mis) in this experiment condition. This value was suggested to be sufficiently large to limit the altered surface area, even if the advection effect of groundwater occurs.