Formation of Secondary Minerals and Uptake of Various Anions Under Naturally-Occurring Hyperalkaline Conditions in Oman

In Japanese transuranic (TRU) waste disposal facilities, ¹²⁹I is the most important key nuclide for the long-term safety assessment. Thus, the Kd values of I to natural minerals are important factor in the safety assessment. However, the degradation of cement materials in the repositories can produce high pH pore fluid which can affect the anion transport behavior. Therefore, it is necessary to understand the behavior of anions such as I⁻ under the hyperalkaline conditions. The natural hyperalkaline spring water (pH>11) in the Oman ophiolite is known to be generated from the partly serpentinized peridotites. The spring water is characteristically hyperalkaline, reducing, low-Mg, Si and $HCO3−$⁠, and high-Ca, while the river water is moderately alkaline, oxidizing, high-Mg and $HCO3−$⁠. The mixing of these spring and river water resulted in the formation of secondary minerals. In the present study, the naturally occurring hyperalkaline conditions near the springs in Oman were used as natural analogue for the interaction between cement pore fluid and natural Mg-$HCO3−$ groundwater. The present aim of this paper is to examine the conditions of secondary mineral formation and the anion uptake capacity of these mineral in this system. Water and precipitate samples were collected from the different locations around the spring vent to identify the effect of mixing ratios between spring and river water on mineral composition and water-mineral distribution coefficient of various anions. On-site synthesis was also carried out to support these data quantitatively. Aragonite was observed in all precipitates, while calcite, brucite and Mg-Al hydrotalcite-like compounds (HTlc) were also determined in some samples. Calcite was observed only closed to the springs. At locations far from the springs, calcite formation was inhibited due to high-Mg fluid from river water. Brucite was observed from the springs with relatively low-Al concentration and HTlc was the opposite. During the formation of the minerals at the mixing points, HCO3- in the river water was fixed as carbonate minerals such as in aragonite and calcite while $H3SiO4−$ in the river water was dominantly fixed into interlayers and surfaces of HTlc. Iodine in spring and river water was mainly fixed in aragonite. Therefore, the uptake I⁻ by secondary minerals can be expected at hyperalkaline conditions as observed at Oman hyperalkaline springs.