No abstract

Monohydrocalcite (CaCO 3 ·H 2 O; MHC) is a rare mineral in geological settings. It is metastable with respect to calcite and aragonite. This metastability of MHC is considered to make it a rare mineral in geological settings. Alteration experiments of MHC in aqueous solutions in a closed system were conducted at temperatures between 10 and 50 °C in order to measure its metastability quantitatively. In the present study, monohydrocalcite transformed to aragonite with time. There are two rate -limiting steps in the transformation of monohydrocalcite to aragonite: the nucleation and crystal growth of aragonite. On the other hand, the dissolution of monohydrocalcite is a faster process than the nucleation and crystal growth of aragonite. The amounts of aragonite were calculated from the X -ray diffraction (XRD) intensity to evaluate the rate of both the processes at different temperatures. The induction times for the nucleation of aragonite were estimated to be 2.7 ± 0.9×10 , and 1.6 ± 0.3×10 -6 mmol·s -1 at 10, 25, 40, and 50 °C, respectively. From Arrhenius plots, the apparent activation energies were estimated to be 108.1 kJ·mol -1 and 80.7 kJ·mol -1 for the nucleation and crystal growth steps, respectively.

The formation conditions, solubility and stability of monohydrocalcite (MHC, CaCO·HO), as well as sorption behaviors of toxic anions on MHC, are reviewed to evaluate MHC as a remediation material for hazardous oxyanions. MHC is a rare mineral in geological settings that occurs in recent sediments in saline lakes. Water temperature does not seem to be an important factor for MHC formation. The pH of lake water is usually higher than 8 and the Mg/Ca ratio exceeds 4. MHC synthesis experiments as a function of time indicate that MHC is formed from amorphous calcium carbonate and transforms to calcite and/or aragonite. Most studies show that MHC forms from solutions containing Mg, which inhibits the formation of stable calcium carbonates. The solubility of MHC is higher than those of calcite, aragonite and vaterite, but lower than those of ikaite and amorphous calcium carbonate at ambient temperature. The solubility of MHC decreases with temperature. MHC is unstable and readily transforms to calcite or aragonite. The transformation consists of the dissolution of MHC and the subsequent formation of stable phases from the solution. The rate-limiting steps of the transformation of MHC are the nucleation and growth of stable crystalline phases. Natural occurrences indicate that certain additives, particularly PO and Mg, stabilize MHC. Laboratory studies confirm that a small amount of PO in solution (>30 μM) can significantly inhibit the transformation of MHC. MHC has a higher sorption capacity for PO than calcite and aragonite. The modes of PO uptake are adsorption on the MHC surface at moderate phosphate concentrations and precipitation of secondary calcium phosphate minerals at higher concentrations. Arsenate is most likely removed from the solution during the transformation of MHC. The proposed sorption mechanism of arsenate is coprecipitation during crystallization of aragonite. The arsenic sorption capacity by MHC is significantly higher than simple adsorption on calcite.

Changes in the hydrochemical conditions of groundwater were evaluated following the construction of a large-scale underground facility at the Mizunami Underground Research Laboratory (MIU), Japan. The facility was constructed to a depth of 500 m in sedimentary and granitic rocks. Drawdown of the groundwater level in the range of several tens to hundreds of meters was observed up to hundreds of meters away from the shafts during the first ten years of facility construction and operation. Subsequent changes in groundwater chemistry occurred due to upconing of high-salinity groundwater from the deepest part of the shaft and the infiltration of low-salinity shallow groundwater. We predict that future deep groundwater chemistry in the vicinity of the MIU facility will resemble that of the present-day shallow groundwater. Multivariate statistical analysis provides fundamental insights into such a site. We found that the extent of hydrochemical variability related to MIU construction and operation was dependent on the distance from the facility shafts and galleries and on hydrogeological compartmentalization resulting from lithological boundaries (such as permeable conglomerates vs. more compact lithological units) and other features (such as faults or clay layers). We conclude that hydrochemical impact assessment of groundwater in low-permeability rock is essential prior to the construction of such a facility. This should include characterization of hydrogeological structures and compartments to propose suitable location of shafts and galleries.

No abstract