10:45 AM - 12:15 PM
[BCG06-P04] U(Ⅵ) uptake behavior during transformation process of monohydrocalcite
Keywords:monohydrocalcite, uranium, coprecipitation
Uranium (U(Ⅵ)) takes various speciation as function of pH and partial pressure of CO2, behaving as a cation under low pH conditions and as an anion under high pH conditions. In general, hazardous heavy metals exist mainly as cations under a relatively wide range of pH conditions and are removed by increasing pH of the solutions. However, since mineral surfaces are negatively charged under high pH conditions, there is concern about the leaching of U(Ⅵ) into the environment, where it takes an anionic forms.
Monohydrocalcite (MHC; CaCO3・H2O) is a type of calcium carbonate that occurs naturally in alkali salt lakes, etc. MHC is metastable with respect to calcite and aragonite and is highly reactive toward trace elements due to its instability (Fukushi et al., 2011). Fukushi et al. (2016) investigated the uptake behavior of arsenate (AsO43-), which exists in anionic form as well as U(Ⅵ), into MHC and reported that arsenate is effectively removed from solution when MHC is transformed to the stable phase. The removal of toxic elements by the transformation of MHC is more effective than the direct formation of calcite or aragonite in solution. This is because the removal of toxic elements proceeds spontaneously only by the addition of MHC, so it is an effective removal material. Therefore, MHC may be a material that can effectively remove U(Ⅵ) under high pH conditions.
In this study, U(Ⅵ) uptake behavior during the transformation process of monohydrocalcite were investigated through kinetics experiments at different Mg concentrations. Observations of the mineral composition and U(Ⅵ) concentration changes with time in a system in which U(Ⅵ) solution was added to a suspension of MHC showed that MHC was transformed to calcite under low Mg concentration conditions and to aragonite under high Mg concentration conditions. The U(Ⅵ) was removed when MHC transformed to aragonite, while it was hardly removed when MHC transformed to calcite. Under high pH as well as water chemistry where MHC is transformed to aragonite, MHC is expected to act as an effective removal material for U(Ⅵ). In the presentation, we will also discuss the uptake mechanism based on the X-ray absorption fine structure (XAFS) results of the reacted solid samples.
Monohydrocalcite (MHC; CaCO3・H2O) is a type of calcium carbonate that occurs naturally in alkali salt lakes, etc. MHC is metastable with respect to calcite and aragonite and is highly reactive toward trace elements due to its instability (Fukushi et al., 2011). Fukushi et al. (2016) investigated the uptake behavior of arsenate (AsO43-), which exists in anionic form as well as U(Ⅵ), into MHC and reported that arsenate is effectively removed from solution when MHC is transformed to the stable phase. The removal of toxic elements by the transformation of MHC is more effective than the direct formation of calcite or aragonite in solution. This is because the removal of toxic elements proceeds spontaneously only by the addition of MHC, so it is an effective removal material. Therefore, MHC may be a material that can effectively remove U(Ⅵ) under high pH conditions.
In this study, U(Ⅵ) uptake behavior during the transformation process of monohydrocalcite were investigated through kinetics experiments at different Mg concentrations. Observations of the mineral composition and U(Ⅵ) concentration changes with time in a system in which U(Ⅵ) solution was added to a suspension of MHC showed that MHC was transformed to calcite under low Mg concentration conditions and to aragonite under high Mg concentration conditions. The U(Ⅵ) was removed when MHC transformed to aragonite, while it was hardly removed when MHC transformed to calcite. Under high pH as well as water chemistry where MHC is transformed to aragonite, MHC is expected to act as an effective removal material for U(Ⅵ). In the presentation, we will also discuss the uptake mechanism based on the X-ray absorption fine structure (XAFS) results of the reacted solid samples.