Amorphous calcium carbonate (ACC) is a precursor of crystalline calcium carbonate; hence, its structural information at the atomic level is very important for controlling the morphology of crystalline calcium carbonate. In this study, we attempted to elucidate the process of Sr extraction from aqueous solution by using ACC for the purpose of removal of radioactive Sr from the contaminated water leaked after the Fukushima Daiichi nuclear accident. The pair distribution functions, g(r) obtained by X-ray and neutron diffraction (ND) measurements show that ACC has a structure partially similar to that of monohydrocalcite, suggesting that ACC is transferred to the crystalline calcium carbonate starting from its crystal nucleus. Rietveld analysis of the ND data showed that the ACC that removed Sr was crystallized to calcite. However, the SrO coordination analyzed using extended X-ray absorption fine structure (EXAFS) implies that the local environment of O around Sr is similar to that in crystalline calcium carbonate aragonite.
The Fukushima nuclear power plant accident caused an outflow of pollutants. Thus, precautionary measures must be taken by using preventive materials for the deterrence of such accidents. Herein, we aimed to develop inexpensive materials that can remove radioactive elements from an aqueous solution. We found that powders obtained from milling scallop shells, which were discarded in large quantities, removed Sr 2+ from aqueous solutions. The Sr 2+ removability of the scallop shell powder improved with milling time, indicating the influence of increase in the specific surface area on Sr 2+ removability. Despite of the same specific surface area, the scallop shell powder exhibited higher Sr 2+ removability than that of the unmilled CaCO 3. Differential scanning calorimetry evaluation for the investigation of factors other than the specific surface area revealed that the milled scallop shell powder contained amorphous calcium carbonate. Moreover, we synthesized amorphous calcium carbonate and found that it exhibited 60-times higher Sr 2+ removability than that of crystalline calcium carbonate. We concluded that the amorphous structure of calcium carbonate significantly affects the Sr 2+ removability from aqueous solutions. It was hypothesized that amorphous calcium carbonate removes Sr 2+ by incorporating Sr 2+ into the structure during crystallization in an aqueous solution.
It is important to efficiently remove radioactive substances contained in polluted waters before they are discharged from nuclear power plants. In particular, there is an urgent need for the development of technology that can adsorb radioactive Sr 2+ , but there are currently no inexpensive Sr 2+ adsorbents with low environmental burden. We found that scallop shell powder adsorbs Sr 2+ in aqueous solutions at various initial concentrations. In this study, to obtain fundamental knowledge of the mechanism of Sr 2+ removal using waste scallop shell, we analyzed the removability of Sr 2+ . Scallop shell showed the same capacity to remove Sr 2+ at a high initial concentration (²0.50 g/dm 3 ) as the reagent CaCO 3 , but a clear difference in removability appeared at a low initial concentration (0.010 g/dm 3 ), where scallop shell proved to be superior. In addition, scallop shell powder had slit-shaped pores and a specific surface area of 4.3 m 2 /g. Measurement of the adsorption isotherm in the low concentration aqueous solution showed that Sr 2+ removal occurred by chemisorption; the adsorbed Sr is present on the surface of the scallop shell powder particles.
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