Thermodynamics of adsorption of Sr with scallop shell powder

Approximately 260,000 tons of scallop shells become industrial waste every year in Japan. So far, attempts to reuse the shells have been mainly limited to the commercial production of CaCO3. Currently, almost all CaCO3 in... [ view full abstract ]

Approximately 260,000 tons of scallop shells become industrial waste every year in Japan. So far, attempts to reuse the shells have been mainly limited to the commercial production of CaCO3. Currently, almost all CaCO3 in Japan is produced from limestone. Each year in Japan, 200 million tons of limestone is mined. CaCO3 produced from limestone is less expensive than that produced from scallop shells. There are no obvious economical benefits to using scallop shells as a source of CaCO3. Therefore, we are attempting to find a new value added use for scallop shells as an advanced functional material. In previous experiments, we have been trying to use calcined shell powder to extract alkaline metal and alkaline-earth metal from seawater.The tsunami of March 11, 2011, caused severe damage to the Fukushima nuclear power plant. Today, many problems still exist, but a particularly serious problem is the leakage of highly toxic contaminated water. Within the storage tanks, there remains a large quantity of contaminated water. This water contains radioactive Sr and Cs. Sr tends to accumulate in bones and is believed to be a cause of bone cancer. Therefore, it is highly desirable to develop a method for the removal of Sr from the contaminated water. Here we will show the potential of scallop shells to adsorb Sr from the solution.We analyzed the crystal structure and the morphology of both the shell and CaCO3. The crystal structure of the sample was examined by using X-ray Diffractometer. The surface morphology of samples were analyzed by Scanning Electron Microscope. The morphology of the scallop shell was unique. A micro column structure was only observed on the inner surface of the shell. All parts of the inner surface of the scallop shells does not have the same morphology, as far as we observed by SEM. We thought this difference of the morphology would affect Sr adsorption performance. Then we put the shell powder into a beaker with a Sr solution and stirred it. We measured the Sr concentration by AAS. Sr adsorptivity is entirely dependent on which part of the scallop shell is used. Therefore, it is possible that structure affects the absorptivity. According to adsorption isotherm studies, the Langmuir isotherm was proved to be the best fit for our experimental data, in comparison to Freundlich. The thermodynamic parameters,  enthalpy, entropy and Gibbs free energy, are evaluated. Thermodynamic parameters showed that the adsorption of Sr onto scallop shell was feasible, spontaneous and endothermic under studied conditions.