The Use of Scallop Shell Powder as a Method of Extracting Alkaline-Earth Metals

Introduction

   Approximately 260,000 tons of scallop shells become industrial waste^[1] 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^[2]. 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^[3], 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 Sr²⁺ and Cs⁺. Sr²⁺ tends to accumulate in bones and is believed to because of bone cancer. Therefore, it is highly desirable to develop a method for the removal of Sr²⁺ from the contaminated water. Here we will show the potential of scallop shells to adsorb Sr²⁺ from the solution.

Experimental

   First, we crushed the shells using a stamp mill and an automatic grinder. Next, we classified the shell powder by sieves according to their diameter. Then, we put the shell powder into a beaker with a solution (including Sr²⁺) and stirred it. Finally, we measured the Sr²⁺ concentration by Atomic Adsorption Spectrometer (AAS) (Thermo, iCE3300). We compared the Sr²⁺concentration in the solution both prior to and then again after it was stirred.

   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 (XRD)(Rigaku, SA-HF3). The surface morphology of samples were analyzed by Scanning Electron Microscope(SEM)(JEOL, JSM-6360A).

Results and discussion

   We calculated the amount of substance from the concentration of solution measured by AAS. The amount of both the Sr²⁺ and the Ca²⁺ was plotted against the stirring time. In Fig.1, which is the solid line, we were able to observe a clear decrease in Sr²⁺ amount. In Fig.1, which is the dotted line, we witnessed a marked increase in Ca²⁺ amount. Both, the decreasing Sr²⁺ and increasing Ca²⁺, were directly related to the amount of time spent stirring. We assumed that Sr²⁺ substituted with Ca²⁺ of the CaCO₃ which is the main component of the shell. We were then able to conclude that this is the reason why the amount of Sr²⁺ was reduced.

   According to the XRD pattern shown in Fig.2, it was clear that the scallop shell and CaCO₃ have almost the same crystal structure.

   There are no differences between scallop shells and CaCO₃ as far as crystal structure is concerned. The morphology of the inner surface of the shell is shown in Fig.3. A micro column structure of the scallop shell was observed on it. This structure is supposed to be an important factor for the adsorption of Sr²⁺.

References

[1] Japanese Ministry of Agriculture, “Fishing and farming mount stats in FY2013” p.1, (2014),

[2] Y. Maki, H. Matsumoto, The nature of the soil news, 54, 25 (2000)

[3] F. Mihara, K. Takeuchi, Y. Kogo, S. Tamura, H. Honda, Proceedings of 2013 Japan-Taiwan Symposium on PT-BMES, p.48 (2013)