Electrochemical Control of Multi Crystal Formation from Clay Mineral in Molten Sodium Chloride and Calcium Chloride
Authors: Honda Mitsunori , Sakanaka Yoshihide , Goto Takuya , Yaita Tsuyoshi , Suzuki Shinichi 
Published: February 2020

A study for decontamination and the reproduction of the contaminated soil which occurred with 1F accident in Fukushima is continuously promoted. We conduct research on detachment mechanism when Cs is removed using mixed salt (NaCl － CaCl2) from clay mineral (weathering biotite :WB) which is highly Cs sorbed. As a result, it was clarified that Cs was taken into salt in a high temperature process revealed by in-situ XAFS measurement. Moreover, it was revealed that WB changed to multiple crystals in a removal process of the Cs. Specifically, it is Hematite (Fe2O3), Calcite (CaCO3), Augite ((Ca,Mg,Fe,Al)2 (Si,Al)2O6), and Wadalite ((Ca, Mg) 6 (Al, Fe) 4 (Si,Al)O4)3O4Cl3). As the next step, we search about the possibility of the separation / collection by using the molten salt electrochemistry method for the multiple crystals in a Cs removal process.

In this study, the change of the product is examined by doing it under electrochemical reducing condition using the molten salt electrochemistry. By this experiment, GC electrode was used for counter electrode, Ag/AgCl electrode in reference electrode, and Pt wire electrode in working electrode. NaCl － CaCl2 mixed salt of mass ratio 1:1 was used for molten salt. To confirm the oxidation-reduction reaction, cyclic voltammogram (CV) measurement were conducted. The CV measurement was carried out in the range of +0.5 V from -2.2 V, and two pairs of oxidation-reduction peaks were confirmed. In this study, it was fixed to the reduction potential of 1.4V this time, and we reacted at 700 °C for two hours. About the sample which returned to room temperature after heating, sonication was carried out for 0.5 hours, and it was dried after having processed washing with 30 ml of water several times after that.

Structure analysis was performed for XRD and XAFS measurements. In this way, the structural change before and after reaction of the electrochemical reduction was examined at the same sample. In the XRD analysis, three kinds of samples were conducted at only WB sample, WB with heat at 700 °C and WB with heat at 700 °C on -1.4V for two hours. WB is oxide comprised of silicate. Therefore it is thought that a [MO +2 e- -> M + O2 -] reaction is promoted by making reduction potential. The element constituting WB is Si, Fe, Al, Mg, Cs, K, Ti, and Ca. When Fe, Al, the ionization tendency of the Ca are compared in that, Fe (-0.447V), Al (-1.662V), Ca (-2.76V) and Ca show the biggest ionization tendency. It is known to be resolved in the cathode by the small order of the ionization tendency. It was considered whether a reduction reaction of Fe was controllable selectively by carrying out a molten salt electrochemical reaction by reduction potential at -1.4V this time. As for Fe in WB, 3 values is dominant. Hematite (Fe2O3) produces wounds in a Cs removal process as it was already shown in the first step to a discussion, but this Fe is 3 values, too. If Fe can be returned to 2 values by the reduction potential by the molten salt electrochemistry method in a Cs removal process, generation of the hematite may be controlled. As a result, creation can do four kinds of crystallization selectively. A new pattern was confirmed in comparison with WB simple substance in the sample after the reaction at -1.4V / 700 °C by the XRD structure analysis result for two hours. It is different from the case (bottom), and this means that a change occurs to a product provided in comparison with the case that is not reducing atmosphere without taking electric potential. Moreover, local structure analysis by XAFS was carried out, and on study changes from 3 values to 2 values were confirmed about an atomic value change of Fe. The above-mentioned result indicates the possibility of the separation / collection by the electrolysis using the molten salt electrochemistry method. The change of the product by the different electric potential is examined in future.