A stepwise change of frayed edge site content in biotite in response to the gradual release of potassium from the interlayers
Ogasawara, Sho | Nakao, Atsushi | Yanai, Junta
Radiocesium (RCs) is selectively adsorbed on weathered micaceous minerals (mica) in soils. Although it is clear that weathered mica has selective adsorption sites for RCs, which have been called ‘frayed edge sites (FES),’ the relationship between the degree of mica weathering and the FES content has not been fully investigated. To evaluate the effect of mica weathering on its FES content, we investigated the changes in the FES content with the release of K⁺ from biotite samples by using sodium tetraphenylborate solution. The FES content was estimated from radiocesium interception potential. The vermiculitic layer charge (Vt charge) was also determined as an indicator of the degree of mica weathering. The amount of K extracted from biotite increased from 154 to 803 mmol kg⁻¹ as the condition of the K extraction was more intensive (i.e., longer time, lower solid/liquid ratio, and higher temperature). As K⁺ was removed to a greater extent, the FES content increased from 3.96 to 11.5 mmol kg⁻¹, whereas the Vt charge value increased from 17.1 to 329 mmol kg⁻¹. At the earlier stage of mica weathering, the formation of FES was proportional to the increasing amount of K⁺ released and to the Vt charges. However, at the later stage of mica weathering, when vermiculite was detected by an X-ray diffraction analysis, FES was not necessarily increased in proportion to the increase in K⁺ released and the amount of Vt charge. These findings indicated that although mica weathering largely increased the FES, the increase was not continuous throughout the weathering stage but evident at the earlier stage of weathering.
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