ABSTRACT

Laboratory experiments on leaching of lanthanoids (Ln's) from andesitic rocks by acidic aqueous solutions were carried out in order to find some clue to better elucidation of Ln patterns of acidic hot spring waters in nature. Ln's are different from major elements but phosphorus (P) in behavior upon leaching: While leaching of the major elements continued even at the reaction time (t_R) of 64 h, that of Ln's seemingly finished by t_R=2 or 4 h. The behavior of P upon leaching was dissimilar to that of other major elements but rather resembled that of Ln's. This indicates that a part of each Ln may exist as phosphates in rocks. Light Ln's were found to be leached out by acidic waters more easily than heavy Ln's. This could be due to the difference in ionic radii of Ln's and/or be attributed to the difference in resistance to dissolution by acidic waters among various Ln-containing minerals in the rocks used in the present experiments. The results obtained are successfully applied to the elucidation of the Ln patterns of the waters from the Kusatsu-Yubatake hot spring, the Kusatsu spa, Gunma and from the Obuki hot spring, the Tamagawa spa, Akita.