Abstract
Up to 100 ppm of As was found in siliceous sinters from Nakabusa Hot Springs, despite a low As concentration (0.16 mg/l) in sinter-depositing thermal waters. In the sinters, As is strongly correlated with Ca, but not with Al, which is considered as being capable of adsorbing As. Evidently As-adsorbing sites on Al were blocked by Na and Ca ions. Excess Ca in the sinters may be present as CaCO3, as the waters are supersaturated with aragonite. CaCO3 behaves as an effective collector of finely dispersed As precipitates, thus leading to the formation of As-enriched sinters. This conclusion is supported by the fact that Ca and As were dissolved simultaneously by mild acid leaching of powdered sinter samples.
