Total arsenic and arsenic species in the acid hot spring water at Iwoyama volcano of Ebino-kougen

Abstract

In order to understand the behavior of arsenic in magmatic fluid, concentrations of total and different chemical species of arsenic were quantitatively analyzed for the hydrothermal waters from the two phreatic eruption vents and low temperature spring water near the vents of Iwo-yama, one of the volcanoes of Kirishima. Total arsenic concentrations of hydrothermal waters from the two vents had been ＞3000 ppb for 22 months at the maximum after the eruption, then dropped to ＜500 ppb. Arsenite was dominant in the former high-arsenic containing waters, while arsenite and monothioarsenate were dominant in the latter low-arsenic containing waters. Arsenic concentrations of low temperature spring water were ＜80 ppb, and arsenate was the dominant species.Arsenite is the primary dissolved species of arsenic in the volcanic fluid of Iwo-yama. Presence of abundant elemental sulfur in the suspended matters and stoichiometric variation of arsenite and monothioarsenate suggest that the monothioarsenate was synthesized in the low-arsenic containing hydrothermal fluids trapped on the top of vents.