Elution behaviour and environmental impact of heavy metals and rare-earth elements from volcanic ashes of Mt. Oyama, Miyake Island

摘要

To obtain information on the environmental impact of materials eluted from volcanic ashes of Mt. Oyama, Miyake Island, which erupted in July 2000, the dissolution behaviours of heavy metals and rare-earth elements from the volcanic ashes were examined. The most important characteristic of the Mt. Oyama eruption is that sulphur dioxide (SO_2) gas has been continuously released, and all persons living on Miyake Island have been required to evacuate. To estimate in terms of the volcanic eruption using SO_2 gas, the ash nature in Mt. Usu, Hokkaido, was also examined and compared with that in Mt. Oyama. When rain water mixed the ashes, the water from the ashes of Mt. Oyama became acidic because of the sulphuric acid. Therefore, SO_2 gas in Mt. Oyama can accelerate the dissolution of protons and heavy metals in the ashes, whereas the rain water in Mt. Usu was not acidic and the dissolution of the heavy metals was not so evident compared with that in the case of Mt. Oyama. With this sulphuric acid, heavy metals such as As, Cd, Pb and Hg in the ashes in Mt. Oyama easily dissolved owing to the low pH. The ashes in Mt. Oyama had been released for eight years and the amount of fallen ashes was estimated to be 33 billion tons. The weights of the harmful heavy metals in the volcanic ashes, such as As, Cd, Pb and Hg, were estimated to be 3.8 × 10~2, 1.3 × 10~3, 1.1 × 10~3 and 29 kg, respectively, and these heavy metals were dissolved and diluted in seawater. Therefore, the concentration and species (chemical form) of these metals should be carefully monitored in the future. Moreover, SO_2 gas, which has a direct effect on human health and has been monitored continuously, causes other effects, such as facilitation of metal ion elution and rock aeration.