Biomineralization of radioactive sulfide minerals in strong acidic Tamagawa Hot Springs
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Bioaccumulation of radioactive sulfide minerals by bacteria in strong acidic hot spring water was found at Tamagawa Hot Springs, Akita prefecture in Japan. The hot spring water produces Hokutolite of radioactive minerals containing high radium and radon. The β-ray measurements of sediments and biofilms indicate 1850-2420 and 5700 cpm, respectively, which are 50-100 times higher than that of the water and the air (50-90 cpm). The characteristics of hot spring water show pH (1.2), Eh (140 mV), EC (29mS/cm), DO (0.8 mg/l), and water temperature (99.5℃), indicating extremely strong acidic and reducing conditions. The hot spring water contains mainly HCl associated with high concentrations of Ca^<2+>, Al^<3+>, Fe^<2+>, HSO_4 and SO_4^<2->. SEM-EDX and TEM demonstrate some insight into how microorganisms affect the chemistry and microbiological characteristics of the strong acidic surroundings with high S, As, Ba, and Ca contents in biofilms. Especially SEM-EDX, ED-XRF, and STEM-EDX elemental content maps illustrate the distribution of sulfur-bearing compounds of barite (BaSO_4), gypsum (CaSO_4・2H_2O), elemental sulfur (S) and orpiment (As_2S_3) in the reddish orange biofilms. The presence of a hydrogen sulfide-rich (H_2S) thermal spring and gypsum deposits suggest the volatilization of H_2S from the spring water, oxidation of the H_2S gas to sulfuric acid, and reaction of the sulfuric acid. TEM micrographs of bacteria in the biofilms reveal in detail the intimate connections between biological and mineralogical processes that the cells are entirely accumulated with spherical grains, 100〜200 nm in diameter. The relationship among sulfide minerals, such as barite, gypsum, sulfur, orpiment, and Hokutolite, associated with bacteria implies that heavy metals have been transported from strong acidic hot spring water to sediments through bacterial metabolism. It is possible that the capability of radioactive sulfide biofims for heavy metal immobilization can be used to counteract the disastrous effects of radio nuclide polluted water and radio pharmaceutical medical treatment.
- The science reports of the Kanazawa University
The science reports of the Kanazawa University 49(1/2), 1-24, 2004