Non-destructive speciation of iron sulfides formed by sulfate-reducing bacteria in river sediments and estimation of the formation process.

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In this study, various investigations were performed in order to clarify the contribution of sulfate-reducing bacteria to the reaction mechanism of iron-sulfur system in estuarine sediments. At first, vertical distributions of sulfate-reducing bacteria and pyrite (FeS_2) in the sediments were examined in several estuarine areas including Tama-river, which is a major and typical urban river. For the detection of pyrite, ^<57>Fe Moessbauer spectroscopy was applied, which is one of the most useful non-destructive means for characterizing the chemical states of iron. The number of living sulfate-reducing bacteria was calculated by a colony counting method on an agar plate. Sediments of Kitajukken-river with large human impact and those of Yatsu-tideland with little human impact were also investigated. The results showed that the vertical distribution of living sulfate-reducing bacteria and that of pyrite agreed well in all areas.Next, the incubation experiment was carried out using sulfate … More -reducing bacteria (Desulfovibrio sp.) isolated from estuarine sediments of Tama-river. The bacteria were incubated in the medium containing sulfate ion and three levels of concentration of ferrous ion (40 ppm,80 ppm and 200 ppm) at 30 degrees under an anaerobic condition. After an appropriate incubation time, the precipitate produced in the medium was separated by centrifugation and bioactivity was stopped by freezing, and then the Moessbauer spectrum was measured. The level of iron concentration affected the chemical change of the products, suggesting that the incubation products with the aid of sulfate-reducing bacteria could not be explained in terms of the simple inorganic reaction of a sulfide ion with a ferrous ion. The result of X-ray absorption fine structure measurements showed that iron atoms were coordinated by sulfur in the incubation products after several days, whereas they were coordinated by oxygen in the initial precipitate. These facts strongly suggest that the sulfate-reducing bacteria are responsible for the pyrite formation in estuarine sediments. Less