| Research Abstract |
Acidophilic sulfur-oxidizing bacterium strain NB1-3 was isolated from corroded concrete of sewage treatment plant and identified as Thiobacillus thiooxidans. The growth of strain NB1-3 in a sulfur medium was strongly inhibited by not only nickel but also tungsten. It was found that nickel and tungsten bind to the plasma membrane of strain NB1-3 and inhibit the enzymes involved in energy generation, sulfur dioxygenase and sulfite oxidase, and as a result, inhibit cell growth. Sulfur-oxidizing bacterium strain RO-1 that can grow under alkaline pH was also isolated from corroded concrete and identified as Thiobacillus versultus. Since strain RO-1 exuded Ca^<2+> from concrete blocks added to thiosulfate medium with pH 9.0, suggesting that strain RO-1 is involved in the early stage of concrete corrosion because concrete structures just after construction contain calcium hydroxide and have a pH of 12-13. Thirty-six strains of iron-oxidizing bacteria were isolated from corroded concrete samples obtained at eight sewage treatment plants in Japan. Since not only T.thiooxidans but also T.ferrooxidans can oxidize reduced sulfur compunds and produce sulfuric acid, author first suggested that T.ferrooxidans as well as T.thiooxidans is involved in concrete corrosion. Portland cement bars (4×4×16 cm) containing nickel and tungsten were prepared and these bars were hung from the lid of the a sludge tank manhole and exposed to 5〜80 ppm hydrogen sulfide atmosphere for 1〜5.5 years. It was found that the surface of the portland cement containing nickel and tungsten was especially smooth, while that without these heavy metals was brittle and rough. From the results, author first suggested that nickel and tungsten are useful as antimicrobial reagents to protect concrete corrosion.
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