The role of sulfate-reducing bacteria on insolubilization of heavy metal ions at waste disposal sites

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Distribution of sulfate-reducing bacteria (SRB) at two sea-based solid waste disposal sites and seven landfill sites were conducted from 1993 to 1995, resulting that SRB widely distributed in the sites. SRBs were accumulated and insolubilization experiments using several heavy metal ions were performed. Insolubilization rate of hexavalent chromium was very low of 16%, neverthless, Cd, Cu, Fe, and Zn ions were precipitated within 48 hr reaction with hydrogen sulfide produced by SRBs. In case of Cd ion, produced precipitate was determined as cadmium sulfide from SEM and EDX analysis. The role of SRBs for insolubilization of heavy metal ions in waste disposal sites turned evident. Among SRBs, Desulfotomaculum sp. was isolated. Methane-producing bacteria (MPB) were also widely distributed in sites and MPB number counted from boring-pits samples were larger than those of from leachates. If samples contained 20mg/l or more of sulfuric ion concentration, SRBs number were greater than MPB number. Two kinds of MPB were isolated. One was Methanosarcina sp., which utilized both methanol and acetic acid and the other was thermophilic Methanomicrobium sp. The difference between MPB and other bacteria was confirmed by fluorescence microscopy using 420nm of exciting wavelength, but it was difficult to differ MPB from SRBs by fluorescence microscopy.