M. S.

Microbial immobilization of soluble zinc (ZnSO₄ ) as ZnS was investigated to reduce the possible hazardous effects of soluble Zn in aquatic systems. Static three-phase microcosms were designed to simulate certain characteristics of natural systems in the laboratory. Microcosms (40, 15, 15 cm in length, width and height, respectively) contained water and sediments collected from a stream which had received metal-rich fly ash effluent from a coal burning power plant. To enhance sulfide production by sulfate reducing bacteria, an organic carbon source (1% cellulose g^-1 dry sediment) and 200 ppm sulfate were added into the microcosms. The rates of microbial respiration were determined by the analyses of carbon dioxide, oxygen, methane and hydrogen sulfide in the head space and water using gas chromatography. Selective chemical leaching techniques were applied to determine the fate of the added Zn as ZnSO₄ in the sediments using atomic absorption spectroscopy. Cellulose amendment resulted in elevated RQ values (M CO₂/M O₂ ) and strongly reduced sediments. Greater volatile sulfide production was observed in these cellulose amended microcosms. The absolute amount of Zn as ZnS in reduced microcosms was significantly greater (p<0.01) than that of non-cellulose amended controls. The amount of Zn as ZnS accented for over 90% when a sufficient amount of sulfate was present. When sulfate limitation was observed, however, the amount of Zn as ZnS accounted for approximately 50% of the total Zn in the sediments. The rest of the added Zn which could not bind with sulfide was mostly recovered as a loosely-bound exchangeable Zn. These results suggest that the addition of a suitable organic carbon source into the fly ash effluent receiving stream would remove soluble Zn from water and consequent precipitation of insoluble ZnS in the sediments. / M.S.