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On-site remediation of metal-contaminated sediments using solid-bed bioleaching technology

In the restoration strategies of river and lake, besides controlling pollution sources and building sewer system, the contaminated sediment may need dredging from the rivers or lakes. Most of sediment dredged from contaminated rivers or lakes often contain substantial amount of heavy metals and thus can not be disposed of on the land and in the water body without any treatment. To date, there are relative few researches for detoxification and decontamination processes of heavy metals in aquatic sediments in Taiwan. In future, it is important to develop the techniques for treatment of the large quantity of dredged sediments in the remediation of contaminated rivers or lakes. Sulfur cycle conversions do not only involve the elemental sulfur, but also directly influence organic matter, nitrogen and heavy metal conversions and fluxes within natural or man-made ecosystems. Therefore, one can also use the sulfur cycle as the driving force behind some specific related environmental biotechnological applications. Sulfur-oxidizing bacteria utilize reduced sulfur as an energy source for chemolithotrophic growth, producing soluble metal sulfates and sulfuric acid. A bioleaching process uses these biological oxidation processes of sulfur-oxidizing bacteria to extract and concentrate metals from polluted soils, sediments or solid waste. The purpose of this study was to develop a microbial process of sulfur cycle for the bioremediation of metal-contaminated sediments. In this project, a bioleaching process for on-site remediation of metal-contaminated sediments was studied. The results showed that the rates of pH reduction, ORP increase, sulfate production and metal solubilization obtained in the solid-bed bioleaching experiment with chloride salts (medium C) were higher that those with sulfate salts (medium S). Based on the results obtained from this study, it was found that the medium C was more suitable for sulfur-oxidizing bacteria in the solid-bed bioleaching process. On the other hand, the rate and efficiency of metal solubilization were enhanced by increasing the flowrate of process water sprinkling into the solid-bed bioreactor. After 40 days of reaction time, the highest efficiencies of heavy metals leached from sediments in this solid-bed bioleaching process were 50%, 60%, 22% and 5% for Zn, Ni, Cu and Pb respectively. Meanwhile, the treated sediment was stable and the residual heavy metals were no longer harmful to the environment after the bioleaching process. Meanwhile, it was found that the sold-bed bioleaching process is not suitable for the treatment of clay-like contaminated sediment.
Sediment; Heavy Metal; Bioleaching; Sulfur-oxidizing bacteria; Solid-bed