Metal Accumulation in Biofilms on Stream Sediments Downstream of Abandoned Coal Mines near Taebaek, South Korea

Coal mines have been operated near the city of Taebaek with maximum production during the 1980s. They were closed in 1990s when the coal mining became unprofitable, with the financial support of Korean government. Since the coal mine closures, some downstream sediments became covered with biofilms showing colors from light gray/light brown to light orange, being an environmental concern for the local residents. The pH conditions, however, were close to neutral in the stream water. The acid mine drainage from the upstream seemed to have been neutralized while flowing on the limestone bedrock. We collected samples of stream sediments, some with biofilms and others excluding the biofilms. When metal contents were analyzed, the biofilm-retaining samples were several times more concentrated with metals, such as zinc and copper, than the samples without biofilms. We also collected biofilm samples, within which we observed aggregates or thin layers of mineral particles. Dark mineral aggregates showed higher zinc contents than other parts. We also present mineral phases identified in the biofilm samples using synchrotron-based X-ray diffraction and electron probe micro-analysis. Under circumneutral conditions, biofilms formed on the stream sediments appear to have entrapped heavy metals from water and accumulated in minerals, presumably by microbial activities. With complete understanding of the metal accumulation mechanism, the natural system can be mimicked and applied in mine or waste water treatment to remove heavy metals.