Characterization and Remediation of Iron(III) Oxide-rich Scale in a Pipeline Carrying Acid Mine Drainage at Iron Mountain Mine, California, USA

摘要

A 3.4 km pipeline carrying acid mine drainage to a treatment plant at the Iron Mountain Mine Superfund Site (California, USA) develops substantial scaling, resulting in occasional spillage and requiring periodic, costly clean-out. Samples of scale and water were collected from four points along the pipeline and water was collected from the mine portal influent. Mineralogy of the scale samples consisted of primarily hydrous ferric oxides including schwertmannite and goethite based on powder X-ray diffraction, wet chemical extractions, and scanning electron microscopy. Laboratory batch experiments with fresh, unfiltered water from the site showed that the scale was formed by microbial Fe(II) oxidation and precipitation of Fe(III) phases. A remediation strategy of lowering the pH of the influent water from 2.7 to 2.0-2.3 was tested using a geochemical model and laboratory batch experiments. Decreasing the pH did not substantially retard the rate of Fe(II) oxidation, but prevented precipitation of Fe(III) phases, suggesting that mixing the pipeline water with low pH water (pH ≈ 1) from the Richmond mine portal may prevent scale formation.